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University of Groningen
Catalytic inulin conversions to biobased chemicalsBoy Arief Fachri, B
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i
Catalytic inulin conversions to biobased chemicals
Boy Arief Fachri
ii
Catalytic inulin conversions to biobased chemicals
Boy Arief Fachri
PhD Thesis
University of Groningen
The Netherlands
The work described in this thesis was conducted at the Department of Chemical Engineering,
University of Groningen.
This PhD project was financially supported by the Directorate General of Higher Education of
the Republic of Indonesia
Cover design by Boy Arief Fachri
Layout by Boy Arief Fachri
ISBN: 978-90-367-7968-5
ISBN: 978-90-367-7967-8 (electronic version)
iii
Catalytic inulin conversions to biobased chemicals
PhD thesis
to obtain the degree of PhD at the University of Groningen on the authority of the
Rector Magnificus Prof. E. Sterken and in accordance with
the decision by the College of Deans.
This thesis will be defended in public on
Tuesday 23 June 2015 at 11.00 hours
by
Boy Arief Fachri
born on 1 September 1974 in Jakarta, Indonesia
iv
Supervisor Prof. H.J. Heeres Co-supervisor Dr. C.B. Rasrendra Assessment committee Prof. A.A. Broekhuis Prof. F. Picchioni Prof. G.J.W. Euverink
v
Table of Contents
1 Introduction .................................................................................................................... 1
2 Experimental and kinetic modeling studies on the sulphuric acid catalysed conversion
of D-fructose to 5-hydroxymethylfurfural and levulinic acid in water ........................ 29
3 Experimental and modeling studies on the uncatalysed thermal conversion of inulin
to 5-hydroxymethylfurfural and levulinic acid ............................................................. 62
4 Experimental and modeling studies on the acid-catalysed conversion of inulin to 5-
hydroxymethylfurfural in water ................................................................................... 86
5 Experimental and modeling studies on the conversion of inulin to 5-
hydroxymethylfurfural using metal salts in water ..................................................... 111
Summary ......................................................................................................................... 142
Samenvatting .................................................................................................................. 145
Acknowledgement .......................................................................................................... 148
List of publications and presentation ............................................................................. 150
1
1 Introduction
2
Abstract In this chapter, the use of biomass as an alternative resource for the production of
biofuels, biobased chemicals and biobased performance materials will be discussed. This
chapter also introduce platform chemical concept and provide an overview of 12
potentially very attractive platform chemicals. 5-Hydroxymethylfurfural is one among
the 12 and is considered a “sleeping giant”. Synthetic procedures to produce HMF from
biomass sources and particularly C6 sugars will be reviewed. The potential of inulin, a
natural biopolymer, for HMF synthesis will be discussed and finally the objective and
scope of the thesis will be provided.
3
1.1 Biomass: drivers, composition and applications
Fossil fuels like oil, coal and natural gas are the prime energy sources in the world
(Figure 1). Yet, their use is under pressure due to the fast depletion and environmental
concerns regarding the emissions of CO2 [1]. As such, there is a need for the
development of alternative, renewable resources for primary energy generation.
Currently, renewables like hydro-electricity, wind, solar and biofuels only supply about
13 % of the primary global energy demand (Figure 1).
Fig. 1 World energy supply [2].
One of the sustainable energy resources is biomass. Biomass is abundantly available on
our planet and has a good application potential. Biomass resources can be categorized
in many ways, a possible distinction involving: i) agricultural residues (e.g. agricultural
processing and production waste, crop residues) ii) forestry products (e.g. wood and
logging residues) and iii) energy crops (e.g. grasses, sugar, starch and oilseed crops) [3].
The annual global biomass production is estimated at 1.70 x 109 ton [4].
Lignocellulosic biomass (wood, grasses) consists mainly of carbohydrates and lignin, with
variable amounts of proteins, oils, minerals and other minor components [5]. The
carbohydrates are generally present in the form of biopolymers like, cellulose,
hemicellulose and starch [6]. The amount of the major biopolymers in the biomass
32%
21%
27%
6%
2%
1%
10%
oil
natural gas
coal
nuclear
hydroelectric
other renewable
biofuels and waste
4
source is determined by the biomass type (species, type of plant tissue), stage of
growth, and growth conditions. Lignocellulosic biomass typically consists of 30–60 % of
cellulose, 20-40 % of hemicellulose and 10-30 % of lignin [5].
Cellulose is a linear polysaccharide consisting of repeating glucose units. It is present in
all plant matter. Wood pulp is a cheap source for cellulose [7, 8]. Hemicellulose is a
branched, relatively low molecular weight polysaccharide containing both C5 (arabinose,
xylose) and C6 (galactose, glucose, mannose) sugars.
Starch is a well-known biopolymer with a wide range of applications in both the food
and non-food industry. It is the main component in grains like corn, rice and wheat.
Starch mainly consists of glucose molecules, which are linked together by glycosidic
bonds. In 2004, about 60 million tons of starch were produced globally [5] and this
amount increased to about 75 million tons in 2012 (Figure 2).
Fig. 2 Global starch production [9].
Biomass may not only be used for primary energy generation but it is also a valuable
feed for the production of biofuels and biobased products. Well known examples of
biofuels from biomass are first generation products, like bioethanol, from sugar feeds
like sugar beets and sugar cane and biodiesel from triglycerides like pure plant oils and
waste cooking oils. This thesis however, is not considering the use of biomass for
biofuels production but particularly focusses on the conversion of biomass to biobased
60
62
64
66
68
70
72
74
76
2008 2009 2010 2011 2012
star
ch p
rod
uct
ion
m, m
io t
on
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year
5
products. A biobased product is defined as a product wholly or partly biobased (derived
from biomass) including biobased chemicals, biobased plastics and biocomposites.
Biobased chemicals are already established products on the market. Examples include
products from the oleochemical industry, starch and cellulose derived products
(chemically or physically modified) and fermentation products like ethanol, high corn
syrup and citric acid. The total global biobased chemical and polymer production level is
estimated to be around 50 million ton per year.
However, major developments are required to replace substantial amounts of the
current petro-chemical products derived from fossil resources by biobased products. In
this respect, two approaches can be considered. The first approach involves
modification of biopolymers like starch, cellulose or proteins while keeping the
biopolymer intact. Examples of this approach are starch modifications by e.g oxidation
to obtain high added value starch products. An alternative approach involves break
down of the bio-polymers to low molecular weight building blocks, also known as
platform chemicals or biomass building blocks. These are subsequently converted to
secondary products, intermediates and finally to end products for introduction in the
market. The value chain from biomass via platform chemicals to end products is given in
Figure 3. An example of a platform chemical is lactic acid, which can be obtained from
the fermentation of D-glucose. Lactic acid is a precursor for polymers like polylactic acid
and solvents (lactate esters).
6
Fig. 3 From biomass to biobased chemicals using the platform chemical concept [4]. (Reproduced with permission).
Researchers from NREL and PNNL have conducted an extensive study to identify sugar
based building blocks from lignocellulosic biomass [10]. About 300 candidates were
screened and 14 were selected by iterative processes. These processes take various
criteria into account (Table 1 and Figure 4). The list is intended to provide guidance to
technology development and commercialisation opportunities.
Table 1 Potential building blocks from sugars.
Compounds Compounds
1,4-diacids (succinic, fumaric and malic) itaconic acid
2,5-furan dicarboxylic acid levulinic acid
3-hydroxy propionic acid 3-hydroxybutyrolactone
aspartic acid glycerol
glucaric acid sorbitol
glutamic acid xylitol/arabinitol
7
Fig.4 Potential building blocks from sugars. 1.2 A sleeping giant: 5-hydroxymethyl furfural (HMF)
As is evident from the top 12 list above, furanics like furfural, hydroxymethylfurfural and
2,5-furandicarboxylic acid (FDCA) are considered attractive biobased molecules from
biomass with good application potential. All three are accessible from the carbohydrate
fraction in ligno-cellulosic biomass. Furfural is obtained from the C5 sugars in e.g.
hemicellulose, whereas the synthesis of HMF and FDCA requires a C6 sugar source
(Scheme 1)[11].
8
Scheme 1 Furanics from the carbohydrate fraction of lignocellulosic biomass.
This thesis focuses on the synthesis of HMF from biomass sources. HMF was selected as
it is among others a precursor for a wide range of interesting derivatives (Figure 5) [12].
9
Fig. 5 Relevant HMF and LA derivatives.
HMF is a solid at ambient conditions and highly water soluble. Some selected properties
of HMF are given in Table 2. Naturally, HMF is stored in a number of plants, such as
magnolia vine (Schisandra) [13], cornelian cherries (Cornus officinalis) [14] and the
marine red algae (Laurencia undulata) [15].
Table 2 Selected physical properties of HMF.
Properties Values
Molecular weight 126.11 Dalton Melting point 28-34
oC
Boiling point 114-116 oC (at 1 mmHg)
Density 1.243 g/mL (at 25 oC)
Refractive index 1.563 ± 0.02 Surface tension 50.0 ± 3.0 dyne/
cm
10
In recent years, research activities on the synthesis of HMF and its derivatives have
intensified dramatically and a number of reviews have been published [16-18]. This is
also clearly expressed by the number of papers and patents published in the field in the
last century (Figure 6).
Fig. 6 HMF publications and patents versus time [18] (Reproduced with permission).
HMF synthesis routes can be conveniently classified by considering the solvent and
catalyst. Regarding solvents, the following distinction can be made: i) aqueous systems,
ii) mixed solvent systems and particularly biphasic systems involving water, and iii) non-
aqueous systems including the application of ionic liquids. Solvent effects on HMF yields
are pronounced, whereas solvent choice also affects product work-up and
solvent/catalyst recycle streams [19]. Cottier and Descotes [20] divided the catalysts
into 5 categories: organic acids, mineral acids, salts, Lewis acids and solid catalysts
(Table 3).
There is general consensus in the literature that D-fructose is the preferred starting C6
sugar for HMF synthesis when considering product yield. However, for economic
reasons, the use of the much cheaper D-glucose is preferred, though yields are in
general considerably lower than for D-fructose [18]. Main byproducts are levulinic acid
11
(LA) and formic acid (FA), formed by a consecutive hydration reaction of HMF (Scheme
2) and insoluble products, known as humins.
Table 3 Catalysts used for HMF synthesis from C6 sugars.
Organic acids Mineral acids Salts Lewis acids Solid catalysts
oxalic acid
levulinic acid
maleic acid
formic acid
acetic acid
p-toluenesulfonic acid
trifluoracetic acid
H3PO4
H2SO4
HCl, HI,
HBr
Ag3PW12O40 AlCl3
ZnCl2
BF3
B(OH)3
Lanthanide salts
ion exchange resins,
zeolites,
Metal oxidesa: ϒ-TiP,
C-ZrP2O7, ZrP, ZrO2,
TiO2, eHTiNbO5-
MgO, NbOPO4,
a TiP = titanium phosphate, C-ZrP2O7 = cubic zirconium pyrophosphate, ZrP = Zr(HPO4)2, eHTiNbO5 =
exfoliated HTiNbO5, obtained by treating the H-compound with Bu4NOH.
Scheme 2 Simplified reaction scheme for the reaction of C6 sugars to HMF and LA/FA.
Tandem glucose to fructose isomerization/dehydration processes are currently actively
being pursued [21, 22] to obtain HMF in high yields from D-glucose.
1.3 Oligo- and polysaccharides for HMF synthesis
In general, HMF synthesis is centered around the use of monosaccharides as the feed.
However, oligo-and polysaccharides are also interesting substrates and may have
benefits from an economical and environmental point of view. Here, the idea is to
hydrolyse the polysaccharides such as inulin, cellulose and hemicellulose in situ to
monosaccharides such as glucose, fructose and xylose. Table 4 gives an overview of
relevant publications regarding the use of oligo- and polysaccharides as substrates for
12
HMF synthesis using various (catalytic) approaches, excluding inulin. The overview will
be discussed in detail in the next paragraph.
1.3.1 Starch
The use of water as the solvent and a heterogeneous catalyst in the form of TiO2 was
reported by McNeff et al. [24]. HMF was obtained in 15% yield after a reaction time of 2
min (180 oC). The use of an ionic liquid (1-octyl-3-methylimidazolium chloride
([OMIm]Cl)) was reported by Chun et al. [25]. A maximum HMF yield of 30 wt % was
reported, obtained after 1 h reaction time at 120 °C. The HMF yield was shown to be
considerably higher (60%) when CrCl2 was added.
Biphasic liquid-liquid systems have also been reported. Chheda et al. [23] performed the
synthesis of HMF from starch in a biphasic liquid-liquid system consisting of water-
dimethylsulfoxide (DMSO) and dichloromethane (DCM) in the absence of a catalyst.
HMF was obtained in 36% yield at 90% starch conversion (140 oC, 11 h). Bhaumik and
Dhepe [26] reported the use of water-methyl isobutyl ketone (MIBK) in combination
with a solid catalyst (a silicoaluminophosphate (SAPO)). The highest HMF yield was 68
mol%, at 175oC and 6 h of reaction time. Recently, Shen et al. [27] reported the use of a
biphasic water(NaCl)/tetrahydrofuran (THF) system in combination with a Lewis acid salt
in the form of InCl3. The HMF yield was 46 mol% at 200°C and 2 h reaction time.
1.3.2 Cellulose
A number of studies have been reported on the use of cellulose as the feed for HMF
synthesis. The majority of the studies involves the use of (sub-critical) water at elevated
temperatures (> 200°C) in the absence of a catalyst [28-30]. In these cases, the highest
HMF yield was about 12%. The use of HCl as a catalyst appeared to have a positive effect
and Yin et al. [29] reported a higher HMF yield (21 %) than in the absence of a catalyst
(300oC). Shi et al, [31] studied the reaction of cellulose in hot compressed steam in the
presence of NaH2PO4. The highest HMF yield was 30.4 mol%, obtained at 280oC, 3 h and
1.8 MPa.
The use of a biphasic system consisting of NaCl-H2O/THF was reported by Shen et al.
[27] using InCl3 as the catalyst and microcrystalline cellulose (MCC) as the feed. The
13
effect of temperature, reaction time and catalyst intake on the HMF yield was
investigated. The highest HMF yield was 39.7 mol% and obtained after 2 h at 200oC.
The use of ionic liquids, either in the presence or absence of a catalyst has also been
reported (Table 5). A landmark is a paper of Zhang et al. [32] showing a 89% yield of
HMF from cellulose when using (EMIM]Cl as the solvent and CrCl2 as the catalyst (120°C,
2 h). Dutta et al. [33] stated that the use of ionic liquids for the conversion of cellulose
to HMF is the best option considering process economics and sustainability.
Besides pure cellulose, the use of biomass sources with a large proportion of cellulose
like corn stalk have also been reported. Yan et al. [34] reacted corn stalk in the presence
of an homemade catalyst (HCSS) prepared by the hydrothermal treatment of corn stalk
followed by sulfonation. The reaction was performed in an ionic liquid (1-butyl-3-
methylimidazolium chloride ([BMIM][Cl]) at different temperatures, catalyst intake and
corn stalk loading. The highest HMF yield was 44.1 mol%, achieved at 150oC and 30 min
of reaction time.
1.3.3 Polysaccharides other than cellulose, starch and inulin
Yi et al. [35] reported HMF synthesis from the kudzu root extract in a batch reactor. The
highest yield was 34.8 % dwt when the reaction was performed in the presence of HCl
(50 % v/v), an ionic liquid ([OMIM]Cl (40 % w/v), ethyl acetate (10 % v/v) and CrF3 (1 %
w/v) as the catalyst at a temperature of 130oC for 12 h.
Wang et al. [36] used chitin as the source for HMF synthesis using ZnCl2 as the catalyst in
water. The highest HMF yield was 21.9% when using a 67 wt % ZnCl2 solution at 120oC.
Agarose, a linear polymer made up of the repeating unit of agarobiose, which is a
disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose, was also used
as a feed for HMF synthesis. Yan et al. [37] studied the effect of metal halides catalyst
for the conversion of agarose to HMF in a tubular reactor. A range of metal halides i.e.
NaCl, CaCl2, MgCl2, ZnCl2, CrCl3, CuCl2 and FeCl3 was tested (180-220oC, catalyst
concentrations between 0.5 - 5% (w/w), reaction times between 0 – 50 min, and a
substrate concentration of 2% (w/w)). NaCl, CaCl2, MgCl2 showed better performance
14
than the other metal halides. The highest HMF yield (40.7 %) and selectivity (49.1%) was
obtained with MgCl2 at 200oC for a reaction time of 35 min.
Kim et al. [38] reported HMF synthesis from agar in the presence of a solid acid (Dowex
50 wt %) and DMSO as the solvent. The reactions were performed at 110 oC for 5 h. The
HMF yield was 10 % in the absence of a catalyst and improved to 20 % when using CrCl2
as the catalyst.
Bahari et al. [39] reported experimental studies using cider lees containing glucan
polymers (10 % of 12 % of cider lees) for the synthesis of HMF in subcritical water (175–
275°C). The highest HMF yield was 12.5 % mole, obtained at 250 oC and a reaction time
less than 2 minutes.
15
Table 4 Overview of studies on the conversion of oligo/polysaccharides to HMF excluding inulin.
Substrate Concentration (wt %)
Solvent Catalyst Conditions HMF Yield (%)
a
Ref.
Starch 10 3:7 water:DMSO (w/w)/ dichloromethane
None 140oC, 11 h, 36
b [23]
Starch 5 water TiO2 180oC, 2 min, extraction solvent
MIBK 15
b [24]
Starch 10 [OMIm]Cl/0.5 M HCl/EtOAc
None 120oC, 1 h 30 (wt%) [25]
Starch 10 Water-MIBK silicoaluminophosphate (SAPO)
175oC, 6 h 68 (mol%) [26]
Starch 10 NaCl-H2O/THF InCl3 200oC, 2 h 46 (mol%) [27]
Cellulose (MCC)
10 NaCl-H2O/THF InCl3 200oC, 2 h 40 (mol%) [27]
Cellulose 4 water None 280oC, 4 min 12
b [28]
Cellulose 3 water None 300oC 10
b [29]
Cellulose 3 water HCl 300oC 21
b [29]
Cellulose 2 water None 350oC, 8.8 s 11
b [30]
Cellulose 5 Steam 1.8 MPa NaH2PO4 280oC, 3 h 30 (mol%) [31]
Cellulose 17 [EMIM]Cl CrCl2 120oC, 6 h 89
b [32]
Cellulose 5 [BMIm]Cl CrCl3 150oC, 10 min 54
b [40]
Cellulose (MCC)
67 LiCl-DMAc [TMG]BF4 140oC, 1 h, microwave irradiation 28 (mol%) [41]
Cellulose (MCC)
5 [BMIm]Cl Cr([PSMIM]HSO4)3 120oC, 5 h 53
b [42]
Chitin 3 water ZnCl2 120oC, 90 min 22 (mol%) [36]
Agarose 10 water MgCl2 200oC, 35 min 41
b [37]
Agarose 10 DMSO Dowex 50 wt % 110oC, 5 h with CrCl2 20
b [38]
Cider lees (glucan polymers)
10 water None 250oC, <2 min 12 (mol%) [39]
a wt or mol in brackets after entries
b wt% or mol% not provided
16
Table 4 Overview of studies on the conversion of oligo/polysaccharides to HMF excluding inulin (continued).
Substrate Concentration (wt %)
Solvent Catalyst Conditions HMF Yield (%)
Ref.
Corn stalk 4 [BMIM][Cl] HCSS 150oC, 30 min 44
(mol%) [34]
Kudzu root extract
10 [OMIM]Cl CrF3 130oC, 12 h, ethyl acetate 1 %(w/v) 35
(dwt%) [35]
a wt or mol in brackets after entries
b wt% or mol% not provided
17
1.3.4 Inulin as a source for biobased chemicals
Inulin is considered an attractive oligosaccharide for biobased chemicals in general and HMF
in particular [33]. Inulin was first isolated from elecampane (Inula helenium L.) in 1804 [43].
Later, it was found that inulin is present in over 30000 plants worldwide, including 1200
native grasses belonging to 10 families and serves as a reserve polysaccharide. Inulin is a
oligosaccharide consisting of mainly β(2-1) fructosyl fructose units, in some cases capped
with a glucopyranose unit (Figure 7) [44-46]. Branching levels by β(2-6) linkages are very low
and typically less than 5%. The degree of polymerisation (DP) of inulin varies between 2-70
and is a strong function of the plant type.
Fig. 7 Molecular structure of inulin.
Well known sources for inulin are the Jerusalem artichoke (Helianthus tuberosus L.), dahlia
tubers (Dahlia sp L.) [44] and chicory (Cichorium intybus L.) [47]. An overview of the inulin
contents of selected plants is given in Table 5.
18
Table 5 Inulin content (% of fresh weight of some plants)[48].
Source Edible parts Dry solids content Inulin content
Onion Bulb 6-12 2-6
Jerusalem artichoke Tuber 19-25 14-19
Chicory Root 20-25 15-20
Leek Bulb 15-20 3-10
Garlic Bulb 40-45 9-16
Artichoke Leaves heart 14-16 3-10
Banana Fruit 24-26 0.3-0.7
Rye Cereal 88-90 0.5-1 (estimated value)
Barley Cereal not available 0.5-1.5 (estimated value)
Dandelion Leaves 50-55 (estimated value) 12-15
Burdock Root 21-25 3.5-4.0
Camas Bulb 31-50 12-22
Murnong Root 25-28 8-13
Yacon Root 13-31 3-19
Salsify Root 20-22 4-11
Native inulin is typically used in the food industry. It serves as a dietary fiber as it is only
hydrolysed to a limited amount in the stomach and small intestine. In addition, a range of
inulin derivatives with a wide application range have been proposed. Examples include
neutral, anionic and cationic polymer derivatives. Inulin may also serve as a feed for
fermentations to produce bioethanol, lactic acid, succinic acid, etc. Finally, (enzymatic)
hydrolysis allows the production of high fructose syrups [46].
In 2003, the inulin production in the Netherlands was 200 kton versus 600.000 kton in
Belgium [49]. Inulin is not only produced in Europe, and production data have also been
reported for India [50] and China [51].
Inulin may also be an important feed for HMF synthesis. In this respect, it requires the initial
effective depolymerisation to (mainly) fructose followed by conversion of D-fructose to
HMF. A number of publications are available on the use of inulin for HMF synthesis and
these are summarised in Table 6. The entries are separated based on the various catalysts
and solvent combinations used.
Water as reaction medium has been explored extensively. Wu et al. [52] used water-CO2
mixtures (0-11 MPa CO2) for the conversion of inulin to HMF in a batch reactor at 180°C.
Highest HMF yields of 53% were obtained after 1.5 h and a CO2 pressure of 6 MPa. Yields in
the absence of CO2 were lower (38%), indicating that the in situ formed H2CO3 is an active
acid catalyst for the depolymerisation of inulin and subsequent dehydration to HMF.
19
The use of solid catalysts in water has been reported by a number of research groups.
Benvenuti et al. [53] conducted research on inulin conversion to HMF using zirconium and
titanium catalysts in water. The reaction was performed in batch at 100oC. The highest yield
was 65 wt% using titanium catalysts (100oC, 2 h). Further, Carlini et al. [54] synthesised HMF
from inulin using vanadyl phosphate as the catalyst in a batch system at 80oC. The highest
yield was 37 mol% using GaVOP (80°C, 2 h). Wu et al. [55] used niobium acid to perform
catalytic inulin conversions to HMF in water. The highest yield was 43 mol% (155oC, 18 min).
The use of miscible water-organic solvent mixtures has also been reported. Nie et al., [56]
performed reactions starting with inulin to HMF in THF-water mixtures. The reactions were
performed at a temperature of 160 oC for 100 min in the presence of graphite oxide (GO)
based catalysts. The highest HMF yield was 61% for a 3 to 1 THF/water ratio.
The use of water-acetone mixtures was demonstrated by Bicker et al. [57] to produce HMF
from inulin in the presence of sulphuric acid. The highest yield was 78 % at 180oC, 20 MPa
and a reaction time of 120 s.
Further improvements were boosted by using biphasic liquid–liquid solvent systems. Chheda
et al. [23] performed the conversion of inulin in the presence of hydrochloric acid in a two-
phase liquid-liquid system consisting of water–DMSO and MIBK–2-butanol. The highest
selectivity of 77% at an inulin conversion of 98% was obtained at 170 oC and a reaction time
of 5 min.
Moreau et al. [58] reported a study on the dehydration of inulin in a biphasic water-MIBK
mixture using a zeolite (a.o. H-ZSM-5) as the catalyst in batch mode at 165 oC. The maximum
HMF yield was 54% at 90% fructose conversion after 1 h reaction time.
Benvenuti et al. [53] used the water-MIBK system to synthesise HMF from inulin using
zirconium and titanium catalysts. The highest yield was 70 wt% (100oC, 1 h, cubic ZrP2O7,
SCR= 1.8).
Yang et al. [59] performed the conversion of inulin to HMF in a biphasic system consisting of
water and 2-butanol with solid acid catalysts such as hydrated niobium pentoxide and
hydrated tantalum oxide. Hydrated tantalum oxide gave 87 % yield of HMF at temperature
of 60 oC, reaction time of 2.5 h.
20
Table 6 Overview of studies on inulin conversion to HMF.
Cload (wt %) Solvent Catalyst Condition Yield (%)a Ref.
Water/homogeneous catalyst
5 water none 160oC, 4 h 38
b [52]
5 water none 200oC, 1 h 48
b [52]
5 water none 180oC, 2 h 41
b [52]
5 water none 200oC, 45 min 41
b [52]
5 water CO2(6 MPa) 160oC, 4 min 45
b [52]
5 water CO2(9 MPa) 160oC, 4 min 42
b [52]
5 water CO2(4 MPa) 180oC, 2 min 45
b [52]
5 water CO2(6 MPa) 180oC, 2 min 50
b [52]
5 water CO2(11 MPa) 180oC, 2 min 52
b [52]
5 water CO2(6 MPa) 200oC, 45 min 53
b [52]
5 water CO2(9 MPa) 200oC, 45 min 49
b [52]
Water/solid catalyst
6 water Cubic ZrP2O7(SCR=1.8)
100oC, 0.5 h 26 (wt%) [53]
6 water Cubic ZrP2O7(SCR=1.8)
100oC, 1 h 35 (wt%) [53]
6 water Cubic ZrP2O7(SCR=1.8)
100oC, 2 h 36 (wt%) [53]
6 water Ti(PO4)(H2PO4).2H2O (SCR=1.8)
100oC, 1 h 41 (wt%) [53]
6 water Ti(PO4)(H2PO4).2H2O (SCR=1.8)
100oC, 2 h 65 (wt%) [53]
6 water FeVOP (SCR=18) 80oC, 2 h 35 (mol%) [54]
6 water CrVOP (SCR=18.1) 80oC, 2h 35 (mol%) [54]
6 water AlVOP (SCR=19.7) 80oC, 2h 34 (mol%) [54]
6 water MnVOP (SCR=22.2) 80oC, 2h 34 (mol%) [54]
6 water GaVOP(SCR=21.2) 80oC, 2h 37 (mol%) [54]
10 water HNb3O8 (SCR=50) 155oC, 18 min 43 (mol%) [55]
One phase water-organic
10 water-THF Graphite oxide SCR= 2.5 wt%
160oC, 100 min 61
b [56]
n/a water-acetone (90/10)
H2SO4 (10mmol/L) 180oC, 20MPa, 120s 78
b [57]
a wt or mol in brackets after entries
b wt% or mol% not provided
21
Table 6 Overview of studies on inulin conversion to HMF (continued).
Cload (wt %) Solvent Catalyst Condition Yield (%)a Ref.
Biphasic water-organic/catalyst
10 water:DMSO (5/5 w/w) MIBK:2-BuOH (7/3 w/w)
HCl 170oC, 5 min 75
b [23]
10 water:DMSO (3/7 w/w) DCM
none 140oC, 2.5 h 70
b [23]
10 water-MIBK various zeolites (30 wt%)
165oC, 1 h 39 [58]
6 water-MIBK Cubic ZrP2O7
(SCR=1.8) 100
oC, 1 h 70 (wt%) [53]
6 water-MIBK Ti(PO4)(H2PO4).2H2O (SCR=1.8)
100oC, 1 h 67 (wt%) [53]
6 water-2-butanol modified hydrated tantalum oxide
160oC, 2.5 h 87
b [59]
organic solvent/catalyst
10 DMSO Graphite oxide 160oc, 100 min 58
b [56]
10 DMSO SnCl4-TBAB 140oC,2 h 62
b [60]
ionic liquids/no catalyst
5 [HMIm][HSO4] none 80oC,3 h 59
b [40]
8 [BMIm]Cl none 130oC, 20 min 53
b [61]
ionic liquid-catalyst
5 ChoCl Oxalic Acid 80oC,2 h 64
b [62]
a wt or mol in brackets after entries
b wt% or mol% not provided
The use of DMSO in combination with catalysts has also been studied for the conversion of
inulin to HMF. Nie et al. [56] investigated the dehydration of inulin to HMF in DMSO using
graphite oxide catalyst. The yield was 58.2 % when the reaction was performed in an
autoclave reactor at 160oC and 100 min. Tian et al. [60] studied the conversion of inulin to
HMF in a batch reactor set up in DMSO using quaternary ammonium salts in combination
with SnCl4 or SnCl2 as the catalyst. Tetra-butylammonium bromide (TBAB) was found to give
the best results. The maximum HMF yield was 62 % at 140 oC, 10 mol % catalyst of SnCl4–
TBAB (1:1).
Finally, the use of ionic liquids has also been reported, both in the presence and absence of
catalysts. Qi et al. [40] used an ionic liquid ([HMIm][HSO4]) to obtain HMF from inulin in the
absence of a catalyst. The HMF yield was 59 % at a temperature of 80 oC for a 3 h reaction
time. Cai et al. [61] performed the reaction of inulin to HMF in the presence of the ionic
liquid [BMIm]Cl. The yield was 53 % at 130oC for 20 min reaction time. Hu et al. [62] showed
22
the potential of the use of an ionic liquid (choline chloride) in combination with oxalic acid as
the catalyst. The HMF yield was 64% when the reaction was performed at 80oC for 2 h
reaction time.
1.4 Thesis Outline
In this PhD thesis, synthetic (catalytic) methodology for the conversion of inulin to HMF is
reported in water with the objective to obtain HMF in high selectivity and, preferably, also at
high conversions. Water was selected as the reaction medium as it is environmentally
benign, safe, non-toxic, readily available and also because it is a good solvent for both the
desired product HMF as well as inulin feed (solubility > 35 g/L at 100°C)[49].
In Chapter 2, a kinetic study on the acid-catalysed conversion of D-fructose to HMF and
levulinic acid (LA) in aqueous solutions using sulphuric acid as the catalyst is reported. A
kinetic model was developed using the power law approach and this model was used to
optimise the reaction conditions for the highest HMF yield and to select the optimum
reactor configuration for both HMF and LA production
In Chapter 3, experimental and modeling studies on the uncatalysed, thermal reaction of
inulin to HMF in water are reported. Optimum reaction conditions were established for
highest HMF yield and the experimental data were modeled using multi-variable non-linear
regression. The results are compared with those obtained for inulin conversions using
Brönsted acid and possible autocatalytic effects of LA and FA will be discussed.
An exploratory study on the conversion of inulin to 5-hydroxymethylfurfural in aqueous
solution using various Brönsted acids is described in Chapter 4. This chapter involves an acid
screening study, determination of optimum condition and the formulation of a statistical
model.
Finally, an exploratory study on the conversion of inulin to HMF in aqueous solution using
simple salts like metal chlorides and sulphates (Al, Sn, Fe, and Cu) is reported in Chapter 5.
The preferred salt was selected for a subsequent study to establish the effect of process
conditions on the HMF yield. The experimental data were modeled using a statistical model
to quantify the effect of process conditions on HMF yield.
23
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29
2 Experimental and kinetic modeling studies on the sulphuric acid catalysed conversion of D-fructose to 5-hydroxymethylfurfural and levulinic acid in water
B.A. Fachri, R.M. Abdilla, H.H. van de Bovenkamp, C.B. Rasrendra and H.J. Heeres
Submitted to ACS Sustainable Chemistry and Engineering
Part of this chapter has been presented at the XIIth Netherlands Catalysis and Chemistry Conference (NCCC12), Noordwijkerhout, Netherlands, March 2011
30
Abstract
Levulinic acid (LA) and 5-hydroxymethylfurfural (HMF) have been identified as promising
biomass-derived platform chemicals. A kinetic study on the conversion of D-fructose to HMF
and LA in water using sulphuric acid as the catalyst has been performed in batch set-ups. The
experiments were carried out in a temperature window of 140−180°C, using sulphuric acid
as the catalyst (0.005−1 M) and an initial D-fructose concentration between 0.1 and 1 M. A
kinetic model for the conversion of D-fructose to HMF and the subsequent reaction of HMF
to LA was developed including the kinetics for the formation of solid by-products (humins)
using a power-law approach. According to the model, the maximum attainable HMF yield in
the experimental window is 56 mol% (Cfruc= 0.1 M; Cacid= 0.005 M; 166oC) which is close to
the highest experimental value within the range (53 mol%) and considerably higher than
reported for D-glucose. The highest modeled LA yield was 70 mol% (Cfruc= 0.1 M; Cacid= 1 M;
140oC), close to the experimental value of 74 mol%. This LA yield is considerably higher than
found for D-glucose within the range of experimental conditions. The model was used to
determine the optimum reactor configuration for highest HMF and LA yields and it was
shown that the highest HMF yields are attainable in a PFR reactor, while a large extent of
backmixing is favorable when aiming for a high LA yield.
Keywords: D-fructose, HMF, levulinic acid, kinetic modeling
31
2.1 Introduction
The steady growth of the use of fossil resources has led to higher prices for fossil energy,
fuels and petrochemical products as well as environmental concerns related to CO2
emissions. As a result, the use of renewables is receiving high attentention [1-10]. Biomass is
considered a very attractive renewable source for the production of bioenergy, biofuels and
biobased chemicals. Biomass is produced at an estimated rate of 170 billion metric tons per
year by photosynthesis, though only 3-4 % is used by humans for food- and non-food
applications [11-14].
Lignocellulosic biomass is an interesting source for five-carbon sugars (D-xylose and L-
arabinose) and six-carbon sugars (like D-glucose, D-mannose, D-fructose and D-galactose)
that can be converted into several interesting platform chemicals [2, 11, 13]. 5-
Hydroxymethylfurfural (HMF) and levulinic acid (LA) have been identified as very attractive
platform chemicals derived from C6 sugars. Both HMF and LA may be converted to a wide
range of derivatives with a broad application range (Figure 1) [2, 3, 10, 15, 16].
Fig.1 Derivatives from 5-Hydroxymethylfurfural and levulinic acid.
32
HMF and LA are accessible from the C6-sugars in lignocellulosic biomass by an acid-catalysed
dehydration process. In the first step, the hexoses are dehydrated to HMF which may react
further to form LA together with formic acid (FA). The simplified reaction scheme can be
depicted as follows:
Scheme 1 Simplified reaction scheme for the conversion of C6 sugars to HMF and LA and FA.
A wide range of catalysts (both homogeneous and heterogeneous), solvents and solvent
combinations have been explored and have been reviewed [5-7, 17, 18]. As part of a larger
program to determine the kinetics of the individual steps in the conversion of C6 sugars to
HMF/LA to be used as input for the development of efficient reactor configurations, we here
report a kinetic study on the conversion of D-fructose to LA and HMF. D-fructose is, though
more expensive than D-glucose, the preferred C6 sugar for HMF synthesis as the yields are at
least 5 times higher than for D-glucose [18,19]. Our interest is in the use of water as the
reaction medium and a simple, cheap, recyclable inorganic acid as the catalyst.
Kinetic studies on the conversion of D-fructose to HMF and LA in monophasic solvents
(water, methanol, acetone/water, acetic acid) using inorganic Brønsted acids have been
reported and relevant studies are summarised in Table 1. Kuster et al., [17] explored the
kinetics of the conversion of D-fructose (0.25-1 M) to HMF in water using HCl as the catalyst
(0.25-1 M) at a fixed temperature of 95°C in a batch set-up. The results were modeled using
a mechanistic scheme involving the conversion of D-fructose to an intermediate (X) and
subsequently to HMF. HMF is rehydrated via a second intermediate (Y) to give LA and FA. All
four reactions are assumed to be first order in substrate. In addition, both X and and Y may
also react to form humins, for which the order was a parameter of the kinetic model and
found to be 1.3 for X.
33
Table 1 Overview of kinetic studies on D-fructose conversion to HMF and LA using inorganic Brønsted acids in water and/or organic solvents. CFRC,0 T,
oC Acid Solvent Ref.
0.25-1 95 HCl (0.5-2 M) water [17] 0.05-0.1 M 210-270 HCl(0.016 M) water [20] 0.03 M 210-270 HCl, pH=1.8 water [20] 5-20 %w/v 70-150 HCl, (2.5-10%w/v) water [21] 0.06 M 120-270 H2SO4 (10 mM) methanol [22] 0.2-1 M 170-220 H3PO4 (0.1-0.5 M) MIBK-water [23] 0.05 M 180-300 H2SO4 (5 mM) acetone-water [24]
Asghari et al., [20] investigated the kinetics of D-fructose conversion to HMF and levulinic
acid at elevated temperatures (210-270°C) in subcritical water using HCl as the catalyst. A
reaction network was proposed including a number of reactions (e.g. to furfural) that are not
occurring to a significant extent below 200°C. For modeling purposes, experiments
performed at a fixed HCl concentration (0.016 M) were used and as such the acid
dependency was not taken into account.
Recently, Swift et al., [21] reported an extensive kinetic study on D-fructose (5-20 %w/v)
dehydration to HMF and the subsequent rehydration to LA at relatively low temperatures
(70-150°C) using HCl as the catalyst (pH: 0.7-1.6) in a batch set-up. A reaction network was
developed involving the five tautomeric forms of D-fructose. It is assumed that only the two
furanose forms give HMF by a two step reaction involving a reversible protonation step at
the C2 position followed by an irreversible, kinetically controlled hydride shift (Scheme 2).
Scheme 2 Proposed two-steps dehydration mechanism for fructose to HMF [21].
34
In addition, a separate reaction from D-fructose to FA and humins is incorporated to account
for the observed deviation of the equimolar FA/LA ratio at low D-fructose conversions.
Additional humin formation pathways include direct reactions starting from D-fructose and
HMF. The proposed mechanism is given in Scheme 3. For modeling purposes, a reaction
order of 1 was used for both HCl and D-fructose in all reactions. The conversion rate of D-
fructose is thus equal to the sum of three individual reactions, the primary reaction to HMF
via an intermediate and two pathways leading to humin formation, of which one also forms
FA.
Scheme 3 Proposed mechanism of the acid-catalysed dehydration of D-fructose in water [21].
Three studies on D-fructose conversion to HMF/LA have been reported using sulphuric acid.
However, all use organic solvents or organic solvent-water combinations are performed in
an extended temperature range (up to 300°C). A kinetic study on D-fructose (0.2-1 M)
dehydration using phosphoric acid as the catalyst (0.1-0.5 M) in water was reported by
Kuster et al., [23] in a stirred tank reactor in a temperature range between 170 and 220°C.
The reactions of D-fructose to HMF and humins and the subsequent reaction of HMF to LA
and humins were modeled used lumped kinetic parameters.
35
Based on this overview, it can be concluded that detailed kinetic studies on the conversion
of D-fructose to HMF and the subsequent reaction of HMF to LA using sulphuric acid in
water have not been reported to date. In this paper, a kinetic study on the conversion of D-
fructose to HMF and LA including the rate of by-product formation for a range of process
conditions is reported using a power law approach instead of assuming first order reactions.
With this model, optimal reaction conditions and reactor configurations can be determined
to i) increase the space time yields (kg product/(m3.s) and thus reduce reactor size and ii)
optimise the yields of HMF and/or LA from d-fructose.
2.2 Experimental Section
2.2.1 Chemicals
D-fructose (99%) and LA (≥ 97%) were purchased from Acros Organic (Geel, Belgium).
Sulphuric acid (96-98 %wt) and formic acid were purchased from Merck KGaA (Darmstadt,
Germany). HMF was obtained from Sigma-Aldrich (Steinheim, Germany). All chemicals were
used without further purification. For all experiments, de-ionized water was used to prepare
the solutions.
2.2.2 Experimental procedures
The experimental methods are based on published work by Girisuta et al., [25]. The
reactions were performed in small glass ampoules with an inside diameter of 3 mm, a wall
thickness of 1.5 mm and length of 15 cm. The ampoules were filled with approximately 0.5
ml of reaction mixture and then sealed by a torch. The ampoules were placed in an
aluminum rack which can hold up to 20 ampoules. The rack was placed inside a convection
oven held at a constant temperature (± 1oC).
At pre-determined reaction times, ampoules were taken from the oven and immediately
submersed in a cold water bath to stop the reaction. The ampoules were opened and the
liquid products were collected. Before analysis, the solids were separated from the liquid by
centrifugation using a micro centrifuge (Omnilab International BV, 10-20 min at 1200 rpm). A
sample of the clear solution was diluted with water and analysed by HPLC.
2.2.3 Analytical methods
High performance liquid chromatography (HPLC) was used to determine the concentration
of products in the liquid phase. The HPLC system consisted of a Hewlett Packard 1050 pump,
a Bio-Rad organic acid column (Aminex HPX-87H) and a Waters 410 differential refractive
36
index detector. Dilute sulphuric acid (5 mM) was used as the eluent at a flow rate of 0.55
ml.min-1. The column was operated at 60 oC. The analysis time for each sample was typically
45 min. The HPLC was calibrated with solutions of compounds with known concentrations.
2.2.4 Definitions and determination of the kinetic parameters
The concentrations of the relevant compounds were determined by HPLC. These
concentrations were used to calculate the conversion of fructose (XFRC), the yield of HMF
(YHMF), LA (YLA) and FA (YFA) according to the following equations:
o,FRC
)FRCo,FRCFRC
C
CCX
(1)
o,FRC
o,HMFHMFHMF
C
CCY
(2)
o,FRC
o,LALALA
C
CCY
(3)
o,FRC
o,FAFAFA
C
CCY
(4)
The space time yields for reactions in the batch reactor were calculated using:
STY =CFRC.0XFRC
t (5)
Where t is the batchtime.
Kinetic parameters for the reactions were determined using the software package MATLAB.
A maximum-likelihood approach, based on minimization of errors between the experimental
data and kinetic model was applied. To minimize the error between the measured values
and the model, the Lsqnonlin method was used.
2.3 Results and Discussion
A total of 23 batch experiments was performed in a temperature range of 140°-180°C, initial D-
fructose concentrations varied between 0.1 and 1 M and sulphuric acid concentrations between
0.005 and 1 M were used. A typical concentration versus time profile is shown in Figure 2. Three
main products are observed, HMF as the intermediate and LA and FA as the final products.
37
Fig.2 Typical concentration profile for D-fructose (T = 140 oC, CFRC,0= 1 M,CH2SO4 = 0.5 M)
Some other by-products were also detected in the liquid phase by HPLC (e.g. furfural and D-
glucose). However, their peak areas were low and as such they were not quantified and
further taken into account in the kinetic modeling. LA and FA were in most cases formed in a
close to 1 to 1 molar ratio.
Insoluble by-products known as humins were formed in all experiments, though the amount
was more pronounced at prolonged reaction times. Elemental analysis on a representative
humin sample showed the presence of 65.99 wt% of carbon, 4.55 wt% of hydrogen and
29.46 wt% of oxygen. These values are well within the range for humins produced from C6-
sugars (64-67 wt% carbon and 28-31 wt% oxygen) [26].
2.3.1 Effect of process conditions on D-fructose conversion and product yields.
The effect of the temperature on the D-fructose conversion (initial D-fructose concentration
0.1 M, 0.01 M sulphuric acid) is given in Figure 3. As expected, the temperature has a
profound effect on the reaction rates. Full D-fructose conversion is obtained after 15
minutes at 180 oC whereas it takes about 300 min at 140oC.
38
1 10
0
20
40
60
80
100
0.1M FRC
0.5 M FRC
1 M FRC
XF
RC (
mo
l %
)
Time (min)
Model
Fig. 3 Effect of temperature on D-fructose conversion (CFRC,0= 0.1 M, CH2SO4 = 0.01 M).
The effect of the initial D-fructose concentration on D-fructose conversion is given in Figure
4. The conversion is essentially independent of the loading, an indication that the order in D-
fructose is close to one.
Fig. 4 Effect of D-fructose loading (0.1- 1 M) on D-fructose conversion (T= 160 oC, CH2SO4 = 0.5
M).
39
The effect of sulphuric acid concentration on the D-fructose conversion is presented in
Figure 5 and clearly implies that high acid concentration leads to enhanced D-fructose
conversion rates. For a first order dependency of both acid and D-fructose, a plot of (ln(1-
XFRC))/(CH+) versus the time should give a straight line. This indeed proved to be the case for
the data reported in Figure 5 (R2 = 0.982, figure not shown for brevity) indicating that the
order in acid should be close to one.
Fig.5 Effect of acid concentration on D-fructose conversion (T= 160 oC, CFRC,0 = 0.1 M).
The highest experimental HMF yield within the experimental window was 53 mol%, obtained
at an initial fructose concentration of 0.1 M, a sulphuric acid concentration of 0.01 M and a
temperature of 180 oC. The HMF yield is considerably higher than for D-glucose, for which
Girisuta et al., [25] reported a maximum yield of 5 mol% in water using sulphuric acid as the
catalyst in a similar window of process conditions. Thus, these findings support the general
consensus in the literature that D-fructose is the preferred feedstock for HMF synthesis in
water when using inorganic acids as the catalyst [5].
For LA, the highest experimental yield (74 mol%) was found at a D-fructose concentration of
0.1 M, a sulphuric acid concentration of 1 M and a temperature of 140oC. These yields are
much higher than found for D-glucose in water using sulphuric acid as the catalyst [8]. In the
latter study, the highest LA yield within the process window was 60 mol% (140oC, CGLC,0: 0.1
M, Cacid: 1 M). Thus, these findings indicate that D-fructose is a better source for LA synthesis
than d-glucose when considering product yield. Detailed studies regarding LA synthesis from
0.1 1 10 100
0
20
40
60
80
100 0.01 M H
2SO
4
0.05 M H2SO
4
0.5 M H2SO
4
1 M H2SO
4
XF
RC (
mo
l %
)
Time (min)
Model
40
D-fructose are scarce and the focus is mainly on the use of d-glucose. An overview of
relevant studies for D-fructose conversion to LA in mono-phasic solvents using
homogeneous or heterogeneous catalysts is provided in Table 2 [17], [27], [28]. Yields
between 25 and 52 mol% have been reported, though a clear comparison with our results
using sulphuric acid is cumbersome due to the differences in temperature and types of
catalyst used.
Table 2 Overview of LA synthesis from D-fructose in mono-phasic solvent systems.
Co, wt% Co, M Solvent Catalyst Conditions LA yield Ref.
18 1 water HCl (7.5 wt%) (2 M) 95oC, 24 h 65
a 17
33 3 water amberlite IR-120 (19 wt%) RT, 27 h 24b 27
4 0.022 none LZY-zeolite (50 wt%) 140oC, 15 h 43
b 28
a mole%
bwt%
2.3.2 Kinetic modeling
2.3.2.1 Model development
Based on literature, the acid-catalysed decomposition of D-fructose is expected to follow a
number of series-parallel reaction pathways [21]. For simplification, it is assumed that the
reaction network involves the direct reaction of D-fructose to HMF which will further react
to LA and FA, though this is not necessarily the correct molecular mechanism (vide supra).
These simplifications were included as the kinetic model would otherwise contain a large
number of parameters which limits the predictive value of the model. It is assumed that D-
fructose and HMF also form humins, see Scheme 4 for details.
41
Scheme 4 Reaction network for the conversion of D-fructose to HMF and LA as used in the kinetic model.
The reaction rates of the individual reactions are expressed based on the power law
approach [25] and the rate expressions are given in equations (6) to (9).
aFFRCF1F1 CkR (6)
bFFRCF2F2 CkR (7)
aHHMFH1H1 CkR (8)
bHHMFH2H2 CkR (9)
Kinetic rate constants are defined in terms of modified Arrhenius equations which take into
account the effect of temperature and acid concentration on the reaction rates.
R
RF1
TT
TT
R
E
RF1
F
HF1 expkCk (10)
R
RF2
TT
TT
R
E
RF1
F
HF2 expkCk (11)
R
RH1
TT
TT
R
E
RH1
H
HH1 expkCk (12)
R
RH2
TT
TT
R
E
RH1
H
HH2 expkCk (13)
Here, TR is the reference temperature, which was set at 140 oC.
The H+ concentration in solution was calculated from the sulphuric acid intake and the 2nd
dissociation constant using the following equation:
42
4424442 HSO,aSOH
2
HSO,aHSO,aSOHHKC4KK
2
1CC (14)
Here 4HSO,a
K represents the dissociation constant of HSO4- for which a value of 10-3.6 was
applied [25].
In a batch set-up, the concentrations of D-fructose, HMF and LA as a function of time are
represented by the following differential equations:
F2F1
FRC RRdt
dC (15)
H2H11FHMF RRR
dt
dC (16)
1HLA R
dt
dC (17)
At the start of the reaction, the reaction takes place non-isothermally due to the heating-up
of the contents of the ampoule from room temperature to the oven temperature. To include
this effect in the kinetic model, the temperature inside the ampoule as a function of time
during the heat-up process was determined experimentally. These experimental profiles at
different temperatures were modeled using a heat balance for the contents in an ampoule:
d(MCpT)
dt= UAt(Toven − T) (18)
When assuming that the heat capacity of reaction mixture is not a function of temperature,
rearrangement of equation (18) gives:
dT
dt=
UAt
MCp(Toven − T) = h(Toven − T) (19)
Solving the ordinary differential equation (19) with an initial value T = Ti at t = 0 leads to:
htiovenoven expTTTT (20)
43
The value of h was determined using a non-linear regression method as described by
Girisuta et al., [25] Equation (20) was incorporated in the kinetic model to describe the
temperature of the reactor at the initial, non-isothermal stage of the reaction.
2.3.2.2 Kinetic modeling results
A total of 23 batch experiments at various conditions was used to develop the kinetic model,
each experiment consisting of on average of 24 datapoints (8 samples were taken during the
batch time and the concentrations of D-fructose, HMF and LA were determined). The
experiments were carried out at temperatures between 140 and 180oC, initial D-fructose
concentrations between 0.1 and 1 M and sulphuric acid concentrations between 0.005 and 1
M.
The kinetic constants, orders in reactants and activation energies were determined using the
MATLAB software package by simultaneous modeling of the complete dataset. The results
for the conversion of D-fructose to HMF and humins (Scheme 2) are given in Table 3.
Table 3 Kinetic parameters for D-fructose conversion to HMF and LA using H2SO4. Parameters Value Units
k1RF 1.1 ± 0.1 (M(1-a
F)(-α
F) min
-1)
a
E1F 123 ± 5 kJ mol-1
k2RF 0.55 ± 0.1 (M
(1-bF
)( –αF
) min
-1)
a
E2F 148 ± 12 kJ mol-1
aF 1.006 ± 0.003 bF 1.179 ± 0.06 αF 0.958 ± 0.02 βF 1.056 ± 0.06
a: TR=140oC
The kinetic parameters for the reaction of HMF to LA and humins were initially taken from a
previous study by Girisuta [25]. However, when implementing these kinetic values, the
model fit was below expectations. This can be due to slightly different experimental
procedures or, alternatively, by different interactions of the starting C6 sugars (D-fructose
and D-glucose) with intermediates, e.g. leading to humins. These interactions are currently
not considered in our reaction network and as such not taken into account in the kinetic
models. Therefore, the kinetic parameters for the reaction of HMF to LA and humins were
determined independently using the experimental dataset. The Girisuta data and the best fit
values for the current dataset are presented in Table 4. The values are in reasonable
44
agreement, the main differences are the activation energy for the conversion of HMF to LA
(E1H) and the orders in acid for LA and humin formation (αH and βH).
Table 4 Kinetic parameters estimation for HMF conversion to LA and FA.
Parameters Previous research4 This work Units
k1RH 0.34 ± 0.01 0.38 ± 0.04 (M(1-a
H)(-α
H) min
-1)
a
E1H 110± 0.7 92 ± 5 kJ mol-1
k2RH 0.117 ± 0.008 0.142 ± 0.04 (M
(1-bH
)( –αH
) min
-1)
a
E2H 111± 2 119± 10 kJ mol-1
aH 0.88 ± 0.01 0.89 ± 0.03 bH 1.23 ± 0.03 1.21 ± 0.08 αH 1.38 ± 0.02 1.16 ± 0.02 βH 1.07 ± 0.04 0.90 ± 0.05
a:TR=140oC
Figure 6 and the parity plot in Figure 7 show that the fit between the model and the
experimental values is good. The order in D-fructose for the reaction to HMF was found to
be close to one (1.006 ± 0.003), whereas the order was 1.179 ± 0.06 for the reaction of D-
fructose to humins. These findings are in line with the near independency of the D-fructose
conversion of the initial D-fructose concentration (Figure 4).
The acid dependency of the experimental D-fructose versus batchtime profiles (Figure 6)
indicated that the order in acid for the two reactions involving D-fructose (to HMF and to
humins) should both be close to one and this was confirmed by the kinetic modeling
activities (0.958 ± 0.02 and 1.056 ± 0.06).
45
0 5 10 15 20 250
0.1
0.2
0.3
0.4
Time (min)
Concentr
ation (
mol/L)
ExpNo: 21 [Fructose]: 0.466 M [H2SO4]: 0.9943 M T:140.2 C
Fig.6 Comparison of experimental data (○: FRC,∆: HMF,□: LA) and kinetic model (solid line) for various reaction conditions.
Fig. 7 Parity plot of experimental data and model predictions.
It is of interest to compare the experimentally determined activation energy for the reaction
of D-fructose to HMF with those reported in the literature. Unfortunately, kinetic studies for
the conversion of D-fructose in water using sulphuric acid as the catalyst have not been
reported to date. However, a number of studies have been reported for HCl, see Table 1 for
46
details, and these will be used as a reference. The activation energy for the reaction of D-
fructose to HMF (R1F) was 123 ± 5 kJ/mol. This value is lower than a previous study from our
group for the reaction of D-glucose to HMF in water with sulphuric acid where an activation
energy of 152 kJ/mol was reported using a power law approach and an essentially similar
reaction network as used here [25]. Thus, D-fructose appears to be more reactive than D-
glucose (vide infra). The activation energy for the first order reaction of D-fructose to HMF
for HCl, as reported by Asghari and Yoshida [20], is higher (161 kJ/mol) than observed in this
study. However, direct comparison is difficult as i) the temperature window for the latter
study starts at a considerably higher temperature (> 210°C) and ii) a different reaction model
with three parallel reactions involving D-fructose was assumed instead of the two in our
study, and all three were reported to have different activation energies (101-161 kJ/mol). As
such, the activation energy as determined in this study using sulphuric acid is in the range as
reported by Asghari and Yoshida [20] for HCl. Comparison of our value with the recently
reported activation energy by Swift et al., [21] for the reaction of D-fructose to HMF using
HCl as the catalyst is also not straightforward as a two step dehydration mechanism is
assumed (Scheme 3). However, as the first step is an equilibrium reaction, the apparent
activation energy of the two step mechanism can be calculated using the temperature
dependency of the first equilibrium constant and the known activation energy of the second
step. By using this approach, Swift et al., [21] reported an activation energy of the HCl
catalysed conversion of D-fructose to HMF of 136 kJ/mol at 150°C, which is slightly higher
than found in this study for sulphuric acid.
2.3.3 Model implications for a batch reactor
2.3.3.1 D-fructose conversion rates
The kinetic model allows determination of the conversion of D-fructose and the yields of LA and
HMF as a function of process conditions. As an example, the modeled batch time required for a
D-fructose conversion of 90 mol% at different acid concentrations and temperatures is provided
(Figure 8). In addition, the results were compared with those obtained for a kinetic model using
D-glucose as the feed and sulphuric acid as the catalyst [25].
47
(a) (b) Fig. 8 Required batch time for 90% of C6-sugar (D-fructose and D-glucose) conversion versus temperature for (a) H2SO4 = 0.1 M (b) H2SO4 = 1 M.
As anticipated, the batchtime is a strong function of the temperature, with times up to 1000
min at 100°C versus less than 1 minute at temperatures above 180 °C (0.1 M) to obtain 90%
D-fructose conversion. For all temperatures and acid concentrations within the window, D-
fructose is by far more reactive than D-glucose, and on average a factor of about 100 was
calculated. The conversion rate increases when using higher acid concentrations (compare
Figure 8a and 8b), as expected based on the positive and close to one order in acid in the
kinetic model.
2.3.3.2 Model predictions for HMF
The highest yield of HMF within the design window was calculated and found to be 56 mol% at
166oC, starting with 0.1 M D-fructose and a sulphuric acid concentration of 0.005 M. The
model allows calculation of the HMF yields as a function of process conditions. The effect of
the acid concentration on the yield of HMF (T = 140oC, CFRC,0 = 0.5 M) is given in Figure 9. The
maximum achievable HMF yield is a function of the acid concentration, with lowest acid
concentrations leading to the highest HMF yields. Clearly, this goes at the expense of
reaction rates and the optimum at lowest acid concentration is obtained after about 200
minutes reaction time compared to 1 minute for the highest acid concentration in the range
(1 M).
100 120 140 160 180 200
0.1
1
10
100
1000
10000
100000
batc
h tim
e (
min
)
T (oC)
d-fructose
d-glucose
100 120 140 160 180 200
0.1
1
10
100
1000
10000
100000
batc
h tim
e (
min
)
T (oC)
d-fructose
d-glucose
48
Fig. 9 HMF yields versus time for different acid concentrations (T = 140oC, CFRC,0 = 0.5 M).
The HMF yield shows a minor dependence on the D-fructose loading, see Figure 10 for
details, though a lower intake has a slight beneficial effect (160oC, acid concentration 0.1
M.). The main reason is a slightly higher order in the reactions leading to humins than the
desired main reactions (Table 3 and 4), indicating that humin formation is retarded at dilute
conditions.
Fig 10 HMF yields versus time for different initial fructose intakes (T= 160oC, acid concentration 0.1 M).
49
The effect of temperature on HMF yields within the temperature range 140-180°C is
relatively limited, see Figure 11 for details (CFRC,0 = 0.5 M, C acid = 0.1 M). These findings may
be explained by considering the differences in activation energy of the three main reactions
with the highest kinetic constants at reference temperature (R1F, D-fructose to HMF, R2F, D-
fructose to humins, R1H, HMF to LA and FA). The activation energy for R2F is the highest of
the three (148 ± 12 KJ/mol) and as such humin formation is expected to be favoured at
higher temperatures, leading to a lowering of the HMF yields. The reaction of HMF to LA has
the lowest activation energy (92 ± 5 KJ/mol) and a lowering of the HMF yield is expected at
lower temperatures. Apparently both effects cancel out and as such the yield of HMF are
about constant in the temperature range used in this study. These findings are not in
agreement with the data reported by Swift et al., [5] reported for D-fructose with HCl as the
catalyst. Here, the HMF yield is a strong function of the temperature, with higher
temperatures leading to higher HMF yields. The maximum HMF yield at 150°C (pH = 0.7) is
about 46 mol%. Though comparison is difficult as the initial D-fructose concentration is not
given and substantial amounts of KCl are present, it is in the range as observed for sulphuric
acid in this study. The reasons for this observed discrepancy in HMF yields versus
temperatures between HCl and sulphuric acid may be due to the difference in temperature
window for both studies (70-150°C for HCl versus 140-180°C for sulphuric acid) and the
presence of KCl in the studies reported in ref.[5]. Extended studies in the same temperature
window will be required to draw definite conclusions.
50
Fig.11 HMF yields versus time at different temperatures (CFRC,0 = 0.5 M, acid concentration
0.1 M).
2.3.3.2.1 Model predictions for the space time yield (STY) of HMF
The STY, also known as the volumetric production rate (mol product/(reactorvolume.time), is
an important parameter for process optimization. A high STY is beneficial as it allows for a
reduction of the reactor size, leading to lower investment costs. The STY in the batch reactor
was calculated using eq 5. The highest STY as a function of the temperature and the initial D-
fructose concentration (0.0 1- 1 M) at a fixed acid concentration of 0.005 M is given in Figure
12. The highest STY (combination of highest HMF yield and reaction rates) is found at the
highest temperature (180°C) and highest initial fructose concentrations (1 M) within the
experimental window.
51
Fig. 12 STY of HMF versus temperature and initial D-fructose concentration (acid
concentration 0.005 M).
2.3.3.2.2 Model predictions for LA yield
The model not only allows for determination of the HMF yield and STY versus process
conditions but may also be used for a similar analyses for LA. The highest modeled LA yield
was 70 mol% (Cfruc= 0.1 M; Cacid= 1 M; 140oC) obtained at the highest acid concentration (1
M), lowest temperature (140°C) and the lowest initial D-fructose concentration (0.1 M) in
the range. The experimental yield was 74 mol% at these conditions.
The modeled LA yield versus the batch time at different acid concentrations at a constant
temperature (140°C) and initial D-fructose concentration (0.5 M) is given in Figure 13 and
Figure 14. The highest yield was obtained with the highest acid concentration in the range (1
M). Another advantage of using a high acid concentration is a higher reaction rate, as is
clearly evident from the profiles in Figure 13 (about 5 min to reach max LA yield at 1 M
versus 1000 min at 0.01 M).
52
Fig. 13 LA yield versus reaction time at different acid concentrations (T = 140oC, CFRC.0 = 0.5 M).
The maximum achievable LA yield is also a function of the initial D-fructose concentration,
with lower concentrations giving higher LA yields (Figure 14).
Fig. 14 LA yield versus reaction time at different initial fructose concentrations (T = 140oC, acid concentration 0.5 M).
53
These finding may be explained by considering the orders in the desired reactions (D-
fructose to HMF and HMF to LA) and the undesired humin formation reactions. The orders in
D-fructose and HMF for the desired reactions are lower than for the humin forming
reactions and as such the former are favoured at lower concentration. Thus, higher LA yields
are attainable at lower D-fructose concentrations.
The LA yield versus the batchtime at three temperatures (140, 160, 180°C) is given in Figure
15. Highest LA yield are attainable at the lowest temperature in the range. These findings
may be rationalised by considering that the reaction of HMF to LA/FA has the lowest
activation energy of all other reactions in the network (Table 3 and 4) and as such will be
favoured at lower temperature.
Fig. 15 LA yield versus reaction time at different temperatures (CFRC,0 = 0.1, acid concentration 0.5 M).
2.3.4 Optimisation of HMF and LA yield in stationary continuous reactor configurations
In continuous reactors, the yield of HMF and LA are a function of process parameters (T,
CFRC,0, CH2SO4 and residence time) as well as the extent of back mixing in the reactor. With the
kinetic model available, the yield of HMF and LA in the two extremes regarding mixing, viz a
PFR reactor and a continuously ideally stirred tank reactor (CISTR) were modeled.
54
For the PFR, yields and selectivities were calculated using the differential equations for the
time-concentration profiles (eq. 15-17) and by substituting the residence time for the
batchtime. The yield was calculated using
in
FRC
in
i
out
i
iC
CCY
(21)
and the selectivity by:
out
FRC
in
FRC
in
i
out
i
iCC
CCS
(22)
The optimum conditions were found by step-wise calculating all possible combinations of
starting concentrations (∆FFRC,0 = 0.01 M), temperature (∆T = 1°C) and catalyst
concentrations (∆CH+ = 0.005 M) and determining the optimum yields of HMF and LA under
these conditions
In the case of a CISTR, the reactor design equation for a stationary situation reads:
i
ini
outi
CISTRR
CC (23)
The relation between XFRC and τCISTR is given by:
F2F1
inFRCFRC
CRR
CXISTR
(24)
Substitution of equation (24) into equation (23) followed by some rearrangement gives:
FRCinFRC
F2F1
H2H1F1outHMF XC
RR
RRRC
(25)
FRCinFRC
F2F1
H1outLA XC
RR
RC
(26)
The HMF yield versus the D-fructose conversion at the highest temperature in the range
(180°C) for both reactor configuratons is given in Figure 16.
55
Fig. 16 HMF yield and selectivity versus D-fructose conversion for a PFR and CISTR (180°C, 0.1 M D-fructose feed, 0.005 M sulphuric acid).
As expected, the yield of HMF shows a clear maximum and the highest achievable yield (56
%) is obtained in a PFR, wheras the yield is considerably lower in the CISTR (42 %). Thus, a
low extent of backmixing is favorable for a high HMF yield.
The LA yield versus the D-fructose conversion for the two reactor configurations at optimum
conditions in the range to achieve high LA yields (140°C, 1.0 M sulphuric acid, 0.1 M D-
fructose, vide supra) is given in Figure 17. As anticipated based on the proposed reaction
network, the yield is increased with D-fructose conversion. Backmixing and thus the use of a
CISTR is favoured, in line with the observation that dilute D-fructose solutions are preferred
to reduce humin formation. The yield of LA in a CISTR goes to a theoretical limit of 99% in
very dilute solutions, thus at high D-fructose conversion levels. However, it should be kept in
mind that the reaction rate will reduce considerably at such highly dilute conditions. For
instance, the model predicts a 99% LA yield in the CISTR after a residence times of 1250 (!)
years. Clearly, this leads to an unacceptable low STY, making these conditions unfavorable
for commercial operation.
56
Fig. 17 LA yield and selectivity versus D-fructose conversion for a PFR and CISTR (140°C, 0.1 M D-fructose feed, 1.0 M sulphuric acid). 2.4 Conclusions
A kinetic model for the acid-catalysed reaction of D-fructose to HMF and LA in water with
sulphuric acid at concentrations between 0.005 and 1 M, initial concentrations of D-fructose
between 0.1 and 1 M and a temperature window of 140−180 °C using the power-law
approach has been developed. A maximum-likelihood approach has been applied to
estimate the kinetic parameters for the main reaction to LA and FA and the side reactions to
humins. A good fit between experimental data and the kinetic model was obtained.
The kinetic model implies that different strategies are required to obtain either a high HMF
or LA yield. The highest HMF yield (about 56 mol%) is attainable at a low acid concentration,
whereas the temperature and the initial D-fructose concentration are of less importance. In
addition, a reactor with a small extent of backmixing (PFR) is the preferred reactor
configuration. For highest LA yields (74 mol%), a high acid concentration, low temperature
and low initial D-fructose concentration are favoured and a CISTR is the most suitable
configuration. The model proposed in this work will aid the rational design and operation of
dedicated reactors for the conversions of various types of biomass feedstock to HMF and LA.
57
The results were compared with earlier kinetic studies in our group on the conversion of D-
glucose to HMF/LA in water using sulphuric acid as the catalyst. d-Fructose is two orders of
magnitude more reactive than D-glucose and HMF yields are a factor of 10 higher. In
addition, the LA yields for D-fructose are also substantially higher (about 10 mol%) than for
D-glucose.
2.5 Nomenclature
aF : reaction order of CFRC in the decomposition of fructose to HMF (−)
αF : reaction order of CH+ in the decomposition of fructose to HMF (−)
aH : reaction order of CHMF in the decomposition of HMF to LA and FA (−)
αH : reaction order of CH+ in the decomposition of HMF to LA and FA (−)
At : heat transfer area (m2)
bF : reaction order of CFRC in the decomposition of fructose to humins (−)
βF : reaction order of CH+ in the decomposition of fructose to humins (−)
bH : reaction order of CHMF in the decomposition of HMF to humins (−)
βH : reaction order of CH+ in the decomposition of HMF to humins (−)
CFRC : concentration of fructose (M)
CFRC,0 : initial concentration of fructose (M)
CH+ : concentration of H+ (M)
CH2SO4 : concentration of sulphuric acid (M)
CHMF : concentration of HMF (M)
CHMF,0 : initial concentration of HMF (M)
Ciin : concentration of the ith compound at the inflow (M)
Ciout : concentration of the ith compound at the outflow (M)
CLA : concentration of LA (M)
CLA,0 : initial concentration of LA (M)
Cp : heat capacity of reaction mixture (J g-1 K-1)
E1F : activation energy of k1F (kJ mol-1)
E1H : activation energy of k1H (kJ mol-1)
E2F : activation energy of k2F (kJ mol-1)
E2H : Activation energy of k2H (kJ mol-1)
h : heat transfer coefficient (min-1)
58
k1F : reaction rate constant of fructose decomposition to HMF
k1RF : reaction rate constant k1F at reference temperature
k1H : reaction rate constant of HMF for the main reaction
k1RH : reaction rate constant k1H at reference temperature
k2F : reaction rate constant of fructose decomposition to humins
k2RF : reaction rate constant k2F at reference temperature
k2H : reaction rate constant of HMF for the side reaction to humins
k2RH : reaction rate constant k2H at reference temperature
Ka,HSO4− : dissociation constant of HSO4
- (-)
R : universal gas constant, 8.3144 (J mol-1 K-1)
R1F : reaction rate of fructose decomposition to HMF (mol L-1 min-1)
R1H : reaction rate of HMF decomposition to LA and FA (mol L-1 min-1)
R2F : reaction rate of fructose decomposition to humins (mol L-1 min-1)
R2H : reaction rate of HMF decomposition to humins (mol L-1 min-1)
t : time (min)
T : reaction temperature (°C)
Ti : temperature of reaction mixture at t = 0 (°C)
Toven : temperature of oven (°C)
TR : reference temperature (°C)
U : overall heat transfer coefficient (W m-2 K-1)
XFRC : conversion of fructose (mol %)
YHMF : yield of HMF (mol %)
YLA : yield of LA (mol %)
Greek symbol
τ CISTR : residence time in a of CISTR (min)
2.6 References
[1] Lichtenthaler, F.W., Peters, S. Carbohydrates as green raw materials for the chemical.
C.R. Chimie, 2004, 7, 65-90.
[2] Bozell, J.J., Petersen, G.R. Technology development for the production of biobased
products from biorefinery carbohydrates—the US department of energy’s “top 10”
revisited. Green Chem., 2010, 12, 539-554.
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[3] Wang, T., Nolte, M.W., Shanks, B.H. Catalytic dehydration of C6 carbohydrates for the
production of hydroxymethylfurfural (HMF) as a versatile platform chemical. Green
Chem., 2014, 16, 548-572.
[4] Ragauskas, A.J., Williams, C.K., Davison, B.H., Britovsek, G., Cairney, J., Eckert, C.A.,
Frederick Jr, W.J., Hallet, J.P., Leak, D.J., Liotta, C.L., Mielenz, J.R., Murphy, R., Templer,
R., Tschaplinski, T. The path forward for biofuels and biomaterials. Science, 2006, 311,
484-489.
[5] Van Putten, R.J., van der Waal, J.C., de Jong, E., Rasrendra, C.B., Heeres, H.J., de Vries,
J.G. Hydroxymethylfurfural, A versatile platform chemical made from renewable
resources. Chem. Rev. 2013, 113, 1499−1597.
[6] Sheldon, R.A. Green and sustainable manufacture of chemicals from biomass: State of
the art. Green Chem. 2014, 16, 950-963.
[7] Delidovich, I., Leonhard, K., Palkovits, R. Cellulose and hemicellulose valorisation: An
integrated challenge of catalysis and reaction engineering. Energy Environ. Sci. 2014, 7,
2803-2830.
[8] de Jong, E., Higson, A., Walsh, P., Wellisch, M. Bio-based chemicals value added
products from biorefineries. IEA Bioenergy, Task 42, Biorefinery. 2013.
[9] Dutta, S., De, S., Saha, B. Advances in biomass transformation to 5-
hydroxymethylfurfural and mechanistic aspects. Biomass and Bioenergy, 2013, 55, 355-
369.
[10] Agirrezabal-Telleria, I., Gandarias, I., Arias, P.L. Heterogeneous acid-catalysts for the
production of furan-derived compounds (furfural and hydroxymethylfurfural) from
renewable carbohydrates: A review. Catalysis Today, 2014, 234, 42–58.
[11] Huber, G.W., Iborra, S., Corma, A. Synthesis of transportation fuels from biomass:
chemistry, catalysts, and engineering. Chem. Rev., 2006, 106, 4044-4098.
[12] Petrus, L., Noordermeer, M.A. Biomass to biofuels, a chemical perspective. Green
Chem., 2006, 8, 861-867.
[13] Corma, A., Iborra, S., Velty, A. Chemical routes for the transformation of biomass into
chemicals. Chem. Rev., 2007, 107, 2411-2502.
[14] Girisuta, B., Danon, B., Manurung, R., Janssen, L.P.B.M., Heeres, H.J. Experimental and
kinetic modeling studies on the acid-catalysed hydrolysis of the water hyacinth plant to
levulinic acid. Bioresource Techn. 2008, 99, 8367-8375.
60
[15] Kuster, B.F.M. 5-Hydroxymethylfurfural (HMF). A review focussing on its manufacture.
Starch/Stärke, 1990, 42, 314-321.
[16] Hayes, D.J., Fitzpatrick, S., Hayes, M.H.B., Ross, J.R.H. The biofine process – production
of levulinic acid, furfural, and formic acid from lignocellulosic feedstocks. in
biorefineries: Industrial processes and products, vol. 1, Kamm, B., Gruber, P. R., Kamm,
M., Wiley-VCH: Weinheim, 2006.
[17] Kuster, B.F.M., van der Baan, H.S. The influence of the initial and catalyst
concentrations on the dehydration of D-fructose. Carbohydr. Res., 1977, 54, 165-176.
[18] Kuster, B.F.M., Temmink, H.M.G. The influence of ph and weak-acid anions on the
dehydration of fructose. Carbohydr. Res. 1977, 54, 185-191.
[19] Asghari, F.S., Yoshida. H. Acid-catalysed production of 5-hydroxymethyl furfural from
D-fructose in subcritical water. Ind. Eng. Chem. Res., 2006, 45, 2163-2173.
[20] Asghari, F.S., Yoshida, H. Kinetics of the decomposition of fructose catalysed by
hydrochloric acid in subcritical water: Formation of 5-hydroxymethylfurfural, levulinic,
and formic acids. Ind. Eng. Chem. Res., 2007, 46, 7703-7710.
[21] Swift, T, D., Bagia, C., Choudhary, V., Peklaris, G., Nikolakis, V., Vlachos, D.G. Kinetics of
homogeneous brønsted acid catalysed fructose dehydration and 5‑hydroxymethyl
furfural rehydration: a combined experimental and computational study. ACS Catal.,
2014, 4, 259−267.
[22] Bicker, M., Kaiser, D., Ott, L., Vogel, H. Dehydration of D-fructose to
hydroxymethylfurfural in sub- and supercritical fluids. J. Supercrit. Fluids, 2005, 36,
118-126.
[23] Kuster, B.F.M., van der Steen, H.J.C. Preparation of 5-hydroxymethylfurfural.
Starch/Starke, 1977, 29, 99-103.
[24] Bicker, M., Hirth, J., Vogel, H. Dehydration of fructose to 5-hydroxymethylfurfural in
sub and supercritical acetone. Green Chem., 2003, 5, 280-284.
[25] Girisuta, B., Janssen, L.P.B.M., Heeres, H.J. A kinetic study on the conversion of glucose
to levulinic acid. Chem. Eng. Res. Des., 2006, 84, 339-349.
[26] van Zandvoort, I., Yuehu, W., Rasrendra, C.B., van Eck, E.R.H., Bruijnincx, P.C.A.,
Heeres, H.J., Weckhuysen, B.M. Formation, molecular structure, and morphology of
humins in biomass conversion: Influence of feedstock and processing conditions.
ChemSusChem., 2013, 6, 1745-1758.
61
[27] Redmon, B.C. Process for the production of levulinic acid. US Patent No. 2738367.
US/Baltimore, 1956.
[28] Jow, J., Rorrer, G.L., Hawley, M.C., Lamport, D.T.A. Dehydration of D-fructose to
levulinic acid over LZY zeolite catalyst. Biomass, 1987, 14, 185-194.
62
3 Experimental and modeling studies on the uncatalysed thermal conversion of inulin to 5-hydroxymethylfurfural and levulinic acid
B.A. Fachri, R.M. Abdilla, C.B. Rasrendra, H.J.Heeres
Accepted in Sustainable Chemical Processes
63
Abstract
5-Hydroxymethylfurfural (HMF), an important biobased platform chemical, is accessible by
the acid catalysed conversion of biopolymers containing hexoses (cellulose, starch, inulin)
and monomeric sugars derived thereof. We here report an experimental study on the
uncatalysed, thermal conversion of inulin to HMF in aqueous solutions in a batch set-up.
The reactions were conducted in a temperature range of 153-187oC, an inulin loading
between 0.03 and 0.12 g/mL and batch times between 18 and 74 minutes using a central
composite experimental design. The highest experimental HMF yield in the process window
was 35 wt% (45 mol%), which is 45% of the theoretical maximum (78 wt%). The HMF yields
were modeled using a statistical approach and good agreement between experimental data
and model was obtained. The possible autocatalytic role of formic acid (FA) and levulinic acid
(LA), two main byproducts, was probed by performing reactions in the presence of these
acids and it was shown that particularly FA acts as a catalyst.
Inulin is an interesting feed for the synthesis of HMF in water. A catalyst is not required,
though autocatalytic effects of FA play a major role and also affect reaction rates and
product yields.
Keywords: Inulin, HMF, thermal reaction, autocatalytic effects
64
3.1 Introduction
Biomass has been identified as an attractive alternative for crude oil, natural gas and coal to
produce fuels and chemicals. When considering biobased chemicals, particularly the
carbohydrate fraction (cellulose and hemicellulose) of lignocellulosic biomass shows high
potential. An example of a biobased chemical is 5-hydroxymethylfurfural (HMF), which has
been classified as one of the top 12 biobased chemicals from biomass by the US department
of Energy (DOE)[1-2]. HMF is a very versatile building block for biofuels or biofuel additives
[3-4], solvents, surface-active agents, fungicides [5] and for interesting monomers for the
plastics industry [1], [6-7].
HMF may be produced from of a wide range of hexoses by elimination of three water
molecules (Scheme 1, example for D-fructose).
Scheme 1 A simplified reaction scheme for HMF formation.
Conventionally, the reaction is carried out in water using a Brönsted acid with D-fructose as
the preferred carbohydrate source [8-20]. Hydrochloric acid and sulfuric acid are the most
commonly used Brönsted acids. Kuster (1990) [21], reported that D-fructose is more reactive
and selective towards to HMF than D-glucose and a HMF yield from D-fructose of 68 % was
reported using HCl as the catalyst (Cfruct= 9 wt%, 1.5 h). Yields in water are less than
quantitative due to the formation of insoluble byproducts (humins) and a subsequent
reaction of HMF to levulinic acid (LA) and formic acid (FA, Scheme 1). Two main reaction
mechanisms have been proposed for the reaction in water, one involving acyclic
intermediates and another with cyclic intermediates [22].
Considerable progress has been made to enhance HMF yields, among others by using
solvents other than water and the use of advanced catalysts [8]. Typical homogeneous
Brönsted catalysts (mineral acids) have the advantage that they are relatively cheap, though
recycling is often cumbersome. As such, heterogeneous catalysts could be beneficial and
65
have been tested in detail. However, catalyst lifetime needs to be established and a major
concern is the deposition of humin substances on the heterogeneous catalysts leading to
irreversible catalyst deactivation. Examples of solvents other than water tested for the
reaction include acetone [17], methanol [17], toluene [23] and dimethyl sulfoxide (DMSO)
[24]. Among them, reactions in DMSO give essentially quantitative HMF yields [8], though
down-stream processing is considerably more complicated than with water and is a critical
issue.
Despite these improvements, efficient, economically viable routes to produce HMF from D-
fructose have not yet been commercialised. Bicker et al., [17] estimated an HMF cost price
of 2 €/kg, using the assumption that D-fructose is available at a price of 0.5 €/kg. Torres et
al., [25] reported that the manufacturing costs of HMF are between 1.97 $/kg and 2.43 $/kg,
depending on the solvent used, based on a D-fructose price of 0.55 $/kg and 7000 ton/year
HMF production unit. An HMF price of around 1.00 $/kg is considered a good starting point
to allow its use in bulk-scale chemical applications.
The major variable cost item in the economic evaluations for HMF manufacture is the cost of
the D-fructose feed. As such, the identification of low priced D-fructose alternatives is of
high importance for the development of techno-economically viable routes to HMF.
Alternative feeds are biopolymers enriched in D-fructose units, which are likely cheaper than
purified D-fructose. An interesting biopolymer is inulin, an oligosaccharide consisting of
mainly fructose units, in some cases capped with a glucose unit [6], [15], [26]. Inulin can be
extracted from plants, examples are jerusalem artichoke [27-31], chicory [28-31] and dahlia
tubers [28-31]. Of particular interest are the Jerusalem artichoke and chicory, which are
reported to have a high inulin content of up to 20% on fresh weight [32].
A number of studies have been performed on the catalysed inulin conversion to HMF in
water, the solvent of choice in the current investigation (Table 1). Temperatures are typically
between 80 and 200°C, reaction times vary between minutes and 3 h, and intakes of inulin
are between 5 and 10 wt% on solvent. A range of catalysts has been applied, varying form
homogeneous (H2CO3, formed in situ by CO2 addition) to heterogeneous catalysts. Reactions
not only have been performed in water but also in water-organic solvent mixtures and ionic
liquids. Highest HMF yields (88 mol% on inulin) were reported by Hu et al., using DMSO as
the solvent [33].
66
Table 1 Overview of HMF yield data for the catalysed conversion of Inulin to HMF in water.
We here report a study on the conversion of inulin to HMF using water as the solvent in the
absence of a catalyst. In this way, catalyst recycle procedures are eliminated and drawbacks
of heterogeneous catalysts (among others deactivation by humin deposition) are avoided.
Water was selected as the solvent of choice, as it is environmentally benign, is a good
solvent for many carbohydrates and it is cheap, nontoxic, and nonflammable [39-42].
A number of studies have been reported on the uncatalysed conversion of D-fructose to
HMF (Table 2). For inulin, very limited information is available in the literature. The only
paper is by Wu et al., [34] who reported an HMF yield of 48% (200oC, inulin loading of 5 wt%
and 1 h reaction time).
Thus, it can be concluded that a detailed study on the effect of process conditions on HMF
yields for the thermal decomposition of inulin in water has not been reported to date. We
here report a systematic study on the effect of process conditions like reaction time, inulin
intake and reaction temperature on the HMF yield. A total of 24 batch experiments were
performed using a composite design. The data were analysed statistically and a model was
developed to describe the HMF yield versus process conditions. Finally, possible
Cinulin, (wt%) T,(oC) t Catalyst Catalyst
loading HMF Yield, (%)
a Ref.
water/homogeneous catalyst 5 160 4 min CO2 6 MPa 45
b [34]
5 160 4 min CO2 9 MPa 42b
[34] 5 180 2 min CO2 4 MPa 45
b [34]
5 180 2 min CO2 6 MPa 50b [34]
5 180 2 min CO2 11 MPa 52b [34]
5 200 45 min CO2 6 MPa 53b [34]
5 200 45 min CO2 9 MPa 49b [34]
water/solid catalyst 6 100 0.5 h Cubic ZrP2O7 0.6 g 26 (wt%) [35] 6 100 1 h Cubic ZrP2O7 0.6 g 35 (wt%) [35] 6 100 2 h Cubic-ZrP2O7 0.6 g 36 (wt%) [35] 6 100 1 h Ti(PO4)(H2PO4).2H2O 0.6 g 41 (wt%) [35] 6 100 2 h Ti(PO4)(H2PO4).2H2O 0.6 g 65 (wt%) [35] 6 80 2 h FeVOP 5 wt% 35 (mol%) [36] 10 155 18 min HNb3O8 SCR=50
c 43 (mol%) [37]
6 100 3 h Niobium phosphate SCR=1.6c
31 (mol%) [38] a: wt or mol in brackets after entries
b: wt% or mol% not provided
c. SCR = substrate to catalyst ratio
67
autocatalytic effects of organic acids (formic acid and levulinic acid) on the rates of reactions
and HMF yields were explored.
Table 2 Overview of studies for the uncatalysed reaction of d-fructose and inulin to HMF in water. Substrate Substrate
intake, (wt%) T, (
oC) t Yield
(%)b
Ref.
D-Fructose
4.5 175 45 min 56c [14]
5 140 1 h 4 (mol%) [16]
9 200 5 min 41 (mol%) [18]
30 160a 5 min 1 (mol%) [43]
30 190a 5 min 36 (mol%) [43]
2 200a 5 min 13 (mol%) [44]
30 170 3 h 43 (mol%) [45]
0.05d 240 3 min 20
c [46]
5 125 5 min 0.8c [47]
11 200 30 min 51c [48]
Inulin
5 160 4 h 38c
[34]
5 180 2 h 41c
[34]
5 200 1 h 48c
[34] a: heating by microwave irradiation
b: wt or mol in brackets after entries
c: wt% or mol% not provided
d: in M
3.2 Experimental section
3.2.1 Chemicals
Inulin from Dahlia tubers was purchased from Acros Organic (Geel, Belgium). D-fructose
(99%) and levulinic acid (≥ 97%) were obtained from Acros Organic (Geel, Belgium). Formic
acid (≥ 95%) and D-glucose (≥ 99.5%) were purchased from Merck KGaA (Darmstadt,
Germany). 5-Hydroxymethylfurfural (HMF) (≥ 99%) was obtained from Sigma Aldrich
(Steinheim, Germany). 2,5-Dihydroxybenzoic acid (DHB) (≥ 99%) was purchased from Fluka
(Deisenhofen, Germany). All chemicals were used without purification. De-ionized water was
used to prepare the solutions.
3.2.2 Experimental procedures
The experimental procedures are based on previous research by our group (Girisuta et al,
[19]. In a typical experiment, the pre-determined amount of inulin and de-ionized water (4
mL) were loaded to glass ampoules with an internal diameter of 5 mm, a length of 15 cm and
thickness of 1.5 mm. The ampoules were sealed with a torch.
For the exploratory experiments, a series of ampoules was placed in a rack in a heating oven
(Heraeus Instruments Type UT6060) at constant temperature. At different reaction times, an
68
ampoule was taken from the oven and quickly quenched in cold water to stop the reaction.
The experiments carried out in the framework of the experimental design were individually
performed in an oven (Heraeus Instruments Type UT6060) at the pre-set temperature. For
the autocatalytic experiments with LA and FA, a series of ampoules were filled with inulin
(0.1 g/ml) and the appropriate amount of each acid (0 or 0.1 M) and placed in an oven
(Binder, APT Line TM FD (E2)) at 180 oC for a predetermined reaction time. The experiments
were repeated at different reaction times allowing construction of an HMF yield versus time
plot for each individual organic acid and the blank experiment. All experiments were
conducted in duplicate and the average value is taken. The outcome of the autocatalytic
experiments cannot be compared directly with that of the screening and experimental
design experiments as the heating up profile (temperature versus time) for both ovens
differs.
After reaction, the ampoules were opened and the reaction mixture was taken out, and
centrifuged for about 10-30 minutes to remove the solids. The liquid product was diluted
with demi water before analysis.
The composition of the inulin sample and particularly the type and amount of C6-sugars was
determined by an acid-catalysed hydrolysis reaction. For this purpose, inulin (2.5 g) was
dissolved at 70oC in 150 mL of water under stirring. The pH was adjusted to 1.4-1.6 by adding
HCl (12 M). Then, the solution was placed in a water-bath for 30 minutes at 90oC. A liquid
sample was taken and analysed by HPLC.
3.2.3 Analysis
Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS)
on a Voyager-DE PRO was used to determine the molecular weight of the inulin sample. 2,5-
Dihydroxybenzoic acid (DHB) was used as the matrix.
HPLC was used to identify and quantify the liquid product from the reactions. The HPLC
system consisted of a Hewlett Packard 1050 pump, a Bio-Rad organic acid column Aminex
HPX-87H and a Waters 410 differential refractive index detector. A sulfuric acid solution (5
mM) was used as the eluent with a constant flow rate of about 0.55 ml.min-1. The column
was operated at 60 oC. The HPLC was calibrated with solutions of the pure compounds at a
range of concentrations. Using the chromatogram peak area and the external calibration
curves, the concentrations of components in the liquid phase were determined.
69
3.2.4 Definitions
The HMF yield (yHMF) is defined according to equation 1 and reported on a weight basis.
YHMF = CHMF ×MHMF × V
WIn × 100 % (wt%) (1)
YLA = CLA ×MLA ×V
WIn x 100 % (wt %) (2)
Here, the CHMF and CLA represents the HMF and LA concentrations (mol L-1), MHMF and MLA
the molecular weights of HMF and LA, respectively (g mol-1), V the reaction volume (L) and
Win the intake of inulin (g/L).
The yield of HMF was converted from wt% to mol% by assuming that inulin consists of linked
glucose/fructose units (C6H10O5) which react to HMF according to the following
stoichiometry:
C6H10O5 → C6H6O3 + 2 H2O (3)
As such, the maximum yield of HMF is 78 wt%. Thus, the yield of HMF in mol% may be
calculated from the yield in wt% by dividing the latter by 0.78.
3.2.5 Statistical modeling
The optimisation experiments were modeled using Design-Expert 7 software (Stat-Ease). The
yield of HMF (YHMF) was modeled using a standard expression as given in equation (4):
yHMF=bo+ ∑ 𝑏𝑖
3
𝑖=1
𝑥𝑖+ ∑ ∑ 𝑏𝑖𝑗𝑥𝑖𝑥𝑗
3
𝑗=1
3
𝑖=1
(4)
The independent variables (inulin intake, temperature and reaction time) are represented by
the indices 1–3. The regression coefficients were obtained by statistical analyses of the data.
Significance of factors was determined by their p-value in the ANOVA analyses. A factor was
considered significant if the p-value was lower than 0.05, meaning that the probability of
noise causing the correlation between a factor and the response is lower than 5%.
70
Insignificant factors were eliminated using backward elimination, and the significant factors
were used to model the data.
3.3 Result and Discussion
3.3.1 Inulin characterization
The molecular weight distribution of the inulin used in this study (Dahlia tubers) was
determined by MALDI-TOF/MS, a technique particularly suited for molecular weight
determinations of oligosaccharides and polysaccharides [49-50]. The 𝑀𝑛 was found to be
2560, the 𝑀𝑤 3680, indicating an average degree of polymerisation (DP) of about 16.
Roberfroid [31] reported that the DP of inulin varies according to plant species, weather
conditions, and the physiological age of the plant. In chicory, DP values range from 2 to 65,
with 15 reported as an average. For inulin from onions, the DP is in the range of 2-12, for
Jerusalem artichoke the DP is reported to be < 40. Thus, the experimentally determined
value of the DP of the inulin sample used in this study is within the ranges reported in the
literature.
The D-fructose and D-glucose content of the inulin used in this study were determined by a
mild hydrolysis of the samples followed by HPLC analyses of the liquid phase. The fructose
content was 94 mol%, the remainder being glucose, giving a fructose to glucose ratio of 15
to 1. The fructose content is in close agreement with the literature for Dahlia tubers (94.1 –
96.7 mol%) [51]. Thus, the inulin sample comprises of oligomers with d-fructoside units, with
each oligomer chain on average capped with a D-glucose molecule, in line with literature
data [31].
3.3.2 HMF synthesis from inulin in the absence of a catalyst
Exploratory experiments on the thermal conversion of inulin were performed at 170oC, using
an inulin intake of 0.1 g/mL. A typical concentration profile is given in Figure 1. Six water
soluble components were identified in the reaction mixtures (HPLC). Three show a clear
maximum in the course of the reaction, viz. D-fructose, D-glucose and HMF, and are as such
intermediates in the reaction sequence. The final products are LA, formic acid (FA) and acetic
acid (AA). In addition, some brown-black insolubles were observed, known as humins, which
are always formed during the acid catalysed conversions of carbohydrates in water, either in
monomeric or polymeric form [8].
71
Fig. 1 Concentration-time profiles for various compounds during the thermal conversion of inulin in water (170oC, CIn = 0.1 g/mL).
The product composition versus time is in agreement with the reaction network provided in
Scheme 2. It involves the saccharification of inulin to the monomeric building blocks (D-
fructose and some D-glucose), followed by the reaction of these C6 sugars to HMF. The
latter is not inert under reaction conditions and reacts further to LA and FA [18-19], [52-55].
Scheme 2 A simplified reaction scheme for the conversion of inulin to HMF.
72
Besides these products, acetic acid was detected in small amounts, though not quantified.
Acetic acid is likely formed by hydrolysis of acetyl side groups in the inulin [56] or
alternatively, from further degradation reactions of the intermediates. For instance, Asghari
et al. [46] reported the formation of acetic acid (5 mol%) from fructose in subcritical water
(200-320oC, residence time of 120 s, Cfruct of 0.05 M, no catalyst).
3.3.3 Effect of process conditions of the uncatalysed conversion of inulin to HMF
To determine and quantify the effect of process variables on the conversion of HMF, 24
experiments were conducted in a batch reactor set up (glass ampoules) using a central
composite design. Three independent variables, the temperature (153-187°C), inulin intake
(0.03-0.12 g/mL) and reaction time (18-74 min), were explored and the yields of HMF and LA
were taken as the dependent variables. The results are provided in Table 3.
Table 3 Overview of experiments for the thermal uncatalysed conversion of inulin to HMF and LA.
Run Cin, g/mL Treaction, oC t, min YHMF, wt % YLA, wt %
1 0.08 170 39 22.8 4.0
2 0.05 160 18 2.7 -
3 0.08 170 39 23.1 3.1
4 0.08 170 39 23.9 3.2
5 0.05 180 18 32.1 5.6
6 0.05 180 18 35.0 6.1
7 0.12 170 39 17.6 6.1
8 0.08 170 39 23.1 3.3
9 0.08 187 39 14.6 13.4
10 0.08 170 39 22.5 1.1
11 0.08 170 39 21.8 1.5
12 0.08 170 39 19.4 1.8
13 0.08 170 39 16.6 2.2
14 0.08 170 74 8.9 1.9
15 0.1 180 18 20.9 5.5
16 0.03 170 39 24.8 3.6
17 0.05 160 60 25.7 3.1
18 0.1 160 60 23.3 3.6
19 0.1 160 18 3.2 -
20 0.05 180 60 10.1 9.4
21 0.05 180 60 10.4 9.0
22 0.1 180 60 3.1 9.5
23 0.1 180 60 2.2 9.8
24 0.08 153 39 12.4 0.3 a The yield of HMF is defined in equation 1;
b The yield of LA is defined in equation 2
73
The center point of the central composite design was measured six times and the HMF yield
was found to be on average 21.4 wt% with a standard deviation of 2.9 wt%. Thus, the
reproducibility of the experimental procedure appears to be good
The highest experimental HMF yield was about 35 wt% (45 mol%, entry 6 in Table 3) and was
achieved at 180 oC, an inulin intake of 0.05 g/mL and a reaction time of 18 min. Wu et al.,
[34] reported an HMF yield of 41% at 180oC, an inulin intake of 0.05 g/mL and a reaction
time of 2 h. Comparison is cumbersome as conditions are not exactly similar and different
starting materials were used (among others, inulin source and related properties like DP).
It is also of interest to compare the results with those obtained for inulin in water with
sulphuric acid as the catalyst [57]. In the latter case, the highest experimental yield of HMF
was 39.5 wt% (50.6 mol%), obtained at 170 oC, 0.17 g/mL inulin intake and an acid
concentration of 0.006 M and 20 min reaction time. Thus, the maximum HMF yield for the
uncatalysed, thermal reaction within the window of process conditions is only slightly lower
than when using a Brönsted acid. The reaction time is in general much longer for the thermal
reaction than for the sulphuric acid catalysed reaction. As such, the use of a Brönsted acid
catalyst has advantages in terms of reaction rates and as such a smaller reactor will be
needed to achieve a certain production rate (kg/h). However, sulphuric acid recycle,
complicating the work-up section, is not required for the uncatalysed reaction. Detailed
process design studies will be required for both options to evaluate the best approach and
these are beyond the scope of the current study.
3.3.4 Statistical modeling
The HMF yield (yHMF) as a function of the independent variables was statistically modeled
using the Design-Expert 7 software package. All independent variables (temperature T,
intake inulin Cin and reaction time t) were shown to be statistically significant and to have an
effect on the HMF yield. The best model for HMF yield is given in equation 5 and includes
both quadratic and interaction terms. An extended version with more significant numbers
for the coefficients to be used for among others reactor engineering studies is given in the
supplementary information. The R-squared of the model is 0.9664, an indication that the
model fits the experimental data well.
74
YHMF = (1466.7)Cin + (11.8)T + (9.3)t – (9.2)CinT – (0.05)tT-(0.03)T2 – (0.009)t2 -1223.9
(5)
Analysis of the model variance is given in Table 4. A good agreement between the model and
the experimental data was observed, as is shown in the parity plot provided in Figure 2.
Table 4 Analysis of variance of the HMF model.
Source Sum of squares df Mean square F-value p-value
Prob > F
Model 1878.14 7 268.31 65.66 < 0.0001
Cin (A) 85.20 1 85.20 20.85 0.0003
T (B) 50.63 1 50.63 12.39 0.0028
t (C) 2.27 1 2.27 0.56 0.4665
AB 55.60 1 55.60 13.61 0.0020
BC 1156.24 1 1156.24 282.96 < 0.0001
B2 107.51 1 107.51 26.31 0.0001
C2 159.51 1 159.51 39.04 < 0.0001
Residual 65.38 16
Fig. 2 Parity plot between the experimental and modelled HMF yields.
With the statistical model available, it is possible to determine the effects of the process
conditions on the HMF yield within the design window. To illustrate this, the model
75
predictions for HMF yield for batch times of 30 min (Figure 3, left) and 40 minutes (Figure 3,
right) are given versus the temperature and inulin intake. The yield of HMF is a complex
function of the independent variables and it is difficult to draw general conclusions. Though,
as expected, the HMF yields after 40 min are lower than for 30 minutes. This is due to the
subsequent reaction of HMF to LA and FA (Scheme 2), leading to a lowering in the HMF yield.
The effect of the inulin intake on the HMF yield is also complex in nature and temperature
depending. At low temperatures, the HMF yield is slightly higher at higher inulin intakes,
whereas the opposite trend is observed at higher temperatures, where a lower inulin intake
is favoured.
Fig. 3 Modeled HMF yield versus temperature and inulin intake for two batch times (left: 30 min; right: 40 min). The occurrence and importance of the consecutive reaction of HMF to LA was also
confirmed by considering the LA yield (yLA) versus the process conditions (Table 3). The LA
yield was modeled and the results are given in Figure 4 and Table 5. The model equation
with an R square of 0.9464 is given in equation 6. An extended version with more significant
numbers for the coefficients to be used for among others reactor engineering studies is
given in the supplementary information. The parity plot (Figure 5) reveals good agreement
between the model and the experimental data.
76
YLA = 437.51 + (0.09)t + (0.17)Cint + (5.88.10-4)Tt + (1422.29)Cin2 + (0.02)T2– (188.89)Cin –
(5.41)T – (0.12)CinT – (1.69.10-3)t2 (6)
Fig. 4 Modeled LA yield versus temperature and reaction time at a fixed inulin intake of 0.09 g/mol.
Table 5 Analysis of variance of the LA model.
Source Sum of squares df Mean square F-value p-value
Prob > F
Model 273.91 9 30.43 27.44 < 0.0001
Cin (A) 1.29 1 1.29 1.16 0.2989
T (B) 157.95 1 157.95 142.42 < 0.0001
t (C) 25.28 1 25.28 22.79 0.0003
AB 8.95 x 10-3
1 8.95 x 10-3
8.07 x 10-3
0.9297
AC 0.08 1 0.09 0.079 0.7829
BC 0.15 1 0.15 0.14 0.7151
A2 14.29 1 14.29 12.88 0.0030
B2 43.84 1 43.84 39.53 < 0.0001
C2 4.97 1 4.97 4.48 0.0527
Residual 15.53 14 1.11
As expected on the basis of the reaction network proposed in Scheme 2, the LA yield is
higher when using a longer batch time and higher temperatures, see Figure 4 for details.
77
Fig. 5 Parity plot between the experimental and modeled LA yield.
3.3.5 Possible role of acid formation on HMF yields: autocatalytic effects
During the course of the reactions, organic acids such LA and FA are formed. These acids
may act as catalysts for all reactions in the proposed reaction network (Scheme 2). To gain
insight in the effect of these organic acids on the reaction rates and product distributions, a
number of additional batch experiment was performed using inulin as the starting material
and with one of the individual acids (LA or FA) and a combination of LA and FA present at the
start of the reaction. The experiments were carried out at a temparature of 180 oC, an acid
concentration of 0.1 M, and an inulin loading of 0.1 g/mL. The results are given in Figure 6.
When considering the fructose concentration versus time profiles, it is evident that the
presence of particularly formic acid has a positive effect on the rate of reaction of fructose
and all fructose reacts away considerably faster when compared to the uncatalysed reaction
and the reaction in the presence of LA. A similar trend was found for the HMF concentration
versus time profiles. Here the presence of FA also leads to a reduction of the time required
to reach the maximum HMF concentration. The autocatalytic effect of LA is by far less
pronounced than for FA and the profiles are essentially similar to the uncatalysed reaction.
This, we may conclude that particularly FA has a positive effect on the reaction rates. These
findings may be explained by considering the pKA values of both acids, 3.75 for FA and 4.6
for LA, indicating that a stronger Brönsted acid leads to higher reaction rates due to a higher
Brönsted acidity of the solution. Thus, the formation of D-fructose and HMF is mainly due to
78
autocatalytic effects of FA. These findings are in line with studies reported in the literature
using fructose as the substrate (Table 2). For instance Ranoux et. al.,[45] showed that the
reaction of fructose to HMF is autocatalysed by formic acid and that the role of LA is by far
less pronounced.
Fig.6 Autocatalytic effect of LA and formic acid on the concentration time profiles for fructose and HMF (180 oC, an acid concentration of 0.1 M, and an inulin loading of 0.1 g/mL).
An interesting observation is that the maximum HMF concentration in the concentration-
time profile is only slightly dependent on the presence of the organic acids, though evidently
the reaction rates are affected. This also seems to be the case for D-fructose, though this is
less clear as the fructose concentration may already have reached a maximum before the
first sampling point (15 min). Both fructose and HMF are intermediates in the reaction
network as given in Scheme 2. The near time independency of the maximum HMF yields as
given in Figure 6 (right) suggests that both the rates of inulin hydrolysis to fructose (with
oligomeric intermediates), the subsequent formation of HMF and the subsequent reaction
to LA and FA are equally affected by the presence of FA, implying a similar reaction order in
H+ (likely close to 1) [57].
However, without detailed kinetic studies (including determination of the amount of
oligomeric sugars during reaction) it is not possible to draw definite conclusions about the
exact mode of action of the organic acids and the way they influence reaction rates and thus
the product yields. In addition, also for the thermal reaction, organic acids are formed in the
course of the reaction, resulting in autocatalytic effects that substantially complicate a
detailed kinetic analysis.
79
3.4 Conclusions
The uncatalysed, thermal conversion of inulin to HMF in water was studied for a wide range
of reaction conditions, including variations in temperature (153-187oC), inulin intake (0.03-
0.12 g/mL) and reaction time (18-74 min). The highest HMF yield was 35 wt%, corresponding
to a 45% yield on a molar basis (180 oC, inulin intake of 0.05 g/mL and a reaction time of 18
min). The experimental data were modelled using a statistical approach. The model shows a
good fit with the experimental data and allows estimation of the HMF yield as a function of
temperature, inulin intake and reaction time. This model may be used for reactor
engineering purposes, for instance to optimise the HMF yield in continuous reactors with
different degrees of mixing.
Autocatalysis of reaction products (FA and LA) and particularly by FA, the strongest acid,
occurs to a significant extent as was shown by separate experiments with inulin in the
presence of organic acids. These findings indicate that it may be advantageous regarding
HMF yield to perform the reaction in a buffer solution at neutral pH values. These
experiments are in progress and will be reported in due course.
Supplementary information.
Supplementary information is provided in the next paragraph
3.5 Nomenclature
CHMF : concentration of HMF (mol L-1)
CLA : concentration of levulinic acid (mol L-1)
MHMF : molecular weight of HMF (g mol-1)
MLA : molecular weight of LA (g mol-1)
t : time (min)
T : temperature (oC)
V : volume of reaction (L)
Win : intake of inulin (g L-1)
YHMF : yield of HMF (wt%)
YLA : yield of LA (wt%)
80
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85
3.7 Supplementary Material
Experimental and modeling studies on the uncatalysed thermal conversion of inulin to 5-
hydroxymethylfurfural and levulinic acid
HMF yield = -1223.87339 1466.69731 x inulin intake 11.75492 x T 9.35425 x time -9.19413 x inulin intake x T -0.05071 x T x time -0.02628 x T^2 -0.00945 x time^2 LA yield = +437.51303 -188.89308 x inulin intake -5.41277 x T +0.092913 x time -0.11725 x inulin intake x T +0.17390 x inulin intake x time +5.88156E-004 x T x time +1422.29493 x inulin intake^2 +0.016862 x T^2 -1.69004E-003 x time^2
86
4 Experimental and modeling studies on the acid-catalysed conversion of inulin to 5-hydroxymethylfurfural in water
B.A. Fachri, R.M. Abdilla, C.B. Rasrendra, H.J.Heeres
Submitted to Chemical Engineering Research and Design
87
Abstract
Inulin is considered an attractive feed for the synthesis of 5-hydroxymethylfurfural (HMF), an
important biobased platform chemical with high application potential. We here report a
systematic study to optimise the HMF yield from inulin a batch reactor for reactions in water
using sulphuric acid as the catalyst. The latter was selected on the basis of a screening study
with seven organic- and inorganic Brönsted acids (H2SO4, HNO3, H3PO4, HCl, trifluoroacetic
acid, maleic acid and fumaric acid). The effect of process conditions such as temperature
(160-184oC), inulin loading (0.05-0.17 g/mL), sulphuric acid concentration (0.001-0.01 M)
and reaction time (0-60 min) on HMF and levulinic acid (LA) yields were determined
experimentally and subsequently modeled using non-linear multivariable regression. The
highest experimental HMF yield was 39.5 wt % (50.6 mol %) and was obtained at 170oC, an
inulin loading of 0.17 g/mL, a sulphuric acid concentration of 0.006 M and a reaction time of
20 min. Agreement between experiments and model for both HMF and LA yield was very
satisfactorily.
Keywords: inulin, acid-catalysts, 5-hydroxymethylfurfural, levulinic acid
88
4.1 Introduction
Biomass is a renewable source for bioenergy, biofuels and biobased chemicals and has
attracted high interest in recent years [1-6]. For instance, biofuels have been commercialised
(bioethanol from sugars, biodiesel from plant oils) and are available on the market.
However, substantial research and development activities will be required to develop and
commercialise efficient chemical processes for the production of a wide range of biobased
chemicals. Bozell & Petersen (2010)[7] compiled a list of 12 biobased chemicals that have
highest techno-economical potential [8]. The top 12 list includes 5-hydroxymethylfurfural
(HMF), which may be converted to versatile building blocks for polymers such as 1,6-
hexanediol [9], 2,5-furandicarboxylic acid [10] and fuel-additives such as 2,5-dimethylfuran
[11].
HMF is typically obtained by reacting C6 sugars in water in the presence of a Brönsted acid
[3, 12-20]. By-products of the reaction are levulinic acid and formic acid [21] and insolubles
known as humines [22]. In water, fructose gives a maximum HMF yield of around 55 mol%,
whereas the yields for glucose are less than 10 mol%. Higher HMF yields from fructose (>80
mol%) have been reported in other solvent systems, especially in ionic liquids and aprotic
polar solvents such as DMSO [3].
Inulin is a biopolymer consisting of mainly fructose and minor amounts of glucose (Figure 1)
and as such is an attractive biopolymer for the synthesis of HMF [15, 23, 24]. The molecular
weight of inulin is by far less than typical biopolymers like cellulose, hemicellulose and starch
and degrees of polymerisation between 2 and 70 have been reported. Inulin is present in
Jerusalem artichoke tubers, Chicory roots, Camas bulbs and Dahlia tubers, with contents of
around 20 wt% on fresh weight [25-27].
Fig. 1 Molecular structure of inulin.
A number of studies have been reported on the synthesis of HMF from inulin in various
solvents [3]. The most commonly used solvent is water. An overview of studies on inulin
89
conversion in water using both homogeneous and heterogeneous catalysts is given in Table
1.
Table 1 Overview of HMF yield data for the catalysed conversion of inulin to HMF in water.
It can be concluded that only a limited amount of information is available for the conversion
of inulin in water using homogeneous Brönsted acids, whereas heterogeneous catalysts have
been explored in more detail, with HMF yields up to 65 wt%. Also of relevance is a patent of
the Süddeutsche Zucker-Aktiengesellschaft who investigated the use of Chicory roots (with
about 18% inulin on dry-matter) as a starting material for a potentially commercial HMF
production process in water. Starting from 20 kg chicory roots, an HMF yield of 13% was
reported. Next to HMF, fructose (30%) and glucose (3.5%) were obtained. The reaction was
carried out with sulfuric acid at a pH of 1.8 at 140 °C for 2 h [24].
We here report a screening study on the conversion of inulin in water using a range of
Brönsted acids. Such studies have not been reported in the literature and are an absolute
novelty of this paper. Water was selected as the solvent as it is environmentally benign and
is more easily removed from the products in the work-up section than polar organic solvents
like DMSO. One of the best catalyst from this screening study (sulphuric acid) was taken and
used for a systematic study to determine the effect of process conditions (temperature,
Cinulin,
(wt%) T,(
oC) t Catalyst Catalyst
loading HMF Yield, (%)
a Ref.
Water/homogeneous catalyst 5 160 4 min CO2 6 MPa 45
b [28]
5 160 4 min CO2 9 MPa 42b
[28] 5 180 2 min CO2 4 MPa 45
b [28]
5 180 2 min CO2 6 MPa 50b [28]
5 180 2 min CO2 11 MPa 52b [28]
5 200 45 min CO2 6 MPa 53b [28]
5 200 45 min CO2 9 MPa 49b [28]
Water/solid catalyst 6 100 3 h Niobium phosphate SCR=1.6
c 31 (mol%) [29]
6 100 0.5 h Cubic ZrP2O7 SCR=1.8c
26 (wt%) [30] 6 100 1 h Cubic ZrP2O7 SCR=1.8
c 35 (wt%) [30]
6 100 2 h Cubic-ZrP2O7 SCR=1.8c 36 (wt%) [30]
6 100 1 h Ti(PO4)(H2PO4).2H2O SCR=1.8c 41 (wt%) [30]
6 100 2 h Ti(PO4)(H2PO4).2H2O SCR=1.8c 65 (wt%) [30]
6 80 2 h FeVOP 5 wt% 35 (mol%) [31] 10 155 18 min HNb3O8 SCR=50
c 43 (mol%) [32]
a: wt or mol in brackets after entries
b: wt% or mol% not provided
c. SCR = substrate to catalyst ratio
90
reaction time, inulin intake, catalyst concentration) on the HMF and LA yield with the main
objective to optimise HMF yields. The experimental data were modeled using multivariable
non-linear regression to quantify the results.
4.2 Experimental Section
4.2.1 Chemicals
Inulin from dahlia tubers was purchased from Acros Organic (Geel, Belgium). Sulfuric acid
(96-98%) was purchased from Merck KGaA (Darmstadt, Germany). D-fructose (99%) and
levulinic acid (≥ 97%) were obtained from Acros Organic (Geel, Belgium). Formic acid (≥ 95%)
and D-glucose (≥ 99.5%) were purchased from Merck KGaA (Darmstadt, Germany).
Hydroxymethylfurfural (≥ 99%) was obtained from Aldrich (Steinheim, Germany). All
chemicals were used without purification and de-ionized water was used to prepare the
solutions.
4.2.2 Experimental procedures
The experiments were performed in small glass ampoules (internal diameter of 5 mm, a
length of 15 cm and thickness of 1.5 mm) according to a procedure reported earlier by our
group [33]. Different ovens were applied for the catalyst screening study and the
optimisation studies with sulphuric acid. This may affect the heating profile (temperature
versus time) for the ampoules when placed in the oven and this may, particularly for the fast
reactions affect the outcome considerably. As such the reported HMF yields for the catalyst
screening study and the experimental design study using sulphuric acid are not directly
comparable.
4.2.3 Typical experiment for the experimental design study using sulphuric acid as the catalyst.
Predetermined amounts of inulin and sulphuric acid in water (4 mL) were loaded in the glass
ampoules. The ampoules were sealed with a torch. A series of ampoules was placed in a rack
in a heating oven at constant temperature. At different reaction times, an ampoule was
taken from the oven and quickly quenched in a cold water bath to stop the reaction. The
ampoules were opened and the reaction mixture was taken out, and centrifuged for about
10-30 minutes to remove the solids. The liquid product was diluted and used as such for
analyses.
91
4.2.4 Screening experiments with a range of Brönsted acids
The experiments were carried out using a similar procedure as described above. To reduce
experimental error, a series of ampoules was filled with inulin (0.17 g/ml) and the
appropriate amount of each Brönsted acids (0.006 M) used in this study. The ampoules were
placed in a metal rack and placed in the oven (170°C). After a pre-determined reaction time,
the metal rack was removed and all ampoules were quickly quenched in cold water.
Products were taken from the ampules and centrifuged at 7000 rpm for 10-30 min to
separate the solid. Supernatants were taken, diluted with demi water and subsequently the
concentration of HMF was determined by HPLC. The experiments were repeated at different
reaction batch times allowing construction of an HMF yield versus time plot for each
individual acid. All experiments were conducted in duplicate and the average value is taken.
4.2.5 Analysis
HPLC was used to identify and quantify the liquid product from the reactions. The HPLC
system consisted of a Hewlett Packard 1050 pump, a Bio-Rad organic acid column Aminex
HPX-87H and a Waters 410 differential refractive index detector. A very dilute aqueous
sulfuric acid solution (5 mM) was used as the eluent with a constant flow rate of about 0.55
ml.min-1. The column was operated at 60 oC. The HPLC was calibrated with solutions of the
pure compounds at a range of concentrations. Using the chromatogram peak area and the
external calibration curve, the unknown concentrations of components in the liquid phase
was determined.
Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS)
MS) on a Voyager-DE PRO was used to determine the molecular weight of the inulin sample.
2,5- Dihydroxybenzoic acid (DHB) was used as the matrix.
The composition of the inulin sample and particularly the type and amount of C6-sugars was
determined by an acid-catalysed hydrolysis reaction. For this purpose, inulin (2.5 g) was
dissolved at 70 oC in 150 mL of water under stirring. The pH was adjusted to 1.4 -1.6 by
adding an aqueous HCl solution. Then, the solution was placed in a water-bath for 30
minutes at 90oC. A liquid sample was taken and analysed by HPLC.
4.2.6 Definitions
The HMF yield (YHMF) and LA yield (YLA) are defined on a wt% basis and determined from the
concentration of HMF and LA after reaction (HPLC) and the inulin intake, see eq. 1 and 2 for
details.
92
YHMF = CHMF ×MHMF ×V
WIn (wt %) (1)
YLA = CLA ×MLA ×V
WIn (wt %) (2)
Here, the CHMF and CLA represents the HMF and LA concentrations (mol L-1), MHMF and MLA
the molecular weights of HMF and LA, respectively (g mol-1), V the reaction volume (L) and
Win the intake of inulin (g/L).
The yield of HMF was converted from wt% to mol% by assuming that inulin consist of linked
glucose/fructose units (C6H10O5) which react to HMF according to the following
stoichiometry:
C6H10O5 → C6H6O3 + 2 H2O (3)
As such, the maximum yield of HMF is 78 wt%. Thus, the yield of HMF in mol% may be
calculated from the yield in wt% by dividing the latter by 0.78.
4.2.7 Statistical modeling
The optimisation experiments were modelled using Design-Expert 7 software (Stat-Ease).
The yield of HMF was modelled using a standard expression as given in equation (4):
YHMF=bo+ ∑ 𝑏𝑖
4
𝑖=1
𝑥𝑖+ ∑ ∑ 𝑏𝑖𝑗𝑥𝑖𝑥𝑗
4
𝑗=1
4
𝑖=1
(4)
The independent variables (inulin intake, H2SO4 concentration, temperature and reaction
time) are represented by the indices 1–4. The regression coefficients were obtained by
statistical analyses of the data. Significance of the factors was determined by their p-value in
the ANOVA analysis. A factor was considered significant if the p-value was lower than 0.05,
meaning that the probability of noise causing the correlation between a factor and the
response is lower than 0.05. Insignificant factors were eliminated using backward
elimination, and the significant factors were used to model the data.
93
4.3 Result and Discussion
4.3.1 Inulin characterisation
Details on the characterisation of the inulin feed used in this study (originating from Dahlia
tubers) are given in Chapter 3. Of relevance are the values for the 𝑀𝑛 (2560) and the 𝑀𝑤
(3680) [34, 35, 36], the D-fructose content (94 mol%), and the fructose to glucose ratio
(15:1) indicating that the sample mainly contains of D-fructose units (DP of 15), with each
oligomer chain on average capped with one D-glucose unit [36].
4.3.2 Brönsted acid catalyst screening
In the first stage of the project, a Brönsted acid screening study was performed using seven
organic and inorganic Brönsted acids to identify the best catalyst for the conversion of inulin
to HMF. All screening experiments were conducted at 170oC, an inulin loading of 0.17 g/mL,
an acid concentration of 0.006 M and a maximum reaction time 60 min. After reaction,
typically a brown solution was obtained accompanied, in some cases, with some dark brown
solids (humins). The HMF yield versus the reaction time for the various acids is given in
Figure 2.
As seen on Figure 2, distinct differences in reaction rates were observed. Highest rates were
observed for the strong acids (H2SO4, HCl, HNO3, TFA) as is evidenced from the slope of the
HMF yield versus time curve between 0-30 min reaction times. For these acids, an optimum
in HMF yield was observed within the timeframe 0-60 min. The maximum HMF yield for
these acids was about 32 wt% (41 mol %). The reaction rate for the other weaker acids
(maleic, fumaric and phosphoric acid) was considerably lower and the maximum HMF yield
was not yet attained within the timeframe of the experiments.
94
Fig. 2 HMF yield versus reaction time for various Brönsted acid catalyst (170oC, inulin loading 0.17 g/mL, acid concentration 0.006 M).
To quantify the data, the HMF yield after 30 minutes (Table 2) is plotted against the pKa
value of the Brönsted acid (Figure 3, left). Two distinct regimes can be observed: i) one with
an almost constant YHMF when the pKA of the acid is below 0 (very strong acids) and ii) a
regime with an inverse linear relation between the HMF yield and the pKa of the acids for a
pKa between 0 and 3. Thus, strong acids give a higher HMF yield after 30 min than the
weaker acids. This indicates that the concentration of H+ plays a major role regarding the
rate of the reactions leading to HMF, see Figure 3 (right). An almost linear relation between
the pH of the solution and the HMF yield is observed, with lower pH values leading to higher
rates. This may be rationalized by positive effects of protons on both the hydrolysis rate of
the inulin ether bonds to fructose and glucose and the subsequent reaction rate of
fructose/glucose to HMF.
95
Table 2 HMF yield after 30 min batch time for the various acids.
Acid Catalyst yHMF, wt %
a
Acidity, (pKa)
b pH
c
Reference for pKa value
Fumaric acid 12 3.03 2.71 [37] Phosporic acid 18 2.12 2.54 [38] Maleic acid 21 1.93 2.35 [37] Trifluoroacetic acid 29 0.23 2.23 [39] Nitric acid 30 -1.3 2.22 [40] Sulfuric acid 29 -3 2.22 [41] Hydrochloric acid 31 -8 2.22 [41] aT=170
oC, Inulin = 0.17 g/mL, Cacid = 0.006 M, 30 min reaction time
bpKa of the first proton dissociation is given
cCalculated at room temperature
Fig. 3 HMF yield after 30 min batchtime versus the pKa of the acid (left) and the pH of the solution (right). For all strong acids, a clear maximum of the HMF yield was observed within the timeframe of
the experiments. This maximum HMF yield is essentially independent of the acid and about
32 wt%. For the weaker acids, the maximum HMF yield was not yet attained after 60 min
reaction time due to the lower reaction rates (vide supra). However, the yields are at least
comparable to those for the strong acids (Figure 2).
4.3.3 Product identification
A typical chromatogram of a representative reaction mixture using sulphuric acid as the
catalyst is given in Figure 4 and shows the presence of glucose, levulinic acid, HMF, and a
small amount of fructose. Minor amounts of benzenetriol, furfural and acetic acid were also
detected (< 1 %).
96
Fig. 4 HPLC chromatogram for a representative reaction mixture (T=170oC, t=30 min, Cin=0.1 g/mL, CH2SO4= 0.005 M).
A typical concentration profile for the reaction is shown in Figure 5. Clear intermediates in
the reaction are fructose, glucose and HMF, the main final products after prolonged reaction
times are LA and formic acid (FA). These findings are consistent with the reaction pathway
provided in Scheme 1. In the initial stage, inulin is depolymerised to sugar monomers (mainly
fructose and some glucose). In the next step, both fructose and glucose react to HMF by a
dehydration reaction. Of interest is the observation that the maximum concentration of
fructose is reached at a much shorter reaction time than that of glucose.
When assuming that the rate of the hydrolysis reaction of ether linkages in inulin is equal for
a glucose-fructose unit and a fructose-fructose unit, this indicates that the conversion of
fructose to HMF is much faster than the reaction of glucose to HMF, in accordance with
kinetic studies reported by our group [33, 42]. Under the prevailing reaction conditions, HMF
is not stable and reacts to LA and the co-product formic acid. Another undesired side
reaction that limits the amount of HMF is the formation of insoluble compounds known as
humins [43-45]. The formation pathways for these condensation products are still under
debate, though are likely formed from reactions between HMF and sugars [22].
glu
cose
fru
ctose
form
ic a
cid
levu
lin
ic a
cid
HM
F
furf
ura
l
ben
zen
etri
ol
97
Fig. 5 Concentration profile for the acid-catalysed decomposition of inulin (T= 160oC, CIn = 0.17 g/mL, CH2SO4=0.006 M).
Scheme 1 Proposed reaction network for the conversion of inulin to HMF and LA/FA.
4.3.4 Systematic investigations for sulphuric acid and statistical modeling
A systematic study on the effect of the main process variables (temperature, inulin intake
and acid concentration) on the yield of HMF was performed using sulphuric acid. This acid
was selected on the basis of the screening study reported above and earlier studies on our
group on the sulphuric acid catalysed conversions of fructose [42] and glucose to HMF [33].
A total of 27 experiments were carried out and the results are given in Table 4.
98
Table 4 Overview of experiments for inulin in water using sulphuric acid.
Run Temperature (T),
oC
Inulin intake (Cin), g/mL
Sulfuric acid concentration, M
Reaction time (t), min
YHMF, wt %
a
YLA, wt %
b
1 170 0.10 0.006 60 13.3 13.2
2 180 0.05 0.010 10 4.7 0.6
3 170 0.10 0.006 20 31.1 2.7
4 180 0.15 0.001 60 1.7 10.1
5 180 0.05 0.001 10 5.3 0.1
6 160 0.05 0.010 10 7.6 0.04
7 160 0.05 0.010 10 7.1 0.06
8 160 0.15 0.010 60 10.1 18.0
9 160 0.15 0.010 60 11.7 16.2
10 156 0.10 0.006 20 21.7 3.5
11 180 0.15 0.010 10 7.0 0.01
12 180 0.05 0.001 60 13.2 10.0
13 170 0.10 0.012 20 25.7 7.1
14 170 0.10 0.006 20 34.5 3.0
15 170 0.17 0.006 20 39.5 2.0
16 170 0.10 0.006 10 10.6 0.06
17 160 0.15 0.010 10 6.5 0.03
18 170 0.10 0.006 20 35.1 2.1
19 160 0.05 0.001 60 33.7 2.4
20 160 0.05 0.001 60 30.1 1.9
21 170 0.10 0.006 20 34.9 3.1
22 160 0.15 0.001 60 24.4 6.8
23 170 0.10 0.006 20 35.1 2.1
24 180 0.15 0.001 10 11.7 0.01
25 184 0.10 0.006 20 34.3 5.6
26 170 0.10 0.006 20 34.8 2.1
27 160 0.05 0.010 60 18.0 13.9 a The yield of HMF is defined in equation 1;
b The yield of LA is defined in equation 2
The center point of the design was measured six times and the HMF yield was found to be
on average 34.2 wt% (43.8 mol%) with a standard deviation of 1.5 wt%. Thus, the
reproducibility of the experimental procedure appears to be good.
The HMF yield as a function of the process variables was statistically modeled using the
Design-Expert 7 software. The best model, shown in eq. 5, includes quadratic and interaction
terms. An extended version with more significant numbers for the coefficients to be used for
among others reactor engineering studies is given in the supplementary information. The R
square of 0.9801 indicates that the model fits the experimental data well.
99
YHMF = (17)T + (56.5)Cload+ (50700.9)Ccat + (16.3)t - (249.9)T Ccat – (0.06)T t – (2.3)
Cload t – (125.2) Ccat t – (0.04)T2– 4.2 x 105 Ccat2 – (0.06) t2-1782.5 (5)
Analysis of variance of the model is given in Table 5. A good agreement between the
empirical model and the experimental data was observed as shown in the parity plot
provided in Figure 6.
Table 5 Analysis of variance for the best model for the yield of HMF.
Source Sum of squares df Mean square F-value p-value
Prob> F Model 3954.59 11 359.51 67.02 < 0.0001 significant
A-T 295.66 1 295.66 55.12 < 0.0001
B-Cload 25.33 1 25.33 4.72 0.0462
C-Ccat. 130.64 1 130.64 24.36 0.0002
D-t 47.93 1 47.93 8.94 0.0092
AC 129.80 1 129.80 24.20 0.0002
AD 294.38 1 294.38 54.88 < 0.0001
BD 124.71 1 124.71 23.25 0.0002
CD 203.95 1 203.95 38.02 < 0.0001
A2 115.41 1 115.41 21.52 0.0003
C
2 146.39 1 146.39 27.29 0.0001
D
2 1660.30 1 1660.30 309.53 < 0.0001
Residual 80.46 15 5.36
Fig. 6 Parity plot between the experimental predicted data from the empirical model.
100
The highest experimental HMF yield is 39.5 wt % (50.6 mol %) and was obtained at 170oC,
Cin= 0.17 g/mL, CH2SO4 = 0.006 M and a reaction time of 20 min. This value is slightly lower
than reported by Wu et al., [28] for the reaction in water under a CO2 atmosphere. Here, in
situ formed carbonic acid catalyses the reaction and a HMF yield of 53 mol% were reported
(200oC, CO2 pressure of 6 MPa, inulin intake of 5 wt % and a reaction time of 45 min).
The modeled HMF yield versus the temperature and catalyst concentration at different
batch times (25-50 min) is given in Figure 7. Highest HMF yield according to the model was
36.1 wt % (39.5 wt% experimental) obtained at 170oC, Cinulin= 0.17 g/ml, CH2SO4= 0.006 M;
and 20 min batchtime. As expected based on the reaction network for HMF formation
involving multiple consecutive reactions (Scheme 1), the batchtime is an important
optimisation parameter. Clearly, batchtimes above 40 minutes lead to a reduction of the
HMF yield in the process window due to the subsequent reaction of HMF to LA and FA (vide
infra). For a certain batchtime, a broad optimum is observed for the HMF yield as a function
of the temperature and catalyst concentration. Highest HMF yields were obtained for a
combination of a high temperature-low acid concentration or low temperature-high acid
concentration. This is rationalised by considering that at low temperature and low acid
concentrations, the rate of HMF formation from D-fructose is still limited whereas at high-
temperature-high acid concentrations, the subsequent reaction to LA is already occurring to
a large extent and the yield of HMF is reduced.
101
Fig. 7 Effect of process conditions on HMF yield according to the statistical model given in eq 4.
For all reactions performed within this study (Table 4), the yield of LA was also determined
experimentally. The LA yield as a function of process variables was quantified using a
statistical model (eq 6) and the results are given in Table 6. An extended version with more
significant digits for the coefficients to be used for among others reactor engineering studies
is given in the supplementary information. A good fit between experiments and model was
obtained as is evident from the R-squared value of 0.9873 and a parity plot of the
experimental versus the modelled LA yields (Figure 8).
102
Table 6 Analysis of variance of the preferred model of LA.
Source Sum of squares df Mean square F-value p-value
Prob> F Model 744.84 11 67.71 106.31 < 0.0001 significant
A-T 31.26 1 31.26 49.08 < 0.0001
B-Cload 7.38 1 7.38 11.59 0.0039
C-Ccat. 94.75 1 94.75 148.76 < 0.0001
D-t 491.77 1 491.77 772.10 < 0.0001
AB 3.34 1 3.34 5.24 0.0370
AD 22.17 1 22.17 34.81 < 0.0001
BD 3.35 1 3.35 5.27 0.0366
CD 70.21 1 70.21 110.23 < 0.0001
A2 3.59 1 3.59 5.64 0.0314
B2 7.00 1 7.00 10.99 0.0047
C2 2.91 1 2.91 4.58 0.0493
Residual 9.55 15 0.64
YLA = 184.2 + (270.3)Cin – (2.3)T – (616.8)CH2SO4 - (0.9)t – CinT + (5.8 x 103)Tt + (0.4)Cint
+ (24.1) CH2SO4t + (6.8 x 10-3)T2 – (468.2)Cin2 + (39425.7)(CH2SO4)
2 (6)
As expected, the LA yield is also a clear function of the reaction time, with longer reaction
times leading to the highest yield within the experimental window (Figure 9). This finding is
in line with the proposed reaction pathway in Scheme 1, where LA is the final product within
the reaction scheme, which is inert under the prevailing conditions. Higher temperature,
particularly at longer reaction times, also leads to higher LA yields, in agreement with the
previous statement.
103
Fig. 8 Parity plot between the experimental predicted data from the empirical model.
Fig. 9 Modeled LA yield versus temperature and reaction time at selected inulin intake. 4.3.5 Comparison between thermal and acid-catalysed reaction of inulin to HMF
In a previous study, we have investigated the non-catalysed, thermal reaction of inulin to
HMF [46]. As such it is of interest to compare the performance of a reaction with a catalyst
in the form of sulphuric acid with a non-catalysed reaction. The HMF yield versus the
batchtime for a catalysed and uncatalysed reaction at otherwise similar conditions (170oC,
Cinulin= 0.17 g/mL) is given in Figure 10. The maximum HMF yield for the catalysed reaction is
occurring at a lower batchtime, indicating that the rate of the various reactions is higher for
the catalysed than the uncatalysed version. For this particular experiment, the maximum
104
0
5
10
15
20
25
30
35
40
15 20 30 60 90 120 180
HM
F yi
eld
, wt
%
time, min
Acid-CatalyzedThermal
HMF yield for the catalysed reaction (40 wt%, 51 mol%) was also considerably higher than
for the non-catalysed version (27 wt%, 35 mol%).
Fig. 10 HMF yield versus reaction time for the thermal and acid-catalysed conversion of inulin to HMF (T=170oC, Cin=0.17 g/mL, CH2SO4 = 0.006 M).
The HMF yield for the thermal reaction was determined for a range of process conditions
(153-187oC, an inulin loading between 0.03 and 0.12 g/mL and batch times between 18 and
74 minutes using a central composite experimental design) was reported by Fachri et al.,
[42] and the highest HMF yield was 35 wt% (45 mol%) at 180 oC, inulin intake of 0.05 g/mL
and a reaction time of 18 min. A comparison with the maximum HMF yield obtained in this
study, viz 39.5 wt % (51 mol %, 170oC, inulin intake of 0.17 g/mL, sulphuric acid
concentration of 0.006 M and reaction time of 20 min) reveals that the catalysed reaction
leads to a higher HMF yield (about 5 wt%). Combined with the fact that the reaction rates
are higher for the acid catalysed version, the space time yields for HMF (kg
HMF/m3reactor.h) are also higher for the catalysed reaction. However, downstream
processing for the uncatalysed version is less complicated than for the catalysed version as
catalyst recycle is not required.
4.4 Conclusions
An in-depth experimental and statistical modeling study on the acid-catalysed reaction of
inulin in water to HMF was studied in a batch reactor. Acid screening studies show that
strong acids like H2SO4 are the best regarding reaction rates. With this particular acid, the
effects of process variables such as inulin intake (0.05-0.17 g/mL), temperature (160-180oC),
sulphuric acid concentration (0.001-0.01 M) and reaction time (0-60 min) on the HMF yields
105
were determined. The highest experimental HMF yield (39.5 wt%, 51 mol%) was obtained at
170oC, an inulin intake of 0.17 g/mL, a sulphuric acid concentration of 0.006 M and a
reaction time of 20 min. The data were modeled using a statistical approach and good
agreement between experiments and model were obtained. The maximum HMF yields (39.5
wt%) as well as the reaction rates for the sulphuric catalysed reactions was shown to be
higher than for the uncatalysed version (35 wt% HMF yield). As such, the catalysed version
seems preferred. However, further process studies are required to draw definite conclusions
as down-stream processing aspects should be considered. For instance, catalyst recycle is
not required for the uncatalysed version and this may have a positive effect on the
economics of the uncatalysed reaction.
Supplementary information.
Supplementary information is provided in the next paragraph
4.5 Nomenclature
CHMF : concentration of HMF (mol L-1)
CLA : concentration of levulinic acid (mol L-1)
MHMF : molecular weight of HMF (g mol-1)
MLA : molecular weight of LA (g mol-1)
t : time (min)
T : temperature (oC)
V : volume of reaction (L)
Win : intake of inulin (g/L)
YHMF : yield of HMF (wt%)
YLA : yield of LA (wt%)
106
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110
4.7 Supplementary Material
Experimental and Modeling Studies on the Acid-Catalysed Conversion of Inulin to 5-
Hydroxymethylfurfural in water
HMF yield = -1782.54913 +17.04244 x T +56.54606 x Cload +50700.93230 x Ccat +16.32121 x t -249.87125 x T x Ccat -0.064363 x T x t -2.33830 x Cload x t -125.24333 x Ccat x t -0.040905 x T^2 -4.18960E+005 x Ccat^2 -0.062532 x t^2 LA yield = +184.23568 -2.28044 x T +270.32156 x Cload -616.76990 x Ccat -0.90558 x t -1.03893 x T x Cload +5.78432E-003 x T x t +0.40560 x Cload x t +24.07840 x Ccat x t +6.86724E-003 x T^2 -468.22481 x Cload^2 +39425.70343 x Ccat^2
111
5 Experimental and modeling studies on the conversion of inulin to 5-hydroxymethylfurfural using metal salts in water
B.A. Fachri, C.B. Rasrendra, H.J.Heeres
Submitted to Biomass Conversion and Biorefinery
112
Abstract
Inulin, a plant polysaccharide consisting of mainly D-fructose units, is considered an
interesting feed for 5-hydroxymethylfurfural (HMF), a top 12 biobased chemical. We here
report an exploratory experimental study on the use of a wide range of homogeneous metal
salts as catalysts for the conversion of inulin to HMF in water. Best results were obtained
using CuCl2. A subsequent screening study on the effect of process conditions on HMF yield
for CuCl2 revealed that the highest HMF yields are attainable at the highest temperature in
the range (180°C) and the lowest inulin intake (0.05 g/ml). The effects of process conditions
on HMF yield for CuCl2 were systematically investigated and quantified using a central
composite design (160-180oC, an inulin loading between 0.05 and 0.15 g/mL, copper (II)
chloride concentration in range of 0.005-0.015 M and a reaction time between 10 and 120
min). The highest experimental HMF yield in the process window was 30.3 wt % (39 mol%,
180oC, 0.05 g/mL inulin, 0.005 M CuCl2 and a reaction time of 10 minutes). The HMF yields
were modeled using non-linear, multi variable regression and good agreement between
experimental data and model were obtained.
Keywords: inulin, metal salts, HMF, levulinic acid, CuCl2
113
5.1 Introduction
The depletion of fossil resources (oil, coal and natural gas) and environmental concerns
regarding the emissions of CO2 have boosted research activities on the development of fuels
and chemicals from renewable resources [1-8]. Biomass is an interesting renewable feed as
it is abundantly available on a sustainable basis [9-18]. Research on biobased chemicals from
biomass and particularly the carbohydrate fractions has intensified the last decades.
Examples of target molecules with high application potential are levulinic acid (LA), lactic
acid and furanics like 5-hydroxymethylfurfural (HMF). HMF has been categorised as a top
(10+4) biobased chemical by the US Department of Energy (DOE) [2, 9] and has a high and
broad application and derivatisation potential. Examples are the use of HMF as a starting
material for renewable monomers for the polymer industry, for solvents and biofuel
additives [13, 19-21]. HMF synthesis involves the use of the C6 sugars in the biomass like D-
glucose and D-fructose. A wide range of typically Brönsted acid catalysts (homogeneous,
heterogeneous) and solvents (mono- and biphasic) have been proposed, for more details see
recent reviews by Wang et al., [3], and van Putten et al., [5]. In general, HMF yields with d-
fructose as a feed are considerably better than for D-glucose [5].
Recently, the use of (cheap) metal salts as catalysts for the conversion of C6 sugars to HMF
has been proposed in water and organic solvents (including ionic liquids) [5]. Our interest in
this field is particularly on the use of water as a reaction solvent as i) it is environmentally
benign and, ii) C6-sugars are reasonably soluble in water. An overview of relevant examples
of conversions of C6 sugars (D-glucose, D-fructose) in water using soluble metal salts, in
some cases in combination with a Brönsted acid (like HCl), is given in Table 1. Highest HMF
yields for D-fructose were 80 mol % using InCl3 as the soluble metal salt. Typical byproducts
are organic acids like levulinic acid (LA), formic acid, lactic acid and insoluble byproducts
known as humins.
114
Table 1 Reactions of D-fructose and D-glucose in the presence of soluble metal salt in water. Substrate Csugar, (M) Catalyst Conditions XC6sugar YHMF max (mol%) Other products Ref.
Monophasic(water)
Fructose 0.3 ZnCl2/HCl = 1/1 mol/mol
120oC, 0-500 min 97.3 53.3 humins [22]
Fructose 0.5 AlCl3 (0.17 M)
/HCl (1 M) 88
oC, 0-500 min 60 20 LA (45 %), humins [23]
Fructose 0.3 DyCl3 2 mM 140oC, 120 min n/a < 1 Humins, LA [24]
Fructose 5a AlCl3 50 mol% 120
oC, 20 min n/a 55.7 LA, FA, humins [25]
Fructose 30a AlCl3 0.87 M/Boric acid 100gL
-1 150
oC, 45 min 100 21
a LA, FA, humins [26]
Fructose 30a FeCl3 0.87 M/Boric acid 100gL
-1 150
oC, 45 min 99 36
a LA, FA, humins [26]
Fructose 30a MgCl2 0.87 M/Boric acid 100gL
-1 150
oC, 45 min 81 52
a LA, FA ,humins [26]
Fructose 10a
CaP2O6 10 wt% 200oC, 5 min 82 34
a n/a [27]
Fructose 10a
α-Sr(PO3)2 10 wt% 200oC, 5 min 88 39
a n/a [27]
Fructose 10a CrCl3 140
oC, 60 min 87 20 LA (17 % at 180 min) [28]
Fructose 10a CrCl3-HCl 140
oC, 25 min 82 30 LA (45 % at 6 h) [28]
Fructose 5a
InCl3 3 wt% 180oC, 15 min 100 79.5 LA (45 % at 1 h) [29]
Monophasic(water)
Glucose 0.3 ZnCl2/HCl =1/1 mol/mol 120oC, 0-500 min 80.5 32.3 humins [22]
Glucose 0.3 DyCl3 2 mM 140oC, 120 min n/a < 1 char, LA [24]
Glucose 5a AlCl3 50 mol% 120
oC, 20 min n/a 40.3 LA, FA,humins [25]
Glucose 10a
CaP2O6 10 wt% 220oC, 5 min 70 20
a n/a [27]
Glucose 10a
α-Sr(PO3)2 10 wt% 220oC, 5 min 60 21
a n/a [27]
Glucose 10a CrCl3 140
oC, 90 min 78 18 LA (17 % at 180 min) [28]
Glucose 10a CrCl3-HCl 140
oC, 50 min 55 16 LA (42 % at 6 h) [28]
Glucose 10a InCl3 3 wt% 180
oC, 10 min 91.4 59.8 LA (27 % at 10 min) [29]
Glucose 0.1 CrSO4, 5 mM 140oC, 6 h 13 13 lactic acid (< 3 %), LA (13 %), fructose (<3 %), humins
c [30]
a in wt %;
b 2-sec-butylphenol;
c water is one of the solvents
115
Table 1 Reactions of D-fructose and D-glucose in the presence of soluble metal salt in water (continued). Substrate Csugar, (M) Catalyst Conditions XC6sugar YHMF max (mol%) Other products Ref.
Monophasic/water
Glucose 0.25 AlCl3.6H2O 160oC, 10 min 98 22 LA (10 %), lactic acid (17%) [31]
Glucose 0.25 AlCl3.6H2O 160oC, 10 min, H2O-NaCl 98 17 LA (29 %) [31]
Glucose 5a MgCl2 0.8 M 160
oC, 70 min 15.6 3.9 n/a [32]
Glucose 5a MgSO4 0.8 M 160
oC, 70 min 45.1 7.7 n/a [32]
Glucose 5a NaCl(0.8M)-HCl 160
oC, 70 min 13.4 3.3 n/a [32]
Glucose 5a KCl (0.8M)-HCl 160
oC, 70 min 14.4 3.4 n/a [32]
Glucose 5a MgCl2(0.8M)-HCl 160
oC, 70 min 11.1 2.7 n/a [32]
Glucose 5a CaCl2(0.8M)-HCl 160
oC, 70 min 18 3.8 n/a [32]
Glucose 10a
CrCl2, 12 mol% 120oC, 3 h 65 <3 % humin [33]
Glucose 10a
AlCl3, 12 mol% 120oC, 3 h 74 <3 % humin [33]
Glucose 10a
FeCl3, 12 mol% 120oC, 3 h 7 0 humin [33]
Glucose 10a
CuCl2, 12 mol% 120oC, 3 h 4 0 humin [33]
Glucose 0.2
CrCl3·6H2O, 12 mol% 140oC, 1 h, 80 bar 99 13 % LA (13 %) [34]
Glucose 0.2
AlCl3·6H2O, 12 mol% 140oC, 1 h, 80 bar 88 19 % LA (6 %) [34]
Glucose 0.2
ZrCl4, 12 mol% 140oC, 1 h, 80 bar 69 5 % LA (13 %) [34]
Glucose 0.2
CuCl2·2H2O, 12 mol% 140oC, 1 h, 80 bar 23 6 % LA (2 %) [34]
Glucose 0.2
BiCl3, 12 mol% 140oC, 1 h, 80 bar 17 5 % LA (3 %) [34]
Glucose 0.2
FeCl3·6H2O, 12 mol% 140oC, 1 h, 80 bar 12 3 % LA (0 %) [34]
Glucose 0.2
MgCl2, 12 mol% 140oC, 1 h, 80 bar 22 0.8 % LA (0 %) [34]
a in wt %;
b 2-sec-butylphenol;
c water is one of the solvents
116
Table 1 Reactions of D-fructose and D-glucose in the presence of soluble metal salt in water (continued). Substrate Csugar, (M) Catalyst Conditions XC6sugar YHMF max (mol%) Other products Ref.
Biphasicc
Glucose 10a CrCl3-HCl 140
oC, 180 min, THF 97 59 LA (5 % at 180 min) [28]
Glucose 0.25 AlCl3.6H2O 160oC, 10 min,
H2O /THF 99 52 LA (trace), Lactic acid (13 %) [31]
Glucose 0.25 AlCl3.6H2O 160oC, 10 min,
H2O-NaCl/THF 99 61 LA (1 %) [31]
Glucose 5a AlCl3-HCl 170
oC, 40 min,SBP
b,c 91 62 LA [35]
Glucose 5a VCl3-HCl 170
oC, 90 min,SBP
b,c 92 49 LA [35]
Glucose 5a GaCl3-HCl 170
oC, 2 h,SBP
b,c 90 45 LA [35]
Glucose 5a InCl3-HCl 170
oC, 150 min,SBP
b,c 86 45 LA [35]
Glucose 5a YbCl3-HCl 170
oC, 2 h,SBP
b,c 93 43 LA [35]
Glucose 5a DyCl3-HCl 170
oC, 160 min,SBP
b,c 93 38 LA [35]
Glucose 5a SnCl4-HCl 170
oC, 45 min,SBP
b,c 90 52 LA [35]
a in wt %;
b 2-sec-butylphenol;
c water is one of the solvents
118
An interesting recent development is the conversion of D-glucose to HMF using a tandem
isomerisation-dehydration reaction with metal salts as the catalysts. Here, it is speculated
that a Lewis acid metal center catalyses the isomerisation of D-glucose to D-fructose,
followed by the conversion of D-fructose to HMF. Whereas the initial focus for the tandem
isomerisation-dehydration reaction was on water only (max. 40% HMF yield), recent
advances show that biphasic systems allow for higher HMF yields (up to 62 mol%, see Table
1 for details).
The metals salts described in Table 1 can be classified in water compatible and water
sensitive compounds [36]. Well known examples of water compatible metal salts are
lanthanide triflates like Y(OTf)3 and La(OTf)3. When dissolved in water, the metal ions
catalyse important chemical transformations by acting as a strong Lewis acid. In contrast,
water sensitive metal salts based on Al, Sn, Fe, Ga and In are hydrolysed in water to various
mono- and oligomeric species and the formation of solutions with a pH < 7. As such, these
water sensitive metal salts in water may both act as a Brönsted and Lewis acid [34].
We here report an experimental study on the conversion of inulin, a D-fructose rich
oligomer, to HMF in water using a range of water soluble metal salts. This study
complements earlier research in our group on the Brönsted acid catalyzed reaction of inulin
to HMF. Inulin is a biopolymer consisting of mainly D-fructose and minor amounts of D-
glucose (Figure 1). It is present in jerusalem artichoke tubers, chicory roots, camas bulbs and
dahlia tubers, with inulin contents of around 15-20 wt % on fresh weight [37-38]. As such, it
is an attractive biopolymer for the synthesis of HMF.
Fig. 1 Inulin structure.
A number of studies have been reported on the synthesis of HMF from inulin in water [39-
42]. Highest HMF yields reported so far in a monophasic system are from Wu et al., [39],
who reported a 53 % HMF yield when performing the reaction in water in the presence of
119
CO2 (11 MPa) at 200°C. Further improvements to 70% HMF were reported by Benvenuti et
al., [40] using a solid acid catalyst in combination with a biphasic system consisting of MIBK
and water [40]. However, to the best of our knowledge, the use of metal salts for the
conversion of inulin to HMF has not been reported to date. A number of metal salts were
screened and the best was selected for a more systematic study to probe the effects of
process conditions on HMF yield. Finally, an experimental design study was initiated to
quantify the effects of process conditions on HMF yield.
5.2 Experimental Section
5.2.1 Chemicals
Inulin isolated from Dahlia tubers was purchased from Acros Organic (Geel, Belgium). D-
fructose (99%) and levulinic acid (≥ 97%) were obtained from Acros Organic (Geel, Belgium).
Formic Acid and D-glucose were purchased from Merck KGaA (Darmstadt, Germany). 5-
Hydroxymethylfurfural (HMF) (≥ 99%) was obtained from Aldrich (Steinheim, Germany).
Copper (II) chloride dihydrate (≥ 99%) was bought from Acros Organics (Geel, Belgium). All
other salts were obtained from Acros Organics (Geel, Belgium). All chemicals were used
without purification. Deionized water was used to prepare the solutions.
5.2.2 Experimental procedures
5.2.2.1 Catalyst screening experiments
For the catalyst screening experiments, 2 mL of a solution of inulin (0.2 g inulin) and the
metal salt (5 mM) in water was placed in a microwave tube. The tube sealed with a plastic
cap and placed in the microwave device (CEM Synthesis Explorer 48), heated to 170oC and
stirred for 1 h at this temperature. After reaction, the tube was rapidly cooled, the contents
were filtered and the liquid phase was analysed by HPLC.
5.2.2.2 Experimental design experiments
A similar experimental procedure as for the screening experiments was applied, the only
difference that solely copper(II) chIoride was used as the catalyst. In addition, a broad
window of process variables was used, viz temperatures in the range of 160-180 oC, initial
inulin loadings in the range of 0.05 – 0.15 g/mL, copper (II) chloride concentrations between
0.005-0.015 M and reaction times between 10 and 120 min.
120
5.2.3 Analyses
HPLC was used to identify and quantify the components in the liquid product after reaction.
An Agilent technologies 1200 series HPLC equipped with an isocratic pump, a Bio-Rad
Aminex HPX-87H organic acid column (300 mm x 7.8 mm) and a Waters 410 differential
refractive index detector was used. Sulfuric acid in water (5 mM) was used as the eluent with
a constant flow rate of 0.55 ml/min. The column was operated at 60 oC. The analysis time
for each sample was 60 minutes. The HPLC was calibrated with solutions of known
compounds. Using the chromatogram peak area and the external calibration curve, the
unknown concentrations of components in the liquid phase was determined.
Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-
TOF/MS) on a Voyager-DE PRO was used to determine the molecular weight of the inulin
sample. 2,5- Dihydroxybenzoic acid (DHB) was used as the matrix.
The composition of the inulin sample and particularly the type and amount of C6-sugars was
determined by an acid-catalyzed hydrolysis reaction. For this purpose, inulin (2.5 g) was
dissolved at 70 oC in 150 mL of water under stirring. The pH was adjusted to 1.4-1.6 by
adding an aqueous HCl solution. Then, the solution was placed in a water-bath for 30
minutes at 90oC. A liquid sample was taken and analysed by HPLC.
5.2.4 Definitions
The HMF yield (YHMF) is reported on a wt% basis and defined as follows:
Y HMF = CHMF × MHMF × V
WIn x 100% (g HMF/g inulin intake) (1)
The yield of LA is defined as:
Y LA = CLA × MLA × V
WIn x 100% (g LA/g inulin ntake) (2)
In equation (1) and (2), CHMF and CLA represent the concentration of HMF and LA, respectively
(mol/L) as measured with HPLC, MHMF and MLA the molecular weight of HMF and LA (g mol),
V the water intake (L) and Win the intake of inulin on a weight basis (g).
121
The yield of HMF was converted from wt% to mol% by assuming that inulin consist of linked
glucose/fructose units (C6H10O5) which react to HMF according to the following
stoichiometry:
C6H10O5 → C6H6O3 + 2 H2O (3)
As such, the maximum yield of HMF is 78 wt%. Thus, the yield of HMF in mol% may be
calculated from the yield in wt% by dividing the latter by 0.78.
5.2.5 Statistical modeling
The yield of HMF as a function of process variables was modelled using the Design-Expert 7
software package(Stat-Ease) using a standard expression given in equation (4):
YHMF=bo+ ∑ 𝑏𝑖
4
𝑖=1
𝑥𝑖+ ∑ ∑ 𝑏𝑖𝑗𝑥𝑖𝑥𝑗
4
𝑗=1
4
𝑖=1
(4)
The process variables (xi: inulin intake, temperature, copper (II) concentration and reaction
time) are represented by the indices 1–4. The regression coefficients were obtained by
statistical analyses of the data. Significance of factors was determined by their p-value in the
ANOVA analyses. A factor was considered significant if the p-value is below 0.05, meaning
that the probability of noise causing the correlation between a factor and the response is
lower than 0.05. Insignificant factors were eliminated using backward elimination, and the
significant factors were used to model the data.
5.3 Result and Discussion
5.3.1 Inulin characterisation
Details on the characterisation of the inulin feed used in this study (originating from Dahlia
tubers) are given in Chapter 3. Of relevance are the values for the 𝑀𝑛 (2560) and the 𝑀𝑤
(3680) [43, 44, 45], the D-fructose content (94 mol%), and the fructose to glucose ratio
(15:1) indicating that the sample mainly contains of D-fructose units (DP of 15), with each
oligomer chain on average capped with one D-glucose unit [45].
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5.3.2 Catalyst screening experiments
A number of homogeneous metal salts containing Al, Sn, Fe and Cu metal ions with chloride
and sulphate anions were tested for the reaction of inulin to HMF. The screening
experiments were conducted at 170oC, a fixed inulin intake of 0.1 g/mL and a salt
concentration of 5 mM. The yield of HMF was determined (HPLC) and the results are shown
in Figure 2. Under these non-optimised conditions, the HMF yield varied between 18 and 30
wt% (23 and 38 mol%).
Fig. 2 HMF yield as a function of the homogeneous metal salt catalyst (170oC, 0.1 g/mL inulin, salt concentration of 5 mM, 1 h).
The best results were obtained for the Cu salts whereas Al salts gave the worse results. In addition,
the results for the sulphate anions were in general less than for the chloride anion, an indication for a
small though significant anion effect. Catalyst performance of the metal salts is expected to be a
function of both the Brönsted and Lewis acidity of the solution. To gain some insight in the relative
importance of both factors, the pH of all solutions was measured at room temperature and the
results are shown in Table 2 and Figure 3.
123
Table 2 pH of the solutions of the metal salts in water.
Salt pHa
YHMF (mol%) Salt pHa YHMF (mol%)
Al2(SO4)3 4.04 18 FeSO4 2.90 25
AlCl3 4.34 19 FeCl3 2.96 27
SnSO4 2.97 21 CuSO4 5.54 28
SnCl2 3.11 22 CuCl2 5.71 30
a Measured at room temperature, 5 mM solution
Fig. 3 HMF yield versus the pH of the solution of the various metal salts in water (170oC , inulin intake of 0.1 g/mL, 5 mM salt, 30 min).
The HMF yield after 30 min is a function of the pH of the solution and shows a clear
minimum at a pH of about 4. All salts, except Cu salts, follow the same trend, i.e. a higher pH
leads to a lowering of the maximum HMF yield as well as the rate of HMF formation (Figure 2
and 3). Thus, it appears that the H+ concentration plays a role in the rate of the reactions,
with lower pH values leading to higher HMF yields after 30 min. Rationalisation of the data is
difficult as the maximum HMF yield is determined by the rate of at least 3 individual
reactions (vide infra), all with a different response regarding the pH. However, it is clear that
Cu salts are an exception in this trend. It shows the highest rate of HMF formation as well as
the highest HMF yield within the series, despite that the pH of the solutions is by far the
lowest. So far we do not have a sound explanation for the good performance of Cu salts. One
of the possibilities is that a Cu cation indeed acts as a Lewis acid and effectively catalyses the
reaction of D-fructose to HMF without having a major effect on the subsequent reaction of
124
HMF to LA, giving rise to the highest HMF yield. Aqueous solutions prepared from copper (II)
chloride contain a range of copper (II) complexes depending on concentration, temperature,
and the presence of additional chloride ions. These include Cu(H2O)6]2+ and halide complexes
of the formula [CuCl2+x]x− and some of them may act as Lewis acids. Based on the results
given in Figure 2, it is clearly that copper (II) chloride gave the highest HMF yield and as a
result this salt was selected for further (systematic) studies.
5.3.3 Exploratory experiments on the conversion of inulin to HMF using CuCl2
Exploratory experiments on the conversion of inulin in presence of CuCl2 in water were
performed at 180oC, an inulin intake of 0.1 g/mL and a CuCl2 concentration of 0.01 M. A
typical concentration versus time profile for the main products is given in Figure 4.
Fig. 4 Concentration versus time profile for the reaction of inulin using CuCl2 (T=180oC, CCu(II)= 0.01 M, Cinulin= 0.1 g/mL).
The main products in the course of the reaction are D-glucose, D-fructose, HMF, levulinic
acid (LA) and formic acid (HPLC, Figure 5). Minor amounts of acetic acid and glyceraldehyde
were also detected. D-fructose, D-glucose and HMF are clearly intermediates in the reaction
sequence and show an optimum in the concentration versus time profile. LA and FA are the
final products.
125
Fig. 5 Typical HPLC spectrum of a reaction product of the reaction of inulin with CuCl2 in aqueous solutions.
On the basis of the concentration versus time profile, a reaction network is proposed and
given in Figure 6. The initial step involves hydrolysis of the inulin to D-fructose, D-glucose
and acetic acid. The latter is likely formed by the hydrolysis of minor amounts of acetyl
groups present in the inulin [46]. This hydrolysis reaction is known to be catalysed by
Brönsted acids though Lewis acids have also been reported to be active [47-48].
Subsequently both the D-fructose and D-glucose are dehydrated to HMF, which in a
consecutive reaction is then rehydrated to LA and FA. At this stage, it is not clear whether
the D-glucose converts to HMF directly or is first isomerized to D-fructose by the action of a
Lewis acids like Cu2+ (tandem isomerization-dehydration) or directly to HMF.
126
Although CuCl2 is reported to be a mediocre catalyst for the isomerization of D-glucose to D-
fructose in water compared to for instance AlCl3 [34], our findings indicate that CuCl2 is a
good active catalyst to promote HMF formation when D-fructose instead of D-glucose is
used as the feedstock. It suggests that Cu-salts are more active catalyst for the dehydration
of D-fructose to HMF rather than for the D-glucose-fructose isomerisation reaction.
Fig. 6 Proposed reaction network for inulin in copper (II) chloride solutions (acetyl
substituents on the inulin structure not shown).
The minor product glyceraldehyde is likely formed from D-fructose via a retro-aldol
condensation [30, 46]. It is known to react further to pyruvaldehyde and subsequently to
lactic acid [30, 49-51]. Also minor amounts of furfural are detected in the reaction and likely
originate from D-fructose [46, 49]. In addition, brown-black insolubles were formed, known
as humins, which are always formed during catalytic conversions of carbohydrates in water
[5, 52].
127
5.3.4 Effect of process conditions on HMF and LA yields for inulin using CuCl2 solutions
To optimise the HMF yield for CuCl2, a number of experiments were carried out at a range of
process conditions (160-180 oC, inulin loading between 0.05-0.15 g/mL, CuCl2 concentrations
between 0.005 and 0.015 M). The reactions were carried in a microwave assisted batch
reactor at different reaction times to obtain time concentration profiles, with a maximum
reaction time of 2 h.
The effect of the temperature on the HMF and LA yield is shown in Figure 7 at a constant
inulin intake (0.1 g/ml) and catalyst concentration (0.01 M). With regards to HMF yield, a
maximum is observed and both the absolute value of this maximum and the time to reach
the maximum yield are a function of the temperature. As expected based on the proposed
network in Figure 6, the optimum is observed at shorter reaction times when using higher
temperatures. The maximum HMF yield is also highest for the highest temperature in the
range, though the effect is not very pronounced (supported by subsequent modeling studies,
vide infra). Rationalisation of this temperature effect on HMF yields is difficult as HMF is
formed by a number of consecutive reactions from inulin and prone to further reactions, all
with their own activation energies. In addition, humin byproducts are also formed, with
temperature depending reaction rates.
Fig. 7 Yield of HMF and LA versus time at different temperatures (Cinulin= 0.1 g/mL, CCuCl2=0.01 M)
The LA yield versus time profile also is a clear function of the temperature, with higher
temperatures leading to higher LA yields (Figure 7). However, it is not possible to determine
128
the temperature at which the LA yield is at its maximum, as not all HMF has been reacted to
LA within the batch time of 2 h.
The effect of the inulin intake on the yield of LA and HMF is shown in Figure 8 at a constant
temperature of 180 °C and a catalyst concentration of 0.01 M. The HMF yield is a function of
the intake, with higher intakes leading to lower HMF yields. These findings are in line with a
kinetic study from our group on the reaction of D-fructose to HMF and LA using a Brönsted
acid (sulphuric acid) [53]. This observation was rationalised by considering that the order of
substrates for the reactions leading to humins is higher than those for the desired main
reactions. As a consequence, dilution is favoured to enhance the yields of LA and HMF and to
reduce polymerisation reactions.
The LA yield is a clear function of the inulin intake with higher inulin intakes leading to higher
yields. However, these data are not conclusive as the HMF and C6 sugar conversion are not
yet quantitative after 2 h reaction time.
Fig. 8 HMF and LA yield versus the time at various inulin intakes (T= 180oC, CCuCl2= 0.01 M).
The effect of the CuCl2 concentration on both the HMF and LA yield is shown in Figure 9
(180oC, inulin intake of 0.1 g/ml). As expected, the catalyst has a positive effect on the
reaction rates. For instance, the HMF conversion is about quantitative after 2 h reaction time
at the highest catalyst concentration, whereas it is considerably lower for the lowest catalyst
concentration. The maximum HMF yield is also a function of the catalyst concentration and it
appears that a higher catalyst concentration has a negative effect on the maximum
129
attainable HMF yield. This suggests that the order in catalyst concentration is not equal for
all reactions in the network.
Fig. 9 HMF and LA yield versus the time at various CuCl2 concentration (T= 180oC, Cinulin= 0.1 g/mL)
A similar trend was observed for a detailed kinetic study by our group on the conversion of
D-fructose to HMF/LA using sulphuric acid as the catalyst [53]. It is not possible to assess the
effect of the catalyst concentration on the LA yield as the HMF conversion is not yet
quantitative for the longest batch time (2 h). However, tentatively, it appears that the
highest catalyst concentration in the range gives the best LA yield. These findings are again in
line with our kinetic study on the conversion of D-fructose to HMF/LA where the highest LA
yield was also obtained with the highest sulphuric acid concentration in the range (1 M).
5.3.5 Quantification of process conditions on HMF and LA yield using experimental design and statistical modeling
A total of 30 experiments were performed at different reaction conditions using a central
composite design to quantify the effect of process conditions on the HMF and LA yield.
Independent variables were the temperature, inulin intake, copper(II) chloride concentration
and reaction time. The dependent variables were the HMF and the LA yield (Table 3). The
center point was measured 6 times, giving an average HMF yield of 20.8 wt% and a standard
deviation of 1.5. As such, the reproducibility is good.
130
Table 3 Overview of experiments for the reaction of inulin in water using CuCl2 as the catalyst.
Run Temperature
(T, oC)
Inulin intake (Cinulin, g/mL)
CuCl2 concentration, (Ccat, M)
Reaction time (t, min)
yHMF
(wt %)a
yLA
(wt %)b
1 170 0.15 0.010 65 18.2 8.3
2 160 0.15 0.005 10 9.8 1.4
3 170 0.10 0.010 65 20.6 9.7
4 170 0.10 0.010 10 21.6 3.0
5 180 0.15 0.005 10 25.8 2.6
6 160 0.05 0.015 10 23.5 5.2
7 160 0.05 0.015 120 25 10.0
8 160 0.05 0.005 10 19.7 6.3
9 170 0.10 0.005 65 18.7 7.8
10 170 0.10 0.010 65 23.0 13.1
11 160 0.15 0.005 120 3.3 7.2
12 180 0.10 0.010 65 18.4 19.0
13 160 0.15 0.015 120 10 8.3
14 170 0.10 0.010 120 7.9 16.1
15 160 0.15 0.015 10 9.7 4.0
16 170 0.10 0.010 65 19.0 9.7
17 180 0.15 0.015 10 17.0 5.3
18 180 0.05 0.015 10 21.4 9.8
19 170 0.10 0.010 65 22.0 11.0
20 160 0.10 0.010 65 23.5 5.2
21 160 0.05 0.005 120 13.6 4.7
22 170 0.10 0.015 65 18.4 14.8
23 180 0.15 0.005 120 1.0 21.6
24 170 0.05 0.010 65 25.4 13.3
25 170 0.10 0.010 65 20.7 10.7
26 180 0.05 0.005 10 30.3 6.5
27 180 0.05 0.015 120 2.0 28.9
28 180 0.15 0.015 120 3.4 27.1
29 170 0.10 0.010 65 19.9 10.6
30 180 0.05 0.005 120 4.4 27.8
a The yield of HMF is defined in equation 1;
b The yield of LA is defined in equation 2
The highest experimental HMF yield within the range was 30 wt % (38.5 mole %) and was
obtained at 180oC, an inulin intake of 0.05 g/mL, a CCu2+ of 0.005 M and 10 min reaction
time. The HMF yield as a function of the processing parameters was modelled using
multivariable regression using the Design-Expert 8 software. The data are best described
with the model provided in equation 5. The model includes both quadratic and interaction
terms of the independent variables.
131
yHMF = (0.46)T + (9258.19)Ccat + (1.46)t + (4.79)T Cload + (7.23)Ccat t – (889.226)Cload -
(48.76)T Ccat – (8.26 x 10-3)T t – 69701.60 Ccat2 – (1.83x 10-3) t2 - 53.94
(5)
An extended version with more significant digits for the coefficients to be used for among
others reactor engineering studies is given in the supplementary information. The analysis of
variance of the model is given in Table 4. The R- square for the model is 0.9764, indicating
that the model fits the experimental data well. This is supported by a parity plot in Figure 10
showing the data and model points.
Table 4 Analysis of variance of the preferred model for the yield of HMF.
Source Sum of squares df Mean square F-value p-value
Prob> F
Model 1831.98 10 183.20 78.50 < 0.0001 significant
A-T 11.81 1 11.81 5.06 0.0365
B-Cload 247.48 1 247.48 106.04 < 0.0001
C-Ccat. 0.92 1 0.92 0.39 0.5373
D-t 647.07 1 647.07 277.25 < 0.0001
AB 91.95 1 91.95 39.40 < 0.0001
AC 95.09 1 95.09 40.74 < 0.0001
AD 330.29 1 330.29 141.52 < 0.0001
CD 63.23 1 63.23 27.09 < 0.0001
C2 10.46 1 10.46 4.48 0.0477
D2 106.06 1 106.06 45.45 < 0.0001
Residual 44.34 19
132
Fig. 10 Parity plot for the HMF yield.
The yield of HMF as a function of the process condition as predicted by the model is given in
Figure 11. Clearly, the reaction time has a profound effect on the HMF yield, rationalized by
assuming that HMF is an intermediate in the reaction sequence and ultimately will be
converted to HMF. Due to the interactions between the independent variables, it is difficult
to draw general conclusions regarding the effect of individual independent variables on the
HMF yield.
Fig. 11 Modeled HMF yield versus temperature and inulin intake at two batch times.
The yield of LA as a function of the process conditions was also modeled and the results are
given in Table 5 and eq 6. An extended version of eq. 6 with more significant numbers for the
coefficients to be used for among others reactor engineering studies is given in the
133
supplementary information. The model describes the data points well as is expressed by an
R-squared value of 0.9651 and a parity plot (Figure 12).
yLA = (0.17)T + (204.63)Cload + (303.82)Ccat – (1.2)t – (1.38)T Cload + (7.73 x 10-3)T t -25.22
(6)
Table 5 Analysis of variance of the preferred model of LA.
Source Sum of squares df Mean square F-value p-value
Prob> F
Model 1538.79 6 256.47 106.20 < 0.0001 significant
A-T 515.98 1 515.98 213.29 < 0.0001
B-Cload 39.77 1 39.77 16.44 0.0005
C-Ccat. 41.54 1 41.54 17.17 0.0004
D-t 644.84 1 644.84 266.56 < 0.0001
AB 7.60 1 7.60 3.14 0.0895
AD 289.06 1 289.06 119.49 < 0.0001
Residual 55.64 23
Fig. 12 Parity plot for LA yield.
The effect of two process conditions (temperature and reaction time) is presented in Figure
13. The LA yield increases at higher temperatures and longer reaction times, in line with the
134
proposed reaction network (Figure 6) where LA is the final product and HMF an
intermediate.
Fig. 13 Modeled LA yield versus temperature and reaction time at a fixed inulin intake (0.1 g/ml) and CuCl2 concentration (0.01 M).
5.4 Conclusions
An exploratory screening study revealed that metal salts may be used as catalysts for the
reaction of inulin to HMF in water. Best results were obtained using CuCl2, giving an HMF
yield of 30.3 wt % (39 mol %) at 180oC, 0.05 g inulin/mL, a 0.005 M CuCl2 solution and a
reaction time of 10 minutes. The special role of Cu2+ salts is likely due to the Lewis acidic
character of Cu2+ and not due to catalysis by Brönsted acids (H+). However, further
investigations using model reactions with intermediates in the reaction sequence (D-
fructose, HMF) and Cu salts will be required to draw definite conclusions. In addition, it was
also shown that metal salts with chloride anions perform better than sulphate anions. Also
this aspect requires further attention and both findings may aid the development of more
efficient catalysts for the conversion of the C6 sugar in biopolymers to HMF and levulinic acid
in water.
135
5.5 Nomenclature
CHMF : concentration of HMF (mol L-1)
CLA : concentration of levulinic acid (mol L-1)
MHMF : molecular weight of HMF (g mol-1)
MLA : molecular weight of LA (g mol-1)
t : time (min)
T : temperature (oC)
V : volume of reaction (L)
Win : intake of inulin (g/L)
YHMF : yield of HMF (wt%)
YLA : yield of LA (wt%)
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[53] Fachri BA, Abdilla RM, Rasrendra CB, Heeres HJ Experimental and modeling studies
on the acid-catalysed conversion of inulin to 5-hydroxymethylfurfural in water.
Submitted to Chemical Engineering Research and Design, 2015.
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5.7 Supplementary Material
Experimental and Modeling Studies on the Conversion of Inulin to 5-
Hydroxymethylfurfural using Metal Salts in Water
HMF yield = -53.93689 +0.46408 x T -889.22578 x Cload +9258.18974 x Ccat +1.46153 x t +4.79451 x T x Cload -48.75759 x T x Ccat -8.26089E-003 x T x t +7.22885 x Ccat x t -69701.59837 x Ccat^2 -1.83443E-003 x t^2 LA yield = -25.22166 +0.17093 x T +204.62752 x Cload +303.82489 x Ccat -1.20496 x t -1.37857 x T x Cload +7.72812E-003 x T x t
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Summary
Biomass is a valuable feed for the production of biofuels and biobased products. The
development of efficient chemical processes for biomass conversion to biobased chemicals is
high on the global research agenda. A number of promising chemicals from biomass have
been identified and are known as platform chemicals. Bozell & Petersen (2010) compiled a
list of 12 biobased chemicals that have the highest techno-economical potential. This top 12
list includes 5-hydroxymethylfurfural (HMF), which may be converted to versatile building
blocks for polymers such as 1,6-hexanediol, 2,5-furandicarboxylic acid and fuel-additives
such as 2,5-dimethylfuran.
HMF is typically obtained by reacting C6 sugars in water in the presence of a Brönsted acid.
By-products of the reaction are levulinic acid (LA) and formic acid and insolubles known as
humins. In water, D-fructose is the C6 sugar feed of choice and gives a maximum HMF yield
of around 55 mol%, whereas the yields for D-glucose are less than 10 mol%.
However, economically viable processes for HMF have not been developed yet. Major
improvements in product selectivity and yield are required. This should not lead to high costs
related to, for instance, expensive catalyst and solvent recycles. In addition, there is a high
incentive to use cheap biomass feeds, preferably with a high amount of D-fructose units to
obtain good HMF yields.
In this thesis, the use of inulin as a carbohydrate biomass feed for HMF synthesis has been
explored. Inulin is a biopolymer mainly consisting of D-fructose and minor amounts of
glucose and as such is an attractive biopolymer for the synthesis of HMF. The molecular
weight of inulin is by far less than typical biopolymers like cellulose, hemicellulose and
starch. Degrees of polymerisation between 2 and 70 have been reported. Inulin is present in
Jerusalem artichoke tubers, Chicory roots, Camas bulbs and Dahlia tubers in amounts of
around 20 wt% on fresh weight. The emphasis has been on water as the solvent as it is
environmentally benign and most carbohydrates and products have a good solubility in
water. A range of catalysts has been explored, examples are cheap inorganic Brönsted acids
and metal salts. In addition, the reaction in the absence of a catalyst has also been explored.
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In Chapter 2, a detailed kinetic study on the conversion of D-fructose to HMF and LA in water
using a simple inorganic acid in the form of sulphuric acid is reported. This is a key step in the
conversion of inulin to HMF. A maximum-likelihood approach was used to estimate the
kinetic parameters for the main reaction to HMF and the side reactions to LA and humins. A
good fit between experimental data and the kinetic model was obtained. The kinetic model
supported by the experiments implies that highest HMF yields from fructose (about 56
mol%) is attainable at a low acid concentration, whereas the temperature and the initial
fructose concentration are of less importance. In addition, a reactor with a small extent of
backmixing (PFR) is the preferred reactor configuration. The results were compared with
earlier kinetic studies conducted in our group on the conversion of D-glucose to HMF/LA in
water using sulphuric acid as the catalyst. D-fructose is two orders of magnitude more
reactive than D-glucose and HMF yields are a factor 10 higher.
In Chapter 3, an experimental study on the synthesis of HMF from inulin in water in the
absence of a catalyst is provided. The reactions were conducted in a temperature range of
153-187°C, an inulin loading between 0.03 and 0.12 g/mL and batch times between 18 and
74 minutes using a central composite experimental design. The highest experimental HMF
yield in the process window was 35 wt% (45 mol%). The HMF yields were modeled using a
statistical approach and good agreement between experimental data and model was
obtained. The possible autocatalytic role of formic acid (FA) and LA, two main by-products,
was probed by performing reactions in the presence of these acids and it was shown that
particularly FA acts as a catalyst.
A systematic study to optimise the HMF yield from inulin for reactions in water using
sulphuric acid as the catalyst is reported in Chapter 4. The latter was selected on the basis of
a screening study with seven organic- and inorganic Brönsted acids (H2SO4, HNO3, H3PO4,
HCl, trifluoroacetic acid, maleic acid and fumaric acid). The effect of process conditions such
as temperature, inulin loading, sulphuric acid concentration and reaction time on HMF and
LA yields were determined experimentally and subsequently modeled using non-linear
multivariable regression. The highest experimental HMF yield was 39.5 wt% (50.6 mol%) and
was obtained at 170°C, an inulin loading of 0.17 g/mL, a sulphuric acid concentration of
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0.006 M and a reaction time of 20 min. Agreement between experiments and model for both
HMF and LA yield was very satisfactorily.
In Chapter 5, an exploratory experimental study on the use of a wide range of homogeneous
metal salts as catalysts for the conversion of inulin to HMF in water is reported. Best results
were obtained using CuCl2. A subsequent screening study on the effect of process conditions
on HMF yield using CuCl2 revealed that the highest HMF yields are attainable at the highest
temperature in the range (180°C) and the lowest inulin intake (0.05 g/ml). The effects of
process conditions on HMF yield were systematically investigated and quantified using a
central composite design. The highest experimental HMF yield in the process window was
30.3 wt% (39 mol%, 180oC, 0.05 g/mL inulin, 0.005 M CuCl2 and a reaction time of 10
minutes). The HMF yields were modeled using non-linear, multi-variable regression and good
agreement between experimental data and model was obtained.
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Samenvatting
Biomassa is een waardevolle grondstof voor de productie van biobrandstoffen en groene
producten. De ontwikkeling van efficiënte chemische processen voor de omzetting van
biomassa naar groene chemicaliën staat wereldwijd hoog op de onderzoek agenda. Een
aantal veelbelovende chemicaliën zijn geïdentificeerd en staan bekend als ‘platform
chemicals’. Bozell & Petersen (2010) hebben een lijst samengesteld van 12 groene
chemicaliën die het grootste techno-economische potentieel hebben. Op deze top 12 lijst
staat ook 5-hydroxymethylfurfural (HMF), dat omgezet kan worden in bouwstenen voor
polymeren, zoals 1,6-hexaandiol en 2,5-furaandicarbonzuur, en in brandstof additieven zoals
2,5 dimethylfuraan.
HMF kan worden gemaakt door C6 suikers te laten reageren in water in de aanwezigheid van
een Brönsted zuur. De bijproducten van de reactie zijn levulinezuur (LA), mierenzuur (FA) en
onoplosbare verbindingen die bekend staan als humines. In water is D-fructose de meest
gebruikte voeding en geeft een maximale HMF opbrengst van ongeveer 55 mol%, terwijl de
opbrengsten bij het gebruikt van d-glucose minder dan 10 mol% zijn.
Economisch haalbare processen voor de productie van HMF zijn nog niet ontwikkeld. Er zijn
forse verbeteringen in HMF selectiviteit en opbrengst nodig. Deze mogen echter niet leiden
tot hoge kosten voor bijvoorbeeld dure katalysatoren en proces stappen voor het hergebruik
van oplosmiddelen. Daarnaast is er een grote drijfveer om goedkope biomassa voedingen te
gebruiken, bij voorkeur met een hoog gehalte aan D-fructose om goede HMF opbrengsten te
realiseren.
In dit proefschrift is het gebruik van inuline als grondstof voor de synthese van HMF
onderzocht. Inuline is een biopolymeer dat voornamelijk uit D-fructose bestaat, naast een
kleine hoeveelheid D-glucose, en als zodanig dus een aantrekkelijk biopolymeer voor de
synthese van HMF. De polymerisatiegraad van inuline (tussen 2 en 70) is aanzienlijk lager
dan die van typische biopolymeren als cellulose, hemicellulose en zetmeel. Aardperen,
cichorei, camas bollen en dahlia knollen bevatten tot ongeveer 20% van het vers gewicht aan
inuline. In dit proefschrift is met name gekeken naar het gebruik van water als oplosmiddel
voor de omzetting van inuline naar HMF omdat het een milieuvriendelijk oplosmiddel is en
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de meeste koolhydraten en producten goed in water oplossen. Een breed scala aan
katalysatoren is onderzocht, bijvoorbeeld goedkope Brönsted zuren en metaal zouten.
Daarnaast is gekeken in hoeverre het mogelijk is om de reactie uit te voeren zonder een
katalysator.
In hoofdstuk 2 wordt een gedetailleerde studie beschreven naar de omzetting van D-
fructose naar HMF en LA in water met het gebruik van een simpel anorganisch zuur
(zwavelzuur). Dit is een belangrijke stap in de omzetting van inuline naar HMF. De kinetische
parameters van de reacties naar HMF en de zijreacties naar LA en humines zijn bepaald. Het
model komt goed overeen met de experimentele data. Het kinetische model geeft aan dat
de hoogste opbrengsten aan HMF (ongeveer 56% mol) bereikbaar zijn bij lage zuur
concentraties. Deze voorspelling wordt ondersteund door de experimentele data. De
temperatuur en de initiële concentratie van D-fructose zijn van minder belang. Daarnaast is
gebleken dat de beste reactor configuratie een reactor zonder backmixing is (PFR). De
resultaten zijn vergeleken met eerdere kinetische studies naar de conversie van D-glucose
naar HMF en LA in water met zwavelzuur als katalysator. D-fructose is een factor 100
reactiever dan D-glucose en HMF opbrengsten zijn een factor 10 hoger.
In hoofdstuk 3 wordt een experimentele studie naar de synthese van HMF uit inuline in
water zonder externe katalysatoren beschreven. De reacties zijn uitgevoerd aan de hand van
een ‘central composite’ ontwerp in een temperatuurgebied van 153oC tot 187oC, met een
inuline hoeveelheid tussen de 0.03 en 0.12 g/ml en reactietijden tussen 18 en 74 minuten.
De hoogste experimentele opbrengst van HMF in deze “process window” was 35 wt% (45
mol%). De HMF opbrengsten zijn gemodelleerd met een statistische benadering en een
goede overeenkomst tussen experimenten en het model is gevonden. De mogelijkheid van
autokatalyse door mierenzuur en LA, de twee belangrijkste bijproducten, is onderzocht door
de reactie uit te voeren in de aanwezigheid van deze zuren. Het is gebleken dat met name
mierenzuur als katalysator kan fungeren.
Een systematische studie om de opbrengst van HMF uit inuline te optimaliseren voor
reacties in water met zwavelzuur als katalysator is beschreven in hoofdstuk 4. Zwavelzuur is
gekozen op basis van de resultaten van een vergelijkend onderzoek met zeven verschillende
organische en anorganische Brönsted zuren (H2SO4, HNO3, H3PO4, HCl, trifluorazijnzuur,
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maleinezuur en fumaarzuur). Het effect van proces condities als temperatuur, inuline
hoeveelheid, zwavelzuur concentratie en de reactietijd op de opbrengsten van HMF en LA is
experimenteel bepaald en vervolgens gemodelleerd. De hoogste experimentele HMF
opbrengst was 39.5 wt% (50.6 mol%) en werd bereikt bij 170oC, een inuline concentratie van
0.17 gr/ml, een zwavelzuur concentratie van 0.006 M en een reactietijd van 20 minuten. Er
werd een goede overeenkomst gevonden tussen de experimenten en het model voor zowel
de HMF als de LA opbrengst.
In hoofdstuk 5 wordt een exploratieve experimentele studie naar het gebruik van een breed
scala aan homogene metaal zouten als katalysator voor de omzetting van inuline naar HMF
in water beschreven. De beste resultaten zijn bereikt met CuCl2. Een daaropvolgende studie
naar het effect van proces condities op de opbrengst van HMF met CuCl2 liet zien dat de
hoogste HMF opbrengsten te verkrijgen zijn bij de hoogste temperatuur (180oC) en de
laagste concentratie inuline (0.05 g/ml) in het bereik. De effecten van proces condities op de
HMF opbrengst zijn systematisch onderzocht en gekwantificeerd met een “central
composite” ontwerp. De hoogste experimentele opbrengt van HMF was 30.3 wt% (39 mol%,
180oC, 0.5 g/ml inuline, 0.005 M. CuCl2, reactietijd 10 minuten). De HMF opbrengt is
gemodelleerd en een goede overeenstemming tussen de experimentele data en het model is
verkregen.
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Acknowledgement
Alhamdulillahirobbil ‘alamiin….
Finally the journey has come to an end. It is my pleasure to express my sincere gratitude to
all those who gave me the possibility to complete this thesis succesfully.
First of all, I would like to thank my supervisor Prof. dr. ir. H.J. Heeres for his valuable
guidance and countless support during my PhD time. Erik, thank you for giving me the
opportunity to pursue my PhD in your group. I appreciate your scientific thinking and
creative ideas to develop and improve my research skills.
I am also deeply indebted to my co-supervisor dr. C.B. Rasrendra for help, for his stimulating
suggestions and encouragement during my time of research, and writing of this thesis.
I would like to thank the members of the reading committee; Prof. A.A. Broekhuis, Prof. F.
Picchioni and Prof. G.J.W. Euverink for spending their precious time to improve my thesis.
I would also like to thank to the Rector of the University of Jember, M.Hassan, PhD and the
Vice Rector for academic affairs, Zulfikar PhD for support and encouragement.
I am thankful to Marya de Jonge and Annette Korringa-de Wit for their support in all
administrative matters during my PhD time.
I also thank Jan Henk Marsman and Leon Rohrbach for helping me with using the analytical
instruments. I also appreciate the support of Anne Appeldoorn, Marcel de Vries and Erwin
Wilbers during my PhD period. Special thanks to Maarten Vervoort for providing me the
ampoule reactors.
My sincere appreciation also goes to Alphons Navest and Tim Zwaagstra for their help
regarding financial and administrative affairs.
I also would like to thank Ria Abdilla (paranymph) and Henk van de Bovenkamp, who gave
valuable contributions regarding writing of the thesis and papers.
I want to thank all people in the chemical engineering department for giving me the sweet
hello: Ignacio Melian Cabrera, Jelle Wildschut, Henk van de Bovenkamp, Arjan Kloekhorst,
149
Claudio Toncelli, M. Bilal Niazi, Jan Willem Miel, Lidia Lopez Perez, Zheng Zhang, Valeriya
Zarubina, Anna Piskun, Maria Jesus Ortiz Iniesta, Diego Wever, Henky Muljana, Buana
Girisuta, M. Chalid, Agnes Ardiyanti, Laura Junistia, Teddy, M. Iqbal, Louis Daniel, Jenny
Soetedjo, Miftahul Ilmi, Angela Kumalaputri (paranymph), Susanti, Martiin Beljaars, Erik
Benjamin, Dennis van der Meulen, Lucas Mevius, Benny Bakker and Joni Arentz.
I also thank de Gromiest, PPIG, Christina Avanti, Surahyo Sumarsono, Awalia Febriana, Kadek
Sutrisna, Laksmi Kusumawati, Aisyah Tiar Arsyad, Abdul Muiz Pradipto, Guntur Fibriansyah,
Intan Windyasari, Aramel, Syarif Riyadi, Desti Alkano, Wahono, Donny K. Wardhono, Nieke
Dewi Sulistiyani, Sri Wahyuni, Mutia Delima, Robby Roswanda, Lia Atwa, Panji Triadyaksa,
Erythrina Stavila, Fanny Lintong, Rizqiya Astri Hapsari, Rachmawati Faturachman, Puri
Handayani, Nur Alia Oktaviani, Tiurma Pitta Alagan, Rosewitha Irawaty and Fean Sarian for
our friendship.
I would also like to thank Widyoseno Estitoyo, Sȇbastien Alexandrȇ (+), Yusuf Hehamahua
and family, Erna Subroto, S. Reza Maulana Ouvaroff, Aulia Tirtamarina, Mackenzie Hadi,
Cyndy Soemadiredja, Ria Abdilla, Yohanes Siem, Fanny Salangka, Adra Karim, Amba Mathilde
Pidada, Inez Taniwangsa, Ysbrand Galama, Bas van der Lee, Martijn Santes and Klara
Kwantoro for spending your time with me.
Finally I would like to thank to HM Zein Harahap families and HM Kariem families and
especially to my parents, brothers and sisters, nieces and nephews for all time support.
150
List of publications and presentation
1 B.A. Fachri, R.M. Abdilla, H.H. van de Bovenkamp, C.B. Rasrendra and H.J. Heeres,
Experimental and kinetic modeling studies on the sulphuric acid catalysed conversion of
D-fructose to 5-hydroxymethylfurfural and levulinic acid in water (submitted to ACS
Sustainable Chemistry and Engineering).
2 B.A. Fachri, R.M. Abdilla, C.B. Rasrendra and H.J.Heeres, Experimental and modeling
studies on the uncatalysed thermal conversion of inulin to 5-hydroxymethylfurfural and
levulinic acid (accepted in Sustainable Chemical Processes).
3 B.A. Fachri, R.M. Abdilla, C.B. Rasrendra and H.J.Heeres, Experimental and modeling
studies on the acid-catalysed conversion of inulin to 5-hydroxymethylfurfural in water
(submitted to Chemical Engineering Research and Design).
4 B.A. Fachri, C.B. Rasrendra and H.J.Heeres, Experimental and modeling studies on the
conversion of inulin to 5-hydroxymethylfurfural using metal salts in water (submitted to
Biomass Conversion and Biorefinery).
5 C.B. Rasrendra, B.A. Fachri, I.G.B.N. Makertihartha, Sanggono Adisasmito and H.J.
Heeres, Catalytic conversion of dihydroxyacetone to lactic acid using metal salts in water
(ChemSusChem., 2011, 4, 1–11).
6 B.A. Fachri, R.M. Abdilla, C.B. Rasrendra and H.J. Heeres, A kinetic study on the acid-
catalyzed conversion of D-fructose to 5-hydroxymethylfurfural and levulinic acid in
aqueous solutions (poster, XIIth The Netherlands Catalysis and Chemistry Conference
(NCCC12), Noordwijkerhout, Netherlands, March 2011).