Aldol condensation of 5-hydroxymethylfurfural to fuel precursor over novel

aluminium exchanged-DTP@ZIF-8

Radhika S. Malkar,a Helen Daly,b Christopher Hardacre,b,* Ganapati D. Yadava,*

aDepartment of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai-400 019, India

bSchool of Chemical Engineering and Analytical Science, University of Manchester, Manchester, M13 9PL, U.K

E-mail address: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in, c.hardacre@manchester.ac.uk

Tel.: +91-22-3361-1001, Fax: +91-22-3361-1020

*Authors to whom correspondence should be addressed

ABSTRACT:

Aldol condensation of 5-hydroxymethylfurfural (HMF) with acetone provides a route to

upgrading of biomass intermediates into value added compounds such as fuel precursors. This

reaction gives C9 and C15 aldol adducts. These adducts can be further converted to jet fuel

additives on hydrogenation and hydrodeoxygenation. Herein, we report a novel catalyst namely

aluminium exchanged heteropolyacid encapsulated inside the cage of ZIF-8 (Al0.66-DTP@ZIF-8)

for selective synthesis of C9 adducts. The acidic nature of the catalyst gives a maximum

conversion of HMF (99%) in 10 h and the smaller pore diameter of ZIF-8 leads to the highly

selective formation of C9 adducts (84.11%). The fresh and spent catalysts were characterized by

various techniques. A kinetic model was developed and the apparent activation energy for both

monomer and dimer formation calculated. Recyclability of catalyst was studied up to 4 cycles

and the catalyst was observed to be stable and reusable.

KEYWORDS: 5-hydroxymethylfurfural (HMF), acetone, aldol condensation, heteropolyacid,

ZIF-8, metal organic frameworks, fuel precursors

2

INTRODUCTION

Increased public awareness of environmental pollution, change in climate and increasing global

temperatures due to greenhouse gas emissions, has led to an increase in the demand for

sustainable and renewable energy sources and fuel precursors. There are a number of steps

involved in converting bio-based feedstocks to a range of chemicals and fuel additives.1 One of

the biomass generated molecules, 5-hydroxymethylfurfural (HMF), is a building block for

synthesis of a range of value added chemicals such as 2,5-dimethyl furan,2,3 furan-2,5-

dicarboxylic acid,4 2,5-diformylfuran,5 etc.

Small chain alkanes are usually used for the synthesis of aromatics,6,7 and fuels such as jet fuel.

Kerosene and gasoline contain C10-22,8 C8-169 and C5-1210,11 alkanes. Conversion of small

chain alkanes to longer chains requires C-C bond formation which is often performed by an aldol

condensation over acid or base catalysts. The aldol condensation of HMF produces HMF

monomer and dimer (Scheme 1), 4-[5-(hydroxymethyl)-2-furanyl]-3buten-2-one (HAc) and 1,5-

bis[ (5-hydroxymethyl)-2-furanyl]-1,4-pentadien-3-one) (HAcH), respectively. These are C9 and

C15 aldol adducts which on further hydrogenation and hydrodeoxygenation produce a range of

linear alkanes, for use as quality jet fuel additives.12,13

Dumesic et al. first time reported a three-step process to convert HMF or furfural into fuel

additives.12 Therein, crossed aldol condensation of HMF with acetone was carried out using Mg-

Al-oxide. The aldol adducts were then hydrogenated over Pd/Al2O3 and then to long chain

alkanes over Pt/SiO2-Al2O3 via dehydration/hydrogenation. Barrett et al. have also developed a

bifunctional catalyst Pd/MgO-ZrO2 for aqueous phase aldol condensation of 5-HMF and furfural

followed by hydrogenation in a single reactor.14 At 326 K they obtained 79% conversion of HMF

with 18% and 61% selectivity to C9 (monomer) and C15 (dimer), respectively, after 26 h.

Overall carbon yield was 94%. Whereas, with the increase in temperature there was a drastic

drop in carbon yield up-to 67%. They concluded that the selectivity and yield of reaction can be

controlled by changing the temperature and molar ratio of HMF and furfural with acetone for

aldol condensation products.

Rong et al. applied a four-step approach to convert hemicellulose to jet fuel alkanes.15 The first

step was dehydration of hemicellulose to furfural, which was then subjected to aldol

condensation to produce a furfural-acetone-furfural (F-Ac-F) dimer. Subsequently, this dimer

3

was hydrogenated and then hydrodeoxygenated to produce C13 and C12 diesel fuel alkanes.15

The same kind of approach has also been applied by many other researchers to produce alkanes

for fuel synthesis.12,16,17 Zhang et al. for the first time prepared a series of renewable triketones

containing repetitive [COCH2CH2] units from aqueous phase aldol condensation of HMF and

acetone over CaO catalyst followed by hydrogenation by using Au/TiO2.18 Thus produced

triketones and diketones can be applied as precursors for the synthesis of branched cycloalkanes

of jet fuel. Also, they are important feedstock for the production of conducting or semi-

conducting polymers. Homogeneous aldol condensation of HMF with acetone was performed

using aqueous NaOH solution at RT. Further hydrogenation of C9 and C15 aldol products was

carried out over Pd/Al2O3 (at 373-413 K and 25-52 bar H2) and Pt/NbPO5 (at 528-568 K and 60

bar of H2) to finally produce a mixture of linear alkanes of C9 and C15 with 73% yield.19

To date, a wide range of catalysts including surface modified zeolite,20 CO2, 21

zirconium

carbonate,22 mixed oxides like MgZr and MgAl 23 and Cu/MgAl2O4,24 have been applied to carry

out the aldol condensation of HMF with acetone; however, these typically required long

synthesis times (Table 1). The reaction carried out over Hie-FAU-ZIF-8 was showing only 68%

conversion with 97.8% selectivity towards monomer by using HMF (0.13 mol/L) and acetone

(13.4 mol/L) over 0.05 g/cm3 of catalyst loading.20 The yield of monomer obtained was very less

as 66.8%.

Table 1: Earlier reports on aldol condensation of HMF with acetone (* % Yield)

Sr No Catalyst Reaction parameters

T / °C Time / hConversion of HMF (%)

Selectivity to HAc (C9) (%) Ref.

1 CO2 200 20 95 95* 21

2 Zr(CO3)x 54 24 100 92* 22

3 MgZr MgAl 50 24 68.0

31.8 20.7 12.1

23

4 Cu/MgAl2O4 140 7 100 85.7 24

5 Hie-FAU-ZIF-8 130 6 68.3 97.8 20

4

6 Nit-NaY MgO-ZrO2

120 24 51.4 68.5

63.6 48.6

25

Lee et al.21 have used CO2 gas as an acid catalyst for aldol condensation of HMF and acetone.

Therein, it was shown that a decrease in CO2 pressure improved the yield of the aldol adduct.

Later two basic catalysts were examined, MgO-ZrO2 and Nit-NaY, for the same reaction and

observed variation in the selectivities towards the aldol adducts with the pore size of the

catalyst.25 MgO-ZrO2, which has a broad range of pore size (5-1000 Å), showing the highest

selectivity towards dimer formation (up to 51.4%) whereas the small pore size of zeolite Nit-

NaY led to monomer formation (up to 63.6%). In addition to the pore size of the catalysts, there

have been reports that at particular ketone to furfural ratios, reaction in the presence of a strong

base leads to double condensation adduct.26 Therefore, the reaction parameters as well as the

properties of the catalyst can play an important role in the selective synthesis of the desired C9

adduct.

Previously we have reported 18% (w/w) dodecatungstophosphoric acid (DTP)

encapsulated ZIF-8 as an effective catalysts for the transesterification of cinnamyl alcohol and

benzoic anhydride.27 The catalyst provided distinct advantages such as reduced leaching of DTP

with 84% conversion of benzoic anhydride. According to the literature, heteropolyacids (HPAs)

are stronger than H3PO4, HCl, H2SO4, HBr, HNO3, HClO4 and CF3SO3H.28 There are various

reports on the modification of HPA with metals to make them insoluble, and increase their

surface area and acidity simultaneously.29–33 Exchanging the protons of HPA in its secondary

structure improves the mobility of protons and thus helps in increasing its acidity.29 Herein, Al

has been used to ion exchange with the protons of DTP which was encapsulated inside the cage

of ZIF-8. The smaller pore diameter of ZIF-8 helps in the selective synthesis of the HMF

monomer (C9) and the exchanged metal ions improves the overall acidity of the catalyst. We

have also explored an acid catalyzed mechanism i.e. enol mechanism to perform the aldol

condensation of HMF with acetone.

5

Scheme 1: Aldol condensation of HMF with acetone over Al-DTP@ZIF-8

EXPERIMENTAL

Reaction procedure and analysis

Experiments were performed in a 100 ml Parr reactor (Parr Instrument, UK) equipped with an

impeller and temperature control unit. HMF (0.14 ml, 0.05 mol/L), acetone (30 ml, 12.9 mol/L)

and 0.15 g (0.005 g/cm3) of catalyst were mixed in the reactor. The total volume of reaction was

31 ml. The reaction mass was agitated at 1100 rpm at 160 °C and samples withdrawn

periodically and analysed by HPLC (Agilent 1260 infinity) with a Thermo Scientific C18

column (250 x 4.6 mm) and gradient elution of 0.1% acetic acid-DI and acetonitrile (70:30). UV

detection at a wavelength of 260 nm with 10 µL of injection volume, 1.0 mL min -1 of phase flow

rate and 35 °C was used. HMF was used as the limiting reagent and the concentrations were

measured using a standard calibration curves. The identity of the products were confirmed by

6

GC-MS (Thermo Scientific Trace 1300 GasChromatograph equipped with an ISQL LT single

quadrupole Mass Spectrometer) using RTX-5 column (150 mm × 0.25 mm, 0.25 μm).

Chemicals

The chemicals were procured from the following sources and were used without further

purification, zinc acetate and 2 methylimidazole (Loba Chemie), dodeca-tungstophosphoric acid

(Thomas Baker, Mumbai, India), cesium chloride (Spectrochem), K-10 clay (Aldrich, USA), 5-

hydroxymethylfurfural, aluminium nitrate nonahydrate and acetone (Sigma Aldrich, UK).

Catalyst synthesis

Cs-DTP/K10 and 18% DTP@ZIF-8 were synthesized using the method reported in the

literature.27,34 The aluminium exchanged DTP was prepared by using the method of Kumar et

al.35 The known amount of DTP (18% w/w) was dissolved in 20 ml of deionized water (DI). To

this solution, the calculated amount of aluminium nitrate nonahydrate was added at 60 oC to

allow the exchange of the protons of DTP. The resulting solution was stirred for 3 h and then

cooled down to room temperature. Afterwards a known quantity of zinc acetate was added. In

another beaker a solution of 2-methyl imidazole in DI water was prepared and added dropwise to

the above mixture. A white precipitate of the catalyst began to form and stirring was continued

for 12 h resulting in the formation crystals of ZIF-8. The precipitate was then separated by

centrifugation and washed several times with distilled water to remove unreacted moieties.

Afterwards the catalyst was calcined at 300 °C for 3 h. The resultant catalyst has the following

formula, Al0.66-DTP@ZIF-8 which is denoted as Al-DTP@ZIF-8.

Catalyst characterization

The prepared catalysts Cs-DTP-K10, DTP@ZIF-8 and Al0.66-DTP@ZIF-8 were characterized by

using surface area analysis and NH3-TPD. The structure of the DTP@ZIF-8 and Al-DTP@ZIF-

8 was also characterized by XRD, HRTEM, SEM, FTIR and TGA to confirm the exchange of Al

and the encapsulation. The results and details of each technique has been provided in

supplementary document.

RESULTS AND DISCUSSION

Catalyst characterization

7

Ammonia- temperature programmed desorption (TPD) and surface area analysis

Ammonia TPD and surface area analysis were carried out to compare the acidity and textural

properties of the catalysts, Cs-DTP-K10, DTP@ZIF-8 and Al-DTP@ZIF-8. Cs-DTP-K10

possessed the highest acidity at 1.51 mmol/g while DTP@ZIF-8 and Al-DTP@ZIF-8 presented

0.44 and 0.54 mmol/g of total acidic sites (Table 2). Metal salts of HPA have higher acidity

compared to the parent HPA, this may be due to generation of Lewis acidic sites (Al3+ ions in

case of Al-DTP@ZIF-8).35,36 However, polyvalent metal salts of the Keggin cation [PW12O40]-3

show greater catalytic activity than the monovalent salts.37 Unsupported and supported

aluminium exchanged HPA salts were previously reported for 2-methoxynaphthalene acylation,38

hydroarylation of vinyl arenes,39 anisole benzylation40 and benzoylation of diphenyloxide33

wherein modified salts always showed greater acidity (Lewis and Brønsted) and higher catalytic

activity. DTP@ZIF-8 and Al-DTP@ZIF-8 have both large surface area and a small pore

diameter whereas CS-DTP-K10 possessed low surface area with a wide pore diameter (Table 2).

Table 2: NH3-TPD and surface analysis of catalysts

Cs-DTP-K10 DTP@ZIF-8 Al0.66-DTP@ZIF-8

NH3-TPD (mmol/g) 1.51 0.44 0.54

Surface area (m2/g) 205.8 1176.3 1116.4

Pore volume (cm3/g) 0.29 0.67 0.65

Pore diameter (nm) 5.9 1.12 1.11

Scanning electron microscopy (SEM)

Fig. 1 shows the SEM images of all the catalysts. Both the ZIF-8 materials before and after

encapsulation of DTP or Al0.66-DTP possessed a dodecahedral rhombic structure41 with all the

crystals found to be of uniform size and shape. The spent catalyst after 3 rd cycle showed some

aggregation after reaction but maintained the same structure.

8

Figure 1. SEM analysis of ZIF-8 (a), 18%DTP@ZIF-8 (b), 18%Al0.66-DTP@ZIF-8 (c) and

reused (3rd cycle) 18%Al0.66-DTP@ZIF-8

Activity and Selectivity for the aldol condensation of HMF

To probe the effect of catalyst acidity and pore diameter on the activity and selectivity to the C9

monomer product from the aldol condensation of HMF and acetone, Cs-DTP-K10, 18%

DTP@ZIF-8 and Al-DTP@ZIF-8 were screened. Cs-DTP-K10, which possessed the highest

acidity, had the highest activity with a conversion of HMF of 71.6% after 6 h of reaction.

However, the selectivity to the C9 monomer was only 43.1% (Figure 2). Interestingly, the Al-

DTP@ZIF-8 which had significantly lower number of total acidic sites, (0.54 compared to 1.51

mmol/g) showed a relatively small drop in the activity compared to the Cs-DTP-K10 catalyst

with conversion of 63.1% achieved in 6 h of reaction (Figure 2). Importantly, the Al-DTP@ZIF-

8 catalyst showed significantly higher selectivity to the C9 adduct compared to the C15 adduct.

The 18% DTP@ZIF-8, which had the lowest acidity of the three catalysts (0.44 mmol/g), had the

lowest conversion of HMF over 4 h (Figure 2) but did have a high selectivity to the C9

9

monomer. The high selectivity to the desired C9 product over the DTP@ZIF-8 and Al-

DTP@ZIF-8 catalysts shows the shape selectivity provided by encapsulation in ZIF-8 where the

small pore diameter of the ZIF-8 can restrict the formation of C15 adduct. Formation of by-

products from the self-condensation of acetone i.e. diacetone alcohol and mesityl oxide was also

significantly reduced for the ZIF-8 encapsulated catalysts. DTP itself can drive the reaction but

as it is highly soluble in polar solvents, the reaction becomes homogeneous. The selectivities

observed for different catalysts, compared at 30% conversion of HMF and presented in Table 3.

The maximum selectivity for C9 monomer was obtained for Al0.66-DTP@ZIF-8 (achieved after

53.7 min of reaction time), whereas 18% DTP@ZIF-8 gave 94% selectivity for the same after

165 min. The highest selectivity of 10.3 % for C15 was shown by Cs-DTP-K10. Therefore, with

good activity and excellent selectivity to the desired C9 product, the Al-DTP@ZIF-8 catalyst

was selected and the effect of HMF concentration and reaction temperature were investigated to

optimize the reaction for the highly selective synthesis of fuel precursor (C9 adduct).

Table 3: Selectivities for C9 and C15 for different catalysts at 30% conversion of HMF

Reaction conditions: HMF concentration 0.005 mol/L, acetone 30 ml, temperature 170 oC, speed

of agitation 1100 rpm, total volume 31 ml, catalyst loading 0.005 g/cm3, conversion of HMF

30%.

10

Selectivity for

C9

Selectivity for

C15 (%)Other (%)

Cs-DTP-K10 81.4 10.3 7.7

Al0.66-DTP@ZIF-8 96.6 2.8 0.6

18% DTP@ZIF-8 94.0 3.9 1.7

Figure 2:

Conversion of HMF and selectivity of products for different catalysts: Reaction conditions: HMF

concentration 0.005 mol/L, acetone 30 ml, temperature 170 oC, speed of agitation 1100 rpm, total

volume 31 ml, catalyst loading 0.005 g/cm3, time 6 h.

Shape selectivity by ZIF-8

It has already been reported by Shen et al. that the pore size of NaY zeolite is about 0.72 nm

which allows access to only smaller molecules like HMF, acetone and the monomer adduct of

aldol condensation while it restricts diffusion for dimer products.25 Hence it provides high shape

selectivity towards the monomer. Likewise, the pore diameter of ZIF-8 is only 1.1 nm and

therefore it favours the monomer formation.

Effect of speed of agitation

The absence of external mass transfer resistance was calculated through the effect of speed of

agitation on conversion of HMF and rate (supplementary document). The reaction was carried

over three different speed of agitation as 1000, 1100 and 1200 rpm (Figure 3). It was observed

that conversion of HMF and rate did not change significantly with the agitation speeds of 1100

11

Cs-DTP-K10 Al0.66-DTP@ZIF-8 18%DTP@ZIF80

102030405060708090

100

Conversion of HMF Selectivity to C9 Selectivity to C15

Con

vers

ion

of H

MF

(%)

rpm and 1200 rpm but a lower rate was found at 1000 rpm indicating the absence of liquid mass

transfer resistance ≥1100 rpm. Therefore 1100 rpm was used thereafter.

Figure 3: Effect of speed of agitation HMF concentration 0.005 mol/L, acetone 30 ml, catalyst

Al0.66 DTP@ZIF-8, temp 170 oC, total volume 31 ml, catalyst loading 0.005 g/cm3, time 4 h.

Effect of catalyst loading

Figure 4 indicates the conversion of HMF with respect to time for different catalyst loading

(0.003 to 0.006 g/cm3) of Al-DTP@ZIF-8. A plot of the initial rate vs catalyst loading (Figure 5)

showed that the initial rate was directly proportional to catalyst loading or number of active sites

present. Therefore intra particle resistance was absent. At this optimum catalyst loading

selectivity for C9 adduct was 72.9% after 6 h of reaction at 170 °C.

12

0 50 100 150 200 250 3000

10

20

30

40

50

60

1000 1100 1200Time (min)

Con

vers

ion

of H

MF

(%)

Figure 4: Effect of catalyst loading: HMF concentration 0.05 mol/L, acetone 30 ml, catalyst

Al0.66 DTP@ZIF-8, temp 170 oC, speed of agitation 1100 rpm, total volume 41 ml, catalyst

loading 0.005 g/cm3, time 6 h.

Figure 5: Plot of initial rate vs catalyst loading: HMF concentration 0.05 mol/L, acetone 30

ml, catalyst Al0.66 DTP@ZIF-8, temp 170 oC, speed of agitation 1100 rpm, total volume 41 ml,

catalyst loading 0.005 g/cm3, time 6 h

13

0 0.001 0.002 0.003 0.004 0.005 0.006 0.0070

0.05

0.1

0.15

0.2

0.25

f(x) = 32.502382000051 xR² = 0.99628312740286

Catalyst loading (g cm-3)

Initi

al r

ate

(mol

cm

-3 m

in-1

)

0 50 100 150 200 250 300 350 4000

10

20

30

40

50

60

70

80

0.003 g/cmᶾ 0.004 g/cmᶾ 0.005 g/cmᶾ 0.006 g/cmᶾTime (min)

Con

vers

ion

of H

MF

(%)

Effect of HMF concentration

The concentration of HMF plays an important role in the selective synthesis of C9 adduct with

increasing concentration reported to result in the formation of C15 adduct. Herein, a range of

HMF concentrations from 0.04 to 0.07 mol/L were tested (Figure 6). At the higher concentration

of HMF, it reacts faster with monomer (C9) than to acetone and leads to an increase in the C15

selectivity. For example, at 0.07 mol/L of HMF 35.1 % C15 selectivity was obtained whereas at

0.05 mol/L and 0.04 mol/L of HMF this dropped to 22.2.6 and 14.2, respectively (Table 4). It has

been already reported in the literature that in the presence of excess acetone (relative to the HMF

concentration), the probability of HMF reacting with acetone increases and thus the rate of

formation of the monomer increases, as expected.16 At 0.04 and 0.05 mol/L the conversion of

HMF was comparable and therefore 0.05 mol/L of HMF concentration was chosen for further

experiments. The maximum conversion of HMF obtained was 67.5% at 170 °C in 6 h of

reaction.

The reaction profiles for each HMF concentration is provided in the supplementary

information (Figure S6-S9). It can be seen that, with the decrease in HMF concentration, the rate

of formation of C9 is increasing significantly. It is also observed that, the rate of increase in the

concentration of C15 is more compared to the by-products.

14

Figure 6: Effect of HMF

concentration Catalyst

loading 0.005 g/cm3,

acetone 40 ml, catalyst

Al0.66 DTP@ZIF- 8, temp 170 oC, speed of agitation

1100 rpm, total volume 41

ml, catalyst loading

0.005 g/cm3, time 6 h.

Table 4: Selectivities for C9 and C15 product at different concentrations of HMF

HMF concentration

(mol/litr)

Selectivity for C9

(%)

Selectivity for C15

(%)

Selectivity for

others (%)

0.07 53.77 35.13 11.1

0.06 61.46 28.65 9.89

0.05 71.95 22.6 5.45

0.04 81.45 14.22 4.33

Effect of Temperature

Further experiments were carried out to investigate the effect of temperature on the aldol

condensation of HMF between 140 to 170 °C (Figure 7) over Al0.66 DTP@ZIF-8. It was noted

that at higher temperatures the conversion of reactants was increased, as expected; however, the

formation of the C15 dimer also increased (Table 5). Between 160 and 170 °C the increase in

conversion was insignificant but the selectivity for C15 was significantly increased at 170 °C. At

140 °C the selectivity for C9 adduct was the highest but the conversion was only reached 41.3%

15

0 50 100 150 200 250 300 350 4000

10

20

30

40

50

60

70

80

0.07 mol/L 0.06 mol/L 0.05 mol/L 0.04 mol/LTime (min)

Con

vers

ion

of H

MF

(%)

after 6 h of reaction. Therefore 160 °C was chosen for this reaction with 68.5 % conversion of

HMF at 6 h. The complete conversion of HMF was obtained at a longer reaction time of 10 h,

where 98% of HMF conversion was obtained into C9 and C15 with a selectivity of 84.11% and

11%, respectively.

Figure 7: Effect of

temperature HMF

concentration 0.005 mol/L, Catalyst loading 0.005 g/cm3, acetone 40 ml, catalyst Al0.66

DTP@ZIF-8, speed of agitation 1100 rpm, total volume 41 ml, catalyst loading 0.005 g/cm3,

time 6 h.

Table 5: Selectivities for C9 and C15 product at different temperature after 6 h reaction time

Temperature (°C) Selectivity for C9 (%) Selectivity for C15 (%)

140 91.25 8.75

150 87.4 12.6

160 85.53 14.47

170 78.33 21.67

16

0 50 100 150 200 250 300 350 4000

10

20

30

40

50

60

70

140 ᵒC 150 ᵒC 160 ᵒC 170 ᵒC

Time (min)

Con

vers

ion

of H

MF

(%)

Development of Kinetic model

Based on the above experimental results, a mathematical model was developed with the best fit

of LHHW model (Langmuir-Hinshelwood–Hougen-Watson) including HMF (A), acetone (B),

HAc/C9 (C), HAcH/C15 (D) and water (W). The formation of HAc and HAcH can be

represented as follows,

(1)

(2)

Considering both the reactants, HMF and acetone adsorb on the catalytic acid site S;

(3)

(4)

The surface reaction of adsorbed species AS and BS gives CS and water as,

(5)

Further, the reaction between CS and AS gives DS and water as,

(6)

The desorption step for two products can be written as,

(7)

(8)

The conversion of HAc to HAcH is slow and is considered as the rate determining step,

therefore, the surface concentrations can be written as,

(9)

(10)

(11)

The rate equations for above reaction can be written as,

(12)

(13)

17

(14)

Putting values from eq. 9, 10 and 11 into eq. 12, 13 and 14,

(15)

(16)

(17)

The total catalytic sites can be given as follows,

Therefore,

(18)

Putting the value of Cs into the eq. 15, 16 and 17 we get,

(19)

(20)

(21)

18

Solving above three equations simultaneously using Polymath 6.0 values of adsorption constants

were obtained (KA=0.09, KB =0.07, KC=0.04, KD=0.0001). They were found to be very small and

hence above equations were reduced to the following form.

(22)

(23)

where,

K1 = w k1 KA KB and K2 = w k2 KA KC

The different rate constants, k1 and k2, were calculated (Table 6) at different temperatures using

the experimental data and Arrhenius plots (Figure 8). From the analysis of these values, the rate

constants for the formation of HAc were found to be greater than for the HAcH. The activation

energies were calculated for both the steps and determined to be 65.3 kJ/mol for HAc and 102.1

kJ/mol for HAcH formation and hence the reaction was kinetically controlled. The concentration

profiles for HMF, HAc and HAcH were plotted at optimised temperature (160 °C) for

experimental data. It can be seen that concentration of HMF (for theoretical and experimental) is

continuously decreasing at the same time formation of HAc and HAcH is increasing (Figure 9).

After 280 min of a reaction a considerable amount of HAcH started forming.

Table 6: Kinetic parameters for the reactions

Temp (K) k1 * 103 (L2mol-g-1sec-

1)

k2 (L2mol-g-1sec-1) Ea (kJ/mol)

413 0.25 0.006

65.3 (1st step)

102.1 (2nd step)

423 0.31 0.01

433 0.63 0.03

443 0.77 0.05

19

Figure 8: Arrhenius

plot for the formation of HAc and HAcH

Figure 9: Theoretical and experimental comparison of concentrations for HMF, HAc and HAcH

at 160°C

20

0.0022 0.00225 0.0023 0.00235 0.0024 0.00245-6

-4

-2

0

2

4

6

8

10

f(x) = − 12293.4538896147 x + 24.7195322439101R² = 0.993420140570966

f(x) = − 7854.40087674846 x + 24.3827079838234R² = 0.987925846296403

HAc formation

1/T (K-1)

lnk

-50 50 150 250 350 450 550 6500

0.01

0.02

0.03

0.04

0.05

0.06

Concentration of HMF theoretical Concentration of HAc theoretical

Concentration of HAcH theoretical Concentration of HMF experimental

Concentration of Hac experimental Concentration of HAcH experimental

Time (min)

Con

cent

ratio

n (m

ol/li

tr)

Reaction mechanism

HMF and acetone are adsorbed on acidic sites of Al-DTP@ZIF-8 via the carbonyl oxygen.

Acetone then converts to an enol and reacts with the electrophilic carbonyl of HMF. Then

condensation of beta-hydroxy group in an acidic environment leads to the formation of HAc

monomer leaving one water molecule. According to LHHW model presented above, acetone and

HMF both gets attached on acidic sites generated by encapsulated DTP. The activity of the

reaction carried by modified DTP@ZIF-8 has increased because of the decrease in the formation

of dimer. Smaller pore size allows only the formation of C9 while restricting the further

interaction of C9 with HMF. Therefore, Al0.66DTP@ZIF-8 proved to be the superior catalyst for

the aldol condensation of HMF with acetone.

Scheme 2: Reaction mechanism for the formation of HAc (C9) over Al-DTP@ZIF-8

21

Reusability of catalyst

Recyclability of Al0.66 DTP@ZIF-8 was studied over four recycles (Figure 10). After the first

reaction, of Al0.66 DTP@ZIF-8 (At 160 °C, each reaction was for 10 h to achieve complete

conversion in each reaction) , the catalyst was separated from reaction mass by simple

centrifugation and then refluxed with acetone 4-5 times to get remove adsorbed moieties from

the catalyst surface. The catalyst was then dried in an oven overnight. The lost catalyst weight

via mechanical losses was made up with the fresh one before each reaction of recyclability (with

a maximum of 5-8%w/w added in each reaction). Again the reaction was performed at the same

optimized conditions and analyzed for the synthesis of C9. It was found that there was 4%

decrease in the conversion for 4th cycle, may be because of some loss in acidic sites.

Characterization of the catalyst after the 3rd experiment showed no alteration in the structure

(Figure S1 and S4). Along with the high conversion, catalyst has also retained high selectivity

for the C9 up to 4th cycle of reusability.

Bohre et al. 22 have shown reusability of Zr(CO3)x catalyst for aldol condensation of HMF

with acetone up to the 5th cycle and there was a drastic drop in the conversion from 91% (1st

cycle) to 84% (5th cycle). The loss in catalyst weight during mechanical recovery and basic sites

were reported as the reasons behind this decreased conversion. In case of two basic catalysts Mg-

Zr and Mg-Al, researchers studied the catalyst stability by recycling it for 3 cycles, without

giving any washing to recycled catalyst.23 There was a 27% and 19% drop in the conversion of

HMF was observed for Mg-Zr and Mg-Al, respectively. The increased deactivation of the

catalysts was because of the interaction of the oligomers with the surface of the catalyst. Shen et

al. 25 studied the catalyst stability over continuous mode for aldol condensation reaction wherein

they reported that MgO-ZrO2 was not affected by reaction moieties and possessed most of its

activity with a slight decrease in the selectivity to the dimer adduct. Whereas, Nit-NaY catalysts

lost activity over recycling due to loss in the basic sites. Therefore, generally it has been

observed that, base catalyst have lost their catalytic activity due to many possible reasons such as

interaction of oligomers with catalyst, change in catalyst morphology because of the in-situ

generated water, loss in active sites, etc.23 As compared to all above catalyst, Al0.66DTP@ZIF-8

have shown excellent retention of catalytic activity with high selectivity towards the C9 product

which may be because of the encapsulated active acidic sites, small pore size and large surface

area.

22

Figure 10: Reusability of catalyst Concentration 0.005 mol/L, temp 160 °C, catalyst loading

0.005 g/cm3, acetone 40 ml, catalyst Al0.66DTP@ZIF-8, speed of agitation 1100 rpm, total

volume 41 ml, catalyst loading 0.005 g/cm3, time 10 h.

CONCLUSIONS

Three different catalysts, Cs-DTP-K10, 18%-DTP@ZIF-8 and Al0.66-DTP@ZIF-8 were

investigated for aldol condensation of HMF. Al0.66-DTP@ZIF-8 was the most selective to the

desired product while providing high conversion of HMF (98%, in 10 h) along with the highest

selectivity for C9 aldol adduct (84.11% at 10 h). Higher acidity of the Al0.66-DTP@ZIF-8 catalyst

along with the smaller pore diameter of ZIF-8 leads to the high selectivity to formation of the C9

adduct. A kinetic model for the formation of the monomer and dimer was developed and the

activation energy calculated as 65.3 and 102.1 kJ/mol respectively. It proves that the catalyst

favours formation of monomer over dimer and that the reaction is kinetically controlled. The

catalyst is thermally stable and was shown to reusable for up to 4 cycles with no change in

properties of the catalyst.

23

Fresh 1st reuse 2nd reuse 3rd reuse 4th reuse60

65

70

75

80

85

90

95

100

Conversion of HMF Selectivity to C9

Con

vers

ion

of H

MF

(%)

ACKNOWLEDGMENTS

R. S. Malkar acknowledges the University Grants Commission (UGC), India for the award of

fellowship in Green Technology and. R.S. Malkar acknowledges the University of Manchester,

UK and Prof. Nancy Rothwell for the award fund. G. D. Yadav acknowledges support from R. T.

Mody Distinguished Professor Endowment and J. C. Bose National Fellowship of Department of

Science and Technology, Government of India. The authors acknowledges the use of the School

of Materials X-ray Diffraction Suite at the University of Manchester and is grateful for the

technical support of Gary Harrison/Dr. John E. Warren.

NOMENCLATURE

A reactant species A, 5-hydroxymethylfurfural

B reactant species B, acetone

C HAc (C9 monomer)

D HAcH (C15 dimer)

W water

S vacant site

w catalyst loading (g)

CA concentration of A mol/L

CB concentration of B mol/L

Cc concentration of C mol/L

CD concentration of D mol/L

CAS concentration of A at catalyst surface (mol g-1cat-1)

CBS concentration of B at catalyst surface (mol g-1cat-1)

CCS concentration of C at catalyst surface (mol g-1cat-1)

CDS concentration of D at catalyst surface (mol g-1cat-1)

CS concentration of vacant sites (mol cm-3)

CT total concentration of the sites (mol cm-3)

K adsorption equilibrium constant for (A,B,C,D and W) (Lmol-1)

k1 reaction rate constant for HAc formation (L2mol-g-1sec-1)

k2 reaction rate constant for HAcH formation (L2mol-g-1sec-1)

24

SUPPORTING INFORMATION

ESI contains detailed description about catalyst characterization methodology and techniques

such as FTIR, powder XRD, HRTEM, TGA. It also includes reaction profile and proof of

absence of external and internal mass transfer resistance.

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TOC:

Synopsis:

29

The present work deals with the conversion of biomass derived 5-hydroxymethylfurfural to a jet

fuel additive precursor novel a novel catalyst.

30