University of Groningen

Multifunctional catalytic systems for the conversion of glycerol to lactatesTang, Zhenchen

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Chapter 1

Achievements and challenges in the design of heterogeneous catalysts

for the conversion of glycerol to valuable products

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1. Introduction

Nowadays, growing concerns about depletion of fossil fuels and global warming are pushing

the scientific and industrial community to consider the production of chemicals and fuels in a

more efficient and sustainable way. Fossil resources such as coal, oil, and natural gas are

currently the main resource for energy generation (heat and power), transportation fuels,

chemicals and materials.1 However, due to high demand, reserves of these non-renewable

resources are slowly depleting and it has been estimated that the worldwide reserves of oil may

be sufficient for only about another 40 years.2 In this context, the substitution of fossil resources

by renewables is of high interest, also to reduce the carbon footprint of fossil resources.

Biomass of vegetable origin is an attractive, renewable alternative to fossil resources and as

such has received considerable attention the last decades. In the current situation in which

technologies for the production of H2 through H2O electrolysis followed by the electrochemical

reduction of CO2 are not yet mature,3, 4 biomass is the dominant renewable source of chemicals.1,

2, 5, 6 In this context, twelve chemicals derived from biomass have been identified as “platform

chemicals”, based on their availability and on their potential to be converted into other valuable

products.7, 8 Among these platform chemicals, glycerol (1,2,3-propanetriol, GLY) plays a

prominent role due to its wide availability and for the range of valuable products that can be

obtained from it (Scheme 1).

Scheme 1. Added-value chemicals from glycerol.

Chapter 1

---3---

Glycerol is the main by-product of the transesterification of triglycerides like pure plant oils

and fats with methanol to produce biodiesel. Typically, the biodiesel processes generate about

10 wt% of glycerol on biodiesel. Moreover, glycerol is also co-generated in the soap and

detergent industry9 and from the conversion of cellulosic biomass10. The steep increase in the

global biodiesel production levels in the last decade has resulted in a surplus of glycerol. The

total European biodiesel production for 2016 was over 11.5 million metric tons, a 4 times

increment from 2004.11 It should be noted that European directives signed in 2015 require that

transportation fuels contain at least 10% of bio-components by 2020.12, 13 This has also led to a

major increase in glycerol production levels and as a consequence, the global price for glycerol

has dropped considerably. Typically, in Europe, the price of crude glycerol is between 330-360

EUR/ton. 14

Biodiesel producing companies have a strong incentive to valorise the glycerol to improve the

economic viability of biodiesel production. However, the quality of the glycerol from the

biodiesel industry represents a challenge. The products contain glycerol (20 to 70%), methanol,

fatty acid methyl esters, fatty acid salts (soap), water free fatty acids and inorganic salts.15, 16

After work-up, the glycerol content in the so-called crude glycerol is typically between 80-

95%.17 The impurities present in crude glycerol can be an issue for subsequent conversions,

particularly when using metal based catalysts, as they can block the active sites of the catalysts

(e.g. mineral impurities). Further refining by distillation is required to produce “pharmaceutical”

grade glycerol (99% purity), which is used in the cosmetics, paints, automotive, food, tobacco,

pharmaceutical, paper, leather and textile industry.18, 19 However, the total market size for these

applications is limited. Therefore, new conversion routes of glycerol to useful products (bulk

or fine chemicals) have been investigated.20, 21 In general, these routes require the use of a

catalyst to achieve the efficient conversion of glycerol with high selectivity towards the selected

target product. Ideally, the catalysts should be compatible with crude glycerol. A large number

of publications and reviews have appeared in the past few years on glycerol conversion using

chemocatalytic, biocatalytic and electrocatalytic methods.2, 10, 20-25 This review focuses on the

design of heterogeneous catalysts for the conversion of glycerol to value-added chemicals

through a wide range of synthetic methodology including oxidation, dehydration,

hydrogenolysis, etherification, esterification, carbonation and acetalization (Scheme 1). The

choice of focussing on heterogeneous catalysis stems from the preference for this type of

catalysts from the point of view of sustainability and industrial applicability, which is a

consequence of their easier separation from the reaction products compared to homogeneous

---4---

catalysts. The originality of this review lies in a systematic analysis and a rationalise of the main

features in terms of composition and other relevant properties of the state-of-the-art catalysts

employed for the efficient and selective conversion of glycerol through each of the above-

mentioned reaction pathways. Through this approach, we identified correlations between

catalytic performance and the design of the catalyst as well as reaction conditions. This critical

comparison will allow the reader to understand why the same catalyst can provide rather

different product selectivity as a function of the reaction conditions, or how two rather different

catalysts can lead to the efficient conversion of glycerol into a same product.

This review is organised along the lines of the main catalytic methodologies reported for

glycerol conversions viz: (i) oxidations, (ii) partial oxidation followed by isomerisation, (iii)

dehydrations and oxidative dehydrations (iv) hydrogenation/hydrogenolysis, (v)

carbonisation, (vi) acetalisation and ketonisations, (vii) etherification, and (viii) esterification.

The conversion of glycerol to epichlorohydrin has been commercialised by Solvay (Epicerol®

process).26, 27 However, this technology involves the use of a carboxylic acid (or derivate) as

homogeneous catalyst28-30 and, therefore, is outside the scope of this review.

2. Conversion of glycerol through oxidation reactions

The oxidation of glycerol is one of the most widely studied conversion path for glycerol. The

possible oxidation products are dihydroxyacetone (DHA), glyceraldehyde (GLAD), glyceric acid

(GLAC), hydroxypyruvic acid (HPA), tartronic acid (TA) and mesoxalic acid (MA), see Scheme 2

for an overview. Moreover, C1- and C2-carbon degradation products such as glycolic acid (GOA),

oxalic acid (OA) and formic acid (FA) are often observed. Some of these compounds are of

industrial interest and thus have market potential. DHA is applied in cosmetics, can be used as

an active ingredient in sunless tanners, and is also an intermediate for the conversion of

glycerol to lactic acid (see section 3). GLAC is reported to have no commercial applications at

the moment, though it can be further oxidized to HPA, TA and MA which have more

opportunities for valorisation. HPA can be used as a substrate for the chemical synthesis of L-

serine and as a flavour component. TA finds applications in pharmaceutical formulations and

as anticorrosive and oxygen-scavenging. Therefore, it has much higher price than glycerol.31-33

MA is a C3-product of glycerol oxidation with the highest oxidation level and is prone to degrade

to CO2. The hydrated form of MA is an expensive pharmaceutical chemical that can be used in

the therapy of diabetes. 34-36

Chapter 1

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Scheme 2. Oxidative routes for glycerol and possible products.

All compounds discussed above are products of the glycerol oxidation, each with a different

degree of oxidation. The primary oxidation products, DHA and GLAD, are actually isomers

which are known to be easily interconverted in the presence of an acidic or basic catalyst

(Scheme 2).37-40 Therefore, the main challenge in the oxidation of glycerol is to achieve high

selectivity towards a specific product, and to prevent the full oxidation to CO2. As such,

selectivity is a key issue and typically mixtures of products are observed for reported glycerol

oxidations.31, 41

2.1 From glycerol to dihydroxyacetone

The first oxidation products of glycerol are two trioses, GLAD and/or DHA. The ratio of the two

is depending whether the primary or the secondary alcohol group undergoes dehydrogenative

oxidation to a carbonyl group. Besides kinetic control by proper choice of the catalyst, the rate

of the equilibrium limited isomerization reaction between the two should also be considered.

Several noble-metal-based catalytic systems have been shown to be good catalysts for the

synthesis of DHA from glycerol. Selectivity control to avoid over oxidation is achieved by

catalyst modifications (introduction of a second metal and careful tuning of the reaction

conditions. An overview of selected catalysts is given in Table 1. The reaction is generally

carried out in the aqueous phase and at mild temperature (≤ 80 oC) to avoid over-oxidation, and

oxygen was used as the oxidant. Pure O2 can be used, probably because the use of water instead

of an organic solvent minimises the risk of explosion.

It should be noted that, different from most glycerol oxidation reactions, the partial oxidation

leading to DHA is often carried out in base-free conditions. Actually, in base-free conditions, the

reaction medium is mildly acidic due to the formation of carboxylic acids during the reaction.

DHA is stable in this acidic medium, whereas it can easily isomerize to GLAD or lactic acid (salt)

---6---

in the presence of a base.42 GLAD can undergo further reactions, such as oxidation to glycerates,

tartronates, or isomerization to lactates at higher reaction temperature (see section 3).43-46

Even in acidic medium, DHA may be further oxidized to carboxylic acids and, eventually, to CO2

under harsh reaction conditions, which accelerate the secondary oxidation and C-C bond

splitting of DHA. In summary, base-free and mild reaction conditions are favourable for DHA

production from glycerol, and they match the concepts of green chemistry at the same time.

Gold catalysts have been widely studied for the oxidation of glycerol. Although in early reports

the yields of DHA were less than 20%47, 48, recent studies using gold nanoparticles supported

on different carbon materials reported higher DHA yield in the oxidation of glycerol under basic

conditions (Entry 1-2, Table 1).42, 49, 50 The presence of NaOH in the reaction media significantly

accelerates the reaction, but this study also demonstrated that DHA is unstable in alkaline

conditions. An acidic environment (pH = 3) is more favourable to limit subsequent conversions

of DHA.42 The pore size of the carbon xerogel (CX) on which the gold was supported affects the

products selectivity. Larger pore sizes (~8 nm, 20CX) are more beneficial than smaller ones (~3

nm, 5CX) (Entry 2, Table 1). It was postulated that narrow pores lead to over-oxidation of the

primary OH group, while large pores provide more space for a glycerol conformation which is

favourable for the adsorption and oxidation of the secondary hydroxyl group. Recently, gold

nanoparticles supported on different metal oxides (Al2O3, TiO2, ZrO2, NiO2 and CuO) were

prepared for the oxidation of glycerol to produce DHA under base-free conditions.44 Among

these oxide-supported catalysts, the Au/CuO catalyst showed the most promising performance

Table 1. State-of-the-art catalysts for the oxidation of glycerol to DHAa

Entry Catalyst Base

Subs./ metal ratio

Reaction conditions

GLY conv. (%)

Selectivity (%)

Ref. OH-/

substrate T (oC) Atmosphere p

(bar) time (h) DHA GLAC TA

1 Au/CNTs NaOH 1300:1 2 60 O2 3 2 93 60 26 1 42

2 Au/20CX NaOH 1300:1 2 60 O2 3 ~3 90 42 40 1 50

3 Au/CuO no 1000:1 0 80 O2 10 2 21 83 1.6 n.g. 44

4 Pt-Bi/AC no 100:1 0 80 O2 2 4 80 60 n.g. n.g. 51

5 Pd-Ag/C no 575:1 0 80 O2 3 24 20 82 8 n.g. 52

Water was the solvent in all listed systems. aCNT: carbon nanotubes; 20CX: carbon xerogel; AC: activated carbon.

n.g. = not given. The yield of glyceraldehyde (GLAD) was not given in these references.

Chapter 1

---7---

by achieving moderate glycerol conversion with remarkable selectivity towards DHA (Entry 3,

Table 1). This study proved that the conversion of glycerol on supported Au nanoparticles is

sensitive to the nature of the support material. However, the reasons for which CuO is the most

suitable support were not elucidated. In line with expectations, increasing the reaction

temperature and/or oxygen pressure promoted further oxidation of DHA and was thus

detrimental for the DHA selectivity. Model studies on oxidation and isotopic labelling using

propanols and propanediols indicate that Au nanoparticles selectively activate the secondary

carbon (C-O and C-H bonds), and then oxidize the corresponding hydroxyl group to a carbonyl

group.

Pt/C was one of the first catalysts used for the production of DHA from glycerol, though with

low DHA yield (4%).53 Several research groups further optimized the catalysts composition by

alloying platinum with bismuth.51, 53, 54 Under the best conditions, the selectivity towards DHA

was 60% at 80% glycerol conversion (Entry 4, Table 1).51 It was proposed that the role of Bi,

which is present as soluble species in the reaction solution,55 is to moderate the high oxidation

activity of Pt, thus inhibiting the over-oxidation of DHA and other parallel side-reactions (e.g.,

degradative reactions).55, 56 Similarly, Pt-Sb alloy supported on multi-walled carbon nanotubes

(MWCNTs) displayed less over-oxidation of DHA to GLAC and much higher activity (based on

TOF) compared to Pt/MWCNTs.57 Alloying proved beneficial, also for palladium-based catalysts

on carbon support. For instance, Pd-Ag/C (Carbon in form of XC72) showed the best

performance in terms of both activity and selectivity towards DHA among several Pd-based

alloys (Entry 5, Table 1).52, 58 A kinetic study indicated that the addition of Ag to Pd improves

the interaction between glycerol and the catalyst surface, whereas Pd is needed to obtain a high

selectivity for the subsequent oxidation. However, this catalyst tends to deactivate, probably

due to poisoning of the Pd sites through adsorption of reaction by-products which block the

oxygen activation sites.

2.2 From glycerol to glyceric acid (GLAC)

Glyceric acid (GLAC) is often observed as the main product of the catalytic oxidation of glycerol.

Many studies have focussed on the selective glycerol oxidation to GLAC (or its salt form). Noble-

metal-based catalysts have been widely used for this reaction, often at 60-100 oC with oxygen

as oxidant (2-10 bar) in a basic aqueous medium (Table 2). Pt/AC and Pd/AC catalysts showed

high glycerol conversion and excellent selectivity towards glyceric acid, with Pd/AC showing

slightly better performance (Entry 1, Table 2).54 However, these Pt- or Pd-based catalytic

---8---

systems have some relevant drawbacks: they operate in the presence of a homogeneous base,

they are expensive and tend to deactivate at longer reaction times, which has been ascribed to

poisoning of active sites by oxygen and/or strongly adsorption of by-products.52, 59 Supported

Au-catalysts showed superior activity compared to platinum group metals in basic medium.

Several factors should be taken into consideration for the design of efficient monometallic Au

catalysts. The particle size of carbon-supported Au catalysts shows remarkable influence on

catalytic performance. Small and highly dispersed Au nanoparticles (~5 nm) are more active

than larger Au nanoparticles (≥ 20 nm).60, 61 On the other hand, the in situ formation of H2O2,

which is responsible for the C-C bond scission and further oxidation of GLAC, was observed on

the small Au particles. Larger Au nanoparticles show higher selectivity to GLAC by suppressing

this C-C bond scission and further oxidation, though at the expenses of catalytic activity.61-63

The optimum balance was achieved for a catalyst with 20 nm Au particles, which lead to 78%

GLAC selectivity at 50% glycerol conversion (Entry 2, Table 2). Moreover, the preparation

method also influences the catalytic performance of Au/AC catalysts. Catalysts prepared by sol-

immobilization (reduction of gold precursors to metallic nanoparticles to a gold-sol in solution

followed by immobilisation of the gold nanoparticles on the support) are more active than those

prepared by conventional surfactant-free precipitation methods.47, 64, 65 The choice of the

reducing agent in the sol-immobilization method is also important for catalytic performance. It

was shown that the catalytic activity increases whereas the selectivity was not affected when

using tetrakis-(hydroxymethyl)-phosphonium chloride (THPC) instead of NaBH4 as reducing

agent.47 Comparison of Au catalysts on different supports showed that carbon-supported gold

has higher activity than gold on oxide supports (Al2O3, Ta2O5, V2O5, MgO, CeO2 and Nb2O5),

though no explanation was provided for this behaviour.66-68 Moreover, the surface properties

of carbon supports, e.g., activated carbon, carbon nanotubes, graphite and carbon ribbon-type

nanofibers have strong effects on the selectivity towards GLAC. It has been proposed that the

presence of oxygenated groups on the surface is not favourable for GLAC.31, 64, 69 It should be

noted that a higher degree of graphitisation and, therefore a lower presence of structural

defects, resulted in smaller metal particles and better catalytic performance. Studies have been

performed using Au nanoparticles on carbon supports which were treated in different ways to

give distinct differences in surface properties.70 The more basic, nitrogen-containing surfaces

turned out to be more active than original and oxidised CNF (Entry 3, Table 2). An increase in

surface hydrophobicity also leads to a higher selectivity to C3 products, probably by reducing

H2O2 formation, which is considered to be responsible for C-C bond cleavage. Indeed, basic and

hydrophobic carbon surfaces enhance both the catalytic activity and selectivity to C3 products

Chapter 1

---9---

of Au catalyst. Both activated carbon and multi-walled carbon nanotubes were treated to obtain

different surface oxygen densities (Entry 4, Table 2). It turns out that basic oxygen-free

supports give higher catalyst activity and selectivity to GLAC, indicating that oxygenated groups

on the surface are not preferred for oxidation.49, 69

Table 2. Typical catalysts for the oxidation of glycerol to GLACa

Entry Catalyst Base

Subs./ metal ratio

Reaction conditions GLY

conv. (%)

Selectivity (%)

Ref. OH-/

substrate T

(oC) Atmosphere p

(bar) time (h) DHA GLAC TA

1 Pd/AC NaOH n.g. pH 11 n.g. Air (flow reactor)

n.g. n.g. 90 n.g. 77 n.g. 54

2 Au/AC NaOH 60000:1 2 60 O2 10 n.g. 50 n.g. 78 2 61

3 Au/N-CNF NaOH 1000:1 4 50 O2 3 n.g. 90 n.g. 68 14 70

4 Au/AC NaOH 2000:1 2 60 O2 3 2 72 18 62 5 49

5 AuPt/

H-mordenite

no 500:1 0 100 O2 3 2 70 n.g. 83 2 71

6 PtCo/RGO no 320:1 0 60 O2 (flow) n.g. 3 70 3 86 n.g. 72

aAC: activated carbon; N-CNF: nitrogen modified carbon nanofibers; RGO: reduced graphene oxide. n.g.

= not given

It should be noted that all catalysts discussed above for the oxidation of glycerol to glyceric acid

operate in alkaline conditions with over-stoichiometric amount of NaOH. Particularly, Au

catalysts show limited activity or are even inactive in the absence of a strong base. The major

role of a base like NaOH is to deprotonate OH groups of glycerol prior to oxidation reactions.

Moreover, the alkaline medium can promote the isomerisation of DHA to GLAD. The latter is an

intermediate in the oxidation of glycerol to GLAC (Scheme 2).44, 54 A process involving over-

stoichiometric amounts of a homogeneous base is undesirable, both from a practical and

environmental point of view. To avoid the use of a homogeneous base, catalysts consisting of

noble metal alloys on solid basic supports were designed. Here, both Au and Pt/Pd based

catalytic systems can be envisaged. In this respect, Au based catalysts typically show excellent

activity in alkaline conditions and are not very prone to poisoning, though Au nanoparticles are

inclined to agglomeration and deactivation. On the other hand, Pt and Pd catalysts show good

activity in acidic and neutral conditions, but are easily poisoned by oxygen and reaction by-

products. Based on these considerations, bimetallic or even trimetallic alloys of gold and

platinum group metals were prepared and studied as catalysts for glycerol oxidation under

base-free conditions. It was shown that these generally showed better catalytic performance

---10---

than their monometallic counterparts.73-75 For example, bimetallic Au-Pt catalysts on basic

supports, such as Mg(OH)2 or hydrotalcite, showed excellent activity and selectivity to glyceric

acid at ambient conditions.76, 77 The catalysts were reusable in consecutive runs, though the

interaction of the basic supports with the acidic products was not investigated. The

performance of monometallic Pt/C catalysts for glycerol oxidation in a base-free aqueous

solution was improved by the addition of Cu, ascribed to synergetic effects. The addition of Cu

accelerates the first dehydrogenation step and reduces the tendency of Pt to oxygen and by-

products poisoning.78 Au-Pt nanoparticles supported on zeolite H-mordenite were reported to

selectively oxidize glycerol to glyceric acid in base free conditions (Entry 5, Table 2).71 It has

been shown that by alloying Au to Pt, the metal leaching tendency is reduced and catalyst

lifetime is improved. Moreover, the acidic H-mordenite support plays an active role during the

glycerol oxidation and, compared to AuPt/AC, improves the selectivity towards GLAC by

preventing H2O2 formation. In a series of AuPt/TiO2 catalysts with different ratio between Au

and Pt, it is found that Au had a clear promoting effect for Pt/TiO2, though Pt had much higher

intrinsic activity than Au. Moreover, higher Pt contents favoured the oxidation of the primary

hydroxyl groups while higher Au contents favoured the oxidation of secondary hydroxyl groups

of glycerol.79 Incorporating Co into Pt/reduced graphene oxide (RGO) largely enhanced

dispersion as well as the stability of Pt nanoparticles thus had a 85% selectivity towards GLAC

at 70% glycerol conversion without adding base (Entry 6, Table 2).72 Recently, bimetallic Au-

Pd/TiO2, Pd-Pt/TiO2, Au-Pt/TiO2 catalysts, and a trimetallic Au-Pd-Pt/TiO2 catalyst were

prepared for the base-free oxidation of glycerol.43 The most active bimetallic catalyst was found

to be Pd-Pt/TiO2 with good selectivity for C3 products and with minimal C-C bond scission

(Selectivity to C1 and C2 products < 7%). The trimetallic catalyst gave a higher initial activity

and comparable selectivity, but was unstable in the long term because of metal leaching and

particle agglomeration.

2.3 From glycerol to tartronic acid and other carboxylic acids

Tartronic acid (TA) is the subsequent oxidation product of GLAC, and is generated when the

primary hydroxyl group of glyceric acid is oxidized into a carboxylic group. TA is typically

observed as a minor product during glycerol oxidation. This highly oxidized C3 molecule with

one central hydroxyl group and two carboxylic groups is rather unstable and tends to undergo

further reactions.80, 81 TA was firstly synthesized by the oxidation of glyceric acid (or its salt)

using 5 wt% Pt-1.9 wt% Bi/C as catalyst. The product selectively was highly dependent on the

reaction pH. At alkaline conditions (pH 10-11), the main product was the salt of tartronic acid

Chapter 1

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(initial selectivity 90%, maximum yield 83%), whereas under acidic conditions (pH 3-4) the

main product was hydroxypyruvic acid (initial selectivity 73%, maximum yield 61%).80 This

indicates that under acidic conditions, Pt-Bi/C selectively catalyses the oxidation of the central

hydroxyl group, similarly to what has been observed for the oxidation of glycerol to DHA over

the same type of catalysts (see section 2.1). At alkaline conditions, the Pt-Bi/C catalyst

preferentially promotes the oxidation of the terminal hydroxyl group to TA, which then reacts

with the base in the solution to form the tartronate. This neutralization reaction shifts the

equilibrium concentration towards the formation of TA.82-85 Supported monometallic Au and

bimetallic Au-based catalysts were used for the direct conversion of glycerol to TA under

alkaline conditions.60, 86, 87 The highest selectivity was up to 45% at nearly full glycerol

conversion. The addition of 0.1 wt% of bismuth to the Au-Pd/AC catalyst lead to a large increase

in TA yield and values up to 78% were reported at full glycerol conversion.56

Mesoxalic acid (MA) is also an interesting dicarboxylic acid derived from glycerol, though

difficult to obtain by the direct oxidation of glycerol due to over-oxidation to CO2. MA is

accessible by using TA as the substrate. Certain bimetallic Bi-Pt/C catalysts are active for this

reaction and a 65% MA yield (at 80% TA conversion) was reported at 60 oC, pH=5 and a

reaction time of 20 min.81 Since both TA and MA are easily oxidised to oxalic acid and CO2,

prolonged reaction times decrease the yield of MA. An alternative route involves the production

of dimethyl mesoxalate by oxidative esterification of glycerol in methanol and 89% selectivity

at full conversion over an Au/Fe2O3 catalyst in alkaline conditions has been reported.88

Glycolic acid (GOA), an α-hydroxy carboxylic acid, is often formed as a side-product in glycerol

oxidations by C-C bond cleavage of C3 intermediates. Using H2O2 as the oxidant, the yield of GOA

was 56% at full glycerol conversion over a 1 wt% Au/graphite catalyst.60 GOA is most likely

formed by the decarboxylation of TA with concomitant cleavage of a C-C bond.

3. Partial oxidation and isomerisation reactions

Lactic acid (2-hydroxy propionic acid, LA) and esters thereof are considered promising bio-

based platform molecules.7, 89 Both can be obtained from glycerol by means of a partial

oxidation followed by an isomerization reaction (Scheme 3).

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Scheme 3. From glycerol to lactic acid or alkyl lactates

Lactic acid is used as a monomer for the production of poly(lactic acid), which is a

biodegradable polyester with a wide range of applications, such as food packaging, protective

clothing and medical uses (e.g., suture, drug coating).89, 90 The market for lactic acid was

estimated to increase by 18.8% from 2014 to 2019 with a turnover of 3.5 billion USD.89, 91 The

alkyl esters of lactic acid, have potential to be used as green solvents.92 Moreover, lactic acid

and its salts are used in the food, cosmetic and pharmaceutical industry.90 Currently, up to 90%

of the lactic acid is produced by fermentation of carbohydrates. This route produces also a large

amount of calcium sulphate as by-product, which is not preferred from a green chemistry point

of view.93, 94 Recently, a process for lactic acid from crude glycerol by using cascade enzymatic

and chemical catalysis in methanol has been reported and it was stated that this route is a good

alternative for conventional fermentative processes from the point of view of sustainability and

operating costs.95

The development of efficient chemocatalytic routes to produce lactic acid from biomass

represents an attractive alternative for the fermentative route. Two possible chemocatalytic

routes have been investigated so far, either with carbohydrates or with glycerol as bio-based

feedstock.40, 45, 96-100 The price gap between glycerol and lactic acid (25-75% higher than

glycerol101) and the growing market for this product underline the relevance of research on the

chemocatalytic conversion of glycerol to lactic acid and alkyl lactates.

The chemocatalytic conversion of glycerol into lactic acid or alkyl lactates involves a multi-step

reaction network (Scheme 3) with high atom efficiency. Lactic acid is the main product when

the reaction is carried out in water, whereas alkyl lactates are obtained in alcohols. The first

step involves the dehydrogenative oxidation of glycerol to produce glyceraldehyde (GLAD)

and/or dihydroxyacetone (DHA), which is usually catalysed by supported noble metal catalysts

(see section 2.1).45, 102 The second main step is the isomerization of the formed triose, GLAD

Chapter 1

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and/or DHA, to produce lactic acid or alkyl lactate. This isomerization actually proceeds via a

number of intermediates. First, the triose undergoes a dehydration step leading to the

formation of pyruvaldehyde. This intermediate rearranges with the addition of an alcohol or

water molecule to yield an alkyl lactate or lactic acid, respectively (Scheme 3). The

rearrangement is anticipated to proceeds via a 1,2-hydride shift and has been described either

as an intramolecular Cannizzaro reaction or as a Meerwein-Ponndorf-Verley-Oppenauer

reaction.103-107 When the reaction is carried out in an alcohol, the reversible formation of

(hemi-)acetals have been observed by reaction of pyruvaldehyde with the alcohol.99, 104

An overview of the various catalysts available for the conversion of glycerol to lactic acid is

given in Table 3. The conversion of trioses to lactates and lactic acid is typically carried out with

high activity and selectivity over various metal based solid acid catalysts (e.g. USY zeolite; Sn-

Beta zeolite; Sn-MCM-41-XS; Sn-, Zr-, Hf-TUD-1) at 80-100 oC in ethanol or water without the

addition of base.99, 103-108 These studies showed that the dehydration of the triose to pyruvic

aldehyde is catalysed by mild Brønsted acid sites, whereas the following rearrangement is

catalysed by Lewis acid sites. Strong Brønsted acid sites should be avoided because they

promote the formation of undesired di-acetals. The use of alcohols is preferred above water,

mainly because the catalysts have shown higher stability in alcohols. High-purity lactic acid can

be obtained from alkyl lactates by distillation followed by hydrolysis.39, 95, 107

The first studies on the direct conversion of glycerol to lactic acid were conducted in harsh

alkaline hydrothermal conditions and yielded sodium or potassium lactate as the main product

(entry 1-2 in Table 3).109, 110 The use of over-stoichiometric amounts of NaOH or KOH and high

temperature is not preferred. Milder reaction conditions are possible when a homogeneous base

is combined with a supported noble metal catalyst (Pt, Ir). The latter promotes the initial

oxidative dehydrogenation of glycerol to trioses (entry 3-5 in Table 3).102, 111 It has been

postulated that the base is required to catalyse the conversion of pyruvic aldehyde to lactic acid

before a subsequent undesired hydrogenation to propanediol. Lactic acid has also been obtained

when the reactions were carried out in a hydrogen atmosphere. However, considering that the

initial step is an oxidative dehydrogenation of glycerol to DHA/GLAD, an inert atmosphere is

favoured (compare entry 4 and 5 in Table 3). In an attempt to substitute noble metals with more

affordable ones, a copper-based catalyst was studied for the synthesis of lactic acid from glycerol

under N2 using a near stoichiometric NaOH/glycerol molar ratio (entry 6, Table 3).112 The high

copper loading of this catalyst (60 wt%) could be significantly lowered (2 wt%) when the copper

was alloyed with 0.05 wt% palladium.118 It has been proposed that the presence of Pd enhances

---14---

both the dehydrogenation activity and the stability of the catalyst (entry 12, Table 3). The

temperature could be significantly reduced by performing the reaction in an oxidative

atmosphere (oxygen or air).

Table 3. Selected catalysts for the conversion of glycerol into lactic acid.

Reaction conditions c LA or alkyl lactate(%)

Entry Catalyst Base

Metal loading (wt%)

Subs./ metal

OH-/ subs.

T (oC) Atmosphere

p (bar)

time (h)

GLY conv. (%) Sel. Yield Ref.

Monometallic catalysts

1 - NaOH - - 4.75 300 air 1 1.5 95 95 90 109

2 - KOH - - 4.75 300 air 1 1.5 >99 90 90 110

3 Pt/C NaOH 3 700 7.4 200 H2 40 5 92 48 44 111

4 Ir/C NaOH ~0.7 ~3000 1.9 180 H2 n.g. n.g. 90 20 18 102

5 Ir/C NaOH ~0.7 ~3000 1.9 180 He n.g. n.g. 95 55 52 102

6 Cu/SiO2 NaOH 60 9 1.1 240 N2 14 6 75 80 60 112

7 Au/CeO2 NaOH 1 n.g. 4 90 air 1 n.g. 98 83 83 113

8 Pt/Sn-MFI - 1.5 350 0 100 O2 6.2 24 90 81 73 114

9 Au/USY b - 0.35 1407 0 160 air 30 10 95 77 73 115

10 Au/Sn-USY b - 95 0 160 O2 5 10 88 90 79 116

11 Au/CuO+Sn-MCM-41-XS

b - 1 985 0 140 air 30 10 95 66 63 117

Bimetallic catalysts

12 CuPd/rGO a NaOH 2.05 519 1.1 200 N2 14 6 85 75 63 118

13 Au-Pt/TiO2 NaOH 1 n.g. 4 90 O2 1 n.g. ~30 86 ~26 45

14 Au-Pt/CeO2 NaOH 0.7 680 4 100 O2 5 0.5 99 80 80 46

15 AuPd/TiO2

+ AlCl3 - 2 2500 0 160 O2 10 2 30 59 18 100

a rGO: reduced graphene oxide; b The solvent is methanol and the product is methyl lactate; c n.g.: not given.

Gold or gold-based alloys are the catalysts of choice for the oxidative dehydrogenation of

glycerol to DHA/GLAD. Supported Au-Pt bimetallic catalysts showed better performance in

terms of activity (TOF and conversion) of glycerol compared to monometallic Au catalysts.

Moreover, incorporation of Pt in the Au catalysts limits the further oxidation of GLAD to GLAC,

thus increasing the selectivity for lactic acid. Alloying Au with Pt also enhances the stability of

Chapter 1

---15---

the catalyst, supported by batch studies, showing that the catalyst could be reused 5 times

without loss of activity and selectivity. This good catalyst stability was explained by considering

a lower tendency of the AuPt particles to agglomerate (compared to Au particles) and a lower

level of O2 poisoning (compared to Pt particles). Support effects were also discussed in this

study.46 Gold supported on nano-ceria showed higher selectivity to lactic acid than when using

titania or carbon supports, where further oxidation of GLAD to GLAC or TA was observed. This

study on gold-based catalysts underlined the importance of tuning the properties of the

supported metal catalysts to control the selectivity of the partial oxidation of glycerol, in order

to avoid further oxidation of the trioses.

A base was used in the studies reported above on the conversion of glycerol to lactic acid. The

function of the base is (1) to deprotonate glycerol, (2) to accelerate the conversion of the

intermediate triose to lactic acid, and (3) to shift the equilibrium towards lactic acid/lactate by

lowering the lactic acid concentration as a result of sodium lactate formation. A such, in the

presence of a base, a lactate salt instead of lactic acid is formed and a subsequent neutralization

step is required, leading to the undesired formation of a salt.

Recent research efforts focussed on the development of a base free version for the oxidation of

glycerol to lactic acid. For this, the rate of the isomerisation reaction of the triose to lactic acid

is key and should be much higher than other reactions involving the triose (e.g., further

oxidations).45, 114, 115 Efficient catalysts were developed and involve the use of noble metal

catalysts for the partial oxidation of glycerol and solid acids that are active for the isomerisation

of the trioses to lactates.96, 97, 99 The solid acid may also acts as the support for the metal particles,

leading to bifunctional heterogeneous catalysts that can be easily separated after the reaction

and recycled. Therefore, these heterogeneous conversions are more desirable from a green

chemistry and technology perspective. The first attempts to develop a base-free catalytic

system involved a homogeneous Lewis acid (AlCl3) in combination with AuPd/TiO2 (entry 15,

Table 3).100 After 2 hours reaction at 160 oC, 10 bar O2, it gave 30% conversion of glycerol and

59% selectivity towards lactic acid. The first report of a fully heterogeneous, bifunctional

catalyst for the one-pot conversion of glycerol into lactic acid employed Pt supported on a

zeolite (Sn-MFI). An excellent selectivity (81%) towards lactic acid at 90% conversion of

glycerol was obtained using O2 (6 bar) at a relatively low temperature of 100 °C (entry 8, Table

3).114 Very recently, an Au supported on zeolite USY catalyst was identified as promising

bifunctional catalyst, achieving 73% yield of methyl lactate at 95% glycerol conversion (30 bar

of air, 160oC, entry 9, Table 1).115 The spent catalyst, which was similar in physicochemical

---16---

properties to the fresh one, was recycled in a second batch run and showed similar activity and

methyl lactate selectivity. Higher yields of ML (79%) were obtained by incorporating Sn in a

related Au/USY zeolite system, though oxygen instead of air was used (entry 10, Table 3).116 A

recent study form our group demonstrated that physical mixtures of Au/CuO and Sn-MCM-41-

XS also efficiently convert glycerol into ML, achieving up to 63% yield at 95% glycerol

conversion (entry 11, Table 3).117 The use of a solid acid containing a combination of mild

Brønsted acid and Lewis acid sites such as Sn-MCM-41-XS is preferable in terms of selectivity

compared to zeolites, which typically contain stronger Brønsted acid sites. An additional

advantage of this catalytic system is that supporting the metal nanoparticles on a different

material from that that providing the Lewis acid sites offers the possibility of modifying and

tuning the nature of the support towards enhanced catalytic performance of the whole system.

4. Dehydration and oxidative dehydration

4.1 Dehydration of glycerol to acrolein

The catalytic dehydration to yield acrolein is an interesting valorisation route for glycerol.

Acrolein is a versatile intermediate with widespread applications in the chemical industry.

Among various valuable chemicals that can be produced from acrolein, the most important and

commercial ones are acrylic acid (and its esters) and methionine.119, 120 Acrylic acid is the

monomer of poly(acrylic acid), which is classified as superabsorbent polymer.121 Methionine is

a sulphur-containing amino acid, which cannot be synthesized by natural organisms, and is

used as supplement in animal feed.122 Acrolein can also be used for the industrial production of

1,3-propanediol, through the hydration of acrolein to 3-hydroxypropanal with subsequent

hydrogenation. Moreover, acrolein is a highly reactive compound and this feature is exploited

in its use a broad-spectrum biocide that is effective at very low concentrations.123 Acrolein is

currently manufactured by the partial oxidation of petroleum-derived propylene over BiMoOx-

based multi-component metal oxides.124 Considering the issues with fossil resources, the

sustainable production of acrolein from glycerol represents an attractive alternative.

Chapter 1

---17---

Scheme 4. From glycerol to acrolein or acetol (B=Brønsted, L=Lewis).

The dehydration of glycerol to acrolein is a double dehydration reaction that is typically

conducted at high temperature (around 300 oC) to perform the reaction in the gas-phase

(Scheme 4A). The acidity of catalyst plays a crucial role in the catalytic performance. One of the

dominant factors is the acid strength of the catalyst. Various solid catalysts with a wide range

of acid-base properties were tested for gas-phase glycerol dehydration to determine the

relation between catalytic performance and acidity and/or basicity.125, 126 It was concluded that

the most selective solid acid-base catalysts for acrolein are those having the largest proportion

of acid sites with acid strengths in the range of -8.2 ≤ H0 ≤ -3.0 (H0, Hammett acidity. Both the

stronger (H0 ≤ -8.2) and weaker acid sites (-3.0 ≤ H0 ≤ 6.8) were less selective or active. For

niobium oxide (calcined at 400 oC), the selectivity to acrolein was 51% at 88% glycerol

conversion (entry 1, Table 4).126 Another study reported on the effects of the acidity and/or

basicity of several (mixed) metal oxides and supported phosphotungstic acid.127 The results

suggested an inverse correlation between selectivity to acrolein and surface density of basic

sites which may leads to the formation of acetol. The (mixed) metal oxides, phosphates and

pyrophosphates were introduced into the gas-phase dehydration of glycerol as the

heterogeneous catalysts. Some of these catalysts, such as niobium oxide, tungsten oxide and

pyrophosphates, offer the possibility of controlling their acid strength via the calcination

step.126, 128-132 In zeolite catalysts, the amount of acid site and acid strength changes with the

Si/Al ratio, this was explored in the case of H-ZSM-5 that therefore offers strong effects on the

catalytic performance.133

The pioneering studies on gas-phase glycerol dehydration discussed above focused on the

effects of acid strength. However, the nature of the acid sites (Lewis or Brønsted) was found to

---18---

Table 4. Selected catalysts for the dehydration and oxidative dehydration of glycerol.

a: GHSV = gas hourly space velocity, TOS = time on stream, n.g. = not given, b: based on weight hourly

space velocity. c: reaction in two consecutive beds

be more important for determining the selectivity. Several solid Brønsted acids (or catalysts

containing Brønsted acids sites as main active species), such as supported heteropolyacid

(HPA), H-type zeolite and silica-alumina materials, (mixed) metal oxides and phosphates, were

studied for the glycerol dehydration to produce acrolein (entry 2-3, Table 4).133-135, 139-144 These

results indicate that the Brønsted acid catalysts are prior to Lewis acid catalysts in the

formation of acrolein. The different role played by Brønsted and Lewis acid sites in the gas-

phase glycerol dehydration have been well clarified (Scheme 4). The Brønsted acid sites are

responsible for the production of acrolein (scheme 4A), whereas the Lewis acid sites catalyse

the partial dehydration of glycerol to produce acetol, which is the main by-product of the

dehydration process from glycerol to acrolein (scheme 4B).134, 135, 139, 145 The Brønsted acid sites

prefer to activate the secondary hydroxyl group to form a more stable carbenium ion as the

transition state, from which H2O is eliminated forming 1,3-propenediol, which then undergoes

keto-enloic tautomerisation and a second dehydration to yield acrolein (Scheme 4A). The last

step (from 3-hydroxylpropionaldehyde to acrolein) is a thermodynamically favourable

dehydration, and may not require the involvement of a Brønsted acid site.123, 146, 147 The reaction

over Lewis acid sites proceeds according to a different path: one of the two terminal hydroxyl

groups is preferentially activated, which leads to elimination of a water molecule to form 2-

propene-1,2-diol. The 2-propene-1,2-diol readily isomerises to the thermodynamically more

Entry Catalyst

Reaction conditionsa

GLY conv. (%)

Selectivity (%)

Ref. T

(oC) Gas flow

GHSV (h-1)

TOS (h)

GLY conc.

(wt %) acrolein acetol Acrylic

acid

1 Nb2O5 315 N2 80 9-10 36 88 51 12 n.g. 126

2 Cs2.5H0.5PW12O40 275 N2 2.8b 1 10 >99 98 n.g. n.g. 134

3 Al/H-ZSM5 315 N2 n.g. 2 36 85 64 n.g. n.g. 135

4 15HPW/ZrO2-AN-650 315 N2 400 9-10 36 76 71 12 n.g. 136

5 VOHPO4·0.5H2O 300 O2/N2 = 4/18

n.g. 10 20 >99 66 4 3 128

6 0.5Pd%/Cs2.5H0.5PW12O40 275 H2 2.8b 5 10 79 96 1 n.g. 134

7 W-V-Nb-O 298 O2/He

= 4/54 18000 2 20 >99 21 n.g. 27 137

8 W-Zr-O + W-V-Mo-Oc 305 O2/Ar = 1/4

n.g. 3-4 20 >99 8 n.g. 44 138

Chapter 1

---19---

stable acetol. It should be noted that Lewis acid sites may be converted into Brønsted acid sites

in the presence of H2O. In such case, the Brønsted acid sites formed in situ can either catalyse

the dehydration reaction to form acrolein or participate in the dehydration of glycerol adsorbed

on the Lewis acid sites nearby.134, 139

For the solid acid catalysts, the textural properties have significant influence on the catalytic

performance. 129-134, 139-141, 148-150 Although glycerol is sufficiently small to enter the micropores

of zeolites, the diffusion of reagents and products is easier in materials containing larger pores,

i.e. in the mesoporous range.151, 152 In addition, the confined space of the micropores are easily

blocked by coke leading to catalyst deactivation as has been observed for the conversion of

glycerol to acrolein.151, 153, 154. Generally, (hierarchical) mesoporous materials undergo less

deactivation by coking compared to microporous solids as a consequence of the larger pores

and thus the better diffusion.148-152

Supported heteropolyacids (HPAs) have been widely studied for the gas-phase dehydration of

glycerol. These catalysts contain very strong Brønsted acid sites and were typically supported

on solid materials, such as metal oxides, silica, carbons and zeolites.134, 136, 155-160 The surface

properties of the support have a strong impact on the catalytic performance. For instance,

silica-supported tungstophosphoric acid are less active, selective and stable for the dehydration

of glycerol to form acrolein than the zirconia based ones. It is concluded that the interaction

between the ZrO2 support and HPA enhances both the thermal stability of the Keggin structure

and the dispersion of HPA, as concluded from XRD and Raman measurements (entry 4, Table

4).136, 155 Moreover, the over-strong interaction of HPA with the surface of ZrO2 leads to a

reduction of the acidity of the supported HPA, and it results in a higher long-term catalytic

performance due to less coking.161

So far, all of the catalysts reported for the gas-phase dehydration of glycerol to acrolein suffer

from rapid deactivation due to coke formation.162 For example, the catalyst SAPO-11 lost more

than 70% activity after 10 h time on stream at 280 °C with GHSV 43 h-1.150 This is an intrinsic

drawback of carrying out a reaction of an organic compound over an acidic catalyst at elevated

temperature. Several methods were developed to regenerate spent catalysts, such as re-

calcination.128, 129, 146 This is not the topic of this review and we will solely discuss catalysts

design activities to improve the intrinsic stability of the catalysts. Catalysts with redox active

sites, such as vanadium phosphates, can be regenerated in situ by co-feeding O2 (entry 5, Table

4).128, 129, 132 This has a positive effect on catalyst stability, though at the expenses of selectivity

---20---

due to oxidation reactions leading to the formation of organic acids, such as acrylic acid and

acetic acid. In addition, safety issues dictate that the O2 amount should not exceed 7 vol% to

avoid explosion risks. Another approach to mitigate deactivation by coking involves the

addition of noble metals to the catalyst formulation combined with H2 or O2 co-feeding (entry

6, Table 4).134, 163, 164 In the case of H2, the noble metals are assumed to catalyse the

hydrogenation of the coke and as such have a positive effect on catalyst stability, which had

only 25% activity loss rather than 60% with unmodified system after 6 h reaction.

4.2 Oxidative dehydration of glycerol to acrylic acid

Acrylic acid, an important monomer in the polymer industry, is an important target chemical

from bio-based glycerol. It is attainable from glycerol by the dehydration of lactic acid (section

3) and oxidation of acrolein (section 4.1). For the latter route, it is preferred to convert glycerol

to acrylic acid in a one-step process, because the intermediate acrolein is toxic and flammable.

For this purpose, O2 is added to the feed to in situ oxidise the formed acrolein at similar reaction

conditions (vide supra).128, 129 Bifunctional catalysts combining acid sites for catalysing the

dehydration and redox sites for the oxidation were designed for this purpose.165 The catalysts

consist of multi-component metal oxides containing metals such as Mo, V, W, Te and Nb (entry

7, Table 4).137, 166-168 These catalysts (W-V-Nb-O, W-V-Mo-O mixed oxides) can display acidity

stemming from the tungsten and niobium oxides and redox sites originating from the vanadium

(or molybdenum) oxides. The balance between acid and redox properties is extremely

important for converting glycerol to acrolein and then oxidising acrolein to acrylic acid while

preventing over oxidation. So far it proved to be difficult to keep a good balance between acid

and redox properties on a single bifunctional catalyst.137, 166, 167 More promising results were

obtained combining two tandem catalytic beds, one with acid sites and the other with redox

sites, in a single reactor.138, 169 For example, H-ZSM-5 zeolite and vanadium-molybdenum oxide

were used for the dehydration and oxidation steps, respectively (entry 8, Table 4).169 In the

view of the yield of acrylic acid, this process is more efficient than the process based on a single

catalyst, but it still needs to face the inconsistency in the reaction conditions of two separate

reactions.

5. Hydrogenolysis

Hydrogenolysis of bio-based substrates rich in oxygen-containing functional groups, such as

glycerol, has been studied extensively as an attractive way to produce useful chemicals and

fuels.21 . The hydrogenolysis of glycerol at elevated temperatures is typically not selective and

Chapter 1

---21---

a wide variety of products is formed. Examples are 1,2-propanediol (1,2-PD), 1,3-propanediol

(1,3-PD), 1-propanol (1-PrOH), 2-propanol (2-PrOH) and ethylene glycol.170 Since 1,2-PD and

1,3-PD are the most common products from glycerol hydrogenolysis, we focus on the catalytic

systems for synthesising them. Currently, 1,2-PD is widely used in antifreeze formulations, as a

monomer for polyester resins and as a solvent.20 1,3-PD is used as a monomer for the synthesis

of polytrimethylene terephthalate (PTT)171, a biodegradable polyester used in carpets and

textile industry. The annual worldwide production of 1,3-PD is about 105 tons.172, 173 Two routes

are currently used for the industrial production of 1,3-PD: hydration of acrolein to 3-

hydroxypropanal with a subsequent hydrogenation step (DuPont) and hydroformylation of

ethylene oxide (Shell process). 1,3-PD can also be produced from glucose fermentation, but at

the time being, the majority of 1,3-PD is produced from chemical synthesis based on fossil

resources. Considering the high demand and price, the global 1,3-PD market is estimated to

reach $621.2 million by 2021, makes it an attractive target compound for production from bio-

based glycerol.174

5.1 From glycerol to 1,2-PD

The synthesis of 1,2-PD from glycerol is a hydrogenolysis reaction. The reaction proceeds

through different pathways in acidic and basic environment. It is widely accepted that the

reaction goes through a dehydration and hydrogenation step in acidic conditions (Scheme 5A).

First, the dehydration of glycerol catalysed by acid sites produces acetol as a key intermediate,

and then the hydrogenation of acetol over metallic sites yields 1,2-PD. In harsh conditions (e.g.

T > 200 oC, pH2 > 4 MPa), further dehydration and hydrogenation of 1,2-PD lead to the formation

of 1-propanol and 2-propanol and even to the totally dehydroxylated product propane, which

is an undesired product.175 On the other hand, in alkaline conditions (Scheme 5B), the reaction

consists of dehydrogenation, dehydration and hydrogenation steps, with glyceraldehyde and

pyruvic aldehyde as intermediates.176 As we discussed in section 3, lactic acid (or lactates) can

be the main product in these conditions, due to the fast transformation of pyruvic aldehyde to

lactic acid (salt) in the presence of a base, which competes with the further.

Based on the route for the formation of 1,2-PD in acidic conditions, a bifunctional catalytic

systems is needed, with acid sites for catalysing the dehydration and metallic sites for the

hydrogenation. The two types of sites can be located on the same material or on two separate

materials. Since the hydrogenolysis of glycerol involves a hydrogenation step, metals that are

noble metals, though they are less stable and active. Various forms of copper- and zinc-based

---22---

Scheme 5. Possible routes for the synthesis of 1,2-PD from glycerol.

catalysts were tested for the aqueous hydrogenolysis of glycerol. Cu-CuxO/ZnO catalysts

showed good catalytic performance in the synthesis of 1,2-PD since the copper catalysts do not

promote the C-C bond cleavage, which would lead to undesired C1 and C2 products.177-181 The

particle size of the active components has a strong influence on the catalytic performance. A

correlation was found between particle size and catalytic performance: smaller ZnO particles

led to higher glycerol conversion, whereas smaller Cu particles gave higher 1,2-PD

selectivity.181 These catalysts tend to deactivate due to sintering of the active copper particles.

Incorporation of Ga2O3 into the catalyst formation (Cu/ZnO/Ga2O3, prepared by co-

precipitation) was reported to help preventing the deactivation by stabilizing the small copper

particles during the hydrogenolysis of aqueous glycerol (entry 1, Table 5).179

Not only the nature of the catalyst, but also a tailored design of the reactor can play an important

role in the two-step conversion of glycerol into 1,2-propanediol. A continuous fixed-bed reactor

was developed to perform the vapour-phase conversion of glycerol into 1,2-PD over a Cu/Al2O3

catalyst with a temperature gradient within the reactor.182, 188-190 The initial dehydration of

glycerol into acetol is carried out at 200 oC, whereas the hydrogenation of acetol into 1,2-PD is

performed at 120 oC. This optimised reactor design led to 97% yield of 1,2-PD (entry 2, Table

5). Catalysts based on copper show good catalytic performance in the 1,2-PD production, but

they need large weight percentage of metal as the effective component due to the relatively low

activity of Cu for hydrogenolysis. On the other hand, noble metals, such as ruthenium, exhibit

higher activity in the hydrogenolysis and can be employed at a lower loading.171, 186, 191-197 It

Chapter 1

---23---

Table 5. Catalysts for the hydrogenolysis of glycerol to synthesis 1,2-PD.

Entry Catalyst

Reaction conditions b GLY

conv. (%)

Selectivity (%)a

Ref. Subs./metal T (oC) pH2

(MPa) time (h)

GLY conc.

(wt %)c 1,2-PD

1,3-PD PrOH others

1 Cu/ZnO/Ga2O3 25 220 2.5 5.5 50 99 80 n.g. n.g. n.g. 179

2 Cu/Al2O3 n.a.

top 200-

bottom 120

flow

LHSV

0.25 h-

1

30 >99 97 n.g. n.g. 2 182

3 Cu-Ru/MWCNTs 52 200 4 6 80 >99 87 n.g. 13 0 183

4 Ru2Fe/CNTs ~234 200 4 12 20 86 52 n.g. n.g. >35 184

5 Cu-

H4SiW12O40/Al2O3 n.a. 240 6

LHSV

0.9 h-1 10 90 90 n.g. 2 7 185

6 Ru/C +

Amberlyst ~30 120 8 10 20 21 77 1 3 19 186

7 Pt/NaY ~395 230 0 15 20 85 64 n.g. 8 13 187

a1,2-PD: 1,2-propanediol; 1,3-PD: 1,3-propanediol; PrOH: propanol; n.g.: not given; n.a.: not applicable;

others: the products involving breaking of a C-C bond, such as ethylene glycol, methanol and methane. b

LHSV: liquid hourly space velocity, WHSV: weight hourly space velocity. Both base on continuous fixed-

bed reactor.

should be noted that the noble metal catalysts are less selective than the copper-based catalysts

because the higher activity also implies the ability to catalyse the C-C bond cleavage with

formation of undesired by-products, especially under harsh conditions (typically, T > 200 oC,

pH2 > 4 MPa). Another strategy for the synthesis of 1,2-PD from glycerol is based on the use of

bimetallic catalysts. This approach aims at exploiting the possible synergy between two metals,

which can bring about positive effects on the catalytic performance. Indeed, the bimetallic

catalyst Cu-Ru/MWCNTs exhibited similar selectivity for 1,2-PD and much higher glycerol

conversion compared to monometallic Cu catalyst. This performance was ascribed to a

hydrogen spill-over effect, which allowed the Cu to benefit from the hydrogen activation by the

tiny Ru clusters (entry 3, Table 5).183 Similar synergetic effects were also observed on Pd-

Cu/Mg-Al-O catalysts.198 In the same context, Ru/CNTs with small Ru nanoparticles is

significantly active for C-C bond cleavage, giving a considerable amount of degradation

products, whereas, the bimetallic RuxFey/CNTs catalyst with a similar particle size was more

efficient for 1,2-PD production (entry 4, Table 5).184 It has been proposed that iron oxide species

also enhance the stability of RuFe bimetallic nanoparticles and, therefore, the reusability of the

catalyst. Nickel-based catalysts were also studied for the glycerol hydrogenolysis.188, 199-202 The

---24---

catalyst Ni/AC modified by cerium showed lower reduction temperature of Ni and smaller

metal particle size. This catalyst achieved higher glycerol conversion and 1,2-PD yield than the

unmodified catalyst.202

To promote the first dehydration step, several kinds of solid acids, such as

supported/unsupported heteropoly acids, acidic resins and metal oxides have been studied as

catalysts.185, 186, 191, 192, 203 Heteropoly acid H4SiW12O40 supported on Cu/Al2O3 was prepared as

catalyst for the continuous hydrogenolysis of glycerol, which leads to both high glycerol

conversion and high selectivity to 1,2-PD (entry 5, Table 5).185 The catalyst showed good long

term stability (250h). It was proposed that the presence of H4SiW12O40 not only enhanced the

acidity but also changed the reduction behaviour of copper oxides. Although the yield of 1,2-PD

can reach 81% at the optimal conditions, the instability of both metallic Cu and H4SiW12O40 in

hot aqueous solutions seriously inhibits practical application of this system. Another

bifunctional catalytic system is provided by the combination of Ru/C with the heat resistant

acidic resin Amberlyst, which exhibited high selectivity to 1,2-PD, and stability in the

hydrogenolysis of glycerol (entry 6, Table 5).186, 191 Compared to the Ru/C catalyst, the addition

of acidic resin Amberlyst largely enhances the glycerol conversion and selectivity to 1,2-PD.

This is consistent with a reaction path involving the dehydration of glycerol to acetol catalysed

by Amberlyst, and the subsequent hydrogenation of acetol to 1,2-PD catalysed by Ru/C. It

should be noted that, different from the gas-phase dehydration of glycerol which mainly

produces acrolein when catalysed by Brønsted acids and yields acetol in the presence of Lewis

acids (See section 4), the dehydrogenation of glycerol in aqueous medium leads to the

preferential formation of acetol also with Brønsted acids. The selectivity towards acetol in the

aqueous system has been ascribed to the higher thermodynamic stability of acetol compared to

the product of the dehydration of the secondary alcohol function of glycerol, i.e. 3-

hydroxypropanal.171

The synthesis of 1,2-PD from glycerol can also proceed in basic conditions (Scheme 5B). The

addition of a homogeneous base can accelerate the initial dehydrogenation step to

glyceraldehyde (see section 3). However, involvement of a homogeneous base will cause

separation problems as well. In basic medium, lactic acid is typically formed as a side-product

or even as main product (see section 3). The selectivity depends on the preferred path from the

pyruvic aldehyde intermediate: further hydrogenation over metal catalyst will lead to 1,2-PD,

whereas base-catalysed isomerization will lead to lactic acid. Metals supported on a solid base

Chapter 1

---25---

tend to catalyse the formation of 1,2-PD,198, 204 while the use of strong homogeneous base

usually leads to lactic acid as product.102, 111, 205

All the catalysts discussed above for the synthesis of 1,2-PD from glycerol operate under H2

pressure (Table 5). In addition, glycerol hydrogenolysis can proceed also by using in situ

generated hydrogen.206-208 This process would be more sustainable than those requiring

addition of external hydrogen (typically obtained from non-renewable resources). It has been

proposed that a suitable balance between metal activity and acidity is important for glycerol

aqueous reforming (leading to in situ hydrogen production) and then glycerol hydrogenolysis

(for 1,2-PD production).187 For the catalyst Pt/NaY, it was assumed that the Pt nanoparticles

first promote glycerol reforming to produce CO2 and H2. The disassociation of the in situ

generated H2CO3 (from CO2 and H2O) provides the protons that catalyse the dehydration of

glycerol to acetol, which is then hydrogenated over Pt to 1,2-PD by the in situ generated H2. The

selectivity towards 1,2-PD can reach 64% at 89% glycerol conversion, which is the highest yield

of 1,2-PD produced in the absence of added hydrogen (entry 7, Table 5). The 1,2-PD production

can also proceed over a combination of two catalysts, such as 5 wt.% Ru/Al2O3 and 5 wt.%

Pt/Al2O3, with the hydrogen from the in situ aqueous phase reforming of glycerol being used

for the conversion of glycerol into 1,2-PD and other products.206 It was proposed that platinum

promotes the aqueous phase reforming of glycerol step, whereas ruthenium is more selective

towards the glycerol hydrogenolysis.

5.2 From glycerol to 1,3-PD

Scheme 6. Possible routes for the synthesis of 1,3-PD from glycerol.209

The hydrogenolysis of glycerol produces preferentially 1,2-PD rather than 1,3-PD as reflected

by the larger number of reports in which the former is the main product. However, 1,3-PD is a

more valuable product and its selective synthesis from glycerol is highly attractive from an

industrial point of view. The synthesis of 1,3-PD requires the selective hydrogenolysis of the

---26---

central C-OH bond, but the exact mechanism of 1,3-PD formation from glycerol is still under

debate.171 In acidic conditions, the first step of the reaction has been proposed to be an acid-

catalysed dehydration with formation of 3-hydroxypropanal, followed by hydrogenation of this

intermediate to generate the target product 1,3-PD (Scheme 6A).171 Although the initial and

crucial dehydration step towards 1,3-PD formation in acidic conditions is similar to the one in

the gas-phase synthesis of acrolein, which is catalysed by Brønsted acid sites (see section 4),

1,2-PD rather than 1,3-PD is the main product of the liquid-phase glycerol hydrogenolysis in

the presence of Brønsted acid catalysts (see section 5.1). If 1,3-PD is the desired product, a co-

catalyst has to be added into the catalytic system in order to selectively activate the central

hydroxyl group of glycerol. Tungsten-based materials are one of the most effective co-catalysts

for increasing the selectivity towards1,3-PD in the hydrogenolysis of glycerol, and have been

extensively investigated.170, 171, 210 However, the mechanism through which these tungsten-

based catalysts promote the selectivity towards 1,3-PD has not been fully understood so far.

Moreover, not all tungsten-based catalysts give high selectivity to 1,3-PD (see for example entry

5, Table 5). Various kinds of tungsten-containing catalysts were prepared for the 1,3-PD

synthesis, including species such as H2WO4,177, 211 supported tungsten polyoxometalates and

WO3.212-219 Catalysts with a combination of tungsten-containing compounds (for the

dehydration of the secondary alcohol group) and supported noble metals (for the

hydrogenation) were studied for the synthesis of 1,3-PD by glycerol hydrogenolysis.177 It is

revealed that the formation of 1.3-PD is well correlated to the amount of super strong Brønsted

acid sites, which is generated from the medium size WOx domains when it interacts with Pt

nanoparticles.220-222 Pt was found to be more selective for the 1,3-PD formation than other

noble metals, such as Pd and Ru that may lead to more C-C bond cleavage reactions.177, 223 A

series of SiO2- or ZrO2-supported H4SiW12O40 and metals were prepared for the glycerol

hydrogenolysis.212-215 The catalyst Pt-H4SiW12O40/SiO2 used for aqueous-phase glycerol

hydrogenolysis that led to quite leaching of the heteropoly acid, but the author did not discuss

the reusability of the catalysts. Among several tested supports (SiO2, ZrO2, AC, TiO2 and γ-Al2O3),

the ZrO2-supported catalysts show the best catalytic performance and stability in the vapour-

phase glycerol hydrogenolysis. This has been ascribed to the strong interaction between Pt and

ZrO2, which favours the formation of stable and highly dispersed Pt nanoparticles.214, 215 The

interaction between H4SiW12O40 and ZrO2 also enhances the stability of Keggin structure. The

support also affects the surface acidic properties that can play a key role in the dehydration

step. Further modification with various alkali metals on the catalyst Pt-H4SiW12O40/ZrO2 leads

to a higher 1,3-PD yield. Particularly, the Li-exchanged catalyst (Pt-Li2H2SiW12O40/ZrO2)

Chapter 1

---27---

exhibited superior activity and 1,3-PD selectivity (entry 1, Table 6) than the unmodified catalyst.

The authors claimed that the Li modification adjusts the amount of Brønsted acid sites that are

responsible for the selective formation of 1,3-dihydroxypropene and 3-hydroxypropanal from

glycerol dehydration. Another class of tungsten-based solid catalysts for the glycerol

hydrogenolysis contains tungsten oxide and noble metal particles.175, 215, 216 The catalyst based

on boehmite-supported Pt nanoparticles and tungsten oxides (Pt/WOx/AlOOH) was found to

act as a highly efficient and reusable solid catalyst for the selective hydrogenolysis of glycerol

to 1,3-PD. The 1,3-PD yield reached 69% from very diluted aqueous glycerol (entry 2, Table

6).217 However, the catalyst loading is rather high compared with other catalytic systems. Apart

from the nature of the catalyst, the reaction medium can have an important influence on the

1,3-PD formation. An impressively high 84% 1,3-PD selectivity was achieved at 67% glycerol

conversion in 1,3-dimethyl-2-imidazolidinone (DMI) solvent over Pt/sulphated-ZrO2 catalyst,

in which the sulphated-ZrO2 is considered to be a “super solid acid” (entry 3, Table 6).224 The

Brønsted acid sites on the catalyst were more advantageous for producing 1,3-PD.222 In this

case, the DMI, which is a highly polar aprotic solvent, is more favourable for the 1,3-PD

formation than water. However, the role that the solvent plays during the reaction was not

explained. On the other hand, it should be kept in mind that using organic solvent and adding

sulphuric acid are less sustainable and costlier than water.

Table 6. Selected catalysts for the hydrogenolysis of glycerol to synthesis 1,3-PD.

Entry Catalyst

Reaction conditions b GLY

conv. (%)

Selectivity (%)a

Ref. Subs./metal T (oC) pH2

(MPa) time (h)

GLY conc.

(wt %)c 1,2-PD

1,3-PD PrOH others

1 Pt-

Li2H2SiW12O40/ZrO2 n.a. 180 5

WHSV

0.09

h-1

10 44 14 54 27 5 215

2 Pt/WOx/AlOOH 108 180 5 12 0.3 >99 2 69 16 n.g. 217

3 Pt/Sulphated ZrO2 e 292 170 7.3 24 ~58d 67 3 84 0 n.g. 224

4 Ir-ReOx/SiO2 700 120 8 36 80 81 5 47 46 2 225

a1,2-PD: 1,2-propanediol; 1,3-PD: 1,3-propanediol; PrOH: propanol; n.g.: not given; n.a.: not applicable;

others: the products involving breaking of a C-C bond, such as ethylene glycol, methanol and methane. b

LHSV: liquid hourly space velocity, WHSV: weight hourly space velocity. Both base on continuous fixed-

bed reactor. c water solutions except entry 3. d reaction in 1,3-dimethyl-2-imidazolidinone (DMI) as

solvent. e 9.7 ml of sulphuric acid (0.5M) was added.

---28---

Another class of catalysts that has been studied in detail for the conversion of glycerol to 1,3-

PD is based on supported rhenium oxide species.225-228 The systematic studies on rhenium-

based catalysts show selective activity in dissociation of the central C-O bond in glycerol. The

initial selectivity of 1,3-PD over Ir-ReOx/SiO2 reached 65%. However, over-hydrogenolysis of

1,3-PD with the formation of 1-PrOH was observed with this catalyst and the final yield of 1,3-

PD was about 38% (entry 4, Table 7).225 The addition of homogeneous or heterogeneous acid

to this system enhanced the conversion, which may be due to the protonation of the surface of

ReOx cluster with consequent increase in the number of hydroxorhenium sites.225, 226 A detailed

mechanism has been proposed for the hydrogenolysis of glycerol over these rhenium-based

catalysts (Scheme 6B).209, 225, 227, 229-232 It was hypothesised that, on the Ir-ReOx/SiO2 catalyst,

the substrates are adsorbed on Re-OH sites leading to the formation of alkoxide species.

Hydrogen is activated at the interface between Ir and Re to form a hydride species. The C-O

bond neighbouring the alkoxide group is attacked by the hydride species via an SN2-type

reaction. Hydrolysis of the produced alkoxide releases the desired 1,3-PD product. This study

was continued by investigating a series of catalysts based on rhenium or iridium, M1-ReOx/SiO2

(M1 = Ir, Ru, Rh, Pd and Pt) and Ir-M2Ox/SiO2 (Re, Cr, Mn, Mo, W and Ag).233 It was found that

Rh-ReOx/SiO2 catalyst is also active for the glycerol hydrogenolysis, but it is less selective for

1,3-PD than Ir-ReOx/SiO2.234-236

6. Carbonation

Glycerol carbonate (GC) is a renewable and versatile building block for organic chemistry that

can be produced by carbonation of glycerol by reaction with CO2, CO, alkylene carbonate,

dimethylcarbonate or urea in the presence of a catalyst.237-239 Glycerol carbonate is currently

industrially produced (Huntsman Corporation, JEFFSOL® Glycerin Carbonate) and has

potential applications as solvent, as intermediate in organic syntheses and for the production

of epichlorohydrin, polycarbonates, polyurethanes and surfactants.237, 240

The synthesis of glycerol carbonate: can be achieved by direct carbonation, in which glycerol

reacts with CO2 or CO and O2.241-245 The choice of the catalyst depends on the compound that is

reacted with glycerol. Pd-based complexes confined into the cages of zeolite Y were employed

to catalyse the carbonation of glycerol with CO, since Pd is able to activate the CO molecule

(Entry 1, Table 7).243-245 The direct reaction between glycerol and CO2 is thermodynamically

unfavourable.246 Therefore, only very low conversions of glycerol are obtained unless the water

Chapter 1

---29---

Scheme 6. Synthesis of glycerol carbonate from glycerol with: dimethyl carbonate (A) or urea

(B).

formed in the reaction is removed (e.g. by reacting with acetonitrile). Using this approach, La-

Zn mixed oxides were identified as promising heterogeneous catalysts, reaching a yield of

glycerol carbonate of 14% at 30% glycerol conversion, under harsh conditions (170 oC, PCO2 =

4.0 MPa, 12h, in CH3CN) (Entry 2, Table 7).246 The authors found that the ZnO sites in the

catalyst are responsible for the activation of glycerol by forming zinc glycerolate. The formation

of La2O2CO3 during the reaction improved the surface basicity, thus promoting the CO2

activation and, consequently, the catalytic activity. Given the thermodynamic limitation and the

need of a sacrificial dehydrating agent, the direct reaction of glycerol with CO2 is not the most

widely investigated route to produce glycerol carbonate. The most studied path for the

synthesis of glycerol carbonate is transcarbonation, which is a carbonate-exchange reaction

between an organic carbonate or urea and glycerol (Scheme 6).237 The carbonate substrate can

be an alkylene carbonate or a dialkyl carbonate.237, 247 The transcarbonation between glycerol

and organic carbonates was estimated to be a thermodynamically favourable reaction for the

production of glycerol carbonate based on thermodynamic calculations of chemical equilibrium

constant.248 This feature makes this route much more viable compared to the direct reaction of

glycerol with carbon dioxide. Several kinds of solid basic catalysts, such as ion exchange resins,

basic oxides and immobilized ionic liquids, were employed as heterogeneous catalysts for the

transcarbonation between ethylene carbonate and glycerol.247, 249, 250 Al-Ca mixed oxide shows

good catalytic performance for this reaction (Entry 3, Table 7).247 The transcarbonation

between dimethyl carbonate and glycerol (Scheme 6A) has been widely studied since the

dimethyl carbonate is an easily accessible chemical which can be produced from methanol and

urea, or from methanol and an alkylene carbonate. Basic solid catalysts, such as (mixed) metal

oxides and carbonates, hydrotalcite and modified metal oxides, were found to be effective for

---30---

this reaction.251-258 Generally, stronger basicity and larger amount of basic sites enhance both

activity and selectivity towards glycerol carbonate. Among a series of transition metal doped

hydrotalcite catalysts, Ni modified hydrotalcite (HTC-Ni) which bears the highest basic site

density shows the highest glycerol carbonate yield (Entry 4, Table 7).253 Another

transcarbonation route is provided by the reaction between glycerol and urea. Although this

reaction has been estimated to be a thermodynamically unfavourable, the spontaneous removal

of gaseous NH3 makes it a feasible path for the synthesis of glycerol carbonate (Scheme 6B).248

Moreover, the main by-product of this reaction, ammonia, can be recycled for the further

production of urea. Based on thermodynamic calculations, high temperature and low pressure

are favourable for the formation of glycerol carbonate from glycerol and urea.248 Various solid

Lewis acids and/or solid bases were found to be effective catalysts for this reaction.247, 259-262

Several zinc-containing catalysts showed very good activity, which may due to the ability of zinc

sites to activate urea by coordination. With a Zn2+-modified heteropoly tungstate catalyst,

complete selectivity towards glycerol carbonate was obtained at 70% glycerol conversion

(Entry 5, Table 7).259 However, it should be noted that the activity might be caused by dissolved

zinc species instead of the heterogeneous ones.263 It was reported that solid catalysts that bear

a good acid-base balance, such as Zn-Al mixed oxide, are particularly suitable for promoting this

reaction since the Lewis acid can activate the carbonyl group in urea, while the basic sites are

able to activate the hydroxyl group of glycerol (Entry 6, Table 7).247 The use of basic sites to

activate of the hydroxyl group of glycerol is similar to the one employed for the oxidation of

glycerol (see section 2).

Table 7. Selected catalysts for the carbonation of glycerol.

Entry Catalyst Reactant

Reaction conditions GC(%)

GLY/ reactant Solvent T (oC) p (bar)

time (h)

GLY conv. (%) Sel. Yield Ref.

1 PdCl2(phen)@Y/CuI/KI CO+O2 - DMF 120 24 3 95 98 93 244

2 La2O2CO3-ZnO CO2 - CH3CN 170 40 12 30 47 14 246

3 AlCaMO Ethylene

carbonate 1:2 - 35 atmosphere 1 89 98 87 247

4 HTC-Ni DMA 1:3 - 100 atmosphere 2 55 100 55 253

5 Zn1TPA urea 1:1 - 140 Reduced 4 69 99 69 259

6 HTc-Zn urea 1:1 - 145 0.04

(pure) 5 82 88 72 247

Chapter 1

---31---

7. Acetalisation

Scheme 7. Synthesis of solketal by the reaction of glycerol with acetone.

The condensation of glycerol and ketones or aldehydes, such as acetone and formaldehyde,

produces ketals or acetals consisting of five- or six-membered rings.10, 264, 265 In the case of

reaction with acetone, the predominant product is solketal (2,2-dimethyl-1,3-dioxolane-4-

methanol) (Scheme 7), a five-membered ring molecule that is considered as an effective fuel

additive.6 The addition of solketal in standard diesel can effectively reduce the harmful

emissions and improve the antifreeze and antiknock properties of the fuel.266-269 Moreover,

solketal can also be used in scents, flavours, as solvent and basis for surfactants.264, 270-272

Conventionally, strong homogeneous Brønsted acids, such as sulphuric acid and p-

toluenesulphonic acid, were tested to be effective catalysts for this reaction.273, 274 A more

environmental friendly and industrially attractive option is represented by solid acid

heterogeneous catalysts such as Amberlyst resins, zeolites, metal containing mesoporous

silicates, HPAs immobilised in a SiO2 matrix, metal oxides and acid-functionalised carbon

materials.269, 271, 272, 275-280 Brønsted acids are known to be active species for this reaction.

Moreover, heterogeneous catalysts with predominantly Lewis acid character, such as Zr-, Hf-

and Sn-TUD-1 and Ga-MCM-41, were also found to be highly active and selective for the solketal

synthesis.108, 276, 277 These reactions were usually conducted at relatively mild conditions (70-

100 oC) with surplus acetone to increase the conversion of glycerol. Moreover, extra acetone

promotes removing water from the reaction system by distillation that is also beneficial to the

yield of solketal. Besides the over-stoichiometric acetone, the hydrophobicity of the catalyst

surface was reported to promote the formation of solketal, as it helps removing water from the

active site region, thus leading to higher solketal yield at equilibrium (Le Châtelier's

principle).108, 271, 277, 281

8. Etherification

Glycerol can react with either alkyl alcohol (such as ethanol) or alkenes (such as isobutylene)

to produce branched ethers. These oxygenated compounds can be used as fuel additives and

---32---

solvents for special purposes.10, 264, 265 The etherification of glycerol with tert-butyl alcohol or

isobutylene yields a mixture of mono-, di- and tri-tert-butyl glycerol ethers (MTBG, DTBG and

TTBG, respectively) (Scheme 8). The DTBG and TTBG lead to a decrease of the emission of

particle matter, hydrocarbons, carbon monoxide and unregulated aldehydes emissions when

they are incorporated into the standard or bio-diesel.20, 282, 283 Besides forming alkyl glycerol

ethers, glycerol can also form oligomers and polyglycerols by inter-molecular etherification.

Further esterification or transesterification of polyglycerols will lead to polyglycerol esters.

Oligo-/poly-glycerol and polyglycerol esters can be used as biodegradable surfactants,

cosmetics, food additives, drug delivery carrier et, al.20, 284-288 Polyglycerols are produced from

different resources now, such as epichlorohydrin, but the direct synthesis from glycerol

etherification is more attractive from an environmental point of view.284

Scheme 8. Etherification of glycerol with tert-butyl alcohol

Strong homogenous acids, e.g. p-toluene sulphonic acid, were applied as catalysts in this

reaction, but the heterogeneous acidic catalysts are more attractive as a result of sustainable

concerns. Several kinds of heterogeneous Brønsted acids, including strong acidic ion-exchange

resins, supported heteropoly acids and acid functionalised porous silicates (e.g. SBA-15), were

studied for the catalytic synthesis of tert-butyl glycerol ethers.289-295 Tert-butyl alcohol was

selected to react with glycerol for the synthesis of tert-butyl glycerol ethers. Glycerol ethers can

be obtained also by etherification of glycerol with isobutylene. Compared to etherification with

alcohols, reaction with isobutylene is more active and also leads to higher yield of DTBG and

TTBG.296 For both reactions, the acid strength and the accessibility of the strong acid sites of the

catalyst play key role in the catalytic performance. These porous silicates functionalised with

sulphuric or phosphoric acid groups were found to display good activity and selectivity towards

di- and tri- ethers in the synthesis of tert-butyl glycerol ethers.293, 294, 297-300 The larger pores of

SBA-15 (7.6nm) compared to those of MCM-41 (3.8) are more favourable for the formation and

Chapter 1

---33---

diffusion of large molecules as DTBG and TTBG. As a consequence, SBA-15 functionalized with

phosphoric acid groups displayed higher selectivity towards di- and tri- ethers.300 Besides the

properties of catalyst, the reaction conditions also have strong influence on the products

distribution. Since the DTBG and TTBG are more valuable chemicals than MTBG, it is necessary

to use excessive isobutylene/tert-butyl alcohol to obtain more DTBG and TTBG and maximize

the conversion of glycerol.297 It was found that the yield of DTBG and TTBG could reach 92% at

full conversion of glycerol at the molar ratio 4 (isobutylene/glycerol) over catalyst

arenesulphonic-acid-functionalized mesostructured silica (Ar-SBA-15) at 75 oC for 4h.297 In the

cases of etherification of glycerol with tert-butyl alcohol, water is a side-product. On the one

hand, the presence of water can inhibit the formation of isobutylene oligomers; on the other

hand, removing water from the catalytic system can shift the equilibrium concentration

towards the desired DTBG and TTBG products.292, 298

The oligomerization of glycerol can yield di-, tri-glycerol and oligomers. Various kinds of solid

acids and bases were applied for this reaction. Two recent reviews have summarized these

studies in detail.284, 288 The acidic zeolites and super acidic resins show the ability to catalyse

the etherification reaction. Over the catalyst Nafion NM-112, the yield of linear diglycerol

reaches 85% at the glycerol conversion 90% at 160 oC and 2 mbar, which is an outstanding

result for this reaction.301 Basic heterogeneous catalysts, such as alkali ion modified zeolites

and alkaline-earth metal oxides, were also used for this reaction. For alkaline-earth metal

oxides, the deprotonation of glycerol by the basic catalyst was found to be a key factor in the

etherification reaction. In the case of CaO and SrO, the metal cations can behave as Lewis acids

and promote the adsorption of one hydroxyl group of glycerol, thus playing a synergistic role

with the basic sites.302, 303 It should be noted that, generally, the longer reaction time/higher

glycerol conversion would lead to consecutive etherification of the desired product linear

diglycerol to higher oligomers.284, 288

9. Esterification

The esterification of glycerol with acetic acid produces a mixture of mono-acetyl-glycerol

(MAG), di-acetyl-glycerol (DAG) and tri-acetyl-glycerol (TAG) (Scheme 9). MAG and DAG can be

used as monomers for the synthesis of bio-based polyesters and as additives in cosmetics and

medicines.10, 274, 304, 305 TAG is a good fuel additive that can improve the anti-knock properties

of gasoline and the anti-freeze properties of biodiesel.10, 304 Other than the esterification with

acetic acid, the esterification with fatty acid is a useful reaction if it selectively produces the

---34---

corresponding monoester, which has applications in the food, pharmaceutical and cosmetic

industries.306 Conventionally, the esterification reactions is catalysed by homogeneous strong

mineral acid, such as sulphuric acid and hydrochloric acid.10 Recently, various heterogeneous

acid catalysts were developed for this reaction in order to increase the greenness of the process.

Acidic ion-exchange resins, acid-functionalised carbon materials, supported heteropoly acids

(leaching was observed) acidic zeolites and acid-functionalised silica were studied as catalysts

for glycerol acetylation.307-315 It was suggested that the catalytic activity is correlated both to

the acid strength and acid amount of the catalytic materials.316 In order to obtain high glycerol

conversion, reaction rate and yield of DAG and TAG, an over-stoichiometric amount of acetic

acid (to shift the equilibrium concentrations towards the products), prolonged reaction time

and high reaction temperature are needed. The cumulative yield of DAG and TAG can reach 90%

at 97% glycerol conversion after 120 min reaction if an acetic acid/glycerol molar ratio of 9:1

is employed at a reaction temperature of 110 oC. 317

Scheme 9. Esterification of glycerol with acetic acid

10. Conclusive Remarks

10.1 Comparison of catalyst and reaction pathway in similar conditions

For many routes of converting glycerol to fine chemicals, such as oxidation, partial oxidation

with isomerisation and hydrogenolysis, (supported) metal catalysts play a key role in these

catalytic reactions. These catalysts have similar form: support and metal, whereas they give

quite different products depending on the specific components and reaction conditions.

Comparing these catalysts and reaction conditions may give us more insight of designing

catalytic system.

For the bifunctional supported metal catalysts (metallic and acidic sites) in hydrogenolysis or

partial oxidation and isomerization, the reaction atmosphere plays a key role in the reaction

catalysed by similar catalysts. Generally, under H2 atmosphere, the reaction gives

Chapter 1

---35---

hydrogenolysed products (such as 1,2-PD, 1,3-PD and PrOH); under O2 atmosphere, lactic acid

or alkyl lactate becomes the main product. Under H2 atmosphere, the acidic support can first

catalyse the dehydration of glycerol to form acetol (Entry 1, Table 5) or 1,3-hydroxypropanal

(Entry 8, Table 5) and the following hydrogenation by metallic sites gives 1,2-PD or 1,3-PD,

respectively (Scheme 5A and Scheme 6A). While under O2 atmosphere, the metal catalyst firstly

accelerates the partial oxidation of glycerol and then the solid Lewis acid transform the

intermediates (DHA and/or GLAD) to lactic acid or alkyl lactate. It should be noted that, under

N2 atmosphere, either hydrogenolysed products or lactic acid could be the main product that

largely depends on reaction conditions and the catalysts. In the acidic conditions (Entry 7, Table

5), it formed hydrogenolysed products. The solid acid accelerates the dehydration step to form

acetol as the intermediate to 1,2-PD. The noble metal catalyst can accelerate both

dehydrogenation of glycerol to release H2 and then hydrogenation of intermediates. In the basic

conditions, the intermediates from the dehydrogenation of glycerol, such as DHA and GLAD,

could be transformed to lactic acid (salt) by the base instead of being hydrogenated.

For the solid acids in dehydration reactions, such as dehydration and etherification,

temperature is the key parameter that affects the selectivity of products. In the high

temperature (around 300 oC), the reaction is conducting in gas phase on a fix-bed reactor

(section 4) that gives acrolein and/or acetol as the main products. While in the low temperature

(around 90 oC), the reaction in liquid phase inclines to give di-, tri-glycerol and oligomers as the

products. It should be noted that the inter-molecular dehydration (between glycerol and

glycerol/other molecule) could conduct at low temperature (below 100 oC) while the intra-

molecular dehydration needs much higher temperature to promote the reaction.

10.2 Conclusions and perspectives

Glycerol is one of the most promising biomass-derived feedstock, and its conversion into useful

chemical products over heterogeneous catalysts is an important research topic that received

increasing attention in recent years. In this perspective article, the several possible reaction

pathways for the conversion of glycerol into valuable chemicals were systematically discussed

from the point of view of catalyst design and catalytic process optimisation.

Since the glycerol molecule contains two primary and one secondary hydroxyl groups, one of

the main challenges in the conversion of glycerol to useful products is to design catalysts that

can selectively activate and convert the chosen hydroxyl group(s) to the desired functional

group(s). The selectivity in activating the central versus the terminal hydroxyl group can

---36---

determine the selectivity towards a specific product (e.g., acrolein versus acetol; 1,2-PD versus

1,3-PD). Two main types of active sites (solid acid and metal) were generally involved in the

conversion of glycerol. To achieve high activity and selectivity towards the chosen target

product, it is important to tune the type/strength/amount of active sites in the case of solid acid

catalysts, and to choose a suitable element/particle size/alloy component/support for metal

catalysts. For example, based on the mechanistic study in glycerol dehydration (see section 4),

the first dehydration step can largely determine the selectivity of products. The elimination of

the central hydroxyl group would preferably lead to the formation of acrolein. Besides the

control of these catalytic features, the reaction conditions (e.g. temperature, pressure,

atmosphere) can strongly affect the yield and selectivity of the desired products. The combined

fine tuning of the composition of catalyst and reaction conditions can be used to depress side

reactions and optimise the yield of the target product. In the alloyed metal catalysts,

incorporating another metal component can largely enhance the activity/selectivity of the

target reaction as well as the stability of catalyst. In partial oxidation of glycerol, further

oxidation could lead to the formation of undesired products, such as CO2 and C-C cleavage

products. By incorporating Bi into AuPd/AC, the relatively unstable intermediate compound TA

could be obtained in the glycerol oxidation (Entry 11, Table 1). More than the designing of

catalyst, optimisation of reaction conditions is also important to achieve higher yield of target

product. For instance, different temperature (in the top and bottom section of catalyst column)

was used for the synthesis of 1,2-PD, which aimed to apply optimised temperature for the

dehydration (high) and hydrogenation (low) steps for enhanced catalytic performance (Entry

2, Table 5).

For many routes for the conversion of glycerol, the reaction pathway and mechanism are still

unclear or under debate. Heterogeneous catalysts generally contain different types of active

sites and the reaction mechanism on their surface is typically complex. Therefore, more

detailed mechanistic and characterisation studies are needed to define the relation between

catalytic activity/selectivity and physicochemical features of the catalytic materials for the

various pathways of glycerol conversion.318, 319 Such understanding, would also be beneficial to

the further development of new catalyst. The elucidation of the reaction pathway is of

particular importance when the conversion of glycerol involves a multi-step reaction (e.g. in

the synthesis of lactic acid, acrolein and propanediol). In such cases, the design of the catalyst

requires identifying which type of site is needed to catalyse each step of the reaction, leading to

a multifunctional system containing two or more types of catalytic sites. Not only the nature

Chapter 1

---37---

but also the relative amount of the active sites (e.g. a metal and an acid site) should be carefully

balanced to achieve an active and selective multifunctional heterogeneous catalyst for one-pot

processes. The excess of one type of active site could cause sub-optimal exploitation of the other

catalytic sites and lead to over-reaction of the desired products. For example, the conversion of

glycerol to lactic acid/alkyl lactate needs partial oxidation of glycerol to a triose and consecutive

isomerisation. The oxidation is catalysed by metal sites, whereas the isomerisation is promoted

by Lewis acid sites. In designing a catalyst for this reaction, an excess of metal sites should be

avoided to prevent undesired further oxidation of the triose intermediates. In addition, the

compatibility between the different types of active sites, both during the preparation of the

catalyst and during its operation, should be taken into consideration when designing

multifunctional catalysts. Finally, the textural and surface properties of catalyst should be taken

into account when designing heterogeneous catalysts for the conversion of glycerol.

Particularly, the surface area, pore size and organisation, the hydrophilicity /hydrophobicity

can influence deeply the catalytic behaviour. Generally, proper pore size (i.e. larger than the

diameter of the transition states) offers larger accessible surface area and better diffusion

properties. As a consequence, zeolites with mesopores showed better properties of anti-coke

and anti-deactivation than the one without mesopores.153, 154 The hydrophobicity of the catalyst

surface can also play an important role by promoting water removal from the active sites, which

can be beneficial in decreasing the undesired C-C bond cleavage reaction in glycerol oxidation

(see section 2.2)70 or for increasing the solketal yield in the ketalisation of glycerol (see section

7).271, 281

11. Aim and scope of this thesis

After the review of all the possible routes of converting glycerol into value-added fine chemicals,

methyl lactate and lactic acid were selected as the target products since they are two of the

most important platform molecules that can be obtained from glycerol. The purpose of this

work was to design and develop multifunctional catalytic systems that would enable the

conversion of glycerol into methyl lactate or lactic acid, thus providing a viable alternative to

the current production based on low-efficiency fermentation processes. The catalytic systems

presented in this thesis displayed improved performance compared to the state of the art.

Moreover, the relation between catalytic results and the structure of catalyst was well

established.

---38---

Two main kinds of reactions were studied in this thesis. The first part of this thesis (Chapters 2

and 3) focuses on the base-free synthesis of methyl lactate from glycerol by multi-functional

catalytic system based on supported noble metal nanoparticles and extra-small Sn-MCM-41.

The second part (Chapters 4 and 5) aims at the synthesis of lactic acid (salt) and hydrogenated

chemicals over supported Pt or Ni-Co catalysts through a transfer hydrogenation and

isomerisation route. Finally, some conclusive remarks and perspectives for future work are

provided in Chapter 6.

Here below, a more detailed outline of the four experimental chapters is provided:

Chapter 2

The purpose of this chapter was to develop a catalytic system providing two types of suitable

active sites for this reaction: (1) metallic sites for partial oxidation of glycerol; (2) acid sites for

the consecutive rearrangement of DHA into methyl lactate. Sn-MCM-41-XS showed excellent

performance in rearranging DHA into methyl lactate. However, it is not a good support for Au

nanoparticles, which are employed to partially oxidise glycerol to DHA/GLAD. Therefore,

various metal oxides were selected as the supports for Au nanoparticles and were used in

combination with Sn-MCM-41-XS (as a physical mixture) to catalyse the synthesis of methyl

lactate. The catalyst preparation, characterisation, catalytic tests and recycling were studied in

this chapter.

Chapter 3

With the knowledge obtained from Chapter 2 (the catalytic system works well with physically

mixed metal catalyst and Sn-MCM-41-XS), several modifications were adopted to improve the

noble metal catalyst: (1) Pd was incorporated into the Au catalyst formulation to improve the

activity; (2) carbon nanotubes (CNTs) were selected as the support to achieve better selectivity;

the CNTs were functionalised by treatment with HNO3-H2SO4 to further enhance the activity.

With these enhancements, the obtained yield of methyl lactate from glycerol is the highest in

the state of the art. Detailed characterisation allowed the understanding of the relation between

improvements in catalytic performance and the modified structure of the catalyst.

Chapter 4

With the aim of utilising the hydrogen made available by the dehydrogenation of glycerol, a

novel catalytic route based on transfer hydrogenation between glycerol and cyclohexene that

Chapter 1

---39---

gives lactic acid (salt) and cyclohexane is established in this chapter. To achieve this target,

Pt/ZrO2 catalysts with extra-fine Pt nanoparticles was prepared by controlling the calcination

and reduction procedures as a novel atomic trapping method. The optimum catalyst allowed to

obtain unsurpassed 96% yield of lactic acid from glycerol under relatively milder reaction

conditions compared to those reported in literature for related systems. The catalysts were

thoroughly characterised to understand the properties of Pt species on ZrO2 support, which

were shown to be connected to the catalytic performance.

Chapter 5

Bimetallic Ni-Co/CeO2 catalysts were developed for the transfer hydrogenation between

glycerol and several hydrogen acceptors, achieving similar catalytic performance but with

much lower cost compared to the Pt catalysts presented in Chapter 4. Though Ni has

intrinsically better catalytic activity, characterisation with a combination of techniques proved

that incorporating Co into the Ni catalysts formulation is critical to obtain well dispersed Ni-Co

catalysts at high metal loading. As a consequence, the bimetallic Ni-Co catalyst displayed

enhanced activity compared to their monometallic counterparts. Several different compounds

were also used as hydrogen acceptors in this study, leading to the successful hydrogenation

into valuable products and showing the broad applicability of this catalytic route.

Chapter 6

The concluding Chapter provides a summary of the main achievements of this PhD project on

the topic of catalytic conversion of glycerol into methyl lactate and lactic acid with different

types of catalysts.

---40---

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