University of Groningen
Multifunctional catalytic systems for the conversion of glycerol to lactates Tang, Zhenchen
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Chapter 1
Achievements and challenges in the design of heterogeneous catalysts
for the conversion of glycerol to valuable products
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).
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
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
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)
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.
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
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
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 (oT C) 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 O 2 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
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
(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
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, 91The
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
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
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. (oT C) 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/TiO+ AlCl 2 3 - 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
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
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
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
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 = 4/54 18000 O2/He 2 20 >99 21 n.g. 27 137
8 W-Zr-O + W-V-Mo-Oc 305 O2/Ar
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
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
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,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
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
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 H
Cu-4SiW12O40/Al2O3 n.a. 240 6
LHSV
0.9 h-1 10 90 90 n.g. 2 7 185
6 Amberlyst Ru/C + ~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
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
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
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
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) p H2 (MPa) time (h) GLY conc. (wt %)c 1,2-PD 1,3-PD PrOH others 1 Li
Pt-2H2SiW12O40/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 ZrO2e 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
