19.3.1 Acid hydrolysis

Cellulose is composed of microcrystalline fibres, hydrogen bonded to each other by a charged water boundary layer formed from dipole–dipole interactions between aligned water molecules and the polar surface of the cellulose fibres.³⁵ To initiate acid hydrolysis, protons must penetrate this charged water boundary layer, and thereby catalyse hydrolysis of the β-glycosidic linkages to release glucose or cellobiose (glucose dimers) from the outermost layer of the cellulose crystallites. However, the initial hydrolysis products tend to remain in close proximity to the cellulose surface due to hydrogen bonding interactions, resulting in a viscous boundary layer that slows further hydrolysis of the underlying cellulose.³⁶ Efficient mixing is thus essential to displace reactively formed glucose from the surface, while the addition of Li⁺, Na⁺, K⁺, Ca^2 + and NH₄⁺ salts has also been reported to help disrupt the crystalline water matrix. While elevated reaction temperatures (via hot compressed water) can accelerate hydrolysis, they may also promote undesired side reactions if products are not continuously removed, with glucose readily undergoing subsequent reaction to by-products such as furfural and 5-HMF.³⁷ Without continuous removal of hydrolysis products from the ‘boundary layer’at the surface of cellulose particles, conventional acid hydrolysis routes can only achieve glucose yields of around 70%, due to glucose degradation under these forcing reaction conditions.³⁶ Furthermore, sulfuric acid-initiated cellulose hydrolysis at acid concentrations ranging from 0.4 to 2 wt%^38–40 poses additional problems due to the requirement for expensive corrosion-resistant reactors and co-production of vast quantities of gypsum waste formed during acid neutralisation via lime addition. There is thus an urgent need for low energy technologies for cellulose conversion to sugars and platform chemicals.

Catalytic aqueous-phase reforming (APR) of cellulose is one such approach which allows the direct conversion of carbohydrates into hydrogen and alkanes (C₁ to C₁₅) and could form a platform for fuels production within an integrated biorefinery.⁴¹ APR typically involves the reaction of cellulose under acid conditions at ∼ 200°C to yield C₁ to C₆ alkanes by aqueous-phase dehydration/hydrogenation (APD/H) of sugars, or C₇ to C₁₅ alkanes by combining aldol condensation with dehydration/hydrogenation (Fig. 19.4).

19.3.2 Heterogeneous catalysts for cellulose conversion to platform chemicals

Cellulose depolymerisation to sugars through acid hydrolysis is the first step in a biorefinery,⁴² hence there is much interest in developing heterogeneous catalysts to replace the conventional mineral acids (e.g., H₂SO₄) currently employed to achieve this.^43,44 Solid acids explored to date include sulfonic acid mesoporous silicas,⁴⁵ sulfonated porous carbons^46–48 and carbon-silicas,⁴⁹ as well as sulfonic acid polystyrene resins.⁵⁰ Sulfonated mesoporous carbons⁴⁸ are reported to produce remarkably high glucose yields (74.5%) following the 150°C hydrolysis of amorphous cellulose obtained from pretreating cellulose in a planetary ball mill at 500 rpm for 48 h. For cellulose hydrolysis by solid acids, efficient solid–solid interfacial contact is critical,⁵¹ with the amount of water playing a key role in controlling the reaction kinetics over carbon-based catalysts. Glucose yields were optimal when the quantity of water was comparable to the weight of solid carbon catalyst; high water content is suggested to hydrate acid sites, decreasing the catalyst Brönsted acidity.

The use of magnetic sulfonated materials affords a novel and facile means to separate catalysts from aqueous media containing water-soluble sugars and solid cellulose. For example, magnetic silica nanoparticulate catalysts comprising a CoFe₂O₄ core with a sulfonic acid derivatised SiO₂ shell⁵² are active for cellulose hydrolysis, while offering facile separation from the reaction media by a magnet, improving their reusability.

Cellulose is only soluble in water at temperatures in excess of > 320°C. However, such temperatures, required to interrupt the hydrogen-bonding between the fibres,⁵³ result in low glucose yields due to subsequent reactions under these aggressive conditions. The hydrolytic hydrogenation of cellulose to sorbitol and sorbitan by mineral acids and supported Ru catalysts under H₂ pressures of 70 bar offer improved sugar yields by hydrogenating glucose as it is formed to sugar alcohols, which have higher chemical stability (Fig. 19.5).⁵⁴ Although promising, the use of mineral acids is undesirable, and the developments of processes employing only solid catalysts are preferred from safety, product recovery and catalyst recycle perspectives.⁵⁵ Ru/CMK-3⁵⁶ and Ru/ZSM-5⁵⁷ have been reported to catalyse cellulose hydrolysis via hydrolytic hydrogenation, with the active Ru species on CMK-3 proposed as a hydrated Ru oxide (RuO_2.2H₂O) postulated to act as a Lewis or Brönsted acid depending on the degree of hydration.

Supported metal catalysts such as Pt/γ-Al₂O₃, Pt/carbon and Ru/carbon are also reported to convert cellulose into sugar alcohols (sorbitol and mannitol) in the presence of hydrogen.⁵⁵ High selectivity to glucose was achieved by choosing appropriate reaction conditions, which include rapid heating (from 25°C to 230°C in 15 min) to hydrolyse cellulose followed by cooling to inhibit glucose degradation.

19.3.3 Cellulose conversion in ionic liquids

Ionic liquids (IL) have shown huge promise for applications in biomass fractionation, and attracted significant academic and industrial interest.⁵⁸ Low temperature ILs are low melting point salts, which form liquids comprising only cations and anions, having low vapour pressures and exceptional solvating properties for diverse compounds. The miscibility of ionic liquids with water or organic solvents can be controlled by varying the side chain lengths on the cation, and by careful anion choice. Some common cation and anion combinations used as ionic liquids are shown in Fig. 19.6. Organic groups on ILs can also be functionalised to impart acid or base character. While ILs are often described as ‘greener’ than conventional solvents, they are not all benign. The toxicity of ILs is mainly ascribed to the alkyl chain, with the toxicity of imidazolium and pyridinium ILs increasing with cation chain length.⁵⁹ Furthermore, bmim (BF₄, PF₆, NTf₂ and N(CN)₂) ILs exhibit poor biodegradability (less than 5% after 28 days), hence large-scale use of such solvents in fuel production would require careful regulation.

As outlined above, the β-glycosidic linkages in cellulose are protected against hydrolysis by tight packing of the cellulose chains into microfibrils, hence cellulose hydrolysis requires severe conditions, such as the use of dilute sulfuric acid at high temperatures. However, cellulose⁵⁸ and wood^60,61 dissolve in alkylmethylimidazolium ILs leaving the cellulose chains accessible to chemical transformations.⁶² ILs have been utilised as reaction media for the synthesis of cellulose derivatives such as carboxymethyl cellulose and cellulose acetate, while the regeneration of cellulose from ionic liquid solutions has been employed in the fabrication of films, gels and composite materials. Even though facile cellulose dissolution in IL solvents represents a significant breakthrough, the use of ILs for cellulose hydrolysis or degradation has only recently been explored.⁶³

Early reports⁵⁸ demonstrated that room temperature ILs, such as 1-n-butyl-3-methylimidazolium (C₄mim⁺) salts with Cl⁻, Br⁻, and SCN⁻ anions, are capable of dissolving cellulose. High molecular weight pulp cellulose (5–10 g -DP 1000) slowly dissolved in ∼ 100 g ionic liquid when heated to 100°C, yielding viscous solutions. Subsequent studies have attempted to improve upon the initial discovery of this solubility. Interactions between the carbohydrate and anion of ILs appear the dominant factor in controlling cellulose dissolution, with Cl⁻ reported to have stronger hydrogen-bonding basicity than Br⁻, SCN⁻, PF₆⁻ and BF₄⁻. However, the chemical structure of cations also affects carbohydrate dissolution, with cellulose solubility decreasing with increasing alkyl chain length in the imidazolium cation, e.g. use of short alkyl chains in the AMIM cation enhances solubility relative to BMIM. The use of oxygenated side chains can also enhance carbohydrate solubility via hydrogen bonding between the oxygen and the carbohydrate. High throughput screening⁶⁴ indicates that EMIM-Ac was the most efficient IL for dissolving cellulose, while AMIM-Cl (1-allyl-3methyl-imidazolium chloride) was the most effective for dissolving wood chips. A proposed mechanism by which ionic liquids break up the hydrogen bonding network in cellulose is shown in Fig. 19.7.⁶⁵ Separation of the resulting sugar from the IL was achieved using ion exclusion chromatography, enabling over 95% recovery of the ionic liquid and 94% recovery of glucose. A commercial process for industrial-scale cellulose conversion to glucose would, however, require alternative separation technologies.

In a recent study by Li et al.,⁶⁶ cellulose hydrolysis was initiated by adding catalytic amounts of H₂SO₄ to a cellulose–C₄min⁺Cl⁻ solution, with a H₂SO₄:cellulose mass ratio of 0.92 producing total reducing sugars (TRS) and glucose in 59% and 36% yields, respectively, within 3 min. Lowering the acid:cellulose mass ratio to 0.46 produced higher yields after 42 min reaction, and for a mass ratio of only 0.11, TRS and glucose yields of 77% and 43%, respectively, were achieved after 9 h. These mild operating conditions and the use of a catalytic amount of H₂SO₄ offer exciting prospects for cellulose conversion.

Incorporation of an acidic function into the ILs has also been investigated through the synthesis of Brönsted acid ILs for the simultaneous dissolution and hydrolysis of cellulose.⁶⁷ Such Brönsted acidic ILs can act as both solvent and catalyst, eliminating waste conventionally produced during mineral acid neutralisation and separation steps. Furthermore, the high concentration of –SO₃H active sites that can be introduced to ILs is expected to accelerate reactions and thus enable lower operating temperatures, facilitating cellulose dissolution and hydrolysis under moderate reaction temperatures and atmospheric pressure. Three types of Brönsted acidic ILs based on methyl imidazolium (1a,b), pyridinium (2), and triethanolammonium (3) have been studied for their ability to dissolve and hydrolyse cellulose at mild reaction temperatures (Fig. 19.8). Cellulose dissolution in Brönsted acidic ionic liquids such as 1-(1-propylsulfonic)-3-methylimidazolium chloride and 1-(1-butylsulfonic)-3-methylimidazolium chloride is achievable up to 20 g/100 g IL upon gentle room temperature mixing.

Solid acids have been reported to selectively catalyse the depolymerisation of cellulose solubilised in 1-butyl-3-methylimidazolium chloride (BMIMCl) at 100°C. Acid strength plays a key role in cellulose depolymerisation,⁵⁰ wherein the resin-based solid acid Amberlyst gave impressive results for cellulose hydrolysis, despite diffusional limitations and poor accessibility of the attendant acid sites. Reactions using solid catalysts preferentially cleave longer cellulose chains to produce oligomers consisting of approximately ten anhydroglucose units (AGU). However, care must be taken with the use of some resin-based catalysts, as these may degrade under reaction conditions leading to homogeneous catalysis.⁶⁸ For example, Amberlyst-15 is reported to be very stable in BMIMCl, but a simple change to BMIM(CH₃COO), a potentially more desirable solvent because of its lower viscosity, resulted in rapid Amberlyst-15 degradation. The poor activity of traditional inorganic solid acids (e.g., Zeolite-Y and ZSM-5) has been attributed to their microporous nature and consequent poor accessibility to bulky cellulose fibres.

Cellulose has been directly converted into environmentally friendly alkyl glycoside surfactants in a one-pot transformation.Utilising BMIMCl in conjunction with an Amberlyst 15Dry (A15) catalyst, and coupling the rates of cellulose hydrolysis and glycosidation of the resultant monosaccharides with C₄-C₈ alcohols, an 82% mass yield of octyl-α-β-glucoside and octylα-β-xyloside was obtained.⁶⁹ The IL alone was unable to effect any cellulose conversion, possibly reflecting the low reaction temperature employed, since ILs are not known to hydrolyse cellulose below 90°C.⁷⁰

Solid acid-promoted hydrolysis of cellulose in ionic liquids can be accelerated by microwave heating, affording similar yields of TRS and glucose to those obtained using liquid mineral acids. Protonated zeolites with a low Si/Al molar ratio and large surface area showed the highest catalytic activity, outperforming the acidic sulfated ion-exchange resin NKC-9.⁷¹ Despite these initial successes, significant breakthroughs in catalyst and process design are essential in order to improve the overall efficiency of glucose production for subsequent bio refining.

19.4.1 Biochemical conversion of carbohydrates

The use of enzymes in biochemical routes allows for selective conversion of sugars via fermentation, but faces several limitations that hinder economic biomass processing. Notably, conventional enzyme catalysed processes are unable to process pentoses, which must therefore be removed from biomass sugar feeds, requiring extra pretreatment steps in order to purify the glucose feedstock. Furthermore, the platform molecules derived from fermentation are often present at low concentrations (typically < 10%) in aqueous solutions, alongside other polar molecules. Purification of such fermentation broths is particularly difficult,^79,80 and energetically unfavourable, hence an ability to directly transform these aqueous solutions would be desirable.⁸¹ Catalysts capable of driving organic chemistry in water to selectively transform platform molecules into useful chemical feedstocks,^5,9,74,82,83 and are resistant to impurities present in the fermentation broth,⁸⁴ therefore require development.

The utilisation of biomass-derived chemicals, whether from fermentation broths or sugars themselves, represents an area with extensive R&D potential for a renewable feedstock-based technology platform. Approaches to handling biomass-derived building blocks will be very different from petroleum processing, e.g. requiring reverse chemical transformations wherein highly functional bio-molecules are deoxygenated to their target product, instead of oxygenated as is usual when starting from crude oil.⁸⁵ Figure 19.10 illustrates a possible biomass synthesis of adipic acid (currently manufactured via the selective oxidation of cyclohexane) involving the selective reduction of glucose. New classes of catalyst are urgently required which are compatible with hydrophilic, bulky substrates to facilitate the move away from existing short-chain hydrocarbon supplies. Improvements and innovations in catalyst and processes design are needed in order to deliver high-value chemicals from biomass-derived building blocks.

These new catalysts will be hydrophilic, stable over a wide pH range and resistant to in-situ leaching.⁸⁶ Catalyst porosity will also be important to enable diffusion of bulky, viscous reactants to active sites; support materials with larger pores (compared to zeolites) are thus a promising starting point. Organic-inorganic hybrid catalysts may also prove interesting, as these allow catalyst hydrophobicity to be readily tuned, and in turn the adsorption strengths of polar molecules.⁸⁷ Mesoporous carbons⁸⁸ are well-suited for biomass conversion since they tend to be highly resistant to acidic and chelating media. Transforming the functional groups on platform chemicals will require catalysts capable of dehydration, hydrogenolysis and hydrogenation chemistry. Suitable catalysts will thus contain acid sites, or exhibit bifunctional character, possessing acid sites for dehydration and metal sites for (de)hydrogenation. Corma et al. have extensively reviewed proposed methods for transforming platform molecules into chemicals,⁹ many of which employ conventional homogeneous reagents or commercial catalysts. There is thus enormous scope for rationally designing improved heterogeneously catalysed processes tailored towards biomass-derived feedstocks.

A number of reports have described pathways to important chemical intermediates from platform molecules.^9,82,83,89 Succinic acid is proposed as a valuable platform chemical, from which a range of chemical intermediates can be derived, as illustrated in Fig. 19.11 for acid catalysed esterification, or metal catalysed reductions. Carbon-based solid acid catalysts have proven effective for succinic acid esterification with ethanol.^90,91 Succinic acid may also afford new biopolymers based on polyesters, polyamides and polyesteramides.⁹²

The purity of crude biorefinery feeds presents a major challenge for catalyst development, with the fermentation broth typically produced as a salty medium containing diammonium succinate rather than pure succinic acid. However, this could be exploited in the production of 2-pyrrolidone or N-methyl-pyrrolidone via hydrogenation of diammonium succinate ⁹³ using catalysts such as Pd/ZrO₂/C and 2.5% Rh-2.5%Re on C, with reaction in the presence of methanol favouring N-methyl-pyrrolidone formation.

19.4.2 Thermochemical conversion of carbohydrates

Carbohydrate conversion by thermochemical routes such as fast pyrolysis, aqueous phase reforming or gasification offers alternative building blocks for chemicals or fuels production. Thermal decomposition of biomass in the absence of oxygen by pyrolysis yields a range of feedstocks depending on the reaction temperature and residence time,⁷⁷ with low temperatures and long residence times favouring charcoal formation, while moderate temperatures and short vapour residence time are optimal for liquids production. High temperatures and long residence times favour gasification,⁹⁴ with the resulting syngas available for well-established catalytic processes such as Fischer–Tropsch and methanol synthesis routes to convert CO/H₂ mixes to fuels and methanol. While complete gasification may be energetically costly, less initial biomass processing is required, so with efficient heat recovery modules, this approach is attractive for its compatibility with existing industrial processes. Low temperature (< 200°C), thermochemical aqueous phase processing of sugars is of particular interest, as this offers a viable method to generate highly functional intermediates such as aldehydes, alcohols and esters via dehydration, hydrogenolysis and isomerisation (Fig. 19.12).^55,74,95

Solid acids or bases are commonly employed in aqueous phase processing, although the application of bi-functional metal-doped catalysts is attracting interest for combined dehydration/hydrogenation of sugar intermediates (HDO). Some examples of key catalytic transformations of platform molecules will now be described.

19.4.3 Conversion of C₆ sugars to 5-HMF

5-Hydroxymethylfurfural (HMF) is a popular platform molecule with huge potential as an important bio-based commodity chemical^96,97 for the synthesis of various commercially useful acids, aldehydes, alcohols, and amines, as well as the promising fuel 2,5-dimethylfuran (DMF)⁹⁸ and renewable monomer furan dicarboxylic acid (FDCA)⁹⁹ as shown in Fig 19.13. HMF also has potential as a building block for the manufacture of commodity chemicals such as caprolactam, the precursor to Nylon 6,6,¹⁰⁰ following hydrogenation and hydrogenolysis to 1,6-hexanediol.

While dehydration of C₆ sugars to 5-hydroxymethylfurfural (5-HMF) can readily be achieved by acid-catalysed dehydration of three water molecules,¹⁰¹ glucose conversion by Brönsted acids often proceeds with low selectivity to 5-HMF due to competing side reactions which form humins.^102,103 In contrast, fructose conversion proceeds with higher selectivity to 5-HMF, hence bifunctional catalysts capable of isomerising glucose to fructose prior to a subsequent acid-catalysed dehydration would be desirable. For example, under high temperature conditions of hot compressed water (200°C), ZrO₂ can promote glucose and fructose isomerisation via a base-catalysed route. Anatase TiO₂, which possesses both acid and base character, promotes glucose isomerisation and dehydration into HMF (Fig. 19.14).¹⁰⁴ For more selective chemistry, tailored solid catalysts capable of working at lower temperatures are sought.

Initial reports of sucrose dehydration over solid acids, including acidic resins,¹⁰⁵ HY-zeolite,¹⁰⁶ aluminium-pillared montmorillonite, MCM-20 and MCM-41,¹⁰⁷ have sparked significant interest in 5-HMF production. Fructose dehydration to 5-HMF can be initiated by Brönsted acids in polar solvents and a range of aprotic polar solvents, such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and sulfolane. A variety of solid acids, including ion-exchange resins,¹⁰⁸ zeolites^109,110 metal oxides,¹¹¹ heteropoly acids,¹¹² niobic acid,¹¹³ niobium phosphate,¹¹⁴ sulfated zirconia,¹¹⁵ and sulfonic acid-functionalised mesoporous silicas¹¹⁶ have been explored for 5-HMF production from fructose. Detailed studies of WO_x/ZrO₂ catalysts reveal¹¹¹ the importance of catalyst amphoteric character in achieving high selectivity to HMF during fructose dehydration. While the overall fructose conversion correlates with acid site density, optimum selectivity towards 5-HMF of ∼ 40% was obtained for catalysts with a base:acid site ratio of 0.3. Solid acid-catalysed conversion of fructose to 5-HMF has also been explored in ILs.^117–121

As discussed above, the direct conversion of glucose to 5-HMF is difficult, requiring isomerisation to fructose, either via proton transfer or an intramolecular hydride shift, respectively base or Lewis acid catalysed. Base-catalysed glucose to fructose isomerisation occurs via deprotonation of the α-carbonyl carbon of glucose to form a series of enolate intermediates, and can be conducted by cation-exchanged zeolites or Mg-Al hydrotalcites.^122,123 High 5-HMF yields are obtained upon subsequent acid-catalysed dehydration of the resulting fructose.

The Lewis-Brönsted acid ratio also plays an important role in directing selectivity during glucose dehydration.¹²⁴ Davis and coworkers showed that tin-containing zeolites are highly active catalysts for glucose isomerisation in water, wherein Sn behaves as a Lewis acid catalysing an intramolecular hydride shift.^125,126 In order to minimise side reactions, biphasic systems, employing Lewis and Brönsted catalysts in conjunction with reactive extraction, have been proposed to improve 5-HMF yields (Fig. 19.15).^97,127 The combination of Lewis and Brönsted acidity is beneficial for 5-HMF formation from glucose, with Sn-β (a Lewis acid) and HCl (a homogeneous Brönsted acid) offering good 5-HMF yield from glucose in a water-tetrahydrofuransolvent mix.¹²⁸ A similar approach was adopted using homogeneous Lewis acid metal chlorides (e.g., AlCl₃) and HCl in a biphasic reactor comprising water and 2-sec butylphenol.¹²⁹ In this respect, amorphous niobium oxide hydrate (Nb₂O₅ · nH₂O; niobic acid) is an interesting candidate for HMF production from glucose in water, as it possesses water-tolerant Lewis acid sites co-existing alongside Brönsted acid sites.¹³⁰

5-HMF yields are also enhanced over mesoporous catalysts with pore diameters of 1–3 nm. Porosity was observed to have a significant effect on 5-HMF and levulinic acid yields obtained from sucrose dehydration over acid exchange resins.¹⁰⁵ Larger pores favour 5-HMF production, with slow 5-HMF diffusion out of smaller pores appearing to promote subsequent reaction and higher selectivity to levulinic acid.

19.4.4 Conversion of C₅ sugars to furfural

Xylose, derived from hemicelluloses, can be readily dehydrated to furfural using Brönsted acids at high temperature. A variety of Brönsted acid catalysts have been examined for furfural synthesis including heteropoly acids^131,132 and mesoporous sulfonic acid silicas,^133,134 H-type zeolites such as H-mordenite and H-Y faujasite,¹⁰⁹ ion-exchange resins,¹³⁵ niobium silicate,¹³⁶ SO₄/ZrO₂¹³⁴ and SO₄/SnO₂.¹³⁸

Oxidative ring opening of furfural using H₂O₂ in the presence of a solid acid offers an opportunity to produce succinic acid along with maleic acid as a by-product (Fig. 19.16).¹³⁹ The yield of succinic acid was optimal using Amberlyst-15 when compared to other resin-based solid acids such as Nafion NR50 and Nafion SAC13, or other inorganic solid acid catalysts such as Nb₂O₅, H-ZSM-5 and ZrO₂.

The catalytic aerobic oxidation of furfural to maleic acid has also been explored using phosphomolybdic acid in biphasic aqueous/organic systems (Fig. 19.17), in which oxidation occurs in the aqueous phase, while the organic phase serves as the reservoir for furfural. Using this approach, maleic acid was obtained with 69% selectivity at a furfural conversion of 50%. Product separation was aided by the fact that furfural and maleic acid predominantly phase separate.¹⁴⁰

19.4.5 Levulinic acid production

Levulinic acid is another valuable precursor to a range of chemical intermediates (Fig. 19.18), which can be used in chiral reagents, inks, coatings and batteries, and can be generated by a combination of acid-catalysed dehydration/esterification or metal-catalysed reduction.¹⁴¹ While a number of studies have investigated reductions, there is surprisingly little work concerning the esterification of platform molecules using solid acid catalysts. To date, levulinic acid esterification is mostly performed using H₂SO₄,¹⁴² with only a few studies employing solid acids such as sulfated TiO₂ and SnO₂,^143,144or heteropoly acids.^145,146

Amberlyst-70 has also been reported to yield 21% of levulinic acid following dehydration of the water-soluble organics obtained via hydrothermal cellulose decomposition at 160°C.¹⁴⁷ When converting saccharides to levulinic acid, resin pore size was also demonstrated to have a significant effect on product selectivity.¹⁰⁵ There is clearly scope for the development of new catalytic systems for the conversion of biorefinery feedstocks to chemicals. However, efforts must focus on water-tolerant catalysts for the direct reaction of aqueous feeds, and the development of tailored porous-solids capable of operating under continuous flow.

19.4.6 Lactic acid synthesis

Sugar fermentation to lactic acid has received much attention in the context of polylactic acid (PLA) synthesis, considered the ‘gold standard’ for renewable polymers and applications including solvents and coatings.¹⁴⁸ Fermentation routes to LA proceed through homolactic or heterolactic fermentation of glucose, via glycolysis to pyruvate,¹⁴⁹ typically yielding > 90% lactic acid as a 100 g.l^− 1 aqueous solution of Ca-lactate. Subsequent H₂SO₄ recovery of lactic acid generates 1 kg CaSO₄ waste per kg lactic acid produced. Homolactic routes are less efficient, with ∼ 44% lactate yields reported¹⁵⁰ at concentrations up to 55–60 g.l^− 1.¹⁵¹ Extraction costs account for 50% of such process economics,¹⁵² due to the high energy consumption for separation and purification. Process improvements have been proposed employing ammonia to isolate an ammonium L-lactate product. ¹⁵³ However, for large-scale processing, a catalytic route would be a desirable alternative.

Lactic acid synthesis has also been proposed via the retro-aldol condensation of fructose or glucose to form glyceraldehyde (GLA) and dihydroxyacetone (DHA).¹⁵⁴ These require Lewis acid or solid base catalysts to achieve high selectivity, with Sn-β,^155,156 Sn-SiO₂,¹⁵⁷ Sn-SiO₂-carbon composite,¹⁵⁸ and Brönsted base hydrotalcite catalysts,¹⁵⁹ reported as potential candidates for lactic acid or lactate synthesis from sugars. Conversion of GLA and DHA is proposed to yield pyruvaldehyde, which is subsequently hydrated to lactic acid via an internal Cannizzaro reaction, Meerwein–Ponndorf–Verley reduction or Oppenauer oxidation (Fig. 19.19).

19.5.1 Challenges and opportunities for introduction of bio-based products

The most significant driver for the production of renewable chemicals is the price of crude oil. When oil prices exceed US$100 per barrel, bio-based feedstocks may compete with petroleum sources.¹⁶⁰ However, there are other motivators for renewable feedstocks such as green marketing initiatives, individual preferences for more sustainable products, and critically their potential to mitigate pollution and CO₂ emissions. There are a number of potential markets for bio-derived chemicals, most notably as identical replacements for current commodity chemicals, so called ‘drop-in’ chemicals (e.g., bio-ethene, nylon or PET) or as completely new types of platform chemical (e.g., PLA, or PEF from FDCA).

Current and potential commercial applications of bio-based chemicals have been reviewed by Erickson et al.,¹⁶¹ and some near-term commercial and future bio-products are summarised in Table 19.1. Succinic acid and 1,4-butanediol are identified as near-term, bio-based chemicals, while longer term targets include levulinic acid and adipic acid, the latter an important precursor to nylon.

Using polyamide synthesis as a case study, a number of building blocks have been developed around natural products using biotechnology. For example, the unsaturated fatty acids sebacic or undecylenic acid derived from castor oil are employed in nylon 6,10 and 10,10 manufacture for use in automobile components (e.g., by Rhodia and Dupont),¹⁶² while Cognis reportedly employ an enzymatic oxidative scission of oleic acids to produce muconic acid (a key precursor to adipic acid).¹⁶³ While these routes produce monomers for new materials, ‘drop in’ chemicals as direct replacements for those currently obtained via crude oil feedstocks are also essential for a sustainable chemicals industry. Promising biochemical routes to adipic acid via fermentation routes to muconic acid, were reported in the early 1990s. However, such approaches remain uneconomic, operating at only a 22% carbon balance, and producing dilute ∼ 3.68 wt% aqueous solutions of muconic acid which impose excessive separation and purification costs.¹⁶⁴ Despite these hurdles, US biotech companies (e.g., Verdezyne) are attempting to scale-up new biocatalytic production routes for commercialisation. These efforts face stiff competition from a recent, cheaper petrochemical route to nylon 6 via butadiene carbonylation, developed by DSM, DuPont and Shell.¹⁶⁵ Selective catalytic routes to bionylon from a dipic acid or caprolactam (that avoid costly separation processes) would thus be particularly attractive.¹⁰⁰

19.5.2 Challenges for process design

Conversion of bio-based feedstocks presents new challenges to the catalytic scientist, as the attendant reaction conditions are very different from those typical of petroleum processing, which occurs mainly through vapour phase processes > 400°C. Biomass processing will be characterised by liquid phase, lower temperature⁴¹ pathways such as hydrolysis, dehydration, isomerisation, oxidation, aldol, condensation, and hydrogenation.¹⁶⁶ The design of catalysts for such biomass transformations requires careful tailoring of pore structure to minimise mass transport limitations, hydrothermal stability under aqueous operation, and tunable hydrophobicity to aid product/reactant adsorption. ¹⁶⁷ Catalyst development should thus focus on the use of tailored porous solids as high area supports to enhance reactant accessibility to active acid/basic groups. The preparation of such templated porous solids has been extensively reviewed^168–171 but generally involves the use of micellar templates to direct the growth of metal oxide frameworks as shown in Fig. 19.20. Subsequent calcination to burn out the organic template, or solvent extraction, yields materials with well-defined meso-structured pores of 2–10 nm and surface areas up to 1000 m².g^− 1.

Macropores can also be introduced if an additional physical template, such as polystyrene microspheres, is added during templating of the mesopore network.¹⁷² Hierarchical macroporous-mesoporous sulfonic acid SBA-15 silicas and aluminas have been synthesised via such dual-templating routes, employing liquid crystalline surfactants and polystyrene beads.^173,174 These materials offer high surface areas and well-defined, interconnected macro- and mesopore networks, with respective narrow size distributions tunable over the range 100–300 nm and 3–5 nm. Such bimodal solid acid architectures offer significantly enhanced activity over mesoporous analogues. A second challenge centres around the need to better understand the selective deoxygenation and hydrogenation of polyols under acid- or base-catalysed conditions. Bifunctional catalysts need to be capable of low temperature dehydration and hydrogenation, and efficient use of H₂ during in situ hydrogenation and/or hydrogenolysis. Surface polarity is another important parameter to control and thereby permit catalyst operation in biphasic systems. As discussed in relation to Fig. 19.15, partitioning of products between aqueous and organic phase solvents via reactive extraction is a promising means to minimise side reactions.^175,176

Hydrophobic catalysts operating in biphasic systems have been explored for ‘bio-oil’ obtained via biomass pyrolysis; bio-oil is a complex liquid that is only partially soluble in water or hydrocarbon solvents. In order to overcome problems of working with such mixtures, materials with tunable hydrophobicity, or even the use of micellar catalyst systems should be considered. This has recently been exploited by the group of Resasco,¹⁷⁶ in which a phase transfer system based upon Pd nanoparticles immobilised on carbon nanotube/silica composite support was employed. Such materials are able to catalyse the transformation of both hydrophilic and hydrophobic substrates to fuels at the oil:water interface, without the need for multiple separation steps or addition of surfactants.¹⁷⁷

The development of new reactor technology will also be critical for efficient biomass processing.¹⁷⁸ Conventional stirred batch reactors are not suited to commodity chemical production scales, hence there is a need to devise slurry reactors that operate continuously. Intensive processing¹⁷⁹ could offer exciting means to improve overall process efficiency. For example, process engineering solutions available for continuous reactions include the use of fixed-bed¹⁸⁰ or microchannel-flow reactors,¹⁸¹ pervaporation methods,^182,183 or reactive distillation.^184–186 Continuous reactors must be carefully designed to utilise the full potential of the associated heterogeneous catalyst; plug flow is a desirable reactor characteristic, as it allows tighter control of product composition and thereby reduces downstream separation processes and capital and running costs. However, conventional plug flow reactors are unsuitable for slow reactions as they necessitate very high length:diameter ratios to achieve the required mixing. Such designs are problematic due to their large footprints, pumping duties and control difficulties.

The oscillatory baffled reactor (OBR) circumvents these problems, offering good mixing and plug flow by oscillating the reaction fluid through orifice plate baffles.¹⁸⁷ In an OBR, mixing is decoupled from the net flow: this allows reactor designs that are very different from conventional plug flow reactors. OBR mixing is also scalable, meaning that longer reaction times can be achieved on industrial scales. Previous work^188,189 has shown that vortical mixing in the OBR is an effective, controllable method of uniformly suspending solid (e.g., catalyst) particles. In this respect, the first such demonstration of a solid acid catalyst incorporated within an OBR for the esterification of organic acids to short chain and fatty methyl esters, pertinent to fine chemicals synthesis and biofuels production, was recently reported. ¹⁹⁰

The development of more efficient separation methods will also be crucial¹⁹¹ with advances in distillation, extraction, adsorption with molecular sieves, filtration, crystallisation and osmosis holding great promise.