and Paresh Laxmikant
a Catalysis & Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, India. E-mail: email@example.com
The synthesis of sugars from disaccharides and polysaccharides sourced from lignocellulosic biomass (agricultural waste, forest residues) is at the onset in the bio-refinery concept. This chapter presents a comprehensive overview of multiple strategies researched from the early 1990s to design and develop various catalysts and catalytic processes to hydrolyse saccharides (cellulose, hemicelluloses) into sugars by both academia and industry. A large body of work is done with mineral acids and enzyme catalysed processes, which are also practiced on an industrial scale. The effects of pH, time, temperature, concentration, substrate type etc. are studied and discussions are focused on those in the chapter along with discussions on kinetics and mechanisms. Recent developments on heterogeneous catalysts (solid acids, supported metals) are also discussed in the chapter. The pros and cons of using isolated saccharides and lignocellulose embedded saccharides as substrates are discussed. An outline of the future path for research in this area is presented for the benefit of researchers.
In the current circumstances, fossil feedstocks (crude oil, coal and natural gas) are utilized for the synthesis of a range of chemicals and fuels. Yet, their sustainability is at stake due to finite reserves, sporadic prices, volatile geopolitical scenarios and unfavourable effects on the environment (global warming) because of the discharge of a major contributor to the greenhouse gas effect, carbon dioxide (CO2) into the atmosphere.1 During World Wars I and II, due to a shortage of crude oil, Germany and a few other countries started extensive research on the production of chemicals and fuels (particularly ethanol and diesel) from alternate sources such as coal and biomass.2 The world's first ethanol production plant (Skutskär sulfite ethanol plant), based on the sulfite process, was started in 1909 in Sweden.3 Although a total of 33 plants were started using the same concept in Sweden, since 1983, just one plant has remained operational.3 After the development of efficient ways throughout the 20th century to explore, extract and process crude oil, research on biomass was decreased. But, following the recent crisis in oil production and for geo-political reasons, there has been a renewed interest in looking for alternative sources for the synthesis of chemicals and fuels. Though, for a long time, Brazil has successfully shown that due to the highest world production of sugarcane (Brazil: 3.3×108–7.7×108 ton per year in 2000–2013, World: 1.3×109–1.9×109 ton per year in 2000–2013),4 it can produce bio-ethanol from bagasse (sugarcane waste after extracting sugar juice) in large quantities for public distribution to run vehicles.5 Conversely, in the rest of the world, after numerous deliberations and considering history, recently, it has been suggested that the only alternative and sustainable resource, biomass should be leveraged for the synthesis of chemicals and fuels by developing environmentally benign pathways. Since biomass is renewable, carbon neutral, abundant, locally accessible in most countries and has a lower impact on the environment, it becomes a natural choice as an alternate resource.1,6 In recent times, several countries and industries have disclosed their interests in developing methods for the conversion of biomass into known and new chemicals and fuels.1 Biomass is a non-fossil and is made up of complex molecules present in plants and animals. It is considered as a rich source of organic products, which have a characteristic chemical composition of C, H, O, N.7 However, until now, much of the work has reported on the conversion of plant-derived biomass into chemicals. Naturally, plant biomass is produced during the photosynthesis pathway using water, carbon dioxide and sunlight and is classified into two categories, namely edible and non-edible, solely based on human consumption ability. For example wheat, rice, corn, potato etc. are made up of a polysaccharide, starch and are considered as edible biomass or a first generation raw material (for the synthesis of fuels and chemicals). Starch is composed of a mixture of linear polysaccharide, amylose (homopolymer of d-glucose linked via a α-1,4 glycosidic bond) and branched polysaccharide, amylopectin (homopolymer of d-glucose linked via a linear α-1,4 glycosidic bond and branched α-1,6 glycosidic bond). Non-edible biomass, for example crop waste or wood, is called lignocellulosic biomass or lignocelluloses and is considered as a second generation raw material. Lignocelluloses have a composition of ca. 45% cellulose (homopolymer or homopolysaccharide of d-glucose linked via a β-1,4 glycosidic bond), ca. 25% hemicellulose (heteropolymer or heteropolysaccharide of several C5 and C6 sugars linked via various bonds), ca. 20% lignin (amorphous 3D network polymer of several aromatic monomers), some macro and micro nutrients (nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, boron, zinc, chloride and molybdenum) and extractives (fats, fatty acids, resins, tannins, volatile oils, proteins etc.).7,8 Typically, saccharides or carbohydrates (hydrates of carbon) have a molecular formula of Cm(H2O)n, where m and n are almost same. For instance, a simple monosaccharide, glucose has a molecular formula of C6H12O6 while deoxyribose has a molecular formula of C5H10O4. This makes saccharides rich in oxygen content with an O/C ratio of ca. 1 and a H/C ratio of 2. Usually, during the formation of disaccharides or polysaccharides for example cellobiose (glucose dimer or disaccharide) with a molecular weight of 342 and cellulose (glucose polysaccharide) with per unit of glucose molecular weight of 162, loss of one mole of water (H2O) with a molecular formula of 18 per two moles of monosaccharides is essential. Hence, the O/C ratio in cellulose and hemicellulose (lignocelluloses) has a slightly lower value (ca. 0.8).8 Nevertheless, for a chemical to be used as a fuel or fuel additive, its O/C ratio should be low (biodiesel: ca. 0.1, ethanol: 0.5).8 Consequently, conversions of saccharides into fuels or fuel additive necessitates extra processing for the reduction in O/C ratio. At the same time, conversions of saccharides into chemicals (sugars and its derivatives) for non-fuel applications exempt the extra process of decreasing the O/C ratio. Hence, it is apparent that lignocelluloses should be used for chemical production. Moreover, economic analysis suggests that while lignocelluloses are obtainable at a price of $50 per ton, glucose has a market price of $450–650 per ton and xylose has a market price of $1000–2500 per ton.9 Further conversion of these monosaccharides (sugars) into various chemicals such as 5-hydroxymethylfurfural (HMF) ($300 000–350 000 per ton), furfural ($2500–3000 per ton), sorbitol ($500–700 per ton) and xylitol ($1000–3000 per ton) adds value to these sugars.9 Hence, it is understandable that suitable transformations of starch, cellulose and hemicellulose to various sugars (C6 and C5) via hydrolysis of glycosidic bonds present in polysaccharides are economical. Nevertheless, use of a first generation biomass (polysaccharide), starch for obtaining sugars as platform chemicals to produce a variety of other essential chemicals is a debatable issue since it is principally used as a food. Hence, use of a second generation biomass, lignocelluloses (non-edible biomass)—with a high energy content (ca. 2×1010 J per ton of dry biomass)10 is desirable for the synthesis of sugars. Additionally, huge worldwide availability (1.8×1012 tones) of plant-derived lignocelluloses including crop (agricultural) wastes and forest residues (90–95% with respect to total plant biomass production)8 might permit those to be used as a feedstock for better rural economy. On the other hand, in an ideal scenario, it can be considered that from non-edible feedstock, one can produce edible products (sugars). The conversion of di/polysaccharides into chemicals can be done by either thermal (combustion, pyrolysis, gasification, supercritical water), thermo-chemical (acid, alkali) or biological (enzyme) methods. Under thermal conditions, substrates are heated at high temperatures (pyrolysis: >350 °C; gasification: >550 °C, supercritical water: ∼300–400 °C) essentially without a catalyst (however, in a few cases such as gasification and treatment in supercritical water, catalysts are added to drive the reaction in a particular direction) to yield sugars, tar, char, gases etc. In most of these studies, gases (CO, CO2, H2, CH4etc.) are formed as the main products with a minor quantity of sugars formed (<20–30%). On the contrary, under thermo-chemical conditions at lower temperatures (<250 °C), catalysts are used to obtain sugars in higher quantities by subjecting substrates to hydrolysis. Considering this, in this chapter, discussions are focused on the conversion of di/polysaccharides into sugars by hydrolysis reactions. In the conversion of lignocelluloses to chemicals, multiple steps are involved and these are depicted in Figure 1.1.
Monosaccharides, or else we call them sugars, are named in two ways: (1) a monosaccharide containing an aldehyde group is called aldose and (2) a monosaccharide containing a ketone group is called ketose. In total, eight C6 aldo-sugars (glucose, mannose, galactose, allose, altrose, gulose, idose and talose) and four C5 aldo-sugars (xylose, arabinose, ribose and lyxose) are structurally possible. Besides these aldo-sugars, two more keto-sugars viz. fructose and xylulose are also well-known in nature. But, among them, idose and talose are not found in nature. Moreover, the presence of allose, altrose, gulose, ribose and lyxose is very rare in nature and hence discussions on those are not made here. The rest of the sugars are generally present in fruits, edible plants, living bodies, bacteria, proteins etc.
In Figure 1.2, likely sources of main C6 sugars (glucose, fructose, mannose, galactose) and C5 sugars (xylose, arabinose, xylulose) are illustrated. In general, these monosaccharides (sugars) can be obtained by the hydrolysis (addition of one mole of water per 2 moles of sugars) of their respective disaccharides [maltose: α-1,4-d-glucose disaccharide (found in potatoes, cereal, beverages etc.), cellobiose: β-1,4-d-glucose disaccharide, sucrose: disaccharide of α-d-glucose and β-d-fructose linked via a 1,2 glycoside bond (found in sugarcane, beet, grains etc.), xylobiose: β-1,4-d-xylose disaccharide etc.]. Further, several polysaccharides such as starch (α-1,4-d-glucose polysaccharide), cellulose (β-1,4-d-glucose polysaccharide), inulin (fructose polysaccharide), hemicellulose (polysaccharide of several C5 and C6 sugars) etc. derived from edible and non-edible parts of plant biomass can yield sugars on hydrolysis. Moreover, lignocelluloses are made up of ca. 75% of polysaccharides (cellulose, hemicellulose, starch and saccharose)11 and hence, it will be beneficial to use plant biomass (lignocelluloses) directly for the synthesis of various sugars. Since cellulose is present as a major component (ca. 45%) in lignocelluloses, its conversion into chemicals (mainly sugars) is considered as foremost in the bio-refinery concept. In recent times, municipal solid wastes (kitchen waste containing cellulose) have also been increasing and their effective utilization to generate chemicals and fuels may prove to be vital in curbing the problems of landfill and incineration, which gives rise to pollution by liberating hazardous chemicals and gases.
Sugars have a variety of applications in fine chemicals, pharmaceuticals, agriculture, cosmetics etc. In most cases, sugars are used as an energy source (glucose), low-calorie sweetener (xylose) and for the synthesis of many industrially important chemicals such as furans (5-hydroxymethylfurufral and furfural; precursors for fuel, resin, plastic, nylon, polyester, fine chemical etc.), sugar alcohols (sorbitol, mannitol, xylitol, arabitol; used as low-calorie sweetener, adhesive, cosmetics, energy source etc.), sugar acids (gluconic acid, xylonic acid, arabinonic acid; used as chelating agent, cement retardant, cosmetics, medicine etc.), acids (succinic acid, itanoic acid, formic acid, glycolic acid; used in the food industry and polymer industry), alcohols (ethanol, butanol; used as fuels, solvents etc.) and alkyl ethers of sugars (alkyl glucoside, alkyl xyloside; used as biomass-derived surfactant etc.)12–14 Because of these extremely significant applications of sugars, it is worth synthesizing sugars from biomass-derived resources.
As suggested in Section 1.1.1, the most favourable way to synthesize sugars is the utilization of lignocellulosic (non-edible) biomass directly as a raw material which consists of polysaccharides (ca. 45% cellulose and ca. 25% hemicelluloses) in large quantities.8 But, in reality, due to very intricate hydrogen bonding such as intra-, inter-molecular and inter-sheet in cellulose (Figure 1.3),15,16 the existence of lignin (aromatic polymer) in lignocelluloses, and multiple bonding between polysaccharides and lignin, it becomes complicated to process lignocelluloses directly into sugars. During the conversion of the polysaccharide part (cellulose and hemicellulose) of lignocelluloses into sugars, lignin remains unconverted because it usually requires high processing temperatures (>250 °C) compared to polysaccharides (<230 °C).17 And if conversions of lignin are also tried simultaneously then degradation reactions of sugars become predominant. In most cases, unconverted lignin is also capable of poisoning the catalytically active sites. As discussed earlier, due to the occurrence of multiple H-bonding in cellulose, its structure becomes very rigid and crystalline and thus becomes difficult to degrade. A representative XRD pattern for commercially available microcrystalline cellulose is presented in Figure 1.4. The pattern shows peaks due to the amorphous (2θ=15.8°) and crystalline phases (2θ=22.5°, 34.7°) of cellulose. Additionally, due to the very strong H-bonding in cellulose, it remains insoluble in many common solvents but is soluble in ionic liquids (ILs), concentrated aqueous ZnCl2 solution and ammoniacal Cu(OH)2 solution.18,19 Cellulose also possesses a very high degree of polymerization (DP), for example in wood pulp, it ranges from 300 to 1700 and in cotton and plant fibres, it ranges from 800 to 10 000.20 Because of these properties of cellulose, its hydrolysis becomes difficult. Furthermore, the presence of lignin (covering polysaccharides), which has the role of protecting the polysaccharides from any chemical or biological attacks, hinders their catalytic conversions. Hence, it becomes indispensable to pre-treat the lignocelluloses before hydrolysis for the removal of lignin and to decrease the crystallinity of cellulose. Other significant aspects in pre-treatment are to avoid the degradation or loss of saccharides and make the overall process economic and environmental friendly. During some of the pre-treatments, the DP of cellulose decreases, which may increase the solubility of cellulose (fractions) in water for efficient hydrolysis.
The available biomass pre-treatment methods are classified roughly into three categories namely: (1) physical (milling, grinding, radiation, ultrasound), (2) physico–chemical (steam explosion, ammonia fiber explosion, carbon dioxide explosion) and (3) chemical (ozonolysis, alkaline hydrolysis, oxidative delignification, organic solvent extraction, acid hydrolysis, enzyme treatment, and ionic liquid treatment).21,22
The main purpose of the physical pre-treatment of lignocellulose is to decrease its particle size and cellulose crystallinity. Use of milling and grinding methods can decrease the size of various biomass to 0.2–2 mm from 10–30 mm.23 The size reduction of lignocellulose is directly related to the energy consumption and time required for pre-treatment processing. Several reports on the ball milling method show a reduction in size (determined by particle size analyser), crystallinity (determined by XRD, NMR studies) and degree of polymerization (DP; determined by anion-exchanged chromatography) in cellulose and hence, increases in its hydrolysis rate.24–26 During ball milling, an increase in temperature is seen, which has an effect on de-crystallization and for this reason, it becomes important to control the temperatures for reproducible results. Recently, an ultrasound technique has been used to decrease cellulose crystallinity within a short time in the presence of water.27 It is shown that Avicel cellulose (particle size=38 µm) can be transformed into 0.1–0.6 µm cellulose with a 12.1% decrease in crystallinity index (without changes in its structure) at 80 °C after subjecting it to ultrasound treatment (optimum amplitude=40%) for 3 h. It is assumed that when cellulose is exposed to ultrasound, which has a higher energy than the H-bonding energy of cellulose (21 kJ mol−1), it breaks the H-bonding in cellulose to form lower crystalline cellulose.28 Microwave irradiation of lignocellulosic biomass causes localized heating of lignocellulose leading to disruption of the lignocellulose structure29 and since it is a harsh process, it leads to very high lignin removal from biomass.30,31 However, in terms of cost and generation of high temperature during treatment, the process is not efficient on a large scale.32 Additionally, treatment of cellulose with γ-rays leads to the reduction of DP and crystallinity in cellulose but this process also faces the drawback of high costs.33
In the widely used steam explosion pre-treatment, biomass is treated with saturated steam (6.9–48.3 bar) at high temperatures (160–260 °C) and after a certain time (few seconds to a few minutes), the pressure is suddenly reduced to atmospheric pressure allowing the biomass to undergo explosive decompression.23,34 This process helps in the removal of hemicellulose and redistribution of lignin. Various factors such as biomass size, moisture content, temperature and time are decisive in designing an effective steam explosion biomass pre-treatment method.35 Moreover, the addition of mineral acid (H2SO4, CO2; 0.3–3 wt/wt) during steam explosion, improves the efficiency of the pre-treatment process by decreasing the temperature and time required.36 The steam explosion treatment has advantages including a lower energy requirement than mechanical treatment (70% less energy requirement)37 and less environmental impact, which allows the process to operate on an industrial scale successfully (Iogen Corporation, Canada; steam explosion using dilute acid pre-hydrolysis of corn stover, barley straw and bagasse38). Biomass pre-treatment is also carried out in liquid hot water instead of steam.39 This process can be done in three types of reactors namely co-current, counter-current and flow-through. In a typical procedure, all the reactors are maintained at desired temperatures (200–230 °C) for a specific time (ca. 0.25 h) to achieve efficient pre-treatment.
Ammonia fibre/freeze explosion (AFEX) is another process for lignocellulose pre-treatment where liquid NH3 is in contact with biomass (typically in a biomass–NH3 ratio of 1 : 1 to 1 : 2) at moderate temperatures (60–100 °C) and high pressures (17–21 bar) for a specific time and subsequently pressure is reduced to make an explosion.21,40 This process is reported to be effective for lignocelluloses with a low lignin content (bagasse: 15%, bermudagrass: 5% etc.) rather than a high lignin content (nutshell: 30–40%, wood: 18–35%, aspen chips: 25% etc.).41
To surmount the problems with high temperature (steam explosion) and high cost (AFEX) associated with these methods, a carbon dioxide explosion pre-treatment, which uses carbon dioxide in a supercritical state (Pc=73.8 bar, Tc=31.1 °C), has been developed.21,42 Moreover, due to the liquid–gaseous state of carbon dioxide under supercritical conditions, it has a lower viscosity and higher diffusivity (than water) and hence, it can easily penetrate inside the architecture of biomass and separate cellulose and hemicelluloses from each other and lignin. This process reduces the prospects of degradation product formation from saccharides because of lower operating temperatures.
Effective lignin removal from lignocellulose can be carried out using ozone treatment to biomass at room temperature and atmospheric pressure (ozonolysis).43,44 This treatment has a minor effect on the hemicellulose part and no effect on the cellulose part of lignocelluloses. Moreover, during this processing, no toxic chemicals are generated. From an environmental point of view, this process is better since the used ozone can easily be catalytically decomposed at increased temperatures.45 However, the requirement of a large amount of costly ozone makes the process expensive.
Another way to remove the lignin part is to use alkali at low temperature and pressure (alkaline hydrolysis).34,46 During this process, in the presence of alkaline reagents such as NaOH, KOH, Ca(OH)2, NH4OH, removal of acetyl and uronic acid substitution in the hemicellulose part is also possible along with removal of lignin.47–49 The treatment of lignocelluloses with alkali mainly hydrolyses the intermolecular ester bonds between lignin and polysaccharides to remove lignin leaving behind the free polysaccharides (cellulose, hemicellulose).23 Use of air/oxygen during alkaline hydrolysis significantly improves the delignification process of biomass with high lignin contents.47 Nonetheless, the chances of polysaccharides undergoing hydrolysis and oxidation reactions under alkaline conditions make this process vulnerable.
Delignification of biomass can as well be carried out using H2O2 as an oxidising agent.46 This process involves removal of both lignin and hemicellulose at a higher extent from lignocellulose. In this method, oxidising chemicals react strongly with the aromatic rings of lignin and generate aromatic carboxylic acids that can act as inhibitors in the later stages of transformations, hence they need to be removed or neutralized before processing the celluloses.50
The organosolv process can also remove lignin from biomass. In this method, several organic solvents (methanol, ethanol, acetone, ethylene glycol, triethylene glycol, tetrahydrofurfuryl alcohol etc.) or organic solvents with water in the presence of mineral and organic acids (H2SO4, HCl, oxalic acid, salicylic acid, acetylsalicylic acid etc.) are used to remove lignin.51–53 The organic solvents used in this method are required to be recoverable to make the process cost efficient. Besides this, the production of very high quality lignin makes this method competent since this lignin can be consumed further for the synthesis of various valuable chemicals.
Dilute acid (<4 wt%) pre-treatment to lignocellulose is another widely used technique. In general, two pathways are undertaken for this purpose: (1) use of a high temperature (>160 °C) continuous flow process for low biomass loading (5–10 wt%) and (2) use of a low temperature (≤160 °C) batch process for high biomass loading (10–40 wt%).54 Typically, dilute acids (H2SO4, HCl, HNO3, H3PO4, maleic acid, fumaric acid etc.) and concentrated acids (H2SO4, HCl) are sprayed over the lignocellulose to remove (selectively) hemicelluloses in the form of sugars and soluble oligomers (2<DP<10).21,23,46,54,55 This method is used on a commercial scale by BlueFire Renewables and Biosulfurol Energy Limited.56,57 However, due to acid treatment, some portions of sugars are further transformed into furans (furfural and HMF) and other by-products, which leads to the loss of sugars. Although this pre-treatment process is quite impressive, due to the use of acids, it becomes hazardous and corrosive and thus requires a high capital cost.23,58
Biological pre-treatment of biomass is a safer and environmentally friendly method which uses various micro-organisms (white-, brown- and soft-rot fungi) to attack the cellulose part selectively.18,21 Although milder conditions are applied for biological pre-treatment, nevertheless the rate of biological hydrolysis is very low and requires a long processing time, which is a great disadvantage of this method.23,59,60
Recently, the use of ionic liquids (ILs) in the pre-treatment of lignocelluloses has also been studied because they have interesting properties such as high thermal stability, low volatility, high activity at low temperatures etc.61,62 ILs are very effective in the de-crystallization of cellulose and in the cleavage of lignin–hemicellulose linkages. As, for example, the anionic part (Cl−) of 1-butyl-3-methylimmidazolium chloride, [BMIM]Cl ionic liquid forms new H-bonds with the sugar (glucose) part of cellulose in a 1 : 1 stoichiometric ratio and thus breaks the earlier H-bonding present in cellulose (Figure 1.5).19,63 This helps in the dissolution of cellulose in ILs, for example [BMIM]Cl, [BMIM]Fm, [BMMIM]Cl, [AMIM]Cl, [AMIM]Br, [AMIM]Fm, [EMIM]Cl, [EMIM]Ac, [BMPyM]Cl etc. have the ability to dissolve 3–39% cellulose with different DP (200–6500) at varying temperatures (45–110 °C).64,65 It is predicted that small cations are helpful in achieving efficient dissolution of cellulose since [AMIM] is proven to be better than [BMIM].66 Although the process may look very attractive for lignocellulose pre-treatment, it can only be possible if the drawbacks (high cost, recycling problems due to homogeneous nature, washing of lignocelluloses etc.) of ILs are minimized.61
In summary, the choice of pre-treatment method is dependent on whether to remove lignin or hemicelluloses or both. However, the foremost aims of these methods are to decrease the size of particles, reduce the crystallinity of cellulose and separate the cellulose, hemicelluloses and lignin parts from each other for their efficient conversions into value-added chemicals. Although these different methods show different effects on the structure of cellulose, these are still not well studied in hydrolysis reactions.
As shown in Figure 1.2, glucose (C6 sugar) can be obtained from various disaccharides (maltose, cellobiose and sucrose (table sugar)) and polysaccharides (starch, cellulose and inulin) by undergoing hydrolysis reactions (addition of one mole of water/two moles of sugars). The presence of acid or base catalysts assists in improving the hydrolysis rates by efficiently cleaving the glycosidic linkages in disaccharides and polysaccharides to form monosaccharides (sugars: glucose, fructose).67
Before discussing the catalytic processes for the cellulose hydrolysis into sugars, it is crucial to know the properties of cellulose and the variety of crystal structures (polymorphs) of cellulose, which are identified as, Iα, Iβ, II, III and IV based on unit cell parameters.68 Undoubtedly, these structures play an imperative role in catalysis since these can restrict or enhance the possibility of interactions between cellulose and catalytically active sites. Nature produces most of the cellulose in the form of cellulose I, which is a composite of two crystalline allomorphs viz. Iα and Iβ in varying ratios depending on the source of cellulose.16,69 In cellulose I crystalline form, two glucosyl units are arranged in a parallel manner and form one-chain triclinic (Iα) and two-chain monoclinic (Iβ) unit cells.70 It is known that the cellulose Iα form is metastable and upon annealing, it transforms to the more stable cellulose Iβ form. Typically, the cell wall of algae and bacterial cellulose contains the Iα crystalline form of cellulose but cotton and wood contain mostly the Iβ type of the cellulose crystalline form.71 Later, treatment to the cellulose Iα and Iβ crystalline forms with concentrated NaOH (∼20%) solution (regeneration) and a further water washing (mercerization) transforms cellulose I into the thermodynamically more stable cellulose II in an irreversible way.69,72 The cellulose II crystalline form (two-chain monoclinic unit cell) consists of two glucosyl units in an antiparallel arrangement. Treatment of both cellulose I and II with amines or liquid ammonia (swelling) and further removal of the swelling agent anhydrously produces the cellulose III crystalline form. Cellulose III is a highly unstable crystalline form and can be sub-classified into cellulose IIII and cellulose IIIII depending on its source (cellulose Iα or Iβ). In cellulose III, two glycosyl units are present in parallel forming a one-chain monoclinic unit cell. Further, when cellulose III is treated with glycerol at 206 °C, it forms the cellulose IV crystalline form. In general, cellulose IV is the disordered form of cellulose I.
Several reagents such as water, organic solvent, alkali and acids have the ability to swell cellulose and, based on the penetration ability of reagents in the structure of cellulose, swelling can be classified as inter-crystalline or intra-crystalline.70 If the reagent only affects the amorphous part of cellulose leaving the crystalline part intact then it is called inter-crystalline swelling and if the reagent affects cellulose completely (amorphous and crystalline part) then it is called intra-crystalline swelling. Due to a hydrophilic cellulose surface (the presence of –OH groups), water molecules interact with cellulose via H-bond formation.73 This leads to the inter-crystalline swelling of cellulose. Because of restrictive swelling due to water, and since water is used as a solvent in hydrolysis reactions, it can be expected that it would be difficult to hydrolyse cellulose. Generally, organic solvents have less ability to swell cellulose (inter-crystalline) than water and the effectiveness of organic solvent swelling depends on the higher H-bonding ability and higher polarity of the solvent.70 In contrast, alkali and mineral acids can strongly affect amorphous and crystalline structures leading to intra-crystalline swelling. This gives an idea that the combined effect of water (as hydrolysis reactant and solvent) and mineral acids or alkalis can pronounce the hydrolysis rates. However, in most of the hydrolysis works, researchers have used commercially (Aldrich, Merck etc.) available microcrystalline (Avicel) cellulose (partial depolymerized form of Iα cellulose by mineral acids74). Besides this, a few researchers have also used several types of isolated celluloses (pre-treated using acid, alkali, supercritical or any other method as discussed in Section 1.2) for the synthesis of sugars (glucose) for which the type of cellulose is not known. All this makes it difficult to really compare the results of all the reactions with each other since, depending on the type of polymorph of cellulose used in the work, activities of the catalysts will vary. Moreover, results obtained with a particular catalyst in the hydrolysis of a particular cellulose are mostly not reproducible with a change of the source of cellulose as the structures of these celluloses are different. In some reports, it has been shown that supercritical ammonia treatment to cellulose I produces cellulose III, which has more enzymatic accessible sites than cellulose I and thus enhances the hydrolysis activity.75,76 However, more detailed investigations are necessary to correlate the catalytic activity with cellulose structures.
Considering these discussions on the changes in cellulose structures after isolation (pre-treatment), it would be beneficial to use lignocelluloses without any pre-treatment for sugar synthesis as cellulose will be present in mostly the Iα form, which is easier to hydrolyse than a few other cellulose structures. Moreover, there is a possibility that results obtained with one lignocellulose cannot be reproduced with other lignocelluloses (at least derived from the same plant species). Nonetheless, very few catalytic reports are known to use raw lignocelluloses for the conversion into chemicals. However, unlike academic research, industries prefer to use lignocelluloses instead of isolated polysaccharides to study the effects of catalysts on their hydrolysis. The academic research is mainly done with isolated polysaccharides because as mentioned earlier (Section 1.2), in the native form, cellulose is present as a very rigid, crystalline material and hence it would be challenging to hydrolyse it. Ideally, when lignocelluloses or cellulose are used as substrates that are not soluble in most of the reaction media (mainly water), all the catalytic systems become heterogeneous. But to simplify the discussions, depending on the solubility of the catalyst in the reaction medium, discussions are made for homogeneous as well as heterogeneous catalytic methods.
Chemical transformations of substrates using homogeneous catalysts are always attractive since these methods prevail over the mass transfer limitations to allow efficient interactions between substrates and catalysts (active sites). An early process (before 1960) depicts that the synthesis of glucose can be achieved from starch in the presence of H2SO4 and HCl.77 Later, simple substrates such as maltose and cellobiose were used to study the reaction mechanism and kinetics in the presence of dilute H2SO4 to yield glucose.78 The activation energy calculated for these substrates hydrolysis was found to be 132–137 kJ mol−1 and it was also observed that the rate of hydrolysis increased with an increase in temperature. The results were confirmed by another research group by observing a similar activation energy of 133 kJ mol−1 in cellobiose hydrolysis.79 However, it was seen that with the use of mineral acids such as HCl and H2SO4, large amounts of degradation products can be formed. To reduce this possibility, dilute H3PO4 was used for starch hydrolysis into glucose although H3PO4 shows a lower activity than HCl.80
The studies were also done with non-edible substrates instead of the edible substrate starch. The first process for the synthesis of glucose from linen (textile) using concentrated H2SO4 was reported as early as 1819.81 In 1937, hydrolysis of cellulose was reported using 40 wt% HCl at room temperature, however, lower yields were observed and also the reaction took a longer time to complete.82 In 1931, the process of cellulose hydrolysis into sugars was commercialized (Scholler process) based on the two step method wherein in the first step, in the presence of a mineral acid (0.5 wt% H2SO4), hydrolysis of wood wastes at 170 °C was carried out to yield sugars and oligomers. In the second step, fermentation was carried out to achieve 50% yield for sugars.2 Around the same time (1935–45) in Russia, using dilute mineral acids and fermentation methods, many industrial processes were developed for the hydrolysis of softwood (corn, grain, molasses) and hardwood into sugars, ethanol and furfural.83 As mentioned earlier, the efficiency of releasing glucose in the solution depends on the structure of cellulose, temperature, time and concentration of acids. With an optimization of reaction conditions, a maximum of ca. 70% glucose formation was reported using dilute H2SO4 from corn stover.84 However, at the same time, if a reaction is not curtailed at an appropriate time, then further conversions of glucose into by-products is feasible due to condensation and degradation reactions.85,86 To avoid the use of higher temperatures (for curbing the degradation reactions of glucose), concentrated acid catalysed low temperature (room temperature) methods were developed at ambient pressures. But the use of concentrated acids again promotes sugar degradation reactions. Nevertheless, with the development of new reactors (continuous, concurrent batch etc.), it is possible to achieve better yields of glucose. But, due to the higher costs of these reactors and the severe problem of corrosion associated with the use of concentrated acids, the commercial viability of this method is in jeopardy. Based on recent developments in this research area, BC International Corporation (BCI), USA, uses agricultural wastes such as rice husk, corn stover, bagasse etc. to yield sugars and subsequently ethanol (by use of microorganisms).70 The concentrated acid hydrolysis method is also very well known for obtaining sugars from raw biomass. In 1948, the Japanese commercialized the method using concentrated H2SO4 and membranes to separate the acid from sugars. A few attempts were also made to carry out hydrolysis in a stepwise manner. For instance, first, a dilute acid treatment is given to biomass to remove the hemicellulose part and then the cellulose and lignin parts are subjected to concentrated acid treatments. This ensures that the degradation products obtained from hemicelluloses under concentrated acid conditions can be avoided. Moreover, during dilute acid hydrolysis of hemicelluloses, loosening of the cellulose structure occurs, which in turn makes it easy to undergo hydrolysis. Arkenol's process is based on this technology wherein 20–30% and ca. 70% sulfuric acid concentrations are used to obtain high yields of C5 and C6 sugars. In the last few years (since 2012), along with US DOE, Masada Resource Group and Arkenol have been working to commercialize dilute–concentrated acid technology.70 It is also mentioned in the literature that the presence of metal salts (LiCl, CaCl2etc.) along with acids helps in achieving a higher hydrolysis rate by the swelling effect as discussed earlier. The same effect is also observed with the use of concentrated acids in higher concentrations (>50 wt%).87,88 The swelling effect is thought to break the hydrogen bonding and gives the catalyst access to internal glycosidic bonds, which ensures an improvement in the rate of hydrolysis. Additionally, numerous studies on the use of liquid mineral acids (H2SO4, HCl, H3PO4, H2CO3etc.) and organic acids (oxalic acid, maleic acid, fumeric acid, p-toluenesulfonic acid etc.) have been done with varying concentrations (0.05–40 wt%) to achieve higher yields of glucose in the hydrolysis of isolated cellulose carried out at 100–260 °C for a few minutes to several hours.14,20,67 Use of gaseous HCl for pre-treatment of wood chips and a further hydrolysis in the presence of dilute HCl yields high amounts of sugars (glucose=80%, xylose=95%).89
It is reported that the acid hydrolysis of cellulose follows first order kinetics and the reaction rate is dependent on several factors such as temperature, acid concentration, physical state of cellulose (polymorph) etc.68,73 In the acid catalysed cellulose hydrolysis reactions, initially, a rapid decrease in the degree of polymerization (DP) is observed. However, after reaching a certain value (depending on cellulose's physical state), the DP value remains almost constant and this phenomenon is termed as ‘levelling-off degree of polymerization (LODP)’.90 This phenomenon can be explained based on the fact that first, the easy to hydrolyse amorphous part of cellulose undergoes rapid conversion into oligomers and sugars and then the difficult to hydrolyse crystalline part starts converting at a much slower rate. When cellulose is heated in the presence of dilute or concentrated mineral acids (HCl, H2SO4), it yields glucose (hydrolysis product) as a primary product along with secondary (degradation) products: 5-hydroxymethylfurfural (HMF), formic and levulinic acids, humins (re-polymerization or condensation products) etc. The kinetic studies have revealed that in the presence of H2SO4, the rate of cellulose hydrolysis into glucose is always higher (Douglas Fir: 1.73×1019 per min, Solka Floc: 1.22×1019 per min, filter paper: 1.22×1019 per min and municipal solid waste: 1.16×1019 per min) than the rate of glucose degradation reactions (Douglas Fir: 2.38×1014 per min, Solka Floc: 3.79×1014 per min, filter paper: 3.79×1014 per min and municipal solid waste: 4.13×1015 per min).91–93 It is known that the activation energies for cellulose hydrolysis reactions are higher (172–180 kJ mol−1) than the glucose degradation reactions (137–142 kJ mol−1). Considering these somewhat contradictory results, it is expected that in the acid hydrolysis of cellulose, glucose degradation products will always be observed. This might be another reason to observe the LODP effect since glucose may undergo repolymerization (condensation) reactions (to achieve equilibrium between hydrolysis and condensation reactions).
Besides mineral acids, homogeneous heteropoly acids (HPAs) are also presented as effective catalysts in the hydrolysis of cellobiose and cellulose. Silicotungstic acid (H4SiW12O40), phosphomolybdic acid (H3PMo12O40) and silicomolybdic acid (H4SiMo12O40) are used for cellobiose hydrolysis reactions to yield 42–53% glucose at 150 °C.94 Moreover, the cellulose hydrolysis reaction has also been reported using H3PW12O40 and H3BW12O40 catalysts to produce 18–77% of glucose and other reducing sugars.94–97
Based on the kinetic data obtained with various substrates and many studies, the cellulose hydrolysis mechanism is proposed. The hydrolysis of cellulose happens by the action of a proton (H+)/H2O or conjugated acid (H3O+) at the glycosidic O-linkages via formation of a positively charged acyclic or cyclic intermediate to yield oligomers and further glucose (Figure 1.6).98
Although several researchers have shown that homogeneous acids can be used as catalysts for the synthesis of C6 sugars (glucose, fructose) from cellulose (saccharides), due to various practical problems such as acid corrosion of reactors (acid concentration=0.05–40 wt%), difficulty in catalyst and product recovery, poor catalyst recyclability (solubility of both, product and catalyst in water), formation of degradation products (at high temperatures and using concentrated acids), generation of neutralization wastes (salt formation), toxicity, health hazards etc., these catalysts may not be suitable for hydrolysis. However, due to a lack of any alternate efficient method, they are still used on an industrial scale.
Cellulose is also hydrolysed using alkalis in two possible pathways: (1) endwise degradation, which is also known as peeling and (2) hydrolysis of glycosidic bonds.68,70,73 The peeling effect literately means peeling off monomers from reducing ends of cellulose below 100–140 °C. Thus, this process eventually will reduce the chain length and will also hydrolyse cellulose completely into glucose. Nonetheless, this process is too slow and does not continue for long. However, if this process was fast or continued until complete hydrolysis was achieved then it would have been practically impossible to subject lignocelluloses to the alkali pre-treatment/Kraft process (Section 1.2.3). Researchers have, however, devised a method by which the peeling effect can be controlled. This can be done by oxidising the reducing ends of cellulose and also by reducing the hemiacetal group of cellulose. If alkali treatment is given above 140 °C, along with peeling, cleavage of glycosidic bonds will start initiating a quick decline in chain length (DP). Since cleaving glycosidic bonds is an arbitrary process, several new reducing ends will be generated and the peeling process will gain momentum. The mechanism of peeling and alkali hydrolysis is illustrated in Figure 1.7. As observed, the mechanistic pathway studied with the help of model compounds proposes the involvement of SN1, SNicB (2) (nucleophilic substitution by an internal nucleophile i.e. conjugate base of C2 hydroxyl group) and SNicB (2)-ro (nucleophilic substitution via intermediate ring opening) pathways. As shown in Figure 1.7, to carry out the SNicB (2) mechanism, first the pyranose ring structure needs to be flipped, but this ring flipping is unfeasible in the case of crystalline cellulose because of severe hydrogen bonding, which makes the structure very rigid. Hence, it is probable that the alkali catalysed cellulose hydrolysis may happen via a SN1, SNicB (2)-ro mechanism. Once the glucose is formed, under alkaline conditions it can undergo an isomerization reaction to yield fructose and mannose (Section 1.5) and thus can hamper the yields of glucose. Although cellulose hydrolysis can be achieved using alkalis, it is also possible for cellulose to undergo oxidation reactions (done purposefully to reduce the peeling off effect) in the presence of alkalis to yield oxycelluloses or carbon dioxide and water (complete oxidation) depending on the severity of the process.
The actions of enzymes on cellulose hydrolysis are explained by ‘inversion’ and ‘retention’ mechanisms.70,99 As illustrated in Figure 1.8, in both mechanisms, cellulose hydrolysis can be carried out with the help of two carboxylic acid groups, wherein one acts as a proton donor (acid) and the other acts as a nucleophile (base).
It is observed that cellulose hydrolysis in the presence of enzymes occurs via two subsequent pathways: (1) dissolution of crystalline cellulose (physical disruption) and (2) hydrolysis of disrupted cellulose glycosidic linkages to form sugar.100 In April 2004, Iogen Corporation of Canada started production of bio-ethanol from wheat straw via the enzymatic hydrolysis of cellulose into sugars and their further fermentation into ethanol. An ethanol producer company from USA, POET, also announced in 2009 the production of bio-ethanol from cellulose (corn cob) in their South Dakota facility under Project Liberty. Abengoa has also shown a possibility for the production of ethanol from corn Stover via sugar formation. In 2013, Raízen Energia SA of Brazil (joint venture of Shell and Cosan) announced the production of bio-ethanol via sugar formation using an enzymatic process. In October 2004, taking into account the high cost of enzymes, Novozymes (Denmark) and Genencor (USA) industries have announced a modified cellulase production method using genetically modified organisms and they showed that the cost of the cellulase enzyme can be reduced to $0.1–0.2 from $5 (cost in 2001).70 This has definitely improved the economics of the enzymatic hydrolysis process and hence, after 2004, several facilities started using enzymatic routes for the conversion of lignocelluloses into sugars and further into ethanol. In these processes, cellulase enzyme (endoglucanase, exoglucanase, β-d-glucosidase etc.) are used to convert cellulose into sugars.101 Due to the specific role of each enzyme, for example while endoglucanase cleaves the internal bonds of cellulose, the exoglucanase and β-d-glucosidase give complete conversion of cellulose into sugars, a mixture of these enzymes is required (endoglucanases and exoglucanases for the hydrolysis of cellulose into cellobiose and β-1,4-glucosidases for the conversion of cellobiose to glucose). One major problem that affects the hydrolysis of cellulose is the inhibitory effect that cellobiose causes on β-1,4-glucosidases. This problem is pertinent with the cellulase derived from Trichoderma viride which has a high efficiency for cellulose hydrolysis to cellobiose. However, if the β-1,4-glucosidases are obtained from any other source then it can partially nullify this inhibitory effect. A few other studies have also shown that compared to ethanol, cellobiose is a potent inhibitor. Although, as discussed, several industrial processes have been developed based on enzymes, other drawbacks due to their homogeneous nature persist.102 To overcome these problems, researchers have worked on the immobilization of enzymes. In this method, several active enzymes are entrapped on an inert and solid material to improve their stability as well as separation ability. It is reported that the cellulase enzymes (celB and β-glucosidase) covalently immobilized on polystyrene or calcium alginate gel particles are reused several times in glucose formation from cellulose.103,104
Cellulose hydrolysis reactions are also catalysed by ionic liquids (ILs) with acidity. Since the role of ILs has been recognized in the pre-treatment of lignocelluloses (as discussed in Section 1.2.3) for quite a few years and their effect on breaking hydrogen bonds is well studied, it was thought that these ILs, if functionalized with acidic groups, may facilitate the hydrolysis of cellulose. Moreover, the ability of ILs to dissolve cellulose (even crystalline), which is a major problem in achieving higher yields of glucose with other catalytic systems, is also an interesting phenomenon that can help in observing higher glucose yields with ILs. Additionally, the advantage of ILs with distinctive properties such as high thermal stability, low vapour pressure, tuneable polarity etc. compared to molecular solvents can be tuned according to the requirements of the reaction. Considering this, first, ILs were used along with mineral acid (H2SO4) to achieve higher yields of glucose. In this way, ILs were responsible for dissolving crystalline cellulose and further hydrolysis was catalysed by H+ from mineral acid. Recently, studies were performed using cellulose +[C4mim]Cl solution along with H2SO4/cellulose with varied mass ratios for several minutes to hours to achieve ca. 50% glucose yield.105 It is also claimed that in the presence of cellulose and [BMIM]Cl, the rate of hydrolysis catalysed by maleic acid was enhanced. The catalytic activity of various acids in the presence of [BMIM]Cl was observed as follows:106 H3PO4<maleic acid<H2SO4<HNO3<HCl.
To prevail over the use of mineral acids, later acidic ILs [1-(1-propylsulfonic)-3-methylimidazolium chloride and 1-(1-butylsulfonic)-3-methylimidazolium chloride] with a –SO3H group attached via the propyl chain to the cation were shown to hydrolyse cellulose into sugars at 70 °C in water.107 Soon after, several research groups studied the effects of various types of acidic ILs on the hydrolysis of cellulose.108–110 However, like mineral acids, ILs also face drawbacks such as a homogeneous nature, recyclability and high costs, which make the overall process commercially unfavorable.111
Overall, the use of homogeneous catalysts in cellulose hydrolysis to yield glucose has long been known and has been practiced on an industrial scale with renewed interest. However, these methods face quite a few drawbacks as discussed. Realising this, researchers have recently started using heterogeneous catalysts which may surmount the problems (non-corrosive, easy recovery and reusability of the catalyst, elimination of neutralization process etc.) faced while using homogeneous catalysts. Moreover, knowing that nearly 80% of the current industrial processes use heterogeneous catalysts, it is interesting for researchers to generate knowledge on the conversion of saccharides into sugars.
As discussed, it will be beneficial to use heterogeneous (solid) catalysts in the hydrolysis of cellulose, because they can be easily separated from the reaction mixture via filtration and their acidity (strength) can be tuned as per the requisite of the reaction.8,112 Synthesis of glucose (C6 sugar) from various feedstocks (mostly from cellulose) using various heterogeneous catalysts can be subcategorised as (1) solid acid catalysed and (2) supported metal catalysed. The details on these processes are discussed next.
Solid acid catalysts defined as ‘solid materials having acidic properties because of the presence of either Brønsted acid sites or Lewis acid sites or both’ are useful in many reactions. Solid acid catalysts such as zeolites, ion-exchanged resins, functionalized mesoporous silicas, functionalized carbons, functionalized and supported metal oxides, heteropoly acids etc. (Figure 1.9) can influence the hydrolysis reaction of disaccharides (cellobiose, maltose and sucrose) and polysaccharides (cellulose, starch, and inulin) into sugar monomers (glucose and fructose). Solid acids have advantages over homogeneous acids (typically mineral acids) in terms of selectivity, stability, recovery and reusability. They can be used as a direct replacement to the homogeneous acids as their acid strengths can be manipulated for achieving optimum concentration of protons in the solution. They also have lower operating costs and avoid any corrosion of reactors as after removal of the catalyst from the solution by simple filtration, the solution becomes neutral. However, they have the drawback of diffusion limitations and thus can slow down the reaction compared to their homogeneous counterparts. Moreover, solid acids can easily be poisoned by water (by forming a solvation layer around acid sites by hydrogen bonding) or they degrade during the hydrothermal treatment involved under biomass conversion conditions and consequently, it becomes absolutely critical to design and develop solid acids that can tolerate water poisoning and are hydrothermally stable.
In the early studies, as an alternative to homogeneous acids, zeolites, a class of solid acid catalysts, were used in the conversion of various disaccharides (maltose, cellobiose and sucrose) and polysaccharides (inulin, starch and cellulose) to sugars (glucose and fructose) in a water medium. Zeolites are microporous, high surface area aluminosilicates with a framework consisting of AlO4 and SiO4 tetrahedra linked with each other via oxygen, are naturally available and have been known for over 200 years following the discovery of stilbite.113 Commercially synthesized zeolites with specific acidity, tuneable hydrophilicity, channel structure, textural property, varying porosity, high thermal stability and Si/Al ratio find much application in oil refining and petrochemistry. Their porous structure developed by 1D, 2D or 3D channel formation can accommodate various substrates depending on the type of zeolites and thus provide the sites for obtaining shape selective catalysis. In the early works, faujasite (HY, Si/Al=2.4, 27, 55, 110) zeolite was employed in the hydrolysis of sucrose and the fact was established that after dealumination (higher Si/Al ratio), the reaction rate enhances.114 Later, in another report, it was shown that the use of sucrose as a substrate in the presence of HY (Si/Al=15) zeolite yields glucose and fructose (invert sugars) at 85 °C.115 The HY (Si/Al=15) zeolite is also used as a catalyst for the hydrolysis of other disaccharides (maltose and cellobiose). When maltose and cellobiose are treated at 150 °C, nearly 90% conversion is achieved to yield ∼83% and ∼88% of glucose, respectively. It is suggested that the catalytic activity is dependent on the ease of adsorption of substrates via two oxygen atoms (ring oxygen and oxygen attached to anomeric carbon) on the catalyst surface. Thereafter, the substrate undergoes hydrolysis over acids sites. Moreover, hydrolysis of inulin and starch has also been reported with HY catalyst to yield 92% fructose and 95% glucose, respectively. The influence of the Si/Al ratio in Hβ is proved with the results that with an increase in Si/Al ratio, the Hβ zeolite becomes more hydrophobic, which promotes an easy maltose adsorption on the catalyst surface and thereby improves the catalytic activity.116 This phenomenon explains the observation of higher amounts (80%) of glucose with Hβ (Si/Al=50) compared to Hβ (Si/Al=12.5) (43%) under similar reaction conditions. The use of dealuminated H-form of mordenite (HMOR) (Si/Al=12) catalyst to produce 66% glucose from 70% maltose conversion at 130 °C is also known. Later, the use of zeolites (HMOR and Hβ) in the hydrolysis of starch was shown to yield 18% glucose.116 However, due to a lower hydrothermal stability, catalysts undergo deactivation via leaching of their active component in water. When cellulose is used as a substrate, due to its insolubility in most of the solvents, the reaction system becomes heterogeneous with respect to the substrate itself. This eventually affects the reaction rate of cellulose hydrolysis in the presence of solid acid catalysts. However, as discussed earlier in this chapter (Section 1.2), pre-treatment of cellulose reduces its crystallinity and helps in solubilizing part of it in hot water. It is shown that the ball milling of microcrystalline cellulose (Avicel) (crystallinity=75%) using ZrO2 balls (mass=1.8 kg, diameter=2 cm) for 48 h (spinning speed=60 rpm) completely diminishes the cellulose crystallinity leading to formation of amorphous cellulose as proven by XRD technique.117 Subsequently, this treated cellulose is hydrolysed over several zeolites such as HMOR (Si/Al=10), Hβ (Si/Al=12, 75) and HZSM-5 (Si/Al=45) at 150 °C. Among all zeolites, Hβ (Si/Al=75) showed the best catalytic activity (glucose yield=13%) due to its higher hydrophobicity compared to other zeolites. Next, to improve the yields with zeolites, reactions were conducted in IL medium as ILs can solubilize cellulose (Figure 1.5). It is seen that with the use of a [BMIM]Cl/H2O system along with HY catalyst, a ca. 50% glucose yield can be obtained at 130 °C within 2 h.118 However, when conventional heating is replaced with microwave heating, a ca. 37% glucose yield at 100 °C within just 0.13 h was observed.119 Although zeolites are active in these reactions, lower hydrothermal stability and leaching of Si and Al in the solution hamper their use. Thus, it is important to synthesize zeolites with higher thermal stability to help achieve recyclable activity in cellulose hydrolysis reactions. Recently, it has been proposed that to increase the stability of zeolites (by avoiding desilication) in hot water, the presence of Al in both forms, lattice and extra framework is helpful.120 It is suggested that lattice Al counteracts the hydrolysis of framework bonds and extra framework Al prevents the solubilization of the framework. Moreover, the authors claim that with a decrease in the Si/Al ratio, an increase in structure stability is possible because of the higher Al content. When H-form of ultra stable Y (HUSY) is subjected to mild steam treatment, the concentration of extra framework Al increases on the external surface of the zeolite, thereby giving it a protective cover, which as mentioned earlier, helps in reducing the solubility of the zeolite.
Typically, (cation) ion-exchanged resins with acidity due to the presence of –SO3H groups are used in many acid catalysed organic transformations. Though they are stable in various organic solvents, their hydrothermal stability is not very high (<150 °C). It is shown that with an increase in the degree of resin cross-linkages, the access of substrate molecules towards the catalyst active centre is more restricted, decreasing the catalytic activity.116 Use of 2% cross-linked resin (Dowex 50x2-100) can afford complete hydrolysis of maltose to yield ca. 95% glucose within 26 h at 120 °C while 8% cross-linked resin (Dowex 50x8-100) catalyst requires 130 °C for complete maltose hydrolysis.116 Amberlyst-15 resin has a very high acid amount (4.65 mmol g−1) compared to Nafion-silica resin (0.17 mmol g−1) and hence, Amberlyst-15 showed higher sucrose hydrolysis activity (conversion=88%, glucose yield=88%, fructose yield=87%) than Nafion-silica (conversion=28%, glucose yield=28%, fructose yield=26%) under identical reaction conditions (80 °C, 4 h).121 These findings are confirmed by another study carried out with Amberlyst-15 (78%), Nafion NR-50 (42%) and Nafion SAC-13 (29%) catalysts for the sucrose hydrolysis reaction at 80 °C.122 However, it should be considered that this activity can be manipulated by charging varying quantities of catalysts in the reaction. Some other reports also discuss the use of ion exchange resins such as Amberlite A120 and Amberlite 200 for the hydrolysis of sucrose to yield 82–98% sugars at 80 °C.123,124 When maltose is used as a substrate, Amberlite 200 resin gives 95% sugar formation at 80 °C. The same catalyst, however, shows lower activity in cellobiose conversion (19%) into sugars (15%).124 At the same time, with a slight increase in temperature to 90 °C and use of Amberlyst-15 catalyst, a maximum of 61% glucose formation with 62% cellobiose conversion was reported.125 Amberlyst-15 catalyst was also evaluated in starch hydrolysis at 130 °C giving 25% glucose along with 12% maltose.121 Although strongly acidic resin catalysts (Amberlyst-15, Nafion NR-50) showed high activity in the hydrolysis reactions of sucrose, maltose and starch, they showed low activity when used in the hydrolysis of microcrystalline cellulose.126 It is suggested that obtaining reproducible activity with resin catalysts in the cellulose hydrolysis reaction is difficult since this process requires temperatures higher than the degradation temperatures of these materials. To overcome these problems, use of ILs along with resins has been suggested by a few research groups. In one report, use of an acidic resin, NKC-9 along with [C4min]Cl ionic liquid, was shown to yield 39% glucose.119 A few other reports also claimed the formation of the hydrolysis product, glucose (35 and 83%), from cellulose using a combination of resin catalysts (Nafion-50 and Dowex 50wx8-100, respectively) and ILs ([BMIM]Cl and [EMIM]Cl, respectively).127,128
In the early 1990s, siliceous mesoporous silicas such as FSM-16 (Folded Sheet Mesoporous/Material), SBA-15 (Santa Barbara Amorphous) and MCM-41 (Mobil Corporation/Composite Mesoporous/Material) with large pore openings (>2 nm) were invented. The discovery of these materials paved the way to overcoming the diffusion limitations faced by bigger molecules to enter the smaller (micro, <2 nm) pores of zeolites.129–131 Though these materials had a 1D channel structure, subsequent developments in the synthesis of many other mesoporous materials with 2D and 3D channel structures have been successful. Characteristically siliceous materials do not have any acidity (or very weak acidity due to surface –OH groups), though by incorporation of heteroatoms such as Al, Ga etc., acid sites can be generated on these materials. Moreover, anchoring sulfonyl groups (−SO3H) on the surface via one-pot or grafting methods also gives rise to acidity in these materials. Because of a large pore diameter and acidity, these materials (MCM-41, SBA-15, FSM-16 and HMM (Hybrid Mesoporous Materials))132 are largely used in several acid catalysed reactions such as etherification, esterification etc. carried out in organic media.133,134 Nevertheless, when these catalysts are used in the presence of water, leaching of –SO3H groups is observed, which makes these catalysts unsuitable for hydrolysis or dehydration reactions. Considering these facts, researchers have used hot water treated (to remove loosely bound –SO3H groups) sulfonic acid functionalized phenylene-bridged mesoporous silica (Ph-HMM) and ethylene-bridged mesoporous silica material (Et-HMM) catalysts in the sucrose reaction.121 These materials with an acid amount of 0.31–0.90 mmol g−1 showed very high catalytic activity for sucrose conversion (81–90%) into glucose and fructose (81–90%) at 80 °C. Moreover, sulfonic acid functionalized FSM-16 (acid amount=1.11 mmol g−1) synthesized by a post-synthesis grafting method was also used to produce glucose and fructose from sucrose.121 Later, these catalysts were employed for the hydrolysis of starch at 130 °C to obtain ∼67% glucose. Another report compared the activities of silica materials after surface functionalizations with various acid groups such as butylcarboxylic, propylsulfonic and arenesulfonic acids in cellobiose hydrolysis.135 It was shown that both propylsulfonic and arenesulfonic acid functionalized silica materials are able to convert >90% of cellobiose at 175 °C, while butylcarboxylic acid functionalized silica exhibited only 15% cellobiose conversion. The ambiguity in results is because of fewer acid sites generated in water for butylcarboxylic acid functionalized silica (pH=4.90) compared to the other two silica materials (pH=2.67–2.89).
This means that pH below 4.0 is mandatory for hydrolysis of cellobiose. Moreover, an almost similar activation energy (110–138 kJ mol−1) for the cellobiose hydrolysis reaction was reported in the presence of all silica catalysts135 and eventually, the values matched well with those reported earlier for H2SO4 (110±29.6 kJ mol−1) and maleic acid (114±9.3 kJ mol−1).136 The good correlation of activation energy values between silica catalysts and homogeneous catalysts revealed that mass transfer limitation is not playing any significant role and the reactions are guided only by hydrated protons. These results are interesting and subsequent studies showed that dispersion of solid acids in water eventually forms H3O+ species in bulk water, which are actually active in the hydrolysis reaction as explained in Figure 1.10.137 These facts elucidate the capability of many solid acid catalysts with a pore diameter <2 nm to hydrolyse bigger molecules such as cellulose or hemicelluloses. Studies have also confirmed that at 175 °C, glucose undergoes only 15% conversion, implying that at lower temperatures, glucose is stable.135
For numerous years, inexpensively available carbon materials (activated carbon, char, graphite, nanotubes etc.) have attracted a lot of attention as adsorbents, catalysts or supports due to their large specific surface area, high porosity, excellent electron conductivity, relative chemical inertness etc.138 These excellent materials with the incorporation of acidic functionality can enhance their performance in several acid catalysed reactions.139,140 The acidity can be incorporated in these materials by washing the carbon with H2SO4 or grafting sulfonic acid groups on its surface. Additionally, activated carbons may have –OH and –CO2H groups on their surface, which can give rise to weak acidity. Sulfonated activated carbon (AC-SO3H) prepared by treatment of AC with concentrated H2SO4 at 150 °C under an Ar gas flow generates high acidity (1.63 mmol g−1), which is responsible for achieving a ∼90% glucose yield in a starch hydrolysis reaction carried out at 120 °C.117 Soon after, various functional groups such as −SO3H, −CO2H and −OH bearing amorphous carbon materials (CH0.64O0.49S0.032) were synthesized by sulfonation (fuming H2SO4, 120 °C, 10 h, N2 gas flow) of carbon obtained from partial carbonization of cellulose (400 °C, 1 h, N2 gas flow).125 During cellobiose hydrolysis, it was suggested that the −OH groups present on the carbon surface form strong H-bonds with cellobiose glycosidic oxygen, allowing it to adsorb on the carbon surface. Subsequently, the −SO3H (acid density=1.5 mmol g−1) group on the carbon surface catalyses the hydrolysis of this anchored cellobiose easily to yield a high amount of glucose (81–83%) at 90 °C. A similar influence of the catalyst surface property is also valid in the hydrolysis of cellohexaose to yield glucose.125 Due to very minor leaching of −SO3H groups from this catalyst, it showed recyclable activity in five runs. Another report showed that partial carbonization of polyvinyl chloride (PVC) at 400 °C in a N2 atmosphere and its subsequent sulfonation using fuming H2SO4 produced a carbon material (PVC-AC-673) with −SO3H group bonded carbon sheets linked via rigid sp2 bonds and flexible aliphatic hydrocarbons linked via sp3 bonds.141 It was proposed that the additional flexible aliphatic hydrocarbons along with planar carbon sheets overcome the diffusion limitations of reactant molecules and hence enhance its catalytic activity to yield 30% glucose from cellobiose. Later, AC-SO3H catalyst was checked in a ball-milled cellulose hydrolysis reaction to yield superior amounts of glucose (41%).117 The better catalytic activity of AC-SO3H is ascribed to the fact that this catalyst has higher hydrophobic graphene planes and strong acidic −SO3H surface functionalization. Furthermore, AC-SO3H catalyst is also shown to be recyclable in three runs without losing its activity. Amorphous carbon bearing −SO3H, −CO2H and −OH groups (CH0.62O0.54S0.05) is known for the successful hydrolysis of microcrystalline cellulose into 64% oligomers (β-1,4-glucan; DP=2–4) and 4% glucose at 100 °C.126 Moreover, the carbon catalyst is shown to be recyclable 25 times with similar activity despite leaching of −SO3H groups after the first run. It is suggested that the better activity of this carbon catalyst is due to the strong adsorption of the substrate on the catalyst surface, which also reduces the activation energy barrier (110 kJ mol−1) for the cellulose hydrolysis reaction compared to H2SO4 (170 kJ mol−1).126,142 Later, from various mathematical and statistical calculations (artificial neural network (ANN) model and response surface methodology (RSM)), authors tried to understand the correlation between reaction physical phenomenons.143 It is concluded that in the hydrolysis of cellulose, the amorphous carbon catalyst follows the same pathway that concentrated H2SO4 follows. One report suggests that due to the presence of aromatic rings in mesoporous carbon nanoparticles (MCN), it forms strong CH–π interactions with hydrogens of glucans (glucose polysaccharide), because of which higher adsorption of glucans on these materials is possible.144 This observation was confirmed by GPC, 13C Bloch Decay NMR and MALDI-TOF-MS analysis. Moreover, it was found that with an increase in glucan chain length, the adsorption free energy decreases (ca. 1.67 kJ mol−1 with each glucose unit in glucan series) due to a higher degree of CH–π interactions. Three sulfonated carbon catalysts with −SO3H, −CO2H and −OH groups were synthesized from bamboo (BC-SO3H-1), cotton (BC-SO3H-2) and starch (BC-SO3H-3) and were used as catalysts in a microcrystalline cellulose (DP=200–1000) hydrolysis reaction under microwave heating.145 Because of the use of a microwave power of 350 W (cellulose crystallinity reduction from 75% to 58%) in the presence of BC-SO3H-1 catalyst, the highest amount (24%) of reducing sugar formation (glucose=17%, cellooligomers=7%) was observed. In contrast to this, only 5% reducing sugar (glucose=3%, cellooligomers=2%) formation was observed under conventional heating (oil bath) at 90 °C. Other carbon catalysts (BC-SO3H-2 and BC-SO3H-3) afforded 28% (glucose=20%, cellooligomers=8%) and 13% (glucose=5%, cellooligomers=8%) of reducing sugars, respectively. Leaching of −SO3H groups in the solution (fresh=1.87 mmol g−1, spent=1.12 mmol g−1) was, however, observed for the BC-SO3H-1 catalyst, which led to a decrease in the catalytic activity in recycle runs (reducing sugar yields; 1st run: 24%, 2nd run: 18%, 3rd run: 16%, 4th run: 15%). Leaching is a major problem with any –SO3H functionalized catalyst and care should be taken to confirm that the activity observed is due to anchored –SO3H groups and not because of leached out –SO3H groups. The influence of sulfonation temperature during the synthesis of sulfonated activated carbon was also studied and it was shown that compared to 150, 200, 280 and 300 °C, 250 °C treatment gave the best yields of glucose (63%) from ball milled cellulose.146 Moreover, the influence of various sources of sulfonated carbons (sulfonation temperature=250 °C), such as acetylene carbon black (ACB), multi-wall carbon nanotube (MWCNT), cell-carbon (carbonization of microcrystalline cellulose), coconut shell active carbon (CSAC), resin carbon (carbonization of mesoporous resin) and mesoporous carbon (CMK-3), was also investigated.146 It was found that the sulfonated CMK-3 catalyst was highly active due to its mesoporous nature and yielded the highest amount of glucose (75%). However, all other sulfonated carbons showed lower activity (glucose yield=15–55%) under similar conditions. Execution of both a strong Brønsted acid site presence and favourable glucan adsorption on the catalyst was explored by using sulfonated silica/carbon nanocomposite catalyst.147 The composite catalyst with a particular composition for Si/C (33 wt%/66 wt%) and particular carbonization temperature (600 °C) showed the best catalytic activity for ball milled cellulose hydrolysis (conversion=61%, glucose yield=50%) compared to other catalysts. This higher catalytic activity is explained by the presence of strong Brønsted acidic sites in the composite catalyst and its ability to adsorb glucan strongly. It is suggested that the silica may provide better mechanical and thermal stability to the catalyst. Nevertheless, when the composite catalyst was reused, it showed a slow but continuous decrease in activity due to leaching of −SO3H groups. From the discussions on carbon catalysts, it is evident that, to achieve higher activity, few hydrophilic groups on the surface are necessary to attract saccharides otherwise they will be repelled by the hydrophobic surface of carbon.
Metal oxides characteristically give Lewis acid sites and due to their higher thermal stability, they are widely used in several organic transformations and in sugar synthesis. With a layered HNbMoO6 catalyst (1.9 mmol g−1) at 80 °C, almost complete conversion of sucrose is observed to achieve selectively glucose and fructose as products. With a HTiNbO5 nanosheet (0.4 mmol g−1) catalyst, only 42% yield was seen, however.122 The very high activity observed with the layered HNbMoO6 catalyst is explained by the strong interactions between sucrose and the interlayer of HNbMoO6 (intercalation; proven by XRD and elemental analysis). However, with the same catalyst, the rate of glucose formation decreased when cellobiose was used as a substrate (1.18 mmol g h−1) instead of sucrose (24.1 mmol g h−1). To understand this difference, the authors studied the intercalation ability of both substrates in the presence of HNbMoO6 catalyst. The results displayed higher substrate adsorption in the case of sucrose (1.12 mol%) compared to cellobiose (0.21 mol%) per mole of catalyst. This difference may arise from the orientation of sugars in these disaccharides. In starch hydrolysis, HNbMoO6 catalyst yielded a maximum of 45% glucose at 100 °C. Cellulose hydrolysis was also performed using layered HNbMoO6 catalyst, however very low yields of glucose were observed.122 Catalytic activity with Nb-W oxide in the hydrolysis of sucrose and cellobiose was also reported.148 With an increase in W content in the catalyst, the formation of strongly acidic Nb3W7 oxide species was observed. This in turn was responsible for improvements in the glucose yields (65%) from sucrose. However, due to the presence of low acid density in the Nb-W catalyst, it showed lower cellobiose hydrolysis activity. Several other metal oxides such as SiO2–Al2O3, SiO2–ZrO2, Nb2O5 and Nb2O5–PO4 have also been evaluated for their activity in sucrose hydrolysis and among them, Nb2O5–PO4 yielded the highest amount (62%) of glucose.124 A bare nano Zn–Ca–Fe oxide catalyst was also shown to be efficient in the hydrolysis of crystalline cellulose to yield 29% glucose.149 Moreover, due to the paramagnetic nature of the Fe oxide present in the catalyst, it can be easily separated from a reaction mixture. Use of −SO3H functionalized mixed metal oxide (CoFe2O4–SiO2) catalyst was demonstrated in the hydrolysis of cellobiose to yield 50% glucose at 175 °C.150 In ball milled cellulose hydrolysis, a sulfonated zirconia (SO42−/ZrO2) catalyst was applied but with limited activity (14% glucose).117 However, the leaching of SO42− in water is very well known and hence these catalysts are not recyclable and thus were not evaluated further.
Heteropoly acids (HPA) or polyoxometalates are another class of acid catalysts that have the composition of various oxoacids (transition metal–oxygen anion clusters).151–153 Structurally, HPAs are of two types, namely Keggin and Dawson with molecular formulae of XYxM(12−x)O40 and X2M18O62, respectively (where X is a heteroatom from the p-block and Y and M are addendum atoms). For example, H3PW12O40 and H5PMo10V2O40 have Keggin structures and H6P2Mo18O62 has a Dawson structure. The protonated forms of HPAs are homogeneous in nature, however, partial replacement of protons by larger ions such as Cs+, Ag+etc. makes HPAs heterogeneous. These catalysts are used in the cellobiose hydrolysis reaction and their activity is shown to be dependent on their Brønsted acid strength.94 Homogeneous HPA and metal ion incorporated phosphotungstic acids (PW12O403−) are also used as catalysts in cellulose (ball milled) hydrolysis reactions to understand the influence of Brønsted and Lewis acid sites.94 From the results obtained with all catalysts, the authors have concluded that stronger Brønsted acid catalysts are more favourable for hydrolysis of β-1,4-glycosidic bonds compared to Lewis acid catalysts. The difference between the activity of Brønsted and Lewis acid sites is because, as mentioned in Figure 1.10, Brønsted acid sites can liberate H+ to give rise to H3O+ which then is available in bulk water. In this way, it can easily interact with the substrate molecule to give rise to protonated oxygen. In the case of Lewis acid sites, the substrate has to interact with them through a lone pair on oxygen (bridged) and only then can the reaction occur. Use of HPA, H3PW12O40 and Sn0.75PW12O40 showed ∼40% formation of total reducing sugars from ball milled cellulose.94 In another report, use of H3PW12O40 as a catalyst for the conversion of microcrystalline cellulose was also shown to produce a maximum of 52% glucose at 180 °C.95 It was discussed that although the H3PW12O40 catalyst is homogeneous in nature, it could be recycled six times after the catalyst was extracted with diethyl ether. However, leaching of Keggin-type PW12O403− ions was seen in the reaction solution as proven by UV-Vis analysis (absorption bands at 200 nm and 265 nm). In another report, micellar HPA, [C16H33N(CH3)3]H2PW12O40 and Cs2.5H0.5PW12O40 catalysts were studied for the hydrolysis of starch and cellulose.154 Higher amounts of glucose formation were demonstrated using micellar HPA catalyst (starch: 82%, cellulose: 39%) compared to Cs2.5H0.5PW12O40 (starch: 43%, cellulose: 21%) since the micellar HPA catalyst attracts cellulose to accumulate around the micellar core, which gives cellulose easy access to catalytically active sites.
Although various methodologies have been developed for the hydrolysis of isolated cellulose into glucose in the presence of several solid acid catalysts, it would obviously be beneficial if a methodology was developed wherein sugars are obtained directly from raw biomass without any pre-treatment (without isolating substrates). This will help to reduce the cost of sugar synthesis (by virtue of reducing capital costs for the isolation process) and will also avoid generation of wastes (generated during isolation). Very few processes for the direct hydrolysis of polysaccharides from lignocelluloses into sugars using heterogeneous catalysts have been reported in the literature. It was demonstrated earlier that carbon catalysts bearing −SO3H, −CO2H and –OH groups have better catalytic influence on the hydrolysis of isolated polysaccharides via strong adsorption capability.126 As an extension to the work, the authors used dried Eucalyptus flakes as a substrate for hydrolysis of its cellulose part along with its hemicellulose part into water soluble saccharides.126 The authors claimed that all the cellulosic material was hydrolysed into water soluble saccharides at 100 °C but a lack of detailed quantitative information on the products’ formation is detrimental to studying this catalyst further. However, from earlier results, it can be assumed that most of the products are water soluble oligomers or polymers. Another research group showed the use of recyclable carbonaceous solid acid (CSA) with −SO3H, −CO2H and –OH groups in the hydrolysis of cellulose and hemicellulose from corn cob to yield C6 and C5 sugars.155
The discussion on solid acid catalysts reveals that it is possible to hydrolyse several di- and polysaccharides into sugars, although it is essential to develop a catalyst that is stable and recyclable even if it gives lower activity. Moreover, poisoning of solid acid catalysts (for example −SO3−H+ → −SO3−K+; −O–Si–O(H)–Al–O– → –O–Si–O(K)–Al–O–) with the impurities present in lignocelluloses (nutrients, waxes) may reduce their activity in recycle runs. To use these catalysts again, treatment of these catalysts would be necessary, which may add the cost to the process and moreover, during these treatments, the catalysts may undergo structural changes by which initial activity may not be retained.
As early as 1957, pioneering work was done by Balandin and co-workers for the one-pot synthesis of sugar alcohols (sorbitol) from cellulosic biomass with the help of dilute acid and Ru/C catalyst.156 The studies were carried out with 1–2 wt% H2SO4 and Ru/C at 70 bar hydrogen pressure to yield ca. 80% of sorbitol. Later, the same group replaced H2SO4 with 0.7 molar phosphoric acid to increase the yield to ca. 90%.157 The increase in yield with phosphoric acid is due to suppression of strong acid (H2SO4) catalysed cyclodehydration reactions of sorbitol to give sorbitan.158 In these studies, homogeneous acids were used to hydrolyse cellulose into glucose and further, its hydrogenation was catalysed by Ru/C catalyst. In 2006, a new method was disclosed to convert cellulose into sugar alcohols (via glucose formation) by avoiding the use of homogeneous acids.159 In the report, it is suggested that by using supported metal catalysts (Pt/Al2O3) under hydrogen pressure and in the presence of water, hetero cleavage of hydrogen is possible. The H+ formed during the process acts as a proton source which helps in the hydrolysis of cellulose to yield glucose. However, under the reaction conditions, it was seen that glucose immediately undergoes a hydrogenation reaction to yield sugar alcohols (sorbitol, mannitol). Around the same time, another group also showed the conversion of cellobiose into sugar alcohols over supported metal catalysts and proposed a similar mechanism.160 These results also point out that when homogeneous acids and Ru/C catalysts were used in early studies, it is possible that Ru/C was also capable of catalysing hydrolysis reactions. Subsequently, several other reports claimed in situ acid site generation over various metals (Pt, Ru, Ni) for the conversion of cellulose.161,162 It is believed that the support material also has an influence on acid site generation. When a carbon nanotube (CNT) was used to support Ru metal, it enhanced the amount of acid sites and thereby catalytic activity since CNT has a higher ability for hydrogen adsorption.163 Based on the concept of in situ acid site generation, recently, it was shown that a Pt/fibrous 3D carbon catalyst can convert lignocelluloses directly into sugar alcohols via a hydrolytic hydrogenation reaction.164 Though supported metal catalysts are probed further in the conversion of cellulose, the main product formed is sugar alcohol. This is because hydrolysis of cellulose is a rate-determining step and requires harsher reaction conditions than the hydrogenation of sugars to yield sugar alcohols. This means that, independent of the type of supported metal catalyst, used along with hydrogen, they will surely yield hydrogenated products of sugars.
Four C5 sugars, namely xylose, arabinose, ribose and lyxose, are present in nature. Invented in 1891, d-ribose is a C-2 epimer of d-arabinose and is present in the backbone of RNA. On the other hand, lyxose is a C-2 epimer of xylose and is available as a component of bacterial glycolipids. Since d-ribose and lyxose are very rarely available in nature and also are not present in plant biomass, discussions on these sugars are not made in this chapter. As shown in Figure 1.2, the abundant sugars, xylose and arabinose can be produced from the hemicellulose part of lignocelluloses. Hemicelluloses are polysaccharides with low molecular weights and are covalently or non-covalently bonded with cellulose and lignin in plant cell walls. They are heteroglycans and are mostly made up of d-xylose, d-mannose, d-galactose, l-arabinose, d-glucose, d-galacturonic acid, glucuronic acid and 4-O-methylglucuronic acid etc.165,166 At times, these sugars also have substituents such as acetyl and methyl.167 Depending on the various sugars linked via different fashions with each other, hemicelluloses are named. For example, hemicelluloses made up of xylose and arabinose are termed as arabinoxylan. Based on the origin of hemicelluloses, either from hardwood or softwood or grasses, differences in their composition and linkages are observed and those are summarized in Table 1.1.
|Arabinogalactan||1,3-, 1,6-, 1,4-, 1,5-||Arabinose, galactose, glucuronic acid|
|Galactoglucomannan||1,4-, 1,6-||Galactose, mannose, glucose, acetyl substitution|
|Arabino-4-O-methylglucuronoxylan||1,4-, 1,2-, 1,3-||Arabinose, xylose, glucuronic acid, methyl substitution|
|Arabinoxylan/ arabinoglucuronoxylan||1,4-, 1,3-, 1,2-, 1,2/3-||Xylose, arabinose, glucuronic acid, other substitution (acetyl, methyl, feruloyl, coumaroyl)|
|O-Acetyl-4-O-methyl-glucuronoxylan||1,4-, 1,2-||Xylose, glucuronic acid, other substitution (acetyl, methyl)|
Typically, hardwood hemicelluloses and grasses contain a high degree of xylans (main backbone is made up of xylose units with branching after 7–10 units). Based on the substituent present on xylose, their names are derived as arabinoxylan and arabinoglucoronoxylan. On the contrary, softwood hemicelluloses are largely galactoglucomannans but along with them, some small portion of xylan is also present. Due to the random and branched structure of hemicelluloses, they do not have any extensive hydrogen bonding (like in cellulose), which makes hemicelluloses amorphous in nature. These amorphous hemicelluloses during isolation from lignocellulosic materials undergo various transformations such as partial hydrolysis (due to high temperatures and pressures) to yield low molecular weight fractions and saponification during alkali treatment. Hence, upon hydrolysis of isolated hemicelluloses, besides the expected products, a few other products might also be detected although on a ppm level. Moreover, due to their varying compositions and linkages, it becomes crucial to develop a universal catalytic method that can hydrolyse various types of hemicelluloses either present in isolated form or present in lignocelluloses (without separation from cellulose and lignin) in an efficient way. It is important to mention that oligosaccharides (2<DP<7) of hemicelluloses (obtained upon hydrolysis) are termed as, “functional food” or “dietary fibres” and added in food to exert health benefits.
Similar to cellulose, for the hydrolysis of hemicellulose, several catalytic systems such as homogeneous and heterogeneous ones were investigated. However, unlike several C6 sugar based disaccharides, which are naturally and commercially available in abundance, almost no C5 sugars containing disaccharides are available and hence in the next section, the focus is only on hemicelluloses.
For the hydrolysis of hemicelluloses, several mineral acids (diluted and concentrated HCl, H2SO4 and H3PO4) and organic acids (oxalic acid, acetic acid, maleic acid, trifluoroacetic acid) are known and details on these are extensively described in a few excellent reviews.12,101,168 Typically, organic acids with lower acid strength (pKa, 3.0–5.0) compared to mineral acids (pKa <1) are sufficient to hydrolyse hemicelluloses into their respective sugars since hemicelluloses are amorphous, branched and have lower DP (<250) compared to cellulose. It has been proved that with the higher strength of mineral acids, formation of degraded products (via sugars) is possible.169 This phenomenon is observed because different linkages in hemicelluloses cleave under different conditions and in most cases, sugars obtained under mild conditions are degraded under the harsher conditions used to cleave the remaining hemicelluloses. Use of trifluoroacetic acid, even if it is somewhat strong, is useful in these reactions since it has a low boiling point (72.4 °C) and thus can be removed from the reaction mixture easily. This also avoids the neutralization required with other acids, which is a major drawback for the mineral acid catalysed method.169 It was also found that, depending upon the source of hemicellulose and the linkages present in the hemicellulose, the activation energy for its hydrolysis to form sugar varies (50–199 kJ mol−1) in the presence of mineral acids (HCl and H2SO4).12 Moreover, studies were also done to check the ease with which various substrates undergo hydrolysis using mineral acids and the following trend was observed:170 arabinoside>xyloside>galactoside>mannoside>glucoside.
Studies on the acid hydrolysis of hemicelluloses (arabinogalactan) were also done and it was established that with an increase in acid concentration (pH) and temperature, the hydrolysis rate increased.171 It was shown that at pH 1 and at 90 °C, complete hydrolysis of arabinogalactan is possible without the formation of any degradation products. A first-order kinetic model inclusive of two parallel reactions for obtaining arabinose and galactose was proposed based on modelling and experimental data.
For the complete hydrolysis of hemicelluloses, a variety of enzymes (hemicellulase: xylanases, arabinofuranosidases, ferulic and coumaric acid esterases, acetylxylan esterases, acetylmannan esterases, acetylgalactan esterases, glucuronidases, xylosidases etc.) are required and details on the enzymes and their modes of action are summarized in Table 1.2.101 Basically, enzymes are classified into two catergories, endo (intracellular) and exo (extracellular)—and both have different roles in the hydrolysis. Because of the complexity of the structure of hemicelluloses, it is essential to use several isoenzymic forms of xylanase for its extensive hydrolysis. Due to the very selective actions of enzymes, many xylanases are not capable of cleaving glycosidic bonds between xylans with substituent(s). However, to overcome this, it is mandatory to use other enzymes that can cleave the substituents on the xylans first and then xylanase can separate various sugars from these treated xylans. Additionally, as discussed for cellobiose/cellulose hydrolysis, in hemicelluloses also, sometimes cleaving a bond between only two xylose units becomes difficult (due to an inhibitory effect). Particularly, the action of enzymes on hemicelluloses in the paper and pulp industry is very important as it can give a bleaching effect without the use of a harmful chlorine treatment.
|Entry no.||Enzyme||Mode of action|
|1||Xylanase||Break down β-1,4 linkages in hemicellulose from non-reducing end and preferably at a region of no-substitution|
|2||Arabinanase||Break down α-1,5 linkages in hemicellulose|
|3||Arabinosidase||Break down α-1,2-, α-1,3- and β-linkages in hemicellulose|
|4||Ferulic and coumaric acid esterase||Break down arabinose linked ferulic and coumaric acids from cereal arabinoxylan|
|5||Acetylxylan esterase, acetylmannan esterase, acetylgalactan esterase||Remove acetyl substitution form xylan, galactoglucomannan, arabinogalactan|
|6||Glucuronidase||Break down α-1,2 linked glucopyranosuric acid or 4-O-methylglucopyranosuric acid side chain substitution from hemicellulose|
|7||Xylosidase||Break down of β-1,4 linkages in hemicellulose from non-reducing end to release xylose|
The rate of acid hydrolysis of hemicellulose is seen to be dependent on the structure (conformation) of anhydrosugar present in the hemicellulose, i.e. pyranose or furanose form and α- anomeric or β-anomeric form.12,170 Due to the higher ring strain in the furanose form of sugar compared to the pyranose form, the hemicellulose with the furanose form of anhydrosugars undergoes an easy hydrolysis reaction. Moreover, hemicellulose with anhydrosugars in the β-anomeric form undergoes a faster hydrolysis reaction (relative rate for β-anomer of d-xyloside: 9.1) than hemicellulose with the α-anomeric form (relative rate for α-anomer of d-xyloside: 4.5).
The superiority of heterogeneous catalysts over their homogeneous counterparts is discussed earlier (Section 1.3.2) in this chapter, however, the selectivity obtained with enzymes is very difficult to achieve with any other catalytic systems. This is particularly important in the case of hemicelluloses since in this substrate, various linkages are present that require varying conditions to cleave them. Subsequently, there is a possibility that sugars extracted at milder conditions can degrade under harsher conditions. Heterogeneous catalysts with acidic functionality such as zeolites, metal oxides, HPAs, carbons, etc., which are known in cellulose hydrolysis, are also used for the hydrolysis of hemicelluloses.
Recently, a one-pot method for the hydrolysis of isolated hemicellulose derived from softwood (oat spelt, xylan) and hardwood (birchwood, xylan) in the presence of various solid acid catalysts was shown.172 Typically, oat spelt derived hemicellulose has a composition of xylose ≥70%, arabinose=10% and glucose=15%. However, birchwood derived hemicellulose with a higher degree of polymerization than oat spelt is made up of >90% xylose. Hence, it is expected that the concentration of the products formed from these two substrates will differ slightly. The authors have studied the effects of several acid catalysts, such as zeolites (HUSY (Si/Al=15), Hβ (Si/Al=19), HMOR (Si/Al=10)), clay (K10), metal oxides (γ-Al2O3, Nb2O5, SO42−/ZrO2), mesoporous materials (Al-MCM-41 (Si/Al=50), Al-SBA-15 (Si/Al=100)) and HPA (Cs2.5H0.5PW12O40), in the reactions.172 Ultimately, it was found that the HUSY catalyst showed the highest amount of C5 sugar (xylose and arabinose) formation (41% yields) from oat spelt derived hemicellulose in water at 170 °C. In these reactions, the presence of oligomers (disaccharide, trisaccharide, tetrasaccharide and pentasaccharide) was confirmed with the help of a LC-MS technique and, based on these observations, the hydrolysis reaction pathway was suggested with hemicellulose (polymer) converting into soluble and detectable oligomers in a stepwise reaction (→ pentasaccharide → tetrasaccharide → trisaccharide → disaccharide →) to yield C5 sugars. Yet, there is always a possibility that tetrasaccharide can cleave in two possible ways: (1) into one mole of trisaccharide and one mole of monosaccharide or (2) two moles of disaccharide. However, due to the very fast reaction rate observed by the authors, it is very difficult to reveal the exact nature of the cleavage. Hence, further studies are required to check the actual pathway of this reaction. Although higher yields are observed with the HUSY catalyst, further characterization of the spent catalyst showed that it undergoes structural changes and hence loses its activity over time. Another research group showed the possibility of the hydrolysis of arabinogalactan with galactose : arabinose : glucoronic acid=5 : 1 : 0.08 and a molar mass of 20 000–100 000 g mol−1 derived from larch wood in a one-pot method.173 Two solid acid catalysts viz. Smopex-101 (fibrous, non-porous catalyst; sulfonic acid functionalized polyethene-graft-polystyrene) and Amberlyst-15 (macroporous resin; sulfonic acid functionalized styrene-divinyl benzene) were used by the authors for the hydrolysis of arabinogalactan. It was seen that Smopex-101 (86% yield for arabinose) gave better activity than Amberlyst-15 (50% yield for arabinose) at 90 °C. This result is interesting since Amberlyst-15 has a higher acid amount (4.7 mmol g−1) than Smopex-101 (3.6 mmol g−1) but the activity was reversed. This observation was explained based on the catalyst structure and diffusion limitations. Due to the fibrous nature of Smopex-101, all the sulfonic acid groups distributed on the catalyst surface became easily accessible to the substrate but, due to presence of acid groups in the pores of Amberlyst-15, the same was not possible with the latter catalyst. Subsequently, an enhancement in arabinose formation (95%) was also achieved by increasing the reaction time to 36 h with Smopex-101. Lower yields of galactose compared with arabinose are possibly due to an easy cleavage of branched arabinose linked via β-1,3 linkages on the galactose backbone. The ease of hydrolysis is also dependent on the source from which arabinogalactan is isolated since in the case of larchwood (which was used in the study), the percentage of branching is very high compared with woody plants and the coffee bean. In soybean seed derived arabinogalactan, branching is at the β-1,3-, α-1,4-, and α-1,5- linkages instead of the β-1,3- or β-1,6- linkages typically observed in other arabinogalactans.166 This difference in branching may affect the catalyst activity in hydrolysis reactions as they may pose the problem of steric hindrance. In another study, a sulfonated biochar catalyst, prepared by pyrolysis of wood chips at 400 °C for 1 h in a nitrogen flow, followed by sulfonation using sulfuric acid, was also used for the hydrolysis of softwood (derived from locust bean gum) and hardwood hemicelluloses (derived from birchwood).174 It was reported that hemicellulose conversion can reach up to 90% in the presence of the sulfonated biochar catalyst at 120 °C while, under similar reaction conditions, sulfonated activated carbon showed lower catalytic activity (hemicellulose conversion=50%). This is due to the fact that sulfonated activated carbon has fewer strong acid sites (2.59 mmol g−1) and lower xylan adsorption capacity (∼145 mg xylan per g catalyst at 23 °C) than sulfonated biochar (acid density=5.65 mmol g−1, xylan adsorption capacity ∼390 mg xylan per g catalyst at 23 °C). However, the sulfonated biochar catalyst exhibited a drastic reduction in hemicellulose conversion activity (1st run: ∼80%, 2nd run: ∼40%, 3rd run: ∼10%, 4th run: 0%) in recycle runs due to leaching of sulfonic acid groups (acid density reduction=3.66 to 0.0 mmol g−1) and its structure deformation (surface area reduction=365 to 5.3 m2 g−1, pore volume reduction=0.20 to 0.004 cc g−1, pore radius reduction=10.5 to 0.0 Å). Various resins (Amberlyst-70, Amberlyst-35, D5081 and D5082), sulfonic acid functionalized silica gel and zeolites (HZSM-5; Si/Al=50 and 80, H-faujasite; Si/Al=5.1 and H-ferrierite; Si/Al=55) were tested in the hydrolysis of beechwood derived hemicellulose (ca. 75% xylose, 2–5% glucose) to achieve C5 sugars (xylose and arabinose) as products.175 Amongst all these catalysts, the resins showed a better activity (55–80% of C5 sugars) at 120 °C because of a higher acid amount. The authors also studied the role of acid density on the catalytic activity and found a direct correlation between activity and acid density. Nonetheless, as discussed earlier for cellulose hydrolysis (Section 184.108.40.206), the resins lost their activity after the first use due to leaching of –SO3H groups. Among the various zeolite catalysts, H-ferrierite offered the best activity for the formation of the highest amount of C5 sugars (41%) at 140 °C, due to its higher acid strength compared to other zeolites.175 Moreover, the catalyst showed recyclable activity up to four runs with a marginal decrease in yields. This indicates that to achieve a recyclable higher activity, it is necessary to have strong acid sites on the catalyst. The use of various acid silicoaluminophosphate (SAPO) catalysts was shown in the hydrolysis of hemicellulose.137,176 Although the final product in the reaction is furfural, which can be obtained directly from hemicellulose via a hydrolysis–dehydration reaction, the results suggested that SAPO catalysts can only be used for the hydrolysis of hemicellulose to produce sugars if the reaction conditions are optimized. This catalyst is interesting to study further since it is stable under the reaction conditions and showed reproducible activity for eight runs at least. This high stability may arise due to the presence of phosphorus in the catalyst, which is responsible for making the catalyst more hydrophilic compared to zeolites. This hydrophilicity is also responsible for better catalyst (active site) and substrate interactions. Supported metal oxide catalysts might also be promising for sugar synthesis from hemicelluloses since they were proven as effective catalysts for hydrolysis–dehydration reactions of hemicellulose.177 The hemicellulose extracted from Miscanthus grass was also used as a substrate for the synthesis of sugars.178 The extracted solution consists of hemicellulose with a molecular weight of 2008 g mol−1 (analysed by GPC), DP of 15, polymer chain length of 7 nm and chain radius of 2 nm (calculated from molecular dynamics simulations). It was demonstrated that hydrothermally treated sulfonated mesoporous carbon nanoparticles (HT5-HSO3-MCN) with a large number of weakly acidic phenolic –OH groups helped in the adsorption of hemicellulose on the catalyst surface effectively via H-bond formation. This in turn was responsible for achieving better catalytic activity. A very high amount of xylose formation (74%) was reported from extracted hemicellulose solution at a buffered pH of 3.9 (using sodium acetate buffer solution) at 150 °C. Moreover, the HT5-HSO3-MCN catalyst was shown to provide a similar activity in three recycle runs after washing (stepwise washing with 0.1 M NaOH, 1 : 1 (v/v) ethanol and water, only ethanol, only water, 4 M HCl, only water) and drying. The complicated washing and drying procedures may hamper their further use in these reactions. It is thus necessary to devise a simple recycling process.
Additionally, some of the developed processes confirm the use of lignocelluloses directly for the synthesis of C5 sugars in the presence of heterogeneous catalysts. In one report, the selective conversion of the hemicellulose part from bagasse into C5 sugars (xylose and arabinose) over HUSY (Si/Al=15) catalyst was shown.172 At 170 °C, 45% of C5 sugar formation was reported while from XRD studies it was shown that the cellulose remained unreacted. Sulfonated allophane catalyst (amorphous aluminosilicate) was also screened for its hydrolysis activity with bamboo, Japanese cedar and rice straw.179 Under the optimized reaction conditions (150 °C), the hemicellulose part was converted selectively to yield 41% xylose from the bamboo powder, 8% xylose and 6% mannose from Japanese cedar powder and 8% arabinose and 2% xylose from rice straw. The product distributions are solely dependent on the type and concentration of hemicellulose present in these lignocelluloses. Very recently, the selective hydrolysis of hemicelluloses from bagasse, rice husks and wheat straw into sugars and furfural was demonstrated over SAPO type catalysts.180 Additionally, a few reports showed that under the operating conditions used, carbon catalysts with various surface functionality can hydrolyse both the hemicellulose and cellulose part of the raw biomass (Eucalyptus flakes and corn cobs) into C5 and C6 sugars.126,155 It was observed that heterogeneous catalysts can catalyse hydrolysis reactions of hemicelluloses. Moreover, depending on the type and source of hemicelluloses, catalytic activity varies. This brings our attention to the fact that it is vital to design and develop a catalyst that can hydrolyse hemicelluloses derived from different sources with good selectivity and recyclability.
Besides obtaining sugars from disaccharides and polysaccharides, it is also possible to obtain them from other monosaccharides by virtue of isomerization reactions. During hydrolysis reactions of disaccharides and polysaccharides, it is possible for sugars to undergo isomerization reactions. For example, it is possible to obtain fructose from glucose once the latter is formed during hydrolysis of various disaccharides and polysaccharides. Similarly, the formation of xylulose from xylose is observed. Nevertheless, in this section, we will discuss the isomerization reactions of sugars (monosaccharides). The formation of two ketonic sugars viz. fructose (C6) and xylulose (C5) from their respective aldonic sugars, such as glucose and xylose, is known to occur via an isomerization reaction. In the presence of a base, aldose sugars transform into ketose sugars via formation of an enediol intermediate and vice versa and this transformation was first discovered in 1885 by Cornelis Adriaan Lobry van Troostenburg de Bruyn and Willem Alberda van Ekenstein and hence is also known as the Lobry de Bruyn–van Ekenstein transformation (Figure 1.11).181 The isomerization of glucose into fructose is a very important industrial reaction since high fructose syrup (HFS) is used as a sweetener. It is known that fructose has a high degree of relative sweetness (110) compared to glucose (74) and sucrose (100). However, since the glucose to fructose isomerization reaction is equilibrium limited, it is necessary to optimize the reaction conditions to achieve higher yields of fructose.
Homogeneous bases, such as NaOH, KOH, sodium aluminate etc., are frequently used for the isomerization of glucose to achieve 30–50% of fructose yields.182–184 The reaction proceeds via formation of a carbanion by extraction of a H from the C2 carbon by the anionic part (OH−, aluminate ion etc.) of bases to form an enolate ion. Further, this enolate ion in the presence of water forms an enediol or can cause an inversion to yield mannose (Figure 1.11). The restricted yields obtained in homogeneous systems may be attributed to the fact that fructose, once formed, is immediately reverted back into glucose and thus equilibrium is achieved. To improve the yields of fructose, it is necessary to shift the equilibrium towards the right hand side. When a resin is used along with a homogeneous base (NaOH), a slight improvement in yield (57%) is observed.185 Subsequently, heterogeneous base catalysts are also known to catalyse this reaction as free hydroxyl anions (OH−) are liberated in the bulk water from these catalysts. It was reported that aluminate resin with a low hydroxide content can produce very high yields (72%) of fructose from 90% glucose conversion at 2 °C (1007 h).186 The formation of a stable complex between fructose and aluminate ion (on the catalyst) helps in achieving higher yields as the reverse reaction of fructose to glucose is thus restricted. These results match well with earlier work when aqueous sodium aluminate (68%) was used.187 It is also possible to obtain mannose as a product when glucose is treated with base, however, the rate of inversion is extremely low compared to enediol formation to yield fructose. Further, in the glucose isomerization reaction, a series of alkali and alkaline earth metals (Li, Na, K, Cs, Ca and Ba) exchanged zeolites (A, X and Y) were screened at 95 °C and the results showed lower conversion (7–34%) with very low fructose formation (4–22%).188 The same report also presented the use of hydrotalcite (HT) catalyst with Mg/Al=3 (atomic) for obtaining 42% glucose conversion with 25% fructose formation. Another report also showed that carbonate and hydroxide forms of HT can produce 14% fructose from 15% glucose conversion at 90 °C.189 When HT was used for the glucose isomerization reaction in N,N-dimethylformamide (DMF) solvent at 100 °C, 38% fructose was formed at 62% glucose conversion.190 Use of HT was also shown by other research groups to yield fructose, however, the main aim was to achieve higher oxidation and hydrogenation activities in the reaction.191 It can be stated from these reports that when HT is used for isomerization, typically, a high selectivity for fructose formation is achievable, which can be again explained by the possible formation of magnesium aluminate (MgAl2O4) or (M+AlO2−) during the synthesis of HT. Moderate yields of 21–39% for fructose were also reported in the presence of various metallosilicates, namely titanosilicates, sodium yttrium silicate, alkali calcium silicate and calcium silicate at 100 °C.192 Although many catalysts are known for the isomerization reaction, depending on the reaction conditions (at high temperatures), glucose also may undergo degradation reactions (to acetic, lactic and propionic acids).
Very recently, a promising pathway for glucose isomerization into fructose via intramolecular 1,2-hydride shift mechanism was shown using Lewis acid catalysts (Figure 1.12).193 Since the process is developed with a Lewis acidic catalyst, it eventually stops sugar degradation reactions. The Sn-β and Ti-β catalysts having Lewis acidity can convert glucose (55% and 26%, respectively) into fructose (32% and 14%, respectively) in water at 110 °C.193 Along with fructose, formation of mannose is also identified (5–9%) as an epimerization (inversion) product in the reaction mixture. Additionally, Sn-β catalyst was demonstrated to be stable and recyclable four times without losing its activity. In an extensive study, with the help of 1H and 13C NMR analysis of isotope labelled glucose, the authors have proven that in the presence of Lewis acidic Sn-β catalyst, an intramolecular 1,2-hydride shift mechanism is operated, which is not possible under alkaline conditions.194 Additionally, diffuse reflectance UV-Vis analysis suggested that a partially hydrolysed Sn framework [(SiO)3Sn(OH)] is the active species in the Sn-β catalyst. The phenomenon of aldose sugar isomerization in the presence of a Lewis acid catalyst is also valid in the case of xylose (C5 sugar) for formation of xylulose. The presence of Sn-β catalyst showed 85% xylose conversion with 18% xylulose formation (isomerization) and 5% lyxose formation (epimerization) at 110 °C in water.195 But the authors have not pointed out the rest of the products. Considering this concept, another research group has prepared Sn incorporated mordenite framework inverted (MFI) and beta (BEA) zeolites and used them for sugar (glucose and xylose) isomerization reactions at 90 °C.196 The small pore diameter (0.56 nm) of the MFI zeolite restricted the access of larger sugar molecules (glucose: 0.73 nm, xylose: 0.66 nm) to the metal centre present inside the MFI zeolite, which thus showed lower isomerization activity (glucose conversion=9%, fructose yield=4% and xylose conversion=40%, xylulose yield=19%). On the contrary, the BEA zeolite with a larger pore diameter (0.77 nm) can easily overcome the diffusion limitations for sugars thereby offering better isomerization activity (glucose conversion=65%, fructose yield=34%, mannose yield=17% and xylose conversion=81%, xylulose yield=24%). Due to the presence of Lewis acid sites in SAPO-44, glucose isomerization into fructose is also possible.86 Since SAPO contains both Brønsted and Lewis acid sites, one-pot synthesis of HMF from glucose via fructose formation is possible.86 In yet another report, use of HT was shown to achieve the acyclic form of glucose to yield higher concentrations of sugar alcohols (sorbitol and mannitol) over supported metal catalysts.191
With the onset of renewed interest in lignocellulose utilization for a sustainable future, it is possible to generate fuels and chemicals from this renewable and abundantly available resource. Though several efforts have been devoted since the 1900s and in recent years to the synthesis of fuels (bio-ethanol, bio-diesel), it is perceived that the synthesis of chemicals from this resource is highly attractive. The estimated worldwide production of bio-fuels (bio-diesel and bio-ethanol) is around 2000 thousand barrels per day, which in comparison to the estimated worldwide production of crude oil is around 2.5% (crude oil production, 79 000 thousand barrels per day).197 This gives the idea that it is practically impossible to match the requirement of world fuel demand through biomass processing. Looking at this scenario, it would be crucial to develop methods for the synthesis of sugars and other chemicals as they can act as platform chemicals to yield many other industrially important, valuable chemicals such as furans, sugar alcohols, sugar acids, glycols etc.
Conversion of plant derived lignocellulosic biomass or lignocelluloses containing cellulose, hemicelluloses and lignin is crucial considering its non-edible property (for humins) compared to food biomass such as starch and various disaccharides. In two possible ways, polysaccharides, cellulose and hemicelluloses, the components of lignocelluloses, can be converted, first, via isolated polysaccharides after pre-treatment to extract them from lignocelluloses and second, via direct hydrolysis of lignocelluloses. For the first path, pre-treatment has to be done with lignocelluloses to separate cellulose, hemicelluloses and lignin from each other as they are linked together through covalent or non-covalent bonding. However, during isolation procedures typically done by acids, bases, steam etc., changes in the structures of these polysaccharides can be observed. In particular, these polysaccharides may undergo such changes as reduction of the chain length, increase in reducing ends, loosening of structures, loss of crystallinity etc., which are helpful in the hydrolysis of cellulose and also hemicelluloses. The other changes such as oxidations, acylation, incorporation of impurities etc., may, however, hamper the hydrolysis of these polysaccharides. Hence it is essential to choose the correct pre-treatment method for efficient isolation. For the second path, lignocelluloses are used directly without giving any pre-treatment. This pathway is beneficial for avoiding the contamination of substrates with impurities (caused during pre-treatment by use of external reagents) and changes in structures possible during pre-treatment procedures. However, due to the native structure of lignocelluloses, it is difficult to hydrolyse the polysaccharides. Moreover, the other components present in lignocelluloses, such as lignin, waxes and nutrients, may poison the catalysts. It is perceived that lignin can strongly adsorb on the catalyst surface through various functional groups present in lignin (–OH, –CHO, –COOH, O etc.). Another possibility is an exchange of H+ with M+ (nutrients such as, K, Mg, Ca etc.) in acidic catalysts, hampering their activity.
Although for the conversion of polysaccharides, many pathways are known, such as thermal (pyrolysis, gasification, application of supercritical water), thermo-chemical (acid, base), or biochemical (enzyme) etc., it is obvious that thermo-chemical and biochemical (enzyme) pathways are preferred to yield the maximum amounts of sugars as with thermal treatments, degradation products are predominant. Mineral acid based methods have been known since 1819 and hence much work is done to optimize the reaction conditions for achieving the highest possible sugar yields. As mentioned in Section 220.127.116.11, many industrial processes use mineral acids for yielding sugars that are later converted into fuels. Base catalysed methods are also known to hydrolyse polysaccharides but the possibility of substrate oxidation and isomerization of the sugars formed may restrict the use of bases on an industrial scale. As with mineral acids, enzyme based biochemical methods have also been known for a long time and they have the advantage of achieving very high yields because of the selective action of enzymes. However, in all these methods, it is important to maintain the pH (either acidic or alkaline), which inherently increases the capital cost as the reactor walls are prone to corrosion. Moreover, due to their homogeneous nature, it is difficult to recover the catalysts for reuse. Considering these drawbacks, recently, heterogeneous catalysts such as solid acids and supported metal catalysts have been employed in these reactions. It is observed that solid acid catalysts can, like mineral acids, promote the reaction via H+, which is available on the surface of the catalyst from Brønsted acid sites. Although solid acids are active in these reactions, they are prone to degradation/decomposition during the course of the reaction because of the active sites leaching in the solution or morphological changes (during hydrothermal treatment). It is widely believed that hydrophilic catalysts may help in avoiding these drawbacks and also may overcome diffusion limitations by enhancing the interactions between the catalyst and the substrate. This enhancement may arise due to hydrophilic interactions between hydrophilic surface –OH groups and hydrophilic –OH groups in substrates. The possibility of using supported metal catalysts is also shown by generating in situ acid sites by splitting molecular hydrogen on the metal surface. Though heterogeneous catalysts are known for these reactions, it would be a real challenge to develop efficient and reusable catalysts and their application on a larger scale.
Worldwide, several governments, private entities and industries are sponsoring various projects on the development of new environmentally benign methods for the conversion of lignocelluloses into chemicals. In the USA, USDA and DOE support most of the biomass related programs on a large scale with NREL being a prime example. Likewise, the EU funds many projects on biomass valorization. In many other countries, their respective ministries and government organizations fund many projects for developing new methods to valorize biomass. Various multi-national corporations such as DuPont, SABIC, Dow, Shell, BP, Haldor Topsøe, Coca-Cola, Solvay, BASF, Honeywell UOP, Reliance, Tata Chemicals, Mitsubishi, Sumitomo, Toyota, Arkema, DSM, P&G, Unilever, Clarient and Asashi Chemicals, along with a range of other industries, are looking into the use of biomass derived products in their portfolio in the near future.
The use of aquatic biomass is also being looked into by various research groups for the synthesis of fuels (green diesel) and chemicals, however, the work is at a very nascent stage at least in the case of chemical synthesis. However, this feedstock may prove to be very interesting in the near future.
While there are many opportunities to work on the synthesis of chemicals or fuels from biomass (undoubtedly from non-edible resources to avoid the food vs. chemical battle), attention to the following few points is required before any methods or technologies are developed: Abundance of resources to produce sufficient amount of product, either bulk, fine or niche Development of catalytic systems that are efficient, recyclable and easy to handle Removal of impurities so that catalyst and product poisoning is avoided Utilization of existing facilities (reactors, delivery, man-power) Process economy compared to fossil feedstock based methods Application of multiple feedstock such as crop wastes (wheat straw, rice husk, corn cob, corn stover, bagasse etc.), forest residues, municipal solid waste etc. Feasibility to scale up the method for commercial purposes
In conclusion, utilization of biomass has thrown up a lot of challenges requiring the development of novel processes to ensure a sustainable future, if not for tomorrow, then for the future.
© The Royal Society of Chemistry 2016 (2015)