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Catalytic Conversion of Structural Carbohydrates and Lignin to Chemicals Abhijit Shrotri, Hirokazu Kobayashi, Atsushi Fukuoka1 Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido, Japan 1 Corresponding author: e-mail address:
[email protected]
Contents 1. Introduction 2. Types of Available Biomass 3. Lignocellulose 3.1 Sources and Availability 3.2 Composition and Structure 3.3 Lignocellulose Pretreatment 3.4 Cellulose 3.5 Hemicellulose 3.6 Lignin 4. Chitin 4.1 Availability and Market 4.2 Composition and Structural Discussion 4.3 Conversion Technologies 5. Future Direction and Prospects References About the Authors
2 3 4 4 5 11 19 34 36 41 41 43 44 53 54 64
Abstract Lignocellulose and chitin are the two most abundant renewable sources of organic carbon available as alternative for chemical and fuel synthesis. Catalytic conversion of these composite polymers to monomers is a multifaceted challenge. Lignocellulose and chitin are inherently unreactive toward chemical attacks as they serve the function of structural materials in plants and animals. A combination of pretreatment and catalytic reaction is necessary to convert these materials into useful small molecules. These upstream reactions involving depolymerization of polymers are the roadblock for realizing future chemicals production based on biomass. In this chapter, we first discuss the pretreatment technologies currently available for lignocellulose and their relevance for catalytic conversion. Catalytic pathways for depolymerization of cellulose, hemicellulose, and lignin are then discussed, highlighting important reactions and mechanism.
Advances in Catalysis ISSN 0360-0564 https://doi.org/10.1016/bs.acat.2017.09.002
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2017 Elsevier Inc. All rights reserved.
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An analogy is derived between mechanism of cellulose hydrolysis using enzymes and heterogeneous carbon catalysts containing acidic functional groups. Finally, the use of chitin as a renewable carbon source is discussed. The chemical structure of chitin is described along with its origin from crab shells and availability. Recent advances in conversion of chitin to N-acetylglucosamine and its derivatives are described.
1. INTRODUCTION The utilization of natural resources has influenced the development of human society throughout our history. Progress in technology and lifestyle is often preceded by the ability to harness a key natural resource. A recent example of such progress and perhaps the most important in current scenario is the industrial revolution powered by the advent of fossil fuels. In two centuries, our society has progressed at breakneck speed, a feat that is difficult to maintain. The unsustainable nature of fossil fuels demands a shift toward use of renewable sources to satisfy the energy and chemical need in the future. Scientific advancement in renewable technologies is promising, but no single technology can replace fossil fuels entirely. Therefore, utilization of each resource is necessary to gradually reduce the dependence of fossil fuels. Biomass is currently the largest source of carbon-based renewable energy in the world as shown in Fig. 1. This fact is often overlooked as most of this biomass is used noncommercially for cooking and heating in developing countries, which is energy inefficient and causes pollution. This biomass can be better utilized in an energy efficient way by synthesis of value-added products. In this context, biomass can occupy the niche for synthesis of chemical and liquid fuels for the future. Use of biomass to produce electricity alone is inefficient, and other direct methods such as solar cells and wind turbines provide a cleaner and longlasting solution. Synthesis of fuels from biomass such as ethanol is an emerging prospect. Currently, biofuels account for 3% of the total liquid fuels used for transport around the world (2). Bioethanol and biodiesel are the two major types of biofuels available today. Complete replacement of fossil fuel with biomass-derived fuels is difficult simply due to the enormous scale of fuel consumption. Biomass is more effectively utilized by converting it to commodity chemicals and chemical precursors used in industry to synthesize polymers and consumer products. The high market value of these chemical precursors enables the use of costly unit operations required for biomass pretreatment, processing, and separation.
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Fig. 1 Share of renewables toward totally energy supply of the world in 2014. Drawn from data reported in Renewables Information 2016 by International Energy Agency (1).
Conversion of biomass to chemical products requires selective conversion of biomass components to monomers that can be purified and further modified to desired products. Therefore, thermochemical methods should be selective in biomass depolymerization. Higher temperatures are unfavorable due to rapid decomposition of polymers to a cocktail of products as in the case of pyrolysis and liquefaction. Low-temperature catalytic methods that target individual bonds in biomass are more useful in this aspect. This chapter focuses on the recent advances in catalytic conversion of lignocellulose and chitin, two of the most abundant biomass sources on our planet.
2. TYPES OF AVAILABLE BIOMASS Biomass is an umbrella term to represent all forms of living and recently dead plants and animals or biological materials derived from them. It comes in many forms and not all of them are available or suitable as feedstock for biorefinery. The essential feature of biomass is that its organic content can either liberate energy or transform into more useful organic chemicals. This definition is too broad, and it encompasses all living organisms and products derived from them. For the biomass to be considered as a feedstock for biorefinery, it should be available in abundance, have little initial value, and must be liable to chemical conversion. These criteria narrow down the list to lignocellulose, chitin, and microalgae in the order of their
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abundance or production capacity. Lignocellulose is unarguably the largest source of renewable carbon, and considerable effort has been devoted for its conversion. Chitin is obtained from the shells of marine animals such as crabs and prawns. It is a linear polymer of N-acetylglucosamine (NAG) and glucosamine, and it is estimated to be the second most abundant natural polymer on earth. Microalgae are microscopic single or multicellular organisms that produce and store biopolymers or oils within their cell wall. In this chapter, we will focus our attention to lignocellulose and chitin, both are primarily carbohydrate-based materials and serve the natural function of structural materials.
3. LIGNOCELLULOSE 3.1 Sources and Availability Source of lignocellulosic feedstock determines its value and composition, and it is a critical classification to make the process economical and sustainable. All naturally occurring lignocellulose from forests, grasslands, and aquatic vegetation is termed as virgin biomass. It is the single largest source of organic carbon in the world. Use of virgin biomass as fuel for domestic heating and cooking is still prevalent in many countries. In developing world, a large fraction (35% in 1987) of total energy used is still derived from burning virgin biomass (3). The current use of virgin biomass provides low heating value and causes pollution due to incomplete combustion. Substituting domestic use with industrial chemical process to improve the energy efficiency may seem attractive. However, use of virgin biomass at industrial scale is not sustainable and it may reduce global forest cover that serves as carbon sink, thereby aggravating the accumulation of CO2 in atmosphere. Agricultural residue is more suitable as a biomass feedstock due to its abundance and low value. A study estimated that about 1.5 109 tons of dry lignocellulose as crop waste is produced globally each year (4). Most of this waste is either burnt in the field to make way for next crop or in low-efficiency solid waste boilers. Open biomass burning of agricultural residues causes air pollution and reduces the air quality (5). The amount of agricultural waste available is bound to increase along with increase in crop production to sustain the growing population of the world. Therefore, exploring alternative methods to utilize agricultural lignocellulose is beneficial not only for the biomass industry but also from an environmental perspective.
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The idea of dedicated energy crops has recently emerged as a method to produce lignocellulosic biomass to feed biorefineries. Energy crops are plants that grow very fast to provide high biomass yield with short rotation. Ideally, these crops would flourish with little need for fertilizers or artificial irrigation. Fast-growing perennial grasses like switchgrass and elephant grass are pegged as alternative to corn for biomass synthesis (6). Deciduous plants of the species Populus, commonly known as poplar, are attractive hardwood species for lignocellulose synthesis (7). Energy crops are attractive due to the ability to supply a steady feed for continuous operation. Furthermore, dedicated energy crop farming will provide consistency in the composition and form of biomass feed. Currently, the debate against energy crop cultivation is centered around land-use issues. It is believed that energy crops would compete with food crops in future, and the advancement in technology or policy shift may make cultivation of energy crop lucrative. Food shortage is already a concern for the growing population and critics argue that unregulated cultivation of energy crops would worsen the situation in future.
3.2 Composition and Structure Lignocellulose in plants is located in their thick secondary cell walls. Dead and dry plant material is almost exclusively composed of lignocellulose. Chemically, lignocellulose is designed to be inert and impermeable. Cellulose, hemicellulose, and lignin are the three main components of lignocellulose that comprise about 80%–95% of dry lignocellulose. Accurate determination of the polymer composition in lignocellulose is challenging as the methods are empirical, based on quantitative analysis of derived monomers. Results obtained by different analytical methods should be compared with caution (8). Even when same method is used, difference in experimental technique can alter the results. Recently, a series of standardized analytical methods were reported by National Renewable Energy Laboratory (NREL) for quantification of various components (9). Adoption of a standard protocol by research groups makes it easier to compare data between laboratories. Quantitatively, cellulose is the largest fraction in lignocellulose followed by lignin and hemicellulose. The polymeric composition of these three components for various biomass types is shown in Table 1. Other components present in small amount are pectin, fats, oils, protein, and extractives. Extractives are so called because they can be extracted by boiling biomass in water or ethanol, and they include terpenes, resins, and tannins. Some
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Table 1 Composition of Typical Lignocellulose Biomass Feedstocks Lignocellulose Source Cellulose (%) Hemicellulose (%) Lignin (%)
Reference
Hardwood
40–55
20–40
18–25
(10,11)
Softwood
35–50
25–35
25–35
(10,11)
Sugarcane
32–44
27–32
19–24
(12–14)
Corn stover
37–45
22–35
15–17
(15,16)
Switchgrass
31–38
20–28
17–23
(15,17)
Elephant grass
38–40
18–24
25
(18,19)
Rice straw
32
28
19
(20)
Rice hulls
28–38
17
16–22
(21,22)
Fig. 2 Structure of cellulose showing a single molecule with degree of polymerization 2n + 2 containing n repeating cellobiose units.
lignocellulosic biomass has high inorganic content as in the case of rice hulls and rice straw, which contain a large amount of silica. 3.2.1 Cellulose Cellulose present in lignocellulose, sometimes referred as native cellulose, is a linear homopolymer of D-glucopyranose monomer units present in its lowest free energy 4C1 chair conformation. The monomers are linked by β-1,4-glycosidic bonds and each second monomer unit is rotated 180° axially (Fig. 2). Degree of polymerization of cellulose is based on its source and can vary from 300–1700 for wood pulp to 10,000 for cotton and flax (23,24). Cellulose is a polymorph that can assume different crystalline structure and at least seven forms are known to exist. Cellulose Iα and Iβ are the two polymorphs that are found naturally (25). Native cellulose contains both forms of cellulose and Iβ is abundant in lignocellulose. The crystalline structure, at least in part, is the result of formation of hydrogen bonds between the glucose monomer units in the cellulose structure. Hydrogen bonds are formed between hydroxyl groups of inter- and intrachain residues. Intrachain hydrogen bonds exist between (O3-H—O5) and (O2-H—O6) groups. Adjacent cellulose chains are linked together by (O6-H—O3)
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hydrogen bonds forming a ribbon-like structure (26,27). These planar structure are held together by van der Waals forces and hydrophobic interaction to form microfibrils (25,27) that are arranged parallel to each other in the lignocellulose matrix (28). Crystalline domain of cellulose is not continuous throughout the structure, and it is disrupted by amorphous domains. Crystallinity index is a term used to represent the abundance of crystalline cellulose in a sample. It is measured as a ratio of abundance of crystalline and amorphous domain. Although different methods such as X-ray diffraction and 13C solid-state nuclear magnetic resonance (NMR) are available, there is little correlation between the reported crystallinity index measured with different method (29). Therefore, at best, use of crystallinity index is limited to comparison of change in degree of crystallinity when a sample is treated and measured under same condition. 3.2.2 Hemicellulose Hemicellulose is a class of sugar polymers that are extracted with alkaline solution after removing pectin from cell walls of plants. The sugar polymer interacts with cellulose by hydrogen bonds and has covalent bonds with lignin, thus connecting cellulose and lignin to provide higher strength (30). In contrast to cellulose, which is a homopolymer of glucose linked with only β-1,4-glycosidic bonds, hemicellulose contains multiple types of sugar units with variety of glycosidic bonds in one molecule. Therefore, hemicellulose is present in amorphous form. Moreover, the composition depends on plant species and parts. A wide variation is found in the structure of hemicellulose, and the details are available in literature (31,32). Hemicellulose is roughly classified into xylan, mannan, xyloglucan, and mixed linkage β-glucan based on its chemical structure (Fig. 3) (31). Xylan consists of a main chain of xylose monomers linked by β-1,4glycosidic bonds and branches of small amounts of other saccharides. The xylan containing 4-O-methyl glucuronic acid branches is called glucuronoxylan, in which the branches are linked by α-1,2-glycosidic bonds (33,34). A polymer of xylose with arabinose branches is named arabinoxylan, in which arabinose units are connected by α-1,3-glycosidic bonds (35,36). Arabinose is in L-form, which is distinct from other monomeric sugars. These xylans are the main hemicellulose in the secondary cell wall of softwood and typical grasses (31). Mannan is divided into galactomannan and glucomannan based on the structure (37). Galactomannan has a mannose chain modified with galactose
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Fig. 3 Structure of different types of hemicelluloses.
connected by α-1,6-glycosidic linkage (38,39), whereas glucomannan possesses both glucose and mannose in the main chain decorated with galactose side chain in varied degree (40,41). In these hemicelluloses, the linkage of main chain is β-1,4-glycosidic bond. Hardwood predominantly has glucomannan as hemicellulose in the secondary cell walls. The main chain of xyloglucan is the same as that of cellulose, but it is mainly modified with xylose by α-1,6-glycosidic bonds (42). Xyloglucan
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is found in the primary cell walls of all higher plants. Hardwood has repeating units of –X–X–X–G–, where X indicates xylose-modified glucose and G shows glucose (31). Mixed linkage β-glucan is a homopolymer of glucose containing both β-1,3- and β-1,4-glycosidic bonds with no branches (31). This type of hemicellulose is specific to Poales. The hemicellulose is present in subaleurone and endospermic cell walls. 3.2.3 Lignin Lignin present in lignocellulose, referred here as native lignin, has a complex polymer structure made from interlinking units of methoxylated hydroxycinnamyl alcohols, also known as monolignols. Primarily, three monolignols p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol are present in varying degree in native lignin (Fig. 4). The abundance of monolignols varies by plant type and among different species. Softwood lignin mainly consists of coniferyl alcohols units (95%) with small amount of p-coumaryl (4%) and sinapyl alcohols units (1%). Hardwood lignin typically contains coniferyl alcohol and sinapyl alcohol units in equal proportion with trace amounts of p-coumaryl alcohol units (43). Monolignols are polymerized during biosynthesis to form a complex disordered three-dimensional structure. These units are linked by various bonds including β–O–4, 5–5, β–5, 4–O–5, β–1, dibenzodioxocin, and β–β linkages (Fig. 5). Lignin in softwood and hardwood consists mainly of β–O–4 linkages. The relative abundance of linkages in softwood and hardwood lignin is shown in Fig. 5. The exact structure of lignin and quantitative measurement of linkages are difficult to determine. Attempts to dissolve lignin for isolation or analysis alter its chemical structure, and the resulting material does not truly represent the native lignin. Recent progress in quantitative NMR analysis provides a good approximation of the native lignin structure (44–46).
Fig. 4 Three major methoxylated hydroxycinnamyl alcohols as building block of lignin.
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Fig. 5 Different structural motifs present in lignin and their abundance in softwood (SW) and hardwood (HW) lignin.
3.2.4 Lignocellulose Superstructure The structure of lignocellulose itself is a complex matrix of the three main polymers cellulose, hemicellulose, and lignin interlinked by covalent and noncovalent bonds. Lignocellulose structure can be compared with the structure of reinforced cement concrete (RCC) used in construction of load-bearing structures (Fig. 6). Long distance strength in lignocellulose comes from bundles of cellulose arranged parallel to each other, a configuration like the metal rebar in RCC. Hemicellulose binds the cellulose microfibrils units together and is analogous to the metal wires used to bind rebar and make the RCC cage. The nature of bonding between cellulose
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Fig. 6 Lignocellulose super structure (left) is analogous to reinforced cement concrete (right) in terms of arrangement and purpose of its components.
and hemicellulose is not fully understood. However, noncovalent attachment of hemicellulose chain with cellulose is the predominant idea (47). Pectin, another polysaccharide present in low concentration, with hemicellulose provides further strength (48). Lignin functions as the concrete by filling up the space in the cell wall. Lignin is bound with hemicellulose through ether- and ester-type covalent linkages (49).
3.3 Lignocellulose Pretreatment Untreated lignocellulose is unsuitable for industrial consumption due to its recalcitrance and heterogeneous composition. Pretreatment of lignocellulose is always required to alter its physical or chemical properties. Physical changes involve particle size reduction and increasing porosity of lignocellulose for penetration of solvents and catalyst. Chemical changes affect the cellulose crystallinity, degree of polymerization, covalent linkage between hemicellulose and lignin, and ether linkages between lignin monomers. Many approaches are available for lignocellulose that vary in effectiveness toward one or multiple pretreatment objectives. It is argued that the cost of pretreatment is the limiting factor for most lignocellulose-derived technologies (50). Therefore, cost effectiveness is the most important aspect for choosing the pretreatment technology. Other factors include the nature of feedstock, tolerance of downstream process for residual components, or chemicals used for pretreatment. These criteria create an interdependence of the pretreatment technology on the downstream process and both must be modified to suit the need of other. Pretreatment of lignocellulosic biomass is widely researched in the context of biochemical conversion of cellulose using enzymes. The aims of pretreatment for biochemical and catalytic conversion are not entirely similar
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and significant differences exist. By-products from pretreatment, even at low concentration, can deactivate the enzymes, a problem not usual for catalytic processes. Simultaneous hydrolysis of cellulose and hemicellulose is possible in catalytic process and their separation is not a prerequisite. Thermally unstable components degrade rapidly under catalytic reaction conditions, and their separation is essential to retain them in useful form. Research in pretreatment technologies is primarily aimed toward enzymatic conversion, and existing methods are adapted for catalytic processes. Pretreatment technologies that are applicable for catalytic conversion of lignocellulose are discussed below. 3.3.1 Mechanical Treatment Mechanical pretreatment refers to chipping, milling, and grinding of lignocellulose and is almost always the first step in all lignocellulose processes. Size reduction of biomass for easier processing and transport is the simplest form of mechanical pretreatment. Size reduction also improves the available surface area of lignocellulose that comes in contact with reactants in the process. Knife mills, hammer mill, and disc mill are some common types of size reduction instruments. The size of feedstock after chipping is in the range of 10–30 mm, and it can be reduced to 2–6 mm by milling and grinding. The relationship between final size and milling energy is not linear, and further reduction of size is energetically demanding (51). Type of feedstock, moisture content, and starting milling size also influence the energy demand. Knife and hammer mills are more economical with energy consumption between 1 and 130 kWh t1 in comparison to disc and ball mills (52). Extensive milling of lignocellulose also alters its physical structure and enhances its reactivity (53,54). Cellulose undergoes the most extensive change as milling disrupts the long-range ordered structure and reduces its crystallinity, making it more susceptible to chemical attacks (55). Amorphous cellulose produced by milling treatment undergoes hydrolysis at a lower temperature and exhibits higher rate of hydrolysis (56). Milling also makes lignin and hemicellulose susceptible to dissolution to facilitate their separation. The simplicity of mechanical pretreatment is attractive as it does not use solvents or corrosive chemicals. However, the high-energy requirement for milling is a major limiting factor for large-scale applications. Mechanocatalysis, involving combined milling of lignocellulose and catalysts, is an attractive method to increase the solubility and reactivity. Combining cellulose with solid acid or small amount of mineral acid catalysts and milling it in a ball mill produced water-soluble oligomers that can be easily
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hydrolyzed to glucose under mild condition (57–59). Acid catalyst directly depolymerized cellulose to yield soluble oligomers. NMR analysis revealed that repolymerization of fragments occurred to form α-1,6 branched oligomers (59) (Fig. 7). These branched oligomers showed high reactivity for hydrolysis and hydrolytic hydrogenation reactions. Mechanocatalysis of lignocellulose produces a composite of depolymerized material that can be reacted directly to produce 5-hydroxymethylfurfural and furfural (60). Deep depolymerization of lignocellulose produces a water-soluble composite that yields sugars after hydrolysis at mild reaction conditions. The residual lignin is obtained as solid material after hydrolysis and can be recovered by simple filtration (61). Depolymerization of lignocellulose is catalyzed by the presence of acids, and influence of radicals was negligible as the presence of lignin, a radical scavenger, did not reduce the rate of depolymerization (62). It can be argued that mechanocatalysis is not merely pretreatment method as the original lignocellulose structure is chemically transformed to a large extent. Energy requirement for mechanocatalysis is analogous to mechanical pretreatment. However, the advantage of deep depolymerization reduces the cost for
Fig. 7 Structure of branched oligomers formed by mechanocatalytic depolymerization of cellulose (A) and (B) 1H NMR of the anomeric region of branched oligomers showing cellulosic β-1,4 linkages and newly formed α-1,6 linkages along with α and β reducing ends.
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subsequent processing of lignocellulose. Energy requirement at gram-scale operation is 200 MWh t1 that can be reduced to 9.6 MWh t1 at kilogram scale (63). Therefore, mechanocatalysis can be feasible at large-scale operation for lignocellulose depolymerization. 3.3.2 Steam Explosion Steam explosion is a physicochemical method that uses high-pressure steam to disrupt bonding between polymeric components and decompression to break the lignocellulose structure. In this method, the lignocellulose is treated with high-pressure steam (433–533 K) for some time and then the vessel is rapidly depressurized to atmospheric pressure (64). The explosive decompression and high temperature causes degradation of hemicellulose, and it is extracted as water-soluble fraction. Cellulose is largely preserved in its original form, and only slight depolymerization occurs at mild reaction condition. Lignin undergoes depolymerization by cleavage of β–O–4 linkages, and condensation of fragments occurs to form a more stable polymer (65). The adiabatic expansion of steam within the cell wall ruptures the lignocellulose structure and results in redistribution of components. Cleavage of glycosidic bonds in stream treatment is catalyzed by hot water or the organic acids released by degradation of hemicellulose (66). Acid catalysts are added to improve the rate of hydrolysis and reduce sugar degradation by reducing the steam retention time and required temperature for the reaction (67). Sulfuric acid is widely used as catalyst for steam explosion as it is inexpensive and has high catalytic activity. SO2 and CO2 are used as gas-phase acid catalysts that do not require an impregnation step prior to steam explosion (68,69). Acid catalysts are also useful for subsequent hydrolysis of carbohydrate polymers to sugars under hydrothermal conditions. Steam explosion is one of the most energy efficient and environmentally friendly pretreatment methods for lignocellulose pretreatment. The lack of organic solvents and corrosive chemicals makes it attractive for industrialscale use. Fractionation of lignocellulose is incomplete in steam explosion as the lignin condenses back to form agglomerated particles decorating the cellulose fiber (70). Degradation of sugars obtained from hemicellulose is also a concern. Application of steam explosion as an upstream process for catalytic conversion of cellulose or lignin is not useful as their chemical reactivity is not improved after the pretreatment step. 3.3.3 Ammonia Treatment Similar to steam explosion, ammonia fiber explosion (AFEX) is a physicochemical process that uses liquid ammonia at high pressure and moderate
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temperature (2–3 MPa, 333–373 K) (71). AFEX causes disruption of lignin–carbohydrate linkage, hydrolyzes hemicellulose, and causes partial decrystallization of cellulose. The rapid explosion caused by evaporation of ammonia ruptures the lignocellulose structure and separates the components. Ammonia-soluble components produced during treatment get deposited on the outer surface of cell wall during evaporation (72). The lower temperature of AFEX makes it more energy efficient compared to steam explosion. The need for recovery and recycle of ammonia is a major drawback of this process. Ammonia recycle percolation (ARP) is another ammonia-based method where aqueous ammonia (5–15 wt%) is passed through a bed of lignocellulose held at high temperature (383–443 K) (73). Lignin and part of hemicellulose are dissolved in the APR solution and removed during the pretreatment (74). The residue in the bed is carbohydrate rich containing cellulose and part of hemicellulose. ARP is more suitable for catalytic application as the carbohydrate-rich fraction can be selectively converted to sugar monomers after APR treatment. However, the requirement for high temperature and longer contact time increases the energy requirement for APR compared to AFEX. 3.3.4 Organosolv Use of organic solvents to produce pulp and high-quality lignin in the 1970s gave birth to the organosolv process. It uses a mixture of organic and aqueous solvents to fractionate and solubilize lignin and hemicellulose. A large number of organic and aqueous–organic systems have been developed that work with or without catalyst in the temperature range of 373–473 K (75). Choice of organic solvent is critical for the treatment efficiency and process design. Low-boiling solvents, like methanol and ethanol, are used to reduce the energy requirement for their recovery by distillation and high-boiling solvents like ethylene glycol and glycerol are useful for high-temperature treatment without the need for pressure vessels (76). Organosolv pretreatment dissolves lignin and hemicellulose, leaving behind solid crystalline cellulose. The solvent is then processed to separate hemicellulose fractions as aqueous solution and lignin as precipitate. Lignocellulose is efficiently fractionated using organosolv method without significant loss of components or monomeric sugars due to degradation. Therefore, the method is useful for catalytic conversion of lignocellulose as all three components can be processed separately after pretreatment. Requirement of large excess of organic solvent to obtain pure fractions is
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a drawback of the process. The distillation of solvents for recycling increases the energy demand of the process. 3.3.5 Acid hydrolysis Simple hydrolysis of carbohydrates is also an effective way to pretreat lignocellulose. Acid hydrolysis is used for both pretreatment and final catalytic hydrolysis of lignocellulose. Therefore, distinct classification of a process as pretreatment or catalytic hydrolysis is difficult. It can be classified as pretreatment when the objective is efficient separation of one or more component. For example, in dilute acid pretreatment, lignocellulose is sprayed with 0.2–2.5 wt% of H2SO4 and then held at elevated temperature (400–483 K) (77,78). The reaction time varies from few minutes to few hours based on the temperature. The mild reaction condition hydrolyzes hemicellulose, and it is removed in solution phase leaving behind solid cellulose and part of lignin. Cellulose is also partially hydrolyzed and the degree of hydrolysis is based on the severity of the condition. Acid hydrolysis is an economical method as it uses inexpensive mineral acid and mild reaction condition. However, practical application is limited by many factors including the cost of acid neutralization and disposal. The yield of sugars derived from hemicellulose is low as the acidic condition causes rapid by-product formation. Acid hydrolysis is not effective for lignin removal, and large amount of acid-insoluble lignin remains with solid cellulose (79). Therefore, acid hydrolysis as a pretreatment method has limited application in the biorefinery scenario. 3.3.6 Ionic Liquid Pretreatment Ionic liquids are salts that have a very low melting point and are preferably liquid at room temperature. The salts have a small anion such as Cl and a large cation like 1-butyl-3-methylimidazolium ion. Ionic liquids have unique solvation properties to dissolve polar and nonpolar compounds. In 1934, Graenacher reported that cellulose could be dissolved in molten anhydrous benzyl-pyridinium chloride kept at 383–388 K (80). This might be the first report of utilizing ionic liquid for lignocellulose treatment. Recently, ionic liquids have emerged again as large-scale lignocellulose pretreatment is required. Today more than 20 ionic liquids are known to dissolve cellulose, and their dissolution capacity and kinetics have been compared (81). Hydrophilic solvents such as 1-butyl-3-methylimidazolium chloride (BMIM-Cl) are most widely used, and they can dissolve cellulose at high concentration (10%–25%) (82).
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Anion effect is predominant in the cellulose dissolution capacity of ionic liquids. The basicity of anions and their ability to make hydrogen bonds with the hydroxyl groups are critical for cellulose dissolution ability (Fig. 8). Halide-based anions such as chlorides are most effective due to their small size and strong electronegativity (83). Halide-based ionic liquids are difficult to handle as they have high melting point and high viscosity near melting point. Formate, acetate, and phosphate variants of imidazolium ionic liquids can also dissolve cellulose. Low viscosity of these ionic liquids at ambient condition facilitates rapid dissolution of cellulose (84). Acetate-type ionic liquids show better thermal stability than formate and are preferred for higher temperature application (85). The role of the cation in ionic liquids is less obvious for cellulose dissolution. It is postulated that the cation can act as an electron acceptor center and interact with the oxygen atoms in the hydroxyl groups (Fig. 8) (86). Presence of allyl group on the imidazolium cation instead of butyl group increases the solubility of the corresponding chloride ionic liquid. The smaller cation size, lower viscosity, and the polarity of the allyl group influence the solubility (84). Cellulose dissolved in ionic liquids is regenerated to separate it as a solid and then used for downstream processing. Regeneration is achieved by addition of an antisolvent like water, ethanol, or acetone. Washed ionic liquid is recovered by removing the volatile antisolvent by distillation. Biphasic systems for recovery of ionic liquids without distillation are another approach (87–89). The cellulose regenerated from ionic liquid is amorphous in nature. The degree of polymerization remains unchanged under mild conditions, and severe conditions can lead to significant depolymerization. Ionic liquid can serve both as a pretreatment agent and reaction medium. Cellulose dissolved in ionic liquid is highly susceptible to hydrolysis. Solid acid resin catalyst can hydrolyze dissolved cellulose selectively to
Fig. 8 Interaction of anion and cation with cellulose during dissolution in BMIM-Cl ionic liquid.
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cello-oligosaccharides and subsequently to glucose (90). Presence of water, an antisolvent for ionic liquid system, is necessary for the hydrolysis reaction. Therefore, low water concentration is essential as high concentration causes precipitation of cellulose and inhibits the hydrolysis reaction (91). Ionic liquids also serve as a medium for derivatization of cellulose to cellulose acetates and other derivatives (92). Lignocellulose itself is partially soluble in ionic liquids used for cellulose dissolution (93). 1-Butyl-3-methylimidazolium chloride and 1-allyl-3methylimidazolium chloride can dissolve softwood and hardwood without any other pretreatment (94). It is postulated that π–π interaction between the cation of ionic liquid and the aromatic moieties in lignin facilitates dissolution of lignin. Dissolved lignocellulose can be regenerated to obtain depolymerized components that easily undergo catalytic reaction. Control over the regeneration method could result in separation of purified cellulose, thereby allowing fractionation of lignocellulose using only ionic liquids (94,95). Ionic liquids are sometimes referred as a green solvent for application in lignocellulose conversion. This notion is derived from the relatively low vapor pressure and lack of requirement for corrosive inorganic or organic solvents in large amounts. However, many challenges remain in the practical application of ionic liquids. Low vapor pressure reduces the explosion hazard of ionic liquid, but it does not make them fire resistant and many ionic liquids are combustible. Furthermore, low vapor pressure makes the separation of ionic liquids by distillation very difficult. Other methods like salting out and extraction must be used for purification and recycle of ionic liquids. Finally, development of technologies for economic synthesis of ionic liquids is necessary for their application in large-scale lignocellulose conversion technologies. 3.3.7 Outlook on Pretreatment Lignocellulose pretreatment for catalytic conversion has different requirements from biological processes. Catalytic conversion of lignocellulose should ideally be an integrated process for utilizing all three major components. Their recovery after pretreatment in a state useful for catalytic application is desired. Features desired in an ideal pretreatment process are optimum recovery of all components either as monomers or polymers that have same or lower recalcitrance than the parent material. Energy required for heating, milling, and recovery should be minimal. Environmentally benign chemicals should be used that do not require a complex disposal
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procedure. Existing pretreatment methods are unable to fulfill these criteria, and further research is necessary to make the pretreatment suitable and cost effective for a catalytic biorefinery scenario.
3.4 Cellulose 3.4.1 Hydrolysis Both biological enzymes and chemical catalysts hydrolyze cellulose to cellooligosaccharides and glucose. Characteristics of enzymes are the low working temperatures below 373 K and high selectivity, while those of artificial catalysts are the variety and tolerance to wide range of reaction conditions. In this section, we focus on artificial catalysts after briefly introducing enzymatic reactions. 3.4.1.1 Enzymatic Hydrolysis
Cellulose-hydrolyzing enzymes, named cellulase, are classified into endoglucanase, cellobiohydrolase, and β-glucosidase (96). Endoglucanase randomly hydrolyzes amorphous part of solid cellulose to increase the number of end groups of cellulose molecules. Cellobiohydrolase captures such an end group and dissociates the substrate by cleaving every alternate glycosidic bond to form cellobiose (97,98). It is noteworthy that even crystalline part of cellulose is hydrolyzed by cellobiohydrolase, while most of catalysts cannot hydrolyze crystalline cellulose. Finally, cellobiose is hydrolyzed by β-glucosidase to glucose in liquid phase. Accordingly, cellulase can convert natural lignocellulose containing crystalline cellulose to glucose, but the rate of reaction is low. Consequently, pretreatment to improve the reactivity of biomass is important for the hydrolysis (99). Many companies are now examining the hydrolysis of lignocellulose by enzymes to produce ethanol in pilot to practical scales. For example, DuPont has operated such a facility with the maximum capacity of 30 million gallons per year in Nevada since October 2015. In the absence of β-glucosidase, endoglucanase and cellobiohydrolase can selectively produce cellobiose. The removal of β-glucosidase is achieved by the decantation after endoglucanase and cellobiohydrolase binds on cellulose. Cellobiose is manufactured by this method, and the product can be used as a prebiotic for better growth of cattle by improving their gut functions (100). The reaction mechanism of cellobiohydrolase is suggestive to design artificial catalysts for the hydrolysis of cellulose. Cellobiohydrolase first adsorbs on bulk solid cellulose with a cellulose-binding domain (101). The binding module consists of aromatic amino acid residues and provides CH–π and
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Fig. 9 Enzymatic hydrolysis of cellulose by (a) retaining mechanism and (b) inverting mechanism. Substituted groups are omitted for better visibility.
hydrophobic interactions with axial face of cellulose (102,103). The enzyme walks on the surface of cellulose due to flexibility of the binding interaction and finds a reducing terminal or nonreducing terminal of cellulose molecule, depending on the type of the enzyme. The cellulose molecule penetrates into the micropore of the enzyme, and sits on the position bearing a hydrolyzing site. The active site containing a carboxylic acid and a carboxylate hydrolyzes the captured cellulose by retaining or inverting mechanisms (Fig. 9) (104). In the retaining mechanism, the glycosidic bond undergoes nucleophilic substitution by the carboxylate, and the alkoxide formed is neutralized by the carboxylic acid. Afterward, a water molecule activated by the carboxylate attacks the ester intermediate. Good balance between weak acidity of carboxylic acids and the basicity of carboxylates and their location is crucial to realize the pathway. As to the inverting mechanism, a water molecule dissociates a glycosidic bond, similar to the second step of the retaining route. Thus, the synergistic mechanisms without free oxocarbenium ions decrease the energy barrier of this reaction. In addition, other moieties of the enzyme also control the energy profile to minimize the activation energy (105). As a result, cellulase can hydrolyze cellulose even at ambient temperatures.
3.4.1.2 Mineral Acid Hydrolysis
At the dawn of artificial hydrolysis of cellulose, people focused on soluble mineral acids as catalysts for good accessibility to solid cellulose (106).
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The most representative two processes used diluted sulfuric acid and concentrated sulfuric acid. Diluted sulfuric acid method was developed in Germany in 1920s, known as Scholler process (107). H2SO4 of 0.5 wt% passed through cellulosic biomass held in a percolator to convert it to glucose in 50% yield at 443 K. Decomposition of glucose was inhibited by the extraction, although it decreased concentration of glucose in the final solution. The acidic product solution was neutralized and used for the ethanol fermentation. Later, Madison process was launched in United States, where plug flow of 0.5–0.6 wt% H2SO4 and wood chip was used (108). In general, a plug flow system is significantly more efficient than a semi-batch-type reaction in terms of productivity. Concentrated sulfuric acid method typically requires >70% concentration, but it gives a higher yield of glucose. The concentrated acid disrupts crystalline cellulose to be amorphous form by the swelling effect, thus resulting in the better hydrolysis performance (109). For instance, in Hokkaido process, solid biomass after the removal of hemicellulose fraction was partially hydrolyzed by 70%–80% sulfuric acid to oligosaccharides (110). Next, the solution was diluted to be H2SO4 concentration of 30%–40% for further hydrolysis to glucose. H2SO4 was separated with an ion-exchange membrane, and glucose was crystallized by double salt method using NaCl (111). Finally, glucose was obtained in 80% yield with 99.5% purity. However, sulfuric acid processes are not working currently because of corrosive property of H2SO4, complicated separation, and mechanical troubles. Recently, heteropolyacids have been tested in the hydrolysis of cellulose (112–114). They are very strong acids (e.g., H0 ¼ 13 for H3PW12O40) (115) and can be easily extracted with organic solvents such as diethylether, which does not dissolve sugars (113,114). In addition, a concentrated heteropolyacid solution can amorphisize cellulose, thus giving high yield of glucose >80%. The reaction mechanism of acid hydrolysis is different from the enzymatic one (Fig. 10) (116). First, an oxygen atom linking two glucose units is protonated in the presence of acid. Although the protonation of the other oxygen atom on C1 position was also hypothesized for the hydrolysis mechanism, it was mostly excluded for glucopyranose-type saccharides based on the kinetic isotope effect studies using 16O and 18O (116). An acid with pKa < 3 is suitable for this process because of low basicity of the oxygen atom (pKa 4) (117,118). Before proceeding to scission of the glycosidic bond, conformation of the glucopyranose ring needs to be envelope or (twisted) boat form to gain overlap of orbitals between σ* orbital of the glycosidic bond and lone pair on the ether oxygen to form a new π bond
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Fig. 10 Reaction mechanism of acid-catalyzed hydrolysis of cellulose. Substituted groups are omitted for better visibility. *The actual conformation is discussed separately.
Fig. 11 Image of antiperiplanar elimination for the dissociation of glycosidic bond. Substituted groups are omitted for better visibility.
(Fig. 11) (118). In other words, the original chair conformation is in gauche form giving a dihedral angle of 60° between the glycosidic bond and lone pair of ether O atom, which should be changed to nearly 180° or 0° to suit the antiperiplanar or synperiplanar elimination. Subsequently, the glycosidic bond is cleaved to form an oxocarbenium ion, which is the rate-determining step. A water molecule rapidly attacks the ion predominantly from the opposite side to which the leaving group is present. Hence, this reaction inverts stereochemistry of glucose unit from beta to alpha anomer in spite of SN1 reaction, although the anomers tautomerize under the hydrolysis conditions. Considering the reaction mechanism, one can presume the reason why hydrolysis of crystalline cellulose is difficult. First, access of acid is limited to the very narrow surface of cellulose. Second, the chair conformation of glucopyranose unit cannot be easily changed due to steric hindrance and intermolecular hydrogen bonds with surrounding glucose units. Third, even if the dissociation of glycosidic bond occurred, water molecules are not
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accessible from the inner side of crystalline cellulose. This limitation might increase the probability of going back to the original glycoside form. Finally, crystalline cellulose has no space to accommodate additional OH groups formed in the hydrolysis. Rinaldi and coworkers remarked that amorphization improves the reactivity due to both better accessibility and relaxation of conformational restrictions using quantum calculations (118). Hama et al. proved this kind of steric effect in the hydrogenation of crystalline benzene as a simple model (119). They compared reactivity of crystalline and amorphous benzene in the quantum tunneling hydrogenation using atomic hydrogen. The reaction temperature was set at 20 K to keep the crystalline/amorphous structure. Amorphous benzene was readily hydrogenated to produce cyclohexane, but crystalline benzene was inert even though the intermolecular interaction is much weaker than that of cellulose bearing hydrogen bonds. Crystalline benzene does not provide room for formation of chair structure cyclohexane from flat benzene due to steric hindrance. 3.4.1.3 Hydrolysis by Solid Acid Catalysts
The first selective conversion of cellulose only using solid catalyst was achieved by hydrolytic hydrogenation reaction in 2006 (120), which triggered the study of heterogeneous catalysis for cellulose depolymerization. In 2008, sulfonated carbon catalysts were found to be active for the hydrolysis of cellulose in the screening of various solid acid catalysts. Hara and coworkers baked cellulose at 723 K under N2 flow to produce a carbon, and the solid was sulfonated by fuming sulfuric acid at 353 K under N2 (121,122). The sample had large amounts of phenolic groups (2.0 mmol g1), carboxylic acids (0.4 mmol g1), and sulfonic groups (1.9 mmol g1). Interestingly, the acid strength of the sulfonic groups was very high (Hammett acid function H0 ¼ 11) due to electron-withdrawing effect of neighboring carboxylic acids, compared to typical organic sulfonic acids (H0 > 3). The catalyst hydrolyzed cellulose to glucose in ca. 16% yield at a substrate/catalyst ratio (S/C) of 3.0 at 373 K in 27 h. It was proposed that phenolic groups adsorbed cellulose (123) and the activated sulfonic acids hydrolyzed glycosidic bonds. Similar to the sulfonated carbon derived from cellulose, sulfonation of a commercial activated carbon produced a catalyst for cellulose hydrolysis (124). Activated carbon was treated with concentrated H2SO4 at 423 K, washed with hot water at 353 K, and boiled at 473 K for 3 h to remove weakly bonded sulfonic groups. The catalyst hydrolyzed ball-milled cellulose to glucose in 40% yield at 423 K in
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24 h. Ionic liquid was used to improve the contact between catalyst and cellulose by dissolving cellulose, but sulfonic groups caused ion exchange with the solvent (90,125). Use of salts is difficult for solid strong acid catalysts. Afterward, many carbon-based materials were studied as sources of sulfonic acid catalysts to improve the activity such as mesoporous carbon CMK-3 (126), mesoporous silica/carbon composite (127), composite of magnetic oxides and organic moieties (128–130), and chloromethylpolystyrene (131). As a result, the yield of glucose has reached >70% in the hydrolysis of amorphous cellulose. After the development of sulfonic acid catalysts, namely, strong acids, carbon materials bearing only weak acid sites were also found to be active for the hydrolysis of cellulose (132). Screening tests of various carbons showed a correlation between hydrolysis activity and content of weak acid sites including carboxylic and phenolic groups (133). The most active catalyst in the screening, an alkali-activated carbon K26, gave glucose in 36% yield from ball-milled cellulose under the rapid heating–cooling condition, which is increasing temperature from 298 to 503 K in 18 min and then quickly cooling down to room temperature. The yield of glucose can be improved up to 88% by the optimization of pretreatment, which is ballmilling of cellulose and catalyst together as described below. Surprisingly, the hydrolysis of crystalline cellulose was also reported using an oxidized nanoporous carbon (134). An advantage of weak acids over strong acids is higher tolerance to minerals involved in real biomass. Indeed, the catalyst was not deactivated by treating it with acetate buffer at pH 4.0. Another merit of the catalyst is the easy preparation. For example, simple air oxidation of activated carbon produces active catalyst with no formation of waste liquid (135). Moreover, this property enables the reuse of weak acid carbon catalyst in the conversion of real biomass. Raw biomass contains lignin, which remains after the hydrolysis reaction together with solid catalyst. Although this is a common issue for solid catalysts, air oxidation of the catalyst and lignin regenerates the weak acid carbon catalyst by converting residual lignin to active catalyst (136). Lignin part is carbonized at the high temperature and simultaneously gains weak acid sites. Other examples of solid catalysts for hydrolysis of cellulose are currently very limited. A layered mixed oxide HNbMoO6 hydrolyzed cellulose to glucose and cellobiose in total 8.5% at 403 K in 12 h (137). Besides, it was reported that a silica material giving an NH3 desorption peak at 860 K converted cellulose to glucose in 50% yield at 433 K in 12 h, although the structure of acid site was not revealed (138).
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3.4.1.4 Influence of Contact Between Solid Catalyst and Solid Cellulose
Both physical and chemical contact between solid catalyst and solid cellulose are crucial to achieve high catalytic performance. Initially, cellulose and solid catalyst interact physically by collision or through continuous physical contact. Then, chemical interaction of cellulose molecules with surface of catalyst occurs at the solid–solid interface. Afterward, the cellulose molecule is hydrolyzed by acid sites. The physical contact between solid catalyst and solid cellulose is usually under control of collision probability, but the degree of contact is not high enough to achieve good catalytic performance. Frequency factor of the ratedetermining step, which is the dissociation of glycosidic bonds, is proportional to the degree of contact. A recent development for enforcing the physical contact is to ball-mill solid catalyst and cellulose together, which is called mix-milling (133). When catalyst and cellulose were individually ball-milled, an alkali-activated carbon K26 hydrolyzed cellulose to glucose in 2.9% and cello-oligosaccharides in 10% at 453 K in 20 min. Contrastingly, when K26 and cellulose were mix-milled prior to the reaction under the same condition, the yields of glucose and oligosaccharides were 20% and 70%, respectively. Thus, the mix-milling pretreatment accelerates hydrolysis of solid cellulose one-order larger than the individual milling. Since the role of mix-milling is the formation of solid–solid contact, it does not affect the rate constants of solid–liquid reactions such as hydrolysis of soluble oligomers and decomposition of glucose (139). Once the physical contact is established, chemical forces attract cellulose molecules onto surface of a solid catalyst. Katz and coworkers studied the effect of such adsorption of cellulose using silica as a model catalyst (140,141). They attached silyl chloride species on the silica surface to chemically immobilize cellulose molecule via ether bonds. The grafted cellulose was hydrolyzed even under mild conditions (pH 4, 378 K), but free cellulose did not undergo hydrolysis under the same conditions. The hydrolysis was further enhanced by an alumina bearing a higher density of surface OH groups with a lower activation energy, compared to silica (142). They proposed that the immobilization increases the chance of attack on glycosidic bonds by acidic OH groups and structural distortion activating the linkages. Hence, the chemical interactions between catalyst and cellulose are considerable factors in this reaction. As noted above, carbon materials uniquely show high catalytic activity for the hydrolysis of cellulose. Therefore, the attracting force has been studied using carbon and model soluble cello-oligosaccharides. The first
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hypothesis was formation of OH–O hydrogen bonds between oxygenated groups such as phenolic OH of carbon and hydroxyl groups of cellulose (122,123,143). However, later, it was found that the polycyclic aromatic domain is the main adsorption site of carbon materials in the adsorption stage (144). A mesoporous carbon with a pore diameter of 3.2 nm adsorbed glucose and cello-oligosaccharides in very large quantity up to 670 mg g1 in water. The adsorption strength was increased with increasing the number of glucose unit. The important discussion on the adsorption site was made using a controlled adsorption experiment in 1 M HCl aqueous solution, where hydrogen bonds by oxygenated groups of carbon potentially compete with H+ and Cl. The adsorption of cellobiose similarly occurred in the presence and absence of HCl, which implies no significant contribution of OH–O hydrogen bonds in the adsorption. Instead, CH–π interactions between CH groups on axial face of cellulose and π electrons of polycyclic aromatics of carbon were proposed (Fig. 12). Perhaps, the CH–π interactions mainly consist of dispersion force, as the dipole moment of a CH group is very low (typically μ ¼ 0.40 D) (145). The thermodynamic analysis showed negative enthalpy change and positive entropy change in the adsorption of cello-oligosaccharides on carbon, e.g., ΔH°ads ¼ 14 kJ mol1 and ΔS°ads ¼ +24 J K1 mol1 for cellobiose (146). The negative enthalpy change was ascribed to the formation of CH–π interactions. Density functional theory calculations with D3 empirical dispersion correction (147) reproduced the adsorption from axial face, in which CH groups directed toward the center of aromatic ring of carbon. The positive entropy change is noticeable as the adsorption generally decreases the degree of freedom on translation and rotation of adsorbate. Indeed, ΔS°ads for the adsorption of cellobiose on polyacrylamide was 3 J K1 mol1 (148). The characteristic result for carbon is ascribed to hydrophobic interactions. It is believed that cellulose is hydrophilic due to many hydroxyl groups on the equatorial face, but the axial face only consists of CH groups. Hydrophobic interaction is not a direct physical force working between hydrophobic materials (149). Instead, it is derived from the presence of water as solvent. Water molecules
Fig. 12 Adsorption of cellulose on polycyclic aromatic rings.
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surrounding hydrophobic materials keep their hydrogen bonding network, but the conformation is highly restricted in that case. Hence, reduction in the surface area of hydrophobic face in materials decreases the number of restricted water molecules, giving a positive entropy change. Consequently, the adsorption driving forces working on carbon material are CH–π and hydrophobic interactions, which is similar to the interaction between cellulose and cellulase enzymes. An important question is if or not carbon materials can pull cellulose molecules in their micropores similarly to which enzymes do. It was found that a zeolite-templated carbon (ZTC) adsorbs long-chain cellulose molecules with molecular weight of 3600 within 2 min even at room temperature (150). ZTC has an ordered pore structure with pore radius of 0.6 nm, which is much smaller than gyration radius of the cellulose molecule (2.9 nm). Nonetheless, the adsorption predominantly occurred in the pore. The adsorbed cellulose molecules were hydrolyzed to glucose in up to 85% yield at 453 K in 3 h, when the material was functionalized with oxygenated groups (151). Accordingly, carbon materials strongly attract cellulose molecules, and the internal surface is also useful for the hydrolysis of cellulose. In addition, the conformation of captured cellulose in the narrow space would be slightly distorted to enhance the cleavage of glycosidic bonds unless the space is too narrow to allow the hydrolysis itself (152–154). This would be the cause for the specifically high catalytic activity of carbon materials in the hydrolysis of cellulose. The attraction force is also applied for the purification of glucose from by-products in the hydrolysis of lignocellulose. A metal–organic framework (MOF) material possessing pyrene units (NU-1000) does not adsorb glucose at all, but adsorbs cello-oligosaccharides and by-products having aromatic rings due to larger degree of dispersion force (155–157). Thus, the property enables purification of concentrated glucose solution by selectively adsorbing 5-hydroxymethylfurfural (5-HMF) and aromatic compounds derived from lignin, which is not achieved by carbon materials with random structures which also adsorb glucose. 3.4.2 Hydrolysis and Successive Reactions 3.4.2.1 Conversion to 5-Hydroxymethylfurfural
5-HMF is likely the most coveted chemical building block derived from cellulose. It is produced by triple dehydration of fructose that is obtained from isomerization of glucose. Therefore, to obtain 5-HMF directly from cellulose, the sequence of reaction involves acid-catalyzed hydrolysis of cellulose
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followed by Lewis acid- or base-catalyzed isomerization of glucose to fructose and finally acid-catalyzed dehydration of fructose (Fig. 13). Direct dehydration of glucose to 5-HMF has been proposed as an alternate pathway for 5-HMF formation (158). 5-HMF easily reacts with water to form levulinic acid and formic acid, making its direct synthesis difficult under hydrothermal conditions required for cellulose hydrolysis. Considerable efforts have been devoted to conversion of monosaccharides to 5-HMF, which are comprehensively summarized in recent review articles (159–161). Heterogeneous catalysts for synthesis of 5-HMF from monosaccharides or polysaccharides can be broadly classified into metal oxides, zeolites, and sulfonated catalysts like resins and carbons (162,163). Catalyst with only Brønsted acid sites, like Amberlyst-15, Amberlyst-70, sulfonated carbon, etc., cannot catalyze the isomerization of glucose and is useful when the reactant is fructose (163–165). On the contrary, Lewis acid containing metal oxides and zeolites or solid base catalysts can isomerize glucose to fructose by intermolecular hydride shift or proton transfer, respectively (Fig. 14) (166). In combination with a homogeneous acid, Lewis acid containing Sn-Beta zeolite can produce 5-HMF glucose with 70% selectivity (167). Only Lewis acid containing phosphated Nb2O5 metal oxide catalyst can also yield 5-HMF in 52% from glucose (168). Suppression of Brønsted acid sites in phosphated Nb2O5 reduced the further reaction of 5-HMF. The conversion of glucose to 5-HMF is not limited to the fructose pathway, and it was revealed that phosphated TiO2 catalyzed direct dehydration of glucose through 3-deoxyglucosone as an intermediate (158). Alternatively, combining a solid
Fig. 13 Plausible pathway for conversion of cellulose to 5-HMF via fructose.
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Fig. 14 Mechanism of glucose isomerization to fructose by proton transfer using base catalyst and by intermolecular hydride transfer by Lewis acid containing Sn-Beta catalyst.
base like Al–Mg hydrotalcite and solid Brønsted acid catalyst like Amberlyst15 is also effective for conversion of glucose to 5-HMF (169). The reaction solvent used for 5-HMF synthesis is very important. 5-HMF easily undergoes rehydration in the presence of water to levulinic acid and formic acid and condenses to form humin polymers. Polar organic solvents or monophasic mixtures of water with organic solvents reduce by-product formation and improve 5-HMF selectivity (170–172).
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Continuous removal of even trace amount of water formed due to dehydration reaction results in 100% selectivity for 5-HMF in dry DMSO with Amberlyst-15 catalyst (173). 5-HMF is easily extracted in organic solvents that are immiscible with water. Therefore, biphasic reaction mixtures are used for continuous removal of 5-HMF from the aqueous phase to prevent its side reactions (174). High partition coefficient of the organic phase is suitable for better 5-HMF extraction (175). Inorganic salts like NaCl can be used to improve the partition coefficient of partially miscible organic solvents like 1-butanol and methyl isobutyl ketone (176). Direct conversion of cellulose to 5-HMF using heterogeneous catalysts is difficult due to the low reactivity of cellulose and high poor stability of 5-HMF. Use of ionic liquids as solvents or homogeneous catalyst is used to increase the reactivity of cellulose (161,177,178). 3.4.2.2 Conversion to Ethylene Glycol
Tungsten-based catalyst can directly synthesize ethylene glycol from cellulose under hydrolysis and hydrogenation conditions (179,180). The reaction proceeds through retro-aldol condensation of glucose to yield glycolaldehyde as the intermediate that is hydrogenated to produce ethylene glycol (181). Zhang reported that all forms of tungsten species are active for ethylene glycol synthesis (182). The flexibility of starting tungsten source is derived from the in situ transformation of tungsten species to tungsten bronze (HxWO3), a soluble but unstable intermediate. Transformation of WO3 to HxWO3 is temperature dependent and upon cooling the reaction mixture and exposing it to air, WO3 precipitates and is deposited back on the catalyst (182). High temperature (above 500 K) is necessary to catalyze the initial hydrolysis of cellulose to synthesize β-1,4-glucans and glucose that undergo retro-aldol condensation. 3.4.3 Hydrolytic Hydrogenation 3.4.3.1 Sugar Alcohols
Direct conversion of cellulose to C6 sugar alcohols is attractive to produce sorbitol, a chemical used in pharmaceutical industry and a precursor to polymers. Current industrial synthesis of sorbitol involves hydrogenation of glucose in the presence of RANEY® Ni catalyst under H2 pressure 10–15 MPa (183). Annual production of sorbitol is exceeding 6.4 105 ton per annum (184), and it is expected to increase with industrial synthesis of isosorbide, a precursor for polycarbonate polymer. Direct conversion of cellulose to
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sorbitol reduces by-product formation as the hemiacetal group in glucose is quickly hydrolyzed to prevent side reactions. The idea to perform hydrolysis and hydrogenation in a single pot is not new. In 1950s, sorbitol was produced by reacting cellulose in the presence of H2SO4, Ru catalyst, and hydrogen pressure (185). The use of mineral acid needing separation and disposal was the drawback of the process. Acid-free hydrolytic hydrogenation of crystalline cellulose to sorbitol in 37% yield was first reported in 2006 using Pt/γ-alumina catalyst in hot water (120). The acid-free reaction enables easy separation of product solution and metal catalyst that could be recycled. The hydrolysis of cellulose itself was catalyzed by Pt/γ-alumina catalyst in the presence of H2. The hydrolysis of cellulose was dependent largely on the metal species (Ru, Pt, Ni) and support materials (γ-alumina, activated carbon, zeolite) (186). Presence of small amount of Cl ions derived from metal precursors (H2PtCl6) also enhanced the hydrolysis of cellulose (187). Alumina support was transformed into boehmite, which reduces its activity for cellulose hydrolysis (187). Therefore, use of carbon support and metal precursor without Cl was useful to prepare stable catalyst (188). The rate-determining step in this reaction is the hydrolysis of cellulose to glucose. To achieve higher yield, the support can be modified by introduction of acid sites such as sulfonic acid groups on activated carbon or oxygenated functional group on carbon nanotubes and nanofibers (189,190). Insoluble salts of heteropoly acids such as Cs3PW12O40 can also be used as support for metal particles (191) or as solid acid catalyst (192). Use of soluble acids in small amounts is still attractive as it increases the hydrolysis rate. Modern pretreatment methods like mechanocatalysis dramatically reduces the amount of acid (0.25–1 mmol g1 cellulose) required to achieve near complete conversion of cellulose and more than 90% yield of sugar alcohols (59,193). The small amount of acid can be neutralized without large environmental burden. Most active metal species used for hydrogenation of cellulose are Pt, Ru, and Ni. Some researchers have also reported Pd, Ir, and Rh monometallic catalysts with reasonable yield of sugar alcohols (188,190,194,195). Ru is frequently used for cellulose conversion as it shows high activity and selectivity for conversion of glucose to sorbitol (196) and it is inexpensive in comparison to Pt. Ni is preferably used in industrial hydrogenation of glucose to sorbitol due to its abundance and low cost. However, activity of Ni to promote hydrolysis of cellulose is poor and high metal loading is required to maximize sorbitol yield (197). Furthermore, Ni tends to leach into the
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reaction solution at high temperature, reducing the life of catalyst (198). Activity of Ni is promoted by addition of another metal to form bimetallic catalyst. Small amount of Pt promotes the activity of Ni by hydrogen spillover from the Pt to Ni site, increasing its hydrogenation activity (199). Similarly, addition of small amount of Ir (1%Ir–5%Ni), another base metal, enhances the activity and stability of Ni catalyst (198). Further improvement in durability and activity of Ni catalyst is required to develop an inexpensive catalyst for direct synthesis of sugar alcohols from cellulose. Requirement of high-pressure hydrogen is a critical drawback of this reaction. The selectivity for sugar alcohols reduces as the hydrogen pressure is lowered and pressure above 2 MPa is usually used (200). Adequate yield of sorbitol can be obtained (30%–33%) with Ru/AC catalyst with 0.8 MPa H2 pressure (201). Pt nanoparticles can yield sorbitol in 54% yield in concentrated solution of H4SiW12O40 with 0.7 MPa of hydrogen pressure (113). Hydrogen donors like 2-propanol (201) and sodium formate (202) are employed to replace the need for high-pressure H2 atmosphere. Mechanocatalytically depolymerized cellulose was converted to sugar alcohols using Ru/AC catalyst with 83% yield using 1:1 ratio of water and 2-propanol (203). Separation of dehydrogenated product of hydrogen donor and its recycle adds to process cost. For example, acetone is produced by dehydrogenation of 2-propanol that must be separated from the product solution and hydrogenated back to 2-propanol for recycle. 3.4.3.2 Sugar Alcohols Derivatives
Sorbitol and its isomer mannitol must be further reacted to produce chemicals that can be used as fuel or polymer precursors. Hydrogenolysis of sugars and alcohol causes cleavage of C–C bonds, producing lower alcohols such as erythritol, glycerol, 1,2-propanediol, and ethylene glycol. Dehydration of sorbitol produces isosorbide, a potential precursor for polymers. Sequential dehydration and hydrogenation without C–C bond cleavage removes oxygen and produces aliphatic hydrocarbons. Synthesis of these downstream chemicals directly from cellulose in one pot is challenging but promising as it reduces production cost by integrating the processes. Direct conversion of cellulose to isosorbide involves three reactions in series: (1) hydrolysis of cellulose to glucose, (2) hydrogenation of glucose to sorbitol, and (3) sequential dehydration of sorbitol to sorbitan and isosorbide (Fig. 15). Therefore, bifunctional catalyst containing acid sites and metal catalyst are necessary for one-pot reaction. Addition of HCl (0.1 M) to promote dehydration of sorbitol produces isosorbide in
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Fig. 15 Reaction pathway for direct conversion of cellulose to isosorbide.
47.9% from microcrystalline cellulose in the presence of Ru/C catalyst at 488 K (204). The acidic condition also promotes rapid hydrolysis of cellulose and eliminates need for pretreatment. Soluble heteropoly acid, H4SiW12O40, is also effective for direct conversion of cellulose and lignocellulose to isosorbide with yield up to 63% (205). The activity of Ru/C catalyst was reduced after the first reaction due to deposition of humintype compounds. Furthermore, presence of lignin and hemicellulose also
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impedes the catalytic activity, and purification of cellulose before reaction increased the isosorbide yield. Ru is the most active hydrogenation catalyst for direct conversion of cellulose to isosorbide. Solid acid catalyst such as niobium phosphate and sulfonic acid resins can also catalyze direct dehydration of cellulose to isosorbide (206–208).
3.5 Hemicellulose 3.5.1 Hydrolysis Xylose is the most attractive sugar obtained from hemicellulose, as it can be converted to ethanol, furfural, and xylitol. Commercially, hydrolysis of xylan by diluted H2SO4 produces xylose at 413–453 K (209). Hemicellulose is soluble in hot-compressed water, and therefore, the hydrolysis is easier than that of cellulose. Corn cob is a typical source of xylan, as it specifically contains a large amount of xylan (28%–35%) (32). The hydrolysis of hemicellulose is also accelerated by conventional solid acid catalysts, which is different from the cellulose hydrolysis that requires specific catalysts. HUSY, H-beta, and HMOR zeolites converted softwood hemicellulose to xylose and arabinose in 40%–50% yield (210). Allophane bearing propyl sulfonic groups hydrolyzed bamboo hemicellulose to xylose in 40% yield (211). A mesoporous carbon functionalized with weak acid sites is also active for the hydrolysis reaction (152). Xylan was extracted from Miscanthus at 463 K, and the xylan extract was hydrolyzed by the carbon catalyst in sodium acetate buffer (pH 4.1) at 423 K for 4 h. The reaction gave xylose in 74.1% yield. 3.5.2 Hydrolysis and Successive Dehydration to Furfural A representative process using hemicellulose is the conversion of xylan to furfural. Furfural is a precursor to furan resins, furfuryl alcohol as an extracting agent to purify lubricating oils, tetrahydrofuran (THF), and furan. THF is a common organic solvent and feedstock for preparing spandex. Recently, the conversion of furfural to 1,5-pentanediol has also been demonstrated in laboratory study (212). Quaker Oats first developed the process using oat husks and diluted sulfuric acid in 1921 (213,214). The reaction at 426 K produced xylose as an intermediate, and xylose was simultaneously dehydrated to form furfural. Currently, annual production amount of furfural is about 360 kilotons according to Kanematsu, and it is mainly synthesized from corncob with diluted sulfuric acid in China. The reaction condition is more severe than that of hydrolysis to produce xylose, which leads to the successive dehydration to furfural. Effects of reaction parameters
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for the hydrolysis of hemicellulose by mineral acids were comprehensively summarized in the review article (215). Similar to the H2SO4 case, solid catalysts also produce furfural using harsher reaction conditions. Zeolites and silicoaluminophosphates have been intensively studied for this reaction (216–219). Dhepe and coworkers found that SAPO-44 showed highest selectivity for the production of furfural from xylose or hemicellulose (220). The yield of furfural reached 63%, and the catalytic activity was sustained for eight times in the reuse experiments. This result is surprising because the pore diameter of SAPO-44 is only 0.43 nm, which does not allow the penetration of sugar molecules into the pores. Aluminosilicate having the same skeleton as SAPO-44 (CHA) was prepared and tested in the furfural synthesis from bamboo hemicellulose (221). The zeolite also showed good selectivity for furfural. Very recently, amorphous Nb2O5 was found to give a high yield of furfural (72%) in the dehydration of xylose (222). The reaction typically occurs after the isomerization to xylulose, but Nb2O5 produces a 1,2-dioxo-3-deoxy intermediate (Fig. 16). This is similar to the manner that observed in the dehydration of glucose by phosphated TiO2 (158). The reaction mechanisms can be distinguished by isotope labeling experiments (158,222).
Fig. 16 Dehydration mechanism of xylose. LA, Lewis acid.
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Use of biphasic systems improves yield of furfural in most cases. Xylose and catalysts are in aqueous phase and the sugar is converted to furfural. Then, furfural is transferred to organic phase such as toluene and methyl isobutyl ketone. The extraction inhibits the decomposition of furfural. For example, Nb2O5 provided 48% yield of furfural in water, but the catalyst gave 72% yield in water/toluene (158). 3.5.3 Hydrolytic Hydrogenation to Sugar Alcohols Hydrolytic hydrogenation of hemicellulose and pectin produces various sugar alcohols. Ru/activated carbon converted arabinan present in beet fiber to arabitol in 83% yield under H2 pressure of 5 MPa (223). Arabinogalactan was also converted to arabitol and galactitol by Ru/MCM-48 or Ru/USY (224,225). Pt/carbon black produced xylitol in ca. 50% yield from silver grass, amur silver grass, and wheat straw after removing salts (197). Basic salts cause retro-aldol reactions and drastically inhibit the formation of sugar alcohols. In all cases, the hydrogenation of sugar in the conversion of hemicellulosic biomass was clearly slower than that of pure sugar. In addition, reuse experiments of the catalysts indicated lower activities in the conversion of real biomass. These results are due to catalyst poisons contained in the biomass such as nitrogen and sulfur compounds derived from protein.
3.6 Lignin 3.6.1 Introduction Lignin is one of the two major components of lignocellulose after cellulose. The word lignin itself was derived from the word lignum, Latin for wood, in 1819 by de Candolle (226). Native lignin, defined here as lignin in its original form present in lignocellulose, is an amorphous polymer made from aromatic building blocks that is different in chemical structure and function from the carbohydrate. Following the lignocellulose matrix model of reinforced concrete, lignin is the concrete itself that acts as a glue that fills space between other components of the cell wall and binds them together to provide structural integrity. It also acts as a barrier to keep chemical and biological foreign elements out. The hydrophobicity of lignin is useful for plant to form vascular channels to conduct water. Separation of lignin from lignocellulose during pulping process alters its chemical structure and adds inorganic impurities. This form of lignin is referred here as technical lignin. From an energy point of view lignin has high C/O ratio and accounts for 40% of carbon-based energy in lignocellulose. The aromatic building blocks
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of lignin make it an attractive feedstock for direct synthesis of specialty and fine aromatic chemicals. However, lignin has always remained secondary to cellulose, a by-product of pulp and paper industry. Only 2% of the 50 million tons of lignin produced by pulp and paper industry was utilized commercially and the remaining was used as a low-value fuel (227). Recently, efforts are made to standardize the lignin extracts obtained by pulping industry and to utilize them for chemical synthesis. 3.6.2 Technical Lignin Technical lignin is obtained after pulping of lignocellulosic biomass by thermochemical method. The objective of pulping is to obtain cellulose free from lignin and hemicellulose. Therefore, the fate of lignin after pulping was only of secondary importance. Technical lignin is usually dissolved or suspended in the pulping liquor from which it is separated and used as a fuel to power the pulping the process. The delignification process depolymerizes the lignin to smaller fragments that condense to form a polymer which closely resembles lignin but has different structural attributes and chemical property. Catalytic conversion of technical lignin is challenging due to its higher recalcitrance and presence of inorganic contaminants that deactivate catalysts. 3.6.2.1 Kraft Lignin
Kraft pulping process is the most dominant lignocellulose pulping process producing about 130 million tons of pulp annually (228). Lignin obtained from kraft pulping process has a complex and recalcitrant structure. The success of kraft pulping is attributed to the recovery of inorganic reactants. The black liquor obtained after pulping contains a mixture of fragmented lignin and Na2SO4, which upon incineration provides Na2S and CO2. Energy released from incineration of the carbonaceous lignin is used to generate steam in a high-pressure boiler (228). Therefore, the residual lignin acts a fuel to power the pulping process. Modern kraft process produces electricity from excess steam that is supplied back to the electrical grid (229). The critical role of lignin in the kraft process prohibits diverting the lignin stream for alternative application and catalytic upgrading to valuable chemicals. Furthermore, the presence of sulfur, a known catalyst poison, is also detrimental for downstream catalytic processes. Nevertheless, the abundance of kraft lignin provides options for utilizing excess lignin residue for catalytic upgrading.
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In kraft process, lignocellulose is cooked in an aqueous solution of NaOH (1 M) and Na2S (0.25–0.75 M) at temperatures of 438–448 K for 2 h (230). Lignin reacts stoichiometrically with hydroxide and hydrosulfide anions and depolymerizes to smaller fragments that dissolve in the alkaline solution. The depolymerization of lignin takes place in three stages during the cooking process. The initial stage where 1%–15% of lignin is removed takes place up to 423 K. The second “bulk stage” where most of the lignin is removed (15%–60%) takes place from 423 to 453 K and further cooking at 453 K. The final stage removes up to 90% lignin by further cooking at 453 K. At this stage the rate of lignin removal is slow and further prolonged cooking causes degradation of carbohydrates in the pulp. Depolymerization of lignin by kraft pulping occurs by multiple reactions over multiple steps. One of the major depolymerization reactions is the cleavage of nonphenolic β-aryl (C–O) linkages via the epoxide mechanism in the presence of OH ion. The second primary reaction involves trapping of quinone methide intermediates in the cleavage of free phenolic β-arylethers. In the presence of HS and OH ions a thioepoxide is formed that stabilizes the γ-CH2OH, which would otherwise be eliminated by retroaldol reaction to form formaldehyde. This reaction is crucial to reduce the condensation of lignin fragments promoted by formaldehyde. The complete reaction pathway leading for formation of kraft lignin is not well understood. It is known that condensation of lignin fragments occurs by repolymerization during the pulping process causing an increase in the molecular weight as the process continues. The regenerated lignin exhibits high abundance of recalcitrant C–C bonds (β–β, β–1, β–5, and 5–5) that have high bond dissociation energy. Therefore, from a perspective of catalytic processing of kraft lignin it is desirable to modify the pulping process to prevent formation of recalcitrant lignin by inhibiting the condensation of lignin fragments. Nevertheless, the production of pulp remains the primary target of kraft process and any modification should merit the overall efficiency and economics of the process without degrading the quality of the pulp. 3.6.2.2 Organosolv Lignin
The advantage of organosolv lignin over kraft lignin is the absence of sulfur in the process that reduces formation of organosulfur compounds, which, apart from being foul smelling, deactivate the catalyst used in subsequent processes. However, the technology to convert the sulfur-free high-quality lignin to useful products is still lacking.
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The organosolv process employs acid-catalyzed solvolysis of β-ether linkages in lignin to dissolve it. The acid is either added (HCl, H2SO4, oxalic acid, formic acid, and acetic acid), or is generated in situ by deacetylation of hemicellulose to release acetic acid in the reaction mixture. The external acid promotes acidolysis of β-ethers, thereby providing cellulose pulp with lower lignin content. On the contrary, the lignin produced by in situ acid generation is thought to retain higher number of β-ether linkages present in the native lignin (231). Comparatively, the abundance of β-ether linkages in organosolv lignin is not radically different from the kraft lignin with similar degree of delignification. Lignin fragments condense back to higher molecular weight polymers at higher acidity as the formation of formaldehyde by the elimination of γ-CH2OH becomes predominant. Therefore, optimization of the process for maximizing the quality of cellulose degrades the quality of lignin and vice versa. 3.6.3 Catalytic Reactions of Lignin The variation in structural components and their bonding makes it difficult to understand the reactions taking place during catalytic conversion of lignin. Therefore, to better understand the reaction pathways and catalytic mechanism many researchers have used model compounds that represent one or multiple components and bonding motifs in lignin. Recent reviews have comprehensively summarized the catalytic conversion of lignin model compounds (232). Here we will briefly discuss lignin conversion by oxidation, catalytic cracking, reduction, and early-stage catalytic conversion. Oxidative depolymerization of lignin produces aromatic compounds with various functional groups. Some of these products are useful as fine chemicals or as precursors for synthesis of organic products. Catalytic side chain oxidation and fragmentation is the primary mode of lignin depolymerization (233). The oxidants used for lignin oxidation include O2, H2O2, ozone, nitrobenzene, and permanganate (234–237). Gaseous O2 is always the preferred oxidant due to its abundance in air and lack of environmental footprint. Vanillin is an important chemical derived from catalytic oxidation of lignin using gaseous O2 (238). Vanillin is the only commodity chemical commercially produced from lignin. Pd/Al2O3 oxidizes lignin obtained from sugarcane bagasse to vanillin and syringaldehyde and p-hydroxybenzaldehyde (239). Metal oxides like CuO under alkaline conditions oxidize milled wood lignin to 25%–50% phenols yield (234,240). Mild oxidation of secondary (benzylic) alcohols in Aspen lignin to benzylic ketones weakens the C–O ether bonds that allow easy depolymerization in
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Fig. 17 Two-step depolymerization of lignin. α Carbon is first oxidized to ketone followed by reaction in formic acid/sodium formate solution to produce low molecular weight aromatics (241).
the presence of formic acid, resulting in formation of up to 61 wt% of low molecular weight aromatic (Fig. 17) (241). Catalytic pyrolysis of lignin in the presence of zeolites produces valuable hydrocarbons (242,243). It has been proposed that the strong acid sites in zeolite catalyze the cleavage of C–O and C–C bonds to produce reactive intermediates of low molecular weight. Consequently, the microspores of zeolite stabilize these intermediates and prevent repolymerization and coke formation (244). However, strong acidity also facilitates side reaction, leading to formation of gaseous species and coke. Addition of cerium to HZSM-5 was reported to reduce acidity and yield valuable oxygenated products (242). Under reductive reaction condition in the presence of hydrogen and metal catalyst lignin undergoes hydrogenolysis, hydrodeoxygenation, and hydrogenation reactions (245). The hydrogenolysis of lignin removes the alkyl chain or functional groups to produce hydrocarbons. This reaction is catalyzed by supported metal catalysts containing Pt, Ru, Ni, Pd, and Cu as typical active metals under basic condition (246,247). Hydrodeoxygenation reaction is used for upgrading liquid products derived from lignin. Bio-oil produced by pyrolysis or liquefaction contains large amount of oxygen that can be removed by hydrodeoxygenation. Transition metal phosphides Ni2P, Fe2P, Co2P, and WP were recently reported to be active for hydrodeoxygenation of lignin and lignin-derived chemicals (248,249). Alkaline lignin could be converted to mixed phenol in the presence of WP supported on carbon at 553 K and 2 MPa H2 pressure (248). Monometallic platinum group catalysts are not suitable for this reaction as they tend to hydrogenate the aromatic ring of lignin monomers. The irreversible degradation of lignin during pulping process is detrimental for the future biorefinery scenario where utilization of all components is important. Recently, lignin separation technologies have
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Fig. 18 Proposed reactions and observed monomers in reductive stabilization of corn stover in methanol using Ru or Ni catalyst supported on acidified carbon (250).
emerged that prevent lignin condensation by stabilizing the components as they are separate. In this approach, termed as Reductive Catalytic Fractionation or Early-Stage Catalytic Conversion of Lignin, solvolytic extraction of lignin is followed by depolymerization and immediate stabilization of reactive intermediates by catalytic reduction (Fig. 18) (251). Metal catalyst like RANEY® Ni (252), Ni/C (253), Pd/C (254,255), and Ru/C (250). The hydrogen is present as gaseous H2, or it can be obtained from hydrogen donors like 2-propanol, methanol, or formic acid (252,253,256). This approach reduces the deposition of condensed lignin on remaining carbohydrates and improves the quality of hemicellulose and cellulose fractions also.
4. CHITIN 4.1 Availability and Market Chitin and chitosan are the second most abundant carbohydrate biomass on Earth, with an estimated natural production of 100 billion tons per year (257). Structurally, they are polymers of NAG and glucosamine linked by
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Fig. 19 Structure of chitin and chitosan. Chitin has a larger ratio of m than chitosan.
β-1,4-glycosidic bonds (Fig. 19). Although there is no clear definition, the polymer with high ratio of NAG/(NAG + glucosamine) is called chitin, and that with a low ratio is denoted chitosan. The ratio is named degree of acetylation (DA). Chitin is insoluble in weak acid, but chitosan is easily soluble due to a larger content of nonprotected amino groups. Chitin is chemically stable and mechanically rigid, and therefore, it is found in shells of crustaceans and insects (258). Crab or prawn shells are the most typical sources of chitin as they are available in large quantity from food industry. Top three catches of crabs were 1.99 million tons in China, 0.15 million tons in United States, and 0.10 million tons in Canada in 2015. In addition, total culture amount of crustaceans was 6.9 million tons in 2014 (259). Price of the shell flakes is typically 0.1–0.5 US dollars per kilogram. Chitosan is present in microbes, but commercial chitosan is also available from crustaceans’ chitin after hydrolyzing its amide bonds by high concentration of NaOH. Chitin, chitosan, and their derivatives have practical application in wide area. In agriculture, crab and prawn shell flakes, which contains N, Ca, and P, are directly sown in the field as fertilizers. Chitin and chitosan oligosaccharides work as elicitors so that plants do not get sick and grow better. Chitosan is a useful agglomeration agent to purify muddy water produced in construction industry (260). Chitosan is positively charged as it possesses NH2 groups, while dispersed clay has negative charge. Thus, chitosan effectively neutralizes charge of clay to precipitate it. The agglomerated clay can be used for planting. Chitosan is more environmentally friendly than conventional agglomeration agents such as aluminum sulfate and polyaluminum chloride with base. In medical use, as chitosan is soluble in alkali solution, it can be easily molded into a film. The film is used for treatment of a burn as chitosan has high affinity with human body and bactericidal action (261,262). Chitin and chitosan oligosaccharides are potential prebiotics to improve gut functions and anticancer health foods (263,264). Although the oligosaccharides are sold by chitin-specialized companies, the medical applications have been in research phase so far. The monomeric units of chitin, NAG and glucosamine, are used as supplements because they are source
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of hyaluronic acid and chondroitin sulfate, which are constituents of synovial fluid and cartilage, respectively. Hence, the oral intake possibly provides relief from osteoarthritis and arthralgia (265). NAG has a sweet taste, whereas glucosamine salts are bitter. However, glucosamine is mainly used for this application because of the high price of NAG up to more than 100 US dollars per kilogram due to difficult production.
4.2 Composition and Structural Discussion The most readily available chitin source is crab shells. The shell has a layered structure composed of epicuticle, pigmented layer, principal layer, and membranous layer from outside to inside (Fig. 20) (266,267). The second and third layers, which are major parts of shells, contain well-aligned chitin microfibrils with ca. 3 nm diameter covered by proteins to form a twisted plywood pattern (266,268). Different from insect’s cuticle, no quinone tanning is found in the protein. Instead, the structure is reinforced by minerals mostly composed of CaCO3 (269,270). The twisted plywood structure and the biomineralization afford high mechanical strength. A red snow crab, Chionoecetes japonicus, shell contained 21.6 wt% chitin, 17.9 wt% proteins, and 52.2 wt% ash in winter in dry basis, according to Toyama Prefectural Agricultural, Forestry & Fisheries Research Center. Three crystal structures are known for chitin (271). In α-chitin, vicinal chitin molecule chains are present in antiparallel form (272,273), but β-chitin has a parallel structure (274,275). γ-Type contains both parallel and antiparallel structures. α-Chitin is the most common form of chitin as found in shells of crabs and prawns, but squids specifically have β-chitin in their cartilage (276). γ-Chitin is found in stomach of squids and octopuses. The crystalline structures can be distinguished by 13C cross-polarization/ magic-angle spinning (CP/MAS) NMR or X-ray diffraction (XRD) (276). Difference in infrared (IR) spectra is not very clear.
Fig. 20 Structure of exoskeletal of crabs.
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DA values of chitin slightly depend on types of animals (276). Crab chitin contains about 80% fraction of NAG units and 20% of glucosamine units, and a prawn has similar DA values of ca. 80%. In contrast, a squid has a very high DA of 95%. DA is determined by IR or ultraviolet–visible (UV–vis) spectroscopy. A UV–vis method measures the amount of acetamido groups by the π–π* transition absorption after the partial hydrolysis of chitin using phosphoric acid to be soluble (277). This method would be reasonably accurate after tuning the hydrolysis condition suited for the specific samples.
4.3 Conversion Technologies The conversion of chitin reported in literatures mostly includes hydrolysis/ alcoholysis to produce polymeric or monomeric sugars (Fig. 21). The monomeric sugars further undergo hydrogenation/hydrogenolysis including retro-aldol reactions to alcohols, oxidation to acids, dehydration to nitrogen-containing cyclic compounds, and dehydration–deamidation to nitrogen-free aromatics. Hydrolysis of chitin to chitosan, chitosan oligosaccharides, chitin oligosaccharides, glucosamine salts, and NAG has been performed in commercial scale, but other technologies are in research phase at this moment. 4.3.1 Hydrolysis 4.3.1.1 Commercial Methods
For synthesizing NAG and glucosamine salts in industry, crab and prawn shell is first treated with HCl aq. to remove CaCO3, followed by NaOH aq. to dissolve proteins. The purified chitin is hydrolyzed with high concentration of HCl or H2SO4 aq. The product shifts from chitin oligosaccharides to NAG, and to glucosamine salts by increasing the reaction time (Fig. 22) (278). Use of diluted acid affords a low yield of NAG due to significant deacetylation even in early hydrolysis stage, the detail of which is described in Section 4.3.1.2. Acetylation of glucosamine with acetic anhydride also produces NAG, as an amino group is a stronger nucleophile than a hydroxyl group. However, use of the NAG synthesized by acetylation is limited. For example, in Japan, NAG of food additive grade must be produced by the enzymatic hydrolysis of chitin oligosaccharides prepared with HCl as determined by the Japan’s Specifications and Standards for Food Additives. The deacetylation and legal issues lead to the higher price of NAG relative to glucosamine salts. Chitosan is produced by deacetylation of the purified chitin using a high concentration of NaOH aq. A strong base easily cleaves amide bonds but
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Fig. 21 Classification of chitin conversion technologies.
hardly dissociates glycosidic bonds. The chitosan with almost 0% DA is hydrolyzed by chitosanases to chitosan oligosaccharides. 4.3.1.2 Mechanism of Acid Hydrolysis
The acid hydrolysis of glycosidic bonds and amide bonds takes place by different mechanisms. Glycosidic bonds are cleaved by an SN1 reaction as
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Fig. 22 Hydrolysis of chitin using a concentrated acid.
discussed in the section of cellulose hydrolysis. In contrast, amide is hydrolyzed by an SN2 reaction, in which the attack of water molecule to protonated carbonyl group is the rate-determining step. Therefore, rate of the two reactions has different dependence on the concentration of acid (279). Diluted HCl (3 M) provided kgly/kami of 2.2 at 298 K, where kgly and kami are pseudo first-order rate constants of the hydrolysis of glycosidic bonds and amide bonds, respectively. In contrast, concentrated HCl (12 M) afforded a very high kgly/kami ratio of 120. Therefore, concentrated HCl is used for the production of NAG. The reactivity of glycosidic bonds in chitin depends on their position in the hydrolysis. A mechanistic study using N,N0 ,N00 ,N000 tetraacetylchitotetraose suggested that the glycosidic bond on nonreducing terminal of chitin was hydrolyzed 2.5 times faster than others (280). A similar result was obtained in the hydrolysis of maltodextrin using radio isotopes to determine the hydrolysis position (281). The higher reaction rate was attributed to a larger entropy in the transition state. The absence of a glycosidic bond at C4 position leads to higher freedom in the conformation. 4.3.1.3 Recent Studies on Hydrolysis
The conventional hydrolysis of chitin uses large excess of mineral acids, leading to environmental load and high cost of monomer production. Therefore, more efficient processes are necessary for the efficient biorefinery.
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Recently, it was found that mechanocatalysis (see Section 3.3.1) is effective for the selective hydrolysis and that some solvents drastically improve the reactivity of chitin. The mechanocatalysis was applied for the conversion of chitin (282). Chitin was impregnated with a catalytic amount of H2SO4, and the sample was treated by planetary ball-mill. The resulting powder was fully soluble in water, and it contained NAG and N-acetylated oligomers. Thus, the hydrolysis of chitin occurs in the presence of mechanical force and an acid catalyst with retention of N-acetyl groups, in which physisorbed water on the sample is used for the reaction. It is speculated that the mechanical force assists the cleavage of the connecting points of polymers, in which polymers are subjected to the mechanical force as tensile stress (283,284). Therefore, the mechanical force predominantly activates glycosidic bonds connecting the NAG units, but not amide bonds in the acetamido groups hanging from the NAG units. The NMR study has clarified that the oligomers contain branched structures such as 1,6-glycosidic bonds (282), similar to the mechanocatalytic hydrolysis of cellulose (59). This might be due to trapping of oxocarbenium intermediates by other sugar molecules in the presence of only small amount of water molecules. Hydrolysis of the mixture of NAG and oligomers containing H2SO4 gave NAG in 53% yield at S/C ¼ 2.0. Later, the mechanocatalytic step was optimized, and the yield of NAG was increased to 61% at S/C ¼ 3.4 (285). The mechanocatalytic process also works under basic conditions (286). Planetary ball-milling of chitin in the presence of a stoichiometric amount of NaOH quantitatively produced low molecular weight chitosan. Hence, the hydrolysis of both glycosidic bonds and amide bonds took place under the reaction conditions, which is different from the mechanocatalysis with H2SO4. Base can directly attack the carbonyl by SN2 reaction, while acid hydrolysis requires a water molecule in the rate-determining step (Fig. 23). This could be the cause why base can hydrolyze amide under the mechanochemical conditions. The production of low molecular weight chitosan was applicable for the conversion of shrimp shells. Product was obtained in 20.9 wt% with a good purity as high as 90%, containing relatively small amounts of proteins and CaCO3. Chitin is very recalcitrant in pure water, but Yan and coworkers have found that the reactivity is greatly improved by adding specific organic solvents (287). Using 100 mM H2SO4, a cosolvent system containing diethylene glycol diethyl ether and water (4:1) converted chitin to glucosamine sulfate in 80% yield, but no glucosamine was obtained in the absence
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Fig. 23 Hydrolysis of amide under (A) basic and (B) strongly acidic conditions.
of the organic solvent. Based on the screening of organic solvents, they proposed two properties that an organic solvent should possess to achieve a high performance. One is low basicity (low β in Kamlet–Taft parameters) (288) to increase the activity of acid. The other is similar degree of hydrogen bonding property (δH ¼ 5.5–7.5 in Hansen solubility parameters) (289) to chitin for the good affinity. 4.3.2 Alcoholysis NAG and glucosamine are produced by the hydrolysis of chitin as described above, and similarly 1-O-substituted NAG and glucosamine are synthesized with an acid catalyst in alcohol instead of water solvent. The alcoholysis of chitin by 8 wt% H2SO4/ethylene glycol produced 1-O-(2-hydroxyethyl)glucosamine in 23.8% yield and 1-O-(2-hydroxyethyl)-NAG in 6% yield at 438 K in 1 h (290). The liquefaction of chitin requires severe reaction conditions that also cause deacetylation except for concentrated acids. Instead, chitin oligomers made by mechanocatalytic hydrolysis were employed for the methanolysis reaction, which gave 70% yield of 1-O-methyl-NAG at S/C ¼ 4.1 (282). 4.3.3 Hydrogenation/Hydrogenolysis Application of hydrogenation conditions to chitin oligomers and NAG gives a series of alcohols. Initially, the hydrogenation/hydrogenolysis of NAG was performed with supported metal catalysts (291). Ru/C converted NAG to 2-acetamido-2-deoxysorbitol in 97.7% yield at 353 K under H2 pressure. The main product was gradually changed to 2-amino-2-deoxysorbitol (<0.1–12.4 %C), 2-acetamido-1,4,5,6-hexanetetrol (0–28.4 %C), Nacetylethanolamine (0–8.7 %C), and butanetriols (0–5.7 %C) with increasing temperature. Time-course analysis of this reaction has suggested the
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Fig. 24 Conversion of NAG under hydrogenation conditions.
following reaction pathways (Fig. 24). NAG undergoes four reactions: (i) hydrogenation of hemiacetal (¼aldehyde), (ii) deacetylation and hydrogenation, (iii) hydrodeoxygenation and hydrogenation, and (iv) retro-aldol reaction and hydrogenation. The direct hydrolytic hydrogenation of chitin gave only a small amount of N-acetylethanolamine (291), and even that of chitin oligomers derived from the mechanocatalytic hydrolysis produced 25% yield of 2-acetamido-2-deoxysorbitol (285). These results are clearly different from those in the conversion of cellulose and its oligomers, which provides up to 90% yield of sorbitol (59,193). The presence of acetamido group instead of hydroxyl group greatly influences the reactivity. Thus, a stepwise conversion of chitin was studied to maximize the yield of 2-acetamido-2-deoxysorbitol and clarify the characteristics of this reaction (285). H2SO4-impregnated chitin (S/C ¼ 3.4) was hydrolyzed by mechanocatalysis, and the resulting solid was subjected to the hydrolysis condition in water. The reaction mixture was heated from 298 K to a designated temperature and then cooled down. Yield of NAG was 61% at 448 K, and a higher temperature or a longer reaction time led to degradation of the product. The product solution was directly used for the subsequent hydrogenation using Ru/TiO2 catalyst at relatively low temperature (393 K) because a higher temperature caused more side reactions (291) as described above. However, the yield of 2-acetamido-2-deoxysorbitol was still 37%. It was found that side reactions easily occur at the low pH (2) due to H2SO4 catalyst in the presence of the hydrogenation catalyst. The partial neutralization of the mixture with NaHCO3 to pH 3–4 prior to
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the hydrogenation reaction improved the yield of 2-aetamido-2deoxysorbitol up to 52%. In contrast, the complete neutralization to pH 7 resulted in the rapid retro-aldol reaction giving N-acetylethanolamine (16 %C) and erythritol (8 %C) due to remaining base (HCO3 ). A slightly acidic condition in addition to low temperature would be the best for the selective hydrogenation of NAG to 2-acetamido-2-deoxysorbitol, which so far conflicts with the requirements for hydrolysis of chitin to NAG. Therefore, the hydrolytic hydrogenation of chitin to the sugar alcohol is difficult. 4.3.4 Oxidation The hemiacetal group of glucosamine and NAG is easily oxidized by supported metal catalysts, which is similar to the oxidation of glucose (186). Au/MgO converted glucosamine to glucosaminic acid in 99% yield, while Au/hydrotalcite produced N-acetylglucosaminic acid in 95% yield from NAG (292). This reaction requires a base to activate aldehyde groups and MgO support provided higher reusability than hydrotalcite, but the weaker basicity of hydrotalcite would be favorable for the selective conversion of NAG with retention of N-acetyl groups. A more severe oxidation condition using CuO catalyst, 5 atm of O2, and 2 M NaOH produces acetic acid from chitin (293). Even shrimp shell was converted to acetic acid in 47.1 %C in the system. The reaction contains deamination, C–C bond dissociation by retro-aldol reactions, isomerization, Cannizzaro reaction, benzilic acid rearrangement, and oxidation of alcohols and aldehydes, which leads to the formation of acetic acid, formic acid, and oxalic acid. Interestingly, the ammonia produced by deamination was used for the formation of a small amount of pyrrole. 4.3.5 Dehydration to Nitrogen-Containing Cyclic Compounds Dehydration of chitin or NAG produces N-containing cyclic compounds (Fig. 25). NAG undergoes dehydration at C3 position probably in the enolate form (294) and subsequent 1,4-cyclization to form chromogen I. Chromogen I is in equilibrium with 2-acetamido-3,6anhydro-2-deoxyglucofuranose (ADGF) and 2-acetamido-3,6-anhydro2-deoxymannofuranose (ADMF). One-water elimination from chromogen I provides chromogen III, and an additional water elimination gives 3-acetamido-5-acetylfuran (3A5AF). Chromogen I and III have been detected in the decomposition of NAG under mild basic conditions, in which the yield of chromogen III increases
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Fig. 25 Plausible mechanism for the dehydration of NAG.
up to 40% (295,296). Potassium tetraborate also accelerated the formation of chromogens in the decomposition of N-acetylhexosamines (297). Based on that, Usui and coworkers obtained 50% yield of chromogen I, 10% yield of ADGF, and 10% yield of ADMF in the conversion of NAG by borate (298). Furanodictins derived from ADGF and ADMF are candidates of medicines (298–300). Besides, hot-compressed water converted NAG to chromogen I in 23.0% yield and chromogen III in 23.1% yield even without a catalyst (301). 3A5AF was found in the pyrolytic products of chitin, although the yield was only 2% at that time (302). The yield can be drastically improved by using catalysts, organic solvents, and NAG. Kerton and coworkers reported 62% yield of 3A5AF in the conversion of NAG with boric acid and NaCl in dimethylacetamide (303). Similarly, a combination of boric acid and 3-butyl-1-methylimidazolium chloride produced 3A5AF in 60% yield (304). The combined use of boric acid and Cl also produced 3A5AF in 7.5% yield from chitin (305) and 28.5% yield from ball-milled chitin (306).
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Although the reaction mechanism is unclear, it is known that boric acid forms esters with diols and Cl interacts with hydroxyl groups of sugars in aprotic organic solvents. Such interactions are possibly involved in the reaction (304). Another type of N-containing cyclic compound formed by the dehydration is fructosazine. The conversion of glucosamine in methanolic HCl produced fructosazine in 12.7% yield (307). Amino groups in two glucosamine molecules attack carbonyl groups of each other to form a cyclic diimine and two water molecules, which further undergoes dehydrogenation to produce fructosazine or dehydration to give deoxyfructosazine (308,309). 4.3.6 Dehydration–Deamidation Conversion of chitin by strong acids under harsh conditions tends to produce N-free furan derivatives. Chitin was converted to 5-(chloromethyl) furfural in 45% yield in a mixture of concentrated HCl and 1,2dichloroethane (310). Up to 10 wt% of substrate can be used in this reaction, which is very important for the economic conversion of sugar biomass. In contrast, it is preferable to develop a more environmentally friendly solvent to facilitate the practical applications. The conversion of glucosamine gave 5-HMF in 21.7% yield in 67% ZnCl2 aq. at 393 K in 90 min (311). This system was applicable to chitosan (13% yield) and chitin (9% yield), but the conversion of NAG gave a very low yield of 5-HMF (2.8%). Deamidation of NAG by Zn2+ was slow, while humin formation rapidly occurred. The yield of 5-HMF was improved to 37.9% in the conversion of NAG by FeCl2 catalyst and DMSO/water mixed solvent (312). An ionic liquid catalyst (S/C ¼ 0.05), N-methylimidazolium hydrogen sulfate, was found to convert NAG, glucosamine, chitin, and chitosan to 5-HMF in yields up to 64.6% in the same solvent (313). Although the reaction mechanism is unclear, the mass spectroscopy detected an intermediate with a molecular weight of NAGacetamide (¼162). The compound may undergo the successive dehydration to 5-HMF. Formic acid solvent transformed ball-milled chitin into 5-(formyloxymethyl)furfural (FMF) in 34.6% yield (314). It was proposed that chitin is partially formylated at C6 position (Fig. 26). Its glycosidic bonds are randomly cleaved, and it leads to the formation of 6-O-formyl NAG. The further formylation gives 3,4,6-O-triformyl NAG. The compound is converted to FMF, although the reaction details are not yet clear.
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Fig. 26 Conversion of chitin to FMF.
5. FUTURE DIRECTION AND PROSPECTS Catalytic conversion of biomass has progressed significantly in the past century. However, many challenges still remain before these processes can be incorporated commercially at global scale. Initially, the structure of lignocellulose and its components is still not fully understood. Despite a century of research there is disagreement in the relationship between structure and chemical inactivity of cellulose. Better understanding of structure would allow design of solvent systems that can dissolve and improve reactivity of cellulose. Similarly, understanding the chemistry of linkages in lignin can help in development of facile process for lignin separation. Furthermore, pretreatment of lignocellulose is the most energy-intensive step in biorefinery process. Chitin has a structure similar to cellulose, and it also requires pretreatment to increase reactivity. Targeted pretreatment reactions based on better understanding of the structure would reduce the energy requirement for biomass pretreatment. The end use of biomass-derived chemicals can be to produce chemical precursors currently produced from fossil fuels. For example, purified terephthalic acid (PTA), currently derived from petroleum and used as monomer for polyethylene terephthalate synthesis, can be produced from biomass through addition of ethylene to 2,5-dimethylfuran and subsequent oxidation. Alternatively, new polymers can be developed that directly utilize the oxygenated monomers. For example, furan dicarboxylic acid is a potential alternative for PTA than can be used for synthesis of polyethylene 2,5furanate.
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Utilizing lignin produced in the pulp and paper industry would add value to the process and reduce carbon footprint. Therefore, it is beneficial to develop pulping methods that extract lignin monomers in tandem instead of producing condensed polymeric product. A process similar to Reductive Catalytic Fractionation can be adopted to achieve this goal. In a purely biorefinery scenario too, utilization of lignin is critical to derive value from all streams to increase economic viability of the process. Therefore, a biorefinery process must be developed that integrates energy and products from various streams.
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Abhijit Shrotri et al.
ABOUT THE AUTHORS Abhijit Shrotri is an Assistant Professor at the Institute for Catalysis in Hokkaido University. He received his PhD in Chemical Engineering from the University of Queensland under the supervision of Dr. Jorge Beltramini in 2014. He served as a postdoctoral fellow at the Catalysis Research Center in Hokkaido University from 2014 to 2016. His research interest includes biomass conversion using heterogeneous catalysts and design of functional carbon catalysts.
Hirokazu Kobayashi earned his PhD in Tokyo Institute of Technology under the supervision of Prof. Ichiro Yamanaka. Then, he moved to Catalysis Research Center, Hokkaido University as an Assistant Professor. He is studying heterogeneous catalysis for the utilization of biomass and catalytic oxidation of alkanes.
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Atsushi Fukuoka is a Professor at the Institute for Catalysis (ICAT) in Hokkaido University. He studied homogeneous catalysis and received a PhD from the University of Tokyo in 1989. Then, he joined Research Institute for Catalysis and Catalysis Research Center (RIC and CRC, predecessors of ICAT) and started the research on heterogeneous catalysis. Since 2010 he had served as Director of CRC and now he is Advisor to the President of Hokkaido University. He received a Society Award from The Catalysis Society of Japan in 2015 and GSC Award from Ministry of Education, Culture, Sports, Science and Technology, Japan in 2015. He is an executive council member, Officer, of the International Association of Catalysis Societies. His current research interests are in biomass conversion by heterogeneous catalysts and the catalysis of mesoporous materials.