Review of Alkali-Based Pretreatment To Enhance Enzymatic

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Review

A review of alkali-based pretreatment to enhance enzymatic saccharification for lignocellulosic biomass conversion Huanfei Xu, Bin Li, and Xindong Mu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01907 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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Industrial & Engineering Chemistry Research

A review of alkali-based pretreatment to enhance enzymatic saccharification for lignocellulosic biomass conversion Huanfei Xu,†,‡ Bin Li,*,‡ Xindong Mu‡ †

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao,

Shandong 266042, China. ‡

CAS Key Laboratory of Bio-Based Material, Qingdao Institute of Bioenergy and Bioprocess

Technology, Chinese Academy of Sciences, Qingdao, Shandong 266101, China. KEYWORDS: lignocellulosic biomass; alkaline pretreatment; mechanisms; enzymatic saccharification; fermentable sugars.

ABSTRACT: Lignocelluloses have been the focus of much attention on their conversion into fermentable sugars for cellulosic ethanol production, both from the viewpoint of energy and the environment. Pretreatment plays a crucial rule in biomass conversion, to overcome the chemical and structural difficulties which have evolved in lignocelluloses, and to produce a cost effective fermentable sugar via enzymatic saccharification. Among the developed pretreatment approaches, alkali-based pretreatment technology, which can utilize the equipment and chemical

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recovery system in pulping industry, has been considered one of the most promising pretreatment methods, due mainly to its high efficiency in delignification and high final total sugar yields. This paper reviews the classification, mechanism, advantages, disadvantages, and the progress of alkali-based pretreatment technologies, in order to better understand the fundamental principles of alkali-based pretreatments. This is of vital importance for the process improvement and commercial production of alkali-based pretreatment for producing cellulosic ethanol.

1. INTRODUCTION. Nowadays, with the improved living standards of human, the energy requirement will be further increased. According to the World Energy Outlook from International Energy Agency, 1 by 2040, global energy demand will increase by about 40%. Meanwhile, with the increased limitations on greenhouse gas emissions, a great deal of effort has been making to reduce the dependence of depleting fossil resources in many countries around the world. Therefore, more and more attentions have been paid on the development of sustainable energy, such as the biomass conversion for producing biofuels and value-added chemicals or materials. 2-3 Global bioenergy resources are greatly sufficient to meet the projected biofuels and biomass supply without any competition with food production. Lignocellulosic biomass can be widely sourced from woody plants, herbaceous plants, agricultural waste (e.g. corn stover, corn cob, wheat stover, sugar cane bagasse, etc.), forest waste (e.g. sawdust, bark, etc.), industrial and municipal solid waste (e.g. office waste paper, waste newspaper, corrugated paper carton, etc.), energy crops (e.g. switchgrass, miscanthus, etc.), and so on, and its annual production is about 200 billion tons all over the world. 4


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Figure 1. The process chart of lignocellulosic biomass conversion for producing biofuel Yet, the complex structure of biomacromolecules of lignocelluloses has strong resistance to microbe or enzyme destruction, leading to a very high cost of biorefinery process. Lignocellulosic biomass conversion for producing biofuels (e.g. bioethanol) mainly includes five unit operations (Figure 1): feedstock preparation, pretreatment, enzymatic hydrolysis, fermentation, and product purification (e.g. separation and distillation). 5 Enzymatic hydrolysis and fermentation can be integrated together, which is known as simultaneous saccharification and fermentation (SSF), while the separated enzymatic hydrolysis and fermentation process is referred as separate hydrolysis and fermentation (SHF). Besides the low efficiency of enzymatic hydrolysis, the major challenge of lignocellulosic biomass conversion is the high cost of pretreatment. 6 As shown in Figure 1, the role of pretreatment is to effectively break the natural recalcitrance, change the biomass chemical composition as well as the macroscopic and microscopic size and structure to significantly ameliorate the efficiency of downstream processes for producing fermentable sugars. 7 Thus, deeper and better understanding of the fundamental principles and process conditions of pretreatment is of significant importance for its process


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improvement/optimization and large scale application (particularly for commercial production in the near future). Up to now, many pretreatment methods have been investigated on a variety of feedstock and still in development. 7 Pretreatment can be generally classified as: physical, biological, chemical (e.g. dilute acid, dilute alkali, organic solvent, ionic liquid, etc.), and multiple/combined pretreatment (e.g. steam explosion, ammonia fibre/freeze explosion (AFEX), CO2 explosion pretreatment, sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL), etc.). 7-8 In general, combined pretreatment approaches like AFEX and SPORL are more effective for the improvement of enzymatic saccharification. 9 Certainly, each pretreatment method has advantages and disadvantages. For example, physical pretreatment like grinding and milling can increase available specific surface area of feedstock, and decrease the crystallinity and degree of polymerization (DP) of cellulose, thus enhancing the digestibility of lignocellulosic biomass, but utilization of physical pretreatment alone will be very expensive with high energy consumption, particularly on large scale application. 9-10 Biological pretreatments are most associated with the action of fungi (like brown-, white-, and soft-rot fungi) which is able to degrade lignin or hemicelluloses in lignocellulosic biomass. 11 Lignin depolymerization by these fungi is very selective and efficient, 9 but the very long residence time (10-14 days), as well as the requirement of large space and careful growth conditions of fungi make biological pretreatment less attractive for industrial purposes. 12 Chemical pretreatments are the most extensively studied pretreatment techniques. For instance, dilute acid pretreatment can efficiently remove hemicelluloses, exposing cellulose for the improvement of enzymatic saccharification, but it has to deal with several key issues, such as


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equipment corrosion, environmental pollution, and high concentration of inhibitors (e.g. 5hydroxymethyl furfural (HMF) and furfural) in hydrolyzate. 5 Alkali-based pretreatment can efficiently remove lignin and various uronic acid substitutions on hemicelluloses with relatively low polysaccharides loss compared to acid or hydrothermal pretreatment. 13 Also, over 90% of pulp mills are alkali-based around the world. 14 Thus, one viable approach is to integrate alkalibased pretreatment into existing pulp mill by using the equipment and chemical recovery system already well developed in pulp industry to lower the capital cost of pretreatment and gain more benefits for pulp mills. 15 Therefore, alkali-based pretreatment is considered as one of the most promising pretreatment methods. 16 In this paper, the mechanisms, status and perspective of alkali-based pretreatment technologies for enhancing enzymatic saccharification were reviewed. For each kind of pretreatment, the typical process, mechanism, recent advances, advantages and disadvantages were discussed. In addition, the strategies for industrial application of alkali-based pretreatment technologies were proposed as well. 2. COMPOSITION AND STRUCTURE OF LIGNOCELLULOSES In order to better understand the behavior of lignocellulosic biomass during pretreatment for the production of fermentable sugars, it is indispensable to have a basic knowledge on the chemical composition and physical structure of lignocellulosic biomass, because the raw material properties have big impact on both the pretreatment conditions used and the effectiveness of pretreatment. 17 As an organic material, lignocellulosic biomass is mainly composed of three elements C, O, and H, and the contents of other elements like N, Na, K, Ca, Mg, Si are very small. 18 For example,


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wood contains about 49% of C, 6% of H, and 44% of O, while the total contents of other elements are less than 1%. 14 These elements form the main cell wall compounds of cellulose, hemicelluloses, and lignin, which are the main compositions of lignocellulosic biomass, 5 as shown in Table S1 and Figure 2. Other substances like extractives and ash also can be found in lignocellulosic biomass. In general, woody biomass (softwood and hardwood) has a higher lignin and cellulose content, and density than herbaceous biomass (e.g. rice straw, wheat straw). Thus, usually, the pretreatment of woody biomass required more serious conditions (like higher temperature, higher chemical charge, and higher energy consumption) compared to herbaceous biomass. Also, woody biomass typically has lower ash content than herbaceous biomass. Lignin content of softwood is higher than that of hardwood. Especially, cotton, hemp and jute have significantly high content of cellulose (60-95%) (Table S1). These high cellulose content lignocelluloses (particularly for cotton) are beneficial for the production of value-added cellulose products as well, like dissolving pulp and special paper products. 19 Cellulose is a strictly linear homopolymer of β-1-4-linked anhydro-D-glucose units, and each unit is corkscrewed 180º to its neighbors (Figure 2a). Because the cellobiose (repeat segment) is the basic unit, to some extent, cellulose is an isotactic polymer of cellobiose. 21 Different from cellulose, hemicelluloses are hetero-polysaccharides, which consist of several different sugar moieties, are mostly branched with lower molecular masses (Figure 2b). In general, the main chain of hemicelluloses can consist of only one sugar unit (i.e. homopolymer like xylans), or of two or more sugar units (i.e. heteropolymer like glucomannans), and some of the units are always (or sometimes) side groups of a backbone of hemicellulose, such as 4-Omethlyglucuronic acid, galactose. 14 Lignin is a complex phenolic polymer formed by radical coupling reactions of mainly three different monolignols: p-coumaryl, coniferyl, and sinapyl. 22


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The three different monolignols are so-called syringyl lignin (S-lignin), guaiacyl lignin (Glignin), and hydroxyl-phenyl lignin (H-lignin), respectively (Figure 2c). Both hemicellulose and lignin are closely associated with cellulose via hydrogen and covalent bonds, or hydrophobic and ionic interactions, 7 forming the strong recalcitrance of lignocelluloses to enzymatic saccharification.

Figure 2. The structure of cellulose (a), the typical molecular structure of hemicellulose (b), and the basic structure of lignin units (c). 3. RECALCITRANCE OF LIGNOCELLULOSES AND THE GOAL OF PRETREATMENT In the past hundred million years, lignocellulosic biomass has evolved complex structure and chemical compositions to protect the structural saccharides from outside attack. Cellulose and


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hemicelluloses are the main source of fermentable sugars, which are compactly associated with lignin, while lignin is the major barrier to enzymatic saccharification of lignocelluloses. 13 Other factors, such as crystallinity, DP, and the strong interchain hydrogen-bonding network of cellulose, available surface area, the content of acetyl groups, the presence of hemicellulose and its bond with cellulose and lignin, the distribution of lignin and hemicelluloses, as well as the type of lignin, 7 also contribute to the hindrance of lignocelluloses to enzymatic saccharification. On the other hand, the natural factors contributing to the resistance of lignocelluloses to chemicals, enzymes or microorganism also include: 23 (1) the epidermal tissue (e.g. the cuticle and epicuticular waxes) of the plant body; (2) the density and arrangement of the vascular bundles; (3) the relative amount of thick wall (sclerenchymatous) tissue; (4) the complexity of cell-wall constituents like microfibrils and matrix polymers as well as the structural heterogeneity; (5) the challenges for cellulolytic enzymes acting on an insoluble substrate; (6) the inhibitors (to the downstream fermentation) existing naturally in cell walls or generated in the conversion of processing. Among of them, the factors (1), (2), (3) and (4) create mass-transport limitations for penetration/delivery of chemicals, as well as the accessibility of enzymes. In addition, the variation of these characteristics accounts for the varying digestibility between different sources of lignocelluloses. 17 Therefore, the conversion cost is strongly affected by the chemical and structural features of lignocellulosic biomass. Up to now, with the great efforts of human, the cost of enzymes has been lowered significantly, and the adaptability and activity of new enzyme products have been improved a lot. 24, 25 However, the cost of pretreatment is still quite high, 26 and hardly meet the requirement of commercial application. Therefore, to a large extent, pretreatment is the main bottleneck for the


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production of biofuel/ biochemical from lignocelluloses. More efforts should be made to develop more cost-effective pretreatment process. The goal of an ideal pretreatment is to cost-effectively break the natural recalcitrance of lignocelluloses to significantly improve the enzymatic digestibility of substrate, and simultaneously get high recovery of hemicelluloses and lignin. 7 More specific requirements for a good alkali-based pretreatment method are as follows: (1) High yields of fermentable sugars and low sugar degradation; (2) Effective delignification or chemical/structural changes of lignin (e.g. sulfonation of lignin); (3) Limited formation of inhibitors and high purity of fermentable sugars; (4) Low chemical consumption or efficient chemical recovery; (5) Low water usage; (6) Low energy consumption; (7) Low cost and environmental benign process; (8) High recovery of hemicelluloses and lignin. 4. CHEMICAL REACTION IN ALKALI-BASED PRETREATMENT Like pulping, to achieve the homogeneous treatment, the pretreatment liquor should be uniformly transported into the porous structure of lignocelluloses. The transportation of pretreatment liquor can be characterized by two main mechanisms: (1) penetration into the capillaries, and (2) diffusion through cell walls, interfaces and pit membranes. 27 Penetration


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refers to the flow of cooking liquor into the air filled voids of the lignocelluloses under the effect of hydrostatic pressure, while diffusion refers to the diffusion of ions or other soluble substance through the water layer of the cell wall, interfaces and pit membrane structure under the effect of a concentration gradient, and thus diffusion is a comparatively slow process. Initially, the cooking liquor needs to penetrate into lignocellulose. In this stage, penetration is the main mechanism. Along with the initial reactions (e.g. lignin removal on the surface of lignocelluloses), new channels/pores will be opened, thereby further enhancing the penetration. On the other hand, swelling may also boost the penetration of chemicals into the matrix of lignocelluloses during pretreatment. After complete penetration, diffusion will be the main mechanism. Molecular diffusion replaces the reactant chemicals because they are consumed by chemical reactions within lignocelluloses, and all transfer of new chemicals and dissolved substances from lignocelluloses will take place via diffusion. 14 Therefore, the degradation reactions of lignocellulosic components are diffusion-controlled. The chemical reactions between the alkaline and lignocelluloses mainly include lignin reactions and carbohydrates (i.e. cellulose and hemicellulose) reactions. Lignin reactions lead to the degradation and dissolution of lignin, which benefits the subsequent enzymatic hydrolysis, while cellulose and hemicellulose reactions result in sugar loss, which should be diminished as much as possible. Different chemicals used in pretreatment may bring different degradation reactions of lignocellulosic components, despite the main mechanism of alkali-based pretreatment is to efficiently break the ester bonds crosslinking lignin and xylan through solvation and saponification. 13 Herein, the common reactions on lignin and carbohydrates in alkali liquor were briefly described.


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Figure 3. Typical lignin reactions and carbohydrate reactions in alkaline conditions (a: The fracture of the phenol type α-aryl ethers; b: The cleavage of the phenol type β-aryl ethers; c: The fracture of the non-phenol type β-aryl ethers; d: cellulose peeling reaction; e: alkaline hydrolysis of cellulose; f: the sulfide fracture of the phenol type β-aryl ethers).


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4.1. Lignin reactions Lignin is a complex and three-dimensional polymer compound on the basis of phenylpropane units, which are cross-linked to each other mainly with ether bonds. For instance, the α-O-4 and β-O-4 ether links taken together represent the most abundant bonds between lignin units (even up to 65%). 14 The reactivity of lignin subunits is different and most notably dependent upon whether the phenolic units are etherified. In general, the reactivity of phenolic lignin moieties is significantly higher than that of nonphenolic lignin, and the dissolution of lignin from lignocellulosic biomass is mainly achieved by the degradation of lignin (via the cleavage of ether bond) and the increased hydrophilicity of lignin. 27 4.1.1. The cleavage of the phenol type α-aryl ethers or α-alkyl ethers In alkali-based pretreatment, the alkali (OH−) first reacts with phenolic hydroxyl (acidic), generating the water soluble phenates. Subsequently, the structural rearrangement of phenate ions facilitate the cleavage of the linkage between the α-C in phenylpropane unit and the O of the aryl ethers or the alkyl ethers, forming the quinone methide intermediates. 14 Once the quinone methide is formed, a number of reactions (like addition, elimination and electron transfer reactions) may proceed. 27 A typical cleavage of phenol type α-aryl ether is shown in Figure 3a, which presents that the molecular weight of lignin will be significantly decreased after the cleavage of phenol type α-aryl ether bond. Usually, it is easy to break the linkage of phenol type α-aryl ether bond, while the link of non-phenol α-aryl ether bond is very stable. 28 4.1.2. The cleavage of the phenol type β-aryl ethers The phenol type β-aryl ethers play a very important role in the variety of connections in lignin macromolecule structures, and the rate of the delignification to a large extent depends on the cleavage of phenol type β-aryl ethers, particularly for softwood. 27 In alkali-based pretreatment,


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hydroxyl can conduct nucleophilic attack on the α-C to form an epoxy compound, thus leading to the cleavage of phenol type β-aryl ethers, as shown in Figure 3b. If there are other stronger nucleophile (like HS−) in pretreatment liquor, the cleavage of phenol type β-aryl ethers (by forming an episulfide compound) can be significantly enhanced (Figure 3f). 28 4.1.3. The cleavage of the non-phenol type β-aryl ethers It has been known that, compared to phenol type β-aryl ether, the most obvious characteristic of non-phenol type β-aryl ether is that it cannot form the quinone methide structure during alkaline treatment. As a result, the non-phenol type β-aryl ether is very stable, and it can only be broken with the existing of α-OH. 27 As shown in Figure 3c, α-OH is easy to be ionized in alkaline condition and the formed oxygen ion can attach the β-C to form the epoxy compound, thus breaking the bonds of non-phenol type β-aryl ethers. 4.1.4. Other lignin reactions Besides the main lignin reactions mentioned above, other reactions (like cleavage of C-C bonds, methylation, condensation, and chromophore formation) may also take place due to the complex nature of lignin and the different pretreatment conditions conducted. For example, C-C bond between aryls is usually very stable, and in most cases, this kind of bond is hard to break. However, the C-C linkage of aryl-alkyl or alkyl-alkyl may be broken under more drastic conditions, which may lower the size of lignin molecular. 14, 27 In addition, condensation occurs after lignin degradation and mainly applies to the residual lignin structures. 14 As reported, the lignin moieties with 5-5ꞌ, β-5, 5-O-4 and diphenylmethane structures are considered as condensed units. 27 Condensation reactions are considered to proceed via the addition of a carbon-centered mesomer of phenoxide anions (donor) to a quinone methide (acceptor), resulting in a new α-5-bond (primary condensation which is a Michael-addition-type reaction), while


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condensation with formaldehyde results in stable diarylmethane units. 14 The condensed lignin are more difficult to cleave. Thus, condensation reactions should be avoided as much as possible by well controlling the pretreatment process. 4.2. Carbohydrates reactions The main function of alkali-based pretreatment is to remove lignin, but the degradation of carbohydrates is unavoidable. Carbohydrates reactions result in the loss of fermentable sugars, while cellulose with highly uniform and crystalline structure is more resistant under alkaline condition and suffers less degradation in comparison with hemicelluloses. In strongly alkali media, all carboxyl groups in carbohydrates are neutralized. The high pH at the beginning of pretreatment may also ionize part of the hydroxyl groups and lead to a de-acetylation of acetyl moieties in hemicelluloses as the temperature is over 70 °C. 14 Initial reactions are solvation of OH groups by hydroxyl ions causing a swollen state. At elevated temperatures the carbohydrates are attacked by strong alkaline solutions and then a large number of reactions take place. The most important reactions responsible for the loss of fermentable sugars and reduction of the DP of cellulose in alkali-based pretreatment are peeling and hydrolysis reactions. 27 4.2.1. Peeling reaction End-wise peeling reaction removes the terminal anhydro-sugar unit, generating a new reducing end group. The peeling reaction (primary peeling) starts with Lobry de Bruyn Alberda van Ekenstein rearrangement, which is an isomerization reaction of polysaccharides under alkaline reaction conditions. 27 The removal of the cellulose chain in the β-C to the anionic intermediate results in a dicarbonyl structure in the leaving fragment. Under the alkaline pretreatment conditions, these leaving fragments are very unstable and various degradation reactions (like


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Cannizzaro reactions or benzilic acid rearrangements) may take place (Figure 3d). The main degradation products are isosaccharinic acid or 2,5-dihydroxypentanoic acid. 14 Peeling reactions mainly occur at 120-130 ºC. 27 Simultaneously, the competitive stopping reaction of reducing end occurs as well, forming a stable saccharinic acid end group. Yet, the speed of peeling reaction is faster than that of stopping reaction. In the case of cellulose, before a competitive stopping reaction occurs, about 50-60 glucose units are peeled off, depending on the reaction conditions. 14 Also, individual reaction rates depend on the type of polysaccharides. 27 Xylans are more stable than glucomannans, which was attributed in the case of birch xylan to the stabilizing effect of the galacturonic acid side-groups adjacent to the reducing end of the xylan chain. The easy cleavage of arabinose side-groups in softwood xylans also has a stabilizing effect against alkaline peeling, in that with the loss of the side-group an alkaline-stable metasaccharinic acid end-group was formed. 4.2.2. Alkaline hydrolysis Alkaline hydrolysis of cellulose starts at the temperature over 140 oC. 14 Alkaline hydrolysis cleaves carbohydrate chain randomly and forms more reducing ends, thus facilitating peeling reactions (secondary peeling). In the alkaline conditions, the hydroxyl groups which is on the position of C2 in cellulose firstly get ionized, and then form an epoxy compound, which leads to the fracture of the cellulose linkage bond and the loss of the cellulose (Figure 3e). The ultrastructure of cellulose plays a very important role in the reaction. To some extent, it can affect the reaction rate. 27 In addition to the above-mentioned acids resulting from peeling and hydrolysis, formic acid, acetic acid and small amounts of dicarboxylic acids are also among the alkaline degradation


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products of cellulose, hexosans and pentosans. 29 Formic acid is liberated during the peeling reaction, while acetic acid results from the cleavage of acetyl groups of xylans and mannans. To reduce the sugar loss caused by peeling reactions in alkali-based pretreatment, reducing end groups can be stabilized by simple oxidation or reduction to carboxyl or alcohol groups, by the supplement of oxidants or reductants, respectively. However, to diminish the degradation of polysaccharides caused by alkaline hydrolysis, perhaps appropriately lowering the pretreatment temperature is a better solution. 17 5. CLASSIFICATION AND DEVELOPMENT OF ALKALI-BASED PRETREATMENT Alkali-based pretreatment involves the utilization of bases, mainly including the hydroxides of sodium, calcium, potassium and ammonium. Also, in this review, the pretreatments by the use of green liquor and the weak base of sodium carbonate were summarized for the first time. 5.1. Sodium hydroxide pretreatment Sodium hydroxide (NaOH), also known as caustic soda or lye, is highly soluble in water, and it is widely used in many industries, mostly as a strong chemical base in pulp industry. NaOH has been extensively studied in pretreatment for many years. It can disrupt the lignin structure, improving the accessibility of enzymes to cellulose and hemicelluloses. 30 As mentioned above, NaOH causes the degradation of lignin mainly by the cleavage of ether bonds, the swelling of cellulose, as well as the partial degradation of cellulose and hemicelluloses by many reactions. The reactivity of remaining polysaccharides increases with the removal of lignin in NaOH pretreatment. Acetyl and other uronic acid substitutions on hemicelluloses are also removed, which also increase the accessibility of enzymes to cellulose surface. 31


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5.1.1. Process description NaOH pretreatment can be classified into low concentration and high concentration processes. 32 In low concentration process, typically 0.5-4wt% NaOH is used, but high temperature/pressure or long pretreatment time is required (Table S2), and no recycling of NaOH occurs. Different lignocellulosic materials may need different conditions to achieve a satisfied pretreatment effectiveness. McIntosh and Vancov found that pretreatment of wheat straw with 2 wt% NaOH at 121 ºC for 30 min improved enzymatic saccharification 6.3-fold in comparison with the control samples, and a 4.9-fold increase of total sugar yields was obtained from the samples pretreated by 2 wt% NaOH at 60 ºC for 90 min. 33 It was also reported that coastal Bermuda grass was pretreated by 0.75 wt% NaOH at 121 ºC for 15 min, 86% lignin was removed, 71% total reducing sugar yields was obtained, and the overall conversion efficiencies for glucan and xylan were 90.43% and 65.11%, respectively. 34 5.1.2. Advantages and disadvantages NaOH pretreatment is usually more effective on agricultural wastes, herbaceous crops, and hardwood, compared to softwood, because softwood has a high content of lignin (especially guaiacyl lignin) and high density.15 Although high NaOH loading may be needed to achieve sufficient lignin removal, the unconsumed alkali can be reused or recovered, so that the chemical consumption can be reduced and the environment impact can be minimized as well. 32, 35 NaOH recovery is expensive, but NaOH pretreatment can be integrated or consolidated with alkalibased pulp mill which has well developed chemical recovery system.15-16 NaOH pretreatment processes use lower temperatures and pressures than other pretreatment methods. However, a large amount of heat is liberated during the dissolution of solid NaOH in water, and this may


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pose a threat to safety. In addition, a neutralization step for the removal of lignin and inhibitors (e.g. phenolic acids and aldehydes) is required before enzymatic saccharification. 36 5.2. Lime pretreatment Lime (calcium hydroxide, Ca(OH)2) is another common alkali which was widely used in alkalibased pretreatment. Lime pretreatment has similar reactions with NaOH pretreatment in the pretreatment process. The main actions of lime is to remove acetyl groups in a relatively milder conditions. 31 The removal of acetyl groups could promote enzymatic hydrolysis efficiency of lignocellulos biomass.37 5.2.1. Process description The solubility of lime in water is very limited (about 1.6 g/L at 20 °C and 0.71 g/L at 100 °C). Thus, as shown in Table S3, the temperature of lime pretreatment is relatively lower (typically in the range of 50-121 °C), but long pretreatment time and more water usage may be needed. For example, after lime pretreatment of switch grass (10 wt% lime charge, 50 °C for 24 h), the obtained total reducing sugar yields, glucose yields and xylose yields were 3.61, 3.15, and 5.78 times higher respectively, compared to the blank samples.38 Moreover, the study showed that under the alkaline conditions, calcium ions which were crosslinked with lignin molecules could decrease lignin solubilization during the pretreatment process, but the high lignin contents of pretreated switchgrass had no clearly harmful impact on the enzymatic digestibility.38 5.2.2 Advantages and disadvantages Lime is a milder and inexpensive base compared with sodium hydroxide, 4 and lime can be easily recovered by the use of carbonated water and it is safe to handle in pretreatment process. Therefore, the cost of lime pretreatment was lower than NaOH pretreatment.39 However, some


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Ca2+ may precipitate as calcium salts during pretreatment, particularly when the dosage of lime was superfluous. The precipitated calcium may cause some issues like the plugging of pipeline, which have to be avoided by suitable solutions (e.g. sufficient washing or the addition of small amount of chelating agent).31 5.3. Sodium carbonate-based pretreatment 5.3.1. Sodium carbonate pretreatment Sodium carbonate (Na2CO3) is the sodium salt of carbonic acid, and it is also known as soda ash, washing soda and soda crystals. Pure Na2CO3 is an odourless and white powder. It has an alkaline taste and can absorb moisture from air. The solubility of Na2CO3 in water is much stronger in comparison with lime. For instance, the solubility of its decahydrate in water is 340.7 g/L (27.8 °C). Na2CO3 is inexpensive and it is also the main product from the chemical recovery process of soda-pulping. 40 As a weak alkali, Na2CO3 can be used as a pretreatment reagent with or without the combination of other reagents (e.g. O2), and it generally has slight alkaline degradation to lignocelluloses. 40 Na2CO3 pretreatment can be operated in the temperature range of 70-180 °C and the reaction time can be extended to a couple of hours, depending on the process used and the properties of feedstock, as shown in Table S4. Salehi et al. confirmed that the pretreatment with Na2CO3 at elevated temperature can significantly decrease the lignin and xylan contents as well as the cellulose crystallinity, which is more amendable for enzymatic hydrolysis.41 Also, cellulose type was converted from type I to type II, and the final ethanol concentration was improved from 90.2 g/L to 351.4 g/L after the pretreatment at 180 °C for 120 min with 0.5 M Na2CO3. 5.3.2. Green liquor pretreatment


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Green liquor is the dissolved chemicals of Na2CO3, Na2S and other compounds (e.g. Na2SO3, Na2SO4, NaCl, Na2SiO3, etc.) from the recovery boiler in the Kraft pulping process, and the main components of green liquor are Na2CO3 and Na2S (The liquor’s eponymous green color is due to the presence of colloidal iron sulfide).14 The green liquor pretreatment was developed based on the concept of repurposing an conventional Kraft pulp mill for the production of ethanol, taking advantage of the recent pulp mill closures (particularly in North America) as a result of the reduction demand in pulp and paper as well as the competition from developing countries with lower cost of raw material and labor. 42 In green liquor pretreatment, the presence of Na2S can facilitate delignification (Figure 3f), as the nucleophilicity of HS− is stronger than OH−, leading to more cleavage of β-aryl ether bonds of lignin.43 Thus, green liquor pretreatment with higher sulfidity at a given total titratable alkali (TTA) charge is effective in increasing enzymatic conversion of the pretreated substrates. 42 It has been reported that green liquor pretreatment of mixed hardwood led to the conversion rate around 75-80% of total carbohydrates in wood with 20 FPU/g-substrate.44 Under similar pretreatment conditions, only 42% of sugar conversion rate for loblolly pine was achieved even with 2-4 times’ higher enzyme loading, 42 but further improvement of sugar conversion can be achieved by the additions of either polysulfide (1%) or sodium borohydride (0.5%) during green liquor pretreatment, as the additions ameliorate the retention of polysaccharides.45 In the pretreatment for wood, generally the temperature of 160-170 °C and TTA charge of over 12 wt% are needed, and this is severe compared to agricultural waste. The latest results of green liquor pretreatment of different lignocellulosic biomass are listed in Table S5. 5.3.3. Sodium carbonate-sodium sulfite pretreatment


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It has been reported that the sulfite pretreatment can enhance the lignin removal in pretreatment and the subsequent enzymatic saccharification of substrate.17 This is because in the pretreatment liquor the nucleophilic bisulfite (HSO31-) and (or) sulfite (SO32-) can lead to the cleavage of αalkyl ether linkages, α-benzyl ether linkages, and β-benzyl ether linkages on phenolic lignin units, as well as particularly the sulfonation of lignin, which can increase the hydrophilicity of lignin and reduce the hydrophobic reactions of lignin with enzymes, thus promoting the downstream enzymatic saccharification. Therefore, the addition of sodium sulfite in Na2CO3 pretreatment can improve enzymatic saccharification as well. Yang et al. investigated the performance of Na2CO3-Na2SO3 pretreatment on the improvement of the enzymatic hydrolysis of rice straw, and they found that a higher ratio of Na2SO3 in pretreatment liquor could achieve better delignification, thus improving the enzymatic saccharification of substrate.46 Under the pretreatment conditions of 140 °C, 60 min, S/L ratio 1:6, molar ratio of Na2CO3 to Na2SO3 (1:1), and chemical charge 16% (as Na2CO3 wt% on raw materials), the degradation ratio of glucan, xylan, and total sugar were 4.7%, 23.6% and 11.2%, respectively. The corresponding lignin removal was 54.2%. 5.3.4. Advantages and disadvantages Na2CO3-based pretreatment are more suitable for herbaceous biomass and agricultural waste compared to woody biomass, although the effectiveness of Na2CO3 pretreatment may be lower than NaOH pretreatment. The advantages of Na2CO3-based pretreatment are: (1) low cost of Na2CO3; (2) Na2CO3 is widely available and it is the main component of green liquor from sodapulping and Kraft pulping; (3) lignin removal can be enhanced by the supplement of Na2S (green liquor pretreatment) or Na2SO3 (Na2CO3-Na2SO3 pretreatment); (4) more polysaccharides can be recovered after Na2CO3-based pretreatment in comparison with NaOH pretreatment; (5) no toxic


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compounds (like furfural) generated during pretreatment; (6) the chemical recovery without causticization and calcination stages will be more economic and simpler. However, in the case of green liquor pretreatment and Na2CO3-Na2SO3 pretreatment, the presence of elemental S will result in the complexity of spent liquor treatment, and it may have negative impact on environment if the scale of treatment is small. 5.4. Aqueous ammonia pretreatment Ammonia is a colorless gas with strong pungent odor, and its solubility in water is 47 wt% (0 °C), 31 wt% (25 °C), and 18 wt% (50 °C), respectively. Aqueous ammonia (NH4OH, also known as ammonium hydroxide, ammonia solution, ammonia water, ammonical liquor, ammonia liquor, or simply ammonia) is a solution of ammonia in water. In aqueous solution, ammonia deprotonates a small fraction of the water to form ammonium and hydroxide as shown in the following equation: NH3+H2O ⇋ NH4++OH-

(1)

5.4.1. Process description In aqueous ammonia pretreatment, with the presence of hydroxide ions, the chemical bonds between lignin and hemicelluloses (e.g. the bonds between lignin and p-coumaric acid) and the bonds between hemicelluloses and ferulic acid (ester bonds) could be broken, 47 leading to the efficient removal of lignin and partial dissolution of hemicelluloses. Also, aqueous ammonia could penetrate into cellulose structure causing the partial breaking of hydrogen bonds, the swelling of cellulose, and the transformation of cellulose type (from type I to type III). The efficient delignification and the swelling of cellulose could significantly increase the digestibility of lignocelluloses after pretreatment.48 Shown in Table S6 are the selected recent results of


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aqueous ammonia pretreatment of different lignocellulosic biomass. To save energy consumption, low temperature (30-70 °C) and long treatment time (several hours to several weeks) could be conducted (Table S6). 5.4.2. Advantages and disadvantages Ammonia is an inexpensive and less corrosive chemical in comparison with sulfuric acid. It can be easily recovered and reused due to its high volatility. Aqueous ammonia pretreatment has high selectivity on lignin removal and effective swelling to cellulose. Compared with other alkaline reagent, aqueous ammonia pretreatment could not leave any irrecoverable salts in the whole process. After evaporation for the recovery of ammonia, the residual ammonia (approximately 1%) was considered as a potential nitrogen source for the downstream fermentation. 48-50 In addition, there was no inhibitors (e.g. furfural, HMF) generated during aqueous ammonia pretreatment for anaerobic digestion. However, aqueous ammonia pretreatment is not suitable for the lignocelluloses with high lignin content (e.g. softwood) and the stench of ammonia has negative impact on environment. 5.5. Alkali-based pretreatment with the supplement of additives The use of additives in alkali-based pretreatment could promote delignification (like the use of O2, and H2O2) and stabilize carbohydrates (e.g. anthraquinone (AQ)), thus enhancing enzymatic saccharification and obtaining more fermentable sugars. 5.5.1. Supplement with O2 Oxygen is a chemical element, existing in air as a diatomic gas (dioxygen, O2). O2 as a green and environmental friendly oxidant, can be used in alkaline pretreatment to improve delignification.51 Oxygen delignification of biomass in pretreatment is a three-phase reaction system (i.e. an


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aqueous phase (pretreatment liquor), suspended substrate (solid phase), and O2 (gas phase). Firstly, O2 dissolves in pretreatment liquor and is then transported through the liquor to the liquid-substrate (e.g. fiber) interface. Subsequently, the dissolved O2 diffuses into the fiber cell wall and reacts with the components (e.g. lignin) of lignocelluloses. Therefore, sufficient mixing is of critical importance to achieve a good pretreatment effectiveness. Based on the general concept of the chemistry of delignification, 14 oxygen delignification starts with the formation of hydroperoxides, which are key intermediates in the oxidation of lignin and carbohydrates. As shown in Figure 4, in the pH range of 10-13, the standard redox potentials of the reactive species (including hydroperoxyl radical (HO2·), superoxide anion radical (O2−·), hydrogen peroxide (H2O2), hydroperoxy anion (HOO−), hydroxyl radical (HO·) and oxyl anion radical (O−·)) are substantially reduced due to the lower potential of the ionized form.52 These radicals or ions play important roles in the oxygen delignification. For example, the HO· is one of the most reactive radicals. It reacts with the main components of lignocelluloses by preferentially attacking electron-rich aromatic and olefinic moieties in lignin, and it also reacts with aliphatic side chains in lignin and carbohydrates, but at a lower rate. The oxygen delignification includes nucleophilic additions and displacements as well as electrophilic additions and displacements. The detailed mechanism of oxygen delignification and the degradation of carbohydrates was previously reviewed by Sixta.14


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Figure 4. The process of dioxygen reductions in four consecutive one-electron steps (E0 standard reduction potential, Adapted from Sixta. 14 Reproduced with permission from reference 14. Copyright 2006 John Wiley and Sons.) As previously reported, the Na2CO3-O2 pretreatment with 12 wt% TTA as Na2O at 110 ºC and 0.5 MPa oxygen pressure could get a higher sugar yield (about 65%) at a relatively lower temperature, 51 compared to Na2CO3-pretreatment 53 due to the higher lignin removal by oxygen delignification. It was also reported that about 46% improvement of sugar conversion could be achieved by green liquor pretreatment (16 wt% TTA, 40% sulfidity, 170 ºC) plus oxygen delignification (5 wt% NaOH, 100 psig oxygen pressure, 110 ºC for 60 min) for Loblolly pine in comparison with the green liquor pretreatment without the supplement of oxygen delignification. 45

Banerjee et al. investigated the effect of O2 dosage on the effectiveness of Na2CO3-O2

pretreatment of rice husk.54 They found that when the oxygen pressure increased from 0.5 MPa to 1.0 MPa under the conditions of 170 ºC for 10 min, the delignification rate could increase about 10%, while the cellulose recovery rate decreased about 5%. With the addition of O2 in alkaline pretreatment, the penetration of chemicals and the kinetic reaction speed could be enhanced under a relatively mild conditions such as a lower temperature 45

but the capital cost will be higher compared with the conventional alkaline pretreatment. The

delignification could be improved, but the radical reactions (particularly for HO·) are unselective, causing some degradation of carbohydrates during pretreatment. To reduce the degradation of carbohydrates, a small amount of chelating agent (e.g. ethylene diamine tetraacetic acid (EDTA), diethylene triamine pentacetate acid (DTPA)) or magnesium ion compounds (e.g. MgSO4, MgCO3) could be added to remove metal ions in pretreatment liquor because transition metal ions (like Mn2+, Fe2+) can promote the decomposition of the formed


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H2O2, thus generating more HO·. By this way, the concentration of HO· in pretreatment liquor could be controlled. In addition, it was indicated that to avoid serious degradation of carbohydrates, oxygen delignification needed to be stopped when about half of the lignin had been removed.51, 55 5.5.2. Supplement with H2O2 H2O2 is a very weak acid and the simplest peroxide. The pure H2O2 is a colorless liquid with a slightly more viscosity than water. Generally, for safety reasons, H2O2 is used as the aqueous solution with a certain concentration between 30 wt% and 70 wt%, 56 which is transparent waterlike liquid. H2O2 is chemically unstable, which has strong oxidizing property. Thus, H2O2 is generally used as surface disinfectants, bleaching agents and oxidizing agents in the daily life, medical field and chemical industry. 57 In alkaline condition (pKa = 11.6), H2O2 will be decomposed to generate HOO− (Figure 4), which is the main oxygen species to degrade lignin in H2O2 delignification.27 HOO− can react with the phenylpropanol/ propiophenone or quinone structures of lignin, as well as double bonds and carbonyl of side chains in lignin to fragment lignin and increase its solubility, thus improving the delignification in pretreatment.57 In addition, in alkaline conditions, O2−· and HO· can also be produced through the decomposition of H2O2 by the following reactions: H2O2 + HOO− → HO·+ O2−·+ H2O

(2)

HO·+ O2-· → O2 + OH−

(3)

The presence of transition metal ions (e.g. Mn2+, Cu2+, Fe2+) can promote decomposition of H2O2 as follows: H2O2 + Me(n-1)+ → Men+ + HO·+ OH− (Fenten-type reaction)

(4)

2H2O2 → 2H2O + O2

(5)


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It was reported that about 7% and 5% improvement of delignification and glucose yield could be obtained respectively when the dosage of H2O2 increased from 0.3 wt% to 0.6 wt% in alkaline peroxide pretreatment (1 wt% NaOH, 30 ºC for 20 h) of sugarcane bagasse.58 Also, alkaline peroxide pretreatment could also result in the increase of carbohydrates loss. 59 When H2O2 dosage increased from 1% to 4% (w/v), the total carbohydrate recovery decreased from 97.4% to 84.9% for rice straw after pretreatment (1:20 S/L ratio, pH 11.5, 30°C, 24 h). 60 The other selected H2O2-assisted alkaline pretreatment results are shown in Table S7. As reported, after alkaline H2O2 pretreatment of Sacrau poplar (20 wt% NaOH, 10 wt% H2O2 , 160 °C), lignin removal could reach about 65% and the final ethanol yield was 132 mg/g-raw material, which was nearly five times higher than the untreated samples.61 Pretreatment with H2O2 has the great advantage of not leaving any residues in the pretreated biomass because H2O2 can decompose into oxygen and water. With the supplement of H2O2, the lignin removal can be efficiently enhanced, thus increasing the enzymatic digestibility of the substrate after pretreatment.57 However, H2O2 is easy to decompose and the generated HO· can also result in the degradation of carbohydrates, lowering the final total sugar yields. Hence, avoiding/reducing the invalid decomposition of H2O2 has to be taken into consideration, as mentioned previously. Either acidic washing before pretreatment, or the addition of small amount of chelating agent (e.g. EDTA, DTPA) or magnesium salts can lower the concentration of transition metal ions in pretreatment liquor, thus reducing the invalid decomposition of H2O2. 5.5.3. Supplement with AQ AQ is a widely used reagent for alkaline pulping process. 62 Due to the similarity between the pulping and biomass pretreatment, AQ can also be used in alkali-based pretreatment for the improvement of enzymatic saccharification.


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The reaction mechanism of AQ in pretreatment is the oxidation reduction reaction in the Quinone ring of AQ, 63 as presented in Figure 5. AQ oxidized the aldehyde end of the carbohydrates to the carboxyl group, which can prevent the peeling reaction and degradation. In this process, AQ is reduced to anthrahydroquinone (AHQ), which can be ionized into the ions of AHQ in alkaline conditions.63 The AHQ ions can react with the methylene quinone structure of the lignin macromolecules, which provide the electronic to lignin molecule and make the aryl ether bonds cleave, leading to the degradation of the lignin macromolecules.27

Figure 5. The reaction mechanism of AQ in alkaline pretreatment (Adapted from Sixta. 14 Reproduced with permission from reference 14. Copyright 2006 John Wiley and Sons.) Corn stover was treated with NaOH and AQ (0.05 wt%) under different pretreatment conditions of temperature (140 °C, 160 °C) and total alkali charge (7 wt%, 10 wt%). The results showed that under the conditions of 140 °C with 10 wt% alkali charge, the glucan and xylan recovery of corn stover were 60.6% and 25.5%, respectively, which were increased by 6.2% and 2.7%, respectively, compared to the pretreatment under the same conditions without AQ.64 For Nordic wood after NaOH-AQ pretreatment, the solid recovery for aspen and pine could increase 2.5-3


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wt% (units) and slightly higher than 3 wt% (units) respectively at an AQ dosage of 0.1 wt%. The corresponding yield improvement of fermentable sugars was about 1% and 2% for aspen and pine wood respectively. 16 Also, the acceleration of delignification by AQ could reduce the pretreatment temperature, time, or alkali charge. 63 For example, at a given pretreatment temperature (170 °C) and time (210 min), 2 wt% alkali charge decrease or 10 °C temperature reduction could be achieved with the addition of 0.1 wt% AQ for aspen wood.16 Therefore, the supplement of AQ in alkaline pretreatment can promote lignin removal and reduce the carbohydrate degradation, 65 which can reduce the dosage of alkali, time or temperature, thus reducing the energy consumption and process cost of pretreatment. However, AQ is water insoluble and this may result in inhomogeneous treatment of feedstock. Therefore, two options could be taken to address this issue: (1) the hydrophilic modification of AQ (to enhance the solubility of AQ in water phase); (2) the use of suitable surfactant to promote the penetration and distribution of AQ during pretreatment. 5.5.4. Supplement with surfactant Surfactants was a class of substances which could significantly change the interface state of the mixed solution system only with small dosage. Surfactants had both the hydrophilic and hydrophobic groups which could reduce the surface tension of the liquid phases. As a result, surfactant could enhance the removal of hydrophobic substance. 66 In pretreatment, surfactant could vary the surface and structure of lignocelluloses, leading to a more exposure of hydrophilic substrate for the following treatment. 67 Alkali pretreatment of softwood spruce and hardwood birch by NaOH + polyethylene glycol (PEG) was studied, 68 and the results showed that PEG molecules could act as hydrogen-bonding


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acceptors and prevent the re-association of cellulose OH groups, which could influence the dissolution process positively, but increasing the temperature reduced the quality of the solvent. Tween 80, Tween 20, PEG 4000 or PEG 6000 was used in the combination with ammonium hydroxide for the pretreatment of sugarcane bagasse. 49 The results indicated that ammonium pretreatment with PEG 4000 and Tween 80 gave the highest cellulose digestibility (62%, 66%) and ethanol yields (73%, 69%), while the cellulose digestibility and ethanol yield for the samples treated by ammonia pretreatment without surfactant was only 38%, and 42%, respectively. As a cationic surfactant, cetyltrimethylammonium bromide (CTAB) was used to assist the NaOH pretreatment of peanut shells, and the results showed that the released reducing sugar was 0.35g/g-biomass, which was clearly higher than the samples pretreated by NaOH without surfactant (0.2 g/g-biomass). 69 The hydrophile-lipophile balance (HLB) value was one of the most important indicators reflecting the hydrophilicity or hydrophobicity of surfactant. For example, the HLB values for PEG 4000, PEG 6000, Tween 20, and Tween 80 were 18.5, 19, 16.7 and 15, respectively. A higher HLB value indicates a higher hydrophilicity of surfactant. It was reported that surfactant with higher HLB had better ability to extract hydrophobic degradation substance of hemicelluloses and lignin. 63 Moreover, we investigated the relationship between the HLB value of the nonionic surfactants and the final total sugar yields of the corn stover pretreated by NaOH pretreatment assisted with AQ and nonionic surfactants (Figure 6). The results showed that the total sugar yields (y) as a function of HLB value (x) of nonionic surfactants can be expressed by the following equation: y = 0.344x + 48.745, (R2 = 0.9336)

(6)


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Figure 6. Total sugar yields as a function of HLB value of nonionic surfactants (The point corresponding to the HLB value of zero was for the sample pretreated without surfactant. Pretreatment conditions: 7 wt% NaOH, 0.1 wt% AQ, 1 wt% surfactant, S/L ratio 1:6, 140 °C, 0.5 h. Enzymatic hydrolysis conditions: 2% substrate consistency, 5 IU β-glucosidase/g-substrate, 20 FPU cellulase/g-substrate, 0.02 % sodium azide, 0.05 M sodium citrate buffer, 50 °C for 48 h) Also, sodium lignosulfonate (SLS) could be used as surfactant to promote the NaOH pretreatment assisted together with AQ. It was reported that SLS produced by directly sulfonating lignin in spent liquor separated from alkali pretreatment could be reused in the next cycle of alkali pretreatment, and sulfonation degree of SLS had linearly positive impact on the final total sugar yields.63 SLS could promote chemical penetration (e.g. AQ) and lignin removal, thus improving effectiveness of pretreatment. After NaOH pretreatment with 2% SLS and 0.1% AQ (at both lab and pilot scale experiments), about 78% of lignin was removed and the final total sugar yields could reach 80% (9% higher compared to blank NaOH pretreatment). 66 Addition of non-ionic or ionic surfactants could increase the delignification during alkali-based pretreatment by increase the solubility of lignin and some extractives. However, surfactants are usually expensive and may also cause some bubble issues during pretreatment. Therefore, the


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cost-effective and green surfactant needs to be developed for pretreatment, and the overall cost of pretreatment (e.g. the balance between the cost of surfactant used and the benefits gained) needs to be carefully estimated when using surfactants. 5.6. Combined alkali-based pretreatment with physical method It has been known that usually the combined pretreatment method (e.g. physiochemical technology) is more cost-effective compared to the one by using chemical or physical method alone. The most studied combined alkali-based pretreatment by physical method mainly included ammonia explosion, extrusion, ultrasound and post mechanical refining. Ammonia explosion mainly includes AFEX and ammonia recycle percolation (ARP) pretreatment technologies, which have been extensively reviewed previously. 7, 9 With regard to ultrasound assisted alkaline pretreatment, ultrasound can improve the contact between biomass and alkaline reagents, and increase the effectiveness of alkali pretreatment. Compared with the control, ultrasound-assisted alkaline pretreatment can shorten the pretreatment time, decrease the pretreatment temperature and provide higher final sugar yields. 70 However, ultrasound is usually energy-consuming particularly at large scale applications. Thus, mechanical refining, and extrusion assisted alkaline pretreatment which have great potential in large scale application were briefly reviewed in the following section. 5.6.1. Mechanical refining-assisted alkaline pretreatment As a traditionally widely used mechanical treatment for pulp and paper making industry, mechanical refining (particularly for disc refining) is a mature technology for improving the fiber performance by decreasing cell wall thickness, fiber coarseness of long fiber fraction, and fiber length, generating fines and increasing specific surface area of fibers and fines. 71-72 Nowadays, with the rapid development of renewable biomass energy conversion, the mechanical refining


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after chemical pretreatment has been extensively studied to further modify the substrate for enhancing the accessibility of substrate and improving final reducing sugar yields. 73-75 Hence, to some extent, the mechanical refining process after pretreatment could partly reduce the selling price of bioethanol and increase the final bioethanol yields. 76 In biomass pretreatment, PFI refiner (a laboratory scale pulp refining instrument) and disc refiner (large scale mechanical refining instrument) are two kinds of widely used post mechanical refining equipment. The selected results regarding the recent studies on the applications of mechanical refining after alkali-based pretreatment are listed in Table S8. It was reported that about 10-20% improvement of biomass digestibility could be achieved by post mechanical refining,

77

while the improvement was highly dependent upon the severity of chemical

treatment. 73 The most recent study found that water retention value (WRV) of the mechanically refined substrate could significantly predict the biomass digestibility for a given substrate (fiber source/composition) and different refining scales generated the comparable biomass digestibility at similar WRV.

78

Also, beating degree of PFI refined substrates was strongly linear with final

total sugar yields for the given pretreated corn stover and it was proven that the effectiveness of enzymatic saccharification could be rapidly predicted by beating degree of PFI refined corn stover.

73

These findings will be of practical importance for the monitoring and controlling of

industrial production in the future application of biomass pretreatment. The advantage of post mechanical refining was that it could efficiently further modify the pretreated substrates and the fiber surface morphology, thereby efficiently promoting the fiber’s enzymatic accessibility and final total sugar yields. However, the energy consumption of post mechanical refining needs to be well controlled and evaluated in large scale application of biomass pretreatment.


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5.6.2. Screw extrusion-assisted alkali pretreatment Screw extrusion was a well-known and widely used industrial technology in plastics and food production. A screw extruder could include forward screw elements (mainly transporting feedstock), kneading screw elements (primarily mixing and shearing), and reverse screw elements (extensively mixing and shearing). 79 The screw configuration can be specially designed and adjusted by arranging the pitches, lengths, stagger angles, positions and the ratio of the different elements (for example, the kneading screw elements might not be included in extruder). The detailed screw configuration affects the throughput of extrusion, extent of mixing and biomass modification, energy consumption and the residence time of biomass during extrusion. With the addition of chemicals, various functions and processes (e.g. materials transportation, mixing, shearing, grinding, heating, chemical reactions and liquid-solid separation) can be conducted in a single continuous step. Therefore, extrusion was also considered as one of the most effectively physical-assisted method for alkaline pretreatment.80 Screw extrusion can be classified as single screw extrusion and twin screw extrusion process. The single screw extrusion consists of an Archimedean screw in a stationary barrel section, while twin screw extrusion consists of two parallel screws placed in a fixed barrel. Their rotation direction of twin extruders can be either co-rotating or counter-rotating. Due to the limited dispersive and distributive mixing of single screw extrusion, the co-rotating twin screw extrusion with extensive mixing and fully-intermeshing is the dominant application in biomass pretreatment process.79, 81


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Figure 7. The schematic chart of the specially designed twin-screw extruder and the photo of pilot scale extrusion machine. A detailed structure of a specially designed twin screw extruder and the pilot twin screw extrusion machine are shown in Figure 7. The two screw extruders were co-rotated intermeshing for each other. Each screw extruder consisted of four transport screw elements (TSE) and four reversed screw elements (RSE). Through feeding inlet, the lignocellulosic biomass was brought into the main part of extruder. By the action of TSE, the biomass was pushed to the RSE with the opposite pitch of threads to the TSE (Figure 7). After crushed by the force of transporting, mixing and grinding, the substrate was pushed to pass through the skewed slots of the RSE. Then, the repeated squeezing and crushing took place when the biomass was delivered to the following sections of TES and RSE.

82

Finally, the biomass was completely crushed and the

fibers were separated and delaminated, leading to a size reduction and the increase of external/internal specific surface area of biomass.

79

Also, with the more efficient chemical

reactions, lignin could be effectively removed. Therefore, the biomass digestibility and finial total sugar yields could be significantly enhanced.


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Compared with other pretreatment technologies, the extrusion-assisted alkali pretreatment has characteristics like: no need of extra heating equipment; high sugar yield (about 80%-90%); no need of severe size reduction of biomass before pretreatment and low water usage (S/L ratio up to 1:2); 82 and no inhibitors generated for the downstream fermentation (e.g. acetone-butanolethanol fermentation of alkali twin screw extrusion pretreated corn stover). 83 Also, extrusionassisted alkaline pretreatment is a continuous process. The configuration of screw extruder can be specially designed and adjusted to meet the requirement of pretreatment effectiveness and properties of raw material. However, the abrasion of extruder and the evaluation of energy consumption have to be taken into consideration for large scale application. 6. DEVELOPMENT TREND OF ALKALI-BASED PRETREATMENT Alkali-based pretreatment is more appropriate for the lignocellulosic biomass with relatively lower lignin content (e.g. agricultural waste, herbal plants, and hardwood), but it is not suited for softwood with higher lignin content. Due to the main drawbacks of the conventional alkaline pretreatment, such as the severe degradation of hemicelluloses (resulting in some waste of fermentable sugars), the limited pretreatment effectiveness, as well as the high cost of chemical recovery and the wastewater treatment, a great deal of effort has been made to improve the effect of alkali-based pretreatment: (1) Suitable additives can be added to assist alkali-based pretreatment to facilitate delignification (like O2, and H2O2), stabilize carbohydrates (like AQ), and promote the penetration of chemicals and dissolution of lignin (like surfactant), leading to the reduction of chemical and energy consumption and the improvement of enzymatic saccharification. (2) Physical methods (e.g. mechanical refining, extrusion) can also be applied to assist alkaline pretreatment. By the assisted physical actions (e.g. grinding, mixing, shearing), the substrates can be efficiently modified to increase the porosity and specific surface area, and


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reduce the particle size of pretreated substrate, which can facilitate the chemical penetration and reactions, and further increase the enzyme accessibility to carbohydrates, thus ameliorating the enzymatic saccharification. (3) Inexpensive, green and easily-recoverable alkaline reagents (e.g. sodium carbonate) has been paid more attention to the alkali-based pretreatment for a given feedstock. (4) Alkali-based pretreatment can also be used as a fractionation method combined with other pretreatment technologies. 7, 84 For instance, hot water pretreatment + alkali pretreatment could separate hemicelluloses, lignin and cellulose sequentially. 85 By this way, the main components of biomass could be better and more efficiently utilized. By developing highvalue products, the feasibility of pretreatment at commercial scale could be highly improved. (5) On the basis of the well land-use management of lignocelluloses, suitable feedstock preparation, and the concept of pulp mill-repurposing, the alkali-based pretreatment can be integrated into the existing pulp mills or even power plants. By this way, the capital cost of pretreatment can be lowered because the existing practical workers, as well as the mature equipment (e.g. digester, disc refining) and processes (e.g. chemical recovery and wastewater treatment system) can be directly utilized; more job positions can be supplied and more benefits can be brought for the mills; by integrated with power plants, surplus electricity and steam from power plant could be utilized for pretreatment and enzymatic saccharification to lower the process cost, and the solid residues from pretreatment or enzymatic saccharification could be subject to power generation. By the better use of raw materials, better understanding of chemical and physical mechanism of pretreatment, process modeling and optimization, as well as the development of value-added products from different component streams after a cost-effective and green alkali-based pretreatment method, the conventional pulp mills or power plants could be converted and upgraded to the modern biorefinery mills with less carbon emission and more beneficial gains in


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the future. In addition, to reach commercial production of alkali based pretreatment, the efficiency and cost of chemical recovery efficiency is another challenge which has to be well addressed particularly for the herbaceous plants and agricultural waste with high content of silicon. 7. CLOSING REMARK Similar to soda- or Kraft- pulping, alkali-based pretreatment can break the cross-linking bonds between lignin and carbohydrates (e.g. ether bonds and ester bonds), swell cellulose, degrade and dissolve lignin, generating more porous structure and improving the enzyme accessibility to substrates. With the supplement of effective and green additives together with the post physical treatment, the enzymatic saccharification can be further enhanced after pretreatment. In order to develop a cost-effective alkali-based pretreatment for a given feedstock for commercial application, on the one hand, it is required to better understand the mechanism of pretreatment and the relationships among the chemical composition and physicochemical structure of lignocelluloses, key process parameters of the used pretreatment method, and the enzymatic digestibility of the pretreated substrates; on the other hand, a rational process design of pretreatment has to match the properties of raw material, the suitable method, process configuration together with the downstream production for value added products besides biofuels. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: **Supplementary Tables. Corresponding experiments.**


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; Phone: +86-532-80662725; Fax: +86-532-80662724. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The financial support of this work was from the National Natural Science Foundation of China (No. 21306216, No. 21433001, and No. 31470609), Shandong Provincial Natural Science Foundation for Distinguished Young Scholar (China) (No. JQ201305), and Doctoral Found of QUST (No. 210-010022706). ABBREVIATIONS SSF, simultaneous saccharification and fermentation; SHF, separate hydrolysis and fermentation; AFEX, ammonia fibre/freeze explosion; SPORL, sulfite pretreatment to overcome recalcitrance of lignocellulose; DP, degree of polymerization; HMF, 5-hydroxymethyl furfural; TTA, total titratable alkali; AQ, anthraquinone; EDTA, ethylene diamine tetraacetic acid; DTPA, diethylene triamine pentacetate acid; AHQ, anthrahydroquinone; PEG, polyethylene glycol; CTAB, cetyltrimethylammonium bromide; HLB, hydrophile-lipophile balance; SLS, sodium lignosulfonate; ARP, ammonia recycle percolation; WRV, water retention value; TSE, transport screw elements; RSE, reversed screw elements. REFERENCES (1) IEA. World Energy Outlook 2014. Paris: Corlet.


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Review of Alkali-Based Pretreatment To Enhance Enzymatic

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