Recent Advances in the Application of Inorganic Salt Pretreatment for

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Review

Recent Advances in Application of Inorganic Salt Pretreatment for Transforming Lignocellulosic Biomass into Reducing Sugars Yu-Loong Loow, Ta Yeong Wu, Khang Aik Tan, Yung Shen Lim, Lee Fong Siow, Jamaliah Md. Jahim, Abdul Wahab Mohammad, and Wen Hui Teoh J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b01813 • Publication Date (Web): 01 Sep 2015 Downloaded from http://pubs.acs.org on September 6, 2015

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Journal of Agricultural and Food Chemistry

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Recent Advances in Application of Inorganic Salt Pretreatment for Transforming

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Lignocellulosic Biomass into Reducing Sugars

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Yu-Loong Loowa, Ta Yeong Wua*, Khang Aik Tana, Yung Shen Lima, Lee Fong Siowb, Jamaliah

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Md. Jahimc, Abdul Wahab Mohammadc, Wen Hui Teohd

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a

Selatan, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia

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b

School of Science, Monash University, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia

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Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon

c

Department of Chemical and Process Engineering, Faculty of Engineering and Built

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Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan,

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Malaysia

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d

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

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ABSTRACT: Currently, the transformation of lignocellulosic biomass into value-added products

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such as reducing sugars is garnering attention worldwide. However, efficient hydrolysis is usually

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hindered by the recalcitrant structure of the biomass. Many pretreatment technologies have been

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developed to overcome the recalcitrance of lignocellulose such that the components can be

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reutilized more effectively to enhance sugar recovery. Among all of the utilized pretreatment

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methods, inorganic salt pretreatment represents a more novel method and offers comparable sugar

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recovery with a potential for reducing costs. The use of inorganic salt also shows improved

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performance when it is integrated with other pretreatment technologies. Hence, this review is

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aimed to provide a detailed overview of the current situation for lignocellulosic biomass and its

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physicochemical characteristics. Furthermore, this review discusses some recent studies using

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inorganic salt for pretreating biomass and the mechanisms involved during the process. Finally,

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some prospects and challenges using inorganic salt are highlighted in this review.

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KEYWORDS: Cellulose, hemicellulose, lignin, inorganic salt, sustainability, valorization, waste

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reuse


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INTRODUCTION

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Lignocellulosic biomass is plant dry matter that can be considered the most abundantly

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available material on the earth. Currently, lignocellulosic material is the only renewable resource

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that contains a carbon source that can be converted into products in solid, liquid and gas forms.1

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As a result of depleting crude oil reserves and increasing global energy consumption, many

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countries are relying on carbon-based biomass as an alternative source for fuel production and

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chemical industries.2 However, the recalcitrant nature of lignocellulosic biomass prevents the

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hydrolysis and fermentation of the biomass by solvents and microorganisms, which represents a

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challenge for the effective utilization of the components contained in the biomass. Hence, a

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pretreatment stage is often required to disrupt the complex structure of the biomass and thereby

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increase the recovery of components and ensure the economic feasibility of the bioconversion

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process. Over the years, various pretreatment methods such as dilute acid hydrolysis,3-7 alkaline

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systems,8-12 autohydrolysis,13-15 steam explosion,16,17 and ionic liquids18 have been heavily

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scrutinized by researchers. A substantial number of reviews have discussed these conventional

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lignocellulose pretreatment technologies in detail.19-21 Among the available biomass pretreatment

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methods, inorganic salt pretreatment is still relatively novel, though it has received considerable

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attention in recent years. Nonetheless, inorganic salt pretreatment has not been critically discussed

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in reviews as the other well-established pretreatments have, and little is known about its

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mechanism, though recent studies22-39 have proven it to be successful. Thus, this review

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investigates the current situation worldwide for various forms of lignocellulosic biomass such as

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agricultural waste, energy grasses, and forest residues and discusses the complex structure of

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lignocellulose to identify the source of its recalcitrant nature. In addition, this review provides a

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critical overview on the recent use of inorganic salts as one of the more novel biomass

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pretreatment methods, the mechanisms involved during the pretreatment process, and the

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prospects and challenges.

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CURRENT SITUATION OF LIGNOCELLULOSIC BIOMASS

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Lignocellulosic biomass can be categorized into (1) agricultural wastes that arise mainly

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from various agricultural and farming activities, (2) energy crops that are grown for biofuel or

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electricity production, and (3) forestry residues from forest logging sites and management

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operations. Figures 1 and 2 depict the major sources of lignocellulosic biomass that are currently

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available as a feedstock to produce value-added products40-43 and the statistical global production

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of potentially available lignocellulosic biomass,41 respectively.

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Agricultural Wastes. Agricultural wastes consist of crop residues such as corn

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stover/stalks, sugarcane bagasse, and crop straws. Table 1 represents some major types of

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lignocellulosic crops and their availability in different countries as of 2013.44,45 Due to the

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extensive agricultural activities in various countries, Menon and Rao estimated that up to 1.5 ×

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1011 tons of agricultural wastes contribute to the yearly global yield of lignocellulosic biomass.46

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For example, approximately 400 million tons of rice and wheat straw along with corn stalks are

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harvested in the United States each year. In mainland China, 700-800 million tons of agricultural

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residues are produced annually, and half of that amount is used as bioresources in the production

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of fuels and chemicals.1,47,48 Brazil, which is the largest producer of sugarcane globally, generates

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185 million tons of sugarcane bagasse per harvest.49 Agricultural residues such as rice husks are

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used to produce electricity in gasification units and have been scrutinized intensely for use in

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silica production.47 Additionally, rice husks were reported to be a potential source of biofertilizer

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after undergoing a biodegradation process.50,51 Moreover, agricultural residues could be

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reprocessed into animal feed and used for farm cultivation.48 However, agricultural residues are

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still underutilized because a large portion of this biomass is discarded as a waste.46 For instance,

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10% of the corn straw in United States is collected for reuse purposes such as mulching, but most

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of the rice straw is disposed of by burning, which results in air pollution.2,49,52,53 Rocha et al. also

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highlighted that certain components of agricultural residues such as sugarcane bagasse are still

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underutilized, though these residue components could potentially serve forty different

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applications.49 ACS Paragon Plus Environment

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Annual and Perennial Dry Energy Grasses. Annual and perennial energy crops are

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produced mainly to serve as the raw materials in the production of biofuel due to their prominent

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characteristic of being low-cost and low-maintenance, which leads to economic and

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environmental advantages over other various forms of biomass.40 For instance, these crops, which

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are normally densely planted, have the distinctive trait of high yields and do not require annual re-

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seeding once established.54 Some of the common forms of energy crop biomass are illustrated in

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Table 2, where switchgrass and Miscanthus straw are among the main contributors annually, with

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5-19 and 5-43 tons/ha/year, respectively.42 Similarly, the cost of both maintenance and investment

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are low for these perennial grasses because they require less harvesting and fertilizers, which

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account for the major portion of the total cost.43 Energy crops are able to grow under poor soil

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conditions without compromising the energy yield. A steady supply flow of these perennial crops

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could be obtained, thus avoiding the concern of requirement for costly storage for large biomass

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volumes.42 In both the United States and Canada, switchgrass serves as a major crop due to its

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abundant availability and its ability to resist both disease and harsh environments.42 The effective

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growth of Miscanthus is well known in Asia, whereas in European regions, Miscanthus is grown

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as a source of combustible energy.19 In African grasslands, minimal amounts of nutrients are

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required for growing Napier grasses, which can swiftly be harvested and yield high amounts of

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lignocellulosic biomass.

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Forestry Waste. Woody raw materials such as saw mill residues and wood chips are more

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flexible in terms of harvesting times. Thus, long latency periods for forestry waste storage can

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also be avoided.43 Conventionally, the lignin content in woody feedstock is greater and contains

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less ash,40 and woody raw materials are thus used in the manufacture of paper and pulp.43 Because

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of these characteristics, high densities of woody biomass are considered to be more attractive for

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cost-effective transportation. In China, approximately 900 million tons of forestry wastes and

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products are produced annually, and one third of the entire amount of waste could be converted

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into renewable energy sources.1 Correspondingly, short-rotation forest crops are cultivated for the



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production of wood chips to serve as a substitute for fossil fuels and other products such as

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geotextiles and building materials in Europe.55

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COMPOSITIONAL

AND

STRUCTURAL

CHARACTERISTICS

OF

LIGNOCELLULOSIC BIOMASS

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Lignocellulosic biomass is basically composed of different layers of plant cell wall (Figure

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3).56 First, the primary wall is formed, followed by the secondary wall layers, through the

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deposition of wall substances into the primary wall, where most cells are elongated and stretched

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during differentiation. The chemical composition of the cell walls is dependent on the species of

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the plants. Some cells contain only non-lignified primary cell wall, whereas others possess a

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thickened secondary cell wall that is normally lignified. This lignified secondary cell wall consists

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of a multilayered structure. As shown in Figure 4,57 the outer part of a softwood bleached kraft

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pulp (SBKP) is the highly lignified compound that consists of middle lamellae and primary wall,

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followed by a thin S1 layer, a thick, less lignified middle S2 layer, a thin inner S3 later and finally,

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a warty layer formed by lignin precursors.58 This lignified secondary wall accounts for the

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majority of the plant biomass and usually comprises 40-50, 15-25, 20-25 and 5-10% of cellulose,

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hemicellulose, lignin and other components, respectively.57,59 Other minor non-structural

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components, such as proteins, ash, extractives and pectin, are dependent on factors such as the

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different species, tissues, plant maturity, harvest times and method of storage and environmental

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factors. Extractives are usually complex mixtures that consist of sugars, terpenoid compounds and

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monolignols.60 Every plant has its own floral cell walls with different amounts of specific

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compounds.59 Some of the components of lignocellulosic biomass that consist primarily of

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agricultural residues are summarized in Table 3.59-63

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Cellulose. Normally, cellulose accounted for 30-40% of floral cell walls. Cellulose is a β-

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1, 4-linked glucose polysaccharide that comprises the larger portion of the cell wall that is

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common to all plants. Cellulose is long, consists of oriented microfibrils, and has the ability to

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coalesce into larger and longer fibrils. These fibrils are hydrophobic and highly crystalline, which ACS Paragon Plus Environment

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enhances the recalcitrance of biomass.60 The FE-SEM (Field Emission Scanning Electron

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Microscopy) image of microfibrils (Figure 5) can be seen clearly from the untreated SBKP.57

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Therefore, decomposition of the fibril is difficult hydrolytically because of its connected area of

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crystalline and amorphous structures.59 Generally, cellulose can withstand higher decomposition

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temperatures than hemicellulose and lignin, with decomposition occurring at a temperature of

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approximately 320-356°C.61

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Hemicellulose. Hemicellulose is known as the second most abundant polymer on earth,

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with an estimated production of 60 billion tons annually, and it comprises 30-35% of floral cell

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walls.59,62 Hemicellulose chains usually interact with one or more cellulose fibrils to form non-

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covalent cross-links between cellulose bundles. As an example, hardwood species are composed

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mainly of glucuronoarabinoxylans, which are formed by the complex pentose sugar chain.

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Conversely, softwood species are the combination of galactoglucomannans with a backbone of β-

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1,4-linked D-mannopyranose and D-glucopyranose units.60 Furthermore, hemicellulose consists of

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different branches of complex carbohydrate structures (xylose, arabinose, mannose, glucose, and

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galactose) that are not recalcitrant to conversion to monomeric sugars because of its amorphous,

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branched and single-chain polysaccharides. Therefore, hemicellulose is more favored for the

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hydrolysis process than cellulose is.59,60 The processing step of extraction of the hemicellulose

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from biomass can produce or liberate compounds such as acetic acid, which eventually will help

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in autohydrolysis pretreatment. However, acetic acid may inhibit subsequent fermentation steps,

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thereby lowering the production of ethanol and other compounds.62

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Lignin. Lignin accounted for 11-25% of floral cell walls. Lignin is known as an

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amorphous phenolic macromolecule that can be found only in the secondary cell wall. Three

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primary units, namely, guaiacyl (G), sinapyl (S) and p-hydroxyphenyl (H) are linked by aryl ether

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or C-C bonds to form lignin.58 The high level of heterogeneity of lignin is synthesized through the

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oxidative coupling of 4-hydrophenylpropanoids during secondary cell wall deposition, which

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results in layers of lignin in different monomers. Lignin acts as a protective barrier with

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by restricting the accessibility of efficient saccharification of cellulosic microfibils.56,59,60

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Additionally, lignin provides mechanical support and water transport for the cell.61

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Overall, cellulose provides strength over the cell walls, whereas hemicellulose acts as a

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wire mesh that circulates around the cellulose and provides enhancement to the strength and

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linkage. Lignin fills the remaining space and sets everything in place and excludes water from the

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polysaccharide environment. This unique combination results in the high resistance of

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lignocellulosic biomass to mechanical and biological degradation.60

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ROLE AND GOAL OF LIGNOCELLULOSIC BIOMASS PRETREATMENTS

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Pretreatment is necessary in the bioconversion process to overcome the complex

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recalcitrant biomass structure. Pretreatment provides higher accessibility for the enzymes to

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convert cellulose and hemicellulose into useful fermentable sugars during the enzymatic

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hydrolysis process. When biomass is exposed to pretreatment, several operations, such as the

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increase in the surface area and porosity, modification of the lignin structure, removal of lignin,

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partial depolymerization of hemicellulose, and reduction of cellulose crystallinity, can be

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accomplished. These result in the improvement of the hydrolysis process such that a higher

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percentage of fermentable sugars can be recovered.64,65 According to Singhvi et al.,66 only 20% of

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the sugar can be recovered without introducing pretreatment, but 80-83% of the fermentable sugar

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can be recovered with introduction of a pretreatment process.67 Generally, pretreatment can be

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categorized into three areas: mechanical, chemical, and biological pretreatments. Mechanical

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pretreatment such as milling, grinding and chipping reduces the size of the biomass and therefore

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provides higher surface area and porosity.64 Another important example of mechanical

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pretreatment is ultrasound pretreatment. Ultrasound pretreatment applies the principle of

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cavitation that forms millions of microscopic vapor bubbles. The explosion of these bubbles will

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create high temperature (1700-4700°C) and pressure (1800 bar) in a microsecond, thus producing

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shockwaves that induce mechanical effects such as breaking larger molecules into smaller

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fractions with particle size reduction.68,69 Chemical pretreatment involves the disruption of ACS Paragon Plus Environment

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biomass recalcitrance by means of chemical reactions. Some of the conventional methods are

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liquid hot water, dilute and concentrated acid hydrolysis, alkaline hydrolysis, organosolv,

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oxidative delignification and room temperature ionic liquids.70 Moreover, certain methods are

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termed “physicochemical” pretreatments because they involve the integration of both mechanical

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and chemical pretreatment, such as steam explosion and CO2 explosion.64 Biological pretreatment

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involves the use of environmentally friendly enzymes such as cellulase and hemicellulase to break

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down complex carbohydrates into simple sugars.70 However, enzymatic hydrolysis is usually

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inefficient if it is applied directly to biomass, unless the biomass has already undergone the

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aforementioned mechanical or chemical pretreatments. The advantages/disadvantages of some

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common biomass pretreatments are tabulated in Table 4.3-18,20,21,35,38,70-75

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USE OF INORGANIC SALTS FOR BIOMASS PRETREATMENT

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One of the most novel pretreatment technologies that has attracted interest is the use of

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inorganic salts either as aqueous solutions in an acid-free environment or as catalysts in an acid

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pretreatment system.22 The integration of inorganic salts has also been implemented with

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organosolv23,24 and microwave irradiation25. This inorganic salt pretreatment method has been

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used to pretreat various types of lignocellulosic biomass, as shown in Table 5.22-24,26-38 Some

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examples of inorganic salts are NaCl, KCl, CaCl2, MgCl2, Fe2(SO4)3, FeCl2, FeSO4, and FeCl3,

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which have been proven to increase the hydrolysis rate and yield of cellulose or

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hemicelluloses.27,30,31 Among all of the inorganic salts, FeCl3 has been proven to be the most

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effective inorganic salt and could be used to replace acid for biomass hydrolysis.70 FeCl3 has the

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capability to solubilize hemicelluloses into monomeric and oligomeric sugars in liquid

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hydrolysate with large amounts of xylose while producing a cellulose-rich solid substrate that is

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highly useful in enzymatic hydrolysis for producing glucose at a later stage.28,32 Therefore,

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inorganic salts possess potential for the production of both cellulosic bioethanol and value-added

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chemicals from hemicellulose-derived sugars.



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Hemicellulosic Sugar Recoveries. Pretreatment of biomass is carried out mostly to

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produce cellulosic bioethanol via fermentation. However, only limited studies have shown a

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remarkable recovery of hemicellulosic sugars from the biomass. One of the pretreatments that has

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been extensively studied is dilute acid hydrolysis, where Fields and Wilson76 found that

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temperatures between 100 and 150°C were usually preferred in xylose production. To compare

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the performance of inorganic salts with dilute acid in the aforementioned temperature range,

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Marcotullio et al.38 investigated the selective production of hemicellulose-derived carbohydrates

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using FeCl3. At 120°C and 120 min, FeCl3 achieved a performance comparable to that of HCl,

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with a total monomeric xylose yield of 20.6 wt% of the initial wheat straw in the hydrolysate.

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Dilute acid hydrolysis of hemicelluloses and cellulose also produced higher amounts of

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fermentable sugar yields when the biomass was first impregnated with FeSO4, again proving the

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positive effect of an inorganic salt in the pretreatment of lignocellulosic biomass.39 Nevertheless,

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the optimal operating conditions reported for hemicellulosic sugar recovery by using inorganic

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salt pretreatment have been somewhat contradictory. According to Linares et al.,33 the use of 0.26

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mol/L of FeCl3 at 153°C and 30 min produced a maximum of 63.2% total hemicellulosic sugar

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recovery from olive tree biomass. Liu et al.27 reported 89.0% xylose recovery when corn stover

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was pretreated with 0.1 mol/L of FeCl3 under almost similar conditions (140°C, 20 min) as

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Linares et al.33 Thus, it can be concluded that the optimal operating conditions for inorganic salt

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pretreatment vary depending on the biomass and the inorganic salt used. Generally, more severe

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operating conditions are not suggested for sugar recovery from the biomass because the conditions

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may cause sugar degradation to furfurals and/or other degradation products. Nevertheless, it is

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important to achieve a balance between pretreatment severity and reducing sugar production by

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using Equation (1).29,77 CS = logI – pH = log [t . exp[(TH – TR)/14.75] – pH

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Equation (1)

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where CS (unitless) is the combined severity parameter, t (min) is the pretreatment time, TH (°C)

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is the reaction temperature, TR (100°C) is the reference temperature, and pH is the final pH of the

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solution. ACS Paragon Plus Environment

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At low severity, hemicelluloses could not be degraded, whereas at high severity, the yield

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of xylose was minimal due to the degradation of xylose into inhibitors, though up to 100% of

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hemicellulose degradation was achieved.29,71 As a case in point, a high FeCl3 concentration >0.10

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mol/L accelerated the dissolution of oligomeric xylose to monomeric xylose and then monomeric

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xylose to degradation products, but not the transformation from xylan into oligomeric xylose.27 At

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a temperature of 180°C, the degradation of xylose and xylotriose was increased by 6- and 49-fold,

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respectively, when the xylose monomer and oligomer mixture were pretreated with 0.8% of FeCl3

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compared with using pressurized hot water.78 However, the hemicellulose structure generally

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forms chemical bonds with cellulose and lignin that are more complicated than the pure monomer

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and oligomer interactions in the synthetic mixture.32 Less severe parameters may be incapable of

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breaking down the hemicellulose structure, despite degrading the monomer and oligomer into

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inhibitors. Sun et al.29 obtained the highest sugar content and a low inhibitor concentration at a

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moderate combined severity of 0.32, when the severity parameter ranged from 0 to 1.81. The

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efficient solubilization of hemicellulose by using inorganic salts could be attributed to the reduced

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activation energy requirements for biomass hydrolysis.39 Hence, in the case of pentose-oriented

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pretreatment, operating at mild temperatures and nearly atmospheric pressure could be a realistic

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option.38

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Cellulosic Sugar Recoveries. In the context of glucose recovery, the removal of

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hemicelluloses and lignin fractions of the lignocellulosic biomass is essential during the

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pretreatment process. FeSO4 was able to selectively hydrolyze up to 95.0% of the hemicelluloses

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into a liquid fraction, with the cellulose still remaining in the biomass residue.22 As a result of

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removing hemicellulose from the solid fraction, the subsequent enzymatic hydrolysis was

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enhanced by promoting cellulose exposure and enzymatic activity of cellulase.22 Liu et al.28

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achieved a high enzymatic hydrolysis of 98.0% for the solid residue after undergoing a

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pretreatment process using 0.1 mol/L of FeCl3, which contributed to 93.0% of the glucose

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recovery. In another notable study, Zhang et al.37 achieved a satisfactory glucose yield of 61.7%

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via the enzymatic hydrolysis of pretreated cattail solid residue. Remarkably, the addition of 0.1 ACS Paragon Plus Environment


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mol/L of FeCl3 during the organosolv pretreatment also removed all hemicelluloses while

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conserving approximately 90% of the initial cellulose in the residue for subsequent enzymatic

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saccharification.23

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The improvement of enzymatic digestibility and the subsequent increase in the cellulosic

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sugar recoveries from the biomass residue was attributed to the effects of the inorganic salt

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pretreatment on the biomass structure. Specifically, the smooth continuous surface of the cellulose

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was damaged for better enzyme access, and the amorphous hemicelluloses existing as irregular

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clumps were almost completely removed from the solid residue after introducing Fe(NO3)3

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pretreatment, as shown in Figure 6.29 Similar changes to the surface morphology of the biomass

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were also observed when NH4Cl or MgCl2 was used to pretreat Miscanthus straw, as shown in

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Figure 7.31 According to Kang et al.,31 both NH4Cl and MgCl2 caused damage to the compact

284

structure of Miscanthus straw, but further studies to determine the cause of the different

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morphology were apparently inconclusive. An application of FeCl3 also exhibited a disruption of

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the recalcitrant structures of the crystalline cellulose that are normally observed in the biomass.

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According to Liu et al.,27 the exposure of the cell internal structure and an improved surface area

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and porosity made the material softer and more readily able to be digested. Hence, the enzymatic

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digestibility of the solid residue pretreated with 0.1 mol/L of FeCl3 and 50% (w/w) ethanol was

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observed to reach as high as 89%.23 Moreover, Yu et al.36 managed to enhance the enzymatic

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digestibility to approximately 86.0% by applying CuCl2 in the flow-through mode. The increase in

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the crystallinity index could possibly be explained by the fact that high amounts of hemicelluloses

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were removed without much alteration to the cellulosic structure due to the milder pretreatment

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conditions.38 Thus, the crystalline structure of the cellulose core was exposed.

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Production of Inhibitors. When lignocellulosic biomass is pretreated with inorganic salts,

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low generation of fermentation inhibitors might occur due to pentose dehydration.23,24 Some

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studies have suggested that inorganic salts might generate fewer inhibitory coproducts compared

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with the acid hydrolysis pretreatments that are well known to produce high amounts of toxic

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degradation compounds, primarily acetic acid, furfural, and HMF (5-hydroxymethylfurfural).20 ACS Paragon Plus Environment

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For instance, low concentrations of furfural (0.011 g/L) and HMF (0.148 g/L) were achieved

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when organosolv was used together with FeCl3.23 The results were in agreement with the results

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of Kamireddy et al.,26 who found that corn stover produced lower amounts of inhibitors when it

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was pretreated with CuCl2 or FeCl3 compared with H2SO4 using the same conditions (160°C, 10

304

min). Specifically, CuCl2 obtained approximately 0.42 g/L HMF and 1.50 g/L furfural, and FeCl3

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produced 0.52 g/L HMF and 1.19 g/L furfural. The HMF and furfural produced during H2SO4

306

hydrolysis were 1.10 and 2.40 g/L, respectively. Nonetheless, the production of inhibitors is

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heavily dependent on inorganic salt characteristics and pretreatment harshness. Increases in the

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Fe(NO3)3 concentration and pretreatment time were proven to intensify furfural production.30 The

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trend of xylose monomer degradation was observed to accelerate significantly when the FeCl3

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concentration was increased from 0.1 to 0.4%.27 When the concentration of FeCl3 was increased

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from 100 to 200 mmol/L, Marcotullio et al.38 observed a growth in acetic acid formation from 1.6

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to 2.6 wt% during the pretreatment of wheat straw at 100°C. Furthermore, temperature had a

313

profound effect on inhibitor production,29,30 where temperatures higher than 160°C favored the

314

conversion of xylose to furfural.23 The increase in pretreatment temperature from 100 to 120°C

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also caused the concentration of acetic acid to rise from 1.6 to 3.2 wt%, though the FeCl3

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concentration was maintained.38

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Nonetheless, some previous studies were also done to test whether inorganic salts could be

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used for intentional conversion of sugars to furfural.78-80 FeCl3 was found to have a peculiar

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catalytic effect on xylose and xylotriose degradation, with the rate constants being 3- and 7-fold

320

greater than those of dilute H2SO4 at the same pH.78 Zhang et al.80 noted that an isomerization of

321

xylose for accelerating sugar dehydration and furfural production was promoted by metal

322

chlorides at 185°C and 100 min. The degradation rates of carbohydrates using inorganic salts were

323

faster than using dilute H2SO4 at the same pH.78 Thus, this phenomenon suggests that the

324

inorganic salts contributed to more efficient carbohydrate dehydration into the furfural compound,

325

which is desirable in the context of using the furfural compound in the fields of chemical industry

326

and liquid fuel production. ACS Paragon Plus Environment


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14 327

REACTION MECHANISM FOR INORGANIC SALT PRETREATMENT

328

Although some studies have reported on the effects of inorganic salts on lignocellulosic

329

biomass pretreatment, little is known about the reaction mechanism involved. To date, several

330

mechanisms have been proposed to explain the behavior of inorganic salts. First, metal salts act as

331

Lewis acid by dissolving in aqueous solvents to produce complex cations. A Lewis acid can be

332

defined as a molecular body that serves as an electron pair acceptor that can react with a Lewis

333

base to form a Lewis adduct.71 Accordingly, coordinate covalent bonds with six water molecules

334

as monodentate ligands are formed around the central metal cation. Leshkov and Davis81 proposed

335

the general expression of these metal ion ligand complexes as [M(H2O)n]z+ (where M is the metal

336

ion, z is the cation oxidation state, and n is the solvation number, typically ranging between 4 and

337

9). Hence, metal chlorides such as Al3+ and Fe3+ are presumed to follow this reaction mechanism

338

to form six coordinate covalent bonds with water molecules, whereas Cu2+ achieves a stable

339

complex ion by coordinating as a tetradentate ligand.26,36

340

The formation of the metal cations ultimately acting as Lewis acids would then aid in the

341

cleavage of the glycosidic linkages. The coordinated water molecules from the hydrated cation

342

play a pivotal role in xylose formation by participating as nucleophiles.82,83 Similarly, the

343

conversion of cellulose to glucose mediated by ZnCl2 has also been examined to understand the

344

mechanism of formation of inorganic salts as Lewis acids. First, the zinc was found to coordinate

345

to the glycosidic oxygen to help break down the glycosidic linkage, resulting in the reduction of

346

reaction energy. Then, in the second step of the mechanism, the coordinated water molecules of

347

the metal cation complex participated as nucleophiles to yield D-glucose, as shown in Figure 8.84

348

The same Lewis acid mechanism pathway can be observed for AlCl3 83 and FeCl3 salts.85

349 350

Second, metal ions can undergo hydrolysis when they are mixed with water to produce H3O+ ion. The hydrolysis reactions are represented by Equations (2) and (3).86

351

[M(H2O)6]2+ + H2O

[M(H2O)5OH]+ + H3O+

Equation (2)

352

[M(H2O)6]3+ + H2O

[M(H2O)5OH]2+ + H3O+

Equation (3)

353

where M is the metal ion. ACS Paragon Plus Environment

Page 15 of 48


15 354

Therefore, the aqueous metal salt solutions present a Brønsted acid character similar to

355

HCl to depolymerize hemicelluloses into monosaccharides,38,85 as shown in Figure 9.26

356

Interestingly, FeSO4 was proposed to facilitate the breakdown of the glycosidic linkage due to the

357

adsorption of Fe2+ to the hydroxyl oxygen atoms and the oxygen of the cellulose pyran ring to

358

form a carbohydrate complex.22 In the dilute acid pretreatment, each proton pairs with one

359

electron to weaken the bond energy for easier bond rupture. Thus, Zhao et al.22 suggested that

360

metal ions could be playing the same role and may possibly be even more effective in assisting

361

bond rupture because they have more positive charges to pair with more electrons.

362

However, the capability of metal salts to accept electrons and the different ionic radii of

363

metal ions contribute to the varying stabilities of the carbohydrate complexes.32 The effect of the

364

inorganic salts on the decomposition of hemicellulose seems to follow this particular order:

365

Transition metal chlorides (FeCl2, FeCl3, CuCl2, ZnCl2) > alkaline earth metal chlorides (MgCl2,

366

CaCl2) > alkaline metal chlorides (KCl, NaCl). Transition metal ions such as Fe3+ and Fe2+ make

367

good electron acceptors compared with the alkaline metal ions, namely K+ and Na+,32 because

368

transition metal ions are able to coordinate with the oxygen donor atoms of carbohydrates and

369

their derivatives without losing the protons that make up the hydroxyl groups of the ligand.36

370

Alkaline metal cations are weak electron acceptors and are unable to coordinate with

371

carbohydrates, thus providing an explanation for the relatively low sugar recovery compared with

372

the sugar recovery observed for transition and alkaline earth metals. Hence, the dissimilar

373

arrangements of extranuclear electrons in the metal elements and the saccharide-metal cation

374

intermediate complex between inorganic salts result in contrasting performance in hemicellulose

375

hydrolysis.36 According to Marcotullio et al.,38 FeCl3 produced filtrates with the pH lower than the

376

HCl pretreatment (1.73–1.81 and 2.25–2.46, respectively). The results concur with those of Liu et

377

al.,27 where the pH after FeCl3 pretreatment reached as low as 1.68. Pretreatment with other

378

inorganic salts, namely NaCl, KCl, CaCl2, and MgCl2, produced filtrates with pH close to 7.

379

When the pH approached neutral, hemicelluloses were mainly solubilized as oligomers, despite

380

the lesser degradation of the hemicelluloses into toxic inhibitors.22 With respect to the findings of ACS Paragon Plus Environment


Page 16 of 48

16 381

Kang et al., the effect of a trivalent salt was reported to be greater than the effect of a di- or

382

monovalent salt during biomass hydrolysis.31 For example, FeCl3 generated 71.6% enzymatic

383

digestibility and almost 100% xylan removal from biomass, which were at least 26.7% and 31.8%

384

higher than di- and monovalent metal salts, respectively.31 Hence, Fe3+ apparently formed a

385

stronger acid in aqueous solution than did other cations because of its pKa (acid dissociation

386

constant) value of 2.46, which was significantly lower than Al3+ (4.85), Cu2+ (6.50), Fe2+ (9.49),

387

Mg2+ (11.4), Ca2+ (12.7), and Na+ (14.1).26

388 389

PROSPECTS AND CHALLENGES OF THE INORGANIC SALT PRETREATMENT

390

Reduction in Operational Cost. Generally, inorganic salts have several advantages over

391

the “current technology” of lignocellulosic biomass pretreatment, such as dilute acid hydrolysis.

392

The dilute acid hydrolysis pretreatment method has been known to cause corrosion issues and

393

requires expensive materials for equipment construction.22 Corrosion occurs easily because

394

inorganic acids such as H2SO4, which is commonly used in dilute acid hydrolysis, produces a

395

strong electrolyte during the pretreatment process. An inorganic salt is less corrosive than an

396

inorganic acid35 because the former undergoes hydrolysis to release hydrogen ions as a weak

397

electrolyte to provide a steadier reactive environment.32 Moreover, Liu et al.27 discovered that

398

certain inorganic salts (NaCl, KCl, MgCl2, CaCl2) generated a hydrolysate with a pH that was

399

more suitable for downstream fermentation. The need for high neutralization requirements may

400

therefore be eliminated. However, stronger inorganic salts such as FeCl3 and Fe2(SO4)3

401

demonstrated higher xylose recovery, despite a low pH of ~2.0 that may require additional

402

neutralization.27 The inorganic salts can be recovered in the form of metal hydroxides via

403

ultrafiltration, and the treatment with conjugated acids is able to convert the metal hydroxides

404

back to metal chlorides, which could be recycled and reused in the later part of the biomass

405

pretreatment process.26

406

Associated Limitations. However, some concerns arise regarding the use of inorganic

407

salts for lignocellulosic biomass pretreatment. In particular, the effect of different inorganic salts ACS Paragon Plus Environment

Page 17 of 48


17 408

on the cellulase hydrolysis is not well understood.71 For instance, excessive Fe and Cl proportions

409

left in the solid fraction might affect the microorganisms or enzymes during bioconversion at the

410

later stage.38 In a study to identify the inhibitory effects of metal ions, Fe3+ and Fe2+ were thought

411

to target cellulose and cause undesirable impacts on cellulolysis.87 Heavy metal elements such as

412

chromium even caused the denaturing and inactivation of cellulase, which led to the

413

unsatisfactory enzymatic hydrolysis of Cr3+ pretreated sugarcane bagasse.35 Kamireddy et al.26

414

reported contradictory results, in which the enzymatic hydrolysis was improved due to the

415

presence of metal complexes that formed lignin-metal complexes. This occurrence eliminated

416

lignin inhibition and granted access to more cellulose sites. This result may explain the huge

417

fluctuation of enzymatic hydrolysis yield, which ranged from 36.6 to 98.0%,28,33 as well as other

418

factors such as biomass characteristics, pretreatment severity, and enzymatic loadings. Kim et al.23

419

argued that FeCl3 could be removed via a washing/filtration process to a point where it was

420

undetected. A novel approach has been developed to generate electricity while removing Fe2+

421

from the liquid fraction by using a fuel cell system.88 Nevertheless, further investigations are

422

required to analyze the mechanism of inorganic salt pretreatments and the effects on downstream

423

applications caused by metal ions in the biomass residues and liquid hydrolysate.

424

Economic Feasibility. At present, the development of second-generation fuels and

425

chemicals from lignocellulosic biomass via a biochemical route may present greater cost

426

reduction potential than the thermochemical route because the latter involves extremely energy-

427

intensive processes. The biotechnological conversion of biomass to second-generation products

428

has not been intensively commercialized, though the subject has been studied for decades, mainly

429

due to the uncertainty of the techno-economic feasibility of large-scale production.46,66

430

Pretreatment of lignocellulosic biomass, which is necessary to achieve high sugar yields in

431

biotechnological operation, has been singled out as the most expensive step, as it represents

432

approximately 20% of the total cost of the whole process.65 Among the available pretreatment

433

technologies, those that employ chemicals currently offer better yields at lower costs, which are

434

essential for economic viability. As part of an initiative of the United States Department of ACS Paragon Plus Environment


Page 18 of 48

18 435

Agriculture, Eggeman and Elander performed a comprehensive economic analysis to evaluate

436

various biomass pretreatment methods (dilute acid, hot water, ARP, AFEX, and lime).89 They

437

concluded that low-cost pretreatment reactors were often counterbalanced by the higher costs

438

associated with catalysts/solvents and product recovery, which resulted in little difference in

439

economic performance among the pretreatments investigated. To date, no literature is available

440

regarding the economics of biomass pretreatment using inorganic salts. Generally, inorganic salts

441

such as FeCl3 and AlCl3 are expensive.90 Nonetheless, the application of inorganic salt has

442

showed promising opportunities for reduced costs because of its less corrosive nature, which

443

allow for the use of cheaper construction materials.35 Furthermore, a recent study conducted by

444

Liu et al.27 proved that inorganic salts could be used directly as an aqueous solution for biomass

445

pretreatment without an addition of acid, which reduced neutralization requirements. Additionally,

446

Marcotullio et al.38 successfully carried out pretreatment of biomass at mild temperatures and at

447

approximately atmospheric pressure using an inorganic salt. Hence, scale-up issues concerning

448

continuous biomass feeding at high pressures and material resistance could be avoided. Silva and

449

Chandel70 also stated that inorganic salts allowed simple process conditions that would reduce

450

operational costs and energy consumption. In conclusion, further research is necessary to

451

investigate the physicochemical changes and mechanisms involved during not only inorganic salt

452

pretreatment but also other lignocellulose pretreatment methods in general on a cellular scale.

453

With an advancement in the field of analytical chemistry, better optimization studies could be

454

performed to develop more cost-effective pretreatment technologies in the future.

455 456

AUTHOR INFORMATION

457

Corresponding Author

458

*(T. Y. Wu) Tel : +60 3 55146258. Fax : +60 3 55146207.

459

Email : [email protected]; [email protected]

460 461 ACS Paragon Plus Environment

Page 19 of 48


19 462

Funding

463

The funding of this research is supported by the Ministry of Higher Education, Malaysia, under

464

Long Term Research Grant Scheme (LRGS/2013/UKM-UKM/PT/01). In addition, the authors

465

would like to thank Monash University, Malaysia, for providing Y.-L. Loow with a postgraduate

466

scholarship.

467 468

Notes

469

The authors declare no competing financial interest.

470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 ACS Paragon Plus Environment


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biomass

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Kim, I.; Rehman, M.S.U.; Kim, K.H.; Han, J.I. Generation of Electricity from FeCl3

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Pretreatment of Rice Straw using a Fuel Cell System. Bioresource Technology. 2013, 135,

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Eggeman, T.; Elander, R.T. Process and Economic Analysis of Pretreatment Technologies.

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Degenstein, J.; Kamireddy, S.R.; Tucker, M.P.; Ji, Y. Oligomer Saccharide Reduction

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during Dilute Acid Pretreatment Co-catalyzed with Lewis Acids on Corn Stover Biomass.

731

International Journal of Agricultural & Biological Engineering. 2013, 6(2), 54-62.

732 733



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30 734

LIST OF TABLES Table 1. Breakdown of worldwide major crops.44,45

735

Crop

Cassava

World production (tonnes) 2.77 × 108

Major Producing Country (tonnes) [% of world total production] Nigeria : 5.3 × 107 [19.1%]

Hemp

1.24 × 105

France : 4.93 × 104 [39.8%]

Maize

1.02 × 109

United States : 3.54 × 108 [34.7%]

Oil Palm

2.66 × 108

Indonesia: 1.2 × 108 [45.1%]

Rapeseed

7.27 × 107

Canada: 1.79 × 107 [24.6%]

Soybean

2.76 × 108

United States: 8.95 × 107 [32.4%]

Sugar Beet

2.46× 108

Russia: 3.93 × 107 [16.0%]

Sugarcane

1.91 × 109

Brazil: 7.68 × 108 [40.2%]

Sorghum

6.23 × 107

United States : 9.88 × 106 [15.9%]

736

737

738

739

740

741


Other Countries (tonnes) Thailand : 3.02 × 107 Indonesia : 2.39 × 107 Brazil : 2.15 × 107 China : 1.6 × 104 Korea : 1.4 × 104 Netherlands : 1.03 × 104 China : 2.19 × 108 Brazil : 8.03 × 107 Argentina : 3.21 × 107 Malaysia : 9.57 × 107 Thailand : 1.28 × 107 Nigeria : 8.00 × 106 China : 1.45 × 107 India : 7.82 × 106 Germany : 5.78 × 106 Brazil : 8.17 × 107 Argentina : 4.93 × 107 China : 1.20 × 107 France : 3.36 × 107 United States : 2.98 × 107 Germany : 2.28 × 107 India : 3.41 × 108 China : 1.29 × 108 Thailand : 1.00 × 108 Nigeria : 6.7 × 106 Mexico : 6.31 × 106 India : 5.28 × 106

Page 31 of 48


31

Table 2. Biomass yields of different energy crops.42

742

Energy Crop Willow

Establishment time (years) 3+

Dry matter biomass yield (ton/ha/year) 5-11

Poplar

3+

2-10

Eucalyptus

4+

10-12

Miscanthus

3+

5-43

Switchgrass

2-3

5-19

Reed Canary Grass

1-2

2-10

Alfalfa

1-2

1-17

Fibre sorghum

1-2

16-43

743

744

745

746

747

748

749

750

751

752

753

754

755



Page 32 of 48

32 756

Table 3. Typical components and particle size of various lignocellulosic biomass,

757

mainly agricultural residues. Lignocellulosic Material Corn stover Corn stover Wheat Straw Rice Straw Switchgrass Poplar Wood

Particle Size (diameter) 0.279 mm N/A N/A N/A N/A N/A 500-1000 µm 212-300 µm

Miscanthus

500-1000 µm 212-300 µm 500-1000 µm 212-300 µm 500-1000 µm 212-300 µm 500-1000 µm 212-300 µm N/A

31.2 27.8 41.6 37.4 72.7 77.7 66.8 74.6 42.19

42.1 45.8 30.0 37.0 5.6 1.8 11.5 6.8 27.60

26.7 26.4 28.4 25.6 21.7 20.5 21.7 18.6 21.56

62

N/A

37.74

27.23

20.57

63

N/A

33.77

27.38

21.28

Straw Distiller Dried Grains Wheat Shorts Sugarcane Bagasse Sugarcane Bagasse Straw

Components of Lignocellulose (%) Cellulose Hemicellulose Lignin 37.99 22.45 18.39 36.4 22.6 16.6 38.2 24.7 23.4 34.2 24.5 11.9 31.0 24.4 17.6 49.9 25.1 18.1 13.9 57.9 28.1 9.1 65.2 25.7

758 759 760 761 762 763 764 765 766 767 768 769 770 ACS Paragon Plus Environment

References 59 60

61

Page 33 of 48


33

Table 4. Comparison of different biomass pretreatments.

771

Pretreatment Dilute acid hydrolysis

Advantages Obtain high yields of fermentable sugars

Concentrated Obtain high yields of acid hydrolysis fermentable sugars Autohydrolysis (Liquid hot water)

Produces less inhibitors, simple operation

Sodium hydroxide (NaOH)

Effectively removes lignin, low temperature and pressure operation, low inhibitor production Low enzyme demands during enzymatic hydrolysis, low inhibitor production

Ammonia fiber expansion (AFEX) Ammonia recycle percolation (ARP) Ionic liquid

Effectively removes lignin, low inhibitor production

Inorganic salt

Less corrosive, low enzyme demands

Steam Explosion

Low enzyme dosages demand, low energy demand. Environmentally friendly, low energy demand

Enzymatic hydrolysis

Effectively dissolves cellulose, minimal environmental impacts

Disadvantages Produces inhibitory by-products, corrosive, requires acid neutralization Sugar recovery rate is slow, highly corrosive, requires acid recovery system for feasibility High temperature operation, high energy and water input. Dissolved pentose appeared mainly in oligomeric form which required further post hydrolysis step to obtain monomeric sugar. Long pretreatment time, expensive chemical, complex recovery process

Reference 3-7, 72

71, 73

13-15, 20, 21

8, 9

Produces high amounts of oligomers which cannot be fermented by microorganism, safety and environmental issues of ammonia Can only handle low solid loadings

10, 21, 70

High costs of reagents, regeneration requirements, lack of knowledge regarding toxicity and causticity of reagent Little understanding of salt residues effect on downstream processing Formation of degradation product that inhibits subsequent hydrolysis and fermentation Inefficient without other pretreatment beforehand, slow process

18, 20, 71, 74, 75


11, 12, 20

35, 38, 71

16, 17

70


Page 34 of 48

34

Table 5. Performance of inorganic salts pretreatment for various biomass. Biomass Corn stover

Initial pretreatment FeSO4 T = 180oC Time = 20 min 10% (w/v) FeSO4

Subsequent pretreatment Enzymatic hydrolysis Cellulase (Celluclast 1.5L) 20 FPU/g substrate T = 50oC, Time = 72 h

Barley straw

Organosolv + FeCl3 T = 170oC Time = 60 min L/S ratio = 7:1 (v/w) 0.1 mol/L FeCl3, 50% (w/w) ethanol Organosolv + MgCl2 T = 210oC Time = 10 min 0.1% (w/v) MgCl2, 50% (v/v) ethanol CuCl2 T = 160oC Time = 10 min 0.125 mol/L CuCl2

Enzymatic hydrolysis Cellulase (Celluclast 1.5L) 20 FPU/g cellulose, β-glucosidase (Novozyme 188) 40 CBU/g cellulose T = 45oC, Time = 72 h

Pitch pine

Corn stover

FeCl3 T = 160oC Time = 10 min 0.125 mol/L FeCl3 Corn stover

FeCl3 T = 140oC

Key findings 1) Liquid fraction: 60.3% xylose recovery 2) Solid fraction: ~79% glucose recovery, ~86% enzymatic hydrolysis yield 1) Solid fraction: 90% cellulose recovery, 89% enzymatic digestibility 2) Liquid fraction: 0.011g/L HMF, 0.148g/L furfural

Enzymatic hydrolysis 1) Solid fraction: 75.8% glucose yield, Cellulase (Celluclast 1.5L) 700 EGU/g 61.2% enzymatic digestibility WIS, β-glucosidase (Novozymes NS50010) 250 CBU/g cellulose T = 50oC, Time = 72 h Enzymatic hydrolysis 1) Liquid fraction: 93% monomeric Cellulase (Accelerase 1500) xylose yield T = 50oC, Time = 72 h 2) Liquid fraction: 3.45 ± 0.34 g/L acetic acid, 0.42 ± 0.09 g/L HMF, 1.50 ± 0.33 g/L furfural Enzymatic hydrolysis 1) Liquid fraction: 94% monomeric Cellulase (Accelerase 1500) xylose yield T = 50oC, Time = 72 h 2) Liquid fraction: 3.30 ± 0.61 g/L acetic acid, 0.52 ± 0.08 g/L HMF, 1.19 ± 0.15 g/L furfural None 1) Liquid fraction: 89.0% xylose recovery


Reference 22

23

24

26

27

Page 35 of 48


35

Time = 20 min 0.1 mol/L FeCl3 Corn stover

FeCl3 T = 160oC Time = 20 min 0.1 mol/L FeCl3

Corn stover silage

Fe(NO3)3 T = 150oC Time = 10 min L/S ratio = 9:1 (w/w) 0.05 mol/L Fe(NO3)3

Corn stover silage

Fe(NO3)3 T = 150oC Time = 21.2 min L/S ratio = 9:1 (w/w) 0.05 mol/L Fe(NO3)3 Miscanthus NH4Cl straw T = 185oC Time = 15 min L/S ratio = 10:1 2% NH4Cl

Enzymatic hydrolysis Cellulase (Celluclast 1.5L) 60 FPU/g glucan, β-glucosidase (Novozyme 188) T = 50oC, Time = 72 h None

None


MgCl2 T = 185oC Time = 15 min L/S ratio = 10:1 2% MgCl2


Miscanthus FeCl3 straw T = 200oC Time = 15 min L/S ratio = 10:1

Enzymatic hydrolysis Cellulase (Celluclast 1.5L) 30 FPU/g cellulose, β-glucosidase (Novozyme 188) 30 CBU/g cellulose


2) Solid fraction: ~90% glucose recovery 1) Solid fraction: 93.0% glucose recovery, 98.0% enzymatic hydrolysis yield

28

1) Liquid fraction: 91.80% xylose recovery, 19.09% glucose recovery, 96.74% arabinose recovery 2) Liquid fraction: 0.03g/L total inhibitor concentration 1) Liquid fraction: 81.66% xylose recovery 2) Liquid fraction: 0.5g/L furfural concentration

29

1) Liquid fraction: 90.2% xylan recovery 2) Solid fraction: 94.4% glucan recovery, 39.7% enzymatic digestibility 1) Liquid fraction: 94.6% xylan recovery 2) Solid fraction: 95.0% glucan recovery, 29.9% enzymatic digestibility 1) Solid fraction: 60.1% glucan recovery, 71.6% enzymatic digestibility

31

30

32


Page 36 of 48

36

Olive tree biomass (OTB)

Softwood chips

Sugarcane bagasse

0.5% FeCl3 FeCl3 T = 153oC Time = 30 min L/S ratio = 5:1 (v/w) 0.26 mol/L FeCl3

T = 50oC, Time = 72 h Enzymatic hydrolysis Cellulase (Celluclast 1.5L) 45 FPU/g WIS, β-glucosidase (Novozyme 188) 15 CBU/g cellulose T = 50oC, Time = 48 h

SO2 + FeSO4 T = 200oC Time = 5 min 0.5 mmol/L FeSO4 H2SO3 + FeSO4 T = 200oC Time = 5 min 0.5 mmol/L FeSO4 FeCl3 T = 170oC Time = 30 min L/S ratio = 10:1 (v/w) 0.1 mol/L FeCl3 CrCl3 T = 170oC Time = 30 min L/S ratio = 10:1 (v/w) 0.1 mol/L CrCl3 AlCl3 T = 170oC Time = 30 min L/S ratio = 10:1 (v/w) 0.1 mol/L AlCl3 FeCl2 T = 170oC Time = 30 min

1) Liquid fraction: 63.2% total hemicellulosic sugars recovery 2) Solid fraction: 36.6% enzymatic hydrolysis yield

33

Enzymatic hydrolysis Cellulase 15 FPU/g WIS, βglucosidase 18 IU/g WIS T = 40oC, Time = 96 h Enzymatic hydrolysis Cellulase 15 FPU/g WIS, βglucosidase 18 IU/g WIS. T = 40oC, Time = 96 h None

1) Solid fraction: 69.1% overall sugar yield

34

None

1) Liquid fraction: ~60% xylose recovery, ~20% glucose recovery 2) Liquid fraction: 1.88 ± 0.47 g/L furfural, 0.667 ± 0.003 g/L HMF

None

1) Liquid fraction: ~46% xylose recovery, ~2% glucose recovery 2) Liquid fraction: 1.70 ± 0.10 g/L furfural, 0.816 ± 0.058 g/L HMF

None

1) Liquid fraction: ~35% xylose recovery 2) Liquid fraction: 3.72 ± 0.10 g/L


1) Solid fraction: 67.7% overall sugar yield 1) Liquid fraction: ~52% xylose recovery, ~16% glucose recovery 2) Liquid fraction: 5.11 ± 0.15 g/L furfural, 0.752 ± 0.030 g/L HMF

35

Page 37 of 48


37

Sweet sorghum bagasse

L/S ratio = 10:1 (v/w) 0.1 mol/L FeCl2 ZnCl2 T = 170oC Time = 30 min L/S ratio = 10:1 (v/w) 0.1 mol/L ZnCl2 CuCl2 T = 184oC Time = 8 min (20 mL/min), 10 min (10 mL/min) 0.1% CuCl2

furfural, 0.268 ± 0.007 g/L HMF None

1) Liquid fraction: ~15% xylose recovery 2) Liquid fraction: 0.870 ± 0.002 g/L furfural, 0.069 ± 0.022 g/L HMF

Enzymatic hydrolysis Cellulase (Imperial Jade Biotechnology) 40 FPU/g dry solid T = 50oC, Time = 48 h

1) Liquid fraction: 2.5% glucan recovery, 19.7% xylan recovery 2) Solid fraction: 42.0% glucan recovery, 2.7% xylan recovery, ~86.0% enzymatic digestibility 1) Liquid fraction: 90% xylose recovery 2) Solid fraction: 61.7% glucose recovery 1) Liquid fraction: 22.9% xylose (20.6% monomeric), 2.6% glucose, 3.1% arabinose (d.b. initial wheat straw), 96.8% total pentose yield

Typha latifolia cattails

MgCl2 T = 180oC Time = 15 min 0.4 mol/L MgCl2

Enzymatic hydrolysis Cellulase 15 FPU/g glucan T = 37oC, Time = 48 h

Wheat straw

FeCl3 T = 120oC Time = 120 min L/S ratio = 5:1 (v/w) 0.2 mol/L FeCl3

None


36

37

38


Page 38 of 48

38

FIGURE CAPTIONS

Figure 1. Major sources of lignocellulosic biomass.40-43 Figure 2. Global production of potentially available lignocellulosic biomass, adapted from Kurian et al.41 Figure 3. Structure and chemical composition of lignocellulose residues.56 Figure 4. TEM images for SBKP (a) show the thin primary cell wall (P) and outermost layer (S1), middle layer (S2) and inner most layer (S3) of the thick secondary cell wall (S) and (b) displays the lamella separation, S1/P and S2.57 Figure 5. FE-SEM image of untreated SKBP (a) shows the microfibrils on the cell wall, (b) displays the exposed P wall layer, whereas (c) presents the exposed secondary cell wall layer (S).57 Figure 6. Scanning electron micrograph of (a) untreated and (b) Fe(NO3)3-pretreated corn stover silage at x1000 magnification.29 Figure 7. Scanning electron micrograph of (a) untreated, (b) NH4Cl-pretreated, and (c) MgCl2-pretreated miscanthus straw at x1000 magnification.31 Figure 8. Conversion of cellulose into glucose via the Lewis acid mechanism.84 Figure 9. Conversion of xylan into xylose via the Brønsted acid mechanism.26


Page 39 of 48


39

Major Sources of Lignocellulosic Biomass

Annual and Perennial Dry Energy Grasses (US$ 100-120/ton)

Forestry Waste Feedstock (US$ 60-80/ton)

Examples i. Switchgrass ii. Miscanthus iii. Canary Grass iv. Giant Reed v. Napier Grass

Examples i. Saw mill ii. Paper mill iii. Wood chips iv. Poplar

Figure 1


Agricultural Waste (US$ 60-80/ton)

Examples i. Corn Stover ii. Sugarcane Bagasse iii. Rice Straw/Husk iv. Wheat Straw


Page 40 of 48

40

Wheat Straw 24% Rice Straw 25% Corn stover 32% Forest wood residues 6% Sorghum stover 2%

Figure 2


Sugarcane bagasse 11%

Page 41 of 48


41

Figure 3



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42

Figure 4


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43

Figure 5



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44

Figure 6


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45

Figure 7



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46

Figure 8


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47

Figure 9



 FOR TABLE OF CONTENTS ONLY


Page 48 of 48

Recent Advances in the Application of Inorganic Salt Pretreatment for

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