Xiaofei Tian, Zhen Fang and Charles (Chunbao) Xu

Abstract Enzymatic saccharification of lignocellulosic biomass encounters many prohibitive factors which make it difficult to be developed on an industrial scale. Pretreatment has been found essentially effective for increasing the susceptibility of substrates to the enzyme, for example, by removing the lignin barrier and breaking down the crystal structure of cellulose in the raw materials. As a green and efficient technique, pretreatment of lignocellulosic biomass employing organic solvents and organic electrolyte solution (OES) is introduced in this chapter. Future prospects and recommended research work for developing these technologies for practical applica­tion, as well as coupling production of high-value bio-products from lignocellulosic biomass, are also discussed.

Keywords Aqueous organic solvent ■ Delignification ■ Decrystallized cellulose ■ Ionic liquids ■ Selective precipitation ■ Hydrolysis yield

14.1 Introduction

In biorefineries of lignocellulosic biomass, especially for cellulosic ethanol pro­duction, there are three key steps in the bioconversion process: (1) lignocellulose pretreatment, to separate the lignin and hemicellulose components from cellulose in the biomass and destroy the recalcitrant cellulosic structure to reactive intermediates; and (2) enzymatic hydrolysis, saccharification of the cellulose and hemicellulose to fermentable sugars (e. g., glucose and xylose) by cellulase-catalyzed hydrolysis; and

Z. Fang (H) ■ X. Tian

Biomass Group, Chinese Academy of Sciences, Xishuangbanna Tropical Botanical Garden,

88 Xuefulu, Kunming, Yunnan province, 650223, China e-mail: zhenfang@xtbg. ac. cn

X. Tian ■ C. (Chunbao) Xu

The Institute for Chemicals and Fuels from Alternative Resources, Faculty of Engineering, Western University, London, Ontario, N6A 5B9, Canada

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries, 309

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_14, © Springer-Verlag Berlin Heidelberg 2013

(3) fermentation, to produce bio-ethanol or other bio-based chemicals (e. g., lactic acid and succinic acid) [1-3].

Effect of the nature of lignocellulosic substrate on the enzymatic attachment and activity of cellulases are two of the critical factors that influence the entire rate and yield during the enzymatic hydrolysis process, and also the big challenges that have to be overcome for the commercial production of lignocellulosic bio-ethanol [4-6]. Consequently, pretreatment is required prior to the hydrolysis to remove the recal­citrance, such as (1) disruption of the carbohydrate-lignin shield by reducing lignin and hemicellulose contents [6-9]; (2) concomitantly decreasing the crystallinity of the cellulosic structure [6, 7]; (3) extension of enzymatic accessible surface area by particle cracking and increasing the volume of micro pores [5]; (4) loosening of the structure of lignin or (and) cellulose by cleavage of beta-1-O-4-aryl-ether bond or (and) beta-1-O-4-glucosidic bond [8]. Since pretreatment accounts for the largest proportion (around 50 %) of bio-ethanol production cost excluding the enzyme and raw material costs [5,9-11], it has great potential for efficiency improvement and cost reduction through research and development [8]. Plenty of work on pretreatment has been conducted and published during the last 10 years. The reported pretreatment methods can be classified into the following categories: (1) physical treatments (i. e., milling, grinding, high energy radiation, etc), (2) chemical treatments (e. g., soaking and boiling by diluted acid or alkalis, ozone oxidation, etc.), (3) physiochemical treatments (steam explosion, ammonia fiber explosion, ammonia recycled percola­tion [8], pyrolysis, hydrothermal and organosolv methods), and (4) biological (white rot fungi) treatments [12].

The nomenclature of organosolv pretreatment is derived from organosolv, which is a pulping technique that uses an organic solvent to solubilize lignin and hemi — cellulose and was invented by Theodore Kleinert [13]. Organosolv pretreatment is generally classified as a physiochemical technique, which employs non-aqueous or aqueous organic solvents, such as various alcohols, acetone, glycerol, dioxane, ethylene glycol, triethylene glycol, and phenol [14], with (or without) catalysts to remove parts of noncellulosic components (mainly lignin), and (or) convert the form of cell walls, as well as structures of natural cellulose, providing effective frac­tions of lignocellulosic biomass for the subsequent enzymatic saccharification step. Though organic acid and organic peracid were used to pretreat biomass as well [15], they differ from the organosolv pretreatment of interest here due to their dissimilar fundamentals.

This chapter presents an overview of the status of organosolv pretreatment tech­niques and their corresponding fundamentals, as well as further processing of the organosolv by-products. In addition, a newly developed technique using organic electrolyte solution (OES) composed of organic solvent and cellulose-soluble elec­trolyte solvent in pretreatment is also introduced. Future prospects and recommended research work on developing these technologies for practical application, as well as coupling production of high-value bio-products from lignocellulosic biomass are also discussed.