14.3.1 Introduction

OES, short for organic electrolyte solution, is defined as non-aqueous or aprotic solvent solution of electrolytes, which usually contains fraction of polar organic solvents and free ions.

Organic solvents mixed with certain lignocellulose-dissolving solutions, such as room temperature ILs [83-86] and concentrated phosphoric acid [87], form new types of homogeneous solutions that can be used to (1) swell and (or) dis­solve the component of cellulose and (or) lignin, as a pure lignocellulose-dissolving solution does, and (2) precipitate the cellulose and lignin selectively. These combina­tions include dimethyl sulfoxide (DMSO) + paraformaldehyde [88, 89], DMSO + [BMIM]Cl [90], DMSO + tetra-n-butylammonium fluoride (TBAF) [91, 92], N, N — dimethylformamide (DMAC) + 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) [93], (acetone, pyridine, or hexamethylphosphoramide) + ILs [94], 3-dimethyl-2- imidazolidinone + 1-ethyl-3-methylimidazolium acetate ([EMIM]AcO) or 1-butyl-

3- methylimidazolium chloride ([BMIM]Cl) [95], DMAC + LiCl [96], and H3PO4 + acetone [97], etc. The addition of common lignocellulose-dissolving solution in or­ganic solvents allows OES to have the equivalent abilities to destroy the crystallinity of natural cellulose (acting as a cellulose solvent) [42]. Meanwhile, with the existence of organic solvents, some OES can remove the lignin component from the lignocel — lulosic homogeneous solutions by precipitating cellulose and hemicellulose (acting as a precipitating agent) [97]. In terms of the selectivity of dissolving abilities, OES — based pretreatment processes can be divided into two categories: OES-dissolving pretreatment and OES-depositing pretreatment.