Many cellulose solvents were employed to treat biomass and accelerate its enzymatic accessibility and digestibility by disrupting linkages among cellulose, hemicellulose, and lignin component as well as breaking up the hydrogen-bond linkages in the or­derly crystalline cellulose to amorphous forms [84,85,107-111]. But this method is still challenging in dealing with real lignocelluloses due to the low efficiency in removal of lignin and hemicellulose from cellulose, resulting in lower cellulose digestibility and slower hydrolysis [42]. It was revealed that cellulose, hemicellu- lose, and lignin have different solubilities in OES composed of aqueous acetone or ethanol with concentrated phosphoric acid or ILs [89,97,112-114]. Utilization of this lignin-soluble OES to deposit cellulose and/or hemicellulose from the lignocellulosic biomass solutions can be a practical approach to resolve the problem.

Zhang et al. [97] developed a lignocellulose pretreatment technique to fractionate lignocelluloses to amorphous cellulose, hemicellulose, lignin, and acetic acid. It is featured by using 8 ml cellulose solvent (pre-calibrated concentrated phosphoric acid 83-85.9 %) to dissolve 1 g biomass under a modest reaction conditions (50 °C and atmospheric pressure). After the reaction, OES was immediately generated by the addition of 20 ml organic solvent (i. e., acetone) into the biomass-phosphoric acid solutions and then well mixed. Coupled with the formation of OES, the cellulose and hemicellulose were rapidly precipitated. Followed by subsequently centrifug­ing, the OES supernatants were collected for separation of lignin, acetic acid, and acetone. With the removal of acetone from the OES by simple evaporation as well as acetic acid, the low molecular lignin was precipitated from aqueous phosphoric acid. In this way, partial lignin was removed from the biomass combined with the decrystallization of cellulose. Consequently, the regenerated amorphous cellulose without lignin was hydrolyzed efficiently. The regenerated corn stover, switchgrass, and poplar, were hydrolyzed, respectively, where a high-hydrolysis efficiency ~94 % and ~96- ~97 % was achieved at the 12th and 24th hour with an enzyme loadings of 15 FPU cellulase and 60 IU beta-glucosidase per gram of glucan, respectively. The data of hydrolysis rates and digestibility were the highest in the literature [2, 8, 115]. Alternatively, although this method seems not to be efficient for treating Douglas Fir (~73 % digestibility at 12 h and ~75 % at 24 h), it was still ~1.7 times of the sample pretreated by SO2 steam explosion [97].

Similar study of OES-precipitating pretreatment for bamboos was conducted us­ing 85 % (w/v) concentrated phosphoric acid as a cellulose solvent and 95 % (v/v) ethanol as an organic solvent [113]. After dissolving by concentrated phosphoric acid and followed by precipitation with simultaneous formation of OES, the glucan recovery yield of the sample was 93.9 % and the delignification yield was 15.3 %. The elevation of hydrolysis yield was due to the increase in cellulose accessibility to cellulase from 0.27 to 9.14 m2 per gram of biomass. Glucan digestibility attained

88.2 % at the cellulase loading of 1 glucan in 72h. The overall glucose and xylose yields were 86.0 % and 82.6 %, respectively.

The advantages of OES-precipitating pretreatment by employing a non-volatile cellulose solvent and a highly volatile organic solvent are:

1. Decrystallized cellulose and hemicellulose can be separated efficiently from dis­solved biomass by precipitation because they have poor solubility in the OES mixture that is able to partially dissolve lignin.

2. Acetone-soluble lignin can be easily recovered after adding water or evaporation of organic solvent from the OES because it is insoluble either in water or specific cellulose solutions, such as phosphoric acid [97].

3. Organic solvents such as acetone and ethanol can be easily recycled and reused by fractional distillation due to their low boiling point against non-volatile ability of the specific cellulose solutions, such as ILs or concentrated phosphoric acids.