In addition to the aforementioned fractionation process, other organic solvents, such as methanol, ethylene glycol, ethanolamine, acetone and dimethyl formam- ide, also attract much attention in fractionation lignocellulosic material in recent years. The typical processes are listed in Table 11.3 [134-145].

11.5.1 Methanol

Methanol fractionation of lignocellulosic material can be carried out without or with the addition of catalysts. In non-catalyzed (auto-catalyzed) methanol frac­tionation of lignocellulosic material, the cooking liquor becomes acidified due to the acetic acid released from the feedstock. In catalyzed processes, the liquor can be acidic, neutral or alkaline depending on the nature of the additives employed. During acid ethanol fractionation process, lignin is mainly dissolved by cleavage of a-aryl ether and arylglycerol-b-aryl ether bonds in the lignin macro­molecule [146]. Whereas the cleavage of b-aryl ether bonds occurs to a lower extent [147]. The cleavage of ether bonds gives rise to new phenolic hydroxyl groups in lignin.

Some lignocellulosic materials, such as wheat straw [148], Eucalyptus globulus [134, 135] and poplar [149] can be delignified by methanol/water without the addition of catalyst. The optimum conditions result in pulps with a high yield and a low kappa number and an acceptable viscosity. By the addition of sulfuric acid as a catalyst, black cottonwood can be delignified in 70% methanol at temperatures ranging from 130 to 210°C. In a typical reaction, pulp with a high yield of 47% and a low kappa number of 8 was obtained in 3 h. The recovered lignin had a high molecular weight, indicating it was a potential chemical feedstock [150]. Aspen (Populus tremuloides) and black cottonwood (Populus trichocarpa) have been fractionated in 30-70% methanol catalyzed with H2SO4 and H3PO4 at pH lower than 3 [151]. After the pretreatment, glucomannan and arablnogalactan were dissolved into liquor and were easily digested by enzymes. The total yields of hydrolysis residues ranged from 40 to 60%, which generated 70-88% of the original six-carbon sugars contained in the wood by further enzyme hydrolysis.

Miscanthus x giganteus was pulped in the alkaline-methanol-anthraquinone process to prepare pulp for thermoplastic composite reinforcement [136]. Under the optimum conditions, methanol concentration 10% (v/v), alkaline concentration 15%, pulping time 25 min, pulping temperature 170°C, the produced pulp had a high thermally stable temperature of 255°C and an aspect ratio of 40, a straightness of 95% and high tensile strength of 890 MPa. The obtained pulp with good strength and thermal properties was an attractive low-weight and low-cost sub­stitute for short glass fiber.