Ethanol fractionation can be operated under low and medium severity as a pre­treatment process to obtain hydrolyzable cellulose. In this case, the hydrolysis reaction mainly occurred at carbon position of the side chains of lignin. Cleavage of a-aryl ether is a main reaction, which lead to the formation of a benzylic carbocation in acidic medium. The benzylic carbocation can react with water or ethanol, or form a bond with an electron-rich carbon atom in the aromatic ring of another lignin unit [27]. This reaction mechanism is supported by lignin model

compound study, in which a-aryl ether linkages are more easily degraded than b-aryl ether linkages [28]. Under highly serious conditions, b-aryl ether linkages are extensively cleaved, which is the controlling reaction in delignification. The extensive cleavage of b-aryl ether linkages results in a substantial increase of phenolic hydroxyl groups, which is confirmed by the low intensity of Cb and Cy signals in the dissolved lignin as compared to MWL [27]. After acidolysis of the ethanol dissolved lignin fraction, the contents of phenolic hydroxyl groups increased significantly, suggesting the presence of intact b-O-4 bonds in the dis­solved lignin [29]. The presence of b-O-4 structures in ethanol lignin was also demonstrated by HMQC 2D NMR [30]. In addition, the presence of carbonyl groups in the dissolved lignin indicated that the formation of Hibbert’s ketones during the fractionation process [31].

During the cleavage of b-O-4 bonds, the homolytic cleavage occurs via methide intermediate thus causes the formation of b-1 inter-linkage through radical cou­pling, which then in turn degrades under the acidic medium to give stilbenes through the loss of the y-methylol group of formaldehyde [28, 32]. In addition, b-5 units are also converted into stilbenes through the same degradation pathway [33]. With respect to cinnamyl alcohol, it is converted into ethyl ether structure [33]. In a recent report, a marked decrease of aliphatic OH and a significant increase of phenolic OH are found in ethanol dissolved lignin of Miscanthus with increase of the severity of the treatment [27]. This observation can be attributed to two simultaneous and opposite reactions: the production of p-hydroxyphenyl OH group due to the scission of b-O-4 bonds involving H units and hydrolysis of a fraction of p-coumaryl ester residues [34].

With respect to the activation energies for cleavages of the two major linkages in lignin, the study of the lignin model compounds indicates that the activation energies for cleavages of a-aryl ethers bonds range from 80 to 118kJ/mol, depending on substituent [35]. These values are slightly higher than those in both auto-catalyzed and acid-catalyzed acetic acid fractionation processes, which are

78.8 and 69.7 kJ/mol, respectively [36]. However, the reported activation energy for b-aryl ether hydrolysis is 150 kJ/mol [27]. Obviously, the high value was not considered to be the controlling reaction in the ethanol fractionation process.

Lignin condensation is an important counterproductive reaction in an acidic or alkaline ethanol fractionation process. The intermediates, i. e., reactive benzyl carbocations or benzyl-linked oxygen atoms, can form a bond with an electron — rich carbon atom in the aromatic ring of another lignin units resulting in the production of condensed products. It has been reported that in a weak acid system, protonation of a benzyl-linked O atom was a SN2 type reaction [37].