Lignins that are currently available or may become available in bulk include industrial lignins, such as kraft lignin from black liquor on pulp manufacturing; organosolv lignin on organosolv pulping using acetone, ethanol, butanol, ethylene glycol, formic acid, or acetic acid; and hydrolysis lignin obtained as residue via acid or enzymatic saccharification of woody materials. Many methods, including hydrogenolysis, pyrolysis, and supercritical water decomposition, have been used to treat such lignins in an effort to produce aromatic chemicals, such as monophenols, which are alternatives to fossil fuels. Generally, decomposition reactions that are performed at high temperatures are very fast and uncontrollable, producing innumerable types of decomposition products. Another important reason for this difficulty is a characteristic of lignin itself. Lignins undergo structural changes under isolation conditions, resulting in the formation of lignin with an unexpected and uncontrollable molecular structure.1 Therefore, it is almost impossible to design a decomposition reaction suitable for this molecular structure. Thus, we have yet to see any successful method of obtaining significant yields of a specific aromatic chemical from lignin.

The derived lignin rich in 1,1-bis(aryl)propane-2-O-aryl ether units is known as lignocresol (Figure 1). Under alkaline conditions at ~140–170 °C, the phenoxide ions of grafted cresols readily undergo nucleophilic attack of the electron-deficient β-carbon, resulting in the selective cleavage of Cβ-aryl-ether linkages (R3-O-Cβ in Figure 1).7, 8, 9 In addition, lignocresol is markedly degraded because half of the native lignin linkages between the monomeric units are Cβ-aryl-ether bonded.10, 11 In principle, after cleavage of the reactive benzyl linkages and the subsequent phenolation and cleavage of Cβ-aryl-ether linkages, syringyl-, and guaiacyl-type phenolic dimers with a coumaran ring (S1 and G1 in Figure 1) could theoretically be obtained. The syringyl unit linked via two Cβ-aryl-ether bonds with neighboring lignin units (R1: OCH3, R3: Aryl, R4: Aryl in Figure 1) potentially produces the phenolic dimer S1. Similarly, the guaiacyl unit (R1: H, R3: Aryl, R4: Aryl in Figure 1) may produce the phenolic dimer G1. Previously, the phenolic dimers S1 and G1 were recovered from the acid-insoluble fraction after alkaline treatment and subsequent acidification. The yield of S1 and G1 reached 21% and 7%, respectively, when raw hardwood (birch) lignocresol was treated at 170 °C for 60 min with 1 m NaOH.12

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