Condensation of Lignins with Carbohydrates in Concentrated Sulfuric Acid

Abstract

Acid-soluble lignin, in addition to acid-insoluble lignin (Klason lignin), is produced when woody material is treated with 72% sulfuric acid. The percentage of acid-soluble lignin (ASL) amounts to less than 1% in softwoods, but can reach several percent in hardwoods (Schoning and Johanson 1965; Musha and Goring 1974). Extensive research on KL provided valuable information on its chemical structure and the mechanism by which it forms (Ito et al. 1981; Yasuda 1984; Yasuda et al. 1980, 1981a, b, 1983, 1985; Yasuda and Ota 1986, 1987; Yasuda and Hirano 1990). However, much remained to be learned about the ASL even as late as 1991. Campbell and McDonald (1952) first isolated beech acid-soluble lignin, a resinous material with fewer methoxy groups and less carbon than KL, features that suggest an oxygen-rich substance. Pearl and Busche (1960) argued that ASL is formed in the initial stage of treatment with 72% sulfuric acid and lignin-carbohydrate complexes (LCCs) are present in it. Musha and Goring (1974) observed an increase in acid-soluble lignin with a rise in the number of syringyl units in wood. Leary et al. (1986) reported a significant hydrolysis of syringyl β-O-4 linkages in the early stage of treatment with 72% sulfuric acid.