Tetramethylammonium hydroxide (TMAH) thermochemolysis: proposed mechanisms based upon the application of 13C-labeled TMAH to a synthetic model lignin dimer

The mechanism by which heated tetramethylammonium hydroxide (TMAH) degrades the lignin biopolymer was investigated by the novel application of 13C-labeled TMAH (13C-TMAH) in the thermochemolysis of a synthetic model guaiacyl lignin dimer. GC-MS analysis of the products showed labeling patterns consistent with a base-catalyzed intramolecular displacement of the β-phenoxy group and the formation of two intermediate guaiacyl propane epoxides, a γ-hydroxy-α,β-epoxide and an α-hydroxy-β,γ-epoxide. Methoxide then functions as a nucleophile to open the epoxide ring. These results substantiate the base-catalyzed reactions previously postulated by Gierer (1970) to explain alkali wood pulping and also explain the facile formation and distribution of lignin derivatives obtained in the TMAH thermochemolysis of natural samples. The absence of substantial numbers of bonds involving propyl-aryl ether linkages with adjacent hydroxyl groups is the limiting factor in the complete decomposition of lignin by TMAH thermochemolysis, as propyl-aryl ether linkages without adjacent hydroxyl groups cannot react via this mechanism. This helps to explain why materials such as highly degraded lignin residues, with significant side chain alteration and type III kerogens above the rank of lignite, where aliphatic-aryl ether linkages are thought to be insignificant, are reported to give low yields of TMAH thermochemolysis products.