Tetramethylammonium hydroxide (TMAH) thermochemolysis: proposed mechanismsbased upon the application of C-13-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 C-13-la beled TMAH (C-13-TMAH) in the thermochemolysis of a synthetic model guaiacy l lignin dimer. GC-MS analysis of the products showed labeling patterns con sistent with a base-catalyzed intramolecular displacement of the beta-pheno xy group and the formation of two intermediate guaiacyl propane epoxides, a gamma-hydroxy-alpha,beta-epoxide and an alpha-hydroxy-beta,gamma-epoxide. Methoxide then functions as a nucleophile to open the epoxide ring. These r esults substantiate the base-catalyzed reactions previously postulated by G ierer (1970) to explain alkali wood pulping and also explain the facile for mation and distribution of lignin derivatives obtained in the TMAH thermoch emolysis of natural samples. The absence of substantial numbers of bonds in volving propyl-aryl ether linkages with adjacent hydroxyl groups is the lim iting factor in the complete decomposition of lignin by TMAH thermochemolys is, as propyl-aryl ether linkages without adjacent hydroxyl groups cannot r eact 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 t hermochemolysis products. (C) 1999 Elsevier Science Ltd. All rights reserve d.