Delignification mechanism during high-boiling solvent pulping - Part 2. Homolysis of guaiacylglycerol-beta-guaiacyl ether

A phenolic beta-O-4 type lignin model compound, guaiacylglycerol-beta-guaiacyl ether, was treated with 70 wt % aq 1,4-butanediol solution at 180degreesC to investigate the delignification mechanism under high-boiling solvent (HBS) pulping conditions. Thirteen compounds including four monomers, six dimers, two trimers and a tetramer were isolated from the reaction products. Most of these products were generated by recombination of phenoxy radicals formed by homolysis of the beta-aryl ether. The results suggest that phenolic beta-O-4 linkages in lignin are cleaved homolytically (radical mechanism) via quinone methide intermediates under HBS pulping conditions.