Delignification mechanism during high-boiling solvent pulping Part 1. Reaction of guaiacylglycerol-beta-guaiacyl ether

A phenolic beta -O-4 type lignin model compound, guaiacylglycerol-beta -guaiacyl ether (1) was treated with 70 wt% aq 1,3-butanediol solution at 160-200 degreesC to investigate the delignification mechanism under HBS (high-boiling solvent) pulping conditions. The following compounds were identified from the reaction products by use of GC-MS: guaiacol (2), coniferyl alcohol (3), gamma -etherified coniferyl alcohols (4) and a-etherified guaiacylglycerol-beta -guaiacyl ethers (5), but acidolysis products, such as Hibbert's ketones were not detected. These results strongly suggest that the phenolic beta -O-4 linkage was cleaved homolytically under HBS pulping conditions. The cleavage of P-ether exhibited a pseudo first-order rate behavior. The pseudo first-order rate constants were as follows: k = 0.94 X 10(-2) min(-1) at 160 degreesC; k = 1.97 X 10(-2) min(-1) at 170 degreesC; k = 3.22 X 10(-2) min(-1) at 180 degreesC ; k = 9.76 x 10(-2) min(-1) at 200 degreesC. The activation energy was 98.3 kJmol(-1). The formation of higher molecular weight compounds was confirmed by GPC. It is highly probable that the oligomeric products were derived from the recombination of phenoxy radicals formed by homolysis of the beta -aryl ether.