Liquefaction mechanism of lignin in the presence of phenol at elevated temperature without catalysts - Studies on beta-O-4 lignin model compound .2. Reaction pathway

Guaiacylglycerol-beta-guaiacyl ether (GG) was used as a starting material to study the liquefaction mechanism of lignin in the presence of phenol under elevated temperature. Based on the structures of the reaction intermediates and their formation and conversion kinetics, a radical reaction pathway for GG is proposed, which was further confirmed by a series of corresponding model reactions. That is, GG first underwent homolytic cleavage at its beta-O-4 linkage to yield coniferyl alcohol and guaiacol radicals as the major elementary intermediates. The coniferyl alcohol radical, which consists of a conjugated double bond in its side chain, reacted extensively, mainly with a phenol radical to yield a series of corresponding phenolated products. This reaction pathway dominated the whole dynamic liquefaction process and contributed to the multiplicity to the reaction products. By contrast, the guaiacol radical was mostly reduced to guaiacol and some of it reacted with the phenol radical to afford the corresponding dimeric coupling products. These dimeric coupling compounds were rather stable and played minor roles in the formation of the final polymeric products.