Thermodynamic parameters governing the stereoselective degradation of arylglycerol-B-aryl ether bonds in milled wood lignin under kraft pulping conditions

Abstract

Abstract During this investigation, softwood (picea mariana) milled wood lignin was subjected to kraft pulping conditions at the temperature range 100-160 oC, for variable lengths of time. After quantitative isolation of the treated lignins, they were subjected to 31PNMR analyses which allowed the absolute rates of scission of the erythro and thewo diastereomers of the arylglycerol-B-aryl ether structures present in lignin to be derived.The plots of the kinetic data revealed that two kinetic regimes operate i.e. an initial fast phase that is followed by another slower phase. In agreement with previous accounts, the rate constants for these scission reactions were found to follow a pseudo-first-order rate law, during both phases. Rate constant data obtained during this effort, invariably indicated that the erythro isomers of the R-0-4 units of softwood milled wood lignin cleave faster than their thr-eo counterparts during both phases. At elevated temperatures (160 "C) the difference was found to be augmented. The general stability of the threo diastereomers toward kraft pulping seems to be the manifestation of a considerably slower reaction of the pulping reagents with the threo diastereomers of the B-aryl ethers. Arrhenius and Eyring plots o i these data supplied the complete profile of thermodynamic parameters governing this commercially important ether scission reaction.