A Detailed Study of the Alkaline Oxidative Degradation of a Residual Kraft Lignin Model Compound

Abstract

The kinetics of alkaline peroxide oxidative degradation of a residual kraft lignin model compound, namely 3,3′-dimethoxy-5,5′-dimethyl-[1,1′-biphenyl]-2,2′-diol (I) were examined. Activation energies for both the consumption of I and the evolution of the main degradation product, 2-hydroxy-3-methoxy-5-methylbenzoic acid were not significantly different (117±2 and 116±3 kJ/mol, respectively). Computational analysis supports the notion that the degradation is driven primarily by reactions with hydroxyl radicals, leading to the formation of radical sites. Subsequent radical couplings yield organic peroxides and then dioxetane structures. Formation of a dioxetane intermediate is a prerequisite for the final fragmentation of the molecule and computations suggest that this is the rate-limiting step in the oxidative degradation pathway of I. A transition structure was computed whose formation requires energy input (+132 kJ/mol) similar to the experimentally determined activation energy.