Pyrolysis mechanisms of methoxy substituted α-O-4 lignin dimeric model compounds and detection of free radicals using electron paramagnetic resonance analysis

Abstract

We report on the pyrolysis of methoxy substituted α-O-4 dimeric phenolic compounds at 500°C using a Frontier Lab micro-pyrolyzer to provide mechanistic insight into lignin pyrolysis. The model compounds were benzyl phenyl ether (BPE), 1-(benzyloxy)-2-methoxybenzene (MBPE) and 1-(benzyloxy)-2,6-dimethoxybenzene (DMBPE). Pyrolysis of model dimers primarily involved homolytic cleavage of the CO linkage in the α-O-4 group. In addition, cleavage of the CO linkage in the methoxy group dominated at 500°C. It was found that the methoxy group substitution on the aromatic ring increases the reactivity toward CO homolysis. Also, methoxy substituted structures can produce many different radical species that can participate in a variety of reaction mechanisms, resulting in complex reaction pathways and pyrolysis products. In addition, free radicals in the condensed phase of the pyrolysis products were detected by electron paramagnetic resonance (EPR) spectroscopy, providing information on the presence of oxygen-centered phenoxy and carbon-centered benzyl radicals. The radical concentrations derived from pyrolysis of the methoxy-substituted dimers, MBPE and DMBPE, were respectively 55 and 30 times higher than the radical concentration derived from BPE. Lastly, it was found that the methoxy group substitution on the aromatic ring significantly promoted oligomerization reactions to form large molecular weight compounds.