Impact of restricted mass transport on pyrolysis pathways for aryl ether containing lignin model compounds

Pyrolysis studies have been conducted at 375 degreesC on several silica-imm obilized phenethyl phenyl ether (PPE) model compounds, representative of re lated beta -O-4 aryl ether linkages in lignin, to explore the impact of res tricted mass transport on reaction pathways. As found previously for fluid- phase PPE, two competitive free-radical decay pathways are operative includ ing a significant rearrangement pathway involving an O,C-phenyl shift for s urface-attached PhCH2CH . OPh radicals. The selectivity for the rearrangeme nt pathway is found to be sensitive to substituents and, in particular, to the structure of neighboring spacer molecules on the surface. In contrast t o solution-phase behavior, dilution of PPE molecules on the surface with ri gid aromatic spacers such as biphenyl or naphthalene hinder the rearrangeme nt path. This phenomenon, attributed to steric constraints that decrease th e rate of the 1,2-phenyl shift, is not observed when a more flexible spacer molecule (diphenylmethane) is employed. An improved knowledge of the pathw ays involved is important since this rearrangement pathway, which was also observed in the pyrolysis of alpha -aryl ether models, can result in the fo rmation of valuable chemicals (aryl aldehydes and ketones) or undesirable r efractory compounds (biphenyls and diphenylmethanes) during the thermochemi cal processing of lignin.