Pyrolysis studies have been conducted at 375 °C on several silica-immobilized phenethyl phenyl
ether (PPE) model compounds, representative of related β-O-4 aryl ether linkages in lignin, to
explore the impact of restricted mass transport on reaction pathways. As found previously for
fluid-phase PPE, two competitive free-radical decay pathways are operative including a significant
rearrangement pathway involving an O,C-phenyl shift for surface-attached PhCH2CH•OPh
radicals. The selectivity for the rearrangement pathway is found to be sensitive to substituents
and, in particular, to the structure of neighboring spacer molecules on the surface. In contrast to
solution-phase behavior, dilution of PPE molecules on the surface with rigid aromatic spacers
such as biphenyl or naphthalene hinder the rearrangement path. This phenomenon, attributed
to steric constraints that decrease the rate of the 1,2-phenyl shift, is not observed when a more
flexible spacer molecule (diphenylmethane) is employed. An improved knowledge of the pathways
involved is important since this rearrangement pathway, which was also observed in the pyrolysis
of α-aryl ether models, can result in the formation of valuable chemicals (aryl aldehydes and
ketones) or undesirable refractory compounds (biphenyls and diphenylmethanes) during the
thermochemical processing of lignin.
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