Pyrolysis studies of silica-immobilized benzyl phenyl ether (approximate to
PhOCH2Ph or approximate to BPE), a model for related ether structures in f
uel resources, have been conducted at 275-325 degrees C to examine the impa
ct of restricted mass transport on the pyrolysis mechanism compared with pr
evious studies in fluid phases. Significant rearrangement chemistry is obse
rved for approximate to BPE occurring through two competitive free-radical
pathways that are both promoted by the diffusional constraints. One path in
volves recombination of incipient benzyl and surface-bound phenoxy radicals
to form benzylphenol isomers, 10. The second, previously unreported rearra
ngement path for approximate to BPE involves a 1,2-phenyl shift in an inter
mediate radical, approximate to PhOCH . Ph, leading to formation of benzhyd
rol (8) and benzophenone (9) as principal products. The rearrangement produ
cts 8-10 typically account for ca. 50% of the pyrolysis products. However,
the path selectivity is a sensitive function of approximate to BPE surface
coverage and the presence of spacer molecules that either facilitate or hin
der hydrogen atom transfer steps on the surface.