Retrogressive pyrolysis pathways for surface-constrained diarylmethanes

Pyrolysis studies of silica-immobilized diarylmethanes, which are models for related structures in fuel resources, have been conducted at 425 degreesC to explore the impact of restricted mass transport on reaction pathways with particular attention to the formation of PAHs. Two retrogressive free radical pathways are observed in the pyrolysis of silica-immobilized diphenylmethane and 2-naphthylphenylmethane that involve the diarylmethyl radical as a key intermediate. The major pathway is cyclization-dehydrogenation that forms fluorene and benzofluorene (two isomers), respectively. This pathway can be attenuated, but not eliminated, by the presence of a neighboring hydrogen donor (tetralin) on the surface in contrast to fluid-phase studies. The second pathway, which has not been reported in fluid phases, is a radical displacement path that forms crosslinked triarylmethane products. The selectivity for these two pathways is found to be a function of the orientation of the diarylmethane on the surface, as well as the presence of neighboring spacer molecules.