Pyrolysis of benzyl phenyl ether confined in mesoporous silica

Benzyl phenyl ether (PhOCH2Ph, BPE), a model for related structural features in lignin and low-rank coal, has been immobilized in MCM-41 hexagonal mesoporous silicas having pore diameters of 2.7, 2.2, and 1.7 nm. Pyrolysis studies conducted at 275 degrees C have revealed that the free-radical reaction pathways previously observed on nonporous Cabosil silica remain operative in the MCM-41 silicas. The BPE pyrolysis rate is slightly slower (ca. 2- fold) in these nanoporous solids than on Cabosil, which is correlated with a corresponding decrease in the rate of radical formation by homolysis of the weak O-C bond. Two rearrangement pathways dominate the chemistry and account for ca. 67 mol % of the products at all pore sizes studied, which is markedly increased compared with the corresponding Cabosil case (ca. 50 mol %). The first pathway is recombination of the incipient benzyl and phenoxyl radicals to form surface-immobilized benzylphenol isomers, 1. The second pathway involves an O-C phenyl shift in surface-attached PhOCH center dot Ph radicals leading to formation of surface-attached benzhydrol (4) and benzophenone (5) products. The selectivity between these two rearrangement pathways shows only a small dependence on pore size, and comparisons with new data obtained for BPE on Cabosil indicate that the density of grafted BPE molecules on the surface is the most important factor in determining this product selectivity.