Remarkable enhancement of pyrolytic carbon deposition on ordered mesoporous silicas by their trimethylsilylation

We propose a novel and simple method to uniformly deposit a thin carbon layer on the pore walls in mesoporous silica (MPS) by the chemical vapor deposition (CVD) at as low a temperature as 600 °C. Four types of MPSs (FSM16, SBA15, TMPS16 and MCM41) were trimethylsilylated and then the silylated MPSs were subjected to the CVD using acetylene. Thanks to the silylation, pyrolytic carbon deposition was significantly enhanced, although only little carbon deposition occurred on the non-silylated MPSs. It is found from the analyses of the former three types of MPSs that the carbon was deposited solely on the mesopore surface as a very thin layer and thereby the carbon-coated MPSs still keep a high surface area and a large pore volume with their mesopore structures intact. The decomposition of the trimethylsilyl groups during the CVD resulted in the formation of Si radicals on the silica surface and their catalysis can explain the observed uniform carbon coating, i.e., the radicals catalyze the trimerization of acetylene to form benzene and induce the carbonization on the silica surface. This mechanism allows the carbonization to occur only on the silica surface and the uniform carbon deposition was achieved as a result.