Development of pore size-controlled silica membranes for gas separation by chemical vapor deposition

Silica membranes were prepared by chemical vapor deposition using oxygen and tetramethoxysilane (TMOS), phenyltrimethoxysilane (PTMS), or dimethoxydiphenylsilane (DMDPS) as the silicon source at 873 K. The pore size was successfully controlled by changing the number of phenyl groups on the silicon precursor. The permeation test of several gases revealed that larger pores were formed upon increasing the number of phenyl groups on the source. The DMDPS-derived membrane showed excellent hydrogen permeance at 573 K of the order of 10−6 mol m−2 s−1 Pa−1, and a high hydrogen/sulfur hexafluoride permselectivity of over 6800, and this excellent performance was constant for 266 h even under moist conditions containing steam at 3.4 kPa and 573 K. Characterization with XPS indicated that the DMDPS-derived membrane was thinner than the TMOS membrane. Additionally, the bond energies of the silicon sources were estimated based on quantum chemical calculations with the CBS-QB3 method. The results showed a strong tendency for gas-phase conversion of TMOS or PTMS to dimethoxysilanone, (MeO)2SiO, and DMDPS to methoxyphenylsilanone, Ph(MeO)SiO, transient intermediates that act as major precursors. It was suggested that whether the intermediate contained phenyl groups greatly influenced the enlargement of pore size of the silica membrane.