Preparation of Amorphous Silica Based Membranes for Separation of Hydrocarbons

Pore size through silica based membranes prepared by using a counter diffusion chemical vapor deposition (CVD) method can be controlled precisely by using silica precursors having organic functional groups. These membranes were applied to hydrocarbon separation especially for propylene/propane system and methane/ethane system. In this paper, ethyltrimethoxysilane (ETMOS), propyltrimethoxysilane (PrTMOS) and hexyl-trimethoxysilane (HTMOS) were used for the silica precursors. According to the thermal decomposition properties of the silica hydrolysis powder, the decomposition temperature of HTMOS (400 degrees C) is higher than those of ETMOS and PrTMOS. The pore sizes through the silica hybrid membranes deposited at 270 degrees C were approximately 0.40 nm that were independent of the sizes of alkyl groups in the silica precursors. The alkyl groups in the membranes can be partially decomposed during the deposition. Methane/ethane permeance ratio of 38 (methane permeance 2.8 x 10(-9) mol m(-2) s(-1) Pa-1) was obtained through the ETMOS derived membrane deposited at 300 degrees C. The HTMOS derived membrane showed the propylene/propane permeance ratio of 414. The activation energy of C3H8 permeation was positive showing that the high permeation ratio was due to molecular sieve permeation mechanism. Amorphous silica based membranes showed the potential for hydrocarbon separation by controlling the deposition conditions.