Propylene/Propane Separation Properties Through Bis (triethoxysilyl) Methane (BTESM)-Derived Silica Membranes With Controlled Silica Network Size

Silica is one of the most attractive inorganic membrane materials for its highly porous structures in a wide temperature range.  Sol-gel and chemical vapor deposition (CVD) methods have been applied to the fabrication of silica membranes on porous substrates.  Amorphous silica networks are formed by hydrolysis and polymerization of silica precursor in sol-gel route.  Tetraethoxysilane (TEOS) is a commonly used precursor for the preparation of sol-gel derived silica membranes.  When TEOS, Si(OC₂H₅)₄, which consists of mono silicon, is used as a precursor for preparation of sol-gel derived silica membranes, TEOS-derived silica membranes consist of amorphous silica networks approximately 0.3 nm in size, which are suitable only for hydrogen (0.289 nm) to nitrogen (0.364 nm) separation [1].  Recently, we proposed the “spacer” technique to control the pore sizes of silica membranes using a bridged alkoxide such as bis (triethoxysilyl) methane (BTESM, Si-C-Si unit) and bis (triethoxysilyl) ethane (BTESE, Si-C-C-Si unit), both of which consist of organic functional groups (“spacer”) between two silicon atoms [2-5].  Pore size distribution, as determined by single-gas permeation, suggested that the order of average pore size is as follows: BTESE-derived silica > BTESM-derived silica > TEOS-derived silica, due to the differences in the minimum units of the silica precursors.  This paper reports on fabrication of BTESM-derived silica membranes and evaluates single and binary-component gas permeation/separation behavior of propylene and propane in a wide temperature range of 50-200°C.  BTESM-derived silica membranes showed superior C₃H₆ permeance with high C₃H₆/C₃H₈ selectivity in a wide temperature range (50-200 ^oC).  The C₃H₆/C₃H₈ permeance ratio obtained by binary-component separation at low temperature was higher than that obtained by single gas permeation, indicating that preferentially adsorbed C₃H₆ molecules block the permeation of C₃H₈molecules.

References

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[5] M. Kanezashi et al., J. Membr. Sci. 2012, 415-416, 478.