Fluorine-induced microporous silica membranes: Dramatic improvement in hydrothermal stability and pore size controllability for highly permeable propylene/propane separation

A molecular sieving membrane was fabricated using triethoxyfluorosilane (TEFS), which contains Si-F bonds and is categorized as a pendant-type alkoxysilane. The hydrothermal stability and hydrocarbon (C3H6, C3H8) permeation properties were evaluated for TEFS membranes. When a fluorine-induced silica membrane had a Si-F bond in the amorphous structure, the reaction of steam and Si-F groups during steam treatment formed Si-OH groups, which slightly decreased the gas permeance. Even though gas permeance slightly decreased under a steam atmosphere, a TEFS membrane calcined at 350 degrees C had networks that were looser and more uniform than those of a conventional SiO2. In addition, the formation of adsorption sites (Si-OH groups) under steam treatment enhanced both interactions with the p-bonds (C=C double bond) of C3H6 and the C3H6/C3H8 permeation properties (C3H6 permeance: 2.2 x 10(-7) mol m(-2) s(-1) Pa-1, C3H6/C3H8 permeance ratio: 42 at 35 degrees C). The hydrothermal stability was dramatically enhanced by calcination temperatures as high as 750 degrees C due to the presence of fewer Si-OH and Si-F bonds in the amorphous structure, although the network pore size of a TEFS membrane was the same whether it was calcined at 750 degrees C or at 350 degrees C.