Modification of Zeolite Membranes for H₂Separation by Catalytic Cracking of Methyldiethoxysilane

Boron-substituted ZSM-5 and SAPO-34 membranes were silylated by the catalytic cracking of methyldiethoxysilane (MDES) to increase their selectivity for H 2 separation from light gases. The MDES reacted in the B-ZSM-5 pores and reduced their effective pore diameter, so that silylation significantly increased their H 2 selectivity. The H 2 /CO 2 separation selectivity at 473 K increased from 1.4 to 37, whereas the H 2 /CH 4 separation selectivity increased from 1.6 to 33. However, silylation decreased the H 2 permeances more than 1 order of magnitude in the B-ZSM-5 membranes. The H 2 permeance and H 2 /CO 2 and H 2 /CH 4 separation selectivities increased with temperature. At 673 K, the H 2 permeance was 1.0 × 10 -7 mol·m -2 ·s -1 ·Pa -1 and the H 2 /CO 2 separation selectivity was 47. Methyldiethoxysilane does not fit into SAPO-34 pores, but silylation apparently decreased the pore size of the nonzeolite pores in the SAPO-34 membranes. After silylation, the H 2 permeances and H 2 /CO 2 and H 2 /N 2 separation selectivities were almost unchanged in the SAPO-34 membranes because H 2, CO 2, and N 2 permeate mainly through SAPO-34 pores. In contrast, H 2 /CH 4 separation selectivity increased from 35 to 59, and CO 2 /CH 4 separation selectivity increased from 73 to 110, apparently because CH 4 permeates mainly through non-SAPO-34 pores.