Preparation and gas separation performance of zeolite T membrane

Abstract

Zeolite T membranes were prepared by hydrothermal synthesis on porous mullite tubes seeded with zeolite T crystals, using milk-like aluminosilicate gel with a molar composition of SiO₂∶Al₂O₃∶Na₂O∶K₂O∶H₂O = 1∶0.05∶0.26∶0.09∶14. A zeolite T crystal layer of about 20 µm in thickness was formed on the outer surface of the support after the synthesis at 373 K for 30 h. Single-gas and mixed-gas permeation experiments through zeolite T membranes were carried out by a vacuum method at 303∼473 K using He, H₂, CO₂, O₂, N₂, CH₄, C₂H₆ and C₃H₈ single-component gases and CO₂/N₂, CO₂/CH₄ and other CO₂/hydrocarbon mixtures, respectively. In single-gas permeation experiments, with increasing kinetic diameter from 0.33 nm for CO₂ to 0.43 nm for C₃H₈, the gas permeance decreased by four orders in magnitude, indicating a kind of molecular sieving behavior for the zeolite T membranes prepared in this study, that is, they had little defects and their permeation behavior was controlled by zeolitic pores of erionite. Permeance of CO₂ was much higher than those of N₂ and CH₄ and the ideal selectivities for CO₂/N₂ and CO₂/CH₄ were 31 and 266 at 343 K, respectively. In mixed-gas permeation experiments, zeolite T membranes showed the high selectivities for CO₂/N₂ and CO₂/CH₄ pairs of 107 and 400, respectively, at 308 K. The selectivity α decreased with an increase in temperature, but was still in a high level of 20 and 52 for CO₂/N₂ and CO₂/CH₄, respectively, even at 473 K. This is due to the synergetic effects of competitive adsorption of CO₂ and molecular sieving of zeolitic pores. Because of the increasing effect of single file diffusion, the selectivities for CO₂/C₂H₆ (α = 61) and CO₂/C₃H₈ (α = 17) were rather low.