Synthesis of all-silica ZSM-58 zeolite membranes for separation of CO2/CH4 and CO2/N2 gas mixtures

We prepared all-silica ZSM-58 zeolite membranes on a-alumina tubular supports and optimized their synthesis conditions. High-purity ZSM-58 zeolite membranes were prepared using a concentrated precursor solution (methyltropinium iodide/SiO2 and NaOH/SiO2 molar ratios > 0.05) with a H2O/SiO2 molar ratio of 52 at 413 K for 24 h. A ZSM-58/DOH mixed layer or a pure DOH layer was generated with ZSM-58 when the synthesis temperature exceeded 413 K and the synthesis time was longer than 24 h. The membranes prepared with a H2O/ SiO2 molar ratio higher than 52 were not covered ZSM-58 zeolite and/or generated DOH zeolite. Further, we investigated the separation performance of the ZSM-58 zeolite membrane for CO2/CH4 and CO2/N-2 gas mixtures as a function of feed pressure. At a pressure drop of 0.2 MPa, the membrane exhibited a CO2 permeance and CO2/CH4 mixture selectivity of 17 x 10(-8) mol s(-1) m(-2) Pa-1 and 290, respectively, for a CO2/CH4 system, and a CO2 permeance and CO2/N-2 selectivity of 29 x 10(-8) mol m(-2) S-1 Pa-1 and 44, respectively, for a CO2/N-2 system. The mixture selectivity was lower than the single gas selectivity in CO2 /CH4 separation due to competitive adsorption since CH4 mainly passes through non-zeolite pores. In contrast, the mixture selectivity was higher than the single gas selectivity in CO2/N-2 separation because N-2 permeation was inhibited by CO2 adsorption in the zeolite pores since N-2 mainly passes through the zeolite pores. Thus, high-purity ZSM-58 zeolite membranes with high CO2 separation performances were obtained by the optimized synthesis conditions.