High-quality AlPO-18 membranes were synthesized using low-cost symmetric alumina supports by a single hydrothermal step, and showed to be useful for light gas separation. Single-gas permeances of CO 2 , N 2 , CH 4 and C 3 H 8 decreased with increasing kinetic diameter. Single CO 2 and CH 4 permeance dependence of pressure drop was predicted by the Maxwell-Stefan diffusion model and was in agreement with experimental data. The best AlPO-18 membrane showed smaller-component permeances of 6.5, 6.3 and 1.0 Â 10 À7 mol (m 2 s Pa) À1 (equal to 1940, 1880 and 300 GPU) and mixture selectivities of 220, 45 and 22 for equimolar CO 2 /CH 4 , CO 2 /N 2 and H 2 /CH 4 binary mixtures at room temperature and 0.2 MPa pressure drop, respectively. The effects of temperature and pressure on the membrane's separation performance in the three binary mixtures were discussed.
Grain boundary defects are normally formed in zeolite membranes during membrane preparation and calcination processes. In this work, a siloxane polymer coating with the imidazole group was grafted on the surface of defective SSZ-13 membranes by chemical liquid deposition to seal the defects. The parameters such as silanization time, polymerization time, monomer type and concentration were optimized.Characterizations including Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM) and energy dispersive x-ray spectroscopy (EDX), showed that siloxane polymers were coated on the surfaces of SSZ-13 crystals and membrane. Six modified membranes showed decreased CO 2 permeance by only 21±5% [average CO 2 permeance of 1.9×10 -7 mol/(m 2 s Pa)] and increased CO 2 /CH 4 selectivity by a factor of 9±3 (average CO 2 /CH 4 selectivity of 108) for an equimolar CO 2 /CH 4 mixture at 298 K. CO 2 /CH 4 and CO 2 /N 2 selectivities of the modified membrane decreased with pressure and temperature. Membrane stability was investigated for a long-time test and exposures to water vapor at temperatures up to 378 K and to some organic solutions. This modification method is also effective in sealing the defects of other zeolite membranes such as AlPO-18 membranes.
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