Influence of the Si/Al ratio on the separation properties of SSZ-13 zeolite membranes

• SSZ-13 (CHA) membranes of varying Si/Al ratio prepared.
• In-detail characterization by various physical–chemical techniques.
• CO2 separation and pervaporation tests.
• Number of defects and polarity increase with Al content.
• Separation performance strongly depends on Si/Al ratio.

SSZ-13 (CHA) zeolite membranes supported by α-alumina hollow fibers were prepared by a hydrothermal secondary growth method. The gel Si/Al ratio was varied between 5 and 100. The water adsorption depended strongly on the Si/Al ratio. Comparatively, ethanol adsorption varied less with membrane hydrophobicity. Consistent with these trends, low-silica membranes showed good performance in the pervaporative dehydration of ethanol/water mixtures. On contrary, high-silica membranes were found to be more effective for CO2/CH4 equimolar gas mixture separation. Together with increasing framework polarity, the concentration of non-zeolitic pathways (intercrystalline defects) increased with the Al content. The net effect of the influence of membrane polarity and defect density makes low-silica SSZ-13 water-selective membranes for alcohol/water pervaporation and high-silica SSZ-13 promising CO2-selective membranes in gas separation (CO2/CH4).

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