Defect-Patching of Zeolite Membranes by Surface Modification Using Siloxane Polymers for CO₂ Separation

Grain boundary defects are normally formed in zeolite membranes during membrane preparation and calcination processes. In this work, a siloxane polymer coating with an 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, field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy showed that siloxane polymers were coated on the surfaces of SSZ-13 crystals and membrane. Six modified membranes showed decreased CO₂ permeance by only 21 ± 5% [average CO₂ permeance of 1.9 × 10^-7 mol/(m² s Pa)] and increased CO₂/CH₄ selectivity by a factor of 9 ± 3 (average CO₂/CH₄ selectivity of 108) for an equimolar CO₂/CH₄ mixture at 298 K. CO₂/CH₄ and CO₂/N₂ selectivities of the modified membrane decreased with pressure and temperature. Membrane stability was investigated by 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.