For the production of high performance inorganic gas separation membranes, membrane supports must be permeable, homogenous, and defect-free. In this study highly-permeable tubular α-Al2O3 membrane supports with a pore size ~1 µm are prepared by a gel-casting method. α-Al2O3 particles (~3 µm) in aqueous suspension are immobilized by the cross-linking of polyvinyl alcohol with 2,5-dimethoxy-2,5 dihydrofuran to form a hydrogel. Upon gelation in an aluminum mold with steel mandrel, a rigid tube is formed, which can then be removed from the casting mold, dried, and heat treated to form a porous tubular support.
Membrane supports are then coated with one or more intermediate layers in order to obtain a substrate with sufficient smoothness and coating properties for thin <100 nm membranes. Coarse α-Al2O3 membrane carriers are coated with a stable dispersion of fine α-Al2O3 to obtain a two layer structure with a smooth deposition surface and surface pores ~80 nm. Further surface smoothing is done by coating of a Boehmite sol and calcination of the layer to form mesoporous γ-alumina with a pore size ~5 nm. The coating quality is highly dependent on the colloid chemistry and coating parameters. The pore structure of γ-alumina membranes prepared by rapid thermal processing was studied using permeation porometry.
Multilayer supports with a mesoporous intermediate layer can then be coated with gas selective microporous silica by a sol-gel method. The deposition of microporous silica on tubular membrane supports by flow coating was investigated. Previous problems with sol infiltration were addressed by the application of a glycerine treatment to the γ-alumina intermediate layer. By filling the pores, silica sol infiltration is minimized, resulting in significant gains in permeance (by a factor of ~20x).
A thin layer of polydimethyl siloxane (PDMS) was applied to defective microporous silica and zeolite Y membranes to improve their selectivity. After application of PDMS, the H2/CO2 and CO2/N2 binary gas separation performance of both silica and zeolite mem-branes was found to improve significantly due to reduction in defect flow. Similar effects were found for supported zeolite Y membranes. Along with the improved separation factors, a reduction in the overall permeance occurred due to reduced defect flow contributions. The decrease in overall permeance due to defect abatement is supported by transport calculations assuming simple expressions for solution-diffusion through the membrane and Knudsen flow through the defects. These calculations, show that the application of PDMS leads to a decrease in the overall permeance but an increase in the H2 selectivity for a wide range of defect area fractions (<10-4).