(370c) Hydrophobic Zeolite Membranes for Gase and Liquid Separation

Most zeolite membranes are inherently hydrophilic and thus preferentially permeate water. This characteristic has led to the commercial application of zeolite membranes in molecular separations, particularly for organic solvent dehydration. However, hydrocarbon-selective membranes are highly desirable for applications such as bioseparations and membrane reactors aimed at enhancing product yields. This paper presents a synthesis strategy to enhance the hydrophobicity—and thereby hydrocarbon selectivity—of zeolite membranes by increasing the Si/Al ratio and crystallinity, while maintaining high permeability through membrane thinning. The approach involves synthesizing zeolite membranes on non-aluminum supports (such as porous yttria-doped zirconia and stainless steel) as well as on alumina supports, incorporating a hierarchical mesoporous pure-silica zeolite intermediate layer between the alumina support and the top zeolite separation layer. This intermediate zeolite layer plays two critical roles: it prevents Al ion migration from the alumina support into the synthesis mixture, and it provides a mechanically robust yet permeable foundation that favors nucleation over crystal growth, enabling the formation of a thin, uniform top layer. These efforts culminate in the synthesis of super-hydrophobic MFI-type zeolite membranes. The hydrophobic nature of these membranes results in negligible impact from the presence of steam during gas separation, as demonstrated using H₂/CO₂ binary and H₂/CO₂/H₂O ternary gas mixtures at elevated temperatures (300–550 °C). In contrast, water vapor significantly degrades the separation performance of hydrophilic zeolite membranes. The super-hydrophobic MFI zeolite membranes exhibit high selectivity for hydrocarbons over water. With 5% alcohol-water liquid feeds in pervaporation separation tests, the best-performing membranes on alumina hollow-fiber supports with an intermediate zeolite layer achieve alcohol/water separation factor of above 180 and 240 at total fluxes of 5 and 3 kg·m⁻²·h⁻¹, respectively, for ethanal/water and methanol/water mixture separation.