Chabazite-Type Zeolite Membranes for Effective CO₂ Separation: The Role of Hydrophobicity and Defect Structure

Chabazite (CHA)-type zeolites are promising for the separation of CO₂ from larger molecules, such as N₂ (relevant to postcombustion carbon capture) and CH₄ (relevant to natural gas/biogas upgrading). In particular, the pore size of CHA zeolites (0.37 × 0.42 nm²) can recognize slight molecular size differences between CO₂ (0.33 nm) and the larger N₂ (0.364 nm) or CH₄ (0.38 nm) molecules, thus allowing separation in favor of CO₂ through CHA membranes. Furthermore, the siliceous constituents in the CHA zeolite can reduce the adsorption capacity toward the smaller H₂O molecule (0.265 nm) and, thus, the H₂O permeation rate. This is highly desirable for securing good molecular sieving ability with CO₂ permselectivity in the presence of H₂O vapor. Indeed, a siliceous CHA film obtained with a nominal Si/Al ratio of 100 (CHA_100) showed high CO₂/N₂ and CO₂/CH₄ separation performance, especially in the presence of H₂O vapor; ∼13.4 CO₂/N₂ and ∼37 CO₂/CH₄ separation factors (SFs) at 30 °C. These SFs were higher than the corresponding values (∼5.2 CO₂/CH₄ SFs and ∼31 CO₂/CH₄ SFs) under dry conditions; such improvement could be ascribed to defect blocking by physisorbed water molecules. Finally, the contribution of molecular transport through zeolitic and nonzeolitic parts was quantitatively analyzed by combining information extracted from image processing of fluorescence confocal optical microscopy images with a one-dimensional permeation model. It appears that ∼19 and ∼20% of the total CO₂ permeance for CHA_100 were reduced due to transport inhibition by the physisorbed water molecules on the membrane surface and defect, respectively.