Chapter 17: Zeolite Membranes for Gas Separations

Intense research efforts are being dedicated to the development of zeolitic membranes for specific gas separations where polymeric membranes cannot be utilized owing to the high operating temperature and the presence of aggressive solvents. Thus, in recent years, different preparation techniques have been developed to obtain almost defect free zeolite membranes with a large surface area. Nowadays, LTA and T zeolite membranes are utilized in organic solvent dehydration by means of pervaporation and vapour permeation processes.

Membranes of different zeolite topologies (FAU, MFI, CHA and DD3R) have been developed with interesting results to separate CO₂ and H₂ from other light gases. For example, with high quality FAU zeolite membranes very high separation factors (20-100) were obtained for the binary system CO₂-N₂. Zeolite membranes (T, DD3R and SAPO-34) characterised by smaller pore size have also been utilized in the separation of small gaseous species. T zeolite membranes showed very high selectivity, that is 104 for CO₂/N₂ and 400 for CO₂/CH₄. DD3R zeolite membrane exhibited a CO₂/CH₄ separation factor of 101 at room temperature.

Recently, modified LTA membranes supported on macroporous carbon discs have been used for the separation of H₂ from CO. The Rb and Cs-forms exhibited the best performance, in which the CO permeation is blocked in the whole temperature investigated and high purity hydrogen is obtained. Tubular Zeolite membranes have been also used to purify hydrogen rich streams from CO by its selective oxidation. A very low CO concentration was measured (down to 10–50 ppm) confirming the good potentiality of these membranes for a deep purification of H₂-rich streams, allowing the hydrogen final use, e.g., in fuel cell applications.