(569b) Chemically and Thermally Stable Pure-Silica Sodalite Membranes for Hydrogen Separation

Hydrogen separation from hydrogen-containing gas mixtures, e.g. syngas, at elevated temperatures has received much attention due to current energy crisis. Ultramicroporous sodalite (SOD) is a highly desirable ultramicroporous structure for the membrane separation of small gas molecules, such as hydrogen, due to the presence of small (~2.8 Å) six-membered ring openings of the sodalite cages. These openings are accessible only to small molecules such as H₂, H₂O and He, resulting in a superior membrane performance. The sodalite structure prepared was pure-silica sodalite (Si/Al = ∞) framework. The disk-shaped, pure-silica sodalite membranes (PSSM), were synthesized in the presence of organic template (ethylene glycol) on α-alumina supports by secondary growth of NA₂O-SiO₂ compositions.

The phase composition, microstructure, the template removal behavior and gas permeation behavior of PSSM were investigated by XRD, SEM, TGA/DSC and unsteady- and steady-state gas permeation tests. The permselectivities of H₂/N₂ and H₂/CO₂ were very low, due to the mismatch between pure-silica sodalite layer and the membrane support, resulting in thermal stress, and hence the formation of intercrystalline defects. These defects were repaired by counter-diffusion Chemical Vapor Deposition (CVD) of TEOS at 350°C for 8 hours. An optimum CVD time was established, at which the permselectivity increased drastically and the permeance was reduced only slightly.

To obtain further increase in hydrogen permeance and further reduce the defect formation, we fabricated thinner, higher flux membranes, with smaller pure-silica SOD seeds, obtained by cryo-grinding. The membranes after seeded secondary growth were subjected to multi-component permeation tests, and, the results obtained showed improved permselectivities and H₂ permeance. Furthermore, pure silica-sodalite membranes were grown on commercial tubular supports. The initial binary permeation tests showed improved H₂ permselectivities and permeance, than the disk-shaped membranes, even before the CVD modification process. Thus, it is possible to obtain highly permeable and permselective, nearly defect free, tubular SOD membranes after CVD modification, to accomplish highly effective hydrogen separation from syngas.