Membrane separation technology has attracted much attention due to its energy efficiency as compared to the other separation technologies, such as distillation. Many attempts have been made to develop membranes for H2 separation to obtain high purity H2 for fuel cell application. Most currently investigated membranes for H2 separation are polymer membranes. However, they suffer from limited stability under harsh environments, such as high temperatures, typically for many H2 process streams. Inorganic microporous membranes are therefore highly promising for high temperature H2 separation because of their high thermal and chemical stabilities. Currently investigated zeolite membranes, such as silicalite-1 membranes, can not provide satisfactory H2 permselectivities because the kinetic diameters of H2, CO2, N2, and CO are smaller than the zeolitic pore diameter of silicalite-1. The lack of suitable zeolite membranes represents a major obstacle in the development of novel inorganic membrane technology for H2 separation.
This dissertation is devoted to developing chemically and thermally stable ultramicroporous sodalite (SOD, pore size: 0.28 nm) and deca-dodecasil 3R (DDR, pore size: 0.36×0.44 nm) membranes for H2 separation. Sodalite membranes were prepared as low-silica (Si/Al=1.0), high-silica (Si/Al=5.0), and more thermally and chemically stable pure-silica sodalite by in-situ crystallization and seeded secondary growth methods. Deca-dodecasil 3R (DDR) membranes were hydrothermally synthesized on porous a-alumina and γ-alumina supports by in-situ crystallization and seeded secondary growth methods using 1-adamantanamine (1-ADA) as the structure-directing agent (SDA).
The X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were applied to investigate the phase composition and microstructure of the membranes and corresponding bulk zeolite phases. The thermal stability, thermal expansion and template removal behavior of sodalite and DDR structures were investigated by high-temperature X-ray diffraction (HTXRD) and thermo-gravimetric analysis (TGA)-differential scanning calorimetry (DSC)-mass spectrometer (MS) system. Confocal microscopy technique was applied to characterize intercrystalline defects present in sodalite membranes. Defective sodalite membranes were successfully modified by chemical vapor deposition (CVD) technique using tetraethylorthosilicate (TEOS) as the silica source. The CVD-modified sodalite membranes exhibited significantly enhanced H2/CO2 permselectivity of 41 and 45 at 550 °C for low- and high-silica sodalite membranes, respectively. H2/CO2 permselectivity of defective DDR membranes obtained by in-situ crystallization increased from 2.6 to ~33 at 550 °C after CVD modification. For largely defect-free DDR membranes obtained by seeded secondary growth, H2/N2 permselectivity increased from ~10 to 14 by the atomic layer deposition (ALD) of thin Al2O3 layer on the surface of the DDR membrane and the H2 permeance slightly decreased from 5.1×10-8 to 2.7×10-8 mol m-2 s-1 Pa-1 at 25 °C. These results indicated that ultramicroporous sodalite and DDR membranes are highly promising for economical H2 separation from various H2-containing process streams, such as syngas from the water-gas-shift reaction.