Hierarchically porous binder-free silicalite-1 discs: a novel support for all-zeolite membraees

摘要

Thermal expansion mismatch between the zeolite film and the support is an important cause for the formation of defects and cracks during the fabrication and use of zeolite membranes. We have studied how silicalite-1 discs with a permeability comparable to commercially available alumina supports can be produced by pulsed current processing (PCP) as a novel substrate for all-zeolite membranes. Hierarchically porous and mechanically strong membrane supports where the surface area and crystallography of the silicalite-1 particles were maintained could be obtained by carefully controlling the thermal treatment during PCP consolidation. In situ X-ray diffraction and dilatometry showed that the coefficient of thermal expansion (CTE) of the silicalite-1 substrate was negative in the temperature range 200-800 ℃ while the commonly used alumina substrate displayed a positive CTE. The critical temperature variation, ΔT, and thicknesses for crack-free supported zeolite films with a negative CTE were estimated using a fracture energy model. Zeolite films with a thickness of 1 μm can only sustain a relatively modest ΔT of 100 degrees when supported onto alumina substrates while the all-zeolite membranes can support temperature variations above 500 degrees.