Template-removal-associated microstructural development of porous-ceramic-supported MFI zeolite membranes

Defect-free thin MFI zeolite films were synthesized on porous alpha-alumina and yttria-doped zirconia (YZ) substrates by an in situ crystallization me thod using tetrapropylammonium hydroxide (TPAOH) as a template. The microst ructure evolution of the supported zeolite films during calcination for tem plate removal was studied by high-temperature X-ray diffraction and in situ gas permeance. Removal of the template from the zeolite films occurs at 35 0-500 degrees C and is accompanied by a substantial shrinkage in the zeolit e framework. After the template is removed from the zeolite at high tempera tures, the zeolite crystals expand on cooling while the support shrinks. A compressive stress develops in the zeolite film during the cooling process when the zeolite crystallites are bound to the support after template remov al. Without annealing prior to template removal, this stress induces cracks in the YZ-supported MFI films. High-quality MFI membranes were obtained on both alpha-alumina and YZ supports by using a suitable calcination tempera ture program. Removal of the template can either create or enlarge intercry stalline gaps as the zeolite crystallites decrease in size and draw away fr om adjacent crystallites. These gaps constitute the microporous non-zeoliti c pores affecting the permselectivity of single-gas permeation for H-2 and SF6. Both alumina- and YZ-supported MFI membranes show very good separation properties for hydrogen and n-butane mixtures, although the YZ-supported M FI films appear to have larger non-zeolitic pores than the alumina-supporte d membranes. (C) 2000 Elsevier Science B.V. All rights reserved.