Alkali-free ZSM-5 membranes: Preparation conditions and separation performance

Alkali-free, H-ZSM-5 membranes were synthesized by in-situ crystallization on porous alpha-alumina, gamma-alumina, and stainless steel tubular supports. Membranes prepared from different Si sources were characterized by X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. Membranes prepared under different conditions were also characterized by single-gas permeances of H-2, N-2, n-C4H10, i-C4H10, and SF6 and by separation selectivities of n-C4H10/i-C4H10 mixtures from 300 to 473 K. The effects of preparation procedure, crystallization time and temperature, number of synthesis layers, gel dilution, Si source, and type of support were studied. The permeation and separation properties of the membranes depend strongly on the preparation procedure. Permeating synthesis solution into the pores of the support before hydrothermal treatment allows zeolites crystal growth within those pores. Increasing the crystallization time and temperature increases the n-C4H10/i-C4H10 separation selectivities, whereas increasing the number of synthesis layers and gel dilution decreases the selectivities. Membranes were prepared with high ideal and separation selectivities for n-C(4)H10/i-C4H10. The highest separation selectivity for n-C4H10/i-C4H10 is 111, which was obtained at 429 K, and the highest separation selectivity at 473 K is 36. Ideal selectivities do not correlate with mixture selectivities for n-butane/i-butanes at low temperature, but they correlate at 473 K.