Template-free secondary growth synthesis of MFI type zeolite membranes

Good quality MFI type zeolite membranes were prepared on porous α-alumina supports by the secondary growth method using pure silica sol without organic template. X-ray photoelectron spectroscopy analysis of various MFI layers prepared by dip-coating and secondary growth indicates transfer of aluminum from the alumina support to the framework of the zeolite layer via two routes. Aluminum diffuses into the zeolite layer during calcination step at high temperature. It also dissolves in the synthesis solution and is incorporated into the zeolite framework during growth of the zeolite layer. Separation properties of an eight component hydrogen-hydrocarbon (C₁-C₄) mixture were measured for the secondary-grown MFI zeolite membranes synthesized without template, and are compared with those of similar in situ synthesized MFI zeolite membranes prepared with template. The secondary-grown zeolite membrane is impermeable to propane, propylene, n-butane and isobutane, and its separation factor decreases with increasing carbon number of the permeating gases. The in situ crystallized zeolite membrane is impermeable only to isobutane and its separation factor increases with increasing carbon number of the permeating gases. These results are discussed in terms of template-removal associated intercrystalline pores in the zeolite layer and the corresponding permeation mechanism of the zeolite membranes prepared with and without template.