Effects of seed crystal type on the growth and microstructures of silicalite-1 membranes on tubular silica supports via gel-free steam-assisted conversion

In this study, silicalite-1 membranes were prepared on seeded tubular silica supports using a gel-free steam-assisted conversion (SAC) method. The effects of seed crystal type on the growth and microstructure of the membranes were investigated. The morphologies of the membranes obtained by this preparation method reflected the orientation of the seed crystal layer covering the tubular support; when spherical seed crystals were used, the membrane surface was composed of randomly oriented crystals; whereas when coffin-type seed crystals were used, the membrane surface was composed of crystals arranged in a relatively b-axis orientation. This suggests that membrane layers can be grown with the same orientation as the seed crystals on the tubular support. That is, the gel-free SAC method is able to suppress changes in the orientation of zeolite crystals during membrane formation, which is a problem in conventional hydrothermal syntheses. This is a very interesting phenomenon that occurs only under gel-free SAC conditions, and accounts for the successful growth of the b-oriented silicalite-1 membrane layer on the tubular support. Our findings successfully demonstrate approaches for the formation of membrane layers that retain the orientation of zeolite seed crystals on tubular supports, and for the synthesis of oriented zeolite membranes with high permeability.