Synthesis of Oriented Zeolite Molecular Sieve Films with Controlled Morphologies

Abstract

Zeolite molecular sieve films (MFI-topology) consisting of one or more layers of pure-silica ZSM-5 crystals have been hydrothermally synthesized on the surfaces of fused silica glass. The resulting films have been characterized by field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray analysis, and Fourier transform infrared spectrometry. At the early stage of reaction a porous gel layer spontaneously condenses on the substrate surface, within which the zeolite crystals are nucleated and grown as mediated by the structure-directing agent. Notable shifts of the C−H stretching vibrations are observed on the tetrapropylammonium cations attached to the gel layer compared to those enclathrated in the crystal frameworks. The zeolite crystals are preferentially oriented with their b axis perpendicular to the substrate surface. The crystal orientation is also strongly dependent on the roughness of the underlayer surface and consequently the thickness of the zeolite film. By control of the synthesis conditions, the individual crystals constituting the zeolite films can be prepared either smaller than 500 nm or as large as 30 μm. Those zeolite films grown in situ on the silica glass also show some unique properties and new potential applications.