Core-shell zeolite microcomposites

Core-shell zeolite composites possessing a core and a shell of different zeolite structure types have been synthesized. A characteristic feature of the obtained composites is the relatively large single-crystal core and the very thin polycrystalline shell. The incompatibility between the core crystals and the zeolite precursor mixture yielding the shell layer has been circumvented by the adsorption of nanoseeds on the core surface, which induced the crystallization of the shell. The pretreated core crystals are subsequently subjected to a continuous growth in a zeolite precursor mixture. The feasibility of this synthetic approach has been exemplified by the preparation of core-shell beta-zeolite-silicalite-1 composites. The synthesized composites have been characterized using X-ray diffraction, high-resolution transmission electron microscopy, and scanning electron microscopy. The integrity of the shell layer has been tested via N-2-adsorption measurements on materials comprising a calcined core (beta-zeolite) and a non-calcined tetrapropylammonium (TPA)-containing shell, the latter being non-permeable for the N-2 molecules. These measurements have shown that 86% of the beta-zeolite crystals are covered with a defect-free TPA-silicalite-1 shell after a single hydrothermal treatment, while after three consecutive crystallization steps this value reaches 99%. The shell integrity of the calcined composite has been studied by the adsorption of butane, toluene, and 1,3,5-trimethylbenzene, which confirmed the superior performance of the,,triple-shell composites.