Factors controlling the formation of core-shell zeolite-zeolite composites

Core-shell zeolite-zeolite composites possessing single-crystal cores and polycrystalline shells of different zeolite structure types were synthesized. The formation of an intergrown shell structure was induced by preliminary seeding of the core crystals and subsequent secondary growth under hydrothermal conditions. The utilization of seeds suppressed the influence of framework particularities of the core crystals on the formation of the shell layer. Hence, the couples of zeolitic materials were chosen in a way to reveal the effect of differences between the chemical compositions and crystallization conditions of core and shell zeolites on the formation of the desired composite. The following couples of microporous materials were involved in the investigation: SOD-LTA, BEA-LTA, FAU-MFI, MFI-BEA, and MFI-MFI (ZSM-5-silicalite-1), the first structure type representing the core and the second the shell material. Complementary techniques, such as XRD, TG/DTA, SEM/TEM, EDS, and X-ray fluorescence analyses, were employed to fully characterize the composites and their intermediates. The integrity of the shell layer was tested by N-2 adsorption measurements on materials comprising calcined core crystals and noncalcined organic template-containing shells thus providing information about the percentage of composites possessing defect-free shells. The results showed the paramount importance of chemical compatibility between shell and core crystals for the successful formation of a core-shell structure. The shell growth was also determined by the match of the crystallization fields of the core and shell materials, that is, at least partial overlapping was required to form a core-shell composite.