Bi-phase MOR/MFI-type zeolite core–shell composite

Core–shell zeolite composites possessing a mordenite core and a shell of intergrown silicalite-1 crystals were synthesized. A classical hydrothermal treatment of mordenite crystals with a silicalite-1 precursor mixture did not result in the formation of an extended silicalite-1 layer on the mordenite surface. The incompatibility between the core crystals and the shell precursor was circumvented by adsorption of nanoseeds on the core surface, which upon a subsequent hydrothermal treatment induced the formation of a well intergrown shell. The kinetics of shell growth was studied at 150 and 200 °C and the optimum conditions further used for the preparation of composites subjected to XRD, SEM, TG/DTA, EDS and X-ray fluorescence analyses. The integrity of the shell layer was tested by N2 adsorption measurements on materials comprising Na- and Na,H-mordenite and a non-calcined tetrapropylammonium (TPA)-containing shell, the latter being non-permeable for the N2 molecules. These measurements showed that 96–98% of the mordenite crystals were covered with a defect-free TPA-silicalite-1 shell after a single hydrothermal treatment. A characteristic feature of the obtained composites is the relatively large single crystal core and the very thin polycrystalline shell. Thus, the relatively large active part of the composite is covered by a tiny shell that ensures fast kinetics.