Zeolites overgrowth

ECR-1 is a true boundary phase between two compatible structures - mazzite and mordenite - which normally crystallize with greatly disparate Si/Al ratios (3.4 and >5 respectively). Yet ECR-1 has an Si/Al ratio identical to mazzite, even though half of its component sheets are structurally characteristic of mordenite (in the structure proposal). This compositional incompatibility is further reflected in the almost complete lack of mixtures of mazzite with mordenite in the experiments so far completed. The ongoing search for ECR-1 with mordenite overgrowths is therefore an important objective in the understanding of crystallization mechanisms in this system, with particular interest in the possible compositional variation between all three component zeolites. 

Results from diverse experimental methods were combined to arrive at structure proposals for ECR-1, a typical example of a microcrystalline zeolite. The zeolite features a 12-ring single channel, formed by a regular alternation of connected sheets of mordenite and mazzite. Crucial clues to the structure came from the HREM observation of a mazzite epitaxial overgrowth on an ECR-1 crystal. Electron and x-ray diffraction, infra-red spectroscopy and synthesis phase relationships were essential additional data sources. 

Three well known zeolite structure types - Linde L(18), mazzite(19) and the hypothetical omega-structure(20) - have essentially identical hexagonal lattice constants with =18A and .=7.5A, but only mazzite has a 7.5A x 18A layer that contains an n-glide plane operator the thickness of this layer is -15.5A. If the hexagonal overgrowth is mazzite, the electron micrograph interpretation implies that ECR-1 comprises mazzite sheets interconnected with some other subunit to form the observed 26A repeat distance. The... 

P-06 - Epitaxial overgrowth of MAZ onto EMT type zeolite crystals... 

The possibility of growing epitaxial zeolite films on micrometer-sized, structurally different zeolite support crystals was explored with the aim to develop polyfunctional zeolite materials with spatially separated adsorptive and/or catalytic functions. Support crystals of one zeolite type were added to a hydrogel for crystallization of a second zeolite type. Tuning of the crystallization conditions led to the formation of a MAZ-on-EMT overgrowth material consisting of an oriented, continuous film of MAZ crystallites completely covering the surface of the EMT support crystals. The epitaxial relations between the two phases were established using HRTEM and SAED. 

In addition to the materials listed in Table 16.1, several silica-based microporous materials were also synthesized using the fluoride route CJS-3[64] (certainly a clathrasil), ITQ-10 [81] (beta like) and ITQ-14 [81] (BEA-BEC overgrowth), PREFER (a lamellar precursor of ferrierite) [82], SSZ-61 (Standard Oil Synthetic Zeolite-61) [12, 83], Nu-86 [12], ZSM-48 [68, 86], ITQ-28 [85], and SSZ-70 (aborosilicate) [86]. 

The Mo-containing MFl-type core-shell HZSM-5-Silicalite-l s (HZ5 S1) materials with various core-shell ratios prepared by the epitaxial growth of Silicalite-1 on HZSM-5 demonstrated the high shape selectivity to aromatics and stability in methane dehydroaromatization [66]. The silicalite-1 layer covering the HZSM-5 core can eliminate the external acid sites, and thus prevent the formation of active Mo species associated with Brpnsted acid sites on the external surface of catalysts. However, the overgrowth of the Silicalite-1 shell may lead to a severe inhibition of the Mo species migration into zeolite pores and consequent anchoring on the Brpnsted acid sites of the HZSM-5 core. Therefore, the catalytic performance is dependent on the core-shell ratio. 

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