Semicrystalline zeolitic mesoporous UL-zeolites of MFI type: Synthesis, characterization and catalytic properties

Titanosilicate and aluminosilicate zeolitic/mesoporous UL-zeolites of MFI-type structure (UL-TS-1 and UL-ZSM-5) were synthesized by a templated solid-state secondary crystallization of zeolites starting from amorphous mesostructured materials. Reference samples of TS-1 and ZSM-5 zeolites were also prepared. Further, different post-synthesis treatments have been applied for UL-zeolites encompassing the coating with TS-1 nanocrystals and trimethylsilylation with hexamethyldisilazane (for UL-TS-1) and leaching with diluted aqueous solutions of NH4OH (for UL-ZSM-5).; These materials were generally characterized using elemental analysis, N2 physisorption, XRD, FTIR of framework vibrations, 29 Si and 27Al MAS NMR spectroscopy, UV-vis spectroscopy, microcalorimetry of ammonia adsorption, TPD of ammonia and FTIR of adsorbed pyridine and 2,6-ditertbutylpyridine. The results were analyzed by comparison with reference samples of amorphous mesoporous precursors and corresponding crystalline zeolites. The characterization results confirmed that UL-zeolite materials are semicrystalline materials with microporous zeolitic components on their mesopore walls. While the microporosity is associated to the MFI-type zeolitic nanophase whose growth gradually takes place from the amorphous mesopore walls of the precursor during the solid-state crystallization process, the mesoporosity originates from the starting mesoporous precursor. Consequently, the studied UL-zeolites showed tetrahedral titanium and aluminium environments, which are essentially the same as in zeolites TS-1 and ZSM-5 respectively. On the other hand, UL-ZSM-5 materials displayed acidity in terms of density, number and strength of the Bronsted acid sites, which was intermediate between amorphous mesoporous precursors and ZSM-5. Additional improvement in Bronsted acidity was obtained by treating UL-ZSM-5 with aqueous solutions of NH4OH. For the distinct amorphous, semicrystalline and fully crystalline aluminosilicate materials under study, it was also demonstrated that the number of Bronsted acid sites which are accessible to bulky molecules like as 2,6-ditertbutylpyridine decreased with the decrease in external surface area.; The catalytic activity of UL-TS-1 has been studied in hydroxylation of 1-naphthol with hydrogen peroxide. UL-ZSM-5 samples were tested in aldol condensation of acetaldehyde with formaldehyde and transalkylation of 1,3,5-trimethylbenzene with toluene. It was found that in line with the characterization results, UL-zeolites display catalytic activity which is usually intermediate between those of mesoporous materials and zeolites. Relationships were established between the catalytic and physico-chemical properties.