Synthesis And Catalytic Properties Of Core/Shell-Structured Or Mesoporous Zeolites

The dissertation has been carried out to develop novel solid acid catalysts based on the structural modification and adjust the active species of MWW and MFI-type zeolites, and to investigate their catalytic properties as solid acid catalysts and their potential applications in the clean processes for producing fine chemicals.In the first part, a shape-selective core/shell-structured Al-MWW@B-MWW composite catalyst has been hydrothermally synthesized through isomorphically overgrowing borosilicate on premade MCM-22aluminosilicate. The secondary growth of borosilicate enlarged obviously the thickness of the platelet crystallites of MCM-22and increased the surface Si/Al ratio from16to222. The Fourier transform infrared (FTIR) spectra of adsorbed2,6-di-tert-butylpyridine indicated that the Bronsted acid sites located on the external surface were virtually covered completely by the generated B-MWW layer, whereas those acid sites within channels were still accessible and detectable by the probing molecules of pyridine or ammonia. When applied to the disproportionation of toluene on a fixed-bed reactor, the Al-MWW@B-MWW composite catalysts exhibited significantly enhanced para-xylene selectivity in comparison with normal MCM-22and its physical mixture with B-MWW. Al-MWW@B-MWW's unique catalytic behaviors were ascribed to an effective suppression of para-xylene isomerization as a result of removal of non-shape-selective acid sites on the external surface.In the second part, based on the shape-selectivity of10-ring channels and acidity of ZSM-5zeolite, we have synthesized a shape-selective core/shell-structured ZSM-5@mesoporous silica composite catalyst through a layer-by-layer approach. The TEM images indicated that the mesopores in the shell arranged disorderly, but were perpendicular to the zeolite core. The acid sites within channels were still accessible and detectable by using pyridine or ammonia as probing molecules. When applied to the alkylation of toluene and methanol on a fixed-bed reactor, the ZSM-5@mesosilica composite catalysts exhibited a higher para-xylene selectivity in comparison to pristine ZSM-5. ZSM-5@mesosilica's unique catalytic behaviors were ascribed to an effective suppression of para-xylene isomerization as a result of partial removal of non-shape-selective acid sites on the external surface by the mesoporous silica shell.In the third part, we have successfully prepared Meso-MCM-22zeolite by treating MCM-22with NaOH in the presence of organic amine. The different post-treated conditions were examined in detail, such as temperature, time, the amount of piperidine and the type of organic amine. The results showed that, compared with it causing the structure collapse by desilication with NaOH only, the method introduced mesopores with size of approximinate20nm. Meanwhile, the3D structure was converted to two-dimensional layered precursor, and the crystallinity of zeolite kept intact. Meanwhile, the acid sites were almost not affected by pyridine or ammonia adsorption-desorption and27Al NMR experiments. Compared with MCM-22, Meso-MCM-22had a larger external surface, which mitigated effectively the steric restrictions to bulky molecules imposed by the intracrystal micropores. The Meso-MCM-22was then superior to MCM-22in the cracking of1,3,5-triisopropylbenzene as well as the alkylation of benzene with isopropanol.