The effect of zeolite architecture and channel dimensionality on p-xylene conversion and selectivity to 1-isopropyl-2,5-dimethyl-benzene was investigated in p-xylene alkylation with isopropyl alcohol over novel zeolites SSZ-33 and SSZ-35. Catalytic behavior of these zeolites was compared with those of zeolites Beta, mordenite, ZSM-11 and ZSM-5. It was found that p-xylene conversion increases with increasing pore size and connectivity of the channel system of individual zeolites with the exception of SSZ-35, which possesses a system of onedimensional 10-ring channels that periodically open into wide, shallow cavities circumscribed by 18-rings. SSZ-35 exhibited the highest conversion among all zeolites at the reaction temperature of 150°C and also the highest selectivity to 1-isopropyl-2,5-dimethyl-benzene. Molecular modeling confirmed the dimensions of the 18-ring cages are optimal for the formation of this alkylation product.
SSZ-33 based-catalyst, after modification with Mo and alumina binder, was evaluated in long-run tests in the toluene disproportionation in the presence of hydrogen as a carrier gas. The activity and selectivity of this catalyst were compared with those of ZSM-5 and mesoporous ZSM-5 prepared with the same concentration of Mo and alumina. Formation of the final catalysts decreased the void volume of micropores in order SSZ-33 [ ZSM-5 [ ZSM-5/Meso. Simultaneously, the concentration of Lewis acid sites increased due to the addition of alumina to the catalyst. The highest toluene conversion was achieved with SSZ-33 catalyst comprising 12-12-10-ring channels, which is the result of high acidity of this zeolite together with increased mass transport through large pores. ZSM-5 zeolite exhibited lower toluene conversion while only a low activity was found for mesoporous ZSM-5 probably due to the limitations of the access to the zeolite channels after modification with alumina.
Modification of textural and chemical properties of SSZ-35 was investigated in respect to the catalytic behavior of this zeolite in p-xylene alkylation with isopropyl alcohol. Acidic treatment of the external surface of SSZ-35 crystals resulted in a decrease in the void volume of the zeolite and in the concentration of acid sites mainly located on the external surface. This was evidenced from the FTIR spectra of adsorbed pyridine and 2,6-di-tertbutylpyridine. The modification caused a significant increase in the selectivity to 2,5-dimethylcumene while the p-xylene conversion decreased only very slightly.
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