Study On ZSM-5 Zeolite-Based Catalysts For Highly Selective Conversion Of Methanol To P-Xylene

As the fundamental raw material,p-xylene has been extensively used in the chemical industry.Currently,the demand still outstrips supplies for the p-xylene market,and alternative p-xylene production ways are urgently needed.With the consumption and depletion of petroleum resources,such an alternative route to p-xylene based on methanol has attracted great attention.In general,zeolites used have proved to be the most promosing catalysts,owing to the high activity.While some side reactions coinstantaneously take place during the reaction process,obtaining the low level of para-selectivity.Therefore,the development of the highly efficient catalyst with adjustable acidity and structural property is the key to improve the shape selectivity and increase the production of p-xylene.In this thesis,systematic research works have been carried out on ZSM-5-based zeolites for the methanol conversion,including high-silica zeolite for toluene alkylation with methanol,mesostructured Zn-Mg-P/AT-ZSM-5,surface modified[Zn,Al]ZSM-5 and nanosized ZSM-5 zeolite catalysts for direct conversion of methanol to p-xylene.The correction between the reaction performance and the physico-chemical property of catalysts has been estabilished through micro-activity evaluation and detailed characterization methods.Meanwhile,the influence factors for the shape-selectivity have been further explored.Reaction performance of toluene alkylation with methanol over ZSM-5 zeolites with different Si/Al2 ratio and different crystal sizes were investigated.It is indicated ZSM-5(HSi)-LC zeolite with few external acid sites exhibited an improved para-selectivity(75.97%),which demonstrating that external acidity plays a dominant role on the catalytic shape-selectivity.Direct conversion of methanol to p-xylene could break the limit of reactant activity to p-xylene selectivity,owing to the easy conversion of methanol.Improving the shape-selectivity during the methanol to aromatics has been a promising method for selective production of p-xylene.Comparing the acidic properties of ZSM-5 and Zn-modified ZSM-5 samples,the generation of new Zn-Lewis acid sites resulted from the interaction between the Zn species and the Br(?)nsted acid sites of ZSM-5 zeolites,which intensifies the dehydrogenation activity.The further modification with Mg and P passivates the acid sites on the pore-opening and external surface,thereby inhibiting the secondary isomerization of p-xylene formed in the micropores and decreasing the formation of o-and m-xylene,and consequently improves the shape-selectivity.The modification of multiple-oxides(Zn,Mg and P)significantly improves the performance of methanol to p-xylene,and Zn-Mg-P/ZSM-5 zeolite sample obtains a high reaction para-selectivity of 88.45%.Althougth Zn-Mg-P/ZSM-5 zeolite samples achieve a high shape-selectivity towards p-xylene.Inevitably,the catalyst activity is diminished during the modification process.To enhance the overall reactivity,additional mesoporosity has been introduced into the catalyst by desilication and the selectivity to total aromatics has been indeed increased.Compared with the contrast samples without desilication,the zeolite samples with additional mesoporosity obtain dramatically enhanced aromatics selectivity,in spite of a little shape-selectivity loss.Meanwhile,the catalyst stability is distinctly improved due to the increased mesoporisity.As a result,continuous fixed bed reaction test indicated that the p-xylene selectivity was maintained around a high level within 50 h over the mesostructured Zn-Mg-P/AT-ZSM-5 sample.In order to intensify the dispersion of introduced Zn species and simplify the preparation step,a series of[Zn,Al]ZSM-5 zeolites have been hydrothermally synthesized by the isomorphous substitution method.All the zeolite samples have been systematically characterized and evaluated for methanol conversion process.The experiment results indicated that the tense interaction between the Zn species and[Zn,Al]ZSM-5 zeolites,producing considerable Zn-Lewis acid sites for improved dehydrogenation activity and contributing to production of aromatics.It is worth nothing that the optimum aromatization performance can be obtained over[Zn-S]ZSM-5 and this catalyst simultaneously exhibits a more effective shape-selectivity than other Zn-substituted zeolites with different Zn precursors.The further doping of Mg into[Zn-S]ZSM-5 greatly improves the shape-selectivity and the outstanding reaction performance of methanol to p-xylene.In general,the p-xylene selectivity was up to 24%for Mg/[Zn-S]ZSM-5,which was higher than that of Zn-Mg/ZSM-5 zeolites modified by Zn and Mg.Owing to the random mesoporous of the desilicaiton process,NZSM-5 with shorter channels and abundant inter-mesopores could improve the diffusion performance and aromatic selectivity in the methanol conversion.And then the fixed-bed catalyst was prepared with the addition of carrier and binder into the Zn-Mg-P/NZSM-5 zeolite,increasing the whole reaction activity and achieving an extremely high p-xylene selectivity of 29.97%.