Study On Modification Of Beta Zeolite And Its Catalysis In The Rearrangement Of Propylene Oxide

Due to the unstable three-membered ring propylene oxide can be rearranged to produce propionaldehyde,acetone and allyl alcohol in the presence of catalyst,which are used in the production of chemicals such as medicine,coatings,and plastics.The rearrangement of propylene oxide is an intramolecular rearrangement reaction with high atom utilization rate.There is no waste during the reaction process,which meets environmental protection requirements.The acid sites of catalysts are closely related to the distribution of products.So it is of great significance to explore the effect of the change in the acid sites of the catalyst on the rearrangement reaction of propylene oxide.In this paper,the influence of acidity change of Beta zeolite on propylene oxide rearrangement was studied,including conversion,selectivity of target product and catalytic stability.For Beta zeolites with different silica to alumina ratios,the acidity change was analyzed by NH₃-TPD characterization techniques.The structure-activity relationship between the acidity and performance of the catalyst was studied.The results showed that the main product of propylene oxide rearrangement catalyzed by Beta zeolite was propionaldehyde.The Lewis acid sites in the catalyst had a positive effect on the reaction activity and can improve the conversion and the selectivity of propionaldehyde.If the proportion of Lewis acid sites in the catalyst was larger,the life of the catalyst was longer.Zn-Beta was prepared by two-step postsynthesis method.The results of NH₃-TPD and Py-IR showed that the acidity of the sample changed greatly,the Br(?)nsted acid sites disappeared,and new Lewis acid sites were created with the introduction of Zn.The ratio of L/B in Zn-Beta was 273.57.The conversion of propylene oxide was 99.1%when Zn-Beta as catalyst,and it could be maintained above 70%at 480 min.However,the conversion of propylene oxide was only 13.3%at 480 min when Beta zeolite as catalyst.The location of the acid sites of Zn-Beta were determined by trimethylpyridine adsorbed IR,the results showed that the new Lewis acid sites were distributed on the inner surface of the pore.The mechanism of propylene oxide rearrangement with Zn-Beta as catalyst was studied by in-situ infrared technique.It was proved that the epoxy bond of propylene oxide interacted with the new Lewis acid sites to form a transition state between the bond of C-O and C=O,and then converts to carbonyl of propionaldehyde.Beta zeolite was combined with ZSM-5 zeolite by recrystallization,and the Beta/ZSM-5zeolite was successfully prepared.The crystal structure and acidity of the samples were changed by adjusting the crystallization time.According to the analysis of the test results,it was found that the crystallization time of the Beta/ZSM-5 was 24 h,the ratio of BEA-type and MFI-type structures in the sample was equivalent.The content of the Lewis acid sites was18.1mmol/g,while that of the Br(?)nsted acid sites were only 6.48 mmol/g,which were more conducive to the reaction.