Modification Of ZSM-5 Zeolite And Its Catalysis In The Rearrangement Of Propylene Oxide To Propionaldehyde

Propylene oxide is an epoxy compound having an unstable three-membered ring which can be rearranged into products such as propionaldehyde,acetone and allyl alcohol in the presence of catalyst.The product distribution of propylene oxide rearrangement is greatly affected by the acidity and alkalinity of the catalyst.In order to obtain different main products,it is of great significance to study the acidity and alkalinity of the catalyst for its specific influence on the product distribution.In this paper,we studied the influence of acidity change of solid acid catalyst ZSM-5 on the reaction of propylene oxide rearrangement.For the synthesis of microporous ZSM-5catalysts with different acidity,we qualitatively and quantitatively analyze the acid amount and acid type by NH₃-TPD and pyridine adsorption infrared and combine the acid characterization results with the catalyst performance.The results show that the main product of ZSM-5 in the catalyzed propylene oxide rearrangement is propionaldehyde and the by-products are mainly acetone and allyl alcohol.The amount of strong Lewis acid and strong Br?nsted acid in the catalyst has a great influence on the reaction conversion and the selectivity of the product propionaldehyde.The greater the ratio of strong Lewis acid amount to strong Br?nsted acid amount,the higher the reaction conversion,the better the selectivity to propionaldehyde and the stability of the reaction.In the modification study of hierarchical ZSM-5 catalyst,it was found that the phosphorus loading significantly reduced the specific surface area and pore volume of the parent ZSM-5 catalyst and also reduced the strong Lewis acid sites in the catalyst.This results in a decrease in the activity of the phosphorus-modified ZSM-5 catalyst and a decrease in the selectivity to propionaldehyde.It was also found that high temperature nitridation modification can introduce new basic sites in ZSM-5 catalyst.The introduction of the basic sites are related to the substitution of nitrogen atom for the oxygen atom in the zeolite backbone.The introduction of the new basic sites increase the selectivity to the product of allyl alcohol but the main product of the reaction remains propionaldehyde.We also studied the mechanism of ZSM-5 catalyzed rearrangement of propylene oxide to propionaldehyde by in-situ infrared technique.An intermediate structure at 1685cm^-1 in the infrared spectrum was determined,and the mechanism of Lewis acid site catalyzing the rearrangement of propylene oxide to propionaldehyde was proposed.At the same time,the precursor of the catalyst carbon deposition during the propylene oxide rearrangement process was determined.