Study On Modified ZSM-5 Zeolite For Rearrangement Of Propylene Oxide

Propylene oxide is an important raw material for organic synthesis.Due to the thermodynamic instability of its three-membered ring,which is easy to open the ring to produce propionaldehyde,acetone,allyl alcoholl,propanol and other products at high temperature or in the presence of catalyst.Among them,propionaldehyde has a wide range of applications in medicine,textiles,coatings,rubber additives and food products.Compared with other methods of propionaldehyde synthesis,the process of propylene oxide rearrangement to propionaldehyde is a one-step process with low cost and abundance of raw materials.furthermore,this reaction shows high effeniency of atomic utilization ratio and is confrimed to be envrionmental friendly.Research advances have demonstrated that the Lewis acid active site on ZSM-5 has an improved catalytic activity on the rearrangement of propylene oxide to propionaldehyde.However,during the process of catalysis of microporous ZSM-5,the catalyst is inevitably deactivated due to carbon deposition.Therefore,we synthesized a series of modified ZSM-5 catalysts.By controlling the amount and pore size of Lewis acid,we screened out the catalysts with excellent catalytic performance.Combined with the characterization of XRD,FTIR,BET,SEM,TG,NH3-TPD and pyridine infrared,we explored the internal reasons for the improvement of catalyst performance and stability.The details are described as follows:First,in order to adjust the acidity of ZSM-5,a series of modified ZSM-5 catalysts with different Mg contents by equal volume impregnation were prepared.The pyridine adsorption infrared and NH3-TPD are used to characterize and quantify the acid type and acid amount of the sample.Mg/ZSM-5-0.1(denoted as the loading of 0.1%Mg in catalyst)significantly increased the amount of Lewis acid.At the same time,the acid strength is also reduced and the deactivation speed of propylene oxide rearrangement into other by-products is limited,thus increasing the selectivity of propionaldehyde,conversion and stability of the reaction.Then,in order to adjust the channel structure of the ZSM-5 zeolite,two different methods are applied to introduce the mesopores in ZSM-5 zeolite to investigate the effect of pore structure on the catalyst performance.One is to remove the framework Si in the zeolite by using different concentrations of NaOH to control the desiliconization.The surface area and acid amount of the samples were analyzed by BET and pyridine adsorption infrared.The results showed that the mesoporous specific surface area of the modified samples increased from 77.72 m2/g to 155.76 m2/g.The Pore size distribution shows that mesopores(3.7 nm)were successfully introduced into all the samples.This indicates that mesopores were successfully introduced into the ZSM-5 zeolite.The treatment of NaOH also reduces the total acid amount of ZSM-5.The amount of Lewis acid is increased,thereby increasing the conversion of propylene oxide and the selectivity of propionaldehyde.The other method is that we synthesized a series of the hierarchical porous ZSM-5 using surfactant CTAB and different amounts of triblock copolymer P123 as co-templates.With the help of two mesoporous templates interaction,we successfully synthesized a hierarchical porous ZSM-5 zeolite with high crystallinity,two types of mesoporous channels and high mesoporous surface area(511.17 m2/g).The introduction of a large number of mesopores improves the diffusion coefficient of the zeolite,inhibits the formation of heavier carbonaceous deposition,and improves the stability of the propylene oxide rearrangement reaction.