Preparation Of Multistage Pore ZSM-5 Molecular Sieve And Its Catalytic Methanol Aromatization Reaction

The development of environmental friendly catalysts is one of the key points of green chemistry research.In order to solve the problem of environmental pollution caused by NOx gas and other catalysts in the synthesis of ZSM-5 zeolite and the limitation of catalyst life in the existing methanol aromatics(MTA)catalyst system,this paper intends to carry out the study on the design,preparation and catalytic performance of the hierarchical ZSM-5 zeolite.A series of ZSM-5 nanozeolites with hierarchical porosity were prepared by a seeding method and the addition of phenylaminopropyltrimethoxysilane(PATMOS)into the medial synthesis system.The influence of the amounts of PATMOS on the pore characteristics,crystallinity,morphology and acidity of the ZSM-5 zeolites were studied by XRD,N2-adsorption,27Al MAS NMR,SEM,TEM and NH3-TPD.The crystal size of the ZSM-5 zeolites could be adjusted in a certain range,and the mesopores were generated by introducing PATMOS.In addition,the catalytic performances of the hierarchical nanocrystalline ZSM-5 were investigated for methanol aromatisation The hierarchical nanocrystalline ZSM-5 catalysts with a small crystal size and numerous mesopores exhibited high stability for the MTA reaction due to an enhanced mass transfer property and reduced diffusion limitations.A series of hierarchical ZSM-5 zeolites with different Si/Al ratios were successfully prepared using a seeddeduced method and the addition of organosilicone reagents into the medial synthesis system.The catalytic performance was evaluated in terms of methanol aromatization to investigate the effects of the Si/Al ratio on the catalyst stability and coke formation.The results indicated that despite their large external surface area and mesopore volume,a decrease in the Si/Al ratio resulted in an increase in the acidity of the zeolites,which significantly increased the coke formation and decreased their catalytic stability.The acid amount exhibits good linear correlation with the coke content,and the generalized and quantitative correlations are reported.The effects of the zeolite structure and acidity on the catalyst life and coke formation in the methanol-toaromatics reaction were investigated using several MFI zeolites with different degrees of mesoporosity and acidity.These zeolites were prepared by post-synthesis desilication,dealumination,and combined desilication and dealumination processes.Dealumination and/or desilication leads to an increase in the mesoporosity by widening the micropores and a decrease in the acid density.The MTA catalyst lifetime is increased by several times due to the enhanced mesoporosity and decreased acid density.The deactivated catalysts were characterized by thermogravimetry and N2 adsorption/desorption experiments.More coke forms inside the micropores than on the external surfaces of the catalysts.The effects of zeolite structure and acidity on catalyst life of methanol to aromatic(MTA)reactions have been investigated using a number of MFI zeolites with different degrees of mesoporosity and acidity,which were obtained via the post-synthetic desilication with TPAOH.The changes of pores by post-treatment were characterised by combined N2 adsorption/desorption technique and the acidity were quantitatively analysed by NH3-temperature programmed desorption technique.Temperature programmed desorptions of 11-octane,n-hexane and acetonitrile were also carried out in order to study the strength and nature of the different acid sites of the zeolites.The results indicated that the desilication increased the mesoporosity via extending of micropores and decreased the strong acid density,which significantly increased coke formation and then decreased their catalytic stability.The MTA catalytic lifetime was extended due to the enhanced mesoporosity and decreased the acid density.