Preparation And Characterization Of MFI-typed Monolithic Catalyst

As one kind of crystalline silicon aluminate crystal,MFI Zeolite has unique pore structure and excellent characteristics of high acid center,large specific surface area,strong ion exchange capacity,high thermal stability and high thermal stability.It has well application prospects in the fields of ion exchange,adsorption separation and industrial catalysis.The monolithic MFI catalyst can not only solve the problems of increasing the pressure drop of fixed bed reactor,the high temperature gradient of fixed bed reactor and the poor mass transfer effect in the current industry,but also realized high conversion and high selectivity of catalytic process to expand the new field for the application of catalyst industrialization.However,there are many problems in the preparation of the monolithic catalyst at present:the crystallization and molding of zeolite are time-consuming and tedious,and the addition of the binder will result in the obstruction of the channel and the mass transfer efficiency.Although there have been a lot of reports on the formation of non-binder,it is rare to achieve crystallization and molding in one step.Therefore,our team uses microcapsule technology to prepare monolithic spherical MFI molecular sieve in one step,effectively combining the physical and chemical properties of organic and inorganic materials,the high filling efficiency of the spherical structure and the uniform resistance of the fluid make it has more excellent catalytic effect.In the first part of this paper,in-situ hydrothermal method is used to prepare different morphologies of Silicalite-1 by using fumed silica(SiO2)as silicon source and Tetrapropylammonium hydroxide(TPAOH)as template.In the second part,spherical self-supported hierarchical ZSM-5 zeolites were prepared by using natural polymer chitosan as pore making agent,silica as silicon source and aluminum sulfate as aluminum source through crystal seed oriented dry-gel method.X-ray diffraction(XRD),scanning electron microscopy(SEM)and N2 adsorption desorption were used to characterize the relative crystallinity,grain size,specific surface area and pore size distribution of molecular sieves.The results are as follows:The first part,lengthening the aging time is beneficial to the growth of crystal,but too long aging time will lead to the two crystallization and obtain the large Silicalite-1 zeolite.With the increase of the fumed SiO2 content,the crystal growth is dominant,the crystal is gradually dispersed from the original spherical aggregation state,the grain size gradually becomes smaller and tends to be uniform.In the process of zeolite synthesis,chitosan can play a structural orientation and change the content of chitosan will influenced the morphology of zeolite With the increase of chitosan content,the growth of zeolite grain size is reduced significantly due to the increase of the content of free hydroxyl groups in the system,and the crystallization temperature can effect on the morphology of zeolite.The temperature is advantageous to the dissolution and polymerization of silicate and the easy to obtain the homogeneous morphology and size of zeolite.The high temperature will produce the transformation of crystal.The addition of ethanol can promote the regulation of nucleation.With the increase of ethanol content,the grain size decreases and the crystal relative crystallinity increases.The Silicalite-1 samples with different morphologies were subjected to fixed-bed reaction.The hexagonal plate-like Silicalite-1 had a regular morphology and the highest crystallinity.It exhibited the best catalytic performance and selectivity in the Beckmann rearrangement of cyclohexanone oxime(50.31%),the conversion rate(84.19%).Therefore,the aging time is 6 h,the content of SiO2 is 0.5 g,the content of chitosan is 0.3g,the precursor composition is TEOS:TPAOH:H2O= 1:0.2:90,170 ? hydrothermal crystallization 24 h as the best synthesis condition.In the second part,the crystal species were added to provide the core of the crystallization growth,which plays the role of induction and acceleration.With the increase of the crystal size,the relative crystallinity of the crystal increases,the specific surface area decreases,the mesoporous volume increases,the pore size distribution is reduced,the pore size is mainly concentrated in the range of 3-5 nm,and in the synthesis of zeolite,Chitosan can play an important role in structure orientation.The mesoporous volume of chitosan content in the system increases slightly and the aperture range is narrowed,mainly in the range of 10-60 nm.With the prolongation of the crystallization time,the specific surface area decreases,the mesoporous volume is basically unchanged and the pore size distribution broadens gradually,with a concentration of 20-50 nm.With the increase of crystallization temperature,the relative crystallinity increases and tends to be stable,the pore size decreases,mesopore increases,and the pore size range widens.