Synthesis Of Porous SAPO-11 Zeolite Molecular Sieve And Its Application In Isomerization Of Long-chain Alkanes

Hydroisomerization of alkanes has been considered as one of the main technologies for producing high quality gasoline in petroleum refining industry.One of the keys of this technology is to develop highly efficient catalysts with high alkane conversion and multi-chain isomer selectivity.The microporous SAPO-11 framework has mild acidity and moderate pores,and is widely used in the fields of adsorption,catalytic cracking and alkane conversion,and its supported metal catalyst shows good catalytic performance in alkane isomerization.However,the small pore size of microporous SAPO-11 zeolite limits the diffusion of reactant molecules,and the weak acidity of the SAPO-11 leads to the reaction performed at high temperature,which leads to aggravate cracking reaction,and the deactivation of catalyst.Therefore,in this project,porous SAPO-11 zeolite with large acid content and mesoporous structure was successfully prepared by adding mesoporous template agent DM or transition metal into the zeolite synthesis gel,and the catalytic performance of porous SAPO-11 zeolite supported Pt catalyst in the hydroisomerization reaction of n-dodecane was investigated.The main research contents are as follows:（1）Synthesis and characterization of microporous SAPO-11 zeolite:SAPO-11 zeolite with different morphologieswas successfully synthesized under hydrothermal conditions using small molecule din-propylamine（DPA）as structural template agent and silica sol,fumed silica and silica gel as silicon source,respectively.On this basis,the effects of Si/Al ratio,crystallization temperature and crystallization time on the synthesis of SAPO-11 molecular sieve were investigated under the condition of the silica sol as silicon source and pseudo-boehmite as aluminum source.The research shows that the microporous SAPO-11 zeolite with better crystallinity and higher BET specific surface area can be obtained under the conditions of silicon-alumina ratio of2.2 in gel,crystallization time of 48 h and crystallization temperature of 200°C.（2）Synthesis and characterization of mesoporous SAPO-11 zeolite:The mesoporous SAPO-11 zeolite with high BET surface area and mesopore volume was successfully synthesized by adding organosilicon quaternary ammonium salt DM as a mesoporous template in the phosphor-aluminum gel.On this basis,the influences of the type of mesoporous template agent（organosilicon quaternary ammonium salt,high polymer containing quaternary ammonium group）,the amount of mesoporous template agent and Si/Al ratio in the gel system on the phase structure,morphology,acidity and texture properties of the synthesized porous SAPO-11 zeolite were investigated.The results show that the pure mesoporous SAPO-11 zeolite can be synthesized if the gel composition is appropriate and the Si/Al ratio is 2.2～3.2.In addition,the mesoporous SAPO-11 zeolite synthesized with DM（SAPO-11-DM）as mesoporous template has the best texture property parameters and crystallinity,and the mesoporous template of DM can better promote the formation of the mesoporous structure of SAPO-11,as compared to that synthesized from high polymer containing quaternary ammonium group.（3）Synthesis and demonstration of metal-doped porous SAPO-11 zeolite:In the phospho-aluminum gel system containing DM mesoporous templates,the transition metal Co and Mo salt were directly added to the gel system,respectively,and metal-doped porous SAPO-11 zeolite was successfully synthesized.The experimental results show that the addition of metal does not change the skeleton structure of zeolite,but changes the morphology of zeolite,and increases the acidity of zeolite.In addition,Mo-doped Mo SAPO-11-M zeolite has higher crystallinity and external specific surface area,indicating that Mo metal is easier to disperse in the skeleton structure or pore of zeolite.（4）Hydroisomerization performance of mesoporous SAPO-11 zeolite supported Pt catalysts:The hydroisomerization performance of SAPO-11,SAPO-11-DM and Mo SAPO-11-M supported Pt catalysts in the hydroisomerization of dodecane was investigated.Combined with the characterization results of XRD,N₂-physical adsorption,H₂-TPR and TEM of the catalyst and NH₃-TPD of the support,it is found that Mo SAPO-11-M zeolite has high BET and external surface area,which is not only conducive to the diffusion of reactant molecules,but also beneficial to the dispersion of active metal Pt;the presence of Mo in Mo SAPO-11-M zeolite not only facilitates the reduction of Pt,but also promotes the dispersion of Pt with small particle size on the surface of zeolite and in mesoporous channels;Mo SAPO-11-M has more weak acid centers and medium strong acid centers.Therefore,the Pt/Mo SAPO-11-M catalyst has higher n-dodecane conversion and double branching at low temperature（340°C）compared to the SAPO-11-S（92.8%）,SAPO-11-DM（38.7%）zeolite-supported Pt catalysts.