Synthesis And Characterisation Of Hierarchical SAPO-34

Ethylene is an important basic raw material in modern chemical industry,and the demand for it is increasing day by day.In the industry,it is mainly derived from the pyrolysis of oil vapor and catalytic conversion of methanol to olefins?MTO?.The wide sources of methanol as well as non-renewability and instability of the oil make the process of MTO particularly important.SAPO-34 is the most commonly used catalyst for MTO due to its and mild acidity and unique framework topology.However,the small pores of SAPO-34 result in the low accessibility to the active sites and mass transfer limitations in the MTO reaction,which is associated with the coke deposition and fast deactivation of SAPO-34 in the process of MTO.Because of the above problems,much attentions have been focused on how to modulate the physical and chemical properties of microporous SAPO-34 and improve its performance in MTO.Synthesis of hierarchical SAPO-34 and SAPO-34 with small crystallite size are two important methods for solving the diffusion limitation of SAPO-34 in MTO and the associated carbon deposition and deactivation.In this paper,the main works were focused on the synthesis of hierarchical SAPO-34 mateials,and improve lifetime of SAPO-34 in MTO by modifying hierarchical porous structure and acid distribution of SAPO-34 materials.The following is main work of this paper:Firstly,hierarchical SAPO-34 is synthesized by using the soft template method with the cheap KH550 as the mesopore directing agent.The effects of the amount of KH550 and the crystallization time in the synthesis system on the synthesis of SAPO-34 and the pore distribution are investigated.The results of XRD,N2-adsorption and SEM show that the morphology of SAPO-34 is spherically aggregates after KH550 is added to the system of synthesis.The maximum size of SAPO-34 is 20 ?m,the total surface area is 761 m2·g-1,and the maximum external surface area is 75 m2·g-1.It also shows if too much KH550?KH550 and the ratio of silica sol molar ratio is greater than 3: 2?is added to the synthesis system,the pure phase SAPO-34 can not be synthesized.Secondly,the hierarchical SAPO-34 is synthesized by using one step gel method with TPOAC and Y5669 as the mesopore directing agent and part of the silicon source.The influences of the TPOAC and Y5669 amount on the synthesis,pore structure as well as acidity of the hierarchical SAPO-34 materials are also studied.The results of XRD,N2-absorption,SEM and NH3-TPD show that the ratio of TPOAC to Silica sol can modify the external surface area of hierarchical SAPO-34.With the increase of TPOAC amount,the total acid amount keeps on decreasing and acid strength weakens.For the hierarchical SAPO-34 synthesized in the TPOAC-containing system,the average pore diameter is 4.9 nm,the maximum external surface area is 100 m2·g-1,and the total surface area is 649 m2·g-1.When the ratio of TPOAC to Silica sol is over 3: 2,the agglomeration of TPOAC occurs.In the Y5669-containing system,the total surface area of the obtained SAPO-34 can reache 761 m2·g-1.When the ratio of Y5669 to Silica sol is 4:1,the morphology of hierarchical SAPO-34 with pore takes on a kind of piece structure.Thirdly,hierarchical SAPO-34 with the size of about 200 nm can be synthesized by using bond blocking method with Y5669 functionalized fumed silica?SiO₂-Y5669?as silica source.The influences of Y5669 amount in SiO₂-Y5669 and the crystallization time on the crystallization and textural structure of hierarchical SAPO-34?especially the surface area and external surface area?were investigated.The as-synthesized samples were characterized by XRD,N2-absorption and SEM.The hierarchical SAPO-34 obtained under the optimized synthesis conditions show 800 m2·g-1 total surface area and 128 m2·g-1 external surface area.This synthesis method can effectively avoid agglomeration of silane coupling agent and make the synthesis easier.Fourthly,in order to decrease synthesis temperature in the industrial production,dynamic crystallization is employed in synthesis hierarchical SAPO-34 using bond blocking method with SiO₂-Y5669 as silica source.The hierarchical SAPO-34 materials with the size of about 200 nm can be successfully synthesized at lower crystallization temperatures,namely,160 ? and 150 ?,under the dynamic crystallization.The influences of the Y-5669 amounts in SiO₂-Y5669 and crystallization time on the textural properties of SAPO-34 synthesized at the relatively lower crystallization temperatures are investigated.For the SAPO-34 samples synthesized at 150 ? or 160 ? for 2 days and 10 days dynamic crystallization,their largest external surface areas of can reach 140 m2·g-1 and 163 m2·g-1,respectively.Compared with the conventional static hydrothermal crystallization method,the dynamic crystallization combined with the adjustion of crystallization temperature and time can modify the pore structure of hierarchical porous SAPO-34 to a larger extent.Lastly,the structure,composition,coordination state of silicon and the acidity of the hierarchical SAPO-34 synthesized under different conditions are compared and analyzed,and their catalytic performance in MTO is also evaluated.For the optimum catalyst,the ethylene-propylene selectivity can be up to 84% and the longest active life of the catalyst can reach 242 min.