A Study On Catalytic Conversion Of Methanol To Olefins Over SAPO-34/ZSM-5 Composite Molecular Sieves

The methanol-to-olefins?MTO?reaction produces high value-added light olefins.Because of the ever-increasing demand on propylene,many efforts have been made to obtain high-efficiency methanol-to-propylene?MTP?catalysts.Composite molecular sieves,as new catalytic materials,which can combine with pore structures and acidic properties of different molecular sieves,had attracted significant attention for their excellent catalytic performance owing to the synergistic effects of their property in recent years.In this paper,firstly,SAPO-34/HZSM-5 composite molecular sieves were obtained by the mechanical mixing method and hydrothermal method.Secondly,HZSM-5 was modified with phosphorus by equal volume impregnation,SAPO-34/P-HZSM-5 composite molecular sieves were prepared by mechanical method and hydrothermal method.The physicochemical properties and acidity of the composite molecular sieves were measured by XRD,SEM,FT-IR,BET,and NH₃-TPD.Finally,the catalytic performance of the composite molecular sieves in the MTO reaction process was investigated.The research contents and experimental conclusions of this paper are as follows:?1?Firstly,SAPO-34 was synthesized by hydrothermal method,the optimum molar ratio was 1Al₂O₃:1P₂O₅:0.4Si O₂:3TEA:50H₂O:1DEA.It was found that adding an appropriate amount of diethylamine can not only adjust the acidic properties and crystal size of the zeolite catalyst but also make the zeolite generate the mesoporous structure.The catalytic performance showed high light olefins selectivity of 88.68%and a lifetime of 6.5 h.?2?The composite molecular sieves of SAPO-34 and HZSM-5?provided by Nankai university?were prepared by physically mixing method with different ratios.The results showed that SAPO-34/HZSM-5 composite molecular sieves can improve the selectivity of propylene.When the mass fraction of SAPO-34 is 90%,i.e.MSZ-90%,the catalyst exhibited excellent catalytic performance and reaction lifetime,with light olefins selectivity of 77.30%,and propylene selectivity of 48.90%for 20 h.Then under the same mass ratio conditions,SAPO-34/HZSM-5 composite molecular sieve was prepared by mechanically and hydrothermally using the self-synthesized SAPO-34 and HZSM-5.It is observed that SAPO-34 crystals with relatively large size are partially covered by many ZSM-5 crystals with smaller size on the composite molecular sieves.The mechanically mixed SAPO-34/HZSM-5exhibited better performance than the synthesized ones in MTO reaction.The mechanical composite molecular sieves with suitable weak acid sites and a higher mesoporous specific surface area can effectively improve the selectivity for light olefins and propylene as well as extend catalytic lifetime.The mechanical composite molecular sieves S/Z-M-9 presented catalytic lifetime over 32 h,light olefins selectivity of 75.10%,and propylene selectivity of 49.96%.?3?HZSM-5 was modified with phosphorus by the method of equal volume impregnation,and then SAPO-34/P-HZSM-5 composite molecular sieves were prepared by mechanical method from P-modified HZSM-5 and self-synthesized SAPO-34.The phosphorus modified HZSM-5 resulted in an extended lifetime of catalyst,as well as higher selectivity towards light olefins to HZSM-5.The experimental results showed that the mechanical composite molecular sieves SAPO-34/P-HZSM-5 exhibited superior catalytic performances with light olefins selectivity of 74.85%and highest propylene selectivity of 51.34%.