Synthesis Of SSZ-13 Molecular Sieve And Its MTO Reaction Performance

With the development of the national economy,China's demand for basic organic chemical raw materials such as ethylene and propylene has increased year by year.The traditional process for preparing low-carbon olefins is naphtha steam cracking on the petroleum route,which does not meet Chinese coal-rich,oil-deficient,and low-gas structural characteristics of energy.As a key link of coal-to-olefins,methanol-to-olefins(MTO)technology has opened up a new way to produce low-carbon olefins from non-petroleum routes,which is helpful to alleviate the current shortage of energy supply in China.SSZ-13 molecular sieve has good hydrothermal stability,and shows good low temperature activity and low carbon olefin selectivity in MTO reaction.However,the traditional synthesis of SSZ-13 has many problems,such as the high price of template and strong acidity.Studies have found that the structural stability of SSZ-13 molecular sieve can be improved by loading Cu with SSZ-13 molecular sieve.At the same time,it has a better inhibitory effect on the side reactions of hydrogen transfer of propylene in the MTO reaction.In this experiment,we first investigated the feasibility of loading Cu on SSZ-13 molecular sieve and then applying it to MTO reaction;Cu-SSZ-13 molecular sieve was prepared by in-situ synthesis using the cheap template Cu-TEPA,and the amount of template,silicon source,alkali source,water source,and copper source in the raw materials was investigated.The prepared molecular sieves were characterized by XRD,NH3-TPD,BET,XRF,FT-IR and the reaction conditions when they catalyzing the MTO reaction were optimized;Investigated the surface modification method of Cu-SSZ-13 molecular sieve prepared by in-situ synthesis method;Introduced mesopores to modify Cu-SSZ-13 molecular sieve prepared by in-situ synthesis.The specific results are as follows:(1)Cu-SSZ-13 molecular sieve prepared by Cu loading on SSZ-13 has lower acidity than SSZ-13.At the same time,the initial low-carbon olefin selectivity increased and the propane by-product selectivity decreased from20%to 10%.(2)When Cu-SSZ-13 molecular sieve was prepared by in-situ synthesis,it is found that the molecular sieve with CHA structure can be synthesized within a certain range,which n(Na2O):n(Al2O3):n(SiO2):n(H2O):n(Cu-TEPA)=(5.50?6.50):1:(8.94?16.59):(250?500):(2.26-4.10),by changing the ratio of the materials.The molecular sieve has a specific surface area of 506 m2/g.When the MTO reaction temperature is 400?,WHSV=0.5 h-1 and catalyst raw powder participates was used in the reaction,the reaction life is 210 minutes,the selectivity of low-carbon olefins is 78.2%,and the initial selectivity of propane by-products is only 5.7%.(3)In the surface modification of Cu-SSZ-13 synthesized in situ,the combination of NH4CI and dilute HNO3 surface modification will reduce the Na and Cu content in the sample and increase the stability of Cu2+ and reduce the content of ANA structural heterocrystals.By surface modification,the catalyst has a more suitable acidity and acid content,and increase the specific surface area,pore volume and pore size.The reaction life of the catalyst is increased by 72%compared with the original sample and the selectivity of low-carbon olefins can reach 83%.The combination of the two modification methods improves the catalytic performance and is superior to the singlemodification.(4)In the mesoporous modification of Cu-SSZ-13 molecular sieve,when n(TPOAC):n(Al2O3)=0.27,mesoporous structure can be introduced into the catalyst.Compared with the catalyst with single pore structure in MTO reaction,its reaction life is increased by 50%,the selectivity of low-carbon olefins reaches 84.5%,and the selectivity of initial propane by-product is only 2.3%.