Study On The Preparation And Performance Of MTO Catalysts With High Diene Selectivity And Long Catalytic Life

In recent years,the demand for these basic chemical raw materials has been increasing every year.Traditionally,the production of ethylene and propylene mainly comes from the thermal cracking of light hydrocarbons and naphtha.The emergence of Methanol To Olefin（MTO）process can solve the shortage of ethylene and propylene caused by the increasingly depleted petroleum in recent years.SAPO-34molecular sieve catalyst has been widely researched and developed as one of the most commonly used catalysts in the MTO process.In recent years,SAPO-18 molecular sieve has been gradually concerned by researchers due to its long catalytic life.The olefins selectivity and catalytic life are two important indexes of catalyst,and we can improve its catalytic performance by regulating the acidity,pore structure and crystal size of the molecular sieve.In this paper,three kinds of SAPO-34 molecular sieves were synthesized by using different sizes of silica sol as silicon source.The differences of the molecular sieves synthesized by different sizes of silica sol were studied by NH₃-TPD and ²⁹Si MAS NMR.It can be seen from the NH₃-TPD characterization results that with the increase of silica sol particle size,the strong and weak acid content of the molecular sieve decreased.From the NMR characterization results of the molecular sieves,it can be found that there were a lot of silicon islands in SAPO-34 molecular sieve synthesized by large particle silica sol.From the MTO test results,it was found that the molecular sieve synthesized by silica sol with particle size of 57 nm has high olefins selectivity and long catalytic life.In order to improve the performance of the catalyst,SAPO-34 gel and SAPO-18gel were used as the main body,and SAPO-18 powder and SAPO-34 powder were added into the gel to synthesize the composite molecular sieve.The optimum ratio of the composite molecular sieve was 85%and 15%,the selectivity of S-SAPO-34/SAPO-18 composite for ethylene and propylene was 85.5%,with a catalytic reaction life of 170 min,the selectivity of S-SAPO-18/SAPO-34 composite for ethylene and propylene was 86.7%,with a catalytic reaction life of 150 min.In addition,the introduction of ultrasonic technology in the synthesis process not only shortens the crystallization time,but also significantly prolongs the life of catalyst.Compared with the composite materials of hydrothermal synthesis,the life of C-SAPO-34/SAPO-18 and C-SAPO-18/SAPO-34 composite molecular sieves synthesized under ultrasonic conditions were extended by 50 and 60 minutes respectively.Different templates can be used to synthesize different types of molecular sieves.It was found that TEAOH and TEA were more conducive to the synthesis of SAPO-34,DIEA was usually used as a template for SAPO-18 synthesis.The combination of DIEA/TEAOH and DIEA/TEA template were used to synthesize SAPO-18/SAPO-34 eutectic molecular sieve with better catalytic performance,the eutectic molecular sieves have excellent performance of two kinds of molecular sieves,the eutectic molecular sieves synthesized by DIEA and TEA have a dilute selectivity of 83.8%and a catalytic lifetime of 150 min,however,the eutectic molecular sieves synthesized by DIEA and TEAOH have a catalytic lifetime of 160 min and a maximum selectivity of 85.5%,which shows that the combination of DIEA and TEAOH templates was more favorable for the synthesis of molecular sieves for MTO reaction.