Study On Preparation And Catalyst For MTO Reaction Of SAPO-34/ZSM-5 Composite Molecular Sieve

Aiming at the disadvantage that exists in MTO process for single ZSM-5 and SAPO-34 molecular sieve and the development of a more stable and efficient MTO catalyst,the study started with the novel catalyst design,prepared of SAPO-34/ZSM-5 composite molecular sieve with composite technology,and the optimization of base phase ZSM-5 preparation system,characterization and catalytic activity of SAPO-34/ZSM-5 composite molecular sieves,process conditions of MTO process and the carbon deposit,structure and physicochemical property of molecular sieves after the reaction were investigated.ZSM-5 molecular sieves were prepared employing microwave-hydrothermal method,and the preparation conditions were discussed by means of XRD,SEM,BET and NH₃-TPD characterization methods.The results showed that template agent,system pH,water content and the crystallization time had a great influence on microstructure,specific surface area,pore structure,and the strength,distribution and content of molecular sieves surface acid;High crystallized and pure ZSM-5 molecular sieves was prepared at the conditions that the raw material molar ratio Na₂O:Al₂O₃:SiO₂:TPAOH:H₂O of 0.45:0.01:1.5:0.53:300,template agent of to four propyl ammonium hydroxide,system pH of 11,crystallization temperature of 180?,crystallization time of 2 h.And the ZSM-5 molecular sieves performed uniform grain crystal size?about 200 nm?,high specific surface area and pore volume(313 m²/g and0.29 mL·g^-1),surface chemical properties of weak acid and no medium and strong acid.SAPO-34/ZSM-5 composite molecular was prepared by the the microwave hydrothermal coating method.Single molecular sieve,mechanical mixing molecular sieve,ZSM-5/SAPO-34 and SAPO-34/ZSM-5 composite molecular were compared.And the influence of silicon aluminum ratio,SAPO-34 feeding mode,ZSM-5 and SAPO-34 mass ratio and other factors on SAPO-34/ZSM-5 composite molecular sieve structure and catalytic properties was studied.The microstructure and physicochemical properties of the composite molecular sieve were characterized and analyzed by means of XRD,SEM,FT-IR,BET and NH₃-TPD characterization methods,and the composite molecular sieve catalytic performance was studied in MTO process.The results showed that the different preparation methods,Si/Al ratio,SAPO-34 feeding mode,mass ratio had a great effect on the composite molecular sieve properties and catalytic performance;Under the condition that Si/Al ratio of 150,adding four propyl ammonium hydroxide after adding SAPO-34,the mass ratio of m?ZSM-5?:m?SAPO-34?=0.48,SAPO-34/ZSM-5 composite molecular sieve with unique microstructure,high specific surface area?BET specific surface area 436 m²/g,micropore specific surface area 211 m²/g,mesopore specific surface area 132 m²/g?and pore volume(micropore volume 0.21 ml·g^-1,mesopore volume0.09 ml·g^-1),surface chemical properties of weak acid(0.209 mmol·g^-1).Under the condition of reaction temperature of 430?,1:2 mole ratio of water to methanol,space velocity of 2.4 h^-1,N₂ flow rate of 23 ml·min^-1,the selectivity for low carbon olefin and the conversion of methanol were 89.93%and 98.9%respectively.The MTO process conditions were optimized with SAPO-34/ZSM-5 composite molecular sieve,and the effect of reaction temperature,molar ratio of methanol and water,space velocity,N₂ flow rate and other factors on the selectivity for olefin and conversion of methanol.The results showed that the selectivity for low carbon olefin,conversion of methanol,the ratio of propylene and ethylene,reaction induction period and the catalyst life were 92.79%,98.2%,1.56,1.5 h,80 h,respectively.under the condition of reaction temperature of 410?,2:1 molar ratio of water to methanol,space velocity of 2.0 h^-1,N₂ flow rate of 20 ml·min^-1.The changes of the carbon deposit,structure and physicochemical property of SAPO-34/ZSM-5 composite molecular sieve after reaction were analyzed,and the effects of different reaction on the carbon deposit,structure and physicochemical property of composite molecular sieve catalyst were investigated.The results showed that the composite molecular sieve catalyst mainly existed low and medium temperature carbon species;the reaction time had a great influence on micro-pore structure and total acid content of the composite molecular sieve catalyst,and micro-pore volume and total acid content of catalyst decreased significantly.