Study On Preparation Of ?-Al₂O₃/SAPO-34 Composite Catalysts And Their MTO Catalytic Performance

For methanol to light olefins over the SAPO-34 molecular sieve with very short lifetime??6h?,a single load modification exists covering the active center,blocking the pore and the load components of the loadability and thermal stability is not excellent.The paper proposes the preparation of the ?-Al₂O₃/SAPO-34 composite catalyst.Based on micro-mesoporous system,it is not only beneficial to promote the synergistic environment of L-B acid required for the dissociation and adsorption of methanol at lower temperature,but also construct micro-mesoporous pore structure to reduce the coke deposition formation rate,reducte the coke molecular size and inhibit the byproduct formation.Based on SAPO-34 molecular sieves,?-Al₂O₃ and the SAPO-34/?-Al₂O₃-M mechanical mixing catalyst,the effects of different preparation techniques on the structural properties and the MTO catalytic performance of the composite catalysts were investigated.And then,the effects of different ?-Al₂O₃ treatment methods and two-phase mass ratio on the structural properties and the MTO catalytic performance of the composite catalysts were investigated.The structure-activity relationship of?-Al₂O₃/SAPO-34 composite catalyst was characterized by XRD,FT-IR,SEM,BET,NH3-TPD and PY-IR.The results indicated that the preparation methods,?-Al₂O₃ treatment methods,two-phase mass ratio and other technical parameters have great influence on the structural properties and the MTO catalytic performance of the composite catalysts.The composite catalyst was not formed by the liquid phase precipitation coating method with the ?-Al₂O₃ calcined at 600? for 3h as the coating phase and the SAPO-34 used as the core phase.The ?-Al₂O₃/SAPO-34 composite catalyst was prepared by hydrothermal coating method with the SAPO-34 as the coating phase and the ?-Al₂O₃ as the core phase under the conditions of ?-Al₂O₃ calcined at 600? for 3h,mass ratio of ?-Al₂O₃:SAPO-34 was 1:1,and hydrothermalcrystallization at 200? for 48 h.The ?-Al₂O₃/SAPO-34 composite catalyst forms a micro-mesoporous structure?microporous specific surface area of 123.49m2·g-1 and microporous pore volume of 0.19cm3·g-1,mesopores specific surface area of95.31m2·g-1 and mesopores pore volume of 0.05cm3·g-1?and L-B acid synergistic center?1450cm-1,1540cm-1,total acid amount of 0.3048mmol/g?which is favorable for improving the MTO reactivity and prolonging the catalyst lifetime under mild conditions.The effects of different reaction temperature and reaction time on the coking species,the coke deposition and the physicochemical properties of the?-Al₂O₃/SAPO-34 composite catalysts were characterized by UV-Raman,O₂-TPO,XRD,BET,NH3-TPD.The coke species and coke quantity of ?-Al₂O₃/SAPO-34 composite catalyst were analyzed by using SAPO-34 molecular sieve as reference.And the physicochemical properties of the fresh and the reacted ?-Al₂O₃/SAPO-34 composite catalyst were investigated.The results indicated that,with the reaction temperature?330-420??increased,the coke species were converted from olefins to hydrogen-rich polyolefin molecules,and the formation rate of coke deposition at the temperature of 380? was 0.0009mmol/?g·min?.With the reaction time?120-600 min,380 ??increased,the coke species were converted from olefins to hydrogen-rich polyolefin molecules for 360 min,and the formation rate of coke deposition increased by 10%.The coke species of ?-Al₂O₃/SAPO-34 composite catalyst is similar to SAPO-34 molecular sieve,but the coke quantity and coke deposition rate of?-Al₂O₃/SAPO-34 composite catalyst decreased by about 5.7% and 9.7% at the same reaction time?360min?,respectively.The specific surface area,pore volume and acid amount of ?-Al₂O₃/SAPO-34 composite catalyst decreased significantly after 990 min reacting at 380?.The effects of reaction temperature,feed space velocity,molar ratio of water to alcohol and nitrogen flow of MTO on methanol conversion,light olefins selectivity and catalytic lifetime were investigated.The results indicated that MTO process parameters have a great impact on the MTO reactivity,the target product selectivity and the catalytic lifetime.The methanol conversion,light olefins selectivity,and thecatalytic lifetime of ?-Al₂O₃/SAPO-34 composite catalyst were 100%,88.10% and990 min,respectively,under the conditions of composite catalyst 1g,molar ratio of water to alcohol 2:1,feed space velocity 2h-1,nitrogen flow 20mL·min-1 and reaction temperature 380 ?.The reaction temperature is lower than that of the SAPO-34 molecular sieve at a temperature of 400? by 20?.