Structure Control Of Zeolites In OX-ZEO Catalyst And Catalytic Performance For Syngas Conversion Into Light Olefins

Light olefins(C⁼_2-4,ethylene,propylene and butene)are important chemical raw materials and the cornerstone of modern chemical industry,which are traditionally obtained from naphtha cracking.Methanol to Olefin（MTO）has made great technological breakthroughs and obtained large-scale industrial application in recent years.However,the light olefins select methanol as raw material,which needs to be produced from coal-based syngas（H₂+CO）.If the syngas,devertited from non-petroleum resources such as coal and biomass,could be directly converted into light olefins（STO）,it will not only simplify the process,save energy and reduce emissions,but also greatly reduce production costs.Fischer-Tropsch synthesis is a traditional route for STO reaction,but the product distribution is limited by the A-S-F mode,where the selectivity of light hydrocarbons（including olefins and alkanes）belows 58%.By using bifunctional catalysts combined with metal oxides and zeolites（OX-ZEO）,the activation of H₂+CO and the coupling of C-C bond can be controlled at different active sites and the selectivity of light olefins is as high as 70～80%,which has been widely studied in recent years.The zeolite catalysis in OX-ZEO is the second step reaction of STO,which converts intermediate products into olefins.The cage structure of zeolites possesses shape selective catalytic effect,and the acidity will affect the activity and olefins selectivity in the reaction process.However,the effects of zeolites in OX-ZEO on the structure-activity relationship of STO remain obscure.Based on this,SAPO-18（SP-18）,SAPO-34（SP-34）and SAPO-17（SP-17）zeolites were hydrothermally synthesized,including the SP-17 zeolites with different crystallization time and SiO₂/Al₂O₃ ratios,and SAPO-17/34（SP-17/34）composite zeolites prepared by co-template method and embedded seed method.The zeolites were characterized by XRD,TEM,SEM,N₂ adsorption-desorption,FTIR,NH₃-TPD and ICP-OES.The effects of zeolites structure,preparation conditions of SP-17zeolites and the preparation methods of SP-17/34 composite zeolites on catalytic performance of STO were studied,after the combination with MnGaO_x metal oxides.The structure-activity relationship was analyzed,and the conclusions are showing as follows:（1）SP-18 and SP-34 had hierarchical pore structure and plate-like morphology,while SP-17 had micropores and columnar morphology.When SP-18 and SP-34 combined with MnGaO_x,the catalyst exhibits no obvious deactivation in the reaction process.The conversion of CO reached 28.7%and 28.5%respectively,and the selectivity of C⁼_2-4 reached 87.0%and87.1%respectively.Due to the large cage size of SP-18 and SP-34,the selectivity of ethylene in the product was lower than propylene and butene selectivity.SP-17 samples deactivated fast during the reaction,the selectivity of C⁼_2-4 is 87.0%at the CO conversion of 19.4%,the selectivity of ethylene reached 45.6%,while the selectivity of butene is only 8.2%.The crystallization time on SP-17 shows that the phase of SP-5 appears when the crystallization time was less than 44 h.Increase the crystallization time to 48 h,only the phase of SP17appears,and the specific surface area and pore volume gradually decreased,the particle size and total acid content increased.Under the reaction conditions of 400℃and 3.0 MPa,SP-17crystallized for 96 h exhibits the highest CO conversion and light olefins space-time yield.（2）When the SiO₂/Al₂O₃ ratio was lower than 0.2,SP-17 zeolite can be successfully synthesized.When the SiO₂/Al₂O₃ ratio higher than 0.2,the competitive growth of SP-17 and SP-34 existed.According to NH₃-TPD results,the acid strength and total acid content of the zeolite were enhanced as the SiO₂/Al₂O₃ ratios increased from 0.05 to 0.3,the Br（?）nsted acid content increased from 0.20 mmol·g^-1 to 0.66 mmol·g^-1.Meanwhile,the CO conversion increased from 16.5%to 25.5%,and the selectivity C⁼_2-4 of decreased from 88.4%to 59.7%.When the crystal form of SP-34 appeared in the SP-17 zeolite,the olefin products distribution gradually changed from ethylene to propylene.The presence of SP-34 significantly improved the stability of SP-17 zeolite.（3）SP-17/34 composite zeolites were successfully prepared by co-template method and embedded seed method.The competitive growth of SP-17 and SP-34 existed during the preparation of the co-template method.The growth of SP-34 was incomplete and the size of SP-17 became smaller.The effect of embedded seed method on the morphology of SP-34seed was small.The activity evaluation results show that the CO conversion of 2.0TEA zeolite prepared by co-template method was 28.5%,C⁼_2-4 selectivity was 84.2%,and the catalytic stability was the highest.As the TEA content increased,the composition of SP-34increased,ethylene content gradually decreased and propylene increased.Finally,the effects of mass ratio of metal oxide to zeolite,space velocity,reaction temperature and pressure on the catalytic performance of MnGaO_x/2.0TEA bifunctional catalyst were investigated.The results showed that the mass ratio of metal oxide to zeolite was 2:1,400℃,3.0 MPa and3000 mL·g^-1_cat·h^-1 were the optimum conditions for synthesis of light olefins from syngas over Mn Ga O_x/2.0TEA bifunctional catalyst.