Study On Structural Regulation Of Mn-based Oxides In OX-ZEO Catalyst And Catalytic Performance For Syngas Conversion Into Light Olefins

Light olefins（C₂-C₄⁼,i.e.,ethylene,propylene,butylene）are very important raw material for the chemical industry.With the rapid development of the chemical industry,the demand for light olefins is increasing.Traditionally,light olefins are mainly produced from petroleum resources.Due to the shortage of petroleum,it becomes key research to develop the non-petroleum route to produce light olefins.The conversion of coal-based syngas to light olefins can realize efficient utilization of coal-based clean energy and alleviate the energy crisis.Starting from syngas（CO/H₂）,the synthesis of high value-added chemical products through the hydrogenation process has gradually become a research hotspot of C1 chemistry in recent years.Being different from traditional Fischer-Tropsch synthesis,the metal oxide and zeolite（OX-ZEO）bifunctional catalysts utilize different reaction mechanism,breaking the limitation of product distribution of ASF.However,CO conversion in reported work is still low,thus it is imminent to develop bifunctional catalysts with higher activity.To convert syngas into light olefins directly over bifunctional catalyst,the adsorption and activation of CO and H₂ mainly occurs on the metal oxides to produce intermediate,subsequently the zeolites convert the generated intermediate to light olefins.Oxygen vacancies on the surface of metal oxides are the sites for CO adsorption and activation,so the concentration of oxygen vacancies determines the ability to active CO.Mn-based oxides have significant activation for CO due to abundant oxygen vacancies,and Ga₂O₃ has good ability for dissociation of H₂.Previous research shows that MnGa/SAPO-34 bifunctional catalyst exhibits high light olefins selectivity in STO reaction,but the catalyst activity is poor.In this study,the effects of preparation method and different precipitants on MnGa oxides were investigated.Combined with characterization results of XRD,BET,TEM,in-situ XRD,and in-situ DRIFTS,the structure-activity relationship between the content of oxygen vacancies on the surface of metal oxides and catalytic activity was revealed.On this basis,XMnGa ternary metal oxides were prepared by introducing various kinds and contents promoters to improve CO conversion by regulating and matching the activation of CO and H₂.The obtained conclusions are shown as follows:（1）By optimizing the preparation methods of MnGa oxides,the co-precipitated MnGa oxide sample exhibited a higher activity and light olefins selectivity than impregnated Ga/Mn O_xand Mn/Ga₂O₃,or the powder mixed MG-（PM）,or the solution precursor mixed MG-（SM）samples.For the precipitants,viz.,MG-AH（NH₃·H₂O）,MG-SH（Na OH）,MG-OA（H₂C₂O₄）and MG-SC（Na₂CO₃）,the MG-AH/SAPO-34 exhibited the best catalytic performance.Under the reaction condition of 400℃,2.5 MPa and 4875 m L·h^-1·g^-1,MG-AH/SAPO-34 exhibited the highest CO conversion of 19.5%and the light olefins space-time yield of 116.1 m L·g^-1_cat·h^-1 with a stable activity in a reaction time of 50 h.The characterization results showed that MG-AH oxide prepared with NH₃·H₂O as the precipitant exhibited in an amorphous structure with high dispersion of Ga and Mn species.Compared with Mn O_x,BET surface area of MG-AH increased to 102 m²·g^-1 with typical mesoporous.The MG-AH showed the highest oxygen vacancy concentration of 39.6%,which enhanced the activation ability of CO.Moreover,MG-AH possessed the largest amount of desorbed CO(309.8μmol g^-1)and moderate H₂ dissociation.The largest BET surface area,the highest oxygen vacancy content and moderate hydrogen dissociation ability of MG-AH oxide were mainly responsible for the enchaned catalytic performance.（2）A series of ternary metal oxide XMnGa（X=Al,Zn,Ce,V）were prepared by the co-precipitation method by using NH₃·H₂O as precipitant,and then combined with SAPO-34 for STO reaction.Zn MnGa oxide sample presented the lowest C₂-C₄⁼selectivity and Ce MnGa oxide sample showed the lowest CO conversion.VMnGa oxide sample had better catalytic activity and VMnGa（0.1）oxide sample with a V/（V+Mn+Ga）atomic ratio of 0.1 exhibited the highest CO conversion of 24.0%and C₂-C₄⁼space time yield of 139.2 m L·g^-1_cat·h^-1 under reaction conditions of 400℃,2.5 MPa and 4875 m L·h^-1·g^-1.The characterization results show that the introduction of proper amounts of V into MnGa oxide formed Mn₂VO₄ spinel structure which enhanced interaction between species and desperation.VMnGa（0.1）formed a nanoflower structure with large specific area.The introduction of V significantly increased the oxygen vacancy and the adsorption for CO/H₂.It is also found that the H₂ pretreatment accelerated the formation of active species in metal oxides,but didn’t change the active phase.Compared with unpretreated catalyst,the catalytic activity exhibited the same catalytic results after a reaction time of 15 hours.In-situ DRIFTS confirmed that*CH₃O was intermediate during the STO reaction.