Construction Of Cu₁/Fe_yMgO_x Catalysts Based On LDHs Layered Precursors And Interface Effects In The Selective Hydrogenation Of Acetylene

Ethylene is an important raw material for the petrochemical industry,and its output is one of the important indicators to measure the petrochemical industry development level of a country.Ethylene is usually produced by steam cracking of naphtha,which inevitably produces 0.5-2.5%acetylene.Traces of acetylene can cause catalysts poisoning during the ethylene polymerization reactions,and even cause explosion accidents.Therefore,it is of great practical significance to effectively remove the acetylene in the ethylene feedstock<1 ppm.Due to the advantages of simple process and environmental friendliness,selective hydrogenation of acetylene is the preferred method for purifying ethylene.Pd-based catalysts which have high catalytic activity are main catalysts generally used in industry.However,Pd-based catalysts still have great challenges.Specifically,it is difficult to control the hydrogenation depth of unsaturated C≡C bonds to generate ethane,and the C-C bonds are easy to polymerize to form oligomers such as green oil,resulting in unsatisfactory ethylene selectivity.In addition,considering the declining reserves and the rapidly rising price of Pd,researchers gradually shifted the research direction to non-noble metal catalysts.However,the activity of non-noble metal catalysts is far lower than Pd-based catalysts.Therefore,there are great challenges in designing and synthesizing non-noble metal catalysts with high activity and selectivity.For supported catalysts,reducible oxides are usually used as supports in the catalytic system.And the metal-oxide interface sites have been revealed as the bifunctional active sites in many reactions.According to literature reports,metal-oxide interface sites exhibit better catalytic performance than surface sites.Its low-coordination oxide support affects the geometric structure and electron transfer of the active metal,therefore improving the catalytic performance.However,during the reduction process,the coordination structure of the oxide support is inevitably uncontrollable and the active metal is easily over-encapsulated.In short,it is of great significance to develop a method to get low coordination interface sites.Based on this background,this thesis closely focuses on the two scientific problems:uncontrollable depth of acetylene hydrogenation and easy polymerization of carbon-carbon bonds.Using LDHs as the precursors,the controllable electronic structure and low coordination Cu-FeMgOx interface catalyst was constructed through structural topological transformation.Furthermore,by adjusting the ratio of Cu/Fe,the mechanism of interaction between the local geometry and electronic structure modulation of the Cu1/FeyMgOx(y=1,0.5,0.16 and 0.12)catalyst interface and the catalytic performance was revealed.This work realized the innovation in the design and construction of bifunctional sites at the Cu1-FeyMgOx interface.And we clarified the cooperative enhancement mechanism of Cu1δ--Feyδ+MgOx site on the catalytic activity and selectivity of the selective hydrogenation of acetylene.The main research contents and results of this thesis are as follows:(1)For the purpose of enhancing the performance of the selective hydrogenation of C≡C bonds in the selective hydrogenation of acetylene,based on the controllability of LDHs laminate elements and the network trapping effect,the Cu-FeMgOx interface catalysts with tunable local electronic structure was prepared through structural topological transformation.And compared with Cu/MgAlOx and FeMgAlOx catalysts,the influence of the structure of Cu-FeMgOx interface structure on the performance of selective hydrogenation of acetylene was systematically discussed.STEM and H2-TPR results proved the construction of Cu-FeMgOx interface structure and the interaction between Cu and Fe.The performance evaluation results show that at 280℃,the single metal Cu/MgAlOx catalyst with no interface structure achieves complete conversion of acetylene.And FeMgAlOx does not show any hydrogenation activity in the temperature range of 120～280℃.It is worth pointing out that Cu1/Fe0.5MgOx catalyst achieves 100.0%acetylene conversion and 93.3%ethylene selectivity when the reaction temperature is 225℃.Its catalytic performance is much higher than that of the reference catalyst.H2-TPD,XPS and XAS clarify that the strong electronic interaction between Cu and FeMgOx species at the Cu-FeMgOx interface is beneficial to the activation and dissociation of the reactant H2.This leads to increasing catalytic activity.(2)Furthermore,take the precise regulation of the local structure of the catalyst interface as the starting point,using the controllability of the composition of LDHs,by adjusting the Cu/Fe,a series of Cu1/FeyMaOx catalysts were prepared.The Cu1-FeyMgOx interface geometry and electronic coordination environment can be precisely controlled.The selective hydrogenation of acetylene results show that Cu1/Fe0.16MgOx with Cu/Fe=6/1 can achieve complete conversion of acetylene at 215℃ while obtaining a high selectivity of 95.0%.The catalyst activity of 1/1 and 8/1 at the same temperature is much lower than that of Cu1/Fe0.16MgOx,and cannot achieved the 100.0%acetylene conversion even when the temperature is increased to 250℃.STEM and H2-TPR were used to illustrate the influence of different Cu/Fe on the strength of the interaction between Cu and Fe at the Cu1-FeyMgOx(y=1,0.16 and 0.12)interface.The DFT calculations confirmed that the bifunctional site Cu1δ--Fe0.16δ+MgOx is the dominant active site of Cu1/Fe0.16MgOx.C2H2 and H2 are easy to adsorb and activate on the interface-Cu sites at the Cu1-FeO.16MgOx interface.The*C2H4 intermediate is adsorbed on the Cu1δ--Fe0.16δMgOx site,realizing the recognition of the bifunctional site.H2-TPD combined with XPS and XAS clarified the influence of different Fe addition on the electronic environment of Cu1δ--Fe0.16δ+MgOx site.It is confirmed that the catalyst has the strongest dissociation ability for the reactant H2 when Cu/Fe=6/1,thereby achieving a significant increase in catalytic activity.Taking the Cu1/Fe0.16MgOx with the best catalytic performance as the research object,DFT calculation is performed.DFT confirmed that C2H4 at Cu1δ--Fe0.16δ+MgOx site is more prone to desorption rather than further hydrogenation.Thereby,it can effectively inhibite the formation of side-products,significantly improving the selectivity of ethylene.Finally,the mechanism of the synergistic enhancement of the catalytic activity and selectivity of Cu1δ--Fe0.16δ+MgOx sites at the interface was clarified.