| As an important part of the chemical industry,the ethylene industry has an extremely significant impact on economic development.In industry,the dominant process for producing ethylene is steam catalytic cracking of naphtha,and the resulting C2 fraction usually contains 0.5-2.4%acetylene.These impurities can poison the Ziegler-Natta catalyst in the olefin polymerization process,so it is necessary to reduce the acetylene content to(<5 ppm).Among the various methods for removing acetylene,the selective hydrogenation of acetylene has become the most effective method due to its simple operation,low cost and environmental friendliness.Pd-based catalysts have attracted widespread attention due to their excellent hydrogenation performance,but they are prone to excessive hydrogenation and severe polymerization in the selective hydrogenation of acetylene,which reduces the yield of ethylene.Studies have found that acetylene will form a strong adsorption transition state on the continuous Pd sites,which is not conducive to desorption of the product,which leads to further hydrogenation of ethylene.Therefore,how to achieve a high degree of dispersion of Pd in the catalyst to increase the selectivity of ethylene is extremely challenging.The supported single-atom catalyst can isolate Pd atoms to the greatest extent and form active sites at the atomic level.Therefore,the performance of the acetylene hydrogenation reaction can be enhanced by the precise construction of the fine structure of the Pd single-atom active site.This thesis takes the selective hydrogenation of acetylene as the target reaction and the MOFs-based single-atom catalyst as the research object.Aiming at the problem that the C≡C bond in the acetylene hydrogenation reaction is easy to hydrogenate to generate ethane and easy to polymerize to generate green oil and other by-products.A new strategy to use the uncoordinated sites in Zr-based MOFs(UIO-66-NH2)to anchor Pd to construct Pd single-atom active sites to increase the selectivity of the carbon-carbon double bond was proposed.In order to further improve the activity of the Pd single-atom catalyst,a new method of using atmosphere induction method to control the local coordination environment and electronic structure of the single-atom site of Pd is proposed to improve the activity of the catalyst while maintaining selectivity.And the mechanism of the catalytic performance of the Pd single-atom active site on the selective hydrogenation of acetylene was revealed.(1)Taking the carbon-carbon double bond ethylene selective enhancement as the starting point,the MOFs-based Pd single-atom catalyst(Pd1/UIO-66-NH2)was prepared by the method of post-synthesis modification.As a comparison,the Pd nano particles was prepared by the impregnation method to systematically discuss the effects of the geometrical arrangement,electronic structure and coordination environment of the Pd single atom sites,which are effect on the performance of the selective hydrogenation of acetylene.The results show that Pd is atomically dispersed on the support.Meanwhile,Pd is coordinated with the N on the support and the electrons on the N transfer to Pd2+,making the valence of the Pd single atom between 0 and +2 valence.The Pd1/UIO-66-NH2 is employed for the selective hydrogenation of acetylene to ethylene.When the acetylene is completely converted,the ethylene selectivity of the Pd1/UIO-66-NH2 catalyst reaches 90%.It is attributed to atomically dispersion of Pd active metal,and the Pdδ+(0<δ<2)single-atom active center is conducive to the desorption of ethylene,thereby increasing the selectivity of ethylene.(2)Taking the high selectivity of ethylene and improving the catalytic activity as the starting point,the Pd1/UIO-66-NH2 prepared above was used as the precursor,and the atmosphere treatment was carried out to induce lattice strain of the MOFs support,so that the fine structure of the Pd single-atom active sites are regulated.Taking the Pd1/UIO-66-NH2 without atmosphere treatment and different ratio atmosphere treatment as comparison samples,the influence of atmosphere induction on the electronic structure and coordination environment of the single-atom active sites was systematically discussed.The Pd1/UIO-66-NH2 catalyst was treated with reaction atmospheres with different ratios of hydrogen to alkyne,and the T-Pd1/UIO-66-NH2-2 catalyst was obtained by treatment with the reaction atmosphere when the ratio of hydrogen to alkyne was 2.It showed the best Catalytic performance,the intrinsic active TOF value is 0.132 s-1,which is higher than the Pd1/UIO-66-NH2 catalyst without reaction gas treatment.The Pd in the T-Pd1/UIO-66-NH2-2 catalyst still exists in the form of single atoms,and the reaction gas treatment process releases a moderate amount of heat to make the carrier UIO-66-NH2 lattice compressively strain,resulting in the Pd on the carrier N transfers electrons,the electron cloud density of Pd decreases.The center of the d-band moves away from the Fermi level,so that the catalytic activity is improved.DFT calculations show that,the desorption energy of ethylene is greater than the energy barrier for further hydrogenation of ethylene,which improves the selectivity of ethylene.The activity and ethylene selectivity of the T-Pd1/UIO-66-NH2-2 catalyst did not change significantly during the 26 hour continuous reaction,which indicated that the T-Pd1/UIO-66-NH2-2 had good stability in acetylene hydrogenation. |
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