Structure Regulation Of Active Center Of Supported Ni-Based Catalyst And Catalytic Performance Of Selective Hydrogenation Of Acetylene

Ethylene is one of the chemical intermediates with the largest annual output and demand,the production capacity of which stands for the petrochemical industry level of a country.Ethylene is mainly produced from naphtha cracking,which usually contains 0.5～2.5%acetylene.A small amount of acetylene impurity could cause catalyst poisoning,and therefore it is of great significance to remove trace acetylene impurities in ethylene-rich gas streams.The conversion of acetylene to ethylene by catalytic hydrogenation has become an important method for ethylene purification.Due to the excellent hydrogenation activity,the noble metal Pd is widely used as active component in industry.However,the scarce resource and expensive price greatly increase the production cost,and therefore the effective replacement of noble metal catalysts has become the crucial issue in industrial catalysis and scientific research.The abundant and inexpensive non-noble metal Ni elements show the good hydrogenation activity due to the unfilled d orbital,attracting much attention in the substitution of noble metals.However,acetylene is tended to polymerize and over hydrogenate on continuous Ni sites to produce green oil and ethane by-products,and thus reduce the selectivity of ethylene.The researchers found that the effective isolation of continuous active metallic Ni sites could improve ethylene selectivity but usually at the expense of activity.Therefore,the development of novel supported Ni-based catalysts and the preparation methods to realize the enhancement of activity and selectivity at mild reaction conditions is of great significance.Focusing on the key scientific problems that acetylene is easy to over-hydrogenate and polymerize,this paper aimed at the simultaneous improvement of acetylene conversion and ethylene selectivity at mild recation conditions.The research on the structural regulation of supported Ni-based catalyst was carried out.The effects of the isolation degree of catalytic active center and the gain or loss number of d-band electrons on the adsorption and desorption type of reactants and product molecules was explored.Combined the various characterizations with DFT calculations,the mechanism of hydrogenation by constructing the isolated Ni active center was clarified,and thus realized the efficient directional conversion of acetylene to ethylene.The detailed research results are summaried as below:（1）Based on the structural characteristics of trinuclear molybdenum-sulfur clusters,a certain percentage of Ni cations with hydrogenation activity was introduced into the unsaturated cubic alkane structures of cluster.The supported Ni₁Mo S/Al₂O₃ catalysts with isolated active Ni sites were prepared after heat treatment.Taking Ni/Al₂O₃ and Ni S/Al₂O₃ as constrast,the effect of isolation degree and the electronic environment of active Ni sites on catalytic performance was systematically explored.The results showed that the Ni₁Mo S/Al₂O₃ catalyst with isolated active Ni sites exhibited the improved activity,selectivity and stability than Ni/Al₂O₃ and Ni S/Al₂O₃.This could be ascribed to the formation of electron-deficient and isolated Ni₁^0.4+Mo S active center in the Ni₁Mo S/Al₂O₃ catalyst,which not only promoted the activation and dissociation of hydrogen molecules,but also facilitates the adsorption of C₂H₄^*intermediates in aπcomplex manner,and thus easily desorb.In addition,the construction of isolated Ni sites could reduce the accumulation of heat,and thus inhibit the agglomeration of active centers and the carbon deposition,improving the catalytic stability.（2）Furthermore,taking reducible metal oxide CeO₂ as support,and Ni as well as m（M=Ga,In,Zn,Sn,Fe,Mn,Mo,Co,Au and Ag）as active components and additives,a series of Ni-based alloy catalysts were prepared by support coordination structure induction method.By adjusting the morphology of CeO₂,the effect of the coordination structure in different morphology supports on the ordering degree of the Ni and M atomic arrangement as well as catalytic performance was investigated.The evaluation results of acetylene hydrogenation showed that Ni Ga/CeO₂,Ni Zn/CeO₂,Ni Sn/CeO₂ and Ni In/CeO₂ catalysts showed the excellent activity and ethylene selectivity,which are much higher than that of monometallic Ni catalysts.Improved activity and ethylene selectivity were due to the highly dispersed Ni-based ordered alloy structure induced by the support coordination environment,which not only isolated the continuous Ni sites and increased the number of active centers,but also weakened the adsorption strength of C₂H₄^*intermediate to promote the rapid desorption from the catalyst surface.