Acetylene Hydrogenation Analysis: A Dendity Functional Theory Study

The ethylene industry is the core of the petrochemical industry and holds an important position in the national economy.At present,ethylene is mainly produced by steam cracking of petroleum hydrocarbons in the petroleum industry.Ethylene products produced by this method usually contain a small amount of acetylene impurities.The acetylene component will reduce the activity of the polymerization catalyst and deactivate the catalyst,thereby affecting the production process of polymers derived from ethylene.Acetylene can be removed by catalytic hydrogenation to convert acetylene to ethylene.Historically,acetylene hydrogenation catalysts that have been used in industrial applications mainly include Pd-based catalysts.Until now,palladium-based catalysts have always dominated industrial applications with their excellent performance.However,the precious metal nature of Pd makes Pd-based catalysts costly.Therefore,it is imperative to look for low-cost,low-toxicity and environment-friendly catalysts.At the same time,in addition to ethylene,this reaction also has side reactions such as over-hydrogenation to ethane and polymerization to produce 1,3-butadiene and other polymers.For this reason,attempts have been made to improve the hydrogenation selectivity and resistance to carbon deposition of palladium catalysts by adding a second component as a cocatalyst to the palladium catalyst.In this paper,the adsorption properties of acetylene,ethylene and 1,3-butadiene on the three low-index surfaces of Pd and Pd₃Sn??111?,?100?,?211?surfaces respectively?and PdSn?010?,acetylene hydrogenation and carbon-carbon coupling reactivity and selectivity were explored.The DFT calculation results show that the addition of Sn reduces the adsorption strength of the adsorbates and improves the selectivity of ethylene.At the same time,the amount of green oil produced on the PdSn?010?surface was reduced compared to the pure Pd catalyst.On the other hand,due to the strong adsorption performance of acetylene on the catalyst surface,it is particularly important to investigate the relationship between adsorption energy and the coverage of adsorbate.Therefore,we investigated in detail the competitive adsorption of acetylene,ethylene,and hydrogen atoms on Pd?111?and Pd₃Sn?111?surfaces.The linear relationship between the adsorption energy and coverage of the three adsorbents was obtained by establishing high coverage model of single species,and the initial coverage of acetylene,ethylene,and hydrogen on the catalyst surface under experimental conditions was calculated using the Langmuir adsorption isotherm equation.Based on this coverage,the acetylene hydrogenation activity and selectivity were further explored.It was found that the acetylene hydrogenation activity of the Pd₃Sn?111?surface is higher than that of the Pd?111?surface,which is contrary to the results of the low coverage model,but is in agreement with the experimental results.In addition,the?111?planes of six representative transition metals Pd,Pt,Ir,Rh,Cu and Ag are used as the calculation models to investigate the hydrogenation performance of acetylene.Firstly,the adsorption energy of acetylene and 1,3-butadiene on all metals were calculated to determine their stable adsorption sites on the metal,and the adsorption regularity of the adsorbate on each metal was defined.Next,a detailed reaction network for the production of main product ethylene,by-product ethane,and1,3-butadiene during the hydrogenation of acetylene was calculated.By comparing the two-step elementary reaction barriers for the generation of ethylene,it was determined that an effective barrier to the generation of ethylene activity could be described and correlated with the acetylene adsorption energy.By calculating the reaction barriers of the three reaction pathways that produce 1,3-butadiene,the reaction pathway with the lowest effective barrier is determined as the dominant pathway,and the generation of green oil is described by the effective potential barrier of this pathway.