Study On Selective Hydrogenation Of Acetylene To Ethylene Over Nickel-Based Catalysts

Ethylene,one of the bulk chemicals,is an important indicator of a country’s petrochemical level.With the development of our country’s economy,our demand for ethylene and its downstream products increase day by day.Although China’s production capacity is increasing every year,it still fails to reverse our import of ethylene and its downstream products.Ethylene is usually produced by catalytic cracking of petrochemical raw materials.Since China is a country with low oil content,abundant coal resources and lack of natural gas,we consider starting from the coal chemical engineering route "coal→calcium carbide→acetylene→ethylene" to develop high-performance catalysts to convert acetylene to ethylene.This route that uses acetylene as a platform molecule to convert coal and natural gas into downstream products through the C2 chemical route will be more efficient than the C1 chemical route.The first part of this thesis is to select Ni as the active metal,and select SiO2,γ-Al2O3 and ZSM-12 as the catalyst supports,and prepare the corresponding catalyst by the incipient wetness impregnation method.The effects of the acidity and basicity of the supports and catalysts on the selective hydrogenation of acetylene to ethylene were studied through optimizing the loading of Ni,the temperature of the catalytic reaction and using SEM,XRD,BET,ICP,TEM,NH3-TPD,Py-FTIR,H2-TPR,XPS,C2H4-TPD and TGA.The results show that 2wt%Ni/ZSM-12 has the best effect on the selective hydrogenation of acetylene at 250℃:the conversion of acetylene is 100%and the selectivity of ethylene is 73.14%.From the results of NH3-TPD and Py-FTIR,it can be concluded that too strong or too weak acidity of the catalysts are not conducive to the selective hydrogenation of acetylene.Moreover,the synergistic effect between Lewis acid and Bronsted acid is more conducive to improving the selectivity of the catalysts for ethylene.In addition,there is a phenomenon of excessive hydrogenation during the reaction,and the stability of the nickel catalyst to this reaction is relatively poor.On the basis of the above work,we introduced the inert metal Sn into the 2wt%Ni/ZSM-12 catalyst to explore the effect of different nickel-tin ratios on the reaction.The results of catalysts evaluation show that the addition of tin has an inhibitory effect on the hydrogenation activity of these catalysts,and the selectivity of ethylene increases first and then decreases as the ratio of nickel to tin increases.When the nickel-tin ratio is equal to 7,the effect of selective hydrogenation of acetylene is best at 250℃.At this time,the conversion rate of acetylene is 100%and the selectivity of ethylene is 92.51%.Furthermore,after the stability test of the catalyst for 100 hs it was found that the conversion rate of acetylene remained at 100%,and the selectivity to ethylene was stable at about 84%.This is because Ni and Sn form a Ni-Sn alloy or a Ni-Sn intermetallic compound,in which the addition of Sn produces geometric effects and electronic effects.This is beneficial to weaken the adsorption of ethylene on the catalyst surface,thereby further inhibiting the excessive hydrogenation of acetylene and hydrogenolysis of the C=C bond,thereby improving the selectivity to ethylene.