The Regulation Of Catalysts Size And Second Metal Doping Of Cu Based Catalysts On The Selectivity And Activity Of Acetylene Hydrogenation To Ethylene

As an important raw material,ethylene can be obtained from the thermal cracking of petroleum in industry.However,during the cracking process,a small amount of acetylene?0.1¹%?and sulfur-containing species?H₂S?are produced.Acetylene will not only affect the properties of the polymer,but also its polymerization to form green leads to the ethylene polymerization catalyst deactivation irreversibly.Therefore,in order to meet the requirements of ethylene polymerization,the content of acetylene must be reduced to less than 1ppm.Nowdays,the noble metal Pd-based catalysts with high activity are widely used for the selective hydrogenation of acetylene in industry,but C₂H₄selectivity and the formation of green oil cannot be regulated simultaneously.Thus,non-noble metal Cu-based catalysts have attracted widely attention due to its high olefin selectivity.In this paper,DFT?Density Functional Theory?method is used to investigate the hydrogenation reaction of acetylene on the non-noble metal Cu.The effects of Cu catalyst size,second metal doping,doping ratio and connection mode of doping metal were fully considered to regulate the catalytic performance?C₂H₄ selectivity and activity?in the reaction of C₂H₂hydrogenation.The main conclusions are shown as follows:?1?The hydrogenation of acetylene on Cu₁₃,Cu₃₈,Cu₅₅ clusters and Cu?111?surface are studied to clarify the effect of Cu cluster size on the activity and ethylene selectivity for acetylene hydrogenation to form ethylene.a)The size of Cu clusters affects the optimal path of acetylene hydrogenation and the selectivity of ethylene,on Cu₁₃,CH₃CH intermediate path is the most favorable path for acetylene hydrogenation,and C₂H₆ is the major product.On Cu₃₈,CH₃CH intermediate and C₂H₄ desorption intermediate paths are two parallel paths;C₂H₄ and C₂H₆ are the major products.On Cu₅₅cluster and Cu?111?surface,C₂H₄ desorption path is the most favorable,and C₂H₄ is the major product.Thus,Cu catalysts with larger size have high ethylene selectivity,which are beneficial for the removal of trace acetylene.b)The size of Cu clusters affects the activity of acetylene hydrogenation to form ethylene,the order is Cu₁₃<Cu₅₅<Cu₃₈<Cu?111?,suggesting that with the increasing of Cu catalyst size,the activity of acetylene hydrogenation to ethylene production is significantly improved.Therefore,for different sizes of Cu catalysts,Cu₅₅ cluster and Cu?111?with the large size have the highest ethylene selectivity,moreover,compared to the Cu₁₃ and Cu₃₈ cluster,Cu₅₅ cluster and Cu?111?surface have higher catalytic activity for the formation of ethylene.Namely,for the acetylene hydrogenation reaction,Cu catalyst should be in a relatively large size in the preparation process,which can improve the activity and selectivity of ethylene in acetylene hydrogenation reaction.?2?The acetylene hydrogenation reaction on Ni,Pd,Pt,Ag,Rh and Au doped-Cu catalysts are examined to elucidate the effects of different metal doped-Cu-based bimetallic catalyst.a)Different doping metal surface affects the optimal path of acetylene hydrogenation and the ethylene selectivity on Cu catalysts,namely,on AuCu catalyst,CH₃CH intermediate path is the most favorable path,the major parduct is C₂H₆.On Cu,PdCu and AgCu catalysts,C₂H₄ desorption path is the most favorable path,the major product is C₂H₄.On PtCu,RhCu and NiCu catalysts,C₂H₄ intermediate path is the most favorable path,the major product is C₂H₆.The ethylene selectivity follows the order:PdCu>Cu>AgCu>PtCu>NiCu>RhCu,namely,Pd-doped Cu catalyst can significantly improve the selectivity of ethylene for the reaction of acetylene hydrogenation.b)Different doping metal affects the activity of acetylene hydrogenation to ethylene on Cu catalyst,the activity follows the order:PdCu>Pt Cu>Ni Cu>Rh Cu>AgCu>AuCu>Cu,suggesting that Pd-doped Cu catalyst significantly improved the activity of acetylene hydrogenation to ethylene.c)For the hydrogenation of acetylene,the catalytic properties of different metal modified-Cu catalysts have a closely relationship with the electronic structure of the surface metal atoms,Pd-doped PdCu catalyst with a medium d band centure has the highest ethylene selectivity and catalytic activity.Therefore,Pd-modified PdCu bimetallic catalyst has the largest C₂H₄selectivity and activity.In industry,Pd-modified Cu catalyst should be used for the hydrogenation of acetylene,which is more conducive to the removel of acetylene at high activity and high selectivity.?3?The hydrogenation of acetylene to form ethylene on Pd Cu catalysts with different Pd_s:Cu ratio and Pd_sub:Cu ratios are studied to clarify the effects of different proportion of surface,sublayer Pd:Cu ratio and the connection mode of Pd on the activity and ethylene selectivity.a)For the surface Pd_s-modified Cu catalysts,different PdCu ratio affects the optimal path of acetylene hydrogenation and the ethylene selectivity,namely,on Cu?111?,Pd₁Cu₈,Pd₃Cu₆ and Pd₆Cu₃,C₂H₄ desorption path is the most favorable,C₂H₄ is the major product,which exhibits high ethylene selectivity,on Pd₉Cu₀,C₂H₄ intermediate path is the most favorable,C₂H₄ and C₂H₄ are the major products,the formation of ethane decreased ethylene selectivity,the order of ethylene selectivity is Pd₁Cu₈>Cu?111?>Pd₃Cu₆>Pd₆Cu₃>Pd₉Cu₀.For the activity of acetylene hydrogenation to form ethylene,the order is Pd₉Cu₀>Pd₃Cu₆>Pd₆Cu₃>Pd₁Cu₈>Cu?111?.For the surface Pd-modified Cu catalysts,the activity increases with the increase of Pd atoms number,and Pd₁Cu₈ has the highest selectivity.Thus,Pd₁Cu₈ catalyst is the best catalyst for the surface Pd-modified Cu catalyst.b)For the Pd_sub-modified Pd₁Cu₈ catalysts,different Pd_sub:Cu ratio and Pd atom connection models affect the ethylene selectivity and the activity clearly.For the ethylene selectivity(?G_a/kJ·mol^-1),when the number of Pd_sub is 1,Pd_sPd_subCu?32.2?>Pd_sPd_subCu-I?31.6?,the connection model of Pd_s and Pd_sub has little effect on the selectivity of ethylene,when the number of Pd_sub is 2,Pd_s2Pd_subCu?45.8?>Pd_s2Pd_subCu-I?21.9?>Pd_s2Pd_subCu-II?19.7?,the connection model of Pd_s and Pd_sub affects the selectivity of ethylene clearly,when the numberofPd_subis3,Pd_s3Pd_subCu-II?41.4?>Pd_s3Pd_subCu-III?36.0?>Pd_s3Pd_subCu?21.4?>Pd_s3Pd_subCu-I?7.3?,the connection model of Pd_s and Pd_subub affects the selectivity significantly,on the other hand,with the increase of the sublayer CuPd ratio,the volcanic curve appears on the selectivity of ethylene,the maximum value appears at Pd_s2Pd_subCu.For the activity(r/s^-1·site^-1)of sublayer Pd-modified Pd₁Cu₈ catalysts,when when the number of Pd_sub is 1,Pd_sPd_subCu?1.06×10⁹?>Pd_sPd_subCu-I?3.85×10²?,the connection model of Pd_s and Pd_sub affects the activity significantly,when the number of Pd_sub is 2,Pd_s2Pd_subCu?9.78×10⁵?>Pd_s2Pd_subCu-II?5.07×10⁰?>Pd_s2Pd_subCu-I?4.93×10⁰?,the connection model of Pd_s and Pd_sub affects the activity significantly,when the number of Pd_sub is 3,Pd_s3Pd_subCu-I?6.30×10¹?>Pd_s3Pd_subCu?9.73×10⁰?>Pd_s3Pd_subCu-II?2.57×10⁰?>Pd_s3Pd_subCu-III(7.00×10^-1),the connection model of Pd_s and Pd_sub affect the activity slightly,when the number of Pd_sub is 6,the activity on Pd_s6Pd_subCu is 1.00×10¹ which close to the surfaces which have three Pd_sub atoms.Thus,increasing with the number of sublayer Pd_sub atoms,the effect of Pd atoms connection mode on the activity are decreased.Therefore,for the surface and sublayer Pd-doped Cu catalysts,the doped promoter Pd in the surface is more favorable for increasing the ethylene selectivity,and the largest selectivity appears on the Pd₁Cu₈.Doping Pd into the sublayer is more beneficial to increase the activity,and the highest ethylene selectivity appears on the Pd_s2Pd_subCu catalyst which the surface Pd_s connects to2 sublayer Pd_sub atoms.Therefore,the interaction between the surface Pd_s and the sublayer Pd_sub increased the selectivity and activity of Pd modified Cu catalyst for the hydrogenation of acetylene to form ethylene.?4?Acetylene and the hydrogenation ofethylene to form ethane are the important factors that affects the stability and catalytic performance for hydrogenation of acetylene to form ethylene,during the hydrogenation of acetylene,Cu catalyst size,a second metal doping and doping ratio canregulate the activity of the catalyst and the selectivity of ethylene to achieve the purpose of efficient removal of trace acetylene in ethylene.In this paper,the catalytic performance of the catalyst is improved by the catalyst modification,and the basic theoretical clues and methods are provided for the design and modification for high efficiency Cu catalyst.