Preparation And Structure-activity Relationship Of Bimetallic Nickel-based Catalyst For Hydroisomerization

With people's environmental awareness improving,the environmental regulations of clean fuel become more and more rigorous.Gasoline,as a kind of staple fuel,is widely used in the engine of automobiles,then the olefin and aromatics components are strictly restricted in the gasoline.Therefore,it is important to produce the high quality and high-octane gasoline in the petroleum industry.Hydroisomerization of n-alkanes,as a valuable process to obtain clean and high-octane gasoline,is attracted by more and more science researchers.Recently,the noble metals,such as Pt and Pd,are usually provided as the metal sites for the hydroisomerization catalysts.However,the noble metals have high cost and limited resources,which restricts their widely industrial application.Therefore,it is valuable to develop a non-noble metal catalyst with low-cost,high activity,and high selectivity for hydroisomerization.Recently,the researchs show that the nickel-based catalyst has high cost performance,but nickel exhibits high activity for hydrogenolysis.Hydrogenolysis,a major side reaction,sacrifices the selectivity of isomerization in hydroisomerization.Thus,restraining the activity of hydrogenolysis becomes more important for the nickel based catalyst in the hydroisomerization process.In this paper,in view of the high activity of hydrogenolysis for nickel,we introduced a second metal(M)to prepare the bimetallic Ni-M based catalysts.The effects of metal modifier,the amount of the second metal,and the preparation methods on the physicochemical properties and catalytic performance of the bimetallic catalysts were studied.The structure-activity relationship between the bimetallic Ni-M/SAPO-11 and catalytic performance was revealed.Hydroisomerization of n-octane was selected as a model reaction,and the reaction parameters were assessed and optimized.The Ni-based and Ni-M based catalysts were assessed at 340 °C,2.0 MPa,H2/n-octane(V/V)ratio of 400,and different weight hourly space velocities(WHSVs).The results suggested that the bimetallic Ni-Cu,Ni-Sn/SAPO-11 catalysts effectively restrained the hydrogenolysis and improved the selectivity of isomerization comparing with the monometallic Ni/SAPO-11 catalyst.The preparation methods of the Ni-Cu/SAPO-11 catalysts were studied,and the co-impregnation method was suggested to be a potential method for the hydroisomerization catalysts.This paper provides a reference for design and industrial application of non-noble catalysts for hydroisomerization,and the research contents are shown as follows.We synthesized SAPO-11 as the support of the catalysts using hydrothermal crystallization method.Four bimetallic Ni-M(M= Cu,Zn,Co,and Sn)/SAPO-11 catalysts were prepared,and the effects of the four metal modifiers on the physicochemical properties and catalytic performance of the catalysts were compared.The suitable metal modifiers were selected from the four metals.The results showed that the synergistic effect between Ni and the second metals altered the metallic properties of Ni.The electronic effect existed in the Ni-M intermetallic compound.Cu,Sn,and Zn,as the electron providers,transferred the electron charge to Ni,improving the electron density of Ni atoms and altering the metallic properties of Ni.The added Co hardly provided electron charge to Ni,and decreased the electron density of Ni atoms.The results of the catalysts assessment showed that the added Co improved the hydrogenolysis activity of Ni,decreasing the selectivity of isomerization.The added Zn slightly restrained the hydrogenolysis reaction and improved the selectivity of isomerization.The added Cu and Sn effectively restrained the hydrogenolysis reaction and increased the selectivity of isomerization.Therefore,Cu and Sn were the appropriate metals to modify the metallic properties of Ni to restrain the hydrogenolysis reaction.The bimetallic NiSn/SAPO-11 catalysts with different Ni/Sn mass ratio were prepared.The effects of the Ni/Sn mass ratio on the catalytic activity and selectivity of the catalysts were discussed.The structure-activity relationship between the bimetallic Ni-Sn alloying effect and catalytic performance was revealed.The results showed that the Ni-Sn alloy was formed in the bimetallic NiSn/SAPO-11 catalysts.The added Sn “diluted” the nickel ensemble size and altered the electronic property of nickel.Then the metallic properties of Ni were changed and the hydrogenolysis reaction was restrained.When the Ni/Sn mass ratio reached 4.5,the bimetallic 3NiSn/SAPO-11 catalyst exhibited higher selectivity of isomerization than the monometallic 3Ni/SAPO-11 catalyst.The monometallic Ni/SAPO-11 catalyst was modified by adding a second metal Cu,then the bimetallic NiCu/SAPO-11 catalysts with different Ni/Cu mass ratio were prepared.The influence of the Cu addition on the physicochemical properties and catalytic performance of the catalysts was discussed.The results showed that the added Cu and Ni formed Ni-Cu alloy.The ensemble effect and the electronic effect of Ni-Cu alloy decreased the ensemble size of Ni and transferred the electron charge to Ni,respectively,which improved the electron density of Ni atoms and altered the metallic properties of Ni.When the Ni and Cu loading were 3 wt% and 1.5 wt%,respectively,the hydrogenolysis reaction was effectively restrained.The bimetallic 3Ni1.5Cu/SAPO-11 catalyst also exhibited high selectivity of isomerization comparing with the noble Pt/SAPO-11 catalyst.Besides,compared with the bimetallic NiSn/SAPO-11 catalyst,the bimetallic NiCu/SAPO-11 catalyst showed higher activity and selectivity of isomerization.The effect of the methods of Cu addition on the the physicochemical properties catalytic performance of the catalysts was investigated.Six bimetallic NiCu/SAPO-11 catalysts were prepared by co-impregnation,sequential impregnation,co-precipitation,and mechanical mixing methods.The effect of the preparation methods on the catalytic performance of the catalysts was compared.The results showed that Ni-Cu alloy was formed in the catalysts prepared by the impregnation and co-precipitation methods.The Ni and Cu atoms were homogeneously dispersed in the catalyst prepared by co-impregnation,while the Ni and Cu atoms were not uniformly dispersed in the catalyst prepared by sequential impregnation.The catalyst prepared by co-precipitation exhibited the smallest metal particles size,but its nickel species had strong interaction with the support.The catalyst prepared by mechanical mixing of NiO and CuO hardly formed Ni-Cu alloy.Besides,the preparation methods also affected the balance between the metal and acid sites.The catalysts' assessment results showed that the formed Ni-Cu alloy effectively restrained the hydrogenolysis reaction.For the different methods of the Cu addition,the catalyst prepared by co-impregnation exhibited both high catalytic activity and selectivity to n-octane isomers among all the catalyst.