Synthesis Of Ni-based Catalysts With Spinel Structure And Its Hydrogenation Saturation Performance Of Phenanthrene

Polycyclic aromatic hydrocarbons（PAHs）is abundant in coal tar.By hydrogenation saturation into cycloalkanes,the combustion performance can be significantly improved to prepare high-quality jet fuel.As a typical PAHs compound,phenanthrene（PHE）was served as model compound in this work.Though the hydrogenation process from phenanthrene to perhydrophenanthrene is thermodynamicly feasible,the saturation of symmetric octahydrophenanthrene becomes the rate determining step.In order to solve this problem,the key point is to develop a kind of saturation catalysts with high activity,which can overcome steric hindrance and promote the hydrogenation of symmetric octahydrophenanthrene（s-OHP）.As the most widely used hydrogenation catalysts in industry,the transition-metal sulfide catalysts have good sulfer resistance,but an insufficient saturation activity.The noble metal catalysts possess high hydrogenation activity but limited by its high cost.Therefore,developing a kind of Ni-based catalyst with high hydrogenation activity is of great significance.By analyzing the adsorption mechanism of aromatics on transition-metal,it is known that the formation of metallic electron-deficient structure is beneficial to the adsorption and activation of aromatics.In addition,increasing the dispersion of Ni can expose more active metals,providing more active sites.In order to achieve the above goals,we introduced nickel-aluminum spinel structure with strong interaction between metal and support（SMSI）to promote the formation of metallic electron-deficient structure and improve Ni dispersion.Furthermore,we introduced bimetallic interaction to improve the stability of electronic structure of active metal Ni.The primary contents and conclusions of this work were summarized as follows:（1）A series of Ni-based ctalysts with nickel-aluminum spinel structure were synthezised by modified sol-gel method with citric acid.By adjusting the calcination temperature,the occurance sites of Ni²⁺species in catalysts can be regulated,which influenced the hydrogenation performance.By evaluated at 300°C,hydrogen pressure of 5 MPa,WHSV of 52h^-1,the initial phenanthrene conversion and perhydrophenanthrene selectivity reached 95%over the reduced Ni/NiAlO_x catalysts.The observed reaction rate(r_obs)and turnover frequency（TOF）over the Ni/NiAlO_x-650 catalysts calcinated at 650°C showed high value of 1.53×10^-3 mol·kg^-1·s^-1 and 14.64×10^-3 s^-1 respectively.This phenomenon indicated that phenanthrene saturation can be achieved efficiently over Ni-based catalysts prepared by this method.Through characterization,there were three different types of Ni²⁺species in Ni/NiAlO_x catalysts after calcination:Ni O in the free form,Ni²⁺occupying octahedral sites and tetrahedral sites of the NiAl₂O₄ spinel phase.When calcinated at 650°C,around 98%nickel entered into octahedral sites in spinel structure,leaving little occurrence in tetrahedral sites or free Ni O.This structure improved the Ni distribution and resulted in SMSI after reduction,forming a metallic electron-deficient state,which improved the adsorption of aromatics.Furthermore,the investigation of s-OHP hydrogenation indicated that the metallic electron-deficient state can overcome the steric hindrance,improving s-OHP hydrogenation.（2）During the reaction process,the perhydrophenanthrene selectivity decreased from 98%to 40%over Ni/NiAlO_x-650 catalysts after phenanthrene hydrogenation for 8 h.Compared with the fresh catalysts,the increase of metallic electron-deffcient state in spent atalysts was unfavourable for the formation ofπback bond,decreasing the perhydrophenanthrene selectivity.Therefore,improving the stability of metallic electronic structure over Ni-based catalyst is the key point to further improve its hydrogenation activity.（3）Based on the Ni/NiAlO_x-650 catalysts,bimetallic interaction was introduced by impregnation of Pt to improve the stability of electron-defficient state of Ni nanoparticles.It was found that the Ni²⁺distribution in Pt-Ni/NiAlO_x catalysts after calcination didn`t change obviously by impregnation of Pt,which was mostly occupied in octahedral sites of nickel alluminate.The optimal loading amont of Pt was 0.5 wt%.By evaluated at 300°C,hydrogen pressure of 5 MPa,WHSV of 52 h^-1,the perhydrophenanthrene selectivity over 0.5Pt-Ni/NiAlO_xcatalysts reached 67%,which maintained 69%of initial activity.From XPS analysis,this phenomenon can be attributed to the improved stability of Ni electronic structure.The introduction of 0.5 wt%Pt relieved the increase of electron deficiency state during reaction.In addition,the intrinsic activity of 0.5Pt-Ni/NiAlO_x catalysts(r_obs=1.81×10^-3 mol·kg^-1·s^-1 and TOF=22.16×10^-3 s^-1)was higher than the sum value of Ni/NiAlO_x-650 catalysts and 0.5wt% Pt/Al₂O₃ catalysts.