SiO₂ Supported Transition Metal Catalysts For The Selective Hydrogenation Of Biphenyl

With the rapid development of modern economic society,the consumption of fossil fuels,mainly including coal,oil and natural gas,has dramatically increased.Aromatic hydrocarbons with carcinogenic effects are produced unexpectedly during the use of fossil fuels.Therefore,reducing the emission of aromatics is of great significance.Hydrogenation dearomatization is one of the most promosing methods to reduce the emissions of the aromatic organic compounds in the industrial processes.Compared with other aromatic hydrocarbons,the hydrogenation of biphenyl is difficult,but the hydrogenated products,cyclohexylbenzene and bicyclohexane,are high addedvalue.In particular,cyclohexylbenzene,as an important chemical intermediate,has attracted widespread attention.Accordingly,this thesis designed a cheap and efficient Ni based catalyst with excellent cycle performance for the selective hydrogenation of biphenyl to cyclohexylbenzene.The detailed investigation included the following parts:A series of Ni/SiO2 catalysts with different loadings were prepared by the overvolume impregnation method,and the effects of Ni loadings on the selective hydrogenation of biphenyl to cyclohexylbenzene were studied.The catalysts were characterized by XRD,N2 physisorption,H2-TPR,SEM,and TEM techniques.The results showed that with the increase of Ni loading,the specific surface area and metal particle size of the catalyst gradually increased.The evaluation results showed that when the loading amount of Ni was 5 wt.%～15 wt.%,biphenyl conversion and cyclohexylbenzene yield gradually increased with the increase of the loading,but the prolonged reaction time i caused the overhydrogenation of cyclohexylbenzene,leading to a decrease in cyclohexylbenzene selectivity.When Ni loading was 20 wt.%and 25 wt.%,biphenyl conversion achieved to 99.5%and cyclohexylbenzene yield to 99.3%.Since there were more active sites on the 25%Ni/SiO2 catalyst,cyclohexylbenzene yield over 25%Ni/SiO2 catalyst decreased more significantly than that over 20%Ni/SiO2 catalyst when enhanced the reaction time.Furthermore,the influences of reaction conditions(solvent,reaction temperature and H2 pressure)on the performance of 25%Ni/SiO2 were explored.The cycle stability revealed that 25%Ni/SiO2 catalyst possessed a high biphenyl conversion(99.8%)and cyclohexylbenzene selectivity(99.4%)even after eight cycles.Furthermore,this paper examined the influence of the support specific surface area on the performance of Ni/SiO2 catalysts for the selective hydrogenation of biphenyl.The catalysts were characterized by XRD,N2 physisorption,H2-TPR and TEM techniques.The results showed that as the support specific surface area increased,the Ni particle size gradually decreased.The evaluation experiments showed that when the specific surface area was 300 m2 g-1,the catalytic performance of the catalyst was the best.At the same time,the effects of active metal species on the performance of the selective hydrogenation of biphenyl to cyclohexylbenzene were explored.The results showed that the selectivity and conversion of biphenyl over the Fe-based and Co-based catalysts were both low.Cyclohexylbenzene yield over the Cu-based catalyst gradually increased from 2.7%(60 min)to 43.0%(240 min)with the extension of the reaction time,which were rather lower than that over the Ni-based catalyst.Based on the abovementioned results,it might be concluded that the Ni-based catalyst was expected to be an efficient and stable catalyst for the selective hydrogenation of biphenyl to cyclohexylbenzene for the industrial application.