Study On The Hydrogenation Of Biphenyl To Cyclohexylbenzene

Cyclohexylbenzene is widely used as lithium ion battery additive to protect the battery from overcharge.It can also be used to produce liquid crystal materials.In this paper,the reaction mechanism and gas-liquid-solid three-phase reaction characteristics of biphenyl hydrogenation into cyclohexylbenzene were studied using catalyst A in a high pressure stirred tank reactor.The influences of solvent,H2 partial pressure,reactant concentration and temperature on the hydrogenation reaction of biphenyl and cyclohexylbenzene were firstly investigated in the low concentration solutions.Then,the effects of temperature and H2 partial pressure on reaction rate and selectivity were observed using pure biphenyl and cyclohexylbenzene as reactants,and the reactionmechanism and behavior were also discussed.The results were summarized as follows:Firstly,biphenyl liquid hydrogenation reactions were carried out in different solvents.It is found that solvent has great influence on reaction rate while has little effect on cyclohexylbenzene selectivity.The influence of solvents embodies in the following aspects:(i)polar solvents are easier to be adsorbed on catalyst surface so inhibit the adsorption of reactants;(ii)solvents with relatively flatter molecular configurations are easier to be adsorbed to block active sites.(iii)proton solvents can decompose to produce H atom,which will improve hydrogen transmission efficiency,so that the reaction rate will increase.Secondly,in order to eliminate the disturbing of internal and external diffusion,the influences of stirring speed and diameter of catalyst particle were studied.It is found that,(i)the mixing of gas-liquid-solid three-phase and the conversion of biphenyl increases with increasing stirring speed.When stirred at 1200 r/min,the catalyst particles and H2 bubbles are well dispersed in liquid phase.The conversion of biphenyl will not increase further when stirring speed is above 1200 r/min.(ii)The conversion of biphenyl increases with decreasing catalyst diameters while the selectivity of cyclohexylbenzene keeps the same.When the catalyst diameter is smaller than 0.3mm,the conversion of biphenyl do not change.Thirdly,the effects of temperature,H2 partial pressure and reactant concentration on hydrogenation of biphenyl and cyclohexylbenzene were researched using cyclohexane as solvent.It is found that:(i)the hydrogenation reaction rates of biphenyl and cyclohexylbenzene increase with increasing temperature,while the selectivity of cyclohexylbenzene has no change.The apparent activation energies for biphenyl and cyclohexylbenzene hydrogenation are approximately the same,which equal to 60kJ/mol.(ii)the reaction rate of biphenyl hydrogenation increases and then decreases with increasing H2 partial pressure.The same regularity is found in the cyclohexylbenzene hydrogenation process but the corresponding H2 partial pressure at highest reaction rate is higher than that during biphenyl hydrogenation.The selectivity of cyclohexylbenzene decreases with increasing H2 partial pressure.(iii)the hydrogenation rate increases and then decreases with increasing biphenyl or cyclohexylbenzene concentration.The selectivity of cyclohexylbenzene keeps the same at different biphenyl initial concentrations.Fourthly,the reaction properties and mechanism were analyzed using Langmuir-Hinshelwood theories.It is presumed that:(i)the biphenyl hydrogenation rate is determined by reaction on catalyst surface at low reactant concentration.There exists competitive adsorption between hydrogen and biphenyl molecular,which leads to a low reaction rate at very high H2 partial pressure.(ii)There exists competitive adsorption between biphenyl and cyclohexylbenzene.The adsorption constant of biphenyl is higher than that of cyclohexylbenzene.The reaction rate equation for biphenyl liquid phase hydrogenation can be given as:The reaction rate equation for cyclohexylbenzene liquid phase hydrogenation can be expressed as:(iii)The determining step for biphenyl hydrogenation may shift to H2 adsorption determination at higher reactant concentration.Finally,the hydrogenation properties for pure biphenyl and cyclohexylbenzene were investigated.It is found that:(i)the liquid phase volume and H2 dissolution increases with increasing conversion,i.e.H2 Henry constants decreases.(ii)the reaction rates for both hydrogenation of pure biphenyl and pure cyclohexylbenzene increase with increasing H2 partial pressure.(iii)the main reaction rate and side reaction rate increase with increasing reaction temperature while the selectivity of cyclohexylbenzene keeps the same.(iv)the determining step for both hydrogenation of pure biphenyl and cyclohexylbenzene may be the adsorption of H2 on catalyst surface.