Kinetic and thermodynamic analysis of liquid-phase benzene hydrogenation

Liquid-phase benzene hydrogenation on Pd/eta-Al2O3 catalysts was studied between 358 and 488 K and over an H-2 pressure range of 7 to 54 atm. Kinetic data were obtained under differential reaction conditions that were free of all transport limitations, thus allowing the development of a kinetic model that could be compared to that for the vapor-phase reaction. In contrast to vapor-phase studies at 1 atm, no activity minimum was observed during liquid-phase hydrogenation between 358 and 493 K and at 54-atm H-2 pressure. A model that invokes dissociative, noncompetitive H-2 adsorption addition of the first H atom as the rate-determining step, and a concurrent dehydrogenated benzene surface species was consistent with experimental observations The fitted parameters were thermodynamically consistent and agreed with those from previous vapor-phase studies. In addition, thermodynamic analysis of the quasi-equilibrated hydrogen adsorption process indicates that in the presence of solvent effects, increasing hydrogen solubility in the liquid phase can increase the surface coverage of hydrogen. In the absence of any solvent effect, however, surface coverage depends only on the partial pressure of hydrogen and is independent of the nature of the solvent, Furthermore, when solvent effects exist, use of the liquid-phase hydrogen concentration in the rate expression yields a solvent-independent adsorption equilibrium constant, while in the absence of solvent effects, P-H2 should be used in the rate expression to acquire this parameter.