Liquid-phase benzene hydrogenation on Pd/eta-Al2O3 catalysts was studied be
tween 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 mode
l 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 li
quid-phase hydrogenation between 358 and 493 K and at 54-atm H-2 pressure.
A model that invokes dissociative, noncompetitive H-2 adsorption addition o
f the first H atom as the rate-determining step, and a concurrent dehydroge
nated benzene surface species was consistent with experimental observations
The fitted parameters were thermodynamically consistent and agreed with th
ose 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 press
ure of hydrogen and is independent of the nature of the solvent, Furthermor
e, when solvent effects exist, use of the liquid-phase hydrogen concentrati
on in the rate expression yields a solvent-independent adsorption equilibri
um constant, while in the absence of solvent effects, P-H2 should be used i
n the rate expression to acquire this parameter.