Dependency of phenol hydrogenation activity on hydrogen chemisorption over supported palladium catalysts

A series of catalysts containing 1 wt.% Pd supported on various oxides of commercial origin have been characterised by hydrogen adsorption at 298 K and evaluated for vapour-phase phenol hydrogenation at 473 K employing an H/phenol ratio of 5.4 at atmospheric pressure. The activity of phenol hydrogenation showed a dependence on the irreversible uptake of hydrogen. Among the catalysts studied, Pd/Al2O3 showed a decrease in activity with increasing temperature while Pd/MgO passed through a maximum at 503 K. The Pd/MgO catalysts exhibited a higher dispersion of smaller Pd crystallites leading to higher activity and total selectivity for cyclohexanone. The higher resistance towards deactivation and better stability of the Pd/MgO catalysts suggest that the acidic sites of alumina may be responsible for deactivation, thereby accounting for the lowering of activity with Pd/Al2O3 catalysts.