Kinetics of m-xylene hydrogenation on Ni/Al2O3

Gas phase hydrogenation of m-xylene to cis- and trans-1,3-dimethylcyclohexane, partially saturated 1,3-dimethylcyclohexene-1 being a by-product, was studied in a differential microreactor at atmospheric pressure and 418-493 K over a Ni/Al2O3 catalyst. Rapid reversible deactivation of the freshly reduced catalyst prompted for reactivation before every kinetic measurement and extrapolation of the steady-state activity to initial activity. Reaction orders were close to zero (up to 0.2) for rn-xylene and ranged from 0.8 (418 K) to 2.6 (493 K) for hydrogen. A rate maximum for the production of the saturated compounds was observed at about 450 K, varying with the pressure ratio of the reactants. Mechanistic modelling of the surface reaction steps, including formation of the cyclic olefin, and thermodynamic aspects suggested that the reaction rate is governed by three steps of pair-wise sequential addition of hydrogen atoms to adsorbed m-xylene. The partially hydrogenated surface intermediates in the first two addition steps were proposed to have aromatic character.