KINETICS OF M-XYLENE HYDROGENATION ON NI AL2O3/

Gas phase hydrogenation of m-xylene to cis- and trans-1,3-dimethylcycl ohexane, partially saturated 1,3-dimethylcyclohexene-1 being a by-prod uct, was studied in a differential microreactor at atmospheric pressur e and 418-493 K over a Ni/Al2O3 catalyst. Rapid reversible deactivatio n of the freshly reduced catalyst prompted for reactivation before eve ry kinetic measurement and extrapolation of the steady-state activity to initial activity. Reaction orders were close to zero (up to 0.2) fo r 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 observ ed at about 450 K, varying with the pressure ratio of the reactants. M echanistic modelling of the surface reaction steps, including formatio n of the cyclic olefin, and thermodynamic aspects suggested that the r eaction rate is governed by three steps of pair-wise sequential additi on of hydrogen atoms to adsorbed m-xylene. The partially hydrogenated surface intermediates in the first two addition steps were proposed to have aromatic character.