DIRECT HYDRODEOXYGENATION AND HYDROGENATION OF 2,6-DIMETHYLPHENOL AND 3,5-DIMETHYLPHENOL OVER SULFIDED COMO CATALYST

Hydrodeoxygenation of 2,6- and 3,5-dimethylphenol (DMP) over a sulphided CoMo/alumina catalyst was carried out at 5 MPa H-2 pressure and over a temperature range of 240 degrees C to 330 degrees C. For 3,5-DMP, the major reaction product was m-xylene, whereas for 2,6-DMP, about equal amounts of m-xylene and 1,3-dimethylcyclohexane were obtained. The reaction is considered to proceed through two pathways: (1) direct cleavage of the C(aromatic)-O bond leading to an aromatic product; and (2) prehydrogenation of the phenol ring, followed by rapid C(aliphatic)-O rupture leading to a saturated cyclic compound. For path (1), 2,6-DMP was approximately ten times less reactive than 3,5-DMP, which was explained in terms of steric hindrance to adsorption by the methyl groups adjacent to the OH group. For path (2), 3,5-DMP was more reactive than 2,6-DMP at temperatures below 290 degrees C, but less reactive above this temperature. The latter results were interpreted in terms of differences in delocalized resonance effects, in which adsorption and intrinsic hydrogenation reactivity compete in opposite directions to yield the different global rates versus temperature.