Catalytic hydrogen treatment of aromatic alcohols

The gas phase hydrogenation/hydrogenolysis of phenol over the temperature range 423 K less than or equal to T less than or equal to 573 K has been studied using a 1.5% w/w Ni/SiO2 catalyst. The effects on reaction rate and product selectivity of varying such process variables as reaction time and temperature, contact time, phenol and hydrogen partial pressures, and the phenol solvent were considered. Hydrogenation was observed to occur in a stepwise fashion where the selective formation of the intermediate, cyclohexanone, was promoted at low hydrogen partial pressures and high temperatures, while Hydrodeoxygenation to benzene proceeded at temperatures in excess of 523 K. The catalytic hydrogen treatment of cyclohexanone and cyclohexanol, under identical reaction conditions, was examined in order to establish the reaction mechanism. The observed conversions and selectivities are compared to those calculated from equilibrium constant data and it is shown that reaction equilibria are prevalent on the catalyst surface where the equilibrium is shifted to complete hydrogenation to cyclohexanol. The overall reaction is first order in phenol and the apparent dependency on hydrogen concentration increased with increasing temperature. Process selectivity is interpreted in terms of reactant/catalyst interactions where phenol adsorption is viewed as occurring via the aromatic pi-electron system and/or the hydroxyl substituent where the latter interaction promotes hydrogenolysis as a direct loss of selectivity with respect to cyclohexanone formation. The hydrogen treatments of the three isomers of cresol were also considered for comparative purposes where the turnover frequency decreased in the order phenol greater than or equal to m-phenol > p-cresol > o-cresol; reactivity and selectivity trends are accounted for in terms of electronic and steric effects. (C) 1998 Academic Press.