Gas-phase hydrogenation/hydrogenolysis of phenol over supported nickel catalysts

The gas-phase hydrogenation/hydrogenolysis of alcoholic solutions of phenol between 423 and 573 K has been studied using a Y zeolite-supported nickel catalyst (2.2% w/w Ni) and Ni/SiO2 catalysts (1.5-20.3% w/w Ni). This is a viable means of treating concentrated phenol streams to generate recyclable raw material. Phenol hydrogenation proceeded in a stepwise fashion with cy clohexanone as a reactive intermediate while a combination of hydrogenolysi s and hydrogenation yielded cyclohexane. Hydrogenolysis to benzene is favor ed by high nickel loadings and elevated temperatures. A catalytic hydrogen treatment of cyclohexanone and cyclohexanol helped to establish the overall reaction network/mechanism. The possible role of thermodynamic limitations is considered and structure sensitivity is addressed; reaction data are su bjected to a pseudo-first-order kinetic treatment. Hydrogen temperature-pro grammed desorption (H-2-TPD) has revealed the existence of different forms of surface hydrogen.. Selectivity is interpreted on the basis of the H-2-TP D profiles and the possible phenol/catalyst interactions. The zeolite sampl e only catalyzed (via the surface Bronsted acidity) anisole formation in th e presence of methanol, but this was suppressed when hexanol was used; the zeolite then promoted hydrogenolysis. The zeolite, however, deactivated and this was not reversed by heating in hydrogen. The results of the hydrogen treatment of aqueous rather than alcoholic phenol solutions are presented, where a switch from methanol to water was, accompanied by a move from highl y selective hydrogenolysis to highly selective hydrogenation.