Gas phase reaction of hydrogen with carboxyl and carbonyl functions in aromatic systems over Ni/SiO2

The hydrogen treatment of methanolic and 2-propanolic solutions of benzoic and o-toluic acid in the gas phase was studied over a Ni/SiO2 catalyst. The reactions were conducted in the absence of diffusion limitations and repro ducible turnover frequencies are presented. The products generated resulted from the hydrogenolysis and esterification (in the case of methanolic solu tions) of the substituent -COOH group and from the hydrogenolytic cleavage of the aryl-carboxyl C-C bond where the aromatic ring remained intact. Este rification proceeded at a greater rate on the pure silica support but was n ot observed when 2-propanol was used as solvent; apparent activation energi es for the esterification step are given. The temperature dependencies of t he specific rates and product selectivities are shown and these reveal the increasing predominance of hydrogen cleavage as the reaction temperature wa s increased. The presence of the ortho-substituted methyl group served to l ower the reactivity of the carboxyl function due to steric and electronic e ffects which are discussed in terms of reactant(s) adsorption/activation. T he reaction of methylbenzoate and methyl 2-methylbenzoate over the same cat alyst was investigated and the overall reaction pathway is identified. The reactivity of the carbonyl group in aromatic aldehydes under identical reac tion conditions was found to be considerably higher where substituent hydro genolysis rather than substituent cleavage was promoted to a greater degree . The response of both carbonyl and carboxyl groups in the transformation o f 2-carboxybenzaldehyde is used to further illustrate the higher reactivity of the carbonyl group. (C) 1999 Elsevier Science B.V. All rights reserved.