Gas phase reaction of hydrogen with carboxyl and carbonyl functions in aromatic systems over Ni/SiO2
✍ Scribed by Mark A Keane
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 248 KB
- Volume
- 138
- Category
- Article
- ISSN
- 1381-1169
No coin nor oath required. For personal study only.
✦ Synopsis
The hydrogen treatment of methanolic and 2-propanolic solutions of benzoic and o-toluic acid in the gas phase was studied over a NirSiO catalyst. The reactions were conducted in the absence of diffusion limitations and reproducible 2 Ž turnover frequencies are presented. The products generated resulted from the hydrogenolysis and esterification in the case of . methanolic solutions of the substituent -COOH group and from the hydrogenolytic cleavage of the aryl-carboxyl C-C bond where the aromatic ring remained intact. Esterification proceeded at a greater rate on the pure silica support but was not observed when 2-propanol was used as solvent; apparent activation energies for the esterification step are given. The temperature dependencies of the specific rates and product selectivities are shown and these reveal the increasing predominance of hydrogen cleavage as the reaction temperature was increased. The presence of the ortho-substituted methyl group served to lower the reactivity of the carboxyl function due to steric and electronic effects which are discussed in terms Ž . of reactant s adsorptionractivation. The reaction of methylbenzoate and methyl 2-methylbenzoate over the same catalyst was investigated and the overall reaction pathway is identified. The reactivity of the carbonyl group in aromatic aldehydes under identical reaction conditions was found to be considerably higher where substituent hydrogenolysis rather than substituent cleavage was promoted to a greater degree. The response of both carbonyl and carboxyl groups in the transformation of 2-carboxybenzaldehyde is used to further illustrate the higher reactivity of the carbonyl group.