Microcalorimetric adsorption and infrared spectroscopic studies of supported Pd, Ru and Pd–Ru catalysts for the hydrogenation of aromatic rings with carboxyl groups

Abstract

The Ru/SiO₂, Pd/SiO₂ and Pd–Ru/SiO₂ catalysts (5% wt) were prepared for the hydrogenation of benzoic acid (BA) and toluene (without and with propionic acid (PA)) to study the effects of –COOH groups on the hydrogenation of aromatic rings. The results showed that PA adsorbed strongly on the catalysts and inhibited the adsorption of toluene and H₂ and thus led to the significant decrease in the conversion of aromatic rings. The strong dissociative adsorption of PA might be responsible for the low selectivity to cyclohexanecarboxylic acid (CCA) on the Ru/SiO₂. Particularly, the adsorption heat of PA was the lowest while that of H₂ was the highest on the Pd–Ru/SiO₂ in the catalysts, indicating its special surface chemical properties that allowed it to adsorb toluene and H₂ still quite strongly even with the pre-adsorbed PA and thus to be highly active and selective for the hydrogenation of BA to CCA. The FTIR spectra of co-adsorbed toluene and H demonstrated that the weakly adsorbed H was highly active for the hydrogenation of toluene on the Ru/SiO₂ even at room temperature, and the higher activity of Ru than Pd for the hydrogenation of aromatic rings might be due to the stronger adsorption of toluene and weaker adsorption of H₂ on Ru than on Pd.