Hydrogenation of vinylbenzenes on Pd–Tm/Al2O3 and Tm/Al2O3 catalysts

Abstract

Pd–Tm/γ-Al ₂ O ₃ catalysts containing 0.1% Pd–1% Tm, 0.3% Pd–1% Tm and 0.5% Pd–1.% Tm, respectively have been obtained by successive impregnation of γ-Al ₂ O ₃ , first with palladium and then with thulium. Introduction of Tm leads to an increase in the conversion of vinylbenzenes compared to that with monometallic Pd/γ-Al ₂ O ₃ catalysts. For monocomponent Pd/γ-Al ₂ O ₃ catalysts, turnover frequency (TOF) is unchanged. For bicomponent Pd–Tm/γ-Al ₂ O ₃ catalysts, if one considers that the reaction occurs only on the Pd particles, then introduction of Tm seems to increase the TOF. However, by taking Tm into consideration when calculating the TOF, then the bicomponent catalysts are seen to behave as the monocomponent catalysts. This behaviour is highlighted also by using the Koros–Nowak criterion. If Tm is not considered to be an active species, vinylbenzene hydrogenation appears to be controlled by mass transfer processes. However, the texture characteristics of the bicomponent catalysts are not sufficiently modified by Tm introduction to support such a hypothesis. On the contrary, by considering the Tm participation in the reaction, the slope of the Koros–Nowak dependence is included in a range typical for kinetically controlled reactions.