Competitive hydrogenation of alkyl-substituted arenes by transition-metal nanoparticles: Correlation with the alkyl-steric effect

The influence of substituents on rate constants of the hydrogenation of monoalkylbenzenes by transition metal nanoparticles or by classical heterogeneous catalysts can be rationalized in terms of the Taft rule. A series of the initial reaction rate constants obtained from various competitive toluene/benzene and toluene/monoalkylbenzene hydrogenation experiments catalyzed by transition-metal nanoparticles prepared in the presence of imidazolium ionic liquids or surfactants [Ir(0), Rh(0) and Ru(0)] or by classical heterogeneous catalysts (PtO2, Rh/C, Rh/Al2O3, Ru/C, Ru/Al2O3 and Pd/C) have been correlated with the Taft equation (log k(B)K(B)/k(A)K(A) = rho E-s). Satisfactory correlation coefficients (r) (between 0.96 and 0.99) and positive slopes (p) between 0.38 and 0.83 have been obtained. The results clearly show that the reaction constants for the alkyl-substituents can be expressed by steric factors and are independent of any other nonsteric factors. It is suggested that bulky alkylbenzene substituents, for both transition metal nanoparticles and classical heterogeneous hydrogenation reactions, lower the overall hydrogenation rate, implying a more disturbed transition state compared to the initial state of the hydrogenation (in terms of the Horiuti-Polanyi mechanism). This competitive method is suitable for the estimation of the constant selectivity for couples of alkylbenzenes in which the difference in hydrogenation rates are very high and experimentally difficult to measure and also useful for the design of more selective "nano" and classical catalysts for hydrogenation reactions.