Experimental and computational kinetics study of the liquid-phase hydrogenation of C[dbnd]C and C[dbnd]O bonds

Solvent effects on adsorption equilibrium and reaction kinetics are evaluated for hydrogenation reactions catalyzed by Pd/alumina in a series of different solvents. Three reactants – cyclohexene, benzene and benzaldehyde – and three solvents – n-heptane (HEP), methylcyclohexane (MCHA), decalin (DL) – have been investigated. Kinetic analysis of hydrogenation of cyclohexene reveals that hydrogen adsorbs on different sites from those where cyclohexene and the solvents adsorb; however, the presence of hydrogen on these separate sites affects the heats of adsorption of the hydrocarbons. When the solvent is a weakly interacting linear alkane (HEP), the rate determining step of the reaction is the first hydrogenation of adsorbed cyclohexene. This conclusion is supported by DFT calculations that show a higher enthalpy barrier for the first hydrogenation than for the second, while statistical thermodynamics analysis validates the physical significance of the entropy of adsorption parameters derived from the kinetic fitting of experimental data. By contrast, with solvents such as MCHA and DL, which interact more strongly with the metal surface and compete for active sites with the reactant and the surface intermediate, the rate limiting step seems to shift to the second hydrogenation step. One of the main conclusions of the study is that the effects observed with non-polar solvents are mostly due to competitive adsorption of the solvent, which causes a decrease in surface coverage of reactants and surface intermediates. Indeed, under identical conditions, the hydrogenation rates of CC bonds in cyclohexene (or the aromatic ring in benzene) follow the order HEP > MCHA > DL, which is consistent with the strongest solvent/metal interaction for DL and the weakest for HEP. Further, binary solvent mixtures of DL-HEP exhibit non-idealities that are clearly evident in the fitting of hydrogenation rates. It is proposed that the higher activity coefficient of liquid DL in the liquid mixture enhances its surface coverage, competing more effectively with the reactant, which causes a decrease in catalytic activity larger than that predicted assuming an ideal mixture of solvents. Likewise, large drops in benzene hydrogenation rates are observed in water-organic co-solvent mixtures due to the competitive adsorption of water on the Pd surface. Water adsorption is further enhanced by the non-ideality of the water-organic mixture. By contrast, in the hydrogenation of the CO bond of benzaldehyde, the presence of water provides a favorable alternative reaction path with lower activation barrier, resulting in higher hydrogenation activity when the reactant is a carbonyl-containing molecule, such as benzaldehyde. In this system, the H-bonded water network assists shuttling a proton from the Pd surface to the hydrophilic O atom in CO, assisted by surface charge separation. Since this H transfer pathway cannot take place with CC bonds or aromatic rings, only site inhibition occurs for these reactions.