Theoretical density functional study of the hydrogenation of maleic acid over Pd and Re surfaces

Non-local density functional (DFT) quantum chemical calculations were used to calculate energies of reaction (ΔE(r x n)), enthalpies of reaction (ΔH(r x n)) and free energies of reaction (ΔG(r x n)) for vapor-phase C = C bond hydrogenation of maleic acid, ethylene, 1,2-ethenediol, vinyl alcohol, 1,1-difluoroethylene and acrolein. The DFT-computed reaction enthalpies were found to be within 10-20 kJ/mol of experiment. While the SCF-based energies of reaction (ΔE(r x n)) were observed to systematically overestimate the enthalpies of reaction (ΔH(r x n)), the calculated correction term was 29 ± 1 kJ/mol for all the reactions of the present homologous set. The computed di-σ binding energy of maleic acid on the Pd(1 1 1) and Re(0 0 0 1) surfaces were found to be - 48 and - 107 kJ/mol, respectively. These values are consistent with expected trends from experiment. The di-σ adsorption of maleic anhydride on Pd(1 1 1), at two different surface coverages, Θ = 0.11 and 0.2 ML, were examined to determine the effect of lateral interactions between the adsorbates. Lateral repulsive interactions predominate as the coverage of di-σ bound maleic anhydride on Pd is increased from Θ = 0.11 to 0.2 ML. This results in significant decrease of the binding energy from - 84 kJ/mol to - 12 kJ/mol. The role of solvent on maleic acid hydrogenation kinetics was investigated, using the β-hydride elimination of ethyl on a Pd₂ cluster as a model for C=C bond hydrogenation. An activation barrier of + 75 kJ/mol and an overall energy of reaction of + 15 kJ/mol for vapor-phase β-hydride elimination of ethyl on the Pd dimer were found to be in reasonable agreement with our earlier results on larger Pd₁₉ clusters (Neurock, 1997). The solvent medium, modeled as a dielectric continuum (ε = 20) surrounding a solute cavity of radius 10 Å, was observed to decrease the activation barrier by less than 1 kJ/mol and decrease the overall endothermicity by 9 kJ/mol, for the β-hydride elimination reaction.