Abstract
The selective hydrogenation of α,β-unsaturated ketones and aldehydes to unsaturated alcohols is an important step in the synthesis of a number of fine chemical and pharmaceutical intermediates as well as in biomass conversion strategies. The ability to selectively hydrogenate C=O over C=C bonds presents a significant challenge as the hydrogenation of the C=C bond is thermodynamically favored over that of the C=O bond. Several factors including the structure of reactants, the composition and structure of the metal, the reaction environment (gas/liquid phase reactions) and the support, however, can be tuned in order to influence the elementary kinetic processes and control the catalytic activity and selectivity in the hydrogenation of unsaturated aldehydes and ketones.
Periodic density functional theoretical (DFT) studies along with ab initio molecular dynamics and microkinetic simulations were used in this work to explore the selective hydrogenation of α,β-unsaturated ketones and aldehydes over model catalytic systems. A wide range of saturated and unsaturated aldehydes and ketones and transition metal surfaces were examined in order to understand and analyze the influence of molecular structure of reactant, metal catalyst, the support and the solvent used in liquid phase hydrogenation reactions on the hydrogenation of the C=C and C=O bonds.
Single substituents such as methyl groups or aromatic rings attached to the C=C bond of an aliphatic unsaturated ketone weaken the adsorption of the reactant to the metal surface which lowers the rate of C=C hydrogenation, and increases the formation of alkoxide intermediates. The change in a single substituent, however, is not enough to fully reverse the trends in selectivity that favor the formation of the unsaturated alcohol (UA) as there is very little change in the hydrogenation of the alkoxide, which is important in production of desired unsaturated alcohol. The addition of multiple substitutions at the unsaturated C=C bonds on the hydrocarbon backbone such as those found on the highly substituted ring structure of ketoisophorone, can alter the selectivity and enhance the rate of C=O hydrogenation over C=C hydrogenation. The calculated adsorption energies and the intrinsic activation barriers establish the effects of metal and help to understand the negligible selectivity to the unsaturated alcohol in the hydrogenation of benzalacetone reported over supported Pt and Ru catalysts as well as the markedly different selectivities found in the hydrogenation of ketoisophorone over supported Pd and Pt catalysts.
Polar and protic solvents such as water and 10% mixed isopropyl alcohol-water solvent were found to lower the activation barriers for hydrogenation by stabilizing the partial charges that form in the transition states and alter the preferred reaction mechanism by directly participating in the hydrogenation of unsaturated aldehyde, acrolein, on model Pt catalysts. The presence and operability of a solvent-mediated proton shuttling pathway depends on the strength of the hydrogen bonds and is calculated to be dominant for the acrolein hydrogenation in water. Non-polar aromatic solvents such as toluene do not provide hydrogen bonding but can influence the reaction through changing the surface property by blocking the surface sites and thus inhibits the adsorption and ring hydrogenation of benzylacetone reactant. At higher concentrations of toluene, the only available path for hydrogenation is via the C=O bond as the adsorbed toluene blocks access to other and such the selectivity to UA increases greatly.
The catalyst support can also act similar to the solvent thus providing additional functionality to enhance C=O hydrogenation. Ab initio DFT studies on the hydrogenation of cinnamaldehyde over novel manganese oxide octahedral molecular sieve (OMS-2) suggest that OMS-2 is highly selective for C=C bond hydrogenation to form the saturated aldehyde, whereas Pt supported on OMS-2 favors C=O hydrogenation resulting in high selectivity to the unsaturated alcohol. The unique selectivity for Pt/OMS-2 can be attributed to the electron donation effects from Pt to the OMS-2 support, leading to an activation of C=O bond to accept the attack from the hydrogen. The selectivity in hydrogenation of unsaturated oxygenates can thus be tailored by the choice of the metal and its interaction with the OMS-2 support.
This work provides fundamental insights into the atomic scale features of the reagents, the metal surface, the support and the solvent that influence the adsorption of different unsaturated aldehydes and ketones and the subsequent hydrogenation of their C=O and C=C bonds.
