(111c) Theoretical Analysis of the Periodic Trends in the Aldol Condensation of Acetaldehyde over Isolated Metal Sites

Aldol condensation of aldehydes and ketones occurs during the heterogeneously catalyzed conversion of bioethanol to commodity chemicals and fuels. This reaction can be catalyzed by isolated metal cations grafted onto a high surface area support. Our interest is in understanding the effects of metal cation identity on the mechanism and the kinetics of aldol condensation. We have investigated this problem using electronic structure calculations (DFT) and statistical mechanics to compute Gibbs free energy profiles for aldol condensation of acetaldehyde over isolated metal cations grafted onto the surface of amorphous silica. As a model of the active center, we use a silsesquioxane cube with one corner replaced by the metal of interest. We consider Zr (IV), and less well known metal ion species for this reaction, such as Ta(V) and Sn(IV) for comparison across three groups on the periodic table. Figure 1 displays the Gibbs free energy landscape for acetaldehyde aldol condensation over isolated Zr, Ta, and Sn sites on silica. We predict enolization (state C^‡) to be rate-limiting for the Ta(V), but C-C bond formation (state D^‡) to be rate-limiting for Zr(IV) and Sn(IV) cations. The similarity in the Gibbs free energies for both transition states (enolization and C-C bond formation) for the Zr site suggests that both steps influence the observed kinetics and apparent activation barrier. We have also considered other metals in groups 4,5, and 14 and identified how the rate-controlling steps vary with the identity of the metal cation. The influence of the Lewis acid/base properties of the active center on the strength of acetaldehyde adsorption and the activation barrier for its enolization will also be discussed.