(111e) Computational Analysis of Kinetics of Ketonization and Aldol Condensation on Isolated Zirconium Sites

Two chemical reactions of interest for upgrading bio-oils to more valuable chemicals and fuels are ketonic decarboxylation (ketonization) and aldol condensation. Experimental work in our group has found that isolated Zr-hydroxy sites grafted onto silica are highly selective catalysts for these reactions. However, the mechanisms by which these reactions occur are not fully understood. Using DFT and statistical mechanics, we calculated the free energy profiles for both reactions. This involved the calculation of enthalpies and entropies for adsorption and reactions. We then used the calculated free energy profile to determine the turnover frequency for each reaction, the apparent activation energy, and the reaction order in reactant partial pressure and then compare these with the values determined from experiments. We also examined the degree of rate control for individual elementary steps as a function of reactant partial pressures. This analysis allowed us to identify limitations in both our computational and experimental methods. By testing the sensitivity of the predicted experimental quantities to potential errors in the assumptions and approximations we used to determine adsorption entropies, we were able to quantify the extent to which the calculated entropy of adsorption may be in error. We found that for our proposed mechanisms, we can replicate experimentally observed activation energies, rate-limiting steps, and zero order rate constants for both reactions within reasonable error. We speculate adsorbates retain significant rotational entropy unaccounted for by harmonic approximations, suggesting that our methods could be improved by explicit treatment of molecular surface rotations.