(176d) The Role of Water in Aldol Condensation of Cyclopentanone on Functionalized-MgO Surface: A DFT Mechanistic Study

Aldol condensation is an effective reaction to produce α,β-unsaturated dimeric ketones, which can be further converted to fuels and chemicals. The promotion or inhibition by water has been observed for the condensation reaction of cyclopentanone (CPO) catalyzed by MgO, which is very sensitive to the presence of water. Hydrophobic functionalized groups (organosilane) could be introduced onto the metal oxide surface to improve the water tolerance of the catalyst. These functional groups can also alter the reaction kinetics, and the exact mechanism remains to be explored. Our DFT calculations suggest that the C-C coupling reaction is facile on pure-MgO on which the reaction is limited by H abstraction. Over the silanes-functionalized MgO surface, the functional groups play multi-faceted roles in the reaction. These groups hinder adsorption of CPO onto the surface and increase the activation barrier of H-abstraction and C-C coupling by 14 and 10 kJ/mol, respectively. We find out a small amount of water can enhance the reaction by stabilizing the CPO on the surface, which increases the coverage of CPO and facilitates the C-C coupling step.