First Principle Insights into Monofunctional Catalytic Deoxygenation and C-C and C-O Bond Formation over Cu

The catalytic pathways involved in C-C (aldol condensation) and C-O (esterification) bond formation reactions of propanal and propanol over Cu are examined in this work by carrying out first-principle density functional theory (DFT) calculations. The reactions of propanal and propanol with minimal hydrogen addition over supported Cu are readily equilibrated via rapid C-H and O-H bond activation to form an equilibrium mixtures. These reactions proceed through the formation of a surface propoxide intermediate (CH3CH2CH2O*) that is formed either via the dehydrogenation of propanol or the hydrogenation of propanal, with activation energies of 120 and 56 kJ/mol, respectively. There is direct charge transfer from Cu surface atoms to the adsorbed propoxide intermediate which results in the formation a surface CH3CH2CH2O(-) intermediate that acts as base which catalyzes condensation as well as esterification reactions. This basic propoxide intermediate can readily abstract the acidic hydrogen atom that resides α to the carbonyl of the propanal (aldehyde) to produce an enolate intermediate with an activation barrier of 54 kJ/mol which initiates aldol condensation. The subsequent C-C coupling between the adsorbed enolate and a second propanal proceeds with an activation barrier of only 9 kJ/mol. The basic propoxide intermediate can also directly react with a surface propanal to form a C-O bond and the propyl propionate intermediate with an activation energy of 57 kJ/mol. The theoretical results indicate that the adsorbed propoxide intermediate reacts with the adsorbed aldehyde in the kinetical relevant step in both the condensation and esterification reactions via α-H abstraction of aldehyde and nucleophilic attack on the carbonyl of the aldehyde, respectively. The comparison with the experimental results indicates that apparent activation energy difference between condensation and esterification reaction is within 7-8 kJ/mol with theory suggesting that esterification is slightly more favored whereas the experiments show a very small preference for the aldol condensation reaction.