(656e) First-Principles Insights into the Mechanisms and Sites for Base Catalyzed Aldol Condensation and Esterification over Copper

Base-catalyzed aldol condensation and esterification are important reactions in the synthesis of large organic molecules from biomass-derived precursors.¹ These molecules can then subsequently be converted to fuels and useful chemicals. Such reactions are typically catalyzed on homogenous catalysts in the presence of a basic promoter or on basic metal oxides, that catalyze C-C and C-O bond formation.^1-3 Recent studies have shown that non-basic materials such as Cu supported on SiO₂ can actively catalyze both condensation and esterification, when equilibrated mixtures of propanol, propanal and H₂ are reacted. ⁴ These supported Cu systems result in the formation of 47% 3-pentanone (C-C formation via condensation) as well as 34% propyl propionate (C-O formation via esterification).⁴ The results suggest that mixtures of propanol, propanal and H₂ readily equilibrated via the formation of surface propoxide intermediates, that can act as a base, thus, catalyzing both condensation and esterification. Ab-initio density functional theoretical calculations were carried along with detailed kinetic analyses to elucidate elementary steps and sites involved in propanal conversion to propanol and the subsequent C-C and C-O bond formation steps. Detailed charge analyses show that alkoxides which readily form on Cu as well as other group 11 metals readily extract electron density from the metal surface and behave as basic intermediates. For the aldol condensation path, the surface bound propoxide abstracts the weakly acidic hydrogen from a bound propanal intermediate to generate an enolate species in a rate-determining step having a Gibbs free energy barrier of 69 kJ/mol. The enolate subsequently undergoes rapid C-C coupling with a second vicinally-bound propanal to generate a β-alkoxide intemediate. The β-alkoxide can either eliminate hydrogen to the Cu or react with a surface propanal to form a 2-formyl-3-pentanone intermediate that subsequently decarbonylate to form 3-pentanone. The same alkoxide intermediates can also catalyze esterification. The calculations show that the bound propoxide can readily carry out a nucleophilic attack on the carbonyl group of a surface bound propanal in a rate-determining step to form a hemiacetalate intermediate with a Gibbs free energy barrier of 66 kJ/mol. The hemiacetalate further reacts on Cu to form the propyl propionate. The theoretical results indicate that the rate limiting steps for both C-C and C-O formation essentially involve the same propoxide and propanal surface intermediates, which is fully consistent with experimental findings. We predict similar activation barriers for both C-C and C-O formation with C-O formation being more favorable by 3 kJ/mol, while experiments report that C-C formation is slightly more favorable than C-O formation. In addition, we show that esterification and condensation preferentially proceed over coordinatively-saturated Cu 111 terrace sites. The alkoxide as well as other critical intermediates bind too strongly to the edge and corner sites and as such are blocked and much less active than the terrace sites. The active alkoxide intermediates bind much more weakly to the terrace sites and thus catalyze condensation and esterification. This is is fully consistent with the experimental results which show that larger particles are more active than the smaller Cu particles.

References

¹Chheda, J.N., and Dumesic, J. A. Catal. Today (2007), 123, 59.

²Hamilton, C. A., Jackson, S.D., and Kelly, G.J. Appl. Catal. A (2004), 263, 63.

³West, R.M., Liu, Z.Y., Peter, M., Gartner, C.A., and Dumesic, J.A. J. Mol. Catal. A: Chem. (2008), 296, 18.

⁴Sad, M.E., Neurock, M., and Iglesia, E. J.Am.Chem.Soc. (2011), 133, 20384.