Theoretical Insight into Cp*Ir-Catalyzed Upgrading of Ethanol to n-Butanol: The Crucial Role of Cs2CO3

The ethanol-to-n-butanol upgrading process catalyzed by a Cp*Ir complex (1-Ir) and mild base Cs2CO3 was investigated using density functional theory calculations. Initially, Cs2CO3 and 1-Ir form an active species 2 with an exothermicity of 12.9 kcal/mol. Ethanol dehydrogenation then occurs through the cooperation of the Ir center and Cs2CO3 to produce an Ir-H complex 3 with the release of acetaldehyde and CsHCO3. Cs2CO3 catalyzes the aldol condensation of acetaldehyde to produce a C4 intermediate crotonaldehyde. Subsequently, the successive hydride and proton migrations occur from 3 and ethanol to crotonaldehyde, respectively, to produce butanal. The proton migration step is the rate-determining step (Delta G double dagger/Delta G = 29.1/-12.1 kcal/mol). Finally, n-butanol is produced via transfer hydrogenation of butanal with ethanol catalyzed by Cs2CO3. High selectivity for n-butanol is due to preferential hydrogenation of crotonaldehyde over its further condensation into C6 species. Cs2CO3 plays a critical role in promoting ethanol dehydrogenation, aldol condensation, and butanal hydrogenation. In contrast, Na2CO3 significantly reduces reaction efficiency mainly due to its weaker basicity in the aldol condensation of acetaldehyde. These findings provide insights into the ethanol-to-n-butanol conversion and offer a foundation for developing milder bases for the reaction.