Ethanol Conversion to Butanol over Small Coinage Metal Clusters: An Experimental and Computational Study

We illustrate the mechanism for the C-O, C-H and O-H bond activation in the coupling of ethanol to butanol over small coinage clusters (copper, silver and gold). It is found that charge transfer interactions between the clusters and the alcohol initiate their reactions allowing a chemisorption step. The binding energy is calculated, whereby ethanol adsorbs very strongly on Au in comparison to Ag and Cu. The nature of bonding is investigated using natural bond orbital (NBO) analysis and quantum theory of atoms-in-molecules (QTAIM). The reactive intermediates, activated complexes, transition states, and bond breaking on icosahedral Au-13,Au- Cu-13, Ag-13 and also triangular Au-3,Au- Cu-3, Ag-3 have been calculated alongside the cycle kinetics. Furthermore, high resolution mass spectroscopy has been used to study the ethanol coupling reactions over small Au cluster catalysts. The observation of the coupling products concurs with the kinetic- and thermodynamic- allowed reaction pathway of Guerbet coupling of ethanol. The highest selectivity for butanol (61%) is obtained after a reaction time of 2 h while the highest ethanol conversion (91%) is obtained after a reaction time of 5 h.

Automated summary

In this study, the mechanism of ethanol coupling to butanol over small metal clusters (copper, silver, and gold) was investigated. It was found that charge transfer interactions between the clusters and the alcohol play a crucial role in initiating the reactions. Ethanol was found to have a stronger adsorption on gold compared to silver and copper, as determined by binding energy calculations. The nature of bonding was studied using natural bond orbital (NBO) analysis and quantum theory of atoms-in-molecules (QTAIM). The reactive intermediates, activated complexes, transition states, and bond breaking on different metal cluster catalysts were calculated along with cycle kinetics. High resolution mass spectroscopy was used to observe the coupling products, which confirmed the kinetic- and thermodynamic-allowed reaction pathway of Guerbet coupling of ethanol. The highest selectivity for butanol (61%) was achieved after 2 hours of reaction time, while the highest ethanol conversion (91%) was achieved after 5 hours of reaction time.