Mechanistic Insight to C–C Bond Formation and Predictive Models for Cascade Reactions among Alcohols on Ca- and Sr-Hydroxyapatites

Biomass-derived light alcohols (e.g., ethanol) may be upgraded via C–C bond formation to form larger alcohols and chemicals. The mechanism for coupling reactions among alcohols (i.e., Guerbet chemistry) is still debated, and the factors that determine the rates of subsequent, and inevitable, reactions among coupling products, and thus the product distributions, are not well understood. Here, the interpretation of the formation rates of products, in situ spectroscopy of surface intermediates, and evidence from isotope labeling experiments are combined to clarify the mechanism of the ethanol coupling over hydroxyapatite (HAP) catalysts. Initial C–C bonds are created by aldol condensation of acetaldehyde, derived in situ from ethanol, and involves a kinetically relevant deprotonation step to form the reactive enolate. In situ infrared spectra show that the coverage of ethanol-derived species far exceeds that of reactive aldehyde intermediates, which is consistent with C–C formation rates that inversely depen...