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 depend on ethanol pressure. Unsaturated aldehyde products are sequentially hydrogenated by the Meerwein-Ponndorf-Verley (MPV) reaction (i.e., C=O bond hydrogenation) and surface-mediated H-transfer (i.e., C=C bond hydrogenation). The MPV reaction simultaneously supplies reactive acetaldehyde needed for the coupling reaction by dehydrogenating ethanol. The rates of self- and cross-coupling reactions among C₂-C₄ alcohols are similar as are the values of the apparent activation enthalpies, which shows that self- and cross-coupling rates depend weakly on the structure of the reactants on HAP catalysts, with few exceptions. The carbon number distribution of the products from ethanol coupling closely matches predictions from an adapted step-growth model. Together, these findings show the mechanism for C-C bond formation between alcohol reactants on HAP catalysts and provide guidance for the production of higher carbon number species from alcohol coupling reactions.