Multiproduct Steady-State Isotopic Transient Kinetic Analysis of the Ethanol Coupling Reaction over Hydroxyapatite and Magnesia

The Guerbet coupling of ethanol into butanol was investigated using multiproduct steady-state isotopic transient kinetic analysis (SSITKA) in a comparative study between stoichiometric hydroxyapatite (HAP) and magnesia (MgO) catalysts at 613 and 653 K, respectively. The steady-state catalytic reactions were conducted in a gas-phase, fixed-bed, differential reactor at 1.3 atm total system pressure. Multiproduct SSITKA results showed that the mean surface residence time of reactive intermediates leading to acetaldehyde was significantly shorter than that of intermediates leading to butanol on both HAP and MgO. This finding may suggest that the dehydrogenation of ethanol to acetaldehyde is fast on these surfaces compared with C–C bond formation. If adsorbed acetaldehyde is a key reaction intermediate in the Guerbet coupling of ethanol into butanol, then SSITKA revealed that the majority of adsorbed acetaldehyde produced on the surface of MgO desorbs into the gas-phase, whereas the majority of adsorbed acetaldehyde on HAP likely undergoes sequential aldol-type reactions required for butanol formation. Adsorption microcalorimetry of triethylamine and CO₂ showed a significantly higher number of acid and base sites on the surface of HAP compared with those on MgO. Diffuse reflectance infrared Fourier transform spectroscopy of adsorbed ethanol followed by stepwise temperature-programmed desorption revealed that ethoxide is more weakly bound to the HAP surface compared with MgO. A high surface density of acid–base site pairs along with a weak binding affinity for ethanol on HAP may provide a possible explanation for the increased activity and high butanol selectivity observed with HAP compared with MgO catalysts in the ethanol coupling reaction.