Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase

An invisible, reversible, previously overlooked catalytic reaction — enolization — occurs consistently when carboxylic acid vapors contact metal oxide surfaces. While the CO₂H group responsible for adsorption readily forms surface carboxylates, the weaker α-C–H acidity becomes evident only through reversible H/D exchange. The enolized carboxylate condensation with another surface carboxylate becomes important in the mechanism of decarboxylative formation of ketones, as part of oxygen removal during biofuels upgrading. In our kinetic study, the rate of approaching equilibrium was measured for H/D isotopic exchange on alpha-carbon of isobutyric acid used in various concentrations in a vapor phase mixture with D ₂ O as well as for reversed D/H exchange between alpha-deuterated isobutyric acid and H ₂ O upon contact with monoclinic zirconia and anatase titania catalysts. Faster rate for H/D vs. D/H exchange points to alpha-deprotonation, i.e., enolization, as the rate determining step of the exchange mechanism. The intrinsic rate of enolization was deduced using McKay equation for equilibrium reactions. Kinetic activation parameters were obtained through temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO ₂ changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 5.8. The opposite effect of KOH doping is observed on anatase TiO ₂ – enolization rates are slightly decreased, k _H /k _D remains relatively constant at 2.6-2.8 indicating that the nature of basic centers on TiO ₂ is unaffected. These results confirm C-C coupling, not enolization, being the rate limiting step of the decarboxylative ketonization mechanism.