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. A relatively strong acidity of the CO2H group in carboxylic acids is responsible for the formation of the conjugate base, carboxylate, adsorbed on Lewis acid centers of metal oxide surfaces. Acidity of the alpha C-H group is much weaker and hidden from direct observations during carboxylic acids interaction with metal oxide surfaces, but it is detectable through hydrogen – deuterium isotopic exchange. Subsequent condensation of the enolized carboxylate with another surface carboxylate comes into play in the mechanism of ketones formation with decarboxylation, an important part of oxygen removal during upgrading of biofuels. In this 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 D2O as well as for the reversed H/D exchange between alpha-deuterated isobutyric acid and H2O 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 the enolization was deduced using McKay equation for equilibrium reactions. The kinetic activation parameters were obtained through the temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO2 changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 8.3. The opposite effect of KOH doping is observed on anatase TiO2 – enolization rates are slightly decreased, the kH/kD remains relatively constant at 2.7-2.8 indicating that the nature of basic centers on TiO2 is unaffected. These kinetic study results confirm that C-C coupling, not enolization, is the rate limiting step of the decarboxylative ketonization mechanism.