Experimental and Theoretical Evidence for the Reactivity of Bound Intermediates in Ketonization of Carboxylic Acids and Consequences of Acid–Base Properties of Oxide Catalysts

Abstract

Ketonization of carboxylic acids on metal oxides enables oxygen removal and the formation of new C–C bonds for increasing the energy density and chemical value of biomass-derived streams. Information about the surface coverages and reactivity of various bound species derived from acid reactants and the kinetic relevance of the elementary steps that activate reactants, form C–C bonds, and remove O atoms and how they depend on acid–base properties of surfaces and molecular properties of reactants is required to extend the range of ketonization catalytic practice. Here, we examine such matters for ketonization of C₂–C₄ carboxylic acids on monoclinic and tetragonal ZrO₂ (ZrO₂(m), ZrO₂(t)) materials that are among the most active and widely used ketonization catalysts by combining kinetic, isotopic, spectroscopic, and theoretical methods. Ketonization turnovers require Zr–O acid–base pairs, and rates, normalized by the number of active sites determined by titration methods during catalysis, are slightly higher on ZrO₂(m) than ZrO₂(t), but exhibit similar kinetic dependence and the essential absence of isotope effects. These rates and isotope effects are consistent with surfaces nearly saturated with acid-derived species and with kinetically limited C–C bond formation steps involving 1-hydroxy enolates formed via α-C–H cleavage in bound carboxylates and coadsorbed acids; these mechanistic conclusions, but not the magnitude of the rate parameters, are similar to those on anatase TiO₂ (TiO₂(a)). The forms of bound carboxylic acids at Zr–O pairs become more stable and evolve from molecular acids to dissociated carboxylates as the combined acid and base strength of the Zr and O centers at each type of site pair increases; these binding properties are estimated from DFT-derived NH₃ and BF₃ affinities. Infrared spectra during ketonization catalysis show that molecularly bound acids and monodentate and bidentate carboxylates coexist on ZrO₂(m) because of diversity of Zr–O site pairs that prevails on such surfaces, distinct in coordination and consequently in acid and base strengths, and that monodentate and bidentate carboxylates are the most abundant species on saturated ZrO₂ surfaces, consistent with their DFT-derived binding strengths. Theoretical assessments of free energies along the reaction coordinate show that monodentate carboxylates act as precursors to reactive 1-hydroxy enolate intermediates, while strongly bound bidentate carboxylates are unreactive spectators. Higher 1-hydroxy enolate coverages, brought forth by stabilization on the more strongly basic O sites on ZrO₂(m), account for the more reactive nature of ZrO₂(m) than TiO₂(a). These findings indicate that the elementary steps and site requirements for ketonization of C₂–C₄ carboxylic acids are similar on M–O site pairs at TiO₂ and ZrO₂ surfaces, a conclusion that seems general to other metal oxides of comparable acid–base strength