Experimental and Theoretical Evidence for the Reactivity
of Bound Intermediates in Ketonization of Carboxylic Acids and Consequences
of Acid–Base Properties of Oxide Catalysts
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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 C<sub>2</sub>–C<sub>4</sub> carboxylic
acids on monoclinic and tetragonal ZrO<sub>2</sub> (ZrO<sub>2</sub>(m), ZrO<sub>2</sub>(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 ZrO<sub>2</sub>(m) than ZrO<sub>2</sub>(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 TiO<sub>2</sub> (TiO<sub>2</sub>(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 NH<sub>3</sub> and BF<sub>3</sub> affinities.
Infrared spectra during ketonization catalysis show that molecularly
bound acids and monodentate and bidentate carboxylates coexist on
ZrO<sub>2</sub>(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 ZrO<sub>2</sub> 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 ZrO<sub>2</sub>(m), account
for the more reactive nature of ZrO<sub>2</sub>(m) than TiO<sub>2</sub>(a). These findings indicate that the elementary steps and site requirements
for ketonization of C<sub>2</sub>–C<sub>4</sub> carboxylic
acids are similar on M–O site pairs at TiO<sub>2</sub> and
ZrO<sub>2</sub> surfaces, a conclusion that seems general to other
metal oxides of comparable acid–base strength