Aldol condensation of carbonyl molecules and ketonization of carboxylic acids form C-C bonds and remove O-atoms via concerted reactions on acid-base site pairs consisting of metal-oxygen (M-O) centers on oxides such as TiO2 and ZrO2. These two reactions involve different reactants, but a similar sequence of elementary steps: (i) α-C-H bond cleavage to form enolate-like species; (ii) C-C bond formation via nucleophilic enolate attack on electrophilic C-atoms in coadsorbed species; and (iii) O-removal from dimers via dehydration (condensation) or decarboxylation (ketonization). For both reactions, deactivation, which impedes rigorous mechanistic analysis and practical use of these reactions, were minimized by using H2 and a metal function mixed physically with the oxides to hydrogenate unsaturated intermediates that block M-O pairs. Such stability allows rigorous mechanistic inquiries using kinetic, isotopic, and spectroscopic methods combined with density functional theory, as well as the titration of sites during catalysis to determine the number and type of sites required for turnovers; the consequent ability to measure turnover rates allows their benchmarking against theoretical treatments and rigorous experimental comparisons among oxide catalysts. Condensation rates on TiO2 and ZrO2 surfaces are limited by enolate formation at M-O pairs that remain essentially uncovered during catalysis. The relevant transition states involve α-C-H activation at the O-center in M-O sites and coordination of the incipiently-formed enolate at the M-center, leading to highest reactivities on M-O pairs with balanced acid-base strength, thus accounting for different condensation turnover rates on rutile/anatase TiO2 and monoclinic ZrO2 and to excellent agreement between their turnover rates an those derived from DFT treatments for a broad range of carbonyl reactants. Ketonization occurs on TiO2 and ZrO2 surfaces nearly saturated by monodentate carboxylates formed by equilibrated dissociation of acid reactants on M-O pairs, with C-C coupling between 1-hydroxy-enolates and coadsorbed acids as the sole kinetically-relevant step. More basic O-atoms at M-O pairs favor hydroxy-enolates over carboxylates and leads to higher ketonization rates on ZrO2 than on TiO2, illustrating how similar catalytic sequences, but limited by different steps, influence the optimal acid-base properties at M-O pairs.

Catalysis for C-C coupling and oxygen removal reactions of oxygenates at acid-base pairs on oxide surfaces

255th National Meeting (2018)

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