Aldol condensations of carbonyl compounds form new C-C bonds and remove O-atoms via subsequent dehydration of aldol to a,�-unsaturated carbonyl compounds. Oxides, such as TiO2 and ZrO2, catalyze aldol condensations, but with rapid deactivation as a result of the formation of unreactive unsaturated residues. The presence of supported metals (Cu, Pt, Pd), even as physical mixtures, leads to hydrogenation of unsaturated intermediates, thus inhibiting deactivation and removing thermodynamic bottlenecks, which also allows the use of either alkanols or alkanals/alkanones as equivalent reactants. Condensation and esterification of C2-C5 oxygenates involve the rate-limiting formation of surface enolates via cleavage of a-C-H bond at acid-base site pairs on TiO2 and ZrO2 surfaces, consistent with kinetic effects of alkanal and H2 pressures, with normal kinetic isotope effects (2.4, ethanol-D6, 523 K), and with titration of propanoic acid during catalysis. A good agreement of the enolate formation ?G� is obtained between the experiments and density functional theory (DFT) calculations using Ti5O19H18 and Zr5O21H22 cluster models, where alkanals are more reactive than alkanones. The formed enolate then couples with another carbonyl to form a new C-C bond followed by sequential dehydration and hydrogenation to saturated carbonyls, or react with an alkanol to form an alkanal and alkoxide, followed by their coupling to a hemiacetal intermediate with a new C-O bond, which then dehydrogenates to an ester on the metal. Difference of ?G� between the branching pathways is determined by the C-C coupling step (between alkanal and enolate) and the C-O coupling step (between alkanal and alkoxide), supported by a secondary kinetic isotope effect (1.4, ethanol-D6, 523 K) and DFT calculations. Acid-base site pairs with intermediate strength are favored for such oxygenate conversions. For instance, rutile TiO2 is less active than anatase TiO2, because stronger basic sites on rutile kinetically limit desorption of the condensation products.

Pathways and site requirements for C-C and C-O bond formations in reactions of oxygenates on TiO<sub>2</sub> and ZrO<sub>2</sub> acid-base site pairs

248th National Meeting (2014)

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Pathways and site requirements for C-C and C-O bond formations in reactions of oxygenates on TiO2 and ZrO2 acid-base site pairs

Pathways and site requirements for C-C and C-O bond formations in reactions of oxygenates on TiO₂ and ZrO₂ acid-base site pairs