Aldol condensation reactions form new C-C bonds and remove O-atoms via subsequent dehydration of aldol to α,β-unsaturated carbonyls. Oxides, such as TiO2 and ZrO2, catalyze condensation reactions, but these unsaturated species reach equilibrium concentrations and undergo subsequent reactions that cause losses in yields and active sites. A metal function (Cu, Pt, Pd), even as separate aggregates, hydrogenates such intermediates and avoids deactivation and thermodynamic bottlenecks, and allow alkanols and alkanals(ones) to be used as equivalent equilibrated reactants. Enolate formation limits condensation and esterification rates for C2-C5 oxygenates and occurs via α-C-H cleavage at acid-base site pairs on TiO2 and ZrO2 surfaces that remain largely uncovered during catalysis, as shown by the kinetic effects of reactant pressures, by normal H/D kinetic isotope effects, and by titration of active sites with CO2, carboxylic acids, and coordinating and non-coordinating pyridines during catalysis. Enolate species form a new C-C bond via reactions with alkanals(ones) and then dehydrate and hydrogenate to form saturated carbonyl products and their corresponding alkanols or react with alkanols to form hemiacetals that dehydrogenate to esters. DFT-derived free energies for enolate formation and for (condensation/esterification) selectivity ratios are in excellent agreement with experiments and confirm the requirement for acid-base pairs of intermediate strength and the underlying basis for the reactivity of different carbonyl compounds. The cleavage of α-C-H bonds in carboxylic acids at acid-base site pairs on TiO2 and ZrO2 also mediates ketonization reactions that involve the formation of enolic species and their rate-limiting reaction with another acid molecule to form α-hydroxy-β-carboxy-alkoxides, and subsequent intramolecular dehydration and sequential decarboxylation to symmetrical alkanones. The kinetic relevance of C-C coupling steps in ketonization was confirmed by the kinetic effects of reactant concentration, by the lack of H/D isotope effects, and by theoretical estimated activation free energies on periodic TiO2 and ZrO2 surfaces.

Oxygen removal and chain growth pathways in the catalytic upgrading of oxygenates

249th National Meeting (2015)

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