Condensation and esterification reactions of alkanals, alkanones, and alkanols on TiO2: Elementary steps, site requirements, and synergistic effects of bifunctional strategies

Rates and selectivity of TiO2-catalyzed condensation of C-3 oxygenates (propanal, acetone) are limited by ubiquitous effects of side reactions, deactivation, and thermodynamic bottlenecks. H-2 together with a Cu function, present as physical mixtures with TiO2, circumvents such hurdles by scavenging unsaturated intermediates. They also render alkanols and alkanals/alkanones equivalent as reactants through rapid interconversion, while allowing esterification turnovers by dehydrogenating unstable hemiacetals. Oxygenates form molecules with new C-C and C-O bonds and fewer O-atoms at nearly complete conversions with stable rates and selectivities. Kinetic, isotopic, and theoretical methods showed that rates are limited by alpha-C-H cleavage from carbonyl reactants to form enolate intermediates, which undergo C-C coupling with another carbonyl species to form alpha,beta-unsaturated oxygenates or with alkanols to form hemiacetals with new C-O bonds, via an intervening H-shift that forms alkoxide-alkanal pairs. Titrations with 2,6-di-tert-butylpyridine, pyridine, CO2, and propanoic acid during catalysis showed that Lewis acid-base site pairs of moderate strength mediate enolate formation steps via concerted interactions with the alpha-H atom and the enolate moiety at transition states. The resulting site-counts allow rigorous comparisons between theory and experiments and among catalysts on the basis of turnover rates and activation free energies. Theoretical treatments give barriers, kinetic isotope effects, and esterification/-condensation ratios in excellent agreement with experiments and confirm the strong effects of reactant substituents at the alpha-C-atom and of surface structure on reactivity. Surfaces with Ti-O-Ti sites exhibiting intermediate acid-base strength and Ti-O distances, prevalent on anatase but not rutile TiO2, are required for facile alpha-C-H activation in reactants and reprotonation of the adsorbed intermediates that mediate condensation and esterification turnovers. (C) 2016 Published by Elsevier Inc.