Substituent Effects and Molecular Descriptors of Reactivity in Condensation and Esterification Reactions of Oxygenates on Acid–Base Pairs at TiO2 and ZrO2 Surfaces

This study reports evidence for the identity and kinetic relevance of the elementary steps that mediate condensation and esterification reactions of C 2 –C 5 alkanals and alkanones and for the number, role, and acid–base properties of active Ti–O and Zr–O site pairs on catalytic surfaces. Kinetic, isotopic, and theoretical methods show that all reactants convert on TiO 2 and ZrO 2 surfaces via similar routes, which involve (i) kinetically relevant α-C–H cleavage in alkanals (alkanones) to enolates on sparsely covered M–O pairs, (ii) selectivity-relevant subsequent enolate reactions with coadsorbed alkanals/alkanones or 1-alkanols to form condensation or esterification precursors, mediated by C–C coupling steps with carbonyl reactants and C–O coupling steps with alkanal–alkoxide pairs formed via H transfer in enolate–1-alkanol pairs, (iii) hydrogenation of C–C coupling products to carbonyl compounds and dehydrogenation of hemiacetals formed via C–O coupling to esters on an interspersed Cu function present as a physical mixture. For mixtures of oxygenate reactants, enolate formation rates from each reactant are unaffected by the other reactants, while C–C and C–O coupling product ratios allow measurements of the relative reactivity of each enolate with different carbonyls or alkanols; these reactivities are unavailable from carbonyl–alkanol mixtures derived from a single reactant but are essential to benchmark theory and experiment. Density functional theory (DFT) treatments on model Ti 5 O 19 H 18 clusters give free energy barriers for enolate formation (Δ G ‡ ) and Δ G ‡ differences for the coupling of each enolate with different alkanals, alkanones, or 1-alkanols in excellent agreement with measured values. Enthalpy and entropy components of DFT-derived activation free energy barriers show that alkyl substituents influence enolate reactivity through their effects on α-C–H bond energies and on steric hindrance at transition states (TS). Substituents influence enolate C–C coupling more strongly than C–O coupling because steric effects predominate at the tighter TS structures that mediate condensation events. Enolate formation turnover rates, based on the number of active M–O pairs measured by titration with propanoic acid during catalysis, are higher on ZrO 2 than TiO 2 . Titrations during catalysis showed that the higher intrinsic reactivity of ZrO 2 reflects its weaker Lewis acid centers and more strongly basic O atoms than on TiO 2, which lead, in turn, to more stable enolate formation transition states. The different properties of the two Zr centers in Zr–O–Zr structures lead to relative C–C and C–O coupling rates that depend on the Zr center that stabilizes the O atom in enolates. This asymmetry contrasts the single-site character of Ti–O–Ti structures and leads to marked differences in the effects of alkanal/alkanol reactant ratios on condensation and esterification selectivities.