Consequences of Intrapore Liquids on Reactivity, Selectivity, and Stability for Aldol Condensation Reactions on Anatase TiO2 Catalysts

Abstract

AbstractThis study provides evidence and mechanistic interpretations for the significant consequences of intrapore non‐polar liquids on acetone aldol condensation turnover rates, selectivity to primary dimer products, and catalyst stability for reactions at Lewis acid‐base site pairs on TiO2 surfaces. These non‐polar liquids confer such benefits through the preferential stabilization of transition states (TS) for adsorption (“entry”) and desorption (“exit”) steps, which place their respective reactants or products within a solvating outer sphere environment. The extent to which non‐polar fluids (n‐heptane) form an intrapore liquid phase within TiO2 voids was obtained from N2 uptakes using established formalisms that consider the different molal volume, surface tension, and volatility between N2 and n‐heptane. Acetone condensation rates are limited by C−H activation, an “entry” step that forms bound prop‐1‐en‐2‐olates via a TS stabilized by intrapore liquids, leading to higher aldol condensation turnover rates as n‐heptane pressure increases and active TiO2 surfaces become increasingly immersed within a non‐polar liquid phase. These liquids solvate the late TS structures that mediate the desorption of primary C6 condensation products even more effectively than those involved in prop‐1‐en‐2‐olate formation or in nucleophilic attack events that later form C−C bonds. Such preferential solvation favors desorption over C−C coupling events, thus disfavoring the formation of larger oligomers that become stranded at active sites, thus leading to much slower deactivation. Moreover, solvation by non‐polar liquids also leads to C6 alkanones as the sole products formed in a single surface sojourn. These effects of a non‐polar dense phase circumvent the inherent stability, reactivity, and selectivity hurdles that have precluded practical aldol condensation catalysis on Lewis acid‐base pairs at oxide surfaces; these consequences are demonstrated here for TiO2 catalysts, acetone aldol condensation reactions, and n‐heptane as the non‐polar liquid but through strategies, concepts, and mechanistic features that extend to other systems. More generally, these observations and their mechanistic origins demonstrate how a contacting liquid preferentially solvates TS structures for elementary steps that involve either reactants arriving from or products entering into an outer sphere environment that contains a dense non‐polar phase.