(729e) Solvent Effects on Intermolecular Mpvo and Cycloaddition Reactions in Zeolites with Different Intrapore Polarity
Solvent polarity plays a critical role for many heterogeneously catalyzed reactions of oxygenated molecules derived from biomass. Here, we investigate the interplay of both solvent and framework polarity on reactions in condensed media using the intermolecular MPVO reaction between 2-butanol and cyclohexanone as a probe reaction. Cyclohexanol site-time-yields (STY), normalized per open Sn, were measured in a 2-butanol solvent as a function of both cyclohexanone activity (ac; 0.01-1 M) and temperature (333-393 K) to extract first-order and zero-order activation enthalpies and entropies. Cyclohexanol STYs are nearly an order of magnitude larger on hydrophobic (Sn-BEA-F) than on hydrophilic (Sn-BEA-OH) Sn-BEA catalysts at high values of ac and all temperatures, but become similar at low values of ac, suggesting that cyclohexanone adsorption plays an important role in governing MPVO rates. Zero-order activation entropies are nearly identical on Sn-BEA-F and Sn-BEA-OH, but zero-order activation enthalpies are smaller on Sn-BEA-F (by ~ 5 kJ mol-1) suggesting that the difference in zero-order rates is primarily due to differences in the enthalpic stabilization of the transition state. First-order activation entropies, however, are more positive (~70 J mol-1 K-1) in Sn-BEA-F than in Sn-BEA-OH, while activation enthalpies are more positive in Sn-BEA-F (~24 kJ mol-1). The differences in first-order activation barriers suggest that polar solvents form ordered structures within hydrophobic voids that become disrupted upon reactant adsorption, while these solvent structures are more liquid-like within polar frameworks resulting in less positive first-order entropic barriers. IR spectra of adsorbed 2-butanol (303 K, P P0-1 = 0.8) confirms the presence of liquid-like 2-butanol in hydrophilic Sn-BEA-OH, while these features are absent in hydrophobic Sn-BEA-F. These results highlight the importance of framework polarity in regulating the intraporous solvent environment confined within Lewis acidic BEA zeolites, which can modify free energy barriers for a variety of different reactions.