(690a) Deconvolution of Solvent Effects on Confined Zeolites: Influences of Partial Solvation and Alkyl Chain Length on Vapor-Phase Epoxidation

Solvents play a significant role in mediating catalysis, especially in confined environments (e.g., microporous zeolites), through interactions with reactive species nearby active sites. However, kinetics within confined structures often reflects the convoluted effects of solvents, including dispersive interactions (“solvation”) and thermodynamic nonideality depending on micropore topology, solvent organization, and the physical properties of reactive species. Here, we demonstrate that an intentional condensation of gaseous acetonitrile (CH₃CN) leads to changes in the vapor-phase alkene epoxidation kinetics, and changes in rate, selectivity, and activation barrier respond differently to the condensed CH₃CN density and the size of substrates (i.e., varying chain length from propylene to 1-decene; C_nH_2n, 3 ≤ n ≤ 10).

We synthesized Ti-BEA zeolites with varying intrapore silanol ((SiOH)_x) densities and controlled CH₃CN partial pressures to introduce different densities of intrapore CH₃CN, varying from 0.4 to 10 molecule∙(unit cell)^-1. Turnover rates of longer-chain alkenes (C₆-C₁₀) epoxidation increase systematically by 3-fold with CH₃CN density. However, rates for short-chain alkenes (C₃-C₄) weakly depend on CH₃CN density. Apparent activation enthalpies and entropies for epoxidations (at a fixed CH₃CN density) decrease with carbon numbers from C₃ to C₆, then increase from C₆ to C₁₀, and these changes range between 30 kJ∙mol^-1 and 80 J∙mol^-1∙K^-1, respectively. These results infer that the stabilization by condensing CH₃CN onto the reactive species depends on the substrate sizes. Solvation by CH₃CN surrounding reactive species leads to an enthalpic stabilization of short-chain alkene substrates, while more significant reorganization of CH₃CN to accommodate bulkier transition states results in entropic benefits to the epoxidation kinetics. Dynamic vapor sorption and in situ infrared spectroscopy reveal that the CH₃CN reorganization depends systematically on the chain length and intrapore CH₃CN densities. Collectively, these results demonstrate the role of solvents within confined structures that strongly depends on the size of reactants and the proximity to active sites.