Probing Regional Hydrophobicity of Zeolite-Confined Cyclopentenyl Cations under Humid Conditions by Solid-State NMR spectroscopy

Cyclopentenyl cations, crucial intermediates in zeolite-catalyzed hydrocarbon transformations, exhibit unique dual hydrophobicity-hydrophilicity due to their polar cationic centers and nonpolar alkyl groups. This study investigates their regional hydrophobicity under moist environments using multidimensional solid-state NMR spectroscopy with propane as a probe molecule. We demonstrate that water enhances noncovalent interactions between cyclopentenyl cations and propane in ZSM-5 channels, primarily through the alkyl groups, confirming their regional hydrophobic character. Competitive adsorption experiments using dichloromethane (dielectric constant, ε = 8.9) further highlight the critical role of solvent polarity (ε_water = 80.1) in promoting hydrophobic interactions. A surfactant-like assembly mechanism is proposed, where water repels propane from Brønsted acid sites in zeolite, driving its accumulation near hydrophobic alkyl regions.