In Situ NMR and Kinetics Reveal Origins of Regioselectivity Differences for Epichlorohydrin Ring-Opening in Lewis and Brønsted Acid Zeolites

Altering the quantities and organization of reactive species at active sites enables control of turnover rates and regioselectivities (rate ratios) for ring-opening of epichlorohydrin (C ₃ H ₅ ClO) across two orders of magnitude. Kinetic analysis suggests that parallel monomolecular (S _N 1) and bimolecular (S _N 2) substitution mechanisms contribute to observed rates of C ₃ H ₅ ClO reactions with methanol (CH ₃ OH) over both Lewis (Sn-BEA) and Brønsted acid (Al-BEA) zeolites in liquid solvents. In situ solid-state ¹³ C-nuclear magnetic resonance spectroscopy (SS-NMR) measurements give direct evidence for the proposed ring-opened carbocations and activated CH ₃ OH intermediates over these catalysts. Interpretation of time-resolved operando ¹³ C-SS-NMR spectra show that C ₃ H ₅ ClO-derived carbocations and CH ₃ OH-derived surface species convert to ring-opening products through S _N 1 and S _N 2 reaction mechanisms and subsequently form distinct produce regioisomers. These NMR spectra also reveal a concomitant shift from S _N 1 to S _N 2 reactions with increases in the coverage of CH ₃ OH-derived reactive intermediates achieved by control of the local concentrations of CH ₃ OH, C ₃ H ₅ ClO, and diluting CH ₃ CN. This knowledge provides new insight into the roles of confinement and coverage on regioselectivity and rates of catalytic reactions of organic species at solid-liquid interfaces.