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 (C3H5ClO) across two orders of magnitude. Kinetic analysis suggests that parallel monomolecular (SN1) and bimolecular (SN2) substitution mechanisms contribute to observed rates of C3H5ClO reactions with methanol (CH3OH) over both Lewis (Sn-BEA) and Brønsted acid (Al-BEA) zeolites in liquid solvents. In situ solid-state 13C-nuclear magnetic resonance spectroscopy (SS-NMR) measurements give direct evidence for the proposed ring-opened carbocations and activated CH3OH intermediates over these catalysts. Interpretation of time-resolved operando 13C-SS-NMR spectra shows that C3H5ClO-derived carbocations and CH3OH-derived surface species convert to ring-opening products through SN1 and SN2 reaction mechanisms and subsequently form distinct product regioisomers. These NMR spectra also reveal a concomitant shift from SN1 to SN2 reactions with increases in the coverage of CH3OH-derived reactive intermediates achieved by control of the local concentrations of CH3OH, C3H5ClO, and diluting CH3CN. This knowledge provides new insight into the role of coverage on regioselectivity and rates of catalytic reactions of organic species at solid-liquid interfaces.