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

Abstract

Abstract 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 shows 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 product 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 role of coverage on regioselectivity and rates of catalytic reactions of organic species at solid–liquid interfaces.