Directing the Rate‐Enhancement for Hydronium Ion Catalyzed Dehydration via Organization of Alkanols in Nanoscopic Confinements. Issue 5 (27th November 2020)

• Record Type: Journal Article
• Authors: Shetty, Manish; Wang, Huamin; Chen, Feng; Jaegers, Nicholas; Liu, Yue; Camaioni, Donald M.; Gutiérrez, Oliver Y.; Lercher, Johannes A.
• Is Part Of: Angewandte Chemie international edition. Volume 60:Issue 5(2021)
• Page Start: 2304
• Page End: 2311
• Publication Date: 2020-11-27
• Subjects: aliphatic alcohols -- confinement effect -- enthalpy-entropy compensation -- hydronium ion -- zeolites; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.202009835
• Languages: English
• ISSNs: 1433-7851

Abstract

Abstract: Alkanol dehydration rates catalyzed by hydronium ions are enhanced by the dimensions of steric confinements of zeolite pores as well as by intraporous intermolecular interactions with other alkanols. The higher rates with zeolite MFI having pores smaller than those of zeolite BEA for dehydration of secondary alkanols, 3‐heptanol and 2‐methyl‐3‐hexanol, is caused by the lower activation enthalpy in the tighter confinements of MFI that offsets a less positive activation entropy. The higher activity in BEA than in MFI for dehydration of a tertiary alkanol, 2‐methyl‐2‐hexanol, is primarily attributed to the reduction of the activation enthalpy by stabilizing intraporous interactions of the Cβ ‐H transition state with surrounding alcohol molecules. Overall, we show that the positive impact of zeolite confinements results from the stabilization of transition state provided by the confinement and intermolecular interaction of alkanols with the transition state, which is impacted by both the size of confinements and the structure of alkanols in the E1 pathway of dehydration. Abstract : The van der Waals stabilization of transition states in smaller‐pore MFI zeolites makes secondary alcohols, 3‐heptanol, and 2‐methyl‐3‐hexanol more reactive toward hydronium ion catalyzed elimination than in the sterically larger pore BEA zeolite. Whereas the stabilization of the Cβ ‐H transition state provided by intraporous alcohol for 2‐methyl‐2‐hexanol provides an additional stabilization in the sterically larger pore BEA zeolite, making hydronium ions in BEA more reactive than in MFI.