Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions

• Published in: Nature communications Vol. 8; no. 1; pp. 15442 - 7
• Main Authors: Shi, Hui, Eckstein, Sebastian, Vjunov, Aleksei, Camaioni, Donald M., Lercher, Johannes A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25.05.2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/638/440/950; 639/638/77/887; Acids; Acids - chemistry; alcohol dehydration; Alcohols - chemistry; biomass conversion; Catalysis; cyclohexanol; Cyclohexanols - chemistry; Dehydration; Desiccation; Humanities and Social Sciences; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ions; Kinetics; Models, Chemical; multidisciplinary; Nanopores; Onium Compounds - chemistry; Pores; Science; Science (multidisciplinary); Solutions; Thermodynamics; transition states; Water; zeolite; Zeolites; Zeolites - chemistry
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms15442

Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions. Here we show that hydronium ion catalysis, exemplified by intramolecular dehydration of cyclohexanol, is markedly influenced by steric constraints, yielding turnover rates that increase by up to two orders of magnitude in tight confines relative to an aqueous solution of a Brønsted acid. The higher activities in zeolites BEA and FAU than in water are caused by more positive activation entropies that more than offset higher activation enthalpies. The higher activity in zeolite MFI with pores smaller than BEA and FAU is caused by a lower activation enthalpy in the tighter confines that more than offsets a less positive activation entropy. Molecularly sized pores significantly enhance the association between hydronium ions and alcohols in a steric environment resembling the constraints in pockets of enzymes stabilizing active sites. The rates of acid-catalysed reactions vary in constrained environments. Here the authors show that molecularly sized pores greatly promote aqueous phase alcohol dehydration by enhancing the association between substrate and hydronium ions, and even by lowering the free energy barrier.