Abstract
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.
Highlights
Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions
We showed previously[23] that the catalytic activity of hydronium ions for aqueous-phase cyclohexanol dehydration is more than an order of magnitude higher in HBEA zeolite than in the acidic solutions
Showed identical rates when normalized to the concentration of hydronium ions
Summary
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. We showed previously[23] that the catalytic activity of hydronium ions for aqueous-phase cyclohexanol dehydration is more than an order of magnitude higher in HBEA zeolite than in the acidic solutions. Understanding the causes and consequences of such rate enhancement induced by confined nanoenvironments is the key to designing novel catalysts that maximize acid-catalysed rates in condensed phase by utilizing inorganic and organic nanoenvironments. The rate of hydronium ioncatalysed elimination reactions can vary by up to two orders of magnitude with changing size of the confines, relative to aqueousphase acid solutions, pointing to the possibility of designing catalysts that can achieve enzyme-like rates for acid-catalysed reactions under mild conditions
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