Role of the ionic environment in enhancing the activity of reacting molecules in zeolite pores

Abstract

Tailoring the molecular environment around catalytically active site allows to enhance catalytic reactivity via a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via formation of hydrated hydronium ions and the negatively charged framework Al tetrahedra. The high density of cation-anion pairs determined by the aluminum concentration of a zeolite induces a high local ionic strength that increases the excess chemical potential of sorbed and uncharged organic reactants. Charged transition states (carbocations for example) are stabilized, reducing the energy barrier and leading to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI in water, we show quantitatively the enhancement of the reaction rate by the presence of high ionic strength as well as potential limitations of this strategy.