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

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Science  28 May 2021:
Vol. 372, Issue 6545, pp. 952-957
DOI: 10.1126/science.abh3418

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Speeding reactions through ionic strength

Brønsted acidity is introduced into microporous zeolites through the addition of framework aluminum. Pfriem et al. show that in the presence of water, the limited volume in the microchannels of zeolite H-MFI leads to a high concentration of hydrated hydronium ions at aluminum sites. The resulting high charge density creates a highly non-ideal solvation environment and, for cyclohexanol dehydrogenation, the charged carbenium-ion transition state was stabilized. A higher rate was maintained with lower-acidity sodium ion–exchanged zeolites that had the same high ionic strength.

Science, abh3418, this issue p. 952

Abstract

Tailoring the molecular environment around catalytically active sites allows for the enhancement of catalytic reactivity through a hitherto unexplored pathway. In zeolites, the presence of water creates an ionic environment via the formation of hydrated hydronium ions and the negatively charged framework aluminum 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, which reduces the energy barrier and leads to higher reaction rates. Using the intramolecular dehydration of cyclohexanol on H-MFI zeolites in water, we quantitatively show an enhancement of the reaction rate by the presence of high ionic strength as well as show potential limitations of this strategy.

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