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.

Influence of Intracrystalline Ionic Strength in MFI Zeolites on Aqueous Phase Dehydration of Methylcyclohexanols

“…As we demonstrated previously, [12] the increasing local ionic strength in the zeolite pores causes the increase in TOFs. This conclusion was drawn unequivocally from a series of Na + partly exchanged H-MFI, in which the ionic strength was kept constant while at the same time the H 3 O + hydr.…”

“…concentration was decreased. [12] Consequently, we conclude that also in the present study, the high ionic strength is responsible for the increasing TOFs by inducing non-ideality to the system. More precisely, the induced ionic environment destabilizes the uncharged sorbed reactant and simultaneously stabilizes the positively charged transition state (carbenium ion), which in turn results in an overall lowering of the free energy barrier and, therefore, in higher TOFs (Table 1).…”

“…Once the void space between the hydronium ions is smaller than the volume of one substrate molecule, the repulsion induced by the sorption of molecules and the partial separation of charge via a rearrangement of the hydronium ions (combination of electrostatic, hydrogen bonding, and dispersion interactions) sets in in constrained systems forcing reorganization in the highly ionic strength environment. [12] The additional work resulting from the partial separation of the negative charge at the zeolite lattice and the positive charge (particularly for the transition state) that has to be overcome, causes an increase of the free energy and, therefore, a decrease of the TOF. In open systems [12] the TOF increased monotonically with increasing ionic strength without passing through a maximum.…”

“…The intracrystalline ionic strength increases proportionally to the H 3 O + concentration, leading to a monotonic increase of the standard free energy of adsorption of nonpolar or less polar substrates in zeolite pores. [12] Using cyclohexanol dehydration to cyclohexene catalyzed by MFI and BEA in water it was shown recently that the ionic environment in zeolite crystallites stabilizes the cationic transition state, thus, decreasing the reaction barrier and enhancing the reaction rate. [12] This provides a new path to influence the catalytic activity of Brønsted acidic zeolites.…”

“…[12] Using cyclohexanol dehydration to cyclohexene catalyzed by MFI and BEA in water it was shown recently that the ionic environment in zeolite crystallites stabilizes the cationic transition state, thus, decreasing the reaction barrier and enhancing the reaction rate. [12] This provides a new path to influence the catalytic activity of Brønsted acidic zeolites.…”