Consequences of Pore Polarity and Solvent Structure on Epoxide Ring-Opening in Lewis and Brønsted Acid Zeolites

Abstract

The structure of solvent molecules within zeolite pores influences the rates and selectivities of catalytic reactions by altering the free energies of reactive species. Here, we examine the consequences of these effects on the kinetics and thermodynamics of 1,2-epoxybutane (C₄H₈O) ring-opening with methanol (CH₃OH) in acetonitrile (CH₃CN) cosolvent over Lewis acidic (Zr-BEA) and Brønsted acidic (Al-BEA) zeolites of varying (SiOH)_x density. Despite ostensibly identical reaction mechanisms across materials, turnover rates depend differently on (SiOH)_x density between acid types. (SiOH)_x-rich Zr-BEA (Zr-BEA-OH) provides ∼10 times greater rates than a (SiOH)_x-poor material (Zr-BEA-F), while Al-BEA-OH and Al-BEA-F give turnover rates within a factor of 2. Zr-BEA-OH shows more positive activation enthalpies and entropies than Zr-BEA-F across the range of [CH₃OH], which reflect the displacement of solvent molecules and lead to greater rates in Zr-BEA-OH due to the dominant role of entropic gains. Measurements of the density and composition of solvent within the pores show that the (SiOH)_x nests within Zr-BEA-OH promote hydrogen-bonded solvent structures distinct from Zr-BEA-F, while the Brønsted acid sites confer interactions similar to (SiOH)_x nests and give solvent structures within Al-BEA-F that resemble those within Al-BEA-OH. Correlations between apparent activation enthalpies and C₄H₈O adsorption enthalpies show that interactions with solvent molecules give proportional changes to both C₄H₈O adsorption and ring-opening transition state formation. The differences in intrapore environment carry consequences for both rates and regioselectivities of epoxide ring-opening, as demonstrated by product regioselectivities that increase by a factor of 3 in response to changes in solvent composition and the type of acid site in the *BEA structure (i.e., Lewis or Brønsted). These results demonstrate the ability to control rates, regioselectivities, and adsorption thermodynamics relevant for industrially relevant liquid-phase reactions through the design of noncovalent interactions among solvating molecules, reactive species, and (SiOH)_x functions.