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

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.

A nucleation-tuned mechanism to prepare centre-crossed zeolite lamellas by the rotating/static switch crystallization strategy

Controllable synthesis of zeolite lamellas with different morphology and property by tuning the nucleation process.

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.