Solvent Polarity and Framework Hydrophobicity of Hf-BEA Zeolites Influence Aldol Addition Rates in Organic Media

Abstract

Solvent identity and pore polarity are known to influence Lewis acidic catalysis in zeolite pores for a variety of liquid-phase chemistries. We investigated how these parameters alter the rates of self-aldol addition of ethyl pyruvate (EP), a model biomass-derived compound, over hydrophobic and hydrophilic Hf-BEA zeolites in both toluene and acetonitrile solvents. Aldol addition rates are of first order across the entire EP activity range (0.02−0.4) for all four systems, consistent with the nucleophilic attack by the enolate as the rate-determining step and a single adsorbed EP as the most abundant reactive intermediate. Apparent first-order rate constants span 2 orders of magnitude across the four systems; at 363 K, the highest rates were observed over hydrophobic Hf-BEA-F in toluene (k app = 0.36 (mmol) (mmol closed Hf) −1 (s) −1 ), while the lowest rates were observed in hydrophilic Hf-BEA-OH in an acetonitrile solvent (k app = 0.0026 (mmol) (mmol closed Hf) −1 (s) −1 ). Apparent reaction enthalpies and entropies for each system, estimated using non-ideal transition-state theory, revealed that despite the substantial rate constant variation across the four systems, apparent enthalpies for Hf-BEA-F in both solvents and Hf-BEA-OH in acetonitrile were within the error of each other (∼70 kJ mol −1 ). Reactions performed using Hf-BEA-OH with toluene featured a higher apparent enthalpic barrier of 83.8 kJ mol −1 . The differences between the systems are attributed to hydrogen-bonding interactions between the EP molecules and polar silanol nests during catalysis in toluene using Hf-BEA-OH, which hinder EP adsorption to the active site in the hydrophilic framework. These hydrogen-bonding interactions are not present when acetonitrile is used as the solvent, as acetonitrile itself binds to and blocks silanol groups. Equilibrium EP absorption measurements indicate that while both toluene and acetonitrile are present in pores during catalysis, neither solvent forms a tight solvation shell around EP in the pores that must be disrupted prior to EP adsorption. These findings show that aldol addition kinetics are not significantly modified by solvent polarity in hydrophobic frameworks beyond site-blocking effects; however, silanol nests in hydrophilic frameworks significantly alter substrate adsorption to the active site.

The Significant Differences in Solvation Thermodynamics of C1-C3 Oxygenates in Hydrophilic versus Hydrophobic Pores of a Hydrophilic Ti-FAU Zeolite Model

“…This suggests that the field created by the adsorbate contributes to 𝑇∆𝑆 !# in hydrophilic pores, with stronger fields having a larger decrease on entropy. Interestingly, experiments have suggested an influence of polarity in hydrophilic zeolites, but this has been attributed to the pore 39,75,76 , whereas our results point to the polarity of the adsorbate. Based on this explanation, entropic gain should be realized in hydrophilic pores through conversion of a less polar species from a more polar one.…”

Heteroatom Lewis acid zeolites: synthesis, characterization and application in the conversion of biomass-derived oxygenates

“…, closed or open sites), and display different acidities, which all together will influence the catalytic performance. 34,35 Moreover, different synthesis methods may lead to different pore structures, crystallite sizes, and concentrations of defect sites (which determines the hydrophobicity or hydrophobicity), which will also affect the catalytic performance. 36–39 Significant advances in the qualification and quantification of the location, fine structure and acidity of heteroatom centers confined in zeolites have also been made in recent years.…”