Understanding Rates and Regioselectivities for Epoxide Methanolysis within Zeolites: Mechanism and Roles of Covalent and Non-covalent Interactions

Abstract

Rates and selectivities for liquid-phase reactions depend on the structure and acidity of active sites within zeolite catalysts and the solvent environment surrounding the active sites. Here, we demonstrate and explain the effects of these interactions on the ring-opening of 1,2-epoxybutane (C 4 H 8 O) with methanol (CH 3 OH) in acetonitrile (CH 3 CN) solvent over *BEA zeolites by varying the identity of the catalytically active element within the framework (Al, Sn, Ti, Zr) and composition of the fluid. Molecular interpretations of kinetics show that the nucleophilic attack to C 4 H 8 O determines product formation rates at sites saturated by C 4 H 8 O-or CH 3 OH-derived intermediates and favors the production of terminal ethers. Among Lewis acid catalysts, turnover rates increase with exponential dependence upon C 4 H 8 O adsorption enthalpies, measured by liquid-phase isothermal titration calorimetry, which reflects covalent and non-covalent interactions. The activation enthalpies for ring-opening and C 4 H 8 O adsorption enthalpies are least endothermic in CH 3 CN and become more endothermic with the addition of CH 3 OH. Greater concentrations of CH 3 OH increase regioselectivities toward the terminal alcohol due to the enrichment of CH 3 OH near active sites and preferential stabilization of transition states by non-covalent interactions (e.g., hydrogen bonding). These findings demonstrate opportunities to leverage cooperative inner-and outer-sphere interactions to control rates and regioselectivities of zeolite-catalyzed ring opening reactions and guide strategies to create desired products.

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

“…Therefore, changes in the stability of reactive species within the zeolite pores (i.e., C 4 H 8 O*, transition state) drive the activation barrier differences with [CH 3 OH] in Figure . Increasingly positive Δ H ‡ and Δ S ‡ values at higher [CH 3 OH] suggest that the displacement of CH 3 OH molecules from the *BEA pores during transition state formation leads to a greater enthalpic cost and corresponding entropic gain than the displacement of CH 3 CN, consistent with our previous interpretations of epoxide ring-opening and alkene epoxidation activation barriers over M-BEA materials.…”

“…These findings indicate that the (SiOH) x structure near Sn atoms influences ring-opening kinetics, which may stem from differences in local solvent structure. Inspired by these findings, our group examined the mechanism of ring-opening 1,2-epoxybutane (C 4 H 8 O) with CH 3 OH over hydrophilic M-BEA zeolites and demonstrated that addition of an aprotic cosolvent (CH 3 CN) increases the preference to form the terminal ether product (from 1 C attack) . Measurements of activation barriers provide evidence that differences in regioselectivity arise from differences in the solvent composition near active sites that affect the stability of transition states for each regioisomers to different extents.…”

“…The interactions between transition states and intermediates with active and interactions with surrounding solvent molecules within zeolites influence rates and barriers for epoxide ring-opening. The distribution of regioisomers for C 4 H 8 O ring-opening with CH 3 OH over M-BEA (see Scheme above) also depends on these intrapore interactions, as shown in our previous work . The regioselectivities are quantified with a rate ratio defined as β β = r 1 M 2 B r 2 M 1 B where r 1M2B and r 2M1B represent the formation rates of 1M2B and 2M1B, respectively.…”

“…The Brønsted–Evans–Polanyi model proposes that the enthalpy of reaction for an elementary step scales linearly with the intrinsic activation energy of the same step, therefore establishing a connection between the intrinsic kinetics and thermodynamics of a reaction. − Hammond later explained this connection by postulating that the transition state for a reaction resembles the stable state (i.e., reactant or product) most similar in energy. , Recent studies demonstrate that the liquid-phase adsorption enthalpies for epoxide molecules over *BEA zeolites correlate positively with Δ H ‡ values for alkene epoxidation as a function of the active metal, (SiOH) x density, , and solvent composition . More recently, we established linear correlations between epoxide adsorption enthalpies (Δ H ads,C 4 H 8 O ) and Δ H ‡ values for C 4 H 8 O ring-opening over M-BEA materials, implicating a transition state for ring-opening that resembles the adsorbed C 4 H 8 O reactant . The measurement of Δ H ads,C 4 H 8 O can provide insight into the differences in ring-opening transition state stability over M-BEA-OH and M-BEA-F materials.…”

ADOR zeolite with 12 × 8 × 8-ring pores derived from IWR germanosilicate

“…Despite the use of zeolites as solid acid catalysts usually suggesting direct or post-synthesis incorporation of three or four valent elements, such as Al, Ti, Sn, and Zr, into the framework positions, the use of the model reaction catalysed by Geassociated active sites allows one to prove the concept with IPC-17 germanosilicate. Thus, the ring-opening of nonsymmetric epoxides, such as epichlorohydrin (1-chloro-2,3epoxypropane), with alcohols of different sizes (such as ethanol and iso-propanol) 36,37 was studied over parent IWR and daughter IPC-17 zeolites. Notably, the distribution between two alcoholysis products that contain ethoxy-or isopropoxy-groups (Fig.…”