(7e) Rates and Regioselectivities of Epichlorohydrin Methanolysis Depend on the Topology of Aluminosilicate Catalysts

The organization of liquid-phase molecules around active sites at solid-liquid interfaces impacts rates and activation barriers for catalytic reaction through non-covalent interactions that include van der Waals interactions and hydrogen-bonds among reactive intermediates, solvent molecules, and pore walls. These effects appear strongly for reactions within microporous materials, and knowledge of these interactions remain crucial to design liquid-phase catalysts. Nucleophilic ring-opening of epoxides with alcohols strongly senses these interactions and the formation rate of distinct regioisomers (i.e., the terminal ether and terminal alcohol) respond differently to spatial constraints and hydrogen bonding interactions with solvent molecules.

Here, we describe differences in rates and regioselectivities of epichlorohydrin ring-opening with methanol (CH₃OH) across a series of Brønsted acidic aluminosilicates of distinct topologies and offer molecular interpretations of the solvating interactions that give rise to these effects. Turnover rates for ring-opening reactions increase at all methanol concentrations with increasing pore size from TON (0.57 nm) to BEA (0.66 nm) before decreasing for larger pore sizes (Figure 1a). The magnitude of rate decreases observed in larger pore materials depends on the kinetic regime (Figure 1b), with rates most sensitive to pore size at high [CH₃OH] when bulky epichlorohydrin reacts in an S_N2 pathway with a CH₃OH-derived surface species. In this regime, apparent activation enthalpies and entropies change non-monotonically with pore size and reflect differences in transition state stabilities due to intrapore solvation of protonated intermediates. These effects selectively stabilize transition states for the sterically accessible terminal ether product as the pore size decreases from H₂SO₄ to TON (Figure 1c). Conversely, regioselectivities remain similar across the examined pore sizes at low [CH₃OH] when an S_N1 pathway involving epichlorohydrin-derived surface species dominates (Figure 1b), which reveals protic solvents confer greater kinetic sensitivity to material topology for liquid-phase zeolite catalysis.