(625c) Influence of Alcohol Chain Length and Zeolite Pore Interactions on the Rate and Regioselectivity of Epoxide Alcoholysis

Confined solvents greatly influence reactive species stabilities during liquid-phase Brønsted and Lewis acid zeolite catalysis. Solvent size, hydrogen bonding ability, and zeolite pore diameter affect the interactions between solvents and reactive species. During regioselective chemistries, these interactions can differentially stabilize transition states to form each regioisomer. Here, we demonstrate 1,2-butylene oxide (C₄H₈O) alcoholysis rates and regioselectivities depend on the alcohol nucleophile (ROH generally; methanol, ethanol, 1-butanol, and 1-hexanol) and solvent composition (through co-solvent acetonitrile (CH₃CN) and H₂O) over Al- and Zr-substituted *BEA zeolites.

Turnover rates in aprotic CH₃CN solvent increase by an order of magnitude with increasing ROH chain length (from C₁-C₆) over both Al- and Zr-BEA (Fig. 1a). Rates increase with protic ROH concentration ([ROH]) as hydrogen-bonding alcohols stabilize reactive intermediates. Rate constants calculated with reagent activities (functions of solvent composition) demonstrate rate differences between ROH arise from changes to intrapore transition state stabilities rather than bulk fluid interactions. Activation enthalpy and entropy decrease linearly with increasing ROH chain length, indicating the enthalpic stabilization of larger transition states by zeolite pore walls dictates rates rather than contributions from solvent rearrangement.

Regioselectivities to the terminal ether decrease from ~80 to 60% with increasing ratios of [ROH]:[C₄H₈O] in the case of methanol over Zr-BEA yet remain largely invariant for longer chain ROH and all ROH over Al-BEA (Fig. 1b). These trends show larger alcohols cannot differentially stabilize regioisomer transition states in BEA zeolite pores. However, the addition of a smaller-sized protic cosolvent like H₂O decreases terminal ether regioselectivities in all ROH except 1-hexanol and to a greater extent in Zr-BEA (Fig. 1c). Regioselectivities are therefore likely limited by the inability of larger alcohol molecules to form hydrogen bonded networks in sterically limited zeolite pores. Collectively, these findings show complex solvent-reactant-zeolite interactions govern rates and regioselectivities in liquid-phase zeolite catalysis.