PNNL

Abstract

In the presence of water, hydronium ions formed within the micropores of zeolite H-BEA significantly influence the sur-rounding environment and the reactivity of organic substrates. The positive charge of these ions, coupled with the zeolite's negatively charged framework, results in an ionic environment that causes strongly non-ideal solvation behavior of cyclohex-anol. This leads to a significantly higher excess chemical potential in the initial state and stabilizes at the same time the charged transition state in the dehydration of cyclohexanol. As a result, the free-energy barrier of the reaction is lowered, leading to a marked increase in reaction rates. Nonetheless, there is a limit to the reaction rate enhancement by hydronium ion concentration. Experiments conducted with low concentrations of reactants show that beyond an optimal concentration, the required spatial rearrangement between hydronium ions and cyclohexanols inhibits further increases in the reaction rate, leading to a peak in the intrinsic activity of hydronium ions. The quantification of excess chemical potential in both initial and transition states for zeolites HBEA, along with findings from HMFI, provides a basis to generalize and predict rates for hydronium-ion catalyzed dehydration reactions in Brønsted zeolites.