PNNL

Abstract

Acid catalysis by hydronium ions, such as aqueous phase dehydration of cyclohexanol, is markedly influenced by the steric nano-environments, enhancing catalytic activity by as much as two orders of magnitude. In contrast to Brønsted-acid-catalyzed gas-phase alcohol elimination reactions, hydronium ions catalyze cyclohexanol dehydration to cyclohexene via an E1-type elimination path with the cleavage of a Cß–H bond in the carbenium-ion as the rate-determining step. The higher catalytic activity of hydronium ions in zeolites with smaller pores is caused mainly by lower activation barriers in such tighter pore confines, which are partly offset by less positive activation entropies. The molecularly sized pores also significantly enhance the association between intraporous hydronium ions and alcohols, leading to a specific binding usually only found in enzymes. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Portions of the NMR experiments were performed at the William R. Environmental Molecular Science Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for DOE by Battelle Memorial Institute.