Zeolite-catalyzed alkane C-H bond activation reactions carried out at room temperature, sub-ambient pressure, and low reagent loadings are used to demonstrate that water can act either to increase or to suppress the observed reaction rates. Isobutane-d10 undergoes regiospecific hydrogen/deuterium exchange with the acidic zeolite HZSM-5 at sub-ambient temperatures, as first reported by us (J. Am. Chem. Soc. 2004  , 126, 11144 and J. Am. Chem. Soc. 2006  , 128, 1847). New experiments demonstrate that the C-H bond activation chemistry is sensitive to the presence of water. Unexpectedly, reaction rate constants increase at water loadings in the range of 1-3 water molecules per catalyst active site relative to the dry catalyst, and an apparent increase in the number of strong acid sites is observed spectroscopically. Conversely, water loadings greater than about 3 water molecules per active site essentially preclude any isobutane reaction. In-situ 1H solid-state NMR results show that water molecules and isobutane molecules are simultaneously proximate to the catalyst active site and to one another, and new signals are observed in the 8-10 ppm region whose intensities are significant relative to the previously reported 4-7 ppm signals characteristic of the HZSM-5 catalyst. These results suggest that water can be an active participant in reactions involving hydrophobic molecules in solid acid catalysts, increasing their reactivity possibly via transition state stabilization, as long as the water concentration is essentially stoichiometric. Such conditions may exist even in well-known high-temperature catalytic reactions, e.g., methanol-to-hydrocarbon conversion, as stoichiometric water is a byproduct of methanol reactions.

Room-temperature catalysis at the solid acid surface: Water can increase or suppress hydrocarbon reaction rates

248th National Meeting (2014)

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