Zeolite Catalysis: Water Can Dramatically Increase or Suppress Alkane C–H Bond Activation

Zeolite-catalyzed alkane C–H bond activation reactions carried out at room temperature, low pressure, and low reagent loadings demonstrate that water can act either to increase or to suppress the observed reaction rates. Isobutane-d10 undergoes hydrogen/deuterium exchange with the acidic zeolite HZSM-5 at subambient temperatures, as first reported by us (Truitt et al. J. Am. Chem. Soc. 2004, 126, 11144 and Truitt et al. J. Am. Chem. Soc. 2006, 128, 1847). New experiments demonstrate that the C–H bond activation chemistry is very sensitive to the presence of water. Isobutane reaction rate constants increase by an order of magnitude at water loadings in the range of ≤1 water molecule per catalyst active site relative to the dry catalyst. Conversely, water loadings greater than about 1–3 water molecules per active site retard isobutane reaction. In situ solid-state NMR data show that water molecules and isobutane molecules are simultaneously proximate to the catalyst active site. These results indicate that water can be an active participant in reactions involving hydrophobic molecules in solid acid catalysts, possibly via transition state stabilization, as long as the water concentration is essentially stoichiometric. Such conditions exist in well-known catalytic reactions, e.g., methanol-to-hydrocarbon chemistries, since stoichiometric water is a first-formed byproduct.