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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-d<sub>10</sub> undergoes hydrogen/deuterium
exchange with the acidic zeolite HZSM-5 at subambient temperatures,
as first reported by us (Truitt
et al. <i>J. Am. Chem. Soc.</i> <b>2004</b>, 126, 11144 and Truitt et al. <i>J. Am. Chem. Soc.</i> <b>2006</b>, 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