We report the encapsulation of platinum species in highly siliceous chabazite (CHA) crystallized in the presence of N,N,N-trimethyl-1-adamantammonium and a thiol-stabilized Pt complex. When compared to Pt/SiO or Pt-containing Al-rich zeolites, the materials in this work show enhanced stability toward metal sintering in a variety of industrial conditions, including H, O, and HO. Remarkably, temperatures in the range 650-750 °C can be reached without significant sintering of the noble metal. Detailed structural determinations by X-ray absorption spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy demonstrate subtle control of the supported metal structures from ∼1 nm nanoparticles to site-isolated single Pt atoms via reversible interconversion of one species into another in reducing and oxidizing atmospheres. The combined used of microscopy and spectroscopy is critical to understand these surface-mediated transformations. When tested in hydrogenation reactions, Pt/CHA converts ethylene (∼80%) but not propylene under identical conditions, in contrast to Pt/SiO, which converts both at similar rates. These differences are attributed to the negligible diffusivity of propylene through the small-pore zeolite and provide final evidence of the metal encapsulation.
We address here the manner in which acid catalysis senses the strength of solid acids. Acid strengths for Keggin polyoxometalate (POM) clusters and zeolites, chosen because of their accurately known structures, are described rigorously by their deprotonation energies (DPE). Mechanistic interpretations of the measured dynamics of alkane isomerization and alkanol dehydration are used to obtain rate and equilibrium constants and energies for intermediates and transition states and to relate them to acid strength. n-Hexane isomerization rates were limited by isomerization of alkoxide intermediates on bifunctional metal-acid mixtures designed to maintain alkane-alkene equilibrium. Isomerization rate constants were normalized by the number of accessible protons, measured by titration with 2,6-di-tert-butylpyridine during catalysis. Equilibrium constants for alkoxides formed by protonation of n-hexene increased slightly with deprotonation energies (DPE), while isomerization rate constants decreased and activation barriers increased with increasing DPE, as also shown for alkanol dehydration reactions. These trends are consistent with thermochemical analyses of the transition states involved in isomerization and elimination steps. For all reactions, barriers increased by less than the concomitant increase in DPE upon changes in composition, because electrostatic stabilization of ion-pairs at the relevant transition states becomes more effective for weaker acids, as a result of their higher charge density at the anionic conjugate base. Alkoxide isomerization barriers were more sensitive to DPE than for elimination from H-bonded alkanols, the step that limits 2-butanol and 1-butanol dehydration rates; the latter two reactions showed similar DPE sensitivities, despite significant differences in their rates and activation barriers, indicating that slower reactions are not necessarily more sensitive to acid strength, but instead reflect the involvement of more unstable organic cations at their transition states. These compensating effects from electrostatic stabilization depend on how similar the charge density in these organic cations is to that in the proton removed. Cations with more localized charge favor strong electrostatic interactions with anions and form more stable ionic structures than do cations with more diffuse charges. Ion-pairs at elimination transition states contain cations with higher local charge density at the sp(2) carbon than for isomerization transition states; as a result, these ion-pairs recover a larger fraction of the deprotonation energy, and, consequently, their reactions become less sensitive to acid strength. These concepts lead us to conclude that the energetic difficulty of a catalytic reaction, imposed by gas-phase reactant proton affinities in transition state analogues, does not determine its sensitivity to the acid strength of solid catalysts.
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