(49b) Strategies for the Encapsulation of Metal Clusters within Zeolites and Consequences for Catalytic Stability, Reactivity and Selectivity

The encapsulation of metal clusters within zeolite voids can protect such clusters from coalescence and growth and also from contact with feed impurities, while allowing reactions to benefit from the product and reactant shape selectivity conferred by the molecular size of zeolite windows and voids [1-3]. We have developed general protocols for the synthesis of metal clusters encapsulated within small and medium-pore zeolites and for the understanding of catalytic consequences of encapsulation [4-6]. The small apertures within these materials preclude post-synthetic encapsulation protocols, which involve solvated metal-oxo oligomers that cannot diffuse into these microporous solids.  Therefore, we have developed encapsulation methods that use ligand-stabilized metal precursors that do not precipitate as colloidal oxyhydroxides at the high pH and temperatures required for hydrothermal zeolite crystallization.  For zeolites requiring temperatures too high for stable ligated precursors, we have achieved encapsulation by first placing clusters within zeolites that form at milder conditions and then retaining encapsulation during their crystalline interconversion into the zeolite framework of interest. Specifically, these combined strategies have led to the successful encapsulation of a wide range of metal clusters (Pt, Pd, Ru, Rh, Ir, Re, and Ag) within small-pore (SOD, LTA, GIS, and ANA) and medium-pore (MFI) zeolites.

      The success of encapsulation, the size uniformity and surface cleanliness of metal clusters were demonstrated by combining electron microscopy, chemisorptive titrations, and catalytic reactions of small and large molecules. Transmission electron microscopy showed that clusters were small (1.1-1.9 nm) and uniform in size, consistent with measurements by H₂ or O₂ chemisorptions (1.2-1.8 nm), indicating that clusters surfaces were accessible and clean and therefore suitable for catalysis on their surfaces. The encapsulated clusters did not change in size during oxidative thermal treatments at temperatures (573–873 K) that led to significant growth for clusters of comparable size and composition dispersed instead on mesoporous SiO₂. The preferential encapsulation of metal clusters within zeolite voids was confirmed by measuring the ratio of oxidative dehydrogenation (ODH) and hydrogenation rates for small (methanol, ethene and toluene) and large (isobutanol, isobutene and trimethyl benzene) reactants on unconstrained clusters dispersed on SiO₂ (Χ_SiO2 = r_{small reactant}/r_{large reactant}) and on clusters of the same metal in zeolites (χ_zeolite). The ratio of these relative reactivities denotes the encapsulation selectivity parameter (Φ = χ_zeolite/χ_SiO2), indicative of the ratio of active surfaces within zeolite crystals to total exposed metal surface area. This encapsulation selectivity parameter would approach unity for clusters with unimpeded access to reactants, such as those at external zeolite surfaces. The encapsulation selectivity (Φ) values were 7-83 for all samples indicating the predominant presence of active sites within zeolite structures. The selective confinement of these clusters within microporous regions also protected cluster surfaces from contact with catalytic poisons, such as thiophene and H₂S, which cannot access such regions, and allowed ethene hydrogenation and H₂-D₂ exchange rates to proceed at rates unperturbed by titrants that fully suppressed reactivity on similar clusters dispersed on mesoporous SiO₂.

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5. Goel, S.; Wu, Z.; Zones, S. I.; Iglesia, E., J. Am. Chem. Soc. 2012, 134 (42), 17688. 

6. Wu, Z.; Goel, S.; Choi, M.; Iglesia, E., J. Catal. 2014, 311, 458.

The authors gratefully acknowledge financial support for these studies from Chevron Energy Technology Company.