Encapsulation of metal clusters within zeolites can protect metals against sintering and prevent their contact with feed impurities, while concurrently allowing active sites to select reactants based on their molecular size. The small apertures in zeolitic materials preclude post-synthetic protocols as the means for encapsulating metal precursors. We have developed general strategies to encapsulate metal clusters within small-pore zeolites by using ligands that stabilize metal precursors against premature precipitation at the high temperatures and pH required for zeolite synthesis. For required synthesis temperatures above 380 K, ligands were not able to prevent precipitation. In such cases, encapsulation was achieved via interzeolite transformation of another zeolite, within which metal precursors or clusters can be encapsulated via post synthesis exchange or ligand protection. 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 zeolites (SOD, LTA, GIS, and ANA).Successful encapsulation was probed using microscopy, chemisorption and catalytic reactions of small and large molecules. TEM images confirmed that metal clusters were small (1.1 -1.9 nm) and uniformly distributed, consistent with the mean cluster sizes obtained from chemisorption (1.2-1.8 nm) indicating that cluster surfaces were free of residues from ligands. Metal clusters were stable against sintering during oxidative thermal treatments (573�873 K). Encapsulation selectivities were measured by comparing the turnover rates of hydrogenation of alkenes and oxidative dehydrogenation of alkanols of small and large sizes, which reflect the ratio of metal clusters within and outside zeolite crystals and ranged from 7 to 83 for all samples, consistent with successful encapsulation. Confinement of metal clusters within zeolites protected them from contact with poison species (e.g. thiophene, H2S) and allowed catalytic reactions to proceed at poison concentrations that otherwise fully suppressed reactivity for unconfined metals dispersed on SiO2.

Synthesis and catalytic consequences of encapsulation of metal clusters within zeolites

248th National Meeting (2014)

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