The selective confinement of metal precursors within nanometer voids can lead to clusters of small and uniform diameter that are stable against coalescence and growth; reactants preferentially access their active surfaces based on their size and larger molecules that often act as poisons are excluded from the void environment. Post-synthetic confinement of precursors or clusters is seldom possible for small-pore and medium-pore zeolites, because their apertures preclude the diffusion of solvated precursors. Here, we describe strategies for encapsulation during hydrothermal zeolite syntheses by using ligand-stabilized precursors that prevent premature colloidal precipitation of precursors at synthesis pH and temperatures. Such strategies succeed for zeolites requiring moderate crystallization temperatures (&lt;400 K), but fail in more demanding conditions, for which encapsulation was achieved instead by first placing precursors in larger-pore zeolites via post-synthesis exchange or ligand protection and then converting parent zeolites into a target structure with smaller apertures. The resulting encapsulated clusters are small and uniform and stable at temperatures much higher than those dispersed in mesoporous supports. Their surfaces are clean and devoid of ligand debris; they catalyze reactions of molecules that can access such surfaces, while excluding larger molecules, including organosulfur poisons. These protocols are successful for many catalytically-relevant elements (Pt, Pd, Ru, Rh, Ir, Re, Ag) and many zeolites (SOD, LTA, GIS, ANA, MFI). The guiding principles and mechanistic insights derived from these successful interzeolite transformations led to the synthesis of high-silica zeolites (MFI, CHA, STF, MTW) from FAU and BEA parent materials without requiring organic templates. These transformations succeed for daughter structures with higher framework densities, when their nucleation becomes possible by the addition of seeds or by the presence of structural building units common to parent and target structures; in the latter case, high-silica zeolites form via spontaneous transformations of parent zeolites at specific synthesis conditions.

Strategies for the synthesis of zeolites and for encapsulating clusters within their voids

251st National Meeting (2016)

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