Hydrothermal synthesis of LTA-encapsulated metal clusters and consequences for catalyst stability, reactivity, and selectivity

Abstract

Noble metal clusters (Pt, Pd, Rh, Ir, Re, and Ag) are selectively encapsulated within LTA voids via hydrothermal synthesis using metal precursors with ligands (NH3 for Pt and Ir; ethylenediamine for Pd, Rh, Re and Ag) that prevent their premature precipitation as colloidal oxyhydroxides. Such stability appears to be necessary and sufficient for successful encapsulation of cationic precursors during nucleation and growth of zeolite frameworks. Mean cluster diameters measured by titration of exposed metal atoms (H-2 on Pt, Pd, Rh, Ir and Re; O-2 on Ag; 1.1-1.8 nm) and by transmission electron microscopy (1.2-1.9 nm) were similar, indicating that cluster surfaces were clean and accessible to molecules used as titrants or reactants. Metal clusters were narrowly distributed in size and stable against sintering and coalescence during oxidative thermal treatments (573-873 K). Encapsulation selectivities were measured from turnover rates for reactions of small and large reactants, specifically hydrogenation of alkenes (ethene and isobutene) and oxidation of alkanols (methanol, ethanol, and isobutanol), which reflect the restricted access to encapsulated clusters by the larger molecules. These encapsulation selectivities, which reflect the ratio of metal surface areas within and outside LTA crystals ranged from 7.5 to 83 for all samples. Confinement within LTA crystals protects clusters from contact with thiophene and allows ethene hydrogenation to proceed at thiophene concentrations that fully suppressed reactivity for metal clusters dispersed on mesoporous SiO2. These protocols provide a general strategy for encapsulating clusters within small-pore zeolite voids, for which post-synthesis exchange is infeasible. Their successful encapsulation protects such clusters from coalescence and growth and allows them to select reactants and reject poisons based on their molecular size.