Engineering Dendrimer-Templated, Metal-Organic Framework-Confined Zero-Valent, Transition-Metal Catalysts.

Abstract

We describe and experimentally illustrate a strategy for synthesizing reactant-accessible, supported arrays of well-confined, sub-nanometer to 2 nm, metal(0) clusters and particles-here, copper, palladium, and platinum. The synthesis entails (a) solution-phase binding of metal ions by a generation-2 poly(amidoamine) (PAMAM) dendrimer, (b) electrostatic uptake of metalated, solution-dissolved, and positively charged dendrimers by the negatively charged pores of a zirconium-based metal-organic framework (MOF), NU-1000, and (c) chemical reduction of the incorporated metal ions. The pH of the unbuffered solution is known to control the overall charges of both the dendrimer guests and the hierarchically porous MOF. The combined results of electron microscopy, X-ray spectroscopy, and other measurements indicate the formation and microscopically uniform spatial distributions of zero-valent, monometallic Cu, Pd, and Pt species, with sizes depending strongly on the conditions and methods used for reduction of incorporated metal ions. Access to sub-nanometer clusters is ascribed to the stabilization effects imposed by the two templates (i.e., NU-1000 and dendrimer), which significantly limit the extent to which the metal atoms aggregate; as the thermal input increases, the dendrimer template gradually decomposes, allowing a further aggregation of metal clusters inside the hexagonal mesoporous channel of the MOF template, which ultimately self-limits at 3 nm (i.e., the mesopore width of NU-1000). Using CO oxidation and n-hexene hydrogenation as model reactions in the gas and condensed phases, we show that the dual-templated metal species can act as stable, efficient heterogeneous catalysts.