Synthetic strategies for the encapsulation of nanoparticles of Ni, Co, and Fe oxides within crystalline microporous aluminosilicates

Abstract

A synthetic strategy is reported here for the selective containment of oxide nanoparticles of base metals within zeolitic voids of molecular dimensions. The technique, though generally applicable, is specifically illustrated to encapsulate Ni, Co, and Fe oxides within LTA, MFI, and FAU zeolites through hydrothermal framework crystallization in the presence of ligand-protected metal cations. Such ligands contain bidentate amine groups that preclude the precipitation of metal precursors in alkaline synthesis gels, and alkoxysilane moieties that form covalent linkages with nucleating zeolite precursors to enforce metal uptake into crystallized solids. These ligands are removed by subsequent oxidative treatments that nucleate oxide nanoparticles without structural degradation of the zeolites. The clusters are small (<2.5 nm) and uniformly distributed in size, reflecting their constrained growth within zeolite crystals. In contrast with exchange strategies for encapsulation, which lead to grafted cations and dense metal aluminosilicates, these methods form oxide nanoparticles, evident from infrared spectra of samples exposed to CO. Oxide nanoparticles undergo more facile redox cycles than grafted cations or dense aluminosilicates, thus rendering oxide domains more effective oxidation catalysts. The dynamics and stoichiometry of nanoparticle reduction in H2 confirmed the presence of NiO, Co3O4, and Fe2O3 clusters and their more facile reducibility relative to metal aluminosilicates. Ethanol oxidation rates on these clusters were essentially unaffected by exposure to bulky thiol poisons that titrate metal oxide surfaces, reflecting the selective placement of the oxide nanoparticles within the confines of microporous voids, where they are protected from contact by large molecules. These synthetic strategies and guiding principles circumvent long-standing hurdles in the selective encapsulation of base metals, and provide enabling routes for the synthesis of the many metal-zeolite systems that confront similar hurdles.