| Single-site catalysts are materials in which all active sites show similar structure and function, irrespective of the density of active sites. The synthesis of these catalysts remains a challenge for high-valent metal-oxo species (e.g., V5+, Mo6+) because the random nature of impregnation processes in aqueous media during synthesis and the sintering of metals and oxides at high temperatures lead to highly non-uniform structures. Zeolite scaffolds provide well-defined structures with specific anchoring sites, which can impose uniform binding sites and prevent agglomeration. Here, we report the synthesis of uniform and stable high-valent metal-oxo species in ZSM5 zeolites and explore their function as catalysts and as catalyst precursors in oxidative and non-oxidative chemical reactions. These studies have led to robust protocols for exchange of high-valent cations and to structural characterization protocols including in situ spectroscopy and mass spectrometry for cations ranging from pentavalent V to heptavalent Re.;Reactions on anchored metal-oxo zeolites were studied in as-prepared materials and after reduction. Many of these materials demonstrate catalytic rates similar to or better than current state-of-art systems. As catalytic precursors, high-valent metal species consistently reduce and/or carburize to form ∼0.8 nm metal (Re) or carbide clusters (V, Mo, W), as detected by X-ray absorption spectroscopy. The size of these clusters resembles that of ZSM5 channel intersections that inhibit agglomeration during high temperature (∼1000 K) reactions of alkanes. It was found that carburization is autocatalytic, pyrolysis products that form on nascent carbide clusters during an induction period increase carburization rates. It was also found that C2 H4 could be added in carburizing feeds to decrease induction period times and increase pyrolysis rates on V-ZSM5 catalysts.;The methodology for precursor selection and cationic exchange into ZSM5 structures demonstrated herein can be more generally applied to other metal and zeolite systems with varying acidic strength, pore structure, and degree of crystallinity of which there are virtually unlimited combinations. The utilization of these methods will help to avoid overly complicated or misapplied synthetic strategies that have been adopted from unrelated materials and will ultimately lead to clearer understanding of a wide range of interesting potentially valuable new materials. |
