Converting Hierarchical to Bulk Structure: A Strategy for Encapsulating Metal Oxides and Noble Metals in Zeolites

Abstract

Encapsulating catalytically active components into zeolites is a way to prepare multifunctional catalysts with unique selectivity and enhanced stability. Previously reported methods for encapsulation were only suitable for encapsulating specific species with a limited loading capacity. Here, we report a general strategy for encapsulating various metal oxides in zeolites. Our strategy is based on the use of directly synthesized hierarchical zeolites with abundant intracrystalline mesopores. Metal oxides that are preloaded in the mesopores by impregnation become encapsulated during a secondary growth process that converts the original hierarchical structure into a bulk structure of zeolite. This method enables the encapsulation of ultrafine particles (2–4 nm) of various metal oxides (CeO2, TiO2, and MnOx) in zeolites with loading as high as >10 wt %. Furthermore, we modify this method to achieve the encapsulation and high dispersion of noble metals (Au and Pt), which would otherwise agglomerate into large particles on the zeolite surfaces, by taking advantage of their strong interactions with metal oxides. The encapsulated metal oxides and metal oxide-supported noble metals demonstrate reactant selectivity, product selectivity, and excellent thermal stability during catalytic oxidation and hydrogenation reactions.