Zeolite Fixed Metal Nanoparticles: New Perspective in Catalysis.

Abstract

ConspectusLoading metal nanoparticles on the surface of solid supports has emerged as an efficient route for the preparation of heterogeneous catalysts. Notably, most of these supported metal nanoparticles still have shortcomings such as dissatisfactory activity and low product selectivity in catalysis. In addition, these metal nanoparticles also suffer from deactivation because of nanoparticle sintering, leaching, and coke formation under harsh conditions. The fixation of metal nanoparticles within zeolite crystals should have advantages of high activities for metal nanoparticles and excellent shape selectivity for zeolite micropores as well as extraordinary stability of metal nanoparticles immobilized with a stable zeolite framework, which is a good solution for the shortcomings of supported metal nanoparticles.Materials with metal nanostructures within the zeolite crystals are normally denoted as metal@zeolite, where the metal nanoparticles with diameters similar to those of industrial catalysts are usually larger than the micropore size. These metal nanoparticles are enveloped with the zeolite rigid framework to prevent migration under harsh reaction conditions, which is described as a fixed structure. The zeolite micropores allow the diffusion of reactants to the metal nanoparticles. As a result, metal@zeolite catalysts combine the features of both metal nanoparticles (high activity) and zeolites (shape selectivity and thermal stability), compared with the supported metal nanoparticles.In this Account, we describe how the zeolite micropore and metal nanoparticle synergistically work to improve the catalytic performance by the preparation of a variety of metal@zeolite catalysts. Multiple functions of zeolites with respect to the metal nanoparticles are highlighted, including control of the reactant/product diffusion in the micropores, the adjustment of reactant adsorption on the metal nanoparticles, and sieving the reactants and products with zeolite micropores. Furthermore, by optimizing the wettability of the zeolite external surface, the zeolite crystals could form a nanoreactor to efficiently enrich the crucial intermediates, thus boosting the performance in low-temperature methane oxidation. Also, the microporous confinement weakens the adsorption of C1 intermediates on the metal sites, accelerating the C-C coupling to improve C2 oxygenate productivity in syngas conversion. In particular, the zeolite framework efficiently stabilizes the metal nanoparticles against sintering and leaching to give durable catalysts. Clearly, this strategy not only guides the rational design of efficient heterogeneous catalysts for potential applications in a variety of industrial chemical reactions but also accelerates the fundamental understanding of the catalytic mechanisms by providing new model catalysts.