Transformation of Stöber Silica Spheres to Hollow Hierarchical Single-Crystal ZSM-5 Zeolites with Encapsulated Metal Nanocatalysts for Selective Catalysis.

Abstract

The activity of zeolite-supported nanocatalysts is dependent on both the dispersion, size, and location of metal nanoparticles around the zeolite and the size and pore structure of the zeolite. In this study, a synthetic approach was developed to encapsulate metal catalysts within hollow interiors of single-crystal ZSM-5. Briefly, Stöber silica spheres were synthesized and then transformed to single-crystal nano-ZSM-5 (Si/Al = 60), followed by growth of embedded metal nanoparticles and subsequently creation of a nanosized (30-50 nm shell thickness) hollow hierarchical zeolite structure. Metal nanoparticles such as Co, Cu, Cu-Zn, Fe, and Ni can be supported on the inner wall of the hollow zeolite and the surrounding satellite mesopores, without any particles present on the external zeolite surface. When evaluated as a catalyst for the Fischer-Trøpsch reaction, the Fe@h-ZSM5 catalyst shows high activity, sintering and coking resistance (50% longer stability than Fe@ZSM5), and secondary cracking reactions in the acid sites in the ZSM-5 shell, which reduce C5+ hydrocarbon selectivity and increase smaller-chain hydrocarbon selectivity. In addition, when Pt was further deposited inside the hollow structure, shape-selective alkene hydrogenation was demonstrated. These configured nanoscale zeolite catalysts have potential applications for reactions that involve supported metal nanoparticle catalysis, shape selectivity, or secondary cracking reactions.