Transition-Metal Nanoparticles in Hollow Zeolite Single Crystals as Bifunctional and Size-Selective Hydrogenation Catalysts

Abstract

Transition-metal nanoparticles (Co, Ni, and Cu) encapsulated in hollow zeolite single crystals were prepared by recrystallization of impregnated bulk MFI crystals in the presence of tetrapropylammonium (TPAOH) solutions. The size and number of particles in hollow MFI depended mainly on the aluminum content. The encapsulation of the nanoparticles prevented them from growing, thus enabling the control of particle size even after high temperature treatments. For low metal loadings (<3 wt %), the mean particle sizes for Co, Ni, and Cu in hollow silicalite-1 were 3.5 ± 0.3, 3.1 ± 0.5, and 1.5 ± 0.2 nm, respectively. In the case of hollow ZSM-5, higher loadings (∼8 wt %) could be obtained with mean particle sizes of 17 ± 2 nm, 13 ± 2 nm, and 15 ± 2 nm for Co, Ni, and Cu systems. The mechanism of transition metal nanoparticle formation was markedly different from that of noble metals. At high pH values, transition-metal cations first reacted with dissolved silica species yielding fibrous metal phyllosilicates that were located inside the crystal cavities. The metal phyllosilicates were then converted into nanoparticles upon reduction under H2 at high temperature (500–750 °C). Silicalite-1 encapsulated Ni particles were used in the catalytic hydrogenation of substituted benzenes and showed an outstanding size-selectivity effect. Ni particles were accessible to toluene but not to mesitylene, confirming that the activity is directly related to the diffusion properties of molecules through the zeolite membrane.

Palladium Nanoparticles Encapsulated in a Silicalite‐1 Zeolite Shell for Size‐Selective Catalysis in Liquid‐Phase Solution

“…reported the synthesis of size‐controlled metal nanoparticles (e.g., Au, Pt, Co, Ni, etc.) encapsulated in hollow silicalite‐1 zeolite yolk–shell materials through a process involving metal‐ion impregnation, tetrapropylammonium hydroxide (TPAOH) treatment, and H 2 reduction . However, by using this method most of the hollow shells contained only one metal nanoparticle.…”

Controlled Pyrolysis of Ni‐MOF‐74 as a Promising Precursor for the Creation of Highly Active Ni Nanocatalysts in Size‐Selective Hydrogenation

“…However, the preparation of catalysts for size‐selective hydrogenation of olefins with different molecular sizes is still a difficult problem. The most common approach to achieve size‐selective catalysis is the fabrication of core–shell type catalysts with molecular sieve properties to avoid the diffusion of huge molecules and/or poisoning of particular active sites to inhibit excess hydrogenation . However, such catalysts frequently suffer from lower reaction rates than conventional catalysts for selective hydrogenation.…”

Characterization of Metal‐zeolite Composite Catalysts: Determining the Environment of the Active Phase

“…Due to the nature of a heterogeneous catalyst, synthesizing a metal‐zeolite composite catalyst with homogeneous distribution of the active phase is challenging. Heterogeneity of the catalyst increases the difficulty of characterization and data interpretation, which can be seen for a recently developed synthesis technique that deposits transition metals within hollow zeolites . Because the active metal phase is located within the zeolite shell, this catalyst can serve as a multifunctional nanoreactor, with many applications including selective hydrogenation.…”