Transition-Metal Nanoparticles in Hollow Zeolite SingleCrystals as Bifunctional and Size-Selective Hydrogenation Catalysts.

Transition-metal nanoparticles (Co,Ni, and Cu) encapsulated inhollow zeolite single crystals were prepared by recrystallizationof impregnated bulk MFI crystals in the presence of tetrapropylammonium(TPAOH) solutions. The size and number of particles in hollow MFIdepended mainly on the aluminum content. The encapsulation of thenanoparticles prevented them from growing, thus enabling the controlof particle size even after high temperature treatments. For low metalloadings (<3 wt %), the mean particle sizes for Co, Ni, and Cuin 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 Cusystems. The mechanism of transition metal nanoparticle formationwas markedly different from that of noble metals. At high pH values,transition-metal cations first reacted with dissolved silica speciesyielding fibrous metal phyllosilicates that were located inside thecrystal cavities. The metal phyllosilicates were then converted intonanoparticles upon reduction under H2at high temperature(500–750 °C). Silicalite-1 encapsulated Ni particles wereused in the catalytic hydrogenation of substituted benzenes and showedan outstanding size-selectivity effect. Ni particles were accessibleto toluene but not to mesitylene, confirming that the activity isdirectly related to the diffusion properties of molecules throughthe zeolite membrane. [ABSTRACT FROM AUTHOR]