Transition-Metal Nanoparticles in Hollow Zeolite Single Crystals as Bifunctional and Size-Selective Hydrogenation Catalysts
Résumé
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 (similar to 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 H-2 at high temperature (500-750 degrees 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.