Infusion of presynthesized iridium nanocrystals into mesoporous silica for high catalyst activity

Traditionally, finely dispersed metal catalysts have been formed by reduction of precursors within mesoporous supports. A new concept for designing catalysts with enhanced activities and selectivities is to infuse presynthesized nanocrystals with well-defined morphologies into ordered mesoporous materials. The decoupling of nanocrystal synthesis and infusion provides exquisite control of the nanocrystal size, morphology, and dispersibility within the pores. A dispersion of indium nanocrystals was infused into mesoporous silica by expanding the solvent toluene with supercritical CO₂. To achieve high nanocrystal loadings, up to 1.3 wt %, we tuned the solvent quality to strengthen the interactions of the nanocrystals with the pore walls, but without precipitating the nanocrystals in the bulk solvent. Z-contrast STEM indicates conclusively that the iridium nanocrystals were located within the pores and not on the external silica surface. High catalytic activity was observed for 1-decene hydrogenation, which is consistent with a high degree of dispersion of the 4.5 nm nanocrystals throughout the pores, as observed by TEM. A maximum turnover frequency (TOP) of 16 s^-1 was measured, which was higher than the initial TOP for homogeneous catalysis with the same nanocrystals in 1-decene. The iridium catalysts do not require pretreatment to remove the tetraoctylammonium bromide ligands to achieve activation, as the ligands bind weakly to the iridium surface. Consequently, the activity was not enhanced when calcined at 500°C in nitrogen or when annealed in supercritical CO ₂ at 275 bar. The ability to predesign nanocrystal morphology and surface properties prior to infusion into the mesoporous silica support offers novel opportunities for enhanced catalyst activity, stability, and reaction selectivity.