Highly Efficient MCM-48-based Template Synthesis of Osmium and Platinum 3-D Nanonetworks

Abstract

Nanostructured materials have attracted a great attention because of their novel sizeand shape-dependent electronic, magnetic, optical, and catalytic properties that differ drastically from those of bulk materials. Metals of nanosize, in particular, are expected to find useful applications in catalysis due to high surface area, and some results using nanoparticles as catalysts were reported. Ni/Pd core/shell nanoparticles show good catalytic activities in Sonogashira coupling reactions, and Pd nanoparticles with chiral ligands are good catalysts for asymmetric catalyses. These nanoparticles contain ligands on their surface, thus possessing very good solubilities in organic solvents. The improved solubility of nanoparticles, however, can impede facile recovery of the catalysts after reaction. Hyeon et al. reported the preparation of micron-sized hollow palladium spheres composed of bare nanoparticles without surfactants and these materials showed excellent catalytic activities in Suzuki cross coupling reactions and a good recoverability, thus calling for further extensive studies on designed 3-D nanostructures of other metals for efficient catalyses and facile catalyst recovery processes. Nanomaterials with smaller dimensions (2-10 nm in diameter) could be obtained through template synthesis using mesoporous silicates such as SBA-15, MCM-41, and MCM-48. Metal precursors, however, do not readily diffuse into the small pore channels of these mesoporous silicates, and thus efficient methods to load the metal inside the pores need to be developed to prepare porous templatebased nanomaterials. Although the insertion of the metal precursors by solution-phase infiltration and thermal vapor infiltration techniques have been successful in the preparation of nanostructured materials, the incorporated metal contents are rather low (3-10 wt % of metal loading based on SiO2 template). Supercritical fluids with large diffusion coefficients and low viscosity have also been employed to increase the amount of precursors loaded into small mesopores. The required harsh conditions to generate supercritical fluids, however, are very cumbersome. Herein we report a facile preparation of 3-D structured Os and Pt nanonetworks by utilizing the capillary action of mesopores of MCM-48 toward molten organometallic precursors. Some preliminary results of the work described herein have been previously reported. A Pyrex glass ampoule (1 × 5 cm, wall thickness = 2 mm) containing C16-MCM-48 (0.2 g) 12 and Os3(CO)12 (0.75 g; Os metal content = 0.47 g) is evacuated by a diffusion pump for 1 h, and then flame-sealed. The ampoule is heated at 265 C in a silicone oil bath. The color change of MCM-48 from white to brown indicates efficient sorption of brown liquid produced by the melting of thermolysis products of Os3(CO)12, such as Os5C(CO)15, Os6(CO)18, and Os7(CO)21. 13