In situ synthesis of gold nanoparticles inside the pores of MCM-48 in supercritical carbon dioxide and its catalytic application

Gold nanoparticles are deposited into the channels of MCM-48 through a simple H-2-assisted reduction of HAuCl4 (aqueous solution) in supercritical carbon dioxide medium at 70 degrees C within 2-4 h. The nanoparticles were characterized by powder Xray diffraction (PXRD), N-2 adsorption-desorption, transmission electron microscopy (TEM), and UV-Vis spectroscopy. The particle size of the synthesized material is tunable with the pressure (density) of the supercritical carbon dioxide medium. At the fixed temperature (70 degrees C) and hydrogen pressure [P(H-2) = 2 MPa], the Au particle size varies from ca. 25 nm to ca. 2 nm with the change in CO2 pressure from 7 MPa to 17 MPa. At the low solvent density conditions, larger particles of similar to 25 nm were obtained. On the contrary, a high solvent density of CO2 slows down particle aggregation, resulting in the small particle size within the range of 2-5 nm. This change in particle size with CO2 pressure and the interaction of the particles with the silica support were correlated well with long-range van der Waals interactions and consequently the Hamaker constant for the gold nanoparticle-CO2 (A(131)) and silica-gold core-CO2 (A(132)), respectively. Supercritical carbon dioxide alone can provide a unique environment for stabilizing gold nanoparticles in the channels of the cubic mesoporous MCM-48 support and exquisite control of the particle size without perturbing the support structure. The synthesized material is highly stable, recyclable and no metal nanoparticle leaching was observed. The selective hydrogenation of crotonaldehyde with the synthesized material provides convincing evidence that the particles are inside the pores and available to the reactant molecules.