Synthesis of Porous Transition Metal Oxides by the Salt-Gel Method
*Department of Chemistry, University of Minnesota, Minneapolis, MN 55455 ** Department of Chemistry, University of California, Berkeley, CA 94720 *** Department of Chemistry, Pennsylvania State University, University Park, PA 16802 ABSTRACT Composites of surfactants with certain transition metal oxides can be arranged into periodic arrays with giant tunnel structures. Transmission electron micrographs of these structures resemble images of mesoporous silicate molecular sieves synthesized in the presence of surfactants. However, data in this study lead to the conclusion that Keggin-ions of the type (H2W12O40)6- are formed during the hydrothermal synthesis of a cetyltrimethyl-ammonium/tungstate composite. These cluster anions are not connected to each other. Removal of the organic template therefore results in the collapse of the tunnel structures. A strategy has been developed which uses pre-arranged surfactant-salt structures as precursors for microporous and mesoporous framework materials. Transition metal silicates with long-range structural order have been synthesized by reacting surfactant salts of niobo-tungstate clusters with tetraethylorthosilicate to form a salt-gel. Infrared and solid-state NMR double resonance spectra indicate that, in addition to linkages between silicate tetrahedra, niobium-oxygen-silicon linkages have been formed in this reaction. Removal of the cationic surfactants by acid-extraction has resulted in porous structures with surface areas up to 265 m2/g. |