SALT GEL SYNTHESIS OF POROUS TRANSITION-METAL OXIDES

Hydrothermal reaction of sodium metatungstate with the surfactant temp late cetyltrimethylammonium (CTA) hydroxide gave the salt (C19H42N)(6) (H2W12O40). Despite the superficial similarity of TEM micrographs and powder X-ray patterns of this material to those of mesoporous silicate s, the salt contains unconnected Keggin ions H2W12O406-. These Keggin ions pack in a puckered layer arrangement and create roughly spherical cavities for the surfactant micelle counterions. Attempts to remove t he template cations and condense the inorganic portion of the structur e invariably lead to dense WO3-x phases. NbxW6-xO19(2+x)(x = 2, 3, 4) clusters also formed layered salts with CTA cations, which were reacte d with tetraethyl orthosilicate (TEOS). The TEOS molecules are absorbe d, presumably into the hydrophobic portion of the structure, and can b e hydrolyzed to form silica within the salt. Infrared and solid-state NMR double-resonance spectra show that the silica network is anchored to the clusters via covalent Nb-O-Si linkages. Extraction of the silic a-modified salts with HCl/ethanol yields materials with both micropore s and mesopores and specific surface areas up to 265 m(2)/g.