Sol-gel synthesis of heterogeneous catalysts from aqueous solutions

The formation of solid compounds from aqueous solutions is based on the hydrolysis and condensation of metal cations. When dissolved in water, metal salts give rise to solvated species in which the metal cation is surrounded by H2O, OH- or O2- ligands. Condensation via elation or oxolation occurs in the pH range where OH Ligands are present. Olation with low-valent cations rather leads to polycations and the precipitation of hydroxides whereas the oxolation of high-valent cations leads to the formation of polyanions and oxides. Complexation can also occur in the presence of foreign ions, allowing some chemical control of condensation reactions. These reactions will be described in the frame of the partial charge model. (C) 1998 Elsevier Science B.V. All rights reserved.