SOL-GEL PROCESSING OF NIOBIUM CHLOROALKOXIDES

The gelation behaviour of niobium chloroalkoxide in alcohol solutions containing excess added water (H2O:Nb > 1) has been investigated as a function of Nb:H2O ratio, alcohol type and concentration and the chlor ide concentration. At an optimum Nb:H2O ratio there is a minimum in th e gelation time which is related to a balance between promotion of hyd rolysis and the concentration of condensing niobium species. The data are fitted to an expression derived from Flory-Stockmeyer theory of ge lation, from which an approximate rate constant for condensation of 80 0 mol-1 dm3 S-1, is calculated. Further water addition causes hydrolys is of residual Nb-Cl bonds accompanied by a decrease in the gelation r ate. This is discussed in terms of (i) the decrease of the chloroxolat ion condensation reaction, (ii) the stabilisation of the oxolation tra nsition state and (iii) the protonation of reactive MOH groups. The Pa rtial Charge Model is used to predict the dependence of hydrolysis rat es on the alcohol type and the nature of the various possible leaving groups. However, the observed trend that the rate of gelation increase s as the size of the alcohol increases appears to be due to the decrea se in precursor substitution with increasing size of the alcohols, rat her than charge effects. This leads to an increase in the number of M- Cl bonds (or M-OH) bonds with a consequently enhanced condensation rat e. Although Cl- is not a good leaving group, chloride ion is not prese nt in the dried gel, i.e. it is completely hydrolysed. Using analytica l and vibrational spectrosopic data we have shown that the product of hydrolysis and gelation is hydrated Nb2O5 rather than NbCl2(OH), as pr eviously proposed.