Sol–gel processing of niobium chloroalkoxides

Abstract

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 chloride concentration. At an optimum Nb : H₂O ratio there is a minimum in the gelation time which is related to a balance between promotion of hydrolysis and the concentration of condensing niobium species. The data are fitted to an expression derived from Flory–Stockmeyer theory of gelation, from which an approximate rate constant for condensation of 800 mol^–1 dm³ s^–1 is calculated. Further water addition causes hydrolysis of residual Nb—Cl bonds accompanied by a decrease in the gelation rate. This is discussed in terms of (i) the decrease of the chloroxolation condensation reaction, (ii) the stabilisation of the oxolation transition state and (iii) the protonation of reactive MOH groups. The Partial Charge Model is used to predict the dependence of hydrolysis rates on the alcohol type and the nature of the various possible leaving groups. However, the observed trend that the rate of gelation increases as the size of the alcohol increases appears to be due to the decrease in precursor substitution with increasing size of the alcohols, rather than charge effects. This leads to an increase in the number of M—Cl bonds (or M—OH) bonds with a consequently enhanced condensation rate. Although Cl^– is not a good leaving group, chloride ion is not present in the dried gel, i.e. it is completely hydrolysed. Using analytical and vibrational spectrosopic data we have shown that the product of hydrolysis and gelation is hydrated Nb₂O₅ rather than NbCl₂(OH)₃ as previously proposed.