NDLI: Sol–gel processing of niobium chloroalkoxides

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Journal of Materials Chemistry Vol - 4 Issue - 6

Journal of Materials Chemistry Vol - 4 Issue - 5

Journal of Materials Chemistry Vol - 4 Issue - 4

Journal of Materials Chemistry Vol - 4 Issue - 3

Journal of Materials Chemistry Vol - 4 Issue - 2

Journal of Materials Chemistry Vol -3

Sol–gel processing of niobium chloroalkoxides

Content Provider	Royal Society of Chemistry (RSC)
Author	Lee, G. Robert Crayston, Joe A.
Copyright Year	1994
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.
Starting Page	1093
Ending Page	1100
Page Count	8
File Format	PDF
ISSN	09599428
Volume Number	4
Issue Number	7
Journal	Journal of Materials Chemistry
DOI	10.1039/JM9940401093
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Open
Subject Keyword	Flory Stockmeyer Niobium Properties of water Chloride Hydrolysis Condensation reaction Dependent and independent variables Leaving group Ion Niobium pentoxide
Content Type	Text
Resource Type	Article
Subject	Chemistry Materials Chemistry

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