Preparation of Transition Metal Oxides By A Nonhydrolytic Sol-Gel Process
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PASCAL ARNAL, ROBERT J.P. CORRIU, DOMINIQUE LECLERCQ, P. HUBERT MUTIN AND ANDRE VIOUX. UMR 44, CNRS/ Rh6ne-Poulenc/ Universit6 Montpellier II, case 007, Universit6 Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, France.
ABSTRACT: Etherolysis of metal chlorides offers a versatile, simple nonhydrolytic sol-gel route to transition metal oxides. The main limitation of the process results from the insolubility of some metal chlorides in non aqueous solvents. The general tendency of this nonhydrolytic process is to delay crystallization of metal oxides.
INTRODUCTION: Classical sol-gel process is based on the hydrolysis of either inorganic salts or metal alkoxides [1]. However transition metal alkoxides are very sensitive to nucleophilic substitution reactions such as hydrolysis; therefore, they readily give rise to precipitates rather than gels by addition of water. This problem has been overcome by chemical modification of metal alkoxides [2]. Another way would be to use inorganic polymerization reactions not involving hydrolysis. We have already reported such a nonhydrolytic sol-gel process, where condensation reactions occur between metal chlorides and either metal alkoxides or ethers (scheme 1) [3]. scheme 1:
MCWn + M(OR)n MCIn + n/2 ROR
ClnIM-O-M(OR)n_1 + RCI . RO-MCln_1 + n RCI
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-
---
--- 2 MOn/2 + n RCl MOn/ 2 + n RC1
Note that the method involving ether (namely etherolysis) is particularly attractive in the case of transition metals, since it avoids the use of metal alkoxides, which are often difficult to synthesize and expensive. It is well-known that metal chlorides adduct with ethers[4]. The thermal stability of complexes depends on the nature of the ether, and a rather general reaction is the formation of chloroalkoxides (scheme 2) [5, 6]. scheme 2: MCIn +x ROR
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0
MCIn , x ROR
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ROMCln_1 + RC1
Futhermore, it has been reported that metal chloroalkoxides themselves react on being heated, giving metal oxo chlorides and alkyl chloride [3, 6, 7]. The purpose of this work is to determine the suitability of etherolysis in the preparation of transition metal oxides, and the influence of the nature of ether on the texture and structure of oxides.
339 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL: The stoichiometric amount of ether (half a mole per Cl groups) was added to the metal chloride either neat or dissolved in a solvent (CS 2 or CH 2C12) in a Schlenk tube. After dissolution, the mixture was transfered in a tube, frozen in liquid nitrogen then sealed under vacuum. The sealed tube was held at 110'C until gelation occured, then opened in a glove bag under argon. The solid and the liquid phases were separated by filtering. The isolated solid was washed with successive fportions of dichloromethane, then vacuum dried at 200'C for 3 h. Thermogravimetric analyses (TGA) were performed in a 20/80 mixture of oxygen and nitrogen at 10 K/min heating rate. Specific surface areas were determined by the BET method. X-ray diffraction (XRD) patterns were rec
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