SO,-ZrO, and SiOz-TiOz mixed oxides with various metal contents have been prepared by a non-hydrolytic sol-gel route involving the condensation between chloride and isopropoxide functions at 110 "C. Well condensed, monolithic gels were obtained in one step, without the use of additives. The Si/M ratio of the oxide may be controlled easily by the composition of the starting mixture. The Si/Zr oxides remained amorphous after calcination for 5 h at 600 "C; IR and 29Si NMR spectroscopy showed a large amount of Si-0 -Zr bonds, indicating a homogeneous distribution of the components on the atomic scale. The crystallization of tetragonal zirconia took place at higher temperature; the transformation of tetragonal to monoclinic zirconia was strongly retarded and did not take place after 2 h at 1300 "C. The crystallization of zircon (for a sample containing 50 mol% Zr) started at 1500 "C and was completed after 20 h at 1500 "C. IR spectroscopy indicated the presence of a limited number of Si-0-Ti bonds in all the Si/Ti oxides after calcination for 5 h at 500 "C. The sample within the stable glass region ( 5 mol% Ti) appeared perfectly homogeneous: it crystallized at 900 "C as single-phase cristobalite oxide, with Ti4+ ions substituting Si4+ ions at random. On the other hand, the precipitation of anatase was observed for the %/Ti oxides with a high Ti content (20-50 mol% Ti), which are outside the stable glass region. The transformation of anatase to rutile was not observed even after 2 h at 1300 "C.The technological interest in silica-titania and silica-zirconia mixed oxides arises from their chemical resistance and their thermomechanical or optical properties: Si0,-TiO, glasses and zircon, SiZrO,, are characterized by very low thermal expansion, which confer them a high thermal shock resistance; Si0,-TiO, and SiO,-ZrO, glasses have high refractive indices, and they are also of great importance as catalysts or as cata!yst supports. Owing to their refractoriness, these oxides are difficult to produce by conventional melting techniques. Accordingly, the sol-gel processing of alkoxides,' which allows the preparation of glasses at low temperature, has been used widely for the preparation of Si02-Ti022-7 and Si0,-Zr028-10 glasses or zircon."-'4The homogeneity of binary oxide gels has a great influence on the structural evolution of the gels during the heat treatment. However, obtaining homogeneous Si0,-Ti02 or Si0,-ZrO, gels by hydrolysis/condensation is not straightforward, owing to the difference in reactivity between silicon alkoxides and transition-metal alkoxides, leading to the fast formation of M-0-M bonds and to the precipitation of the metal oxide. To overcome this limitation several procedures have been elaborated, such as the pre-hydrolysis of the less reactive silicon a l k ~x i d e ~. ~. 'or the stabilization of the metal alkoxide by c o m p l e ~a t i o n . ~~' ~The aim of these procedures is to promote the formation of mixed Si-0-M bonds, i.e. homogeneity on the atomic scale.In recent years, we have been developi...

The efficiency of the nonhydrolytic sol−gel process based on the condensation between metal chlorides and metal alkoxides has been studied in the preparation of aluminum titanate gels. Solutions of aluminum and titanium precursors in the ratio Al/Ti = 2 gave gels with the same metal ratio. The structural evolution of the samples is unusual; they directly crystallize at T > 600 °C into metastable pseudo-brookite β-Al2TiO5 and are reluctant to decompose into stable TiO2 (rutile) and α-Al2O3 at 1150 °C. This behavior must be connected to the high homogeneity of the samples originating from the nonhydrolytic sol−gel process.

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Abstract. The effectiveness of nonhydrolytic Sol-Gel process to the synthesis of homogeneous bicomponent gels has been studied in the case of aluminium titanate and zirconium titanate. The direct crystallization of A12TiO5 and ZrTiO4 at low temperature without the intermediate formation of Tit2 or A1203 in the former case and Tit2 or ZrO2 in the latter indicates that homogeneous bicomponent gels have been formed by this route.

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