Titanium oxide particles were treated using six organophosphorus compounds chosen as model coupling molecules: phenylphosphonic and diphenylphosphinic acids, their ethyl esters, and their trimethylsilyl esters. The ability of all of these coupling molecules to modify the surface of the TiO2 particles was demonstrated by elemental analysis, thermogravimetric analysis, and nitrogen adsorption. The bonding modes on the surface were investigated by means of diffuse reflectance IR Fourier transform (DRIFT) and 31P solid-state MAS NMR spectroscopy. Upon irradiation in water, a marked trend to the photooxidative degradation of the anchored organophosphorus groups was evidenced, especially in the case of phosphinate groups.
The surfaces of alumina particles were modified by grafting with phenylphosphonic acid and its organic-soluble ester derivatives (diethyl phenylphosphonate and bis(trimethylsilyl) phenylphosphonate). Solid-state 31 P NMR spectroscopy indicated that in aqueous media the formation of bulk aluminium phosphonate phases could be avoided by using phenylphosphonic acid at pH 6. The formation of such phases was detected in organic media in the presence of phenylphosphonic acid or its silyl ester. On the other hand the use of the dialkyl ester derivative in organic media allowed controlled grafting, excluding the formation of phosphonate phases even under prolonged heating. Alternatively a two-step sol-gel process was carried out, which involved first the nonhydrolytic condensation between aluminium alkoxide and phenylphosphonic acid (or the parent bis(trimethylsilyl) ester), then the hydrolysis-condensation of the remaining Al-OR groups. 31 P and 27 Al NMR spectroscopy proved the homogeneity of the solids obtained, even for P/Al ratios as high as 1.
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...
Titania samples prepared by different non-hydrolytic sol-gel methods, mainly based on the etherolysis and alcoholysis of titanium tetrachloride, have been found to differ in both structure and texture. Thus, the reaction of diethyl ether with TiCl, at 110 'C affords anatase, which begins to convert into rutile only around 1000 "C. The reaction of TiC1, with ethanol leads to rutile as early as 110 "C, whereas the reaction of tert-butyl alcohol at 110 "C leads to the singular formation of brookite.The development of low-temperature routes to transitionmetal oxide processable materials underlies the recent works in the field of sol-gel synthesis.' In these methods based on the hydrolysis of molecular precursors, such as metal alkoxides, the major problem is control of the reaction rates which are generally too fast. An attractive solution is to use organic additives which act as chelating ligands (carboxylic acids, pdiketones, etc.) and modify the reactivity of the precursors.2 Here we propose various non-hydrolytic sol-gel routes tested in the case of titanium, chosen as a representative transition metal.As reported previously, a novel sol-gel route is provided by the thermal condensation of metal halides with metal alkoxides:3 MX, + M(OR), -t2MO,,, + nRX (1) Alternatively, it is possible to generate the alkoxyl groups in situ by the action of alcohol4 or dialkyl ether5 on metal halides: =M-X + ROH-M-OR + HX or EM-X + R O R -t E M-OR + RX (2) Another possible variation is the direct reaction of anhydride with alkoxide precursors,6 which leads to acylation, then condensation, via the formation of an ester: M(OR),+n/2 (R'C=O),O+MO,,,+nR'COOR (3)In these non-hydrolytic reactions, the metal alkoxide, dialkyl ether, alcohol and anhydride act as oxygen donors, instead of water. The main features of these methods are: (i) low synthesis temperatures (cu. 100-150°C); (ii) a simple reaction of TiC1, [or Ti (OR),] with readily available compounds; (iii) an easily removed by-product (hydrogen halide, alkyl halide or acetate); (iv) no cosolvent required (otherwise needed to dissolve water). This paper falls into two parts. First, four typical nonhydrolytic routes to titania were compared [systems TiC1, plus Ti(OPr'),, TiCl, plus diisopropyl ether, TiC1, plus isopropyl alcohol, Ti(OPr'), plus acetic anhydride]; the structures and textures of the different samples were studied by means of Xray diffraction (XRD) and BET measurements. Secondly, the influence of the nature of the oxygen donor on the structure and crystallisation behaviour of the TiO, precursors was investigated in the etherolysis and alcoholysis methods.
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