Development of Acidity on Sol-Gel Prepared TiO 2 -SiO 2 Catalysts
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ABSTRACT Three different TiO 2-SiO 2 gels (Xerogel, Carbogel and Aerogel) are more active acid catalysts than other reference samples used here. As deduced from FTIR, XRD and XANES studies, the structural properties of these gels are quite different to each other, thus revealing the strong influence of the drying treatment. It is found that the degree of Si-O-Ti linking and the surface acidity follows the same trend (X ) C ) A). We conclude that supercritical drying at 600 K and 190 bars can induce Ti leaching followed by redeposition in the narrower pores of the gel. These effects modify both the textural and surface chemical properties of the resulting material. INTRODUCTION The design of active solids with well developed textures and controlled acid-base and redox properties constitutes a major challenge in heterogeneous catalysis [1]. The sol-gel methods are suitable to obtain active solids with high specific surface areas and tailored porosities [2,3]. One way of modulating the acidity of an oxide is the incorporation of a second oxide into the host structure of the first one [4]. The dispersion of titania in a silica matrix is one model example of generation of Lewis acid sites [5]. The aim of this work is the preparation of TiO 2 -SiO2 samples by a sol-gel method to obtain acidic catalysts with convenient textures. The dehydration of tert-butanol to isobutene has been used as a test reaction to check the surface acidity of the catalysts [6]. It is currently assumed that, the higher the dispersion of titania into the silica matrix the higher is the acidity of the mixed oxide [5]. The segregation limit for titania is close to 10% molar ratio TiO 2 /Si0 2 [7]. Beyond this loading anatase inclusions would be expected to occur. Accordingly, TiO 2 (10%)/Si0 2 was the standard chemical composition of the phases investigated here. After gelling, we have studied the effect of several drying treatments on the structural, textural and acidic properties of the TiO 2 -SiO 2 mixed oxides. Drying procedures affect not only the textural properties of these materials, but also may induce titanium migration as well as severe structural transformations. In parallel, the acidic properties of the resulting gels would also be altered. The trends of structural changes correlate well with the catalytic performance of the investigated samples. EXPERIMENTAL Tetraethoxysilane (TEOS) and tetrabutylorthotitanate (TBOT) were the precursors used in the preparations. TBOT was dissolved in n-butanol and modified by the addition of acetic acid to decrease its reactivity [8] (TBOT/n-BuOH/Acetic Acid = 1/3.5/5.5 moles). TEOS was dissolved in ethanol (1:1 vol) and the hydrolysis water added to this solution after being acidified with HCI up to pH 1.5. The molar water/alkoxides ratio is four. Solutions containing both 685 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
precursors were mixed at 273 K under continuous stirring. Gelling of the sol took place in hermetic glass containers, at 323 K, after one week. The alcogel
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