Langmuir 1996, 12, 4385-4390
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Surface Acidity of Sulfated TiO2-SiO2 Sol-Gels J. Navarrete, T. Lopez,* and R. Gomez Universidad Autonoma MetropolitanasIztapalapa, Department of Chemistry, P.O. Box 55-534, D. F. 09340, Mexico
F. Figueras Institut de Recherches sur la Catalyse du CNRS, 2 Avenue A. Einstein, 69626 Villeurbanne Cedex, France Received October 23, 1995. In Final Form: May 9, 1996X Sulfated titania-silica gels have been obtained from tetraethoxysilane and titanium isopropoxide gelled either in acid medium (pH 3, HCl) or basic medium (pH 9, NH4OH). These gels were sulfated using sulfuric acid or ammonium sulfate. Three different methods have been used to study the acidity of these solids: formation of H-bonds with benzene, selective H/D exchange with deuterobenzene, and adsorption of pyridine. The nonsulfated gels show only weak Lewis acidity, the strength of which increases with sulfation. The solid obtained by in situ sulfation, i.e. gellation using sulfuric acid, shows a strong Bronsted acidity. Their acid strength evaluated by the shift of the infrared bands of the hydroxyls when contacting the solid with benzene is comparable to that of HY zeolites. H-D exchange at low temperature can be observed between deuterated benzene and the gels obtained with sulfuric acid, which confirms the high acid strength of sulfated TiO2-SiO2 samples. Titanium is at least partly incorporated in the lattice of silica and can easily be extracted by self-steaming.
Introduction The replacement of liquid acids by solids is now considered as highly desirable in order to design clean processes for better protection of the environment. Many kinds of binary oxides are well-known to possess acid sites at the surface,1 and the titania-silica system has recently attracted much attention, since it was reported to show higher acid strength than alumino silicates and gave excellent results as catalyst for different acid catalyzed reactions (2-7) or as support for NO reduction.8-10 The mechanism of acidity generation is however still uncertain and two different theories accounting for the creation of these acid sites have been proposed, assuming dilute solid solutions of one cation in a matrix of the main oxide. The model of Tanabe et al.11 predicts that Bronsted acidity will appear on silica-rich mixed oxides, whenever Ti retains the octahedral coordination normally found in titanium oxides. By contrast the model proposed by Kung12 predicts Lewis acidity when incorporating titanium in silica. The acidity of the resulting solid depends on the coordination of titanium, and there is little agreement in the literature on the strength and number of acid sites on mixed oxides, as discussed recently by Liu et al.,7 because it appears * Corresponding author. E-mail:
[email protected]. Fax: +52 57 24 46 66. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases, Their Catalytic Properties; Studies on Surface Science Catalysis 51; Kodansha-Elsevier: Amsterdam, 1989; p 27. (2) Itoh, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225. (3) Ko, E. I.; Chen, J. P.; Weissmann, J. G. J. Catal. 1987, 105, 511. (4) Nakabayashi, H. Bull. Chem. Soc. Jpn. 1992, 65, 914. (5) Sohn, J. R.; Jang, H. J. J. Catal. 1991, 132, 563. (6) Imamura, S.; Tarumoto, H.; Ishida, S. Ind. Eng. Chem. Res. 1989, 28, 1449. (7) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 149, 117. (8) Handy, B. E., Baiker, A., Schraml-Marth, M., Wokaun, A., J. Catal. 1992, 133, 1. (9) Bjorklund, R. B.; Odenbrand, C. U. I.; Brandin, J. G. M.; Andersson, L. A. H.; Liedberg, B. J. Catal. 1989, 119, 187. (10) Odenbrand, C. U. I.; Lundin, S. T.; Andersson, L. A. H. Appl. Catal. 1985, 18, 335. (11) Tanabe, K., Sumiyoshi, T., Shibata, K., Kiyoura, T., and Kitagawa, J. Bull. Chem. Soc. Jpn. 1974, 47, 1064. (12) Kung, H. H. J. Solid State Chem. 1984, 52, 191.
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difficult to control the homogeneity of the solid by usual precipitation methods. Homogeneous TiO2-SiO2 mixed oxides can be obtained by sol-gel. These solids combine the mechanical properties of silica with the chemical properties of titania, and the addition of small quantities of titanium to silica confers to this system a high thermal stability. Compositions containing about 10 and 12% TiO2 in silica are close to the eutectic point in the phase diagram, though silica can accommodate statistically up to 15% titania.13 Sandstrom et al.14,15 demonstrated that, at low Ti content, Ti4+ cations are tetrahedrally coordinated, with a small quantity of Ti4+ cations octahedrally coordinated, the proportion of which increased with the ratio Ti/Si. The analysis of highly homogeneous samples with small particle size, obtained by sol-gel, showed only tetrahedral Ti cations.13 Sulfated titania and zirconia have been proposed as super acids, with acid strengths measured by Hammett indicators reaching values of H0 ) -14.6 for TiO2-SO42(ref 16) and -16 for ZrO2-SO4 2- (ref 17). The acidity on the solid evacuated at 500 °C was only of Lewis type,18,19 but other authors have reported the presence of both Lewis and Brønsted sites.20-22 The structure of this acid site was determined by infrared spectroscopy:18 the generation of strong acidity was related to the appearance of an intense band at 1375 cm-1 characteristic of SdO.18,19,23 (13) Evans, D. L. J. Non Cryst. Solids 1982, 52, 115. (14) Sandstrom, D. R.; Lytle, F. W. J. Non Cryst. Solids 1980, 41, 201. (15) Greegor, B.; Lytle, F. W.; Sandstrom, R. D.; Wong, J.; Schultz, P. J. Non Cryst. Solids 1983, 55, 27. (16) Hino, M.; Arata, K. J. Chem Soc. Chem. Commun. 1979, 1148. (17) Hino, M.; Arata, K. J. Chem Soc. Chem. Commun. 1980, 851. (18) Yamaguchi, T.; Jin, T.; Tanabe, K. J. Phys. Chem. 1986, 90, 3148. (19) Bensitel, M.,;Saur, O.; Lavalley, J. C.; Morrow, B. A. Mater. Chem. Phys. 1988, 19, 147. (20) Nascimento, P.; Akratapoulou, C.; Oszgyan, M.; Coudurier, G.; Travers, C.; Joly, J. P.; Vedrine, J. C. In Proceedings of the 10th International Congress on Catalysis, Budapest 1992; Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Akademiai Kiado: Budapest, 1993; Vol. B, p 1185. (21) Komarov, V. S.; Sinilo, M. F. Kinet. Catal. (Engl. Ed.) 1988, 29, 701. (22) Kustov, L. M., Kazansky, V. B., Figueras, F.; Tichit, D. J. Catal. 1994, 150, 143.
© 1996 American Chemical Society
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Figure 1. Scheme of the preparation of the sol-gel TiO2-SiO2 mixed oxides.
Few works have been devoted to the study of sulfated mixed oxides. Sohn and Jang studied coprecipitates of zirconia24 or titania5,25 with silica obtained from basic aqueous solutions. The resulting solid showed surface areas in the range 200-550 m2/g after calcination at 400 °C. These samples were sulfated by H2SO4 and then calcined at 400 °C. The acid properties determined by the adsorption of Hammett indicators in benzene solution showed that either TiO2-SiO2-SO42- (73% TiO2) or ZrO2SiO2-SO42- (85% ZrO2) converted 2,4-dinitrofluorobenzene (pKa ) -14.5) to the acid form and therefore exhibited strong acidity. By pyridine adsorption both Lewis and Brønsted sites were observed. From the structural point of view, these coprecipitates are most probably multiphasic systems, and titanium oxides were indeed observed by X-ray diffraction. It was then attempted in the present work to study the surface acidity of the TiO2 -SiO2 mixed oxides obtained by sol-gel, at low Ti content (
