0 5 10 15 20 2q / deg; (Cu-Ka) Intensity / a.u. ( h) ( g) ( f ) ( e) ( d) ( c) ( b) ( a) Synthesis of silica-pillared layered titanium niobium oxide Wenfeng Shangguan, Kozo Inoue and Akira Yoshida*dagger; Inorganic Materials Department, National Industrial Research Institute, Shuku-machi, Tosu-shi, Saga-ken, 841, Japan A silica-pillared layered titanium niobium oxide with high thermal stability has been synthesized and characterized for the first time. The successful methods for preparation of pillared materials that were first developed from pillaring clays and clay minerals,1ndash;3 led to many attempts to prepare new classes of porous materials, which can be used as shape-selective catalysts and molecular sieves.4,5 There are also a wide variety of layered metal oxides that have the potential to undergo ion-exchange reactions similarly to clays, but the pillaring procedures developed for smectite clays are not generally applicable to these laminar metal oxides that do not spontaneously delaminate in aqueous media owing to their high charge densities on the frameworks.Recently, the preparation of alumina-pillared layered sodium trititanate, Na2Ti3O7 6 and silica-pillared layered tetratitanate K2Ti4O9 7 have been reproted. Silica is one of the most commonly used pillars. Silica-pillared layered lanthanumndash; niobium oxides were also prepared and investigated as catalysts.8,9 As for titaniumndash;niobium oxides, Wadsley10 first prepared potassium titanoniobate KTiNbO5 and found that it had a layered structure.Raveau and coworkers11ndash;13 studied the ion-exchange properties of potassium titanoniobate KTiNbO5 and the intercalation of organic materials into titanoniobic acid. Nakato et al.14 reported the preparation of a methyl viologenndash; HTiNbO5 intercalation compound. We report here the preparation of silica-pillared layered titanoniobate with high porosity and good thermal stability by a novel pillaring method, in which n-hexylamine and tetraethylorthosilicate Si(OEt)4 are employed as an interlayer exchange guest and a pillar precursor, respectively.The starting material, layered potassium titanoniobate KTiNbO5 was prepared by heating a mixture of K2CO3 (Katayama), TiO2 (Katayama) and Nb2O5 (Wako) (molar ratio = 1 : 2 : 1) at 1060 deg;C in air. Ion exchange of KTiNbO5 was carried out with 6 m HNO3 at room temperature for 72 h to afford HTiNbO5. Since HTiNbO5 cannot react directly with tetraethylorthosilicate to form an intercalate, the n-hexylaminendash; HTiNbO5 intercalation compound was prepared first in order to increase the interlayer distance and lower the charge density.n-Hexylamine-intercalated titanoniobate was obtained by adding HTiNbO5 to a 50 n-hexylamine (Wako)ndash;ethanol (Wako) solution and stirring at room temperature for one week, followed by filtering off the product and washing successively with ethanolndash;water (1 : 1) and distilled water. The obtained n-hexylamine-intercalated titanoniobate (3.0 g) was then added to 120 ml of tetraethylorthosilicate (Wako).Having failed to obtain a silica-pillared titanoniobate by reaction between n-hexylamine-intercalated titanoniobate and tetraethylorthosilicate at room temperature, the present operation was performed by stirring for one week at ca. 65 deg;C with two additions of tetraethylorthosilicate (40 ml) during this period. After the reaction, the product was separated and washed as above and finally dried at room temperature.Obtained products were studied by X-ray diffraction (Gigaku Geigerflex, Cu-Ka radiation). In KTiNbO5, units consisting of two MO6 (M = Ti, Nb) octahedra are linked in the b-direction to form layers with the potassium ions in the interlayer spaces. XRD analysis shows Fig. 1(a), 2(b) that obtained KTiNbO5 has an interlayer distance of 9.01 Aring; by measuring the first diffraction peak (2q = 9.82deg;) which decreases to 8.28 Aring; (2q = 10.69deg;) upon exchanging K+ with H+ in HNO3 solution, which is comparable to the value of 8.35 Aring;11 but is smaller than 8.5 Aring;4 published previously.These differences may be due to the differential preparation conditions. As shown in Fig. 1(c), n-hexylamine readily intercalates layered titanoniobate and leads to an extremely strong peak at 2q = 4.08 Aring;. This interlayer distance is 21.7 Aring;, which is more than twice that of HTiNbO5. The n-hexylamine intercalated titanoniobate had rather low thermal stability, and collapses after calcination at 400 deg;C Fig. 1(d). However, the opened layers can facilitate the reaction of layered-hexylamine intercalated titanoniobate with tetraethylorthosilicate, which gives rise to a lower 2q (3.68deg;) and a larger layer distance 24.01 Aring;, Fig. 1(e). Fig. 1(f) shows that the interlayer distance of the product calcined at 400 deg;C decreases remarkably to 15.61 Aring; (2q = 5.66deg;) as a result of decomposition of organic matter. The peak is still retained upon heat treatment at 600 deg;C with only slightly increased 2q and lower intensity Fig. 1(h). TGndash;DTA analysis at a heating rate of 5 deg;C min21 for the product intercalated with tetraethylorthosilicate indicated that the interlayer organic matter was decomposed between 300 and 440 deg;C, and no mass loss was observed at higher temperatures. From these results, it is concluded that silica-like clusters are formed in the interlayer, which act as pilalrs to prop up the titanoniobate layers after interlayer Fig. 1 Cu-Ka X-ray diffraction patterns of (a) KTiNbO5 synthesized at 1060 deg;C; (b) as (a), proton-exchanged by 6 m HNO3 at room temperature for 72 h; (c) reaction product of HTiNbO5 with n-hexylamine solution; (d) as (c), calcined in air at 400 deg;C for 2 h; (e) reaction product of n-hexylamineintercalated titanoniobate with tetraethylorthosilicate; (f) as (e), calcined in air at 400 deg;C for 2 h; (g) as (f), calcined in air at 500 deg;C for 2 h; (h) as (g), calcined in air at 600 deg;C for 2 h.Chem. Commun., 1998 779organic matter is removed. The interlayer spacing of the product calcined at 500 deg;C is found to be 14.05 Aring; 2q = 6.29deg;, Fig. 1(g), which is nearly twice that of HTiNbO5. Fig. 2 shows transmission electron microscopy (TEM, recorded with a JEOL JEM-200CX microscope) images of untreated silica-pillared layered titanoniobate Fig. 2(a) and a sample calcined at 500 deg;C Fig. 2(b). The layer structures are clearly visible, and the interlayer separations observed by TEM are consistent with those obtained from XRD.While the interlayer spacing of the material increases, the BET surface area of the titanoniobates increases from 1.8 m2 g21 for the original KTiNbO5 to 20.0 m2 g21 for pillared SiO2ndash;TiNbO5 calcined at 500 deg;C, and decreases to 11.3 m2 g21 at 600 deg;C. Elemental analysis results showed that the SiO2 content in this silica-pillared material is ca. 56 mass. The BET surface area is perhaps lower than expected due to a large amount of SiO2 incorporated into the interlayers and the weak Bronsted acidity of HTiNbO5 as compared to some layered compounds.15 The UVndash;VIS spectra of SiO2ndash;TiNbO5 shifted into the visible region as compared to those of KTiNbO5 and HTiNbO5.Hydrogen evolution from the photocatalytic decomposition of water on the pillared layered titanoniobate was observed under a mercury lamp and sunlight. Thus, the new material is expected to show photocatalytic activity. Future investigation on its systematic characterization and catalytic properties is being undertaken.Notes and References dagger; E-mail: yosidaa@kniri.go.jp 1 R. M. Barrer and D. M. Macload, Trans. Faraday Soc., 1955, 52, 1290. 2 G. W. Brindley and R. E. Sepples, Clay Miner., 1977, 12, 229. 3 S. Yamanaka and G. W. Brindley, Clay Miner., 1979, 27, 119. 4 F. Figueras, Catal. Rev. Sci. Eng., 1988, 30, 457. 5 T. J. Pinnavaia, Science, 1983, 220, 365. 6 M. W. Anderson and J. Klinowski, Inorg. Chem., 1990, 29, 3260. 7 W. Hou, Q. Yan and X. Fu, J. Chem. Soc., Chem. Commun., 1994, 1371. 8 T. Matsuda, M. Udagawa and I. Kunou, J. Catal., 1997, 168, 26. 9 C.-X. Guo, W.-H. Hou, M. Guo, Q.-J. Yan and Y. Chen, Chem. Commun., 1997, 801. 10 A. D. Wadsley Acta Crystallogr., 1964, 17, 623. 11 H. Rebbah, G. Desgardin and B. Raveau, Mater. Res. Bull., 1979, 14, 1125. 12 H. Rebbah, M. M. Borel, M. Bernard and B. Raveau, Rev. Chim. Miner., 1981, 18, 109. 13 A. Grandin, M. M. Borel and B. Raveau, Rev. Chim. Miner., 1987, 24, 351. 14 T. Nakato, H. Miyata, K. Kuroda and C. Kato, React. Solids, 1988, 6, 231. 15 J. Gopalakrishnan, V. Bhat and B. Raveau, Mater. Res. Bull., 1987, 22, 413. Received in Cambridge, UK, 22nd December 1997; 7/09108G Fig. 2 TEM image of layered silica-pillared titanium niobium oxide with no calcination (a) and calcined at 500 deg;C for 2 h (b) 780 Chem. Commun., 1998
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