J. Phys. Chem. B 2001, 105, 2897-2905
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ARTICLES A Novel Titanosilicate with MWW Structure. I. Hydrothermal Synthesis, Elimination of Extraframework Titanium, and Characterizations Peng Wu, Takashi Tatsumi,* Takayuki Komatsu,† and Tatsuaki Yashima† DiVision of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed: August 3, 2000; In Final Form: January 18, 2001
A novel titanosilicate with MWW topology, Ti-MWW, has been prepared by an acid treatment on a corresponding lamellar precursor which is hydrothermally synthesized with the coexistence of boron and titanium using piperidine (PI) or hexamethyleneimine (HM) as a structure-directing agent. The MWW precursor can be synthesized to have a Si/Ti ratio as low as 10 when the Si/B ratio of the gel is maintained at 0.75. Both the materials synthesized using PI and HM exhibit the crystal form of thin platelets, while the latter shows a larger crystal size. Besides the tetrahedral Ti species, the precursor always contains the octahedral Ti species showing a UV-vis band at 260 nm, regardless of the Ti content. Calcination of the precursor results in a partial condensation of the octahedral Ti species to form the anatase phase, which is hardly removed by the acid treatment. MWW-type titanosilicate nearly free of both anatase and boron, however, is successively prepared by a cyclic treatment on the lamellar precursor, that is, an acid treatment, subsequent calcination, and a further acid treatment. After evacuation at 773 K, the titanosilicate thus prepared shows a characteristic IR band at 960 cm-1 not observed for the Ti-free sample. The intensity of the 960 cm-1 band increases linearly with the Ti content up to a level corresponding to Si/Ti ) 40, indicating the limitation of incorporating Ti into the framework of MWW by the present method.
Introduction The success of titanium-containing silicate with MFI structure (TS-1), which is able to catalyze the selective oxidation of various organic substrates in liquid phase using hydrogen peroxide as an oxidant, is a great breakthrough in the field of zeolite catalysis.1-3 However, the medium-size pores of TS-1, as well as TS-24 with MEL analogue, restrict their use to substrates and oxidants both with relatively small molecular diameters. To overcome the disadvantage of TS-1 and TS-2, other Ti-containing crystalline silicates with 12-ring channels, such as Ti-Beta,5,6 TAPOS-5,7 Ti-MOR,8 and more recently ITQ-7,9 have been prepared by either direct or postsynthesis method. Among these materials, Ti-Beta prepared both by direct hydrothermal synthesis5 and by dry gel conversion (DGC)6 has shown remarkable activity for the oxidation of cyclic substrates, even using the bulky organic oxidant tert-butyl hydroperoxide (TBHP); postsynthesized Ti-MOR is used successfully as a catalyst in the hydroxylation of bulky aromatics with H2O2;8 and Ti-ITQ-7 with a three-dimensional system of a 12membered ring pore is reported to be a more interesting catalyst than Ti-Beta.9 Nevertheless, these materials still do not satisfy the demands from the fine chemical and pharmaceutical fields, where more accessible reaction space is desirable. This stimulates the researchers to prepare Ti-containing catalysts with * Correponding author. Tel: +81-45-339-3943. Fax: +81-45-339-3941. E-mail:
[email protected]. † Department of Chemistry, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku,Tokyo 152-8551, Japan.
larger pore sizes. From this viewpoint, mesoporous molecular sieves developed in recent years have opened the possibility for preparing such catalysts. In fact, Ti-MCM-41, hydrothermally synthesized10 or prepared by grafting with titanocene,11 is an active and selective catalyst for olefin oxidation with TBHP. However, significant problems occur on Ti-MCM-41, such as low mechanical and hydrothermal stability, easy leaching of Ti species in the liquid phase, and lower intrinsic activity than TS-1 or Ti-Beta for the substrates without diffusion problems. These problems are mainly because the walls of MCM-41 lack crystalline linkages and have abundant silanol groups, whose hydrophilicity makes the Si-O-Si bonds easily cleaved in the aqueous solution. These problems can be resolved partially by silylation with organic silanes to increase the hydrophobicity of Ti-MCM-41,12,13 but the silylation seems to be a provisional but not a final method. There is still much room for researchers to incorporate Ti into highly stable materials having the benefits of mesopores as well. A novel zeolite with MWW topology,14 having a unique and unusual crystalline structure, is expected to serve as such a candidate for preparing Ti-containing catalysts that are both highly stable and accessible. The MWW structure is constructed from a lamellar precursor undergoing dehydroxylation upon calcination between the layered sheets. The dehydroxylation causes a recrystallization to form crystals of hexagonal thin plates. Besides two-dimensinal sinusoidal channels of 10memebered rings (MR) running throughout the structure parallel to the ab-plane, the MWW structure contains also an independ-
10.1021/jp002816s CCC: $20.00 © 2001 American Chemical Society Published on Web 03/24/2001
2898 J. Phys. Chem. B, Vol. 105, No. 15, 2001 ent channel system which is comprised of large supercages (0.7 × 0.7 × 1.8 nm). The supercages turn to be pocket or cup moieties (0.7 × 0.7 nm) at the crystal exterior.14-16 The unique pore system of intracrystalline supercages and abundant pockets covering the hexagonal faces of thin crystals have already proved to serve as an open reaction space in the disproportionation of toluene17 and also in the alkylation of benzene.18 Furthermore, the MWW lamellar precursor can be converted into a micromesoporous hybrid, MCM-36,19,20 by swelling with organic surfactant followed by pillaring with polymeric silica, and it can also be delaminated into ITQ-2,16 a material consisting of very thin silica sheets of 2.5 nm height, by forcing apart the swollen precursor with ultrasound. Both MCM-36 and ITQ-2 have extremely high surface area and are highly accessible for bulky molecules. Unfortunately, despite the structural diversity, the incorporation of catalytically active sites into the MWW structure has been focused on trivalent cations such as Al,18 Fe,21 and B22 but little on Ti. The only report dealing with this issue is the postsynthesis of Ti-ITQ-2 by grafting titanocene onto the surface of pure silica ITQ-2.23 Recently, we have communicated a new approach for preparing titanosilicate of MWW topology by direct hydrothermal synthesis and have verified that the material thus prepared is a potential catalyst for the bulky oxidation reactions.24 Here we report the detailed synthesis conditions for incorporation of Ti into the MWW structure under alkali-free conditions using piperidine or hexamethyleneimine as an structure-directing agent (SDA) and using boric acid as a structure-supporting agent. The main issues will deal with the removal of framework B and extraframework Ti and the characterization of the Ti species. Experimental Section Hydrothermal Synthesis and Acid Treatment. Ti-containing MWW precursor was synthesized using fumed silica (Cabo-sil M7D), tetrabutyl orthotitanate (TBOT) (95%, Wako), boric acid (99.5%, Wako), and deionized water, with piperidine (PI) (98%, Wako) or hexamethyleneimine (HM) (98%, Wako) as an SDA. The procedures followed those reported for MWW type borosilicate, ERB-1,22 but a significant modification was made. The details are given herein using the sample of Si/Ti ) 50 as a representative in the case of PI. PI (182.5 g) was dissolved in 513 g of deionized water at room temperature. This solution was divided into two equal parts, to one of which was added 10.8 g of TBOT under vigorous stirring, and to the other 124.2 g of H3BO3 was added under vigorous stirring. The stirring was maintained for 30 min to hydrolyze TBOT. Silica (90 g) was also divided into two equal parts which were added gradually to the solutions containing Ti and B, respectively. A further stirring for 1 h was allowed to form two homogeneous gels. The gels were then mixed together and stirred for 1.5 h to obtain a gel with a molar composition of 1:0.02:0.67:1.4:19 for SiO2:TiO2:B2O3:PI:H2O. The resulting gel was transferred into a Teflon-lined autoclave of 2 L and heated with a stirring rate of 100 rpm at 403 K and then 423 K each for 1 day and further at 443 K for 5 days. After cooling, the solid product was filtered off and washed with deionized water to pH 40, and this linear relationship fits well
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Wu et al. SCHEME 2
Figure 10. IR spectra of Ti-MWW-PI with Si/Ti ratio of (a) ∞, (b) 203, (c) 136, (d) 107, (e) 59, (f) 53, and (g) 25 after the evacuation at 773 K for 1 h. The Si/B ratio was over 450 for all the samples. The dotted lines show the difference spectra extracted from spectrum a.
Figure 11. The dependence of the intensity of the 960 cm-1 band on the Ti content.
SCHEME 1
with that of TS-1. Since we adopted the isotope exchange technique to confirm that the Ti species in the TS-1 samples mainly occupy the tetrahedral framework sites as depicted in Scheme 1, it is reasonable to conclude that at Si/Ti > 40 the Ti species remaining within the Ti-MWW after various treatments are mainly located in the framework sites. The band intensity came to deviate from the linear line at higher Ti contents. This is because these samples, prepared from the precursors of Si/Ti )10 and 20, contained some condensed Ti species, as evidenced by earlier UV-visible spectra. As a result, highly crystalline Ti-MWW material has been prepared successively to contain mainly the framework Ti as low as Si/Ti ) 40. The corresponding Ti amount is similar to the maximum level reported for anatase-free TS-1.2,3 Conclusion A novel MWW-type titanosilicate containing the tetrahedral Ti species in the framework has been successfully prepared by combining a conventional hydrothermal synthesis and an acid
treatment. The formation process is described graphically in Scheme 2, where for clarity, Si, B, O, and organic species are omitted, but with an emphasis on Ti sites. The lamellar precursor containing Ti can be synthesized with PI or HM as an SDA in alkali-free conditions, but boric acid is necessary as a structuresupporting agent. Ti species incorporated occupy both the tetrahedral framework sites within the sheets and the sites on the terminal surface of the sheets. A surface Ti may share the same oxygen with a neighboring Ti to have a Ti-O-Ti linkage and maintain its electronic balance by bonding with other coordinated groups such as hydroxyl groups or water. This kind of Ti thus shows an octahedral coordination, i.e. [TiO6]. A direct calcination on the precursor causes crystallization between the sheets to form the three-dimensional MWW structure and simultaneously a condensation of octahedral Ti species to form an anatase phase, which is hardly removed by acid treatment. However, by treating directly the lamellar precursor with an acid solution, the octahedral Ti species are removed readily together with a large amount of B, while the tetrahedral Ti species remain to a high level. Subsequent calcination leads to the total formation of the MWW structure with tetrahedral Ti species probably existing within the supercages, the exterior pockets, and the channels of the 10-ring. A further acid treatment can extract nearly all the residual B to yield the objective TiMWW. References and Notes (1) (a) Taramasso, M.; Perego, G.; Notari, B. U.K. Patent 2,071,071, 1981. (b) Taramasso, M.; Perego, G.; Notari, B. U.S. Patent 4,410,501, 1983. (2) Bellussi, G.; Rigguto, M. S. Stud. Surf. Sci. Catal. 1994, 85, 177. (3) Notari, B. AdV. Catal. 1996, 41, 253. (4) (a) Reddy, J. S.; Kumar, R.; Ratnasamy, P. Appl. Catal. 1990, 58, L1. (b) Reddy, J. S.; Kumar, R. J. Catal. 1990, 130, 440.
A Novel Titanosilicate with MWW Structure (5) (a) Camblor, M. A.; Corma, A.; Martı´nez, A.; Pe´rez-Pariente, J. J. Chem. Soc. Chem. Commun. 1992, 589. (b) Corma, A.; Camblor, M. A.; Esteve, P., A.; Martı´nez, A.; P’erez-Pariente, J. J. Catal. 1994, 145, 151. (c) Corma, A.; Esteve, P.; Martı´nez, A.; Valencia, S. J. Catal. 1995, 152, 18. (6) (a) Tatsumi, T.; Xia, Q. H.; Jappar, N. Chem. Lett. 1997, 677. (b) Jappar, N.; Xia, Q. H.; Tatsumi, T. J. Catal. 1998, 180, 132; Tatsumi, T.; Jappar, N. J. Phys. Chem. 1998, 102, 7126. (7) Tule, A. Zeolites 1995, 15, 228. (8) (a) Wu, P.; Komatsu, T.; Yashima, T. J. Phys. Chem. 1996, 100, 10316. (b) Wu, P.; Komatsu, T.; Yashima, T. J. Catal. 1997, 168, 400. (c) Wu, P.; Komatsu, T.; Yashima, T. Stud. Surf. Sci. Catal. 1997, 105, 663. (d) Wu, P.; Komatsu, T.; Yashima, T. J. Phys. Chem. B 1998, 102, 9297. (9) Diaz-Cabanas, M.; Villaescusa, L. A.; Camblor, M. A. Chem. Commun. 2000, 761. (10) Corma, A. Chem. ReV. 1997, 97, 2373. (11) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 278, 159. (12) (a) Tatsumi, T.; Koyano, K. A.; Tanaka, Y.; Nakata, S. Stud. Surf. Sci. Catal. 1998, 117, 143. (b) Tatsumi, T.; Koyano, K. A.; Igarashi, N. Chem. Commun. 1998, 325. (13) Corma, A.; Domina, M.; Gaona, J. A.; Jorda´, J. L.; Navarro, M. T.; Rey, F.; Pe´rez-Pariente, J. Chem. Commun. 1998, 2211. (14) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (15) Lawton, S. L.; Leonowicz, M. E.; Partridge, P. D.; Chu, P.; Rubin, M. K. Microporous Mesoporous Mater. 1998, 23, 109. (16) Corma, A.; Forne´s, V.; Pergher, S. B.; Maesen, Th. L. M.; Buglass, J. G. Nature 1998, 396, 353. (17) Wu, P.; Komatsu, T.; Yashima, T. Microporous Mesoporous Mater. 1998, 22, 343. (18) Cheng, J. C.; Degnan, T. F.; Beck, J. S.; Huang, Y. Y.; Kalyanaraman, M.; Kowalski, J. A.; Loehr, C. A.; Mazzone, D. N. Stud. Surf. Sci. Catal. 1999, 121, 53.
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2905 (19) Roth, W. J.; Kresge, C.; Vartuli, J. C.; Leonowicz, M. E.; Fung, A. S.; McCullen, S. B. Stud. Surf. Sci. Catal. 1995, 94, 301. (20) He, Y. J.; Nivarthy, G. S.; Eder, F.; Seshan, K.; Lercger, J. A. Microporous Mesoporous Mater. 1998, 25, 207. (21) Wu, P.; Lin. H.; Komatsu, T.; Yashima, T. Chem. Commun. 1997, 663. (22) Millini, R.; Perego, G.; Parker, W. O.; Bellussi, G.; Carluccio, L. Microporous Mater. 1995, 4, 221. (23) Corma, A.; Dı´az, U.; Forne´s, V.; Jorda´, J. L.; Domine, M.; Fey, F. Chem. Commun. 1999, 779. (24) Wu, P.; Tatsumi, T.; Komatsu, T.; Yashima, T. Chem. Lett. 2000, 774. (25) Corma, A.; Corell, C.; Pe´rez-Pariente, J. Zeolites 1995, 15, 2. (26) Zones, S. I. U.S. Pat. 4,826, 667, 1989. (27) Camblor, M. A.; Corell, C.; Corma, A.; Dı´az-Caban˜as, M. J.; Nicolopoulos, S.; Gonza´lez-Calbet, J. M.; Vallet-Regı´, M. Chem. Mater. 1996, 8, 2415. (28) Wu, P.; Tatsumi, T.; Komatsu, T.; Yashima, T. Unpublished data. (29) Khouw, C. B.; Davis, M. E. J. Catal. 1995, 151, 77. (30) (a) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Phillippou, A.; MacKay, S. P.; Ferreria, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (b) Lamberti, C. Microporous Mesoporous Mater. 1999, 30, 155. (31) Boccuzzi, F.; Coluccia, S.; Ghiotti, G.; Morterra, C.; Zecchina, A. J. Phys. Chem. 1978, 82, 1298. (32) Bellussi, G.; Carati, A.; Clerici, M. G.; Maddinelli, G.; Millini, R. J. Catal. 1993, 133, 220. (33) (a) Sayed, M. B.; Ve´derin, J. C. J. Catal. 1986, 101, 43. (b) Coudurier, G.; Auroux, A.; Ve´derin, J. C.; Farlee, R. D.; Abrams, L.; Shannon, R. D. J. Catal. 1987, 108, 1. (c) Cornaro, U.; Wojciechowski, B. W. J. Catal. 1989, 120, 182. (34) Hunger, M.; Ernst, S.; Weitkamp, J. Zeolites 1995, 15, 188.
