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Self-Assembly of a Metallosupramolecular Coordination Polyelectrolyte in the Pores of SBA-15 and MCM-41 Silica Dilek Akcakayiran,† Dirk G. Kurth,‡,§ Stefan Ro¨hrs,† Gu¨nther Rupprechter,| and Gerhard H. Findenegg*,† Institut fu¨ r Chemie, Technische Universita¨ t Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany, Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam, Germany, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Fritz-Haber-Institut der Max Planck Gesellschaft, D-14195 Berlin, Germany Received January 26, 2005. In Final Form: May 12, 2005 It is shown that intrinsically stiff chain aggregates of a metallosupramolecular coordination polyelectrolyte (MEPE) can form in the cylindrical nanopores of MCM-41 and SBA-15 silica by self-assembly of its constituents (metal ions and organic ligand). The UV/vis spectra of the resulting MEPE-silica composites exhibit the characteristic metal-to-ligand charge transfer band of the MEPE complex in solution. For the MEPE-silica composite in SBA-15 an iron content of 1.2 wt % was found, corresponding to ca. 10 MEPE chains disposed side by side in the 8 nm wide pores of the SBA-15 matrix. In the case of MCM-41 (pore width < 3 nm), where only one MEPE chain per pore can be accommodated, an iron content of 0.3 wt % was obtained, corresponding to half-filling of the pores. It was also found that MEPE chains spontaneously enter the pores of SBA-15, when a solution of MEPE is exposed to the silica matrix.
* Corresponding author. Telephone: +49 30 314 24171. E-mail:
[email protected]. † Technische Universita ¨ t Berlin. ‡ Max-Planck-Institut fu ¨ r Kolloid- und Grenzfla¨chenforschung. § National Institute for Materials Science. | Fritz-Haber-Institut der Max Planck Gesellschaft.
mesoporous molecular sieves, such as MCM-4112 and SBA15,13 which constitute two-dimensional (2D) hexagonal arrangements of cylindrical pores (space group P6mm) of uniform diameter in the size range 2-4 nm (MCM-41) and 5-10 nm (SBA-15).14 Incorporation of functional molecular components or metal ions into such mesoporous oxide structures can be achieved directly in the hydrothermal synthesis,15,16 or by an ion-exchange reaction with as-synthesized material,16,17,18 or by a grafting/ionexchange19 procedure. Wet preparation techniques such as incipient wetness and impregnation20,21 have also been used for incorporating functional molecular components or metal compounds in pores. The present work was motivated by the question of whether an intrinsically stiff chain aggregate can be introduced into the nanometer-sized pores of MCM-41 and SBA-15 from the surrounding solution by a diffusioncontrolled process, and if the aggregate can form spontaneously in the pore by self-assembly of its constituents. It is well-established that in aqueous acidic solutions divalent metal ions such as Fe2+ react with stoichiometric amounts of ditopic organic ligands with terpyridine groups22 to form metallosupramolecular coordination
(1) Bru¨hwiler, D.; Calzaferri, G. Microporous Mesoporous Mater. 2004, 72, 1. (2) Taguchi, A.; Schu¨th, F. Microporous Mesoporous Mater. 2005, 77, 1. (3) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (4) Wark, M.; Rohlfing, Y.; Altindag, Y.; Wellmann, H. Phys. Chem. Chem. Phys. 2003, 5, 5188. (5) Schomburg, C.; Wark, M.; Rohlfing, Y.; Schulz-Ekloff, G.; Wo¨hrle, D. J. Mater. Chem. 2001, 11, 2014. (6) Mintova, S.; De Waele, V.; Ho¨lzl, M.; Schmidhammer, U.; Mihailova, B.; Riedle, E.; Bein, T. J. Phys. Chem. A 2004, 108, 10640. (7) Scott, B. J.; Bartl, M. H.; Wirnsberger, G.; Stucky, G. D. J. Phys. Chem. A 2003, 107, 5499. (8) Yatskou, M. M.; Meyer, M.; Huber, S.; Pfenniger, M.; Calzaferri, G. ChemPhysChem 2003, 4, 567. (9) Vietze, U.; Krauss, O.; Laeri, F.; Ihlein, G.; Schu¨th, F.; Limburg, B.; Abraham, M. Phys. Rev. Lett. 1998, 81, 4628. (10) Marlow, F.; McGehee, M. D.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1999, 11, 632. (11) Wirnsberger, G.; Yang, P. D.; Huang, H. C.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F.; Stucky, G. D. J. Phys. Chem. B 2001, 105, 6307.
(12) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (13) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (14) Selvam, P.; Bhatia, S. K.; Sonvane, C. G. Ind. Eng. Chem. Res. 2001, 40, 3237. (15) Wingen, A.; Anastasievic, N.; Hollnagel, A.; Werner, D.; Schu¨th, F. J. Catal. 2000, 193, 248. (16) Holland, B. T.; Walkup, C.; Stein, A. J. Phys. Chem. B 1998, 102, 4301. (17) Zhu, J.; Ko´nya, Z.; Puntes, V. F.; Kiricsi, E.; Miao, C. X.; Ager, J. W.; Alivisatos, A. P.; Somorjai, G. A. Langmuir 2003, 19, 4396. (18) Ganschow, M.; Wark, M.; Wo¨hrle, D.; Schulz-Ekloff, G. Angew. Chem. 2000, 112, 167. (19) Hess, C.; Hoefelmeyer, J. D.; Tilley, T. D. J. Phys. Chem. B 2004, 108, 9703. (20) Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (21) Vinu, A.; Murugesan, V.; Hartmann, M. J. Phys. Chem. B 2004, 108, 7323. (22) Schu¨tte, M.; Kurth, D. G.; Linford, M.; Mo¨hwald, H. Angew. Chem. 1998, 110, 3058.
Introduction The incorporation of metal clusters or organic molecules in solid matrixes is of widespread interest in materials science as it allows construction of functional materials with well-defined spatial arrangements on the nanoscale.1 Ordered mesoporous silicas are widely used as support materials for nanostructured metal or metal oxide catalysts2 and for the supramolecular organization of dye molecules and transition metal complexes exhibiting optical or related functionalities.3 Such structures have great potential in the field of optical gas sensing,4 UV sensing and molecular switching,5,6 photonic antenna host-guest systems,7,8 and microlasers.9-11 Initially research was focused on zeolites, but their small pore sizes turned out to be a limiting factor in some applications. An important step forward was the development of periodic
10.1021/la050230x CCC: $30.25 © 2005 American Chemical Society Published on Web 07/06/2005
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Scheme 1. Metallosupramolecular Coordination Polyelectrolyte (MEPE) Formed by Metal Ion Induced Self-Assembly of Fe2+ Ions and the Ditopic Ligand 1,4-Bis(2,2′:6′2′′-terpyridine-4′-yl)benzene (1)
Akcakayiran et al. Table 1. Characterization of SBA-15 and MCM-41 by Nitrogen Adsorption and SAXDa sample
D/nm
SBA-15 MCM-41
8.2 2.9
N2 sorption vp/cm3 g-1 1.07 0.9
as/m2 g-1
SAXD d10/nm
790 1120
9.01 3.94
a BJH pore diameter D (based on the Harkins-Jura t-layer equation), specific mesopore volume vp, BET specific surface area as, and separation of (10) lattice planes d10.
polyelectrolyte (MEPE) chains, as indicated in Scheme 1. Integrating such supramolecular architectures into periodic mesoporous oxide materials may open up routes to new functional materials for application in electronics, photonics, and catalysis. For example, it will be possible in this way to implant metal ions at well-defined separations uniformly along the pores. Two methods for embedding MEPE complexes in the pores were tried: (i) The first was preparation of the complex in solution and contacting the silica particles with the solution. In this case, incorporation of MEPE may occur either by direct transfer of extended stiff chain aggregates from solution into the pores, or by diffusion of the constituent monomers coexisting with the complex in solution. (ii) The second method was the introduction of the constituents into the matrix consecutively by adsorption from solution, first the organic ligand and then, after appropriate rinsing, the Fe2+ ions, so that the assembly of the MEPE complex can occur only in the pore space. Experiments were made with MCM-41 of pore width < 3 nm, in which only a single MEPE chain can be accommodated, and SBA-15 of pore width ) 8 nm, in which several chains may be placed side by side in the pores. The resulting samples were characterized by a combination of different techniques including X-ray fluorescence (XFA), transmission electron microscopy (TEM), energy-dispersive X-ray fluorescence analysis (EDX), and UV/vis spectroscopy. Experimental Section Preparation of MCM-41 and SBA-15. MCM-41 was synthesized as described in ref 23, using hexadecyltrimethylammonium bromide (C16TAB) as the structure-directing agent. A 50 mL volume of aqueous ammonia (25 wt %) was added to a solution of 5 g of C16TAB (Fluka, purity 99%) in 550 mL of Milli-Q water. A 25 mL volume of TEOS (ABCR, purity 97%) was added to this solution as a silica precursor under vigorous stirring at 30 °C. The surfactant-silica composite formed as a fine precipitate after a few minutes and was kept in the reaction solution for 4 h at 30 °C under constant stirring and then transferred into an autoclave for 48 h at 105 °C. The material was filtered, washed with Milli-Q water and dried at 60 °C for 6 h, and finally calcined in air at 550 °C. SBA-15 was synthesized by a method reported in ref 13 using technical grade poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic P103, BASF USA, Mount Olive, NJ) as the structure-directing agent. A 13 mL volume of H2SO4 (97%) was added to a solution of 4 g of P103 in 240 mL of Milli-Q water to reach pH 1 wt % in the case of SBA-15 (Table 2). Apparently the metal ions in the pores remain in a very finely distributed state. In the MEPE-silica materials the MEPE chains are aligned in the direction of the pore axes and the Fe2+ ions have well-defined distances c ≈ 1.5 nm along the chain. This repeat distance of the MEPE chains should be detectable by SAXS as a correlation peak centered at q ) 2π/c ≈ 4.2 nm-1. In MCM-41, where the pores accommodate only a single MEPE chain, this peak should be
Figure 4. Nitrogen sorption isotherms for native and MEPE-containing SBA-15 (A) and MCM-41 (B). The results in (B) show that calcination of the MEPE-silica composite (samples M2 and M2c) causes no further decrease in pore size and pore volume. Results for several fresh sample specimens are shown to indicate the repeatability of the measurement.
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more pronounced than in SBA-15, where the presence of several MEPE chains at short separations within each pore will cause broadening of this peak. However, for the present sample of MCM-41 the expected position of this correlation peak coincides with that of the (21) Bragg peak of the matrix, as shown in Figure 2B, and thus we cannot decide if a correlation peak resulting from the MEPE chains exists. In the case of SBA-15, for which the Bragg peaks appear at smaller q, the SAXS curve exhibits a monotonic decay in the region at which the correlation peak is expected at 4.2 nm-1, as shown in Figure 2A. Hence from the present results we cannot decide if the individual Fe ions, if still existing in separate form in the calcined sample, are arranged in a regular way or not. Summary and Conclusions The present work has shown that intrinsically stiff metallosupramolecular polyelectrolyte chains can be introduced into the cylindrical nanopores of MCM-41 and SBA-15, either by directly exposing the porous silica matrix to a solution of MEPE, or by assembling the MEPE in the pores in a two-step process in which the adsorption of the organic ligand at the pore walls forms the first step. This conclusion is based on the following observations: (1) The characteristic metal-to-ligand charge transfer band in the UV/vis absorption spectrum of MEPE is retained in the spectra of the MEPE-silica composite samples. (2) From the iron content of the MEPE-silica composite samples before and after calcination, it has been established that metal ions and the organic ligand are introduced into the pores in stoichiometric amounts. (3) The amount of MEPE assembled in the 8 nm pores of SBA-15 by the two-step process corresponds to ca. 11 MEPE chains disposed side by side in each pore. From the size and geometry of the MEPE complex this is about the maximum number of chains that can be accommodated in a single layer at the pore walls. This result indicates
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that the first step of the process involves the adsorption of an almost complete monolayer of the ligand at the pore wall. Self-assembly of MEPE in the pores is then induced by the exposure to metal ions in the second step of this process. (4) In the case of MCM-41 (pore diameter 2.9 nm) only one MEPE chain per pore can be accommodated for geometric reasons, but only about half of this maximum loading was attained under conditions equivalent to those of the two-step assembly in SBA-15. (5) Direct transfer of MEPE from solution into the pores (one-step process) leads to a significantly lower loading than the two-step process under the experimental conditions of this study. However, a study at several solution concentrations is needed to establish the distribution of MEPE between the pore space and the bulk solution in a quantitative way. Covalent polymers generally do not absorb in nanopores due to entropic reasons. The fact that MEPE is found in the pores is attributed to the dynamic character of this type of self-assembling macromolecule. Smaller units can diffuse into the pores. Our results show that self-assembly in the pores is possible. (6) The organic ligand of MEPE can be removed from the pores by calcination without detectable loss of the metal. Transmission electron microscopy indicates that the metal remains in the matrix in finely divided form as no metal clusters are detectable. More work is needed to characterize the state and distribution of iron in the matrix after calcination. Also, it will be of interest to extend this study to other transition metals and to study the catalytic activity of these materials. We are planning to perform such studies in the future. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft in the framework of SFB 448 “Mesoscopically organized composites”. LA050230X
