© Copyright 1998 by the American Chemical Society
VOLUME 102, NUMBER 36, SEPTEMBER 3, 1998
LETTERS Imaging the Pore Structure and Polytypic Intergrowths in Mesoporous Silica Wuzong Zhou,*,† Hazel M. A. Hunter,‡ Paul A. Wright,‡ Qingfeng Ge,† and John Meurig Thomas*,§ The UniVersity Chemical Laboratories, Lensfield Road, Cambridge CB2 1EW, U.K., School of Chemistry, The UniVersity of St. Andrews, Fife KY16 9ST, Scotland, U.K., and DaVy-Faraday Research Laboratories, The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, U.K. ReceiVed: June 10, 1998
Mesoporous silica, SBA-2, has been investigated by using high-resolution transmission electron microscopy supported by computer image simulations. The complete pore structure connecting the discrete supercages of which the silica is composed has been determined. In addition, unexpected well-defined cubic-hexagonal (polytypic) intergrowths have been uncovered, involving a hitherto unknown mesoporous structure that we designate STAC-1. It was found that both SBA-2 and STAC-1 contain a two-dimensional pore system and that the symmetry of the SBA-2 structure must be lower than that (space group P63/mmc) determined previously on the basis of X-ray powder diffraction methods. Polytypic irregularities in these mesoporous materials indicate that, similar to microporous zeolitic systems, a range of solids with structures intermediate between end members SBA-2 and STAC-1 might be prepared.
Mesoporous silicas with channels of diameter in the range of 15 to ca. 100 Å have attracted much attention as catalyst supports1-4 and also because of their potential as size- and shape-selective chemical sensors and membranes for molecular separations.5,6 They are accessible to molecules too large for diffusion in to and out of zeolitic molecular sieves, which have diameters ranging from 4 to ca. 12 Å. X-ray-based methods are not well-suited for their structural elucidationschiefly because of the paucity of the hkl reflections that they yields and, usually, all that may be deduced from X-ray diffraction (XRD) patterns of mesoporous silicas, apart from proof of their crystallinity, is the crystallographic phase and space group to which they belong. Mesoporous silicas, typified by those designated MCM-416 * To whom correspondence should be addressed. Fax/phone: 44-1714957395 (J.M.T). Email:
[email protected] (W.Z.). † University of Cambridge. ‡ University of St. Andrews. § Royal Institution.
and FSM-16,7,8 first reported in the early 1990s, have a cylindrical (1D) system of channels. Many recipes are now available9 for the production of these mesoporous silicas (and mesoporous metallic oxides), typical examples of which have the acronyms KIT-1, MCM-48, and SBA-1, -2, -3, and -15. The detailed ultrastructural characteristics of only very few of these silicas are known: MCM-41 has nonintersecting straight channels, and MCM-48 is a cubic phase consisting of two interweaving, nonintersecting channels.10 But the pore structures of many of the reported mesoporous silicas are incompletely understood. Since future applications of these novel, thermally stable, high-area solids depend crucially upon a detailed knowledge of their pore structure, we have carried out a high-resolution transmission electron microscopic (HRTEM) study11 of a typical mesoporous silica, so-called SBA-2. Partly because of previous reports,12,13 it is thought to consist of discrete large cages that are, to an extent and degree unclear, connected in ways that could make SBA-2 as a catalyst support superior to both MCM-
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Figure 1. XRD patterns of SBA-2 prepared using the C16H33Nx(CH3)2(CH2)3Nx(CH3)2 cationic surfactants. (a) and (b) are from samples as prepared, with better counting statistics on (b) to show lower d spacing peaks. (c) and (d) are from samples calcined, with better counting statistics on (d). Patterns c and d are indexed according to the hexagonal unit cell.
41 and MCM-48 silicas. The other reason for this study is to investigate whether HRTEM can uncover the existence of polytypic and related structural variants as it has done for numerous microporous zeolites14-16 and thereby greatly elucidate structure-property relationships in this large class of uniform heterogeneous catalysts. Specimens were prepared using the so-called “two-headed” (or gemini) quaternary ammonium surfactants as templates with a formula [H3C(CH2)15N(CH3)2C3H6N(CH3)3]Br2. The silica was produced from a gel consisting of tetramethylammonium hydroxide (TMAOH), tetraethyl orthosilicate (TEOS), and water with a ratio TMAOH:TEOS:H2O of 0.05:0.5:1:150. The reaction pH was adjusted to 11 with 1 M HCl. After 2 h stirring at room temperature, the materials were recovered by filtration, washed with distilled water, and dried in air at room temperature. The surfactant molecules could be removed by calcination at 500 to 600 °C for between 2 and 8 h, giving solids that adsorbed 290 cm3 (stp) of nitrogen per gram at 77 K and p/p0 ) 0.75. XRD patterns of as-synthesised and subsequently calcined mesoporous silica (Figure 1) could be indexed on the basis of hexagonal unit cells with a ) 55.5 and c ) 91.4 Å (assynthesized) and a ) 49.0 and c ) 80.4 Å (calcined). These give c/a ratios of 1.65 and 1.64, respectively, compared with 1.63 for perfect hexagonal close packing (hcp). We estimate the supercages (that are close-packed) to have a diameter of ca. 40 Å. These patterns are similar to those of Huo et al.13 except that the (100), (101), and (103) peaks are not as distinct in our patterns.
Letters HRTEM images were recorded using a JEOL JEM-200CX electron microscope operating at 200 kV with a modified specimen stage with objective lens parameters Cs ) 0.41 mm and Cc ) 0.95 mm, giving an interpretable point resolution of ca. 1.85 Å. SBA-2 samples were prepared by crushing the particles between two glass slides and spreading them on a holey carbon film supported on a Cu grid. The samples were briefly heated under a tungsten filament light bulb in air before transfer into the specimen chamber. HRTEM images were recorded along all the high-symmetry zone axes at magnifications of 24 000× to 49 000×. It is immediately apparent from the images shown in Figure 2a that the linked supercages (that form layers) are not stacked solely in the “ABAB...” sequence characteristic of hcp structure. Observed sequences frequently include “ABCABC...” stacking that signifies the presence of polytypic intergrowths of the cubic close-packed (ccp) structure. Such intergrowths are reminiscent of the ones that occur in zeolites, where extensive HRTEM studies14-16 have revealed that the so-called FAU (ccp) polytype, characteristic of zeolite Y, may intergrow in various ways with the hcp polytype known17 as EMT. It is relevant to note that Vaughan18 and others16,19,20 have shown that zeolites such as CSZ-1, CSZ-3, ECR-30, ECR-35, ZSM-3, and ZSM-20, all at one time (on the basis of XRD patterns) thought to be structurally distinct, are simply different kinds of polytypic intergrowths of FAU and EMT end members. We may gain deeper insights into the nature of polytypic intergrowths as well as channels connecting the supercages in SBA-2 by recording HRTEM images from underfocus (Figure 2a) to overfocus (Figure 2b), a procedure that greatly highlights ultrastructural irregularities. For example, the large white dots seen in the real-space image taken down the a axis (Figure 2a) change into black ones in the overfocused image. These large dots represent the “pores” along the a-axis and are similar in appearance to those seen in comparable6 HRTEM images of the 1D pores in MCM-41. For clarity, we designate these pores type I. Evidently, the type I pores connecting supercages in the ab planes run along the a direction, and this is confirmed by the images down the [001] projection shown in Figure 2c, where 1D lines appear in the thin region. In hcp regions of SBA-2 there are obviously no straight pores connecting the supercages in the [001] direction: this is clearly evident from all the HRTEM images taken either parallel or perpendicular to the c axis. The large white dots of Figure 2c signify the existence of a linear array of discrete supercages, but the contribution from the cages to the image contrast in Figure 2a is concealed by that from the channels. In Figure 2b, on the other hand, the zigzag stretches along the c axis appear in the thicker hcp regions, strongly indicating that there is a second group of channels (designated type II) that intersects the type I channels. Further analysis of through focus HRTEM images along various directions enables us to draw a complete channel system in SBA-2 as presented in Figure 3. In a single hexagonal unit cell of SBA-2 (Figure 3a), each cage, marked B as an example, is in contact with six-coordinated cages (marked by A1-A6) from its neighboring layers. These connections should be identical and all six linkages should exist simultaneously if the space group P63/mmc13 is applicable. However, since there are 1D type I channels, the symmetry of the structure is reduced and these interlayer connections fall into two groups according to their relationships with the type I channels. In detail, the first group channels connecting cages B and A3 would be perpendicular to the type I channels, and such a connection was
Letters
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Figure 3. Schematic drawing of the channel systems in SBA-2 with hcp stacking, showing (a) a single unit cell, (b) a view along the [001] direction, and (c) a single sheet containing the supercages, type I (straight) and type II (zigzag) channels.
Figure 2. (a) Underfocus HRTEM image of SBA-2 viewed down the a axis of the hexagonal unit cell. The top inset shows corresponding Fourier transform optical diffraction pattern with Miller-Bravais indices. The bottom inset is a simulated image using a model of Figure 3a, with specimen thickness of 98 Å and lens focus of -300 Å. The arrangement of close packed layers along the [001] zone axis are marked. (b) Overfocus HRTEM image recorded from the same crystal region as in (a). The close packed layers are also marked, indicating the same arrangement as in (a). A pathway of channels along the c axis is highlighted by a white zigzag line. All three types of channels are visible, i.e., type I, straight ones along the view direction of the projected image, type II, zigzag channels along the c axis in the hcp region with a turning angle of 141°, and type III, channels run straight in the ccp region. The insets show simulated images based on the model of Figure 3a with specimen thicknesses of 98 Å (top) and 49 Å (bottom) and len focuses of 2200 Å (top) and 2000 Å (bottom). (c) HRTEM image of SBA-2 viewed down the [001] direction. The insets show corresponding Fourier transform optical diffraction pattern with MillerBravais indices (left), simulated images with specimen thicknesses of 334 Å (top right) and 84 Å (bottom), and lens focuses of -375 Å (top right) and -250 Å (bottom). 1D type I channels along the a axis are visible at the thin region.
not observed in the present work. Otherwise, we should see an extra zigzag line in Figure 2b. On the other hand, the corresponding angle for the second group channels between cages B and A1 or cages B and A2 is about 60°. Coexistence of channels of A1-B-A4 and A2-B-A5 is unlikely, because a formation of triangle channels connecting three neighboring supercages will result in a very thin silica wall at the center.
Figure 4. Underfocus (a) and overfocus (b) HRTEM images of STAC-1 viewed down the a axis of a hexagonal unit cell (indicated by [hkl]h) or the [110] direction of a cubic unit cell (indicated by [hkl]c). The crystal is dominated by “ABCABC” close packing (indicated on (a)) with one stacking fault (marked by a horizontal line on (a)). A Fourier transform optical diffraction pattern with both Miller-Bravais indices to the hexagonal unit cell and Miller indices (in parentheses) to the cubic unit cell is inserted in (b). Simulated images are also inserted with specimen thickness of 300 Å, and lens focuses of -300 Å (a) and -100 Å (b).
HRTEM images viewed down various directions never show double zigzag channels along the c axis. Consequently, in the final proposed model (Figure 3) of SBA-2, there are only two types of channels, type I and type II, the latter being single zigzag channels lying on the c axis. The two-dimensional network consisting of supercages and channels is shown in Figure 3c. Consequently, the pore structure in SBA-2 can be regarded as MCM-41-like channels (type I) connected to each other by secondary channels (type II) with a supercage at each intersection, and there are no cross-connections between these 2D sheets. From Figure 2b, we see that, in the regions of SBA-2 where there is regular “ABAB” stacking along [001], the type II channels change direction at every layer running the shortest way along the [001] zone axis. However, in regions where the stacking is “ABCABC”, there is no zigzag character to the intersecting channels. Such channels, designated type III, stand out clearly in Figure 4b, where the whole particle is dominated by the ABCABC stacking arrangement. This indicates a new
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Figure 5. Schematic drawing of the channel systems in STAC-1 with ccp stacking. (a) shows a single hexagonal unit cell for a comparison with the hcp structure of SBA-2, (b) a cubic unit cell, and (c) a single sheet in a 2 × 2 × 2 cubic supercell containing the supercages, type I and type III channels. In an ideal case, these two types of channels become equivalent.
mesoporous phase, which we designate STAC-1 (St. AndrewsCambridge-1). The structural principle of channel formation in STAC-1 is similar to that in SBA-2; i.e., the secondary channels are never perpendicular to the type I channels, and there is no connection between the 2D channel-supercage sheets (Figure 5). The 2D straight channel system (type I and type III), which is impossible in SBA-2, makes the pore structure of STAC-1 even more attractive as a catalyst support since it may allow faster diffusion of large molecules. To further confirm our conclusions, computer image simulations were performed using the proposed models for SBA-2 (Figure 3a) and STAC-1 (Figure 5b), which have been built on the basis of amorphous SiO2. For the model of SBA-2, it started with a hexagonal unit cell filled by SiO2 with a ) 49.0 Å, c ) 80.4 Å, and γ ) 120°. The atoms inside the spheres centered at (0, 0, 0) and (1/3, 2/3, 1/2) with a radius of 20 Å were removed first to form hexagonal close-packed supercages. Then, the atoms inside the cylinders of radius 15 Å along the a axis connecting the empty spheres were removed, forming the type I channels. Finally, the atoms inside the cylinders connecting spheres at (0, 0, 0) to (1/3, 2/3, 1/2) and then to (0, 0, 1) were removed, forming the zigzag type II channels. For the model of STAC-1, a cubic unit cell of amorphous SiO2 with a ) 69.3 Å was created. The atoms inside the spheres at face-centered sites with a radius of 20 Å were removed. Pores were created by removing atoms inside the cylinders of radius of 15 Å connecting the empty spheres along [110] and [101h] directions. Computer simulation of the HRTEM images using proposed models were performed according to the multislice method, using the CERIUS HRTEM program developed by Cambridge Molecular Design Ltd. The experimental image contrast patterns at different conditions have been successfully reproduced as shown in the insets of Figures 2 and 4. In summary, using HRTEM technique, we uncovered the complete pore system in SBA-2, which is two-dimensional consisting of type I channels running straight along the a axis and type II channels in zigzag fashion along [001]. The type I channels in the (ab) planes must introduce a slight difference in the unit cell dimensions of the a and b axes, but the XRD and HRTEM are insufficiently accurate to show this. Zigzag mesopores selectively connect the supercages along the c axis.
Letters Both these facts imply that the symmetry of the structure must be lower than that (space group P63/mmc) determined previously on the basis of XRD studies.13 A new mesoporous phase, STAC-1, with ccp stacking of supercages connected by 2D straight channels (type I and type III) has also been discovered. Although the formation mechanism is not yet fully understood, it has been found that the two types of channels in either SBA-2 or STAC-1 are never perpendicular to each other. The 2D mesopore structures together with the supercages ensure that the materials will demonstrate better properties than MCM-41 in future applications as catalyst supports. In term of structural chemistry, the observation of polytypic irregularities in these mesoporous materials indicates that, similar to microporous zeolitic systems, a range of solids with structures intermediate between end members SBA-2 and STAC-1 might be prepared. Synthesis of monophasic STAC-1 and study of detailed formation mechanisms of these 2D pore systems are being carried out, and it is likely that by varying the surfactant species a wide range of mesoporous structures may be synthesized.21 Acknowledgment. The work was supported by EPSRC rolling grant (J.M.T.) and EPSRC studentship (H.H.). P.A.W. wishes to thank the University of St. Andrews. W.Z. thanks Dr. D. A. Jefferson for many helpful discussions. References and Notes (1) Corma, A. Top. Catal. 1997, 4, 249-260. (2) Sayari, A. Chem. Mater., 1996, 8, 1840-1852. (3) Tudor, J.; O’Hare, D. J. Chem. Soc., Chem. Commun. 1997, 603604. (4) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. Nature 1995, 378, 159-162. (5) Bein, T. Chem. Mater. 1996, 8, 1636-1653. (6) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (7) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. (8) Inagaki, S.; Fukushima, F.; Kuroda, K. Stud. Surf. Sci. Catal. 1994, 84, 125-132. (9) Zhao, D.; Yang, P.; Huo, Q.; Chmelka, B. F.; Stucky, G. D. Curr. Opin. Solid State Mater. Sci. 1998, 3, 111-121 and references therein. (10) Alfredsson, V.; Anderson, M. W. Chem. Mater. 1996, 8, 11411146. (11) Jefferson, D. A.; Thomas, J. M.; Millward, G. R.; Harriman, A.; Brydson, R. D. Nature 1986, 323, 428-431. (12) Tolbert, S. H.; Schaffer, T. E.; Feng, J.; Hansma, P. K.; Stucky, G. D.Chem. Mater. 1997, 9, 1962-1967. (13) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147-1160. (14) Thomas, J. M.; Audier, M.; Klinowski, J. J. Chem. Soc. Chem. Commun. 1981, 1221-1222. (15) Newsam, J. M.; Treacy, M. M. J.; Vaughan, D. E. W.; Strohmaier, K. G.; Mortier, W. J. J. Chem. Soc., Chem. Commun. 1989, 8, 493-495. (16) Ohsuna, T.; Terasaki, O.; Alfredsson, V.; Bovin, J. O.; Watanabe, D.; Carr, S. W.; Anderson, M. W. Proc. R. Soc. London 1996, A452, 715740. (17) Meier, W. M.; Olson, D. H. In Atlas of zeolite structure types, 2nd ed.; Butterworths: London, 1987. (18) Vaughan, D. E. W. Composition and process for preparing ECR30. U.S. Patent 4,879,103, 1989; 11 pp. (19) Thomas, J. M.; Vaughan, D. E. W. J. Phys. Chem. Solids 1989, 50, 449-467. (20) Vaughan, D. E. W.; Strohmaier, K. G.; Treacy, M. M. J.; Newsam, J. M. Crystalline ECR-35 zeolites, and their manufacture. U.S. Patent 5,116,590, 1992; 18pp. (21) Antonietti, M.; Go¨ltner, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 910-928.
