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Superstructures of mesoporous silicas
174
Superstructures of mesoporous silicas Ferdi Schuth During the past year, the major breakthroughs in the field of ordered mesoporous silicas have been the development of various techniques to control the morphologies of the materials on the macroscale and to align the pore-s of the materials on the microscale. The quality of the materials has also greatly improved, and the ability to control pore size in the size range up to 50 nm is on the horizon.
Addresses Institut fOrAnorganische Chemie, Johann Wolfgang Goethe-Universital, Marie Curie Stra~e 11, 60439 Frankfurt, Germany; e-mail: [email protected] Current Opinion in Colloid & Interface Science 1998, 3:174-180 Electronic identifier: 1359-0294-003-00174
© Current Chemistry Ltd ISSN 1359-0294
Introduction Research in the field of ordered mesoporous oxides, predominantly silica, expanded dramatically after the discovery by researchers from Mobil Oil Corporation in 1992 that silica can be ternplated, not only by single molecules as in the case of zeolites, but also by larger entities consisting of many molecules, such as those found in liquid crystals [1]. These silica oxides, known under the group name M41S, with the hexagonal MCM-41 being the most prominent member, dramatically expanded the range of pore sizes accessible in the form of an ordered pore system. Interestingly, Di Renzo et al. [2e ] recently discovered a patent from 1971 in which synthesis procedure similar to the one used by the Mobil group was described as yielding 'low-bulk density silica'. The patent procedure was reproduced, and the product had all the features of a well-developed J'vICM-41 structure, as shown by transmission electron microscopy, X-ray diffraction and nitrogen adsorption.
a
Up to the end of 1996, four years after Mobil had announced J'vICM-41, tremendous progress had already been made. Various types of molecules had been used as templates, as reviewed elsewhere [3]; the approach had been generalized from silicas and aluminosilicates to different transition metal oxides, as predicted in one of the early publications [4]; and the mechanisms of formation, although still a matter of debate [5], were considered as essentially understood. However, methods to control the development of these ordered oxides on larger scales such as the micron to millimeter scale, and to orient the pores of the materials macroscopically were still to be found along with applications which could lead to new technologies. In this review the advances in the synthesis and characterization of M41S materials and
the developments in the areas mentioned above will be highlighted.
Synthesis and characterization of mesostructured silica The quality of the materials prepared in different laboratories is still a matter of concern. The various synthetic procedures developed so far certainly do not result in the formation of identical materials with respect to perfection of the structure and thermal and hydrothermal stability. This is exemplified in two publications in which different synthesis conditions [6] or template removal procedures [7e] have been compared. With respect to the quality of the MCM-41 type materials, the procedures involving sulfuric acid titration during the synthesis developed by Edler and White [8] probably produce the best ordered materials with the largest domain size. The X-ray patterns of these materials show up to seven peaks in the low angle range (Figure 1), which is about the maximum that can be expected for a perfect MCM-41 structure, according to simulation of X-ray data performed recently in my laboratory (unpublished data) and by Edler et al. [ge ] . Also published in this paper [ge ] was a new pore model that was inferred from fitting the form factor to X-ray data obtained from high quality materials with synchrotron radiation. The best fit was achieved by assuming that a hollow pore with a relatively small diameter (-I.4nm) was surrounded by a low density material (-1.3 nm thickness) and eventually a denser wall (-0/6 nm). This is in contrast to sorption data where higher void volumes are determined. The analysis of pore sizes from adsorption experiments in this size range is certainly not easy (a careful adsorption study with different adsorbents was carried out by Boger et al. [10», since most of the algorithms usually fail in the regime of the smaller mesopores. Great advances, however, have been made using density functional theory calculations [11e ] , and, using this theory, pore sizes close to 4 nm for the materials synthesized with CI6 templates can be calculated. The discrepancy between the X-ray model and sorption data is thus not resolved, yet. Up until the end of 1996 the adjustment of pore sizes was primarily possible through the use of different surfactants or the addition of auxiliaries, although small changes had been achieved by other means. Simpler, very effective procedures, however, have been developed for J\1CM-41-type materials [12] and MSU-type materials [13], where controlled swelling or temperature variations could lead to desirable pore size changes by about a factor of two with the same surfactant. This still left the upper limit of pore sizes that are possible in ordered mesoporous silica at around 10 nm. A breakthrough towards larger
Superstructures of mesoporous silicas Schuth
The synthesis method for mesoporous silica, making use of surfactants, was generalized to other oxides soon after the discovery in 1992. Two recent exceptional examples shall be mentioned here: the manganese oxides, which have a spectacular thermal stability of 1000'C [15-], and the nanostructured metals produced with liquid crystal templates [16,17-], The high surface areas of these catalyst materials suggest applications in various processes, such as catalytic combustion over the manganese oxides, or hydrogenation reactions over the nanostructured metals,
Figure 1
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(a)
2000 x-10
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1000 Controlling the macrostructure of mesostructured silica
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175
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(b)
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500
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2 theta Synchrotron X-ray diffraction patterns of a high quality MCM-41 material from a synthesis quring which precipitation was controlled by titration with sulfuric acid. (a) Calcined sample, (b) uncalcined sample. Reproduced with permission from [9'1,
pores was recently achieved by Stucky's group [14"] and Goltner et 01. [53,54--; see Note added in proof]. The key improvement in both approaches is the use of block copolymers as liquid crystal templates, yielding mesoporous silica. The procedure developed by Stucky's group uses Pluronic® type block copolymers in the acid synthesis yielding haxagonally ordered silica. The materials formed have astonishing. properties: the wall thickness can be up to about 6 nm and the pore sizes can be as high as 30 nm if mesitylene is used as an auxiliary. The approach by Antonietti and Goltner [54--; see Note added in proof], which uses ionic block copolymers, has not resulted in the formation of a highly ordered haxagonally packed pore system so far, but the pore sizes of up to 50 nm and wall thicknesses or up to 10 nm achieved are even bigger than for the materials mentioned above. These synthetic processes open up another dimension in ordered porous materials, and the sizes come close to those which can be prepared by physical means from solid bulk materials.
For many of the envisaged applications of mesoporous oxides it is highly desirable to control the assembly of the primary particles into macroscopic objects. The highest degree of freedom in determining morphologies is achieved if the mesoporous materials can be molded into the form of monoliths, Two different techniques are available so far to achieved this form: the' first is based on the 'true liquid ternplating' technique developed by Attard et 01. [18]. If amphiphilic block copolymers are used, their properties enhance the ductility and elasticity of the composites, which can therefore be molded into monolithic blocks [19], The other approach relies more on conventional sol-gel chemistry, in that conditions are used where mixing of the reagents results in the formation of highly viscous gels, which can be shaped into the form of monoliths if carefully processed [20,21--].
Figure 2
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Example of gyroid morphology obtained from an acidic synthesis with cetyltrimethylammonium and tetrabutoxysilane. Reproduced with permission from [251,
Morphology control on the particle level was first demonstrated by Schacht et 01. [22]. The interfaces, which are formed between an aqueous phase and an oil phase,
176
Materials aspects
containing the siliconalkoxide, control the morphology of the resulting rnesophase, Shear stress introduced via stirring determined the details of the morphology. The acidic synthesis used here, which had been introduced by Huo et 01. [23], seems to be the most suitable method to prepare desired macroscopic shapes. Carrying our this synthesis under milder acidic conditions and using well-adjusted concentrations without stirring results in the formation of beautiful architectures, resembling various types of shells [24-,25] (Figure 2). The mechanisms controlling this assembly process are not clear yet ; the beauty of the particles, however, is striking. Simpler morphologies can be prepared in a more controlled "manner. A modified Stober procedure using surfactants during the polymerization of the silica results in the formation of spheres in the micrometer range [26]. Larger spheres that also incorporate transition metals can be produced using tetrabutoxysilane as a precursor [27]. These spheres are transparent and very hard -which means that no shaping is required ~so they might be directly used as catalyst particles. More difficult than the production of approximately spherical particles is the preparation of fibers. Under specific synthetic conditions, the formation of hollow tubules [28]or fibers consisting of agglomerated particles has already been observed [22]. Better defined fibers, however, have been prepared using a static interface from a similar synthetic system [29-], Figure 3. These fibers then grow into the aqueous phase from an oil-water interface. The specific morphologies developed in this system are not controlled by shear forces but rather by diffusion limitations. A totally different, and so far unique, approach is either spinning or spray drying [30-] the precursors to produce fibers or hollow spheres. Crucial in this process is the evaporation of the solvents used in the synthesis. A highly original approach was chosen by Mann and co-workers [31]. A fiber-type superstructure, consisting of bacteria (Bacillus subtilis), was used as a template for the deposition of MCr-.1-41 type material. The MCM-41 precipitated on the cell walls of the bacteria strands, thus forming a hollow fiber of !\IClvl-41 after removal of the bacteria. In the early publications on macroscopic morphology control, the preparation of either supported or free standing films was a major objective because of their potential applications in sorption or membrane catalysis [22,32-34]. The growth of these films on substrates and at the air/water interface has subsequently beenstudied in more detail [35-37]. Using a double headed surfactant instead of the conventional qu aternary ammonium cations leads to the formation of a new structure with space group P63/mmc [35]. It is by now quite clear that ordered micellar or hemimicellar arrangements of the surfactants on the substrate serve as the nucleation sites for film growth. In these coating processes, however, many of the
parameters, such as concentrations or the type of substrate, cannot be freely chosen in order to -influence the film properties. Thus, one important step forward was the development of dip [38-] or spin [39] coating techniques to produce such films in a more controlled manner. In the dip coating approach the surfactant-silica self-assembly is brought about by the evaporation of the solvent leaving a critical concentration of nonvolatile components in the mixture which subsequently forms the mesostructure configuration. If the conditions for film formation are well tuned, films with either cubic or cylindrical pores can be prepared. Surface acoustic techniques [39] reveal that the pores of the film are fully accessible to molecules from the gas phase.
Orientation of the pore system in mesoporous silicas Along with control of the morphology of the macrostructure, large scale control of the pore orientation is still one of the most important goals in 'the field. Some of the appro aches discussed above can also be used to macroscopically align the pore system. Several forces can bring about orientation of the domains in the composites; these forces are also known through their use in aligning liquid crystals. They include shear fields, electric fields, magnetic fields and the influence of surfaces. With the exception of electric field alignment, all these possibilities have been exploited for the orientation of the pores of meso porous silicas. Chmelka and co-workers [21--,40-] used high magnetic fields produced by an NMR magnet to macroscopically orient a silicate-surfactant liquid crystal prior to full condensation of the silica framework. Because of the high viscosity of the liquid crystal the sample can be removed from the magnet and polymerization can be induced by gaseous HCI or aqueous HBr while the orientation is maintained. This is also the case after calcination of the material, where the pores in the mesoporous solid are macroscopically oriented in the resulting monolith (Figure 4). With auxiliary organics present in the liquid crystals , different orientations can be achieved depending on which molecules couple strongest with the magnetic field. Most of the fibers presented in the previous section have pores aligned preferentially along the fiber axis. This characteristic is certainly closely related to the growth mode of the fibers. If fiber-type morphology is achieved by shear, such as in [22,30-], the shear stress provides the director for the pore orientation on the nanometer scale. Even the very first examples of films presented pores in the preferred orientation. Unfortunately, most of the films have pores aligned parallel to the film plane, which makes transport across the film rather difficult. The directing field in film formation is mostly provided by the presence of surfaces, although shear flow might be
Superstructures of mesoporous silicas SchUth
177
Figure 3
0.4 mm
Scanning electron microscopy image of mesoporous silica fibers grown from a film at the oil/water interface. (a) Two sides of the film. (b) Fast growth fiber. (e) Slow growth fiber. (d) Fiber with knots. (e) long fibers. (f) Fiber with helix morphology. All fibers were obtained from the aqueous side of the film. Reproduced with permission from [29·].
178
Materials aspects
Figure 4
As zeolites are mostly used in petroleum chemistry, MCi"f-41 is also tested in such applications. For the hydrogenation of aromatics in diesel fuels, Pt/MCM-41 has been compared with other supports, such as zeolites or amorphous aluminosilicates [46]. It was found that MCM-41 is superior; this was attributed to the high dispersion of the platinum [46]. The catalysts in this case were prepared by impregnation. An alternative is the introduction of preformed noble metal clusters which also results in a high dispersion and very active catalysts for CO oxidation [47]. The clusters are stabilized by entrapment in the mesopores. Although l\ICl\I-41 is weakly acidic, a basic catalyst can be prepared by its impregnation with caesium and lanthanum [48]. The small caesium-lanthanum oxide clusters in the channel system are thermally stable and can be used for the Knoevenagel condensation in aqueous media. Many researchers have incorporated transition metal ions into the silica framework in order to prepare redox catalysts. The potential of various transition metal ions has been investigated in a comparative study [49]. When compared to TS-1, several of the catalysts had higher activities for the epoxidation of cyclohexane, although only the chromium sample exhibited a comparable peroxide efficiency together with higher activity.
Two-dimensional X-ray diffraction pattern obtained from (a) oriented silica surfactant composite and (b) oriented mesoporous silica. For an isotropic distribution of pore orientations, a ring around the primary beam would be expected. The nonuniform distribution indicates the preferred orientation. The sketches show the orientation of the pore system with respect to the beam. Reproduced with permission from [21·].
used as well [41]. In the case of the films formed at the oil/water or air/water interface, flow caused by mass transport between the phases might have an influence, too. Surfactant molecules present at solid surfaces could form micellar or hemimicellar arrangements, which might act as nuclei for film formation [42,43]: The surface layer, however, preferentially orders the pores parallel to the film plane and not perpendicular. Still unclear are the factors responsible for alignment of the pores of MCivf-41 type material formed in small diameter capillaries [44].
Applications of ordered mesoporous materials Many catalytic processes, which have been discussed in a recent comprehensive review [45], have been investigated using M41S-type materials. So far no ind~strial application seems to have emerged, however, and the reports on the catalytic properties do not appear to be very promising.
Apart from catalysis, two other interesting applications have been reported recently. Thiol-functionalized MCM41 has been used as an adsorptive for heavy metals from solution [50·,51]. The supports with ordered pores are assumed to have advantages over amorphous silica. The sorption capacities were found to be an order of magnitude higher [51], and no conversion of the adsorbed heavy metals to highly toxic organometallic compounds by bacteria is possible, since the pores are too small for bacteria [SO·]. A final application, where the sharp pore size distribution of MCM-41 type materials is exploited, is its use as a matrix for semiconductor quantum dots [52]. SiGe was deposited on a silica film" by molecular beam epitaxy. The buried quantum dots have improved luminescence characteristics, which suggests their use in optoelectronic devices.
Conclusions It has been shown that a remarkably high degree of control over the properties of ordered mesoporous silicas has been achieved over the past few years. There is a wide variety of compositions available now, and morphology and orientation of the pore system can be adjusted by various techniques. There are, however, no real technical applications, and it can be predicted that research efforts will eventually decrease if rio applications emerge". To explain this lack ofapplications one should bear in mind that one of the main advantages of zeolites, the shape
Superstructures of mesoporous silicas Schuth
179
selectivity, can not be expected to be important in the pore size range of 4 nm as most molecules of interest arc much smaller and size restrictions arc therefore highly improbable. Moreover, for most of the applications, a high surface area is cruci al, and this can be achieved for silica by less costly synthetic strategies. The strict control of pore sizes and structure is only necess ary for special applications. In more conventional fields, i\(QM-4"l-type materials will have to compete with amorphous silicas prepared by other pathways.
Ravikovitch PI, W ie 0, Chueh WT, Haller GL, Neimark AV: Evaluation of pore structure parameters of MCM-41 catalyst supports and catalysts by means of nitrogen and argon adsorption. J Phys Chern B 1997, 101:3671-3679. A careful adsorption study in which an advanced density functional theory model has been used to extract pore structure parametersform the isotherm data
Note added in proof
14. ••
The two references [53,54··) have been ' .accepted for publication since this review was submitted.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: •• 1.
of special interest of outstand ing interest Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS: Ordered meso porous molecular sieves synthesized by a liquid crystal templating mechanism. Nature 1992,359:710-712.
Oi Renzo F, Cambon H, Outarte R: A 28-year '0Id synthesis of micelle·templated mesoporous silica. Micropor Mater 1997, 10:283·286. The authors reproduced a patented procedure (filed in 1969) which gave a low bulk density silica. In the patent procedure, cetyltrimethylammonium had been added during the hydrolysis of tetrabutoxysilane. Rigorous characterization by X·ray diffraction, transmission electron microscopy and sorption analysis revealed that the materiqJ has properties identical to those of MCM· 41. .. 2.
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Corma A, Kan C, Navarro MT, Perez-Parienta J, Rey F: Synthesis of MCM-41 with different pore diameters without addition of auxiliary organics. Chern Mater 1997, 9:2123-2126.
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Prouzet E, Pinnavaia TJ: Assembly of mesoporous molecular sieves containing wormhole motifs by a non ionic surfactant pathway: control of pore size by synthesis temperature. Angew Chern Int Ed 1997, 36:516-518.
Zhao D, Feng J, Huo C, Melosh W, Frerickson GH, Chmelka SF, Stucky GO: The cooperative self-assembly of period ic large pore structures. Science 1998, in press. The authors used commercially available block copolymers which can form liquid crystals to obtain hexagonallyordered silica with wall thicknesses between 3 and 6 nm and pore sizes up to 30 nm. Pore volumes amount up to 2.2 mllg. The approach chosen seems to be very generally applicable; for the first time the high wall thickness presents the possibility to convert the walls to zeolitic structures without losing the mesostructure. TIan ZR, Tong W, Wang JY, Ouan NG, Krishnan W, Suib SL: Manganese oxide mesoporous structures: mixed valent semiconducting catalysts. Science 1997,.276:926-930. Manganese oxide microcrystals of different manganese valency can be arranged into a hexagonal structure by an approach similar to the synthesis of MCM-41. The materials are thermally stable up to 1OOO·C. There seems to be potential for catalytic applications making use of the redox active framework. Oxidation of cyclohexane and hexane have been investigated by the authors. 15.
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17.
Allard GS, Geitner CG, Corker JM, Henke S, Templer RH: Liquid crystals templates for nanostructured metals. Angew Chern Int Ed 1997, 36:1315-1317. Hexagonally structured, mesoporous platinum is formed by impregnating the hexagonal lyotropic phase of octaethyleneglycolmonohexadecylether with hexachloroplatinic acid and subsequent reduction with less noble metals. The possibility of forming a hexagonal structure not only in oxidic materials, but also in pure metals, is very surprising and opens interesting possibilities.
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Hitz S, Prins R: Influence of template extraction on structure, activity, and stability of MCM-41 catalysts. J Cata/1997, 168:194 -206. A very careful study of the influence of template extraction procedures on the quality of the resulting mesoporous material. Pronounced differences in quality and catalytic performance of the porous oxides were observed, depending on the procedure used. A recommended procedure consists of extraction with acidic ethanol followed by a calcination treatment. The findings of this study highlight the importance of controlled sample preparation to obtain reliable and comparable data 8.
Edler KJ, Reynolds PA, White JW, Cookson 0 : DiHuse wall structure and narrow mesopores In highly crystalline l.lCM-41 materials studied by X-ray diHraction. J Chern Soc - Faraday Trans 1997, 93:199-202. One of the few publications in which modeling of X'ray diffraction data has been attempted. The authors had a very high quality MCM-41 material, so that synchrotron diffraction data could be used to fit the form factor. A good fit for the pores could only be achieved if a three shell model was assumed: the center of the pores is hollow and is surrounded by a low density shell and finally by the dense wall. 10.
18.
Attard GS, Glyde JC, Geitner CG: Liquid crystalline phases as templates for the synthesis of mesoporous silica. Nature 1995, 378:366·368.
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Anderson MT, Martin JE, Odinek JG, Newcomer PP, Wilcoxon JP: Monolithic periodic mesoporous silica gels. Micropor Mater 1997, 10:13-24.
Tolbert SH, Firouzi A, Stucky GO, Chmelka BF: Magnetic field alignment of ordered silicate-surfactant composites and mesoporous silica . Science 1997, 278:264-268. By apply'.ng a strongmagnetic field of 11.7 T, a noncondensed surfactant-sil ica liquid crystal can be macroscopically oriented if heated above the anisotropic to isotropic phase transition temperature and then cooled in the field. Condensation of the silica by gaseous HCI or aqueous HBr does not disturb the ordering. Also, calcination is possible without losing the macroscopic order. Oriented monolithic mesoporous silica could have applications in manyfields where the anisotropy is important, for instance in optical applications. 22.
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26.
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Lin HP, Mou CY: 'Tubules-within-a-tubule' hierarchical order of mesoporous molecular sieves in MCM-41. Science 1996, 273:765-768.
diHerent orientations, depending on the species in the liquid crystal which couples strongest with the magnetic field, is very.interesting. 41.
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Bruinsma PJ, Kim AY, Liu J, Baskaran S: Mesoporous silica synthesized by solvent evaporation: spun fibers and spray dried hollow spheres. Chern Mater 1997, 9:2507-2512. An interesting approach for the formation of mesostructured silica. By a spinning procedure fibers can be formed, whereas spray drying results in the formation of hollow spheres. If bulk processing of MCM-41-type materials should become necessary at some point, these techniques could become useful, since bulk quantities can in principle be processed.
46.
Corma A, Martinez A, Martinez-SoriaV: Hydrogenation of aromatics in diesel fuels on PlIMCM-41 catalysts. J Ca1a/1997, 169:4880-4891.
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Firouzi A, Schafer OJ, Tolbert SH, Stucky GO, Chmelka BF: Magnetic-field induced orientational ordering of alkaline lyotropic silicate-surfactant liquid crystals. J Am Chern Soc 1997, 119:9466-9477. A thorough study of the possibilities of orienting silicate-surfactant liquid crystals by an external magnetic field. This study laid the foundation for the preparation of oriented mesoporous silica [20]. The possibility of achieving
Feng X, Fryxell GE, Wang La, Kim AY, Liu J, Kemner KM: Functionalized monolayers on ordered mesoporous supports. Science 1997, 276:923·926. Thiol·functionalized MCM-41, obtained by reacting the silanol groups of MCM·41 with tris(methoxy)mercaptopropylsilane, is a highly effective adsorbent for heavy metal ions. Distribution coefficients as high as 340,000 have been achieved for mercury at pH =9. Technicalapplication in remediation of water pollution seems to be possible because of the stability of the material. 50.
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Mercier L, PinnvaiaTJ: Access In mesoporous materials: advantages of a uniform pore structure in the design of a heavy metal ion adsorbent for environmental remediation. Adv Mater 1997, 9:500-503.
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Tang YS, Cai S, Jin G, Duan J, Wang KL, Soyez HM, Dunn BS: SiGe quantum dots prepared on an ordered mesoporous silica coated Si substrate. Appl Phys Lett 1997, 71:2448·2450.
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Geitner CG, Henke S, Weipnerberger MC, Antonietti M: Mesoporous Silicate aus Iytrop-fliissigkristallinen BlockcoPOlymertemplaten. Angew Chern 1998, 110:633636. (Title translation: Mesoporous silicates from lyotropic liquid crystalline block copolymer templates.]
54. ••
Kramer E, Forster S, Geitner CG, Anotnietti M: Synthesis of nanoporous silica with new pore morphologies by templating the assemblies of ionic block copolymers. Langmuir 1998, inpress. The authors report the use of cationic polybutadiene-b'poly(vinylpyridinium), or anionic poly(ehtylethylene)-b'polystyrenesulfonate block copolymers to direct the synthesis of silica via hydrolysis of tetramethoxysilane. The result· ing silica gel network, after calcination, was found to be a precise copy of the self·assembly structure which in turn can be adjusted by the nature of the block copolymer. Pore sizes up to 50 nm and wall thicknesses up to 10 nm are accessible, albeit not perfectly ordered. The authors claim that the templating is so precise that silica casting can be used to depict unknown aggregation structures of block copolymers, such as the transition between the dispersed micellar and the lamellar phase.
Superstructures of mesoporous silicas Ferdi Schuth During the past year, the major breakthroughs in the field of ordered mesoporous silicas have been the development of various techniques to control the morphologies of the materials on the macroscale and to align the pore-s of the materials on the microscale. The quality of the materials has also greatly improved, and the ability to control pore size in the size range up to 50 nm is on the horizon.
Addresses Institut fOrAnorganische Chemie, Johann Wolfgang Goethe-Universital, Marie Curie Stra~e 11, 60439 Frankfurt, Germany; e-mail: [email protected] Current Opinion in Colloid & Interface Science 1998, 3:174-180 Electronic identifier: 1359-0294-003-00174
© Current Chemistry Ltd ISSN 1359-0294
Introduction Research in the field of ordered mesoporous oxides, predominantly silica, expanded dramatically after the discovery by researchers from Mobil Oil Corporation in 1992 that silica can be ternplated, not only by single molecules as in the case of zeolites, but also by larger entities consisting of many molecules, such as those found in liquid crystals [1]. These silica oxides, known under the group name M41S, with the hexagonal MCM-41 being the most prominent member, dramatically expanded the range of pore sizes accessible in the form of an ordered pore system. Interestingly, Di Renzo et al. [2e ] recently discovered a patent from 1971 in which synthesis procedure similar to the one used by the Mobil group was described as yielding 'low-bulk density silica'. The patent procedure was reproduced, and the product had all the features of a well-developed J'vICM-41 structure, as shown by transmission electron microscopy, X-ray diffraction and nitrogen adsorption.
a
Up to the end of 1996, four years after Mobil had announced J'vICM-41, tremendous progress had already been made. Various types of molecules had been used as templates, as reviewed elsewhere [3]; the approach had been generalized from silicas and aluminosilicates to different transition metal oxides, as predicted in one of the early publications [4]; and the mechanisms of formation, although still a matter of debate [5], were considered as essentially understood. However, methods to control the development of these ordered oxides on larger scales such as the micron to millimeter scale, and to orient the pores of the materials macroscopically were still to be found along with applications which could lead to new technologies. In this review the advances in the synthesis and characterization of M41S materials and
the developments in the areas mentioned above will be highlighted.
Synthesis and characterization of mesostructured silica The quality of the materials prepared in different laboratories is still a matter of concern. The various synthetic procedures developed so far certainly do not result in the formation of identical materials with respect to perfection of the structure and thermal and hydrothermal stability. This is exemplified in two publications in which different synthesis conditions [6] or template removal procedures [7e] have been compared. With respect to the quality of the MCM-41 type materials, the procedures involving sulfuric acid titration during the synthesis developed by Edler and White [8] probably produce the best ordered materials with the largest domain size. The X-ray patterns of these materials show up to seven peaks in the low angle range (Figure 1), which is about the maximum that can be expected for a perfect MCM-41 structure, according to simulation of X-ray data performed recently in my laboratory (unpublished data) and by Edler et al. [ge ] . Also published in this paper [ge ] was a new pore model that was inferred from fitting the form factor to X-ray data obtained from high quality materials with synchrotron radiation. The best fit was achieved by assuming that a hollow pore with a relatively small diameter (-I.4nm) was surrounded by a low density material (-1.3 nm thickness) and eventually a denser wall (-0/6 nm). This is in contrast to sorption data where higher void volumes are determined. The analysis of pore sizes from adsorption experiments in this size range is certainly not easy (a careful adsorption study with different adsorbents was carried out by Boger et al. [10», since most of the algorithms usually fail in the regime of the smaller mesopores. Great advances, however, have been made using density functional theory calculations [11e ] , and, using this theory, pore sizes close to 4 nm for the materials synthesized with CI6 templates can be calculated. The discrepancy between the X-ray model and sorption data is thus not resolved, yet. Up until the end of 1996 the adjustment of pore sizes was primarily possible through the use of different surfactants or the addition of auxiliaries, although small changes had been achieved by other means. Simpler, very effective procedures, however, have been developed for J\1CM-41-type materials [12] and MSU-type materials [13], where controlled swelling or temperature variations could lead to desirable pore size changes by about a factor of two with the same surfactant. This still left the upper limit of pore sizes that are possible in ordered mesoporous silica at around 10 nm. A breakthrough towards larger
Superstructures of mesoporous silicas Schuth
The synthesis method for mesoporous silica, making use of surfactants, was generalized to other oxides soon after the discovery in 1992. Two recent exceptional examples shall be mentioned here: the manganese oxides, which have a spectacular thermal stability of 1000'C [15-], and the nanostructured metals produced with liquid crystal templates [16,17-], The high surface areas of these catalyst materials suggest applications in various processes, such as catalytic combustion over the manganese oxides, or hydrogenation reactions over the nanostructured metals,
Figure 1
3000
(a)
2000 x-10
....
~
1000 Controlling the macrostructure of mesostructured silica
VI
C
~ ......
....c
175
0
(b)
1000
500
2
4
8
10
12
2 theta Synchrotron X-ray diffraction patterns of a high quality MCM-41 material from a synthesis quring which precipitation was controlled by titration with sulfuric acid. (a) Calcined sample, (b) uncalcined sample. Reproduced with permission from [9'1,
pores was recently achieved by Stucky's group [14"] and Goltner et 01. [53,54--; see Note added in proof]. The key improvement in both approaches is the use of block copolymers as liquid crystal templates, yielding mesoporous silica. The procedure developed by Stucky's group uses Pluronic® type block copolymers in the acid synthesis yielding haxagonally ordered silica. The materials formed have astonishing. properties: the wall thickness can be up to about 6 nm and the pore sizes can be as high as 30 nm if mesitylene is used as an auxiliary. The approach by Antonietti and Goltner [54--; see Note added in proof], which uses ionic block copolymers, has not resulted in the formation of a highly ordered haxagonally packed pore system so far, but the pore sizes of up to 50 nm and wall thicknesses or up to 10 nm achieved are even bigger than for the materials mentioned above. These synthetic processes open up another dimension in ordered porous materials, and the sizes come close to those which can be prepared by physical means from solid bulk materials.
For many of the envisaged applications of mesoporous oxides it is highly desirable to control the assembly of the primary particles into macroscopic objects. The highest degree of freedom in determining morphologies is achieved if the mesoporous materials can be molded into the form of monoliths, Two different techniques are available so far to achieved this form: the' first is based on the 'true liquid ternplating' technique developed by Attard et 01. [18]. If amphiphilic block copolymers are used, their properties enhance the ductility and elasticity of the composites, which can therefore be molded into monolithic blocks [19], The other approach relies more on conventional sol-gel chemistry, in that conditions are used where mixing of the reagents results in the formation of highly viscous gels, which can be shaped into the form of monoliths if carefully processed [20,21--].
Figure 2
2 • I
e
k V
X 6 •
e ek ' '5 :
e'e'''; m
Example of gyroid morphology obtained from an acidic synthesis with cetyltrimethylammonium and tetrabutoxysilane. Reproduced with permission from [251,
Morphology control on the particle level was first demonstrated by Schacht et 01. [22]. The interfaces, which are formed between an aqueous phase and an oil phase,
176
Materials aspects
containing the siliconalkoxide, control the morphology of the resulting rnesophase, Shear stress introduced via stirring determined the details of the morphology. The acidic synthesis used here, which had been introduced by Huo et 01. [23], seems to be the most suitable method to prepare desired macroscopic shapes. Carrying our this synthesis under milder acidic conditions and using well-adjusted concentrations without stirring results in the formation of beautiful architectures, resembling various types of shells [24-,25] (Figure 2). The mechanisms controlling this assembly process are not clear yet ; the beauty of the particles, however, is striking. Simpler morphologies can be prepared in a more controlled "manner. A modified Stober procedure using surfactants during the polymerization of the silica results in the formation of spheres in the micrometer range [26]. Larger spheres that also incorporate transition metals can be produced using tetrabutoxysilane as a precursor [27]. These spheres are transparent and very hard -which means that no shaping is required ~so they might be directly used as catalyst particles. More difficult than the production of approximately spherical particles is the preparation of fibers. Under specific synthetic conditions, the formation of hollow tubules [28]or fibers consisting of agglomerated particles has already been observed [22]. Better defined fibers, however, have been prepared using a static interface from a similar synthetic system [29-], Figure 3. These fibers then grow into the aqueous phase from an oil-water interface. The specific morphologies developed in this system are not controlled by shear forces but rather by diffusion limitations. A totally different, and so far unique, approach is either spinning or spray drying [30-] the precursors to produce fibers or hollow spheres. Crucial in this process is the evaporation of the solvents used in the synthesis. A highly original approach was chosen by Mann and co-workers [31]. A fiber-type superstructure, consisting of bacteria (Bacillus subtilis), was used as a template for the deposition of MCr-.1-41 type material. The MCM-41 precipitated on the cell walls of the bacteria strands, thus forming a hollow fiber of !\IClvl-41 after removal of the bacteria. In the early publications on macroscopic morphology control, the preparation of either supported or free standing films was a major objective because of their potential applications in sorption or membrane catalysis [22,32-34]. The growth of these films on substrates and at the air/water interface has subsequently beenstudied in more detail [35-37]. Using a double headed surfactant instead of the conventional qu aternary ammonium cations leads to the formation of a new structure with space group P63/mmc [35]. It is by now quite clear that ordered micellar or hemimicellar arrangements of the surfactants on the substrate serve as the nucleation sites for film growth. In these coating processes, however, many of the
parameters, such as concentrations or the type of substrate, cannot be freely chosen in order to -influence the film properties. Thus, one important step forward was the development of dip [38-] or spin [39] coating techniques to produce such films in a more controlled manner. In the dip coating approach the surfactant-silica self-assembly is brought about by the evaporation of the solvent leaving a critical concentration of nonvolatile components in the mixture which subsequently forms the mesostructure configuration. If the conditions for film formation are well tuned, films with either cubic or cylindrical pores can be prepared. Surface acoustic techniques [39] reveal that the pores of the film are fully accessible to molecules from the gas phase.
Orientation of the pore system in mesoporous silicas Along with control of the morphology of the macrostructure, large scale control of the pore orientation is still one of the most important goals in 'the field. Some of the appro aches discussed above can also be used to macroscopically align the pore system. Several forces can bring about orientation of the domains in the composites; these forces are also known through their use in aligning liquid crystals. They include shear fields, electric fields, magnetic fields and the influence of surfaces. With the exception of electric field alignment, all these possibilities have been exploited for the orientation of the pores of meso porous silicas. Chmelka and co-workers [21--,40-] used high magnetic fields produced by an NMR magnet to macroscopically orient a silicate-surfactant liquid crystal prior to full condensation of the silica framework. Because of the high viscosity of the liquid crystal the sample can be removed from the magnet and polymerization can be induced by gaseous HCI or aqueous HBr while the orientation is maintained. This is also the case after calcination of the material, where the pores in the mesoporous solid are macroscopically oriented in the resulting monolith (Figure 4). With auxiliary organics present in the liquid crystals , different orientations can be achieved depending on which molecules couple strongest with the magnetic field. Most of the fibers presented in the previous section have pores aligned preferentially along the fiber axis. This characteristic is certainly closely related to the growth mode of the fibers. If fiber-type morphology is achieved by shear, such as in [22,30-], the shear stress provides the director for the pore orientation on the nanometer scale. Even the very first examples of films presented pores in the preferred orientation. Unfortunately, most of the films have pores aligned parallel to the film plane, which makes transport across the film rather difficult. The directing field in film formation is mostly provided by the presence of surfaces, although shear flow might be
Superstructures of mesoporous silicas SchUth
177
Figure 3
0.4 mm
Scanning electron microscopy image of mesoporous silica fibers grown from a film at the oil/water interface. (a) Two sides of the film. (b) Fast growth fiber. (e) Slow growth fiber. (d) Fiber with knots. (e) long fibers. (f) Fiber with helix morphology. All fibers were obtained from the aqueous side of the film. Reproduced with permission from [29·].
178
Materials aspects
Figure 4
As zeolites are mostly used in petroleum chemistry, MCi"f-41 is also tested in such applications. For the hydrogenation of aromatics in diesel fuels, Pt/MCM-41 has been compared with other supports, such as zeolites or amorphous aluminosilicates [46]. It was found that MCM-41 is superior; this was attributed to the high dispersion of the platinum [46]. The catalysts in this case were prepared by impregnation. An alternative is the introduction of preformed noble metal clusters which also results in a high dispersion and very active catalysts for CO oxidation [47]. The clusters are stabilized by entrapment in the mesopores. Although l\ICl\I-41 is weakly acidic, a basic catalyst can be prepared by its impregnation with caesium and lanthanum [48]. The small caesium-lanthanum oxide clusters in the channel system are thermally stable and can be used for the Knoevenagel condensation in aqueous media. Many researchers have incorporated transition metal ions into the silica framework in order to prepare redox catalysts. The potential of various transition metal ions has been investigated in a comparative study [49]. When compared to TS-1, several of the catalysts had higher activities for the epoxidation of cyclohexane, although only the chromium sample exhibited a comparable peroxide efficiency together with higher activity.
Two-dimensional X-ray diffraction pattern obtained from (a) oriented silica surfactant composite and (b) oriented mesoporous silica. For an isotropic distribution of pore orientations, a ring around the primary beam would be expected. The nonuniform distribution indicates the preferred orientation. The sketches show the orientation of the pore system with respect to the beam. Reproduced with permission from [21·].
used as well [41]. In the case of the films formed at the oil/water or air/water interface, flow caused by mass transport between the phases might have an influence, too. Surfactant molecules present at solid surfaces could form micellar or hemimicellar arrangements, which might act as nuclei for film formation [42,43]: The surface layer, however, preferentially orders the pores parallel to the film plane and not perpendicular. Still unclear are the factors responsible for alignment of the pores of MCivf-41 type material formed in small diameter capillaries [44].
Applications of ordered mesoporous materials Many catalytic processes, which have been discussed in a recent comprehensive review [45], have been investigated using M41S-type materials. So far no ind~strial application seems to have emerged, however, and the reports on the catalytic properties do not appear to be very promising.
Apart from catalysis, two other interesting applications have been reported recently. Thiol-functionalized MCM41 has been used as an adsorptive for heavy metals from solution [50·,51]. The supports with ordered pores are assumed to have advantages over amorphous silica. The sorption capacities were found to be an order of magnitude higher [51], and no conversion of the adsorbed heavy metals to highly toxic organometallic compounds by bacteria is possible, since the pores are too small for bacteria [SO·]. A final application, where the sharp pore size distribution of MCM-41 type materials is exploited, is its use as a matrix for semiconductor quantum dots [52]. SiGe was deposited on a silica film" by molecular beam epitaxy. The buried quantum dots have improved luminescence characteristics, which suggests their use in optoelectronic devices.
Conclusions It has been shown that a remarkably high degree of control over the properties of ordered mesoporous silicas has been achieved over the past few years. There is a wide variety of compositions available now, and morphology and orientation of the pore system can be adjusted by various techniques. There are, however, no real technical applications, and it can be predicted that research efforts will eventually decrease if rio applications emerge". To explain this lack ofapplications one should bear in mind that one of the main advantages of zeolites, the shape
Superstructures of mesoporous silicas Schuth
179
selectivity, can not be expected to be important in the pore size range of 4 nm as most molecules of interest arc much smaller and size restrictions arc therefore highly improbable. Moreover, for most of the applications, a high surface area is cruci al, and this can be achieved for silica by less costly synthetic strategies. The strict control of pore sizes and structure is only necess ary for special applications. In more conventional fields, i\(QM-4"l-type materials will have to compete with amorphous silicas prepared by other pathways.
Ravikovitch PI, W ie 0, Chueh WT, Haller GL, Neimark AV: Evaluation of pore structure parameters of MCM-41 catalyst supports and catalysts by means of nitrogen and argon adsorption. J Phys Chern B 1997, 101:3671-3679. A careful adsorption study in which an advanced density functional theory model has been used to extract pore structure parametersform the isotherm data
Note added in proof
14. ••
The two references [53,54··) have been ' .accepted for publication since this review was submitted.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: •• 1.
of special interest of outstand ing interest Kresge CT, Leonowicz ME, Roth WJ, Vartuli JC, Beck JS: Ordered meso porous molecular sieves synthesized by a liquid crystal templating mechanism. Nature 1992,359:710-712.
Oi Renzo F, Cambon H, Outarte R: A 28-year '0Id synthesis of micelle·templated mesoporous silica. Micropor Mater 1997, 10:283·286. The authors reproduced a patented procedure (filed in 1969) which gave a low bulk density silica. In the patent procedure, cetyltrimethylammonium had been added during the hydrolysis of tetrabutoxysilane. Rigorous characterization by X·ray diffraction, transmission electron microscopy and sorption analysis revealed that the materiqJ has properties identical to those of MCM· 41. .. 2.
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Corma A, Kan C, Navarro MT, Perez-Parienta J, Rey F: Synthesis of MCM-41 with different pore diameters without addition of auxiliary organics. Chern Mater 1997, 9:2123-2126.
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Prouzet E, Pinnavaia TJ: Assembly of mesoporous molecular sieves containing wormhole motifs by a non ionic surfactant pathway: control of pore size by synthesis temperature. Angew Chern Int Ed 1997, 36:516-518.
Zhao D, Feng J, Huo C, Melosh W, Frerickson GH, Chmelka SF, Stucky GO: The cooperative self-assembly of period ic large pore structures. Science 1998, in press. The authors used commercially available block copolymers which can form liquid crystals to obtain hexagonallyordered silica with wall thicknesses between 3 and 6 nm and pore sizes up to 30 nm. Pore volumes amount up to 2.2 mllg. The approach chosen seems to be very generally applicable; for the first time the high wall thickness presents the possibility to convert the walls to zeolitic structures without losing the mesostructure. TIan ZR, Tong W, Wang JY, Ouan NG, Krishnan W, Suib SL: Manganese oxide mesoporous structures: mixed valent semiconducting catalysts. Science 1997,.276:926-930. Manganese oxide microcrystals of different manganese valency can be arranged into a hexagonal structure by an approach similar to the synthesis of MCM-41. The materials are thermally stable up to 1OOO·C. There seems to be potential for catalytic applications making use of the redox active framework. Oxidation of cyclohexane and hexane have been investigated by the authors. 15.
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17.
Allard GS, Geitner CG, Corker JM, Henke S, Templer RH: Liquid crystals templates for nanostructured metals. Angew Chern Int Ed 1997, 36:1315-1317. Hexagonally structured, mesoporous platinum is formed by impregnating the hexagonal lyotropic phase of octaethyleneglycolmonohexadecylether with hexachloroplatinic acid and subsequent reduction with less noble metals. The possibility of forming a hexagonal structure not only in oxidic materials, but also in pure metals, is very surprising and opens interesting possibilities.
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Hitz S, Prins R: Influence of template extraction on structure, activity, and stability of MCM-41 catalysts. J Cata/1997, 168:194 -206. A very careful study of the influence of template extraction procedures on the quality of the resulting mesoporous material. Pronounced differences in quality and catalytic performance of the porous oxides were observed, depending on the procedure used. A recommended procedure consists of extraction with acidic ethanol followed by a calcination treatment. The findings of this study highlight the importance of controlled sample preparation to obtain reliable and comparable data 8.
Edler KJ, Reynolds PA, White JW, Cookson 0 : DiHuse wall structure and narrow mesopores In highly crystalline l.lCM-41 materials studied by X-ray diHraction. J Chern Soc - Faraday Trans 1997, 93:199-202. One of the few publications in which modeling of X'ray diffraction data has been attempted. The authors had a very high quality MCM-41 material, so that synchrotron diffraction data could be used to fit the form factor. A good fit for the pores could only be achieved if a three shell model was assumed: the center of the pores is hollow and is surrounded by a low density shell and finally by the dense wall. 10.
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Anderson MT, Martin JE, Odinek JG, Newcomer PP, Wilcoxon JP: Monolithic periodic mesoporous silica gels. Micropor Mater 1997, 10:13-24.
Tolbert SH, Firouzi A, Stucky GO, Chmelka BF: Magnetic field alignment of ordered silicate-surfactant composites and mesoporous silica . Science 1997, 278:264-268. By apply'.ng a strongmagnetic field of 11.7 T, a noncondensed surfactant-sil ica liquid crystal can be macroscopically oriented if heated above the anisotropic to isotropic phase transition temperature and then cooled in the field. Condensation of the silica by gaseous HCI or aqueous HBr does not disturb the ordering. Also, calcination is possible without losing the macroscopic order. Oriented monolithic mesoporous silica could have applications in manyfields where the anisotropy is important, for instance in optical applications. 22.
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diHerent orientations, depending on the species in the liquid crystal which couples strongest with the magnetic field, is very.interesting. 41.
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Corma A, Martinez A, Martinez-SoriaV: Hydrogenation of aromatics in diesel fuels on PlIMCM-41 catalysts. J Ca1a/1997, 169:4880-4891.
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Firouzi A, Schafer OJ, Tolbert SH, Stucky GO, Chmelka BF: Magnetic-field induced orientational ordering of alkaline lyotropic silicate-surfactant liquid crystals. J Am Chern Soc 1997, 119:9466-9477. A thorough study of the possibilities of orienting silicate-surfactant liquid crystals by an external magnetic field. This study laid the foundation for the preparation of oriented mesoporous silica [20]. The possibility of achieving
Feng X, Fryxell GE, Wang La, Kim AY, Liu J, Kemner KM: Functionalized monolayers on ordered mesoporous supports. Science 1997, 276:923·926. Thiol·functionalized MCM-41, obtained by reacting the silanol groups of MCM·41 with tris(methoxy)mercaptopropylsilane, is a highly effective adsorbent for heavy metal ions. Distribution coefficients as high as 340,000 have been achieved for mercury at pH =9. Technicalapplication in remediation of water pollution seems to be possible because of the stability of the material. 50.
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Mercier L, PinnvaiaTJ: Access In mesoporous materials: advantages of a uniform pore structure in the design of a heavy metal ion adsorbent for environmental remediation. Adv Mater 1997, 9:500-503.
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Tang YS, Cai S, Jin G, Duan J, Wang KL, Soyez HM, Dunn BS: SiGe quantum dots prepared on an ordered mesoporous silica coated Si substrate. Appl Phys Lett 1997, 71:2448·2450.
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Geitner CG, Henke S, Weipnerberger MC, Antonietti M: Mesoporous Silicate aus Iytrop-fliissigkristallinen BlockcoPOlymertemplaten. Angew Chern 1998, 110:633636. (Title translation: Mesoporous silicates from lyotropic liquid crystalline block copolymer templates.]
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Kramer E, Forster S, Geitner CG, Anotnietti M: Synthesis of nanoporous silica with new pore morphologies by templating the assemblies of ionic block copolymers. Langmuir 1998, inpress. The authors report the use of cationic polybutadiene-b'poly(vinylpyridinium), or anionic poly(ehtylethylene)-b'polystyrenesulfonate block copolymers to direct the synthesis of silica via hydrolysis of tetramethoxysilane. The result· ing silica gel network, after calcination, was found to be a precise copy of the self·assembly structure which in turn can be adjusted by the nature of the block copolymer. Pore sizes up to 50 nm and wall thicknesses up to 10 nm are accessible, albeit not perfectly ordered. The authors claim that the templating is so precise that silica casting can be used to depict unknown aggregation structures of block copolymers, such as the transition between the dispersed micellar and the lamellar phase.