Nanocasting: A Versatile Strategy for Creating ... - Wiley Online Library

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DOI: 10.1002/adma.200600148

Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials** By An-Hui Lu and Ferdi Schüth* Nanocasting is a powerful method for creating materials that are more difficult to synthesize by conventional processes. We summarize recent developments in the synthesis of various structured porous solids, covering silica, carbon, and other nonsiliceous solids that are created by a nanocasting pathway. Structure replication on the nanometer length scale allows materials’ properties to be manipulated in a controlled manner, such as tunable composition, controllable structure and morphology, and specific functionality. The nanocasting pathway with hard templates opens the door to the design of highly porous solids with multifunctional properties and interesting application perspectives.

1. Introduction Porous materials are of great interest in various applications, ranging from catalysis, adsorption, sensing, and separation to biotechnology, owing to their high surface area, tunable pore size, adjustable framework, and surface properties. The specific surface areas can reach values of up to several thousand square meters per gram, depending on the material. Many synthetic pathways have been reported for the synthesis of porous materials, either with a disordered pore system or ordered with various structures, which can meet the demands of the target application.[1] In particular, research in the synthesis of ordered porous materials has seen tremendous growth since the discovery of the ordered mesoporous silica of the M41S family[2] or related materials,[3] which are synthesized with the help of cooperative surfactant templating. Since these pioneering studies, significant progress has been made in terms of structural, compositional, and morphological control. Several reviews covering synthesis, properties, and applications of mesoporous materials are available.[4–14]

– [*] Prof. F. Schüth, Dr. A.-H. Lu Max-Planck-Institut für Kohlenforschung 45470 Mülheim an der Ruhr (Germany) E-mail: [email protected] [**] The authors thank the Leibniz Program and the FCI for support in addition to the basic funding provided by the Institute.

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As important as siliceous materials are, both from a fundamental and from an applications point of view, the design and synthesis of nonsiliceous materials with controlled composition and structural ordering are even more important from an academic as well as an industrial perspective, since they have a much broader application range and their syntheses provide additional challenges. Compared to the silicon alkoxides, the hydrolysis and polymerization of transition-metal alkoxides are more difficult to control precisely. Consequently, the obtained metal oxides usually exhibit very poor structural ordering and low thermal stability after removal of the surfactant templates.[15] Only special precursors, such as atranes, allow these problems to be circumvented in selected cases.[16] Moreover, it is very difficult to obtain mesoporous carbon materials with an ordered structure via a sol–gel process involving a surfactant templating strategy, owing to the complexity of the carbon-structure evolution.[17,18] Only four recent reports are available concerning the synthesis of ordered mesoporous carbon, mainly as a thin film, by the use of a rigid polymer resorcinol-formaldehyde as the carbon source in a sol–gel process.[19–22] As an alternative to cooperative surfactant templating in solution, the nanocasting pathways developed over the last five years, which use hard templates to create ordered replicas, provide promising routes for the preparation of mesostructured materials with novel framework compositions.[23] The first report by Ryoo’s group on this pathway described the synthesis of mesoporous carbon with an ordered structure, where the replication of the MCM-48 structure led to the formation of a new type of mesoporous carbon material (CMK-1).[24] In the following, we will discuss the recent devel-

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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opments in the field of nanocasting for the creation of porous materials. The basic principles of nanocasting are introduced, and the various replicated porous materials with their different framework compositions, structures, and properties will be described. An interesting possibility is the direct creation of additional functionality during the nanocasting process, and this topic will be addressed before we will, in the last part, attempt to highlight the perspectives and possibilities created by this synthetic approach for the generation of structured materials.

2. The Concept of Nanocasting In a casting process on the macroscopic scale, a rigid mold, made of wax, plaster, metal, or other material, is normally needed.[10] By filling the void of the mold with the material to be cast, or a precursor for it, subsequent optional processing, and final removal of the mold, a replica structure can be obtained, which is the negative replica, if the casting process is done only once. If this procedure is conceptually scaled down to the nanometer scale, “nanocasting” would be the most suitable word to describe this process. Nanocasting is thus the process in which a mold with relevant structures on the length scale of nanometers is filled with another material, and the initial mold is afterwards removed. In order to avoid any misunderstanding, the word “template” is used to describe the concept of a “mold” when the casting process proceeds on the nanometer scale. The structures and properties of the templates play a crucial role with respect to the properties of the replicated porous materials. Generally, two kinds of templates, defined as hard and soft templates, have been described as molds for nanocasting processes. Nanocasting from soft templates was devel-

oped first, and organic precursor species, often polymers, which allow the formation of liquid crystals, can be used as soft templates.[25] For clarity of terminology, one should keep in mind that not all surfactant-assisted synthesis pathways are nanocasting routes, since many syntheses rely on a cooperative assembly between the surfactant and inorganic phase, and they do not replicate a preformed surfactant structure.[5] For instance, in the synthesis of mesoporous silica, the concentration of surfactant can be varied from low (even below the critical micelle concentration) to high (fully developed liquidcrystal phase). The synthetic mechanisms range from cooperative mechanisms to a true liquid-crystal-phase templating (TLCT) mechanism. However, even in the TLCT mechanism, the organic mesophase can be temporarily destroyed, for instance, by the methanol released during the hydrolysis of the silica source, tetramethoxysilane.[26] Consequently, templating via TLCT is often not a real nanocasting process, because the soft templates do not really provide a rigid framework, but rather are nanoreactors. In the confined spaces that are provided by the soft template, the liquid phase structured by the surfactant is solidified by a chemical reaction, for instance a sol–gel reaction or a reductive coupling, thus leading to a mesostructured solid. In contrast, in cases where a hard template is used, the synthesis indeed corresponds to a directcasting mechanism, where a relatively precise negative replica of the template is created.[24] Another advantage of using a hard template is the fact that the syntheses are relatively easy to control, since the template structures are fixed. Soft template structures are often much more flexible, and can be dependent on temperature, solvent, ionic strength, and other parameters, which makes the prediction of the resulting negative replica more difficult. The nanocasting pathway to create nanostructured materials involves three main steps: i) formation of the template;

Ferdi Schüth studied Chemistry and Law at the Westfälische-Whilhelm Universität in Münster, where he received his Ph.D. in Chemistry in 1988 and the State Examination in Law in 1989. After post-doctoral work in the group of L. D. Schmidt at the University of Minnesota, he joined the group of K. Unger at the University of Mainz. After Habilitation in 1995 he became Professor at the Johann-Wolfgang-Goethe Universität Frankfurt. In 1998 he was appointed Director at the Max-Planck-Institut für Kohlenforschung, Mülheim. His research interests include porous solids and high-throughput experimentation in catalysis.

An-Hui Lu studied Chemical Engineering at Taiyuan University of Technology (P.R. China), where he received his B.S. in 1996. In 2001 he received his Ph.D. from the Institute of Coal Chemistry, Chinese Academy of Sciences. After post-doctoral work (as Max-Planck research fellow and Alexander von Humboldt fellow) in the group of F. Schüth at the Max-Planck-Institut für Kohlenforschung, he was promoted to group leader in 2005. His research interests include synthesis and functionalization of nanostructured materials and magnetically separable catalysts, and the use of these materials in heterogeneous catalytic reactions. 1794

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moved by chemical (leaching, combustion) or physical (thermal treatment) methods to obtain the true replicas.

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ii) the casting step with target precursors, including the conversion of the precursor—which is typically molecular—to a solid; and iii) removal of the template, as shown in Figure 1. Inorganic, ordered porous solids are mostly used as the nanoscale hard template in the first step. For instance, zeolites,[27]

3. Nanostructured Porous Materials Created by Nanocasting 3.1. Porous Carbon

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Figure 1. Schematic illustration of the nanocasting pathway.

If one traces back the history of the hard-templated synthesis of porous carbon, Knox et al. were probably the first to synthesize porous carbon via such a pathway, using silica gel or porous glass as templates.[33] However, as the templates had a disordered mesostructure, so had the replicas. The successful synthesis of carbon with an ordered pore structure was first achieved by Ryoo’s group in 1999, whereby MCM-48 was used as a template to create a carbon material (CMK-1).[24] Previous attempts to replicate the narrower pore systems of zeolites had given indications that a replication should be possible, but the materials lacked the perfection of the CMK-1 structure.[34] Starting with the initial publication from the Ryoo group, many studies were carried out to synthesize mesoporous carbons with ordered structures. Table 1 gives a survey over the types of carbon materials so far generated by nanocasting. Generally, the synthetic procedure for ordered mesoporous carbon (OMC) can be described as follows: Mesoporous silica with a specific structure (as a template) is impregnated with a carbon precursor(s) (including monomer and polymer) to give

alumina membranes,[28] ordered mesoporous silica,[24] ordered mesoporous carbon,[10] or, for somewhat larger structure sizes, the assembly structure of colloidal spheres[29] have been employed as a true template to replicate other materials. The pore structures of these parent materials can be transferred to the solid structure of the generated porous materials, while the walls of the parent become the voids of the replica. In order to be able to control the morphology and structural parameters of the replicas, templates with a controllable morphology and structure are highly advantageous. One of the most versatile hard templates is ordered mesoporous silica, since it can be prepared in shapes as diverse as noodlelike, spherical, fibrous, rodlike, and even with chiral morphologies.[30–32] Another requirement for the template is the ability to remove it without affecting the cast. Possibilities are leaching with different agents, melting (although this has not been used yet, to the best of our knowledge), Table 1. Summary of the reported ordered mesoporous carbons (OMCs) generated by nanocasting. or combustion, which is possible with a carbon template. OMC Space group Template Space group Precursor Reference MCM-48 Ia3d sucrose, phenol resin [24] CMK-1 I41/a The target material is usually not or lower (SNU-1) incorporated in the pore system of CMK-2 Unknown SBA-1 Pm3n sucrose [35] the template as such, but in the form cubic of a precursor that subsequently has CMK-3 p6mm SBA-15 p6mm sucrose, [53] to be converted to the final material. CMK-3 p6mm HMS p6mm sucrose, phenol resin, [36–40] This precursor needs to meet several analogue MSU-H furfuryl alcohol SBA-3 requirements: As it must enter the MCM-41 template structure, it must either be CMK-4 Ia3d MCM-48 Ia3d acetylene [35] gaseous, highly soluble, or liquid at CMK-5 p6mm SBA-15 p6mm furfuryl alcohol [45,57,58, moderate conditions, so that it can 61,63,64] be infiltrated into the voids of the NCC-1 p6mm SBA-15 p6mm furfuryl alcohol [67] N-OMC p6mm SBA-15 p6mm acrylonitrile, pyrrole [50,51] template while achieving sufficiently G-CMK-3 p6mm SBA-15 p6mm acenaphthene, benzene, [72–76] high loadings. Conversion to the demesophase pitches, sired composition should be simple pyrrole, and with as little volume shrinkage poly(vinyl chloride) as possible. Finally, it should not OMC (cubic) Ia3d KIT-6, FDU-5 Ia3d sucrose, furfuryl alcohol [41,70] chemically react with the hard OMC (cubic) unknown FDU-12 Fm3m sucrose [42] OMC (cubic) Im3m SBA-16 Im3m sucrose, furfuryl alcohol, [43,44] template. In addition, the templates acenaphthene should be easily and completely re-

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the desired composition. Subsequent polymerization (in some cases, curing or stabilization steps are needed) and carbonization of the precursor in the pore system result in a carbon–silica composite. Finally, a replica mesoporous carbon can be obtained after removing the silica template by leaching. One should keep in mind that carbon precursors should be selected such that they have a high carbon yield and do not simply decompose during the carbonization step. This ensures that the pores of the template remain relatively well filled, and that the final product is really a replica of the silica templates. Suitable carbon precursors were found to be sucrose,[24] furfuryl alcohol,[45] phenol resin,[46] mesophase pitch,[47,48] polydivinylbenzene,[49] acrylonitrile,[50] pyrrole,[51] etc. All available data indicate that the structure of the resulting carbon is indeed determined by the parent template.[52] For instance, CMK-n carbons were templated from different mesoporous silicas, such as CMK-1 (I41/32 or lower) from MCM-48, CMK-2 from SBA-1, and CMK-3 (p6mm) from SBA-15.[53–55] Nanocasting, however, is not only a method for producing novel materials. The finding of a nanocast carboncopy (CMK-3) made with SBA-15 as a template allows conclusions to be made with respect to the structure of the SBA-15: it incidentally proves that the mesopores in SBA-15 are most probably interconnected through the walls via micropores, because otherwise the stability of the carbon replica could not be explained. An unconnected packing of carbon rods that would result from the replication of an unconnected honeycomb pore structure should fall apart, and should not retain its ordered arrangement as soon as the template structure is removed. This interpore connectivity of the mesopores in SBA-15 must be rather well developed, because it is not only the structure on the nanometer scale that is reproduced: even the morphology of the resulting CMK-3 corresponds to the rodlike or noodlelike shapes of the original SBA-15 template.[31] Simultaneous control of pore size and morphology was recently demonstrated for the example of SBA-15 replicated by a chemical vapor deposition (CVD) process with acetonitrile.[56] The resulting carbon was graphitic, the extent of carbonization dependent on the processing conditions, most importantly on the treatment temperature. Interestingly, by varying the filling degree of the carbon precursor in the pore system of a mesoporous silica, the structure of the resulting carbon can be varied. If the pore system of the SBA-15 is only coated by the carbon precursor instead of completely filling it, a surface-templated mesoporous carbon, named CMK-5, with an array of hollow carbon tubes is obtained.[45] A transmission electron microscopy (TEM) image of CMK-5 is presented in Figure 2 a. The removal of the silica template then results in two different types of pores in the CMK-5 matrix. One type of pore is generated in the inner part of the channels that are not filled with carbon precursor. The other type of pore is obtained from the spaces where the silica walls of the SBA-15 template had previously been. Since there are two different mechanisms for pore generation, it should be possible to control the properties of the two pore systems independently. Moreover, due to the fact that the

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Figure 2. a) TEM image of CMK-5 with a tubular structure. b) Schematic depiction of this structure. Reprinted with permission from [45]. Copyright 2001 (Nature Publishing Group).

tubular structure exhibits two—an inner and an outer—surfaces, CMK-5 can reach very high surface areas and large pore volumes, thus making this a potential material for use in adsorption and catalyst-support applications. A first example is described in the initial publication on CMK-5, where it had been used as a support for the anode catalyst in a proton exchange membrane (PEM) fuel cell.[45] To explore the synthesis space of CMK-5-type carbon more comprehensively, several groups have studied its synthesis and have reported their synthetic procedures, such as controlling the polymerization temperature and time,[57–62] introducing the carbon precursor by catalytic CVD,[63] and varying the concentration of furfuryl alcohol.[64] To synthesize bimodal porous carbons, a new method was reported, i.e., a combination of the nanocasting and imprinting strategies.[65,66] In principle, the pore sizes of the resulting carbons can be tuned by choosing different silica colloid particles and mesoporous silica. To create mesoporous carbons with larger pore diameters (> 4 nm), we synthesized NCC-1 carbon, which is essentially similar to CMK-5 and has a bimodal pore size distribution and high pore volume.[67] The crucial factors for the synthesis of such carbons are an aging temperature of 140 °C for the template SBA-15, a relatively low concentration of furfuryl alcohol (25 vol %), and a carbonization temperature higher than 750 °C.[68] In Figure 3, the TEM images shows that two pore systems can be clearly identified for NCC-1. The isotherms of such carbons have a pronounced double hysteresis loop, as seen in Figure 3. This demonstrates the existence of a bimodal pore system, with the step at lower relative pressure corresponding to the pores left by the silica template, and the step at higher pressure to the pores in the inner part of the nanotubes. To synthesize porous carbon with larger pores, Hyeon and co-workers reported the synthesis of mesocellular silica foam with uniform and large mesocells (> 20 nm) by using cellular, foamlike molecular-sieve silica as the template.[69] Structures other than the ones initially used have now been replicated. Large-pore ordered silica, KIT-6, with cubic Ia3d symmetry was synthesized in a triblock-copolymer–butanol mixture.[70] The pore size of this material can be easily tuned from 4 to 12 nm via hydrothermal treatment. Using this silica as a hard template, rodlike (CMK-8) or tubelike (CMK-9)


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size porous carbon with graphitic framework structures (Fig. 4) through in situ conversion of an aromatic compound—acenaphthene—to mesophase pitch inside the silica templates.[72] Later, Pinnavaia’s group also synthesized a

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Figure 3. Nitrogen sorption isotherm (inset pore size distribution; Vad: volume, D: pore diameter, p/p0: partial pressure, STP: standard temperature and pressure) and TEM images of the mesoporous carbon NCC-1. Reprinted from [67].

mesoporous carbons, maintaining the same symmetry (Ia3d) as the parent silica, were synthesized, depending on the carbon precursor. This is in contrast to the carbon synthesized using MCM-48 as the template, where typically the symmetry is reduced upon replication.[35] This maintenance of the original symmetry was the first indication that the walls in this block-polymer-templated silica also have pores connecting the mesopore systems, as had previously been observed for SBA-15. More direct proof was later obtained by the Terasaki group using electron crystallography.[71] In contrast, however, to the case of SBA-15, where the micropores seem to be basically disordered, the micropores connecting the two enantiomeric pore systems in the large-pore Ia3d silica are ordered and are located on special flat points on the G-surface,[71] which separates the two pore systems. The carbon copy was found to be an exact replica of the silica pore system. Other syntheses of cubic mesoporous carbons are compiled in Table 1. The first reported examples of nanocast carbons—and also most of the other nanocast carbon materials described in the literature—have amorphous carbon frameworks. However, a graphitic carbon structure on the atomic scale is highly desirable for some possible applications. For instance, the electronic conductivity is much higher for graphitic carbon than for the amorphous material. Ryoo’s group was the first to synthe-

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Figure 4. TEM images of CMK-3-type graphitic carbon (left) and its photomagnification (right), and the corresponding electron diffraction pattern (inset). Reprinted from [72]. 10 nm

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CMK-3-type carbon with graphitic walls.[73] The obtained pure-carbon replica shows high electronic conductivity compared to normal CMK-3 carbon with amorphous characteristics. Using mesophase pitch as the carbon precursor, Zhao’s group prepared OMC replicas with ordered graphitized structures (2D hexagonal (p6mm) or 3D bicontinuous cubic (Ia3d) structures) via a one-step melting-impregnation method.[74] These carbon types show relatively low surface areas of 350 m2 g–1 and pore volumes of 0.4 cm3 g–1, owing to their graphitized frameworks. Better textural properties (surface areas up to 1560 m2 g–1) were achieved when polypyrrole was used as the carbon precursor, as this allows the creation of graphitic structures.[51] The pyrrole is oxidatively polymerized with FeCl3, the amount of which can be used to control the loading before pyrolysis. However, one should bear in mind that the presence of nitrogen in the precursor will lead to nitrogencontaining carbon materials, and not to pure carbons (see below). Fuertes and Centeno reported the synthesis of mesoporous carbon with improved graphitization using mesoporous silica as the template, pyrrole as the carbon precursor, and FeCl3 as both an oxidant and catalyst for the graphitization of an amorphous carbon.[75] As in other graphitic mesoporous carbons, the electrical conductivity was markedly improved compared to the nongraphitic case (0.14 S cm–1 compared to 0.003 S cm–1). In addition, Fuertes and Alvarez reported that mesoporous carbon with well-developed graphitic order and a surface area of 260 m2 g–1 can be prepared using poly(vinyl chloride) as the carbon precursor. The poly(vinyl chloride) was infiltrated into the pores of mesoporous silica, followed by carbonization, removal of the template, and graphitization of this carbon at 2300 °C.[76] However, even though mesoporosity with pores in the size range of the ordered material was retained, the long-range order had collapsed in samples treated under such harsh conditions. In addition, graphitic porous carbons with a wide variety of textural properties were obtained by using a silica xerogel as


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the template, a phenolic resin as the carbon precursor, and metals (Fe, Ni, or Mn) as the catalyst.[77]

3.1.2. Ordered Mesoporous Carbon with Additional Framework Components OMCs with other frameworks, i.e., nitrogen-containing skeletons, also attract research interest.[50,51] One might find many new properties and applications for such carbons, owing to their special chemical and physical features. OMCs containing nitrogen groups have been synthesized, using mesoporous silica SBA-15 as the template, polyacrylonitrile (PAN) as the carbon source, and 2,2′-azobisisobutyronitrile as the initiator.[50] A series of steps, i.e., stabilization, carbonization, and removal of the silica template, leads to the formation of PAN-based ordered mesoporous carbon. By changing the stabilization and carbonization temperatures, PAN-based OMCs with either monomodal or bimodal pore size distributions can be prepared. In the case of PAN-based OMCs with a bimodal pore size distribution, the connectivity between adjacent pores was improved owing to the randomly distributed, incompletely coated pore walls. By combining the pore connectivity and the surface functionality, such PAN-based OMCs could become promising materials for use as adsorbents and catalysts. Interestingly, as is known for other PAN-based but disordered carbon materials, the nitrogen content and the type of nitrogen species present in the material can be controlled by the treatment temperature.[78] On increasing the treatment temperature, the nitrogen content strongly decreases, and the major species changes from a pyridine-like nitrogen species to pyridinium ions. An alternative approach to nitrogen-containing carbons is provided by using pyrrole vapor as the precursor.[51] Since the polymerization proceeds oxidatively, the loading with polypyrrole can be rationally determined by controlling the amount of pre-impregnated FeIII species, which acts as the oxidant to induce the formation of radical cations (C4NH5+.). A fluorinated carbon with an ordered mesoporous structure was synthesized by reacting OMC obtained from nanocasting with fluorine at room temperature.[79] Strictly speaking, this fluorinated carbon framework was synthesized by post-treatment with fluorine gas, rather than directly from nanocasting; since this is an interesting surface functionality, the material should nevertheless be mentioned here. On increasing the reaction temperature, the color of the fluorinated carbon gradually changed from black to dark brown to gray to white. The mesostructure of the carbon gradually degrades during the treatment, because the fluorine reacts with unsaturated carbon atoms to form sp3-hybridized carbon atoms, which inevitably leads to an increase in bond length as well as the dimension of carbon framework. The fluorinated carbon could have potential applications in electrochemistry or in batteries. Ordered carbons that possess order on longer scales cannot be prepared by casting from surfactant-templated silica, since

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the templates are no longer accessible. If repeat distances of the order of several tens of nanometers in the carbons are desired, then colloidal-crystal templating has to be chosen. Colloidal crystals consisting of silica spheres have thus been used as templates to prepare ordered macroporous carbons with 3D structures and lower degrees of graphitization by CVD at 750–850 °C.[80,81] Highly graphitized, ordered nanoporous carbon (Fig. 5) was achieved by graphitization (2500 °C) of the pitch-based carbon obtained by using silica colloidal crystals as template.[82] This carbon has spherical pores on the borderline between mesopores and macropores, i.e., 40–

Figure 5. TEM images of ordered nanoporous carbon graphitized under argon at 2500 °C. Reprinted with permission from [82]. Copyright 2005 (American Chemical Society).

100 nm, indicating that it retains some of the features of the template. However, the disadvantage of colloidal-crystal templating to date is the fact that a much lower diversity of template structure is accessible, since, at least for extended colloidal crystals, they mostly have densely packed structures.

3.1.3. Monolithic Carbon From the viewpoint of many practical applications, monolithic carbons are easier to handle than powdered materials. The shaping of carbon materials is often rather difficult, owing to the elastic properties of the grains, and, therefore, direct shaping of the materials during their generation can be highly advantageous. In general, carbon monoliths are fabricated by extrusion or, directly, by wet chemistry (sol–gel process).[1,83] However, fine tuning of the pore structure of the carbon monoliths is difficult to achieve using these pathways, and they also have the disadvantage of needing binders and additives in the cases of extrusion, or supercritical drying/freezedrying during the sol–gel process.[84,85] In comparison, the nanocasting pathway from shaped precursors provides an opportunity to create monolithic carbons with an ordered or hierarchical structure, and the pore sizes are tailorable to some extent, if the integrity of the template can be maintained in the cast carbon.[86–88] Monolithic mesoporous carbon with a bicontinuous cubic structure (Ia3d symmetry) was prepared by using mesoporous silica monoliths as the hard template (Fig. 6).[85] This monolithic carbon shows a uniform pore size of 4.6 nm and a surface area of 1530 m2 g–1, and was described as a promising


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Figure 6. Photographs of monolithic mesoporous silica (left, 2.1 cm diameter; 0.3 cm thick) and mesoporous carbon (right, 1.7 cm diameter; 0.2 cm thick). Reprinted with permission from [85]. Copyright 2002 (Royal Society of Chemistry).

electrode for electrochemical double-layer capacitors. The difficulty in preparing such carbon monoliths is, in fact, the preparation of the noncracked silica monoliths needed as templates. Owing to the stresses involved in the preparation, cracking of the structures can occur at different stages, the most sensitive ones being the drying stage, due to capillary forces, and the calcination stage, due to the temperature-induced stress and/or pressure developing inside the monoliths during the decomposition and combustion of the organic material in the pore system. Thus, the preparation of monolithic mesoporous carbon with an ordered mesostructure is still a great challenge. Related to the synthesis proposed by Nakanishi,[89] silica monoliths with a hierarchical structure containing macropores and mesopores can be prepared by adding poly(ethylene glycol) and/or hexadecyltrimethylammonium bromide as a porogen.[90] Using such silica monoliths as templates and furfuryl alcohol or sucrose as a carbon precursor, carbon monoliths with well-developed porosity are accessible.[88–93] Interestingly, the pore system of the nanocast carbon monoliths can be varied to three- or four-modal porosity by varying the loading amount of furfuryl alcohol in the one-step impregnation. Regardless of the loading with the carbon precursor, the obtained carbon monolith is a positive replica of the silica monolith on the micrometer scale, and a negative replica on the nanometer scale, as shown in Figure 7. Combined volume and surface templating, together with the controlled synthesis of the starting silica monoliths used as the scaffold, provides a flexible means of pore-size control on several length scales simultaneously.[87,88,91]

3.2. Metal Oxides, Metals, and Other Inorganic Materials 3.2.1. Ordered Silica as the Template As discussed in the Introduction, using a surfactant as a template to synthesize metal oxides often leads to the loss of the ordered structure after removal of the template. Using mesoporous silica as a hard template to create an ordered metal oxide is an alternative method that helps to circumvent the problems in templating with surfactants. The synthesis of

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Figure 7. Scanning electron microscopy (SEM) images (upper), photograph (lower, left), and TEM image (lower, right) of silica and carbon monoliths.

a wide variety of materials by this technique is a very interesting topic, both for fundamental research, concerning the basics of a replication on the nanometer length scale, and for the production of materials with desired properties. The requirements for such a casting process to be successful have already been addressed in the Introduction. The surface functionality of the silica template seems to be crucial for obtaining a high-quality replica structure. This is probably due to the wetting behavior of the parent material, since only if the precursor—and also the primary products of the precursor conversion towards the final materials—has a favorable interaction with the wall of the template will a fully coherent material be formed. After all, one should bear in mind that a high interfacial area is created in the nanocasting process. Even if the interfacial energies are only of the order of some hundred joules per square meter, then energy contributions of some tens of kilojoules per mole will result, if the interface area is 1000 m2 g–1 and the molar mass is above 100 g mol–1, as is frequently found for oxides. Thus, in many cases, unmodified silica was found to be unsuitable for the replication of certain materials. For instance, for the production of porous chromium oxide single crystals it was necessary to use aminopropyltriethoxysilane-functionalized SBA-15 as the template and H2Cr2O7 as the chromium precursor, which was chemically adsorbed. Subsequent thermal treatment and template removal by an aqueous hydrofluoric acid (HF) solution led to the desired product.[94] In this synthesis, calcination temperatures exceeding 350 °C were needed in order to create the crystalline phase. This material is believed to have potential as a catalyst with high activity, owing to its relatively large surface area (58 m2 g–1) and possible shape-selective properties. Zhao and co-workers have demonstrated that microwave-digested 3D


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mesoporous silica can be used as a hard template to fabricate various ordered crystalline gyroidal metal oxides, such as In2O3, Co3O4, Mn2O3, and CeO2.[95–97] The microwave digestion treatment probably leads to an enhanced number of silanols, which provides the surface functionality needed for the incorporation of the precursor species. A representative TEM image of cubic In2O3 is shown in Figure 8. These ordered metal oxides have large surface areas of 50–140 m2 g–1 and exhibit single crystallinity in larger domains. Recently in our group,

nanocasting process using mesoporous SBA-15 as a hard template.[100] The incorporation of metal and sulfur precursors can be carried out by one-step impregnation. These nanowires are polycrystalline and are discussed due to their potential application in optical and electronic devices. However, the use of silica as the hard template is limited, because it can only be leached under strongly alkaline conditions or by HF. However, such a leaching process is not compatible with many different oxides, since they are also attacked by these reagents. Carbon templates provide an interesting alternative to silica, since carbon can be removed by combustion or treatment with other highly reactive gases.

3.2.2. Carbon as the Template

Figure 8. TEM images of the cubic Ia3d mesostructured In2O3 framework along the a) [100], b) [111], and c) [311] directions. d) TEM image of the cubic (possibly I432) mesostructured In2O3 framework along the [111] direction. Reprinted with permission from [95]. Copyright 2003 (American Chemical Society).

nanocast ordered mesoporous Co3O4 with the spinel structure was synthesized, wherein vinyl-functionalized large-pore Ia3d silica was used as the template and Co(NO3)2 was the cobalt precursor.[98] Although replication of neat silica was meanwhile also found to be possible, the vinyl-functionalized precursor species provided an easier processing method to the cast cobalt oxide. It is believed that the vinyl groups in the silica play a role in bonding the Co2+ ions in the pores. This material is antiferromagnetic, shows a weak ferromagnetic transition at low temperatures, and a negative exchange bias that is found in ferromagnetic/antiferromagnetic coupled systems. This process seems to be extendable to other metal oxides, and has recently been used to produce ordered mesoporous ferromagnetic CoFe2O4. It is also possible to synthesize the hexagonally ordered Co3O4 by casting from SBA-15 (the product also having interesting magnetic behavior).[99] In addition, metal sulfide nanowires, such as CdS, ZnS, and In2S3, with ordered mesostructure were synthesized by a

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Porous carbons can be used as templates to generate highsurface-area inorganic solids, owing to their special properties, such as high porosity, high thermal stability, and easy removal by combustion.[10,101] In principle, an ordered mesoporous carbon, such as CMK-3, could be used as a template to construct other compositions with ordered mesoporous structures. The realization of this idea was reported almost simultaneously by two independent groups.[102,103] Tetraethoxysilane (a silicon source) can be conveniently infiltrated into the pore system of CMK-3-type carbon, and hydrolysis can be initiated by treatment with a solution of HCl. A repeated impregnation procedure is necessary in order to achieve the desired loading. After inducing silanol condensation to an as-complete-as-possible extent by thermal treatment at 700 °C under a nitrogen flow, the composites are calcined at 550 °C in air, which removes the carbon, producing a white powder, designated as NCS-1. Figure 9 shows the nanocasting procedure and the corresponding TEM images of the template and final product. Combining the X-ray diffraction (XRD) and nitrogen sorption analyses, it can be verified that NCS-1 does replicate the ordered structure of CMK-3 and exhibits strong structural similarities to SBA-15, even if there are differences in detail.[104] Another example is the synthesis of a new silica mesostructure, HUM-1, by nanocasting from ordered mesoporous carbon that had been obtained via replication of the MCM-48 silica. The structure of the final silica had a different symmetry than the parent silica.[105,106] While this multistep route is inefficient for the production of mesoporous silicates because they are more easily accessible by the cooperative pathway, it is a viable method for the synthesis of other compositions that are more difficult to template directly with surfactants. Roggenbuck and Tiemann have demonstrated the flexibility of this type of carbon templating for the production of magnesium oxide. Using CMK-3 carbon as a template, obtained via a nanocasting pathway, they synthesized mesoporous magnesium oxide with hexagonal p6mm symmetry by repeated nanocasting from the carbon.[107] One should note that the synthesis of basic oxides, such as MgO, is difficult following the solution-mediated pathway using surfactants, since the solubilities of these more basic oxides are typically rather


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template.[112] Single-component (ZrO2, TiO2, and Al2O3) and multicomponent mesoporous oxides (TiSiO3Oy and Ti2ZrOy), and metal phosphates 1 2 (ZrP and AlP) can be prepared with surface areas ranging from 100 to 400 m2 g–1. Another approach to preparing inorganic materials with mesoporosity following a nanocasting pathway is the use of carbon aerogels as templates. Although their porosity CMK-3 SBA-15 NCS-1 is not ordered, they can be synthesized with adjustable pore size distributions that are rather narrow compared to many other materials. Such aerogels have been employed to prepare porous MgO,[113] glassy Al2O3,[114] and monolithic zeolite sam100 nm 100 nm 100 nm ples[115] via the nanocasting process. The successful synthesis of inorganic porous materials verifies that Figure 9. Illustration of the nanocasting procedure for NCS-1 and the corresponding carbon aerogel is an alternative template. Since the TEM images of SBA-15, CMK-3, and NCS-1. Reprinted from [104]. pore structure of carbon aerogel can be varied to some extent during the synthesis, it is expected that inorganic porous materials with many other compositions will high under the synthetic conditions used. Subsequently, using be prepared in the near future. tri(methylamino)borazine as the boron nitride source and CMK-3 as the template, ordered mesoporous boron nitride with a specific surface area of 500 m2 g–1 was synthesized, 3.2.3. Colloidal Crystals as the Template where the carbon template was removed by a high-temperature ammonia treatment.[108] We have attempted to use CMKMacroporous solids have potential applications as optical 3 as a template to synthesize mesoporous alumina. However, crystals, catalysts, supports, sensors, and porous electrodes or such alumina does not maintain the ordered structure, as is electrolytes. The synthesis of ordered macroporous colloidal the case in NCS-1, even though the resulting alumina is mesocrystals via the replication of ordered array structures of polyporous.[109] This is mainly due to the complicated phase transistyrene or silica lattices has received much attention in phystion of alumina during the calcination process. Recently, ics, chemistry, and materials science. Covering the progress porous metal oxides, such as Al2O3, TiO2, ZrO2, V2O5, etc., achieved in this field would exceed by far the scope of this with high thermal stabilities as well as crystalline frameworks review; however, the processes used are in fact nanocasting were prepared via the above nanocasting technique.[110] Howsteps. More details can be found in the reviews by Velev and ever, characterization of the materials showed that, although Lenhoff[116] and Stein.[117] some of the structural features of the parent templates could Here we only wish to highlight the principle that such synbe transferred to the casts, the degree of structural order was theses can also be considered and generalized as nanocasting appreciably lower than in the parent materials. processes. As seen from Figure 10, colloidal crystals are first A catalytically quite interesting material is ZnO, since it is a formed by packing uniform spheres into 3D or 2D arrays. major component in catalysts for methanol synthesis and Then the interstitial space of the colloid crystals is filled with methanol steam reforming. Direct synthesis of mesostrucliquid precursor that is subsequently converted into a solid tured ZnO by solution techniques is very difficult, and so far skeleton. Removal of the spheres leads to the generation of a only thin films have been obtained. Synthesis of ZnO by the solid skeleton in the location of the former interstitial spaces nanocasting technique from carbon is not straightforward and interconnected voids where the spheres were originally either, since ZnO is relatively easily to reduce, and the carbon located. Periodic porous solids with various compositions, inin contact with ZnO at high temperatures could lead to intercluding silicates and organosilicates, metal oxides, metals, mediate reduction. Recently, Polarz et al. succeeded in metal chalcogenides, and carbon allotropes, have been presynthesizing an ordered mesoporous ZnO by replication from pared by nanocasting from colloidal crystals. The resulting a PAN-based CMK-3-type material. TEM analysis revealed materials could be interesting for various applications in phothe ordered structure, and surface areas came close to tonics.[116,117] 200 m2 g–1.[111] A substantial fraction of the publications on ordered mesoporous materials is devoted to the control of morphology, i.e., 3.3. Functionalized Porous Solids by Nanocasting the synthesis of films, spheres, fibers, etc. This interest has also led to similar work following nanocasting strategies. MonodisAs discussed above, many new materials have been created perse mesoporous inorganic spheres have been synthesized by by the nanocasting strategy. Removal of the template leads to the nanocasting route by using mesoporous carbon spheres, the negative replica of the primary template. However, the which were nanocast from mesoporous silica spheres, as the SBA-15

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Figure 10. General synthesis scheme for ordered macroporous solids and the corresponding SEM images for a polystyrene/silica system prepared with tetraethylorthosilicate. Reprinted with permission from [117]. Copyright 2001 (Elsevier).

nanocasting pathway can be modified to also provide the possibility for functionalization of the inner or outer surface of porous materials in a controlled manner. If the pores of such a material can be filled reversibly, then one has the equivalent of the protection-group strategy used in organic synthesis for the functionalization of porous solids. A pore could thus be filled with a blocking agent, then the material could be modified in the unblocked part, and finally the blocking agent could be removed to make the unmodified part of the surface accessible again. Ryoo’s group has confirmed that the surface of ordered nanoporous carbon (CMK-3) can be nanocast by an organic polymer, for example polystyrene.[118] The resultant materials, exhibiting surface properties of the polymers, as well as the electrical conductivity of the carbon framework, could provide new possibilities for advanced applications. Such a strategy can furthermore be extended to other inorganic templates, such as mesoporous silicas.[119] Most of the as-made porous carbon materials have particle sizes in the sub-micrometer range. These carbons are notoriously difficult to separate from solution, and thus magnetic separation is an attractive alternative to filtration or centrifugation and has therefore been high on the wish list in catalysis for a long time.[120] We have established a series of consecutive manipulation steps to fabricate magnetically separable ordered mesoporous carbons, on which magnetic nanoparticles were selectively deposited on the outer surface of the carbons—the pore system was left blocked during the modification, and the nanoparticles were protected by a nanometerthick carbon layer.[121] The overall synthetic strategy and typical TEM images of such a magnetically separable carbon are presented in Figures 11 and 12, respectively. Such magnetic nanocomposites have very high surface areas, large pore volumes, and uniform pore sizes. Applications of this porous carbon as magnetically separable adsorbents and catalysts have been demonstrated. However, one may also envisage other applications; for example, as a magnetically directable drug carrier. If a drug could be loaded onto the porous carbon, one

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could possibly accumulate the magnetic particles in the target area in the organism, and then induce release of the drug by magnetic heating (in an alternating magnetic field) of the particles.[122] To deposit magnetic nanoparticles spatially on the outer surface of mesoporous silica while simultaneously keeping the

a

(1)

b

(2)

c

(3)

d

(4)

e

f (5)

Figure 11. Illustration of the synthesis procedure of magnetically separable carbon: a) ordered mesoporous silica SBA-15; b) carbon/SBA-15 composite; c) (b) with surface-deposited cobalt nanoparticles; d) protected cobalt nanoparticles on (c); e) magnetically ordered mesoporous carbon; f) Pd on (e). 1) Carbonization of the carbon precursor in SBA15; 2) incorporation of cobalt nanoparticles on (b); 3) coating of carbon on cobalt nanoparticles; 4) dissolution of silica to create pore system; 5) loading of Pd in pores to introduce catalytic function. Reprinted from [121].

Figure 12. TEM images of magnetically separable carbon at a) low and b) high magnifications. Reprinted from [121].


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4. Conclusion and Outlook Nanostructured porous materials created by the nanocasting strategy, especially using hard templates, can be produced as monoliths or powders with ordered or disordered structures, which mainly depend on the structures of the primary templates. The range of the templates applied in nanocasting already extends from silica to carbon, the latter being easily removed by simple combustion. Using the nanocasting strat-

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egy, one can create negative—or, after repeated nanocasting, positive—replicas that can preserve the fine structural details of the template. It has been demonstrated in principle that this technique allows the generation of porous materials on the nanometer scale with partly variable textural parameters. The systematic exploration of the nanocasting pathway will add a new dimension to the fabrication of many porous inorganic materials by using different hard templates. In particular, in the synthesis of multicomponent metal oxides, sulfides, or alloys, the nanocasting strategy is probably the most promising approach—as compared to the commonly employed surfactant-assisted strategies—for the synthesis of ordered porous solids. Many compositions will not be accessible in mesostructured form by techniques other than nanocasting. In addition, the mostly expensive alkoxides used in solutionbased processes can be replaced by metal nitrates, chlorides, sulfides, or acetates as raw materials, thus also avoiding the difficulties in the control of the hydrolysis rate of the alkoxides. However, if ordered mesoporous carbon is used as the template, one still needs to synthesize the carbon template in the first synthetic step, which is a major drawback. There are still a number of remaining challenges: For many target compositions, the chemistry of the target material is not compatible with the conditions of the template-removal process, be it leaching or combustion. Increasing the loading of the template with as much precursor as possible is also a challenge. This is necessary to ensure a rigid structure, thus avoiding collapse of the pore system after removal of the mold. Basically, a very high concentration of the precursor solution, or, if at all possible, the neat precursor, is highly recommended. Although the replication of non-silica materials does not seem to be as straightforward as for silica, it can be foreseen that these obstacles will be overcome in the coming years by continued research effort in this rapidly developing field. It is almost certain that the nanocasting pathway will be extended to many other compositions that are not accessible by solutionbased methods, and that this technique will become a standard tool in the tool box for the synthesis of ordered porous materials.

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pore system open, we have developed and used the strategy of reversible polymer protection of the silica pore system, which was briefly mentioned in the introductory paragraph to this section.[123] This also creates the possibility of functionalizing the inner surface of the mesoporous silica. CMK-5 has a bimodal pore system, the pores of which are generated at different stages, i.e., during the carbonization and silica-removal steps. This provides the possibility for independently modifying the two pore systems in order to create a material with specific physical and chemical properties. Recently, we have chemically modified the surfaces of the inner and outer sides of the CMK-5 tubes selectively by nitric acid oxidation under moderate conditions followed by esterification or alkylation. The results will be reported in detail in the near future. An alternative possibility to keep the pore system open is the modification of the pore walls by introducing foreign atoms, molecules, or nanoclusters. This strategy has been widely used for the synthesis of organosilica materials. We have synthesized, by the nanocasting pathway, Pd supported on ordered mesoporous carbon, as revealed by EDX analysis, where highly temperature-stable, molecular-level dispersed Pd clusters (below the detection limit of approximately 1 nm) are uniformly embedded in the carbon walls.[124] Although this material was pyrolyzed at temperatures up to 750 °C, no visible Pd clusters were formed in the carbon walls, as revealed by high-resolution TEM. The catalytic activity of Pd-OMC was tested in the oxidation of an alcohol (to an aldehyde) using supercritical CO2 as the reaction medium. Alcohols, including benzyl alcohol, 1-phenylethanol, and cinnamyl alcohol, were used as the substrates. Under the conditions used, the selectivity to the corresponding aldehyde was in all cases higher than 99 %. No acid was detected as a reaction product, indicating that the catalyst is highly selective for the conversion of the alcohols to the corresponding aldehydes. Another development in the field of nanocast silica is the homogeneous incorporation of metallic nanoparticles (Pd, Pt, Ru) into the solid-silica skeleton by nanocasting from a 3D array of polystyrene spheres. Cyclodextrin as an additive in this process plays two roles: inclusion of the metallic nanoparticles and homogeneous dispersion in the silica sol through hydrogen bonding.[125] The resulting silica material shows a bimodal pore structure, which might be interesting for catalysis because it allows fast mass transfer owing to the larger pores, while also maintaining the smaller pore system.

Received: January 23, 2006

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