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Mixed metal oxide aerogels from tailor-made precursors
Accepted Manuscript Title: Mixed Metal Oxide Aerogels from Tailor-Made Precursors Author: Andrea Feinle Nicola H¨using PII: DOI: Reference:
S0896-8446(15)30068-1 http://dx.doi.org/doi:10.1016/j.supflu.2015.07.015 SUPFLU 3391
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
14-2-2015 1-7-2015 14-7-2015
Please cite this article as: A. Feinle, Mixed Metal Oxide Aerogels from Tailor-Made Precursors, The Journal of Supercritical Fluids (2015), http://dx.doi.org/10.1016/j.supflu.2015.07.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract (for review)
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*Highlights (for review)
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Overview over synthetic strategies towards mixed oxide aerogels is given. Nanoparticle assemblies are discussed Sol-gel processes are summarized Prehydrolysis approaches versus complexation to deliberately tailor the reaction rate Application of single-source precursors in the synthesis of mixed oxide aerogels
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*Manuscript Click here to view linked References
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Mixed Metal Oxide Aerogels from Tailor-Made Precursors Andrea Feinle1, Nicola Hüsing*1 1
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Materials Chemistry, Paris Lodron University Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria. [email protected]; [email protected] Tel: 0043 662 8044 5404
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Abstract Highly porous mixed oxides and metal-doped materials are relevant for many applications including filters, sorption media or photocatalysts, in medical, or electrochemical and optical applications, etc. Mixed oxides are of special interest for several reasons: the chemical properties of an otherwise “inert” support can be improved by the second component, i.e. comprising characteristics, such as higher acidity, larger surface areas, thermal stability, etc. However, the properties of the final material strongly depend on the chemical homogeneity or degree of demixing of the various components. The development of general, cost-effective routes allowing for the synthesis of such mixed metal oxides with deliberately designed structural features on all length scales (from the atomic level to the macroscopic morphology) is still a demanding task. In this paper, we will provide an overview over different solution-based chemical approaches, e.g. sol-gel processes, to highly porous (monolithic) mixed metal oxide aerogels. Keywords: mixed metal oxide, aerogel, sol-gel processing, single source precursor
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1. Introduction Aerogels are solid materials that are characterized by extremely low densities combined with high porosities, and large inner surface areas [1]. One intriguing feature of aerogels is that all constituting elements are in the nanoscale regime, providing a unique combination of properties that make the materials attractive for a variety of applications, such as catalysis, adsorbents, drug delivery, thermoresistors, piezoelectrics, or sensors. Although the first aerogel was already prepared in 1931 by Kistler [2], the development and application-oriented research of aerogels only started in the late 1960s, concomitant with the rapid development of the sol-gel process. Since then, many different chemical compositions, including purely inorganic gels, e.g., metal oxides and sulfides (tellurides and selenides), carbides, metal-doped or mixed metal oxides, as well as inorganic−organic hybrid gels, organic polymeric gels, including resorcinol/formaldehyde, polyurethane, polyamide, conducting polymers (e.g. polythiophenes) and many more, graphene and carbon aerogels, have been prepared. There are some excellent review articles summarizing the recent progress in aerogel science [1, 3−7]. In this review, we will focus on aerogels that are composed of two or more different metal/semimetal oxide centers. These binary of multinary oxides (e.g. ZrO2/SiO2, TiO2/SiO2, TiO2/ZrO2, Al2O3/SiO2, CoTiO2/TiO2, etc.) are of significant technological importance, however, the prop-
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erties strongly depend on parameters such as 1) the MaOx/MbOy ratio, 2) the dispersion of one component in the other and with that the chemical homogeneity of the network, 3) the accessibility of the different metal/semimetal centers as well as 4) the textural properties of the material, all of which are governed not only by the synthetic approach, but also the drying procedures [8-14]. The rapid development of sol-gel processes during the past decades has led major steps forward in the deliberate synthesis of porous, mixed metal oxide networks. Sol-gel procedures complement the broad range of other conventional methods used for the preparation of mixed metal oxide nanostructures, such as ceramic synthesis, flame hydrolysis, ion exchange on supported oxides, precipitation or impregnation methods, typically followed by high temperature treatment [15-24]. Most of these procedures, are however not suited for the production of highly porous aerogel monoliths. The advantages of sol-gel processing are that (i) the hydrolysis and condensation reactions already proceed at room temperature and (ii) by the use of liquid precursor molecules molecular mixing of the different metal/semimetal centers is in principle possible, (iii) particle sizes can be easily controlled by the manifold processing variables, as well as (iv) the application of templates allows for a deliberate tailoring of the porous network architecture. Developing and designing materials from a molecular/ atomic level requires the ability to deliberately control the Page 3 of 18
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2. Chemical Homogeneity What exactly is meant with chemical homogeneity: Looking at the atomic level, the he degree of homogeneity is commonly associated with the relative amount of Si-O-Ti Si linkages in the mixed oxide. Depending on the reaction condicond tions, different scenarios have to be distinguished (see Figure 1, top and middle). ). Titanium can be dispersed in the silica framework on an atomic level (isomorphic substitusubstit tion), resulting in tetrahedrally coordinated metal centers as it is found for the zeolitic system titanosilicalite 1 (TS-1) (TS [25]. This is not the regular case, since one has to consider that for transition metals the coordination oordination number (c.n.) can be higher than the valency, resulting in a coordination exe pansion, i.e. for titanium the valency is typically four but up to six ligand are octahedrally coordinated to the metal center as present in the crystalline titanium dioxide oxide phases, anatase or rutile (Figure 1, top right) [13].
If larger segregated titania domains are present, those can either be homogeneously distributed in the silica matrix (Figure 1A, bottom) or demixed domain structures can be formed to different extent (Figure 1B B and 1C, bottom). The size of these domains strongly depends on the reaction and drying conditions and can vary from a few nanometers up to micrometer-sized domains. For these phase separated systems one also has to consider that the particles can eeither her be crystalline or amorphous, depending on the synthe synthetic approach. Unfortunately, in many cases, no or only very limited information on the chemical homogeneity, neither on the atomic nor on the particulate level is given. Starting from solution as in sol-gel gel processes, the key to design the desired dispersion and mixing of the different metal centers lies in an understanding of the chemical rea reactivity and the structural chemistry of the precursors. Typ Typical precursors for sol-gel gel processes are metal alk alkoxides, e.g. Ti(OR)4 for titania, or Si(OR)4 for silica. A major challenge in sol-gel processes es in the presence of water is the different reaction rate for the hydrolysis reaction. This reaction typically proceeds via a nucleophilic substitution mechanisms and therefore depends on the partial charge, δ+, on the metal atom (Si or Ti). The titanium atom, e.g. Ti4+ in Ti(OiPr)4, is a strong Lewis acid with a significant positive partial charge imparting a much higher reactivity towards hydrolysis than the Si atoms in tetraalkoxysilanes (Si4+) [26]. This can easily be observed by the vigorous reaction of titanium alkoxides with water resulting in ill illdefined titanium-oxo/hydroxo oxo/hydroxo compounds.
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positioning of the building blocks within a material. This is also an essential requirement in the synthesis of porous materials, such as aerogels, in which the arrangement of different building blocks (particles) forming the solid framework determines the pores’ size, shape and arrangearrang ment. In the following a strong focus will be given to SiO2/TiO2 mixed oxide aerogels as a model system, system however, the underlying concepts are very general and can easily be transferred to other mixed oxide and even chalcochalc genide systems.
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Figure 1: Isolated metal species in tetrahedral (top, left) or octahedral (top, right) coordination;; atomic dispersion for mixed metal oxides in tetrahedral (c.n. = 4) or octahedral (c.n.. = 6) geometry for titanium centers by Si-O-Ti bridges (middle) and segregated titania domains either homohom geneously dispersed in the silica matrix (A) or phase separated to different extents (B,C); (note that in the bottom images A-C, C, the coloured spheres represent silica or titania particles not atoms).
In 2001, Gash et al. developed a new sol-gel gel route based on metal ion salts in the presence of epoxides as gelation agents to synthesize different transition metal and main group metal oxide aerogels [27]. The versatility of this procedure has been shown with the synthesis of a large variety of metal/Si mixed oxide aerogels [28]. The choice of epoxide proved to give a good control over composition regarding the M/Si ratio,, even allowing for the synthesis of gels with the metal oxide – not silica – as the major co component. Due to the compositional generality of the method, the obtained nanocomposites are built-up up from a variety of metal oxides with several oxidation states. For these gels not only the structure, but also the dispersion of the diffe different elements in the gels has as been investigated by using electron energy-loss loss spectroscopy (EELS) in tandem with TEM. The authors have shown that the different elements are uniformly dispersed throughout the materials and no large silica domains appear to be present. Recent progresss in the synthesis of mixed oxides applying this epoxide-based approach,, e.g. for ZrO2/SiO2 mixtures, starting from low-cost cost commercially available precursors, such as water glass or metal salts, has been en shown by Ratke
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provides local coordinationn symmetry and oxidation states or EXAFS providing ding more detailed information, such as MO bond length or the coordination sphere of the cations. These methods are complementary to other powerful spectroscopic methods, such as Raman or infrared vibrational spectroscopy,, solid state NMR spectroscopy or UV-vis (diffuse reflection, DR) spectroscopy. Recently, signifi significant advances have been made in analyzing the structure of disordered or partially ordered materials using atomic pair distribution function (PDF) analysis of powder diffraction data. The possibilities bilities of this approach have been shown for chalcogels and silica/titania mixed oxide aerogels.[34,35]
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et al.. However, no information on the chemical homogenehomogen ity of the mixed oxides is given [29]. As seen above, porous mixed metal oxide materials are easily accessible from a synthesis point of view. view However, a detailed characterization, especially with a focus on the molecular/ atomic level chemical homogeneity of the material remains notoriously difficult. To get a complete picture of the structure, morphology, composition and a chemical homogeneity typically a combination of different analytical techniques has to be applied (see Figure 2). 2)
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Approaches to control chemical homogeneity If different alkoxides, e.g. silicon and metal alkoxides, are processed separately and the sols are combined at a later stage in the synthesis, heterogeneous systems comprising SiOx(OH)y(OR)z species as well as the corresponding metal oxide particles are obtained after drying as schematically depicted in Figure 3.. Depending on the dimensions of the sol particles the homogeneity of the final material can be adjusted. However, the amount of Si-O-Ti Ti bridges will be very low.
Figure 2: Schematic overview of some typical methodologies available for the characterization of mixed metal oxide aerogels
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Regarding structural features, such as microstructure, morphology, specific surface areas, pore sizes and pore size distributions, etc., standard methods including X-ray X scattering or diffraction, electron microscopy, gas sorption or porosimetry have been used. However, one has to keep in mind that due to the amorphous or nanocrystalline character of aerogels, the obtained values often show rather broad distributions that by no means can be compared with the precise information that is given for analogous crystalline samples. Several excellent review articles summarize the recent advances in the characterization of mixed oxide materials, e.g. by X-ray ray absorption spectroscopy (XAS), Raman or infrared nfrared spectroscopy, solid state NMR spectroscopy, UV-vis vis diffuse reflectance spectroscopy, HR TEM, etc.[30-33] While it is straighforward to analyze the chemical composition of a material, chemical hemical homogeneity remains a materials als characteristic that is difficult to address due to the lack of spatially resolved analytical techniques. Most methods provide information on a rather large sample area; thus the atomic level information is always an average over many atoms/molecules. Typical ypical methods that give information on the homogeneity or heterogeneity of the porous material on very short length scales are X-ray absorption spectroscopy tools, such as XANES that
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Figure 3: SiO2 and metal oxide sols prepared separately resulting in heterogeneous solids
A wide variety of aerogels prepared from preformed nan nanoparticles has already been synthesized, including PbS, ZnS, or BaTiO3 and Au, Ag, Pt [36-38]. The feasibility of this process in the production of binary and multinary aerogels has recently been shown by the group of Niederberger et al. by reacting amorphous silica and crystalline titania nan nanoparticles to yield mixed oxide aerogels [339]. This approach
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Given this rather simple sol-gel approaach, phase homogeneity or the dispersion of the metal ions on an atomic scale can be improved by the following measures as also depicted in Figure 5.. These three approaches will be di discussed in more detail in the following sections, starting with thee prehydrolysis of the slower reacting component, followed by the complexation reaction of the faster reacting alkoxide and the application of stable single single-source precursors. All synthetic pathways have been used in the syn synthesis of mixed oxide aerogels and selected examples are presented below. It would be beyond the scope of this co contribution to give a comprehensive review, therefore, the choice of examples is somewhat arbitrary.
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What happens, if the metal alkoxides are just mixed togethtoget er? Even being a very straightforward approach, it is very probable that rather inhomogeneous materials erials with a low a b number of heterocondensed M -O-M bridges will result due to the above mentioned differences in the hydrolysis reaction rates (Figure 4).
the material, the system evolves into a more thermodynamically stable state with formation ation of Ti and Si rich domains [41].
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not only offers to produce gels from amorphous and cryscry talline nanoparticles simultaneously, but also gives gels with a unique combination of properties, e.g. photocatalytic behavior by anatase particles combined with the rich sursu face chemistry of silica. In another modular approach, they processed gold and anatase nanoparticles via controlled coco gelation in the presence of preformed tungsten oxide nan nowires to obtain macroscopic aerogel monoliths with high specific surface areas of 473 m2g-1, transparency ncy and excelexce lent crystallinity [40]. Collapse of the filigree pore system is in all cases prevented by supercritical drying with carbon dioxide. Even if the chemical homogeneity cannot be ada justed on the atomic level, this co-gelation gelation approach opens fascinating possibilities for the creation of aerogels with unprecedented architectural complexity.
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Figure 4: Simultaneous processing of silicon and metal alkoxides.
However, not only the reaction rates will determine the final dispersion and structure, but also solubility limits for the various mixtures have to be considered, in addition to the hetero- (Ma-O-Mb) versus homo-condensation condensation (Ma-Oa b b M , M -O-M ) rates. Interestingly, previous studies dies by solid 17 29 state O and Si MAS NMR spectroscopy have ha shown that by reacting Si-OH groups with M-OR OR groups the forfo mation of Si-O-M bonds (with M = Ti or Zr) seems kinetikinet cally favored, so that the addition of a titanium alkoxide to a sol containing silanol species will immediately lead to a significant formation of such bonds. However, upon ageing
Figure 5: Different synthetic strategies to increase the homogeneity of the final materials.
Prehydrolysis of the slower reacting alkoxide precursor as proposed by Yoldas;[42] Inn the case of silica/titania mixed oxides that would e.g. be the reaction of tetraalkoxysilanes with aqueous acids. To name just a few examples:: Aravind and coworkers synth synthesized monolithic silica-titania aerogels with prehydrolysis of tetraethoxysilane with acidified water (HCl) with a surface area up to 685 m2 g−1 and modified th the surface of the material with trimethylchlorosilane for ambient pressure drying.[43] Miller et al. reported studies in which they compared TiO2/SiO2 aerogels that were prepared either with prehydrolysis or without prehydrolysis of the silica precursor molecules.[44, 45] Here, independent of the Ti/Si ratio, the prehydrolyzed samples exhibit exhibited higher pore volumes and higher specific surface areas than the non nonprehydrolyzed samples, whereas the pore size distributions
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networks, the mixed oxide walls are not strong enough to withstand the high pressures and temperatures during the supercritical drying. The TiO2/SiO2 aerogel prepared by the ethanol supercritical drying contains hexagonally arranged microdomains embedded within an amorphous phase. Furthermore, the mixed oxide aerogels contain anatase domains of approximately 9 nm in size.
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Modification of the transition metal alkoxide with chelating ligands; Modification of the transition metal alkoxide, here exemplary shown for Ti(OR)4 (eqn. 1 and Figure 6), can be done with organic groups, e.g. multi- or bidentate complexing ligands (L), such as organic acids, diketones (L-H) or other anionic ligands with neutral donor groups X (e.g., -NH2) [48]. M(OR)x-yLy + y ROH
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M(OR)x + y L-H
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remained mostly unaffected. Furthermore, they described that for both approaches the Ti/Si ratio has a strong influence on the crystallization temperature: the lower the Ti/Si ratio, the higher the crystallization temperature of TiO2 to anatase or rutile. This is also reflected in the significant difference between the two prepared samples in the catalytic activity in 1-butene isomerization. Both mixed oxide samples were indeed catalytically more active than pure titania, but in comparison to the non-hydrolyzed counterparts, the prehydrolyzed samples showed again a threetimes higher activity. Further differences, indicating structural differences are that the non-hydrolyzed samples lead to high cis/trans isomerization product ratios, low total acid site densities and low fractional Brønsted site populations. For these reasons, the authors suggested that silica and titania domains are segregated in the non-prehydrolyzed samples, and that prehydrolysis of the silicon precursor represents an effective strategy for promoting homogeneity of TiO2/SiO2 aerogels. In 2009, Yeung et al. reported the possibility to prepare freestanding TiO2/SiO2 monoliths with ultra-low densities and well-defined hierarchical pore structures [46]. For this purpose they mixed a solution of titanium isopropoxide dissolved in ethanolic nitric acid with a pre-hydrolyzed silica precursor solution containing the triblock polymer Pluronic P123 as structure directing agent. The time of gelation increased concomitantly with the titanium content. The synthetic approach for the materials differs only by the type of supercritical drying. Alcogels with Ti/Si < 0.75 were immersed in liquid CO2 and supercritically dried at 323 K, whereas for the samples with Ti/Si ≥ 0.75 ethanol was used as supercritical extraction medium. The reason for the change of the supercritical treatment condition (from CO2 to ethanol) is that the aerogels dried by supercritical CO2 displayed numerous cracks at high Ti contents (Ti/Si ≥ 0.75). Although the samples suffered from high shrinkages (20%) after the use of the high temperature (543 K) supercritical drying process with ethanol, crack-free TiO2/SiO2 aerogels with high titanium content (50-60 wt%) have been obtained. Irrespective of the type of supercritical drying, all samples were calcined at 723 K for 5 h to remove the remaining polymer and exhibited high specific surface areas between 400 to 800 m2g-1. Analogous to the results described by Brodzik et al., the specific surface area decreased with increasing titanium content [47]. In addition, X-ray diffraction measurements showed that the introduction of Ti atoms in the SiO2 host perturbs the long range ordered hexagonal pore arrangement, which can be observed in analogously prepared pure silica aerogels. The introduction of titanium sites in the SiO2 host increases the number of Si-O-Ti bonds, thus weakening the mechanical stability of the network walls. In contrast to pure Si-O-Si
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Figure 6: The structure of dimeric derivatives Ti(OR)3(OOCR`) (left), [Ti(OR)3(acac)]2 (middle), and Ti2(OR)6(O-X)2 (right).
With the replacement of one or more of the alkoxy groups by the organic ligand L, the reactivity of the metal center is significantly decreased. Multidentate ligands are more strongly bonded than the corresponding monodentate ligands because of the chelate effect, resulting in a lower reactivity towards hydrolysis. In addition, steric effects typically hinder hydrolysis. As a result of this coordination, precursor molecules with modified structures and lower reactivities are obtained. Besides this change in the reactivity, the replacement of alkoxy groups by complexing ligands L has several other chemical and structural effects that are discussed in detail in an excellent review article published by Schubert [49]. Brodzik et al. reported a comparison of different synthesis strategies for mixed oxide aerogels. They used four different synthetic methodologies for preparing TiO2/SiO2 aerogels and compared the materials with respect to differences in the photocatalytic activity and physicochemical properties [46,49]. Two methods were based on a prehydrolyzed silica precursor solution to which (a) an unmodified titanium alkoxide precursor or (b) a titanium precursor modified with acetylacetone was added. The third method (c) was a direct co-condensation method in which both alkox-
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carbon dioxide and are therefore spatially confined within the matrix [53]. In many examples, the authors do not explicitly mention the coordination of the reactive titanium centers by chelating or bridging ligands. In example, the synthetic approach reported by Aravind et al. is not only based on prehydrolysis of the less reactive silicon alkoxide, but the titanium alkoxide is processed in the presence of acetic acid [43]. With that a concomitant slowing down of the hydrolysis reaction of the titanium alkoxide is highly probable.
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Development of novel precursor molecules, e.g. singlesource precursors (SSP); In a single-source precursor, a stable (mostly covalent or coordinative) linkage between two or more metal centers prevents phase separation in different domains [54-56]. Error! Reference source not found.7 shows the schematic structure of single-source precursors consisting of a silicon center bearing hydrolysable alkoxy groups, and organic functionalities with which the titanium center can be coordinatively linked to the silane. The use of singlesource precursors offers the possibility to adjust the reaction rates and thus increase the probability of developing homogeneously distributed mixed oxides. This singlesource precursor approach, linking two different alkoxides via an organofunctional silane is in principle comparable to the processing of organobissilyl-species, such as 1,2bis[tris-(2-hydroxyethoxy)silyl]ethane (bEtGMS) and 1,4bis[tris-(2-hydroxyethoxy)silyl]benzene bPhGMS), which have been described in detail as precursors for hybrid porous materials [56].
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ides are mixed and allowed to gel, and the fourth method (d) is a liquid impregnation of a silica alcogel with a modified titanium precursor. With all applied methods highly porous aerogels could be obtained. However, the materials showed some interesting differences: With synthesis methods (a), (b), and (d), monolithic, partially translucent aerogels have been obtained, whereas simple mixing of both precursors without any modification of the titanium alkoxide leads to the formation of opaque, white powders. Similarly, the materials prepared by the direct cocondensation (c) have a somewhat lower specific surface area than the materials prepared by the other methods (a, b, and d). Overall, all materials exhibit very high BET surface areas ranging from 460 m2g-1 to 1000 m2g-1. For synthetic approaches (a) and (b) a clear trend to a lower surface area with higher titania contents can be detected. XRD measurements show the presence of titania in the anatase form, whereas silica is present in amorphous form indicating phase separated materials. The presence of anatase crystallites is particularly important from the photocatalytic point of view. Surprisingly, the first two synthesis methods resulted in aerogels with the lowest photocatalytic activities. Brodzik et al. explained this fact by the poorer accessibility of active titania crystals in the amorphous silica wall. Therefore, they are less accessible on the silica surface. Aerogels prepared with the method (c) showed the highest photocatalytic activity due to the presence of isolated titania particles. In this context, the studies of Brodsky et al. and Beghi et al. must be mentioned [50,51]. They demonstrated that the drying temperature and the supercritical drying agent significantly influence the average pore diameter and the crystallinity of the resulting oxidic materials. An increase in the drying temperature results in larger pore sizes and increased crystallization tendencies during the calcination step. This suggests that high temperatures during the supercritical drying retard a homogeneous distribution in the mixed oxide TiO2/SiO2 aerogels. High temperatures lead to phase separation into silica-rich (titania poor) and titaniarich (silica poor) domains. Supercritical drying procedures can be avoided by following an ambient pressure drying protocol. This has been presented by Liu et al for Si/Ti binary mixed oxide aerogels by a two-step surface modification with decamethyltetrasiloxane/ trimethylchlorosilane or hexamethyldisiloxane/ trimethylchlorosilane followed by treatment with trimethylchlorosilane in hexane [52]. Sun et al. presented an approach relying on hydrolysis and condensation reactions in the highly viscous polymer matrix of polypropylene. Phase separation into silica and titania domains is prevented as the precursors are impregnated on a molecular level into the polymer using supercritical
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Figure 7: Schematic representation of a single-source precursor with different hydrolysable metal/semimetal centers; examples of Si/Ti single source precursors with coordinatively tethered centers.
The group of Schubert synthesized a single-source precursor with 3-(3-trimethoxysilylpropyl)acetylacetone and tetraisopropylorthotitanate with a Si/Ti ratio = 1/1 and 2/1 (see Figure 7) and after base catalyzed hydrolysis silicatitania aerogels with specific surface areas up to 230 m2 g−1
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Figure 8: Sol-gel processing of tetrakis(2-hydroxyethyl)orthosilicate and a single-source precursor/organofunctional silane (the molecular structure of the different precursor molecules are presented as schematic sketch).[59]
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A different single-source precursor approach is based on the synthesis of mixed alkoxides as has been shown by Coles et al. [60]. They applied Ti[OSi(OtBu)3]4 in toluene to a pyrolytic conversion at 225 °C to obtain aerogels and xerogels with fourfold coordinated titanium centers and a specific surface area of 500–700 m2 g−1 [60]. Miller et al. used a solution of diethoxysiloxaneethyltitanate [Si(OEt)2OTi(OEt)2O]x polymer in methanol and consecutively added water and ammonium hydroxide [61]. Within a few minutes they received a gel that was subsequently aged, supercritically dried and calcined. They observed that small amounts of titania (6.25 mol%) do not change the pore characteristics dramatically. A significant difference could be observed, however, in the crystallization behavior, for which a very low tendency of titania to crystallize has been observed. This implies that crystalline fractions of rutile or anatase can be observed only at higher temperatures as typically expected. The disadvantage of their prepared samples was, however, that they were catalytically inactive for 1-butene isomerization due to the lack of Brønsted acid sites. Klabunde et al. synthesized starting materials of the following composition, M[OTi(OnPr)4]2 (M = divalent metal), by thermal condensation of titanium alkoxides with metal (M) acetates or refluxing of Ti(OnPr)4 with Mg(OAc)2. These precursors were than hydrolytically converted to give gels, which were supercritically dried with nitrogen. Aerogellike structures with rather high specific surface areas of 78320 m2g-1 and after thermal treatment crystalline MgTi2O5 or composites such as MTiO3/TiO2 for M = Fe, Co, Zn have been obtained [14].
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and 4- and 6-coordinated titanium centers were obtained [54,55]. An interesting difference between the use of these singlesource precursor and the use of a mixture of (propyl)Si(OMe)3 and (acac)Ti(OiPr)3 is that gelling of a mixture containing the SSP occurred after about 30 min, whereas no gelation over several days has been observed for the equimolar mixture of the non-linked precursor molecules. From this, it can be concluded that the fast gelation is a consequence of the linkage of both moieties and not due to electronic effects. Upon supercritical drying of the wet gels with CO2, monolithic but opaque aerogels were obtained. However, a disadvantage of this singlesource precursor approach is that the stoichiometry is fixed. It can only be changed by varying the ligand to Ti ratio. Two possibilities have already been shown in Figure 7. To obtain further Si/Ti ratios, an additional silicon or titanium source must be added. An unusually high crystallization temperature (800 – 850 °C) of anatase is discussed for these systems, indicating a homogeneous distribution of titanium and silicon. For the anatase–rutile transformation, the anatase particles obviously need to exceed a certain size, which explained that rutile is observed at lower temperatures for samples with a higher titanium content. In agreement with this, 4-fold coordinated titanium centers can still be observed in the samples with high silica content, even at high temperatures. Only recently, novel carboxylic acid derivatized silanes have been successfully synthesized allowing for coordinative linkage of a wide variety of metal centers as has been shown for porous europium(III)/silica mixed oxides [57,58]. Silica-titania monoliths with a hierarchical organization of the pore structure (meso- and macropores) have been prepared from an aqueous solution of the ethylene glycolmodified single-source precursor, 3-[3-{tris(2hydroxyethoxy)silyl}propyl]acetyl-acetone, in the presence of P123 as structure-directing agent and tetrakis(2hydroxyethyl)orthosilicate, followed by extraction with supercritical carbon dioxide for drying (Figure 8). With that a very homogeneous dispersion of tetrahedrally coordinated titanium centers in the silica matrix is obtained even for high titanium portions [59].
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3. Summary Highly porous, mixed oxides are of interest for a variety of technical applications, e.g. including photocatalysis, biomaterials, heterogeneous catalysis, etc. Many different synthetic approaches will give highly porous mixed oxide aerogels. However facts, such as i) the Ma/Mb ratio (solubility limits), ii) dispersion of the different components, iii) tendency of crystallization (phase separation), iv) accessibility of the active centers, v) chemical and structural homogeneity, and vi) textural properties have to be considered very carefully for all synthetic steps, starting from processing of the precursors, necessary thermal treatments, but also supercritical extraction methods for drying of the material. Sol-gel processing in combination with supercritical drying is an ideal route towards chemically well-defined mixed metal oxides, due to the low temperatures and the possibili-
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* Nicola Hüsing: [email protected]
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Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft within the Priority Programme 1570 ‘‘Poröse Medien mit definierter Porenstruktur in der Verfahrenstechnik – Modellierung, Anwendungen, Synthese’’ (Hu 1427/6-1).
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ty to prepare amorphous materials over a large compositional range. The adjustment of the precursor solution chemistry allows for the synthesis of a wide variety of morphologies, textural properties and chemical compositions and dispersions. With respect to the textural features of the network, the final porosity is adjustable from low to high porosities and pore sizes from the nanometer up to the micrometer level, by (i) simply changing the processing parameters for sol formation (particle sizes), (ii) addition of templates or (iii) the drying procedures (xerogel vs aerogel). The chemical composition is easily adjusted by the choice of the precursors. For many applications, control over the degree of cocondensation (Ma-O-Mb bridges), thus the chemical homogeneity of the product is desirable. The ability to manipulate the chemical reactivity by changing the precursor chemistry opens a wide spectrum of possibilities for the final network homogeneity. E.g., for a well-dispersed metal oxide phase in a silica matrix, different processing options can be followed when alkoxide-based sol-gel processes are applied: Adjustment of the rates for the hydrolysis reaction by either prehydrolysing the slower reacting alkoxide, in this case the silicon alkoxide, or applying ethylene glycolmodified silanes that show an instantaneous hydrolysis in aqueous media, or slowing down the faster reacting species – the titanium alkoxide – by coordination to bi- or multidentate ligands. Application of a single-source precursor seems to be an elegant approach to catch several birds with one stone: synchronization of the reactivity of the different precursors, in addition to processing to deliberately tailored pore architectures. Depending on the synthesis parameters and the applied post-synthesis treatments different levels of homogeneity of silicon and titanium in the network can be achieved.
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S0896-8446(15)30068-1 http://dx.doi.org/doi:10.1016/j.supflu.2015.07.015 SUPFLU 3391
To appear in:
J. of Supercritical Fluids
Received date: Revised date: Accepted date:
14-2-2015 1-7-2015 14-7-2015
Please cite this article as: A. Feinle, Mixed Metal Oxide Aerogels from Tailor-Made Precursors, The Journal of Supercritical Fluids (2015), http://dx.doi.org/10.1016/j.supflu.2015.07.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract (for review)
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*Highlights (for review)
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Overview over synthetic strategies towards mixed oxide aerogels is given. Nanoparticle assemblies are discussed Sol-gel processes are summarized Prehydrolysis approaches versus complexation to deliberately tailor the reaction rate Application of single-source precursors in the synthesis of mixed oxide aerogels
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*Manuscript Click here to view linked References
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Mixed Metal Oxide Aerogels from Tailor-Made Precursors Andrea Feinle1, Nicola Hüsing*1 1
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Materials Chemistry, Paris Lodron University Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria. [email protected]; [email protected] Tel: 0043 662 8044 5404
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Abstract Highly porous mixed oxides and metal-doped materials are relevant for many applications including filters, sorption media or photocatalysts, in medical, or electrochemical and optical applications, etc. Mixed oxides are of special interest for several reasons: the chemical properties of an otherwise “inert” support can be improved by the second component, i.e. comprising characteristics, such as higher acidity, larger surface areas, thermal stability, etc. However, the properties of the final material strongly depend on the chemical homogeneity or degree of demixing of the various components. The development of general, cost-effective routes allowing for the synthesis of such mixed metal oxides with deliberately designed structural features on all length scales (from the atomic level to the macroscopic morphology) is still a demanding task. In this paper, we will provide an overview over different solution-based chemical approaches, e.g. sol-gel processes, to highly porous (monolithic) mixed metal oxide aerogels. Keywords: mixed metal oxide, aerogel, sol-gel processing, single source precursor
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1. Introduction Aerogels are solid materials that are characterized by extremely low densities combined with high porosities, and large inner surface areas [1]. One intriguing feature of aerogels is that all constituting elements are in the nanoscale regime, providing a unique combination of properties that make the materials attractive for a variety of applications, such as catalysis, adsorbents, drug delivery, thermoresistors, piezoelectrics, or sensors. Although the first aerogel was already prepared in 1931 by Kistler [2], the development and application-oriented research of aerogels only started in the late 1960s, concomitant with the rapid development of the sol-gel process. Since then, many different chemical compositions, including purely inorganic gels, e.g., metal oxides and sulfides (tellurides and selenides), carbides, metal-doped or mixed metal oxides, as well as inorganic−organic hybrid gels, organic polymeric gels, including resorcinol/formaldehyde, polyurethane, polyamide, conducting polymers (e.g. polythiophenes) and many more, graphene and carbon aerogels, have been prepared. There are some excellent review articles summarizing the recent progress in aerogel science [1, 3−7]. In this review, we will focus on aerogels that are composed of two or more different metal/semimetal oxide centers. These binary of multinary oxides (e.g. ZrO2/SiO2, TiO2/SiO2, TiO2/ZrO2, Al2O3/SiO2, CoTiO2/TiO2, etc.) are of significant technological importance, however, the prop-
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erties strongly depend on parameters such as 1) the MaOx/MbOy ratio, 2) the dispersion of one component in the other and with that the chemical homogeneity of the network, 3) the accessibility of the different metal/semimetal centers as well as 4) the textural properties of the material, all of which are governed not only by the synthetic approach, but also the drying procedures [8-14]. The rapid development of sol-gel processes during the past decades has led major steps forward in the deliberate synthesis of porous, mixed metal oxide networks. Sol-gel procedures complement the broad range of other conventional methods used for the preparation of mixed metal oxide nanostructures, such as ceramic synthesis, flame hydrolysis, ion exchange on supported oxides, precipitation or impregnation methods, typically followed by high temperature treatment [15-24]. Most of these procedures, are however not suited for the production of highly porous aerogel monoliths. The advantages of sol-gel processing are that (i) the hydrolysis and condensation reactions already proceed at room temperature and (ii) by the use of liquid precursor molecules molecular mixing of the different metal/semimetal centers is in principle possible, (iii) particle sizes can be easily controlled by the manifold processing variables, as well as (iv) the application of templates allows for a deliberate tailoring of the porous network architecture. Developing and designing materials from a molecular/ atomic level requires the ability to deliberately control the Page 3 of 18
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2. Chemical Homogeneity What exactly is meant with chemical homogeneity: Looking at the atomic level, the he degree of homogeneity is commonly associated with the relative amount of Si-O-Ti Si linkages in the mixed oxide. Depending on the reaction condicond tions, different scenarios have to be distinguished (see Figure 1, top and middle). ). Titanium can be dispersed in the silica framework on an atomic level (isomorphic substitusubstit tion), resulting in tetrahedrally coordinated metal centers as it is found for the zeolitic system titanosilicalite 1 (TS-1) (TS [25]. This is not the regular case, since one has to consider that for transition metals the coordination oordination number (c.n.) can be higher than the valency, resulting in a coordination exe pansion, i.e. for titanium the valency is typically four but up to six ligand are octahedrally coordinated to the metal center as present in the crystalline titanium dioxide oxide phases, anatase or rutile (Figure 1, top right) [13].
If larger segregated titania domains are present, those can either be homogeneously distributed in the silica matrix (Figure 1A, bottom) or demixed domain structures can be formed to different extent (Figure 1B B and 1C, bottom). The size of these domains strongly depends on the reaction and drying conditions and can vary from a few nanometers up to micrometer-sized domains. For these phase separated systems one also has to consider that the particles can eeither her be crystalline or amorphous, depending on the synthe synthetic approach. Unfortunately, in many cases, no or only very limited information on the chemical homogeneity, neither on the atomic nor on the particulate level is given. Starting from solution as in sol-gel gel processes, the key to design the desired dispersion and mixing of the different metal centers lies in an understanding of the chemical rea reactivity and the structural chemistry of the precursors. Typ Typical precursors for sol-gel gel processes are metal alk alkoxides, e.g. Ti(OR)4 for titania, or Si(OR)4 for silica. A major challenge in sol-gel processes es in the presence of water is the different reaction rate for the hydrolysis reaction. This reaction typically proceeds via a nucleophilic substitution mechanisms and therefore depends on the partial charge, δ+, on the metal atom (Si or Ti). The titanium atom, e.g. Ti4+ in Ti(OiPr)4, is a strong Lewis acid with a significant positive partial charge imparting a much higher reactivity towards hydrolysis than the Si atoms in tetraalkoxysilanes (Si4+) [26]. This can easily be observed by the vigorous reaction of titanium alkoxides with water resulting in ill illdefined titanium-oxo/hydroxo oxo/hydroxo compounds.
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positioning of the building blocks within a material. This is also an essential requirement in the synthesis of porous materials, such as aerogels, in which the arrangement of different building blocks (particles) forming the solid framework determines the pores’ size, shape and arrangearrang ment. In the following a strong focus will be given to SiO2/TiO2 mixed oxide aerogels as a model system, system however, the underlying concepts are very general and can easily be transferred to other mixed oxide and even chalcochalc genide systems.
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Figure 1: Isolated metal species in tetrahedral (top, left) or octahedral (top, right) coordination;; atomic dispersion for mixed metal oxides in tetrahedral (c.n. = 4) or octahedral (c.n.. = 6) geometry for titanium centers by Si-O-Ti bridges (middle) and segregated titania domains either homohom geneously dispersed in the silica matrix (A) or phase separated to different extents (B,C); (note that in the bottom images A-C, C, the coloured spheres represent silica or titania particles not atoms).
In 2001, Gash et al. developed a new sol-gel gel route based on metal ion salts in the presence of epoxides as gelation agents to synthesize different transition metal and main group metal oxide aerogels [27]. The versatility of this procedure has been shown with the synthesis of a large variety of metal/Si mixed oxide aerogels [28]. The choice of epoxide proved to give a good control over composition regarding the M/Si ratio,, even allowing for the synthesis of gels with the metal oxide – not silica – as the major co component. Due to the compositional generality of the method, the obtained nanocomposites are built-up up from a variety of metal oxides with several oxidation states. For these gels not only the structure, but also the dispersion of the diffe different elements in the gels has as been investigated by using electron energy-loss loss spectroscopy (EELS) in tandem with TEM. The authors have shown that the different elements are uniformly dispersed throughout the materials and no large silica domains appear to be present. Recent progresss in the synthesis of mixed oxides applying this epoxide-based approach,, e.g. for ZrO2/SiO2 mixtures, starting from low-cost cost commercially available precursors, such as water glass or metal salts, has been en shown by Ratke
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provides local coordinationn symmetry and oxidation states or EXAFS providing ding more detailed information, such as MO bond length or the coordination sphere of the cations. These methods are complementary to other powerful spectroscopic methods, such as Raman or infrared vibrational spectroscopy,, solid state NMR spectroscopy or UV-vis (diffuse reflection, DR) spectroscopy. Recently, signifi significant advances have been made in analyzing the structure of disordered or partially ordered materials using atomic pair distribution function (PDF) analysis of powder diffraction data. The possibilities bilities of this approach have been shown for chalcogels and silica/titania mixed oxide aerogels.[34,35]
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et al.. However, no information on the chemical homogenehomogen ity of the mixed oxides is given [29]. As seen above, porous mixed metal oxide materials are easily accessible from a synthesis point of view. view However, a detailed characterization, especially with a focus on the molecular/ atomic level chemical homogeneity of the material remains notoriously difficult. To get a complete picture of the structure, morphology, composition and a chemical homogeneity typically a combination of different analytical techniques has to be applied (see Figure 2). 2)
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Approaches to control chemical homogeneity If different alkoxides, e.g. silicon and metal alkoxides, are processed separately and the sols are combined at a later stage in the synthesis, heterogeneous systems comprising SiOx(OH)y(OR)z species as well as the corresponding metal oxide particles are obtained after drying as schematically depicted in Figure 3.. Depending on the dimensions of the sol particles the homogeneity of the final material can be adjusted. However, the amount of Si-O-Ti Ti bridges will be very low.
Figure 2: Schematic overview of some typical methodologies available for the characterization of mixed metal oxide aerogels
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Regarding structural features, such as microstructure, morphology, specific surface areas, pore sizes and pore size distributions, etc., standard methods including X-ray X scattering or diffraction, electron microscopy, gas sorption or porosimetry have been used. However, one has to keep in mind that due to the amorphous or nanocrystalline character of aerogels, the obtained values often show rather broad distributions that by no means can be compared with the precise information that is given for analogous crystalline samples. Several excellent review articles summarize the recent advances in the characterization of mixed oxide materials, e.g. by X-ray ray absorption spectroscopy (XAS), Raman or infrared nfrared spectroscopy, solid state NMR spectroscopy, UV-vis vis diffuse reflectance spectroscopy, HR TEM, etc.[30-33] While it is straighforward to analyze the chemical composition of a material, chemical hemical homogeneity remains a materials als characteristic that is difficult to address due to the lack of spatially resolved analytical techniques. Most methods provide information on a rather large sample area; thus the atomic level information is always an average over many atoms/molecules. Typical ypical methods that give information on the homogeneity or heterogeneity of the porous material on very short length scales are X-ray absorption spectroscopy tools, such as XANES that
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Figure 3: SiO2 and metal oxide sols prepared separately resulting in heterogeneous solids
A wide variety of aerogels prepared from preformed nan nanoparticles has already been synthesized, including PbS, ZnS, or BaTiO3 and Au, Ag, Pt [36-38]. The feasibility of this process in the production of binary and multinary aerogels has recently been shown by the group of Niederberger et al. by reacting amorphous silica and crystalline titania nan nanoparticles to yield mixed oxide aerogels [339]. This approach
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Given this rather simple sol-gel approaach, phase homogeneity or the dispersion of the metal ions on an atomic scale can be improved by the following measures as also depicted in Figure 5.. These three approaches will be di discussed in more detail in the following sections, starting with thee prehydrolysis of the slower reacting component, followed by the complexation reaction of the faster reacting alkoxide and the application of stable single single-source precursors. All synthetic pathways have been used in the syn synthesis of mixed oxide aerogels and selected examples are presented below. It would be beyond the scope of this co contribution to give a comprehensive review, therefore, the choice of examples is somewhat arbitrary.
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What happens, if the metal alkoxides are just mixed togethtoget er? Even being a very straightforward approach, it is very probable that rather inhomogeneous materials erials with a low a b number of heterocondensed M -O-M bridges will result due to the above mentioned differences in the hydrolysis reaction rates (Figure 4).
the material, the system evolves into a more thermodynamically stable state with formation ation of Ti and Si rich domains [41].
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not only offers to produce gels from amorphous and cryscry talline nanoparticles simultaneously, but also gives gels with a unique combination of properties, e.g. photocatalytic behavior by anatase particles combined with the rich sursu face chemistry of silica. In another modular approach, they processed gold and anatase nanoparticles via controlled coco gelation in the presence of preformed tungsten oxide nan nowires to obtain macroscopic aerogel monoliths with high specific surface areas of 473 m2g-1, transparency ncy and excelexce lent crystallinity [40]. Collapse of the filigree pore system is in all cases prevented by supercritical drying with carbon dioxide. Even if the chemical homogeneity cannot be ada justed on the atomic level, this co-gelation gelation approach opens fascinating possibilities for the creation of aerogels with unprecedented architectural complexity.
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Figure 4: Simultaneous processing of silicon and metal alkoxides.
However, not only the reaction rates will determine the final dispersion and structure, but also solubility limits for the various mixtures have to be considered, in addition to the hetero- (Ma-O-Mb) versus homo-condensation condensation (Ma-Oa b b M , M -O-M ) rates. Interestingly, previous studies dies by solid 17 29 state O and Si MAS NMR spectroscopy have ha shown that by reacting Si-OH groups with M-OR OR groups the forfo mation of Si-O-M bonds (with M = Ti or Zr) seems kinetikinet cally favored, so that the addition of a titanium alkoxide to a sol containing silanol species will immediately lead to a significant formation of such bonds. However, upon ageing
Figure 5: Different synthetic strategies to increase the homogeneity of the final materials.
Prehydrolysis of the slower reacting alkoxide precursor as proposed by Yoldas;[42] Inn the case of silica/titania mixed oxides that would e.g. be the reaction of tetraalkoxysilanes with aqueous acids. To name just a few examples:: Aravind and coworkers synth synthesized monolithic silica-titania aerogels with prehydrolysis of tetraethoxysilane with acidified water (HCl) with a surface area up to 685 m2 g−1 and modified th the surface of the material with trimethylchlorosilane for ambient pressure drying.[43] Miller et al. reported studies in which they compared TiO2/SiO2 aerogels that were prepared either with prehydrolysis or without prehydrolysis of the silica precursor molecules.[44, 45] Here, independent of the Ti/Si ratio, the prehydrolyzed samples exhibit exhibited higher pore volumes and higher specific surface areas than the non nonprehydrolyzed samples, whereas the pore size distributions
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networks, the mixed oxide walls are not strong enough to withstand the high pressures and temperatures during the supercritical drying. The TiO2/SiO2 aerogel prepared by the ethanol supercritical drying contains hexagonally arranged microdomains embedded within an amorphous phase. Furthermore, the mixed oxide aerogels contain anatase domains of approximately 9 nm in size.
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Modification of the transition metal alkoxide with chelating ligands; Modification of the transition metal alkoxide, here exemplary shown for Ti(OR)4 (eqn. 1 and Figure 6), can be done with organic groups, e.g. multi- or bidentate complexing ligands (L), such as organic acids, diketones (L-H) or other anionic ligands with neutral donor groups X (e.g., -NH2) [48]. M(OR)x-yLy + y ROH
(1)
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M(OR)x + y L-H
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remained mostly unaffected. Furthermore, they described that for both approaches the Ti/Si ratio has a strong influence on the crystallization temperature: the lower the Ti/Si ratio, the higher the crystallization temperature of TiO2 to anatase or rutile. This is also reflected in the significant difference between the two prepared samples in the catalytic activity in 1-butene isomerization. Both mixed oxide samples were indeed catalytically more active than pure titania, but in comparison to the non-hydrolyzed counterparts, the prehydrolyzed samples showed again a threetimes higher activity. Further differences, indicating structural differences are that the non-hydrolyzed samples lead to high cis/trans isomerization product ratios, low total acid site densities and low fractional Brønsted site populations. For these reasons, the authors suggested that silica and titania domains are segregated in the non-prehydrolyzed samples, and that prehydrolysis of the silicon precursor represents an effective strategy for promoting homogeneity of TiO2/SiO2 aerogels. In 2009, Yeung et al. reported the possibility to prepare freestanding TiO2/SiO2 monoliths with ultra-low densities and well-defined hierarchical pore structures [46]. For this purpose they mixed a solution of titanium isopropoxide dissolved in ethanolic nitric acid with a pre-hydrolyzed silica precursor solution containing the triblock polymer Pluronic P123 as structure directing agent. The time of gelation increased concomitantly with the titanium content. The synthetic approach for the materials differs only by the type of supercritical drying. Alcogels with Ti/Si < 0.75 were immersed in liquid CO2 and supercritically dried at 323 K, whereas for the samples with Ti/Si ≥ 0.75 ethanol was used as supercritical extraction medium. The reason for the change of the supercritical treatment condition (from CO2 to ethanol) is that the aerogels dried by supercritical CO2 displayed numerous cracks at high Ti contents (Ti/Si ≥ 0.75). Although the samples suffered from high shrinkages (20%) after the use of the high temperature (543 K) supercritical drying process with ethanol, crack-free TiO2/SiO2 aerogels with high titanium content (50-60 wt%) have been obtained. Irrespective of the type of supercritical drying, all samples were calcined at 723 K for 5 h to remove the remaining polymer and exhibited high specific surface areas between 400 to 800 m2g-1. Analogous to the results described by Brodzik et al., the specific surface area decreased with increasing titanium content [47]. In addition, X-ray diffraction measurements showed that the introduction of Ti atoms in the SiO2 host perturbs the long range ordered hexagonal pore arrangement, which can be observed in analogously prepared pure silica aerogels. The introduction of titanium sites in the SiO2 host increases the number of Si-O-Ti bonds, thus weakening the mechanical stability of the network walls. In contrast to pure Si-O-Si
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Figure 6: The structure of dimeric derivatives Ti(OR)3(OOCR`) (left), [Ti(OR)3(acac)]2 (middle), and Ti2(OR)6(O-X)2 (right).
With the replacement of one or more of the alkoxy groups by the organic ligand L, the reactivity of the metal center is significantly decreased. Multidentate ligands are more strongly bonded than the corresponding monodentate ligands because of the chelate effect, resulting in a lower reactivity towards hydrolysis. In addition, steric effects typically hinder hydrolysis. As a result of this coordination, precursor molecules with modified structures and lower reactivities are obtained. Besides this change in the reactivity, the replacement of alkoxy groups by complexing ligands L has several other chemical and structural effects that are discussed in detail in an excellent review article published by Schubert [49]. Brodzik et al. reported a comparison of different synthesis strategies for mixed oxide aerogels. They used four different synthetic methodologies for preparing TiO2/SiO2 aerogels and compared the materials with respect to differences in the photocatalytic activity and physicochemical properties [46,49]. Two methods were based on a prehydrolyzed silica precursor solution to which (a) an unmodified titanium alkoxide precursor or (b) a titanium precursor modified with acetylacetone was added. The third method (c) was a direct co-condensation method in which both alkox-
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carbon dioxide and are therefore spatially confined within the matrix [53]. In many examples, the authors do not explicitly mention the coordination of the reactive titanium centers by chelating or bridging ligands. In example, the synthetic approach reported by Aravind et al. is not only based on prehydrolysis of the less reactive silicon alkoxide, but the titanium alkoxide is processed in the presence of acetic acid [43]. With that a concomitant slowing down of the hydrolysis reaction of the titanium alkoxide is highly probable.
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Development of novel precursor molecules, e.g. singlesource precursors (SSP); In a single-source precursor, a stable (mostly covalent or coordinative) linkage between two or more metal centers prevents phase separation in different domains [54-56]. Error! Reference source not found.7 shows the schematic structure of single-source precursors consisting of a silicon center bearing hydrolysable alkoxy groups, and organic functionalities with which the titanium center can be coordinatively linked to the silane. The use of singlesource precursors offers the possibility to adjust the reaction rates and thus increase the probability of developing homogeneously distributed mixed oxides. This singlesource precursor approach, linking two different alkoxides via an organofunctional silane is in principle comparable to the processing of organobissilyl-species, such as 1,2bis[tris-(2-hydroxyethoxy)silyl]ethane (bEtGMS) and 1,4bis[tris-(2-hydroxyethoxy)silyl]benzene bPhGMS), which have been described in detail as precursors for hybrid porous materials [56].
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ides are mixed and allowed to gel, and the fourth method (d) is a liquid impregnation of a silica alcogel with a modified titanium precursor. With all applied methods highly porous aerogels could be obtained. However, the materials showed some interesting differences: With synthesis methods (a), (b), and (d), monolithic, partially translucent aerogels have been obtained, whereas simple mixing of both precursors without any modification of the titanium alkoxide leads to the formation of opaque, white powders. Similarly, the materials prepared by the direct cocondensation (c) have a somewhat lower specific surface area than the materials prepared by the other methods (a, b, and d). Overall, all materials exhibit very high BET surface areas ranging from 460 m2g-1 to 1000 m2g-1. For synthetic approaches (a) and (b) a clear trend to a lower surface area with higher titania contents can be detected. XRD measurements show the presence of titania in the anatase form, whereas silica is present in amorphous form indicating phase separated materials. The presence of anatase crystallites is particularly important from the photocatalytic point of view. Surprisingly, the first two synthesis methods resulted in aerogels with the lowest photocatalytic activities. Brodzik et al. explained this fact by the poorer accessibility of active titania crystals in the amorphous silica wall. Therefore, they are less accessible on the silica surface. Aerogels prepared with the method (c) showed the highest photocatalytic activity due to the presence of isolated titania particles. In this context, the studies of Brodsky et al. and Beghi et al. must be mentioned [50,51]. They demonstrated that the drying temperature and the supercritical drying agent significantly influence the average pore diameter and the crystallinity of the resulting oxidic materials. An increase in the drying temperature results in larger pore sizes and increased crystallization tendencies during the calcination step. This suggests that high temperatures during the supercritical drying retard a homogeneous distribution in the mixed oxide TiO2/SiO2 aerogels. High temperatures lead to phase separation into silica-rich (titania poor) and titaniarich (silica poor) domains. Supercritical drying procedures can be avoided by following an ambient pressure drying protocol. This has been presented by Liu et al for Si/Ti binary mixed oxide aerogels by a two-step surface modification with decamethyltetrasiloxane/ trimethylchlorosilane or hexamethyldisiloxane/ trimethylchlorosilane followed by treatment with trimethylchlorosilane in hexane [52]. Sun et al. presented an approach relying on hydrolysis and condensation reactions in the highly viscous polymer matrix of polypropylene. Phase separation into silica and titania domains is prevented as the precursors are impregnated on a molecular level into the polymer using supercritical
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Figure 7: Schematic representation of a single-source precursor with different hydrolysable metal/semimetal centers; examples of Si/Ti single source precursors with coordinatively tethered centers.
The group of Schubert synthesized a single-source precursor with 3-(3-trimethoxysilylpropyl)acetylacetone and tetraisopropylorthotitanate with a Si/Ti ratio = 1/1 and 2/1 (see Figure 7) and after base catalyzed hydrolysis silicatitania aerogels with specific surface areas up to 230 m2 g−1
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Figure 8: Sol-gel processing of tetrakis(2-hydroxyethyl)orthosilicate and a single-source precursor/organofunctional silane (the molecular structure of the different precursor molecules are presented as schematic sketch).[59]
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A different single-source precursor approach is based on the synthesis of mixed alkoxides as has been shown by Coles et al. [60]. They applied Ti[OSi(OtBu)3]4 in toluene to a pyrolytic conversion at 225 °C to obtain aerogels and xerogels with fourfold coordinated titanium centers and a specific surface area of 500–700 m2 g−1 [60]. Miller et al. used a solution of diethoxysiloxaneethyltitanate [Si(OEt)2OTi(OEt)2O]x polymer in methanol and consecutively added water and ammonium hydroxide [61]. Within a few minutes they received a gel that was subsequently aged, supercritically dried and calcined. They observed that small amounts of titania (6.25 mol%) do not change the pore characteristics dramatically. A significant difference could be observed, however, in the crystallization behavior, for which a very low tendency of titania to crystallize has been observed. This implies that crystalline fractions of rutile or anatase can be observed only at higher temperatures as typically expected. The disadvantage of their prepared samples was, however, that they were catalytically inactive for 1-butene isomerization due to the lack of Brønsted acid sites. Klabunde et al. synthesized starting materials of the following composition, M[OTi(OnPr)4]2 (M = divalent metal), by thermal condensation of titanium alkoxides with metal (M) acetates or refluxing of Ti(OnPr)4 with Mg(OAc)2. These precursors were than hydrolytically converted to give gels, which were supercritically dried with nitrogen. Aerogellike structures with rather high specific surface areas of 78320 m2g-1 and after thermal treatment crystalline MgTi2O5 or composites such as MTiO3/TiO2 for M = Fe, Co, Zn have been obtained [14].
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and 4- and 6-coordinated titanium centers were obtained [54,55]. An interesting difference between the use of these singlesource precursor and the use of a mixture of (propyl)Si(OMe)3 and (acac)Ti(OiPr)3 is that gelling of a mixture containing the SSP occurred after about 30 min, whereas no gelation over several days has been observed for the equimolar mixture of the non-linked precursor molecules. From this, it can be concluded that the fast gelation is a consequence of the linkage of both moieties and not due to electronic effects. Upon supercritical drying of the wet gels with CO2, monolithic but opaque aerogels were obtained. However, a disadvantage of this singlesource precursor approach is that the stoichiometry is fixed. It can only be changed by varying the ligand to Ti ratio. Two possibilities have already been shown in Figure 7. To obtain further Si/Ti ratios, an additional silicon or titanium source must be added. An unusually high crystallization temperature (800 – 850 °C) of anatase is discussed for these systems, indicating a homogeneous distribution of titanium and silicon. For the anatase–rutile transformation, the anatase particles obviously need to exceed a certain size, which explained that rutile is observed at lower temperatures for samples with a higher titanium content. In agreement with this, 4-fold coordinated titanium centers can still be observed in the samples with high silica content, even at high temperatures. Only recently, novel carboxylic acid derivatized silanes have been successfully synthesized allowing for coordinative linkage of a wide variety of metal centers as has been shown for porous europium(III)/silica mixed oxides [57,58]. Silica-titania monoliths with a hierarchical organization of the pore structure (meso- and macropores) have been prepared from an aqueous solution of the ethylene glycolmodified single-source precursor, 3-[3-{tris(2hydroxyethoxy)silyl}propyl]acetyl-acetone, in the presence of P123 as structure-directing agent and tetrakis(2hydroxyethyl)orthosilicate, followed by extraction with supercritical carbon dioxide for drying (Figure 8). With that a very homogeneous dispersion of tetrahedrally coordinated titanium centers in the silica matrix is obtained even for high titanium portions [59].
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3. Summary Highly porous, mixed oxides are of interest for a variety of technical applications, e.g. including photocatalysis, biomaterials, heterogeneous catalysis, etc. Many different synthetic approaches will give highly porous mixed oxide aerogels. However facts, such as i) the Ma/Mb ratio (solubility limits), ii) dispersion of the different components, iii) tendency of crystallization (phase separation), iv) accessibility of the active centers, v) chemical and structural homogeneity, and vi) textural properties have to be considered very carefully for all synthetic steps, starting from processing of the precursors, necessary thermal treatments, but also supercritical extraction methods for drying of the material. Sol-gel processing in combination with supercritical drying is an ideal route towards chemically well-defined mixed metal oxides, due to the low temperatures and the possibili-
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Author Information
Corresponding Author
* Nicola Hüsing: [email protected]
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Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft within the Priority Programme 1570 ‘‘Poröse Medien mit definierter Porenstruktur in der Verfahrenstechnik – Modellierung, Anwendungen, Synthese’’ (Hu 1427/6-1).
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ty to prepare amorphous materials over a large compositional range. The adjustment of the precursor solution chemistry allows for the synthesis of a wide variety of morphologies, textural properties and chemical compositions and dispersions. With respect to the textural features of the network, the final porosity is adjustable from low to high porosities and pore sizes from the nanometer up to the micrometer level, by (i) simply changing the processing parameters for sol formation (particle sizes), (ii) addition of templates or (iii) the drying procedures (xerogel vs aerogel). The chemical composition is easily adjusted by the choice of the precursors. For many applications, control over the degree of cocondensation (Ma-O-Mb bridges), thus the chemical homogeneity of the product is desirable. The ability to manipulate the chemical reactivity by changing the precursor chemistry opens a wide spectrum of possibilities for the final network homogeneity. E.g., for a well-dispersed metal oxide phase in a silica matrix, different processing options can be followed when alkoxide-based sol-gel processes are applied: Adjustment of the rates for the hydrolysis reaction by either prehydrolysing the slower reacting alkoxide, in this case the silicon alkoxide, or applying ethylene glycolmodified silanes that show an instantaneous hydrolysis in aqueous media, or slowing down the faster reacting species – the titanium alkoxide – by coordination to bi- or multidentate ligands. Application of a single-source precursor seems to be an elegant approach to catch several birds with one stone: synchronization of the reactivity of the different precursors, in addition to processing to deliberately tailored pore architectures. Depending on the synthesis parameters and the applied post-synthesis treatments different levels of homogeneity of silicon and titanium in the network can be achieved.
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