Hybrid Materials. Synthesis, Characterization, and Applications. Edited by Guido KickelbickCopyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31299-3
5Porous Hybrid MaterialsNicola Hsing
5.1General Introduction and Historical Development
Porosity is ubiquitous to most known materials, with the exception of metals andceramics that are red at high temperatures. Even in nature, many materials areporous, including wood, cork, sponge, bone, or the skeleton structures of very sim-ple organisms such as diatoms, radiolaria, etc.
Mankind has been using porous materials for a long time, certainly dating backto prehistoric times, e.g. as charcoal for drawings in ancient caves, or for puri-cation of water or medical treatment. However, it was only in the rst half of the20th century that the deliberate design of porous materials, i.e. their composition,pore structure and connectivity, became possible. Early examples include materi-als such as aerogels with porosities above 95%, or the development of novel syn-thetic routes to crystalline zeolite lattices with dened pore size and structure. Themost promising, namely template-based approaches towards porous materials,have been advancing rapidly since the end of the 20th century, a typical examplebeing the M41S type of materials or inverse opal structures (see below). In addi-tion, the range of compositions has been extended dramatically from purely inor-ganic, i.e. metals or metal oxides through carbons to porous organic materials, i.e. polymers such as poly(styrene)poly(divinylbenzene) or organic foams such aspoly(urethanes). A large variety of inorganicorganic hybrid porous materials areaccessible today one prominent recent example being three-dimensional (3-D)metalorganic frameworks, so-called MOFs.
For chemical routes to porous materials, deliberate control over the positioningof molecular network-forming building blocks within a material is crucial, sincethe arrangement of the different building blocks forming the solid framework de-termines not only the chemical composition, but also the size, shape and arrange-ment of the pores (Fig. 5.1).
Within this chapter, a selection of different hybrid inorganicorganic porous ma-terials will be presented, focusing on porous inorganic matrices with organic func-tions. However, the reader is reminded that this covers the eld only to some
extent, since many organic materials are porous and can easily be modied withinorganic species. Since most textbooks do not even mention porous materials,despite their technological importance in many different areas, this chapter startswith a short introduction to porosity, followed by a brief overview of different typesof porous solids. At the end, the reader is introduced to the different options forsynthesizing hybrid porous materials, focusing on the problems and challengesfor the given materials. The chosen examples are somewhat arbitrary, but to cover all types of porous hybrid materials would be far beyond the scope of thischapter.
This chapter gives an introduction to: Materials that are characterized by different types of
porosity regarding the size and arrangement of the pores. Hybrid materials that differ in the way the organic part is
incorporated; e.g. inclusion compounds, materials withcovalently anchored organic functions, or even materials in
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Fig. 5.1 Porosity: interparticle versus intraparticle porosity(top left and right) and statistic versus periodic arrangement(top and bottom) of the pores.
which the organic entity is an integral part of the porousnetwork structure.
Different synthetic strategies for hybrid materials, rangingfrom post treatment of a preformed matrix, in situ synthesisby mixing all the inorganic and organic components, orapproaches in which the precursors already contain theinorganic and organic part within the same molecule.
Some of the major applications of the different materials.
5.1.1Denition of Terms
Porous materials A solid is called porous, when it contains pores, i.e. cavities,channels or interstices, which are deeper than they are wide. A porous materialcan be described in two ways: a) by the pores or b) by the pore walls. Some porousmaterials are based on agglomerated or aggregated powders in which the poresare formed by interparticle voids, while others are based on continuous solid networks.
For most applications the pore size is of major importance. However, pore sizesare not susceptible to precise measurement, because the pore shape is usuallyhighly irregular, leading to a variety of pore sizes within one single material. Nev-ertheless, the use of three different pore size regimes was recommended by IU-PAC and this terminology will also be used throughout this chapter:
Micropores, with diameters smaller than 2nm; Mesopores, with diameters between 2 and 50nm; Macropores, with diameters larger than 50nm.
This nomenclature is not arbitrarily chosen, but is associated with the differenttransport mechanisms occurring in the various types of pores, i.e. molecular dif-fusion and activated transport in micropores; while in mesopores Knudsen trans-port, surface diffusion and capillary condensation are the major mechanisms(Knudsen diffusion occurs when the mean free path is relatively long comparedto the pore size, so the molecules collide frequently with the pore wall); and inmacropores, bulk diffusion and viscous ow dominate.
As already mentioned, a wide variety of porous inorganic frameworks is known(Fig. 5.2). Today, zeolites or MOFs are the most prominent examples for micro-porous materials. Mesoporous solids with pore sizes between 2 and 50nm can befound for example in aerogels, pillared clays and M41S materials, while macro-porous solids are for example glasses, foams or inverse opal structures. In addi-tion, these materials can be distinguished by the arrangement of the pores periodic or random and the pore radii distribution, which can range from eithernarrow with a rather uniform pore size distribution to quite a broad distribution.
In the discussion of porous materials, not only pore size distribution and porediameters are of interest for later applications, but also the connectivity of the poresystem or its dimension is of high interest. Porous channel systems, e.g. the ones
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in M41S phases, may be one-dimensional (1-D) as found for the hexagonally organized pore systems with their long channels, or 3-D as found for a cubic or-dered pore structure. Two-dimensional (2-D) systems are layered materials whichwill not be discussed in this context (Fig. 5.3). In addition to the dimensionalityof the pore system, two different surfaces must be distinguished in porous mate-rials. The outer or exterior surface is an outward curving surface (convex) with acompletely different reactivity as the inward curving surface (concave) that is typ-ically found in the interior of the pores. This effect is of importance for function-alization reactions as discussed in the later sections of this chapter.
Nanocomposite Traditionally composites have been fabricated from preformedcomponents in a process that organizes them in a matrix and with a particulararrangement. The integration of the different components is often a top-down ap-proach and therefore, the structure and composition of the interfaces between theconstituent parts is typically not under molecular scale control. In addition, thematerial may be divided into macroscopic domains with sizes of the order of milli- or micrometers (see also macroscopic phase separation). The bottom-up approach is an appealing solution to the interface problem, by co-assembling molecular inorganic and organic precursors into a nanocomposite material withmolecular level command over interfaces, structure and morphology.
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Fig. 5.2 Different porous materials classied according totheir pore size and pore size distribution (insert).
Phase separation An inherent problem in the synthesis of inorganicorganic hybrid materials is the incompatibility of many organic moieties with the aque-ous-based synthesis of inorganic matrices. This incompatibility may result inmacroscopic phase separation during the synthesis if the organic and inorganiccomponents are mixed together. Typically this type of phase separation is not desired and synthetic pathways are developed to circumvent this problem, e.g. by linking the inorganic and organic moiety within one molecule such as in organically-modied trialkoxysilanes as already discussed in Chapter 1.
Nevertheless, the processing of such inorganicorganic hybrid precursors maystill result in microphase separation, where two phases are formed on the nanome-ter scale.
However, in some cases phase separation is deliberately induced, especially inthe formation of porous materials, such as M41S materials (see Sections 18.104.22.168;22.214.171.124). Here, an organized texture is formed via self-assembly of template mole-cules. This texture is based on microphase separation which divides the reactionspace into a hydrophilic and a hydrophobic domain.
5.1.2Porous (Hybrid) Matrices
With an organic modication of porous solids a wide eld of porous hybrid ma-terials can be obtained that, by the combination of inorganic and organic buildingblocks, benet from the properties of both parts; an approach which already hasbeen performed on a wide variety of different matrices. The organic groups canbe placed selectively on the internal and/or the external pore surfaces or even
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Fig. 5.3 Hexagonal, cubic and lamellar packings resulting in1-D and 3-D pore dimensionalities and description of interiorversus outer (exterior) surfaces.
within the pore walls. The organic modication in principle permits a ne tuningof materials properties, including surface properties such as hydrophilicity/hydrophobicity or potential interaction to guest molecules. In addition, the surfacereactivity can be altered and the surface can be protected by organic groups withrespect to chemical attack, but also bulk properties, e.g. mechanical or optical prop-erties can be changed. This exibility in choosing organic, inorganic or even hybrid building blocks allows one to control the materials properties to optimizethem for each desired applications.
The selection of porous matrices that are discussed in the following sections of this chapter are chosen somewhat arbitrary, but can be taken as representativeexamples for the general reaction schemes and the problems associated to the different pathways in the synthesis of hybrid materials.
126.96.36.199 Microporous Materials: ZeolitesZeolites are microporous crystalline oxides, typically composed of silicon, oxygenand aluminum with cavities that are interconnected by smaller windows. Sincetheir rst discovery in the middle of the 18th century, zeolites had been generallyregarded as microporous crystalline aluminosilicates having ion-exchangeablecations and reversibly desorbable water molecules (analog to natural zeolites). Today this denition has been extended to quite some extent for several reasons,i.e. in 1978, a purely siliceous zeolitic material, silicalite, was synthesized, whichdoes not have an ion-exchanging ability (it is an aluminum-free material) or in1982, the rst aluminophosphates as microporous crystalline molecular sieves,again with an electrostatically neutral framework, were prepared. The progressmade can be related to some extent to the better understanding of the synthesismechanism, which relies typically on hostguest reactions, with inorganic or organic cations as structure-directing agents. It would be far beyond the scope ofthis chapter to cover all aspects of zeolite chemistry and their microporous analogs.The reader is referred to the Bibliography at the end of this chapter.
As a denition, zeolites can be described as open 4-connected 3-D nets whichhave the general (approximate) composition AB2, where A is a tetrahedrally con-nected atom and B is any 2-connected atom, which may or may not be shared be-tween two neighboring A atoms (Fig. 5.4). For classical zeolites this means thatA is either a SiO4- or AlO4-tetrahedron and two tetrahedra are linked by a corner-sharing oxygen atom.
In zeolites, the pores are formed as an inherent feature of the crystalline inor-ganic framework thus they are also periodically arranged. When discussing poresin zeolites, the reader should be aware of the fact that one has to distinguish be-tween a cage, in which molecules can be accommodated, and the windows to thiscage that are typically smaller than the actual cage. Therefore, molecules that tinto the cage are not necessarily able to cross the windows, thus diffusion withinthe material can be drastically limited. The size of the cage (pore) must be spa-cious enough to accommodate at least one molecule. For the accommodation ofwater molecules the pore diameter must exceed 0.25nm which is the lower limitfor the pore size in zeolites. Today a wide variety of zeolitic structures either nat-
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ural or synthetic is well known, covering the pore size regime from 0.25 to 1.5nm.In addition to the different pore sizes, zeolites can be classied as uni-, bi- andtridirectional zeolites, depending on whether the channel system is arranged alongone, two, or the three Cartesian axes. This directionality is extremely importantwith respect to the ability of guest molecules to diffuse within the zeolite matrix.
Zeolites owe their importance not only to the presence of active moieties, e.g.acid centers, in the matrix, but to their general use as catalysts in gas-phase, large-scale petrochemical processes, such as catalytic cracking, FriedelCrafts alkylationand alkylaromatic isomerization and disproportionation. In addition to their importance in heterogeneous catalysis, it is likely that these solids will also attractinterest in the development of functional materials and in nanotechnology, forwhich zeolites provide an optimal rigid matrix which allows for inclusion of someactive components.
Modication of zeolites can be performed by different approaches. A very com-mon way is the substitution of framework atoms by heteroelements, e.g. Co, Mg,B, Ga, Ge, Fe, and many more, to add new properties to the microporous frame-work. Another possibility relies on the intrinsic ability of zeolites to exchange
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Fig. 5.4 Scheme of the zeolite synthesis forming a three-dimensional network.
cations, which is due to the isomorphic substitution of silicon as a tetravalentframework cation by trivalent cations (typically Al) resulting in a net negativecharge of the network.
With respect to the synthesis of inorganicorganic hybrid zeolites, ve differentapproaches for the modication should be considered and they will be discussedin the following sections:
Post-synthetic ion exchange...