1.1. Introduction
In the past twenty years, sol-gel science1,2 has undergone a spectacular development. The various stages of the sol-gel process have been scrutinized in considerable detail and established a sound basis for technological development. A turning point was reached with the emergence of organic-inorganic nanocomposite materials via the sol-gel process that opened gateway to whole classes of new materials. In 1985, Wilkes et al.3 initiated the work to develop novel organic-inorganic hybrid network materials by reacting metal alkoxides with end- functionalized condensational polymeric/oligomeric species through a sol- gel process. Their first successful example has been the incorporation of poly(dimethylsiloxane) (PDMS) oligomers into the silica matrix.3 In 1990, Wei et al.4 pioneered the synthesis of vinyl polymer- metal oxide hybrid materials.
In the early 1990s, another new material was made by Kresge et al.5 by templating silica species with surfactant molecules leading to the formation of ordered mesoporous silica oxides. These new products, known under the group name M41S, with the hexagonal MCM-41 being the most prominent member, dramatically expanded the range of pore sizes accessible in the form of an ordered pore system. Soon enough, research in this area has been extended to many metal oxides systems other than silica and also the novel organic-inorganic hybrid mesoporous materials etc. Our lab at Drexel has developed a nonsurfactant templating (i.e., small organic molecules) pathway in making mesoporous materials via the sol- gel process. In the following sections, we will briefly review the work on mesoporous materials and organic- inorganic hybrid sol- gel materials.
1.2. Mesoporous Molecular Sieves
Meso, the Greek prefix, meaning "in between", has been adopted by IUPAC to define porous materials with pore sizes between 2 and 50 nm.6 Along with two other classes of porous solids (defined by pore size), microporous (<> 50 nm),7 they have been used technically as adsorbents, catalyst and catalyst supports owing to their high surface areas and large pore volumes. Typical macroporous example include porous vicor and hydrogels. Mesopores are present in aerogels, pillared layered clays which show disordered pore systems with broad pore-size distributions. Well known members of the microporous class are the zeolites, which provide excellent catalytic properties by virtue of their crystalline inorganic, e.g., aluminosilicate network. However, their applications are limited by the relatively small pore openings.8,9 Thus, a persistent demand has developed for larger pores with well-defined pore structures.
1.2.1. Periodic Mesoporous Silicates
The design and synthesis of organic, inorganic and polymeric materials with controlled pore structure are important academic and industrial issues. In many applications, the precise control of pore dimensions, sometimes to a fraction of an angstrom, is the dividing line between success and failure. Zeolites and zeolite-like molecular sieves (zeotypes) often fulfill the requirements of an ideal porous materials such as narrow pore size distribution and a readily tunable pore size in a wide range.
However, despite the many important commercial applications of zeolites where the occurrence of a well-defined micropore system is desired, there has been a persistent demand for crystalline mesoporous materials because of their potential applications as catalysts in processes for heavy oil cracking and catalytic conversion for large molecules, separation media or hosts for bulky molecules for advanced materials applications. Until recently, most mesoporous materials were amorphous and often with broad pore size distributions.
The breakthrough came in 1992 with the discovery of MCM (Mobil Composition of Matter)-41 mesoporous materials.5 These new (alumino) silicate materials, with welldefined pore sizes of about 2-10 nm, break past the pore-size constraint (<> 1000 m2 g-1) and narrow pore size distributions. Instead of using small organic molecules as the templating compound as in the case of zeolites, Mobil scientists employed long chain surfactant molecules as the structure-directing agent during the synthesis of these highly ordered materials.5 Rather than individual molecular directing agent participating in the ordering of the reagents of forming the porous materials, assemblies of molecules, dictated by solution energetics, are responsible for the formation of these pore systems. This supramolecular directing concept has led to a family of materials whose structure, composition, and pore size can be tailored during synthesis by variatio n of the reactant stoichiometry, nature of the surfactant molecule, or by post-synthesis functionalization techniques. For example, MCM-41 contains regular arrangements of hexagonal pores in a honeycomb arrangement; MCM-48 which exhibits cubic symmetry that can be envisaged as a gyroid minimal surface10,11 and MCM-50 is a layered silicate (Figure 1-1).
Following the initial announcement of MCM-41, there has been a surge in research activity, echoed by published literature (For recent reviews, see Reference 12). Interestingly, Di Renzo et al.13 recently discovered a patent from 1971 in which a synthesis procedure similar to the one used by the Mobil group was described as yielding "low-bulk density silica". The patent procedure was reproduced, and the product had all the features of a well-developed MCM-41 structure, as shown by transmission electron microscopy, X-ray diffraction and nitrogen adsorption. However, in the original patent only few of the remarkable properties of the materials were actually discovered. It was the Mobil scientists who really recognized the spectactular features of these ordered mesoporous oxides.
Scientists have postulated that the formation of these molecular sieve materials concerns the concepts of structural directing agent or template. Templating has been defined, in a general sense, as a process in which an organic species functions as a central structure about which oxide moieties organize into a crystalline lattice.14-16 Strictly speaking, a template is a structure (us ually organic) around which a material (often inorganic) nucleates and grows in a "skin- tight" fashion, so that upon the removal of the templating structure, its geometric and electronic characteristics are replicated in the (inorganic) materials.12f The above definition has also been elaborated to include the role of the organic molecules as:14
(1) space- filling species,
(2) structural directing agents,
(3) templates.
In the simplest case of space filling, the organic species merely serves to occupy a void about which the oxide crystallizes. Therefore, the same organic molecule can be used to synthesize a variety of structures or vice versa. Structural direction requires that a specific framework is formed from a unique organic compound. And this does not imply that resulting oxide structure mimics identically the form of the organic molecule. In true templating, however, in addition to the structural directing component, there is an intimate relationship between the oxide lattice and the organic form such that the synthesized lattice contains the organic "locked" into position. Thus, the lattice reflects the identical geometry of the organic molecule.
In M41S materials, a liquid crystal templating (LCT) mechanism has been proposed in which supramolecular assemblies of alkyltrimethylammonium surfactants serve as one component of the operative template for the formation of mesophase.5,17 This mechanism behind the composite mesophase formation is best, although not fully, understood for the synthesis under high pH conditions. Under these conditions, anionic silicate species, and cationic surfactant molecules, cooperatively organize to form hexagonal, lamellar or cubic structures. The earliest model relied on the liquid crystal templating of the surfactant.17 Here the composite hexagonal mesophase is suggested to form by condensation of silicate species around a preformed hexagonal surfactant array or by adsorption of silicate on organic arrays thus initiating the hexagonal ordering in both the surfactant template molecules and the final product. Surprisingly, hexagonal mesostructures may also be synthesized using a cationic surfactant such as cetyltrimethylammonium chloride under extremely acidic conditions where the silicate species are positively charged. The pore size in MCM-41 materials can be controlled from 1.5 to 10 nm by the hydrophobic alkyl chain length of ionic surfactants or with the aid of auxiliary organic compounds (i.e., trimethylbenzene) as spacers and fillers. Strong electrostatic interactions between the ionic surfactants and the inorganic species result in MCM-41 matrix with limited pore wall thickness of 0.8-1.3 nm that are influenced little by the pH conditions.
Neutral template molecules, such as primary amines (with carbon tail lengths between C8 and C18) have also been employed to direct mesoporosity in silicates.18 It is suggested that a neutral silicate would interact with micellar aggregates through hydrogen bonding between hydroxyl groups of hydrolyzed silicate species and the polar surfactant headgroups. The resultant framework structures are shown to have thicker silicate walls (i.e., 1.7-3.0 nm) and therefore enhanced thermal and hydrothermal stability compared to MCM-41. However, the final mesoporous materials (namely, HMS) has only short-range hexagonal ordering. The use of these surfactants, although offering advantages, does suffer from several drawbacks also associated with cationic surfactants, being expensive and toxic and therefore not optimal for large-scale production. Polyethylene oxide neutral templates (PEO, C11-15EO9-30), are a convenient alternative to primary amines being nontoxic, low cost and biodegradable.19 Named MSU, these PEO template materials again show only short-range order. However, later on, ordered mesoporous materials with large wall thickness values of 3.1-6.4 nm have been made using alkyl PEO oligomeric surfactants in strongly acidic media.19d
Other newly developed methods include the use of nonsurfactant templates20 and copolymer precursor pathways.21 The nonsurfactant templated synthesis discovered in our lab utilize small organic molecules such as D-glucose, D- fructose, dibenzoyl tartaric acid (DBTA) etc., as the structure-directing agent. By simply varying the concentration of the template molecules, mesoporous materials with different pore sizes (i.e., 2-6 nm) can be obtained. The template can be easily removed by washing with water or by solvent extraction. These products possess high surface areas of ~1000 m2 g-1, pore volumes as large as ~1.0 cm3 g-1, and narrow pore size distributions. In addition to low cost, environmental friendliness, and easy removal of templates, this new approach also provide many other advantages such as mild synthesis conditions and good compatibility with enzymes which has led to a novel technology for immobilization of enzymes and other bioactive substances.22
1.2.2. Non-Silica Periodic Mesostructured Materials
Stucky and co-workers first extended the surfactant templating strategy to the synthesis of non-silica based mesostructure, mainly metal oxides.23 It was found that for the formation of an mesophase to be successful, three conditions should be satisfied:
1) the inorganic precursor should have the ability to form polyanions or polycations allowing multidendate binding to the surfactant;
2) the polyanions or polycations should be able to condense into rigid walls;
3) a charge density matching between the surfactant and the inorganic species is necessary to control the formation of a particular phase.
The complex solution chemistry often observed for transition metal ions as a result of the multitude of possible coordination numbers and oxidation states has a pronounced influence on the observed composite mesoporous materials as expected.
Depending also on the surfactant concentration, lamellar, cubic and hexagonal mesostructure could be synthesized in the pH range of 6-7. However, the overwhelming majority of products were layered. Regardless of the composite symmetry, removal of the surfactant caused structural breakdown probably due to the lack of complete condensation of the inorganic framework. The first stable mesoporous transition metal oxide was prepared by controlled hydrolysis of titanium isopropoxide in the presence of tetradecylphosphates using acetylacetone as a chelating agent.24 Through this "modified sol- gel method", titania mesostructure (named TMS for "tech molecular sieves) with a narrow pore size distribution and a high surface area of about 200 m2 g-1 has been obtained. However, IR spectroscopy measurements revealed the presence of phosphate groups after calcinations at 623 K. Mesoporous niobium oxide (Nb-TMS) and tantalum oxide (Ta-TMS) have also been made through a ligand-assisted templating (LAT) path.25-27 In this approach, the surfactants were pretreated with the metal alkoxides in the absence of water to form metal- ligated surfactants. Upon addition to the alkoxide-surfactant solution, water actedas both a solvent and a reactant, to initiate surfactant self-assembly and alkoxide hydrolysis/condensation, respectively.
Wormlike mesoporous alumina have been prepared with surface areas greater than 400 m2 g-1 using PEO surfactants.19c Higher surface areas (i.e., 710 m2 g-1) were obtained by mixing aluminum alkoxides with carboxylic acids in low molecular weight alcohols in the presence of a controlled amount of water.28 A narrow pore size distribution centered around 20 Å was observed. However, the material showed no longrange order.
Zirconium oxide is an interesting material catalytically and much effort has been devoted to the generation of a mesoporous form. Studies showed that both Zrisopropoxide29 and zirconium sulfate30 can be used as the inorganic precursor, while the former seemed to result in more stable but less ordered materials. In the Zr(SO4)2 case, a hexagonal form was successfully stabilized by post-synthesis treatment with potassium phosphate at room temperature followed by air calcination at 623 K.30 Many other non-silica mesostructured systems have been studied extensively in the recent years. A list of successful synthesis includes V,31 W,31-33 Zr,28,29,34-36 Sn,37 Hf38 oxides, etc. However, it still remains a major challenge to stabilize their structure or to discover new or improved pathways for these mesoporous molecular sieves.
1.2.3. Organic-Inorganic Hybrid Mesoporous Materials
Besides the extension from silicate to non-silica mesoporous materials, one other important way of modifying the physical and chemical properties of mesoporous silica materials has been the incorporation of organic components, either on the silicate surface, inside the silicate wall, or trapped within the channels. Organic functionalization of these solids permits the tuning of surface properties (e.g., hydrophilicity, hydrophobicity, binding to guest molecules), alteration of the surface reactivity, protection of the surface from attack, and modification of the bulk properties of the materials, and at the same time stabilizing the materials towards hydrolysis. For example, mesoporous silica having thiol groups on the pore surface showed high adsorption efficiency for heavy metals such as Hg, Ag, and Cr ion.39 Sulfonic acid groups grafted mesoporous materials exhibited high catalytic activity for selective formation of bulky organic molecules.40
These hybrid materials are generally synthesized via two methods.41 The first is the post-synthesis grafting method in which pore wall surface of the pre-fabricated inorganic mesoporous materials is modified with organosilane compounds after the surfactant removal. The original structure of the mesoporous support is usually maintained after grafting. Another method is the direct co-condensation of a tetraalkoxysilane and one or more organoalkoxysilanes with Si-C bonds through sol-gel process.
Grafting of the mesopore surface with both passive17,42 (i.e., alkyl and phenyl) and reactive43 (i.e., amines, nitriles etc.) surface groups have been studied. The former can be used to tailor the accessible pore sizes, increase surface hydrophobic ity, while the latter permits further functionalizations. Multiple grafting has also been demonstrated. In order to minimize involvement of the external surface in reaction processes and to optimize selectivity, researchers have tried to graft external surface first through passive groups, before functionalizing the internal silanol groups.44 The "one-spot" cocondensation approach has been studied extensively. Co-condensation using ionic,45 neutral surfactant46 and non-surfactant templates47 have all been demonstrated. Each of the two functionalization methods has certain advantages. If uniform surface coverage with organic groups is desired in a single step, the direct method may be the first choice.
It also provides better control over the amount of organic groups incorporated in the structure. However, products obtained by post-synthesis grafting are often structurally better defined and hydrolytically more stable. Although pore sizes can be controlled to some extent by both methods, it is more easily achieved by grafting.
A recent development in this field has been the study of organic- inorganic species covalently bonded inside the mesoporous wall structure. The surfactant template synthesis of these materials use a precursor that has two trialkoxysilyl groups connected by an organic bridge.48-50 The new technique allows stoichiometric incorporation of organic groups in silicate networks, resulting in higher loading of organic functional groups than by grafting or direct synthesis method. One can envision further tunability of the mechanical, surface chemical, electronic, optical, or magnetic properties of the hybrid composite by introducing suitable functional groups into the walls.
1.3. Organic-Inorganic Hybrid/Nanocomposite Materials by the Sol-Gel Process
Organic-inorganic hybrid composites are one of the most important classes of synthetic engineering materials. By definition, a composite is a material composed of two or more physically distinct components, the intent of which is to achieve better properties than can be obtained by a single homogeneous materials.51 Aside from the intrinsic physical properties of the components, composite materials can also display special new properties as a result of the nature and degree of interfacial interaction between the two components. One of the frontiers in composites engineering is the development of viable methods for the efficient design and synthesis of organic-inorganic composites with nanometer-scale architecture. The term "nanocomposites" was proposed for the first time by Theng in 1970.52 According to Rustum Roy's definition,53 a nanocomposite is an interacting mixture of two phases, one of which is in the nanometer size range, typically, 1-20 nm in at least one dimension.
Organic and inorganic materials are usually quite different from each other in their properties. Inorganic materials such as glass and ceramics are hard but not impact resistant, that is, they are brittle, whereas organic polymer/oligomers are resilient. However, organic polymers generally suffer from some of the inherent drawbacks such as instability to heat and tendency of natural degradation upon aging. In general, inorganic species usually have good mechanical and thermal stability as well as optical properties. Organic moiety would provide flexibility, toughness, hydrophobicity and new electronic or optical properties. Organic-inorganic hybrid materials could then have desired combinations of the features of both organic and inorganic components. Because of the new and different properties of these nanocomposite materials that the traditional macroscale composites do not have, the preparation, characterization, and applications of organic/inorganic hybrid materials have become a fast expanding area of research in materials science. However, since the traditional processing conditions for inorganic materials usually involve high temperature, the survival of organic compounds is completely impossible. Thus, the mild-conditioned sol- gel process slowly established its popularity in making organic- inorganic hybrid materials. The sol- gel process, which is mainly based on inorganic polymerization reactions, is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and ceramics. Their unique low temperature synthesis condition allows it to be well adopted for the preparation of organic/inorganic hybrid materials and have proven to be effective. It brings both inorganic and organic components into intimate mixing that lead to new morphologies and unique properties. The extent of phase mixing can vary, but domain sizes are typically on the nanometer scale. Even though many of the network systems are comprised of components having very different refractive indexes, the resulting materials can often be prepared optically transparent due to the small scale lengths over which phase separation may exist. As a result, these composites can find applications in many fields which are far beyond the scope of application of traditional composite materials which tend to be opaque. Several different methods can be used in making these hybrid materials via the sol-gel process based on their macromolecular structures and phase connectivities:3
1. Hybrid networks synthesized by using low molecular weight organoalkoxysilanes as one or more of the precursors to introduce the Si-C bond;
2. Co-condensation of functionalized organic species with metal alkoxides and establishing covalent bonding;
3. Infiltration of preformed oxide gels with polymerizable organic monomers/polymer or vise versa, entrap organic molecules as guest within inorganic gel matrix (as host);
4. In situ formation of the inorganic species within a polymer matrix, especially for inorganic particles;
5. Hybrid networks can also be formed by interpenetrating networks and simultaneous formation of organic and inorganic phases.
The properties of a composite product depend not only on the properties of the individual components, but are also greatly influenced by such factors such as the phase's size, shape and interfacial properties. According to the nature of the interface between the organic and inorganic components, these composites can also be simply classified into two major families:54 16
(1) Class I---organic and inorganic components are linked through covalent bond;
(2) Class II---does not contain covalent bond between organic and inorganic components.
The scope of research and applications of these composite materials have ranged from inorganically modified organic polymers to inorganic glasses modified by organic compounds.55 Potential applications for these materials include: scratch and abrasiveresistant coatings;56 electrical and nonlinear optical (NLO) properties;57 adhesives and contact lens materials;58 reinforcement of elastomers and plastics;59 catalyst and porous supports, adsorbents;60 hybrid solid-state dye laser and chemical/biomedical sensor materials;61 photochromic hybrid material62 etc. To date, even though the number of commercial sol-gel hybrid products is comparatively small, but the promises of new technological uses are still high.
Asignatura: CRF
Fuente: idea.library.drexel.edu/bitstream/1860/39/1/feng_thesis.pdf
