Professor Pagliaro is a research chemist and science methodology professor at Italy’s National Research Council based in Palermo where he leads a research group collaborating with researchers of 10 countries and the new Institute for Scientific Methodology. His research interests lie at the interface of materials science, chemistry and biology. Reported in 60 scientific papers, book chapters and several patents, his achievements include chemical technologies now on the market.

Silica-Based Materials for Advanced Chemical Applications

By Mario Pagliaro

The Royal Society of Chemistry

Copyright © 2009 Mario Pagliaro
All rights reserved.
ISBN: 978-1-84755-898-5

Contents

Acknowledgements, xii,
About the Author, xiv,
Chapter 1 Functionalized Silicas: the Principles,
Chapter 2 Controlled Release,
Chapter 3 Purification and Synthesis,
Chapter 4 Coatings,
Chapter 5 Catalysis,
Chapter 6 Sensing,
Chapter 7 Hybrid Silica-Based Composites,
Chapter 8 Strategic Aspects of Functional Silicas,
Subject Index, 187,

CHAPTER 1

Functionalized Silicas: the Principles

1.1 Functionalized Silicas

The formation of a sol–gel functional silica gel takes place by hydrolytic polycondensation of suitable precursors in the presence of a dopant in solution. An example is given in the following (unbalanced) equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)

All of the oxygen in the final solid comes from the water molecules which cause hydrolysis of the alkoxide. In general, the hydrolysis process enables control of the monomer [right arrow] oligomer [right arrow] sol [right arrow] gel [right arrow] xerogel (dry gel) transition at a molecular level. The values of m, n and p are in fact dictated by several factors, including the concentration of H+ or OH- employed as catalysts, co-solvent, the existence of additives like surfactants, the water:silane ratio (=r), temperature, drying method and even the size and shape of the final product that can be obtained as thin film, powder, capillary, monolith, etc. Up to the xerogel stage, the low temperatures employed in the process enable one to close the traditional gap between organic and ceramics chemistry, and the chemistry and physics of organic molecules becomes applicable within ceramic matrices.

In general, these oxides show excellent optical quality including high transparency in the visible region that allows fluorescence as well as charge separation processes. Applications are numerous and range from highly sensitive photochemical sensors to photochromic glasses. The enormous versatility of the sol–gel process exemplified in the choice of process parameters affords a potentially enormous class of doped materials, either as glassy solids (amorphous or periodic) or as crystalline solids in which the host–guest interaction can be tailored.

Doped sol–gel silica oxides (Figure 1.1), and organically modified silicates (ORMOSIL) in particular, are thus used in a number of impressive applications that range from tailored organic light-emitting diodes (OLEDs) of enhanced durability and efficiency to promising efficient delivery of genes for gene therapy and fast drug assessment for toxicity; from self-ordered silica helices to highly sensitive photo-chemical oxygen sensors; and to “biochemical reactors” made up of entrapped enzymes, whole cells and even bacteria. In brief, doped ORMOSIL are functional materials with a multifaceted and exceptional chemistry, which are repeating the revolution that plastics caused in the 1940s and eventually finding a number of commercial, practical applications.

Amongst other advantages, these materials offer the opportunity to utilize in a positive way geometric imperfections. In other words, similar to what happens in nature, complicated structures can be constructed with the sol–gel process allowing “correlations and disorder to compete and to come to terms with each other through an optimal solution”. What are thus the physico-chemical bases originating the ORMOSIL’s chemistry superior performance in so many applications? How, furthermore, shape and structural effects in silica-based funtional materials are capable to dictate function?

1.2 Shapes Dictating Function

In a variety of powerful functional silica-based materials shape controls function and utility. Large efforts in contemporary research in materials science and biology are aimed to prepare materials with functionally powerful shapes, based on the understanding of the constructional processes that give rise to complex inorganic structures under the mild, wet conditions typical of biological processes. One general finding of these studies is that porosity is a fundamental part of any nanos-tructured materials that does chemistry, as the void phase – i.e., nothing – ensures both accessibility, dispersion and effective confinement of any entrapped molecules. To paraphrase Davis, beyond the atoms and molecules that define the porous space, the challenge in the field of porous materials aims to control their shape. In other words, void space and deliberate disorder are used as design components, as disorder and geometrical imperfection of solid structures have long been known for their unique relevance to heterogeneous chemical processes. Eventually, the overall objective is to develop what has been called “a chemistry of form” in the laboratory.

Silica-based materials obtained by the sol–gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol–gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic poly-condensation at room temperature. Called “soft chemitry” or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin.

These and several other researchers extended the methodology with the aim to widen functionality, using dopant molecules and silicon precursors derivatized with organic moities giving place to a vast class of hybrid organic-inorganic organosilica nanocomposites capable to meet numerous, advanced requirements in fields as diverse as catalysis, chromatography, surface coating, sensing, drug release and biotechnology. In general, from periodic mesoporous organosilicas (PMO) to Brinker’s evaporation-induced self-assembly (EISA), sol–gel processing is coupled to molecular self-assembly as a simple, general means to prepare porous and composite nanostructures. Hence, by generalizing into chemistry the biominerals growth principles elucidated in the late 1910s, one may recognise how organics, and especially soft matter (lyotopic mesophases, foams, emulsions and beyond), are used to template all types of porous materials (Scheme 1.1).

Such emerging approach to functional materials has been named by Ozin “nanochemistry”, and refers to a basic chemical strategy for making nanomaterials using molecular or nanometre scale building blocks (with a wide range of shapes, compositions and surface functionalities) that are further chemically processed to organize into structures serving as tailored functional materials. Indeed, solid state synthesis strategies in materials preparation are rapidly being supplanted by molecular methodologies, particularly the self-assembly of materials with structures that mimic the complexity of those observed in Nature. Almost inevitably, then, concepts such as anisotropy or symmetry become key parameters when considering “form” effects on the chemistry of these functional materials.

1.3 The Nature of Sol–Gel Entrapment

In 1995, commenting on the physical nature of the sol–gel entrapment of molecules in porous oxides, the inventor of sol–gel doped materials (first reported in the Journal of Physical Chemistry ten years before)emphasized how it was “really remarkable to see how many applications of the entrapment have been reported, without fully understanding the picture at molecular level”.

Today of course this understanding has evolved and we have a broader picture of the factors governing the chemical behaviour of these materials, whose mild preparation conditions were soon shown to be compatible with the effective entrapment of biomolecules (with no loss, and often with enhancement, of biological activity) opening the route to the merger of chemistry, biology and materials science (Figure 1.2).

Encapsulation of molecules into the inner porosity of a sol–gel matrix affords unprecedented molecular dispersion in a solid phase. The question of dopant aggregation in sol–gel cages emerged in 1984 in the very first report of molecular sol–gel entrapment describing the photophysical behaviour of Rhodamine 6G confined in a SiO2 matrix: whereas it was originally assumed that single-molecule caging was taking place, it was later established that at the high concentrations (of the order of several mmolg-1) typical of doped sol–gel applications some dopant aggregation was actually occurring.

The physical and chemical properties of the entrapped dopants are generally retained. Yet, the efficient isolation of one molecule from another and the active role played by the sol–gel cage, for instance in dictating accessibility, gives place to a vast new chemistry and physics of sol–gel entrapped molecules which largely encompasses and goes beyond traditional solution chemistry.

Thus, for example, new optical applications become possible since entrapped excited molecules cannot diffuse (giving place to the thermal energy dissipation typical of molecules in the liquid phase) such as in the case of photoinduced electron transfer in SiO2-co-entrapped pyrene and methylviologen. Or, in catalysis, much higher selectivities are achieved in delicate organic syntheses as the reactants approaching the entrapped catalyst are forced to assume preferred configurations. In general, confinement of a molecule in solid microporous cages of a solid modifies the electron energy of the molecule, altering in particular the frontier molecular orbital energy. Moreover, entrapped molecules interact with the matrix and its surface by covalent or by non-covalent interactions (van der Waals interaction, π-stacking, electrostatic attractions and hydrogen bonding) depending on the specific structure of the matrix and of the dopant molecule and on the (chemical or physical) nature of the entrapment. For example, dyes physically encapsulated in SiO2 glasses normally show a red shift in the positions of the absorption and emission spectra due to interaction of the dye molecules with the internal surface of the porous matrix.

Typically, at dopant concentrations > 10-3 M molecular aggregation starts to occur as shown for example by fluorescence studies of silica-entrapped rhodamines (rhodamines tend to form fluorescent aggregates at the adsorbed state; Figure 1.3). Hence, whilst at low concentration neither the absorption nor the excitation spectra show signs of aggregation, the excitation spectra of xerogels of increasing Sulforhodamine B (SB) concentration clearly show (Figure 1.4) the formation of fluorescent J-dimers.

High dopant loads (for instance, starting from a sol molar ratio composition TEOS:PhTES:dopant = 1:1:0.4 (TEOS, tetraethylortho-silicate; PhTES, phenyltriethoxysilicate)) are useful in photophysical applications where enhanced absorption is sought such as in the case of the UV protective coatings made of transparent phenyl-modified silica films doped with entrapped 2,2-dihydroxy-4-methoxybenzophenone protecting organic materials from light damage. In the latter case, the use of an organically modified silica matrix enhances the solubility of the UV absorber in the matrix and allows the preparation of highly loaded coatings. We recall here briefly that in SiO2 samples prepared from TEOS or tetramethylorthosilicate (TMOS), the surfaces of the pores in the resulting matrix consist mainly of uncondensed OH groups, which confer a very polar environment on the pore (57.9 kcalmol-1 in the Reichardt ET(30) scale). Incorporation of R groups into the structure dramatically decreases the cage polarity with large organic groups hindering further the influence of the residual OH groups at the pore surfaces. The organic groups are located at the cage’s interface, with profound consequences for the homogeneity of the entrapment and the chemical reactivity of the resulting material. The latter is indeed a property that has large effect on the ability of doped sol–gel materials to work as photochemical sensors or efficient catalysts. This is immediately revealed by the first-order decay profiles of entrapped dyes (Figure 1.5) as nonlinear kinetic plots arise when the dopant chromophore report simultaneously from more than one microenvironment that exhibit different chemical properties.

Furthermore, silica-entrapped molecules are physically and chemically protected. For example, organic fluorophores encapsulated via a sol–gel in ORMOSIL nanoparticles (20–30 nm in diameter) become 20 times brighter and more photostable than their constituent fluoro-phore due to the cage protecting the fluorophore from bleaching due to oxygen dissolved in the solvent.

Figure 1.6 shows indeed evidence that the silica nanoparticles are largely impermeable to solvent as the spectra show little spectral red shift in the excitation and emission spectra upon solvent exchange from ethanol to water.

Such good protection afforded by the sol–gel cage has tremendous consequences for biochemical applications using entrapped enzymes. For example, by entrapment in (surfactant-modified) silica sol–gel matrices, alkaline phosphatase remains functioning in extreme acidic environments, and acid phosphatase works smoothly in extreme alkaline environments. This is due to the unique fact that large pH changes in very small local environments — such as the free space between the outer surface of the protein and the silica surface of the cage — actually mean very small variations in the actual number of protons (Figure 1.7).

Supposing that the water layer is a small reservoir of 100 water molecules, and that the external pH is 0, then the hydronium ions penetrate that reservoir until equilibrium is reached and a nominal “pH = 0” is also obtained in the layer’s volume. From the point of view of the protein, this means that the protein gets protonated by only two protons (“pH = 0” means 2 moles H3O+ for each 100 moles water) which are enough to compensate for the extreme pH gradient while clearly posing no stress at all for the encapsulated protein.

The effects of the hydrophilicity–lipophilicity balance (HLB) of the sol–gel matrix on the chemical reactivity of the resulting material are large and mostly due to the enhanced cage flexibility. Referring to the photophysical behaviour of ORMOSIL-entrapped naphthopyrans (Figure 1.8), the shape and larger size of the isobutyl groups is also responsible for the increased flexibility as compared with methyl and phenyl groups, which facilitates the movement of the photochromic molecules inside the pore resulting in faster isomerization kinetics.

Getting back to photochemistry, photochemical reactions are kinetically controlled conversions ubiquitous in nature where phenomena far from equilibrium are the rule, rather than the exception. They are generally categorized into two groups: those from equilibrated excited molecules (with reactive species with lifetimes usually in nanoseconds or microseconds); and those from short-lived unequilibrated molecules (with short-lived vibrationally excited species) in which reactivity is relatively insensitive to minor environmental perturbation.

In the early 1960s it became evident that the reaction environment had an important role in dictating the course of photochemical conversions acting on the course of the relaxation processes and stabilizing photo-products. A constrained medium such as that of a porous matrix or a micelle provides the restricted environment to stop any bimolecular processes that could lead to degradation of products. These effects, however, are subtle. For instance, confinement of a molecule within a host instead of leading to inhibition of reactions of the trapped substrate often results in enhanced reactivity and selectivity because confinement does not mean steric inhibition of all motions of the entrapped host molecule which may eventually enjoy less restriction of some motions than in common solvents.

Remarkably analogous findings have been established in recent years from the study of a number of catalytic species entrapped in sol–gel glasses. In particular, molecular entrapment in hybrid organic–inorganic ceramic matrices such as organically modified silicates resulted in enhanced reactivity to transition metal, organo- and enzymatic catalysts, providing clear examples of heterogeneous catalysts in which the solid organic–inorganic surface participates actively in the reaction mechanism.

For example, consider organic reactions in water. Beyond a general methodology for carrying out catalytic conversions in H2O mediated by doped ORMOSIL in the presence of a modest amount of surfactant, another recent method for the waste-free oxidation of alcohol affords high yields of commercially valued carbonyl compounds in water with complete selectivity and remarkable stability.

The method uses a simple electrode made of a thin film of sol–gel organosilica doped with nitroxyl radicals deposited on the surface of an indium tin oxide (ITO) electrode. Thus, whereas in water benzyl alcohol is rapidly oxidized to benzoic acid, the use of the hydrophobic sol–gel molecular electrode TEMPO@DE affords benzaldehyde only (Figure 1.9), with an unprecedented purity, which is highly desirable for the fragrance and pharmaceutical industries where this aromatic aldehyde is employed in large amounts.

1.4 The Nature of the Sol–Gel Cage

Normally, the organic groups of ORMOSIL are located at the cage interface. The hydrolysis of organosilanes is slower compared to fully hydrolysable silicon alkoxides, and the slowly generated R-Si(OH)3 monomers rapidly condense in micellar-like structures typical of the very early stages of the sol–gel process. These hydrolysed monomers tend to arrange themselves with the polar -Si(OH)3 head groups at the front of the growing sol–gel material, and the hydrophobic non-poly-merizable residue R orientated away from the water–alcohol solvent interfacial (strongly hydrogen bonding).

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