Monolithic Sol-Gel Materials

The physical and chemical principles of the sol-gel process were presented extensively in Ref. [3] in 1990 and an updated presentation is given in the current Sol-Gel Handbook 2014. Following the definition in Ref. [3] by Brinker and Scherer If the smallest dimension of the gel is greater than a few millimeters, the object is generally called a monolith, the common definition for sol-gel 

The Sol-Gel Handbook Synthesis, Characterization, and Applications, First Edition. 

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

Hydrolysis and polycondensation reactions usually occur simultaneously and the reaction rates depend on the type of precursor as well as on reaction conditions such as pH, temperature, and ionic strength [8]. Hydrolysis of alkoxy precursors (=M-OR , where M is the metal atom and R is the alkyl group) occurs by water due to electrophilic reaction in the presence of acid catalyst or by nucleophilic substitution of alkoxy groups in the presence of base catalyst [3,9]. The hydrolyzed precursors (=M-OH) can react either with alloy precursors (alcoxolation) or with other hydrolyzed precursors (oxolation). In both cases the mechanism is nucleophilic substitution or nucleophilic addition for which the result is polycondensation [3,10]. The reactions that describe the sol-gel process are as follows  

When liquid evaporates from the porous gels, the pore walls are subjected to a stress equal to the capillary pressure. The stress can cause gel shrinkage and collapse of the pore walls. Moreover, the material can be exposed to pressure gradients between the wet and dry parts of the pores. This can also cause cracking of the monolithic structure. The capillary pressure depends on the properties of the material and the solvent, and is expressed with the Young-Laplace equation [3]  

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Along this line, sol-gel materials based on Si02find increasing use in solid-phase assisted synthesis. The sol-gel synthesis creates well ordered porous glasses which retain a rigid and exposed surface area (300-1000 m g ) [56]. Gelation occurs after a sol is cast into a mold so that the monolithic samples can be tailored to a desired size or shape [57]. 

Hoveyda and co-workers immobilized an olefin metathesis catalyst on monolithic sol-gel and claimed that the catalytic material is easily recyclable. Barrett and co-workersprepared a recyclable boomerang polymer supported catalyst for olefin methathesis by grafting the preformed catalyst to a polystyrene... 

Sol-gel materials such as monolithic silicas or silica films are transparent well into the UV optical applications are possible. 

Other properties which have contributed to the attractiveness and versatility of the sol-gel doping approach are the chemical, photochemical and electrochemical inertness as well as the thermal stability of the matrix the ability to induce electrical conductivity16 the richness of ways to modify chemically the matrix and its surface as well as the above-mentioned controllability of matrix structural properties the enhanced stability of the entrapped molecule1,17 the ability of employing the chromatographic properties of the matrix for enhanced selectivity and sensitivity of reactions with the dopant4 the simplicity of the entrapment procedure the ability to obtain the doped sol-gel material in any desired shape (powders, monoliths, films, fibers) and the ability to miniaturize it18,19. 

The sol-gel process is the name given to a number of processes in which a solution, or sol, undergoes a sol-gel transition. In this broadest sense, the term sol-gel refers to the preparation of inorganic oxides by wet chemical methods, irrespective of final form product—monolith, crystalline, or amorphous (1). Using sol-gel materials for mechanical entrapment of enzymes permitted stabilization of the proteins, tertiary structure owing to the tight gel network (2). Moreover, the easy insertion of substituent groups into... 

The great interest in sol-gel chemistry arises not only because of the simplicity of the procedure required to generate the material and the ease of doping, but also for several technological reasons. It is straightforward to obtain a sol gel material in a particular shape, be it in the form of powders, monoliths, thick/thin films or fibers. 

L6pez T., G6mez R. Catalyst doped sol-gel materials. In Sol-Gel Optics Processing and Applications, Klein L.C., ed. Norwell, USA. Kluwer Academic Publishers, 1994, pp. 345-371 Lubda D., Cabrera K., Nakanishi K., Minakuchi H. Sol-Gel products news. Monolithic HPLC silica columns. J. Sol-Gel Sci. Technol. 2002 23 185-187 MacChesney J.B., Johnson D.W., Bhandarkar S., Bohrer M.P., Fleming J.W., Monberg., Trevor D.J. Optical fibers by a hybrid process using sol-gel silica over cladding tubes. J.Non-Cryst. Sol. 1998 226 232-238... 

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