Nonhydrolytic sol-gel reactions

Nanocrystalline anatase TiOj photocatalysts prepared via a facile low temperature nonhydrolytic sol-gel reaction of TiCl4 and benzyl alcohol. 76 (2007)... 

Many different metal oxide nanoparticles have been produced in the last couple of years applying nonhydrolytic sol-gel reactions. One of the most used methods is the so-called benzyl alcohol route, in which various metal oxide precursors are... 

In contrast to conventional sol-gel processes, which are based on the hydrolysis and condensation of metal precursors in an aqueous alcohol-mixed solvent, the nonhydrolytic sol-gel reaction proceeds by reactions of the metal precursors in organic media [16]. This nonhydrolytic sol-gel process has unique advantages... 

Nonhydrolytic sol-gel synthesis is a more recent variation on the sol-gel method. A metal halide is reacted with an oxygen derivative of the metal (Eq. 2-234). The reaction is catalyzed by Lewis acids such as FeCl3 and alkyl halide is a by-product [Hay and Raval, 2001], The reaction has been used to produce alumina (A1203) and titania (Ti02) as well as silica [Corriu and Leclercq, 1996]. 

This method is closely related to the nonhydrolytic sol-gel method. For example, titania is prepared by etherolysis/condensation of TiCl4 by diisopropyl ether (Equation 2.4) or by direct condensation between TiCl4 and Ti(0- Pr)4 (Equation 2.5). Detailed chemistry of the reaction was examined by means of nuclear magnetic resonance (NMR), and it has been reported that the tme precursors are titanium chloroisopropoxides in equilibrium through fast ligand exchange reactions. A variety of metal oxides, " nomnetal oxides," multicomponent oxides" " were studied, and the nonhydrolytic sol-gel method was surveyed by Vioux and Leclercq. ... 

A relatively new nonhydrolytic sol-gel method, based on the reaction between alkoxides and halides of Al, Si, Ti, etc., or between metal halides and oxygen containing organic compounds (ethers, aldehydes, ketones, etc.), was developed by Vioux and coworkers [133-138]. Structural investigations of the obtained material using Al NMR have revealed the presence of Al Al, and metastable Al sites. 

Both stages are controlled by condensation chemistry that can include, as a first step, hydrolysis of hydrated metal ions or metal alkoxide molecules [6, 7] (hydrolytic sol-gel processing). The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. The hydroxylated species for further condensation can be formed also by a non-hydrolytic route, that is, by reactions between metal chlorides and alcohols with electron-donor substituents [8]. The nonhydrolytic sol-gel processing may... 

Another class of reactions leading to polycondensation of oxygenmetal precursors is the so-caUed aprotic condensation that proceeds during nonhydrolytic sol-gel processing where water is not required for precursor activation and not produced by condensation [9, 10]. It includes reactions between alkoxides of dissimilar metals differing in the polarity of M—O bonds, or of alkoxides with metal esters and metal chlorides. The condensation rate is strongly dependent on the OR/M ratio in mixed precursors, as well as on temperature, the presence of catalyst and structure of the R derivative. 

Joo, J., Kwon, S.G., Yu, T., Cho, M., Lee, J., Yoon, J. and Hyeon, T. (2005) Large-scale synthesis of Ti02 nanorods via nonhydrolytic sol-gel ester elimination reaction and their application to photocatalytic inactivation of E. coli. The Journal of Physical Chemistry E, 109, 15297-302. 

After several decades of intense research on the development of synthesis routes, it is nowadays possible to prepare nanoparticles with amazing structural, compositional, and morphological sophistication. But in comparison to the highly advanced synthesis know-how, knowledge about mechanistic aspects of nanoparticle formation has not yet reached the same leveL The reasons for this are not lack of interest or research efforts, but may be foimd in both the complexity of the processes involved in nanoparticle formation and the difficulty to monitor the reactions from the dissolution of the precursor to the formation of the final nanocrystalline product However, in the case of nonhydrolytic sol-gel processes, chemical reaction mechanisms are relatively well established. The reaction rates are typically slower than those in aqueous systems due to the moderate reactivity of the C—O bond (which plays the major role in nonhydrolytic reactions in contrast to the O—H bond in aqueous systems), and it is relatively straightforward to investigate the organic reactions that are correlated to nanoparticle formation and thus provide information about possible formation mechanisms [5]. 

Nonhydrolytic sol-gel routes are not necessarily completely water-free. Although the initial reaction mixture might be anhydrous, specific organic reactions are able to produce water in situ (e.g., aldol condensation and esterification reactions), rendering the system in principle hydrolytic. Therefore, nonhydrolytic sol-gel processes are often also called nonaqueous. But even in the absence of water, it is possible to have hydroxylation reactions. 

There are only a few reports on nonaqueous sol-gel process applied for the formation of silica. The published literature describes the formation of monolithic silica structures [25], mixed silica metal oxide compounds [26], and the generation of a silica phase within a polymer matrix [27]. There are no reports that describe the synthesis of silica nanoparticles via the nonhydrolytic sol-gel process. One of the inherent problems might be that this type of reaction does not generate charged silica surfaces, which are usually required for the stabilization of the particles. Contrarily, a large variety of binary and ternary metal oxides can be formed. 

Since the initial reaction step represents a hydrolysis of the metal alkoxide bond, it may be defined as hydrolytic sol-gel synthesis in order to point out the similarity but also distinct difference to Hhefluorolytic sol-gel synthesis that will be reflected in more detail here. As a matter of fact, both the hydrolytic and the nonhydrolytic sol-gel syntheses result in the formation of M-X-M (X O or F) bridges that - when properly performed - results in the formation of nanoscopic particles. 

Alkoxide sol-gel precursors can be converted to the same hydroxylated activated substances by nonhydrolytic routes. The thermal intermolecular hydroxylation was used as a first step for gelation of Zr02 at 200-300°C in a conventional reactor [23] or inside the channels of mesostmctured SBA-15 siKca [24]. Partially hydroxylated alkoxides can be synthesized by reactions between metal chlorides and alcohols with electron-donor substituents or reactions between basic alkoxides and carbonyl compounds like ketones [9]. 

Noteworthy features of this method are that nonhydrated oxides without residual hydroxo groups are obtained, due to the aprotic conditions, and that in bimetallic systems the metals M and M have an alternate order (no phase separation), due to the reaction mechanism (Eq. (1.7)). A limitation of nonhydrolytic processes is that the M/M ratio is not freely selectable if fully condensed products are targeted. Eor this reason, sol-gel processes of mixed metal systems are sometimes initiated by nonhydrolytic reactions (to obtain a high homogeneity) and then completed by hydrolytic reactions (to obtain complete hydrolysis and condensation). 

The aim of this chapter is to highlight the contribution of the nonaqueous sol-gel approach not only toward nanoparticle synthesis, but also for the mechanistic understanding of the organic and inorganic reactions involved in nanoparticle formation. In addition, we provide selected examples on the assembly of such nonhydrolytically prepared nanoparticles into one-, two-, three-dimensional structures and their application in selected fields of technology. 

Figure 26.2 (a) Aligned hollow barium ferrite nanoparticles prepared by nonhydrolytic sol-fibers prepared by aqueous sol-gel route. gel reaction coated with oleic acid, (c) Mag-... 

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