Nonaqueous synthesis approach

Figure 2.1 Upper part Schematic of the nonaqueous sol-gel synthesis approach to metal oxide nanoparticles using autoclave, oil bath, or microwave heating. Lower part TEM...

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

The kinetics and mechanism of synthesis and decomposition of macrocyclic compounds are regarded as one of the most important aspects in the chemistry of these compounds. The majority of papers concern metal ions complexing with preliminarily synthesized macrocyclic ligands and metal ion substitutions by other metal ions in the preliminarily prepared complexes. Template synthesis, the most promising approach to the directed preparation of macrocyclic compounds with desired structures [17], plays a still more decisive role in the chemistry of macrobicyclic complexes with encapsulated metal ion. However, the literature contains only scarce data on the kinetics and the mechanism of the template synthesis of macrocyclic compounds because of the difficulties encountered in experimental determinations of kinetic and thermodynamic parameters, such as low product yields, nonaqueous media, high temperatures, and side reactions. 

In the aqueous synthesis of multicomponent oxides, a common approach is to first prepare a coprecipitate that can then be calcined to produce the desired complex oxide as the final product. When the component metal ions exhibit widely different aqueous solubilities, effecting coprecipitation becomes a challenge. Chhabra et al. [109] overcome this difficulty in the case of the ferrite BaFei20i9 by using a nonaqueous microemulsion system based... 

A new class of water soluble polypyrrole has been prepared by selfdoping of the polymer. These self-doped polypyrroles can be prepared electrochemically or chemically, using various dopant anions covalently bound to the polymer backbone. The self-doped sulfonated polypyrrole is most commonly synthesized electrochemically in nonaqueous media. Electrochemical synthesis in aqueous media and chemical synthesis are not typically used, presumably due to issues with overoxidation. The postpolymerization modification of polypyrrole, in a manner similar to that used to form sulfonated polyaniline is rare [41]. The various synthetic approaches and properties of the polymer are discussed in the following sections. 

NiederbergerM., Bartl M.H., Stucky G.D. Benzyl alcohol and titanium tetrachloride—a versatile reaction systan for the nonaqueous and low-temperature preparation of crystalline and luminescent titania nanoparticles. Chem. Mate. 2002 14 4364-4370 Niederberger M., Bartl M.H., Stucky G.D. Benzyl alcohol and transition metal chlorides as a versatile reaction system for the nonaqueous and low-tenpraature synthesis of crystalline nano-objects with controlled dimensionaUly. J. Am. Chem. Soc. 2002 124 13642-13643 Parala H., Devi A., Bhakta R., Fischa- RA. Synthesis of nano-scale Ti02 particles by a non-hydrolytic approach. J. Mate. Chem. 2002 12 1625-1627 Park M., Komameni S., Choi Y. Effect of substituted alkyl groups on textural properties of OR-MOSILs. J. Mater. Sci. 1998 33 3817-3821... 

However, the above does not answer the main question how can one employ isolated enzymes for the preparation of surfactants In fact, the answer is simple Use hydrolytic enzymes in nonaqueous media. Indeed, many hydrolytic enzymes, such as lipases, proteases, and glycosidases, available in large quantities, are very robust and inexpensive, and do not require any cofactors to manifest their catalytic activity. As any other catalyst, enzymes cannot influence the equilibrium of a chemical reaction and therefore the removal of water from the reaction medium forces them to work in reverse, i.e., to synthesize a chemical bond rather than to break it. Consequently, there is a principal difference between microbial and enzymatic synthesis of surfactants regarding the type of enzymes involved and the reaction medium. The former is a biosynthetic process catalyzed by living microorganisms and as such dependent solely on their viability, whereas the latter is an organic synthesis whereby enzymes are used as substitutes for chemical catalysts. The two approaches are complementary not only in terms of the production methods but because the surfactant structures amenable to both methodologies are quite different. 

Hydrolyzable salts of metal ions are used for synthesis of corresponding oxides in colloidal form by their forced hydrolysis under hydrothermal conditions [322] or in high-boiling solvents (polyols) [323], Hydrolysis in nonaqueous solutions has been applied also to metal alkoxides [324] and diketonates [325], offering a convenient route to the uncapped nanoparticles. Synthesis of oxide nanocrystals has been directed to nonaqueous approaches [326-328] mostly inspired by the success of the synthesis of high quality semiconductor nanocrystals in nonaqueous media [329]. The quality of the nanocrystals yielded by these nonaqueous solution methods is generally better than that of the nanocrystals synthesized in aqueous solutions. 

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. 

If metal salts are heated in a solvent or solvent mixtures in the presence of thiourea, then metal sulfides are accessible. Based on this idea, thiourea has been widely used as a sulfur source in nonaqueous syntheses of ternary and quaternary metal chalcogenides [139-142]. Ai and Jiang presented the synthesis of hierarchical porous Cu2FeSnS4 hollow chain microspheres via a surfactant-assisted microwave-solvothermal approach using ben2yl alcohol [143]. Ionic liquids have also been used as solvents for the formation of metal sulfides MS (M = Cd, Zn, and Pb) by the reaction of thioacetamide with metal acetates in imidazolium-based ionic liquids [144]. 

The main problem of mechanistic studies is the broad variety of synthesis systems even within the nonaqueous sol-gel approach, which makes it very difficult to establish generally valid rules and concepts. Every precursor-solvent combination behaves differentiy and slight variations of the synthesis conditions might change the reaction mechanism. Accordingly, it is impossible to exhaustively cover the whole field. Instead, we present selected ex2unples from the recent literature, which, in our opinion, provide a representative overview of the activities in the area of nanoparticle formation mechanisms in nonaqueous environment. 

The application of microwave radiations for the preparation of metal oxides (under aqueous and nonaqueous conditions) and polymer/ZnO nanocomposites, has been shown to be a versatile approach to the design novel morphologies of nanoparticles. Particularly the faster reaction rates (shorter reaction times), better product yields and the possibility to automatically combine different experimental parameters makes microwave-assisted synthesis suitable for the studies of the influences of the reaction conditions on the morphology and sizes of ZnO particles. 

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