Multicomponent gels synthesis

It is certainly beyond the scope of this book to discuss in any detail the solution chemistry pertinent to the large number of multicomponent silicate systems described in the literature. However, it is necessary to address at the minimum the synthetic strategies employed in multicomponent gel synthesis. In aqueous systems coprecipitation is so commonly used that it warrants no further discussion here. In alkoxide systems, there are two general approaches 1) hydrolysis of mixed-alkoxide or metal organic precursors and 2) sequential addition of alkoxides to partially hydrolyzed precursors. 

Hydrolysis and condensation rates depend on the molecular structure of metal alkoxides and alkoxide precursors have to be chosen as a function of the desired material final product. In the case of Ti02, for instance, monomeric precursors such as Ti(OPF)4, in which Ti is fourfold coordinated, react very quickly with water leading to the uncontrolled precipitation of polydispersed Ti02. The reaction is much slower with oligomeric precursors such as [Ti(OEt)4] in which Ti has a higher coordination number. Spherical monodispersed Ti02 powders can be produced via the controlled hydrolysis of diluted solutions of Ti(OEt)4 in EtOH. On the contrary, monomeric precursors are more convenient for the sol-gel synthesis of multicomponent oxides. The perovskite phase BaTiOs is formed upon heating around 800 °C when [Ti(OEt)4] is used as a precursor. This temperature decreases down to 600 °C with the monomeric precursor Ti(OPT)4 which favors the formation of Ti-O-Ba bonds. ... 

An enormous number of possible multicomponent systems, makes it impossible to di.scuss them here in detail. However it should be mentioned that there are two general approaches in the sol-gel synthesis of multicomponent systems hydrolysis of mixed-alkoxide or metal organic precursors, and sequential addition of alkoxides to partially hydroly/.ed precursors (I9j. The first method was invented by Dislich [20], based on the idea to form complex via alcolation that contains all metals in proper stoichiometry. The second approach which is based on the sequential addition of alkoxides in the reverse order of their respective reactivities, was introduced by Thomas [18] and Yoldas [21,22]. The idea is that the newly added unhyhdrolyzed alkoxide will condense with partially hydrolyzed sites on the polymeric species fonned by the preceding hydrolysis and condensation, rather than reacting with themselves. The homogeneity of the product will depend on the size of the polymeric species to which the last component is added. T)ie most widely studied multicomponent systems are AI O,-SiO.. B.O,-SiO and TiO-SiO. ... 

Alkoxide groups can also bridge two different metal atoms, leading to the formation of heteroalkoxides, which are often used a precursors for the sol-gel synthesis of multicomponent ceramics [15], Condensation can even go one step further, leading to p,-oxo bridges via ether elimination. 

Strongly complexing P-diketones are currently employed to stabilize highly reactive metal alkoxides, such as W(OEt)6 [25]. Aluminum sec-butoxide, modified by ethylacetoacetate (etac), appears to be quite attractive as a precursor for the sol-gel synthesis of multicomponent ceramics, such as cordierite. Al(OsBu)2(etac) is more soluble and less reactive than the corresponding alkoxide [26]. 

The classic procedure for the sol-gel synthesis of high purity glasses and ceramics starts from a mixture of alkoxides and water in a common solvent, usually an alcohol. Tarasevich [17] proposed an alternative route to obtain silica gels, in the absence of a solvent, by means of ultrasonic activation. This procedure has been extended to the preparation of multicomponent systems [18]. Materials obtained in this unconventional way are called sonogels. 

In the field of sol-gel derived materials, multicomponent gel evolves, after densification process and thermal annealing, to mixed oxide. However, in the synthesis of multicomponent... 

Four different sol-gel routes can be used to prepare various kinds of sulfide materials. Sol-gel synthesis of sulfides usually follows colloidal chemical processing, except route A. It is very important to control the sizes of colloidal particles. As-S and Ge-S glass films could be prepared by sol-gel processing for planar waveguides, for IR optical applications. The sol-gel synthesis of multicomponent sulfide glasses needs further study, perhaps based on the Ge-S system. Route D can also be used for synthesis of multicomponent sulfides. 

It should be noted that the sol-gel process is particularly attractive for the synthesis of multicomponent particles with binary or ternary compositions using double alkoxides (two metals in one molecule) or mixed alkoxides (with mixed metaloxane bonds between two metals). Atomic homogeneity is not easily achieved by coprecipitating colloidal hydroxides from a mixture of salt solutions, since it is difficult to constmct double metaloxane bonds from metal salt. Hybrid... 

The synthesis of a polymer by a free-radicjd or polycondensation mechanism usually leads to a product with a rather wide distribution of molecular weights. These polydisperse samples must be considered as multicomponent mixtures rather than as chemically pure substances. Using analytical methods like gel permeation chromatography (GPC) it is possible to analyze such mixtures. 

A. Douy, Polyacrylamide gel an efficient tool for easy synthesis of multicomponent oxide precursors of ceramics and glasses. Int. J. Inorg. Mater. 3(7), 699-707 (2001). 

Ozin and coworkers recently extended the supra-molecular I S+ assembly into the synthesis of binary mesoporous yttrium oxide-stabilized-zirconium oxide materials.These materials were synthesized by a modified sol-gel method under basic conditions, where zirconium ethoxide and yttrium acetate were used as the precursors for the transition metal oxides, and CTAB was used to form the supramolecular templates. The use of ethylene glycol with coordinating capability as a cosolvent may play a role in controlling the hydrolysis rate and solubility of zirconium(IV) and yttrium(III). This synthesis strategy is similar to that of so-called polymerizable-complex method, which was widely used to prepare multicomponent single-phase oxides. The yttrium content in these binary materials can be tuned from 12-56 wt%, and no phase segregation of yttrium and zirconium oxides was observed. These materials could be applied in designing new solid oxide fuel-cell electrode materials. 

The synthesis of xanthene derivatives using silica-supported add was also investigated by Karimi et al. (2011). They reported a facile procedure for the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[fl]xanthen-ll-one derivatives through a multicomponent reaction using Caro s add-siUca gel (CA-Si02) as a nontoxic, inexpensive, and easily obtained catalyst. Benzaldehyde, P-naphthol 8, and dimedone (12) were allowed to react at different temperatures and with different amounts of catalyst to find the optimal reaction conditions for the preparation of xanthenone (11) (Scheme 1.8). 

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