Titanium- citric acid complexes

Figure 25.7 Model for the formation of mesostructured 7102 Si02 materials by the use of titanium-citric acid complexes as templates. (Adapted with permission from J. Phys, Chem. C 2009, 113, 9345 [8]. Copyright 2009, American Chemical Society).

An additional advantage of this synthetic approach is that the mesopores of silica restrict the size of the TiO species in the mixed oxide materials. Furthermore, Liu et al. were able to control the homogeneity of the mixed oxide materials by changing the pH value of the initial sol-gel mixture. pH values lower than the isoelectric point of sihca (2.0) lead to uniformly dispersed titania domains, whereas at higher pH values phase separation is observed due to the instability of the titanium-citric acid complexes. A similar approach has been reported by Oki et al. [56] for the synthesis of Ti02—Si02 mixed oxides by addition of various alk)d amines such as hexylamine, dodecylamine, and octadecylamine or even... 

FIGURE 5.2 Single crystal of water-soluble titanium complex ammonium salt (left) citric acid complex (right) lactic acid complex. 

Tile Eu(III)-citric acid complex is reduced at 1/2= -0.89 V SCE and can be used to determine Eu in rare earth phosphates which contain titanium (Ignatova, 1977). Eu(II) and Eu(III) also form complexes with ethylenediamine, inalonic acid, succinic acid, glutaric acid, etc., and yield well-defined waves suitable for analysis. 

Arima M., Kakihana M., Nakamura Y., Yashima M., Yoshimura M. Polymerized conplex route to barium titanate powders using barium-titanium mixed-metal citric acid complex. J. Am. Ceram Soc. 1996 79 2847-2856... 

A nonaqueous workup is also possible for those substrates that are water soluble. The procedure advanced by Sharpless serves to remove the titanium as its insoluble citrate complex. To the cold reaction mixture is added 30 mM of citric acid monohydrate dissolved in acetone/ diethyl ether (1 9) or 30 mM of citric acid dissolved in diethyl ether42. The cooling bath is removed and the mixture is stirred for 20 - 30 minutes then filtered through a bed of Celite. The filtrate is concentrated to yield the crude epoxy alcohol containing tartrate and rert-butyl hydroperoxide42. 

Hydroxycarboxylic acids, which include citric acid, malic acid, lactic acid, etc., are benign to the environment and very convenient for the solution processing. Moreover, since these reagents can form stable complexes with other cations, they rarely yield a precipitate. For several complexes single crystals of well-defined composition suitable for the X-ray structural analysis were isolated. Thus, these water-soluble titanium complexes of hydroxycarboxylic acids are promising precursors for the synthesis of ceramics from an aqueous solution and their industrial utilization is expected in the future. In this chapter we decribe the method of synthesis, structural analysis, and stability of these complexes. The examples of multicomponent oxide materials preparation using these compounds are presented. 

Ba Srj TiOj is synthesized, barium and strontium carbonates or acetates are dissolved in the aqueous solution containing water-soluble titanium complex. Although strontium salts can be dissolved in water relatively easily (there are numerous water-soluble strontium salts), strontium forms precipitate with citric acid that is hardly soluble in water. Therefore to utilize citratoperoxotitanium complex for the synthesis of Ba Srj. Ti03 is difhcult. In such a case, the com-plexing agent should be changed. 

As described above, titanium forms a stable complex with not only citric acid, but also lactic acid. Lactate ion does not form precipitate with strontium or barium. Therefore all of the titanium, barium, and strontium ions can be kept in an aqueous solution with use of lactic acid, that is, synthesis of Ba Srj. TiOj is possible. The ratio of the metallic elements can be arbitrarily changed, and thus all compositions of Ba Sri Ti03 can be synthesized. 

Simultaneous presence ofbarium and titanium in the solution of citric acid causes the effect that can not be described as the superposition of the solutions of barium citrate and titanium citrate. In addition to the familiar Ti-0 bonds and shift of carboxylic groups NMR peaks, the central carbon peak in the C-NMR spectra is displaced to higher frequency (Fang, 2001 Kakihana, 1999b), which means a deprotonation of-OH group and most likely formation of metal-oxygen bonds. This observation is particularly interesting if one takes into account p/Q = 10.96 and low pH of citric acid solution. Barium and titanium participate in the formation of the metal complex with Ba Ti CA ratio of... 

Kakihana et al. introduced a new complex, citratoperoxotitanate(IV), based on the titanium peroxo complex (Kakihana et al., 2001). This complex is synthesized hy adding citric acid into a titanium peroxo solution, where yellow ammonium salt ((NH4)8[TU(C6H407)4(02)4]-8H20) is obtained. This salt is water soluble (more than 30 g salt in 100 g water at room temperature), and stable in a wide range of pH. This... 

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