Zirconium alkoxides hydrolysis

Employing silicon alkoxides, the hydrolysis has to be catalyzed by the addition of an acid or a base, and an excess of water is often used. Employing zirconium alkoxides, the hydrolysis reaction proceeds much faster than the condensation so that the product is obtained as a precipitate rather than a gel. 

The zirconium alkoxides Zr(OR)4 are reported as inactive for the epoxidation of olefins under the conditions recommended with the titanium analogs. When synthesized by reaction of Zr(CH2CMe3)4 with silica, followed by hydrolysis or calcination, a solid as active as the related Ti-based catalyst is obtained. The low selectivity for the formation of the epoxide is related to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide.46... 

Randon et al. reported on an alternative approach for the preparation of zirconia monoliths [96]. The sol-gel process is initiated by hydrolysis of an ethanolic zirconium alkoxide solution, on addition... 

Peercy, P.S., Dosch, R.G., and Morosin, B., "Preparation and Structural Studies of the Hydrolysis Products of Titanium, Niobium and Zirconium Alkoxides," SAND76-O556, Sandia Laboratories, Albuquerque, NM (1976). 

It has been traditionally assumed that condensation is a much slower reaction than substitution of the alkoxide group by the hydroxide however, quite recendy it was shown that the rates of these processes are actually very close. Kinetic study of hydrolysis of titanium and zirconium alkoxides by means of a quick mixing technique with FIIR, SAXS, and conductivity measurements monitoring has shown that hydrolysis is a very quick reaction followed by condensation, which is also a very fast process and occurs after from 25 to 50% of alkoxide groups are substituted by hydroxides (under the experiment conditions this occurs in 80 milliseconds after the beginning of mixing of the reagents) [709]. 

As pointed out above, many studies on TOP "decomposition" emphasize the influence of water formed in a dehydration reaction of alcohol present either as solvent or as traces. Such a chain mechanism involving a first alcohol molecule producing one water molecule which, by the hydrolysis reaction, gives two additional alcohol molecules has been shown by Bradley and al. studying zirconium alkoxides [11]. Curiously, such a "hydrolytic" decomposition of titanium derivatives has not been taken into account in the most recently reported studies on CVD experiments from TOP [8-10]. 

Three classes of hybrid HPA are known to be stable to hydrolysis 1. Organometallic derivatives of the type RM (M = Si, Ge, Sn, Pb and R = alkyl or aryl). 2. Cyclopentadienyl-titaninm derivatives. 3. Zirconium alkoxide or phosphate derivatives, all of which are illustrated in Figure 2. We have tested phenyl model compounds of all of these for stability by boiling them in 6M HCl or H2O2 solution. This study showed that only PhP-O-HPA moieties are stable under conditions likely to be encountered in a fuel cell. Never the less we continue to study model compounds of the type RSi-O-HPA due to the large diversity of available ethoxy- and chloro- silanes. 

A further complication concerning the involvement of water in the decomposition reactions has been reported for the decomposition of tertiary alkoxide compounds of zirconium. Kinetic studies on the decomposition of Zr(0-f-Bu)4 at 200-250°C showed that decomposition occurs by a chain reaction mechanism involving the hydrolysis of the zirconium alkoxide by water produced by the dehydration of the tertiary alcohol (Eq. 5.19 and 5.20) [129]. The dehydration of the tertiary alcohol is surface catalyzed, and the overall decomposition to Zr02 is given in Eq. 5.21. 

The preparation of nanocomposite membranes by intra-membrane growth within a proton exchange membrane was first described by Mauritz et al. [45-47]. The then novelty of this approach and the breadth and depth of these studies warrant the following discussion of the results, which in many ways laid the foundation for future work in this area. This group made use of the hydrophilic ionic cluster regions of Nafion for confined, sulfonic acid group catalysed, hydrolysis/condensation reactions of impregnated alkoxides. Nafion membranes were first swollen in ethanol/water, then tetraethoxy-silane (or aluminium, titanium and zirconium alkoxides) permeated from one side of the membrane. In addition to the concentration profile of in-... 

Zirconia based fibers can be produced from aqueous sols of zirconia particles. Such sols are prepared, e.g., through the hydrolysis of a zirconium alkoxide, by the simplified overall following equations [87]. 

Zirconia aerogels were among the first to be prepared by controlled hydrolysis of zirconium alkoxides in nonaqueous media followed by SCD [18] (Chap. 1). Later this method became a standard and was extended to many other metal oxides [19, 20]. 

The effects of different synthesis parameters on the properties of zirconia aerogels prepared by sol-gel process have been investigated by many research groups. In fact, taking into account that the hydrolysis of zirconium alkoxides is very fast, it has been noted that the rate of the condensation process can be influenced by the concentration of an acid catalyst leading to zirconia aerogels with different textural properties. The amount of added water... 

Research works made until now on the sol-gel preparation of AI2O3, Ti02 and Zr02 (Y2O3, CaO or Ce02-stabilized) fibers are summarized in Table 18-2. As mentioned above, very carefully designed hydrolysis of titanium or zirconium alkoxide in alcohols... 

As was discussed for silicon precursors (Figure 1.2), closed structures (oxo clusters) can be formed as well by the condensation reactions, and the same remarks apply with regard to their importance in sol-gel processes. Figure 1.8 shows an example of a cluster obtained by hydrolysis of an organically substituted zirconium alkoxide derivative. 

Zirconia monoliths can be prepared from alkoxy precursors however, obtaining mechanically stable zirconia monolithic materials is a challenging task [8]. Zirconium alkoxides have faster hydrolysis rates compared with aluminiun or titanium alkoxides. This is due to larger positive partial charge of the zirconium atom, which enhances nucleophilic attacks on the zirconium atoms [52]. 

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