Oxolation reaction

Figure 13.19 summarizes the reaction mechanism starting from Sn(OH) + or Sn(OH)e in acidic media in alkaline media. As in the case of Pd, Sn02 oxide is spontaneously formed by dehydration due to an internal oxolation reaction promoted by a strong polarization of the O-H bond of the hydroxide. Thermodynamically stable species with respect to pH are presented in Fig. 13.20. Various molecular cationic species with different hydroxylated levels are possible in an acidic medium, whereas only Sn(OH)g is expected for a basic pH.

Another way of obtaining suspensions of anisotropic mineral moieties is by the spontaneous condensation of dissolved molecular species. A typical example of this process is the synthesis of V2O5 ribbons by using chimie douce (soft chemistry) techniques (Fig. 6) [35,36]. At pH=2, V(V) species exist in an octahedral coordination with a V=0 bond pointing along the Oz axis, three V-OH bonds in the xOy plane, and two bonded H2O molecules to fill the coordination sphere. Beyond a concentration threshold of 10 mol 1 , these vanadate species spontaneously condense in the xOy plane by two different reactions respectively called olation and oxolation reactions (olation V-0H-l-V-0H2 V-0H-V-i-H20 oxola-tion V-OH-l-V-OH—>V-0-V-i-H20) to form ribbons 1 nm thick, about 25 nm... 

Fig. 6. a The molecular species that condense through olation and oxolation reactions into V2O5 ribbons b whose molecular structure is shown in c. (Reprinted from [4b], copyright (2000) from John Wiley and Sons)... 

We have previously underlined the possible role of olation and oxolation reactions in the formation of metal hydroxide/metal oxide nudei. In fmther steps of catalyst pretreatment, such nudei may actually be reduced to metal(O) particles without significant size changes thus, metal dispersion may be determined from the earliest stages of supported metal preparation , and particularly before the final reduction stage, a fad that is not often realized. 

Polyanions may be formed from oxo hydroxo species via oxolation reactions, followed by deprotonation. The initially formed oligomers are also called polyacids. It depends on the metal at which stage of oligomerization 6qh becomes positive. For example, polycondensation of V0(0H)3 ( oh = —0.09) stops at the stage of Vio022(OH)6 (6oh =+0.003), which is spontaneously deprotonated to [Vio028-x(OH)J (x = 0-2) in the pH range 2-6, i.e., 10 vanadium atoms are required to make 6qh positive. 

Near the isoelectric point, neutral precursors h = z) are able to condense indefinitely via olation and/or oxolation reactions to form metal hydroxo or oxo-hydroxo species, depending on When 5h2O<0, hydroxide products are isolated, and oxyhydroxides are formed as metastable intermediates to fully condensed metal oxides, MO /2, when 6h,o > 0. 

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... 

Polyanions are formed at rather high pH from oxo-hydroxo precursors via oxolation reactions, for example ... 

Subsequent oxolation reactions between chains convert unstable 2(OH)j bridges into stable 3(0), bridges forming double chains ... 

Further oxolation reactions between chains lead to the fibrous structures evident by TEM (Fig. 7). Electron and X-ray diffraction studies have shown that the fibers are actually flat ribbons —lOnrn wide and Inm thick [66]. These ribbons in turn are composed of fibrils —2.7 nm wide linked side by side. Since compact decavanadate species are formed by the proton exchange method, these polyions must restrueture to form the fibrous structures portrayed in Fig. 7. The mechanistie details of this restructuring process are currently not understood. 

The h = 1 [Fe(OH)(OH2)5] precursor undergoes dimerization via olation-oxolation reactions to nucleate y-FeOOH (lepidocrocite), a phase isomorphous with y-AIOOH (boehmite), comprising double chains of... 

Flynn [79] suggests that the polymer forms by oxolation reactions between chains of edge-shared Fe(0, OH, OH2)f, octahedra resulting in double chain structures of a or -FeOOH. (See Fig. 9.) The formation of y-FeOOH cannot proceed as simply from the double chains consistent with its formation only from low-molecular-weight precursors. 

This reaction is analogous to olation and oxolation reactions between hydrolyzed species. It involves, at pH PZCi Oxo or hydroxo ligands from the surface acting as nucleophiles within the coordination sphere of the adsorbed ion. This reaction is also responsible for the adsorption of mixed aquo-amino complexes of nickel. Washing silica suspensions which have adsorbed hexamino nickel(ll) complexes leads to the formation of tetra-amino complexes. The shift in the UV-visible absorption towards lower frequencies makes it possible to follow the level of replacement of NH3 by H2O, and then of water by SiO [4,10] ... 

Assuming tliat there are no labile groups within the coordination sphere of the reactants, the oxolation reaction mechanism is associative. The coordination of the transition state must increase by one unit before a leaving group can be released. 

Although qualitative, the proposed mechanism stresses that the formation of vanadium oxide does not stem from an oxolation reaction alone. This is also true for other elements [Sn(lV), Sb(V), W(VI), etc.). The formation of solid phases of metal oxides by precipitation always involves at least one olation step, because oxolation alone leads only to the formation of polyanipns (see Chapter 2). 

Elements such as Si(IV), Sn(IV), Ge(IV), Sb(V) and TefVI), for example, have sizes comparable with Mo(Vl) and W(V1). However, they cannot form tt bonds with oxygen (no terminal oxygen) since they lack accessible d orbitals. Some polyoxoanions formed under specific acid conditions may protonate upon increased acidification, and may increase their condensation by aggregation or oxolation between particles. This leads to large chains or planar structures, or even networks of tetrahedra (Si) or octahedra (Sn, Sb, Te). There is no short M-0 bond in the coordination polyhedron to hinder protonafion and condensation. This might be an opportune time to point out the case of P(V) the presence of the P=0 double bond decreases the electrophilic character of the cation and prevents its condensation in solution. In addition, if the oxolation reaction is the only one to occur in solution, condensation is always limited and causes the formation of polyacids. The formation of solid phases is usually the result of a double condensation process, oxolation and olation. 

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