Topochemical dehydration

M0O3 in the ReOj structure can be prepared by the dehydration of the hydrate [11]. WOj-l/SH O undergoes topochemical dehydration to yield different phases of WO3 as shown in Figure 10.1.4 [7]. 

Topochemical dehydration has been used for sometime to prepare new metastable solids (e.g. the synthesis of Ti Nb O, from HTiNbOj [12]). This strategy has been extended to perovskites [13]. Thus, upon heating of a solid acid with... 

The majority of cluster technetium compounds are subject to thermal decomposition topochemically (i.e. their decomposition reaction occurs in the solid phase), [H(H20)2]2 [Tc8Br4( -Br)8]Br2 being an exception. This compound melts before decomposition (at 610-620 °C), which is good evidence in favour of the molecular crystalline structure of its dehydrated form [Tc8Br4(/i-Br)8]Br2. ... 

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

In custom-designing materials with tailored properties, it is often necessary to s)m-thesize metastable phases that will be kinetically stable under the temperature and conditions of use. These phases are obtainable only through kinetic (chemical) control. In many cases, kinetic control has been achieved via the soft chemical low-temperature (e.g. electrochemical synthesis, sol-gel method) and/or topochemical routes (e.g. intercalation, ion exchange, dehydration reactions), since these routes use nuld synthetic conditions. It should be noted that not all soft chemical routes are topochemical. A reaction is said to be under topochemical control only if it follows the pathway of minimum atomic or molecular movement (Elizabe et al., 1997). Accordingly, topochemical reactions are those in which the lattice of the solid product shows one or a small number of... 

Soft-chemistry routes, which the French call chimie douce, are indeed desirable because they lead to novel prodncts, many of which are metastable and cannot otherwise be prepared. Soft-chemistry rentes essentially make nse of simple reactions such as intercalation, ion exchange, hydrolysis, dehydration and rednction that can be carried out at relatively low temperatures. The topochemical nature of certain... 

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