Dehydration, topotactic

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

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 ... 

Figure 8.2 Topotactic dehydration of Mo03-2H20 (a) (101) projection of M0O3 2H2O (b) (001) projection of M0O3 H2O and (c) (100) projection of M0O3. Open circles denote interlayer water and filled circles, coordinated water. (After GUnter, 1972.)...

A recent kinetic study [108] of the overall dehydration rates of KA1(S04)2.12H20 and KCr(S04)2.12H20 showed that measured ur-time data were well described by the Avrami-Erofeev equation with = 2. This was not consistent with expectation for the growth of three dimensional nuclei for which A = 3 and = 0 or 1, respectively. In accordance with the reaction models described above, there must be limited water losses from all surfaces together with an overall reaction controlled by product recrystallization that confers the apparent topotacticity on the overall... 

The layered phosphonates M(OjPCH3).H20(M=Mg,Zn,Co) undergo topotactic dehydrations [122]. Rehydration is shape selective to the incoming molecule, the Mg salt rehydrates rapidly, Zn is unreactive to H2O and the behaviour of the Co salt is intermediate. 

The dehydration of copper(Il) formate tetrahydrate is a topotactic process [145]. The anhydrous crystalline product of dehydration retains two-dimensional features of the reactant structure but the stacking mode is changed. This structure is different from that obtained by the direct preparation of anhydrous Cu(HCOO)2. 

Product structure. Dehydration may result in relatively minor changes in structure such as decreases in some or all unit cell dimensions in topotactic processes. Alternatively, reaction may result in fundamental reorganization through recrystallization, or conversion to an amorphous or zeolitic material. Dehydration. Dehydration mechanisms encompass a wide variety of routes, summarized in Figure 7.4. The following features require comment. 

Giovanoli et al. [23] conclude that there is a structural relationship between y-Zn(OH)2.0.5H2O and P-Zn(OH)2, into which it is converted topotactically on dehydration. These hydroxides and y-Zn(OH)2 all decompose in an electron beam to yield highly-oriented ZnO. The crystal structure of the original hydroxide appears to exert a strong influence on the rate of oxide sintering at about 1070 K. 

From a photomicrographic study, Figlarz and Vincent [41] concluded that the dehydration of Co(OH)2 is initiated at randomly-distributed sites on the surfaces of the approximately hexagonal crystals of reactant. Following the topotactic advance of the reaction interface, alignment of the (111) fece of CoO with (0001) of Co(OH)2 preserves the external form of the reactant particles to yield pseudomorphic particles of product. Appreciable quantitites of water are retained in the residual material. 

The reaction of Ni(OH)2 resembled [39] that of Fe(OH)2 in that the contracting area equation fitted the data and the value of was 95 kJ mol. The rate was appreciably decreased by water vapour. The textural changes that accompany water removal have been studied [41] by electron microscopy which identified rapid initial nucleation at crystallite edges to form a continuous interface. The dehydration is topotactic to yield particles of product which are pseudomorphs of the reactant. These textural changes are consistent with the earlier conclusions based on kinetic evidence. 

The Re03 structure of M0O3 is metastable and is produced only by topotactic dehydration under mild conditions. We believe that the preparation of Re03 like M0O3 by mild chemical processing is significant. 

The topotactic dehydration reaction [215] transforming goethite into hematite was studied in situ at room temperature and shown to... 

Like the natural aluminophosphates variscite and metavariscite, these as-synthesized aluminophosphates are hydrates and contain octahedral aluminum where two of the coordination sites are occupied by water. Dehydration converts the octahedral aluminum to tetrahedral aluminum as shown in Fig. 3. This process, however, is limited in reversibility. For some structures such as HI, an irreversible topotactic transformation occurs resulting in the generation of a new phase, AIPO4-8 [30 - 35]. H2 can be reversibly dehydrated at room temperature but with further heat treatment converts to AlP04-tridymite [27]. H3, too, can be dehydrated to form AIPO4-C. Thermal treatment results in a topotactic transformation to AIPO4-D which can be hydrated to produce a different hydrate, H6. None of these phases have counterparts in the aluminosilicate zeolite system. 

The topotactic character of the thermal dehydration of VOHPO4 O.5H2O to (V0>2P207 was studied by Torardi and Calabrese and by Bordes et The... 

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