Solid-state reactions examples

The application of RBS is mostly limited to materials applications, where concentrations of elements are fairly high. RBS is specifically well suited to the study of thin film stmctures. The NMP is usefiil in studying lateral inliomogeneities in these layers [30] as, for example, in cases where the solid state reaction of elements in the surface layers occur at specific locations on the surfaces. Other aspects, such as lateral diffusion, can also be studied in tluee-dimensions. 

The reactivity of the transition metals towards other elements varies widely. In theory, the tendency to form other compounds both in the solid state (for example reactions to form cations) should diminish along the series in practice, resistance to reaction with oxygen (due to formation of a surface layer of oxide) causes chromium (for example) to behave abnormally hence regularities in reactivity are not easily observed. It is now appropriate to consider the individual transition metals. 

Classification of solid state reactions according to Boldyrev [100] Initial step Reaction mechanism Examples... 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

There have been few discussions of the specific problems inherent in the application of methods of curve matching to solid state reactions. It is probable that a degree of subjectivity frequently enters many decisions concerning identification of a best fit . It is not known, for example, (i) the accuracy with which data must be measured to enable a clear distinction to be made between obedience to alternative rate equations, (ii) the range of a within which results provide the most sensitive tests of possible equations, (iii) the form of test, i.e. f(a)—time, reduced time, etc. plots, which is most appropriate for confirmation of probable kinetic obediences and (iv) the minimum time intervals at which measurements must be made for use in kinetic analyses, the number of (a, t) values required. It is also important to know the influence of experimental errors in oto, t0, particle size distributions, temperature variations, etc., on kinetic analyses and distinguishability. A critical survey of quantitative aspects of curve fitting, concerned particularly with the reactions of solids, has not yet been provided [490]. 

The dissociation of Ag20 in oxygen was an early (1905) example (Lewis [640]) of a kinetic study of a solid state reaction and interest in... 

Some of the reactions which yield MM0O4 or MW04, referred to in Sect. 4.1.4, are closely related to those discussed here in which a carbonate or higher oxide is used as reactant. For example, the solid state reaction... 

As an example, consider the following. Suppose we have a crucible half-filled with a powder. We now fill the crucible with another powder of different composition and then heat the filled crucible. Any solid state reaction which does occur can only do so at the boundary of the two layers of powders. If the reaction is A -t- B = AB, then we find that the reaction product, which is also a solid, forms as a phase boundary between the two layers. The same condition exists in a solid state reaction between two crystalline particles having differing compositions. That is, they can only react at the interface of each particle. This is illustrated in the following diagram, which is a model of how the components react through a phase boundary ... 

Let us now turn to diffusion in the general case, without worrying about the exact mechanism or the rates of diffusion of the various species. As an example to illustrate how we would analyze a diffusion-limited solid state reaction, we use the general equation describing formation of a compound with spinel (cubic) structure and stoichiometry ... 

It should be clear by comparing the examples for calcium silicate and barium silicate that one cannot predict how the diffusion-controlled solid state reactions will proceed since they are predicated upon the relative thermodsmamic stability of the compounds formed in each separate phase. 

Sintering of particles occurs when one heats a system of particles to an elevated temperature. It Is caused by an interaction of particle surfaces whereby the surfaces fuse together and form a solid mass. It Is related to a solid state reaction In that sintering is governed by diffusion processes, but no solid state reaction, or change of composition or state, takes place. The best way to illustrate this phenomenon is to use pore growth as an example. 

The 0-silylation reaction of alcohols is important as a protection method of hydroxyl groups. 0-Silylations of liquid or crystalline alcohols with liquid or crystalline silyl chlorides were found to be possible in the solid state. For example, when a mixture of powdered L-menthol (26), ferf-butyldimethylsilyl chloride (27), and imidazole (28) was kept at 60 °C for 5 h, 0-tert-butyldi-methylsilyl L-menthol (29) was obtained in 97% yield [8] (Scheme 4). Similar treatments of 26 with the liquid silyl chlorides, trimethyl- (30a) and triethylsilyl chloride (30b), gave the corresponding 0-silylation products 31a (89%) and 31b (89%), respectively, in the yields indicated [8] (Scheme 4). However, 0-silylation of triisopropyl- (30c) and triphenylsilyl chloride (30d) proceeded with difficultly even at 120 °C and gave 31c (57%) and 31d (70%), respectively, in relatively low yields. Nevertheless, when the solvent-free silylation reactions at 120 °C were carried out using two equivalents of 30c and 30d, 31c (77%) and 31d (99%) were obtained, respectively, in relatively high yields. 

Since solid-state reactions can easily be monitored by continuous measurement of spectra, it is easy to study the mechanism of the reactions. For this purpose, IR spectroscopy is the most useful, because IR spectra can be measured simply as Nujol mulls or directly for any mixture of solid-solid, solid-liquid, or liquid-liquid by using the ATP (attenuated total reflection) method. Some such examples of the mechanistic study are described. 

Enantioselective Br2 addition to cyclohexene (11) was accomplished by the solid-state reaction of a 2 1 inclusion complex of 10b and 11 with 7, although the optical yield was low (Sect. 2.1). However, some successful enantioselective solid-state reactions have been reported. For example, reaction of a 1 1 complex of 68 and acetophenone (64a) with borane-ethylenediamine complex (130) in the solid state gave the (i )-(+)-2-hydroxyethylbenzene (65a) of 44% ee in 96%... 

Mixed-phase oxide pigments are manufactured by high temperature (800-1000 °C) solid state reactions of the individual oxide components in the appropriate quantities. The preparation of nickel antimony titanium yellow, for example, involves reaction of Ti02, NiO and Sb203 carried out in the presence of oxygen or other suitable oxidising agent to effect the necessary oxidation of Sb(m) to Sb(v) in the lattice. 

When the course taken by a given solid-state reaction is determined by geometrical details of the crystal lattice, the reaction type falls under the general category of topochemistry. In a topochemical reaction, the reaction takes place in the solid state with a minimum amount of molecular motion. For example, bimolecular reactions are expected to take place between nearest neighbors, which then suggests that the product of the reaction would be a function of the geometric relation in the crystal structure of the reactant molecules. 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

Monitoring solid state reactions that play a role in catalyst activation forms a useful application of XRD. The example discussed above concerns a catalyst with large iron oxide particles as is used in the water gas shift reaction, and represents a particularly favorable system for XRD analysis. Similar studies with small particles are certainly also feasible, although it may be advisable to use laboratory X-ray sources of higher energy, such as Mo Ka, or a synchrotron [13]. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

Increasing the size of PAHs makes their deposition on surfaces difficult because they can neither be sublimed nor made sufficiently soluble for solution processing. A precursor route has thus been designed according to which molecules are deposited on a surface and transformed into the final disc-type adsorbate structures in a thermal solid-state reaction with the substrate surface acting as a template.1261 An exciting example is the hexaether 41 (scheme 11) which is sublimed onto a Cu-(1U) sur-... 

The simple solid state reaction may involve either (i) the interfacial reaction or (ii) the diffusion of reactants to each other, as the rate determining step. For example, decomposition of a solid giving rise to another solid... 

Figure 2.19. Examples of systems in which intermediate phases corresponding to small composition ranges are formed. These are SnTe (congruent melting), HfRu (congruent melting), ZrV2 (peritectic formation) and TaV2 (formed through a solid-state reaction).

These examples illustrate a number of points. Solid-state reactions can, in many cases, be performed very easily. Their courses are frequently interpretable in terms of straightforward geometrical principles concerning conformation and/or packing of the reactant molecules. It is sometimes possible to design the reactant molecule with suitable substituents to ensure a desired crystal structure... 

To the organic chemist, the most striking feature of solid-state reactions is the stereochemical purity of die product obtained in most cases. This feature allows conversion by conventional methods of the solid-state product to other materials of desired stereochemistries. We illustrate this by some examples of reactions starting from the cyclobutanes obtained by solid-state (2 + 2) photodimerization. 

As described above, most solid-state reactions are heterogeneous, in the sense that reactant and product are in different solid phases. In many of these, product crystals first appear as nuclei that grow at the expense of the parent crystal. On the other hand, there are some solid-state reactions that are not accompanied by a phase change and for which, therefore, analogy with a solid-state transformation is not plausible. Such reactions are of particular interest in several respects They make possible conversion of a single crystal of reactant to a single crystal of product they enable study, for example by X-ray diffraction, of the structures of the parent and product molecules as functions of the degree of conversion in more or less constant environments and one can elucidate from them the constraints that the parent crystal imposes both on the reaction pathway and on the conformation of the product. It is in connection with the latter that this subject is of particular interest in the present context. This class of processes has been discussed by Thomas (183). 

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