Solid-state reactions intermediates

In this case, we have given both the starting conditions and those of the intermediate stage of solid state reaction. It should be clear that A reacts with B, and vice versa. Thus, a phase boundary is formed at the interface of the bulk of each particle, i.e.- between A and AB, and between B and AB. The phase boundary, AB, then grows outward as shown above. Once the phase boundary is established, then each reacting specie must diffuse through the phase AB to reach its opposite phase boundary in order to react. That is- A must difiuse through AB to the phase boundary... 

Note carefully the sequence of intermediate reactions that have occurred, as depicted in this diagram. We see that the first solid state reaction involves formation of the orthosilicate. But this is very quickly transformed into meta- and pyro-sllicates. However, it is the metasilicate which predominates as the solid state reaction reaches its maturity. Finally, it predominates over all other forms of silicate present. 

When solids react, we would like to know at what temperature the solid state reaction takes place. If the solid decomposes to a different composition, or phase, we would like to have this knowledge so that we can predict and use that knowledge In preparation of desired materials. Sometimes, intermediate compounds form before the final phase. In this chapter, we will detail some of the measurements used to characterize the solid state and methods used to foUow solid state reactions. This will consist of various types of thermal analysis (TA), including differentlEd thermal analysis (DTA), thermogravimetric analysis (TGA) and measurements of optical properties. 

However, if we set the furnace temperature just slightly greater than T2, we would obtain a reaction limited to that of A - B, and thus could identily the intermediate reaction product, B. This technique is called isothermal thermogravimetry. Thus, we can follow a solid state reaction by first surveying via d3mamic TGA. If there are any intermediate products, we can isolate each in turn, and after cooling (assmning each is stable at room temperature) cam identify it by x-ray analysis. Note that we can obtain an assay easily ... 

Until recently the products of all nitrile cyclizations by the Thorpe reaction had been formulated as imines, although the products were found in 1955 to be better written as the enamine structure. In order to verify the reaction mechanism of the Thorpe reaction, the solid-state reaction of 84 and Bu OK was monitored by measurement of IR spectra in Nujol mulls. As the reaction proceeds (Scheme 14), the CN absorption of 84 at 2250 cm" decreases and a new CN absorption of the imine intermediate (87) arises at 2143 cm As 87 is converted into 88 by a proton migration, the CN absorption of 87 at 2143 cm" disappears, and only the CN absorption of 88 at 2189 cm remains finally [13]. 

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).

Unfortunately the authors argue that they were performing mechanochemical reactions with mechanical energy input for the salt formation or complexation to occur, rather than just creating the required contacts between reacting crystals. Furthermore, they did not exclude moisture, reported intermediate liquid phases in various cases, and did not separate out any real solid-state reactions that might have been achieved. It is therefore not possible to discuss the results in more detail here. 

Here, the term no-solvent means the absence of a traditional solvent—the reactants are neat and may well be sohds. Where one reactant is in sufficiently large excess to qualify as a solvent, for example in Friedel-Crafts alkylations or acylations with excess benzene or toluene, the reactions are not normally classified as no-sol-vent. The phrase solid-phase (solid-state) reaction today often describes a reaction carried out on a solid phase, like a resin, to which the reaction intermediates are bound by adding reagents in solution. These reactions have become very important in combinatorial chemistry, but they do not meet the definition of no-solvent. The nosolvent reactions refer only to the primary reactions themselves and not to workup conditions which may or may not involve solvents (Dittmer, 1997). 

Raman spectroscopy can be used in similar ways to study transient intermediates, but it can also play a different role in mechanistic studies of crystals. Prasad and his collaborators have used phonon spectra of the lattice itself to probe solid-state reactions [46], They have used this technique to study whether reaction occurs uniformly throughout the solid, whether phase transformation accompanies reaction, and whether the starting lattice allows easy deformation in a direction that could lead toward reaction. 

Solid-state reaction in all cases implies the introduction of disorder into a crystal. Even for a perfectly homogeneous reaction the initial product sites are randomly distributed. Thus it is frequently not possible to refine atomic positions in the structures of crystalline reaction intermediates. In rare cases where atomic refinement of intermediates can be done it gives extremely precise information about how a solid-state reaction occurs. Examples of such cases will be discussed below [70,71]. 

G. M. J. Schmidt, with his ground-breaking work on the solid-state reactions of cinnamic acids, was one of the first to look at intermediate stages of a reacting organic solid (Scheme 17) [63,108,109]. He did this, in the early 1960s, with the... 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

Fullerenes have also been chemically modified. The reactivity of Ceo is closer to that of an olefin rather than a benzene ring. Solid-state reaction of the fullerene with potassium cyanite results in a fullerene dimer (a dumbbellshaped fullerene see Fig. 3.3). Because the fullerene intermediate anion (with a CN substituent) is reactive in the solid state, dimerization of the fullerene occurs. In contrast, this anion is stabilized by solvation, and so dimerization does not proceed. 

In the solid-state reaction of 3.5 BaO + 1 Am02 at 800°-900°C. the compound Ba(Bao.5 Amo.5 )03 (17) with ordered perovskite structure is formed. At higher temperatures there is a loss of oxygen, transforming Ba(Bao.5, Amo 5)03 into BaAm Os with the cubic perovskite lattice in such a way that no intermediate stages of this transition can be recog-... 

Greigite has been repeatedly implicated as an intermediate in pyrite formation (Benning et al., 2000 Schoonen and Barnes, 1991b Sweeney and Kaplan, 1973 Wang and Morse, 1994 Wilkin and Barnes, 1996). Evidence that the greigite to pyrite transformation can be a solid-state reaction comes from observations that intermediate stages of this... 

Materials such as aluminum titanate and silicon carbide appear to be promising for high-temperature catalytic combustion. However, problems such as extrudability, the application of washcoats, and reaction with deposited washcoats are not solved yet. For instance, when hexa-aluminate, presented in the introduction to this section, was applied to silicon carbide monoliths, solid-state reactions occurred at 1200-1400 C [76], causing exfoliation of the coating and the formation of new phases. The application of an intermediate mullite layer was suggested as an approach to hinder these solid-state reactions. 

Any solid state reaction, however complex, must resolve itself into interactions between pairs of solid phases, the elementary processes occurring successively or simultaneously to give a variety of intermediate and final products. Because the entropy change is small, all solid state reactions are exothermic. This property forms the basis of the heating curve method for detecting reactivity in solid mixtures. 

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