Topochemical kinetics

Frank-Kamenetskii D. A. Report at a meeting on topochemical kinetics. Leningrad, May 11 (1938). 

Phase transformations in heterogeneous catalysis have been described recently by topochemical kinetic models [111-115]. These models were taken from solid chemistry, where they had been developed for "gas-solid reactions. The products of such reactions are solids. When gas is in contact with the initial solid, the reaction rate is negligible. But as nucleates of the phase... 

Mg(N03)2 is stable to nearly 600 K. Its decomposition was studied from 575 to 725 K.72 Initially, N02 is liberated and the decomposition reactions are inferred from the N02 liberated. The curves follow first-order topochemical kinetics. Thermodynamic data are given in Tables 5.45 to 5.47. 

CH4-TPR. Typical curves of product evolution in CH4 temperature-programmed reduction of LSFNo 3-GDC composite calcined at 1200°C are shown in Fig. 107. First CO2 appears at 700 °C, then CO and H2 simultaneously evolve. Isothermal experiments (Fig. 108) revealed that CO and H2 formation is described by typical topochemical kinetics even at 700 °C, i.e. reduction of nanocomposite by methane proceeds via nucleation and growth of reduced phases (Ni or Ni alloy particles). CO2 appears immediately after CH4 contact with the catalyst surface evidencing a high reactivity of the surface oxygen of these composites. 

If the flow is accompanied with CBA decomposition, the G value in Eq. (5) should be substituted with its time function, G(t). In the general case, thermal decomposition of a solid substance with gas emission is a heterogeneous topochemical reaction [22]. Kinetic curves of such reactions are S -shaped the curves representing reaction rate changes in time pass a maximum. At unchanging temperature, the G(t) function for any CBA is easily described with the Kolrauch exponential function [20, 23, 24] ... 

In order to rationalize such characteristic kinetic behaviour of the topochemical photoreaction, a reaction model has been proposed for constant photoirradiation conditions (Hasegawa and Shiba, 1982). In such conditions the reaction rate is assumed to be dependent solely on the thermal motion of the molecules and to be determined by the potential deviation of two olefin bonds from the optimal positions for the reaction. The distribution of the potential deviation of two olefin bonds from the most stable positions in the crystal at OK is assumed to follow a normal distribution. The reaction probability, which is assumed to be proportional to the rate constant, of a unidimensional model is illustrated as the area under the curve for temperature Tj between 8 and S -I- W in Fig. 7. 

Shortly after, Doetschman and Hutchison reported the first example of a reactive carbene in the crystalline solid state, by preparing diphenylcarbene from diphenyldi-azomethane in mixed crystals with 1,1-diphenylethylene 84 (Scheme 7.23). When the mixed crystals were irradiated, carbene 85 was detected by electron paramagnetic resonance (EPR) and the disappearance of the signal was monitored to determine its kinetic behavior. Two reactions were shown to take place under topochemical... 

The physical organic chemistry of very high-spin polyradicals, 40, 153 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-slate chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and 27, 1... 

Stability and decomposition kinetics of aspirin both as a solid and in solution continue to be studied. The topochemical decomposition pattern of aspirin tablets has been explored.175 The degradation of aspirin in the presence of sodium carbonate and high humidity was studied by x-ray diffraction.176 The activation energy of decomposition by water vapor in the solid state was found to be 30 kcal/mol.177 The effect of common tablet excipients on aspirin in aqueous suspension was also studied.178... 

Several reviews deal with the solid-state reactions of simple inorganic salts and of organic compounds.1-8 The essential differences between solid-state reactions and reactions in solution can be ascribed to the fact that solid-state reactions occur within the constraining environment of the crystal lattice. The reactant crystal lattice can control both the kinetic features of a reaction, and the nature of the products. In many solid-phase reactions the separation distances and mutual orientations of reactants in the solid determine the product. Such reactions are said to be topo-chemically controlled.9 Topochemical control of a reaction product is analogous to kinetic control in solution. The product is not necessarily the thermodynamically most stable product available to the system, but is rather the one dictated by the reaction pathway available in the constraining environment of the solid. 

H-atoms (Mertens and von Sonntag 1994). In contrast to BLM, where H4 is attacked, it is mainly the H5 that is abstracted by NCS. This points to a highly stereoselective topochemical reaction, since the tertiary HI and H4 would be the thermodynamically, and at equal accessibility, also the kinetically favored targets. This is supported by marked differences between esperamicin A and esperamicin C (Epstein et al. 1997). The former cleaves only at C(5 ) while the latter also cleaves at C(4 ). 

The initial period of chemical kinetics (1860-1910) is the key to the understanding of the further progress in this science. It is during this period that formal kinetics was created. The lucidity (and the small number) of the basic conceptions and the integrity of its subject are characteristic of this period of chemical kinetics. Later, that initial integrity was lost, giving way to many forms of "kinetics gas- and liquid-phase reactions, catalytic, fermentative, electrochemical, topochemical, plasmachemical, and other kinetics. These "kinetics differ in their experimental techniques and special languages. 

It should be said that at present the available literature concerning the kinetic models which account for the topochemical character of catalyst surface processes is limited, but reference can be made to refs. 119 and 120. In ref. 119, a kinetic model for the oxidation of hydrogen on platinum is... 

The topochemical model, however, describes the origination and growth of macrostructures. In principle one could construct kinetic models accounting for the kinetics of cluster (or nucleate) formation as a model for the system or reverse consecutive reactions [114, 121]. 

A.Ya. Rozovskii, Kinetics of Topochemical Reactions, Khimiya, Moscow, 1974 (in Russian). 

A correlation between surface and volume processes is described in Section 5. The atomic-molecular kinetic theory of surface processes is discussed, including processes that change the solid states at the expense of reactions with atoms and molecules of a gas or liquid phase. The approach reflects the multistage character of the surface and volume processes, each stage of which is described using the theory of chemical kinetics of non-ideal reactive systems. The constructed equations are also described on the atomic level description of diffusion of gases through polymers and topochemical processes. 

In contrast to the thermodynamics of decomposition, where a few parameters permit the calculation of the equilibrium properties of the system, the determination of decomposition rates is largely an experimental problem, i.e., there are no standard kinetic data from which these rates can be calculated. This is particularly true for the decomposition reactions of solids which are topochem-ical," i.e., where the rate depends on structural factors. One reason for this situation is that it does not yet seem possible to prepare duplicate samples of any solid inorganic salt that are identical in all the properties that may determine the rate of decomposition, e.g., the dislocation density of the crystals. 

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... 

Tetrahedral intermediates, derived from carboxylic acids, spectroscopic detection and the investigation of their properties, 21, 37 The physical organic chemistry of very high-spin polyradicals, 40, 153 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-slate chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and 27, 1 Transition stale structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1 Transition states, theory revisited, 28, 139... 

Microscopy is the most appropriate technique for studying the kinetics of nucleation. The shapes, sizes, textures and distributions of nuclei can be determined and the kinetics of nucleation can be distinguished fi om the kinetics of growth. Details of the intranuclear material, which is often porous with small crystallites separated by fine channels that provide routes for escape of product gas, may be discemable. Changes in particle-size, topochemical relationships and the possibility of melting of the solid reactant can also be recognized. 

Ingraham and Marier [88] identified the intermediate Zn0.2ZnS04 in the decomposition of ZnS04. This is a topochemical process, following linear kinetics and the rate is decreased by the adsorption of SO3 on the product oxide. The reverse reaction (ZnO + SO3 ZnS04) is very slow if the solid is not finely divided. 

The essence of this phenomenon is that properties of defect clustered centers and kinetic features of the oxidation reactions depend upon stoichiometry of the surface layer. For oxides studied, surface reduction is a topochemical type process and proceeds via spreading of the reduced zone from the extended surface defects accompanied by a cations redistribution between the regular and the interstitial positions. Reoxidation as well as hydroxylation/carbonization causes shrinkage of this zone. 

---