Oxidation in the Solid Phase

The mechanism of solid-phase oxidation, in particular, the oxidation of polymers has been still less investigated. Work in this field has begun comparatively recently. 

Since the known polymers are obtained from monomers belonging to various classes of organic compounds, they differ not only in the mechanisms of oxidation, but even in the character of the gross processes. 

In the oxidation of polyvinyl chloride, for example, a vital role is played by splitting out of HCl molecules and the formation of double bonds. The oxidation of polyamides proceeds without any appreciable induction period and differs sharply in character from the oxidation of hydrocarbons. The thermooxidative destruction of polysiloxanes also proceeds uniquely. Condensation polymers - epoxide resins, polyary-lates, and polycarbonates begin to be oxidized at comparatively high temperatures, and thus far no methods of stabilizing them are known. 

The oxidation of raw and cured rubbers also proceeds extremely uniquely. The available material on the oxidation of the enumerated polymers is described in Chapters IV-IX of this book. 

In this chapter we shall consider only the results of investigations of the oxidation of polypropylene, since the process of oxidation of this polymer has been investigated in the greatest detail moreover, it has been found that its mechanism is extremely close to the mechanism of the oxidation of hydrocarbons in the liquid phase. 

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IR spectra of thiolane oxides in the solid phase were shown to be most outstandingly different in the sulfoxide region depending on the particular crystalline state/structure a fact which can be used to advantage for conformational analysis. Also, as one could expect, the sulfoxide absorptions indicate strong hydrogen bonding. 

Compounds (M) Known compounds (M) and their oxidation products should be analyzed in both the water and solvent phases separately with methods which allow individual quantification if possible. While the oxidation progress can be described by overall parameters like DOC and COD in the aqueous phase, in the solvent phase these concentrations are normally dominated by the one of the solvent itself. In gas/water/solid systems a method should be developed to measure a compound on the particles, e. g. by extraction or dissolution of the particles, and to describe the oxidation in the solid phase. 

Lignin thermally decomposes to char and contributes little to flaming combustion. Most of the flaming combustion from wood is attributed to the hemicellulose and cellulose. However, lignin does support oxidation in the solid phase. Some fire retardants, such as phosphorus and boric acid, inhibit oxidation in the solid phase other additives, such as sodium compounds, may promote it. 

The kinetics of polymer oxidation in the solid phase (films) and in solutions of films, and of the polymers themselves, has been investigated. On account of the extremely large amount of theoretical and experimental data, only two fundamental cases, namely, autocatalytical oxidation in the presence of high and low concentrations of oxygen and the experimental principles in the investigation of degradation and cross-linking kinetics, are discussed in this chapter. 

In a comparison of pore water profiles of red clays and hemipelagic sediments, Sawlan and Murray (1983) showed that Mn and Fe are below the detection limit in the pore waters of red clays, Ni is present in the same concentation as in ocean bottom water and Cu shows a pronounced maximum at the sediment-water interface. In hemipelagic clays, denitrification becomes important and remobilization of Mn and Fe takes place. Ni correlates with Mn in the pore waters suggesting that it is associated with the Mn oxides in the solid phase. Cu is regenerated very rapidly at the sediment-water interface. The diffusive flux of Mn in hemipelagic sediments was determined to be in the range 2,200-... 

Ammonium nitrate decomposes into nitrous oxide and water. In the solid phase, decomposition begins at about I50°C (302°F) but becomes extensive only above the melting point (I70°C) (338°F). The reaction is first-order, with activation energy about 40 kcal/g mol (72,000 Btii/lb mol). Traces of moisture and Cr lower the decomposition temperature thoroughly dried material has been kept at 300°C (572°F). All oxides of nitrogen, as well as oxygen and nitrogen, have been detected in decompositions of nitrates. 

The trichloroethylene is oxidized, the gaseous products are removed by the flowing air, and the ehlorine is eaptured by the solid soda and forms salt. The solid salt is removed by diseharging the used OXITOX at the bottom of the reaetor. This is a relatively slow reaetion and the central interest is in removing the last traees of toxic chlorinated compounds (for which TCE is only a model eompound), therefore a very simple model was used. Based on conservation prineiples, it was assumed that chloride removed from the gas phase ends up in the solid phase. This was proven in several material balanee ealeulations. No HCl or other ehlorinated compound was found in the gas phase. The eonsumption rate for TCE was expressed as ... 

Thermal decomposition of [Cu0Si(0 Bu)3]4 in the solid phase begins at ca. 100 °C under argon (by TGA) and results in formation of an amorphous material until roughly 600 °C, at which temperature Cu metal was detected (by PXRD) [105]. Conversely, decomposition under oxygen led initially to a material with Cu crystalhtes and small amounts of CU2O and CuO, and subsequent heating beyond 800 °C resulted in oxidation of all the copper to CuO. 

Soft acids (SA) are strongly polarizable, have low or zero oxidation numbers, the softness decreases with increasing oxidation number and if groups with a large negative charge are also bonded to the central atom, then the softness increases they bind bases primarily through covalent bonds. Typical examples are Cu+, Ag+, Hg+, Cs+, Pd2+, Cd2+, Hg2+, Tl3+, T1(CH3)3, BH3, I+, Br+, H02+, I2, Br2, O, Cl, Br, I, metal atoms, metals in the solid phase. 

In order to elucidate the causes of the increased stability of the hydrolyzed cluster ions compared with the unhydrolyzed ions, further studies were made of the behaviour of [Te2X8]3 (where X = Cl,Br, or I) in solutions of hydrogen halides [43,52,80,87]. The studies were performed mainly in relation to the most stable and most readily synthesized [Tc2C18]3- ion (Fig. la) kinetic methods with optical recording were employed. The identity of the reaction products was in most cases confirmed by their isolation in the solid phase. The studies showed that the stability of the [Tc2X8]3 ions (where X = Cl, Br, or I) in aqueous solutions is determined by the sum of competing processes acid hydrolysis complex formation with subsequent disproportionation and dissociation of the M-M bonds, and oxidative addition of atmospheric oxygen to the Tc-Tc multiple bond. 

The interstellar dust was shown to contain quinone derivatives as well as oxygen-rich condensed aromatic compounds the quinones were present in both hydrated and carboxylated form. Very little nitrogen was present in the compounds detected. The cometary material, however, contained condensed nitrogen heterocycles. Hardly any oxygen was detected in the solid phase of the cometary dust it possibly evaporates from the tail of the comet in the form of water or oxidized carbon compounds. The authors assume that these analytical results could lead to a reconsideration of the current biogenesis models (Kissel et al 2004 Brownlee, 2004). 

The total emission In the commercial heat treatment of 5 to 8 hours at 170 to 160°C varied from 0.4 to 1.2% for CO2 and 0.05 to 0.2% for CO and 0.04 to 0.1% for total acids based on dry board. Some of this emission might emanate from pyrolysis of higher molecular weight material condensed and deposited on the walls of the heat treatment chamber. The heat of formation of this CO2 and CO Is about half the total heat release measured. Part of the oxidation products might remain in the solid phase within the board material, e.g. as bound carbonyl and carboxylic groups, partly followed by heat consuming dehydration reaction. 

At 373 K, the ratio of the rate constants is kis/kp = 8x 10 5 L-1 mol, where kis and kp refer to the reactions of isomerization and addition of oxygen. Isomerization competes with the reaction of the macroradical P with 02 therefore, intense hydroxylation during the oxidation of PP may imply that isomerization in the solid phase is slower than in the liquid phase. The experimentally measured ratio A[02]/[ROOH] at different partial pressure of oxygen helps to estimate the ratio k jkp in the oxidized polymer. Since the kinetics of chain PP oxidation are characterized by the rate v... 

Iodine oxides and bromine oxides are solid compounds which are beyond the scope of this article and will therefore not be discussed in any detail. Clyne and Coxon410 have found that BrO decays in a similar manner to CIO, with the second order rate coefficient of the order of 2x 108 1.mole-1.sec-1. In view of the important role of CIO in the decomposition of chlorine oxides, it is conceivable that BrO may play a similar role in the decomposition of bromine oxides. However, no kinetic information on the decomposition of bromine oxides in the gas phase appears to be available at the present time. 

It is interesting to juxtapose the reduction of a solid Fe(III)(hydr)oxide with a Fe(II) complex, e.g., with an oxalato complex of Fe(Il) (FenOx) on one hand with the oxidation of a Fe(II)bearing solid phase, e.g., a Fe(II) silicate with Fe(III) on the other hand (compare reactions (3) and (8), Table 9.1). In both cases the electron transfer (ET) occurs heterogeneously between Fe(III) and a Fe(II) complex. In one case the oxidant is the solid phase, in the other case the reductant is the solid phase. In simplified schematic notation ... 

There are only few examples for oxidation reactions on solid supports, because most linkers or polymeric supports are sensitive towards some reagents suitable for classical transformations. Classical oxidation reagents are not soluble in most solvents used in the solid-phase organic synthesis step (but e.g. Scheme 3.10). 

The Reformatsky type of reaction with Zn(0) was performed in situ and led to somewhat unstable phosphonodiamidite (step a) which was coupled with 5 -DMTr-thymidine to give the intermediate mononucleoside phospho-noamidite (step b). The latter was further coupled with 3 -acetyl-thymidine (step c). Couplings described in steps b and c were activated by tetrazole. The intermediate dinucleoside phosphonite was oxidized with (lS)-(+)-(10-camphorsulphonyl)oxaziridine (step d) or sulfurized with Beaucage reagent. The phosphonoamidites mentioned above were used in the solid-phase chemical synthesis of phosphonoacetate and thiophosphonoacetate oligonucleotides. 

All elements that form solid, liquid, and gaseous compounds stable over a wide temperature range are likely to have variations in isotopic composition. Generally, the heavy isotope is concentrated in the solid phase in which it is more tightly bound. Heavier isotopes tend to concentrate in molecules in which they are present in the highest oxidation state. 

II) Solid-phase reaction zone Nitrogen dioxide and aldehydes are produced in the thermal degradation process. This reaction process occurs endothermically in the solid phase and/or at the burning surface. The interface between the solid phase and the burning surface is composed of a solid/gas and/or soUd/Uquid/gas thin layer. The nitrogen dioxide fraction exothermically oxidizes the aldehydes at the interface layer. Thus, the overall reaction in the solid-phase reaction zone appears to be exothermic. The thickness of the solid-phase reaction zone is very small, and so the temperature is approximately equal to the burning surface temperature, T. ... 

The boron particles are thermally inert in the solid phase beneath the burning surface of the pyrolant The oxidation of the boron particles occurs just above the burning surface. This implies that the temperature gradient in the gas phase, (j), increases and hence the burning rate is increased accordingly. 

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