Decomposition and Gas Evolution

As far as ammonium is concerned, it is normally present in zeolites as the cation, taken up by cation-exchange, especially for preparing intermediate compounds, to be transformed by heating into the corresponding catalytically active H-forms. DTA of NH4-exchanged zeolites usually results in an overlapping of the dehydration and deammoniation effects, which are not resolvable by TG. Ammonia evolution, the amount of which is a measure of the cation-exchange completeness, may be monitored by the thermo-gas titrimetric (TGT) technique, which allows quantitative evaluation of NH3 to be obtained by a potentiometric acid-base titration [23]. 

The common practice in thermal analysis, confirmed here, is to use TG essentially for quantitative analytical purposes, namely the rapid and accurate estimation of the organic matter present in ZSM-5. On the contrary DTA proved to be an useful tool for collecting information on the state and the nature of the organic species inside the zeolite framework. 

An analogous analysis can be done on the DTA curves of the samples prepared in the presence of F with similar results, the only important difference being that minima connected with TPA F decomposition are shifted towards higher temperatures, because of the higher thermal stability of the fluoride, compared to the hydroxide [30]. Differences are found in the decomposition mechanisms and therefore in the number and positions of the endotherms, when the synthesis is performed in the presence of TMA+ OH or TEA+ OH (tetraethy-lammonium hydroxide) [34]. 

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PVC should not be melt-mixed with acetal polymers. These polymers are chemically incompatible mixing could cause rapid decomposition and gas evolution. 

The thermal behaviour of zeolites has thoroughly been investigated. When heated, a zeolite powder undergoes a series of physical and chemical changes, which include water loss, decomposition and gas evolution, phase transition, structure breakdown, re-crystallisation, melting, and others [75J. The thermal characterisation of natural zeolites has been carried out by various techniques and the relevant data may be found in several publications [44,76-78]. 

Abstract. After a brief introduction on zeolite constitution, structure and properties, the suitability of thermal analysis in characterizing the zeolite materials and in investigating their potential behavior in different application fields is analyzed. Kinetics and thermodynamics of water desorption, thermal stability, phase transformations, occluded phase decomposition and gas evolution, structure collapse and recrystallization, change in electrical properties, all in relation to thermal treatments, are the specific subjects reviewed. Use of thermal analysis in the evaluation of zeolite content in multicomponent mixtures and in the characterization of zeolite catalysts are the two additional main topics discussed. 

The action of heat on samples has long been a useful dry test to determine some of the qualitative characteristics of the material. Dehydration occurs with hydrates and with materials such as cellulose. Chemical decomposition and gas evolution are observed with carbonates, sulfates and nitrates. Occasionally, explosive reactions happen and all of these can be characterized by DTA and DSC. 

Mixtures of nitrobenzene and tin(IV) chloride undergo exothermic decomposition with gas evolution above 160°C. 

Interaction can lead to spontaneous exothermic decomposition of the fluoride accompanied by a bright flash and gas evolution. Safety precautions are required for this reaction system. 

The liquid explosives of the type outlined above may serve not only as high explosives but also as propellants for rocket propulsion. Liquids which are not explosives in the strict sense, but which undergo violent decomposition under certain conditions, with heat emission and gas evolution, may also be used for this purpose. The liquids employed for rocket propulsion are called propergols. 

Such a reaction is controlled by the rate of addition of the acid. The two-phase system is stirred throughout the reaction the heavy product layer is separated and washed thoroughly with water and alkaU before distillation (Fig. 3). The alkaU treatment is particularly important and serves not just to remove residual acidity but, more importantiy, to remove chemically any addition compounds that may have formed. The washwater must be maintained alkaline during this procedure. With the introduction of more than one bromine atom, this alkaU wash becomes more critical as there is a greater tendency for addition by-products to form in such reactions. Distillation of material containing residual addition compounds is ha2ardous, because traces of acid become self-catalytic, causing decomposition of the stiU contents and much acid gas evolution. Bromination of alkylthiophenes follows a similar pattern. 

To a solution of 130 g. (0.6 mole) of arsanilic acid (Org. Syn. 3, 13) in 600 cc. (0.6 mole) of normal sodium hydroxide is added 52 g. (0.62 mole) of sodium bicarbonate and 70 g. (0.75 mole) of chloroacetamide (Org. Syn. 7, 16). The mixture is heated 011 a water bath to 90-1000 and a steady evolution of carbon dioxide occurs. At the end of two hours, when gas evolution has practically ceased, the mixture is cooled to 40° C., stirred vigorously and 150 cc. of 1 1 hydrochloric acid poured in rapidly. /i-Arsonophenylglycinamide crystallizes at once and, after cooling to room temperature, is filtered by suction and washed once with 2 per cent hydrochloric acid (Note 1), then with cold water. The crude product thus obtained is contaminated with some arsanilic acid and possibly other products. These are removed during purification. The crude product is suspended in about 400 cc. of water and with vigorous stirring, treated carefully with 25 per cent aqueous sodium hydroxide until solution is just complete. At this point the mixture is still acid to litmus and an excess of sodium hydroxide is to be avoided to prevent decomposition of the product. About 15 g. of boneblack... 

Carbide decompositions yield no volatile product and, therefore, many of the more convenient experimental techniques based on gas evolution or mass change cannot be applied. This is a probable reason for the relative lack of information about the kinetics of reaction of these and other compounds which are correctly classifed under this heading, such as borides, silicides, etc. 

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