Sulfate decompositions, mechanisms

Due to the fact that the sulfate and selenate oxyanions have the same spatial configuration, the corresponding rare earth compounds are isostructural with only a few exceptions. Their physical and chemical properties are also largely similar. On the other hand, the relationship between rare earth sulfites and selenites is not as close as one would expect. Besides the larger size of the selenite ion, possible factors leading to structural and other differences between the sulfites and selenites include the different relative stabilities of the tetra- and hexavalent states and the lower stability of the selenium-selenium bond compared to the sulfur-sulfur bond. The first factor is responsible for the different observed thermal decomposition mechanisms while the latter contributes, for instance, to the formation of a diselenite ion which has an oxygen-bridged structure. This is much different from the disulfite ion, which is nonsymmetrical and contains a sulfur-sulfur bond. 

Because of the delay in decomposition of the peroxide, oxygen evolution follows carbon dioxide sorption. A catalyst is required to obtain total decomposition of the peroxides 2 wt % nickel sulfate often is used. The temperature of the bed is the controlling variable 204°C is required to produce the best decomposition rates (18). The reaction mechanism for sodium peroxide is the same as for lithium peroxide, ie, both carbon dioxide and moisture are required to generate oxygen. Sodium peroxide has been used extensively in breathing apparatus. 

A direct attack on the oxazirane ring occurs with ferrous salts. Oxaziranes are decomposed by aqueous solutions of ferrous ammonium sulfate even at room temperature. The reaction follows a radical-chain mechanism because le.ss than stoichiometric amounts of the ferrous salt cause the decomposition, 2-icrt-Butyl-3-phenyloxazirane (9) and 1 equivalent of ferrous salt give ferf-butylbenzamide in 98%... 

The thermal decomposition of binuclear technetium sulfate clusters also occurs according to the disproportionation mechanism, but in this case, (a) other technetium-containing products are formed, and (b) a weight loss due to the evolution of gaseous products is also observed (36) [59]. 

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

The fact that the decomposition of ozonides is catalyzed by finely divided metals (silver, platinum, palladium) and metal salts such as ferrous sulfate suggests a free-radical mechanism.23 2-Butene ozonide is broken down with dilute ferrous sulfate solution into acetic acid and acetaldehyde.23... 

Due to the intrinsic difficulties of sulfuric acid decomposition, we have introduced two new classes of solar driven TCWSCs by modifying the original S-NH3 cycle. They include (I) 12 metal sulfate-ammonia (MSO4-NH3) based TCWSCs and (II) 3 metal pyrosulfate-ammonia (M2S2O7-NH3) based TCWSCs. Our preliminary experimental results of the ammonia released from the ZnO + (NH4)S04 mixtures show the feasibility of these new cycles. More experiments are currently underway to determine the reaction mechanisms and the nature of the reaction intermediates and products formed. These experimental and thermodynamic analyses are expected to lead to development of a highly efficient, solar driven water splitting cycle. 

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