Oxygen metal salt catalysts

Two further observations are relevant to a discussion of photodegradation of polyurethanes. Industrial production normally uses metal salt catalysts which cannot be removed from the final product and which diminish the photostability of the material. Irradiation in the presence of oxygen of the type of polymer structure shown in the above scheme results in discolouration as well as deterioration in mechanical properties. The reaction scheme shown in Scheme 13 has been proposed. 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Chemical Properties. On thermal decomposition, both sodium and potassium chlorate salts produce the corresponding perchlorate, salt, and oxygen (32). Mixtures of potassium chlorate and metal oxide catalysts, especially manganese dioxide [1313-13-9] Mn02, are employed as a laboratory... 

In summary, the oxidation of thiols to disulfides is quantitative in aqueous alkaline solution and may best be effected at high oxygen pressures in the presence of a catalyst. The catalyst should dissolve in the alkaline solutions, and of the simple metal salts, the addition of copper, cobalt, and nickel results in the most effective catalysis. 

In recent years, much effort has been spent on developing both selective and environmentally friendly oxidation methods using either air or oxygen as the ultimate, oxidant. One of the most selective and efficient catalyst systems reported to date is based on the use of stable nitroxyl radicals as catalysts and transition metal salts as co-catalysts (15). The most commonly used co-catalysts are (NH4)2Ce(N03)6 (16), CuBr2-2,2 -bipiridine complex (17), RuCl2(PPh3)3 (18,19), Mn(N03)2-Co(N03)2 and Mn(N03)2-Cu(N03)2 (20). However, from an economic and environmental point of view, these oxidation methods suffer from one common drawback. They depend on substantial amounts of expensive and/or toxic transition metal complexes and some of them require the use of halogenated solvents like dichloromethane, which makes them unsuitable for industrial scale production. 

The performance of a catalyst is well known to be sensitive to its preparation procedure. For this reason, ideally an oxide-supported metal catalyst should be subjected to a number of characterization procedures. These may include measurements of the metal loading within the overall catalyst (usually expressed in wt%), the degree of metal dispersion (the proportion of metal atoms in the particle surfaces), the mean value and the distribution of metal particle diameters, and qualitative assessments of morphology including the particle shapes and evidence for crystallinity. These properties in turn can depend on experimental variables used in the preparation, such as the choice and amounts of originating metal salts, prereduction, calcination or oxygen treatments, and the temperature and duration of hydrogen reduction procedures. 

In contrast to the lighter feedstocks that may be subjected to the hydrodesulfurization operation, the heavy oils and residua may need some degree of pretreatment. For example, the process catalysts are usually susceptible to poisoning by nitrogen (and oxygen) compounds and metallic salts (in addition to the various sulfur-compound types) that tend to be concentrated in residua (Chapter 3) or exist as an integral part of the heavy oil matrix. 

Reactions of methane and ethane with HF recyclable metal fluorides to give fluorocarbons have been reported briefly in the patent and journal literature [16-18], Reaction of methane with hydrogen fluoride in the presence of oxygen and the salt or oxide of a variable valency metal as catalyst yielded small amounts of fluoromethane and difluoromethane at temperatures above 500°C. Olsen et al. [17] reacted copper(ll) fluoride with methane at high temperatures (>600°C) and found products that always included copper metal, hydrogen fluoride, fluoromethane and carbon. Although activity was first detected around... 

Alternatively TEMPO can be reoxidized by metal salts or enzyme. In one approach a heteropolyacid, which is a known redox catalyst, was able to generate oxoammonium ions in situ with 2 atm of molecular oxygen at 100 °C [223]. In the other approach, a combination of manganese and cobalt (5 mol%) was able to generate oxoammonium ions under acidic conditions at 40 °C [224]. Results for both methods are compared in Table 4.9. Although these conditions are still open to improvement both processes use molecular oxygen as the ultimate oxidant, are chlorine free and therefore valuable examples of progress in this area. Alternative Ru and Cu/TEMPO systems, where the mechanism is me-... 

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