Preparation zinc chromite

Catalysts suitable specifically for reduction of carbon-oxygen bonds are based on oxides of copper, zinc and chromium Adkins catalysts). The so-called copper chromite (which is not necessarily a stoichiometric compound) is prepared by thermal decomposition of ammonium chromate and copper nitrate [50]. Its activity and stability is improved if barium nitrate is added before the thermal decomposition [57]. Similarly prepared zinc chromite is suitable for reductions of unsaturated acids and esters to unsaturated alcohols [52]. These catalysts are used specifically for reduction of carbonyl- and carboxyl-containing compounds to alcohols. Aldehydes and ketones are reduced at 150-200° and 100-150 atm, whereas esters and acids require temperatures up to 300° and pressures up to 350 atm. Because such conditions require special equipment and because all reductions achievable with copper chromite catalysts can be accomplished by hydrides and complex hydrides the use of Adkins catalyst in the laboratory is very limited. 

Zinc chromite (ZnCr204), which is used to make magnetic tape for cassette recorders, can be prepared by the thermal decomposition of (NH4)2Zn(CrC>4)2. The other reaction products are N2 and water vapor. 

Oleyl alcohol occurs naturally in fish oils. Synthetically, it can be prepared from butyl oleate by a Bouveault-Blanc reduction with sodium and butyl alcohol. An alternative method of manufacture is by the hydrogenation of triolein in the presence of zinc chromite. 

The dehydrogenation of piperazines to pyrazine was first achieved by Stoehr (32), who heated piperazine (87) or its hydrochloride with zinc dust or, better, zinc dust and lime to give a yield of approximately 10% pyrazine (88). Since that time a number of publications and patents has described the conversion of piperazines to pyrazines by heating at elevated temperatures with various catalysts usually containing copper chromite (464) but also with palladium-charcoal (465) and platinum on alkali-washed firebrick (466), and also with other reagents. Some of these preparations are summarized in Table II.11 (464-475). 

Fixed bed processes can be used for the l drogenation of fatty acid methyl esters. The methyl esters can be prepared direcdy from the fatty acid or by tra 5 -esterification of the triglyceride with methanol. The hydrogenation is carried out in a bed of solid copper chromite catalyst, which usually loses activity after operating for 3-6 months. Copper oxide/zinc oxide catalysts have also been used. 

Lazier of Du Pont published many patents covering the preparation of metal chromites." These were formed by precipitation from a solution of zinc nitrate and chromic acid with ammonia at pH 6.8. The zinc ammine complex obtained was decomposed at about 400°C to give the mixed oxides. As Adkins noted, his copper chromite equivalent was extracted with dilute acetic acid solution to adjust the copper content. It is not clear whether the same treatment was ever used in producing methanol catalysts or even whether the zinc ammine intermediate was produced commercially. One problem with the Lazier preparation was the difficulty in controlling the exothermic decomposition of the ammine that could affect the catalyst activity. 

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