Catalysts 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. 

The influence of the C02/C0 ratio on the synthesis has been embedded in kinetic equations only recently. The early kinetic equations for the high-pressure Zn0/Cr203 catalyst did not contain a C02-dependent term at all, perhaps because the effects of C02 were not significant when zinc chromite catalysts were used Natta et al. (57) proposed the rate equation for the Zn0/Cr203 catalyst at temperatures 300-360°C as follows ... 

The investigation of zinc oxide-zinc chromite of the composition ZnO-ZnO-CraOs (77), which reveals an x-ray pattern of zinc oxide plus a spinel structure, led to the result that 5obs. does not agree with any of the values of evaluated for n = 1, 2, and 3 (Fig. 18). The differential heat of adsorption of carbon dioxide on this catalyst was found to be 43 kcal./ mole, i.e., equal to that on zinc oxide alone but by far greater than that... 

Zn-. [M THarshaw] Zinc chromite or zinc oxide catalysts. 

The concentration theory completely fails to explain the selective nature of catalysis. Why, for example, does formic acid decompose into hydrogen and carbon dioxide with a zinc oxide catalyst, whereas with titanium oxide, it breaks down to carbon monoxide and water Or, to quote another example, why do carbon monoxide and hydrogen form methane in the presence of nickel, whereas quantitative yields of methanol are produced with a zinc chromite catalyst.6... 

Conventional technology of the hydrogenolysis of fatty acid methyl esters to the corresponding fatty alcohols uses copper chromite or zinc chromite based catalysts and the manufacturing process requires high pressures (200-300 bar) and temperatures (250-300 °C). The activity of copper chromite catalysts was significantly increased by the addition of zinc. ... 

These reactions are accelerated by zinc chromite catalyst. The rate of conversion and heat of formation increase with pressure. Typical optimum conditions are about 250 atmospheres and 300°C. 

First methanol synthesis plant, opened by BASF at Merseburg, using a zinc chromite catalyst... 

Following the introduction of a copper chromite catalyst based on the DuPont recipe for zinc chromite, a further copper catalyst was developed from experimental work related to the high-pressure methanol synthesis process." " ... 

Precipitated copper oxide/zinc oxide catalysts were more active for a range of reactions than zinc chromite but lost activity as the copper was poisoned by gaseous impurities in the synthesis gas. The two oxides were found to be mutually promoting in methanol synthesis because the mixture of very small crystallites was more active than the individual oxides. 

Catalysts used for the hydrogenation step are usually copper chromite formulations, although copper oxide/zinc oxide catalysts have also been used. The process accounts for about half of the copper chromite catalysts used commer-dally. Both acid group and double bonds in the long carbon chain are hydrogenated during the reaction, which produces a saturated alcohol. When an unsaturated fatty alcohol is required, a more selective zinc chromite catalyst may be used. 

The catalyst copper chromite, used in all three processes, also hydrogenates all double bonds in the chain, and completely saturated alcohols are the result. With use of a zinc-containing mixed catalyst, the double bonds in unsaturated starting materials are retained to an extent of 98%. Highly unsaturated alcohols are recovered. 

Hydrogenation. Gas-phase catalytic hydrogenation of succinic anhydride yields y-butyrolactone [96-48-0] (GBL), tetrahydrofiiran [109-99-9] (THF), 1,4-butanediol (BDO), or a mixture of these products, depending on the experimental conditions. Catalysts mentioned in the Hterature include copper chromites with various additives (72), copper—zinc oxides with promoters (73—75), and mthenium (76). The same products are obtained by hquid-phase hydrogenation catalysts used include Pd with various modifiers on various carriers (77—80), Ru on C (81) or Ru complexes (82,83), Rh on C (79), Cu—Co—Mn oxides (84), Co—Ni—Re oxides (85), Cu—Ti oxides (86), Ca—Mo—Ni on diatomaceous earth (87), and Mo—Ba—Re oxides (88). Chemical reduction of succinic anhydride to GBL or THF can be performed with 2-propanol in the presence of Zr02 catalyst (89,90). 

High pressure processes P > 150 atm) are catalyzed by copper chromite catalysts. The most widely used process, however, is the low pressure methanol process that is conducted at 503—523 K, 5—10 MPa (50—100 atm), space velocities of 20, 000-60,000 h , and H2-to-CO ratios of 3. The reaction is catalyzed by a copper—zinc oxide catalyst using promoters such as alumina (31,32). This catalyst is more easily poisoned than the older copper chromite catalysts and requites the use of sulfiir-free synthesis gas. 

Copper—cadmium and zinc—chromium oxides seem to provide most selectivity (38—42). Copper chromite catalysts are not selective. Reduction of red oil-grade oleic acid has been accompHshed in 60—70% yield and with high selectivity with Cr—Zn—Cd, Cr—Zn—Cd—Al, or Zn—Cd—A1 oxides (43). The reduction may be a homogeneously catalyzed reaction as the result of the formation of copper or cadmium soaps (44). 

Hydrogenolyses of carboxylic acids and esters to the corresponding aldehydes seems very attractive due to their simplicity. Copper chromites are the most widely used catalysts.15 Raney copper and zinc oxide-chromium oxide have also been used for this process.16-18 The hydrogenolysis of methyl benzoate to benzaldehyde was studied on various metal oxides at 300-350°C. ZnO, Zr02 and Ce02 presented high activities and selectivities (Scheme 4.8). 

Pure decarbonylation typically employs noble metal catalysts. Carbon supported palladium, in particular, is highly elfective for furan and CO formation.Typically, alkali carbonates are added as promoters for the palladium catalyst.The decarbonylation reaction can be carried out at reflux conditions in pure furfural (165 °C), which achieves continuous removal of CO and furan from the reactor. However, a continuous flow system at 159-162 °C gave the highest activity of 36 kg furan per gram of palladium with potassium carbonate added as promoter. In oxidative decarbonylation, gaseous furfural and steam is passed over a catalyst at high temperatures (300 00 °C). Typical catalysts are zinc-iron chromite or zinc-manganese chromite catalyst and furfural can be obtained in yields of... 

Most synthetic camphor (43) is produced from camphene (13) made from a-pinene. The conversion to isobomyl acetate followed by saponification produces isobomeol (42) in good yield. Although chemical oxidations of isobomeol with sulfuric/nitric acid mixtures, chromic acid, and others have been developed, catalytic dehydrogenation methods are more suitable on an industrial scale. A copper chromite catalyst is usually used to dehydrogenate isobomeol to camphor (171). Dehydrogenation has also been performed over catalysts such as zinc, indium, gallium, and thallium (172). 

Tishchenko (79), using a modified form of Raney nickel, obtained a 95.7 % yield of camphor from the dehydrogenation of borneol. Rutovskii, (80) received a 93.5% yield of camphor with Raney alloy. Reeves and Adkins (81), studying the dehydrogenation of primary alcohols, removed the hydrogen with ethylene. It was found that, though Raney nickel could be used for a catalyst for the reaction, the yields were low and, in general, the Raney nickel was inferior to a catalyst composed of copper, zinc, nickel, and barium chromite. 

Fatty alcohols are obtained by direct hydrogenation of fatty acids or by hydrogenation of fatty acid esters. Typically, this is performed over copper catalysts at elevated temperature (170°C-270°C) and pressure (40-300 bar hydrogen) [26], By this route, completely saturated fatty alcohols are produced. In the past, unsaturated fatty alcohols were produced via hydrolysis of whale oil (a natural wax occurring in whale blubber) or by reduction of waxes with sodium (Bouveault-Blanc reduction). Today, they can be obtained by selective hydrogenation at even higher temperatures (250°C-280°C), but lower pressure up to 25 bar over metal oxides (zinc oxide, chromium oxide, iron oxide, or cadmium oxide) or partially deactivated copper chromite catalysts [26],... 

The decarbonylation of furfural to give furan is best carried out at rather high temperatures. The following catalysts have been described Pd or Pd on charcoal,30 calcium oxide,31 32 zinc and iron chromite,33 or zinc, chromium, and manganese oxide (from ammonium chromate and manganese nitrate).34 The optimum reaction temperature with... 

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