Chromic oxide gel

Formula Cr(0H)3 3H20 MW 157.06 occurs only as hydrates Synonyms chromic hydroxide chromic oxide hydrous chromic oxide gel chromium hydrate chromic hydrate. 

Chromic oxide gel, chromium on aluminum, nickel-aluminum oxide Phosphoric acid on kieselguhr... 

Clarke, Kassel, and Storeh5 interpret the slow adsorption on chromic oxide gel as in part a diffusion of an already adsorbed layer to secondary centres of adsorption but Burwell and Taylor criticize the whole diffusion hypothesis, partly on the grounds that the temperature coefficient of the rate of adsorption is not that to be expected for diffusion.6 It may not, however, be easy to distinguish between the gross characteristics of an activated diffusion along the surface, and of an activated adsorption indeed it seems just admissible to consider the former as a localized form of the latter. 

Chromic Hydroxide. Chromium hydroxide, chromic oxide gel chromic oxide, hydrous. CrH3Oj mol wt 103.03. Cr 50.48%. H 2-93%, O 46.59%. Cr(OH)j. Occurs only as hydrates. Prepn Ruthroff, Inorg. Syn. 2, 190 (1946) Hein, Herzog in Handbook of Preparative Inorganic Chemistry vol. 2, G- Brauer, Ed, (Academic Press, New York, 2nd ed- 1965) pp 1345-1346. 

Thermograms of precipitated chromic oxide gels show endothermic effects indicating that part of the water is loosely bound and part rig-dly bound. Maximum surface area is found at a temperature at which complete expulsion of water seems to take place. These endothermic effects are followed by an exothermic peak due to crystallization of Cr20s. D.T.A. of gels heated up to 600° in nitrogen atmosphere fails to show any exothermic peak. X-ray diffraction and surface area results are in conformity with those of D.T.A. 

The technique of differential thermal analysis (D.T.A.) has been extensively employed in the study of clay and other minerals for elucidating their structures for more than three decades. The application of D.T.A. as a tool has not been widely made to the systematic study of solid catalysts. Only a few references on the subject could be foimd 1-6). In the present article, differential thermal studies of a number of solid catalysts like chromic oxide gels, ferric oxide gels, and chromic oxide-ferric oxide are reported. An attempt has also been made to correlate the data with x-ray... 

The chromic oxide gels were prepared by precipitation followed by careful washing. In all cases, both air-dried and oven-dried (dried at 110° for 24 hrs.) samples were prepared. 

Fig. 1. Differential thermal analysis of ammonia-precipitated chromic oxide gel-----, Air dried.----, Oven dried. 

Fig. 2. Differential thermal analysis of chromic oxide gels prepared from various...

Chromic oxide gel heated in air shows that the gel remains amorphous up to a temperature of 300°. At 350°, a very faint x-ray pattern is obtained. The gel heated to 400° shows a distinct pattern of crystalline chromic oxide. It has been reported that ammonia-precipitated gel crystallizes at 350° and the sodium hydroxide-precipitated gel crystallizes at 400°C 10). The gel heated in vacuum up to a temperature of 500° fails to show a clear pattern of crystalline Cr203, as may be expected from the results of D.T.A. 

Table I gives the surface areas of chromic oxide gels heated to different temperatures. The specific surface of the gel heated in vacuum to higher temperatures progressively decreases from 235.9 m. /g. at 100° to 74.6...

Surface Areas of Thermally Treated Chromic Oxide Gels... 

It may be deduced from the thermograms of chromic oxide gels that the complete expulsion of water takes place just below 345°, and hence this temperature should correspond to maximum surface area. 

An analogous third series of gels was prepared in which the ferric oxide gel and chromic oxide gel were separately precipitated and mixed in moist condition. 

Mixed gels of hydrous ferric and chromic oxides were prepared by the addition of an equivalent amount of ammonium hydroxide to mixtures of the solutions of ferric nitrate (0.5M with respect to Fe20j) and chromic nitrate (0.5M with respect to Cr203). The dual gels were washed free of nitrate ions and air- or oven-dried. A series of mixtures corresponding to 20, 40, 60, and 80% Fe203 was prepared. 

The effect will be clear from a comparison of the magnetic properties of chromium oxide gel with those of massive crystalline chromic oxide. Chromium oxide gel may be made by precipitation of the hydroxide from a nitrate solution, followed by slow dehydration. Several other processes are available, of which slow reduction from a basic chromate solution is one. On ignition, these gels generally undergo the glow-phenomenon during which they revert to Crystalline chromic oxide. 

Table II shows the magnetic susceptibility of a chromium oxide gel compared with that of crystalline chromic oxide. The gels always contain some water, hence a more striking comparison is made by calculating the susceptibility of the chromium ions in each substance.

Chromic oxide on silica gel is inert. Cobalt chromite is very weak. 

The catalyst preparation appeared to be a very simple procedure and consisted of impregnation a silica support, made by precipitation of silica gel, with a soluble chromium salt. It was found that siloxyl chromium complexes were formed by the reaction of chromic oxide with the hydroxyl groups on the silica surface, as the catalyst was activated prior to operation. The stracture of the... 

Compounds containing susceptible C—H bonds can be oxidized to alcohols.134 Nearly always, the C—H bond involved is tertiary, so the product is a tertiary alcohol. This is partly because tertiary C—H bonds are more susceptible to free-radical attack than primary and secondary bonds and partly because the reagents involved would oxidize primary and secondary alcohols further. In the best method the reagent is ozone and the substrate is absorbed on silica gel.135 Yields as high as 99% have been obtained by this method. Other reagents, which often give much lower yields, are chromic acid,136 alkaline permanganate,137 potassium... 

Chromic acid deposited on silica gel from the anhydride CrOs in aqueous (or in aqueous acidified) solutions affords a useful oxidant. 

Colloidal Chromium Hydroxide.—The hydrosol is obtained as a deep greeir solution by the peptisation of the hydroxide by means of chromic chloride, or by a solution of copper oxide in ammonia. As already stated, the freshly precipitated hydroxide forms an apparently clear green solution with excess of an alkali hydroxide. That the chromic hydroxide is peptised and not dissoh ed is shown by the fact that it can be completely filtered out by means of a collodion, filter, leaving a colourless filtrate. The colloidal solution is stable while hot, but slowly yields a gel on keeping at ordinary temperatures. ... 

Oxidation of Alcohols to Aldehydes and Ketones with Chromic Acid Adsorbed on Silica Gel [5J7]... 

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