Chromium oxide sols

The overhydrogenation of aromatic aldehydes beyond benzylic alcohols is rarely important with copper-chromium oxide and Raney nickel unless the reaction conditions are too vigorous. Thus, over copper-chromium oxide o- and p-methoxybenzaldehydes were hydrogenated to the corresponding methoxybenzyl alcohols in high yields in methanol at 110-125°C and 22-24 MPa H2 (eq. 5.18).42 At 185°C, however, / -methoxybenzyl alcohol was hydrogenolyzed to give p-cre-sol methyl ether (eq. 5.18). 

Chromic Oxide. (Chromium Oxide, Chromium Sesquioxide, Chromium Hemitrioxide or Green Cinnabar) [Chrom (lll)-oxyd in Ger], Cr20, mw 152,02, dk-grn amorphous pdr or hex crysts, mp 1990 d5.2l insol in w, s 1 sol in ale can be prepd by heating Amm dichromace or Na dichromate with sulfur and washing out the Na sulfate (Refs 1, 2,3 5). 

To a suspension of tin(IV) oxide gel in triply distilled water was added a solution of chromium(VI) oxide and the solution stirred for 24h. The resulting yellow mixture was filtered and the yellow solid air dried at 60°C for 24h. If desired, tlie powdery material was then washed with distilled water imtil the washings were colourless. The resulting material was again air dried. Copper(II)-doped tin(lV) oxide was either prepared in a similar manner using a solution of copper(II) nitrate, or by coprecipitation from an aqueous solution containing tin(IV) chloride and copper(II) nitrate in the required ratio. Both chromiun(VI)- and copper(II)-doped catalysts were also prepared by treatment of tin(VI) oxide sol-gel using aqueous solutions of cliromium(VI) oxide or copper(II) nitrate. Mixed Sn-Cu-Cr-0 catalysts were obtained by the sol-gel route. 

E. Barouch, Heterocoagulation. VI. Interactions of monodispersed hydrous aluminum oxide sol with polystyrene latex, /. Colloid Interf. Sci. 1980, 76,319-329 (c) K. Csoban, E. PefFerkorn, Perikinetic aggregation induced by chromium hydrolytic polymer and sol,/. Colloid Interf. Sci. 1998, 205,516-527. 

With Acyl Halides, Hydrogen Halides, and Metallic Halides. Ethylene oxide reacts with acetyl chloride at slightly elevated temperatures in the presence of hydrogen chloride to give the acetate of ethylene chlorohydrin (70). Hydrogen haUdes react to form the corresponding halohydrins (71). Aqueous solutions of ethylene oxide and a metallic haUde can result in the precipitation of the metal hydroxide (72,73). The haUdes of aluminum, chromium, iron, thorium, and zinc in dilute solution react with ethylene oxide to form sols or gels of the metal oxide hydrates and ethylene halohydrin (74). 

Chromium adds oxidatively not only to vinylic iodides, but also to allylic, aryl, and alkynyl iodides, as well as to CHE (see Chap ter 13).18 The corresponding triflates can be employed equally well. On the other hand the reactivity of bromides and chlorides is usually too limited. Reaction is carried out in the polar aprotic sol vent DMF because this is capable of dissolving both of the salts, and homogeneous conditions accelerate the reaction. 

At several points within the text we have referred to instances in which isotope studies have furnished information concerning the possible path of a reaction. For example, the oxidation of SOl with ClOj in water enriched with O18 (p. 366) yields SO " having practically no enrichment with O18, indicating that the reaction has taken place by direct oxygen transfer from CIO to SQ. Similarly, the oxidation of Or(II) with Co(NH3)5Cl2+ must proceed through transfer of a chlorine atom from the cobalt ion to the chromium ion, for it has been shown that one Cr—Cl bond is formed for each chromium(II) ion oxidized. Representing the reaction schematically ... 

In general, both cordierite and metallic monoliths are unsuitable as catalytic supports. To process a monolith into an active monolithic catalyst, a layer of porous catalytic support must be deposited on the walls between channels. y-Alumina appeared to be the most effective support for automotive catalysts. The alumina layer is deposited by sol-gel technique (so called washcoating). Adherence of 7-alumina to cordierite is relatively strong. However, to form the stable 7-alumina layer on a metallic surface, we need to use an appropriate alloy that is appropriately processed before the layer is deposited. Stainless steel containing chromium, aluminum, and yttrium subjected to thermal treatment under oxidizing conditions meets requirements of automotive converters. Aluminum in the steel is oxidized to form 7-alumina needles (whiskers) protruding above the metal... 

Soudan, P., Pereira-Ramos, J.P, Farcy, J., Gregoire, G., and Baffler, N. 2000. Sol-gel chromium-vanadium mixed oxides as lithium insertion compounds. Solid State Ionics 135, 291-295. 

Colour systems suitable for use in the spectrophotometric method may also be formed in redox reactions. Some examples of such reactions are the oxidation of Mn(II) to Mn04" or Cr(III) to Cr04, oxidation of dimethylnaphthidine with vanadium(V) or chromium(VI), oxidation of o-tolidine with cerium(lV) or with chlorine. Examples of oxidation reactions are also the iodide methods, in which iodide ions are oxidized with bromine to give iodate ions which, in turn, react with the excess of iodide anions to form free iodine (see Chapter 25). A colour effect of reduction also occurs, for example, in determinations of Se and Te in the form of coloured sols produced in the reduction of Se(lV) or Te(IV) to their elementary forms. 

Until comparatively recently, the only reported stable inorganic hydrosols were primarily sols of elements such as gold, sulphur, selenium, etc. and compoimds such as silica, lead iodate, silver halides, etc. A considerable amount of attention is now being paid, however, to the preparation of mono-dispersed hydrous metal oxides, which are chemically considerably more complex than other crystalline or stoichiometrically well-defined materials and are of interest as potential catalysts. Examples include the hydrous oxides of chromium and aluminium (spheres) and copper and iron (polyhedra) with particle sizes < 1 pm. One manufacturing procedure consists of ageing aqueous... 

Properties Semi-gray hard brittle metal sol. in acids (except nitric) and strong alkalis insol. in water a.w. 51.996 dens. 7.1 m.p. 1900 C b.p. 2200 C corrosion resist, to air and water attacked by nonoxidizing acids and alkalis Toxicology TLV (dust and fume) 0.5 mg/m of air metal dust is an irritant hexavalent chromium compds. are suspect human carcinogens human poison by ing. may cause Gl disturbances, kidney damage, and circulatory shock TSCA listed Precaution Powd. will explode spontaneously in air ignites and is potentially explosive in CO2 atm. other violent reactions possible incompat. with oxidants... 

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