Cerium salts, oxidations

Alternative cerium salt oxidants with limited suitability have been discussed. Whereas cerium-IV sulfate Ce(S04)2 resulted in reasonable conductivities, but Ce-contaminated PEDOT, (NH4)2Ce(N03)6 yielded PEDOT with only low conductivity. So cerium-based oxidants do not represent advantages compared to iron-III. 

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

Cerium Oxide. The most stable oxide of cerium is cerium dioxide [1306-38-3] Ce02, also called ceria or ceric oxide. When cerium salts are calcined in air or if oxygen is present, this tetravalent Ce(IV) oxide is formed, cerium sesquioxide [1345-13-7] can be prepared in strongly reducing... 

Ceri-oxyd, n. ceric oxide, cerium(IV) oxide, -salz, rt. ceric salt, cerium(IV) salt. sulfat, n. ceric sulfate, cerium(IV) sulfate. 

M sulphuric acid at 25 °C is 1.43 0.05 volts. It can be used only in acid solution, best in 0.5M or higher concentrations as the solution is neutralised, cerium(IV) hydroxide [hydrated cerium(IV) oxide] or basic salts precipitate. The solution has an intense yellow colour, and in hot solutions which are not too dilute the end point may be detected without an indicator this procedure, however, necessitates the application of a blank correction, and it is therefore preferable to add a suitable indicator. 

As it was shown in73, 74), methods that can be used to synthesize these copolymers of PAN are those of radical AN block copolymerization in the presence of an oxidation-reduction system in which the hydroxyl end groups of polyethylene oxide) (PEO)73) and polypropylene oxide) (PPO)74- oligomers serve as the reducing agents and tetravalent cerium salts as the oxidizing agents. 

The oxidation of an anthracene suspension in sulfuric acid conducted in the presence of cerium salts can serve as an example of mediated oxidation. In the bulk solution the Ce" ions chemically oxidize anthracene to anthraquinone. The resulting Ce ions are then reoxided at the anode to Ce". Thus, the net result of the electrochemical reaction is the oxidation of anthracene, even though the electrochemical steps themselves involve only cerium ions, not anthracene. Since the cerium ions are regenerated continuously, a small amount will suffice to oxidize large amounts of anthracene. 

The work on the electrochemical generation of a solution of ceric sulphate from slurry of cerous sulphate in 1-2 M sulphuric acid was abandoned by BCR due to difficulties encountered in handling slurried reactants. A 6kW pilot reactor operated at 50 °C using a Ti plate anode and a tungsten wire cathode (electrolyte velocity about 2ms 1) produced 0.5 M Ce(S04)2 on the anode with a current efficiency of 60%. The usefulness of Ce(IV) has been limited by the counter anions [131,132], Problems include instability to oxidation, reactivity with organic substrates and low solubility. Grace found that use of cerium salts of methane sulfonate avoids the above problems. Walsh has summarized the process history, Scheme 6 [133],... 

Cerium(IV) oxide may be obtained by heating cerium oxalate, carbonate or other salts at elevated temperatures ... 

Europeum generally is produced from two common rare earth minerals monazite, a rare earth-thorium orthophosphate, and bastnasite, a rare earth fluocarbonate. The ores are crushed and subjected to flotation. They are opened by sulfuric acid. Reaction with concentrated sulfuric acid at a temperature between 130 to 170°C converts thorium and the rare earths to their hydrous sulfates. The reaction is exothermic which raises the temperature to 250°C. The product sulfates are treated with cold water which dissolves the thorium and rare earth sulfates. The solution is then treated with sodium sulfate which precipitates rare earth elements by forming rare earth-sodium double salts. The precipitate is heated with sodium hydroxide to obtain rare earth hydrated oxides. Upon heating and drying, cerium hydrated oxide oxidizes to tetravalent ceric(lV) hydroxide. When the hydrated oxides are treated with hydrochloric acid or nitric acid, aU but Ce4+ salt dissolves in the acid. The insoluble Ce4+ salt is removed. 

A typical NH3 synthesis catalyst (10) contains iron oxide plus 1% K2O, 1-2% AI2O3, and may contain v l% CaO on the surface. After fusion and reduction the surface is largely metallic iron plus reduced promoters concentrated on the surface ( ). Sze and Wang (11) have shown that a catalyst washed with Ce(N03)3 and subsequently reduced is much more active than the conventional catalyst, Table II, Mischmetal salts may be substituted for the cerium salt. 

K bicarbonate, K chloride, K iodide, K metaphosphate, K perchlorate, K phosphate, K silico-fluoride, K urea oxalate, sodium ammonium sulfate, sulfur, zinc sulfate and Zr oxide It was claimed that methylene urea reduced the flash to a far greater extent than any of the organic compounds used, ft was also stated that cerium salts were much more effective than any other metallic salts investigated (Ref 4)... 

In order to generate SOI in acidic media, the reaction between sodium bisulfite and cerium ammonium nitrate should be employed. In (NH4)2Ce(N03)6, cerium has the oxidation state 4+. In acids, valence 4 cerium salts are strong oxidants. With the sulfite ion, the... 

Addition of cerium salts accelerates the electrolytic oxidation of anthracene, naphthalene, and phenanthrene, which yield the corresponding quinones. The hydrocarbons may be in solution or in the form of a finely divided suspension. Anthracene, for example, is oxidised to Anthraqtjinone in 20 per cent, sulphuric acid anode current density is about 5 amps, per dm.2, and by the addition of 2 per cent, of cerium sulphate the current efficiency 8 is stated to reach nearly 100 per cent. 

Pr(N03)3 is exceptional in that the final oxide is Pr6On. The hydrated cerium salt shows no stable intermediates, but goes to Ce02. The above uncertainties could probably be cleared up by TGA studies of anhydrous nitrates. The above data indicate that these should be stable to about 300°C. 

Malonyl radicalsThiophenes and furans undergo substitution at C2 when treated with a dialkyl malonate and ceric sulfate at 25°. The reaction is considered to involve oxidative generation of malonyl radicals by the cerium salt. Yields are... 

Cerous oxalate, Ce2(C2O4)3-10H2O, is formed in the Welsbach treatment of Monazite sands in the production of thorium nitrate, and is the starting point in the preparation of nearly all cerium salts. Since ceric oxide is more easily acted upon by common reagents, it is often prepared as the first step in making other salts from the oxalate. 

Acetyl- and 3-benzoylisoxazoles have been conveniently obtained by one-pot reactions of alkynes with ammonium cerium(IV) nitrate or ammonium cerium(III) nitrate tetrahydrate in acetone or acetophenone these processes probably involve 1,3-DC of nitrile oxides formed by nitration of the carbonyl compound by cerium salts <02TL7035>. 

Cerium dioxide is preferred over other oxides because platinum is expensive and vanadium and iron oxides are poisons for FCC catalysts. Usually an aqueous solution of a cerium salt is used to impregnate a spinel base which is then calcined to obtain the Ce02 active form. We have found that a 10 to 12% Ce02 is sufficient for effective S02 oxidation(1). 

The particles shown in Figure 3.3 can be prepared from tetravalent cerium salt solution (CeS04-4HjO, (NH4)4Ce(S04)4 2H20. and (NH4)2Ce(NO )< ) in low concentrations by low temperature aging in a sealed vesseL The metal ions are solvated by water molecules which can be deprotonated to give hydroxide or oxide particles. This method is very sensitive to the concentration, temperature, and pH value of the solution. 

It maj- also be separated from solutions of uranium salts bj the addition of a little cerium salt and precipitation with aqueous hydrofluoric acid, or with oxalic acid or bj adding a small quantity of thorium nitrate and precipitating with t-nitrobenzoic acid. It may also be absorbed bj- charcoal,iDasic ferric acetate, and by various oxides, sulphides, sulphates, and gelatinous silica. In the case of charcoal the uranium X is completely removed from a solution of a... 

Acetyl- and 3-benzoylisoxazoles 389 (and isoxazolines) have been prepared by one-pot reactions of alkynes (and alkenes) with ammonium cerium(iv) nitrate (CAN(lv)) or ammonium cerium(lll) nitrate tetrahydrate (CAN(m))-formic acid, in acetone or acetophenone. These processes probably involve 1,3-dipolar cycloaddition of nitrile oxides produced via nitration of the carbonyl compound by cerium salts. The existence of nitrile oxides as reaction intermediates was proved by the formation of the dimer furoxan 390 when the above reaction was carried out in absence of any dipolarophile (Scheme 95) <2004T1671>. An analogous improved procedure has been applied to alkynyl glycosides as dipolarophiles for the preparation of carbohydrate isoxazoles <2006SL1739>. 

Figure 15.11 shows x-ray diffraction (XRD) profiles of ceria powders produced by precipitation. The XRD data for the synthesized particles show characteristics of Ce02 with a typical fluorite stracture. Since the starting cerium salt was Ce(N03)3, it required the oxidation of Ce + to Ce + in the solution. In this system, there is a possible cause for this oxidation. According to the Lewis definition of acids and bases, Ce is a Lewis base and Ce + is a Lewis acid. Basic solution therefore favors Ce compared with Ce . The crystallite size was calculated from the Scherrer formula ... 

Continuing, one carbon homologation of 96 was easily achieved by oxidation of the alcohol to the corresponding ketone and a subsequent enolisation-formylation sequence. The last lacking carbon was introduced by nucleophilic addition on the complex keto-sugar 97 using vinyl magnesium bromide in the presence of cerium salts. The diol 98 was further elaborated into a relay compound already prepared from squalestatin [88]. 

Among electron carriers used for indirect oxidation reactions, cerium salts [Ce -t- e Ce E° = -t-1.44 V vs. NHE] appear to be of particular interest when a mild oxidation has to be considered. Substituted toluenes and methylaryl compounds are easily functionalized to the corresponding aldehydes in high yields [125-129]. Acidic solutions are required (such as aqueous AcOH, aqueous methane sulfonic acid, or aqueous trifluorosulfonic acid). The conversion of aromatic compounds into quinones may also be conducted by means of electrogenerated ceric ions (see Table 3). Let us stress the example... 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

A series of manganese-cerium oxide catalysts with different Mn/Ce atomic ratios was prepared by thermal decomposition of amorphous citrate precursors [5]. The samples were prepared by varying the ratio between the manganese and cerium salts. 

Use Production of cerium salts and ceric oxide, opa-cifier in glasses and enamels (imparts yellow color), shielding glass. 

The use of electricity in reactions is clean and, at least in some cases, can produce no waste. Toxic heavy metal ions need not be involved in the reaction. Hazardous or expensive reagents, if needed, can be generated in situ where contact with them will not occur. The actual oxidant is used in catalytic amounts, with its reduced form being reoxidized continuously by the electricity. In this way, 1 mol% of ruthenium(III) chloride can be used in aqueous sodium chloride to oxidize benzyl alcohol to benzaldehyde at 25°C in 80% yield. The benzaldehyde can, in turn, be oxidized to benzoic acid by the same system in 90% yield.289 The actual oxidant is ruthenium tetroxide. Naphthalene can be oxidized to naphthoquinone with 98% selectivity using a small amount of cerium salt in aqueous methanesulfonic acid when the cerium(III) that forms is reoxidized to cerium(IV) electrically.290 Substituted aromatic compounds can be oxidized to the corresponding phenols electrically with a platinum electrode in trifluoroacetic acid, tri-ethylamine, and methylene chloride.291 With ethyl benzoate, the product is a mixture of 44 34 22 o/m/fhhy-... 

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