Lanthanides exchange

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

Probably the biggest use in terms of quantity is that of lanthanide-exchanged zeohtes as cracking catalysts in the petrochemical industry (mainly La and Ce), using about 5001 a day in the USA alone. 

Heterogeneous catalysis, with lanthanides Zeolites, lanthanide exchanged, in catalysis Cracking catalysts, lanthanide Activity-stabilization, lanthanide Homogeneous catalysis, with lanthanides... 

No reports of studies of metal ions with high co-ordination numbers have appeared apart from the lanthanide exchange reactions noted in Sections 7 and 8. 

No study of pH dependent and independent rat of lanthanide exchange has been reported for DTPA or DTPA-BMA. The values for are estimates, based on the assumption that the pH dependent pathway will dominate at pH = 1.0. 

Evidence other than that of ion-exchange favours the view of the new elements as an inner transition series. The magnetic properties of their ions are very similar to those of the lanthanides whatever range of oxidation states the actinides display, they always have -1-3 as one of them. Moreover, in the lanthanides, the element gado-... 

Fig. 3. The elution of tnpositive actinide and lanthanide ions. Dowex-50 ion-exchange resin was used with ammonium a-hydroxyisobutyrate as the eluant.

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

Because of the technical importance of solvent extraction, ion-exchange and precipitation processes for the actinides, a major part of their coordination chemistry has been concerned with aqueous solutions, particularly that involving uranium. It is, however, evident that the actinides as a whole have a much stronger tendency to form complexes than the lanthanides and, as a result of the wider range of available oxidation states, their coordination chemistry is more varied. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

Certain lanthanide alkoxides, such as r-BuOSmL, have also been found to catalyze hydride exchange between alcohols and ketones.194 Isopropanol can serve as the reducing agent for aldehydes and ketones that are thermodynamically better hydride acceptors than acetone. 

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