Lanthanide cycling

Lanthanide complexation by organic 7. Lanthanide cycling in anoxic marine basins, ... 

To reinforce the arguments for the role of surface coatings in lanthanide cycling, the compositional patterns of fig. 16 can be considered in more detail. The filtered seawater... 

When this analysis was first attempted [9-11] very few values of 1 had been obtained from series limits in the third spectra of the lanthanides, and the first comprehensive sets were calculated from Born-Haber cycles [9]. Subsequent spectroscopic values [12] confirmed the early work and are plotted in Eig. 1.1. In all cases they refer to the ionization process... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

Low-valent lanthanides represented by Sm(II) compounds induce one-electron reduction. Recycling of the Sm(II) species is first performed by electrochemical reduction of the Sm(III) species [32], In one-component cell electrolysis, the use of sacrificial anodes of Mg or A1 allows the samarium-catalyzed pinacol coupling. Samarium alkoxides are involved in the transmet-allation reaction of Sm(III)/Mg(II), liberating the Sm(III) species followed by further electrochemical reduction to re-enter the catalytic cycle. The Mg(II) ion is formed in situ by anodic oxidation. SmCl3 can be used in DMF or NMP as a catalyst precursor without the preparation of air- and water-sensitive Sm(II) derivatives such as Sml2 or Cp2Sm. 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

The alloy can also be used as a reductant to recycle samarium(iii) to samarium(ii). Because pinacolate ligand exchange from samarium to light lanthanides proceeds smoothly, addition of MesSiCl is not necessary to complete the catalytic cycle (Schemes 4 and 5). 

The unique properties of lanthanide-based materials, e.g., lanthanide-silicates and lanthanide-doped silicas, can be related to the special properties of the 4f" orbitals. Among lanthanide oxides, only Ce, Pr and Tb form dioxides, which crystallize in one simple structure with M4+ ions showing octahedral coordination [17]. For instance, cerium dioxide exhibits an 8 4 catiomanion coordination [18]. Its characteristic feature is the ability to undergo oxidation-reduction cycles in a reversible way [19], It was shown that the presence of Ce and La additives in mesoporous silicas, e.g., MCM-41 [10,11] and MSU-X [12], improves their thermal and hydrothermal stability. 

Peterman, D.R., Law, J.D., Todd, T.A., Tillotson, R.D. 2006. Use of Cyanex-301 for separation of Am/Cm from lanthanides in an advanced nuclear fuel cycle. In Separations for the Nuclear Fuel Cycle in the 21st Century. Lumetta, G.J. et al. Eds. ACS Symposium Series Vol. 933, American Chemical Society, Washington, DC, pp. 251-259. 

Di-iso-decylphosphoric Acid The DIDPA Process An(III) and Ln(III) can be partitioned using the DIDPA solvent (DIDPA and TBP, respectively dissolved at 0.5 and 0.1 M in n-dodecane) in a two-step process approach. First coextracted and costripped in a 4 M nitric acid solution in a first DIDPA cycle (see Section 3.3.1.1.4), the An(III) + Ln(III) fraction is partitioned in a second cycle after denitration of the An(III) + Ln(III) product by formic acid to reduce the nitric acid concentration to at least 0.5 M. In this second DIDPA cycle, An(III) and Ln(III) are first coextracted by the DIDPA solvent, and the An(III) are selectively stripped by DTPA (0.05-0.1 M) in a solution buffered at pH 3 with lactic acid (1 M). The triva-lent lanthanides are further stripped with a 4 M nitric acid solution (134). 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

Meridiano, Y., Berthon, L., Lagrave, S., Crozes, X., Sorel, C., Testard, F., Zemb, T. 2008. Correlation between aggregation and extracting properties in solvent extraction systems Extraction of actinides (IB) and lanthanides (III) by a malonamide in non acidic media. ATALANTE 2008 Nuclear Fuel Cycles for a Sustainable Future, May, Montpellier, France. 

Modolo, G., Asp, H., Vijgen, H., Malmbeck, R., Magnusson, D., Sorel, C. 2007. Demonstration of a TODGA/TBP process for the recovery of bivalent actinides and lanthanides from a PUREX raffinate. Global 2007 Advanced Nuclear Fuel Cycles and Systems, September, Boise, ID. 

The flowsheet of the UREX process, developed in the United States, includes the following extraction cycles (1) separation of uranium and technetium, (2) separation of plutonium, (3) separation of cesium and strontium, (4) separation of MAs and Rare Earth Elements (REE), and (5) group separation of MA from REE metals.9,10 Flowsheet development in Europe11 includes a modified PUREX process and, after that, the DIAMEX process for separation of MAs and lanthanides, the SANEX process for separation of MAs from lanthanides, and a special cycle for Am/Cm separation. Cesium and strontium will be in the raffinate of the DIAMEX process, and this raffinate will be vitrified, or cesium can be preliminarily extracted.12... 

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