Andrea Leoncini

Alkylation and pre-organization of diglycolamide ligands on flexible platforms for nuclear waste treatment

The research topic of this thesis concerns the synthesis and evaluation of new diglycolamide (DGA) ligands for the extraction and (potential) separation of actinide and lanthanide ions from bulk spent nuclear fuel. This work was aimed at the development of new pre-organised ligands with improved extraction and separation efficiency. This was done by focusing on the preparation of rather simple, easily accessible and completely incinerable ligands to be used in different processes in the nuclear waste industry. Chapter 3 describes two new simple methods for the synthesis of DGAs, based on the Schotten-Baumann reaction and the Al(III)-catalysed direct amidation of esters. These reactions were also employed for the synthesis of some of the ligands studied in Chapter 4, which deals with small modifications of the backbone of TODGA (tetraoctyldiglycolamide) and its influence on the extraction and degradation behaviour of the ligands. Functionalisation of the TODGA backbone with alkyl chains exhibited increased selectivity and better stripping properties; comparison of the extraction properties of two diastereomeric ligands highlighted that even steric interactions between small methyl groups can have a significant impact on the performance of the ligands. In Chapters 5 through 7, the synthesis of pre-organised DGA ligands and their extraction behaviour in molecular solvents and imidazolium-based ILs is reported. Tripodal ligands based on the flexible tris(N-alkylaminoethyl)amine platforms are described in Chapter 5, whereas generation-0 through generation-2 poly(propylene imine) dendritic platforms are used in Chapter 6, and DGA ligands based on the more rigid 1,3,5-trifunctionalised benzene platform are described in Chapter 7. The results of the tripodal DGA ligands showed that the interplay between the platform and the solvent system is a fundamental contributor to the extraction properties and that, while pre-organisation provides a general increase in extraction efficiency, flexible platforms have the tendency to afford higher selectivity than rigid ones. RTIL systems proved their importance as a tool to modify the extraction behaviour, also via a change in extraction mechanism. The results described in this thesis have the potential to lead to the development of more selective ligands for the separation of actinides and lanthanides and, ultimately, to more environmentally friendly reprocessing processes and long-term storage strategies.

Liquid-liquid extraction and facilitated transport of f-elements using an N-pivot tripodal ligand

Diglycolamide (DGA)-functionalized tripodal ligands offer the required nine-coordinated complex for effective binding to a trivalent lanthanide/actinide ion. A N-pivot tripodal ligand (TREN-DGA) containing three DGA pendant arms was evaluated for the extraction and supported liquid membrane transport studies using PTFE flat sheets. Solvent extraction studies indicated preferential extraction of 1:1 (M:L) species, while the metal ion extraction increased with increasing HNO3 concentration conforming to a solvated species extraction. Flat sheet-supported liquid membrane studies, carried out using 4.0 × 10-3 M TREN-DGA in 95% n-dodecane + 5% iso-decanol indicated faster mass transport for Eu3+ ion as compared to Am3+ ion. The determined transport parameters indicated slow diffusion of the M-TREN-DGA (M = Am or Eu) complex being the rate-determining step. The transport of lanthanides and actinides followed the trend: Eu3+ > Am3+∼ Pu4+ >> UO22+ and Am can be selectively separated from a mixture of U and Pu by oxidizing the latter to its +6 oxidation state. The liquid membrane stability was not encouraging and was deteriorating the transport efficiency with time, which was attributed to carrier loss into the aqueous phases.

Unique selectivity reversal in actinide-lanthanide extraction in a tripodal tren-based diglycolamide in ionic liquuid: extraction, luminescence, complexation and structural studies

An N-pivot diglycolamide extractant (DGA-TREN) was synthesized for the first time and its complexation behaviour was studied towards trivalent lanthanide/actinide ions. The solvent extraction studies suggested a unique selectivity reversal in the extraction of trivalent actinides versus trivalent lanthanides which was observed performing extraction studies in an ionic liquid vis-à-vis a molecular diluent for a tripodal TREN-based diglycolamide ligand (DGA-TREN) vs. a tripodal diglycolamide ligand (T-DGA) which may have great significance in radioactive waste remediation. The nature of the bonding to Eu(3+) ion was investigated by EXAFS as well as by DFT calculations.

Wall-Coated Polymer Brushes as Support for Chiral Organocatalysts in Microreactors

Cinchona alkaloid and proline derivatives as enantioselective catalysts were covalently attached onto the inner walls of a microreactor using glycidyl methacrylate polymer brushes. The successful formation of the organocatalystfunctionalized brush layers on flat silicon oxide surfaces was confirmed by several techniques such as Fourier transform infrared (FT-IR), ellipsometry, and X-ray photoelectron spectroscopy (XPS). The applicability of the cinchona alkaloid (cinchonidine or quinidine)- and proline-containing polymer brushes in a microreactor was demonstrated for the Diels-Alder reaction between anthrone and N-substituted maleimides, and the aldol reaction between 4-nitrobenzaldehyde and cyclohexanone, respectively, which showed moderate conversions (up to 55% and 23%, respectively) and moderate to good enantioselectivities (up to 55% and 93%, respectively). The pristine catalytic activity of the microreactor was intact even after 1 month.

Complexation thermodynamics of tetraalkyl diglycolamides with trivalent f-elements in ionic liquids: spectroscopic, microcalorimetric and computational studies

The thermodynamics of lanthanide complexation with a series of diglycolamides (DGAs) containing varying alkyl chain lengths (methyl, ethyl, n-butyl and n-hexyl) was studied in [C4mim][Tf2N], a room temperature ionic liquid. The stability constants, enthalpies and entropies of complexation were determined by spectrophotometry and calorimetry. All the DGA ligands formed a 1 : 3 (Nd3+/DGA) complex as the limiting species, and their stability constants increased linearly with increasing alkyl chain length. The stability constants of the Nd3+/DGA complexes are orders of magnitude higher in [C4mim][Tf2N] as compared to those observed in aqueous medium. For all the complexes, the enthalpy of complexation was negative with a positive entropy change, indicating that the complexation was driven by both enthalpy and entropy. The enthalpy changes observed in [C4mim][Tf2N] medium were more exothermic than those in the aqueous medium. Fluorescence lifetime data indicated that the complexation proceeded via the replacement of water molecules from the primary coordination sphere of the metal ion. DFT calculations were performed on the structures of the different Nd3+/tetramethyl DGA (TMDGA) complexes, and the corresponding free energies in both gas and solution phases were calculated. Insights into the structural features by DFT studies confirmed that the nature of Ln3+/DGA complexes formed in [C4mim][Tf2N], n-dodecane, and aqueous medium and that in the crystalline state is identical.