Next-Generation 3,3'-AlkoxyBTPs as Complexants for Minor Actinide Separation from Lanthanides: A Comprehensive Separations, Spectroscopic, and DFT Study.

Progress toward the closure of the nuclear fuel cycle can be achieved if satisfactory separation strategies for the chemoselective speciation of the trivalent actinides from the lanthanides are realized in a nonproliferative manner. Since Kolarik's initial report on the utility of bis-1,2,4-triazinyl-2,6-pyridines (BTPs) in 1999, a perfect complexant-based, liquid-liquid separation system has yet to be realized. In this report, a comprehensive performance assessment for the separation of 241Am3+ from 154Eu3+ as a model system for spent nuclear fuel using hydrocarbon-actuated alkoxy-BTP complexants is described. These newly discovered complexants realize gains that contemporary aryl-substituted BTPs have yet to achieve, specifically: long-term stability in highly concentrated nitric acid solutions relevant to the low pH of unprocessed spent nuclear fuel, high DAm over DEu in the economical, nonpolar diluent Exxal-8, and the demonstrated capacity to complete the separation cycle with high efficiency by depositing the chelated An3+ to the aqueous layer via decomplexation of the metal-ligand complex. These soft-N-donor BTPs are hypothesized to function as bipolar complexants, effectively traversing the organic/aqueous interface for effective chelation and bound metal/ligand complex solubility. Complexant design, separation assays, spectroscopic analysis, single-crystal X-ray crystallographic data, and DFT calculations are reported.

Prediction of the structures and heats of formation of MO2, MO3, and M2O5 for M = V, Nb, Ta, Pa.

Structures for the mono-, di-, and tri-bridge isomers of M2O5 as well as those for the MO2 and MO3 fragments for M = V, Nb, Ta, and Pa were optimized at the density functional theory (DFT) level. Single point CCSD(T) calculations extrapolated to the complete basis set (CBS) limit at the DFT geometries were used to predict the energetics. The lowest energy dimer isomer was the di-bridge for M = V and Nb and the tri-bridge for M = Ta and Pa. The di-bridge isomers were predicted to be composed of MO2+ and MO3- fragments, whereas the mono- and tri-bridge are two MO2+ fragments linked by an O2-. The heats of formation of M2O5 dimers, as well as MO2 and MO3 neutral and ionic species were predicted using the Feller-Peterson-Dixon (FPD) approach. The heats of formation of the MF5 species were calculated to provide additional benchmarks. Dimerization energies to form the M2O5 dimers are predicted to become more negative going down group 5 and range from -29 to -45 kcal mol-1. The ionization energies (IEs) for VO2 and TaO2 are essentially the same at 8.75 eV whereas the IEs for NbO2 and PaO2 are 8.10 and 6.25 eV, respectively. The predicted adiabatic electron affinities (AEAs) range from 3.75 eV to 4.45 eV for the MO3 species and vertical detachment energies from 4.21 to 4.59 eV for MO3-. The calculated MO bond dissociation energies increase from 143 kcal mol-1 for M = V to ∼170 kcal mol-1 for M = Nb and Ta to ∼200 kcal mol-1 for M = Pa. The M-O bond dissociation energies are all similar ranging from 97 to 107 kcal mol-1. Natural bond analysis provided insights into the types of chemical bonds in terms of their ionic character. Pa2O5 is predicted to behave like an actinyl species dominated by the interactions of approximately linear PaO2+ groups.

Assignment of Vibrational Bands of Critical Surface Species Containing Nitrogen in the Selective Catalytic Reduction of NO by NH3.

The selective catalytic reduction (SCR) of NO by NH3 on metal oxides plays a key role in minimizing NOx emissions. Electronic structure calculations at the density functional theory level have been performed to predict the vibrational modes of NH3/NH4+ bound to validated cluster models of vanadium oxide bound to a TiO2 surface. Excellent agreement of the scaled calculated values with the observed bands attributed to surface-bound species is found. The presence of NH3 bound to Lewis acid sites and NH4+ bound to Brønsted acid sites when VOH groups are present is supported by our predictions. NH4+ is expected to dominate the spectra even at low concentrations, with predicted intensities 5 to 30 times greater than those predicted for surface-bound NH3. This is particularly evident in the lowest-energy N-H stretches of surface NH4+ due to partial proton transfer interactions with the vanadium oxide surface model. The current work is consistent with experimental vibrational spectroscopy results and does not support the presence of a significant amount of NH2 on the catalyst surface for the SCR reaction on VOx/TiO2. The combined experimental and computational results support the presence of both NH3- and NH4+-type species bound to the surface.

Electronic Structure Investigation of NO2 and NO Binding on Vanadium Oxides.

NO2 and NO, which are generated in combustion processes, binding to vanadium oxide clusters including TiO2-supported catalyst models in the selective catalytic reduction (SCR) of NO has been studied by density functional theory and coupled cluster methods. NOx binding on vanadium oxides is predicted to depend on several factors, including the excitation energy of the oxide, ionization energies of both the unbound oxide and the deoxygenated reduced oxide, and the strength of the molecular V-O bonds. NO2 chemisorption occurs either through covalent bond formation in a HONO-like pattern or through abstraction of a metal oxide oxygen leading to the formation of NO3-. Nitrate formation is more favorable than what was predicted for group IVB or group VIB oxides [except (CrO3)n] and is either the lowest energy binding mode or within a few kcal/mol of the lowest mode in all clusters, likely due to the stability of V in the +4 oxidation state. Physisorption on V oxides is very weak. V with 2 oxo groups have a lower excitation energy and a more sterically open geometry which results in strong chemisorption as predicted for group IVB oxides. Tetrahedrally coordinated vanadia with a single oxo group and 3 V-O single bonds are predicted to have significantly higher excitation energies and behave like group VIB oxides such that chemisorption is unlikely and weak physisorption dominates the interaction. In larger clusters, including SCR catalyst models, only tetrahedrally coordinated vanadia are present and NO2 binding is not expected to occur. NO adsorption is weaker overall than NO2 binding and occurs either as physisorption or as chemisorption through the formation of NO2- analogous to nitrate formation in NO2 binding. The ability of NO to bind reflects the patterns predicted for NO2, such that NO is strongly bound vanadia with two V═O groups and only weakly physisorbed when there is a single V═O or none at all.

Observation of Selectively Populated Monohalide Excited States from the Reactions of Group 3 Metal (Sc, Y, and La) Monomers and Dimers with Halogen-Containing Molecules.

The chemiluminescent reactions of the group 3 metals Sc and Y with F2, Cl2, Br2, ClF, ICl (Sc), IBr (Y), and SF6 and La with F2, SF6, Cl2, and ClF have been studied at low pressures (6 × 10-6 to 4 × 10-4 Torr) using a beam-gas arrangement and extended to the 10-3 Torr multiple collision pressure range. Contrary to previous reports, the observed chemiluminescent spectra are primarily attributed to emission from the metal monohalides. Extensive pressure and temperature dependence studies and high-level correlated molecular orbital theory calculations of the bond dissociation energies support this conclusion and the attribution of the chemiluminescence. Evidence for the "selective" production of a monohalide excited electronic state is obtained for several of the Sc and Y reactions. All reactions producing the metal monofluorides are first order with respect to the oxidant, while reactions producing the monochlorides and monobromides are found to be "faster than first order" with respect to the oxidant. This difference is associated with the metal halide bond dissociation energies and the metal halide product internal density of states. Analysis of the temperature dependence for six representative reactions indicates that the "selective" excited-state formation of the metal monohalides proceeds via a direct mechanism with negligible activation energy. We compare and contrast the present results with previous experiments and interpretations which have assigned the selective emission from these systems to the group 3 dihalides produced in a two-step reaction sequence analogous to an electron jump process. The current results suggest a distinctly different interpretation of the observed processes in these systems. The observed selectivity observed in these studies is remarkable given the significant number of known and potential excited states in the scandium and yttrium halides as well as their different electronic configurations.

Prediction of An(III)/Ln(III) Separation by 1,2,4-Triazinylpyridine Derivatives.

The effect of frustrated Lewis donors on metal selectivity between actinides and lanthanides was studied using a series of novel organic ligands. Structures and thermodynamic energies were predicted in the gas phase, in water, and in butanol using 9-coordinate, explicitly solvated (H2O) Eu, Gd, Am, and Cm in the +III oxidation state as reactants in the formation of complexes with 2-(6-[1,2,4]-triazin-3-yl-pyridin-2-yl)-1H-indole (Core 1), 3-[6-(2H-pyrazol-3-yl)pyridin-2-yl]-1,2,4-triazine (Core 2), and several derivatives. These complexations were studied using density functional theory (DFT) incorporating scalar relativistic effects on the actinides and lanthanides using a small core pseudopotential and corresponding basis set. A self-consistent reaction field approach was used to model the effect of water and butanol as solvents. Coordination preferences and metal selectivity are predicted for each ligand. Several ligands are predicted to have a high degree of selectivity, particularly when a low ionization potential in the ligand permits charge transfer to Eu(III), reducing it to Eu(II) and creating a half-filled f7 shell. Reasonable separation is predicted between Cm(III) and Gd(III) with Core 1 ligands, possibly due to ligand donor frustration. This separation is largely absent from Core 2 ligands, which are predicted to lose their frustration due to proton transfer from the 2N to the 3N position of the pyrazole component of the ligands via tautomerization.