A Framework for Lanthanide Speciation Analysis in an Aqueous Carboxylate Ionic Liquid

Carboxylate-based ionic liquids have been proposed as promising candidates for rare earth element (REE) separation. However, identifying the chemical basis for separation is challenging due to multiple complexes forming between the REEs and the carboxylate ligands. We developed two semi-quantitative methods to identify changes in speciation across the lanthanide series: (ⅰ) paramagnetic NMR (pNMR) that exploits lanthanide magnetic properties applying the Bleaney theory with the Evans method, and (ⅱ) scoring thermal decomposition curves obtained from thermogravimetric analysis (TGA) using dynamic time warping (DTW). Herein, we investigated Ln3+ speciation in aqueous protonated betaine bis(trifluoromethylsulfonyl)imide ([HBet][Tf2N]) ionic liquid systems using these methods in concert with cloud point temperatures of Ln3+ containing aqueous [HBet][Tf2N] systems and density function theory (DFT) calculations for isomerization energies between two known lanthanide/betaine complexes ([Ln2(Bet)4(H2O)8]6+ with coordination numbers of 8 and 9). Descriptors from pNMR and TGA classify solutions of lanthanides in aqueous [HBet][Tf2N] into three groups with structural transitions at Gd3+ and Er3+, consistent with the UCST trends. Small differences in energies for the complexes with heavy lanthanides suggest thermodynamics governs the exchange between multiple different complexes, whereas larger energy differences for the light lanthanides complexes kinetically controlled changes in speciation, according to the Bell-Evans-Polanyi relation. Thus, this multimodal framework provides unique insight into lanthanide speciation in complex carboxylate-based extraction systems.