Experimental and theoretical insight into biphasic extractive mass transfer of thorium into ionic liquid phase using chloroamide ligands

Abstract

2-chloro-N-methyl-N-phenylacetamide (L1) and 2-chloro-N-(3-methylpyridin-2-yl)acetamide (L2) have been explored for the efficient separation of tetravalent thorium ion (Th4+) from aqueous phase to the ionic liquid phase. Due to the positive inductive effect (+I effect) of alkyl group, the electron density on N in L1 was more compared to that in L2; as evidenced from the distribution ratio values of Th4+ (DThL1 ∼ 180 and DThL2 ∼ 107 at 1 M HNO3). The extraction proceeds via 'cation exchange' mechanism involving 1:3 metal ligand stoichiometry for both the ligands. Each thorium complex was associated with three nitrate ions; thus, making the complex uni-positive in nature. The charge compensation during extractive mass transfer was done by dissolution of one hexyl methylimidazolium cation from ionic liquid phase to aqueous phase. The mass transfer followed slower kinetics, which was attributed to the high viscosity coefficient of ionic liquid. The extraction processes were spontaneous (ΔGL1300K ∼ −16.31 kJ mol−1 and ΔGL2300K ∼ −15.75 kJ mol−1) and endothermic in nature with a significant enhancement in the entropy (ΔSL1300K ∼ 107.98 kJ mol- 1K−1 and ΔSL2300K ∼ 103.54 kJ mol-1K−1). Density functional theory (DFT) calculation revealed the presence of two types of Th-O bond associated with nitrate ion and ligands (Th-OnitrateL1 ∼ 2.511A˚, Th-OligandL1 ∼ 2.411A˚, Th-OnitrateL2 ∼ 2.518A˚ and Th-OligandL2 ∼ 2.422A˚. The higher extraction efficiency of Th4+ for L1 was corroborated from the negative ΔG values calculated for L1 in gas and ionic liquid phase compared to that for L2.