Tailoring the pore size of expanded porphyrinoids for lanthanide selectivity

Abstract

Despite increase in demand, capacity for the recycling of rare earth elements remains limited, partly due to the inefficiencies with processes currently utilised in the separation of lanthanides. This study highlights the potential use of expanded porphyrinoids in lanthanide separation through selective binding, dependent on the tailored pore size of the macrocycle. Each emerging trend is subjected to multi-factored analysis to decompose the underlying source. Results promote the viability of size-based separation with preferential binding of larger lanthanum(iii) ions to amethyrin and isoamethyrin macrocycles, while smaller macrocycles such as pentaphyrin(0.0.0.0.0) present a preferential binding of lutetium(iii) ions. Additionally, the porphyrin(2.2.2.2) macrocycle shows a selectivity for gadolinium(iii) ions over both larger and smaller ions. An upper limit of applicable pore size is shown to be ≈2.8 Å, beyond which the formed complexes are predicted to be less stable than the corresponding nitrate complexes.

Fig. 1. Schematic of expanded porphyrinoid macrocycles studied throughout this work. Nomenclature of (n.n.n.⋯.n), where n = 0, 1, 2, is used to denote the number of –CH– groups bridging each pyrrole group within a given macrocycle.

Fig. 2. Schematic representation of circular approximation to the core size of an expanded porphyrin.

Fig. 3. Pore sizes of selected porphyrinoids as determined by a circular approximation.

Fig. 4. Selection of La(iii) complexes utilised throughout this study: (A) La(NO₃)₃(H₂O)₃; (B) La(Pent0)(H₂O)₄; (C) La(Porph1122)(H₂O)₃NO₃; (D) La(Porph1212)NO₃(H₂O)₃. Yellow – carbon; blue – hydrogen; pale red – oxygen; purple – nitrogen; bright red – lanthanum.

Fig. 5. Pore sizes of selected EP macrocycles both at their respective geometric minima (red), and complexed to each Ln(iii) ion, as determined by a circular approximation, as in eqn (1).

Fig. 6. Energy changes (kcal mol⁻¹) for the exchange reactions shown in eqn (5) in which the smaller ion replaces the larger ion within the EP macrocycle. As such, negative reaction energies denote a preference for EP complexation with the smaller lanthanide, while positive reaction energies denote selectivity preference for EP complexation of the larger ion.

Fig. 7. Energy differences (kcal mol⁻¹) required to distort each EP macrocycle from its geometric minima to the geometry achieved when complexing a given Ln(iii) ion.