Effects of Phosphonic Acid Groups and Benzene Ring Content on Crystallization Behavior and Thorium(IV) Adsorption Performance

In this study, we exploit the selective adsorption affinity of phosphonate groups toward thorium (Th) to synthesize three ionic organic crystals (IOCs) with systematically varied phosphonate group coverages. Using a green and energy-efficient synthetic strategy, we successfully prepared three IOCs with tunable properties, namely, TP-Py, BPBP-Py, and PMBP-Py, each crystallizing in the triclinic system with the formulas [C₇₅H₈₈N₃O₂₃P₆S₄], [C₄₈H₄₈N₂O₁₈P₆], and [C₃₆H₅₆N₂O₂₂P₆]. The findings revealed how the number of phosphonate groups and benzene rings significantly affected the adsorption performance of the IOCs by regulating the hydrophilicity and occupancy of adsorption sites. The experimental data demonstrate the exceptional performance of the BPBP-Py ([4-(4-phosphonophenyl)phenyl]phosphonic acid-4,4'-dipyridyldimethiodide), exhibiting a remarkable separation (S_Th-U = 6.77) and enrichment capabilities (Q_e = 467 mg/g), in addition to its notable thermal stability (300 °C) and durability (≥3 cycles). Moreover, it was confirmed by utilizing various characterization methods, including EDX mapping (energy-dispersive X-ray spectroscopy), FTIR (Fourier transform infrared spectroscopy), XPS (X-ray photoelectron spectroscopy), and DFT (density functional theory) calculations, which indicate that the oxygen and phosphorus atoms in the phosphonate groups may be the primary coordination adsorption sites. What's more, as the DFT calculation results show, the presence of highly enriched phosphonate groups on the surface of BPBP-Py offers a substantial number of active sites for Th(IV) adsorption, thus providing theoretical and practical evidence for the existence of selective adsorption by phosphonate groups and the special adsorption site in the phosphonate group.