A Zn-based MOF with honeycomb topology for highly efficient iodine uptake from vapor and liquid phases: synthesis, crystal structure, topology and dual-phase sorption performance

Abstract

Effectively capturing volatile radioiodine released during nuclear fission is vital for long-term environmental protection. Here we report a zinc-based metal-organic framework, [Zn(tba)0.5(H2O)2]n (FV-Zn1), synthesized hydrothermally from Zn(NO₃)₂•6H₂O and a tetratopic benzene-1,2,4,5-tetracarboxylate ligand (tba) in the presence of 2-methylimidazole as a structure-directing base. Single-crystal X-ray diffraction shows that FV-Zn1 crystallizes in the monoclinic space group P121/n1 and features five-coordinate {ZnO5} nodes linked by the rigid tetra carboxylate ligand into corrugated layers, which assemble into a three-dimensional framework. Topological analysis reveals that the underlying coordination network is a uninodal 3-connected hcb (honeycomb) net with point symbol 6³, providing well-defined hexagonal channels. The material is thermally robust and its bulk phase matches the single-crystal model, as confirmed by PXRD, FTIR, TGA, SEM, TEM, Raman and XPS analyses. FV-Zn1 exhibits excellent iodine sorption performance in both vapour and liquid phases. In iodine vapour at 70 °C it reaches an equilibrium uptake of ~470 mg g⁻¹, while from n-hexane solution it removes ≈97% of dissolved I2, corresponding to a capacity of 301.3 mg g⁻¹. The adsorption kinetics follow a pseudo-second-order model and the framework retains most of its capacity over multiple adsorption-desorption cycles, indicating chemisorption-dominated binding and good recyclability without loss of crystallinity. Spectroscopic measurements confirm the presence of I2/polyiodide species confined within the channels of FV-Zn1. DFT calculations further disclose the π-rich linker environment as the preferred iodine binding site (short I2⋯π contact, 2.826 Å) and predict markedly stronger binding for this mode than for carbonyl-associated configurations, supporting the experimentally observed strong host-guest interactions. These results identify FV-Zn1 as an efficient and durable dual-phase iodine sorbent and highlight Zn-carboxylate honeycomb MOFs as promising candidates for radioactive iodine management.