Two novel nitrogen-rich metal–organic nanotubes: syntheses, structures and selective adsorption toward rare earth ions†

The construction and development of metal–organic nanotubes (MONTs) with nanoscale interior channel diameters for potential applications is of great interest. An angular nitrogen-rich ligand, 3,6-bis(2-ethylimidazole)-2-methylpyrimidine (beim-CH₃), was designed to construct MONTs by coupling with the V-shaped carboxylate ligands of benzophenone 4,4′-dicarboxylic acid (H₂bpndc) and 4,4′-oxybisbenzoic acid (H₂obba). Two new MONTs were synthesized and named NCD-166 ([Zn(bpndc)(beim-CH₃)]·H₂O) and NCD-167 ([Zn(obba)(beim-CH₃)]·H₂O), and they were isostructural and have almost identical tube inner diameters of approximately 1.76 nm. Benefiting from the abundantly exposed nitrogen and oxygen atoms in their tube walls and open nanoporous channels, they display superior adsorption capacities for Eu³⁺ (150.90 mg g⁻¹) and high adsorption selectivity (>96%) in the low-concentration solutions. Additionally, it was revealed that the adsorption effect of ether oxygen on rare earth elements was significantly better than that of carbonyl oxygen. The adsorption isotherm conformed to the Langmuir model and the adsorption kinetics obeyed the pseudo-second-order model. These results clearly indicate that such novel MONTs are favorable sorbents for REEs.