Gel self-assembly of lanthanum aminopolycarboxylates with skeleton structures and adsorptions of gases

Abstract

Three lanthanum aminopolycarboxylates [La(Hida)₂(H₂O)₂]_n·nCl·4nH₂O (1), K_2n[La(nta)₂]_n·nH₂O (2) and (H₂en)_n[La(edta)(H₂O)]_2n·10nH₂O (3) (H₂ida = iminodiacetic acid; H₃nta = nitrilotriacetic acid; H₄edta = ethylenediaminetetraacetic acid; en = ethylenediamine, NH₂CH₂CH₂NH₂) were isolated by gel self-assembly and were fully characterized. Structural analyses reveal that 1–3 have infinite 2D and 3D structures with different coordination modes. Topological analyses of 1–3 were also performed. Solid state ¹³C NMR spectra show that the coordinated β-CO₂ groups give obvious downfield shifts compared with free β-CO₂ groups. Interestingly, 3 possesses 5.8 Å diameter tunnels inside the crystals, which can selectively adsorb very small amounts of O₂ at 5.49 mg g⁻¹ and 29.9 bar, while no adsorption of H₂, N₂, CH₄ and CO₂ is observed. Compared with lanthanum 1,3-propylenediaminetetraacetate {La(H₂O)₄[La(1,3-pdta)(H₂O)]₃}_n·11.25nH₂O (4) with 10.0 Å diameter tunnels, the latter can adsorb CO₂, O₂ and N₂ at 55.33, 18.01 and 4.41 mg g⁻¹ and 29.9 bar, respectively. This shows that smaller diameter tunnels have better selectivity towards the adsorption of specific gases. When the diameter of the tunnels increases, the adsorption amounts of numerous gases increase significantly with a decrease in selectivity.