Four anionic Ln-MOFs for remarkable separation of C2H2–CH4/CO2–CH4 and highly sensitive sensing of nitrobenzene

Abstract

3-(3,5-Dicarboxylphenyl)-5-(4-carboxylphenyl)-1,2,4-triazole (H₃DBPT) as a ligand was used to synthesize four porous anionic lanthanide-based metal–organic frameworks {[Me₂NH₂][Ln₃L₃(HCOO)]·7DMF}_n (Ln-MOFs, Ln = Gd, Tb, Ho, Dy, L = H₃DBPT) under solvothermal reaction conditions. Single-crystal X-ray diffraction reveals that the four Ln-MOFs are isostructural and belong to the monoclinic C2/m space group. Three Ln(III) ions are connected through ten carboxylic groups to form a trinuclear [Ln₃(COO)₁₀] SBU. The SBU polyhedra share edges with each other generating a 1D inorganic chain along the a-axis, and the chains are further connected by the L³⁻ ligands to construct the final 3D framework with tunnels parallel to the a-axis. The luminescence intensity of the Tb-MOF can be modulated using small molecule organic solvents, and the Tb-MOF exhibits significant quenching in nitrobenzene, which indicates that the Tb-MOF is a potential luminescent sensing material for nitro explosives. Furthermore, the Gd-MOF after desolvation exhibits potential for industrially important C₂H₂/CH₄ and CO₂/CH₄ separation. The highly selective adsorption of C₂H₂ and CO₂ over CH₄ might be attributed to the synergistic interplay of the tunnels, the functional active sites such as open metal sites and the Lewis basic nitrogen atoms on the tunnel surface.