Robust lanthanide metal–organic frameworks with “all-in-one” multifunction: efficient gas adsorption and separation, tunable light emission and luminescence sensing

Abstract

Combining the porous structure and fluorescence properties of metal–organic frameworks (MOFs) with rational design to realize multifunction application is of great significance from an energy and environmental standpoint. Herein, a new series of lanthanide metal–organic frameworks (Ln–MOFs), {[Ln(L)(H₂O)]·4H₂O }_n (1-Ln, where Ln = Eu, Tb, Gd, and Eu_xTb_1−x), were constructed by using a rigid 5-(2′,5′-dicarboxylphenyl) picolinic acid ligand and Ln³⁺ ions. 1-Ln was isostructural and exhibited a similar porous framework. Thermogravimetry analysis (TGA), powder X-ray diffraction (PXRD) and N₂ adsorption measurements revealed that 1-Eu exhibits excellent thermal, water and chemical stabilities. As expected, 1-Eu-a (activated 1-Eu) shows a high adsorption capacity for C₂H₂ (109.2 cm³ g⁻¹) as well as excellent selective separation with ideal adsorbed solution theory (IAST) selectivity for C₂H₂/CO₂ (4.1–3.3), C₂H₂/CH₄ (23–17) and CO₂/CH₄ (5.6–5.4) at room temperature. More importantly, density functional theory (DFT) calculations and breakthrough experiments were carried out, which highlight its great potential to address challenging industrially important separation. Moreover, a series of bimetallic MOFs 1-Eu_xTb_1−x were constructed by changing the ratio of Eu³⁺ and Tb³⁺ and showed excellent linear color tunability (from green to red), indicating that they are suitable for barcode applications. Furthermore, luminescence studies revealed that 1-Eu is an excellent fluorescent probe for sensing Fe³⁺ (LOD: 0.57 μM) and CrO₄²⁻/Cr₂O₇²⁻ (LOD: 0.79/0.42 μM) ions in water, with a fast response, high sensitivity, selectivity and recyclability. To the best of our knowledge, 1-Eu is the first reported Ln–MOF material with all-in-one multifunction, including excellent gas adsorption and separation, tunable light emission and efficient luminescence sensing.