Designing Suitable Pore Size and Anionic Framework of MOFs for Improving Proton Conductivity and Separation of Organic Dyes

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Reasonable design of metal–organic frameworks (MOFs) with multifunction pore environment and anionic structure for proton conduction and dye adsorption has important application value. Herein, we report two novel and stable isomorphic three-dimensional (3D) lanthanide metal–organic frameworks (LCUH-121 and LCUH-122). Both MOFs have unique pore sizes, which are respectively made up of smaller proton-conducting channels (LCUH-121: 6.8 × 11.4 Å2, LCUH-122: 6.1 × 11.4 Å2) and larger dye adsorption cavities (LCUH-121: 10.2 × 11.4 Å2, LCUH-122: 10.1 × 11.4 Å2). Remarkably, the high-density dimethylamine cations ([(CH3)2NH2]+) in the one-dimensional channel also endow them with dual functions, exhibiting both efficient proton conduction pathways and excellent cationic dye adsorption performance. The abundance of dimethylamine cations and the special environment of small pores give LCUH-121 and LCUH-122 proton conductivity (σ) values as high as 1.62 × 10–2 and 1.46 × 10–2 S·cm–1 (80 °C, 100% RH), which is the highest reported anionic MOFs to date. Meanwhile, the synergistic effect of the optimal large pore size and anion framework resulted in adsorption capacities of 1.23 and 1.20 g·g–1 for methylene blue (MB) by LCUH-121 and LCUH-122, respectively, and exhibited record-breaking adsorption rate constants (0.0698 and 0.0662 g·mg–1·min–1). More importantly, the Grand Canonical Monte Carlo (GCMC) elucidated that the proton conduction mechanism and dye adsorption mechanism mainly rely on a unique size dual pathway mechanism, in which protons are conducted through the small-pore framework and dye molecules are accommodated in a large pore framework.