Isostructural fcu Rare-Earth Metal–Organic Frameworks Based on a Zigzag Dicarboxylate Ligand: One-Step Ethylene Purification from C2 Mixtures and Turn-On Sensing of a Nerve Agent Simulant

Abstract

Efficient ethylene purification and real-time detection of nerve agents are of great significance in industrial separation and national security protection, respectively. Rare-earth metal–organic frameworks (RE-MOFs) have emerged as promising candidates for both gas separation and fluorescence sensing. Typically, MOFs with an fcu topology are assembled from linear dicarboxylate linkers and rare-earth ions. In this study, using a zigzag dicarboxylate ligand, 4,4′-H2BPyDC (2,2′-Bipyridine-4,4′-dicarboxylic acid), and 2-fluorobenzoic acid as a modulator, we successfully constructed an isostructural series of fcu-type RE-MOFs, denoted as fcu-4,4′-BPyDC-RE (RE = Yb, Y, Eu, Gd), under solvothermal conditions. Gas adsorption and separation studies reveal that fcu-4,4′-BPyDC-Yb/Y exhibit markedly elevated adsorption capacities for acetylene and ethane than for ethylene under equivalent conditions, enabling the simultaneous removal of acetylene and ethane from a ternary C2 mixture and thus one-step purification of ethylene with substantially reduced energy consumption. The separation performance was validated by breakthrough experiments on mixed gases, and the underlying mechanism was elucidated by grand canonical Monte Carlo (GCMC) simulations. In terms of sensing performance, fcu-4,4′-BPyDC-Eu shows a turn-on fluorescence response to the nerve agent simulant DCP (diethyl chlorophosphate) in both solution and vapor phases. Remarkably, a gas-phase sensor based on this material enables rapid (5 s) and highly selective detection of DCP with a low detection limit of 3.2 ppb. The sensing mechanism, specifically, protonation-induced energy-level modulation of the ligand, was systematically elucidated by PXRD, FT-IR, XPS, 1H/31P NMR, and phosphorescence spectroscopy of fcu-4,4′-BPyDC-Gd. Collectively, this fcu-type RE-MOF platform integrates competitive hydrocarbon separation capability with highly sensitive chemical sensing, offering an alternative structural route for the design of multifunctional MOF materials.