A Trifunctional Luminescent 3D Microporous MOF with Potential for CO2 Separation, Selective Sensing of a Metal Ion, and Recognition of a Small Organic Molecule

Abstract

A 3D microporous luminescent metal–organic framework {[Zn(oba)(L)0.5]·dma}n (IITKGP-9, IITKGP represents the Indian Institute of Technology Kharagpur; L = 3,3′-azodipyridine, H2oba = 4,4′-oxydibenzoic acid) with 1D rhombus-shaped channels has been synthesized by employing a bent organic linker with an unexploited azo-functionalized N,N′ spacer. The metal–organic framework (MOF) exhibits potential for highly selective sorption of CO2 with excellent selectivity of CO2 from CO2/N2 (291 at 273 K and 118 at 295 K) and CO2/CH4 (17.5 at 273 K and 4.8 at 295 K) at a pressure of 1 bar, as demonstrated by ideal adsorbed solution theory (IAST) calculations. Such high separation ability, especially at 273 K for CO2/N2, can be attributed to the microporous (5.8 × 6.6 Å2) nature of this MOF and the enhanced interactions of the adsorbed CO2 molecules with the functional azo groups decorating the pore channels. This MOF is highly emissive at 374 nm upon excitation at 310 nm. The free azo groups exposed in the channels serve as functional sites for the sensing of metal ions and the recognition of small organic molecules through the quenching of fluorescence. Fluorescence measurements revealed that this MOF can selectively sense Fe3+ and also detect toxic nitromethane with high sensitivity. Elemental mapping by energy-dispersive X-ray spectroscopy and powder XRD analysis of the Fe3+-loaded MOF material, recyclability tests, and a plausible quenching mechanism are presented. Thus, this MOF demonstrates potential as a trifunctional MOF material for three different applications, namely selective gas separation, selective ion sensing, and selective recognition of organic molecules.