Coordination Flexibility Aided CO2 -specific Gating in an Iron Isonicotinate MOF

Abstract

Coordination flexibility assisted porosity has been introduced into an Iron-isonicotinate metal-organic framework (MOF), (Fe(4-PyC)₂ ⋅ (OH). The framework showed CO₂ -specific gate opening behavior, which gets tuned as a function of temperature and pressure. The MOF's physisorptive porosity towards CO₂ , CH₄ , and N₂ was investigated; it adsorbed only CO₂ via a gate opening phenomenon. The isonicotinate, representing a borderline soft base, is bound to the hard Fe³⁺ centre through monodentate carboxylate and pyridyl nitrogen. This moderately weak binding enables isonicotinate to spin like a spindle under the CO₂ pressure opening the gate for a sharp increase in CO₂ uptake at 333 mmHg (At 298 K, the CO₂ uptake increases from 0.70 to 1.57 mmol/g). We investigated the MOF's potential for CO₂ /N₂ and CO₂ /CH₄ gas separation aided by this gating. IAST model reveals that the CO₂ /N₂ selectivity jumps from 325 to 3131 when the gate opens, while the CO₂ /CH₄ selectivity increases three times. Interestingly, this Fe-isonicotinate MOF did not follow the trend set by our earlier reported Hard-Soft Gate Control (established for isostructural M²⁺ -isonicotinate MOFs (M=Mg, Mn)). However, we account for this discrepancy using the different oxidation state of metals confirmed by X-ray photoelectron spectroscopy and magnetism.

Keywords: CO2 capture; Isonicotinic; flexible MOF; gating.