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.

High-Performance Selective CO2 Capture on a Stable and Flexible Metal–Organic Framework via Discriminatory Gate-Opening Effect

“…Besides separation performance, stability is also an important factor that determine the industrial utility of adsorbents. , Though some flexible MOFs demonstrate potential for selective CO 2 capture, their industrial employment still remains a challenge. On one hand, the actual feed gases usually contain a trace amount of water, so water/moisture robustness is requisite for industrial adsorbents. , However, some MOFs with excellent CO 2 capture performance exhibit poor chemical stability or remain elusive on their chemical stability. − For example, ELM-11 holds excellent CO 2 volumetric uptake and high theoretical CO 2 working capacity under PVSA operations, but its structure is easily damaged and the CO 2 capacity is lost dramatically after a few cycles with contact of moist air. ,,, On the other hand, some stable MOFs such as ZIF-7 , only show moderate volumetric uptake and working capacity toward CO 2 . Hence, there is still a great demand to rationally develop more innovative flexible materials with both excellent separation performance and high water/moisture stability.…”

Ultramicroporous iron-isonicotinate MOFs combining size-exclusion kinetics and thermodynamics for efficient CO2/N2 gas separation

“…14,32,33 This affinity has been proved in different porous materials formed by isonicotinic acid, which exhibit effective CO 2 /N 2 separation despite the absence of a size exclusion mechanism. 34–37…”

Gate Opening without Volume Change Triggers Cooperative Gas Interactions, Underpins an Isotherm Step in Metal–Organic Frameworks

“…Such a feature of the isotherm is desirable in pressure swing adsorption . However, breathing materials are not suitable for applications using tanks of fixed volume . In such situations, a step can be introduced via capillary condensation in mesoporous materials.…”

“…62 However, breathing materials are not suitable for applications using tanks of fixed volume. 63 In such situations, a step can be introduced via capillary condensation in mesoporous materials. But in general, MOFs with larger pore dimensions are less stable mechanically, 64 and the isotherms are usually hysteretic.…”

“…53,64,65 Researchers have devised various strategies to overcome the limitations of breathing or meso-porous MOFs by designing MOFs that are both microporous and nonbreathing. 63,66,67 For instance, based on a statistical model, Simon et al 68 showed that the flexibility of ligands under a specific setting can give rise to a step in adsorption isotherm. Abrahams et al 69 reported MOFs that exhibited a step in the CO 2 isotherm at 298 K and the same was rationalized using density functional theory (DFT) calculations to have arisen from an increase in the accessible surface area through small swings in the framework aromatic rings at high CO 2 loading.…”