Selective CO2 uptake and inverse CO2/C2H2 selectivity in a dynamic bifunctional metal–organic framework

Abstract

The unique bifunctional porous metal-organic framework, [Co(HL dc )]$1.5MeOH$dioxane, incorporates both free-standing carboxyl and pyridyl groups within its pores. Gas adsorption measurements on the desolvated framework reveal unusual selective CO 2 adsorption over C 2 H 2 and CH 4 linked to a framework phase change from a narrow pore (np) to a large pore (lp) form, mediated by CO 2 uptake at 195 K. This phase transition has been monitored by in situ powder X-ray diffraction and IR spectroscopy, and modelled by Grand Canonical Monte Carlo simulations revealing that the reversible np to lp transition is linked to the rotation of pyridyl rings acting as flexible ''pore gates''. Scheme 1 In situ decarboxylation of H 4 L to give [HL dc ] 2À in 1 and 2, and view of binding of [HL dc ] 2À to Co(II) showing non-coordinated pendant acidic and basic sites (Co: turquoise; O: red; N: blue; C: grey; H: white).

Statistical mechanical model of gas adsorption in porous crystals with dynamic moieties

“…Several reported MOFs possess dynamic constituents, e.g., bridging ligands that can rotate, while maintaining an approximately rigid unit cell (18,(24)(25)(26)(27)(28)(28)(29)(30)(31)(32)(33)(34). Nuclear magnetic resonance (NMR) spectroscopy revealed that aromatic rings in the ligands of MOF-5 (26), IRMOF-3 (27), and MIL-47(V) (35) rotate, resembling an ordered assembly of molecular rotors (36).…”

“…When the dominant conformations of these flexible parts in MOFs are different in the evacuated solid than in the presence of gas, the dynamic moieties can qualitatively affect the adsorption behavior, e.g., by inducing inflections (30), steps (29), and hysteresis (18) in the adsorption isotherms. Linkers in ZIF-7 are believed to rotate and cause steps and hysteresis in the adsorption isotherms of several hydrocarbons (39,40).…”

“…[10][11][12][13][14]. Reported modes of guest-induced structural changes in MOFs include breathing (15), swelling (16), and subnetwork displacement (17), as well as less dramatic changes, such as the rotation of a ligand (18) or deformation (19). In molecular recognition and enzyme catalysis in biology, a conformation change of the enzyme is often involved in substrate binding (20,21).…”

Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity

“…11,[14][15][16][17] This behaviour results in stepped adsorption isotherms, which may exhibit a hysteresis that can be exploited in gas capture or separation applications. [18][19][20] The most well-studied MOFs that exhibit such defined transitions are those with a pillared or columnar structure that contain 1D channels with a diamond or square cross-section, which compress or expand upon breathing in a manner analogous to that of a wine-rack. Such MOFs include the paddlewheel MOF DMOF, [21][22][23] [Zn 2 (BDC) 2 (dabco)] (BDC = 1,4-benzenedicarboxylate or derivatives; dabco = 1,4-diazabicyclooctane) and azolate MOFs [M(BDP)] (M = Fe, Co; BDP = 1,4-benzenedipyrazolate).…”

Methane Adsorption in Metal–Organic Frameworks Containing Nanographene Linkers: A Computational Study

“…[11][12][13][14][15][16][17][18] MOFs are compounds containing metal nodes (metal ions or clusters) coordinated to organic linker molecules that form the extended network structures with unique physical and chemical properties. The ability to tailor and control the density, internal pore volume and the internal surface area of MOFs 19,20 is crucial for utilizing these materials in high capacity gas uptake applications.…”