Optimizing Pore Space for Flexible-Robust Metal–Organic Framework to Boost Trace Acetylene Removal

Abstract

Isoreticular principle has been employed to realize a flexible-robust metal–organic framework (MOF) with extended pore structure for the adsorptive removal of trace acetylene from ethylene under ambient conditions. The substitution from zinc­(II) to copper­(II) of high coordination distortion leads to elongated Cu–F bonds that expand the closed pore cavities in the prototypical MOF from 3.5 × 3.9 × 4.1 to 3.6 × 4.3 × 4.2 Å3. The optimal cavity size together with strong binding sites thus endows the new Cu analogue to possess open pore space accessible for trace C2H2 within a substantial low-pressure range while excluding C2H4 molecules, as validated by gas isotherms and single-crystal structure of its partially C2H2-loading phase. In contrast to the Zn prototype, at 298 K and 1.0 bar, the guest-free Cu analogue shows significant C2H2 uptake increase with a total capacity of 4.57 mmol g–1, and gains an over two orders of magnitude jump in IAST selectivity for C2H2/C2H4 (1/99, v/v). These results are higher than the benchmark MOFs for molecular sieving of C2H2/C2H4, leading a high C2H4 productivity of 14.9 mmol g–1. Crystallography studies, molecular modeling, selectivity evaluation, and breakthrough experiments have comprehensively demonstrated this flexible-robust MOF as an efficient adsorbent for C2H2/C2H4 separation.

Molecular Sieving of Acetylene from Ethylene in a Rigid Ultra‐microporous Metal Organic Framework.

“…Benefitting from the absolute molecular sieving effect and very low uptake of C 2 H 4 , the uptake ratio of C 2 H 2 /C 2 H 4 at 298 K and 100 kPa represents the second high value (63.6), which is higher than other benchmark materials for C 2 H 2 /C 2 H 4 (e. g., 5.79 for SIFSIX‐14‐Cu‐i, [12] 8.1 for NTU‐69, [13] 62.8 for NTU‐65, [20] 26.75 for GeFSIX‐dps‐Cu, [19] 5.39 for GeFSIX‐14‐Cu‐i, [16] 14.28 for NCU‐100a [17] ) and only lower than the value of 75 in UTSA‐300a (Figure 3d and Table S1) [15] . Moreover, the adsorption selectivity of C 2 H 2 /C 2 H 4 for F−PYMO−Cu was also calculated based on the ideal adsorbed solution theory (IAST), [23] after fitting adsorption data of C 2 H 2 and C 2 H 4 at 298 K to the Langmuir‐Freundlich equation.…”

“…[14] Moreover, framework flexibility is also an unfavorable factor under this context because of co-adsorption behavior. Until now, there are six flexible MOFs (UTSA-300, [15] GeFSIX-14-Cu-i/ ZU-33, [16] NCU-100a, [17] bnn-1-CaÀ H 2 O, [18] GeFSIX-dps-Cu, [19] NTU-65 [20] ) reported showing the sieving effect for C 2 H 2 over C 2 H 4 by differential gate-opening behavior. However, though quite low or negligible C 2 H 4 uptake is observed based on the adsorption data of single-gas isotherms, co-adsorption of C 2 H 2 and C 2 H 4 would still occur in dynamic gas mixture separation process once the closed or narrower pore system fully opened by C 2 H 2 (Figure 1).…”

Two-Effects-in-One: Kinetic Quantum and Molecular Sieving Effect in a Robust-Flexible [Th6Co2] Cage for both Isotope and Isomer Separation

“…41 Seem form the adsorption isotherms, n-butane adsorbed steeply at low pressure below 0.02 bar, then an abrupt junction was observed, and following adsorption also becomes steep, fully agreeing with the recently de ned robust-exible adsorption. 43,44 However, this phenomenon is not observed for iso-butane adsorption. As we konw, n-butane gives dynamic diameter of 4.7 Å, slightly larger than the window of [Th 6 Co 2 ] cage (big window of 4.4Å), and thus occurrence of robust-exible adsortion is reasonable.…”

Review on Flexible Metal‐Organic Frameworks

“…Hence slight modification in flexible MOF results in the robust pore at guest Free State with compatible sizes which enables the MOF to absorb acetylene under low concentration and low‐pressure conditions. In this way, the principle of isoreticular (similar topology) chemistry comes into play which is defined as the building of metal‐organic frameworks with expanded pore sizes and volume from the organic ligands of different sizes with common geometry or symmetry to develop particular MOF for tough gas separation processes [63] …”