Hydrogen‐Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

Abstract

Hybrid hydrogen‐bonded (H‐bonded) frameworks built from charged components or metallotectons offer diversified guest‐framework interactions for target‐specific separations. We present here the first study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F‐, Cl‐, Br‐, and I‐, for developing hybrid H‐bond synthons, enabling the controllable construction of microporous H‐bonded frameworks exhibiting fine‐tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl‐, Br‐, and I‐, are found to readily participate in the charge‐assisted H‐bonding assembly with well‐defined coordination behav‐iors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ~ 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record‐high separation factor (SF = 7.0) among H‐bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2‐containing gas stream exactly mimicking the off‐gas of spent nuclear fuel (SNF) reprocessing.

Porous isoreticular non-metal organic frameworks

“…TAPT.Cl and TT.Br adsorb 4.0 mmol g −1 and 4.6 mmol g −1 CO 2 , respectively, at 195 K as saturation is approached. TT.Br in particular adsorbs more CO 2 than do the other porous organic salts at this temperature 5,34,41,42 . By contrast, TTBT.Cl adsorbed around 2.4 mmol g −1 despite having the lowest predicted crystal density and the largest nominal pore volume (Fig.…”

“…The second pore (B) is diamond shaped, is less polar and is defined by the aromatic linkers; this pore diameter is larger in TTBT.Cl (14.3 Å × 8.5 Å) than in TAPT.Cl (7.9 Å × 4.6 Å) or TT.Br (7.0 Å × 5.8 Å), but the dimensions of the ionic pore (A) are almost the same in all three predicted structures. This dual-channel structure leads to predicted pore volumes in these trigonal amine salts that are higher than for 4,4′,4′′,4′′′-(ethene-1,1,2,2-tetrayl)tetraaniline (ETTA) salts 34 . For example, using a probe radius of 1.2 Å, the calculated solvent-accessible pore volume for TAPT.Cl is 31.9% of the unit-cell volume, 31.4% for TT.Br , and 43.2% for the larger-pore isoreticular framework, TTBT.Cl .…”

“…For example, a wide variety of other counter-ions can be considered, such as nitrates, sulfates and hydrogen sulfates, tetrafluoroborates, hydrogen carbonates, phosphates, cyclic phosphates, arsenates, carboxylates and tetrafluoroborates, all of which are known to form salts with ammonium cations. Mixed-anion frameworks are also possible 34 , although predicting the most stable salt composition a priori could be computationally expensive. Just as for MOFs, a range of organic amine linkers can be conceived, including aliphatic amines, if they are sufficiently rigid, as well as pyridinium or imidazolium analogues.…”

“…A different dense ammonium salt has been reported 33 that could efficiently catalyse the reduction of U(IV) to U(VI). And last year, porous ammonium halide salts were reported that could adsorb gases such as krypton and xenon 34 , although those materials were synthesized without any computational structural design.…”

“…The equivalent calculated pore volume calculated for the chloride salt of ETTA (ETTA_Cl) is 18.2% (ref. 34 ). This four-arm linker, ETTA, is closer in structure and geometry to TAPM (Fig.…”