Krypton/Xenon separation at room temperature in a flexible coordinative framework sorbent

Domain: neuroscience
Published: 2026-01-26
Editorial outcome: minor_revision

Abstract
The separation of xenon from krypton holds significant importance across various fields, including high-tech industries, national defence, and aerospace. Adsorptive separation via porous solids is considered one of the most promising alternatives to conventional energy-intensive cryogenic processes. While traditional adsorbents selectively capture xenon over krypton under ambient conditions, their desorption energy penalties for producing pure xenon impede practical implementation. Here, we present a metal‒organic framework characterized by synergistic structural and local flexibility that reverses conventional selectivity, thereby enabling preferential adsorption of krypton at room temperature. This material demonstrates a krypton uptake of 36.8 cm3 cm-3 (298 K, 1 bar) and achieves a Kr/Xe selectivity of 10.4 in 1/99 Kr/Xe mixture breakthrough experiments. Moreover, this material exhibits commendable radioactive stability and effectively captures trace amounts of krypton (40 ppm). Mechanistic studies indicate that dynamic adjustments in cavity windows and localized ligand vibrations work synergistically to exploit subtle size differences between krypton and xenon. This enables kinetically controlled sieving of krypton through transiently expanded channels. With an energy-efficient approach to krypton-centric separation, our research redefines the design paradigm for noble gas purification and offer new possibilities for separating other dynamically matched molecules through adaptive host‒guest interactions.

Introduction
Xenon (Xe) is a highly valuable noble gas that is experiencing increasing demand across various high-tech industries, medical therapies, national defences, aerospace, nuclear energy, fundamental research, and even everyday applications1–5. While air serves as a primary source for Xe production through energy-intensive cryogenic processes, exhaust gases from the nuclear sector also represent an important alternative6,7. In both scenarios, krypton (Kr) is present at varying concentrations and must be effectively removed from the Xe/Kr mixture. Consequently, the cost-effective and energy-efficient separation of Xe from Kr is essential to meet the growing market demand for Xe across diverse applications.

With significant advancements in purification technologies, the adsorptive separation of Xe/Kr using porous solids has become a promising alternative to conventional methods8–10. However, the comparable sizes of Xe (4.047 Å) and Kr (3.655 Å) make size-selective molecular sieving challenging for most porous materials. These solids typically favor thermodynamic Xe-selective adsorption because of their higher polarizability (40.44×10−25 cm3) than Kr (24.84×10−25 cm3)11,12. While some Xe-selective adsorbents can achieve selectivity greater than 100, producing high-purity Xe still requires considerable energy because preferentially adsorbed Xe must be desorbed for recovery13–16. Additionally, capturing low concentrations of radioactive 85Kr isotopes (40 ppm), which have a long half-life of 10.8 years and are found in nuclear exhaust gases, is crucial to minimize environmental impact6,17,18. Rapid sequestration and transfer of this isotope to storage facilities are essential. To facilitate the direct recovery of pure Xe during adsorption, Kr must be preferentially adsorbed through a kinetic-driven sieving mechanism.

To date, only a few porous materials can sieve Kr from Xe: the densified CuBTC to form FS-CuBTC sorbent19, the porous carbon C-UM-Suc-750 material with an appropriate pore size of approximately 4.0 Å20 and three MOFs, i.e., FMOFCu, Mn(ina)2 and CoNi-DAB with attractive separation performance at low temperatures below 273 K through kinetic-controlled molecular sieving21–23 (Fig. 1a). In addition to inherent local flexibility like bond rotation and stretching, guest-responsive dynamic MOF frameworks with narrow pore windows show promise in distinguishing between these gases due to significant differences in diffusion energy through small apertures24–26. These findings suggest that fine-tuning structural flexibility and pore size in this class of porous materials could enable selective Kr adsorption at room temperature via kinetic-controlled sieving.

Herein, we demonstrate the preferential adsorptive separation of Kr over Xe at room temperature using the ultrasmall-pore MOF MIP-203-Suc (developed by the Institute of Porous Materials in Paris and based on succinic acid)27. This zirconium MOF exhibits synergistic structural flexibility with temperature- and guest-responsive dynamics, as well as local bond rotation and stretching (Fig. 1b). Thermal activation induces notable pore shrinkage, creating optimal apertures for Kr/Xe separation. The inherent flexibility of the aliphatic chain in succinate further fine-tunes accessibility to Kr and Xe. We observe a significant difference in the diffusion energy barriers: 12.8 kJ mol-1 for Kr versus 21.0 kJ mol-1 for Xe, indicating a strong kinetic preference for Kr transport through the narrow pores of MIP-203-Suc. This molecular discrimination results in excellent separation performance, confirmed by breakthrough experiments with a 1/99 Kr/Xe mixture at room temperature, yielding a selectivity factor of 10.4. In addition to its kinetic-controlled sieving capabilities, MIP-203-Suc shows excellent Kr uptake of 36.8 cm3 cm−3 from single-component sorption isotherms. Notably, this MOF maintains its separation efficacy across various Kr/Xe ratios, facilitating high-purity Xe production. Furthermore, it demonstrates exceptional trace-capture ability by selectively removing Kr even at concentrations as low as 40 ppm from Xe-rich mixtures typical of nuclear reprocessing off-gases.

Results
Temperature-driven pore structure transition and single-gas sorption isotherms

To achieve kinetic-driven sieving of Kr over Xe through adsorptive separation, it is essential to use porous materials with precisely sized and shaped pore apertures that can effectively distinguish the subtle dimensional differences between Kr and Xe. This requires the development of highly ordered porous frameworks with exceptional pore size control. Conventional inorganic porous solids, such as modified oxides and porous carbons, often have broad pore size distributions, making them unsuitable for this application. Additionally, the local flexibility of microporous MOFs with well-ordered pores significantly aids in the selective adsorption of Kr over Xe under cryogenic conditions21–23. These insights help identify MOF materials capable of achieving Kr/Xe separation at room temperature. Our comprehensive examination led us to explore MIP-203-Suc, a zirconium-based MOF made from 10-coordinated Zr6 clusters and succinate linkers featuring an inherently flexible C4 backbone. This framework combines optimal pore dynamics and local flexibility27, making it highly suitable for room-temperature Kr/Xe separation.

The reduced connectivity of Zr6 clusters in MIP-203-Suc (10-connected versus 12-connected in rigid analogues) enhances framework flexibility. This structural adaptability stems from lower node connectivity, promoting greater ligand mobility and dynamic responses to external stimuli. The as-synthesized structure of MIP-203-Suc, modeled via density functional theory (DFT) geometry optimization and powder X-ray diffraction (PXRD) data27, exhibits anisotropic modulation of pore apertures upon thermal activation at 373 K. Along the a-axis of the crystal structure, the effective window size, estimated from the carbon–carbon distance between succinate linkers on neighboring Zr6 clusters within the 1D channels, considering the van der Waals radii of carbon atoms (2×1.7 Å), shrinks from 4.1 Å (in the as-synthesized structure) to 3.7 Å upon thermal activation. Conversely, the aperture along the b-axis expands slightly from 2.5 to 2.8 Å after activation (Fig. 2a and b, Supplementary Table 1), indicating anisotropic stress relaxation via ligand reorientation along different crystallographic directions.

To validate the permanent porosity of activated MIP-203-Suc, CO₂ adsorption measurements were conducted at 195 K (Supplementary Fig. 3), yielding a Brunauer-Emmett-Teller (BET) area of 161 m2 g⁻1. Pore size distribution analysis employing the Horvath-Kawazoe (H-K) method revealed bimodal characteristics with peaks centered at about 4.0 Å and 5.0 Å (Supplementary Fig. 4). These pore dimensions are consistent with the crystallographically resolved one-dimensional channels composed of bottle-shaped cavities along the a-axis direction. While static structures have constrained pores, succinate linker flexibility through torsional motion is expected to facilitate dynamic aperture adaptation during gas diffusion, potentially enabling kinetic Kr/Xe separation through kinetically controlled sieving. Specifically, transient narrowing of pore apertures along the a-axis may preferentially restrict larger Xe atoms while allowing faster permeation of smaller Kr atoms—even when equilibrium adsorption selectivity favors Xe.

To assess the efficacy of the activated MIP-203-Suc sample in selectively adsorbing Kr from Xe and other gases, initial single-component sorption measurements were conducted (Supplementary Fig. 5). The corresponding results were subsequently plotted against volumetric uptake, as the volumetric adsorption capacity is commonly utilized when different sorbents with varying densities are compared in practical applications13. The Kr isotherms collected at temperatures above 268 K exhibit a standard decrease in the adsorption amount and a weaker affinity as the temperature increases (Fig. 3a). This behavior aligns with that observed for other gases, where the aperture size of the porous materials does not limit the entrance of guest molecules26,28–30. However, the Kr uptake decreased as the measurement temperature dropped below 268 K, this observation is in line with the flexible nature of the activated MIP-203-Suc structure: a significant steric hindrance due to limited bond stretching and rotation at low temperature impedes Kr to enter into the pore structure, resulting in a decreased Kr adsorption uptake (Fig. 3b). Notably, the Kr uptake observed in MIP-203-Suc at 298 K and 1 bar (36.8 cm3 cm−3) ranks among the highest values reported to date for porous materials (Fig. 3d), substantially surpassing some Xe-selective materials, such as Ag-MOF-30331, Co(HCOO)232, HOF-FJU-4633 and CC334. However, owing to the rather small pore volume of MIP-203-Suc, its Kr uptake is slightly lower than that of MOFs with larger pore volumes, such as MOF-Cu-H18 and ZJU-74a-Pb14 (Supplementary Table 4).

As for the Xe adsorption isotherms, MIP-203-Suc exhibited an inverse trend in uptake with temperature under the same testing conditions. When the temperature increases, local flexibility within the succinate linkers becomes increasingly pronounced; this notably enhances the ability to permit Xe passage through confined pore windows, thereby leading to significantly increased Xe uptake at elevated temperatures, such as those observed at 348 K (Fig. 3b and Supplementary Fig. 6). Consequently, across the temperature range from 253 to 348 K, Kr uptake consistently surpasses that of Xe by a substantial margin, suggesting an excellent separation performance of this material for Kr over Xe. In comparison to the previously reported MOFs, i.e., FMOFCu21, Mn(ina)222, and CoNi-DAB23, MIP-203-Suc demonstrates a broader sieving temperature range and superior Kr/Xe uptake ratio (Supplementary Fig. 7). Other molecules present in air, such as argon (Ar), oxygen (O2), and nitrogen (N2), exhibit negligible adsorption uptakes (Fig. 3c). These findings highlight the significant potential of activated MIP-203-Suc for selectively adsorbing Kr. In view of the anomalous temperature-changes of the Xe and Kr adsorption uptakes of MIP-203-Suc, the temperature-dependent pore structural flexibility and the corresponding temperature-dependent diffusion processes of Xe or Kr within the narrow pore apertures in MIP-203-Suc are likely the main causes. Therefore, we pursued further investigation into the kinetic adsorption performance of Kr and Xe on MIP-203-Suc.

As shown in Supplementary Figs. 8–10, the diffusion time constants and kinetic selectivity of Xe and Kr on MIP-203-Suc were calculated using the classical micropore diffusion model35. The diffusion time constants were calculated to be 7.38×10⁻4 min⁻1 for Xe and 9.37×10⁻3 min⁻1 for Kr at 298 K and 50 kPa, indicating a notably faster diffusion rate of Kr relative to Xe and yielding a high kinetic Kr/Xe selectivity of 12.7. Temperature-dependent diffusion studies reveal that, at 1 kPa, the diffusion time constants of Xe at 273 K (3.44×10⁻4 min⁻1) increased significantly to 6.13×10⁻4 min⁻1 at 298 K and 8.77×10⁻4 min⁻1 at 318 K. While for Kr at the same pressure, it only slightly increased from 1.72×10⁻3 min⁻1 at 273 K to 2.03×10⁻3 min⁻1 at 298 K. The increase in temperature can enhance the molecular diffusivity and adsorption kinetics, with Xe exhibiting substantially stronger temperature-responsive behavior compared to Kr. This confirms the theoretical expectation that larger molecular gases are more sensitive to thermal activation due to their higher intrinsic diffusion barriers.

Breakthrough experiments and kinetic-driven sieving mechanism

To evaluate the dynamic separation performance of MIP-203-Suc, we conducted breakthrough experiments utilizing an equimolar Kr/Xe mixture. The gas mixture was introduced into a packed column containing the activated sorbent at 298 K, with a constant flow rate of 1.5 mL min−1 (Fig. 4a and Supplementary Fig. 11). Consistent with the results of preliminary single-component adsorption measurements, a distinct separation of the Kr/Xe mixture was observed. Pure Xe eluted first from the column at approximately 5 minutes, while Kr remained for approximately 15 min. Following this period, Kr began to break through and approached equilibrium conditions by approximately 90 minutes. The robust regeneration and cycling capabilities of MIP-203-Suc were demonstrated through five consecutive breakthrough cycles conducted under identical conditions (Supplementary Fig. 12). The dynamic adsorption capacity of Kr on MIP-203-Suc, as determined from the breakthrough curve, was calculated to be 0.40 mmol g-1. This value is slightly lower than the corresponding equilibrium uptake derived from the adsorption isotherms (0.62 mmol g-1), which is likely due to mass transfer limitations and the relatively slow diffusion of Kr into the MOF pores under dynamic conditions. Notably, the selectivity for Kr/Xe obtained from these breakthrough experiments reached a value of 6.3 at room temperature, underscoring the exceptional capacity of the material for kinetically controlled sieving (Supplementary Note 2).

With the dynamic Kr selectivity of MIP-203-Suc established, we subsequently evaluated its capacity for Xe purification from trace Kr contaminants through a breakthrough experiment utilizing a 99:1 Xe/Kr mixture to simulate relevant conditions. As illustrated in Fig. 4b, Xe eluted from the packed column in less than 5 min, whereas Kr was retained for more than 22 min, demonstrating the exceptionally high selective behavior of MIP-203-Suc under trace-level separation conditions. This process resulted in post treated Xe gas with significantly enhanced purities exceeding 99.99%, as determined by chromatography data (Supplementary Figs. 14 and 15). Consequently, the purity of Xe can be increased by a factor of 100 (from 99 to 99.99%) within a single cycle of dynamic separation at room temperature, effectively removing trace amounts of Kr—an outcome that holds considerable promise for practical applications. Notably, the calculated Kr/Xe selectivity under these conditions reaches 10.4 at room temperature, representing a 65% increase compared with the value obtained from a Kr/Xe mixture with a ratio of 50:50 (Supplementary Note 2).

Furthermore, to address the urgent challenge of rapidly removing radioactive Kr isotopes from exhaust gases produced by nuclear industrial waste, a simulated mixture of nuclear exhaust gases—comprising 400 ppm Xe and 40 ppm Kr—was subjected to dynamic separation testing. As illustrated in Fig. 4c and Supplementary Fig. 16, Xe is eluted from the column first, followed by argon (Ar), nitrogen (N2), and oxygen (O2). Notably, Kr remains trapped within the column for up to 32 min before breakthrough occurs, underscoring the exceptional trapping efficiency of MIP-203-Suc in dilute separation regimes, with an extraction rate of 0.054 mmol kg-1 Kr from simulated nuclear exhaust gas streams. Importantly, this material retains its structural integrity and superior separation performance under γ-irradiation at doses up to 100 kGy (Supplementary Figs. 17–19). This advancement signifies substantial potential for the reprocessing of nuclear off-gas streams.

To elucidate the kinetics-driven sieving mechanism of the activated MIP-203-Suc material and its guest-induced structural response, we integrated advanced DFT simulations with comprehensive experimental characterization. The energy barriers for Kr and Xe diffusion through the pore apertures of the activated MIP-203-Suc were computed via DFT-based climbing-image nudged elastic band (CI-NEB) calculations. The central ellipsoidal porosity of MIP-203-Suc connects to eight triangular apertures along the crystallographic a- and b-axes, with two bridging formates contributing to an asymmetric pore landscape (Supplementary Note 4 and Supplementary Fig. 20). Depending on their orientations, these bridging formates induce a distortion of the central porosity into an asymmetrical ellipsoidal cage where one side, along with its corresponding apertures, becomes significantly narrower than the other side of the Zr6 cluster (Supplementary Tables 1 and 2). All crystallographic possible pathways were systematically identified, leading to eight potential diffusion pathways, with only six are geometrically viable (Supplementary Note 5 and Supplementary Fig. 21).

Along the a-axis, four diffusion paths traverse triangular apertures framed by succinate linkers and terminal OH-H2O groups of Zr6 clusters. These paths organize into two geometrically distinct pairs (Path-a/Path-a’) with different aperture dimensions (C‒C (2nd C in the aliphatic chain) distance = 5.64 Å vs 4.70 Å; Supplementary Table 2) on opposing sides of the Zr6 clusters. Given this measurable geometric disparity, we analysed one path from each size category (Path-a and Path-a’) to capture the full range of a-axis diffusion behavior. The b-axis shows even greater heterogeneity. While four potential routes exist, synergistic occlusion by bridging formates CH wedges, and adjacent succinate linkers sterically block Path-b2 and Path-b2’ (Supplementary Fig. 22; Supplementary Videos 1 and 2). The remaining Path-b and Path-b’ retain viable diffusion channels with significant aperture differences (5.4 Å vs. 4.2 Å; Supplementary Table 2). Therefore, four distinct paths were investigated to characterize the entire framework’s anisotropic diffusion landscape.

The CI-NEB-calculated minimum energy paths (MEPs) reveal significant differences in the migration behavior of the two guest molecules. Along the crystallographic a-axis, Path-a (Fig. 5a) exhibited moderate energy barriers of 12.8 kJ mol-1 for Kr and 21.0 kJ mol-1 for Xe, i.e., ΔE Xe-Kr ≈ 8 kJ mol-1 (Fig. 5b and Supplementary Videos 3 and 4). As anticipated, the asymmetric pore architecture creates distinct diffusion environments even among structurally related pathways. For example, Path-a’, with a slightly narrower constriction of 4.7 Å (separating the distance between the second carbons in the aliphatic chains along the a-axis, vs. 5.64 Å for Path-a (see illustration in Fig. 5d)) increases the energy barriers to 33.1 kJ mol-1 for Kr and 43.8 kJ mol-1 for Xe, resulting in a ΔE Xe-Kr of ~11 kJ mol-1 (Supplementary Fig. 23). Diffusion along the b-axis through Path-b and Path-b’ results in a significantly tighter constriction formed by succinate linkers and bridging formates, resulting in substantially higher energy barriers: 31 kJ mol−1 and 77 kJ mol−1 for Kr and 56 kJ mol−1 and 108 kJ mol−1 for Xe (Fig. 5c, Supplementary Fig. 24 and Supplementary Videos 5 and 6).

All the computed diffusion pathways reveal substantially higher energy barriers for Xe than for Kr, which aligns with the experimental observations and supports a kinetically controlled sieving mechanism. Notably, our Cl-NEB calculations reveal that transport is most favorable along the a-axis, with Path-a identified as the most energetically favorable trajectory and thus expected to dominate the separation mechanism. Accordingly, it is important to clarify how the cooperative structural flexibility of the succinate linkers facilitates the kinetic-driven separation of Kr from Xe along Path-a. As demonstrated in Supplementary Videos 3 and 4, the migration of Kr and Xe through this path reveals aperture adaptability mediated by the coordinated motions of the succinate linkers. As molecules approach the window, this dual-mode flexibility mechanism, which combines linker displacement and rotational freedom, enables diffusion while preserving framework integrity. During Kr passage, the succinate linkers—which exhibit a dihedral angle of 20.5° between the carboxylate (O-C-O) plane and the aliphatic (C-C-C) plane in the activated MIP-203-Suc structure—are significantly rotated. This distortion increases the same interplanar angle to 34.8°, representing a 14.3° deviation from equilibrium (Fig. 5d–f). The linker twisting effect is more pronounced for Xe, where the angle reaches 40.9°, forcing the linker to sacrifice its preferred geometry to accommodate this bulkier guest molecule (Supplementary Fig. 25). These rotational motions—clearly visible in Supplementary Videos 3 and 4 — occur synergistically with pore aperture expansion. The characteristic C‒C distance separating the opposite aliphatic chain initially measured at 5.6 Å expands to 6.66 Å when Kr crosses. Interestingly, these deformations are fully reversible; evidence lies in the smooth elastic recovery of linkers back to their equilibrium configurations (both in separating distances and dihedral angles) following guest passage.

Furthermore, Raman spectroscopy was applied to monitor the MOF dynamic characteristics before and after loading Kr into the pore structure (Supplementary Fig. 26). Notably, two absorption peaks corresponding to the symmetrical and asymmetrical vibrations of succinate carboxylates exhibited pronounced redshifts—from 1604 and 1468 cm−1 in their activated form to 1614 cm−1 and 1479 cm−1 in their Kr-loaded state. In contrast, a peak related to -CH2 wagging and C-C-C stretching blue-shifted by 8 cm−1 (from 1284 to 1276 cm−1) after Kr loading27. These three significant shifts in the Raman spectra support that local flexibility within succinate is Kr-assisted and most likely contributes to the migration of Kr throughout the MOF pores.

Importantly, the PXRD pattern of the fully Kr-loaded phase differs from that of the activated empty phase; however, it becomes nearly identical to that observed in the as-synthesized phase (Supplementary Fig. 27). This observation suggests that the MIP-203-Suc structure is reversibly flexible upon exposure to Kr-adsorption or a thermal stimulus. This findings sharply contrast with the relatively rigid structural behavior of this MOF upon CO2 adsorption even at high gas pressure27. This suggests that the activated MIP-203-Suc structure has a selective framework and linker flexibility in response to Kr, thereby facilitating precise diffusion-driven molecular sieving. Powder refinement of high-resolution PXRD data collected from the Kr-fully loaded sample was performed (Supplementary Fig. 28), resulting in a compelling structural model responsive to adsorbed Kr atoms. The Kr atoms are positioned in proximity to the succinate linkers, with distances between the linker hydrogens and Kr ranging from 3.52 to 3.70 Å. Remarkably, these atoms are located at a significant distance from the terminal hydroxyl groups of the Zr6 clusters (Supplementary Fig. 29). This spatial configuration optimizes the interatomic distances among adjacent Kr atoms, thereby minimizing repulsive interactions and culminating in a stabilized Kr-loaded structure at ambient temperature.

Methods
Synthesis of MIP-203-Suc
A mixture of ZrCl4 (233 mg, 1 mmol), succinic acid (500 mg, 4.23 mmol), and formic acid (3.8 mL) was transferred into a Teflon-lined autoclave (25 mL) and heated at 393 K for 72 h. Upon cooling to room temperature, a white powder product designated MIP-203-Suc was obtained. The product was then washed with water and subsequently exchanged with methanol several times for further purification.

Gas adsorption measurement
The as-synthesized MIP-203-Suc samples (minimum of 200 mg) were activated under vacuum at 393 K for 12 h. Single-component gas adsorption and desorption isotherms were measured via a 3Flex instrument (Micromeritics) at various temperatures, specifically, 273 K, 288 K, 298 K, 318 K, 333 K, and 348 K. The pressure range during these measurements ranged from 0 to 100 kPa. Ultra high-purity gases (99.999%), including He, N2, O2, CO2, Ar, Xe, and Kr, were utilized in the experiments. To maintain the desired temperatures throughout the process, a Micromeritics ISO Controller (Thermoelectric Cooled Dewar) was employed.

Powder X-ray diffraction analysis
A Bruker D8 Advance powder diffractometer (Cu-Kα, 40 kV and 40 mA) was used to acquire powder X-ray diffraction (PXRD) profiles. The measurement parameters included a scanning speed of 1.0° min−1 and a step size of 0.026°.

Synchrotron powder X-ray diffraction analysis
The MIP-203-Suc sample was introduced into a borosilicate glass capillary with an inner diameter of 1 mm. The material underwent activation at 393 K under vacuum for 12 h, after which Kr gas was introduced via a custom-built apparatus. The capillary was subsequently sealed utilizing the flame from a lighter. PXRD data were collected at BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF), employing a radiation wavelength of 0.6887 Å. For comparative analysis, the synchrotron-radiation XRD patterns were calibrated to the 2θ scale corresponding to Cu-Kα radiation (λ=1.5406 Å), thereby enabling direct comparison of peak positions with those obtained from laboratory X-ray sources. Structural refinement of the PXRD powder data was conducted via TOPAS software.

Raman spectroscopic analysis
The activated and Kr-loaded MIP-203-Suc samples were prepared via the same methodology employed for synchrotron PXRD analysis. Raman spectra were obtained via a Horiba LabRAM HR Evolution Raman spectrometer, which was equipped with a laser wavelength of 633 nm.

Column breakthrough experiments
Column breakthrough experiments for various gas mixtures were conducted via a custom-designed dynamic breakthrough system. A copper tube with an internal diameter of 5.0 mm and a length of 300 mm was packed with approximately 4.0 g of activated MIP-203-Suc adsorbent. Prior to testing, the column underwent thermal activation at 393 K under vacuum for 12 h and was subsequently pre-saturated with helium. The effluent gas composition was monitored in real time via a Hiden Analytical HPR-20 mass spectrometer. Quantitative analysis of gas purity was performed via a Scion 456-GC gas chromatograph equipped with a PESTEK-MXT-SA-MS PLOT separation column. Following calibration with certified standard gases, component purity values were calculated on the basis of integrated peak areas; specifically, xenon purity was determined through residual subtraction from the krypton concentration data.

DFT calculations
The minimum energy paths (MEPs) for Kr and Xe migration through the triangular apertures of MIP-203-suc were investigated by climbing-image nudged elastic band (CI-NEB) calculations. To map the diffusion pathways, we systematically constructed initial (IS) and final state (FS) configurations by placing individual Kr/Xe atoms outside and inside the pores, respectively. These configurations were DFT geometry-optimized with fixed experimental cell parameters for the activated MIP-203-Suc sample before CI-NEB calculations.

All the periodic DFT calculations were performed via the Vienna Ab Initio Simulation Package (VASP, version 6.4)36 with plane-wave basis sets and projector-augmented wave (PAW) potentials37,38. The Perdew–Burke–Ernzerhof (PBE)39 generalized gradient approximation (GGA) was employed with Grimme’s empirical dispersion correction (DFT-D3/BJ)40. A plane-wave cut-off energy of 600 eV and Γ-point Brillouin zone sampling were used.

For geometry optimizations of the MIP-203-suc and initial/final states (IS/FS) of the Kr/Xe migration paths, convergence criteria of 10−6 eV for energy and 0.03 eV/Å for forces were applied. The transition states were located via the VTST module41 in VASP with the following criteria: 10−5 eV and 0.03 eV/Å for the energy and forces, respectively. NEB paths were constructed and analysed via the Atomic Simulation Environment (ASE)42.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript reports that MIP-203-suc metal–organic framework (MOF) exhibits excellent performance for Kr adsorption and Kr/Xe selectivity at room temperature. The authors attribute this performance to a synergistic combination of structural and local flexibility within the MOF. Furthermore, they demonstrate that the material maintains good radiative stability under gamma irradiation and is capable of efficiently capturing trace amounts of Kr. The results are impressive, and the authors have carried out a comprehensive set of experiments.\nHowever, MIP-203-suc MOF was previously synthesized by other groups (J. Am. Chem. Soc. 2019, 141, 43, 17207–17216), and the mechanistic interpretation presented here is not particularly novel. In fact, as shown in Fig. 2(b), there are other MOFs that outperform MIP-203-suc in Kr adsorption at 298 K and 1 bar. Therefore, I find the novelty and broader significance of this work insufficient to merit publication in Nature Communications. Here are my comments:\n1. While Figures 1 and 4 include atomic geometries, it would be nicer to include higher-resolution, zoomed-in views—such as those in Figure S1—in the main text.\n2. The authors have conducted NEB simulations with DFT to understand Kr/Xe diffusion behavior. However, I recommend that they consider investigating longer timescale diffusion dynamics, potentially through molecular dynamics simulations or other relevant techniques.\n3. Although the selectivity of this MOF is excellent, the manuscript would be strengthened by a more thorough comparison with other state-of-the-art MOFs in terms of Kr/Xe selectivity.", "reviewer_2": "The manuscript describes a metal-organic framework (MOF) exhibiting synergistic structural and local flexibility, which enables preferential adsorption of krypton (Kr) over xenon (Xe) at room temperature. The reported material demonstrates commendable Kr uptake, exceptional Kr/Xe selectivity, robust radioactive stability, and effective capture of trace Kr from simulated nuclear exhaust gases. While the work represents a significant experimental advancement with promising results, substantial clarifications and methodological improvements are required to fully support the claims. Key concerns include:\n1.In Figure 1, representation of framework flexibility progression via arrows is misleading, as these flexibility modes should occur concurrently, not sequentially.\n2.Critical analyses such as pore structure characterization and thermogravimetric analysis (TGA) are absent.\n3.The consistent hysteresis observed in the isotherms requires explanation. Additionally, the observation that Kr uptake increases while Xe decreases at 273 K raises questions about the temperature dependence of separation efficiency. Investigating lower temperatures could provide further insights.\n4.The initial Xe breakthrough co-adsorption phenomenon (figure 3) needs clarification. Is this behavior mediated by framework flexibility, or does it stem from other factors? This aspect must be explicitly addressed.\n5.While guest-induced flexibility is emphasized, the temperature-responsive flexibility, particularly the high-temperature Xe preference, lacks a detailed mechanistic explanation. The study should also confirm whether all realistic diffusion paths are accounted for in computational models and whether temperature's effect on diffusion barriers is experimentally validated.\n6.The manuscript does not detail the kinetic methods employed, nor does it quantify the differences in Xe and Kr diffusion rates. Kinetic measurements at pressures such as 1 kPa or 50 kPa would complement the existing breakthrough tests and strengthen the study's conclusions.\n7.A comprehensive table comparing the performance of the reported MOF with other relevant materials is essential for contextualizing its advancements.\n8.The assertion that \"only three MOFs separate Kr/Xe\" is incorrect. The authors must revise this statement to acknowledge other material classes (e.g., references: 10.1021/acsami.4c01833; 10.1002/adma.202409474) that have demonstrated Kr/Xe separation capabilities.", "reviewer_3": "Dong et al report the efficient separation of Kr/Xe at room temperature using a Metal- Organic Framework, MIP-203-Suc which is based on zirconium and succinate. The authors attribute the separation performance to a \"kinetic-driven molecular sieving\" mechanism due to a local structural flexibility of the MIP-203-Suc framework.\nIn comparison with the traditional adsorptive separation methods for Xe/Kr, Kr/Xe separation offers the advantages of providing high-purity xenon while enabling the disposal of krypton, thereby enhancing economic efficiency and operational effectiveness. Therefore this original work represents an interesting advancement in achieving effective molecular sieve Kr/Xe separation using microporous materials with uniform and appropriately sized pores. The study is well conducted, supported by thorough investigation into the adsorption and separation performance of krypton and xenon on MIP-203-Suc, as well as an exploration of the underlying mechanisms that govern this separation process. I thus feel the work is highly suitable for publication in Nature Communication providing a few comments are addressed. In particular, the authors are kindly encouraged to further conduct and discuss comparative analysis to even better illustrate the benefit and separation mechanism of their material.\n1. MIP-203-F (fumarate)\nIn a previous work (AIChE J. 2023, 69, e18169, DOI:10.1002/aic.18169), MIP-203-F (fumarate) was shown to exhibit an adsorptive separation for Xe/Kr, whereas MIP-203-Suc exhibits effective separation performance for Kr/Xe. Could the authors discuss about these two isostructural MOFs and investigate the factor influencing the kinetic sieving effect of Kr/Xe in MIP-203-Suc?\n2. Enhanced the comparison with three MOFs that exhibit Kr/Xe separation below 273 K\nAs indicated by the authors, three MOFs have previously been reported to separate Kr from Xe at temperature below 273 K. It would be interesting to indicate and compare their performance (selectivity, Kr uptake, etc) with that of the room temperature separation performed in this study.\n3. Enhanced the discussion on the comparison of the Kr uptake to several benchmark materials\na) A brief description in Supplementary of the composition/structure/porosity of those benchmark materials would be beneficial for the readers.\nb) While MIP-203-Suc surpassed most of the other materials, 3 materials (Co-Suqarate, MOF-Cu-H, ZJU-74a-PD) exhibited higher or equivalent Kr uptakes. Their features leading to high Kr uptake should be discussed in the text.\nc) Also, how do these materials perform in term of selectivity in adsorbing Kr from Xe?\nd) The adsorption capacities for Kr uptake are presented with inconsistent units throughout the manuscript (mg/cm³ in Lines 41 and 144 versus cm³/cm³ or cm*/g in other sections and figures). To ensure uniformity in data presentation and facilitate direct comparison of adsorption performance, it is recommended that the authors adopt a standardized unit system throughout the text.\n4. Xe adsorption\na) Considering the extremely low adsorption rate of Xe, how are the Xe adsorption isotherms assessed? is the adsorption capacity obtained reasonable?\nb) The scale bar of the Y axis of Figure S4 could be modified to improve the quality of the figure.\nSome other minor comments\n5. In Lines 97-100, the author should provide proper citation support for this claim.\n6. In Lines 142–145, the depiction of the channels along the a-axis would greatly benefit from the addition of a supplementary figure. Specifically, incorporating a visualization of these channels using the Connolly surface would significantly enhance the clarity and structural presentation of this information.\n7. In Lines 297 ‘thermal stimulus’, the author should provide data or citation to support this claim, or rephrase this sentence.\n8. The legend of figure S21 does not correspond to the data provided in the Figure."}, "author_response": "We would like to express our sincere gratitude to the reviewer for their positive evaluation and insightful comments aimed at enhancing our work. As noted by the reviewer, we reported the structure of MIP-203-Suc in J. Am. Chem. Soc. in 2019. The objective of that previous paper was to demonstrate the feasibility of synthesizing Zr-MOFs using natural C4 linkers, specifically fumaric acid, succinic acid, and malic acid. Additionally, we conducted a preliminary exploration into the flexibility of these MOFs at a fundamental level through thermal treatment and exposure to carbon dioxide, which served as a model quadrupolar molecule; however, this study did not delve into mechanisms related to applicable properties. In contrast, our current paper focuses on Kr/Xe separation. This involves a systematic elucidation through Xe/Kr adsorption isotherms, synchrotron radiation X-ray diffraction, Raman spectroscopy, and comprehensive theoretical simulations.\nIn addition, we agree with the reviewer that the adsorption uptake of Kr is a critical metric for assessing the performance of a sorbent in Kr/Xe separation. A comparative analysis between MIP-203-Suc and other benchmark MOFs regarding their Xe and Kr adsorption isotherms is illustrated in Figure R1. It is indeed accurate that the Kr adsorption uptake by MIP-203-Suc at 298 K and 1 bar is comparable to those reported for established MOFs such as ZJU-74a-Pd, MOF-Cu-H, and Co-squarate, as highlighted by the reviewer.\nMore importantly, minimal Xe adsorption uptake is essential for an effective Kr-selective sorbent; in this regard, MIP-203-Suc fulfills this crucial requirement, as evidenced in Figure R1. Ultimately, high selectivity for separating Kr from Xe serves as a primary criterion for identifying good Kr-selective sorbents. As demonstrated in Figure 4b of the manuscript, MIP-203-Suc effectively removes Kr from Xe, thereby facilitating efficient Xe purification. This characteristic renders MIP-203-Suc more energy-efficient and economically viable compared to other MOFs that require additional desorption processes to achieve effective Xe purification. Notably, this feature sharply contrasts with many benchmark MOFs that exhibit significantly higher uptakes of Xe relative to Kr, resulting in an adsorptive separation selectivity that favors Xe over Kr.\nFigure R1. Comparison of Kr and Xe adsorption isotherms on MIP-203-Suc, ZJU-74a-Pd, MOF-Cu-H and Co-squarate.\n* Data acquired at 296 K.\nQuestion 1:. While Figures 1 and 4 include atomic geometries, it would be nicer to include higher-resolution, zoomed-in views—such as those in Figure S1—in the main text.\nResponse: We sincerely appreciate the valuable suggestions provided by the referee. In response to these recommendations, we have revised the structural description diagrams to improve the clarity of both the crystal and pore structures of MIP-203-Suc. The relevant results are presented in Figure R2 and have been incorporated as Figure 2 in the revised manuscript.\nFigure R2. Crystal structures of MIP-203-Suc. a. and b. Description of axial structural change and Connolly surface for as-synthesized and activated MIP-203-Suc.\nQuestion 2: The authors have conducted NEB simulations with DFT to understand Kr/Xe diffusion behavior. However, I recommend that they consider investigating longer timescale diffusion dynamics, potentially through molecular dynamics simulations or other relevant techniques.\nResponse: We sincerely appreciate the reviewer's suggestion. The objective of our computational study was not to elucidate the mechanistic origin of the experimentally-observed kinetic sieving of MIP-203-Suc. As such, the separation is governed by the relative diffusion rates of the guests, which are exponentially dependent on the activation energy (E ) of passing through the pore aperture, as described by the standard Arrhenius relationship: 𝑘 ∝ 𝑒−𝐸a/𝑘𝑇 . To this end, we employed DFT-based NEB calculations. This method is the standard ab initio technique for precisely locating the minimum energy path and quantifying the E for such a \"rare-event\" phenomenon. Our CI-NEB calculations revealed a substantial difference in the diffusion barriers for the dominant pathway (Path-a): 12.8 kJ·mol-1 for Kr versus 21.0 kJ·mol-1 for Xe. This calculated energy difference (ΔE Xe−Kr ≈ 8.2 kJ·mol-1) provides a clear, fundamental explanation for the kinetic-sieving separation mechanism. Crucially, the CI-NEB method directly captures and quantifies the \"guest-induced flexibility\" that is central to our paper's claim. As shown in Figures 5d-f, the calculations explicitly visualize the requisite distortion of the succinate linkers (e.g., a 14.3° change in dihedral angle) and the cooperative pore aperture expansion (from 5.6 Å to 6.66 Å) required for Kr to pass the transition state. Indeed, we would like to also stress that while classical MD simulations could probe the guest diffusion in the pores at longer- time and length-scale, they would not deliver any more insights into the kinetic-sieving mechanism which is fundamentally governed by the rare-event gate crossing phenomenon where the rate-limiting step might occur on timescales far beyond the reach of conventional MD simulations.\nQuestion 3: Although the selectivity of this MOF is excellent, the manuscript would be strengthened by a more thorough comparison with other state-of-the-art MOFs in terms of Kr/Xe selectivity.\nResponse: We appreciate the referee's valuable suggestion. We have supplemented the comparison with other state-of-the-art MOFs (see Table R1). Additionally, this information has been included in the revised Supplementary Information as Supplementary Table 3. Relevant literature has also been incorporated into the updated Supplementary Information.\nTable R1. Comparison of separation performance of MIP-203-Suc with reported porous crystalline materials.\nDensity\nXe uptake Kr uptake Kr/Xe Uptake\nAdsorbent Ref.\n(mmol/g) (mmol/g) ratio (g/cm3)\nMIP-203-Suc 0.075 0.90 12.0 1.81 This work\nFMOFCu 0.45 0.44 1.0 1.69 [1]\nMn(ina) 1.75 0.13 0.0741 1.53 [2]\nCo-squarate 1.35 0.80 0.588 2.19 [4]\nNKMOF-1-Ni 2.16 1.15 0.526 1.75 [5]\nMIP-203-F 2.14 1.15 0.526 1.80 [6]\nMOF-Cu-H 3.18 1.63 0.5 1.27 [7]\nZr-Fum-Me 1.9 0.72 0.385 1.67 [8]\nATC-Cu 5.0 2.7 0.526 1.41 [9]\nNU-1107-Ag(I) 2.79 0.82 0.294 0.789 [10]\nCC3 2.44 0.95 0.385 0.97 [11]\nHOF-FJU-46[a] 2.51 0.95 0.385 1.087 [12]\nECUT-60 4.3 1.45 0.333 N.A. [13]\nZUL-C2 2.56 1.34 0.526 1.26 [14]\nZJU-74a-Pd[a] 2.81 1.07 0.385 1.67 [15]\nSBMOF-2 2.78 0.92 0.333 1.19 [16]\nCo (HCOO) 2.0 0.77 0.385 1.82 [17]\n3 6\nZIF-4 2.32 0.89 0.385 1.22 [18]\nZUL-530 3.13 1.43 0.455 1.44 [19]\nHOF-ZJU-210a 3.01 1.65 0.556 1.41 [20]\nUSTC-740\n3.27 1.45 0.435 1.32 [21]\nAl-Fum-Me 3.01 1.18 0.385 1.08 [22]\nAg-MOF-303 3.35 1.12 0.333 1.44 [23]\nCo-MOF-74[b] 6.71 1.66 0.25 1.17 [24]\nCROFOUR-1-Ni 1.77 0.49 0.278 1.17 [25]\nSBMOF-1 1.4 1.0 0.714 1.55 [26]\nUiO-66-NH (OMe) [c] 1.0 0.31 0.313 1.51 [27]\n2 2\nZU-62 3.76 2.33 0.625 1.62 [28]\n[a] Measured at 296 K;\n[b] Measured at 293 K;\n[c] Measured at 283 K;\nReviewer #2:\nGeneral Comment: The manuscript describes a metal-organic framework (MOF) exhibiting synergistic structural and local flexibility, which enables preferential adsorption of krypton (Kr) over xenon (Xe) at room temperature. The reported material demonstrates commendable Kr uptake, exceptional Kr/Xe selectivity, robust radioactive stability, and effective capture of trace Kr from simulated nuclear exhaust gases. While the work represents a significant experimental advancement with promising results, substantial clarifications and methodological improvements are required to fully support the claims. Key concerns include:\nResponse: We appreciate the referee’s support and positive assessment of our work.\nQuestion 1: In Figure 1, representation of framework flexibility progression via arrows is misleading, as these flexibility modes should occur concurrently, not sequentially.\nResponse: We appreciate the referee's valuable suggestions. We have revised the structural description diagrams to enhance the understanding of the framework flexibility of MIP-203-Suc. The relevant results are presented in Figure 1 of the revised manuscript.\nQuestion 2: Critical analyses such as pore structure characterization and thermogravimetric analysis (TGA) are absent.\nResponse: We appreciate the insightful comments provided. In response, we have incorporated the results of physisorption and TGA analyses of MIP-203-Suc into the supplementary information to enhance the characterization of the material. The relevant findings are now presented as Supplementary Figures 2-4 in the revised manuscript.\nQuestion 3: The consistent hysteresis observed in the isotherms requires explanation. Additionally, the observation that Kr uptake increases while Xe decreases at 273 K raises questions about the temperature dependence of separation efficiency. Investigating lower temperatures could provide further insights.\nResponse: The observed sorption hysteresis loop can be attributed to the structural flexibility of MIP-203-Suc during the adsorption and desorption processes of Kr or Xe. MIP-203-Suc features 1D channels composed of interconnected bottle-like cavities with narrow apertures. During the adsorption phase, Kr or Xe atoms infiltrate these pore cavities, creating \"nano-traps\" that are associated with the expansion of the 1D channels as well as the rotation and stretching of succinate linkers. In contrast, during desorption, the adsorbed Kr or Xe atoms must escape from these nano-traps within the 1D channels while undergoing structural restoration. This process significantly hinders the diffusion of adsorbed Kr or Xe from the cavities in MIP-203-Suc into the surrounding environment. This phenomenon is likely a primary reason for the observed adsorption-desorption hysteresis loop in MIP-203-Suc. As temperature increases, it is noted that the hysteresis loop of the adsorption-desorption isotherm gradually diminishes, indicating that the flexible ultra-microporous structure of MIP-203-Suc forms diffusional traps for both Kr and Xe. Such sorption hysteresis loops have also been reported in other flexible MOFs, including Mn(ina) (J. Am. Chem. Soc., 2020, 142, 2 20088–20097) and Flex-Cd-MOF-2a (J. Am. Chem. Soc., 2024, 146, 14357−14367).\nIn response to the reviewers' comments, we conducted sorption experiments for Kr and Xe on MIP-203-Suc at lower temperatures (253 K, 263 K, and 268 K) to investigate the temperature dependence of separation efficiency. The results indicate that within the temperature range of 368 K to 268 K, there is a gradual decrease in Xe adsorption capacity accompanied by a concurrent increase in Kr adsorption capacity. This trend leads to a continuous enhancement of the Kr/Xe adsorption capacity ratio (at 100 kPa), thereby improving separation efficiency. However, further reduction of the temperature from 268 K to 253 K results in a significant decline in Kr adsorption, which diminishes the Kr/Xe ratio. This observation suggests that cooling below 268 K does not enhance Kr/Xe separation performance. Additionally, decreased temperatures lead to reduced diffusion rates of Kr during dynamic adsorption, compromising its dynamic adsorption capacity and overall separation effectiveness. The relevant data from original Figures R3 and R4 have been integrated into Supplementary Figures 5 and Figure 3b in the revised manuscript as well as in the updated supplementary information.\nFigure R3. Single-component adsorption-desorption isotherms of Kr and Xe on MIP-203-Suc at different temperature.\nFigure R4. Correlation plots between the Xe and Kr uptake at 1 bar and the indicated temperatures.\nQuestion 4: The initial Xe breakthrough co-adsorption phenomenon (figure 3) needs clarification. Is this behavior mediated by framework flexibility, or does it stem from other factors? This aspect must be explicitly addressed.\nResponse: We would like to express our gratitude for the insightful comments provided by the reviewer. To investigate the co-adsorption of Xe during breakthrough experiments, we conducted temperature programmed desorption (TPD) experiments on the MIP-203-Suc column under adsorption equilibrium conditions using a Xe-Kr gas mixture in a 50:50 volume ratio. The TPD curve revealed that MIP-203-Suc continues to adsorb a certain amount of Xe throughout the dynamic breakthrough process. This phenomenon may be attributed to the framework flexibility of MIP-203-Suc, which allows Xe to compete with Kr for entry into its pores under high partial pressure during the breakthrough phase. The TPD curve for MIP-203-Suc is presented in Figure R5 and has been incorporated as Supplementary Figures 13 in the revised supplementary information.\nFigure R5. Desorption of the Kr and Xe from MIP-203-Suc with 1.5 mL min-1 of He from RT to 393 K.\nTo further investigate the structural response of MIP-203-Suc in a high-concentration Xe atmosphere, in situ PXRD measurements were conducted within a pure Xe environment utilizing a custom-designed laboratory-scale in-situ PXRD cell. To ensure the acquisition of high-quality PXRD data, our measurement setup incorporated a 2 μm-thick polyimide film with excellent X-ray transmittance to seal the in-situ sample cell (Fig. R6a). The MIP-203-Suc sample was contained within the chamber and subjected to the following sequence: (1) thermal activation at 393 K under vacuum for 12 hours to eliminate residual solvents; (2) exposure to pure Xe gas at 298 K and 100 kPa (approximately 1 bar) for 5 hours to achieve thermodynamic adsorption equilibrium; and (3) rapid transfer to a Bruker D8 Advance diffractometer for immediate PXRD characterization.\nThe PXRD results demonstrate a significant reduction in the intensities of low-angle diffraction peaks (2θ: 7~12°) when subjected to a Xe atmosphere, as compared to the activated sample (Fig R6b). This attenuation can be attributed to the flexible pore-channel architecture of MIP-203-Suc. Under conditions of high Xe concentration, the narrow pore apertures impose considerable diffusion constraints on Xe atoms, potentially limiting adsorption primarily to surface-accessible cavities—consistent with the observed limited Xe uptake capacity. This localized incorporation of Xe induces framework expansion within the long-range periodic structure, resulting in partial disruption of the ordered lattice. Such structural disorder diminishes the coherence of X-ray scattering at low angles, consequently reducing the intensity of low-angle diffraction peaks (J. Appl. Cryst. 1999, 32, 36-50). The similar characteristic attenuation of low-angle XRD peaks during gas-adsorption-induced phase transitions has also been reported in previous reported flexible MOFs such as [Ni(bpy) (NCS) ] 2 2 n (ACS Materials Lett. 2024, 6, 666−673). These findings indicate the dynamic structural adaptability of MIP-203-Suc in response to interactions with Xe, emphasizing its responsive framework behavior under gas loading.\nFigure R6. (a) custom-designed in situ PXRD cell. (b) PXRD comparison of activated MIP-203-Suc and Xe loaded MIP-203-Suc.\nQuestion 5: While guest-induced flexibility is emphasized, the temperature-responsive flexibility, particularly the high-temperature Xe preference, lacks a detailed mechanistic explanation. The study should also confirm whether all realistic diffusion paths are accounted for in computational models and whether temperature's effect on diffusion barriers is experimentally validated.\nResponse: We sincerely thank the reviewer for raising this question.\na) Temperature-responsive flexibility and high-temperature Xe preference:\nThe anomalous observation that Xe uptake increases with temperature is the classic experimental signature of an activated diffusion process, where sorption is kinetically limited. Our NEB calculations directly explain this: Xe faces a substantial diffusion barrier of 21.0 kJ·mol-1. At low temperatures, Xe is kinetically trapped; as temperature rises, thermal energy (kT) enables barrier crossing, increasing uptake. The temperature-responsive flexibility of the framework is confirmed by non-monotonic Kr uptake maximum at 268 K and below this temperature framework rigidity increases diffusion barriers even for smaller Kr. Our NEB calculations also revealed that Xe passage requires larger linker distortions (40.9° vs. 34.8° dihedral angle for Kr), making Xe disproportionately benefit from thermally-enhanced framework flexibility at elevated temperatures.\nb) Comprehensiveness of diffusion pathways:\nWe confirm that all crystallographic viable pathways were systematically evaluated. The structure contains eight triangular apertures, but only six are physically accessible. Along the a-axis, symmetry reduces four apertures to two distinct types (Path-a: 5.64 Å; Path-a': 4.70 Å). Along the b-axis, Path-b2/b2' are sterically blocked by bridging formates. In summary, all the remaining viable pathways (Path-a, Path-a', Path-b and Path-b') have been fully characterized, with Path-a dominating transport — consistent with experimental PXRD showing preferential a-axis expansion upon Kr loading.\nc) Experimental validation of temperature effects on diffusion barrier:\nWe systematically investigated the temperature-dependent effects on diffusion barriers through adsorption kinetic studies across varying temperatures, enabling direct observation of how thermal energy modulates diffusion time constants—key indicators of diffusion energy barriers. The diffusion time constants of Xe at 1kPa, derived from temperature-dependent kinetic profiles, exhibit a significant and progressive increase with rising temperature: 3.44×10-4 min-1 at 273 K, 6.13×10-4 min-1 at 298 K, and 8.77×10-4 min-1 at 318 K, respectively (Fig R8). In contrast, Kr shows only a modest increase over the same range, with the diffusion time constants at 1kPa rising from 1.72×10-3 min-1 at 273 K to 2.03×10-3 min-1 at 298 K (Fig R7). These results confirm that elevated temperatures reduce diffusion energy barriers for both gases, thereby enhancing molecular diffusivity, accelerating adsorption kinetics, and promoting faster equilibration. Notably, the temperature dependence is markedly stronger for Xe than for Kr, consistent with the expectation that larger gas molecules exhibit greater sensitivity to thermal activation due to their inherently higher diffusion barriers.\nRelevant results are illustrated in Figure R7-R8 and have been included as Supplementary Figure 8-9 in the revised supplementary information.\nFigure R7. Time-dependent gas uptake profiles (a) and fitting of diffusion time constants (b) for Kr in MIP-203-Suc at 1 kPa under different temperatures. The diffusion time constants of Kr were calculated to be 1.72 × 10-3 min-1 at 273 K and 2.03 × 10-3 min-1 at 298 K, respectively.\nFigure R8. Time-dependent gas uptake profiles (a) and fitting of diffusion time constants (b) for Xe in MIP-203-Suc at 1 kPa under different temperatures. The diffusion time constants of Xe were calculated to be 3.44 × 10-4 min-1 at 273 K, 6.13 × 10-4 min-1 at 298 K, and 8.77 × 10-4 min-1 at 318 K, respectively.\nQuestion 6: The manuscript does not detail the kinetic methods employed, nor does it quantify the differences in Xe and Kr diffusion rates. Kinetic measurements at pressures such as 1 kPa or 50 kPa would complement the existing breakthrough tests and strengthen the study's conclusions.\nResponse: We appreciate the valuable comments from the referee. We have supplemented kinetic measurements and methods in the revised manuscript. Relevant results are illustrated in Figure R7-R9 and have been included as Supplementary Figure 8-10 in the revised supplementary information.\nFigure R9. Time-dependent gas uptake profiles (a) and fitting of diffusion time constants (b) for Kr and Xe in MIP-203-Suc at 298 K and 50 kPa. The diffusion time constants were calculated to be 7.38 × 10⁻4 min⁻1 for Xe and 9.37 × 10⁻3 min⁻1 for Kr.\nQuestion 7: A comprehensive table comparing the performance of the reported MOF with other relevant materials is essential for contextualizing its advancements.\nResponse: We appreciate the referee's valuable suggestion. We have supplemented the comparison with other state-of-the-art MOFs (see Table R1). Additionally, this information has been included in the revised Supplementary Information as Supplementary Table 3. Relevant literature has also been incorporated into the updated Supplementary Information.\nQuestion 8: The assertion that \"only three MOFs separate Kr/Xe\" is incorrect. The authors must revise this statement to acknowledge other material classes (e.g., references: 10.1021/acsami.4c01833; 10.1002/adma.202409474) that have demonstrated Kr/Xe separation capabilities.\nResponse: We have made revisions to the relevant descriptions, and the materials cited by the referee have been duly incorporated into the main text. Additionally, pertinent literature has been integrated into the revised manuscript.\n“To date, only a few porous materials can sieve Kr from Xe: the densified CuBTC to form FS-CuBTC sorbent,19 the porous carbon C-Suc-750 material with an appropriate pore size of UM approximately 4.0 Å20 and three MOFs, i.e. FMOFCu, Mn(ina) and CoNi-DAB with attractive 2 separation performance at low temperatures below 273 K through kinetic-controlled molecular sieving21, 22, 23 (Fig. 1a).”\nReferences:\n19. Mohamed MH, Elzeny I, Samuel J, Huang Y, Helal AS, Galanek M, et al. Trailblazing Kr/Xe Separation: The Birth of the First Kr-Selective Material. ACS Applied Materials & Interfaces 2024, 16(22): 29364-29373.\n20. Chen F, Zheng F, Huang X, Chu Z, Sun H, Yang L, et al. The First Kr-Selective Carbon Molecular Sieve for Inverse Adsorption of Krypton Over Xenon at Ambient Temperature. Advanced Materials 2025, 37(4): 2409474.\nReviewer #3:\nGeneral comment: Dong et al report the efficient separation of Kr/Xe at room temperature using a Metal- Organic Framework, MIP-203-Suc which is based on zirconium and succinate. The authors attribute the separation performance to a \"kinetic-driven molecular sieving\" mechanism due to a local structural flexibility of the MIP-203-Suc framework.\nIn comparison with the traditional adsorptive separation methods for Xe/Kr, Kr/Xe separation offers the advantages of providing high-purity xenon while enabling the disposal of krypton, thereby enhancing economic efficiency and operational effectiveness. Therefore this original work represents an interesting advancement in achieving effective molecular sieve Kr/Xe separation using microporous materials with uniform and appropriately sized pores. The study is well conducted, supported by thorough investigation into the adsorption and separation performance of krypton and xenon on MIP-203-Suc, as well as an exploration of the underlying mechanisms that govern this separation process. I thus feel the work is highly suitable for publication in Nature Communication providing a few comments are addressed. In particular, the authors are kindly encouraged to further conduct and discuss comparative analysis to even better illustrate the benefit and separation mechanism of their material.\nResponse: We sincerely appreciate the referee's support and their favorable assessment of our work.\nQuestion 1: MIP-203-F (fumarate) In a previous work (AIChE J. 2023, 69, e18169, DOI:10.1002/aic.18169), MIP-203-F (fumarate) was shown to exhibit an adsorptive separation for Xe/Kr, whereas MIP-203-Suc exhibits effective separation performance for Kr/Xe. Could the authors discuss about these two isostructural MOFs and investigate the factor influencing the kinetic sieving effect of Kr/Xe in MIP-203-Suc?\nResponse: We appreciate the referee's valuable suggestion. This can be attributed to the structural characteristics of MIP-203-F, which is constructed from a comparatively more rigid 2D fumarate-based linker and features larger pore channels in comparison with MIP-203-Suc. The rigidity of the 2D fumarate linker somewhat restricts cooperative dynamic motions, such as ligand rotation or vibrational flexibility within the framework. In contrast, MIP-203-Suc, which is built from a relatively more flexible 3D succinate-based linker, exhibits narrower pores and facilitates synergistic structural flexibility that includes both framework expansion and ligand rotational dynamics. These distinct structural and dynamic attributes enable MIP-203-Suc to effectively separate Kr and Xe at ambient temperature, whereas MIP-203-F primarily functions as a Xe-selective adsorbent.\nThe analysis of PXRD patterns for the MIP-203-F and MIP-203-Suc samples, which were soaked in various solvents as reported in J. Am. Chem. Soc. 2019, 141, 43, 17207–17216, indicates that MIP-203-F possesses larger pores compared to those of MIP-203-Suc. DMF with a molecular kinetic diameter of approximately 4.9 Å is capable of opening the pore channels of MIP-203-F. In contrast, solvent molecules with larger sizes or branched structures (e.g., DMF, acetone, and ethanol) can scarcely penetrate the dynamic pores of MIP-203-Suc.\nQuestion 2: Enhanced the comparison with three MOFs that exhibit Kr/Xe separation below 273 K. As indicated by the authors, three MOFs have previously been reported to separate Kr from Xe at temperature below 273 K. It would be interesting to indicate and compare their performance (selectivity, Kr uptake, etc) with that of the room temperature separation performed in this study.\nResponse: We appreciate the referee's valuable suggestion. We compared the uptake ratio of the reported Kr/Xe separation materials at different temperatures, and the adsorption capacities of Kr/Xe at 298K. Relevant results are illustrated in Figure R10 and have been included as Supplementary Figure 7 in the revised supplementary information.\nFigure R10. Comparison of MOFs of Kr/Xe separation. (a) Uptake ratio at different temperature; (b) Kr (Xe) uptake at 298 K.\nQuestion 3: Enhanced the discussion on the comparison of the Kr uptake to several benchmark materials.\na) A brief description in Supplementary of the composition/structure/porosity of those benchmark materials would be beneficial for the readers.\nResponse: We appreciate the referee's valuable suggestion. We have supplemented the comparison with other state-of-the-art MOFs (see Table R1). Additionally, this information has been included in the revised supplementary information as Supplementary Table 3. Relevant literature has also been incorporated into the updated Supplementary Information.\nb) While MIP-203-Suc surpassed most of the other materials, 3 materials (Co-Suqarate, MOF-Cu-H, ZJU-74a-PD) exhibited higher or equivalent Kr uptakes. Their features leading to high Kr uptake should be discussed in the text.\nResponse: The saturated adsorption capacity of materials is fundamentally determined by two critical parameters: the gas-material binding energy and the structural porosity. In the case of MIP-203-Suc, the observed limitations in adsorption arise from its restricted pore volume—a correlation that we have systematically validated through comparative benchmarking with established porous crystalline materials (refer to Table R2, included as Supplementary Table 4 in the revised manuscript). This structural constraint necessitates a thorough analysis of porosity, which we have comprehensively provided alongside literature-derived reference data in the updated Supplementary Information. Relevant comparative methodologies and interpretations of datasets have been appropriately cited in accordance with international standards for materials characterization.\nAlthough the Kr adsorption capacity of MIP-203-Suc is slightly lower than that of Co-Suqarate, MOF-Cu-H, and ZJU-74a-Pd, this discrepancy can be attributed to the significantly higher specific surface areas and pore volumes exhibited by these benchmark materials. Notably, despite its comparatively lower overall porosity, MIP-203-Suc demonstrates a comparable—or only marginally reduced—Kr adsorption performance. This observation suggests a more efficient utilization of its pore space. Such findings indicate that the pore architecture of MIP-203-Suc is particularly effective for Kr adsorption, reflecting superior structural suitability and enhanced adsorption efficiency.\nTable R2. Comparison of pore volume and porosity with some reported porous crystalline materials.\nPore volume Porosity[b]\nAdsorbent Ref.\n(cm3 g-1) (cm3 cm-3)\nMIP-203-Suc 0.098 0.177 This work\nCo-squarate 0.106[a] 0.232 [4]\nMOF-Cu-H 0.460[a] 0.584 [7]\nHOF-FJU-46 0.200 0.217 [12]\nZJU-74a-Pd 0.250[a] 0.418 [15]\nCo (HCOO) 0.170[a] 0.309 [17]\n3 6\nCROFOUR-1-Ni 0.200[a] 0.234 [25]\nSBMOF-1 0.094[a] 0.146 [26]\n[a] the pore volume is calculated theoretically based on the equation: pore volume = porosity / crystal density. The porosity is calculated using Platon software with the Connolly diameter of 1.8 Å (the diameter of N probe 2 molecule). (Materials with no pore volume reported in the original literatures are marked with the superscript a, while other materials without special explanation are reported with specific pore volume shown in the original literatures)\n[b] Porosity = pore volume × crystal density.\nc) Also, how do these materials perform in term of selectivity in adsorbing Kr from Xe?\nResponse: In fact, these materials demonstrate a higher adsorption capacity for Xe compared to Kr, indicating a stronger preference for the adsorption of Xe. Consequently, their Kr/Xe saturation adsorption ratios are all less than 1. We have provided a comparative chart and data table that illustrate the Kr/Xe ratio. It is evident that for materials co-adsorbing Xe and Kr, selectivity rarely exceeds 5; however, MIP-203-Suc achieves a selectivity greater than 10. Relevant results are depicted in Figure R11.\nFigure R11. Comparison of uptake ratio of Xe/Kr or Kr/Xe at 298 K and 1.0 bar among various materials.\nd) The adsorption capacities for Kr uptake are presented with inconsistent units throughout the manuscript (mg/cm³ in Lines 41 and 144 versus cm³/cm³ or cm*/g in other sections and figures). To ensure uniformity in data presentation and facilitate direct comparison of adsorption performance, it is recommended that the authors adopt a standardized unit system throughout the text.\nResponse: We appreciate the referee's valuable suggestion. The units of adsorption capacities have been uniformed in the revised manuscript.\nQuestion 4:\nXe adsorption\na) Considering the extremely low adsorption rate of Xe, how are the Xe adsorption isotherms assessed? is the adsorption capacity obtained reasonable?\nResponse: The Xe adsorption isotherms were measured using a 3Flex high-performance gas adsorption analyzer (Micromeritics), which represents the most advanced instrument in this field for providing high-resolution adsorption and desorption isotherms. Although the adsorption rate of Xe is relatively slow at 298 K and 50 kPa, as determined by the kinetic measurements of Xe on MIP-203-Suc, the obtained Xe adsorption isotherms for MIP-203-Suc using the 3Flex are both convincing and reasonable. The data points collected from the Xe adsorption isotherm indicate that dynamic adsorption equilibrium has been nearly fully achieved. This equilibrium state, established within a closed and isothermal system, occurs when sufficient equilibration time allows the adsorbate gas to reach a steady-state distribution between its gaseous and solid phases. Macroscopically, this condition is characterized by an insignificant change in system pressure over time. In the experimental procedure, data acquisition was conducted only when the rate of pressure variation remained below 0.01% over a 20-second interval. Under these criteria, the resulting adsorption isotherm can be considered to accurately represent equilibrium conditions.\nb) The scale bar of the Y axis of Figure S4 could be modified to improve the quality of the figure.\nResponse: We appreciate the referee's valuable suggestion. We have adjusted the scale of the Y-axis to render the adsorption data curve for Xe more clearly. The relevant results are presented in Figure R12 and have been incorporated as Figure Supplementary Figure 6 in the revised supplementary information.\nFigure R12. Xenon adsorption isotherms for activated MIP-203-Suc samples obtained at different temperatures.\nSome other minor comments\nQuestion 5: In Lines 97-100, the author should provide proper citation support for this claim.\nResponse: We appreciate the referee's valuable suggestion. The proper citation which supports the claim has been added in the revised manuscript. The corresponding revisions are presented as follows.\n“In addition to inherent local flexibility like bond rotation and stretching, guest-responsive dynamic MOF frameworks with narrow pore windows show promise in distinguishing between these gases due to significant differences in diffusion energy through small apertures24, 25, 26.”\nReference：\n24. Xiong H, Peng Y, Liu X, Wang P, Zhang P, Yang L, et al. Topology Reconfiguration of Anion-Pillared Metal–Organic Framework from Flexibility to Rigidity for Enhanced Acetylene Separation. Advanced Materials 2024, 36(29): 2401693.\n25. Parent LR, Pham CH, Patterson JP, Denny MS, Jr., Cohen SM, Gianneschi NC, et al. Pore Breathing of Metal–Organic Frameworks by Environmental Transmission Electron Microscopy. Journal of the American Chemical Society 2017, 139(40): 13973-13976.\n26. Zeng H, Xie M, Wang T, Wei R-J, Xie X-J, Zhao Y, et al. Orthogonal-array dynamic molecular sieving of propylene/propane mixtures. Nature 2021, 595(7868): 542-548.\nQuestion 6: In Lines 142–145, the depiction of the channels along the a-axis would greatly benefit from the addition of a supplementary figure. Specifically, incorporating a visualization of these channels using the Connolly surface would significantly enhance the clarity and structural presentation of this information.\nResponse: We appreciate the referee's valuable suggestion. We have supplemented the description of the Connolly surface along the a-axis to facilitate a better understanding of the crystal and pore structures of MIP-203-Suc. Relevant results are illustrated in Figure R2 and have been included as Figure 2 in the revised manuscript.\nQuestion 7: In Lines 297 ‘thermal stimulus’, the author should provide data or citation to support this claim, or rephrase this sentence.\nResponse: We appreciate the referee's valuable suggestion. The term of ‘thermal stimulus’ is not suitable; we have removed this sentence in the revised manuscript.\nQuestion 8: The legend of figure S21 does not correspond to the data provided in the Figure.\nResponse: We appreciate reviewer’s comment to improve our work. The legend of figure S21 has been corrected in the revised supplementary information. Relevant results are illustrated in Figure R13 and have been included as Supplementary Figure 26 in the revised supplementary information.\nFigure R13. Raman spectra of the activated MIP-203-Suc and Kr-loaded MIP-203-Suc."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Although I had reservations at first, I now acknowledge the importance of this study given its excellent Xe/Kr selectivity.", "reviewer_2": "After the manuscript has been revised and supplemented, it demonstrates sufficient rigor in mechanism research, providing a comprehensive evidence encompassing characterization, theoretical calculations, and spectroscopy validation. This manuscript effectively supports the core conclusion that “synergizing structural flexibility with local flexibility enables room-temperature kinetic sieving of Kr/Xe”. Nevertheless, certain details remain amenable to further refinement. Such as in Supplementary Figure 18，the names of MIP-203-Suc and “Zr-SUC” are inconsistent, and the expressions of xenon-krypton concentrations vary (e.g., Xe/Kr=50/50, Xe/Kr=1/1).", "reviewer_3": "The authors have satisfactorily addressed the concerns raised during the initial review. They have performed a substantial number of additional experiments that significantly strengthen the conclusions of the study. The authors have also clarified mechanistical and conceptual aspects. The responses to reviewers’ comments are detailed and well-justified, demonstrating careful consideration of the feedback provided. The revised version represents a significant improvement and, in my opinion, meets the standards expected for publication."}, "author_response": "We appreciate the referee's support and positive feedback on our work.\nWe appreciate the referee's support and positive feedback on our work. The name of Zr-SUC of figure S18 has been deleted in the revised supplementary information. And the expressions of xenon-krypton concentrations have been unified.\nWe appreciate the referee's support and positive feedback on our work."}]

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