Introduction

The separation of xenon (Xe) and krypton (Kr) is one of the many challenging gas separations in chemical, petrochemical, metallurgical, and environmental processes due to their similar sizes and physicochemical properties (Table S1)1,2,3,4. Due to the higher polarizability of Xe (4.01 ų) compared to Kr (2.46 ų), most current Xe/Kr separation research relies on equilibrium-based strategies using porous materials such as metal–organic frameworks (MOFs)4,5,6,7,8,9,10,11, hydrogen-bonded organic frameworks (HOFs)12, coordinated metal compounds13,14, and zeolites15. Various strategies have been pursued to achieve highly selective Xe adsorption, including constructing an optimal pore size of around 4–5 Å, close to the kinetic diameters of Xe (4.05 Å) and Kr (3.65 Å), to optimize their accommodation and/or tuning local pore surfaces and polarity to enhance Xe binding. However, progress in developing high-performance porous materials for highly selective Xe/Kr separation has been slow, as this mechanism inherently limits separation performance, as evidenced by the highest reported Xe/Kr IAST selectivity of only 103.48. Simultaneously, kinetic-based approaches exploiting the small differences in diffusion rates through pore channels have been attempted16,17,18, but these too have met with limited success due to the minimal difference between the two gases and the impractical requirement for large bed volumes to recover the minor component Xe from the gas mixtures (e.g., Xe/Kr = 20/80, v/v). It is thus imperative to develop transformative approaches to resolve this challenging separation.

To achieve Xe-over-Kr separation with high selectivity, or even at the molecular sieving level, the molecular trapdoor effect, or cation-tuned gating effect, offers a promising approach19,20,21,22,23,24,25,26. In these systems, unlike size-based sieving, extraframework cations block pore apertures, regulating cavity accessibility through their interaction with adsorbates. Gases are admitted only when they induce a temporary and reversible cation displacement above a threshold admission temperature, referred to as ({T}_{C}), resulting in temperature-regulated adsorption behavior. This mechanism allows for exceptionally high selectivity by tuning ({T}_{C}) to permit exclusive admission of one gas over another at specific operating temperatures.

We observed the Kr/Xe gas size sieving behavior in the primitive Na-exchanged Linde Type A (LTA) zeolites, a phenomena simultaneously discovered by Liu et al. 15 We figured out the separation mechanism, so-called trapdoor mechanism, in which Na⁺ cations, residing at the eight-membered ring (8MR) apertures and acting as dynamic “door-keepers”, require the system temperature to exceed a ({T}_{C}) to allow gas entry, and thus favor the admission of the smaller Kr molecules over Xe. This result itself is not so encouraging due to the minimal Kr uptake and limited selectivity, but it does provide us with the promise to target cation-exchanged LTA zeolite materials for the effective Xe/Kr gas separation. Given the fact that Ag⁺ has strong coordination bonding with the highly polarizable Xe27,28,29, we hypothesized that replacing Na⁺ with Ag⁺ to construct the Ag⁺-exchanged LTA zeolite and make use of the cation-tuned gating effect can realize Xe-over-Kr sieving (Fig. 1a). Herein, we present comprehensive studies of the adsorption isotherms, breakthrough experiments, synchrotron powder X-ray diffraction, X-ray absorption spectra, and ab initio density functional theory calculations to exclusively establish the cation-tuned gating effect for the unusual size-inverse molecular sieving Xe/Kr gas separation, meaning that larger Xe can be exclusively admitted and adsorbed while smaller Kr is excluded for admission and adsorption. Benefiting from the interplay of size and interaction of gas molecules and door-keeping cations, Ag-exchanged LTA achieves a high Xe/Kr IAST selectivity over 1600 at ambient conditions. Further tuning of cation density via Ca²⁺ incorporation yields a high dynamic Xe/Kr selectivity of 30 and a high Xe uptake of 1.65 mmol/g.

Fig. 1: Schematic of the cation-tuned gating mechanism and the structure of Ag-LTA zeolites.

a The unusual size-inverse sieving of Ag-LTA enabled by the cation-tuned gating mechanism. b Crystallographic locations of Ag+ in LTA zeolites. Xe (purple), Kr (pink), Ag (gray), Si (dark blue), Al (light blue), O (red); elements are shown as spheres or sticks as indicated.

Results

Reversing Kr/Xe sieving selectivity via door-keeping cation modulation in LTA adsorbents: achieving size-inverse sieving

To study Na+ and Ag+ as door-keeping cations for regulating selective gas admission through the cation-tuned gating effect, it is essential that all 8MR apertures in small-pore zeolites are fully occupied by the target cations. LTA zeolite (Si/Al = 1) is particularly suited for this purpose due to its unique cation site distribution and occupancy. Structurally, LTA zeolite features a body-centered cubic arrangement of LTA and sodalite (SOD) cages, with 8MRs (4.1 (\times)4.1 Å) serving as the only access for guest molecules to the supercavities. Three distinct sites exist within the framework: site I (located at 6MRs), site II (at 8MRs), and site III (at 4MRs) as shown in Fig. 1b30. Among these sites, Na+ and Ag+ at site II, typically residing in the plane of 8MRs, play a critical role in regulating gas accessibility, rendering LTA a suitable scaffold for developing Na+- and Ag⁺-based cation-tuned gating adsorbents.

To develop our proposed cation-tuned gating adsorbent, Ag-form LTA, capable of selectively admitting Xe, we began with a Na-form precursor. Commercial 4 A zeolite (Na12A, where 12 denotes the number of cations per unit cell) served as the starting material has been reported to show Kr-over-Xe selectivity, albeit with low uptake15. We confirmed that selective Kr adsorption in Na12A is governed by the a dynamic gating effect (Figs. S1–7, Table S2), wherein Na+ temporarily and reversibly move (door-opening) to admit guest molecules with different threshold admission temperatures (({T}_{c})) despite initially blocking each pore opening19,30,31.

However, the relatively low Kr uptake is understandable given the weak binding affinity of Kr on the adsorbent surface. Moreover, since Xe is the minor component in its source, i.e., air separation byproducts (Xe/Kr = 20/80, v/v), relying on Kr-selective materials would necessitate large bed volumes to recover sufficient Xe. Consequently, materials that exclusively admit and adsorb Xe would offer a more efficient solution for the separation of Kr and Xe.

Given the strong interaction between Ag+ and Xe27,28, we hypothesized that replacing Na+ with Ag+ as the door-keeping cations would invert the ({T}_{c}) and flip the selectivity. To test this hypothesis, we synthesized Ag-exchanged LTA zeolites with varying Ag+ contents, denoted as Ag5Na7A and Ag12A (where the numbers represent the respective cation counts per unit cell), via ion-exchange from Na12A. The resulting Ag-form LTA samples exhibited excellent crystallinity, purity, morphology, thermal stability, and requisite elemental composition, as confirmed by PXRD (Fig. S8), SEM (Figs. S9–11), TGA (Fig. S12), EDS, and ICP-OES (Table S3). All three samples show negligible N2 uptake at 77 K (Fig. S13), indicative of pore-blockage and significant CO2 uptake at 273 K (Figs. S14, S15, Table S4), indicative of pore-opening. In contrast to the apparently opposite pore accessibility status, Rietveld refinement of synchrotron powder X-ray diffraction (PXRD) data for the dehydrated LTA samples at 298 K (Fig. 2a, Figs. S7, S16 and Table S2) confirms that cations fully reside in the plane of 8MRs (site II) with nearly 100% occupancy, suggesting pore-blockage at ambient temperature. Combining gas adsorption and XRD results reveals characteristic of dynamic gating behavior is responsible for CO2 admission in these LTA zeolites20.

Fig. 2: Xe and Kr adsorption and selectivity on Ag-LTA zeolites.

a Crystal structure of Ag12A refined from synchrotron PXRD data via Rietveld analysis: Ag (gray), Si (dark blue), Al (light blue), and O (red) are shown as sticks. b Xe and Kr adsorption isotherms on Ag12A at 298 K. c Xe and Kr adsorption isobars on Ag12A at 100 kPa. Shaded areas denote the threshold admission temperature ({T}_{C}) for each gas. Lines are guides to the eye**. d** Bar chart comparing Xe/Kr (20/80, v/v) IAST selectivity of Ag12A at ambient conditions with other prominent adsorbents (NiCo@C-70047, ATC-Cu10, ZJU-74a-Pd, ZJU-74a-Ni8, Ca-CHA-2.515, Ni(4-DPDS)2CrO414, CROFOUR-1-Ni, CROFOUR-2-Ni5, HOF-ZJU-201a, HOF-ZJU-202a12, HCP-Co, and HCP-Fe13).

The Xe and Kr adsorption isotherms of Ag12A at 298 K (Fig. 2b) reveal that Ag+ incorporation significantly increases Xe uptake (2.05 mmol/g) while maintaining a negligible Kr uptake (0.096 mmol/g), thereby demonstrating a size-inverse sieving effect that favors Xe over Kr. Isobar analyses of Ag5Na7A (Figs. S17–20) and Ag12A (Fig. 2c, Figs. S21, S22) further indicate that Ag+ incorporation effectively lowers ({T}_{c})(Xe) while raising ({T}_{c})(Kr) relative to Na12A. Specifically, ({T}_{c})(Xe) decreases from > 373 K in Na12A to 323 K in Ag5Na7A and further to 298 K in Ag12A, whereas ({T}_{c})(Kr) increases from 273 K in Na12A to 286 K in Ag5Na7A and finally to 373 K in Ag12A. Given the cation size, the 8MR apertures should be effectively blocked for both Xe and Kr, enabling a gate-opening process facilitated by cation movement, a mechanism we coin as cation-tuned gating effect. Therefore, the exclusive admission of Xe over Kr at ambient temperature enables an unusual size-inverse Xe/Kr separation, achieving an IAST selectivity over 1600 (Fig. 2d, Figs. S23, S24 and Table S5)—a value that far surpasses previously reported Xe-selective adsorbents.

To rule out alternative mechanisms—such as pore aperture expansion due to framework thermal changes and/or door-opening due to permanent migration of door-keeping cations—we performed in-situ synchrotron PXRD measurements on dehydrated AgNa-LTA samples over the temperature range used for the isotherm studies (Fig. 3a, Figs. S25–29 and Tables S6–11). The PXRD patterns of Ag12A clearly showed a uniform peak shift to the larger angle with increasing temperature (Fig. 3b), indicating a contraction of the framework. Interestingly, Na-exchanged and Ag-exchanged LTA exhibit opposite trends in lattice parameter changes (Figs. 3c,d). Specifically, the pore apertures of Na12A expand slightly for 0.045 Å, whereas those of Ag5Na7A and Ag12A contract for 0.027 Å and 0.130 Å, respectively. Despite these variations, the 8MR aperture remains within 4.8 – 5 Å—a range too narrow to permit gas admission. Further analysis confirmed that both Na+ in Na12A and Ag+ in Ag12A maintain nearly 100% occupancy of the 8MR centers even at elevated temperatures (Fig. 3e). These observations strongly support that the cation-tuned gating effect—rather than framework expansion or permanent cation migration–governs gas admission and separation in these LTA zeolites.

Fig. 3: Confirmation of the cation-tuned gating mechanism by synchrotron PXRD and XAS analyses.

a,** b** Synchrotron PXRD patterns of dehydrated Ag12A under vacuum from 298 K to 423 K, showing a rightward shift at higher temperatures. c Lattice parameters, (d) 8MR pore aperture dimensions, and (e) site II occupancy of Na+ and Ag+ as a function of temperature. Lines are guides to the eye. The error bars are smaller than the symbols. f XANES spectra and (g) Fourier transforms of EXAFS spectra of the dehydrated Ag12A at 298 K, shown alongside reference materials Ag metal and Ag2O. Before measurements, Ag12A underwent 10 cycles of static Xe adsorption-desorption, stored for 1 month, and then activated at 473 K for 8 h.

Given the common concerns regarding Ag+ aggregation and leaching in Ag-containing materials32,33,34, we systematically evaluated the long-term stability of Ag⁺ in Ag12A using synchrotron PXRD, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). The Ag12A sample was subjected to 10 static Xe adsorption-desorption cycles and stored for one month prior to analysis. Synchrotron PXRD patterns showed no evidence of metallic Ag or Ag2O formation under 0.1 wt% detection limitation (Figs. S30, S31), and Rietveld refinement confirmed that Ag⁺ remains stably located at Site I (near the 6MRs) and Site II (at the 8MRs) (Fig. S32). X-ray absorption near-edge structure (XANES) analysis revealed an absorption edge distinct from that of metallic Ag and Ag2O (Fig. 3f), indicating a different coordination environment with the two reference materials. The local environment of Ag⁺ within the LTA framework interacts with the zeolite’s SiO4/AlO4 network rather than forming typical oxide or metallic bonds. This different environment alters the electronic structure and shifts the absorption edge. The Ag(3 d) XPS measurement further confirmed that Ag12A exhibits peak positions (e.g., 3d5/2 and 3d3/2) similar to those of Ag2O (Fig. S33), indicating that silver predominantly exists in the monovalent state. In addition, extended X-ray absorption fine structure (EXAFS) spectra showed a prominent peak at approximately 1.8 Å (Fig. 3g), corresponding to Ag+-framework oxygen coordination, with negligible Ag–Ag contributions, excluding the possibility of silver clustering. Quantitative fitting of the Ag K-edge EXAFS spectra showed precise Ag-O bond lengths of 2.25 Å and 2.38 Å (Fig. S34, Table S12), corresponding to 6MR and 8MR oxygen atoms, respectively. These results collectively demonstrate the excellent chemical and structural stability of Ag⁺ in Ag12A, with no evidence of leaching or aggregation under the tested conditions.

Underlying factors for invertible Xe/Kr selectivity in LTA zeolites: the roles of size and interaction capability

After confirming that the cation-tuned gating effect underpins the invertible Xe/Kr selectivity, we investigated the mechanisms governing gas-induced door-opening in these systems. In dynamic gating zeolites, successful admission of a guest molecule requires overcoming an energy barrier depending on two factors19,25: interaction capability – a stronger attractive interaction between the gas molecule and the door-keeping cation lowers the energy barrier; size—larger gas molecules and/or door-keeping cations necessitate a greater extent of cation deviation, thereby increasing the energy barrier.

Note that Ag+ (ionic diameter 2.58 Å, polarizability 4.3 Å3) is slightly larger and significantly more polarizable than Na+ (2.32 Å, 2.1 Å3). Consequently, in Ag12A, the stronger interaction strength between Ag+ and guest molecules is expected to dominate, whereas in Na12A the size effect is more pronounced. Similarly, Xe (kinetic diameter 4.1 Å, polarizability 4.01 ų) is slightly larger and significantly more polarizable than Kr (3.7 Å, 2.46 Å3), implying that Xe benefits more from strong gas-cation interactions, while Kr is more affected by size constraints.

These hypotheses align with our experimental observations: in Na12A, the smaller Kr more readily opens the gate than the larger Xe, reflected by ({T}_{c})(Kr) being lower than ({T}_{c})(Xe) (Fig. S4), resulting in Kr-over-Xe selective admission at intermediate temperatures. In contrast, in Ag12A, the stronger interaction between Xe and Ag⁺ lowers ({T}_{c})(Xe) below ({T}_{c})(Kr) (Fig. 2c), leading to Xe-over-Kr selective admission. Specifically, replacing Na+ with Ag+ results in an increase in ({T}_{c})(Kr) (Fig. S35) due to the dominant size effect for Na+ and Kr, and a decrease in ({T}_{c})(Xe) (Fig. S36) because the interaction effect between Ag+ and Xe facilitates Xe admission. We refer to this unique invertible sieving mechanism in a gas pair, facilitated by the manipulation of door-keeping cations, as the “cation-tuned gating effect”.

To quantify these interactions, we calculated the adsorption energies of Xe and Kr with Na+ and Ag+ located at 8MR (site II) using ab initio DFT (Fig. 4a, Figure S37). The adsorption energy for Xe on Ag+ is −0.49 eV, significantly stronger than on Na+ (−0.30 eV), whereas the difference for Kr is minimal (an increase of 0.05 eV when switching from Na+ to Ag+).

Fig. 4: Simulation of gas-cation interactions and associated cation deviations during gas admission in LTA zeolites.

a Bar charts depicting the calculated adsorption energies of Xe and Kr with Ag+ and Na+ at 8MR (Site II) on Ag12A and Na12A. Panels (b–e) present side-view snapshots illustrating transient deviations of door-keeping cations from the 8MR during gas admission, with corresponding variations in gas-cation distances. Color code: Kr (pink), Xe (purple), Ag (gray), Na (yellow), Si (deep blue), Al (light blue), O (red). Panels (f) and (g) display the energy profiles for the deviation of Ag+ and Na+, respectively, along the direction perpendicular to the 8MR plane towards the gas-rich side. Lines are guides to the eye.

Additional calculations of gas adsorption on cation sites at 6MRs (site I) (Fig. S38) (confirmed by synchrotron PXRD, Fig. 2a, Figs. S7, S16) reveal that the 8MR sites yield higher adsorption energies for both gases, underscoring the role of cations at these sites (Table S13).

Experimental heat of adsorption measurements on Ag-form LTA, conducted at temperatures above their respective ({T}_{c}) to avoid the influence of the dynamic gating effect (Figs. S39, S40, Table S13), agree with our DFT calculations and further support these findings. Replacing Na+ with Ag+ increases the heat of adsorption for Xe by approximately 10 kJ/mol (up to 28 kJ/mol), while only marginally increasing that for Kr by 2–3 kJ/mol. This substantial enhancement confirms that the Xe-Ag+ interaction is markedly stronger than the Xe-Na+ interaction, consistent with literature27,28 and the observed reduction in ({T}_{c})(Xe) in Ag-form LTA.

To further elucidate the size-inverse sieving mechanism, we performed DFT simulations of gas admission with Xe or Kr fixed at the 8MR while all other atoms were fully relaxed (Figs. 4b–e). The calculated gas-cation distances reveal that the separation between Kr and Ag+ (2.724 Å) is slightly greater than that between Kr and Na+ (2.697 Å), likely reflecting the larger size of Ag+. In contrast, the distance between Xe and Ag+ (2.810 Å) is smaller than that between Xe and Na+ (2.902 Å), indicating a stronger Xe-Ag+ interaction that reduces the extent of cation deviation—and thus the energy barrier—for Xe admission.

To investigate the cation deviation pathway further, we conducted DFT simulations to determine the most plausible route for door-keeping cations initially located at 8MR centers (site II). Analysis of synchrotron PXRD data of Na12A and Ag12A (Fig. 2a, Fig. S7) confirms that the adjacent 6MR cation sites (site I) in LTA cages are fully occupied, ruling out cation deviation in that direction, while deviation toward the vacant 4MR (site III) is energetically unfavorable due to strong repulsive forces. The only viable route is a perpendicular movement from the 8MR towards the gas side, where the LTA cavity provides sufficient space for cation movement. DFT-derived energy profiles (Fig. 4f, g, Fig. S41) and corresponding snapshots (Figs. S42–47) show that the presence of a gas molecule significantly reduces the energy barrier for cation deviation for both Ag+ and Na+, due to attractive gas-cations interactions. Notably, the change in energy barrier for Xe on Ag⁺ ((\triangle E)Ag+(Xe)) is clearly smaller than that for Kr on Ag⁺ ((\triangle E)Ag+(Kr)) (Fig. 4f), indicating that interaction capability dominates in Ag-form LTA. Conversely, in Na-form LTA, (\triangle E)Na+(Xe) is nearly identical to (\triangle E)Na+(Kr), indicating that it is not the interaction capability effect—but the size effect—that leads to the discriminative admission for the two gases.

In summary, our results demonstrate that in Na-form LTA, the size effect governs trapdoor behavior—favoring the admission of smaller Kr molecules—whereas in Ag-form LTA, the enhanced interaction capability between Xe and Ag⁺ predominates, facilitating Xe admission and yielding a size-inverse sieving effect. This dual influence of size and interaction capability is central to the invertible Xe/Kr selectivity observed in our LTA zeolites, underpinned by a cation-tuned gating effect.

Demonstration of size-inverse sieving for Xe/Kr separation by breakthrough tests

Motivated by the promising size-inverse sieving behavior observed in static adsorption isotherms on Ag12A, we conducted dynamic breakthrough experiments under industrially relevant conditions (298 K, 1 bar, Xe/Kr = 20/80, with Ar as the carrier gas). However, Ag12A exhibits negligible uptake of both Xe and Kr (Fig. S48), a discrepancy that reveals a key limitation of the cation-tuned gating mechanism: an “excessive” density of door-keeping cations can hinder sorption kinetics by introducing multiple activated admission steps.

To address this limitation, we proposed incorporating divalent cations into Ag12A. Specifically, Ca2+ was selected as the representative divalent cation, as it is among the most common in LTA frameworks (e.g., zeolite 5 A). Divalent cations are expected to reduce the number of extraframework cations required to balance the charge of the zeolite framework, thereby creating vacant cation sites. Considering the site distribution of Ag+ and Ca2+, we hypothesize that these vacant cation sites may appear at either Site II on the 8MRs or Site I near the 6MRs, both of which could enhance sorption kinetics because: (1) Reducing the density of door-keeping cations at the 8MRs decreases the number of events for cation deviation; (2) Reducing the density of non-door-keeping cations at the 6MRs would alleviate repulsive cation-cation interactions and lower the energy barrier for cation deviation.

Accordingly, we synthesized a series of AgCa-LTA zeolites with varying Ca2+ content, denoted as Ag10Ca1A, Ag9Ca1.5A, and Ag6Ca3A, via ion-exchange. PXRD patterns (Fig. S49), TGA curves (Fig. S50), EDS, and ICP-OES results (Table S14, Fig. S51) confirmed that these samples retain high crystallinity, thermal stability, and the expected elemental composition.

Gas adsorption studies reveal that Ag10Ca1A and Ag9Ca1.5A exhibit negligible N2 uptake at 77 K (Fig. S52) and significant CO2 uptake at 273 K (Fig. S53), indicating that the cation-tuned gating effect remains intact. In contrast, Ag6Ca3A, with higher Ca2+ content (50%), shows substantial N2 uptake at 77 K and a BET surface area of 423.32 m2/g (Table S15), suggesting that excessive Ca2+ incorporation weakens the cation blockage and effectively disrupts the cation-tuned gating effect.

To determine cation site preference after Ca2+ incorporation, synchrotron PXRD tests were conducted on a series of dehydrated AgCa-LTA zeolites (Figs. S54–56, Table S16). By analyzing changes in electron density at different cation sites and considering charge balance, specific cation site preferences can be determined. Using Ag10Ca1A as an illustrative example, the refined electron density at the 8MR site is identical to that of Ag12A (Fig. S57, Table S16), indicating that 8MR remains fully occupied by Ag+. Any Ca2+ substitution would reduce the local electron density due to its lower scattering factor. In contrast, the electron density at the 6MR site decreases to 94% of that in Ag12A (Fig. S57, Table S16), attributed to (1) reduced total cation numbers from introducing divalent Ca2+, and (2) partial substitution of Ag+ by Ca2+ at the 6MRs.

DFT calculations confirm that Ca2+ preferentially occupies 6MR sites over 8MR sites, with an energy difference of 0.84 eV (Table S18), consistent with the 6MR site preference of Ca2+ observed in XRD results. Furthermore, Ca K edge XAS gives an average Ca–O distance of 2.38 Å (Figs. S58–60, Table S19), reasonably close to the DFT predicted Ca–O distances at 6MR (2.29–2.31 Å) and 8MR (2.26–2.28 Å) (Fig. S61), supporting the reliability of the DFT structural models.

Overall, the combined PXRD, DFT, and XAS evidence indicates that Ca2+ resides preferentially at 6MR in AgCa-LTA, whereas Ag+ still occupies the 8MR site, acting as a door-keeping cation.

Based on the resolved synchrotron PXRD results (Fig. S57, Table S16), the Ag+ occupancy at the 8MR sites was quantified as follows: 99.7% for Ag10Ca1A, 96.0% for Ag9Ca1.5A, and 58.6% for Ag6Ca3A. The near-complete occupancy of the 8MR sites in the first two materials aligns with their negligible N2 uptake at 77 K (Fig. S52, Table S15). In contrast, the significant incorporation of Ca2+ in Ag6Ca3A drastically reduces the density of door-keeping Ag+, effectively negating the trapdoor effect. As a result, N2 molecules gain access at 77 K, leading to measurable N2 uptake and a corresponding increase in BET surface area (Fig. S52, Table S15).

After confirming that Ag10Ca1A and Ag9Ca1.5A retain the cation-tuned gating effect, we determined ({T}_{c}) for Xe and Kr via static adsorption isotherms at different temperatures to construct isobars (Figs. 5a, b, Figs. S62–69). Gradual incorporation of Ca2+ leads to a decreasing trend in ({T}_{c})(Kr): ({T}_{c})(Kr) drops from 373 K in Ag12A to 348 K in Ag10Ca1A and 323 K in Ag9Ca1.5A, while ({T}_{c})(Xe) decreases from 298 K in Ag12A to 286 K in Ag10Ca1A and 273 K in Ag9Ca1.5A. The persistence of ({T}_{c}) confirms that the cation-tuned gating effect remains functional for both gases.

Fig. 5: Adsorptive separation performance of AgCa-LTA zeolites.

a Adsorption isotherms of Xe and Kr at 298 K on Ag9Ca1.5A. b Adsorption isobars of Xe and Kr at 100 kPa on Ag9Ca1.5A. The shaded area indicates the threshold admission temperature ({T}_{C}) of for each gas. Lines are guides to the eye. c Dynamic breakthrough curves for Xe/Kr (20/80, v/v) using Ar as a carrier gas on Ag9Ca1.5A at 298 K and 1 bar, with a total flow rate of 10 cc/min (Xe 1 cc/min, Kr 4 cc/min, Ar 5 cc/min**). d Comparison of dynamic Xe/Kr selectivity (20/80, v/v) versus IAST selectivity at ambient conditions, alongside other prominent adsorbents (NiCo@C-70047, ATC-Cu10, Ca-CHA-2.515, Ni(4-DPDS)2CrO414, HOF-ZJU-201a, CROFOUR-1-Ni, CROFOUR-2-Ni5, HOF-ZJU-202a12, HCP-Co, HCP-Fe13, ZJU-74a-Pd48). e** Recycling performance of Ag9Ca1.5A for Xe/Kr (20/80, v/v) separation, demonstrating effective regeneration via He flushing at 473 K for 30 min.

At 298 K, Ag9Ca1.5A exhibits a Xe uptake of 2.33 mmol/g – comparable to Ag12A – while its Kr uptake is slightly higher (0.33 mmol/g), yielding a high IAST selectivity of 266 (Figs. S70, S71, Table S5). Sorption kinetics tests at 298 K further demonstrate that Ag9Ca1.5A has a significantly improved Xe adsorption rate compared to both Ag12A and Ag10Ca1A (Figs. S72–74).

To investigate how Ca2+ incorporation enhanced adsorption kinetics, we further analyzed the cation site occupancy using synchrotron PXRD of AgCa-LTA. Taking Ag9Ca1.5A as a representative example, the refined electron density at the 6MR site is only 91% of that observed in Ag12A (Fig. S57, Table S16). Based on charge balance, cation site preference, and electron density, it is determined that Ca2+ occupies 18.8% of the 6MR sites, leaving 5% of these sites vacant. Note that in Ag12A with fully occupied 6MR sites, DFT calculations predicted that the only viable route for the movement of door-keeping cations was a perpendicular shift from the 8MR towards the gas side (Fig. S41). The presence of 5% vacant 6MR sites introduces an alternative pathway, allowing door-keeping movement towards the 6MR. This would reduce the difficulty of door-keeping cation displacement, lower the energy barrier for door-opening, and thereby enhance adsorption kinetics.

Furthermore, the electron density at the 8MR site in Ag9Ca1.5A decreases to 96% of that in Ag12A (Fig. S57, Table S16), indicating a 4% reduction in door-keeping Ag+ cations with 4% vacant 8MR sites. This directly reduces the number of events in which gas molecules must overcome the energy barrier for admission and adsorption, thus improving adsorption kinetics.

Therefore, the enhanced adsorption kinetics after Ca2+ incorporation primarily arise from two aspects: (1) the reduced cation density of non-door-keeping cations near the 6MRs, which alleviates the difficulty of moving the door-keeping cations at adjacent 8MR sites, thereby lowering the energy barrier of the door-opening process; and (2) the decreased density of door-keeping cations at 8MRs, which reduces the number of events for cation deviation.

To evaluate performance under practical Xe/Kr conditions, dynamic breakthrough experiments were conducted using a Xe/Kr/Ar mixture (10/40/50, v/v/v) at 298 K. While Ag10Ca1A exhibited negligible gas uptake (similar to Ag12A, Fig. S75), Ag9Ca1.5A and Ag6Ca3A demonstrated Xe-selective adsorption, achieving Xe capacities of 1.65 and 1.37 mmol/g, respectively, and dynamic selectivities of 30.04 and 22.33 (Fig. 5c, Fig. [S76](https://www.nature.com/articles/s41467