Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation

Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation Heping Ma, Yongzheng Wang, Shanshan Wang, Yinhui Li, Shizhen Liu, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7885074/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The development of dynamically responsive soft porous crystals (SPCs) is crucial for advanced gas separation due to their stimuli-triggered structural transitions. Herein, we report a novel 3D covalent organic framework (FCOF-XJ) constructed from flexible tetrahedral (F-3D-Td) and rigid tetrahedral (3D-Td) building blocks, exhibiting unique temperature-dependent selective adsorption of xenon (Xe). FCOF-XJ features thermoresponsive -O-C-C-C-O- single bonds that enable temperature-dependent conformational switching. The adaptive pore structure of FCOF-XJ enables an unprecedented Xe-triggered gate-opening effect, with 4 times increase of Xe adsorption and a record Xe/Kr selectivity (36.9 at 298 K, 1 bar) among porous organic materials, surpassing most of metal-organic frameworks (MOFs). Breakthrough experiments confirm > 99% Xe recovery purity from Xe/Kr mixtures, enabled by thermoresponsive pore-gating dynamics. This work establishes a new benchmark for stimuli-responsive COFs for noble gas separations. Physical sciences/Chemistry/Supramolecular chemistry/Self-assembly Physical sciences/Materials science/Materials for energy and catalysis/Porous materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Flexible porous crystals that undergo guest-triggered structural transitions offer transformative potential for gas adsorption and separation. 1 – 24 Polar guests (CO₂, C₂H₂) can induce pore structure changes of flexible crystals via hydrogen bonding or dipole interactions, 25, 26 but nonpolar gas -- especially for monatomic noble gas xenon (Xe) and krypton (Kr) gas induced flexibility change remains unexplored. The chemical inertness and high symmetry monatomic nature of Xe and Kr make the commonly molecular recognition sites (hydrogen bonds, electrostatic interactions, etc.) in flexible porous crystals unsuitable for Xe and/or Kr recognition and separation. Recently, some flexible MOFs and COFs can be triggered by external stimuli, such as temperature, light, electricity, magnetic field, etc. 27 – 31 As external physical stimuli can be finely regulated, so the predictable structure change in flexible MOFs/COFs may realize fully controllable target guest’s adsorption, separation, or release. 32 , 33 The most reported external stimulus response is light triggered structural isomerization, such as azo or imine bond cis/trans isomerization. Although light triggered conformational isomerism have been realized in some imide linked COFs for CO 2 adsorption and separation, but the inhibited rotation of the C = N double bond interconversion cannot cause significant pore changes. 34 – 37 Moreover, due to the inherently limited penetration depth of light, photoresponsive COFs face significant operational complications in practical adsorption processes. In contrast, thermal energy offers distinct advantages of homogeneous propagation and precise mouldability, making it a more desirable external physical stimulus for practical application. However, temperature stimuli triggered flexible COF remains rare, hindered mainly by the scarcity of monomers incorporating unambiguous thermoresponsive functional units. 38 , 39 Herein, we reported a highly flexible 3D COF (FCOF-XJ) with a 5-fold interpenetrating dia topology, which integrated with flexible tetrahedral units with consecutive ether bonds (-O-C-C-C-O-) that can undergo temperature-dependent conformational changes. FCOF-XJ achieves Xe triggered gate-opening via van der Waals interactions in 257 K-298 K, which is the first demonstration of thermoresponsive and Xe-triggered gate-opening in flexible porous crystals. In addition, FCOF-XJ exhibits 4 times increase in Xe adsorption capacity with record Xe/Kr selectivity (36.9) among porous organic materials at 273 K-298 K and atmospheric pressure. Moreover, FCOF-XJ has excellent acid-base stability and can be used as a stable adsorbent for dynamics Xe/Kr separation validated by fixed-bed breakthrough experiments. Results Design principle and synthesis As shown in Fig. 1 , FCOF-XJ was synthesized from the flexible tetrahedral building block Tetrakis[(4-formylphenoxy)methyl]methane (TFPM) and the rigid tetrahedral building block tetra(p-aminophenyl)methane (TAPM) via a [4 + 4]imine condensation reaction. TFPM and TAPM were added to a mixed solution of mesitylene/chloroform/12 M acetic acid (v:v:v = 7:3:1) and reacted at 120 o C for 7 days to successfully obtain light gray powder FCOF-XJ with a yield of 75%. Material characterization The successful synthesis of FCOF-XJ was firstly demonstrated by Fourier transform infrared (FT-IR) spectroscopy. The amino absorption peak of TAPM and the characteristic peak of the aldehyde group of TFPM at 1692 cm − 1 disappeared in FT-IR, while a characteristic absorption peak attributed to the C = N bond appeared at 1598 cm − 1 (Figure S3). At the same time, the 13 C solid nuclear magnetic resonance spectrum showed that there was no signal peak of aldehyde carbon at a chemical shift of 190 ppm, indicating that TAPM and TFPM successfully co-polymerization (Fig. 1 b, S4, S5). As shown in Fig. 1 a and 1 e, f, scanning electron microscopy images show that the microstructure of FCOF-XJ is irregular crystals with sizes ranging from 1 to 3 µm. Transmission electron microscopy (TEM) images revealed uniformly arranged lattice fringes, indicating high crystallinity of FOCF-XJ, which is further supported by the pronounced diffraction peaks observed in the experimental powder X-ray diffraction (PXRD) pattern. In addition, thermogravimetric analysis (TGA) revealed that FCOF-XJ exhibited no significant weight loss below 400 o C, indicating excellent thermal stability. The differential scanning calorimetry (DSC) curve displayed an endothermic transition attributed to a structural rearrangement of the flexible framework, confirming the successful integration of the thermoresponsive ether units (-O-C-C-C-O-) into the network (Figure S6). Furthermore, FCOF-XJ demonstrated outstanding chemical stability, retaining its structural integrity after exposure to 1 M HCl and 1 M NaOH aqueous solutions (Figures S7 and S8). a) PXRD pattern of FCOF-XJ (experimental curve: black; Pawley refined curve: red; Background curve: yellow; Bragg line: green line; difference curve: blue; simulated curve: pink). b) 13 C solid-state NMR spectrum of FCOF-XJ. c) Gas adsorption-desorption isotherms (CO 2 at 195 K: blue; CO 2 at 273 K: red; CO 2 at 298 K: purple; N 2 at 77 K: orange; Ar at 87 K:pink). d) PXRD pattern of FCOF-XJ after vacuum drying (black); exposure to tetrahydrofuran (red) and N,N-Dimethylformamide (blue). e) SEM image of FCOF-XJ. f) TEM image of FCOF-XJ. N 2 , Ar and CO 2 were employed as probe molecules to evaluate the porosity of FOCF-XJ. Interestingly, as shown in Fig. 1 c, only CO 2 was able to diffuse into the framework, exhibiting a unique S-shaped adsorption isotherm at 195 K. The pores opened at relative pressures (P/P 0 ) above 0.6, with CO 2 uptake gradually increasing to a maximum adsorption capacity of 245 cm 3 g − 1 . In contrast, N 2 and Ar displayed negligible adsorption, suggesting restricted accessibility at their respective measurement temperatures. The Brunauer-Emmett-Teller (BET) specific surface area of FCOF-XJ was calculated to be 499 m 2 g − 1 . In addition, FCOF-XJ exhibited a solvent-stimulated response when immersed in tetrahydrofuran (THF) and dimethylformamide (DMF), as evidenced by a leftward shift in the PXRD peaks (Fig. 1 d). The first diffraction peak at 10.86 of the activated sample shifts to 8.96 (after immersion in DMF), which reflects a structural change of the framework. Structural characterization Materials Studio Crystals module was used for structural modeling and optimization of FCOF-XJ crystal structure. FCOF-XJ was finally built under the I-4 space group with a 5-fold interpenetrating dia network (Scheme 1c). 41 Pawley refinement based on experimental PXRD profiles showed matching peak assignments. A high consistency factor was obtained (R p = 3.20%, R wp = 4.41%) and the parameters a = 16.279(1) Å, b = 16.279(1) Å, c = 9.148(2) Å, α = β = γ = 90 o . Ball and stick model of the 3D structure of FCOF-XJ-CP ( a ) and FCOF-XJ-OP ( d ) viewed along the z-axis. The simulated pore size distribution of FCOF-XJ-CP ( b ) and FCOF-XJ-OP ( e ) and the intuitive diagram of the distribution of pores and skeleton. PXRD patterns for the FCOF-XJ-CP ( c ) and FCOF-XJ-OP ( f ) phases. C gray; N blue; O red; H white. To investigate the structural flexibility and porosity changing of FCOF-XJ in different phases, ball-and-stick models of both the closing-phase (CP) and opening-phase (OP) were constructed and systematically analyzed (Fig. 2 a and 2 d). The framework undergoes a transition from a CP phase (cell dimension:16.28 Å × 16.28 Å × 9.15 Å) to an OP phase (cell dimension: 18.30 Å × 18.30 Å × 8.59 Å) configuration after activation. The angle of the -O-C-C-C-O- bond widens from 77.69 o to 87.46 o . Pore size distribution (PSD) analyses were conducted using the ZEO + + software package, which performs geometric evaluations on static models. As shown in Fig. 2 b and 2 e, the CP phase exhibited pore sizes primarily centered at 2.6 Å and 3.0 Å, whereas the OP phase showed a broader and more uniform distribution, with a dominant pore size around 3.9 Å. These differences were further visualized through pore density maps generated in Materials Studio. The CP phase displayed sparse and discontinuous pore domains, while the OP phase revealed interconnected, uniformly distributed channels (The illustration in Fig. 2 b and 2 e). Powder X-ray diffraction (PXRD) patterns further confirmed the crystallinity and phase integrity of both states. As shown in the Fig. 2 c and 2 f, the experimental PXRD patterns for the CP and OP phases closely match the corresponding simulated profiles, verifying the accuracy of the structural models. Single-component gas adsorption properties The single-component Xe and Kr adsorption isotherms of FCOF-XJ were investigated at various temperatures. At 195 K, negligible adsorption of both Xe and Kr was observed as shown in Figure S9 (Xe, 1bar: 13.1 cm 3 g − 1 ; Kr, 1bar: 13.3 cm 3 g − 1 ). However, at 273 K and 298 K, Xe atoms were able to open the skeleton to produce significant adsorption, with values of 67.1 cm 3 g − 1 at 1 bar and 273 K, and 45.8 cm 3 g − 1 at 1 bar and 298 K, respectively (Fig. 3 a, b), while Kr adsorption remained negligible (273 K, 1bar: 5.4 cm 3 g − 1 ; 298 K, 1bar: 2.0 cm 3 g − 1 ). In order to investigate the Xe triggered “gate-opening” properties of FCOF-XJ, Xe adsorption isotherms were investigated over a wide range of temperatures (195 K-313 K). As shown in Fig. 3 c and Figure S10, the adsorption of Xe increased gradually with decreasing temperature from 257 K to 313K (257 K, 1 bar: 85.2 cm 3 g − 1 ; 263 K, 1bar: 74.8 cm 3 g − 1 ; 273 K, 1bar: 67.1 cm 3 g − 1 ; 283 K, 1bar: 64.0 cm 3 g − 1 ; 298 K, 1bar: 45.8 cm 3 g − 1 ; 318 K, 1 bar: 27.0 cm 3 g − 1 ). And for a given temperature, there is essentially no adsorption before the structural transition, which suggests that the pore size of the structure is too small at this point, resulting in the inability to introduce Xe atoms. The adsorption isotherm gradually rises and levels off when the Xe pressure reaches the “gate-opening” pressure of the skeleton, and the “gate-opening” pressure decreases with decreasing temperature. This is a very interesting result because as mentioned before when the temperature was reduced to 195 K, the adsorption of Xe by FCOF-XJ was also negligible. This temperature-dependent Xe triggered gate-opening in FCOF-XJ is affected by both the structural transitions and the Xe diffusion limitation. 42 At low temperatures, FCOF-XJ skeleton is shrinkage and the gas diffusion rate is extremely slow, which makes the Xe/Kr adsorption is negligible at 195 K. With the increase of temperature, heightened thermal motion induces stronger vibrational amplitudes in the COF framework. At the same time, the gas diffusion rate is accelerated, which can trigger the “gate-opening” of FCOF-XJ by Xe. The zero-coverage correlation adsorption enthalpy of Xe calculated by the Clausius-Clapeyron equation is 18.9 kJ mol − 1 , which is higher than the zero-coverage correlation adsorption enthalpy value of Kr (14.8 kJ mol − 1 ), which suggests that the affinity of FCOF-XJ with Xe is stronger than that of Kr (Figure S12 and S13). As shown in Figure S14, The Xe/Kr adsorption selectivity calculated based on the S(DIH) equation is 5.7 at 273 K and pressures lower than 30 kPa. When the pressure exceeds 30 kPa, the Xe/Kr selectivity increases significantly to a maximum of 27.9, and then decreases gradually. This interesting change may be due to the S-type adsorption isotherm caused by the “gate-opening” effect of the material skeleton. At 298 K, the change of Xe/Kr adsorption selectivity has the same trend as that at 273 K. The difference is that when the pressure is greater than 60 kPa, the Xe/Kr adsorption selectivity gradually increases from 9.2 to a maximum value of 36.9, which is higher than most of the currently reported porous materials as shown in Fig. 3 d. Single-component adsorption isotherms for Xe and Kr at 273 K ( a ) and 298 K ( b ). c) . Single-component adsorption isotherms for Xe at 195 K-313 K. The solid and hollow centers represent the adsorption and desorption curves, respectively. d) . Comparison of Xe/Kr uptake ratio at room temperature and Xe/Kr selectivity for the indicated materials. Multicomponent gas competitive adsorption In order to assess whether the Xe triggered “gate-opening” effect is effective under dynamic/practical conditions for Xe/Kr separation, we performed breakthrough experiments at different temperatures (257 K, 263 K, 273 K and 298 K) and two Xe/Kr ratios (3:7 and 5:5 by volume). As shown in Fig. 4 a-d, a plateau Xe adsorption period corresponding to a single-component open/closed pore pressure was observed for all four sets of breakthrough experiments at 257 K and 263 K. However, at 273 K, the breakthrough curve does not show a plateau period when the Xe/Kr flow ratio is 3:7 because 30% of the Xe partial pressure was not sufficient to make FCOF-XJ open. When the Xe/Kr flow ratio changes from 3:7 to 5:5, the partial pressure of Xe gas is large enough to open the pores of FCOF-XJ, and the breakthrough curve also shows a plateau as shown in Fig. 4 e, 4 f. Similarly, at 298 K, 50% of the Xe partial pressure is still not enough to open the pores of FCOF-XJ according to Fig. 3 b, and no Xe plateau period is observed in the two sets of breakthrough experiments (Fig. 4 g, 4 h). In addition, the dynamic adsorption changes of Xe/Kr for the eight sets of experiments can also well demonstrate the pore-opening performance of FCOF-XJ for Xe. Adsorbed Xe can be easily desorbed by purging inert gas and gentle heating. After the breakthrough experiments were completed and switched to Ar for purging, the Kr flow out quickly and became undetectable, suggesting that the Kr was mainly in the dead space rather than adsorbed in the adsorbent. FCOF-XJ can easily accomplish complete regeneration by heating to 353 K and high purity Xe can be collected (257 K/ 263 K/ 273 K: >99%; 298 K: >93%), further demonstrating the ultra-high Xe dynamic adsorption selectivity of FCOF-XJ. Xe/Kr dynamic breakthrough experiments and blowup plots at different temperatures with different gas ratios. a) . 257 K, Xe/Kr = 3:7; b) . 257 K, Xe/Kr = 5:5; c) . 263 K, Xe/Kr = 3:7; d) . 263 K, Xe/Kr = 5:5; e) . 273 K, Xe/Kr = 3:7; f) . 273 K, Xe/Kr = 5:5; g) . 298 K, Xe/Kr = 3:7; h) . 298 K, Xe/Kr = 5:5. (Xe flow rate: red; Kr flow rate: blue, total flow rate: 1.0 mL min − 1 ). Discussion In conclusion, we report a temperature-dependent flexible 3D-COF (FCOF-XJ) with interesting “gate-opening” properties for Xe and Kr separation. FCOF-XJ possesses thermoresponsive consecutive ether units (-O-C-C-C-O-) that can undergo Xe-triggered pore expansion under 257 K-298 K temperature range. This unique structural transition enables thermoregulatory Xe-specific gate-opening, which is the first demonstration of nonpolar gases induced flexible crystal pore changes. FCOF-XJ exhibits record Xe/Kr separation performance at gate-opening phases with 4 times higher Xe adsorption capacity than that of closed pore phase. Breakthrough experiments have also confirmed the dual control of temperature and Xe partial pressure for dynamic Xe/Kr separation performance in FCOF-XJ. This work not only demonstrates a robust flexible porous crystal with novel thermoresponsive unit for Xe/Kr separation, but also inspires future studies aimed at external stimuli-specific adsorbent for the separation of other gases. Methods Materials All reagents and solvents were purchased from commercial sources and were used without further purification. Tetrakis[(4-formylphenoxy)methyl]methane was synthesized according to the reported procedure. 40 Synthesis of FCOF-XJ FCOF-XJ was synthesized by the literature method. 1 Tetrakis[(4-formylphenoxy)methyl]methane (TFPM) (44.2 mg, 0.08 mmol) and tetra(p-aminophenyl)methane (TAPM) (30.4 mg, 0.08 mmol) were placed in a Pyrex tube with mesitylene (2.8 mL), chloroform (1.2 mL), and 12 M aqueous acetic acid (0.4 mL). The tube was flash frozen in a liquid nitrogen bath (77 K), degassed through three freeze − pump − thaw cycles, sealed under vacuum, and then heated at 120°C for 7 days. The mixture was cooled to room temperature, and the resulting precipitate was filtered, exhaustively washed by Soxhlet extractions with tetrahydrofuran and dichloromethane, and dried at 90°C under vacuum for 24 h. The activated FCOF-XJ was isolated as a light gray powder (60 mg, 75% yield). Characterization PXRD measurements were recorded on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation by depositing the powder on a glass substrate with 2θ from 5° to 40° in 0.02° increments. TG-DSC data collected by STA449F5. Field emission scanning electron microscopy was performed using a MALA3 LMH at accelerating voltages ranging from 0.1 to 20 kV. Collecting Sample Transmission Electron Microscope Images with Talos F200X. 13 C NMR spectra measured on an AVANCE NEO NMR spectrometer. Nitrogen isotherms at 77 K, Ar isotherms at 87 K, CO 2 isotherms at 195 K/273 K/298 K, and Xe, Kr isotherms at different temperatures were collected on a BSD-PM High Performance Specific Surface Area and Microporous Analyzer. Prior to the measurements, the samples were degassed in a vacuum at 120°C for 12 hours. Column Breakthrough Measurement Experiments Breakthrough separation experiments were performed in the BSD-MAB apparatus. In a typical breakthrough experiment, 0.45 g of FCOF-XJ was placed into a glass column (6 mm diameter, 6 cm long) to form a fixed bed. First, the adsorbent bed was purged under a 10 mL min − 1 flow of He gas for 10 h at 120°C for the activation process. After cooling to room temperature, the gas flow was switched to the desired Xe/Kr gas mixture at a flow rate of 1.0 mL min − 1 . The outlet composition was continuously monitored by mass spectrometry until complete breakthrough was achieved. After switching the valve to allow Ar to purge the penetration column, when the gas flow signal of Kr was not monitored, the penetration column was warmed up to 80°C and continued to purge until no Xe gas flow signal was monitored. After each breakthrough experiment, the packed column bed was regenerated at a constant He flow rate (10 mL min − 1 ) for 10 h at 120°C to ensure complete sample regeneration. the total gas flow rate for Xe/Kr was 1.0 mL min − 1 . Declarations Ethics declarations Competing interests The authors declare no competing interests. Acknowledgment The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (grant nos. 22178280), Supported by the Programme of Introducing Talents of Discipline to Universities (B23025), the Natural Science Basic Research Plan in Shaanxi Province of China (grant nos. 2024JC-JCQN-20 and 2024JC-YBMS-097), the Shaanxi Province Qin Chuangyuan "Scientist + Engineer" Team Construction Project (grant nos. 2023KXJ-058), and the Fundamental Research Funds for the Central Universities (grant nos. xzy012023171). We also thank the Instrument Analysis Center of Xi’an Jiao Tong University for the assistance test. The authors also gratefully acknowledge Prof. Huanyu Zhao at Jilin University for help in getting access to Materials Studio. Data Availability Experimental data are provided with this paper and in the supplementary materials. 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Supplementary Files Sourcedata.xlsx Dataset 1 SupportingInformation.docx Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7885074","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":533807660,"identity":"94bb6940-cfd2-409e-8274-fd6ec6ef21e8","order_by":0,"name":"Heping 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06:29:51","extension":"html","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":122733,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/4d95c1bc0598b835f8d33e90.html"},{"id":94249770,"identity":"b191053d-6870-4d38-b4ce-9cd4b262769d","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":5970597,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis route and gate-opening effect of FCOF-XJ.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ea)\u003c/strong\u003eSchematic representation of the synthesis of FCOF-XJ. \u003cstrong\u003eb)\u003c/strong\u003e Schematic representation of thermoresponsive and xenon triggered gate-opening effect in FCOF-XJ. \u003cstrong\u003ec)\u003c/strong\u003e Schematic representation of the structural unit changes of FCOF-XJ in different phases (Close Pore phase and Open Pore phase).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/5d522dc9b385c384d5ad4802.png"},{"id":94249775,"identity":"ff887265-2092-4315-a252-fbec3e19030b","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":7016810,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMaterial characterizations of FCOF-XJ.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ea)\u003c/strong\u003e PXRD pattern of FCOF-XJ (experimental curve: black; Pawley refined curve: red; Background curve: yellow; Bragg line: green line; difference curve: blue; simulated curve: pink). \u003cstrong\u003eb)\u003c/strong\u003e \u003csup\u003e13\u003c/sup\u003eC solid-state NMR spectrum of FCOF-XJ. \u003cstrong\u003ec)\u003c/strong\u003e Gas adsorption-desorption isotherms (CO\u003csub\u003e2\u003c/sub\u003e at 195 K: blue; CO\u003csub\u003e2\u003c/sub\u003e at 273 K: red; CO\u003csub\u003e2\u003c/sub\u003e at 298 K: purple; N\u003csub\u003e2\u003c/sub\u003e at 77 K: orange; Ar at 87 K:pink). \u003cstrong\u003ed)\u003c/strong\u003e PXRD pattern of FCOF-XJ after vacuum drying (black); exposure to tetrahydrofuran (red) and N,N-Dimethylformamide (blue). \u003cstrong\u003ee)\u003c/strong\u003e SEM image of FCOF-XJ. \u003cstrong\u003ef)\u003c/strong\u003e TEM image of FCOF-XJ.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/6676caa24d8fc7ce9ef78e70.png"},{"id":94249772,"identity":"a3828e34-f590-4725-b87a-12c4215e5343","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4094087,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCrystal structures and pore properties.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBall and stick model of the 3D structure of FCOF-XJ-CP (\u003cstrong\u003ea\u003c/strong\u003e) and FCOF-XJ-OP (\u003cstrong\u003ed\u003c/strong\u003e) viewed along the z-axis. The simulated pore size distribution of FCOF-XJ-CP (\u003cstrong\u003eb\u003c/strong\u003e) and FCOF-XJ-OP (\u003cstrong\u003ee\u003c/strong\u003e) and the intuitive diagram of the distribution of pores and skeleton. PXRD patterns for the FCOF-XJ-CP (\u003cstrong\u003ec\u003c/strong\u003e) and FCOF-XJ-OP (\u003cstrong\u003ef\u003c/strong\u003e) phases. C gray; N blue; O red; H white.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/84a3b6d32011cb5dd79064fd.png"},{"id":94249971,"identity":"032b4e24-8da4-4754-a87c-1ab6acc10d06","added_by":"auto","created_at":"2025-10-24 06:37:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1396869,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSingle-component gas adsorption properties.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSingle-component adsorption isotherms for Xe and Kr at 273 K (\u003cstrong\u003ea\u003c/strong\u003e) and 298 K (\u003cstrong\u003eb\u003c/strong\u003e). \u003cstrong\u003ec)\u003c/strong\u003e. Single-component adsorption isotherms for Xe at 195 K-313 K. The solid and hollow centers represent the adsorption and desorption curves, respectively. \u003cstrong\u003ed)\u003c/strong\u003e. Comparison of Xe/Kr uptake ratio at room temperature and Xe/Kr selectivity for the indicated materials.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/856a44bd2de6fad458f1ddad.png"},{"id":94249781,"identity":"b17b0b4c-aade-421c-9b16-9a8eb9bfee19","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1967706,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMulticomponent gas competitive adsorption.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eXe/Kr dynamic breakthrough experiments and blowup plots at different temperatures with different gas ratios. \u003cstrong\u003ea)\u003c/strong\u003e. 257 K, Xe/Kr = 3:7; \u003cstrong\u003eb)\u003c/strong\u003e. 257 K, Xe/Kr = 5:5; \u003cstrong\u003ec)\u003c/strong\u003e. 263 K, Xe/Kr = 3:7; \u003cstrong\u003ed)\u003c/strong\u003e. 263 K, Xe/Kr = 5:5; \u003cstrong\u003ee)\u003c/strong\u003e. 273 K, Xe/Kr = 3:7; \u003cstrong\u003ef)\u003c/strong\u003e. 273 K, Xe/Kr = 5:5; \u003cstrong\u003eg)\u003c/strong\u003e. 298 K, Xe/Kr = 3:7; \u003cstrong\u003eh)\u003c/strong\u003e. 298 K, Xe/Kr = 5:5. (Xe flow rate: red; Kr flow rate: blue, total flow rate: 1.0 mL min\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/c99ee0a601d0a765c0ff396d.png"},{"id":94250762,"identity":"594b8ecd-ba5b-4647-9793-16d7ea9ee76b","added_by":"auto","created_at":"2025-10-24 06:46:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":24856679,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/2567c637-48c8-471e-be8e-9f420672e6af.pdf"},{"id":94249771,"identity":"3242283f-3932-4adc-afd4-75ea8b6d44bc","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1487733,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"Sourcedata.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/e9661aba3b94d87d63e4c3d5.xlsx"},{"id":94249777,"identity":"384d032b-1511-42ea-a443-424c8d7d99eb","added_by":"auto","created_at":"2025-10-24 06:29:51","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":10053088,"visible":true,"origin":"","legend":"Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7885074/v1/80db7c33e5607e11c4649c35.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFlexible porous crystals that undergo guest-triggered structural transitions offer transformative potential for gas adsorption and separation.\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5 CR6 CR7 CR8 CR9 CR10 CR11 CR12 CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22 CR23\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e Polar guests (CO₂, C₂H₂) can induce pore structure changes of flexible crystals via hydrogen bonding or dipole interactions,\u003csup\u003e25, 26\u003c/sup\u003e but nonpolar gas -- especially for monatomic noble gas xenon (Xe) and krypton (Kr) gas induced flexibility change remains unexplored. The chemical inertness and high symmetry monatomic nature of Xe and Kr make the commonly molecular recognition sites (hydrogen bonds, electrostatic interactions, etc.) in flexible porous crystals unsuitable for Xe and/or Kr recognition and separation.\u003c/p\u003e\u003cp\u003eRecently, some flexible MOFs and COFs can be triggered by external stimuli, such as temperature, light, electricity, magnetic field, etc.\u003csup\u003e\u003cspan additionalcitationids=\"CR28 CR29 CR30\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e As external physical stimuli can be finely regulated, so the predictable structure change in flexible MOFs/COFs may realize fully controllable target guest\u0026rsquo;s adsorption, separation, or release.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e The most reported external stimulus response is light triggered structural isomerization, such as azo or imine bond cis/trans isomerization. Although light triggered conformational isomerism have been realized in some imide linked COFs for CO\u003csub\u003e2\u003c/sub\u003e adsorption and separation, but the inhibited rotation of the C\u0026thinsp;=\u0026thinsp;N double bond interconversion cannot cause significant pore changes.\u003csup\u003e\u003cspan additionalcitationids=\"CR35 CR36\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e Moreover, due to the inherently limited penetration depth of light, photoresponsive COFs face significant operational complications in practical adsorption processes. In contrast, thermal energy offers distinct advantages of homogeneous propagation and precise mouldability, making it a more desirable external physical stimulus for practical application. However, temperature stimuli triggered flexible COF remains rare, hindered mainly by the scarcity of monomers incorporating unambiguous thermoresponsive functional units.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eHerein, we reported a highly flexible 3D COF (FCOF-XJ) with a 5-fold interpenetrating dia topology, which integrated with flexible tetrahedral units with consecutive ether bonds (-O-C-C-C-O-) that can undergo temperature-dependent conformational changes. FCOF-XJ achieves Xe triggered gate-opening via van der Waals interactions in 257 K-298 K, which is the first demonstration of thermoresponsive and Xe-triggered gate-opening in flexible porous crystals. In addition, FCOF-XJ exhibits 4 times increase in Xe adsorption capacity with record Xe/Kr selectivity (36.9) among porous organic materials at 273 K-298 K and atmospheric pressure. Moreover, FCOF-XJ has excellent acid-base stability and can be used as a stable adsorbent for dynamics Xe/Kr separation validated by fixed-bed breakthrough experiments.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eDesign principle and synthesis\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, FCOF-XJ was synthesized from the flexible tetrahedral building block Tetrakis[(4-formylphenoxy)methyl]methane (TFPM) and the rigid tetrahedral building block tetra(p-aminophenyl)methane (TAPM) via a [4\u0026thinsp;+\u0026thinsp;4]imine condensation reaction. TFPM and TAPM were added to a mixed solution of mesitylene/chloroform/12 M acetic acid (v:v:v\u0026thinsp;=\u0026thinsp;7:3:1) and reacted at 120 \u003csup\u003eo\u003c/sup\u003eC for 7 days to successfully obtain light gray powder FCOF-XJ with a yield of 75%.\u003c/p\u003e\u003cp\u003eMaterial characterization\u003c/p\u003e\u003cp\u003eThe successful synthesis of FCOF-XJ was firstly demonstrated by Fourier transform infrared (FT-IR) spectroscopy. The amino absorption peak of TAPM and the characteristic peak of the aldehyde group of TFPM at 1692 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e disappeared in FT-IR, while a characteristic absorption peak attributed to the C\u0026thinsp;=\u0026thinsp;N bond appeared at 1598 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Figure S3). At the same time, the \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC solid nuclear magnetic resonance spectrum showed that there was no signal peak of aldehyde carbon at a chemical shift of 190 ppm, indicating that TAPM and TFPM successfully co-polymerization (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, S4, S5). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, f, scanning electron microscopy images show that the microstructure of FCOF-XJ is irregular crystals with sizes ranging from 1 to 3 \u0026micro;m. Transmission electron microscopy (TEM) images revealed uniformly arranged lattice fringes, indicating high crystallinity of FOCF-XJ, which is further supported by the pronounced diffraction peaks observed in the experimental powder X-ray diffraction (PXRD) pattern. In addition, thermogravimetric analysis (TGA) revealed that FCOF-XJ exhibited no significant weight loss below 400 \u003csup\u003eo\u003c/sup\u003eC, indicating excellent thermal stability. The differential scanning calorimetry (DSC) curve displayed an endothermic transition attributed to a structural rearrangement of the flexible framework, confirming the successful integration of the thermoresponsive ether units (-O-C-C-C-O-) into the network (Figure S6). Furthermore, FCOF-XJ demonstrated outstanding chemical stability, retaining its structural integrity after exposure to 1 M HCl and 1 M NaOH aqueous solutions (Figures S7 and S8).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003ea)\u003c/b\u003e PXRD pattern of FCOF-XJ (experimental curve: black; Pawley refined curve: red; Background curve: yellow; Bragg line: green line; difference curve: blue; simulated curve: pink). \u003cb\u003eb)\u003c/b\u003e \u003csup\u003e13\u003c/sup\u003eC solid-state NMR spectrum of FCOF-XJ. \u003cb\u003ec)\u003c/b\u003e Gas adsorption-desorption isotherms (CO\u003csub\u003e2\u003c/sub\u003e at 195 K: blue; CO\u003csub\u003e2\u003c/sub\u003e at 273 K: red; CO\u003csub\u003e2\u003c/sub\u003e at 298 K: purple; N\u003csub\u003e2\u003c/sub\u003e at 77 K: orange; Ar at 87 K:pink). \u003cb\u003ed)\u003c/b\u003e PXRD pattern of FCOF-XJ after vacuum drying (black); exposure to tetrahydrofuran (red) and N,N-Dimethylformamide (blue). \u003cb\u003ee)\u003c/b\u003e SEM image of FCOF-XJ. \u003cb\u003ef)\u003c/b\u003e TEM image of FCOF-XJ.\u003c/p\u003e\u003cp\u003eN\u003csub\u003e2\u003c/sub\u003e, Ar and CO\u003csub\u003e2\u003c/sub\u003e were employed as probe molecules to evaluate the porosity of FOCF-XJ. Interestingly, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec, only CO\u003csub\u003e2\u003c/sub\u003e was able to diffuse into the framework, exhibiting a unique S-shaped adsorption isotherm at 195 K. The pores opened at relative pressures (P/P\u003csub\u003e0\u003c/sub\u003e) above 0.6, with CO\u003csub\u003e2\u003c/sub\u003e uptake gradually increasing to a maximum adsorption capacity of 245 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. In contrast, N\u003csub\u003e2\u003c/sub\u003e and Ar displayed negligible adsorption, suggesting restricted accessibility at their respective measurement temperatures. The Brunauer-Emmett-Teller (BET) specific surface area of FCOF-XJ was calculated to be 499 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. In addition, FCOF-XJ exhibited a solvent-stimulated response when immersed in tetrahydrofuran (THF) and dimethylformamide (DMF), as evidenced by a leftward shift in the PXRD peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). The first diffraction peak at 10.86 of the activated sample shifts to 8.96 (after immersion in DMF), which reflects a structural change of the framework.\u003c/p\u003e\u003cp\u003eStructural characterization\u003c/p\u003e\u003cp\u003eMaterials Studio Crystals module was used for structural modeling and optimization of FCOF-XJ crystal structure. FCOF-XJ was finally built under the I-4 space group with a 5-fold interpenetrating dia network (Scheme 1c).\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e Pawley refinement based on experimental PXRD profiles showed matching peak assignments. A high consistency factor was obtained (R\u003csub\u003ep\u003c/sub\u003e = 3.20%, R\u003csub\u003ewp\u003c/sub\u003e = 4.41%) and the parameters a\u0026thinsp;=\u0026thinsp;16.279(1) \u0026Aring;, b\u0026thinsp;=\u0026thinsp;16.279(1) \u0026Aring;, c\u0026thinsp;=\u0026thinsp;9.148(2) \u0026Aring;, α\u0026thinsp;=\u0026thinsp;β\u0026thinsp;=\u0026thinsp;γ\u0026thinsp;=\u0026thinsp;90\u003csup\u003eo\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBall and stick model of the 3D structure of FCOF-XJ-CP (\u003cb\u003ea\u003c/b\u003e) and FCOF-XJ-OP (\u003cb\u003ed\u003c/b\u003e) viewed along the z-axis. The simulated pore size distribution of FCOF-XJ-CP (\u003cb\u003eb\u003c/b\u003e) and FCOF-XJ-OP (\u003cb\u003ee\u003c/b\u003e) and the intuitive diagram of the distribution of pores and skeleton. PXRD patterns for the FCOF-XJ-CP (\u003cb\u003ec\u003c/b\u003e) and FCOF-XJ-OP (\u003cb\u003ef\u003c/b\u003e) phases. C gray; N blue; O red; H white.\u003c/p\u003e\u003cp\u003eTo investigate the structural flexibility and porosity changing of FCOF-XJ in different phases, ball-and-stick models of both the closing-phase (CP) and opening-phase (OP) were constructed and systematically analyzed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). The framework undergoes a transition from a CP phase (cell dimension:16.28 \u0026Aring; \u0026times; 16.28 \u0026Aring; \u0026times; 9.15 \u0026Aring;) to an OP phase (cell dimension: 18.30 \u0026Aring; \u0026times; 18.30 \u0026Aring; \u0026times; 8.59 \u0026Aring;) configuration after activation. The angle of the -O-C-C-C-O- bond widens from 77.69\u003csup\u003eo\u003c/sup\u003e to 87.46\u003csup\u003eo\u003c/sup\u003e. Pore size distribution (PSD) analyses were conducted using the ZEO\u0026thinsp;+\u0026thinsp;+\u0026thinsp;software package, which performs geometric evaluations on static models. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, the CP phase exhibited pore sizes primarily centered at 2.6 \u0026Aring; and 3.0 \u0026Aring;, whereas the OP phase showed a broader and more uniform distribution, with a dominant pore size around 3.9 \u0026Aring;. These differences were further visualized through pore density maps generated in Materials Studio. The CP phase displayed sparse and discontinuous pore domains, while the OP phase revealed interconnected, uniformly distributed channels (The illustration in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). Powder X-ray diffraction (PXRD) patterns further confirmed the crystallinity and phase integrity of both states. As shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, the experimental PXRD patterns for the CP and OP phases closely match the corresponding simulated profiles, verifying the accuracy of the structural models.\u003c/p\u003e\u003cp\u003eSingle-component gas adsorption properties\u003c/p\u003e\u003cp\u003eThe single-component Xe and Kr adsorption isotherms of FCOF-XJ were investigated at various temperatures. At 195 K, negligible adsorption of both Xe and Kr was observed as shown in Figure S9 (Xe, 1bar: 13.1 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Kr, 1bar: 13.3 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). However, at 273 K and 298 K, Xe atoms were able to open the skeleton to produce significant adsorption, with values of 67.1 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 1 bar and 273 K, and 45.8 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 1 bar and 298 K, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b), while Kr adsorption remained negligible (273 K, 1bar: 5.4 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 298 K, 1bar: 2.0 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). In order to investigate the Xe triggered \u0026ldquo;gate-opening\u0026rdquo; properties of FCOF-XJ, Xe adsorption isotherms were investigated over a wide range of temperatures (195 K-313 K). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and Figure S10, the adsorption of Xe increased gradually with decreasing temperature from 257 K to 313K (257 K, 1 bar: 85.2 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 263 K, 1bar: 74.8 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 273 K, 1bar: 67.1 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 283 K, 1bar: 64.0 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 298 K, 1bar: 45.8 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; 318 K, 1 bar: 27.0 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). And for a given temperature, there is essentially no adsorption before the structural transition, which suggests that the pore size of the structure is too small at this point, resulting in the inability to introduce Xe atoms. The adsorption isotherm gradually rises and levels off when the Xe pressure reaches the \u0026ldquo;gate-opening\u0026rdquo; pressure of the skeleton, and the \u0026ldquo;gate-opening\u0026rdquo; pressure decreases with decreasing temperature. This is a very interesting result because as mentioned before when the temperature was reduced to 195 K, the adsorption of Xe by FCOF-XJ was also negligible. This temperature-dependent Xe triggered gate-opening in FCOF-XJ is affected by both the structural transitions and the Xe diffusion limitation.\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e At low temperatures, FCOF-XJ skeleton is shrinkage and the gas diffusion rate is extremely slow, which makes the Xe/Kr adsorption is negligible at 195 K. With the increase of temperature, heightened thermal motion induces stronger vibrational amplitudes in the COF framework. At the same time, the gas diffusion rate is accelerated, which can trigger the \u0026ldquo;gate-opening\u0026rdquo; of FCOF-XJ by Xe. The zero-coverage correlation adsorption enthalpy of Xe calculated by the Clausius-Clapeyron equation is 18.9 kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is higher than the zero-coverage correlation adsorption enthalpy value of Kr (14.8 kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), which suggests that the affinity of FCOF-XJ with Xe is stronger than that of Kr (Figure S12 and S13). As shown in Figure S14, The Xe/Kr adsorption selectivity calculated based on the S(DIH) equation is 5.7 at 273 K and pressures lower than 30 kPa. When the pressure exceeds 30 kPa, the Xe/Kr selectivity increases significantly to a maximum of 27.9, and then decreases gradually. This interesting change may be due to the S-type adsorption isotherm caused by the \u0026ldquo;gate-opening\u0026rdquo; effect of the material skeleton. At 298 K, the change of Xe/Kr adsorption selectivity has the same trend as that at 273 K. The difference is that when the pressure is greater than 60 kPa, the Xe/Kr adsorption selectivity gradually increases from 9.2 to a maximum value of 36.9, which is higher than most of the currently reported porous materials as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSingle-component adsorption isotherms for Xe and Kr at 273 K (\u003cb\u003ea\u003c/b\u003e) and 298 K (\u003cb\u003eb\u003c/b\u003e). \u003cb\u003ec)\u003c/b\u003e. Single-component adsorption isotherms for Xe at 195 K-313 K. The solid and hollow centers represent the adsorption and desorption curves, respectively. \u003cb\u003ed)\u003c/b\u003e. Comparison of Xe/Kr uptake ratio at room temperature and Xe/Kr selectivity for the indicated materials.\u003c/p\u003e\u003cp\u003eMulticomponent gas competitive adsorption\u003c/p\u003e\u003cp\u003eIn order to assess whether the Xe triggered \u0026ldquo;gate-opening\u0026rdquo; effect is effective under dynamic/practical conditions for Xe/Kr separation, we performed breakthrough experiments at different temperatures (257 K, 263 K, 273 K and 298 K) and two Xe/Kr ratios (3:7 and 5:5 by volume). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-d, a plateau Xe adsorption period corresponding to a single-component open/closed pore pressure was observed for all four sets of breakthrough experiments at 257 K and 263 K. However, at 273 K, the breakthrough curve does not show a plateau period when the Xe/Kr flow ratio is 3:7 because 30% of the Xe partial pressure was not sufficient to make FCOF-XJ open. When the Xe/Kr flow ratio changes from 3:7 to 5:5, the partial pressure of Xe gas is large enough to open the pores of FCOF-XJ, and the breakthrough curve also shows a plateau as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef. Similarly, at 298 K, 50% of the Xe partial pressure is still not enough to open the pores of FCOF-XJ according to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, and no Xe plateau period is observed in the two sets of breakthrough experiments (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eh). In addition, the dynamic adsorption changes of Xe/Kr for the eight sets of experiments can also well demonstrate the pore-opening performance of FCOF-XJ for Xe. Adsorbed Xe can be easily desorbed by purging inert gas and gentle heating. After the breakthrough experiments were completed and switched to Ar for purging, the Kr flow out quickly and became undetectable, suggesting that the Kr was mainly in the dead space rather than adsorbed in the adsorbent. FCOF-XJ can easily accomplish complete regeneration by heating to 353 K and high purity Xe can be collected (257 K/ 263 K/ 273 K: \u0026gt;99%; 298 K: \u0026gt;93%), further demonstrating the ultra-high Xe dynamic adsorption selectivity of FCOF-XJ.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eXe/Kr dynamic breakthrough experiments and blowup plots at different temperatures with different gas ratios. \u003cb\u003ea)\u003c/b\u003e. 257 K, Xe/Kr\u0026thinsp;=\u0026thinsp;3:7; \u003cb\u003eb)\u003c/b\u003e. 257 K, Xe/Kr\u0026thinsp;=\u0026thinsp;5:5; \u003cb\u003ec)\u003c/b\u003e. 263 K, Xe/Kr\u0026thinsp;=\u0026thinsp;3:7; \u003cb\u003ed)\u003c/b\u003e. 263 K, Xe/Kr\u0026thinsp;=\u0026thinsp;5:5; \u003cb\u003ee)\u003c/b\u003e. 273 K, Xe/Kr\u0026thinsp;=\u0026thinsp;3:7; \u003cb\u003ef)\u003c/b\u003e. 273 K, Xe/Kr\u0026thinsp;=\u0026thinsp;5:5; \u003cb\u003eg)\u003c/b\u003e. 298 K, Xe/Kr\u0026thinsp;=\u0026thinsp;3:7; \u003cb\u003eh)\u003c/b\u003e. 298 K, Xe/Kr\u0026thinsp;=\u0026thinsp;5:5. (Xe flow rate: red; Kr flow rate: blue, total flow rate: 1.0 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn conclusion, we report a temperature-dependent flexible 3D-COF (FCOF-XJ) with interesting \u0026ldquo;gate-opening\u0026rdquo; properties for Xe and Kr separation. FCOF-XJ possesses thermoresponsive consecutive ether units (-O-C-C-C-O-) that can undergo Xe-triggered pore expansion under 257 K-298 K temperature range. This unique structural transition enables thermoregulatory Xe-specific gate-opening, which is the first demonstration of nonpolar gases induced flexible crystal pore changes. FCOF-XJ exhibits record Xe/Kr separation performance at gate-opening phases with 4 times higher Xe adsorption capacity than that of closed pore phase. Breakthrough experiments have also confirmed the dual control of temperature and Xe partial pressure for dynamic Xe/Kr separation performance in FCOF-XJ. This work not only demonstrates a robust flexible porous crystal with novel thermoresponsive unit for Xe/Kr separation, but also inspires future studies aimed at external stimuli-specific adsorbent for the separation of other gases.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eMaterials\u003c/p\u003e\u003cp\u003eAll reagents and solvents were purchased from commercial sources and were used without further purification. Tetrakis[(4-formylphenoxy)methyl]methane was synthesized according to the reported procedure.\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eSynthesis of FCOF-XJ\u003c/p\u003e\u003cp\u003eFCOF-XJ was synthesized by the literature method.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e Tetrakis[(4-formylphenoxy)methyl]methane (TFPM) (44.2 mg, 0.08 mmol) and tetra(p-aminophenyl)methane (TAPM) (30.4 mg, 0.08 mmol) were placed in a Pyrex tube with mesitylene (2.8 mL), chloroform (1.2 mL), and 12 M aqueous acetic acid (0.4 mL). The tube was flash frozen in a liquid nitrogen bath (77 K), degassed through three freeze\u0026thinsp;\u0026minus;\u0026thinsp;pump\u0026thinsp;\u0026minus;\u0026thinsp;thaw cycles, sealed under vacuum, and then heated at 120\u0026deg;C for 7 days. The mixture was cooled to room temperature, and the resulting precipitate was filtered, exhaustively washed by Soxhlet extractions with tetrahydrofuran and dichloromethane, and dried at 90\u0026deg;C under vacuum for 24 h. The activated FCOF-XJ was isolated as a light gray powder (60 mg, 75% yield).\u003c/p\u003e\u003cp\u003eCharacterization\u003c/p\u003e\u003cp\u003ePXRD measurements were recorded on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation by depositing the powder on a glass substrate with 2θ from 5\u0026deg; to 40\u0026deg; in 0.02\u0026deg; increments. TG-DSC data collected by STA449F5. Field emission scanning electron microscopy was performed using a MALA3 LMH at accelerating voltages ranging from 0.1 to 20 kV. Collecting Sample Transmission Electron Microscope Images with Talos F200X. \u003csup\u003e13\u003c/sup\u003eC NMR spectra measured on an AVANCE NEO NMR spectrometer. Nitrogen isotherms at 77 K, Ar isotherms at 87 K, CO\u003csub\u003e2\u003c/sub\u003e isotherms at 195 K/273 K/298 K, and Xe, Kr isotherms at different temperatures were collected on a BSD-PM High Performance Specific Surface Area and Microporous Analyzer. Prior to the measurements, the samples were degassed in a vacuum at 120\u0026deg;C for 12 hours.\u003c/p\u003e\u003cp\u003eColumn Breakthrough Measurement Experiments\u003c/p\u003e\u003cp\u003eBreakthrough separation experiments were performed in the BSD-MAB apparatus. In a typical breakthrough experiment, 0.45 g of FCOF-XJ was placed into a glass column (6 mm diameter, 6 cm long) to form a fixed bed. First, the adsorbent bed was purged under a 10 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e flow of He gas for 10 h at 120\u0026deg;C for the activation process. After cooling to room temperature, the gas flow was switched to the desired Xe/Kr gas mixture at a flow rate of 1.0 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The outlet composition was continuously monitored by mass spectrometry until complete breakthrough was achieved. After switching the valve to allow Ar to purge the penetration column, when the gas flow signal of Kr was not monitored, the penetration column was warmed up to 80\u0026deg;C and continued to purge until no Xe gas flow signal was monitored. After each breakthrough experiment, the packed column bed was regenerated at a constant He flow rate (10 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e ) for 10 h at 120\u0026deg;C to ensure complete sample regeneration. the total gas flow rate for Xe/Kr was 1.0 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003e\u003cb\u003eEthics declarations\u003c/b\u003e\u003c/h2\u003e\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e\u003cp\u003eThe authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (grant nos. 22178280), Supported by the Programme of Introducing Talents of Discipline to Universities (B23025), the Natural Science Basic Research Plan in Shaanxi Province of China (grant nos. 2024JC-JCQN-20 and 2024JC-YBMS-097), the Shaanxi Province Qin Chuangyuan \"Scientist\u0026thinsp;+\u0026thinsp;Engineer\" Team Construction Project (grant nos. 2023KXJ-058), and the Fundamental Research Funds for the Central Universities (grant nos. xzy012023171). We also thank the Instrument Analysis Center of Xi\u0026rsquo;an Jiao Tong University for the assistance test. The authors also gratefully acknowledge Prof. Huanyu Zhao at Jilin University for help in getting access to Materials Studio.\u003c/p\u003e\n\u003ch3\u003eData Availability\u003c/h3\u003e\n\u003cp\u003eExperimental data are provided with this paper and in the supplementary materials. Any additional data that support the findings of this study are available from the corresponding authors upon request. Source data are provided with this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKrause, S.; Hosono, N.; Kitagawa, S. Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties. \u003cem\u003eAngewandte Chemie International Edition \u003c/em\u003e\u003cstrong\u003e2020\u003c/strong\u003e, \u003cem\u003e59\u003c/em\u003e (36), 15325-15341. 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DOI: 10.1126/science.aar6833.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7885074/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7885074/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe development of dynamically responsive soft porous crystals (SPCs) is crucial for advanced gas separation due to their stimuli-triggered structural transitions. Herein, we report a novel 3D covalent organic framework (FCOF-XJ) constructed from flexible tetrahedral (F-3D-Td) and rigid tetrahedral (3D-Td) building blocks, exhibiting unique temperature-dependent selective adsorption of xenon (Xe). FCOF-XJ features thermoresponsive -O-C-C-C-O- single bonds that enable temperature-dependent conformational switching. The adaptive pore structure of FCOF-XJ enables an unprecedented Xe-triggered gate-opening effect, with 4 times increase of Xe adsorption and a record Xe/Kr selectivity (36.9 at 298 K, 1 bar) among porous organic materials, surpassing most of metal-organic frameworks (MOFs). Breakthrough experiments confirm\u0026thinsp;\u0026gt;\u0026thinsp;99% Xe recovery purity from Xe/Kr mixtures, enabled by thermoresponsive pore-gating dynamics. This work establishes a new benchmark for stimuli-responsive COFs for noble gas separations.\u003c/p\u003e","manuscriptTitle":"Thermoresponsive and Xenon Triggered Gate-opening in Flexible Three-dimensional Covalent Organic Framework for Xenon/Krypton Separation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-24 06:29:46","doi":"10.21203/rs.3.rs-7885074/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"83b0730f-ba4d-4291-8cdd-4b1d60dc6701","owner":[],"postedDate":"October 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":56752008,"name":"Physical sciences/Chemistry/Supramolecular chemistry/Self-assembly"},{"id":56752009,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Porous materials"}],"tags":[],"updatedAt":"2026-04-23T07:18:10+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-24 06:29:46","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7885074","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7885074","identity":"rs-7885074","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}