Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope Separation

Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope 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 Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope Separation Chuanqin Xia, Fenglei Wang, Zining Wang, Chaofan Jiang, Wenting Liu, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7838780/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 selective separation of hydrogen isotopes under mild cryogenic conditions remains a formidable challenge due to their nearly identical physicochemical properties. Here, we report a dual strategy of pore topology design and bimetallic synergistic engineering to amplify chemical affinity quantum sieving (CAQS). Among three tailored Cu(II)-MOFs (Cu-bptc, Cu-mbtc, Cu-ATC), Cu-ATC exhibited exceptional performance, achieving a D₂/H₂ selectivity of 20 at 50 K (10 mbar) and 1.8 in breakthrough experiments at 77 K, surpassing all reported Cu-MOFs. The ultramicroporous topology of Cu-ATC fixes a Cu···Cu distance of 5.98 Å within one-dimensional channels (~5.6 Å), while Jahn–Teller distortion induces axial elongation at each Cu(II) center, stabilizing the d z 2 orbitals and enhancing their availability for interaction with hydrogen isotope molecules. This dual effect creates two closely spaced open metal sites that can simultaneously interact with H 2 and D 2 , amplifying their differential interactions and thereby driving isotope separation via CAQS. The distinct binding strength is evidenced by in situ DRIFTS (H–H stretch red-shift of 682 cm -1 ) and by DFT calculations showing stronger adsorption of D₂ (−15.8 kJ mol -1 at the primary site). These microscopic differences account for the observed D 2 /H 2 selectivity, highlighting a new paradigm for CAQS-based isotope separation under mild cryogenic conditions. Physical sciences/Chemistry/Materials chemistry/Metal–organic frameworks Physical sciences/Chemistry/Nuclear chemistry Hydrogen isotopes separation Metal-Organic Frameworks dual metal open metal sites confined channel synergistic effects Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 INTRODUCTION Deuterium enrichment from hydrogen isotope mixtures represents a critical bottleneck in nuclear fusion fuel cycles. In current tokamak reactors, the utilization of D 2 and T 2 fuels is typically below 10%, with most discharged as waste streams, leading to tritium loss and the requirement for continuous deuterium enrichment to sustain the fuel cycle. 1,2 Despite decades of research, conventional separation technologies, including cryogenic distillation, thermal diffusion, and chromatographic methods, suffer from high energy consumption and low separation factors (~ 1.5), rendering them economically unsustainable for large-scale implementation, 3 underscoring the urgent need for new approaches that combine higher selectivity with lower energy demand. The intrinsic difficulty in hydrogen isotope separation lies in their nearly identical physicochemical properties, with the primary difference being a mass ratio of two, necessitating the exploitation of quantum effects for efficient separation under mild conditions. Metal–organic frameworks (MOFs) have emerged as promising platforms for hydrogen isotope separation by exploiting quantum sieving effects arising from zero-point energy differences. 4–7 Kinetic quantum sieving (KQS) can deliver remarkable selectivity—D 2 /H 2 = 41.4 at 20 K 8 —through diffusion discrimination in confined pores, but this requires impractically low temperatures. To extend applicability, chemical-affinity quantum sieving (CAQS) was introduced, 9 relying on Kubas-type σ donation and π back-donation between H 2 /D 2 and open metal sites. 10–17 At 77 K, however, achieving high selectivity (> 10) remains challenging, particularly regarding stability and capacity for practical application. Building on this, researchers have attempted to combine the two mechanisms. Imidazole-modified MOF-74 achieved a selectivity of 26 at 77 K, 18 while Cu(I)-exchanged ZSM-5 reached 24.9 at 100 K. 19 These studies illustrate the potential of KQS–CAQS synergy, yet they also highlight persistent challenges including the instability of Cu(I) sites, the complexity of post-synthetic modification, and the rapid loss of selectivity once the temperature rises above 77 K. The central challenge therefore remains to establish isotope–metal interactions that are both strong and durable, capable of sustaining high selectivity under practical cryogenic conditions without compromising framework integrity. Among MOF architectures, Cu(II)-based paddlewheel frameworks are particularly attractive because their robust [Cu 2 (O 2 CR) 4 ] secondary building units(SBUs, Fig. 1 (c)) allow facile generation of open metal sites upon activation and systematic modulation of pore environments. In addition, the Jahn–Teller distortion intrinsic to Cu(II) enhances the accessibility of d z 2 orbitals for guest–molecule interactions, 20,21 offering potential advantages for isotope binding. Despite these features, benchmark Cu-BTC (HKUST-1) delivers only modest performance, with a D₂/H₂ selectivity of 1.3 at 77 K, 22–27 as its large ~ 1 nm cages permit molecules to bypass OMSs and diminish the CAQS effect. Attempts to improve performance by reducing Cu(II) to Cu(I) temporarily boosted selectivity to 37.9 at 30 K, 28 but this advantage quickly disappeared above 40 K because the reduction disrupted d z 2 orbital alignment and undermined framework stability. A similar issue was observed in FJI-Y11, 29 where DFT calculations revealed that adsorption occurred near oxygen atoms rather than Cu centers, as the d z 2 orbitals were geometrically misaligned with the pore channels, limiting effective interaction with hydrogen molecules and resulting in low selectivity (α ≈ 1.4). These observations show that OMS density alone is not sufficient, and that isotope separation critically depends on how pore confinement and orbital orientation cooperate. A promising direction is therefore to design frameworks with ultramicroporous channels that enforce close contact between isotopes and appropriately oriented, Jahn–Teller–stabilized Cu(II) sites, while preserving the structural integrity of the paddlewheel motif. Guided by this rationale, we synthesized three tetracarboxylate Cu(II) MOFs with distinct topologies including cage-type Cu-bptc, 30 wide-channel Cu-mbtc (~ 10 Å), 31 and ultramicroporous Cu-ATC (~ 5 Å), 32 to examine how pore architecture affects isotope separation. Comparative evaluation showed that Cu-bptc and Cu-mbtc displayed only limited selectivity, indicating that pore confinement alone cannot account for high performance. In contrast, Cu-ATC exhibited outstanding separation, achieving a D 2 /H 2 selectivity of 20 at 50 K and 1.8 in breakthrough experiments at 77 K, surpassing all reported Cu(II) MOFs. Subsequent analysis revealed that its ultramicroporous channels not only constrain molecular motion but also position isotopes within reach of two closely spaced paddlewheel Cu(II) sites, where accessible d z 2 orbitals enable cooperative dual-site interactions that markedly amplify the CAQS effect. These findings establish Cu-ATC as a benchmark system and highlight a generalizable design principle in which pore topology and site-specific electronic structure act in concert to realize practical hydrogen isotope separation under mild cryogenic conditions. RESULTS AND DISCUSSIONS Structure Characterization The structures of the three Cu(II)-based MOFs were characterized by powder X-ray diffraction (PXRD) and nitrogen adsorption-desorption experiments, with detailed results provided in the Supporting Information. The PXRD patterns confirmed that the crystal structures of these materials aligned with literature reports, indicating successful synthesis and high purity (Figure S1 -3). The nitrogen adsorption isotherms exhibited IUPAC Type I characteristics for all three MOFs, verifying their microporous nature (Figure S4-6). Pore size distributions (Figure S7-9), calculated using nonlocal density functional theory (NLDFT), revealed distinct topologies: Cu-ATC and Cu-bptc possess narrow ultramicropore (5.6 Å and 5.8 Å, respectively), whereas Cu-mbtc showed a larger pore size (10.0 Å). This variation reflects the designed optimization from cage-like Cu-bptc to one-dimensional channels in Cu-mbtc and Cu-ATC (Fig. 1 ). Brunauer-Emmett-Teller (BET) surface areas were 606 m 2 g -1 for Cu-ATC, 1811 m 2 g -1 for Cu-bptc, and 498 m 2 g -1 for Cu-mbtc (Figure S4-6). These values correlate with framework compactness: Cu-bptc's high surface area arises from its interconnected cage network, whereas Cu-ATC's lower value stems from denser ultramicroporous channels. Such topological tuning establishes the structural basis for subsequent hydrogen isotope adsorption studies, consistent with the dual strategy of pore topology modulation and bimetallic synergy. Adsorption properties of H 2 and D 2 Single-component H₂ and D₂ adsorption isotherms for the three MOFs were measured at 77 K and 87 K (Fig. 2 a,b). At 77 K and 101 kPa, Cu-bptc exhibited high uptake capacities (q): 248.907 cm 3 g -1 for D₂ and 219.596 cm 3 g -1 for H₂, outperforming many reported MOF-based hydrogen storage materials. In contrast, Cu-ATC showed lower capacities (154.2 cm 3 g -1 for D₂ and 146.6 cm 3 g -1 for H₂), attributable to its narrow one-dimensional channels (~ 5.6 Å) limiting accessible volume, while Cu-bptc's cage-like structure (~ 8–10 Å) offers larger voids. Cu-mbtc displayed even lower uptakes (85.6 cm 3 g -1 for D₂ and 76.8 cm 3 g -1 for H₂), despite a comparable surface area to Cu-ATC, highlighting the adverse effect of channel expansion (~ 10 Å) in topologically similar frameworks. To evaluate the adsorption density of hydrogen isotope molecules on the surface of MOFs, BET surface area-normalized capacities (q SA , q SA =q/S BET ) were calculated (Fig. 2 c). At 77 K, Cu-ATC's q SA values (0.257 cm 3 m -2 for D₂ and 0.244 cm 3 m -2 for H₂) were approximately twice those of Cu-bptc (0.136 cm 3 m -2 for D₂ and 0.120 cm 3 m -2 for H₂) and Cu-mbtc (0.154 cm 3 m -2 for D₂ and 0.172 cm 3 m -2 for H₂), demonstrating superior adsorption utilization per unit area in Cu-ATC's ultramicroporous topology, despite its lower absolute capacity. The isotherm steepness in the low-pressure region ( Cu-bptc > Cu-mbtc (Fig. 2 a), reflecting stronger gas-framework interactions and faster adsorption kinetics in Cu-ATC. At 87 K (Fig. 2 b), Cu-ATC's capacities decreased by only 8–9% (at 101 kPa), whereas Cu-bptc and Cu-mbtc decreased by 25–28% and 36–37%, respectively, underscoring Cu-ATC's enhanced adsorption stability at elevated temperatures. Ideal adsorbed solution theory (IAST) selectivity for D₂/H₂ (equimolar mixture), computed using a dual-site Langmuir-Freundlich model, was 1.84 for Cu-ATC at 77 K, surpassing those of Cu-bptc (1.42) and Cu-mbtc (1.42). At 87 K, Cu-ATC retained 1.60, while the others dropped to 1.17 and 1.38. These results affirm Cu-ATC's superior separation performance under mild conditions. Isosteric heats of adsorption (Q st ), derived via the virial method (Fig. 2 d), further elucidated the mechanism. Cu-ATC's zero-coverage Q st values were 12.47 kJ mol -1 for D₂ and 12.09 kJ mol -1 for H 2 —the highest among copper paddlewheel MOFs—exceeding those of Cu-bptc (7.26/6.70 kJ mol -1 ), Cu-mbtc (7.26/6.88 kJ mol -1 ), FJI-Y11 (7.88/7.13 kJ mol -1 ), and HKUST-1 (7.32/6.41 kJ mol -1 ). Cu-ATC also outperformed Co-MOF-74 (12.1/11.0 kJ mol -1 ) and Zn-MOF-74 (9.2/8.3 kJ mol -1 ), being only slightly below Ni-MOF-74 (13.7/12.1 kJ mol -1 ). The maximum D₂-H₂ Q st difference of 1.4 kJ mol -1 ranks among the highest for copper paddlewheel MOFs, indicating that paired OMSs and ultramicropores synergistically enhance isotope affinity differences, bolstering the CAQS mechanism. In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) To probe the H₂ adsorption mechanism in the MOFs, a stepwise hydrogen dosing protocol (2.6 mL min -1 per dose) was employed with In situ DRIFTS monitoring at room temperature (298 K) (Fig. 3 ). In Cu-ATC, increasing H 2 exposure produced a progressively intensified broad absorption band centered at 3479 cm -1 , red-shifted by 682 cm -1 relative to free H 2 vibrations (~ 4161 cm -1 ). 33,34 This substantial shift confirms a strong interaction between H 2 and OMSs, supporting dihydrogen bond formation even at ambient temperature. In contrast, Cu-bptc and Cu-mbtc showed no detectable H₂-related IR signals (Fig. 3 b and c), indicating negligible chemisorption at 298 K. This observation is consistent with Cu-ATC's elevated adsorption enthalpies (Q st >12 kJ mol -1 ), highlighting topological differences: Cu-ATC's ultramicroporous one-dimensional channels (~ 5.6 Å) facilitate optimized H 2 interactions with paired OMSs (symmetric d z 2 orbitals), whereas the larger cavities/channels (> 8 Å) in Cu-bptc and Cu-mbtc allow molecules to bypass OMSs, weakening the interaction. These DRIFTS results directly substantiate Cu-ATC's CAQS advantage through structural synergy, extending the applicability of copper paddlewheel MOFs under mild conditions. Cryogenic Thermal Desorption Spectroscopy (TDS): Adsorption Sites and Isotope Preferential Binding To identify adsorption sites in the three MOFs and their affinity for H 2 /D 2 , TDS measurements were conducted. Samples were exposed to an equimolar H 2 /D 2 mixture (10 mbar) at 26 K for 10 min, evacuated to 20 K, and heated at 0.1 K s -1 (6 K min -1 ) over 20–150 K (Fig. 4 ). Cu-ATC's TDS spectrum revealed two desorption peaks (Fig. 4 a): a higher-temperature peak assigned to OMSs (Site I) and a lower-temperature peak associated with framework oxygen atoms (Site II). D₂ peaks shifted to higher temperatures than H₂, indicating preferential D₂ binding at OMSs due to lower zero-point energy, thereby enhancing CAQS, consistent with the Q st difference (1.4 kJ mol -1 ) and DRIFTS redshift. Cu-bptc showed three peaks (Fig. 4 c): OMSs (I), oxygen atoms (II), and small pores (III), but with smaller D₂/H₂ shifts, reflecting diluted site-specific interactions in its cage-like topology. Cu-mbtc exhibited two peaks (Fig. 4 e): OMSs (I) and large pores (II), with the weakest D₂ preference, attributed to channel expansion (~ 10 Å) diminishing interactions. These TDS results quantify topology-dependent site diversity: Cu-ATC's ultramicroporous channels amplify OMS preference for D₂ (largest peak shift), providing the basis for its high selectivity and distinguishing it from conventional copper MOFs. Separation Performance To assess the practical impact of the MOF topologies on separation, samples were exposed to an equimolar H 2 /D 2 mixture (10 mbar) at varying temperatures (26, 50, 77, 87 K), with selectivity (S D2/H2 = D 2 /H 2 desorption area ratio) evaluated via TDS (Fig. 5 and Supporting Information Figures S10–12). At 26 K, all adsorption sites (strong and weak) were occupied, and the combined KQS and CAQS effects yielded a high D₂ preference. Above 50 K, adsorption was confined to OMSs, yet CAQS continued to favor D₂. Cu-bptc and Cu-mbtc selectivities declined sharply with temperature, reflecting their weak OMS affinity, whereas Cu-ATC exhibited superior values: 20 at 50 K (the highest among all tested Cu(Ⅱ)-MOFs) and 8.0 at 87 K. This performance originates from its ultramicroporous channels (~ 5.6 Å) and paired OMS synergy enhancing isotope affinity differences, consistent with Q st and DRIFTS results. These findings underscore Cu-ATC's CAQS potential above liquid nitrogen temperatures, supporting its promise for industrial hydrogen isotope separation (Fig. 6 ). Dynamic column breakthrough experiments To simulate industrial H₂/D₂ separation, dynamic column breakthrough experiments were performed using an H₂/D₂/Ne mixture (10/10/80 vol.%) at a flow rate of 5 mL min -1 , 77 K, and 100 kPa (Fig. 7 ). In all materials, H₂ broke through first (reflecting its lower binding affinity), whereas D₂ was retained longer. Cu-mbtc showed nearly simultaneous H₂/D₂ breakthrough (Fig. 7 c), yielding a low selectivity of 1.35, due to channel dilation that weakens isotope discrimination separation. In contrast, Cu-ATC exhibited an H₂ breakthrough time of 823 s g -1 (Fig. 7 a), much shorter than Cu-bptc's 2593 s g -1 (Fig. 7 b), reflecting accelerated displacement kinetics in its ultramicroporous framework. For D₂, the breakthrough time was 5400 s g -1 in Cu-ATC, significantly longer than that of Cu-bptc (2483 s g -1 ), demonstrating enhanced retention. Curve integration yielded dynamic parameters: Cu-ATC showed a D₂ capacity of 1.04 mmol g -1 with a selectivity of 1.8, outperforming Cu-bptc (1.2) and Cu-mbtc (1.35). Although dynamic values were lower than those from static isotherms, owing to incomplete equilibration under flow, Cu-ATC’s performance corroborates its industrial potential, consistent with its TDS selectivity (20 at 50 K). Adsorption mechanism study To elucidate the hydrogen adsorption and separation mechanisms in the three MOFs, Grand Canonical Monte Carlo (GCMC) simulations were conducted in Materials Studio at 77 K and constant pressure and probability density maps of H 2 were obtained as shown in Fig. 8 , the results showed two primary adsorption sites in Cu-ATC (Fig. 8 a): (I) near copper paddlewheel OMSs and (II) proximal to oxygen atoms in the clusters. Adsorption at Site II was attributed to the compact structure, which facilitated significant interactions between H₂ and oxygen atoms within the framework. In contrast, Cu-bptc (Fig. 8 b) exhibited uniform H₂ distribution across cage-like cavities, lacking OMS localization; its spacious voids (~ 8–10 Å) dilute interactions, weakening the CAQS effect. Cu-mbtc (Fig. 8 c) showed a preference for wall-bound OMSs due to the excessively large pore size (~ 10 Å). The above three patterns confirm topological control: Cu-ATC's confined channels (~ 5.6 Å) enforce OMS engagements, boosting CAQS. Periodic density functional theory (DFT) calculations, based on GCMC-identified sites, assessed H₂ adsorption energies (Fig. 9 ). Cu-ATC's primary Site I (between adjacent paddlewheels) yielded a H₂ adsorption energy of -11.01 kJ mol -1 , matching experimental Q st , far stronger than the values at the comparable OMS in Cu-bptc (-8.52 kJ mol -1 ) and Cu-mbtc (-7.30 kJ mol -1 ). This enhancement arises from Cu-ATC's short Cu-Cu distance (5.98 Å). As shown in Fig. 10 a, the charge density difference (CDD) calculated using vaspkit further demonstrates that D₂ simultaneously engages in strong synergistic orbital interactions with the facing Cu d Z ² orbitals in Cu-ATC, the calculated binding energy for this interaction was as high as -15.8 kJ mol -1 . Integrated analyses reveal key structural determinants: although Cu-ATC and Cu-bptc share similar apertures, Cu-bptc's oversized cages and OMS-diffusion path mismatch limit interactions to single OMSs. Cu-mbtc's dilated channels hinder d z 2 orbital engagement. Conversely, Cu-ATC's ultramicropores synergize three features: (i) optimal vacant d z 2 orbital alignment along diffusion trajectories, (ii) confined spaces promote the trapping of hydrogen isotope molecules by OMS, and (iii) proximate OMS spacing for dual-site cooperativity. These enable rapid, simultaneous dual-OMS engagements, accelerating and intensifying isotope interactions to amplify CAQS, conferring superior H₂/D₂ separation even at 77 K and positioning Cu-ATC as an innovative benchmark in stable Cu(II)-MOFs. CONCLUSION In summary, we demonstrate that pore topology modulation combined with bimetallic synergistic engineering enables Cu-ATC, a paddlewheel-type MOF with one-dimensional ultramicroporous channels, to achieve record hydrogen isotope separation performance. Cu-ATC exhibits steep adsorption isotherms at low pressures and a TDS selectivity of 20 at 50 K, the highest among reported Cu(II)-MOFs. Importantly, at 77 K, its dynamic breakthrough selectivity of 1.8 is maintained, arising from synergistic interactions between paired Cu sites and ~ 5.6 Å channels that amplify CAQS (ΔQ st = 1.4 kJ mol -1 ). These findings establish the structural principles by which confinement and OMS arrangement cooperate to determine isotope separation, providing a general framework for designing efficient MOFs under mild conditions and advancing deuterium enrichment technologies. Declarations ASSOCIATED CONTENT Supporting Information: XRD patterns of samples. Experimental details. GCMC and DFT calculation details. Thermal desorption spectra of samples. Summary of hydrogen isotope separation factors for various porous frameworks. AUTHOR INFORMATION Corresponding Authors : Chuanqin Xia- College of Chemistry, Sichuan University, Chengdu 610064, P.R. China. E-mail: [email protected] Lijian Ma- College of Chemistry, Sichuan University, Chengdu 610064, P.R. China. E-mail: [email protected] Notes : The authors declare no competing financial interest. Acknowledgments : This work was supported by the National Natural Science Foundation of China (Grant No. 22176136, U21A20296, 22376149，U1867205), and the Key Research and Development Program of Sichuan Province, China (Project No. 2020YFS0070, 2021YFH0170). We are grateful for the technical support from the Comprehensive Training Platform of the Specialized Laboratory (College of Chemistry, Sichuan University) and Analytical & Testing Center of Sichuan University. References Glugla, M. et al. 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Supplementary Files Supportinginformation.docx Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope 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. 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05:59:16","extension":"html","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":96662,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/3441d2fdf5d85c38c9f9f3c7.html"},{"id":94164444,"identity":"18f78aa5-7605-4fcb-9d9e-e9cd862048c5","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":285754,"visible":true,"origin":"","legend":"\u003cp\u003eIllustrated structure of (a) Cu\u003csub\u003e2\u003c/sub\u003e(CO\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e units; (b) 3,3',5,5'-biphenyltetracarboxylate ; (c) 4,4',4'',4'''-methanetetrayltetrabenzoate; (d) 1,3,5,7-adamantanetetracarboxylate ; (e) (f) Schematic illustrations of two cage-like structures in Cu-bptc; Three-dimensional structural schematic of (g) Cu-mbtc and (h) Cu-ATC.（Atom colors: C: gray, O: red, Cu: cyan, H: white, and the yellow sphere represents the largest sphere that would occupy the cavity without contacting the interior van der Waals surface）\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/263be84e95f2e18264e3f80d.png"},{"id":94164445,"identity":"5004b234-e29f-423e-aa55-08d6ac4ce733","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":233918,"visible":true,"origin":"","legend":"\u003cp\u003eSingle-component H\u003csub\u003e2\u003c/sub\u003e and D\u003csub\u003e2\u003c/sub\u003e adsorption isotherms of Cu-ATC (blue), Cu-bptc (red), and Cu-mbtc (green) at 77 K (a) and 87 K (b). Single-component H\u003csub\u003e2\u003c/sub\u003e and D\u003csub\u003e2\u003c/sub\u003e isotherms based on unit BET surface area for Cu-ATC, Cu-bptc, and Cu-mbtcat 77 K (c). The isosteric heat of D\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e for Cu-ATC, Cu-bptc, and Cu-mbtc (d).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/305cf0df086ba982497fabc1.png"},{"id":94164447,"identity":"b47bf64b-5e53-49fd-8e13-356123ca83c7","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":89517,"visible":true,"origin":"","legend":"\u003cp\u003eIn situ DRIFT spectra of pure H\u003csub\u003e2\u003c/sub\u003e adsorption on Cu-ATC (a), Cu-bptc (b), and Cu-mbtc (c) dosing at 298 K with each dosage being 2.6 mL/min.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/e515fd618359b87a0fbd5ba6.png"},{"id":94164623,"identity":"7c135840-8ba2-492b-b60e-1df9d4fcab0a","added_by":"auto","created_at":"2025-10-23 06:07:15","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":268895,"visible":true,"origin":"","legend":"\u003cp\u003eThermal desorption spectra and corresponding adsorption sites of Cu-ATC (a) (b), Cu-bptc (c) (d), and Cu-mbtc (e) (f). The samples were exposed to 1:1 D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e mixture gas at 26 K for 10 min and then evacuated before heating at a rate of 6 K/min.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/ae168c49d0cc4cde8b9b16ba.png"},{"id":94164451,"identity":"9a7d24a8-ace5-42b1-96a8-0587e3899761","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":63846,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorptive separation for an equimolar D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e isotope mixture.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/588846cb209c5f5202112435.png"},{"id":94164626,"identity":"680e458b-8b60-4d10-ae9d-83654f00d1c6","added_by":"auto","created_at":"2025-10-23 06:07:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":98699,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e selectivity between the results in the literature and our work.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/807ae3605731d2bc6a75447f.png"},{"id":94164455,"identity":"b660ca61-04d2-4ca3-8f98-df1705e59212","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":95034,"visible":true,"origin":"","legend":"\u003cp\u003eThe dynamic breakthrough curves of Cu-ATC (a), Cu-bptc (b), and Cu-mbtc (c) at 77 K for the mixed gases of H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e/Ne (10/10/80 vol.%).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/6b97596dc254fb614f5640ee.png"},{"id":94164457,"identity":"6f60abc5-02f0-44a5-b7b5-33a5fff2c494","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":614242,"visible":true,"origin":"","legend":"\u003cp\u003eThe probability distribution density diagram of H\u003csub\u003e2\u003c/sub\u003e molecules adsorption sites around the Cu-ATC (a), Cu-bptc (b), and Cu-mbtc (c). Color code: C, gray; O, red; Cu, orange.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/0dfcbc1706eeead5dea74621.png"},{"id":94164637,"identity":"f7482d8c-0e34-4aaa-baef-e56850a4d381","added_by":"auto","created_at":"2025-10-23 06:07:16","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":179755,"visible":true,"origin":"","legend":"\u003cp\u003eA molecular illustration of the H\u003csub\u003e2\u003c/sub\u003e binding site (a) between the Cu OMSs of adjacent paddlewheels in Cu-ATC and about a single Cu OMS in Cu-bptc (b) and Cu-mbtc (c) as determined from simulation. The adsorbate molecules are shown in yellow.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/0a90c335d974c3e54467b69a.png"},{"id":94164632,"identity":"166f8a69-7fb2-4cab-88f7-9fe3169c0bec","added_by":"auto","created_at":"2025-10-23 06:07:16","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":187475,"visible":true,"origin":"","legend":"\u003cp\u003eThe 3D charge density difference (a) and electron density distribution (b) of Cu-ATC-D\u003csub\u003e2\u003c/sub\u003e. The yellow zone and cyan zone denote the charge accumulation and depletion.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/c6d267c8d72efb6e9b1ea346.png"},{"id":94165608,"identity":"862cc44f-7cff-4f87-917e-faaf28899d9f","added_by":"auto","created_at":"2025-10-23 06:23:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2370721,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/df90f93b-3e2e-44d0-ab73-1c836cebe8c0.pdf"},{"id":94164448,"identity":"edf722ad-e6f4-4047-b928-c95f4ec54f76","added_by":"auto","created_at":"2025-10-23 05:59:15","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3763539,"visible":true,"origin":"","legend":"Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope Separation","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7838780/v1/b8d5a46846d385f553b02ca8.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope Separation","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eDeuterium enrichment from hydrogen isotope mixtures represents a critical bottleneck in nuclear fusion fuel cycles. In current tokamak reactors, the utilization of D\u003csub\u003e2\u003c/sub\u003e and T\u003csub\u003e2\u003c/sub\u003e fuels is typically below 10%, with most discharged as waste streams, leading to tritium loss and the requirement for continuous deuterium enrichment to sustain the fuel cycle. \u003csup\u003e1,2\u003c/sup\u003e Despite decades of research, conventional separation technologies, including cryogenic distillation, thermal diffusion, and chromatographic methods, suffer from high energy consumption and low separation factors (~\u0026thinsp;1.5), rendering them economically unsustainable for large-scale implementation, \u003csup\u003e3\u003c/sup\u003e underscoring the urgent need for new approaches that combine higher selectivity with lower energy demand. The intrinsic difficulty in hydrogen isotope separation lies in their nearly identical physicochemical properties, with the primary difference being a mass ratio of two, necessitating the exploitation of quantum effects for efficient separation under mild conditions.\u003c/p\u003e\u003cp\u003eMetal\u0026ndash;organic frameworks (MOFs) have emerged as promising platforms for hydrogen isotope separation by exploiting quantum sieving effects arising from zero-point energy differences. \u003csup\u003e4\u0026ndash;7\u003c/sup\u003e Kinetic quantum sieving (KQS) can deliver remarkable selectivity\u0026mdash;D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;41.4 at 20 K \u003csup\u003e8\u003c/sup\u003e\u0026mdash;through diffusion discrimination in confined pores, but this requires impractically low temperatures. To extend applicability, chemical-affinity quantum sieving (CAQS) was introduced, \u003csup\u003e9\u003c/sup\u003e relying on Kubas-type σ donation and π back-donation between H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e and open metal sites. \u003csup\u003e10\u0026ndash;17\u003c/sup\u003e At 77 K, however, achieving high selectivity (\u0026gt;\u0026thinsp;10) remains challenging, particularly regarding stability and capacity for practical application. Building on this, researchers have attempted to combine the two mechanisms. Imidazole-modified MOF-74 achieved a selectivity of 26 at 77 K, \u003csup\u003e18\u003c/sup\u003e while Cu(I)-exchanged ZSM-5 reached 24.9 at 100 K. \u003csup\u003e19\u003c/sup\u003e These studies illustrate the potential of KQS\u0026ndash;CAQS synergy, yet they also highlight persistent challenges including the instability of Cu(I) sites, the complexity of post-synthetic modification, and the rapid loss of selectivity once the temperature rises above 77 K. The central challenge therefore remains to establish isotope\u0026ndash;metal interactions that are both strong and durable, capable of sustaining high selectivity under practical cryogenic conditions without compromising framework integrity.\u003c/p\u003e\u003cp\u003eAmong MOF architectures, Cu(II)-based paddlewheel frameworks are particularly attractive because their robust [Cu\u003csub\u003e2\u003c/sub\u003e(O\u003csub\u003e2\u003c/sub\u003eCR)\u003csub\u003e4\u003c/sub\u003e] secondary building units(SBUs, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(c)) allow facile generation of open metal sites upon activation and systematic modulation of pore environments. In addition, the Jahn\u0026ndash;Teller distortion intrinsic to Cu(II) enhances the accessibility of d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals for guest\u0026ndash;molecule interactions, \u003csup\u003e20,21\u003c/sup\u003e offering potential advantages for isotope binding. Despite these features, benchmark Cu-BTC (HKUST-1) delivers only modest performance, with a D₂/H₂ selectivity of 1.3 at 77 K, \u003csup\u003e22\u0026ndash;27\u003c/sup\u003e as its large\u0026thinsp;~\u0026thinsp;1 nm cages permit molecules to bypass OMSs and diminish the CAQS effect. Attempts to improve performance by reducing Cu(II) to Cu(I) temporarily boosted selectivity to 37.9 at 30 K, \u003csup\u003e28\u003c/sup\u003e but this advantage quickly disappeared above 40 K because the reduction disrupted d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbital alignment and undermined framework stability. A similar issue was observed in FJI-Y11, \u003csup\u003e29\u003c/sup\u003e where DFT calculations revealed that adsorption occurred near oxygen atoms rather than Cu centers, as the d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals were geometrically misaligned with the pore channels, limiting effective interaction with hydrogen molecules and resulting in low selectivity (α\u0026thinsp;\u0026asymp;\u0026thinsp;1.4).\u003c/p\u003e\u003cp\u003eThese observations show that OMS density alone is not sufficient, and that isotope separation critically depends on how pore confinement and orbital orientation cooperate. A promising direction is therefore to design frameworks with ultramicroporous channels that enforce close contact between isotopes and appropriately oriented, Jahn\u0026ndash;Teller\u0026ndash;stabilized Cu(II) sites, while preserving the structural integrity of the paddlewheel motif.\u003c/p\u003e\u003cp\u003eGuided by this rationale, we synthesized three tetracarboxylate Cu(II) MOFs with distinct topologies including cage-type Cu-bptc, \u003csup\u003e30\u003c/sup\u003e wide-channel Cu-mbtc (~\u0026thinsp;10 \u0026Aring;), \u003csup\u003e31\u003c/sup\u003e and ultramicroporous Cu-ATC (~\u0026thinsp;5 \u0026Aring;), \u003csup\u003e32\u003c/sup\u003e to examine how pore architecture affects isotope separation. Comparative evaluation showed that Cu-bptc and Cu-mbtc displayed only limited selectivity, indicating that pore confinement alone cannot account for high performance. In contrast, Cu-ATC exhibited outstanding separation, achieving a D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e selectivity of 20 at 50 K and 1.8 in breakthrough experiments at 77 K, surpassing all reported Cu(II) MOFs. Subsequent analysis revealed that its ultramicroporous channels not only constrain molecular motion but also position isotopes within reach of two closely spaced paddlewheel Cu(II) sites, where accessible d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals enable cooperative dual-site interactions that markedly amplify the CAQS effect. These findings establish Cu-ATC as a benchmark system and highlight a generalizable design principle in which pore topology and site-specific electronic structure act in concert to realize practical hydrogen isotope separation under mild cryogenic conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSIONS","content":"\u003cp\u003eStructure Characterization\u003c/p\u003e\u003cp\u003eThe structures of the three Cu(II)-based MOFs were characterized by powder X-ray diffraction (PXRD) and nitrogen adsorption-desorption experiments, with detailed results provided in the Supporting Information. The PXRD patterns confirmed that the crystal structures of these materials aligned with literature reports, indicating successful synthesis and high purity (Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e-3). The nitrogen adsorption isotherms exhibited IUPAC Type I characteristics for all three MOFs, verifying their microporous nature (Figure S4-6). Pore size distributions (Figure S7-9), calculated using nonlocal density functional theory (NLDFT), revealed distinct topologies: Cu-ATC and Cu-bptc possess narrow ultramicropore (5.6 \u0026Aring; and 5.8 \u0026Aring;, respectively), whereas Cu-mbtc showed a larger pore size (10.0 \u0026Aring;). This variation reflects the designed optimization from cage-like Cu-bptc to one-dimensional channels in Cu-mbtc and Cu-ATC (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Brunauer-Emmett-Teller (BET) surface areas were 606 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Cu-ATC, 1811 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Cu-bptc, and 498 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Cu-mbtc (Figure S4-6). These values correlate with framework compactness: Cu-bptc's high surface area arises from its interconnected cage network, whereas Cu-ATC's lower value stems from denser ultramicroporous channels. Such topological tuning establishes the structural basis for subsequent hydrogen isotope adsorption studies, consistent with the dual strategy of pore topology modulation and bimetallic synergy.\u003c/p\u003e\u003cp\u003eAdsorption properties of H\u003csub\u003e2\u003c/sub\u003e and D\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSingle-component H₂ and D₂ adsorption isotherms for the three MOFs were measured at 77 K and 87 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea,b). At 77 K and 101 kPa, Cu-bptc exhibited high uptake capacities (q): 248.907 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for D₂ and 219.596 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for H₂, outperforming many reported MOF-based hydrogen storage materials. In contrast, Cu-ATC showed lower capacities (154.2 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for D₂ and 146.6 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for H₂), attributable to its narrow one-dimensional channels (~\u0026thinsp;5.6 \u0026Aring;) limiting accessible volume, while Cu-bptc's cage-like structure (~\u0026thinsp;8\u0026ndash;10 \u0026Aring;) offers larger voids. Cu-mbtc displayed even lower uptakes (85.6 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for D₂ and 76.8 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for H₂), despite a comparable surface area to Cu-ATC, highlighting the adverse effect of channel expansion (~\u0026thinsp;10 \u0026Aring;) in topologically similar frameworks.\u003c/p\u003e\u003cp\u003eTo evaluate the adsorption density of hydrogen isotope molecules on the surface of MOFs, BET surface area-normalized capacities (q\u003csub\u003eSA\u003c/sub\u003e, q\u003csub\u003eSA\u003c/sub\u003e=q/S\u003csub\u003eBET\u003c/sub\u003e) were calculated (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). At 77 K, Cu-ATC's q\u003csub\u003eSA\u003c/sub\u003e values (0.257 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for D₂ and 0.244 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for H₂) were approximately twice those of Cu-bptc (0.136 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for D₂ and 0.120 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for H₂) and Cu-mbtc (0.154 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for D₂ and 0.172 cm\u003csup\u003e3\u003c/sup\u003e m\u003csup\u003e-2\u003c/sup\u003e for H₂), demonstrating superior adsorption utilization per unit area in Cu-ATC's ultramicroporous topology, despite its lower absolute capacity. The isotherm steepness in the low-pressure region (\u0026lt;\u0026thinsp;10 kPa at 77 K) followed the order Cu-ATC\u0026thinsp;\u0026gt;\u0026thinsp;Cu-bptc\u0026thinsp;\u0026gt;\u0026thinsp;Cu-mbtc (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), reflecting stronger gas-framework interactions and faster adsorption kinetics in Cu-ATC. At 87 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), Cu-ATC's capacities decreased by only 8\u0026ndash;9% (at 101 kPa), whereas Cu-bptc and Cu-mbtc decreased by 25\u0026ndash;28% and 36\u0026ndash;37%, respectively, underscoring Cu-ATC's enhanced adsorption stability at elevated temperatures.\u003c/p\u003e\u003cp\u003eIdeal adsorbed solution theory (IAST) selectivity for D₂/H₂ (equimolar mixture), computed using a dual-site Langmuir-Freundlich model, was 1.84 for Cu-ATC at 77 K, surpassing those of Cu-bptc (1.42) and Cu-mbtc (1.42). At 87 K, Cu-ATC retained 1.60, while the others dropped to 1.17 and 1.38. These results affirm Cu-ATC's superior separation performance under mild conditions.\u003c/p\u003e\u003cp\u003eIsosteric heats of adsorption (Q\u003csub\u003est\u003c/sub\u003e), derived via the virial method (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), further elucidated the mechanism. Cu-ATC's zero-coverage Q\u003csub\u003est\u003c/sub\u003e values were 12.47 kJ mol\u003csup\u003e-1\u003c/sup\u003e for D₂ and 12.09 kJ mol\u003csup\u003e-1\u003c/sup\u003e for H\u003csub\u003e2\u003c/sub\u003e\u0026mdash;the highest among copper paddlewheel MOFs\u0026mdash;exceeding those of Cu-bptc (7.26/6.70 kJ mol\u003csup\u003e-1\u003c/sup\u003e), Cu-mbtc (7.26/6.88 kJ mol\u003csup\u003e-1\u003c/sup\u003e), FJI-Y11 (7.88/7.13 kJ mol\u003csup\u003e-1\u003c/sup\u003e), and HKUST-1 (7.32/6.41 kJ mol\u003csup\u003e-1\u003c/sup\u003e). Cu-ATC also outperformed Co-MOF-74 (12.1/11.0 kJ mol\u003csup\u003e-1\u003c/sup\u003e) and Zn-MOF-74 (9.2/8.3 kJ mol\u003csup\u003e-1\u003c/sup\u003e), being only slightly below Ni-MOF-74 (13.7/12.1 kJ mol\u003csup\u003e-1\u003c/sup\u003e). The maximum D₂-H₂ Q\u003csub\u003est\u003c/sub\u003e difference of 1.4 kJ mol\u003csup\u003e-1\u003c/sup\u003e ranks among the highest for copper paddlewheel MOFs, indicating that paired OMSs and ultramicropores synergistically enhance isotope affinity differences, bolstering the CAQS mechanism.\u003c/p\u003e\u003cp\u003e\u003cem\u003eIn Situ\u003c/em\u003e Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo probe the H₂ adsorption mechanism in the MOFs, a stepwise hydrogen dosing protocol (2.6 mL min\u003csup\u003e-1\u003c/sup\u003e per dose) was employed with \u003cem\u003eIn situ\u003c/em\u003e DRIFTS monitoring at room temperature (298 K) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In Cu-ATC, increasing H\u003csub\u003e2\u003c/sub\u003e exposure produced a progressively intensified broad absorption band centered at 3479 cm\u003csup\u003e-1\u003c/sup\u003e, red-shifted by 682 cm\u003csup\u003e-1\u003c/sup\u003e relative to free H\u003csub\u003e2\u003c/sub\u003e vibrations (~\u0026thinsp;4161 cm\u003csup\u003e-1\u003c/sup\u003e). \u003csup\u003e33,34\u003c/sup\u003e This substantial shift confirms a strong interaction between H\u003csub\u003e2\u003c/sub\u003e and OMSs, supporting dihydrogen bond formation even at ambient temperature. In contrast, Cu-bptc and Cu-mbtc showed no detectable H₂-related IR signals (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and c), indicating negligible chemisorption at 298 K. This observation is consistent with Cu-ATC's elevated adsorption enthalpies (Q\u003csub\u003est\u003c/sub\u003e \u0026gt;12 kJ mol\u003csup\u003e-1\u003c/sup\u003e), highlighting topological differences: Cu-ATC's ultramicroporous one-dimensional channels (~\u0026thinsp;5.6 \u0026Aring;) facilitate optimized H\u003csub\u003e2\u003c/sub\u003e interactions with paired OMSs (symmetric d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals), whereas the larger cavities/channels (\u0026gt;\u0026thinsp;8 \u0026Aring;) in Cu-bptc and Cu-mbtc allow molecules to bypass OMSs, weakening the interaction. These DRIFTS results directly substantiate Cu-ATC's CAQS advantage through structural synergy, extending the applicability of copper paddlewheel MOFs under mild conditions.\u003c/p\u003e\u003cp\u003eCryogenic Thermal Desorption Spectroscopy (TDS): Adsorption Sites and Isotope Preferential Binding\u003c/p\u003e\u003cp\u003eTo identify adsorption sites in the three MOFs and their affinity for H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e, TDS measurements were conducted. Samples were exposed to an equimolar H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e mixture (10 mbar) at 26 K for 10 min, evacuated to 20 K, and heated at 0.1 K s\u003csup\u003e-1\u003c/sup\u003e (6 K min\u003csup\u003e-1\u003c/sup\u003e) over 20\u0026ndash;150 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Cu-ATC's TDS spectrum revealed two desorption peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea): a higher-temperature peak assigned to OMSs (Site I) and a lower-temperature peak associated with framework oxygen atoms (Site II). D₂ peaks shifted to higher temperatures than H₂, indicating preferential D₂ binding at OMSs due to lower zero-point energy, thereby enhancing CAQS, consistent with the Q\u003csub\u003est\u003c/sub\u003e difference (1.4 kJ mol\u003csup\u003e-1\u003c/sup\u003e) and DRIFTS redshift. Cu-bptc showed three peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec): OMSs (I), oxygen atoms (II), and small pores (III), but with smaller D₂/H₂ shifts, reflecting diluted site-specific interactions in its cage-like topology. Cu-mbtc exhibited two peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee): OMSs (I) and large pores (II), with the weakest D₂ preference, attributed to channel expansion (~\u0026thinsp;10 \u0026Aring;) diminishing interactions. These TDS results quantify topology-dependent site diversity: Cu-ATC's ultramicroporous channels amplify OMS preference for D₂ (largest peak shift), providing the basis for its high selectivity and distinguishing it from conventional copper MOFs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eSeparation Performance\u003c/p\u003e\u003cp\u003eTo assess the practical impact of the MOF topologies on separation, samples were exposed to an equimolar H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e mixture (10 mbar) at varying temperatures (26, 50, 77, 87 K), with selectivity (S\u003csub\u003eD2/H2\u003c/sub\u003e = D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e desorption area ratio) evaluated via TDS (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e and Supporting Information Figures S10\u0026ndash;12). At 26 K, all adsorption sites (strong and weak) were occupied, and the combined KQS and CAQS effects yielded a high D₂ preference. Above 50 K, adsorption was confined to OMSs, yet CAQS continued to favor D₂. Cu-bptc and Cu-mbtc selectivities declined sharply with temperature, reflecting their weak OMS affinity, whereas Cu-ATC exhibited superior values: 20 at 50 K (the highest among all tested Cu(Ⅱ)-MOFs) and 8.0 at 87 K. This performance originates from its ultramicroporous channels (~\u0026thinsp;5.6 \u0026Aring;) and paired OMS synergy enhancing isotope affinity differences, consistent with Q\u003csub\u003est\u003c/sub\u003e and DRIFTS results. These findings underscore Cu-ATC's CAQS potential above liquid nitrogen temperatures, supporting its promise for industrial hydrogen isotope separation (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDynamic column breakthrough experiments\u003c/p\u003e\u003cp\u003eTo simulate industrial H₂/D₂ separation, dynamic column breakthrough experiments were performed using an H₂/D₂/Ne mixture (10/10/80 vol.%) at a flow rate of 5 mL min\u003csup\u003e-1\u003c/sup\u003e, 77 K, and 100 kPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). In all materials, H₂ broke through first (reflecting its lower binding affinity), whereas D₂ was retained longer. Cu-mbtc showed nearly simultaneous H₂/D₂ breakthrough (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), yielding a low selectivity of 1.35, due to channel dilation that weakens isotope discrimination separation. In contrast, Cu-ATC exhibited an H₂ breakthrough time of 823 s g\u003csup\u003e-1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), much shorter than Cu-bptc's 2593 s g\u003csup\u003e-1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), reflecting accelerated displacement kinetics in its ultramicroporous framework. For D₂, the breakthrough time was 5400 s g\u003csup\u003e-1\u003c/sup\u003e in Cu-ATC, significantly longer than that of Cu-bptc (2483 s g\u003csup\u003e-1\u003c/sup\u003e), demonstrating enhanced retention. Curve integration yielded dynamic parameters: Cu-ATC showed a D₂ capacity of 1.04 mmol g\u003csup\u003e-1\u003c/sup\u003e with a selectivity of 1.8, outperforming Cu-bptc (1.2) and Cu-mbtc (1.35). Although dynamic values were lower than those from static isotherms, owing to incomplete equilibration under flow, Cu-ATC\u0026rsquo;s performance corroborates its industrial potential, consistent with its TDS selectivity (20 at 50 K).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAdsorption mechanism study\u003c/p\u003e\u003cp\u003eTo elucidate the hydrogen adsorption and separation mechanisms in the three MOFs, Grand Canonical Monte Carlo (GCMC) simulations were conducted in Materials Studio at 77 K and constant pressure and probability density maps of H\u003csub\u003e2\u003c/sub\u003e were obtained as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, the results showed two primary adsorption sites in Cu-ATC (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea): (I) near copper paddlewheel OMSs and (II) proximal to oxygen atoms in the clusters. Adsorption at Site II was attributed to the compact structure, which facilitated significant interactions between H₂ and oxygen atoms within the framework. In contrast, Cu-bptc (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) exhibited uniform H₂ distribution across cage-like cavities, lacking OMS localization; its spacious voids (~\u0026thinsp;8\u0026ndash;10 \u0026Aring;) dilute interactions, weakening the CAQS effect. Cu-mbtc (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec) showed a preference for wall-bound OMSs due to the excessively large pore size (~\u0026thinsp;10 \u0026Aring;). The above three patterns confirm topological control: Cu-ATC's confined channels (~\u0026thinsp;5.6 \u0026Aring;) enforce OMS engagements, boosting CAQS.\u003c/p\u003e\u003cp\u003ePeriodic density functional theory (DFT) calculations, based on GCMC-identified sites, assessed H₂ adsorption energies (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Cu-ATC's primary Site I (between adjacent paddlewheels) yielded a H₂ adsorption energy of -11.01 kJ mol\u003csup\u003e-1\u003c/sup\u003e, matching experimental Q\u003csub\u003est\u003c/sub\u003e, far stronger than the values at the comparable OMS in Cu-bptc (-8.52 kJ mol\u003csup\u003e-1\u003c/sup\u003e) and Cu-mbtc (-7.30 kJ mol\u003csup\u003e-1\u003c/sup\u003e). This enhancement arises from Cu-ATC's short Cu-Cu distance (5.98 \u0026Aring;). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003ea, the charge density difference (CDD) calculated using vaspkit further demonstrates that D₂ simultaneously engages in strong synergistic orbital interactions with the facing Cu d\u003csub\u003eZ\u003c/sub\u003e\u0026sup2; orbitals in Cu-ATC, the calculated binding energy for this interaction was as high as -15.8 kJ mol\u003csup\u003e-1\u003c/sup\u003e. Integrated analyses reveal key structural determinants: although Cu-ATC and Cu-bptc share similar apertures, Cu-bptc's oversized cages and OMS-diffusion path mismatch limit interactions to single OMSs. Cu-mbtc's dilated channels hinder d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbital engagement. Conversely, Cu-ATC's ultramicropores synergize three features: (i) optimal vacant d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbital alignment along diffusion trajectories, (ii) confined spaces promote the trapping of hydrogen isotope molecules by OMS, and (iii) proximate OMS spacing for dual-site cooperativity. These enable rapid, simultaneous dual-OMS engagements, accelerating and intensifying isotope interactions to amplify CAQS, conferring superior H₂/D₂ separation even at 77 K and positioning Cu-ATC as an innovative benchmark in stable Cu(II)-MOFs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"CONCLUSION","content":"\u003cp\u003eIn summary, we demonstrate that pore topology modulation combined with bimetallic synergistic engineering enables Cu-ATC, a paddlewheel-type MOF with one-dimensional ultramicroporous channels, to achieve record hydrogen isotope separation performance. Cu-ATC exhibits steep adsorption isotherms at low pressures and a TDS selectivity of 20 at 50 K, the highest among reported Cu(II)-MOFs. Importantly, at 77 K, its dynamic breakthrough selectivity of 1.8 is maintained, arising from synergistic interactions between paired Cu sites and ~\u0026thinsp;5.6 \u0026Aring; channels that amplify CAQS (ΔQ\u003csub\u003est\u003c/sub\u003e= 1.4 kJ mol\u003csup\u003e-1\u003c/sup\u003e). These findings establish the structural principles by which confinement and OMS arrangement cooperate to determine isotope separation, providing a general framework for designing efficient MOFs under mild conditions and advancing deuterium enrichment technologies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eASSOCIATED CONTENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupporting Information: XRD patterns of samples. Experimental details. GCMC and DFT calculation details. Thermal desorption spectra of samples. Summary of hydrogen isotope separation factors for various porous frameworks.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR INFORMATION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Authors\u003c/strong\u003e\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eChuanqin Xia- College of Chemistry, Sichuan University, Chengdu 610064, P.R. China. E-mail: [email protected]\u003c/p\u003e\n\u003cp\u003eLijian Ma- College of Chemistry, Sichuan University, Chengdu 610064, P.R. China. E-mail: [email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e The authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e: This work was supported by the National Natural Science Foundation of China (Grant No. 22176136, U21A20296, 22376149，U1867205), and the Key Research and Development Program of Sichuan Province, China (Project No. 2020YFS0070, 2021YFH0170). We are grateful for the technical support from the Comprehensive Training Platform of the Specialized Laboratory (College of Chemistry, Sichuan University) and Analytical \u0026amp; Testing Center of Sichuan University.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGlugla, M.\u003cem\u003e et al.\u003c/em\u003e ITER fuel cycle R\u0026amp;D: Consequences for the design. \u003cem\u003eFusion Engineering and Design\u003c/em\u003e \u003cstrong\u003e81\u003c/strong\u003e, 733-744, doi:https://doi.org/10.1016/j.fusengdes.2005.07.038 (2006).\u003c/li\u003e\n\u003cli\u003eSmith, R.\u003cem\u003e et al.\u003c/em\u003e Hydrogen isotope separation for fusion power applications. \u003cem\u003eJournal of Alloys and Compounds\u003c/em\u003e \u003cstrong\u003e645\u003c/strong\u003e, S51-S55, doi:https://doi.org/10.1016/j.jallcom.2015.01.231 (2015).\u003c/li\u003e\n\u003cli\u003eRae, H. 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Unusually strong adsorption of molecular hydrogen on Cu+ sites in copper-modified ZSM-5. \u003cem\u003ePhysical Chemistry Chemical Physics\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 5250-5255, doi:10.1039/B409848J (2004).\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":"Hydrogen isotopes separation, Metal-Organic Frameworks, dual metal open metal sites, confined channel, synergistic effects","lastPublishedDoi":"10.21203/rs.3.rs-7838780/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7838780/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe selective separation of hydrogen isotopes under mild cryogenic conditions remains a formidable challenge due to their nearly identical physicochemical properties. Here, we report a dual strategy of pore topology design and bimetallic synergistic engineering to amplify chemical affinity quantum sieving (CAQS). Among three tailored Cu(II)-MOFs (Cu-bptc, Cu-mbtc, Cu-ATC), Cu-ATC exhibited exceptional performance, achieving a D₂/H₂ selectivity of 20 at 50 K (10 mbar) and 1.8 in breakthrough experiments at 77 K, surpassing all reported Cu-MOFs. The ultramicroporous topology of Cu-ATC fixes a Cu···Cu distance of 5.98 Å within one-dimensional channels (~5.6 Å), while Jahn–Teller distortion induces axial elongation at each Cu(II) center, stabilizing the d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals and enhancing their availability for interaction with hydrogen isotope molecules. This dual effect creates two closely spaced open metal sites that can simultaneously interact with H\u003csub\u003e2\u003c/sub\u003e and D\u003csub\u003e2\u003c/sub\u003e, amplifying their differential interactions and thereby driving isotope separation via CAQS. The distinct binding strength is evidenced by in situ DRIFTS (H–H stretch red-shift of 682 cm\u003csup\u003e-1\u003c/sup\u003e) and by DFT calculations showing stronger adsorption of D₂ (−15.8 kJ mol\u003csup\u003e-1\u003c/sup\u003e at the primary site). These microscopic differences account for the observed D\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e selectivity, highlighting a new paradigm for CAQS-based isotope separation under mild cryogenic conditions.\u003c/p\u003e","manuscriptTitle":"Dual-Orbital Synergy in Paired Cu(II) Open Metal Sites for Enhanced High-Temperature Hydrogen Isotope Separation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-23 05:59:10","doi":"10.21203/rs.3.rs-7838780/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":"198c87c1-5fa1-4aeb-9d6a-8c3b544bbd74","owner":[],"postedDate":"October 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":56367471,"name":"Physical sciences/Chemistry/Materials chemistry/Metal\u0026#x2013;organic frameworks"},{"id":56367472,"name":"Physical sciences/Chemistry/Nuclear chemistry"}],"tags":[],"updatedAt":"2026-04-30T16:31:05+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-23 05:59:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7838780","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7838780","identity":"rs-7838780","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}