Lone-pair-electron Governed Gas-recognizable Flexibility in an Ultra-stable MOF for Boosting C3H6/C3H8 Separation and Its Aqueous Scalable Synthesis

Lone-pair-electron Governed Gas-recognizable Flexibility in an Ultra-stable MOF for Boosting C3H6/C3H8 Separation and Its Aqueous Scalable Synthesis | 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 Lone-pair-electron Governed Gas-recognizable Flexibility in an Ultra-stable MOF for Boosting C3H6/C3H8 Separation and Its Aqueous Scalable Synthesis Zhiyong Lu, Yuhang Liu, Bufeng Wang, Lilei Zhang, Banghao Wei, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8403378/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 separation of propylene (C 3 H 6 ) from propane (C 3 H 8 ) constitutes one of the most energy-demanding processes in the petrochemical industry, a consequence of their nearly identical molecular dimensions and physicochemical properties. Metal-organic frameworks (MOFs) have emerged as a promising adsorptive alternative to traditional distillation. Among these, flexible MOFs, which undergo selective guest-induced structural transformations, offer a unique mechanism to amplify subtle molecular differences into distinct adsorption behaviors. Nevertheless, the precise molecular-level modulation of framework flexibility to elicit a selective response to one specific gas over another remains a great challenge. This study confronts this challenge through the rational structural evolution of a hydrolytically stable, pillar-layered Cu-MOF, NJU-Bai5. Our strategy involves the strategic engineering of the organic pillar ligand, specifically replacing pyridyl termini with imidazolyl groups. This modification introduces accessible nitrogen lone-pair electrons, which function as molecular anchors capable of forming selective interactions with hydrogen-bonding donors possessing higher acidity. These interactions, in turn, drive a gas-recognizable flexibility of the framework architecture. The resulting material, NJU-Bai5-bib, exhibits a well-defined, selective gate-opening transition that is triggered preferentially by C 3 H 6 at low pressures. This specific response enables the framework to achieve highly selective propylene/propane separation under ambient conditions. Furthermore, by inheriting exceptional hydrolytic stability from its progenitor and featuring a scalable, aqueous synthesis, NJU-Bai5-bib demonstrates not only fundamental excellence in guest-specific recognition but also considerable potential for practical, energy-efficient propylene purification. Physical sciences/Chemistry/Coordination chemistry Physical sciences/Materials science/Soft materials Physical sciences/Chemistry/Green chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 INTRODUCTION Propylene is a pivotal chemical intermediate, essential for manufacturing plastics and materials. 1-2 Its global demand is soaring, with over 60% consumed by polypropylene production, which requires ultra-pure (99.5%) propylene. However, propylene is predominantly co-produced with propane, and their separation is a major industrial challenge, owing to their nearly indistinguishable molecular sizes (difference < 0.5 Å) and boiling points (226 K vs 231 K). 3 The conventional method, cryogenic distillation, is exceptionally energy-intensive. It requires columns with over 200 trays and operates under conditions with very low relative volatility, leading to massive energy consumption for heating and cooling. 4-6 In fact, olefin/alkane separations contribute substantially to the energy footprint of petrochemical operations. Therefore, the development of adsorptive separation materials capable of discriminating between C 3 H 6 and C 3 H 8 at ambient temperatures and moderate pressures is of both scientific and industrial urgency. 6-9 Metal-organic frameworks (MOFs), as one of the most promising classes of porous materials, have shown great advantage in gas separation, owing to their modular building blocks towards adaptive pore geometries and surface properties for target gases. 10-19 The good controllability of MOFs in isoreticular synthesis as well as functionalization, especially for rigid structures, have made them well-explored in olefin/alkane separation, via the strategies of unsaturated metal sites construction, π-electron ligands, or polar functional groups to preferentially bind olefins. 20-24 For MOFs constructed by flexible linkers, structural changes (e.g., gating, breathing, or phase transitions) in response to guest molecules or external stimuli were ingeniously exploited in kinetic gas separation. 25-28 In such systems, the framework resides in a closed or compressed conformation in the absence of guests or at low pressure, but expands or opens in response to a specific guest molecule, thereby enabling selective uptake. 29-30 This dynamic adaptability offers a route to amplify subtle molecular differences (size, electronic structure, polarizability) into measurable separation performance. 31-36 Nevertheless, realizing precise, robust, and selective flexible behavior in MOFs for olefin/alkane separation is still nontrivial. A central challenge lies in the precise modulation of framework flexibility: excessive rigidity can impede structural responsiveness, while excessive compliance may induce structural collapse or irreversible phase transitions accompanied by hysteresis. 25, 37-40 An additional process-related concern arises when the partial pressure of the target gas falls below the gate-opening threshold, which can compromise the purity of the resulting product. 29, 41 Consequently, an optimal framework requires a balanced integration of binding affinity, mechanical deformability, and kinetic accessibility. 42-44 Over a decade ago, we reported the evolution process of a pillar-layered Cu-MOF, NJU-Bai5, which was one of the earliest MOFs showing extremely high hydrolytic stability. This work revealed how a simple two-dimensional layer was progressively transformed into an advanced stable three-dimensional porous structure for CO 2 capture through connecting the terminal-anchoring pyridine by alklyl into bis-connected pillars. Taking its advantage of high hydrolytic stability, a critical pre-requisite for the practical application of MOFs in current stage, a further evolution of NJU-Bai5 to NJU-Bai5-bib was illustrated in this contribution ( Scheme 1 ). By judicious engineering of pillar ligands without altering the underlying topology of NJU-Bai5—particularly substituting pyridyl with imidazolyl—the uncoordinated nitrogen atoms of imidazolyl can function as anchors with lone pair electrons (LPE) that could either interact with hydrogen-bonding donors or molecules possessing positive charge center, for further leading to the contraction/expansion of the framework. Such lone-pair-nitrogen-triggered flexibility combining alkly extension establishing a continuum from original rigid to highly dynamic and enabling molecular recognition events to be translated into macroscopic separations. The ultimate conformation of evolution, NJU-Bai5-bib enables a well-defined gate-opening transition that is triggered preferentially by C 3 H 6 at low pressure. A combination of crystallographic, gas adsorption, and computational analyses demonstrates that π-π and multiple C-H···N/O interactions stabilize C 3 H 6 in the expanded pore, whereas C 3 H 8 binding is comparably weaker. The evolution from NJU-Bai5 to NJU-Bai5-bip and ultimately NJU-Bai5-bib delineates a pillar-directed flexibility gradient, providing a robust yet precisely guest-responsive platform for demanding separations. Inheriting from NJU-Bai5, NJU-Bai5-bib constructed in a same motif shows high water stability, and it can be efficiently synthesized at a large scale in aqueous solution. The above outstanding features of NJU-Bai5-bib make it not only an excellent laboratory-scale material for propylene purification but also a promising candidate for potential industrial applications. RESULTS AND DISCUSSION Structural Evolution of NJU-Bai5 from Rigid to Flexible In our previous study, NJU-Bai5 was a final product obtained by the evolution process from a 2D-layered structure to a 3D pillar-layered water stable Cu-MOF. It is assembled by Cu(II) and TCMBT/bpp mixed linkers with typical pillar-layered structure showing permanent porosity (estimated structural porosity 33.7%). Tetranuclear [Cu 4 ( μ 3 -OH) 2 N 4 (COO) 6 ] cluster connected with six TCMBT linkers to form two-dimensional layers. Between neighbouring layers, bpp linkers act as pillars connecting to Cu 4 nodes from both sides to support layers and create nano-space. Due to the V-shaped configuration of bpp in the crystal structure, only the microporous channels along a axis with a diameter of 0.6 × 0.24 nm exist between layers ( Figure 1a ). Upon solvent-removal by thermo-vacuo treatment, it preserves a permanently open phase ( Figures 1b ). Single-crystal structure analysis of activated NJU-Bai5 reveal only subtle dihedral adjustments of the bpp pillar (from 106.4° to 99.9° and 127.7°) and minor variations of the coordinated N···N distance (from 9.64 Å to 9.72 and 9.14 Å), without appreciable distortion or secondary building unit (SBU) reorganization ( Figures S 4c ). Consequently, the framework maintains its open-pore configuration with negligible contraction (porosity 30.3%), demonstrating that the bpp pillar possessing limited rotation freedom to induce a structural contraction. Substituting the pyridyl unit with a bis-imidazole pillar generates NJU-Bai5-bip (porosity 26.2%), which preserves the iso-reticular pillar-layered structure yet acquires distinct flexibility. Guest removal induces obvious contraction (porosity 13%) accompanied by pronounced pillar torsion (N···N distances reduced from 8.47 Å to 6.02 and 6.64 Å) and near-planar imidazole dihedrals (from 53.4° to 8.2° and 6.5°; Figure 1c&1d and Figure S5 ). These configuration changes of bip ligands created high stress to the [Cu 4 ( μ 3 -OH) 2 N 4 (COO) 6 ] clusters, causing them twisting with concomitant Cu-O bond cleavage and reformation ( Figure S 5 c ), which would surely push up the energy barrier for subsequent gas-induced pore-expansion. Due to the contracted phase it adopts after solvent removal, the accessible volume of NJU-Bai5-bip is significantly reduced yet not fully eliminated. Further elongating the bis-imidazole pillar with an extra carbon increasing the degree of rotation freedom would expect an release of the stress towards metal clusters, and decline the energy barrier for gas-induced pore-expansion. The resultant analogue NJU-Bai5-bib (porosity 27.7%) undergoes complete structural contraction upon activation, yielding a closed-pore phase (porosity 7.2%) while preserving the parent topology. The elongated pillar backbone allows large-amplitude rotations (dihedral from 126.6° to 84.3°; N···N distance from 8.70 Å to 6.71 Å), coupled with TCMBT linker contraction and interlayer sliding that shortens diagonal Cu···Cu distances (from 16.76 to 13.49 Å) and reduces layer torsion (from 97.3° to 66.9°) ( Figure 1e&1f and Figure S6 ). These cooperative deformations contributed to a more contracted phase without obvious sacrifice of the bondings in metal clusters, thus ensure its easier structural responsiveness under external stimuli. Variable-temperature PXRD (VT-PXRD) under vacuum further confirmed the enhanced structural dynamics from the originally rigid NJU-Bai5 to the highly dynamic NJU-Bai5-bib. As shown in Figure S10 , vacuum induces a slight shift of the (1-10) peak to a higher angle, which subsequently reverts upon heating, indicating a minor lattice adjustment triggered by guest removal followed by thermal stress release ( Figure S10a ). For NJU-Bai5-bip, a stepwise lattice contraction is observed, with vacuum directly inducing the initial structural transformation and elevated temperature driving a second contraction step ( Figure S10b ). In contrast, NJU-Bai5-bib undergoes a complete transition from an open to a closed phase under vacuum alone; further temperature increase produces no significant peak changes ( Figure S10c ). These distinct responses confirm that: 1) none pronounced contraction is triggered by vacuum or heating in NJU-Bai5; 2) the contraction in both NJU-Bai5-bip and NJU-Bai5-bib is initially induced by solvent removal; 3) the contraction in NJU-Bai5-bip is comparatively more hindered than in NJU-Bai5-bib, requiring additional thermal energy to complete the phase transition. These observations align well with the expected behavior summarized in Scheme 1 : 1) methanol molecules interact weakly with bpp, insufficient to induce its configurational change; 2) strong solvent interactions with LPEs in either bip or bib can trigger configurational change in both linkers, though the extended carbon chain in bib facilitates this process. Besides the different interaction with solvent molecules, the rotation freedom of these pillars was also a key point for framework contraction ( Figure 1g ), which was further confirmed by theoretical calculations. At a molecular level, Figure 1h systematically illustrates the conformational variability and rotational energy barriers of the three pillar ligands (bpp, bip, and bib), providing direct insight into the intrinsic origin of framework flexibility in this MOF series. Potential energy surface (PES) scan based on density functional theory (DFT) calculations reveals pronounced differences in conformational stability as the torsion angle increases ( Figure S11 ). Specifically, the pyridyl ligand bpp exhibits a high deformation barrier during rotation, suggesting that its conformational transformation is strongly constrained, thereby suppressing large-amplitude molecular torsion. In contrast, bis-imidazole ligand bip shows a substantially reduced rotational barrier, although the energy rises monotonically with increasing torsion angle. This behavior indicates that, despite the higher rotation freedom grafting by imidazole rings, the relatively short bridge still imposes significant steric and torsional strain between the two imidazole rings. Upon further extension of the bridging chain, the bib ligand displays the lowest energy barrier, with a Gaussian-distribution-shaped energy profile characterized by a gentle energy variation at small torsion angles followed by a marked decrease at larger angles. This indicates that the system maintains a low energetic penalty across a broad conformational range, reflecting its intrinsic flexibility and minimal deformation barrier. Gas-responsive Flexibility of NJU-Bai5 Analogues Notably, pore contraction in both NJU-Bai5-bip and NJU-Bai5-bib is induced by the removal of polar solvent molecules such as MeOH. Subsequent investigations focused on the ability of these LPEs to recognize different gas molecules to reopen their pores. Several conventional gases—N 2 , Ar, and CO 2 —were selected for porosity characterization. Both N 2 and Ar are nonpolar, nonquadrupolar molecules, whereas CO 2 possesses a quadrupole moment with a positively charged carbon center. Consequently, CO 2 is expected to exhibit stronger interactions with the bip and bib ligands than Ar or N 2 , thereby triggering pore expansion in the contracted phases of NJU-Bai5-bip and NJU-Bai5-bib. To experimentally confirm this gas-selective behavior, adsorption measurements were conducted using N 2 (77 K), Ar (87 K), and CO 2 (195 K). The rigid pristine framework NJU-Bai5 displays a Type-I isotherm for all gases, indicative of a permanent microporous structure. Saturated uptakes reach 206.2 cm 3 g -1 for N 2 , 208.4 cm 3 g -1 for Ar, and 199.0 cm 3 g -1 for CO 2 ( Figure 2a ), yielding a BET surface area of 815.3 m 2 g -1 and a Langmuir surface area of 1177 m 2 g -1 based on the CO 2 isotherm, in excellent agreement with the previously reported value. 45 Its CO 2 isotherm shows a slight pore expansion around P / P 0 = 0.05 ( Figure 2a , inset), suggesting a mild configurational adjustment of the bpp ligands from the activated phase to an open-pore phase, consistent with the overall rigid character of NJU-Bai5 ( Figure S4 ). In contrast, both NJU-Bai5-bip and NJU-Bai5-bib exhibit negligible N 2 and Ar uptake, confirming that neither gas can induce pore reopening. As anticipated, their CO 2 adsorption isotherms reveal pronounced gate-opening behavior ( Figure 2b&2c ). NJU-Bai5-bip exhibits a saturation uptake of 178.6 cm 3 g -1 and a Langmuir surface area of 1134 m 2 g -1 , with multiple incremental steps at P / P 0 ~ 0.04, 0.10, 0.20, and 0.25. This multi-step profile suggests a gradual, pressure-dependent gate-opening process, likely arising from partial cleavage and reformation of Cu–O bonds within the tetranuclear copper cluster, combined with significant torsion of the bis-imidazole pillar ( Figure S5 ), which stabilizes a series of metastable intermediate states. In comparison, NJU-Bai5-bib shows a single steep transition at P / P 0 ~ 0.10, with a CO 2 uptake of 151.6 cm 3 g -1 and a Langmuir surface area of 914 m 2 g -1 . This sharp, one-step inflation indicates a cooperative and well-defined gate-opening mechanism. The extended pillar backbone facilitates large-amplitude rotation and interlayer sliding while maintaining the integrity of the [Cu 4 (μ 3 -OH) 2 N 4 (COO) 6 ] cluster ( Figure S6c ), thereby establishing a facile pore-reopening pathway. Another group of gases comprising strong hydrogen-bonding donor, weak hydrogen donor, and non-donor were also investigated. As a prototypical hydrogen-bonding donor, water can form robust hydrogen bonds with the LPE motifs present in both bip and bib ligands. In contrast, methane, as a weak, nonpolar, non-quadrupolar hydrogen-bonding donor, induces only feeble interactions with the LPEs. Meanwhile, SF 6 , being highly symmetric, nonpolar, non-quadrupolar, and devoid of hydrogen atoms, is expected to remain essentially inert toward the LPE motifs. Accordingly, their experimental adsorption isotherms were measured at 298 K. As shown in Figure S13a , both NJU-Bai5-bip and NJU-Bai5-bib exhibit significant water vapor uptake, with pore inflation initiating at approximately 20–50% RH, similar to NJU-Bai5, achieving saturation capacities of 195.0 cm 3 g -1 and 216.7 cm 3 g -1 , respectively. Conversely, both materials show negligible CH 4 and SF 6 uptake, as expected ( Figures S13b & S13c ). The distinct discrimination against CH 4 and SF 6 compared to water demonstrates that LPEs in these frameworks can selectively respond to hydrogen-bonding donors with different acidity. Therefore, according to the acidity of hydrogen in the donor, they have great potential to further differentiate between various hydrocarbon species. The separation of propylene and propane presents a significant challenge due to their nearly identical molecular sizes and boiling points. However, the difference in the acidity of their hydrogen atoms—vinyl versus alkyl hydrogens—offers an opportunity for discrimination by LPEs. Therefore, we further measured the propylene and propane adsorption isotherms for NJU-Bai5-bip and NJU-Bai5-bib, using NJU-Bai5 as a control. As established, NJU-Bai5 represents a rigid archetype, where the bpp pillar effectively locks the framework in a permanently open pore state. At 298 K, both C 3 H 6 and C 3 H 8 exhibit sharp uptake at low pressure, with no significant difference in their saturation capacities (52.59 cm 3 g -1 for C 3 H 6 and 43.99 cm 3 g -1 for C 3 H 8 ). At 0.5 bar, the uptake is 50.90 cm 3 g -1 for C 3 H 6 and 43.75 cm 3 g -1 for C 3 H 8 , corresponding to an adsorption selectivity of 1.16. In contrast, NJU-Bai5-bip shows negligible uptake for C 3 H 8 . For C 3 H 6 , a limited pore-opening occurs around 0.85 bar, though the uptake remains below 10 cm 3 g -1 . This indicates that C 3 H 6 , with its more acidic vinyl hydrogen, can interact with LPEs to induce a minor structural expansion at room temperature, whereas C 3 H 8 cannot. However, the framework’s responsiveness remains kinetically hindered, rendering it currently inefficient for practical. 46-47 Lowering the temperature to 273 K enhanced the pore expansion of NJU-Bai5-bip induced by C 3 H 6 , while C 3 H 8 triggered only a comparatively smaller response ( Figure S14 ). However, the C 3 H 6 uptake at 0.5 bar remained minimal, reaching only 3.9 cm 3 g -1 —far below the threshold required for practical application. Although reduced temperatures are conventionally employed to improve C 3 H 6 /C 3 H 8 separation in certain MOFs, 48-49 such conditions conflict with the requirement for ambient-temperature processing and are therefore unsuitable for NJU-Bai5-bip. NJU-Bai5-bib, as a linker-length-extending product of NJU-Bai5-bip, manifests a well-defined and highly selective guest-responsive gate-opening behavior ( Figure 2f ). C 3 H 6 adsorption inflates at low pressure range, displaying uptake capacities of 0.37, 1.86, 42.62, and 47.18 cm 3 g -1 at 0.01, 0.1, 0.5, and 1 bar, respectively. Comparably, C 3 H 8 shows negligible uptake below ~ 0.6 bar and triggers a small extent of gate-opening at higher pressure. At the pressure points of 0.01, 0.1, 0.5, and 1 bar, NJU-Bai5-bib uptakes C 3 H 8 with the values of 0.13, 0.48, 1.24, and 20.15 cm 3 g -1 . As a consequence, NJU-Bai5-bib achieves a remarkable C 3 H 6 /C 3 H 8 uptake ratio of ~ 34.36 at 0.5 bar, far surpassing that of top-outperforming adsorbents such as Co-gallate (12.78) 4 , HKUST-7(20) 6 , ZU-609 (27.80) 7 , Y-abtc (16.17) 50 , CALF-20 (1.25) & NCU-20 (22.2), 51 HIAM-301 (11.03) 8 , Y-dbai (25.70) 52 , NTU-85-WNT (12.8) 53 , UTSA-400 (34.00) 54 , lower than NTU-65-CoTi (37.05) 55 and HAF-1 (35.6) 56 ( Figure 2g )( Table S2 ). These distinct sorption behaviors demonstrate that LPEs in NJU-Bai5-bib, which function similarly to those in NJU-Bai5-bip, can discriminate C 3 H 6 from C 3 H 8 via vinyl-hydrogen recognition. Combined with the greater rotational freedom of the bib ligand compared to bip, this enables an effective and responsive gate-opening transition specifically for C 3 H 6 . In-situ Characterization for Gas-responsive Flexibility In‑situ variable‑pressure PXRD measurements ( Figure 2h&2i and Figure S15 ) were performed to directly monitor the structural evolution of these analogues during gas adsorption. Throughout the entire pressure range of both C 3 H 6 ( Figure S15a ) and C 3 H 8 ( Figure S15b ) adsorption, NJU‑Bai5 retains its structural integrity. Only minor, fully reversible peak shifts are observed; for instance, the (011) reflection shifts from 2θ ≈ 7.58° to 7.34°, and the (002) reflection moves slightly from 10.26° to 10.04°, with no emergence of new reflections. These subtle low‑angle shifts indicate a reversible elastic expansion of the framework upon gas loading, rather than a phase transition. Consistently, both C 3 H 6 and C 3 H 8 adsorption isotherms display typical Type‑I profiles with nearly identical uptake capacities across all pressures, showing no stepwise or gate‑opening features. This inherent rigidity of NJU‑Bai5 ensures exceptional structural robustness but inherently limits its molecular discrimination capability for C 3 H 6 /C 3 H 8 separation. For NJU-Bai5-bip ( Figures S15c & S15d ), the diffraction peak positions remain nearly unchanged upon progressive exposure to either C 3 H 6 or C 3 H 8 , with only a slight attenuation in peak intensity. This indicates that neither low- nor high-pressure dosing of C 3 H 6 or C 3 H 8 is sufficient to induce a transition from the closed-pore phase to an open framework. Such structural inertness aligns fully with the adsorption isotherms, in which NJU-Bai5-bip exhibits minimal C 3 H 6 uptake and virtually no adsorption of C 3 H 8 . Together, these results confirm that NJU-Bai5-bip persists in a metastable, non‑responsive state, reflecting insufficient framework flexibility to permit a guest‑induced structural transformation. In contrast, NJU-Bai5-bib—an optimized framework exhibiting a differential gate-opening response to propylene and propane—reveals distinct diffraction features associated with a transient intermediate phase between 0 and 0.2 bar ( Figure 2h ), capturing the incipient expansion from the closed-pore to the open-pore phase. This indicates that even trace amounts of C 3 H 6 can effectively initiate a structural response. Above 0.1 bar, the PXRD patterns progressively evolve from the contracted phase toward a nearly fully expanded phase, consistent with the sharp gate-opening step in the adsorption isotherm. By comparison, C 3 H 8 induces no discernible structural change until ca. 0.8 bar. Notably, multiple new diffraction peaks emerge during C 3 H 8 -induced expansion (0.6-1.0 bar, Figure 2i ), indicating coexistence of closed and open phases and a gradual, multi-step structural transition. In contrast, the C 3 H 6 -triggered transformation occurs rapidly within a narrow pressure window (0-0.1 bar), also through a mixed-phase region. This pronounced contrast unequivocally demonstrates a C 3 H 6 -selective gate-opening mechanism in NJU-Bai5-bib, where subtle host-guest interactions direct a well-defined, reversible structural transition. Such selective responsiveness fundamentally underlies its excellent C 3 H 6 /C 3 H 8 separation performance. Computational simulation and Gas-loaded Structure Analysis To elucidate the distinct responsiveness of NJU-Bai5-bib toward propylene and propane, we further investigated the transport properties of C 3 H 6 and C 3 H 8 within its framework. Specifically, diffusion along a diagonal migration pathway inside the pore channels was examined, as this trajectory reflects both the directional movement of guest molecules under confinement and the associated diffusion energy barriers. Subsequently, DFT calculations were performed to construct the corresponding potential energy profiles and quantify the energetic cost of migration along this path, thereby assessing the relative ease of gas diffusion through the pore. The migration of C 3 H 6 from position 1 to position 10 involves variable energy levels ( Figure 3a ), with an energy difference of 15.7 kJ mol -1 between the high-energy state (position-3) and the low-energy state (position-1). For C 3 H 8 , the ten corresponding positions in the same pore were also evaluated ( Figure 3b ). The energy difference between its high-energy state (position-6) and low-energy state (position-1) is 50.1 kJ mol -1 , substantially higher than that for C 3 H 6 . The significantly lower diffusion barrier for C 3 H 6 unequivocally indicates much easier mobility compared to C 3 H 8 , thereby rationalizing the instantaneous pore expansion observed in both the C 3 H 6 adsorption isotherm and the in situ variable-pressure PXRD measurements. Further evaluation of the adsorption differences between C 3 H 6 and C 3 H 8 within NJU-Bai5-bib framework was based on gas-loaded single-crystal structures ( Figures 3c&3d ). Due to the ultra-stability of NJU-Bai5-bib, both C 3 H 6 -loaded and C 3 H 8 -loaded single crystals can be successfully obtained and characterized. Upon C 3 H 6 dosing, well-defined electron densities were observed within the one-dimensional channels, precisely confined between the TCMBT linkers and the bib pillars ( Figures 3c ). Each C 3 H 6 molecule adopts a nearly coplanar orientation with the adjacent aromatic ring, forming a π···π stacking interaction of 3.79 Å, accompanied by multiple C-H···O (3.02-3.64 Å) and C-H···N (3.74-3.88 Å) hydrogen bonds, together with multiple C-H···π contact (2.95-3.94 Å) ( Figure S18a ). In contrast, the crystal structure of C 3 H 8 @NJU-Bai5-bib ( Figures 3d ) reveals a different scenario. C 3 H 8 molecule lacks π-orbitals for creating efficient π-related interactions as C 3 H 6 does, and only weak C-H···O/N contacts (2.99-3.5 Å and 3.9-3.99 Å) could be identified ( Figure S18b ). Therefore, compared with C 3 H 6 interaction, the weak interaction between C 3 H 8 and the framework can be qualitatively identified. We further employed DFT calculations to quantify the binding energies of C 3 H 6 and C 3 H 8 and probe the energetic contributions governing their adsorption preferences. The results reveal that the static binding energy of C 3 H 8 @NJU-Bai5-bib is 64.19 kJ mol -1 , whereas that of C 3 H 6 @NJU-Bai5-bib reaches 66.32 kJ mol -1 , indicating a stronger interaction between C 3 H 6 and the NJU-Bai5-bib framework. As shown in Figure s 3e&3f , multiple hydrogen-bonding interactions are formed between C 3 H 6 and the NJU-Bai5-bib, in good agreement with the C 3 H 6 -loaded single-crystal structure. The shorter C-H···π distances and the greater number of host-guest interaction sites together account for the stronger binding affinity of C 3 H 6 , underscoring the preferential stabilization of C 3 H 6 within the pore. Collectively, the crystallographic and computational analyses demonstrate that the selective adsorption of C 3 H 6 by NJU-Bai5-bib stems from a synergistic interplay of π-π and multipoint hydrogen-bonding interactions, which cooperatively stabilize the open-pore phase exclusively in the presence of propylene. The comparatively higher initial isosteric heat of adsorption for C 3 H 6 on NJU-Bai5-bib, coupled with its lower intracrystalline diffusion barrier, jointly triggers a rapid, stimuli-responsive transition to the open-pore phase. This elucidated molecular-level mechanism accounts for the exceptional separation selectivity and macroscopic reversibility observed experimentally. Water stability test Given that NJU-Bai5 exhibits ultrahigh water stability due to its saturated metal nodes and hydrophobic pore environment, NJU-Bai5-bib—which shares an isoreticular structure with NJU-Bai5, is anticipated to display comparably high hydrolytic stability. Immersion of as-synthesized NJU-Bai5-bib in water for over one month resulted in essentially unchanged PXRD patterns and CO 2 uptake ( Figure S19 ). In practical gas separation applications, moisture tolerance under long-term cyclic operation is of particular importance. Therefore, we further evaluated the water vapor adsorption capacity and cycling stability of NJU-Bai5-bib. As shown in Figure 4a , the framework exhibits a stepwise uptake with an inflation point near 30 % RH and a saturation capacity of 216.67 cm 3 g -1 . Although the saturated uptake decreased by 3.7 % (to 208.6 cm 3 g -1 ) in the second cycle, subsequent cycles exhibited no significant decline. Importantly, the porosity could be fully restored by reactivating the sample at 100 °C under vacuum for 10 h, as confirmed by the recovered CO 2 adsorption isotherm at 195 K ( Figure S20 ). Thus, NJU-Bai5-bib demonstrates both high hydrolytic stability and excellent cycling stability under water vapor, positioning it as a promising candidate for practical gas-separation applications. Breakthrough Experiment To comprehensively assess the practical separation performance of NJU-Bai5-bib in C 3 H 6 /C 3 H 8 purification, dynamic breakthrough experiments were carried out using an equimolar C 3 H 6 /C 3 H 8 (v/v, 50/50) mixture at a controlled flow rate of 0.5 mL min -1 . As shown in Figure 4b , C 3 H 8 elutes sharply at 40.2 min g -1 , whereas C 3 H 6 does not break through until 144.2 min g -1 , delivering a broad wide separation window. This performance surpasses most state-of-the-art sorbents evaluated under similar breakthrough conditions, including NTU-85-WNT (7 min g -1 ) 53 , GeFSIX-2-Cu-i (58 min g -1 ) 57 , thereby highlighting the remarkable C 3 H 6 /C 3 H 8 separation capability of NJU-Bai5-bib. Notably, even after five consecutive breakthrough cycles, the retention time remains as high as ~70 min g -1 ( Figure S21 ), indicating excellent recyclability and structural resilience of the flexible framework during repeated adsorption-desorption operation. Moreover, NJU-Bai5-bib preserves its separation performance under humid conditions (65 % and 85 % RH, Figure 4c ), consistent with its outstanding hydrolytic resistance observed in water vapor sorption studies. Such humidity tolerance is rarely achieved in flexible MOFs and strongly supports its suitability for industrial mixtures containing moisture. Considering all these findings, NJU-Bai5-bib demonstrates an exceptional combination of environmental robustness, cycling durability, and high separation efficiency, establishing its strong potential for energy-efficient C 3 H 6 /C 3 H 8 separation processes. Scalable Aqueous Synthesis NJU-Bai5-bib can be synthesized on the gram scale through a fully aqueous and environmentally benign route, affording a crystalline product whose PXRD pattern and CO 2 sorption behavior are essentially identical to those of the small-scale material ( Figure 4d ). This scalable protocol preserves both structural integrity and adsorption performance ( Figure 4e ), while delivering an exceptionally low synthetic cost of 158.3 $ kg -1 . By contrast, the practical deployment of many high-performance MOFs remains impeded by prohibitive manufacturing costs and harsh synthetic conditions. ZU-609, UTSA-400, HAF-1, and NTU-65-Co-Ti exhibit synthesis costs ranging from 389 to 19173.69 $ kg -1 and require extended solvothermal or diffusion processes lasting 12 ~ 336 hours, resulting in substantial energy, equipment, and safety demands. A radar plot ( Figure 4f ) highlights the comprehensive advantages of NJU-Bai5-bib in synthesis efficiency, large-scale cost, green synthesis, water-related stability and 0.5 bar C 3 H 6 /C 3 H 8 uptake ratio. Its aqueous preparation relies solely on inexpensive precursors—CuCl 2 ·2H 2 O, TCMBT, and bib—enabling production at ~158.3 $ kg -1 , two to three orders of magnitude lower than typical high-performance MOFs. Moreover, NJU-Bai5-bib delivers competitive C 3 H 6 /C 3 H 8 uptake ratios at 0.5 bar and maintains its CO 2 capacity after multiple water-vapor cycling tests ( Figure S20 ), underscoring its structural robustness under industrially relevant humidity. Crucially, the scalability of NJU-Bai5-bib is supported by the high-yield, low-cost availability of both key ligands. TCMBT can be prepared on a 190 g scale in 99.7 % isolated yield through a simple water-based amide coupling, while bib is obtained in 95 % yield via an efficient imidazole substitution reaction. The inexpensive and readily accessible ligand production ensures that the entire synthetic sequence remains economically viable and industrially practical, further reinforcing the promise of NJU-Bai5-bib for real-world adsorption-separation applications. Conclusion Based on a strategically evolved pillar-layered platform, this study establishes a delicate design principle for achieving highly selective olefin/paraffin separation through precise, pillar-directed modulation of framework flexibility. Beginning with the hydrolytically stable yet rigid prototype NJU-Bai5, we systematically engineered its organic pillars to introduce and finely tune guest-responsive dynamics. The pivotal advancement was achieved by substituting terminal pyridyl groups with imidazolyl units, and further facilitating the ligand rotation freedom with alkyl elongation, to get the ultimate product NJU-Bai5-bib. This modification embeds accessible nitrogen lone-pair electrons that function as specific molecular anchors, which engage in selective, cooperative interactions with propylene, while interactions with propane remain energetically insufficient. Consequently, NJU-Bai5-bib exhibits a well-defined, low-pressure gate-opening transition triggered exclusively by C 3 H 6 , translating a subtle molecular distinction into macroscopic separation performance with exceptional selectivity under ambient conditions. This evolution from rigidity to controlled flexibility delineates a clear structure-property relationship. Intermediate structures, such as NJU-Bai5-bip, provided critical insights into the instability of moderate flexibility but underscored the necessity for a fully realized dynamic response. The final material, NJU-Bai5-bib, successfully balances scalable cost, exceptional hydrolytic stability, and environment-friendly aqueous synthesis with its precise dynamic behavior. Its performance demonstrates that framework mechanics can be harnessed as a functional tool, amplifying minor differences in guest electronic structure and polarizability into effective kinetic or thermodynamic separation pathways. Declarations ASSOCIATED CONTENT Supporting Information . Additional materials synthesis and characterization data, including additional gases adsorption isotherms, powder X-ray diffraction patterns, 1 H NMR, and IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] ACKNOWLEDGMENT We thank for the financial support from the National Natural Science Foundation of China (21601047 and 22573048). References Roy A, Venna SR, Rogers G, Tang L, Fitzgibbons TC, Liu J, McCurry H, Vickery DJ, Flick D, Fish B (2021) Membranes for olefin–paraffin separation: An industrial perspective. Proc. Natl. Acad. Sci. 118 (37), e2022194118 Sadegh F, Sadegh N, Wongniramaikul W, Choodum A (2024) Recent advances in metal–organic frameworks for C 3 H 6 -and C 3 H 8 -selective separation of C 3 H 6 /C 3 H 8 binary natural gas mixtures: A review. 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Nat Chem 17(1):141–147 Wang X, Zhang P, Zhang Z, Yang L, Ding Q, Cui X, Wang J, Xing H (2020) Efficient Separation of Propene and Propane Using Anion-Pillared Metal–Organic Frameworks. Ind Eng Chem Res 59(8):3531–3537 Scheme 1 Scheme 1 is available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files image1.jpeg Scheme 1. Introducing nitrogen lone-pair electrons into a pillar-layered Cu-MOF (NJU-Bai5) by substituting pyridyl with imidazolyl groups in the pillar ligand, which enhances host–guest interactions and thereby triggers selective gas-responsive structural flexibility. SupportingInformation.docx SUPPLEMENTARY INFORMATION 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. 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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-8403378","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":570345124,"identity":"112dc739-545c-4c3d-97a4-a49641059f15","order_by":0,"name":"Zhiyong Lu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCElEQVRIiWNgGAWjYDACCcYGMM3HwHyAQQLIMAAJEqWFjYEtgUEigSgtUJqNgQeomBgt8rOb2yQ+7qhlYJPI+fbA8oeNvDkD88HbPAx2ebi0MM452CY588xxoJbc7QYSCWmGOxvYkq15GJKLcWlhlkhsk+ZtOwbSsk1CIuFwgsEBHjNpHoYDiQ04tLAhtOQ8A2r5D9TC/w2vFh6IlhqQFjaglgMgW9jwapGQSGy2nNkGVMbzzExCIi3ZcMNhNmPLOQbJOLXIz0h/eONjW50cP3vyM2kJGzt5g+PND2+8qbDDqQUIWICxcJgHEhZgEkQY4FYPUvKBgaEOzGL8gFfhKBgFo2AUjFQAAKnbS5l+zl+dAAAAAElFTkSuQmCC","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":true,"prefix":"","firstName":"Zhiyong","middleName":"","lastName":"Lu","suffix":""},{"id":570345125,"identity":"0a9e6515-844b-40a5-9a20-d656df530856","order_by":1,"name":"Yuhang Liu","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Yuhang","middleName":"","lastName":"Liu","suffix":""},{"id":570345126,"identity":"ec311372-bcb8-494d-afdb-7106dde2ddd3","order_by":2,"name":"Bufeng Wang","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Bufeng","middleName":"","lastName":"Wang","suffix":""},{"id":570345127,"identity":"c7f2b95c-b892-43f6-8a90-de1353a94ad6","order_by":3,"name":"Lilei Zhang","email":"","orcid":"","institution":"Luoyang","correspondingAuthor":false,"prefix":"","firstName":"Lilei","middleName":"","lastName":"Zhang","suffix":""},{"id":570345128,"identity":"d99fe65d-325e-423a-8bc7-4bc4dcb7cba3","order_by":4,"name":"Banghao Wei","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Banghao","middleName":"","lastName":"Wei","suffix":""},{"id":570345129,"identity":"d0a8cf27-0ebb-40f6-a20d-6626cee84ef5","order_by":5,"name":"Yingpeng Jiang","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Yingpeng","middleName":"","lastName":"Jiang","suffix":""},{"id":570345130,"identity":"cbf16a30-3c44-4c4a-8c2a-25e52934af6e","order_by":6,"name":"Junfeng Bai","email":"","orcid":"","institution":"Nanjing Tech University","correspondingAuthor":false,"prefix":"","firstName":"Junfeng","middleName":"","lastName":"Bai","suffix":""}],"badges":[],"createdAt":"2025-12-19 10:06:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8403378/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8403378/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":100756629,"identity":"9e96fed5-8b95-4a2e-9fad-03e3487edf36","added_by":"auto","created_at":"2026-01-21 06:38:01","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1326094,"visible":true,"origin":"","legend":"\u003cp\u003ePhase transformation of NJU-Bai5 analogues from the as-synthesized to the activated phase. a)-b) Crystal structure transition of NJU-Bai5 from the as-synthesized open phase (left) to the solvent-removed activated phase (right), showing the pillar ligand configuration change. c)-d) Corresponding transition of NJU-Bai5-bip from the open to the closed phase upon solvent removal. e)-f) Corresponding transition of NJU-Bai5-bib from the open to the closed phase upon solvent removal. g) Schematic diagram of the rotational freedom for the bpp, bip, and bib ligands. h) Rotational energy profiles of the bpp, bip, and bib ligands at different dihedral angles based on DFT calculations.\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/e03586ce6d0ca765c75f6ee2.jpeg"},{"id":100756663,"identity":"66bf0052-a7c8-486a-8739-e5e1afd68648","added_by":"auto","created_at":"2026-01-21 06:38:17","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":908247,"visible":true,"origin":"","legend":"\u003cp\u003ea)-c) N\u003csub\u003e2\u003c/sub\u003e (77 K), Ar (87K) and CO\u003csub\u003e2\u003c/sub\u003e (195 K) isotherms for NJU-Bai5, NJU-Bai5-bip, and NJU-Bai5-bib, respectively; d)-f) C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e isotherms at 298 K for NJU-Bai5, NJU-Bai5-bip, and NJU-Bai5-bib, respectively; g) Comparison of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e uptake ratios at 0.5 bar with typical MOFs; h)-i) \u003cem\u003eIn-situ\u003c/em\u003e variable-pressure PXRD patterns based on adsorption isotherm plots for NJU-Bai5-bib under C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e atmospheres, respectively.\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/67aef651cabaa3a363b81932.jpeg"},{"id":100756623,"identity":"ea1bd18e-9786-4300-9e20-a049b0e3b05c","added_by":"auto","created_at":"2026-01-21 06:37:51","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":949724,"visible":true,"origin":"","legend":"\u003cp\u003eDiffusion energy barriers evaluated by ten different positions of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e a) and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e b) within the channels of NJU-Bai5-bib, and illustrated C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e@NJU-Bai5-bib and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e@NJU-Bai5-bib structures are those positions with the highest and lowest energy; c) Crystal structure of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e@NJU-Bai5-bib; d) Crystal structure of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e@NJU-Bai5-bib; e)-f) DFT calculated binding configurations of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e in NJU-Bai5-bib, respectively.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/755c051ec11557f363144e99.jpeg"},{"id":100756627,"identity":"c86b92c9-ec35-427c-a204-aab5ea4d4b46","added_by":"auto","created_at":"2026-01-21 06:38:00","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":560090,"visible":true,"origin":"","legend":"\u003cp\u003ea) Water vapor adsorption-desorption isotherms of NJU-Bai5-bib for five cycles at 298 K; b) C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e (50:50 v/v) breakthrough curves at 298 K and 1 bar; c) Breakthrough curves measured after five recycling, at 65% and 85% relative humidity, and after subsequent recycling following humidity tests. d) Comparison of PXRD patterns for large-scale aqueous-synthesized NJU-Bai5-bib sample with small-scale solvo-thermal synthesized sample, inset: photograph of large-scale synthesis; e) Comparison of CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms at 195 K for large-scale aqueous-synthesized NJU-Bai5-bib sample with small-scale solvo-thermal synthesized sample; f) A radar chart comparing the comprehensive performance of NJU-Bai5-bib to that of various representative C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e adsorbents.\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/892086c21a2c385c57726ed8.jpeg"},{"id":100756857,"identity":"a4e9e1a1-8068-47a4-ab21-671359b97a7a","added_by":"auto","created_at":"2026-01-21 06:41:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4633559,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/c8222f58-e2e7-48a7-8f8c-7253a4433b64.pdf"},{"id":100756619,"identity":"4f602d2b-edb0-46c1-8f19-cdd3875072dd","added_by":"auto","created_at":"2026-01-21 06:37:45","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1969676,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1.\u003c/strong\u003e Introducing nitrogen lone-pair electrons into a pillar-layered Cu-MOF (NJU-Bai5) by substituting pyridyl with imidazolyl groups in the pillar ligand, which enhances host–guest interactions and thereby triggers selective gas-responsive structural flexibility.\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/f68029ff150a1443dd43ec50.jpeg"},{"id":100756628,"identity":"8e90781c-cd1f-4fad-b641-e14203cbf161","added_by":"auto","created_at":"2026-01-21 06:38:01","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":14960736,"visible":true,"origin":"","legend":"SUPPLEMENTARY INFORMATION","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8403378/v1/e5e91d7c56696ee8173a0b90.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Lone-pair-electron Governed Gas-recognizable Flexibility in an Ultra-stable MOF for Boosting C3H6/C3H8 Separation and Its Aqueous Scalable Synthesis","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003ePropylene is a pivotal chemical intermediate, essential for manufacturing plastics and materials.\u003csup\u003e1-2\u003c/sup\u003e Its global demand is soaring, with over 60% consumed by polypropylene production, which requires ultra-pure (99.5%) propylene. However, propylene is predominantly co-produced with propane, and their separation is a major industrial challenge, owing to their nearly indistinguishable molecular sizes (difference \u0026lt; 0.5 \u0026Aring;) and boiling points (226 K vs 231 K).\u003csup\u003e3\u003c/sup\u003e The conventional method, cryogenic distillation, is exceptionally energy-intensive. It requires columns with over 200 trays and operates under conditions with very low relative volatility, leading to massive energy consumption for heating and cooling.\u003csup\u003e4-6\u003c/sup\u003e In fact, olefin/alkane separations contribute substantially to the energy footprint of petrochemical operations. Therefore, the development of adsorptive separation materials capable of discriminating between C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e at ambient temperatures and moderate pressures is of both scientific and industrial urgency.\u003csup\u003e6-9\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003eMetal-organic frameworks (MOFs), as one of the most promising classes of porous materials, have shown great advantage in gas separation, owing to their modular building blocks towards adaptive pore geometries and surface properties for target gases.\u003csup\u003e10-19\u003c/sup\u003e The good controllability of MOFs in isoreticular synthesis as well as functionalization, especially for rigid structures, \u0026nbsp;have made them well-explored in olefin/alkane separation, via the strategies of unsaturated metal sites construction, \u0026pi;-electron ligands, or polar functional groups to preferentially bind olefins.\u003csup\u003e20-24\u003c/sup\u003e For MOFs constructed by flexible linkers, structural changes (e.g., gating, breathing, or phase transitions) in response to guest molecules or external stimuli were ingeniously exploited in kinetic gas separation.\u003csup\u003e25-28\u003c/sup\u003e In such systems, the framework resides in a closed or compressed conformation in the absence of guests or at low pressure, but expands or opens in response to a specific guest molecule, thereby enabling selective uptake.\u003csup\u003e29-30\u003c/sup\u003e This dynamic adaptability offers a route to amplify subtle molecular differences (size, electronic structure, polarizability) into measurable separation performance.\u003csup\u003e31-36\u003c/sup\u003e Nevertheless, realizing precise, robust, and selective flexible behavior in MOFs for olefin/alkane separation is still nontrivial. A central challenge lies in the precise modulation of framework flexibility: excessive rigidity can impede structural responsiveness, while excessive compliance may induce structural collapse or irreversible phase transitions accompanied by hysteresis.\u003csup\u003e25, 37-40\u003c/sup\u003e An additional process-related concern arises when the partial pressure of the target gas falls below the gate-opening threshold, which can compromise the purity of the resulting product.\u003csup\u003e29, 41\u003c/sup\u003e Consequently, an optimal framework requires a balanced integration of binding affinity, mechanical deformability, and kinetic accessibility.\u003csup\u003e42-44\u003c/sup\u003e \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOver a decade ago, we reported the evolution process of a pillar-layered Cu-MOF, NJU-Bai5, which was one of the earliest MOFs showing extremely high hydrolytic stability. This work revealed how a simple two-dimensional layer was progressively transformed into an advanced stable three-dimensional porous structure for CO\u003csub\u003e2\u003c/sub\u003e capture through connecting the terminal-anchoring pyridine by alklyl into bis-connected pillars. Taking its advantage of high hydrolytic stability, a critical pre-requisite for the practical application of MOFs in current stage, a further evolution of NJU-Bai5 to NJU-Bai5-bib was illustrated in this contribution (\u003cstrong\u003eScheme 1\u003c/strong\u003e). By judicious engineering of pillar ligands without altering the underlying topology of NJU-Bai5\u0026mdash;particularly substituting pyridyl with imidazolyl\u0026mdash;the uncoordinated nitrogen atoms of imidazolyl can function as anchors with lone pair electrons (LPE) that could either interact with hydrogen-bonding donors or molecules possessing positive charge center, for further leading to the contraction/expansion of the framework. Such lone-pair-nitrogen-triggered flexibility combining alkly extension establishing a continuum from original rigid to highly dynamic and enabling molecular recognition events to be translated into macroscopic separations.\u0026nbsp;The ultimate conformation of evolution, NJU-Bai5-bib enables a well-defined gate-opening transition that is triggered preferentially by C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e at low pressure. A combination of crystallographic, gas adsorption, and computational analyses demonstrates that \u0026pi;-\u0026pi; and multiple C-H\u0026middot;\u0026middot;\u0026middot;N/O interactions stabilize C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e in the expanded pore, whereas C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e binding is comparably weaker. The evolution from NJU-Bai5 to NJU-Bai5-bip and ultimately NJU-Bai5-bib delineates a pillar-directed flexibility gradient, providing a robust yet precisely guest-responsive platform for demanding separations. Inheriting from NJU-Bai5, NJU-Bai5-bib constructed in a same motif shows high water stability, and it can be efficiently synthesized at a large scale in aqueous solution. The above outstanding features of NJU-Bai5-bib make it not only an excellent laboratory-scale material for propylene purification but also a promising candidate for potential industrial applications.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cp\u003e\u003cem\u003eStructural Evolution of NJU-Bai5 from Rigid to Flexible\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eIn our previous study, NJU-Bai5 was a final product obtained by the evolution process from a 2D-layered structure to a 3D pillar-layered water stable Cu-MOF. It is assembled by Cu(II) and TCMBT/bpp mixed linkers with typical pillar-layered structure showing permanent porosity (estimated structural porosity 33.7%). Tetranuclear [Cu\u003csub\u003e4\u003c/sub\u003e(\u003cem\u003e\u0026mu;\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e-OH)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e(COO)\u003csub\u003e6\u003c/sub\u003e] cluster connected with six TCMBT linkers to form two-dimensional layers. Between neighbouring layers, bpp linkers act as pillars connecting to Cu\u003csub\u003e4\u003c/sub\u003e nodes from both sides to support layers and create nano-space. Due to the V-shaped configuration of bpp in the crystal structure, only the microporous channels along \u003cem\u003ea\u003c/em\u003e axis with a diameter of 0.6 \u0026times; 0.24 nm exist between layers (\u003cstrong\u003eFigure 1a\u003c/strong\u003e). Upon solvent-removal by thermo-vacuo treatment, it preserves a permanently open phase (\u003cstrong\u003eFigures 1b\u003c/strong\u003e).\u0026nbsp;Single-crystal\u0026nbsp;structure\u0026nbsp;analysis of activated NJU-Bai5\u0026nbsp;reveal only subtle dihedral adjustments of the bpp pillar (from 106.4\u0026deg; to\u0026nbsp;99.9\u0026deg; and 127.7\u0026deg;) and minor variations\u0026nbsp;of the\u0026nbsp;coordinated\u0026nbsp;N\u0026middot;\u0026middot;\u0026middot;N\u0026nbsp;distance\u0026nbsp;(from 9.64 \u0026Aring; to 9.72 and 9.14 \u0026Aring;), without appreciable distortion or secondary building unit (SBU) reorganization (\u003cstrong\u003eFigures S\u003c/strong\u003e\u003cstrong\u003e4c\u003c/strong\u003e). Consequently, the framework maintains its open-pore configuration with negligible contraction (porosity 30.3%), demonstrating that the bpp pillar possessing limited rotation freedom to induce a structural contraction. Substituting the pyridyl unit with a bis-imidazole pillar generates NJU-Bai5-bip (porosity 26.2%), which preserves the iso-reticular pillar-layered structure yet acquires distinct flexibility. Guest removal induces obvious contraction (porosity 13%) accompanied by pronounced pillar torsion (N\u0026middot;\u0026middot;\u0026middot;N distances reduced from 8.47 \u0026Aring; to 6.02 and 6.64 \u0026Aring;) and near-planar imidazole dihedrals (from 53.4\u0026deg; to 8.2\u0026deg; and 6.5\u0026deg;; \u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;1c\u0026amp;1d\u003c/strong\u003eand\u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;S5\u003c/strong\u003e).\u0026nbsp;These\u0026nbsp;configuration changes of bip ligands created high stress to the [Cu\u003csub\u003e4\u003c/sub\u003e(\u003cem\u003e\u0026mu;\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e-OH)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e(COO)\u003csub\u003e6\u003c/sub\u003e]\u0026nbsp;clusters, causing\u0026nbsp;them\u0026nbsp;twisting with concomitant Cu-O bond cleavage and reformation (\u003cstrong\u003eFigure S\u003c/strong\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003cstrong\u003ec\u003c/strong\u003e), which would surely push up the energy barrier for subsequent gas-induced pore-expansion.\u0026nbsp;Due to the contracted phase it adopts after solvent removal, the accessible volume\u0026nbsp;of NJU-Bai5-bip\u0026nbsp;is significantly reduced yet not fully\u0026nbsp;eliminated.\u0026nbsp;Further elongating the\u0026nbsp;bis-imidazole pillar\u0026nbsp;with an extra carbon increasing the degree of rotation freedom would expect an release of the stress towards metal clusters, and decline the energy barrier for gas-induced pore-expansion.\u0026nbsp;The resultant analogue NJU-Bai5-bib (porosity 27.7%) undergoes complete structural\u0026nbsp;contraction\u0026nbsp;upon activation, yielding a closed-pore\u0026nbsp;phase\u0026nbsp;(porosity 7.2%) while preserving the parent topology. The elongated pillar backbone allows large-amplitude rotations (dihedral from\u0026nbsp;126.6\u0026deg; to 84.3\u0026deg;; N\u0026middot;\u0026middot;\u0026middot;N distance from 8.70 \u0026Aring; to 6.71 \u0026Aring;), coupled with TCMBT linker contraction and interlayer sliding that shortens diagonal Cu\u0026middot;\u0026middot;\u0026middot;Cu distances (from 16.76 to 13.49 \u0026Aring;) and reduces layer torsion (from 97.3\u0026deg; to 66.9\u0026deg;) (\u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;1e\u0026amp;1f\u0026nbsp;\u003c/strong\u003eand\u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;S6\u003c/strong\u003e). These cooperative deformations contributed to a more contracted phase without obvious sacrifice of the bondings in metal clusters, thus ensure its easier structural responsiveness under external stimuli.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eVariable-temperature PXRD (VT-PXRD) under vacuum further confirmed the enhanced structural dynamics from the originally rigid NJU-Bai5 to the highly dynamic NJU-Bai5-bib. As shown in \u003cstrong\u003eFigure S10\u003c/strong\u003e, vacuum induces a slight shift of the (1-10) peak to a higher angle, which subsequently reverts upon heating, indicating a minor lattice adjustment triggered by guest removal followed by thermal stress release (\u003cstrong\u003eFigure S10a\u003c/strong\u003e). For NJU-Bai5-bip, a stepwise lattice contraction is observed, with vacuum directly inducing the initial structural transformation and elevated temperature driving a second contraction step (\u003cstrong\u003eFigure S10b\u003c/strong\u003e). In contrast, NJU-Bai5-bib undergoes a complete transition from an open to a closed phase under vacuum alone; further temperature increase produces no significant peak changes (\u003cstrong\u003eFigure S10c\u003c/strong\u003e). These distinct responses confirm that: 1) none pronounced contraction is triggered by vacuum or heating in NJU-Bai5; 2) the contraction in both NJU-Bai5-bip and NJU-Bai5-bib is initially induced by solvent removal; 3) the contraction in NJU-Bai5-bip is comparatively more hindered than in NJU-Bai5-bib, requiring additional thermal energy to complete the phase transition. These observations align well with the expected behavior summarized in \u003cstrong\u003eScheme 1\u003c/strong\u003e: 1) methanol molecules interact weakly with bpp, insufficient to induce its configurational change; 2) strong solvent interactions with LPEs in either bip or bib can trigger configurational change in both linkers, though the extended carbon chain in bib facilitates this process.\u003c/p\u003e\n\u003cp\u003eBesides the different interaction with solvent molecules, the rotation freedom of these pillars was also a key point for framework contraction (\u003cstrong\u003eFigure 1g\u003c/strong\u003e), which was further confirmed by theoretical calculations. At a molecular level, \u003cstrong\u003eFigure 1h\u0026nbsp;\u003c/strong\u003esystematically illustrates the conformational variability and rotational energy barriers of the three pillar ligands (bpp, bip, and bib), providing direct insight into the intrinsic origin of framework flexibility in this MOF series. Potential energy surface (PES) scan based on\u0026nbsp;density functional theory\u0026nbsp;(DFT) calculations reveals pronounced differences in conformational stability as the torsion angle increases (\u003cstrong\u003eFigure S11\u003c/strong\u003e). Specifically, the pyridyl ligand bpp exhibits a high deformation barrier during rotation, suggesting that its conformational transformation is strongly constrained, thereby suppressing large-amplitude molecular torsion. In contrast, bis-imidazole ligand bip shows a substantially reduced rotational barrier, although the energy rises monotonically with increasing torsion angle. This behavior indicates that, despite the higher rotation freedom grafting by imidazole rings, the relatively short bridge still imposes significant steric and torsional strain between the two imidazole rings. Upon further extension of the bridging chain, the bib ligand displays the lowest energy barrier, with a Gaussian-distribution-shaped energy profile characterized by a gentle energy variation at small torsion angles followed by a marked decrease at larger angles. This indicates that the system maintains a low energetic penalty across a broad conformational range, reflecting its intrinsic flexibility and minimal deformation barrier.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eGas-responsive Flexibility of NJU-Bai5 Analogues\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNotably, pore contraction in both NJU-Bai5-bip and NJU-Bai5-bib is induced by the removal of polar solvent molecules such as MeOH. Subsequent investigations focused on the ability of these LPEs to recognize different gas molecules to reopen their pores. Several conventional gases\u0026mdash;N\u003csub\u003e2\u003c/sub\u003e, Ar, and CO\u003csub\u003e2\u003c/sub\u003e\u0026mdash;were selected for porosity characterization. Both N\u003csub\u003e2\u003c/sub\u003e and Ar are nonpolar, nonquadrupolar molecules, whereas CO\u003csub\u003e2\u003c/sub\u003e possesses a quadrupole moment with a positively charged carbon center. Consequently, CO\u003csub\u003e2\u003c/sub\u003e is expected to exhibit stronger interactions with the bip and bib ligands than Ar or N\u003csub\u003e2\u003c/sub\u003e, thereby triggering pore expansion in the contracted phases of NJU-Bai5-bip and NJU-Bai5-bib. To experimentally confirm this gas-selective behavior, adsorption measurements were conducted using N\u003csub\u003e2\u003c/sub\u003e (77 K), Ar (87 K), and CO\u003csub\u003e2\u003c/sub\u003e (195 K). The rigid pristine framework NJU-Bai5 displays a Type-I isotherm for all gases, indicative of a permanent microporous structure. Saturated uptakes reach 206.2 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for N\u003csub\u003e2\u003c/sub\u003e,\u0026nbsp;208.4 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for Ar, and 199.0 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for CO\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eFigure 2a\u003c/strong\u003e), yielding a BET surface area of 815.3 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e and a Langmuir surface area of 1177 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e based on the CO\u003csub\u003e2\u003c/sub\u003e isotherm, in excellent agreement with the previously reported value.\u003csup\u003e45\u003c/sup\u003e Its CO\u003csub\u003e2\u003c/sub\u003e isotherm shows a slight pore expansion around \u003cem\u003eP\u003c/em\u003e/\u003cem\u003eP\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e = 0.05 (\u003cstrong\u003eFigure 2a\u003c/strong\u003e, inset), suggesting a mild configurational adjustment of the bpp ligands from the activated phase to an open-pore phase, consistent with the overall rigid character of NJU-Bai5 (\u003cstrong\u003eFigure S4\u003c/strong\u003e). In contrast, both NJU-Bai5-bip and NJU-Bai5-bib exhibit negligible N\u003csub\u003e2\u003c/sub\u003e and Ar uptake, confirming that neither gas can induce pore reopening. As anticipated, their CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms reveal pronounced gate-opening behavior (\u003cstrong\u003eFigure 2b\u0026amp;2c\u003c/strong\u003e). NJU-Bai5-bip exhibits a saturation uptake of 178.6 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e and a Langmuir surface area of 1134 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e, with multiple incremental steps at \u003cem\u003eP\u003c/em\u003e/\u003cem\u003eP\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e ~ 0.04, 0.10, 0.20, and 0.25. This multi-step profile suggests a gradual, pressure-dependent gate-opening process, likely arising from partial cleavage and reformation of Cu\u0026ndash;O bonds within the tetranuclear copper cluster, combined with significant torsion of the bis-imidazole pillar (\u003cstrong\u003eFigure S5\u003c/strong\u003e), which stabilizes a series of metastable intermediate states. In comparison, NJU-Bai5-bib shows a single steep transition at \u003cem\u003eP\u003c/em\u003e/\u003cem\u003eP\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e ~ 0.10, with a CO\u003csub\u003e2\u003c/sub\u003e uptake of 151.6 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e and a Langmuir surface area of 914 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e. This sharp, one-step inflation indicates a cooperative and well-defined gate-opening mechanism. The extended pillar backbone facilitates large-amplitude rotation and interlayer sliding while maintaining the integrity of the [Cu\u003csub\u003e4\u003c/sub\u003e(\u0026mu;\u003csub\u003e3\u003c/sub\u003e-OH)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e(COO)\u003csub\u003e6\u003c/sub\u003e] cluster (\u003cstrong\u003eFigure S6c\u003c/strong\u003e), thereby establishing a facile pore-reopening pathway.\u003c/p\u003e\n\u003cp\u003eAnother group of gases comprising strong hydrogen-bonding donor, weak hydrogen donor, and non-donor were also investigated. As a prototypical hydrogen-bonding donor, water can form robust hydrogen bonds with the LPE motifs present in both bip and bib ligands. In contrast, methane, as a weak, nonpolar, non-quadrupolar hydrogen-bonding donor, induces only feeble interactions with the LPEs. Meanwhile, SF\u003csub\u003e6\u003c/sub\u003e, being highly symmetric, nonpolar, non-quadrupolar, and devoid of hydrogen atoms, is expected to remain essentially inert toward the LPE motifs. Accordingly, their experimental adsorption isotherms were measured at 298 K. As shown in \u003cstrong\u003eFigure S13a\u003c/strong\u003e, both NJU-Bai5-bip and NJU-Bai5-bib exhibit significant water vapor uptake, with pore inflation initiating at approximately 20\u0026ndash;50% RH, similar to NJU-Bai5, achieving saturation capacities of 195.0 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e and 216.7 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e, respectively. Conversely, both materials show negligible CH\u003csub\u003e4\u003c/sub\u003e and SF\u003csub\u003e6\u003c/sub\u003e uptake, as expected (\u003cstrong\u003eFigures S13b\u003c/strong\u003e\u0026amp;\u003cstrong\u003eS13c\u003c/strong\u003e). The distinct discrimination against CH\u003csub\u003e4\u003c/sub\u003e and SF\u003csub\u003e6\u003c/sub\u003e compared to water demonstrates that LPEs in these frameworks can selectively respond to hydrogen-bonding donors with different acidity. Therefore, according to the acidity of hydrogen in the donor, they have great potential to further differentiate between various hydrocarbon species.\u003c/p\u003e\n\u003cp\u003eThe separation of propylene and propane presents a significant challenge due to their nearly identical molecular sizes and boiling points. However, the difference in the acidity of their hydrogen atoms\u0026mdash;vinyl versus alkyl hydrogens\u0026mdash;offers an opportunity for discrimination by LPEs. Therefore, we further measured the propylene and propane adsorption isotherms for NJU-Bai5-bip and NJU-Bai5-bib, using NJU-Bai5 as a control. As established, NJU-Bai5 represents a rigid archetype, where the bpp pillar effectively locks the framework in a permanently open pore state. At 298 K, both C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u0026nbsp;\u003c/sub\u003eand C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e exhibit sharp uptake at low pressure, with no significant difference in their saturation capacities (52.59 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and 43.99 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e). At 0.5 bar, the uptake is 50.90 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and 43.75 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e, corresponding to an adsorption selectivity of 1.16. In contrast, NJU-Bai5-bip shows negligible uptake for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e. For C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, a limited pore-opening occurs around 0.85 bar, though the uptake remains below 10 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e. This indicates that C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, with its more acidic vinyl hydrogen, can interact with LPEs to induce a minor structural expansion at room temperature, whereas C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e cannot. However, the framework\u0026rsquo;s responsiveness remains kinetically hindered, rendering it currently inefficient for practical.\u003csup\u003e46-47\u003c/sup\u003e Lowering the temperature to 273 K enhanced the pore expansion of NJU-Bai5-bip induced by C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, while C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e triggered only a comparatively smaller response (\u003cstrong\u003eFigure S14\u003c/strong\u003e). However, the C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e uptake at 0.5 bar remained minimal, reaching only 3.9 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e\u0026mdash;far below the threshold required for practical application. Although reduced temperatures are conventionally employed to improve C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e separation in certain MOFs,\u003csup\u003e48-49\u003c/sup\u003e such conditions conflict with the requirement for ambient-temperature processing and are therefore unsuitable for NJU-Bai5-bip. NJU-Bai5-bib, as a linker-length-extending product of NJU-Bai5-bip, manifests a well-defined and highly selective guest-responsive gate-opening behavior (\u003cstrong\u003eFigure 2f\u003c/strong\u003e).\u0026nbsp;C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e adsorption inflates at low pressure range, displaying uptake capacities of 0.37, 1.86, 42.62, and 47.18 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e at 0.01, 0.1, 0.5, and 1 bar, respectively. Comparably, C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u0026nbsp;\u003c/sub\u003eshows negligible uptake below ~ 0.6 bar\u0026nbsp;and triggers a small extent of gate-opening at higher pressure. At the pressure points of 0.01, 0.1, 0.5, and 1 bar, NJU-Bai5-bib uptakes C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u0026nbsp;\u003c/sub\u003ewith the values of 0.13, 0.48, 1.24, and 20.15 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e. As a consequence, NJU-Bai5-bib achieves a remarkable C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e uptake ratio of ~ 34.36 at 0.5 bar, far surpassing that of top-outperforming adsorbents such as Co-gallate (12.78)\u003csup\u003e4\u003c/sup\u003e, HKUST-7(20)\u003csup\u003e6\u003c/sup\u003e, ZU-609 (27.80)\u003csup\u003e7\u003c/sup\u003e, Y-abtc (16.17)\u003csup\u003e50\u003c/sup\u003e, CALF-20 (1.25) \u0026amp; NCU-20 (22.2),\u003csup\u003e51\u003c/sup\u003e HIAM-301 (11.03)\u003csup\u003e8\u003c/sup\u003e, Y-dbai (25.70)\u003csup\u003e52\u003c/sup\u003e, NTU-85-WNT (12.8)\u003csup\u003e53\u003c/sup\u003e, UTSA-400 (34.00)\u003csup\u003e54\u003c/sup\u003e, lower than NTU-65-CoTi (37.05)\u003csup\u003e55\u003c/sup\u003e and HAF-1 (35.6)\u003csup\u003e56\u003c/sup\u003e (\u003cstrong\u003eFigure 2g\u003c/strong\u003e)(\u003cstrong\u003eTable S2\u003c/strong\u003e). These distinct sorption behaviors demonstrate that LPEs in NJU-Bai5-bib, which function similarly to those in NJU-Bai5-bip, can discriminate C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e from C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e via vinyl-hydrogen recognition. Combined with the greater rotational freedom of the bib ligand compared to bip, this enables an effective and responsive gate-opening transition specifically for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eIn-situ Characterization for Gas-responsive Flexibility\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eIn‑situ\u003c/em\u003e variable‑pressure PXRD measurements (\u003cstrong\u003eFigure 2h\u0026amp;2i\u003c/strong\u003e and \u003cstrong\u003eFigure S15\u003c/strong\u003e) were performed to directly monitor the structural evolution of these analogues during gas adsorption. Throughout the entire pressure range of both C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e (\u003cstrong\u003eFigure S15a\u003c/strong\u003e) and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e (\u003cstrong\u003eFigure S15b\u003c/strong\u003e) adsorption, NJU‑Bai5 retains its structural integrity. Only minor, fully reversible peak shifts are observed; for instance, the (011) reflection shifts from 2\u0026theta; \u0026asymp; 7.58\u0026deg; to 7.34\u0026deg;, and the (002) reflection moves slightly from 10.26\u0026deg; to 10.04\u0026deg;, with no emergence of new reflections. These subtle low‑angle shifts indicate a reversible elastic expansion of the framework upon gas loading, rather than a phase transition. Consistently, both C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u0026nbsp;\u003c/sub\u003eadsorption isotherms display typical Type‑I profiles with nearly identical uptake capacities across all pressures, showing no stepwise or gate‑opening features. This inherent rigidity of NJU‑Bai5 ensures exceptional structural robustness but inherently limits its molecular discrimination capability for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e separation.\u003c/p\u003e\n\u003cp\u003eFor NJU-Bai5-bip (\u003cstrong\u003eFigures S15c\u003c/strong\u003e\u0026amp;\u003cstrong\u003eS15d\u003c/strong\u003e), the diffraction peak positions remain nearly unchanged upon progressive exposure to either C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e or C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e, with only a slight attenuation in peak intensity. This indicates that neither low- nor high-pressure dosing of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e or C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e is sufficient to induce a transition from the closed-pore phase to an open framework. Such structural inertness aligns fully with the adsorption isotherms, in which NJU-Bai5-bip exhibits minimal C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e uptake and virtually no adsorption of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e. Together, these results confirm that NJU-Bai5-bip persists in a metastable, non‑responsive state, reflecting insufficient framework flexibility to permit a guest‑induced structural transformation.\u003c/p\u003e\n\u003cp\u003eIn contrast, NJU-Bai5-bib\u0026mdash;an optimized framework exhibiting a differential gate-opening response to propylene and propane\u0026mdash;reveals distinct diffraction features associated with a transient intermediate phase between 0 and 0.2 bar (\u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e2h\u003c/strong\u003e), capturing the incipient expansion from the closed-pore to the open-pore phase. This indicates that even trace amounts of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e can effectively initiate a structural response. Above 0.1 bar, the PXRD patterns progressively evolve from the contracted phase toward a nearly fully expanded phase, consistent with the sharp gate-opening step in the adsorption isotherm. By comparison, C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e induces no discernible structural change until ca. 0.8 bar. Notably, multiple new diffraction peaks emerge during C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e-induced expansion (0.6-1.0 bar, \u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e2i\u003c/strong\u003e), indicating coexistence of closed and open phases and a gradual, multi-step structural transition. In contrast, the C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e-triggered transformation occurs rapidly within a narrow pressure window (0-0.1 bar), also through a mixed-phase region. This pronounced contrast unequivocally demonstrates a C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e-selective gate-opening mechanism in NJU-Bai5-bib, where subtle host-guest interactions direct a well-defined, reversible structural transition. Such selective responsiveness fundamentally underlies its excellent C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u0026nbsp;\u003c/sub\u003eseparation performance.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eComputational simulation and Gas-loaded Structure Analysis\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTo elucidate the distinct responsiveness of NJU-Bai5-bib toward propylene and propane, we further investigated the transport properties of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e within its framework. Specifically, diffusion along a diagonal migration pathway inside the pore channels was examined, as this trajectory reflects both the directional movement of guest molecules under confinement and the associated diffusion energy barriers. Subsequently, DFT calculations were performed to construct the corresponding potential energy profiles and quantify the energetic cost of migration along this path, thereby assessing the relative ease of gas diffusion through the pore. The migration of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e from position 1 to position 10 involves variable energy levels (\u003cstrong\u003eFigure 3a\u003c/strong\u003e), with an energy difference of 15.7 kJ mol\u003csup\u003e-1\u003c/sup\u003e between the high-energy state (position-3) and the low-energy state (position-1). For C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e, the ten corresponding positions in the same pore were also evaluated (\u003cstrong\u003eFigure 3b\u003c/strong\u003e). The energy difference between its high-energy state (position-6) and low-energy state (position-1) is 50.1 kJ mol\u003csup\u003e-1\u003c/sup\u003e, substantially higher than that for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e. The significantly lower diffusion barrier for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e unequivocally indicates much easier mobility compared to C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e, thereby rationalizing the instantaneous pore expansion observed in both the C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e adsorption isotherm and the \u003cem\u003ein situ\u003c/em\u003e variable-pressure PXRD measurements.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurther evaluation of the adsorption differences between\u0026nbsp;C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e within NJU-Bai5-bib framework was based on gas-loaded single-crystal structures (\u003cstrong\u003eFigures\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e3c\u0026amp;3d\u003c/strong\u003e). Due to the ultra-stability of NJU-Bai5-bib, both C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e-loaded and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e-loaded single crystals can be successfully obtained and characterized. Upon C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e dosing, well-defined electron densities were observed within the one-dimensional channels, precisely confined between the TCMBT linkers and the bib pillars (\u003cstrong\u003eFigures\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e3c\u003c/strong\u003e). Each C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e molecule adopts a nearly coplanar orientation with the adjacent aromatic ring, forming a \u0026pi;\u0026middot;\u0026middot;\u0026middot;\u0026pi; stacking interaction of 3.79 \u0026Aring;, accompanied by multiple C-H\u0026middot;\u0026middot;\u0026middot;O (3.02-3.64 \u0026Aring;) and C-H\u0026middot;\u0026middot;\u0026middot;N (3.74-3.88 \u0026Aring;) hydrogen bonds, together with multiple C-H\u0026middot;\u0026middot;\u0026middot;\u0026pi; contact (2.95-3.94 \u0026Aring;) (\u003cstrong\u003eFigure S18a\u003c/strong\u003e). In contrast, the crystal structure of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e@NJU-Bai5-bib (\u003cstrong\u003eFigures\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;3d\u003c/strong\u003e) reveals a different scenario. C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e molecule lacks \u0026pi;-orbitals for creating efficient \u0026pi;-related interactions as C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003edoes, and only weak C-H\u0026middot;\u0026middot;\u0026middot;O/N contacts (2.99-3.5 \u0026Aring; and 3.9-3.99 \u0026Aring;) could be identified (\u003cstrong\u003eFigure S18b\u003c/strong\u003e). Therefore, compared with C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e interaction, the weak interaction between C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e and the framework can be qualitatively identified. We further employed DFT calculations to quantify the binding energies of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e and probe the energetic contributions governing their adsorption preferences. The results reveal that the static binding energy of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e@NJU-Bai5-bib is 64.19 kJ mol\u003csup\u003e-1\u003c/sup\u003e, whereas that of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e@NJU-Bai5-bib reaches 66.32 kJ mol\u003csup\u003e-1\u003c/sup\u003e, indicating a stronger interaction between C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and the NJU-Bai5-bib framework. As shown in \u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003es\u003c/strong\u003e\u003cstrong\u003e3e\u0026amp;3f\u003c/strong\u003e, multiple hydrogen-bonding interactions are formed between C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e and the NJU-Bai5-bib, in good agreement with the C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e-loaded single-crystal structure. The shorter C-H\u0026middot;\u0026middot;\u0026middot;\u0026pi; distances and the greater number of host-guest interaction sites together account for the stronger binding affinity of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, underscoring the preferential stabilization of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e within the pore. Collectively, the crystallographic and computational analyses demonstrate that the selective adsorption of C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e by NJU-Bai5-bib stems from a synergistic interplay of \u0026pi;-\u0026pi; and multipoint hydrogen-bonding interactions, which cooperatively stabilize the open-pore phase exclusively in the presence of propylene. The comparatively higher initial isosteric heat of adsorption for C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e on NJU-Bai5-bib, coupled with its lower intracrystalline diffusion barrier, jointly triggers a rapid, stimuli-responsive transition to the open-pore phase. This elucidated molecular-level mechanism accounts for the exceptional separation selectivity and macroscopic reversibility observed experimentally.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eWater stability test\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eGiven that NJU-Bai5 exhibits ultrahigh water stability due to its saturated metal nodes and hydrophobic pore environment, NJU-Bai5-bib\u0026mdash;which shares an isoreticular structure with NJU-Bai5, is anticipated to display comparably high hydrolytic stability. Immersion of as-synthesized NJU-Bai5-bib in water for over one month resulted in essentially unchanged PXRD patterns and CO\u003csub\u003e2\u003c/sub\u003e uptake (\u003cstrong\u003eFigure S19\u003c/strong\u003e). In practical gas separation applications, moisture tolerance under long-term cyclic operation is of particular importance. Therefore, we further evaluated the water vapor adsorption capacity and cycling stability of NJU-Bai5-bib. As shown in \u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003e4a\u003c/strong\u003e, the framework exhibits a stepwise uptake with an inflation point near 30 % RH and a saturation capacity of 216.67 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e. Although the saturated uptake decreased by 3.7 % (to 208.6 cm\u003csup\u003e3\u003c/sup\u003e g\u003csup\u003e-1\u003c/sup\u003e) in the second cycle, subsequent cycles exhibited no significant decline. Importantly, the porosity could be fully restored by reactivating the sample at 100 \u0026deg;C under vacuum for 10 h, as confirmed by the recovered CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm at 195 K (\u003cstrong\u003eFigure\u003c/strong\u003e\u003cstrong\u003eS20\u003c/strong\u003e). Thus, NJU-Bai5-bib demonstrates both high hydrolytic stability and excellent cycling stability under water vapor, positioning it as a promising candidate for practical gas-separation applications.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eBreakthrough Experiment\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eTo comprehensively assess the practical separation performance of NJU-Bai5-bib in C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e purification, dynamic breakthrough experiments were carried out using an equimolar C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e (v/v, 50/50) mixture at a controlled flow rate of 0.5 mL min\u003csup\u003e-1\u003c/sup\u003e. As shown in \u003cstrong\u003eFigure 4b\u003c/strong\u003e, C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e elutes sharply at 40.2 min g\u003csup\u003e-1\u003c/sup\u003e, whereas C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e does not break through until 144.2 min g\u003csup\u003e-1\u003c/sup\u003e, delivering a broad wide separation window. This performance surpasses most state-of-the-art sorbents evaluated under similar breakthrough conditions, including NTU-85-WNT (7 min g\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e53\u003c/sup\u003e,\u0026nbsp;GeFSIX-2-Cu-i (58 min g\u003csup\u003e-1\u003c/sup\u003e)\u003csup\u003e57\u003c/sup\u003e, thereby highlighting the remarkable C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e separation capability of NJU-Bai5-bib. Notably, even after five consecutive breakthrough cycles, the retention time remains as high as ~70 min g\u003csup\u003e-1\u003c/sup\u003e (\u003cstrong\u003eFigure S21\u003c/strong\u003e), indicating excellent recyclability and structural resilience of the flexible framework during repeated adsorption-desorption operation. Moreover, NJU-Bai5-bib preserves its separation performance under humid conditions (65 % and 85 % RH, \u003cstrong\u003eFigure 4c\u003c/strong\u003e), consistent with its outstanding hydrolytic resistance observed in water vapor sorption studies. Such humidity tolerance is rarely achieved in flexible MOFs and strongly supports its suitability for industrial mixtures containing moisture. Considering all these findings, NJU-Bai5-bib demonstrates an exceptional combination of environmental robustness, cycling durability, and high separation efficiency, establishing its strong potential for energy-efficient C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e separation processes.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eScalable Aqueous Synthesis\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNJU-Bai5-bib can be synthesized on the gram scale through a fully aqueous and environmentally benign route, affording a crystalline product whose PXRD pattern and CO\u003csub\u003e2\u003c/sub\u003e sorption behavior are essentially identical to those of the small-scale material (\u003cstrong\u003eFigure 4d\u003c/strong\u003e). This scalable protocol preserves both structural integrity and adsorption performance (\u003cstrong\u003eFigure 4e\u003c/strong\u003e), while delivering an exceptionally low synthetic cost of 158.3 $ kg\u003csup\u003e-1\u003c/sup\u003e. By contrast, the practical deployment of many high-performance MOFs remains impeded by prohibitive manufacturing costs and harsh synthetic conditions. ZU-609, UTSA-400, HAF-1, and NTU-65-Co-Ti exhibit synthesis costs ranging from 389 to 19173.69 $ kg\u003csup\u003e-1\u003c/sup\u003e and require extended solvothermal or diffusion processes lasting 12 ~ 336 hours, resulting in substantial energy, equipment, and safety demands. A radar plot (\u003cstrong\u003eFigure 4f\u003c/strong\u003e) highlights the comprehensive advantages of NJU-Bai5-bib in synthesis efficiency, large-scale cost, green synthesis, water-related stability and 0.5 bar C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e uptake ratio. Its aqueous preparation relies solely on inexpensive precursors\u0026mdash;CuCl\u003csub\u003e2\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO, TCMBT, and bib\u0026mdash;enabling production at ~158.3 $ kg\u003csup\u003e-1\u003c/sup\u003e, two to three orders of magnitude lower than typical high-performance MOFs. Moreover, NJU-Bai5-bib delivers competitive C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e/C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e uptake ratios at 0.5 bar and maintains its CO\u003csub\u003e2\u003c/sub\u003e capacity after multiple water-vapor cycling tests (\u003cstrong\u003eFigure S20\u003c/strong\u003e), underscoring its structural robustness under industrially relevant humidity. Crucially, the scalability of NJU-Bai5-bib is supported by the high-yield, low-cost availability of both key ligands. TCMBT can be prepared on a 190 g scale in 99.7 % isolated yield through a simple water-based amide coupling, while bib is obtained in 95 % yield via an efficient imidazole substitution reaction. The inexpensive and readily accessible ligand production ensures that the entire synthetic sequence remains economically viable and industrially practical, further reinforcing the promise of NJU-Bai5-bib for real-world adsorption-separation applications.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eBased on a strategically evolved pillar-layered platform, this study establishes a delicate design principle for achieving highly selective olefin/paraffin separation through precise, pillar-directed modulation of framework flexibility. Beginning with the hydrolytically stable yet rigid prototype NJU-Bai5, we systematically engineered its organic pillars to introduce and finely tune guest-responsive dynamics. The pivotal advancement was achieved by substituting terminal pyridyl groups with imidazolyl units, and further facilitating the ligand rotation freedom with alkyl elongation, to get the ultimate product NJU-Bai5-bib. This modification embeds accessible nitrogen lone-pair electrons that function as specific molecular anchors, which engage in selective, cooperative interactions with propylene, while interactions with propane remain energetically insufficient. Consequently, NJU-Bai5-bib exhibits a well-defined, low-pressure gate-opening transition triggered exclusively by C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, translating a subtle molecular distinction into macroscopic separation performance with exceptional selectivity under ambient conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis evolution from rigidity to controlled flexibility delineates a clear structure-property relationship. Intermediate structures, such as NJU-Bai5-bip, provided critical insights into the instability of moderate flexibility but underscored the necessity for a fully realized dynamic response. The final material, NJU-Bai5-bib, successfully balances scalable cost, exceptional hydrolytic stability, and environment-friendly aqueous synthesis with its precise dynamic behavior. Its performance demonstrates that framework mechanics can be harnessed as a functional tool, amplifying minor differences in guest electronic structure and polarizability into effective kinetic or thermodynamic separation pathways.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eASSOCIATED CONTENT\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e. Additional materials synthesis and characterization data, including additional gases adsorption isotherms, powder X-ray diffraction patterns, \u003csup\u003e1\u003c/sup\u003eH NMR, and IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAUTHOR INFORMATION\u003c/p\u003e\n\u003cp\u003eCorresponding Author\u003c/p\u003e\n\u003cp\u003e* [email protected]\u003c/p\u003e\n\u003cp\u003e* [email protected]\u003c/p\u003e\n\u003cp\u003eACKNOWLEDGMENT\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe thank for the financial support from the National Natural Science Foundation of China (21601047 and 22573048).\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRoy A, Venna SR, Rogers G, Tang L, Fitzgibbons TC, Liu J, McCurry H, Vickery DJ, Flick D, Fish B (2021) Membranes for olefin\u0026ndash;paraffin separation: An industrial perspective. \u003cem\u003eProc. Natl. Acad. 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Ind Eng Chem Res 59(8):3531\u0026ndash;3537\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\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-8403378/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8403378/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe separation of propylene (C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e) from propane (C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e8\u003c/sub\u003e) constitutes one of the most energy-demanding processes in the petrochemical industry, a consequence of their nearly identical molecular dimensions and physicochemical properties. Metal-organic frameworks (MOFs) have emerged as a promising adsorptive alternative to traditional distillation. Among these, flexible MOFs, which undergo selective guest-induced structural transformations, offer a unique mechanism to amplify subtle molecular differences into distinct adsorption behaviors. Nevertheless, the precise molecular-level modulation of framework flexibility to elicit a selective response to one specific gas over another remains a great challenge. This study confronts this challenge through the rational structural evolution of a hydrolytically stable, pillar-layered Cu-MOF, NJU-Bai5. Our strategy involves the strategic engineering of the organic pillar ligand, specifically replacing pyridyl termini with imidazolyl groups. This modification introduces accessible nitrogen lone-pair electrons, which function as molecular anchors capable of forming selective interactions with hydrogen-bonding donors possessing higher acidity. These interactions, in turn, drive a gas-recognizable flexibility of the framework architecture. The resulting material, NJU-Bai5-bib, exhibits a well-defined, selective gate-opening transition that is triggered preferentially by C\u003csub\u003e3\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e at low pressures. This specific response enables the framework to achieve highly selective propylene/propane separation under ambient conditions. Furthermore, by inheriting exceptional hydrolytic stability from its progenitor and featuring a scalable, aqueous synthesis, NJU-Bai5-bib demonstrates not only fundamental excellence in guest-specific recognition but also considerable potential for practical, energy-efficient propylene purification.\u003c/p\u003e","manuscriptTitle":"Lone-pair-electron Governed Gas-recognizable Flexibility in an Ultra-stable MOF for Boosting C3H6/C3H8 Separation and Its Aqueous Scalable Synthesis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-21 06:35:00","doi":"10.21203/rs.3.rs-8403378/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":"c38b9bd0-6f14-45d6-8076-b1057ebde859","owner":[],"postedDate":"January 21st, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":60689723,"name":"Physical sciences/Chemistry/Coordination chemistry"},{"id":60689724,"name":"Physical sciences/Materials science/Soft materials"},{"id":60689725,"name":"Physical sciences/Chemistry/Green chemistry"}],"tags":[],"updatedAt":"2026-01-21T06:35:01+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-21 06:35:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8403378","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8403378","identity":"rs-8403378","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}