Molecular‐Rotor‐Enhanced Pore‐Space Partition in Metal–Organic Frameworks for Boosting One‐Step Ethylene Purification

ABSTRACT

One‐step ethylene purification from the ternary acetylene/ethane/ethylene (C₂H₂/C₂H₆/C₂H₄) mixture is a pivotal yet unresolved challenge in industrial petrochemical processes. Herein, a molecular‐rotor‐enhanced pore‐space partition (MR‐enhanced PSP) strategy is proposed, which successfully achieves synergistic optimization of pore environments in metal–organic frameworks for boosting one‐step ethylene separation. The insertion of pore partitioners through open metal sites in typical MIL‐88 framework precisely tunes the local pore apertures, while aromatic molecular rotors step‐by‐step regulate the pore windows and progressively enriches the π–π and C–H⋯π interactions with gas molecules. Optimized SNNU‐636 adsorbent achieves a C₂H₂/C₂H₄ selectivity of 4.17, outperforming its counterparts without molecular rotors (SNNU‐630: 1.78, SNNU‐631: 3.29, SNNU‐632: 1.42) and additionally exhibiting preferential adsorption of C₂H₆ over C₂H₄. Fixed‐bed breakthrough experiments confirm that SNNU‐636 enables one‐step production of ultrahigh‐purity C₂H₄ (> 99.9999%) from the ternary mixture with a record‐high yield of 7.8 mmol·g⁻¹, surpassing all benchmark adsorbents under similar conditions. Mechanistic studies further validate this MR‐enhanced PSP strategy, revealing that C₂H₂ and C₂H₆ are stabilized via multiple π⋯π and C–H⋯π interactions, underpinning the state‐of‐the‐art one‐step C₂H₄ purification performance.

The molecular‐rotor‐enhanced pore‐space partition is proposed for metal–organic framework adsorbents, which enables a boosting one‐step ethylene purification from ternary acetylene/ethylene/ethane mixture with a record‐breaking yield of 7.8 mmol g⁻¹ and ultra‐high purity of 99.9999%.

1. Introduction

Ethylene (C₂H₄) is the cornerstone of the global petrochemical industry, serving as the primary feedstock for the synthesis of polyethylene, ethylene glycol, and a wide range of high‐value‐added chemicals [1]. Industrial C₂H₄ is predominantly produced via naphtha steam cracking, but the crude product stream is invariably contaminated with acetylene (C₂H₂) and ethane (C₂H₆) [2]. C₂H₂ is highly reactive and acts as a catalyst poison in polymerization processes, while C₂H₆, an inert saturated hydrocarbon, cannot be polymerized and reduces the selectivity of downstream reactions [3, 4]. Consequently, the development of efficient and energy‐saving separation technologies to remove C₂H₂ and C₂H₆ is indispensable for meeting the purity requirements (>99.99%) of industrial C₂H₄ [5].

Metal–organic frameworks (MOFs) have achieved significant advances in adsorption and separation due to their highly designable structures and tunable pore environments [6, 7]. By precisely controlling pore sizes, channel shapes, and the chemical environment within the pores, highly specific adsorption can be engineered [8, 9]. This enables the efficient recognition and separation of gas molecules exhibiting extremely minor differences in physicochemical properties [10]. However, overcoming the limitations of conventional pore design to maximize the density of binding sites within the pores and the host–guest interactions, thereby achieving optimal utilization of the pore space, remains a key challenge in the development of high‐performance MOF adsorbents [11, 12].

Various design strategies have been explored for MOF adsorbents including the incorporation of open metal sites (OMSs), polar functional groups and pore size control to target C₂H₂/C₂H₄/C₂H₆ separations [13, 14]. Compared to C₂H₄, C₂H₂ exhibits higher acidity (pK_a≈25) and a larger quadrupole moment, enabling strong hydrogen bonding with electronegative sites or stable coordination with OMSs. (Table S1) [15, 16] As a result, the selective removal of C₂H₂ from C₂H₄ is thermodynamically favorable and has been widely studied (Scheme 1, left). In contrast, the separation of C₂H₆, a chemically inert molecule, remains a critical bottleneck. C₂H₆ lacks significant charge distribution and π‐bonds, precluding strong interactions with conventional MOF pore surfaces. Instead, its adsorption relies on weak van der Waals forces, spatial confinement, and induced dipole‐π interactions. To achieve selective C₂H₆ adsorption, MOFs must therefore combine finely tuned pore sizes with abundant π‐electron density in the pore environment, which strengthens C─H⋯π interactions to compensate for the inherently weak affinity of C₂H₆.

SCHEME 1.

The MR‐enhanced PSP strategy for one‐step ethylene purification proposed in this work.

Pore‐space partition (PSP) has emerged as a powerful strategy for engineering MOF pore environments to enhance gas separation performance. By reducing the effective pore size and enhancing the confinement effect, this strategy significantly increases the strength of interactions between gas molecules and the pore surface [17, 18]. Although the PSP process induced by OMS sites demonstrates advantages in enhancing adsorption capacity and selectivity, it essentially creates a rigid pore. Once the structure is formed, the shape of the pore channels and the interaction sites are fixed, and adsorption relies primarily on static confinement (Scheme 1, medium) [19, 20]. However, for gas molecules like C₂H₆ whose adsorption relies primarily on weak forces or C─H⋯π interactions, a single static pore structure often fails to achieve perfect host–guest matching, making it difficult to fully realize the adsorption potential of the pore space [21, 22].

To overcome the limitation of rigid pores introduced by typical OMS‐induced PSP process, the introduction of dynamic adaptability is proposed. Previous studies have attempted to achieve dynamic pore regulation through the framework interpenetration or topological restructuring, but such approaches are often constrained by demanding geometric matching requirements, and precise control over synthesis and the microenvironment is extremely difficult to achieve [23]. In contrast, the incorporation of large π‐conjugated systems into the pore‐partitioned framework as molecular rotors not only acts as a spatial barrier to further regulate the pore size but also provides a highly controllable dynamic regulation through their inherent rotational freedom. The self‐driven rotation of rotors may modulate the microenvironment within the pore, enabling a dynamic and controllable perfect match between the binding site and specific guest molecules, thereby adaptively optimizing the adsorption environment [24, 25].

Keep above‐mentioned considerations, this work proposes a promising molecular‐rotor‐enhanced pore space partition (MR‐enhanced PSP, Scheme 1, right) strategy with static pore partitioners help to precisely construct ultra‐microporous confined spaces and large π‐conjugated molecular rotors provide dynamic adaptive effects and modulate the pore windows. Such dynamic‐static synergy may not only impart rigid spatial confinement to the framework but also creates a dynamic π‐environment, providing an ideal pathway for maximizing binding site density and dynamic host‐guest interactions and thus boost the one‐step C₂H₄ separation challenging.

The feasibility of this strategy is validated by a series of static pore‐space‐partitioned MOF adsorbents incorporated with dynamic molecular rotors ranging from 1,4‐benzenedicarboxylic acid (BDC), 1,4‐naphthalenedicarboxylic acid (NDC) to 9,10‐anthracenedicarboxylic acid (ADC). Among them, the optimized SNNU‐636 material exhibited significantly enhanced C₂H₂/C₂H₄ selectivity (4.17) outperforming its counterparts without molecular rotors (SNNU‐630: 1.78, SNNU‐631: 3.29, SNNU‐632: 1.42) and a preference for adsorbing C₂H₆ over C₂H₄. Critically, SNNU‐636 achieves a record‐high C₂H₄ yield (7.8 mmol·g⁻¹) and ultrahigh purity (>99.9999%) in one‐step ternary separation. In situ FT‐IR spectroscopy, combined with Grand Canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations, confirms that C₂H₂ and C₂H₆ are stably anchored in the π‐rich pores via multiple π⋯π and C─H⋯π interactions, while C₂H₄ interacts only weakly with the framework. These findings demonstrate the power of molecular‐rotor enhanced synergistic pore‐space partition and provide a new structural paradigm for the rational design of high‐performance adsorbents for hydrocarbon separation.

2. Results and Discussion

The classical MIL‐88B platform was employed as a proof‐of‐concept model to valid the proposed MR‐enhanced PSP strategy, which exhibits a framework with one‐dimensional hexagonal channel through the coordination of six trinuclear clusters with six dicarboxylate ligands. Nitrogen‐containing ligands serving as pore partitioners to coordinate with open metal sites on the trinuclear clusters, thereby partitioning the hexagonal channel along the c‐axis into multiple uniform cylindrical spaces (called OMS‐induced PSP herein) (Figure S1). The partitioned structure features two types of cages: one is the original triangular bipyramidal cage (t‐cage) formed by five trinuclear clusters and six dicarboxylate ligands; the other is a new octahedral cage (o‐cage) formed by six trinuclear clusters and six dicarboxylate ligands (Figures S2–S19; Tables S2–S22).

Since the t‐cage is consistently present before and after the OMS‐induced PSP, the regulation of molecular rotors on t‐cage is first discussed. In parent MIL‐88B framework, the BDC ligand exhibits an almost planar conformation (θ = 0°), forming a large cage with a size of approximately 8.0 Å. After OMS‐induced pore partitioning, MIL‐88B was converted to SNNU‐632, with its pore size significantly reduced to 4.5 Å, validating the effectiveness of the static PSP strategy in space confinement (Figure 1a–c). Then, molecular rotors with larger molecular volumes and extended π‐conjugated systems were introduced by replacing BDC with NDC or ADC. NDC and ADC are chemically coordinated to the trinuclear clusters as part of the MOF framework, affording SNNU‐630 and SNNU‐631 (neutral PCN‐19) [26], respectively (Figure 1d–h). Unlike the breathing behavior of MIL‐88B, the introduction of these large aromatic ligands effectively transformed the originally unconfined flexible structure into a controllable dynamic microenvironment. The introduction of ADC and NDC significantly reduced the pore sizes of SNNU‐630 and SNNU‐631 to 5.0 Å and 4.2 Å, respectively. After partitioning SNNU‐630, the pore size of SNNU‐633 further reduced to 4.1 Å (Figure 1f). Furthermore, introducing various static separators to SNNU‐631 framework, the SNNU‐634‐636 series was synthesized (Figure 1i–k).

FIGURE 1.

The regulation of t‐cages in MIL‐88B and SNNU‐630‐636 by the MR‐enhanced PSP strategy.

By comparing the changes in pore size, it can be observed that while the molecular rotor alone reduces the initial 8.0 Å pore to 4.2–5.0 Å through its rotation, coupling it with the static partition can precisely and stably lock the pore size within an ultra‐narrow range of 4.0–4.3 Å (Figure 1l,m). These results indicate that OMS‐induced PSP and molecular rotors play different but complementary roles: the former creates the partitioned confined pores, while the latter further tunes the pore size and flexibility. Taking the optimized SNNU‐636 as an example, the static 2,4,6‐tris(4‐pyridyl)pyridine (TPP) provides a rigid 4.2 Å space that precisely matches the kinetic diameter of C2‐hydrocarbon molecules, while the dynamic ADC rotor enhances the affinity between the pore and the guest molecule through its flexible conformation and rich π electrons. The combination of rigid spatial confinement and dynamic rotors provides an ideal structural foundation for the subsequent efficient adsorption of C₂H₆ and the one‐step purification of C₂H₄.

The pore window sizes (d) and rotor rotation angles (α and β) for o‐cages and t‐cages were further analyzed to clarify how the molecular rotors control the cage environments (Figure 2a–c). In MIL‐88B, BDC ligands are arranged symmetrically, with identical rotor angles (α = β = 52.1°) (Figure 2d,e). After PSP to MIL‐88B, SNNU‐632 adopt same rotor angles (α = β = 46.5°) (Figure 2f,g) After the introduction of rotors, NDC or ADC in SNNU‐630 and SNNU‐631 make the cage crowded. To fit, the ADC rotor rotates significantly, causing asymmetric angles of α = 65.9° and β = 15.6°. This proves the rotors are highly flexible (Figure 2h–m). After pore partitioning, the incorporation of TPP partitioners in SNNU‐633 and 4,4′‐Dipyridylamine (DPA) partitioners in SNNU‐634 divides the pores and compresses the rotors, further decreasing the window distance (Figure 2j–o). Notably, SNNU‐635, featuring the smallest Tri(pyridin‐4‐yl)amine (TPA) separator, severely restricts ADC rotation, as reflected by its larger α = 70.2° and smaller β = 10.3°. This restricted rotation brings neighboring rotors closer together, sharply reducing the pore window size to 3.5 Å (Figure 2p,q). Although PLATON analysis indicates that SNNU‐635 retains a crystallographic porosity of 17.4%, such a narrow window kinetically restricts the diffusion of C₂H₂, C₂H₄ and C₂H₆ into the pores.

FIGURE 2.

The regulation of pore window sizes (d) and rotor rotation angles (α and β) for o‐cages and t‐cages in MIL‐88B and SNNU‐630‐636 by the MR‐enhanced PSP strategy.

Finally, when the TPP introduced, the ADC rotor demonstrated excellent adaptability. The angles were adjusted from the extreme values in the SNNU‐635 to a relatively balanced state (α = 66.7°, β = 19.4°). This angle adjustment is crucial, creating space for the partition while preventing the rotors from crowding too closely together (Figure 2r,s). Consequently, the windows in SNNU‐636 were not completely closed off as in SNNU‐635 but remained at an appropriate size. This not only keeps the window open but also ensures that the opening is precisely sized to allow the gas molecules amid to separate to pass through. Clearly, the MR can flexibly adjust the angles (α and β) on the shared surfaces of the cage, thereby directly controlling whether the window size (d) increases or decreases (Figure 2b,c). They flexibly adjust their angles (α and β) based on their surroundings to control the window size (d). Such flexible adjustment creates ideal environments for targeted gas separation.

The permanent porosity of the MOFs was evaluated via N₂ (77 K) and CO₂ (195 K) adsorption isotherms (Figure 3a,b). SNNU‐631, SNNU‐632, and SNNU‐633 exhibited typical Type I N₂ adsorption isotherms, demonstrating that TPP (pore partitioner) and ADC (MR) successfully tune the pore environment. Their saturated N₂ uptakes and Brunauer–Emmett–Teller (BET) surface areas were 188.2 cm³ g⁻¹ (523.2 m² g⁻¹), 286.4 cm³ g⁻¹ (779.6 m² g⁻¹), and 209.0 cm³ g⁻¹ (577.9 m² g⁻¹), respectively. In contrast, MIL‐88 showed negligible N₂ uptake due to its high flexibility, while SNNU‐630, 634, 635, and 636 also had negligible uptakes because their bulky organic ligands blocked the pores.

FIGURE 3.

(a) N₂ adsorption isotherms at 77 K. (b) CO₂ adsorption isotherms at 195 K. (c) Pore size distributions. Compared C₂H₂/C₂H₄/C₂H₆ adsorption at 298 K for (d) MOFs with MR‐enhanced mode: MIL‐88, SNNU‐630 and SNNU‐631; (e) MOFs with OMS‐induced mode: MIL‐88, SNNU‐630‐633, and SNNU‐636. (f) Low‐pressure adsorption behavior of MOFs with MR and OMS synergistic mode at 298 K for SNNU‐630‐633 and SNNU‐636.

To further probe the pore characteristics, CO₂ adsorption was tested at 195 K. The maximum CO₂ uptakes and corresponding BET areas were of SNNU‐630 (68.7 cm³ g⁻¹, 316.87 m² g⁻¹), ‐631 (128.3 cm³ g⁻¹, 580.0 m² g⁻¹), ‐632 (265.5 cm³ g⁻¹, 1144.6 m² g⁻¹), ‐633 (195.7 cm³ g⁻¹, 865.7 m² g⁻¹), ‐634 (77.5 cm³ g⁻¹, 326.13 m² g⁻¹), and ‐636 (90.0 cm³ g⁻¹, 402.7 m² g⁻¹). MIL‐88B and SNNU‐635 exhibited lower CO₂ uptake capacities of 9.1 cm³ g⁻¹ and 4.7 cm³ g⁻¹, respectively. Pore size distribution analysis revealed that SNNU‐632 has a pore size of 4.5 Å (Figure 3c). By introducing different partitions, the pore sizes were finely modulated: SNNU‐630 (4.38 Å), SNNU‐631 (4.11 Å), SNNU‐633 (4.16 Å), SNNU‐634 (4.23 Å), and SNNU‐636 (4.45 Å). Clearly, the introduction of dynamic molecular rotors, static partitioners, and their synergistic combination significantly modifies the pore structures. Notably, the 4.45 Å pore size of SNNU‐636 is precisely larger than the kinetic diameter of C₂H₆ (4.4 Å). This ideal spatial match is highly beneficial for enhancing the host–guest interaction forces during C₂H₆ adsorption.

The single‐component adsorption isotherms of C₂H₂, C₂H₄, and C₂H₆ were measured for MOF compounds at 273, 283, and 298 K (Table S23; Figures S20–S35). Compared to original MIL‐88 material, SNNU‐630 and SNNU‐631 regulated by the MR exhibited higher adsorption capacities for C₂H₂, C₂H₄, and C₂H₆ at 298 K (Figure 3d). At 1 bar, their capacities reached 45.18/30.59/26.22 cm³ g⁻¹, demonstrating a clear trend of capacity enhancement. The increasement was most pronounced for C₂H₂, reflecting its stronger affinity for molecules with high quadrupole moments. Meanwhile, the adsorption capacities of SNNU‐632, SNNU‐633, and SNNU‐636 for three gases under the same pressure were further enhanced through OMS‐induced treatment (Figure 3e). Across the full pressure range, the adsorption capacities for three gases increased. The saturated adsorption capacities of the three compounds for C₂H₂, C₂H₄, and C₂H₆ were of 106.60/90.05/81.31 cm³ g⁻¹ for SNNU‐632, 86.99/71.82/68.38 cm³ g⁻¹ for SNNU‐633, and 65.39/44.52/50.34 cm³ g⁻¹ for SNNU‐636, demonstrating the enhanced gas storage capacity of the materials.

In addition to the saturated adsorption values at room temperature, the adsorption properties at the low‐pressure region are also critical for evaluating the affinity and selectivity of adsorbents. Here, the adsorption behaviors of five MOFs at 0.1 bar were further compared (Figure 3f). SNNU‐630 exhibits a lower adsorption capacity, with the order of adsorption following C₂H₂> C₂H₄ ≈ C₂H₆. SNNU‐631 exhibits the stronger adsorption for C₂H₂ 18.2 cm³ g⁻¹, with nearly equivalent adsorption for C₂H₄ and C₂H₆ (9 cm³ g⁻¹), maintaining the order C₂H₂> C₂H₄ ≈ C₂H₆. SNNU‐632 and SNNU‐633 exhibit remarkably similar low‐pressure adsorption capacities for the three components. SNNU‐636 material exhibits a significant adsorption order of C₂H₂> C₂H₆> C₂H₄ in the low‐pressure region. At 0.1 bar, the adsorption capacity of C₂H₆ is 18.2 cm³ g⁻¹, significantly higher than that of C₂H₄ at 13.9 cm³ g⁻¹. This result highlights the stronger affinity of SNNU‐636 toward C₂H₆ in the low‐pressure region and supports its C₂H₆‐selective adsorption behavior for C₂H₄ purification.

Subsequently, further analysis was conducted on introducing different static partitioners into the SNNU‐631 framework in the presence of ADC molecular rotors to achieve various dynamic‐static synergistic effects and evaluate their impacts on adsorption separation. SNNU‐636 demonstrates significantly superior thermodynamic performance compared to other materials in the separation of acetylene and ethylene. Compared to SNNU‐631 and SNNU‐634, SNNU‐636 exhibits increased adsorption capacities for C₂H₂, C₂H₄, and C₂H₆ at 298 K by 44.7%/109.3%, 45.5%/101.2%, and 92.0%/139.7%, demonstrating the significantly enhanced adsorption capacity resulting from the introduction of the TPP ligand (Figure 4a–c).

FIGURE 4.

Gas adsorption isotherms of SNNU‐631, SNNU‐634 and SNNU‐636 at 273 and 298 K for (a)C₂H₂, (b) C₂H₄ (c) C₂H₆. (d) Isosteric heats of adsorption (Q_st) for C₂H₂, C₂H₄, and C₂H₆. (e) IAST selectivity values for C₂H₂/C₂H₄ at 298 K. (f) IAST selectivity values for C₂H₆/C₂H₄ at 298 K. (g) Comparison of IAST and ΔQ_st (C₂H₂‐C₂H₄) for this study and representative MOFs reported in the literature. (h) Comparison of Q_st for C₂H₆ and ΔQ_st (C₂H₆‐C₂H₄) among SNNU‐636 and MOFs reported in the literature.

To further evaluate the interactions of MOFs with gas molecules, the virial equation is used to fit the isothermal adsorption data and derive the heats of adsorption (Q_st) (Table S24 and Figures S36–S59). The ‐Q_st values of SNNU‐630, SNNU‐632 and SNNU‐633 are generally low, with C₂H₂ slightly higher and neglectable differences between C₂H₄ and C₂H₆, indicating weaker selectivity. The ‐Q_st of C₂H₂, C₂H₄ and C₂H₆ for SNNU‐631 and SNNU‐634 is slightly enhanced, but both SNNU‐631 and SNNU‐634 follow the same trend for C₂H₂ > C₂H₄> C₂H₆. However, SNNU‐636 significantly enhances the interaction with C₂H₂, with ‐Q_st as high as 44.49 kJ mol⁻¹, significantly higher than that of C₂H₆ and C₂H₄ (39.46 kJ mol⁻¹ and 35.38 kJ mol⁻¹), forming the order C₂H₂> C₂H₆> C₂H₄. This indicates that the introduction of TPP significantly enhances the material's thermodynamic recognition ability for acetylene, facilitating high‐selectivity separation at low pressure (Figure 4d).

To evaluate the separation ability of MOFs for C₂H₂/C₂H₄ and C₂H₆/C₂H₄ mixtures, the selectivity values at 273–298 K were calculated using ideal adsorption solution theory (IAST) (Table S25; Figures S60–S83). The C₂H₂/C₂H₄ IAST selectivity of SNNU‐636 at 1 bar is 4.17, significantly higher than that of SNNU‐630 (1.78), SNNU‐631 (3.29), SNNU‐632 (1.42) SNNU‐633 (1.65), and SNNU‐634 (2.99), indicating its most pronounced preferential adsorption of acetylene (Figure 4e). SNNU‐630, SNNU‐632, and SNNU‐633 exhibit a selectivity of C₂H₆/C₂H₄ = 1.06, 1.01, and 1.13, respectively. While SNNU‐631 and SNNU‐634 exhibit a C₂H₄/C₂H₆ selectivity. Notably, SNNU‐636 exhibits a reverse C₂H₆/C₂H₄ selectivity of 1.51, indicating a preferential adsorption for C₂H₆ (Figure 4f). SNNU‐636 achieves double‐preferred adsorption of C₂H₂ and C₂H₆ over C₂H₄, which is a key feature for purifying C₂H₄.

Comparing the separation potential of various MOF materials for C₂H₂/C₂H₄ at 298 K, the thermodynamic recognition ability of MOFs was evaluated based on IAST and ΔQ_st = Q_st(C₂H₂)–Q_st(C₂H₄) (Figure 4g; Table S26). The C₂H₂/C₂H₄ IAST selectivity of SNNU‐636 reaches as high as 4.17, with ΔQ_st of 9.1 kJ mol⁻¹, far exceeding that of SNNU‐631 (IAST = 3.29, ΔQ_st = 2.26 kJ mol⁻¹), SNNU‐634 (IAST = 2.99, ΔQ_st = 1.22 kJ mol⁻¹ and SNNU‐632 (IAST = 1.42, ΔQ_st = 0.55 kJ mol⁻¹), demonstrating stronger gas discrimination ability and separation potential. Notably, compared to high‐performance MOFs reported in the literature, such as HIAM‐210 (IAST = 2, ΔQ_st = 11.5 kJ mol⁻¹) [27], Al‐Pydc (IAST = 4.3, ΔQ_st = 7.5 kJ mol⁻¹) [28], CuTiF6‐TPPY (IAST = 5.03, ΔQ_st = 6.9 kJ mol⁻¹) [29], MOF‐303 (IAST = 2.4, ΔQ_st = 7.4 kJ mol⁻¹) [30], UiO‐67‐(NH₂)₂ (IAST = 2.1, ΔQ_st = 2.9 kJ mol⁻¹) [16] and CAU‐23 (IAST = 1.4, ΔQ_st = 7.5 kJ mol⁻¹) [31], SNNU‐636 remains in the optimal region, only slightly below [Ni(tzba)_0.5(F)(bpy)]DMF_0.5H₂O (IAST = 4, ΔQ_st = 13.6 kJ mol⁻¹) [32] and Zr‐TCA (IAST = 5.64, ΔQ_st = 19.9 kJ mol⁻¹) [33], making it one of the few materials that satisfy both high selectivity and high thermodynamic recognition ability.

In addition, to evaluate the strength of the interaction between MOFs and C₂H₆, the adsorption heat (Q_st) of MOF materials toward C₂H₆ was compared (Figure 4h). The Q_st value of SNNU‐636 is 39.5 kJ mol⁻¹, higher than that of SNNU‐631 (37.69 kJ mol⁻¹), SNNU‐634 (33.97 kJ mol⁻¹) and SNNU‐632 (28.85 kJ mol⁻¹) in this study. When compared with some high‐performance MOF materials reported in the literature, the Q_st value of SNNU‐636 is higher than that of equivalent materials, such as NUM‐9(35.75 kJ mol⁻¹) [34], Zr‐TCA (35.3 kJ mol⁻¹) [33], CuTiF6‐TPPY (34.2 kJ mol⁻¹) [29], Zn(ad)(int) (33.2 kJ mol⁻¹) [35], Zn(BDC)(H₂BPZ) (31.8 kJ mol⁻¹) [36], HIAM‐210 (31.24 kJ mol⁻¹) [27], NKU‐301 (30.4 kJ mol⁻¹) [37]. Even approaching certain framework structures with exceptionally strong adsorption capacity for ethane, such as Zn‐FBA (42.8 kJ mol⁻¹) [38]. Additionally, the difference in intermolecular forces between C₂H₆ and C₂H₄, that is ΔQ_st of C₂H₆‐C₂H₄, also serves as a significant indicator for evaluating a material's separation performance. For SNNU‐636, this value is 4.08, which is higher than those reported for NKU‐301 (3.7) [37], NUM‐9 (3.43) [34], Zn‐FBA (3) [38], CAU‐23 (3) [31], and Uio‐67‐(NH₂)₂ (2) [16]. The Q_st value is an important indicator for assessing the microporous confinement effect and non‐polar gas recognition capability of materials. The high Q_st value of SNNU‐636 for C₂H₆ provides the thermodynamic basis for its high‐selective adsorption in one‐step C₂H₄ purification.

To fully evaluate actual separation performance of MOFs for one‐step C₂H₄ purification, the breakthrough performance of the activated samples further was evaluated (Figures 5 and S84–S106). First, the separation capabilities of three MOFs for C₂H₂/C₂H₄ = 10/90 mixed gas were evaluated. In SNNU‐632, C₂H₂ began to elute at 13 min g⁻¹ after C₂H₄ had eluted, while SNNU‐631, SNNU‐633 and SNNU‐634 exhibited stronger ability to adsorb C₂H₂, with a separation time of 30, 22, and 34 min g⁻¹. Notably, SNNU‐636 extended the separation time to 44 min g⁻¹, significantly surpassing the previous two materials, indicating its capability to separate C₂H₂/C₂H₄ gas mixtures (Figure 5a). Subsequently, the ability of the three materials for a C₂H₆/C₂H₄ = 10/90 mixed gas was evaluated. Neither SNNU‐631 nor SNNU‐632 could achieve separation of C₂H₆/C₂H₄, while SNNU‐633 and SNNU‐634 can only separate for 6 min g⁻¹. SNNU‐636 demonstrated the strongest separation ability, with a separation time of up to 27 min g⁻¹ (Figure 5b).

FIGURE 5.

(a) Breakthrough curves for C₂H₂/C₂H₄ = 10/90 at 298 K. (b) Breakthrough curves for C₂H₆/C₂H₄ = 10/90 at 298 K. (c) Breakthrough curves for C₂H₂/C₂H₆/C₂H₄ = 2/3/95 of SNNU‐631, SNNU‐632, SNNU‐633, SNNU‐634, and SNNU‐636. (d) Cycling breakthrough tests of SNNU‐636. (e) Comparison of C₂H₄ productivity among representative materials. (f) Overall performance comparison with representative high‐performance materials. (g) PXRD of SNNU‐636 after 12 h of soaking in different pH solutions. (h) C₂H₂ adsorption isotherm of SNNU‐636 after soaking in different pH solutions. (i) Breakthrough curves for C₂H₂/C₂H₆/C₂H₄ = 2/3/95 of SNNU‐636 under humid conditions.

The ability of MOFs to purify C₂H₄ in a three‐component mixed gas was further evaluated (Figure 5c). In SNNU‐631, C₂H₄ and C₂H₆ eluted at the same time, while C₂H₂ eluted 45 min g⁻¹ after the former two. SNNU‐631 exhibits medium selectivity toward C₂H₂ but low selectivity toward C₂H₆/C₂H₄, thus unable to achieve one‐step purification of C₂H₄. In SNNU‐632, C₂H₆ and C₂H₂ eluted at 7 and 21 min g⁻¹, respectively, after C₂H₄ elution. In SNNU‐633 and SNNU‐634, C₂H₆ and C₂H₂ eluted at 11/22 min g⁻¹ and 22/97 min g⁻¹, after C₂H₄ elution. Clearly, the separation performance of SNNU‐636 surpasses that of other MOF materials. The breakthrough curves of SNNU‐636 demonstrate its strong selectivity toward C₂H₆ and C₂H₂, with separation times of up to 103 min g⁻¹ and 129 min g⁻¹, respectively. Based on this, SNNU‐636 can achieve one‐step purification of C₂H₄ at 298 K, with C₂H₄ purity exceeding 99.9999% (Figures 5c and S107). Additional breakthrough experiments using C₂H₂/C₂H₆/C₂H₄ compositions covering C₂H₄ contents of 33.3%‐90% afforded 99.9%–99.9999% C₂H₄, further confirming the excellent one‐step purification performance of SNNU‐636 (Figures S108–S112). This performance is attributed to the π‐electron‐rich confined space formed by the MR‐enhanced PSP strategy effects of ADC and TPP, which enhances interactions with C₂H₂ and C₂H₆, thereby enabling highly accurate separation.

Additionally, five cycles of C₂H₂/C₂H₆/C₂H₄ mixed gas separation tests were conducted on SNNU‐636, demonstrating its excellent reversibility and cycling stability, making it suitable for industrial recycling (Figure 5d). SNNU‐636 exhibits an excellent C₂H₄ yield of 7.8 mmol g⁻¹ at 298 K, far surpass all reported MOFs and other adsorbents, such as UPC‐66‐a (2.01 mmol g⁻¹) [39], Al‐PyDC (1.61 mmol g⁻¹) [28], Zn(ad)(int) (1.43 mmol g⁻¹) [35], MOF‐303 (1.35 mmol g⁻¹) [30], and Azole‐Th‐1 (1.34 mmol g⁻¹) [40] (Figures 5e and S113; Table S27). The above breakthrough curves and cycle tests have sufficiently demonstrated the excellent separation performance of SNNU‐636. To comprehensively evaluate its overall performance in practical applications, a systematic comparison was conducted using multi‐dimensional benchmarks. SNNU‐632, SNNU‐631, SNNU‐636, and representative materials from the literature, such as ZJU‐300 and UIO‐67‐NH₂, were compared across five key performance dimensions (Figure 5f). Among these, SNNU‐636 demonstrated superior performance in five dimensions: Q_st of C₂H₂ and C₂H₆, separation time of C₂H₂/C₂H₄ and C₂H₆/C₂H₄, and C₂H₄ yield. It demonstrates an obvious advantage in terms of overall performance and is verified to be the most balanced MOF material with the best C₂H₄ purification performance.

Subsequently, the stability of SNNU‐636, which exhibited the best performance, was investigated to verify its potential for practical separation applications. Samples were soaked in different pH solutions to verify their acid–base stability and water stability. The PXRD patterns of the samples after soaking treatment showed good correspondence with the original samples (Figure 5g). Further adsorption tests were conducted on the samples. At 298 K, the C₂H₂ adsorption capacity of the samples treated with different conditions remained unchanged, indicating that the treatment did not affect the structure or adsorption capacity of the samples (Figure 5h). Separation performance under humid conditions is also a key indicator for its application in actual production. Even under conditions of up to 90% humidity, the separation performance of SNNU‐636 remains unchanged, with a separation time of up to 95 min g⁻¹, demonstrating its excellent separation performance in humid and complex environments. SNNU‐636 exhibits outstanding stability and holds promise for application in actual industrial separation processes (Figure 5i).

In situ IR spectroscopy was performed to investigate C₂H₂, C₂H₄, and C₂H₆ adsorption in these MOFs (Figure 6 and Figures S114–S158). Due to low adsorption capacities, the IR spectra of MIL‐88, SNNU‐630, and SNNU‐635 showed no significant changes. In contrast, for SNNU‐632, SNNU‐631, SNNU‐634, and SNNU‐636, introducing C₂H₂ caused significant peak shifts, indicating strong host–guest interactions. Specifically, the enhanced C≡H stretching peak (ν(C≡H), ∼3300 cm⁻¹) confirms that C₂H₂ entered the pores [41]. The C≡H bending (δ(C≡H), ∼1350 cm⁻¹) and aromatic C–H deformation (γ(C‐H_ph), ∼700 cm⁻¹) peaks also increased (Figure 6a). However, with C₂H₄, spectral changes were less pronounced: the C═C─H stretching (ν(= CH₂), ∼3150 cm⁻¹), C═C vibration (ν(C = C), 1850 cm⁻¹), and C─H rocking (γ(= C–H), 950 cm⁻¹) peaks were visible (Figure 6b) [42]. When C₂H₆ was introduced, the C‐H stretching peak (ν(C–H), 2950 cm⁻¹) was significantly enhanced. Additionally, the C–H deformation vibrations (γ(C‐H)) at ∼1460 cm⁻¹ and ∼1220 cm⁻¹ exhibited strong responses (Figure 6c).

FIGURE 6.

In situ FT‐IR spectra of MOFs after adsorption (a) C₂H₂. (b) C₂H₄. (c) C₂H₆. Changes in SNNU‐636 highlighting gas‐induced responses (d) C₂H₂. (e) C₂H₄. (f) C₂H₆.

To investigate host–guest interactions in SNNU‐636, pressure‐dependent in situ IR spectroscopy was performed (Figure 6d–f). With increasing C₂H₂ pressure, key peaks showed clear intensity changes. C₂H₂ stretching (ν(C≡H), 3300 cm⁻¹) and bending (δ(C≡H), 1250‐1700 cm⁻¹), alongside the ligand's out‐of‐plane bending (γ(C‐H_ph), 730 cm⁻¹). This indicates interactions between C₂H₂ and the ligand's aromatic rings, likely via π–π stacking or enhanced hydrogen bonding. For C₂H₄, intensity changes occurred at ∼3100, 1650, and 950 cm⁻¹. However, these are solely characteristic gas peaks with no MOF framework shifts, confirming weak C₂H₄‐framework interactions. Notably, C₂H₆ adsorption revealed much stronger interactions. Alongside C₂H₆ vibrations (ν(C‐H), ∼2950 cm⁻¹; γ(C–H), 1150–1250 cm⁻¹), several framework peaks changed simultaneously: the framework C–H bending (γ(C–H_ph), 750–850 cm⁻¹), the ADC carboxyl peak (ν(C═O), 1600–1650 cm⁻¹), and the Ni─O─C coordination peak (ν(Ni─O─C), 1150–1250 cm⁻¹). Overall, these in‐situ IR results clearly demonstrate that SNNU‐636 π‐rich confined space interacts strongly with C₂H₂ and C₂H₆.

To understand the adsorption behaviors at the molecular level, GCMC simulations and DFT calculations were conducted (Figures S159–S171). The basic MIL‐88 and SNNU‐630 show weak single‐point interactions (>4.0 Å) and low adsorption energies (18.7 kJ·mol⁻¹, 16.6 kJ·mol⁻¹ and 15.5 kJ·mol⁻¹ for C₂H₂, C₂H₄ and C₂H₆ in MIL‐88). After MR induction, SNNU‐631 shortens C₂H₂ distances (3.21–3.99 Å) and shows stronger interactions. With TPP partitioned, SNNU‐632 and SNNU‐633 form multi‐point interactions (3.00–3.98 Å) and enhance C₂H₂ binding (29.1 kJ·mol⁻¹/33.5 kJ·mol⁻¹). However, their energies for C₂H₄ and C₂H₆ remain similar (23.7‐28.0 kJ·mol⁻¹), showing limited selectivity. The DPA‐induced show moderate confinement, while SNNU‐635 restricts adsorption due to its small space. Ultimately, SNNU‐636 combines ADC‐MR and TPP‐PSP to form an effective dynamic‐static synergistic framework. It achieves the strongest multi‐site and guest‐guest interactions for C₂H₂ (−43.3 kJ·mol⁻¹) and strong C‐H···π interactions for C₂H₆ (−39.9 kJ·mol⁻¹), but only weak interactions for C₂H₄ (−34.7 kJ·mol⁻¹). These results clearly demonstrate the synergistic recognition of C₂H₂ and C₂H₆ by SNNU‐636, underscoring the effectiveness of the MR‐enhanced PSP strategy in improving selective adsorption.

3. Conclusion

The molecular‐rotor‐enhanced pore‐space partition strategy (MR‐enhanced PSP) is proposed in this work, which enables fine‐tuned control over the pore structure and adsorption microenvironment of MOF adsorbents. Specifically, PSP achieves static partition of the pore space by constructing rigid ultra‐microporous cages, while MR enables dynamic regulation of the pore adsorption environment through the introduction of π‐conjugated ligands with rotor‐like properties. The synergistic interaction between two factors significantly enhances the spatial confinement effect and the ability to regulate weak interactions within the channels, thereby effectively improving pore space utilization efficiency and led to benchmark one‐step ethylene purification performance. Overall, this study provides a promising pore engineering principle for the development of advanced MOFs capable of achieving precise and efficient gas separation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73228‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.