Temperature-dependent rearrangement of gas molecules in ultramicroporous materials for tunable adsorption of CO₂ and C₂H₂

Abstract

The interactions between adsorbed gas molecules within porous metal-organic frameworks are crucial to gas selectivity but remain poorly explored. Here, we report the modulation of packing geometries of CO₂ and C₂H₂ clusters within the ultramicroporous CUK-1 material as a function of temperature. In-situ synchrotron X-ray diffraction reveals a unique temperature-dependent reversal of CO₂ and C₂H₂ adsorption affinities on CUK-1, which is validated by gas sorption and dynamic breakthrough experiments, affording high-purity C₂H₂ (99.95%) from the equimolar mixture of C₂H₂/CO₂ via a one-step purification process. At low temperatures (<253 K), CUK-1 preferentially adsorbs CO₂ with both high selectivity (>10) and capacity (170 cm³ g⁻¹) owing to the formation of CO₂ tetramers that simultaneously maximize the guest-guest and host-guest interactions. At room temperature, conventionally selective adsorption of C₂H₂ is observed. The selectivity reversal, structural robustness, and facile regeneration of CUK-1 suggest its potential for producing high-purity C₂H₂ by temperature-swing sorption.

Introduction

Host-guest chemistry is fundamental to the selectivity of many molecular recognition systems^1,2,3,4,5. The optimization of cooperative interactions, such as electrostatic interactions and hydrogen bonding, plays a crucial role in the design of efficient molecular recognition systems, particularly in porous materials. These cooperative interactions are essential for achieving high performance in gas adsorption, sensing, and catalysis applications^{1,3,6,7,8,9,10,11,12,13,14,15,16,17}. On the other hand, guest-guest interactions or the formation of guest clusters also play an important role in molecular recognition. However, the direct observation and control of guest-guest interactions within confined nanovoids of porous materials is highly challenging and remains poorly explored^18,19,20. Screening new host-guest and guest-guest interactions can promote the design of new functional porous materials^1,21.

Ultramicroporous metal-organic frameworks (MOFs), featuring highly inerratic porosity, tunable pore chemistry, and designable structures, provide a unique platform to explore host-guest interactions^{11,22,23,24,25,26,27}. In particular, the modular nature and reticular structure endow ultramicroporous MOFs with the possibility to precisely control the host-guest and guest-guest interactions within the pores^28,29. Great advances in host-guest chemistry have been achieved in ultramicroporous MOFs with tailor-made properties for gas adsorption and separation, owing to the confinement effect from the strong host-guest interactions^30,31,32. Currently, the major interest in gas adsorption and separation using ultramicroporous MOFs is focused on enhancing recognition selectivity by tuning the host-guest interactions^{19,29,33,34,35,36}. This is pronounced for selective adsorption of acetylene (C₂H₂) from carbon dioxide (CO₂), as C₂H₂ is one of the most important industrial precursors, and the CO₂ contaminant would be coproduced during the production of C₂H₂ via partial combustion of natural gas^34,35,36. However, the understanding of the impact of guest-guest interactions or guest clusters on selectivity remains lacking due to the difficulties in the direct observation of such dynamic and weak interactions.

Herein, we report the modulation of geometries of guest-clusters as a function of temperature (Fig. 1) for the normal and inverse selectively and separation of CO₂ and C₂H₂ within the robust ultramicroporous M-CUK-1 (M = Co, Ni, and Mg) materials. The guest-guest interactions and binding domains within CUK-1 with different metal nodes have been observed by in-situ synchrotron X-ray diffractions and molecular simulations. The efficient packing of well-organized CO₂ clusters with T-shaped dimers gives rise to notably higher crystallographic occupancy and capacity of CO₂ (106 vs. 86 cm³ g⁻¹ of C₂H₂ in Co-CUK-1 at 298 K), while the stronger host-guest interactions between C₂H₂ and CUK-1 at room temperature lead CUK-1 to preferentially adsorb C₂H₂ over CO₂ (Fig. 1). Notably, a much larger increment of CO₂ capacities at low temperatures was observed compared with those of C₂H₂, which is resulted from the highly efficient packing of CO₂ clusters with tetramers and the significantly stronger host-guest interactions between CO₂ and CUK-1. This finally leads to much higher CO₂ capacities (170 vs. 119 cm³ g⁻¹ of C₂H₂ at 233 K) and clear sorption inversion of CO₂ over C₂H₂. Such an inverse CO₂/C₂H₂ adsorption behavior is more desirable for industrial production of C₂H₂ via a one-step CO₂ adsorption process but is rarely investigated^{18,29,37,38,39,40}. The temperature-dependent reversal of sorption behavior for CO₂ and C₂H₂ is demonstrated by gas sorption isotherms and dynamic breakthrough experiments at various temperatures. High-purity C₂H₂ (99.995%) can be directly obtained in a one-step process, and the low energy input for the regeneration suggests that CUK-1 is a promising adsorbent for C₂H₂ production via the temperature-swing adsorption (TSA) process.

Fig. 1: Illustration of the temperature-dependent packing geometries of guest clusters for the normal and inverse adsorption behavior.

At a high temperature (T1, purple), the strong host-guest and guest-guest interactions result in preferential adsorption of C₂H₂, but the efficient packing of molecular chains formed by CO₂ molecules through strong guest-guest interactions leads to the higher uptake at the high-pressure range (>P_cross). After decreasing the temperature to T2 (blue), the CO₂ clusters with T-shaped dimers exhibit higher occupancy of the pore channels than that of C₂H₂ clusters packed together via π⋯π interactions, coupled with the strong host-guest interactions, leading to the inverse CO₂ preferential sorption.

Results

Materials and characterization

M-CUK-1 (M = Co, Ni, and Mg) were hydrothermally synthesized by reacting 2,4-pyridinedicarboxylic acid (2,4-H₂pdc) and M²⁺-containing salts (M = Co, Ni, and Mg) with KOH in water at 210 °C for 24 h^41,42,43. The CUK-1 materials are isostructural. The edge- and vertex-sharing M₃(µ₃-OH)₂ chains serve as undulating pillars connecting the 2,4-pdc ligands in an orthogonal fashion, forming a ‘wine-rack’ topology with one-dimensional diamond-shaped and corrugated channels (Supplementary Fig. 1)^41,42,43. All three CUK-1 materials show excellent chemical and structural stability, and are entirely stable upon air exposure for two years (Supplementary Figs. 2–5). Desolvated CUK-1 exhibits an ultramicroporous structure, as evidenced by the negligible N₂ uptakes and typical type-I CO₂ isotherms at 77 K and 196 K, respectively (Supplementary Figs. 6–9). The calculated Brunauer–Emmett–Teller (BET) surface areas are 500~600 m² g⁻¹ based on the CO₂ isotherms. Upon desolvation, the exposed μ₃-OH groups reside orderly in the channels (8.1 × 10.6 Ų, Supplementary Fig. 1), acting as potential binding sites to guest molecules through electrostatic interactions^41,42,43. This is highly desirable for the adsorption and separation of hydrocarbons.

Analysis of gas adsorption isotherms and selectivity

Adsorption isotherms of CO₂ and C₂H₂ on desolvated M-CUK-1 (M = Co, Ni, and Mg) at 298 K indicate the preferential adsorption of C₂H₂ at low pressure but higher saturation capacity of CO₂ upon increasing the pressure (Fig. 2a–c). This behavior results in the intersection of the two isotherms at moderate pressure. Another ultramicroporous compound, SIFSIX-3-Ni, exhibits similar isotherm crossing but with a stronger affinity to CO₂ at low pressures²⁹. After the intersection of CO₂ and C₂H₂ isotherms at 0.42 bar on Co-CUK-1, the CO₂ isotherm is above that of C₂H₂, and CO₂ uptake at 1 bar can reach 106 cm³ g⁻¹, much higher than that of C₂H₂ (86 cm³ g⁻¹, Fig. 2a). To the best of our knowledge, such an intersection between CO₂ and C₂H₂ isotherms is rarely observed on porous materials²⁹. Similarly, Ni- and Mg-CUK-1 show stronger sorption affinities to C₂H₂ in the low-pressure range, and the CO₂ and C₂H₂ isotherms also intersect but with relatively higher intersecting pressures of 0.6 and 0.95 bar on Ni- and Mg-CUK-1, respectively (Fig. 2b, c).

Fig. 2: CO2 and C2H2 adsorption and separation performance on CUK-1 materials.

The CO₂ and C₂H₂ adsorption isotherms on desolvated Co-CUK-1 (a), Ni-CUK-1 (b), and Mg-CUK-1 (c) at 298 (purple), 253 (blue), and 233 K (orange). d The comparison of CO₂ and C₂H₂ uptakes at 0.5 bar on CUK-1 materials at different temperatures. e The inverse CO₂/C₂H₂ (1/1) (top) and normal C₂H₂/CO₂ (1/1) (bottom) selectivities at 233 and 298 K, respectively, on CUK-1 materials. f Comparison of the zero-coverage heat of adsorption of CUK-1 materials for CO₂ with those of other materials for inverse CO₂/C₂H₂ separation.

Considering that the inversed CO₂/C₂H₂ selectivity is more desirable for industrial C₂H₂ production, the respective guest loadings were measured at progressively lower temperatures to decrease the intersecting pressures. Upon reducing the temperature, there are significant enhancements for CO₂ uptakes but only a slight increase in C₂H₂ uptakes, finally leading to much higher CO₂ uptakes, even at very low pressures (Fig. 2a–c and Supplementary Figs. 7–9). Specifically, the CO₂ uptakes at 233 K are 170, 142, and 144 cm³ g⁻¹ on Co-, Ni-, and Mg-CUK-1 (ca. 4.15, 3.43, and 3.50 CO₂ molecules per cell, respectively), which notably exceed those of C₂H₂ (119, 97, and 89 cm³ g⁻¹, respectively; ca. 2.89, 2.29, and 2.32 C₂H₂ molecules per cell on Co-, Ni-, and Mg-CUK-1, respectively). The densities of adsorbed CO₂ molecules (based on the structural pore volume) in Co-, Ni-, and Mg-CUK-1 at 233 K were estimated to be 1.40, 1.27, and 1.25 g cm⁻³, respectively. Notably, these densities are higher than that of liquid CO₂ (1.1 g cm⁻³) but lower than that of dry ice (1.55 to 1.7 g cm⁻³)⁴⁴, indicating the highly efficient packing of CO₂ in the pores. However, the densities of adsorbed C₂H₂ molecules were recorded as only 0.56, 0.50, and 0.45 g cm⁻³ in Co-, Ni-, and Mg-CUK-1, respectively, lower than that of liquid C₂H₂ (0.69 g cm⁻³)⁴⁵. At 253 K and 233 K, there is a clear inversion in the adsorption selectivity from C₂H₂ to CO₂ on the CUK-1 materials. The uptake gap between CO₂ and C₂H₂ at 0.5 bar on Co-CUK-1 can reach 40 and 45 cm³ g⁻¹ at 253 and 233 K (Fig. 2d), respectively.

State-of-the-art C₂H₂/CO₂ separation is mainly realized by cryogenic distillation and solvent absorption with high energy penalty. The adsorptive separation using CO₂-selective other than C₂H₂-selective materials is preferable in the industry for producing pure C₂H₂ via one-step sorption procedures. To see whether such a temperature-induced adsorption inversion behavior can be used for inverse CO₂/C₂H₂ separation, we evaluated CUK-1 materials for separating the equimolar mixture of CO₂/C₂H₂ by analyzing the single-component isotherms via ideal adsorbed solution theory (IAST). At 298 K, CUK-1 only shows a moderate C₂H₂/CO₂ selectivity of ca. 2. However, at 233 K, CUK-1 exhibits the inversed CO₂/C₂H₂ selectivity of 9.5, 8.4, and 12.1 for Co-, Ni-, and Mg-CUK-1, respectively (Fig. 2e). The inversed selectivities are comparable with those of the state-of-the-art materials for inversed CO₂/C₂H₂ separation, such as [Mn(bdc)(bpe)] (9)⁴⁶, Ce(IV)-MIL-140-4F (9.6)³⁷, PCP-NH₂-ipa (6.4)³⁵, and SIFSIX-3-Ni (7.5)²⁹, but lower than the benchmark material Cu-F-pymo (> 10⁵)³⁸. Furthermore, the isosteric heats of adsorption (ΔH_ads) of CO₂ on Co- and Ni-CUK-1 were calculated to be 20.8 and 21.7 kJ mol⁻¹, respectively (Supplementary Fig. 10), much lower than that of other materials (Fig. 2f), such as PCP-NH₂-ipa (26.8 kJ mol⁻¹)³⁵, [Mn(bdc)(bpe)] (29.5 kJ mol⁻¹)⁴⁶, MUF-16 (32 kJ mol⁻¹)³⁹, and Tm₂(OH-bdc) (45 kJ mol⁻¹)⁴⁰.

Guest configurations determined by in-situ synchrotron X-ray powder diffraction

In-situ synchrotron X-ray powder diffraction data on CO₂- and C₂H₂-loaded CUK-1 materials were collected as a function of temperature (Supplementary Figs. 11–12). Full refinements of the data indicate two binding sites in the asymmetric unit: site I is close to the μ₃-OH group, and site II locates near the pore surface (Figs. 3–4 and Supplementary Figs. 13–18). At 298 K, the total crystallographic occupancy of C₂H₂ molecules (2.05 per cell) in Co-CUK-1 is in excellent agreement with that obtained from the isotherm (2.07 C₂H₂ per cell). C₂H₂ molecules at site I locate almost perpendicular to μ₃-OH groups, forming O-H⋯π_C2H2 H-bonds (2.92 Å, dotted green lines), supplemented by additional interactions via C-H_C2H2⋯O_ligand H-bonding (dotted green lines, 2.67-2.71 Å, Fig. 3a and Supplementary Fig. 13a). C₂H₂ molecules at site II sit close to the aromatic rings on the pore surface and form weak interactions with the framework through multiple C-H_C2H2⋯O_ligand H-bonding (3.37–3.82 Å) and π_C2H2⋯H_ligand (3.44–3.67 Å) interactions. Moreover, at high loading, the neighboring C₂H₂ molecules synergistically interact with each other through multiple H_C2H2⋯π_C2H2 interactions (dotted pink lines, 2.33–2.95 Å), forming the tetramer-clusters of C₂H₂ (Fig. 3b).

Fig. 3: Configurations of adsorbed C2H2 and CO2 molecules within Co-CUK-1 from refinements of in-situ synchrotron X-ray powder diffraction data at 298 K.

Views of host-guest interactions of C₂H₂ (a) and CO₂ (c) in Co-CUK-1. Packing geometries of C₂H₂ (b) and CO₂ (d) clusters. Color code: C, gray; H, gray-25%; O, red; N, blue; Co, pink.

Fig. 4: Configurations of adsorbed C2H2 and CO2 molecules within Co-CUK-1 from refinements of in-situ synchrotron X-ray powder diffractions at 233 K.

Views of host-guest interactions between C₂H₂ (a) and CO₂ (c) in Co-CUK-1. Packing geometries of C₂H₂ (b) and CO₂ (d) clusters. Color code: C, gray; H, gray-25%; O, red; N, blue; Co, pink.

In contrast, CO₂ molecules show different geometries of packing (Fig. 3c and Supplementary Fig. 14a). CO₂ molecules at site I exhibit an end-on interaction to μ₃-OH group via hydrogen bonds (dotted lime lines, 2.42 Å of O-H⋯O_CO2), but no interactions between CO₂ and the ligand of CUK-1 were observed. CO₂ molecules at site II interact with the pore surface via weak O⋯H_ligand interactions (3.88–3.91 Å, Supplementary Fig. 14a). Thus, adsorbed CO₂ molecules at both sites show much weaker interactions compared with C₂H₂, entirely consistent with the adsorption results at room temperature. However, at high loading, two one-dimensional chains of CO₂ (dotted azure lines) running along the channel were formed via strong guest-guest interactions (2.93 and 2.74 Å, Fig. 3d). These chains are stabilized by multiple weak intermolecular dipole interactions between monomer-to-dimer and dimer-to-dimer of CO₂. Furthermore, two chains interact with each other via multiple synergistic host-host interactions (dotted green lines, 3.27–3.28 Å). Notably, the neighboring CO₂ molecules exhibit a head-to-center (C = O⋯C) geometry (Supplementary Fig. 14a), thus leading to the efficient packing of CO₂ in the pore channels. Similar binding sites of CO₂ in Ni-CUK-1 were also observed (Supplementary Fig. 18). Compared with C₂H₂ clusters, the efficient packing of CO₂ molecules near the center of pore channels via strong guest-guest interactions but with less host-guest interactions is the main reason for the high adsorption of CO₂ in CUK-1 at high pressures.

At 233 K, remarkable changes in the packing geometry of CO₂ and C₂H₂ were observed (Fig. 4), and the crystallographic occupancy of C₂H₂ molecules increased to 2.77 per cell. Meanwhile, the CO₂ occupancy increased to 3.47 per cell (vs. 2.07 at 298 K), indicating the high capacity of Co-CUK-1 for CO₂ at 233 K compared with that for C₂H₂. This is entirely consistent with the isotherms. C₂H₂ molecules at site I interact with bridging μ₃-OH groups via π_C2H2⋯H-O H-bond (2.96 Å) that is supplemented by weak C-H⋯O_ligand H-bonding (dotted green lines, 2.75 Å, Fig. 4a and Supplementary Fig. 13b). CO₂ molecules at site I are stabilized by C-O_CO2⋯H_μ3-OH H-bonding (2.43 Å) and C-O_CO2⋯H_ligand interactions (2.80 and 3.64 Å, Fig. 4c and Supplementary Fig. 14b). C₂H₂ molecules at site II reside near the center of pore channels with fewer host-guest interactions (π⋯H_ligand 3.31 Å), similar to that of CO₂ in the channels at 298 K. However, CO₂ molecules are located in the corner of the pore channels and stabilized by multiple weak host-guest interactions (C-O_CO2⋯H_ligand, 2.40–2.87 Å), thus leading to the strong binding affinities of host framework for CO₂. Meanwhile, in Co-CUK-1, C₂H₂ clusters are formed with C₂H₂ molecules via π_C2H2⋯π_C2H2 interactions (dotted orange lines, 2.95 and 3.18 Å, Fig. 4b). The neighboring clusters synergistically interact with each other via weak C-H⋯π H-bonding (3.05 and 3.17 Å), leading to the efficient packing of C₂H₂ molecules. By contrast, the isolated CO₂ clusters are formed with four CO₂ molecules by closely interacting with each other (distances of 2.53 and 3.14 Å) with the head-to-center configurations, forming the quasi-T-shaped geometry (C = O⋯C, dotted azure lines, Fig. 4d). This is similar to that in dry ice, indicating the highly efficient packing of CO₂ molecules (thus packing densities) in Co-CUK-1. Similar CO₂ clusters with quasi-T-shaped dimers were also observed in Ni-CUK-1 at 233 K (Supplementary Fig. 16d). Thus, the notably stronger guest-guest interactions between adsorbed CO₂ molecules at low temperature promote the unusually selective adsorption of CO₂ over C₂H₂ at 233 K. Importantly, to the best of our knowledge, such guest-guest packing geometries and host-guest interactions at different temperatures have not been observed in porous materials yet.

To quantitatively compare the binding affinities of CUK-1 to CO₂ and C₂H₂ at different temperatures, the static binding energies (ΔE) were further estimated by first-principles density functional theory (DFT) calculations (Supplementary Tables 2 and 3). The results show that after decreasing the temperature from 298 to 233 K, there are significant increases of ΔE at site I for CO₂. Especially, ΔE for CO₂ at site I on Co-CUK-1 is 43.5 kJ mol⁻¹, much higher than that for C₂H₂ (33.3 kJ mol⁻¹), directly validating the preferential adsorption of CO₂ over C₂H₂. There is a subtle difference in host-guest interactions when varying the metal nodes in CUK-1 (Supplementary Tables 2 and 3), and this has little influence on the formation of guest clusters and the tunable CO₂ and C₂H₂ sorption behavior. Thus, the guest-guest interactions and/or the arrangement of guest clusters play the dominant role in the inverse CO₂ sorption of CUK-1 materials.

Dynamic breakthrough tests

Dynamic breakthrough experiments on CUK-1 materials using mixtures of CO₂/C₂H₂ were conducted (Fig. 5). For the equimolar CO₂/C₂H₂ mixture at 298 K, Ni- and Mg-CUK-1 show typical C₂H₂-preferential sorption over CO₂ with a clear separation of C₂H₂ and CO₂, but Co-CUK-1 shows very poor separation (Fig. 5a and Supplementary Fig. 19). These are consistent with the isotherm results at 298 K. At 273 K, a clear inversed CO₂/C₂H₂ separation was observed on Co-CUK-1 (Fig. 5b and Supplementary Fig. 20), and there is an obvious deterioration in C₂H₂/CO₂ separation performance on Ni- and Mg-CUK-1 (Supplementary Fig. 21). At 233 K, an evident inversed CO₂/C₂H₂ separation was observed on CUK-1 materials, and all materials exhibit highly selective adsorption of CO₂ over C₂H₂ (Fig. 5b, c). The dynamic CO₂ uptake capacities at 233 K were calculated to be 140, 110, and 122 cm³ g⁻¹ on Co-, Ni-, and Mg-CUK-1, respectively, much higher than those of C₂H₂ (62, 67, and 78 cm³ g⁻¹, respectively). To mimic the industrial processes for C₂H₂ production, we further studied a gas mixture of CO₂/C₂H₂ (1/2). A complete inversed CO₂/C₂H₂ separation was realized with CO₂ retained in the fixed bed for a longer duration (Fig. 5d). Significantly, the productivity of pure C₂H₂ (99.995%) can reach 62 and 41.7 L kg⁻¹ on Co-CUK-1 and Mg-CUK-1, respectively, much higher than that on MUF-16 (27 L kg⁻¹)³⁹ and Cu-F-pymo (3.7 L kg⁻¹)³⁸. It is worth noting that these materials show excellent cycling separation performance and can be easily regenerated by purging helium (He) at 298 K (Supplementary Figs. 22–23). The notably high CO₂ uptake and facile regeneration of CUK-1 are particularly desirable for practical applications to reduce the energy footprint compared with state-of-the-art cryogenic distillations.

Fig. 5: Breakthrough curves of CUK-1 materials for CO2/C2H2 mixtures.

a Breakthrough curves of CO₂/C₂H₂ (1/1) mixture on CUK-1 materials at 298 K with a flow rate of 2.0 mL min⁻¹. b Breakthrough curves of CO₂/C₂H₂ (1/1) mixture on Co-CUK-1 at 273 and 233 K with flow rates of 2.0 and 3.0 mL min⁻¹, respectively. c Breakthrough curves of CO₂/C₂H₂ (1/1) mixture on Ni-CUK-1 and Mg-CUK-1 at 233 K with a flow rate of 3.0 mL min⁻¹. d Breakthrough curves of CO₂/C₂H₂ (1/2) mixture on CUK-1 materials at 233 K with a flow rate of 3.0 mL min⁻¹.

Discussion

In summary, we report the direct observation of packing geometry rearrangement of gas clusters as a function of temperature to control the adsorption selectivity of CO₂ and C₂H₂ on ultramicroporous MOFs. The strong host-guest interactions of CUK-1 for C₂H₂ at ambient conditions led to the preferential adsorption of C₂H₂. However, the efficient packing of CO₂ molecules via strong guest-guest interactions forms CO₂ clusters, leading to higher CO₂ capacity. Impressively, after decreasing temperature, the host-guest interactions between CO₂ and host framework became stronger than that for C₂H₂. Furthermore, the highly ordered arrangement of CO₂ clusters with the T-shaped dimers endows CUK-1 with remarkably higher capacities for CO₂ over those for C₂H₂. Such host-guest interactions, guest-guest interactions, and gas cluster formation were elucidated by in-situ synchrotron X-ray powder diffraction and molecular simulations. This idiosyncratic inversion of the adsorption behavior of C₂H₂ and CO₂ was verified by dynamic breakthrough experiments with high-purity C₂H₂ (99.995%) obtained in a one-step process. Furthermore, the fixed-bed packed with CUK-1 can be easily regenerated at room temperature by purging an inert gas, indicating that the TSA process using CUK-1 materials is highly efficient for C₂H₂ production.

Methods

Gas adsorption and separation experiments

CO₂ and C₂H₂ sorption isotherms were collected at different temperatures on a Micromeritics ASAP 2020 instrument equipped with commercial software for data calculation and analysis. The test temperatures were controlled by soaking the sample cell in a circulating water bath (298 K), ice/methanol mixture (233–273 K), dry ice/acetone mixtures (196 K), or liquid N₂ (77 K). Before measurement, the sample (80–100 mg) was degassed at 423 K for 24 h. The breakthrough experiments were performed in a stainless-steel fixed bed (4.6 mm inner diameter × 50 mm length) packed with ~0.6 g of CUK-1 powder. Before the breakthrough experiment, the fixed bed was heated at 423 K under a flow of He for complete activation. The fixed bed was then cooled to room temperature and immersed in a water/methanol bath with different temperatures. Then, the gas mixtures (C₂H₂/CO₂) were introduced, and the outlet gas was monitored by mass spectrometry (Hidden QGA quantitative gas analysis system).

In-situ synchrotron powder X-ray diffraction and structure determination

Fresh samples of Co-CUK-1 or Ni-CUK-1 were pre-activated under a dynamic vacuum at 200 °C, then loaded into a 0.7 mm borosilicate capillary under an inert atmosphere. Then the capillary was further activated by heating to 100 °C under a dynamic vacuum for 2 h before cooling down to room temperature. For samples measured under 233 K, synchrotron X-ray powder diffraction was carried out on the ID22 high-resolution powder diffraction beamline at the European Synchrotron Radiation Facility (ESRF). C₂H₂ or CO₂ was introduced into the capillary, and diffraction data were collected after one-hour stabilization. Data were measured between 0 and 35° with a 13-channel multi-analyzer stage under the wavelength of 0.354267(4) Å. Data were binned using a step size of 0.002°. For samples measured under 298 K, high-resolution powder X-ray diffraction patterns were collected on the powder diffractometer (λ = 0.825829(1) Å) at room temperature on beamline I11 (Diamond Light Source, UK). C₂H₂ or CO₂ was dosed offline and then sealed for measurement. Data were collected between 0 and 150° using a step size of 0.001° with five multi-analyzing crystal (MAC) detectors without further re-binned.

Pawley and Rietveld refinements of the structure were carried out using the TOPAS software package (roughly between 18–0.90 Å in d-spacing). Due to the high flexibility of the framework, index with constraints was used to get the information on cell parameters and space groups. Stephen fitting was applied to describe the diffraction peaks and their anisotropic broadening. Approximate positions of the guest molecule under a rigid body model were found using the simulated annealing approach before further refinement was used to find the optimal orientation of the guest molecules. The final refined structural parameters include cell parameters, the fractional coordinates (x, y, z) and isotropic displacement factors for all atoms except hydrogen, and the site occupancy factors (SOF) for guest molecules. The accuracy of the final model was verified by the convergence of the weighted profile factor (R_wp), the chemical sense of the model, and the good correlation between the observed and calculated diffraction patterns.