Insights into the thermodynamic–kinetic synergistic separation of propyne/propylene in anion pillared cage MOFs with entropy–enthalpy balanced adsorption sites

Abstract

Propyne/propylene (C₃H₄/C₃H₆) separation is an important industrial process yet challenged by the trade-off of selectivity and capacity due to the molecular similarity. Herein, record C₃H₄/C₃H₆ separation performance is achieved by fine tuning the pore structure in anion pillared MOFs. SIFSIX-Cu-TPA (ZNU-2-Si) displays a benchmark C₃H₄ capacity (106/188 cm³ g⁻¹ at 0.01/1 bar and 298 K), excellent C₃H₄/C₃H₆ IAST selectivity (14.6–19.3) and kinetic selectivity, and record high C₃H₄/C₃H₆ (10/90) separation potential (36.2 mol kg⁻¹). The practical C₃H₄/C₃H₆ separation performance is fully demonstrated by breakthroughs under various conditions. 37.8 and 52.9 mol kg⁻¹ of polymer grade C₃H₆ can be produced from 10/90 and 1/99 C₃H₄/C₃H₆ mixtures. 4.7 mol kg⁻¹ of >99% purity C₃H₄ can be recovered by a stepped desorption process. Based on the in situ single crystal analysis and DFT calculation, an unprecedented entropy–enthalpy balanced adsorption pathway is discovered. MD simulation further confirmed the thermodynamic–kinetic synergistic separation of C₃H₄/C₃H₆ in ZNU-2-Si.

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Introduction

Propylene (C₃H₆) is the world's second largest volume hydrocarbon with the global production capacity exceeding 140 million tons in 2020. It is a basic olefin feedstock for the manufacture of various polymers and chemicals such as polypropylene and propylene oxide.¹ Originating from the cracking of crude oil, C₃H₆ is inevitably mixed with a small amount of propyne (C₃H₄), which must be reduced to a ppm level before further processing as it can severely poison the C₃H₆ polymerization catalysts.² The state-of-the-art industrial technologies for the removal of C₃H₄ from C₃H₆ rely on noble-metal catalyst based selective hydrogenation, which suffers from several drawbacks such as high cost, low efficiency and potential secondary pollution. On the other hand, C₃H₄ that can be manufactured from the catalytic or thermal pyrolysis of C₃H₆ is also a fundamental material for speciality fuel and chemical intermediates.³ To recover C₃H₄, solvent extraction is the current dominant technology, which is not only energy intensive but also associated with pollution. Thus, it is of urgent importance to develop new technologies for efficient C₃H₄/C₃H₆ separation.

Adsorptive separation based on porous solid adsorbents has been recognized as a promising alternative technology for gas/vapor separation because of its eco-friendly nature and energy efficiency.^4–10 However, due to the great similarity in the molecular size (C₃H₄: 4.16 × 4.01 × 6.51 ų, C₃H₆: 4.65 × 4.16 × 6.44 ų) and polarizability (C₃H₄: 55.5 × 10⁻²⁵ cm³, C₃H₆: 62.6 × 10⁻²⁵ cm³), the adsorptive separation of C₃H₄/C₃H₆ by molecular recognition is still very challenging.¹¹ Only two examples of zeolites are reported and their C₃H₄ capacity is relatively low.¹⁰ Recently, metal–organic frameworks (MOFs) with tuneable pore size/shape and chemistry have emerged as a new class of porous materials for the separation of C₃H₄/C₃H₆.^12–27 Among them, pcu type anion pillared MOFs (APMOFs) with strong Lewis basic functional sites display the benchmark separation performance.^12–18 Nonetheless, the trade-off between the capacity and selectivity is still a critical problem to overcome. For example, SIFSIX-3-Ni (pore size = 4.2 Å, Scheme 1a) as a single-molecule trap for C₃H₄ can afford extremely high C₃H₄/C₃H₆ selectivity (>200), but the capacity of C₃H₄ is only 67 cm³ g⁻¹; SIFSIX-1-Cu (pore size = 8.0 Å, Scheme 1b) can accommodate a large amount of C₃H₄ (201 cm³ g⁻¹) by cooperative host–guest interactions, but the separation selectivity is <10. Besides, these pillared layered SIFSIX MOFs are not chemically stable and some are even sensitive to humid air, which hinders the practical applications.^8d,28 On the other hand, the kinetic separation of C₃H₄/C₃H₆ has never been reported.

Scheme 1 Strategies to overcome the trade-off of capacity and selectivity in C₃H₄/C₃H₆ separation in a stable cage-like APMOF by the thermodynamic–kinetic synergism mechanism. (a) Structure of SIFSIX-3-Ni. (b) Structure of SIFSIX-1-Cu. (c) Structure of SIFSIX-Cu-TPA. (d) Illustration of the thermodynamic–kinetic cooperation for C₃H₄/C₃H₆ separation.

Anion pillared cage-like MOFs with ith-d topology are a new class of APMOFs.^27,29 The rational combination of anion pillars and tridentate organic linkers provides anion sustained cage-like APMOFs with ultrahigh chemical stability. In 2021, Wu et al. discovered the first pillar-cage ith-d MOF SIFSIX-Cu-TPA (Scheme 1c) with a complete SiF₆²⁻ cross-link for CO₂/C₂H₂ separation.²⁹ Soon after, our group reported independently an isomorphic MOF termed ZNU-2 (TIFSIX-Cu-TPA).²⁷ Considering the slight change of the organic linker, the metal ion or anion pillar in pcu-type APMOFs can lead to a dramatic separation difference, we envision that tuning the pore aperture and pore window in cage-like APMOFs can also be applied to tune the gas adsorption properties. Nonetheless, the length and angle matching between the tridentate ligand and anion pillar is very significant to construct the pillar embedding structures. Pillar-cage Tripp-Cu-SIFSIX with the overlong organic linker 2,4,6-tris(4-pyridyl)pyridine is not stable upon guest removal since the mononuclear Cu ion center is only half sustained by SiF₆²⁻ and coordination unsaturated.³⁰ [Cu₃(SiF₆)₃(TMTPB)₄] (FJI-W1) with triangular ligand 1,3,5-trimethyl-2,4,6-tris(4-pyridyl)benzene belongs to pillar-layer APMOFs that feature 1D hexagonal channels.²⁶ Thus, only the modification of the anion pillar or metal ion (size difference < 0.1 Å) is a good alternative to fine-tune the pore structure and chemistry while retaining the topology. Furthermore, the integration of large cages and narrow interlaced channels has the potential to show a kinetic difference for C₃H₄ and C₃H₆ molecules with a slight diameter difference (Scheme 1d), which has not been explored in theory yet.

With this in mind, herein we prepared three isomorphic APMOFs using SiF₆²⁻, TiF₆²⁻, and NbOF₅²⁻ as the pillars, and investigated the C₃H₄/C₃H₆ adsorption and separation performance. To our delight, these three reticular MOFs exhibit quite distinctive but ordered C₃H₄ adsorption capacity as well as C₃H₄/C₃H₆ selectivity. The pore size follows the sequence of SIFSIX-Cu-TPA < TIFSIX-Cu-TPA < NbOFFIVE-Cu-TPA while the C₃H₄ adsorption capacity and the separation selectivity are both SIFSIX-Cu-TPA > TIFSIX-Cu-TPA > NbOFFIVE-Cu-TPA. Benchmark high uptakes of C₃H₄ are observed both at low pressure (106 STP cm³ g⁻¹ at 0.01 bar and 298 K) and normal pressure (188 STP cm³ g⁻¹ at 1 bar and 298 K) on SIFSIX-Cu-TPA. The C₃H₄ storage density reached 0.60 and 0.65 g cm⁻³ at 298 and 278 K, 89% and 97% of the liquid C₃H₄ density. The calculated IAST selectivity is 14.6–19.3 depending on the ratio of C₃H₄/C₃H₆ (1/99–50/50). Record high C₃H₄/C₃H₆ (10/90) separation potential (36.2 mol kg⁻¹) is obtained, which is 65% higher than the previous benchmark of NKMOF-11 without anion functionalities. The modestly high C₃H₄ adsorption heat of 43.2 kJ mol⁻¹ is advantageous for both C₃H₄ adsorption and facile regeneration. Simulated breakthroughs indicated SIFSIX-Cu-TPA displays the best separation performance for C₃H₄/C₃H₆ (10/90) mixtures. Practical separations of C₃H₄/C₃H₆ (50/50, 10/90, 1/99) mixtures were also confirmed by breakthrough experiments. Notably, the practical separation performance is even superior to that of simulation due to the kinetic enhancement, which has never been reported in C₃H₄/C₃H₆ separation. 37.8 and 52.9 mol kg⁻¹ of C₃H₆ is produced from the 10/90 and 1/99 C₃H₄/C₃H₆ mixtures, respectively. The productivity is increased to 79.2 mol kg⁻¹ when the process temperature decreased to 278 K. Such high productivity has never been achieved by chemically stable porous materials. 4.7 mol kg⁻¹ of >99% purity C₃H₄ can be recovered. Repeated breakthrough experiments under dry or humid conditions showed the complete retention of separation performance, confirming the high stability of SIFSIX-Cu-TPA for practical separations. The in situ single crystal structure of C₃H₄-loaded SIFSIX-Cu-TPA directly demonstrates the C₃H₄ binding configuration under near-saturation conditions, which is distinct from the previous study.²⁷ Comprehensive modelling studies including Grand Canonical Monte Carlo (GCMC) simulations, Molecular Dynamics (MD) simulations and Density Functional Theory (DFT) calculations were completely applied to investigate the adsorption/separation process, which indicated that the contracted channel serves as a single molecule flow channel that differentiates C₃H₄/C₃H₆ kinetically while the large cage provides high affinity for C₃H₄ adsorption by cooperative host–guest and guest–guest interactions. To the best of our knowledge, the kinetic separation of C₃H₄/C₃H₆ has for the first time been revealed by MD simulations. The obvious thermodynamic–kinetic synergism in breakthroughs has never been reported in porous materials for C₃H₄/C₃H₆ separation. Moreover, our study unprecedentedly disclosed the important role of entropy effects on C₃H₄ adsorption and gas cluster assembly in the pores while the GCMC and DFT based gas binding configuration may not reflect the practical gas binding sites due to the neglect of the entropy effect.

Results and discussion

The single crystals of isostructural SIFSIX-Cu-TPA (ZNU-2-Si), TIFSIX-Cu-TPA (ZNU-2-Ti) and NbOFFIVE-Cu-TPA (ZNU-2-Nb) are all produced by layering a MeOH solution of TPA onto an aqueous solution of CuX (X = SiF₆²⁻, TiF₆²⁻, NbOF₅²⁻). ZNU-2-Nb has been reported for the first time (Fig. 1a and b). All of these three coordination complexes crystallize in three-dimensional (3D) frameworks with the cubic Pm3̄n space group (Table S1†). The frameworks consist of large icosahedral cages (∼8.5 Å, Fig. 1c) with 12 outlets and narrow interlaced channels (∼4 Å, Fig. 1d) that connect four independent cages (Fig. S6 and S7†). The large cages have abundant Lewis basic F binding sites in the surface for C₃H₄ adsorption and storage. Such interconnected 3D channel pores (Fig. 1e) are distinct from those of pillar-layer MOFs (e.g. SIFSIX-3-Ni) with straight 1D channels. Due to the tiny size difference of anion pillars, the pore aperture and channel diameter also show a very slight difference (<0.05 Å), which is reflected in the N⋯N and Cu⋯Cu distances (Fig. 1f). As the channel diameter is very close to the cross-sections (4.01 × 4.16 Ų for C₃H₄ and 4.65 × 4.16 Ų for C₃H₆), a slight shrinking of the channel may provide a much enhanced kinetic difference in C₃H₄/C₃H₆ adsorption. Thus, ZNU-2-Si with a reduced channel diameter has the potential to show a remarkable kinetic difference in C₃H₄ and C₃H₆ adsorption. In brief, ZNU-2-Si features the most promising structure to offer benchmark C₃H₄/C₃H₆ separation performance by thermodynamic–kinetic synergism.

Fig. 1 Porous structure and stability test of ZNU-2-M (M = Si, Ti, Nb). (a) Basic units to construct ZNU-2-M. (b) Structure of ZNU-2-M with cage-like pores and interlaced channels. (c) Structure of the icosahedral Cu^II cage with the N⋯N distance (L1) of TPA highlighted. (d) Structure of the interlaced channel between four cages with the Cu⋯Cu distance (L2) highlighted. (e) The voids of ZNU-2-Si illustrating the interlinked 3D channels. (f) Comparison of the L1 and L2 among ZNU-2-M. (g) Optical microscopy of single crystals of ZNU-2-M (M = Si, Ti, Nb) after different treatments.

Before gas adsorption experiments, the chemical and thermal stability of ZNU-2-M is fully studied since stability is a prerequisite for practical use in real-world systems. To our delight, all three materials are highly stable in humid air and water as indicated by the PXRD patterns (Fig. S12, S14 and S15†). To provide straightforward evidence, we take photographs of the single crystals of ZNU-2-M after different treatments (Fig. 1g, S86, S88 and S90†). As shown, the single crystals are still of high quality after being left in humid air for 6 months, soaking in water for 2 months, soaking in acidic and basic aqueous solutions or exposure to humid HCl vapor at 323 K for 3 h. Heating at 393 K under vacuum provides desolvated ZNU-2-M with the same crystal structure as indicated by single crystal and powder XRD analysis (Fig. 1g, S11, S14, S15 and S86–S90†). TGA curves showed that the framework of ZNU-2-M is stable at 523, 581, and 573 K (250, 308 and 300 °C), respectively (Fig. S13–S15†). The weight reduction between room temperature and 423 K is ascribed to the loss of solvents (MeOH/H₂O) in the pores of ZNU-2-M.

Encouraged by the ultrahigh stability of ZNU-2-M, we are interested in investigating their permanent porosity as well as unary gas adsorption difference between C₃H₄ and C₃H₆. At first, N₂ gas adsorption experiments at 77 K were conducted (Fig. S16–S19†), which indicated their microporous character with pore size distribution in the range of 6.27–9.84 Å, 6.56–9.40 Å, and 7.85–9.40 Å, respectively, very close to the pore aperture of ∼8.5 Å calculated from the single crystal structure. The BET surface areas and pore volumes are 1339/1380/1281 m² g⁻¹ and 0.565/0.575/0.521 cm³ g⁻¹, for ZNU-2-Si, ZNU-2-Ti and ZNU-2-Nb respectively. These BET surface areas are all superior to the benchmark of SIFSIX-1-Cu (1128 m² g⁻¹) in pillar-layer APMOFs.¹⁴

Single-component C₃H₄ adsorption isotherms were then collected at 298 K (Fig. 2a). At 1 bar, the C₃H₄ uptakes were 188, 171 and 162 cm³ g⁻¹ for ZNU-2-Si, ZNU-2-Ti and ZNU-2-Nb, corresponding to 4.52, 4.25, and 4.34 C₃H₄ molecules adsorbed per anion (Fig. 2b). Such a high C₃H₄/anion ratio means every free F site can bind 1.13, 1.06, and 1.09 C₃H₄ molecules, much higher than those of SIFSIX-2-Cu-i (2.57), TIFSIX-14-Cu-i (2.31), ZU-62 (2.30) and SIFSIX-3-Ni (1.09) (Fig. 2b). The adsorption capacities under 0.01 and 0.1 bar were further compared with those of other MOFs (Fig. 2c). The C₃H₄ uptake of ZNU-2-Si at 0.01 bar is record high at 106 cm³ g⁻¹. This uptake is even much higher than the saturated capacities (1 bar) of most MOFs, such as ELM-12 (61.4 cm³ g⁻¹),¹⁹ SIFSIX-3-Ni (66.8 cm³ g⁻¹),¹⁴ NKMOF-11 (69.4 cm³ g⁻¹),²⁰ GeFSIX-14-Cu-i (75.3 cm³ g⁻¹),¹⁷ Ca-based MOF (67.4 cm³ g⁻¹),²⁵ UTSA-200 (81.1 cm³ g⁻¹),¹⁵ ZU-62 (82.0 cm³ g⁻¹)¹⁸ and TIFSIX-14-Cu-i (86.5 cm³ g⁻¹).¹⁷ Interestingly, a good negative linear relationship between the C₃H₄ uptakes under low pressure (0.1 bar) and the N⋯N/Cu⋯Cu distances is observed (Fig. 2d). Such a structure–capacity relationship has never been reported before. Then C₃H₄ and C₃H₆ adsorption isotherms on ZNU-2-M at 278, 298 and 308 K were all collected (Fig. 2e). The C₃H₆ adsorption capacities are much lower than those of C₃H₄, especially in low pressure regions. The C₃H₄/C₃H₆ selectivity on ZNU-2 at 298 K was calculated by using Ideal Adsorbed Solution Theory (IAST). As shown in Fig. 2f, the selectivity of ZNU-2-Si for 1/99 C₃H₄/C₃H₆ is 14.64, which is higher than that of ZNU-2-Ti (12.53), ZNU-2-Nb (9.84), ZIF-8 (1.9),¹⁵ FJI-W1 (2.2),²⁶ Cu-BTC (3.2),¹⁵ MIL-100(Cr) (4.5),¹⁵ and SIFSIX-1-Cu (9.0)¹⁴ (Fig. 2f). The increased ratio of C₃H₄ in the gas mixture results in increased C₃H₄/C₃H₆ selectivity, which is 16.05 and 19.29 for 10/90 and 50/50C₃H₄/C₃H₆ mixtures, respectively. The simultaneous increase of the C₃H₄/C₃H₆ selectivity along the uptakes or C₃H₄ ratios suggests the existence of cooperative interactions inside ZNU-2-Si.

Fig. 2 (a) C₃H₄ adsorption isotherms in the ZNU-2 family at 298 K. (b) Comparison of the C₃H₄ adsorption isotherms of the ZNU-2 family with fluorinated anion hybrid microporous materials. (c) Comparison of the low pressure C₃H₄ uptake and stability among top-performing MOFs. (d) Plot of C₃H₄ uptake at 0.1 bar vs. N⋯N distance (L1)/Cu⋯Cu distance (L2). (e) C₃H₄ and C₃H₆ adsorption isotherms for ZNU-2-Si at 278, 298 and 308 K. (f) Comparison of IAST selectivity of the ZNU-2 family with other MOFs showing high C₃H₄ capacity (>100 cm³ g⁻¹). (g) Comparison of ZNU-2's IAST based separation potential (Δq = C₃H₄ uptake × 9 − C₃H₆ uptake) for C₃H₄/C₃H₆ (10/90) mixtures with reported top performing MOFs. (h) The isosteric heat of adsorption, Q_st, for C₃H₄ and C₃H₆ in ZNU-2-Si. (i) Adsorption kinetic curves of C₃H₄ and C₃H₆ in ZNU-2-Si.

The static C₃H₄ and C₃H₆ uptakes from the 10/90 mixture of C₃H₄/C₃H₆ were calculated for the ZNU-2 family and other leading materials (Fig. S30, S35, S40 and S42–S48†). The separation potential (Δq = q₁y₂/y₁ − q₂)³¹ as a combined metric of both selectivity and capacity was utilized here for further comparison, which showed a trend of ZNU-2-Si (36.2 mol kg⁻¹) > ZNU-2-Ti (31.0 mol kg⁻¹) > SIFSIX-1-Cu (29.0 mol kg⁻¹)¹⁴ > TIFSIX-14-Cu-i (27.3 mol kg⁻¹)¹⁷ > GeFSIX-14-Cu-i (26.3 mol kg⁻¹)¹⁷ > ZNU-2-Nb (25.0 mol kg⁻¹) > SIFSIX-14-Cu-i (24.6 mol kg⁻¹) > SIFSIX-3-Ni (24.1 mol kg⁻¹)¹⁴ > SIFSIX-2-Cu-i (23.4 mol kg⁻¹)¹⁴ > ZU-62 (22.4 mol kg⁻¹)¹⁸ > NKMOF-11 (22.0 mol kg⁻¹)²⁰ > ELM-12 (21.2 mol kg⁻¹)¹⁹ at 1 bar and 298 K (Fig. 2g). The isosteric enthalpies of adsorption (Q_st) for ZNU-2-M were then calculated with the Clausius–Clapeyron equation. Q_st values at near-zero loading for C₃H₄ and C₃H₆ were 43.2/43.0/41.6 and 35.5/34.5/32.4 kJ mol⁻¹, respectively (Fig. 2h, S28, S33 and S38†). The Q_st values for C₃H₄ in the ZNU-2 family are lower than those of most MOFs for C₃H₄/C₃H₆ separation such as ZU-62 (71.0 kJ mol ⁻¹),²⁵ SIFSIX-3-Ni (68.0 kJ mol⁻¹),¹⁴ NKMOF-1-Ni (65.1 kJ mol ⁻¹),²⁵ Ca-based MOF (55.4 kJ mol ⁻¹),²⁵ UTSA-200 (55.3 kJ mol⁻¹),¹⁵ ELM-12 (60.6 kJ mol ⁻¹)¹⁹ and SIFSIX-2-Cu-i (46.0 kJ mol ⁻¹),¹⁴ but slightly higher than that of SIFSIX-1-Cu (37.2 kJ mol⁻¹)¹⁴ (Table S18†). Such modestly high Q_st not only facilitates preferential C₃H₄ adsorption, but also allows the facile recovery of C₃H₄ by desorption under mild conditions. To further compare the adsorption difference of C₃H₄ and C₃H₆ on ZNU-2-Si, we studied the kinetic adsorption behavior. The adsorption rate of C₃H₄ in ZNU-2-Si is faster than that of C₃H₆. This means that the intra-crystalline diffusion of C₃H₄ is faster than that of C₃H₆ (Fig. 2i). To the best of our knowledge, such kinetic difference has not been reported in pillar-layered APMOFs for C₃H₄/C₃H₆ separations. Besides, adsorption thermodynamics and diffusion are usually anti-synergistic as stronger adsorption often implies reduced mobility.³² Therefore, ZNU-2-Si with thermodynamic–kinetic synergism is highly promising to provide benchmark practical C₃H₄/C₃H₆ separation performance.

To obtain direct host–guest interaction information between ZNU-2-Si and adsorbed gases, we introduced C₃H₄ and C₃H₆ into the desolvated ZNU-2-Si and measured it in the single crystal X-ray diffractometer. Due to the high stability of ZNU-2-Si, the C₃H₄ and C₃H₆ loaded structures are ambiguously resolved (Fig. 3 and S10†). 24 C₃H₄ molecules and 18 C₃H₆ molecules are adsorbed per cell, equal to 4 C₃H₄ and 3 C₃H₆ molecules for every SIFSIX anion, consistent with the experimental results. After adsorption, the framework remained in the same cubic Pm3̄n space group with negligible bond or angle changes (Table S2†). From the in situ crystals, C₃H₄ showed disorder into two overlapping configurations. The configuration with the alkynyl C–H end closer to SiF₆²⁻ is chosen to be discussed in the following text. The hydrogen bond distances between the terminal C₃H₄ hydrogen and F atom of SiF₆²⁻ are all 2.576 Å. Interestingly, no single C₃H₄ molecule is completely loaded in the narrow interlaced single molecule channel, which was previously considered as the best energy favored single molecule binding site.²⁷ Instead, four C₃H₄ molecules are equally close to the interlaced channel while their alkynyl C–H ends are inside (Fig. 3b). On the other hand, all C₃H₄ molecules can be considered to locate in the large cage with their C–H end reaching outside (Fig. 3c). Therefore, every large cage can accommodate 12 C₃H₄ molecules. The C₃H₆ adsorption sites are very close to that for C₃H₄. Due to the high symmetry, every free F atom is able to bind 0.75 C₃H₆ molecules (Fig. S10†). Strangely, the hydrogen bond distance (2.108 Å) between terminal =CH₂ and the F atom is even shorter than the ≡C–H⋯F distance. Thus, DFT calculations are applied to directly compare their binding energies, which will be discussed in the next part.

Fig. 3 Single crystal structure of C₃H₄ loaded ZNU-2-Si. (a) A holistic view. (b) View around the interlaced channel. (c) View around the cage.

The structure of ZNU-2-Si with less C₃H₄ loading is also obtained by controlling the adsorption pressure at 0.01 bar. However, due to the ultrahigh adsorption uptake of C₃H₄ under low pressure, there is still a large amount of C₃H₄ observed in the cage of ZNU-2-Si and the binding sites are the same. The only difference is the occupancy of every C₃H₄ molecule is only ca. 50%, equal to 6 C₃H₄ molecules in every cage. Such uptake is close to the experimental adsorption capacity (106 cm³ g⁻¹) under 0.01 bar.

In most of the literature, bond length is used to compare the interaction strength. However, in our case, the ≡C–H⋯F distance (2.576 Å) is longer than the =CH⋯F distance (2.108 Å), making it difficult to judge which interaction is stronger as the acidity of the ≡C–H hydrogen is stronger than that of =CH₂. Thus, crystallography based DFT calculation is applied to calculate the bonding energy. First of all, we calculate the bonding energy between the framework and single gas molecule. To our delight, the results indicated the binding energy between a single C₃H₄ molecule and ZNU-2-Si is −39.35 kJ mol⁻¹ (Fig. 4a) while that for C₃H₆ is only −34.26 kJ mol⁻¹ (Fig. 4b), indicating the interaction between C₃H₄ and ZNU-2-Si is stronger. The binding energy difference (5.09 kJ mol⁻¹) is also close to the experimental Q_st difference (7.7 kJ mol⁻¹).

Fig. 4 The DFT-D calculated interaction energy of ZNU-2-Si and gas molecules under different loadings based on the single crystal structure. (a and b) A C₃H₄ or C₃H₆ molecule located in a cage. (c and d) Six C₃H₄ or C₃H₆ molecules located in a cage. (e) 8 C₃H₄ molecules located near two neighbouring interlaced channels. (f) 24 C₃H₄ molecules in a unit cell.

The binding energies of ZNU-2-Si and six gas molecules were also calculated, which are −42.57 and −36.93 kJ mol⁻¹ for C₃H₄ and C₃H₆ (Fig. 4c and d), respectively. These binding energies can be separated into two parts: ZNU-2-Si⋯gas (host–guest) interaction energy and gas⋯gas (guest–guest) interaction energy. For C₃H₄, the ZNU-2-Si⋯C₃H₄ binding energy is still −39.35 kJ mol⁻¹ and the interaction energy of six C₃H₄⋯C₃H₄ molecules is −3.22 kJ mol⁻¹ (Fig. 4c). For C₃H₆, the ZNU-2-Si⋯C₃H₆ binding energy is −34.27 kJ mol⁻¹ and the interaction energy of six C₃H₆⋯C₃H₆ molecules is −2.66 kJ mol⁻¹ (Fig. 4d). These results indicated that the C₃H₄⋯C₃H₄ interactions are stronger than C₃H₆⋯C₃H₆ interactions in the confined cavity while the ZNU-2-Si⋯gas molecules remained nearly unchanged with the loading increase.

To gain more insight into the C₃H₄⋯C₃H₄ cluster, we choose another two models with different C₃H₄ molecules for comparison. Fig. 4e displays the structure of 8 C₃H₄ molecules in two neighboring interlaced channels, where the C₃H₄⋯C₃H₄ interaction energies increased to −4.89 kJ mol⁻¹. Fig. 4f displays the complete loading of C₃H₄ molecules in the cages (i.e. 24 C₃H₄ molecules in a unit cell), where the C₃H₄⋯C₃H₄ interaction energies are further increased to −8.46 kJ mol⁻¹. These results unambiguously revealed the boosted C₃H₄ adsorption behavior in ZNU-2-Si through cooperative guest⋯guest interactions.

To gain more insight into the gas adsorption behavior, GCMC simulations were performed, which indicated two distinct binding sites: one located completely in the interlaced channel and the other completely inside the cage. Moreover, the results indicated that 30 C₃H₄ molecules can be adsorbed in a single unit cell at 298 K and 1 bar (Fig. S68†), equal to 209 cm³ g⁻¹ for ZNU-2-Si, similar to the experimental value of 188 cm³ g⁻¹. DFT calculations were then applied to identify the adsorption configuration and binding energies of C₃H₄ in ZNU-2-Si. Fig. 5a shows that the C₃H₄ molecule in the first binding site is completely in the interlaced channel. The three hydrogen atoms from the methyl group in C₃H₄ strongly interact with three F atoms at the sharing edges of four different cages. The hydrogen bond distances are 2.24, 2.73, 2.86 and 2.89 Å. Besides, multiple additional weak van der Waals interactions exist with the C⋯H distances of 2.68, 2.77 and 2.85 Å (Fig. S56†). All of these interactions contribute to a high binding energy of −55.31 kJ mol⁻¹. The second binding site located inside the cage adsorbs C₃H₄ by two strong hydrogen bonds between the terminal hydrogen of C₃H₄ and two adjacent F atoms with distances of 2.29 and 2.31 Å (Fig. 5b). This binding energy is −42.87 kJ mol⁻¹. The binding energy for the second C₃H₄ molecule inside the cage increases to −46.45 kJ mol⁻¹. Thus, the average binding energy of two C₃H₄ molecules inside the cavity is −44.66 kJ mol⁻¹. In addition, the binding energies increase to −48.98, −49.72, and −50.55 kJ mol⁻¹ for accommodation of 6, 10 and 13 C₃H₄ molecules in a cage, respectively (Fig. 5c–f). To provide direct comparison, the GCMC simulation result with 24 C₃H₄ molecules located both in the narrow channel and the large cage was chosen as a model for optimization. DFT calculation indicated the average bonding energy is −50.85 kJ mol⁻¹ (Fig. S69a†), which is still higher than that (−47.81 kJ mol⁻¹) based on the single crystal structure. Moreover, the GCMC optimized C₃H₄ configurations (i.e. 6 C₃H₄ molecules completely in the 6 narrow channels and 18 C₃H₄ molecules in two large cages) do not display distinct changes under DFT optimization.

Fig. 5 The DFT-D optimized C₃H₄ adsorption configuration based on GCMC simulation and bonding energy of C₃H₄ in ZNU-2-Si. (a) Binding site I inside the interlaced channel. (b) Binding site II inside the cage. (c–f) 2, 6, 10, and 13 C₃H₄ molecules adsorbed inside the cage.

As described above, the GCMC based DFT calculation obviously provided more energy favorable binding sites for C₃H₄ molecules compared to those based on the single crystal structure. Then why do C₃H₄ molecules not follow this pathway for accommodation? Analysis of the cage-channel structure indicates that the narrow interlaced channel is the only passage that connects cages. Gas molecules in cage I must pass through the intersection to reach cage II. Thus, once the intersection is occupied, the diffusion is limited. Moreover, the entropy of C₃H₄ in the interlaced channel is the lowest because the rotation is highly restricted in the narrow channel. The diffusion of C₃H₄ from the intersection to the large cages is entropy favorable. Therefore, the experimentally observed C₃H₄ binding configuration is an entropy–enthalpy balanced result. As GCMC simulations neglect the influence of diffusion or entropy effect, they may not reflect the real binding sites in biporous materials.³³ Binding site I (Fig. 5a) can be the exact location for the adsorption of the first C₃H₄ molecule under extremely low pressure. Once the pressure or number of C₃H₄ molecules increases, the diffusion or entropy effect becomes obvious, and the symmetrical binding sites in Fig. 4 to provide higher entropy are favored. On the other hand, the final C₃H₄ adsorption configuration can be considered as the result of competitive adsorption of C₃H₄ from different cages. Due to the high symmetry of the framework, four C₃H₄ molecules in the neighbouring large cages show the same potential to enter the interlaced channel to be strongly trapped but this narrow channel can only accommodate a single C₃H₄ completely. Thus, four C₃H₄ molecules squeeze their smaller ≡C–H ends into the interlaced channel but leave their larger ≡C–CH₃ ends outside of the channel. We further calculate the bonding energy between 24 C₃H₄ molecules and ZNU-2-Si based on single crystal structures with all molecules relaxed (Fig. S69b†). In this case, the binding energy of −50.42 kJ mol⁻¹ is only slightly inferior to the GCMC result (−50.85 kJ mol⁻¹), which is easy to be covered by the entropy penalty. In brief, GCMC based DFT calculations can provide some information on the initial adsorption while in situ single crystal structures give the direct adsorption behavior under the measured conditions.

To gain some insight into the distinct adsorption kinetic difference of C₃H₄ and C₃H₆ in ZNU-2-Si as well as to provide more evidence for the entropy effect, MD simulations were carried out. The configurations of C₃H₄ and C₃H₆ molecules are based on the GCMC simulations and the whole framework is considered flexible except the Cu atoms. Fig. 6a–c illustrate the MSD in the x, y and z directions for 1, 4, and 7 C₃H₄ or C₃H₆ molecules per cage of ZNU-2-Si respectively. These graphs show that within the period of 5000 ps, the C₃H₄ molecules can migrate to other cages through the interlaced channels freely independent of the pressure, namely the number of C₃H₄ molecules located in a cage (Fig. 6d, S70, S71 and S73†). However, the C₃H₆ molecules can only move inside the original cage and are not able to spread to other cages until the number of molecules accommodated in a single cage reaches 5 (Fig. 6e, S70, S72 and S74†). MD-derived C₃H₄ and C₃H₆ diffusion coefficients in ZNU-2-Si were further calculated. The values are 4.72 × 10⁻¹¹/6.79 × 10⁻¹⁴, 4.89 × 10⁻¹¹/4.64 × 10⁻¹³, and 7.55 × 10⁻¹¹/2.50 × 10⁻¹¹ m² s⁻¹ for 1, 4 and 7 C₃H₄ or C₃H₆ molecules located inside a cage. Therefore, the diffusion coefficient of C₃H₄ is much higher than that of C₃H₆, especially under low pressure with the number of the gas molecules in a cage less than 5 (Table S20†). In other words, the diffusion rate of C₃H₄ in ZNU-2-Si is much faster than that of C₃H₆. The C₃H₄/C₃H₆ kinetic selectivity is as high as ∼695 under low pressures and ∼3.0 under high pressures. Such high kinetic selectivity has never been found in porous materials for C₃H₄/C₃H₆ separation, which is highly related to its unique framework structure with large cavities and narrow channels. Since the cages are connected by narrow interlaced channels, the gas molecules must pass through the channels when they need to diffuse from one cage to another. As the cross-section of C₃H₆ (4.65 × 4.16 Ų) is larger than that of C₃H₄ (4.01 × 4.16 Ų), larger pressure is needed to expand the channel sizes (original size ∼ 4 Å) by the rotation of the pyridine ring. Therefore, the narrow interlaced channels can be regarded as molecular sieves to allow the C₃H₄ molecules to pass through while prohibiting the migration of the C₃H₆ molecules under certain pressures. Only when the pressure increased to a higher degree did the gate opening (i.e. ligand rotation) allow C₃H₆ to diffuse fast within different cages.

Fig. 6 MD simulations. (a–c) MSD plot of C₃H₄ and C₃H₆ molecules in ZNU-2-Si with 1, 4 and 7 molecules in a single cage. (d and e) Snapshots of MD simulation of C₃H₄ (d) and C₃H₆ (e) molecules at 0, 2500, and 5000 ps.

We also tried MD simulation considering the framework is completely rigid. In this case, it is very difficult for both C₃H₄ and C₃H₆ molecules to diffuse from one cage to another due to the limitation of the over-contracted intersection (∼4 Å). Therefore, the free energies of C₃H₄ and C₃H₆ moving from the narrow channels to the large cages were compared by calculating the potential of mean force (PMF). The results are presented in Fig. S75† which showed that C₃H₄ has a lower free energy barrier than C₃H₆, suggesting the diffusion of C₃H₄ is much easier than C₃H₆.

To evaluate the practical separation performance of ZNU-2-Si for selective C₃H₄/C₃H₆ separation, transient breakthrough simulations were conducted for the 10/90 C₃H₄/C₃H₆ mixture. The results showed that highly efficient separations could be accomplished by ZNU-2-Si (Fig. 7a). The productivity of C₃H₆ (>99.996% purity) in a single adsorption process is also calculated for ZNU-2-Si and other benchmark materials, which showed ZNU-2-Si has the record C₃H₆ productivity of 30.8 mol kg⁻¹ (Fig. 7b), consistent with the separation potential Δq_IAST based on the static gas adsorption isotherms. Experimental breakthrough studies with the C₃H₄/C₃H₆ (10/90) mixture flowed over a ZNU-2-Si packed column with a flow rate of 4 mL min⁻¹ at 298 K were then carried out. The experimental results were superior to the simulated one and 37.8 mol kg⁻¹ of high purity C₃H₆ can be produced (Fig. 7c). Such enhancement can be attributed to the existence of the kinetic effect, which has never been reported in C₃H₄/C₃H₆ separation. For isomorphic ZNU-2-Ti, the kinetic enhancement is not obvious. The experimental C₃H₄ productivity (25.50 mol kg⁻¹) is even slightly lower than that of the simulation (25.93 mol kg⁻¹). The difference between ZNU-2-Si and ZNU-2-Ti can be accounted for by the reduced channel aperture in ZNU-2-Si that increases the diffusion difference in C₃H₄/C₃H₆ adsorption.

Fig. 7 (a) Simulated breakthrough curves of ZNU-2-Si and other top-performing materials for C₃H₄/C₃H₆ (10/90) at 298 K. (b) Plots of the calculated productivity of C₃H₆ in >99.996% purity and separation potential Δq_IAST. (c) Comparison of the experimental and simulated breakthrough curves of ZNU-2-Si and ZNU-2-Ti for C₃H₄/C₃H₆ (10/90). (d) Experimental breakthrough curves and desorption curves of ZNU-2-Si for C₃H₄/C₃H₆ (50/50) at 298 K. (e) Experimental breakthrough curves of ZNU-2-Si for C₃H₄/C₃H₆ (1/99) at 278, 298, and 308 K. (f) Experimental breakthrough curves of ZNU-2-Si for C₃H₄/C₃H₆ (1/99) at 298 K under dry and humid conditions (activation conditions of ZNU-2-Si between cycles: Ar flow rate 20 mL min⁻¹ at 393 K).

To thoroughly identify the separation performance of ZNU-2-Si, we conducted more breakthrough experiments under various conditions. C₃H₄/C₃H₆ mixtures containing a higher ratio (50%) or lower ratio (1%) of C₃H₄ were tested. In both cases, clean C₃H₄/C₃H₆ separations were achieved. For the 50 : 50 C₃H₄/C₃H₆ mixture, the retention time of C₃H₄ is over twice that of C₃H₆. 7.06 mol kg⁻¹ of C₃H₄ was captured in the column with a purity of ∼86% (Fig. 7d). Controlling the desorption conditions, 4.7 mol kg⁻¹ of >99% purity C₃H₄ can be recovered from the column by evacuation after blowing C₃H₆ out firstly (Fig. 7d and S81†). This record high dynamic productivity of C₃H₄ is impossible to obtain by other APMOFs due to their low C₃H₄ capacity. For the 1 : 99 C₃H₄/C₃H₆ mixture, C₃H₆ broke out at ∼18 min and became saturated immediately while C₃H₄ was not detected until ∼156 min and reached saturation slowly (Fig. 7e). The calculated experimental productivity of C₃H₆ from the 1 : 99 C₃H₄/C₃H₆ mixture at 298 K is 52.9 mol kg⁻¹, much higher than those of SIFSIX-1-Cu (5.0 mol kg⁻¹), ELM-12 (15.0 mol kg⁻¹), SIFSIX-3-Ni (20.0 mol kg⁻¹), SIFSIX-2-Cu-i (25.5 mol kg⁻¹) and ZNU-2-Ti (42.0 mol kg⁻¹). Upon lowering the experimental temperature to 278 K, the productivity of C₃H₆ increased to 79.20 mol kg⁻¹, exceeding that of UTSA-200 (62.9 mol kg⁻¹, 298 K)¹⁵ and NKMOF-11 (74.4 mol kg⁻¹, 298 K)²⁰ (Fig. 7e and S79†). Due to its extremely high water stability, we further carried out the breakthrough experiments under humid conditions. The C₃H₄/C₃H₆ (1 : 99) mixture was firstly bubbled into a bottle full of water and then introduced into the column packed with ZNU-2-Si. The humidity was measured constantly, which was stable at ∼60% after reaching equilibrium. From the repetitive humidity tests, it can be concluded that the influence of moisture is negligible for C₃H₄/C₃H₆ separation in ZNU-2-Si (Fig. 7f). Finally, the breakthrough experiments were conducted for six cycles, and the excellent separation capacity of ZNU-2-Si was retained, indicating that ZNU-2-Si possesses a high cycling stability (Fig. S84 and S85†). In summary, ZNU-2-Si sets a new record for practical simultaneous C₃H₆ purification and C₃H₄ recovery/storage by the combination of high productivity of polymer grade C₃H₆, large amount recovery of C₃H₄, retention of separation performance under humid conditions, outstanding recycling capacity and facile regeneration conditions.

Conclusions

In conclusion, we reported a chemically stable MOF, ZNU-2-Si, with large three-dimensional pores and narrow interlaced channels for record propyne storage and propyne/propylene separation. Notable features of this work include: (1) benchmark C₃H₄ capacity of 106 cm³ g⁻¹ under a low pressure of 0.01 bar; (2) extremely high C₃H₄ storage capacity (188 cm³ g⁻¹, 298 K) and storage density (0.60/0.65 g cm⁻³ at 298/278 K) at 1.0 bar; (3) record high C₃H₄/C₃H₆ (10/90) separation potential (36.2 mol kg⁻¹); (4) record high experimental C₃H₆ productivity (37.81 mol kg⁻¹) from 10/90 C₃H₄/C₃H₆ mixtures; (5) record high >99% purity C₃H₄ recovery (4.7 mol kg⁻¹) from a 50/50 C₃H₄/C₃H₆ mixture by a stepped desorption process; (6) benchmark experimental C₃H₆ productivity (52.9/79.2 mol kg⁻¹ at 298/278 K) from 1/99 C₃H₄/C₃H₆ mixtures; (7) excellent breakthrough recyclability and performance retention under humid conditions; (8) unprecedented revelation of the adsorption and separation mechanism by in situ single crystal analysis and GCMC/MD simulations and DFT calculations. In general, our work not only proposes a strategy of using MOFs with large cages and narrow channels for thermodynamic–kinetic synergistic separation, but also highlights the importance of combining the in situ single crystal structure analysis and theoretical studies to investigate the adsorption/separation mechanism. These cage-like APMOFs with optimal pore chemistry and pore structures are supposed to be promising for many other challenging gas separations.

Data availability

All the data supporting this article have been included in the main text and the ESI.†

Author contributions

Y. J.: synthesis, characterization, adsorption experiments, draft preparation; L. W.: single crystal structure measurement and analysis, funding; T. Y.: GCMC simulation, DFT calculation, MD simulation; J. H.: GCMC simulation, DFT calculation; W. S.: breakthrough experiments; R. K.: IAST calculation, breakthrough simulation; D. W.: discussion, advice; Z. G.: PMF calculation; D. L.: supervision of the theoretical study; X. C.: supervision of the theoretical study; H. X.: supervision of the theoretical study; Y. Z.: concept, supervision, draft preparation, funding.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21908193 and 22205207) and Jinhua Industrial Key Project (2021A22648).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2190367–2190372. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc05742e

‡ These authors contributed equally to this work.

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