Acetylene (C₂H₂), as an important industrial gas, has been widely used as feedstock for the production of commercial chemicals including acrylic acid, vinyl compounds and polyester plastic [1]. Typically, C₂H₂ comes from hydrocarbon cracking or methane combustion, thus inevitably coexisting with some of carbon dioxide (CO₂). Separation of C₂H₂ from C₂H₂/CO₂ mixtures is thus of great importance to obtain high-purity C₂H₂ for the subsequent manufacture processes. Due to their very similar molecular sizes/shapes (C₂H₂, 3.34 × 3.32 × 5.70 ų; CO₂, 3.33 × 3.18 × 5.36 ų, Fig. S1 in Supporting information) associated with close boiling points (C₂H₂, 189.3 K; CO₂, 194.7 K) [2], currently used cryogenic distillation and solvent extraction are energy-consuming and environment-unfriendly. Adsorptive separation by using porous materials thus offers a promising alternative approach for such C₂H₂ purification.

Microporous metal-organic frameworks (MOFs) also known as porous coordination polymers (PCPs) have attracted increased attention on various fields [3,4] including catalysis [5,6], sensing [7,8], biomedicine [9,10], especially for gas separation [11–13] owing to their readily tunable pore structure and functionality. Kitagawa's group reported the first MOF for C₂H₂/CO₂ separation in 2005 [14]. After this pioneering work, several MOFs that can selectively capture C₂H₂ [15–18] or CO₂ [19–22] from C₂H₂/CO₂ mixtures have been developed. The gas capture is generally driven by MOF network-gas interactions originating from the pore size and pore chemistry. Since C₂H₂ and CO₂ feature very similar molecular sizes/shapes, the pore confinement strategy of MOFs alone is quite limited to achieve high C₂H₂/CO₂ separation performance [23–26]. Hence, great efforts have been made to create functional sites including open metal sites [27], hydrogen-bonding acceptors [28–31], to selectively enforce the binding affinity towards C₂H₂. Those functionalized MOFs indeed provide large amount of C₂H₂ adsorption, while result in quite a few co-adsorption of CO₂ due to their enough large pore space. Optimizing pore size and functional surface simultaneously is thus pivotal for construction of the targeted MOFs for C₂H₂/CO₂ separation [32–34].

Recently, the well-known SIFSIX-type materials featuring anionic fluorinated groups (such as SiF₆²⁻ and NbOF₅²⁻) have been demonstrated as very promising materials for gas separation including CO₂/N₂ [35], C₂H₂/C₂H₄ [36], C₃H₄/C₃H₆ [37] and C₃H₆/C₃H₈ [38] separation, since their pore environments can be finely engineered. Whereas, such series of materials normally afford similar adsorption towards C₂H₂ and CO₂, as the fluoride atoms can form strong affinity to both of them, thus leading to moderate C₂H₂/CO₂ selectivity [39,40]. Compared with fluoride atoms, atoms with less negative charges might show increased recognition for C₂H₂ versus CO₂. In particular, oxygen atoms have been well-established to form strong hydrogen-bonding with C₂H₂ molecules, but relatively weaker electrostatics with CO₂ molecules [41,42].

With this in mind, we have firstly introduced ClO₄⁻ as the inorganic linker, imidazole derivative dmpbi (dmpbi = 1,1′-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole)) as the organic linker, and successfully constructed a new perchlorate-based hybrid ultra-microporous material ZJU-194 [Cu(dmpbi)_1.5(ClO₄)(C₂H₅O₂)]_n (CCDC No. 2,050,515). Notably, ZJU-194 possesses the flexible-robust network with one-dimensional tight channels lined with rich bare oxygen atoms. By integrating the refined pore space as well as specific binding sites, the activated ZJU-194 (ZJU-194a) enables a selective two-step gate-opening behavior toward C₂H₂ over CO₂. It thus affords very high adsorptive selectivity for C₂H₂/CO₂ (22.4) at ambient conditions, which is even comparable to the benchmark FeNi-M'MOF (24.0) [32] and MOF-OH (25.0) [43], and superior to most MOFs. Dynamic breakthrough experiments further suggest that the complete C₂H₂/CO₂ separation and high-purity C₂H₂ production can be achieved by ZJU-194a.

Reaction of dmpbi ligand with Cu(ClO₄)₂·nH₂O yielded blue sheet-shaped crystals ZJU-194, and the phase purity was further confirmed by powder X-ray diffraction analysis (PXRD) (Fig. S2 in Supporting information). Crystal analyses clearly show that ZJU-194 crystallizes in triclinic P_-1 space group (Fig. S3 and Table S1 in Supporting information). Both dmpbi ligands and ClO₄⁻ groups are two-connected with Cu atoms, while all Cu centers serve as six-connected nodes, but fall into two different coordination models (Fig. 1a). One type Cu atom connects with four independent dmpbi ligands and two ClO₄⁻ anions, and the other is coordinated by two dmpbi ligands, two ClO₄⁻ anions and two glycol molecules. Both Cu nodes arrange alternatively and are linked adjacently by one ClO₄⁻ anion, thus resulting in one-dimensional (1D) chains (Fig. 1d). Such chains are further bridged by dmpbi ligands to form the 3D network, generating the rhomboid channels with the cavity size of ~4.0 Å (Figs. 1b and c). Moreover, the pore walls are lined by rich bare oxygen atoms originated from ClO₄⁻ anions and the coordinated glycol molecules. These rich bare oxygen atoms within tight cavities can induce strong electrostatics field that would benefit for further gas storage and separation.

We first evaluated the stability of ZJU-194, since the industrial C₂H₂ streams typically contain some moisture. As shown in Fig. S5 (Supporting information), ZJU-194 can keep its structure stable even after soaking in water for 1 day, thus revealing its remarkable chemical stability. It also suggests the good thermostability of ZJU-194, as no decomposition of framework occurs up to 305 ℃ (Fig. S6 in Supporting information). After solve-exchange, ZJU-194 was evacuated at room temperature for 1 day to yield the activated sample (termed as ZJU-194a). PXRD analyses reveal that ZJU-194a still holds its crystalline nature, but shows different patterns from the original ones (Fig. S2). Such similar changes have been widely observed in some MOFs with flexible networks like MIL-53 [44], UTSA-300 [45] and ZJU-196 [46]. It means a probably flexible network of ZJU-194a. To proof such speculation, the molecular probe CO₂ and C₂H₂ were utilized for gas sorption experiments at 196 K (Fig. S7 in Supporting information). Specifically, ZJU-194a shows a typical two-step gate-opening adsorption for CO₂, of which the uptake at the first step (0–0.46 bar) is up to 34.7 cm³/g, and the second step finally affords the adsorption capacity of 43.0 cm³/g at 1.0 bar. The corresponding Brunauer–Emmett–Teller (BET) was estimated to be 108.7 m²/g. Notably, such well-defined plateau of the adsorption isotherm clearly reveals the flexible-robust network of ZJU-194a. Conversely, fast adsorption of C₂H₂ in ZJU-194a was achieved at 196 K with the saturated capacity of ~60 cm³/g at 0.03 bar. It thus indicates the much stronger C₂H₂-ZJU-194a interaction than that of CO₂.

Single-component equilibrium sorption isotherms of C₂H₂ and CO₂ were obtained at 273 K and 296 K respectively (Fig. 2a). Notably, both C₂H₂ adsorption isotherms exhibit two-step uptake behaviors. For 273 K, the C₂H₂ uptake at the first step (0–0.12 bar) is up to 0.91 mmol/g, probably driven by the intrinsic microporous cavies of the robust dense-phase structure [47]. The second step starts at around 0.12 bar and finally affords a C₂H₂ adsorption capacity of 2.42 mmol/g at 1.0 bar. As expected, the gate-opening pressure shifts to higher value (0.32 bar) when the temperature rises up to 296 K. The C₂H₂ adsorption amount of ZJU-194a (1.74 mmol/g at 296 K and 1.0 bar) is comparable to the benchmark CPL-1-NH₂ (1.84 mmol/g) [48] and JNU-1 (2.10 mmol/g) [49], and greatly outperforms that of iMOF-6C (1.11 mmol/g) [50]. Conversely, CO₂ is incapable to trigger the gate-opening adsorption at both 273 K and 296 K, with quite minor uptake capacity of ~0.5 mmol/g at 1.0 bar. For 50/50 C₂H₂/CO₂ separation, the C₂H₂/CO₂ uptake ratio at 0.5 bar is also important. Due to the exclusion effect towards CO₂, ZJU-194a shows very high C₂H₂/CO₂ uptake ratio (4.03) at 0.5 bar, which is superior to most of best-performing MOFs, such as UTSA-74 (2.03) [27], BUT-85 (3.56) [29], FeNi-M'MOF (2.01) [32], NKMOF-1-Ni (1.25) [33], TCuCl (1.96) [51], ZNU-1 (2.46) [52], SNNU-45 (2.03) [53] and MUF-17 (1.25) (Fig. 2b) [54]. Such well-controlled gate-opening adsorption of C₂H₂ versus CO₂ endows ZJU-194a as a desired C₂H₂-selective adsorbent.

Encouraged by the obvious uptake differences of ZJU-194a, we employed the well-established Ideal Adsorbed Solution Theory (IAST) to estimate its adsorption selectivity for 50/50 (v/v) C₂H₂/CO₂ mixtures (Fig. 3a and Figs. S9-S12 in Supporting information). For the very challenging C₂H₂/CO₂ separation, ZJU-194a shows an ultrahigh selectivity up to 125.0 at 0.01 bar (Fig. 3d). With the increase of pressure, the selectivity gradually decreases down to 22.4 at 1.0 bar. We further made comparison of C₂H₂/CO₂ selectivity at ambient conditions among reported MOFs with various separation mechanisms. Clearly, as shown in Fig. 3b, it is hardly for those MOFs to achieve high C₂H₂/CO₂ selectivity (> 20) based on a single separation mechanism including OMS, H-binding and pore confinement [55–60]. On the contrary, the combined effects of OMS or H-binding with pore confinements afford the increased C₂H₂/CO₂ selectivity [61–66]. Actually, most leading MOFs for C₂H₂/CO₂ separation are those with tight binding sites within small pores. Especially, those ultramicroporous MOFs with high density OMS exhibit the benchmark C₂H₂/CO₂ selectivity including ZJU-74a [32] and ATC-Cu [15]. Whereas, it seems more challenging for MOFs to achieve benchmark C₂H₂/CO₂ selectivity via H-binding combined with pore confinement effects, as most of them show moderate selectivity. In particular, although some MOFs with large pores exhibit extremely high C₂H₂ uptake capacity such as SNNU-45 [53], MOF-160 [63,64] and ZJNU-117 [67], their C₂H₂/CO₂ selectivity are still lower than that of ZJU-194a. Moreover, the C₂H₂/CO₂ selectivity of ZJU-194a is even comparable to the benchmark MOFs with high density OMS such as FeNi-M'MOF (24) [32] and NKMOF-1-Ni (~22) [33], and much higher than most reported promising materials including BUT-85 (6.1) [29], MUF-17 (6.0) [54], JCM-1 (13.7) [56], CAU-10-NH₂ (10.8) [58], TCuCl(16.9) [51], UTSA-74 (9.0) [27] and FJU-6-TATB (4.3) (Fig. 3e) [24]. We further obtained the static uptake isotherms of C₂H₂ and CO₂ from equimolar C₂H₂/CO₂ mixtures based on IAST calculations. As shown in Fig. 3c and Fig. S13 (Supporting information), ZJU-194a gives high adsorption capacity of C₂H₂ (1.97 mmol/g and 1.32 mmol/g for 273 K and 296 K respectively) at 1.0 bar while almost completely excluding CO₂, further indicating its particular promise for C₂H₂/CO₂ separation. Dynamic breakthrough test was thus conducted to evaluate the feasibility of ZJU-194a for equimolar C₂H₂/CO₂ mixtures. Clearly, the efficient separation can be accomplished by ZJU-194a. As shown in Fig. 3d, CO₂ eluted first from the column, while C₂H₂ could be detected after 23 min. The regeneration curves of ZJU-194a were obtained via purging by He flow (5 mL/min) at 308 K. As illustrated in Fig. S15 (Supporting information), 96% CO₂ could be desorbed within 5 min, while it needs 35 min to totally remove C₂H₂ from the breakthrough column. In addition, the C₂H₂ production is estimated to be 0.77 mmol/g for one cycle, and the recovered purity reaches up to 80%.

The C₂H₂-selective capture from C₂H₂/CO₂ mixtures by ZJU-194a could be explained by the different host-guest affinity, which is reflected in the isoteric heat of adsorption (Q_st). We thus evaluated the Q_st of C₂H₂ and CO₂ in ZJU-194a by calculating the gas adsorption isotherms at different temperature (Figs. S16 and S17 in Supporting information). As shown in Fig. 3e, the Q_st values of C₂H₂ at near zero loading is as high as 58.1 kJ/mol, notablely higher than that of CO₂ (34.4 kJ/mol). It thus reveals the much stronger interaction between the empty network of ZJU-194a and C₂H₂ than that of CO₂. Particularly, such high Q_st value of ZJU-194a for C₂H₂ is comparable to and even higher than those of MOFs with high density OMS, such as Fe-MOF-74 (47 kJ/mol) [68], NKMOF-1-Ni (60.3 kJ/mol) [33], and ZJU-74a (45 kJ/mol) [34].

In summary, we have designed a new perchlorate-based hybrid ultramicroporous material for high C₂H₂/CO₂ separation. By integrating the refined pore space as well as rich bare oxygen atoms, ZJU-194a enables a selective two-step gate-opening behavior toward C₂H₂, but blocks off the adsorption of CO₂. It thus affords a very high adsorptive selectivity for C₂H₂/CO₂ (22.4) at ambient conditions, which is superior to most MOFs. Both single-component equilibrium sorption and dynamic breakthrough experiments reveal its high C₂H₂ capture and C₂H₂/CO₂ separation performance. Our work here provides a new view of regulating the pore chemistry and the flexible-robust network comprehensively within MOF adsorbents to address the very challenging C₂H₂ separation.

The authors declare no competing financial interest.

This work was supported by the National Natural Science Foundation of China (No. 52003069) and the Hainan Provincial Natural Science Foundation of China (No. 520QN220).

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2022.108039.