Efficient acetylene/carbon dioxide separation with excellent dynamic capacity and low regeneration energy by anion-pillared hybrid materials

Abstract

Adsorptive separation of acetylene/carbon dioxide mixtures by porous materials is an important and challenging task due to their similar sizes and physical properties. Here, remarkable acetylene/carbon dioxide separation featuring a high dynamic breakthrough capacity for acetylene (4.3 mmol·g^–1) as well as an ultralow acetylene regeneration energy (29.5 kJ·mol^–1) was achieved with the novel TiF₆^2–-pillared material ZU-100 (TIFSIX-bpy-Ni). Construction of a pore structure with abundant TiF₆^2– anion sites and pores with appropriate sizes enabled formation of acetylene clusters through hydrogen bonds and intermolecular interactions, which afforded a high acetylene capacity (8.3 mmol·g^–1) and high acetylene/carbon dioxide uptake ratio (1.9) at 298 K and 1 bar. Moreover, the NbO₅^2– anion-pillared material ZU-61 investigated for separation of acetylene/carbon dioxide. In addition, breakthrough experiments were also conducted to further confirm the excellent dynamic acetylene/carbon dioxide separation performance of ZU-100.

Key words： adsorption; acetylene/carbon dioxide separation; dynamic capacity; anion-pillared hybrid material

1 Introduction

Acetylene (C₂H₂) is one of the fundamental raw materials used for manufacturing various organic chemicals and is generally produced by partial combustion of natural gases or decomposition of calcium carbide. To obtain pure C₂H₂, CO₂ impurities must be removed. However, due to the similarities in physical properties (boiling points: C₂H₂, 198.3 K; CO₂, 194.7 K) and molecular sizes (C₂H₂, 3.3 Å × 3.3 Å × 5.7 Å; CO₂, 3.2 Å × 3.3 Å × 5.4 Å) [1], it is very difficult and challenging to separate these two molecules. In industry, C₂H₂/CO₂ separation currently depends primarily on bulk extraction by organic solvents or cryogenic distillation, which are costly and energy intensive. Physisorption with porous materials has been demonstrated to be an alternative enabling C₂H₂/CO₂ separation in an efficient and eco-friendly manner.

Metal organic frameworks (MOFs), also known as porous coordination polymers, are considered as some of the most promising physical adsorbents due to their diverse topologies and tunable structures [2–15]. Studies on the use of MOFs for C₂H₂/CO₂ separation are widely reported [16–25]. To achieve high C₂H₂/CO₂ separation selectivity, strong adsorption sites, such as amino groups [26], uncoordinated metal sites [27], and Cu^I ion sites [28], have been used to enhance binding of C₂H₂ over that of CO₂. However, strong binding leads required energy consumption for regeneration. Therefore, construction of porous materials with appropriate adsorption sites and suitable pore sizes is essential for C₂H₂/CO₂ separation. Electrostatic anion adsorption sites are selective and have been widely investigated. In previous studies, the SiF₆^2– anion pillared hybrid materials SIFSIX-3-Ni (3.8 Å) and SIFSIX-2-Cu-i (5.1 Å) have been investigated for separation of C₂H₂/CO₂ [29–32]. Interestingly, SIFSIX-3-Ni and SIFSIX-2-Cu-i showed opposite C₂H₂/CO₂ separation selectivities, which demonstrates that subtle tuning of the pore structures and geometries of binding sites has a great influence on separation performance. However, the C₂H₂ capacities of SIFSIX-3-Ni and SIFSIX-2-Cu-i are relatively modest due to their low surface areas and pore volumes. To further explore and promote C₂H₂/CO₂ separation by anion-pillared hybrid materials, we introduced anion adsorption sites into the target materials and precisely designed the pore sizes and geometries of binding sites. Finally, different pore structures were constructed by using the ligand 4,4’-bipyridine. Specifically, two anion-pillared hybrid materials, NbOFFIVE-bpy-Ni (NbOFFIVE = NbOF₅^2–, bpy = 4,4’-bipyridine, also termed ZU-61) and TIFSIX-bpy-Ni (TIFSIX = TiF₆^2–, also termed ZU-100), showed relatively large pore sizes of 7.8 Å. Adsorption experiments confirmed the increased C₂H₂ uptake capacities of ZU-61 (6.4 mmol·g^–1) and ZU-100 (8.3 mmol·g^–1) at 298 K and 1 bar. Moreover, the C₂H₂/CO₂ selectivity of ZU-100 (7.6) was higher than those of SIFSIX-3-Ni (0.13) and SIFSIX-2-Cu-i (6.5) [31]. Detailed analysis revealed that four C₂H₂ molecules were strongly adsorbed in every unit cell through C–H···F hydrogen bonds as well as multiple H^δ+···C^δ– dipole–dipole interactions between C₂H₂ molecules. However, CO₂ molecules only formed relatively weak van der Waals interactions with a single F atom due to expansion of the pore. Breakthrough experiments confirmed the excellent dynamic separation efficiency and improved dynamic capacity for C₂H₂ adsorption.

2 Experimental

Synthesis of NiNbOF₅. Firstly, 0.7471 g NiO, 1.33 g Nb₂O₅ and 4 mL HF were added into a PTFE reactor and stirred until fully dissolved, then the mixture was kept at 100 °C for 24 h. Take the supernatant into non-glass container and heated to 90 °C for evaporation crystallization. Then we got the NiNbOF₅. The infrared spectrum of NiNbOF₅ was shown at Fig. S1 (cf. Electronic Supplementary Material, ESM).

Synthesis of ZU-61 powder. An ethylene glycol solution (40 mL) of 4,4’-bipyridine (0.35 g) and an aqueous solution (20 mL) of NiNbOF₅ (0.41 g) were mixed and heated at 65 °C for 1 h under stirring. The obtained powder was filtered and washed with methanol.

Synthesis of ZU-100 powder. An ethylene glycol solution (40 mL) of 4,4’-bipyridine (0.35 g) and a methanol solution (20 mL) of (NH₄)₂TiF₆ (0.41 g) and Ni(BF₄)₂ (0.19 g) were mixed and heated at 65 °C overnight under stirring. The obtained powder was filtered and washed with methanol.

Synthesis of ZU-61 single crystal. The single crystals of ZU-61 were synthesized by slow diffusion of a methanol solution (4.0 mL) of NiNbOF₅ (0.03 g) into an ethylene glycol solution (4.0 mL) of 4,4’-bipyridine (0.03 g) after a week.

Characterization of ZU-100 and ZU-61. Powder X-ray diffraction (PXRD) and single crystal X-ray diffraction data were collected on a SHIMADZU XRD-6000 diffractometer and a Bruker APEX-II CCD diffractometer, respectively. Surface area was based on the nitrogen adsorption and desorption isotherms at 77 K using micromeritics ASAP 2460 adsorption apparatus. The thermal gravimetric analysis was performed on an instrument of TGA Q500 V20.13 Build 39. More details could be seen in ESM.

Gas adsorption measurements. C₂H₂ and CO₂ gas adsorption measurements were performed on the Micromeritics ASAP 2460. Before gas adsorption measurements, the sample of ZU-61 and ZU-100 were evacuated at 65 °C for 12 h until the pressure dropped below 1 Pa. The sorption isotherms were collected at 283–313 K on activated samples.

Column breakthrough experiments for C₂H₂/CO₂ gas mixture. The breakthrough experiments were accomplished by a dynamic gas breakthrough equipment. The experiment was conducted using a stainless-steel column (4.6 mm inner diameter × 50 mm). The weight of ZU-61 and ZU-100 powder packed in the column was 0.2816 and 0.2588 g. The column packed with sample was activated with N₂ flow (15 mL·min^–1) for 12 h at 65 °C. After activation, the mixed gas (C₂H₂/CO₂=50/50, v/v) flow was introduced at 2 mL·min^–1. Outlet gas from the column was monitored using gas chromatography (GC-490) with a thermal conductivity detector. After the breakthrough experiment, the sample was regenerated with N₂ flow (15 mL·min^–1) for about 12 h at 65 °C. Detailed calculation methods for dynamic capacity and separation factors were shown in supplementary information and Fig. S2 (cf. ESM).

Dispersion-corrected density-functional theory (DFT) calculations. To further understand the interactions between guests and host framework, DFT method was utilized to calculate the binding energies of C₂H₂ and CO₂ molecules. More details could be seen in ESM.

3 Results and discussion

ZU-100 is a new material synthesized from 4,4’-bipyridine, TiF₆^2– and Ni²⁺ through a hydrothermal reaction. As shown in Fig.1, two-dimensional (2D) nets of organic ligands (4,4’-bipyridine) and metal nodes (Ni²⁺) were pillared with TiF₆^2– anions in the third dimension to form three-dimensional coordination networks with a pcu topology (Fig.1(a) and 1(b)). The maximum F–F distance of the pore aperture in ZU-100 was 12.15 Å, while the minimum H–H distance was 9.91 Å (Fig.1(c)), which is much larger than the pore sizes of SIFSIX-3-Ni (3.8 Å) and SIFSIX-2-Cu-i (5.1 Å) [31]. ZU-61 is isostructural with ZU-100 containing NbOF₅^2– anions [33]. The TiF₆^2– anion is smaller than the NbOF₅^2– anion, which led to a ZU-100 pore size larger than that of ZU-61 (F–F distance: 12.11 Å; H–H distance: 9.25 Å). The Ni–Ni distances between the two 2D nets of the two materials were also different, which were 7.931 and 7.836 Å for ZU-100 and ZU-61, respectively. Moreover, the stronger electrostatic potential of TiF₆^2– anions provided stronger adsorption sites for C₂H₂ molecules. The N₂ adsorption–desorption isotherms of ZU-61 and ZU-100 at 77 K were measured for further investigation, as depicted in Figs. S3 and S4 (cf. ESM). The Brunauer–Emmett–Teller (BET) surface areas were calculated to be 1022 and 1161 m²·g^–1 for ZU-61 and ZU-100, respectively, much higher than the BET surface areas of SIFSIX-3-Ni (368 m²·g^–1), SIFSIX-2-Cu-i (503 m²·g^–1) [31] and benchmark materials UTSA-300 [35] (311 m²·g^–1) and CPL-1-NH₂ [26] (103 m²·g^–1), which indicated the potential for high C₂H₂ capacity. Relevant stability analyses, including thermogravimetric analysis (Fig. S5, cf. ESM) and water stability analysis (Fig. S6, cf. ESM), were undertaken. The thermogravimetric curve of ZU-100 showed that ZU-100 underwent no significant loss of quality until 250 °C, which is lower than the decomposition temperature of ZU-61 (320 °C) [33]. PXRD data demonstrated the integrity of the crystal structure of ZU-100 after exposure to wet air for one day.

Fig.1 Schematic representations of ZU-61 and ZU-100. (a) Sketch of metal node, inorganic pillar and ligand with the pyridine H atoms omitted for clarity; (b) the perspective view of the skeleton structure of ZU-61 and ZU-100; (c) the skeleton structure of ZU-61 and ZU-100 viewed along the c axis, the F–F and H–H distance of the two structures (Color code: F red; C and H gray; Ni purple; Nb and Ti cyan; O green; N blue).

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The single-component adsorption isotherms of C₂H₂ and CO₂ on ZU-61 and ZU-100 were collected to further investigate the adsorption properties. As depicted in Fig.2, the C₂H₂ isotherms for adsorption on ZU-61 and ZU-100 at 298 K both exhibited type-I microporous adsorption behavior (Fig.2(a)). The saturated C₂H₂ uptake of ZU-100 at 1 bar and 298 K was 8.3 mmol·g^–1, while ZU-61 showed a lower C₂H₂ uptake of 6.4 mmol·g^–1 under the same conditions; these values were higher than those of SIFSIX-3-Ni (3.3 mmol·g^–1) [30], SIFSIX-2-Cu-i (4.1 mmol·g^–1) [31] and other benchmark materials, such as UTSA-300 (3.1 mmol·g^–1) [34], CPL-1-NH₂ (1.8 mmol·g^–1) [27] and JCM-1 (3.4 mmol·g^–1) [35]. Both materials exhibited strong affinity for C₂H₂, which was demonstrated by high C₂H₂ uptake levels at low pressures. The C₂H₂ uptake capacities on ZU-61 and ZU-100 at 0.1 bar and 298 K were 3.0 and 2.3 mmol·g^–1, respectively. The difference in C₂H₂ capacities between the two materials was mainly attributed to the larger BET area and lower cell density of ZU-100 (0.88 cm³·g^–1) relative to ZU-61 (0.95 cm³·g^–1). The CO₂ adsorption affinities of the two materials were much weaker than the C₂H₂ affinities. The CO₂ uptake levels increased slowly with increasing pressure and reached 4.4 and 2.7 mmol·g^–1 for ZU-100 and ZU-61 at 1 bar and 298 K, respectively. The C₂H₂/CO₂ uptake ratios at 1 bar were calculated to compare the C₂H₂/CO₂ selectivities of different materials. The C₂H₂/CO₂ uptake ratios for ZU-61 and ZU-100 at 1 bar were 2.3 and 1.9, which were higher than those of SIFSIX-3-Ni (1.2) [29] and SIFSIX-2-Cu-i (0.95) [31]. Comparisons of the C₂H₂ capacities and C₂H₂/CO₂ uptake ratios for the two materials and other reported materials are also exhibited in Fig.2(b) [26–28,36–38]. Both ZU-61 and ZU-100 were located in the best performance region.

Fig.2 (a) Single-component adsorption isotherms of C₂H₂ (red) and CO₂ (blue) on ZU-61 and ZU-100 at 298 K; (b) the comparison of C₂H₂ capacity and C₂H₂/CO₂ uptake ratio at 1 bar and 298 K with other materials; (c) ideal adsorbed solution theory (IAST) calculations of C₂H₂/CO₂ (50/50, v/v) adsorption selectivity for ZU-61 and ZU-100; (d) calculated C₂H₂ adsorption isotherms from C₂H₂/CO₂ (50/50, v/v) mixtures at 298 K [34–36,39]; (e) calculated isosteric heats of adsorption for C₂H₂ at different C₂H₂ loadings on ZU-61 and ZU-100; (f) the comparison of Q_st (isosteric heats of adsorption) among ZU-61, ZU-100 and other materials.

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IAST selectivities were also calculated to describe the different C₂H₂/CO₂ separation abilities of the two materials, which were 5.7 and 7.6 at 1 bar (C₂H₂/CO₂ = 50/50, v/v), respectively (Fig.2(c)). These results demonstrated that the electrostatic potential and affinity of C₂H₂ for TiF₆^2– were higher than those for NbOF₅^2– due to the stronger alkalinity of TiF₆^2–. Then, we calculated the C₂H₂ uptakes of the two materials from C₂H₂/CO₂ (50/50, v/v) mixtures. The results seen at varying pressures are shown in Fig.2(d) [34–36,39]. At 298 K and 1 bar, the C₂H₂ capacity of ZU-100 was the highest, 6.3 mmol·g^–1, which exceeded most of the capacities reported for MOFs. The C₂H₂ capacity of ZU-61 (4.8 mmol·g^–1) was slightly lower than that of ZU-100 but was still considerable. The calculated C₂H₂ capacities for C₂H₂/CO₂ (1/99, v/v) mixtures are also shown in Fig. S7 (cf. ESM). ZU-100 (0.34 mmol·g^–1 at 100 kPa) and ZU-61 (0.19 mmol·g^–1 at 100 kPa) still performed better than other reported MOFs.

To better understand the interactions between the adsorbent materials and guest molecules, we calculated the Q_st of ZU-61 and ZU-100 for C₂H₂ and CO₂ based on the adsorption isotherms collected at 283, 298 and 313 K (Figs. S8–S11, cf. ESM). The curves for Q_st at all gas loadings are shown in Fig.2(e) and S12 (cf. ESM). The Q_st values for C₂H₂ gradually increased with increasing C₂H₂ uptake, which be attributed to enhancement of guest-guest interactions. The Q_st values of ZU-61 and ZU-100 for C₂H₂ were 27.5 and 29.5 kJ·mol^–1 at zero loading, respectively; these were higher than their Q_st values for CO₂ at zero loading (18.5 kJ·mol^–1 for ZU-61 and 20.4 kJ·mol^–1 for ZU-100), which indicated the stronger affinities of the two materials for C₂H₂. We noted that the Q_st values of C₂H₂ in ZU-61 and ZU-100 were notably lower than those of most known materials, such as SIFSIX-2-Cu-i (46.3 kJ·mol^–1) [31], SIFSIX-3-Ni (36.7 kJ·mol^–1) [29], UTSA-74a (45 kJ·mol^–1) [14] and SNNU-45 (40 kJ·mol^–1) [15], and only slightly higher than the Q_st of FJU-90 (25.1 kJ·mol^–1) [36]. The data highlighted the low generation energy requirements of ZU-61 and ZU-100 for C₂H₂, which is a crucial advantage for industrial production.

Dynamic breakthrough experiments were carried out for a C₂H₂/CO₂ (50/50, v/v) mixture at 298 K to examine the separation ability of the two materials with the C₂H₂/CO₂ mixture. As shown in Fig.3, the breakthrough curve of ZU-61 indicated that CO₂ first eluted at 14 min·g^–1 and then immediately approached a plateau without detectable C₂H₂, which was detected until 37 min·g^–1 and reached a balance at 65 min·g^–1 (Fig.3(a)). ZU-100 showed a better performance (Fig.3(b)). During the breakthrough test, CO₂ was first detected at 25 min·g^–1, while C₂H₂ was detected at 70 min·g^–1 and reached a balance at 140 min·g^–1. The dynamic C₂H₂ capacities also calculated according to the breakthrough results. ZU-61 had a relatively low dynamic capacity of 1.9 mmol·g^–1. However, the dynamic C₂H₂ capacity of ZU-100 was stable at 4.3 mmol·g^–1, and this exceeded the capacities of the vast majority of MOFs, such as ZJU-280a (4.1 mmol·g^–1) [40], Cu^I@Uio-66-(COOH)₂ (2.9 mmol·g^–1) [28] and JCM-1 (2.2 mmol·g^–1) [35], which are only lower than those of SIFSIX-Cu-TPA (5.7 mmol·g^–1) [41] and SNNU-45 (5.2 mmol·g^–1) [37] (Fig.3(c)). Breakthrough experiments under higher flows also conducted, and the dynamic C₂H₂ capacities were almost identical (Figs. S13 and S14, cf. ESM). The excellent dynamic C₂H₂ capacity of ZU-100 is vital to improving the separation efficiency, which improves the potential for industrial applications of ZU-100. Another key point for industrial application is the cycling ability. The outstanding cycling abilities of the two materials were confirmed by continuous adsorption−desorption experiments, and the results are depicted inFig.3(d).

Fig.3 Experimental dynamic breakthrough curves for C₂H₂/CO₂ (50/50, v/v) separations with (a) ZU-61 and (b) ZU-100 at 298 K and 1 bar (mixed gas flow: 2 mL·min^–1); (c) the comparison of dynamic C₂H₂ capacities between ZU-61, ZU-100 and some other representative MOFs; (d) cycle breakthrough tests for C₂H₂/CO₂ (50/50, v/v) separation with ZU-61 and ZU-100.

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To demonstrate the detailed interactions occurring between guest molecules and host frameworks, DFT method [42] was utilized (Fig.4). The optimized adsorption binding sites of C₂H₂ and CO₂ molecules in the pores of ZU-100 are given in Fig.4(a) and 4(b). For C₂H₂ molecules, there were altogether four C₂H₂ molecules located in one unit cell, which was consistent with the experimental result (3.7 C₂H₂ molecules per unit cell). The strong adsorption of C₂H₂ molecules was mainly attributed to C–H···F hydrogen bonds with a distance of 1.950 Å as well as van der Waals interactions between C₂H₂ molecules and 4,4’-bipyridine linkers. Intermolecular interactions between guest C₂H₂ molecules also played an important role. The distance between neighboring adsorbed C₂H₂ molecules was ideal for them to synergistically interact with each other through formation of multiple H^δ+···C^δ– dipole−dipole interactions with distances of 2.716 and 2.820 Å. All the interactions together determined the static adsorption energy (ΔE) of C₂H₂, which was 46.9 kJ·mol^–1. However, CO₂ molecules only formed relatively weak van der Waals interactions with one F atom in the pores, and this exhibited a distance of 2.696 Å. Two CO₂ molecules at the diagonal positions in one unit cell were too far apart to form guest−guest interactions. The ΔE was calculated to be 28.7 kJ·mol^–1. DFT calculations were also conducted with ZU-61 and led to a similar conclusion, with ΔE values of 44.6 and 29.8 kJ·mol^–1 for C₂H₂ and CO₂, respectively; these are shown in Fig. S15 (cf. ESM).

Fig.4 Schematic pictures showing the DFT optimized (a) C₂H₂ and (b) CO₂ adsorption configurations and the distances between atoms in ZU-100 (Color code: F red; Ni and O purple; Nb and Ti cyan; N blue; C orange; H white).

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4 Conclusions

In summary, through introduction of strong adsorption sites and reasonable tuning of the pore structure and geometry, we synthesized two anion-pillared hybrid materials, ZU-100 and ZU-61, for separation of C₂H₂/CO₂ mixtures. The appropriate pores enhanced the C₂H₂ storage density (8.3 mmol·g^–1 for ZU-100 and 6.4 mmol·g^–1 for ZU-61 at 298 K and 1 bar) and provided sufficient space for formation of C₂H₂ clusters, which enhanced C₂H₂ adsorption in the pores. The IAST selectivity and C₂H₂/CO₂ uptake ratio were calculated to verify the perfect C₂H₂/CO₂ separation ability. The extremely low C₂H₂ isosteric heats of adsorption (27.5 kJ·mol^–1 for ZU-61 and 29.5 kJ·mol^–1 for ZU-100) and the excellent dynamic breakthrough capacities for C₂H₂ (4.3 mmol·g^–1 for ZU-100) were two highlights of ZU-100 and ZU-61 and were essential for decreasing energy consumption and improving the separation efficiencies. DFT calculations revealed the binding sites for C₂H₂ molecules in the pores, which involved both C–H···F hydrogen bonds and intermolecular interactions. CO₂ molecules formed only relatively weak van der Waals interactions with single F atoms. This work revealed the importance of appropriate adsorption sites and reasonable pore structures for hydrocarbon separation.

Acknowledgements

This work was financially supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No. LR20B060001), the National Natural Science Foundation of China (Grant Nos. 22122811, 21938011, and 21890764), and the Research Computing Center in College of Chemical and Biological Engineering at Zhejiang University.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://dx.doi.org/10.1007/s11705-022-2183-x and is accessible for authorized users.

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