Boosting Adsorption and Selectivity of Acetylene by Nitro Functionalisation in Copper(II)‐Based Metal–Organic Frameworks

Abstract

Purification and storage of acetylene (C₂H₂) are important to many industrial processes. The exploitation of metal–organic framework (MOF) materials to address the balance between selectivity for C₂H₂ vs carbon dioxide (CO₂) against maximising uptake of C₂H₂ has attracted much interest. Herein, we report that the synergy between unsaturated Cu(II) sites and functional groups, fluoro (−F), methyl (−CH₃), nitro (−NO₂) in a series of isostructural MOF materials MFM‐190(R) that show exceptional adsorption and selectivity of C₂H₂. At 298 K, MFM‐190(NO₂) exhibits an C₂H₂ uptake of 216 cm³ g⁻¹ (320 cm³ g⁻¹ at 273 K) at 1.0 bar and a high selectivity for C₂H₂/CO₂ (up to ~150 for v/v = 2/1) relevant to that in the industrial cracking stream. Dynamic breakthrough studies validate and confirm the excellent separation of C₂H₂/CO₂ by MFM‐190(NO₂) under ambient conditions. In situ neutron powder diffraction reveals the cooperative binding, packing and selectivity of C₂H₂ by unsaturated Cu(II) sites and free −NO₂ groups.

The isostructural MOF materials, MFM‐190(R) (R=−F, −CH₃, −NO₂) exhibit exceptional acetylene (C₂H₂) adsorption and selectivity. MFM‐190(NO₂) shows a high C₂H₂ uptake of 216 cm³ g⁻¹ at 1 bar, 298 K and excellent C₂H₂/CO₂ selectivity (up to ~150 for v/v = 2/1). Dynamic breakthrough experiments confirm efficient separation under ambient conditions, supported by in situ neutron powder diffraction studies revealing cooperative binding of C₂H₂ at unsaturated Cu(II) and −NO₂ sites.

Acetylene (C₂H₂) is a crucial precursor for the manufacturing of many chemicals and materials, such as vinyl chloride, synthetic rubber and polyester plastics.[ ¹ , ² , ³ , ⁴ ] State‐of‐the‐art production of C₂H₂ involves the partial combustion of natural gas (>80 % methane) or hydrocarbon cracking, where carbon dioxide (CO₂) is a key byproduct that must be removed to produce high purity C₂H₂ for downstream applications.[ ⁵ , ⁶ ] However, the separation of C₂H₂ from CO₂ is highly challenging due to their similar properties (same kinetic diameter of 3.3 Å; molecular size: 3.2×3.3×5.4 Å for CO₂ and 3.3×3.3×5.7 Å for C₂H₂; boiling point: 194.7 K for CO₂; 189.3 K for C₂H₂).[ ⁷ , ⁸ ] Current technologies rely on solvent extraction or cryogenic distillation, which are energy‐intensive and associated with safety hazards. ^[9] In addition, the explosive nature of C₂H₂ raises concerns regarding its transportation and storage with the storage pressure being limited to below 2.0 bar. ^[10] There is, therefore, a critical need to explore new adsorption‐based technologies to enable energy‐efficient purification and safe storage of C₂H₂. ^[11]

Metal–organic framework (MOF) materials have been investigated widely for gas adsorption and separation owing to their adjustable pore environment.[ ⁹ , ¹⁰ , ¹¹ , ¹² ] However, the optimisation of selectivity between C₂H₂ and CO₂ against the C₂H₂ uptake remains a challenging task.[ ¹³ , ¹⁴ ] For example, MOFs with large pores often exhibit high gas uptakes, but with limited selectivity for C₂H₂/CO₂. ^[15] Conversely, narrow‐pored MOFs can achieve high selectivity at the expense of low C₂H₂ uptake. ^[16] Various strategies have been developed to address this conundrum, including introduction of unsaturated metal sites, ^[17] regulation of pore size, ^[14] and ligand functionalisation. ^[7] Pyridyl functionalised NbO‐type MOFs constructed from [Cu₂(O₂CR)₄] paddlewheels and organic linker are particularly appealing for C₂H₂ adsorption because the N‐donors in the pyridyl groups can bind C₂H₂ via H−C≡C−H⋅⋅⋅N hydrogen bonding, thus improving C₂H₂ uptake.[ ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ ] For example, ZJU‐5 that incorporates Lewis basic pyridyl sites exhibits a high C₂H₂ uptake of 193 cm³ g⁻¹ at 298 K and 1.0 bar. ^[17] Additionally, the introduction of functional groups within the pore interior can strengthen the host–guest interactions, thus promoting C₂H₂ uptake. ^[22] However, NbO‐type MOFs have been rarely explored for the separation of C₂H₂/CO₂ owing to their high porosity and thus poor selectivities.

Herein, we report a strategy to develop and utilise the synergy between unsaturated Cu(II) sites, pyridyl centres and pendant functional groups in a series of NbO‐type MOFs, denoted as MFM‐190(R) (R=−F, −CH₃, −NO₂), to boost the adsorption of C₂H₂ while simultaneously promoting the separation of C₂H₂/CO₂. At 273 K and 1.0 bar, MFM‐190(NO₂) exhibits an exceptional C₂H₂ uptake capacity of 320 cm³ g⁻¹, surpassing that of MFM‐190(CH₃) (300 cm³ g⁻¹) and MFM‐190(F) (282 cm³ g⁻¹). More importantly, at 298 K and 1.0 bar, MFM‐190(NO₂) demonstrates simultaneously high C₂H₂ uptake (216 cm³ g⁻¹) and a high selectivity for C₂H₂ vs CO₂ (at v/v = 2/1, relevant to industrial cracking stream) of up to ~150 based on analysis using ideal adsorbed solution theory (IAST). This compares favourably with state‐of‐the‐art materials. Furthermore, breakthrough experiments show excellent separation performance for MFM‐190(NO₂), affording clear separation of C₂H₂/CO₂, while cycling breakthrough experiments confirm the stability and recyclability of MFM‐190(NO₂). Direct visualisation of binding of C₂H₂ and CO₂ molecules within the pores of MFM‐190(NO₂) has been achieved using in situ neutron powder diffraction (NPD) analysis, which affords important insights into the high adsorption and selectivity of C₂H₂ at a molecular level.

Isostructural MFM‐190(R) (R=−F, −CH₃, −NO₂) were obtained by introducing −F, −CH₃, and −NO₂ groups onto the parent ligand used for the preparation of MFM‐190(H) (Figure S1).²³ Powder X‐ray diffraction (PXRD) analysis confirmed their phase purity (Figure S2), and their crystal structure derived from NPD data shows two types of metal‐ligand cages, which are alternately stacked in a 1 : 1 ratio to afford an open NbO‐type structure. The cylindrical cage (size of ca. 18.6×27.4 Å) is encapsulated by twelve [Cu₂(O₂CR)₄] moieties and six organic ligands. The spherical cage (size of 14.7×14.7 Å) is surrounded by six [Cu₂(O₂CR)₄] paddlewheels and twelve organic ligands (Figure 1). The interior walls of these cages, upon de‐solvation, are decorated with unsaturated Cu(II) sites, pyridyl centres as well as pendant functional groups, which can promote the efficient binding and packing of C₂H₂ molecules by facilitating their close contacts. MFM‐190(F), MFM‐190(CH₃) and MFM‐190(NO₂) show Brunauer–Emmett–Teller (BET) surface areas of 2530, 2550 and 2300 m² g⁻¹, respectively, as determined by N₂ isotherms at 77 K (Figure S3).

Figure 1.

Views of the crystal structures MFM‐101 and MFM‐190(R) derived from the NPD studies (Cu, cyan; C, black; O, red; N, blue; H, grey; R, green; R represents −F, −CH₃, and −NO₂). Two types of metal‐ligand cages are interconnected by sharing three [Cu₂(O₂CR)₄] paddlewheel units and three iso‐phthalate moieties.

Adsorption and desorption isotherms of C₂H₂ and CO₂ for MFM‐190(R) were recorded at 273–298 K (Figure 2a–b and Figure S4–S6). Of these three materials, MFM‐190(NO₂) shows the highest C₂H₂ uptake of 320 and 216 cm³ g⁻¹ at 273 and 298 K, respectively, compared with MFM‐190(CH₃) (300 and 194 cm³ g⁻¹) and MFM‐190(F) (282 and 174 cm³ g⁻¹) (Figure 2a and Figure S7). MFM‐190(NO₂) also demonstrates an excellent cyclability for C₂H₂ as confirmed by pressure‐swing adsorption and desorption (0–500 mbar) at 298 K (Figure S8). The incorporation of relatively bulker functional group (−NO₂) leads to the reduced pore sizes and surface area compared with −CH₃ and −F groups, which can facilitate enhanced interaction between C₂H₂ and the MOF. By comparison, at 298 K and 1.0 bar, the isostructural pyridyl‐free ZJNU‐36(NO₂) ^[22] and −NO₂‐free ZJU‐5 ^[15] show lower C₂H₂ uptakes of 176 and 193 cm³ g⁻¹, respectively, suggesting the positive impacts of these functional groups on C₂H₂ uptakes. Notably, the C₂H₂ uptake of MFM‐190(NO₂) compares favourably with leading MOFs (Table S2).[ ¹ , ² , ⁷ , ⁹ , ¹⁹ , ²¹ , ²² , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² ] Using the crystal density (0.92 g cm⁻³) of MFM‐190(NO₂), the C₂H₂ packing density in the pores is calculated to be 230 g L⁻¹ at 298 K and 1.0 bar, which is 196 times that of gaseous C₂H₂ (1.17 g L⁻¹) and surpasses that of ZJNU‐47 (170 g L⁻¹ at 295 K) ^[21] and NJU‐Bai17 (204 g L⁻¹ at 296 K), ^[19] and is slightly lower than FJU‐90a (256 g L⁻¹ at 298 K), ^[14] indicating the highly efficient packing of C₂H₂ molecules within MFM‐190(NO₂). In contrast, the CO₂ uptakes of MFM‐190(F), MFM‐190(CH₃), and MFM‐190(NO₂) are recorded as 82.9, 90.9 and 86.4 cm³ g⁻¹, respectively, at 1.0 bar and 298 K (Figure 2b ). Interestingly, at 0.1 bar and 298 K, adsorption of C₂H₂ in MFM‐190(NO₂) displayed a rapid and steep increase up to 87 cm³ g⁻¹, while that for CO₂ increases linearly with pressure and exhibits a much lower uptake of 12 cm³ g⁻¹, demonstrating the great potential for their separation. The IAST selectivity for C₂H₂/CO₂ mixtures (v/v = 1/1, 2/1) was calculated using the single‐component isotherms at 298 K (Figure 2c and Figure S10–S12). Notably, MFM‐190(NO₂) demonstrates a drastically elevated C₂H₂/CO₂ selectivity of 150 at low pressure for C₂H₂/CO₂ mixtures (at v/v = 2/1), surpassing that of MFM‐190(F) (7.2) and MFM‐190(CH₃) (20). With the increase of pressure, the selectivity gradually stabilises at 11, 2.7 and 5.1 at 1.0 bar for MFM‐190(NO₂), MFM‐190(F), and MFM‐190(CH₃), respectively. The IAST selectivity (150–11) of MFM‐190(NO₂) compares favourably with that of benchmark MOFs, such as CAU‐10(H) (24.2–4.0), ^[12] FJI−H8 (10.4–5.4), ^[33] FJU‐112a (4.2), ^[34] MIL‐160 (10), ^[7] ZJU‐40 (17–11.5), ^[35] and ZJU‐50a (30–12) (Figure 2d and Table S2). ^[36] It is worth noting that MOF materials exhibiting simultaneously high adsorption and selectivity for C₂H₂ are exceedingly rare.

Figure 2.

(a) Adsorption isotherms of C₂H₂ in MFM‐190(NO₂), MFM‐190(CH₃) and MFM‐190(F) at 273–298 K. (b) Adsorption isotherms of C₂H₂ and CO₂ in MFM‐190(NO₂), MFM‐190(CH₃) and MFM‐190(F) at 298 K. Desorption isotherms are omitted for clarity and can be found in the Supporting Information . (c) IAST selectivities of C₂H₂/CO₂ (v/v = 1/1, 1/2) of MFM‐190(NO₂) at 298 K. (d) Comparison of state‐of‐art sorbents for C₂H₂ capacity and C₂H₂/CO₂ selectivity at 298 K and 1.0 bar.

We investigated the adsorption kinetic profiles of MFM‐190(NO₂) at 298 K (Figure S13). MFM‐190(NO₂) shows rapid diffusion for both C₂H₂ and CO₂, reaching adsorption equilibrium within approximately 6 minutes reflecting the large pores in this material. The binding affinity of MFM‐190(R) toward C₂H₂ and CO₂ was further elucidated by measurement of the isosteric heats of adsorption (Q _st) (Figure S14–S19). Notably, MFM‐190(NO₂) exhibits an exceptional Q _st value for C₂H₂ (90 kJ mol⁻¹) at low surface coverage, significantly higher than that of MFM‐190(F) (37 kJ mol⁻¹) and MFM‐190(CH₃) (38 kJ mol⁻¹). The values of Q _st for CO₂ for MFM‐190(F) and MFM‐190(CH₃) are similar, and are slightly lower than that of MFM‐190(NO₂). The Q _st value for C₂H₂ in MFM‐190(NO₂) is comparable with MOFs incorporating high‐density unsaturated metal sites, such as Cu^I@UiO‐66‐(COOH)₂ (74.5 kJ mol⁻¹), ^[1] NKMOF−Ni (60.3 kJ mol⁻¹), ^[37] ZJU‐74a (45 kJ mol⁻¹), ^[38] and ATC−Cu (79.1 kJ mol⁻¹), ^[16] indicating the critical role of Cu(II) sites and functional groups (−NO₂ and −N) in binding C₂H₂ molecules in MFM‐190(NO₂). More importantly, the substantial difference in Q _st values between C₂H₂ and CO₂ in MFM‐190(NO₂) is consistent with the steep isotherms of C₂H₂ at low pressure.

The high static C₂H₂ adsorption and C₂H₂/CO₂ selectivity of MFM‐190(NO₂) motivated us to evaluate its separation performance under dynamic conditions. Initially, single‐component breakthrough experiments of CO₂ and C₂H₂ were conducted using a diluted He flow with a total flow rate of 20 mL min⁻¹ through a fixed‐bed packed of activated MFM‐190(NO₂) at 298 K (Figure S20–S21). The dynamic adsorption capacities for C₂H₂ and CO₂ were calculated to be 3.6 and 0.4 mmol g⁻¹, respectively. These values are slightly lower than the static capacities obtained from the isotherms at 298 K at 0.1 bar (4.1 and 0.5 mmol g⁻¹, respectively), and such discrepancy is consistent with literature reports. ^[39] Dynamic breakthrough experiments were performed using mixtures of C₂H₂/CO₂ (v/v = 2/1, 1/1) at 298 K and 1.0 bar (Figure 3a). Clear separation of the equimolar mixture of C₂H₂/CO₂ was achieved, with CO₂ eluting first at 10 min g⁻¹, while no C₂H₂ was detected until breakthrough at 55 min g⁻¹. The separation factor for MFM‐190(NO₂) was calculated as 5.5, which is higher than the top‐performing MOF materials, such as JCM‐1 (4.4), ^[40] ZJU‐50a (4.2), ^[36] Cu^I@UiO‐66‐(COOH)₂ (3.4), ^[1] CuZn₃(PDDA)₃(OH) (3.3), ^[41] but lower than that of BSF‐3 (16.3) ^[15] and JNU‐4a (12.8). ^[9] Importantly, MFM‐190(NO₂) displays simultaneously high C₂H₂ uptake (216 cm³ g⁻¹) and high separation factor (5.5), surpassing those of benchmark MOFs, such as SNNU‐27‐Fe (182 cm³ g⁻¹, 2.8), ^[27] MIL‐160 (191 cm³ g⁻¹, 1.7), ^[7] Cu^I@UiO‐66‐(COOH)₂ (48 cm³ g⁻¹ and 3.4). ^[1] High‐purity C₂H₂ (>99.9 %) has been harvested from MFM‐190(NO₂), resulting in a productivity of 4.0 mol kg⁻¹ at 298 K, which is also higher than those of benchmark materials, such as ZJU‐74a (3.6 mol kg⁻¹), ^[38] Cu^I@UiO‐66‐(COOH)₂ (2.9 mol kg⁻¹), ^[1] and CAU‐10 (3.3 mol kg⁻¹). ^[3] Moreover, MFM‐190(NO₂) shows excellent separation of a mixture of C₂H₂/CO₂ (v/v = 2/1), which is relevant to separations for industrial cracking streams (Figure 3b). ^[15] When purging with He at 298 K, the fixed‐bed can be regenerated completely, and the recyclability of the medium for separation of C₂H₂/CO₂ (v/v = 2/1) was confirmed for three cycles with excellent stability (Figure 3c).

Figure 3.

(a, b) Breakthrough curves for mixtures of C₂H₂/CO₂ (a) v/v = 1/1 and (b) 2/1 diluted in He at a flow rate of 20 mL min⁻¹ at 298 K and 1.0 bar over a fixed‐bed of MFM‐190(NO₂) (sample weight: 0.55 g) and regeneration of sorbent. (c) The recyclability of MFM‐190(NO₂) for the separation of C₂H₂/CO₂ (v/v = 2/1) over three cycles (the saturated sorbent was regenerated by heating at 373 K under a flow of He for 30 mins between cycles).

In situ NPD analysis was performed to determine the preferred binding domains for adsorbed gas (C₂D₂, CO₂) molecules within MFM‐190(NO₂) (Figure S22–S24 and Table S3–S8). Rietveld refinements revealed seven distinct binding sites for C₂D₂ (Figure 4) and four sites for CO₂ (Figure 5). In the structure of MFM‐190(NO₂)⋅(C₂D₂)_5.2, site I (C₂D₂/Cu=0.359) is within the spherical cage B and exhibits strong binding interactions with the unsaturated Cu(II) sites [Cu⋅⋅⋅C≡C_C2D2=3.05(2) Å]. In addition, site I is stabilised by intermolecular interactions with site II and site IV [C_I⋅⋅⋅C_II=3.39(1) Å and C_I⋅⋅⋅C_IV=2.55(1) Å]. Site II (C₂D₂/Cu=0.192) is situated within both cages and binds to −NO₂ groups and phenyl rings via electrostatic interactions [D_C2D2⋅⋅⋅O_NO2=2.54(1) Å; D_C2D2⋅⋅phenyl rings=2.81(3) Å]. Sites III and IV are located only in cage B where C₂D₂ molecules bind to unsaturated Cu(II) sites [Cu⋅⋅⋅D_C2D2=2.75(4) Å; Cu⋅⋅⋅C_C2D2=3.35(1) Å]. Sites V–VII are found in the cylindrical cage A and stabilised by intermolecular interactions [C_V⋅⋅⋅C_VI=2.68(2) Å and by relatively weak electrostatic interactions with pyridyl sites [D_C2D2⋅⋅⋅pyridyl sites=2.99(1) Å] and phenyl rings [C_VII⋅⋅⋅phenyl rings=3.79(2) Å and 3.96(1) Å]. Notably, the smaller spherical cage B plays an important role in C₂D₂ adsorption, and intermolecular interactions between each site stabilises further the packing of C₂D₂ molecules to give an enhanced overall uptake (Figure 4c).

Figure 4.

Views of host–guest interactions in C₂D₂‐loaded MFM‐190(NO₂); all structural models were determined from Rietveld refinements of in situ NPD data collected at 10 K. The interatomic distances are quoted in angstroms (Å). The occupancy of each site has been converted into C₂D₂ per Cu for clarity. The radii of the coloured spheres are proportional to the corresponding crystallographic occupancies. Cu, cyan; C, black; O, red; N, blue; H, grey. (a) Views of the distribution of C₂D₂ in MFM‐190(NO₂)⋅(C₂D₂)_5.2. (b) Detailed views of host–guest interactions between MFM‐190(NO₂) and C₂D₂. (c) Views of the packing of adsorbed C₂D₂ molecules within cage B . (site I: yellow, site II: purple, site III: orange, site IV: green, site V: pink, site VI: violet, site VII: black)

Figure 5.

Views of host–guest interactions in CO₂‐loaded MFM‐190(NO₂); all structural models were determined from Rietveld refinements of in situ NPD data collected at 10 K. The interatomic distances quoted in angstroms (Å). The occupancy of each site has been converted into CO₂ per Cu for clarity. The radii of the coloured spheres are proportional to the corresponding crystallographic occupancies. Cu, cyan; C, black; O, red; N, blue; H, grey. (a) Views of the distribution of CO₂ in MFM‐190(NO₂)⋅(CO₂)_3.1. (b) Detailed views of host–guest interactions between MFM‐190(NO₂) and CO₂ (site I: yellow, site II: purple, site III: orange, site IV: green).

For CO₂‐loaded MFM‐190(NO₂), perhaps surprisingly, the most favourable adsorption site I (CO₂/Cu=0.394) was located at the centre of the [Cu₂(O₂CR)₄] paddlewheels, stabilised by electrostatic interactions between phenyl rings and O_CO2 [phenyl rings⋅⋅⋅O_CO2=3.94(1) Å]. Site II (CO₂/Cu=0.378) is situated in the spherical cage B and binds to the unsaturated Cu(II) site [Cu⋅⋅⋅O_CO2=3.32(1) Å], and is further stabilised by the hydrogen bonding between H_{phenyl rings} and O_CO2 [H_{phenyl rings}⋅⋅⋅O_CO2=2.99(1) Å] and intermolecular interactions with site III [O_III⋅⋅⋅C_II=3.57(7) Å]. Site III (CO₂/Cu=0.167) is stabilised by the interactions involving pyridyl sites and CO₂ [N⋅⋅⋅C_CO2=3.68(4) Å]. Site IV (CO₂/Cu=0.165), located at the centre of cylindrical cage A , features C atoms in close proximity to phenyl rings [C_CO2⋅⋅⋅phenyl rings=3.99(2) Å], and is also stabilised by weak interactions with the Cu(II) site [Cu⋅⋅⋅ C_CO2=4.37(1) Å]. Thus, the NPD study confirms unequivocally the presence of multiple binding sites and strong affinity for C₂D₂ in MFM‐190(NO₂) compared with CO₂, directly supporting the observed high adsorption and selectivity of C₂H₂.

In summary, we have confirmed that the synergy between unsaturated metal sites, pyridyl groups and pendant functional groups promote the adsorption and selectivity of C₂H₂ in NbO‐type MFM‐190(R) (R =−F, −CH₃, −NO₂) materials. MFM‐190(NO₂) shows an exceptional C₂H₂/CO₂ selectivity up to 150 (v/v = 2/1) and high C₂H₂ uptake of 216 cm³ g⁻¹ at 298 K and 1.0 bar. MFM‐190(NO₂) can harvest a polymer‐grade C₂H₂ stream (>99.9 %) with an exceptional productivity of 4.0 mol kg⁻¹. Dynamic breakthrough experiments confirmed excellent separation performance for mixtures of C₂H₂/CO₂. The molecular mechanism of host–guest and guest–guest interactions in MFM‐190(NO₂) has been elucidated by in situ NPD studies as a function of gas loading for both C₂D₂ and CO₂. Overall, the judicious combination of accessible Cu(II) sites, pyridyl groups, functional groups (particularly −NO₂) and high porosity is key to the development of efficient sorbents for C₂H₂.

Data and code availability

The crystallographic data in this work have been deposited in the Cambridge Crystallographic Data Centre (CCDC). They can be accessed free of charge from https://www.ccdc.cam.ac.uk/structures/. under accession numbers CCDC: 2310235 [bare MFM‐190(NO₂)], 2310228 [MFM‐190(NO₂)⋅(C₂D₂)_5.2], and 2310234 [MFM‐190(NO₂)⋅(CO₂)_3.1]. Synthetic procedures, characterisation, and additional analysis of crystal structures and adsorption results have been presented as Supporting Information. Any additional information required in this paper is available from the corresponding authors upon reasonable request.

The authors declare no competing financial interests.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Supporting Information

Supporting Information

Supporting Information

Guo L., Han X., Li J., Li W., Chen Y., Manuel P., Schröder M., Yang S., Angew. Chem. Int. Ed. 2025, 64, e202417183. 10.1002/anie.202417183

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.