Spatial disposition of square-planar mononuclear nodes in metal-organic frameworks for C2H2/CO2 separation

Abstract

The efficient separation of acetylene (C₂H₂) from its mixture with carbon dioxide (CO₂) remains a challenging industrial process due to their close molecular sizes/shapes and similar physical properties. Herein, we report a microporous metal-organic framework (JNU-4) with square-planar mononuclear copper(ii) centers as nodes and tetrahedral organic linkers as spacers, allowing for two accessible binding sites per metal center for C₂H₂ molecules. Consequently, JNU-4 exhibits excellent C₂H₂ adsorption capacity, particularly at 298 K and 0.5 bar (200 cm³ g^-1). Detailed computational studies confirm that C₂H₂ molecules are indeed predominantly located in close proximity to the square-planar copper centers on both sides. Breakthrough experiments demonstrate that JNU-4 is capable of efficiently separating C₂H₂ from a 50 : 50 C₂H₂/CO₂ mixture over a broad range of flow rates, affording by far the largest C₂H₂ capture capacity (160 cm³ g^-1) and fuel-grade C₂H₂ production (105 cm³ g^-1, ≥98% purity) upon desorption. Simply by maximizing accessible open metal sites on mononuclear metal centers, this work presents a promising strategy to improve the C₂H₂ adsorption capacity and address the challenging C₂H₂/CO₂ separation.

Scheme 1. A simple concept to enhance C₂H₂ adsorption via spatial disposition of square-planar mononuclear metal centers with tetrahedral spacers.

Fig. 1. (a) MPTBDC organic linker as a tetrahedral building unit, Cu(COO)₂(N-pyridine)₂ and Cu(COO)₂(N-triazole)₂ complexes as square-planar 4-connected building units, respectively. (b) Simplified pts topology of JNU-4, highlighting that every copper center has both OMSs orientated toward the channels accessible for C₂H₂ interaction. Color code: green, Cu; red, O; blue, N; gray, C. C₂H₂ molecules are shown in a ball-and-stick model depicted in gray.

Fig. 2. (a) N₂ adsorption/desorption isotherms of JNU-4a at 77 K. Filled/empty squares represent adsorption/desorption. (Inset) Calculated pore-size distribution using the Non-Localized Density Functional Theory (NL-DFT) method. (b) C₂H₂ and CO₂ single-component adsorption/desorption isotherms of JNU-4a at 298 K up to 1 bar. (c) Comparison of C₂H₂ gravimetric uptake in JNU-4a and some representative MOF materials at room temperature (0.5 bar and 1 bar). (d) Differential scanning calorimetry of JNU-4a upon introducing C₂H₂ or CO₂ at a flow rate of 20 mL min⁻¹ under ambient conditions (298 K and 1 bar).

Fig. 3. (a) Experimental and simulated adsorption isotherms of C₂H₂ (red) and CO₂ (blue) at 298 K and up to 1 bar. (b) GCMC simulated adsorption density distributions of C₂H₂ in JNU-4a at 298 K and 1 bar. (c) GCMC simulated adsorption density distributions of CO₂ in JNU-4a at 298 K and 1 bar. (d) DFT-D-calculated binding configurations of C₂H₂ at site I. (e) DFT-D-calculated binding configurations of C₂H₂ at site II. (f) DFT-D-calculated binding configurations of CO₂ at site I (green, Cu; dark gray, C; blue, N; red, O; white, H. C₂H₂ and CO₂ are represented in ball-and-stick models in (d)–(f), and the distance unit is Å).

Fig. 4. (a) Breakthrough curves (left y-axis) of C₂H₂ and CO₂ on JNU-4a for an equimolar mixture of C₂H₂/CO₂ (4.0 mL min⁻¹) at 298 K. Empty squares depict the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column (right y-axis). (b) Comparison of the estimated amount of C₂H₂ captured on the breakthrough column for an equimolar mixture of C₂H₂/CO₂ (JNU-4a and other top-performing materials). (c) Desorption curves of C₂H₂ and CO₂ after breakthrough equilibrium with helium gas (10.0 mL min⁻¹) sweeping at 298 K. Solid green squares represent the ratios of C₂H₂/CO₂ in the desorbed gas mixture. The yellow area highlights C₂H₂ with over 98% purity (105 cm³ g⁻¹) collectible during the desorption. (d) Continuous breakthrough experiments on JNU-4a for an equimolar mixture of C₂H₂/CO₂ at 298 K and the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column of nine cycles. In situ regeneration was carried out with helium gas (10.0 mL min⁻¹) sweeping at 298 K.

Fig. 5. (a) Continuous breakthrough curves of C₂H₂ and CO₂ on JNU-4a for an equimolar C₂H₂/CO₂ mixture (6.0 mL min⁻¹) under dry (0% RH) and humid conditions (10% RH). In situ regeneration was carried out with helium gas (10.0 mL min⁻¹) sweeping at 298 K. (b) One set of breakthrough curves (left y-axis) of C₂H₂ and CO₂ on JNU-4a for an equimolar C₂H₂/CO₂ mixture (6.0 mL min⁻¹) under humid conditions (10% RH). Empty squares depict the estimated amount of C₂H₂ and CO₂ captured on the breakthrough column (right y-axis). (c) Desorption curves of C₂H₂ and CO₂ after breakthrough equilibrium with helium gas (10.0 mL min⁻¹) sweeping at 298 K. Solid green squares represent the ratios of C₂H₂/CO₂ in the desorbed gas mixture. The yellow area represents C₂H₂ with over 98% purity (85 cm³ g⁻¹) collectible during the desorption. (d) Comparison of the C₂H₂ productivity estimated from desorption curves for an equimolar C₂H₂/CO₂ mixture.