Surface engineering on a microporous metal-organic framework to boost ethane/ethylene separation under humid conditions

Abstract

Recently, examples of metal-organic frameworks (MOFs) have been identified displaying ethane (C₂H₆) over ethylene (C₂H₄) adsorption selectivity. However, it remains a challenge to construct MOFs with both large C₂H₆ adsorption capacity and high C₂H₆/C₂H₄ adsorption selectivity, especially under humid conditions. Herein, we reported two isoreticular MOF-5 analogues (JNU-6 and JNU-6-CH₃) and their potential applications in one-step separation of C₂H₄ from C₂H₆/C₂H₄ mixtures. The introduction of CH₃ groups not only reduces the pore size from 5.4 Å in JNU-6 to 4.1 Å in JNU-6-CH₃ but also renders an increased electron density on the pyrazolate N atoms of the organic linker. JNU-6-CH₃ retains its framework integrity even after being immersed in water for six months. More importantly, it exhibits large C₂H₆ adsorption capacity (4.63 mmol g^-1) and high C₂H₆/C₂H₄ adsorption selectivity (1.67) due to the optimized pore size and surface function. Breakthrough experiments on JNU-6-CH₃ demonstrate that C₂H₄ can be directly separated from C₂H₆/C₂H₄ (50/50, v/v) mixtures, affording benchmark productivity of 22.06 and 18.71 L kg^-1 of high-purity C₂H₄ (≥99.95%) under dry and humid conditions, respectively.

Fig. 1. (a) Isostructural frameworks of JNU-6 and JNU-6-CH₃ assembled with two six-connected Zn₄O SBUs and their respective organic linkers. (Color code: Zn, cyan; C, dark gray; N, blue; O, red; H, white). (b) Connolly surface analysis of JNU-6 and JNU-6-CH₃, depicting the reduced pore size upon the introduction of CH₃ groups. (c) Electrostatic potential mapping of JNU-6 and JNU-6-CH₃, depicting the increased electron density on pyrazolate N atoms upon the introduction of CH₃ groups.

Fig. 2. (a) N₂ adsorption/desorption isotherms of JNU-6 and JNU-6-CH₃ at 77 K. Inset shows the difference in their pore size distribution. (b) C₂H₆ and C₂H₄ adsorption/desorption isotherms of JNU-6 and JNU-6-CH₃ at 298 K. (c) Comparison of N₂ adsorption isotherms at 77 K and PXRD patterns of the as-synthesized JNU-6 and water-treated JNU-6 (being soaked in water for 3 days). (d) Comparison of N₂ adsorption isotherms at 77 K and PXRD patterns of the as-synthesized JNU-6-CH₃ and water-treated JNU-6-CH₃ (being soaked in water for 6 months).

Fig. 3. Primary adsorption sites for C₂H₆ (a) and C₂H₄ (d) in JNU-6-CH₃ determined by Monte Carlo (GCMC) simulations. C–H⋯π interactions (green dashed lines) for C₂H₆ (b) and C₂H₄ (e) at the primary adsorption site of JNU-6-CH₃. Independent gradient model based on Hirshfeld partition (IGMH) analysis for C₂H₆ (c) and C₂H₄ (f) at the primary adsorption site of JNU-6-CH₃ (green surfaces represent vdW interactions). (Color code: Zn, cyan; C, dark gray; N, blue; O, red; H, white. The distance unit is in Å).

Fig. 4. (a) Differential scanning calorimetry (DSC) measurements of heat flow upon introducing C₂H₆, C₂H₄, and water vapor on JNU-6-CH₃ at a flow rate of 10 mL min⁻¹ at 298 K. (b) Water vapor adsorption isotherms of JNU-6 and JNU-6-CH₃ at 298 K. (c) Experimental breakthrough curves on JNU-6 (1.0 g) for a C₂H₆/C₂H₄ (50/50, v/v) mixture at a flow rate of 2.0 mL min⁻¹ and 298 K under 0% RH and 98% RH conditions. (d) Experimental breakthrough curves on JNU-6-CH₃ (0.85 g) for a C₂H₆/C₂H₄ (50/50, v/v) mixture at a flow rate of 2.0 mL min⁻¹ and 298 K under 0% RH and 98% RH conditions. (e) Three cycles of breakthrough experiments on JNU-6-CH₃ for a C₂H₆/C₂H₄ (50/50, v/v) mixture at a flow rate of 2.0 mL min⁻¹ and 298 K under 98% RH conditions. (f) Comparison of the C₂H₄ productivity estimated from breakthrough curves for JNU-6-CH₃, JNU-6, and other reported porous materials.