Gate-Opening Effect in a Flexible Metal-Organic Framework for Sieving Acetylene

Abstract

Adsorptive separation employing porous materials is one of the most promising alternative technologies for C₂H₂ purification due to its energy-efficient and environmentally friendly advantages. Herein, we present the design and synthesis of a dicopper-paddle-wheel-based metal-organic framework (termed JNU-5-Me) with a carboxylate-azolate organic linker. The use of such a linker results in the axial positions of the dicopper paddle wheels being occupied by azolates, and therefore, a much-improved chemical stability of the framework structure. JNU-5-Me shows negligible adsorption of C₂H₄, C₂H₆, and CO₂ at 1.0 bar and 298 K, while a gate-opening effect for C₂H₂ and a large C₂H₂ adsorption (4.7 mmol g^-1) at 1.0 bar and 298 K. Dynamic breakthrough studies on JNU-5-Me demonstrate its excellent C₂H₂ separation performance from C₂H₂/CO₂ (50/50, v/v) and C₂H₂/CO₂/C₂H₄/C₂H₆ (70/10/10/10, v/v/v/v) mixtures. Additionally, in-situ infrared spectroscopy and Grand canonical Monte Carlo (GCMC) simulation reveal that the carboxylate oxygens and methyl groups on the framework are involved in the strong binding of C₂H₂, which may be attributed to the gate-opening effect of JNU-5-Me.

Figure 1

(a) The traditional coordination mode of Cu paddle-wheel SBU with carboxylates as linkers. (b) The coordination mode of Cu paddle-wheel SBU in JNU-5-Me with carboxylate-azolate as linker, showing the axial positions were occupied by azolates. (c) The perspective view of the 1D channels of JNU-5-Me in the a-axis direction. (d) The PXRD patterns of JNU-5-Me after treatment with water and 0.1 M HCl aqueous solution for a duration of 24 h.

Figure 2

(a) N₂ and CO₂ adsorption/desorption isotherms of JNU-5-Me at 77 and 196 K, respectively. (b) Adsorption/desorption isotherms of JNU-5-Me for C₂H₂, CO₂, C₂H₄, and C₂H₆ at 298 K and 1.0 bar. (c) Adsorption isotherms of JNU-5-Me for C₂H₂ at different temperatures, showing the gate-opening pressures increase with temperatures. (d) Differential scanning calorimetry measurements of heat flow upon introducing C₂H₂, CO₂, C₂H₄, and C₂H₆ on JNU-5-Me at 298 K and 1 bar.

Figure 3

(a) GCMC calculated density distribution of C₂H₂ in JNU-5-Me at 100 kPa. (b) DFT calculated binding of C₂H₂ in JNU-5-Me at its optimized adsorption sites (distance unit is Å). (c) In-situ FT-IR spectra of the activated JNU-5-Me samples upon being exposed to C₂H₂ gas at 298 K.

Figure 4

(a) Experimental breakthrough curves for a C₂H₂/CO₂ (50/50, v/v) mixture with a flow rate of 2.0 mL min^–1 at 298 K and 2.0 bar. (b) Three consecutive breakthrough curves for a C₂H₂/CO₂ (50/50, v/v) mixture with a vacuum at room temperature as the regeneration method. (c) Experimental breakthrough curves for a C₂H₂/CO₂/C₂H₄/C₂H₆ (70/10/10/10, v/v/v/v) mixture. (d) Three consecutive breakthrough curves for a C₂H₂/CO₂/C₂H₄/C₂H₆ (70/10/10/10, v/v/v/v) mixture with a vacuum at room temperature as the regeneration method.