Microporous MOF for CO2 capture at different process conditions

https://doi.org/10.1039/D1MA00919B

“The isotherms of CO₂, N₂, and CH₄ at ambient temperatures were measured on the activated sample of [Cu₃(μ₃-OH)(PCA)₃] to investigate its gas adsorption properties. As shown in Fig. 2a, [Cu₃(μ₃-OH)(PCA)₃] exhibits a CO₂ uptake of 2.93 mmol g⁻¹ (65.6 cm³ g⁻¹) at 1 bar and 298 K. With increasing temperature, the CO₂ uptake gradually decreases. In contrast to the high uptake of CO₂, [Cu₃(μ₃-OH)(PCA)₃] exhibits a near-linear N₂ isotherm and displays a negligible uptake of N₂ at 298 K (as shown in Fig. 2b). The significant difference between the isotherms of CO₂ and N₂ indicates the high adsorption selectivities for CO₂/N₂. Similar to N₂, [Cu₃(μ₃-OH)(PCA)₃] also displays a linear isotherm for CH₄ at 298 K, exhibiting a CH₄ uptake of 0.67 mmol g⁻¹ at 298 K and 1 bar.”

“Based on CO₂ isotherms at different temperatures, the CO₂ adsorption enthalpy (Q_st) was calculated by using the Clausius–Clapeyron relation (see the calculated procedure in the ESI,† S3). As shown in Fig. 2c, the Q_st of CO₂ adsorption is in the range of 31.5–36.1 kJ mol⁻¹, with minor differences with increasing adsorption amounts. These Q_st values are apparently larger than that of common MOFs, such as HKUST-1 (25.26 kJ mol⁻¹)²² and NiDABCO (20 kJ mol⁻¹).²³ The large value of Q_st suggests the high adsorption potential of [Cu₃(μ₃-OH)(PCA)₃] to CO₂ molecules. The high adsorption potentials to CO₂ molecules could be attributed to the synergistic effect of nano-cages and the high density of open metal sites.”

“Based on adsorption isotherms, the equilibrium adsorption selectivity for the CO₂/N₂ mixture can be evaluated by the ideal adsorbed solution theory (IAST), of which several studies have widely validated the reliability.^10,24 As shown in Fig. 2d, the IAST selectivity for the CO₂/N₂ mixture is calculated as 70 at 1 bar and 298 K (see calculation procedures in the ESI,† S4). This value is among the high ranks of adsorbents and is significantly higher than that of the common MOFs (see comparison Table in the ESI,† S5).¹⁸ Induced by the higher adsorption potential for CO₂, [Cu₃(μ₃-OH)(PCA)₃] shows a high CO₂/CH₄ selectivity. The equilibrium CO₂/CH₄ selectivity calculated from IAST is in the range of 11.8–15.9, which is also superior to the bench-marking MOFs, such as HKSUT-1 (5.5)^18,22 and NiDABCO.²³

The outstanding CO₂ separation performance of [Cu₃(μ₃-OH)(PCA)₃] also encourages us to investigate the separation performance for light hydrocarbons, which play crucial roles in petroleum chemistry. As shown in Fig. 2e, [Cu₃(μ₃-OH)(PCA)₃] exhibits type-I isotherms for C₂H₄, C₂H₆, C₃H₆, and C₃H₈, which is in clear contrast with CH₄. At 298 K and 1 bar, adsorption uptakes of 2.2, 1.9, 2.5, and 2.1 mmol g⁻¹ are observed for C₂H₄, C₂H₆, C₃H₆, and C₃H₈, respectively. We can clearly see that the adsorption uptake increases with the chain length of light hydrocarbons. Furthermore, the adsorption uptakes of C₂H₄ and C₃H₆ are higher than those of C₂H₆ and C₃H₈, implying that alkenes are more preferentially adsorbed on [Cu₃(μ₃-OH)(PCA)₃] than alkanes. Based on the isotherms at different temperatures, the Q_st values of adsorption for C₂H₄, C₂H₆, C₃H₆, and C₃H₈ are calculated as 35.4, 33.8, 54.6, and 45.8 kJ mol⁻¹, respectively. The higher Q_st values for C3 molecules compared to C2 molecules indicate that [Cu₃(μ₃-OH)(PCA)₃] displays a higher interaction strength with the hydrocarbons with longer chain length. Besides, [Cu₃(μ₃-OH)(PCA)₃] shows a higher interaction strength with alkenes than alkanes. The preferential adsorption of alkenes to alkanes could be attributed to the π-electrons of alkenes, which could give back donations to open metal sites of MOFs. Due to the higher adsorption potentials, [Cu₃(μ₃-OH)(PCA)₃] exhibits higher adsorption selectivity for C2/C3 hydrocarbons than CH₄. Based on their adsorption isotherms, the equilibrium adsorption selectivities for C₂H₄/CH₄, C₂H₆/CH₄, C₃H₆/CH₄, and C₃H₈/CH₄ at 298 K are in the range of 22–167, 13–18, 124–737, and 73–550, respectively (as shown in Fig. 2f). Notably, at 298 K and 1 bar, high IAST selectivities of 124 and 73 have been reached for C₃H₆/CH₄ and C₃H₈/CH₄, outperforming common adsorbents and common MOFs.^25,26 The excellent adsorption uptake and separation selectivity demonstrates the promising potential of [Cu₃(μ₃-OH)(PCA)₃] in separating light hydrocarbons.”

“Fig. 2 (a) CO₂ adsorption isotherms on [Cu₃(μ₃-OH)(PCA)₃] at 298, 308 and 318 K and 0–1 bar; (b) the comparison of adsorption isotherms for CO₂, CH₄ and N₂ on [Cu₃(μ₃-OH)(PCA)₃] at 298 K; (c) the isosteric adsorption enthalpy for CO₂ adsorption on [Cu₃(μ₃-OH)(PCA)₃]; (d) the equilibrium separation selectivity for CO₂/N₂ and CO₂/CH₄ mixtures calculated from the IAST method at a feed gas ratio of 50/50; (e) the comparison of adsorption isotherms for CH₄, C₂H₄, C₂H₆, C₃H₆ and C₃H₈ on [Cu₃(μ₃-OH)(PCA)₃] at 298 K; (f) the equilibrium separation selectivity for equimolar C₂H₄/CH₄, C₂H₆/CH₄, C₃H₆/CH₄ and C₃H₈/CH₄ mixtures calculated from the IAST method.”