CO2 capture using ZIF-94 at different temperatures

https://doi.org/10.3390/molecules27175608

“Based on the pore structure and configuration of ZIF-94, the adsorption isotherms of CO₂/N₂ were studied at different temperatures (273 and 298 K) and 1 bar. Figure 2a shows the adsorption isotherms of CO₂ and N₂ at 298 K and 1 bar. ZIF-94 showed high CO₂ uptake (53.30 cm³/g) and low N₂ uptake (6.39 cm³/g), giving an excellent uptake ratio of CO₂ over N₂ (8.34), indicating that the structure had a unique adsorption affinity for CO₂. An adsorption test was performed at 273 K under the same conditions to study the adsorption performance of the material further, as presented in Figure 2b. Notably, lower temperatures led to higher adsorption capacity for the same component, and for N₂ under the same conditions, the adsorption capacity of CO₂ was still higher than that of the former. In addition, the adsorption and desorption isotherms of CO₂ and N₂ at the two temperatures overlapped completely, and it could be clearly seen that the adsorption–desorption process of the material could be accomplished perfectly, which could reduce energy consumption and provide the possibility of implementing the adsorption–desorption cycle of the adsorbent in industry. Subsequently, ideal adsorbed solution theory (IAST) calculations were performed using the fitted parameters of the two-site Langmuir–Freundlich isotherm model (Tables S1 and S2) to predict the selectivity of the CO₂/N₂ mixture at different temperatures (273 and 298 K) and 1 bar. As shown in Figure 2c, the calculated CO₂/N₂ (15:85, v/v) selectivities were 67.11 and 54.12 at 273 and 298 K, and the calculated CO₂/N₂ (50:50, v/v) selectivities were 87.44 and 95.29 at 273 and 298 K, respectively. The Langmuir–Freundlich parameters are shown in Figure S3 and gave satisfactory agreement (R² > 0.999). Therefore, the CO₂-selective adsorbents were compared in terms of CO₂ adsorption capacity and selectivity, as shown in Table S3, outperforming previously reported benchmark materials with GME topology (such as ZIF-78 and ZIF-82), which showed the great potential of ZIF-94 in CO₂/N₂ separation. In addition, the heats of adsorption of CO₂ and N₂ of ZIF-94 as shown in Figure 2d were calculated using the Clausius–Clapeyron equation. In the near-zero adsorption region, the adsorption heats of ZIF-94 for CO₂ and N₂ were 28.23 kJ/mol and 15.71 kJ/mol, respectively. On the one hand, this showed that ZIF-94 was a CO₂-selective adsorbent, and on the other hand, the relatively low adsorption heat was beneficial to the desorption regeneration of the adsorbent during the separation process, and the view was also confirmed that the above adsorption–desorption isotherms were completely coincident (Figure 2a,b), further reducing the energy consumption. In addition, ZIF-94 showed high adsorption capacity and high IAST selectivity for CO₂/N₂ mixtures, making it an excellent potential adsorbent for high-humidity applications.”

“Figure 2. (a) The adsorption isotherms of CO₂ and N₂ at 298 K and 1 bar. (b) The adsorption isotherms of CO₂ and N₂ at 273 K and 1 bar. (c) The calculated CO₂/N₂ selectivity at 273 and 298 K. (d) The isosteric heats of adsorption (Q_st) for ZIF-94.”