Improved structure of Zr-BTC metal organic framework using NH₂ to enhance CO₂ adsorption performance

Modified mesoporous NH₂-Zr-BTC mixed ligand MOF nanocomposites were synthesized via the hydrothermal method as a novel adsorbent for CO₂ capture. The newly modified MOF-808 with NH₂ demonstrated a similar mesoporous morphology as MOF-808, whereas the specific surface area, pore volume, and average particle size, respectively, increased by 15%, 6%, and 46% compared to those of MOF-808. The characterization analyses exhibited the formation of more active groups on the adsorbent surface after modification. In addition, a laboratory adsorption setup was used to evaluate the effect of temperature, pressure, and NH₂ content on the CO₂ adsorption capacity in the range of 25-65 °C, 1-9 bar, and 0-20 wt%, respectively. An increase in pressure and a decrease in temperature enhanced the adsorption capacity. The highest equilibrium adsorption capacity of 369.11 mg/g was achieved at 25 °C, 9 bar, and 20 wt% NH₂. By adding 20 wt% NH₂, the maximum adsorption capacity calculated by the Langmuir model increased by about 4% compared to that of pure MOF-808. Moreover, Ritchie second-order and Sips models were the best-fitted models to predict the kinetics and isotherm data of CO₂ adsorption capacity with the high correlation coefficient (R² > 0.99) and AARE% of less than 0.1. The ΔH°, ΔS°, and ΔG° values were - 17.360 kJ/mol, - 0.028 kJ/mol K, and - 8.975 kJ/mol, respectively, demonstrating a spontaneous, exothermic, and physical adsorption process. Furthermore, the capacity of MH-20% sample decreased from 279.05 to 257.56 mg/g after 15 cycles, verifying excellent stability of the prepared mix-ligand MOF sorbent.