Investigating the selective adsorption of CO2 by MIL-101(Cr)-NH2 and modeling the equilibrium data using a new three-parameter isotherm

Abstract

Compared to MIL-101(Fe)-NH2, MIL-101(Cr)-NH2 has considerably higher adsorption of CO2. On the other hand, the adsorption of CH4 and N2 by MIL-101(Cr)-NH2 is lower than the adsorption of these gases by MIL-101(Fe)-NH2 leading to a higher selectivity of CO2 to CH4 and N2 for MIL-101(Cr)-NH2. The chemisorption mechanism is more influential in higher temperatures and pressure, which can be attributed to the performance of amines which can adsorb more CO2 at higher temperatures and pressure. The isosteric heat of CO2 adsorption on MIL-101(Cr)-NH2 was estimated equal to − 31 kJ mol-1.MIL-101(Cr)-NH2 was synthesized and characterized by the BET, XRD, FESEM, TGA, and DTA methods. The BET surface area of the synthesized MOF was achieved equal to 1768 m2 g-1. The adsorption capacity of pure CO2, CH4, and N2 was estimated using the volumetric method. A new three-parameter equation was presented to model the experimental adsorption data including both types I and II isotherm shapes. Using the Ideal Adsorbed Solution Theory (IAST), the selectivity of CO2 to CH4 at high pressure, and CO2 to N2 at lower pressures was estimated equal to 4.1 and 4.7, respectively. In addition to 25 °C, the adsorption tests were performed at 18 and 30 °C to estimate the adsorption heat of CO2 on MIL-101(Cr)-NH2 achieved equal to − 31 kJ mol-1. The adsorption isotherm of CO2 at 25 and 30 °C have been almost identical showing the role of the chemisorption mechanism of CO2 by the amine functional group, especially at higher temperatures and pressures. Furthermore, the resistance of synthesized MOF to water was investigated by putting it in contact with water for 24 h. The result showed that MIL-101(Cr)-NH2 can be classified as thermodynamically stable to water.