Investigation of synergistic effects using alkanolamines on post-synthetic modification of metal-organic framework and CO2 adsorption capacity

Abstract

The net global carbon balance is a key modern and even existential environmental issue to humans. Therefore, developing highly efficient gas separation technologies, e.g. based on the application of solid adsorbents, is critically important. Among the advanced solid adsorbents metal-organic frameworks (MOFs), especially MIL-101(Cr), which offers superior physicochemical properties, have proven to be highly promising porous materials for such purposes. In this approach, MIL-101(Cr) nanoadsorbent was functionalized using four alkanolamines (MIL-101(Cr)-alkanolamine) to enhance its CO2 adsorption capacity by increasing the polarity of its pores. Additionally, the synergistic effects of steric hindrance (-CH3 and –CH2CH2OH) and the basicity strength (pKa) of alkanolamines on post-synthetic modification (PSM) of MIL-101(Cr) were investigated. Alkanolamines were selected as the organic ligands because their nitrogen atoms (amino group) coordinate with the chromium ion in MIL-101(Cr), while the hydroxyl groups increase pore polarity and improve the CO2 adsorption capacity through creating weak hydrogen bonding (H-bonding) interactions, resulting in reduced energy consumption during regeneration. In this study, PXRD, IR, and FE-SEM results demonstrated that the crystal structure and morphology of nanoadsorbents remained intact after modification. CHN and ICP analyses were additionally employed to calculate the Nalkanolamine/Cr ratios. CO2 adsorption capacities of all MIL-101(Cr)-alkanolamine nanoadsorbents (∼30–∼45 %) and their selectivity profile (CO2:CH4 and CO2:N2) exhibited significant improvement as opposed to MIL-101(Cr). Investigations revealed that the adsorption capacities of all nanoadsorbents negligibly changed after five adsorption-desorption cycles.