Dual positive charging sites for MIL-101 enhanced adsorption of toluene under high humidity conditions: Experimental and theoretical studies

Highly efficient capture of volatile organic compounds (VOCs) under humid conditions is a significant yet formidable task for metal–organic frameworks (MOFs). Based on the material simulation technique, we proposed implanting a significantly positive-density ligand (DABA) to modify MIL-101 and constructing dual positive-charging centers to improve MIL-101 competitive adsorption of benzene VOCs over H2O vapors both thermodynamically and kinetically. The introduction of DABA containing two –NH2 groups with electrostatically positive charges into MIL-101 resulted in the formation of an open tetrahedral cage with a dual positive charging center. This facilitated strong N-H…π interactions with toluene molecules, leading them to accumulate firmly in MOF pores, and thus dramatically enhanced its adsorption capacity from 17 to 31 toluene molecules per unit cell compared to pristine MIL-101. Moreover, the open tetrahedral cage in D-MIL-101 preferentially captured toluene and occupied the strongest adsorption sites first, thus effectively preventing H2O entry into MOF pores to contact with hydrophilic metal sites. This realized high water resistance and toluene adsorption uptake under high humidity conditions. Results showed that D-MIL-101 had a high specific surface area of 2558 m2/g and a new microporous structure of 9.5 Å size. It presented higher toluene uptake of 4.69 mmol/g at lower vapor pressure of P/P0 = 0.013 and 298 K temperature, as well as 3.3 folds higher working capacity of toluene compared to pristine MIL-101 under 80 % RH. This study presents a novel approach for constructing hydrophobic MOFs using new positive charging ligands and delves deeper into the competitive adsorption of VOCs/H2O in MOF composites for large-scale applications.