Selective adsorption mechanisms of pharmaceuticals on benzene-1,4-dicarboxylic acid-based MOFs: Effects of a flexible framework, adsorptive interactions and the DFT study

The synergetic effects of benzene-1,4-dicarboxylic acid (BDC) linker structure and the metal cluster of MOFs on adsorption mechanisms of carbamazepine, ciprofloxacin and mefenamic acid were investigated in single and mixed solutions. A 1D flexible framework MIL-53(Al), 3D rigid framework UiO-66(Zr) and 3D flexible framework MIL-88B(Fe) were applied as adsorbents. The breathing effect of MIL-53(Al) caused by its flexible structure can enhance intraparticle diffusion for all pharmaceuticals and perform a critical role in excellent adsorption performances. The 3D rigid BDC structure of UiO-66(Zr) caused a steric effect that reflected low or negligible adsorption. Unless concerning accessibility through the internal structure of the MOFs, the binding strengths calculated by the DFT study were in the following order: MIL-88B(Fe) > MIL-53(Al) > UiO-66(Zr). The Fe cluster in MIL-88B(Fe) seems to have the highest affinity for the carboxylic group of pharmaceuticals compared with Al and Zr; however, the lower porosity of MIL-88B(Fe) might limit the adsorption capacity. Moreover, in mixed solutions, the higher acidity of mefenamic acid can enhance competitive performance in interactions with the metal cation cluster of each MOF. Together with the breathing effect, H-bonding and π-π interaction were shown to be the alternative interactions of synergetic adsorption mechanisms.