Adsorptive removal of o-phenylenediamine from water using dinitro-functionalized metal-organic framework: Beneficial contribution of the loaded nitro group to adsorption

Metal-organic frameworks (MOFs) composed of various functional groups, excluding nitro group, have been frequently studied for water purification via effective adsorption. In this study, the adsorption of an organic compound over a dinitro-functionalized MOF was investigated for the first time. o-Phenylenediamine (OPD) and DUT-52(Zr)-(NO₂)₂ were applied as the representative adsorbate and MOF with dinitro group, respectively. Despite decreased porosity, the dinitro-functionalization improved noticeably the adsorptive performance compared to the pristine MOF. The favorable contribution of dinitro-functionalization to OPD adsorption could be interpreted via both experimental and theoretical approaches. The electron-withdrawing nature of the nitro groups helped in increasing the efficiency of hydrogen bonding (between -NH₂ of OPD and μ₃-O⁻ of the MOF) because of the ready deprotonation of μ₃-OH. Similarly, the nitro group increases the π-π interactions between the aromatic rings of MOF and OPD having an electron-rich ring. Additionally, the nitro group can make another hydrogen bond with an amino group of OPD. Furthermore, the DUT-52(Zr)-(NO₂)₂ was recyclable after simple solvent washing. Based on these results, it can be concluded that nitro-functionalization is an effective method for improving the adsorptive performance of MOFs for aromatic amines.