Adsorptive removal of aromatic diamines from water using metal-organic frameworks functionalized with a nitro group

Adsorptive removal of aromatic diamines such as methylenedianiline (MDA) and p-phenylenediamine (PPD) was firstly investigated with nitro-functionalized metal-organic frameworks (MOFs, MIL-101(Cr)-NO2). The MIL-101 (Cr)-NO2 showed much better performances in the removal of MDA and PPD, in both adsorption capacity and kinetics, than any other adsorbents. For example, MIL-101(Cr)-NO2 had a much higher maximum adsorption capacity for MDA (1111 mg.g(-1)) than activated carbon (208 mg.g(-1)) or a reported adsorbent (391 mg.g(-1)). Based on experimental results, hydrogen bonding (especially, via the formation of a 6-membered ring (6-MR) between -NO2 of the adsorbent and -NH2 of the adsorbates) could be suggested as the main mechanism to interpret the noticeable adsorption of the diamines. Importantly, this is the first example to confirm that MOFs with nitro group can be a competitive adsorbent to remove organics composed of amino group, especially via making 6-MR through hydrogen bonding. Higher adsorption of MDA than that of PPD over MIL-101(Cr)-NO2 might be explained with pi-pi interaction between aromatic rings (pi-lean aromatics of MOF and pi-rich aromatics of the adsorbates). Moreover, MIL-101(Cr)-NO2 could be recycled after simple washing, suggesting the potential use of the MOF in adsorptive purification of contaminated water with organics with amino groups.