Adsorption of Phenol and p-Nitrophenol from Aqueous Solutions on Metal-Organic Frameworks: Effect of Hydrogen Bonding

Three metal organic frameworks (MOFs), MIL-100(Fe, Cr) and NH2-MIL-101(Al), were prepared, and their adsorption equilibria for phenol and p-nitrophenol (PNP) from water were investigated. All three MOFs show similar and limited adsorption capacities for phenol, but NH2-MIL-101(Al) reveals exceptional adsorption capacity for PNP, greatly exceeding those of MIL-100(Fe, Cr). MIL-100(Fe, Cr) possess similar adsorption affinity for phenol and PNP, which suggests that the effect of metal ions and the coordinatively unsaturated sites in MOFs show negligible effect for phenol and PNP adsorption from water. NH2-MIL-101(Al) exhibits superior adsorption capacity for PNP and uniquely higher adsorption selectivity for PNP over phenol than a benchmark activated carbon. The remarkable adsorption affinity is attributed to the hydrogen bonding between PNP and the amino groups in NH2-MIL-101(Al). Phenol and PNP displayed a fast adsorption kinetics on NH2-MIL-101(Al) and followed a pseudo-second-order kinetic model. This work highlights that introducing functional groups into MOFs through an organic linker is a promising way to tailor MOFs for aqueous adsorption and separation.