Fast and Selective Heavy Metal Removal by a Novel Metal-Organic Framework Designed with In-Situ Ligand Building Block Fabrication Bearing Free Nitrogen

Fast and effective adsorbents for the selective removal of Hg^II and Pb^II ions were prepared by the reaction of Zn(NO₃ )₂ ⋅6H₂ O, H₂ BDC, and N₁ ,N₂ -bis(pyridin-4-ylmethylene) ethane-1,2-diamine (L) that yields an unprecedented two-dimensional layer-based supramolecular framework, {Zn(BDC)(L*)}⋅DMF (TMU-40), by solvothermal reaction. The formation of this framework involved an in situ C=C coupling of L to L* [L*=5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine]. As L* contains free nitrogen atoms, direct reaction of L* and metals led to metallated products. Post-synthetic modification of this novel MOF (TMU-40) with H₂ O₂ gives a new framework (O-TMU-40) by same structure and different ligand, which also bears free nitrogen atoms. FTIR spectra, TGA analysis, X-ray diffraction, Zeta Potential analysis and ¹ H NMR spectroscopy were used to characterize the prepared frameworks. The TMU-40 and O-TMU-40 frameworks were used for heavy-metal removal from aqueous solutions. Maximum adsorption values of 269 mg g^-1 for Hg^II with TMU-40 and 215 mg g^-1 for Pb^II with O-TMU-40 were achieved in 10 min at 298 Kwithout changes in the pH of the adsorption medium with pseudo-second order kinetics based on the Langmuir model. The extremely fast kinetics of TMU-40 means this adsorbent can reduce heavily contaminated water containing Hg^II concentrations of 40 ppm down to the acceptable limit of 2 ppb.