Fabrication of thiophene decorated side chain entanglement free COFs for highly regenerable mercury extraction

Abstract The accumulation of mercury in the food chain and environment is harmful to living beings. This raises the need for high affinity, reusable cost-efficient adsorbents for mercury. Herein, we report a side chain entanglement free hetero atom effect bearing covalent organic framework MTF prepared by integrating melamine and thiophene linker. The AA stacked MTF-2 was prepared via transformation strategy from its AB stacked MTF-1 congener. The MTF-1 and MTF-2 were comprehensively analyzed through PXRD, SEM, TEM, 13C CP/MAS, N2 sorption and mercury removal efficiency. The adsorption capacity of MTF-2 revealed a value of 540 mg g-1, which is greater than that of MTF-1 (328 mg g-1). The adsorption mechanism was studied through XPS, 13C CP/MAS and XAFS indicating probable coordination of Hg2+ with both the triazine and sulfur moieties in a cooperative manner. To demonstrate the role of aromatic triazine ring, the analogous aliphatic triaminoguanidine was selected and the removal efficiencies between triaminoguanidine-thiophene COF (TAG-TF) and MTF-2 were compared. It revealed that the triazine unit containing MTF-2 showed 2.2 times greater removal efficiency than TAG-TF. The results reveal the structure-performance relationship, where the high surface area and relatively ordered functional moieties both paly a synergistic role in efficient mercury adsorption.