Removing Perfluoro Pollutants PFOA and PFOS by Two-Pronged Design of a Ni₈-Pyrazolate Porous Framework

Abstract

Inspired by the practical need to remove persistent perfluoro pollutants from the environment, we leverage cutting-edge crystal engineering approaches. For this, we set our eyes on a recent porous coordination framework system based on the Ni₈-oxo cluster and pyrazolate linkers as it is known for its stability to bases and other harsh environmental conditions. Our designer linker molecule here features (1) pyrazole donors masked by <i>t</i>-butyloxycarbonyl and (2) ethynyl side units protected by triisopropylsilyl groups. The former solvothermally demasks to assemble the Ni₈-pyrazolate framework, in which the triisopropylsilyl groups can be post-synthetically cleaved by guest fluoride ions to unveil the terminal alkyne group (-CCH). The ethynyl groups of the framework solid offer versatile reactions for functionalization, as with perfluorophenyl azide (via a click reaction) to afford the two prongs of the 1,2,3-triazole base unit and the perfluoro unit. Together, these two functions make for an effective adsorbent for the topical acid pollutants of perfluorooctanoic acid and perfluorooctanesulfonic acid, with a high apparent rate constant (<i>k</i>_obs) of 0.99 g mg^-1 h^-1 and large maximum uptake capacity (<i>q</i>_max) of 268.5 mg g^-1 for perfluorooctanoic acid and <i>k</i>_obs of 0.77 g mg^-1 h^-1 and <i>q</i>_max of 142.1 mg g^-1 for perfluorooctanesulfonic acid.