Encapsulated anion-dominated photocatalytic and adsorption performances for organic dye degradation and oxoanion pollutant capture over cationic Cu(i)-organic framework semiconductors

Abstract

Decontamination of industrial wastewater containing toxic organic dye molecules and oxoanions is urgently desirable for environmental sustainability and human health. Water-stable porous metal–organic frameworks (MOFs) have emerged as highly efficient photocatalysts and/or adsorbents for water purification through controllable integration of the constitutive requirements. To reveal the inclusion anion effect of microporous MOFs on wastewater treatment, two isostructural MOFs incorporating positive charge and semiconductive characteristics, {[Cu(tpt)]·3H₂O·0.5SO₄}_n (1) and {[Cu(tpt)]·2H₂O·ClO₄}_n (2, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine), have been synthesized and employed as dual-functional materials for both dye photodegradation and oxoanion removal. The two MOFs possess the same 3-fold interpenetrating cationic backbones but are encapsulated by highly disordered sulfate or perchlorate in the open channels. These included anions have significantly tuned the hydrophilicity of the channels, extended the visible-light absorption, optimized the bandgap and decreased the conduction band potential. Under the low-energy irradiation of a 30 W LED lamp, MOF 1 has selectively and efficiently degraded rhodamine B compared to 2 with accelerated kinetics, resulting from the stronger reduction ability and less migration resistance of the photogenerated electrons. Instead, MOF 2 can quickly capture harmful MnO₄⁻ and Cr₂O₇²⁻ by exchanging with the entrapped ClO₄⁻, with maximum adsorption amounts of 557 and 168 mg g⁻¹, respectively, under ambient conditions. The improved decolorization of the aqueous solution over 2 benefits essentially from the shape and charge memory effect and the smaller hydration energy of ClO₄⁻ than SO₄²⁻. These interesting observations highlight the importance of the included anions inside the porous MOF semiconductors on wastewater treatment.