Photoactive hourglass-type M{P₄Mo₆}₂ networks for efficient removal of hexavalent chromium

The exploration of high-efficiency photocatalysts to drive the conversion of highly toxic heavy metal hexavalent chromium (Cr(VI)) in wastewater to low-toxic trivalent chromium (Cr(III)) is of great significance for purifying water that contains emerging contaminants. Herein, four hourglass-type phosphomolybdate-based hybrid networks—(H₂bpe)₂[M(H₂O)₃]₂{M[P₄Mo₆O₃₁H₇]₂}·8H₂O (M = Mn for 1, Co for 2) and (Hbpe)(H₂bpe)Na[M(H₂O)₃]₂{M[P₄Mo₆O₃₁H₇]₂}·9H₂O (M = Mn for 3, Co for 4; {M[P₄Mo₆O₃₁H₇]₂}⁸⁻ (abbr. M{P₄Mo₆}₂); bpe = 1,2-di(4-pyridyl)ethylene)—were hydrothermally synthesized as heterogeneous photocatalysts for Cr(VI) reduction. A structural analysis showed that the four hybrids 1–4 exhibited two-dimensional inorganic sheet-like structures with a 3,6-connected kgd topology built of hourglass phosphomolybdate clusters having different central metal ions, which further interacted with organic bpe cations via abundant hydrogen-bonding interactions to extend the structure to a three-dimensional (3D) supramolecular network. The four hybrids displayed excellent redox properties and wide visible-light absorption. When used as heterogeneous photocatalysts, hybrids 1–4 exhibited excellent photocatalytic activity for Cr(VI) reduction under 10 W white light irradiation, with reduction rates of 91% for 1, 74% for 2, 90% for 3, and 71% for 4, respectively, within 80 min. The Cr(VI) reduction reaction over hybrids 1–4 followed the pseudo first-order kinetics model with reaction rate constants k of 0.0237 min⁻¹ for 1, 0.0143 min⁻¹ for 2, 0.0221 min⁻¹ for 3 and 0.0134 min⁻¹ for 4, respectively. The Mn{P₄Mo₆}₂-based hybrids 1 and 3 showed better photocatalytic performance than the Co{P₄Mo₆}₂-based hybrids 2 and 4, along with excellent recycle stability. This mechanism study shows that the different central metals M in the M{P₄Mo₆}₂ cluster have a considerable impact on photocatalytic performance due to their regulation effect on the electronic structure. This work provides evidence for the important role of the central metal in hourglass-type phosphomolybdate in the regulation of photocatalytic performance, and it brings inspiration for the design of highly efficient photocatalysts based on polyoxometalates.