Facile fabrication and characterization of rare earth complexes based on Keggin-type polyoxometalate with highly efficient activity for photocatalytic degradation of MO

Abstract

Using phosphotungstic acid, tetraethyl ethylenebisphosphonate (L) and rare earth chloride as raw materials, a series of Keggin-type POM-based rare earth complexes were synthesized by a heating and stirring method: [LnL₃(H₂O)]PW₁₂O₄₀·CH₃CN (1–6) (Ln = Dy, Ho, Er, Tm, Yb, Lu). The molecular structures of the complexes were determined by X-ray single-crystal diffraction, and the influence of weak forces on the molecular packing structures were studied by calculating the presence of hydrogen bonds inside the complexes using PLATON. By analyzing the experimental results, it is concluded that tetraethyl ethylenebisphosphonate is coordinated with rare earth ions through bidentate chelation, and POMs which are not involved in coordination exist as anions. Complexes 1–6 are three-dimensional supramolecular structures constructed by hydrogen bonds. Powder X-ray diffraction proved that the structures of the synthesized complexes are consistent with the results of single-crystal analysis, and the purity is high. Thermogravimetric analysis showed that the series of complexes have good thermal stability above 200 °C. Notably, the complexes 1–6 showed good photocatalytic degradation ability to methyl orange (MO) solution under UV irradiation; their degradation rate reached more than 95% within 15 min. The membrane material of complex 1 was successfully prepared by combining complex 1 with polyvinylidene fluoride (PVDF). Scanning electron microscopy (SEM) showed that complex 1 was supported on the surface of the PVDF film. For photocatalytic degradation of MO solution, the MO degradation rate reached 97.4% within 50 min. Compared with the powder-type catalyst, the degradation efficiency is lower, but the membrane material catalysis solves the problem of catalyst recovery and has better recyclable performance. Using the method of adding different active species scavengers, the mechanism of the photocatalytic reaction was studied. Finally, terahertz time-domain spectroscopy (THz-TDS) was performed, which provided a spectroscopic basis for the rapid detection of the weak forces contained in the complexes.