Synthesis of Hybrid Phosphomolybdates and Application as Highly Stable and Effective Catalyst for the Reduction of Cr(VI).

Three hybrid phosphomolybdates were successfully synthesized by hydrothermal method and characterized by a series of physicochemical analysis techniques. X-ray single-crystal structural analysis revealed that three compounds with the reductive polyanionic clusters (MoV) were wrapped by protonated organic component bpp cations (bpp = 1,3-bi(4-pyridyl)propane) through the complex supramolecular hydrogen bonding network. They also have similar molecular formulas: (H2bpp)3[PbM(H2O)2]2H6{M[Mo6O12(OH)3(HPO4)3(PO4)]2}2· nH2O (M = Fe in 1, Zn in 2, n = 4; or M = Mn in 3, n = 6). The oxidation states of all Mo centers in these polyanions are in the form of +5, presenting clusters with the higher negative charge. The feature showed that they were easy to be modified by transition metal and organic moieties, so as to form a high-dimensional structure and produce functional materials with specific properties. Comparison of catalytic ability of three crystals to reduce CrVI using formic acid as reductant, found that crystal 1 was effectively active to this redox reaction. The conversion of CrVI can reach 99% after 120 min of heating in 55 °C water bath, and the conversion of above 95% can still be achieved after 5 recycles of applications.