Degradation of Rhodamine-B By Heteropolyoxometalate Ionic Liquid Under Visible Light

The photo-catalytic degradation of Rhodamine-B(RhB) dye was successfully achieved using different heteropolyoxometalate phosphonium ionic liquids(PWPILs) under visble light irradiation. A series of heteropolyoxometalate phosphonium ionic liquids were first prepared from phosphotungstic acid and different quaternary phosphonium salts trihexyl(tetradecyl)phosphonium bromide, tributyl(octyl)phosphonium bromide and tetrabutyl phosphonium bromide with different molar ratioa. The melting points of PWPILs, all lower than 100°C,were characterized by differential scanning calorimeter(DSC). The results show that the characterized peaks of phosphotungstic anion in IR spectra of PWPILs are consistent with that of phosphotungstic acid. This indicates that the quaternary phosphonium cations do not change the Keggin structure of heteropoly acid anion. The influences of experimental parameters, such as the carbon chain length in phosphoniums, the number of free hydrogen in phosphotungstic anions ,addition of different catalysts, irradiation time and the concentration of RhB solution on the photo-catalytic degradation of RhB were investigated. The photo-degradation efficiency was characterized by decolourization rate of RhB. The results indicated that [P(C₁₄H₂₉(C₆H₁₃)₃]₃[PW₁₂O₄₀] showed the best photo-catalytic degradation efficiency. The RhB solution (2×10-4mol/L) was almost decomposed catalyzed by [P(C₁₄H₂₉)(C₆H₁₃)₃]₃[PW₁₂O₄₀] under UV-Vis light irradiation for 3 hours. The degradation rate was more than 95%.