Tungstophosphoric acid immobilized on ammonium Y and ZSM5 zeolites: Synthesis, characterization and catalytic evaluation

Abstract

Two series of materials based on tungstophosphoric acid (TPA) immobilized on NH4Y and NH4ZSM5 zeolites were prepared by wet impregnation of the zeolite matrix with TPA aqueous solutions. Their concentration was varied in order to obtain TPA contents of 5, 10, 20 and 30% (w/w) in the solid. The materials were characterized by N2 adsorption–desorption isotherms, XRD, FT-IR, 31P MASNMR, TGA-DSC, DRS–UV–vis, and the acidic behavior was studied by potentiometric titration with n-butylamine. The specific surface area (SBET) decreased when the TPA content was raised as a result of the zeolite pore blocking. The X-ray diffraction patterns of the solids modified with TPA only presented the characteristic peaks of NH4Y and NH4ZSM5 zeolites, and an additional set of peaks assigned to the presence of (NH4)3PW12O40. According to the Fourier transform infrared and 31P magic angle spinning-nuclear magnetic resonance spectra, the main species present in the samples (except for NH4YTPA05) is the [PW12O40]3− anion, which was partially transformed into [P2W21O71]6− anion during the synthesis and drying steps. The thermal stability of the NH4YTPA and NH4ZSM5TPA materials is similar to that of their parent zeolites. Moreover, the samples with the higher TPA content present band gap energy values similar to those reported for TiO2. The immobilization of TPA on NH4Y and NH4ZSM5 zeolites is a good method to obtain catalysts with high photocatalytic activity in the 4-chlorophenol degradation. They can be reused at least three times without an important decrease in the degradation degrees.