Tungstophosphoric acid heterogenized onto NH₄ZSM5 as an efficient and recyclable catalyst for the photocatalytic degradation of dyes

Abstract

Materials based on tungstophosphoric acid (TPA) immobilized on NH<SUB>4</SUB>ZSM5 zeolite 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, <SUP>31</SUP>P MAS-NMR, TGA.-DSC, DRS-UV-Vis, and the acidic behavior was studied by potentiometric titration with n-butylamine. The BET surface area (S<SUB>BET</SUB>) decreased when the TPA content was raised as a result of zeolite pore blocking. The X-ray diffraction patterns of the solids modified with TPA only presented the characteristic peaks of NH<SUB>4</SUB>ZSM5 zeolites, and an additional set of peaks assigned to the presence of (NH<SUB>4</SUB>)<SUB>3</SUB>PW<SUB>12</SUB>O<SUB>40</SUB>. According to the Fourier transform infrared and <SUP>31</SUP>P magic angle spinning-nuclear magnetic resonance spectra, the main species present in the samples was the [PW<SUB>12</SUB>O<SUB>40</SUB>]<SUP>3-</SUP> anion, which was partially transformed into the [P<SUB>2</SUB>W<SUB>21</SUB>O<SUB>71</SUB>]<SUP>6-</SUP> anion during the synthesis and drying steps. The thermal stability of the NH<SUB>4</SUB>ZSM5TPA materials was similar to that of their parent zeolites. Moreover, the samples with the highest TPA content exhibited band gap energy values similar to those reported for TiO<SUB>2</SUB>. The immobilization of TPA on NH<SUB>4</SUB>ZSM5 zeolite allowed the obtention of catalysts with high photocatalytic activity in the degradation of methyl orange dye (MO) in water, at 25 °C. These can be reused at least three times without any significant decrease in degree of degradation.