Synthesis of a phosphomolybdic acid/nanocrystalline titanium silicalite-1 catalyst in the presence of hydrogen peroxide for effective adsorption-oxidative desulfurization

Abstract

Phosphomolybdic acid (HPMo) supported on microporous nanocrystalline titanium silicalite-1 zeolite (Nano-TS-1) catalysts were prepared in the absence or presence of hydrogen peroxide via the impregnation method. The catalysts were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV-visible diffuse reflectance spectroscopy (UV-vis), N₂ adsorption–desorption, ³¹P and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results of FT-IR, UV-vis and ³¹P MAS-NMR characterization confirm that the Keggin framework structure of the HPMo catalyst supported by the Nano-TS-1 is maintained, and a small amount of peroxomolybdatephosphate species is generated after the introduction of H₂O₂. The results of the kinetics research experiments show that the adsorption desulfurization activity of various aromatic sulfur-containing compounds over Nano-TS-1 and supported HPMo catalysts decreases in the order thiophene (TH) > benzothiophene (BT) > dibenzothiophene (DBT). Besides, the time to reach adsorption saturation is extended in turn, which is the result of their diffusion effect in microporous zeolites. The catalysts exhibit high catalytic activity in the oxidation of DBT and TH under mild conditions. Under selected conditions, i.e. m(oil)/m(catalyst) = 8.0 g/0.15 g, moL(O/S) = 10, V(model oil/ethanol) = 10.0 mL/10.0 mL, the desulfurization rates of TH and DBT are 99.8% and 58.6%, respectively. The results show that the supported HPMo or HPMo-H₂O₂ catalysts exhibit oxidative desulfurization efficiency in the order TH > DBT > BT, which may be attributed to the combined effect of the electron cloud density on the sulfur atoms, the pseudo liquid phase of HPMo and shape selectivity over Nano-TS-1.