Controlled synthesis of efficient NiWS active phases derived from lacunary polyoxometalate and the application in hydrodesulfurization

Abstract

Efficient catalysts with abundant NiWS active phases for hydrodesulfurization (HDS) were prepared based on structural diversity as well as unique electrostatics property of lacunary Keggin polyoxometalates (POMs). Owing to super nucleophilicity and coordinative ability, the trivacant Keggin [PW9O34]9- (PW9) can accurately capture and anchor Ni2+ promoters into its skeleton to form nanosized Nix@PW9 molecule in impregnating solution, thereby achieving ultra-dispersion of Ni promoters, high Ni/W ratio, and atomic-scale Ni–W proximity. The superior structure of Nix@PW9 promoted the simultaneous reduction and sulfuration of Ni and W species, thus obtaining shorter slab length, less stacking number, highly efficient synergism, and high concentration of NiWS active phases. Therefore, the resulted S-NiPW9/Al2O3 catalyst exhibits efficient HDS activity for dibenzothiophene (DBT) removal compared to that of reference catalysts prepared from monovacant [PW11O39]7- (PW11), saturated [PW12O40]3- (PW12), and conventional oxide precursors. The structure-directing effects of oxidic POM precursors on the texture and performance of HDS active phases were revealed by employing a variety of characterization methods, including transmission Fourier transform infrared (TS-FTIR), attenuated total reflectance Fourier transform infrared (ATR-FTIR), ultraviolet–visible spectroscopy (UV–Vis), single crystal X-ray diffraction, H2 temperature programmed reduction (H2-TPR), CO adsorbed Fourier transform infrared spectroscopy (IR/CO), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). This POMs-based molecular methodology opens a new avenue to better understand the evolution behavior of precursors and the formation mechanism of active species during the preparation of catalysts.