Preparation, Characterization and Desulfurization of the Supported Nickel Phosphide Catalysts

Introduction

The exhaust emission, especially the SO₂ emission, has attracted more and more attentions by the governments all over the world due to the developments of automobile industry and increasingly grievous situation in environmental protections. Accordingly, more and more stringent environmental regulations have been enacted to limit the S content in the fuels. To meet the regulations, most of the S should be eliminated by the deep hydrodesulfurization (HDS) during the petroleum refining processes. It is difficult to remove the S content to ultra-low levels using a conventional Mo(W)Co(Ni)/Al₂O₃ HDS catalyst. The transition metal phosphides are reported as a novel catalytic material with exceptional HDS performances, showing great prospect as the new generation of catalysts in deep HDS. Among the transition metal phosphides, nickel phosphide showed the best activity. Previously the nickel phosphides were usually supported on Al₂O₃ or SiO₂. The specific surface areas of Al₂O₃ or SiO₂ supported catalysts were relatively low, which was unfavorable for the dispersion of the active phases, leading to an unsatisfactory activity or stability. The mesoporous molecular sieve SBA-15 was regarded as a superior support due to the higher specific surface area, higher pore volume and narrower pore size distribution.

In the present work the mesoporous molecular sieve SBA-15 was synthesized via hydrothermal method using the cheap sodium silicate as the silicon source, the triblock copolymer (P123, EO₂₀PO₇₀EO₂₀) and cetyltrimethyl ammonium bromide (CTAB) as the co-template reagents. The Ni_xP/SBA-15 catalysts with a fixed Ni content of 20 wt% and a varying initial P/Ni molar ratio (0.25, 0.5, 0.75, 1.0, 1.25, 1.5, 2.0) were prepared by a co-impregnation method using Ni(NO₃)₂⋅6H₂O as the nickel source, (NH₄)₂HPO₄ as the phosphorus source and SBA-15 as the support, and following by the H₂ temperature-programmed reduction (H₂-TPR) treatment.

The phase formation, specific surface area and pore structure of the prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ sorption analysis and Fourier transform infrared spectrometry (FTIR) techniques. The nature of the active sites on the surface of the catalysts was investigated by CO adsorption using in situ diffuse reflectance infrared Fourier transfer spectroscopy (DRIFTS). The HDS of dibenzothiophene (DBT) was employed as a model reaction to test the HDS performances of the catalyst. The effects of the external diffusion, the internal diffusion, the total pressure, the volume ratio of H₂ to liquid feed, and the reaction temperature on the activity of the catalyst was examined systematically. The stability of the catalyst and the evolution of the structure during the stability test were investigated as well.

The obtained results showed that with the cheap sodium silicate as the silicon source, the prepared mesoporous molecular sieve SBA-15 had the specific surface area (S_BET) of 674.0 m²/g, the pore volume (V_P) of 1.12 m³/g and average pore diameter (D_BJH) of 6.7 nm. For the prepared Ni_xP/SBA-15, Ni₂P and Ni₁₂P₅ phases existed at P/Ni = 0.5. Only Ni₂P phase was detected at P/Ni ≥ 0.75. Both Ni₂P and Ni₁₂P₅ phases showed catalytic activity in HDS reactions while Ni₂P was more active. The optimal P/Ni molar ratio was found at 0.75. Either too high or too low P/Ni molar ratio would lead to a diminished HDS activity. The mesoporous structure was mainly maintained with the P/Ni molar ratio ranging from 0.25 to 2.0. All of the Ni_xP/SBA-15 catalysts had high S_BET and V_P as well as uniform pore size distribution, which would benefit the dispersion of the active phases on the catalysts and the diffusion of reactants during HDS reactions.