Hydrodesulfurization (HDS) of petroleum feedstocks is an area of great importance in the refining industry because the sulfur content of available crude feedstocks is steadily increasing, while at the same time the oil is becoming heavier. Transition metal phosphides have attracted attention as new catalysts for hydrodesulfurization with activity higher than conventional sulfides. Early work showed that the activity of common phosphides follows the order: Ni2P &gt; WP &gt; MoP &gt; CoP &gt; Fe2P and the high activity of Ni2P is the subject of this presentation. A study of Ni and Fe phosphide alloys allows identification of the active site. The bulk structure of Ni2P consists of tetrahedral Ni(1) sites and square pyramidal Ni(2) sites in Ni2P and the role of the sites was studied by substitution of Ni with Fe. The Fe component was deemed as a good probe because Ni2P and Fe2P adopt the same hexagonal crystal structure, yet Fe2P is completely inactive for HDS. For this purpose a series of NiFeP/SiO2 catalysts were prepared with different Ni:Fe molar ratios (1:0, 3:1, 1:1, 1:3, and 0:1) and investigated in the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) at 300 and 340 oC. The uniformity of the NiFe series was demonstrated by x-ray diffraction analysis and by Fourier transform infrared (FTIR) spectroscopy of adsorbed CO. The position of substitution of Fe was determined by extended X-ray absorption fine structure (EXAFS) analysis. It was found that at 300 oC the HDS activity of the catalysts decreased with increasing Fe content and that this could be explained by the substitution of Fe at the more active Ni(2) sites. As temperature was raised to 340 oC, the activity of the Fe-containing samples increased dramatically almost to the level of Ni2P, and this could be understood from a reconstruction of the NiFe phase to expose more Ni(2) sites. This was likely driven by the formation of surface Ni-S bonds, which could be observed by EXAFS in spent samples. The extremely high activity of the NiFe catalyst was accompanied with an unprecedented increase in the selectivity of HDS toward the direct desulfurization pathway, with the major product being dimethylbiphenyl.

Highly active FeNi bimetallic phosphide catalyst gives unprecedented selectivity to the direct desulfurization pathway

2019 Spring National Meeting

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