Transition metal phosphides have being investigated as new catalysts for hydroprocessing. The most active phosphide in found to be Ni2P and its reactivity can be understood from studies of changes in exposed sites with nano-sized crystallites. Tetrahedral M(1) sites are responsible for direct desulfurization while pyramidal M(2) sites carry out hydrogenation. A series of NiFeP/SiO2 catalysts with different Ni:Fe molar ratios (1:0, 3:1, 1:1, 1:3, 0:1) was investigated for the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene. The Fe component is a good probe for active sites because Ni2P and Fe2P adopt the same hexagonal crystal structure, yet Fe2P is completely inactive for HDS. X-ray diffraction analysis and FTIR spectroscopy of adsorbed CO indicated the formation homologous alloys. At 3.1 MPa and 613 K (340 oC) the activity of the alloys was similar to that of Ni2P, which was very high. There was also unprecedented selectivity towards direct desulfurization (DDS). A reconstruction of the NiFe phase occurred to expose more Ni sites, likely driven by the formation of surface Ni-S bonds as observed by EXAFS. The analysis showed that Ni(2) pyramidal sites responsible for hydrogenation were largely replaced by Fe. This left behind Ni(1) tetrahedral sites which favor DDS and explains the reactivity results.

Hydrodesulfurization studies on phosphide catalysts

251st National Meeting (2016)

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