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Synthesis, characterization and hydroprocessing activity of modified transition metal phosphides Abu, Ibrahim Inamah


In the present study, modified transition metal phosphides (bulk and supported catalysts) were prepared by the temperature programmed reduction (TPR) o f the corresponding phosphate precursors. The catalysts were characterized by X-ray diffraction (XRD), BET surface area, X-ray Photoelectron Spectroscopy (XPS), n-propyl amine chemisorption, CO uptake and Transmission Electron Microscopy (TEM). The activity of the prepared catalysts were then tested using 4,6-dimethyldibenzothiophene (4,6-DMDBT) for hydrodesulftrrisation (HDS) and carbazole for hydrodenitrogenation (HDN). As well, selected catalysts were tested using Light Gas Oil (LGO) derived from Athabasca bitumen. The HDS of 4,6-DMDBT showed that small amounts of Co used to modify Ni₂P and - MoP produced high selectivity for the direct desulfurization (DDS) product dimethylbiphenyl (DMBP). Co[sub 0.4]Ni₂P/Al₂O₃ showed high conversion with little cracked products. Supported Co[sub 0.4]Ni₂P/MCM showed almost complete conversion of 4,6-DMDBT with cracked products. Fluorination marginally increased the conversion of Co[sub 0.4]Ni₂P/Al₂O₃ with insignificant changes in the product distribution and the addition of Pt enhanced hydrogenation. The Ni[sub x]MoP (0 ≤ x ≤ 1.1) bulk series and supported Ni[sub O.33]MoP were tested for the hydrodenitrogenation (HDN) of carbazole at 523-583 K , 3 MPa and a range of space velocity of 8.6-61 x 10⁻² mol/h gcat. Ni[sub 0.07]MoP showed the highest bicyclohexyl (BCHX) selectivity among the Ni[sub x]MoP. The improved selectivity was attributed to the enhanced CO uptake and acidity that resulted in increased hydrogenation of carbazole to tetrehydrocarbazole which readily undergoes C-N bond cleavage on acid sites to produce BCHX. The HDS conversion obtained over the Ni[sub 0.33]MoP/Al₂O₃ using the LGO showed higher conversion than a commercial sulfided NiMo/AI₂O₃. The results from the present study therefore indicate that addition of small amounts of Co to metal phosphides is beneficial as enhanced selectivity to the hydrogenolysis route of 4,6-DMDBT is promoted. Similarly, addition of small amounts of Ni is beneficial for the HDN of carbazole. However in this case, the presence of Ni increased the hydrogenation of carbazole making it easier for the hydrogenolysis to occur.

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