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UBC Theses and Dissertations

A study of molybdenum carbide catalysts supported on carbon derived from petroleum coke for hydrotreating Wang, Haiyan

Abstract

Mo₂C catalysts supported on carbon have been investigated for use in hydrotreating reactions that remove S, N and O from oil fractions. The thesis reports on the stability of the catalysts in the presence of different model reactants. The synthesis of mesoporous carbons derived from petroleum coke (petcoke), a by-product of Canadian oilsand upgrading, is described. The impact of the mesoporous carbon as a support of the Mo₂C catalysts is also examined. An activated charcoal (AC) was initially used as the carbon source to prepare Mo₂C/AC and Ni-Mo₂C/AC catalysts by carbothermal hydrogen reduction (CHR). The most active catalyst for 4-methylphenol (4-MP) hydrodeoxygenation (HDO) was obtained at a CHR temperature of 650 ℃. The direct deoxygenation selectivity of this catalyst was > 78%, indicative of high O removal with low H₂ consumption. The effect of a Ni promoter on the synthesis and activity of Ni-Mo₂C/AC catalysts was also assessed. The presence of Ni significantly reduced the CHR temperature required for Mo₂C formation by 100 ℃. However, the Ni accelerated catalyst sulfidation during hydrodesulphurization (HDS) and formed a unique core-shell Mo₂C-MoS₂ structure. Additionally, there was an improved activity in HDS of dibenzothiophene (DBT) in the presence of Ni, provided the Ni:Mo < 0.44. Extending these results to petcoke, the transition of Mo species and the corresponding changes to the activated petroleum coke (APC) morphology that occur during CHR were determined. A maximum mesoporosity of 37% was achieved for a sample reduced to 750 ℃. The activity of the Mo₂C/APC catalysts for the HDO of 4-MP was > 3x’s higher than that of Mo₂C/AC because of the high surface area (~2000 m²/g) of the Mo₂C/APC catalyst, and the high dispersion of the Mo₂C nanoparticles. Finally, the stability of the Mo₂C/APC catalysts during the HDS, hydrodenitrogenation and HDO of DBT, carbazole and dibenzofuran, respectively, was determined as a function of the Mo₂C average particle size. DFT calculations were combined with experimental data to explain the selectivity change from hydrogenation to DDS observed during the HDS of DBT. Both S and N irreversibly deactivated the catalysts; whereas, the effect of O was reversible.

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