(715g) The Activity and Stability of Molybdenum Carbide Supported on Activated Petroleum Coke in Hydrotreating Reactions

Mo₂C, a novel catalyst that has attracted attention due to its Pt-like behavior, ¹ has potential as an effective catalyst for heteroatom (S, N and O) removal from crude oils and bio-oils. However, the stability of Mo₂C in the presence of these heteroatoms is not well established. ^{2, 3} In the present study, we report on the stability of β-Mo₂C catalysts with varying particle sizes, during hydrodesulfurization (HDS), hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) of the model reactants dibenzothiophene (DBT), dibenzofuran (DBF) and carbazole (CBZ), respectively. Transformation of the Mo₂C in the presence of the heteroatoms is investigated. Petroleum coke was converted into a high surface area activated carbon (APC) by chemical activation and then used to prepare supported β-Mo₂C by carbothermal hydrogenation reduction. This study demonstrates that S and N have a strong, irreversible effect on Mo₂C, while O partially modifies the catalyst based on the O and H₂ concentrations. The catalyt performance was assessed in a down-flow trickle bed reactor using 0.2 wt% DBF, DBF or CBZ in decalin as reactant. The experiments were operated at 350 ^oC and 4.1 MPa under H₂. The liquid products collected periodically were analyzed by GC-MS. Fresh and used catalysts were characterized using XRD, XPS, TEM and BET.

β-Mo₂C formation was confirmed by XRD, XPS and TEM. The physical properties of the obtained Mo₂C/APC catalysts changed during CHR, compared to APC, with surface areas of ca. 2000 m²/g and pore volumes of 1.2 cm³/g, with up to ca. 50% being mesoporous. TEM results showed that the particle size of the Mo₂C increased with increased Mo loading from 2% to 10 wt%. The deconvolution of the Mo_3d XPS spectra indicated that the ratio of Mo₂C plus oxycarbide to Mo-oxide species was similar (~ 75:15) for all Mo loadings. Also, a good correlation between the actual Mo loading after CHR and the XPS Mo:C ratio (I_Mo/I_C) was achieved, indicating good dispersion of Mo species on the APC when the Mo loading ≤ 10 wt%. Experiments were conducted for ~500 min time-on-steam (TOS) to investigate catalyst stability in the presence of the S, O and N containing reactants. The conversion of DBT and CBZ decreased and the product selectivity changed with reaction time before stabilizing; whereas, the conversion of DBF remained constant and the product selectivity changed. Increasing the DBF concentration 10 x’s, resulted in catalyst deactivation. Catalyst regeneration by hydrogenation at 400 ^oC and 1 MPa for 2 h was possible after HDO, but the deactivation was irreversible in the case of HDS and HDN. Catalyst deactivation is also shown to be closely related to particle size. The larger the particle size, the faster the deactivation during HDS and HDN.The present study demonstrates different mechanisms of Mo₂C deactivation during HDS, HDN and HDO reactions. S/N atoms have an irreversible effect on Mo₂C; whereas, the effect of O is reversible.

References

1. Furimsky, E., Metal carbides and nitrides as potential catalysts for hydroprocessing. Applied Catalysis A: General 2003, 240, (1), 1-28

2. Aegerter, P. A.; Quigley, W. W. C.; Simpson, G. J.; Ziegler, D. D.; Logan, J. W.; McCrea, K. R.; Glazier, S.; Bussell, M. E., Thiophene hydrodesulfurization over alumina-supported molybdenum carbide and nitride catalysts: adsorption sites, catalytic activities, and nature of the active surface. Journal of Catalysis 1996, 164, (1), 109-121

3. Stellwagen, D. R.; Bitter, J. H., Structure–performance relations of molybdenum-and tungsten carbide catalysts for deoxygenation. Green Chemistry 2015, 17, (1), 582-593.