Hydrodesulfurization of dibenzothiophene on NiMo/γ-Al2O3 washcoated monoliths

• Optimum metal loading was determined for hydrodesulfurization of dibenzothiophene on NiMo/γ-Al2O3 catalyst.
• Higher catalyst size showed pore diffusional resistances.
• Catalyst dispersion decreased with metal loading.
• NiMo/γ-Al2O3 washcoated on monoliths was fully utilized with no external or internal mass transfer resistances.

Hydrodesulfurization of dibenzothiophene, dissolved in n-hexadecane, was conducted in the temperature range of 548–593 K at 10 MPa pressure in a conventional packed bed reactor for two different catalyst sizes and on monoliths washcoated with different metal loadings. The monolith catalysts were made by washcoating using NiMo/γ-Al2O3 catalysts prepared by incipient wetness and uniform well-adhered washcoats were obtained by calcination after each dipping–air blowing–drying cycle. The MoO3 loading of the powder catalyst was varied from 12.0 to 24.3 wt.%, keeping the MoO3/NiO wt. ratio fixed at 6. With an increase in metal loading, the surface area, pore volume and the concentration of weak acidic sites decreased, whereas the metal particle size and the concentration of acidic sites of intermediate strength increased. The highest conversion was obtained with the catalyst containing 18.6 wt.% MoO3 and 3.21 wt.% NiO. The effectiveness factor of the larger size catalyst (average size: 1.5 mm) varied from 0.61 to 0.71 at 593 K. The rate constants (calculated on the basis of metal loading) for the monoliths were in good agreement with those obtained in a packed bed, implying that all the catalyst was utilized in the monoliths. After a 72 h run at 593 K and 10 MPa, the concentration of strong acidic sites increased but there was no change in the activity or product distribution.

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