Simultaneous hydrodesulfurization of dibenzothiophene and substituted dibenzothiophenes over phosphorus modified CoMo/Al ₂O ₃ catalysts

Phosphorous modified CoMo/γ-Al ₂O ₃ catalysts, containing 0.0 to 1.5 wt.% P ₂O ₅, were prepared by incipient wetness co-impregnation method. X-ray diffraction results showed increase in MoO ₃ and CoMoO ₄ phases with the addition of phosphorous. Pseudo-first order hydrodesulfurization (HDS) rates via direct desulfurization (DDS) and hydrogenation (HYD) pathways were determined for simultaneous conversion of dibenzothiophene (DBT) and 4-methyl DBT (4-MDBT) as well as DBT and 4,6-dimethyl DBT (4,6-DMDBT) in an autoclave reactor at 6.2 MPa and 573-623 K. The k _DBT/k _4-MDBT ratio was 2-3, while the k _DBT/k _4,6-DMDBT ratio was about 6-7. Modification of CoMo/γ-Al ₂O ₃ catalysts by phosphorus addition strongly increased the HDS activity and the maximum enhancement was achieved with 1.0 wt.% P ₂O ₅. Results show that HDS of DBT and 4-MDBT proceed mainly via DDS pathway, whereas 4,6-DMDBT exhibits preference towards HYD pathway. Due to competition for the DDS active sites, the DBT conversion was lower in the presence of 4-MDBT than 4,6-DMDBT. Enhancement in HDS rates due to 1.0 wt.% P ₂O ₅ addition at 623 K was in the following order: 4,6-DMDBT (51%) > 4-MDBT (38%) > DBT (26%). Since 4,6-DMDBT is one of the most refractive sulfur compounds in the diesel fuel, enhancement in its HDS rate by about 50% will substantially contribute in achieving the 10-15 ppm sulfur level.