Simultaneous hydrodesulfurization of dibenzothiophene and substituted dibenzothiophenes over phosphorus modified CoMo/Al2O3 catalysts

Phosphorous modified CoMo/γ-Al2O3 catalysts, containing 0.0 to 1.5 wt.% P2O5, were prepared by incipient wetness co-impregnation method. X-ray diffraction results showed increase in MoO3 and CoMoO4 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 kDBT/k4-MDBT ratio was 2–3, while the kDBT/k4,6-DMDBT ratio was about 6–7. Modification of CoMo/γ-Al2O3 catalysts by phosphorus addition strongly increased the HDS activity and the maximum enhancement was achieved with 1.0 wt.% P2O5. 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.% P2O5 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.