Kinetics of simultaneous HDS of DBT and 4-MDBT/4,6-DMDBT over CoMoP/γ-Al₂O₃ catalysts

A series of CoMo/γ-Al₂O₃ catalysts were prepared with an addition of 0%, 0.5%, 1% or 1.5% P₂O₅ to study the influence of phosphorus addition on the simultaneous HDS of refractory sulfur compounds. Two sets of HDS experiments were performed using sulfur bearing model compounds, dibenzothiophene (DBT) with 4-methyl dibenzothiophene (4-MBDT) and DBT with 4,6-dimethyl dibenzothiophene (4,6-DMDBT). The Langmuir Hinshelwood mechanism based kinetics model was developed from the experimental conversion and product distribution data for simultaneous HDS of the two sulfur compounds via direct desulfurization (DDS) and hydrogenation (HYD) routes. The kinetic model fits the experimental data quite adequately for all the species. The HDS rate for DBT was approximately 2 and 7 times higher than that estimated for 4-MDBT and 4,6-DMDBT, respectively. Enhancement of HDS rate by P addition was observed up to 0.1 g/g (1.0 wt%) P₂O₅ while higher amount of P is not advantageous. A marginal preference towards HYD pathway was observed for the HDS of 4,6-DMDBT while DDS route was predominant during the HDS of DBT and 4-MDBT. The trends in values of the rate constants estimated by the Langmuir-Hinshelwood kinetic model compared well with pseudo-first rate constants.