Effect of Methyl Groups at 4- and 6-Positions on Adsorption of Dibenzothiophenes over CoMo and NiMo Sulfide Catalysts

The major sulfur compounds existing in the current commercial diesel fuels are alkyl dibenzothiophenes (DBT), especially those with one or two alkyl groups at the 4- and/or 6-positions, e.g., 4-methyldibenzothiophene (4-MDBT) and dimethyldibenzothiophene (4,6-DMDBT), which are difficult to remove by the conventional HDS processes. The adsorption of DBT, 4-MDBT, and 4,6-DMDBT on two sulfided commercial catalysts, i.e., Cr344 (CoMo/Al₂O₃) and Cr424 (NiMo/Al₂O₃), was studied using a liquid-phase adsorption device. By comparing DBT, 4-MDBT, and 4,6-DMDBT, the increases of adsorption selectivity in the order of 4,6-DMDBT < 4-MBBT < DBT indicated that the methyl groups at the 4- and 6-positions inhibited the interaction between the S atom in DBT and the active sites on the catalyst, which led to the decrease of adsorption capacities of 4,6-DMDBT and 4-MDBT on the catalyst. The adsorption capacities corresponding to the saturation point of 4,6-DMDBT, 4-MDBT, and DBT were ∼ 0.062, ∼ 0.064, and ∼ 0.068 mmole/g, respectively. At least one tenth of the adsorptive sites on the catalyst adsorbed DBT, but failed to adsorb 4,6-DMDBT due to the steric hindrance of the methyl groups at the 4- and 6-positions.