Desulfurization of fuels is commonly achieved by catalytic hydrodesulfurization (HDS), in which the organic sulfur species are converted to H2S and the corresponding hydrocarbon, as in the following reaction: R-SH + H2 → R-H + H2S. Here R represents an alkyl group, such as methyl (CH3–), or ethyl (C2H5–). Figure 9.2 shows the type and relative reactivity of different sulfur species in HDS reactions. The reactivity of R-SH (mercaptan, or thiol) compounds is higher than that of disulfides (R-S-S-R). The H2S is easily removed from the desulfurized oil by absorption in a gas treatment unit and subsequently converted to elemental sulfur by the Claus process, as will be discussed in Lesson 10.
Some HDS reactions are shown in Figure 9.3. Note that the objective of hydrotreatment reactions is to take the heteroatom out with minimum hydrogen consumption. The ideal scenario is to get the hydrogen atoms to break the carbon-sulfur bonds and to remove sulfur as H2S. This may not always be possible because of problems with the accessibility of sulfur atoms to hydrogen atoms. Sulfur ring compounds such as methylated dibenzothiophenes have the lowest reactivity in HDS reactions because they are planar compounds, and in some methylated DBTs the S atom is shielded by the methyl groups (See Figure 9.4). Removing sulfur from these compounds requires more hydrogen consumption in order to saturate the aromatic rings in dibenzothiophene (DBT) to form non-planar compounds. Remember that, as opposed to aromatic compounds, cycloalkanes are not planar compounds. Therefore, saturation of the aromatic rings in methylated DBTs eliminates the steric hindrance (shielding) of methyl groups to make the S atom accessible to active sites on the catalyst surface for removal as H2S. The geometry of this proposition is better understood when one looks at the configuration of active sites on the HDS catalysts shown in Figure 9.5.
The HDS catalysts consist of sulfided molybdenum supported on Al2O3. Sulfided molybdenum surfaces have vacancies (with missing S atom in the sequence) that act as active sites on the catalyst surface (Figure 9.5). In addition to Mo, HDS catalysts also contain other metals, such as cobalt (Co) as promoters, the main function of which is believed to dissociate H2 to atomic species that readily react with S. The HDS mechanism involves inviting the sulfur atom at the HDS target to sit in the vacancy and once the S is in the vacancy, dispatching H atoms to clip C-S atoms to make H2S that will leave the vacancy and make it available for the next action. You can see that this scenario would get into trouble if S cannot sit in the vacancy, as would happen with the compound 4,6 dimethyldibenzothiophene (Figure 9.4 on the previous page and Figure 9.5 below). This calls for saturating the aromatic rings to twist the methyl groups aside to make the S atom fit in the vacancy to be taken away as H2S. The list below lists the relative HDS reactivities of methylated DBT compounds relative to HDS of unsubstituted DBT.
One can see above that the HDS reactivity of 4,6 dimethyldibenzothiophene (4,6 dimethylDBT) is one-tenth of unsubstituted dibenzothiophene (DBT), because of the shielding of the S atom by methyl groups as discussed above. Moving the methyl groups away from the sulfur atom to 3,7 positions (See Figure 9.4) increases the HDS reactivity 15- fold to 1.5 times of DBT. Interestingly, having methyl groups away from the S atom (as in 3,7 and 2,8 positions) increases the HDS reactivity relative to that of DBT. Methyl substitution on DBT away from the S atom increases the HDS reactivity by promoting adsorption of the compounds on the catalyst surface and may weaken the C-S bonds on DMDBTs. In contrast, even one methyl group shielding the S atom (4-methyldibenzothiphene in list above) reduces the reactivity of the methylatedDBT to one-fifth of the reactivity of DBT.