Aids to Mechanistic Understanding

Calculation of Metal-Sulfur Coordination Bond Strength (Adsorption) 

The reactivity of an organic sulfur compound in an HDS process depends on several factors. The compound must first be adsorbed onto the catalyst 

Enthalpies of S-Containing Molecule Adsorption on SBMS of Different Compositions in Terms of the Method of Interacting Bonds (2) 

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

It is clear from the data presented in Table XIV that H2S, though adsorbed competitively with dibenzothiophene, is not a major inhibitor for dibenzothiophene adsorption. Dibenzothiophene was shown to be preferentially adsorbed relative to biphenyl on both the cr and r sites. It is surprising that no adsorption of H2S was noted on the hydrogenation site (r) since it is known to be a strong inhibitor for many aromatic hydrogenations. A 

---