Direct desulfurization

If one takes, as a reference, the rate constant for direct desulfurization of unsubstituted dibenzothiophene (kDg) and renormalizes this value to be... 

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). 

Compound Direct desulfurization site (cr) Hydrogenation site (t)... 

Perhaps the largest discrepancies in reported results are the relative values for the adsorption constants of H2S and thiophene molecules (THs, including thiophene, benzothiophene, and dibenzothiophene). The reported preference for adsorption on the direct desulfurization site ranges from H2S THs (122,123,125) to about the same (104) to H2S < c THs (125). 

Satterfield s studies indicated that, as the temperature was increased, the preference for adsorption of THs becomes larger (125), but the differences between authors is far more than can be explained by the different temperatures of their experiments. The various parameters are summarized in Table XV. The report of Froment may provide the best guidelines at present (104). That report indicates the following relative preferences for adsorption on the direct desulfurization site (cr) ... 

Examining Table XVII, one can see that it is true that, for dibenzothiophene, the direct desulfurization route (/cDo) suffers the greatest inhibition by H2S only 7.5% of the original activity remained in the inhibited case. However, the hydrogenative route (kHs,) was also severely inhibited by H2S only 25% of the original activity remained. The largest inhibition was, in fact, in the hydrogenation of biphenyl (A f,P ). 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Fig. 33. Reaction pathways of 4,6-DMDBT conversion in the presence of zeolite containing CoMo/Al203 catalyst (a) HDS with isomerization route (b) hydrocracking route (c) direct desulfurization route.

Hydroprocesses offer direct desulfurization of heavy feedstocks and high conversion. These processes were not originally designed for heavy feedstocks... 

Additional information about the catalytic performance of the catalysts can be obtained from the analysis of the product distribution, which is affected by the metallic and acid functionalities. Tables 4 and 5 compare the product distributions obtained in the DBT and 4,6-DMDBT reactions with the NiMo/Al203, NiMo/HNaY and NiMo catalysts with 20% of HNaY in their formulation. In the case of DBT, zeolite incorporation into the catalyst changes the contributions of the direct desulfurization (DDS) pathway, which yields biphenyl-type compounds, and of the desulfurization through hydrogenation (HYD) pathway, which gives cyclohexylbenzene-type compounds. Also, the proportion of CHB in the reaction products and the liquid yield decrease with the number of accessible zeolite acid sites in the catalyst. This effect is due to the cracking of CHB on the zeolite acid sites. On the other hand, the formation of DCH is enhanced on the catalysts where Mo precursor phase is more polymerized (NiMo/HNaY-Al203(P) and NiMo/HNaY formulations). 

Despite the different mechanistic interpretations that have been advanced, it is deal from the experimental facts that the presence of alkyl substituents on the arene rings has a major inhibiting effect on the rate of the direct desulfurization route, and therefore the molecules of interest in the context of deep desulfurization are desulfurized predominantly via a hydrogenative pathway. This is very important in relation to the design of new effective HDS catalysts, which must display a high activity for the hydrogenation of arene rings. 

Determined at 30 % of total 4,6-DMDBT conversion. DDS, direct desulfurization pathway ... 

DFT has also allowed to obtain knowledge in the sites involved in HDS reactions. For instance, Moses and co-workers (145) have reported a DFT study on HDS of thiophene, and they have fovmd that the thiophene adsorption and subsequent hydrogenation of thiophene to 2,5-dihydrothiophene occurred at the Mo(lOiO) edge brim site, while direct desulfurization took place at the sulfur vacancies at the S(iOlO) edge. 

Perot et al. proposed the formation of dihydro intermediates during the HDS of DBT and dialkyl-DBT, regardless of whether the route involves hydrogenolysis or direct desulfurization as seen in Fig. 19. They ascribed die inertness of 4,6-DMDBT to hindered P-elimination by substituted methyl groups. 

CoMo catalysts desulfurize primarily via the direct desulfurization route. NiMo catalysts, which exhibit a higher hydrogenation activity, have a relatively higher selectivity for desulfurization via the hydrogenation route. The extent to which a given catalyst desulfurizes via one route or the other determines the effect of H2 partial pressure, H2S partial pressure and feed properties on the catalyst activity. 

In the expression for the rate of desulfurization (-dCs/dt), the first term represents the direct desulfurization route, which is enhanced by an increase of the hydrogen partial pressure and inhibited by the presence of H2S. The second term represents the hydrogenation route, which is also enhanced by an increase of the hydrogen partial pressure and inhibited by certain nitrogen-... 

Chughtai, Y. M., Linneweber, K. W and Schmid, C., 1990, Direct Desulfurization in Combination with Polishing Reactor," paper presented at the EPRI/EPA 1990 SO2 Control Symposium, New Orleans, LA, May 8-11. 

Figure 10.5 The Two Reaction Pathways for the Hydrodesulfurization of Dibenzothiophene (DBT) Leading Either to Cyclohexylbenzene Through the Hydrogenating (HYD) Route or to Biphenyl Through the Direct Desulfurization (DDS) Route.

Addition of quinoline in the reaction mixture slightly decreased the activity of all studied catalysts in HDS of DBT, but completely inhibited the 4,6-DMDBT transformations. In presence of quinoline, DBT ehmination occurred almost completely through the direct desulfurization (DDS) route, indicating that quinoUne poisoned the sites active for the hydrogenation pathway of the reaction. Since HDS of 4,6-DMDBT oeeurs preferentially through the hydrogenation route [3], it e>q)lains the complete loss of the catalysts activity for this reaction in presence of quinohne. 

NiMo catalysts supported on SBA-15 were prepared by coimpregnation and successive impregnation methods with the addition of citric acid in the impregnation solutions. Addition of citric acid resulted in an increase in both catalysts activity in dibenzotMophene HDS and selectivity towards the direct desulfurization route, which was due to an increase in the M0S2 dispersion and in the amount of Ni-Mo-S species. 

Catalytic activity tests (Table 2) show that citric acid addition in NiMo/SBA-15 catalysts leads to an increase in the activity in DBT HDS and selectivity towards the direct desulfurization (DDS) route of the reaction. A comparison of the catalytic behavior of the NiMoCA/SBA-15 catalysts prepared by coimpregnation (C) and successive impregnation (S) methods indicates that the catalyst prepared by the C method resulted to be more active and more selective for the DDS route than its counterpart prepared by the S procedure. In general, activity and selectivity trends... 

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