Dibenzothiophene hydrodesulfurization

Figure 1. Volcano plot showing the effect of the heat of formation of metal sulfides on the dibenzothiophene hydrodesulfurization activity of various mono- and bimetallic catalysts. Adapted from Chianelli et al. [6] and reprinted with permission of John Wiley Sons from L. L. Hegedus, ed., Catalyst Design— Progess and Perspectives, p. 1. Wiley, New York (1987) [1]. Copyright 1987, John Wiley Sons.

In industrial practice, catalytic surfaces are often very complex, not only structurally but also chemically. An example is shown in Fig. 1 from Chianelli et al. [6] for hydrodesulfurization catalysts. The data indicate that maximum dibenzothiophene hydrodesulfurization activity is achieved at intermediate heats of formation of metal sulfides, i.e., at intermediate metal-sulfur bond strengths. Again, while such surface energetic considerations do not have ab initio predictive ability, they are valuable tools for catalyst synthesis and prescreening. 

Nagai, M. Kabe, T. Selectivity of molybdenum catalyst in hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation effect of additives on dibenzothiophene hydrodesulfurization. J. Catal. 1983, 81, 440-449. 

Alonso and collaborators have reported the synthesis of CoMo, NiMo and NiW catalysts using tetraalkylammonium thiometallate precursors using either ex situ activation method under flow of H2/H2S or through in situ decomposition under dibenzothiophene hydrodesulfurization conditions (52-55). In situ activation led systematically to more efficient catalysts. However, a balance between structural carbon (see below) and the formation of carbon in excess blocking active sites must be optimized. This situation depends on the reducibility of the initial sulfide phase. Use of tetraalkylammonium precursors is then beneficial for M0S2-based systems, particularly promoted by Co or Ni. The situation is reversed for WS2-based catalysts due to the more difficult reducibility of tungsten sulfide. Alonso and co-workers have more recently developed a modified ex situ activation method, that generates active catalysts (56-58). The method consists of the partial hydrothermal decomposition of the precursor at 523 K followed by complete activation in a tubular furnace at 673 K. 

Orozco EO, Vrinat M (1998) Kinetics of dibenzothiophene hydrodesulfurization over M0S2 supported catalysts modelization of the H2S partial pressure effect. Appl Catal Gen 170 195-206... 

It should be noted, however, that this reaction sequence may be different from what may actually be occurring in the reactor. The reactions proceed at different rates depending on the process variables. Hydrodesulfurization of complex sulfur compounds such as dibenzothiophene also occurs under these conditions. The desulfurized product may crack to give two benzene molecules ... 

Can you think of reasons why substituted dibenzothiophenes are more difficult to desulfurize than thiophene or simple thiols (see Fig. 9.2) Depending on the choice of catalyst, hydrodesulfurization can be accompanied by hydrogenation to various extents. In which of the product streams in the refinery would you choose hydrogenative HDS and in which would you not ... 

Folsom, B. R. Schieche, D. R. DiGrazia, P. M., et al., Microbial Desulfurization of Alkylated Dibenzothiophenes From a Hydrodesulfurized Middle Distillate by Rhodococcus Erythropo-lis 1-19. Applied and Environmental Microbiology, 1999. 65(11) pp. 4967-4972. 

Figure 4. Hydrodesulfurization reactivity of dibenzothiophene using Co/Mo MSC catalysts.

J-Bonded metal complexes, hydrodesulfurization models with benzothiophene, 1, 769 with dibenzothiophene, 1, 769 Bonding studies energetics, 1, 285 overview, 1, 573—603 ring size effects, 1, 396 strength, 1, 609... 

Thus, it has become possible to define certain general trends that occur in the hydrodesulfurization of petroleum feedstocks. One of the more noticeable facets of the hydrodesulfurization process is that the rate of reaction declines markedly with the molecular weight of the feedstock (Figure 4-6) (Scott and Bridge, 1971). For example, examination of the thiophene portion of a (narrowboiling) feedstock and the resulting desulfurized product provides excellent evidence that benzothiophenes are removed in preference to the dibenzothiophenes and other condensed thiophenes. The sulfur compounds in heavy oils and residua are presumed to react (preferentially) in a similar manner. 

In spite of all of the work, the kinetics and mechanism of alkyl-substituted dibenzothiophene, where the sulfur atom may be sterically hindered, are not well understood and these compounds are in general very refractory to hydrodesulfurization. Other factors that influence the desulfurization process such as catalyst inhibition or deactivation by hydrogen sulfide, the effect of nitrogen compounds, and the effect of various solvents need to be studied in order to obtain a comprehensive model that is independent of the type of model compound or feedstock used. 

Under the usual commercial hydrodesulfurization conditions (elevated temperatures and pressures, high hydrogen-to-feedstock ratios, and the presence of a catalyst), the various reactions that result in the removal of sulfur from the organic feedstock (Table 4-3) occur. Thus, thiols as well as open chain and cyclic sulfides are converted to saturated and/or aromatic compounds depending, of course, on the nature of the particular sulfur compound involved. Benzothio-phenes are converted to alkyl aromatics, while dibenzothiophenes are usually converted to biphenyl derivatives. In fact, the major reactions that occur as part of the hydrodesulfurization process involve carbon-sulfur bond rupture and saturation of the reactive fragments (as well as saturation of olefins). 

Figure 2.15 Examples of volcano plots, describing the reaction rate as a function ofthe heat of adsorption (left), and the activity of the second-row and third-row transition metal sulfides in the hydrodesulfurization of dibenzothiophene (right).

Reaction network for the hydrodesulfurization of dibenzothiophene and for the hydrogenation of acenaphthene and biphenyl... 

Hydrodesulfurization (HDS) is an important process in petrochemical refinery as it allows deaeasing the sulfur content in diesel fuel. Compounds such as alkylated dibenzothiophene (DBT) raise some problems in HDS. The alkyl groups, especially when located in the 4 and 6 positions of the ring (see Figure 18) make alkylated DBT resistant to classical HDS catalysts because of the methyl groups that prevent the thiophenic sulfur atom to be in contact with the active site of the catalyst. 

Activity Tests with Model Compounds. Activity tests with model compounds were also carried out for the fresh, regenerated, and aged catalysts in a fixed bed reactor under a vapor phase condition at 5.0 MPa. 3 cm of crushed catalyst (0.35 - 0.5mm) was diluted with 9 cm of inactive alumina particles. Catalyst activities, such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrogenation (HG), were measured, feeding a mixture of 1 wt% carbon dioxide, lwt% dibenzothiophene, 1 wt% indole, and 1 wt% naphthalene in n-heptane. The catalysts were presulfided with a 5% H2S/H2 mixture at 400 °C for two hours and aged with a liquid feed at a reaction condition for 24 hours. Tests for HDS and HDN reactions were conducted at 275 °C, while those for a HG reaction were done at 325 °C. Condensed liquid products were analyzed with gas chromatography. Since all the reactions took place with the crashed catalysts in the vapor phase, we assumed that effectiveness factors were unity (9). 

Kwak C., Lee J. J., Bae J. S. and Moon S. H., Poisoning effect of nitrogen compounds on the performance of C0M0S/AI2O3 catalyst in the hydrodesulfurization of dibenzothiophene 4-methyldibenzothio phene and 4-6-dimethyldibenzothiophene, Appl. Catal. B Environ. 35 (2001) pp. 59-68. 

Figure 2 Effect of hydrogen sulfide on hydrodesulfurization of dibenzothiophene (DBT) over a Co/Mo/Alumina catalyst. (Based on experimental data from Ref. 2.)...

Farag, H., Mochida, I., and Sakanishi, K. Fundamental comparison studies on hydrodesulfurization of dibenzothiophenes over CoMo-based carbon and alumina catalysts. Applied. Catalysis. A, General, 2000, 194, 147. 

Olguin, E., Vrinat, M., Cedeno, L., Ramirez, I., Borque, M., and Lopez-Agudo, A. The use of Ti02-A1203 binary oxides as supports for Mo-based catalysts in hydrodesulfurization of thiophene and dibenzothiophene. Applied. Catalysis. A, General, 1997, 165, 1. 

Song, C. and Reddy, K.M. Mesoporous zeolite-supported Co-Mo catalyst for hydrodesulfurization of dibenzothiophene in distillate fuels. American Chemical Society, Division of Petroleum Chemistry, 1996, 41, 567. 

Hydrodesulfurization of Dibenzothiophene on a Nitrided Supported Molybdena-Alumina Catalyst... 

The activity and selectivity of 12.5% M0/AI2O3 nitrided at various temperatures for the hydrodesulfurization (HDS) of dibenzothiophene and the effect of re-treatment of NH3 on dibenzothiophene HDS were studied. The nitrided catalyst was significantly more active toward the scission of the C-S bond from dibenzothiophene with hydrogenation of dibenzothiophene. The sulfur species accumulated on the surface of the nitrided M0/AI2O3 catalysts by replacement of nitrogen species after reaction was analyzed by XPS measurement. The formation of molybdenum sulfide during the HDS dibenzothiophene led to a decrease in the activity of the nitrided catalyst, which approached that of the sulfided catalyst. 

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