2. Hydrodesulfurization (HDS)

Hydrodesulfurization is one of the most important processes among the oil refining industry, whose purpose is to reduce pollutants in the fraction of the petroleum distillates. The operation conditions of HDS reaction are in a pressure range between 60 and 200 atm and temperatures higher than 280°C. On the other hand, the sulfur vacancies, called coordinatively unsaturated sites (CUS), present at the edge of the Mo(W)S2 crystals are the active sites [13, 14].

Crude oil contains a complex mixture of sulfur compounds, which in turn have a different reactivity. It is known that for each type of fuel, the sulfur-containing molecules are different and the degree of reactivity of these sulfur compounds in HDS depends on its structure. The degree of reactivity for the removal of sulfur can vary in several magnitudes. Generally, acyclic sulfides such as sulfides, disulfides, and thiols are highly reactive in HDS compared to thiophene. The reactivities of sulfur compounds with 1–3 rings decrease in the following order thiophenes > benzothiophenes > dibenzothiophenes [14]. Similarly, the reactivity of alkyl-substituted compounds such as 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene (4,6-DMDBT) is much less reactive than other compounds. Figure 1 qualitatively represents the relationship between the type and size of sulfur compound in different fractions of distillate and their relative reactivity. In the range of diesel, molecules such as dibenzothiophenes and alkyldibenzothiophenes predominate, which present low reactivity for HDS.

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions DOI: http://dx.doi.org/10.5772/intechopen.84724

countries have opted to make stringent environmental regulations to reduce the levels of air pollutant emissions. Worldwide legislations have been issued to reduce the amount of sulfur contents in the transport fuels close to zero sulfur ppm (ultralow sulfur fuels) [5]. This represents a challenge due to the declining trend of light oil supplies [6], leading to process heavy crudes with higher concentrations of sulfur, nitrogen, and metals. In this context, the hydrodesulphurization (HDS) is considered the most effective used process to produce ultralow sulfur transport fuels, for which the design and preparation of catalysts of high HDS performance are believed to be central factors [7, 8]. In the last decade, traditional HDS catalysts have usually been based on Mo or W sulfides promoted by Co or Ni supported on γ-alumina [9, 10]. Nonetheless, due to the need to process increasingly heavy crudes and in consequence with higher concentrations of sulfur as mention before, it is becoming more difficult to produce ultralow-sulfur transport fuels using traditional HDS catalysts [11, 12]. Therefore, it is necessary to design novel catalysts with higher performance in the HDS reaction. In order to achieve it, the HDS catalysts require (1) complete sulfidation of the molybdenum or tungsten and Co(Ni) precursor oxides phases, (2) high extent of promotion, and (3) high dispersion of the Co(Ni)Mo(W)S active phase. It is well known that the strength of interaction between the support and the Co(Ni)Mo(W)S active phase has an important effect on the above three parameters. Alumina interacts strongly with the Co, Ni, Mo, and W oxide-supported phases; therefore, new support materials with weaker metalsupport interaction must be identified. This chapter will explain which factors can lower the reactivity for HDS in some sulfur compounds such as 4,6-DMDBT, as well as the models proposed to explain the nature of the active sites, and briefly summarize recent advances in the use the silicon in catalyst support with the aim of understanding the difference in HDS performance compared with the traditional catalyst supported on γ-alumina from the point of view of the strength of the

interaction between the support and the active phase.

the edge of the Mo(W)S2 crystals are the active sites [13, 14].

sulfur compounds with 1–3 rings decrease in the following order thio-

alkyl-substituted compounds such as 4-methyldibenzothiophene and

phenes > benzothiophenes > dibenzothiophenes [14]. Similarly, the reactivity of

4,6-dimethyldibenzothiophene (4,6-DMDBT) is much less reactive than other compounds. Figure 1 qualitatively represents the relationship between the type and size of sulfur compound in different fractions of distillate and their relative reactivity. In the range of diesel, molecules such as dibenzothiophenes and alkyldibenzothiophenes predominate, which present low reactivity for HDS.

Hydrodesulfurization is one of the most important processes among the oil refining industry, whose purpose is to reduce pollutants in the fraction of the petroleum distillates. The operation conditions of HDS reaction are in a pressure range between 60 and 200 atm and temperatures higher than 280°C. On the other hand, the sulfur vacancies, called coordinatively unsaturated sites (CUS), present at

Crude oil contains a complex mixture of sulfur compounds, which in turn have a different reactivity. It is known that for each type of fuel, the sulfur-containing molecules are different and the degree of reactivity of these sulfur compounds in HDS depends on its structure. The degree of reactivity for the removal of sulfur can vary in several magnitudes. Generally, acyclic sulfides such as sulfides, disulfides, and thiols are highly reactive in HDS compared to thiophene. The reactivities of

2. Hydrodesulfurization (HDS)

Silicon Materials

8

Figure 1. Relative reaction rate of organic sulfur compounds.

### 2.1 Reaction routes

The activity study of the catalysts for deep desulfurization has focused mainly on the most refractory molecules as can be observed in the literature [14, 15]. These molecules such as 4,6-dimethyldibenzothiophene (4,6-DDMBT), 4-methyldibenzothiophene (4-MDBT), and dibenzothiophene (DBT) have different reaction routes [16, 17], considering mainly the direct desulfurization route (DDS) and the hydrogenation route (HYD).

Desulfurization route (DDS). In the case of 4,6-DMBT, the DDS route is one in which the sulfur atom is removed from the structure and replaced by hydrogen, without carrying out the hydrogenation of any of the carbon-carbon double bonds present in the molecule. Some authors [17, 18] proposed that once it is adsorbed, the molecule on the active site, the DDS route begins with hydrogenation of one of the double bonds adjacent to the sulfur atom, to obtain a dihydrogenated product and then, the opening of the C▬S bond through a process of elimination. Figure 2 shows the mechanism for the C▬S cleaving. According to the studies of the structure reactivity, the activity for the DDS reactions is attributed to sites located at the edges of the MoS2 crystal [19], and it is suggested that the sites are sulfur anionic vacancies, called coordinatively unsaturated sites (CUS), which are created and regenerated during the reaction in the presence of hydrogen.

Hydrogenation route (HYD). In the HDS, the HYD route involves the hydrogenation of one of the aromatic rings, prior to the cleaving of the C▬S bond: assuming that the cleavage of the C▬S bond occurs through the β-elimination process, several explanations for the low reactivity of 4,6-DMDBT have been proposed and discussed by Bataille et al. [17] such as: (a) steric hindrance of the methyl groups for the adsorption of the molecule hydrogenated, (b) steric hindrance of the methyl groups for the cleavage of the C▬S bond, (c) the fact that only one atom of H is available for the cleavage of the C▬S bond, and (d) an effect of the methyl group on the acidity of the H atom involved in the elimination process.

The mechanism of elimination E2 (β-elimination) is described as following: a group S: (nucleophile) subtracts a proton from the molecule with sulfur atom and the leaving group is the S atom from that molecule (Figure 3).

2.2 Active phase models

DOI: http://dx.doi.org/10.5772/intechopen.84724

Figure 4.

Figure 5.

11

Model "Rim-edge" [21].

molecule to the catalytic center.

In the literature, there are some models that try to explain the structure and operation of the active phase. The models proposed by Daage and Chianelli [20] and Chianelli et al. [21] have named the sites located in the upper and lower edges of the crystal as "Rim site," which are reactive to the reactions of HYD and the break of the C▬S bond. While the "edge" sites are active only in the cleaving of the C▬S.

Ramos et al. [22] have also showed for unsupported systems the existence of strong electron donation from Co to Mo and an enhanced metallic character associated to the Co9S8/MoS2 interface. Berhault et al. [23] studied the structural role of cobalt, and the influence of support interactions on the morphology and catalytic

On the other hand, another model called of the mixed phase "Co(Ni)-Mo(W)-S"

Figure 5 depicts the location of these sites in the MoS2 crystal.

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions

properties of Mo and CoMo catalysts supported on alumina and silica.

combines studies of tunneling microscopy (STM) with calculations of density functional theory (DFT), identifying an area with high electron density in the upper part of the MoS2 crystal that was called "BRIM site," which have metal properties capable of efficiently carrying out the hydrogenation reactions [19, 24–26]. In the present chapter, the mixed phase model was used, since it is the most widely

Approaching of: (a) DBT, (b) methyl group of the 4,6-DMDBT, and (c) hydrogen atom of the 4,6-DMDBT

The favorable configuration for an elimination E2 is to have both the sulfur atom and the hydrogen-β atom interacting with the surface of the active phase (Mo(W)S2) at the same moment (Figure 4a). Then, the methyl group can hinder the process of elimination by blocking either the sulfur atom or the hydrogen-β atom as it approaches to catalytic center (Figure 4b). Moreover, the methyl group in 4,6-DMDBT molecule can also cause the hydrogen-β atom involved in the elimination process to be less acidic than the DBT molecule (Figure 4c). On the other hand, in the case of 4,6-DMDBT molecule, only one hydrogen atom is available for the elimination instead of two H atoms as occurs in DBT molecule. All of these factors can lower the reactivity of the 4,6-DMDBT compared to the DBT molecule.

Figure 2. Mechanism of the cleaving of the C▬S bond in the DDS route.

Figure 3. Mechanism of elimination E2 (β-elimination).

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions DOI: http://dx.doi.org/10.5772/intechopen.84724
