2.2 Active phase models

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 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.

the leaving group is the S atom from that molecule (Figure 3).

Silicon Materials

Figure 3.

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Figure 2.

Mechanism of elimination E2 (β-elimination).

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

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

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

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

Figure 4.

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

Figure 5. Model "Rim-edge" [21].

utilized in the literature. Based on this model, the catalytic properties of the sulfur vacancies at the edges of Mo(W)S2 crystallites are strongly enhanced by the close presence of a promoter atom (Co or Ni) in the so-called Co(Ni)-Mo(S)-S structures [23, 26–28]. It has been reported that there are two types of structures that involve molybdenum or tungsten Co(Ni)-Mo(W)-S crystal, these structures were named types I and II [23, 27–29]. Type I structure has a strong interaction in the γ-alumina, since there is the presence of Mo-O-Al linkages and presents poor sulfidation. Type II structures are characterized by a weaker interaction with the γ-alumina, allowing to be full sulfided and exhibits high HDS activity. Therefore, it is important to find a way to weaken the interaction between the active phase and the support. One option is to use supports that present weak interaction with the active phase such as silica or carbon. On these supports (silica or carbon) "multilayer" MoS2 structures are generated, thus the superior crystallites in the structure have a lower interaction with the support and form type II structures, which means that stacked or multilayer structures are type II and that single layer structures are type I, as shown in the Figure 6.

support for HDS catalysts is the presence of a large specific area in which the active phase Mo(W)S2 presents a very high dispersion and/or present weak interaction support—active phase to generate more active structures (type II). Therefore, it is important to find a way to weaken the interaction between the active phase and the support. Therefore, the purpose of the next section is to make an overview of the recent investigations into the role of silicon in the generation of the type II structures (weak metal-support interaction), which present better HDS conversion.

The mesoporous silicates such as MCM-41 and SBA-15 have received great attention in the last decades due to their excellent properties as catalyst support in hydrotreating reactions [30–36]. These catalysts have reported a high catalytic activity in the HDS of DBT than their counterpart supported on alumina. In the case of MCM-41, its low structural stability has limited its industrial use as support. It is also reported that the addition of Al2O3 stabilizes the structure of the MCM-41,

In order to obtain a deep insight into how the preparation conditions of the MCM-41 have influence on the performance of the catalyst, Hernandez Cedeño et al. [38] did a detail study of UV-vis spectroscopy in order to find out how the pH (7 and 9) during the preparation of MCM-41 (MCM41) and Al2O3 (Al) supports as well as the effect of varying the molar ratio of Si/Al (10, 25, and 50) affect the coordination of the supported metals, and thus the metal-support interaction. The catalysts NiW were evaluated in the HDS of DBT. For the catalysts, W/Al2O3 at different pH presented well-defined bands at 423, 720–722 nm, which can be related

On the other hand, the catalysts NiW/MCM41 prepared at pH 7 with different

At pH of 9, the NiW/AMS50 prevails the band at 818 nm, related to nickel with distorted octahedral symmetry, while for the NiW/AMS10 using the same pH, no octahedral species were observed. An opposite behavior was observed with the pH 7 catalyst, which means the catalyst NiW/AMS10 presents more define bands related to nickel species in octahedral coordination with respect to the NiW/AMS 50.

Then, the catalyst with the highest reaction rate at pH 7 and pH 9 was the NiW/

AMS10 and NiW/AMS50 catalysts, respectively. Therefore, there is a trend between the HDS activity and the bands in the range of 804–820 nm as well as the bands between 710 and 730 nm, which are related to the interaction of Ni-W and Ni species in octahedral coordination, respectively. Meanwhile, on the catalyst NiW/Al2O3 prevailed the well-defined band between 720 and 775 nm associated to Ni2+ in octahedral symmetry, which is caused by the interaction between the metal and promoter, and leads to the formation of species NiWS type II, resulting in a

Although the incorporation of Al in the MCM41 framework improved the hydrothermal stability, the reaction rate decreases. According to the results of that work, the incorporation of Al in the MCM41 framework has a negative effect on the catalyst performance. On the other hand, in the NiW/AMS catalyst, both the pH

Si/Al (10, 25 and 50) molar ratio presented bands in the range of 710–730 nm ascribed to octahedral nickel species [11, 49–51]. But also a band between 807 and 818 nm were detected, which was associated to nickel species with octahedral symmetry distorted; this latter may be related to an interaction Ni-W [41, 42]. In addition, the catalysts prepared at pH 9 bands at 804 and 820 nm ascribed to nickel with octahedral distortion were also detected similarly to the catalyst synthesized at

3. HDS catalysts supported on mesoporous silicates

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions

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

but do not achieve the promoting effect of Ni [37].

to nickel in octahedral coordination [39, 40].

pH 7 [43].

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better conversion of the DBT.

Nevertheless, it is also true that single layer structures that are type II can be obtained through a complete sulfidation of the oxidized phase and weaken the electronic interaction with the support. Figure 7 illustrates the aforementioned.

The choice of a suitable support material is often dictated by the process conditions in which a catalyst has to operate. Although, one of the key features of a

Figure 6. Structures Co(Ni)-Mo(W)-S types I and II.

Figure 7. Sulfidation reaction in Mo/Al2O3 catalysts.

support for HDS catalysts is the presence of a large specific area in which the active phase Mo(W)S2 presents a very high dispersion and/or present weak interaction support—active phase to generate more active structures (type II). Therefore, it is important to find a way to weaken the interaction between the active phase and the support. Therefore, the purpose of the next section is to make an overview of the recent investigations into the role of silicon in the generation of the type II structures (weak metal-support interaction), which present better HDS conversion.
