1. Introduction

The air pollution is one of the main problems that the governments around the world have been concerned to mitigate in recent decades. The harmful gases found in the atmosphere are the product of the generation of energy through the combustion of hydrocarbons [1–3]. However, the hydrocarbons will continue to be used as the main source of energy in the next decades [4]; thus, it represents an environmental problem. The main pollutants generated by the combustion of fuels are SOx, CO, NOx, and traces of some heavy elements. The SOx is considered particularly dangerous since SO2 can be oxidized to SO3 by several routes, depending on the particular conditions of the atmosphere. Once the SO3 is formed, it is diluted in the water droplets that are present in the atmosphere, therefore generating the sulfuric acid (H2SO4), leading to the acid rain. Moreover, it produces a direct environmental damage to humanity, such as eye irritation and constriction of the respiratory tract, causing harm to the entire population, but especially to asthmatics and other sensitive people. It also contributes to vegetation damage, causing discoloration and lesions on the leaves. In addition, sulfur dioxide has been associated with steel corrosion, deterioration of concrete structures, paper, leather, historical monuments, and certain textiles. Based on the described problems,

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.

2.1 Reaction routes

Figure 1.

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hydrogenation route (HYD).

Relative reaction rate of organic sulfur compounds.

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions

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

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

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

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

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

explanations for the low reactivity of 4,6-DMDBT have been proposed and

regenerated during the reaction in the presence of hydrogen.

on the acidity of the H atom involved in the elimination process.
