3. HDS catalysts supported on mesoporous silicates

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

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 to nickel in octahedral coordination [39, 40].

On the other hand, the catalysts NiW/MCM41 prepared at pH 7 with different 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 pH 7 [43].

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 better conversion of the DBT.

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

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

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 condi-

tions in which a catalyst has to operate. Although, one of the key features of a

type I, as shown in the Figure 6.

Silicon Materials

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

Sulfidation reaction in Mo/Al2O3 catalysts.

Figure 6.

Figure 7.

12

and the amount of alumina affect the reaction rate. In the case of the catalysts impregnated at pH 7, the highest reaction rate was achieved with the catalyst that contains the highest amount of alumina in its structure (NiW/AMS10). On the contrary for the catalysts impregnated at pH 9, the NiW/AMS50 catalyst (lowest amount of alumina) presented the highest reaction rate of this series.

(Table 1). One reason for the poor activity in the catalysts CAT/40TiS15 is the higher amount of tetrahedral species compared to their counterparts. In contrast, the catalyst CAT/60TiS15 (Si/Ti = 60 molar ratio) presented a lower amount of the tetrahedral species. Tetrahedral species are difficult to be reduced and sulfide, and therefore are not susceptible to develop the HDS active sites. Then, CAT/60TiS15 exhibited the highest HDS activity (96.98% of DBT conversion), the better performance for this catalyst is due to the small amount of tetrahedral species, the low staking, and high dispersion of Mo(W)S2 phases. As it is observed, there is an optimal relationship of Si/Ti molar ratio in order to improve the HDS performance. Furthermore, the incorporation of certain amount of Ti into the structure of SBA-15 generates a high dispersion of the active phase, and a large number of

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions

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

As mentioned above, the type I structure with poor sulfidation has strong Mo-O-Al linkage with γ-alumina and presents low activity, whereas the type II structures with full sulfidation possess weak interaction with γ-alumina and exhibit high HDS activity. The surface modification of alumina with silica is an efficient way to weaken the metal-support interaction. Sanchez-Minero et al. [46] studied the effect of incorporate SiO2 with a nominal loading of 10 wt% (SAC 10) onto the surface of alumina in NiMo/Al2O3-SiO2(x) catalysts for the hydrotreatment of mixtures of 4,6-DMDBT-naphthalene-carbazole. An infrared analysis of the hydroxyl region was carried out, characteristics bands of Al2O3 hydroxyl groups were observed at 3790, 3775, 3740, 3730 and 3680 cm<sup>1</sup> [47], and it can be noted from the IR

spectrum of the alumina support (SAC). The most basic hydroxyl groups in alumina

(SAC 10), some changes in the bands intensity are observed. A new band localized in the region at 3725–3750 cm<sup>1</sup> appears, which is assigned to isolated silanol groups [48, 49]. Furthermore, the bands corresponding to the most basic hydroxyl groups

with SiO2 eliminates the most basic hydroxyl groups in alumina promoting that the sulfided NiMoSAC10 catalyst presented highly stacked MoS2 crystallites with more than two layers (type II structure). These sites favor the hydrogenation route,

Recently, Romero-Galarza et al. [50] carried out a systematic study of the change in activity, selectivity, dispersion, sulfidation, and extent of promotion for CoMo and NiMo HDS catalysts supported on Al2O3 and SiO2/Al2O3. They found for CoMo and NiMo catalysts that the grafting of the surface of alumina support with a 4.0 wt% of silica was enough to eliminate the most basic hydroxyl groups

have a higher proportion of Mo and Co(Ni) in octahedral coordination (DRS-UVvis results), resulting in a better catalytic performance in the HDS of 4,6-DMDBT. It was also found that the extent of promotion, determinated by the XPS ratio of NiMoS/NiT, is larger for the Ni-promoted catalysts than for Co-promoted catalyst, which is in line with the fact that NiMo catalysts can incorporate the Ni promoter on three different edges of a dodecagonal NiMoS particle, in contrast to CoMo catalysts where the copromoter is incorporated only on the sulfur edge of a hexagonal CoMoS cluster [26, 27]. The origin of the better performance of the NiMoSAC catalyst over their alumina-supported counterparts, NiMoAl and CoMoAl, seems to be mainly related to the higher extent of promotion and sulfidation achieved in the catalysts with SiO2 (type II structures). This fact is reflected with a good

) disappear. This behavior indicates that modifying the alumina surface

. When silica is incorporated to alumina

), and thus, this induces to

structure type II.

(3775 cm<sup>1</sup>

15

4. HDS catalysts supported on SiO2-Al2O3

give rise to an IR band at 3775 cm<sup>1</sup>

being more active for 4,6-DMDBT HDS.

bonded to tetrahedral aluminum (IR band at 3767 cm<sup>1</sup>

Continuing with the influence of the preparation conditions of the support with Si on the performance of the HDS catalysts in terms of metal-support interaction, Gómez-Orozco et al. [44] analyzed the effect of modify the SBA-15 support with Ti on its physicochemical properties and its sulfidation behavior using NiMoW/SBA-15 catalysts in the HDS reaction of DBT. The amounts of Ti4+ ions incorporated during the direct synthesis of the SBA-15 support were varied using an Si/Ti molar ratio of 60, 40, and 20, and the nominal metals loading were 3.84, 13.83, and 17.33 wt% of Ni, Mo, and W, respectively. The supported catalysts were labeled as CAT/S15, CAT/60TiS15, CAT/40TiS15, and CAT/20TiS15 in agreement with the nominal Si/Ti ratio of 60, 40, or 20, respectively.

The coordination of Ni, Mo, and W ions was analyzed by UV-vis-DRS. It was observed that all the samples present a strong band in the range of 210–280 nm, which is ascribed to Mo and W ions in tetrahedral coordination, such as Mo(W) O4<sup>2</sup>. The intensity of this band (210–280 nm) was observed to follow the next trend: CAT/40TiS15 > CAT/20TiS15 > CAT/S15 > CAT/60TiS15. The catalysts CAT/ 40TiS15 and CAT/60TiS also showed an intense band at about 350 nm, which is related to Mo or W ions with octahedral coordination [45]. Furthermore, these catalysts present a band at 750 nm assigned to Ni2+ ions in octahedral coordination. In general, the population of octahedral W species and the W species in tetrahedral coordination was increased significantly upon Ti incorporation into S15. In addition, the incorporation of Ti did not decrease the catalytic activity in the HDS of DBT reaction except for Si/Ti = 40. The UV-vis analysis of the catalyst CAT/40TiS15 indicates a higher amount of tetrahedral species than its counterpart CAT/60TiS15. The lower amount of Ti in the catalyst (CAT/60TiS15) presented the highest hydrogenation capability among the catalysts studied (HYD/DDS = 0.81) (Table 1) despite the main route of DBT reaction was the direct desulfurization (DDS) pathway. Hence, the superior activity for CAT/60TiS15 and CAT/S15 samples was related to a higher dispersion of Mo(W)S2 phase and a lower amount of tetrahedral species, which are not easy to reduce and sulfide (type I structure), which means that the catalysts CAT/60TiS15 and CAT/S15 possess a higher population of Mo(W)S2 phase with type II structure.

The incorporation of Ti into the structure of the SBA-15 affected the catalytic properties of the sulfide catalysts. However, the Ti effect depends on its loading. In this study, the moderate Ti loading (Si/Ti = 40 molar ratio) was observed and had a negative effect in the catalytic activity, presenting the lower DBT conversion


THDBT, tetrahydrodibenzothiophene; BF, biphenyl; CBH, cyclohexylbenzene; and BCH, bicyclohexyl; calculated for batch reactor operating at T = 320°C and PH2 = 800 psi for 5 h.

#### Table 1.

DBT conversion and selectivity HYD/DDS (at 40% of DBT conversion) [39].

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

(Table 1). One reason for the poor activity in the catalysts CAT/40TiS15 is the higher amount of tetrahedral species compared to their counterparts. In contrast, the catalyst CAT/60TiS15 (Si/Ti = 60 molar ratio) presented a lower amount of the tetrahedral species. Tetrahedral species are difficult to be reduced and sulfide, and therefore are not susceptible to develop the HDS active sites. Then, CAT/60TiS15 exhibited the highest HDS activity (96.98% of DBT conversion), the better performance for this catalyst is due to the small amount of tetrahedral species, the low staking, and high dispersion of Mo(W)S2 phases. As it is observed, there is an optimal relationship of Si/Ti molar ratio in order to improve the HDS performance. Furthermore, the incorporation of certain amount of Ti into the structure of SBA-15 generates a high dispersion of the active phase, and a large number of structure type II.
