4. HDS catalysts supported on SiO2-Al2O3

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 give rise to an IR band at 3775 cm<sup>1</sup> . When silica is incorporated to 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 (3775 cm<sup>1</sup> ) disappear. This behavior indicates that modifying the alumina surface 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, being more active for 4,6-DMDBT HDS.

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 bonded to tetrahedral aluminum (IR band at 3767 cm<sup>1</sup> ), and thus, this induces to 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

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

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

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

DBT reaction except for Si/Ti = 40. The UV-vis analysis of the catalyst

addition, the incorporation of Ti did not decrease the catalytic activity in the HDS of

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 popula-

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

BF (%) CBH (%) BCH (%) THDBT (%) HYD/DDS Conversion (%)

Catalysts DDS HYD DBT

CAT/S-15 22.34 13.47 1.11 3.08 0.79 91.57 CAT/60Ti-S15 22.16 13.05 1.05 3.75 0.81 96.86 CAT/40Ti-S15 23.99 12.3 0.1 3.6 0.65 80.15 CAT/20Ti-S15 22.86 13.23 0 3.58 0.75 82.36 THDBT, tetrahydrodibenzothiophene; BF, biphenyl; CBH, cyclohexylbenzene; and BCH, bicyclohexyl; calculated for

amount of alumina) presented the highest reaction rate of this series.

nominal Si/Ti ratio of 60, 40, or 20, respectively.

Silicon Materials

tion of Mo(W)S2 phase with type II structure.

batch reactor operating at T = 320°C and PH2 = 800 psi for 5 h.

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

Table 1.

14

Figure 8. Relationship between global HDS rate constant vs. Ni or Co atoms involved in the Co(Ni)MoS phase.

correlation between the degree of promotion with the hydrodesulfurization rate constant displayed in Figure 8, and agrees with the literature reports that indicate that promotion favors the appearance or brighter (more metallic) brim site, which can perform hydrogenating reactions [51], which is the main route for the HDS of 4,6-DMDBT.

structures Co(Ni)-Mo-S type II, unlike the catalysts supported in γ-alumina where the structures type I predominate, which are not well sulfided due to a strong metalsupport interaction. On the other hand, a different Mo precursor is used (gemini surfactant-linked) in conjunction with the use of a mixed Al2O3-SiO2 support with a composition of 96.4 and 3.6 wt%, respectively, resulting in the formation of sulfide molybdenum (MoS2) crystals with higher stacking, generating the so-called

The metal-support interaction is one of the most important parameters in the design of HDS catalysts. The use of silicon in the preparation of HDS catalyst support has been showed to weak the metal interaction, generating type II Co(Ni)- Mo(W)-S structures, which are characterized by: (i) a complete sulfidation of the

(ii) stacked structures so that the upper crystallites in the structure Mo(W)S2 have a low interaction with the support. The so-called type II Co(Ni)-Mo(W)-S structures are more active than the partially sulfide type I. The use of mesoporous silicates such as MCM-41 and SBA-15 has been proposed in the literature, with the intention to increase the active phase, acidity, the type II structures, and the dispersion of the active phase (Mo(W)S2). In the case of the MCM-41 support, the preparation conditions such as pH and the Si/Al molar ratio increase the number of oxidized species in octahedral coordination, which are precursors of the type II structures. However, these materials did not show better HDS performance in the DBT molecule compared with the alumina support. It would be very useful to evaluate this type of catalyst support (MCM-41) in a molecule more refractory to HDS, such as 4,6-DMDBT, and see if it is possible to increase the catalytic performance compared to the catalyst supported in alumina, since the 4,6-DMDBT molecule is more sensitive to the geometry of the Co(Ni)-Mo(W)-S structure than the DBT molecule. On the other hand, SBA-15 mesoporous silicate doped with a certain amount of Ti (Si/Ti = 60 molar ratio) showed to improve the catalytic performance in the HDS of the DBT molecule, through generating a greater population of type II

Ni-Mo-S type II, which are more active in HDS of 4,6-DMDBT molecule.

Catalysts kHDS<sup>a</sup> TOFb <sup>10</sup><sup>4</sup> Product ratioc

The Silicon on the Catalysis: Hydrodesulfurization of Petroleum Fractions

) (s<sup>1</sup>

Number of the reacted 4,6-DMDBT molecules per second and per Mo atom at the edge surface.

Determinated at about 50% of the total 4,6-DMDBT conversion by changing liquid hourly space velocity.

Mo-IM 0.32 1.72 0.33 1.85 Mo-CTHD 0.47 2.24 0.3 2.41 Mo-GSHD 0.7 2.83 0.27 3.07 NiMo-IM 3.16 5.62 0.16 2.04 NiMo-CTHD 4.18 7.34 0.13 2.72 NiMo-GSHD 5.78 9.21 0.11 3.67 TH, tetrahydrodimethylbenzothiophene; HH, hexahydrodimethyldibenzothiophene; MCHT, dimethylbicyclohexyl;

) (TH + HH)/MCHT MCHT/DMDBP

s 1

(10<sup>7</sup> molg<sup>1</sup>

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

Calculated with the 4,6-DMDBT conversion at about 30%.

HDS results of 4,6-DMDBT on different catalysts [47].

oxidized phase, weakening the electronic interaction with the support or

structures; therefore, these materials are useful as HDS catalyst support.

5. Conclusions

17

and DMDBP, dimethylbiphenyl.

a

b

c

Table 2.

On the other hand, Xu et al. [11] prepared a novel NiMo/SiO2-Al2O3 catalysts with the improved stacking and good dispersion of supported active phase via gemini surfactant-assisted synthesis. In this method, polymolybdates anions were transformed into gemini surfactant-linked Mo precursor (GSMP), dispersing Mo species well and weakening the strong Mo-support interaction. The GSMP-based NiMo/SiO2-Al2O3 (NiMo-GSHD) catalyst presents higher activity for the HDS of 4,6-DMDBT than its counterparts prepared via impregnation (NiMo-IM) and the cetyltrimethylammonium bromide-assisted hydrothermal method (NiMo-CTHD). To understand their different activities (Table 2), the HDS activities of the catalysts were correlated with the structure of their metal phase.

The reason for the higher HDS activity of 4,6-DMDBT (Table 2) exhibited in the catalyst NiMo-GSHD is related to the greater MoS2 dispersion, a superior average stacking number determinated by HRTEM and the higher extent of promotion (NiMoS) calculated by XPS and by NO-IR characterization, thus generating more Ni-Mo-S active sites with sufficient brim sites (type II structures). Such that, the prehydrogenation activity of NiMi-GSHD for 4,6-DMDBT with steric hindrance is markedly improved. The prehydrogenated products (4,6-THDMDBT and 4,6- HHDMDBT) without steric hindrance are much easier to be desulfurization via hydrogenolysis on the edge sites of Ni-Mo-S phases than initial 4,6-DMDBT. Therefore, NiMo-GSHD, with more edge sites due to its better metal dispersion, possesses higher 3,3´-MCHT selectivity than NiMo-IM and NiMo-CTHD.

According to the results showed in this section, the use of supports SiO2-Al2O3 results in an improvement in the performance of the catalysts in the HDS reaction of the 4,6-DMDBT molecule. Grafting SiO2 on the surface of γ-alumina generates two main effects. Increasing the extent of sulfidation and promotion, generating


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

TH, tetrahydrodimethylbenzothiophene; HH, hexahydrodimethyldibenzothiophene; MCHT, dimethylbicyclohexyl; and DMDBP, dimethylbiphenyl.

a Calculated with the 4,6-DMDBT conversion at about 30%.

b Number of the reacted 4,6-DMDBT molecules per second and per Mo atom at the edge surface.

c Determinated at about 50% of the total 4,6-DMDBT conversion by changing liquid hourly space velocity.

#### Table 2.

correlation between the degree of promotion with the hydrodesulfurization rate constant displayed in Figure 8, and agrees with the literature reports that indicate that promotion favors the appearance or brighter (more metallic) brim site, which can perform hydrogenating reactions [51], which is the main route for the HDS of

Relationship between global HDS rate constant vs. Ni or Co atoms involved in the Co(Ni)MoS phase.

On the other hand, Xu et al. [11] prepared a novel NiMo/SiO2-Al2O3 catalysts with the improved stacking and good dispersion of supported active phase via gemini surfactant-assisted synthesis. In this method, polymolybdates anions were transformed into gemini surfactant-linked Mo precursor (GSMP), dispersing Mo species well and weakening the strong Mo-support interaction. The GSMP-based NiMo/SiO2-Al2O3 (NiMo-GSHD) catalyst presents higher activity for the HDS of 4,6-DMDBT than its counterparts prepared via impregnation (NiMo-IM) and the cetyltrimethylammonium bromide-assisted hydrothermal method (NiMo-CTHD). To understand their different activities (Table 2), the HDS activities of the catalysts

The reason for the higher HDS activity of 4,6-DMDBT (Table 2) exhibited in the catalyst NiMo-GSHD is related to the greater MoS2 dispersion, a superior average stacking number determinated by HRTEM and the higher extent of promotion (NiMoS) calculated by XPS and by NO-IR characterization, thus generating more Ni-Mo-S active sites with sufficient brim sites (type II structures). Such that, the prehydrogenation activity of NiMi-GSHD for 4,6-DMDBT with steric hindrance is markedly improved. The prehydrogenated products (4,6-THDMDBT and 4,6- HHDMDBT) without steric hindrance are much easier to be desulfurization via hydrogenolysis on the edge sites of Ni-Mo-S phases than initial 4,6-DMDBT. Therefore, NiMo-GSHD, with more edge sites due to its better metal dispersion, possesses higher 3,3´-MCHT selectivity than NiMo-IM and NiMo-CTHD.

According to the results showed in this section, the use of supports SiO2-Al2O3 results in an improvement in the performance of the catalysts in the HDS reaction of the 4,6-DMDBT molecule. Grafting SiO2 on the surface of γ-alumina generates two main effects. Increasing the extent of sulfidation and promotion, generating

were correlated with the structure of their metal phase.

4,6-DMDBT.

16

Figure 8.

Silicon Materials

HDS results of 4,6-DMDBT on different catalysts [47].

structures Co(Ni)-Mo-S type II, unlike the catalysts supported in γ-alumina where the structures type I predominate, which are not well sulfided due to a strong metalsupport interaction. On the other hand, a different Mo precursor is used (gemini surfactant-linked) in conjunction with the use of a mixed Al2O3-SiO2 support with a composition of 96.4 and 3.6 wt%, respectively, resulting in the formation of sulfide molybdenum (MoS2) crystals with higher stacking, generating the so-called Ni-Mo-S type II, which are more active in HDS of 4,6-DMDBT molecule.
