**3.1 Determination of species in solution**

**Figure 2** shows the theoretical diagrams for Co, Mo and Al species as determined from the MEDUSA program for the C3 CoMo/Al2O3 sample. A total of 28 possible reactions were considered, including 5 soluble compounds (H<sup>+</sup> , Co2+, MoO4 <sup>2</sup>−, Al3+, C4H9NH2), 3 solid species (Al(OH)3(cr), Co(OH)2(c), H2MoO4(c)), and the other species were Co, Mo or Al complexes.

**Figure 2a** depicts the distribution of two Co species. One Co2+ species appeared at acidic pH and it decreased after a pH value higher than 7. At this value, the formation of Co(OH)2 began to increase, reaching maximum concentration at pH higher than 8.5. Some authors have reported cobalt species in solution pH values higher than 6 [19, 20], in agreement with theoretical calculations. Moreover, these compounds exhibited also high stability.

Seven Mo species were identified in **Figure 2b**. Six of them were observed at acidic pH, among them HMoO4<sup>−</sup> y Mo7O24 <sup>6</sup><sup>−</sup> species were found at pH values between 4 and 6. MoO4 <sup>2</sup><sup>−</sup> species began to be noticeable at pH values around 4, reaching maximum concentration at pH higher than 6. Furthermore, this was the only remaining species at pH higher than 8. Regarding MoO4 <sup>2</sup><sup>−</sup> species, it has been reported that they exist primarily at low concentrations and at pH = 10 [11]. [MoO4] <sup>2</sup><sup>−</sup> has also been determined as the predominant oxo-molybdenum (VI) species in the catalyst MoO3/Al2O3, in which case this species is found in solution and adsorbed on the alumina surface depending on the concentration of Mo [21–23].

Regarding Al, **Figure 2c** shows three species. Al3+ ions began to disappear at pH values near 2.8 and Al(OH)3 concentration increased. This crystalline species reached its maximum concentration at pH = 4.3 and it decreased at pH = 10.5. The Al(OH)4 <sup>−</sup> species began to appear at pH = 11 and its concentration reached a maximum value at pH = 13.7.

Several authors have reported that Al(OH)3 in solution was obtained at pH values between 8 and 11 [17, 24].

**Table 2** gives those reactions favoring the most common complexes involved in CoMo catalysts synthesis. These were formed at pH 10. The results from reactions in **Table 2** were considered to establish concentration and pH ranges in which the formation of suitable species within the nanodroplets of the microemulsion may occur. Moreover, these values were taken into account to avoid the release and precipitation of solid particles. This can be observed when an excessive growth of particles within of the micelles takes place. For catalysts with different metal loading (C1, C2 C4 and C5 samples), analogous diagrams were obtained, showing the same species distribution.

Overall, the selected species to be obtained at the conditions fixed experimentally have been those needed for the formation of the precursors of the active species in HDS catalysts [9, 19, 20, 25].

#### **3.2 Atomic absorption**

**Table 3** gives the results for metallic loadings as determined by atomic absorption for catalysts after calcination. According to **Table 3**, it can be observed that the Co/Mo ratio was around 0.236 for all the samples. Therefore, it was shown that the method of reverse microemulsion allowed us to obtain a Co/Mo ratio with the same average value for all samples, independently of the metal content. Furthermore, the ratio Co/Mo is comparable to metal contents in an industrial catalyst (CoMoind), taken as reference.

**Figure 2.**

*Species distribution diagrams for the synthesis for CoMo/γ-Al2O3 catalysts (C3): (a) cobalt species, (b) molybdenum species (c), aluminum species.*

**99**

**Table 3.**

*CoMo/γ-Al2O3 Catalysts Prepared by Reverse Microemulsion: Synthesis and Characterization*

**Ions Formed species log K**

+ Co(OH)2 (c) −18.6

+ Al(OH)3(cr) −8.11

<sup>2</sup><sup>−</sup> −52.99

+ 7MoO4

CoMo/γ-Al2O3 catalysts have been synthesized by several methods and previous papers [4] agree that the content of metals on the support, including an average ratio of Co/Mo of 0.24, is adequate to prepare active HDS catalysts. However, it has also been reported that metal loading on the support depends on the method of preparation [4]. In some cases, it has been determined that the number of slabs on the support (alumina) increases and the edges on the slabs decrease with increasing Mo content. These edges are generally decorated by deposition of cobalt species as promoter [7]. Therefore, the metal loading must be controlled to an extent, that is, 15 or 20 wt.% of Mo, since high loadings lead to the formation of inactive structures [4]. Furthermore, some studies have been aimed at obtaining a monolayer of molybdenum species on the surface of the carrier and Co/(Co + Mo) ratios corresponding to high dispersion of Co on the edges of the slabs of molybdenum [25, 26]. In our case, the results indicate that precipitation of the particles out of the micelles of the microemulsion systems did not occur during catalyst preparation, since it would have resulted in a heterogeneous distribution of the catalyst composition.

Results for nitrogen physisorption for the catalysts in this work and for an industrial catalyst are given in **Table 4**. Catalysts C1, C2 and C3 exhibited higher surface areas than the industrial catalyst (CoMoind). Catalyst C4 showed a comparable surface area as that of the CoMoind sample, while the C5 catalyst exhibited a significantly lower surface area. Furthermore, pore diameter values increased from catalyst C1 to C5, showing a comparable value between C2 and CoMoind. One of the fundamental aspects of the analysis of physisorption that has been suggested for HDS supports is that their surface area must be high enough to ensure a high

Thus, we observed high surface areas for the synthesized catalysts by using microemulsions. It is possible that nanosized particles were formed inside the micelles systems as reported in the literature [27–30]. However, high Mo loading such as those in C4 and C5 catalysts could lead to different porous structures and

**Key Co (wt.%) Mo (wt.%) Co/Mo** C1 3.7 15.3 0.236 C2 4.6 18.6 0.234 C3 5.6 24.0 0.238 C4 8.0 33.1 0.238 C5 13.2 54.6 0.235 CoMoind 5.7 24.5 0.233

*DOI: http://dx.doi.org/10.5772/intechopen.82586*

*Hydrolysis reactions for Al, Co and Mo species.*

2H2O + Co2+ = 2H+

3H2O + Al3+ = 3H+

4H2O + Mo7O24<sup>6</sup><sup>−</sup> = 8H+

**3.3 Nitrogen physisorption**

**Table 2.**

dispersion of the active species [2].

one cannot rule out pore blocking by the metals.

*Metal content for the catalysts as determined by atomic absorption.*

*CoMo/γ-Al2O3 Catalysts Prepared by Reverse Microemulsion: Synthesis and Characterization DOI: http://dx.doi.org/10.5772/intechopen.82586*


#### **Table 2.**

*Microemulsion - A Chemical Nanoreactor*

**98**

**Figure 2.**

*molybdenum species (c), aluminum species.*

*Species distribution diagrams for the synthesis for CoMo/γ-Al2O3 catalysts (C3): (a) cobalt species, (b)* 

*Hydrolysis reactions for Al, Co and Mo species.*

CoMo/γ-Al2O3 catalysts have been synthesized by several methods and previous papers [4] agree that the content of metals on the support, including an average ratio of Co/Mo of 0.24, is adequate to prepare active HDS catalysts. However, it has also been reported that metal loading on the support depends on the method of preparation [4]. In some cases, it has been determined that the number of slabs on the support (alumina) increases and the edges on the slabs decrease with increasing Mo content. These edges are generally decorated by deposition of cobalt species as promoter [7]. Therefore, the metal loading must be controlled to an extent, that is, 15 or 20 wt.% of Mo, since high loadings lead to the formation of inactive structures [4]. Furthermore, some studies have been aimed at obtaining a monolayer of molybdenum species on the surface of the carrier and Co/(Co + Mo) ratios corresponding to high dispersion of Co on the edges of the slabs of molybdenum [25, 26]. In our case, the results indicate that precipitation of the particles out of the micelles of the microemulsion systems did not occur during catalyst preparation, since it would have resulted in a heterogeneous distribution of the catalyst composition.

#### **3.3 Nitrogen physisorption**

Results for nitrogen physisorption for the catalysts in this work and for an industrial catalyst are given in **Table 4**. Catalysts C1, C2 and C3 exhibited higher surface areas than the industrial catalyst (CoMoind). Catalyst C4 showed a comparable surface area as that of the CoMoind sample, while the C5 catalyst exhibited a significantly lower surface area. Furthermore, pore diameter values increased from catalyst C1 to C5, showing a comparable value between C2 and CoMoind. One of the fundamental aspects of the analysis of physisorption that has been suggested for HDS supports is that their surface area must be high enough to ensure a high dispersion of the active species [2].

Thus, we observed high surface areas for the synthesized catalysts by using microemulsions. It is possible that nanosized particles were formed inside the micelles systems as reported in the literature [27–30]. However, high Mo loading such as those in C4 and C5 catalysts could lead to different porous structures and one cannot rule out pore blocking by the metals.


**Table 3.**

*Metal content for the catalysts as determined by atomic absorption.*
