**2. Experiment**

*Microemulsion - A Chemical Nanoreactor*

MoO4

helps to avoid the formation of undesired species such as cobalt or nickel aluminates or segregated sulfides [2]. Nevertheless, different preparation sequences or procedures have been investigated since this is not a simple task [3, 4]. Furthermore, Co and Mo form the so-called "CoMoS" phase which has been reported as the active phase in this reaction to remove sulfur from compounds such as dibenzothiophene (DBT) and 4, 6-dimethyl dibenzothiophene (4,6-DMDBT) [4, 5]. This means to synthesize sulfides of 3 slabs stacking (CoMoS type II) [4, 6, 7]. It has also been reported that Co(Ni) MoS species can be obtained, with the use of thiomolybdate precursors in the presence of nonionic surfactants. Moreover, the morphology can be controlled, resulting in an increased activity as compared with more conventional synthesis using ammonium polymolybdates [8]. To carry out the impregnation of the support, solutions of ammonium heptamolybdate tetrahydrate and nickel(II) nitrate hexahydrate are generally used. Firstly, monomeric species such as

<sup>2</sup><sup>−</sup> are deposited on the carrier. These ions are obtained at pH values between 10 and 12 and MoO3 species are formed after calcination [9, 10]. After impregnation of molybdenum, Co2+ species are impregnated at pH between 2 and 5.96 [11, 12] and CoOx species are obtained after calcination [12]. Alternative synthesis methods have been tested. For instance, spray pyrolysis allowed the formation of nanosized spherical particles of CoMo sulfides supported on Al2O3 that increased the activity

A method capable of obtaining nanoparticles with interesting applications as heterogeneous catalysts is inverse microemulsion [13]. The water/oil microemulsion (reverse microemulsion) uses surfactant molecules to stabilize the water/oil interface of nanosized water droplets that are dispersed in an organic solvent. These water droplets consist of an aqueous solution of metal precursors [13, 14]. Recently, reverse microemulsion has been used to prepare NiMo catalysts by precipitation; after calcination and sulfidation, nickel was found decorating the edges of MoS2. However, after HDS reaction, a significant amount of nickel was segregated, provoking a low activity in comparison with a NiMo/γ-Al2O3 catalyst taken as reference [15]. Reverse microemulsion is attractive to extend the actual studies to obtain CoMo/γ-Al2O3 catalysts with active species as required for HDS, preparing

In the microemulsion systems, the interaction between ions in solution and the interface where the surfactant and the organic agent coexist is an important issue. 13C NMR studies have shown that Co (II) is retained at the CTAB-hexanol-water interface, with a 1:1 interaction between Co(II) and hexanol [16]. Also, at low concentrations, octahedral Co(II) complexes are formed. On the other hand, in the preparation of alumina, sodium dodecyl sulfate (SDS, anionic) and cetyltrimethylammonium bromide (CTAB, cationic) have been used as mixtures in different proportions. It was found that the SDS head remains in the alumina network with a decrease in its surface area [17]. Some reports describe the synthesis of unsupported Co(Ni)-Mo-S catalyts for HDS reactions, using surfactants and chelating agents as textural promoters, and the materials obtained show bigger surface areas and a

The effect of the surfactant on the preparation of CoMo/γ-Al2O3 by the microemulsion method was reported [18]. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were used and compared. Microemulsions synthesized with SDS provided larger size nanodrops than those obtained with CTAB, with a lower amount of surfactant added. After calcination, the solids prepared with SDS showed the presence of sodium sulfate and had surface areas

In this work, the preparation of catalysts by a reverse microemulsion method has been undertaken to provide nanostructured-supported metals used in HDS. A

of the catalysts, due to weak interaction of CoMo and alumina [2].

structured nanoparticles in only one step.

higher catalytic activity than commercial catalysts [8].

50% lower than those obtained with CTAB.

**94**

### **2.1 Preparation of reverse microemulsions**

Five samples were prepared using 1-butanol, water and cetyltrimethylammonium bromide (CTAB), according to the mass percentages shown in **Table 1**. All chemicals were reagent grade from Sigma Aldrich, otherwise it is indicated. **Figure 1** shows a diagram for material synthesis. For each catalyst, the organic phase was 1-butanol (C4H9OH), and the aqueous phase was composed of cobalt nitrate (Co(NO3)2 6H2O), ammonium heptamolybdate (AHM, (NH4)6Mo7O24 4H2O) and aluminum nitrate Al(NO3)3 9H2O (J. T. Baker). For each microemulsion, the required amount of CTAB (CH3(CH2)15NBr (CH3)3) cationic surfactant was added. During the microemulsion formation, the mixture was continuously stirred with a magnetic stirrer, the CTAB surfactant was added slowly, until it turned from turbid to translucent. The electric conductivity was measured continuously with a LabPro Vernier (model CON-BTA) coupled to a conductivity probe with a sensitivity of ±0.001 S/m. Measurements were conducted at a constant temperature of 20 ± 0.1°C. Critical micelle concentration (cmc) was established for all the samples. This occurred at a point where the conductivity showed a sharp inflection point. Each microemulsion was adjusted to pH = 10 with n-butylamine to generate the required species in solution. Their molar concentrations are shown in the last three columns of **Table 1**. The use of pH 10 in the microemulsions was determined with the development of species distribution diagrams obtained through the Medusa (Make Equilibrium Diagrams Using Sophisticated Algorithms) program that generates distribution curves for species in solution, depicted as the logarithm of the concentration versus pH.


#### **Table 1.**

*Nominal compositions for the synthesis of microemulsions.*

**Figure 1.**

*Diagram for the synthesis method used for preparing the reverse microemulsion.*

Subsequent to the formation of the microemulsions, the wet solids were maintained at room temperature for 48 h to evaporate the solvents. The obtained solids were calcined at 500°C for 6 h, with an air flow of 20 ml/min.

#### **2.2 Characterization**

The concentrations of the metallic elements in calcined samples were determined in a Varian SpectrAA 220 FS Atomic Absorption Spectrometer equipment. The spectrophotometer was calibrated with certified standards.

The nitrogen physisorption analysis for calcined catalysts was developed in an Autosorb 1 gas sorption system (Quantachrome). Samples were outgassed at 477 K under vacuum for 6 h. Then, nitrogen physisorption experiments were carried out at 77 K. The determination of surface area and volume and pore diameter were carried out using the BET equation and the BJH method, respectively.

To determine the coordination number and crystallographic arrangements of the synthesized support (alumina), a magic angle spinning-nuclear magnetic resonance equipment (MAS-NMR) Bruker Avance II 300, equipped with a multinuclear detector of 4-mm CPMAS with a frequency range 31P to 15N was utilized. Analyses were carried out with a rotational speed of 10 kHz.

The crystalline phases of the calcined samples were identified using a Siemens D-500 Kristalloflex diffractometer, with a CuKα radiation, λ = 0.15406 nm, and with primary and secondary monochromators. The equipment was operated at 35 kV, 20 mA, with a time interval of 1 s and scan rate of 0.03°/s.

Raman spectra for the calcined samples were recorded using Raman HORIBA Jobin Yvon T64000 equipment. The excitation laser source wavelength was 532.1 nm, with a power of 20 mW at the laser head; 100 scans of each sample were performed and accumulated at room temperature, and the spectra were recorded in the range 100–1300 cm<sup>−</sup><sup>1</sup> .

**97**

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

**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

C4H9NH2), 3 solid species (Al(OH)3(cr), Co(OH)2(c), H2MoO4(c)), and the other

Seven Mo species were identified in **Figure 2b**. Six of them were observed

reaching maximum concentration at pH higher than 6. Furthermore, this was

been reported that they exist primarily at low concentrations and at pH = 10 [11].

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.

Several authors have reported that Al(OH)3 in solution was obtained at pH

**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

**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),

<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

<sup>−</sup> species began to appear at pH = 11 and its concentration reached a

the only remaining species at pH higher than 8. Regarding MoO4

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

**Figure 2a** depicts the distribution of two Co species. One Co2+ species appeared

, Co2+, MoO4

<sup>6</sup><sup>−</sup> species were found at pH values

<sup>2</sup><sup>−</sup> species began to be noticeable at pH values around 4,

<sup>2</sup>−, Al3+,

<sup>2</sup><sup>−</sup> species, it has

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

**3.1 Determination of species in solution**

species were Co, Mo or Al complexes.

compounds exhibited also high stability.

between 4 and 6. MoO4

[MoO4]

[21–23].

The Al(OH)4

maximum value at pH = 13.7.

values between 8 and 11 [17, 24].

species in HDS catalysts [9, 19, 20, 25].

**3.2 Atomic absorption**

taken as reference.

at acidic pH, among them HMoO4<sup>−</sup> y Mo7O24

reactions were considered, including 5 soluble compounds (H<sup>+</sup>

**3. Results and discussion**
