**1. Introduction**

Regarding sulfur content in diesel fuels, more stringent environmental regulations have motivated research on new catalysts and novel synthesis methods, to produce highly active hydrodesulfurization (HDS) catalysts [1]. The commercial HDS catalysts are based on MoS2 promoted by Co or Ni, supported on high-surface area γ-alumina. The most common synthesis procedure involves impregnation of aqueous solutions of Mo and Co (or Ni), followed by drying and calcination steps prior to the activation by a sulfur-containing agent. Generally, it is found that Mo could form a monolayer to prevent Ni or Co species to interact with the support. This

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 MoO4 <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 of the catalysts, due to weak interaction of CoMo and alumina [2].

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 structured nanoparticles in only one step.

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 higher catalytic activity than commercial catalysts [8].

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 50% lower than those obtained with CTAB.

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

**95**

**Table 1.**

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

series of CoMo/γ-Al2O3 catalysts were synthesized and characterized, and the influence of concentration of metals on the calcined solid materials was evaluated. In order to determine the solution conditions to obtain Co and Mo on Al2O3, chemical species distribution calculations were made. These calculations allowed us to establish pH and concentrations to employ for the microemulsion preparation and the proper chemical species to be obtained in the solid materials. In a first attempt to correlate the catalyst synthesis parameters with the resultant structural and textural characteristics, characterization of the oxidic phases by atomic absorption, N2 physisorption, X-ray diffraction (XRD), magic angle spinningnuclear magnetic resonance (MAS-NMR) and Raman spectroscopy is presented

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

**Microemulsions Species concentration**

**Co(II) (mM)**

**Mo(VI) (mM)**

**Al(III) (mM)**

**CTAB (%)**

C1 31.7 47.2 21.1 8.05 20.95 197.61 C2 31.9 47.4 20.7 8.50 22.14 162.43 C3 32.2 47.9 19.9 9.21 23.96 125.52 C4 32.6 48.4 19.0 9.65 25.11 78.95 C5 33.0 49.0 18.0 10.03 26.09 27.34

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

**2.1 Preparation of reverse microemulsions**

and discussed.

**2. Experiment**

concentration versus pH.

**Key Aqueous phase** 

**(%)**

*Nominal compositions for the synthesis of microemulsions.*

**1-Butanol (%)**

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

series of CoMo/γ-Al2O3 catalysts were synthesized and characterized, and the influence of concentration of metals on the calcined solid materials was evaluated. In order to determine the solution conditions to obtain Co and Mo on Al2O3, chemical species distribution calculations were made. These calculations allowed us to establish pH and concentrations to employ for the microemulsion preparation and the proper chemical species to be obtained in the solid materials. In a first attempt to correlate the catalyst synthesis parameters with the resultant structural and textural characteristics, characterization of the oxidic phases by atomic absorption, N2 physisorption, X-ray diffraction (XRD), magic angle spinningnuclear magnetic resonance (MAS-NMR) and Raman spectroscopy is presented and discussed.
