**3.5 X-ray diffraction**

**Figures 5** and **6** show the diffractograms for the calcined catalysts. As it can be seen, no XRD lines were observed for almost all catalysts, except for the C5 solid, pointing out to amorphous solids. Thus, cobalt and molybdenum oxides may be highly dispersed on the support (γ-Al2O3). This has also been observed by other authors who detected a broad band between 5 and 50° (2Ө) describing amorphous catalysts. **Figure 6** (extended from **Figure 5**) shows the diffractogram for the C5 catalyst. Well-defined XRD lines were detected and they were associated with CoMoO4 and Al2(MoO4)3 [2]. This finding is consistent with MAS-RMN results for the highly loaded CoMo catalyst (C5).


**101**

**Figure 4.**

**Figure 3.**

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

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

*27Al MAS-NMR spectrum for the C3 CoMo/γ-Al2O3 uncalcined catalyst.*

*27Al MAS-NMR spectra for the CoMo/γ-Al2O3 calcined catalysts series.*

#### **Table 4.**

*Nitrogen physisorption results for the calcined catalysts.*

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

**Figure 3.** *27Al MAS-NMR spectrum for the C3 CoMo/γ-Al2O3 uncalcined catalyst.*

**Figure 4.** *27Al MAS-NMR spectra for the CoMo/γ-Al2O3 calcined catalysts series.*

*Microemulsion - A Chemical Nanoreactor*

methods [3, 32, 33].

[Al(OH)6Mo6O18]

**3.5 X-ray diffraction**

temperature reached 500°C [36].

**3.4 Spinning-nuclear magnetic resonance equipment (MAS-NMR)**

**Figure 3** shows the 27Al MAS-NMR spectrum for the C3 catalyst before calcination. One can observe a broad band with a maximum at around 0 ppm, that can be ascribed to the formation of the Al(OH)3 (gibbsite) compound. This species has low electronegativity and, therefore, it can interact more easily with more electronegative elements [31], such as molybdenum species, rather than with cobalt species. This is relevant for the preparation method, considering that the latter has a tendency to form cobalt aluminate with the support when using impregnation

**Figure 4** displays the 27Al MAS-NMR spectra for the calcined catalysts. The CoMoind sample exhibited a broad band at 0 ppm that can be assigned to octahedral species of the support. Besides, formation of tetrahedral species was determined as a weak band at around 60 ppm. These two bands are characteristic of γ-Al2O3 [34, 35]. It is likely that during calcination up to 250°C, the gibbsite formed boehmite (α-AlO(OH)). This compound was transformed to γ-Al2O3, when the

Additionally, **Figure 4** shows that pentahedral species between 40 and 50 ppm appeared for the C5 and CoMoind catalysts. These species have been identified as defects in the support structure, as originated by the replacement of oxygen in the network of octahedral symmetry by hydroxyl groups [37]. For C5 sample, a band around −14 ppm was detected. This band has been related to the presence of the Al2(MoO4)3 species which distorts the octahedral network of the support [35].

Some authors have reported that the Al2(MoO4)3 species can be due to the dissolution of the Al3+ species, which subsequently react with heptamolybdate complexes during the impregnation step, forming the Anderson-type heteropolymolybdate

(around −15 ppm) could be due to the formation of an Anderson-type heteropoly-

**Figures 5** and **6** show the diffractograms for the calcined catalysts. As it can be seen, no XRD lines were observed for almost all catalysts, except for the C5 solid, pointing out to amorphous solids. Thus, cobalt and molybdenum oxides may be highly dispersed on the support (γ-Al2O3). This has also been observed by other authors who detected a broad band between 5 and 50° (2Ө) describing amorphous catalysts. **Figure 6** (extended from **Figure 5**) shows the diffractogram for the C5 catalyst. Well-defined XRD lines were detected and they were associated with CoMoO4 and Al2(MoO4)3 [2]. This finding is consistent with MAS-RMN results for

<sup>3</sup><sup>−</sup> [35]. In this study, the appearance of the molybdate species

**100**

**Table 4.**

**Catalysts Tc (500°C) SBET (m2**

the highly loaded CoMo catalyst (C5).

molybdate obtained in the synthesis mixture.

*Nitrogen physisorption results for the calcined catalysts.*

**/g) Pore volume (cm3**

C1 294 0.74 55.8 C2 286 0.62 56.2 C3 261 0.41 68.3 C4 212 0.28 71.2 C5 125 0.19 76.2 CoMoind 204 0.49 56.6

**/g) Dp (Ǻ)**

#### **3.6 Raman analysis**

Raman spectra for the CoMo/γ-Al2O3 catalysts series and the CoMoind catalyst are given in **Figure 7**. For C1, C2 and C3 CoMo catalysts, more attenuated and wider bands were observed, as compared with C4 and C5 spectra. It could be due to more microcrystalline particles in low-content Mo samples. Thus, particle sizes for highly loaded catalysts C4 and C5 were larger, as expected.

**Table 5** summarizes all the Raman bands indicating the type of species as assigned for the catalysts in oxidic state. To assign the species present in the samples, the signals were deconvoluted and identified according to those reported in the literature.

One can notice bands between 500 and 700 cm<sup>−</sup><sup>1</sup> for C1 and C2 catalysts. These peaks can be assigned to the stretching mode vibration for bridged Mo-O-M links [38]. Specifically, the presence of Mo-O-Co bonds between 540 and 560 cm<sup>−</sup><sup>1</sup> was identified. This type of band corresponds to the interval of heteropolymolybdate structures and they indicate a strong interaction between cobalt and molybdenum oxides. This means that a weak interaction with the support occurs and, thus, it can induce a high degree of sulfidation of the catalyst as published by others [2, 39]. Besides, bands at 817–818 cm<sup>−</sup><sup>1</sup> were observed for samples with Mo loadings >20% in catalysts C4 and C5. These peaks are generated by MoO3 species, indicating that a monolayer of MoO3 on the surface of the support has been exceeded [40, 41]. Moreover, these MoOx species have been identified as orthorhombic molybdate species [42, 41]. Bands between 850 and 875 cm<sup>−</sup><sup>1</sup> were attributed to Mo-O-Mo bonds, assigned to the asymmetric vibrational stretching mode [2, 41, 43]. Other bands located between 930 and 960 cm<sup>−</sup><sup>1</sup> were assigned to Mo=O links, as the vibrational stretching mode for the dioxo groups in oxomolybdate species. This indicates the formation of MoO2t species, where t indicates terminal oxygen atoms. This type of species was present in all studied catalysts, as tetrahedral MoOx structures on alumina [40].

Furthermore, it was found that the bands corresponding to the Mo-O-Mo links increased proportionally when increasing the bands corresponding to the MoO2t links [2, 41, 43]. This suggests that there was an increase in the number of bridges of Mo, which implied high dispersion of Mo on the support [40, 44].

**103**

**Figure 7.**

*Raman spectra for the CoMo/γ-Al2O3 calcined catalysts series.*

**Figure 6.**

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

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

*XRD results for the CoMo/γ-Al2O3 calcined C5 catalyst.*

**Figure 5.** *XRD results for the CoMo/γ-Al2O3 calcined catalysts series.*

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

**Figure 6.** *XRD results for the CoMo/γ-Al2O3 calcined C5 catalyst.*

*Microemulsion - A Chemical Nanoreactor*

loaded catalysts C4 and C5 were larger, as expected.

One can notice bands between 500 and 700 cm<sup>−</sup><sup>1</sup>

as published by others [2, 39]. Besides, bands at 817–818 cm<sup>−</sup><sup>1</sup>

Raman spectra for the CoMo/γ-Al2O3 catalysts series and the CoMoind catalyst are given in **Figure 7**. For C1, C2 and C3 CoMo catalysts, more attenuated and wider bands were observed, as compared with C4 and C5 spectra. It could be due to more microcrystalline particles in low-content Mo samples. Thus, particle sizes for highly

**Table 5** summarizes all the Raman bands indicating the type of species as assigned for the catalysts in oxidic state. To assign the species present in the samples, the signals were deconvoluted and identified according to those reported

These peaks can be assigned to the stretching mode vibration for bridged Mo-O-M links [38]. Specifically, the presence of Mo-O-Co bonds between 540

heteropolymolybdate structures and they indicate a strong interaction between cobalt and molybdenum oxides. This means that a weak interaction with the support occurs and, thus, it can induce a high degree of sulfidation of the catalyst

for samples with Mo loadings >20% in catalysts C4 and C5. These peaks are generated by MoO3 species, indicating that a monolayer of MoO3 on the surface of the support has been exceeded [40, 41]. Moreover, these MoOx species have been identified as orthorhombic molybdate species [42, 41]. Bands between 850

vibrational stretching mode [2, 41, 43]. Other bands located between 930 and

in all studied catalysts, as tetrahedral MoOx structures on alumina [40].

of Mo, which implied high dispersion of Mo on the support [40, 44].

was identified. This type of band corresponds to the interval of

were attributed to Mo-O-Mo bonds, assigned to the asymmetric

 were assigned to Mo=O links, as the vibrational stretching mode for the dioxo groups in oxomolybdate species. This indicates the formation of MoO2t species, where t indicates terminal oxygen atoms. This type of species was present

Furthermore, it was found that the bands corresponding to the Mo-O-Mo links increased proportionally when increasing the bands corresponding to the MoO2t links [2, 41, 43]. This suggests that there was an increase in the number of bridges

for C1 and C2 catalysts.

were observed

**3.6 Raman analysis**

in the literature.

and 560 cm<sup>−</sup><sup>1</sup>

and 875 cm<sup>−</sup><sup>1</sup>

960 cm<sup>−</sup><sup>1</sup>

**102**

**Figure 5.**

*XRD results for the CoMo/γ-Al2O3 calcined catalysts series.*

**Figure 7.** *Raman spectra for the CoMo/γ-Al2O3 calcined catalysts series.*


#### **Table 5.**

*Raman bands and the species associated for the calcined CoMo/γ-Al2O3 catalysts.*

Additionally, bands between 905 and 918 cm<sup>−</sup><sup>1</sup> for the C5 and CoMoind catalysts were attributed to the presence of molybdate CoMoO4 species, either isolated or polymerized, involving a strong interaction with the support. As reported by some authors [33, 42, 45, 46], the formation of aluminum molybdate species occurs at high Mo loadings. It has been published also that some amount of Mo reacts with Co during calcination.

For C1 and C2 catalysts, a band in the range between 970 and 1100 cm<sup>−</sup><sup>1</sup> was detected and highly dispersed MoO4 tetrahedral species can be assigned in agreement with MAS-NMR results. It is likely that these metal loadings did not reach the MoOx monolayer formation [44, 47]. Moreover, the bands appearing between 200 and 400 cm<sup>−</sup><sup>1</sup> were attributed to Mo monomeric species [33].

Regarding Co species, no bands associated with this oxide were identified. However, since no segregation of Co species was noticed, one can propose that there exists an interaction of Mo and Co.

#### **3.7 A model for the interaction of species in solution and the micelle**

The results obtained in this work for the characterization of the catalytic materials demonstrate that these have: high surface areas, amorphous γ-Al2O3, a MoO3 type species on the surface of the support and a constant Co/Mo ratio independent of the metal loading. Overall, it is possible to establish an interaction between Co species at the interface with the micelles, preventing the migration of Co into the alumina network. There is also an interaction between Mo and AlOOH in solution which hindered the Co-Al interaction and promoted the formation fo Mo-O-Co, as shown by Raman results. It is possible to depict a representative scheme of the species inside the micelles as can be observed in **Figure 8**. In this diagram, 1-butanol replaces water in the first coordination sphere of Co ion, so that Co is retained at the interface of the micelles [16].

**105**

**Acknowledgements**

**4. Conclusions**

*Mo species, Co species located at the interface.*

**Figure 8.**

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

CoMo/γ-Al2O3 catalysts were synthesized at several metal (Co + Mo) contents employing a reverse microemulsion method. The microemulsion was formed using 1-butanol as organic agent, cetyltrimethylammonium bromide as surfactant and water. A study of chemical species distribution in solution as a function of pH was performed and provided the pH and precursor salt concentrations to be used in the synthesis to obtain the desired final material. The study allowed to obtain stable micelles, no precipitation of the metallic particles outside the micelles occurred and a constant Co/Mo ratio in all samples independent of the metal loading was observed. The solids calcined at 500°C showed large surface areas which decrease as the metal content was increased. All the calcined samples were amorphous for X-ray diffraction and only at the highest Co + Mo concentration some crystalline phases were found. On these samples, species such as Mo-O-Mo, MoO2t, Mo-O-Co, Al-O-Mo were detected. These species are considered precursors of highly active catalytic sites in HDS reactions. Based on these results, a schematic model for the micelle formed was produced. In this model, Al and Mo species in solution interact whereas Co species interact with 1-butanol at the interface. With this model, it is possible to envisage the formation of the solid material with Mo covering the surface of Al2O3 and Co interacting with Mo on the surface of the aluminum oxide.

*Schematic diagram for the interaction between species in solution within the micelle. Al species interacting with* 

The authors would like to thank CONACYT-Mexico and UAM for the financial

support. J. L. Munguía thanks CONACYT-Mexico for the scholarship granted.

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

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

#### **Figure 8.**

*Microemulsion - A Chemical Nanoreactor*

**Range (cm<sup>−</sup><sup>1</sup> )**

300– 400

800– 900

900– 1000

1000– 1100

**Table 5.**

Additionally, bands between 905 and 918 cm<sup>−</sup><sup>1</sup>

320 320 360 335–

850 850 870 818

950

*Raman bands and the species associated for the calcined CoMo/γ-Al2O3 catalysts.*

937 930 930,

during calcination.

and 400 cm<sup>−</sup><sup>1</sup>

exists an interaction of Mo and Co.

interface of the micelles [16].

were attributed to the presence of molybdate CoMoO4 species, either isolated or polymerized, involving a strong interaction with the support. As reported by some authors [33, 42, 45, 46], the formation of aluminum molybdate species occurs at high Mo loadings. It has been published also that some amount of Mo reacts with Co

**C1 C2 C3 C4 C5 CoMoind Assigned species**

346– 369

> 817 875

1085 1086 1090 1045 (MoO4)

200–300 215 Heptamolybdates or

365

870

939, 950 978

500–700 570 570 560 Mo-O-Co

For C1 and C2 catalysts, a band in the range between 970 and 1100 cm<sup>−</sup><sup>1</sup>

 were attributed to Mo monomeric species [33]. Regarding Co species, no bands associated with this oxide were identified. However, since no segregation of Co species was noticed, one can propose that there

**3.7 A model for the interaction of species in solution and the micelle**

detected and highly dispersed MoO4 tetrahedral species can be assigned in agreement with MAS-NMR results. It is likely that these metal loadings did not reach the MoOx monolayer formation [44, 47]. Moreover, the bands appearing between 200

The results obtained in this work for the characterization of the catalytic materi-

als demonstrate that these have: high surface areas, amorphous γ-Al2O3, a MoO3 type species on the surface of the support and a constant Co/Mo ratio independent of the metal loading. Overall, it is possible to establish an interaction between Co species at the interface with the micelles, preventing the migration of Co into the alumina network. There is also an interaction between Mo and AlOOH in solution which hindered the Co-Al interaction and promoted the formation fo Mo-O-Co, as shown by Raman results. It is possible to depict a representative scheme of the species inside the micelles as can be observed in **Figure 8**. In this diagram, 1-butanol replaces water in the first coordination sphere of Co ion, so that Co is retained at the

for the C5 and CoMoind catalysts

was

octamolybdates

Heptamolybdates or octamolybdates

Al-O-Mo

Mo-O-Mo

CoMoO4 \*MoO2t Al2(MoO4)3

2−

354 Monomers

850 MoO3

918 951

**104**

*Schematic diagram for the interaction between species in solution within the micelle. Al species interacting with Mo species, Co species located at the interface.*

### **4. Conclusions**

CoMo/γ-Al2O3 catalysts were synthesized at several metal (Co + Mo) contents employing a reverse microemulsion method. The microemulsion was formed using 1-butanol as organic agent, cetyltrimethylammonium bromide as surfactant and water. A study of chemical species distribution in solution as a function of pH was performed and provided the pH and precursor salt concentrations to be used in the synthesis to obtain the desired final material. The study allowed to obtain stable micelles, no precipitation of the metallic particles outside the micelles occurred and a constant Co/Mo ratio in all samples independent of the metal loading was observed. The solids calcined at 500°C showed large surface areas which decrease as the metal content was increased. All the calcined samples were amorphous for X-ray diffraction and only at the highest Co + Mo concentration some crystalline phases were found. On these samples, species such as Mo-O-Mo, MoO2t, Mo-O-Co, Al-O-Mo were detected. These species are considered precursors of highly active catalytic sites in HDS reactions. Based on these results, a schematic model for the micelle formed was produced. In this model, Al and Mo species in solution interact whereas Co species interact with 1-butanol at the interface. With this model, it is possible to envisage the formation of the solid material with Mo covering the surface of Al2O3 and Co interacting with Mo on the surface of the aluminum oxide.

#### **Acknowledgements**

The authors would like to thank CONACYT-Mexico and UAM for the financial support. J. L. Munguía thanks CONACYT-Mexico for the scholarship granted.

*Microemulsion - A Chemical Nanoreactor*
