Synthesis of the silatrane-alumatrane complex: MCF precursor

Before the synthesis of mesoporous supports MCF, the atrane complex of silicon and aluminum was prepared according to S. Cabrera et al. ⁹. Initially, triethanolamine (TEAH₃) was heated to 140°C, then, tetraethyl orthosilicate (TEOS) and Aluminium isopropoxide (AIP) were added. After that, it was heated to 140°C, keeping this temperature for 30 minutes.

MCF with 10% of structural aluminum

The synthesis method is similar to SBA-15Al reported by Pardo-Tarifa et al. and Li et al. ^{1), (5} with the following modifications: The amount of ammonium fluoride is 2.67g and the amount of 1,3,5-trimethylbenzene (TMB) is 16.8 ml. The molar composition of the reagent mixture is: SiO₂: Al₂O₃: TEAH₃: P-123: TMB: NH₄F: HCl: H₂O equal to 0.89: 0.054: 3.44: 0.019: 4.38: 0.87: 5.76: 178.90. The support thus obtained was named as MCF10Al.

Synthesis of MCF with different content of structural aluminum: 5% Al, 10% Al and 15% Al

For the MCF-type silicoaluminate series with different proportions of Al₂O₃, the synthesis was performed in a similar manner as the MCF synthesis mentioned above. The molar composition is the same as in the MCF10Al support, however, the molar ratio Si: Al for the MCF5Al support is 0.94: 0.030 and for the MCF15 support is 0.83: 0.087.

Cobalt impregnation on mesoporous supports

For all the synthesized mesoporous silatrane-alumatrane supports, cobalt was added by the incipient wetness impregnation method ^{5), (11}. A nominal weight percentage of 12% Co was added to the supports from an aqueous solution of cobalt nitrate. The impregnated supports were dried at 70°C overnight, then, they were calcined as follow: till 120°C for 4 hours, heated up till 220°C for 4 hours, and finally heated up till 350°C for 6 hours. All the mentioned steps were done at a rate of 2°C/min. Thus, the catalysts obtained were named as Co/MCF5Al, Co/MCF10Al and Co/MCF15Al.

Support and catalyst characterization

The nitrogen adsorption/desorption was analyzed at a constant temperature, 77 K, in a surface analyzer Autosorb iQ from Quantachrome Instruments. The surface area was obtained by means of the BET (Brunauer-Emmet-Teller) method, and the volume and pore size distribution were obtained by means of the BJH (Barrett-Joyner-Halenda) method. The SEM images were obtained in a Philips XL-30 Electron Microscope Equipment SEM that operates at a voltage of 15 KV with last minute resolution of 1 μm and a magnification range of 200X to 5000X. The X-ray diffraction analysis (XRD) was performed on a model of the equipment from Panalytical X'Pert Pro X-ray reflectometer, equipped with a goniometer and a Cu lamp (Kα line, λ=1.5406 Å). The analysis program includes a sweep angle: 10° ≤ 2θ ≤ 90°.

The analysis of desorption of ammonia at a programmed temperature, TPD, was carried out in a ChemBET TPR/TPD from Quantachrome Instruments. About 0.1g of sample was used in a U-quartz tube, the sample was degassed with a Nitrogen flux (N₂ 5.0 ultra-High Purity) of 70 ml/min at 250°C for about 1 hour. This sample was then activated with a flow of Helium (He 5.0 ultra-High Purity) of 70 ml/min with a heating ramp of 10°C/min up to 500°C, maintaining this temperature for 30 minutes. Subsequently, the temperature is lowered to 100 °C and a flow of ammonia (5% V/V of NH₃/He) of 50 ml/min is passed, allowing the ammonia to adsorb in the sample for 1 hour. The physically adsorbed ammonia was then removed for 1 hour at 100°C with a He flow. Finally, the TPD analysis with helium flow of 70 ml/min was performed, increasing the temperature 10 °C/min up to 800°C and keeping it at this temperature for approximately 60 min. For the TPD analysis of the catalysts, prior analysis, the catalyst was activated with dilute Hydrogen flux (5% V/V H₂/N₂) of 70 ml/min with a temperature ramp of 10 °C/min up to 500 °C and kept at this temperature for 20 min before the analysis.

The TPR programmed temperature reduction analysis was carried out on a ChemBET TPR/TPD equipment from Quantachrome instruments. About 0.1 g of sample was used in a U-quartz tube. The sample was degassed with nitrogen flow (N₂ 5.0 ultra-High Purity) of 70 ml/min at 250°C for about 1 hour. Afterwards, the temperature was lowered to 20°C and then, the TPR analysis of the cobalt catalysts was carried out with diluted hydrogen flow (5% V/V H₂/N₂) of 70 ml/min with a temperature ramp of 10°C/min up to 900°C and kept at this temperature for 20min.

Mesoporous cellular Foam (MCF) with different Alumina content

Nitrogen Physisorption (BET-BJH)

The adsorption/desorption isotherms of MCF in different aluminum content supports are shown in figure 1. The shape of the isotherms is divided into four parts: The first part of the isotherm is at a relative pressure (P°/P) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P°/P) of 0.15 to 0.7 for MCF5Al and MCF10Al; and 0.15 to 0.6 for MCF15Al, which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P°/P) from: 0.7 to 0.94 for the MCF10Al support; from 0.7 to 0.96 for MCF5Al support and from 0.6 to 0.9 for the MCF15Al support. According to the hysteresis loop type ¹² (see figure 2). It is of the H1 and H2 type. These isotherms tend slightly to the H3 type, generating a deformation of the isotherm compared to the MCF support isotherm that may be due to the content of Aluminum. Finally, it is possible to observe the last part of the isotherm, which is at a relative pressure (P°/P) between 0.94 and 0.96 to 1. The shape of this last part of the isotherm is similar to the H1 for MCF10Al and MCF15Al; and similar to H3 for MCF5Al.

As can be seen in figure 1, the slope of the hysteresis of the MCF10Al support may be due to the existence of tetrahedral Aluminum inside the structure, and to octahedral aluminum that would be found in the commonly called extra-framework Aluminum (EFA) ^{13), (15}]. This wider variation in nitrogen capillary condensation (hysteresis inclination) implies a greater extension in the pore size distribution and in turn, this also affects the result of pore volume and surface area.

The supports of the type MCF-Al present two types of porosity, window and cell. These refer to the internal diameter of the pore (cell) and the diameter of entrance to the pore (window). Besides, in general, the MCF (without Al) supports have 3D spherical pores, moderately high surface area, and uniform pore size distribution ^{4), (7), (16}. Thus, the specific surface area of the MCF-Al support is smaller than MCF because it has a larger pore sizes. However, the specific area and pore size distribution has a variation according to the amount of aluminum added. In table 1 the MCF5Al support has the largest pore size distribution, and the smallest specific surface area. Inversely, the MCF15Al support has the smallest pore size distribution, and the largest specific area, because of their smallest pores. This shows a greater distortion of the mesostructure when a higher proportion of aluminum is added to the support.

Figure 1 N₂ Adsorption-Desorption isotherms and pore size distribution of a series of MCF supports containing alumina. The surface area (see table 3) of the supports has the following sequence: MCF15Al˃ MCF10Al˃ MCF5Al; 643.4 m²/g; 408.5 m² /g; 349.4 m²/g, respectively. The window pore size distribution of the supports (see table 3) MCF5Al˃ MCF10Al ˃ MCF15Al are: 12.6 nm; 12.1 nm and 8.2 nm respectively. In the same way, the cell pore size distribution of the supports MCF5Al˃ MCF10Al ˃ MCF15Al are: 16.8nm, 15.9nm and 12.5nm. 

Small Angle X-Ray Diffraccion (XRD)

The XRD patterns of mesoporous MCF supports with aluminum content are shown in figure 3. For the mesostructured supports type MCF-Al, the characteristic signal of the MCF support is observed at the 2θ (2 theta) ≈ 0.5° position. These results agree with the results of the nitrogen adsorption analysis (BET-BJH). However, there is some displacement of the signal from right to left when the amount of Aluminum in the structure increases from 5% for the MCF5Al support up to 15% for the MCF15Al support. This could also show that while the amount of aluminum increases in the structure, the ordering of the pores decreases.

Table 1 Surface area, pore volume and pore size distribution of MCF5Al, MCF10Al, MCF15Al, 

Figure 2 IUPAC Classification of adsorption-desorption hysteresis loops [12, 17]. 

Figure 3 XRD patterns of MCF Supports with 2θ from 0.5-4.0 MCF5Al, MCF10Al y MCF15Al 

Temperature Programed Desorption (TPD)

TPD of mesoporous MCF5Al, MCF10Al and MCF15Al

The deconvolution of TPD profiles for the mesoporous silicoaluminate MCF-Al different supports are presented in Figure 4. Figure 4a, 4b, and 4c show the deconvolution profiles of MCF5Al, MCF10Al, and MCF15Al, respectively. Each figure shows three principal signals that were identified and compared based in the literature ^{8), (15), (18), (20}. The first signal at 222, 215, and 242 °C, respectively, which might correspond to Brönsted and Lewis weak acid sites such as: the silanols (Si-O-H) with low number of aluminum atoms near to the silicon atom (NNN’s) respectively and the species seen in figure 5(d) from Weitkamp, J. et. al ¹⁵.

Figure 4 Temperature-Programmed Desorption TPD Profile of support: a) MCF5Al, b) MCF10Al and c) MCF15Al 

The second signal at 412ºC, 450°C and 380ºC corresponds to the deconvolution profiles of MCF5Al, MCF10Al, and MCF15Al, respectively. They may correspond to Brönsted and Lewis medium acid sites such as: the silanols (Si-O-H) with medium number of aluminum atoms near to the silicon atom (NNN’s) and alumina species Extra-framework Aluminum such as AlO(OH) (see figure 5(c)) respectively. The third signal at 753°C, 795ºC and 720ºC, respectively, corresponds to an unidentified peak that may be strong acidity, which might agree to Brønsted and Lewis strong acid sites. These sites may be Si-O-H, Si-OH-Al bonds type (see figure 5(a) and 6(b)) with high number of aluminum atoms near to the silicon atom (NNN’s), and species extra framework such as AlO⁺, Al(OH)²⁺, respectively. Finally, an unidentified signal at 800°C is found for MCF10Al only.

Figure 5 Scheme representing the different types of hydroxyl groups and acid sites in a silicoaluminate [15]. 

Table 2 shows the study database of the deconvolution of the TPD analyzes of the mesoporous supports according to the amount of aluminum in the mesoporous MCF type (MCF5Al, MCF10Al and MCF15Al). There, the silicon to aluminum ratio was 16.2; 7.6 and 4.8 for MCF5Al, MCF10Al and MCF15Al, respectively. The supports have a higher amount of ammonia desorbed per gram according to the following order MCF10Al> MCF5Al> MCM15Al. The dispersion of acid sites per square meter of mesoporous support area (μmol NH₃/m²) has the following order MCF10Al> MCF5Al> MCM15Al. The percentage of accessibility of the acid sites ((μmol NH₃ desorbed/μmol of Al in the sample) ^* 100) has the following sequence from highest to lowest MCF5Al> MCF10Al> MCM15Al.

Table 2 Adsorption capacity of the mesoporous supports and catalysts type MCF 

¹μmol of NH₃ per gram/ specific surface area (g/m²); ²Aluminium in the surface of support/total aluminum; ³μmol of metal acidity sites per gram of catalyst

Hydrolysis and condensation

The hydrolysis and condensation process for MCFAl materials with different concentrations of aluminum are typically performed in acidic medium (pH = 1-2). Pluronic-123 triblock micelles are formed by liquid crystal mechanisms or by cooperative arrangement, depending on system conditions and surfactant concentration. A micelle expander agent is added so that the shape of the surfactant micelles passes from hexagonal cylinders in 2D to 3D spheres ^{7), (12}. Generally, the difference in hydrolysis and condensation rates with aluminum and silicon complexed with alkoxides is very high. However, when it comes to silatrane and alumantrane complexes, the hydrolysis and condensation rates decrease considerably, mainly due to steric hindrance. ^{2), (9}. This would facilitate a greater uniformity of deposition of hydrolyzed species on the surface of the surfactant. Although the surfactant of P-123 is anionic, it is characterized by a negatively charged surface ⁶. This property causes regions with higher densities of positive charges of hydrolyzed aluminum and silicon species to be attracted to this surface.

Acid sites formation

Thus, in this way, the negative charges on the surface of the micelles act similarly to a nucleophilic attack on the positively charged areas of the hydrolyzed species of the Aluminum and silicon atoms. Consequently, with less amount of Aluminum content, there will be less strength. This is because there will not be strong charges of an anion as such, but with a negative charge density, the interaction will be similar to a weak adsorption (physisorption). Thus, the hydrolyzed species will be attracted and adsorbed to these surfaces. As a result, high amounts of strong Lewis and Brønsted acid sites will be obtained. However, by restructuring and stabilizing the charges, weak and medium acid sites of Lewis and Brønsted can also be formed, but in smaller proportions.

Dried and calcined

Drying and calcining are also important stages so that mesostructured spherical 3D pores do not suffer damage or cracking, due to rough thermal conditions. In addition, it is at this stage, where the intensity and type of acidity on the surface finish forming.

Textural and structural characterization

Characterization was carried out by the methods of Nitrogen Adsorption (BET-BJH), and X-ray Diffraction at low angles (DRX). The spherical structure in 3D of the support could be evidenced, in addition, it was observed that the pore size distribution and the adsorption and desorption isotherm correspond to the MCF type material ^{5), (6), (10}. However, there is some distortion of the uniformity of the pores. This may be due to the fact that during the deposition of hydrolyzed Silicon and Aluminum species, there is a rearrangement of the species due to the formation of Aluminum in the tetrahedral and octahedral state. Thus, this rearrangement deforms the uniformity of the porous structure ^{3), (5), (6}. When the Si/Al ratio decreases, there is a greater formation of octahedral aluminum on the surface, while increasing a greater number of extra-framework aluminum species. This further distorts the uniformity of the pores of the support.

According to the Ammonia Programmed Temperature Desorption (TPD) analysis, a high formation of strong acid sites of Brønsted and Lewis can be evidenced. In addition, a lower proportion of strong Lewis and Brønsted acids can be observed. The MCF10Al supports have a higher proportion of total acidity than the MCF5Al and MCF15Al supports. With low Al contents, the concentration of strong Brønsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Furthermore, at low Al Contents, the amount of desorbed ammonia increases with growing content of aluminum. With increasing Al contents, the total amount of desorbed ammonia reaches a maximum value when the content of Al₂O₃ is approximately 10%. A further increase of the aluminum content, which is accompanied by structural losses, causes a distinct decrease of ammonia desorption. This might be due to the existence of an acidity limit achieved due to the Al Content. These results are similar with Kosslick, H et. al ⁶.

Cobalt catalysts on silicate aluminates: MCF with different Alumina content

Nitrogen Phisisorption (BET-BJH)

MCF5Al and Co/MCF5Al

The adsorption/desorption isotherms of MCF5Al and Co/MCF5Al with 12% of Co content are shown in figure 6. The shape of the isotherms is divided into four parts: The first part of the isotherm is at a relative pressure (P°/P) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P°/P) of 0.15 to 0.7 for MCF5Al; and 0.15 to 0.68 for Co/MCF5Al, which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P°/P) of 0.7 to 0.96 for the MCF5Al support; and 0.68 to 0.9 for Co/MCF5Al catalyst. The hysteresis loop types are of the H1 and H2 type. These isotherms tend slightly to the H3 type. Finally, the last part of the isotherm is at a relative pressure (P°/P) between 0.96 and 0.9 to 1. This ending is similar to the H1 for MCF10Al, Co/MCF10Al, MCF15Al and Co/MCF15Al; and H3 for MCF5Al and Co/MCF5Al.

MCF10Al and Co/MCF10Al

The adsorption/desorption isotherms of MCF10Al and Co/MCF10Al with 12% of Co content are shown in figure 6. There are four parts that the isotherms form: The initial part of the isotherm is at a relative pressure (P°/P) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P°/P) of 0.15 to 0.7 for MCF10Al; and 0.15 to 0.55 for Co/MCF10Al, which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P°/P) of 0.7 to 0.94 for the MCF10Al support; and 0.55 to 0.85 for Co/MCF10Al catalyst. The hysteresis loop type is of the H1 and H2 type. Finally, the last part of the isotherm is at a relative pressure (P°/P) between 0.94 and 0.85 to 1. This isotherm ending is similar to the H1 for both MCF10Al and Co/MCF10Al.

MCF15Al and Co/MCF15Al

The adsorption/desorption isotherms of MCF15Al and Co/MCF15Al with 12% of Co content are shown in figure 6. The shape of the isotherms is divided into four parts: The first part of the isotherm is at a relative pressure (P°/P) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P°/P) of 0.15 to 0.6 for MCF15Al; and 0.15 to 0.67 which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P°/P) between: 0.6 and 0.9 for the MCF15Al support; and between 0.67 and 0.94 for the Co/MCF15Al. In this part, the hysteresis loop type is of the H1 and H2 type, which tend slightly to the H3 type. Finally, the last part of the isotherm, which is at a relative pressure (P°/P) between 0.9 and 0.94 to 1, has terminations that are similar to the H1 type.

As in the MCF-Al type supports, the inclination of the hysteresis of the Co/MCF-Al catalysts may be due to the existence of tetrahedral aluminum that is inside the structure, and to octahedral extra-framework aluminum ^{8), (15), (18), (20}. In addition, the cobalt has an influence on the morphological nature of the pores of the catalyst. This affects the specific surface area and the pore size distribution. A wider variation in nitrogen capillary condensation (hysteresis inclination) implies a greater extension in the pore size distribution, and in turn, this also affects the results of pore volume and surface area ⁴.

As previously, the influence of the Al content on the textural properties of the MCF-Al type supports was observed. Similarly, when cobalt is added in the form of oxide, the surface area and the pore size distribution vary (see table 3). For the Co/MCF15Al catalyst, the pores of the MCF15Al support are smaller. This would imply that the Co₃O₄ particles could have covered a larger number of small pores of the support and the larger pores might have been left mostly free. For this reason, the average pore size distribution increases and the specific surface area decreases. For the Co/MCF10Al catalyst compared to the fresh MCF10Al support, the specific surface area increases and the pore size distribution decreases. This may be due to the pore size that is larger and some pores are partially covered, increasing the surface area and decreasing the pore size distribution. In the case of the Co/MCF5Al catalyst compared to the fresh MCF5Al support, this phenomenon is similar to Co/MCF10Al. However, this material has pores of a larger size, thus, decreasing the number of pores totally blocked.

Figure 6 N₂ Adsorption-Desorption isotherms and pore size distribution of supports and catalysts of alumina content MCF series. a) Adsorption isotherms and pore size distribution of supports. b) Adsorption isotherms and pore size distribution of catalysts 

In table 3 and according to figure 6, a summary of the results of the nitrogen desorption analysis of the cobalt catalysts Co/MCF5Al, Co/MCF10Al and Co/MCF15Al can be observed. The surface area, the pore volume, the internal pore size distribution are shown. The average window pore sizes of Co/MCF5Al, Co/MCF10Al and Co/MCF15Al are 9.8 nm; 7.9 nm and 9.5 nm respectively. The average internal pore sizes of Co/MCF5Al, Co/MCF10Al and Co/MCF15Al are 11.3 nm; 9.8 nm and 12.8 nm, respectively. The surface area calculated by the BET method, for Co/MCF5Al, Co/MCF10Al and Co/MCF15Al is 414.7 m²/g; 422.1 m²/g and 538.9 m²/g, respectively.

Table 3 Surface area, pore volume and pore size distribution of MCF5Al, MCF10Al, MCF15Al, Co/MCF5Al, Co/MCF10Al and Co/MCF15Al 

X-Ray Diffraccion (XRD)

X-Ray Diffraction of cobalt supported catalysts

Figure 7 shows the XRD patterns of the cobalt support catalysts in silicoaluminate mesoporous, which are Co/MCF5Al, Co/MCF10Al, Co/MCF15Al. In each of these materials the presence of cobalt oxide Co₃O₄ is observed as the only crystalline phase of cobalt, with signals at angles 2θ (2 theta) ≈ 31.1°; 36.9°, 45.0°; 59.4° and 65.4°. These reflection signals are associated with the planes (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) respectively ^{4), (21), (22}, and these correspond to the cubic system centered on the face of the spinel Co₃O₄^{4), (11}. The average crystallite size of Co₃O₄ and CoO were calculated using the Scherrer equation ^{1), (5}. The Cobalt Co/MCF5Al catalyst has the most intense and sharp cobalt oxide characteristic signals compared to the Co/MCF10Al and Co/MCF15Al catalyst. Moreover, the Co/MCF10Al catalyst has the cobalt oxide (Co₃O₄) characteristic signals more intense and acute than the Co/MCF15Al catalyst. This could indicate that the catalysts have a crystalline domain size of cobalt particles according to the following sequence from highest to lowest: Co/MCF5Al> Co/MCF10Al > Co/MCF15Al. The most intense signal for these materials is at 2θ ≈ 36.9°. This peak is associated with the plane (3 1 1) ¹³. Figure 7 and table 4 show the comparative sequence: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al.

Figure 7 XRD patterns of cobalt catalysts with 2θ from 10-70: Co/MCF5Al, Co/MCF10Al y Co/MCF15Al 

In table 4 and according to figure 7, the crystalline microdomain size of the cobalt particles found in the mesoporous silicoaluminate supports is observed. This parameter was calculated with the Scherrer equation ²² which is represented as the diameter of the cobalt crystals in nanometers (nm). Figure 7 shows the cobalt catalysts Co/MCF5Al, Co/MCF10Al and Co/MCF15Al.

The crystalline microdomain size increased sequence is directly proportional to the increase in the pore size distribution of the mesoporous supports (See table 3. The microdomain crystalline size sequence of Co₃O₄ is as follows: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al: 17.7nm> 13.3nm> 9,8nm, respectively. The microdomain crystalline size sequence of Co⁰ is as follows: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al; 13.3nm> 10.0nm> 9.8nm respectively. However, the crystal size distribution of Co₃O₄ and Co⁰ is larger than the average size of each of the mesostructure types. This may be due to the particle size distribution of Co₃O₄ and Co⁰ that increases because there are particles of the largest metal clusters found on the surface of the support particles. ¹⁴

Table 4 Crystallite size from XRD analysis of Cobalt particles of the Co/MCF5Al, Co/MCF10Al and Co/MCF15Al catalysts 

* d(Co⁰)=0,75d(Co₃O₄). [14]

Temperature Programmed Reduction (TPR)

Figure 8 shows the signs of TPR profiles, of the Co/MCF5Al, Co/MCF10Al and Co/MCF15Al catalysts. The reduction signals of the three catalysts are divided into two well-marked regions, the first at low temperatures between 237°C and 603°C, and the second is between 610°C and 900°C.

The signals at low temperatures represent the reduction reactions of Co₃O₄ to CoO and from CoO to Co⁰. The signals at high temperatures represent the cobalt oxide and minor phases such as cobalt silicates and cobalt silicoaluminates that have high interaction with the support ^{14), (22}. The deconvolution at low temperature shows a signal for the reduction from Co₃O₄ to CoO in agreement to ^{11), (21), (22}, and two signals for the reduction from CoO to Co⁰. CoO is reduced in two stages; In one hand, this is due to the reduction of CoO on the surface of the cobalt particles (weak metal-support interactions); in the other hand, it is due to the reduction of CoO that is close to or those CoO that has an interaction with the surface of the support (strong metal-support interactions) ^{6), (14}.

The consumption of H₂ in the region of low temperature follows as: Co/MCF5Al > Co/MCF10Al > Co/MC15Al. The signal for the Co/MCF5Al is displaced at lower temperatures compared to the other two catalysts. The reduction signal at low temperatures of the Co/MCF5Al catalyst represents the highest H₂ consumption and also has the widest peak and a high temperature displacement. From the consumption of H₂ in the region at high temperature, the catalysts are ordered as: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al.

Regarding the extent of each reduction reaction, the temperature at which the reduction from Co₃O₄ to CoO occurs is similar in the three catalysts as shown in figure 8. More CoO is reduced to Co⁰ in the Co/MCF15Al catalyst as in contrast to Co/MCF10Al and Co/MCF15Al catalysts (see table 5), specially, in the reduction at low temperature (L). Thus, the reducibility degree decreases in the following order: Co/MCF15Al> Co/MCF10Al> Co/MCF15Al. The latter is an important parameter for the application in catalytic synthesis such as the Fischer-Tropsch synthesis, where the higher reducibility the higher the activity of the catalyst.

Table 5 and Figure 8 show the summary of the H₂ consumption data of the integrals of the conversion of the TPR results of the cobalt catalysts Co/MCF5Al, Co/MCF10Al and Co/MCF15Al. For the reduction of Co₃O₄ →CoO, there are no differences in brands between the three catalysts. For the CoO→ Co⁰ reduction at low temperatures (L), there is an order for the H₂ consumption of the catalysts as follows: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al. For the CoO →Co⁰ reduction at high temperatures (H) there is an order for H₂ consumption of the catalysts as follows: Co/MCF5Al> Co/MCF10Al> Co/MCF15Al. For the cobalt silicoaluminous zone, it does not have a marked sequence that is the following: Co/MCF5Al> Co/MCF15Al> Co/MCF10Al. The cobalt catalyst Co/MCF5Al has the highest degree of reduction of 54.1%, followed by the Co/MCF10Al catalyst with 44.1%, and the Co/MCF15Al catalyst that has a lower reduction percentage of 29.8%.

Figure 8 Temperature- Programmed Profile for reduction of catalysts: a)Co/MCF5Al, b)Co/MCF10Al, c)Co/MCF15Al 

Table 5 Hydrogen consumption of cobalt catalysts: Co/MCF5Al, Co/MCF10Al and Co/MCF15Al 

Temperature-Desorption Programmed (TPD)

c) TPD of mesoporous Co/MCF15Al catalyst

The deconvolution of TPD profiles for the mesoporous silicoaluminate MCF15Al support and Co/MCF15Al catalysts (see figure 9c) shows similar TPD profiles for both materials. For MCF15Al, there are three identified signals as compared to literature: at 242°C a weak acidity, at 380°C a medium acidity, and at 720°C an unidentified peak that it may be strong acidity. The latter signal is markedly increased by the presence of Cobalt (Co/MCF15Al catalyst), which might be explained by a Lewis weak, medium and strong acidity of Co^{0 (13}. Also, when cobalt is added to MCF15, the total amount of acid sites is increased by 2.5 fold as is shown in table 4. In addition, the dispersion of acid sites is increased by approximately three times when cobalt is added.

Table 6 shows the study database of the deconvolution of the TPD analysis of the mesoporous supports according to the amount of aluminum in the mesoporous MCF type (MCF5Al, MCF10Al and MCF15Al). The silicon/aluminum ratio was 16.2; 7.6 and 4.8 for MCF5Al, MCF10Al and MCF15Al respectively. The supports have a higher amount of ammonia desorbed per gram according to the following order MCF10Al> MCF5Al> MCM15Al. The dispersion of acid sites per square meter of mesoporous support area (μmol NH₃/m²) has the following order MCF10Al> MCF5Al> MCM15Al. The percentage of accessibility of the acid sites ((μmol NH³ desorbed/μmol of Al in the sample) * 100) has the following sequence from highest to lowest MCF5Al> MCF10Al> MCM15Al.

Table 6 Adsorption capacity of the mesoporous supports and catalysts type MCF 

¹μmol of NH₃ per gram/ specific surface area (g/m²); ²Aluminium in the support surface/ total aluminum; ³μmol of metal acidity sites per gram of catalyst

For the series of supports and catalysts type MCF with aluminum variation (MCF5Al, MCF10Al and MCF15Al), the results of nitrogen physisorption show high specific surface area, and a narrow pore size distribution in the order of the mesopores. According to these results, these materials could be suitable as support for Fisher Tropsch catalysts, and catalytic cracking among others. The variation of surface area and pore size distribution in this series depends on the amount of Aluminum inserted in the matrix. In the XRD diffractograms of the cobalt catalysts in this series, it is observed that the Cobalt particles were inserted homogeneously in the supports, since there is a single phase of cobalt in the surface of the matrix which is the double oxide of cobalt (Co₃O₄). This property is good for catalysis because the reduction of this cobalt phase is expected at low temperatures.

Figure 9 Desorption Temperature-Programmed TPD Profile of support and catalysts: a) MCF5Al and Co/MCF5Al, b)MCF10Al and Co/MCF10Al, c)MCF15Al and Co/MCF15Al 

The mesoporous supports and catalysts synthesized by the atrane route show different weak, medium and strong acid species of Lewis and Brønsted, depending on the amount of Aluminum added (MCF5Al, MCF10Al and MCF15Al). Addition of cobalt to these matrices results in an increase of the acidity of the medium and weak acids of Lewis, which varies according to the type of mesostructure. These Lewis acids are due to the presence of Co⁰ that could be related to the catalytic performance of the cobalt active sites. The TPR reduction profiles of the 12% Cobalt catalysts are similar to the series according to mesoporous type. However, the Co/MCF5Al catalyst has a greater degree of relative reducibility. The total amount of reduction depends on the type of support and the amount of aluminum inserted in the matrix. This property is important for Fischer-Tropsch Synthesis.