Synthesis of MCM-41 with 10% of structural aluminum

This new method has minor modifications as compared with previously published in S. Cabrera et al. [10]. Initially, the triethanolamine (C₆H₁₅NO₃, TEAH₃) was heated to 140°C, then it was added to a 1M NaOH solution. Then, the mixture was stirred at a constant temperature of 140°C. After, the tetraethyl orthosilicate (Si(OC₂H₅)₄, TEOS) and aluminum isopropoxide (Al(C₃H₆OH)₃, IPA) were added. The mixture was then heated to 140°C maintaining this temperature for 30 minutes. Subsequently, the temperature was lowered to 80°C to add Cethyl trimethylammonium bromide (C₁₉H₄₂BrN, CTAB). After that, the temperature was reduced to 50°C, and water was added and stirred for 2 hours. The mixture was then subjected to a hydrothermal treatment at 100 °C for 24 hours. The molar composition of the reaction mixture was SiO₂: Al₂O₃: TEAH: CTAB: NaOH: H₂O equal to 0.89: 0.054: 3.46: 0.27: 0.23: 82.34.

As a result of the reaction, the solid mixture was then filtered at room temperature, then, the solid was washed with methanol and water, and allowed to dry overnight. A thermal treatment was applied afterwards as follows: The filtered and dried solid was calcined with a ramp of 2°C/min up to 80°C maintaining that temperature for 4 hours. Then, it was heated with the same speed to 120°C for 3 hours. Subsequently, it was heated with the same speed to 220°C for 4 hours. After that, the temperature was raised to 350°C for 4 hours, and then, the temperature was raised to 600°C and kept for 5 hours. The support thus obtained was named as MCM-41Al.

Synthesis of the silatrane-alumatrane complex: precursor of SBA-15 and MCF

Before the synthesis of mesoporous supports SBA-15 and MCF, the atrane complex of silicon and aluminum was prepared [10]. Initially TEAH₃ was heated to 140°C, then, tetraethyl orthosilicate (TEOS) and (IPA) were added. After that, the mixture was heated to 140°C keeping this temperature for 30 minutes.

SBA-15 with 10% of structural aluminum

The method has minor modifications as compared with previously published in Pardo et al. [5]. Initially, 9.38 g of Pluronic triblock copolymer (HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H) (P-123) and 40 ml of concentrated HCl were dissolved in 270 ml of distilled H₂O. Then, the solution was stirred for 3 hours at room temperature. After that, 0.89 g of Ammonium fluoride (NH₄F) was added. After, the solution was stirred for 30 minutes at room temperature, 5.6 ml of trimethylbenzene (C₆H₃(CH₃)₃) (TMB) was added and stirred for 1 hour at 40°C. Then, silicoalumatrane was added and stirred for 30 minutes at 40°C. Subsequently, it was aged at 100°C for 24 hours without stirring. The molar composition of the reactive mixture was SiO₂: Al₂O₃: TEAH₃: P-123: TMB: NH₄F: HCl: H₂O equal to 0.89: 0.054: 3.43: 0.019: 0.58: 0.11: 5.74: 178.39. After the aging stage, the batch mixture was cooled down to room temperature and washed with distillated water. Then, it was washed with a 50/50 V/V of water and ethanol, and lastly, it was washed only with ethanol. Afterwards, the mixture was filtered, and the remaining solid was dried overnight at 60°C. Finally, the solid product was calcined following the same thermal treatment as described above in the section for the production of MCM-41, with 550°C as the final temperature. The material thus obtained was named as mesoporous support SBA-15Al.

MCF with 10% of structural aluminum

The synthesis method was similar to SBA-15Al with the following modifications: The amount of ammonium fluoride is 2.67g and the amount of (TMB) is 16.8ml. The molar composition of the reagent mixture was: 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 obtained was named as MCF10Al.

Cobalt impregnation on mesoporous supports

For all the synthesized mesoporous silatrane-alumatrane supports, cobalt was added by the pore volume technique ^2),(8. A nominal weight percentage of 12% of Co was added to the supports from an aqueous solution of cobalt nitrate (1.2 M Approximately). The impregnated supports were dried at 70°C overnight. Then, they were calcined as follow: till 120°C for 4 hours, heated up to 220°C for 4 hours, and finally heated up to 350°C for 6 hours. All the mentioned steps were done at a rate of 2°C/min. The catalysts obtained were named as Co/MCM-41Al, Co/SBA- 15Al, and Co/MCF10Al for each of the prepared supports, respectively.

Support and catalyst characterization

The nitrogen adsorption/desorption isotherms were performed at a constant temperature of -196°C 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 was 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 Panalytical instrument equipped with a goniometer and a Cu lamp (Kα line, λ=1.5406 A). The analysis program includes a sweep angle: 10° ≤ 2θ ≤ 90°. The average particle size of Co₃O₄ in calcined samples was calculated using the Scherrer equation from the most intense Co₃O₄ peak at 2θ= 36.9° in the XRD pattern. The obtained Co₃O₄ particle size could be used to calculate the diameter of metallic Co⁰ crystallite after reduction by the following formula ³.

(𝐶𝑜⁰) = 0.75 (𝐶𝑜₃𝑂₄)

The analysis of desorption at programmed temperature TPD of ammonia was carried out in a ChemBET TPR/TPD from Quantachrome Instruments. About 0.1g of sample was placed 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 is 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, and keeping 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 ammonia physically adsorbed is then removed for 1 hour at 100°C with the He flux. Finally, the TPD analysis with helium flow of 70 ml/min is performed, increasing the temperature with 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 is 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.

The route of the atranes for different support type MCM-41, SBA-15 and MCF

MCM-41 and MCM-41Al

The adsorption/desorption isotherm of MCM-41Al support is shown in figure 1, where the completion of the monolayer is between 0.0 to 0.2 in P/P₀. Multilayer formation is between the 0.2 to 0.4 fraction. This support is compared with the isotherm of MCM-41 previously reported in literature ^5),(12. The constant curves of the completely filled pores can be observed from 0.4 to 0.95 for MCM-41Al, and from 0.4 to 0.9 for MCM-41. The 0.95 and 0.90 P/P₀ fraction corresponds to the content of interparticle pores. The addition of water promotes the hydrolysis processes that are necessary for the creation of these supports in basic media (Thermodynamically favored) ^6),(10. However, the polymerization decreases when silatrane and alumatrane are used as sources of Si and Al, mainly due to the steric effect of the atran complexes. Subsequently, the polymerization of Si and Al slows down. Then, due to the loss of ethanol in the silatrane and alumantrane complexes, these final oligomers would be negatively charged. Thus, they would be placed on the surface of the surfactant formed by CTA⁺ aggregates that agree with negatively charged oligomers. This would form a micellar surfactant coated with silica and/or alumina ^5),(11),(13. However, in the case of MCM-41Al support, during this process, monomers, dimers, trimers and oligomers might be formed from only aluminum atoms. Because of their negative charge density, they might be added more quickly to the micellar surface of CTA⁺, which would result in the final product having extra-framework regions of aluminum oxides. The addition of aluminum inside and outside the structure could have decompensated the charges during the formation of the final pores. This could be the cause of the variation in the hysteresis of the adsorption/desorption curves in the MCM-41Al support in relation to the MCM-41 support.

The addition of aluminum in the structure of the mesostructured support would involve a decrease in surface area, and an increase in the size of the walls of the structure, as well as a greater distortion in the uniformity of the pore size distribution (See table 1). This may be because of two reasons: The first, to compensate for the surfactant load, the adequate inorganic polyanion would increase as the Al content increases. The second reason may be that as much as the system involves two different elements, there will be a diversity of inorganic polyanions that could contribute to such a load balance. ⁽⁶

Hysteresis loops were observed in most isotherms of the mesoporous materials studied in the present study (Figure 1) and (table 1), even though there might not be a clear reason in every case. However, it is generally attributed to a thermodynamics factor and the network effect of the pore system. The thermodynamics factor is related to the metastability of the adsorption and desorption branches. In other words, capillary condensation and evaporation shift towards lower and higher pressures, respectively, relative to the one at which gas and liquid nitrogen (N₂) coexist. For example, for a large pore connected to several other small pores, adsorbates could not desorb at the pressure at which its pore size is. This is because the small connected pores have been filled with the liquid containing the adsorbates. Indeed, the network effect is considered to be the main cause of the hysteresis loops ¹⁴.

Another illustration of this may happen in capillary condensation within the so-called "ink-bottle" formed pores, i.e. having an entrance distance across littler than the estimate of the pore body. Condensation happens continuously in such pores until the pores are totally filled by the condensable vapor at high relative pressures. However, when the relative pressure decreases, the fluid should go out of the pores and thus has to pass through the narrow necks. This happens at a lower relative pressure than what was required for filling the pores, and when the critical relative pressure is come to, the pores empty quickly. Subsequently, a loop in the isotherms shows up ¹⁴.

Figure 1 Nitrogen adsorption-desorption isotherms and pore size distribution of mesoporous supports: MCF10Al, SBA-15Al, MCM-41Al. 

SBA-15 and SBA-15Al

The adsorption/desorption isotherms of the SBA-15Al support is shown in figure 1. This isotherm is compared with the previously reported SBA-15 support ^{15), (17}. The shape of the isotherms is divided into four parts: The first part of the isotherm of this material is at a relative pressure (P/P₀) between 0 and 0.2, where there is a completion of the monolayer. The second part of the isotherm is at a relative pressure (P/P₀) of 0.2 to 0.4, which is due to the formation of multilayers. The third part of the isotherm is at a relative pressure (P/P₀) between 0.4 and 0.9 for the SBA-15Al support and 0.70 to 0.90 for the SBA-15 support. The type of hysteresis loop that is observed in this material is between types H2 for SBA-15Al and H1 for SBA-15. The deformation of the isotherms of the SBA-15Al support compared to the SBA-15 material might be due to the presence of Aluminum. Finally, the last part of the isotherm, which is at a relative pressure between 0.9 and 1, is similar to the terminations of the H1 and H2 type loops ¹⁴. The deformation of the hysteresis of the support may be due to the existence of tetrahedral aluminum found inside the structure, and octahedral aluminum that would be found outside the structure. ⁸. This variation also affects the result of pore volume and surface area distributions.

The micellization of the triblock copolymers (P-123) is driven by the hydrophobic polyethylene oxide (PO) block. The triblock copolymer consists of a core PO block and an ethylene oxide, (EO), block crown. The hydrophilic part, EO, will attract the anionic oligomers of oxohydroxo-Si, thus, forming the condensation of silicon and/or aluminum in the micelles. When TMB is added to the synthesis, it prefers to self-assemble into the hydrophobic core of PO, which causes the micelles to swell ¹⁶. Finally, the addition of the swelling agent (TMB) could affect the equilibrium formation of surfactant micelles and produce disorganized materials. Hence, in an acid medium it is suggested to use silicon and aluminum atrane complexes as metallic precursors ⁵. However, as in the case of MCM- 41, part of the aluminum might enter the structure and another part of the aluminum may form aluminum oligomers that would be added more quickly. Therefore, this could occasionally cause variations in the formation of the pores of the SBA-15.

For this same reason, there is also a slight inclination in the hysteresis of nitrogen adsorption and desorption branches (see Fig. 1).

MCF and MCFAl

The adsorption/desorption isotherm of MCF10Al is shown in figure 1. This isotherm is compared to the adsorption/desorption isotherm of MCF support previously reported in literature ^1),(5),(18. The shape of the isotherms is divided into four parts: The first part of the isotherm is at a relative pressure (P/P₀) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P/P₀) of 0.15 to 0.7, which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P/P₀) between 0.7 and 0.94 for the MCF10Al support and 0.7 to 0.98 for the MCF support. The hysteresis loops type is of the H1 type for both supports (See Fig. 2). This isotherm tends slightly to the H3 type for the MCF10Al support, generating a deformation of the isotherm compared to the MCF support isotherm that may be due to the presence of Aluminum. Finally, the last part of the isotherm, which is at a relative pressure (P/P₀) between 0.94 and 1, corresponds to a termination similar to the H1 type loops. As in the SBA-15Al support, the inclination of the hysteresis of the MCF10Al support may be due to the existence of tetrahedral Aluminum that is inside the structure, and to octahedral aluminum that would be found in the extra- framework ⁸. 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 distributions ¹⁴.

According to these nitrogen adsorption/desorption curves (BET-BJH), the use of atranes as precursors is important. Since they may have controlled hydrolysis and condensation rates, they decrease and resemble hydrolysis rates and condensation of silatrane and alumatrane. This would have favored the introduction of Aluminum in the structure. ^6),(10.

The supports SBA-15Al and MCF present two types of porosity, window and cell, that refer to the internal diameter of the pore (cell), and the diameter of entrance to the pore (window), respectively ¹⁹. The SBA-15 support has 2D hexagonal pores ²⁰, and the MCF has 3D spherical pores ¹. Thus, the specific surface area of the MCF10Al support is smaller than the SBA-15Al because it has a larger pore size.

The specific surface area (see table 1) of the supports have the following sequence: MCM-41Al˃ SBA-15Al˃ MCF10Al; 1045.1 m²/g; 974.9 m²/g; 408.5 m²/g, respectively. This is similar to the sequence of the supports: MCM- 41 ˃ SBA-15 ˃ MCF: 1200 m²/g; 940 m²/g; 860 m²/g. In the same way, the pore size distribution of the supports (see table 1) MCF10Al˃ SBA-15Al ˃ MCM-41Al are: 12.1 nm; 4.4 nm and 3.0 nm, respectively. This sequence is similar for the series: MCF ˃ SBA -15Al ˃ MCM-41Al, with the average pore size distribution of: 29.3nm; 7.5nm; and 2.6nm.

Table 1 Specific surface area, pore volume, and pore diameter size distribution of MCM-41Al, SBA-15Al, MCF10Al, and MCM-41* [23], SBA-15* [22] and MCF* [21]. 

Table 1 shows the BET surface area and BJH pore size distribution data. These materials were also prepared by the route of the atranes. According to this technique, it can be observed that the materials synthesized by the route of the atranes with the addition of 10% aluminum, retain their typical mesostructured morphology. This may be due to the fact that during the synthesis, the hydrolysis and condensation of Si and Al was controlled, since the deposition rate of these metals on the surfactants was similar [5].

Figure 2 Classification of adsorption-desorption hysteresis loops. Image reproduced by creative commons license [11]. 

Small Angle X-Ray Diffraction (XRD) MCM-41 and MCM-41Al

For the MCM-41Al support (see Fig. 3), the characteristic signals can be observed in the 2 θ (2 theta) ≈ 2° position. This was compared to the data previously reported for the MCM-41[14, 17]. MCM-41 pattern exhibits the higher signal in 2° (1 0 0). Its secondary signals are in 3.55° (110), 4° (2 0 0) and 5.3° (2 1 0). Analyzing the data, it can be stated that the signals of MCM-41Al has some alteration with respect to the MCM-41 pattern ²⁰ (see Fig. 3). This could be due to the Aluminum load that distorts in some way the defined structure of the pores. For instance, the replacement of silicon by aluminum in the framework favors the formation of tetrahedral aluminum with Al-O(H)- Si bridges, allowing the formation of Brønsted acid sites ⁹. For MCM-41Al, secondary signals are also present, although with less intensity.

SBA-15 and SBA-15Al

For the SBA-15Al support (see Fig. 3), the characteristic signal is observed in the 1θ (1theta) ≈ 1 ° position. This support was compared to the data previously reported for the SBA-15 [14, 22]. Its pattern shows the higher signal in 1° (1 0 0), and the secondary signals are in 1.7° (110) and 1.9° (2 0 0). Comparing both supports it is possible to see that the signals of SBA-15Al have some alterations with respect to the SBA-15 pattern. It is possible that the Aluminum load distorts in some way the defined structure of the pores, as explained in the prior part. For SBA-15Al, secondary signals are also observed, although with less intensity.

X-Ray MCF and MCFAl

The MCF pattern, analyzed from literature ^14),(18, shows the unique higher signal in 2θ ≈ 0.55°. On the other hand, for MCF10Al (Se Fig. 3), there is a characteristic signal of the MCF support observed at position 2θ (2 theta) ≈ 0.5. With this signal, it can be verified that the structure of these supports is in accordance with the results of the nitrogen adsorption analysis (BET-BJH). However, there is a noticeable distortion between the signals of both supports due to the distortion caused by Al that is produced during the synthesis process.

Figure 3 DRX Low angles of MCM-41Al, SBA-15Al and MCF10Al 

Temperature Programed Desorption (TPD) TPD of mesoporous MCM-41Al

The deconvolution of TPD profiles for the mesoporous silicoaluminate MCM-41Al support (see Figure 6.(a)) shows four signals. At low temperature (≤ 450°C) three signals are identified as compared to literature: at 217 °C a Lewis weak acidity site which may correspond to the species seen in figure 4 (d); at 285°C, Brønsted weak and medium acidity sites that may correspond to the silanol (Si-O-H) (see figure 4 (b)) with low and medium number of aluminum atoms near to the silicon atom (NNN’s); at 365°C a Brønsted strong acidity site, corresponding to Si-O-H, and Si- OH-Al bonds type (see figure 4 (b) (c)) with high number of aluminum atoms near to the silicon atom (NNN’s). At high temperature (≥ 450°C) a signal corresponding to Lewis strong acidity site is found at 600°C that might correspond to bonds of aluminum species totally or partially outside of the framework, such as (AlO)⁺ and Al(OH)²⁺. Finally, an unidentified signal at 750 °C is found.

Figure 4 Schematic representation of the different types of hydroxyl groups and acid sites in zeolites. [15] 

In aluminosilicate-type zeolites, the 4⁺ charge on framework silicon atoms and the 2⁻ charges on the four coordinating oxygen atoms lead to a neutral tetrahedral framework (SiO₂). If, however, the silicon cation in the framework is substituted by a cation with a 3⁺ charge, typically with an aluminum cation, the formal charge on that tetrahedron changes from neutral to 1⁻ (AlO ^-). This negative charge is balanced by a metal cation or a hydroxyl proton forming a weak Lewis acid site or a strong Brønsted acid site, respectively. Hydroxyl protons acting as Brønsted acid sites, i.e., as proton donors, are located on oxygen bridges. They connect a tetrahedrally coordinated silicon and aluminum cation on framework positions (Figure 4(a)). These OH^- groups are commonly referred to as structural or bridging OH^- groups (SiOHAl) ²³.

The best description of Brønsted acid sites in zeolites is a weakly bound proton of a bridging hydroxyl group between two tetrahedrally coordinated atoms, typically Si and Al. On the other hand, Brønsted acid sites in amorphous materials are silanol groups involved in a weak interaction with neighboring atoms acting as Lewis acid sites, i.e., as electron pair acceptors, such as Al atoms. ²³

In addition, the impact of the Si−O−T bond angle on the partial charge and the acid strength of the hydroxyl proton has to be considered. In zeolites, the Si−O−T bond angles vary from 137° to 177° for zeolite ZSM-5, from 143° to 180° for mordenite and from 138° to 147° for zeolite Y. Theoretical studies indicate that at a given Si−O−Al bond angle in the local structure of SiOHAl groups, a corresponding partial charge and, therefore, a corresponding acid site with a specific acid strength occurs. ⁶.

In another theory describing the chemical behavior of aluminosilicate-type zeolites, the aluminum site distribution is considered to be the primary factor affecting the acid strength of SiOHAl groups. The key property is the lower electronegativity of aluminum atoms in comparison with silicon atoms. For example, in FAU-type zeolites, each framework aluminum atom is linked via oxygen bridges with four silicon atoms, which in turn, are connected with nine further T atoms in the next coordination sphere. These nine T atoms in the latter coordination sphere are called next nearest neighbors (NNNs). According to the NNN concept, the acid strength of SiOHAl groups in aluminosilicate-type zeolites depends on the number of framework aluminum atoms on NNN positions. ²³

Up till now, the precise chemical nature of Lewis acid sites in zeolites is a subject of research. Lewis acid sites can be attributed to extra-framework aluminum (EFAL) species of octahedral or tetrahedral coordination as well as tri- coordinated aluminum atoms partially dislodged in the framework. Scherzer and co-workers suggested AlO⁺, Al(OH)²⁺ and AlO(OH) as EFAL species on extra-framework positions in dealuminated zeolites. Similarly, Kühl et. al concluded from X-ray spectrometry that [AlO]⁺ units removed from the zeolite framework are transformed into cationic extra-framework species, which act as so-called “true” Lewis acid sites. Framework Lewis acid sites have been suggested to consist of positively charged silicon ions in the neighborhood of tricoordinated aluminum atoms. Elanany et al. ²⁴ studied the formation of framework Lewis sites via dehydroxylation routes (a) and (b) in figure 5. Route (b), which contains a defect SiOH group in the vicinity of the bridging OH group, is significantly less endothermic than and, hence, preferred over route (a), which starts with two neighboring bridging OH^- groups ^6),(23.

Figure 5 Dehydroxilation mechanisms of zolites and the formation of Lewis acid sites. Mechanism extracted from 15. Weitkamp et. al [15] 

TPD of mesoporous SBA-15Al

The deconvolution of TPD profiles for the mesoporous silicoaluminate SBA-15Al support (see figure 6 (b)) shows tree signals that were identified by comparison to literature: at 255°C a weak acidity signal, which might correspond to Brønsted and Lewis weak acid sites such as the silanoles (Si-O-H) with low number of aluminum atoms near to the silicon atom (NNN’s), and the species seen in figure 4(d), respectively. Then, at 425°C a medium acidity signal, which might correspond to a Brønsted and Lewis medium acid site such as the silanoles (Si-O-H) with medium number of aluminum atoms near to the silicon atom (NNN’s), and alumina species extra-framework such as AlO(OH), respectively. At 750°C, an unidentified peak that may correspond to a strong acidity site, which might agree with a Brønsted and Lewis weak and strong sites such as: Si-O-H, Si-OH-Al bonds type (see figure 4 (a) (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, a no identified signal at 800°C was found.

TPD of mesoporous MCF10Al

The deconvolution of TPD profiles for the mesoporous silioaluminate MCFAl support (see figure 6 (c)) shows three signals, that were identified as compared to literature: at 215°C a weak acidity signal, which might correspond to Brønsted and Lewis weak acid sites. These may be silanoles (Si-O-H) with low number of aluminum atoms near to the silicon atom (NNN’s), and the species seen in figure 4 (d), respectively. At 450°C, a medium acidity signal is observed. This might correspond to Brønsted and Lewis medium acid sites such as the silanoles (Si-O-H) with medium number of aluminum atoms near to the silicon atom (NNN’s), and alumina species extra framework such as AlO(OH), respectively. At 795°C, an unidentified peak, that might correspond to Brønsted and Lewis strong acid sites such as Si-O-H, Si-OH-Al bonds type (see figure 4 (a) (b)). These have high number of aluminum atoms near to the silicon atom (NNN’s). In addition, another peak may correspond to extra framework species such as AlO⁺, and Al(OH)²⁺ respectively. Finally, an unidentified signal at 800°C was found.

According to the Ammonia Programmed Temperature Desorption (TPD) analysis (Figure 6), 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.

Table 2 shows the data base of the deconvolution study of the TPD analyzes of the mesoporous supports according to the mesoporous type (MCM-41Al, SBA-15Al and MCF10Al). The silicon/aluminum ratio, remained constant at 7.6. The MCM-41Al support has a higher proportion of weak acids sites than the supports SBA-15Al and MCF10Al according Fig. 6. and table 2. The supports have a higher amount of ammonia adsorbed per gram according to the following order MCF10Al> SBA-15Al> MCM-41Al. In the same way, the dispersion of acid sites per square meter of mesoporous support area (μmol NH₃/m²) has the following order MCF10Al> SBA-15Al> MCM-41Al. 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 MCF10Al> SBA-15Al> MCM-41Al. This sequence may be due to the species that generate strong, medium and weak Lewis and Brønsted acid sites that are formed in the synthesis process for each one of the supports (seen in this and in the previous section).

Table 2 Adsorption capacity of the mesoporous supports and catalysts 

¹μmol of NH3 per gram/ specific surface area (g/m²) ²Aluminium in the surface of support / total aluminum

Figure 6 Desorption Temperature-Programmed TPD Profile of support: a)MCM-41Al, b)SBA-15Al and c)MCF10Al 

Nitrogen Physisorption (BET-BJH) MCM-41Al and Co/MCM-41Al

The adsorption/desorption isotherms of the MCM-41Al and Co/MCM-41Al supports are shown in figure 1 and figure 7, where the completion of the monolayer is between 0.0 to 0.2 in P/P₀. Multilayer formation is between 0.2 to 0.4. The constant curves of the completely filled pores go from 0.4 (for both) to 0.95 and 0.9 for MCM-41Al and Co/MCM-41Al, respectively. And of 0.95 and 0.90 P/P₀ corresponding to the content of interparticle pores. There is a difference in the distribution of pore size and surface area. This may be due to the arrangement of the cobalt particles on the surface of the support particles at the time of the addition of Cobalt and calcining of this material at 350 ° C. Another reason may be the pore size and the support-metal interactions. Therefore, this could cause its different surface properties in the material.

SBA-15Al and Co/SBA-15Al

The adsorption/desorption isotherms of the SBA-15Al and Co/SBA-15Al supports are shown in figure 1 and figure 7. The shape of the isotherms is divided into four parts: The first part of the isotherm of these materials is at a relative pressure (P/P₀) between 0 and 0.2, where there is the completion of the monolayer. The second part of the isotherm is at a relative pressure (P/P₀) of 0.2 to 0.4 for MCM-41Al and 0.2 to 0,55 for Co/MCM-41Al, which is due to the formation of multilayers. The third part of the isotherm is at a relative pressure (P/P₀) between 0.4 and 0.9 for the SBA-15Al support, and 0.55 to 0.92 for the Co/SBA-15Al catalyst. The type of hysteresis loop that is observed in both of the materials is of H2 type. The deformation of the isotherms of the SBA-15Al support compared to the Co/SBA-15Al material might be due to the presence of uniform pore mesostructures.

MCF10Al and Co/MCF10Al

The adsorption/desorption isotherms of MCF10Al support is shown in figure 1 and figure 7. The shape of the isotherms is divided into four parts: The first part of the isotherm is at a relative pressure (P/P₀) between 0 and 0.15. The second part of the isotherm is at a relative pressure (P/P₀) of 0.15 to 0.7 for MCF10Al and 0.15 to 0.6 for Co/MCF10Al, which belongs to the formation of multilayers. The third part of the isotherm is at a relative pressure (P/P₀) between 0.7 and 0.94 for the MCF10Al support, and 0.6 to 0.88 for the Co/MCF10Al catalyst. The hysteresis loop is similar to a H1 type. These isotherms tend slightly to the H3 type, generating a deformation of the isotherm compared to the Co/MCF10Al support isotherm that may be due to the arrangement of cobalt particles.

Figure 7 N2 Adsorption-desorption isotherms and pore size distribution: mesoporous supports with Co 

In table 3 and according to figure 7, a summary of the results of the nitrogen desorption analysis of the cobalt catalysts Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al can be observed, i.e., the surface area, the pore volume, the internal pore size distribution are shown. The average window pore sizes of Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al are 2.2 nm; 5.4 nm and 7.7 nm respectively. The average internal pore size of Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al is 2.6 nm; 8.9 nm and 9.8 nm, respectively. The surface area calculated by the BET method, for Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al is 664.6; 647.5 and 422.1 m²/g respectively. These surface area results are consistent with the slope in stage 2 of each of the catalyst isotherms. The increase in the specific surface area of the Co/MCF10Al support with respect to MCF10Al might be due to the breakdown of the support structure. And also, this may be due to Cobalt oxide Co₃O₄ nanoparticles on the surface of the support that are not large enough to cover the pores completely, as occurs with Co/MCM-41Al and Co/SBA-15Al. This, increases the surface area of the support with the surface of the Cobalt oxide Co₃O₄ nanoparticles.

Table 3 Surface area, pore volume and pore diameter size distribution of MCM-41, SBA-15Al, MCF10Al, Co/MCM-41Al, Co/SBA-15Al and CoMCF10Al 

X-Ray Diffraction (XRD)

X- Ray Diffraction of cobalt supported catalysts

Figure 8 shows XRD profiles of the cobalt supported catalysts in mesoporous silicoaluminate, which are Co/MCM- 41Al, Co/SBA-15Al, Co/MCF10Al. 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) of ≈ 31.1°; 36.9°, 45.0°; 59.4° and 65.4° (JCPDS: 073-1701). 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 ^2),(5. These correspond to the cubic system centered on the face of the spinel Co₃O₄^15),(20. The Cobalt Co/MCF10Al catalyst has the most intense and sharp cobalt oxide characteristic signals, compared to the Co/SBA-15Al catalyst. In turn, the Co/SBA-15Al catalyst has the cobalt oxide Co₃O₄ characteristic signals more intense and acute than the Co/MCM-41Al 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/MCF10Al> Co/SBA-15Al > Co/MCM-41Al. The most intense signal for these materials is at 2θ ≈ 36.9°. This peak is associated with the plane (3 1 1) ²⁰. In effect, the crystallite sizes of Co₃O₄ and Co⁰ calculated from the Scherrer equation ^25),(26 follows the sequence: Co/MCF10Al>Co/SBA-15Al>Co/MCM-41Al (see table 4).

Figure 8 XRD profiles of mesoporous silicoaluminate cobalt catalysts: Co/MCM-41Al, Co/SBA15Al and Co/MCF10Al 

In table 4 and according to figure 8, the crystalline microdomain size of the cobalt particles found in the mesoporous silicoaluminate supports is observed. This parameter was calculated with the Scherrer equation ^25),(26, which is represented as the diameter of the cobalt crystals in nanometers (nm). This parameter was calculated for the Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al catalysts. The size of the crystalline microdomain increases when the pore size of the mesoporous supports is larger (see table 4[25]. The crystalline microdomain size distribution has the following sequence: Co/MCF10Al> Co/SBA-15Al> Co/MCM-41Al; 13.3nm> 11.8nm> 10.0nm, 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 because the particles of larger metal clusters found on the surface of the support particles produce an increase in the particle size distribution of Co₃O₄ and Co^{0 (3}.

Table 4 Crystallite size from XRD of Cobalt particles from the Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al catalysts 

Temperature Programmed Reduction (TPR)

Figure 9 shows the Programmed Temperature Reduction signals of the Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al catalysts. The reduction signals of the three catalysts are divided into two well-marked regions. The first at low temperatures between 280°C and 580°C, and the second is between 610°C and 900°C. The signals at low temperatures represent the reduction reactions from Co₃O₄ to CoO and from CoO to Co⁰. On the other hand, the signals at high temperatures represent the cobalt oxide and minor phases such as silicates and cobalt silicoaluminates that have a strong interaction with the support ^1),(2. The deconvolution at low temperature shows one signal for the reduction from Co₃O₄ to CoO in agreement to the study performed by Claure M. et al. ⁴, and also, two signals for the reduction from CoO to Co⁰. Cobaltous oxide, CoO, is reduced in two stages: the first reduction is produced on the surface of the cobalt particles, and the second reduction stage is produced on the cobalt that is close to or that has an interaction with the surface of the support ^1),(4.

The degree of H₂ consumption in the catalysts in the region of low temperature has the following order: Co/MCF10Al > Co/SBA-15Al > Co/MCM-41Al. The signal for the Co/SBA-15Al is displaced at lower temperatures compared to the other two catalysts. The reduction signal at low temperatures for the Co/MCF10Al catalyst represents the highest H₂ consumption and also, this catalyst has the widest peak at a high temperature. The order of the catalysts for the H₂ consumption in the region at high temperature, from highest to lowest is: Co/MCM-41Al> Co/MCF10Al> Co/SBA-15Al. The reduction signal for Co/SBA-15Al has the lowest intensity. The reduction peak for the Co/MCF10Al has the widest signal at a high temperature compared to the other catalysts..

Regarding the extent of each reduction reaction, the reduction from Co₃O₄ to CoO (temperatures) is similar in the three catalysts as is shown in Table 5. More CoO is reduced to Co⁰ in the Co/MCF10Al catalyst compared to Co/SBA-15Al and Co/MCM-41Al catalysts (table 5) specially at low temperature (L) (see Fig. 9). Thus, the reducibility degree decreases in the following order: Co/MCF10Al> Co/SBA-15Al> Co/MCM-41Al. The latter is an important parameter for the application in catalytic reactions such as the Fischer-Tropsch synthesis, i.e., the higher reducibility the higher the activity of the catalyst

Table 5 and figure 9 show the summary of the H₂ consumption data and the deconvolution of the TPR results of the cobalt catalysts Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al. For the reduction of Co₃O₄→CoO, there is no difference of the signal intensity between the three catalysts. For the CoO→ Co⁰ reduction at low temperatures (L), there is an order of the catalysts for the H₂ consumption as follows: Co/MCF10Al> Co/SBA-15Al> Co/MCM-41Al. For the CoO →Co⁰ reduction at high temperatures (H), there is an order of the catalysts for H₂ consumption as follows: Co/MCF10Al> Co/SBA-15Al> Co/MCM-41Al. For the cobalt silicoaluminous zone, the reducibility degree decreases in the following order: Co/MCM-41Al> Co/MCF10Al> Co/SBA-15Al. The cobalt catalyst Co/MCF10Al has the highest degree of reduction with 44.1%, followed by the Co/SBA-15Al catalyst with 39% and the Co/MCM- 41Al catalyst that has the lowest reduction percentage of 31%.

The SBA-15 catalyst has larger pore sizes of hexagonal cylindrical shapes, and also a smaller surface area. These pores have a lower proportion of aluminum silicoaluminates than the MCM-41 support. These differences are attributed to the higher surface area and small pore diameter size of the MCM-41, which favors the formation of very small particles that can easily interact with -OH species present in MCM-41 silica. Cobalt-support interactions in Co/MCF are attributed to the more open network of the silica support which favors the diffusion of Co ions during impregnation and calcination stage. Thereafter, these cobalt ions might reach the window pores in the cell spheres where the probability of interaction with the silanol groups is higher due to the small window pore sizes.

Co/MCM-41Al and Co/SBA-15Al catalysts showed different degrees of reduction. One explanation might be the pore diameter of the silica support. It was reported that smaller pore diameters in mesoporous silica likely form and stabilize smaller cobalt oxide particles with higher dispersion. Nevertheless, the pore diameter of the silica support material is not the only possible explanation for diverse reduction properties ^2),(3. Some authors tried to explain the difference between MCM-41Al and SBA-15Al and found that the pore surface consists of regularly arranged isolated surface SiOH groups ^{8), (9}. These OH species, in parallel direction to the cylindrical pore axis, are absent in MCM-41. On the contrary, the inner surface of SBA-15 contains isolated and interacting SiOH groups able to form hydrogen bonds with other OH groups as well as geminal Si(OH)₂ groups with all of them pointing in all directions of space. Pore models have shown that the inner surface of SBA-15 is substantially “rougher” than the pore surface of MCM-41 ²⁷. Therefore, it is reasonable to think that due to the rougher inner surface of the SBA- 15 support, the interactions of cobalt particles with silica surface sites in the Co/SBA-15 catalyst are weaker and, as a result, less cobalt-silicate is formed.

Figure 9 Temperature- Programmed Reduction (TPR) Profile for reduction of catalysts: a) Co/MCM-41Al, b) Co/SBA-15Al y c) Co/MCF10Al 

Consequently, the Degree of Reduction (%Reducibility degree) is higher for Co/SBA-15Al than for the MCM-41Al support. It is interesting to note that the degree of reduction is also high for Co/MCF10Al even though it has a high cobalt-silicate formation. The explanation given to this fact is the pore structure and size of the silica. It was reported that water produced during the reduction process (TPD) oxidizes the Co⁰. This effect is more pronounced in catalysts with small and long pores due to water accumulation inside the pores. However, this is not the case for the Co/MCF10Al which has large 3D spherical pores. Moreover, the produced water is easier to diffuse through this network and possibly, there might be less Co° oxidation, which can increase the degree of reduction. ¹¹

Table 5 Hydrogen consumption of cobalt catalysts: Co/MCM-41, Co/SBA-15Al y Co/MCF10Al (based on Co°) 

Cluster accumulation on the catalyst surface and by pore size

Cobalt clusters are accommodated in two different ways on mesostructured or amorphous materials by the method of incipient Wetness ⁸. There is a deposition of cobalt clusters on the surface of the support particles, and another fraction is deposited according to the pore size. There are also small clusters that can be deposited indoors inside the pore interstices. So, according to the diffractograms, and using the Scherrer equation, it can be seen that the average size of the cobalt particles is somewhat larger than the diameter of the pores. Ordering the catalysts as a function of their particle size distribution from highest to lowest we have: Co/MCF10Al˃ Co/SBA-15Al˃ Co/MCM-41Al

Cobalt-support TPR interaction

The support-metal interactions mainly depend on the size of the pores, the type of pore, the angle of the porous cavity, the type of surface acidity.

Temperature Desorption Programmed (TPD) TPD of Co/MCM-41Al catalyst

The TPD profiles for the mesoporous silioaluminate Co/MCM-41Al catalyst (see figure 10 (a)) are similar to the profile of the MCM-41Al support (see figure 6 (a)). At low temperature (≤ 450°C) three signals were identified as compared to literature: at 217 °C a signal for Lewis weak acidity sites, at 285 °C a signal for Brønsted weak and medium acidity sites, at 365°C a signal for Brønsted strong acidity sites. At high temperature (≥ 450°C) a signal corresponding to Lewis strong acidity sites was found at 600°C, and an unidentified signal at 750C was found. The latter signal is notably increased by the presence of Cobalt (Co/MCM-41Al catalyst), which might be attributable to Lewis strong acidity of Co° ²¹. Also, when cobalt is added to MCM-41Al, the total amount of acid sites is increased by three hold as is shown in Table 4. In addition, the dispersion of acid sites is increased by approximately six times when cobalt is added

According to the TPD of the MCM-41Al support (see figure 6 a), this material has a majority fraction of weak Brønsted acid sites and Lewis weak and medium acid sites. This would favor a lower metal-support interactions. Because of this, it would be expected to have a reduced metallic cobalt fraction at a lower temperature, which is true and is shown in Figure 10. However, there are also particles or clusters that were deposited in internal pores cavities and/or had a strong interaction with strong acids. This may be verified by the second band of reduction present at temperatures greater than 600 °C. These signals may be due to cobalt silicoaluminates formed by these strong interactions.

TPD of Co/SBA-15Al catalyst

The TPD profiles for the mesoporous silioaluminate Co/MCM-41Al catalyst (see figure 10 (a)) are similar to the profile of the MCM-41Al support (see figure 6 (a)). At low temperature (≤ 450°C) three signals were identified as compared to literature: at 217 °C a signal for Lewis weak acidity sites, at 285 °C a signal for Brønsted weak and medium acidity sites, at 365°C a signal for Brønsted strong acidity sites. At high temperature (≥ 450°C) a signal corresponding to Lewis strong acidity sites was found at 600°C, and an unidentified signal at 750C was found. The latter signal is notably increased by the presence of Cobalt (Co/MCM-41Al catalyst), which might be attributable to Lewis strong acidity of Co° ²¹. Also, when cobalt is added to MCM-41Al, the total amount of acid sites is increased by three hold as is shown in Table 4. In addition, the dispersion of acid sites is increased by approximately six times when cobalt is added.

Unlike MCM-41Al, this support has a higher proportion of acidic sites on the surface, with similar pore size, but with a larger size. This acidity causes greater metal-support interactions. For this reason, the reduction in TPR is shifted to the right, requiring a higher temperature for its reduction. However, it has a greater degree of reducibility. The reason may be that at larger pore sizes, the metal particles are larger and easier to reduce.

TPD of Co/MCF10Al catalyst

The TPD profiles for the mesoporous silioaluminate Co/MCF10Al catalyst (see figure 10 (c)) shows similar profile to the one of the MCF10Al support (see figure 6 (c)). Three signals are identified as compared to literature: at 215°C a peak for weak acidity sites, at 450°C a peak for medium acidity sites, at 795°C an unidentified peak that it may be of strong acidity sites, and an unidentified signal at 800°C was found. The latter signal is notably increased by the presence of Cobalt (Co/MCFAl catalyst), which might be attributable to Lewis strong acidity of Co°. Also, when cobalt is added to MCF, the total amount of acid sites is increased by 63% as is shown in Table 6.

Co/MCF10Al catalyst as in the case of Co/SBA-15Al catalyst, has a large number of strong acids, which increases the metal-support interactions. However, according to the average size of Co particles, Co/MCF10Al has the largest average pore size distribution among the two previous supports. What makes it possible is that despite having a strong support-metal interactions, its cobalt particles are larger, and mostly reducible at a lower temperature and with a greater degree of reducibility.

Table 6 shows the data base of the deconvolution study of the TPD analyzes of the catalysts according to the type of mesoporous (Co/MCM-41Al, Co/SBA-15Al and Co/MCF10Al). The catalysts have a higher amount of ammonia adsorbed per gram according to the following order Co/MCF10Al> Co/MCM-41Al> Co/SBA-15Al. The contribution of acidity due to the presence of cobalt (Co⁰) active sites has the following order: Co/MCM-41Al> CoMCF10Al> Co/SBA-15Al. Similarly, the dispersion of acid sites per square meter of catalyst area (μmol NH₃/m²) has the following order: Co/MCF10Al > Co/MCM-41Al > Co/SBA-15Al.

Table 6 Adsorption capacity of the mesoporous supports and catalysts 

¹μmol of NH3 per gram/ specific surface area (g/m²) ²μmol of metal acidity sites per gram of catalyst

For the series of mesoporous type materials supports (MCM-41Al, SBA-15Al, and MCF10Al) and Catalysts (Co/MCM-41, Co/SBA-15, and Co/MCF), 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 these series depends on the amount of Aluminum inserted in the matrix and the route of synthesis for each one. In the XRD diffractograms of the cobalt catalysts in these 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. The mesoporous supports and catalysts synthesized by the atrane route show different weak, medium and strong acid species of Lewis and Brønsted. This depends on the amount of Aluminum added, the pore size and the structure being of 1-Dimention, 2-Dimention and/or of 3-Dimention.

Figure 2. Desorption Temperature-Programmed TPD Profile of catalysts: a)Co/MCM-41Al, b)Co/SBA-15Al and c)Co/MCF10Al

The addition of cobalt to these matrices results in an increase of the acidity of the strong, 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 Co/MCF10Al 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.