- Citado por SciELO
versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003
J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324
MESOPOROUS ALUMINOSILICATE MOLECULAR SIEVE: CONTROL
OF TEXTURAL PROPERTIES BY POST-SYNTHESIS HYDROTHERMAL REATMENT
A. Hidrobo, J. Retuert, P. Araya
Centro para la Investigación Multidisciplinaria Avanzada en Ciencia de los Materiales (CIMAT) and
Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile,
Av. Beaucheff 850, Casilla 2777, Santiago, Chile. firstname.lastname@example.org
(Received: February 28 2003 - Accepted: April 30 2003)
By means of sol-gel synthesis with silicon and aluminium alkoxides, Triton X-114, and hydrothermal stabilization at various times and temperatures, were obtained aluminosilicates which have a porous structure (MSA) and are thermally stable and partially ordered. The as-synthesized MSA samples were subjected to hydrothermal treatment (HT) from 6 to 96 hours at 150 °C and at 110 to 175 °C for 6 hours to determine the effect of time and temperature on the structure of the final aluminosilicate. All the materials obtained are mesoporous, showing changes in their structural properties: surface area, pore diameter, and mesopore area and volume in direct relation to temperature and HT time.
Key Words: Mesoporous molecular sieve, aluminosilicate, sol-gel, hydrothermal treatment, catalytic support.
Mesoporous molecular sieve silicas with wormhole framework structures (MSU-X and HMS)1 are generally more active catalysts compared to their hexagonally arranged analogs (MCM-41, SBA-15).2,3 Their reactivity is partly attributed to the three-dimensionally connected porous framework, which allows enhanced access of the molecules into the structure of the material.
These catalysts have been synthesized using alkoxides as precursors of the silicon component of the framework, and surfactants as pore modeling agents. In recent years similar materials have been obtained with the participation of primary amines and block copolymers as tools for the production of mesoporous structures.4,5 However, siliceous materials have low acidity, and therefore the framework is modified by incorporating heteroatoms into the electrically neutral silicate structure. These elements can be aluminium, titanium, magnesium, etc. Materials that include, for example, aluminium in their structural framework have acidic sites with medium and high acid strength, so they are considered for application as potential acid catalysts in reactions such as cracking, hydrocracking, etc.6
In the literature there is abundant information on mesoporous materials made from Si10-15 or with Ti incorporated into the silicate framework.16-18 Similarly, several materials have been synthesized recently which incorporate Al into the structure of the mesoporous materials. Such are the mesoporous aluminosilicates (Al-MMS) obtained using primary amines as templates19,20 and those synthesized with block copolymers or with polyethylene oxide surfactants.21 These mesoporous aluminosilicates (Al-MSU, Al-HMS) are obtained by gel formation at room temperature or by hydrothermal synthesis at autogenous pressure and low temperature.
This paper continues this line of research on the synthesis of mesoporous aluminosilicates using polyethylene oxide surfactants (Triton X-114). This work attempts to clarify the role of the hydrothermal treatment (HT) on the synthesis of aluminosilicates. The solids obtained by the sol-gel method are subjected to HT, varying either the temperature or the time of treatment.
2.1. Synthesis of materials
Aluminium tri-sec-butyrate (ATB) and tetraethyl orthosilicate (TEOS) were used as sources of aluminium and silicon, respectively. TEOS is hydrolized over the foam formed by the surfactant and water. The pH is adjusted to 1-2 by adding drops of nitric acid, and the mixture is stirred for 1 hour. When the system has produced abundant foam, NH4OH is added dropwise, increasing the stirring speed to 900 rpm, until pH 9 is reached, and then the ATB is added. The solution is stirred for 20 additional hours, when a white fine gel is formed. The system is allowed to rest overnight and then the sample is filtered, washed with distilled water until pH 6 is reached, and allowed to dry. The sample is divided into two parts: one of them is calcined directly at 550 °C and the other is subjected to HT at specified temperatures and times.
In order to study the effect of time and temperature on the structure of the final aluminosilicate, the MSA samples are subjected to HT in two groups. One is treated for periods of 6 to 96 hours maintaining the temperature constant at 150 °C, and the other is treated at temperatures between 110 and 175 °C, keeping the treatment time constant.
Nitrogen adsorption/desorption isotherms at 196 °C were obtained on a Micromeritics ASAP 2010 apparatus. Specific surface area was determined from the linear part of the BET equation (P/P0 = 0.05 - 0.15). Mesopore size distribution was calculated from the desorption branch of the N2 adsorption/desorption isotherms and the BJH formula. Mesopore surface area, SBJH, and mesopore volume, VBJH, were obtained from the pore size distribution curves. Average mesopore diameter, DBJH, was calculated as 4 VBJH / SBJH.
X-ray diffraction patterns were obtained with a Siemens D5000 (40 KV, 30 mA) diffractometer using CuKa radiation (l = 1.542 Å).
3. Results and Discussion.
With the purpose of determining whether the amount of aluminium present in the samples had an influence on the properties of the final solid, mesoporous aluminium silicates were synthesized with two different proportions of aluminium in the framework. The Si/Al ratios with which the solids were prepared were 7.5, labeled as SA1-0, which incorporates an important Al proportion, and 55, labeled as SA2-0, with a much smaller proportion of Al in the framework.
Powder XRD patterns of the calcined samples, which have a single peak at low angles, are shown in figure 1. Similar XRD spectra have also been observed for this type of materials.4,20,22,23 Additional peaks at higher angle reflections are not detected. The d100 spacings corresponding to reflections at low angles, are 4.2 and 4.3 for SA1-0 and SA2-0, respectively (2q = 2.05 and 2.1). It is seen that the d100 spacing of the samples decreases by increasing the aluminium incorporation. Further, SA1-0 presents a reduction in intensity and a broadening of the peak, suggesting a decrease in the order. This behavior has been reported also for aluminosilicate MCM-41, Al-MMS (aluminosilicate mesoporous molecular sieve) and Al-MTS (Al-micelle templated silica).20,23-25 The unit cell dimension (a0) of the materials, assuming hexagonal symmetry, also decreases with the aluminium content; a0 decreases from 5 to 4.8 nm. Thus, we conclude that sample SA2-0, with the lowest aluminium content, has a higher degree of partial order. However, when the synthesized solids (not calcined) are subjected to HT treatment, no XRD peak is seen, showing that this treatment breaks the structural order of the aluminosilicate framework. This probably occurs because the HT involves a structural change by a dissolution/reprecipitation mechanism, which causes the collapse of the arrangement.11
Table 1 shows the surface area, pore volume and average pore size of the aluminosilicates. In agreement with previously reported similar mesoporous aluminosilicates, it is seen that as the proportion of aluminium incorporated increases, the surface area decreases, while pore volume and average pore diameter increase. This is probably due to partial collapse of the structure during calcination to remove the template, and is caused by the instability associated with the presence of increasing amounts of framework aluminium, so pore size increases and therefore surface area decreases.20,26,27
* a0 = the lattice parameter. Calculated from the XDR data using a0 = 2 d100 / (3)1/2
If the materials are subjected to HT treatment, substantial changes occur in their structural properties. For SA1, with Si/Al = 7.5, the surface area goes from 1045 m2/g (0 h of HT) to 582 m2/g (6 h), 470 m2/g (12 h), 367 m2/g (24 h) and 367 m2/g (48 h of HT) (see Fig 2.b). The HT was carried out at a constant temperature of 150 °C. As stated earlier, the surface area decrease implies increased pore size, which occurs in the range of 38 to 113 Å, while pore volume remains essentially constant at 1.4 cm3/g.
On the other hand, the SA2 sample (Si/Al = 55) shows very similar results, except that the surface area of the untreated material is slightly higher than that of SA1 due to its lower Al content. Table 1 shows the changes in surface area, pore size and pore volume.
Figure 2 illustrates the N2 adsorption/desorption isotherms for the synthesized materials. The calcined samples SA1-0 and SA2-0 (Fig 1.a.1) display a well-defined step in the adsoption isotherm at P/P 0 0.45 0.6 and a hysteresis loop in the desorption isotherm over the same relative pressure range. This feature result from the condensation of the adsorbate within the framework-confined mesopores.13 This hysteresis loop is indicative of mesoporosity. The hysteresis loop at high partial pressure (p/p0 > 0.8) is associated with textural mesoporosity or macroporosity. In the samples, therefore, the lack of textural mesoporosity is indicated by the absence of a hysteresis loop at p/p0 > 0.8. However, when the samples are subjected to HT they have a type IV isotherm. The isotherms show large type H1 hysteresis (ink-bottle-type connected pores), and a development of some macroporosity occurs; this is in agreement with the hysteresis loop at medium and high partial pressure (see Fig. 2.a.2 and 2.a.3) and suggests a shift of pore size to higher values. 10,20
Fig. 2. a) Nitrogen adsorption and desorption isotherms for SA2:
2) hydrothermally treated 12 h, and 3) hydrothermally treated 96 hours.
b) Influence of HT (150 ¥C) time on surface area of mesoporous aluminosilicates.
In Fig. 3 it is seen that the materials undergo an important increase in pore diameter, starting with 31 Å when SA2 is calcined directly (Fig. 3a) and going to 99 Å when it is subjected to a 48-h HT. It is also seen that as HT time is increased beyond 48 h, the aluminosilicate (SA2H150) tends to stabilize, since it does not undergo further increase in pore size, which finally reaches 106 Å (Fig. 3c and Table 1). Looking at the comparative pore size vs. HT time curves for SA1H150 and SA2H150 (Fig. 3 insert), it is seen that the two curves have the same tendency, indicating that the materials behave similarly in spite of having different proportions of Al in their framework. The same conclusion is reached with respect to the influence of HT time on the surface area of SA1H150 and SA2H150, for which it is seen (Fig. 2b) that the area decreases sharply during the first hours of HT, reaching a plateau after 48 hours of treatment. Thus, SA1H150-48 has a surface area close to 400 m2/g and SA2H150-X (where X is 48 or 96, depending on the HT time) exceeds 400 m2/g (see table 1).
Fig. 3. Pore size distribution determined from the N2 desorption isotherms for SA2: a) calcined. b) hydrothermally treated 12 h, and c) hydrothermally treated 96 hours.
This paper shows the substantial influence of hydrothermal treatment on mesoporous aluminosilicates synthesized by the sol-gel method with different aluminium proportions, using polyethylene oxide type surfactant (triton X-114) as template. It was found that both HT temperature and time have a large influence on the structural properties of the solids. The study shows that it is possible to model pore size and volume of the aluminosilicates by changing the time and/or temperature variables of the HT. This can be of a great importance for the design of selective supports, molecular sieves or catalysts. In any case the materials obtained have large surface area, with pore sizes between 30 and 110 Å and pore volumes between 0.6 and 1.6 cm3/g.
It was also possible to obtain a material with high hydrothermal stability, since it retains a surface area greater than 450 m2/g even after 96 h of HT. It also has an extraordinary pore diameter (106 Å) and volume (1.6 cm3/g), making it potentially useful as a support for either metals or zeolites in applications such as catalytic cracking of heavy petroleum fractions. It is believed that the material would allow an adequate diffusion of the species to the active sites of the incorporated zeolites to achieve the cracking of high molecular weight molecules. Active research along these lines is currently taking place in our laboratory.28
The authors acknowledge the financial support of Conicyt (Fondap Project 1198000-2, and Fondecyt Projects 8000015 and 1010525). A.H. thanks the German Academic Exchange Service (DAAD) for a Ph.D. scholarship.
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