Characterization and acidic properties of Al-SBA-15 materials prepared by post-synthesis alumination of a low-cost ordered mesoporous silica
Graphical abstract
Al KLL spectra of Al-SBA-15 materials with different Si/Al ratios.
Introduction
Ordered mesoporous silica was first reported independently by Mobil scientists [1] and by Kuroda's group [2], [3] in the early 1990s. These materials are characterized by a regular array of pores, in the 2.0–10.0 nm range, with uniform diameter, high specific surface area, and pore volume which are advantageous for the adsorption and catalytic conversion of bulky molecules. The number of research papers dealing not only with mesoporous silica, but also with other oxides, such as alumina, titania, and zirconia, has grown tremendously during the last decade [4]. The vast majority of these papers deal with syntheses from organosilicates as silica source, which are quite expensive. As a result, economic considerations have recently triggered an interest in the use of an inexpensive inorganic silicate as starting material, in order to increase the number of viable applications for the material.
On another front, a great variety of cationic, anionic, and neutral surfactant molecules have been used in the synthesis of ordered mesoporous silica- or non-silica [5], [6]. Non-ionic surfactants give important commercial advantages in comparison to ionic surfactants. They are easily removable, non-toxic, biodegradable and relatively cheap. Pinnavaia et al. [7] have reported on the synthesis of MSU-X mesoporus materials with several non-ionic surfactants. Stucky et al. [8] have studied the synthesis of a highly ordered hexagonal mesoporous silica SBA-15 with ultralarge d(100) spacings of 10.4–32.0 nm by using poly(alkylene oxide) triblock copolymer. SBA-15 has BET surface areas of 690–1040 m2 g−1, large pore sizes of 4.6–30.0 nm, and unusually large pore volumes of up to 2.5 cm3 g−1, with silica wall thicknesses ranging from 3.1 to 6.4 nm. The improved hydrothermal and thermal stability make them some of the most promising catalytic materials. However, the synthetic procedures are not as commercially viable due to the use of tetraethyl orthosilicate as a silica source. Roziere et al. [9] prepared mesoporous monolithic silicas and aluminosilicates using commercial non-ionic surfactants as structure-directing agents; and large mesoporosus organosilicas using triblock copolymers as structure-directing species [10]. Guth et al. [11] have, recently, reported the synthesis of mesoporous silica material based on the use of non-ionic surfactant and sodium silicate, but the materials obtained exhibit irregular or disordered channel connectivity and broad pore size distributions. More recently, Kim and Stucky [12] have developed a new synthetic method for highly ordered mesoporous silica using sodium metasilicate as a silica source and amphiphilic block copolymers as the structure-directing agents.
Moreover, the substituents such as aluminum, titanium and zirconium can be incorporated into the silica framework to obtain materials for applications such as catalysis and ion exchange. For this reason, we recently prepared a series of zirconium-doped silica with different Si/Zr molar ratios using a sol–gel methodology, starting from inorganic Zr and Si precursors, different to alkoxides, and using polyethyleneoxide as a non-ionic surfactant template structure-directing agent [13]. These materials are excellent candidates to be used in heterogeneous catalysis as both acid catalysts and supports [14], [15]. Among the metal-substituted mesoporous materials, aluminum-incorporated mesoporous materials have great potential in moderating acid-catalyzed reactions for large molecules [16], [17], [18]. However, it is very difficult to introduce the metal ions directly into SBA-15 due to the easy dissociation of metal-O–Si bonds under strong acidic conditions. In fact, to date, only a few studies on the direct synthesis of Al-SBA-15 have been reported [19], [20], [21], [22], [23], [24]. Thus, the post-synthesis method for the alumination of the mesoporous silicas, that are obtained under strongly acid conditions, becomes an appealing alternative [25]. Many studies have shown that aluminum can be effectively incorporated into siliceous MCM-41 and MCM-48 materials via various post-synthesis procedures. The authors claimed that the materials produced via the post-synthesis method have superior structural integrity, acidity, and catalytic activity than those of materials having aluminum incorporated during synthesis [26], [27], [28], [29]. However, as of present, few post-synthesis alumination methods for SBA-15 have been reported [30], [31], [32]. Recently Kao et al. [33] have shown an efficient route for synthesizing SBA-15 with high-framework aluminum contents (a Si/Al ratio close to 5), without any significant loss in textural properties of SBA-15, by the treatment of pure silica SBA-15 with an aqueous solution of (NH4)3AlF6 at room temperature. Zeng et al. [34] have also recently shown that the pH and aluminum concentration in the aqueous solution are important parameters in the preparation of Al-SBA-15 materials by post-synthesis method because they have a direct effect on the acidity of the resulting material.
Here we present the synthesis, characterization and the evaluation of the acid properties of a series of Al-SBA-15 materials with different aluminum content. The preparation of the mesoporous silica was based on the procedure proposed by Kim and Stucky [12] by using sodium silicate as a cheap inorganic silica source and a triblock copolymer (Pluronic P123) as a non-ionic structure-directing agent, but introducing some modifications. A series of Al-SBA-15 materials, with different Si/Al ratio were obtained by post-synthesis alumination as proposed by Zeng et al. [34]. The materials were fully characterized by different physico-chemical methods showing a high-framework aluminum content (up to a bulk Si/Al ratio of 5.5) and good structural integrity. The concentration of acid centers was found to depend on the amount of aluminum incorporated into the siliceous framework. The acid nature of this family of mesoporous solids has been studied by the adsorption of two-probe basic molecules and by catalytic tests of 2-propanol and 1-butene conversion.
Section snippets
Reactants
The silicon source was a sodium silicate solution (Na2Si3O7 with 27% SiO2 and 14% NaOH from Aldrich). Aluminum was incorporated with aluminum chloride (AlCl3·6H2O) from Aldrich. The non-ionic surfactant was triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer Pluronic P123 (Mav=5800, EO20PO70EO20) from Aldrich. Analytical-grade sodium hydroxide from Prolabo and tetramethylammonium hydroxide (TMAOH) 25 wt% solution in water from Aldrich were also used.
Samples preparation
In a
Elemental analysis, XRD, and morphology
The aluminum content of the samples, determined by atomic absorption, is listed in Table 1. The Si/Al ratios obtained were higher than those added in the initial synthesis mixture. The high Al content in the Al-2.5-SBA-15 sample (Si/Al ratio of 5.5) is comparable to that obtained by other post-synthesis methods recently published [23], [32], but by using a Si-SBA-15 obtained with TEOS.
The small-angle powder XRD patterns of calcined Si-SBA-15 and Al-x-SBA-15 materials are shown in Fig. 1. All
Conclusions
A series of low-cost SBA-15 with a high-framework Al content were prepared by treating a pure silica, obtained from sodium silicate, with AlCl3 and TMAOH aqueous solutions. XPS and 27Al MAS-NMR studies have confirmed that this post-synthesis method is a good route to incorporate aluminum to a mesoporous silica SBA-15. Pyridine adsorption results show that this series of materials has both Lewis and Brönsted acid sites and, as shown by NH3-TPD results, the acid centers are mainly medium acidic
Acknowledgments
We gratefully acknowledge the funding of this work by the Ministry of Science and Technology (Spain) Project MAT03-2986 and Project MAT06-02465.
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