Structural incorporation of titanium and/or aluminum in layered silicate magadiite through direct syntheses
Graphical abstract
Highlights
► Well-crystallized magadiite containing structural aluminum and titanium were hydrothermically synthesized. ► Systematical studies on crystallization times and metal amount. ► Use of multivariate analyses methods for improving synthetic data.
Introduction
Crystalline metalosilicate materials have many different applications in sorption and catalysis, mainly when the precursor silicate combines other metals in a silicon oxide structure, to yield products that enhance involvement in several applications, not only from the scientific but also from the technological point of view. The high degree of improvement comes from the self-organized structured material, in which the attached layers have inserted metals with introduction of active centers, producing, for example, selective catalytic properties [1], [2], [3], [4].
Depending on the metal and silica source and the synthetic methodology used, a great variety of structures can be obtained, with many different properties and, consequently, many potential applications [5], [6], [7]. So, these new materials can be designed for specific purposes, such as contaminant degradation or sorption from aqueous solutions and also catalytic applications, which is an important feature for this kind of material, concerning their environmental and economic relevance [1], [2], [3], [8].
Among many possible inorganic structures, synthetic layered materials are of particular interest, owing to their properties, including thermal and mechanical stabilities and swellability in polar solvents. Other advantages associated with these synthetic materials over natural ones are high purity, homogeneity and controlled textural properties [9], [10].
Magadiite, a crystalline hydrated lamellar silicate with Na2Si14O29.9H2O composition occurs naturally [11] and was successfully synthesized in laboratory a half century ago [12], but its self-organized structure has not yet been completely solved [13]. This material is an important scientific and technological interest, due to properties related to ionic exchange, sorption, lamellar swelling etc [14]. The isomorphic substitution of silicon by a metal provides measurable changes in the silicate's properties [15] and the distribution of the metals is very important for several applications [16]. Layered hydrated silicates were also used as reactants in the synthesis of zeolites by recrystallization processes, as in the transformation of kanemite into beta zeolite with various aluminum concentrations [17]. On the other hand, titanosilicate layered materials such as JDF-L1 and AM-4 have good performance for actinide removal in salt solutions [18] and for catalytic oxidation [19].
Other investigations in the same direction applied layered silicate materials to yield zeolites and zeotypes [20], [21]. Also, some metalosilicates were directly synthesized with isomorphic substitution methodology [22], [23]. The importance of this approach is to obtain zeolite crystals with unusual morphologies, some of them displaying better access to the internal parts and, consequently, more effective catalytic sites. Surprisingly, in some cases, the layered materials themselves present even better catalytic properties than the corresponding analogous zeotypes [23].
In attempting to optimize materials for some applications and also to build a complete series of properties map for each condition, a large number of synthetic samples are normally required. From many characterization techniques there are information not easily detected by usual data treatments. To better interpret the multivariate approaches are powerful tools when associated with other kinds of characterization and technical analysis in many fields, such as food sciences, analytical chemistry, biology etc [24], [25], [26]. On the other hand, material science seems to be a forsaken area concerning the association of these results with multivariate analysis methods as there are only rare publications that apply this kind of procedure [27].
The present investigation deals with the synthesis of magadiite derivatives containing titanium and/or aluminum atoms in the crystalline network through a very simple route, without template, using a systematic study. The main focus is related to the short time of crystallization at 423 K, for 18–72 h, and the amount of metal source. The new layered compounds obtained from this original and simple synthetic procedure have been characterized with many techniques and their physicochemical properties are also discussed. The structural features were supported by chemometric analysis from X-ray diffraction and fluorescence data. To our knowledgement this is the first study associating these characterization techniques, applying the PCA method, for materials science.
Section snippets
Syntheses
Magadiite in the sodium form was synthesized by reacting silica in alkaline conditions by the hydrothermal procedure [14]. Briefly, a stirred suspension of silica gel (Aldrich) (8.0 g, 0.13 mol) and sodium hydroxide (2.4 g, 0.060 mol) in 54.0 cm3 (3.0 mol) of water was allowed to react under static conditions at 423 K for 18 up to 72 h in a Teflon-lined stainless steel autoclave. The solid (Na-mag) obtained was washed with water to remove the excess of base and air-dried at room temperature.
The
Structural features
The slow addition rate of reagents is an important feature to improve silicate crystallization. The resulting apparent material consisted of the magadiite phase formed in good yields. However, sometimes fragile monoliths were obtained. This type of irregular crystallization is associated with the short hydrolysis time of titanium isopropoxide under basic conditions [28]. Due to this behavior, a careful and slow addition is required to obtain a pure magadiite phase with titanium incorporated
Conclusions
The set of data supplied for the diverse analyses for magadiite samples demonstrates that the isomorphic substitution of silicon of well-structured magadiite by titanium and aluminum can be carried out. However, when higher titanium contents are used, the same success was not observed. A crystallization kinetics study demonstrated that phase transitions, amorphous, magadiite and more stable thermodynamic phases, such as trydimite and crystobalite and mixtures of them, can be formed. Metal
Acknowledgments
The authors are indebted to CAPES, FAPESP and CNPq for fellowships and financial support, and also to LME-LNLS for TEM measurements.
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