Emulsion-mediated synthesis of hierarchical mesoporous-macroporous Al-Mg hydrotalcites
Graphical abstract
Introduction
As well known, hydrotalcites exhibit many valuable features that enable their use as supports, catalysts, or catalyst precursors [1], [2]. These substances are lamellar hydroxides that have the common formula [M2+1-xM3+x(OH)2]x+Az-x/z, where M2+ stands for a divalent cation, M3+ for a trivalent cation, and Az− for the charge compensating anion. Considering the usual Mg2+ and Al3+ hydrotalcites, changes in the relative contents of Al and Mg alter the chemical acid-base and textural properties of the material [1], [3], [4], [5], [6], [7], [8], [9], with established pure and crystalline hydrotalcite structures having compositions in which the molar value of x ranges from around 0.2 to 0.55 [7], [10]. The Al and Mg cations distribute themselves randomly into the layers of crystalline brucite-like structure [3], [10], [11], [12], [13], [14], [15], [16]. At higher values of x, a greater quantity of Al3+ octahedrons leads to the intercalation of larger amounts of anions, and as a consequence, the high density of Al3+ leads to a mixture of low crystallinity oxides. As hydrotalcite-like materials have several applications as support or catalyst, the fine-tuning of their porous structure is highly desirable [17], [18], [19], [20], [21], [22], [23].
These materials are simple and inexpensive to synthesize using a mixture of di- and trivalent salts and aging of the solution until precipitation occurs. The co-precipitation is generally performed with Na2CO3 solutions [17], [18], [19]. Another method, which produces materials with diverse textural properties, involves precipitation of the metal cations in a solution containing urea or acetate anions [17], [18], [22], [24]. Thermal decomposition of the organic precursors used in the synthesis leads to gaseous products that break through the stacked brucite layers and cause exfoliation, consequently creating a mesoporous structure [22], [24]. Alternatively, the synthesis can be performed by a sol-gel transition method. The versatility of the sol-gel method lies in the possibility of the combined use of organic pore templates such as polymers, surfactants, and emulsions [20], [21], [25], [26], [27], [28].
An important feature of the emulsion-mediated synthesis of materials is that in an oil-in-water (O/W) type emulsion, it is possible to use an aqueous dispersion of solids in combination with the sol-gel transition. The entrapment of the dispersed oil droplets by the continuous phase occurs during the sol-gel transition of the system, induced, for example, by a rise in pH [25], [26]. Furthermore, the addition of a surfactant to the emulsion can not only control the sizes of the droplets [27], [28], [29], but also intermediate the kinetic stability of the emulsion due to its presence between the oil-water interfaces, avoiding agglomeration of the isolated droplets [25], [26], [27], [28], [29]. It is easy to see that the method offers a mean of tuning pores of a predetermined size range, controlled by the droplet size. After the removal of the oil droplets by firing, a material with a structured pores arrangement could be obtained.
Emulsion-mediated syntheses are considered by many authors as a great tool for structural control because it is easy to manufacture a wide range of pore sizes. Materials as porous silica, metal oxides, carbons, and metals have been templated with emulsions and demonstrate the flexibility of this technique to form solid porous materials. The system which has been most extensively studied involves a W/O emulsion containing styrene and divinylbenzene as the continuous phase [30], [31], [32].
The present work describes a new sol-gel emulsion-mediated methodology for the synthesis of Al-Mg hydrotalcites with hierarchical mesoporous and macroporous structure, varying the relative proportions of Al and Mg and the amount and chain size of the paraffin used as an emulsifier, with the aim of independently control the size and frequency of the mesoporous and macroporous arrangements of the produced hydrotalcites. The impact of that resulting porous structure in the textural properties in comparison with hydrotalcites conventionally prepared, it is also discussed.
Section snippets
Synthesis of the hierarchical mesoporous and macroporous hydrotalcites
The synthesis of the hierarchical hydrotalcites involved the use of aluminum tri-sec-butoxide (Al(i-But)3) and magnesium nitrate as sources of the hydrotalcite framework. The assembly of the emulsified system employed linear alkanes (n-C6H14, n-C12H26, or n-C18H38), the non-ionic block copolymer Pluronic P123 (20 EO: 70 PO: 20 EO, 5826.38 g/mol), and ethanol as solvent. Pluronic P123 and the linear alkanes are known organic pore structure directing agents (PSDAs). Water was not a suitable
Preliminary study of the formation of emulsion and its kinetic stability
A series of emulsions prepared using different mass ratios of Pluronic P123, paraffin, and ethanol were studied in order to determine the most suitable proportions for the synthesis of the hydrotalcites (Fig. 1 and Supplementary Material Fig. S1). As can be seen from Fig. 1, the transparent L1 region occurred at the extremes of the obtained diagrams, corresponding to paraffin-poor (<20%) and paraffin-rich (>80%) conditions. The area of the opaque L2 region increased according to increase of the
Conclusions
Novel Al-Mg mesoporous and macroporous hydrotalcites were obtained by a new one-step synthesis method based on the use of paraffin emulsions and surfactant as pore structure directing agents (PSDA), combined with sol-gel transition. The hydrotalcites produced in the emulsion-mediated synthesis presented well-defined hierarchical structures of mesoporous and macropores, exhibiting high specific surface areas and pore volumes, compared to their reference counterparts synthesized in the absence of
Acknowledgements
This work was supported by the Brazilian agencies CNPq (grant 470094/2013-3) and FAPESP (grants 2013/01328-0 and 2015/05321-5). The authors also thank the Brazilian Synchrotron Light Laboratory (LNLS) in Campinas for use of the XPD beamline (proposal XPD-17839).
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