Abstract
Pillared clays are interesting materials with applications in catalysis and adsorption processes. To obtain these materials, several preparation procedures are necessary and must be optimized to tune the final properties of the resulting pillared clay. Therefore, this article reports the influence of synthesis parameters (temperature and concentration) of Keggin-Al13 polycations and different intercalation times (0.5 up to 72 h) on the structural properties of Al-pillared clays. The natural clays are from Brazil, and they are composed mainly of montmorillonite. By XRD, N2 sorption, XRF and 27Al NMR results of the Al-PILCs, we verified that the pillaring solution could be prepared at room temperature with an aging time of 24 h. For the cation exchange process, a period of at least 2 h is necessary to ensure the formation of pillared materials. The concentration of the Keggin-Al13 polycations was evaluated by using diluted pillaring solutions followed by applying re-pillaring procedures. After submitting the pillared clay to another pillaring process, the number of pillars in the interlamellar space increased; however, the micropore volume decreased concomitantly. Thus, by optimizing the synthesis conditions of the Keggin-Al13 polycations, Al-PILCs could be obtained with good values of basal spacing and specific surface area.
Highlights
Published: 29 October 2021Pillared interlayered clays (PILCs) are thermally stable porous materials obtained from the intercalation of inorganic polycations within the interlayer space of a natural clay.Typically, the clay mineral used is montmorillonite due to its high cation exchange capacity and swelling properties of the lamellae [1]
Other Al species can be formed depending on the pH solution [10]. Several synthesis parameters, such as OH/Al ratio, temperature, and time under stirring, have been studied to improve the preparation of Keggin-Al13 polycations to further use as a pillaring agent [10]
The degassed first at micropore volumes were determined by the α-plot model, values of specific surfacewere area were calculated by Brunauer-Emmett-Teller (BET)
Summary
Pillared interlayered clays (PILCs) are thermally stable porous materials obtained from the intercalation of inorganic polycations within the interlayer space of a natural clay. Zhu et al [25] reported a procedure using the Keggin-Al30 polycation as a pillaring agent to generate higher basal spacing and porosity in the resulting PILC. When the temperature is increased up to 60 ◦ C, an aging period of 24 h is sufficient to form Keggin-Al13 polycations [10,25] Another critical parameter during the synthesis of the pillaring agent that affects the formation of Al species is the OH/Al molar ratio, which usually is in the range of 2.0–2.4 [10]. Considering the exposed above, in this work, we evaluated three key parameters to optimize the pillaring procedure: (i) the temperature of both dripping and aging steps during the preparation of the Keggin-Al13 polycations, (ii) the intercalation time through cationic exchange, and (iii) the pillar insertion within the clay layers by using diluted pillaring solutions, followed by re-pillaring procedures to control the porosity and density of pillars within clays
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