Reactivity of MgeAl hydrotalcites in solid and delaminated forms in ammonium carbonate solutions Georgiana Stoica a , Marta Santiago a , Sònia Abelló a , Javier Pérez-Ramírez a, b, c, * a Institute of Chemical Research of Catalonia (ICIQ), Avinguda dels Països Catalans 16, 43007 Tarragona, Spain b Catalan Institution for Research and Advanced Studies (ICREA), Passeig de Lluís Companys 23, 08010 Barcelona, Spain c Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, HCI E 125, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland article info Article history: Received 24 December 2009 Received in revised form 20 July 2010 Accepted 4 August 2010 Available online 13 August 2010 Keywords: Hydrotalcite LDH Delamination Colloid Ammonium carbonate Composite Dawsonite abstract Treatment of MgeAl hydrotalcites (LDHs, layered double hydroxides) in aqueous (NH 4 ) 2 CO 3 at 298 K leads to composites of dawsonite, hydrotalcite, and magnesium ammonium carbonate. The mechanism and kinetics of this transformation, ultimately determining the relative amounts of these components in the composite, depend on the treatment time (from 1 h to 9 days), the Mg/Al ratio in the hydrotalcite (2- 4), and on the starting layered double hydroxide (solid or delaminated form). The materials at various stages of the treatment were characterized by inductive coupled plasma-optical emission spectroscopy, X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetry, and nitrogen adsorption at 77 K. The progressive transformation of hydrotalcite towards crystalline dawsonite and magnesium ammonium carbonate phases follows a dissolutioneprecipitation mecha- nism. A gradual decrease of the Mg/Al ratio in the resulting solids was observed in time due to magnesium leaching in the reacting medium. Dawsoniteehydrotalcite composite formation is favored at high aluminum contents in the starting hydrotalcite, while the formation of magnesium ammonium carbonate is favored at high Mg/Al ratios. The synthetic strategy comprising hydrotalcite delamination in formamide prior to aqueous (NH 4 ) 2 CO 3 treatment is more reactive towards composite formation than starting from the bulk solid hydrotalcite. Ó 2010 Elsevier Masson SAS. All rights reserved. 1. Introduction Hydrotalcite-like compounds (HTlcs) or layered double hydrox- ides (LDHs) have a broad spectrum of applications, as catalyst precursor or support, in separation and membrane technologies, filtration, scavenging and controlled release of anions, etc. LDHs consist of layers of positively charged nanosheets with brucite-type structure neutralized by anions in the interlayer space, where water molecules are also present [1]. In addition to the direct synthesis by coprecipitation from salt precursors, HTlcs have been subject of many post-synthesis trans- formations involving their well-known memory effect property and ion-exchange capacity [2e7]. That is, by recovering of the layered structure through calcination of the starting hydrotalcite and subsequent contact of the derived mixed oxide with solutions containing many different anions, or by direct exchange of the anions located in the interlayer space by others, an impressive variety of hydrotalcite-derived compounds have been synthesized. Focusing on the layered nature of these compounds, LDHs act like the host structure for polymers, bio-active compounds, pillar- ing agents, etc., leading to nanocomposite materials [7,8]. The use of intercalating agents by ion-exchange, like sodium dodecyl sulfate or alkyl carboxylates [7], followed by exfoliation induces the separation of the layers into unilamellar positive nanosheets (colloids) [9e11]. Polar [9,12e16] or nonpolar [17,18] solvents aided by heat [18], ultrasounds [18], or electric current [19] were also applied for delamination of HTlcs. Hibino and Jones [12] introduced the exfoliation of intercalated LDHs in formamide without the need of heat or reflux treatments, based on a two-stage mechanism: rapid swelling due to the ability of HCONH 2 for hydrogen bonding followed by slow exfoliation [20e22]. The subsequent restacking of the hydrotalcite nanosheets enables to fabricate nanostructured materials, offering an elegant alternative to the traditional intercalation of anionic species in their interlayer space by ion-exchange [23]. The restacking of colloidal LDHs, leading to the recovery of the native hydrotalcite structure, * Corresponding author. Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, HCI E 125, Wolfgang-Pauli- Strasse 10, CH-8093 Zurich, Switzerland. Fax: þ41 44 633 14 05. E-mail address: jpr@chem.ethz.ch (J. Pérez-Ramírez). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.08.003 Solid State Sciences 12 (2010) 1822e1830