Please cite this article in press as: T. Coelho, et al., Influence of the Mg 2+ or Mn 2+ contents on the structure of NiMnAl and CoMgAl hydrotalcite materials with high aluminum contents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.07.015 ARTICLE IN PRESS G Model CATTOD-9172; No. of Pages 8 Catalysis Today xxx (2014) xxx–xxx Contents lists available at ScienceDirect Catalysis Today j our na l ho me page: www.elsevier.com/locate/cattod Influence of the Mg 2+ or Mn 2+ contents on the structure of NiMnAl and CoMgAl hydrotalcite materials with high aluminum contents Tiago Coelho a , Renan Micha a , Santiago Arias a , Yordy E. Licea a , Luz Amparo Palacio b , Arnaldo C. Faro Jr. a,∗ a Instituto de Química, Universidade Federal do Rio de Janeiro, Ilha do Fundão, CT bloco A, 21941-909 Rio de Janeiro, Brazil b Instituto de Química, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier, 524, 20550-013 Rio de Janeiro, Brazil a r t i c l e i n f o Article history: Received 20 January 2014 Received in revised form 1 July 2014 Accepted 17 July 2014 Available online xxx Keywords: Hydrotalcite Terephthalate Cobalt Nickel Manganese Magnesium a b s t r a c t In this work, layered double hydroxides with high aluminum content (aluminum atom fractions 0.3 and 0.5, relative to total metal cations) were successfully synthesized in the systems Co Al, Co Mg Al, Ni Al and Ni Mn Al, using a co-precipitation method with controlled pH. Terephthalate was used as the charge compensation anion. Different characterization techniques (XRD, FTIR, ICP, AAS, TGA, CHN and EXAFS) showed that for these high-aluminum LDHs, the terephthalate ions are vertically placed within the interlayer galleries and the cations were effectively included in the layers. The unusually large limiting value for the intralayer aluminum proportion is possibly associated with the larger length and polariz- ability of the terephthalate anion as compared to the usual small inorganic anions (as carbonate, nitrate or hydroxyl). However, in the trimetallic samples (Co Mg Al and Ni Mn Al) a segregated aluminum phase was found for higher Mg or Mn contents. With high Mn content a manganese terephthalate phase was also observed. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Layered double hydroxides (LDHs) with the hydrotalcite struc- ture have the general formula M 1-x 2+ M x 3+ (OH - ) 2 A x/n n- ·mH 2 O, and consist of brucite-like layers where a partial divalent (M 2+ )/trivalent (M 3+ ) substitution has taken place, thus providing a positive charge to the layers, which is balanced by anions (A n- ) located, together with water molecules, in the interlayer space. The anions can be easily replaced by ion exchange. Many ternary LDHs involving mixtures of different M 2+ and/or M 3+ cations can also be prepared [1]. Terephthalate LDHs are useful as catalyst precursors, as the terephthalate anion (TA) provides a larger expansion of the inter- layer galleries, being effective for the incorporation of oxometalates by ion-exchange [2]. The corresponding mixed oxides may be obtained by calcining the exchanged LDHs, and they can be used as precursors for several catalysts, such as the ones used in the hydrotreating of petroleum fractions (NiMo [3], CoMo and NiW mixed-sulphides) or in propane oxidative dehydrogenation (VMg mixed-oxides [4]). ∗ Corresponding author. Tel.: +55 21 39387821. E-mail addresses: farojr@gmail.com, farojr@iq.ufrj.br (A.C. Faro Jr.). It is often said that pure LDH phases can only be formed with alu- minum atom fractions in the range 0.20 < x < 0.33. It has been argued that for x > 0.33, the presence of M 3+ O M 3+ linkages would be required and these are unfavorable from the point of view of charge repulsion (the so-called cation avoidance rule) [1]. In our previous work with the NiAl-TA system [5], we found that the x values within the brucite layers increase with increasing overall aluminum con- tent up to a limiting value in the range 0.5–0.6, significantly higher than predicted by the cation avoidance rule. The proportion of a segregated aluminum tri-hydroxide phase increased continuously with increasing overall aluminum content, but only became signif- icant for overall aluminum atom fractions equal to or higher than 0.6. This naturally raises the question as to whether it is possi- ble to prepare terephthalate LDHs with x values above the one predicted by the cation avoidance rule where the divalent cation is other than Ni 2+ (Co 2+ , Mg 2+ and Mn 2+ in the present case). The possibility of preparing trimetallic LDHs with high aluminum atom fraction is also interesting, from the standpoint of composi- tional flexibility of these materials. These systems are important from the catalytic standpoint, as catalysts containing Co, Mg and Al have been studied, for example, in the selective oxidation of styrene [6], reforming and decomposition of methane [7,8], gasi- fication of biomass [9], partial oxidation of methane to syngas [10], http://dx.doi.org/10.1016/j.cattod.2014.07.015 0920-5861/© 2014 Elsevier B.V. All rights reserved.