Job/Unit: I20174 /KAP1 Date: 10-07-12 16:09:08 Pages: 8 FULL PAPER DOI: 10.1002/ejic.201200174 Well-Crystallized CO 3 2– -Type LiAl–LDH from Urea Hydrolysis of an Aqueous Chloride Solution Hongliang Kang, [a,b] Matteo Leoni, [c] Hongmei He, [a] Gailing Huang, [a] and Xiaojing Yang* [a] Keywords: Hydroxides / Crystallization / Urea / Hydrolysis / Imbibition / Layered compounds / Lithium / Hydrothermal synthesis An LiAl–layered double hydroxide (LiAl–LDH) was synthe- sized by autoclaving an aqueous solution of LiCl and AlCl 3 with urea at 100–160 °C. Chemical and structural analyses show the resulting powder to be pure and well-crystallized LiAl 2 (OH) 6 (CO 3 ) 0.5 ·yH 2 O (1  y  2). X-ray powder diffrac- tion reveals an apparent monoclinic C2/m symmetry and the Introduction In recent years, layered double hydroxides (LDHs) [1] have received increasing attention because of their potential ap- plications in a wide range of important fields such as cataly- sis, [2] photochemistry, [3] electrochemistry, [4] magnetiza- tion, [5] biomedicine [6] and the environment. [7] The general formula for these compounds is [M z+ 1–x - M 3+ x (OH) 2 ] q+ (A n– q/n )·yH 2 O; the structure is characterized by the presence of layers formed by the metal hydroxides, with water and anions in the interlayer space. In this paper a given LDH will be identified as MM–A–LDH where the two Ms specify the metals and A the anion; the latter will possibly be omitted (leading to MM–LDH) whenever a dis- cussion that is valid for a broad class of compounds is pre- sented. Two main classes of LDHs can be defined on the basis of z, the valence of one of the metal cations: (i) For z = 2, the charge balance requires that q = x, and the general formula is [M 2+ 1–x M 3+ x (OH) 2 ] x+ (A n– x/n )·yH 2 O. The symbols M 2+ and M 3+ identify one or more types of divalent and trivalent metal cations, respectively. Partial substitution of M 3+ for M 2+ results in positive charged lay- ers. The most common material of these is certainly hydro- talcite, i.e. MgAl–CO 3 –LDH where the layers are based on [a] College of Chemistry, Beijing Normal University, Beijing 100875, China Fax: +86-10-5880-2075 E-mail: yang.xiaojing@bnu.edu.cn [b] Beijing National Laboratory for Molecular, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China [c] Department of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38123 Trento, Italy Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 presence of disorder in the arrangement of the layers. A de- tailed study of the reaction is also proposed, demonstrating that the LiAl–LDH does not form by homogeneous pre- cipitation, but is the result of a one-pot imbibition of the Al(OH) 3 gel precursor by Li 2 CO 3 , favored by an increase in the pH. brucite [i.e. Mg(OH) 2 ]. These LDHs are therefore also called brucite-type LDHs. The spatial distribution of the cations in the layers of MgAl–LDH (at least in the NO 3 type) is fully ordered if Mg/Al = 2 and still nonrandom (in particular with no Al 3+ –Al 3+ close contacts) for those with Mg/Al  2. [8] Coprecipitation is a common synthetic route to brucite- type LDHs; one or more M 2+ and M 3+ species are mixed together with the anion that is to be incorporated into the LDHs, and the material is directly precipitated from the aqueous solution. In recent years, homogeneous hydrother- mal precipitation [by urea or hexamethylenetetramine (HMT) hydrolysis] is also widely applied (see, for example MgAl–LDH and NiAl–LDH [9] ) as it provides uniform sizes and platelet-like primary particles with a well-defined hex- agonal shape. [9a,10] (ii) For z = 1, the most common choice being LiAl–LDH with the formula [LiAl 2 (OH) 6 ] + (A n– 1/n )·yH 2 O, [11] the vacant octahedral sites in the Al(OH) 3 layer are occupied by the Li + cations, resulting again in a net positive charge. The arrangement of these Li and Al cations in the layers is regu- lar, in analogy to the MgAl–LDH having Mg/Al = 2. Di- valent cations can also be inserted in the octahedral sites, leading to compounds of the formula [MAl 4 (OH) 12 ] + - (A n– 1/n )·yH 2 O. [12] According to the literature, LiAl–CO 3 –LDH was first synthesized by hydrolysis of aluminum tri-sec-butoxide in the presence of Li 2 CO 3 . [13] However, the imbibition method is a more general route to the synthesis of materials with a large variety of A n– interlayer anions (e.g. Cl – , Br – ,I – , OH – , NO 3 – , CO 3 2– , SO 4 2– , or CH 3 COO – ). Imbibition involves the direct reaction of gibbsite or bayerite [two polymorphs of Al(OH) 3 ] with concentrated solutions of Li salts; [14] because