Synthesis, Structure, Properties, and Application of Aluminium Hydroxide Intercalation Compounds

Chemistry for Sustainable Development 8 (2000) 121–127 121 Synthesis, Structure, Properties, and Application of Aluminium Hydroxide Intercalation Compounds VITALY P. ISUPOV, LYUDMILA E. CHUPAKHINA, RAISA P. MITROFANOVA and KONSTANTIN A. TARASOV Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Ul. Kutateladze 18, Novosibirsk 630028 (Russia) Abstract The possibility to use intercalation processes to modify physicochemical properties of gibbsite, a crystal modification of aluminium hydroxide, is considered. The structure and the mechanism of formation of alu- ⋅ minium hydroxide intercalation compounds with lithium salts [LiAl2(OH)6]nX pH2O are investigated. The possibility to apply these compounds and intercalation processes to develop low-waste methods of the preparation of sorbents, fine aluminium hydroxide and corundum, lithium aluminates and aluminosilicates, nanophase and low-dimensional systems is demonstrated. INTRODUCTION THE SYNTHESIS AND STRUCTURE OF THE INTERCALATION COMPOUNDS OF ALUMINIUM HYDROXIDE Intercalation processes are reversible topo- taxial chemical reactions involving the insertion The structure of all the crystal modifications of guest molecules into the host matrix of a solid of aluminium hydroxide including gibbsite is which remains unbroken during the synthesis formed by two-dimensional layers linked to [1]. These processes are interesting for re- each other by hydrogen bonds [2–4]. In turn, searchers in connection with the possibilities to the layers are composed of two nets of tightly synthesize, under soft conditions, novel com- packed hydroxide ions. Aluminium ions occupy pounds with a set of valuable physicochemical two thirds of octahedral voids in this packing, properties. So, the investigation of possibilities therofore one third of voids is empty (Fig. 1). to use intercalation processes to modify The radius of octahedral voids is close to 0.6 Å physicochemical properties of matrices with which is sufficient to hold small cations (lithi- layered structures is surely of interest for prac- um, magnesium, transition metals). Interlayer tice. A typical example of a layered matrix, alu- space also contains rather large voids with the minium hydroxide {Al(OH)3}, exists as three radius of 0.7–0.8 Å. They are connected with crystal modifications, gibbsite, bayerite, and each other by channels [5, 6]. Such a structure nordstrandite. The most important among these allows aluminium hydroxide modifications to modifications is gibbsite which is produced on exhibit the properties of a host matrix in the in- industrial scale and used for the preparation of tercalation of metal salts. For example, the for- metal aluminium and its compounds. All these mation of intercalation compounds in the inter- compounds are widely used to produce cata- action of well soluble lithium salts with gibbsite lysts, supports, ceramics, materials for elec- follows the equation tronics and medicine. The use of intercalation Li X+2nAl(OH) + pH O processes to modify the properties of alumi- n 3 2 = [LiAl (OH) ] X⋅pH O (LADH-Õ) (1) nium hydroxide has been described in litera- 2 6 n 2 whereX=Cl–,Br–,I–, SO2–, NO–. ture poorly. 4 3 122 VITALY P. ISUPOV et al. The structure of the compounds [LiAl2(OH)6]X – – – ⋅ (X=Cl ,Br , NO3 ) and [LiAl2(OH)6]Cl H2O was studied using powder X-ray and neutron diffraction analyses (Rietweld method), as well as NMR on aluminium, lithium and hydrogen nuclei. It was demonstrated that, similarly to the initial hydroxide, the structure of the formed intercalation compounds involved hydroxide layers. However, unlike gibbsite, octahedral voids in these layers are occupied by lithium cations. To conserve zero charge, anions intercalate into the solid matrix. Together with water molecules they occupy positions in the interlayer space (Fig. 2). The intercalation of lithium cations into the voids leads to some chan- ges in the positions of oxygen atoms of hydroxide ions that form these voids; they are also re- oriented. The intercalation of anions and water molecules into the interlayer space leads to a sharp increase of the distance between the layers depending on the anion. More complicated inorganic and organic anions can also act as in- tercalating ions. It is convenient to use a two- stage process to synthesize intercalates com- prising these anions [6, 18]. At the first stage, the intercalate containing a single-charged anion, most often chloride, is synthesized according to the reaction (1). At the second stage, the synthesized intercalate is treated with aqueous solutions of salts containing the necessary anion Yn–; the process follows the equation ⋅ n[LiAl2(OH)6]Cl pH2O+NanÕ+aq Fig. 1. Gibbsite structure: a – projection to the (100) plane; ⋅ b – projection to the (001) plane. The position of octahedral = [LiAl2(OH)6]nÕ pH2O+nNaCl (2) voids is shown schematically. Organic anions in the intercalates are ori- Individual character of the synthesized com- ented in a manner providing direct contact be- pounds was proved by means of chemical, tween oxygen atoms of the acid groups (carbo- X-ray, and crystal optical analyses [6–11]. The + xylic, etc.) and the surface of [LiAl2(OH)6] synthesis of these compounds can be performed layer. The change of the charge and geometry using nonaqueous solvents [6, 12, 13], as well as low-temperature melts of lithium salts. The investigation of intercalation kinetics dependence on temperature, lithium salt concentrations, solvent type, dispersity, and defect content of the initial gibbsite was reported in [6, 14]. It was discovered that rather high concentration of lithium salt (more than 20 %) and temperature above 90 oC are required to achieve high degree (more than 90 %) of gibbsite transformation into intercalation compounds within reasonable Fig. 2. Schematic structure of the intercalation compound time (several hours). of aluminium hydroxide with lithium salts. SYNTHESIS, STRUCTURE, PROPERTIES, AND APPLICATION OF INTERCALATION COMPOUNDS 123 of the anion allows to vary in a wide range the formation of partial dislocations. Their move- distance not only between the layers but also ment leads to the formation of packing defects between the nearest anions in the layer starting in gibbsite prior to the reaction front. from molecular contact till the distance of about Not only gibbsite but also another crystal 5–6 Å. modification, bayerite, is able to act as intercalation matrix [6]. INTERCALATION OF LITHIUM SALTS INTO GIBBSITE POSSIBLE APPLICATION AREAS FOR INTERCALATION The in situ investigation of the interaction of PROCESSES AND INTERCALATION COMPOUNDS gibbsite single crystals with lithium salts by means of polarization microscopy is the evi- Selective recovery of lithium dence of anisotropy of the intercalation process. from highly mineralized solutions The boundary between the initial gibbsite and intercalation compound moves in the direction The size of octahedral voids in the hydroxide parallel to basal planes of the initial compound layer is sufficient to place small cations (lithium, [6, 8, 9, 14, 19]. The interaction of gibbsite single magnesium, transition metals). To place larger crystals with lithium salts leads to the forma- cations (sodium, calcium, barium, strontium) tion of block intercalate crystals that conserve inside the layer means to deform it substantial- the morphology of the initial gibbsite single ly. So, the salts of these metals do not intercalate into crystal aluminium hydroxide. This not crystals. The investigation of orientation rela- only explains a well known fact that amorphous tions between the initial gibbsite and reaction aluminium hydroxides are highly selective to products containing lithium chloride, bromide, lithium salts [20, 21] but also allows to propose iodide and sulphate showed that the interaction new ways to synthesize selective sorbents of li- of gibbsite with lithium salts is characterized by thium on the basis of structurally disordered the following orientation relations: aluminium hydroxide forms obtained from tech- || ≈ [010]g [010]LADH-Õ (ag aLADH-Õ); nical gibbsite. One of the possible synthesis rou- tes is connected with the preliminary mechani- [100] || [100] (b ≈ b ) g LADH-Õ g LADH-Õ cal activation of gibbsite in high energy plane- tary activators. The activation of gibbsite leads This means that the (001) planes of LADH-X not only to disordering of hydrogen bonds bet- are parallel to the (001) planes of gibbsite. ween hydroxide layers but also to their partial These data, as well as the comparison of the dehydroxylation with the formation of X-ray structure of initial gibbsite with reaction pro- amorphous phase [6, 21–27]. The rate of lithium ducts allow us to propose a scheme to describe salt intercalation into such a defect-containing the intercalation of lithium salts into gibbsite structure increases sharply [6, 21, 28, 29]. This [5, 6]. According to this scheme, at the first allows to use X-ray amorphous hydroxide both stage lithium cations, anions, and water mole- directly to recover lithium from liquors and to cules get into the interlayer space. Then lithium synthesize selective adsorbents of lithium cations move from the interlayer space inside [30–34]. the octahedral voids of the aluminium hydroxide layer. The intercalation of anions and water Preparation of fine aluminium hydroxide molecules causes the increase of the distance be- and corundum tween aluminium hydroxide packings by 2.8 Å and more. As a result, substantial elastic strain Intercalation of lithium salts into aluminium appears at the boundary gibbsite – intercala- hydroxide is reversible [6, 35–37]. The treat- tion compound. The relaxation of this strain ment of intercalation compounds with water le- leads both to the dispersion of initial gibbsite ads to the release of lithium salts into the li- and to plastic deformation of the initial gibbsite quid phase and to the appearance of aluminium before the reaction front. As it was shown in trihydroxide in the solid phase according to the [14], plastic deformation is connected with the equation 124 VITALY P.

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