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Anion incorporation into layered solids

Posted on Feb 01 2011 by Hanna Driessen, Mark I. Ogden, Daniel C. Southam*

6768 views | 2 comments | MESSAGE
Tags: asell chemistry, analytical chemistry, inorganic chemistry, macromolecular and materials chemistry

Experiment Overview

Many inorganic solids have spaces or voids in their structure. Where these voids are of the appropriate size and geometry molecules (guests) can be captured by the inorganic lattice (host). In a layered structure guests can be incorporated as guests in the gaps between layers. These 2-dimensional voids are flexible, and the gap between layers or interlamellar spacing depends on the size of the guest molecule that is incorporated. 

The layered compounds of interest here are those termed layered double hydroxides. As the name suggests, these compounds are hydroxides, have a layered structure, and include two different metal cations. The most widely studied class of these materials is the hydrotalcite group of minerals: these have the general formula [Mg1-xAlx(OH)2]•[An-x/n•zH2O], where the ratio of Mg:Al varies from 4:1 to 2:1 (0.20<=x<=0.33), and An- can be a range of different anionic species. The anions in these structures can be readily exchanged with alternative anions, to give new intercalation compounds. Hydrotalcite type materials are typically poorly crystalline, as well as having a variable stoichiometry, which can complicate investigation of their properties. 

In contrast, layered double hydroxide materials with well defined structures and highly crystalline structures can be synthesised from gibbsite, one of the structural modifications of Al(OH)3. Gibbsite itself is a layered compound, and reaction with LiCl yields the layered double hydroxide [LiAl2(OH)6]Cl.H2O. This material is a well defined crystalline solid, and the structure of the dehydrated compound has been determined using X-ray diffraction methods (Fig 1). 



Fig 1   Structure of [LiAl2(OH)6]Cl, showing chloride anions between the layers; dark spheres are lithium, aluminium cations shown as octahedral polyhedra.

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