Another reason is that ni2 can substitute mg2 in

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Unformatted text preview: methane. Another reason is that Ni2+ can substitute Mg2+ in hydrotalcite-type anionic clays, and hence the Ni dispersion and the basicity of the catalysts can be varied, without significant structural changes in the hydrotalcite-type precursor or the calcined catalyst. This allows studying the influence of the basicity and the Ni dispersion on the coke resistance of the catalysts. 1. Introduction 5 1.2. Hydrotalcite-type compounds Hydrotalcite, Mg6Al2(OH)16CO3· 4H2O, is a natural occurring anionic clay. It was first discovered in Sweden around 1842 and its name derives from the fact that it can be easily crushed into a white powder similar to talc [1, 25]. The structure of hydrotalcite can be derived from brucite, Mg(OH)2, which exhibits layers of edge-sharing hydroxide octahedra centred by magnesium according to Mg(OH)6/3 (Fig. 1). The sheets are stacked one on top of the other and are held together by weak interactions through hydrogen bonds. Isomorphic replacement of Mg2+ ions by Al3+ results in an overall positive charge of the layers. The positive charge is compensated by the intercalation of CO32- in the interlayer space. Also the equilibrium products of aqueous CO32solutions (OH-, HCO3-) are present in the interlayer space as water molecules are also intercalated. The water molecules participate in hydrogen bonding to the hydroxide layers [1, 2, 25, 26]. Fig. 1 Structure of hydrotalcite [27] 6 1. Introduction At the same time as hydrotalcite, a compound of the composition Mg6Fe2(OH)16CO3· 4.5H2O was found, called pyroaurite due to its likeness to gold when heated. Later pyroaurite was recognised to be isostructural with hydrotalcite. In nature many compounds have been found, which have the approximate composition M(II)6M(III)2(OH)16CO3· 4H2O and which are isomorphous to hydrotalcite. Also minerals like sjögrenite, Mg6Fe2(OH)16CO3· 4.5H2O, have been found that have also the approximate formula M(II)6M(III)2(OH)16CO3· 4H2O and the same layered structure but a different stacking sequence, corresponding to a hexagonal instead a rhombohedral symmetr...
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This document was uploaded on 10/07/2013.

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