Chromium solid-state structures

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

Although no solid-state information is available for the pentaaqua-chromium(III) alkyl cations, crystal structures are available for chelated analogues, e.g., [(H20)(2,3,3,3-tet)CrR](C104)2, where 2,3,3,3-tet is the N4 macrocycle, 1,4,8,12-tetraazacyclopentadecane and R is... 

The structure of tris(l,l,l,5,5,5-hexafluoro-2,4-pentanedionato)chromium(III) has been determined by gas-phase electron diffraction.767 A most interesting feature of this study was that the normalized ligand bite angle [(O—O/M—O) 30.1°] was very close to the value found in the solid state by X-ray diffraction (30.8°) but quite different to the value predicted by Kepert s model,768 The results of this and related work on vapour phase structures are summarized in Table 80. 

The X-ray crystal data are often used in relation to solution chemistry, and the general question of the identity of the species in solution and those present in the crystals has to be considered, since rapid interconversion between different bridged species is frequently seen for both chromium(III) and cobalt(III). Supplementary solution measurements are therefore generally required in order to be reasonably certain that the solid-state structure corresponds to the structure in solution. 

Chromium is the only metal of the first transition series to form a hexafluoride (3). The compound is so unstable that no solid-state characteristics have been recorded. The metals of the second and third transition series form hexafluorides which, structurally as a group, are the most closely related of all the transition metal fluorides. These compounds are molecular species, with an essentially regular, octahedral arrangement of fluorine atoms around the metal atom. This leads... 

Several attempts have been made to stabilize the layered LiMn02 phase by substitution of manganese ions with small amounts of chromium and aluminum by conventional solid state reaction and the introduction of cobalf and nickel on the manganese sites via the chimie douce reaction. Although these substituted layered compounds resulted in improvements in capacity retention in comparison to pure LiMn02, they have been found to be thermodynamically unstable with respect to their transformation to the less desired spinel structure. 

An X-ray structural analysis of pentacarbonyl (A,A diethyl-A,A -dimethylenetelluro-urea)chromium indicated that the tellurolate form is preferred in the solid state. However, these complexes are described on page 520. 

In solid-state laser materials, such as ruby (chromium doped alumina, AljOjiCr " ) (1) and emerald (chromium doped beryl, Be,Al,(Si03)5 Cr ) (2), transitions between multiplets of impurity states are utilized. These states mainly consist of 3d orbitals of the impurity chromium ions. For the analysis of these multiplet structures, the semi-empirical ligand-field theory (LFT) has been frequently used (3). However, this theory can be applied only to the high symmetry systems such as O, (or T ). Therefore, the effect of low symmetry is always ignored in the analysis based on the LFT, although most of the practical solid-state laser materials actually possess more or less distorted local structures. For example, in ruby and emerald, the impurity chromium ions are substituted for the aluminum ions in the host crystals and the site symmetry of the aluminum ions are C, in alumina and D, in beryl. Therefore, it is important to clarify the effect of low symmetry on the multiplet structure, in order to understand the electronic structure of ruby and emerald. 

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