Silicates metal complexes

The viscosity and non-Newtonian flooding characteristics of the polymer solutions decrease significantly in the presence of inorganic salts, alkali silicates, and multivalent cations. The effect can be traced back to the repression of the dissociation of polyelectrolytes, to the formation of a badly dissociating polyelectrolyte metal complex, and to the separation of such a complex fi"om the polymer solution [1054]. 

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

In general, there appears to be considerable scope for the application of ordered mesoporous structures in such fields as biochemical separations and nanocomposite materials. The potential value of MCM-41 -based catalysts has been stressed by Thomas (1994,1995). In principle, metal complexes, enzymes and other species can be attached to the channel walls to give high concentrations of structurally well-defined active sites. It should be possible therefore to take full advantage of die regularity of both the channel and the surface structure. Furthermore, the incorporation of heteroatoms such as Ti into the siliceous framework of MCM-41 should provide an elegant way of controlling catalytic selectivity (Thomas, 1995). 

Anchoring of active catalysts to insoluble materials such as oxides, silicates, and zeolites often reduces the loss of catalyst during the catalytic process (66). The fixation of the active centers can be achieved either by means of their interaction with hydroxyl groups on a solid surface or, alternatively, by means of interactions between the CO ligands of the metal complex and a Lewis acidic center of the surface. Zeolite-supported cobalt catalysts have been reported for hydroformylation reactions (67). 

This situation, clear in the case of more ionic structures, is less stringent in graphite intercalates where, presumably, there is electron transfer to (in the case of alkali-metal intercalates) or from (in the case of metal halide intercalates) the half-filled conduction bands of the graphite layers (produced by overlap of the 7t orbitals). Similarly, the periodicity requirements are less stringent for the alternating composite layers of layer silicates with complex intralayer and interlayer charge balance. 

Decomposition of the metal complex intercalate in inert gas liberates the coordinated molecules and forms the nude metal ions confined in the interlayer region. The metal ion is surrounded by oxide ions of the silicate sheet, but the coordinative bonding of oxygen is unusually weak considering the geometry of the oxide ions. It is therefore expected that the material (d. Fig. 1) shows the characteristic activity of the metal ion when the silicate sheet is catalytically inactive. Some findings obtained in recent years using a catalyst of this type are introduced herein. 

The Sorption of Cations to Layer Silicates Metal-Ion Binding by Ion Exchange and Surface Complex Formation... 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

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