Silicate intercalates reactions

Clay minerals are typically layered oxides or silicates and are the products of the weathering of primary oxides. Many members of this gtoup are able to undergo intercalation reactions and they can be divided into two subgroups, the kandite minerals and the smectites and vermicalites. 

In the most reactive silicic acids the layers of the air-dried samples are separated by water molecules. The intercalation of guest molecules is then a displacement of interlamellar water molecules and does not require the high energy needed for layer separation of the anhydrous acids. Therefore, compounds with dipole moments below 4 Debye are also intercalated, for instance, ethylene glycol, with 2.28 Debye, or alcohols (1.7 Debye). Nevertheless, the interplay between formation of hydrogen bonds and arrangements with optimal van der Waals energy also determines the intercalation reaction. 

It has been also reported that the layer silicate intercalates can undergo an organic solid-state reaction within the interlayer region, such a reaction can be considered as highly regiospecific due to the distinct surface properties such as surface acidity or layer charge [5-7]. Moreover, size- and shape-selective layer silicates have been prepared [8,9] and the catalytic properties investigated in recent years [10]. 

Reactions in sheet-silicate intercalates Anilines from diaminostilbenes... 

Reactions between certain solids possessing layered structures (e.g. graphite, silicates, metal dichalcogenides such as TaS2) and Lewis bases such as ammonia and pyridine, forming intercalation compounds, are discussed in the next section. 

It is important to distinguish these novel reactions, which take place between the individual sheet silicates and involve the intercalation of one or more of the reactants, from those catalysed by clay mineral surfaces. While details of the mechanism of reactions involving intercalation in sheet silicates are not fully understood, it is generally known that acid sites present in sheet silicates (Bronsted, as well as Lewis) are involved in the reactions. Protonated intermediates such as carbonium or oxonium ions play a crucial role in such reactions. 

The catalysts preparable with swelling layer lattice silicates are classified into four types, as illustrated in Fig. 1. The intercalate of hydrated metal ion (a. Fig. 1) is easily obtained by a simple ion-exchange reaction in an aqueous medium. The intercalate acts as a Br0nsted acid catalyst because... 

The constant activity of Cu Pd " -TSM for the selective formation of acetone was achieved in a few hours and was maintained at least for 20 h. The results demonstrate that a Pd " and Cu " pair in the interlayer region of TSM catalyzes the Wacker reaction as well as that in the aqueous solution. As illustrated in Fig. 7, the interlayer spaces of TSM are considered to be filled with an aqueous solution similar to that used in the Wacker reaction, although the counteranions are now negatively charged silicate sheets instead of Cl ions. Evidence for the presence of water in the interlayer space has been obtained by TPD and XRD studies on the catalyst samples. The TPD spectrum shows two peaks at around 100 and 200°C, corresponding to the desorption of weakly adsorbed water and that of intercalated water, respectively. The latter peak temperature is higher than the reaction tempera-... 

Table IV compares for a series of dienes the yields of 1,2 addition products obtained with Rh(NBD)(dppe)+ as the catalyst precursor under intercalated and homogeneous reaction conditions. The yields of terminal olefins are consistently higher for the intercalated catalyst. The deviation from solution yields are larger when the intercalated catalyst is solvated with methanol than with acetone.0 Methanol swells the interlayers to an average thickness of 12 A, whereas acetone swells the interlayers to w 15 A. Since the more constricted methanol solvated interlayers provide the higher yields of terminal olefins, spacial factors as well as polarization effects induced by the charged silicate sheets may be contributing to the deviations from solution behavior. In this reaction system polarization effects may well be more important than spacial factors in directing hydrogenation transfer because the spacial requirements of the transition states derived from or r 3 allyl intermediates should be very similar.

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