Zeolites alkali metal clusters

Alkenes may also be used as reagents in side-chain alkylation. Zeolites with occluded alkali metal and alkali metal oxide clusters were tested in alkylation of toluene with ethylene439 440 Alkali metal clusters created by decomposing Na or... 

The number of probe molecules suitable for studying the basicity of zeolites by application of IR spectroscopy is much more limited than in the case of acidity. The sites of basicity are most likely oxygen atoms of the framework or basic extra-framework species such as CS2O or alkali metal clusters (cf. Volume 3, Chapters 5 and 6 of the present series). 

Blake NP, Stucky GD (1995) Alkali-metal clusters as prototypes for electron solvation in zeolites. In Herron N, Corbin DR (eds) Inclusion chemistry with zeolites nanoscale... 

Although stable indefinitely under vacuum or inert atmosphere, alkali metal clusters in zeolites are nevertheless air- and moisture-sensitive. This inevitably poses problems with regard to characterization. Samples prepared as described above can be returned to the inert atmosphere of a glove box and loaded into airtight cells. Alternatively, the reaction tubes themselves may be designed with sealable side arms appropriate for structural or spectroscopic studies (Fig. 3). A number of groups have found it more convenient to use Pyrex instead of... 

Fig. 3. Experimental set-up for the preparation of alkali metal clusters in zeolites...

In spite of the variety of methods now available for the synthesis of alkali metal clusters in zeolites, the range of zeolites in which clusters have been identified remains small. On the basis of diffuse reflectance spectra, Liu and Thomas [36] reported the presence of K + in potassium zeolite L (LTL), and NaJ and Na + in both sodium mordenite (MOR) and the clay laponite, after irradiation with far-UV light. Ikemoto et al. [37] used microoptical spectroscopy to examine the adsorption of potassium in potassium mordenite. With regard to frameworks other than aluminosilicates, Na has been reported in aluminogermanate and gallosilicate sodalites (SOD) [38,39] and in gallosilicate zeolite Y (FAU) [36]. 

Although to date there are no clear examples of mixed alkali metal clusters in zeolites, the knowledge that the featureless singlet ESR lines often observed from alkali metals in zeolites derive from interacting clusters rather than metal particles [67] provides evidence that such clusters may exist Blatter et al. [68], for instance, reported ESR signals with a range of g-values characteristic of electrons interacting with both sodium and caesium nuclei. 

The question of the nature of alkali metal clusters in zeolites was first examined by Anderson and Edwards [14], who compared the hyperfine splitting of a series of zeolite-based clusters with a variety of neutral and charged sodium... 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

The section above described how alkali metal species such as solvated M+ ions and M ions can be formed and isolated within the lattices ofcertainhquid solvents. It is also possible to obtain single alkah metal atoms (M) and clusters (usually cationic ones, (M) +) in various solid hosts such as zeolites and graphite. There is considerable interest in clustered species since they are especially relevant to the question of what has been called the "metal/nonmetal transition, that is, how many atoms need to be gathered together before metallic (rather than molecular, nonmetalhc) properties become apparent The few electron alkah elements lend themselves to state-of-the-art... 

Alkali metal ion exchanged variants of both zeolites X and Y react with Na vapour to form cluster cations such as Na4 and Na6, with the production of intensely coloured products (reminiscent of the colour (F-) centres in alkali halides). 

If empty zeolite LTA is exposed to cesium gas, and redox reaction with the exchangeable cations of the zeolite can occur, then the product Cs" " ions may seal the zeolite crystals. Everything within the zeolite may be encapsulated, including the product atoms, perhaps as neutral clusters. Well, nearly all cations, including Na" ", are reduced by Cs(g) only some of the other alkali-metal cations may resist reaction. Depending upon the mobility of the Cs" " ions at the reaction temperature, the surfaces of the crystals may be closed initially. If the reaction proceeds, the entire volume of the zeolite may be sealed. The ultimate outcome of the process will depend upon the mobilities of all the atoms and ions in the structure at the reaction temperature, and upon considerations of the stability and lability of any structural subunits which may form. 

Arrangements of atoms which are unfamiliar to chemistry (because they are unstable) may be found in intrazeolitic space (because they are stable within the zeolite). The cationic clusters of the alkali metals (Section 6.2.1.2.) are an example of this. Within the zeolite, such new species may be studied and put to use. Allotropes of elements and compounds (continua), some charged, will continue to be found within zeolites substances must adopt new structures if they choose to conform to the geometrical requirements of intrazeolitic space, with different structures for different zeolites. In all cases, the properties of these allotropes must be different from those of the native substances in their original crystal structures. 

In 1984, Edwards and coworkers reported the formation of Na43+ in zeolites Y and A and the formation of K43 1 in K+-exchanged zeolite Y. Later on, they found that if the zeolite was Na-Y, the formed metal ion clusters are Na43+ no matter what (Na or K) the reaction vapor was whereas if the zeolite used was K-Y, the obtained metal clusters will be K43+. That is, the formed metal cluster species is not related with the vapor but simply depends on the type of cation in the zeolite used. In fact, through variation of the reaction condition, various M,(i ion clusters can be prepared. If M = Na, n = 2 6, whereas if M = K, n 3,4. After the alkali metals enter the channels or cages of zeolites to form metal ion clusters, the electrons on the original metal atoms will be released to be shared by more than one metal atom. It has been confirmed that these free electrons actually occupy the holes formed by the metal atoms (ions). Therefore, these electrons are also called solid solvated electrons (in analogy with the solvated electrons formed by alkali metals in solvents such as liquid ammonia),[7] and the formed compounds are called solid electrides. 

Because cation zeolites such as Na-X can adsorb a large amount of alkali metals to form metal clusters, a large number of extra electrons in unit volume can be produced in the compounds. When the concentration of these extra electrons reaches a certain value, it is possible for the metal-cluster-containing host-guest material to undergo transition from an insulator to a conductor. In particular, when there are crystallographically onedimensional channels, the formed one-dimensional metal clusters may become quantum wires (Figure 9.7). lx 211... 

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