Ion exchange alkali metal

ESR and ESEM studies of Cu(II) in a series of alkali metal ion-exchanged Tl-X zeolites were able to demonstrate the influence of mixed co-cations on the coordination and location of Cu(II) (60). The presence of Tl(l) forces of Cu(II) into the -cage to form a hexaaqua species, whereas Na and K result in the formation of triaqua or monoaqua species. In NaTl-X zeolite, both species are present with the same intensity, indicating that both cations can influence the location and coordination geometry of Cu(II). The Cu(II) species observed after dehydration of Tl-rich NaTl-X and KT1-X zeolites was able to interact with ethanol and DMSO adsorbates but no such interaction was observed with CsTl-X zeolites. This interaction with polar adsorbates was interpreted in terms of migrations of the copper from the -cages. 

Alkali metal ion-exchanged zeolites and occluded alkali metal oxide zeolites have been investigated extensively and applied as basic catalysts for a variety of organic transformations (1,41,221,222). Zeolites modified with alkaline earth compounds have been applied much less frequently as base catalysts for organic reactions. 

Infrared spectral studies of pyridine adsorbed on alkali metal ion-exchanged faujasites have demonstrated the absence of Brpnsted acidity, as reported by Eberly (151), Ignat eva et al. (208), and Ward (156, 209-211). Pyridine is adsorbed weakly by coordination to the alkali metal ions (151, 156). Addition of small amounts of water does not result in formation of Br0nsted acid sites, indicating that the coordinate bound pyridine is not associated with Lewis acid sites in the zeolite framework (210). 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

However, the zeolite is not a unique substrate for this reaction, as is indicated in a recent patent (180), where it is shown that a Cu+-exchanged mont-morillonite clay and synthetic amorphous aluminosilicate will also catalyze butadiene cyclodimerization with high selectivities to VCH (>95%). Preexchange of these aluminosilicates with Cs+ ions was claimed to increase catalyst stability. This is most probably explained by a reduction in surface acidity resulting from the alkali metal ion exchange. 

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. 

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). 

Figure 16. Electronic spectra of dehydrated MV +-alkali metal ion-exchanged zeolite Y as a function of alkali metal ion.

The reaction over alkali metal ion exchanged X, Y, and ZSM-5 zeolites was also studied by Wierzchowski and Zatorski using trioxane as the source of HCHO. It is concluded that the alkali-metal-ion-exchanged ZSM-5 zeolite catalysts are not suitable as catalyst for this reaction. 

The suggested ionic mechanism was used to explain the activity and selectivity behaviour in the oligomerization of olefins over alkali metal ion-exchanged zeolites. In olefin oligomerization, the activity pattern for the alkali metal ion forms in zeolites is inversely proportional to the ionic radius, e.g. Li > Na" >... 

Cerium(iv) phosphate sulphates have been reduced to their Ce " counterparts. CeH2P20g,1.33H20 was synthesized in the reaction of Ce ions with H3PO4, and has been used as an alkali-metal ion exchanger. Fusion of... 

In another study, two series of alkali-metal ion-exchanged zeolites have been investigated in order to analyze the possible correlations between the acidity and basicity of the X and Y zeolite structures and their catalytic properties [90]. The catalytic results for the conversion of 4-methylpentan-2-ol show that the activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The main reaction that takes place is dehydration, giving 4-methylpent-l-ene and 4-methylpent-2-ene in variable amounts. Skeletal isomers of Ce-alkenes were also formed in some cases. Simultaneous dehydrogenation to 4-methylpenten-2-one may also occur. 

For a series of alkali metal ion-exchanged zeolites the N-H frequency of pyrrole is found to shift to lower wavenumbers as the Sanderson intermediate electronegativity (Sin of the zeolite increases. Since, the charge on the oxygen (Sq) is directly correlated with the intermediate electronegativity by,... 

Modified Zeolites. As described above, alkali ion-exchanged zeolites are weak bases. Various efforts have been made to increase the base strength of alkali metal-ion exchanged zeolites. Metallic sodium particles in zeolites are formed by the decomposition of occluded sodium azide (59). These sodium particles are capable of performing base-catalyzed reactions. These catalysts catalyze the isomerization of butenes at 300 K and the side-chain alkylation of toluene with ethylene at 523 K. 

Li and Na zeolites presented much higher heats of NH3 adsorption and greater coverage at the same pressure in comparison with the other zeolites. The acid-base properties of alkali-metal ion exchanged X and Y zeolites have also been investigated by ammonia and sulphur dioxide adsorption microcalorimetry, in parallel with the study of a catalytic reaction, viz. 4-methylpentan-2-ol conversion [111]. 

A. Auroux, P. Artizzu, I. Ferine, R. Monad, E. Rombi, V. Sohnas et al.. Conversion of 4-methylpentan-2-ol over alkali-metal ion-exchanged X and Y zeolites a microceilorimetric and catalytic investigation. Micropor. Mater. 11, 117-126 (1997)... 

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