Alkali Metals in Zeolites

Alkali metal inclusion gives rise to very strongly basic zeolites, with strong electron donating power (Lewis bases). These are very active as basic catalysts, but their reactivity is too high to permit useful application. Where reducible cations are present in the framework such as in the titanosilicate, ETS-10, inclusion of sodium metal results in the reversible reduction of the framework 

The beautiful blue and turquoise colours observed in natural forms of sodalite (examples of which are known as lapis lazuli and ultramarine) are known to result from the inclusion of sulfide 83 (blue) and smaller concentrations of 82 (yellow) chromophores within the sodalite cages. 8imilar materials based 

Careful analysis of both powder X-ray dilfraction and EXAFS spectroscopic data located the cadmium sulfide as (CdS)4 cubes occupying the space within sodalite cages, with the Cd ions coordinated to framework oxygen atoms (Fig. 6.10). Furthermore, the clusters were observed to order between adjacent sodalite cages, to give superclusters or a superlattice structure. In subsequent work, a variety of compounds and elements have been prepared as well-defined clusters within zeolite frameworks, including metal oxides, selenides and phosphides, and these have been studied mainly with the view of determining the effects of cluster size on optical and electronic properties. 

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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 next homologues are 1- and 2-butyne, where similar isomerizations have been observed [20] a recent report describes the reaction on a basic, alkali metal-exchanged zeolite [21]. As an unexpected product, an allene was obtained in reactions with hydrogen and a samarium catalyst [16, 22]. 

Pyrrole has also been applied as a probe molecule in FTIR specroscopic studies. Upon interaction with a base site, the N—H stretching vibration is found to shift to lower wavenumber and in alkali metal-exchanged zeolites this behavior has been found to correlate with both N Is XPS data and the negative charge calculated from Sanderson electronegativities [4, 26]. 

Huang and Kaliaguine interpret spectra such as those in Figure 10 to mean that the strongly basic sites in alkali metal exchanged zeolites are framework oxygen atoms immediately adjacent to the alkali metal cations, acting as Lewis bases. Since the v(NH) frequency of pyrrole adsorbed on MgO surfaces is at 3320 cm, those zeolites for which the... 

Parallels have been proposed between the dissolution of the alkali metals in nonaqueous solvents and the interactions of alkali metals with zeolites.The sorption of sodium or potassium vapor into dehydrated zeolites produces intensely colored compounds, ranging from burgundy red to deep blue, depending upon the metal concentration. A combination of EPR,... 

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. 

R. Schenkel, A. Jentys, J.A. Lercher S.F. Parker (2004). J. Phys. Chem. B, 108, 15013-15026. INS and IR and NMR spectroscopic study of C1-C4 alcohols adsorbed on alkali metal-exchanged zeolite X. 

As shown in Table 1, the alkali metal-exchanged zeolites were significantly... 

The samples can be prepared by alkali metal vapor treatment of the zeolite, treatment of the molecular sieve with a solution of the alkali metal in liquid ammonia or in primary amines, treatment of the zeolite with organolithium compounds, or decomposing sodium azide previously impregnated on sodium zeolites [59-62]. 

Alkali metal-exchanged zeolites have been used to prepare activated alkenes of interest as prepolymers by Knoevenagel condensation of malononitrile with ketones having different positive charge density on the carbon of the carbonyl group benzophenone, cyclohexanone, and p-aminoacetophenone [85]. The reactivity depends both on ketone structure (the order of reactivity was benzophenone > cyclohexanone > p-aminoacetophenone) and on the catalyst used. For instance, when malononitrile is condensed with cyclohexanone in the presence of a CsY zeolite conversion was very low. CsX zeolite, however, which has a substantial number of basic sites with pX in the range 9 < P a < 10.7 and some with 10.7 < < 13.3 could be used to perform... 

Hydrogen Transfer. - Both experimental and theoretical approaches have been used to study the reactivity of w-butyrophenone included in alkali-metal-exchanged zeolites. The results indicate that with smaller cations the Norrish Type I process is enhanced over the Norrish Type II reaction. " Others have reported that the photochemical decomposition of w-butyrophenone in a variety of solvents follows first-order kinetics. ... 

Alkali metal exchanged zeolites have also been shown to catalyse the breakdown of methyl halides, liberating hydrocarbons. In situ NMR studies by the group of suggest that the reaction proceeds by nucleophilic... 

An investigation of eight classes of catalysts (exchange resins, zirconium, titanium, and tin homogeneous catalysts, Group VB and VIB compounds, alkali metal silicates, zeolites, acidic resins, tertiary phosphine polymer catalysts) for DMC production from EC is given in [700]. The relative performance advantages and mechanistic pathways of these different classes of catalysts are compared and discussed. 

Until recent work by Zhen et al. [ 149,150], there has been little evidence for the formation of mixed metal clusters in zeolites. Through single crystal XRD, they studied the reaction of zinc vapour with Cd-X and Tl-X (FAU) and identified a unique range of possible cadmium-zinc and thallium-zinc clusters. Some evidence has also been presented that the reaction of various alkali metals with zeolites containing the cations of a different metal may result in the formation of mixed metal clusters [68, 71, 151], but, for the most part, these products remain poorly characterized. 

A new dimension to acid-base systems has been developed with the use of zeolites. As illustrated in Fig. XVIII-21, the alumino-silicate faujasite has an open structure of interconnected cavities. By exchanging for alkali metal (or NH4 and then driving off ammonia), acid zeolites can be obtained whose acidity is comparable to that of sulfuric acid and having excellent catalytic properties (see Section XVIII-9D). Using spectral shifts, zeolites can be put on a relative acidity scale [195]. An important added feature is that the size of the channels and cavities, which can be controlled, gives selectivity in that only... 

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