Alkali metal zeolites

Cation emitters The alkali metal zeolites, and other alkali metal aluminosilicates, are efficient emitters of alkali metal cations. The cation emitters have been known for a much longer time than the anion emitters, but the anion emitters are better understood from a chemical perspective hence they are discussed here. Both types of emitters, however, can be scaled up in intensity readily to be used for the primary ion guns in static SIMS instruments. Ion beams of 50 pA to 1 nA focused to a 1-mm spot size are routinely produced by using these emitters. These emitters are primarily used in SIMS guns, as opposed to being used for isotope ratio analyses. 

EF material free, alkali exchanged zeolites are used as quite mild basic catalysts. Light alkali and alkaline earth metal zeolites, such as Na-X, Na-Y [165], alkali-MOR, Na-A and Ca-A [166], have a mild Lewis acid behavior and do not appear to have strong basic character. The same occurs for Na-silica-alumina [167]. However, heavy alkali metal zeolites such as Cs-Y actually act as base catalysts, or rather as acid-base catalysts, for example for toluene side-chain alkylation. Stronger basic character arises from impregnation of alkali zeolites with alkali salts, later... 

The recent studies on the relationship between activation temperature and carbonium ion type catalytic activity of both decationized and cation exchanged zeolites show that at arid above the temperature required for the removal of all observable hydroxyls with vibrational frequencies between 3700-3500 cm" the activity sharply declines. The lowest concentration of acidic lattice hydroxyl required for carbonium ion activity seems to depend on the reaction involved. For example, dehydroxylation of La-exchanged Y to a level at which hydroxyl content was unobservable by currently-used infrared techniques led to total loss of activity to crack n-butane, but only partial loss of activity to crack cumene (vide infra) and to alkylate toluene with propylene (74). The activity and hydroxyl content lost on dehydroxylation can be restored upon subsequent treatment with water (11). Furthermore, alkali metal zeolites, which have little or no carbonium ion type activity can be made to show strong activity by the addition of a proton source, such as alkyl chlorides (51, 58). The similarity of the products obtained with the... 

H2O + Si-O-Al M(OH) + Si-0(H )-Al where Si-O-Al represents a part of the zeolite firework (10). It has also been shown that Bronsted acid sites can be present in low concentrations in alkali-metal zeolites. For example, Ramamurthy and coworkers have established the presence of low levels of Bronsted acidity in NaY and NaX zeolites using the color change of a base indicator (10, 11, 19). The acidic forms of retinyl acetate, retinol and retinyl Schiff bases are colored blue and are easily identified by both visual inspection and spectrophotometrically. Using these bases, they were able to determine whether or not Bronsted acids were present in low concentrations in various zeolite samples. Importantly, these studies showed that the presence of small quantities of acid sites... 

Basic catalysts also show very different behaviour from acid catalysts for the alkylation of aromatics. Whereas acid catalysts promote alkylation of the aromatic ring, with high shape selectivity in the important case of ZSM-5 (Chapter 8), alkali metal zeolites catalyse side chain alkylation. In the case of the reaction of toluene with methanol over Cs-X, for example, the products include ethylbenzene and styrene. The side chain alkylation proceeds by the following base-catalysed steps, (i) formation of formaldehyde from methanol, (ii) activation of the toluene by polarisation of the methyl group (tending towards carbanion formation) and (iii) nucleophilic attack of the carbanion of toluene on the carboxyl group of formaldehyde. Side chain alkylation of aromatics is therefore a special case of aldol condensation. Reactions of this... 

In accordance with this assumption, a variety of alkali-metal zeolites (Li, Na, K, Rb, Cs) are shown to be active and selective (50). Addition of alkali metals suppressed by-product formation, possibly by a more effective neutralization of residual Bronsted sites. The NiX merits, when it comes to dimerization, have also been observed in the case of butene where octene was the major product. Yet isomerisation of butenes appeared to be much more rapid than its dimerization (51). 

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... 

Molecular sieves are an adsorbent that is produced by the dehydration of naturally occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates). The dehydration leaves inter-crystalline cavities into which normal paraffin molecules are selectively retained and other molecules are excluded. This process is used to remove normal paraffins from gasoline fuels for improved combustion. Molecular sieves are used to manufacture high-purity solvents. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

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]. 

Extremely high ion exchange affinities are however sometimes observed for alkali metals (e.g. Cs) and transition metal ion complexes in clay minerals and zeolites. The objective of this paper is to give an account of the factors which are involved in these high selectivity phenomena. The discussion will be focussed mostly on montmorillonites and faujasites as representatives of the phyllosilicate and tectosilicate groups. 

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. 

Under the same conditions (batch or GL-PTC) discussed for CHg-acidic compounds, primary aromatic amines also react with DMC. In this case, although the reaction yields selectively the mono-A-methylated amines with no dimethylated by-products, sizable amounts of methyl carbamates (ArNHCOgMe) are formed. ° Much better results can be gathered in the presence of zeolites, particularly alkali metal exchanged Y and X faujasites. These aluminosilicates posses pseudospheri-cal cavities (supercavities) of 11-8 A in diameter, which can be accessed through channels whose size is 7.4 kP ... 

Then, contrary to our previous hypothesis, the reaction proceeds via a Bai2 displacement of aniline on DMC. The product, mono-A -methyl aniline (PhNHMe), plausibly adsorbs into the zeohte in a different way with respect to anihne, because different H-bonds (N H — O-zeolite) take place with the solid. As recently reported by Su et al., A-methyl amines also may interact with NaY by H-bonding between the protons of the methyl group and the oxygen atoms of the zeolite this probably forces the molecule a bit far from the catalytic surface in a fashion less apt to meet DMC and react with it. This behavior can account for the mono-A-methyl selectivity observed, which is specific to the use of DMC in the presence of alkali metal exchanged faujasites in fact, the bis-A-methylation of primary aromatic amines occurs easily with conventional methylating agents (i.e., dimethyl sulfate). ... 

The method developed by Milton in the late 1940s, involves the hydrothermal crystallization of reactive alkali metal aluminosilicate gels at high pH and low temperatures and pressures, typically 100°C and ambient pressure. Milton, Breck and coworkers synthesis work led to over 20 zeolitic materials with low to intermediate Si/Al ratios (1-5) [86]. Chapter 3 and references [1] and [25] provide more detailed discussion of synthesis. 

Similarly, reactive oxide mixtures are also used to synthesize aluminophosphate molecular sieves, usually starting from phosphoric acid along with the addition of alumina and silica sources analogous to those used in zeolite synthesis with a notable exception alkylammonium salts and amines were ultilized in structure-direchng and space filling to the exclusion of alkali hydroxide solutions and alkali metal salts. 

McCulloch, B. and Gatter, M.G. (1991) Process for extracting meta-dichlorobenzene from isomer mixtures with mixed alkali metal exchanged X zeolite adsorbents. U.S. Patent 4,995,380. 

---