Zeolite alkali-exchanged

The production of ketene by this method has no significant environmental impact. The off-gases from the ketene furnace are either circulated to the furnace and burned to save energy or led to a flare system. The reaction can also be carried out at 350—550°C in the presence of alkali-exchanged zeolite catalysts (54). Small quantities of ketene are prepared by pyrolysis of acetone [67-64-1] at 500—700°C in a commercially available ketene lamp (55,56). 

The spectra of alkaline earth ion-exchanged samples, with the exception of the barium form (211), have hydroxyl absorption bands at 3645 and 3540 cm-1, similar to those found in H—Y zeolite. The barium form behaves like the alkali-exchanged zeolites. The similarity of the spectra of the alkaline earth forms with that of the hydrogen form suggests that the acidic hydroxyls are associated with the same structural features (151). Band frequencies in the region of 3600 to 3560 cm-1 vary with the cations and are thought to result from hydroxyl groups associated with the divalent cations (211). They are weakly acidic or inaccessible to adsorbate molecules since the band intensity is not affected by adsorption of pyridine (209). 

In a different application of Raman spectroscopy to studies of zeolites, Cooney and Tsai (6) have investigated the adsorption of bromine on alkali exchanged zeolites X, NaY and NaA, and the... 

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... 

Alkali-exchanged zeolites can be used for selective condensation reactions [32-37], double bond isomerization, alkylation of toluene by methanol, mono-methylation of phenylacetonitrile with methanol and dimethyl carbonate (DMC) [38], and selective N-alkylation with methanol, ethanol, and 2-propanol as alkylating agents [39-44]. However, the basicity of alkaline exchanged zeolites is rather low. 

Under heterogeneous conditions, the Knoevenagel reaction has been used as a well-adopted test reaction to check the activity of the basic sites of different solids, mainly basic zeolites such as alkali-exchanged zeolites [40] or zeolites containing occluded metal oxides [41]. The basic activity of alkali-containing MCM-41 [42] or binary cesium-lanthanum oxide supported on MCM-41 [43] has, moreover, also been evaluated in the Knoevenagel condensation. 

H stretching and deformation bands of methanol adsorbed on alkali-exchanged zeolites [165] ... 

Catalytic behaviour of Lewis acid-base sites on alkali-exchanged zeolites... 

The Lewis acid-base properties of zeolites received a growing interest recently. A series of works " suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in Si/AI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchanged zeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. 

Three different reactions are shown to be catalyzed by alkali-exchanged zeolites. The results revealed that Lewis acid-base sites on zeolites possess potential application in the production of final chemicals. 

There has been some work on compressing the above process to one step using a solid base whose dehydration capability may be due to Lewis acid sites, e.g., alkali-exchanged zeolites. These attempts do not appear to have resulted in reasonable selectivities, because further dehydration and condensation reactions occur. An exception is the synthesis of methyl or phenyl vinylketones by condensation of a precursor methylketone with methanol/02 or formaldehyde. This process can take place with >95% selectivity at <20% conversion on Fe/Ag/AhOs at 600-650 K, or at >70% conversion with a secondary amine hydrochloride ( 40/1 aldehyde/amine)/Nb205 catalyst mixture at 390-420 K, or at lower selectivity ( 40-50%) but almost complete conversion using typical H-form zeolites at 670-730 K. ... 

Finally, as far as alkyne isomerization is concerned. Lavalley and coworkers j 147] used the isomerization of but-I-yne (yielding but-2-ync and. in some cases, buta-1.2-diene) as a probe to characterize basicity of several alkali-exchanged zeolites. 

Some other materials are AIPO4 [293, 302], AIP04-Ti0 [294], magnesium phosphates [26], alkali-exchanged zeolites [148, 286], hydrotalcite-like materials [295], lithium aluminaies and HAIO2 [202]... 

Lavailey. JC l.amotte, J Travert, A Czyzniewska. J Ziolek. M, Use of but- l-ync as u probe for the characierizaiion of the basicity of alkali-exchanged zeolites. Journal of the Chemical Society Faradav Transactions, 1998 94. 331-3.15. 

The main families of amphoteric solids are l)bulk oxides such as Y-AI2O3, Z1O2, Ti02, Ga203, etc, 2) alkali-exchanged zeolites (Li, Na, K, Rb, Cs-X or Y zeolites,. .,), 3) doped and supported oxides (V-MgO,. ..), 4) oxynitrides (ZrPON, AlPON, AIVON). 

Alkylation of toluene (pKa = 37) with methanol over alkali exchanged zeolites gives styrene and ethylbenzene over basic sites at 700 K. Thus, the basic... 

Fig. 5. Catal3rtic activities of Eu, Yb supported on alkali-exchanged zeolites (35). Conditions Catalyst 0.5 g, reaction temperature = 273 K, 1-butene = 20.5 kPa.

The ring exchange of tetrahydrofuran to tetrahydrothiophene is also catalyzed by basic sites and showed a decrease in activity with addition of HCl [8, 235]. However, pyridine adsorption also decreased the catalytic activity of the reaction on alkali-exchanged zeolites. In this case, acid and basic sites seem to be necessary for catalysis. 

Zeolites are very useful catalysts for a large variety of reactions, from acid to base and redox catalysis [27]. Hutchings et al. reported that bis(oxazoline)-modified Cu (II)-HY catalysts are effective for asymmetric carbonyl- and imino-ene reactions and aziridination of styrene [28, 29]. Recently Djakovitch and Kohler [30-34] found that Pd(II)-NaY zeolite activates aryl halides towards Heck olefination, a-arylation of malonate, and amination reactions. It is well known that alkali-exchanged faujasite zeolites are solid base catalysts [35]. Owing to the usefulness of zeolites in organic chemistry, and our interest, we recently reported the use of modified alkali-exchanged zeolite Y, NaY zeolite [36] with electron rich copper catalyst in the Y-arylation of nitrogen heterocycles with aryl halides to afford A -arylheterocycles in excellent yields under mild conditions without the use of any additive. 

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