Alkali-exchanged

Alkali ions in solution increase the pH, with two consequences. The rate of silica extraction increases with the rise in pH value above 9 and the rate of alkali exchange is reduced. However, since the higher alkalinity favours solution (of silica) further alkali is released by the lattice. The quantity of alkali extracted can be used as a measure of the resistance of glass to attack... 

The adsorbent is an alkali exchanged faujasite bound into a bead. Because m-xylene is by far the most basic of the four mixed xylene isomers, the selectivity of the zeolite is tailored for this chemical trait. 

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... 

Nkosi, B., Ng, F. T. T. and Rempel, G. L. The oligomerization of butenes with partially alkali exchanged NiNaY zeolite catalysts. Appl. Catal. A, 1997, 158, 225-241. 

Alkali-exchanged mesoporous molecular sieves are suitable solid base catalysts for the conversion of bulky molecules which cannot access the pores of zeolites. For example, Na- and Cs-exchanged MCM-41 were active catalysts for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate (pKa=10.7) but low conversions were observed with the less acidic diethyl malonate (pKa=13.3) [123]. Similarly, Na-MCM-41 catalyzed the aldol condensation of several bulky ketones with benzaldehyde, including the example depicted in Fig. 2.38, in which a flavonone is obtained by subsequent intramolecular Michael-type addition [123]. 

Figure 15. Thermodynamic activities (TMS data) for NaCl (-0-) and KCl in the Ki slag-alkali exchange process. The indicated reference state partial pressures were obtained from Ref. 36. Key P NaCl = 5 X atm P° KCl = 5 X...

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... 

Williams T. J., Candela P. A., and Piccoli P. M. (1997) Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures an experimental investigation. Contrib. Mineral. Petrol. 1282/3, 114-126. 

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. 

We have used pyrrole as an XPS probe molecule and monitored the line of this molecule chemisorbed on alkali exchanged X and Y zeolites [26,57]. We also studied this adsorbed species on the same samples by IR [58] and microcalorimetry [59]. A similar study was also performed using chloroform as the probe molecule in IR [60] and XPS [61]. 

Njs binding energies for pyrrole chemisorbed on alkali exchanged X zeolites [57]... 

Ethylbenzene can also be formed by side-chain alkylation of toluene with methanol over basic catalysts, such as alkali-exchanged X and Y zeolites [229,230]. The cross-coupling of toluene and methane to produce F.B/styrene at high temperatures (> 700 °C) in the presence of oxygen has also been attempted by using basic zeolites, such as CsX and BaO/NaX [231, 232,233],... 

Molecular sieves belonging to alumino phosphate with an AEL structure and its derivatives were prepared by hydrothermal synthesis as per the literature [10]. Composition of these samples was established by wet chemical analysis methods by using atomic absorption (Hitachi, Japan ) and inductively coupled plasma spectroscopy ( Jubin Yuan, France ). Different degrees of alkali exchanged Y type zeolites were also prepared from Linde NaY by the ion-exchange procedure. Characterization of the catalysts and the experiments are carried out as discussed in the publication [11]. 

Highly selective monoalkylated products of the side chain are formed at high aniline conversion levels with alkali exchanged Y zeolite. 

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. 

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