Alkali metal-exchanged zeolites

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]. 

Propene Alkali-metal exchanged zeolites X and Y T chemical shifts, NOE enhancements, effects of paramagnetics (301)... 

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... 

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... 

Torres F J, Vitillo J G, Civalleri B, Ricchiardi G and Zecchina A (2007a), Interaction of H2 with alkali-metal-exchanged zeolites a quantum mechanical study , J Phys Chem C, 111, 2505. 

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 lead to a mixture of... 

Typical base-catalysed reactions that occur over alkali metal-exchanged zeolites include dehydrogenations, double bond isomerisations, side-chain alkylation of aromatics, conversion of methyl halides and a range of condensations. The reaction of alcohols over zeolites can be used to determine whether acid or base catalysis predominates. Whereas acid forms of zeolites catalyse dehydrations, leading to alkenes and the products of their subsequent reactions, basic sites catalyse dehydrogenations, leading to aldehydes and ketones. 

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... 

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). ... 

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. 

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

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. 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

Figure 18. Low frequency Raman spectra of dehydrated alkali metal exchanged faujasite zeolites, (a) Cs, (b) Rb, (c) K, (d) Na, (e) Li. Reproduced with permission from reference 51.

The transformation of n-hexane was carried out at 723K and 773K at atmospheric pressure over dl the metal exchanged catalysts at identical conditions to compare their performances. The results for the alkali metal exchanged samples are presented in Table 2. Both the conversion of n-hexane and the yield of benzene increase in the order Li < Na < K < Rb < Cs. We have already noted above that the acidity of the zeolites decrease in the same order (Li to Cs). Besoukhanova et al. [12] have already shown that in case of Pt-L zeolites... 

Alkene oxidation over transition metal exchanged zeolites has been of recent interest. Yu and Kevan have studied the partial oxidation of propene to acrolein over Cu2+ and Cu2+/alkali-alkaline earth exchanged zeolites.33 In both... 

The development of environmentally friendly solid catalysts for the production of fine chemicals has known a recent growing interest and recent review articles have been devoted to catalysis by solid bases. Several solid bases have been proposed such as alkali ion-exchanged zeolites [1], alkaline oxides supported on microporous [2] and mesoporous solids [3], sodium metal clusters in zeolites... 

On the basis of an infrared study of the adsorption and reaction of methanol and dimethyl ether over alkali metal cation exchanged zeolites, we propose a reaction mechanism for the decomposition of methanol over alkali cation exchanged zeolites. Additionally, formaldehyde adsorption is performed on these molecular sieves and attempts will be made to correlate its adsorption structure with the surface reactivity. 

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