Zeolites direct protonation with

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The dealumination and realumination of low sodium P-zeolite have been studied by IR spectroscopy and powder X-Ray diffraction. The calcination-dealuminated HP was easily realuminated by direct treatment with aluminate sodium solution. After transformation into protonic HP, the framework Bronsted sites (IR band at 3610 cm ) were restored and the acidity was the same as in the original Hp. The realumination of acid dealuminated Hp was difficult, but could be achieved with aluminate sodium solution by hydrothermal treatment in autoclave. This incorporation of A1 into zeolite framework increased the number of acid sites, in particular that of weak acid sites which in comparison with parent Hp. 

The coordination complex chemistry in zeolites provides a very useful conceptual bridge to coordination-chemistry controlled catalysis in the liquid phase. This is discussed in this chapter for the oxidation of ethylene to produce vinyl acetate from acetic acid and ethylene. The catalytic system appears to consist of dimeric or trimeric Pd complexes. The elementary reaction steps can take place in the direct contact with the metal centers, the so-called inner-sphere mechanism. The reaction can also proceed through an outer-sphere mechanism in which proton transfer between reactants and acetate plays an essential role. 

Another aim of the research is the understanding of the water-zeolite interaction. Stahl et al. [36] have shown that in the channels of a natural zeolite, Bikitaite, water molecules are bonded in chains (H...O bond). The water chain is held in place by electrostatic interactions with the channel walls. They suggest the possibility of directional proton conduction. 

With this in view, the acidic strength of zeolites with reference to the stability of their framework and the presence of proton sites in the crystal structure of zeolites can be directly correlated with their intermediate electronegativity. 

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

Partially proton-exchanged Na faujasite X, in turn, is the best catalyst for selective monochlorination with tert-butyl hypochlorite.258 NaX, NaY, and NaKL zeolites used in the chlorination of toluene with sulfuryl chloride undergo rapid deactivation because of the accumulation of polychlorinated toluenes in the pores of the catalysts and dealumination.259, 260 Direct electrophilic fluorination of aromatics can be effected by using Selectfluor in the presence of triflic acid.261 Electrophilic fluorination may also be carried out by R2NF and R3N+FA reagents.262 Elemental fluorine may also act as a powerful electrophile in acidic media (sulfuric acid, trifluoroacetic acid, or formic acid), but monosubstituted aromatics give isomeric mixtures.263-265... 

The intensity of an NMR signal is directly proportional to the total number of protons. It is possible to measure a minimum value of 1020 hydroxyl groups per cm3 with an accuracy of 10% (1). Table I gives the number of OH groups per supercage for various zeolites as determined from the intensities of proton signals (14,15,16). For Y-zeolites no ammonium ions could be detected by both NMR or IR techniques after pretreatment... 

Reaction mechanism It is generally admitted that, over zeolites, acetylation of arenes with AA is catalysed by protonic acid sites. Comparison of the activity of a series of dealuminated HBEA samples allows one to exclude any direct participation of Lewis acid sites in 2-MN acetylation with AA. Indeed, two HBEA samples with similar protonic acidities but with very different concentrations of Lewis acid sites (170 and 16 pmol g ) have practically the same acylating activity.1271 The role of Brpnsted sites is also clearly expressed in Spagnol et a/.131... 

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