Zeolites carbonium ions

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

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... 

For the sake of simplicity, carbenium ions, carbonium ions or protonated cyclopropane rings were used as reaction intermediates, omitting the anionic zeolite framework in the illustrahon of the reaction mechanisms for the reactions discussed here. Furthermore, it is conceivable that many such reachon paths involve alkoxide intermediates, instead of carbenium and carbonium ions. 

Boronat, M. and Corma, A. (2008) Are carbenium and carbonium ions reaction intermediates in zeolite-catalyzed reactions Sci. Dir. Appl. Catal. A,... 

At the time of maximum rate of hydrocarbon formation the composition of reaction product shows the pattern of a carbonium ion mechanism (much i-butane, i-pentane, Cg and Cy monomethyl paraffins, some aromatics and olefins C3, (Table 1). The zeolite hosts a large amount of non volatile hydrocarbons. 

Sommer (130, 130a) and Hall (131) have independently described the low-temperature H/D exchange of isobutane on zeolites. The traditional mechanism involves a five-coordinate carbonium ion intermediate yet no exchange occurred for the methine position, and this is inconsistent with a carbonium ion. This surprising result was explained by Sommer with a reaction sequence beginning with hydride abstraction by an unknown route... 

Do these results also suggest that five-coordinate carbonium ions are not essential to explain alkane cracking The evidence is mixed. Kazansky and van Santen (132) reported low-level calculations and found a metastable carbonium ion (CH3-H-CH1) formed from ethane and a zeolite Brpnsted site, but this species was so high in energy that it did not appear to be thermally accessible. More extensive work by van Santen (133) shows, however, that the transition states leading from this species do not relate to ethane cracking Blaszkowski, Nascimento, and van Santen (134) found other transition states for ethane cracking (Fig. 26) that are similar to carbenium ions albeit with stabilization from the lattice. 

It is clear that suggestions of carbonium ion intermediates in zeolite catalysis should be viewed with caution pending extension and confirmation of the work by Sommer, van Santen, Kazansky, and others. 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Only a few attempts have been made to relate the catalytic activity to the properties of cations on the transition metal-exchanged zeolites. Cross, Kemball, and Leach (5) studied the isomerization of 1-butenes over a series of the ion-exchanged X zeolites. Their results with CeX zeolite and the majority of other zeolites indicated a carbonium ion mechanism however a radical mechanism was operative with NiX and in some cases with ZnX. 

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... 

Carbonium ions and isoparaffins are formed by hydride ion abstraction and hydride ion transfer reactions. This mechanism has been described for HF.SbFg (5). Isomerization of n-paraffins over monofunctional acidic catalysts has also been claimed for mordenite (6, 7), for sieve Y (8), and for the base of the catalyst of undisclosed composition applied in the isomerization process using a noble metal on an acidic zeolite base (3). 

The scheme implies that in the presence of a metal which establishes the olefin-paraffin equilibrium, the carbonium ion concentration on the surface depends on the hydrogen partial pressure. The stabilizing effect of a given metal load will depend on its dispersion and distribution and on the prevailing hydrogen pressure. Similar experiments show that for zeolite Y based catalysts the reaction mechanism is identical with that discussed above for mordenite. 

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