Zeolites interaction with carbenium ions

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... 

A primary carbenium ion is less stable than a secondary and a tertiary ion is most stable. The interaction of various carbenium ions with the zeolite wall decreases in the same order. A primary carbenium ion forms covalently bonded ethoxy species with the zeolite wall. As illustrated in Fig. 4.64, when an ethylene molecule approaches a proton or weak n hydrogen bond is formed initially. Upon proton transfer a stable a ethoxy species is formed. 

The elementary reaction steps of the hydrocarbons considered in this section are summarized in Fig. 8. Tlie occurrence of monomolecular reactions with linear hydrocarbons that produce hydrogen and alkane fragments was first demonstrated by Haag and Dessau [94], For convenience, the zeolite lattice to which the proton is attached is not explicitly shown in the scheme. However, it will become clear later that proton activation cannot be understood properly without explicitly taking into account the interaction of the carbonium and carbenium ion intermediates with the negatively charged zeolite wall. 

Early IR and UV-VIS spectroscopic studies on the formation of carbonium ions from triphenyl methyl compounds on zeolites, titania and alumina were carried out by Karge [111]. In 1979, upon interaction of olefins Hke ethene and propene with zeoHtes CoNaY, NiCaNaY, PdNaY and HY, the appearance of electronic bands between 230 and 700 nm was observed by Garbowski and PraHaud and attributed to an allylic carbenium ion which upon thermal treatment transforms into polyenyl carbenium ions and/or aromatic compounds [112]. These findings were corroborated and extended by studies of the interaction of propene, cyclopropane and frans-butene on zeoHtes NaCoY and HM [30]. In spite of the obscuration of the spectrum in the range between 450 and 700 nm by the threefold spHt d-d band of tetrahedraUy coordinated Co(II) ions in the case of zeoHte NaCoY,the development of bands near 330,385 and 415 nm was assigned to unsaturated carbocations. 

The product distributions of acid-catalysed reactions over acidic zeolites have long been interpreted in terms of the reactions of short-lived carbenium ion intermediates in line with observed reactions in superacid solutions. Information from NMR studies and theoretical calculations has, since the early 1990s, indicated that a different interpretation is required. Alkoxy species bound to the framework are the observed intermediates in many of these reactions, rather than carbenium ions, and carbenium-ion-like species, strongly stabilised by interaction with the framework, are postulated high-energy transition states. In addition, the observation of a reactive hydrocarbon pool is gaining acceptance as an important part of the mechanism in reactions such as the conversion of... 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

In the first part of this section we have shown for zeolite solid acids that carbenium or carbonium ion intermediates are typically present as transition states or unstable intermediates. The activation energies depend on the deprotonation energy of the zeolite, the stabilization of the charged cationic intermediates by screening effects and by their interaction with the negative charge left on the zeolite lattice. 

In the first instance UV-VIS and Raman spectroscopy are the traditional complementary methods yielding additional support to IR spectroscopic studies. Besides its application in exploring the coordination sphere of transition metal-loaded zeolites [3] and the interaction of zeolites with SO2 [136], UV-VIS spectroscopy has been successfully applied to the detection of carbenium ions formed during the reaction of hydrocarbons on the zeolite surface (see e.g. [135,137,138] and the references cited therein). 

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