Carbonium ion cracking

The carbonium ion thus formed can isomerize to a more stable species (a tertiary carbonium ion), crack to form a smaller ion and an olefin, or desorb as a normal olefin. 

In contrast, at higher reactant partial pressures, higher conversion, and lower temperatures the monomolecular carbonium ion cracking is gradually replaced by the bimolecular carbenium ion cracking described above for... 

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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. 

The carbonium ion s charge is not stable and the acid sites on the catalyst are not strong enough to form many carbonium ions. Nearly all the cat cracking chemistry is carbenium ion chemistry. 

The unstable carbonium ion decomposes to a carbenium ion [CnH2 3r [CnH2n.ir+H2 and, in a cracking step... 

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... 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

Protonated cyclopropane ring closure and ring opening steps are discussed in Section 13.8.1 within the context of alkene skeletal siomerization. Carbonium ion decomposition is further discussed in Section 13.8.4.2 within the context of mono-molecular cracking of alkanes. 

In later work by Haag and Dessau product selectivity data were provided for n-butane cracking at 426-523 °C over HZSM-5 with Si/Ah = 70 [90]. The selectivity results at 496 °C and 1-10 kPa for n-butane were extrapolated to zero percent conversion in Table 13.6 to be able to identify the primary products and to assess the decomposition pattern of the n-butyl carbonium ion. Similar selectivities to methane and propylene implied, as expected, that the decomposition of the car-... 

Peter Hervey Given was bom in 1918. He was educated at Oxford University, receiving a B.A. in Chemistry in St. Peter s Hall, Oxford, and the M.A. and D.Phil. in the Dyson Perris Laboratory under the direction of Professors D. LI. Hammick and Sir Robert Robinson (who was the Nobel laureate in chemistry for 1947). Given s thesis research dealt with carbonium ion reactions of aromatic hydrocarbons on cracking catalysts (1-. ... 

Cracking reactions can take place on either the acid site or the platinum site. Acid cracking is characterized by the formation of C3 and C4 paraffins due to the carbonium ion mechanism. Metal cracking (hydrogenolysis), as shown by Sinfelt (19), is random and forms more C, and C2 gases relative to acid cracking. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

The acid-catalyzed reactions of olefin polymerization and aromatic alkylation by olefins have been very well explained by the carbonium ion mechanism developed by Whitmore (21). This mechanism provides the basis of the ensuing discussion, which is devoted to the application of such concepts (7,17) to catalytic cracking systems and to the provision of much added support in terms of recently developed structural energy relationships among hydrocarbons and new experimental evidence. 

To complete this picture, it is necessary to show how the carbonium ion intermediate is formed in the catalytic cracking of hydrocarbons. For olefins, it is the reaction of proton addition ... 

For paraffins and naphthenes, the important reaction of hydride ion exchange (2) is postulated, which is in turn initiated by carbonium ions derived from small amounts of thermally produced olefins in the cracking system. 

Aromatics are in a sense unique in their catalytic cracking reactions. The aromatic ring contains the equivalent of six double bond or pi electrons, which are, however, mutually stabilized by strong resonance energy. We may postulate an association between a carbonium ion and these electrons in a generalized sense ... 

All group I hydrocarbons (paraffins, olefins, and naphthenes) crack to give an olefin and a carbonium ion by the generalized mechanism ... 

---