Cracking of alkylbenzenes

The cracking of alkylbenzenes can be treated as a case of aromatic electrophilic substitution (for recent views on this type of reaction see ref. 241) where the attacking agent is either a proton from a surface Br0nsted site or a coordinatively unsaturated surface cation acting as a Lewis site (cf. ref. 238)... 

Watson BA, Klein MT, Harding RH. Catalytic cracking of alkylbenzenes Modeling the reaction pathways and mechanisms. Appl Catal A-Gen 1997 160 13-39. 

Fio. 1. Correlation of cracking of alkylbenzenes over silica-alumina catalyst at 500° (24) by the Taft equation. 

Intercalation of smectite clays with polyoxycations provides a new class of porous materials. Intercalated clays are called pillared clays and they have high thermal stability and large surface area. Vaughan and Lussier(ref. 1) were the first to point out the shape selective sorption property of pillared montmorillonite using various probe hydrocarbons. The shape selective catalysis in cracking of alkylbenzenes was demonstrated by Shabtai and co-workers(ref. 2). 

Raw materials for obtaining benzene, which is needed for the production of alkylbenzenes, are pyrolysis gasoline, a byproduct of the ethylene production in the steam cracking process, and coke oven gas. Reforming gasoline contains only small amounts of benzene. Large amounts of benzene are further produced by hydrodealkylation of toluene, a surplus product in industry. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

Thus hydrochloric acid is a derivative of chlorine. About 93% of it is made by various reactions including the cracking of ethylene dichloride and tetrachloroethane, the chlorination of toluene, fluorocarbons, and methane, and the production of linear alkylbenzenes. It is also a by-product of the reaction of phosgene and amines to form isocyanates. 

A cracking process, the dealkylation of alkylbenzenes, became an established industrial synthesis for aromatics production. Alkylbenzenes (toluene, xylenes, tri-methylbenzenes) and alkylnaphthalenes are converted to benzene and naphthalene, respectively, in this way. The hydrodealkylation of toluene to benzene is the most important reaction, but it is the most expensive of all benzene manufacturing processes. This is due to the use of expensive hydrogen rendering hydrodealkylation too highly dependent on economic conditions. 

Fragmentation of alkylbenzenes over silica-alumina occurs exclusively by acid-catalyzed cracking. The reaction selectively cleaves the bond between the phenyl ring and the a-carbon of the side-chain. This occurs more than 100 times more often than the cracking of all the other bonds combined. Cracking rates of secondary alkylbenzenes are about an order of magnitude higher than those of w-alkylbenzenes. 

Herlem et al463 have observed that asphaltene is dissolved in fluorosulfuric acid and the process is accompanied by strong redox reactions (SO2 and HF evolution). The products are mainly functionalized by SO3H groups, but SO2F groups were also detected by XPS. Indeed, model studies with benzene showed the formation of benzenesulfonic acid, diphenylsulfone, and benzenesulfonyl fluoride. For alkylbenzenes, sulfonation was not accompanied by cracking of the alkyl chain. 

Conventional thermal cracking of pure hydrocarbons has been studied extensively in the past, and well-substantiated mechanistic proposals have been outlined (7-19). In contrast, hydropyrolysis of pure model compounds has been studied to a lesser extent and has been confined mostly to low (Ci-C5) paraffins, and simple alkylbenzenes and naphthenoaro-... 

Alkylbenzenes with Side Chains of Three or More Carbons. The products from hydrocracking of alkylbenzenes containing side chains of three to five carbon atoms are relatively simple. Direct dealkylation is the primary cracking reaction. For example, ferf-amylbenzene gives benzene... 

Rase and Kirk 24) have compared the adsorption coefficients of a series of alkylbenzenes calculated for cracking of these hydrocarbons on a silica-alumina catalyst from Eq. (1) with the bond strength of structurally related alkanes and have obtained a linear relation. The correlation coefficient is again high, 0.98. As has been shown in Table I, the corresponding rate constants can be correlated by the Taft equation. However, the plot of log Kj vs a gives a curve. 

Mochida, I. Yoneda, Y., Linear Free Energy Relationships in Heterogeneous Catalysis I. Dealkylation of Alkylbenzenes on Cracking Catalysts, J. Catal, 1967, 7,386-392. 

Important structural information was also obtained from pyrolysis of PRB A at 400 °C carried out under an helium flow so as to minimize secondary reactions (67). GC-EIMS of the pyrolysate fractions obtained by CC and AgN03-Si02 TLC showed the predominance of hydrocarbons ca 55% of the total pyrolysate). Regular series of C13 to C31 n-alkanes and n-alk-l-enes, formed by cracking of C-C bonds, are the major constituents of this fraction. They are accompanied by minor series of n-alkylbenzenes, n-alkyl- and n-alkenylcyclohexanes and n-trans-alkenes. Pyrolysis also provided a complex mixture of unidentified ketones and a series of n-fatty acids dominated by palmitic and oleic acids. The recovery of fatty acids on pyrolysis of PRB A, although isolation of this resistant material required drastic basic and acid treatments, indicates that the corresponding esters are sterically protected in the polymeric network. 

Where relatively high concentrations of H atoms exist, addition of H followed by loss of an alkyl group is an efficient way of reducing alkylbenzenes and polyaromatics to benzene which, of course, is of considerable significance in aromatic cracking processes. 

Of the n-alkylbenzenes the effect of toluene equalled that of benzene. The higher alkylbenzenes inhibited both cracking and isomerization. 

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