Aromatic Hydrocarbons, Steam Dealkylation

Aromatic Hydrocarbons, Steam Dealkylation.—Research on steam dealkylation has thrown light on the mechanism of reforming. Rabinovich and other Russian workers, following their earlier research in this area which is summarized by Grenoble, concluded that the toluene reaction with steam over precious metal/alumina catalysts proceeds by simultaneous-consecutive reactions  

The catalysts are bifunctional with dealkylation of toluene taking place on the metal, producing intermediates such as the methylene radical =CH2, which reacts with hydroxyl groups produced by water adsorption on the alumina support  

As indicated the adsorbed CO can be desorbed or undergo the shift reaction to an extent depending on the catalyst. In kinetic studies, the reaction orders for both toluene and water were almost zero, indicative of strong adsorption of both reactants. Toluene conversion was retarded by the addition of CO, particularly the rupture of the aromatic ring where the benzene ring is w-bonded flat to the surface. The dealkylation which precedes this involves a a-bonded adsorbed toluene compound where the ring is not in the catalyst surface plane. CO competing for the metal surface would impede the formation of the -complex more than the cr-bonded molecule. 

The work of Kikuchi et supports this view that benzene is an inter- 

No n-hexane was produced from n-heptane so step 2 is irreversible and benzene must have been produced from the adsorbed toluene intermediate. At 600 °C, n-heptane and toluene were gasified almost exclusively via the 6-membered intermediates, i.e., steps 1, 2, 5, and 6. 

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Steam Dealkylation. - The steam dealkylation of aromatic hydrocarbons, such as toluene ... 

Duprez, D., Pereira, P., Miloudi, A., and Maurel, R. (1982) Steam dealkylation of aromatic hydrocarbons II Role of the support and kinetic pathway of oxygenated species in toluene steam dealkylation over group VIII metal catalysts. /. Catal, 75 (1), 151-163. 

More than 90% of today s petrochemicals are produced from refineiy products. Most are based on the use of C2-C4 olefins and aromatics finm hydrocarbon steam cracking units, which are even more closely linked to refineries. In North America, the feedstock for steam cracker units have generally been ethane, propane, or LPG. As a result, most of the propylene and aromatics have been provided by FCC units and catalytic reformers. In maity other parts of the world where naphtha feed has been more readily available, suppUes of propylene and aromatics have been produced directly by steam cracking. When necessary, the catalytic dehydrogenation of paraffins or dealkylation of toluene can balance the supply of olefins or benzene. In Table 7.2 some of the catalytic processes that convert olefins and benzene from a steam cracker into basic petrochemicals for the modem chemical industry are shown. 

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