Dehydrocyclization, paraffins oxide

The formation of aromatics by the catalytic dehydrocyclization of paraffins with chains of six or more carbon atoms has been known for some time. Certain oxides of the 5th and 6th subgroups of the periodic table, such as chromia and molybdena, were shown early to be particularly effective catalysts for the reaction. Consequently, most of the reported studies of the kinetics and mechanism of the reaction have been carried out using these catalysts (P6, H4, H5). Since the available data on the kinetics of dehydrocyclization over oxide catalysts have been reviewed by Steiner (S9) in 1956, only a brief summary of the work will be made here, primarily for the purpose of orientation. The relatively few kinetic data which have been reported for dehydrocyclization over the bifunctional platinum on acidic oxide catalysts will be discussed subsequent to this. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Concurrently with the discovery and development in this country of the catalytic conversion of paraffins to aromatics (131) three different groups in the U.S.S.R. discovered this reaction independently of each other. Moldavskil and co-workers (238,239) showed that paraffins with six or more carbon atoms form aromatics by closure of a six-membered ring. For example, n-octane gives xylene and some ethylbenzene over amorphous chromia at about 470°C. Olefins also undergo this reaction. In subsequent publications, the group headed by Moldavskil demonstrated that molybdenum sulfide, titanium dioxide, and other oxides as well as activated carbon also may be used for dehydrocyclization (237,239). 

In his 1940 review Plate subjected the experimental material on dehydrocyclization of paraffins published to that time to a critical analysis (304) and concluded that aromatization of paraffins at the temperatures employed will depend upon the selection of proper catalysts in order to suppress the competing reactions, that the multiplet theory satisfactorily explains the mechanism of cyclization, and that intermediate formation of olefins is conceivable on oxide catalysts but can hardly occur on platinum. 

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