Haag-Dessau mechanism

Operando DRIFTS measurements suggest that bridged hydroxyl groups are in extensive interaction with hexane molecules during the reaction even at 553 K. However adsorbed alkene or surface alkoxide could not be detected. These findings questions, whether the Haag-Dessau mechanism [4] gives true description of the alkane activation process over zeolite catalysts. 

S. Kotrel, H. Knozinger, B. C. Gates, The Haag-Dessau mechanism of protolytic cracking of alkanes, Micropor. Mesopor. Mater, 2000, 35-36, 11-20. 

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

Haag, W.O., Dessau, R.M., and Lago, R.M. (1991) Kinetics and mechanism of paraffin cracking with zeolite catalysts. Stud. Surf Sd. Catal., vol. 60, Elsevier, Amsterdam, pp. 255-265. 

Haag, W.G. and Dessau, R.M. (1984) Duality of mechanism in add catalyzed paraffin cracking, in Proceedings of the Eighth International Congress on Catalysis, vol. 2, Verlag Chemie, Weinheim, p. 305. 

Pore size may also affect the reaction order. Cracking of small (i.e., less than C ) paraffins over amorphous acid catalysts and large-pore zeolites may proceed either by a bimolecular or by a monomolecular mechanism. In medium- and small-pore zeolites the space is insufficient to form bulky bimolecular transition states. This makes a monomolecular path more likely. Low reactant partial pressure, low acid site density, and high temperatures (above 450-500 C) also favor the monomolecular mechanism. According to Haag and Dessau [24] and Kranilla, Haag, and Gates [25], the transition state of the monomolecular reaction involves a penta-coordinated carbonium ion. 

Aceording to the mechanism proposed by Haag and Dessau, the initial activation takes place by protonation of the reactant alkane at the most highly substituted carbon atom. In the case of methylcyclohexane, this yields the following structure ... 

Haag WO, Dessau RM (1984) Duality of mechanism for acid-catalyzed paraffin cracking. In Proceedings of the 8th international congress on catalysis, Berlin, 1984, vol 2. Verlag Chemie-Dechema, Heidelberg, p 305... 

The direct formation of carbenium ions from alkane molecules has been the subject of much discussion. Haag and Dessau showed that a monomolecular mechanism via a penta-coordinated carbonium ion intermediate as well as a hydride transfer mechanism make important contributions under certain conditions. 

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