Activity-acidity relationship for alkane cracking over zeolites:: n-hexane cracking over HZSM-5

The acidity and the catalytic activity for n-hexane cracking of three related HZSM-5 catalysts have been determined. A method is described for obtaining the distribution of acid strength by deconvoluting ammonia TPD spectra and it is shown that a linear relationship exists between the enthalpy of the reaction and the activation energy for the desorption of ammonia. Both the Polanyi and the Marcus models were used to derive relationships between the reaction activation energy and the ammonia desorption activation energy. As expected, the former decreases when the latter increases; the reaction activation energy is lower on acidic sites of greater strength. Similarly, equations were derived relating the catalytic activity to the acid strength of the catalytic sites expressed by the activation energy for the desorption of ammonia. Observed activities for the cracking of n-hexane were correlated satisfactorily to theoretical ones evaluated using the Polanyi and the Marcus models, considering the distribution of acid site strength. For the cracking of n-hexane and the range of experimental conditions used, the Marcus model does not present a significant improvement relative to the Polanyi model. The strategy described in this work indicates that it is possible to predict the activity of a zeolite for a given reaction, provided the acid sites strength distribution is known; one simple method for obtaining the latter being ammonia TPD. (C) 2004 Elsevier B.V. All rights reserved.