Influence of zeolite composition and structure on hydrogen transfer reactions from hydrocarbons and from hydrogen

The cracking of n-heptane proceeds at 623 K and a reaction pressure of 2.5 MPa preferentially according to the bimolecular cracking route. This was confirmed for MFI- and FAU-type zeolites with varying density of acid sites. The activity was found to be linearly dependent on the number of Bronsted acid sites for MFI-type zeolites. In the case of FAU-type zeolites it was observed that the activity per Bronsted acid site decreases with increasing concentration of acid sites. This was explained by differences in acid strength. All the investigated acid zeolite samples were able to activate hydrogen under the actual reaction conditions. The activated hydrogen was found to participate in bimolecular hydrogen transfer reactions during n-heptane cracking. The action of hydrogen is discussed on the basis of its influence on the rate-determining step during cracking and was confirmed by a kinetic study of n-heptane cracking on a FAU-type sample. Hydrogen transfer from hydrogen as well as from hydrocarbons was influenced in the same way by zeolite structure and acid site density. (C) 1996 Academic Press, Inc.