A THEORETICAL DESCRIPTION FOR THE MONOMOLECULAR CRACKING OF C-C BONDS OVER ACIDIC ZEOLITES

The pathway to monomolecular C-C bond cracking over Bronsted acid sites has been characterised theoretically using the AM1 molecular orbital method. The model describes not only the transition of the proton from the acid site to the paraffin, but also the collapse of the resulting carbonium ion directly to a smaller alkene and alkane. This model thus predicts that the protolysis is not driven by an acid-base pair type reaction, as witnessed with ammonia and alkene activation. The data amassed allowed a quantitative description of the primary product distributions for the alkanes butane and hexane which shows good agreement with experimental data. (C) 1995 Academic Press, Inc.