The Role of Reaction Temperature and Cracking Catalyst Characteristics in Determining the Relative Rates of Protolytic Cracking, Chain Propagation, and Hydrogen Transfer

The cracking of isobutane on USY zeolites with different unit cell size has been studied in the temperature range 400-500°C, using an experimental apparatus which makes it possible to follow the reaction at very short times on stream. By measuring product initial selectivities it has been found that protolytic cracking and bimolecular reactions take place on Brølnsted acid sites. In this way the contributions of bimolecular and monomolecular processes have been established. Bimolecular reactions involving hydride transfer have been separated from those responsible for chain transfer and those producing hydrogen transfer. Chain transfer accounts for the chain propagation in paraffin cracking, while hydrogen transfer produces the extra paraffin amounts obtained in these reactions. Hydrogen transfer reactions increase, but chain transfer reactions decrease when the unit cell size increases. From energetic considerations, the influence of the zeolite catalyst and reaction conditions on the controlling step in isobutane cracking can be suggested.