Catalytic cracking of C4 alkenes to propene and ethene: Influences of zeolites pore structures and Si/Al2 ratios

Catalytic cracking of butene to propene and ethene was carried out in a continuous plug-flow stainless steel reactor over various zeolites with different pore structures and Si/Al2 ratios. It was found that the porous structure and acidity of zeolites play important roles in the zeolite performance in the butene cracking process. The smaller the pore size of the zeolite catalysts, the higher the initial selectivities towards propene and ethene and the lower the initial selectivities to C3-C4 alkanes. This is attributed to a suppression of the hydrogen transfer reaction of the alkenes. Although high initial selectivities to propene and ethene could be gained on medium-pore zeolites of ZSM-22, ZSM-23, ZSM-35 and on small-pore zeolite SAPO-34, the stabilities of all these zeolites were poor. On the other hand, due to its special structure characteristic, the ZSM-5 zeolite exhibited the best stability in the catalytic cracking of butene among the tested zeolites. MCM-22 also presented a better catalytic performance. Besides the pore structure, the acidity, which was influenced by the Si/Al2 ratio of the zeolite, was another important factor affecting the performance of the zeolites. By increasing the Si/Al2 ratio of the ZSM-5 zeolite, the selectivities to propene and ethene and the stabilities were remarkably increased owing to a suppression of secondary reactions.