Acid strength controlled reaction pathways for the catalytic cracking of 1-butene to propene over ZSM-5

The influence of the acid strength of P-modified and of HNO ₃-dealuminated HZSM-5 on the selectivity for the formation of propene was examined in the conversion of 1-butene. Under the appropriate amount of total acid sites, the reaction pathways of dimerization cracking of 1-butene, which involve the mole ratio of propene to ethene (P/E ratio) and conversion of butenes, could be controlled by acid strength distribution. The results showed that the P/E ratio increased with the decreased amount of strong acid sites. For the ZSM-5 with relatively large amount of strong acid sites, the butene conversion was improved via increasing reaction temperature. In contrast, the butene conversion decreased with the increased reaction temperature over ZSM-5 bearing small amount of strong acid sites. It was because the activation energies of different reaction pathways were influenced by acid strength to a different extent. Strong acid sites could promote the pathways I (2C4→C8I→2C4) and II (2C4→C8II→C2+C6) favorably, while weak acid sites preferred the pathway III (2C4→C8III→C3+C5) since pathway III underwent some energetically favorable forms (tertiary-secondary, secondary-secondary) of cracking (C8I,C8II,C8III, represent octyl carbenium ions). According to designing acid strength distribution on ZSM-5, P/E ratio and selectivity of propene could be significantly improved.