Mechanistic origin of transition metal modification on ZSM-5 zeolite for the ethylene yield enhancement from the primary products of n-octane cracking

In this work, n-octane has been selected to represent the alkane content in the naphtha for the catalytic cracking over Mn-ZSM-5 and Fe-ZSM-5 catalysts. Various characterization techniques are applied to quantify the possible active sites. The introduction of transition metal cations does not influence the strength of BAS. Therefore, the change on kinetic behavior is the contribution from newly generated LAS. Upon the elucidation of their roles in different adsorption scenarios, it is found that the new LAS establishes new adsorption equilibrium with olefin molecule formed from the thermal cracking, making the remaining alkane molecules adsorb and react on the BAS with minimum coverage of olefin. It is therefore favorable for the enhancement of ethylene yield in the primary cracking products. Furthermore, the unfavorable energy requirement of the carbonium ion pathway is compensated by the boosted activation entropy via the pathway change, as suggested by the kinetic analysis.