Reaction mechanism and microkinetics of 1-hexene catalytic pyrolysis on HZSM-5: A first-principles study

Abstract

Catalytic pyrolysis is an important way of integrating refining and petrochemical processes, as it efficiently converts FCC gasoline into propylene. Enhancing the selectivity of propylene production remains a major focus in this field. To enhance the selectivity of propylene in catalytic pyrolysis of olefin, the reaction mechanism, the microkinetics of the catalytic pyrolysis of 1-hexene, as well as the effect of acid strength were investigated using the density functional theory. The optimal paths to light olefins are determined, in which β-scission reaction is the rate controlling step to produce propylene in 1-hexene cracking paths. Meanwhile, increasing the acid strength will reduce the apparent activation energy of the reaction and facilitate the path to propylene production than the path to ethylene production. The microkinetic modeling shows that the system rate controlling step gradually shifts from a high-energy-barrier reaction to an adsorption reaction of reactants as the reaction temperature increases. In addition, the proportion of monomolecular mechanisms is larger than the bimolecular mechanism under high temperature conditions and the yield of propylene reaches maximum at 750 K, which helps optimize operating temperature for catalytic pyrolysis of 1-hexene. The work can offer new insights into the mechanism of catalytic pyrolysis of olefins and provide a theoretical guide for designing highly active catalysts for such reactions.