Investigating the Activation Mechanism for Efficient Catalyst Design in the Catalytic Pyrolysis of Hexane: A DFT-based Microkinetics and Experiments

Promoting hexane activation to further improve propylene selectivity has been a challenge. In this study, the activation mechanisms of hexane on Brønsted acid sites (BAS) of ZSM-5, on metal-doped Lewis acid sites (LAS) and on mixed systems with olefins have been investigated by density functional theory (DFT), microkinetics and experiments. DFT results show strong BAS are more favorable for the activation of hexane compared to weak BAS. The existence of two mechanisms for the activation of hexane under LAS. The activation of hexane to generate hexene and hydrogen on the LAS under M−I type metal doping, Ni and Cu belong to the M−I type metals, where Ni metal promotes C–H bond breaking more effectively (136.11 kJ/mol). On the other hand, under M−II type metal doping, the activation of hexane to produce hexyl carbenium ions, Co, Mn, Fe, and Zn belong to the M−II type metals, with Co metal exhibiting the highest efficiency in promoting C–H bond cleavage (18.56 kJ/mol). Among the mixed systems of olefin and hexane, the mixed system of pentene and hexane was chosen to be more effective. Microkinetics revealed that reactions occurred at lower temperatures on LAS compared to BAS. Furthermore, the capability to promote hexane activation was ranked as follows: LAS>olefin addition > BAS, which is consistent with the experimental results. To further promote hexane activation, it is necessary to dope the catalyst with Co metals and reduce the reaction temperature appropriately. Additionally, a small amount of olefin can be added in the process. This study provides a theoretical basis for the design of catalysts and optimization of operation conditions.