Abstract
A series of HZSM-5 catalysts with similar Si/AlF mole ratio, textual properties and morphology, but different contents of AlF pairs, were synthesized by controlling the Na/Al molar ratios in the precursor gel and used for propane aromatization. It is shown that the catalyst with a Na/Al molar ratio of 0.8 in the synthetic gel possesses the highest paired AlF concentration (64.4%) and shows higher propane conversion (38.2%) and aromatics selectivity (19.7 wt.%). Propane pulse experiments, micro reactor activity estimation, Operando diffuse reflectance ultraviolet-visible (DR UV-vis) spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) analysis of coke species deposited on the catalysts provide evidence that AlF pairs in the ZSM-5 framework promote oligomerization and cyclization reactions of olefins, and then produce more aromatics. Density Functional Theory (DFT) calculations demonstrate that the cyclization of olefins and hydride transfer reaction occurring on AlF pairs in HZSM-5 zeolite show a lower free energy barrier and a higher rate constant than those on single AlF, indicating that the structure of AlF pairs in the HZSM-5 zeolite has a stronger electrostatic stabilization effect on the transition states than that of single AlF.
Highlights
IntroductionThe aromatization of light alkanes (mainly propane and butane) has been extensively studied for almost three decades due to its importance in the economic and strategic fields [1,2]
The aromatization of light alkanes has been extensively studied for almost three decades due to its importance in the economic and strategic fields [1,2]
The obtained ZSM-5 catalysts similar Si/AlF mole ratio, textual properties, morphology, but different AlF pairs concentration were with similar Si/AlF mole ratio, textual properties, morphology, but different AlF pairs concentration applied to propane aromatization reactions
Summary
The aromatization of light alkanes (mainly propane and butane) has been extensively studied for almost three decades due to its importance in the economic and strategic fields [1,2]. Few studies have examined the influence of the framework aluminum (AlF ) distribution of HZSM-5 zeolites on its propane aromatization catalytic performance. Sazama and co-workers [20] found that AlF pairs in HZSM-5 framework were beneficial to 1-butene oligomerization in 1-butene conversion, whereas single AlF was favorable to 1-butene cracking and gave higher selectivity to lower olefins. The structure of AlF pairs in the ZSM-5 zeolite shows stronger electrostatic stabilization effect on the transition states than that of single AlF. These results gave new insights into the impact of AlF distribution in HZSM-5 on propane aromatization, which should facilitate the design of high performance catalysts
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