The direct transformation of ethane, the second largest component of natural gas, to benzene, toluene, and xylene (BTX) has become increasingly important due to the recent shale gas revolution. Although, various papers and patents have demonstrated the successful conversion of ethane to BTX using metals, such as Pt, Zn, Ga and Re, functionalized zeolites as the catalyst, the fundamental knowledge, such as the underlying catalytic mechanisms and the properties of the catalytic active sites for this reaction has been rarely discussed. Generally, a synergy between the metal function for dehydrogenation and zeolite acid function for aromatization has been considered as vitally important for ethane dehydroaromatization, however, the state of the metal species (metallic, oxidic or partially carbonized) remains unclear. Unfortunately, such information is difficult to obtain due to the fact that 1) fast coke deposition during the reaction, especially during the early-stage, and 2) significant transform of the metal/zeolites interaction through, such as redistribution and solid-state ion exchange during the high-temperature reaction process. Therefore, our recent study focuses on the Transient Kinetic Analysis of ethane dehydroaromatization, which provides the early-stage (induction period) catalytic performance data (activity and products distribution). We found that the early-stage catalytic behaviors for the fresh and regenerated Zn/HZSM-5 catalysts were significantly different. While the transient period in terms of benzene formation is ~400 s for the fresh Zn/HZSM-5 catalyst, it takes only ~150 s for the regenerated catalyst after coke burn-off. For both catalysts, a sharp peak for methane formation was observed during the initial time, which then decreased to the steady state selectivity after ~100 s. The different transient behavior between the fresh and regenerated catalysts suggested that the catalytically active phase is undergoing reconstruction, which supposedly to involve the solid-state ion exchange, and/or redistribution of the metal particles into the microchannel of the ZSM-5, during the reaction process. In terms of the steady state catalytic performance, both fresh and regenerated Zn/HZSM-5 catalysts shown similar ethane conversion (~32%) and aromatics selectivity (the sum selectivity of BTX is more than 50% and the total selectivity of BTX + olefins is more than 90%).

Early-stage catalytic behavior of ethane dehydroaromatization over zinc modified HZSM-5 catalyst via transient kinetic methods

255th National Meeting (2018)

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