ACS Catalysis p. 7984 - 7997 (2019)
Update date:2022-08-11
Topics:
Han, Seung Ju
Lee, Sung Woo
Kim, Hyun Woo
Kim, Seok Ki
Kim, Yong Tae
For a stable methane to olefins, aromatics, and hydrogen (MTOAH) reaction, 0.27-0.43 wt % Fe-containing silica catalysts were synthesized through various preparation methods and tested. The presence of Fe species in SiO2 mixtures increased the true and apparent densities of the catalysts during the melt-fusing process at 1700 °C. Several characterizations (i.e., H2-TPR, TEM, and XAS) revealed that partially reduced iron oxide (Fe3O4) predominantly existed in cristobalite (CRS) in the melt-fused catalysts. The FeCRS catalyst prepared from fayalite and quartz by the melt-fusing method showed a higher resistance to structural sintering and coke deposition than other Fe catalysts during MTOAH at 1020 °C. It also showed a 40% higher apparent activation energy for coke formation than for methane consumption in the temperature range of 1000 to 1040 °C. Increased CRS surfaces increased the coke selectivity, indicating that even the pure CRS surface acts as a chain reaction terminator to form coke. At the same space velocity (9400 h-1), the FeCRS catalyst was more selective in producing C2 (ethane, ethylene, and acetylene), C3-C5 olefins, and aromatics than pure CRS and other Fe catalysts. At a steady state, the FeCRS surface was most suitable for methane conversion, being 2.3 times more efficient than without a catalytic surface. The FeCRS catalyst exhibited a stable activity and low coke selectivity, even for 50 h, in the MTOAH reaction. EXAFS profiles showed that highly dispersed Fe carbide with Fe-Si coordination was formed in the FeCRS catalyst, and electronic structure calculations indicated that these confined Fe sites were more favorable for methyl radical formation and a high coke resistance than Fe3C clusters. By optimizing reaction parameters, the FeCRS catalyst exhibited 6.9-5.8% methane conversion and 86.2% C2 selectivity for 100 h with cofeeding of 50% H2 at 1080 °C.
View MoreDoi:10.1039/jr9600000502
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