Abstract
A series of large-pore mesoporous silica (LPMS)-supported CrOx catalysts were synthesized by hydrothermal and impregnation methods and tested for ethane dehydrogenation in the presence of CO2 as an oxidant. To assess the effect of hydrothermal temperature treatment on the characteristics of LPMS support, different hydrothermal temperatures (100–160 °C) were studied and optimized. The optimum support was then loaded with different amounts of chromium (0, 2, 4, 8, and 11 wt % Cr). The obtained catalysts were characterized by different techniques such as XRD, BET, TEM, SEM, XPS, FTIR, and diffuse reflectance UV-Vis spectroscopy. The characterization results indicated that the sample hydrothermally treated at 130 °C exhibited the highest pore volume, a narrow pore size distribution, and a moderate BET surface area. Chromium species with various oxidation states including Cr3+, Cr6+, and α-Cr2O3 were detected in all synthesized Cr(y)/LPMS-130 catalysts. A lower Cr content resulted in the formation of Cr6+, whereas a higher Cr content dominated the α-Cr2O3 on the surface of the catalyst. Among the synthesized catalysts, the Cr(4)/LPMS-130 catalyst showed the highest Cr6+/Cr3+ ratio, indicating a good dispersion of chromium species along with a fine particle size. The ethane conversion and ethylene selectivity were 50.5 and 91.1% for Cr(4)/LPMS-130, respectively. Carbon dioxide was believed to supply enough lattice oxygen to maintain the Cr species at a higher oxidation state and to consume the hydrogen resulting from ethane cracking by a reverse water gas shift reaction.
Highlights
The high market demand for ethylene derivatives such as polyethylene, ethylene oxide, ethylene glycol, ethylene dichloride, and ethylbenzene, has made ethylene one of the essential intermediate feedstocks in the petrochemical processes [1,2,3]
Hydrothermal treatment is an essential step in the synthesis of mesoporous material and is a major bottleneck that defines surface area, pore size, and structures of the prepared materials
Illustrate the N2 sorption isotherms and the distribution of pore size of the prepared large-pore mesoporous silica (LPMS)-x samples treated at different hydrothermal temperatures
Summary
The high market demand for ethylene derivatives such as polyethylene, ethylene oxide, ethylene glycol, ethylene dichloride, and ethylbenzene, has made ethylene one of the essential intermediate feedstocks in the petrochemical processes [1,2,3]. Kim et al established that, by including n-butanol as a co-solvent and by changing the silicate/n-butanol ratio in the MCM48 synthesis procedure, various silicate structures can be obtained, whereas the change in temperature treatment widens the pore diameter in the range of 4.5–10 nm [20] Amphiphilic triblock copolymers such as P123 have been employed as very effective templates for the synthesis of uniform large-pore (~50 nm) mesoporous silicate materials such as SBA15 and MCM48 [21]. Most studied catalysts have been based on nano-silica and porous MCM41 as support for the catalyst, where its BET surface area was correlated with the obtained catalytic activity. In this work, large-pore mesoporous silica nanomaterials were synthesized and loaded with chromium oxide to produce a series of supported CrOx catalysts that were further tested for the ODH of ethane (Supplementary Materials). The structural properties of the synthesized catalysts were investigated
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