Cr-MIL-101 derived nano-Cr₂O₃ for highly efficient dehydrogenation of n-hexane

Nano Cr₂O₃ (n-Cr₂O₃) was prepared by the thermolysis of the mesoporous Cr-MIL-101, and its catalytic performance for n-hexane dehydrogenation was investigated and compared with Cr₂O₃ obtained by traditional method. It is found that dehydrogenation of n-hexane on n-Cr₂O₃ catalyst can produce n-hexenes and benzene efficiently, and the catalytic performance is related to the calcination temperature. The optimal n-hexane conversion can be obtained on n-Cr₂O₃ calcinated under 600 ℃, is 40.6%, and the selectivities to n-hexenes and benzene are 20.1% and 69.3%, respectively. The conversion of n-hexane for n-Cr₂O₃ catalyst is decreased with calcination temperature, while the catalyst stability in dehydrogenation reaction is enhanced. n-Hexane conversion of p-Cr₂O₃-1 (obtained by precipitation method) and p-Cr₂O₃-2 (calcinating Cr(NO₃)·9H₂O directly) catalysts are very low (<7.5 %), and their specific activity for n-hexane dehydrogenation are 1.5 and 1.7 g/(m²·h) respectively, lower than that of n-Cr₂O₃-600 (2.0 g/(m²·h)). The results of BET, XRD, TEM and FT-IR reveal that n-Cr₂O₃ is the nanoparticles with large specific surface area that more dehydrogenation active sites are exposed, while p-Cr₂O₃ is the large particles with extremely low surface area that few dehydrogenation active sites are presented. By contrast, industrial Cr₂O₃/Al₂O₃ catalyst possesses the highest specific activity of 2.4 g/(m²·h) due to the dispersion effect of Al₂O₃. Therefore, highly catalytic activity of n-Cr₂O₃ for n-hexane dehydrogenation is attributed to the unique properties of small particle, large specific surface area and more exposed active sites. This work not only explains the high dehydrogenation activity of nano-Cr₂O₃ derived by Cr-MIL-101, but also provides guidance for the precise design and synthesis of high-performance CrO_x-based catalyst for the dehydrogenation of alkanes.