Cr-MIL-101 derived nano-Cr2O3 for highly efficient dehydrogenation of n-hexane
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摘要: 通过热解大比表面Cr-MIL-101制备纳米Cr2O3(n-Cr2O3),考察其催化正己烷脱氢反应性能,并比较与沉淀法p-Cr2O3-1、焙烧铬盐得到的p-Cr2O3-2以及工业Cr2O3/Al2O3催化正己烷脱氢活性差异。n-Cr2O3能够催化正己烷高效脱氢为己烯和苯,并且其催化脱氢活性与焙烧温度有关。600 ℃焙烧的n-Cr2O3催化正己烷脱氢转化率最高40.6%,对产物己烯和苯的选择性分别为20.1%和69.3%。提高焙烧温度,n-Cr2O3催化正己烷脱氢活性下降但稳定性增强,催化剂积炭量减少。p-Cr2O3-1和p-Cr2O3-2催化正己烷脱氢转化率很低(<7.5%),比活性分别为1.5和1.7 g/(m2·h),低于n-Cr2O3-600的(2.0 g/(m2·h))。通过BET、XRD、TEM和FT-IR等表征发现,n-Cr2O3为具有较大比表面的纳米颗粒(10−20 nm),多暴露晶面和脱氢活性位,而p-Cr2O3是比表面非常小的大颗粒,所暴露脱氢活性位少。相比之下,Cr2O3/Al2O3催化剂由于大比表面Al2O3的分散作用,催化正己烷脱氢效率更高(2.4 g/(m2·h))。因此,由Cr-MIL-101焙烧得到的n-Cr2O3催化正己烷脱氢的高活性源于这种纳米Cr2O3所具有的独特性质:小颗粒,大比表面,多暴露活性位。Abstract: Nano-Cr2O3 (n-Cr2O3) was prepared by calcining the mesoporous Cr-MIL-101, and the catalytic performance for n-hexane dehydrogenation was investigated. It was found that dehydrogenation of n-hexane on n-Cr2O3 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-Cr2O3-600, is 40.6%, and the selectivities to n-hexenes and benzene are 20.1% and 69.3%, respectively. Increasing the calcination temperature, the conversion of n-hexane is decreased while the stability is enhanced. The n-hexane conversion of p-Cr2O3-1 (obtained by precipitation method) and p-Cr2O3-2 (obtained by calcinating Cr(NO3)·9H2O directly) catalysts are relative 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 nanoparticle with large specific surface area that more crystal planes and dehydrogenation active sites are exposed, while p-Cr2O3 is the large particle with extremely low surface area that the dehydrogenation active sites are less exposed. By contrast, industrial Cr2O3/Al2O3 catalyst possesses high specific activity of 2.4 g/(m2·h) due to the dispersion effect of Al2O3. Therefore, the highly catalytic activity of n-Cr2O3 for n-hexane dehydrogenation is attributed to the unique properties of n-Cr2O3: small particle, large specific surface area and more exposed active sites. This work not only explains the highly dehydrogenation performance of nano-Cr2O3 derived by Cr-MIL-101, but also provides guidance for the precise design and synthesis of high-performance CrOx-based catalyst for the dehydrogenation of alkanes.
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Key words:
- n-hexane /
- dehydrogenation /
- n-hexenes /
- benzene /
- nano-Cr2O3
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表 1 Cr2O3催化剂的织构性质
Table 1 Textural properties of Cr2O3 catalysts
Sample SBET/(m2·g−1) vpore/(cm3·g−1) Dpore/nm n-Cr2O3-600 62.5 0.28 16.4 n-Cr2O3-700 57.0 0.22 18.3 n-Cr2O3-800 39.1 0.14 17.4 n-Cr2O3-950 13.4 0.09 19.7 p-Cr2O3-1 5.2 0.03 26.1 p-Cr2O3-2 6.1 0.03 33.5 Cr2O3/Al2O3 117.5 0.21 4.9 表 2 常压、不同温度下正己烷脱氢为单己烯热力学平衡数据
Table 2 Thermodynamic equilibrium data for n-hexane dehydrogenation to n-hexenes at different temperatures
t/℃ △G/(kJ·mol−1) Ke Equilibrium conversion/% 400 165.75 1.57×10−2 11.76 450 133.55 6.64×10−2 22.66 500 96.85 0.27 40.36 550 66.65 0.80 57.88 600 30 2.33 75.51 表 3 反应1 h时不同焙烧温度n-Cr2O3催化正己烷脱氢反应
Table 3 Catalytic results of n-Cr2O3 catalyst for n-hexane dehydrogenation at TOS of 1 h
Sample n-Cr2O3-600 n-Cr2O3-700 n-Cr2O3-800 n-Cr2O3-950 Conversion of n-C6H14/% 40.6 37.8 22.8 11.3 Selectivity to product/% n-C6H12 20.1 21.5 25.1 28.2 C6H6 69.3 68.1 64.4 60.6 2,4-C6H10 2.9 3.0 3.1 3.2 Cracking products 3.2 2.9 2.8 2.9 Isomerization products 2.8 2.9 2.9 3.0 Others 1.7 1.6 1.7 2.1 -
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