Catalytic dehydrogenation of propane over iron-silicate zeolites
Catalytic dehydrogenation of propane over iron-silicate zeolites
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DOI：
10.1016/j.jcat.2014.02.007
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发表时间：
2014-04
影响因子：
7.3
通讯作者：
J. Yun;R. Lobo
中科院分区：
文献类型：
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作者：
J. Yun;R. Lobo
制备了同晶取代的铁硅酸盐分子筛，并考察了其对单分子丙烷反应的催化性能。合成了Si/Fe比分别为26和48的[Fe]ZSM-5（MFI）和Si/Fe比为15的[Fe] β沸石（BZ *），并用X射线粉末衍射（XRD）、N2吸附和紫外-可见（UV/Visible）光谱对其进行了表征。使用XRD确定的晶胞体积用于监测框架中Fe的存在以及Fe物质从框架向框架外位置的迁移。在753 K的空气中温和煅烧后，大部分Fe原子仍保留在骨架位置上，但在973 K下蒸汽处理后，大部分Fe物种迁移到骨架外位置。Fe物种从框架到框架外位置的迁移也可以在单分子丙烷反应期间发生。孤立的骨架Fe位是最重要的烃类转化位点，即使在框架外位置的Fe物种也表现出脱氢催化活性。铁硅酸盐分子筛的脱氢反应速率比裂解反应速率高近20倍，而硅铝比相近的H-[Al]ZSM-5分子筛的脱氢反应选择性较低（裂解比为1.26）。H-[Fe]ZSM-5的活化能也低于H-[Al]ZSM-5的活化能。一个氧化还原催化循环，建议通过形成丙烷自由基阳离子的烃反应在[Fe]沸石中的孤立的Fe骨架网站，因为质子机制似乎无法解释所观察到的活化能和选择性的差异。
Isomorphously substituted iron-silicate zeolites were prepared and their catalytic properties for the monomolecular propane reaction were investigated. [Fe]ZSM-5 (MFI) samples with Si/Fe ratios of 26 and 48, and [Fe]beta zeolite (BEA*) with Si/Fe ratio of 15 were synthesized and characterized using X-ray powder diffraction (XRD), N2adsorption and ultraviolet–visible (UV/Visible) spectroscopy. Unit cell volumes, determined using XRD, were used to monitor the presence of Fe in the framework and the migration of Fe species from framework to extra-framework positions. Most of the Fe atoms remain in framework positions after mild calcination (753 K in air), but most of the Fe species migrate to extra-framework positions after steaming at 973 K. Migration of Fe species from framework to extra-framework positions can occur during the monomolecular propane reaction as well. Isolated framework Fe sites are the most important sites for hydrocarbon conversion even though Fe species in extra-framework positions also showed catalytic activity for dehydrogenation. The iron-silicate zeolites exhibited nearly 20 times higher dehydrogenation rates than cracking rates while H-[Al]ZSM-5 with similar Si/Al ratio (∼26) showed low dehydrogenation selectivity (dehydrogenation–to-cracking ratio ∼0.36). The activation energy for H-[Fe]ZSM-5 is also lower than the activation energy for H-[Al]ZSM-5. A redox catalytic cycle is suggested for the hydrocarbon reaction over isolated Fe framework sites in [Fe] zeolites via the formation of propane radical cations since a protolytic mechanism seems to be unable to explain the observed differences in activation energy and selectivity.