Aqueous-Phase Methane Oxidation over Fe-MFI Zeolites; Promotion through Isomorphous Framework Substitution
Aqueous-Phase Methane Oxidation over Fe-MFI Zeolites; Promotion through Isomorphous Framework Substitution
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DOI：
10.1021/cs400288b
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发表时间：
2013-08-01
期刊：
影响因子：
12.9
通讯作者：
Hutchings, Graham J.
中科院分区：
文献类型：
--
作者：
Hammond, Ceri;Dimitratos, Nikolaos;Hutchings, Graham J.
含铁和含铜的沸石最近被证明是在50摄氏度的水介质中，以H2O2作为绿色氧化剂，甲烷一步选择性转化为甲醇的有效催化剂。在此之前，我们已经观察到单独的Fe物种能够催化这种高度选择性的转化。然而，进一步的催化测试和光谱研究表明，尽管这些框架外的铁是催化剂的活性成分，但在将其他三价阳离子（如Al3+或Ga3+）掺入mfi框架时，可以观察到显著的促进作用。虽然这些额外的框架物种不构成活性催化中心，但它们的加入可以观察到促进作用，因为它们(1)促进了铁从沸石框架中提取，从而增加了活性铁物种的形成；(2)提供了一个相关的带负电的框架，能够稳定和维持负责催化活性的阳离子框架外铁物种的分散。通过了解这些现象并随后控制催化剂（Fe和Al）的整体组成，我们随后能够制备一种具有相同内在活性（即TOF）的催化剂，但与迄今为止报道的该反应的最佳催化剂相比，生产率（即时空产率）提高了五倍。
Fe- and Cu-containing zeolites have recently been shown to be efficient catalysts for the one-step selective transformation of methane into methanol in an aqueous medium at only 50 degrees C, using H2O2 as green oxidant. Previously, we have observed that Fe species alone are capable of catalyzing this highly selective transformation. However, further catalytic testing and spectroscopic investigations demonstrate that although these extra-framework Fe species are the active component of the catalyst, significant promotion is observed upon the incorporation of other trivalent cations, e.g., Al3+ or Ga3+, into the MFI-framework. While these additional framework species do not constitute active catalytic centers, promotion is observed upon their incorporation as they (1) facilitate the extraction of Fe from the zeolite framework and hence increase the formation of the active Fe species and (2) provide an associated negatively charged framework, which is capable of stabilizing and maintaining the dispersion of the cationic extra-framework Fe species responsible for catalytic activity. By understanding these phenomena and subsequently controlling the overall composition of the catalyst (Fe and Al), we have subsequently been able to prepare a catalyst of equal intrinsic activity (i.e., TOF) but five-times higher productivity (i.e., space-time-yield) compared with the best catalysts reported for this reaction to date.