Bols Max L, Hallaert Simon D, Snyder Benjamin E R, Devos Julien, Plessers Dieter, Rhoda Hannah M, Dusselier Michiel, Schoonheydt Robert A, Pierloot Kristine, Solomon Edward I, Sels Bert F
Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis , KU Leuven , Celestijnenlaan 200F , B-3001 Leuven , Belgium.
Department of Chemistry , KU Leuven , Celestijnenlaan 200F , B-3001 Leuven , Belgium.
J Am Chem Soc. 2018 Sep 26;140(38):12021-12032. doi: 10.1021/jacs.8b05877. Epub 2018 Sep 13.
The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from NO to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV-vis-NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with FeO being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology's 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties.
本文展示了在CHA沸石拓扑结构中形成的单中心α-Fe。该中心在从NO中夺取氧原子形成高活性α-O方面表现出活性，能够在室温下活化甲烷生成甲醇。甲醇产物随后可在200°C下通过在线蒸汽脱附。对于反应循环的中间步骤，通过紫外-可见-近红外光谱和穆斯堡尔光谱监测Fe活性中心的演变。构建了CHA中α-Fe中心的B3LYP-DFT模型，并通过CASPT2计算配体场跃迁。该模型通过α-Fe相对于其他Fe物种的优先形成、成对骨架铝的需求以及表明单核活性中心的甲醇/铁比在实验上得到了证实。简单的CHA拓扑结构被证明可以减轻在其他Fe沸石上发现的铁物种的不均匀性，除了在Fe-CHA中形成的α-Fe之外，FeO是唯一可识别的相。α-Fe中心在d6r复合构建单元中形成，这在合成和天然沸石中经常出现。最后，通过比较Fe-CHA和Fe-*BEA中的α-Fe，发现拓扑结构的6MR几何形状以可预测的方式影响α-Fe中心的结构、配体场以及光谱。因此，沸石拓扑结构的变化可用于合理调节活性中心的性质。
