Mahyuddin M Haris, Shiota Yoshihito, Staykov Aleksandar, Yoshizawa Kazunari
Engineering Physics Research Group, Bandung Institute of Technology , Bandung 40132, Indonesia.
Inorg Chem. 2017 Sep 5;56(17):10370-10380. doi: 10.1021/acs.inorgchem.7b01284. Epub 2017 Aug 15.
While the most likely structure of the active site in iron-containing zeolites has been recently identified as [FeO] (Snyder et al. Nature 2016, 536, 317-321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d isoelectronic systems, [FeO] and [MnO] zeolites. We found that [FeO] zeolites favor the direct-radical-rebound mechanism with low CH activation energies, while [MnO] zeolites prefer the nonradical mechanism with higher CH activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and O (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO] zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for NO decomposition over [Fe]-MFI and -AEI zeolites were also performed.
虽然含铁沸石中活性位点最可能的结构最近已被确定为[FeO]（斯奈德等人，《自然》，2016年，536卷，317 - 321页），但在这种活性物种上甲烷直接转化为甲醇的机制在直接自由基反弹机制或非自由基（协同）机制之间仍存在争议。利用周期系统的密度泛函理论，我们计算了[FeO]和[MnO]沸石这两种d等电子体系上的两种反应机制。我们发现，[FeO]沸石有利于直接自由基反弹机制，具有较低的CH活化能，而[MnO]沸石则倾向于具有较高CH活化能的非自由基机制。尽管它们具有等电子结构，但这些差异主要是由于金属配位数和O（氧代）自旋密度的不同。此外，分子轨道分析表明，沸石的空间位阻进一步降低了[MnO]沸石对甲烷的反应活性。我们评估了两种类型的沸石骨架，即中孔ZSM - 5（MFI骨架）和小孔SSZ - 39（AEI骨架）沸石，但未发现反应活性有显著差异。发现速率决定反应步骤是甲醇脱附而非甲烷活化。我们还仔细研究了承载活性物种的最稳定位点，并计算了[Fe] - MFI和 - AEI沸石上的NO分解。
