Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
J Am Chem Soc. 2023 Jul 5;145(26):14365-14378. doi: 10.1021/jacs.3c03351. Epub 2023 Jun 20.
The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal-organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C-H activation via a terminal metal-oxo species. We carried out density functional theory calculations of the radical rebound mechanism for methane-to-methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energies─a key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane-to-methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.
甲烷直接部分氧化制甲醇的挑战促使人们有针对性地寻找金属有机骨架（MOFs）作为这一转化的一类有前途的材料，因为它们具有孤立的金属和可调节的配体环境。已经合成了数千种 MOFs，但只有相对较少的 MOFs 因其在甲烷转化方面的潜力而被筛选出来。我们开发了一种高通量的虚拟筛选工作流程，可以从大量尚未研究过催化作用的实验性 MOFs 中识别出 MOFs，这些 MOFs 具有热稳定性、可合成性，并且通过末端金属-氧物种具有有前途的不饱和金属位点用于 C-H 活化。我们对 87 种选定的 MOFs 的次级建筑单元（SBU）模型进行了甲烷到甲醇转化的自由基回弹机制的密度泛函理论计算。虽然我们表明，与之前的工作一致，随着 3d 填充的增加，氧形成的有利性降低，但之前观察到的氧形成与氢原子转移（HAT）之间的比例关系被我们的 MOF 集合中的更大多样性所破坏。因此，我们专注于 Mn MOFs，它们有利于形成中间态的氧，而不会不利地影响 HAT 或导致甲醇释放能量升高——这是甲烷羟化活性的一个关键特征。我们确定了三种包含不饱和 Mn 中心的 Mn MOFs，这些中心与平面或弯曲几何形状的弱场羧酸盐配体结合，具有有前途的甲烷到甲醇动力学和热力学性质。这些 MOFs 的能量跨度表明它们具有有前途的甲烷到甲醇的周转率，值得进一步进行实验催化研究。
