甲烷的活化及其高效转化是一个重要的科学问题。在最近与厦大化院汪骋课题组合作研究中，我们介绍了一种利用二氧化氮作为光媒介和分子氧作为终端氧化剂将甲烷转化为甲醇的方法（已发表于Inorg. Chem.）。该过程可以实现17%的甲烷转化率，同时对CH₃ONO₂的选择性高达78%。HCl催化光硝化途径的理论与实验结果吻合。

Activation of methane and its conversion to added-value products is an important topic which requires chemical solutions with high yields and selectivity. In our recent collaborative study, we present the oxidation of methane to methanol using nitrogen dioxide as a photo-mediator and molecular oxygen as the terminal oxidant. This process achieves the methane conversion of 17% while the selectivity towards CH₃ONO₂ is as high as 78%. Joined experimental and theoretical investigation support the HCl-catalyzed photo-nitration pathway.

提出这个方法的动机是为了解决选择性转化高惰性甲烷的挑战。甲烷在大气中通过涉及NO₂的复杂光反应自然发生活化，但NO₂在大规模甲烷转化中的潜力尚未实现完全探索。

The motivation behind the presented proposal is to address the challenges in selectively converting methane which is highly inert. Methane activation occurs naturally in the atmosphere through complex photoreactions involving NO₂, but the potential of NO₂ in large-scale methane conversion is not fully explored.

我们计划建立一个化学循环实现光驱动NO₂介导的CH₄转化，将其转化为CH₃OH，其中CH₃ONO₂作为气体中间体。该策略可以避免过氧化，而且可以通过硝酸甲酯的水解获得甲醇这个重要的平台化学品。

The plan is to construct a chemical looping for the photo-driven NO₂-mediated CH₄ conversion to CH₃OH via CH₃ONO₂ as a gas intermediate. This strategy can prevent over-oxidation and obtain methanol as an important platform chemical through the hydrolysis of methyl nitrate.

激发态NO₂具有活化烷烃的能力，这已被它在低温（-10°C）下与乙烷的光硝化反应所证实。然而，NO₂和甲烷之间的反应直到180°C才开始发生，在这个温度下甲烷主要被转化为CO_x（该现象与理论计算结论一致）。我们发现了CH₄与基态²A₁态下NO₂反应的三条不同途径（见下图）。所有这些途径的吉布斯自由能垒都在37-44 kcal/mol的范围内，比低温（-10°C）反应的能垒高的多。我们没有找到任何CH₄与激发态²B₂态下NO₂反应的反应路径。

Excited NO₂ has demonstrated the ability to activate alkanes, as evidenced by the successful photo-nitration of ethane at a low temperature of -10°C. However, the reaction between NO₂ and CH₄ does not take place until 180°C, at which temperature the methane is converted to CO_x as the main products. Theoretical investigations also support this experimental phenomenon. We found three different pathways for the reaction of CH₄ with NO₂ in the ground, ²A₁ state (see figure below). Nevertheless, all of them have the Gibbs free energy barriers of 37-44 kcal/mol, which are too high for a reaction to take place at a low temperature of -10°C. We could not locate any reaction paths for CH₄ and NO₂ in the excited, ²B₂ state.

幸运的是，我们发现HCl可以通过氢原子转移（HAT）反应催化甲烷的光硝化。理论研究表明，HCl可以以两种不同的方式与NO₂（²A₁）反应，其自由能垒（26-33 kcal/mol）比CH₄与NO₂（²A₁）反应自由能垒低，但仍然没有低到可在-10°C下轻易进行。在NO₂分子被450 nm LED照射激发后，HCl可以与NO₂（²B₂）发生反应，其自由能垒仅为20.4 kcal/mol（见下图）。因此，在可见光的帮助下， HCl与NO₂发生氢提取反应产生Cl•自由基。在线质谱（MS）证实了Cl•自由基的存在，此外，光激发Al(NO₃)/AlCl₃混合物后大量的HONO被检测到。

Fortunately, we found that HCl can catalyze methane photo-nitration via relay hydrogen atom-transfer (HAT) reactions. Theoretical investigations show that HCl can react with NO₂ (²A₁) in two different ways with lower free energy barriers (26–33 kcal/mol) than CH₄ but the energy barriers are still not low enough to make these reactions happen easily at -10°C. After NO₂ molecules are excited by the 450 nm LED illumination, HCl can react with NO₂ (²B₂) species with the low 20.4 kcal/mol free energy barrier (see figure below). Thus, the free Cl• radical can be readily generated from the HCl molecule via the H-abstraction by NO₂ with the assistance of visible light. The existence of Cl• radical is supported by the online MS and a substantial amount of HONO is also detected after photoexciting a mixture of Al(NO₃)/AlCl₃.

随后，Cl•自由基与CH₄反应，重新生成HCl并产生CH₃•自由基。CH₄与Cl•自由基之间的氢原子转移反应（HAT）的能垒（约为1.67 kcal/mol）要比CH₄与NO₂之间反应的能垒（37–44 kcal/mol）低得多。接着，通过CH₃•自由基与O₂、NO₂、NO之间的反应，可以在相对较低的温度下选择性生成CH₃ONO₂（可以水解生成CH₃OH）。

The Cl• radical can then react with CH₄ to regenerate HCl and produce CH₃• radical. The HAT between CH₄ and Cl• radical has a much lower barrier (about 1.67 kcal/mol) than that between CH₄ and NO₂ (37–44 kcal/mol). Then, CH₃ONO₂ can be selectively produced at a relatively low temperature via the reaction between CH₃• radical and O₂, NO₂, NO. The produced CH₃ONO₂ can be hydrolyzed in water to produce CH₃OH.

这项工作为烷烃升级领域打开了新的机遇。我们所提出的策略可以激发对其它烷烃转化和官能化新途径的探索。

This work has opened up new opportunities in the field of alkane upgrading. The strategy proposed here can inspire the exploration of new pathways for the conversion and functionalization of other alkanes.

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