Direct methane conversion to methanol via chemical looping using copper-exchanged zeolites has attracted considerable attention during the last decades and is one of the most-actively studied processes. Despite the significant progress that has been made in the design of active systems and the elucidation of active sites, the effects of zeolite topology and the structure of copper species on the nature of the reaction products are yet unclear. Herein, we show that oxygen-activated copper-exchanged zeolites of different framework types, namely, MOR, MFI, BEA, and FAU, yield different products, as detected by in situ Fourier-transform infrared and nuclear magnetic resonance spectroscopy. Molecular methanol, methoxy species, and dimethyl ether prevail at lower reaction temperatures (<473–523 K), and Cu^I carbonyls and gaseous carbon oxides were detected above 573 K. Methane coupling to C₂ and C₃ hydrocarbons was shown for the first time over CuMOR, CuBEA, and CuFAU. The nature and relative fraction of formed products strongly depend on the structure of the copper active sites, which is governed by the topology of the zeolite host. Several pathways of methane transformation over copper-exchanged zeolites are identified, opening opportunities for tuning the properties of the materials to achieve the best performance.

中文翻译：

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铜交换沸石上的甲烷转化：从部分氧化到 C-C 偶联和碳氢化合物的形成

在过去的几十年中，使用铜交换沸石通过化学循环将甲烷直接转化为甲醇引起了相当多的关注，并且是研究最活跃的过程之一。尽管在活性系统的设计和活性位点的阐明方面取得了重大进展，但沸石拓扑结构和铜物种结构对反应产物性质的影响尚不清楚。在此，我们展示了不同骨架类型的氧活化铜交换沸石，即 MOR、MFI、BEA 和 FAU，产生不同的产物，如原位检测傅里叶变换红外光谱和核磁共振光谱。分子甲醇、甲氧基物质和二甲醚在较低的反应温度 (<473–523 K) 下占主导地位，并且在 573 K 以上检测到Cu ^I羰基化合物和气态碳氧化物。甲烷与 C ₂和 C ₃烃的偶联首次被显示时间超过 CuMOR、CuBEA 和 CuFAU。形成的产物的性质和相对分数在很大程度上取决于铜活性位点的结构，这由沸石主体的拓扑结构控制。确定了铜交换沸石上甲烷转化的几种途径，为调整材料的性能以实现最佳性能提供了机会。