Selective synthesis of transition metal ion nanoclusters embedded in zeolites for methane oxidation at low temperatures

沸石微孔的结晶性质提供了能够容纳过渡金属作为阳离子、金属氧化物或金属簇的多种位点。沸石框架中的Cu和Fe阳离子或金属-氧代纳米团簇的稳定化已经导致在低温下用于烷烃的选择性氧化的成功催化剂，模拟酶中的金属中心。特别地，具有挑战性的用O2将甲烷选择性氧化成甲醇可以在150-200 °C下在Cu沸石上成功地进行。然而，这些材料在实际应用中仍有局限性，例如需要萃取步骤来释放强吸附的甲醇。对于基于知识的氧化催化剂设计，有必要了解不同氧化态的氧和金属阳离子的纳米结构的组合如何导致特定的催化功能。已经发现Cu在几种沸石拓扑结构中是活性的，但是在大多数情况下，由于材料中非活性Cu的浓度大，因此关于活性物质的结构的信息仍然缺乏。首次报道的沸石基甲烷氧化催化剂中的Cu-氧代活性中心是嵌入ZSM-5中的[Cu 2 O]2+簇。我们最近发现CuMOR中的活性中心是一个三铜氧代络合物[Cu_3 O_3]~（2+），并发现这种活性中心也可以在CuZSM-5中形成。我们假设，自由基性质的Cu-O-Cu桥，一个共同的特点，发现在这两个积极的铜氧簇，是至关重要的活化甲烷。在此背景下，本项目的主要目标是产生的材料中的金属离子嵌入沸石分子筛具有明确的活性位点的选择性氧化的低碳烷烃。这需要控制那些影响活性金属-氧簇合物的最终结构的合成参数。因此，该项目的一个重要部分是针对深入了解在沸石中形成活性金属氧簇的基本步骤（即，金属离子交换，脱水，热处理和活性氧物种的产生中的化学过程）以及基质的活化机制。此外，我们的Cu-MOR和Cu-MFI的经验表明，相同的Cu-氧代簇表现出不同的催化活性，如果稳定在不同的框架。因此，局部环境的作用和潜在的约束效应的能力，以激活甲烷将通过探索沸石与不同的孔径和连接性进行研究。 该项目的另一个目标是找到释放强烈吸附的甲醇的策略，以关闭这种类型的催化剂上甲烷氧化的催化循环。最后，我们将探讨沸石中Ni和Co阳离子在甲烷氧化中的活性，从而揭示金属阳离子对氧化C-H活化的特异性。

The crystalline nature of zeolite micropores provide a wide variety of sites able to host transition metals as cations, metal oxides or metal clusters. The stabilization of Cu and Fe cations or metal-oxo nanoclusters in zeolite frameworks has led to successful catalysts for the selective oxidation of alkanes at low temperatures, mimicking the metal center in enzymes. In particular, the challenging selective oxidation of methane to methanol with O2 can be successfully performed at 150-200 °C on Cu-zeolites. However, these materials have still limitations for practical applications, such as the necessity of an extraction step to release the strongly adsorbed methanol. For a knowledge-based design of oxidation catalysts, it is necessary to understand how the combination of nano-architectures of oxygen and metal cations in different oxidation states lead to a particular catalytic function. Cu has been found to be active in several zeolite topologies, but information on the structure of the active species is still lacking in most of the cases because of the large concentration of inactive Cu in the materials. The first reported Cu-oxo active site in zeolite-based methane oxidation catalysts was a [Cu2O]2+ cluster embedded in ZSM-5. We have recently identified the active site in CuMOR as a tricopper oxo complex, [Cu3O3]2+, and found that such active site can also be formed in CuZSM-5. We hypothesize that the radical character of O in the Cu-O-Cu bridges, a common feature found in both active Cu-oxo clusters, is crucial for the activation of methane.With this background, the main objective of this project is to generate materials consisting in metal ions embedded in zeolites with well-defined active sites for selective oxidation of light alkanes. This requires controlling those synthesis parameters that affect the final structure of active metal-oxo clusters. Thus, an important part of this project is directed to gain insight into the elementary steps involved in the formation of active metal-oxo clusters in zeolites (namely, the chemical processes in metal ion exchange, dehydration, thermal treatments and generation of active oxygen species) as well as into the mechanism of activation of the substrate. Furthermore, our experience with Cu-MOR and Cu-MFI have shown that identical Cu-oxo clusters exhibit different catalytic activity if stabilized in different frameworks. Thus, the role of the local environment and potential confinement effects in the ability to activate methane will be studied by exploring zeolites with different micropore size and connectivity. An additional objective of the project is to find strategies to release the strongly adsorbed methanol in order to close the catalytic cycle for methane oxidation on this type of catalysts. Finally, we will explore the activity in methane oxidation of Ni and Co cations in zeolites, so that the specificity of the metal cation for the oxidative C-H activation can be revealed.