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各种各样的位置，能够容纳过渡金属，如阳离子、金属氧化物或金属团簇。沸石框架中Cu和Fe阳离子或金属-氧纳米团簇的稳定导致了在低温下烷烃选择性氧化的成功催化剂，模拟了酶中的金属中心。特别是，在150-200°C的cu -沸石上，可以成功地用O2将甲烷选择性氧化为甲醇。然而，这些材料在实际应用中仍然存在局限性，例如需要一个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如何结合导致特定的催化功能。Cu已被发现在几种沸石拓扑结构中具有活性，但由于材料中存在高浓度的非活性Cu，大多数情况下仍缺乏活性物质的结构信息。在沸石基甲烷氧化催化剂中首次报道的Cu-oxo活性位点是嵌入在ZSM-5中的[Cu2O]2+簇。我们最近确定了CuMOR中的活性位点为三铜氧配合物[Cu3O3]2+，并发现CuZSM-5中也可以形成这种活性位点。我们假设在Cu-O-Cu桥中O的自由基特征是甲烷活化的关键，这是两种活性Cu-oxo簇的共同特征。在此背景下，该项目的主要目标是生成由嵌入沸石中的金属离子组成的材料，这些材料具有明确的活性位点，用于选择性氧化轻烷烃。这就要求控制那些影响活性金属氧簇最终结构的合成参数。因此，该项目的一个重要部分是深入了解沸石中活性金属-氧簇形成的基本步骤（即金属离子交换、脱水、热处理和活性氧生成的化学过程）以及底物活化的机制。此外，我们对Cu-MOR和Cu-MFI的研究表明，如果在不同的框架中稳定，相同的Cu-oxo簇表现出不同的催化活性。因此，将通过探索具有不同微孔大小和连通性的沸石来研究局部环境和潜在约束效应对甲烷活化能力的作用。该项目的另一个目标是找到释放强吸附甲醇的策略，以关闭这类催化剂上甲烷氧化的催化循环。最后，我们将探索沸石中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.