Thermodynamics Perspective on the Stepwise Conversion of Methane to Methanol over Cu-Exchanged SSZ-13
Thermodynamics Perspective on the Stepwise Conversion of Methane to Methanol over Cu-Exchanged SSZ-13
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DOI：
10.1021/acscatal.1c00691
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发表时间：
2021-06-11
期刊：
影响因子：
12.9
通讯作者：
Mavrikakis, Manos
中科院分区：
文献类型：
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作者：
Goltl, Florian;Bhandari, Saurabh;Mavrikakis, Manos
众所周知，过渡金属交换沸石可以高选择性地将甲烷转化为甲醇，这是一个循序渐进的过程，包括暴露于氧化剂，然后暴露于甲烷，最后暴露于水蒸气。然而，对于这一过程中可能的活性位点的性质及其各自的变化，目前还缺乏全面的理论研究。在这里，我们结合了密度泛函数理论计算的广义梯度近似（DFT-GGA）和后dft方法，确定了cu交换沸石SSZ-13在甲烷逐步转化为甲醇过程中的热力学优先位点。我们开发了一个热力学模型，用于广泛的可能的活性位点，即铜单体，二聚体和三聚体，它们锚定在不同的环结构中，并由一系列不同的局部Al分布支持。随后，构建相图并用于确定甲烷逐步转化为甲醇过程中每个步骤的热力学有利位点。我们发现，在暴露于O-2时，羟基化二聚体- cu2o2h2和cu2oh（取决于局部Al结构）是首选。一旦暴露于甲烷，就会形成位点结合的甲醇分子。随着水蒸气压的增加，观察到单原子Cu的热力学偏好和甲醇的释放。此外，我们将我们的预测结果与文献中发表的实验测量结果进行了比较，发现在铜配位数和一些考虑的位点的键距方面非常一致。我们期望在这里获得的见解可以用来提高我们对反应机理的理解，并优化甲烷逐步转化为甲醇。
Transition-metal exchanged zeolites are known to convert methane to methanol with high selectivity, in a stepwise process, involving exposure to oxidants, followed by exposure to methane, and finally by exposure to water vapor. However, a comprehensive theoretical study on the nature of the possible active sites and their respective changes during this stepwise process is still lacking. Here, we use a combination of density functional theory calculations in its generalized-gradient approximation (DFT-GGA) and post-DFT methods to identify the thermodynamically preferred sites in Cu-exchanged zeolite SSZ-13 during the stepwise conversion of methane to methanol. We develop a thermodynamic model for an extensive set of possible active sites, that is, Cu monomers, dimers, and trimers, which are anchored in different ring structures and supported by a series of different local Al distributions. Subsequently, phase diagrams are constructed and used to identify thermodynamically favored sites at each step during the stepwise conversion of methane to methanol. We find that during exposure to O-2, hydroxylated dimers-Cu2O2H2 and, depending on the local Al configuration, Cu2OH-are preferred. Upon exposure to methane, site-bound methanol molecules are formed. With the subsequent increase in water vapor pressure, a thermodynamic preference for monoatomic Cu and the release of methanol are observed. Furthermore, we compare our predicted results to experimental measurements published in the literature and find close agreement in terms of Cu coordination number and bond distances for some of the sites considered. We expect that the insights obtained here can be used to improve our understanding of the reaction mechanism and to optimize the stepwise conversion of methane to methanol.