Cu-modified zeolites have enormous potential as the catalysts facilitating the conversion of methane to methanol. It becomes important to investigate the active sites and the reaction mechanisms involved. In this paper, several spectroscopic methods such as UV-vis diffuse reflectance spectroscopy (UV-vis DRS), pulse electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared spectroscopy, and solid-state (13C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-to-methanol transformation on Cu/H-ZSM-5 zeolite with low (0.10 wt %) Cu content. UV-vis DRS and EPR data imply the presence of two types of Cu2+ cations bound to the zeolite framework Si-O--Al sites (Z). One of them is a species of the type Z[Cu(II)O] or Z[Cu(II)(OH)] with extra-framework O- or OH- ligands. The other one refers to Z2Cu(II) species without extra-framework O-containing ligands. CW EPR studies reveal that the Z2Cu(II) species are the major part of the Cu(II) sites present in the zeolite. 1H HYSCORE and DRIFTS data are supportive of the formation of a molecular complex of methane and Z2Cu(II) species, with a strongly polarized C-H bond and a 3.3 Å separation between the hydrogen atom of methane and Cu. 13C MAS NMR provides evidence for the formation of both the surface methoxy intermediate and physisorbed methanol. It is suggested that experimentally identified Z[Cu(II)O] or Z[Cu(II)(OH)] are those sites that provide a homolytic cleavage of the methane C-H bond to yield surface bound methoxy species and/or methanol molecule, the possibility that has been recently justified with density functional theory ( Kulkarni et al. Catal. Sci. Technol. 2018 , 8 , 114 ). The comparison of the amount of the surface methoxy intermediates formed and the number of different Cu(II) sites present in the zeolite allowed us to conclude the involvement of Z2Cu(II) sites in methane C-H bond activation. The mechanism of methane activation on Z2Cu(II) sites has been proposed. It includes two steps: (1) the formation of the molecular complex of methane with Z2Cu(II); (2) heterolytic dissociation of the polarized C-H bond affording surface copper(II) hydride and methoxy species, both bound to zeolite framework Si-O--Al sites.

中文翻译：

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低铜含量的H-ZSM-5沸石上的甲烷活化。结合光谱法鉴定的活性部位和中间体的性质。

铜改性的沸石作为促进甲烷向甲醇转化的催化剂具有巨大的潜力。重要的是要研究活性位点和涉及的反应机理。在本文中，已采用了几种光谱方法，例如紫外可见漫反射光谱（UV-vis DRS），脉冲电子顺磁共振（EPR），漫反射傅里叶变换红外光谱和固态（13C MAS）NMR表征了Cu含量低（0.10 wt％）的Cu / H-ZSM-5沸石上的催化剂活化和甲烷转化为甲醇的过程中形成的铜位点和中间体的状态。紫外可见DRS和EPR数据表明存在两种与沸石骨架Si-O-Al位点（Z）结合的Cu2 +阳离子。其中之一是具有框架外O-或OH-配体的Z [Cu（II）O]或Z [Cu（II）（OH）]类型。另一类是指不含骨架外含O配体的Z2Cu（II）物种。连续EPR研究表明，Z2Cu（II）物种是沸石中存在的Cu（II）部位的主要部分。1H HYSCORE和DRIFTS数据支持甲烷和Z2Cu（II）物种的分子复合物的形成，具有强极化的CH键以及甲烷和Cu的氢原子之间存在3.3Å的间隔。13 C MAS NMR提供了表面甲氧基中间体和物理吸附甲醇形成的证据。建议实验确定的Z [Cu（II）O]或Z [Cu（II）（OH）]是那些位点，这些位点可提供甲烷CH键的均质裂解，从而产生表面结合的甲氧基和/或甲醇分子，最近已经用密度泛函理论证明了这种可能性（Kulkarni et al.Catal.Sci.Technol.2018，8，114）。比较形成的表面甲氧基中间体的数量和沸石中存在的不同Cu（II）部位的数量，可以得出结论，Z2Cu（II）部位参与甲烷CH键的活化。已经提出了在Z 2 Cu（II）位点上甲烷活化的机理。它包括两个步骤：（1）甲烷与Z2Cu（II）的分子络合物的形成；（2）极化CH键的杂化解离提供了表面氢化铜（II）和甲氧基物质，它们都结合在沸石骨架Si-O-Al位上。比较形成的表面甲氧基中间体的数量和沸石中存在的不同Cu（II）部位的数量，可以得出结论，Z2Cu（II）部位参与甲烷CH键的活化。已经提出了在Z 2 Cu（II）位点上甲烷活化的机理。它包括两个步骤：（1）甲烷与Z2Cu（II）的分子络合物的形成；（2）极化CH键的杂化解离提供了表面氢化铜（II）和甲氧基物质，它们都结合在沸石骨架Si-O-Al位上。比较形成的表面甲氧基中间体的数量和沸石中存在的不同Cu（II）部位的数量，可以得出结论，Z2Cu（II）部位参与甲烷CH键的活化。已经提出了在Z 2 Cu（II）位点上甲烷活化的机理。它包括两个步骤：（1）甲烷与Z2Cu（II）的分子络合物的形成；（2）极化CH键的杂化解离提供了表面氢化铜（II）和甲氧基物质，它们都结合在沸石骨架Si-O-Al位上。（1）甲烷与Z2Cu（II）的分子络合物的形成; （2）极化CH键的杂化解离提供了表面氢化铜（II）和甲氧基物质，它们都结合在沸石骨架Si-O-Al位上。（1）甲烷与Z2Cu（II）的分子络合物的形成; （2）极化CH键的杂化解离提供了表面氢化铜（II）和甲氧基物质，它们都结合在沸石骨架Si-O-Al位上。