Methane Transformation over Copper-Exchanged Zeolites: From Partial Oxidation to C-C Coupling and Formation of Hydrocarbons

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 C-2 and C-3 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.

Automated summary

Direct methane conversion to methanol via chemical looping using copper-exchanged zeolites is a highly researched area, where different zeolite topologies and copper species structures can influence the nature of reaction products. The structure of copper active sites in zeolites strongly determines the type and relative fraction of formed products, leading to opportunities for tuning material properties for optimal performance. Multiple pathways of methane transformation over copper-exchanged zeolites have been identified, showing potential for further development in this field.