Significant quantities of methane occur in remote locations where the economy of scale for commercial reformers is not feasible. Selective oxidation of methane offers an attractive approach to convert these stranded methane reserves to condensable alcohols that can be transported to a central processing facility in pipelines. However, highly selective catalysts and well-controlled reaction conditions are needed to steer the reaction towards alcohols rather than the thermodynamically favored CO2. This contribution describes the use of in-situ IR spectroscopy to analyze surface species of selective oxidation and undesired side reactions like complete combustion to CO2. The work focuses on bifunctional catalysts consisting of a redox-active ceria zirconia (CZ) support and well-dispersed Lewis acidic metal oxide clusters. Lewis acid sites serve as the primary sites for methane activation while CZ is capable of activating O2. In-situ IR experiments are performed with self-supported wafers of the catalysts that are placed in a flow-through cell, and the effluent of the cell is studied by MS. These experiment show that methane activation by metal oxide clusters initially leads to the formation of a methyl and a hydroxyl group. Depending on the distribution of active oxygen species, the methyl groups can be converted to various products. Highly reactive adsorbed oxygen species lead to the formation of surface formates and carbonates that are further transformed to CO2. Less reactive oxygen species from CZ allow for the formation of methoxy intermediates that can be hydrolyzed to methanol. In addition, aromatic deposits are formed that deactivate some of the active sites.

Understanding selective oxidation of methane based on in-situ IR spectroscopy

ACS Fall 2021

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