Active Sites in Low-Loaded Copper-Exchanged Mordenite: Spectroscopic and Stability Study for Methane Oxidation Using Mild Conditions

Low-loaded copper-exchanged mordenite samples (3 wt.% of copper) were prepared by a solid-state milling method using controlled conditions. The milled samples were then submitted to a calcination process where trimeric copper active species were formed, according to XPS, EPR, IR, and UV–vis recorded spectra. To verify the interaction of the active site with methane at mild conditions, a test experimental design was developed in a batch reactor configuration using mild two-step conditions: (1) activation temperature at 400 °C in an air atmosphere, and (2) isothermal conversion process at 200 °C with 6 bar methane. The analyzed samples were active in methanol conversion in batch conditions, nonetheless less efficient than the usually reported copper mono $\mu$-oxo sites using harder experimental conditions. The herein reported copper active sites are as follows: a trinuclear copper active cluster [Cu₃($\mu$-O)₃]²⁺ and a possible intermediate during methane contact detected as bis($\mu$-oxo) dicopper species were identified and studied on each reaction step. This study revealed that trinuclear copper active sites can be obtained through grinding. Nonetheless, they stabilize after a calcination stage in an air atmosphere. Their stability is then maintained during the whole cyclic experimental test, suggesting their potential use for multicyclic processes.