Conversion of methane to methanol over Cu-MAZ (zeolite omega): an oxygen-free process using H2O and CO2 as oxidants

Strong oxidants have been reported to be required in the catalytic oxidation of CH4 to methanol in cyclic processes employing CuO-zeolites. Here, using Cu-exchanged zeolite omega (Cu-MAZ), it was demonstrated that O2 could be replaced by CO2, enabling the development of a new process. CuO-MAZ thermally treated in Ar, O2, or CO2 was investigated using in situ DRS-UV-Vis analysis and spectra calculated using density functional theory. The active species were characterized at different temperatures of reaction with CH4. The species of the type Zn-[CuxOy]n+ and main species Z-[CuOH-HOCu]2+-Z were formed on Cu-MAZ. After the reaction with CH4, the Cu+ formed from Zn-CuxOy could be reoxidized with CO2, while the Cu+ formed from Z-CuOH-HOCu-Z was only reoxidized with H2O, and not with CO2. After reoxidation with H2O, the activity of the material was fully restored by thermal treatment in O2 or CO2. The highest CH3OH yield was achieved using CO2.