Cu based zeolites: A UV–vis study of the active site in the selective methane oxidation at low temperatures

An extensive series of 30 Cu exchanged zeolites and Cu impregnated silicas and aluminas have been tested in their capacities to stabilize the bis(μ-oxo)dicopper core. This core shows a remarkably activity towards methane, as it selectively hydroxylates methane into methanol at the low temperature of 125 °C. UV–vis spectroscopy is an easy approach to detect the presence of this bis(μ-oxo)dicopper core since it is characterized by an intense charge transfer band at 22 700 cm−1. In this way it was found that after calcination, only the Cu exchanged zeolites ZSM-5 and MOR are capable of stabilizing this core. In addition, an optimum in the Si/Al ratio and in the calcination temperature were observed, indicating that this core requires a rather specific coordination environment. For ZSM-5, the optimal Si/Al ratio for bis(μ-oxo) dicopper core formation is between 12 and 30 and the amount of this core increases with increasing copper loading above Cu/Al = 0.2. Calcination in O2 should be done at temperatures higher than 280 °C and lower than 700 °C. After reaction with methane at low temperature (150 °C), it was found that only Cu-ZSM-5 and Cu-MOR yielded methanol, whereas all the other Cu based materials yielded almost no methanol. At higher temperatures (200 °C) however, Cu-FER and Cu-BEA showed comparable methanol yields as Cu-ZSM-5 and also the methanol yield of Cu-MOR increased at this higher reaction temperature, indicating that a second not yet identified Cu-oxygen species is activated in the FER, BEA and MOR zeolites at higher temperatures.