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

• Published in: Catalysis today Vol. 110; no. 3; pp. 303 - 309
• Main Authors: Smeets, Pieter J., Groothaert, Marijke H., Schoonheydt, Robert A.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 30.12.2005; Elsevier Science
• Subjects: Alumina; Bis(μ-oxo) dicopper core; Catalysis; Chemistry; Exact sciences and technology; General and physical chemistry; Ion-exchange; Oxidation; Silica; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; UV–vis spectra; Zeolites; Zeolites: preparations and properties; Oxidation; Bis(μ-oxo) dicopper core; Zeolites; Silica; Alumina; UV–vis spectra; Methane; Heterogeneous catalysis; UV-vis spectra; Active site; Zeolite; Molecular sieve; Low temperature
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2005.09.028

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 O 2 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.