Spectroscopic Identification of Copper Active Sites in Cu-Zeolites

Methane is mainly valorized via a conversion to syngas, a mixture of H2 and CO, which serves as an intermediate towards useful chemicals such as methanol and hydrocarbons. These processes operate under harsh and energy-intensive conditions and are therefore only profitable when carried out on a large scale. This impedes an on-site valorization of small-scale, remote natural gas sources. The alternative solution of transporting methane to large centralized units is unecomical, due to its low energy density as a gas. As methane has a higher global warming potential than CO2 it is usually opted to flare this methane to CO2 prior to atmospheric release, wasting a valuable resource and contaminating the environment with CO2. An ideal solution for these stranded gas reserves would be an on-site, direct conversion to methanol, an easily transportable liquid. Copper-exchanged zeolites are promising materials for this selective oxidation of methane to methanol. In addition to the active copper/oxygen (Cu/O) sites on these zeolites, inactive spectator copper sites are also present. They complicate the identification of the active sites and the reaction mechanism, impeding the optimization of the catalyst and the reaction conditions. Additionally, they preclude the determination of structure-reactivity relationships that could be translated to other transition metal ion exchanged zeolites and other reactions. Until now, a [CuOCu]2+ species on Cu-MFI and two slightly differing [CuOCu]2+ species on Cu-MOR zeolites are the only active Cu/O species that have been thoroughly characterized in terms of structure and reactivity by using site-selective spectroscopic techniques (ignoring the spectator sites). For Cu-CHA zeolites, trans-μ-1,2-peroxo dicopper(II) and copper(II) superoxo species were proposed in the literature based on resonance Raman (rR) vibrations. In this thesis, a more in-depth spectroscopic study is performed on Cu-CHA, combining DR-UV-Vis, rR profiling and 18O isotope perturbation to refute the previous assignments and attribute these rR vibrations to another [CuOCu]2+ active site. Kinetic experiments show that the Cu-CHA species has a lower activation energy for methane activation than the previously identified [CuOCu]2+ on Cu-MFI and Cu-MOR zeolites. DFT models comparing MFI and CHA show that the higher reactivity of Cu-CHA is due to a destabilization of the [CuOCu]2+ core in CHA imposed by lattice constraints originating in the relative orientation of the two planes of the coordinating bidentate O-Al-O T-sites. In the subsequent chapters, the MOR topology is revisited, where a novel [CuOH]+ and a third [CuOCu]2+ active species are identified. Their structure and kinetics in methane oxidation were determined and compared to previously identified active sites.

Jaar van publicatie:2021

Toegankelijkheid:Closed