For decades Fe-exchanged zeolites such as FeBEA*, FeFER and FeZSM-5 are known to be active in the direct, selective oxidation of methane to methanol at room temperature, an interesting reaction given the emerging feedstock transition from oil to gas. Recently our group elucidated the structure of the α-Fe site and the reactive α-O species, formed after oxidation with N2O, on FeBEA*, using site-selective spectroscopic techniques (Nature 536, 317–321, 2016). α-Fe was assigned to a mononuclear high-spin S=2 square planar Fe(II) species and α-O to a high-spin S=2 square pyramidal Fe(IV)=O species. These α-species are stabilized in a six-membered ring (6MR), containing an Al-Si-Si-Al sequence, i.e. paired Al atoms (see Figure).
Recently we proved that the chabazite (CHA) zeolite topology is also able to stabilize α-Fe and α-O species (JACS, 140 (38), 12021–12032, 2018). Spectroscopy (UV-Vis-NIR and Mössbauer) combined with theoretical models (DFT and CASPT2) proved that the α-species are again stabilized in a six-membered ring (6MR), containing an Al-Si-Si-Al sequence (see Figure). The simple CHA topology (only 1 type of T-atom) is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 the only identifiable phase other than α-Fe formed in Fe-CHA at more elevated Fe loadings. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology’s 6MR geometry is found to influence the structure and the ligand field and consequently the spectroscopy of the α-Fe site in a predictable manner.
In our current research we are investigating the different steps of the reaction cycle in more detail, both mechanistically and kinetically (N2O activation, reaction path of α-O with methane) to get an even better understanding of the requirements for an efficient reaction cycle. Then we are using this information to try increasing the methanol yield by for instance investigating the introduction method of Fe in the zeolite.
Alongside Fe-exchanged zeolites, Cu-exchanged zeolites are another promising candidate for direct methane oxidation to methanol, attracting a lot of research since 2005. Our group managed to identify and characterize a mono-(μ-oxo) dicopper(II) species (Cu-O-Cu) in ZSM-5 zeolites (PNAS, 106 (45), 18908-18913, 2009) and two slightly differing Cu-O-Cu species in mordenite (MOR) zeolites (JACS, 137 (19), 6383–6392, 2015) as active sites for methane conversion at temperatures around 423 K (see Figure). These species can be formed with either O2 or N2O as the oxidant. Their structure (bond angle, bond lengths) differs slightly based on the location in the zeolite channels and cavities. This leads to different reactivity with methane. In addition, we have shown that the zeolite framework itself has also a relevant influence on the reaction coordinate (confinements effect). This is comparable to the workings of an enzyme, where second-sphere effects are directing the approach of methane to the active sites and stabilize the transition state.
Currently we are hunting for different copper active sites in other zeolite frameworks (or porous structures in general). We are also trying to understand the influence of every step in the activation procedure on the copper speciation in the zeolite. To this end we are again using a combination of site-selective spectroscopy (DR-UV-Vis, resonance Raman, EPR...) backed-up by computational studies, and our in-house knowledge of zeolite properties and zeolite synthesis.
In conclusion we hope to present our most recent research on Fe- and Cu-exchanged zeolites for selective methane oxidation to methanol on the 12th Natural Gas Conversion Symposium.
