Reactive intermediates in Cu MOR zeolites for alkane oxidation

Abstract

The Cu^(2+)-O-Cu^(2+) core in ZSM-5 zeolite is, thus far, the only well-characterized copper intermediate in the methane to methanol reaction. This recent discovery has had a broad impact in the field of heterogeneous catalysis and bioinorg. chem. Next to its identification, its formation pathway both from N_2O and O_2 was studied in detail recently. O_2 activation proceeds through the formation of a µ-η^2:η^2-peroxodicopper(II) site. Remarkably, almost at the same time, it was reported that also pMMO enzyme is also able to form such a peroxo dicopper site. Studying the activation mechanism of N_2O is important in view of N_2O decompn. and redn. of greenhouse gases. The N_2O activation on Cu-ZSM-5 proceeds via a bridged µ-1, 1-O binding fashion if two Cu + atoms are closely sepd. (<4.2 Å). This pathway is in line with that recently discovered for the N_2O reductase enzyme. In our latest work, we studied the influence of the zeolite lattice on the identity of the Cu-O-Cu site. Similar to our previous work on ZSM-5 zeolite, we investigated Cu-O-Cu formation in Cu-MOR type using a combination of (operando) spectroscopic tools and kinetic studies of the reactivity. The lecture will presented new data showing firstly the overall picture of the copper distributions and secondly the Cu redox chem. in the CuMOR zeolites. Exptl. spectroscopic proof of square planar mononuclear copper sites was found. Next to the monomers, a large quantity of reactive dinuclear Cu sites, viz. 60% for CuMOR vs. 5% for Cu-ZSM-5, were detd. and identified. Closer inspection on the identity of the sites using kinetic data of the reactivity and spectroscopic studies (e.g., resonance Raman, UV-VIS, XRD-Rietveld refinement and reactivity study (with alkanes) reveals the presence of two Cu^(2+)-O-Cu^(2+) sites in CuMOR, with slightly different geometry, location and reactivity. It is suggested that one dinuclear Cu-O-Cu site is located in the 8-membered oxygen ring (8MR) at the intersection with the 12MR channel; the other site resides in the 8MR channel.