(630f) Shared Space: Interactions between Metal Cations in Cu/Zn-ZSM-5 Systems

This work investigates the spectroscopic and catalytic behavior of ZSM-5 containing two industrially-interesting metal substitutions, Cu and Zn. Presented material introduces synthetic techniques and exchanges for Cu and Zn guest ions as well as early research into the series exchange of the dual-metal derivative via titration. Characterization of the zeolite utilizes EDS, in situ UV-vis, DRIFTS, and adsorption analysis. The catalytic potential of these species is evaluated in a model reaction: the partial oxidation of methane. Cu uptake kinetics were favorable relative to Zn according to decreases in micropore volume following metal exchanges. However, either metal partially exchanges with the other under high concentration gradients. Selective titration provided evidence that the two metals exchange at a Cu:Zn ratio near 1:2, where Cu:Z is stoichiometrically limited at 1:2 but Zn:Z exceeds this ratio. Nitric oxide binding revealed differential coordination capacities between the two metal ions; Cu formed strong geminal di-nitrosyl species and possible nitrates, whereas Zn ions form weak mononitrosyl complexes that are easily reversed above ambient temperatures. From this, we can gather that Cu remains highly coordinatively unsaturated in its Lewis acidity, while Zn is electronically more satisfied. Operando anaerobic calcination also confirms that Cu undergoes an easily reversible partial autoreduction from divalent to monovalent species. Cu-ZSM-5 species displayed characteristic methane conversion, which was not observed with Zn counterparts.