(521bj) Influence of Cu-CHA Catalyst Composition and Structure on Rates and Selectivities of Catalytic Partial Methane Oxidation to Methanol

Copper-exchanged zeolites catalytically convert methane to methanol (533-573 K) with high selectivity (50-100%) at low conversions (<0.1%). Prior studies determined that increasing the framework Al content and decreasing the extraframework Cu content increases the selectivity towards methanol, but have not identified how these bulk compositional variables affect the number and structure of active Cu sites formed during steady-state partial methane oxidation (PMO) catalysis. Herein, we studied how the distribution of Cu active sites in CHA was affected by increasing the extraframework Cu site density and by altering the density and arrangement of framework Al atoms in chabazite (CHA) zeolites, and their influences on continuous PMO rates, selectivity, and apparent reaction orders. The number of 6-MR paired Al sites in CHA was varied by adapting established methods to use different combinations of organic and inorganic structure-directing agents during crystallization and quantified through Co(II) titration. All Cu-CHA samples showed PMO rates that were nearly first-order in methane pressure, consistent with prior reports that suggest C-H activation in methane is the rate limiting step in continuous PMO. Cu-CHA samples with different framework Al arrangements showed that formation rates of over-oxidation products (e.g., CO₂) had different apparent reaction orders in O₂, implying differences in the Cu structures formed during reaction. The number of redox-active Cu sites were quantified through linear combination fitting of XANES spectra collected under in-situ and transient conditions after reactant (O₂) cut-off. Characterization of Cu sites using in-situ and transient XAS and UV-visible spectroscopies indicate different types of binuclear O_x-bridged Cu active sites are formed during reaction and are influenced by the arrangement of framework Al atoms in CHA. These findings highlight the effects of Al density, Al arrangement, and Cu density on the structures of extraframework Cu complexes that form during partial oxidation catalysis to influence rates and selectivity.