Copper-exchanged zeolites catalytically convert methane to methanol (533-573 K) with high selectivity (50-100%) at low conversions (&lt;0.1%). Prior studies have determined that increasing the framework Al content and decreasing the extraframework Cu content increased the selectivity towards methanol but have not identified how these bulk compositional variables affected the number and structure of active Cu sites formed during steady state catalysis. Herein, we studied how the distribution of Cu active sites in CHA was affected by increasing the extraframework Cu site density and altering the arrangement of framework Al atoms in chabazite (CHA) zeolites and their effect on continuous partial methane oxidation (PMO) kinetics, apparent reaction orders, and apparent activation energies. The number of 6-MR paired Al sites in CHA was varied using previously reported 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. 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 active sites 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-vis indicate different types of binuclear oxo-bridged Cu active sites are formed during reaction and are influenced by the arrangement of framework Al atoms in CHA zeolite. This investigation highlights the effect of Al siting in zeolites on the structures of extraframework Cu complexes that form and, in turn, on partial oxidation rates and selectivity.

Investigation of Cu-CHA catalyst composition and structure on catalytic partial methane oxidation

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