Methane is an abundant and inexpensive alkane that can be sourced from natural and shale gas and upgraded to chemicals of higher value, including methanol. The direct conversion of methane to methanol is challenging, however because of its high C-H bond strength, low polarizability, and low electron and proton affinity [1]. Cu-zeolites have received renewed attention as materials that may facilitate partial methane oxidation (PMO), with many proposed active site motifs, including monomeric and multimeric copper species [2,3,4]. More precise identification of the copper structures that activate CH4 can be facilitated by studying model heterogeneous supports that contain well-defined copper speciation, as a result of the synthetic and treatment procedures used to prepare them. Chabazite (CHA) zeolites, composed of double 6-membered ring building units that connect to form 8-membered ring windows, are high-symmetry frameworks that contain a single, unique tetrahedral site (T-site). CHA serves as a model support that can be used to simplify the interpretation of experimental characterization data of exchanged Cu ions and complexes, which can be compared more faithfully to structural models studied using computational approaches. Synthetic control of the framework aluminum distribution in chabazite zeolites between isolated and paired Al sites (Al-(Si-O)x)Al, x>2 for isolated and x=1,2 for paired), influences the speciation of extraframework Cu cations between monovalent [CuOH]+ (ZCuOH) and divalent Cu2+ (Z2Cu) ion sites, respectively, whose concentrations are quantified by ammonia titration after Cu-exchange and through a variety of spectroscopic probes [5]. Monomeric ZCuOH sites in close proximity can condense to form binuclear species of the ZCuOCuZ type, and the combination of H2 and CO temperature programmed reduction [6] and in-situ UV-visible spectroscopy were used to probe the presence of mononuclear and binuclear Cu site types. Distinct spectral differences between Z2Cu and ZCuOH sites observed experimentally by UV-visible spectroscopy were corroborated and assigned with ab-initio molecular dynamics (AIMD) combined with time-dependent density functional theory (TD-DFT). Calculations that only consider DFT-optimized single geometries neglect the dynamic changes in structure and coordination present among Cu sites in experiment, and the incorporation of site dynamics was found to be required for proper assignments of spectral features to specific Cu site types. The synthesized model Cu-zeolites were used to probe the oxidation of methane to methanol in a stoichiometric step-wise procedure involving activation at high temperature in air (723-773 K, 0.167 K s-1 ramp rate, 1-6 h), methane exposure at low temperatures (473-503 K, 20%-99% CH4, 0.5-2 h), and then water exposure (473-503 K, 3% in He, 1 h) to form methanol. These results suggest that dimeric copper sites are the dominant precursors to PMO active sites, and we will discuss zeolite synthesis and treatment strategies to generate larger fractions of PMO-active multinuclear copper sites.
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[2] Ipek B. et al., ACS Catal., 2017, 7, 4291-4303.
[3] Grundner S. et al., Nature Comm., 2015, 6, 1-9.
[4] Ravi M. et al., Angew. Chem. Int. Ed., 2017, 56, 16464-16483.
[5] Paolucci C. et al., J. Am. Chem. Soc. 2016, 138, 6028-6048.
[6] Da Costa P. et al., Phys. Chem. Chem. Phys., 2002, 4, 4590-4601.
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