Synthesis and Characterization of Mononuclear and Binuclear Copper Species in Cu-Exchanged Zeolites for Redox Reactions including Partial Methane Oxidation
Cu-zeolites have received renewed attention as catalytic materials that facilitate partial methane oxidation (PMO) to methanol, with a variety of mononuclear, binuclear, and multinuclear Cu active site motifs that have been proposed in prior literature. Our approach to more precisely identify and probe the Cu structures that activate O2 and reduce in CH4 relies on the synthesis of model supports with varying composition and well-defined Cu speciation, which also facilitates connections between experimental data and theoretical models. Chabazite (CHA) zeolites are high-symmetry frameworks that contain a single lattice tetrahedral site (T-site), in which Cu2+ ions exchange at paired Al sites in a six-membered ring (6-MR) while CuOH+ species exchange at isolated 6-MR Al sites, the latter of which can react to form binuclear O/O2-bridged Cu structures. In this work, Cu-CHA zeolites were synthesized to contain predominantly Cu2+ (Z2Cu) or CuOH+ (ZCuOH) species of varying density, or a mixture of Z2Cu and ZCuOH sites. Z2Cu and ZCuOH sites were quantified by titration of residual Brønsted acid sites with NH3, which respectively exchange with 2:1 or 1:1 H+:Cu2+ stoichiometry. Stoichiometric PMO reaction cycles on Cu-zeolites involved high-temperature (723 K) activation in O2, and then moderate-temperature (473 K) reduction in CH4 and treatment in H2O (473 K) to extract CH3OH. In-situ UV-Visible spectroscopy under oxidizing (O2, 723 K) and reducing (CO, 523 K; CH4, 473 K; He, 723 K) conditions detected the presence of mononuclear and binuclear Cu site types, while in-situ Cu K-edge X-ray absorption spectroscopy after such treatments was used to quantify Cu(I) and Cu(II) contents and in situ Raman spectroscopy was used to identify the Cu structures formed. ZCuOH, but not Z2Cu sites, are precursors to binuclear O/O2-bridged Cu sites that form upon O2 activation and subsequently produce methanol after stoichiometric PMO cycles, at yields (per total Cu) that increased systematically with ZCuOH site density. The fraction of Cu(II) sites that undergo auto-reduction in inert at high temperatures (He, 723 K) is identical, within experimental error, to the fraction that reduces in CH4 at temperatures relevant for PMO (473 K), providing a quantitative link between the binuclear Cu site motifs involved in both reaction pathways and motivating refinement of currently postulated PMO reaction mechanisms. These Cu-CHA zeolites were also studied for other redox chemistries including the selective catalytic reduction (SCR) of NOx with NH3. In situ UV-Visible and X-ray absorption spectroscopies were used to monitor and quantify the transient partial reduction of Cu(II) to Cu(I) during exposure to NH3 (473 K), in concert with titration methods that use NO and NH3 co-reductants to fully reduce all Cu(II) ions that remain after treatment in NH3 alone to the Cu(I) state, providing quantitative evidence that both Z2Cu and ZCuOH sites are able to reduce in NH3 alone to similar extents as a function of time. These findings provide new insight into the reaction pathways and mechanisms in which NH3 behaves as a reductant of mononuclear Cu(II) sites in zeolites, which are undesired side-reactions that occur during steady-state NOx SCR and that often unintendedly result in Cu(II) reduction prior to spectroscopic or titrimetric characterization. Overall, the strategy in this dissertation employs synthetic methods to control framework Al density and arrangement in zeolite supports to emphasize extra-framework Cu site motifs of different structure and at different spatial densities, and to interrogate these model materials using a combination of in situ spectroscopic techniques together with theory, in order to elucidate active site structure and proximity requirements in redox catalysis. This work demonstrates how quantitative reactivity and site titration data, brought together with an arsenal of tools available in contemporary catalysis research, can provide detailed mechanistic insights into transition metal-catalyzed redox cycles on heterogeneous catalysts.
- Doctor of Philosophy
- Chemical Engineering
- West Lafayette