Synthesis and Characterization of Mononuclear and Binuclear Copper Species in Cu-Exchanged Zeolites for Redox Reactions including Partial Methane Oxidation

Description

<p>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 CH₄relies 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. <i>I</i><i>n-situ</i> 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 <i>in-situ</i> Cu K-edge X-ray absorption spectroscopy after
such treatments was used to quantify Cu(I) and Cu(II) contents and <i>in situ</i> 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 NO_x
with NH₃. <i>In situ </i>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 NO_x 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 <i>in situ</i>
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.</p>

Links & Downloads

1. 10.25394/pgs.16803931.v1

Tags

in situ characterization

redox chemistry

partial methane oxidation

Binuclear active sites

Cu-CHA zeolites

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/16803931
Date	13 October 2021
Creators	Laura Wilcox (7534151)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/thesis/Synthesis_and_Characterization_of_Mononuclear_and_Binuclear_Copper_Species_in_Cu-Exchanged_Zeolites_for_Redox_Reactions_including_Partial_Methane_Oxidation/16803931