Complexation of NpO2+ with Amine-Functionalized Diacetamide Ligands in Aqueous Solution: Thermodynamic, Structural, and Computational Studies

Complexation of Np(V) with three structurally related amine-functionalized diacetamide ligands, including 2,2'-azanediylbis( N, N'-dimethylacetamide) (ABDMA), 2,2'-(methylazanediyl)bis( N, N'-dimethylacetamide) (MABDMA), and 2,2'-(benzylazanediyl)bis( N, N'-dimethylacetamide) (BnABDMA), in aqueous solutions was investigated. The stability constants of two successive complexes, namely, NpO2L+ and NpO2L2+, where L stands for the ligands, were determined by absorption spectrophotometry. The results suggest that the stability constants of corresponding Np(V) complexes follow the trend: MABDMA > ABDMA ≈ BnABDMA. The data are discussed in terms of the basicity of the ligands and compared with those for the complexation of Np(V) with an ether oxygen-linked diacetamide ligand. Extended X-ray absorption fine structure data indicate that, similar to the complexation with Nd3+ and UO22+, the ligands coordinate to NpO2+ in a tridentate mode through the amine nitrogen and two oxygen atoms of the amide groups. Computational results, in conjunction with spectrophotometric data, verify that the 1:2 complexes (NpO2(L)2+) in aqueous solutions are highly symmetric with Np at the inversion center, so that the f-f transition of Np(V) is forbidden and NpO2(L)2+ does not display significant absorption in the near-IR region

Spectrophotometric analysis of ternary uranyl systems to replace tri-N-butyl phosphate (TBP) in used fuel reprocessing

In this report, the interaction of monoamide/diamide and monoamide/diglycolamide mixtures with UO2+2 are investigated in pH = 1 methanolic nitric acid media. These monoamides include N,N-dimethylacetamide (DMAA), N,N-diethylacetamide (DEAA), N,N-dibutylacetamide (DBAA) and N,N-dibutylbutanamide (DBBA). N,N,N′N′-tetraethylmalonamide (TEMA) and N,N,N′,N′-tetraethyldiglycolamide (TEDGA), which were chosen as model diamides and diglycolamides, respectively. Complex stability constants for each ligand were modelled using the Stability Quotients Using Absorbance Data program using UV–visible data. Complex stoichiometry of ligand mixtures was determined using Job plots and UV–Vis spectrometry. Monoamides were confirmed to produce only disolvate complexes with UO2+2 in solution. The log10(K) values for monoamides were found to be independent of amine-side chain length, but were slightly dependent on the carbonyl-side chain length. TEDGA was found to produce multiple uranyl complexes in solution. Job plot data indicated that the uranyl cation strongly prefers to bond either only with the monoamide or diamide in ternary monoamide–diamide–UO2 systems. Monoamide–diglycolamide–UO2 systems were more complicated, with Job plot data indicating the potential for multiple ternary species being present is dependent on the monoamide structure

Structural incorporation of Neptunyl(V) into Calcite: Interfacial Reactions and Kinetics

In this experimental work the calcite-water interface is characterized by means of zetapotential and surface diffraction measurements. Based on the experimental results a new Basic Stern Surface Complexation model for calcite is developed. Neptunyl(V) adsorption at the calcite surface and incorporation into the calcite structure is studied by batch type adsorption- and mixed flow reactor experiments. Adsorption and incorporation species of Neptunyl are investigated by EXAFS spectroscopy

Relations entre structure et réactivité dans l’interaction entre les substances humiques, les polluants métalliques du cycle du nucléaire et les surfaces minérales

This document proposes an analysis of the structure-reactivity relationships in the interaction between humic substances, metallic pollutants from the nuclear cycle, and mineral surfaces. It composes the scientific document, which allowed the author to defend a habilitation degree. It is mainly focused on the research works into which the author have been involved in on this particular thematic. Humic substances are issued from the degradation of the living. They have an important influence onto migration of metals in the environment. They are showing particular intrinsic physic and chemical, metal complexation, and adsorption onto mineral surfaces properties, which render the global comprehension of the different mechanisms somehow difficult. These three aspects are covered in this document. The first part is dedicated to the studies on composition, structure, and organization of humic substances, which cannot be considered as a well-defined type of chemical. They are a heterogeneous degradation product with a supramolecular organization, which is showing fractal properties from fractions up to several nanometers. Second part is on the complexation reactions. The different modelling strategies come from the difficulties on apprehending composition, structure, and organization of humic substances. The different models used are showing more or less strongly empiric characteristics. They can be derived from the mass action law, or explicitly account for heterogeneity, acido-basic, or ionic strength related parameters. The third and latter part covers the adsorption studies. The main property is adsorptive fractionation, which induces modification of chemical composition of humic substances between the surface and the solution. It also induces modification of complexation properties between the adsorbed and non-adsorbed fractions. Because of adsorptive fractionation, and the particular influence of ionic strength on humic substances, and of complexed metals, adsorption, the studies on simple model molecules to represent humic substances reactivity are an illusion. The global comprehension will mainly be obtained from the understanding of dedicated studies.Ce document présente une analyse des relations entre structure et réactivité dans l’interaction entre les substances humiques, les polluants métalliques du cycle du nucléaire et les surfaces minérales. Il reprend le document scientifique qui a permis la soutenance d’une habilitation à diriger les recherches, et à ce titre se concentre principalement sur les recherches menées ou animées par l’auteur sur cette thématique. Les substances humiques sont issues de la dégradation du vivant, et possèdent une influence importante sur la migration des métaux dans l’environnement. Elles présentent des propriétés physico-chimiques intrinsèques, des capacités de complexation des métaux et d’adsorption sur les surfaces minérales qui font qu’une compréhension globale des différents mécanismes en jeu est souvent difficile. Ces trois aspects sont abordés dans ce document. La première partie est consacrée aux études sur la composition, la structure et l’organisation des substances humiques. Les substances humiques ne constituent pas un type de molécules chimiques bien définies, mais plutôt un mélange hétérogène de produits de dégradation, ayant une organisation supramoléculaire et fractale sur une grande échelle de taille, depuis la fraction de nanomètre jusqu’à plusieurs dizaines de nanomètres. La deuxième partie aborde les réactions de complexation. Les différentes stratégies de modélisation sont la conséquence des difficultés de compréhension de la composition, de la structure et de l’organisation. Les modèles utilisés possèdent des caractéristiques plus ou moins fortement empiriques. Ils peuvent être dérivés de la loi d’action des masses avec des paramètres extra-thermodynamiques, ou bien prendre en compte implicitement des paramètres liés à l’hétérogénéité, aux réactions acido-basiques, à la force ionique, et aux réactions de compétitions. La troisième et dernière partie couvre les études d’adsorption et les comportements particuliers des substances humiques. La propriété principale est le fractionnement par les surfaces qui induit des modifications de compositions des substances humiques entre la surface et la solution, et par la même des différences de propriétés de complexation des métaux entre fraction à la surface et en solution. Ce fractionnement, ainsi que l’influence particulière de la force ionique sur l’adsorption des substances humiques, et des métaux complexés, font qu’il est illusoire de vouloir représenter la réactivité des substances humiques par des composés organiques simples, et que la compréhension globale de ces systèmes ne peut être atteinte que par des études dédiées

Organometallic Neptunium Chemistry

Fifty years have passed since the foundation of organometallic neptunium chemistry, and yet only a handful of complexes have been reported, and even fewer fully characterised. Yet increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently binding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal – ligand bonding. Understanding the subtleties are key to the safe handling and separations of the highly radioactive nuclei. This review describes the complexes that have been synthesised to date, presents a critical assessment on the successes and difficulties in their analysis, and the bonding information they have provided. Because of the recent work to start new Np air-sensitive inorganic chemistry labs, the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochemical synthesis of one organometallic) and the available anhydrous starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chemical behaviour between Np and its closest neighbours, uranium and plutonium, are found.JRC.G.I.5-Advanced Nuclear Knowledg

The sorption of uranium(VI) and neptunium(V) onto surfaces of selected metal oxides and alumosilicates studied by in situ vibrational spectroscopy

The migration behavior of actinides and other radioactive contaminants in the environment is controlled by prominent molecular phenomena such as hydrolysis and complexation reactions in aqueous solutions as well as the diffusion and sorption onto minerals present along groundwater flow paths. These reactions significantly influence the mobility and bioavailability of the metal ions in the environment, in particular at liquid-solid interfaces. Hence, for the assessment of migration processes the knowledge of the mechanisms occurring at interfaces is crucial. The required structural information can be obtained using various spectroscopic techniques. In the present study, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of titania, alumina, silica, zinc, and alumosilicates has been investigated by the application of attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. Moreover, the distribution of the hydrolysis products in micromolar aqueous solutions of U(VI) and Np(V/VI) at ambient atmosphere has been characterized for the first time, by a combination of ATR FT-IR spectroscopy, near infrared (NIR) absorption spectroscopy, and speciation modeling applying updated thermodynamic databases. From the infrared spectra, a significant change of the U(VI) speciation is derived upon lowering the U(VI) concentration from the milli- to the micromolar range, strongly suggesting the dominance of monomeric U(VI) hydrolysis products in the micromolar solutions. In contradiction to the predicted speciation, monomeric hydroxo species are already present at pH ≥ 2.5 and become dominant at pH 3. At higher pH levels (> 6), a complex speciation is evidenced including carbonate containing complexes. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3, and they are comparatively discussed with U(VI). For both actinides, the formation of similar species is suggested at pH ≤ 4, whereas at higher pH, the infrared spectra evidence structurally different species. At pH 5, the formation of a carbonate-containing dimeric complex, that is (NpO2)2CO3(OH)3−, is strongly suggested, whereas carbonate complexation occurs only under more alkaline conditions in the U(VI) system. The results from the experiments of the sorption processes clearly demonstrate the formation of stable U(VI) surface complexes at all investigated mineral phases. This includes several metal oxides, namely TiO2, Al2O3, and SiO2, serving as model systems for the elucidation of more complex mineral systems, and several alumosilicates, such as kaolinite, muscovite and biotite. From a multiplicity of in situ experiments, the impact of sorbent characteristics and variations in the aqueous U(VI) system on the sorption processes was considered. A preferential formation of an inner-sphere complex is derived from the spectra of the TiO2 and SiO2 phases. In addition, since the in situ FT-IR experiments provide an online monitoring of the absorption changes of the sorption processes, the course of the formation of the U(VI) surface complexes can be observed spectroscopically. It is shown that after prolonged sorption time on TiO2, resulting in a highly covered surface, outer-sphere complexation predominates the sorption processes. The prevailing crystallographic modification, namely anatase and rutile, does not significantly contribute to the spectra, whereas surface specific parameters, e.g. surface area or porosity are important. A significant different surface complexation is observed for Al2O3. The formation of inner-spheric species is assumed at low U(VI) surface coverage which is fostered at low pH, high ionic strength and short contact times. At proceeded sorption the surface complexation changes. From the spectra, an outer-spheric coordination followed by surface precipitation or polymerization is deduced. Moreover, in contrast to TiO2, the appearance of ternary U(VI) carbonate complexes on the γ-Al2O3 surface is suggested. The first results of the surface reactions on more complex, naturally occurring minerals (kaolinite, muscovite and biotite) show the formation of U(VI) inner-sphere sorption complexes. These findings are supported by the spectral information of the metal oxide surfaces. In this work, first spectroscopic results from sorption of aqueous Np(V) on solid mineral phases are provided. It is shown that stable inner-sphere surface species of NpO2+ are formed on TiO2. Outer-sphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4 – 7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2, and ZnO indicate structurally similar bidentate surface complexes. The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FT-IR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V/VI), in the environment. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.Das Migrationsverhalten von Aktiniden und anderen radioaktiven Schadstoffen in der Umwelt wird von wichtigen molekularen Prozessen entlang der Grundwasserfließwege reguliert. Dazu gehören sowohl die Hydrolyse und Komplexierung in wässrigen Lösungen als auch Diffusion und Sorption der Schwermetalle an Mineralen. Diese Reaktionen beeinflussen entscheidend die Mobilität und Bioverfügbarkeit der Metallionen in der Umwelt, insbesondere an den fest-flüssig Grenzflächen. Genaue Kenntnisse über die an diesen Grenzflächen stattfindenden Mechanismen sind somit entscheidend, um Migrationsprozesse verlässlich abschätzen zu können. Die benötigten strukturellen Informationen können mit verschiedenen spektroskopischen Techniken ermittelt werden. Das Ziel der vorliegenden Arbeit war die Untersuchung der Speziation von Uran(VI) und Neptunium(V) an umweltrelevanten Grenzflächen von Oxiden des Titans, Aluminiums, Siliziums und Zinks und von Alumosilikaten mittels ATR FT-IR Spektroskopie. Des Weiteren wurde die Verteilung aquatischer Spezies in mikromolaren Lösungen des U(VI) und Np(V/VI) unter Normalbedingungen charakterisiert. Diese erstmalige Untersuchung wurde mit einer Kombination aus Speziationsmodellierung unter Anwendung aktueller thermodynamischer Daten und ATR FT-IR und NIR Absorptionsspektroskopie realisiert. Die Infrarotspektren zeigen eine deutliche Änderung der Speziesverteilung im Konzentrationsverlauf vom millimolaren zum mikromolaren Bereich. Dies verweist auf die Bildung monomerer U(VI) Hydrolyseprodukte. Im Gegensatz zu berechneten Speziationen werden diese monomeren Komplexe schon bei pH ≥ 2,5 gebildet und dominieren die Speziation bei pH 3. Bei höheren pH-Werten (> 6) konnte eine komplexe Speziesverteilung mit Anteilen von Karbonatkomplexen nachgewiesen werden. Erstmals konnten im Rahmen dieser Arbeit spektroskopische Befunde der Hydrolysereaktionen des Np(VI) im submillimolaren Konzentrationsbereich bis pH 5,3 erhalten werden. Diese wurden im Vergleich mit der U(VI) Speziation diskutiert. Obwohl im sauren Bereich (pH ≤ 4) die Bildung ähnlicher Komplexe nachgewiesen wurde, zeigen die bei höheren pH-Werten erhaltenen Spektren eine unterschiedliche Speziesverteilung. Im Gegensatz zum U(VI) bildet das Np(VI) schon bei pH 5 karbonathaltige aquatische Spezies wie (NpO2)2CO3(OH)3−. Die Ergebnisse der Sorptionsexperimente von U(VI) zeigen die Bildung stabiler Oberflächenkomplexe an allen untersuchten Mineralphasen. Dies umfasst mehrere als Modellsystem dienende Metalloxide wie TiO2, Al2O3 und SiO2, als auch komplexere Alumosilikate wie Kaolinit, Muskovit und Biotit. Für eine detaillierte Charakterisierung der Oberflächenkomplexe wurde eine Vielzahl von in situ Sorptionsexperimenten durchgeführt, die den Einfluss unterschiedlicher Parameter der mineralischen Phase als auch des wässrigen U(VI) Systems berücksichtigen. Die bevorzugte Bildung von innersphärischen Komplexen an TiO2 und SiO2 wird aus den spektroskopischen Daten abgeleitet. Da die in situ FT-IR Spektroskopie eine kontinuierliche Registrierung der Absorptionsänderungen während der ablaufenden Sorptionsprozesse erlaubt, kann somit der Verlauf dieser Prozesse quasi in Echtzeit spektroskopisch verfolgt werden. Es konnte gezeigt werden, dass mit fortschreitender Sorptionsdauer, d.h. bei hohen Beladungsdichten, die Bildung einer weiteren außersphärischen Spezies die Sorption dominert. Die vorliegende kristallographische Modifikation, Anatas und Rutil, ist nicht maßgeblich für das Auftreten unterschiedlicher Sorptionsprozesse verantwortlich, obwohl Parameter wie die spezifische Oberfläche und die Porosität für den Sorptionsprozess von Bedeutung sind. Deutlich verschiedene Oberflächenreaktionen werden für Al2O3 beobachtet. Aus den Spektren kann die Ausbildung einer innersphärischen Spezies bei sehr niedrigen U(VI) Beladungen, niedrigen pH-Werten, hohen Ionenstärken und kurzen Kontaktzeiten abgeleitet werden. Bei fortschreitender Sorption ändert sich die Art der Oberflächenkomplexe. Zunächst bilden sich außersphärische Spezies, während im weiteren Verlauf die Spektren auf eine beginnende Oberflächenausfällung bzw. Polymerisation hinweisen. Weiterhin wird das Auftreten von ternären U(VI) Karbonatkomplexen an γ-Al2O3 aus den spektroskopischen Daten abgeleitet. Die ersten Ergebnisse der Sorptionsexperimente an komplexeren, natürlich auftretenden Mineralphasen (Kaolinit, Muskovit und Biotit) zeigen eine bevorzugte Ausbildung von innersphärischen U(VI) Komplexen. Diese Resultate werden durch die spektralen Befunde der Experimente der Metalloxide gestützt. Erstmalig werden in dieser Arbeit spektroskopische Ergebnisse der Sorptionsprozesse von wässrigen Np(V) an verschiedenen Mineralphasen präsentiert. Wie U(VI) bildet Np(V) stabile innersphärische Oberflächenkomplexe an TiO2. Die Speziesverteilung an der TiO2 Oberfläche ist im pH Bereich 4 – 7,6 konstant. Daher ist zu erwarten, dass eine außersphärische Komplexierung hier nur eine untergeordnete Rolle spielt. Der Vergleich von Spektren der Np(V) Sorptionskomplexe an TiO2, SiO2 und ZnO weist auf die Bildung strukturell ähnlicher bidentater Komplexe hin. Die Vielzahl der hier vorgestellten infrarotspektroskopischen Experimente bietet eine fundierte Sammlung spektroskopischer Daten, die für zukünftige Untersuchungen komplexer aquatischer und mineralischer Systeme unerlässlich ist. Gleichzeitig wurde der Anwendungsbereich der ATR FT-IR Technik auf dem Gebiet der Radioökologie umfassend erweitert. Die im Rahmen dieser Arbeit gewonnenen Ergebnisse tragen zu einem besseren Verständnis der geochemischen Wechselwirkungen von Aktiniden, im Speziellen von U(VI) und Np(V) in der Umwelt bei. Damit unterstützen sie den Aufklärungsprozess der Migration von radioaktiven Kontaminationen und dienen als Grundlage für zuverlässige Prognosen für die Sicherheitsbewertung von Endlagern für nukleare Abfälle

DFT+U study of the structures and properties of the actinide dioxides

The actinide oxides play a vital role in the nuclear fuel cycle. For systems where current experimental measurements are difficult, computational techniques provide a means of predicting their behaviour. However, to date no systematic methodology exists in the literature to calculate the properties of the series, due to the lack of experimental data and the computational complexity of the systems. Here, we present a systematic study where, within the DFT+U formulism, we have parametrized the most suitable Coulombic (U) and exchange (J) parameters for different functionals (LDA, PBE, PBE-Sol and AM05) to reproduce the experimental band-gap and lattice parameters for ThO2, UO2, NpO2, PuO2, AmO2 and CmO2. After successfully identifying the most suitable parameters for these actinide dioxides, we have used our model to describe the electronic structures of the different systems and determine the band structures, optical band-gaps and the Bulk moduli. In general, PBE-Sol provides the most accurate reproduction of the experimental properties, where available. We have employed diamagnetic order for ThO2, PuO2 and CmO2, transverse 3k antiferromagnetic order for UO2 and AmO2, and longitudinal 3k antiferromagnetic order for NpO2. The Fm m cubic symmetry is preserved for diamagnetic ThO2, PuO2 and CmO2 and longitudinal 3k NpO2. For UO2 and AmO2, the transverse 3k antiferromagnetic state results in Pa symmetry, in agreement with recent experimental findings. Although the electronic structure of ThO2 cannot be reproduced by DFT or DFT+U, for UO2, PuO2, NpO2, AmO2 and CmO2, the experimental properties are very well represented when U = 3.35 eV, 6.35 eV, 5.00 eV, 7.00 eV and 6.00 eV, respectively, with J = 0.00 eV, 0.00 eV, 0.75 eV, 0.50 eV and 0.00 eV, respectively

DFT plus U study of the structures and properties of the actinide dioxides

The actinide oxides play a vital role in the nuclear fuel cycle. For systems where current experimental measurements are difficult, computational techniques provide a means of predicting their behaviour. However, to date no systematic methodology exists in the literature to calculate the properties of the series, due to the lack of experimental data and the computational complexity of the systems. Here, we present a systematic study where, within the DFT+U formulism, we have parametrized the most suitable Coulombic (U) and exchange (J) parameters for different functionals (LDA, PBE, PBE-Sol and AM05) to reproduce the experimental band-gap and lattice parameters for ThO2, UO2, NpO2, PuO2, AmO2 and CmO2. After successfully identifying the most suitable parameters for these actinide dioxides, we have used our model to describe the electronic structures of the different systems and determine the band structures, optical band-gaps and the Bulk moduli. In general, PBE-Sol provides the most accurate reproduction of the experimental properties, where available. We have employed diamagnetic order for ThO2, PuO2 and CmO2, transverse 3k antiferromagnetic order for UO2 and AmO2, and longitudinal 3k antiferromagnetic order for NpO2. The Fm m cubic symmetry is preserved for diamagnetic ThO2, PuO2 and CmO2 and longitudinal 3k NpO2. For UO2 and AmO2, the transverse 3k antiferromagnetic state results in Pa symmetry, in agreement with recent experimental findings. Although the electronic structure of ThO2 cannot be reproduced by DFT or DFT+U, for UO2, PuO2, NpO2, AmO2 and CmO2, the experimental properties are very well represented when U = 3.35 eV, 6.35 eV, 5.00 eV, 7.00 eV and 6.00 eV, respectively, with J = 0.00 eV, 0.00 eV, 0.75 eV, 0.50 eV and 0.00 eV, respectively

Activating Methane and Other Small Molecules: Computational study of Zeolites and Actinides

Exploring the catalytic properties and reactivity of actinide complexes towards activation of small molecules is important as human activities have led to the increased distribution of these species in nature. Toward this end, it is important to have a computational protocol for studying these species, in this thesis we provide details on the performance of multiconfigurational pair-density functional theory (MC-PDFT) in actinide chemistry. MC-PDFT and Kohn-Sham Density Functional Theory (KS-DFT) perform well for these species with indications that the former can be used for species with even greater static electron correlation effect. In addition, we study the activity of organometallic trans-uranium complexes towards the electrocatalytic reduction of water. We conclude that, with a guided choice of ligand, neptunium complexes can provide similar reactivity when compared to organometallic uranium complexes.Conversion of methane to methanol has been a major focus of research interest over the years. This is largely due to the abundance of natural gas, of which methane is the major constituent. Copper-exchanged zeolites have been shown to be able to kinetically trap activated methane as strongly-bound methoxy groups, preventing over-oxidation to CO2, CO and HCOOH. In this stepwise process, there are three cycles; an initial activation step to form the copper oxo active site, methane C-H activation and lastly simultaneous desorption of methanol and re -activation of the active site.. We provide detailed description of the pathway for the formation of over oxidation products. It is observed that to ensure high selectivity to methanol and prevent further hydrogen atom abstraction by extra-framework species, the methyl group must be stabilized from the copper-oxo active sites. There is a temperature gradient between the steps in the methane-to-methanol conversion cycle which is an impediment to industrial adoption of this approach for methane-to-methanol conversion. To mitigate this, we have investigated the impact of heterometallic extra-framework motifs on the temperature gradients of each step. Using periodic DFT, we provide detailed descriptions of the mechanistic pathways for each of the three steps. We were subsequently able to design motif(s) with great methane C-H activities as well as the abilities to be formed and regenerated at nearly the same temperatures. We found [Cu-O-Ag] and [Cu-O-Pd] to be potential candidates for isothermal or near-isothermal operations of the methane-to-methanol conversion cycle. Finally, we provide insights to the changes in optical spectra of activated copper-exchanged zeolites, gaining an understanding of the evolution of these systems on a molecular level will provide opportunities to achieve improved reactivity