Methane-to-Methanol via Chemical Looping: Economic Potential and Guidance for Future Research

Nature and Redox Properties of Iron Sites in Zeolites Revealed by Mössbauer Spectroscopy

Iron-containing zeolite-based catalysts play a pivotal role in environmental processes aimed at mitigating the release of harmful greenhouse gases, such as nitrous oxide (N2O) and methane (CH4). Despite the rich iron chemistry in zeolites, only a fraction of iron species that exhibit an open coordination sphere and possess the ability for electron transfer are responsible for activating reagents. In addition, the splitting of molecular oxygen is facilitated by bare iron cations embedded in zeolitic matrices. Mössbauer spectroscopy is the ideal tool for investigating the valency and geometry of iron species in zeolites because it leaves no iron forms silent and provides insights into in-situ processes. This review is dedicated to the utilization of Mössbauer spectroscopy to elucidate the nature of the extra-framework iron centers in ferrierite (FER), beta-structured (*BEA), and ZSM-5 zeolite (MFI) zeolites, which are active in N2O decomposition and CH4 oxidation through using the active oxygen derived from N2O and O2. In this work, a structured summary of the Mössbauer parameters established over the last two decades is presented, characterizing the specific iron active centers and intermediates formed upon iron's interaction with N2O/O2 and CH4. Additionally, the impact of preparation methods, iron loading, and the long-term stability on iron speciation and its redox behavior under reaction conditions is discussed.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C1 -C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with CuI sites, formed upon reduction of CuII -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Assessing the Productivity of the Direct Conversion of Methane-to-Methanol over Copper-Exchanged Zeolite Omega (MAZ) via Oxygen Looping

The methane-to-methanol (MtM) conversion via the oxygen looping approach using copper-exchanged zeolites has been extensively studied over the last decade. While a lot of research has focussed on maximizing yield and selectivity, little has been directed toward productivity-a metric far more meaningful for evaluating industrial potential. Using copper-exchanged zeolite omega (Cu-omega), a material highly active and selective for the MtM conversion using the isothermal oxygen looping approach, we show that this material exhibits unprecedented potential for industrial valorization. In doing so, we also present a novel methodology combining operando XAS and mass spectrometry for the screening of materials for the MtM conversion in oxygen looping mode.

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide-Metal Interfaces

Direct methane conversion to methanol has been considered as an effective and economic way to address greenhouse effects and the current high demand for methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs at high temperatures and retaining of produced methanol that requires mild conditions. This Perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which build a strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH₄-to-CH₃OH conversion.

Gas-Phase Selective Oxidation of Methane into Methane Oxygenates

Methane is an abundant resource and its direct conversion into value-added chemicals has been an attractive subject for its efficient utilization. This method can be more efficient than the present energy-intensive indirect conversion of methane via syngas, a mixture of CO and H2. Among the various approaches for direct methane conversion, the selective oxidation of methane into methane oxygenates (e.g., methanol and formaldehyde) is particularly promising because it can proceed at low temperatures. Nevertheless, due to low product yields this method is challenging. Compared with the liquid-phase partial oxidation of methane, which frequently demands for strong oxidizing agents in protic solvents, gas-phase selective methane oxidation has some merits, such as the possibility of using oxygen as an oxidant and the ease of scale-up owing to the use of heterogeneous catalysts. Herein, we summarize recent advances in the gas-phase partial oxidation of methane into methane oxygenates, focusing mainly on its conversion into formaldehyde and methanol.

Partial Methane Oxidation in Fuel Cell-Type Reactors for Co-Generation of Energy and Chemicals: A Short Review

The conversion of methane into chemicals is of interest to achieve a decarbonized future. Fuel cells are electrochemical devices commonly used to obtain electrical energy but can be utilized either for chemicals’ production or both energy and chemicals cogeneration. In this work, the partial oxidation of methane in fuel cells for electricity generation and valuable chemicals production at the same time is reviewed. For this purpose, we compile different types of methane-fed fuel cells, both low- and high-temperature fuel cells. Despite the fact that few studies have been conducted on this subject, promising results are driving the development of fuel cells that use methane as a fuel source for the cogeneration of power and valuable chemicals.

Facilitating green ammonia manufacture under milder conditions: what do heterogeneous catalyst formulations have to offer?

Ammonia production is one of the largest industrial processes, and is currently responsible for over 1.5% of global greenhouse gas emissions. Decarbonising this process, yielding 'green ammonia', is critical not only for sustainable fertilizer production, but also to unlocking ammonia's potential as a zero-carbon fuel and hydrogen store. In this perspective, we critically assess the role of cutting-edge heterogeneous catalysts to facilitate milder ammonia synthesis conditions that will help unlock cheaper, smaller-scale, renewables-coupled ammonia production. The highly-optimised performance of catalysts under the high temperatures and pressures of the Haber-Bosch process stands in contrast to the largely mediocre activity levels reported at lower temperatures and pressures. We identify the recent advances in catalyst design that help overcome the sluggish kinetics of nitrogen activation under these conditions and undertake a categorized analysis of improved activity achieved in a range of heterogeneous catalysts. Building on these observations, we develop a 'catalyst efficiency' analysis which helps uncover the success of a holistic approach - one that addresses the issues of nitrogen activation, hydrogenation of adsorbed nitrogen species, and engineering of materials to maximize the utilization of active sites - for achieving the elusive combination of high-activity, low-temperature formulations. Furthermore, we present a discussion on the industrial considerations to catalyst development, emphasising the importance of catalyst lifetime in addition to catalyst activity. This assessment is critical to ensuring that high productivities can translate into real advances in commercial ammonia synthesis.

Methane-to-Methanol on Mononuclear Copper(II) Sites Supported on Al2 O3 : Structure of Active Sites from Electron Paramagnetic Resonance*

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu‐exchanged zeolites have been shown promote this reaction under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono‐, di‐ to trimeric Cu^II. Herein, we report the formation of well‐dispersed monomeric Cu^II species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in γ‐ and η‐alumina phase, and that their EPR signature can be attributed to species having a tri‐coordinated [(Al₂O)CuIIO(OH)]⁻ T‐shape geometry. Overall, the selective conversion of methane to methanol, a two‐electron process, involves two monomeric Cu^II sites that play in concert.

Methane to Methanol through Heterogeneous Catalysis and Plasma Catalysis

Direct oxidation of methane to methanol (DOMTM) is attractive for the increasing industrial demand of feedstock. In this review, the latest advances in heterogeneous catalysis and plasma catalysis for DOMTM are summarized, with the aim to pinpoint the differences between both, and to provide some insights into their reaction mechanisms, as well as the implications for future development of highly selective catalysts for DOMTM.

Representing Catalytic and Processing Space in Methane Oxidation Reaction via Multioutput Machine Learning

Multioutput support vector regression (SVR) is implemented to simultaneously predict the selectivities and the CH₄ conversion against experimental conditions in methane oxidation catalysts. The predictions unveil the details of how each selectivity and CH₄ conversion behaves in each catalyst. In particular, the selectivity and the CH₄ conversion of Mn-Na₂WO₄/SiO₂, Ti-Na₂WO₄/SiO₂, Pd-Na₂WO₄/SiO₂, and Na₂WO ₄/SiO₂ are predicted, and the effects of Mn, Ti, and Pd are unveiled. In addition, the trade-off points of CO and C₂H₆ are identified for each catalyst, leading to maximization of the C₂H₆ yield. Thus the simultaneous prediction of the reaction trend with catalysts not only will help with the understanding of the catalyst activities but also will provide guidance for designing the experimental conditions.

Copper-exchanged large-port and small-port mordenite (MOR) for methane-to-methanol conversion

Zeolite mordenite (MOR) is one of the most studied zeolites for the stepwise direct conversion of methane to methanol, but it also can exist in two forms: large port and small port. Here we report that the synthesis and selection of the parent mordenite is critical for optimizing productivity, and that large-port mordenite outperforms small-port mordenite for the stepwise conversion of methane to methanol.

The synthesis and selection of large-port mordenite is critical for optimizing productivity for the direct conversion of methane to methanol.

Catalytic oxidation of methane to methanol over Cu-CHA with molecular oxygen

Catalytic production of CH3OH in a CH4–O2–H2O flow reaction is improved using Cu-CHA having improved redox property involved in the C–H activation of CH4.

Water Is the Oxygen Source for Methanol Produced in Partial Oxidation of Methane in a Flow Reactor over Cu-SSZ-13

Direct oxidation of methane to methanol is a long-standing challenge in the heterogeneous catalysis community. This Communication demonstrates that water, not dioxygen, is the main source of the oxygen present in the methanol produced in the partial oxidation of methane to methanol over Cu-SSZ-13 in a continuous-flow reactor. This is confirmed by experiments performed in the absence of molecular oxygen and with the use of ¹⁸O-labeled water. These findings should lead to new approaches for improving the partial oxidation properties of copper zeolites.

Approaches for Selective Oxidation of Methane to Methanol

Methane activation chemistry, despite being widely reported in literature, remains to date a subject of debate. The challenges in this reaction are not limited to methane activation but extend to stabilization of the intermediate species. The low C-H dissociation energy of intermediates vs. reactants leads to CO2 formation. For selective oxidation, nature presents methane monooxygenase as a benchmark. This enzyme selectively consumes methane by breaking it down into methanol. To assemble an active site similar to monooxygenase, the literature reports Cu-ZSM-5, Fe-ZSM-5, and Cu-MOR, using zeolites and systems like CeO2/Cu2O/Cu. However, the trade-off between methane activation and methanol selectivity remains a challenge. Density functional theory (DFT) calculations and spectroscopic studies indicate catalyst reducibility, oxygen mobility, and water as co-feed as primary factors that can assist in enabling higher selectivity. The use of chemical looping can further improve selectivity. However, in all systems, improvements in productivity per cycle are required in order to meet the economical/industrial standards.

Comparative performance of Cu-zeolites in the isothermal conversion of methane to methanol

A series of zeolites were screened for the direct conversion of methane to methanol under isothermal low-temperature stepwise conditions; of the screened zeolites, omega zeolite (MAZ) showed superior performance.