The structure of copper sites formed under an oxidative environment and their evolution in the course of the reaction with methane at elevated temperature was investigated by means of Cu K-edge X-ray absorption spectroscopy for a series of copper-containing MFI, MOR, and FAU zeolites. The pretreatment in oxygen at 723 K leads to the formation of copper(II)-oxo sites, whose nature depends on the framework type. Dimeric species are formed in CuMFI material, dimeric and monomeric sites coexist in CuMOR, and agglomerated copper-oxo nanoclusters are found in large-pore copper-containing faujasite (CuFAU). For all studied materials, the reaction with methane resulted in the exclusive formation of copper(I) species; no formation of metallic copper was detected even at 748 K. The nature of formed copper(I) species is governed by the structure of corresponding copper(II) centers. In particular, monomeric and dimeric copper(II)-oxo sites hosted in CuMOR and CuMFI are transformed into isolated copper(I) cations coordinated to ion-exchange positions of the zeolite. Contrarily, copper(II)-oxo clusters present in CuFAU undergo restructuring with only a partial loss of extra-framework oxygen and form aggregated species with a structure similar to that of bulk copper(I) oxide.

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In this work, adsorption of nitrogen monoxide (NO) and carbon monoxide (CO) probe molecules on various copper sites in a range of zeolites is studied. The structures of copper sites, binding energies, and vibrational frequencies of adsorbed probe molecules are calculated using density functional theory (DFT). This allows mapping vibrational spectra regions to specific copper species as a function of the zeolite topology and Si/Al ratio. CO can adsorb on Cu⁺ions by forming mono- and dicarbonyls or on copper ions bonded to methoxy species by forming methoxy-monocarbonyls, which exhibit a blue shift in wavenumbers. The stretching frequencies of adsorbed NO generally increase in the following order: [CuOH]⁺< [Cu₂O]²⁺/[Cu₂O₂]²⁺< [Cu²⁺] < [Cu_n + 1On]²⁺/[CunOn]²⁺(n> 3) < [Cu₃O₂]²⁺/[Cu₃O₃]²⁺. The shift values between different species vary between 5 and 20 cm^-1, showing the possibility for structure assignment based on infrared frequencies. Zeolite frameworks with smaller pores exhibit a shift of vibrational bands of adsorbed NO toward lower frequencies because of the confinement effect of the zeolite pore structure. Zeolites with larger pores stabilize the copper species of higher nuclearity. Our data indicate that the tabulated infrared frequencies of adsorbed CO and NO may be used to assign zeolitic copper speciation from experimental data.

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Copper-exchanged zeolites are a class of redox-active materials that find application in the selective catalytic reduction of exhaust gases of diesel vehicles and, more recently, the selective oxidation of methane to methanol. However, the structure of the active copper-oxo species present in zeolites under oxidative environments is still a subject of debate. Herein, we make a comprehensive study of copper species in copper-exchanged zeolites with MOR, MFI, BEA, and FAU frameworks and for different Si/Al ratios and copper loadings using X-ray absorption spectroscopy. Only obtaining high quality EXAFS data, collected at largek-values and measured under cryogenic conditions, in combination with wavelet transform analysis enables the discrimination between the copper-oxo species having different structures. The zeolite topology strongly affects the copper speciation, ranging from monomeric copper species to copper-oxo clusters, hosted in zeolites of different topologies. In contrast, the variation of the Si/Al ratio or copper loading in mordenite does not lead to significant differences in XAS spectra, suggesting that a change, if any, in the structure of copper species in these materials is not distinguishable by EXAFS.

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In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol. We consider the varied experimental evidence arising from the application of X-ray diffraction, and vibrational, electronic, and X-ray spectroscopies that exist, along with the results of theory. We aim to establish both what is known regarding these elusive materials and how they function, and also where gaps in our knowledge still exist, and offer suggestions and strategies as to how these might be closed such that the rational design of more effective and efficient materials of this type for the selective conversion of methane might proceed further.

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In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper–zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.

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A direct route to convert methane into high-value commodities, such as methanol, with high selectivity is one of the primary challenges in modern chemistry. Copper-exchanged zeolites show remarkable selectivity in the chemical looping process. Although multiple copper species have been proposed as active, an in situ spectroscopic investigation is difficult, because of their similar fingerprints. We used ambient pressure X-ray photoelectron spectroscopy to investigate an actual powder sample. We could discriminate between different types of active species involved in the conversion of methane to methanol over two different copper-exchanged zeolites, namely, mordenite and mazzite. After activation at 400 °C in oxygen, we followed the reaction in situ at 200 °C, switching from methane to water, and followed by a second cycle with anaerobic activation. Our experimental results, combined with theoretical calculations, prove that Cu(II) sites bound to extra-framework oxygen are involved in the reaction, and that their structure, formation, and stabilization depend on the type of zeolite and on the Si/Al ratio. ©

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Direct methane functionalization and, in particular, the selective partial oxidation to methanol, remains an eminent challenge and a field of competitive research. The conversion of methane to methanol over transition-metal-containing zeolites using molecular oxygen is a promising and extensively studied process. Herein, we scrutinize some oft-cited assumptions in this topic—which include the labelling of the process as biomimetic, the debate regarding the industrial viability of direct methane-oxidation systems and the claim that methane is difficult to activate—and delineate the extent to which these are scientifically robust. We highlight both the merits and pitfalls of such statements and point out the hazards associated with their improper use. By examining these misconceptions, we build an outlook for future research, highlighting the need to optimize materials and process conditions for the stepwise approach and to further explore catalytic processes that explicitly employ strategies for the preservation of methanol.

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Development of a suitable mild-condition process for direct conversion of methane to methanol faces multiple challenges, the principal ones being the higher reactivity of the primary oxidation products and the need for temperature swings in the typically employed chemical looping procedures. To circumvent these problems, the use of water as a mild oxidant has been recently suggested, leading to the concurrent formation of molecular hydrogen. By means of ab initio calculations, we address the experimentally observed features of the reaction to identify possible reaction pathways of such hydrogen release. We propose that, along with a strong stabilizing effect of water, short-lived [Cu-H] intermediate species play a crucial role in the mechanism of the reaction. Proton transfer from the Brønsted acid site of the zeolite framework via an adsorbed water molecule to the Cu^I species generates a [Cu-H] intermediate, which then facilitates the release of molecular hydrogen. This allows the reaction to proceed over a relatively low-energy transition state configuration. At the same time, excess of water leads to increased complexity of the concerted transition state, which results in hindering of the hydrogen transfer and increase of the corresponding energy barrier.

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The application and quantification of in situ copper K-edge X-ray absorption near-edge structure (XANES), when linked to independently made reactor-based studies of methanol production, result in a majority relation between the production of CuI and methanol from methane that complies with the expectations of a two-electron mechanism founded upon CuII/CuI redox couples.@en

Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.

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Labinger argues that stepwise reaction of methanewithwater to producemethanol and hydrogen will never be commercially feasible because of its substoichiometric basis with respect to the active site and the requirement of a large temperature swing. This comment is not touching any new ground, beyond describing the thermodynamic feasibility, thermal cycling, and the role of water as discussed previously. Most important, it does not have a solid numerical basis.@en

Direct functionalization of methane in natural gas remains a key challenge. We present a direct stepwise method for converting methane into methanol with high selectivity (∼97%) over a copper-containing zeolite, based on partial oxidation with water. The activation in helium at 673 kelvin (K), followed by consecutive catalyst exposures to 7 bars of methane and then water at 473 K, consistently produced 0.204 mole of CH₃OH per mole of copper in zeolite. Isotopic labeling confirmed water as the source of oxygen to regenerate the zeolite active centers and renders methanol desorption energetically favorable. On the basis of in situ x-ray absorption spectroscopy, infrared spectroscopy, and density functional theory calculations, we propose a mechanism involving methane oxidation at Cu^II oxide active centers, followed by Cu^I reoxidation by water with concurrent formation of hydrogen.

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Periana argues that the stepwise reaction of methane with water is thermodynamically unfavorable and therefore impractical. We reply by presenting an in-depth thermodynamic analysis of each step in the process and show that the surface concentrations of the reactants and products as well as the stabilizing effect of additional water molecules, as discussed in the original paper, fully support the feasibility of the proposed reaction.

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MBEA zeolites are known to catalyze carbon-carbon coupling reactions such as acetaldehyde condensation, which is an important step in a range of industrially relevant processes, e.g., a sustainable butadiene synthesis. The widely accepted mechanism of the reaction includes a separate enolization step via an α-proton transfer to zeolite. By combining the H-D exchange activity studies, FTIR spectroscopy, and DFT calculations, we show that such a mechanism is indeed feasible for the SnBEA zeolite. For the ZrBEA and TiBEA zeolites, on the other hand, experimental evidence suggests that a separate enolization step is unlikely. We propose the possibility of an alternative concerted single-step mechanism that involves coadsorption of two aldehyde molecules at the open M(IV) Lewis acid site and a subsequent proton transfer between the adsorbates in a collective transition state stabilized by the M-OH group of the open site. The study suggests that the nature of a zeolite dopant can thus be used to control the activity of the catalyst by modifying the reaction mechanism.@en

Fourier transform infrared spectroscopy and density functional theory calculations have been used to elucidate the nature of active sites of ZrBEA zeolite responsible for the catalytic synthesis of butadiene. We show that the content of open Zr(IV) Lewis acid sites, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three -O-Si linkages and one OH group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene. The higher catalytic activity of the open sites is attributed to their higher acid strength and steric accessibility. The study suggests that the control of such open sites plays a crucial role for the further design of the optimal multifunctional zeolite-based catalysts. (Figure Presented).

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