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.

The direct conversion of methane to methanol using oxygen is a challenging but potentially rewarding pathway towards utilizing methane. By using a stepwise chemical looping approach, copper-exchanged zeolites can convert methane to methanol, but productivity is still too low for viable implementation. However, if the nature of the active site could be elucidated, that information could be used to design more effective catalysts. By employing anomalous X-ray powder diffraction with support from theory and other X-ray techniques, we have derived a quantitative and spatial description of the highly selective, active copper sites in zeolite omega (Cu-omega). This is the first comprehensive description of the structure of non-copper-oxo active species and will provide a pivotal model for future development for materials for methane to methanol conversion.

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.

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. ©

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.

Copper-containing zeolites exhibit high activity in the direct partial oxidation of methane into methanol at relatively low temperatures. Di- and tricopper species have been proposed as active catalytic sites, with recent experimental evidence also suggesting the possibility of the formation of larger copper oxide species. Using density functional theory based global geometry optimization, we were able to identify a general trend of the copper oxide cluster stability increasing with size. For instance, the identified ground-state structures of tetra- and pentamer copper clusters of Cu_nO_n²⁺ and Cu_nO_n-1²⁺ stoichiometries embedded in an 8-ring channel of mordenite exhibit higher relative stability compared to smaller clusters. Moreover, the aluminium content and localization in the zeolite pore influence the cluster's stability and its geometrical motif, which offers a perspective of tuning the properties of copper-exchanged zeolites by creating copper oxide clusters of a given structure and size. With the activity of the cluster towards methane being connected to its stability, such tuning will potentially allow the design of catalysts with engineered properties.