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.

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.

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.

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.

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.