Advanced X‐ray Absorption Spectroscopy Analysis to Determine Structure‐Activity Relationships for Cu‐Zeolites in the Direct Conversion of Methane to Methanol

Abstract

An industrial process for direct conversion of methane to methanol (DMTM) would revolutionize methane as feedstock for the chemical industry and it would be a cherished contribution to climate change mitigation. At the present stage, it is a rather remote perspective, but the search for the materials that would make it possible and economically viable, is very active. Cu‐exchanged zeolites have been shown to cleave the C−H bond of methane at low temperatures (≤200 °C), and has been extensively studied for the stepwise DMTM conversion over the last decades. The determination of the speciation of CuxOy‐species in zeolites and the understanding of their role in the reaction mechanism has been heavily debated. Lately, advanced X‐ray absorption spectroscopy (XAS) analysis has been standing out as a powerful technique for investigating the behavior of Cu in zeolites. In this review we focus on the in situ and operando studies of changes in the electronic and structural properties of Cu, during the different steps of the DMTM conversion uncovered by XAS, which have led to new and insightful information about the complex behavior of Cu‐zeolites in the DMTM process.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

“…No signals due to any Cu III species were detected [64] . The small shoulder at 8983 eV is due to the 1s→4p transitions in Cu I species, [19, 30, 33, 61–63] formed via autoreduction, as was also observed in FTIR experiments. Upon the reaction with methane, the intensity of this signal significantly increases due to the increasing fraction of Cu I .…”

“…Activation in oxygen and reaction with methane induce the change of copper oxidation state and local environment, which was analyzed by X‐ray absorption spectroscopy (Figure 1 B). In the spectra of oxygen‐activated samples, two main signals at 8977 and 8986 eV are present, corresponding to dipole forbidden 1s→3d and 1s→4p transitions in tetra‐coordinated Cu II species, respectively [19, 30, 33, 61–63] . These peaks appear in the copper‐exchanged zeolites after the high‐temperature activation, indicating the transformation of hydrated octahedral Cu(H 2 O) 6− x (OH) x (2− x )+ to the extra‐framework copper species bond to the ion‐exchange positions.…”

“…Apart from the nature of support for copper species, the silicon to aluminum ratio of the parent zeolite, copper loading, reaction temperature and methane pressure are the parameters that have significant effect on the final methanol productivity [17, 18, 32, 33, 39–47] . These empirical observations were rationalized in terms of a different nature of copper species, present on different copper‐containing materials [29–31] …”

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

“…Over the past decade, significant progress in the design and engineering of novel copper‐containing materials for the direct conversion of methane to methanol was achieved [28–31] . It was reported that copper hosted in zeolites of different topology (MFI, [16, 17] MOR, [18, 19] CHA, [24, 32] FAU, [33] FER, [34] ERI, [35] and MAZ [36] ), and supported on alumina or silica [37, 38] can activate methane.…”

Element-doped graphitic carbon nitride: confirmation of doped elements and applications

“…In general, XAS measurement exhibits the following advantages: 176 temporal resolutions for an in-situ experiment. Hence, XAS has been extensively utilized in the study of material science in recent years, 182,183 especially in the research of some element-doped materials, such as Cu-exchanged zeolites, 184 P-doped hematite, 185 Ta-doped α-Fe 2 O 3 nanorods, 186 and P-B co-doped amorphous porous NiFe-based material. 187 Therefore, the future looks bright for the use…”