Understanding and Optimizing the Performance of Cu‐FER for The Direct CH 4 to CH 3 OH Conversion

Competition between Mononuclear and Binuclear Copper Sites across Different Zeolite Topologies

A key challenge for metal-exchanged zeolites is the determination of metal cation speciation and nuclearity under synthesis and reaction conditions. Copper-exchanged zeolites, which are widely used in automotive emissions control and potential catalysts for partial methane oxidation, have in particular evidenced a wide variety of Cu structures that are observed to change with exposure conditions, zeolite composition, and topology. Here, we develop predictive models for Cu cation speciation and nuclearity in CHA, MOR, BEA, AFX, and FER zeolite topologies using interatomic potentials, quantum chemical calculations, and Monte Carlo simulations to interrogate this vast configurational and compositional space. Model predictions are used to rationalize experimentally observed differences between Cu-zeolites in a wide-body of literature, including nuclearity populations, structural variations, and methanol per Cu yields. Our results show that both topological features and commonly observed Al-siting biases in MOR zeolites increase the population of binuclear Cu sites, explaining the small population of mononuclear Cu sites observed in these materials relative to other zeolites such as CHA and BEA. Finally, we used a machine learning classification model to determine the preference to form mononuclear or binuclear Cu sites at different Al configurations in 200 zeolites in the international zeolite database. Model results reveal several zeolite topologies at extreme ends of the mononuclear vs binuclear spectrum, highlighting synthetic options for realization of zeolites with strong Cu nuclearity preferences.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Cu and Zn Bimetallic Co-Modified H-MOR Catalyst for Direct Oxidation of Low-Concentration Methane to Methanol

The direct oxidation of low-concentration methane to value-added chemicals can not only reduce carbon emission but also provide an alternative production route for fossil fuels. Herein, we proposed a novel catalyst for the direct oxidation of low-concentration methane to methanol via the impregnation method, which selected copper and zinc as co-modifiers to modify the MOR catalyst. The highest methanol yield of 71.35 μmol·gcat-1·h-1 was obtained over a bimetallic Cu0.5Zn0.35-MOR catalyst. The catalyst retained good activity after three cycles of testing experiments, indicating good recyclability. Based on the results of performance tests and characterization studies, it was confirmed that Cu species bound to the zeolite framework were the main active sites for methane oxidation. The introduction of Zn decreased the generation of the octahedrally coordinated extra-framework aluminum, which promoted the dispersion of Cu within the zeolite framework. In other words, more tetrahedrally coordinated FAl-stabilized Cu species were presented in our CuZn-MOR catalyst system in comparison to the monometallic Cu-MOR catalyst. Benefiting from the aforementioned modification, the agglomerative sintering of the metal during the reaction was effectively prevented. This work may provide a feasible guide for the future optimization of Cu-based catalysts designed for the selective oxidation of methane.

Copper-zeolites Prepared by Solid-state Ion Exchange - Characterization and Evaluation for the Direct Conversion of Methane to Methanol

Direct conversion of methane to methanol (MTM) over Cu-zeolites is a so-called “dream reaction” for the chemical industry. There is still a lot that can be done in order to optimize the reaction by e.g. achieving a deeper understanding of the reaction mechanism and the nature of the Cu-sites. In this study, we investigated a solid-state ion exchange method to incorporate CuI ions into zeolites (MOR, BEA, ZSM-5 and FAU), as a more scalable technique. The solid-state ion exchange led to a Cu/Al ration of about 0.8, however with a heterogeneous distribution of Cu. Regardless, Fourier transform-infrared spectroscopy still revealed that most Brønsted acid sites were exchanged in all four samples. Further, CH4-temperature programmed reaction experiments showed that some Cu-sites formed were reactive towards CH4, with CuI-MOR and CuI-FAU having the largest CH4 consumption. Ultimately, the CuI-zeolites were tested in the MTM reaction and proved capable of producing methanol, even without the presence of Brønsted sites. A MOR with lower Cu/Al ratio (0.30) was also tested for comparison, and as this sample obtained a much higher productivity than the CuI-MOR with high Cu-loading (0.10 vs. 0.03 molMeOH/molCu), it was demonstrated that some fine-tuning is necessary to obtain the active Cu sites for methane activation.

Activity of Cu-Al-Oxo Extra-Framework Clusters for Selective Methane Oxidation on Cu-Exchanged Zeolites

Cu-zeolites are able to directly convert methane to methanol via a three-step process using O₂ as oxidant. Among the different zeolite topologies, Cu-exchanged mordenite (MOR) shows the highest methanol yields, attributed to a preferential formation of active Cu-oxo species in its 8-MR pores. The presence of extra-framework or partially detached Al species entrained in the micropores of MOR leads to the formation of nearly homotopic redox active Cu-Al-oxo nanoclusters with the ability to activate CH₄. Studies of the activity of these sites together with characterization by ²⁷Al NMR and IR spectroscopy leads to the conclusion that the active species are located in the 8-MR side pockets of MOR, and it consists of two Cu ions and one Al linked by O. This Cu-Al-oxo cluster shows an activity per Cu in methane oxidation significantly higher than of any previously reported active Cu-oxo species. In order to determine unambiguously the structure of the active Cu-Al-oxo cluster, we combine experimental XANES of Cu K- and L-edges, Cu K-edge HERFD-XANES, and Cu K-edge EXAFS with TDDFT and AIMD-assisted simulations. Our results provide evidence of a [Cu₂AlO₃]²⁺ cluster exchanged on MOR Al pairs that is able to oxidize up to two methane molecules per cluster at ambient pressure.

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

Copper-exchanged zeolites of different topologies possess high activity in the direct conversion of methane to methanol via the chemical looping approach. Despite a large number of studies, identification of the active sites, and especially their intrinsic kinetic characteristics remain incomplete and ambiguous. In the present work, we collate the kinetic behavior of different copper species with their spectroscopic identities and track the evolution of various copper motifs during the reaction. Using time-resolved UV/Vis and in situ EPR, XAS, and FTIR spectroscopies, two types of copper monomers were identified, one of which is active in the reaction with methane, in addition to a copper dimeric species with the mono-μ-oxo structure. Kinetic measurements showed that the reaction rate of the copper monomers is somewhat slower than that of the dicopper mono-μ-oxo species, while the activation energy is two times lower.

A potential catalyst for continuous methane partial oxidation to methanol using N2O: Cu-SSZ-39

Continuous catalytic methanol production from methane is reported on Cu-SSZ-39 using N₂O as an oxidant. Through optimization of CH₄, N₂O and H₂O partial pressures, a methanol formation rate of 499 μmol_CH3OH g^-1 h^-1 and a methanol selectivity of 34% is achieved.

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.

Single-Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside

Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "inside" thanks to the subatomic resolution of synchrotron X-ray absorption spectroscopy (XAS). The extended X-ray absorption fine structure and associated methods will be shown to be powerful tools in identifying the single-atom site and can provide details into the coordination environment and bonding disorder of SACs. The X-ray absorption near-edge structure will be demonstrated as a valuable method in probing the electronic properties of SACs by analyzing the white line intensity, absorption edge shift, and pre-/postedge features. Emphasis is also placed on in situ/operando XAS using state-of-the-art equipment, which can unveil the changes in structure and properties of SACs during the dynamic catalytic processes in a highly sensitive and time-resolved manner.

Active sites and mechanisms in the direct conversion of methane to methanol using Cu in zeolitic hosts: a critical examination

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.

Cu oxo nanoclusters for direct oxidation of methane to methanol: formation, structure and catalytic performance

Cu oxo nanoclusters hosted in microporous solids have emerged in the past decades as promising materials for catalyzing the selective conversion of methane to methanol.

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.