Hermann Gies

Upgrading methane to value-added chemicals is significant but still challenging. Well-designed catalysts are required to activate methane. Extensive efforts have been dedicated to the catalytic conversion of methane over transition-metal-containing catalysts. A simple aluminosilicate FER-type zeolite has been found to activate methane and nitrous oxide to methanol stably. To further diversify the catalytic properties, zeolites with different topologies were screened in the methane oxidation reaction. Here, we show that small-pore zeolites have unique advantages in the direct upgrading of methane to light olefins. Among them, the CHA-type zeolite (SSZ-13) exhibited an outstanding performance, with a total selectivity of 90.5% to light olefins and a CH₄ conversion of 5.3% at 350 °C, accompanied by excellent regeneration. Penta-coordinated Al species as one of the active centers was figured out for methane activation and further identified as a non-framework tetra-coordinated Al species by density functional theory (DFT) calculations. The acidity of the zeolite was confirmed to influence the tandem conversion of methanol to light olefins. Our study revealed a novel possibility that transition-metal-free aluminosilicate small-pore zeolites can serve as bifunctional catalysts directly and continuously upgrade methane to light olefins via methanol as the intermediate. This approach avoided energy consumption and cumbersome operations and achieved high-efficiency and low-energy production of light olefins.

Fe-containing zeolites are effective catalysts in converting the greenhouse gases CH₄ and N₂O into valuable chemicals. However, the activities of Fe-containing zeolites in methane conversion and N₂O decomposition are frequently conflated, and the activities of different Fe species are still controversial. Herein, Fe-containing aluminosilicate CHA zeolites with Fe species at different spatial distances affected by the arrangement of framework Al atoms were synthesized in a one-pot manner in the presence or absence of Na. The arrangement of framework Al atoms was identified by ²⁷Al and ²⁹Si MAS NMR spectra and thermogravimetry-differential thermal analysis (TG-DTA) curves. Ultraviolet (UV)-vis, X-ray absorption spectroscopy (XAS), and NO adsorption fourier transform infrared spectroscopy (FTIR) spectra were adopted to analyze Fe speciation. The higher proportion of Fe species in the 6 MR of Fe-CHA zeolites in the presence of Na was confirmed by the NO adsorption FTIR spectrum. The activities of proximal and distant Fe sites in reactions including direct oxidation of methane to methanol, methanol to hydrocarbon, and N₂O decomposition were compared at different temperatures to provide the corresponding active sites and reaction pathways. The distant, isolated Fe and isolated proton were more active in the direct oxidation of methane to methanol and tandem conversion of methanol to hydrocarbon reactions than the proximal, isolated Fe and paired protons, respectively. Additionally, proximal, isolated Fe sites afforded higher activity in N₂O decomposition. These findings guide the design of highly active catalysts in methane oxidation, methanol to hydrocarbon, and N₂O decomposition reactions, addressing energy and environmental concerns.

Direct oxidation of methane to methanol was reported to be highly dependent on the transition- or noble-metal-loading catalysts in the past decades. Here, we show that the transition-metal-free aluminosilicate ferrierite (FER) zeolite effectively catalyzed methane and N₂O to methanol for the first time. The distorted tetracoordinated Al in the framework and pentacoordinated Al on the extra framework formed during calcination, activation, and reaction processes were confirmed as the potential active centers. The possible reaction pathway similar to the Fe-containing zeolites was advocated based on the reaction results using different oxidants, N₂O adsorption FTIR spectra, and ²⁷Al MAS NMR spectra. The stable and efficient methanol production capacity of FER zeolite was ascribed to the two-dimensional straight channels and its distinctive Al distribution of FER zeolite (CP914C) from Zeolyst. The transition-metal-free FER zeolite performed better than the record in the literature and our recent results using transition-metal-containing catalysts in terms of selectivity and formation rate of methanol and stability. This work has great significance and prospects for utilizing CH₄ and N₂O as resources and will open new avenues for methane oxidation.

The spatial distribution of framework Al (Al_F) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special Al_F distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique Al_F distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA⁺ and tetrahedral [AlO₄]^- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA⁺. This effect leads to Al_F atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate Al_F distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

Unraveling the effect of spatially separated bifunctional sites on catalytic reactions is significant yet challenging. In this report, we investigate the role of spatial separation on the oxidation of methane in a series of Cu-exchanged aluminosilicate zeolites. Regulation of the bifunctional sites is done either through studying a physical mixture of Cu-exchanged zeolites and acidic zeolites or by systematically varying the Cu and acid density within a family of zeolite materials. We show that separated Cu and acid sites are beneficial for the formation of hydrocarbons while high-density Cu sites, which are closer together, facilitate the production of CO₂. By contrast, a balance of the spatial separation of Cu and acid sites shows more favorable formation of methanol. This work will further guide approaches to methane oxidation to methanol and open an avenue for promoting hydrocarbon synthesis using methanol as an intermediate.

The discovery of new zeolite framework types plays an important role in producing new porous materials for applications such as adsorption, catalysis, separation, etc.