Understanding the Preparation and Reactivity of Mo/ZSM-5 Methane Dehydroaromatization Catalysts

Methane dehydroaromatization is a promising reaction for the direct conversion of methane to liquid hydrocarbons. The active sites and the mechanism of this reaction remain controversial. This work is focused on the operando X‐ray absorption near edge structure spectroscopy analysis of conventional Mo/ZSM‐5 catalysts during their whole lifetime. Complemented by other characterization techniques, we derived spectroscopic descriptors of molybdenum precursor decomposition and its exchange with zeolite Brønsted acid sites. We found that the reduction of Mo‐species proceeds in two steps and the active sites are of similar nature, regardless of the Mo content. Furthermore, the ZSM‐5 unit cell contracts at the beginning of the reaction, which coincides with benzene formation and it is likely related to the formation of hydrocarbon pool intermediates. Finally, although reductive regeneration of used catalysts via methanation is less effective as compared to combustion of coke, it does not affect the structure of the catalysts.

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4 catalytic active sites resulting in surface WO5 sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH4 than the poisoned surface WO5 sites or the original WO4 sites. However, the surface reaction intermediates formed from CH4 activation over the Mn2O5 surface sites are more stable than those formed over the Na2WO4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na2WO4 surface sites towards CH4 activation but can yield MnO x surface sites that activate CH4 faster than Na2WO4 surface sites, but unselectively.

Unraveling Hydrocarbon Pool Boosted Propane Aromatization on Gallium/ZSM-5 Zeolite by Solid-State Nuclear Magnetic Resonance Spectroscopy

Propane aromatization on metal-modified zeolites provides a promising route to produce valuable chemicals such as benzene, toluene and xylene via non-petroleum feedstocks. The mechanistic understanding of propane conversion to aromatics is still challenging due to the complexity of the aromatization process. Herein, by using solid-state NMR spectroscopy and GC-MS, it is shown that cyclopentenyl cations are formed as active intermediates during propane aromatization on Ga/ZSM-5 zeolite. Autocatalysis of propane to aromatics is identified in the induction period. The cyclopentenyl cations serve as key hydrocarbon pool species to co-catalyze propane conversion and promote aromatics formation, revealing a dominant hydrocarbon pool process in propane aromatization.

Dual Active Sites on Molybdenum/ZSM-5 Catalyst for Methane Dehydroaromatization: Insights from Solid-State NMR Spectroscopy

Methane dehydroaromatization (MDA) on Mo/ZSM-5 zeolite catalyst is promising for direct transformation of natural gas. Understanding the nature of active sites on Mo/ZSM-5 is a challenge for applications. Herein, using 1 H{95 Mo} double-resonance solid-state NMR spectroscopy, we identify proximate dual active sites on Mo/ZSM-5 catalyst by direct observation of internuclear spatial interaction between Brønsted acid site and Mo species in zeolite channels. The acidic proton-Mo spatial interaction is correlated with methane conversion and aromatics formation in the MDA process, an important factor in determining the catalyst activity and lifetime. The evolution of olefins and aromatics in Mo/ZSM-5 channels is monitored by detecting their host-guest interactions with both active Mo sites and Brønsted acid sites via 1 H{95 Mo} double-resonance and two-dimensional 1 H-1 H correlation NMR spectroscopy, revealing the intermediate role of olefins in hydrocarbon pool process during the MDA reaction.

Methane activation by ZSM-5-supported transition metal centers

This review focuses on recent fundamental insights about methane dehydroaromatization (MDA) to benzene over ZSM-5-supported transition metal oxide-based catalysts (MO_x/ZSM-5, where M = V, Cr, Mo, W, Re, Fe). Benzene is an important organic intermediate, used for the synthesis of chemicals like ethylbenzene, cumene, cyclohexane, nitrobenzene and alkylbenzene. Current production of benzene is primarily from crude oil processing, but due to the abundant availability of natural gas, there is much recent interest in developing direct processes to convert CH₄ to liquid chemicals. Among the various gas-to-liquid methods, the thermodynamically-limited Methane DehydroAromatization (MDA) to benzene under non-oxidative conditions appears very promising as it circumvents deep oxidation of CH₄ to CO₂ and does not require the use of a co-reactant. The findings from the MDA catalysis literature is critically analyzed with emphasis on in situ and operando spectroscopic characterization to understand the molecular level details regarding the catalytic sites before and during the MDA reaction. Specifically, this review discusses the anchoring sites of the supported MO_x species on the ZSM-5 support, molecular structures of the initial dispersed surface MO_x sites, nature of the active sites during MDA, reaction mechanisms, rate-determining step, kinetics and catalyst activity of the MDA reaction. Finally, suggestions are given regarding future experimental investigations to fill the information gaps currently found in the literature.

Insight into interactions among P, Zn and ZSM-5 during bi-component modification on ZSM-5

The formation of Zn₃(PO₄)₂ and Zn₂SiO₄ in Zn/P/ZSM-5 during the hydrothermal procedure deteriorates the stability of the framework and the aromatization capability.

Reversible Nature of Coke Formation on Mo/ZSM-5 Methane Dehydroaromatization Catalysts

Non-oxidative dehydroaromatization of methane over Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM-5 catalysts and a wide range of reaction temperatures and space velocities. High-pressure operando X-ray absorption spectroscopy demonstrated that the structure of the active Mo-phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass-spectrometry and 13 C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space.