Non-oxidative Conversion Of Methane Research Articles

Photocatalytic non-oxidative conversion of methane.

The direct conversion of methane to hydrogen and high-value hydrocarbons under mild conditions is an ideal, carbon-neutral method for utilizing natural gas resources. Compared with traditional high-temperature thermal catalytic methods, using clean light energy to activate inert C-H bonds in methane can not only significantly reduce the reaction temperature and avoid catalyst deactivation, but also surpass the limitations of thermodynamic equilibrium and provide new reaction pathways. This paper provides a comprehensive review of developments in the field of photocatalytic non-oxidative conversion of methane (PNOCM), while also highlighting our contributions, particularly focusing on catalyst design, product selectivity, and the underlying photophysical and chemical mechanisms. The challenges and potential solutions are also evaluated. The goal of this feature article is to establish a foundational understanding and stimulate further research in this emerging area.

Mechanistic Insight into the Direct Nonoxidative Conversion of Methane to Ethylene over Structure-Sensitive RhO2/TiO2 Catalysts

Mechanistic Insight into the Direct Nonoxidative Conversion of Methane to Ethylene over Structure-Sensitive RhO₂/TiO₂ Catalysts

Methyl radical chemistry in non-oxidative methane activation over metal single sites

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Single-atom catalysts: In search of the holy grails in catalysis

This perspective assesses the recent progress using single-atom catalysts for five “holy grail” reactions, including partial methane oxidation to methanol, non-oxidative methane conversion, photocatalytic CO2 reduction, photocatalytic water splitting, and nitrogen activation. Using selected case studies, we discuss promising efforts in these domains and at the same time identify the remaining challenges in using SACs for selective and efficient chemical transformations across thermal, electrical, and light-driven catalysis. Looking forward, we highlight the significant potential of SAC research in areas like mechanistic studies, high-throughput screening, and novel catalytic system design.

Nonoxidative methane coupling in a micro‐DBD with enhanced secondary electron emission

AbstractThe conversion of methane into transportable and storable materials is crucial in the petrochemical sector. In this study, a specially constructed AC‐driven dielectric barrier discharge (DBD) that has charge injector pyramids on one of the electrodes and runs at ambient temperature and pressure was used to evaluate noncatalytic methane conversion. The obtained result was compared with the traditional flat electrode DBD. It was discovered that the product selectivity in direct nonoxidative methane conversion depended on the discharge conditions. Pyramid electrode plasma sources convert methane up to 50 more than flat electrode plasma due to the appearance of more microdischarges. Pyramid electrode plasma generally has a greater production efficiency than flat electrode plasma, while requiring more operational power. The turnkey solutions offered by the sustainable methane coupling method discussed here may be advantageous for the long‐term small‐scale ethylene exploration scenario.

Unveiling the complexity of non-oxidative coupling of methane: A simplified kinetics approach

This study presents the generation and optimization of a detailed mechanism for the non-oxidative coupling of methane (NOCM), consisting of 1,112 species and 106,877 gas-phase reactions using Reaction Mechanism Generator. A mechanism reduction, performed in the Ansys Workbench, focused on four target products and employed various reduction methods, yielding an optimal skeletal mechanism of 178 species and 9,695 gas-phase reactions. Methane conversion and species concentration were evaluated using Chemkin-pro software, accounting for six independent variables, with temperature and pressure exhibiting the most significant impact. We observed that the formation rate of light hydrocarbons is higher at the initial position of the reaction, whereas aromatic hydrocarbons form more readily as reactor length increases and temperatures rise. Therefore, tailored reactor size and reaction conditions could improve the yield of C2 products under NOCM conditions. Further, a smaller reactor size at high temperatures and low pressure may optimize the formation of light hydrocarbons. Through sensitivity analyses, we identified critical reactions for product formation and aromatic coke precursors, thereby providing insights into gas-phase non-oxidative methane conversion. Further optimization studies are needed to investigate parameter interactions, aiming for optimal conversion with higher yields of target species and minimal aromatic coke precursors. These findings can inform effective reactor design and optimization strategies.

Non-oxidative conversion of methane into various petrochemical grades over tunable tri-metallic Fe-W-Mo/HZSM-5 catalyst systems

Most mono-metallic catalysts applied in non-oxidative conversion of methane exhibit low catalyst activity and limited selectivity towards useful petrochemicals. In this study, a series of thermally stable and tunable 5.4 wt% metal/support Fe-W-Mo/HZSM-5 catalyst systems were synthesized, characterized, and applied in non-oxidative conversion of methane in a custom-made stainless-steel reactor at various process conditions. Analysis of products from the reactor was done using Shimadzu 2014 gas chromatograph. Varying the amount of Fe, W, and Mo on HZSM-5 greatly influenced catalyst activity in terms of methane conversion and product distribution. When the quantities of Fe and W were increased to 2.25 wt% each and the quantity of molybdenum reduced to 0.9 wt% in the overall 5.4 wt% metal/ HZSM-5 catalyst, the resultant catalyst system became most active in methane conversion (17.4%) at 800 °C. Reducing the quantity of Fe and W each to 1.35 wt% and increasing Mo to 2.7 wt% in the overall 5.4 wt% catalyst, the resultant catalyst system became less selective towards C2 hydrocarbons and coke, but highly selective towards xylene and benzene. Therefore, this study demonstrates that varying metal loading presents an opportunity to tune the 5.4 wt% binary Fe, W, and Mo on HZSM-5 to achieve desired methane conversion and product distribution.

Investigation of the Properties of Mo/ZSM-5 Catalysts Based on Zeolites with Microporous and Micro–Mesoporous Structures

The dehydroaromatization of methane (MDA) is of great interest as a promising process for processing natural and associated petroleum gases, the main component of which is methane. The rapid loss of catalyst activity because of coke formation hinders the introduction of the DHA methane process into the industry. Therefore, the aim of this research was to find ways to improve Mo/ZSM-5 catalysts for MDA. The paper presents the results of the synthesis of high-silica zeolites of the ZSM-5 type with microporous and micro–mesoporous structures, the preparation of Mo/ZSM-5 catalysts based on them, and the study of the physicochemical and catalytic properties of the obtained samples during the non-oxidative conversion of methane into aromatic hydrocarbons. Zeolite catalysts were investigated using IR spectroscopy, X-ray diffraction, TPD-NH3, SEM, HR-TEM, and N2 adsorption. It was found that the addition of carbon black in the stage of the synthesis of zeolite type ZSM-5 did not lead to structural changes, and the obtained samples had a crystallinity degree equal to 100%. The creation of the micro–mesoporous structure in Mo/ZSM-5 catalysts led to an increase in their activity and stability in the process of methane dehydroaromatization. The highest conversion of methane was observed on a 4.0%Mo/ZSM-5 catalyst prepared based on zeolite synthesized using 1.0% carbon black and was 13.0% after 20 min of reaction, while the benzene yield reached 7.0%. It was shown using HR-TEM that a more uniform distribution of the active metal component was observed in a zeolite catalyst with a micro–mesoporous structure than in a microporous zeolite.

Plasma catalytic non-oxidative methane conversion to hydrogen and value-added hydrocarbons on zeolite 13X

Non-thermal plasma has unfolded highly efficient, safe to operate novel routes for methane conversion to hydrogen. In this work, methane conversion is performed under atmospheric dielectric barrier discharge (DBD) plasma with and without 13X zeolite-based catalysts i.e., 13X, Ga/13X, Pd/13X, and Pd-Ga/13X. Experimental results indicate that the plasma catalytic process delivered almost twofold higher product yield than the plasma only route. The binary Pd-Ga catalyst possesses highest catalytic performance with about 40 % CH4 conversion at an input flowrate of 5 cm3min−1 and 2 W due to the formation of the Pd-Ga alloy, which acts as catalytic active centre for activating C–H bonds. Product yield can be tailored by the catalyst design where the bimetallic Pd-Ga/13X preferably favours the hydrocarbon formation while H2 is the dominant product obtained over the Pd/13X. The cleavage of C–H bonds of methane molecule over the plasma only route is mainly governed by energetic electrons in the gaseous phase the catalyst activity, and plasma-catalyst synergism play a significant role in the plasma catalysis process. The findings from this work provide significant insights into the methane activation for subsequent optimization of the methane conversion processes operated on floating production, storage, and offloading vessels (FPSOVs). Exploiting untapped offshore natural gas reserves, where conventional pipeline systems are less economical.

Activity Origin of the Nickel Cluster on TiC Support for Nonoxidative Methane Conversion.

Designing an active and selective catalyst for nonoxidative conversion of methane under mild conditions is critical for natural gas utilization as a chemical feedstock. Here, we demonstrate that the origin of the selective nonoxidative conversion of methane by the titanium carbide supported nickel cluster arises from the formation of a nickel carbide site under the reaction conditions, which could stabilize the CHx intermediate to facilitate the C-C coupling, but further coking is rather limited. The reaction mechanism reveals that the C2 products can be formed via a key -CHx-CH3 intermediate. In addition, we demonstrate that boration of the nickel cluster site can improve the methane conversion toward C2 products. That higher activity and selectivity from the moderate rise in d orbital energy levels can therefore be considered as a descriptor of the catalyst effectiveness. These findings provide an understanding of the dynamic behavior of the single nickel cluster toward methane conversion to C2 products and guidance for their future rational design.

Effect of silicon carbide-based iron catalyst on reactor optimization for non-oxidative direct conversion of methane

The conversion of methane to olefins, aromatics, and hydrogen (MTOAH) can be used to stably obtain hydrocarbons when the effect of the catalytic surface is optimized from the reaction engineering perspective. In this study, Fe/SiC catalysts were packed into a quartz tube reactor. The catalytic surfaces of SiC and the impregnated Fe species decreased the apparent activation energies (Ea) of methane consumption in the blank reactor between 965 and 1020 °C. Consequently, the hydrocarbon yield increased by 2.4 times at 1020 °C. Based on the model reactions of ethane, ethylene, and acetylene mixed with hydrogen in the range of 500–1020 °C, an excess amount of Fe in the reactor favored the C–C coupling reaction over the selective hydrogenation of acetylene; consequently, coke formation was favored over the hydrogenation reaction. The gas-phase reactions and catalyst properties were optimized to increase hydrocarbon yields while reducing coke selectivity. The 0.2Fe catalyst-packed reactor (0.26 wt% Fe) resulted in a hydrocarbon yield of 7.1% and a coke selectivity of <2% when the ratio of the void space of the post-catalyst zone to the catalyst space was adjusted to be ≥2. Based on these findings, the facile approach of decoupling the reaction zone between the catalyst surface and the gas-phase reaction can provide insights into catalytic reactor design, thereby facilitating the scale-up from the laboratory to the commercial scale.

On design of plasma jet reactor for non-oxidative methane conversion.

Non-oxidative methane coupling in an atmospheric pressure plasma jet reactor with an internal diameter of 3.0 mm has been studied. The jet reactor consisted of a quartz tube surrounded by a copper ring and a stainless-steel tube, which were separated by a variable distance. The stainless-steel tube (inner diameter: 1.0 mm, outer diameter: 1.5 mm) served as high voltage electrode and gas inlet. The jet characteristics with different tip angles of the high voltage electrode were investigated using voltage-current waveforms and gas analysis at a constant methane flow rate of 100 ml min-1. The effect of the length of the ground electrode and the gap between the electrodes on conversion, C2 selectivity and energy efficiency has been studied. The methane conversion was nearly tripled with a sharp angle of the electrode (15°), while the energy consumption was reduced from 150 to 55 kJ molC2H y -1. The carbon deposition was also reduced.

Direct conversion of methane to zeolite-templated carbons, light hydrocarbons, and hydrogen

Efficient direct valorization of methane to value-added chemicals and materials remains an unsolved challenge for modern chemistry and materials science. In this work, we explored direct non-oxidative conversion of methane to hydrogen, hydrocarbons, and valuable zeolite-templated carbon materials. First, using a set of spectroscopy and microscopy characterization tools, we investigated how the reaction conditions influence the process of carbon formation inside the typical zeolite-based catalyst Mo/ZSM-5. Then, we explored the effect of the zeolite topology on the growth of carbon materials from methane decomposition over Mo/MFI, Mo/MOR, Mo/BEA, and Mo/FAU templates. Finally, we applied the obtained insights to prepare high-quality zeolite-templated carbons directly from methane using Fe/BEA and Fe/FAU templates. Altogether, our results represent a carbon-economic method of methane conversion to COx-free hydrogen gas, a mixture of light aliphatic and aromatic hydrocarbons, and valuable carbon materials.

Methane decomposition by plasma-packed bed non-thermal plasma reactor

A dielectric barrier discharge reactor (DBD) was employed as a non-thermal plasma (NTP) source for non-oxidative conversion of methane at ambient condition. The reactor was later operated as packed-bed DBD and also served with catalyst coupling. In the bed, dielectric materials (glass beads, BaTiO3) as well as some catalytic support materials (TiO2, SiO2 and γ-Al2O3 beads) were introduced in order to find out the non-catalytic reaction mechanism. Typical results indicated that the non-catalytic mode could favorably activate the strong C-H bond to produce H2 and light hydrocarbons without applying any oxidant or thermal energy. Apart from the materials property (shape, size, surface area, dielectric constant) different plasma parameters has a strong influence on the reaction performance. The existence of highest methane conversion and products yield was experimentally observed form the catalyst coupled plasma mode which could be reasonably described by their dominant catalytic surface properties. Moreover, the present reactor model found to be able enough in limiting the solid coke formation on catalyst surface.

High-performance photocatalytic nonoxidative conversion of methane to ethane and hydrogen by heteroatoms-engineered TiO2

Nonoxidative coupling of methane (NOCM) is a highly important process to simultaneously produce multicarbons and hydrogen. Although oxide-based photocatalysis opens opportunities for NOCM at mild condition, it suffers from unsatisfying selectivity and durability, due to overoxidation of CH4 with lattice oxygen. Here, we propose a heteroatom engineering strategy for highly active, selective and durable photocatalytic NOCM. Demonstrated by commonly used TiO2 photocatalyst, construction of Pd–O4 in surface reduces contribution of O sites to valence band, overcoming the limitations. In contrast to state of the art, 94.3% selectivity is achieved for C2H6 production at 0.91 mmol g–1 h–1 along with stoichiometric H2 production, approaching the level of thermocatalysis at relatively mild condition. As a benchmark, apparent quantum efficiency reaches 3.05% at 350 nm. Further elemental doping can elevate durability over 24 h by stabilizing lattice oxygen. This work provides new insights for high-performance photocatalytic NOCM by atomic engineering.

Nonoxidative coupling of methane to olefins and aromatics over molten W-In bimetal catalyst

Direct nonoxidative conversion of methane to ethylene, aromatics and hydrogen has been recognized as an attractive utilization of natural gas, but qualified catalyst faces huge challenges. Most catalysts reported for this reaction are deactivated fast by carbon deposition. Herein, we demonstrated an active, selective and stable molten tungsten-indium bimetal catalyst entrapped in a silica gel, offering 11% methane conversion in single pass, with C2/C3-C5/aromatics hydrocarbon distributions of 36%/4%/54% at 950 °C. A little carbon was formed during reaction, but spontaneously floated onto tungsten-indium liquid surface and moved onto silica surface thereby leaving the tungsten-indium liquid surface clean. No deactivation was thus observed even after 120-hour testing. Theoretical calculations and experimental results reveal that methane is converted into ethane on indium clusters, followed by gas-phase thermal cracking to ethylene and tandem aromatization on tungsten clusters to aromatics. The high stability and controllable hydrocarbon distribution promise our catalyst great potential of practical applications.

甲烷无氧直接制备烯烃/芳烃研究进展（II）

Direct, nonoxidative conversion of methane towards olefins/aromatics is a hot topic in the background of “carbon peak, carbon neutrality”, owing to zero CO2 emissions, high carbon atom utilization efficiency and hydrogen production. In the present paper, the advances of methane dehydroaromatization (MDA) and direct nonoxidative conversion of methane to olefins, aromatics, and hydrogen (MTOAH) are reviewed, based on our research works and the publications from 2018 to 2021. The determination of active sites, reaction intermediates, reaction mechanism, catalyst modification and improvement were considered. Finally, the future prospect was given for the direct nonoxidative conversion of methane to olefins/aromatics.

Understanding the Impact of Hydrogen Activation by SrCe0.8Zr0.2O3-δ Perovskite Membrane Material on Direct Non-Oxidative Methane Conversion.

Direct non-oxidative methane conversion (DNMC) converts methane (CH4) in one step to olefin and aromatic hydrocarbons and hydrogen (H2) co-product. Membrane reactors comprising methane activation catalysts and H2-permeable membranes can enhance methane conversion by in situ H2 removal via Le Chatelier's principle. Rigorous description of H2 kinetic effects on both membrane and catalyst materials in the membrane reactor, however, has been rarely studied. In this work, we report the impact of hydrogen activation by hydrogen-permeable SrCe0.8Zr0.2O3−δ (SCZO) perovskite oxide material on DNMC over an iron/silica catalyst. The SCZO oxide has mixed ionic and electronic conductivity and is capable of H2 activation into protons and electrons for H2 permeation. In the fixed-bed reactor packed with a mixture of SCZO oxide and iron/silica catalyst, stable and high methane conversion and low coke selectivity in DNMC was achieved by co-feeding of H2 in methane stream. The characterizations show that SCZO activates H2 to favor “soft coke” formation on the catalyst. The SCZO could absorb H2 in situ to lower its local concentration to mitigate the reverse reaction of DNMC in the tested conditions. The co-existence of H2 co-feed, SCZO oxide, and DNMC catalyst in the present study mimics the conditions of DNMC in the H2-permeable SCZO membrane reactor. The findings in this work offer the mechanistic understanding of and guidance for the design of H2-permeable membrane reactors for DNMC and other alkane dehydrogenation reactions.

Resolving one of the holy grails of catalysis: Direct nonoxidative methane conversion to ethylene over plasma-assisted atomically dispersed Pt catalyst

One of the holy grails of catalysis - direct nonoxidative methane conversion to ethylene is resolved in this study using dielectric barrier discharge plasma (DBDP) assisted catalysis. Motivated by the formation of ethane and very low amount of ethylene in the DBDP over acid or base treated kaolin supported Ni and oxalate ligand functionalized Ni in CO2 reforming of methane wherein Ni catalysts that were calcined at 800 °C showing higher methane conversion (XCH4), ethylene to ethane (C2 = /C2–) ratio, and hydrogen yield (YH2). A novel CeO2 confined atomically dispersed Pt supported on oxalate ligand functionalized Cu modified zeolite Y catalyst (PtCe/CuX-ZY) was synthesized to exhibit strong-metal-support interaction, which rendered the d-band center far away from the Fermi level, densification, and localization of d electrons thus weakening the adsorption of C2H4 upon formation to prevent subsequent hydrogenation to ethane. The novel PtCe/CuX-ZY catalyst showed an impressively high average XCH4, YC2H4, and YH2 of 73.5%, 23.9%, and 33.8%, respectively with a high C2 = /C2– ratio of ca. 12.3 compared to an average of 0.032 observed over Ni-based catalysts. This finding is impressive and opened up a further investigation in both academia and the industry.

Non-oxidative methane conversion by Fe single site catalysts: quantifying temperature limitations imposed by gas-phase pyrolysis

A combined heterogeneous and homogeneous model of methane conversion reveals the temperature limitations of methane to olefins, aromatics, and hydrogen (MTOAH).