Methane elimination from n‐pentane

The quantum chemical calculations at the different levels of theory were performed with the target being to determine the vibration frequencies and to estimate the barriers to internal rotations of n‐pentane molecules. In connection with the observed losses of CH₃ and CH₄ from the n‐pentane in gas phase, the calculations at the B3LYP level of theory with the 6‐31G(d) basis set were used to study the ground‐state potential energy surface of the n‐pentane. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A Metal–Organic Framework Based Methane Nano‐trap for the Capture of Coal‐Mine Methane

As a major greenhouse gas, methane, which is directly vented from the coal‐mine to the atmosphere, has not yet drawn sufficient attention. To address this problem, we report a methane nano‐trap that features oppositely adjacent open metal sites and dense alkyl groups in a metal–organic framework (MOF). The alkyl MOF‐based methane nano‐trap exhibits a record‐high methane uptake and CH₄/N₂ selectivity at 298 K and 1 bar. The methane molecules trapped within the alkyl MOF were crystalographically identified by single‐crystal X‐ray diffraction experiments, which in combination with molecular simulation studies unveiled the methane adsorption mechanism within the MOF‐based nano‐trap. The IAST calculations and the breakthrough experiments revealed that the alkyl MOF‐based methane nano‐trap is a new benchmark for CH₄/N₂ separation, thereby providing a new perspective for capturing methane from coal‐mine methane to recover fuel and reduce greenhouse gas emissions.     

Fine Tuning of MOF‐505 Analogues To Reduce Low‐Pressure Methane Uptake and Enhance Methane Working Capacity

We present a crystal engineering strategy to fine tune the pore chemistry and CH₄‐storage performance of a family of isomorphic MOFs based upon PCN‐14. These MOFs exhibit similar pore size, pore surface, and surface area (around 3000 m² g⁻¹) and were prepared with the goal to enhance CH₄ working capacity. [Cu₂(L2)(H₂O)₂]_n (NJU‐Bai 41: NJU‐Bai for Nanjing University Bai's group), [Cu₂(L3)(H₂O)₂]_n (NJU‐Bai 42), and [Cu₂(L4)(DMF)₂]_n (NJU‐Bai 43) were prepared and we observed that the CH₄ volumetric working capacity and volumetric uptake values are influenced by subtle changes in structure and chemistry. In particular, the CH₄ working capacity of NJU‐Bai 43 reaches 198 cm³ (STP: 273.15 K, 1 atm) cm⁻³ at 298 K and 65 bar, which is amongst the highest reported for MOFs under these conditions and is much higher than the corresponding value for PCN‐14 (157 cm³ (STP) cm⁻³).     

Non‐Oxidative Methane Conversion Using Lead‐ and Iron‐Modified Albite Catalysts in Fixed‐Bed Reactor

Raw and modified albite catalysts, including Pb/Albite and Fe/Albite catalysts, have been investigated for methane conversion to C₂ hydrocarbons under non‐oxidative conditions. Introduction of Pb to albite improved the activity and selectivity to non‐coke products. Based on characterization, it was found that Pb entered into the alkali and alkaline‐earth metal sites of albite, while partial Fe doped in the tetrahedron sites and the other loaded on the surface of albite. At the reaction temperature of 1073 K, methane gas hourly space velocity (GHSV) of 2 L·gcat^–1·h^–1, catalyst dosage of 0.25 g (300 mesh), the methane conversion catalyzed by raw albite in the fixed‐bed micro reactor exhibited a methane conversion of 3.32%. Notably, introducing a Pb content of 3.4 wt% into albite greatly enhanced the conversion of methane up to 8.19%, and the selectivity of C₂ hydrocarbons reached 99% without any coke under the same reaction conditions. While Fe‐doping could weakly heighten the methane conversion to 3.97%, and coke was formed. Thus, a comparison of Pb/Albite and Fe/Albite catalysts demonstrates that the catalytic activity of albite is mainly decided by alkali and alkaline‐earth metal sites, and lead‐modification can effectively improve the catalytic activity of albite.     

Dual‐Function Cobalt–Nickel Nanoparticles Tailored for High‐Temperature Induction‐Heated Steam Methane Reforming

The tailored chemical synthesis of binary and ternary alloy nanoparticles with a uniform elemental composition is presented. Their dual use as magnetic susceptors for induction heating and catalytic agent for steam reforming of methane to produce hydrogen at temperatures near and above 800 °C is demonstrated. The heating and catalytic performance of two chemically synthesized samples of CoNi and Cu ? CoNi are compared and held against a traditional Ni‐based reforming catalyst. The structural, magnetic, and catalytic properties of the samples were characterized by X‐ray diffraction, elemental analysis, magnetometry, and reactivity measurements. For induction‐heated catalysts, the conversion rate of methane is limited by chemical reactivity, as opposed to the case of traditional externally heated reformers where heat transport limitations are the limiting factor. Catalyst production by the synthetic route allows controlled doping with miniscule concentrations of auxiliary metals.     

H2S‐Mediated Thermal and Photochemical Methane Activation

Sustainable, low‐temperature methods for natural gas activation are critical in addressing current and foreseeable energy and hydrocarbon feedstock needs. Large portions of natural gas resources are still too expensive to process due to their high content of hydrogen sulfide gas (H₂S) mixed with methane, deemed altogether as sub‐quality or “sour” gas. We propose a unique method of activation to form a mixture of sulfur‐containing hydrocarbon intermediates, CH₃SH and CH₃SCH₃, and an energy carrier such as H₂. For this purpose, we investigated the H₂S‐mediated methane activation to form a reactive CH₃SH species by means of direct photolysis of sub‐quality natural gas. Photoexcitation of hydrogen sulfide in the CH₄+H₂S complex resulted in a barrierless relaxation by a conical intersection to form a ground‐state CH₃SH+H₂ complex. The resulting CH₃SH could further be coupled over acidic catalysts to form higher hydrocarbons, and the resulting H₂ used as a fuel. This process is very different from conventional thermal or radical‐based processes and can be driven photolytically at low temperatures, with enhanced control over the conditions currently used in industrial oxidative natural gas activation. Finally, the proposed process is CO₂ neutral, as opposed to the current industrial steam methane reforming (SMR).     

Spin‐Selective Thermal Activation of Methane by Closed‐Shell [TaO3]+

Thermal reactions of the closed‐shell metal‐oxide cluster [TaO₃]⁺ with methane were investigated by using FTICR mass spectrometry complemented by high‐level quantum chemical calculations. While the generation of methanol and formaldehyde is somewhat expected, [TaO₃]⁺ remarkably also has the ability to abstract two hydrogen atoms from methane with the elimination of CH₂. Mechanistically, the generation of CH₂O and CH₃OH occurs on the singlet‐ground‐state surface, while for the liberation of ³CH₂, a two‐state reactivity scenario prevails.     

Catalytic and Mechanistic Insights of the Low‐Temperature Selective Oxidation of Methane over Cu‐Promoted Fe‐ZSM‐5

The partial oxidation of methane to methanol presents one of the most challenging targets in catalysis. Although this is the focus of much research, until recently, approaches had proceeded at low catalytic rates (<10 h^?1), not resulted in a closed catalytic cycle, or were unable to produce methanol with a reasonable selectivity. Recent research has demonstrated, however, that a system composed of an iron‐ and copper‐containing zeolite is able to catalytically convert methane to methanol with turnover frequencies (TOFs) of over 14 000 h^?1 by using H₂O₂ as terminal oxidant. However, the precise roles of the catalyst and the full mechanistic cycle remain unclear. We hereby report a systematic study of the kinetic parameters and mechanistic features of the process, and present a reaction network consisting of the activation of methane, the formation of an activated hydroperoxy species, and the by‐production of hydroxyl radicals. The catalytic system in question results in a low‐energy methane activation route, and allows selective C₁‐oxidation to proceed under intrinsically mild reaction conditions.     

Didehydroaspartate Modification in Methyl‐Coenzyme M Reductase Catalyzing Methane Formation

All methanogenic and methanotrophic archaea known to date contain methyl‐coenzyme M reductase (MCR) that catalyzes the reversible reduction of methyl‐coenzyme M to methane. This enzyme contains the nickel porphinoid F₄₃₀ as a prosthetic group and, highly conserved, a thioglycine and four methylated amino acid residues near the active site. We describe herein the presence of a novel post‐translationally modified amino acid, didehydroaspartate, adjacent to the thioglycine as revealed by mass spectrometry and high‐resolution X‐ray crystallography. Upon chemical reduction, the didehydroaspartate residue was converted into aspartate. Didehydroaspartate was found in MCR I and II from Methanothermobacter marburgensis and in MCR of phylogenetically distantly related Methanosarcina barkeri but not in MCR I and II of Methanothermobacter wolfeii, which indicates that didehydroaspartate is dispensable but might have a role in fine‐tuning the active site to increase the catalytic efficiency.     

Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces

Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li‐doped MgO that are much smaller (12±6 kJ mol^?1) than the barriers inferred from different experimental studies (80–160 kJ mol^?1). This raises further doubts that the Li⁺O^.? site is the active site as postulated by Lunsford. From temperature‐programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH₄ on both Li‐doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol–gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH₄ binds heterolytically on Mg²⁺O^2? sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O₂.     

Outstanding Methane Oxidation Performance of Palladium‐Embedded Ceria Catalysts Prepared by a One‐Step Dry Ball‐Milling Method

By carefully mixing Pd metal nanoparticles with CeO₂ polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd‐O‐Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO₂ particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO₂ and creates highly reactive and stable sites constituted by PdO_x embedded into CeO₂ surface layers. This specific arrangement outperforms conventional Pd/CeO₂ reference catalysts in methane oxidation by lowering light‐off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.     

Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM‐5‐Catalyzed Methane Dehydroaromatization

Non‐oxidative methane dehydroaromatization is a promising reaction to directly convert natural gas into aromatic hydrocarbons and hydrogen. Commercialization of this technology is hampered by rapid catalyst deactivation because of coking. A novel approach is presented involving selective oxidation of coke during methane dehydroaromatization at 700 °C. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas; a H₂/CO ratio close to two is the main side‐product of coke combustion. Using ¹³C isotope labeling of methane it is demonstrated that oxygen predominantly reacts with molybdenum carbide species. The resulting molybdenum oxides catalyze coke oxidation. Less than one‐fifth of the available oxygen reacts with gaseous methane. Combined with periodic regeneration at 550 °C, this strategy is a significant step forward, towards a process for converting methane into liquid hydrocarbons.     

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.