Sorption charateristics of methane among various rank coals: impact of moisture

Enhanced coalbed methane (ECBM) in deep coal seams is being actively investigated around the world. Since the in situ coal seams are always saturated with water, methane sorption behavior on coal in the presence of water can help accurately assess the amount of recoverable methane. Thus, methane sorption isotherms have been measured on a high-rank anthracite, a low-volatile bituminous, a middle-volatile bituminous and a high-volatile bituminous coal with the manometric technology at 30 °C under six different moisture contents. The Dubinin–Astakhov (D–A) equation was used to fit the experimental sorption isotherm data. In all cases, the moisturized coals exhibited lower sorption capacity than the corresponding dry materials and moisture has a significant effect on CH₄ sorption capacity. The maximum sorption capacity, V ₀, displays a linear decline with the moisture content for the Changcun and Malan samples, but it is nonlinear for the other two coal samples. The net heat of CH₄ sorption, βE, is also reduced by the presence of water, but varies only slightly between a relatively small span of about 8.8 and 10.0 kJ mol^?1 for the dry samples studied, despite the difference in coal rank. In addition, the maximum sorption capacity of CH₄ in dry coals presents the typical “U-shape” trend with coal rank. Moisture has a greater impact on the sorption capacity in low-rank coals than that in high-rank coals. The mechanisms responsible for the effect of moisture on CH₄ sorption among various rank coals are also presented. The pore-blocking effect is the main influencing factor for high-rank anthracite, whereas, the competition sorption is dominant for low-rank coals.     

Study on gas adsorption and desorption characteristics on water injection coal

To study the desorption mechanism of methane in coal by H₂O injection and establish the Wiser molecular structure model of bituminous coal, the Grand Canonical Monte Carlo method was used to study the desorption behavior of CH₄ in coal with different amounts of H₂O injection at molecular scale. The results showed that at 293 K, the maximum adsorption capacity of H₂O was about 16 mmol/g, and that of CH₄ was about 8 mmol/g, which was about twice that of CH₄. This indicates that H₂O has a stronger adsorption capacity than CH₄. For methane-bearing coal, when the amount of water injected is 100, the average relative concentration of CH₄ is 0.5446, and the average relative concentration of CH₄ decreases by 33.77% compared to the water content of 20. Under the same time conditions, the root mean square displacement and diffusion coefficient of CH₄ decrease with the increase of H₂O injection quantity. With the increase of H₂O injection, the motion velocity of CH₄ in vacuum layer decreased. When water was injected, methane was trapped in the coal by water. The more H₂O injected, the more methane trapped in the coal, and the less methane desorption. This research lays a theoretical foundation for further research involving coal-water interaction.     

Theoretical study of model elementary reactions of dissociative addition of light hydrocarbons to the Ti-doped aluminide cluster Al<Subscript>12</Subscript>Ti

The potential energy surfaces (PES), energies E, and activation barriers h of elementary reactions of dissociative addition of CH₄ and C₂H₆ molecules to the Al₁₂Ti cluster with a marquee structure in the singlet and triplet states were calculated within the B3LYP approximation of the density functional theory using the 6-31G* basis set. The first stage of the reaction Al₁₂Ti + CH₄ leads to the adsorption complex CH₄ · Al₁₂Ti with the R(TiC) distance of ～2.4 Å. The methane molecule is coordinated as a tridentate ligand the singlet state and as a bidentate ligand in the triplet state, although both coordination modes are close in energy. In the transition state, the CH₄ molecule is coordinated through its active C-H bond to an inclined Ti-Al edge of the cluster, and the C-H bond is significantly elongated and weakened. The activation barrier height h referenced to the CH₄ complex is ～9 and ～19 kcal/mol for the singlet and triplet, respectively, and that referenced to the primary products Al₁₂Ti(CH₃)(H) is ～21 kcal/mol. The barrier to migration of the CH₃ group around the metal cluster is estimated at ～10 kcal/mol. At the initial stage of the reaction Al₁₂Ti + C₂H₆, two types of C₂H₆ · Al₁₂Ti adsorption complexes are formed. In one of them, the ethane molecule is coordinated through a methyl group (as the methane molecule); and in the other type, the coordination is through the C-C bond. This reaction can proceed through two paths by means of insertion into C-H or C-C bonds to give Al₁₂Ti(C₂H₅)(H) or Al₁₂Ti(CH₃)₂, respectively. The second path is impeded by a high barrier (～30 kcal/mol) and is possible, if at all, only at high temperatures. Conversely, the insertion into a C-H bond in ethane is somewhat more favorable than in methane. Analogously, the PES of addition of the second methane molecule to Al₁₂Ti(CH₃)(H) was calculated. The second molecule is adsorbed and dissociates by the same mechanism as the first CH₄ molecule, but with somewhat lower barriers and energy effect of formation of Al₁₂Ti(CH₃)₂(H)₂. The addition of propane and longer hydrocarbons is briefly considered. The results are compared with the results of previous analogous calculations of the PES of related reactions of dissociative adsorption of dihydrogen on the Al₁₂Ti cluster, which are more exothermic, have lower barriers, and can occur under milder conditions.     

Selective Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate

A catalytic process is demonstrated for the selective conversion of methane into carbon monoxide via oxychlorination chemistry. The process involves addition of HCl to a CH₄–O₂ feed to facilitate C?H bond activation under mild conditions, leading to the formation of chloromethanes, CH₃Cl and CH₂Cl₂. The latter are oxidized in situ over the same catalyst, yielding CO and recycling HCl. A material exhibiting chlorine evolution by HCl oxidation, high activity to oxidize chloromethanes into CO, and no ability to oxidize CO, is therefore essential to accomplish this target. Following these design criteria, vanadyl pyrophosphate (VPO) was identified as an outstanding catalyst, exhibiting a CO yield up to approximately 35 % at 96 % selectivity and stable behavior. These findings constitute a basis for the development of a process enabling the on‐site valorization of stranded natural‐gas reserves using CO as a highly versatile platform molecule.     

Back Cover: Illustrating the Fate of Methyl Radical in Photocatalytic Methane Oxidation over Ag−ZnO by in situ Synchrotron Radiation Photoionization Mass Spectrometry (Angew. Chem. Int. Ed. 32/2023)

Photocatalysis has emerged as an ideal method for the direct activation and conversion of methane under mild conditions. In this reaction, methyl radical (⋅CH₃) was deemed a key intermediate that affected the yields and selectivity of the products. However, direct observation of ⋅CH₃ and other intermediates is still challenging. Here, a rectangular photocatalytic reactor coupled with in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) was developed to detect reactive intermediates within several hundred microseconds during photocatalytic methane oxidation over Ag−ZnO. Gas phase ⋅CH₃ generated by photogenerated holes (O⁻) was directly observed, and its formation was demonstrated to be significantly enhanced by coadsorbed oxygen molecules. Methoxy radical (CH₃O⋅) and formaldehyde (HCHO) were confirmed to be key C1 intermediates in photocatalytic methane overoxidation to CO₂. The gas-phase self-coupling reaction of ⋅CH₃ contributes to the formation of ethane, which indicates the key role of ⋅CH₃ desorption in the highly selective synthesis of ethane. Based on the observed intermediates, the reaction network initiated from ⋅CH₃ of photocatalytic methane oxidation could be clearly illustrated, which is helpful for studying the photocatalytic methane conversion processes.     

Light‐Driven C−H Oxygenation of Methane into Methanol and Formic Acid by Molecular Oxygen Using a Perfluorinated Solvent

The chlorine dioxide radical (ClO₂^.) was found to act as an efficient oxidizing agent in the aerobic oxygenation of methane to methanol and formic acid under photoirradiation. Photochemical oxygenation of methane occurred in a two‐phase system comprising perfluorohexane and water under ambient conditions (298 K, 1 atm). The yields of methanol and formic acid were 14 and 85 %, respectively, with a methane conversion of 99 % without formation of the further oxygenated products such as CO₂ and CO. Ethane was also photochemically converted into ethanol (19 %) and acetic acid (80 %). The methane oxygenation is initiated by the photochemical Cl?O bond cleavage of ClO₂^. to generate Cl^. and O₂. The produced Cl^. reacts with CH₄ to form a methyl radical (CH₃^.). Finally, the oxygenated products such as methanol and formic acid were given by the radical chain reaction. A fluorous solvent plays an important role of inhibiting the deactivation of reactive radical species such as Cl^. and CH₃^..     

Theoretical study on the gas‐phase reaction mechanism between nickel monoxide and methane for syngas production

The comprehensive mechanism survey on the gas‐phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6‐311++G(3df, 3pd)//B3LYP/6‐311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH₃ is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH₃NiOH and HNiOCH₃, locate on the deep well, while CH₃NiOH is more energetically favorable than HNiOCH₃ on both the triplet and singlet states. The main products shall be syngas once HNiOCH₃ is created on the singlet state, whereas the main products shall be methyl radical if CH₃NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate‐determining step (RDS) is the reaction step for the singlet intermediate HNiOCH₃ formation involving an oxidative addition of NiO molecule into the C? H bond of methane, with an energy barrier of 120.3 kJ mol^?1. The syngas formation would be more effective under higher temperature and photolysis reaction condition. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Molecular Simulation of the Adsorption Characteristics of Methane in Pores of Coal with Different Metamorphic Degrees

In order to study differences in the methane adsorption characteristics of coal pores of different metamorphic degrees, 4 nm pore structure models based on three typical coal structure models with different metamorphic degrees were constructed. Based on the molecular mechanics and dynamics theory, the adsorption characteristics of methane in different coal rank pores were simulated by the grand canonical Monte Carlo (GCMC) and molecular dynamics methods. The isothermal adsorption curve, Van der Waals energy, concentration distribution, and diffusion coefficient of methane under different conditions were analyzed and calculated. The results showed that at the same pore size, the adsorption capacity of CH₄ is positively correlated with pressure and metamorphic degree of coal, and the adsorption capacity of CH₄ in high metamorphic coal is more affected by temperature. The relative concentration of CH₄ in high-order coal pores is low, and the relative concentration at higher temperature and pressure conditions is high. The CH₄ diffusion coefficient in high-rank coal is low, corresponding to the strong Van der Waals interaction between CH₄ and coal. The research results are of great significance for further exploration of the interaction mechanism between CH₄ and coal with different metamorphic degrees and can provide theoretical support for the selection of gas extraction parameters.     

A comment on the dependence of the C-H proton isotropic hyperfine constants on carbon hybridization in planar radicalS

Properties of the McConnell Q_CH parameter relating proton isotropic hyperfine coupling constants to spin densities in planar nng radicals are examined. Q_CH is a sensitive function of the hybridization of the C—H bond attached to radical carbocycles C_nH_n (n = 3,5,6,7,8). The hybridization effect is most pronounced in the highly strained cyclopropenyl radical. Q_CH spans a narrow range in most of the common π-radicals, including moderately strained fused systems like azulene, biphenylene, acenaphthylene and acepleiadylene anions.     

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

Reaction of triisobutylaluminum with SBA15₇₀₀ at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [(SiO)₂Al–CH₂CH(CH₃)₂] 1a, silicon isobutyl [Si–CH₂CH(CH₃)₂] 1b and a silicon hydride [Si–H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (¹H, ¹³C, ²⁹Si, ²⁷Al and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [SiO–Al–[CH₂CH(CH₃)₂]₂], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, Si–O–Si but with two different mechanisms, showing that the reality of “single site” catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O–Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a β-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al–H bond and a O–Si bond. Thermal treatment of 1a (at 250 °C) under high vacuum (10^–5 mbar) generates Al–H through a β-H elimination of isobutyl fragment. These supported well-defined Al–H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and ²⁷Al–¹H J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system.     

Cleavage of benzyltrimethylstannanes by aqueous methanolic perchloric acid

The rates of cleavage of some XC₆H₄CH₂SnMe₃ bonds by aqueous-methanolic perchloric acid have been measured spectrophotometrically, and the rate of cleavage of the MeSn bonds of PhCH₂SnMe₃ and Me₄Sn by monitoring the methane evolution. The results indicate that for X = H, p-Me, o-Me, p-Bu^t, o-, m- and p-F and -Cl, and o-Br, the cleavage of the CH₂SnMe₃ bond involves attack of the acid at the benzylic atom, and is not much faster than that of the SnMe bonds, but that a mechanism involving ring-protonation is important for X = m-Me, and greatly predominant for X = m-OMe.     

Propylene Radical Copolymerization

Unlike ethylene, which is quite active in radical poly- and co-polymerization, the next member of the olefinic row–propylene (Pr)–is much less efficient in such reactions. Thermodynami-cally, the polymerization of Pr is permitted as is evidenced by reactions with Ziegler-Natta catalysts (The free energy change δG for the conversion of liquid Pr into amorphous poly-Pr at 25°C is -12.2 kcal/mol [1].), but the strong chain transfer activity of the Pr-monomer prevents formation of high molecular weight polymers via radical mechanism. Pr easily gives up H″ atoms with the formation of inactive and probably resonance stabilized allyl radicals CH₂[sbnd]CH[sbnd]CH₂ .(The high resonance stabilization of the Pr-radi-cal 'CH₂ CH[dbnd]CH₂, formed after separation of the H″ atom, should not be confused with a very low resonance stabilization of the Pr-radical 'CH₂ [sbnd]CH[sbnd]CH₃, formed after opening of the double bond. The Q–e scheme deals with the second type of radical and Pr is characterized by a low resonance factor, Q_pr 0.002.) As a result, Pr radical homopolymerization gives low-molecular weight polymer (even at a pressure of 15,000 atm the molecular weight of poly-Pr is only ～3000 [2, 3]).     

Thermal Conversion of Methane to Formaldehyde Promoted by Gold in AuNbO3+ Cluster Cations

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO₃⁺ heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass‐selected to react with CH₄ in an ion‐trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au?O bond into an Au?Nb bond during the reaction.     

MNDO/PM3 Study of the Reaction Mechanism between Methane and Complexes Generated in Br2·AlBr3 System

Potential energy surface (PES) of systems AlBr₅ and AlBr₅ + CH₄ ⁺ were investigated by MNDO/PM3 method. All the five donor-acceptor complexes Br₂·AlBr₃ with no barrier add to methane providing multiple adducts with various localization of interactions and with different conformations. However further transformations occur only with adducts of two complexes Br₂·AlBr₃ possessing considerable ionic character. On the reaction path resulting in CH₃Br and HBr as intermediates function bromonium type complexes CH₃BrH⁺·AlBr₄ ^- and the intermediates on the path leading to CH2Br2 and H₂ are the complexes with 3c-2e bond of H2 quasi-molecule with the C atom of bromomethyl cation H₂C(H-H)Br⁺·AlBr₄ ^-. Potential barriers on both reaction paths are about 30 kcal mol-1, and the transition states (TS) are analogous to the classical 3c-2e TS (Olah scheme) with an electrophile attack on a CH bond and to the recently suggested TS with an electrophile attack on an unshared electron pair of the carbon atom in the nonclassical methane H2C(H-H) respectively.     

H2气氛下褐煤快速热解过程中CH4逸出规律的分析

在带有输送煤样的管式反应器上进行了霍林河褐煤加压快速氢解实验,分析了H₂对煤/半焦的化学键断裂和对CH₄生成规律的影响。在加压快速氢解条件下,CH₄产率随着热解温度升高、压力的增大而增大;在50% H₂气氛下,操作压力为1.0 MPa、温度为900 ℃时,CH₄产率为8.08%,达到最大,较N₂气氛下的提高了72.5%。H₂或H·自由基诱发了芳环的开裂、侧链、脂肪链和醚键的断裂,促进了煤热解。CH₄产率的增加主要是由于外部供H的结果;热解温度低于700 ℃时,H₂对煤结构中活性基团的作用促进了煤热解,导致了CH₄产率的增加;而热解温度高于700 ℃后,煤/半焦加氢气化促进了CH₄产率的增加。     

Temperature dependences of J(C,H) and J(C,D) in 13CH4 and some of its deuterated isotopomers

The nuclear spin—spin coupling constants J(C,H) and J(C,D) have been measured over the temperature range 200–370 K for the methane isotopomers ¹³CH₄, ¹³CH₃D, ¹³CHD₃ and ¹³CD₄. The coupling constants increase with increasing temperature for any one isotopomer and decrease with increasing secondary deuterium substitution at any one temperature. The results are entirely attributable to intramolecular effects and the data have been fitted by a weighted least-squares regression analysis to a spin—spin coupling surface thereby yielding a value for ¹J_e(C,H), the coupling constant at equilibrium geometry, and values for the bond length derivatives of the coupling. We find that ¹J_e(C,H) = 120.78 (±0.05) Hz which is about 4.5 Hz smaller than the observed value in ¹³CH₄ gas at room temperature. Results are also reported for J(H,D) in ¹³CH₃D and ¹³CHD₃ for which no temperature dependence was detected.     

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.     

O–H···C hydrogen bond in the methane–water complex

Quantum chemical calculations were performed at different levels of theory (SCF, DFT, MP2, and CCSD(T)) to determine the geometry and electronic structure of the HOH···CH₄ complex formed by water and methane molecules, in which water is a proton donor and methane carbon (sp³) is an acceptor. The charge distribution on the atoms of the complex was analyzed by the CHelpG method and Hirshfeld population analysis; both methods revealed the transfer of electron charge from methane to water. According to the natural bond orbital (NBO) analysis data, the charge transfer upon complexation is caused by the interaction between the σ orbital of the axial С–H bond of methane directed along the line of the O–H···C hydrogen bridge and the antibonding σ* orbital of the О–H bond of the water molecule. Topological analysis of electron density in the HOH···CH₄ complex by the AIM method showed that the parameters of the critical point of the bond between hydrogen and acceptor (carbon atom) for the O–H···C interaction are typical for Н-bonded systems (the magnitude of electron density at the critical point of the bond, the sign and value of the Laplacian). It was concluded that the intermolecular interaction in the complex can be defined as an Н bond of O–H···σ(С–H) type, whose energy was found to be 0.9 kcal/mol in MP2/aug-cc-pVQZ calculations including the basis set superposition error (BSSE).     

A non-radical mechanism for methane hydroxylation at the diiron active site of soluble methane monooxygenase

We propose a non‐radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOH_Q intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(μ‐oxo)diiron(IV ) complex in which one of the iron atoms is coordinatively unsaturated (five‐coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOH_Q involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken‐symmetry singlet state by a non‐radical mechanism; in the first step a methane C? H bond is dissociated via a four‐centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three‐centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane–methanol conversion by the bare FeO⁺ complex and relevant transition metal–oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.     

Role of sodium decoration on the methane storage properties of BC3 nanosheet

First-principles calculations including dispersion correction are carried out to investigate pristine and Na-decorated graphene-like BC₃ (h-BC₃) for their application as methane storage materials. Structural optimization shows that the methane is physisorbed on the pristine sheet via van der Waals forces with adsorption energy of ?2.7 kcal/mol. It was found that unlike the pristine graphene, sodium decorated sheet can effectively interact with the CH₄ molecule, so that each metal atom bound on sheet may adsorb up to four CH₄. Furthermore, no bond dissociation was observed for the adsorption of CH₄ on Na-decorated h-BC₃, which means that decorated sheet can act as a storage device for methane safety storage. The results indicate that decoration of the Na atom on surface of sheet induces significant changes in electronic properties of the sheet and its E _g is unchanged after adsorption of CH₄ molecules. Theoretical methane storage capacity of Na-decorated BC₃ nanosheet could approach 18.1 wt%.