methane

[jiǎ wán]

The simplest organic matter

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This entry is made by College of Chemical Sciences, University of Chinese Academy of Sciences Participate in editing and review Science Popularization China · Science Encyclopedia authentication.

Methane is a nonpolar molecule with regular tetrahedron structure, which is the simplest organic compound 。 As the main component of conventional natural gas, shale gas and combustible ice, methane is a very important carbon based resource^[1] 。 It is the most important non CO two Greenhouse gas. In the stratosphere of the atmosphere, methane will be decomposed into water vapor (cloud), which will lead to the destruction of the ozone layer^[2] 。

Chinese name: methane
Foreign name: methane
Alias: Carbonane
chemical formula: CH₄
molecular weight: sixteen point zero four three
CAS login number: 74-82-8
EINECS login number: 200-812-7
Melting point: -182.5 ℃
Boiling point: -161.5 ℃

Water solubility: Difficult (normal temperature and pressure 0.03)
Density: 0.42 (- 164 ℃) (standard condition) 0.717g/L
Appearance: Colorless and odorless gas under normal temperature
Flash point: -188 ℃
Security description: S9；S16；S33
Hazard symbol: R12
UN dangerous goods number: one thousand nine hundred and seventy-one
Classification: Chain aliphatic hydrocarbon (GB/T 37885-2019)
Discovery: The first discovery of methane in the air in the 1940s^[2]

catalog

▪ Direct conversion to high-value chemicals
▪ Indirect transformation
eight exploitation

source

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In today's atmosphere, about 20% of methane comes from ancient times, which existed under coal seams, seabed, natural gas deposits and melted permafrost millions of years ago and has only been released today. The methane produced in modern times mainly comes from cattle, bogs, asphalt, paddy fields, newly reclaimed land, decayed garbage and termites. Regardless of the source, the organic matter is decomposed by bacteria under anoxic conditions. If there is oxygen, CO will be produced two^[2] 。

physical property

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Colorless, flammable, non-toxic gas, boiling point is - 161.49 ℃. The weight ratio of methane to air is 0.54, and the solubility is poor.

Methane explosion limit

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Under normal pressure, the LEL of methane is 5-6%, and the UEL is 15-16%; When the concentration of methane in the air reaches 9.5%, the strongest explosion will occur. Among them, the lower explosion limit has little change when the oxygen concentration decreases, while the upper explosion limit decreases significantly; When the oxygen concentration is lower than 12%, the mixed gas will lose its explosiveness^[3] 。

chemical property

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(1) It does not react with strong oxidants such as potassium permanganate, nor with strong acids and bases.

(2) Methane can substitute with chlorine^[4]

CH three Cl+Cl two CH two Cl two +HCl

CH two Cl two +Cl two CHCl three +HCl

CHCl three +Cl two CCl four +HCl

(3) Flammable and catalytic combustion^[5]

CH four +2O two CO two +2H two O

catalyzer Generally, Pd based catalysts, Pt based catalysts, Au based catalysts or multi-component noble metal catalysts such as Pd Pt, Pd Rh, Pd Au and Pd Pt Rh are used in noble metal catalysts.

Non noble metal catalysts include Metal oxide catalyst 、 Perovskite catalyst Spinel catalyst, hexaaluminate catalyst, etc.

Noble and non noble metal catalysts for methane catalytic combustion^[5]

preparation

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Laboratory methane production

CH three COONa CH three ·+·COONa

CH three ·+NaOH CH four +·ONa

·ONa+CH three COONa CH three ·+Na two CO three

CH three ·+CH three COONa CH three COCH three +·ONa

CH three ·+CH three · C two H six^[6]

Experimental device^[7]

Coal conversion to methane

Coal CO+H two

CO+H two O CO+H two

CO+H two CH four +CO two +H two O

Methane generation from coal pyrolysis

(1) Mechanism^[8]

Direct primary pyrolysis of coal sample and active H * Methane generation: Coal CH three +H * Coal+CH four

Hydrogenation of pyrolysis solid products: C (solid)+2H two CH four

The secondary pyrolysis of liquid products generated from the previous pyrolysis is also one of the ways to produce methane.

Methane formation mechanism

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Raw coal: divided into four stages.

The first stage belongs to the removal of adsorbed methane.

In the second stage, there are two reactions, namely, the shedding of methyl in methoxy group, the shedding of carbon dioxide and methane, or the shedding of methyl in alcohol functional group, and the generation of methane and water.

The third stage is the pyrolysis of toluene to generate methane and benzene, followed by the cleavage of methylene bridge bonds and the demethylation of hydrogenated aromatic rings.

The fourth stage is the result of dehydrogenation of aromatic system and the reaction of residual C.

application

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Direct conversion to high-value chemicals

(1) Oxidative coupling of methane to ethylene (OCM)^[9]

KELLER et al., 1982^[10] The oxidative coupling of methane to C was first reported two Hydrocarbon reaction process. In 1985, DRISCOLL et al^[11] Firstly, it is proposed that OCM follows the reaction mechanism of heterogeneous and homogeneous reactions. The C-H bond of methane is first catalysed to generate vapor phase methyl radical, and then coupled to ethane , ethane is further dehydrogenated to ethylene 。 LUO, etc^[12-13] The synchronous radiation vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technology was used to detect the gas phase methyl radical, ethyl radical, peroxymethyl radical, methyl hydrogen peroxide, ethyl hydrogen peroxide and other important gas phase intermediates in the oxidation of methane and ethane catalyzed by Li MgO for the first time, providing direct experimental evidence for the study of the mechanism of the oxidative dehydrogenation of OCM and ethane.

(2) Oxygen free aromatization of methane to aromatics (MDA)

1993, WANG et al^[14] This paper reports the reaction of Mo/ZSM-5 catalytic methane anaerobic aromatization to benzene and other aromatics, and proposes a dual functional mechanism, namely, the active Mo species and Br ø nsted acid sites act as methane activation and C two The active sites of hydrocarbon aromatization have been widely recognized. However, some researchers believe that the active Mo species have inherent aromatization characteristics, the molecular sieve framework provides an anchor point for the active Mo site, and the shape selection environment of the ten membered ring promotes the formation of aromatics^[15-16] 。 KOSINOV, etc. in 2018^[17] A breakthrough has been made in the research of, and a "hydrocarbon pool mechanism" has been proposed, that is, the primary product after methane activation reacts with the aromatic carbon deposit in the molecular sieve limit region to form aromatic hydrocarbons such as benzene. VOLLMER, etc^[18] A similar Mars van Krevelen mechanism (MvK) was proposed. The active Mo site contains carbon atoms, which participate in the formation of benzene ring in the reaction. Ç A Ğ LAYAN, etc^[19] And KOSINOV, etc^[20] It is believed that there are two independent C-H bond activation paths in the MDA process.

(3) Direct production of ethylene from methane without oxygen

GUO, etc^[21] HAADF-STEM, in situ XAS and other techniques were used to characterize the reaction ©SiO two The active site of FeSiC is a single atom iron coordinated by two carbon atoms and one silicon atom to form a single iron center (FeSiC two ）; XIE, etc^[22] Report monatomic Pt one @CeO two Catalytic MTOAH process. DRIFTS research results show that Pt after reaction one @CeO two There are adsorbed species such as ethylene and acetylene with π bond, which indicates that the single Pt site may have stable C two The ability of hydrocarbons to adsorb species. EGGART, etc^[23] Pt CeO was prepared by flame spray pyrolysis (FSP) two Single atom catalyst, operando XAS results show that Pt and Ce bond under reaction conditions, due to CeO two The coating modification of Pt or the formation of Pt Ce alloy. HAO, etc^[24] Using the hydrogen atom Rydberg identification time-of-flight spectroscopy cross molecular cyclization device, the hydrogen free radicals produced during the MTOAH process were detected for the first time in the experiment, and their formation rate increased with the increase of reaction temperature. It was also found that the hydrogen free radicals produced by the thermal cracking of hydrogen donor molecules such as 1,2,3,4-tetrahydronaphthalene (THN) and benzene can improve the methane conversion rate and the yield of olefins and aromatics, The activation temperature of ethylene formation was reduced, and the mechanism of methane activation promoted by gaseous hydrogen radicals (· H+CH four → H two + ·CH three ）^[25] 。

Indirect transformation

Syngas (mixed gas of carbon monoxide and hydrogen) is produced through reforming, and the syngas obtains high-value chemicals through methanol path, Fischer Tropsch synthesis or new OXZEO process.^[26]

(1) Methane steam reforming (SRM)

CH four +H two O = CO+3H two ……Δ Δ =206kJ·mol -1

(2) Catalytic partial oxidation of methane (POM)

2CH four +O two = 2CO+4H two ……Δ Δ =-44·kJ·mol -1

(3) Methane carbon dioxide reforming (CDM)

CH four +CO two = 2CO+2H two ……Δ Δ =+247 ·kJ·mol -1

(4) CO of methane two -O two Joint reorganization

CO two -O two The combined reforming CH reaction combines the exothermic process of catalytic partial oxidation of methane with the endothermic process of carbon dioxide reforming of methane to produce syngas.

exploitation

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Main development stages of coal mine gas development and utilization in China
time	Development and utilization stage	Representative gas drainage mode	Basic development
Late 1950s and 1970s	Gas drainage+prevention stage	Represented by Liaoning Fushun model, Shanxi Yangquan model and Chongqing Tianfu model, gas drainage technology for high permeability and high gas extra thick coal seams, pressure relief gas drainage technology for adjacent layers of cross seam drilling, and protective layer mining technology for high gas outburst coal seams have been formed respectively	For the purpose of preventing and controlling mine gas disasters, most of the extracted gas is directly discharged into the atmosphere
Late 1970s and early 1990s	Development stage of gas drainage and utilization	The technologies such as pressure relief gas drainage in protective layer and hydraulic enhanced gas drainage in low-permeability coal seam have been continuously promoted, and China's comprehensive gas drainage technology system has been initially established	With the continuous development of gas drainage technology, underground gas drainage and utilization projects have been promoted, and high concentration gas drainage has been used for industrial and civil purposes
1990s - early 21st century	Gas large-scale extraction and utilization stage	Taking the Huainan model in Anhui Province as a representative, a technical system of pressure relief gas drainage by the method of boring in the remaining roadway of complex coal seams is formed	With the further development of gas drainage technology and mode, gas drainage has gradually become large-scale, and the amount of gas drainage and utilization has increased rapidly
2006-2021	Gas resource extraction and utilization stage	Represented by Chongqing Songzao model and Shanxi Jincheng model, the "three zone matching three advanced enhanced permeability drainage" and "three zone linkage coalbed methane (gas) well up and down three-dimensional progressive drainage" models have been formed respectively	The technology and mode of gas drainage are gradually mature, the utilization of low concentration and ultra-low concentration coal mine gas has made progress, and the scope of gas utilization has continued to expand
After 2021	Synergistic stage of gas resource extraction and emission reduction utilization	Gradually build and form a technical system of gas drainage and emission reduction in the whole life cycle of coal development "pre mining, mining and post mining"	Emphasize the management and control of greenhouse gas emissions, and further develop the gas drainage technology in the direction of intelligence and precision; Encourage the comprehensive and efficient utilization of low concentration gas and low concentration gas, and finally form a stepped comprehensive utilization mode of full concentration gas

(Table data source^[27] ）

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