Kinetic analysis of the oxidative coupling of methane over Na sup + -doped MgO

Journal Article · Thu Jun 01 00:00:00 EDT 1989 · Journal of Catalysis; (USA)

DOI:https://doi.org/10.1016/0021-9517(89)90352-7· OSTI ID:6467684

Iwamatsu, Eiji; Aika, Ken-Ichi ^[1]

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1. Tokyo Institute of Technology (Japan)

The reaction rate of oxidative coupling of methane was studied kinetically using 0.0165 or 0.05 g of 15% Na{sup +}-MgO catalyst at 923, 973, and 1023 K in a flow reactor under a CH{sub 4} pressure of 1.36 to 13 kPa and an O{sub 2} pressure of 0.36 to 4.7 kPa. The reaction rate was well expressed by the following mechanism. The kinetic data were analyzed by the Rideal-redox-type rate equation assuming methyl radical and active surface oxygen as the steady-state intermediates: (1) O{sub 2} + site 2k1{yields} active oxygen, (2) CH{sub 4} + active oxygen k2{yields} CH{sub 3} + OH(a) + site, (3) CH{sub 3} + xO ke{yields} CO, CO{sub 2}, (4) 2 CH{sub 3} k4{yields} C{sub 2}H{sub 6}. The constants k{sub 1}, k{sub 2}, x, and k{sub 4}/k{sub 3}{sup 2} were obtained for every temperature. The activity is related to k{sub 1} and k{sub 2}, while the selectivity (C{sub 2}/C{sub 1}) is related to k{sub 4}/k{sub 3}{sup 2}. The activation energies are 18 and 36 kcal/mol for k{sub 1} and k{sub 2}, respectively. By considering the negative activation energy of k{sub 3} ({minus}7 kcal/mol), step 3 is inferred to contain the equilibrium reaction in which CH{sub 3} and O{sub 2} form methyl peroxide if the value of x is 2.0. The actual smaller value of x suggest that part of the methyl radical is oxidized by the surface oxygen. The specific surface area effect is also explained by this reaction mechanism if we assume that k{sub 3} occurs on the surface (methyl peroxide decomposition) and k{sub 4} occurs in the gas phase. The Langmuir-Hinshelwood mechanism can also be applied; however, it leaves several ambiguous points. These conclusions are only valid at high temperature (923 to 1023 K) and under the low conversion, whereas the consecutive oxidation of C{sub 2} compounds should be taken into account under the high conversion condition.

• OSTI ID: 6467684
• Journal Information: Journal of Catalysis; (USA), Vol. 117:2; ISSN 0021-9517
• Country of Publication: United States
• Language: English

Related Subjects

03 NATURAL GAS
MAGNESIUM OXIDES
CATALYTIC EFFECTS
METHANE
OXIDATION
SODIUM
ACTIVATION ENERGY
CATALYSIS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DOPED MATERIALS
ETHANE
HIGH TEMPERATURE
KINETIC EQUATIONS
METHYL RADICALS
OXYGEN
REACTION INTERMEDIATES
SURFACE AREA
SURFACE PROPERTIES
SYNTHESIS
ALKALI METALS
ALKALINE EARTH METAL COMPOUNDS
ALKANES
ALKYL RADICALS
CHALCOGENIDES
ELEMENTS
ENERGY
EQUATIONS
HYDROCARBONS
KINETICS
MAGNESIUM COMPOUNDS
MATERIALS
METALS
NONMETALS
ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
RADICALS
REACTION KINETICS
030300* - Natural Gas- Drilling
Production
& Processing