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

Journal Article · Thu Jun 01 04:00:00 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]

---

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

Related Subjects

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