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Elementary steps of radical-surface interactions in oxidative coupling of methane
561
catalysisToduy#13(1992)561-564 ElaevierSciencePublishersB.V.,Am&edam
ELEMENTARY STEPS OF RADICAL-SURFACE
INTERACTIONS IN OXIDATIVE
COUPLING OF METHANE M.Yu.Sinev Institute of Chemical gina 4. Moscow 117334.
Physics, USSR.
USSR
Academy
of
Sciences,
Kosy-
Abstract. Three types of elementary radical-surface interactions CRSI3, CH-atom transfer, O-atom transfer. radical are postulated Method of the RSI rate constants evaluation basing on capture). Polyani-Semenov correlation is suggested.
The comparison of the free radicals formation data in oxidative coupling of methane COCIQ with general kinetic features of this process in presence of a series of oxide catalysts led to the creation of a heterogeneous-homogeneous kinetic model of to this model the formation of an initial OCM Cll. According - takes place in a gas phase by the recomOCM product - ethane with the participation bination of methyl radicals; processes of methylperoxide radicals lead to the homogeneous formation of total oxidation products. The mentioned model describes qualitatively the dependence of G-selectivity on temperature at low conversions. However, all the attempts to model the complete process using the conventional homogeneous schemes of methane oxidation supplemented by were unof heterogeneous methyl radicals formation the steps Obviously,one of the main reasons of this failures successful. is a neglect of the radical-surface interactions CRSI) which the products selectivity and the kinetics of can influence on stoichiometover-all process C21. Unfortunately,information ry and kinetics of RSI as a rule is absent. The most of experireactions investigation are not methods for radical mental adapted for RSI studies. The present paper is devoted to the analysis of the elementaPossible ways of ry steps which may take place in OCM process. and transformation of different free radicals the formation at methane oxidation conditiCCHa, CHaOz, CHaO. HOz, OH etc.1 ons are analysed. Three forms of surface active site Coxidized COIS, reduced and dehydroxilated C Is> and three types hydroxilated [OH18 of RSI are considered: - hydrogen atom transfer CH-type>
rols
+ RH -->
rOHl*
+
17
or
1992-E1sevierSciencePublishersB.V.
tOHI=
+ R'
-->
COls + R'H
562 R. R'
- CHs.
- oxygen C Is
+
atom
R*O
R* - CHa,
C 3s + R'O
103~
OH,
CHaO,
etc.
CO-type>
+
R*
OH
capture -->
CHaOa,
transfer
-->
CHsO.
- radical
R*
Oz. HOa.
or
CR'011
adsorption or
CA-type> #
COls
+ R
step
may
-->
IOR%
- H, CHs The
rate
of RSI
elementary
be offered
as
W = P*Z*u*S*N*prexpC-Eo/RTl*PR where
Cl1
P - steric
coefficient, gas-surface impact frequency Cper active site cross-section, catalyst specific surface area, active sites number Cper 1 m*>, - catalyst density, Lactivation energy, PR- radical partial pressure; z0 SN-
or in terms
of bi-molecular
gas
1 m* at 1 at.&,
molecule/surface
site
reaction
W = k*T"'= *expC-E~~/RTI*CL*CR
ce>
k = P*Z*+R*T"'"
c33
rate constant Cml/mole+s.>, k - bi-molecular gas constant, Ractive sites and radicals volume CL and CR - surface concentrations. The evaluation of P and Eo values was carried out in a propocorresponding homogeneous gas sition of analogy of RSI and correlation phase reactions. As a rule, Polyani-Semenov where
Eo = Eo
+
aCAH>
C4>
for elemenis valid for the evaluation of activation energies tary gas phase reactions. The analysis of experimental data C3. 41 shows that eqation C43 can be used with EO = 20 kJ/mole. 01 = 1 and 0.15 for endo- and exothermic H-type reactions respectia = 0 for exothervely. Equation C4> with EO = 10 kJ/mole and maI O-type reactions also can be used. The data on kinetics and thermochemistry of OCM model catacorrelysts interactions with methane C5.61 demonstrated that lation C41 for homogeneous reactions of H-atom abstraction from CHr molecule can be applied for heterogeneous process Csee Fig.
563 Table
1. Thermochemical catalysts
characteristics
Energy,
Li/MgO
of
some
Pb/AlaOa
OCM
model
K/A1203
__--__------~~--~--~--_---_--_~~-~~-_--_~___-___-__-_____
E o H, E
COI'
Table
2.
kJ/mole
320
kJ/mole
Evaluated reactions
250
535
270
407
kinetic parameters on Li/MaO catalvst
450
of
RSI
elementary
Reaction
k E. Cmljmole*s.> kJ/mole --__-___-___________~~~~~~-~--~~~~~--~~-~~-_----~-_--_~ H-type: LO+CH4=LOH+CHB LOH+O2=LO+HO2 LOH+H=LO+Hi! LOH+HC2=LO+H202 LOH+OH=LO+H20 LO+H202=LOH+HCZ? LO+H==LOH+O2 LOH+CH3=LO+CH4 LOH+CH3C2=LO+CH3OOH LOH+CHBO=LO+CHBOH LO+CH30=LOH+CH20 LO+CH2OH=LOH+CHBO LOH+CH2OH=LO+CHBOH LO+CHO=LOH+CO LO+CH20=LOH*CHO LO+CHBOH=LOH+CHBOH LO+CHSOH=LOH+CH30 -_---------O-type: L+CHBOE=LO+CHBO L+CHBO=LO+CHB L+HO2=LO+OH --_-----_--_ A-type: LO+CH3=LCCH3 L+CHBO=LCCHB L+CHBO=LOCHB LO+H=LOH L+OH=LOH L+L+O2=Lo+Lo
1.22Ell 8.67ElO 4.QEll 8.4E8 1.2Ell 8.4ElO 8.4EQ 1.26EQ ?.OSEB 8.868 1.20EQ 8.8EQ 8.8E8 Q.OE7 Q.OE8 8.7E8 8.7E8 _ _ __-_--_ 7.05E8 8.8E8 8.4E8 _ _ ____--1.25EQ 8.8E8 Q.OE8 4.9Ell 1.2Ell 8.7ElO
--~~----__--_--__-__--_~-__--__-__--_~~__--_~~--~---~-_~ * - calculated from experimantally measured catalyst reduction with methane
90.0" 114.5 14.5 0 0 44.5 0 14.5 14.5 0 21.0 12.5 21.0 0 12.5 42.0 89.5 -_____ 10 10 10 -_-_-_ 0 0 0 0 0 0 rates
of
554
Fig.1). Therefore, the values of Ea for RSI elementary steps were calculated using C4> and thermochemical data for model catalysts CLi/MgO, Pb/AlaOs, K/Al2091 reduction and re-oxidation CS-71 Csee Table 13. For A-type reactions Ea=O was proposed. The values of P in Cl> were evaluated by the comparison of the pre-exponential factors of analogous gas reactions basing on P = 1 for Hz, CH4, H and 0. The evaluated constants for some RSI elementary reactions are collected in Table 2. The RSI elementary steps and gas elementary reactions of methane oxidation were included into heterogeneouso H, kJ/mols homogeneous kinetic model of OCM. The calculations based on this model allowed to excorrelation Figl. plain high values of activa-- _- Polyani-Semenov X + CH4 --> XH + CH2 tion energies of steady-stafor reactions CX - F, OH, H, Cl, te catalytic process; the 0, SH, CHaO, HOz, Br. CH202, features of catalysts reA, B and C - Li/MgO, oxidation without previous J. 02; K/A1202 and Pb/A120s> dehydroxilation; the influence of hydrogen peroxide on catalytic methane oxidation. Evaluated values of rate constants allow to choose the conditions for successful experimental studies of RSI. REFERENCES l.M.Yu.Sir~ev.V.N.Korchak.O.V.Krylov. Kinet.Katal.,28ClQ87>1376. 2.Y.Tong and J.H.Lunsford. J.Am.Chem.Soc., 113 ClQQl> 4741. 3.V.N.Kondrat'ev. Rate Constants of Gas Phase Reactions, Nau1970. ka. Moscow, 15ClQ86>1086. 4.W.Tsang and R.F.Hampson, J.Phys.Chem.Ref.Data. O.V.KryS.V.Yu.Bychkov, M.Yu.Sinev, V.N.Korchak. E.L.Aptekar. lov. Kinet.Katal.. 3OClQ89>1137. O.V.Kry6.M.Yu.Sinev, V.Yu.Bychkov, V.N.Korchak. E.L.Aptekar. 3OClQ89>1421. 10-v. Kinet.Katal., Catal. 7.M.Yu.Sinev. V.Yu.Bychkov, V.N.Korchak and O.V.Krylov, Today. 6 ClQQOI 543.
catalysisToduy#13(1992)561-564 ElaevierSciencePublishersB.V.,Am&edam
ELEMENTARY STEPS OF RADICAL-SURFACE
INTERACTIONS IN OXIDATIVE
COUPLING OF METHANE M.Yu.Sinev Institute of Chemical gina 4. Moscow 117334.
Physics, USSR.
USSR
Academy
of
Sciences,
Kosy-
Abstract. Three types of elementary radical-surface interactions CRSI3, CH-atom transfer, O-atom transfer. radical are postulated Method of the RSI rate constants evaluation basing on capture). Polyani-Semenov correlation is suggested.
The comparison of the free radicals formation data in oxidative coupling of methane COCIQ with general kinetic features of this process in presence of a series of oxide catalysts led to the creation of a heterogeneous-homogeneous kinetic model of to this model the formation of an initial OCM Cll. According - takes place in a gas phase by the recomOCM product - ethane with the participation bination of methyl radicals; processes of methylperoxide radicals lead to the homogeneous formation of total oxidation products. The mentioned model describes qualitatively the dependence of G-selectivity on temperature at low conversions. However, all the attempts to model the complete process using the conventional homogeneous schemes of methane oxidation supplemented by were unof heterogeneous methyl radicals formation the steps Obviously,one of the main reasons of this failures successful. is a neglect of the radical-surface interactions CRSI) which the products selectivity and the kinetics of can influence on stoichiometover-all process C21. Unfortunately,information ry and kinetics of RSI as a rule is absent. The most of experireactions investigation are not methods for radical mental adapted for RSI studies. The present paper is devoted to the analysis of the elementaPossible ways of ry steps which may take place in OCM process. and transformation of different free radicals the formation at methane oxidation conditiCCHa, CHaOz, CHaO. HOz, OH etc.1 ons are analysed. Three forms of surface active site Coxidized COIS, reduced and dehydroxilated C Is> and three types hydroxilated [OH18 of RSI are considered: - hydrogen atom transfer CH-type>
rols
+ RH -->
rOHl*
+
17
or
1992-E1sevierSciencePublishersB.V.
tOHI=
+ R'
-->
COls + R'H
562 R. R'
- CHs.
- oxygen C Is
+
atom
R*O
R* - CHa,
C 3s + R'O
103~
OH,
CHaO,
etc.
CO-type>
+
R*
OH
capture -->
CHaOa,
transfer
-->
CHsO.
- radical
R*
Oz. HOa.
or
CR'011
adsorption or
CA-type> #
COls
+ R
step
may
-->
IOR%
- H, CHs The
rate
of RSI
elementary
be offered
as
W = P*Z*u*S*N*prexpC-Eo/RTl*PR where
Cl1
P - steric
coefficient, gas-surface impact frequency Cper active site cross-section, catalyst specific surface area, active sites number Cper 1 m*>, - catalyst density, Lactivation energy, PR- radical partial pressure; z0 SN-
or in terms
of bi-molecular
gas
1 m* at 1 at.&,
molecule/surface
site
reaction
W = k*T"'= *expC-E~~/RTI*CL*CR
ce>
k = P*Z*+R*T"'"
c33
rate constant Cml/mole+s.>, k - bi-molecular gas constant, Ractive sites and radicals volume CL and CR - surface concentrations. The evaluation of P and Eo values was carried out in a propocorresponding homogeneous gas sition of analogy of RSI and correlation phase reactions. As a rule, Polyani-Semenov where
Eo = Eo
+
aCAH>
C4>
for elemenis valid for the evaluation of activation energies tary gas phase reactions. The analysis of experimental data C3. 41 shows that eqation C43 can be used with EO = 20 kJ/mole. 01 = 1 and 0.15 for endo- and exothermic H-type reactions respectia = 0 for exothervely. Equation C4> with EO = 10 kJ/mole and maI O-type reactions also can be used. The data on kinetics and thermochemistry of OCM model catacorrelysts interactions with methane C5.61 demonstrated that lation C41 for homogeneous reactions of H-atom abstraction from CHr molecule can be applied for heterogeneous process Csee Fig.
563 Table
1. Thermochemical catalysts
characteristics
Energy,
Li/MgO
of
some
Pb/AlaOa
OCM
model
K/A1203
__--__------~~--~--~--_---_--_~~-~~-_--_~___-___-__-_____
E o H, E
COI'
Table
2.
kJ/mole
320
kJ/mole
Evaluated reactions
250
535
270
407
kinetic parameters on Li/MaO catalvst
450
of
RSI
elementary
Reaction
k E. Cmljmole*s.> kJ/mole --__-___-___________~~~~~~-~--~~~~~--~~-~~-_----~-_--_~ H-type: LO+CH4=LOH+CHB LOH+O2=LO+HO2 LOH+H=LO+Hi! LOH+HC2=LO+H202 LOH+OH=LO+H20 LO+H202=LOH+HCZ? LO+H==LOH+O2 LOH+CH3=LO+CH4 LOH+CH3C2=LO+CH3OOH LOH+CHBO=LO+CHBOH LO+CH30=LOH+CH20 LO+CH2OH=LOH+CHBO LOH+CH2OH=LO+CHBOH LO+CHO=LOH+CO LO+CH20=LOH*CHO LO+CHBOH=LOH+CHBOH LO+CHSOH=LOH+CH30 -_---------O-type: L+CHBOE=LO+CHBO L+CHBO=LO+CHB L+HO2=LO+OH --_-----_--_ A-type: LO+CH3=LCCH3 L+CHBO=LCCHB L+CHBO=LOCHB LO+H=LOH L+OH=LOH L+L+O2=Lo+Lo
1.22Ell 8.67ElO 4.QEll 8.4E8 1.2Ell 8.4ElO 8.4EQ 1.26EQ ?.OSEB 8.868 1.20EQ 8.8EQ 8.8E8 Q.OE7 Q.OE8 8.7E8 8.7E8 _ _ __-_--_ 7.05E8 8.8E8 8.4E8 _ _ ____--1.25EQ 8.8E8 Q.OE8 4.9Ell 1.2Ell 8.7ElO
--~~----__--_--__-__--_~-__--__-__--_~~__--_~~--~---~-_~ * - calculated from experimantally measured catalyst reduction with methane
90.0" 114.5 14.5 0 0 44.5 0 14.5 14.5 0 21.0 12.5 21.0 0 12.5 42.0 89.5 -_____ 10 10 10 -_-_-_ 0 0 0 0 0 0 rates
of
554
Fig.1). Therefore, the values of Ea for RSI elementary steps were calculated using C4> and thermochemical data for model catalysts CLi/MgO, Pb/AlaOs, K/Al2091 reduction and re-oxidation CS-71 Csee Table 13. For A-type reactions Ea=O was proposed. The values of P in Cl> were evaluated by the comparison of the pre-exponential factors of analogous gas reactions basing on P = 1 for Hz, CH4, H and 0. The evaluated constants for some RSI elementary reactions are collected in Table 2. The RSI elementary steps and gas elementary reactions of methane oxidation were included into heterogeneouso H, kJ/mols homogeneous kinetic model of OCM. The calculations based on this model allowed to excorrelation Figl. plain high values of activa-- _- Polyani-Semenov X + CH4 --> XH + CH2 tion energies of steady-stafor reactions CX - F, OH, H, Cl, te catalytic process; the 0, SH, CHaO, HOz, Br. CH202, features of catalysts reA, B and C - Li/MgO, oxidation without previous J. 02; K/A1202 and Pb/A120s> dehydroxilation; the influence of hydrogen peroxide on catalytic methane oxidation. Evaluated values of rate constants allow to choose the conditions for successful experimental studies of RSI. REFERENCES l.M.Yu.Sir~ev.V.N.Korchak.O.V.Krylov. Kinet.Katal.,28ClQ87>1376. 2.Y.Tong and J.H.Lunsford. J.Am.Chem.Soc., 113 ClQQl> 4741. 3.V.N.Kondrat'ev. Rate Constants of Gas Phase Reactions, Nau1970. ka. Moscow, 15ClQ86>1086. 4.W.Tsang and R.F.Hampson, J.Phys.Chem.Ref.Data. O.V.KryS.V.Yu.Bychkov, M.Yu.Sinev, V.N.Korchak. E.L.Aptekar. lov. Kinet.Katal.. 3OClQ89>1137. O.V.Kry6.M.Yu.Sinev, V.Yu.Bychkov, V.N.Korchak. E.L.Aptekar. 3OClQ89>1421. 10-v. Kinet.Katal., Catal. 7.M.Yu.Sinev. V.Yu.Bychkov, V.N.Korchak and O.V.Krylov, Today. 6 ClQQOI 543.