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Uncharged atomic oxygen in oxidative conversion of C1-C2 alkanes
495
catalysis To&~,13(1992) 495-501 ElsevierSciencePubliahersB.V., Amsterdam
UNCHARGED ATOMIC OXYGEN IN OXIDATIVE CONVERSION OF Cl-C2
ALKANES
A.G.Anshits
Institute of Chemistry of Natural Organic Materials,Siberian Branch of the USSR Academy of Sciences, 42, K.Marx Str., Krasnoyarsk,660049, USSR
Abstract The,role of uncharged atomic oxygen species in oxidative conversion of CH and C H has been considered.It is shown that active atomic oxygen species p%+icipate in C -C alkanes conversionin the presence of N 0 on zeolites, in the CH -N O'an8 CH -0 systems on transition metal silfcides and nitrides, as wef1 & hydroc&b&r oxidationduring N 0 and 0 photolysis by excimer laser light. Participationof uncharged oxyggn speci& in conversion of light alkanes makes it possible to explain not only hydrocarbon formation pathways, but those of oxygen-containingcompounds as well. INTRODUCTION The hetero-homogeneousmechanism of oxidative coupling of methane is at present universally accepted [1,21. The only points the authors disagree on are the role of different oxygen species in the reaction. The most popular is interpretationprovided by Lunsford 121, CH4 with
who showed that interactionof
Li/MgO sy
rects of structures such as O- or [M+O-I gave CH radicals on 3 :m. Recombinationof the two radicals on the surface or in the
gas phase
!ads to the formation of ethane as a major product. At the same
time C2H6
lrmationfrom methane is also ascribed to peroxide ion O2 2- [31.
l
The gi\ I review focuses on the contributionof uncharged atomic oxygen species ta .he conversionof light alkanes. ALKANE CONVERSION ON ZEOLITES
Unlike alkali-earthand rare-earthmetal oxides, zeolites and their catalytic role in
Cl-C2 alkane oxidative conversionhave not been given broad
consideration.Only a few articles are devoted to the problem. For example, it was reported that the zeolite activity in CH and C H oxidative conver4 26 sion in the presence of oxygen was mainly determined by the activation of hydrocarbon on acid sites [4,51. The process in the N20-containingsystems is characterized by considerable quantity of heavy hydrocarbons and no
0920~5361/92/$05.00 0 1992-ElaevierSciencePubliiheraB.V.Allrightareaervecl.
496
correlation between zeolite activity and the number of strong acid sites was found [5]. The absence of the correlation between the acidity and the activity of the zeolites in C2H6-N20 system becomes especiallyevident for samples with different content of sodium (Table 1). Table 1 Catalytic properties and acidity of zeolite ZSM-5 (SiO /Al 0 -41) with different Na content CT=660 K, reaction mixture (vol.%?:CiHi-36,6; NzO-3,6)
Catalyst
NaZSM-5 HZSM-5 Na,HZSM-5
Content Rate of ethane Selectivity,% Acidity, mmol/g of Na, conversion, Total High tempera% CO2 CH4 C2H4 C3H6 lO'*molec/g*s ture form I,7 O,l 1,5
091 9,7 13,o
-_0,7 1,3 91 0,3 4,O 90
_ 7 6
0,89 0,74 0,98
0,38 0,08
The results obtained indicate that a parent sample NaZSM-5, which is completely devoid of a capabilityto adsorb NH3 at high temperature,is characterized by low rate of C2H6 oxidation.Decationationof the parent sample results in the increase of both the activity and the number of strong acid sites. Reintroductionof 1,5 (wt.%) Na into HZSM-5 results in suppressing the strong acid sites whereas its activity is even higher than that of HZSM-5(41). High activity of ZSM-5 zeolite in C2H6-N20 system can be associated with the active oxygen species which were formed on the surface by N20 decomposition. The interactionof nitrous oxide at 620 K with the surface of zeolite was accompaniedby an evolutionof dinitrogen into the gas phase, while oxygen remained bound with the surface. The linear correlationwas found between the initial rate of ethane conversionand the amount of oxygen held by the surface for all examined zeolites of ZSM-5 type and mordenite. The fact that ethane interactswith surface oxygen to form stoichiometricquantities of ethylene and water gives strong evidence for the atomic character of active oxygen species [51. Equally interesting is the nature of N20 decompositionand oxygen stabilization sites. Zeolites are known to possess the sites capable of one-electron transfer. According to Slinkin 161 these sites stabilize oxygen in atomic and anion-radicalforms (0-I during N20 decompositionon H-mordenite. On the base of spectral and quantum-chemicaldata, Zholobenko [71 sug-
497
gested that at 423 K
N20
decompositioncould be attributed to high reacti-
vity of Lewis acid sites on
HZSM-5(401 zeolite. Decompositionresults in
dinitrogen evolution into the gas phase, with oxygen being chemisorbed on the zeolite. The number of Z-Ochem sites was equal to the value of 1O"g-', which is in good agreement with data 151. The sites decompose at temperatures higher than 623 K with oxygen evolution.Since molecular oxygen is not known to be chemisorbedon dehydroxylatedzeolite the authors believe that sites are formed by chemisorption of atomic oxygen and possess '-'them strong oxidative properties. Defect structures and Lewis acid sites may not be
the only contributors
to N20 decomposition. The sites formed at the stage of zeolite synthesis and containing transitionmetal ions may take part in the process as well. This assumption was carefully considered during the experiments to study the specific properties of surface oxygen species obtained on FeZSM-5 [8]. The major characteristicsof oxygen species, obtained on FeZSM-5 and supported vanadium, molybdenum and tungstenoxides are listed in Table 2. Table 2 Comparison of surface oxygen forms 181 Properties
(OI-FeZSM-5
+
Formation from O2 N2° Formation after: Reduction Oxidation Behavior under heating
0-
+
+
+ +
+
desorption
reoxidation
Reactivity at room temperature: interactionwith CH4,C0
+
+
heteroexchangeO2
+
+
homoexchangeO2
+
+
EPR signal
+
O2 +
+ reoxidation, desorption
+
+
According to the data presented, the reactivity of surface oxygen (0) on FeZSM-5 is similar to that of radical species O- found on supported oxide catalysts. Both these species interact with CH4, CO and participate in hetero- and homoexchangeof oxygen.
498 However,
active oxygen species bound with FeZSM-5 surface are different
from O- radical. (01 FeZSM-5 sites are stable in the presence of oxygen under heating, while O- radicals disappear on supported MO- and Vcontaining catalysts under this treatment. The increase of temperature up .. to 573-603 K results in desorptionof oxygen from FeZSM-5, whereas O- radicals disappear with reoxidationof the surface. SPECIFIC QUALITATIVEREACTIONS OF ATOMIC OXYGEN The data published suggest that N20 decompositionon ZSM-5 zeolites should be ascribed to active oxygen species, which could not be detected by means of physical techniques.Chemical reactions can be used to prove the involvementof these species in the process. It is known that minor amount of carben :CH is formed as a result of the reaction of methane with atomic 2 oxygen in triplet state 191 (Scheme 1). Scheme 1
CH4
+
OC3P1 W
The intermediatespecies :CH2 may react with ethylen to glve cyclopropane. Thus, the formation of cyclopropanein CH4-N20-C2H4system may indicate the existence of atomic oxygen. Catalytic reactions on the most active catalysts, such as HZSM-5(41) and l,S% Na,HZSM-S(411. in pulse experimentswith the use of the reaction mixture of CH4:N20:C2H4=9:6:24in helium showed that cyclopropane could be only found on 1,5X Na,HZSM-5(411sample. However, no cyclopropanewas detected on HZSM-St411 zeolite, which may be attributed to high rates of cyclopropane isomerizationon zeolite acid sites. Cyclopropanewas also formed (0,2X mol.1 under condition of oxidativedimerizationof methane on titanium silicide catalyst at 1073 K in CH4-air system [lo]. Participation of adsorbed atomic oxygen at the stage of CH: radical formation was suggested in 1111 on the basis of a study of catalyticpropertiesof transition metal borides, silicides and nitrides in oxidativecoupling of methane. Consequently, the participationof atomic oxygen in Cl-C2 alkane condensation, with either nitrous oxide or oxygen present, can be proved in terms of cyclopropane formation as one of the products of hydrocarbonconversion. Different reactivityof ion-radicaland uncharged oxygen species can also tell on their behavior in ethane conversion. According to the scheme of
Lunsford El21 oxidative conversion of ethane on Li/MgO involving reaction of O- species does not lead to methane as a product of the reaction. Unlike the authors mentioned, Aika [13] emphasizesthat C2H6 conversion results in formation of methane on Co/MgO surface, provided there are oxygen species with the properties of the gas phase atomic oxygen. According to the authors, CH4 is a product of thermal decompositionof carboxylates,the latter having been formed by oxidation of surface ethoxyde with atomic oxygen: Scheme 2 C2H50-
+20
CH3COO- +
w OH- w
CH3COO-
+
H20
CH 4
+
co;-
Hence, the difference between the ion-radicalsand atomic oxygen species lies in the pathways of CH4 generation.The low rate of methane formation may be attributed to anion-radicalnature of active oxygen species, whereas the high rate may be ascribed to uncharged oxygen species with the properties of the gas phase atomic oxygen. The above mentioned dissimilarities are expected to hold good for 6% Mo/ZSM-SC1481and 13% MO/T-A1203 catalysts as well, which are known to generate two different species of atomic oxygen [5,141. Table 3 presents relative rates of C2H6 oxidation on MO-containing catalysts. Table 3 Relative rates of product formation from C2H6 on MO-containingcatalysts =1,5 ml) (T=630 K; mixture: 36,4% C2H6, 3,6% 02(N20) in He. V pulse C2H6-N20
C2H6 Catalyst
Relative rates of formation CH4
6% Mo/ZSM-5(148) 13% MO/~-A1203
C2H6-02
0,l O,l
C2H4
CH4
CO2
C2H4
CH4
CO2
120
15 -
-
260 120
1,2 0.2
187 11
C2H4 78 180
The given results indicate that the low rate of CH4 formation is associated with alumina-molibdenacatalyst,which is known to activate both of the two oxidants in anion-radicalform. As far as zeolite catalysts are concerned, the relative rate of CH4 conversion in C2H6-N20 system is one or two order of magnitude greater than that in other cases. This may be taken as one more evidence of uncharged species of atomic oxygen being involved in ethane conversion on zeolite catalysts.
500 Participation of various surface oxygen species in the process of deep oxidation can also be proved by comparing activities of the two Mo-containing catalysts. No CO2 formation occures without an oxidizing agent (i.e. a pulse of pure C2H61, that indicatesthe role of stepwise mechanism in the process of total oxidation (Table 31. Acceleration of total oxidation in the presence of oxygen allows to suggest that CO2 formation on Mo-containing catalysts is caused by molecular oxygen species (e.g. 02-l' whereas dehydrogenationand condensationare associatedwith atomic forms.
ALKANE CONVERSION IN GAS PHASE Oxidation of CH4 and C2H6 with atomic oxygen during N20 photolysis by ArF excimer laser light at 300-600 K was studied by Japanese scientists. It was shown [151 that alkane activation proceeded through atomic oxygen O(lDl insertion into the C-H bond of hydrocarbon,giving rise to either [CH30Hl* or
[C2H50HI* species, which are convert to the products reflected in
Scheme 3. Scheme 3 N2°
c3!!*
hv CH4
+
1 O(ID) + I N2°
N2
+
O2
CH; CH30H*-
OH'+ CHj -
C2s +
O21 . CH 00 + 3
CH' T .3 C2H5 + CH4
. CHj CH30 -
CHF
CH OCH --3-3
OH' JI HCHO +
HCO'+ _' CO
CO -2
It is shown that with oxidizing agents other than oxygen in the gas phase, CH4 and C2H6 conversion results in C2-C4-hydrocarbonsand CO as their major products. Upon the increase of temperatureand pressure of the reaction mixture, the selectivityof C2-C4 hydrocarbonformation is also observed to noticeably grow, whereas the formation selectivityof different oxygen-containing (includingCO) products is reported to decrease. The above mentioned authors [15] also showed that atomic oxygen O(lD1, which was obtained by O3 photolysiswith KrF excimer laser light, underwent quick deactivation to form 0t3Pl radicals.These radicals are active in the
501 reaction on hydrogen abstraction to give hydroxyl and hydrocarbon radicals (Scheme 1) and are inert in the reaction of oxygen insertion. This fact also influences the compositionof the reaction products. Thus, the pathway analysis of light alkane conversionshows the uncharged species of atomic oxygen are likely to participatein the formation of partial oxidation products. To prove the fact that oxidation processes are associated with the forms mentioned above one should resort to specific qualitative reactions. ACKNOWLEDGEMENTS. The author is gratefully indebted to Dr. S.Vereshchaginfor experimental part of this work. REFERENCES
9 10 11 12 13 14 15
G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse,Chem. Sot. Rev., 18 (19891 251. D.J. Driscoll, W. Martir, J.-X. Wang and J.H. Lunsford, J. Amer. Chem. sot., 107 (19851 58. M.Yu. Sinev, V.N. Korchak and O.V. Krylov, Kinet. katal., 27, No 5 (19861 1274. S.S. Shepelev and K.G. Ione, Kinet. katal., 25, No 2 (1984) 347. S.N. Vereshchagin,L.I. Baikalova and A.G. Anshits, Izv. AN SSSR, ser. him., No 8 (19881 1718. A.A. Slinkin, T.K. Lavrovskaya,I.V. Mishin and A.M. Rubinshtein,Kinet. katal., 19, No 4 (19781 922. V.L. Zholobenko, I.N. Senchenya,L.M. Kustov and V.B. Kazanskii, Kinet. katal., 32, No 1 (19911 151. G.I. Panov, V.I. Sobolev and A.S. Kharitonov,Preprints of paper First Tokyo Conference on Advanced CatalyticScience and Technology,Tokyo, Japan, July 1-5, 1990. S.D. Razumovskii,Oxygen: its elementaryspecies and properties, M Khimiya, 1979. N:;. Ilchenko,Yu.1. Shmyrko and G.I. Golodets, Teoret. i exper. himiya, No 2 (1991) 231. N.I. Ilchenko,L.N. Raevskaya,A.I.Bostanand G.I. Golodets, Kinet. Katal., 32, No 4 (19911 873. D.J. Driscoll and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415. K. Aika, M. Tadzima and T. Onishi, Preprints of papers for the 7-th Soviet-JapanCatalysis Seminar, Novosibirsk (1983) 58. V.A. Khalif, B.V. Rozentuller,A.M. Frolov et al., Kinet. Katal., 19, No 5 (19781 1234. Y. Oshima, M. Saito, S. Koda and H. Tominaga,Sekiyu Gakkaishi, 32, No 2 (19891 59; 32, No 4 (19891 216.
catalysis To&~,13(1992) 495-501 ElsevierSciencePubliahersB.V., Amsterdam
UNCHARGED ATOMIC OXYGEN IN OXIDATIVE CONVERSION OF Cl-C2
ALKANES
A.G.Anshits
Institute of Chemistry of Natural Organic Materials,Siberian Branch of the USSR Academy of Sciences, 42, K.Marx Str., Krasnoyarsk,660049, USSR
Abstract The,role of uncharged atomic oxygen species in oxidative conversion of CH and C H has been considered.It is shown that active atomic oxygen species p%+icipate in C -C alkanes conversionin the presence of N 0 on zeolites, in the CH -N O'an8 CH -0 systems on transition metal silfcides and nitrides, as wef1 & hydroc&b&r oxidationduring N 0 and 0 photolysis by excimer laser light. Participationof uncharged oxyggn speci& in conversion of light alkanes makes it possible to explain not only hydrocarbon formation pathways, but those of oxygen-containingcompounds as well. INTRODUCTION The hetero-homogeneousmechanism of oxidative coupling of methane is at present universally accepted [1,21. The only points the authors disagree on are the role of different oxygen species in the reaction. The most popular is interpretationprovided by Lunsford 121, CH4 with
who showed that interactionof
Li/MgO sy
rects of structures such as O- or [M+O-I gave CH radicals on 3 :m. Recombinationof the two radicals on the surface or in the
gas phase
!ads to the formation of ethane as a major product. At the same
time C2H6
lrmationfrom methane is also ascribed to peroxide ion O2 2- [31.
l
The gi\ I review focuses on the contributionof uncharged atomic oxygen species ta .he conversionof light alkanes. ALKANE CONVERSION ON ZEOLITES
Unlike alkali-earthand rare-earthmetal oxides, zeolites and their catalytic role in
Cl-C2 alkane oxidative conversionhave not been given broad
consideration.Only a few articles are devoted to the problem. For example, it was reported that the zeolite activity in CH and C H oxidative conver4 26 sion in the presence of oxygen was mainly determined by the activation of hydrocarbon on acid sites [4,51. The process in the N20-containingsystems is characterized by considerable quantity of heavy hydrocarbons and no
0920~5361/92/$05.00 0 1992-ElaevierSciencePubliiheraB.V.Allrightareaervecl.
496
correlation between zeolite activity and the number of strong acid sites was found [5]. The absence of the correlation between the acidity and the activity of the zeolites in C2H6-N20 system becomes especiallyevident for samples with different content of sodium (Table 1). Table 1 Catalytic properties and acidity of zeolite ZSM-5 (SiO /Al 0 -41) with different Na content CT=660 K, reaction mixture (vol.%?:CiHi-36,6; NzO-3,6)
Catalyst
NaZSM-5 HZSM-5 Na,HZSM-5
Content Rate of ethane Selectivity,% Acidity, mmol/g of Na, conversion, Total High tempera% CO2 CH4 C2H4 C3H6 lO'*molec/g*s ture form I,7 O,l 1,5
091 9,7 13,o
-_0,7 1,3 91 0,3 4,O 90
_ 7 6
0,89 0,74 0,98
0,38 0,08
The results obtained indicate that a parent sample NaZSM-5, which is completely devoid of a capabilityto adsorb NH3 at high temperature,is characterized by low rate of C2H6 oxidation.Decationationof the parent sample results in the increase of both the activity and the number of strong acid sites. Reintroductionof 1,5 (wt.%) Na into HZSM-5 results in suppressing the strong acid sites whereas its activity is even higher than that of HZSM-5(41). High activity of ZSM-5 zeolite in C2H6-N20 system can be associated with the active oxygen species which were formed on the surface by N20 decomposition. The interactionof nitrous oxide at 620 K with the surface of zeolite was accompaniedby an evolutionof dinitrogen into the gas phase, while oxygen remained bound with the surface. The linear correlationwas found between the initial rate of ethane conversionand the amount of oxygen held by the surface for all examined zeolites of ZSM-5 type and mordenite. The fact that ethane interactswith surface oxygen to form stoichiometricquantities of ethylene and water gives strong evidence for the atomic character of active oxygen species [51. Equally interesting is the nature of N20 decompositionand oxygen stabilization sites. Zeolites are known to possess the sites capable of one-electron transfer. According to Slinkin 161 these sites stabilize oxygen in atomic and anion-radicalforms (0-I during N20 decompositionon H-mordenite. On the base of spectral and quantum-chemicaldata, Zholobenko [71 sug-
497
gested that at 423 K
N20
decompositioncould be attributed to high reacti-
vity of Lewis acid sites on
HZSM-5(401 zeolite. Decompositionresults in
dinitrogen evolution into the gas phase, with oxygen being chemisorbed on the zeolite. The number of Z-Ochem sites was equal to the value of 1O"g-', which is in good agreement with data 151. The sites decompose at temperatures higher than 623 K with oxygen evolution.Since molecular oxygen is not known to be chemisorbedon dehydroxylatedzeolite the authors believe that sites are formed by chemisorption of atomic oxygen and possess '-'them strong oxidative properties. Defect structures and Lewis acid sites may not be
the only contributors
to N20 decomposition. The sites formed at the stage of zeolite synthesis and containing transitionmetal ions may take part in the process as well. This assumption was carefully considered during the experiments to study the specific properties of surface oxygen species obtained on FeZSM-5 [8]. The major characteristicsof oxygen species, obtained on FeZSM-5 and supported vanadium, molybdenum and tungstenoxides are listed in Table 2. Table 2 Comparison of surface oxygen forms 181 Properties
(OI-FeZSM-5
+
Formation from O2 N2° Formation after: Reduction Oxidation Behavior under heating
0-
+
+
+ +
+
desorption
reoxidation
Reactivity at room temperature: interactionwith CH4,C0
+
+
heteroexchangeO2
+
+
homoexchangeO2
+
+
EPR signal
+
O2 +
+ reoxidation, desorption
+
+
According to the data presented, the reactivity of surface oxygen (0) on FeZSM-5 is similar to that of radical species O- found on supported oxide catalysts. Both these species interact with CH4, CO and participate in hetero- and homoexchangeof oxygen.
498 However,
active oxygen species bound with FeZSM-5 surface are different
from O- radical. (01 FeZSM-5 sites are stable in the presence of oxygen under heating, while O- radicals disappear on supported MO- and Vcontaining catalysts under this treatment. The increase of temperature up .. to 573-603 K results in desorptionof oxygen from FeZSM-5, whereas O- radicals disappear with reoxidationof the surface. SPECIFIC QUALITATIVEREACTIONS OF ATOMIC OXYGEN The data published suggest that N20 decompositionon ZSM-5 zeolites should be ascribed to active oxygen species, which could not be detected by means of physical techniques.Chemical reactions can be used to prove the involvementof these species in the process. It is known that minor amount of carben :CH is formed as a result of the reaction of methane with atomic 2 oxygen in triplet state 191 (Scheme 1). Scheme 1
CH4
+
OC3P1 W
The intermediatespecies :CH2 may react with ethylen to glve cyclopropane. Thus, the formation of cyclopropanein CH4-N20-C2H4system may indicate the existence of atomic oxygen. Catalytic reactions on the most active catalysts, such as HZSM-5(41) and l,S% Na,HZSM-S(411. in pulse experimentswith the use of the reaction mixture of CH4:N20:C2H4=9:6:24in helium showed that cyclopropane could be only found on 1,5X Na,HZSM-5(411sample. However, no cyclopropanewas detected on HZSM-St411 zeolite, which may be attributed to high rates of cyclopropane isomerizationon zeolite acid sites. Cyclopropanewas also formed (0,2X mol.1 under condition of oxidativedimerizationof methane on titanium silicide catalyst at 1073 K in CH4-air system [lo]. Participation of adsorbed atomic oxygen at the stage of CH: radical formation was suggested in 1111 on the basis of a study of catalyticpropertiesof transition metal borides, silicides and nitrides in oxidativecoupling of methane. Consequently, the participationof atomic oxygen in Cl-C2 alkane condensation, with either nitrous oxide or oxygen present, can be proved in terms of cyclopropane formation as one of the products of hydrocarbonconversion. Different reactivityof ion-radicaland uncharged oxygen species can also tell on their behavior in ethane conversion. According to the scheme of
Lunsford El21 oxidative conversion of ethane on Li/MgO involving reaction of O- species does not lead to methane as a product of the reaction. Unlike the authors mentioned, Aika [13] emphasizesthat C2H6 conversion results in formation of methane on Co/MgO surface, provided there are oxygen species with the properties of the gas phase atomic oxygen. According to the authors, CH4 is a product of thermal decompositionof carboxylates,the latter having been formed by oxidation of surface ethoxyde with atomic oxygen: Scheme 2 C2H50-
+20
CH3COO- +
w OH- w
CH3COO-
+
H20
CH 4
+
co;-
Hence, the difference between the ion-radicalsand atomic oxygen species lies in the pathways of CH4 generation.The low rate of methane formation may be attributed to anion-radicalnature of active oxygen species, whereas the high rate may be ascribed to uncharged oxygen species with the properties of the gas phase atomic oxygen. The above mentioned dissimilarities are expected to hold good for 6% Mo/ZSM-SC1481and 13% MO/T-A1203 catalysts as well, which are known to generate two different species of atomic oxygen [5,141. Table 3 presents relative rates of C2H6 oxidation on MO-containing catalysts. Table 3 Relative rates of product formation from C2H6 on MO-containingcatalysts =1,5 ml) (T=630 K; mixture: 36,4% C2H6, 3,6% 02(N20) in He. V pulse C2H6-N20
C2H6 Catalyst
Relative rates of formation CH4
6% Mo/ZSM-5(148) 13% MO/~-A1203
C2H6-02
0,l O,l
C2H4
CH4
CO2
C2H4
CH4
CO2
120
15 -
-
260 120
1,2 0.2
187 11
C2H4 78 180
The given results indicate that the low rate of CH4 formation is associated with alumina-molibdenacatalyst,which is known to activate both of the two oxidants in anion-radicalform. As far as zeolite catalysts are concerned, the relative rate of CH4 conversion in C2H6-N20 system is one or two order of magnitude greater than that in other cases. This may be taken as one more evidence of uncharged species of atomic oxygen being involved in ethane conversion on zeolite catalysts.
500 Participation of various surface oxygen species in the process of deep oxidation can also be proved by comparing activities of the two Mo-containing catalysts. No CO2 formation occures without an oxidizing agent (i.e. a pulse of pure C2H61, that indicatesthe role of stepwise mechanism in the process of total oxidation (Table 31. Acceleration of total oxidation in the presence of oxygen allows to suggest that CO2 formation on Mo-containing catalysts is caused by molecular oxygen species (e.g. 02-l' whereas dehydrogenationand condensationare associatedwith atomic forms.
ALKANE CONVERSION IN GAS PHASE Oxidation of CH4 and C2H6 with atomic oxygen during N20 photolysis by ArF excimer laser light at 300-600 K was studied by Japanese scientists. It was shown [151 that alkane activation proceeded through atomic oxygen O(lDl insertion into the C-H bond of hydrocarbon,giving rise to either [CH30Hl* or
[C2H50HI* species, which are convert to the products reflected in
Scheme 3. Scheme 3 N2°
c3!!*
hv CH4
+
1 O(ID) + I N2°
N2
+
O2
CH; CH30H*-
OH'+ CHj -
C2s +
O21 . CH 00 + 3
CH' T .3 C2H5 + CH4
. CHj CH30 -
CHF
CH OCH --3-3
OH' JI HCHO +
HCO'+ _' CO
CO -2
It is shown that with oxidizing agents other than oxygen in the gas phase, CH4 and C2H6 conversion results in C2-C4-hydrocarbonsand CO as their major products. Upon the increase of temperatureand pressure of the reaction mixture, the selectivityof C2-C4 hydrocarbonformation is also observed to noticeably grow, whereas the formation selectivityof different oxygen-containing (includingCO) products is reported to decrease. The above mentioned authors [15] also showed that atomic oxygen O(lD1, which was obtained by O3 photolysiswith KrF excimer laser light, underwent quick deactivation to form 0t3Pl radicals.These radicals are active in the
501 reaction on hydrogen abstraction to give hydroxyl and hydrocarbon radicals (Scheme 1) and are inert in the reaction of oxygen insertion. This fact also influences the compositionof the reaction products. Thus, the pathway analysis of light alkane conversionshows the uncharged species of atomic oxygen are likely to participatein the formation of partial oxidation products. To prove the fact that oxidation processes are associated with the forms mentioned above one should resort to specific qualitative reactions. ACKNOWLEDGEMENTS. The author is gratefully indebted to Dr. S.Vereshchaginfor experimental part of this work. REFERENCES
9 10 11 12 13 14 15
G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse,Chem. Sot. Rev., 18 (19891 251. D.J. Driscoll, W. Martir, J.-X. Wang and J.H. Lunsford, J. Amer. Chem. sot., 107 (19851 58. M.Yu. Sinev, V.N. Korchak and O.V. Krylov, Kinet. katal., 27, No 5 (19861 1274. S.S. Shepelev and K.G. Ione, Kinet. katal., 25, No 2 (1984) 347. S.N. Vereshchagin,L.I. Baikalova and A.G. Anshits, Izv. AN SSSR, ser. him., No 8 (19881 1718. A.A. Slinkin, T.K. Lavrovskaya,I.V. Mishin and A.M. Rubinshtein,Kinet. katal., 19, No 4 (19781 922. V.L. Zholobenko, I.N. Senchenya,L.M. Kustov and V.B. Kazanskii, Kinet. katal., 32, No 1 (19911 151. G.I. Panov, V.I. Sobolev and A.S. Kharitonov,Preprints of paper First Tokyo Conference on Advanced CatalyticScience and Technology,Tokyo, Japan, July 1-5, 1990. S.D. Razumovskii,Oxygen: its elementaryspecies and properties, M Khimiya, 1979. N:;. Ilchenko,Yu.1. Shmyrko and G.I. Golodets, Teoret. i exper. himiya, No 2 (1991) 231. N.I. Ilchenko,L.N. Raevskaya,A.I.Bostanand G.I. Golodets, Kinet. Katal., 32, No 4 (19911 873. D.J. Driscoll and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415. K. Aika, M. Tadzima and T. Onishi, Preprints of papers for the 7-th Soviet-JapanCatalysis Seminar, Novosibirsk (1983) 58. V.A. Khalif, B.V. Rozentuller,A.M. Frolov et al., Kinet. Katal., 19, No 5 (19781 1234. Y. Oshima, M. Saito, S. Koda and H. Tominaga,Sekiyu Gakkaishi, 32, No 2 (19891 59; 32, No 4 (19891 216.