Experimental Verification of Theoretically Calculated Transition Barriers of the Reactions in a Gaseous Selective Oxidation of CH4-O2-NO2

Article abstract of DOI:10.1021/jp9926239

The selective oxidation of methane with oxygen to C₁ oxygenates (methanol and formaldehyde) is an important process. However, the precise reaction mechanism in a gaseous chain reaction has not been clarified. Methane activation and the selectivity of C₁ oxygenates (CH₃OH, CH₂O) in a gaseous selective oxidation of CH₄O₂-NO₂ have been examined under atmospheric pressure with both theoretical and experimental approaches. Theoretically calculated transition barrier of hydrogen abstraction from CH₄ of the reaction CH₄ + NO₂ → CH₃ + HNO₂ was lower than that of the reaction with O₂, i.e., CH₄ + O₂ → CH₃ + O₂H. This decrease of the transition barrier was experimentally verified by the linear enhancement of CH₄ conversion with NO₂ concentration in CH₄-O₂-NO₂. The experimental varied results of selectivity of C₁ oxygenates on various reaction conditions (NO₂ concentration, CH₄O₂ ratio, space velocity) showed the appropriateness of the consideration on the selectivity with the calculated values of transition barriers and rate constants of the selected reaction routes from CH₃O to CH₃OH and CH₂O. After considering the transition barriers and rate constants of each elemental reaction route, we attained ca. 7% yield of C₁ oxygenates.

Full text of DOI:10.1021/jp9926239

2648
J. Phys. Chem. A 2000, 104, 2648-2654
Experimental Verification of Theoretically Calculated Transition Barriers of the Reactions
in a Gaseous Selective Oxidation of CH4-O2-NO2
Kenji Tabata,* Yonghong Teng, Yoichi Yamaguchi,† Hiroaki Sakurai, and Eiji Suzuki
Research Institute of InnoVatiVe Technology for the Earth (RITE), Kizugawadai, Kizu-cho, 9-2, Soraku-gun,
Kyoto, 619-0292 Japan
ReceiVed: July 27, 1999; In Final Form: January 11, 2000
The selective oxidation of methane with oxygen to C1 oxygenates (methanol and formaldehyde) is an important
process. However, the precise reaction mechanism in a gaseous chain reaction has not been clarified. Methane
activation and the selectivity of C1 oxygenates (CH3OH, CH2O) in a gaseous selective oxidation of CH4-
O2-NO2 have been examined under atmospheric pressure with both theoretical and experimental approaches.
Theoretically calculated transition barrier of hydrogen abstraction from CH4 of the reaction CH4 + NO2 f
CH3 + HNO2 was lower than that of the reaction with O2, i.e., CH4 + O2 f CH3 + O2H. This decrease of
the transition barrier was experimentally verified by the linear enhancement of CH4 conversion with NO2
concentration in CH4-O2-NO2. The experimental varied results of selectivity of C1 oxygenates on various
reaction conditions (NO2 concentration, CH4/O2 ratio, space velocity) showed the appropriateness of the
consideration on the selectivity with the calculated values of transition barriers and rate constants of the
selected reaction routes from CH3O to CH3OH and CH2O. After considering the transition barriers and rate
constants of each elemental reaction route, we attained ca. 7% yield of C1 oxygenates.
Introduction
of C1 oxygenates (CH3OH and CH2O) reached 7% at atmo-
spheric pressure. They suggested that the contribution of gas-
phase reactions for the formation of C1 oxygenates and the
presence of NO must alter the equilibria in the gas-phase
reactions.
The selective oxidation of methane with oxygen to C1
oxygenates (methanol and formaldehyde) is an important process
for the effective use of natural gas resources. In recent years,
many researchers have studied the selective oxidation of natural
gas with various types of catalysts. However, the products
mostly comprise CO, CO2, and H2O with only trace formation
of CH3OH, CH2O, and C2H6.1-3 Accordingly, the metal-oxide
catalysts that have been examined cause successive oxidation.
The gas-phase reaction without catalysts seems to have the
advantage of yielding C1 oxygenates because the difficulty of
the desorption stage from the surface of a catalyst could be
liberated but the control of a chain reaction could be difficult.
Formation of methanol and/or formaldehyde with methane and
oxygen in a gaseous reaction has been reported.2,4-14 Kinetic
and thermodynamic models of these gaseous reactions have been
discussed. The rate-determining step of the selective oxidation
of methane is the first hydrogen abstraction from methane.
Therefore, initiators and sensitizers have been examined in order
to reduce the activation energy of the first hydrogen abstraction
from methane.14-16 The promotion effect of nitrogen oxides for
methane selective oxidation in a gaseous reaction has been
reported.13,14,17-20 Bromly et al. prepared the kinetic models in
CH4-O2-NOx, based on the heats of formation and entropies.14
The predictions of their kinetic models for the oxidation
reactions in an atmospheric pressure were in good agreement
with the experimental data over the entire range of conditions,
though the main product of the reaction was CO, and the
formation of C1 oxygenates was not reported. Recently, Ban˜ares
et al.21 reported a high yield of methanol and formaldehyde over
V2O5/SiO2 catalysts in the presence of NO. The highest yield
Very recently, we proposed a reaction model for the conver-
sion of methane to methanol and formaldehyde with CH4-NOx
(x ) 1 or 2) in the gas phase by means of theoretical calculations
at the MP2 (frozen core) and CCSD(T) levels.22 It was found
that in both NO and NO2, a nitrogen atom showed a higher
activity for the cleavage of the C-H bond of methane than did
an oxygen atom in NOx. Furthermore, the activation energies
were calculated as 62 kcal/mol for a nitrogen atom of NO and
37.6 kcal/mol for that of NO2, indicating that NO2 had higher
activity for the hydrogen abstraction from methane than NO.
Through the theoretical analysis of the CH4-NO2 reaction
system in the previous paper, we suggested a possible reaction
path leading to yield in both methanol and formaldehyde within
all the barriers of less than 40 kcal/mol via CH3O.22 However,
we did not consider the contribution from the coexistent oxygen
to the reactions for the formation of C1 oxygenates in CH4-
O2-NO2.22
In this study, we will examine exclusively the gaseous
selective oxidation of methane with CH4-O2-NO2 under
atmospheric pressure. We will examine the effects of NO2 on
methane activation and the selectivities of methanol and
formaldehyde with both experimental and theoretical ap-
proaches. Some of the experimental results were submitted as
a letter.23 We will study theoretically the role of coexistent
oxygen on the reaction path leading to the selective formation
of CH3OH and CH2O. We will examine on the applicability of
our calculated transition barriers and rate constants of each
elemental reaction route in order to consider the selectivities to
CH3OH and CH2O under various reaction conditions, since the
* To whom correspondence should be addressed.
† Kansai Research Institute, Kyoto Research Park 17, Chudoji Minami-
machi, Shimogyo-ku, Kyoto, 600-8813 Japan.
10.1021/jp9926239 CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/26/2000

Experimental Verification of Transition Barriers
J. Phys. Chem. A, Vol. 104, No. 12, 2000 2649
detailed mechanisms of the selectivities to CH3OH and CH2O
for the gaseous chain reactions in CH4-O2-NO2 have not been
made clear.
Experimental Section
The reaction test was carried out using a single-pass flow
reactor made of a quartz tube with an inside diameter of 7.0
mm and a heated length of 200 mm. The total flow rate was
120 cm3 min-1. The standard gas composition (CH4: 55.6%,
O2: 27.7%, NO2: 0.5%, He: 16.2%) was controlled with a mass
flow controller. The concentration of NO2 ranged from 0.125%
to 1.0%; however, the ratio of CH4 to O2 was 2.0, unless
otherwise indicated. The products were analyzed with two on-
line gas chromatographs serially connected. A thermal conduc-
tivity detector (Molecular Sieve 5A) and a flame ion detector
(Porapack Q), using helium as a carrier gas, were used. The
carbon balance before and after the reaction exceeded 95%.
Measurements were carried out after 30 min of reacting at each
experimental condition, and all experimental data were taken
at least three times to check the reproducibility.
Method of Calculation. All of the calculations were carried
out with the Gaussian 94 ab initio program package.24 The
geometrical optimization for all of the present molecules was
performed with the MP2 (frozen core) level of theory and
6-311++G(2d, p) basis set. On the basis set of the optimized
geometries, the single point calculations of the energies were
calculated at the CCSD(T)/6-311++G(2d, p) level with the
zero-point energy corrections of the MP2 level. Thermal rate
constants for all of the present elementary reactions at 800 K
and 1 atm were also estimated using the calculated results of
MP2 and CCSD(T) levels of theory. A Silicon Graphics Origin
2000 R10000 workstation was used for calculations in this study.
Figure 1. CH4 conversion variation at different NO2 concentration as
a function of reaction temperature. Reaction gas: CH4 (55.6%) + O2
(27.7%) + NO2 in He (balance). Flow rate: 120 mL/min.
conversion. This value is close to the reported experimentally
obtained value (49.7 kcal/mol).20 This increase of reactivity
seems to be led by the decrease of the transition barrier of
hydrogen atom abstraction from methane for the reaction path
of CH4-NO2 instead of CH4-O2 as we expected from the
theoretical calculation. Therefore, the reaction of hydrogen atom
abstraction from methane mainly progresses on the reaction
route of CH4-NO2.
Methoxide Formation. We calculated the formation route
of CH3O, which is an important intermediate because only CH3O
can lead to yield CH3OH and CH2O.22 The coupling CH3 with
NO2 directly produces CH3NO2 and trans-CH3ONO. Each
stabilization energy was 57.0 and 55.7 kcal/mol, respectively.
The thermal decomposition of trans-CH3ONO into CH3O and
NO via [TS3] was calculated.22
trans-CH3ONO f [TS3] f CH3O + NO
(3)
The transition barrier of this decomposition reaction was
calculated as 38.4 kcal/mol.
Results
We did not consider the contribution of oxygen to CH3O
formation in the previous paper.22 However, the methyl radical
is assumed to react easily with oxygen. We calculated the
transition barriers of the following four reaction routes of CH3O
formation. The calculation method is the same as with our
previous paper.22
Methane Activation. The rate-determining step of the
selective oxidation of methane is the first abstraction of
hydrogen atom. The high barrier value as 55.2 kcal/mol has
been calculated theoretically for the O2 +CH4 f CH3 + O2H
reaction.22 We calculated the methane activation barriers of the
optimized transition structures with NO2 as follows:
O2 + CH3 f [TS4] f CH3OO
(4)
(5)
NO2 + CH4 f [TS1] f HNO2 + CH3
(1)
(2)
CH3OO + CH4 f [TS5] f CH3OOH + CH3
NO2 + CH4 f [TS2] f trans-HONO + CH3
CH3OOH f [TS6] f CH3O + OH
(6)
(7)
where the total energy of CH4 + NO2 is taken as a standard.
The transition barrier for eq 1 is calculated as 37.6 kcal/mol.
Equation 2 has a barrier of 45.8 kcal/mol. These barriers for
the reactions in eqs 1 and 2 are significantly lower by 9-18
kcal/mol than the above-mentioned O2 + CH4 f O2H + CH3
reaction, which indicates a higher activity of CH4 in the presence
of NO2 for the reaction of hydrogen abstraction from methane
in comparison with O2.
Figure 1 shows the emergent region of methane conversion
as a function of reaction temperature. The conversion without
NO2, i.e., CH4 + O2 shows a quite low activity even at 966 K.
The addition of a small quantity of NO2 in the reaction gas
enhances the conversion greatly. The reactivity increases with
the concentration of NO2 to 0.75% in CH4-O2-NO2 mixed
gas. The calculated transition barrier in the presence of NO2
from this experiment is 45.7 kcal/mol at the level of 10% CH4
CH3OO + NO f [TS7] f CH3O + NO2
Each calculated transition barrier is shown in Figure 2. Here
the state of the separated reactants is taken as a standard. The
optimized geometries of transition states [TS4] and [TS7] with
the CS structures and [TS5] and [TS6] with the C1 structures
are illustrated in Figure 3. The highest transition barrier in eqs
4-7 is 37.8 kcal/mol, and it is calculated as that of [TS6] in eq
6. Since the value of the transition barriers of [TS6] and those
of [TS1] and [TS2] are almost the same, all of the reactions in
eqs 4-6 are assumed to be possible to make progress. If the
concentration of NO in the reactant gas is sufficient, CH3OO
will react with NO to form CH3O through [TS7] in eq 7. This
route is energetically favorable in comparison with the routes

2650 J. Phys. Chem. A, Vol. 104, No. 12, 2000
Tabata et al.
Figure 2. Potential energy diagram for the formation of CH3O through CH3OO. The total energies for the separated reactants O2 + CH4 (I),
CH3OO + CH4 (II), CH3OOH (III), and CH3OO + NO (IV) are -189.7868, -230.1974, 190.4562, and -319.4922 hartrees, respectively. Relative
energies are given in kcal/mol.
Figure 3. Optimized geometries for TS4 of the CH3 + O2 reaction, TS5 of the CH3OO + CH4 reaction, TS6 of the decomposition of CH3OOH,
and TS7 of the CH3OO + NO reaction. All bond lengths and bond angles are given in Å and degrees, respectively. Imaginary frequency modes
for the forward reactions are given in cm-1
.
TABLE 1: Calculated Rate Constants, k, and Activation Energies, EA, at 800 Ka
reaction
k
EA (kcal/mol)
b
CH4 + NO2 f CH3 + H-NO2
5.4 × 10-20
3.3 × 10-22
2.5 × 10-5
37.6
45.8
35.6
CH4 + NO2 f CH3 + trans-HONOb
trans-CH3ONO f CH3O + NOb
CH3 + O2 f CH3OO
1.3 × 10-15 (2.0 × 10-12
)
)
14.8 (0.0)
29.9 (21.5)
33.0 (43.0)
-9.7 (-0.4)
16.9 (11.0)
27.5 (30.0)
21.7 (2.1)
6.7
CH3OO + CH4 f CH3OOH + CH3
CH3OOH f CH3O + OH
3.3 × 10-19 (1.3 × 10-18
1.5 × 106 (7.2 × 103)
CH3OO + NO f CH3O + NO2
1.5 × 10-10 (5.3 × 10-12
1.8 × 10-15 (9.9 × 10-16
2.2 × 106 (6.4 × 105)
1.7 × 10-17 (1.7 × 10-14
1.1 × 10-15
)
)
b
CH3O + CH4 f CH3OH + CH3
CH3O f CH2O + Hb
CH3O + O2 f CH2O + HO2
)
b
CH3O + NO2 f CH2O + H-NO2
CH3O + NO f CH2O + HNOb
3.1 × 10-14 (2.6 × 10-12
)
5.6 (-0.4)
a Values in parentheses represent the experimental data taken from refs 4 and 14. Rate constants are in s-1 and cm3 molecule-1 s-1 for unimolecular
and bimolecular reactions, respectively. Activation energies are in kcal/mol. From ref 22.
b
through eqs 5 and 6. The calculated rate constants and activation
energies at 800 K are listed in Table 1. The data were calculated
using the conventional transition-state theory and using the
calculated results of MP2 and CCSD(T) levels of theory. The
reported values of activation energies and rate constants are
listed in Table 1 also for comparison.
Reaction Routes to CH3OH and CH2O. The elementary
reactions for yielding CH3OH and CH2O from CH3O are shown

Experimental Verification of Transition Barriers
J. Phys. Chem. A, Vol. 104, No. 12, 2000 2651
reactant is taken as a standard. The highest transition barrier is
25.7 kcal/mol for the thermal decomposition of CH3O in eq 9.
All of the transition barriers in eqs 8-12 are lower than that of
hydrogen atom abstraction in eq 1. Therefore, all of the reactions
of eqs 8-12 are energetically possible to proceed. The calculated
rate constants of these reactions are listed in Table 1 also.
Regarding the selectivities of CH3OH and CH2O, one of the
important points is that CH3OH is produced only from reaction
8. The second important point is that the transition barriers of
CH2O formation from CH3O and NO2 or NO (assumed to be
produced during the reaction in the reactor) in eqs 11 and 12
are lower than [TS8] for the CH3OH formation.
We show our suggested reaction routes up to the formation
of CH3OH and CH2O (Figure 6). The largest value of the
calculated transition barrier is 45.8 kcal/mol, which is given
from the reaction for the first hydrogen abstraction from CH4.
Decomposition Route from CH3OH and CH2O. We ad-
dressed the reaction routes for the formation of CH3OH and
CH2O, but the reaction routes for the decomposition of CH3-
OH and CH2O are assumed to affect strongly the selectivities
of those. The decomposition routes of CH3OH and CH2O have
been suggested widely.4,5,14 In the suggested routes, we focused
on the route with OH species:
Figure 4. Potential energy diagram for the CH3O + O2 reaction with
the optimized geometry for TS10. The total energy for the separated
reactants is -264.8679 hartrees. Relative energies are given in kcal/
mol.
CH3OH + OH f CH2OH + H2O
CH2OH + O2 f CH2O + HO2
(13)
(14)
CH2O + OH f CHO + H2O
CHO + O2 f CO + HO2
(15)
(16)
The transition barrier in eq 13 was reported as 1.4 kcal/mol,4
and the others in eqs 14-16 were less than this value. Therefore,
if OH species are sufficient in the gaseous phase, the reaction
for the decomposition of CH3OH and CH2O proceeds easily.
The decomposition routes of CH3OH and CH2O with coexistent
NO2 also should be considered:
Figure 5. Potential energy diagram for the formation of C1 oxygenates.
The total energies for the separated reactants CH3O + CH4 (I), CH3O
(II), CH3O+O2 (III), CH3O + NO2 (IV), and CH3O + NO (V) are
-155.1623, -114.7925, -264.8679, -319.5106, and -244.4570
hartrees, respectively. Relative energies are given in kcal/mol.
CH3OH + NO2 f CH2OH + HONO
CH2O + NO2 f CHO + HONO
(17)
(18)
as follows:
The transition barriers of these reactions are reported as 21.4
and 16.1 kcal/mol, respectively.14 We calculated also the
transition barriers of the following reactions with coexistent NO2
as 28.3 and 21.4 kcal/mol, respectively:
CH3O + CH4 f [TS8] f CH3OH + CH3
CH3O f [TS9] f CH2O + H
(8)
(9)
CH3O + O2 f [TS10] f CH2O + HO2
(10)
CH3OH + NO2 f CH2OH + HNO2
CH2O + NO2 f CHO + HNO2
(19)
(20)
CH3O + NO2 f [TS11] f CH2O + H-NO2 (11)
CH3O + NO f [TS12] f CH2O + H-NO (12)
Experimental Results of the Selectivities of CH3OH and
CH2O. NO2 Concentration. The concentration of NO2 in a
reactant gas should affect not only the conversion of CH4 but
also the selectivities of CH3OH and CH2O. Figure 7 shows the
selectivities of the products as a function of NO2 concentration
at the same 10% methane conversion. The selectivity of CH3-
OH is higher than that of CH2O in the range of 0.25-1.0%
NO2 concentration. This is the first report that the selectivity
The transition barriers except for [TS10] were reported in the
previous paper,22 and we calculated [TS10] in order to examine
the contribution to CH2O formation from coexistent O2 with
the same calculation procedures. The calculated transition barrier
and the optimized geometries of transition states are shown in
Figure 4. The complete potential energy diagram for eqs 8-12
is illustrated in Figure 5, where the total energy of each separated

2652 J. Phys. Chem. A, Vol. 104, No. 12, 2000
Tabata et al.
Figure 6. Suggested reaction pathways for the formation of CH3OH and CH2O in CH4-O2-NO2.
Figure 8. Selectivity variation at the level of 10% CH4 conversion as
a function of CH4/O2 ratio. The percentages of CH4 and O2 in the feed
gas were changed so as to change the ratio of CH4/O2 from 1 to 10.
The percentages of NO2 and He were fixed at 0.5% and 16.2%,
respectively except for the case at CH4/O2 ) 1. The percentages of
NO2 and He were 0.5% and 44.0%, respectively, at CH4/O2 ) 1.
Figure 7. Selectivity variation as a function of NO2 concentration at
10% CH4 conversion with CH4 (55.6%) + O2 (27.7%) + NO2 in He
(balance).
of CH3OH exceeds that of CH2O in the gaseous selective
oxidation of CH4-O2-NO2 under atmospheric pressure. Both
selectivities in Figure 7 increased from 0.25% to 0.5%, then
decreased slightly to 1.0%. The selectivity of CO decreased until
NO2 reached 0.75%, then increased again. The ratio of CH2O
to CH3OH increased gradually in the region of 0.25-1.0% NO2
concentration.
CH4/O2 Ratio. The ratio of coexistent methane to oxygen in
the reactant gas is theoretically assured to affect the selective
yields of CH3OH and CH2O (Figure 5). We experimentally
studied the effects of the CH4/O2 ratio on each selectivity of
CH3OH and CH2O. Figure 8 shows each selectivity of products
at the 10% conversion of CH4 as a function of the ratio of CH4:
O2 in CH4-O2-NO2 reactant gas. The conversion of CH4 as a
function of reaction temperature on each ratio from 1 to 10 is
shown in Figure 9. The selectivity of CH3OH is higher than
that of CH2O in the range of 1 to 10. One feature of the results
in Figure 8 is the highest yield of both CH3OH and CH2O at
CH4/O2 ) 2.
Space Velocity (SV). We studied the effects of space velocity
(SV) on the selectivities. The SV was calculated by dividing
the gas flow volume per 1 h at 298 K and 1 bar by the volume
of the vacant heated zone of the reactor. Figure 10 shows the
variation of each selectivity with SV at 10% methane conver-
sion. CH2O increases with SV but CO decreases. These changes
of both CH2O and CO agree well with the obtained results by
Irusta et al.18 This means that CO is a secondary product
following the formation of CH2O. The selectivities of CH3OH
and CO2 are less affected by the SV variation in comparison
with CH2O and CO. All of these data were detected close to
800 K, in other words, the difference of each detected reaction
temperature at the 10% methane conversion is less than 10 K
(Figure 11).
Discussion
It has been known for more than 70 years that the oxidation
of CH4 with O2 can be accelerated by the addition of NO.25
Recently, Otsuka et al. examined the partial oxidations of CH4,
C2H6, C3H8, and iso-C4H10 with O2 by addition of NO in the
gas phase.20 Regarding the contribution of NOx to the activation
of methane, they suggested that NO2 would work as an initiator
or oxidant in the oxidations because the reaction between NO
and alkane did not take place in the absence of O2. Thus they
speculated that some NO2-containing hydrocarbons could be
the reaction intermediate for the formation of oxygenates. They
examined the decomposition reactions of 1-C3H7NO2, 2-C3H7-
NO2, and tert-C4H9ONO in the flow of O2 or a mixture of NO
and O2 and detected the formation of HCHO, CO, CO2, and
CH3NO2 from only the decomposition of tert-C4H9ONO. They
suggested that alkyl nitrite could be the intermediate for the
formation of oxygenates. From these experimental results, they

Experimental Verification of Transition Barriers
J. Phys. Chem. A, Vol. 104, No. 12, 2000 2653
Figure 9. CH4 conversion under different CH4/O2 ratios as a function
of reaction temperature. The percentages of CH4 and O2 in the feed
gas were changed so as to change the ratio of CH4/O2 from 1 to 10.
The percentages of NO2 and He were fixed at 0.5% and 16.2%,
respectively, except for the case at CH4/O2 ) 1. The percentages of
NO2 and He were 0.5% and 44.0%, respectively, at CH4/O2 ) 1.
Figure 10. Selectivity variation at the level of 10% CH4 conversion
as a function of space velocity (SV) with CH4 (55.6%) + O2 (27.7%)
+ NO2 (0.5%) in He (balance).
suggested a reaction scheme. Their suggested scheme is listed
as follows:
NO + 1/2 O2 f NO2
CH4 + NO2 f CH3 + HNO2
CH3 + NO2 f CH3ONO
CH3 + NO2 f CH3NO2
CH3ONO f CH3O + NO
(21)
(22)
(23)
(23′)
(24)
(25)
CH3O f CH2O + H
O2, NO2
CH3
8 HCHO, CO, CO2
(26)
The details in eq 26 were not clear. Furthermore, the reaction
route to the formation of CH3OH was not discussed. Especially,
the details of the transition barriers and rate constants for
addressing the selectivities of CH3OH and CH2O have not been
made clear yet.
Figure 11. CH4 conversion at different space velocity (SV) as a
function of reaction temperature with CH4 (55.6%) + O2 (27.7%) +
NO2 (0.5%) in He (balance).
The selectivity of CO decreased rapidly in the same range.
Therefore, we considered that the increase of the selectivities
of CH3OH and CH2O resulted from the decline of decomposition
reactions of CH3OH and CH2O with OH in eqs 13-16. The
reaction route for the formation of OH is known from eq 6.
The transition barrier of [TS6] is 37.8 kcal/mol (Figure 2), and
this value is much higher than that of [TS7] for the formation
of CH3O. Additionally, there is no transition barrier for the
reverse reaction from CH3O to CH3OOH. Therefore, the reaction
route for the decomposition of CH3OO in eq 7 is favorable to
form CH3O. Furthermore, since the rate constant of eq 7 is larger
than that of eq 5, CH3OO is assumed to react with NO.
Therefore, the concentration of OH in the gas phase of CH4-
O2-NO2 could be low in comparison with that in the gas phase
of CH4-O2. This low concentration of OH could bring the
decline of decomposition reaction, then the selectivities of CH3-
OH and CH2O increased in the range of 0.1-0.5% NO2
concentration. Furthermore, we must point out the effect of
We have tried to connect the theoretically calculated transition
barriers and rate constants of elemental reaction routes for the
formation of CH3OH and CH2O and to the experimentally
observed selectivities of those. The effects of NO2 concentration,
the ratio of CH4/O2, and SV on the selective yields of CH3OH
and CH2O were experimentally examined. We addressed the
theoretically obtained transition barriers and rate constants and
the variations of observed selectivities of CH3OH and CH2O
under several reaction conditions at the same time. In Table 1,
the calculated values of [TS4] and [TS8] are much different
from the reported experimental values.4,14 We assumed that the
main reason of for this difference came from the reactions
between a doublet state (CH3, CH3O) and a triplet state (O2)
species, which caused a large spin contamination in the results
at the level of our calculations. Regarding the effects of NO2
concentration on the selectivities of CH3OH and CH2O (Figure
7), both selectivities increased in the range from 0.25% to 0.5%.

2654 J. Phys. Chem. A, Vol. 104, No. 12, 2000
Tabata et al.
reaction temperature. We compared the product selectivities at
the level of 10% CH4 conversion. We therefore compared the
data at different temperatures in order to assess the effect of
NO2 concentration. The reaction temperature at the level of CH4
conversion decreased by the addition of NO2 (Figure 1). This
decrease may suppress the subsequential oxidation to CO.
Additionally, since higher NO2 concentration over 0.5% brings
a slight decrease of both selectivities of CH3OH and CH2O,
the decomposition reaction of CH3OH and CH2O with NO2 in
eqs 17-20 could affect their selectivities.
Regarding the CH4/O2 ratio, we expected an increase of
selectivity of CH3OH and a decrease of selectivity of CH2O
from eqs 8 and 10 as the ratio increased. The variations on the
selectivities of CH3OH and CH2O are small except for CH4/O2
) 2 during the region of our experiments (Figure 8). Further-
more, another feature of the experimental result is the parallel
movement of the selectivities of CH3OH and CH2O in that
region. The selectivities of CO and CO2 decrease at CH4/O2 )
2 and that of CO increases slightly after the ratio exceeds 2.
Therefore, we considered that CH3OH produced through eq 8
could be decomposed subsequently to CO through eqs 13-16.
The increase of the reaction temperatures at the 10% CH4
conversion in Figure 9 is assumed to enhance the subsequential
oxidation to CO. CH4/O2 ) 2 is optimum for the formation of
CH3OH and CH2O.
Regarding the variation with SV, the selectivities of CH2O
and CO seem to move in conjunction (Figure 10). The decrease
of CO and the increase of CH2O in the selectivities are assumed
to be the result of the retardation of the subsequent oxidation
from CH2O to CO. The less variation of CH3OH selectivity
with SV shows that CH3OH is more stable. The transition
barriers of the decomposition route in eqs 13 and 15 are 1.4
and 0.2 kcal/mol, respectively.4 The small difference between
the transition barriers is assumed to lead to the large difference
of selectivities of CH3OH and CH2O under a higher SV region.
We obtained ca. 7% yield of C1 oxygenates at SV ) 7500 h-1
(Figure 10).
experimentally detected selectivities of C1 oxygenates by using
the calculated transition barriers and rate constants of the
reaction routes.
Acknowledgment. This study was financially supported by
the New Energy and Industrial Technology Development
Organization (NEDO, Japan). K.T. acknowledges Prof. J. L.
G. Fierro and his colleagues for their kind discussions.
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Conclusion
The anticipation of the selectivity of reaction products in a
gaseous chain reaction is still difficult, and the precise reaction
mechanisms have not been cleared. We examined several
possible contributing reactions for the production of CH3OH
and CH2O in gaseous selective oxidation with CH4-O2-NO2
with both theoretical and experimental approaches. We theoreti-
cally calculated several transition barriers and rate constants of
our suggested reaction routes. We can appropriately explain the
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