Direct conversion of ethane to acetic acid over H-ZSM-5 using H 2O2 in aqueous phase

Article abstract of DOI:10.1016/j.apcata.2013.02.009

Partial oxidation of ethane on protonated pentasil-type zeolite (H-ZSM-5) was studied by using hydrogen peroxide (H₂O₂), an environment friendly oxidizing agent. Acetic acid (CH₃COOH) and formic acid (HCOOH) were obtained with high yield and selectivity from ethane. In addition, the influence of reaction parameters such as temperature, pressure, amount of oxidizing agent, and reaction period was investigated. Yields of acetic acid and formic acid (based on ethane) were achieved up to values of 18.7% and 14%, respectively, under the optimized reaction condition of 393 K and 3.0 MPa. Selectivity to both compounds is 84% under this condition. The amount of oxidant (H₂O₂) was optimized to be 279 mmol for the reaction. A low silica to alumina ratio in H-ZSM-5 was favorable for the partial oxidation of ethane. CO₂ formation was observed as a deep oxidation product at initial period. Partial oxidation was observed to occur within a short reaction period and was associated with the active oxygen species produced from H₂O₂ and CO₂ formed through the deep oxidation of ethane. Since the yield of CH₃COOH from ethane is much higher than that from ethylene, partial oxidation on H-ZSM-5 proceeds through a reaction mechanism different from that of a Wacker-type oxidation reaction.

Full text of DOI:10.1016/j.apcata.2013.02.009

Applied Catalysis A: General 456 (2013) 82–87
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Direct conversion of ethane to acetic acid over H-ZSM-5 using H O in
2
2
aqueous phase
Abul Kalam Md. Lutfor Rahmana,1, Rie Indo , Hidehisa Hagiwara , Tatsumi Ishihara
a
a
a,b,∗
a
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
International Institute for Carbon Neutral Research Energy (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Partial oxidation of ethane on protonated pentasil-type zeolite (H-ZSM-5) was studied by using hydro-
gen peroxide (H2O2), an environment friendly oxidizing agent. Acetic acid (CH3COOH) and formic acid
Received 12 December 2012
Received in revised form 2 February 2013
Accepted 9 February 2013
Available online xxx
(HCOOH) were obtained with high yield and selectivity from ethane. In addition, the influence of reaction
parameters such as temperature, pressure, amount of oxidizing agent, and reaction period was investi-
gated. Yields of acetic acid and formic acid (based on ethane) were achieved up to values of 18.7% and 14%,
respectively, under the optimized reaction condition of 393 K and 3.0 MPa. Selectivity to both compounds
is 84% under this condition. The amount of oxidant (H2O2) was optimized to be 279 mmol for the reaction.
A low silica to alumina ratio in H-ZSM-5 was favorable for the partial oxidation of ethane. CO2 formation
was observed as a deep oxidation product at initial period. Partial oxidation was observed to occur within
a short reaction period and was associated with the active oxygen species produced from H2O2 and CO2
formed through the deep oxidation of ethane. Since the yield of CH3COOH from ethane is much higher
than that from ethylene, partial oxidation on H-ZSM-5 proceeds through a reaction mechanism different
from that of a Wacker-type oxidation reaction.
Keywords:
Acetic acid
Liquid phase oxidation
Ethane
H-ZSM-5
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
the chemical industry, is presently synthesized by oxidizing ethyl-
ene (CH2 CH ). Because of the cost of ethylene, extensive research
2
Ethane (C H ) is the second largest component in natural gas
into the conversion of ethane to acetic acid through partial oxida-
tion using gaseous oxygen and a conventional catalyst such as a
heteropoly acid has been undertaken. However, the yield of acetic
acid is still low. Therefore, a new catalytic process for converting
ethane into more useful chemicals such as acetic acid and formic
acid is especially desirable.
2
6
after methane (CH ). Depending on its source and location, nat-
4
ural gas can contain anywhere from 1% to 20% ethane. Similar
to methane, ethane is also a least reactive hydrocarbon that is
very difficult to convert into useful organic chemicals. Because
of its natural abundance, selective conversion of C H into use-
2
6
ful oxygenated products is an important issue and the reaction is
of particular interest in the field of catalysis. Thus, considerable
research has been conducted on the partial oxidation of ethane to
In our previous study [4], partial oxidation of CH4 with H O2
was investigated and it was found that H-ZSM-5 is highly active and
2
selective for the conversion of CH into formic acid (HCOOH) by the
4
valuable chemical feedstocks such as ethylene (CH2 CH ), acetic
addition of triphenyl phosphine. The yield of HCOOH achieved was
2
acid (CH COOH), acetaldehyde (CH CHO), ethanol (CH CH OH),
13% (based on CH ). The present study focuses on the partial oxida-
3
3
3
2
4
and methanol (CH OH). However, high reaction temperatures,
tion of ethane to acetic acid (CH COOH) and formic acid (HCOOH).
3
3
low conversion ratios, and limited yield still restrict its utility
CH COOH is a very useful chemical reagent as well as a raw mate-
3
[
1–3]. Therefore, an efficient catalytic pathway with high yield and
rial for the production of many other chemical compounds such
as acid anhydride and esters. HCOOH is a promising fuel for alka-
line electrolyte-type fuel cells as a byproduct compound. Although
the synthesis gas route for acetic acid is currently an economically
attractive process for large-scale acetic acid production (Monsanto
process), several energy-intensive intermediate steps increase the
overall production costs. Therefore, there is a need for a simplified
and less expensive route to produce acetic acid. Thorsteinson et al.
selectivity is desirable. Among the potential valuable oxygenated
compounds, acetic acid, which is a fundamental chemical reagent in
∗ Corresponding author at: Department of Applied Chemistry, Faculty of Engi-
neering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
Fax: +81 92 802 2871.
[
5] first reported the selective oxidation of ethane to acetic acid
E-mail address: ishihara@cstf.kyushu-u.ac.jp (T. Ishihara).
1
on oxide catalysts containing Mo, V, and other elements (Nb, Sb,
Ti, Ta, Sn, As, W, Fe). They reported that ethylene is formed as a
Present address: Department of Chemistry, Jagannath University, 8-9 Chittaran-
jan Avenue, Dhaka 1100, Bangladesh.
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.02.009

A.K.Md.L. Rahman et al. / Applied Catalysis A: General 456 (2013) 82–87
83
Table 1
Product yields for the partial oxidation of ethane using H2O2 oxidant and various catalysts.
Catalyst
Yield (%)
Selectivity to CH3COOH (%)
Conversion of ethane (%)
CH3COOH
HCOOH
CO2
CH3CHO
CH3CH2OH
CH3OH
H-TS-1
H-beta zeolite
7.1
16.0
9.2
4.2
18.7
0.3
5.0
7.9
1.3
14.0
0.8
2.7
3.3
0.8
4.6
0.2
–
0.3
0.5
0.8
0.06
0.63
0.08
0.06
0.43
–
83.5
65.2
44.2
62.0
48.5
19.4
28.7
27.3
31.3
35.1
0.12
0.04
0.01
0.1
5
%W–H-ZSM-5
H4PVMo11O40
H-ZSM-5
Catalyst 1.5 g; triphenylphosphine 0.3 g; H2O2 279 mmol; H2O 70 mL; C2H6 pressure 3.0 MPa; temperature 393 K; reaction time 2 h; selectivity was calculated on the basis
of the total amount of detected carbon-containing products.
main product and acetic acid is formed only at higher pressure as
a product of the subsequent oxidation of ethylene. Recently, Rah-
man et al. [6] reported the partial oxidation of ethane to ethylene
and acetic acid over MoV type catalysts at a temperature range of
after 10 min later, sample was cooled down in liquid N2 and then
ESR measurement was performed.
IR measurement of the adsorbents on H-ZSM-5 was performed
using a Fourier transform infrared (FT-IR) spectrometer (JASCO type
610) with an ATR measurement set-up. After reaction started at 1,
3, or 5 h, autoclave type reactor was quenched to room temperature
and then the catalyst was sampled with reaction media and set into
ATR measurement cell. FT-IR measurement was performed at room
temperature.
5
13–553 K using O as the oxidant. Many researchers have investi-
2
gated direct partial oxidation of ethane to acetic acid using various
homogeneous or heterogeneous catalysts [1,7–9]; however, nei-
ther the yield nor C H conversion is high. Thus, if a new catalyst for
2
6
converting ethane efficiently into useful oxygen-containing com-
pounds such as acetic acid and formic acid is developed, it would
contribute considerably to the efficient use of feedstocks. However,
the direct synthesis of HCOOH remains primarily focused on the
partial oxidation of methane. Several reports exist on the selective
oxidation of methane to formic acid, but yields are still low [10]. In
3. Results and discussion
In order to obtain a selective and efficient catalyst for partial oxi-
dation of ethane, several catalysts were examined for C H6 partial
2
this study, partial oxidation of ethane with H O2 on H-ZSM-5 was
2
oxidation under similar reaction conditions. Table 1 summarizes
studied as a route for the direct synthesis of acetic acid. Formic acid
was obtained as a minor product in this process. Although H-ZSM-5
has not been studied as a partial oxidation catalyst, it was expected
that the strong acidity of H-ZSM-5 would be effective for the acti-
vation of lower alkanes [11–14]. Therefore, we chose to investigate
the yield of all detected oxygenated compounds using H O2 oxi-
2
dant. For all catalysts examined, the partial oxidation products are
CH COOH, HCOOH, CH CHO, CH CH OH, CH OH, and the deep oxi-
3
3
3
2
3
dation product CO . When the reaction is carried out using titanium
2
silicate (TS-1), a well-known catalyst for partial oxidation of lower
alkanes with H O , acetic acid is formed as the major product but
H-ZSM-5 for the liquid-phase oxidation of ethane using H O as the
2
2
2
2
oxidant, a reaction that has not heretofore been studied in detail.
the yield is lower than that of H-ZSM-5. Since tungsten-loaded
ZSM-5 has been reported as an active catalyst for conversion of
lower alkanes [11], we also examined W (5%)-ZSM-5. However, a
W-loaded ZSM-5 catalyst shows poor activity in the partial oxida-
tion of ethane, less than that of simple H-ZSM-5. Tungsten species
easily enters or partially blocks ZSM-5 zeolite’s channels and thus
reduces surface area, micropore volume, as well as the number of
acid sites of the catalyst [11]. Therefore, it is reasonable to assume
that further treatment of H-ZSM-5 with another co-catalyst or just
by loading any species would decrease the oxidation activity. A
high yield of acetic acid was also obtained using H-beta zeolite with
larger pore size as the catalyst. However, among the catalysts exam-
2
. Experimental
Protonated zeolites were obtained via ion exchange of Na-type
zeolite (supplied by TOSOH Corp.). Initially, Na-zeolite was ion-
exchanged into its ammonium form by NH NO3 aqueous solution
at approximately 368 K for 2 h. The sample was then dried overnight
at 333 K and calcined at 773 K for 2 h. The catalyst (typically 1.5 g)
and triphenylphosphine (0.3 g) were fed into a 200-mL reactor
4
with H O and H O. The liquid phase volume was kept at 80 mL
2
2
2
to ensure smooth mechanical stirring during the reaction period.
The reactor was flushed several times with N2 to remove air. Pure
ethane was fed into the reactor vessel at the designated pres-
sure. The temperature of the reaction was maintained at 373 K.
The reaction was performed for 5 h. The gas phase was analyzed
by thermal conductivity detector gas chromatography with a col-
umn containing a molecular sieve (4 mm diameter × 5 m length)
for O , N , CO, and active carbon (4 mm diameter × 2 m length) for
ined, H-ZSM-5 (SiO /Al O = 23.8) still yielded the highest amount
2
2
3
of CH COOH and HCOOH, indicating that H-ZSM-5 shows strong
3
acidity, which might be effective in the activation of ethane for
partial oxidation with H O2 as oxidant.
2
Our previous study [4] suggested that acid sites are an impor-
tant parameter for the direct synthesis of HCOOH from methane.
Therefore, we investigated the effect of the SiO /Al O ratio of
2
2
2
2
3
ethane and ethylene, respectively. Acetic acid was detected using a
DIONEX ICS-1000 ion chromatography system with an IonPacICE-
AS6 ion-exclusion column (9 mm × 250 mm). The yield of formic
acid was measured by a DIONEX DX-120 ion chromatograph
enclosed in an IonPacAS9-HC Analytical, 4 mm × 250 mm col-
the H-ZSM-5 catalyst on product distribution. It is well known
that the SiO /Al O ratio determines the acidity as well as acid
2
2
3
amounts of the zeolite. Fig. 1 shows the yield of CH COOH and
3
HCOOH as a function of the SiO /Al O ratio and for the conversion
2
2
3
of ethane. Evidently, the yield of HCOOH increases as the ratio of
Si/Al decreases and decreases as the ratio increases. This tendency
is almost the same as the methane partial oxidation reported in our
previous study [4]. Han et al. [11a] reported that, for the H-ZSM-5
catalyst with higher alumina content (lower ratio, more acid sites),
the yields of C2+ and C5+ liquid hydrocarbons in the partial oxi-
dation of methane are higher than those of the reaction using the
H-ZSM-5 catalyst with lower alumina content. Farizul et al. [15] also
reported that higher conversions of palm oil were achieved using
umn. Ethanol (CH CH OH), acetaldehyde (CH CHO), and methanol
3
2
3
(
CH OH) were detected by gas chromatography–mass spec-
3
troscopy (GC–MS) using a Shimadzu GCMS-QP2010 Plus with a
0
.32 mm × 60 m Stabilwax column. The amount of H O before and
2
2
after the reaction was estimated through automatic redox titration.
In order to detect OH radical, ESR measurement was performed at
7
7 K by using Bruker EMX-8/2.7S. After reaction at 353 K for 1 h, 5,5-
dimethyl-1-pyrroline-N-oxide (DMPO) was added for spin trap and

8
4
A.K.Md.L. Rahman et al. / Applied Catalysis A: General 456 (2013) 82–87
2
1
1
0
5
0
4
3
2
1
0
2
4
6
a)
b)
CH COOH
3
HCOOH
5
0
8
0
CO
2
10
100
Si/Al ratio
1000
1
0
100
Si/Al ratio
1000
Fig. 1. (a) Yield of CH3COOH, HCOOH, and CO2 and (b) conversion of ethane as a function of the ratio of silica to alumina.
2
1
1
0
5
0
1
0
a)
b)
8
100
90
CH COOH
3
6
4
2
HCOOH
5
80
CO2
0
70
3
40
360
380
400
420
440
3
40
360
380
400
420
440
Temperature /K
Temperature /K
Fig. 2. (a) Yield of CH3COOH and HCOOH and (b) conversion of C2H6 and H2O2 as a function of reaction temperature.
H-ZSM-5 with a lower SiO /Al O ratio. Similar to HCOOH, the
the production of all oxygenated compounds of 57.6%, estimated by
a mole of oxygen in the detected oxygenated compound per mole
2
2
3
yield of CH COOH also decreases as the SiO /Al O ratio increases.
3
2
2
3
A lower Si/Al ratio is associated with higher alumina content,
which is allied with the stronger acid sites of the zeolite catalyst.
of H O , subtracting the O produced from the initial amount of
2 2 2
H O .
2
2
Since H-ZSM-5 with SiO /Al O = 23.8 showed the highest yield of
Fig. 4(a) shows the formation rate of CH COOH and HCOOH
2
2
3
3
CH COOH, this catalyst was studied in greater detail, focusing on
the role of the acid sites.
as a function of the total reaction pressure. The formation rates
increase continuously with increasing total reaction pressure up to
3.6 MPa. The rate increases slowly in the 3–3.6 MPa range. Higher
3
Fig. 2 shows the yield of CH COOH and HCOOH as a function of
3
reaction temperature. The yield of both acetic acid and formic acid
increases with increasing temperature. The highest yield is reached
at 393 K. At higher temperatures, the yield tends to decrease
reaction pressure seems to be effective in preventing simple H O2
2
decomposition into H O and increasing the amount of active
2
oxygen and ethane in the aqueous reaction media, which could
because of the decomposition of HCOOH and CH COOH into CO .
improve the yield of CH COOH and HCOOH. However, maintaining
3
2
3
Simple decomposition of H O2 (H O → H O + (1/2)O ) increases
higher pressure incurs significant additional production costs,
which are always avoided in industrial scale operations. Therefore,
2
2
2
2
2
with increasing temperature and H O conversion reaches 100% at
2
2
3
93 K, as shown in Fig. 2 (right). Although large amounts of molec-
ular O2 still remain in the gas phase, the partial oxidation reaction
is saturated (ethane conversion) when H O is fully decomposed,
2
0
5
4
5
2
2
suggesting that, for partial oxidation, the active oxygen species
are obtained from H O and not from the O2 gas phase. This
40
35
30
25
2
2
CH COOH
3
1
may suggest the contribution of hydroxy radical which is formed
from H O . Therefore, the optimum reaction temperature seems
2
2
to be 393 K.
We examined the effect of the amount of the oxidant H O
10
5
2
2
on the yield of CH COOH and HCOOH and the conversion of
3
HCOOH
ethane, as shown in Fig. 3. The yield of both CH COOH and HCOOH
3
increases with increasing amounts of H O , and reaches the maxi-
2
2
mum of 17.6% and 11.2% for CH COOH and HCOOH, respectively, at
3
2
0
2
79 mmol of H O . The yields decrease rapidly at higher amounts
CO2
200
Amount of H O /mmol
2
2
of H O because further oxidation of CH COOH and HCOOH to CO
15
400
2
2
3
2
0
becomes dominant. Production of CO2 as a deep oxidation product
increases slowly with increasing amounts of H O up to 279 mmol.
100
150
250
300
350
2
2
2
2
At that point, it increases very sharply, indicating deep oxidation
due to the excess oxidant. Under the reaction conditions used, the
Fig. 3. Yield of CH3COOH, HCOOH and conversion of C2H6 as a function of the
amount of oxidant.
decomposition of H O is 100%; this yields an efficiency factor for
2
2

A.K.Md.L. Rahman et al. / Applied Catalysis A: General 456 (2013) 82–87
85
1
5
0
5
0
60
b)
a)
5
0
C H
2
6
1
40
30
20
HCOOH
CH COOH
3
1
0
CO2
0
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Pressure /MPa
Pressure /MPa
Fig. 4. (a) Rate of CH3COOH, HCOOH, CO2 formation and (b) conversion of C2H6 as a function of total reaction pressure.
the optimum reaction pressure can be considered to be 3 MPa for
the present reaction systems.
partial oxidation of ethanol and ethylene catalyzed by H-ZSM-5
using H O2 as the oxidant. When ethanol is used for partial oxida-
2
Fig. 5(a) shows the yield of CH COOH, HCOOH, CH CHO, and
tion, acetic acid and formic acid are mainly produced. Therefore,
3
3
C H5OH as a function of the reaction period. Conversion of C H
CH COOH and HCOOH might be produced using ethanol as an
2
2
6
3
and H O and the yield of CO2 are plotted in Fig. 5(b). As shown
intermediate. However, because of a low carbon balance, unknown
products (may be ketone or aldehyde) seem to be formed from
ethanol partial oxidation (about 20% yield). On the other hand,
when ethylene is used as the reactant, the main product is HCOOH
2
2
in Fig. 5(a), the yield of CH COOH and HCOOH initially increases
3
monotonously with reaction time and reaches the maximum of
1
8.7% and 14.0% for CH COOH and HCOOH respectively. However,
3
after 120 min, the formation of CH COOH and HCOOH is almost
and the amount of CH COOH is quite small. Therefore, it seems
3
3
terminated because no H O2 remains in the solution (shown in
likely that CH COOH may not be formed through ethylene, which
2
3
Fig. 5(b)). Therefore, apparently CH COOH and HCOOH are formed
is generally considered to be an intermediate in the production
3
by the partial oxidation of ethane using H O2 as the oxidant.
of CH COOH from ethane in the Wacker-type reaction. From this
2
3
Ethanol, which is presumed to be the intermediate, initially
increases with time and then decreases radically at longer reaction
periods. Several researchers have reported the partial oxidation of
ethane to oxygenated compounds through both ethanol and ethyl-
viewpoint, the reaction mechanism of the partial oxidation of
ethane on H-ZSM-5 with H O2 becomes extremely interesting.
2
On the other hand, HCOOH, which is primarily formed by
the subsequent oxidation of ethanol, may also be formed from
methanol, an intermediate product in the partial oxidation of
methane [14,10a,18]. Note that a small amount of methane was
detected as the reaction product in the present study. Bone and co-
authors [19,20] reported that ethanol is an intermediate product
of ethane partial oxidation. Acetaldehyde (CH CHO) and CH OH
ene [4,5,16,17]. Fig. 5(b) shows the complete conversion of H O at
2
2
a reaction period of approximately 120 min, which suggests that,
in the present system, partial oxidation occurs within a reaction
period of 2 h. Therefore, the yield of CO2 remains almost the same
value even at longer reaction times. However, despite the decrease
3
3
of ethanol, the amount of CH COOH and HCOOH at longer reac-
tion period does not increase, which suggests that ethanol is not
have also been detected in the reaction product by GS-MS. Conant
and Tongberg speculated that HCOOH was formed by the oxidation
3
the single intermediate for the production of CH COOH or HCOOH.
of CH CHO [21]. However, C H5OH, which is formed directly from
3
3
2
Therefore, the partial oxidation of ethane described in this study
could be quite unique compared to the reports of partial oxidation
with molecular oxygen. In order to better understand the reaction
mechanism, instead of ethane, we investigated model reactions
using ethanol, ethylene, and acetaldehyde, separately as raw mate-
rials. In the conventional study [4–9], it has been reported that
ethane, is not considered to be the intermediate for CH COOH for-
mation in ethane partial oxidation. This is because the amount of
3
CH COOH and even HCOOH does not increase simply as a function
3
of reaction period, while C H5OH decreases when H O is com-
2
2
2
pletely converted, as shown in Fig. 5. Therefore, if molecular oxygen
reacts with C H5OH, deep oxidation rather than partial oxidation
2
CH COOH was obtained from ethane through subsequent oxida-
mainly occurs.
3
tion of ethylene as an intermediate product in ethane oxidation
Although the details of the reaction mechanism of ZSM-5 are
still not clear, the following scheme can be tentatively proposed for
(
Wacker-type reaction). Table 2 shows the product yields from the
2
1
1
1
1
1
0
8
6
4
2
0
1
.0
0.8
.6
0.4
a)
b)
1
00
CH COOH
3
CH3CHO
H O
2
2
8
0
HCOOH
CH3CH2OH
0
60
40
CH3CH3
8
CH4
CO2
6
4
2
0
2
0
0
0
.2
.0
CH OH
3
0
0
100
200
300
400
500
0
100 200 300 400 500
Reaction time (min.)
Reaction time (min)
Fig. 5. (a) Yield of CH3COOH, HCOOH, C2H5OH, CO2 and (b) conversion of C2H6 and H2O2 as a function of reaction period.

8
6
A.K.Md.L. Rahman et al. / Applied Catalysis A: General 456 (2013) 82–87
Table 2
Effect of ethanol and ethylene in the production of CH3COOH and HCOOH.
Input
Yield (%)
Conversion (%)
CH3COOH
HCOOH
CO2
CH3CHO
CH3CH2OH
CH3OH
Ethanol
Ethylene
Methanol
Acetaldehyde
35.3
2.6
0
19.4
25.3
68.9
27.7
4.2
5.5
20.0
10.1
5.3
0.7
0
–
0.5
0
0.4
0.04
–
84.1
38.6
98.1
92.4
49.2
7.6
0
0
Catalyst 1.5 g; triphenylphosphine 0.3 g; H2O2 279 mmol; H2O 70 mL; pressure 3.0 MPa; temperature 393 K; reaction time 2 h; selectivity was calculated on the basis of the
total amount of detected carbon-containing products.
ethane partial oxidation on the H-ZSM-5 catalyst for the production
of CH COOH. Hydroxy or oxygen radical formed from H O may
contribution of OH radical to the oxidation reaction of ethane into
acetic acid seems to be reasonable as discussed above.
3
2
2
react with C H resulting in the formation of intermediate like ethyl
In order to further study intermediates on H-ZSM-5 in this reac-
tion, IR spectra of ZSM-5 were measured by ATR unit. Fig. 7 shows IR
spectra of H-ZSM-5 in reaction media before reaction and after 1, 3,
and 5 h reaction started. Obviously, after 1 h later, absorption peak
2
6
+
or CH3 radical. Here, triphenylphosphine (Ph3P) and H in ZSM-5
may be related to form hydroxyl radicals from H O , which is highly
2
2
active to oxidation. Although Ph3P is a large molecule and cannot
enter micro-pore of ZSM-5, Ph3P contributes formation of active
oxidation species like hydroxyl radical and hydroxyl radical reacts
with ethane in micro-pore of ZSM-5. The oxygenated intermedi-
ate easily converts into alcohol or aldehyde followed by carboxylic
acid or aldehyde or through carboxylic compound. Acid catalyst is
requested for the conversion of alcohol or aldehyde to carboxylic
acid. H-ZSM-5 seems to catalyze the formation of hydroxy rad-
−
1
−1
at 1577 cm and broad peaks around 3400–3700 cm appeared.
−
1
−1
Absorption peaks around 1577 cm
and 3400 cm are typical
ones for C O in aldehyde and O H band, respectively. Therefore,
as discussed, reaction seems to be pass through aldehyde like
acetaldehyde or glyoxal by partial oxidation with OH radicals.
−
1
After 3 h later, peak at 1577 cm
disappeared and new peak
−
1
appeared at 1715 cm which is also C O bond in carbonate, and
that at 1070 cm for O H. These peaks became more significant
with reaction period. In addition, two peaks were also observed
around 3000 cm , which are typical peaks assigned to C HX.
Therefore, acetic acid was mainly observed after 3 h. Therefore, up
to now, detailed reaction mechanism is still not clear; however,
−
1
ical from H O and convert alcohol or aldehyde into carboxylic
2
2
•
•
acid. On the other hand, if ethyl radical (C H + OH = C H5 + H O)
2
6
2
2
−
1
forms from ethane, then C H5OH seems to be easily formed by
2
•
•
reaction with hydroxyl radicals (C H5 + OH = C H5OH) resulting
2
2
in formation of acetic acid as shown in Table 1. On the other
hand, if methyl radical forms from C H with oxygen radicals from
considering the high yield of HCOOH and CH COOH, acetaldehyde
2
6
3
•
•
H O (C H + 2O = 2CH + O ), it seems to react with CO2 result-
seems to be the intermediate for CH COOH formation. In order
2
2
2
6
3
2
3
ing in the formation of acetic acid, which is a reverse reaction of
Kolbe reaction [22]. Since CO2 yield decreases after 60 min but
to confirm the reactivity of acetaldehyde, partial oxidation of
acetaldehyde was performed and the results are shown in Table 2.
Conversion of acetaldehyde was achieved at ca. 93%, and acetic
acid and formic acid were formed selectively. Therefore, partial
CH COOH yield increases up to 120 min, mechanism with reverse
3
Kolbe reaction is also suspected. In order to detect radical for-
mation, electron spin resonance measurement was performed by
trapping spin with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) at
room temperature.
oxidation of C H6 to CH COOH or HCOOH on H-ZSM-5 may
2
3
proceed through acetaldehyde.
Considering the ESR and IR data, the following reaction route is
Fig. 6 shows the ESR spectrum after reaction at 353 K for 1 h.
Although, signal is not strong and slightly noisy, four ESR signals
were observed around g = 2.045, 2.031, 2.010, and 1.991. This
ESR signals could be assigned to DMPO/OH adduct suggesting
the formation of OH radical during reaction [23,24]. Therefore,
the most reasonable main route for CH COOH formation at initial
period.
3
•
H O = 2OH
(1)
(2)
2
2
•
C H + 4OH = CH CHO + 3H O and then CH CHO + (1/2)O
2
6
3
2
3
2
g=2.031
g=2.045
=
CH COOH or CH CHO + (3/2)O = 2HCOOH
3
3
2
0
.25
.20
.15
.10
.05
O-H
1070
0
0
0
0
CO
2
C-HX
C=O
OH
g=1.991
g=2.010
1
715
577
5h aꢁer
1
3h aꢁer
0.00
1h aꢁer
Before reacꢀon
3250
3300
3350
Magneꢀc field /G
3400
3450
-0.05
000
4
3000
2000
1000
-1
Wavenumber[cm ]
Fig. 6. ESR spectra of DMPO adducts at 77 K after 1 h reaction at 1.0 MPa over H-
Fig. 7. FT-IR spectra of ZSM-5 in reaction media for C2H6 partial oxidation after 1,
ZSM-5 and triphenylphosphine.
3, 5 h later.

A.K.Md.L. Rahman et al. / Applied Catalysis A: General 456 (2013) 82–87
87
Since small amount of acetaldehyde was observed in Fig. 5, reactiv-
ity of acetaldehyde is high on H-ZSM-5 and easily converted into
acetic acid or formic acid as shown in Table 2.
Another route is considered using an alcohol like ethanol as
discussed:
the formation of CO2 is limited (4.6% yield, 11% selectivity). At
present, the reaction mechanism on H-ZSM-5 is not clearly under-
stood; however, OH radical could play an important role for the
formation of acetic acid and formic acid.
Acknowledgement
•
C H + OH = C H5OH and then C H5OH + O
2
6
2
2
2
=
CH COOH + H O
(3)
The authors express sincerely thanks to Prof. Tsutomu Katsuki,
International Institute for Carbon Neutral Energy Research, Kyushu
University for useful discussions on reaction mechanism.
3
2
CH3 + OH• = CH OH and then CH OH + O = HCOOH + H O
•
3
3
2
2
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(
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[
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2
2
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[
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2
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(
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