Direct conversion of methane to methanol over nano-[Au/SiO2] in [Bmim]Cl ionic liquid

Article abstract of DOI:10.1016/j.apcata.2011.03.028

This article describes a green chemical process employing nano-particle gold as the catalyst and ionic liquids (IL) as solvent for the methane oxidation. The catalytic reaction was carried out in a 100 ml autoclave filled with 2 MPa of CH₄ gas, together with nano-particle gold supported on SiO₂ as the catalyst, [Bmim]Cl as the solvent, trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA) as the acidic reagents, and K ₂S₂O₈ as the oxidant. The influence of the amounts of Au/SiO₂ and the ionic liquid on the conversion of methane was investigated at reaction temperature of 90 °C. The main product is methanol, which exists as the methyl group of the methyl trifluoroacetate. In presence of 0.01 g Au/SiO₂ and 1 g IL, the methane conversion is 24.9%, the selectivity of product is up to 71.5% and the yield is 17.8%. The selectivity of carbon dioxide is 1.6% and the yield is 0.6%. The selectivity of hydrogen is 0.4% and the yield is 0.1%. In the reaction system, the gold particles and IL can be recycled, which recovery is about 96.9%. The conversion of methane in the recycled system remains as high as 21.75%. The mechanism of methane to methano conversion, as well as the catalytic action of the nano-gold, was also discussed.

Full text of DOI:10.1016/j.apcata.2011.03.028

Applied Catalysis A: General 398 (2011) 150–154
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Direct conversion of methane to methanol over nano-[Au/SiO2] in [Bmim]Cl
ionic liquid
T. Li, S.J. Wang∗, C.S. Yu, Y.C. Ma, K.L. Li, L.W. Lin
School of Chemical Engineering and Materials, Dalian Polytechnic University, Dalian 116034, China
a r t i c l e i n f o
a b s t r a c t
Article history:
This article describes a green chemical process employing nano-particle gold as the catalyst and ionic liq-
uids (IL) as solvent for the methane oxidation. The catalytic reaction was carried out in a 100 ml autoclave
filled with 2 MPa of CH4 gas, together with nano-particle gold supported on SiO2 as the catalyst, [Bmim]Cl
as the solvent, trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA) as the acidic reagents, and
K2S2O8 as the oxidant. The influence of the amounts of Au/SiO2 and the ionic liquid on the conversion of
methane was investigated at reaction temperature of 90 ◦C. The main product is methanol, which exists
as the methyl group of the methyl trifluoroacetate. In presence of 0.01 g Au/SiO2 and 1 g IL, the methane
conversion is 24.9%, the selectivity of product is up to 71.5% and the yield is 17.8%. The selectivity of
carbon dioxide is 1.6% and the yield is 0.6%. The selectivity of hydrogen is 0.4% and the yield is 0.1%. In the
reaction system, the gold particles and IL can be recycled, which recovery is about 96.9%. The conversion
of methane in the recycled system remains as high as 21.75%. The mechanism of methane to methano
conversion, as well as the catalytic action of the nano-gold, was also discussed.
Received 5 September 2010
Received in revised form 11 March 2011
Accepted 15 March 2011
Available online 22 March 2011
Keywords:
Oxidation of methane to methanol
Ionic liquid
Nano-gold catalyst
Green chemistry
© 2011 Published by Elsevier B.V.
1. Introduction
In a system of trifluoroacetic acid, Bao has used organic qui-
nine [15] as co-catalyst to enhance the selective oxidation of
Methane as the main component of natural gas is an important
chemical raw material for C1 chemistry. Conventionally, methanol
is synthesized at high temperatures and pressures. Direct oxida-
tive conversion of methane is characterized by less greenhouse
gas emissions under mild conditions, which is becoming a research
focus in green C1 chemical technology. Since 1983, the Pt(II)/Pt(IV)
system [1] has been used in studies of methane oxidation, and
the main concern is the selective oxidation of methane. For liquid-
phase oxidation of methane, Hg (OSO3H)2 [2], Pt (bmpy)Cl2 [3] and
Au salts [4] were employed as catalysts in 100% H2SO4. When Pt
(bmpy)Cl2 was used as the catalyst, methane conversion was up to
90%, the selectivity of methyl bisulfate was 81%, and the yield was
70%. Originally, HgSO4 [5] was used as the catalyst for the selective
oxidation of methane in sulfuric acid. Later, Pd (II) [6,7], Co (III) [8],
Pd (OAc)2/Cu (OAc)2 [9,10], Rh (III) [11], Eu(III)/Zn [12], V2O5 [13],
and I2 [14] in concentrated sulfuric acid were employed as the cat-
alysts. However, the use of sulfuric acid or heavy metals as catalyst
systems will bring problems of strong corrosion and serious pol-
lution. Accordingly, the exploring of green processes for methane
conversion has attracted considerable interest.
methane over a Pd catalyst, in order to avoid the adverse effects
of the strong acids and heavy metals. In concentrated sulfu-
ric acid, methane was oxidized over Pt compounds as catalysts,
and ionic liquids were used as solvents [16]. Ionic liquids (IL) in
homogeneous catalytic transformations of methane to methanol
not only acted as a dissolution media for those otherwise insol-
uble Pt salts/oxides, but also played a key role in promoting
the reactivity of Pt. The system appeared to be more water tol-
erant than the conventional catalytic reactions. When IL and
metallic nanoparticles were employed to form a homogenous sys-
tem [17], the former could promote the activity of the metallic
nanoparticles. For example, in our previous work [18], IL and the
trifluoroacetic acid were used as the reaction medium, and partial
oxidation of methane was investigated with PdCl2 as the cata-
lyst. Results showed that the IL could promote the activity of the
PdCl2.
In this article, we are introducing a green chemical process of
methane conversion under mild conditions. In the present work,
SiO2-supported gold nanoparticles were used as the catalyst, which
was dispersed in [Bmim]Cl. The catalyst system containing acidic
reagents was introduced into an autoclave, which was then filled
with methane of a definite pressure [18]. The effects of the IL and
the Au/SiO2 catalyst on the conversion of methane were investi-
gated. The heterogeneous catalyst and the IL could be recycled. The
mechanism of methane oxidation was also discussed.
∗
Corresponding author. Tel.: +86 0411 86323725.
E-mail address: wgsoju@163.com (S.J. Wang).
0926-860X/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.apcata.2011.03.028

T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
151
2. Experimental
2.1. Instruments and materials
2.1.1. Instruments
The D/Max-3B X-ray diffraction (XRD) equipment was sup-
plied by Ricoh Co., the Tecnai G2 Spirit transmission electron
microscopy (TEM) was from FEI Co., the PE2000 Fourier trans-
form infrared spectrophotometer (FT-IR) was from Hitachi Co., the
GC 6820 gas chromatography was from Agilent Co., the 7890A
gas chromatography–mass spectrometer (GC–MS) was provided
by Agilent Co., and the DRX-400 nuclear magnetic resonance spec-
trometer (NMR) was from Bruker Co.
2.1.2. Materials
Methane (97% volume fraction), with argon gas (3% in volume)
added as an internal standard, was supplied by the Dalian Hede-
tech Co., China. Nitrogen (99.999% in volume) was received from
the Dalian Zhonghuida Co., China. Hydrazine hydrate (80% con-
centration) and silica sol (30% concentration) were obtained from
the Dalian Shenlian Chemicals Co., China. Chloroauric acid, trifluo-
roacetic acid (TFA), potassium persulfate, as well as trifluoroacetic
anhydride (TFAA), were also purchased from the Dalian Shen-
lian Chemicals Co., China. All of these reagents were analytically
pure. The materials were used without any further treatments. IL,
[Bmim]Cl, was synthesized according to the procedures described
in the literature [19].
Fig. 1. XRD pattern of Au/SiO2.
area of methane after the reaction, and A2[Ar] the peak area of
argon after the reaction. The liquid product was analyzed by FT-
IR (PE 2000), Agilent GC–MS 7890A, and NMR (Bruker DRX-400).
The weight of methyl trifluoroacetate was obtained by GC–MS,
which was corrected by a standard curve; the selectivity [3] to
CF3COOCH3, H2 and CO2 were calculated (CO was ignored because
its content was very low) as follows:
2.2. Preparation of nano-particle Au on SiO2
[CF3COOCH3]
[CH4]initial − [CH4]final
% selectivity to CF3COOCH3
=
HAuCl4 (0.01 mol/l, 5 ml) and a given amount of IL (gold/IL mole
ratio of 1:1) as well as 5 ml of silica sol were charged into a 3-
neck round-bottom flask 100 ml). Hydrazine hydrate (0.01 mol/l)
was added dropwise to the mixed solution. The reaction was stirred
for 3 h, aged for 12 h, then dried and calcined at 500 ◦C to give the
product. The properties of the product were characterized by means
of XRD and TEM.
% yield to CF3COOCH3 = % methane conversion
× % selectivity to CF3COOCH3
[H2]
% selectivity to H2
=
[CH4]initial − [CH4]final
2.3. Liquid phase oxidation of methane
% yield to H2 = % methane conversion × % selectivity to H2
In a 100 ml autoclave reactor which was protected with poly-
tetrafluoroethylene lining and provided with a Teflon-coated
magnetic stirrer, the catalyst of nano-particle Au on SiO2 (0.01 g)
and the IL [Bmim]Cl (1 g) were heated under stirring for 15 min.
Then, K2S2O8 (1.36 g), TFA (5 ml) and TFAA (1.5 ml) were intro-
duced. The reactor was purged with CH4 of 20 atm for three times,
and then pressurized with CH4 of 20 atm pressure (with argon as an
internal standard). It was then heated to 90 ◦C in an oil bath and kept
for 20 h under stirring. After the reaction, the reactor was cooled to
3 ◦C in ice water, and the pressure was slowly reduced. The liquid
phase reactant was distilled after the reaction, and the liquid prod-
uct fraction at 43 ◦C was collected. The product was analyzed by
FT-IR, GC–MS, and NMR, and quantified by GC.
[CO2]
[CH4]initial − [CH4]final
% selectivity to CO2
=
% yield to CO2 = % methane conversion × % selectivity to CO2
3. Results and discussion
3.1. Characterization of nano-particle Au on SiO2
The purple-colored solid product of nano-particle Au on SiO2,
as described in Section 2.2, was characterized by the following
procedures.
2.4. Quantitative and qualitative analysis of methane oxidation
3.1.1. Analysis by X-ray diffraction
Fig. 1 shows the X-ray diffraction (XRD) pattern of the powder
samples of nano-particle Au on SiO2.
Methane conversion rate was calculated on the basis of the CH4
raw material. The gas product was analyzed by Agilent GC 6820,
using HayesepD as the packed column in TCD detection. Methane
conversion rate was calculated by using argon gas (3%) as an inter-
nal standard (by the area normalization method).
Fig. 1 shows the XRD profile of Au/SiO2. By comparing with the
standard cards, the diffraction peak of SiO2 is at 2ꢀ = 22.24◦, and
the typical diffraction peaks of gold are at 2ꢀ = 38.08◦, 44.26◦ and
64.50◦, corresponding to the gold crystal of face-centered cubic
(FCC), which is composed of the (1 1 1), (2 0 0) and (2 2 0) planes.
From Fig. 4, we see that gold exhibits a high degree of crystallization
when loaded on SiO2. Estimated by the Sherrer formula, the size of
the gold particles is about 20.44 nm, belonging to the nano-scale.
A2[CH4] × A1[Ar]
% Methane conversion = 1 −
A1[CH4] × A2[Ar]
where A1[CH4] is the peak area of methane before the reaction,
A1[Ar] the peak area of argon before the reaction, A2[CH4] the peak

152
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
Table 2
Influence of the amount of nano-Au/SiO2 catalyst on methane conversion.
Nano-Au/SiO2/g p0CH /MPa T/◦C Selectivitya/% Conversiona/% Yielda/%
4
0.0
0.5
1.0
1.5
2.0
2
2
2
2
2
90
90
90
90
90
0
0.8
17.2
24.9
13.9
6.0
0
8.6
17.8
10.9
4.7
50.2
71.5
78.6
78.7
Reaction conditions: in a 100 ml autoclave, 1 g [Bmim]Cl, 5 ml (64.60 mmol)TFA,
1.5 ml (10.55 mmol) TFAA, 1.36 g (5 mmol) K2S2O8, p0 /MPa, T = 90 ◦C, t = 20 h.
CH
4
a
Calculated from Section 2.4.
The experimental results show that the IL amount influences the
selectivity limitedly, but influences the methane conversion and
yield greatly. The conversion and yield of methane oxidation with
the nano-Au/SiO2 catalyst is 15.8% and 11.3% in the absence of IL.
With an increasing ionic liquid amount, the conversion and yield of
methane increases. In the case of 1.0 g ILs, the methane conversion
and yield reaches a maximum of 24.9% and 17.8%. We inferred that
ILs can improve the dispersion and can stabilize the nano-Au/SiO2
catalyst. It leads to an increase in the methane conversion. With an
increase of IL amount from 1.0 g to 2.0 g, the methane conversion
and yield decrease to 12.6% and 9.0%, which is lower than that with-
out the IL. From the literatures [3,5,13,14,20], the acid medium not
only can provide a good environment for electrophilic substitution,
but also can inhibit the poisoning of the catalyst. However, as indi-
cated in Supplement information [2], the trifluoroacetic acid would
exchange their anions with the IL addition. Moreover, the concen-
tration ofthe trifluoroacetic acidwould bediluted, andthe acidityof
the reaction system would be decreased, which is not favorable for
electrophilic substitution. Consequently, the methane conversion
rate and yield would decrease.
Fig. 2. TEM photograph of Au/SiO2.
3.1.2. Analysis by transmission electron microscope
Fig. 2 shows the TEM photograph of Au/SiO2.
The transmission electron microscopy (TEM) photograph of the
morphology of the gold particles is shown in Fig. 2. The image in
Fig. 2 clearly displays that the gold particles have a spherical shape,
a good dispersion and a rather uniform particle size. There are also
some smaller particles, as indicated by the arrow, the size of which
is about 2–3 nm. The average gold particle size is about 20–30 nm,
in consistent with the results of XRD (Fig. 1).
3.2.2. The influence of the nano-Au/SiO2 catalyst on the
conversion of methane
The influence of the amount of the nano-Au/SiO2 catalyst on the
conversion of methane is shown in Table 2.
These results indicate that the gold particles were in the nano-
scale when supported on SiO2.
3.2. Methane conversion in the presence of nano-Au/SiO2 catalyst
and IL [Bmin]Cl
The results of Table 2 show that, when no nano-Au/SiO2 was
present in the reaction system, no conversion of methane could
be detected. With the increasing of the amount of nano-Au/SiO2
in the reaction system, the conversion rate, selectivity, and yield
increased correspondingly. As the amount of the nano-Au/SiO2
reached 0.01 g, the conversion rate, selectivity, and yield increased
to 24.85%, 71.5%, and 17.8%. Further addition of the nano-Au/SiO2
catalyst increased the selectivity limitedly, but lowered the con-
version rate and yield rapidly. One possible reason is that, as the
amount of the nano-Au/SiO2 was further increased, side reactions
will become more significant and this will accelerate the exchange
of the anions of trifluoroacetic acid with the anions of the ionic liq-
uid ([Bmim]Cl), which will then lead to a decrease in the acidity of
the reaction system, thus decreasing the methane conversion rate
and yield dramatically.
3.2.1. Influence of the IL ([Bmim]Cl) on the conversion of methane
In the test procedure described in Section 2.3, the amount of
the nano-catalyst Au/SiO2 was fixed to be 0.01 g in each test. Then,
the influence of the amount of ionic liquid on the conversion of
methane was investigated, and the results are shown in Table 1.
As reported in the literature [17], the IL with a chain longer
than one –CH3 unit would be oxidized quickly by Pt catalysts in
the concentrated H2SO4. In our work, no methyl trifluoroacetate
was detected in the liquid products in the blank test in the absence
of methane in Supplement information [1]. We are sure that the
[Bmim]Cl with long alkyl chains has good chemical stability and is
not oxidized in our experimental conditions.
3.3. Characterization of the products of methane conversion
Table 1
Influence of amount of IL on methane conversion.
3.3.1. IR characterization of the liquid products
[Bmim]Cl/g P/MPa Gas
T/◦C Conversiona/% Selectivitya/% Yielda/%
The IR spectra are shown in Fig. 3, in which 1# is the IR spectrum
of the liquid product after the reaction, and 2# is the IR spectrum
of methyl trifluoroacetate.
1# IR spectrum of the liquid product 2# IR spectrum of home-
made methyl trifluoracetate.
In Fig. 3, the IR absorption band of the liquid product at
3479.35 cm−1 is assigned to the –OH group, while the strong
absorption peaks at 2922.45 cm−1 and 1448.82 cm−1 indicate the
presence of the –CH3 group. The absorption band at 1779.16 cm−1
is assigned to the carbonyl of the ester, and the absorption peaks at
1.0
0
0.5
1.0
1.5
2.0
2
2
2
2
2
2
N2
90
0
0
0
CH4 90
CH4 90
CH4 90
CH4 90
CH4 90
15.8
17.6
24.9
14.9
12.6
71.6
71.6
71.5
71.4
71.4
11.3
12.6
17.8
10.6
9.0
Reaction conditions: in
a 100 ml autoclave, 0.01 g nano-gold catalyst, 5 ml
(64.60 mmol) TFA, 1.5 ml (10.55 mmol) TFAA, 1.36 g (5 mmol) K2S2O8, P = 2 MPa,
T = 90 ◦C, t = 20 h.
a
Calculated from Section 2.4.

T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
153
1#
1448.8
1332.9
2922.4
1175.0
2#
3479.3
1779.1
1000
4000.
3000
2000
1500
400.
/cm-1
Fig. 3. Comparison of IR spectra of the liquid product with home-made methyl
trifluoroacetate.
Fig. 5. HNMR spectrum of the liquid product.
1332.90 cm−1 and 1175.00 cm−1 signify the presence of the ether
of ester and –CF3.
It can be seen that the two spectra are quite similar, which indi-
cates that the liquid product contained methyl trifluoroacetate.
vated and oxidized to form methanol in our reaction system. The
methanol thus formed then reacts with trifluoroacetic acid to pro-
duce methyl trifluoroacetate.
3.3.2. GC–MS characterization of the liquid products
The GC–MS spectra of the liquid products are shown in Fig. 4.
Comparison of the mass spectra of Fig. 4 with the standard cards
confirmed that the compound in the liquid product was methyl
trifluoroacetate, showing that the liquid product contained methyl
trifluoroacetate.
3.4. Recycling of the catalyst and the ionic liquid ([BMIM][Cl])
It was reported in the literature [17] that nanoparticles could
form colloidal systems with the solvents, and this made it difficult
to recycle the nanoparticles for repeated utilization. In the present
work, however, since the nano-gold (Au/SiO2) catalyst was dis-
persed in an ionic liquid to form a heterogeneous catalyst system
in acidic medium, the catalyst can be recycled easily.
3.3.3. NMR characterization of the liquid product
The HNMR of the liquid product is shown in Fig. 5.
The recycling procedure can be described as follows: after
distilling of the overall reaction products, the bottom materials
containing the catalyst and the ionic liquid are collected to extract
the soluble materials. Then water is removed by a rotary evapo-
rater. With the presence of added ethanol, the mixture is filtered.
The filtrate is then collected, and rotary-evaporated at 70 ◦C to
remove the ethanol. The final mixture, which contains dissolved
nano-Au/SiO2 in IL ([Bmim]Cl), can be employed as catalyst for
recycling. The recovery of [Bmim]Cl and gold was approximately
96.91%.
The chemical shift of methyl ester is at 3.7 ppm. The single peak
signifies the chemical structure of the methyl trifluoroacetate in
the liquid product, in which no H is present in the adjacent carbon.
The peak at 3.9 ppm shows a pronounced upfield shift due to the
electro negativity of F. Thus, the typical peak at 3.9 ppm is a unique
characterization of the product methyl trifluoroacetate.
Thus, from determinations of IR, GC–MS and HNMR, it is fully
confirmed that the liquid phase product is methyl trifluoroacetate.
Moreover, by comparing with the blank test experiment discussed
in Table 1, one can conclude that the liquid product methyl trifluo-
roacetate is actually transformed from the raw material methane.
Furthermore, our results have proved that methane can be acti-
It was found that the conversion of methane with the recycled
catalyst system could still remain at 21.75%, which was slightly
lower than that for the fresh catalyst system. The recycling of the
s
59.0
69.0
22000
20000
18000
16000
14000
12000
10000
8000
F
F
O
O
F
6000
4000
50.0
2000
99.0
78.0
109.0
40.0
0
126.9
3
4
5
6
7
8
9
10
11
12
13
M/Z
Fig. 4. Mass spectra of methyl trifluoroacetate.

154
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
H+
4. Conclusion
MN+
CH3
H
MN+
CH3
+
Over the catalyst system of nano-gold (Au/SiO2) and IL, the pro-
cess of methane to methanol was investigated. The results showed
that IL acted as a dissolution medium for the nano-Au/SiO2, while
in the meantime played a key role in promoting the reactivity
of gold, and rendered a reaction with high selectivity. The sys-
tem of nano-Au/SiO2 and IL could be recycled, and conversion
of methane in the recycled system still remained at a high level.
This process of methane oxidation did not use concentrated sul-
furic acid and heavy-metal catalysts, thus we can eliminate the
impacts on the environment. A reaction mechanism was pro-
posed, which indicated that molecular oxygen was consumed in
the oxidation–reduction cycle. Consequently, methane oxidation
to methanol can be achieved as a green chemical process. The sys-
tem can also be used in other green chemical processes of liquid
phase or gas phase oxidation.
Nu: -
OX
M(N-2)+
CH3
+
N
Fig. 6. The mechanism of electrophilic substitution.
2Au+/IL + 2KHSO4
CF3COOCH3
2H+ +2e-
2Au/IL + 2H+ + K2S2O8
CH4 + CF3COOH
2Au+/IL + 2e-
1
2
+
3
2Au/IL
Fig. 7. Mechanism of methane oxidation over nano-gold catalysts in ionic liquid.
Acknowledgement
catalyst system (nano-gold and ionic liquid) not only can meet
the requirements of green chemistry, but can also lower operation
costs, save resources, and reduce waste emissions. Therefore, it is
an environmentally friendly process.
We acknowledge support from the State Key Projects for Basic
Research & Development, China, through Grant No. 2005CB221406.
We thank X. Hou and Dr. L.G. Wei for supporting.
Appendix A. Supplementary data
3.5. Discussion on reaction mechanism
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.03.028.
Methane can be activated and oxidized by means of three
modes: (1) via formation of a methyl radical, (2) via oxidative addi-
tion, (3) via electrophilic substitution. Presently, the mechanism
of electrophilic substitution is widely accepted, which is shown in
Fig. 6.
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Refs. [3,4,15], we can propose the reaction mechanism which is
shown in Fig. 7.
In our reaction system, Au/IL, which is formed by nano-gold
dispersed in [Bmim]Cl, was oxidized to Au+/IL by K2S2O8 (Eq.
(1)). Au+/IL was an active catalyst for the oxidation of methane in
CF3COOH to generate methyl trifluoroacetate (Eq. (2)). Afterwards,
the catalyst is reduced back to Au/IL (Eq. (3)).
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V.R. Ziatdinov, J. Mol. Catal. A: Chem. 220 (2004) 7–25.