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SiO2] in [Bmim]Cl ionic liquid
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
Article history: 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
a b s t r a c t 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 K2 S2 O8 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. © 2011 Published by Elsevier B.V.
1. Introduction 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 oxidative 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 liquidphase oxidation of methane, Hg (OSO3 H)2 [2], Pt (bmpy)Cl2 [3] and Au salts [4] were employed as catalysts in 100% H2 SO4 . 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], V2 O5 [13], and I2 [14] in concentrated sulfuric acid were employed as the catalysts. However, the use of sulfuric acid or heavy metals as catalyst systems will bring problems of strong corrosion and serious pollution. Accordingly, the exploring of green processes for methane conversion has attracted considerable interest.
∗ Corresponding author. Tel.: +86 0411 86323725. E-mail address: [email protected] (S.J. Wang). 0926-860X/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.apcata.2011.03.028
In a system of trifluoroacetic acid, Bao has used organic quinine [15] as co-catalyst to enhance the selective oxidation of methane over a Pd catalyst, in order to avoid the adverse effects of the strong acids and heavy metals. In concentrated sulfuric 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 insoluble Pt salts/oxides, but also played a key role in promoting the reactivity of Pt. The system appeared to be more water tolerant than the conventional catalytic reactions. When IL and metallic nanoparticles were employed to form a homogenous system [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 catalyst. 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 investigated. The heterogeneous catalyst and the IL could be recycled. The mechanism of methane oxidation was also discussed.
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
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2. Experimental 2.1. Instruments and materials 2.1.1. Instruments The D/Max-3B X-ray diffraction (XRD) equipment was supplied by Ricoh Co., the Tecnai G2 Spirit transmission electron microscopy (TEM) was from FEI Co., the PE2000 Fourier transform 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 spectrometer (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 Hedetech Co., China. Nitrogen (99.999% in volume) was received from the Dalian Zhonghuida Co., China. Hydrazine hydrate (80% concentration) and silica sol (30% concentration) were obtained from the Dalian Shenlian Chemicals Co., China. Chloroauric acid, trifluoroacetic acid (TFA), potassium persulfate, as well as trifluoroacetic anhydride (TFAA), were also purchased from the Dalian Shenlian 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]. 2.2. Preparation of nano-particle Au on SiO2 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 3neck 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. 2.3. Liquid phase oxidation of methane In a 100 ml autoclave reactor which was protected with polytetrafluoroethylene 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, K2 S2 O8 (1.36 g), TFA (5 ml) and TFAA (1.5 ml) were introduced. 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 product fraction at 43 ◦ C was collected. The product was analyzed by FT-IR, GC–MS, and NMR, and quantified by GC.
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 FTIR (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 CF3 COOCH3 , H2 and CO2 were calculated (CO was ignored because its content was very low) as follows: % selectivity to CF3 COOCH3 =
[CF3 COOCH3 ] [CH4 ]initial − [CH4 ]final
% yield to CF3 COOCH3 = % methane conversion × % selectivity to CF3 COOCH3
% selectivity to H2 =
[H2 ] [CH4 ]initial − [CH4 ]final
% yield to H2 = % methane conversion × % selectivity to H2 % selectivity to CO2 =
[CO2 ] [CH4 ]initial − [CH4 ]final
% 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 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 internal standard (by the area normalization method). % Methane conversion = 1 −
A2 [CH4 ] × A1 [Ar] 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
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 . Fig. 1 shows the XRD profile of Au/SiO2 . By comparing with the standard cards, the diffraction peak of SiO2 is at 2 = 22.24◦ , and the typical diffraction peaks of gold are at 2 = 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.
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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 /%
0.0 0.5 1.0 1.5 2.0
2 2 2 2 2
90 90 90 90 90
0 50.2 71.5 78.6 78.7
0.8 17.2 24.9 13.9 6.0
0 8.6 17.8 10.9 4.7
4
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) K2 S2 O8 , p0CH /MPa, T = 90 ◦ C, t = 20 h. 4
a
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). These results indicate that the gold particles were in the nanoscale when supported on SiO2 . 3.2. Methane conversion in the presence of nano-Au/SiO2 catalyst and IL [Bmin]Cl 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 H2 SO4 . 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.
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 without 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 indicated in Supplement information [2], the trifluoroacetic acid would exchange their anions with the IL addition. Moreover, the concentration of the trifluoroacetic acid would be diluted, and the acidity of the reaction system would be decreased, which is not favorable for electrophilic substitution. Consequently, the methane conversion rate and yield would decrease. 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. 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 conversion 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 liquid ([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. 3.3. Characterization of the products of methane conversion
Table 1 Influence of amount of IL on methane conversion. [Bmim]Cl/g
P/MPa
Gas
T/◦ C
Conversiona /%
Selectivitya /%
Yielda /%
1.0 0 0.5 1.0 1.5 2.0
2 2 2 2 2 2
N2 CH4 CH4 CH4 CH4 CH4
90 90 90 90 90 90
0 15.8 17.6 24.9 14.9 12.6
0 71.6 71.6 71.5 71.4 71.4
0 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) K2 S2 O8 , P = 2 MPa, T = 90 ◦ C, t = 20 h. a Calculated from Section 2.4.
3.3.1. IR characterization of the liquid products 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 homemade 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
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
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1#
1448.8
T( %)
1332.9
2#
4000.
3479.3
2922.4
3000
1175.0
1779.1
2000
1000
1500
400.
-1
/cm
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 indicates 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 produce 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 dispersed in an ionic liquid to form a heterogeneous catalyst system in acidic medium, the catalyst can be recycled easily. 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 evaporater. 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%. 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
3.3.3. NMR characterization of the liquid product The HNMR of the liquid product is shown in Fig. 5. 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 trifluoroacetate is actually transformed from the raw material methane. Furthermore, our results have proved that methane can be actis 59.0
22000
69.0
20000 18000
abundance
16000 14000
O
F
O
F
F
12000 10000 8000 6000 4000 50.0
2000
99.0 78.0
40.0
3
4
5
6
7
0 8
9
109.0
10
M/Z Fig. 4. Mass spectra of methyl trifluoroacetate.
11
126.9
12
13
154
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
MN+
+
CH3
H
H+
MN+
4. Conclusion
CH3
Nu: -
OX
M(N-2)+
CH3
+
N
Fig. 6. The mechanism of electrophilic substitution.
2Au/IL + 2H+ + K2S2O8 CH4 + CF3COOH 2Au+/IL + 2e-
2Au+/IL + 2KHSO4
1
+
2
CF3COOCH3 + 2H 2Au/IL
+2e
-
3
Over the catalyst system of nano-gold (Au/SiO2 ) and IL, the process 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 system 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 sulfuric acid and heavy-metal catalysts, thus we can eliminate the impacts on the environment. A reaction mechanism was proposed, 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 system can also be used in other green chemical processes of liquid phase or gas phase oxidation.
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.
3.5. Discussion on reaction mechanism
Appendix A. Supplementary data
Methane can be activated and oxidized by means of three modes: (1) via formation of a methyl radical, (2) via oxidative addition, (3) via electrophilic substitution. Presently, the mechanism of electrophilic substitution is widely accepted, which is shown in Fig. 6. In our case, methane is first attacked by the metal ions (or other ions such as H+ ) as a strong electrophilic reagent to form a metallic intermediate. Then, this intermediate is attached to the nucleophilic reagent to form a stable species of CH3 –N, which is a low valence state metallic intermediate that can be oxidized by an oxidant to a high valence state. In this way, a complete catalytic-cycle will be formed. In the present study, the catalyst as an electrophilic reagent not only attacks the methane, but also activates the C–H bond. The trifluoroacetic acid, as a co-catalyst, provides the acid medium for the removing of the protons in methane, and then reacts with methanol to form methyl trifluoroacetate. Because of the strong electron effect of the CF3 COO− , it is difficult to be oxidized to form the product methyl trifluoroacetate, so the reaction can exhibit good selectivity. Potassium persulfate acts as the oxidant in the system. Gold is a uniquely efficient electrophilic catalyst for methane conversion [4]. According to the above-mentioned mechanism and 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 K2 S2 O8 (Eq. (1)). Au+ /IL was an active catalyst for the oxidation of methane in CF3 COOH to generate methyl trifluoroacetate (Eq. (2)). Afterwards, the catalyst is reduced back to Au/IL (Eq. (3)).
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2011.03.028. References [1] Y.V. Geletil, A.E. Shilov, Catal. Kinet. 24 (1983) 413–419. [2] R.A. Periana, D.J. Taube, E.R. Evitt, D.G. Löffler, P.R. Wentrcek, G. Voss, T. Masuda, Science 259 (1993) 340–343. [3] R.A. Periana, J.T. Doube, S. Gamble, H. Taube, T. Satoh, H. Fujii, Science 280 (1998) 560–564. [4] C.J. Jones, D. Taube, V.R. Ziatdinov, R.A. Periana, R.J. Nielsen, J. Oxgaard, W.A. Goddard III, Angew. Chem. Int. Ed. 43 (2004) 4626–4629. [5] G.Ch. Ying, Z.W. Xi, G.Q. Cao, X.F. Zhang, M. Li, Chin. J. Catal. 18 (1997) 402–405. [6] L.C. Kao, A.C. Hutson, A. Sen, J. Am. Chem. Soc. 113 (1991) 700–701. [7] M. Lin, A. Sen, J. Am. Chem. Soc. 114 (1992) 7307–7308. [8] M.N. Vargaftik, I.P. Stolarov, I.I. Moiseev, J. Chem. Soc., Chem. Commun. (1990) 1049–1050. [9] T. Nishiguchi, K. Nakata, K. Takaki, Y. Fujiwara, Chem. Lett. (1992) 1141–1142. [10] K. Nakata, Y. Yamaoka, T. Miyata, Y. Taniguchi, K. Takaki, Y. Fujiwara, J. Organomet. Chem. 473 (1994) 329–334. [11] M. Lin, A. Sen, Nature (London) 368 (1994) 613–615. [12] I. Yamanaka, M. Soma, K. Otsuka, J. Chem. Soc., Chem. Commun. (1995) 2235–2236. [13] L.Y. Chen, B.L. Yang, X.Ch. Zhang, W. Dong, K. Cao, Chin. J. Catal. 26 (2005) 1027–1030. [14] L.Y. Chen, B.L. Yang, X.Ch. Zhang, W. Dong, J. Chem. Eng. Chin. Univ. 19 (2005) 54–58. [15] Z.J. An, X.L. Pan, X.M. Liu, X.W. Han, X.H. Bao, J. Am. Chem. Soc. 128 (2006) 16028–16029. [16] J.H. Cheng, Z.W. Li, M. Haught, Y.C. Tang, Chem. Commun. (2006) 4617–4619. [17] C.X. Xiao, N. Yan, Y. Kou, Chin. J. Catal. 30 (2009) 753–764. [18] S.J. Wang, Y. Zhang, Y. Ch.M, L.W. Lin, Nat. Gas Chem. Ind. 35 (2010) 10–12. [19] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Chem. Commun. 3 (2001) 156–164. [20] R.A. Periana, G. Bhalla, W.J. Tenn III, K.J.H. Young, X.Y. Liu, O. Mironov, C.J. Jones, V.R. Ziatdinov, J. Mol. Catal. A: Chem. 220 (2004) 7–25.
Contents lists available at ScienceDirect
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
Article history: 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
a b s t r a c t 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 K2 S2 O8 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. © 2011 Published by Elsevier B.V.
1. Introduction 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 oxidative 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 liquidphase oxidation of methane, Hg (OSO3 H)2 [2], Pt (bmpy)Cl2 [3] and Au salts [4] were employed as catalysts in 100% H2 SO4 . 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], V2 O5 [13], and I2 [14] in concentrated sulfuric acid were employed as the catalysts. However, the use of sulfuric acid or heavy metals as catalyst systems will bring problems of strong corrosion and serious pollution. Accordingly, the exploring of green processes for methane conversion has attracted considerable interest.
∗ Corresponding author. Tel.: +86 0411 86323725. E-mail address: [email protected] (S.J. Wang). 0926-860X/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.apcata.2011.03.028
In a system of trifluoroacetic acid, Bao has used organic quinine [15] as co-catalyst to enhance the selective oxidation of methane over a Pd catalyst, in order to avoid the adverse effects of the strong acids and heavy metals. In concentrated sulfuric 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 insoluble Pt salts/oxides, but also played a key role in promoting the reactivity of Pt. The system appeared to be more water tolerant than the conventional catalytic reactions. When IL and metallic nanoparticles were employed to form a homogenous system [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 catalyst. 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 investigated. The heterogeneous catalyst and the IL could be recycled. The mechanism of methane oxidation was also discussed.
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2. Experimental 2.1. Instruments and materials 2.1.1. Instruments The D/Max-3B X-ray diffraction (XRD) equipment was supplied by Ricoh Co., the Tecnai G2 Spirit transmission electron microscopy (TEM) was from FEI Co., the PE2000 Fourier transform 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 spectrometer (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 Hedetech Co., China. Nitrogen (99.999% in volume) was received from the Dalian Zhonghuida Co., China. Hydrazine hydrate (80% concentration) and silica sol (30% concentration) were obtained from the Dalian Shenlian Chemicals Co., China. Chloroauric acid, trifluoroacetic acid (TFA), potassium persulfate, as well as trifluoroacetic anhydride (TFAA), were also purchased from the Dalian Shenlian 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]. 2.2. Preparation of nano-particle Au on SiO2 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 3neck 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. 2.3. Liquid phase oxidation of methane In a 100 ml autoclave reactor which was protected with polytetrafluoroethylene 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, K2 S2 O8 (1.36 g), TFA (5 ml) and TFAA (1.5 ml) were introduced. 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 product fraction at 43 ◦ C was collected. The product was analyzed by FT-IR, GC–MS, and NMR, and quantified by GC.
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 FTIR (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 CF3 COOCH3 , H2 and CO2 were calculated (CO was ignored because its content was very low) as follows: % selectivity to CF3 COOCH3 =
[CF3 COOCH3 ] [CH4 ]initial − [CH4 ]final
% yield to CF3 COOCH3 = % methane conversion × % selectivity to CF3 COOCH3
% selectivity to H2 =
[H2 ] [CH4 ]initial − [CH4 ]final
% yield to H2 = % methane conversion × % selectivity to H2 % selectivity to CO2 =
[CO2 ] [CH4 ]initial − [CH4 ]final
% 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 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 internal standard (by the area normalization method). % Methane conversion = 1 −
A2 [CH4 ] × A1 [Ar] 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
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 . Fig. 1 shows the XRD profile of Au/SiO2 . By comparing with the standard cards, the diffraction peak of SiO2 is at 2 = 22.24◦ , and the typical diffraction peaks of gold are at 2 = 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.
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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 /%
0.0 0.5 1.0 1.5 2.0
2 2 2 2 2
90 90 90 90 90
0 50.2 71.5 78.6 78.7
0.8 17.2 24.9 13.9 6.0
0 8.6 17.8 10.9 4.7
4
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) K2 S2 O8 , p0CH /MPa, T = 90 ◦ C, t = 20 h. 4
a
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). These results indicate that the gold particles were in the nanoscale when supported on SiO2 . 3.2. Methane conversion in the presence of nano-Au/SiO2 catalyst and IL [Bmin]Cl 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 H2 SO4 . 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.
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 without 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 indicated in Supplement information [2], the trifluoroacetic acid would exchange their anions with the IL addition. Moreover, the concentration of the trifluoroacetic acid would be diluted, and the acidity of the reaction system would be decreased, which is not favorable for electrophilic substitution. Consequently, the methane conversion rate and yield would decrease. 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. 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 conversion 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 liquid ([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. 3.3. Characterization of the products of methane conversion
Table 1 Influence of amount of IL on methane conversion. [Bmim]Cl/g
P/MPa
Gas
T/◦ C
Conversiona /%
Selectivitya /%
Yielda /%
1.0 0 0.5 1.0 1.5 2.0
2 2 2 2 2 2
N2 CH4 CH4 CH4 CH4 CH4
90 90 90 90 90 90
0 15.8 17.6 24.9 14.9 12.6
0 71.6 71.6 71.5 71.4 71.4
0 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) K2 S2 O8 , P = 2 MPa, T = 90 ◦ C, t = 20 h. a Calculated from Section 2.4.
3.3.1. IR characterization of the liquid products 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 homemade 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
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
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1#
1448.8
T( %)
1332.9
2#
4000.
3479.3
2922.4
3000
1175.0
1779.1
2000
1000
1500
400.
-1
/cm
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 indicates 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 produce 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 dispersed in an ionic liquid to form a heterogeneous catalyst system in acidic medium, the catalyst can be recycled easily. 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 evaporater. 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%. 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
3.3.3. NMR characterization of the liquid product The HNMR of the liquid product is shown in Fig. 5. 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 trifluoroacetate is actually transformed from the raw material methane. Furthermore, our results have proved that methane can be actis 59.0
22000
69.0
20000 18000
abundance
16000 14000
O
F
O
F
F
12000 10000 8000 6000 4000 50.0
2000
99.0 78.0
40.0
3
4
5
6
7
0 8
9
109.0
10
M/Z Fig. 4. Mass spectra of methyl trifluoroacetate.
11
126.9
12
13
154
T. Li et al. / Applied Catalysis A: General 398 (2011) 150–154
MN+
+
CH3
H
H+
MN+
4. Conclusion
CH3
Nu: -
OX
M(N-2)+
CH3
+
N
Fig. 6. The mechanism of electrophilic substitution.
2Au/IL + 2H+ + K2S2O8 CH4 + CF3COOH 2Au+/IL + 2e-
2Au+/IL + 2KHSO4
1
+
2
CF3COOCH3 + 2H 2Au/IL
+2e
-
3
Over the catalyst system of nano-gold (Au/SiO2 ) and IL, the process 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 system 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 sulfuric acid and heavy-metal catalysts, thus we can eliminate the impacts on the environment. A reaction mechanism was proposed, 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 system can also be used in other green chemical processes of liquid phase or gas phase oxidation.
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.
3.5. Discussion on reaction mechanism
Appendix A. Supplementary data
Methane can be activated and oxidized by means of three modes: (1) via formation of a methyl radical, (2) via oxidative addition, (3) via electrophilic substitution. Presently, the mechanism of electrophilic substitution is widely accepted, which is shown in Fig. 6. In our case, methane is first attacked by the metal ions (or other ions such as H+ ) as a strong electrophilic reagent to form a metallic intermediate. Then, this intermediate is attached to the nucleophilic reagent to form a stable species of CH3 –N, which is a low valence state metallic intermediate that can be oxidized by an oxidant to a high valence state. In this way, a complete catalytic-cycle will be formed. In the present study, the catalyst as an electrophilic reagent not only attacks the methane, but also activates the C–H bond. The trifluoroacetic acid, as a co-catalyst, provides the acid medium for the removing of the protons in methane, and then reacts with methanol to form methyl trifluoroacetate. Because of the strong electron effect of the CF3 COO− , it is difficult to be oxidized to form the product methyl trifluoroacetate, so the reaction can exhibit good selectivity. Potassium persulfate acts as the oxidant in the system. Gold is a uniquely efficient electrophilic catalyst for methane conversion [4]. According to the above-mentioned mechanism and 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 K2 S2 O8 (Eq. (1)). Au+ /IL was an active catalyst for the oxidation of methane in CF3 COOH to generate methyl trifluoroacetate (Eq. (2)). Afterwards, the catalyst is reduced back to Au/IL (Eq. (3)).
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