5940
J. Phys. Chem. B 2000, 104, 5940-5944
Reaction Mechanism of Oxidation of Methane with Hydrogen Peroxide Catalyzed by 11-Molybdo-1-vanadophosphoric Acid Catalyst Precursor Yasuhiro Seki, Joon Seok Min, Makoto Misono, and Noritaka Mizuno* Department of Applied Chemistry, Graduate School of Engineering, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: February 2, 2000; In Final Form: April 6, 2000
The reaction mechanism for the selective oxidation of methane with hydrogen peroxide was investigated for a H4PV1Mo11O40 catalyst precursor, which has been reported to be the most active among various Keggintype heteropolyacids and vanadium complexes in trifluoroacetic acid anhydride. The conversion vs selectivity relationships, the comparison of reactivities of products with methane, and kinetic results show that the first step, selective oxidation of methane into methanol or methyltrifluoroacetate, is rate-determining. The facts that the oxidation was much suppressed by the addition of a radical scavenger, that chlorocyclohexane was formed by the oxidation of cyclohexane in the presence of carbon tetrachloride, and that the epoxidation of cis-stilbene proceeded without retaining the stereochemistry show that the reaction includes a radical path. UV-vis data revealed that monoperoxomonovanadate is an active species, which would promote the selective oxidation of methane into methanol or methyltrifluoroacetate.
Introduction The activation and functionalization of methane has attracted much attention because of its abundance in natural gas and the low reactivity.1-11 Recently, there has been a renewed interest in the direct oxidation of methane into oxygenates. The use of various catalysts has been attempted with molecular oxygen and various peroxides in homogeneous systems.1-31 Hydrogen peroxide is a useful oxidant because of the simplicity of handling, the environmentally friendly nature of its coproduct (water), the high oxygen atom efficiency, and the versatility.32,33 Therefore, the oxidation of methane with hydrogen peroxide is interesting. Peroxovanadium complexes perform a variety of oxidation reactions.9,34 Mimoun et al. synthesized a covalent vanadium oxo peroxo complexes and applied them to “stoichiometric” oxidation reactions.35 Shul′pin et al. used them as “catalysts” for oxidation of alkanes (except for light alkanes) with hydrogen peroxide.36 Recently, we reported that Keggin-type vanadiumcontaining H4PV1Mo11O40 is an active catalyst precursor for selective oxidation.15 Vanadium-containing catalysts have also been reported to be active for the selective liquid-phase oxidation of methane with hydrogen peroxide.12-15 However, little is known of the active chemical species for the reaction catalyzed by vanadium-containing catalysts. In this study, we attempted to clarify the reaction mechanism including active species for the oxidation. Experimental Section Catalysts and Reagents. Keggin-type heteropolyacids (H3+xPVxMo12-xO40 (x ) 0, 1, 2, 3), H4PV1W11O40, and H5SiV1Mo11O40 (Nippon Inorganic Color and Chemical Co., Ltd.), sodium metavanadate (Nacalai Tesque, Inc.), vanadium metal (Mitsuwa Chemical Co., Inc.), vanadium oxytrifluoride (Fluka), vanadium acetylacetonate (Kanto Chemical Co., Inc.), vanadyl * To whom correspondence should be addressed.
sulfate pentahydrate (Nacalai Tesque, Inc.), and vanadium (V) oxide (Wako Pure Chemical Industries, Ltd.) were commercially obtained and used as received. Hydrogen peroxide (Wako Pure Chemical Industries, Ltd.), trifluoroacetic acid anhydride (Tokyo Chemical Co., Ltd.), perchloric acid (Wako Pure Chemical Industries, Ltd.), sodium perchlorate (Wako Pure Chemical Industries, Ltd.), chloric acid (Nacalai Tesque, Inc.), nitric acid (Nacalai Tesque, Inc.), sulfuric acid (Nacalai Tesque, Inc.), and the other solvents and reagents were also commercially obtained and used as received. Preparation of Monoperoxovanadium Complex, VO(O2)+, and Diperoxovanadium Complex, VO(O2)2-. Aqueous solutions of VO(O2)+ and VO(O2)2- were prepared according to refs 37 and 38, respectively, as follows: Sodium vanadate (2.5 × 10-3 M) was used as a V5+ source and perchloric acid (0.5 M) and sodium perchlorate (0.3 M) were used to control pH and to keep ionic strength 0.3 M at 25 °C. Initial concentrations for VO(O2)2- were [VO3-] ) 1.0 × 10-3 M, [H2O2] ) 2.0 × 10-3 M, pH ) 5.4, and those for VO(O2)+ were [VO3-] ) 1.0 × 10-3 M, [H2O2] ) 2.0 × 10-3 M, pH ) 0.3. Reaction. The oxidation of methane was carried out with the apparatus; the wall inside the stainless tube was coated with Teflon to avoid the progress of oxidations on the wall.39 The standard procedure was as follows. Specified amounts of catalyst precursors (5 µmol) and 36% hydrogen peroxide (2.4 mmol) were dissolved in 1.8 mL of trifluoroacetic acid anhydride in a Teflon vessel. Next, this Teflon vessel was quickly attached inside an autoclave. The gas phase above the liquid was removed by evacuation and then the autoclave was pressurized with methane to 50 atm. The autoclave was heated to 80 °C in an oil bath for 24 h. After the reaction, the autoclave was cooled to room temperature. Then the gas phase was sampled with the aid of a sampler directly connected to a gas chromatograph with Porapak QS and Molecular Sieve 5A columns.40 The liquid was sampled with the aid of a gastight microsyringe and analyzed on a gas chromatograph with Porapak QS and HayeSep DB columns. No increase in the concentration of methanol was
10.1021/jp000406y CCC: $19.00 © 2000 American Chemical Society Published on Web 05/31/2000
Oxidation of Methane with Hydrogen Peroxide
J. Phys. Chem. B, Vol. 104, No. 25, 2000 5941
TABLE 1: Selective Oxidation of Methane with Hydrogen Peroxide Catalyzed by Various V-containing Compoundsa selectivity (%)
b
no.
catalyst
conv. (%)
CH3OH
HCOOH
HCO2CH3
CF3CO2CH3
C2H6
CO2
yieldb (%)
1 2 3 4 5 6 7 8 9
H4PV1Mo11O40 V(metal) VOSO4‚5H2O VO(acac)2 VOF3 V2O5 H3PMo12O40 H5PV2Mo10O40 H6PV3Mo9O40
4.7 1.6 0.1 0.3 8.5 1.0 2.7 3.0 1.6
1 0 0 0 tr. tr. 0 0 0
15 0 0 0 0 0 0 0 0
72 59 0 51 7 87 0 24 41
5 8 43 49 tr 1 14 7 17
tr. 0 tr. tr. 92 0 tr. 2 0
7 33 57 tr. 1 12 86 67 42
4.4 1.1 tr. 0.3 0.5 0.9 0.4 0.9 0.9
a Reaction temp., 80 °C; reaction time, 24 h; methane, 50 atm (30.9 mmol); (CF3CO)2O, 1.8 mL; hydrogen peroxide, 2.4 mmol; catalyst, 5 µmol. Sum of yields of selective oxygenates (see text).
observed by the reduction of the solution with NaBH4 or PPh3, showing that no alkylperoxides were formed, whereas formation of methylhydroperoxide was reported for a [NBu4]VO3-pyrazine-2-carboxylic acid system.12-14 Conversions were calculated according to the following equation: conversion (%) ) 100 × (N1 + N2 + 2 × N3 + N4 + N5)/NCH4, where N1 - N5 and NCH4 were the concentrations in moles of methanol, formic acid, methylformate, methyltrifluoroacetate, carbon dioxide formed, and methane introduced, respectively. For NCH4, the solubility of CH4 in each solvent was taken into account.41 It was confirmed for a trifluoroacetic acid anhydride solvent that the carbon balance was above 95%. All runs with H4PV1W11O40 in a trifluoroacetic acid anhydride solvent were repeated twice and the error bars roughly estimated based on these two runs were within 7% of the averaged values. Titration of Hydrogen Peroxide. The titration of hydrogen peroxide was carried out according to ref 42. One to 2 g of solution were accurately weighed and quickly dissolved in 200 mL water. The solution was stirred with a magnetic stir bar at 23 °C. Titration data were obtained with an HM-30 pH meter (TOA Electrochemical Measuring Instruments). The potential was monitored as a solution of Ce(NH4)(SO4)4‚2H2O in water (0.1 M) was added with a buret into the solution in 0.1 mL intervals. Measurements of UV-vis Spectra. UV-vis spectra were recorded on a UV-vis scanning spectrophotometer (Shimadzu UV-2100PC). A 2.5 mM reaction solution of each catalyst was sampled and the solution was diluted with 400 µL acetonitrile (0.5 mM). Then the UV-vis spectrum of the solution was measured at 25 °C. Results and Discussion Oxidation of Methane Catalyzed by H4PV1Mo11O40. Products were methylformate, formic acid, methyltrifluoroacetate, methanol, and carbon dioxide. Similar products were observed for the other V-containing catalysts used. The main products for H4PV1Mo11O40 were methylformate and formic acid within 24 h. The conversion linearly increased and reached the constant value after 24 h. Conversions and selectivities for various V-containing catalysts are summarized in Table 1. The sum of yields of selective oxygenates (i.e., CH3OH, HCOOH, HCOOCH3, and CF3COOCH3), for H4PV1Mo11O40 was the highest. The conversion and selectivity depended on reaction temperatures for H4PVMo11O40. The amounts of products significantly increased with an increase in the reaction temperature from 50 to 80 °C. The amount of each product was decreased by the increase of the reaction temperature to 100 °C. The decomposition of hydrogen peroxide to form dioxygen and water
Figure 1. Arrhenius plots for the selective oxidation of methane. Reaction conditions: reaction time, 24 h; H4PV1Mo11O40, 2.5 mM; hydrogen peroxide, 1.2 M.
was dominant. This may result in the decrease of the amounts of products. Figure 1 shows the Arrhenius plots for H4PVMo11O40. A fairly good linear correlation was observed. The apparent activation energy was 65 kJ‚mol-1. The value is fairly close to 64 kJ‚mol-1 for the liquid-phase oxidation of methane catalyzed by CuCl2 + K2PdCl4 with molecular oxygen reported by Sen et al.27 For comparison, the activation energy for hydrogen abstraction from methane in the oxidative coupling reactions was in the range 105-209 kJ‚mol-1.43 The dependence of rates on concentrations of H4PVMo11O40 is shown in Figure 2. The conversion increased up to 1.3 mM of [H4PVMo11O40] and then decreased. Such catalyst-inhibitor inversion is attributable to chain radical oxidation processes and is explained by the participation of the catalyst in chain termination.44 Reaction Scheme. Figure 3 shows dependences of selectivities of products on percent conversion of methane. The selectivity to CF3COOCH3 + CH3OH extrapolated to 0% conversion was ca. 100%, showing that there is one path in the first step, CH4 f CF3COOCH3 + CH3OH. The selectivity sharply decreased with an increase in conversion, while the selectivities to HCOOH increased, showing the progress of oxidation of CF3COOCH3 and CH3OH to HCOOH. Then the selectivity to HCOOCH3 increased while that to HCOOH slightly decreased, showing the progress of reaction of HCOOH with CF3COOCH3 or CH3OH to produce HCOOCH3. Finally, the selectivity to CO2 gradually increased. This shows the progress of oxidation of HCOOH and its derivative to CO2.
5942 J. Phys. Chem. B, Vol. 104, No. 25, 2000
Seki et al. TABLE 2: Oxidation of CH3OH, HCHO, and HCOOHa temp (°C)
time (h)
conv. (%)
CH3OH
25 80
0.5 24
93 100
HCOOH
25 80
0.5 24
0 100
substrate
product (selectivity) (%) HCOOH (90) HCHO (10) CH2 (tr.) HCOOH (69) CO2 (10) HCOOCH3 (8) CO (3) Othersb (10) CO2 (tr.) CO2 (81) C2H6 (5) CO (2)
a He, 50 atm; substrate, 1400 µmol; solvent, 1.8 mL; hydrogen peroxide, 2.4 mmol; H4PV1MO11O40, 5 µmol. b CF3COOCH3, C2H5OH, and C2H6.
Figure 2. Dependence of conversion on [H4PV1Mo11O40]. Reaction conditions: 80 °C, 24 h.
Figure 4. UV-vis spectrum of solutions before and after reaction. (a) Before reaction; (b) after reaction. Reaction conditions: 80 °C, 24 h. The reaction solution was diluted with CH3CN (reaction solution/ CH3CN ) 4/1).
SCHEME 1. Reaction Scheme for Oxidation of Methane in a H4PVMo11O40 + H2O2 + (CF3CO)2O System
Figure 3. Dependence of selectivities to products on percent conversion of methane. B, n, 9, and 4 are selectivities to CH3OH + CF3COOCH3, HCOOH, HCOOCH3, and CO2 respectively. The solid lines were obtained by the simulation (see text).
Table 2 compares reactivities of CH3OH and HCOOH. Even at 25 °C, CH3OH was completely oxidized to HCOOH with hydrogen peroxide. Formaldehyde was also completely oxidized to HCOOH with hydrogen peroxide at 25 °C. HCOOH was hardly oxidized with hydrogen peroxide at 25 °C, but mainly oxidized to CO2 at 80 °C (i.e., the standard reaction temperature for CH4 oxidation). These facts show that the oxidation reactions of CH3OH to HCOOH are much faster than that of HCOOH to CO2, and that these two oxidation reactions are faster than that
of CH4. On the basis of the above results, the following Scheme 1 could be proposed. The relative rate constants (k1 - k5 in Scheme 1) were estimated from the conversion-selectivity correlation in Figure 3, assuming the first-order reactions with respect to organic molecules for all steps in Scheme 1 and neglecting the small change of [H2O2] by its consumption. The kinetic simulation gave the solid lines with k1:k2:k3:k4:k5 ) 1.0 (taken as unity): 1.5 × 102:6.6:2.4 × 104:1.7 × 101, respectively, and the good fits with experiment data well support the idea that the oxidation proceeds according to Scheme 1 and that the first step is rate-determining. Characterization of Active Species. Active species for the oxidation of methane catalyzed by the H4PV1Mo11O40 catalyst precursor have been characterized by UV-vis spectroscopy because measurements of 31P and 51V NMR spectra had been unsuccessful due to the low concentration. Figure 4a and b shows UV-vis spectra of solutions of H4PV1Mo11O40 before and after reactions, respectively. In Figure 4a a band was observed at 308 nm characteristic of PV1Mo11O404( ) 21 600 M-1cm-1). The band intensity much decreased and a new band appeared at 447 nm with a shoulder at ca. 290 nm
Oxidation of Methane with Hydrogen Peroxide
J. Phys. Chem. B, Vol. 104, No. 25, 2000 5943
Figure 5. Changes in band intensity at 447 nm and concentration of hydrogen peroxide with reaction time. Reaction conditions: 80 °C, 24 h. B, Band intensity at 447 nm; n, [H2O2].
in Figure 3b. The band at 447 nm was also observed for the other V-containing catalysts such as H5PV2Mo10O40, VO(acac)2, and V metal active for the selective oxygenation of methane. The most intense band was observed for H4PV1Mo11O40. The band at 447 nm disappeared when H2O2 was completely consumed, as shown in Figure 5, suggesting that the band is assigned to a peroxo species. No mono-, di-, and tri-peroxomonomolybdates showed bands in the region 400-500 nm.38,43,45 Tetraperoxomonomolybdate shows the UV-vis band at 450 nm.45 However, this compound is stable only at pH 5-12, much higher than pH 1-2 of the present system, suggesting that tetraperoxomonomolybdate is unstable in the present case. Therefore, the band at 447 nm is most probably assigned to a peroxovanadate species. Vanadium (V5+) exists as VO2+ in an acidic solution (pH e 2).38,44-49 It has also been reported that peroxovanadates of VO(O2)+ and VO(O2)2- are formed by the addition of H2O2 to VO2+ according to the eqs 1 and 2.38,47,49
VO2+ + H2O2 f VO(O2)+ + H2O
(1)
VO(O2)+ + H2O2 f VO(O2)2- + 2H+
(2)
These compounds show absorption bands at 455 and 330 nm, respectively.38,44-49 In addition, VO(O2)+ is stable at the pH value of the present reaction solution.38,44-49 The band position of VO(O2)+ is very close to 447 nm, as shown in Figure 4b. The slight change of the band position is probably caused by the coordination of trifluoroacetate or water. In fact, the band position of VO(O2)+ at 453 nm was shifted to 446 nm by the addition of trifluoroacetic acid. These facts show that the band at 447 nm in Figure 4 is assigned to VO(O2)+ formed according to eq 3.
PVMo11O404- + H2O2 f PMo11O397- + VO(O2)+ + 2H+ (3) The PMo11O397- polyoxometalate shows the characteristic band at 290 nm ( ) 20 600 M-1cm-1).50 The band position is in agreement with that of a shoulder band in Figure 4b. Therefore, the shoulder in Figure 4b is likely assigned to PMo11O397and the concentration calculated was 0.41 M. The calculated concentration of VO(O2)+ was ca. 0.5 M.35 These values were
Figure 6. Correlation between conversions and UV-vis band intensities at 447 nm. For the sample numbers attached, see Table 1. Reaction conditions: 80 °C, 24 h, catalyst; 5 µmol. There are two data for H4PV1Mo11O40 with the weights of 5 and 2.5 µmol.
in approximate agreement with the initial concentration of H4PV1Mo11O40, 0.5 M. Therefore, most of PVMo11O404- polyoxometalates decompose to form PMo11O397- and VO(O2)+. A solid sample was prepared by evaporation of the used reaction solution to dryness. The infrared spectrum (KBr disk) also shows the presence of PMo11O397- (1079, 1052, 950, 909, 869, 830, 815, and 760 cm-1),50 supporting the idea. The band at 447 nm steeply increased and then reached the constant value in Figure 5. These facts suggest that the reaction of H4PV1Mo11O40 with H2O2 to form PMo11O397- and VO(O2)+ is fast and that the reaction of VO(O2)+ with methane is slow. Figure 6 shows correlation between yields of the selective oxygenates and the concentration of VO(O2)+ calculated by the band intensities at 447 nm for vanadium-containing catalysts. The concentration of VO(O2)+ increased with an increase in the yields, supporting the idea that chemical species producing the band at 447 nm correspond to an active species for the selective oxygenation of methane. The lower solubilities of V2O5, VOSO4‚5H2O, V(metal), H5PV2Mo10O40, H6PV3Mo9O40, H4PV1W11O40, and H5SiV1Mo11O40 than H4PVMo11O40 into the reaction solution are probably related to the weaker band intensities. The solubilities of VOF3 and VO(acac)2 were rather high, but showed low activities. This is because VO(O2)+ could not be formed at pH 2-3 of reaction solutions of VOF3 and VO(acac)2, respectively. Next, the role of coexisting heteropolyanion is discussed. Aqueous solutions containing sodium metavanadate and heteropolyacids or various acids were prepared. The pH of solutions was kept at 2.0. The band intensities of VO(O2)+ decreased in the order of H3PMo12O40 > HClO4 . HNO3 ≈ H2SO4 > HCl ) 0. Only a band of VO(O2)2- was observed for HCl. The results suggest that heteropolyanions can promote the formation of VO(O2)+ from VO3- and H2O2 or stabilize VO(O2)+. Here we used PMo12O403- instead of PMo11O397- because of the stability of polyanions in this aqueous solution. But PMo11O397is considered to be stable in organic reaction solution.50 To provide evidence for or against radical mechanism, oxidation of cis-stilbene with hydrogen peroxide was performed. The oxidation gave cis-stilbene oxide, trans-stilbene oxide, benzaldehyde, benzophenone, benzil, and benzyl phenyl ketone, and cis-stilbene with percent selectivities of 11:45:9:3:8:24,
5944 J. Phys. Chem. B, Vol. 104, No. 25, 2000
Seki et al.
TABLE 3: Radical Trapping Experiment with CCl4a b
c
conversion (%)
product
12
Chlorocyclohexane Cyclohexanol Cyclohexanone Bicyclohexyl
b
selectivity (%) 83 8 8 1
a Reaction temp, 30 °C; reaction time, 24 h; substrate, cyclohexane (1400 µmol); solvent, (CF3CO)2O (1.8 mL); hydrogen peroxide, 2.4 mmol; H4PV1Mo11O40, 5 µmol; carbon tetrachloride, 1.4 mmol. b Cyclohexane-basis. c 510 µmol carbon dioxide was produced.
respectively. The approximately 19:81 cis/trans epoxide ratio shows that the present system is rather nonstereospecific, supporting the idea that radical processes prevail to a major degree.51 The reaction of methane was much suppressed by the addition of a radical scavenger, hydroquinone, also supporting the idea.52,53 Cyclohexyl chloride was formed when the oxidation of cyclohexane by H4PV1Mo11O40 was carried out in a 1:1 (CF3CO)2O-CCl4 mixture (Table 3), indicating that cyclohexyl radicals are produced and intercepted by chlorine atoms coming from CCl4.35,54 This also supports the idea that radical processes prevail to a major degree for the oxidation of methane in the present system. The radicals formed may be [VO(O2)]•+, •OH, or the derivatives from the reaction with methane, according to the literature.13,34,35 Conclusions To summarize, the above results demonstrate: (1) The catalytic oxidation of methane with hydrogen peroxide on the H4PVMo11O40 catalyst precursor; (2) that the oxidation mainly proceeds in the following scheme including a radical-chain mechanism, CH4 f CH3OH, or CF3COOCH3 f HCOOH f CO2, where the first step is rate-determining; (3) that H4PVMo11O40 decomposes to form VO(O2)+ and PMo11O397during the catalytic test; (4) and that VO(O2)+ is an active species. Acknowledgment. We acknowledge Mr. I. Kiyoto (The University of Tokyo) for the experimental assistance of oxidation of cis-stilbene. This work was supported in part by a Grantin-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. References and Notes (1) Gesser, H. D.; Hunter N. R.; Prakash, C. B. Chem. ReV. 1985, 85, 235. (2) Pitchai R.; Klier, K. Catal. ReV.-Sci. Eng. 1986, 28, 13. (3) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Chem. Soc. ReV. 1989, 18, 251. (4) Srivastava, R. D.; Zhou, P.; Stiegel, G. J.; Rao, V. U. S.; Cinquegrane, G. Catalysis (London) 1992, 9, 1993. (5) Fierro, J. L. G. Catal. Lett. 1993, 22, 67. (6) Axelrod, M. G.; Gaffney, A. M.; Pitchai, R.; Sofranko, J. A. Natural Gas ConVersion II; Elsevier: New York, 1994; p 93. (7) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (8) Srivastava, R. D.; Gollakota, S. V.; Stiegel, G. J.; Bose, A. C. Methane and Alkane ConVersion Chemistry; Plenum: New York, 1995; p 291. (9) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley: New York, 1995. (10) Labinger, J. A. Fuel Process Technol. 1995, 42, 325. (11) Shilov, A. E.; Shul′pin, G. B. Chem. ReV. 1997, 97, 2879. (12) Nizova, G. V.; Su¨ss-Fink, G.; Shul′pin, G. B. Chem. Commun. 1997, 397.
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