Energy & Fuels 2006, 20, 915-918
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Methane Oxidation over a V2O5 Catalyst in the Liquid Phase Liyu Chen,*,†,‡ Bolun Yang,*,† Xiucheng Zhang,‡ Wu Dong,‡ Kai Cao,† and Xiaoping Zhang‡ School of Energy and Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, People’s Republic of China, and School of Chemical Engineering, Northwest UniVersity, Xi’an 710069, People’s Republic of China ReceiVed August 27, 2005. ReVised Manuscript ReceiVed March 11, 2006
The selective oxidation of methane by V2O5 catalysts in oleum has been studied. The effects of the reaction temperature, the concentration of V2O5, and the residence time on the conversion of methane and the yield of methanol were investigated. We have found that methane was transformed to methyl bisulfate first, and the methyl bisulfate was then hydrolyzed to methanol. The process of the selective oxidation of methane was involved in electrophilic alternative mechanisms by V2O5 catalysts, and the sulfur trioxide in oleum was necessary for the reaction systems. The methane oxidation reaction was determined to be a first-order reaction, based on the pressure-time relationship. The maximum methanol yield of 45.5% and the methane conversion of 54.5% were obtained at a V2O5 concentration of 0.0175 mol, a reaction temperature of 453 K, a methane pressure of 4.0 MPa, a reaction time of 2 h, and the sulfur trioxide content in oleum is ∼50%.
1. Introduction The selective catalytic oxidation of methane under mild conditions is one of the most challenging industrial catalysis problems, because of the great practical importance of this reaction. The economic process that involves the selective oxidation of methane to its oxygenated derivatives is rather limited the literature. Shilov et al. studied the oxidation of methane by platinum complexes in water.1 After that, various catalyst systems, such as Pd(II), Co(III), Cu(II), Rh(III), and Eu(III), have been investigated in aqueous solution.1-5 The conversion is low in aqueous solution, despite the high selection. Recently, several new processes that utilize electrophilic late transition metals in strong acid media have been developed. Periana et al. found that methane was converted to methanol by Hg2SO4 catalyst in sulfuric acid.6 The yield of methanol attained was 43% (85% * Author to whom correspondence should be addressed. Tel.: +86-2988302633. E-mail:
[email protected] (L.C.). † Xi’an Jiaotong University. ‡ Northwest University. (1) Shilov, A. E.; Shulpin, G. B. Activation of C-H bonds by metal complexes. Chem. ReV. 1997, 97, 2879-2932. (2) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities. Chem. ReV. 2001, 101, 953-996. (3) Istva, T. H.; Rayamond, A. C.; John, M. M. Low-temperature methane chlorination with aqueous platinum chlorides in the presence of chlorine. Organometallics 1993, 12, 8-10. (4) Lin, M.; Hogan, T. E.; Sen, A. Catalytic carbon-carbon and carbonhydrogen bond cleavage in lower alkanes. J. Am. Chem. Soc. 1996, 118, 4574-4580. (5) Bjerrum, N. J.; Xiao, G.; Hjuler, A. A process for the catalytic oxidation of hydrocarbons. World Patent Application No. WO 24,383, 1999. (6) Chen, L.; Yang, B.; Zhang, X.; Dong, W. Study of catalysis oxidation of methane to methanol in liquid phase. J. Chem. Eng. Chin. UniV. (in Chin.) 2005, 19, 54-58.
selectivity at 50% conversion). It is reported that Pt(bmpy)Cl2 complexes can efficiently catalyze methane to methanol in oleum.7 The selectivity of methanol reached 80% at a conversion of 90%. Powdered palladium and iodine catalysts in oleum also have been discussed.8-11 The similar reactions can be conducted in a variety of strong acid media (for example, in 100% sulfuric acid and trifluoroacetic acid). However, some problems were observed in liquid-phase systems: the conversion is low in aqueous solution; serious environmental pollution will occur when Hg2SO4 is used as a catalyst in sulfuric acid; the yield is not satisfactory for the powdered palladium catalyst in oleum; and the catalyst complexes for the Pt(bmpy)Cl2 system in oleum are very expensive. In this work, a new method for the partial oxidation of methane using transition-metal oxides in oleum was developed. The methane was oxidized by V2O5 in oleum. The reaction order was confirmed, and the mechanism of reaction with the V2O5 catalysis was studied. The effect of parameters, such as the amount of catalyst, reaction temperature, reaction time, and oleum concentration on the oxidation of methane was investigated. 2. Experiment 2.1. Material. Methane (99.99%) and argon were purchased from Beijing AP Gases Co., Ltd. (PRC). Oleum (with 50 wt % and 20 (7) Periana, R. A.; Doube, J. T.; Evitt, R. E.; Lottler, D. G.; Wentrcek, P. R.; Voss, G.; Masuda, T. A mercury-catalyzed high-yield system for the oxidation of methane to methanol. Science 1993, 259, 340-343. (8) Periana, R. A.; Doube, J. T.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Platinum catalysts for the high-yield oxidation of methane to methanol derivation. Science 1998, 280, 560-564. (9) Michalkiewicz, B.; Kalucki, K.; Sosnicki, J. G. Catalytic system containing metallic palladium in the process of methane partial oxidation. J. Catal. 2003, 215, 14-19. (10) Lin, M.; Hogan, T.; Sen, A. A highly catalytic bimetallic system for low-temperature selective oxidation of methane and lower alkanes with dioxygen as oxidant. J. Am. Chem. Soc. 1997, 119, 6048-6053. (11) Xiao, G.; Yimin, Z.; Henning, B. Iodine as catalyst for the direct oxidation of methane to methyl sulfates in oleum. Appl. Catal., A 2004, 261, 91-98.
10.1021/ef050280+ CCC: $33.50 © 2006 American Chemical Society Published on Web 04/18/2006
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Figure 1. Methane conversion versus the amount of V2O5 catalyst: (O) conversion and (4) selectivity. Conditions: pressure, 4.0 MPa; temperature, 473 K; 70 mL oleum (50 wt % SO3); stirred for 3 h.
wt % SO3), as well as H2SO4 (98 wt %), were purchased from Beijing Chemical Reagent Co., Ltd. (PRC). V2O5 was purchased from the Reagent Factory of the Hunan Coal Institute (PRC). 2.2. Experimental Methods. The oxidation reaction of methane was conducted in a 250-mL autoclave that was equipped with a magnetic stirring. The reactor was loaded with transition-metal oxide catalysts (3.5-24.5 mmol) and 70 mL of oleum. The autoclave was degassed with methane three times and then was pressurized to 4.0 MPa with methane. The reactor was maintained at 453 K for 3 h before cooling. A known volume of the reaction mixture then was hydrolyzed at 363 K for 3 h in a sealed vial by the addition of 20-times-distilled water to one part crude reaction solution. 2.3. Product Identification. The reaction products were divided into two parts: methyl bisulfate and methanol in the liquid phase, and some compounds in gaseous phase (such as CH4, CO2, and CO).12 It is difficult to use methane as the basis for the conversion calculation. Thus, argon was added to methane as the inert gas. Both the gaseous products and liquid products were analyzed, respectively. Moreover, the hydrolysate was also analyzed as the reference. The yield was calculated based on methane. The gas products were analyzed via gas chromatography (GC), using a TCD detector (Tianmei model GC7890II, equipped with a Hayesep D column). The liquid products were identified via the use of a 1H nuclear magnetic resonance (1H NMR) spectrometer (Bruker model AM 500). A known amount of CH3NO2 was added to an aliquot of reaction solution as an internal standard and D2O was used as an external lock and reference.12 The hydrolysate was analyzed via GC, using a TCD detector (Tianmei GC7890II, equipped with a GDX102 column).
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Figure 2. Methane conversion versus temperature: (O) conversion and (4) selectivity. Conditions: pressure, 4.0 MPa; 70 mL oleum (50 wt % SO3); V2O5 amount, 0.0175 mol; stirred for 3 h.
Figure 3. Methane conversion versus reaction time: (O) conversion and (4) selectivity. Conditions: pressure, 4.0 MPa; temperature, 473 K; 70 mL oleum (50 wt % SO3); V2O5 amount, 0.0245 mol. Table 1. Effect of SO3 Concentration on the Oxidation of Methanea w(SO3) (%)
V2O5 amount (mol)
conversion (%)
selectivity (%)
50 20 0
0.0175 0.0175 0.0175
54.5 12.1 3.0
83.5 81.0 80.0
a The reaction conditions are as follows: T ) 453 K, P CH4 ) 4.0 MPa, stirred for 3 h.
3.1. Effect of Process Condition on Methane Oxidation. The V2O5 catalyst was used for the oxidation of methane, because it is one of the most common catalysts for oxidation reactions. The process conditions, including the catalyst and oleum concentrations, reaction temperature, and time for oxidation of methane in oleum were studied in this work. The effect of the amount of V2O5 catalyst, reaction temperature, and reaction time on the oxidation of methane are shown in Figures1-3, and Table 1 shows the effects of the amount of V2O5 catalyst and the oleum concentration on the oxidation of methane, in terms of conversion and selectivity. As shown in Figure 1, the methane conversion increased as the amount of V2O5 increased, and obviously, the selectivity of
methanol was decreased. The optimal conversion and selectivity was obtained at a V2O5 amount of 0.0175 mol. As shown in Figure 2, the effect of reaction temperature on the selective oxidation of methane is considerably significant. As the reaction temperature increased, the conversion of methane increased and the selectivity of products decreased. The yield of methanol was highest at a temperature of 453 K. The results showed that, as the reaction time increased, the conversion of methane was increased and the selectivity of products was decreased. The optimal reaction time was determined to be 2 h. As shown in Table 1, there is a huge effect of oleum concentration on methane oxidation. As the oleum concentration decreases, the conversion decreases rapidly. The function of free sulfur trioxide in oleum is very important, in regard to the oxidation of methane. 3.2. Proposed Methane Selective Oxidation Mechanism. The electrophilic alternative mechanism for methane oxidation in strong acid was proposed by Periana7 and it has been widely accepted.13 The V2O5 catalyst is a typical transition-metal
(12) Guochuan, Y.; Zuwei, X.; Guoying, C. Selective oxidation of methane by some inorganic salts in liquid phase. J. Catal. (in Chin.) 1997, 18, 402-405.
(13) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Homogeneous oxidation of alkanes by electrophilic late transition metal. Angew. Chem., Int. Ed. 1998, 37, 2180-2192.
3. Results and Discussion
Methane Oxidation oVer V2O5 in Liquid Phase
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Figure 4. Catalytic cycle for the oxidation of methane by V2O5 in oleum.
catalyst. The catalytic process of methane in oleum by V2O5 can be considered to belong to an electrophilic alternative mechanism. The mechanism for the oxidation of methane by V2O5 in oleum may be presented as shown in Figure 4. The intermediate CH3OSO3H, produced as depicted in Figure 4, will deactivate because of the electron-withdrawing OSO3H group under the reaction conditions.14 In strong acid media, the reaction of methane and acid leads to the methyl ester, which does not readily undergo oxidation. Subsequently, as a result of the ester hydrolysis, methanol is formed and the systems avoid the over-oxidation of methane. A key role of the selective oxidation of methane is to bond the methane that is formed as methyl ester, thereby retarding its further oxidation. Oleum that contains free sulfur trioxide is a stronger oxidant than sulfuric acid. The catalytic process relies on the formation of the alkyl-V5+ intermediate, which reacts with sulfuric acid. The C atom undergoes oxidation to methyl bisulfate, while the V5+ cation is reduced to V3+. The sulfur trioxide that is presented in the reaction mixture causes the reoxidation of V3+ to V5+. Thus, the selective oxidation of methane to methanol can be represented by the following equation:
Figure 5. Reactor pressure versus reaction time: (A) heating stage, (B) reaction stage, and (C) point at which the reaction has reached equilibrium.
Figure 6. Plot of ln(P - Pe) versus reaction time t at a temperature of 473 K.
V2O5
CH4 + H2SO4 + SO3 98 CH3OSO3H + H2O + SO2 3.3. Model of the Methane Oxidation Reaction. Generally, the kinetics of methane oxidation can be described as
-
dpCH4 dt
n ) kC moleum P CH 4
where PCH4 is the pressure of the methane and Coleum is the concentration of oleum at time t. The parameter k represents a constant that is associated with the reaction rate. The Coleum term can be treated as a constant, because the amount of oleum is present in excess; thus, the concentration of oleum can be lumped into the reaction rate constant. Methane is the major component in the gas mixture inside the autoclave. Therefore, the partial pressure of methane can be treated as a constant times the total pressure of the gas mixture, and this constant can also be lumped into the reaction rate constant k′; thus, the kinetics can be reduced to
-
dpCH4 dt
n ) k′P CH 4
Figure 5 shows a typical plot of reaction pressure versus reaction time. Initially, the reaction pressure increases with the reaction temperature. After the agitation begins, the reaction pressure decreases, because methane is consumed by the (14) Sen, A.; Benvenuto, M. A.; Lin, M.; Hutson, A. C.; Basickes, N. Activation of Methane and Ethane and Their Selective Oxidation to Alcohols in Protic Media. J. Am. Chem. Soc. 1994, 116, 998-100.
Figure 7. Arrhenius plot for the dependency of k on temperature T.
reaction. Finally, reaction equilibrium is attained and the pressure curve becomes flat. The experiment is terminated by stopping the heating and stirring, and then cooling the reactor with cooling air. The reaction pressure could be described by the relation P - Pe, where Pe is the pressure when the reaction reaches the equilibrium. Our data agrees with the work of Sen et al.,14 that methane oxidation is a first-order reaction. The plot of ln(P Pe) vs t at a temperature of 473 K is shown in Figure 6 in the reaction stage B, where a straight line is obtained. (See Figure 5.) The straight line can be represented by the equation y ) -0.0245x + 1.2038. The slope of the straight line, k′ ) 0.0245,
918 Energy & Fuels, Vol. 20, No. 3, 2006
is the reaction rate value for the respective reactions. The methane oxidation reaction model is given by
-
dp dt
) 0.0245P
The activation energy was determined by the equation (K-1)
( )
k ) k0 exp -
Ea RT
The value of k increased from 0.0077 h-1 to 0.0256 h-1 as the temperature was increased from 433 K to 483 K. The dependency of k on temperature followed an Arrhenius-type function (see Figure 7). The values of the Arrhenius constant (k0) and
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the activation energy (Ea) were determined to be 2.22 × 103 h-1 and 45.2 kJ K-1 mol-1, respectively. 4. Conclusion We have demonstrated that methane can be selectivity oxidized to methanol over a V2O5 catalyst in an oleum solution. The optimum process conditions are as follows: CH4 pressure, 4.0 MPa; reaction temperature, 453 K; reaction time, 2 h; concentration of V2O5, 0.0175 mol; and the concentration of SO3 in oleum should be no less than 50%. Free sulfur trioxide in oleum is necessary for the oxidation of methane by a V2O5 catalyst to occur. Our data support the electrophilic alternative mechanism for this reaction. The lumped kinetic model of methane oxidation has been developed. EF050280+
