Pd(OAc)2 and chloranil-catalyzed oxidation of low-concentration coal mine gases to methanol
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Abstract: A self-made experimental system was used to study the catalytic oxidation of low-concentration coal mine gases to methanol in acetic acid solution.With Pd(OAc)2 as catalyst, addition of p-benzoquinone or chloranil in the reaction system provides a beneficial environment for activation of methane, and chloranil shows a more positive effect.Chloranil amount and reaction temperature and pressure are the key factors influencing the catalytic properties.The target product yield has a positive relationship with these three factors.CH3OH is formed from the reaction of generated H2O2 and CH4.CH3COOCH3 is generated via two routes;one is direct oxidation of Pd2+ and CH4, the other is esterification of CH3OH and CH3COOH.
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Key words:
- palladium acetate /
- chloranil /
- mine gases /
- methanol
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Although mine gases, as one of coal accessories, may cause destructive disaster for coal mines and severe air pollution, they can be used to serve as clean energy and manufacture chemicals[1-3]. The mine gases explosion accident is often avoided by enhancing their extraction. However, this makes the extracted mine gases with low concentration of CH4 (＜30%), thus, majority of which are directly discharged into air due to lack of economic utilization technology. This not only leads to severe air pollution, but also waste significant amounts of carbon resource[1,4,5]. Therefore, it is necessary and important to utilize these low-concentration mine gases. An interesting approach is to convert them into methanol by liquid-phase catalytic oxidation[6,7]. Liquid-phase catalytic oxidation allows the reaction to be conducted under mild conditions by means of solvation. It has been shown that noble metal catalysts, such as Pt, Pd and Rh, give high methane conversion in acidic medium[8-11]. Mukhopadhyay et al[12] synthesized CH3SO3H by reacting SO2 and CH4 in H2SO4 solution in the presence of PdCl2-CuCl2 catalyst. Periana et al[13] obtained a high yield of methanol through liquid-phase partial oxidation of methane in oleum by using SO3 and Pt(bpym)Cl2 as oxidant and catalyst respectively. The catalytic effect of metallic palladium dissolved in oleum on the conversion of methane was investigated by Michalkiewicz et al[14]. With the biological oxidation knowledge, An et al[15] proposed an electron-transfer loop with Pd2+ as active site for oxidation of methane in CF3COOH. In this way, methane was selectively oxidized to methanol at 80℃ and 5.5MPa. Yuan et al[16] found that methane can also be selectively oxidized to methanol in CF3COOH with Pd(OAc)2/benzoquinone/H5PMo10V2O40 as catalyst. Xu et al[17-20] preliminarily studied the liquid-phase oxidation of methane or mine gases to methanol over Pd(OAc)2 or Pd-CuPc/Y catalyst in CH3COOH solution. However, the above studies center on the conversion of pure methane or mine gases with high concentration of methane. Liquid-phase catalytic oxidation of mine gases having low content of methane has not been reported yet. Thus, catalytic oxidation of mine gases with low concentration of methane (20%, volume ratio) to methanol over palladium acetate and chloranil are studied here.
1. Materials and methods
1.1 Materials
The model mine gases (φ(CH4) = 20%, φ(O2) = 5%, φ(N2) = 75%) were purchased from Tongda Industrial Gas Sales Department, Lanxi County, Heilongjiang Province. Carbon monoxide (CO, 99.999%) was bought from Harbin Liming Gas Group. Palladium acetate (AR) and chloranil (AR) were purchased from Sinopharm Chem. Reagent Co., Ltd.. Acetic acid (AR) and 3%Pd/C were ordered from Tianjin Regent Chem. Co., Ltd. and Shaanxi Kaida Chem. Eng. Co., Ltd. respectively. Chloranil (98%) was provided by Aladdin Ind. Co..
1.2 Experimental apparatus
The self-made experimental apparatus for liquid-phase catalytic oxidation of model mine gases is shown in Figure 1. The effective volume, agitation speed, and maximum working temperature and pressure of the reactor are 100mL, 0-1000r/min, and 230℃ and 15MPa. The agitation torque is 1.2Nm.
Figure 1. Schematic diagram of experimental apparatus1.3 Experimental
First, a certain amount of palladium acetate or 3% Pd/C, stoichiometric p-benzoquinone or chloranil, and 30mL of acetic acid were added and sealed into the reactor. Then, it was purged with model mine gases for three times. After that, 0.1MPa carbon monoxide was filled. This was followed by filling model mine gases until reaching the required pressure. Finally, the reaction mixture was heated to the designed temperature, and reacted for 3h under agitation conditions. After reaction, the reactor was cooled to room temperature. The gas product was analyzed by a GC9790 gas chromatograph equipped with a packed column (TDX-01) and a thermal conductivity detector (TCD). The liquid product was analyzed by another GC9790 gas chromatograph equipped with a capillary column (KB-5, 50m × 0.25μm × 0.25μm) and a flame ionization detector (FID). The external standard method was used for analyses.
2. Results and discussion
2.1 Effects of p-benzoquinone and chloranil
In the Pd(OAc)2-catalyzed reaction system, p-benzoquinone is a co-oxidant. This is supported by out result obtained in the acetic acid system that p-benzoquinone is an indispensable additional oxidant for liquid-phase catalytic oxidation of methane to methanol[17,20]. An et al[21]found that chloranil was more stable than p-benzoquinone in the acidic system, and also beneficial to the catalytic conversion of methane. To investigate the effects of p-benzoquinone and chloranil on the catalytic oxidation of low-concentration mine gases, the reaction was carried out with two palladium catalysts, viz. 3%Pd/C and Pd(OAc)2, under the following conditions: reaction temperature of 140℃, reaction time of 3h, 1000μmol of p-benzoquinone or chloranil, 100μmol of Pd(OAc)2 or 2g of 3% Pd/C, and gas pressure of 4 MPa. Table 1 summarizes the catalytic results. CH3COOCH3 and CH3OH were detected in the liquid product. The target product was expressed in the form of CH3OH as CH3COOCH3 can be hydrolyzed into equivalent amounts of CH3OH. Thus, the CH3OH yield displayed in Table 1 includes both CH3COOCH3 and CH3OH.
Table 1. CH3OH yield obtained over the Pd(OAc)2 and 3%Pd/C in the oxidation of mine gasesEntry Reaction system CH3OH yield m/μmol 1 3%Pd/C-p-benzoquinone -CO 105 2 3%Pd/C- chloranil -CO 120 3 Pd(OAc)2-p-benzoquinone -CO 352 4 Pd(OAc)2- chloranil -CO 385 | Show Table
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Table 1 shows that selective oxidation of low-concentration mine gases to methanol can occur in all the four reaction systems, and chloranil has more positive effect than p-benzoquinone regardless of the used catalyst.
2.2 Effect of chloranil amount
Figure 2 shows the methanol yield obtained in the Pd(OAc)2-chloranil-CO reaction system containing different amounts of chloranil. In the experimental range, the methanol yield increased with increasing amount of chloranil. This may be because chloranil provide a beneficial environment for activation of methane.
Figure 2. Catalytic results obtained in the Pd(OAc)2-chloranil-CO reaction system containing different amounts of chloranil (reaction conditions: 100μmol of Pd(OAc)2, 140℃, 4MPa of mine gases)2.3 Effect of gas pressure
Figure 3 shows the methanol yield obtained in the Pd(OAc)2-chloranil-CO reaction system filled with different amounts of mine gases. Generally, in the liquid-phase reaction, the target product yield is highly dependent on the solubility of reactive gas in the solvent. The solubility of mine gases in solvent increases with increasing their partial pressure. Thus, more mine gases would approach the catalyst, consequently increasing methanol yield.
Figure 3. Catalytic results for oxidation of mine gases at different gas pressure (reaction conditions: 100μmol of Pd(OAc)2, 140℃, 1500μmol of chloranil)2.4 Effect of reaction temperature
Figure 4 shows the methanol attained in the Pd(OAc)2-chloranil-CO reaction system at different reaction temperature. In the experimental range, the methanol yield sharply increases with increasing temperature up to 140℃. A further increase in the temperature has no positive effect. This shows that reaction temperature is a key factor influencing the oxidation of mine gases. An appropriate increase in the reaction temperature promotes the activation of methane.
Figure 4. Catalytic results for oxidation of mine gases at different temperature (1500μmol of chloranil, 100μmol of Pd(OAc)2, gas pressure of 5MPa )2.5 Reaction mechanism
The above results show that only CH3COOCH and CH3OH are detected at the end of reaction. It was reported that CH3COOCH3 was formed in the CH4-O2-CH3COOH system when Pd(OAc)2 was added[17], while no CH3COOCH3 was detected in the absence of Pd(OAc)2. This shows that the following reaction occurs in CH4-O2-Pd(OAc)2-CH3COOH system:
$${\rm{C}}{{\rm{H}}_4} + {\rm{P}}{{\rm{d}}^{2 + }} + {\rm{C}}{{\rm{H}}_3}{\rm{COOH}} \to {\rm{C}}{{\rm{H}}_3}{\rm{COO}}{{\rm{H}}_3} + {\rm{P}}{{\rm{d}}^0} + {{\rm{H}}^ + }$$ 1 Pd(OAc)2 is an oxidative-reductive catalyst in the oxidation of CH4, but it failed to catalyze circularly[17]. This is because O2 in the reaction system can oxidize Pd0 to form only HOOPdOOCH3 but not Pd2+, as shown in the following:
2 However, when chloranil was added into the reaction system, Pd0 was oxidized to Pd2+ by chloranil through the following reaction (3), realizing the catalytic cycle.
3 The formed 2, 3, 5, 6-tetrachloro-1, 4-dihydroxybenzene can be oxidized to chloranil and H2O2 by O2 (reaction(4)). The generated H2O2 can further oxidize CH4 and Pd0 to CH3OH and Pd2+ through the reactions of reactions of (5) and (6).
4 $${\rm{C}}{{\rm{H}}_4} + {{\rm{H}}_2}{\rm{O}} \to {\rm{C}}{{\rm{H}}_3}{\rm{OH}}$$ 5 $${\rm{P}}{{\rm{d}}^0} + {{\rm{H}}_2}{{\rm{O}}_2} + {{\rm{H}}^ + } \to {\rm{P}}{{\rm{d}}^{2 + }} + {{\rm{H}}_2}{\rm{O}}$$ 6 In order to confirm the above reaction route, 1, 4-dihydroxybenzene was added into the reaction system, and methanol was indeed detected. It was reported that the loss of 1, 4-dihydroxybenzene decreased when its H-group in the benzene ring was replaced with R-group[21]. Therefore the 2, 3, 5, 6-tetrachloro-1, 4-dihydroxybenzene system is more stable than the 1, 4-dihydroxybenzene system. This is why more CH3OH was produced in the Pd(OAc)2-2, 3, 5, 6-tetrachloro-1, 4-dihydroxybenzene-CO system than in the Pd(OAc)2-1, 4-dihydroxybenzene-CO system (Table 1).
H2O2 can also be formed from the reactions of (7), (8) and (9)：
$${\rm{CO}} + {\rm{P}}{{\rm{d}}^{2 + }} + {{\rm{H}}_2}{\rm{O}} \to {\rm{C}}{{\rm{O}}_2} + {\rm{P}}{{\rm{d}}^0} + {{\rm{H}}^ + }$$ 7 $${\rm{CO}} + {{\rm{H}}_2}{\rm{O}}{{\rm{H}}_2} + {\rm{C}}{{\rm{O}}_2}$$ 8 $${{\rm{O}}_2} + {{\rm{H}}_2} \to {{\rm{H}}_2}{{\rm{O}}_2}$$ 9 Oxidation of mine gases in CH3COOH solution generates CH3COOCH3 through the following esterification reaction of (10).
$${\rm{C}}{{\rm{H}}_3}{\rm{OH}} + {\rm{C}}{{\rm{H}}_3}{\rm{COOH}} \to {\rm{C}}{{\rm{H}}_3}{\rm{COO}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}}$$ 10 Analysis of gas samples before and after reaction reveals that O2 is consumed during the reactions (Table 2). The O2 conversions at different experimental conditions support above-mentioned reaction mechanism.
Table 2. O2 conversions at different experimental conditionsEntry Temperature t/℃ Pressure p/MPa Amount of chloranil m/μmol Conversion of O2 x/% 1 140 4 0 1.8 2 140 4 500 4.2 3 140 4 1000 4.6 4 140 4 1500 7.5 5 140 2 1500 9.1 6 140 3 1500 7.7 7 140 5 1500 6.9 8 130 5 1500 2.3 9 150 5 1500 6.8 10 160 5 1500 6.5 | Show TableDownLoad:
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3. Conclusions
This work studies the catalytic oxidation of low-concentration mine gases to methanol in acetum. With Pd(OAc)2 as catalyst, both p-benzoquinone and chloranil can oxidize mine gases to methanol, and chloranil has stronger effect. This is because addition of chloranil in the reaction system improves the reaction environment of methane activation. As a result, the methanol yield increases with the amount of chloranil. Both reaction pressure and reaction temperature have a great effect on the catalytic oxidation of mine gases, and methanol yield increased with increasing reaction pressure and temperature. The oxidation of CH4 by the formed H2O2 generates CH3OH. The CH3COOCH3 in the product is formed by directly oxidizing Pd2+ and CH4 and by the esterification of CH3OH with CH3COOH.
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Figure 2. Catalytic results obtained in the Pd(OAc)2-chloranil-CO reaction system containing different amounts of chloranil (reaction conditions: 100μmol of Pd(OAc)2, 140℃, 4MPa of mine gases)
Figure 3. Catalytic results for oxidation of mine gases at different gas pressure (reaction conditions: 100μmol of Pd(OAc)2, 140℃, 1500μmol of chloranil)
Figure 4. Catalytic results for oxidation of mine gases at different temperature (1500μmol of chloranil, 100μmol of Pd(OAc)2, gas pressure of 5MPa )
Table 1. CH3OH yield obtained over the Pd(OAc)2 and 3%Pd/C in the oxidation of mine gases
Entry Reaction system CH3OH yield m/μmol 1 3%Pd/C-p-benzoquinone -CO 105 2 3%Pd/C- chloranil -CO 120 3 Pd(OAc)2-p-benzoquinone -CO 352 4 Pd(OAc)2- chloranil -CO 385 
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Table 2. O2 conversions at different experimental conditions
Entry Temperature t/℃ Pressure p/MPa Amount of chloranil m/μmol Conversion of O2 x/% 1 140 4 0 1.8 2 140 4 500 4.2 3 140 4 1000 4.6 4 140 4 1500 7.5 5 140 2 1500 9.1 6 140 3 1500 7.7 7 140 5 1500 6.9 8 130 5 1500 2.3 9 150 5 1500 6.8 10 160 5 1500 6.5
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