Electrosynthesis of Dimethyl Carbonate from Methanol and Carbon

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Electrosynthesis of Dimethyl Carbonate from Methanol and Carbon Monoxide under Mild Conditions Yuting Yu, Xiaohui Liu, Wen Zhang, Yao Zhang, Lijun Li, Zhenzhu Cao, Zihan Guo, Hong Wang, Gan Jia, Yongshen Pan, and Yanfang Gao* College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, P.R. China ABSTRACT: Electrochemical carbonylation of methanol to dimethyl carbonate (DMC) at room temperature and atmospheric pressure in a self-fabricated two-compartment electrolytic cell was achieved using CuCl2−2,2′-bipyridyl as catalyst. The reaction mechanism was analyzed, and the influence factors such as supporting electrolyte, anode potential, and concentration of supporting electrolyte and CuCl2−2,2′-bipyridyl were evaluated. The proper conditions for the electrosynthesis of DMC were obtained, in which the content of DMC was 0.1279%, the current efficiency of DMC was 76.4%, and the amount of DMC was 0.3161 mmol. In the distilled anolyte after reaction no other species but CH3OH and DMC were found via gas chromatography.

1. INTRODUCTION With the development of “green chemistry”, nontoxic chemical materials and environmental benign production processes have received considerable attention. Dimethyl carbonate (DMC) is considered as an environmentally benign chemical due to its negligible ecotoxicity and low bioaccumulation.1,2 DMC is widely used as a safe carbonylation reagent to substitute for the strong toxic reagents, dimethyl sulfate and phosgene for polymer syntheses, such as polycarbonate and isocyanate.3,4 In addition, DMC can be used as a fuel additive and a polar solvent5,6 and is a very important synthetic intermediate and building block7,8 in the current chemical industry. Hence, the production of DMC has received increasing attention. DMC is produced by the phosgene method in the early stage, but phosgene is highly toxic.9 Therefore, phosgene-free methods have been studied and developed. The current DMC synthesis techniques mainly include oxidative carbonylation of methanol and transesterification of cyclic carbonate with methanol.10−13 These processes have a great advantage of not using phosgene, but they have numerous drawbacks. The oxidative carbonylation of methanol method suffers from harsh reaction conditions, the need for corrosion resistant reactors, the potential for explosion of carbon monoxide (CO), and a large number of byproducts. The major disadvantages of the transesterification method are high energy consumption and high investment and production costs due to the requirement of intermediate separation. Therefore, research and development of a new method for DMC synthesis are significant. Compared with the oxidative carbonylation of methanol method, the electrochemical process can solve the problem of byproducts and potential for explosion of CO. Furthermore, this process is an environmentally friendly method. Early approaches for the electrosynthesis of DMC by carbonylation of methanol were based on the redox couple Br−/Br2;14 however, Br2 is highly toxic and rather volatile. Otsuka et al.15−17 reported that Pd and Cu elements are active for the electrochemical carbonylation of CH3OH to DMC in a gas system at 70 °C. However, the selectivity of these systems is rather poor, and numerous byproducts exist, such as CO2, © 2013 American Chemical Society

dimethoxy methane (DMM), and methyl formate (MF). Galia et al.18,19 found that CuCl(bipy) has electrochemical activity for the carbonylation of CH3OH to DMC at room temperature and atmospheric pressure. However, CuCl is unstable in air and thus not suitable for industrial production. In addition, its low solubility in methanol is not favorable to the formation of active intermediates generated from reactants and catalyst. PdCl2/ vapor-grown carbon fiber (VGCF) anode and CuCl2/VGCF anode have also been studied in the three-phase-boundary of the electrolysis system at 25 °C; CuCl2/VGCF anode was not active in this system.20 In the recent years, Au20,21 and Pd22−24 as electrocatalysts have been analyzed in the forms of Au/ VGCF anode and Pd/VGCF anode at room temperature and atmospheric pressure. However, Au and Pd are very expensive, and when using Au or Pd as electrocatalyst, the selectivity is rather poor. In the current work, CuCl2 was chosen as the donor of Cu2+/Cu+ couple to analyze the CuCl2−2,2′bipyridyl-catalyzed electrochemical carbonylation of CH3OH to DMC at room temperature and atmospheric pressure in the two-compartment electrolytic cell. The cost of this process is low, and its operation is relatively simple. Therefore, this process has potential for industrial production.

2. EXPERIMENTAL SECTION 2.1. Reagents and Instruments. All chemicals and solvents used were purchased from Fine Chemical Research Institute of Tianjin Guangfu, PR China. All products were used as received without further purification, except that CuCl2·5H2O was placed into the oven at 105 °C for 3 h to remove the crystal water and obtain CuCl2. All materials are of analysis grade, except for CH3OH (chromatographic grade). CO (99.99%), N2 (99.999%), and H2 (high purity) were purchased from Beijing Tianyoushun Gas Co., Ltd., PR China. Received: Revised: Accepted: Published: 6901

January 20, 2013 April 17, 2013 April 25, 2013 April 25, 2013 dx.doi.org/10.1021/ie400220y | Ind. Eng. Chem. Res. 2013, 52, 6901−6907

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the anode and cathode compartments, respectively. Then, CO was bubbled into the anode compartment using a tubule for 10 min to remove the oxygen (CO passed through the drying tower with CaCl2 at first, and then into a buffer bottle with methanol, and was finally bubbled into the anode compartment). Lastly, anhydrous copper chloride and 2,2′-bipyridyl(bipy) were added into the anode compartment to begin the electrolysis, which was carried out under potentiostatic condition. CO was bubbled into the anode compartment in the whole electrolysis process. The potentiostatic condition was provided by the electrochemical analyzer. 2.3. Purification and Analysis of Products. After electrolysis, the anolyte was placed into a round-bottom flask and underwent a simple distillation to remove the solid ions. The distillate was analyzed by gas chromatography.

Electrochemical analyzer (CHI660C, Shanghai Chenhua Instrument Co., Ltd., PR China), gas chromatograph (GC17A with flame ionization detector and 30 m × 0.25 mm capillary column, Shimadzu.), two-compartment electrolytic cell (self-fabricated, as shown in Figure 1), anion-exchange

3. RESULTS AND DISCUSSION 3.1. Analysis of the Electrosynthesis System. The solution of the electrosynthesis system consisted of CH3OH, supporting electrolyte, and catalyst. CO as a reactant was bubbled into the solution; the catalyst was an equimolar mixture of CuCl2 and 2,2-bipyridyl(bipy), in which 2,2bipyridyl is a ligand. The electrosynthesis was carried out under potentiostatic condition. Table 1 shows the stoichiometric reaction among CO, CuCl2 or CuCl, CH3OH, and bipy in the presence of KOH at 25 °C. Table 1. Influences of Reactant Composition on the Formation of DMC in No Electricitya sequence number 1 2 3 4

components of reaction system CuCl2 + bipy + CO + KOH + CH3OH CuCl + bipy + CO + KOH + CH3OH CuCl2 + CO + KOH + CH3OH bipy + CO + KOH + CH3OH

content of DMC (%) 0.02908 0 0 0

General conditions: CH3OH (10 mL) + KOH (0.1 mol L−1) + catalyst (0.07 mol L−1 CuCl or CuCl2) + bipy (0.07 mol L−1), θ = 25 °C, Pco = 1 atm, t = 2 h. a

Figure 1. Image (a) and cross-sectional view (b) of the self-fabricated two-compartment cell used for the electrosynthesis of DMC from CH3OH and CO. (b): 1-PTFE end plate; 2-PTFE inner sleeve; 3PTFE inner sleeve; 4-anion membrane; 5-positioning screw; 6-M6 nut.

The reaction conditions are extremely similar to the electrosynthesis conditions, except for the application of the electrochemical potential. As shown in entry 1, CuCl2 and bipy appeared simultaneously in the system, and a small amount of DMC was observed. However, when the system contains CuCl2 in the absence of bipy (entry 3), or contains bipy in the absence of CuCl2 (entry 4), or contains CuCl (rather than CuCl2) and bipy (entry 2), no DMC formation was observed. The results suggest that only the formation of complex CuCl2−bipy was active in the oxidative carbonylation of methanol to DMC at room temperature and atmospheric pressure. This phenomenon may be attributed to the fact that 2,2′-bipyridyl and CuCl2 formed CuCl2−bipy complex “in situ” in the anolyte, and 2,2′-bipyridyl as a ligand can donate lone pair electrons, which can change the electronic structure and spatial structure of Cu2+. CuCl was only the reduction product of the CuCl2 oxidative carbonylation of methanol to DMC, and 2,2′-bipyridyl was only a ligand of the catalyst and had no catalytic effect. In order to obtain the approximate potential, the extent of Cu+(bipy) was oxidized to Cu2+(bipy) in the anode surface with the KOH solution in CH3OH, cyclic voltammogram (CV)

membrane (Tianjin Blue Crystal Purification Refrigeration Equipment Co., Ltd., PR China), digital viscometer (SNB-1, Shanghai Nirun Intelligent Technology Co., Ltd., PR China), Ag/Ag2O reference electrode (made in-house), carbon rod (made in-house from waste dry battery.), and copper wire were used in the experiment. 2.2. Electrosynthesis Using a Two-Compartment Cell. The electrocatalytic carbonylation of methanol was carried out in the self-fabricated two-compartment electrolytic cell made of polytetrafluoroethylene. The anodic compartment was equipped with a gas inlet and an outlet as well as reference (Ag/ Ag2O) and working electrodes (carbon rod). The cathodic compartment was equipped with a counterelectrode (copper wire). Anode and cathode compartments were divided by an anion-exchange membrane. Each compartment had a volume of 20 cm3. The main reaction steps are as follows. First, the electrolyte solution which consisted of a certain amount of supporting electrolyte and 10 mL of methanol was added into 6902

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measurements of CuCl2 + 2,2′-bipyridyl (0.07 mol L−1) and CaCl2 + 2,2′-bipyridyl (0.07 mol L−1) in KOH (0.1 mol L−1)/ CH3OH solution were conducted using glassy carbon electrode as the working electrode, with the scan rate of 100 mV s−1, as shown in Figure 2.

Figure 3. Cyclic voltammograms of the system for the electrosynthesis of DMC from CH3OH and CO, with scan rate = 200 mV s−1:  -  before reaction, the system without CuCl2;  before reaction, the system with CuCl2; − − − − after reaction. General conditions: CH3OH (10 mL) + electrolytes (0.1 mol L−1) + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O.

Figure 2. Cyclic voltammograms with glassy carbon electrode. Voltage scan rate = 100 mV s−1; electrolyte, KOH (0.1 mol L−1)/CH3OH under nitrogen atmosphere. (a) CuCl2 (0.07 mol L−1) + 2,2′-bipyridyl (0.07 mol L−1), (b) CaCl2 (0.07 mol L−1) + 2,2′-bipyridyl (0.07 mol L−1), (c) CH3OH + KOH (0.1 mol L−1).

electrolytic cell, except CuCl2. When CuCl2 was added into the system, an obvious oxidation peak was observed in the CV spectra. However, the oxidation peak in the CV spectra of the system before reaction almost disappeared when the reaction was carried out after 2 h. No current peak in the CV spectra was observed again. The results in Figure 3 correspond to the inferences made from Table 1 that 2,2′-bipyridyl was only a ligand of the catalyst and CuCl2 was the active species. The formation of complex CuCl2−bipy was active in the oxidative carbonylation of methanol to DMC under mild conditions. Based on the CV spectra of the system when the reaction was carried out after 2 h, the system was almost deactivated when the reaction was ended. 3.2. Reaction Mechanism Research. On the basis of the earlier discussion, a new tentative reaction mechanism was proposed as follows. In the process of electrocatalytic carbonylation of CH3OH to DMC, Cu2+/Cu+ redox couple likely has the role of electron transfer mediator. The following process was predicted. First, Cu 2+ and 2,2′-bipyridyl formed the complex Cu2+(bipy) “in situ” in the reaction solution. CH3OH was dissociated into H+ and CH3O−. CO was oxidized to carbonyl by Cu2+(bipy), and Cu2+(bipy) was reduced to Cu+(bipy) in the anolyte simultaneously. Second, two CH3O− ions attacked the carbonyl to cause a nucleophilic addition reaction and formed a DMC in the anolyte. Cu+(bipy) was diffused to the anode electrode surface and was oxidized to Cu2+(bipy) simultaneously. Cu2+(bipy) was then diffused to the anolyte anew to further oxidize CO. Through this process, the electrosynthesis reaction was cycling. The overall reaction scheme is illustrated in Figure 4. In this system, the addition molar ratio of CuCl2 and 2,2′-bipyridyl is 1:1. The oxidation potential (+0.3 V) of Cu+(bipy) to Cu2+(bipy) is much lower than the oxidation potential (+1.4 V) of CH3OH to DMM and MF, which avoided the formation of DMM and MF effectively. Due to no O2 existing, the formation of CO2 was also avoided.

The CV(a) spectra showed the redox properties of CuCl2 in the solution of KOH, 2,2′-bipyridyl, and CH3OH with glassy carbon electrode swept with a wide window from −0.5 V to +2.0 V in N2. Two oxidation peaks and one reduction peak were observed. The Ox(1) was identified at +0.18 V, Ox(2) at +1.4 V, and Red(1) at −0.13 V. Ox(1) and Red(1) seemed to be a redox couple, but no reduction peak corresponding to the Ox(2) in the CV(a) was found. The CV(b) spectra showed the redox properties of CaCl2 under the same conditions as with CuCl2. Only one oxidation peak, Ox(3) at +1.4 V, was observed, and no reduction peak corresponding to the Ox(3) in the CV(b) was identified. By comparing CV(a) with CV(b), we determine that Ox(1) and Red(1) were the current peaks of the Cu2+/Cu+ redox couple. The minimum potential of Cu+ to be oxidized to Cu2+ under this condition was +0.18 V (vs Ag/ Ag2O). Ox(2) and Ox(3) at +1.4 V might be the same oxidation peaks of the electrochemical oxidation of CH3OH to DMM and MF, which would undoubtedly decrease the selectivity and yield of the oxidative carbonylation of methanol to DMC. In order to prove the above conclusion, the CV of the background was scanned, as shown in Figure 2c. The oxidation peak, Ox(4) at +1.4 V, was observed, which corresponded to the Ox(2) and Ox(3). Thus, it was demonstrated that the Ox(2), Ox(3), and Ox(4) were the same oxidation peaks of the electrochemical oxidation of CH3OH to DMM and MF. Hence, the supplied potential of the working electrode must be lower than +1.4 V in the succeeding experiments. In this study, CV measurements of the reaction system were carried out, as shown in Figure 3. The CV curves showed redox properties of the electrosynthesis system in a wide window, from −1.0 V to +1.0 V, at the scan rate of 200 mV s−1, in CO. Before reaction, the CV spectra of the system had almost no current peak when other reagents were all added to the 6903

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from the gas chromatography data directly, whereas the CE was calculated as follows: CE =

Q theoretical Q actual

=

nDMCZF Q actual

(5)

nDMC ADMC ADMC = ≈ nCH3OH(total) A CH3OH(total) A CH3OH(measured) + 2ADMC (6)

Q actual = Q consumption + Q Cu2+provided

Figure 4. Reaction scheme of CuCl2−bipy catalyzing the electrosynthesis of DMC from CH3OH and CO.

= Q consumption + nCu2+ZF

where A is the peak area of gas chromatogram, F is Faraday constant, Z is the transfer electron number of the reaction, n is the mole number of the substance, and Qconsumption can be measured using an electrochemical analyzer. 3.4.1. Effects of Supporting Electrolytes. The results of the electrocatalytic carbonylation of methanol to DMC with various supporting electrolytes are summarized in Table 2.

The electrochemical reaction equations of the experiment were estimated as follows: anode: Cu +(bipy) → Cu 2 +(bipy)

(1)

cathode: 2CH3OH + 2e− → 2CH3O− + H 2

Table 2. Influences of Electrolytes on the CuCl2−bipyCatalyzed Electrosynthesis of DMC from CH3OH and COa

(2)

anolyte: Cu 2 + + 2,2′‐bipyridyl → Cu 2 +(bipy)

(3)

2CH3OH + 2Cu 2 +(bipy) + CO → (CH3O)2 CO + 2H+ + 2Cu+(bipy)

(7)

(4)

An anion-exchange membrane was used between the two compartments to avoid the migration of cations in the anodic compartment toward the cathodic compartment. The electric neutrality of the anolyte was kept by the transfer of anions from the catholyte to the anolyte. 3.3. Analysis of Selectivity. For all experiments, the analytical results of gas chromatography showed that the anolyte consisted of two substances only, namely, methanol and DMC. Only two peaks were identified in the gas chromatography map. The qualitative analyses of the two peaks were carried out in contrast to the peak retention time of the standard samples. Given the absence of the common peaks of byproducts generated from the oxidation of methanol, such as MF and DMM, the selectivity of DMC based on methanol was almost 100%. The high selectivity of the reaction was mainly attributed to the advantages of the electrochemical method. Compared with traditional chemical methods, the electrochemical method is more accurate for the control of the oxidizing power by controlling the electrochemical potential applied to the working electrode, which can suppress the formation of byproducts to a great extent. Instead of Au and Pd, the suitable electrocatalyst, CuCl2−2,2′-bipyridyl, was another critical factor for the high selectivity of the reaction. In addition, the mild reaction conditions, room temperature, and atmospheric pressure contributed significantly to the high selectivity. 3.4. Study on the Influence Factors of the Electrosynthesis of DMC. The effects of the influence factors, such as supporting electrolyte, anode potential, and the concentrations of supporting electrolyte and CuCl2(bipy) on the electrosynthesis of DMC, were evaluated to increase the yield of DMC. The results were characterized by the content of DMC and the current efficiency (CE). The DMC content was obtained

electrolytes

Qconsumption (C)

content of DMC (%)

current efficiency (%)

KOH NaOH CH3OK K2CO3 CH3ONa CH3COOK NaF NaClO4 KCl TBAB

12.28 19.79 1.707 8.680 4.533 0.8634 0.5541 0.4448 0.5340 0.2350

0.1279 0.1119 0.08117 0.02659 0.02022 0.01857 0 0 0 0

76.4 61.1 55.9 16.7 13.4 13.0 0 0 0 0

General conditions: CH3OH (10 mL) + electrolytes (0.1 mol L−1) + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), anode potential = 0.3 V, θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O. a

When acidic electrolyte, tetrabutyl ammonium bromide (TBAB), and neutral electrolytes, NaClO4 and KCl, were used, no DMC was formed in the anolyte. When alkaline electrolytes were used, with the increase of basicity, both the DMC content and the CE increased gradually. The increasing trend of the CE corresponds to the increasing trend of the DMC content. KOH was the best supporting electrolyte for the electrosynthesis of DMC in this study based on the content and the CE of DMC shown in Table 2. Table 2 strongly suggests that strong alkaline supporting electrolytes are effective for the electrosynthesis of DMC in this experimental condition. Stronger basicity of the supporting electrolyte is more favorable to the formation of DMC. Acidic and neutral electrolytes were not effective for the electrosynthesis of DMC in this study. Hence, we deduced that CO was oxidized to carbonyl by Cu2+, a carbonyl made a nucleophilic addition reaction with two CH3O− to form a DMC, and CH3O− was produced through the dissociation of CH3OH in the anolyte. With strong alkaline supporting electrolyte in the anolyte, CH3OH can easily dissociate into CH3O− and H+, which facilitated the synthesis of DMC. With the increase of the basicity of the supporting electrolyte, the effect on the 6904

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of the reaction system. Thus, +0.3 V is the proper anode potential for the electrocatalytic carbonylation of methanol to DMC for further experiments. 3.4.3. Effects of the Concentration of KOH. The concentration of KOH from 0.05 mol L−1 to 0.25 mol L−1 was examined to obtain a suitable concentration of the supporting electrolyte. Figure 6 shows that 0.10 mol L−1 was the suitable concentration of KOH for the formation of DMC in the general conditions. When the concentration was lower than 0.1 mol L−1, the content and the CE of DMC were both increased with the increase in concentration. When the concentration was higher than 0.1 mol L−1, the content and the CE of DMC were both reduced drastically with the increase in KOH concentration, and then changed insignificantly. The content and the CE of DMC have a similar variation tendency to the change of the KOH concentration. The effects of KOH concentration on the formation of DMC are mainly related to the role of KOH in the electrosynthesis system. In Table 2, KOH was not only a supporting electrolyte of the system but a cocatalyst. As a cocatalyst, when the concentration of KOH was lower than 0.1 mol L−1, KOH had little promotion effect on the dissociation of CH3OH to CH3O− and H+. As a supporting electrolyte, the lower concentration of KOH dissociated a small amount of ions that had a disadvantage for the transfer of electrons. Thus, neither as a cocatalyst nor a supporting electrolyte was the lower concentration of KOH favorable to the formation of DMC. When the concentration of KOH was higher than 0.1 mol L−1, KOH was favorable to the dissociation of CH3OH. However, as a supporting electrolyte, the viscosity of the synthesis system increased with KOH concentration, as shown in Table 3. With the increase of viscosity, the diffusion of reactants was limited, thus further influenced the formation of DMC. Therefore, 0.1 mol L−1 was considered as the KOH concentration in further experiments. 3.4.4. Effects of the Concentration of CuCl2−bipy. Figure 7 shows that the effects of the concentration of CuCl2−bipy on the formation of DMC were studied in the general conditions of 0.03 mol L−1 to 0.14 mol L−1 . The 0.07 mol L −1 concentration of CuCl2−bipy enhanced the formation of DMC. The content and the CE of DMC increased sharply with the increase in the concentration of CuCl2−bipy when concentration of CuCl2−bipy was lower than 0.07 mol L−1. The content and the CE of DMC decreased drastically with the increase in the concentration of CuCl2−bipy when the concentration of CuCl2−bipy was higher than 0.07 mol L−1. Hence, the suitable concentration of CuCl2−bipy for the formation of DMC was 0.07 mol L−1. A relatively low or high concentration of CuCl2−bipy as a catalyst was unfavorable to the formation of DMC in our research. The results were mainly related to the active centers of CuCl2−bipy, which had significant influence on the formation of DMC. When the concentration of CuCl2−bipy was lower than 0.07 mol L−1, only a few CuCl2−bipy molecules and active centers were observed; thus, the catalytic action of CuCl2−bipy on the oxidative carbonylation of methanol to DMC was weak. When the concentration of CuCl2−bipy was higher than 0.07 mol L−1, a large number of CuCl2−bipy molecules were observed in the solution; the coupling among the CuCl2−bipy molecules increased, which reduced the amount of active centers, thereby affecting the catalytic action of CuCl2−bipy. The proper concentration of CuCl2−bipy in our study, 0.07 mol L−1, provided a relatively large number of

dissociation reaction of CH3OH becomes more obvious, and DMC formation was greatly affected. Therefore, the strong alkaline supporting electrolyte is not only an electrolyte in the system, but also a cocatalyst. By contrast, acidic and neutral electrolytes do not have the same function. Therefore, selecting a suitable supporting electrolyte is critical for the electrosynthesis of DMC. In further experiments, KOH was considered as supporting electrolyte to avoid introducing more new ions into the reaction system. 3.4.2. Effects of Anode Potential. For constant potential electrolysis, the electrode potential determined both the reaction type and reaction rate that occurred in the electrode/solution interface. Thus, selecting a suitable electrode potential value for the electrosynthesis reaction is the key to control the direction of the electrode reaction to ensure the quantity and quality of the products. Figure 2 shows that methanol was oxidized to DMM and MF under the potential of +1.4 V, and Cu+ was oxidized to Cu2+ at +0.18 V. Thus, the anode potential from +0.2 V to +0.6 V has been for this study. In this anode potential range, DMC was the only product of the electrocatalytic carbonylation of methanol. Figure 5 presents the effects of anode potential on the electrosynthesis of DMC in the general conditions. The

Figure 5. Influences of anode potential on the CuCl2−bipy-catalyzed electrosynthesis of DMC from CH3OH and CO. General conditions: CH3OH (10 mL) + KOH (0.1 mol L−1) + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O.

potential of +0.3 V showed excellent performance for DMC formation. When the system was under other electric potential, the content and the CE of DMC were all less compared with that of +0.3 V. When the anode potential was lower than +0.3 V, the content and the CE of DMC both increased with the increase of potential. When the anode potential was higher than +0.3 V, the content and the CE of DMC both decreased drastically with the increase of potential. It was just that when the anode potential was lower than +0.3 V, the reaction rate that occurred in the electrode/solution interface was slow, accompanied with the deactivation of the reaction system, the formation of DMC was limited. When the anode potential was higher than +0.3 V, with the increase of electric potential, the speed of the deactivation of the reaction system became faster. This phenomenon drastically decreased the formation of DMC, as shown in Figure 5. When the system was under +0.3 V, higher reaction rate occurred in the electrode/solution interface, resulting in relatively lower speed of the deactivation 6905

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Figure 6. Influences of KOH concentration on the CuCl2−bipy-catalyzed electrosynthesis of DMC from CH3OH and CO. General conditions: CH3OH (10 mL) + KOH + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), anode potential = 0.3 V, θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O.

gradually up to a limit, and then decreased and became constant, given the progression in the electrosynthesis reaction.

Table 3. Viscosity of the Synthesis System with Different KOH Concentrationa KOH concentration (mol/L)

viscosity (mPa·s)

0.05 0.10 0.15 0.20 0.25

164.1 174.1 183.3 199.6 225.0

a General conditions: CH3OH + KOH + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), θ = 25 °C, Pco = 1 atm.

Figure 8. Current−time curve of the entire process of CuCl2−bipycatalyzed electrosynthesis of DMC by CH3OH and CO. General conditions: CH3OH (10 mL) + KOH (0.1 mol L−1) + CuCl2 (0.07 mol L−1) + bipy (0.07 mol L−1), anode potential = 0.3 V, θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O.

The increase in the current intensity was due to the increasing amount of Cu+, which needed to be oxidized to Cu2+ in the electrode surface as the reaction progressed. Cu+ was produced by Cu2+ oxidized CO to carbonyl, which was reduced to Cu+. The current intensity reached a maximum after the reaction had occurred for some time, and then, the current intensity decreased. The decrease of current intensity during potentiostatic electrolyses indicated the progressive deactivation of the reaction system, which corresponded with the results shown in Figure 3. Figure 3 shows that, when the reaction ended, the CV spectra of the system almost had no oxidation peak. The system was almost deactivated. The formation of water in the reaction process may have caused the deactivation of the reaction system. The deactivation may also have been caused by other reasons. This problem is another challenging subject.

Figure 7. Influences of the concentration of CuCl2−bipy on the CuCl2−bipy-catalyzed electrosynthesis of DMC from CH3OH and CO. General conditions: CH3OH (10 mL) + KOH (0.1 mol L−1) + CuCl2−bipy, anode potential = 0.3 V, θ = 25 °C, Pco = 1 atm, t = 2 h. CE: copper wire, WE: carbon rod (2 cm2), RE: Ag/Ag2O.

active centers for the electrocatalytic carbonylation of methanol to DMC. 3.4.5. Electrosynthesis at Optimum Conditions. The content and the CE of DMC at optimum conditions were 0.1279% and 76.4%, respectively. The calculated amount of DMC was 0.3161 mmol based on eq 6. The current−time curve of the entire process at optimum conditions is shown in Figure 8. The current intensity of the system increased 6906

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4. CONCLUSION A two-compartment electrolytic cell with a volume of 20 cm3 for each compartment was fabricated and utilized to demonstrate that the electrocatalytic carbonylation of methanol to DMC can be achieved at atmospheric pressure and room temperature with a catalytic system based on CuCl2−2,2′bipyridyl complexes. No other species but CH3OH and DMC were found by GC in the distilled anolyte after the reaction. The reaction mechanism and influence factors were also studied. The information obtained indicated that only the formation of complex Cu2+(bipy) was active in the oxidative carbonylation of methanol to DMC at room temperature and atmospheric pressure. The strong basicity of the supporting electrolyte was favorable for DMC formation. Anode potential and the concentration of KOH and CuCl2−bipy also had significant influences on the formation of DMC. The reaction mechanism was as follows: in the reaction solution, CH3OH was first dissociated into H+ and CH3O−; the complex Cu2+(bipy) was then formed in situ. CO was oxidized to carbonyl by Cu2+(bipy), and two CH3O−’s attacked the carbonyl to cause the nucleophilic addition reaction. Thereafter, DMC was formed in the anolyte. The reduced Cu2+(bipy) was oxidized to Cu2+(bipy) anew on the surface of the anode electrode, and then diffused to the anolyte to oxidize CO again. The proper conditions for the electrosynthesis of DMC were obtained. The anode potential was 0.3 V vs (Ag/Ag2O), the concentration of CuCl2−bipy was 0.07 mol L−1, and the concentration of KOH was 0.1 mol L−1. The DMC content was 0.1279%, the CE of DMC was 76.4%, and the amount of DMC was 0.3161 mmol under the aforementioned conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +86 471 6575722. Fax +86 471 6503298. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (No. 21266018), Natural Science Foundation of Inner Mongolia, P.R.C. (No. 2010MS0218), the Scientific Research Foundation for the Returned Overseas Chinese Scholars State Education Ministry, and the “Xibuzhiguang” Foundation for Fostering Personal Ability.

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REFERENCES

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dx.doi.org/10.1021/ie400220y | Ind. Eng. Chem. Res. 2013, 52, 6901−6907