NMI in Homogeneous Carbonylation for Synthesis of

Nov 3, 2009 - Kartikeya ShuklaVimal Chandra Srivastava. Industrial & Engineering Chemistry Research 2018 57 (38), 12726-12735. Abstract | Full Text ...
0 downloads 0 Views 2MB Size
Download PDF
Ind. Eng. Chem. Res. 2009, 48, 10845–10849

10845

CuCl/phen/NMI in Homogeneous Carbonylation for Synthesis of Diethyl Carbonate: Highly Active Catalyst and Corrosion Inhibitor Hui Xiong, Wanling Mo, Jianglin Hu, Rongxian Bai, and Guangxing Li* School of Chemistry & Chemical Engineering, Huazhong UniVersity of Science & Technology, Hubei Key Laboratory of Materials and Chemistry & SerVice Failure, Wuhan 430074, People’s Republic of China

The effects of N-donor ligands on CuCl-catalyzed oxidative carbonylation of ethanol were investigated. It was found that 1,10-phenanthroline (phen) and N-methyl imidazole (NMI) exhibited a synergic effect on the catalytic activity. Catalytic efficiency of the CuCl system was enhanced dramatically, and corrosion of the reaction system was effectively inhibited when phen and NMI were simultaneously used as the ligands. Under the reaction conditions of cCuCl ) 0.2 mol/L, nCu:nphen:nNMI ) 2:1:1, T ) 393 K, P ) 2.4 MPa with PCO/PO2 ) 2:1, and time ) 3 h, conversion of ethanol and selectivity to diethyl carbonate (DEC) were 15.2% and 99.0%, respectively. As compared to pure CuCl, the catalytic activity of CuCl/phen/NMI is much higher (3.6-fold). Furthermore, the high catalytic activity could be kept for a long time of service during the reaction. Corrosion of CuCl was also inhibited by using phen and NMI as ligands. For example, the inhibition efficiencies of CuCl/phen/NMI catalyst system on stainless steel 304 and Hastelloy 5923 were 98.1% and 98.2%, respectively. Introduction Diethyl carbonate (DEC) has been attracting wide attention as an environmentally benign chemical raw material.1 It is an excellent carbonylation reagent for the manufacturing of pharmaceutics, plastics, fine chemicals, paint, and fragrance. It is also gaining attention as a safe solvent and an additive in the lithium cell electrolyte.2-5 High octane number of the carbonate makes it an excellent candidate to replace MTBE as a gasoline additive. Meanwhile, it has a more favorable gasoline/water distribution coefficient and a lower volatility as compared to dimethyl carbonate or ethanol as a promising substitute for MTBE.6,7 There are several methods to prepare DEC, such as the phosgene process, oxidative carbonylation of ethanol,8-13 carbonylation of ethyl nitrite,14,15 and transesterification.16 Among those processes, oxidative carbonylation of ethanol with carbon monoxide and oxygen to produce DEC matches best the principles of “green chemistry”. Although PdCl2 catalyst has been proven to be an efficient catalyst for the synthesis of DEC through oxidative carbonylation, commercialization of this process was strictly restricted by some disadvantages, such as it is easy to deactivate and is of high cost. In our group, we have found that a CuCl/Schiff-base catalyst exhibits high catalytic activity in dimethyl carbonate (DMC) synthesis by oxidative carbonylation.17,18 The CuCl/Schiff-base catalyst has also been used in the synthesis of DEC by the same carbonylation method.13 However, our reported preliminary result is still far from satisfactory. The goal of the present work is to improve the catalytic activity and minimize the corrosion of our previous system. We focused particularly on the influence of various N-donor ligands on the catalytic activity and the corrosion of the CuCl catalyst system. Experimental Section The reaction was carried out in a 0.25 L stainless steel autoclave lined with Teflon, equipped with a magnetic stirrer. All of the reagents were used as received without further purification. In a typical experiment, ethanol, N-donor ligand, and cuprous chloride * To whom correspondence should be addressed. Tel.: +86-2787543732. Fax: +86-27-87544532. E-mail: [email protected].

were added into the autoclave. The autoclave was then purged three times with CO and pressurized to 2.4 MPa with CO and O2 (PCO/ PO2 ) 2:1) at room temperature. When the temperature went up to 363 K, the magnetic stirrer was turned on and the temperature was held at 393 K. During the reaction, the CO and O2 mixture was pumped periodically to keep the pressure constant (2.4 MPa). After the reaction, the reactor was cooled to room temperature and depressurized. The liquid product was collected and distilled under atmosphere pressure. The product analysis of the distillates was conducted on a gas chromatograph (Agilent GC112A) using an FID detector. Conversion and selectivity were calculated by the following equations: conversion (XEtOH) ) 2 ×

selectivity (SDEC) )

nDEC + nb × 100% nEtOH

nDEC × 100% nDEC + nb

where nDEC is the molar amount of the DEC, nEtOH is the molar amount of the ethanol, and nb is the total molar amount of the byproduct. The catalyst lifetime was estimated according to the procedure described in the literature.17 At the end of each run, the gas was vented and the liquid phase was evaporated and collected. Next, fresh ethanol, the used catalyst, CO, and O2 were introduced into the autoclave for performing the next run. During the reaction, the rate of corrosion, in mm/a, was measured by a standard method described in ASTM No. B575. Test pieces (about 20 mm × 10 mm × 3 mm) of Hastelloy 5923 (Cr 23%, Mo 16%, Fe e 1.5%, C e 0.01%, Ni base; from ThyssenKrupp Corp., Germany) were introduced into the autoclave using a special sample cage. Corrosion data were obtained at 393 K and 2.4 MPa after 24 h of reaction. The inhibition efficiency (η) was calculated using the following equation: η) corrosion rate of CuCl - corrosion rate of CuCl/Schiff base corrosion rate of CuCl

10.1021/ie901139e CCC: $40.75  2009 American Chemical Society Published on Web 11/03/2009

10846

Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009

Table 1. Effects of N-Ligands on the Catalytic Activitiesa

a

entry

ligand

Cligand (mol/L)

XEtOH (%)

SDEC (%)

TON

pKa19-21

1 2 3 4 5 6 7 8 9

none imidazole 2-methylimidazole 1,2-dimethylimidazole NMI 5-NO2-phen 2,2′-bipyridine phen phen/NMI

0.4 0.4 0.4 0.4 0.2 0.2 0.2 0.1/0.2

3.9 4.8 6.9 7.5 10.8 6.6 4.5 8.4 13.6

98.6 99.7 96.4 98.6 99.5 96.9 98.5 98.2 99.1

1.6 2.1 2.9 3.2 4.6 2.7 1.9 3.6 5.8

6.95 7.86 8.20 7.06 2.80 4.35 4.86 4.86

Conditions: ethanol 80 mL, [CuCl] ) 0.2 mol/L, PCO/PO2 ) 2:1, P ) 2.4 MPa, T ) 393 K, time 3 h, and stirring speed 1000 rpm.

Electrochemical studies were performed at 298 K with a three-electrode potentiostatic system (Electrochemical Workstation CS300) using a Pt paste electrode as working electrode and a Pt coil as counter electrode. The potentials are expressed with reference to an aqueous saturated calomel electrode (SCE) placed in a separate compartment containing the supporting electrolyte. Current-voltage curves were scanned at 0.050 V/s. The electrolytic solutions, 0.1 mol/L Bu4NBF4 (TBABF4) in tetrahydrofuran (THF), were routinely deoxygenated with argon.

Table 2. Influence of Different Ratios of phen/NMIa

Results and Discussion

Table 3. Recycle Use of the Catalystsa

Effects of N-Donor Ligands on the Catalytic Activity. Initially, effects of various N-donor ligands on the catalytic performance of CuCl for the synthesis of DEC were investigated, and the results are listed in Table 1. It was found that among all of the N-ligands examined, the combination of CuCl/ phen/NMI showed the highest catalytic activity, and under the given condition, a maximal ethanol conversion, 13.6%, was obtained in entry 9 of Table 1. This value is 3.6 times higher than the results obtained with pure CuCl system (without any additives). It is well-known that the substituent on the heterocyclic ring of the N-donor ligands affects its basicity or σ-donor ability. The relationships between the σ-donor ability of the N-donor ligands can be expressed by their pKa values.19 Therefore, their catalytic performance was also studied. As shown in entry 2 of Table 1, the catalytic activity of CuCl/imidazole is very low. It has been reported that imidazole molecules could form a long chain structure in solution by means of intermolecular hydrogenbond formation, which might prevent the coordination of reactants, C2H5O- and CO, to the Cu(I), and, as a result, restrict the formation of active intermediate.22 In the N-N bidentate ligands, the promoting effect of phen is superior as compared to 2,2′-bipyridine. This might be attributed to the stronger basicity, chelating capability, and rigidity of phen.23 The σ-donor ability of 5-NO2-phen is significantly weaker than that of phen, due to an electronwithdrawing effect of the NO2 group, which is detrimental for the coordination of CO and C2H5O- with the Cu(I) complex. According to the reported mechanism of CuCl-catalyzed oxidative carbonylation of alcohols,24-26 carbon monoxide reacts with CuCl to form a monocarbonyl complex, COCuCl, which could then interact with copper ethoxide to generate ethoxycarbonyl. The main factors that might affect the reaction rate are the rates of (i) generating copper ethoxide and (ii) CO insertion. As described above, the coordination of CO and C2H5O- to Cu(I) might be affected by N-donor ligand. Thus, different catalytic activities were obtained by altering the N-donor ligands. Yamamoto27,28 found that the π-conjugated character of the N-ligands played an important role in accelerating the Cu(II)/Cu(I) redox cycle, which is in good agreement with our experimental results.

entry

cphen/cNMI (mol/L)

XEtOH (%)

SDEC (%)

yield (%)

TON

1 2 3 4 5 6

0.2/0.1 0.1/0.1 0.1/0.2 0.1/0.4 0.05/0.2 0.05/0.1

8.9 15.2 13.6 10.8 12.0 8.5

98.6 99.0 99.1 99.5 99.0 98.5

8.8 15.0 13.5 10.7 11.9 8.4

3.8 6.5 5.8 4.6 5.0 3.7

a Conditions: ethanol 80 mL, [CuCl] ) 0.2 mol/L, T ) 393 K, PCO/ PO2 ) 2:1, P ) 2.4 MPa, time 3 h, and stirring speed 1000 rpm.

runs

XEtOH (%)

SDEC (%)

TON

XCO (%)

SCO (%)

1 2 3 4 5 6

15.2 13.6 14.2 13.8 13.3 12.9

99.0 99.2 99.6 99.0 99.3 99.5

6.5 5.8 6.1 5.9 5.7 5.5

56.9 55.6 55.9 55.8 55.5 55.4

80.0 78.8 79.5 80.5 81.2 81.1

a Conditions: ethanol 80 mL, [CuCl] ) 0.2 mol/L, nCu:nphen:nNMI ) 2:1:1, T ) 393 K, P ) 2.4 MPa, PCO/PO2 ) 2:1, time 3 h, and stirring speed 1000 r/min. Note: Conversion (XCO) ) (the number of moles for consumed CO)/(the number of moles for added CO) × 100%. Selectivity (SCO) ) (the number of moles for produced DEC)/(the number of moles for consumed CO) × 100%.

Effect of the Concentration of the N-Ligands. Influences of the molar ratio of phen/NMI on the conversion of ethanol and TON were also investigated, and the results are presented in Table 2. It is clear that the conversion and TON were increased by changing the ratio of phen/NMI from 2/1 to 1/1 in entries 1 and 2 of Table 2, and then decreased with increase of NMI in entries 2-4 of Table 2. Although the conversion changes dramatically, no significant change in terms of the selectivity to DEC occurs. These results indicate that phen and NMI have a synergic effect on the catalytic performance of the CuCl catalyst. It is well-known that NMI and phen can coordinate with copper to form two kinds of complexes: monochelate ring or bis-chelate ring complexes. When the concentration of phen or NMI is too high, it would not be unreasonable to expect that the copper metal center tended to coordinate with the N-donor ligand strongly and to form bis-chelate ring complex preferably; as a result, leaving of the ligand from the metal center was not easy, and, therefore, it is difficult to offer a coordination vacancy for CO and C2H5O-. When the concentration of ligands especially for NMI was reduced, the catalyst activity obviously decreased as listed in entries 5 and 6 of Table 2. It would be possible that ligands were not sufficient for coordination with CuCl. Lifetime of the Catalyst. A catalyst with a reasonably long lifetime is critical for its commercialization. The stability of CuCl/phen/NMI catalyst was also tested, and the results are shown in Table 3. After five times of reuse, it was found that,

Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009

10847

a

Table 4. Corrosion Inhibition of N-Ligand Promoters in the CuCl Catalyst System for Alloys weight-loss (mg)

corrosion rate (mm/a)

η (%)

entry

catalyst

304

5923

304

5923

304

5923

1 2 3 4

CuCl CuCl/phen CuCl/NMI CuCl/phen/NMI

139.9 11.2 44.0 4.5

9.8 1.2 4.0 0.2

9.1607 0.8722 2.9130 0.1698

0.8652 0.1178 0.1672 0.0152

90.5 68.2 98.1

86.4 80.7 98.2

a Conditions: ethanol 1000 mL, [CuCl] ) 0.2 mol/L, [phen] ) 0.1 mol/L, [NMI] ) 0.1 mol/L, PCO/PO2 ) 2:1, P ) 2.4 MPa, T ) 393 K, time 24 h, and stirring speed 800 rpm.

Figure 1. EPMA photographs of hastelloy 5923 surface corrosion in different catalytic systems: (a) CuCl, (b) CuCl/NMI, (c) CuCl/phen, and (d) CuCl/ phen/NMI.

although the catalytic activity was slightly decreased, the selectivity to DEC still remains unaltered. Corrosion Inhibition. In our system, the corrosion of the catalytic component Cu+ and Cl- ions to the structure materials of the reactor has also to be considered before its application in large scale. Here, the corrosion rates of the CuCl and CuCl/ phen/NMI catalysts in the liquid phase of the C2H5OH/H2O system under the reaction conditions have been determined by the weight-loss method, and the results are presented in Table 4. It was found that the corrosion of CuCl in the reaction system to stainless steel 304 and hastelloy 5923 is very severe, and the pitting corrosion could be clearly observed on the metal surface in this catalytic system. With the addition of the N-ligand, the corrosion was significantly inhibited, and no pitting corrosion was observed under the identical conditions. Somasundaran and co-workers reported that the severe corroding effect of halogen ions can be significantly restricted by adsorbing organic molecules on the metal surfaces.29,30 Our results in entries 2 and 3 of Table 4 showed that phen is more effective than NMI for inhibiting the corrosion under the given conditions. The remarkable ability of phen might have resulted from (i) strong hydrophobicity of the formed film, and (ii) the decrease of

oxidation reduction potential of Cu2+/Cu+ coordinated by phen and NMI in the liquid system. With increase of carbon atoms in the hydrophobic side chain, inhibitor was changed from the anodic type to the cathodic type, indicating the fact that the length of the hydrophobic chain plays an important ro1e in controlling the active type and inhibition mechanism. When phen was used with the aid of NMI, a synergic effect was observed as a maximum inhibition was achieved. Also, in this case, the inhibition efficiencies for 304 and 5923 in the CuCl/ phen/NMI catalyst system are 98.1% and 98.2%, respectively. Although we cannot offer an explanation yet for this outstanding result, the small size of NMI might be favorable for inhibiting the corrosion of nonoverlay area. Figure 1 clearly demonstrates the corrosion status of hastelloy 5923 surface in the different catalytic system. In the pure CuCl catalytic system, the corrosion of 5923 is very serious. Figure 1a is a picture with 120 times magnification and shows the surface of hastelloy 5923 treated in the presence of CuCl. It is noted that many corrosive pits and corrosion products can be clearly observed on the alloy surface. By using phen or NMI as inhibitor, the corrosion can be significantly restricted. In the case of the CuCl/phen/NMI system, the alloy surface is clean,

10848

Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009

Figure 2. Cyclic voltammograms of CuCl/N-ligand in THF/0.1 M TBABF4 solution, potential sweep rate ) 0.050 V s-1, [CuCl] ) 2 mmol/L, nCu:nphen:nNMI ) 2:1:1, T ) 303 K.

and no obvious corrosion pitting could be observed even though the sample was magnified about 450 times. Electrochemical Behavior. The corrosion result suggests also that nickel-base alloys alone are inadequate for making the reactor when CuCl was used as a catalyst. Nevertheless, when phen and NMI were added simultaneously to the CuCl catalytic system, corrosion of the catalyst to the nickel-base alloys could be restricted significantly. Therefore, it might be a suitable material for making the reactor. Decrease of the corrosion rate may not only be attributed to the adsorption of the N-ligand on the metal surface but also due to the decrease of disproportionation of Cu+ to Cu0 and Cu2+ that can reasonably restrict the pitting corrosion of the material. To investigate the intrinsic redox performance in the reaction medium, electrochemical behavior of the CuCl/Ln catalyst systems was measured in THF/TBABF4 solutions, and the results are shown in Figure 2. Because of the fact that the ligands have no redox reaction in the region from -0.3 to 1.0 V vs SCE,31 the oxidation-reduction peaks observed here should be ascribed to the metal-centered redox process, in which the redox potential of the Cu+/Cu2+ couple would be located between 0.3 and 0.7 V vs SCE. The cyclic voltammogram of Cu2+ (line a in Figure 2) shows two redox processes.

It is obvious that N-ligand is a perfect inhibitor to restrict the corrosion of CuCl to the alloys. On the basis of the known mechanism of the corrosion inhibition, the inhibition behavior of the N-ligand mainly represents two respects. First, the N-ligand could coordinate with Cu+ making CuCl stable, decreasing the disproportionation of Cu+ and consequently restricting deposition of Cu0 on the alloy surface. Second, a close absorption of the N-ligand forms a film on the alloy surface, which separates the alloy from the reaction medium and blocks the progress of corrosion.33 The coordination of phen and the absorption of NMI might synergically effect the inhibition performance. Conclusions

Cu2++ e- f Cu+ at Epc ) 0.36 V

(1)

In this work, we have demonstrated that N-ligand promoters with different donor abilities and molecular structures could significantly affect the catalytic performance of CuCl for oxidative carbonylation of ethanol to DEC. Among all of the N-ligands studied, the combination of phen/NMI was proved to be a dual functional promoter for CuCl catalyst. With this combination, high catalytic activity and excellent corrosion inhibition were achieved. This new catalyst system, CuCl/phen/ NMI, would be highly expected to have a potential application in the commercial synthesis process of DEC by oxidative carbonylation.

Cu++ e- f Cu0 at Epc ) 0.06 V

(2)

Acknowledgment

As shown in Figure 2, the quasi-irreversible redox system observed with voltammograms of CuCl/phen and CuCl/phen/ NMI is obviously different from that of CuCl and CuCl/NMI. This difference in quasi-irreversible redox system could be attributed to a coordination of phen to CuCl that prevents reduction of CuCl to Cu0. Therefore, the dissolving process of the anode was limited, and, as a result, the metal is not easy to corrupt.32 In addition, it was found that the redox process (1) in Figure 2, which might be attributed to the Cu2+/Cu+ system, was shifted to negative potential values after addition of N-ligand in the electrolytic solution, indicating the fact that addition of N-ligand has an effect on the redox process (1). Meanwhile, the redox process (2) remains unchanged. These facts imply that the suspected interaction between CuCl and phen is stronger than the complexation of the CuCl by NMI.

We gratefully acknowledge financial support from the Torch High Technology Program for Industry Development, Ministry of Science and Technology, China [2003EB030282]. We are thankful for the financial support from the Research Foundation for Faculty of Science, Huazhong University of Science & Technology. We are also grateful to the Analytical and Testing Center of HUST for their help with EPMA measurement and the ThyssenKrupp Corp. of Germany for their kind supply of the sample of Hastelloy 5923. Literature Cited (1) Ouk, S.; Thiebaud, S.; Borredon, E.; Le Gars, P. Dimethyl carbonate and phenols to alkyl aryl ethers via clean synthesis. Green Chem. 2002, 5, 431. (2) Jeong, S. K.; Inaba, M.; Iriyama, Y.; Abe, T.; Ogumi, Z. AFM study of surface film formation on a composite graphite electrode in lithium-ion batteries. J. Power Sources 2003, 119-121, 555.

Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009 (3) Moumouzias, G.; Ritzoulis, G.; Siapkas, D.; Terzidis, D. Comparative study of LiBF4, LiAsF6, LiPF6, and LiClO4 as electrolytes in propylene carbonate-diethyl carbonate solutions for Li/LiMn2O4 cells. J. Power Sources 2003, 122, 57. (4) Herstedt, M.; Stjerndahl, M.; Gustafsson, T.; Edstrom, K. Anion receptor for enhanced thermal stability of the graphite anode interface in a Li-ion battery. Electrochem. Commun. 2003, 5, 467. (5) Gnanaraj, J. S.; Zinigrad, E.; Asraf, L.; et al. On the use of LiPF3(CF2CF3) 3 (LiFAP) solutions for Li-ion batteries, electrochemical and thermal studies. Electrochem. Commun. 2003, 5, 946. (6) Pacheco, M. A.; Marshall, C. L. Review of dimethyl carbonate (DMC) manufacture and its characteristics as a fuel additive. Energy Fuels 1997, 11, 2. (7) Roh, N. S.; Dunn, B. C.; Eyring, E. M.; et al. Production of diethyl carbonate from ethanol and carbon monoxide over a heterogeneous catalytic flow reactor. Fuel Process. Technol. 2003, 83, 27. (8) Dunn, B. C.; Guenneau, C.; Hilton, S. A.; et al. Production of diethyl carbonate from ethanol and carbon monoxide over a heterogeneous catalyst. Energy Fuels 2002, 16, 177. (9) Roh, N. S.; Dunn, B. C.; Eyring, E. M.; et al. Continuous production of diethyl carbonate over a supported CuCl2/PdCl2/KOH catalyst. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 2002, 47, 142. (10) Punnoose, A.; Seehra, M. S.; Dunn, B. C.; Eyring, E. M. Characterization of CuCl2/PdCl2/activated carbon catalysts for the synthesis of diethyl carbonate. Energy Fuels 2002, 16, 182. (11) Zhang, Z.; Ma, X. B.; Zhang, J.; He, F.; Wang, S. P. Effect of crystal structure of copper species on the rate and selectivity in oxidative carbonylation of ethanol for diethyl carbonate synthesis. J. Mol. Catal. A: Chem. 2005, 227, 141. (12) Liu, T. C.; Chang, C. S. Vapor-phase oxidative carbonylation of ethanol over CuCl-PdCl2/C catalyst. Appl. Catal., A: Gen. 2006, 304, 72. (13) Mo, W. L.; Li, G. X.; Zhu, Y. Q.; Xiong, H.; Mei, F. M. A novel catalyst CuCl/1,10-phenanthroline/N-methylimidazole for the oxidative carbonylation of ethanol to diethyl carbonate. Chin. J. Catal. 2003, 24, 3– 4. (14) Jiang, X. Z.; Su, Y. H.; Chien, S. H. Novel synthesis of diethyl carbonate over palladium/MCM-41 catalysts. Catal. Lett. 2001, 72, 245. (15) Jiang, X. Z.; Su, Y. H.; Lee, B. J.; Chien, S. H. A study on the synthesis of diethyl oxalate over Pd/Al2O3 catalysts. Appl. Catal., A: Gen. 2001, 211, 47. (16) Luo, H. P.; Xiao, W. D. A reactive distillation process for a cascade and azeotropic reaction system: carbonylation of ethanol with dimethyl carbonate. Chem. Eng. Sci. 2001, 56, 403. (17) Mo, W. L.; Liu, H. T.; Xiong, H.; Li, G. X. Preparation of CuCl/ 1,10-phenanthroline immobilized on polystyrene and catalytic performance in oxidative carbonylation of methanol. Appl. Catal., A: Gen. 2007, 333, 172. (18) Mo, W. L.; Xiong, H.; Li, T.; Guo, X. C.; Li, G. X. The catalytic performance and corrosion inhibition of CuCl/Schiff base system in

10849

homogeneous oxidative carbonylation of methanol. J. Mol. Catal. A: Chem. 2006, 247, 227. (19) Katritzky, A. R. Physical Methods in Heterocyclic Chemistry; Academic Press: New York, 1963; p 97. (20) Dean, J. A. Lang’s Handbook of Chemistry, 15th ed.; McGrawHill Book Co.: New York, 1999; Section 8, p 45. (21) Charton, M. Substituent effects in 1,10-phenanthrolines. I. Equilibria. J. Org. Chem. 1966, 31, 3739. (22) Hua, W. T. Heterocycle Chemistry; Beijing University Press: Beijing, 1991; p 176. (23) Bontempi, A.; Alessio, E.; Chanos, G.; Mestroni, G. Reductive carbonylation of nitroaromatic compounds to urethanes catalyzed by (di1,10-phenanthroline)palladium bis(hexafluorophosphate) and related complexes. J. Mol. Catal. 1987, 42, 67. (24) Anderson, S. A.; Root, T. W. Investigation of the effect of carbon monoxide on the oxidative carbonylation of methanol to dimethyl carbonate over Cu+X and Cu+ZSM-5 zeolites. J. Mol. Catal. A: Chem. 2004, 220, 247. (25) Sato, Y.; Kagotani, M.; Yamamoto, T.; Souma, Y. Novel effective poly(2,2′-bipyridine- 5,5′-diyl)-CuCl2. Appl. Catal., A: Gen. 1999, 185, 219. (26) Raab, V.; Merz, M.; Sundermeyer, J. Ligand effects in the copper catalyzed aerobic oxidative carbonylation of methanol to dimethyl carbonate (DMC). J. Mol. Catal. A: Chem. 2001, 175, 51. (27) Sato, Y.; Yamamoto, T.; Souma, Y. Poly(pyridine-2,5-diyl)-CuCl2 catalyst for synthesis of dimethyl carbonate by oxidative carbonylation of methanol: catalytic activity and corrosion influence. Catal. Lett. 2000, 65, 123. (28) Maruyama, T.; Yamamoto, T. New copper complex with π-conjugated electrically conductive polymer chelating ligand, poly(6,6′-dialkyl2,2′-bipyridine-5,5′-diyl). Preparation and optical and electrochemical properties of the complex. Inorg. Chim. Acta 1995, 238, 9. (29) Schmid, G. M.; Huang, H. J. Spectroelectrochemical studies of the inhibition effect of 4,7-diphenyl-1,10-phenanthroline on the corrosion of 304 stainless steel. Corros. Sci. 1980, 20, 1041. (30) Wei, Z. Q.; Duby, P.; Somasundaran, P. Pitting inhibition of stainless steel by surfactants: an electrochemical and surface chemical approach. J. Colloid Interface Sci. 2003, 259, 97. (31) Kitagawa, S.; Munakata, M.; Miyaji, N. The affinity for carbon monoxide and electrochemical and spectral properties of binuclear copper (I) complexes. Bull. Chem. Soc. Jpn. 1983, 56, 2258. (32) Lin, F.; Zhang, S. Z.; Shi, R. H. Electrochemical characteristics of corrosion of stainless steel. Mater. Prot. (in Chinese) 2000, 33, 50. (33) Moshier, W. C.; Davis, G. D. Interaction of molybdate anions with the passive film on aluminum. Corrosion 1990, 46, 43.

ReceiVed for reView July 16, 2009 ReVised manuscript receiVed October 15, 2009 Accepted October 17, 2009 IE901139E