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Articles Synthesis of Dimethyl Carbonate by Oxidative Carbonylation Using an Efficient and Recyclable Catalyst Co-Schiff Base/Zeolite Dajian Zhu, Fuming Mei, Lijuan Chen, Tao Li, Wanling Mo, and Guangxing Li* Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and SerVice Failure, Huazhong UniVersity of Science and Technology, 1037 Luoyu Road, Wuhan 430074, P.R. China ReceiVed December 18, 2008. ReVised Manuscript ReceiVed March 3, 2009
Dimethyl carbonate (DMC) has been synthesized by oxidative carbonylation of methanol over the Co-Schiff base complexes, which have been encapsulated in situ in zeolite Y by a “ship-in-the-bottle” approach. These hybrid materials Co-Schiff base/zeolite Y have been characterized by FT-IR, UV-vis, XRD, BET, and TG/ DTA techniques. Analysis of the hybrid materials indicates the formation of complexes in the cavity without affecting the zeolite framework structure. The catalytic activities and corrosion behavior of the encapsulated complexes and their homogeneous analogues were examined. Zeolite-encapsulated Co complexes were found to be more active and stable than the neat Co complexes. When the reaction was carried out using 1.0 g of encapsulated catalyst, 40 mmol of methanol, CO/O2 ratio of 2:1, and at 3.0 MPa and 120 °C for 4 h, zeoliteencapsulated Co-salophen shows the highest activity; and the conversion of methanol and selectivity to DMC were 25.4 and 99.5%, respectively. It was also demonstrated that zeolite-encapsulated Co-salophen catalyst can be reused five times without loss of activity. Both neat and encapsulated Co-salophen exhibit noncorrosive behavior to the reactor made by stainless steel.
1. Introduction Dimethyl carbonate (DMC) is an environment-friendly chemical for many potential applications.1,2 Research has shown that DMC could be used as an oxygenated fuel additive of gasoline or diesel oil, to replace the methyl-tert-butyl ether (MTBE).3 Moreover, DMC can reduce the surface tension of diesel boiling range fuels and provide an improved diesel fuel, thereby giving better injection delivery and spray. A further significant advantage of DMC over other fuel additives is that DMC could slowly decompose to CO2 and methanol, which have no serious environmental impact when released into the environment.4 There are several methods available for the preparation of DMC.2,5 The oxidative carbonylation of methanol is the most prospective phosgene-free route for industrial production of DMC. A lot of catalytic systems have been extensively investigated in this process.6-13 Among those transition metal * Corresponding author. Phone: +86-27-87543732; fax: +86-27 87544532; e-mail:
[email protected]. (1) Delledonne, D.; Rivetti, F.; Romano, U. Appl. Catal., A 2001, 221, 241–251. (2) Pacheco, M. A.; Marshall, C. L. Energy Fuels 1997, 11, 2–29. (3) Lu¨, X. C.; Yang, J. G.; Zhang, W. G.; Huang, Z. Energy Fuels 2005, 19, 1879–1888. (4) Crandall, J. W.; Deitzler, J. E.; Kapicak, L. A.; Poppelsdorf, F. US Patent 4663477, 1987. (5) Filippis, P. D.; Scarsella, M.; Borgianni, C.; Pochetti, F. Energy Fuels 2006, 20, 17–20. (6) Delledonne, D.; Rivetti, F.; Romano, U. J. Organomet. Chem. 1995, 488, C15-C19. (7) Chin, C. S.; Shin, D.; Won, G.; Ryu, J.; Kim, H. S.; Lee, B. G. J. Mol. Catal. A: Chem. 2000, 160, 315–321.
catalysts, palladium and copper are most frequently used.7-13 Palladium catalyst is very expensive and easy to deactivate.7 CuCl catalyst is highly corrosive to metallic vessels due to the existence of Cl- and the redox reaction between Cu(II)/Cu(I) and Cu(0).12 For many years, modification of the copper chloride catalyst has been sought by different research groups in the effort to improve the catalyst performance,9,11 but the catalytic activity and corrosion are hard to compromise. Co-Schiff base complexes are known to catalyze the oxidation of numerous organic substrates,14-16 but examples of application of this catalyst in oxidative carbonylation of methanol are few.6 Co-Schiff base systems appear very attractive because they are halogen-free and are noncorrosive compared to systems containing chloride, especially to copper chlorides. However, the Co macrocyclic complex undergoes (8) Raab, V.; Merz, M.; Sundermeyer, J. J. Mol. Catal. A: Chem. 2001, 175, 51–63. (9) Hu, J. C.; Cao, Y.; Yang, P.; Deng, J. F.; Fan, K. N. J. Mol. Catal. A: Chem 2002, 185, 1–9. (10) Mo, W. L.; Xiong, H.; Li, T.; Guo, X. C.; Li, G. X. J. Mol. Catal. A: Chem 2006, 247, 227–232. (11) Mo, W. L.; Liu, H. T.; Xiong, H.; Li, M.; Li, G. X. Appl. Catal., A 2007, 333, 172–176. (12) Yasushi, S.; Masahiro, K.; Yoshie, S. J. Mol. Catal. A: Chem. 2000, 151, 79–85. (13) Zhang, Y. H.; Briggs, D. N.; Smit, E.; Bell, A. T. J. Catal. 2007, 251, 443–452. (14) Baleizao, C.; Garcia, H. Chem. ReV. 2006, 106, 3987–4043. (15) Kervinen, K.; Korpi, H.; Leskela¨, M.; Repo, T. J. Mol. Catal. A: Chem. 2003, 203, 9–19. (16) Amarasekara, A. S.; Oki, A. R.; McNeal, I.; Uzoezie, U. Catal. Commn. 2007, 8, 1132–1136.
10.1021/ef801115e CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
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Scheme 1. Preparation process of zeolite-encapsulated Co-Schiff base complexes
Scheme 2. Zeolite-encapsulated Co-Schiff base complex as an oxidative carbonylation catalyst
dimerization, and this will severely shorten the lifetime of the Co-Schiff base catalytic system.6 So far few attempts have been made to improve the stability of Co complexes in the oxidative carbonylation of methanol. Indeed, there are some ways to avoid dimerization and degradation of the Co-Schiff base complexes. One is to encapsulate it into the supercage of zeolite. This strategy has previously been used in many reactions such as partial oxidation,17,18 selective hydrogenation,19,20 and electro- catalytic reactivity.21 Immobilization of those complexes within zeolite has surely enhanced their stability and reactivity. Recently, we have successfully synthesized several neat and encapsulated Co-Schiff base complexes and have studied the structural and catalytic properties in the oxidative carbonylation of aniline.22,23 Presently, we are extending our study to the oxidative carbonylation of methanol catalyzed by the encapsulated Co-Schiff base complexes in zeolite Y. The confined cages of zeolite Y can effectively inhibit the formation of the µ-oxo dimers of the Co complexes that are inactive for the oxidative carbonylation.6 In the present work, the details of characterization, catalytic activity, and corrosion behavior of the zeolite-encapsulated Co complex catalyst are presented.
the integration sphere diffuse reflectance attachment. Thermogravimetric analysis of the samples was conducted on a Perkin-Elmer TGA-7 instrument at a heating rate of 10 °C min-1 under an Ar atmosphere. The cobalt leaching content in the filtrate was measured by AAS using a Perkin-Elmer AA-300 spectrophotometer. The surface area and the pore volume of zeolite-encapsulated catalysts were determined by the Brunauer-Emmett-Teller (BET) method on a micrometrics ASAP 2020-M apparatus. X-ray diffraction (XRD) profiles were recorded in a Philip X-Pert Pro X-ray diffractometer using Cu KR radiation in the 2θ range 4-50°, at a scan rate of 13° min-1. 2.4. Catalytic Activity and Stability Tests. Catalytic oxidative carbonylation of methanol was performed in a 0.1 L stainless steel autoclave lined with Teflon and equipped with a magnetic stirrer. In a typical experiment, the appropriate amount of Co complexes or zeolite-encapsulated Co complexes, 40 mmol of methanol, and 25 mL of acetonitrile were loaded into the autoclave. The autoclave was purged three times with CO and then pressurized to 3.0 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 then the temperature was held at 393 K. The reaction process in the presence of zeolite-encapsulated Co complexes was shown in Scheme 2. After the reaction, the reactor was cooled down to room temperature and depressurized over a period of 10 min via an ice-salt bath. The heterogeneous catalyst was thoroughly washed with acetonitrile after filtration, and dried at 80 °C under vacuum, then reused in the next run without changing the reaction conditions. The products were analyzed by GC equipped with a SE-30 (30 m × 0.32 mm × 0.25 µm) capillary column and flame ionization detector. 2.5. Determination of Co Leaching. To determine the Co content in solution, part of the filtrate was placed in a 50 mL crucible and heated to 600 °C with a heating rate of 10 °C/min, then calcined at 600 °C for 3 h. The residue was dissolved in aqua regia and diluted. The Co leaching in solution was detected by AAS. 2.6. Corrosion Behavior Tests. During the reaction the corrosion rate in the presence of different catalysts was measured by method of weight loss. Test pieces (20 mm × 10 mm × 3 mm) of stainless steel (1Cr18Ni9Ti: Ni 9.43%, Cr 17.19%, C 0.09%, Mn 1.10%, Ti 0.34%, Fe base) were introduced into the autoclave using a special sample cage. The corrosion data were obtained at 393 K after reaction for 72 h under a pressure of 3.0 MPa. The corrosion rate (CR) was calculated using the following equation:
2. Experimental Section 2.1. Reagents. Na-Y zeolite was purchased from Nanjing Nanda Surface and Interface Chemical Engineering and Technological Research Center Co. Ltd., China. Salicylaldehyde, cobalt acetate tetrahydrate, ethylenediamine, diethylenetriamine, 1,2cyclohexanediamine, o-phenylenediamine, and acetonitrile were analytical grade reagents. All the solvents were refined prior to use. Carbon monoxide with a purity of 99.99% and oxygen with a purity of 99.9% were purchased from the local manufacturer. 2.2. Catalysts Preparation. The four Schiff base ligands, denoted as salen, saldien, salcyen, and salophen, respectively, were synthesized according to the procedures described in our previous article.23 According to the literature,23 the encapsulated catalysts were prepared by a stepwise method shown in Scheme 1. 2.3. Characterizations of the Catalysts. Infrared spectra were recorded on an Equinox 55 FT-IR spectrophotometer in the range 4000-400 cm-1. The UV-vis spectroscopy experiments were performed on a Shimadzu UV-2550 spectrophotometer with
CR ) (17) Xavier, K. O.; Chacko, J.; Mohammed Yusuff, K. K. Appl. Catal., A 2004, 258, 251–259. (18) Joseph, T.; Halligudi, S. B.; Satyanarayan, C.; Sawant, D. P.; Gopinathan, S. J. Mol. Catal. A: Chem. 2001, 168, 87–97. (19) Mo¨llmann, E.; Tomlinson, P.; Ho¨lderich, W. F. J. Mol. Catal. A: Chem. 2003, 206, 253–259. (20) Kahlen, W.; Wagner, H. H.; Ho¨lderich, W. F. Catal. Lett. 1998, 54, 85–89. (21) Bessel, C. A.; Rolison, D. R. J. Am. Chem. Soc. 1997, 119, 12673– 12674. (22) Chen, L. J.; Bao, J.; Mei, F. M.; Li, G. X. Catal. Commun. 2008, 9, 658–663. (23) Li, G. X.; Chen, L. J.; Bao, J.; Li, T.; Mei, F. M. Appl. Catal., A 2008, 346, 134–139.
24 × 365 ∆W × S × 72 F
(1)
CR: corrosion rate (mm/a); ∆W: wight loss of the test piece (mg); S: surface area of the test piece (mm2); F: density of the test piece (g/cm3).
3. Results and Discussion 3.1. FT-IR of the Catalysts. The FT-IR spectra of Na-Y and encapsulated Co complexes are shown in Figure 1. Na-Y zeolite presents the characteristic bands at 460, 577, 726, 790, 1019, and 1118 cm-1 (Figure 1I, curve f). These bands are not
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Figure 2. UV-vis spectra of the samples: (a) Na-Y, (b) Co-Y, (c) Co(salen)-Y, (d) Co(salcyen)-Y, (e) Co(salophen)-Y, and (f) Co(saldien)-Y.
Figure 1. FT-IR spectra of the samples: (a) Co(salen)-Y, (b) Co(salcyen)-Y, (c) Co(saldien)-Y, (d) Co(salophen)-Y, (e) Co-Y, (f) Na-Y, (g) Co(salophen).
Figure 3. XRD patterns of the samples (a) Na-Y and (b) Co(salophen)-Y. Table 1. The Textural Properties of Samples
significantly modified following ionic exchange with Co2+ or by supporting the Co complexes (Figure 1I, curves a-e), proving that the structure of the zeolite is preserved.23 This also indicates that the structure of metal complexes fit nicely within the cavity of the zeolite. The IR bands of all encapsulated complexes are weak due to their low concentration in the zeolite cages and thus can only be observed in the regions where the zeolite matrix does not absorb, that is, from 1200 to 1600 cm-1. As an example, Figure 1II shows the IR spectra of the Co(salophen) complex, the encapsulated catalyst, and the NaY zeolite, which is even more emphasized on the extended spectrum in the range of 1600-1200 cm-1. The IR spectra of the Co(salophen) complex show major bands around 1521, 1460, 1443, 1378, 1325, and 1243 cm-1. Similar frequencies were observed in the case of Co(salophen)-Y catalyst,24 indicating the incorporation of the complex in the zeolite. The results showed that the Co-Schiff base complex was indeed present in the cavity of the zeolite Y. 3.2. UV-vis Spectra of the Catalysts. The formation and encapsulation of Co complexes inside zeolite Y are further confirmed by UV-vis spectra shown in Figure 2. It is evident that after complexation and further extraction, a strong band at 325 nm is present in the spectra of the Co(salen)-Y, Co(salcyen)-Y, Co(salophen)-Y, and Co(saldien)-Y samples, while it is not observed for the Na-Y and Co-Y samples. The
parent NaY showed a spectrum similar to that of Co-Y, which has no absorption band above 300 nm. Poltowicz reported that the band at 325 nm was attributed to ligand-to-metal charge transfer, giving a strong evidence for the formation of Co-Schiff base complex molecules inside the cavity of zeolite Y.25 3.3. XRD of the Catalysts. XRD patterns of Na-Y and the encapsulated Co(salophen)-Y are shown as examples in Figure 3. Those of other catalyst samples were similar. Essentially similar diffraction patterns were noticed in the encapsulated Co(salophen)-Y and Na-Y, except the zeolite with encapsulated Co(salophen) has a slightly weaker intensity. These observations indicate that the framework of the zeolite does not suffer any significant structural changes during encapsulation. Nevertheless, a slight modification occurred, as confirmed by the alteration of the relative intensities of the 220 and 311 reflections (2θ of about 10 and 12°, respectively). For Na-Y, I220 was slightly higher than I311, whereas after ion exchange
(24) Johansson, M.; Purse, B. W.; Terasaki, O.; Ba¨ckvall, J. AdV. Synth. Catal. 2008, 350, 1807–1815.
(25) Połtowicz, J.; Pamin, K.; Tabor, E.; Haber, J.; Adamski, A.; Sojka, Z. Appl. Catal., A 2006, 299, 235–242.
sample
Co content (wt%)
SBET (m2/g)
specific pore volume (ml/g)
Na-Y Co(salen)-Y Co(saldien)-Y Co(salcyen)-Y Co(salophen)-Y
1.36 1.42 1.39 1.40
845.3 454.6 435.7 446.3 443.2
0.443 0.241 0.219 0.232 0.223
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No.
cat
1 2 3 4 5 6 7 8 9 10
CuCl CuCl/Phenc Co(salen) Co(salcyen) Co(salophen) Co(saldien) Co(salen)-Y Co(salcyen)-Y Co(salophen)-Y Co(saldien)-Y
0.60b 0.60b 0.60 0.60 0.60 0.60 0.58 0.60 0.59 0.59
5.4 15.5 16.7 18.2 21.2 5.7 19.6 22.9 25.4 7.0
96.1 98.2 99.5 99.5 99.5 90.3 99.5 99.5 99.5 91.0
5.2 15.2 16.6 18.1 21.1 5.1 19.5 22.8 25.3 6.4
a Reaction conditions: 1.0 g of encapsulated catalyst, 40 mmol of methanol, 25 mL of acetonitrile, P(CO) ) 2.0 MPa, P (O2) ) 1.0 MPa, 120 °C, 4 h. b n(CuCl)/n(methanol) ) 0.60%. c Phen: 1,10-phenanthroline, n(CuCl)/n(Phen) ) 1:1.
Figure 4. TG and DTA patterns of Co(salophen) (a and a′) and Co(salophen)-Y (b and b′).
with Co2+ and coordination to salophen I220 became lower than I311. This is typical for the formation of large transition metal complex in the supercages of zeolite Y,23 which can also previously be verified by FT-IR spectroscopy and UV-vis spectroscopy. 3.4. Textural Property. Table 1 lists the textural properties of the encapsulated Co complexes. The Co contents of different encapsulated samples were almost the same in the range of 1.3-1.5 wt %. Considerable loss of surface area and pore volume was observed on zeolite-encapsulated complexes. Since the zeolite crystallinity was retained, such loss of surface area and pore volume was a direct evidence for the presence of complexes in the zeolite cages and not on the external surface. A similar reduction of surface area was reported in the case of encapsulated salen complex in the previous literatures.17,18 3.5. TG/DTA of the Catalysts. The TG/DTA patterns of the prepared Co(salophen) and Co(salophen)-Y are displayed in Figure 4. The neat Co complex showed weight loss at about 330 °C. However, for the corresponding encapsulated complex (Figure 4, traces b and b′), the weight loss extends up to 400 °C, which indicated the thermal stability was greatly enhanced. This gives another piece of strong evidence for the inclusion of Co(salophen) in Na-Y. A similar enhancement of the thermal stability of metal complexes by encapsulation has been observed earlier.18,23 On the basis of the thermal analysis data, we may conclude that zeolite-encapsulated Co(salophen) may be treated thermally in the catalytic reaction (for example, in 120 °C) without any significant decomposition. 3.6. Activity and Stability of the Catalysts. The catalytic activities of the encapsulated Co complexes and their homogeneous analogues in the oxidative carbonylation of methanol to DMC are shown in Table 2. Co-Schiff base complexes except Co(saldien) showed high activity and selectivity toward formation of DMC. Even the catalyst CuCl/Phen, which exhibited the highest catalytic activity among the homogeneous Cu-Schiff base complex catalysts,10 is less active in comparison with Co complexes (entries 2-5). From the data in Table 2, one can notice that the encapsulated catalysts show better activity than their homogeneous analogues at the same conditions. A similar observation has been reported in our earlier article for the oxidative carbonylation of aniline.23 The results indicated that immobilization of complexes in the supercages of zeolite was advantageous over neat complexes in the site isolation of complex molecules. The reason is that the encapsulation prevents the formation of dimerization and improves the catalytic efficiency of Co complexes. This site isolation by
Table 3. Recycle Tests of Co(salophen)-Y in the Oxidative Carbonylation of Methanola recycle times
MeOH conv, %
DMC selectivity, %
DMC yield, %
1 2 3 4 5
25.4 23.5 23.2 22.9 22.0
99.5 99.5 99.5 99.0 99.0
25.3 23.4 23.1 22.7 21.8
a Reaction conditions: 1.0 g of encapsulated catalyst, 40 mmol of methanol, 25 mL of acetonitrile, P(CO) ) 2.0 MPa, P(O2) ) 1.0 MPa, 120 °C, 4 h.
encapsulation was proved to be an effective strategy to preclude dimerization in many reactions and to enhance the lifetime and activity of catalysts.26,27 Among the homogeneous catalysts, the complex bearing phenyl group (in Table 2, entry 5) shows the best catalytic activity. For the conjugated effect of phenyl group, the electron density of the N atom coordinated with Co was enhanced, which may be contributed to increase the catalytic activity. Co(saldien) (in Table 2, entry 6) with a square-pyramidal coordination performed poorly compared with the other complexes with a square-plane coordination (entries 3-5). These effects of encapsulated catalysts are in agreement with the neat catalysts. Zeolite-encapsulated Co-salophen catalyst shows the highest activity, and the conversion of methanol and selectivity to DMC are 25.4 and 99.5%, respectively. The important point concerning heterogeneous catalysis is the deactivation and reusability of the catalyst, which is very vital in the commercialization of a new catalytic system. To test this, a series of five consecutive runs of the reaction were carried out. The catalytic activities and recyclability of the encapsulated Co-salophen as an example are shown in Table 3. Romano et al.6 reported that the catalytic activity after the first cycle of neat Co-Schiff base complexes had a sharp decrease because of the dimerization of the Co complexes. In contrast, there is no significant change in the activity of the encapsulated catalyst during five runs except a slight decrease after the first run. This slight decrease of activity is accompanied with a small amount of Co loss in the first run, present in Table 4. Bessel et al.28 confirmed that small amounts of Co2+ and excess ligands were trapped in zeolite in the catalyst Co(salen)/ Na-Y owing to diffusion being hindered by Co(salen) complexes in the cages. Such small amounts of Co2+ and ligand, (26) Balkus, K. J.; Eissa, M.; Levado, R. J. Am. Chem. Soc. 1995, 117, 10753–10754. (27) Wo¨ltinger, J.; Ba¨ckvall, J.; Zsigmond, Á. Chem.sEur. J. 1999, 5 (5), 1460–1467. (28) Bessel, C. A.; Rolison, D. R. J. Phys. Chem. B 1997, 101, 1148– 1157.
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Table 4. Co Leaching of Co(salophen)-Y Catalyst run
1
2
3
4
5
Co lossa (%)
0.89
