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Novel Route for the Synthesis of Methyl Propionate from 3-Pentanone with Dimethyl Carbonate over Solid Bases Zhi Chen* and Dudu Wu Center of Analysis, Guangdong Medical College, Dongguan 523808, China ABSTRACT: A novel and simple route for the synthesis of methyl propionate by the methoxycarbonylation of 3-pentanone with dimethyl carbonate in the presence of solid base catalysts was developed. Solid bases with moderate strength (such as MgO) facilitated the formation of methyl propionate, and the yield of methyl propionate could be linearly correlated with the amount of moderate basic sites. The function of basic catalysts was mainly attributed to the activation of 3-pentanone through the abstraction of the α-H by base sites.
1. INTRODUCTION Methyl propionate (MP) is an important fine chemical for the preparation of perfumes, dyes, and agrochemicals.1 Conventionally, it is synthesized through the esterification of propionic acid with methanol with sulfuric acid as a catalyst, which suffers from drawbacks such as corrosion, pollution, and separation problems.2 An alternative route is through the hydroesterification of ethylene in the presence of methyl formate or methanol with carbon monoxide, but it requires toxic and expensive catalysts.3,4 Dimethyl carbonate (DMC), as an effective additive to gasoline, has been attracting much attention in the energy field in recent years because of its high oxygen content and high octane value.5 It could also be an environmentally benign building block in fine chemistry because of its versatile chemical properties and low toxicity. DMC is a safe substitute for methyl halides or dimethyl sulfate as a methylating and carboxylating agent.6,7 In the presence of a base, it can also react with a number of nucleophilic anions generated from different substrates such as ketones, amines, oximes, indoles, and phenols to give the methoxycarbonylation products.8 11 In the 1970s, Ruset first reported the reaction of cyclohexanone with DMC to form a methoxycarbonylation product (carbomethoxycyclohexanone) over NaH.12 Later, the reaction of alicyclic ketones with DMC was found to give dimethyl esters in the presence of homogeneous catalysts such as Cs2CO3crown ether, CH3ONa, and organic amines.13 15 However, few works have focused on the reaction of aliphatic ketones with DMC,16 and to the best of our knowledge, the reaction of 3-pentanone with DMC has not been investigated previously, even though it might be an attractive synthetic route for the production of MP because it is a safe and clean chemical process. Moreover, research on the synthesis of fine chemicals using solid bases as catalysts has increased over the past decade because of its economic and ecological significance.17 Thus, a novel and facile one-pot approach to the production of methyl propionate was developed through the methoxycarbonylation of 3-pentanone with dimethyl carbonate over solid base catalysts, and the function of the solid base catalysts is discussed in the present work. r 2011 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. CaO, MgO, and ZrO2 were prepared by the thermal decomposition under a nitrogen atmosphere of calcium carbonate at 800 °C for 2 h, magnesium hydroxide at 500 °C for 5 h, zirconium hydroxide at 500 °C for 5 h, respectively. Lanthana (La2O3, Tianjin Kermel Chemical Reagent Co. Ltd.) and alumina (Al2O3, Tianjin Kermel Chemical Reagent Co. Ltd.) were purchased and treated in N2 at 700 and 500 °C, respectively, for 5 h before use. Hβ was obtained by calcining the ammonium form (NH4-beta, Shanghai Chemical Reagent Co.) in air at 550 °C for 3 h. A series of Al/MgO samples with molar ratios of 0.01 0.3 were prepared using the impregnation method. A certain amount of aluminum isopropoxide (molar ratio of Al/Mg was from 0.01 to 0.3) was dissolved in benzene and then added to Mg(OH)2 to form the impregnated samples. After impregnation, each sample was dried at 100 °C for 12 h and finally treated under flowing N2 at 500 °C for 5 h. The specific surface areas of the samples were obtained by the Brunauer Emmett Teller (BET) method. The samples were degassed at 350 °C for 5 h under a vacuum (10 6 Torr) prior to the measurement, and nitrogen adsorption/desorption isotherms were obtained at 196 °C on a Micromeritics ASAP2000 instrument (Norcross, GA). CO2 temperature-programmed desorption (CO2 TPD) measurements were performed using Ar as a carrier gas. Catalyst samples (0.10 g, 40 60 mesh) were pretreated in Ar at 500 °C for 2 h. After the samples had cooled to room temperature, CO2 was pulsed to the reactor until saturation was reached. Once physically adsorbed CO2 had been purged off by the carrier gas, CO2 TPD experiments were carried out from 20 to 800 °C at a heating rate of 10 °C/min under an Ar flow (50 mL/min), and the effluent was detected with a gas chromatograph that employed a thermal conductivity detector. Received: March 20, 2010 Accepted: October 6, 2011 Revised: October 3, 2011 Published: October 06, 2011 12343
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2.2. Catalytic Test. The reaction was carried out in an autoclave reactor composed of a 150 mL stainless-steel autoclave and a magnetic stirrer (revolving at a rate of 1000 rpm).
The reaction conditions were as follows: 3-pentanone/DMC = 1:6 (molar ratio), time = 5 h, T = 260 °C, catalyst content = 1.5 wt % (based on the total mass of reactant). Typically, 3-pentanone (4.00 g), DMC (25.15 g), and the catalyst (0.44 g) were charged into the autoclave, and the resulting mixture was heated to 260 °C within 30 min. After running at 260 °C for 5 h under autogenous pressure (about 1 3 MPa in the temperature range from 200 to 280 °C), the reactor was cooled in situ to room temperature. The catalysts were removed by rapid filtration on a vacuum, and the liquid products were analyzed with a gas chromatograph (GC-920, Shanghai Haixin Chromatograph Instrument Co. Ltd.) with a flame ionization detector and an HP-5 column. Moreover, the amount of moderate basic sites was determined by calculating the areas of the desorption peaks in the CO2 TPD profiles using Microcal Origin software.
3. RESULTS AND DISCUSSION 3.1. Basicity. Figure 1 presents TPD profiles of CO2 adsorbed on ZrO2, MgO, La2O3 , and CaO. CaO exhibited a major Figure 1. CO2 TPD profiles of solid base catalysts.
Figure 3. Relationship between methyl propionate yield and amount of moderate basic sites.
Figure 2. CO2 TPD profiles of MgO with different Al contents.
Table 1. CO2 Uptake and Synthesis of Methyl Propionate over Solid Catalysts selectivityc (%) runa
catalystb
BET surface area (m2/g)
CO2 uptake (mmol/m2)
3-pentanone conversion (%)
MP
MO
ACPs
MLPs
73.6
26.4
1 2
Hβ
10.1
3
Al2O3
21.5
Traces
Traces
55.8
44.2
4
NaOCH3
46.4
43.2
16.8
35.0
5.04
5
NaOH
6 7
ZrO2 La2O3
150 6.9
2.01 10 1.36 10
5
8
MgO
118.5
2.54 10
9
CaO
10.7
4.38 10
47.7
38.1
23.7
36.0
2.23
12.1 22.7
27.4 61.0
13.7 31.3
40.2 5.94
18.7 1.76
4
40.9
53.9
27.6
11.5
7.00
3
30.3
54.7
28.3
10.8
6.22
4
Reaction conditions: 3-pentanone/DMC = 1:6, time = 5 h, T = 260 °C, catalyst content = 1.5 wt % (based on the total mass of reactant). b Amount of NaOH and NaOCH3 equaled the amount of basic sites of MgO. c MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2pentene). a
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Figure 4. Effects of temperature on the conversion of 3-pentanone and selectivities of products. Reaction conditions: time = 5 h, catalyst content = 1.5 wt %, 3-pentanone/DMC = 1:6. MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2-pentene).
Figure 5. Effect of reaction time on the 3-pentanone conversion and product selectivities. Reaction conditions: 3-pentanone/DMC = 1:6, T = 260 °C, catalyst content = 1.5 wt %. MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2-pentene).
desorption at 550 °C, indicating strong basic sites on its surface. La2O3 showed a relatively strong basic strength, with a CO2 desorption peak at 450 °C. For MgO, three desorption peaks at 100, 200, and 300 °C were observed, suggesting that MgO has weak and moderate basic sites, but for ZrO2, only weak basic sites with a CO2 desorption peak at 170 °C was detected. The basic strengths followed the order CaO > La2O3 > MgO > ZrO2, but the basic numbers of catalysts ranked in the order CaO > MgO > La2O3 > ZrO2 (see Table 1). The addition of alumina did not change the basic strength of MgO (see Figure 2), but the basic numbers decreased with increasing Al content. 3.2. Catalytic Performance. The reaction was carried out in the temperature range from 200 to 280 °C, and the main byproduct was methyl 2-methyl-3-oxopentanoate, which was the intermediate methoxycarbonylation product. The self-condensation product
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Figure 6. Effects of the amount of catalyst on the 3-pentanone conversion and product selectivities. Reaction conditions: 3-pentanone/DMC = 1:6, T = 260 °C, time = 5 h. MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2-pentene).
Figure 7. Effects of molar ratio on the 3-pentanone conversion and product selectivities. Reaction conditions: T = 260 °C, time = 5 h, catalyst content = 1.5 wt %. MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2-pentene).
of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one) and the methylation products (2-methyl-3-pentanone and 3-methoxy-2-pentene) were also detected. The catalytic performances of the solid catalysts in the synthesis of MP from the reaction of 3-pentanone with dimethyl carbonate are reported in Table 1. It can be seen that no products were detected when the catalyst was not present and that only the aldol condensation products of 3-pentanone and methylation product were detected over solid acids such as Al2O3, which suggests that solid acid catalysts are inactive toward the synthesis of MP. However, the reaction was efficiently catalyzed by solid bases. Among those bases, MgO, which has a moderate basic strength, exhibited the highest performance. ZrO2 and La2O3 gave lower MP yields, which might be due to their relatively weak basic numbers 12345
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Table 2. Recycling of MgO in the Reaction of DMC and 3-Pentanone selectivity (%)d 3-pentanone catalysta
conversion (%)
MP
MO
ACPs
MLPs
MgO
40.9
53.9
27.6
11.5
MgO (r1)b
35.4
42.3
21.5
17.5
18.7
7.00
MgO (r2)b
32.6
40.7
20.6
17.1
21.6
MgO (r4)b
25.4
35.8
18.7
19.2
26.3
MgO (r6)b
19.7
31.4
16.5
21.7
30.4
MgO (r1)*c
39.8
51.4
28.7
11.2
MgO (r2)*c MgO (r4)*c
40.5 39.2
52.3 50.6
24.3 26.9
13.2 11.1
10.2 11.4
MgO (r6)*c
40.3
49.5
25.3
11.5
13.7
8.70
Reaction conditions: 3-pentanone/DMC = 1:6, T = 260 °C, time = 5 h, catalyst content = 1.5 wt % (based on the total mass of reactant). b Used catalysts obtained from the filtration of the reaction mixture. c Used catalysts obtained by calcination at 500 °C for 5 h under a nitrogen atmosphere after filtration from the reaction mixture. d MP, methyl propionate; MO, methyl 2-methyl-3-oxopentanoate; ACPs, aldol condensation products of 3-pentanone (5-ethyl-4-methyl-4-hepten-3-one); MLPs, methylation products (2-methyl-3-pentanone + 3-methoxy-2pentene). a
compared to that of MgO. CaO provided slightly lower 3-pentanone conversion and MP selectivity despite its high basic numbers. This can be attributed to the acid strength of Ca cations being too weak to stabilize the carbanion species.18 It should be noted that MgO even showed a higher MP selectivity than homogeneous catalysts (NaOH and NaOCH3) with the same amount of basic sites. As a result, both basic numbers and basic strength played important roles in the reaction, and basic sites with moderate strength were effective for the synthesis of methyl propionate by the methoxycarbonylation of 3-pentanone with dimethyl carbonate. To further investigate the effect of moderate basic sites on catalytic performance, a series of MgO catalysts with different Al contents were prepared to supply different amount of moderate base sites (see Figure 2). It was found that the addition of alumina did not change the basic character of MgO, but the density of moderate basic sites decreased with increasing Al content. For MgO-based catalysts, a good linear correlation was observed between the methyl propionate yield and the amount of moderate basic sites (see Figure 3). Thus, the synthesis of methyl propionate over solid base catalysts should be closely related to the moderate basic sites. 3.3. Effects of the Reaction Parameters. The effects of reaction parameters on this reaction over MgO were investigated. As shown in Figure 4, the conversion of 3-pentanone increased sharply with the temperature, suggesting that high
Scheme 1. Plausible Mechanism for the Reaction of DMC with 3-Pentanone
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dx.doi.org/10.1021/ie1006933 |Ind. Eng. Chem. Res. 2011, 50, 12343–12348
Industrial & Engineering Chemistry Research temperature favored 3-pentanone conversion. At a temperature of 260 °C, the selectivity of MP reached a maximum of 53.9%. This could be attributed to the production of aldol condensation products and methylation products at high temperature. On the other hand, the thermal decomposition of DMC also increased with increasing temperature, especially beyond 260 °C.19 As a result, the optimum temperature for the production of MP was about 260 °C. Figure 5 shows the effect of the reaction time, and it can be seen that both the 3-pentanone conversion and the product selectivities changed slightly after 5 h, indicating that the reaction almost reached equilibrium. Furthermore, a similar tendency was also observed when the amount of catalyst changed (see Figure 6). The conversion of 3-pentanone remained unchanged after the amount of catalyst reached 1.5 wt %. Moreover, excess 3-pentanone was found to be favorable for the formation of aldol condensation products (see Figure 7). However, both 3-pentanone conversion and MP selectivity increased with the rise of DMC. When the molar ratio of DMC to 3-pentanone was over 6, the reaction almost reached equilibrium. The recycling of MgO in the reaction of 3-pentanone with DMC is presented in Table 2. It was found that the 3-pentanone conversion decreased by 21.2% and the MP selectivity decreased by 22.5% after the catalysts had been used for six times. Meanwhile, the selectivity of methylation products (including 2-methyl3-pentanone and 3-methoxy-2-pentene) reached 30.4%. These results might indicate that Mg salts of carbonates were generated, because of the formation of carbon dioxide in the methylation reaction. Fortunately, the activity of used catalysts could be recovered through calcination under the preparation conditions even after the catalysts had been used several times (see Table 2). 3.4. Plausible Reaction Mechanism. The reaction of 3-pentanone with DMC was probably started with the activation of ketones through the abstraction of the α-H (Hα) by basic sites, because DMC was hardly activated by solid bases.20 Moreover, the selectivity of methyl 2-methyl-3-oxopentanoate always changed in opposition to the selectivity of MP despite the variety of reaction conditions (see Figures 4 7). Thus, the mechanism of methyl propionate formation is proposed as follows (see Scheme 1): An anion of 3-pentanone is generated by the abstraction of the α-H by the solid base, and then the anion and proton are absorbed on acidic sites (Mn+) and basic sites (O2 ) on the solid base surface, respectively. Afterward, the carbanion reacts with the carbonyl group of DMC, which gives methyl 2-methyl-3-oxopentanoate as the intermediate. Finally, the resulting intermediate reacts through the methoxy anion to yield the final product methyl propionate (see Scheme 1a). Similarly, the aldol condensation product (5-ethyl-4-methyl-4hepten-3-one) and the methylation products (2-methyl-3-pentanone and 3-methoxy-2-pentene) were also produced as byproducts. After the initial abstraction of the α-H from 3-pentanone, the carbanion could attack a second 3-pentanone molecule to produce 5-ethyl-4-methyl-4-hepten-3-one (see Scheme 1b). Likewise, the 3-pentanone carbanion and its enolate resonance isomer can react with the methyl group of DMC to produce 2-methyl3-pentanone and 3-methoxy-2-pentene, respectively, which form methanol and carbon dioxide as methylation byproducts (see Scheme 1c).
4. CONCLUSIONS A novel benign route was developed for synthesis of methyl propionate from the reaction of dimethyl carbonate with
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3-pentanone. The synthesis of methyl propionate was found to be closely related to the moderate base sites, and the formation of methyl propionate presumably was carried out through the abstraction of the α-H from 3-pentanone by basic sites. Under optimal conditions, the 3-pentanone conversion and the MP selectivity reached 40.9% and 53.9%, respectively.
’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT The authors acknowledge financial support from the Medical Science and Technology Development foundation of Guangdong, PR China, No. B2009190, and the Doctor Start foundation of Guangdong Medical College, No. XB0811. ’ REFERENCES (1) Mamoru, A. Formation of methyl methacrylate from methyl propionate and methanol. Catal. Today 2006, 111, 398–402. (2) Xue, X.; Liu, Z.; Ma, C.; Chen, S. Synthesis of methyl propionate in ionic liquid. Chem. World 2006, 47, 667–668. (3) Hidai, M.; Koyasu, Y.; Chikanari, K.; Uchida, Y. Synthesis of ketones and esters from olefins, carbon monoxide and alcohols by using ruthenium-iodide catalysts. J. Mol. Catal. 1987, 40, 243–254. (4) Keim, W.; Becker, J. Catalytic reactions of methyl formate with olefins. J. Mol. Catal. 1989, 54, 95–101. (5) Pacheco, M. A.; Marshall, C. L. Review of dimethyl carbonate (DMC) manufacture and its characteristics as a fuel additive. Energy Fuels 1997, 11, 2. (6) Jyothi, T. M.; Raja, T.; Talawar, M. B.; Rao, B. S. Selective O-methylation of catechol using dimethyl carbonate over calcined Mg Al hydrotalcites. Appl. Catal. A 2001, 211, 41. (7) Tundo, P.; Moraglio, G.; Trotta, F. Gas liquid phase-transfer catalysis: A new continuous-flow method in organic synthesis. Ind. Eng. Chem. Res. 1989, 28 (7), 881. (8) Tundo, P.; Selva, M. The chemistry of dimethyl carbonate. Acc. Chem. Res. 2002, 35, 706. (9) Tundo, P.; Trotta, F.; Moraglio, G.; Ligorati, F. Continuous-flow processes under gas liquid phase-transfer catalysis (GL-PTC) conditions: The reaction of dialkyl carbonates with phenols, alcohols, and mercaptans. Ind. Eng. Chem. Res. 1988, 27 (9), 1565. (10) Shieh, W. C.; Dell, S.; Bach, A.; Blacklock, T. J. Dual nucleophilic catalysis with DABCO for the N-methylation of indoles. J. Org. Chem. 2003, 68, 1954. (11) Mei, F.; Pei, Z.; Li, G. The transesterification of dimethyl carbonate with phenol over Mg Al hydrotalcite catalyst. Org. Process. Res. Dev. 2004, 8, 372. (12) Ruest, L.; Blouin, G.; Dislongchamps, P. A convenient synthesis of 2-carbomethoxycyclohexanone. Syn. Commun. 1976, 6, 169. (13) Fisher, R. Preparation of α,ω-dicarboxylic acid diesters. U.S. Patent 5,453,535, 1995. (14) Tundo, P.; Memoli, S.; Selva, M. Synthesis of α,ω-diesters. WO Patent 0,214,257, 2002. (15) Koehler, G.; Metz, J. Process for preparing diesters of higher α,ω-dicarboxylic acids. U.S. Patent 5,786,502, 1998. (16) Selva, M.; Marques, C. A.; Tundo, P. The addition reaction of dialkyl carbonates to ketones. Gazz. Chim. It. 1993, 123, 515. (17) Hattori, H. Heterogeneous basic catalysis. Chem. Rev. 1995, 95, 527. (18) Zhang, W.; Wang, H.; Wei, W.; Sun, Y. Solid base and their performance in synthesis of propylene glycol methyl ether. J. Mol. Catal. 2005, 231, 83. 12347
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(19) Fu, Y.; Zhu, H.; Shen, J. Thermal decomposition of dimethoxymethane and dimethyl carbonate catalyzed by solid acids and bases. Thermochim. Acta 2005, 434, 88–92. (20) Wu, D.; Fu, X.; Li, J.; Zhao, N.; Wei, W.; Sun, Y. Methoxycarbonylation of ketones with dimethyl carbonate over solid base. Catal. Commun. 2008, 9, 680–684.
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