Polyethylene Glycol Enfolded KBr Assisted Base Catalyzed Synthesis

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Polyethylene Glycol Enfolded KBr Assisted Base Catalyzed Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide Subodh Kumar and Suman L. Jain* Chemical Sciences Division, CSIR-Indian Institute of Petroleum Mohkampur, Dehradun 248005, India ABSTRACT: Polyethylene glycol enfolded potassium bromide, that is, [K+{PEG}Br−] promoted the synthesis of DMC from the reaction of methanol and carbon dioxide (CO2) using alkali metal based catalyst. The experimental results demonstrated that [K+{PEG}Br−] acted as a promoter and provided up to 12.8% yield with 98% selectivity of dimethyl carbonate (DMC). After completion of the reaction, the reaction products were isolated by distillation and the resulting PEG layer containing potassium bromide (KBr) and potassium carbonate (K2CO3) was efficiently reused for next six runs without significant loss in DMC yield. After the six run, the yield of DMC was found to be 12.1%. In the absence of [K+{PEG}Br−], the reaction between methanol and CO2 using K2CO3 base catalyst provided only 1−2% yield of DMC.

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INTRODUCTION Dimethyl carbonate owing to its nontoxic and biodegradable nature has been recognized to be a promising substitute for dimethyl sulfate and phosgene, which are toxic and corrosive methylating or carbonylating agents.1−5 In addition, dimethyl carbonate (DMC) is considered to be a better choice for meeting the oxygenate specification for transportation fuels.6 The conventional route for the synthesis of DMC involves the reaction of methanol and phosgene.7 However, the highly toxic and corrosive nature of phosgene makes this process of less synthetic utility. Subsequently, ester exchange8,9 and oxidative carbonylation of methanol10 has been developed to be efficient processes for the synthesis of DMC. Among them, oxidative carbonylation of methanol with carbon monoxide (CO) has extensively been studied for the production of DMC. A number of homogeneous and heterogeneous catalysts including transition metal complexes, zeolites, and solid supported metal catalysts have been investigated.11−20 However, these methods are associated with certain limitations such as use of highly toxic carbon monoxide (CO), poor performance of catalyst, low yield, and higher cost. Recently, carbon dioxide (CO2) chemistry (particularly, capture and conversion) has received significant interest due to the global warming and other environmental problems caused by CO2. Such environmental consequences have suggested for the development of effective approaches to reducing this harmful greenhouse gas. In this regard, transformation of carbon dioxide into industrially valued chemicals is considered to be an excellent approach due to its significant values in both environmental preservation and resource utilization.21 Thus, the direct synthesis of DMC from the reaction of CO2 and methanol has become highly important in recent years. In this context, a number of metallic catalysts such as metallic magnesium powder, Sn(IV), and Ti(IV) alkoxides have been used as catalysts.22−29 However, the yield of DMC was low even in the presence of the promoters and dehydrating agents mainly due to the decomposition of the catalysts by water, and due to the reverse hydrolysis of DMC. Several attempts have been made in order to avoid the detrimental effect of water.30−32 However, the unsatisfactory © 2014 American Chemical Society

results and poor yield of DMC still remained as a major challenging issue. Herein, we describe a simple and cost-effective approach for the synthesis of DMC from methanol and CO2 in the presence of potassium carbonate (K2CO3) base as catalyst and polyethylene glycol enfolded potassium bromide [K+{PEG}Br−] as a promoter (Scheme 1). Scheme 1. Base Catalyzed Synthesis of DMC

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EXPERIMENTAL SECTION Materials and Techniques. Dehydrated methanol and polyethylene glycol (PEG400) were purchased from SigmaAldrich and used as received without any further purification. Anhydrous KBr and K2CO3 were purchased from Acros organics. The reactions were carried out in a 25 mL parr reactor under carbon dioxide pressure in a batch manner. Synthesis of [K+{PEG} Br−] 2. In a round-bottomed Flask (100 mL) equipped with a magnetic stirrer, KBr (0.95 g, 8 mmol) was added to a solution of PEG400 (2 g, 5 mmol) in water (30 mL), and the mixture stirred for 4 h at room temperature. After completion, the obtained residue was extracted with dichloromethane and the combined organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give brownish yellow viscous oil. Typical Experimental Procedure for the DMC Synthesis. The DMC synthesis experiments were carried out in a high pressure reactor equipped with magnetic stirrer. In a typical experiment, methanol (10 mL), [K+{PEG}Br−] (2 mL), and Received: Revised: Accepted: Published: 15798

June 27, 2014 September 16, 2014 September 20, 2014 September 20, 2014 dx.doi.org/10.1021/ie502579m | Ind. Eng. Chem. Res. 2014, 53, 15798−15801

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significantly and provided 12.8% yield of DMC under similar experimental conditions (Table 1, entry 2). The reaction did not occur in the absence of K2CO3 under otherwise identical experimental conditions (Table 1, entry 3). The use of PEG400 in place of 2 did not influence the reaction and afforded only 2% yield of the DMC (Table 1, entry 4). However, a lower yield of DMC (8.8%) was obtained when a physical mixture of PEG400 and KBr (1:1) was used under described experimental conditions (Table 1, entry 5). Effect of Reaction Temperature. We investigated the effect of reaction temperature on the synthesis of DMC by varying the temperature from 25 to 130 °C under the reaction conditions: CH3OH (10 mL), excess CO2, K2CO3 (1.2 g), and 2 mL of 2 at 20 bar for 2 h. Figure 1 shows that the yield of DMC was found to

K2CO3 (1.2 g) were taken in a 25 mL volume capacity vessel. The vessel was sealed and pressurized by carbon dioxide up to 20 bar. Then, the mixture was heated at 100 °C for 2 h. After being cooled the vessel at room temperature, the reaction mixture was extracted with diethyl ether and analyzed by GC-FID. The yield and selectivity of DMC was found to be 12.8% and 98%, respectively, which is calculated with the help of calibration curves generated with commercially available standards using GC. The products were isolated by distillation under reduced pressure and the resulting PEG layer containing KBr and K2CO3 was reused for recycling experiments without any purification step.

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RESULT AND DISCUSSION Liquid range PEGs such as PEG400 and PEG200 are well-known to form crown ether-like host guest complexes with the alkali metal salts.33 Furthermore, as reported in the literature, potassium and sodium salt containing PEG’s incorporating 7−9 units are considered to be more effective catalysts due to their liquid property and two hydroxyl groups.34 Thus, for the present study, we have chosen PEG400 mainly due to its liquid nature. The required polyethylene glycol enfolded potassium bromide, that is, [K+{PEG}Br−], was easily obtained by stirring of KBr and PEG400 1 in water at room temperature. The resulting mixture was extracted with dichloromethane and subjected to usual workup to give [K+{PEG}Br−] 2 as a brownish yellow viscous oil in quantitative yield (Scheme 2). The successful formation of [K+{PEG}Br−] was confirmed by UV−vis and ESI-MS analyses. The characterization of 2 is mentioned in our previous literature report.35 Scheme 2. Synthesis of [K+{PEG}Br−] 2

Figure 1. Effect of reaction temperature and carbon dioxide pressure on DMC yield.

be increased with an increase in the reaction temperature up to 100 °C. Further increase in reaction temperature decreases the DMC formation rate significantly. It is most likely due to the poor solubility of CO2 in methanol at high temperature and the decomposition of DMC (Figure 1). Based on these results, 100 °C was chosen as the optimum reaction temperature for this reaction. Effect of Reaction Pressure. To investigate the effect of pressure of carbon dioxide on the synthesis of DMC, we performed the reactions at 5, 10, 15, 20, and 25 bar pressure of carbon dioxide by keeping other parameters constant. The DMC formation rate was found to be increased with an increase in pressure up to 20 bar. It is most likely due to the higher solubility of the CO2 in reaction medium at high pressure. Further increase in pressure did not improve the yield of DMC to any significant extent (Figure 1). Thus, we have chosen 20 bar as the optimum pressure for the present reaction. Effect of Different Base Catalysts. To evaluate the effect of various base catalysts, DMC synthesis was performed using different bases under the identical reaction conditions (Figure 2). The reactivity order of the different alkali metal catalysts was found to be K > Na > Li. Among the different catalysts studied, potassium carbonate was found to be best choice for the synthesis of DMC from methanol and CO2. Effect of Catalyst Amount. We investigated the effect of catalyst amount on the yield of DMC using methanol (10 mL), excess CO2, K2CO3 (varying from 0.5 to 2 g) at 100 °C, and 20 bar pressure for 2 h. Figure 3 shows the yield and the selectivity of

At first to obtain the optimum reaction conditions, the effect of various parameters (promoter, temperature, pressure, reaction time, and catalyst concentration) on the synthesis of DMC from CO2 and methanol was studied. Effect of Promoter and Catalyst. To evaluate the effect of 2, we carried out the blank experiment without promoter for the synthesis of DMC from methanol with excess carbon dioxide using K2CO3 as catalyst at 100 °C and 20 bar pressure of CO2 for 2 h. The reaction was found to be very slow and afforded only 2% yield of DMC (Table 1, entry 1). However, the addition of 2 (2 mL) in the reaction mixture, enhanced the reaction rate Table 1. Effect of Promotera entry

catalyst

1 2 3 4 5

K2CO3 K2CO3 K2CO3 K2CO3

promoter [K+{PEG}Br−] [K+{PEG}Br−] PEG400 PEG400 + KBr

DMC yield (%)b 2 12.8 2 8.8

a

Reaction condition: methanol (10 mL), catalyst (1.2 g), promoter (2 mL), CO2 at 100 °C and 20 bar pressure for 2 h. bDetermined by GC. 15799

dx.doi.org/10.1021/ie502579m | Ind. Eng. Chem. Res. 2014, 53, 15798−15801

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Figure 2. Activity of different bases on the yield of DMC.

Figure 4. Recycling of promoter and catalyst.

Scheme 3. Possible Mechanistic Pathway

abstraction of proton led to the formation of methoxy species, which subsequently reacted to the CO2 to give methoxy carbonate. This intermediate carbonate then reacted with the methyl bromide generated from the reaction of HBr and methanol to give DMC and regenerated the promoter 2 and base catalyst, as shown in Scheme 3.

Figure 3. Effect of catalyst and time on DMC yield.

DMC with an increase in the catalyst amount. The maximum yield and selectivity of DMC was obtained at 1.2 g of the catalyst (K2CO3) as shown in Figure 3. Further increase in catalyst amount promoted the side reactions and gave increased yields of the byproducts such as dimethyl ether (DME) and C1−C2 hydrocarbons.36,37 Effect of Reaction Time. The effect of reaction time was investigated by varying the time from 0.5 to 4 h under similar experimental conditions. As shown in Figure 3, DMC formation rate was found to increase with an increase in reaction time from 1 to 2 h. However, further increase in reaction time affected the DMC formation adversely and provided significant decrease in DMC yield along with the formation of byproducts. Reusability of the Promoter and Base Catalyst. After completion of the reaction, the product was separated by distillation under reduced pressure. The resulting PEG layer containing base catalyst (K2CO3) and KBr was subjected as such for subsequent experiments by adding fresh methanol and CO2. The reusability of the system was checked for subsequent six runs (Figure 4). Almost similar activity with slightly decrease yield of DMC established the efficient recycling and reusability of the recovered PEG containing KBr and K2CO3. Although the exact mechanism of the reaction is not known at this stage, a probable mechanism of the reaction is shown in Scheme 3. In analogy to the literature report,36,38 the possible mechanism of the reaction may involve the three essential steps, that is, activation of methanol, carbon dioxide insertion, and production of DMC. Base assisted activation of methanol and

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CONCLUSION We have demonstrated an improved base catalyzed synthesis of DMC from methanol and carbon dioxide using polyethylene glycol enfolded potassium bromide, that is, [K+{PEG}Br−] as a promoter without adding any dehydration agent. The presence of both base as well as promoter was found to be essential for this reaction. When the reaction was performed either using K2CO3 alone without [K+{PEG}Br−] or using [K+{PEG}Br−] without K2CO3 under otherwise identical experimental conditions, the yield of DMC was found to be very low (0−2%). Under the optimized reaction conditions (1.2 g of base for 10 mL methanol at 100 °C and 20 bar CO2 pressure) maximum 12.8% yield of the DMC was obtained. After the reaction, the product was isolated by distillation, and the resulting PEG layer containing base and KBr was subsequently used for recycling experiments. The almost similar DMC yield for six runs established the efficient recycling ability of the developed catalytic system. The developed catalytic system is simple, cost-effective and recyclable, which can be explored further for the large scale synthesis of DMC from methanol and CO2.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. 15800

dx.doi.org/10.1021/ie502579m | Ind. Eng. Chem. Res. 2014, 53, 15798−15801

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Director, CSIR-IIP for granting permission to publish these results. S.K. is also thankful to CSIR, New Delhi, for providing research fellowship.

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dx.doi.org/10.1021/ie502579m | Ind. Eng. Chem. Res. 2014, 53, 15798−15801