One-Pot Synthesis of Dimethyl Carbonate over a Binary Catalyst of an Ionic Liquid and an Alkali Carbonate under Low Pressure.

A mild and highly efficient approach has been developed for the one-pot synthesis of dimethyl carbonate (DMC) from epoxide, carbon dioxide, and methanol under low initial pressure. The key to the successful transformation is the use of a bicomponent catalytic system composed of a hydroxyl-functionalized ionic liquid and an alkali carbonate. This bicomponent catalytic system demonstrated excellent reusability in four runs. Under the optimal reaction conditions, a 64% yield of DMC from propylene oxide and an 81% yield of DMC from ethylene oxide were obtained.

Introduction

Carbon dioxide is mainly produced by the metabolism of the organisms in nature and the burning of fossil fuels. Despite being accused of global warming, CO2 is an attractive C1 building block, as it is naturally abundant, economical, nontoxic, and nonflammable.[1,2] Therefore, CO2 capture and chemical fixation[3] have been gaining much attention; one example is the synthesis of dimethyl carbonate (DMC). It has been recognized that DMC is an eco-friendly solvent and a versatile intermediate. It can be used as a precursor for the synthesis of polycarbonate, a carbonylation reagent, and a useful methylation agent.[4] It is deemed as a gasoline additive in place of methyl tert-butyl ether (MTBE) due to its high oxygen content.[5] Besides, DMC can be used as an electrolytic solution for lithium batteries.[6] Therefore, the development of efficient and green synthetic methods for DMC has attracted extensive attention.

The technical routes of industrialization include phosgenation,[7] methanol oxidative carbonylation,[8] and two-step transesterification.[9−11] Recently, the urea alcoholysis process for the synthesis of DMC has been developed.[12] Direct preparation of DMC from methanol and CO2 is one of the most preferable reactions.[13−18] Nonetheless, in most cases, the yield of DMC was far from satisfactory due to thermodynamic confinement and the decomposition of the catalysts by concomitant water.[19] Naturally, CO2 is inert and epoxide is reactively active. The one-pot synthesis of DMC from epoxide, carbon dioxide, and methanol has been considered an extremely attractive alternative (Scheme ).

Scheme 1

One-Pot Synthesis of DMC (1) and Its Side Reaction (2)

Researchers at Bayer AG reported this approach for the first time in 1979.[20] Since that, numerous attempts have been made to develop efficient catalysts for the one-pot synthesis of DMC, mainly including one-component and bicomponent catalysts[21−28] (see Table S1 in the Supporting Information (SI)). However, the reported reaction systems still encountered high reaction pressures and relatively low DMC yields so far. For instance, Bhanage et al.[21] reported that a 26.9% yield of DMC was obtained using 20.0 mol % MgO as the catalyst at 150 °C under 8 MPa CO2 in the case of ethylene oxide (EO). A bicomponent catalyst of [bmim]BF4/CH3ONa was developed by Chen et al.[25] and a 67.6% yield of DMC was reached at 150 °C under 4 MPa CO2 using propylene oxide (PO) as the starting material. Wang et al.[28] reported a two-step synthesis of DMC with no need for separation of ethylene carbonate (EC) catalyzed by K2CO3-based binary salts in the presence of H2O. Near 100% conversion of EO and an 82% yield of DMC were obtained under 2.5 MPa CO2. Recently, we found that a lower initial pressure (the optimal pressure of only 0.5 MPa) was suitable for the one-pot synthesis of DMC catalyzed by an alkali carbonate.[29] In 2008, Sun et al.[30] found that hydroxyl-functionalized ionic liquids (HFILs) showed excellent performances for the chemical fixation of CO2 to cyclic carbonate since the hydroxyl group has a positive effect on the ring opening of the epoxide. Inspired by the results mentioned above, we speculate that there might be a more effective combination between HFILs and alkali carbonates for the one-pot synthesis of DMC under low pressure. Herein, we report the catalytic performance of a novel bicomponent catalytic system composed of HFILs and alkali carbonates for the one-step synthesis of DMC.

Results and Discussion

PC stands for the abbreviation of propylene carbonate, and PG stands for the abbreviation of 1,2-propanediol. In the side reaction, PM is used to represent both 2-methoxy-1-propanol (2-ME-1-PA) and 1-methoxy-2-propanol (1-ME-2-PA) due to their same boiling points.

Effect of the Base Catalyst on the One-Pot Synthesis of DMC

The reaction of PO, CO2, and methanol was selected as a template reaction to screen the base catalyst at an initial pressure of 0.5 MPa. The results are shown in Table . No DMC was generated in the absence of base and (2-hydroxyethyl)trimethylammonium bromide (IL) (Table , entry 1). The conversion of PO was 54% and the main byproduct was PM (52%). The yield of DMC was only 4% and the conversion of PO reached up to 97% in the presence of IL (1 mol %), which demonstrated that IL was efficient to the cycloaddition of PO, providing a 31% yield of PM and a 62% yield of PC (Table , entry 2). When only Na2CO3 (5 mol %) was added as the catalyst, the yield of DMC reached 37% and the conversion of PO was 90%, which showed that Na2CO3 was catalytical active to both the cycloaddition of PO and the transesterification of PC with methanol (Table , entry 3). As expected, the yield of DMC increased up to 58% and the conversion of PO significantly reached 99% (Table , entry 4), when a binary catalyst system of IL and Na2CO3 was used. These results exhibit a synergistic effect of the HFIL and the alkali carbonate in the one-step synthesis of DMC. In the case of different bases used, the conversions of PO exceeded 93% and the yields of DMC varied from 41 to 58% (Table , entries 5–9) when catalyzed by a binary catalyst. The catalytic activity of strong bases such as KOH and CH3ONa is poor than those of weaker alkalis. The reason may be that the transesterification can be inhibited as the alkalinity increases.[31] Besides, the organic base DBU also showed good catalytic activity, and a 54% yield of DMC was obtained (Table , entry 9). Based on these results, Na2CO3 was selected as the base catalyst for further investigation.

Table 1

Screening of the Base Catalyst for the One-pot Synthesis of DMCa

Reaction conditions: 5 mmol of PO, 15 equiv of CH3OH, 1.0 mol % IL, 5.0 mol % base, initial pressure: 0.5 MPa, 140 °C, and 6 h.

The sum of the yields of PC, PG, and PM.

Yields are the average of two runs, determined by GC using an internal standard technique.

Effect of IL Loading on the One-Pot Synthesis of DMC

The effect of IL loading has been investigated and the results are shown in Table . The amount of IL had little effect on the PO conversion. PO was nearly completely consumed when the loading of IL was 1 mol %. The DMC yield increased from 37 to 61% in the range of IL loadings increasing from 0 to 2.0 mol %. On further increasing the IL loading, the PO conversion and the DMC yield did not increase anymore. The PM yield decreased with the increase of IL and became constant after 2.0 mol %. The results show that 2.0 mol % is the optimal IL loading.

Table 2

Effect of IL Loading on the One-Pot Synthesis of DMCa

			yieldb (%)
entry	IL loading (mol %)	conversion of POc (%)	DMC	PM	PG	PC
1	0	90	37	30	38	22
2	0.5	96	52	23	53	20
3	1	99	58	18	59	22
4	1.5	99	59	16	60	23
5	2	99	61	13	62	24
6	2.5	99	61	13	62	24
7	3	99	61	12	63	24

Reaction conditions: 5 mmol of PO, 15 equiv of CH3OH, 5.0 mol % Na2CO3, initial pressure: 0.5 MPa, 140 °C, and 6 h.

Yields are the average of two runs, determined by GC using an internal standard technique.

The sum of the yields of PC, PG, and PM.

Effect of Initial Pressure on the One-Pot Synthesis of DMC

Using PO as the starting material to investigate the effect of initial pressure on the one-pot synthesis of DMC. The results are shown in Figure . When the initial pressure was less than 0.5 MPa, the conversion of PO increased as the initial pressure increased. The conversion of PO reached 99% at 0.5 MPa and then became stable. The yields of DMC and PG increased and reached the maximum value at 0.5 MPa (61 and 62%, respectively). After that, the yields showed a downward trend as the initial pressure increased from 0.5 to 3.5 MPa. It can be conjectured that CO2 was dissolved in propylene oxide or ‘liquefied’’ through the formation of a CO2–PO complex.[32] Very high CO2 pressure may retard the transesterification between PC and CH3OH as well as the CO2 dilution effect, thus resulting in a low yield of DMC.[33] The yield of PC generally showed an upward trend, which indicated that the high initial pressure restrains the transesterification step. The results illustrate that the yields of PC and DMC show a competitive relationship, which is also consistent with the reaction formula. Hence, 0.5 MPa was chosen as the optimal initial pressure for the one-pot synthesis of DMC.

Figure 1

Effect of initial pressure on the one-pot synthesis of DMC. Reaction conditions: 5 mmol of PO, 15 equiv of CH3OH, 2.0 mol % IL, 5.0 mol % Na2CO3, 140 °C, and 6 h. Yields are the average of two runs, determined by GC using an internal standard technique.

Effect of Oil Bath Temperature on the One-Pot Synthesis of DMC

The effect of oil bath temperature is shown in Table . A pronounced effect on the yield of DMC was observed when the temperature varied from 100 to 160 °C. At 100 °C, the conversion of PO was 82%, while the yield of DMC was only 6% and the yield of PC reached 66%. The results illustrate that a lower temperature is not conducive to the progress of transesterification. The conversion of PO was above 95% at temperatures between 110 and 150 °C, then decreased to 86% at 160 °C. The yield of PM increased with the increase of temperature, which indicated that a higher temperature was favorable for the alcoholysis of PO. When the temperature was lower than 140 °C, the DMC yield gradually increased with the increase of temperature. At 140 °C, the yield of DMC reached a maximum of 61%. When the temperature was higher than 140 °C, the yields of DMC, PG, and PC decreased. Therefore, an oil bath temperature of 140 °C was picked for further investigations.

Table 3

Effect of Oil Bath Temperature on the One-Pot Synthesis of DMCa

			yieldb (%)
entry	bath temperature (°C)	conversion of POc (%)	DMC	PM	PG	PC
1	100	82	6	10	6	66
2	110	99	19	12	19	68
3	120	99	42	12	44	43
4	130	99	59	13	61	25
5	140	99	61	13	62	24
6	150	95	56	15	58	22
7	160	86	48	16	51	19

Reaction conditions: 5 mmol of PO, 15 equiv of CH3OH, 2.0 mol % IL, 5.0 mol % Na2CO3, initial pressure: 0.5 MPa, and 6 h.

Yields are the average of two runs, determined by GC using an internal standard technique.

The sum of the yields of PC, PG, and PM.

Effect of Na2CO3 Loading on the One-Pot Synthesis of DMC

The effect of Na2CO3 loading is shown in Table . A slight effect of the base catalyst on the PO conversion was observed, and conversions higher than 97% were obtained at loadings from 0 to 7.0 mol %. The DMC yield increased quickly from 4 to 54% in the range of Na2CO3 loadings increasing from 0 to 2.0 mol % and then increased smoothly from 54 to 61% in the range of Na2CO3 loadings from 2.0 to 5.0 mol %. A further increase in the Na2CO3 loading to 7.0 mol % did not lead to an increase in DMC yield. A Na2CO3 loading of 5.0 mol % was chosen for further studies.

Table 4

Effect of Na2CO3 Loading on the One-Pot Synthesis of DMCa

			yieldb (%)
entry	Na2CO3 (mol %)	conversion of POc (%)	DMC	PM	PG	PC
1	0	97	4	31	4	62
2	1	99	32	16	32	51
3	2	99	54	15	56	28
4	3	99	59	14	60	25
5	4	98	59	14	61	23
6	5	99	61	13	62	24
7	6	98	61	13	62	23
8	7	98	60	14	62	22

Reaction conditions: 5 mmol of PO, 15 equiv of CH3OH, 2.0 mol % IL, initial pressure: 0.5 MPa, 140 °C, and 6 h.

Yields are the average of two runs, determined by GC using an internal standard technique.

The sum of the yields of PC, PG, and PM.

Effect of Methanol Loading on the One-Pot Synthesis of DMC

The effect of methanol loading has been investigated and the results are shown in Table . A trivial influence of methanol loading on the PO conversion was noticed within the range of 6–23 molar equiv. The DMC yield increased from 44 to 64% as the loading of methanol increased from 6 to 18 molar equiv. The PM yield increased with the increase of methanol, which showed that the increase of methanol dosage was beneficial to the alcoholysis of PO. On further increasing the methanol amount, the yield of DMC decreased due to dilution of the reaction system and a considerable portion of PO had been alcoholized. Therefore, 18 molar equiv of methanol was considered to be the optimal reaction condition.

Table 5

Effect of the Methanol Amount on the One-Pot Synthesis of DMCa

			yieldb (%)
entry	methanol amount (equiv)	conversion of POc (%)	DMC	PM	PG	PC
1	6	98	44	7	48	43
2	9	97	51	9	55	33
3	12	98	58	13	59	26
4	15	99	61	13	62	24
5	18	99	64	14	65	20
6	21	99	61	18	62	19
7	23	99	56	20	57	22

Reaction conditions: 5 mmol of PO, 2.0 mol % IL, 5.0 mol % Na2CO3, initial pressure: 0.5 MPa, 140 °C, and 6 h.

Yields are the average of two runs, determined by GC using an internal standard technique.

The sum of the yields of PC, PG, and PM.

Effect of the Reaction Time on the One-Pot Synthesis of DMC

Reaction times in the range of 2–8 h have been tested and the results are shown in Table . When the reaction was carried out for 2 h, the conversion of PO was 89%. It increased to 99% after 3 h, while the DMC yield was only 34%. The yield of DMC increased from 29 to 64% when the reaction time was prolonged from 2 to 6 h. On further prolonging the reaction time to 8 h, the yield of DMC remained roughly constant. In summary, a reaction time of 6 h was considered to be the optimal condition. The template reaction was scaled up in a 75 mL stainless-steel autoclave under the optimum conditions (Table , entry 8), and a yield of 58% was obtained.

Table 6

Effect of the Reaction Time on the One-Pot Synthesis of DMCa

			yieldb (%)
entry	reaction time (h)	conversion of POc (%)	DMC	PM	PG	PC
1	2	89	29	14	30	45
2	3	99	34	15	35	49
3	4	99	53	15	55	29
4	5	99	60	16	61	22
5	6	99	64	14	65	20
6	7	99	63	15	65	19
7	8	99	64	15	64	20
8d	6	98	58	16	59	23

Reaction conditions: 5 mmol of PO, 18 equiv of CH3OH, 2.0 mol % IL, 5.0 mol % Na2CO3, initial pressure: 0.5 MPa, and 140 °C.

Yields are the average of two runs, determined by GC using an internal standard technique.

The sum of the yields of PC, PG, and PM.

25 mmol of PO, carried out in a 75 mL stainless-steel autoclave.

Recycling of the Binary Catalyst

The recycling experiment has been carried out to further evaluate the performance of the binary catalyst system. At the end of the reaction, the products were separated by distillation under reduced pressure and then the residue containing IL and Na2CO3 was reused directly for the next run. The results are shown in Figure . It can be seen that the yield of DMC still reaches 56% after three times of reuse, which indicates that the catalytic system is practically reusable. The reason for the decrease of the DMC yield might be that the accumulation of the high-boiling compounds such as PG and PC in the consecutive recycles inhibits the reaction.

Figure 2

Reusability of the binary catalyst. Reaction conditions: 5 mmol of PO, 18 equiv of CH3OH, 2.0 mol % IL, 5.0 mol % Na2CO3, initial pressure: 0.5 MPa, 140 °C, and 6 h. Yields are the average of two runs, determined by GC using an internal standard technique.

Study on the Influence of the Molecular Structures of HFILs on Catalytic Performances

The effects of the structures of HFILs on the one-step synthesis of DMC have been investigated and the results are shown in Table . The binary catalyst of IL and Na2CO3 showed obvious catalytic activity on this reaction and a 64% yield of DMC was obtained (Table , entry 1). The number of hydroxyl groups and the distance between the hydroxyl group and the nitrogen atom have little effect on the reaction; high PO conversions and DMC yields can be obtained (Table , entries 2–6). The DMC yield decreased when the anion of the HFIL was changed from Br– to OH–, while the PM yield increased (Table , entry 7). The reason may be that the strong alkalinity of the reaction system inhibits the transesterification and accelerates the alcoholysis of the side reaction. To some extent, the alcoholysis activity of the epoxide is proportional to the strength of the base.[34]

Table 7

Effect of the Structure of Hydroxyl-Functionalized Ionic Liquids on the One-Pot Synthesis of DMCa

Reaction conditions: 5 mmol of PO, 2.0 mol % HFIL, 5.0 mol % Na2CO3, 18 equiv of CH3OH, initial pressure: 0.5 MPa, 140 °C, and 6 h.

The sum of the yields of PC, PG, and PM.

Yields are the average of two runs, determined by GC using an internal standard technique.

Kinetic Study of Various HFILs on the One-Pot Synthesis of DMC

The kinetic study of four HFILs containing different numbers of hydroxyethyl groups (the structures of IL1–IL4 showed in Table ) has been investigated and the results are shown in Figure . As the reaction time progressed, both the PO conversion and the DMC yield increased and reached a maximum after 6 h and then remained steady. However, both of them decreased as the number of hydroxyl groups in HFILs increases at the same reaction time. The reason may be that the extra hydroxyl groups in HFILs preferentially form hydrogen bonds with the halogen anion, which reduces the nucleophilicity of the anion, thereby retarding the ring opening of the epoxide.[35]

Figure 3

Kinetic study of HFILs in the one-pot synthesis of DMC. Reaction conditions: 5 mmol of PO, 18 equiv of CH3OH, 2.0 mol % HFIL, 5.0 mol % Na2CO3, 0.5 MPa, 140 °C, and 6 h. Yields are the average of two runs, determined by GC using an internal standard technique.

Study on the Suitability of Ethylene Oxide

The suitability of ethylene oxide has been studied under the optimal reaction conditions and the result is showed in Figure . The activity of EO is superior to PO; the difference in activity is mainly due to differences in the standard enthalpies of formation and the standard free energies of the formation of the chemicals.[36] An 81% yield of DMC was achieved in the case of ethylene oxide. Besides, the yields of 2-methoxyethanol, ethylene glycol (EG), and EC were 8, 81, and 10%, respectively.

Figure 4

Study on the suitability of ethylene oxide. Reaction conditions: 5 mmol of EO, 18 equiv of CH3OH, 2.0 mol % IL, 5.0 mol % of Na2CO3, initial pressure: 0.5 MPa, 140 °C, and 6 h. Yields are the average of two runs, determined by GC using an internal standard technique.

Plausible Reaction Mechanism

According to the previously reported results in the literature,[26,28−30] as well as the results obtained in this research, a plausible mechanism by the synergistic effect of a binary catalyst of an HFIL and an alkali carbonate was proposed (Scheme ). First, the hydroxyl group at the HFIL forms a hydrogen bond with the O atom of the epoxide to activate the epoxide; on the other hand, the anion of the HFIL or the carbonate ion of the alkali carbonate attacks the C atom connected to the O atom of the epoxide. The synergy between the two aspects facilitates the ring opening of the epoxide (step 1). Then, intermediate I or/and II acts as a nucleophile to attack and activate CO2 (Step 2). Next, the active intermediate formed in the previous step undergoes charge transfer to form a stable cyclic carbonate III (step 3). On the other hand, methanol reacts with sodium carbonate (step 4), and CH3ONa formed by this equilibrium reaction is consumed by III to form intermediate IV containing the first methyl group for DMC (step 5). Finally, intermediate IV is attacked by the nucleophile again to form the final product DMC (step 6).

Scheme 2

Proposed Mechanism for the One-Pot Synthesis of DMC

Conclusions

In summary, a mild and efficient binary catalyst system of HFILs and an alkali carbonate was developed for the one-pot synthesis of DMC from epoxide, CO2, and methanol under a low initial pressure of 0.5 MPa. The reaction conditions were systematically investigated and the optimal conditions were obtained as follows: oil bath temperature, 140 °C; initial pressure, 0.5 MPa; reaction time, 6 h; 18 equiv of methanol; 2.0 mol % HFILs; and 5.0 mol % sodium carbonate. An 81% yield of DMC from EO and a 64% yield of DMC from PO were obtained under the optimal reaction conditions. This binary catalyst was reusable for four runs. The kinetic study of various combinations of sodium carbonate with HFILs showed that the increasing number of hydroxyl groups in an HFIL had a negative effect on both the PO conversion and the DMC yield in the initial reaction period.

Experimental Section

Typical Synthesis Procedure of Hydroxyl-Functionalized Ionic Liquids[37,38]

For the synthesis of N,N,N-triethyl-2-hydroxyethanaminium bromide (IL1), a mixture of triethylamine (5 mmol), 2-bromoethanol (7 mmol), and dry toluene (2 mL) was heated at 110 °C for 12 h in a 10 mL single flask under vigorous stirring. Then, the mixture was cooled down to room temperature, and a white solid formed rapidly. The resultant crude solid was filtered off, washed with acetone (5 × 6 mL), and dried at 80 °C for 12 h under vacuum. Other hydroxyl-functionalized ionic liquids IL2–IL5 were prepared by the same method. Basic ionic liquid choline hydroxide (IL6) was synthesized by the following procedure: 2 mL of methanol (50 mmol), 0.9 g of choline bromide (IL, 5 mmol), and 0.28 g of potassium hydroxide (5 mmol) were charged into a three-neck flask. The mixture was stirred at 60 °C for 12 h. After cooling to room temperature, the mixture was filtered to remove solid KBr. The filtrate was evaporated under reduced pressure and IL6 was obtained.

General Procedure for the One-Pot Synthesis of DMC

For the one-pot synthesis of DMC, the reactions were conducted in a 25 mL stainless-steel autoclave equipped with a magnetic stirrer and an automatic temperature control system. A typical reaction was carried out as follows: an appropriate amount of CO2 was charged to an autoclave containing a mixture of PO (5 mmol), IL catalyst (2 mol %), base catalyst (5 mol %), and methanol (18 equiv) at room temperature. Then, the autoclave was placed in an oil bath preheated to the designated temperature. After stirred for 6 h, the autoclave was cooled down to room temperature, and the remaining CO2 was removed slowly. The reaction mixture was analyzed by gas chromatography using biphenyl as an internal standard. Taking DMC as an example, the DMC yield was calculated according to formula (. Among them, SDMC represents the peak area of DMC, Sbiphenyl represents the peak area of biphenyl, and mDMC represents the theoretical mass of DMC. The conversion of PO is calculated by the sum of the yields of PM, PG, and PC.