One-Pot, Metal-Free Synthesis of Dimethyl Carbonate from CO2 at Room Temperature

Herein, we report on the metal-free, one-pot synthesis of industrially important dimethyl carbonate (DMC) from molecular CO2 under ambient conditions. In this process, initially the CO2 was chemisorbed through the formation of a switchable ionic liquid (SIL), [DBUH] [CH3CO3], by the interaction of CO2 with an equivalent mixture of organic superbase 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and methanol. The obtained SIL further reacted with methyl iodide (CH3I) to form DMC. The synthesis was carried out in both dimethyl sulfoxide (DMSO) and methanol. Methanol is preferred, as it not only served as a reagent and solvent in CO2 capture and DMC synthesis, but it also assisted in controlling the side reactions between chemical species such as CH3I and [DBUH]+ cation and increased the yield of DMC. Hence, the use of methanol avoided the loss of captured CO2 and favored the formation of DMC with high selectivity. Under the applied reaction conditions, 89% of the captured CO2 was converted to DMC. DBU was obtained, achieving 86% recovery of its salts formed during the synthesis. Most importantly, in this report we describe a simple and renewable solvent-based process for a metal-free approach to DMC synthesis under industrially feasible reaction conditions.


Introduction
Dimethyl carbonate (DMC) is a commercially important organic carbonate, considering its valuable and widespread industrial applications. DMC has attracted much attention from researchers at both the academic and industrial levels, as it is an important and environmentally friendly precursor for phosgene-free polycarbonate synthesis, whereas polycarbonates are extensively used in construction materials, automobiles, and certain medical devices [1,2]. Additionally, due to its high dielectric constant, DMC is also used as an electrolyte in lithium ion batteries and could emerge as a sustainable alternative to fossil fuels [3]. As a result of having a high oxygen content (53%), high octane number, high biodegradability, and low toxicity, DMC is considered to be an efficient and alternative fuel additive compared to the environmentally hazardous methyl tert-butyl ether [4]. Hence, due to DMC's various green and sustainable applications, the annual demand for this chemical has increased rapidly during the past decade. Therefore, several industrially applied processes produce enormous amounts of DMC every year.
Until now, several catalytic routes have been applied in order to synthesize DMC, whereby phosgenation, transesterification, oxidative carbonylation, and urea methanolysis have been extensively studied and industrially applied [4][5][6][7]. However, these catalytic routes are accompanied CH3I-involved, base-catalyzed reaction approach. In contrast to previous CH3I-involved processes, DMC synthesis in this report was carried out at ambient reaction conditions by implementing a single-solvent system such as methanol. As shown in the reaction Scheme 1, upon the actual synthesis, an equivalent mixture of organic superbase such as DBU and methanol activates CO2 and the obtained [DBUH] [CH3CO3], i.e., SIL further interacts with an equivalent amount of CH3I to form DMC. In this report, we described the synthetic protocol for DMC in terms of type of solvent, amount of CH3I, reaction time, recovery of DBU, etc. The progress as well as the amount of chemical species formed in the reaction mixture were monitored and explained by means of nuclear magnetic resonance (NMR) spectroscopy analysis.

NMR Analysis
The composition of the reaction mixtures was confirmed by NMR analysis using a Bruker Avance 600 MHz spectrometer operated at 1 H and 13 C frequencies of 600.17 and 150.91 MHz, respectively. NMR analyses were carried out in different solvents such as with crude reaction mixture and glass capillary filled with D2O or in neat D2O or CDCl3. NMR data were further processed with TopSpin 3.2 software. After conducting the NMR analysis, the chemical species that were identified in the reaction composition for DMC synthesis process are shown in Figure 1.

Synthesis of SIL [DBUH] [CH3CO3]
SIL, i.e., [DBUH] [CH3CO3], was synthesized by the interaction of CO2 with DBU (0.75 mL, 4.9 mmol) and methanol (0.2 mL, 4.9 mmol) in DMSO solvent (4 mL). During the actual synthesis, CO2 gas (100 mL/min) was bubbled under stirring, at room temperature, in DMSO solution containing equimolar amounts of DBU and methanol for 20 min. In the DMSO-free reaction mixture, CO2 gas was bubbled in alcoholic solutions of DBU (25 or 50 vol.% of DBU in methanol, 0.5 mL or 1 mL of DBU in 1.5 or 1 mL of methanol, respectively), at room temperature for 20 min. In all cases, pale yellow colored clear solutions were obtained, and their compositions were further confirmed by NMR analyses.

Synthesis of DMC
In this synthesis, the reaction mixtures with different reaction compositions were used in order to improve the yield of DMC. In these regards, in the first approach DMC synthesis was accomplished with addition of CH3I (based on equivalents of DBU) to DMSO solution containing SIL. In another reaction approach, the DMSO solution containing SIL was diluted initially with different amounts of methanol, and then CH3I was added to initiate the formation of DMC. Finally, in the case of DMSOfree synthesis, the CH3I was added to the alcoholic solution of SIL containing different amounts of methanol. As per requirement, in all reaction approaches, the reactions were quenched after the desired period, and the progress of the reaction was monitored by NMR analysis. All experiments for DMC synthesis were carried out at room temperature. In all experiments, CH3I was added in amounts ranging from 1 to 5 equivalents based on the amount of DBU used in the reaction mixture.  [27,28]. Initially, NaCl saturated aqueous solution was prepared by mixing NaCl in water under stirring, and the required amount of solution was taken to make 4 wt% solution of NaOH in NaCl-saturated solution. The mixture of DBU salts comprising [DBUH] [I] and [DBUCH3] [I] was added to the alkaline solution of NaCl, and the reaction mixture was further stirred for 30 min at room temperature. Ethyl acetate was used further to extract DBU from the aqueous phase, whereby DBU was further obtained through the removal of ethyl acetate from the organic phase. The purity of the as-recovered DBU was confirmed by means of NMR analysis, whereas the recovery percentage (weight %) of DBU was calculated from its recovered amount and the initial amount used for CO2 capture in the reaction.

Results and Discussion
Being chemically inert, CO2 was initially activated via the synthesis of SIL, which is considered as a well-practiced process for the capture of CO2. In a consecutive step, the obtained SIL further reacted with CH3I to form DMC. In order to achieve the maximum yield upon DMC synthesis, reaction parameters such as the type of solvent system, amount of CH3I, and reaction time were examined. In case of variations in the solvent system, the two-step DMC synthesis approach mentioned in the reaction Scheme 1 was carried out. Initially, this was tried in the DMSO solvent and further explored in a similar synthesis protocol in methanol. In the case of SIL, i.e., [DBUH] [CH3CO3] synthesis, after bubbling the CO2 in the DMSO solution of DBU and methanol, the composition of the obtained pale-yellow solution was confirmed by 1 H and 13 C NMR analysis. As shown in Figure 2, the carbon atoms at position 6 and 9, respectively, in DBU were shielded by 5-6 ppm, whereas the peak for carbon at position 7 was de-shielded by 5 ppm after CO2 capture. In addition, the peak at chemical shift 47.44 ppm for the methyl group disappeared, corresponding to methanol, and new peaks for the methyl group and the carbonyl carbon in the [CH3CO3] − anion appeared at 50.06 and 155.97 ppm, respectively. This confirmed the formation of SIL, i.e., [DBUH] [CH3CO3], after interaction of DBU and methanol with CO2. Regarding the 1 H NMR spectrum, it also confirmed the formation of SIL since changes in the chemical shifts for the protons, especially at positions 6 and 9, were observed in the case DBU, and a new peak for the protons in the methyl group of [CH3CO3] − anion was also observed ( Figure S1). As shown in the reaction Scheme 1, after the synthesis of SIL, CH3I was added to the DMSO solution of SIL to synthesize DMC. In this case, in order to gain insight into the progress of the reaction, CH3I was added as 0.25, 0.5, 0.75, and 1 equivalents based on the amount of DBU used previously during the SIL synthesis. After 15 min of reaction time, for each and every reaction, the reaction mixture was further analyzed by 13 C NMR analysis, and the obtained spectra are shown in Figure S2 and Figure 3, respectively. As shown in Figure 3, after addition of different equivalents of CH3I to the reaction mixture, the peaks belonging to the methyl group and carbonyl carbon in the [CH3CO3] − anion, at the chemical shifts 30.45 and 155.98 ppm, respectively, steadily decreased. On the other hand, new peaks appeared at the chemical shifts 40.09, 47.44, 53.74, 154.51, 164.15, and 164.87 ppm, respectively, whereby the intensities of these peaks increased with increasing amounts of CH3I. As observed during the synthesis of SIL, the peak with the chemical shift 47.44 ppm is related to the methyl group of methanol. Further, after comparing the NMR spectra of both reaction mixtures (with addition of 1 eq. CH3I) and commercially available DMC, signals at 53.74 and 154.51 ppm, respectively, belonging to the carbon atoms in CH3-and carbonyl group of the DMC were observed ( Figure S2). Hence, as mentioned in the proposed reaction Scheme 1, it is possible to synthesize DMC after the synthesis of SIL followed by its equivalent interaction with CH3I. 1 H NMR spectra of the reaction mixture also confirmed the formation of DMC in the reaction mixture. Similar to the case of 13 C NMR analysis, upon an increase in the amount of CH3I, the amount of DMC steadily increased ( Figures S3 and S4). Simultaneously, the peak intensity for the free protons of the methyl group in methanol, at 3.45 ppm, along with an unknown signal at 3.55 ppm, also increased in the reaction mixture upon increasing the amount of CH3I. In order to identify the unknown peaks in both 1 H and 13 C NMR, as well as to gain insights into the formation of methanol in the reaction mixture, two-dimensional NMR (2D NMR) analysis of the reaction mixture was carried out further.
In Figure 4a, the correlation peak observed in the HSQC analysis at 3.46/47.44 ppm belongs to the methyl group of methanol; hence, as shown in 1 H and 13 C NMR previously, 2D NMR also confirmed that a free methanol molecule formed during DMC synthesis. Usually, interaction of SIL with protic acids is the pathway for the room temperature decomposition of [CH3CO3] − anions upon regeneration of methanol through the release of CO2. This was further confirmed by the equivalent interaction of para-toluene sulfonic acid and SIL in a DMSO solution. After the addition of acid in the DMSO solution of SIL, CO2 bubbles were suddenly released in the reaction mixture, and the formation of methanol was confirmed by 1 H NMR analysis ( Figure S5). Hence, during DMC synthesis, it seems that the acidic moieties formed in-situ in the reaction mixture, thus allowing further decomposition of the [CH3CO3] − anion in the SIL composition.
The unknown peak at chemical shift 3.56 ppm appeared as a singlet and showed an integration value identical to that of the methyl group in methanol. Thus, it seems that the unknown peak at this position belongs to the methyl group. Further, it was also observed that this methyl group showed correlation peaks at 3.56/40.13 and 3.56/164.87 ppm in HSQC and HMBC analyses, respectively ( Figure 4). As shown in both 1 H and 13 C NMR analyses, two types of DBU salts formed in the reaction mixture after addition of 1 eq. of CH3I in the reaction mixture (Supplementary Materials S2 and S4). The peaks for the carbon atoms at position 7 (i.e., C-7) in these two salts appeared at 164.15 and 164.87 ppm, respectively, in the 13 C NMR spectra. One of the two DBU salts is protonated (i.e., [DBUH] [I]) since DMC formed in the reaction mixture as shown in the reaction Scheme 1b. Further, since the newly appearing methyl group at the chemical shift 3.56 ppm has a long range coupled with the carbon atom C-7 at chemical shift 164.87 ppm in HMBC spectra, the peak appearing at 164.  Further, in order to confirm that the captured CO2 during SIL synthesis became a part of DMC, labeled CO2 (i.e., 13 C-enriched CO2 (tracer technique)) was used during the two-step DMC synthesis process. The existence of 13 C-enriched CO2 in SIL as well as DMC was further confirmed by 13 C NMR and HMBC analyses, respectively. As shown in Figure 5b, SIL formed after the reaction of DBU and methanol with 13 C-enriched CO2, and the high-intensity peak for the carbonyl carbon in [CH3CO3] − anion was observed at the chemical shift of 154.56 ppm. After an addition of CH3I in the DMSO solution of SIL, the high-intensity peak for the carbonyl carbon atom in DMC was observed, and this was further confirmed with the help of 13 C NMR of the commercially available DMC (Figure 5c,d). Being 13 C-enriched, the intensities of the peaks for the carbonyl carbon atoms in both [CH3CO3] − and DMC were found significantly higher than the corresponding intensities of the C-7', C-7 # , and C-7 ## in DBU cations formed in the reaction mixture. On the other hand, as shown in Figure 3, the intensity of the peak for the carbonyl carbon was comparable with the intensities of these sp 2 -hybridized carbons in the DBU cation when no 13 C-enriched CO2 was used. Further, HMBC analysis also revealed that a correlation peak was observed for the protons of the methyl group and 13 C-enriched carbonyl carbon atom in DMC ( Figure S6a). In addition, due to having a 13 C-enriched carbon atom, the doublet peak was observed for the protons for the methyl group in DMC, whereas when no 13 C-enriched CO2 was used, a singlet peak was observed for these protons ( Figure S6b) [29]. Therefore, these abovementioned NMR analysis studies revealed that the 13 C-enriched CO2 became a part of DMC formed after the reaction of 13 CO2 enriched SIL with CH3I. Hence, it can be concluded that DMC can be synthesized from CO2 following a two-step synthetic approach, mentioned in the reaction Scheme 1. Besides confirming the origin of carbonyl carbon in DMC, in the 13 C NMR spectra of the reaction mixture, the peak for molecular CO2 at 123.01 ppm was also observed (Figure 5c) [30]. Hence, as shown in the reaction Scheme 3, this control experiment further proved that CO2 was released in the reaction mixture through the decomposition of [CH3CO3] − anions upon DMC synthesis. The amounts (%) of DMC, methanol, and [CH3CO3] − anions in the reaction mixture after each addition of CH3I were calculated using Equations (1)-(3), respectively. As described in the experimental section, the equations correspond to the integrated peak area of the carbon atom in the methyl group of these chemical species obtained from the 13 C NMR spectra. The number of [CH3CO3] − anions was also taken into consideration because both DMC and methanol formed upon consumption of the [CH3CO3] − anion (reaction Scheme 2). Representative integrations of the required signals in 13 C NMR spectra are shown in Figure S7. The obtained values for the amounts of DMC, methanol, and [CH3CO3] − anions after calculation are shown in Figure 6. After sequential addition of CH3I in DMSO solution of SIL, the number of [CH3CO3] − anions steadily decreased and were completely consumed after addition of 1 eq. of CH3I. At the same time, the amounts of both DMC and methanol increased and reached values corresponding to 62 and 38% in the reaction mixture, respectively. In this case, it seems that as a result of the direct interaction between CH3CO3 − anions and CH3I, a higher amount of DMC can be obtained. On the other hand, methanol synthesis possibly proceeds through a three-step process, namely interaction of CH3I with DBUH + , intermolecular proton transfer, and decarboxylation of [CH3CO3] − anions to methanol. These intermediate steps might have slowed down the formation of methanol compared to DMC. Further, after addition of 1 eq. of CH3I and complete conversion of [CH3CO3] − anions, the reaction mixture was added to the ethyl acetate, and the thus-obtained white solid was analyzed by 1 H NMR analysis after separation from the reaction mixture. As shown in Figure 7, the NMR spectra confirm that the obtained white solid is a physical mixture of iodide salts of protonated and methylated DBU, i.e.,  Hence, as shown in the reaction Scheme 1, DMC formed after equivalent interaction between SIL and CH3I. In this case, even though CO2 is a chemically inert species, the highly basic (Brønsted base) characteristics of DBU further facilitated the activation of CO2 under ambient reaction conditions. Further, CH3I has negligible steric hindrance, and since I − is a good leaving group, its interaction with the [DBUH] + cation and [CH3CO3] − anion of the SIL was further promoted. Hence, as a result of having the reagents with these special chemical characteristics, the overall synthesis proceeded with lower energy consumption in terms of temperature and pressure. However, as described previously, the reaction is not atom efficient, as simultaneous formation of methanol and CO2 also proceeds after decomposition of [CH3CO3] − anions (reaction Schemes 2 and 3). This sidereaction not only involves the loss of CH3I reagent, but it also releases equally important CO2, which is needed upon DMC synthesis. Since this side-reaction occurred due to the interaction between the [DBUH] + cation and CH3I, it is necessary to avoid their interaction in order to increase the yield of DMC in this synthetic approach.
Considering these aspects, prior to the addition of CH3I, the DMSO solution of SIL was initially diluted with methanol, whereby the total reaction mixture volume of methanol was maintained as 6, 12, 25, or 50 vol.%. In these diluted reaction mixtures, three equivalents of CH3I (based on DBU in the composition) were added, and after 1 h the reaction mixture was analyzed by NMR analysis. As shown in Figure 8, peaks related to the methylated DBU cation steadily decreased as the amount of methanol in the reaction mixture increased from 6 to 50 vol.%. The amounts of DMC, methanol, and [CH3CO3] − anions in the reaction mixture were calculated using Equations (1)-(3), respectively. As the reaction mixture comprised an excess quantity of methanol, the integrated value of the peak belonging to CH3-group in [DBUCH3] + cation was used for methanol. This was decided based on the fact that equal integrated values were observed for the methyl group in both methanol and [DBUCH3] + cations when the reaction was previously carried with only DMSO as a solvent ( Figure  S7). The calculated amounts of DMC, methanol, and [CH3CO3] − anions observed in the reaction mixture are shown in Figure S9.  Figure S9 depicts that dilution imparts a positive effect on the DMC formation, where the amount of DMC increased from 62% (reaction proceeded with no dilution with methanol) to 81 and 86% with 12 and 25 vol.% of methanol used in the reaction composition, respectively. However, further dilution with 50 vol.% methanol did not improve the DMC yield, and only 16% yield was obtained and 76% of [CH3CO3] − anions remained unreacted. In this case, having 50 vol.% of methanol in the reaction mixture could not allow efficient interaction between the [CH3CO3] − anion and CH3I. Most importantly, upon an increase in the amount of methanol, the intensity of the peaks for the carbon atoms in DBU and the methyl group in [DBUCH3] [I] salt steadily decreased. In other words, instead of using neat DMSO solution of SIL, its diluted solution using methanol allowed more selective interaction between CH3I and [CH3CO3] − anions, whereas it seems that the side-reaction for the interaction between [DBUH] + and CH3I was significantly hampered.
The influence of the amount of CH3I on the formation of DMC in the reaction mixture was further examined in a diluted reaction mixture, whereupon excess methanol was used with SIL and DMSO. In the actual study, the different equivalents of CH3I (on the basis of DBU in the composition) such as 1, 3, or 5 were added to the reaction mixture in which 88 vol.% of DMSO solution of SIL and 12 vol.% of methanol were present in the reaction composition. As shown in Figure S10, with increasing amounts of CH3I from 1 to 3 equivalents, the amount of DMC also increased from 62 to 83% in the reaction mixture. This could be observed since it resulted in a shift in the equilibrium towards DMC with more equivalents of CH3I compared to SIL amount. However, further increase in the CH3I to 5 equivalents did not improve the amount of DMC in the reaction mixture.
As it was observed that dilution with methanol resulted in an increase in the amount of DMC, this synthesis approach was further upgraded by replacing DMSO with methanol. During the actual experimental process, 25 or 50 vol.% solutions of DBU in methanol were prepared and used for CO2 capture through SIL formation. Further, DMC was synthesized after addition of CH3I (3 equivalents based on amount of DBU) in the resultant alcoholic solution of SILs, and the obtained 13 C NMR spectra are shown in S11. Here, along with the influence of the amount of DBU in its alcoholic solution (25 and 50 Vol.%), the influence of the reaction time was also examined in terms of the formation of DMC. As shown in Figure 9a,b in 1 h of reaction time, 13 and 58% of DMC formed when 25 and 50 vol.% of DBU were used, respectively. As observed previously, the use of excessive amounts of methanol resulted in lower amounts of DMC in the reaction mixture. On the other hand, in a comparatively concentrated system, i.e., with 50 vol.% of DBU, a higher amount of DMC was obtained as a result of effective interaction between [CH3CO3] − anions and CH3I. In addition, a similar trend was also observed in the case of methanol formation, whereby higher amounts of methanol were found (7%) with 50 vol.% DBU compared to 25 vol.% DBU-containing reaction mixture. Further, for 5 h reaction period, with 25 vol.% DBU-containing reaction mixture, merely 54% of the [CH3CO3] − anion reacted and 49% of DMC was formed. On the other hand, in the reaction mixture in which 50 vol.% of DBU was used, 96% of [CH3CO3] − anion was consumed, whereas 10 and 86% of methanol and DMC were formed, respectively. DBU is a well-known organocatalyst, and various organic transformations have been performed where it acts either as a strong Brønsted base or good nucleophile. In non-protic solvents the [DBUH] + cation is usually involved in a proton exchange with DBU, and there DBU can perform both as a nucleophile as well as a base in the reaction. Since DMSO is a non-protic solvent (not acting as a proton source), during this proton exchange it further allows the interaction between DBU formed in situ with CH3I. On the other hand, in protic solvents, good nucleophilic characteristics of DBU can be diminished, and it transforms to a weak base as the nucleophilic nitrogen becomes engaged in hydrogen bonding with protic solvents [31]. Since methanol is a protic solvent, there is a lower probability of DBU forming from its protonated form, and nucleophilic interaction of DBU (and hence [DBUH] + cation) with CH3I is kept under control. Hence, instead of DMSO, the use of methanol as a solvent efficiently controlled the side-reactions and further improved the yield of DMC in the reaction mixture. In order to complete conversion of [CH3CO3] − anions in the reaction mixture with 50 vol.% of DBU, the reaction time was further increased from 5 h to 7 and 10 h. The corresponding 13 C NMR spectra for this study are shown in Figure S12. As shown in Figure S13, the [CH3CO3] − anion was completely consumed in the reaction mixture, whereas the amount of DMC increased from 86 to 89% in 10 h of reaction time. Simultaneously, the amount of methanol did not significantly increase, and 11% was found in the reaction mixture. Since no formation of other methylated products (such as dimethyl ether) except methanol was observed, DMC formation reached 89% selectivity in 10 h. Hence, in this reaction approach, as mentioned previously, methanol not only became a part of the CO2 capture process, but it also assisted in the synthesis of DMC with high selectivity. In this regard, excess methanol in the reaction composition significantly influenced the interactions between the chemical species and favored the formation of more DMC.
The reaction mixture in which 50 vol.% of DBU was used was further added to ethyl acetate after 10 h reaction to separate the salts of DBU forming during the reaction. The composition of these salts was confirmed by 13 C NMR analysis. As shown in Figure 10a [I], whereas its methylated analogue was present in much lower quantities. In order to recover the DBU from the mixture of salts, 4 wt.% NaOH in saturated solution of NaCl was used. The purity and recovery of the DBU was controlled by 1 H NMR analysis and gravimetric measurements, respectively. As shown in Figure S14, a pure form of DBU was recovered following alkaline solvent treatment. Besides that, 86% recovery of DBU was achieved from the recovered DBU salts. Hence, as a part of capture and storage, CO2 is stored chemically through the formation of DMC, where initially the SIL was prepared to activate chemically inactive CO2; subsequently, it reacted with CH3I to form DMC. In this reaction approach, a reaction yield >85% was obtained under mild reaction conditions. Even though the use of alcoholic solution further led to slow reaction kinetics, it simultaneously improved the yield of DMC through control over loss of chemisorbed CO2 through side-reactions. Previous reports based on metal carbonates and oxides were accompanied with the use of high reaction temperature and pressure to reach the optimized DMC yield. In addition, in these cases, the catalyst and product decomposition (due to formation of water during the reaction) and formation of by-products (e.g., dimethyl ether in metal carbonate based process) significantly influenced DMC yield. Simultaneously, the regeneration of catalysts or promoters was not carried out in these metal carbonate and oxide-based processes. On the other hand, in this report, the industrially feasible, single-solvent-based and metal-free synthetic approach was established to obtain DMC with high yields and selectivity, at room temperature and under atmospheric pressure. This synthetic approach also demonstrated the recovery of organic superbase, which was considered as an important moiety in the synthesis. The current organic superbase-induced DMC synthesis approach is also accompanied with the use of equivalent amounts of CH3I as the methylating reagent. This substrate is unfortunately considered as a toxic chemical moiety to the ecosystem as well as human beings. However, CH3I is considered as a more active reagent and a more powerful methylating agent compared to others such as DMC, methanol, and dimethyl sulphate [32]. Additionally, the reaction with CH3I usually proceeds under mild reaction conditions and is frequently active without any catalyst, whereas in the case of other methylating agents, the reaction proceeds under harsh reaction conditions and also requires catalysts and sometimes results in low yields. If CH3I is handled safely during its use by considering all the necessary safety precautions, then it can be explored in bulk implementation for DMC synthesis, at industrial scale. Further, this reaction approach involving an organic superbase was carried out with easily available reagents as well as a solvent system, and this fundamental study for DMC synthesis can be extended to synthesize a wide range of industrially important alkyl carbonates. In this case, several alcoholic moieties as well as alkyl halides ranging from lower to high molecular weight can be applied.

Conclusions
The industrially important dimethyl carbonate (DMC) was synthesized from CO2 following switchable ionic liquid (SIL) involved in a one-pot and metal-free reaction approach under ambient conditions. CO2 was activated through its equivalent interaction with organic superbase DBU and methanol, and the resultant SIL reacted with CH3I and formed DMC. Initially, this synthetic approach was proven in DMSO solvent, where 62% of chemisorbed CO2 was converted to DMC. The yield of DMC was further increased to 89% with the DMSO-free reaction approach where synthesis was carried out with methanol as a solvent. Based on NMR analysis, it was observed that with DMSO as a solvent, along with DMC synthesis, as a side reaction, the CH3I also simultaneously reacted with protonated DBU in SIL and formed methylated DBU cations. Hence, released protons further decarboxylated [CH3CO3 − ] anions in unreacted SIL to form methanol and released CO2 in the reaction mixture. However, the use of methanol as a solvent further improved the yield of DMC where the loss of CO2 was avoided by control over the side reactions. Therefore, in this regard, methanol not only performed as a reagent in CO2 capture and DMC synthesis processes, but it also simultaneously assisted in the efficient conversion of chemisorbed CO2 to DMC. DBU salts such as [DBUH] [I] and [DBUCH3] [I] were also converted to pure DBU, where 86% recovery was achieved. Hence, metalfree, single-solvent-based, and one-pot synthesis of DMC was performed under ambient conditions. As a main achievement, in this fundamental study anthropogenic CO2 was not only captured but also converted to a valuable chemical.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1. 1 Figure S9. Amounts (%) of DMC, CH3OH. and CH3CO3 − anions in the reaction mixture containing different amounts of methanol or DMSO solution of SIL; 3 eq. of CH3I was added (based on the amounts of DBU), 1h of reaction time; Figure S10. Amounts (%) of DMC, CH3OH, and [CH3CO3]anions in the reaction mixture after addition of different amounts of CH3I (1, 2, and 5 eq. based on amount of DBU). The reaction performed with 12 vol.% of methanol with 88 vol.% of SIL in DMSO for 1h; Figure S11. 13 C NMR spectra of the reaction mixture for the different reaction times after addition of 3 eq. of CH3I (based on amount of DBU) in the solution of SIL ([DBUH][CH3CO3]) in methanol. The 25 or 50 vol.% of DBU in methanol was taken during SIL synthesis, (a) 0 h (only SIL in methanol), (b) 1h, and (c) 5h (NMR analysis with D2O capillary); Figure S12. 13 C NMR spectra for the synthesis of DMC in reaction mixtures with 50 vol.% of DBU in alcoholic solution with different reaction times: (a) 0 h (only SIL in methanol), (b) 1h, (c) 5h, (d) 7h, and (e) 10h; 3 eq. of CH3I was added (NMR analysis with D2O capillary); Figure S13. Amounts of DMC, methanol, and [CH3CO3 − ] anions formed in the reaction mixture with different reaction times with reaction composition having 50 vol.% of DBU in alcoholic solution; 3 eq. of CH3I was added; Figure S14. 13