Communication Chemical Adsorption Strategy for DMC-MeOH Mixture Separation †

Fucan Zhang 1,2, Ping Liu 1, Kan Zhang 1 and Qing-Wen Song 1,*

molecules1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China; [email protected] (F.Z.); [email protected] (P.L.); [email protected] (K.Z.) 2 State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering and Environment, Communication Chinese China University of Petroleum, Beijing 102249, China * Correspondence: [email protected] Chemical Adsorption† Dedicated Strategy to the 100th anniversary for DMC-MeOH of Chemistry at Nankai MixtureUniversity. Separation † Abstract: The effective separation of dimethyl (DMC) from its mixture through Fucan Zhang 1,2 , Ping Liu 1simple,, Kan Zhang inexpensive1 and Qing-Wen and low Song energy-input1,* method is a promising and challenging field in the process of organic synthesis. Herein, a reversible adsorption strategy through the assistance of 1superbaseState Key Laboratoryand CO2 of for Coal DMC/methanol Conversion, Institute separation of Coal Chemistry, at ambient Chinese Academy condition of Sciences, was described. The process Taiyuan 030001, China; [email protected] (F.Z.); [email protected] (P.L.); [email protected] (K.Z.) was demonstrated effectively via the excellent CO2 adsorption efficiency. Notably, the protocol was 2 State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering and Environment, alsoChinese suitable China to University other ofalcohol Petroleum, (i.e., Beijing monohydric 102249, China alcohol, dihydric alcohol, trihydric alcohol) mix- *tures.Correspondence: The study [email protected] provided guidance for potential separation of DMC/alcohol mixture in the † Dedicated to the 100th anniversary of Chemistry at Nankai University. scale-up production. Abstract: The effective separation of (DMC) from its methanol mixture through simple,Keywords: inexpensive dimethyl and low carbonate; energy-input methodsuperbase; is a promising carbon and di challengingoxide; reversible field in the ad processsorption; separation ofmethod organic synthesis. Herein, a reversible adsorption strategy through the assistance of superbase Citation: Zhang, F.; Liu, P.; Zhang, and CO2 for DMC/methanol separation at ambient condition was described. The process was demonstrated effectively via the excellent CO adsorption efficiency. Notably, the protocol was also K.; Song, Q. Chemical Adsorption 2 suitable to other alcohol (i.e., monohydric alcohol, dihydric alcohol, trihydric alcohol) mixtures. The Strategy for DMC-MeOH Mixture study provided guidance for potential separation of DMC/alcohol mixture in the scale-up production. Separation. Molecules 2021, 26, x. 1. Introduction https://doi.org/10.3390/xxxxx Keywords: dimethyl carbonate; superbase; ; reversible adsorption; separation method Dimethyl carbonate (DMC), defined as a green chemical, has many applications in  Academic Editor: Bhanu P.S. synthetic chemistry, and it is also used as a , an additive in gasoline, a depurative, Chauhan Citation: Zhang, F.; Liu, P.; Zhang, and a surfactant [1–3]. During the past ten years, much attention has been paid to the 1. Introduction K.; Song, Q.-W. Chemical Adsorption carbon dioxide (CO2)-based synthetic routes to DMC, such as transesterification of cyclic Dimethyl carbonate (DMC), defined as a green chemical, has many applications in Received:Strategy 21 February for DMC-MeOH 2021 Mixture carbonate and methanol [4–7] and of CO2 [8–12]/ [13] with methanol synthetic chemistry, and it is also used as a solvent, an additive in gasoline, a depurative, Accepted:Separation 16 March . Molecules 2021 2021, 26, 1735. (Scheme 1). Notably, in these processes, because of the thermodynamic equilibrium and https://doi.org/10.3390/molecules and a surfactant [1–3]. During the past ten years, much attention has been paid to the Published: 17 March 2021 26061735 carbonlimited dioxide catalytic (CO2 )-basedability, synthetic excess methanol routes to DMC, was such generally as transesterification used for obtaining of cyclic the high effi- carbonateciency. Therefore, and methanol the [4 –target7] and carbonylationproduct was of COalways2 [8–12 mixed]/urea [with13] with methanol, methanol and the easily Publisher’sAcademic Note: Editor: MDPI Bhanu stays neu- (Schemeformed1 ) DMC/MeOH. Notably, in these binary processes, azeotrope because of(appr the thermodynamicoximate methanol/DMC equilibrium and composition of P.S. Chauhan tral with regard to jurisdictional limited70%:30%) catalytic increased ability, excess the methanolcomplexity was generallyof DMC used separation for obtaining [14–16]. the high Until efficiency. now, the available claims in published maps and insti- Therefore, the target product was always mixed with methanol, and the easily formed Received: 21 February 2021 separation methods have included extractive distillation, liquid–liquid extraction, selec- tutional affiliations. DMC/MeOH binary azeotrope (approximate methanol/DMC composition of 70%:30%) Accepted: 16 March 2021 increasedtive adsorption, the complexity lower of DMCtemperature separation crystalliz [14–16]. Untilation, now, and the availablethe membrane/distillation separation inte- Published: 19 March 2021 methodsgrated haveprocess, included etc. extractive Among distillation, them, pressure liquid–liquid distillation extraction, technique selective adsorption, is used in industrial lower temperature crystallization, and the membrane/distillation integrated process, etc. processes, and the energy input is very high. Therefore, improvement and innovation are Publisher’s Note: MDPI stays neutral Among them, pressure distillation technique is used in industrial processes, and the energy Copyright:with regard © 2021 to jurisdictionalby the authors. claims in inputurgently is very required high. Therefore, for the improvement enhancement and innovationof separation are urgently efficiency required and for the the decrease in en- Licenseepublished MDPI, maps Basel, and Switzerland. institutional affil- enhancementergy consumption. of separation efficiency and the decrease in energy consumption. iations. This article is an open access article O distributed under the terms and CO2 CO CH OH O O 2 OO 3 -H2O conditions of the Creative Commons CH3OH R O O Attribution (CC BY) license R Copyright: © 2021 by the authors. OH urea -NH (http://creativecommons.org/licensesLicensee MDPI, Basel, Switzerland. OH 3 R /by/4.0/).This article is an open access article Indirect route Direct route distributed under the terms and conditions of the Creative Commons Scheme 1. 1.CO CO2-sourced2-sourced routes routes to dimethyl to dime carbonatethyl carbonate (DMC). (DMC). Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ In 2005, Jessop et al. reported a reversible nonpolar-to-polar solvent [17]. In their 4.0/). work, a non-ionic liquid including an alcohol and an organic base was converted into

Molecules 2021, 26, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/molecules

Molecules 2021, 26, 1735. https://doi.org/10.3390/molecules26061735 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x 2 of 7

Molecules 2021, 26, 1735 2 of 7 In 2005, Jessop et al. reported a reversible nonpolar-to-polar solvent [17]. In their work, a non-ionic liquid including an alcohol and an organic base was converted into a

salta salt in inliquid liquid form form upon upon exposure exposure to to a a CO CO2 2atmosphere,atmosphere, and and then then reverted reverted back back to its non-ionic form when exposed to nitrogen or argon gas. After this, the reversible process was employed as an effective strategy for CO 2 capturecapture [18–20]. [18–20]. Referring Referring to to these these studies, studies, we speculatedspeculated thatthat performing performing the the reversible reversible adsorption adsorption strategy strategy with with CO2 ,CO a suitable2, a suitable base baseand MeOHand MeOH could could also also be abe potential a potential candidate candidate for for the the separation separation of of the the MeOH/DMC MeOH/DMC mixture. AfterAfter thethe adsorption adsorption reaction, reaction, DMC DMC stays stays in the in liquidthe liquid phase phase and the and new the formed new formedsolid adduct solid precipitatesadduct precipitates (Scheme 2(Scheme). For the 2). mechanism, For the mechanism, methanol methanol is initially is activated initially activatedby 1,5-diazabicyclo(4.3.0)non-5-ene by 1,5-diazabicyclo(4.3.0)non-5-ene (DBN) and (DBN) subsequently and subsequently attacks on attacks the CO on2 molecule the CO2 moleculewith the generationwith the generation of ionic solid of ionic adduct solid which adduct precipitates which precipitates from the system.from the system.

Scheme 2. ReversibleReversible CO CO22 adsorptionadsorption applied applied in in CH CH33OH/DMCOH/DMC separation. separation.

Herein, a reversible adsorption strategy utilising superbase, CO , and methanol for Herein, a reversible adsorption strategy utilising superbase, CO2, and methanol for DMC separation from its methanol mixture at ambient condition was proposed, and the detailed parameters beingbeing relatedrelated to to the the adsorption adsorption efficiency efficiency such such as component,as component, solvent, sol- vent,temperature temperature and pressure and pressure were studied. were studied. The work The aims work to aims provide to provide a separation a separation protocol protocolfeaturing featuring as low energy as low input, energy renewable, input, renewable, simple and simple high and separation high separation efficiency. efficiency. 2. Results and Discussion 2. Results and Discussion Initially, various bases in combination with stoichiometric methanol were exam- Initially, various bases in combination with stoichiometric methanol were examined ined on the absorption capacity under the neat conditions (Scheme3). The conjugated onand the strong absorption organic capacity bases such under as 1,8-diazabicyclo(5.4.0)undec-7-enesthe neat conditions (Scheme 3). The (DBU), conjugated DBN and strong1,1,3,3-tetramethylguanidine organic bases such as (TMG) 1,8-diazabicyclo(5.4.0)undec-7-enes belonging to superbase [21] showed (DBU), high DBN absorp- and 1,1,3,3-tetramethylguanidine (TMG) belonging to superbase [21] showed high absorption tion ability for 1 h under ambient pressure of CO2. In addition, the common bases abilitysuch as for pyridine 1 h under derivative, ambientN pressure,N,N’,N ’-tetramethyl-1,6-hexanediamineof CO2. In addition, the common (TMHDA) bases such and as pyridine1-methylimidazole derivative, (MeIM) N,N displayed,N’,N’-tetramethyl-1,6-hexanediamine weak absorption ability. The basicity(TMHDA) and conju-and 1-methylimidazole (MeIM) displayed weak absorption ability. The basicity and conju- gated structure of base candidates were both key factors for high and fast CO2 absorption. gated structure of base candidates were both key factors for high and fast CO2 absorp- Among them, 7.4 mmol CO2 was captured through 10 mmol methanol and 10 mmol DBN tion.under Among identical them, conditions. 7.4 mmol CO2 was captured through 10 mmol methanol and 10 mmol DBN under identical conditions. The solvent effect is one of most key factors for CO2 capture in the CH3OH/DMC mixture, and therefore was investigated using DBN as a base here (Table1). As seen from the results for the first 1h (entries 1–4), the initial absorption rate decreased sharply. Furthermore, the absorptive amount increased a lot after 16 h. These results reveal that the organic media could reduce the reaction rate, and the reason is probably that the low CO2 solubility limited the molecule transfer. In addition, polar solvent is beneficial for the enhancement of CO2 absorption rate. To verify these speculations, the solubility of CO2 in the solvent used in Table1 was explored (Supplementary Materials, Table S1). The order of solubility under ambient conditions is as follows: S(DMF, 58.02 mg/L) > S(DMC, 19.92 mg/L) > S(, 14.58 mg/L) > S(hexane, 6.63 mg/L). The results support the hypothesis that the CO2 diffusion process was hindered by the medium. Meanwhile, the trend of the absorption capacity for 16 h was also consistent with the solubility rule (entries 1–4). Moreover, the increase in the temperature could lead to the further decrease in absorption rate (entry 3 vs. 5 and entry 6 vs. 7). Consider the absorption efficiency

Molecules 2021, 26, 1735 3 of 7

(entry 6) and operation convenience, the next experiments were carried out at room temperature. With an increase in the pressure, the absorption process was sharply enhanced and saturated absorption loading was achieved even in a shorter time (entries 9–11). Under the pressured conditions, the absorption process also quickly reached the balance for other Molecules 2021, 26, x 3 of 7 but with an absorption capacity slightly lower than the maximum absorption capacity (entries 12–14).

Scheme 3. CO22 absorption usingusing variousvarious bases. bases. Conditions: Conditions: MeOH MeOH (10 (10 mmol, mmol, 0.32 0.32 g), g), base base (10 (10 mmol), ◦ COmmol),2 balloon, CO2 balloon, 60 min, 60 12 min,C. 12 °C.

a TheTable solvent 1. Parameters effect investigationis one of most on thekey absorption factors for. CO2 capture in the CH3OH/DMC mixture, and therefore was investigated using DBN as a base here (Table 1). As seen from CO2 Absorption Loading/mmol Entry Solventthe Pressure/MPa results for theTemp./ first ◦1hC (entries 1–4), the initial absorption rate decreased sharply. Furthermore, the absorptive amount20 min increased a lot 1 h after 16 h. 100 These min results reveal 16 h that 1 hexanethe organic 0.1 media could 12 reduce the reaction - rate, and 1.05 the reason is - probably that 6.01 the low 2 tolueneCO2 solubility 0.1 limited 12 the molecule transfer. - In addition, 1.65 polar solvent - is beneficial 7.56 for the 3 DMCenhancement 0.1 of CO2 12absorption rate. - To verify these 1.34 speculations, - the solubility 8.78 of CO2 4 DMFin the 0.1solvent used in 12 Table 1 was -explored (Supplementary 1.92 Materials, - Table 9.02 S1). The 5 DMC 0.1 25 - 0.56 1.60 - 6 -order 0.1of solubility under 25 ambient conditions - is as 9.50follows: S(DMF, 9.60 58.02 mg/L) > -S(DMC, 7 -19.92 mg/L) 0.1 > S(toluene, 35 14.58 mg/L) - > S(hexane, 6.63 0 mg/L). The 0.25 results support - the hy- 8 DMFpothesis 0.1 that the CO 252 diffusion process - was hindered 1.89 by the medium. 2.70 Meanwhile, 9.41 the 9 DMFtrend 1.0of the absorption 25 capacity for 9.42 16 h was also consistent - with -the solubility rule - (en- 10 DMFtries 1–4). 2.0 Moreover, 25the increase in 9.45 the temperature - could lead to -the further decrease - in 11 DMFabsorption 3.4 rate (entry 25 3 vs. 5 and entry 9.47 6 vs. 7). Consider - the absorption - efficiency - (entry 12 toluene 1.0 25 8.63 - - - 13 toluene6) and 1.8 operation convenience, 25 the 8.72next experiments - were carried -out at room tempera- - 14 DMCture. With 1.0 an increase 25 in the pressure,8.21 bthe absorption- process was -sharply enhanced 8.22 and a saturated absorption loading was achieved even in a shorterb time (entries 9–11). Under Conditions: MeOH (10 mmol, 0.32 g), DBN (10 mmol, 1.24 g), solvent (3 mL), CO2 balloon (0.1 MPa). 40 min. Blank absorption was deducted for all the data. N,N-dimethylformamidethe pressured conditions, (DMF) the absorption process also quickly reached the balance for other solvents but with an absorption capacity slightly lower than the maximum absorp- tion capacity (entries 12–14).

Molecules 2021, 26, x 4 of 7

Table 1. Parameters investigation on the absorption.a.

CO2 Absorption Loading/mmol Entry Solvent Pressure/MPa Temp./°C 20 min 1 h 100 min 16 h 1 hexane 0.1 12 - 1.05 - 6.01 2 toluene 0.1 12 - 1.65 - 7.56 3 DMC 0.1 12 - 1.34 - 8.78 4 DMF 0.1 12 - 1.92 - 9.02 5 DMC 0.1 25 - 0.56 1.60 - 6 - 0.1 25 - 9.50 9.60 - 7 - 0.1 35 - 0 0.25 - 8 DMF 0.1 25 - 1.89 2.70 9.41 9 DMF 1.0 25 9.42 - - - 10 DMF 2.0 25 9.45 - - - 11 DMF 3.4 25 9.47 - - - 12 toluene 1.0 25 8.63 - - - 13 toluene 1.8 25 8.72 - - - 14 DMC 1.0 25 8.21b - - 8.22 a Molecules 2021 26 b Conditions: MeOH, , 1735 (10 mmol, 0.32 g), DBN (10 mmol, 1.24 g), solvent (3 mL), CO2 balloon (0.1 MPa). 40 min. Blank absorption was4 of 7 deducted for all the data. N,N-dimethylformamide (DMF)

The effect of DMC concentration on the absorption was also investigated under at- The effect of DMC concentration on the absorption was also investigated under at- mospheric pressure (Figure 1). With the increase in DMC content in the mixture, the CO2 mospheric pressure (Figure1). With the increase in DMC content in the mixture, the CO absorption loading decreased quickly during the same time. In total, the absorption effi- 2 absorption loading decreased quickly during the same time. In total, the absorption effi- ciency is not enough for application in the separation of CH3OH/DMC mixture. There- ciency is not enough for application in the separation of CH3OH/DMC mixture. Therefore, fore, further improvement was obtained through increasing CO2 pressure. Under 1.0 further improvement was obtained through increasing CO2 pressure. Under 1.0 MPa CO2 MPa CO2 for only 20 min, high CO2 absorption capacity was gained (Figure 1b). The for only 20 min, high CO absorption capacity was gained (Figure1b). The separation separation efficiency of DMC/MeOH2 is up to 98.7% (Figure 1c). efficiency of DMC/MeOH is up to 98.7% (Figure1c).

FigureFigure 1. 1. AdsorptionAdsorption separation separation strategy strategy for for DMC DMC an andd MeOH mixture. Conditions:Conditions: MeOHMeOH (10(10 mmol, ◦ mmol,0.32 g), 0.32 DBN g), (10DBN mmol, (10 mmol, 1.24 g), 1.24 DMC g), (15–90DMC ( wt%,15–90 0.63–32 wt%, 0.63–32 mmol, mmol, 0.057–2.88 0.057–2.88 g), 25 C,g), (25a) CO°C,2 (aballoon,) CO2 balloon,2 h; (b, c2) h; CO (b2,c1.0) CO MPa,2 1.0 20 MPa, min. 20 Blank min. absorptionBlank absorption was not was deducted not deducted for the for data. the data.

Here, the freezing-centrifugation technology was used in the separation process. First of all, the adducts are sensitive to the temperature and water, and they will decompose under the heat condition (approximate 35 ◦C, see entry 7 of Table1). In addition, in the separation process, one of the most crucial factors is the solubility of the adducts in the

DMC or other solvents. The lower solubility will give a higher proportion or purity of DMC in the mother liquid. Taken an example, with the assistance of PhCH3, the ratio of DMC to CH3OH (most in adduct form) in the mother liquid is up to 20. The DBN, ◦ CH3OH and CO2 from the adduct were successively recovered with heat (> 35 C). The study provided a new and potential method for DMC and CH3OH mixture separation by a low energy-input manner. In the process of the “two step” method, namely the transesterification of cyclic carbonate ( carbonate or ) and methanol [22], several alcohols coexist, which further increases the complexity of separation. Therefore, typical alcohols including monohydric alcohol, dihydric alcohol, trihydric alcohol and their mixtures were also verified here. As seen in Table2, both monohydric alcohol and polyhydric alcohol showed high CO2 absorption efficiency (entries 1–4). However, with the increase in hydroxyl quantity in alcohol molecule, the absorption rate reduced. The reason is probably that the elevated hinders CO2 molecule transfer. Furthermore, the mixtures of methanol and dihydric alcohols were also demonstrated effectively for CO2 absorption with high rate and capacity (entries 5–8). Molecules 2021, 26, 1735 5 of 7

Table 2. The absorption investigation on the mixture of DMC and various alcohol candidates.

Alcohol Candidate CO2 Separation Efficiency of Entry DMC/wt% Loading (mmol) Alcohol Type Absorption/mmol MeOH/% or Mole Ratio 1 22.9 EtOH 10 9.52 95.2 2 30.6 EG 5 8.92 89.2 3 26.5 PG 5 8.45 84.5 4 30.9 Glycerol 3.33 7.28 72.8 5 30.3 MeOH and EG 2:1 9.64 96.4 6 28.1 MeOH and PG 2:1 9.78 97.8 7 10 MeOH and PG 2:1 9.76 97.6 8 80 MeOH and PG 2:1 9.15 91.5 ◦ Conditions: DBN (10 mmol, 1.24 g), 10 mmol –OH group in alcohol or mixture, CO2 1.0 MPa, 20 min, 25 C. Blank absorption was not deducted for the data; (EG), 1,2- (PG).

3. Materials and Methods 3.1. General Information General analytic methods. 1H-NMR spectra were recorded on 400 MHz spectrom- eters (Bruker AVANCE IIITM 400 MHz, Baden, Switzerland) using DMSO-d6 as solvent 13 referenced to DMSO-d6 (2.50 ppm). C-NMR was recorded at 100.6 MHz in DMSO-d6 (39.52 ppm). Multiplets were assigned as singlet, doublet, triplet, doublet of doublet, multiplet, and broad singlet.

3.2. Materials

CH3OH (analytical reagent, >99.5%, Tianjin Fengchuan Chemistry Reagent Limited Company, Tianjin, China). Carbon dioxide (99.999%, Shanxi Yihong Gas Industry Limited Company, Taiyuan, Shanxi, China). The reagents, i.e., 1,8-diazabicyclo(5.4.0)undec-7-enes (DBU, 99%), 1,5-diazabicyclo(4.3.0)non-5-ene (DBN, 98%), 1,1,3,3-tetramethylguanidine (TMG, 99%), N,N,N0,N0-tetramethyl-1,6-hexanediamine (TMHDA, 99%) and 1-methylimid- azole (MeIM, 99%), were obtained from Aladdin (Aladdin Industrial Corporation, Shanghai, China) and were used as received.

3.3. General Procedure for the Absorption Reaction The absorption process was performed in a 50 mL autoclave with a glass vessel inside equipped with magnetic stirring under 1.0 MPa CO2 (10 mL Schlenk tube under atmospheric pressure of CO2). After introducing DBN (10 mmol, 1.24 g), CH3OH (10 mmol, 0.32 g), and DMC (0–90 wt%), the autoclave was sealed and filled with CO2 to keep ◦ the pressure of CO2 under 1.0 MPa. Then, the reaction mixture was stirred at 25 C for 20 min. When the absorption reaction was completed, the residual CO2 was carefully released. The absorbed CO2 was calculated by the increased weight of the glass vessel. Note that these adducts are very sensitive to the water and temperature. NMR technology was selected for the characterization of these adducts. During the process, trace water molecules were introduced, and a small part of the adducts was decomposed. However, the characterization was not obviously affected, as indicated by the spectrum.

3.4. General Procedure for the Separation Freezing-centrifugation technology was used in the separation process. We took an example with the conditions of DBN (10 mmol, 1.24 g), CH3OH (10 mmol, 0.32 g), DMC ◦ (27 wt%), and PhCH3 (10 mL) under 1.0 MPa CO2 at 25 C for 20 min. After completing the absorption reaction (see Procedure 3.3.), the tube was sealed and stored at −7 ◦C for one hour, and then centrifuged at 3000 revolutions per minute for one minute. The upper liquid was brought out and analyzed by gas chromatography. The precipitate was decomposed over heat and recovered. Molecules 2021, 26, 1735 6 of 7

4. Conclusions In conclusion, we have demonstrated an unprecedentedly reversible adsorption strat- egy using a superbase, CO2, and methanol for DMC separation at ambient conditions. The low temperature and viscosity, high CO2 pressure, and medium are verified to be beneficial for the adsorption process. This strategy is compatible with a wide range of DMC concentrations, with excellent separation efficiency. The effective separation of DMC from its methanol mixture through simple, inexpensive, and low energy input method is also suitable for other alcohols (i.e., monohydric alcohol, dihydric alcohol, trihydric alcohol) or their mixture. The exploration of the whole separation technology is underway in our laboratory.

Supplementary Materials: The following are available online, Table S1: Solubility of CO2 in different solvents; Part 2: NMR Data of the Adducts; Part 3: Spectral Copies of the Adducts. Author Contributions: Conceptualization, Q.-W.S.; methodology, Q.-W.S.; software, F.Z.; validation, F.Z., P.L., K.Z. and Q.-W.S.; formal analysis, F.Z. and Q.-W.S.; investigation, F.Z.; resources, Q.-W.S.; data curation, F.Z.; writing—original draft preparation, F.Z.; writing—review and editing, Q.-W.S.; visualization, Q.-W.S.; supervision, P.L. and K.Z.; project administration, P.L. and K.Z.; funding acquisition, P.L., K.Z. and Q.-W.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (21602232), the Natural Science Foundation of Shanxi (201701D221057), Leading Talents Projects of Emerging Industries of Shanxi Province (Y9SW9U62, E0SW9862) and CAS Key Technology Talent Program (YB2021001). Data Availability Statement: All data included in this study are available upon request by contact with the corresponding author. Acknowledgments: Thanks for the Public Technical Service Centre of Institute of Coal Chemistry, Chinese Academy of Sciences. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compounds for this experiment are available upon request by contact with the corresponding author.

References 1. Shaikh, A.A.G.; Sivaram, S. Organic . Chem. Rev. 1996, 96, 951–976. [CrossRef] 2. Ono, Y. Catalysis in the production and reactions of dimethyl carbonate, an environmentally benign building block. Appl. Catal. A: Gen. 1997, 155, 133–166. [CrossRef] 3. Huang, S.; Yan, B.; Wang, S.; Ma, X. Recent advances in dialkyl carbonates synthesis and applications. Chem. Soc. Rev. 2015, 44, 3079–3116. [CrossRef] 4. Wang, J.-Q.; Sun, J.; Sun, W.-G.; Cheng, Shi, C.-Y.; Dong, K.; Zhang, X.-P.; Zhang, S.-J. Synthesis of dimethyl carbonate catalyzed by carboxylic functionalized imidazolium salt via transesterification reaction. Catal. Sci. Technol. 2012, 2, 600–605. [CrossRef] 5. Yang, Z.-Z.; Dou, X.-Y.; Wu, F.; He, L.-N. NaZSM-5-catalyzed dimethyl carbonate synthesis via the transesterification of with methanol. Can. J. Chem. 2011, 89, 544–548. [CrossRef] 6. Kim, D.-W.; Lim, D.-O.; Cho, D.-H.; Koh, J.-C.; Park, D.-W. Production of dimethyl carbonate from ethylene carbonate and methanol using immobilized ionic liquids on MCM-41. Catal. Today 2011, 164, 556–560. [CrossRef] 7. Juárez, R.; Corma, A.; García, H. Gold nanoparticles promote the catalytic activity of ceria for the transalkylation of propylene carbonate to dimethyl carbonate. Green Chem. 2009, 11, 953–958. [CrossRef] 8. Honda, M.; Tamura, M.; Nakagawa, Y.; Sonehara, S.; Suzuki, K.; Fujimoto, K.-I.; Tomishige, K. Ceria-Catalyzed Conversion of Carbon Dioxide into Dimethyl Carbonate with 2-Cyanopyridine. Chem. Sus. Chem 2013, 6, 1341–1344. [CrossRef] 9. Zhao, T.; Hu, X.; Wu, D.; Li, R.; Yang, G.; Wu, Y. Direct Synthesis of Dimethyl Carbonate from Carbon Dioxide and Methanol at Room Temperature Using Imidazolium Hydrogen Carbonate Ionic Liquid as a Recyclable Catalyst and Dehydrant. Chem. Sus. Chem 2017, 10, 2046–2052. [CrossRef] 10. Zhang, Z.; Liu, S.; Zhang, L.; Yin, S.; Yang, G.; Han, B. Driving dimethyl carbonate synthesis from CO2 and methanol and production of acetylene simultaneously using CaC2. Chem. Commun. 2018, 54, 4410–4412. [CrossRef][PubMed] 11. Xuan, K.; Pu, Y.; Li, F.; Li, A.; Luo, J.; Li, L.; Wang, F.; Zhao, N.; Xiao, F. Direct synthesis of dimethyl carbonate from CO2 and methanol over trifluoroacetic acid modulated UiO-66. J. CO2 Util. 2018, 27, 272–282. [CrossRef] Molecules 2021, 26, 1735 7 of 7

12. Liao, Y.; Li, F.; Dai, X.; Zhao, N.; Xiao, F. Dimethyl carbonate synthesis over solid base catalysts derived from Ca–Al layered double hydroxides. Chem. Pap. 2018, 72, 1963–1971. [CrossRef] 13. Yang, B.; Wang, D.; Lin, H.; Sun, J.; Wang, X. Synthesis of dimethyl carbonate from urea and methanol catalyzed by the metallic compounds at atmospheric pressure. Catal. Commun. 2006, 7, 472–477. [CrossRef] 14. Kumar, R.; Bhakta, P.; Chakraborty, S.; Pal, P. Separating Cyanide from Coke Wastewater by Cross Flow Nanofiltration. Sep. Sci. Technol. 2011, 46, 2119–2127. [CrossRef] 15. Liu, Z.; Lin, W.; Li, Q.; Rong, Q.; Zu, H.; Sang, M. Separation of dimethyl carbonate/methanol azeotropic mixture by pervaporation with dealcoholized room temperature-vulcanized silicone rubber/nanosilica hybrid active layer. Sep. Purif. Technol. 2020, 248, 116926. [CrossRef] 16. Hu, C.-C.; Cheng, S.-H. Development of alternative methanol/dimethyl carbonate separation systems by extractive distilla-tion— A holistic approach. Chem. Eng. Res. Des. 2017, 127, 189–214. [CrossRef] 17. Jessop, P.G.; Heldebrant, D.J.; Li, X.; Eckert, C.A.; Liotta, C.L. Green chemistry: Reversible nonpolar-to-polar solvent. Nature 2005, 436, 1102–1103. [CrossRef] 18. Heldebrant, D.J.; Yonker, C.R.; Jessop, P.G.; Phan, L. Organic liquid CO2 capture agents with high gravimetric CO2 capacity. Energy Environ. Sci. 2018, 1, 487–493. [CrossRef] 19. Yang, Z.-Z.; He, L.-N.; Zhao, Y.-N.; Li, B.; Yu, B. CO2 capture and activation by superbase/polyethylene glycol and its subsequent conversion. Energy Environ. Sci. 2011, 4, 3971–3975. [CrossRef] 20. Polshettiwar, V.; Varma, R.S. Green chemistry by nano-catalysis. Green Chem. 2010, 12, 743–754. [CrossRef] 21. Caubere, P. Unimetal super bases. Chem. Rev. 1993, 93, 2317–2334. [CrossRef] 22. Tian, J.-S.; Miao, C.-X.; Wang, J.-Q.; Cai, F.; Du, Y.; Zhao, Y.; He, L.-N. Efficient synthesis of dimethyl carbonate from methanol, propylene oxide and CO2 catalyzed by recyclable inorganic base/phosphonium halide-functionalized polyethylene glycol. Green Chem. 2006, 9, 566–571. [CrossRef]