Study of Superbase-Based Deep Eutectic Solvents as the Catalyst in the Chemical Fixation of CO₂ into Cyclic Carbonates under Mild Conditions

Abstract

Superbases have shown high performance as catalysts in the chemical fixation of CO₂ to epoxides. The proposed reaction mechanism typically assumes the formation of a superbase, the CO₂ adduct as the intermediate, most likely because of the well-known affinity between superbases and CO₂, i.e., superbases have actually proven quite effective for CO₂ absorption. In this latter use, concerns about the chemical stability upon successive absorption-desorption cycles also merits attention when using superbases as catalysts. In this work, ¹H NMR spectroscopy was used to get further insights about (1) whether a superbase, the CO₂ adduct, is formed as an intermediate and (2) the chemical stability of the catalyst after reaction. For this purpose, we proposed as a model system the chemical fixation of CO₂ to epichlorohydrin (EP) using a deep eutectic solvent (DES) composed of a superbase, e.g., 2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine (TBD) or 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine (DBU), as a hydrogen acceptor and an alcohol as a hydrogen bond donor, e.g., benzyl alcohol (BA), ethylene glycol (EG), and methyldiethanolamine (MDEA), as the catalyst. The resulting carbonate was obtained with yields above 90% and selectivities approaching 100% after only two hours of reaction in pseudo-mild reaction conditions, e.g., 1.2 bars and 100 °C, and after 20 h if the reaction conditions of choice were even milder, e.g., 1.2 bars and 50 °C. These results were in agreement with previous works using bifunctional catalytic systems composed of a superbase and a hydrogen bond donor (HBD) also reporting good yields and selectivities, thus confirming the suitability of our choice to perform this study.

Keywords: CO2 absorption; CO2 fixation; eutectic solvents; superbases.

Figure 1

¹H and ¹³C NMR spectra (left and right columns, respectively) of the deep eutectic solvent (DES) composed of 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine (DBU) and benzyl alcohol (BA) in a 1:1 molar ratio, e.g., DBU(1):BA(1), before (A,C) and after (B,D) CO₂ absorption. The chemical structures of both the DES resulting from H-bond interaction between DBU and BA in the DES and the salt formed upon CO₂ absorption on DES are also included.

Figure 2

¹H and ¹³C NMR spectra (left and right columns, respectively) of the DES composed of 2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine (TBD) and BA in a 1:1 molar ratio, e.g., TBD(1):BA(1), before (A,C) and after (B,D) CO₂ absorption. The chemical structures of both the DES resulting from H-bond interaction between TBD and BA in the DES and the salt formed upon CO₂ absorption on DES are also included.

Figure 3

(A) ¹H and ¹³C NMR spectra (left and right columns, respectively) of both the main product, e.g., 4-(chloromethyl)-1,3-dioxolan-2-one, and the sub-product, e.g., 1,3-dichloropropan-2-ol, resulting after CO₂ fixation to EP using either DBU (top panel) or DBU(1):BA(1) DES (bottom panel) as the catalyst. The catalyst to EP molar ratio was 1:100. Symbols assign peaks to their respective molecules, i.e., ■ for 4-(chloromethyl)-1,3-dioxolan-2-one, ♦ for 1,3-dichloropropan-2-ol and ● for EP. (B) Chemical structures of 4-(chloromethyl)-1,3-dioxolan-2-one and 1,3-dichloropropan-2-ol, as well as chemical shifts assigned to their respective peaks at the ¹H and ¹³C NMR spectra. The reaction conditions were 100 °C, 1.2 bars and 2 h for these particular spectra, but similar spectra were obtained in the different reaction conditions described in Table 2.

Figure 4

Plausible mechanisms of the chemical fixation of CO₂ into epichlorohydrin (EP) catalyzed by DBU in the presence of a hydrogen bond donor (HBD), according to those proposed in previous works (A,B) and according to the NMR results obtained in this work (C).

Figure 5

¹H NMR spectra of a mixture of BA and EP in a molar ratio of 1:4. The chemical structures of EP and the H-bond complex formed between EP and BA are also included.

Figure 6

(A) ¹³C NMR spectra of the intermediate observed in the reaction mixture composed of EP and DBU, in either its bare (left column) or DES-form (right column), and in the absence (top panel) or the presence (bottom panel) of CO₂; (B) The chemical structure proposed for the intermediate, as well as the chemical shifts assigned to the structure in the ¹³C NMR spectra are also included for better visualization of the corresponding peaks at the NMR spectra.

Figure 7

¹³C NMR spectra of the reaction mixture before and after submission of the reaction mixture to 100 °C over 2 h in a 1.2-bar CO₂ atmosphere using either (A,B) DBU or (C,D) DBU(1):BA(1) DES as catalysts. The region of chemical shifts displayed in the spectra just ranged from ca. 20–30 ppm because neither EP, nor BA exhibit peaks at that region.