The DESs are consistent with HBA and HBD, and the number of possible systems is huge. Here, we choose ChCl as the HBA and various compounds as the HBDs, such as urea, ZnCl2, OA, AA, propionic acid (PA), butyric acid (BA), and malonic acid (MaA) as the HBD, and then as the solvent for furfural oxidation with NaClO3 and V2O5 system, and the products were analyzed utilizing HPLC. Figure 2 shows the typical HPLC curves of the products when various DES were employed for the reaction. The products are relatively simple, and MA and FA are the main products, indicating the DESs system can be used as the solvent for furfural oxidation.
Figure 2. High performance liquid chromatography (HPLC) curves of products obtained from furfural oxidation with NaClO3 and V2O5 system in various deep eutectic solvent (DES): a) Urea/choline chloride (ChCl), b) Butyric acid/ChCl (BA/ChCl), c) Propionic acid/ChCl (PA/ChCl); d) AA/ChCl.
Table 1 lists the results of the reactions with various DESs. It is found that for urea/ChCl DES, nearly no products formed while for ZnCl2/ChCl DES only little amount of the MA and FA appeared, indicating that weak alkalinity and Lewis acid-based DES are not suitable for the oxidation of furfural with NaClO3 and V2O5 system. Based on the mechanism of chlorate oxidation, acidity may promote the reaction. So a series of organic acids were used to prepare the DESs for the reaction. The oxidation of furfural took place in all the organic acids system to form MA and FA, and the yield of the MA and FA for diacid is lower than that of single acid, and the best is AA (50.1%) while butyric acid is also a potential choice (21.2%). In this study, AA/ChCl DES has been chosen as the green solvent for the oxidation of furfural.
Solvent Yield of MA (%) Yield of FA (%) Yield of MA and FA (%) Urea/ChCl DES Trace Trace Trace ZnCl2/ChCl DES 0.4 0.6 1.0 Oxalic acid/ChCl DES 9.5 10.0 19.5 Acetic acid/ChCl DES 20.1 30 50.1 Propionic acid/ChCl DES 7.8 11.2 19.0 Butyric acid/ChCl DES 5.7 15.5 21.2 Malonic acid/ChCl DES 12.2 6.9 19.1 Water 8.2 32.5 40.7 Notes: reaction conditions are 2 mmol furfural, 5 mL DES (HBD꞉HAD = 1꞉2), 2.6 mmol of NaClO3 with 0.07 mmol of V2O5, 50 ℃ and 24 h. MA, maleic acid; FA, fumaric acid.
Table 1. MA and FA yield from furfural by oxidation in various DES
The mechanism of furfural oxidation with NaClO3 and V2O5 system had been suggested (Fig. 3) (Milas, 1928; Tachibana et al., 2015), and the amount of oxidant used is a key parameter. As shown in Fig. 4, the yield of MA and FA has a direct relationship to the amount of NaClO3. At the reaction temperature is 70 ℃, the yield of the MA decreased from 31% to 15% with the increase of the molar ratio of NaClO3 to furfural. However, the yield of the FA increased from 52.5% to 57.5% with more NaClO3 added, indicating a transmutation of the MA to FA under such condition. The total yield of MA and FA increased gradually.
Figure 4. Effect of molar ratio of NaClO3 to furfural on yield of MA and FA in AA/ChCl DES solution. Reaction condition: furfural (2 mmol), 5 mL DES, 70 ℃, 12 h.
When the reaction temperature was increased to 80 ℃, it may promote the reaction. As shown in Fig. 5, the yield of the FA increased quickly with the increase of the oxidant and catalyst, and the total yield of the MA and FA also increased up to 70% when 2.3 fold of NaClO3 was used. However, the yield of the MA decreased with the increase of the oxidant, which may be due to the temperature increase since more NaClO3 was added. Interestingly, with the increase of the formation of the FA, crystalline can be found in the system, which results from the low solubility of the FA in AA, even in the presence of the ChCl.
Figure 5. Effect of molar ratio of NaClO3 to furfural on yield of MA and FA in AA/ChCl DES solution. Reaction condition: furfural (2 mmol), 5 mL DES, 80 ℃, 12 h.
Reaction time is another key point for the reaction, and the reaction was carried out at a temperature from 6 h to 24 h with other conditions kept the same. As shown in Fig. 6, at 6 h, the yield of the MA and FA are 20.1% and 30.0%, respectively. The yield of the FA increased at the beginning and then decreased with longed reaction time, and the maximum yield of the FA is 49.2% at 12 h while the total yield of the MA and FA is 66.1%. The reason may be the furthermore oxidation of the FA to carbon dioxide at a long reaction time with bubble generation. However, for the MA, the yield decreased with the prolonged reaction time, indicating much MA should take place transmutation or degraded to carbon dioxide.
Figure 6. Effect of reaction time on yield of MA and FA in AA/ChCl DES with NaClO3 and V2O5 system. Reaction condition: furfural (2 mmol), NaClO3 (3.4 mmol), 5 mL DES, 80 ℃.
The effect of reaction temperature on the oxidation of furfural was also studied, and the results were shown in Fig. 7. Increasing reaction temperature promotes the formation of the FA while the yield of the MA decreases. At high temperature, the transmutation of the MA to FA occurs easily (Tachibana et al., 2015). Interestingly, at 100 ℃, the yield of the FA reaches 66.7% while the yield of MA is only 4.2%, indicating that the FA can be selectively synthesized in the system by controlling the reaction temperature. For the reaction, there are two distinct stages, a violent one followed by a mild one, while in the first one furfural is oxidated and the second one for the transmutation of the MA to FA. So, it is easy to increase the reaction temperature for the DES solvent compared with the aqueous system, and the reaction can be completed in a short time at high temperature.