2. Catalytic Application of Deep Eutectic Solvents in Upgrading Biomass

Table 1 lists a collection of processes where DESs are employed as either Lewis acid-type or Brønsted acid-type catalysts to convert biomass to value-added chemicals.

 

 

 

2.1. Processes Catalyzed by Lewis Acid-Type DESs

Lewis acid-based transition-metal chlorides can catalytically activate the electron-rich substrates due to their electron deficiency ^[111][78]. The DESs containing metal chlorides are Lewis acidic. In catalytic upgrading of biomass, there are a few cases with catalytic DESs composed of metal chlorides. However, the most widely used metal chlorides are ZnCl₂ [67], FeCl₃ [68], CrCl₃ [69], MnCl₂ [77] and CuCl₂ [77] to form DESs often with ChCl. This part discusses the application of DESs as Lewis acid catalysts in biomass conversion.

Long et al. [67] studied the transesterification of soybean oil to biodiesel using (ChCl:xZnCl₂) DES as the catalyst, where methanol was used as a solvent. They found that the strength of the Lewis acidity of the DES increased upon increase of x. They suggested that Zn₂Cl₅⁻ mainly catalyzed the transesterification, as Zn₂Cl₅⁻ was the predominant species in the mixture. However, the yield of biodiesel was not satisfying because of the weak acidity of Zn₂Cl₅⁻. With the addition of 10% (ChCl:2ZnCl₂) catalyst and a 16:1 molar ratio of methanol to soybean oil, the conversion rate reached 54.52% after 72 h at 70 °C. Finally, they proposed the transesterification mechanism as shown in Figure 2.

Figure 2. The proposed transesterification mechanism. Reprinted from reference [67] with permission.

Mondal et al. [68] produced Fe₃O₄/Fe-doped graphene nanosheets (GNs) by pyrolysis of granules of seaweed biomass and used (ChCl:FeCl₃) DES as catalyst. The reaction was performed at 700–900 °C under an atmosphere of 95% N₂ and 5% H₂. The resulted Fe₃O₄/Fe-doped GN had favorable properties such as high surface area (220 m²·g⁻¹) and high electrical conductivity (2384.6 mS·m⁻¹). It was found that the nanosheets had highly stable oxygen reduction reaction (ORR) electrocatalytic activity after 30,000 cycles. This would make the produced graphene sheets a sustainable substitution for the existing catalysts composed of expensive metal-based ORRs. Figure 3 displays the proposed flowchart for production of Fe₃O₄/Fe-GN from Sargassum tenerrimum biomass.

Figure 3. The flowchart for production of Fe₃O₄/Fe-GN from Sargassum tenerrimum biomass.

The Lewis acid-type DESs have also been employed as catalysts for chemical modifications of cellulose to value-added chemicals. For example, Sirvio et al. [79] found that a reactive DES composed of dimethyl-urea and ZnCl₂ (10:3) could be used to synthesize cellulose methyl carbamate (CMeC) at elevated temperature (150 °C). The CMeC had a degree of substitution (DS) of 0.17 after 3 h at 150 °C. The original cellulose fibers had a very poor alkaline solubility, whereas the product with a DS of 0.17 exhibited a good alkaline solubility. In this process, dimethyl-urea degraded to methylamine and methylisocyanate to react with the hydroxyl groups of cellulose. The heat-driven reaction was significantly catalyzed by ZnCl₂. Figure 4 illustrates the proposed mechanism for CMeC synthesis.

Figure 4. The proposed reaction mechanism for CMeC synthesis. Reprinted from reference [79] with permission.

Liu et al. [66] prepared a series of FeCl₃·6H₂O-based catalytic DESs with ethylene glycol, glycerol, malonic acid, L-alanine, L-serine, xylitol, pentaerythritol, and glycine as HBDs for transformation of cellulose to gluconic acid where the ferric chloride behaved as the catalyst. Most of the DESs had low viscosities, low melting points, and high conductivities. The high conductivities of the prepared DESs compared to other DESs and ILs are important for their electrochemical applications. Among the DESs, the (FeCl₃·6H₂O:ethylene glycol) DES showed the best performance, with 100% cellulose conversion and 53% gluconic acid yield in 1 h at 120 °C. The interesting point was that gluconic acid could self-precipitate, and the process of product separation could be skipped. Therefore, the employed method had the advantage of integrating solvent and catalyst as well as reaction and separation. Figure 5 illustrates the experimental procedure to obtain gluconic acid from glucose or cellulose.

Figure 5. The experimental procedure to obtain gluconic acid from glucose or cellulose.

Recently, Yang et al. ^[81][82] fabricated cellulose nanocrystals (CNCs) from bleached eucalyptus kraft pulp (BEKP) using a FeCl₃-catalyzed DES with a composition of (ChCl:oxalic acid·2H₂O:FeCl₃·6H₂O) in a 1:4.43:0.1 molar ratio. This composition had the best swelling ability and strongest hydrolysis activity. Based on the cellulose content in BEKP, more than 90% CNC yield was obtained by a one-step DES treatment where the reaction took 6 h at 80 °C. Under these conditions, glucose and xylose were also obtained. They found that the resultant CNCs had higher thermal stability than the traditional H₂SO₄-hydrolyzed ones.

3. Recyclability of Catalytic DESs

In the chemical industry, recycling technology is an important stage towards sustainability ^[85][86]. Recyclability usually involves separation, purification and reusing a solvent or catalyst in successive steps, namely recovering step, refining step and recycling (reusing) step [47]. The whole process greatly affects the solvent or catalyst’s utilization in industrial processes from economic and environmental aspects. In general, the more recyclable a solvent or catalyst, the better it is. In most cases, the chemical efficiency is directly correlated with the recyclability. The recyclability of the applied catalytic DESs have been evaluated in several studies. For example, Song et al. [77] performed a reusability experiment of ChCl:2urea DES in methanolysis of polycarbonate under the optimum reaction conditions. They found that the DES could be reused five times with no significant reduction in polycarbonate conversion, indicating the good reusability of the DES. In the process of conversion of cellulose to gluconic acid, Liu et al. [66] reported no loss of the catalytic activity of 2FeCl₃·6H₂O:ethylene glycol DES, which was due to the self-precipitation of gluconic acid from the reaction system. Liu et al. ^[80][81] used the catalytic DES (1,3-dimethylurea:ZnCl₂) in 4:1 molar ratio to convert polyethylene terephthalate (PET) to bis(hydroxyalkyl) terephthalate (BHET). As is shown in Figure 6, the catalyst worked efficiently after being recycled five times.

Figure 6. Recycling efficiency of the employed catalyst. Reprinted from reference ^[80][81] with permission.

The recyclability of (ChCl:oxalic acid·2H₂O) DES, used as a catalyst in the conversion of inulin to 5-HMF, was investigated. The results indicated that activity of the recycled DES was very high even after utilization six times. The same DES was investigated for its reusability as a catalyst in the epoxidation of soybean oil. It was found that ChCl:oxalic acid·2H₂O DES had good catalytic activity, with 90% selectivity after five recycle runs under optimum conditions. Hayyan et al. studied the recyclability of (ATPPB:3p-TSA) DES employed as a catalyst in biodiesel production from LGCPO. They found that the DES had a high conversion yield in the first three recycle runs, while the efficiency reduced in the fourth recycle run. They suggested that the main advantage of using the DES was its reusability andthat using p-TSA alone as a catalyst would not facilitate acid recycling, because of the loss of the acid in the product. The same research group performed the recyclability experiment of the employed catalytic DES, ChCl:3p-TSA, in the process of FFA conversion to FAME. The results revealed that first and second runs of recycling had high efficiencies, while for the third run, a slight reduction in the conversion yield was observed. Therefore, a lengthened reaction time was suggested to have the same yield as the first two runs. However, the successful reusability of the DES was dependent on the efficiency of the catalyst separation by centrifugation. Alhassan et al. examined the recycling of ChCl:4p-TSA catalytic DESs (So-DES and Un-DES). A high-speed centrifuge was used to regenerate the catalysts. Then, the biodiesel production was performed, employing the recycled DESs under optimum conditions. For Un-DES, marginal reduction in the catalytic activity was observed for the first four runs. For So-DES, after eight successive experiments, catalytic activity reduced after, which the catalyst became deactivated. Deactivation was proposed to be due to the decrease in the density of the acidic sites.

The catalytic (ATPPB:3p-TSA) DES was used to reduce the oleic acid in low-grade oil via esterification process. The results showed that for five repeated recycle runs, the catalytic efficiency of the DES did not change significantly, implying that the DES catalyst regained its high catalytic activity even after several usages. Compared to the study by Hayyan et al. where the same DES catalyst was employed, the DES catalyst in this study worked more efficiently (five recycle runs compared to three recycle runs). However, it should be noted that in the present study, more catalyst was used (5.0 wt%) than 1.0 wt% used by Hayyan et al. Yu et al. used (taurine:3TfOH) DES dissolved in PEG-200 to alkylate isobutane and isobutene for production of gasoline. After the reaction was completed, the alkylate oil was simply separated from taurine:3TfOH + PEG-200 catalytic system via decantation. They found that after eight times of using the catalyst, the isobutane and isobutene conversion rates and octane selectivity did not drop significantly. Additionally, compared to pure TfOH, taurine:3TfOH or TfOH/PEG-200, the taurine:3TfOH + PEG-200 catalytic system worked remarkably better in terms of reusability and octane selectivity.

In order to investigate the reusability of (TBAB:2p-TSA) catalyst in a process of biodiesel production from yellow horn seed biomass, Shen et al. examined the FAMEs conversion yield and oil extraction yield in several runs. They found that the oil extraction yield was nearly unchanged and the conversion yield of FAME reached 80% after four recycle runs. They did not recommend more than four runs of recycling because glycerol, as the by-product of the process, hindered the transesterification reaction. Zhang et al. tested the reusability of the (ChCl:2ethylene glycol) + CrCl₃ catalytic system employed for 5-HMF production. After the decomposition of glucose, ethyl acetate was employed as an extraction solvent to separate 5-HMF, levulinic acid and formic acid from the reaction mixture. Because of the very low solubility of (ChCl:2ethylene glycol) and CrCl₃ in ethyl acetate, 5-HMF was readily separated from the mixture. The remaining liquid was heated to remove the residual solvent from the catalytic system. The catalytic system could be used up to four times, with 38% yield of 5-HMF. Therefore, the catalytic system could be successfully recycled for decomposition of glucose to 5-HMF for four times.

4. Limitations of DESs

DESs are superior over other common organic solvents, due to their features mentioned above, for sustainability. However, despite the many remarkable properties of DESs, acting either as catalysts and/or reaction media in biomass transformations, there are a number of limitations restricting their use. For instance, the biomass conversions are usually carried out at high temperatures. Some DESs may decompose or undergo side reactions under those conditions. For example, there are studies reporting the decomposition of the DES constituents and formation of unwanted chemicals at high temperatures. Moreover, it has been reported that some types of DESs composed of ChCl and carboxylic acids underwent esterification reactions as a consequence of interactions between hydroxyl groups of the salt and the acids. For similar reasons, the suitability of DESs with high melting points, such as type I DESs, which are composed of an organic salt and a non-hydrated metal salt, for industrial applications is highly limited.

The other concern about the applicability of DESs in biomass conversions is their recyclability. As given in Table 1, the recyclability of DESs of both types of Brønsted acids and Lewis acids is hardly greater than 6 times, after which their efficiencies drop significantly. Difficulties in purification of DESs after several recycling processes has created serious constraints towards scaling up. There are therefore only a few studies reporting scaling-up processes containing DESs. The usually high viscosities of DESs along with high melting points of some of them would result in reduced mass transfer during the pretreatment process, hampering their applications and recyclability. However, as a well-practiced approach, water addition would circumvent the high viscosities of DESs. This practice may have also a positive influence on the efficiency of the process.

Finally, it has been argued that the impurities and minerals present in feedstock can change the physicochemical properties of the DESs negatively and impact their catalytic properties.