Study on the Solubility of Industrial Lignin in Choline Chloride-Based Deep Eutectic Solvents
, Shuangquan Yao 2 and Hailong Yu 1,2,*
Round 1
Reviewer 1 Report
This manuscript used different ChCl-based DES to investigate the solubilization of two types of lignin, but it needs more data to really study solubility patterns. With this title, I expect different types of DES, not only ChCl-based DES, be explored, and more types of lignin too, comparing with molecular modeling. Besides that, all figures has a poor resolution. Authors also describes DES as salts [A]n[B]m, what is not necessarily correct, they are just a mixture.
Author Response
We thank the editor and reviewers for their careful read and thoughtful comments on our manuscript. We have revised the manuscript carefully according to the comments. The point-by-point responses to the comments are listed below.
Reviewer 1:
This manuscript used different ChCl-based DES to investigate the solubilization of two types of lignin, ut it needs more data to really study solubility patterns. With this title, I expect different types of DES, not only ChCl-based DES, be explored, and more types of lignin too, comparing with molecular modeling. Besides that, all figures has a poor resolution. Authors also describes DES as salts [A]n[B]m, what is not necessarily correct, they are just a mixture.
Response: Thanks very much for your kind suggestions.
Alkali lignin and sodium lignosulfonate are two valuable industrial lignin materials. Dissolution in a suitable solvent is required for further use of them. Deep eutectic solvents (DESs) have emerged as a promising class of solvents for lignin dissolution, which could significantly expand the application potential of lignin. However, there is lack of study regarding the solubility of above industrial lignin materials in DES. The ChCl-based DES has gained a lot of attention due to its easy availability; therefore we choose this type of DES to investigate the solubilization of two common industrial lignin materials. Therefore, we mainly investigated the dissolution of above two industrial lignins in ChCl-based DES. In our manuscript, we also expanded the application of ChCl-based DES in the separation of lignins from poplar wood and corn stover based on our results, the results has been indicated in Fig. 9. Of course, it is very significant to explore other types of DES and lignins. Our original title is indeed inappropriate, and we have revised the title as “Study on the solubility of industrial lignin in choline chloride-based deep eutectic solvents”.
We appreciate the reviewer's suggestion and agree with your opinion: comparing with molecular modeling. We have added some references on molecular modeling to further explanation in the revised manuscript (References 28,29 and 38), which are specifically marked. In the next, we will conduct an in-depth study following your kind suggestions.
We have improved the clarity of pictures in the manuscript. Here, we have not listed the changes, but the pictures in the revised documents have been replaced.
The name of DES is indeed not correct. We have revised it as A/ B(n:m) according to the references.
(1) Peeters, N.; Janssens, K.; de Vos, D.; Binnemans, K.; Riaño, S. Choline Chloride–Ethylene Glycol Based Deep-Eutectic Solvents as Lixiviants for Cobalt Recovery from Lithium-Ion Battery Cathode Materials: Are These Solvents Really Green in High-Temperature Processes? Green Chem. 2022, 24 (17), 6685–6695.
(2) Kohli, K.; Katuwal, S.; Biswas, A.; Sharma, B. K. Effective Delignification of Lignocellulosic Biomass by Microwave Assisted Deep Eutectic Solvents. Bioresour. Technol. 2020, 303, 122897.
In addition to the modifications mentioned above, we further refined the language of the article to make it clearer and more understandable. In the revised manuscript, we reviewed the format of the article to ensure that it meets the requirements of the journal. At the same time, we provided more detailed descriptions of some content to help readers better understand our research methods and results.
Reviewer 2 Report
The submitted manuscript (ID: sustainability-2341937) prepared three major types DESs and investigate the relationship between DESs structure and lignin solubilization. Two of each of the three types of DESs with good solubilization effect on lignin were selected to pretreat poplar wood and straw to investigate the solubilization effect on lignin. Meanwhile, the characterization of the DESs are sufficient, and the reaction part is also reasonable. However, there are a number of issues needing to clarify before the manuscript can be considered for possible publication.
1. In my opinion, preparation and characterization of the material is typical and quite complete. It may be appropriate to discuss the recyclability of DESs : is the efficiency of recycling high within the experimental error range ?
2. The high temperature mentioned in the manuscript contributes to the dissolution of lignin by DES. Please explain why the solubility of the temperature [ChCl][EG]3 group is reduced in Figure 2b.
3. In the section on the effects of water on DESs, the author may consider reference (1) 10.1016/j.foodchem.2015.03.123 ; (2) 10.1002/anie.201702486 ; (3) 10.1021/la900552b. In order to better explain the experimental phenomena.
4. Please explain what “Y” in Figure 9a and “J” in Figure 9c represent respectively..
5. There are many format errors in the manuscript, which should be corrected. For example, for the first time, the eutectic solvent needs to be abbreviated (31 lines), the hydrogen bond acceptor needs to be abbreviated (36 lines), and the hydrogen bond donor needs to be abbreviated (48 lines). 128 lines, unified full-text symbol format.For other errors, please review and modify the full text.
Author Response
We thank the editor and reviewers for their careful read and thoughtful comments on our manuscript. We have revised the manuscript carefully according to the comments. The point-by-point responses to the comments are listed below.
Reviewer 2:
- Comment: In my opinion, preparation and characterization of the material is typical and quite complete. It may be appropriate to discuss the recyclability of DESs: is the efficiency of recycling high within the experimental error range ?
Response: Thanks for your kind suggestion. Indeed，the recovery of DES is important. The references have indicated several methods on recovering DES. Thulluri et al. illustrated that rotary evaporation was the simplest and most commonly used to recover DES, and recovery rate was over 74%. In addition, Mamilla et al. recovered and purified DES by extraction in yields of up to 75%. We will further explore the recovery of DES in subsequent studies according to your kind suggestion.
(1) Mamilla, J. L. K.; Novak, U.; Grilc, M.; Likozar, B. Natural Deep Eutectic Solvents (DES) for Fractionation of Waste Lignocellulosic Biomass and Its Cascade Conversion to Value-Added Bio-Based Chemicals. Biomass Bioenergy. 2019, 120, 417–425.
(2) Thulluri, C.; Balasubramaniam, R.; Velankar, H. R. Generation of Highly Amenable Cellulose-Iβ via Selective Delignification of Rice Straw Using a Reusable Cyclic Ether-Assisted Deep Eutectic Solvent System. Sci Rep. 2021, 11 (1), 1591.
- Comment: The high temperature mentioned in the manuscript contributes to the dissolution of lignin by DES. Please explain why the solubility of the temperature [ChCl][EG]3 group is reduced in Figure 2b.
Response: Thanks. An error in the preparation of the graph caused the data to be reversed. We apologise for our oversight and have updated the results in Figure 2b. Specifically, our revised findings showed that high temperatures promote the dissolution of lignin by [ChCl][EG]3 (which was later corrected to ChCl/EG(1:3)).
- Comment: In the section on the effects of water on DESs, the author may consider reference (1) 10.1016/j.foodchem.2015.03.12; (2) 10.1002/anie.201702486; (3) 10.1021/la900552b. In order to better explain the experimental phenomena.
Response: We sincerely thank you for your valuable suggestion. We have carefully reviewed the literature and have added relevant sections to the revised manuscript (references 25, 36, 37).
- Comment: Please explain what “Y” in Figure 9a and “J” in Figure 9c represent respectively.
Response: Thanks. In the Figure 9, "Y" and "J" represent unprecedented poplars and corn straws, respectively. It has been included in the manuscript.
- Comment:There are many format errors in the manuscript, which should be corrected. For example, for the first time, the eutectic solvent needs to be abbreviated (31 lines), the hydrogen bond acceptor needs to be abbreviated (36 lines), and the hydrogen bond donor needs to be abbreviated (48 lines). 128 lines, unified full-text symbol format.For other errors, please review and modify the full text.
Response: We are very sorry for our careless mistakes, thank you for your reminder. These errors have been modified and the full text has been checked. The changes have been marked in the revised manuscript. We hope that the manuscript can be accepted by you.
Reviewer 3 Report
Lignin is a highly attractive renewable material, which can be used to produce valuable aromatic compounds, fuel products and functional materials. However, further applications of lignin are limited by the poor solubility of existing conventional solvents. The deep eutectic solvent (DES) is a new environmentally friendly solvent that is an attractive alternative to many of the conventional solvents used for lignin dissolution. The authors summarized the solubility pattern of lignin in different types of DES, providing favorable guide for the high-value utilization of lignin. In general, the experimental is well designed, and the conclusion is well supported by the findings. Therefore, this reviewer would like to recommend the manuscript for publication after minor revision.
1. Line 134: Irregular use of punctuation. 2. Figure 4. should be made clearer.
3. More comments should be provided on how the length of the carbon chain inhibits lignin from being dissolved. 4. Line 200: Specific representation of lignin dissolution data for six selected DES. 5. 3.4 Structural change of lignin: Inaccurate representation. 6. Relationship between acidic, neutral and basic hydrogen bond donors in line 49 and hydrogen bond donors containing carboxyl groups over those containing hydroxyl, amino/amides later in the text.
Author Response
We thank the editor and reviewers for their careful read and thoughtful comments on our manuscript. We have revised the manuscript carefully according to the comments. The point-by-point responses to the comments are listed below.
Reviewer 3:
- Comment: Line 134: Irregular use of punctuation.
Response: Thank you for your careful inspection and correct it according to your opinions, and thank you for your correction.
- Comment: Figure 4. should be made clearer.
Response: Thank you for your suggestion. The clear Figure 4 has been updated to the corrected document, and I hope to get your recognition.
- Comment: More comments should be provided on how the length of the carbon chain inhibits lignin from being dissolved.
Response: Thank you for your suggestion. The research content is supplemented according to your suggestion. The content is as follows: The reason for the length of the carbon chain to inhibit lignin dissolution may be the following two points: the first may be due to the growth of the carbon chain. The second is that the reduction of the length of the HBD carbon chain will increase the ability to provide protons and promote the solubility of wood quality.
- Comment: Line 200: Specific representation of lignin dissolution data for six selected DES.
Response: Thank you very much for your suggestion. We have added to the content based on your suggestion and hope to receive your approval. The specific additions are “The solubility of [ChCl][FA]3, [ChCl][LA]3, [ChCl][Gly]3, [ChCl][EG]3, [ChCl][Urea]2, [ChCl][TEOA]3 at 120°C for alkaline lignin was 4.54 wt%, 6.34 wt%, 7.55 wt%, 7.60 wt%, 7.49 wt% and 6.14 wt%, and for sodium lignosulfonate dissolution rates of 4.35 wt%, 6.76 wt%, 6.34 wt%, 8.25 wt%, 7.79 wt% and 6.00 wt%, respectively.”
- Comment: 3.4 Structural change of lignin: Inaccurate representation.
Response: Thanks for reminding, this title has been corrected to 3.4 FT-IR characterization of lignin.
- Comment: Relationship between acidic, neutral and basic hydrogen bond donors in line 49 and hydrogen bond donors containing carboxyl groups over those containing hydroxyl, amino/amides later in the text.
Response: Thank you for your careful consideration. DES containing carboxyl groups are acidic, DES containing hydroxyl groups are neutral and DES containing amino/amides are basic and correspond to each other.
Round 2
Reviewer 1 Report
No more comments
