Deep Eutectic Solvents for the Valorisation of Lignocellulosic Biomasses towards Fine Chemicals
Akihiko Sakurai
Round 1
Reviewer 1 Report
The submitted manuscript have comprehensively reviewed the deep eutectic solvents for the valorization of lignocellulosic biomasses towards fine-chemicals. The manuscript is well-organized and the writing is good. I appreciate the current review regarding to the physiochemical properties of DES and their applications. I think the review is worthy of publication. To better improve the guiding significance of this manuscript, I think the following issues should be considered. 1. The author lists many types of DES and gives many examples of DES dissolution. However, for readers, they would like to know more about which DES is more efficient for lignin, hemicellulose and cellulose. Therefore, it is recommended that the author give some comments and suggestions in the corresponding places. 2. The utilization of hemicellulose in DES pretreatment is generally not well solved. What is the author's comment on this? Please include a discussion in the appropriate space and give some suggestions. 3. As for the industrial application of DES in the utilization of wood fiber, the author thinks how to better realize its industrial application? This will guide researchers to better design the DES system.
Author Response
Reviewer #1
The submitted manuscript have comprehensively reviewed the deep eutectic solvents for the valorization of lignocellulosic biomasses towards fine-chemicals. The manuscript is well-organized and the writing is good. I appreciate the current review regarding to the physiochemical properties of DES and their applications. I think the review is worthy of publication.
Answer:
To better improve the guiding significance of this manuscript, I think the following issues should be considered.
- The author lists many types of DES and gives many examples of DES dissolution. However, for readers, they would like to know more about which DES is more efficient for lignin, hemicellulose and cellulose. Therefore, it is recommended that the author give some comments and suggestions in the corresponding places.
Answer: Thank you for your suggestion. In this revised version of the manuscript, the following comment was added soon after Table 2 to address this lack.
“As stated above, the pretreatment of lignocellulosic biomass with DES is usually finalised to solubilize lignin and hemicellulose, leaving the cellulose intact. The comparison of pretreatment operated by using different DES is complicated due to the large number and complexity of the biomasses studied, which can interact in a different way to the same type of DES. However, three experimental parameters, such as delignification yields, hemicellulose solubilization and cellulose recovery, can be considered to evaluate and compare the efficacy of a DES treatment. In general, the best performances are recorded by using acid DES, for example those synthesized starting from choline chloride and lactic acid, which reach near 90 % delignification yield and complete solubilisation of the hemicellulose [65,66,68]. Neutral or basic DES do not achieve comparable performance. Amine- and amide-based DESs are less investigated as compared to poly-alcohol- and acid-based DES. As reported in the literature, lignin is soluble in an alkaline medium, and this can be exploited to remove lignin from raw biomass, breaking the ether linkages in lignin and ester linkages between lignin and hemicellulose, and thus favouring the consequent enzymatic degradation of cellulose. Lignin and xylan removal by ChCl:Urea from different biomass sources (rice straw and oil palm empty fruit bunch) were respectively around 30 % and 20 % [46,80]. Zhao et al. evaluated the wheat straw fractionation efficiency of various amine- (monoethanolamine, diethanolamine and methyldiethanolamine) and amide based (acetamide and urea) DES: compared to DES based on urea and acetamide, the DES based on amines show better performances for the biomass fractionation. This behaviour can be linked to the high basicity nature of the HBD component. The amide-based DES (pH 7.3-8.2) show worst performances than amine-based DES (pH 10.4-10.9). Ethanolamine has more potential in biomass fractionation over amide groups due to exhibiting high pH near to 10. Lignin and xylan removal of 81 % and 47 % were reported, respectively. onwheat straw pretreated with monoethanolamine-based DES [48]. The high delignification yields recorded for basic DES are attributable to forming strong hydrogen bonds with the phenolic groups present in the lignin. The cellulose and xylan conversions are only 20.9 % and 8.9 % respectively for untreated wheat straw and enormously improved after monoethanolamine-DES pretreatment, 92.4% and 75.8% respectively.”
- The utilization of hemicellulose in DES pretreatment is generally not well solved. What is the author's comment on this? Please include a discussion in the appropriate space and give some suggestions.
Answer: Thank you again
This biomass fractionation approach with DES is mainly focused on removing lignin from cellulose. However, hemicellulose plays a fundamental role in the purpose of valorizing the initial biomass. Being a polymer easily hydrolysable, made up of C5 (mainly) and C6 sugars, it can be chemically exploited by separating it from the other biomass components as hydrolysed sugars. The quantitative solubilization of the hemicellulosic fraction will allow xylans to be recovered together with lignin, for a final conversion into other compounds with a high value (e.g. furfural [59]).
- As for the industrial application of DES in the utilization of wood fiber, the author thinks how to better realize its industrial application? This will guide researchers to better design the DES system.
Answer: Thank you for this question and suggestion. In the final section, besides some points already reported, the following final remark was added:
“Especially the dissolution of cellulose, as a sort of emulation of some ionic liquid, could help in developing new routes of valorization. Intensification of processes may represent the new frontiers for this kind of processes, if the pretreatment step can be combined with the conversion of polymeric subunits into the different target molecules in a single step, a greener valorization process could be achieved.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Reviewer’s report
Title: Deep Eutectic Solvents for the Valorization of Lignocellulosic 2 Biomasses Towards Fine-Chemicals .
Comments and Suggestions for Authors
Dear Authors,
In my opinion, the article requires a significant reconstruction. I admit that when I read the article, I did not know what the purpose of the authors was. Please specify clearly what the purpose of conducting the literature research is. A clear answer to the question of what is the purpose of delignification is particularly important. In my opinion, the authors should indicate in the introduction section what is the problem with the separation of compounds in ligniocellulosic materials?
After all, separating lignin from biomass has been known and carried out in paper mills for a long time. It should be clearly emphasized what will be a significant novelty in the case of applying deep eutetic solvents. Please highlight the novelty of research presented in the manuscript.
Detail comments:
Line 135-137: What is the purpose of pretreatment of lignocellulosic materials? Is the main task of this treatment to separate cellulose from lignin? This should be clearly stated.
Line 78-85: The authors have indicated different types of lignin depending on their origin as obtained from hardwood, softwood, and monocots. To which group can lignin obtained from dicotyledonous herbaceous plants be included?
Line 156: Please use a one type of temperature unit (° C or Kelvin). The note applies to the entire manuscript.
Line 224-226: What are the disadvantages of DES compared to ILs?
Chapter 3: I suggest organizing the information by introducing a table showing the effectiveness of biomass fractionation. The authors described the effectiveness of biomass fractionation using ethanolamine, and how effective are DES compounds with amide groups in biomass fractionation?
Line 561: There is a reference to table 1 on this manuscript line, shouldn't there be a reference to table 3?
Table 4: The title of the manuscript indicates the use of DES for the processing of lignocellulosic materials, why did the authors include glucose and fructose as feedstock in Table 4? Are they listed as hemicellulose base ingredients? If so, it should be explained in the description to the table.
Line 646-647: "In conventional processes, cellulose carbamates with a nitrogen content (N%) of 1–2.5% are prepared by using organic solvent and harsh operative conditions (> 408 K)." - please standardize the units.
So my decision is: Major revision, which you should made according the Reviewer's indications.
Best regards
Reviewer
Author Response
Reviewer #2
Dear Authors,
- In my opinion, the article requires a significant reconstruction. I admit that when I read the article, I did not know what the purpose of the authors was. Please specify clearly what the purpose of conducting the literature research is. A clear answer to the question of what is the purpose of delignification is particularly important. In my opinion, the authors should indicate in the introduction section what is the problem with the separation of compounds in ligniocellulosic materials?
Answer: Thank you very much for this comment. The introduction section has been implemented in this revised version of the manuscript by focusing the main purposes of the delignification. The main objective of this review is to present a detailed description of how a chemical valorization of lignocellulosic biomass through the obtainment of simple sugars (namely glucose) and aromatics could be achieved. After a preliminary description of the biomasses, their constituents and complexity, a specific description of the use of DES was introduced for demonstrating that they are able to promote the valorization of the lignocellulosic materials.
- After all, separating lignin from biomass has been known and carried out in paper mills for a long time. It should be clearly emphasized what will be a significant novelty in the case of applying deep eutetic solvents. Please highlight the novelty of research presented in the manuscript.
Answer: Thank you for the pertinent comment. The conventional treatment of delignification already operated in paper mills cannot be used for the purpose of this review, since the conditions are too drastic and the formation of inhibitors to the enzymatic hydrolysis cannot be avoided. In addition, the conventional pulping of lignocellulose would generate a huge amount of wastewater to remove residual reactants and partially degraded sugars (levulinic acid, HMF, etc.). The revised text was also implemented by focusing this point.
- Line 135-137: What is the purpose of pretreatment of lignocellulosic materials? Is the main task of this treatment to separate cellulose from lignin? This should be clearly stated.
Answer: Thank you for the clear suggestion. In the present version of the manuscript, a wide description of the final aim of the work was developed along with the introduction. The final objective is to achieve from biomasses possible renewable platform molecules capable of feeding a modern biorefinery for the obtainment of liquid fuels, bio-material and fine-chemicals.
- Line 78-85: The authors have indicated different types of lignin depending on their origin as obtained from hardwood, softwood, and monocots. To which group can lignin obtained from dicotyledonous herbaceous plants be included?
Answer: Softwood lignin is mainly composed of G units (>95%), while hardwood lignin is made up of both G and S units. According to our knowledge dicotyledonous angiosperms are classified as hardwoods having a dominance of G-S groups.
- Line 156: Please use a one type of temperature unit (° C or Kelvin). The note applies to the entire manuscript.
Answer: Thank you very much for your comment. Temperatures along with the manuscript have been coherently reported as x° C.
- Line 224-226: What are the disadvantages of DES compared to ILs?
Answer: Thank you for this pertinent question. Criticisms related to DES were implemented in the final section. The following text was also added:
“DES systems appear to be the greenest solution to replace ionic liquids in various applications of industrial interest, however they have significant disadvantages compared to ILs. Being constituted by hydrogen interactions, much weaker than the ionic bonds present in ILs, the presence of impurities in the components, or of water and/or gases commonly present in the air may negatively affect the stability and the properties of DES even at moderate temperatures.”
- Chapter 3: I suggest organizing the information by introducing a table showing the effectiveness of biomass fractionation. The authors described the effectiveness of biomass fractionation using ethanolamine, and how effective are DES compounds with amide groups in biomass fractionation?
Answer: Thank you very much for this comment. In the present version a wide comment on Table 2 was reported with the aim of introducing the complexity of the topic and a qualitative comparison among the different DES activity. As for effectiveness of biomass fractionation using ethanolamine the following text was also reported:
“Neutral or basic DES do not achieve comparable performance. Amine- and amide-based DESs are less investigated as compared to poly-alcohol- and acid-based DES. As reported in the literature, lignin is soluble in an alkaline medium, and this can be exploited to remove lignin from raw biomass, breaking the ether linkages in lignin and ester linkages between lignin and hemicellulose, and thus favouring the consequent enzymatic degradation of cellulose. Lignin and xylan removal by ChCl:Urea from different biomass sources (rice straw and oil palm empty fruit bunch) were respectively around 30 % and 20 % [46,80]. Zhao et al. evaluated the wheat straw fractionation efficiency of various amine- (monoethanolamine, diethanolamine and methyldiethanolamine) and amide based (acetamide and urea) DES: compared to DES based on urea and acetamide, the DES based on amines show better performances for the biomass fractionation. This behaviour can be linked to the high basicity nature of the HBD component. The amide-based DES (pH 7.3-8.2) show worst performances than amine-based DES (pH 10.4-10.9). Ethanolamine has more potential in biomass fractionation over amide groups due to exhibiting high pH near to 10. Lignin and xylan removal of 81 % and 47 % were reported, respectively. onwheat straw pretreated with monoethanolamine-based DES [48]. The high delignification yields recorded for basic DES are attributable to forming strong hydrogen bonds with the phenolic groups present in the lignin. The cellulose and xylan conversions are only 20.9 % and 8.9 % respectively for untreated wheat straw and enormously improved after monoethanolamine-DES pretreatment, 92.4% and 75.8% respectively.”
- Line 561: There is a reference to table 1 on this manuscript line, shouldn't there be a reference to table 3?
Answer: Thank you very much for your help. You are right, in that line it was referred to Table 3.
- Table 4: The title of the manuscript indicates the use of DES for the processing of lignocellulosic materials, why did the authors include glucose and fructose as feedstock in Table 4? Are they listed as hemicellulose base ingredients? If so, it should be explained in the description to the table.
Answer: Thank you for your comment. In the first part of section 5.4, the following sentence that introduce the topic is reported:
“Once that lignocellulosic biomass was pretreated, cellulose can be reacted to produce glucose/fructose as conceptually shown in Figure 7. DES offer a valid alternative to complete the valorization of biomasses…”
As for Table 4, the following title was reported:
“Table 4. DESs as catalyst/solvent for the synthesis of platform molecule: glucose and fructose were also included since they can be potentially obtained from biomasses.”
- Line 646-647: "In conventional processes, cellulose carbamates with a nitrogen content (N%) of 1–2.5% are prepared by using organic solvent and harsh operative conditions (> 408 K)." - please standardize the units.
Answer: Thank you for your suggestion. The units were standardized as required.
So my decision is: Major revision, which you should made according the Reviewer's indications.
Best regards
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper summarized the application of deep eutectic solvents to lignocellulosic biomass processing. I believe it deserves to be published in Biomass.
One typographical error in line 59 on page 20 should be corrected; "Rent. Et al." must be "Ren et al"
Author Response
Reviewer #3
This paper summarized the application of deep eutectic solvents to lignocellulosic biomass processing. I believe it deserves to be published in Biomass.
One typographical error in line 59 on page 20 should be corrected; "Rent. Et al." must be "Ren et al"
Answer: Thank you very much for your comment. The typographical error was corrected.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Dear Authors, thank you for introducing corrections in the manuscript in accordance with my comments and for clarifying the ambiguities in the content of the reviewed article. I believe that it can be published in this form.
Best regards
Reviewer
