Kraft Lignin Electro-Oxidation under Ambient Temperature and Pressure

Round 1

Reviewer 1 Report

The manuscript entitled, “Kraft lignin electro-oxidation under ambient temperature and pressure” describes the electrocatalytic pathway for the depolymerization of kraft lignin to the monophenolic compounds. Six different metals in two different sizes were evaluated by authors. Vanillin and vanillic acid were obtained in 50 – 60 % yield. They also noticed that oxygen evolution reaction enhances the efficiency of the product yields. Article in general is well written and described. Abstract and conclusion are well expressed. Results and discussion section can be improved. I have few suggestions/comments and the article can be published after Major Revisions.

There are lot of previously published articles in this research field which has been published by several scientists on electrocatalytic depolymerization of kraft lignin. What is the novelty and uniqueness in their results? How is it different from the previous reports?

Reference 9 should be moved from line 45 to the lines 58 – 60 instead. Moreover, some of the recent reviews on electrolytic depolymerization of lignin are missing and should be cited, e.g., International Journal of Molecular Sciences, 2022, 23, 3767; Green Energy & Environment, 2023, 8, 383; Journal of Bioresources and Bioproducts, 2023, 8, 1.

I suggest authors to give turn over number of their Nickel electrodes vs. Nickel foam?

Authors reported enhanced yields in the presence of oxygen. They also performed blank experiment with only presence of oxygen and no voltage. What happens if they perform electrolytic depolymerization reaction in the continuous supply of oxygen. Also, what will be results in the presence of ozone as a continuous oxygen supply? How much increase in reaction yield can be observed in the presence of continuous supply from oxygen source?

Authors observed only formation of vanillin and vanillic acid as major products. They do observe guaiacol and other products but in too low amounts to quantify. In the experimental description they wrote that they evaporated solvent at 47 ^oC water bath. Phenols from lignin have low boiling points and might be removed during solvent evaporation. I suggest authors to evaporate solvent at 35 – 40 ^oC and record GCMS to confirm if they did not produce Guaiacol or other products in good quantities. Moreover, Guaiacol is also water soluble so it might go in the aqueous layer as well. I suggest authors to determine the total organic carbon (TOC) of the aqueous layers they have separated.

Table 3 should include error bars for each ECSA measurements.

Besides few minor comments;

Typo errors observed a few places;

Page 9, Table 1 should be Table 3.

Page 10, Figure 2 should be Figure 5.

Page 4, line 183, Error! Reference source not found should provide correct reference number.

Page 5, line 201, Error! Reference source not found should provide correct reference number.

Page 8, line 277, Error! Reference source not found should provide correct reference number.

Figure S4 should be removed. It is trivial information.

Figures S6 – S11 Change the outline colour of the markers from blue to brown (filled colours of markers). The colour of markers and outline should be same.

Finally, quality of chemdraw structures is very poor and should be improved. Table 2 have some structure with thin line thicknesses while others with thicker lines. All chemdraw structures should have same line thickness.

Figure 2a have too thin lines in comparison to Figure 2b. All Figures should use same style.

I recommend manuscript to be published after Major Revision.

Minor English Language editing required.

Author Response

Thanks for taking your time to review our manuscript and for valuable constructive feedback.

Author Response File: Author Response.docx

Reviewer 2 Report

This article is devoted to the processes of lignin oxidation. The topic is relevant, since the oxidation of lignin into aromatic substances with high added value has been an actively developing area for many years. An “optimal” method for lignin oxidation has not yet been found, but this work brings its own perspective to this issue. In terms of the level of material, volume and subject matter, this work meets the requirements of the Journal. The article is written qualitatively and at a good level. There are some points for improvement:

1. How efficient is the electro-oxidation of lignin compared to hydrothermal oxidation?

2. It would be good if the yield of aromatic compounds was also presented in mass percentage.

3. There are a lot of related and important publications on this topic. Please cite: 10.3390/catal11040467f, 10.1007/s11244-023-01826-3.

4. Overall, the paper could make more comparisons with the literature.

5. Conclusions can be made more concise.

Author Response

Thanks for taking your time to review our manuscript and for valuable constructive feedback.

Author Response File: Author Response.docx

Reviewer 3 Report

My key point is about the lack of detailed information regarding the chemical composition of Kraft lignin in the study. It's essential for scientific research to acknowledge the complexity of the materials the author are working with, especially when dealing with natural substances like Kraft lignin, which is a complex mixture of various aromatic compounds and polymers.

I suggest the authors to acknowledge the Complexity of Kraft Lignin. They should provide a more comprehensive understanding of the starting material, Kraft lignin, by at least recognising its complexity and diverse chemical composition. This would set realistic expectations for the readers about the limitations of the study.

Then, they should highlight the Importance of Characterization: I suggest that the authors could include characterization data, such as the more types of aromatic compounds present in the starting material – qualitatively and quantitatively, to give readers a better idea of what they are working with. This would help other researchers assess the generalizability of the study's findings to different sources of Kraft lignin.

Then, they should Emphasize the Need for Comprehensive Studies and to Mention that future research should focus on comprehensive studies that consider the diversity of lignin sources and their chemical compositions, which can vary significantly depending on the source (e.g., softwood, hardwood, agricultural residues). This could lead to more robust and widely applicable electrochemical valorization methods.

By addressing these points, the authors can make some improvements for future studies in lignin electrocatalysis.

If not, then they should limit their findings and to remove all the mention about Kraft lignin in the text and refine the research only to somme molecules…

none

Author Response

Thanks for taking your time to review our manuscript and for valuable constructive feedback.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thanks authors for revising the manuscript and taking into consideration my comments. I recommend manuscript to be published after correction of two typo errors.

Page 5 Line 198, as shown in Figure S4 should be removed as there is no Figure S4 in the SI file because it is removed.

Page 7, Line 280, Figure S5 should be Figure S4.

Author Response

We would like to thank the reviewer for recommending this manuscript to be published after minor revision.

For the errors pointed out, we fixed this in the updated manuscript.

Reviewer 3 Report

I appreciate the detailed changes made in the manuscript. However, I would like to suggest that the presentation of the data be more explicit and less bombastic.

Instead of general statements such as "the yield doubled from 1 to 2 ppm," it would be more informative and scientifically rigorous to provide the actual values. For instance, stating that the yield increased from 1 ppm to 2 ppm provides readers with a clear understanding of the quantitative change.

Including specific values (product yields obtained in relation to the raw material, %) will enhance the transparency of your findings and allow for a more precise interpretation of the results. It aligns with the principles of scientific communication and ensures that readers can accurately assess the significance of the observed changes.

I encourage you to consider this suggestion in your revision, and I look forward to seeing the refined presentation of your data.

none

Author Response

We appreciate the suggestions by the reviewer. We understand that the conventional presentation on yield is a mass percentage. However, since our goal was to study the intrinsic performance of the catalysts, preferably in the surface reaction (charge transfer) limit regime, we intentionally used a large reaction volume (40 mL) and a small unit area of catalyst in a relatively short time scale (1500 s). This helped us avoid bulk mass transport effect on the electrocatalysis due to diminishing lignin concentration as the reactant. In other words, the conversion of the starting materials i.e., lignin, was kept insignificant compared to the initial amount. Presenting the yield in a mass percentage therefore would be minimizing the difference between the catalysts and the comparison would be not as clear. The paper aims to emphasis the normalization metrics and provide a more fairly comparison on the performance using the ECSA-normalized yield and the charge efficiency. These key parameters provide more valuable insights than evaluating only the absolute yield by mass. The objective of this study is not to optimize the absolute product generation in a batch mode reactor.

In addition, depolymerization of the lignin also consumes charges and the resulting depolymerized lignin structures that still have a large molecular weight are not included in our desired product analysis. Using the proposed charge efficiency can provide a better understanding on how much target products are produced per Coulomb of electric charge supplied. Closing the gap in the literature where a Faradaic efficiency is lacking.

We hope that this addresses the reviewer’s concern.