Decomposition Mechanisms of Lignin-Related Aromatic Monomers in Solution Plasma

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study examines the decomposition of lignin model compounds (phenol, guaiacol, syringol) through solution plasma treatment. The authors analyzed the liquid and gas phase products, proposing potential decomposition mechanisms that highlight the role of OH radicals and the formation of dicarboxylic acids. This work is informative and relevant to the fields of plasma science and sustainable chemistry. I recommend minor revisions based on the following comments.

1) In section 2.2, the description of the plasma power system lacks detail. To enhance clarity, I suggest including representative voltage and current waveforms in the Appendix, particularly given the authors' use of probes.

2) In section 2.2, the authors report an increase in discharge power from 16 W to 28 W as the electrode gap increases from 0.5 mm to 1.5 mm. Two points require clarification: First, please provide details on the method used to precisely control the electrode gap within such a narrow range. Second, it is counterintuitive that a larger gap would result in higher discharge power, as plasma breakdown typically occurs more readily at shorter gaps. Please elaborate on the factors contributing to this observation.

3) In figures 3 and 4, the markers in the plot are not sufficiently clear. Please improve the visibility of the markers for better data interpretation.

4) In section 3, in addition to liquid and gas products, did the authors observe solid product formation on the electrodes? Solution plasma is known to cause electrode degradation and contamination by organic and inorganic compounds, as demonstrated in the study (10.1002/ppap.201900159). Did the author any observe changes in voltage and current over time, as these would be useful to note.

5) In section 3.3, beyond plasma-generated gases, electrolysis of water can contribute to hydrogen and oxygen production, as shown in the studies cited (10.1088/0022-3727/45/44/442001). Please discuss the potential contribution of electrolysis to the observed gas products.

6) In section 3.4 figure 14, the product yields, including unreacted feedstocks, are consistently below 60%. Please provide an explanation for the observed carbon imbalance.

7) In section 3.4, the authors compare product distributions from the decomposition of various model compounds. Did they also investigate the influence of discharge power (i.e., electrode gap) on product distribution? If so, please include a discussion of any observed differences.

Author Response

Reply to reviewer 1:

Thank you very much for your kind and thoughtful comments. We have revised the manuscript according to your comments and provided our response below.

 

1. In section 2.2, the description of the plasma power system lacks detail. To enhance clarity, I suggest including representative voltage and current waveforms in the Appendix, particularly given the authors' use of probes.

 

We have added the voltage and current waveforms as Figure A1 in Appendix A, along with relevant descriptions in the main text (Lines 129–130 and 460–468). The probes used for these measurements are described in Lines 126–128 of the main text.

 

2. In section 2.2, the authors report an increase in discharge power from 16 W to 28 W as the electrode gap increases from 0.5 mm to 1.5 mm. Two points require clarification: First, please provide details on the method used to precisely control the electrode gap within such a narrow range. Second, it is counterintuitive that a larger gap would result in higher discharge power, as plasma breakdown typically occurs more readily at shorter gaps. Please elaborate on the factors contributing to this observation.

 

We precisely adjusted the electrode gap through iterative manual fine tuning while observing magnified camera images using the electrode diameter (1.0 mm) as a reference scale. We have added this explanation to the main text (Lines 107–109). In practice, we developed a Windows-based software that enables camera capture, image magnification, and measurement of the gap length. Using this software, we manually adjusted the electrode gap.

The power supply used in this study employs a leakage magnetic transformer, which exhibits constant-current characteristics, resulting in nearly identical current waveforms regardless of electrode gap settings. In contrast, the voltage increases with the electrode gap. This is because a larger gap results in higher impedance between the gap, requiring a higher voltage to maintain the same current. Consequently, the discharge power increased with the electrode gap. We have added this explanation in Appendix A (Lines 460–468).

 

3. In figures 3 and 4, the markers in the plot are not sufficiently clear. Please improve the visibility of the markers for better data interpretation.

 

To improve clarity, we have modified the marker shape and size, and enlarged the overall Figures 3 and 4. However, in Figure 3, the marker size could not be sufficiently increased due to the visibility of the error bars. Therefore, legends have been added to all panels (a), (b), and (c).

 

4. In section 3, in addition to liquid and gas products, did the authors observe solid product formation on the electrodes? Solution plasma is known to cause electrode degradation and contamination by organic and inorganic compounds, as demonstrated in the study (10.1002/ppap.201900159). Did the author any observe changes in voltage and current over time, as these would be useful to note.

 

No solid deposition was observed on the electrode surfaces under the conditions of this study, as explained in Lines 379–381. We reviewed the article you cited; however, since the experimental conditions differ in many respects, we do not yet fully understand what factors contribute to solid product formation in that case.

On the other hand, electrode degradation was observed. For example, when the initial electrode gap was set to 1.0 mm, it increased to approximately 1.05 mm after 60 minutes of plasma treatment. However, this slight increase in gap distance did not contribute to the observed increase in discharge power. As shown in Figure A2 of Appendix A, the discharge power increased slightly over the course of plasma treatment, which is attributed to the rise in water temperature. For instance, when the water was forcibly heated, the discharge voltage also increased, resulting in higher power. This is presumably because the elevated temperature promoted the formation of water vapor bubbles in the gap, leading to an increase in gap impedance. We have added this explanation to Appendix A (Lines 474–476).

 

5. In section 3.3, beyond plasma-generated gases, electrolysis of water can contribute to hydrogen and oxygen production, as shown in the studies cited (10.1088/0022-3727/45/44/442001). Please discuss the potential contribution of electrolysis to the observed gas products.

 

As you correctly pointed out, H₂ was also produced in the blank test without aromatic model compounds (approximately 45 mL). In the original manuscript, we subtracted this amount of H₂ when preparing Figure 11 and explained it. However, to place greater emphasis on the presence of water-derived H₂, we have revised Figure 11 to show the total amount of gas collected in the gas bag without subtraction, and now explain that approximately 45 mL of the hydrogen originates from water (Lines 330–333).

It should be noted that almost no H₂ was produced when an electric current was applied without discharge. This suggests that the formation of H₂ from water is not due to electrolysis, but rather to the generation of H radicals via homolytic cleavage of H₂O induced by plasma.

 

6. In section 3.4 figure 14, the product yields, including unreacted feedstocks, are consistently below 60%. Please provide an explanation for the observed carbon imbalance.

 

We also recognize the carbon imbalance as an important concern. To investigate the potential presence of undetected products, we rinsed the gas bag with water and methanol and analyzed the rinsates; however, no additional products were detected. In addition, no formation of solid precipitates or phase-separated products from the aqueous phase was observed under the experimental conditions. Therefore, the reason for the missing carbon balance remains unclear at this stage. These points were described in the original manuscript (Lines 373–384). Accordingly, the decomposition mechanism proposed in this study may not account for all the reaction pathways, and the existence of other decomposition mechanisms cannot be excluded. To emphasize this point, we have added a corresponding sentence in Lines 384–386.

 

7. In section 3.4, the authors compare product distributions from the decomposition of various model compounds. Did they also investigate the influence of discharge power (i.e., electrode gap) on product distribution? If so, please include a discussion of any observed differences.

 

We presented the product distribution only for the case with an electrode gap of 1.5 mm (28 W), because the distributions did not change significantly at electrode gaps of 0.5 mm (16 W) and 1.0 mm (21 W). At least within this power range (16–28 W), the decomposition behavior appears to remain largely unchanged. As shown in Figure 3, the discharge power seems to affect only the reaction rate. We have added this explanation in Lines 387–391.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

1. Conclusions

"The decomposition rate in aqueous solution plasma followed the order syringol > guaiacol > phenol, which may be attributed to the greater number of electron-donating methoxy groups."

should be corrected to:

"The decomposition rate in aqueous solution plasma is following the order syringol > guaiacol > phenol, which may be attributed to the greater number of electron-donating methoxy groups."

2. Abstract

"Lignin is an abundant natural aromatic macromolecule in wood, yet it remains underutilized."

should be replaced by:

"Lignin is a natural macromolecule, well abundant in wood, however it is not easy to be utilized completely.

3. The authors should correct the literature according to journal requirements. Please, add DOIs.

Author Response

Reply to reviewer 2:

Thank you very much for your kind and thoughtful comments. We have revised the manuscript according to your comments and provided our response below.

 

1. Conclusions

"The decomposition rate in aqueous solution plasma followed the order syringol > guaiacol > phenol, which may be attributed to the greater number of electron-donating methoxy groups."

should be corrected to:

"The decomposition rate in aqueous solution plasma is following the order syringol > guaiacol > phenol, which may be attributed to the greater number of electron-donating methoxy groups."

 

We agree that, since this is a general statement based on our findings, the use of the present tense is appropriate. However, as the present progressive tense may not be suitable for an academic article, we have revised the sentence to use the simple present tense (Line 429).

 

2. Abstract

"Lignin is an abundant natural aromatic macromolecule in wood, yet it remains underutilized."

should be replaced by:

"Lignin is a natural macromolecule, well abundant in wood, however it is not easy to be utilized completely.

 

We intended to refer to lignin as a globally abundant resource, not its abundance within wood itself. We agree that the original sentence was ambiguous in this regard, and we appreciate your suggestion. To clarify the meaning, we have revised the sentence accordingly (Lines 9–10).

We hope that the above and this revision align with the intent of your suggestion.

 

3. The authors should correct the literature according to journal requirements. Please, add DOIs.

 

We reviewed the journal's guidelines and found no explicit requirement to include DOIs in the reference list. As DOIs are not consistently included in other published articles either, we did not add them.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript describes the reaction of phenolic monomers related to lignin under high voltage plasma conditions. I'm not very familiar with plasma chemistry, but this seems a novel approach to the breakdown of lignin monomers. The work is well documented and well illustrated, the manuscript reads very well, and is interesting. I have a few minor comments for consideration by the authors:

1. A general comment is that lignin degradation often occurs by oxidative cleavage of the C3 alkyl side chain, which these compounds lack. Therefore it would be interesting in the future to apply this approach to lignin dimers, some of which are commercially available.
2. Arising from point 1, I would therefore suggest to modify slightly the wording in the title. I would suggest to describe these compounds as "lignin-related aromatic monomers".
3. The proposed mechanisms seem correct to me. For the dicarboxylic acid products, I agree for the formation of the C6 and C4 di-acid products, but I wonder how the C3 diacid (malonic acid) could be formed? This would seem to require an aromatic intermediate with meta-hydroxyl groups to undergo oxidative cleavage. Therefore my suggestion is that catechol is further hydroxylated to form hydroxyquinol (observed in this study), and that oxidative cleavage of hydroxyquinol could give rise to malonic acid.
4. The authors might be interested that there is a report of an unusual manganese superoxide dismutase enzyme that can break down lignin that generates hydroxyl radical, in which some similar kinds of reaction such as phenol hydroxylation and demethylation have been observed (see ACS Chem Biol 2018, 13, 2920-2929). So there is some biological precedent.

Author Response

Reply to reviewer 3:

Thank you very much for your kind and thoughtful comments. Our responses are provided below.

 

1. A general comment is that lignin degradation often occurs by oxidative cleavage of the C3 alkyl side chain, which these compounds lack. Therefore it would be interesting in the future to apply this approach to lignin dimers, some of which are commercially available.

 

We fully agree that propyl side chains and various linkage structures play important roles in lignin decomposition. Indeed, in other reaction systems, we have synthesized or purchased a variety of lignin dimer models and utilized them in our studies. However, these compounds are either costly or require considerable effort to synthesize. Therefore, we chose to begin our investigation with more affordable, basic lignin aromatic models.

 

2. Arising from point 1, I would therefore suggest to modify slightly the wording in the title. I would suggest to describe these compounds as "lignin-related aromatic monomers".

 

We agree that the title you suggested is a more appropriate and less misleading expression, and we have revised it accordingly.

 

3. The proposed mechanisms seem correct to me. For the dicarboxylic acid products, I agree for the formation of the C6 and C4 di-acid products, but I wonder how the C3 diacid (malonic acid) could be formed? This would seem to require an aromatic intermediate with meta-hydroxyl groups to undergo oxidative cleavage. Therefore my suggestion is that catechol is further hydroxylated to form hydroxyquinol (observed in this study), and that oxidative cleavage of hydroxyquinol could give rise to malonic acid.

 

We agree with your consideration regarding the formation of C3 diacid. Although we do not currently have direct evidence to support this specific pathway, we recognize it as an important perspective for future investigation and have therefore refrained from discussing it explicitly in the current manuscript.

 

4. The authors might be interested that there is a report of an unusual manganese superoxide dismutase enzyme that can break down lignin that generates hydroxyl radical, in which some similar kinds of reaction such as phenol hydroxylation and demethylation have been observed (see ACS Chem Biol 2018, 13, 2920-2929). So there is some biological precedent.

 

We appreciate your introduction of the related study. The biochemical approach is also of great interest, and we will keep it in mind for future investigations in comparison with solution plasma.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I am satisfied with the authors' response.

Reviewer 3 Report

Comments and Suggestions for Authors

I am happy with the revised manuscript.