Lignin Hydrogenolysis over Bimetallic Ni–Ru Nanoparticles Supported on SiO₂@HPS

Round 1

Reviewer 1 Report

The manuscript described the synergy between two metals and support surface acidity on lignin hydrogenolysis. However, I found some problems in the discussion section. Moreover, extensive studies have been conducted on lignin hydrogenolysis using Ru-Ni catalysts. After carefully read this manuscript, I recommend a major revision to improve the research integrity and the readability of this manuscript.

1. The determination of the acidic sites is not available when comparing the preparation of the support, but only after impregnation of the metal. It needs to be added.

2. The acidity has a role in the target reaction. Therefore, the amount of acidic sites among the tested catalysts in this study should be further discussed.

3. Line 151~153, “The addition of Ru to Ni leads to a decrease in the particle size with the narrow size distribution as well as to its stabilization avoiding aggregation.” How do you count the size distribution of the metal particles? Because this is supporting evidence for the reduction of metal particle size. More evidence is needed, such as the statistical distribution of particle size by TEM images.

4. The stabilization of metals to avoid aggregation requires more visual evidence, such as TEM.

5. The catalyst characterization section does not highlight the interactions between Ru and Ni metals and requires additional discussion.

6. Line 169~171, “An increase in the lignin conversion over the bimetallic catalyst can be assigned to a synergy between two metals, which facilitates metal reducibility due to increased hydrogen and electron transfer.” Bimetallic interactions need to be discussed in the context of characterization analysis,such as the electron transfer in the reaction.

Author Response

Answers to the comments of Reviewers:

We thank the reviewers for the careful reading of our manuscript and for their valuable remarks, which are helpful for the improvement of paper quality. All comments/questions were fully addressed, and the necessary changes have been made in the revised manuscript.

Reviewer 1

Comments:

The manuscript described the synergy between two metals and support surface acidity on lignin hydrogenolysis. However, I found some problems in the discussion section. Moreover, extensive studies have been conducted on lignin hydrogenolysis using Ru-Ni catalysts. After carefully read this manuscript, I recommend a major revision to improve the research integrity and the readability of this manuscript.

1. The determination of the acidic sites is not available when comparing the preparation of the support, but only after impregnation of the metal. It needs to be added.
2. The acidity has a role in the target reaction. Therefore, the amount of acidic sites among the tested catalysts in this study should be further discussed.

Response: Agree.

Action taken: The information about the support acidity was added in the Section 2.1.

3. Line 151~153, “The addition of Ru to Ni leads to a decrease in the particle size with the narrow size distribution as well as to its stabilization avoiding aggregation.” How do you count the size distribution of the metal particles? Because this is supporting evidence for the reduction of metal particle size. More evidence is needed, such as the statistical distribution of particle size by TEM images.

Response: The distribution of the metal particle size was estimated only by SAXS analysis. Unfortunately, the statistical distribution of particle size based on TEM is not available since we don’t have high resolution TEM images. We obtained by TEM at the moment only a general overview of the catalysts' surface.

4. The stabilization of metals to avoid aggregation requires more visual evidence, such as TEM.

Response: Agree. Unfortunately, HR-TEM images are not available at the moment.

5. The catalyst characterization section does not highlight the interactions between Ru and Ni metals and requires additional discussion. Line 169~171, “An increase in the lignin conversion over the bimetallic catalyst can be assigned to a synergy between two metals, which facilitates metal reducibility due to increased hydrogen and electron transfer.” Bimetallic interactions need to be discussed in the context of characterization analysis, such as the electron transfer in the reaction.

Response: Agree.

Based on the characterization results we do confirm the influence of Ru on the formation of smaller particles and diminished polydispersity of active phase. This probably leads to the formation of supplemental surface defects as compared to pure Ni-NPs. These surface defects are known to facilitate the bond cleavage. Moreover, according to the literature, for the bimetallic catalyst the promoting effect of the second metal is also related to the formation of more active electron-rich sites. All of this leads to higher catalytic activity.

Action taken: The discussion and additional references were introduced in the revised manuscript.  

Reviewer 2 Report

The work studies the performance of Ni-Ru catalysts supported on SiO₂@HPS for the hydrogenolysis of lignin. The process aims the production of aromatics, which is a very interesting research topic. Furthermore, the results reported are interesting and exposed that synthesized catalysts are quite promising. However, some aspects must be improved prior to be published:

1. The auhotrs need to dig deeper in the literature to improve the Introduction section. The originality and novelty of the work must be explained more clearly.

2. The catalysts have been characterized through some techniques, both for the metallic and acidic phases. However, the acidic phase must be characterized throughly. It suposses that the authors need to perform an FTIR TPD to determine the strength of the acidic sites, as well as an i-propyl amine TPD to calculate the number of acid sites. These properties are crucial since the acidic sites are key in the hydrogenolysis mechanisms.

3. The authors have not mentioned the eactivation of the catalysts. Probably, coke will be formed during the reaction and will be deposited on the catalysts blocking, at least, partially the chaneels of the catalysts. Therefore, the authors need to provide information about the deposited coke. The work by Cordero-Lanzac et al. (Applied Catal. B. Enviromental 239 (2018) 513-524) will be useful for them to understand the coke formation routes.

4. English must be carefully proofreaded.

Author Response

Reviewer 2

Comments:

The work studies the performance of Ni-Ru catalysts supported on SiO₂@HPS for the hydrogenolysis of lignin. The process aims the production of aromatics, which is a very interesting research topic. Furthermore, the results reported are interesting and exposed that synthesized catalysts are quite promising. However, some aspects must be improved prior to be published:

Response: We thank the reviewer for the careful reading of the manuscript and for the appreciation of the results obtained. We addressed all his comments and made the necessary changes in the revised manuscript.

1. The authors need to dig deeper in the literature to improve the Introduction section. The originality and novelty of the work must be explained more clearly.

Response: Agree

Action taken: The introduction section was revised by adding more information about the catalytic behavior of frequently used metals as well as their interaction with the acidic supports. The originality and novelty of the work were highlighted.

2. The catalysts have been characterized through some techniques, both for the metallic and acidic phases. However, the acidic phase must be characterized thoroughly. It supposes that the authors need to perform an FTIR TPD to determine the strength of the acidic sites, as well as an i-propyl amine TPD to calculate the number of acid sites. These properties are crucial since the acidic sites are key in the hydrogenolysis mechanisms.

Response: Agree

Indeed, the strength and number of the acid sites is crucial for the better understanding of the mechanism of this reaction. However, in this work we focused on the attaining of high activity and especially high stability of the developed catalysts to show their industrial potential. We continue actually to investigate the mechanistic aspects of lignin depolymerization and the results will be reported elsewhere. Regarding the determination of the number of acid sites, the ammonia chemisorption allowed estimation of total acidity.

Action taken: No

3. The authors have not mentioned the deactivation of the catalysts. Probably, coke will be formed during the reaction and will be deposited on the catalysts blocking, at least, partially the channels of the catalysts. Therefore, the authors need to provide information about the deposited coke. The work by Cordero-Lanzac et al. (Applied Catal. B. Enviromental 239 (2018) 513-524) will be useful for them to understand the coke formation routes.

Response: We thank the reviewer for the helpful reference. We agree that the deactivation of the catalyst can take place because of the coke formation on the catalyst surface. By analyzing the catalysts after the reaction, we have observed a slight decrease in the acidity, but no coke deposition (or formation of the carbonized species) on the metal-containing phase was found according to the XPS. We also demonstrated the stability of the developed catalysts observed in 10 consecutive reaction runs as it is shown in Table 5.

Action taken: The reference has been added and the discussion extended.

4. English must be carefully proofreader.

Response: Agree

Action taken: English was checked thoroughly for grammar and stylistic mistakes.

Reviewer 3 Report

The manuscript is very interesting, because it concerns biomass (lignin) processing into chemical products. Studies on stability of catalyst in successive cycles is particularly interesting for me. The samples have been sufficiently tested, but it would be interesting to examine surface of catalyst e.g. by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). It would allow for better understanding of mechanism of the process.

Author Response

Reviewer 3

Comments:

The manuscript is very interesting, because it concerns biomass (lignin) processing into chemical products. Studies on stability of catalyst in successive cycles is particularly interesting for me. The samples have been sufficiently tested, but it would be interesting to examine surface of catalyst, e.g. by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). It would allow for better understanding of mechanism of the process.

Response: We thank the reviewer for the appreciation of the results obtained and fully agree that some additional data (i.e. the Time-of-Flight Secondary Ion Mass Spectrometry) could be helpful in the understanding of the reaction mechanism. However, in this work we focused on the attaining of high activity and especially high stability of the developed catalysts to show their industrial potential. We continue actually to investigate the mechanistic aspects of lignin depolymerization over newly developed catalysts and the results will be reported elsewhere.

Round 2

Reviewer 1 Report

The manuscript has been carefully revised based on the reviewers' comments. I think it can be accepted for publication.