Depolymerization and Hydrogenation of Organosolv Eucalyptus Lignin by Using Nickel Raney Catalyst

Round 1

Reviewer 1 Report

The manuscript reports on the depolymerisation and hydrogenation of organosolv Eucaliptus Lignin by using nickel Raney catalyst. Overall, it is written well. However, some detailed comments as below need to be addressed. Also, English needs some improvement by an expert in the area of process technology/engineering.

1)L21: Is this hydrocracking? Water is used or hydrogen in the reaction?

2)What is the advantage of recovery of monomers instead of recovery of polymer lignin?

3)Do authors mean a 0.3 l reactior in this statement? If yes, a correction is required (The conversion of lignin was carried out in a 0.3 l stirred reactor …) : “The conversion of lignin was carried out with a batch reactor equipped with a 0.3 l vessel with adjustable internal stirrer and heat control.”

4)Section 2.3. What type of experimental design is this? Central composite design based on what?

5)Table 1: What is the exact definition of yield for the phenols obtained? Not indicated!

6)What is the total recovery of lignin in extraction process? Not indicated.

7)Table 4: IS this mass balance or overall mass recovered?

 

 

Author Response

1)L21: Is this hydrocracking? Water is used or hydrogen in the reaction?

The authors thank the reviewer for the remark that allows clarifying the text and improving understandability. In this paper, hydrocracking means cracking in the presence of hydrogen. In our work, we explained that the reaction takes place on lignin dissolved in butanol since it is a fraction of the organosolv process; Pressurized hydrogen was used. Water is not present.

2)What is the advantage of the recovery of monomers instead of the recovery of polymer lignin?

The monomers can be used for example as fuels, sustainable aviation fuel for example. In general, monomers can be upgraded or functionalized to produce fine chemicals, fuels, etc, while the lignin is hard to be used, but in a boiler, are reported in the introduction. 

3)Do the authors mean a 0.3 l reactor in this statement? If yes, a correction is required (The conversion of lignin was carried out in a 0.3 l stirred reactor …) : “The conversion of lignin was carried out with a batch reactor equipped with a 0.3 l vessel with an adjustable internal stirrer and heat control.”

The authors thank the reviewer for the remark, the volume is now specified as of the reactor, line 126 (Hydrocracking tests were performed by using 300 mL Parr® Reactor (from Büchi, model Limbo-li) equipped with a 0.3 litres vessel)

4)Section 2.3. What type of experimental design is this? Central composite design based on what?

The authors thank the reviewer for the remark,  the information about the type of the experimental design is given in section 2.3, line 161 the type of design (subtype Randomized), the centre point is the average of the extremes of considered variables.

5)Table 1: What is the exact definition of yield for the phenols obtained? Not indicated!

The authors thanks to the reviewer for the remark, in the revised text it is specified corresponding to the ratio: (weight of monomer/ weight of loaded lignin ) x100. The weight of the monomer is determined by its concentration x the volume of the liquid phase. This is reported in the note 1 of Table 1.

6)What is the total recovery of lignin in the extraction process? Not indicated.

The authors thank the reviewer for the remark, this information is now added at the beginning of the "Results and discussions" paragraph, line 69-70.

 

7)Table 4: IS this mass balance or overall mass recovered?

The authors thank the reviewer for the remark, new information has been added in Table 4 to clarify this aspect i.e. solution recovery, produced gas, mass balance, the definition of recovery.

Author Response File: Author Response.docx

Reviewer 2 Report

The followings are some of the concerns:

1. The originality of the paper needs to be further clarified.
2. There are many published articles about hydrocracking of lignin and lignin depolymerization. An updated and complete literature review should be conducted.
3. What are the C balances of the experiments?
4. line 346-347, for “The elaborations are shown in Figure 19 where the ordinates show the average specific rate of formation in the time 0-90 minutes”, the “Figure 19” should be Figure 9.

Author Response

1. The originality of the paper needs to be further clarified.

The authors thank for the remark and to further clarify the originality of the paper lines 95 and 97-98 have been changed and added.

1. There are many published articles about hydrocracking of lignin and lignin depolymerization. An updated and complete literature review should be conducted.

The authors thank for the remark. Indeed, hydrocracking of lignin and lignin depolymerization are topics on which a lot has been written in recent years and this led the authors to choose to represent it through the choice of a limited number of significant articles in the previous version. In the revised text several new references have been added, all thematic reviews, on Page 2 lines 16, 17, 20, and 25 to meet the right expectations.

1. What are the C balances of the experiments?

The authors thank for the remark. Unfortunately, this analysis has not been carried out. The authors did not consider it because of the presence of unresolved organic molecules in the liquid phase (by GC MS). On the other side, drying the organic phase would involve losses of volatile components, so making the mass balance (and C balance as well) difficult to close. Above all this information is not central to the scope of the paper.

1. line 346-347, for “The elaborations are shown in Figure 19 where the ordinates show the average specific rate of formation in the time 0-90 minutes”, the “Figure 19” should be Figure 9.

The authors thank the reviewer for the remark, the number of the figure is now corrected, it is Figure 14.

Author Response File: Author Response.docx

Reviewer 3 Report

Major points:

1. Authors claim that the technology they used is more economical for lignin depolymerization than the studied technologies so far. However, they don't present any evidence for it. I recommend authors to either remove this claim, or submit a detailed economical analysis of their method with comparisons of other methods out there.
2. What are the potential products that can be produced from the degraded lignin? The draft lacks the clear end objective for their study. The draft needs clear path for upstream processing and product streams. Authors only briefly mention the the potential use of the depolymerized lignin in the biofuel production. However, they use a lot of fuel (butanol) to produce very small amounts of degraded lignin that can be used for producing fuel.
3. Yield of degraded lignin is very small (about 9% combined) just like most other methods used in other available publications. What makes this method advantageous compared to the others? 
4. What is unique, novel or exciting about the method used in this study that would take the interest of the readers? There is no compelling discussion or evidence provided to the audience from this perspective.   Do the authors see their method ever commercialized ? If yes, why? All of that discussion need to be included in the draft. 

 

Author Response

1. Authors claim that the technology they used is more economical for lignin depolymerization than the studied technologies so far. However, they don't present any evidence for it. I recommend authors to either remove this claim, or submit a detailed economical analysis of their method with comparisons of other methods out there.

The authors based the statement on the fact of using commercial, and low-cost catalysts, like Ni Raney. However, they recognize this statement, if taken in absolute means, would need a detailed economic analysis that is out of the scope of the paper. Indeed, the statement at line 93 94 reported our view: “The aim of this work was to use this catalyst to improve the economic sustainability of the lignin conversion into monomeric phenols.” Accepting the suggestion of the reviewer we have changed into: “The aim of this work was to use this catalyst that potentially has characteristics to improve the economic sustainability of the lignin conversion into monomeric phenols, because of its relatively low cost and large availability.”

1. What are the potential products that can be produced from the degraded lignin? The draft lacks the clear end objective for their study. The draft needs clear path for upstream processing and product streams. Authors only briefly mention the the potential use of the depolymerized lignin in the biofuel production. However, they use a lot of fuel (butanol) to produce very small amounts of degraded lignin that can be used for producing fuel.

The authors are well aware of the solvent (butanol) consumption and investigated the conditions to minimize this. Indeed, the optimized (minimized) butanol consumption was a constraint of the design (Table 2, Criteria used for reaction parameters optimization), moreover, the issue was cited in the Conclusion paragraph to highlight the importance of finding a solution. In the current study, the used lignin was the stream obtained from the pretreatment, so it was solubilized in the same solvent used in that step. An alternative strategy would be the complete recovery of the solvent (butanol) to be recycled in the pretreatment step and solubilized dried lignin in another solvent not converted into the hydrogenation reactions, so being in principle recoverable and recyclable. Though reasonably identified, this plan would imply repeating all tests. Moreover and above all, the scope of the paper was to investigate the lignin fragmentation, so the evidence of the issues linked to the type of solvent is reported mostly to offer a view as complete as possible of the process. Different solvents are under consideration and will be the object of other work.

1. Yield of degraded lignin is very small (about 9% combined) just like most other methods used in other available publications. What makes this method advantageous compared to the others? 

The authors highlighted as the scope of the work investigating the use of cheap and common catalyst when applied to a relatively new type of lignin (organosolv), as explained before the level of study is not yet sufficient to sustain the superiority of our approach as it would be necessary to deal economic aspects and a complete process design, including recycling of the streams with unconverted lignin to increase yield. These aspects are beyond the scope of the paper and the authors deliberately avoid to deal a scale of efficiency or conveniences among the available treatments.

1. What is unique, novel or exciting about the method used in this study that would take the interest of the readers? There is no compelling discussion or evidence provided to the audience from this perspective.   Do the authors see their method ever commercialized? If yes, why? All of that discussion need to be included in the draft. 

The authors deal with these aspects at the end of the introduction (lines 89-96), after a short referencing to the available papers, studies, and reviews that overall is truly vast literature. As explained in the above points, the paper reports lab experiments to demonstrate the feasibility of depolymerizing organosolv lignin by hydrogenation catalyzed by a cheap and common catalyst, with quantitative data concerning also the solvent. These are basic data that would be useful for further studies, including an economic assessment, however far beyond the scope of the paper.

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors, for the most part, only answered my questions as a private note to me. The reviewer comments need to be addressed and discussed in the draft as much as possible without repeating the experiments. Also, please avoid using languages other than English in the comments and in other editing in the draft. This should be an all English draft since it will be published in English. 

Author Response

replies to Reviewer 3

2 round revision:  The autors thank the reviewer for the remarks: in this 2nd submission, to avoid misunderstandings, the manuscript has been cleaned from the non-English comments that appeared at side. Below, please find enclosed the comments, also attached as file that was missed in previous submission.

1. Authors claim that the technology they used is more economical for lignin depolymerization than the studied technologies so far. However, they don't present any evidence for it. I recommend authors to either remove this claim, or submit a detailed economical analysis of their method with comparisons of other methods out there.

The authors based the statement on the fact of using commercial, and low-cost catalysts, like Ni Raney. However, they recognize this statement, if taken in absolute means, would need a detailed economic analysis that is out of the scope of the paper. Indeed, the statement at line 93 94 reported our view: “The aim of this work was to use this catalyst to improve the economic sustainability of the lignin conversion into monomeric phenols.” Accepting the suggestion of the reviewer we have changed into: “The aim of this work was to use this catalyst that potentially has characteristics to improve the economic sustainability of the lignin conversion into monomeric phenols, because of its relatively low cost and large availability.”

1. What are the potential products that can be produced from the degraded lignin? The draft lacks the clear end objective for their study. The draft needs clear path for upstream processing and product streams. Authors only briefly mention the the potential use of the depolymerized lignin in the biofuel production. However, they use a lot of fuel (butanol) to produce very small amounts of degraded lignin that can be used for producing fuel.

The authors are well aware of the solvent (butanol) consumption and investigated the conditions to minimize this. Indeed, the optimized (minimized) butanol consumption was a constraint of the design (Table 2, Criteria used for reaction parameters optimization), moreover, the issue was cited in the Conclusion paragraph to highlight the importance of finding a solution. In the current study, the used lignin was the stream obtained from the pretreatment, so it was solubilized in the same solvent used in that step. An alternative strategy would be the complete recovery of the solvent (butanol) to be recycled in the pretreatment step and solubilized dried lignin in another solvent not converted into the hydrogenation reactions, so being in principle recoverable and recyclable. Though reasonably identified, this plan would imply repeating all tests. Moreover and above all, the scope of the paper was to investigate the lignin fragmentation, so the evidence of the issues linked to the type of solvent is reported mostly to offer a view as complete as possible of the process. Different solvents are under consideration and will be the object of other work.

1. Yield of degraded lignin is very small (about 9% combined) just like most other methods used in other available publications. What makes this method advantageous compared to the others? 

The authors highlighted as the scope of the work investigating the use of cheap and common catalyst when applied to a relatively new type of lignin (organosolv), as explained before the level of study is not yet sufficient to sustain the superiority of our approach as it would be necessary to deal economic aspects and a complete process design, including recycling of the streams with unconverted lignin to increase yield. These aspects are beyond the scope of the paper and the authors deliberately avoid to deal a scale of efficiency or conveniences among the available treatments.

1. What is unique, novel or exciting about the method used in this study that would take the interest of the readers? There is no compelling discussion or evidence provided to the audience from this perspective.   Do the authors see their method ever commercialized? If yes, why? All of that discussion need to be included in the draft. 

The authors deal with these aspects at the end of the introduction (lines 89-96), after a short referencing to the available papers, studies, and reviews that overall is truly vast literature. As explained in the above points, the paper reports lab experiments to demonstrate the feasibility of depolymerizing organosolv lignin by hydrogenation catalyzed by a cheap and common catalyst, with quantitative data concerning also the solvent. These are basic data that would be useful for further studies, including an economic assessment, however far beyond the scope of the paper.

Author Response File: Author Response.docx