Catalytic Conversion of Glycerol to Lactic Acid Over Cu-Based Catalysts

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors In this paper, copper (I, II) oxide powders were prepared and tested by different methods for the conversion of glycerol to lactic acid under alkaline conditions. It was found by characterization that in all cases, almost complete in situ reduction of Cu(I, II) oxides to metal Cu was observed even after a single catalytic run.Moreover, all samples of spent catalysts exhibited similar catalytic activity independent of initial form and particle size.  Compared to commercial copper powders and prepared copper powders, the in situ reduced copper catalysts had similar activity to the specially prepared copper powders and possessed higher activity than the commercial copper powders . At the same time, the in situ reduced copper catalysts also showed considerable stability. After ten catalytic cycles, glycerol conversion and lactic acid selectivity were about 98% and 70%, respectively. Here are some specific opinions:

 

1.       In Section 2. Results and Discussions , The paper mentions that The quantity of by-produced diglycerol using Cu-based catalysts was significantly reduced in comparison to using only sodium hydroxide (25% versus 2÷6%)，which should actually be (25% versus 2~6%). Please modify it.

2.       The Figure 2. SEM photos of catalysts is not clear, and the morphology and structure of the catalyst cannot be clearly displayed. it is recommended that the authors increase the clarity of the picture so that readers can observe relevant information.

3.       In Figure11, please explain what catalysts are referred to by the red and blue lines, respectively.

4.       The introduction of the X-ray Photoelectron Spectroscopy(XPS) test is recommended when examining changes in the chemical valence and composition of the catalyst before and after the reaction.

5.       Please add the performance test results of specially prepared copper powders and commercial copper powder catalysts under optimal reaction conditions and compare it with the performance of in situ Cu catalyst. Some pioneering work should be discussed (J. Am. Chem. Soc. 2023, 145, 48, 26133–26143; ACS Catal. 2023, 13, 13920–13930)

6.       Please provide appropriate proof and explanation of the reaction pathway and mechanism for the conversion of glycerol to lactic acid.

Comments on the Quality of English Language

N/A

Author Response

1. In Section 2. Results and Discussions , The paper mentions that The quantity of by-produced diglycerol using Cu-based catalysts was significantly reduced in comparison to using only sodium hydroxide (25% versus 2÷6%)，which should actually be (25% versus 2~6%). Please modify it.

Correction was made.

2. The Figure 2. SEM photos of catalysts is not clear, and the morphology and structure of the catalyst cannot be clearly displayed. it is recommended that the authors increase the clarity of the picture so that readers can observe relevant information.

Corrections of SEM photos were made.

3. In Figure11, please explain what catalysts are referred to by the red and blue lines, respectively.

Both reactions (marked by red and blue) were performed over the same in situ Cu catalyst

4. The introduction of the X-ray Photoelectron Spectroscopy (XPS) test is recommended when examining changes in the chemical valence and composition of the catalyst before and after the reaction.

Authors agree with your recommendation. Unfortunately, we can’t do XPS tests during the time given to article revision. At the same time, we suppose that analyses presented in the article are enough to prove recovery processes proceeding. Moreover, references in the article are also in common with our assumptions.

5. Please add the performance test results of specially prepared copper powders and commercial copper powder catalysts under optimal reaction conditions and compare it with the performance of in situ Cu catalyst. Some pioneering work should be discussed (J. Am. Chem. Soc. 2023, 145, 48, 26133–26143; ACS Catal. 2023, 13, 13920–13930)

Test results of specially prepared copper powders, commercial copper powder catalysts and in situ Cu catalyst (Cu2O-1 Cu2O-1and Cu2O-3) under close to optimal reaction conditions are presented in table 1.

References are added.

6. Please provide appropriate proof and explanation of the reaction pathway and mechanism for the conversion of glycerol to lactic acid.

Proof and explanation of the reaction pathway and mechanism for the conversion of glycerol to lactic acid was presented in our previous article (Zavrazhnov, S. A. Mechanism analysis and kinetic modelling of Cu NPs catalysed glycerol conversion into lactic acid / S. A. Zavrazhnov, A. L. Esipovich, S. Y. Zlobin, A. S. Belousov, A. V. Vorotyntsev // Catalysts. 2019, Vol. 9, № 3. 21 p.) and other works. References are made throughout the text.

Reviewer 2 Report

Comments and Suggestions for Authors

Catalytic conversion of glycerol to lactic acid is an interesting and sustainable route for the production of lactic acid. A series of copper (I, II) oxides powders were synthesized and employed for this process, and highly catalytical performance was obtained over the in situ reduced copper catalyst under optimal reaction conditions. However, there are still some issues should be addressed before publication.

1.     It should be noted that there of typical Cu catalysts (Cu₂O-1 Cu₂O-1and Cu₂O-3) possessed different average size but endow the similar catalytic performance, please giving the reason.

2.     The catalytic performance indicated that all the Cu species (Cu⁰, Cu⁺, Cu²⁺) could realize the process of glycerol to lactic acid, please giving the possible reaction route for these Cu species.

Author Response

1. It should be noted that there of typical Cu catalysts (Cu2O-1 Cu2O-1and Cu2O-3) possessed different average size but endow the similar catalytic performance, please giving the reason.

Obviously, some tendencies can be found while comparing the results from table 1, 2 and 3. It is seen, that glycerol conversion decreases with the increase of catalyst average size, while catalysts are fresh. In case of spent catalysts, results of different catalysts are similar because of catalysts agglomeration.

2. The catalytic performance indicated that all the Cu species (Cu⁰, Cu⁺, Cu²⁺) could realize the process of glycerol to lactic acid, please giving the possible reaction route for these Cu species

Proof and explanation of the reaction pathway and mechanism for the conversion of glycerol to lactic acid was presented in previous article (Zavrazhnov, S. A. Mechanism analysis and kinetic modelling of Cu NPs catalysed glycerol conversion into lactic acid / S. A. Zavrazhnov, A. L. Esipovich, S. Y. Zlobin, A. S. Belousov, A. V. Vorotyntsev // Catalysts. 2019, Vol. 9, № 3. 21 p.). Additional references are made throughout the text.

Reactions schemes are presented in supplementary file

Shen, H. Yin, H. Yin, S. Liu, A. Wang / Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu₂O Nanoparticles in NaOH Aqueous Solution // J. Nanosci. Nanotechnol. 2017, Vol. 17. № 1 pp 780-787.

Y. Yang, Y. H. Ke, H. F. Ren, C. L. Liu, R. Z. Yang, W. S. Dong. The conversion of glycerol to lactic acid catalyzed by ZrO₂-supported CuO catalysts // Chemical Engineering Journal, 283, 2016, pp. 759–767

Yin, H. Yin, A. Wang, L. Shen, Y. Liu, Y. Zheng. Catalytic Conversion of Glycerol to Lactic Acid Over Metallic Copper Nanoparticles and Reaction Kinetics // J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 2 pp. 1255–1266

Suna, Y. Yamadaa, S. Satoa, W. Ueda. Glycerol as a potential renewable raw material for acrylic acid production J. Green Chemistry DOI: 10.1039/C7GC00358G

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

T   This article is interesting and nicely written and it brings some useful information. Thus, I recommend the revising of the article based on the main points mentioned below.

 

-        The novelty of the study is not properly highlighted in the ”Introduction” section.

-        Please replace in all manuscript ”ml” with ”mL”; ”hours” with ”h”; ”mol/mL” with ”mol·mL^-1”.

-        Page 7, line 162: please replace ”cm-1” with ”cm^-1”.

-        In cases where the mean was calculated, please provide the standard deviation or standard error (Figures 6-12).

-        Page 12, line 289: please replace ”CH3COO^-”  with ”CH₃COO^-”.

-        Page 13, line 300: please replace ”NaBH4”  with ”NaBH₄”.

-        Page 14, line 366, please describe what it means each terms from equation.

 

Author Response

1. The novelty of the study is not properly highlighted in the ”Introduction” section.

Done. Corrections were made. New references were added.

2. Please replace in all manuscript ”ml” with ”mL”; ”hours” with ”h”; ”mol/mL” with ”mol•mL-1”.

Page 7, line 162: please replace ”cm-1” with ”cm-1”.       

Done.

4. In cases where the mean was calculated, please provide the standard deviation or standard error (Figures 6-12).

“Relative deviations did not exceed ± 5%” – written in section 3.2.

5. Page 12, line 289: please replace ”CH3COO-” with ”CH3COO-”.

Page 13, line 300: please replace ”NaBH4”  with ”NaBH4”.       

Page 14, line 366, please describe what it means each terms from equation.       

Done.

 

Reviewer 4 Report

Comments and Suggestions for Authors

The authors present here a research on the activtiy and selectivity of copper based catalysts on the formation of lactic acid from glycerol. The authors tested different copper(I) and copper(II) oxides and copper(0) catalysts and compared their performance in this catalytic reaction. Even though the authors adequately characterized the materials, and performed detailed catalytic evaluation, I propose the article to undergo a major revision before being accepted for publication. I have found several issues with the paper, some of them are more important, while others not, I will outline them in no particular order:

1. The introduction to the paper needs to be improved. There is only a few citations on the copper based catalysts for this reaction, and the authors need to expand the introduction with previous literature. Namely, I believe it would be of interest to list at least a few comparisons with regard to selectivity and conversion percentage of glycerol to lactic acid. The authors also claim there is practically no information in the literature about the stability of copper oxide catalytic systems, however, I have found some papers on this topic.

2. The title headings don't follow one another. For example, the GC-MS results start right below the heading 2. Results and discussions, this is followed by 2.1. Catalysts characterization, and 2.1.2. SEM analysis. Where is 2.1.1.?

3. SEM images between fresh and spent catalysts show quite large differences in magnification. Usually, a small difference is not a problem, however, here it is very difficult for the reader to compare the particles by themselves, due to such a large difference in scale. I would advise the authors to replace some images to at least similar magnifications.

4. The authors noticed an increase in particle size, and decrease in surface area before and after catalysis. However, the average crystallite size slightly decreases before and after catalysis. Usually, in most cases, the particle size change should accompany the change in crystallite size; this is not the case in this paper. I believe it would be interesting to comment this in the paper. Perhaps one reason is due to the structural changes induced in the crystal system changing from the oxide to metallic copper. Also, the decrease in the surface area of the catalyst could be explained by the adsorption of organic species, which hinder the adsorption of nitrogen molecules.

5. I would encourage the authors to not use 3D graphs as in Figs. 7, 9, 10, 12. These kinds of graphs might be aesthetically pleasing, but they are much worse at conveying information to the readers. I believe the graphs should be remade as 2D versions. Also some graphs contain boxes around them, which are inconsistent with the rest of the graphs (for example, Fig. 7).

6. The arrows in Fig. 8 are unnecessary, it would be better to color the left y-ax in blue, and the right y-ax in red in my opinion.

7. Line 236: Do the authors mean "...further increase in catalyst concentration..." instead of "...further increase in glycerol concentration..."?

8. Why were two different copper salts used for the synthesis of different copper oxides? For the synthesis of CuO, copper acetate was used. After hydrolysis the sample was annealed at 400 degrees for 4 h. On the other hand, Cu₂O was synthesized using copper sulfate with different reducing agents. This sample was not annealed. Annealing the sample of CuO could have improved the crystallinity of the sample, as well as removed the potentially unwashed acetates from the surface of the catalyst, which could have improved its catalytic properties. Even though the samples Cu₂O were washed some sulfate anions, organic contaminats or borates could have left on the surface of the catalysts. In my opinion, both samples should have been annealed (in N₂ atmosphere at least to not oxidize Cu(I).)

9. NaBH4 was used for both the synthesis of Cu₂O and metallic Cu. How did the authors control the phase formation between the oxide and the metallic Cu? In the experimental section there are only small differences in the concentration of precursors?

10. In equation S1. the shape factor is denoted as capital K, while in the text below, the shape factor is denoted as lowercase k so this typo should be corrected.

11. There is quite a large difference between the crystallite sizes calculated for different reflections, depending on the scattering angle (supplementary information). Is it possible there are other factors influencing the crystallite size calculations, for example crystal anisotropy in different crystallographic directions?

12. I would advise against the use of "SEM photos... (line 121)" and instead the correct wording would be SEM micrographs or SEM images. Please fix this throughout the manuscript.

13. I think if possible it would be interesting to see the XPS analysis of samples before and after catalytic testing.

14. Were the catalyst powders washed after each catalytic cycle or used as is?

Comments on the Quality of English Language

Some wording could be improved. For example lines 133 to 135. 

The authors should check the use of "increasing" vs. "increase", for example in line 196, and similar words throughout the manuscript.

Author Response

1. The introduction to the paper needs to be improved. There is only a few citations on the copper based catalysts for this reaction, and the authors need to expand the introduction with previous literature. Namely, I believe it would be of interest to list at least a few comparisons with regard to selectivity and conversion percentage of glycerol to lactic acid. The authors also claim there is practically no information in the literature about the stability of copper oxide catalytic systems, however, I have found some papers on this topic.

New references are added. Sentences are rewritten.

2. The title headings don't follow one another. For example, the GC-MS results start right below the heading 2. Results and discussions, this is followed by 2.1. Catalysts characterization, and 2.1.2. SEM analysis. Where is 2.1.1.?

Corrections were made.

3. SEM images between fresh and spent catalysts show quite large differences in magnification. Usually, a small difference is not a problem, however, here it is very difficult for the reader to compare the particles by themselves, due to such a large difference in scale. I would advise the authors to replace some images to at least similar magnifications.

Obviously, SEM images presentation looks better when scales are the same. Unfortunately, powders of used catalysts are looking rather unclear while using scales, different from what was presented in the article, due to its agglomeration. We really tried to.

4. The authors noticed an increase in particle size, and decrease in surface area before and after catalysis. However, the average crystallite size slightly decreases before and after catalysis. Usually, in most cases, the particle size change should accompany the change in crystallite size; this is not the case in this paper. I believe it would be interesting to comment this in the paper. Perhaps one reason is due to the structural changes induced in the crystal system changing from the oxide to metallic copper. Also, the decrease in the surface area of the catalyst could be explained by the adsorption of organic species, which hinder the adsorption of nitrogen molecules.

There are no differences in crystallite size for cooper powders (CP). Differences are detected only in case of cooper oxides, and crystallite size of spent catalysts is practically similar to crystallite size of СР

5. I would encourage the authors to not use 3D graphs as in Figs. 7, 9, 10, 12. These kinds of graphs might be aesthetically pleasing, but they are much worse at conveying information to the readers. I believe the graphs should be remade as 2D versions. Also some graphs contain boxes around them, which are inconsistent with the rest of the graphs (for example, Fig. 7).

Based on previous articles published in this journal and others journals by MDPI, authors suppose that 3D graphs are welcome. Boxes are deleted.

6. The arrows in Fig. 8 are unnecessary, it would be better to color the left y-ax in blue, and the right y-ax in red in my opinion.

Based on previous articles published in this journal and others journals by MDPI, authors suppose that this way of data performing is welcome.

7. Line 236: Do the authors mean "...further increase in catalyst concentration..." instead of "...further increase in glycerol concentration..."?

Surely, there is a mistake in the text. Thank you.

8. Why were two different copper salts used for the synthesis of different copper oxides? For the synthesis of CuO, copper acetate was used. After hydrolysis the sample was annealed at 400 degrees for 4 h. On the other hand, Cu2O was synthesized using copper sulfate with different reducing agents. This sample was not annealed. Annealing the sample of CuO could have improved the crystallinity of the sample, as well as removed the potentially unwashed acetates from the surface of the catalyst, which could have improved its catalytic properties. Even though the samples Cu2O were washed some sulfate anions, organic contaminats or borates could have left on the surface of the catalysts. In my opinion, both samples should have been annealed (in N2 atmosphere at least to not oxidize Cu(I).)

CuO was annealed according to the synthesis methodic. All Cu2O catalyst samples were prepared as nanoparticles (see Table 3). Copper oxide (1) nanoparticles are capable of sintering at a lower temperature of about 215-230 ºC. Burning (even in N2 atmosphere) would lead to inevitable sintering and hence deterioration of their catalytic properties.

9. NaBH4 was used for both the synthesis of Cu2O and metallic Cu. How did the authors control the phase formation between the oxide and the metallic Cu? In the experimental section there are only small differences in the concentration of precursors?

The main difference is the reaction time. Thank you for your review, we added this information in experimental section.

10. In equation S1. the shape factor is denoted as capital K, while in the text below, the shape factor is denoted as lowercase k so this typo should be corrected.

Correction was made.

11. There is quite a large difference between the crystallite sizes calculated for different reflections, depending on the scattering angle (supplementary information). Is it possible there are other factors influencing the crystallite size calculations, for example crystal anisotropy in different crystallographic directions?

Obviously, there are lots of factors influencing the crystallite size calculation. But, unfortunately, estimation of its influence in current situation is a rather difficult investigation and takes a lot of time. There is some difference between the crystallite sizes calculated for different reflections (depending on the scattering angle). On the other hand, peaks (Figure 3) are rather thin what confirms rather good mathematical distribution. Thus, authors suppose that final data presented in Figure 3 looks like validated.

12. I would advise against the use of "SEM photos... (line 121)" and instead the correct wording would be SEM micrographs or SEM images. Please fix this throughout the manuscript.

Correction was made. Thank you for your advice.

13. I think if possible it would be interesting to see the XPS analysis of samples before and after catalytic testing. Authors agree with your recommendation.

Unfortunately, we can’t do XPS tests during the time given to article revision. At the same time, we suppose that analyses presented in the article are enough to prove recovery processes proceeding. Moreover, references in the article are also in common with our assumptions.

14. Were the catalyst powders washed after each catalytic cycle or used as is?

Catalyst powders were not washed after each catalytic cycle. Liquid phase was removed, then fresh reactants were added and new cycle started.

15 Comments on the Quality of English Language. Some wording could be improved. For example lines 133 to 135. The authors should check the use of "increasing" vs. "increase", for example in line 196, and similar words throughout the manuscript.

Done.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Accept in present form

Reviewer 2 Report

Comments and Suggestions for Authors

Accept. 

Author Response

Accept. 

Thank you for your review.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have somewhat updated the manuscript based on the suggestions presented in the review. However, any bigger issue I have found the authors didn't consider, such as no update to the introduction, redrawing of the images, not fully answering question 8, etc. Some of it is valid (like doing the figures in 3D format), while other points are important (improvement of the introduction, answering fully to question 8) and without them I would not accept the manuscript. These are not changes that require a lot of time, and can be done in the timeframe given.

Author Response

The authors have somewhat updated the manuscript based on the suggestions presented in the review. However, any bigger issue I have found the authors didn't consider, such as no update to the introduction, redrawing of the images, not fully answering question 8, etc. Some of it is valid (like doing the figures in 3D format), while other points are important (improvement of the introduction, answering fully to question 8) and without them I would not accept the manuscript. These are not changes that require a lot of time, and can be done in the timeframe given.

Introduction has been revised.  New data was added. New references were also added.

Answering question 8

Why were two different copper salts used for the synthesis of different copper oxides? For the synthesis of CuO, copper acetate was used. After hydrolysis the sample was annealed at 400 degrees for 4 h. On the other hand, Cu2O was synthesized using copper sulfate with different reducing agents. This sample was not annealed.

Obviously, both CuO and Cu2O can be synthesized using cooper acetate or sulfate as reactants with rather similar yields and product purity. These methodic are well known and used for a long time. During the numbers of experiments we used both of them to synthesize both CuO and Cu2O, but comparison of these methods was not the goal of the article.

The main reason for the choice of copper acetate in preparation of CuO was the easy decomposition of acetate residues on the catalyst surface during calcination. In addition, sodium acetate is more easily washed off with water (solubility 760 g•L^-1) than sodium sulfate (190 g•L^-1).

Annealing the sample of CuO could have improved the crystallinity of the sample, as well as removed the potentially unwashed acetates from the surface of the catalyst, which could have improved its catalytic properties. Even though the samples Cu2O were washed some sulfate anions, organic contaminats or borates could have left on the surface of the catalysts. In my opinion, both samples should have been annealed (in N2 atmosphere at least to not oxidize Cu(I).

Cu₂O nanoparticles are capable of sintering at a lower temperature of about 215-230 ºC. Burning (even in N₂ environment) would lead to inevitable sintering. We added some references of Cu2O usage in investigated process while Introduction Section revision. All authors use oxidation-reduction process to synthesize Cu2O (as we did) and no one use annealing of Cu2O.

Cu2O samples were washed with aqua-ethanol mixture after synthesis. No side-products were detected in XRD or any other analysis.