Evaluation of Cobalt, Nickel, and Palladium Complexes as Catalysts for the Hydrogenation and Improvement of Oxidative Stability of Biodiesel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work is devoted to the study of the transfer hydrogenation of methyl linoleate and jatropha biodiesel in the presence of imine and amide complexes of Pd, Ni and Co. The authors claim that in the presence of the these complexes, complete conversion of the substrates is achieved, and depending on the composition of the complexes, it is possible to regulate the selectivity to C18:1 and C18:0 products. In addition, according to the presented data, hydrogenation improves the oxidative stability of biodiesels. Despite the fact that the authors have obtained a sufficient number of new results, the manuscript in this form cannot be recommended for publication, primarily due to the extremely low level of presentation and discussion of the results, which makes it difficult to understand and raises doubts about the reliability of the results. Moreover, the submitted manuscript is not an article, but rather a compilation and description of the results, which resembles a report on the work, which is unacceptable for publication in a scientific journal. Below are the main comments that the authors should take into account if they want to publish the work.

1. In the “Results and discussion” section, an analysis of the obtained results should be carried out, and not just a description (in many cases there is no answer to the question “why?”). This concerns, in particular, the reasons for the differences in catalyst activity, i.e. the authors do not analyze the reasons for the influence of the catalyst composition on their activity, but only states such an influence. What happens to the catalysts during the reaction? Do they destruct to metals and free ligands? These are very important questions, especially from a practical point of view, but there are no explanations in the text.

From Table 3 it is clear that the content of the C16:0 component after 3 hours of hydrogenation (entry 7) is almost 2 times higher than after 6 hours of hydrogenation (entry 6). How can this be explained?

At the end of the “Results and discussion” section (pp. 13-14), the authors paid some attention to the comparison of the hydrogenation results with literature data. However, this comparison is unexpectedly presented in a subsection devoted to the study of oxidative stability.

2. The text is written very carelessly, there are a large number of various mistakes, including grammatical and terminological ones. As noted above, this makes it difficult to understand and raises doubts about the reliability of the results.

- Mistakes in the numbering of sections and subsections.

- Mistakes in the numbering of schemes.

- There is a caption to scheme 2, but there is no scheme itself.

- Extremely low quality of figures and schemes (low resolution), low level of their design, carelessness.

- Examples of some strange mistakes in the text: “photograph in Fig. S12” (Fig. S12 shows not a photograph, but a dependence between conversion and temperature), “GC spectra” (should be “gas chromatograms”, since GC is not a spectral method), “example of the gas chromatograph”, “Figure 3. Gas chromatograph” (the figure shows not a device, but a chromatogram) and many others.

- “…Figure S12 shows the changes in the viscosity of the oil after hydrogenation”. In fact, this figure demonstrates a dependence between conversion and temperature.

- Figure S8 is not described in the text at all.

- “Other mass fragments are shown in Figure S15” (line 245). Figure S15 shows NMR spectra, not mass spectra.

- “…round bottom flask (Figure S18)” (line 268). Figure S18 shows a mass spectrum, not a flask.

3. Other serious mistakes that cause confusion and raises doubt about the reliability of the results:

- Lines 164-167: “Table 2 entries 1-3 show the catalytic performance of the imine catalysts in the transfer hydrogenation of methyl linoleate (ML) [C18:2], where the observed conversions are 97%, 94% and 96%, respectively, for the C1, C2, and C3 catalyst, with 100% selectivity towards reduction of the C=C bonds”. According to Table 2, the conversion in all cases (except entry 7) is complete, since the content of the C18:2 component is zero. In this regard, the data in Table 5 (entries 9-13) are incorrect (if we believe the data in Table 2), including the selectivity values, which must be checked and adjusted.

- In some places it is stated that the hydrogenation conditions were optimized (for example, in the Abstract, captions to Tables 2 and 3). However, nowhere in the text are there results of experiments on optimization of the hydrogenation conditions (except Fig. S12), which should include varying not only the temperature, but also the nature of the solvent and hydrogenating agent, reaction time, the catalyst-to-substrate ratio, etc.

- The experimental section does not contain a description of the methods for catalyst characterization (NMR, ESI-MS, IR spectroscopy, elemental analysis) and for analysis of hydrogenation products (NMR, IR spectroscopy, GC, GC-MS), which is necessary to enable the results to be reproduced by anyone interested.

Comments on the Quality of English Language

 Extensive editing of English language required.

Author Response

Response to comments and queries by Reviewer 1

Comment 1: In the “Results and discussion” section, an analysis of the obtained results should be carried out, and not just a description (in many cases there is no answer to the question “why?”). This concerns, in particular, the reasons for the differences in catalyst activity, i.e. the authors do not analyze the reasons for the influence of the catalyst composition on their activity, but only states such an influence. What happens to the catalysts during the reaction? Do they destruct to metals and free ligands? These are very important questions, especially from a practical point of view, but there are no explanations in the text.

Response: An explanation on the plausible reasons for the differences in catalytic activity between the catalysts, and electronic effects are included on page 6, lines 164-182 of the revised manuscript.

Comment 2: From Table 3 it is clear that the content of the C16:0 component after 3 hours of hydrogenation (entry 7) is almost 2 times higher than after 6 hours of hydrogenation (entry 6). How can this be explained?

Response: Thank you for your observation regarding the C16:0 content in Table 3. Given that C16:0 is a fully saturated fatty acid, it is chemically stable and cannot be converted further during hydrogenation reactions. It is indeed rather odd that the composition of C16:0 appears to have increased for a component of the product that is saturated, which is likely due to an unknown impurity that may have been in the sample analyzed. All other peak compositions at reaction time 3 h are accurate. This explanation has been included as a footnote in Table 3.

Comment 3: At the end of the “Results and discussion” section (pp. 13-14), the authors paid some attention to the comparison of the hydrogenation results with literature data. However, this comparison is unexpectedly presented in a subsection devoted to the study of oxidative stability.

Response: The comparison and Table 5 have been moved from section 2.2.4 to section 2.2.3 as suggested.

Comment 4: The text is written very carelessly, there are many various mistakes, including grammatical and terminological ones. As noted above, this makes it difficult to understand and raises doubts about the reliability of the results.

Response: Thank you for for highlighting the issues with the text. We apologize for the careless errors, including the grammatical and terminological mistakes. We hve now carefully proof-read the revised manuscript and corrections and edited texts are highlighted throughout the revised manuscript.

Comment 5: Extremely low quality of figures and schemes (low resolution), low level of their design, carelessness. Mistakes in the numbering of sections and subsections. - Mistakes in the numbering of schemes.- There is a caption to scheme 2, but there is no scheme itself.

Response: We sincerely apologize for the issues you identified with the quality and design of the figures and schemes, as well as the errors in the numbering of sections, sub-sections, and schemes. Scheme 1 has been separated into two on page 3 and is now captioned as Scheme 1a and Scheme 1b (page 3). Scheme 2 has been included with its appropriate caption.

Comment 6: Examples of some strange mistakes in the text: “photograph in Fig. S12” (Fig. S12 shows not a photograph, but a dependence between conversion and temperature), “GC spectra” (should be “gas chromatograms”, since GC is not a spectral method), “example of the gas chromatograph”, “Figure 3. Gas chromatograph” (the figure shows not a device, but a chromatogram) and many others. “…Figure S12 shows the changes in the viscosity of the oil after hydrogenation”. In fact, this figure demonstrates a dependence between conversion and temperature. Figure S8 is not described in the text at all. “Other mass fragments are shown in Figure S15” (line 245). Figure S15 shows NMR spectra, not mass spectra. “…round bottom flask (Figure S18)” (line 268). Figure S18 shows a mass spectrum, not a flask.

Response: Fig. S10 has now been included, which is a picture of the oil before and after hydrogenation. The word chromatograph has been changed to chromatogram, and for all the GC images, the word spectra has been changed to chromatogram. Original Figure S8 showed a cyclic voltammetry study, but the analysis was removed from the paper as it was not relevant for the applications of the synthesized complexes. Figure S8 has now been removed from the supporting information and the ESI and the main text have been aligned.

Comment 7: Other serious mistakes that cause confusion and raises doubt about the reliability of the results: - Lines 164-167: “Table 2 entries 1-3 show the catalytic performance of the imine catalysts in the transfer hydrogenation of methyl linoleate (ML) [C18:2], where the observed conversions are 97%, 94% and 96%, respectively, for the C1, C2, and C3 catalyst, with 100% selectivity towards reduction of the C=C bonds”. According to Table 2, the conversion in all cases (except entry 7) is complete, since the content of the C18:2 component is zero. In this regard, the data in Table 5 (entries 9-13) are incorrect (if we believe the data in Table 2), including the selectivity values, which must be checked and adjusted.

Response: For clarity, the results are discussed based on the percentage of the C18 components after the reaction, in Table 2 and Table 5, instead of the % conversion.

Comment 8: In some places it is stated that the hydrogenation conditions were optimized (for example, in the Abstract, captions to Tables 2 and 3). However, nowhere in the text are there results of experiments on optimization of the hydrogenation conditions (except Fig. S12), which should include varying not only the temperature, but also the nature of the solvent and hydrogenating agent, reaction time, the catalyst-to-substrate ratio, etc.

Response: We thank the reviewer for drawing attention to this issue. We have included a statement on page 5, lines 160-162, indicating that the best conditions for C1 and C2 were established for a similar hydrogenation reaction in a previous report by us where variations of catalyst loading, solvent and temperatures gave us the conditions used for the hydrogenation reactions in this study. We trust that this explanation addresses the query by the Reviewer.

Comment 9: - The experimental section does not contain a description of the methods for catalyst characterization (NMR, ESI-MS, IR spectroscopy, elemental analysis) and for analysis of hydrogenation products (NMR, IR spectroscopy, GC, GC-MS), which is necessary to enable the results to be reproduced by anyone interested.

Response: Where necessary, the characterisation methods and sample preparation of the catalysts, and analysis of the reaction mixture is now included in a paragraph in the supplementary material, on pages 15 and 16.

Reviewer 2 Report

Comments and Suggestions for Authors

This work presents a strategy to prepare metal-based homogenous catalysts for the hydrogenation of C=C bonds in several fatty acids.  It has also reported the possibility of kinetically controlling the reaction conditions for partial hydrogenation.   The systematic design of metal-based catalysts and establishing optimal hydrogenation reaction conditions could provide interesting aspects to the organometallic chemistry and materials science communities.  However, a few important pieces of information listed below should be addressed in the manuscript.

For a clearer presentation, labeling the type of catalyst in Table 2 and Table 3 would be helpful (e.g., C1-Co, C2-Ni, etc.).  Based on the results presented in Table 2 and Table 3, it would be informative if the authors could explain the metal-dependent reactivity (e.g., instead of simply explaining the experimental observations, the authors could speculate why C1 (Co) and C2 (NI) gave lower percentage of C18:0 compared to amide catalysts C4 (Co) and C5 (Ni), and imine catalyst C3 (Pd).  Please also check line 176: “The Pd catalyst (which one?) shows high activity…”

 The quality of some figures should be improved (Figure 2 and Figure 3).

 Please enlarge IR graph in Figure S3 and Figure S6 and label vibrational groups and wavenumbers for clear presentation.

 Including schematic drawings could great help understand the synthetic process in the 4.2 section.

 Characterization methods should be clearly presented (GC, MS, CV, NMR, etc).  Example: GC characterization conditions including the type of column, pressure, oven temperature, etc.  It is also recommended to explain how samples were treated/prepared for these analyses.

Comments on the Quality of English Language

There are no major concerns with English.

Author Response

Response to comments and queries by Reviewer 2

Comment 1: For a clearer presentation, labelling the type of catalyst in Table 2 and Table 3 would be helpful (e.g., C1-Co, C2-Ni, etc.).  

Response: As suggested by the Reviewer, the catalysts C1, C2, C3, C4, C5 and C6 have been renamed to C1-Co, C2-Ni, C3-Pd, C4-Co, C5-Ni and C6-Pd, respectively.

Comment 2: Based on the results presented in Table 2 and Table 3, it would be informative if the authors could explain the metal-dependent reactivity (e.g., instead of simply explaining the experimental observations, the authors could speculate why C1 (Co) and C2 (NI) gave lower percentage of C18:0 compared to amide catalysts C4 (Co) and C5 (Ni), and imine catalyst C3 (Pd).  Please also check line 176: “The Pd catalyst (which one?) shows high activity…”

Response: An explanation on the possible causes of the differences in catalytic activity between the catalysts (metal-dependent reactivity) and electronic effects are now included on page 6, lines 164-182, of the revised manuscript.

 Comment 3: The quality of some figures should be improved (Figure 2 and Figure 3).

Response: Thank you for suggesting that the quality of Figures 2 and 3 should be improved. We recognize the importance of high-quality figures and Figures 2 and 3 have now been reproduced using Mestrenova and Chem draw software respectively, shown on page 8 of the revised manuscript.

 Comment 4: Please enlarge IR graph in Figure S3 and Figure S6 and label vibrational groups and wavenumbers for clear presentation.

Response: The IR images have been enlarged and relabelled to include the wavenumbers of the expected functional groups. For the IR of complex C5, the functional groups are appended to the caption of Figure S6b.

 Comment 5: Including schematic drawings could great help understand the synthetic process in the 4.2 section.

Response: The schematic diagrams for the synthesis procedure in section 4.2 is outlined on page 3, labelled Schemes 1a and 1b.

 Comment 6: Characterization methods should be clearly presented (GC, MS, CV, NMR, etc).  Example: GC characterization conditions including the type of column, pressure, oven temperature, etc.  It is also recommended to explain how samples were treated/prepared for these analyses.

Response: The experimental procedure and sample analysis for all the characterisation techniques and sample preparation is outlined on page 16 of the Supporting Information.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have significantly improved the article. The level of presentation of results and discussion has become significantly higher compared to the previous version. Now, in addition to the description, explanations of the data obtained are also provided. The authors have answered all the questions and comments that were raised in the previous review. However, before publishing, some remaining shortcomings need to be corrected.

1. Unfortunately, the authors did not provide an analysis of what happens to the catalysts during the reaction. Do they destruct to metals and free ligands? I repeat, this is a very important question, especially from a practical point of view.

2. Table 2. The reaction temperature should be given in the footnote.

3. The data in Table 2 do not correspond to those in Table 5. The authors indicated in their response that the results are now given as “the percentage of the C18 components after the reaction, in Table 2 and Table 5, instead of the % conversion”. In Table 2, we see that the balance of the components matches. For example, the C1-Co catalyst gives 86% of C18:0, 14% of C18:1, and 0% of C18:2 (i.e., complete conversion of C18:2). However, in Table 5, for the same catalyst, the authors provide the selectivities: 100% of C18:0, 0% of C18:1, and 85.6% of C18:2. Firstly, the sum of the selectivities should be 100%. C18:2 is the starting compound, and the selectivities should be given only for the reaction products. The selectivity for C18:0 cannot be 100%, since, according to the data of Table 2, it is 86% (at full conversion, the selectivity corresponds to the percentage of the product in the mixture). All this applies to other catalysts as well.

Thus, Table 5 should be corrected accordingly, and to avoid confusion, I recommend that the authors provide in the experimental section the formulas for calculating all the catalytic parameters discussed (percentage content, selectivity, conversion).

4. In the Supplementary materials (page 14), the authors descibe a method of cyclic voltammetry. The results of this method are not presented or analyzed in the work. Therefore, this description of cyclic voltammetry should be removed.

Comments on the Quality of English Language

I recommend minor editing of English language.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf