Liquid-Phase Selective Oxidation of Methane to Methane Oxygenates

Round 1

Reviewer 1 Report

Reviewer’s comments

 Manuscript number: catalysts-2878613

Manuscript title: Liquid-phase selective oxidation of methane into methane oxygenates

The present manuscript includes the critical considerations of several recent advances in the liquid-phase conversion of methane in to valuable oxygen-containing products.  

The review includes the main following parts:

1. Introduction

2. Liquid-phase partial oxidation of methane in strong acids

3. Liquid-phase partial oxidation of methane in water

In Introduction part the authors presented the main processes of methane transformations into different semi-products. On my opinion the representation of the main processes of methane transformation in terms of Free Gibbs Energy gives the cleaner picture of problem in comparison with terms of enthalpy.

Additionally, it is important to add the thermodynamic analysis of the methane transformation processes in liquid phase conditions with using of H2O2, O2 and other oxidizing agents.

Conclusion part (Summary and Outlook) is very short. Unfortunately, the authors did not formulate the base principles, which the best process of POM must be corresponded. Here, on my opinion it is important to use the “green chemistry” view for analysis of the described processes. In the manuscript a lot of attention was paid to the process of POM with using of H2O2 as oxidant. However, in conclusion part the authors forgot to give final analysis of peroxide method with underlying of advantages and disadvantages.

Finally, the analysis of the perspective catalytic systems must be added in Conclusion part (Summary and Outlook).

English may be improved.

Author Response

Dear a reviewer,

Thank you for your kind and valuable comments. Please see the attached file on our responses to your comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript (catalysts-2878613) described the recent advances for liquid phase methane conversion. I think the topic is of good significance and will be interesting to a wide reader. Before the consideration for publications, the following comments and suggestions should be concerned.

Comment 1: Why the review focus on the liquid phase methane conversion? Please make more explanation of the superiority of the liquid phase methane conversion compared to the gas phase methane conversion in the introduction part.

Comment 2: The comparsion of the performance of methane oxidation in acid solvent and water should be offered. Is acid offered a promotion effect for the catalyst?

Comment 3: There are some format mistakes, e.g. Line 185 and many other places. Please carefully check.

Comment 4: The author put emphasis on the metal catalyst, however, the catalyst support may also play important role. The metal oxide such as Co3O4, TiO2 etc. with plenty of oxygen vacancies may facilitate the alkane activation. Please make comments on this. The following manuscript is suggested for the citation: Molecular Catalysis 553 (2024) 113768,  DOI: 10.1021/acscatal.8b03924.

Comment 5: The full name should be provided if the abbreviation is used for the first time, e.g. HTFA etc.

Comment 6: Please unify the format of all the references, since there is deviation for different references, such as the format of journal name.

Comment 7: The auther stated the directe conversion of CH4 to O2 in Line 627, how can this happen?

Comment 8: It is strange to have citation in the summary part. It is suggested to move the citation in the main article. The summary should concisely conclude the whole manuscript and make a prosepect.

Comment 9: What is the opinion of the auther for the practical appliction of liquid phase CH4 conversion in industry？What is the most promising conversion avenue? And what is the main restricting factor?

Comment 10: It is suggested to make more analysis on the reaction mechanism like the Figure 10, therefore it will be more instructive for the understanding of the reaction pathway.

Comment 11: There is lack of the comparison between the different section, e.g. the different oxidant. The author should give the reader clues which is better.

Minor english modification is suggested.

Author Response

Dear a reviewer,

Thank you for your kind and valuable comments. Please see the attached file on our responses to your comments.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors corrected manuscript according to reviewer's comments. I think that the revised manuscript may be recommended for publication in the Catalysts journal. 

Author Response

This reviewer has not asked for any changes to our manuscript, so we do not need to respond to this reviewer.

Reviewer 2 Report

Thank the author for addressing my comments, however, there are still some points that is not satisfied. 

Comment 1: The comparison of liquid phase POM and gas phase POM is not sufficient enough. "the selectivity to the value-added methane oxygenates decreases with increasing methane conversion" may also a problem for liquid phase POM. Please dig more into the supeirority of liquid-phase POM, e.g. the product tunability etc. Meanwhile, to have a fair evaluation, the weakness of liquid-phase POM should also be addressed since the solubility of methane is indeed not high.

Comment 2: The author stated that the reducible metal oxide only play as catalyst support in liquid phase POM. I'm not totally agree, since the liquid phase POM also involves C-H bond activation. The oxygen vacancies may function importantly for the adsorption and activation of methane. Please still refer to the reference: Molecular Catalysis 553 (2024) 113768. It is suggested to make the prospect for the study on the metal oxide as support for enhancing the catalytic performance, which will be important for the following work in this reaction.

The english is ok.

Author Response

Dear a reviewer,

We appreciate your kind comments. Please see the attached file in which we have tried to respond to your comments in detail.

Author Response File: Author Response.pdf