Identifying Monomeric Fe Species for Efficient Direct Methane Oxidation to C₁ Oxygenates with H₂O₂ over Fe/MOR Catalysts

Round 1

Reviewer 1 Report

Perfect description of the nature of iron in Fe/MOR is presented, with every detail both in the paper and in the supporting information. 

 

However, due to the insoluble nature of the material, recycling studies should be carried out. Besides, the heterogenous nature of the system needs to be confirmed by hot-filtration experiments to detect possible metal leaching, ICP analysis of reaction liqours are required. Also, some chromatograms of the reaction tested have to be introduce in the supplementary information.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. A modified liquid ion exchange method was used to control Fe loading in MOR-type zeolite catalyst using H₂O₂ as the oxidant for direct methane oxidation to C₁ The results of turnover frequencies (TOFs), yield of products, UV-vis DRS, H₂O₂ conversion (%), and transmission electron microscopy (TEM), etc. after the Fe loading by modified liquid ion exchange method (mIE) were compared with those by sIE, SSIE and IWI. The experimental results of this study might provide helpful references for preparing C₁ oxygenates from direct CH₄ oxidation. A few modifications are suggested before further being consideration for possible publication in this journal.
2. Whole words should appear when their corresponding abbreviations first present, for example: MOR, TOFs, sMMO, DFT, MFI, HPLC, GC-FID, etc.
3. A few sentences are totally repeated, for example: the sentences at lines 83-87 are wholly repeated with those sentences at lines 92-97. Adequate modification for the latter repeated sentences should be made.
4. Terminology expression should be consistent, for example: X-ray absorption spectroscopy (XAS) at line 16 is different from X-ray absorption spectra (XAS) at lines 250 and 509.

Author Response

Response: We would like to thank the reviewer for his/her kind evaluation, positive comments and constructive suggestions. We all correted it in our manuscript according to your suggestions.  The point-to-point responses are listed below:

Comment 1: “Whole words should appear when their corresponding abbreviations first present, for example: MOR, TOFs, sMMO, DFT, MFI, HPLC, GC-FID, etc..”

Response: We checked the whole manuscript and corrected it.

Comment 2: “A few sentences are totally repeated, for example: the sentences at lines 83-87 are wholly repeated with those sentences at lines 92-97. Adequate modification for the latter repeated sentences should be made.”

Response: We corrected it according to your suggestion. Please see Line 112-113.

Comment 3: “Terminology expression should be consistent, for example: X-ray absorption spectroscopy (XAS) at line 16 is different from X-ray absorption spectra (XAS) at lines 250 and 509.

Response: We corrected it according to your suggestion.

Reviewer 3 Report

Major comments:

The study comprised of “Identifying monomeric Fe species for efficient direct methane oxidation to C1 oxygenates with H2O2 over Fe/MOR catalysts.” The authors have investigated a modified liquid ion exchange method to achieve better control of Fe loading in MOR-type zeolite. The optimized Fe/MOR catalyst possesses superior performance of direct methane oxidation with TOFs up to 555 h-1 to C1 oxygenates. Though the study contains a lot of results but does contain some major loopholes. Insufficient literature was reviewed, which can eventually lead to lack of novelty. There is no comparison of the results of the present study with the existing literature that makes the results unauthentic and unique. The results shown in this study are lack of innovation and novelty. One of the major flaws is that the experimental design and results are rather simple, making the manuscript lack of strong elucidation to support their novelty. The literature review is not strong enough to provide research gaps. The discussion on the different parameters was so intermixed that it was confusing at many points. There is no clear understanding of what the authors want to elaborate in terms of different parameters. Furthermore, the English language used needs major improvements as there are many punctuation and grammatical mistakes present throughout the manuscript. Sentences need more clarity, better construction and should be written in a symmetrical way. It is obvious the quality of the manuscript does not meet the standards of the Methane Journal in its present form, therefore, needs major revisions or should be rejected in its present form.

Introduction:

The introduction is lack of sufficient background information, which is unable to give the reader detailed background knowledge and possible wide application of this study. The introduction needs to be more emphasized on the research work with a detailed explanation of the whole process considering past, present and future scope. The conventional methods to be explained well to indicate the relevance of the research work. It needs to be strengthened in terms of recent research and updated literature review in this area with possible research gaps. It is strongly recommended to add a recent literature survey about different types of novel materials, catalysts, methods and techniques that are used in order to study efficient direct oxidation. Research gaps should be highlighted more clearly and future applications of this study should be added.

Specific comments:

1. Abstract: Though this section contains detailed findings, it lacks major parameters such as the background information is not given as well as the novelty of the study is missing. Furthermore, future applications need to be addressed. These points need to be catered to make it a standard abstract. All the objectives of the study must be mentioned in the beginning. Also, the novelty statement must be given in the abstract rather under a separate heading and should clearly describe its uniqueness. Therefore, achieving suggested highlighting the novelty of this work clearly in the abstract to make it a standard abstract.
2. More specific keywords can be added such as zeolite, catalyst.
3. Page 1, Introduction: “Exploiting an efficient way to convert methane to methanol and other oxygenates, 28 which is transportable, storable, and highly valuable in the chemical market, would be a 29 promising strategy, compared to flaring. The main obstacle to achieve applicable trans- 30 formation yet lies in the difficulty in activating the robust C-H bond (440 kJ mol−1).” Is there any recent research that could be used as more relevant? It is suggested to add more recent literature to these important claims.
4. Page 2, Introduction: “ In 2012, Hammond et al. first reported that a H-ZSM-5 catalyant with 0.014 wt% Fe impurity was found to exhibit unique activity with TOFs up to 2200 h-1 towards C1 oxygenates 48 at 50 °C using H2O2 as the oxidant in a batch reactor. “Is there any recent research that could be used as more relevant? It is suggested to add more recent literature to these important claims.
5. Page 2, Materials and methods: “The source and purity of the chemicals mentioned in the text are listed in Table S1. “ Where is that Table S1? Table S1 is not inserted.
6. Lines 82, 91, 99, 107, 113 Materials and methods: “ … a certain amount of Fe salts … “ What is that certain amount of Fe salt? Explain.
7. Page 3, Materials and methods: “For the FeCl3 sublimation method, the catalyst was prepared by chemical vapor 120 depositions using FeCl3·6H2O as the iron precursor? Is there any recent research that contains a modified method for more relevant results?
8. Page 3, Materials and methods: “ Prepared catalysts are denoted as MOR(NH4) (without activation procedure), 131 MOR(NH4)-500, MOR(NH4)-700, and MOR(NH4)-900, respectively according to the calcination temperatures 500 °C, 700 °C and 900 °C.” Why this temperature range? What would happen if higher or lower temperatures than these were used?
9. Page 6, section 2.3 Textural characterizations: Authors have not mentioned what standard methods have been followed, please compare FTIR/SEM/XRD techniques/methodology with the following recent studies: Chemical Engineering Journal, 2014;235:100-108. Industrial & Engineering Chemistry Research, 2020;59(51):22092-106. Journal of Molecular Liquids, 2019;293:111442.
10. Page 4, Materials and methods: “Commercial H-MOR (n-Si/Al = 12, ZEOCAT FM-8/25 H) with only 0.003 wt% Fe impurity was used as support to load different metals to reduce the interference from Fe impurity in NH4-MOR (n-Si/Al = 9, Alfa Aesar).” Whether a higher percentage of Fe impurities can be used and what would be the maximum of impurity?
11. Page 4 Experimental methods: “The gas product was collected by an aluminum gas bag and analyzed by GC-FID”? Can the product be collected in bags made of other materials? Why aluminum bags?
12. Page 5 Experimental methods: “The liquid phase products were analyzed and quantified by 1H NMR on a 400 MHz 199 Bruker AVANCE III NMR spectrometer.”Write the full forms of the abbreviation (NMR) as it is not stated above in the text. Can any other methods and devices be used for this analysis?
13. The discussion presented is very weak no strong comparison has been made with the literature to support the authenticity of the obtained results. Therefore, the authors are suggested to discuss their results with the following recent researches about types of advanced catalysts, nanomaterials, synthesis methods and novel materials to make the background and discussion more strong: Chemosphere, 2021;270:129523. Scientific Reports, 2020;10(1):1-2. Case Studies in Chemical and Environmental Engineering, 2020; 2:100037. Catalysis Today, 2019;335:437-51. 
14. Results and discussions: All the results under this section are required to be clearly stated and discussed with the current research works of other authors. Authors are suggested to discuss the result critically and properly.
15. Figures need to be revised, please make sure these are High Definition Quality.
16. Conclusion: The conclusions only talk about some studied parameters, which is insufficient to depict the whole picture of the contribution of this study. The authors are advised to write the conclusions in a comprehensive way and should contain the novelty and suitability of the applied method.
17. The authors are advised to revise references, including the latest references. Please see some suggestions in the comments for the ‘introduction’ section.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have addressed most of the comments; they have also tried to make changes according to the reviewers’ suggestions. After revisions, the quality of the manuscript has been adequately enhanced. Therefore, the manuscript could be considered for publication in the Journal. However, there are still some editing/ syntax errors present in the manuscript which need to be corrected, hence the publishing team is advised to read the manuscript carefully before publishing.