Simple Fabrication of Hydrophobicity-Controlled Fe-ZSM-5 for Aqueous-Phase Partial Oxidation of Methane with Hydrogen Peroxide

Round 1

Reviewer 1 Report

In this study, the authors synthesized carbon-coated ZSM-5 and Fe-ZSM-5 materials with varying carbon components to modulate surface hydrophobicity. Subsequently, they characterized these materials and assessed their efficacy for methane oxidation with H2O2.

The authors attributed the increased reactivity of carbon-containing materials to enhanced surface hydrophobicity, a claim with which I disagree. Notably, the C-Fe-ZSM-5 material with a carbon content of 0.62% exhibited the highest product yield, nearly doubling that of Fe-ZSM-5. However, the water contact angle of C0.62-Fe-ZSM-5 was very similar to that of Fe-ZSM-5 (18.1 vs. 17.2°). Similarly, the C-ZSM-5 material with a carbon content of 0.3% showed approximately 1.5 times higher product yield compared to H-ZSM-5, despite similar water contact angles (11.6 vs. 11.8°). Thus, surface hydrophobicity may not be the primary factor influencing catalytic performance.

Additional comments:

1.     The font size in lines 96-110 appears different from the rest of the manuscript.

2.     It is suggested that the authors provide pore size distribution profiles to offer clearer evidence of how the carbon coating affects pore size.

Based on the above, I am unable to recommend this manuscript for publication in Catalysts.

Author Response

Thank you for your kind and valuable comments. Please see the attached file in which we tried to answer all questions raised by a reviewer.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript, authors modified the surface hydrophobicity of Fe-ZSM-5 via polycondensation of furfural alcohol, giving higher yields of oxygenates. 

However, I remained several concerns of this work:

Q1. The catalytic yields from H-ZSM-5 and Fe-ZSM-5 were shown in Figure 4 and Figure 5, how much difference of activity between the parent H-ZSM-5 and parent Fe-ZSM-5? 

The y-axis is not clearly shown to compare the activities of two catalysts. Scales of Figure 4 and 5 should be identical or similar.

Since the conversion of H2O2 are not identical, authors may calculate TOF or productivity of two catalysts to make fair comparison. 

As authors claim the isolated Fe sites as active sites, why parent H-ZSM-5 looks to provide similar yields or selectivity of HCOOH?

Q2. In table 1, authors may also show external surface area, which may be more straightforward to show the FA deposited/reacted on acid sites on external surface and block the pores. 

Q3. The FA looks possible to squeeze into micropores. From table 1, the micropore volume decreases obviously even adding small amount of FA. In contrast, the surface area dramatically decreases when adding higher amount of FA. Such findings probably indicate that FA reacted/adsorbed on the strong acid site inside pores firstly and then deposited on the external. Did authors test other larger or longer chain coating carbon precursors? 

Q4. Both isolated Fe sites and FeOx small clusters could be formed on acid sites on external surface, affecting the coating of carbon. Did authors test other loading of Fe? 

Q5. Since the coated material were calcined at high temperature under inert gas, characterization such as FTIR, NMR, TGA, TEM or other approaches are needed to confirm the existence of carbon of coated materials.

Q6. As authors present a new simple coating method to synthesize the materials, more details of preparation should be provided. For instance, "a certain amount of FA" means how many FA was added? A typical process should be written as an example.

Some sentences are confusing. For example, in line 83-89, it may be better to rephrase.
Typos and formate need to be corrected. For example, the paragraph in line 96-110. The font looked different with other paragraph. 

Author Response

Thank you for your kind and valuable comments. Please see the attached file in which we tried to answer all questions raised by a reviewer.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper by Hwang et al. presents a study on aqueous-phase partial oxidation of methane. The effect of surface hydrophobicity on the partial oxidation of methane was analyzed. Although this article provides detailed catalyst characterization results and performance studies, I still have some confusion about catalysts.

1. As mentioned in the title, the focus of this study is on Fe-ZSM-5. So the authors need to explain why Fe-ZSM-5 performs better than H-ZSM-5.

2. Due to the very small amount of carbon deposition, the authors need to explain whether the hydrophobicity of the catalyst originates from the deposited carbon or the ZSM-5 after surface passivation by the deposited carbon.

3. As shown in figure 6, the recycling test with Fe-ZSM-5 and C1.08-Fe-ZSM-5 exhibits deactivation phenomenon even at very low reaction temperatures. The author needs to explain the reasons for the instability of the catalyst after only three uses.

This article needs major revisions before being accepted.

The paper by Hwang et al. presents a study on aqueous-phase partial oxidation of methane. The effect of surface hydrophobicity on the partial oxidation of methane was analyzed. Although this article provides detailed catalyst characterization results and performance studies, I still have some confusion about catalysts.

1. As mentioned in the title, the focus of this study is on Fe-ZSM-5. So the authors need to explain why Fe-ZSM-5 performs better than H-ZSM-5.

2. Due to the very small amount of carbon deposition, the authors need to explain whether the hydrophobicity of the catalyst originates from the deposited carbon or the ZSM-5 after surface passivation by the deposited carbon.

3. As shown in figure 6, the recycling test with Fe-ZSM-5 and C1.08-Fe-ZSM-5 exhibits deactivation phenomenon even at very low reaction temperatures. The author needs to explain the reasons for the instability of the catalyst after only three uses.

This article needs major revisions before being accepted.

Author Response

Thank you for your kind and valuable comments. Please see the attached file in which we tried to answer all questions raised by a reviewer.

Author Response File: Author Response.pdf

Reviewer 4 Report

In this manuscript, authors reported some interesting results on the preparation of carbon-coated H-ZSM-5 and Fe-ZSM-5 catalysts with enhanced hydrophobicity for the application in aqueous-phase oxidation of methane with H2O2. The whole manuscript is well organized, and I recommend it can be accepted for publication after minor revision.

Comments and questions:

1)  Authors should provide some information on describing the chemical state of the carbon species. 

2) How about the stability and recyclability of the carbon-coated catalysts?

The English expression of the manuscript is fine.

Author Response

We appreciate the reviewer's kind and valuable comments. We have tried to revise the manuscript based on the reviewer's comments. Please see the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I appreciate the authors' response trying to address my concern and for including the error bars for the contact angle measurement. The inclusion of error bars reinforces that the difference in contact angle measurements between C0.62-Fe-ZSM-5 and Fe-ZSM-5 (and between C0.30-H-ZSM-5 and H-ZSM-5) falls within the error bar. However, it's clearly that their catalytic performances exhibit significant discrepancies, with a difference of approximately 1.5-2 times. This further supports my concern that surface hydrophobicity may not be the primary determinant influencing catalytic performance. catalytic performance. Therefore, I cannot agree the publication of the current manuscript.

Author Response

We appreciate the reviewer's kind and valuable comments. Please see the attached file in which we tried to respond the reviewer's comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors provided more information and solved parts of questions. The promotion of carbon coating is confirmed through authors' work. 

What I am concerned is the statement authors claimed methane oxidation via Fe-ZSM-5 provide more yields of oxygenates products.

If compare Figure 4 and Figure 5, Fe-ZSM-5 produced ~300 umol products while H-ZSM5 produced ~400 umol total products. For example, C0.45-H-ZSM-5 produced >600 umol while C0.62-Fe-ZSM-5 produced similarly >600 umol products.

I am worried the results authors present in Figure 4 and Figure are not convinced to indicate the higher activity. As authors mentioned their previous work, how result in this work compared with previous work? A fair comparison such as TOF or productivity should be provided or cited.

In addition, why is the yield/consumed H2O2 via Fe-ZSM-5 is generally lower than H-ZSM-5?

Language is acceptable.

Author Response

We appreciate the reviewer's kind and valuable comments. Please see the attached file in which we tried to respond the reviewer's comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have correctly responded the reviewer's questions one by one, and the revised manuscript, I think, can be accepted in this journal.

Author Response

We appreciate the reviewer's kind comments. We are not responding to this reviewer's comments as they have not requested any further corrections.

Round 3

Reviewer 2 Report

New data authors provided are convinced. Please check typos. 

It is okay.

Author Response

We appreciate the reviewer's kind and valuable comments. We have tried to revise the manuscript based on the reviewer's comments. Please see the attached file.

Author Response File: Author Response.pdf