Review Reports

Round 1

Reviewer 1 Report

I suggest the following corrections to the authors:

1. Did the use of high temperatures (150 °C) form undesirable by-products? The authors should use milder temperatures (around 70 °C) to avoid the formation of undesirable by-products. In the literature, results are found with high conversions using this type of catalyst with the use of a temperature of 70 °C.

2. The authors should carry out a characterization of the ester (methyl tryptophan) obtained. How to determine the viscosity, density, and water content of the obtained product and compare it with commercial methyl tryptophan.

3. Authors should cite similar works from the literature in section "3.7. Catalytic Performance". They should compare the results of this study with other studies in the literature.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Interesting manuscript on S₂O₈^2-/CeO₂ solid superacid catalyst prepared by RF plasma-assisted hydrothermal method. The manuscript evaluates a unique RF plasma-based approach to synthesizing CeO2-based solid superacid and further tests it in the production of tryptophan methyl ester. The data provided is good, however, it needs to be revised before this manuscript can become suitable for publication. My questions/suggestions are as below:

My questions/suggestions:

1.  In line 79, a word is missing ‘catalytic activity … solid superacid’. For e.g., ‘of’.

2.     In section 2.3, authors should describe what mode of FTIR was used: ATR/Transmission/DRIFTS.

3.     Equation 1 uses the Greek term ‘ν’ whereas line 154 states ‘V’ for volume. Make the formula consistent like in the text.

4.     Figure 3, make the TEM picture sizes uniform for all (a to h). Currently, some are bigger, some are smaller.

5.     Even though in section 3.3, based on O=S=O and S-O vibrations authors confirm superacid structure formation, can authors use any other techniques to support it? For e.g., Hammett indicators (as mentioned in lines 24, 25).

6.     Also, often in literature, for confirming superacidity, a test reaction such as ‘n-butane/n-pentane isomerization’ is performed. Have the authors performed any such reaction?

 

7.     In section 3.4, authors use pyridine FTIR and calculate the total acid site density to find SC-RF-600 to have the highest. However, increasing the total acid site density does not necessarily constitute the formation of superacid sites. Unless the peaks are shifted to higher wavenumbers (stronger pyridine adsorption), it is difficult to claim superacidity. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

See the attached file.

Comments for author File: Comments.pdf

English is fine. Just some minor aspects to be corrected.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The following revisions are proposed before the publication in this journal:
1. Introduction: the synthesized catalysts S2O28-/CeO2 were tested in the esterification reaction of tryptophan. The authors explain more in detail the choice of this application, the system currently used as catalysts in this reaction and the advantages of their use. In addition, it would also be useful to insert a diagram of the reaction.

2. Caracterizzation and testing: What is the advantage of synthesizing S2O28-/CeO2 catalysts with plasma compared to a simple impregnation? It would be necessary to make a comparison in terms of XRD, Py-IR and reactivity compared to the synthesized S2O28-/CeO2 without the use of plasma.

3. These catalysts require high temperature and pressure conditions to be used (150 C and 1 MPa). Why should these catalysts be used compared to the other systems?

4. Figure 8: It is necessary to insert the reactivity profile in absence of the catalyst. What is the reaction yield in absence of the catalyst?

 

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

1.      It’s not usual to use “IR, and so forth” in the abstract section. Please rephrase.

2.      The manuscript still lacks some clarity: see page 2, lines 83-84: “Finally, a series of properties such as pore structure, surface morphology, crystal phase and surface acidity of the catalyst were evaluated the of characteristics of SC-RF-600”.

3.      According to authors response, Figure 2 should present some particle size statistical data, but it contains currently contains Hammett titration of SC RF 600: (a) before titration, (b). Please correct the comment to be taken into account.

4.      The results provided in Figure R2 and Table R1 should be presented in the manuscript or in the Supplementary file, along with a proper comment and justification of the results.

5.      I agree that the BET is consistent with TEM analyses. But according to some references  [1], there is not quite a common practice to analyse the materials just once. In the presented reference, the error bars of BET analysis are presented. Please find some other argument.

6.      According to authors cover letter:  „see the reaction system where tryptophan was completely converted at 3 h in Figure S2”. Unfortunately, Figure S2 presents just a picture with a flask containing some liquid. No information about a conversion of 99 % is provided.

7.      The authors are stating (page 10, lines 281-291that „because both substrate and product were destroyed under the long time and strong acid environment”. Given this argument, the synthesized material can not be used as a catalyst in industrial conditions, as one of the main characteristics of a catalyst should be the mechanical resistance in the reaction environment. Please justify!

8.      Given that the reaction mixture is composed from sevaral high molecular weight compounds, is the rotation speed of 500 rpm in the reactor enough to avoid the external diffusion influence on the reaction rate? Please bring more arguments.

9.      According to relation (2) the yield is expressed in weight %, and not mol %.

10.  The authors are stating that the reaction mixture analysis was carried out by HPLC. Some information about the calibration of the instrument should be provide. A typical chromatogram should be presented at least in Supplementary file.

11.  The authors are giving the GC-MS chromatogram in Figure S1, but no information about the instrument and method  to get it is provided. Please ammend the document.

 

References

 [1] D.P. Lapham, J.L. Lapham, BET surface area measurement of commercial magnesium stearate by krypton adsorption in preference to nitrogen adsorption, International Journal of Pharmaceutics 568 (2019) 118522.

Please recheck the manuscript for some small issues.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The authors have responded comprehensively to the reviewers' comments, so publication is recommended.

Author Response

Thanks so much to the reviewer for the endeavors to review the manuscript again.