Direct Dimethyl Carbonates Synthesis over CeO₂ and Evaluation of Catalyst Morphology Role in Catalytic Performance

Round 1

Reviewer 1 Report

To Authors

General comments

The reviewed manuscript deals with the synthesis of ceria catalysts featuring different grain morphology. The ultimate materials synthesized via simple method of precipitation of cerium hydroxide by the reaction of cerium chloride with urea (varying in the precipitation duration) followed by annealing at 500 °C towards cerium oxide with different particle shapes. The calcined materials were in-depth characterized using well-chosen techniques. The materials were tested in the synthesis of dimethyl carbonate (DMC) by the reaction of CO2 and MeOH.

It was shown that depending on the morphology of ceria grains, the catalytic activity in the reaction of synthesis of DMC varies significantly. The best catalytic performance was observed for the spindle-shaped CeO2 thank to the textural parameters (highest specific surface) and the crystallographic features, namely the exposition of the (1 1 1) crystal plane, which is believed to be responsible for the attraction of the reactants. Moreover, the influence of the type of dehydrating agent used in the catalytic reaction was also studied. Finally, the likely mechanism of DMC synthesis over CeO2 was proposed.

The topic of this work may be interesting for the community of researchers focused on the development of advanced catalytic materials.

I find this work interesting, the main body of the text is in general well-elaborated. The layout of the manuscript is clear. The manuscript poses to be a logical whole. With this, it may be good reference for the researchers focused on synthesis of new catalytic materials. However, there are some missing/incorrect elements to be corrected/supplemented/addressed. As the subject of the work is in line with the scope of the journal, it deserves to be published in the Catalysts journal after moderate/major corrections as described in details below.

Substantive aspects

1. Abstract: “Here we report a 15 production of DMC from highly toxic greenhouse gases, CO2 using”. In fact, CO2 is non-toxic gas, it is only suffocating.
2. Experimental: “nitrogen adsorption-desorption method using BET method”. BET is only MODEL of calculation, not a method.
3. Details on the measurements should be provided in the Experimental section (N2 – outgassing parameters, models of pore size/volume calculations).
4. Synthesis of catalyst section: „The ultrafine soni-114 cation applied to reaction mixture tomake it uniform and heated in heating oven to 120 oC 115 for 3 h.” but next, the authors state that „Depending on their synthesis time (heating in an oven) i.e. 8 h, 10 h, 12 119 h and 16 h, CeO2 samples”. I think, this should be clarified – probably, the heating times were different, not each time 3 h?
5. 3. In the cases of such poor porous samples, the results of nitrogen adsorption (isotherms) should be discussed carefully. I mean, in case of such poor nitrogen uptakes, the sorption measurements may be burdened with large inaccuracies. Hence, the discussion of isotherm's shapes and pore size distribution curves seem to be a bit exaggerated in my opinion.
6. In Fig. 3. Caption, the BJH model is mentioned, while in the text, the NLDFT model (being in fact closer to the reality) is discussed. Which one was employed for these calculations?
7. Table 1. Surface areas should be displayed without decimal places; pore volumes at most to the second decimal, while pore size up to one decimal place. It is senseless to display more in view of the accuracy of the measurement.
8. The IR regions are in fact the same for all samples, so it should be removed from Table 1.
9. What was the TON unit? For clarity, it should be mentioned.
10. The catalytic run with Molecular sieve 4A desiccant was quite interesting. Maybe it would be expedient to perform, let say, „blank” test over Molecular sieve 4A without ceria catalyst? I am afraid this Molecular sieve 4A may play as self-contained catalyst in this reaction.
11. Please, discuss the issue of CO2 TPD in view of FT-IR results. Is it not interesting that although the ceria catalysts feature desorption of CO2 in the TPD results, the same samples do not feature the characteristic CO2 bands in IR spectra?
12. 8. What is the fate of the hydrogen atom (this one from this one OH group, which forms ultimately C=O group) just before the formation of the ultimate DMC molecule?

Other details

1. In general, there are numerous grammar problems. Some phrases needs verbs! Numerous typos, lacking spacebars. Please, read the text again carefully.
2. 1. Caption: “and CeO2 nanorods (J-H)” there is no J part.
3. Please, use uniform temperature scale (Celsius of Kelvin, consequently).
4. 9. Typo in the “spheres” word in the figure.

Author Response

Please see the attachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work, Tamboli et al. investigated the effect of catalyst morphology on the DMC synthesis reaction over CeO₂ catalysts and demonstrated that the spindle shaped CeO₂ catalyst performs better than the sphere and nanorod shaped CeO₂ catalysts.  In general, this work is of interest to the audience of the Catalysts. However, there are some major concerns that need to be addressed before acceptance.

1. Significant grammatical errors occur nearly throughout the entire manuscript.
   1. The passive voice was not correctly used in numerous places, to name a few, line 92-102, line 115, line 125-132, line 337-339, line 361-362, line 378, line 387-388, line 400, line 404-405…
   2. Line 74-75, Line 84-89: these are not completed sentences.
2. Figure 1: Clarifying the names of different CeO₂ catalysts, i.e., 8h-CeO₂, 10h-CeO₂, 12h-CeO₂, and 16h-CeO₂, as declared in section 2.2 , will enable Figure 1 caption clearer.
3. Figure 2: Please add the names of experimental technology, i.e., TEM, HRTEM, and SAED in Figure 2 caption.
4. Figure 8, the proton is not balanced in the proposed reaction mechanisms. Specifically, a proton is missing in both the following two reactions: CeO₂ + MeOH -> CeO(OME) + H₂O and OCeOC(OMe)₂(OH) -> O(OMe)₂ + CeO
5. Marciniak et al. (Marciniak, Molecular Catalysis 493, 2020, 111053) and Liu et al. (Liu ACS Catalysis 2018, 8, 10446-10456) have demonstrated that the effect of surface oxygen vacancy on DMC synthesis reaction. I would recommend the authors to revise their proposed reaction mechanism in Figure 8 after reviewing more literature.

Author Response

Please See the Attachmet. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

To Authors

The changes introduced in the manuscript are satisfactory. The author’s responses displayed in the cover letter clarified my doubts. Thus, I perceive this version of the article as deserving for publication in the Catalysts journal.

Author Response

Response to the Reviewers

         We would like to thank you for your kind and great efforts in reviewing our manuscript. Please see the for our responses on your comments.

General comments

<Comment 1>

The changes introduced in the manuscript are satisfactory. The author’s responses displayed in the cover letter clarified my doubts. Thus, I perceive this version of the article as deserving for publication in the Catalysts journal.

<Response>

Thank you very much for recommending our manuscript for the esteem journal "Catalysis"

Reviewer 2 Report

My 1^st comment “Significant grammatical errors occur nearly throughout the entire manuscript” was not addressed at all. The authors have not correctly used the passive voice in this manuscript. For example, new line 94-103 (original line 92-102): “obtained” should be “were obtained”. Similarly, “purchased” --> “were purchased”, “utilized”--> “were utilized”, “investigated”--> “were investigated”. Please refer to my original comment #1 to fix all these.

“The passive voice was not correctly used in numerous places, to name a few, line 92-102, line 115, line 125-132, line 337-339, line 361-362, line 378, line 387-388, line 400, line 404-405…”

Author Response

Please See the attachment.

Author Response File: Author Response.pdf