Versatile Polyoxometalates of Different Structural Dimensionalities for Liquid Phase Catalytic Oxidation

Round 1

Reviewer 1 Report

I am sending you the evaluation of the manuscript titled: Versatile polyoxometalates of different structural dimensionalities for liquid phase catalytic oxidation. Manuscript ID: catalysts-2836243

This work can be considered the continuation of others previously published by the group. However, the work reviewed here involves the quite rational synthesis of five new polyoxomolybdates, and a serious comparative study, between them and others previously published, in terms of relationship structure/properties and the behaviour as catalysts in different transformations of interest. These facts manage to achieve interest in the publication of this work.

Although the article is interesting for publication, however, I believe that some small reform and the correction of some errors in the text are necessary.

In this context, my first proposal to the authors would be to review Figures 1, 2 and 3, since part (a) of them (the essential building units with labelling scheme) in my opinion are very little visual if you compare them with how the text describes those figures. The authors could look for another view of these structures that would show more information and a clearer labelling scheme.

Also, in the text I think there are some errors that should be corrected:

- I think line 109 is not necessary, since (a) in my opinion it is an error and should not be written

- On line 386 the quantity 01 mol is an error

- On line 392 there is an error in the text where it says (0.01 The structures of The structure of)

Author Response

In this context, my first proposal to the authors would be to review Figures 1, 2 and 3, since part (a) of them (the essential building units with labelling scheme) in my opinion are very little visual if you compare them with how the text describes those figures. The authors could look for another view of these structures that would show more information and a clearer labelling scheme.

RESPONSE:

Following the recommendation of the reviewer, the labellings have been added to the three figures (1, 2 and 3), and representations of how the new crystalline structures of compounds 1 and 2 are built have been included (new Figure 1c and Figure 2c). Compound 5 (Figure 3) is a trimolybdate, which has a comparable construction to that shown in Fig. 2c for compound 2 (which is also trimolybdate); this is mentioned in the discussion. Hence, the authors considered that it may not be necessary to repeat the representation of Fig. 2c (for 2) in Fig. 3 (for 5).

Also, in the text I think there are some errors that should be corrected:

- I think line 109 is not necessary, since (a) in my opinion it is an error and should not be written

RESPONSE:

The authors apologise and have deleted “(a)”.

- On line 386 the quantity 01 mol is an error

RESPONSE:

The authors apologise and have corrected the information.

- On line 392 there is an error in the text where it says (0.01 The structures of The structure of)

RESPONSE:

The authors apologise and have corrected the information.

Reviewer 2 Report

The paper reports on different polyoxomolybdates being hybridized with different ligands, ie the physicochemical characterization of the solids formed, and their performance in (ep)oxidation reactions. The paper is interesting and well written, but in my opinion it should not be accepted in the present form ... by far.

Indeed, unless my deep misunderstanding,

1) the solids formed are characterized

2) the performance in the epoxidation of cyclooctene are told to be (at least partially / mostly) due to catalysis in the homogeneous phase

3) the nature of the really active (thus dissolved) species are not addressed.

In this context, the authors correlate the performance observed for the different catalysts in the different reactions on the basis of the physico-chemical features of the initially synthesized materials. But, (again) unless my deep misunderstanding, this does not make any sense !

The authors should make sure which species make the catalytic job in each reaction. It could be that it is not the same in the respective 3 reactions tested. They should characterize the active (dissolved) species carefully by tools like NMR on solutions, and/or on solids recovered from the solutions after proper recrystallization. The recent works of G. Hidalgo is a good example.

The authors should for example acquire information on the fate of the ions building the solids (once they have dissolved) when they are put in contact "activated" with tBHP.

And when the dissolved species are recrystallized from the catalytic media, the authors should prove that the recovered solids are really the reflect of what was making the job in solution.

For the moment, unless my misunderstanding (again), it sounds to me that the correlation the authors try to establish is a very naive shortcut.

Besides some minor typos, english sounds fine for me.

Author Response

Following the recommendation of the reviewer, the authors have carried out additional work to identify . Attempts were made to precipitate metal species from the catalytic reaction mixtures, which involved the addition of solvents such as pentane, hexane and acetone, and keeping the solutions for some days at 4 °C, which unfortunately did not lead to precipitation of measurable amounts of solid. Nevertheless, at least for the used solids, the authors have carried out characterisation studies by ATR FT-IR spectroscopy and powder XRD. The results suggested that the chemistry (ATR FT-IR spectroscopy) and crystalline structures (XRD) were preserved, since the fresh and used catalysts exhibited similar characterisation results (new Figure S4.8). This parallels that reported in the literature for compound 6 [doi:10.1016/j.apcata.2018.07.001]. Hence, the constituent ions of the IPOMs seem to withstand the catalytic reaction conditions. The new results have been added to the discussion in page 9 and the new Figure S4.8 has been included in the Supplementary Materials. Additionally, a comment has been added to the Conclusions regarding the challenging identification of dissolved metal species for future studies, which may require advanced characterisation techniques and computational chemistry.

Reviewer 3 Report

Comments on paper “Versatile polyoxometalates of different structural dimensionalities for liquid phase catalytic oxidation”

The paper reports on synthesis and application of ionic polyoxomolybdate compounds. The work was well performed and clearly described, with a correct and readable language..
Characterization of new species is complete.
Also, catalytic application was accurately carried on. Noticeably, kinetic investigation was well done, which is not usual nowadays.
The authors underline that synthesis of substrates was eco-friendly, but avoid to discuss the solvent used in epoxidation reactions. I do not think that trifluoromethylbenzene is a “green” solvent.

The paper deserves publication.  

Author Response

The authors underline that synthesis of substrates was eco-friendly, but avoid to discuss the solvent used in epoxidation reactions. I do not think that trifluoromethylbenzene is a “green” solvent.

Response:

The “eco-friendly” concerned the synthesis procedures of the catalysts (in water). For the catalytic epoxidation reactions, TFT was used as solvent, which is not classified as a substance of very high concern (according to ECHA, Europe). Nevertheless, the authors share the same thoughts of the reviewer in that greener (e.g., less volatile) solvents may be preferred. However, the authors respectfully prefer to consider further optimisation studies of the reaction conditions in future works of IPOMs, since the present manuscript already contemplates several new structures of catalysts, their characterisation and detailed catalytic studies (substrate scope, reaction scope, etc.).

The paper deserves publication. 

Response:

The authors are grateful for the positive comments.

Reviewer 4 Report

The paper is properly written, the results are clearly and logically presented. The experimental part supports conclusions.

-   I would put Table 1 closer to the beginning of the part 2.1. The first sentence starts with ‘Table 1 summarises’ and then a reader needs to scroll several pages down to actually see this Table

-      I would recommend to emphasize in the text more that authors mean 100% conversion of starting compound. ‘Reaction reached 38-100% conversion’ sounds unclear

-      It would be helpful to show a selection of GC chromatograms to support the reaction conversion results

The English quality has to be moderately improved. Some sentences are very long and not properly structured. Punctuation has to be checked as well.

Author Response

Reviewer 4: The paper is properly written, the results are clearly and logically presented. The experimental part supports conclusions.

-   I would put Table 1 closer to the beginning of the part 2.1. The first sentence starts with ‘Table 1 summarises’ and then a reader needs to scroll several pages down to actually see this Table

Response:

The authors are grateful for the reviewer´s comments and suggestions. Following the recommendation of the reviewer, Table 1 has been moved closer to the beginning of section 2.1. The remaining text has been formatted according to the template.

-      I would recommend to emphasize in the text more that authors mean 100% conversion of starting compound. ‘Reaction reached 38-100% conversion’ sounds unclear

Response:

Following the recommendation of the reviewer, for the sake of clarity, the types of substrates has been specified for the conversion values indicated throughout the manuscript.

-      It would be helpful to show a selection of GC chromatograms to support the reaction conversion results

Response:

Following the recommendation of the reviewer, examples of chromatograms have been included in the Supplementary Materials. A comment has been added to the experimental section (page 18).

The English quality has to be moderately improved. Some sentences are very long and not properly structured. Punctuation has to be checked as well.

Response:

Following the recommendation of the reviewer, the manuscript has been checked with respect to English usage, punctuation and typing errors. Long sentences have been divided. These improvements have also been highlighted in yellow in the manuscript.

Round 2

Reviewer 1 Report

The author has followed the proposed recommendations and has improved the manuscript.

I agree with the current form of the article.

Author Response

The authors are grateful to the reviewer for agreeing with the current form of the manuscript for publication.

Reviewer 2 Report

The modifications brought by the authors in the revised ms, and the experiments they seems to have attempted, do not at all answer to the criticism I made for the initial version.

In the revised version, the authors mentioned that they could not recrystallize the dissolved fraction of the catalysts, and still at the same time they consider that this fraction is at least in part responsible for the catalytic performance they observe. So we have no information about the nature of the active sites. Crystallization is sometimes a very difficult task, and sometimes require several weeks/months of trial and errors and patience, and I am not surprised that the authors did not succeed that in only a few days. In my initial scoring of the ms, I pointed "major revisions" because I told that the authors had this kind of data already available. But now that I realize that they dont have, I think rejection of the ms, and resubmission when they have the data is more appropriate.

The fact that the solids having remaining solid during the tests, and recovered after catalytic tests, are not modified does not tell anything on whether they are responsible of the part of the performance observed. Actually, I would almost say that the fact that they are not modified more likely means that they are not involved in the catalytic mechanism (under the form they are characterized). This is because the POM are super prone to change their molecular structure in the contact of peroxides ... so it could be that the fact that the solid POM remain unchanged during the reaction, means that it could be that they never "react" with the peroxides, and if so they would thus not play any active role. In short it could be that the solids that the authors characterize (as unchanged before and after reaction) is actually the INactive form of the catalysts !

Hence the correlation they attempt between these unchanged solids and the performance is non sense in my opinion ... I confirm my comment made at the initial version stage, and confirm my scoring of "rejection".

/

Author Response

The authors are very grateful for the reviewer´s comments and kindly ask to please see the attached PDF file with responses.

Author Response File: Author Response.pdf