Keggin Heteropolyacid Salt Catalysts in Oxidation Reactions: A Review
Round 1
Reviewer 1 Report
This manuscript reviews the preparation and characterization of heteropolyacids and the study of their catalytic activity. Polyoxometalates are a rather popular class of compounds in terms of their application in catalysis, which has been the subject of several reviews. In this review, the authors focused on a number of compounds (salts with different metal cations, lacunar POMs with d-metals, etc.) and their participation in the catalysis of the oxidation of monoterpenes, olefins, aldehydes, terpene alcohols or aromatics. The work is well presented, but needs significant improvements:
1) The language needs to be completely rewritten. It is sometimes difficult to understand.
2) Schemes 1, 2 and 3: All the synthesis of salts can be presented in one scheme. In addition, the color of the salts usually depends on the cation and the POM. Thus, showing dried powders in a certain color causes misunderstanding.
3) Schemes 8, 9 and 10 can also be combined into one.
4) Figure 6: the PXRD patterns are shown for Al salts, while the caption says "-iron-exchanged salts".
5) Figures 9 and 10 can be combined into one. Please use the same color as used in diagram 11 for better presentation and to have a link between diagram and figure.
6) Figures 12 and 13: the picture of POM doesn't fit with the listed formulas, which are usually lacunar POMs.
7) Part 2.1: please add more information about cations that can be used to make salts with appropriate links.
8) Part 2.3: same comment, please add more information about which metals can be included in POMs with appropriate links.
9) Part 3.2: The comparison of powder diffraction patterns of different compounds is absolutely wrong. What is known about the crystal structures of these salts? Do they crystallize in the same space group?
10) Part 4: More discussion is needed. Why was this or that POM studied in certain reactions? Why do different POMs show different catalytic activity?
11) Conclusion: "Keggin heteropolyacid salts are efficient catalysts with performance superior to conventional catalysts used in oxidation processes". In which of the presented papers is this demonstrated? What catalysts are typically used in these reactions? It is necessary to add this information as a demonstration of really more efficient catalysis.
I believe that this work can be published in the journal Inorganics after a major revision.
Author Response
This manuscript reviews the preparation and characterization of heteropolyacids and the study of their catalytic activity. Polyoxometalates are a rather popular class of compounds in terms of their application in catalysis, which has been the subject of several reviews. In this review, the authors focused on several compounds (salts with different metal cations, lacunar POMs with d-metals, etc.) and their participation in the catalysis of the oxidation of monoterpenes, olefins, aldehydes, terpene alcohols or aromatics. The work is well presented, but needs significant improvements:
1)The language needs to be completely rewritten. It is sometimes difficult to understand.
Response: The authors agree with the reviewer. The paper's language was carefully revised. In addition, paragraphs were rewritten aiming for an easier understanding. All the corrections and new paragraphs are highlighted in bold. We hope that it has been significantly improved and that now it can be published.
2) Schemes 1, 2 and 3: All the syntheses of salts can be presented in one scheme. In addition, the colour of the salts usually depends on the cation and the POM. Thus, showing dried powders in a certain colour causes misunderstanding.
Response: The authors thank the reviewer's suggestion. The schemes were combined and the colour of the salt is equal to the metal cation.
3) Schemes 8, 9 and 10 can also be combined into one.
Response: The authors thank the reviewer's suggestion. The schemes were combined.
4) Figure 6: the PXRD patterns are shown for Al salts, while the caption says "-iron-exchanged salts".
Response: The authors thank the reviewer's observation. The mistake was corrected.
5) Figures 9 and 10 can be combined into one. Please use the same colour as used in diagram 11 for better presentation and to have a link between the diagram and figure.
Response: The authors thank the reviewer's suggestion. As previously suggested by the reviewer, Figures 8 -10 were combined. Therefore, the correction mentioned is not necessary.
6) Figures 12 and 13: the picture of POM doesn't fit with the listed formulas, which are usually lacunar POMs.
Response: The authors kindly would like to disagree with the reviewer. At first view, truly, the Figure seems to represent the saturate anion (i.e., PW12O403-). However, if the reviewer compares the topside of heteropolyanion with their underside, it will be possible to realize that there is one octahedron absent.
7) Part 2.1: please add more information about cations that can be used to make salts with appropriate links.
Response: The authors are grateful to the reviewer for the suggestion. The information suggested was included in the revised version.
8) Part 2.3: the same comment, please add more information about which metals can be included in POMs with appropriate links.
Response: The authors thank the reviewer for the suggestion. The information suggested was included in the current version.
9) Part 3.2: The comparison of powder diffraction patterns of different compounds is absolutely wrong. What is known about the crystal structures of these salts? Do they crystallize in the same space group?
Response: The authors partially agree with the reviewer. The straight comparison of XRD patterns of these salts with acid precursor it is not the best option. Nonetheless, the authors kindly would like to highlight some aspects that hamper the determination of the crystallographic structure of these salts. In general, when large cations are the counterions in the heteropolyacids (i.e., Cs+, K+, TBA) the salts are solid and consequently easily crystallized. The literature has described these data. For instance, Viabrille et al. verified that the XRD pattern of vanadium-doped cesium phosphomolybdate showed its principal diffraction angles at 8.1, 8.9, 9.3, 27.8, 28.2, and 28.88 of 2q angles [P. Villabrille et al. / Applied Catalysis A: General 324 (2007) 69–76] These data agree with the literature authors (M. Fournier, C. Feumi-Jantou, C. Rabia, G. Herve´, S. Launay, J. Mater. Chem. 2 (1992) 971–978; T. Ilkenhans, B. Herzog, T. Braun, R. Schlo¨gl, J. Catal. 153 (1995) 275– 292). They conclude that the diffractograms show diffraction lines that belong to those of triclinic symmetry for the crystal lattice.
However, in this review, the protons of silicotungstic acid were exchanged with small metal cations. There are scarce data about the XRD patterns of these salts. Kumar et al. (C. Ramesh Kumar et al. / Applied Catalysis A: General 485 (2014) 74–83) described that in the FePW12O40 salt, the characteristic peaks related to Keggin ion were shown at 2 of 10.5◦, 14.7◦, 18.1◦, 21◦, 23.4◦, 25.7◦, 29.8◦, 35.2◦ and 38.3◦ which confirmed that the presence of Keggin ion in Fe exchanged TPA (TPA-Fe) catalyst. Lingaiah et al. used the XRD patterns to assure that Zinc was incorporated into the secondary structure of the Keggin anion (Zn3-xPW12O40) (Catal. Sci. Technol., 2014, 4, 2969–2977). Those authors stated that typical diffraction lines of Keggin anion 2θ values of 10.28°, 17.93°, 23.13°, 25.42°, 29.49°, and 37.74°.
There is an important point to be highlighted n all of these references. All the authors stated that hydration molecules and cation size strongly impact the XRD patterns. Therefore, how typical diffraction lines of silicotungstic anion were determined and were present in the diffractograms of salts, we carried the discussion showed in the paper.
10) Part 4: More discussion is needed. Why was this or that POM studied in certain reactions? Why do different POMs show different catalytic activity?
Response: The authors thank the reviewer for the suggestion. In the current version, the paragraph below was included.
“Keggin heteropolyacids such as phosphomolybdic acid are active in oxidation reactions even in the absence of metal as countercations or dopants. However, depending on the substrate selected, (i.e., aldehydes, alcohols, olefins), the presence of cation metal may trigger a synergism and increase its efficiency. From a practical viewpoint, most of the reactions require an initial screening to select the most active Keggin HPA salt. The salts reported are soluble, therefore, their activity should be related to the electronic aspects, which are modified in the presence of metal cations in determined positions.”
11) Conclusion: "Keggin heteropolyacid salts are efficient catalysts with performance superior to conventional catalysts used in oxidation processes". In which of the presented papers is this demonstrated? What catalysts are typically used in these reactions? It is necessary to add this information as a demonstration of really more efficient catalysis.
Response: The authors are grateful to the reviewer for the suggestion. The information suggested was included in the revised version. Notwithstanding, their questions can be answered herein too. The sentence “conventional catalysts used in oxidation processes” is linked to the use of noble metal catalysts (i.e., Au, Pd, Ag) that are frequently used in oxidation reactions. These catalysts are used under oxygen pressure, or high temperatures, due to low surface area. For these reasons, nanoparticles of these metals have been synthesized to increase their activity. These and other comments were included in the conclusion. Moreover, these catalysts decompose the hydrogen peroxide and are not efficient with this oxidant.
I believe that this work can be published in the journal Inorganics after a major revision.
The authors sincerely thank the reviewers for their comments and suggestions. Significant improvement has been achieved with a revision of the paper. We hope that this current version deserves to be published in “Inorganics”.
Author Response File: 
Author Response.docx
Reviewer 2 Report
da Silva et. al. summarized the syntheses and characterizations of doped Keggin polyoxometalates and their catalytic applications in oxidation reactions. This review is well written, and it is enjoyable for me to read. The listed examples are complete, and the discussion of catalytic applications are inspired. I would recommend this review to be published after addressing few following minor suggestions.
1) Can the authors explain more on the characterization of Keggin heteropolyanions by powder XRD technique (section 3.2)? How the introduction of heteroatoms influences the crystal packing of the POM? And can we distinguish different Keggin heteropolyanions by comparing their powder XRD patterns? To my knowledge, the introduction of heteroatoms only affects the local environment of the POM rather than the global packing. Maybe the size of ionic radii of metal cations will change the packing or some certain crystal packing mode. If so, the authors should discuss more on this issue.
2) Can the authors give more explanation or discussion in section 3.3 concerning about the acidity properties of Keggin polyoxoanions about the distinguishing the nature of the acid site? It makes me confused that the reason of failure of identifying Lewis or Bronsted acids of a certain (type) of Keggin polyoxoanions, and the reason that causes this failure.
3) In line 88-89, the authors claim the ionic radium greater than 1.3 A causes the insolubility of the Keggin HPA in water. Is this an empirical conclusion or does it have any hidden mechanism to explain it, or is there any aggregates forming with the large metal cation?
4) For Figures 14-15, the authors compare the conversion and selectivity of the borneol oxidation reaction by using different transition metal doped HPA. It is unclear of the role of these transition metals play in the oxidation reaction and how can they influence the conversion and selectivity. And how to choose a good doping metal that would benefit the target oxidation reactions.
Author Response
da Silva et. al. summarized the syntheses and characterizations of doped Keggin polyoxometalates and their catalytic applications in oxidation reactions. This review is well-written, and it is enjoyable for me to read. The listed examples are complete, and the discussion of catalytic applications is inspired. I would recommend this review be published after addressing a few following minor suggestions.
1) Can the authors explain more on the characterization of Keggin heteropolyanions by powder XRD technique (section 3.2)?
Response: The authors thank the reviewer´s comments. Your questions will be separately answered, and the comments will be added to the review.
1.1. How the introduction of heteroatoms influences the crystal packing of the POM?
Response: The authors thank the reviewer´s comments. A figure with three Keggin structures was included in the revised version (i.e., primary, secondary, and tertiary structures). In addition, the Bragg equation was also added. A paragraph was included, aiming to show that if a large cation replaces the protons of HPAs, the interplanar distance “d” is modified, consequently, the diffraction patterns are also altered.
1.2. And can we distinguish different Keggin heteropolyanions by comparing their powder XRD patterns? To my knowledge, the introduction of heteroatoms only affects the local environment of the POM rather than the global packing. Maybe the size of ionic radii of metal cations will change the packing or some certain crystal packing mode. If so, the authors should discuss more on this issue.
Response: The authors agree with the reviewer´s comments. This aspect was addressed in several parts of the revised version. All the comments are in bold in the current version.
2) Can the authors give more explanation or discussion in section 3.3 concerning about the acidity properties of Keggin polyoxoanions about the distinguishing the nature of the acid site? It makes me confused that the reason of failure of identifying Lewis or Bronsted acids of a certain (type) of Keggin polyoxoanions, and the reason that causes this failure.
Response: The authors agree with the reviewer; sometimes, there is a little confusion about the nature of the acidic sites of Keggin HPAs salts. Initially, we would like to say that the potentiometric titration technique is very simple and cheap; however, it provides total acidity, without distinguishing the acid site's nature. The Ei value gives the strength of acidity sites, while the plateau region allows the calculation of the number of acid sites. The main challenge is that Keggin HPAs are hydrated compounds. Thus, Bronsted acid sites are predominant in the HPAs. However, the literature describes that other techniques (i.e., FT-IR spectroscopy of adsorbed pyridine and ammonium) showed that there is a Lewis acidity, due to the presence of Mo6+ or W6+ cations in the pristine heteropolyacids. On the other hand, when metal cations likewise iron, aluminium, copper, or iron are included in the Keggin anion, it is expected that Lewis acid sites appear. However, due to hydrolyses undergone by these metal cations, Bronsted acid sites are also generated (see Equation 1).
Therefore, when Lewis acid metals are added to the Keggin anion, Bronsted acid sites will be also generated.
3) In lines 88-89, the authors claim that ionic radium greater than 1.3 A causes the insolubility of the Keggin HPA in water. Is this an empirical conclusion or does it have any hidden mechanism to explain it, or is there any aggregates forming with the large metal cation?
Response: The authors congratulate the reviewer for the excellent question. This conclusion comes from the literature, as described in the review. We believe that this value was experimentally determined. However, it is true that it probably depends on the other aspects. For instance, how the metal cation will react with water molecules present in heteropolyanion, or even how many water molecules are present. Therefore, we suppose that probably, more studies should be carried out to elucidate this question.
4) For Figures 14-15, the authors compare the conversion and selectivity of the borneol oxidation reaction by using different transition metal doped HPA. It is unclear of the role of these transition metals play in the oxidation reaction and how can they influence the conversion and selectivity. And how to choose a good doping metal that would benefit the target oxidation reactions.
Response: Once more, the authors congratulate the reviewer for another excellent question. These two works were carried out in our group. We sincerely believe that electronic questions are playing a key role in these reactions. Nonetheless, when these works were published, the novelty was the more important aspect. To answer this question and others that the reviewer asks us will be necessary hard work, involving theoretical and experimental investigations. However, in this review, the intention was to show how solid heteropolyacid salts can be useful catalysts in oxidation reactions and try to stimulate more work in this fascinating research field.
The authors sincerely thank the reviewers for their comments and suggestions. Significant improvement has been achieved with a revision of the paper. We hope that this current version deserves to be published in “Inorganics”.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors have done a good job of improving the article. Necessary data has been added and errors have been corrected. However, some minor corrections are still needed:
1) The Figure 7 caption still have description “iron-exchanged salts” while figure contain only the data for Al-contained compounds.
2) Line 209 change “important t note” to “important to note”
I believe that this work can be published in the journal Inorganics after a minor revision.
Author Response
Response to Reviewer 1 Comments
Point 1: The Figure 7 caption still have the description “iron-exchanged salts” while the figure contains only the data for Al-contained compounds.
Response 1: The authors thank the reviewer. The correction was done on the Figure 7 caption.
Point 2: Line 209 change “important t note” to “important to note”
Response 2: The authors are grateful to the reviewer. The correction was performed.
The authors sincerely thank the reviewers for their comments and suggestions. Significant improvement has been achieved with a revision of the paper. We hope this current version deserves to be published in “Inorganics”.
"Please see the attachments"
Author Response File: Author Response.docx
