Oxyfunctionalization of Benzylic C-H Bonds of Toluene Mediated by Covalently Anchored Co-Schiff Bases

Round 1

Reviewer 1 Report

This is a resubmission, and although the authors did not include a reply to the comments I provided in the previous review round, it seems that the main points were addressed. In my assessment, the paper is now deemed acceptable for publication in the Molecules journal. 

Author Response

Thank you very much your comments.

Reviewer 2 Report

I would like to thank the authors for the response to my suggestions. I feel that they have addressed all the point I raised.

 

I would suggest that there are still numerous grammatical errors throughout the manuscript and it would benefit from further language correction.

Author Response

This manuscript has been carefully checked and corrected grammar errors before we can resubmit it as you suggested.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this manuscript the oxyfunctionalization of benzylic C-H bonds of toluene catalyzed by anchored Co-Schiff based complexes is described.

It is not clear why the authors discuss the catalyst and the spent catalyst (for example the textural properties, AE,…) only for EASA-Co species. this becomes evident when they discuss the catalytic performances, but as a reader, I believe that the organization of the manuscript should be revised. First the catalytic results must be reported, then a discussion on the properties of the catalysts, as made and after catalysis.

The catalytic performances of the supported species should be compared in my opinion with those of the homogeneous species (the corresponding Co-Schiff complexes, at least one complex should be included for comparison).

Other points:

-there is no correspondence between the names in figure 2 and the caption. In this regard, all the three anchoring molecules should be included in Scheme 1, not only EASA.

 

Reviewer 2 Report

The manuscript by Shi et. al. entitled ‘Oxyfunctionalization of benzylic C−H bonds of toluene mediated by covalently anchored Co-Schiff bases’ outlines their studies on the immobilisation of a cobalt catalyst onto silica particles for the oxidation of toluene. Although the study may be of interest to some, I cannot currently recommend this article for publication, as I feel that there is a significant amount of experimental data omitted, specifically related to the thorough characterisation of the immobilised catalysts.

 

My main concern is that despite the assertion in the manuscript that catalysts were ‘methodically characterised and the details are included in the supplementary information’ there is a significant lack of data for 3 out of 4 catalysts (SA, HAP and NSA). All 4 synthesised catalysts should have the microscope images, TGA, BET, ICP-OES/elemental analysis and any other collected data included with raw data shown where possible. The systems physical properties should be compared based on the collected data and Co loading for each catalyst should be listed, not just for EASA. This is crucial as, For example, for discussion related to the conversions in Table 2, as the low conversion for NSA derivative could be related to loading or physical properties.

 

The introduction lists several challenges in the field of toluene oxidation with molecular oxygen, including challenges with substrate scope (P2, L37-42). However, no studies into substrate are carried out in this study, severely limiting is usefulness/interest over currently reported systems highlighted in the introduction. I also notice that the catalyst used in this manuscript had been reported in previous studies (P2, L53-57) but there was no discussion or comparison to the solution based previous studies and the authors current results. Also the previous similar studies were not included in Table S2, despite them arguably being the most relevant to this work.

 

Minor comments:

1. The manuscript needs language and grammar correction before publication.

2. There are points (especially in figure captions and legends, such as in Table 3) where units are missing from values related to the amounts of materials used. Please thoroughly check that all relevant units/masses/amounts are correct.

3. P10, L308 – You use the word ‘inhibited’ but do you mean ‘exhibited’?

4. The figures that include labels (A,B,C,etc) have become misaligned (possible during PDF conversion). These need to be corrected.

Reviewer 3 Report

The paper describes the synthesis, characterization, and catalytic tests of a cobalt-based amino-silicate solid catalyst applied to toluene's catalytic conversion into aromatic oxygen-containing species. The authors report the elemental composition (C, N, and Co), textural properties (surface area, pore-volume, and pore size), images (SEM/EDS), and chemical (FTIR, UV-Vis, XPS, and NMR) and physical (TG) characterization of the synthesized catalysts, including materials obtained in intermediate synthesis steps. Then, the catalytic experimental data is presented. The authors report toluene conversion and selectivity to 4 primary products (benzaldehyde, benzyl alcohol, benzoic acid, and dibenzyl ether) at an oxygen pressure of 26 bar and 100^oC. The authors also investigated the effect of different catalyst ligands and toluene to co-catalyst ratios on conversion and selectivity. Finally, catalyst reusability tests are reported.

The authors provided substantial characterization data for the as-synthesized catalysts, focusing on assessing the anchoring of the amino cobalt ligand on the catalyst's surface. Overall, the results agree with the presence of the chemical species shown in the proposed schematic depicted in Scheme 1. Regarding the catalytic tests, the authors should better present and discuss the results since some of the conclusions are not well supported by the data presented in Tables 3-5 and Figure 7. I think the points below must be addressed before the paper is deemed acceptable for publication in the Molecules journal. Please, find below my specific comments.   

Major comments:

• In several instances, the authors unnecessarily repeat definitions for the same acronyms used throughout the paper. For instance, the definition of the acronym SA (salicylaldehyde) is repeated several times. If the manuscript employs acronyms for simplicity, the authors don't need to define the acronym every time they use it. Also, I found the number of acronyms excessive, making it difficult for the reader to follow the discussion.
• Page 4, line 160: It should be noted in the text the theoretical value for the C/N ratio. Also, the authors state that "The atomic ratio of C/N (8.9) is higher than the theoretical value calculated according to Scheme 1, which can be ascribed to the adsorbed 4-(diethylamino) salicylaldehyde (EASA) with a higher C/N ratio." If EASA is bound to APTES and Co, the C/N should be 8, as pointed out in the manuscript, regardless of the higher C/N of EASA. Do the authors mean that there are EASA species adsorbed to the catalyst without forming APTES-EASA complexes? If so, the sentence should be clarified, and a better justification should be provided for the higher C/N ratio.
• Page 5, line 179: The authors state that the reduction in catalyst surface area after synthesis indicates "successful surface functionalization". However, a reduction in catalyst surface area may be due to the collapse of the catalyst's pores due to the mechanical agitation used during catalyst synthesis. Also, textural changes are not a suitable rationale to justify the introduction of surface functions on the catalyst's surface.
• Figure 1: I suggest that the authors remove the carbon mapping from the EDS analysis since the catalyst likely contains carbon from other carbon-containing species adsorbed on the catalyst's surface. Also, the authors need to state the material of the sample grid used for TEM analysis in the experimental section.
• The authors state that "The distributions of C and Co elements are almost same, suggesting a successful anchorig of the Co-Schiff base on the surface of the support SiO₂". This analysis should not be done in terms of C (see comment #4). Instead, the paper should depict the EDS spectra and the mapping shown in Figure 1.
• Figure 2 is problematic because of the poor resolution, and it makes it difficult for the reader to identify any of the peaks discussed in the manuscript. For instance, it is stated that "The aminated SiO₂ (SiO₂-APTES) exhibited asymmetric and symmetric stretching vibration modes of saturated C−H bonds at 2931 cm-1 and 2878 cm^-1, and their bending mode at 1470 cm^-1." These vibration modes are barely visible in Figure 2. The resolution of the image should be improved.
• Page 7, line 218: According to the discussion, the aminated SiO₂ catalyst does not exhibit UV-Vis peaks, but Figure 3 shows an intense peak at 240 nm for the SiO₂-APTES intermediate. Hence, the sentence should be corrected.
• Figure 4 provides insightful data, but the discussion should be better justified with references. As a suggestion, the authors could include a table with the assignment of the chemical shift, the specific functional group, and the reference for the assignment. Also, the chemical shift closer to 0 ppm is assigned to 6 groups (1,2,3,4,27, and 28) – 1-4 are methyl groups, and 27-28 are methylene groups – while groups 31 and 32 (also methyl) are assigned to a higher chemical shift ~25. Could the authors comment on that? Although it is stated that the C-13 NMR spectrum of the fresh catalyst is similar to that of the spent one, there are significant changes that need to be addressed, g., the appearance of chemical shifts at 125-150 ppm after regeneration.
• It is stated that " The survey XP spectra of both the fresh and the retrieved catalysts inhibited the presence of the Co, N, O and Si atoms and a similar intensity, indicating a successful anchoring of the organic moiety Co-Schiff base and its relative stability under the applied reaction conditions (Fig. S1)." The figure in the SI does not show similar intensities for Co and the other species mentioned. Co has a much lower intensity, as shown in Figure S1.
• The authors identify Co²⁺ on the catalyst, but some CoO particles may have formed during the catalyst synthesis step. So, the discussion should also point to this possibility since the Co in the CoO particles would also produce a component at 780.3 eV in the XP spectrum. Also, a decrease in the binding energy of 0.3 eV is too small, and not too many conclusions regarding the nature of the cobalt on the catalyst should be drawn from Figure 6.
• Section 3.4 is problematic for several reasons: 1- equations showing how the conversion and selectivities were calculated should be provided, 2- there is no information about mass balance closure for the catalytic experiments, and 3- no replicates or reproducibility data was provided for the data in Tables 3-5 and Figure 7. As an example, can we really distinguish a conversion of 19.2% from being different than a conversion of 20.0 or 20.8%? Reproducibility data could help answer that. The authors should include that for the validity of the experimental data in section 3.4.
• It seems that the cobalt-based amino silicate catalyst requires an activator, NHPI. This information is crucial, but it does not seem to be described in the abstract, giving the impression that the synthesized catalyst can be used standalone. It should be made clear in the manuscript that the catalyst requires a co-catalyst to work because this is only discussed in the last sections of the manuscript.
• It needs to be better elaborated on how benzyl alcohol is converted into benzaldehyde via an oxidative pathway. A possible path is the dehydrogenation of benzyl alcohol into benzaldehyde, which may involve oxygen or not. Please, refer to the following reference: https://doi.org/10.1039/C2CY20222K.
• The temperature of 50^oC for catalyst regeneration is too mild, and tests at a higher temperature ~550^oC would be more helpful. However, I understand that such temperatures would likely degrade the catalyst, as shown in the TG data in Figure 5. This discussion should be included in the manuscript. Also, it should be mentioned if carbon deposits were formed after the reaction.

Minor comments:

• Overall, the English presented in the manuscript is acceptable, but I still would recommend the authors improve the quality of the English presented.
• It should be noted in Scheme 1 that 2 molecules of EASA are necessary to form the SiO₂-APTES-EASA material. Also, it should be noted that conversion of SiO₂-APTES into SiO₂-APTES-EASA involves the removal of 2 water molecules.
• In subsection 2.3 Catalytic tests, the numbering is missing for the Catalytic activity and Reusability subsection.
• Page 5, line 194: anchoring, not anchorig.
• Figure 5 depicts temperatures of 460 and 575^oC, but the relevance of these temperatures does not seem to be discussed in the paper. Why are these temperatures important?
• The reacted catalyst should be referred to as "spent" instead of "retrieved".
• The Supplementary Information provided the experimental procedures, but the main manuscript should also contain a sentence stating that more details about the experimental procedures can be found in the Supplementary Information.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

In the manuscript by Guojun Shi et al., Co-Schiff base catalysts were immobilized on commercial silica gel by covalent anchoring and used to catalyze oxidation of toluene. However, novelty and importance of this work remain under question as Co complexes with various ligands immobilized on silica gel are well documented. Unfortunately, the obtained results are not properly analyzed and discussed in the Results and Discussion section. Catalytic efficacy of the prepared catalytic systems (conversion, selectivity and recyclability) is not compared with that of known immobilized catalysts of the liquid-phase toluene oxidation reactions with molecular oxygen. The reaction conditions (temperature, pressure, reaction time, etc.) were not optimized. Particular role of N-hydroxyphthalimide (NHPI) additive in the catalytic system is not specified. The authors succeeded in SiO2-APTES-EASA-Co recycling, but no information on the fate of the NHPI additive after workup is given. It is not clear if the reaction is applicable to toluene derivatives bearing substituents in the aromatic ring. Supplementary information is obviously insufficient.

Minor shortcomings:

• – Page 3, line 96 and page 10, line 290: It is not clear if NSA contains the nitro or the nitryl group.
• – Page 2, lines 48-51: Ishii is not among co-authors of ref. 19.
• – References 2 and 3 are not relevant judging from the titles.
• – Reference 7 and subsequent references have double numerations.
• – English should be improved.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Please, see attached file. 

Comments for author File: Comments.pdf

Reviewer 4 Report

The authors made some technical corrections in the text which improved the manuscript a bit. Though, its quality and potential impact on further academic and industry-oriented researches in this area remain moderate, Nevertheless, I think it may be published in Molecules as an additional experimental example of catalytic aerobic oxidation reaction.