NbCl5 Functionalized Perlite: A Potent and Recyclable Catalyst for Synthesis of Pyrans
, Monisha Sivanandhan 1, Subramaniyan Ramasundaram 2,†, Tae Hwan Oh 2, Kinjal J. Shah 3,†, Kumaravel Saranraj 4, Amutha Parasuraman 1,* and Krishnakumar Balu 5,*Round 1
Reviewer 1 Report
The subject is interesting and deserves attention from the audience. Some reformulations and changes are needed to get the paper in its best form, for publication.
Please use the highlighted and commented version of the paper to undergo corrections.
Comments for author File: 
Comments.pdf
Author Response
Responses to the reviewers’ comments
The subject is interesting and deserves attention from the audience.
Dear Reviewer,
Thank you for your comments. Your comments will help us to improve the quality of our manuscript.
Some reformulations and changes are needed to get the paper in its best form, for publication. Please use the highlighted and commented version of the paper to undergo corrections.
Ans: Thank you for your valuable comments. We have revised the manuscript according to your comments. The suggested corrections have been carried out.
Author Response File: 
Author Response.doc
Reviewer 2 Report
The work presented by Lakshminarayanan et al can be interesting to the readers but it is not adequately supported by experimental results, and the conclusions are not always correct, in my opinion. For all these reasons the manuscript cannot be accepted at the moment but below the authors can find some of my suggestions to improve it in the future.
1) Please rewrite the abstract. It is not well underlined the aim, the best obtained results and their relevance.
2) Line 18: correct to FT-IR
3) Why this catalyst has been adopted for the synthesis of pyrans? This is catalysed by Lewis acids? Which is the mechanism of this reaction? If the acidity of the catalyst is an important property, this study lacks of this characterization.
4) How perlite was pretreated?
5) In the text sometimes there is & instead of and. Please correct.
6) The authors should report also the FT-IR spectrum of pure perlite and pure NbCl5 in Figure 1.
7) The authors should report also the XRD of pure perlite in Figure 2.
8) Cl was not detected by EDAX? The authors are sure that after calcination NbCl5 was still in this form or it is as oxide? This aspect should be better investigated.
9) I think that after calcination some of NbCl5 is as oxide. The weight loss at 550-590°C could be the degradation of NbCl5 still present after calcination? In fact the calcination was carried out at 250 °C that probably is too low to convert all the loaded NbCl5. The authors should calcine the catalyst at about 600 °C and perform again the TGA.
10) The authors should report also the TGA of pure perlite in Figure 4.
11) Figure 5: I think that these different values can be due to the experimental error. The 5 wt% NbCl5 allowed to achieve analogous yield but with a lower amount of NbCl5, thus adopting a cheaper catalyst. In my opinion these are the best results. The same conclusions are for the amount of catalyst. The authors gained a 7% of yield but with ten times as much catalyst, this is not suitable.
12) Note Tables 3,4 and Table 5, specify ethanol instead of alcohol.
13) Why the reaction was so activated in ethanol?
Author Response
Responses to the reviewers’ comments
The work presented by Lakshminarayanan et al can be interesting to the readers but it is not adequately supported by experimental results, and the conclusions are not always correct, in my opinion. For all these reasons the manuscript cannot be accepted at the moment but below the authors can find some of my suggestions to improve it in the future.
Dear Reviewer,
Thank you for your comments. Your comments will help us to improve the quality of our manuscript.
1) Please rewrite the abstract. It is not well underlined the aim, the best obtained results and their relevance.
Ans: Thank you for your valuable comment. The abstract has been revised.
2) Line 18: correct to FT-IR
Ans: Thank you for your insightful feedback. The correction has been carried out.
3) Why this catalyst has been adopted for the synthesis of pyrans? This is catalysed by Lewis acids?
Ans: Pyrans were generally prepared using a Lewis acid catalyst. The major drawback of Lewis acids is that they are corrosive in nature, the reaction takes more time, an inert atmosphere is required, the process is expensive and time-consuming. As a result, incorporating NbCl5, which is also a Lewis acid, in solid support perlite, overcame all of the drawbacks of Lewis acids.
3.1 Which is the mechanism of this reaction? If the acidity of the catalyst is an important property, this study lacks of this characterization.
Ans: Thank you for your suggetions. In this reaction, a plausible mechanism which involves Knoevenagel condensation, Michael addition, and then intermolecular ring closure causes the formation of substituted-4H-pyran derivatives catalyzed by NbCl5/Perlite (Tetrahedron Lett. 2016, 57, 5050–5052 [ref.35]). This mechanism was well establihed, and these lines had been added in the manuscript with suitable reference. Pyrans were generally prepared using a Lewis acid catalyst. As a result, incorporating NbCl5, which is also a Lewis acid, in solid support perlite. We had already prepared BiCl3-loaded montmorillonite K10: a new solid acid catalyst for solvent-free synthesis of bis(indolyl)methanes (Res Chem Intermed (2015) 41:5353–5364 [ref. 38]), The surface acidity of BiCl3–MK10 was determined by temperature-programmed desorption (TPD), using ammonia as probe molecule, in the temperature range 120–600 oC. The acidity measurement was not performed at present, since it is a same type material, and the proper reference is cited. The suggestions will be carried out in future endures.
4) How perlite was pretreated?
Ans: In order to eliminate organic impurities, about 10 g of perlite powder was suspended in 200 mL of ethanol by vigorous stirring for 10 h. The obtained residue was filtered and washed with ethanol and heated in aqueous solution of 5 N NaOH for 30 min. The resulting bare perlite was then filtered and rinsed with excess water to remove NaOH and allowed to dry at 120 °C for 48 h. We have already explained this process in our previous paper (Research on Chemical Intermediates (2019) 45:4367–4381 [ref. 30]), the reference is cited in the revised manuscript.
5) In the text sometimes there is & instead of and. Please correct.
Ans: Thank you for your wise suggestions. It has been rectified.
6) The authors should report also the FT-IR spectrum of pure perlite and pure NbCl5 in Figure 1.
Ans: As suggested, the FT-IR spectra of pure Perlite and NbCl5 are included in Figure. 1, and discussed in the revised manuscript.
7) The authors should report also the XRD of pure perlite in Figure 2.
Ans: As suggested, the XRD of the pure perlite has been included in Figure 2. Previously, we have given the XRD of NbCl5/Perlite using smoothen peak options, the main peak (Perlite peak) was removed. Now, we have given the original file, and corrected in the revised manuscript.
8) Cl was not detected by EDAX? The authors are sure that after calcination NbCl5 was still in this form or it is as oxide? This aspect should be better investigated.
Ans: Thank you for your comment. Our primary aim was to immobilize NbCl5 over Perlite surface, not for into its oxide form. We did calcination at 250 °C for efficient binding of NbCl5 over Perlite surface. Of Couse, the partial oxidation happened, however, TGA confirms that the major degradation of NbCl5 took place at 550-590°C. This has been explained in XRD and TGA discussion. The Cl was not detected by EDAX in our previous work when BiCl3 loaded over montmorillonite K10 (Res Chem Intermed (2015) 41:5353–5364 [ref. 38]), even the composite was prepared at 110 °C, and may be due to very low concentration in the composite.
9) I think that after calcination some of NbCl5 is as oxide. The weight loss at 550-590°C could be the degradation of NbCl5 still present after calcination? In fact the calcination was carried out at 250 °C that probably is too low to convert all the loaded NbCl5. The authors should calcine the catalyst at about 600 °C and perform again the TGA.
Ans: Thank you for your comment. Our primary aim was to immobilize NbCl5 over Perlite surface, not for into its oxide form. We did calcination at 250 °C for efficient binding of NbCl5 over Perlite surface. Of Couse, the partial oxidation happened, however, TGA confirms that the major degradation of NbCl5 took place at 550-590°C. This has been explained in XRD and TGA discussion.
10) The authors should report also the TGA of pure perlite in Figure 4.
Ans: As suggested, the TGA of pure perlite has been reported in Figure 4.
11) Figure 5: I think that these different values can be due to the experimental error. The 5 wt% NbCl5 allowed to achieve analogous yield but with a lower amount of NbCl5, thus adopting a cheaper catalyst. In my opinion these are the best results. The same conclusions are for the amount of catalyst. The authors gained a 7% of yield but with ten times as much catalyst, this is not suitable.
Ans: Thank you for your comment. However, the optimization of the process parameters is very important in any heterogeneous catalysis process, especially in organic transformation/synthetic reactions. Even 1% of the impurity will affect the purity of the synthesized compounds, the 7% enhancement in the yield is not the subject of negotiation. Moreover, in our previous work, when bare TiO2 and sulfated TiO2 were utilized for N-formylation reaction (J. Mol. Catal. A, 2011, 334, 98-102), both gave same yield (>99%) for aniline used as reactant. However, the substituted anilines and cyclic amines used, the bare TiO2 was not active. Furthermore, the bare TiO2 lose its activity in the cyclic runs, but sulfated titania performed well in multiple runs. The optimization of different wt% of sulfated titania was performed for the green synthesis of quinoxaline and dipyridophenazine derivatives at room temperature (Catal. Commun., 2010, 11, 997-1002), the difference in the yield between the different sulfated loaded catalysts was very low, however in order to get 100% pure product, we performed optimization.
12) Note Tables 3,4 and Table 5, specify ethanol instead of alcohol.
Ans: Thank you for your valuable suggestions. Corrections have been made.
13) Why the reaction was so activated in ethanol?
Ans: The solubility is less in water and dichloromethane. The reaction was completed in 15 min and 8 h with 95% and 70% yield in solvents ethanol and acetonitrile, respectively. It was identified that the same reaction in water and dichloromethane required a longer time to give 65% and 80% yield of product. Owing to yield of product in minimum time, ethanol was selected as a suitable solvent for other reactions.
Author Response File: Author Response.doc
Reviewer 3 Report
This manuscript reports a potent and recyclable catalyst, NbCl5/perlite, for synthesis of pyrans. The heterogeneous catalyst possesses good catalytic performance for synthesis of pyrans in high yields without side-products under optimized conditions and acceptable reusability. This provides a green methodology to develop new type of hetero-catalysts applying in organic synthesis. It is recommended for publishing in Sustainability.
Comments:
-The IR spectra of NbCl5 and perlite should be provided.
-The XRD patterns of NbCl5 and perlite should be provided.
-The EDX spectrum of perlite should be provided, and element percentages should be provided by so-obtained EDX analysis instead of reference data (reference 24).
-Page 5, line 171. The unit should be provided for “… peaks at 2.1 and 2.2 for Nb”.
-An independent experiment should be carried out by using NbCl5 alone as catalyst under the same conditions in the work instead of reference data (reference 34) for comparison purpose.
-The influence of substitutes on the yield should be explained.
Author Response
Responses to the reviewers’ comments
This manuscript reports a potent and recyclable catalyst, NbCl5/perlite, for synthesis of pyrans. The heterogeneous catalyst possesses good catalytic performance for synthesis of pyrans in high yields without side-products under optimized conditions and acceptable reusability. This provides a green methodology to develop new type of hetero-catalysts applying in organic synthesis. It is recommended for publishing in Sustainability.
Dear Reviewer,
Thank you for your comments. Your comments will help us to improve the quality of our manuscript.
Comments:
-The IR spectra of NbCl5 and perlite should be provided.
Ans: As suggested, the FT-IR spectra of pure Perlite and NbCl5 have included in Figure 1, and discussed in the revised manuscript.
-The XRD patterns of NbCl5 and perlite should be provided.
Ans: As suggested, the XRD of pure Perlite has been included in Figure 2, and discussed in the revised manuscript. The XRD of NbCl5 is not provided, it is a highly deliquescent solid.
-The EDX spectrum of perlite should be provided, and element percentages should be provided by so-obtained EDX analysis instead of reference data (reference 24).
Ans: The EDX spectrum of pure perlite was already reported in our previous paper (Research on Chemical Intermediates (2019) 45:4367–4381 [re. 30]). Now, we are providing the EDX figure in supporting information. We wrongly cited reference 24, now it has been removed.
-Page 5, line 171. The unit should be provided for “… peaks at 2.1 and 2.2 for Nb”.
Ans: Thank you for your feedback. The corrections have been rectified.
-An independent experiment should be carried out by using NbCl5 alone as catalyst under the same conditions in the work instead of reference data (reference 34) for comparison purpose.
Ans: Thank you for your comment, Under the same condition, the reaction was performed with pure NbCl5 gave 78% of yield. This has been included in the section 3.2. Reference 34 (now ref 35), was used for literature comparison purpose.
-The influence of substitutes on the yield should be explained.
Ans: As suggested, the discussion of influence of substituents effects has been improved in the revised manuscript.
Author Response File: Author Response.doc
Round 2
Reviewer 1 Report
The revised paper has addressed the issues raised and now it meets the requirements for publication
Author Response
Dear Reviewer,
Thank you for your comments. As suggested, the English corrections have been made throughout the manuscript. We hope, now it can be acceptable for publications.
Reviewer 2 Report
The manuscript has been partially modified according to the reviewer's comments, thus it can be published.
Author Response
Dear Reviewer,
Thank you for your comments. As suggested, the English corrections have been made throughout the manuscript. We hope, now it can be acceptable for publications.
Reviewer 3 Report
This manuscript is accepted for publishing in Sustainability after minor revision as follow.
-For the purpose of comparison, it is duggested that Figures 1a-d should be merged in the same one. Also, Figures 4a and 4b should be merged, and the y-axis should be rescaled.
Author Response
Responses to Reviewers comments.
-For the purpose of comparison, it is suggested that Figures 1a-d should be merged in the same one.
Ans: Thank you for your suggestion. However, Due to the different in intensity, they are not merged and they have been given as separate Figures. Since Figure 1a and 1b are the FT-IR spectrum of pure Perlite in different region, and they cannot be merged together (in order to better understanding, and distinguish the peaks clearly, we have given as separate spectrum).
Also, Figures 4a and 4b should be merged, and the y-axis should be rescaled.
Ans: Thank you for your suggestion. As suggested, Figure 4a and 4b have been merged, and y axis rescaled.
Author Response File: Author Response.doc
