Copper Oxide Nanoparticles over Hierarchical Silica Monoliths for Continuous-Flow Selective Alcoholysis of Styrene Oxide

Round 1

Reviewer 1 Report

Dear authors,

I found your manuscript "Copper Oxide Nanoparticles Over Hierarchical Silica Monoliths for Continuous-Flow Selective Alcoholysis of Styrene Epoxide" very well prepared and I suggest only few minor changes:

1/ I would avoid using NH4OH and use NH3 aqueous solution instead (throughout the manuscript).

2/ Styrene epoxide should be styrene oxide according to English nomenclature  (throughout the manuscript)

3/ You are presenting productivity and space time yield in conclusion, however these numbers do not appear in the main text of the manuscript. I suggest to include a table presenting catalytic performance including productivity and space time yield. It will enable an easy comparison with other catalysts (only mentioned in the conclusion).

4/ Typos: Row 133 - leading to (twice)
                Row 186  - by divided dividing
                Row 200 - syrene
                Row 236 - significative
                

Author Response

Authors Answers:

Reviewer 1: I found your manuscript "Copper Oxide Nanoparticles Over Hierarchical Silica Monoliths for Continuous-Flow Selective Alcoholysis of Styrene Epoxide" very well prepared and I suggest only few minor changes:

1/ I would avoid using NH4OH and use NH3 aqueous solution instead (throughout the manuscript).

We agree with reviewer. Following the suggestion we used NH₃ aqueous solution instead NH₄OH into the manuscript.

2/ Styrene epoxide should be styrene oxide according to English nomenclature  (throughout the manuscript)

Thank you for the revision. Following the suggestion we properly included styrene oxide instead of styrene epoxide into the manuscript.

3/ You are presenting productivity and space time yield in conclusion, however these numbers do not appear in the main text of the manuscript. I suggest to include a table presenting catalytic performance including productivity and space time yield. It will enable an easy comparison with other catalysts (only mentioned in the conclusion).

According to the reviewer suggestion we include the values of the productivity and space time yield referred to the catalyst and the comparison with a previously reported Cu on silica catalyst in the main text. Notably, in case of the previous catalyst (ref. 35 in the manuscript) only the productivity value (by using ethanol as solvent, instead 2-propanol) were reported, considering that the catalytic tests were carried out in batch reaction conditions  (see page 5, line 248-251 of the revised manuscript).

 

4/ Typos: Row 133 - leading to (twice)
                Row 186  - by divided dividing
                Row 200 - syrene
                Row 236 – significative

Thank you. They were revised.

 

 

Reviewer 2 Report

The authors describe the synthesis of Cu-based heterogeneous catalysts and their use in the selective alcoholysis of styrene epoxide. I would encourage authors to proofread the manuscript before the final submission. I also have a few comments, and I would like the authors to address these before the article can be accepted for publication.

Regarding the characterization of the CuO@SiO2-MN, I suggest including the FT-IR spectra and powder  XRD for SiO2-MN and CuO@SiO2-MN. In addition, the thermal stability for CuO@SiO2-MN should also be included and discussed.

 

Regarding the catalytic studies, the catalyst CuO@SiO2-MN suffered a decrease in catalytic activity from 14 h to 34 h. I suggest adding some characterization data, namely FT-IR spectra and powder XRD, and commenting on the differences when comparing it with the as-synthesized material.

Author Response

Authors Answers:

Reviewer 2: The authors describe the synthesis of Cu-based heterogeneous catalysts and their use in the selective alcoholysis of styrene epoxide. I would encourage authors to proofread the manuscript before the final submission. I also have a few comments, and I would like the authors to address these before the article can be accepted for publication.

We thank the reviewer.

1/ Regarding the characterization of the CuO@SiO2-MN, I suggest including the FT-IR spectra and powder XRD for SiO2-MN and CuO@SiO2-MN.

By FTIR, it is very difficult to observe CuO bands (1114 cm-1 and 480 cm-1) [ref. J. Sol-gel Science, 2016] as they are hidden by the bands of SiO₂ (1100 cm-1 with shoulder 1200 cm-1) [ref. Scientific Reports, 2014]. As requested by the reviewer FTIR analysis both of the fresh CuO@SiO₂-MN (orange colour, in Figure below) and the spent one (blue colour in Figure below) were carried out. However, the analysis gives no additional information about the structure of the catalysts and therefore it was not included in the revised version of the manuscript.

 

Figure: Please, see the attached file

 

The best way to observe CuO NPs on silica is XRD as silica is amorphous, giving a broad band centered at 2 theta = 20° and CuO NPs give rise to thin peaks at 2 theta = 35.4° (002) and 38.8° (111). CuO NPs of 5 nm have been identified by XRD [particuology, 2011; Xu, Petroleum Chemistry, 2020].

 

XRD was performed on CuO@SiO2-MN and no CuO peaks were observed confirming the very small size of CuO nanoparticles (< 5 nm). Only the broad peak relative to amorphous SiO2 was obtained as for the parent silica monolith. Analogously to the fresh catalyst, XRD on the spent CuO@SiO2-MN showed no CuO diffraction peaks, confirming the small size of the CuO nanoparticles (< 5 nm) even after catalysis, as evidenced by TEM. The XRD analysis of the fresh and the spent catalysts were reported in the new Fig. S1 and discussed in the main text of the revised manuscript (pag. 3, line 137-138 and page 7, line 254-256)

Following the reviewer we went deeper insight in the study of the CuO phase. With this aim we carried out also a TPR analysis of both the fresh and spent catalyst (new Fig. S2). In both catalysts a very symmetrical and sharp peak, with a maximum located at a relatively low temperatures (about 230 °C) was observed according to the presence of a uniform and highly dispersed copper phase, easy to be reduced to metallic Cu (see page 3, line 139-142 and page 7, line 256-259 of the main text).

 

 

 

3682. Huo, J. Ouyang, H. Yang, Scientific Reports, 2014, 4, 3682. DOI: 10.1038/srep03682

I.M. El-Nahhal et al., J. Sol-Gel Sci. Technol., 2016. DOI: 10.1007/s10971-016-4034-z

S.J. Ahmadi, Particuology, 2011, 9, 480-485. DOI: 10.1016/j.partic.2011.02.010

880. Xu, Petroleum Chemistry, 2020, 60, 858-880. DOI: DOI : 10.1134/S0965544120080150

 

2/ In addition, the thermal stability for CuO@SiO2-MN should also be included and discussed.

Prior of the Cu deposition the monoliths was calcined at 550 °C for 8 h to remove organics from the surface and to obtain a stable hierarchical structure of the silica monolith. Moreover, after the Cu chemisorption hydrolysis deposition the monolith was treated at 350°C for 4 h in air in order to obtain the formation of stable CuO NPs. The catalytic tests were carried out at significantly lower temperatures (i.e. 60°C) as well as the regeneration (220°C for 3 hour), thus we assumed that the CuO@SiO2-MN is stable under the experimental conditions used.

3/ Regarding the catalytic studies, the catalyst CuO@SiO2-MN suffered a decrease in catalytic activity from 14 h to 34 h. I suggest adding some characterization data, namely FT-IR spectra and powder XRD, and commenting on the differences when comparing it with the as-synthesized material.

FTIR and XRD spectra were performed on the spent catalyst and added to Figure S1 and S2. FTIR spectrum shows no difference with the one of fresh catalyst. XRD pattern of spent catalyst shows no peaks of CuO, relative to the formation of larger particle. The fresh catalyst is blue, while the spent catalyst is grey (Figure S1). The main reason for the decrease in catalytic activity should be the presence of carbon residues poisoning the Cu active sites (< 4 wt%), as evidenced by TGA of spent catalyst (Figure S5). Moreover, as reported above additional TPR analysis of both the fresh and spent catalyst pointed out the presence of a uniform and highly dispersed copper phase, easy to be reduced to metallic Cu.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors present an novel adaptation of methodology for production of a CuO/Silica catalyst and provide experimental data on its performance. The synthesis methods make the paper of potential strong interest.  Some minor edits are needed to the paper to fully convey its positives and make the work suitable for publicaiton:

 

1.     The authors should revise section 2.2 to better separate out the new results from past results. The first part of the section reads more like a lit review that should be part of the introduction rather than the

2.     It is highly desirable that the authors provide a secondary validation of the CuO particle size.  “representative” TEM samples are always dubious without other support. Something as simple as Schulz-Flory xrd analysis would help in establishing that TEM/STEM give sized of the correct order of magnitude.

3.     The authors should briefly justify the importance of the probe reaction used to the borader audience early in the introduction.

4.     Given the structure as shown in micrographs, the authors should better explain what they mean by “monolith “as it does not aper to have the uniform directionality of pores normally expected for that description.

5.     The numbering of sections skips form 3 to 5

Author Response

Authors Answers:

Reviewer 3: The authors present an novel adaptation of methodology for production of a CuO/Silica catalyst and provide experimental data on its performance. The synthesis methods make the paper of potential strong interest.  Some minor edits are needed to the paper to fully convey its positives and make the work suitable for publicaiton:

 

We thank the reviewer for the very positive comments.

 

1. The authors should revise section 2.2 to better separate out the new results from past results. The first part of the section reads more like a lit review that should be part of the introduction rather than the

 

Thank you for the proper suggestion. We moved the description of the past results into the introduction section (see introduction section of the revised manuscript).

 

 

2. It is highly desirable that the authors provide a secondary validation of the CuO particle size. “representative” TEM samples are always dubious without other support. Something as simple as Schulz-Flory xrd analysis would help in establishing that TEM/STEM give sized of the correct order of magnitude.

 

XRD was performed on both fresh and spent CuO@SiO2-MN and no CuO peaks were observed confirming the very small size of CuO nanoparticles (< 5 nm). Only the broad peak relative to amorphous SiO₂ was obtained as for the parent silica monolith. A new Figure S1 was added in Supplementary Information.

Additionally, TPR analysis of both the fresh and spent catalyst (new Fig. S2) were carried out. In both catalysts a very symmetrical and sharp peak, with a maximum located at a relatively low temperatures (about 230 °C) was observed according to the presence of a uniform and highly dispersed copper phase, easy to be reduced to metallic Cu (see page 3, line 139-142 and page 7, line 256-259 of the main text).

 

 

3. The authors should briefly justify the importance of the probe reaction used to the borader audience early in the introduction.

 

According to the reviewer’s suggestion a new sentence on the importance of the probe reaction was included into the introduction section (see revised manuscript).

 

 

4. Given the structure as shown in micrographs, the authors should better explain what they mean by “monolith “as it does not aper to have the uniform directionality of pores normally expected for that description.

 

The name “monolith” has been given to this type of materials from the IUPAC definition (explained in ref. 17 Sachse, New J. Chem., 2011) as “a shaped, fabricated intractable article with a homogeneous microstructure that does not exhibit any structural components distinguishable by optical microscopy”

 

There is different types of monoliths, and effectively the more known are the ones obtained by extrusion giving honeycomb with uniform straight channels, but there are also foams, and others…

 

 

5. The numbering of sections skips form 3 to 5

 

Yes, 5. Conclusions, has been replaced by 4. Conclusions

Thank you

Round 2

Reviewer 2 Report

All the suggestions commented on by the reviewer were adequately addressed and supported by characterization data.