Glycerol Valorization over ZrO₂-Supported Copper Nanoparticles Catalysts Prepared by Chemical Reduction Method

Round 1

Reviewer 1 Report

This manuscript concerns “Glycerol Valorization over ZrO2-Supported Copper Nanoparticles Catalysts Prepared by Chemical Reduction Method”. The work is very basic in this research work and has already been discussed widely. However, I am summarizing below some of my concerns regarding the manuscript that should be addressed properly before publication:

1. The novelty of this work is not enough better to clarify with more detail and how the Author distinguishes this work from others as a lot of work is published previously?
2. The authors should cite recent advanced and promising research work in the introduction section. Other cited references should also be revised with publications in recent years especially in the last 5 years. (All 61 references are very old).

3 The author discussed the Surface atomic percentages in Table 3. Did the author also verify this composition with TEM results or perform ICP measurements? I think only XPS atomic composition is not enough.

4. The author discussed ZrO2-Supported Copper Nanoparticles as Catalysts however, showed TEM images for Cu nanoparticles in the main text why? The authors should present TEM images for Cu-loaded ZrO2.
5. Why the author did not consider catalytic stability. Stability tests are required to evaluate catalyst properly. It is better to show SEM/TEM images before and after the activity evaluation in the dehydration of glycerol.
6. ZrO2-Supported Copper Catalysts are widely discussed previously hence it is recommended to show a comparison table with other published work to make a difference and clarity of your work.
7. It is noticed that on Page 11; line from 309-328 and Page 17;18, line from 487-515 the paragraphs look like a copy-paste and should be re-write to avoid plagiarism.
8. All figures should follow the same parameters to understand well for example in Figure 1 TEM images, the author used to apply alphabets however in figure 2 and 3 numbers. It is recommended to keep one standard to follow in all figures to keep the connectivity and clarity.
9. a, b, c labels are not shown in figure 6 and is so confusing. It is recommended to label properly.

After careful reading the manuscript, the overall suggestion is for a major revision to remove typo errors and with proper arguments.

Author Response

 POINT-TO-POINT ANSWER FOR MAJOR REVISION

 REVIEWER #1:

1. The novelty of this work is not enough better to clarify with more detail and how the Author distinguishes this work from others as a lot of work is published previously?

Answer: Thanks you for your comments about our manuscript.

 

We added in the revised version of the Manuscript file the following text marked in yellow color (line 372-381) with the aim to remark the relevance of our work:

 

Most of the related investigation of glycerol conversion to 1,2 propanediol (1,2-PDO) have been performed under hydrogen pressure employing H₂ from an external supply. In this respect, the need of external hydrogen resource and the requirement of high hydrogen pressures is the main disadvantage of reported studies. To overcome this inherent drawback one promising alternative is to produce the required hydrogen in situ directly in the reaction mixture under the process conditions. Here, we reported the formation of 1,2-propanediol from glycerol under inert atmosphere and autogenous pressure. This is a promising alternative approach to produce value added compounds from glycerol without the use of hydrogen. It is reported that the hydrogen is generated in-situ by dehydrogenation paths of glycerol conversion [39] These hydrogen species may hydrogenate the formed acetol producing 1,2-PDO.

 

• The authors should cite recent advanced and promising research work in the introduction section. Other cited references should also be revised with publications in recent years especially in the last 5 years. (All 61 references are very old).

Answer: Thank you for your comments about our manuscript. However, at this point we need to emphasize that not all 61 references are very old. Based on the original version of the submitted manuscript the publications of the last 5 years (from 2016) are:

Reference [7] (2016); Reference [15] (2019); Reference [16] (2017); Reference [17] (2016); Reference [19] (2018); Reference [22] (2020); Reference [23] (2018); Reference [24] (2021); Reference [25] (2018); Reference [26] (2021); Reference [28] (2016); Reference [32] (2016); Reference [35] (2017); Reference [46] (2019); Reference [47] (2016); Reference [48] (2017); Reference [50] (2017)

 

On the other hand, we have also revised the introduction with new additional updated references:

 

New references added in the revised version of the manuscript:

 

10. Sun, Y. Yamada, S. Sato, W. Ueda, Glycerol hydrogenolysis into useful C3 chemicals, Appl. Catal. B Environ. 193 (2016) 75–92, https://doi.org/10.1016/j. apcatb.2016.04.013.

 

10. Basu, V. Shree, A.K. Sen, Role of cerium as a promoter and process optimization studies for dehydrogenation of glycerol to acetol over copper chromite catalyst, https://doi.org/10.1016/j.jre.2021.01.013

 

51. Azri, I. Ramli, U. I. Nda-Umar, M. R. Shamsuddin, M. I. Saiman, Y. H. Taufiq-Yap, Copper-dolomite as effective catalyst for glycerol hydrogenolysis to 1,2-propanediol, Journal of the Taiwan Institute of Chemical Engineers, 112 (2020) 34-51.

 

Liu, Y., Guo, X., Rempel, G.L., Ng. F.T.T., The promoting effect of Ni on glycerol hydrogenolysis to 1,2-propanediol with in situ hydrogen from methanol steam reforming using a Cu/ZnO/Al₂O₃ catalyst, Catalysts 9 (2019) 412.

 

2. Mitta, N. Devunuri, Jyothi Sunkarin Suresh Mutyala, Putrakumar, Balla, Vijayanand Perupogu, A highly active dispersed copper oxide phase on calcined Mg₉Al_2.7-Ga_2.3O₂catalysts in glycerol hydrogenolysis, Catalysis Today 375 (2021) 204-215.

 

319. Mitta, P. K. Seelam, S. Ojala, R. L. Keiski, P. Balla, Tuning Y-zeolite based catalyst with copper for enhanced activity and selectivity in vapor phase hydrogenolysis of glycerol to 1,2-propanediol, Applied Catalysis A: General 550 (2018) 308-319.

 

Naveen Kumar Mishra, Praveen Kumar, Vimal Chandra Srivastava, Urška Lavrenčič Štangar, Synthesis of Cu-Based catalyst for hydrogenolysis of glycerol to 1,2-propanediol with in-situ generated hydrogen, Journal of Environmental Chemical Engineering, 9 (2021) 105263,

3. The author discussed the Surface atomic percentages in Table 3. Did the author also verify this composition with TEM results or perform ICP measurements? I think only XPS atomic composition is not enough.

Answer: Thank you for your comments about our manuscript. Due to the unprecedent context of the Pandemic the accessibility to the HR-TEM & ICP analysis is under restriction. In this regard, the only facility currently available in our Campus is SEM-Elemental mapping to get more insights about the copper composition in our catalysts. The results of energy-dispersive X-ray (EDS) matches well respect the nominal values of the copper loading. In conclusion, it is sufficient evidence for the scope of this piece of work.

Please check the Figure 9 (highlighted ESI-file). Also, SEM-EDS after glycerol reaction in Figure 10 (highlighted ESI-file).

 

1. The author discussed ZrO2-Supported Copper Nanoparticles as Catalysts however, showed TEM images for Cu nanoparticles in the main text why? The authors should present TEM images for Cu-loaded ZrO2.

Answer: Thank you for your comments about our manuscript. We move the TEM of CuNPs/ZrO₂ Catalyst from ESI-file to highlighted Main Manuscript, please check Figure 2 (line 141).

1. Why the author did not consider catalytic stability. Stability tests are required to evaluate catalyst properly. It is better to show SEM/TEM images before and after the activity evaluation in the dehydration of glycerol.

Answer: Thank you for your comments about our manuscript. The focus of our work was the application of the copper nanoparticles prepared by chemical reduction method as a strategy to disperse copper in the catalyst. We have investigated recycling in previous work (Chimentão, R.J.; Hirunsit, P.; Torres, C.S.; Ordoño, M.B.; Urakawa, A.; Fierro, J.L.G.; Ruiz, D. Selective dehydration of glycerol on copper based catalysts. Catal. Today 2021, 367, 58–70, doi:https://doi.org/10.1016/j.cattod.2020.09.031.), but considering the scope of the current piece of work and the limits impose for the Pandemic conditions in Chile we propose the follow complementary SEM-EDS information for this work:

The analysis of SEM-EDS data obtained from Cu NPs/ZrO₂ pH = 7 catalyst after reaction was added to the highlighted main manuscript (please check line: 400-409) and Figure S9 (highlighted ESI-file). Also, SEM-EDS after glycerol reaction in Figure S10 (highlighted ESI-file).

 

1. ZrO₂-Supported Copper Catalysts are widely discussed previously hence it is recommended to show a comparison table with other published work to make a difference and clarity of your work.

Answer: Thank you for your comments about our manuscript. We have added in the revised version of the manuscript the following Table 6 ,(line 470), and respective  text in the manuscript (line: 467). 

5. It is noticed that on Page 11; line from 309-328 and Page 17;18, line from 487-515 the paragraphs look like a copy-paste and should be re-write to avoid plagiarism.

 

Answer: We improved this point in the revised version of the manuscript. We declare that in the submitted version the text was written in an original way inside of our group. It is referred to the experimental protocol reported by our previous publication (Chimentão, R.J.; Hirunsit, P.; Torres, C.S.; Ordoño, M.B.; Urakawa, A.; Fierro, J.L.G.; Ruiz, D. Selective dehydration of glycerol on copper based catalysts. Catal. Today 2021, 367, 58–70, doi:https://doi.org/10.1016/j.cattod.2020.09.031.). Focusing on the commentary of the reviewer we re-write the sentences in the revised version to reinforce this observation citing the respective paper:

 

line from 309-328: (309-323 in revised version)

 

The Zr 3d spectra are shown in Figure 3 (b). All spectra are well described with one doublet with the Zr3d_5/2 binding energy at about 182.5 eV. Based on the biding energy values of Zr 3d_5/2 and Zr 3d_3/2 it is suggested that the zirconium oxide is present in a single type of oxide with an oxidation state of +4 [46]. The difference between the Zr 3d_3/2 and Zr 3d_5/2 binding energies is 2.43 eV and the integral intensities of its components relate as 3:2. 

The total quantity of acid sites was determined by NH₃-TPD. NH₃-TPD profiles (Figure S7) exhibit a maximum temperature at about 400 °C for Cu NPs/ZrO₂ pH = 7 and Cu NPs/ZrO₂ pH = 9 samples. The NH₃-TPD of the Cu NPs/ZrO₂ pH = 4 catalyst exhibits a peak at 300 °C together a tail suggesting a wide distribution of strength of acidic sites. The Cu NPs/ZrO₂ pH = 9 presented the highest total amount of acid sites as shown in Table 2. The strength of the solid acid sites can be distinguished by the different range of temperatures of NH₃ desorption such as weak (120-300 ⁰C), moderate (300-450 ⁰C) and strong (above 450 ⁰C) [52].  The results of the total acidity determined for the Cu nanoparticles catalysts quite relate well with typical catalysts for glycerol dehydration such as H-ZSM-5 and aluminosilicate supported heteropoly acids with the total acidity values of 0.2-0.9 mmol g^-1 [53] and 0.2-0.4 mmol g^-1 [51], respectively.

 

Line from 487 -515:

 

Re-write in main manuscript: line 514 – 544 (543-554 in the revised version)

The textural properties of copper nanoparticles catalysts were analyzed by N₂-physisorption with a Micromeritrics ASAP 2010 equipment. Before the analysis at about 100 mg of the sample was degassed under vacuum at 120 °C for 4 h. The specific surface area of the catalysts was calculated by BET equation. The total pore volume was calculated at P/P₀ equal to 0.99.  The average pore size was determined by Barret-Joyner-Halenda (BJH) method using the desorption isotherm branch.

X-ray diffraction was performed in a Bruker diffractometer model D4Endeavor equipped with a nickel filter and a Cu-Ka-ray source (l = 0.154 nm). The analysis conditions were 40 kV and 20 mA. The diffractograms were measured in a range of Bragg angles (2θ) between 10 ° and 90 ° at 0.02 counts per second.

Before the temperature programmed of reduction (TPR) at about 100 mg of catalyst was oxidized at 200°C with air flow (50 mL/min) for 2 hours. The TPR was carried out using 10%H₂/Ar with a flow of 50 mL/min raising the temperature from room temperature to 800°C with a heating rate of 5°C/min. TCD detector was used to monitor the TPR. The water formed during the reduction process was trapped by a salt/ice cooling bath.

The catalysts were reduced at 200 °C with a H₂ flow (50 ml/min) for 2 hours and then cooled to 40 °C under He flow (50 mL/min) before the temperature-programmed desorption of ammonia (NH₃-TPD). A flow of 50 mL/min of NH₃ was used to saturate the sample surface for 10 minutes. The physisorbed NH₃ was removed with a He flow (50 mL/min) and the system was heated up from 40 to 75°C under He flow. Finally, in the TPD analysis the system was heated from 75 to 800°C at rate of 5°C/min. TCD detector monitored the desorption of NH₃.  

XPS chemical analysis was carried out on a Kratos Axis Ultra HAS spectrometer, with a hemispheric analyzer of Mg Kα X-rays radiation source (hv = 1253.6 eV), conducted at 10 mA and 15 kV. The binding energies of the XPS spectra were referred to the C1s component (BE = 285 eV).

The copper catalyst was reduced with 5% H₂/Ar (20 mL/min) at 5 °C/min from 25 °C to 200 °C holding the maximum temperature for 2 hours before the N₂O analysis. The adsorption capacity and dispersion of copper was determined by N₂O chemisorption with an Autochem II 2920 equipment. Selective oxidation of the copper surface to Cu₂O was performed under 20% N₂O/Ar flow (20 mL/min) at 40 °C (N₂O +2Cu_surface ® Cu₂O_surface + N₂). Then Cu₂O surface was reduced with 5% H₂/Ar (20 mL/min) at 5 °C /min from 25 °C to 900 °C (H₂+Cu₂O_surface ® 2Cu_surface + H₂O). Surface copper was calculated considering the stoichiometry of N₂O/Cu = 0.5  and dispersion (D) was determined as the ratio between the amount of surface copper and total nominal content of copper. The copper nanoparticle diameter (  were determined by:

1. All figures should follow the same parameters to understand well for example in Figure 1 TEM images, the author used to apply alphabets however in figure 2 and 3 numbers. It is recommended to keep one standard to follow in all figures to keep the connectivity and clarity.

Answer:

 We understand that labelling the Figures with a, b and c it will be easier to distinguish the cases where there is a group of profiles. In those cases, it is easier to name each profile by number such as 1, 2 and 3 instead of a, b and c. In this regard we prefer keep the manuscript in the original form.

1. a, b, c labels are not shown in figure 6 and is so confusing. It is recommended to label properly.

Answer:

 The same remark expressed in the previous answer we emphasize here.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

 

The authors prepared Cu/ZrO₂ catalysts for dehydration of glycerol. While the method is simple and an indecent catalytic result was attained. The work suffered obvious drawbacks in catalysts characterization. As the authors discussed in line 221-228, bulk Cu may have formed. TEM only captures a tiny area of catalyst. There was no clear proof that Cu NPs are supported on ZrO₂. I advise using SEM/EDS to get a general picture of the catalyst.

(1) Abstract and Introduction

Revise sentences line 16-17, Line 25-26, 30-32

The introduction is poorly organized and contains much information less relative to the work. The introduction should be revised to be concise.

Line 107-110, line 110-111

“The distribution of metal NPs of a colloidal solution onto …” Please give proper references for each case.

Please list literature proof for line 110-111.

Line 100-105 and line 113-116 are redundant. Line 125-129 should not be mentioned in the last paragraph of introduction. The utilization of ZrO₂ should be mentioned in the introduction of glycerol dehydration.

(2) Figure 1 and Figure S1, “TEM images of the copper nanoparticles catalysts”

Figure 1 (a)(c)(e) show the TEM images of pure copper NPs. However, copper NPs are not stable and easily aggregated within minutes. The authors admitted in Section 2.2 the Cu NPs got easily oxidized. How the authors prepared the sample grid (e.g. How long the reduction was carried out before dropping the colloidal Cu NPs on lacey carbon grids)? The authors should revise the sentence in line 150-152 as the size distribution is not narrow.

The Cu/ZrO₂ TEM images (Figure S1) are of low resolution and the readers cannot discern Cu NPs and ZrO₂ support. The authors need to show high-quality TEM images (e.g. STEM). Moreover, the TEM image of ZrO₂ support should be given.

(3) Section 2.2 UV-visible spectroscopy of Cu-NPs

Line 160-161, line 169-170

Where is the work “described in section 2.2.1”? The authors work is to prepare “support Cu₂O (instead of Cu) nanoparticles on ZrO₂ for catalytic applications”.

(4) Line 202-209

The authors proposed that “the appearance of CuO bulk would occur for copper loading equal to or higher than at about 7 wt%”. The discussion is not convincing. Please give more proof.

Author Response

POINT-TO-POINT ANSWER FOR MAJOR REVISION

REVIEWER #2:

The authors prepared Cu/ZrO₂ catalysts for dehydration of glycerol. While the method is simple and an indecent catalytic result was attained. The work suffered obvious drawbacks in catalysts characterization. As the authors discussed in line 221-228, bulk Cu may have formed. TEM only captures a tiny area of catalyst. There was no clear proof that Cu NPs are supported on ZrO₂. I advise using SEM/EDS to get a general picture of the catalyst.

Answer:.

 

Due to the unprecedent context of the Pandemic the accessibility to the ICP analysis is under restriction. In this regard, the only facility currently available in our Campus is SEM-Elemental mapping to get more insights about the copper composition in our catalysts. The results of energy-dispersive X-ray (EDS) matches well respect the nominal values of the copper loading. In conclusion, it is sufficient evidence for the scope of this piece of work.

Please check the Figure S9 (highlighted ESI-file). Also, SEM-EDS after glycerol reaction in Figure S10 (highlighted ESI-file).

 

(1) Abstract and Introduction

Revise sentences line 16-17, Line 25-26, 30-32

Answer: Thank you for your comments about our manuscript. The following main manuscript modifications were carried out and highlighted:

 

Line 16-17:

An hypsocromic shift was noticed by UV-vis spectroscopy as the pH of the synthesis increased from pH=4 to pH=9.

 

Line 25-26:

TPR patterns showed two main peaks for Cu NPS/ZrO₂ pH=9 catalyst. The first peak between 125 and 180°C (region I) is ascribed to more dispersed copper species and the second one between 180 and 250°C (region II) is assigned to bulk CuO.

 

Line 30-32 revised:

The different pH applied to the synthesis medium of the copper nanoparticles determined the resultant copper dispersion on the ZrO₂ support providing active domains for the glycerol conversion.

The introduction is poorly organized and contains much information less relative to the work. The introduction should be revised to be concise.

Answer:

We have worked to re-organize the introduction in a concise way and focused to the importance of chemical reduction method in the synthesis of copper nanoparticles and its application in the glycerol conversion. The new version of the introduction with new paragraphs is marked in yellow color in the revised version of the manuscript (line 37-125).

Line 107-110, line 110-111

“The distribution of metal NPs of a colloidal solution onto …” Please give proper references for each case.

Answer: Thanks for your comment. Three appropriate references were incorporated to the main manuscript, line 107 highlighted.

Please list literature proof for line 110-111.

Answer: Thanks for your comment. Three appropriate references were incorporated to the main manuscript, line 110 highlighted.

Line 100-105 and line 113-116 are redundant. Line 125-129 should not be mentioned in the last paragraph of introduction. The utilization of ZrO₂ should be mentioned in the introduction of glycerol dehydration.

Answer: Thank you very much for your commentary. We agree. The new version is marked in yellow color in the revised version of the manuscript as follow (line 71-76):

The Lewis acid sites of ZrO₂ are water tolerant [20], therefore it is a highly favorable support for the glycerol transformation which occurs in water medium. ZrO₂ is a material with remarkable hydrothermal stability, it has acid-base properties and oxidation-reduction capacity. Tolerant Lewis acid sites in the catalytic system is necessary for systems in contact with aqueous environment. It should also be mentioned that ZrO₂ is a low-cost material.

 

(2) Figure 1 and Figure S1, “TEM images of the copper nanoparticles catalysts”

Thank you very much for your commentary. We have revised this point in the revised version of the manuscript. as follow (line 141):

Figure 2. TEM images of the copper nanoparticles/ZrO2 catalysts. (a) Cu NPs/ZrO₂ pH = 4, (b) Cu NPs/ZrO₂ pH = 7, (c) Cu NPs/ZrO₂ pH = 9.

Figure 1 (a)(c)(e) show the TEM images of pure copper NPs. However, copper NPs are not stable and easily aggregated within minutes. The authors admitted in Section 2.2 the Cu NPs got easily oxidized. How the authors prepared the sample grid (e.g. How long the reduction was carried out before dropping the colloidal Cu NPs on lacey carbon grids)? The authors should revise the sentence in line 150-152 as the size distribution is not narrow.

Answer: Thank you very much for your comment. We have worked this issue in the new version of the manuscript. You can find the TEM image of isolated CuNPs and supported CuNPs/ZrO₂ materials between lines 134 and 142. The additional information was incorporated in ESI-file.

About the TEM image of the copper nanoparticles:

The following statement was added in line (503-505) of the revised version:

The as-prepared nanoparticles by the chemical reduction method were washed with ethanol and finally stored also in ethanol. Before TEM analysis one drop of the copper nanoparticles in ethanol was added in 10 mL of ethanol (10 times of dilution) and sonicated before to be dropped on the carbon grids.

 

The Cu/ZrO₂ TEM images (Figure S1) are of low resolution and the readers cannot discern Cu NPs and ZrO₂ support. The authors need to show high-quality TEM images (e.g. STEM). Moreover, the TEM image of ZrO₂ support should be given.

Answer:

About the TEM image of the copper nanoparticles catalysts:

In the Campus of the University, we have accessed to TEM Transmission Electron Microscopy (TEM-EDX) using a JEOL transmission electronic microscope model JEM 1200 II. Due to the unprecedent time of the Pandemic it is not possible to access higher resolution electron transmission macroscopy (HRTEM) from external collaboration due to the short deadline for major revision. However, we present SEM-EDS data in the new version of the manuscript SI) with the aim to demonstrate the presence of Cu and ZrO₂ in the catalyst.  

(3) Section 2.2 UV-visible spectroscopy of Cu-NPs

Line 160-161, line 169-170

Where is the work “described in section 2.2.1”? The authors work is to prepare “support Cu₂O (instead of Cu) nanoparticles on ZrO₂ for catalytic applications”.

Answer:

Thank you very much for your commentary. There was a mistake in the original submitted manuscript edition. The right phrase is: “The samples were prepared following the protocol details described in section 4.3.1”. This revised phrase was included in the highlighted manuscript (line 156).

(4) Line 202-209

The authors proposed that “the appearance of CuO bulk would occur for copper loading equal to or higher than at about 7 wt%”. The discussion is not convincing. Please give more proof.

Answer:

This statement was based on the following literature report:

reference 52 in the new version (Lopez, A. Bueno, M. Illán, Appl. Catal. B-Environ. 84 (3-4) (2008) 651–658).

To support this statement, we added new references in the following text marked in yellow color in the revised version of the manuscript (line 209):

At 7 wt.% of copper it is stated to be possible to identify Cu peaks in the XRD analysis [F. López, A. Bueno, M. Illán, Appl. Catal. B-Environ. 84 (3-4) (2008) 651–658.]. In addition, previous work reported that the highly crystalline CuO is founded from around 4 wt.% of Cu for 100 m²g^{− 1} of γ-Al₂O₃ [G.Aguila, F. Gracia, P. Araya, CuO and CeO2 catalysts supported on Al₂O₃, ZrO₂ and SiO₂ in the oxidation of CO at low temperature, Applied Catalysis A:General 343 (2008) 16-24].

Author Response File: Author Response.pdf

Reviewer 3 Report

After careful revision of the manuscript entitled Glycerol valorization over ZrO2-Supported Copper Nanoparticles Catalysts Prepared by Chemical Reduction Method by J. Garcés, R. Arrué, N. Novoa, A. F. Peixoto and R. J. Chimentão this referee suggests major revision. In this paper, Chimenão et. al present a methodology to synthesize spherical nanoparticles using glycerol as medium and hydrazine as reducing agent. These nanoparticles are, at the same time, used as catalysts for the dehydration of glycerol.

While the research might be interesting for the readership of the journal Catalysts, there are aspects that require attention prior to consideration of acceptance. In general terms, the presentation of the introduction and results needs to be revised prior to acceptance in this journal and some experiments done or further clarified. A compelling story should be presented and not a compendium of results. To clarify this general comment, I will present my comments and concerns by sections.

Introduction:

There is a disconnection between the main achievement of the paper, i.e. preparation of nanoparticles using glycerol as medium and hydrazine as reducing agent (as is stated in the materials and methods, section 4.1) and subsequent application to the dehydration of glycerol and the presentation of the results. The initial part of the introduction pivots mainly about glycerol and its positives and barely shows why the dehydration of this species is of interest to the scientific community. In this regard, perhaps deleting the first three paragraphs of the introduction and incorporating a paragraph on this topic will very much help to focus the introduction on the topic.

Results:

There is a significant disconnection between the introduction section and the description of the results. The rationale behind the experiments discussed should be advanced to the reader in this section.

An explanation of the selection of the pHs would be a plus for this scientific contribution.

In line 152 the authors use one decimal to refer to the diameter of the nanoparticles, however, in Figure 1 the pictures use no decimals. The authors should be consistent in the number of decimals used. This inconsistency on the decimal use is extended to the rest of the paper, note that at lines 279-282 this inconsistency appears again.

In line 161 the authors refer to section 2.2.1…. I am afraid I cannot find this section in the paper.

In section 2.3, line 182: a description of the chemical reduction method must be indicated. Whether this is in the main text or the SI it is necessary for the sake of clarity.

At lines 184-186 the authors say: “This decrease could be caused by the consumption of the surface hydroxyl groups of the support through a superficial reaction with the oxidized species and the agglomeration process of the copper species inside the pores of ZrO2 during preparation of catalysts”, what evidence do they have for making this suggestion? This needs to be further clarified.

The text on page 7/23 is convoluted, the authors clarify the relevance of regions of temperature and their appearance after they are discussed. This is not clear. The regions should be clarified in the Figure 2, as it is written now is not straightforward to infer which temperature corresponds to which region.

The authors should also clarify what is the composition of the buffers they use and how this composition affects the outcome of the process.

At section Catalyst activity, the authors claim that they found first-order reaction. Regarding this statement, it should be indicated the order with respect to which reagent (catalyst/glycerol) and also it would be beneficial for the paper to have the order for the other reagent.

There is a quite dramatic change in the k value when going from pHs 4 to 7 to pHs 7 to 9, an explanation on this would increase the value of these results.  How can the authors be sure that without the nanoparticles the dehydration of glycerol would not happen at pH9? These tests should be incorporated into the text.

In the Discussion section, the authors mention that water can deactivate the catalyst, have the authors tried to do the reaction in other polar non-protic solvents?

Author Response

REVIEWER #3:

After careful revision of the manuscript entitled Glycerol valorization over ZrO2-Supported Copper Nanoparticles Catalysts Prepared by Chemical Reduction Method by J. Garcés, R. Arrué, N. Novoa, A. F. Peixoto and R. J. Chimentão this referee suggests major revision. In this paper, Chimenão et. al present a methodology to synthesize spherical nanoparticles using glycerol as medium and hydrazine as reducing agent. These nanoparticles are, at the same time, used as catalysts for the dehydration of glycerol.

While the research might be interesting for the readership of the journal Catalysts, there are aspects that require attention prior to consideration of acceptance. In general terms, the presentation of the introduction and results needs to be revised prior to acceptance in this journal and some experiments done or further clarified. A compelling story should be presented and not a compendium of results. To clarify this general comment, I will present my comments and concerns by sections.

Introduction:

There is a disconnection between the main achievement of the paper, i.e. preparation of nanoparticles using glycerol as medium and hydrazine as reducing agent (as is stated in the materials and methods, section 4.1) and subsequent application to the dehydration of glycerol and the presentation of the results. The initial part of the introduction pivots mainly about glycerol and its positives and barely shows why the dehydration of this species is of interest to the scientific community. In this regard, perhaps deleting the first three paragraphs of the introduction and incorporating a paragraph on this topic will very much help to focus the introduction on the topic.

Answer:

Done. We re-organize the introduction in a concise way and focused to the importance of chemical reduction method in the synthesis of copper nanoparticles and its application in the glycerol conversion. Also, new related references have been incorporated.

The new version of the introduction is marked in yellow color in the revised version of the manuscript.

Results:

There is a significant disconnection between the introduction section and the description of the results. The rationale behind the experiments discussed should be advanced to the reader in this section.

An explanation of the selection of the pHs would be a plus for this scientific contribution.

Answer: The pH =4, 7 and 9 synthesis conditions were selected to explore the control of the nanoparticle size respect to the literature reports. Some examples are reported for instance by ref 43 (Johnson, B.F.G. Nanoparticles in Catalysis. Top. Catal. 2003, 24, 147–159, doi:10.1023/B:TOCA.0000003086.83434.b6.).

This reference is already incorporated to the highlighted manuscript.

In line 152 the authors use one decimal to refer to the diameter of the nanoparticles, however, in Figure 1 the pictures use no decimals. The authors should be consistent in the number of decimals used. This inconsistency on the decimal use is extended to the rest of the paper, note that at lines 279-282 this inconsistency appears again.

Answer:

We have worked to express consistence number of decimals along the revised version of the manuscript. Also, the figure 1 was modified in order to unified the format in the work.

In line 161 the authors refer to section 2.2.1…. I am afraid I cannot find this section in the paper.

Answer:

Thank you very much for your commentary. There was a mistake in the original submitted manuscript edition. The right phrase is: “The samples were prepared following the protocol details described in section 4.3.1”. This revised phrase was included in the highlighted manuscript (line 156).

In section 2.3, line 182: a description of the chemical reduction method must be indicated. Whether this is in the main text or the SI it is necessary for the sake of clarity.

Answer:

Thanks for your comment. The method was described in the section 4, materials, and methods, in the highlighted manuscript and a couple of representative reference was added in the respective section. This section was referrer in the line 178 (highlighted manuscript). In this regard, we propose not add additional information to ESI-file

At lines 184-186 the authors say: “This decrease could be caused by the consumption of the surface hydroxyl groups of the support through a superficial reaction with the oxidized species and the agglomeration process of the copper species inside the pores of ZrO2 during preparation of catalysts”, what evidence do they have for making this suggestion? This needs to be further clarified.

Answer: To this point we added the following text marked in yellow color for clarification (line 185-190):

Previous work reported the adsorption of Cu at ZrO₂ surface hydroxyl sites upon deposition. Such deposition predominates at lower copper weight loading and facilitates higher levels of dispersion [Okamoto, Y., and Gotoh, H., Catal. Today 36, 71 (1997)]. Apart of this, it is reported that oxygen vacancies stabilize Cu⁺ which is often invoked as key specie for activity in catalytic reactions [Tada, S.; Katagiri, A.; Kiyota, K.; Honma, T.; Kamei, H.; Nariyuki, A.; Uchida, S.; Satokawa, S. Cu species incorporated into amorphous ZrO2 with high activity and selectivity in CO₂ -to-methanol hydrogenation. J. Phys. Chem. C 2018, 122, 5430–5442]. Evidence that copper is interacting with the pore walls of the ZrO₂ is the decrease of the pore volume of zirconia support when the copper nanoparticles were introduced by the chemical reduction method (Table 1).

The text on page 7/23 is convoluted, the authors clarify the relevance of regions of temperature and their appearance after they are discussed. This is not clear. The regions should be clarified in the Figure 2, as it is written now is not straightforward to infer which temperature corresponds to which region.

Answer:

We clarified this point. Two main peaks were observed for Cu NPS/ZrO₂ pH=9 catalyst was noticed. The first peak between 125 and 180°C (region I) ascribed to more dispersed copper species and the second one between 180 and 250°C (region II) assigned to bulk CuO.

This new text to clarify this point was added in the line 212 and 223 of the revised version of the manuscript as follow:

Two main peaks were observed for Cu NPS/ZrO₂ pH=9 catalyst was noticed. The first peak between 125 and 180°C (region I) ascribed to more dispersed copper species and the second one between 180 and 250°C (region II) assigned to bulk CuO.

The authors should also clarify what is the composition of the buffers they use and how this composition affects the outcome of the process.

Answer:

Done. The appropriate information was added in the highlighted main manuscript, line 493.

At section Catalyst activity, the authors claim that they found first-order reaction. Regarding this statement, it should be indicated the order with respect to which reagent (catalyst/glycerol) and also it would be beneficial for the paper to have the order for the other reagent.

Answer:

We have clarified this point. The reaction is first order with respect to glycerol. We included this statement in line 341 on the revised version of the manuscript as follow:

The reaction was identified as first order referred to glycerol.

There is a quite dramatic change in the k value when going from pHs 4 to 7 to pHs 7 to 9, an explanation on this would increase the value of these results.  How can the authors be sure that without the nanoparticles the dehydration of glycerol would not happen at pH9? These tests should be incorporated into the text.

Answer:

We have added new text in line 328 in the revised version of the manuscript to clarify this point:

It is important to emphasize that pure ZrO₂ did not give noticeable glycerol conversion in absence of Cu NPs under the same experimental conditions.

 

In the Discussion section, the authors mention that water can deactivate the catalyst, have the authors tried to do the reaction in other polar non-protic solvents?

Answer:  

The use of polar proctic and non-proctic solvent is considered for future work. The use of hydrogen donor molecules can be of industrial importance. It is relevant to develop catalytic transformation of glycerol without the external use of hydrogen. Molecular hydrogen is produced in energy intensive processes from fossil supplies [A. Martin, U. Armbruster, I. Gandarias, P.L. Arias, Eur. J. Lipid Sci. Technol 115 (2013) 9.]. To overcome this inherent drawback one promising alternative is to produce the required hydrogen in situ directly in the reaction mixture under the process conditions. Here, in the present work (under aqueous medium) it was observed the formation of 1,2-propanediol. This suggests that acetol is being hydrogenated to the formation of 1,2-PDO. 

Concerning to this issue we added the following text in the revised version of the manuscript (line 122-125)

Crude glycerol is obtained as by-product in biodiesel production and represents 10 wt.% of the produced biodiesel. Crude glycerol contains impurities such as water. In this respect it is relevant to investigate the catalytic conversion of aqueous solution of glycerol as a strategy to develop active copper catalysts.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The present form of the manuscript after revision following the reviewers' comments seems much better. I agree with its publication.

Reviewer 3 Report

After revising the manuscript, this referee suggests acceptance in the present form.