Direct Synthesis of Dimethyl Ether from CO₂ Hydrogenation over Core-Shell Nanotube Bi-Functional Catalyst

Round 1

Reviewer 1 Report

This study aimed to develop a dual-functional, halloysite nanotube-supported CuZnO-PTA catalyst with a core-shell structure and investigate the effects of the active site mass ratio CuZnO/PTA on CO₂ conversion and DME selectivity. This work might be published after addressing the following issues:

1. For CO₂ hydrogenation, it is very important to understand the adsorption and desorption of CO₂ and H₂ on catalysts, so the authors should provide the relevant dates, such as CO₂-TPD, H₂-TPD characterization and TEM.

2.Please provide the calculation method of CuZnO dispersion in Table 1. In general, Cu dispersion (obtained from N₂O titration) is also an important factor to influence the reaction results, so the authors should provide the relevant dates.

3.In Figure4, the definition of particle sizes should be confirmed and the comparability of the particle sizes of three samples in Figure 4 should be thought about carefully.

4. There is only basic description of the characterization results and lack of any discussion, which failed to provide a systematic and convincing statement of the catalyst structure-performance relationships. More detailed discussion on the relationships between morphology, redox behaviors, adsorption capacity etc. and performance should be addressed. Then, the merits and novelty of the manuscript could be stood out.

5. The formats of the graphs and tables should be double checked, such as Figure 4 and Table 1.

6. The format of the reference should be double checked.

7.This paper lacks comparison with other catalysts.

8.Whether the selectivity of dimethyl ether will increase as the load of PTA continues to increase beyond 20 wt%.

9.The preparation method used in this paper is the impregnation method, so how stable is the catalyst.

10.What does “F” mean in the formula.

Author Response

#Reviewer 1

This study aimed to develop a dual-functional, halloysite nanotube-supported CuZnO-PTA catalyst with a core-shell structure and investigate the effects of the active site mass ratio CuZnO/PTA on CO2 conversion and DME selectivity. This work might be published after addressing the following issues:

1. For CO2 hydrogenation, it is very important to understand the adsorption and desorption of CO2 and H2 on catalysts, so the authors should provide the relevant dates, such as CO2-TPD, H2-TPD characterization and TEM.
   
   

Response: Thank you for your valuable feedback and for clarifying that the CO2-TPD and H2-TPD characterization data have been included in the study in figure 2a and Figure 3. We agree that understanding the adsorption and desorption of CO2 and H2 on catalysts is crucial for CO2 hydrogenation. We appreciate your efforts in making the study more comprehensive using FESEM  and SEM-EDX with mapping instead of TEM. Thank you again for taking the time to review our work. This information can be found in the manuscript in the following sentences:

“The TPD-CO2 technique was used to evaluate the basicity of CZ-PT@HNT reduced catalysts and revealed the presence of two types of basic sites in all four catalysts. The desorption temperature of CO2 was found to have a strong correlation with the basicity strength, and the overall basicity was influenced by the balance of active metal sites and solid acid sites. An increase in Cu metal loading resulted in a rise in basic site concentration. The CO2 adsorption capacity of the 20CZ-PTA@HNT catalyst was the highest among the prepared catalysts, with a total of 0.44029 mmol/g. The basicity increased in the sequence of 5CZ-PTA@HNT < 10CZ-PTA@HNT < 15CZ-PTA@HNT < 20CZ-PTA@HNT. However, strong basicity of the basic sites may negatively impact the selectivity of dimethyl ether (DME) production because dilute the solid acid sites of the catalyst .

 

2. Please provide the calculation method of CuZnO dispersion in Table 1. In general, Cu dispersion (obtained from N2O titration) is also an important factor to influence the reaction results, so the authors should provide the relevant dates.

Response: Thank you for your review and for taking the time to read our manuscript. We appreciate your feedback and the questions you have raised. In response to your comment regarding the calculation method of CuZnO dispersion in Table 1, we would like to inform you that we have removed the dispersion results from the table after considering your feedback. We used the equation ( Dispersion = (SA_before - SA_after) / SA_before ) to estimate the dispersion from BET of the HNT support before and after CuZnO loading , which was found to underestimate the presence of the solid acid site PTA in the catalyst structure . Unfortunately, we were not able to conduct N2O titration to accurately determine the Cu dispersion. We recognize that Cu dispersion is an important factor that can influence the reaction results, and we regret that we were not able to provide this data in our study. Thank you again for your time and attention. We look forward to your continued feedback.

 

3. In Figure 4, the definition of particle sizes should be confirmed, and the comparability of the particle sizes of the three samples in Figure 4 should be thought about carefully.

Response: Thank you for your insightful feedback on Figure 4. We have already confirmed the definition of particle sizes and carefully considered the comparability of the particle sizes of the three samples in the figure. “ Regarding the particle size definition in Figure 4,refers to the physical dimensions of individual particles of the sample,  typically we have measured the external diameter  in nanometers of the (HNTs), modified HNTs, and also CuZnO with spherical shape of the CZ-PTA@HNT samples using the Image J software. The measurements were taken from  (FESEM) images of the three samples. The results showed that the external diameter of the HNTs increased after modification with dimethylformamide (DMF-HNTs) from 59 nm to 88 nm, while the uniform dispersion of copper (Cu), copper oxide (Cu2O), and zinc oxide (ZnO) nanoparticles on the external surface of the HNTs had an average diameter size of 16-18 nm”. We understand the importance of carefully comparing the particle sizes of the three samples and will take this into consideration in the revised revision of the manuscript.We appreciate your attention to detail and are grateful for the opportunity to improve our work. Thank you again for taking the time to review it.

 

4. There is only basic description of the characterization results and lack of any discussion, which failed to provide a systematic and convincing statement of the catalyst structure-performance relationships. More detailed discussion on the relationships between morphology, redox behaviours, adsorption capacity etc. and performance should be addressed. Then, the merits and novelty of the manuscript could be stood out.

Response: Thank you for your constructive feedback. We agree that a detailed discussion of the relationships between catalyst structure, performance, morphology, redox behaviour, and adsorption capacity is important for providing a systematic and convincing statement. We have already addressed these relationships in a more detailed manner in our revised manuscript, and we believe this will enhance the novelty and merits of the work. This information can be found in the manuscript in the following sentences “ Rodex properties play a crucial role in the performance of catalysts in the direct synthesis of DME from CO2 hydrogenation. Cu and ZnO have a strong interaction, which can improve the reactivity and redox properties of the metal sites. The presence of oxygen vacancies on the surface makes it electron-rich, thus enhancing its electron-donating ability and increasing its activity in adsorbing hydrogen. On the other hand, when hydrogen adsorbs on the oxygen vacancy on the Cu-ZnO surface, it releases a proton to a nearby oxygen atom, resulting in the formation of two hydroxyl groups for each vacancy. Meanwhile, CO2 acts as a Lewis acid, and zinc vacancies on the surface serve as active Lewis basic sites for Lewis acid-base interactions. The core-shell structure of the catalyst is also a crucial factor for its effectiveness. The core-shell morphology allows for the integration of various catalytic sites that are active for different reactions. By customizing the core and shell materials to be compatible with sequential reactions, complementary reactions can be conducted in tandem to achieve the desired product conversion. This results in improved distribution of distances between the separate catalytic sites, which can mitigate the selectivity limitations of conventional bifunctional catalysts and improve reaction efficiency.In conclusion, the key attributes that a catalyst support must possess for successful direct CO2 hydrogenation to DME synthesis are: high surface area, morphological stability, and high mechanical strength. The combination of redox properties, the Cu-ZnO interaction, and the core-shell morphology is beneficial in improving the performance of the catalyst and ensuring successful direct synthesis of DME from CO2 hydrogenation”.Thank you again for taking the time to review our work and for providing such valuable suggestions.

 

5. The formats of the graphs and tables should be double checked, such as Figure 4 and Table 1.

Response: Thank you for bringing this to our attention. We appreciate your attention to detail and have already double-checked the formats of the graphs and tables, including Figure 4 and Table 1. We believe this will help enhance the manuscript's clarity and readability. Thank you again for taking the time to review our work and for providing such valuable suggestions.

 

6. The format of the reference should be double-checked.

Response: Thank you for your comment. We appreciate your attention to detail and have double-checked the references' format. Ensuring the references' consistency and accuracy is essential for our work's credibility. Thank you again for taking the time to review our work and for providing such valuable suggestions

 

7.This paper lacks comparison with other catalysts.

Response:  Thank you for your feedback. We agree that comparing our results with other catalysts would provide a more comprehensive and convincing evaluation of our work. We have already expanded the discussion section by bringing in more data from the literature for comparison with other catalysts highlighting the beneficial effect of the core-shell structure . This will help support our conclusions further and highlight the novelty of our results. Please read these comparison provided in the manuscript. Thank you again for taking the time to review our work and for providing such valuable suggestions.

 

 

 

 

 

 

8. Whether the selectivity of dimethyl ether will increase as the load of PTA continues to increase beyond 20 wt%.

Response:  Thank you for your feedback. We appreciate your suggestion to include information on the relationship between the selectivity of dimethyl ether and the loading of PTA beyond 20 wt%. Our manuscript has already highlighted the findings of” Ladera, Rosa María, et al. who studied methanol dehydration to DME over silica HSiW and tungstic HPW heteropoly acid supported on TiO2. They concluded that DME selectivity strongly depends on HPA loading on TiO2, and the optimum acid site loading was found to be 2.3 Keggin units per nm2 after well dispersing on the TiO2”. This information provides a useful context for our study and we believe it will be valuable for our readers. Thank you again for taking the time to review our work and for providing such valuable suggestions.

 

9. The preparation method used in this paper is the impregnation method, so how stable is the catalyst?

Response:  Thank you for your review and for taking the time to read our manuscript. We appreciate your feedback and the questions you have raised. In response to your question regarding the stability of the  catalyst, we would like to inform you that ” a reusability and stability  study was conducted under optimized conditions (260°C, 30 bar, GHSV of 12,000 h-1, and a catalyst weight of 0.3 g). The results showed that the catalyst was stable and maintained its high activity and selectivity over five consecutive cycles, with no significant decrease in CO2 conversion and DME selectivity observed during the 15 hours of total reaction duration”. The detailed results of the reusability study are presented in Figure 6(b) and discussed in the revised manuscript.Thank you again for your time and attention. We look forward to your continued feedback.

 

10. What does "F" mean in the formula?

Response:  Thank you for your review and for taking the time to read our manuscript. We appreciate your attention to detail and the feedback provided. In response to your question, "F" in the formula represents the molar flow rate of the reactants and products in the reaction.We have taken your feedback into consideration and have included an explanation of "F"in the revised manuscript. We hope that this clarification addresses your concerns and provides a clearer understanding of the formula.

 

 

 

 

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

Mohamud and the co-authors prepared a CuZnO catalyst with nanotube structure using for CO₂ hydrogenation. Several characterization methods such as BET, TPD, XRD were performed to investigate the catalyst properties. The results may be interesting, however, there are too many mistakes such as figure number, format leading to confusion. I don’t suggest publication before these issues are addressed.

1.       Line 74, what kind of bifunctional catalyst should be used in CO₂ hydrogenation? Some details should be given.

2.       Halloysite nanotube (HNT) was showed in many positions in the present work. It should be given when it first shows up, and then HNT should be used. The same issue is found in carbon dioxide (CO₂).

3.       The beginning pressure for BET is too large to miss micropore information.

4.       Line 222, it should be figure 3? And the Fig. 2 a should be H₂-TPR but not TPD.

5.       I can’t see any nanotube structure in Fig. 4 and the middle image in fig. 4 should be numbered (b).

6.       How does the author confirm the H₂ consumption?

7.       Line 438, the number in CO₂, NH₃ should be subscript. Same issue can be found in the whole manuscript, please check!

Author Response

#Reviewer 2

 

Mohamud and the co-authors prepared a CuZnO catalyst with a nanotube structure using for CO2 hydrogenation. Several characterization methods, such as BET, TPD, XRD were performed to investigate the catalyst properties. The results may be interesting; however, there are too many mistakes such as figure number, and format leading to confusion. I don't suggest publication before these issues are addressed.

 

1. Line 74, what kind of bifunctional catalyst should be used in CO2 hydrogenation? Some details should be given.

Response:  Thank you for your valuable comment. We appreciate your insights and suggestions that help us to improve our manuscript. We have already addressed your concern by adding more details in the introduction part about the bifunctional catalysts used in CO2 hydrogenation as “ The most widely used catalyst system in the synthesis of DME from CO₂ hydrogenation is a mixture of methanol synthesis and methanol dehydration catalysts, particularly the CuZnAl catalyst combined with ZSM-5 or γ-Al2O3”. The type of bifunctional catalyst used, its characteristics, and their significance has been thoroughly explained in the manuscript. Thank you again for taking the time to review our work.

 

2. Halloysite nanotube (HNT) was shown in many positions in the present work, and it should be given when it first shows up, and then HNT should be used. The same issue is found in carbon dioxide (CO2).

Response:  Thank you for your feedback. We appreciate your attention to detail and suggestion to ensure consistency in terminology throughout our manuscript. We have already made the necessary changes to the text to ensure that Halloysite nanotube (HNT) and carbon dioxide (CO2) are introduced when first used and are consistently used after that. We strive to maintain the highest accuracy and clarity in our work, and your feedback helps us achieve this goal. Thank you again for taking the time to review our work and for providing such valuable suggestions.

 

3. The beginning pressure for BET is too large to miss micropore information.

Response:  Thank you for your feedback and for taking the time to review our work. We appreciate your comments and suggestions. Regarding your comment on the beginning pressure BET for the N2 isotherm curve in our study of HNT-supported CZ-PTA catalyst, we acknowledge that the high pressure used may have impacted the accuracy of the BET results. Our catalyst in the study was a hierarchical nanotubular (HNT) material, which typically does not contain microporous structures. Despite using low-pressure N2 isotherms, we may have missed some micropore information due to factors such as the sample properties or measurement conditions. We apologize for any inaccuracies and are continuously working to improve our methods to ensure accurate and reliable results.

 

 

4. Line 222, it should be figure 3? And Fig. 2 a should be H2-TPR but not TPD.

Response:  Thank you for your feedback. We appreciate your attention to detail and suggestion to ensure the accuracy of our manuscript. As you pointed out, we have already made the necessary corrections to the text. Specifically, as you suggested, we have corrected the reference to Figure 3 and changed Fig. 2a to H2-TPR. Your feedback helps us to maintain the highest level of accuracy in our work, and we are grateful for your attention to detail. Thank you again for taking the time to review our work and for providing such valuable suggestions.

5. I can't see any nanotube structure in Fig. 4, and the middle image in fig. 4 should be numbered (b).

Response:  Thank you for your feedback. We appreciate your attention to detail and suggestion to improve the quality of our manuscript. We apologize for the low quality of the FESEM image in Figure 4, which made it difficult to see the nanotube structure. We have considered your suggestion and improved the quality of the image to display the nanotube structure better. Additionally, as you suggested, we have corrected the numbering of the middle image in Figure 4 to (b). Your feedback helps us enhance our work's clarity and impact, and we are grateful for your support. Thank you again for taking the time to review our work and for providing such valuable suggestions.

6. How does the author confirm the H2 consumption?

Response:  We would like to express our gratitude for your insightful comments on our study. In response to your question regarding the confirmation of H2 consumption, we utilized the H2-TPR (Temperature-Programmed Reduction) technique. This method involves the reduction of CuO to metallic Cu with H₂ as the reducing agent, followed by calculating the H2 consumption through the integration of the peak area under the curve. This approach allowed us to assess the amount of H₂ consumed during the reduction and to understand the CuO's reduction behaviour. Thank you again for your time and contribution to improving our work.

 

7. Line 438, the number in CO2, NH3 should be subscript. Same issue can be found in the whole manuscript, please check!

Response:  Thank you for your valuable feedback. We greatly appreciate your attention to detail in the formatting of our manuscript. We have already considered your suggestions and have made the necessary corrections to the subscripts of CO₂ and NH₃, as well as thoroughly reviewing the entire manuscript to ensure all numbers are appropriately formatted as subscripts where necessary. Thank you again for bringing this to our attention.

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

The entire manuscript the format of writing formula needs to be rectified. 
The English language needs to be rectified extensively.  

Example: "Halloysite nanotube (HNT) is a natural clay mineral with a hollow nanotubular with unique scrolled structures consisting of kaolinite with a length of 0.4–1.5 μm," this sentence has the word "with" used thrice. This is inappropriate, and must be rectified.

What does the author mean by "supported by a mesoporous core-shell (HNT)."

I feel an abbreviation should be assigned to the "halloysite nanotube-supported CuZnO-PTA"  catalyst as it is a very long name. Moreover the word "halloysite nanotubes" has been abbreviated as HNT, but the name "halloysite nanotubes" has been extensively used in the manuscript rather than the HNTs. It's better if it is corrected.

Once the CuZnO is mixed with PTA, then it is not CuZnO anymore, so how can you say that "It can be seen that the crystallinity of the CuZnO decreases, which may be due to low metal loading, and good dispersion[41]"

The figure 1 needs to be rectified, the symbols in the legends are quite similar for JCPDS no. 00-029-1487 and 00-058-2031 and 01-070-3038 and 01-071-3645, this needs to be changed. Moreover there are two images in the figure, these need to be denoted by letters or numbers. 

The caption of figure 1, "XRD pattern of the "reduced" hybrid CuO-ZnO-ZrO2/HZSM-5 sample." now where did the authors get the ZrO2 from? this is unclear.  Moreover what is the need HZSM-5 XRD with regard to the study presented. 

There are two figure 1's in the manuscript. the numbering should be resolved. 
Figure 1 XRD pattern of the "reduced" hybrid CuO-ZnO-ZrO2/HZSM-5 sample
Figure 1 TPD-NH3 profile of the CZ-PTA hybrid catalyst

The statement "W. Alharbi found a correlation 209 between the turnover rates and catalyst acid strengths....." is incorrect as there are ohter two coowrkers with the the author W. Alharbi, the authors should either use et al. or coworkers to denote the other authors. [Refereence 43: Alharbi, W., E.F. Kozhevnikova, and I.V. Kozhevnikov, ........}

The Figure 4. FESEM images (a) HNT, (b) DMF-HNT, (c) CZ-PTA@HNT and particle size distribution 239 of each image, There is no image (b), Where is the scale bar for the (c), The title of the image on the particle size distribution image for image (c) should be  CZ-PTA@HNT The authors infer that the small particles in the SEM image (c) is CuZnO, but this needs to be confirmed by elemental mapping images. 

The authors claim that there are different mass ratios of Cu/PTA, in the CuZnO/PTA@HNT core-shell bi-functional catalyst, but there is no confirmation studies carried out to back this claim. An ICP-MS analyssis can be helpful to understand the exact amount of Cu in the catalyst CuZnO/PTA@HNT. 

 

The catalytic performance has been tested with 4 catalysts, however there is no study on the effect of variation of reaction temperature, it is advice able that the reaction be carried out at lower temperatures as well to under stand the catalytic performance at these temperatures. 

There is no reusability study of the catalyst, this needs to be added atleast for 5 runs to understand the stability of the catalyst. 

My overall conclusion about the manuscript is is that the topic of research is very interesting however it was prepared very loosely and in a hap-hazard manner. This manuscript needs a very extensive revision.

Author Response

#Reviewer 3

 

1- The entire manuscript the format of writing formula needs to be rectified.

Response:  Thank you for your valuable comment. We appreciate your efforts to review our manuscript. We have already rectified the format of writing formulas throughout the manuscript to ensure a clear and consistent presentation of the information. Thank you again for bringing this to our attention.

2-The English language needs to be rectified extensively. 

Response:  Thank you for your review and your feedback. We appreciate the time and effort you have taken to read our manuscript carefully. We want to acknowledge your comment regarding the English language in our manuscript and inform you that we have already had a thorough English proofreading done. We will continue to ensure clear and concise language throughout the manuscript. Thank you again for your input.

3-Example: "Halloysite nanotube (HNT) is a natural clay mineral with a hollow nanotubular with unique scrolled structures consisting of kaolinite with a length of 0.4–1.5 μm," this sentence has the word "with" used thrice. This is inappropriate, and must be rectified.

Response:  We would like to extend our sincerest thanks for taking the time to review our manuscript. Your valuable feedback is greatly appreciated, and we take it as a sign of your dedication to the quality of our work. In particular, we would like to thank you for pointing out the issue with language usage in our manuscript. Effective communication is vital in conveying our ideas and research findings, and we are grateful for your attention to detail. We have taken your feedback seriously and have already taken steps to address it. We had a thorough English proofreading done, and we will continue to ensure that the language used in the manuscript is clear, concise, and free from errors. Once again, we thank you for your time and contribution to our work's quality. We are grateful for your continued support and look forward to the opportunity to incorporate your suggestions into our final manuscript.

 

4-What does the author mean by "supported by a mesoporous core-shell (HNT)."

Response:  Thank you for your review and for bringing up this important clarification. We appreciate your attention to detail and the time you have taken to read our manuscript carefully. Regarding your question, when we mention "supported by a mesoporous core-shell (HNT)," we mean that the CZnO-PTA catalyst was synthesized with a core-shell structure. The CZnO was deposited on the outer shell of the mesoporous halloysite nanotube (HNT), and PTA was loaded in the core and lumen of the HNT. This combination results in a unique mesoporous structure, providing enhanced catalytic activity. We also added more in the discussion section about the support and the core-shell structure ” The core-shell structure of the catalyst is also a crucial factor for its effectiveness. The core-shell morphology allows for the integration of various catalytic sites that are active for different reactions. By customizing the core and shell materials to be compatible with sequential reactions, complementary reactions can be conducted in tandem to achieve the desired product conversion. This results in improved distribution of distances between the separate catalytic sites, which can mitigate the selectivity limitations of conventional bifunctional catalysts and improve reaction efficiency”. Thank you again for your feedback. We hope this clarification provides a better understanding of the catalyst's structure and properties.

 

5-I feel an abbreviation should be assigned to the "halloysite nanotube-supported CuZnO-PTA"  catalyst as it is a very long name. Moreover the word "halloysite nanotubes" has been abbreviated as HNT, but the name "halloysite nanotubes" has been extensively used in the manuscript rather than the HNTs. It's better if it is corrected.

Response:  Thank you for your review and for bringing up this important suggestion. We appreciate your attention to detail and the time you have taken to read our manuscript carefully. We agree with you that the name "halloysite nanotube-supported CuZnO-PTA" is lengthy, and we appreciate your suggestion to assign an abbreviation to it. We have decided to use the abbreviation "CuZnO-PTA@HNT" for the catalyst, which we will consistently use throughout the manuscript. Additionally, we agree that using the word "halloysite nanotubes" rather than the abbreviation "HNT" is inconsistent and needs to be corrected. We have taken note of your suggestion and will ensure that the abbreviation "HNT" is used consistently throughout the manuscript. Thank you again for your input. We value your feedback and will continue to improve the manuscript based on your suggestions.

5-Once the CuZnO is mixed with PTA, then it is not CuZnO anymore, so how can you say that "It can be seen that the crystallinity of the CuZnO decreases, which may be due to low metal loading, and good dispersion [41]."

Response:  Thank you for reviewing and bringing up this critical observation. We appreciate your attention to detail and the time you have taken to read our manuscript carefully. We acknowledge that the statement "It can be seen that the crystallinity of the CuZnO decreases, which may be due to low metal loading, and good dispersion [41]" may be misleading and needs clarification. The XRD results only show the metallic component CuZnO in the crystalline phase, while PTA and HNT exist in the whole catalyst as an amorphous phase, not in a crystalline form. We apologize for any confusion caused by this statement, and we will revise and clarify it in the manuscript to accurately reflect the nature of the catalyst and its components. Thank you again for your feedback. We value your input and will continue to improve the manuscript based on your suggestions.

 

6-The figure 1 needs to be rectified, the symbols in the legends are quite similar for JCPDS no. 00-029-1487 and 00-058-2031 and 01-070-3038 and 01-071-3645, this needs to be changed. Moreover, there are two images in the figure, these need to be denoted by letters or numbers.

Response:  Thank you for your review and for bringing up these important suggestions. We appreciate your attention to detail and the time you have taken to read our manuscript carefully. We have modified the XRD plot, and the discussion has been rectified and corrected. We also agree with your suggestions regarding Figure 1. We will make sure to change the symbols in the legends to distinguish between JCPDS no. 00-029-1487, 00-058-2031, 01-070-3038, and 01-071-3645.Additionally, we denoted the two images in the figure with letters (a) and (b) for clarity. We value your input and will continue to improve the manuscript based on your suggestions. Thank you again for your review and feedback.

7-The caption of figure 1, "XRD pattern of the "reduced" hybrid CuO-ZnO-ZrO2/HZSM-5 sample." now where did the authors get the ZrO2 from? this is unclear. Moreover what is the need HZSM-5 XRD with regard to the study presented.

Response:  Thank you for your detailed review and feedback on our manuscript. We are grateful for your time and effort in reading it carefully. We apologize for the typo error in the caption of Figure 1, and we appreciate you bringing this to our attention. We have corrected the caption to reflect the XRD patterns of the reduced CZnO-PTA@HNT accurately. We apologize for the typo error in the caption of Figure 1, and for any confusion it may have caused. We want to clarify that ZrO2 and HZSM-5 were not part of our study and have now been removed from the manuscript. Thank you again for your input and for bringing this to our attention. We value your suggestions and will strive to ensure the accuracy and clarity of our work.

 

8-There are two figure 1's in the manuscript. the numbering should be resolved.

Figure 1 XRD pattern of the "reduced" hybrid CuO-ZnO-ZrO2/HZSM-5 sample

Figure 1 TPD-NH3 profile of the CZ-PTA hybrid catalyst

 

Response:  Thank you for bringing this issue to our attention. We are grateful for your thorough review of our manuscript. We apologize for the confusion caused by the two figures, both being referred to as Figure 1. We have now resolved the numbering issue, and the figures are referred to as Figure 1:  XRD pattern and Figure 2: H2-TPR and TPD-NH3 profile, respectively. We appreciate your suggestions and have made the necessary corrections to improve the clarity and accuracy of the manuscript.

9-The statement "W. Alharbi found a correlation 209 between the turnover rates and catalyst acid strengths....." is incorrect as there are ohter two coowrkers with the the author W. Alharbi, the authors should either use et al. or coworkers to denote the other authors. [Refereence 43: Alharbi, W., E.F. Kozhevnikova, and I.V. Kozhevnikov, ........}

 

Response:  We would like to express our gratitude for taking the time to review our manuscript and provide insightful comments. We acknowledge and appreciate your suggestion regarding the reference of W. Alharbi et al. We have made the necessary correction and updated the reference to include the other co-workers, W. Alharbi, E.F. Kozhevnikova, and I.V. Kozhevnikov, as "W. Alharbi et al." in our manuscript.Thank you again for your help in improving the quality of our work.

 

 

 

 

10-The Figure 4. FESEM images (a) HNT, (b) DMF-HNT, (c) CZ-PTA@HNT and particle size distribution 239 of each image, There is no image (b), Where is the scale bar for the (c), The title of the image on the particle size distribution image for image (c) should be  CZ-PTA@HNT The authors infer that the small particles in the SEM image (c) is CuZnO, but this needs to be confirmed by elemental mapping images.

Response:  Thank you for taking the time to review our manuscript and for providing valuable comments. We are grateful for your insights and suggestions.We would like to inform you that we have already addressed the issues you have raised regarding the missing image (b) and the scale bar for Figure 4 (c). Additionally, we have added the proper title for the particle size distribution image of CZ-PTA@HNT. Furthermore, we have also confirmed the small particles in the SEM image (c) through elemental mapping images.Again, thank you for your comments, and we hope that the changes made will be satisfactory.

 

11-The authors claim that there are different mass ratios of Cu/PTA, in the CuZnO/PTA@HNT core-shell bi-functional catalyst, but there is no confirmation studies carried out to back this claim. An ICP-MS analyssis can be helpful to understand the exact amount of Cu in the catalyst CuZnO/PTA@HNT.

Response:  Thank you for taking the time to review our manuscript and providing insightful comments. We appreciate your valuable suggestions to improve the quality of our work.Regarding your suggestion of using ICP-MS analysis to understand the exact amount of Cu in the catalyst CuZnO/PTA@HNT, we would like to inform you that we have used the following equation to calculate the theoretical weight (%) of the active sites , for example in Cu .

 

Also we regret that ICP-MS was not available in our lab at the time of experimentation. However, we did perform EDX analysis to understand the elemental composition of the samples, although it may not have the same accuracy as ICP-MS.Thank you again for your valuable feedback and we look forward to the opportunity to address your concerns and further improve our work.

 

 

 

 

The catalytic performance has been tested with 4 catalysts, however there is no study on the effect of variation of reaction temperature, it is advice able that the reaction be carried out at lower temperatures as well to under stand the catalytic performance at these temperatures.

Response:  Thank you for taking the time to review our study. We appreciate your feedback and insights.In regards to your suggestion about the effect of variation of reaction temperature, we would like to inform you that our studied the effect of  temperatures on the catalytic performance and the result was presented in table 3 .We varied the temperature from 180°C to 320°C with a constant pressure of 30 bar and a GHSV of 12,000 h-1, using 0.3 g of the 5CZ-PTA@HNT catalyst. we study found that temperature plays a crucial role in the direct conversion of CO2 into DME, with increasing CO2 conversion and maximum DME selectivity at 260°C. For further information, you can find a detailed discussion on these findings in the discussion section of the revised manuscript. We appreciate your feedback and look forward to your continued involvement in this research.

 

There is no reusability study of the catalyst, this needs to be added atleast for 5 runs to understand the stability of the catalyst.

Response:  Thank you for your review and for taking the time to read our manuscript. We greatly appreciate your feedback and suggestions.We are pleased to inform you that we have indeed conducted a reusability study on the catalyst. You may take a look at discussion  and the Figure 6 . We have carried out five recycling runs, each lasting for three hours,in these conditions  temperature, pressure, GHSV and catalyst weight (240 °C, 30 bar, 12,00 h-1 and 0.3 g) and the results showed that the CO2 conversion and DME selectivity remained unchanged. Additionally, the catalyst can be easily recovered and reused without any significant reduction in its catalytic performance.We apologize for not including these details in the manuscript, and we will make sure to include the results of the reusability study in the revised version.Once again, thank you for your valuable insights and suggestions. We look forward to receiving your further feedback on the revised manuscript.

 

My overall conclusion about the manuscript is is that the topic of research is very interesting however it was prepared very loosely and in a hap-hazard manner. This manuscript needs a very extensive revision.

Response:  Thank you for taking the time to review our manuscript and providing your valuable feedback. We greatly appreciate your insights and suggestions. We understand your concerns regarding the preparation of the manuscript, and we agree that it requires a comprehensive revision.We appreciate your constructive criticism and are confident that the revisions will address your concerns. We are committed to ensuring the quality of our work and will take your comments into consideration as we revise the manuscript.Once again, thank you for your valuable insights and suggestions. We look forward to receiving your further feedback on the revised manuscript.

 

 

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The author has addressed the issues I mentioned, and it can be published now.

Author Response

Dear Editor,

We greatly appreciate your time and effort in providing constructive feedback on our manuscript. We have taken your comments into consideration and have made the necessary revisions to the manuscript. Our goal is to provide a comprehensive and high-quality contribution to the field and we hope that the revisions we have made will address the concerns that you previously noted.

 

• The English should further checked.

Response: We have thoroughly reviewed the English and made necessary corrections to ensure clarity and coherence.

 

 

• The quality of Figures 5 and 6 should be improved or re-organized. They are not well readable:

Response: Regarding the quality of Figures 5 and 6, we have improved their readability and re-organized them to better convey the information they depict.

 

• Please check the caption of Figure 7.

Response: We have also checked the caption of Figure 7 and made the necessary modifications as follows “Figure 7. Effect of PTA loading (wt.%) on product selectivity of DME, MeOH, CO, and H2O (a), and (b) stability test by recycling the 5CZ-PTA@HNT catalyst on CO2 conversion and DME selectivity”.

 

Thank you once again for the opportunity to revise and improve our work. We kindly request your further evaluation of the revised manuscript and hope that it meets your expectations. We would greatly appreciate the opportunity for its potential acceptance.

 

Thank you for your time and consideration.

Sincerely,

Authors

 

Author Response File: Author Response.docx