Novel Ionic Liquid Synthesis of Bimetallic Fe–Ru Catalysts for the Direct Hydrogenation of CO₂ to Short Chain Hydrocarbons

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

It seems to me to be a very novel way of preparing catalysts and using two different procedures to prepare them and compare the results. However, I have some observations regarding the results.
1. The difference in the results is clearly indicative that there is an influence of the exposed X-ray diffraction planes. In this regard, I think that the authors need to carry out a more in-depth study to see what happens in the nanoparticles prepared depending on the preparation method. This simply implies an in-depth analysis of the results presented.
2. The results regarding the conversion of CO2 to the different compounds is very simple. I believe that the efficiency of the catalysts should be mentioned and shown clearly, using different characterization and conversion measurement techniques.
On the other hand, the following points must be taken into account:
1. The paragraphs on lines 270 to 285 appear to be repeated.
2. The quality of the X-ray diffraction figure of the different materials must be improved.

Comments on the Quality of English Language

Regarding ther English, there are some words, like sprinkled and leverage need to be changed for more appropriate language.

Author Response

Dear Reviewer,

Thank you for your thoughtful and constructive feedback on our scientific paper. We appreciate your time and efforts in reviewing our work. We have carefully considered each of your observations and have made the necessary revisions to address them. Please find below a point-wise response to the comments. We believe that the revised manuscript addresses the comments satisfactorily and is now suitable for publication in MDPI Catalysts.    

 

 

Observations on Results:

1. In-depth Study on X-ray Diffraction:

The difference in the results is clearly indicative that there is an influence of the exposed X-ray diffraction planes. In this regard, I think that the authors need to carry out a more in-depth study to see what happens in the nanoparticles prepared depending on the preparation method. This simply implies an in-depth analysis of the results presented.

We agree with your recommendation to undertake a more comprehensive investigation into the impact of exposed X-ray diffraction planes on the outcomes. To address this, we conducted supplementary analyses of the catalysts, specifically focusing on the nanoparticles' characteristics in relation to the preparation method (refer to lines 263-298). Notably, we have incorporated X-ray diffraction (XRD) analyses of the catalysts at varying Fe/Ru ratios (1:1, 3:1, and 9:1) for both the COL method (Figure 1.a) and IL Method (Figure 1.b). This supplementary analysis aims to provide a thorough understanding of the discerned differences.

 

1. Efficiency of Catalysts:

The results regarding the conversion of CO₂ to the different compounds is very simple. I believe that the efficiency of the catalysts should be mentioned and shown clearly, using different characterization and conversion measurement techniques.

We acknowledge your comment regarding the simplicity of the results concerning the conversion of CO₂ to different compounds. In response, we have enriched the discussion by providing a more detailed exploration of the catalysts' efficiency. This expanded discussion encompasses additional characterization techniques, including the XRD analysis of spent catalysts, the graphs of which are available in the Supplementary Information. Furthermore, our examination now includes conversion and selectivity data specifically investigating the catalysts' activity under different pressures (6 and 20 bar) at a gas hourly space velocity (GHSV) of 5400 mL/h/gcat. We have also explored the impact of varying space velocities (5400 mL/h/gcat and 1800 mL/h/gcat) at 20 bar. These enhancements aim to offer a more comprehensive and elucidating depiction of the catalysts' performance.

 

Points to be Taken into Account:

1. Repetition of Paragraphs:

The paragraphs on lines 270 to 285 appear to be repeated.

We have addressed the issue of repeated paragraphs on lines 270 to 285 by eliminating the redundancy. We apologize for any confusion caused by this oversight.

 

1. Improvement of X-ray Diffraction Figure Quality:

The quality of the X-ray diffraction figure of the different materials must be improved.

We understand the importance of clear figures in scientific communication. We have taken steps to enhance the quality of the X-ray diffraction figures for better visualization and interpretation.

 

 

Comments on the Quality of English Language:

Regarding the English, there are some words, like sprinkled and leverage need to be changed for more appropriate language.

We appreciate your input regarding certain words like "sprinkled" and "leverage." In response, we have revised the manuscript to use more appropriate language, ensuring clarity and precision in our descriptions.

 

Once again, we thank you for your valuable feedback, which has significantly contributed to the improvement of our paper.

 

Reviewer 2 Report

Comments and Suggestions for Authors

comments:

1-In the absence of the H2-TPR profiles of Fe2O3 alone and RuO2, it is difficult to identify the characteristic reduction peaks of Fe2O3 and RuO2.

In Figure 2a you say that the low temperature reduction peak appearing around 380 K was attributed to the reduction of Fe2O3 to Fe3O4, this is only valid for the 1wt% Fe-Ru 3:1/Al2O3 (COL) compound, however, Fe-Ru 3:1/Al2O3 (IL) catalyst, does not display any reduction peak at this temperature (380K).

You also say that the reduction peak at high temperature, 620K (no peak at this temperature), was attributed to the subsequent reduction of Fe3O4 to FeO and FeO, but here we clearly see a peak before this temperature around 500K, which you did not attribute.

2- Why the kinetic experiments were carried out at a single temperature of 320 °C and at two pressures 6 and 20 bar?

3- How did you follow the evolution of CO2 conversions and product selectivities using FT-IR gas analyzer?

4-The Fe2O3/Al2O3 catalyst, which are selective for CH4 and hydrocarbons at 20 bars, have not been the subject of any physicochemical characterization. For what?

5- The reaction conditions (total pressure and reaction temperature) are decisive parameters affecting the rates of overall CO2 conversion and formation of C2-C5 hydrocarbons. Other key factors that influence the selectivity to C2-C5 hydrocarbons were: the catalyst design, its pretreatment time, and the mass of nominal Fe2O3 in the samples. No study has been carried out in this direction!!

6- In your study, Catalytic experiments were carried at 6 bar and a space speed of 5400 mL/h/gcat of bar, at 20 the space speed is reduced to 1800 mL/h/gcat. Does space velocity have no effect on catalytic performance?

7- In line 381 you say that the absence of long-chain hydrocarbons (C2-C5) in the 1wt% Fe/Al2O3 sample, observed at 20 bar, is likely due to the insufficient pressure to facilitate chain growth.   It is at 6 bar that C2-C5 are not observed.

8- The catalytic composition 4% by weight Fe-Ru 3:1/Al2O3 (IL) has a high C2-C5 selectivity at 6 bars compared to 20 bars. Isn't this a contradiction with the pressure effect?

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The quality of the English language is appreciable

Author Response

Dear Reviewer,

 

Thank you for your detailed and insightful comments on our manuscript. We appreciate the thorough examination of our work, and we have carefully addressed each of your concerns. Please find below a point-wise response to the comments. We believe that the revised manuscript addresses the comments satisfactorily and is now suitable for publication in MDPI Catalysts.

 

 

1- H₂-TPR profiles

In the absence of the H₂-TPR profiles of Fe₂O₃ alone and RuO₂, it is difficult to identify the characteristic reduction peaks of Fe₂O₃ and RuO₂.

We acknowledge the absence of H₂-TPR profiles for Fe₂O₃ alone and RuO₂. To rectify this, we included the description of these profiles in the revised manuscript (lines 297-397) from the reference papers:

DOI: 10.7763/IJCEA.2015.V6.519 for Fe₂O₃ and DOI:10.3390/catal9020108  to RuO₂ to aid in identifying the characteristic reduction peaks of Fe₂O₃ and RuO₂ in the bi-metallic catalysts presented.

 

In Figure 2a you say that the low temperature reduction peak appearing around 380 K was attributed to the reduction of Fe2O3 to Fe3O4, this is only valid for the 1wt% Fe-Ru 3:1/Al₂O₃ (COL) compound, however, Fe-Ru 3:1/Al2O3 (IL) catalyst, does not display any reduction peak at this temperature (380K).

Concerning the variation in reduction peaks for Fe-Ru 3:1/Al₂O₃ (IL), we have revisited our interpretation and rectified any discrepancies to faithfully reflect the observed reduction temperatures (lines 312-318). Additionally, we have amended the graph, correcting the y-axis temperatures, which were inaccurately reported in K rather than in °C.

 

You also say that the reduction peak at high temperature, 620K (no peak at this temperature), was attributed to the subsequent reduction of Fe3O4 to FeO and FeO, but here we clearly see a peak before this temperature around 500K, which you did not attribute

Regarding this aspect of the TPR discussion, we intended to highlight that in a typical H₂-TPR profile of Fe₂O₃, a second peak around 620°C is commonly observed, attributed to the further reduction of Fe₃O₄ to FeO and Fe⁰. In our case, the reduction peak position associated with Fe₂O₃ in bimetallic catalysts shifted to a lower temperature compared to the pure Fe₂O₃ catalyst. This shift resulted from the introduction of RuO₂, which accelerates the reduction of Fe species. This observation is consistent with findings reported in the literature ("Effect of different RuO₂ contents on selective catalytic oxidation of ammonia over RuO2-Fe2O3 catalysts," J Fuel Chem Technol, 2019, 47(2), 215-223).

 

2. Kinetic Experiments:

2- Why the kinetic experiments were carried out at a single temperature of 320 °C and at two pressures 6 and 20 bar?

Thank you for bringing up the question regarding the choice of experimental conditions in our kinetic experiments. We appreciate the opportunity to clarify our rationale behind this decision.

The decision to conduct kinetic experiments at a single temperature of 320 °C and at two pressures, 6 and 20 bar, is based on a thoughtful consideration of factors influencing CO₂ hydrogenation to hydrocarbons. We acknowledge the significance of high-pressure conditions for this reaction to be representative of industrial scenarios. Our choice of 20 bar is intended to simulate conditions closer to industrial settings, where high pressures are often employed.

The use of 6 bar is a common practice in the literature (DOI:10.1002/anie.201910579), serving a screening purpose for new catalytic routes. Although CH₄ is the primary expected product at this lower pressure, the information gathered under these conditions is invaluable for comparing the activity and selectivity of different catalysts. This screening step allows us to discern the performance of catalysts in a preliminary stage, providing insights that contribute to the overall understanding of their catalytic properties.

Recognizing the importance of temperature as a crucial parameter, we want to assure you that we are fully conscious of its impact on the reaction. In our future investigations, we plan to explore the effect of temperature on the CO₂ hydrogenation process on IL catalysts. This will contribute additional insights into the temperature dependence of our catalysts' performance.

We hope this clarification addresses your concerns, and we committed to incorporating a more detailed explanation of our experimental choices in the revised manuscript (Lines 351-438).

 

 

3. FT-IR Gas Analyzer:

How did you follow the evolution of CO₂ conversions and product selectivities using FT-IR gas analyzer?

We employed a comprehensive approach by connecting two instruments online: an infrared gas analyzer (MATRIX MG5, Bruker) and a GC Agilent (model 8860). The infrared gas analyzer provided real-time monitoring of CO₂ conversions and product formation , while the GC Agilent equipped with a TCD detector connected to a G35591-81187 packed column along with a FID detector connected to CP-Sil PONA column, allowed for detailed product speciation and quantification. The combination of these instruments enabled us to capture both the temporal evolution of CO₂ conversions and the selectivities of different products during the catalytic process. However, it's important to note that the calculations for CO₂conversions and product selectivities presented in the manuscript were primarily based on the GC analysis. This analytical technique was chosen for its ability to offer a more accurate and detailed speciation of the products, ensuring precise quantification.

We appreciate your attention to this aspect of our methodology, and we will emphasize these details more explicitly in the revised experimental section of the manuscript (LINES 239-248) to provide a clearer understanding of our analytical approach.

 

4. Physicochemical Characterization of Fe₂O₃/Al₂O₃ Catalyst:

The Fe2O3/Al2O3 catalyst, which are selective for CH4 and hydrocarbons at 20 bars, have not been the subject of any physicochemical characterization. For what?

We appreciate your observation regarding the absence of physicochemical characterization for the Fe₂O₃/Al₂O₃catalyst, particularly its selectivity for CH₄ and hydrocarbons at 20 bars. We acknowledge this oversight and have taken corrective measures in the revised manuscript. Specifically, we have incorporated pertinent XRD characterization conducted both before and after testing the sample (LINES 262). This addition aims to provide a more comprehensive analysis of the impact of the synthesis method and the formation of active species on the catalyst. We believe that this enhancement will contribute valuable insights to the understanding of the Fe₂O₃/Al₂O₃ catalyst's behavior under the specified conditions.

 

5. Factors Affecting Selectivity:

The reaction conditions (total pressure and reaction temperature) are decisive parameters affecting the rates of overall CO₂ conversion and formation of C2-C5 hydrocarbons. Other key factors that influence the selectivity to C₂-C₅hydrocarbons were: the catalyst design, its pretreatment time, and the mass of nominal Fe2O3 in the samples. No study has been carried out in this direction!!

We appreciate your insightful comments regarding the critical parameters influencing selectivity in CO₂ conversion. In our study, we have introduced a novel catalyst design approach, incorporating Ionic Liquids in the synthesis. This innovative strategy aims to enhance the interplay between iron and ruthenium, thereby influencing the selectivity of the reaction towards desired products. Specifically, we have explored the impact of varying the mass of Fe₂O₃/ RuO₂ ratio on the catalyst surface, ranging from 1:1, 3:1, to 9:1 (the mass ratios are reported in Table 1 of the revised manuscript). This investigation allowed us to assess the effects of these variables on catalyst activity and selectivity. We have also included the comparison of gas hourly space velocity (GHSV), 5400 and 1800 mL/h/gcat; and effect of pressure by comparing at 20 bar and 6 bar total pressure (reported in figures 4, 5 and 6 in the revised manuscript).

While our current focus has been on this catalyst design approach, we acknowledge the importance of other factors such as catalyst pretreatment. In the subsequent phases of our study, we plan to delve into these aspects, providing a more comprehensive understanding of the interplay between various parameters and their influence on selectivity. It's essential to note that these investigations will be specifically conducted on the best-formulated catalysts prepared with Ionic Liquid solvents presented in this paper.

 

6. Space Velocity Effect:

In your study, Catalytic experiments were carried at 6 bar and a space speed of 5400 mL/h/gcat of bar, at 20 the space speed is reduced to 1800 mL/h/gcat. Does space velocity have no effect on catalytic performance?

We acknowledge the importance of space velocity as a critical parameter influencing catalytic performance. In response to your comment, we have re-evaluated and thoroughly discussed the impact of space velocity in the revised manuscript.

To provide a more comprehensive analysis, we have conducted additional experiments comparing the activity and selectivity of the catalyst at 20 bar under two different values of Gas Hourly Space Velocity (GHSV): 5400 and 1800 mL/h/gcat (reported in figure 4 and 5, respectively). Furthermore, we have extended our investigation by comparing the effects of total pressure at a consistent space velocity of 5400 mL/h/gcat, specifically at both 20 and 6 bar.

These supplementary experimental data have enabled us to systematically analyze the catalyst's behavior, allowing us to assess the influence of process variables such as space velocity and pressure on its activity. We appreciate your feedback, and these additions will contribute to a more thorough understanding of the experimental conditions in our study.

 

7. Pressure and C₂-C₅ Hydrocarbons:

In line 381 you say that the absence of long-chain hydrocarbons (C2-C5) in the 1wt% Fe/Al2O3 sample, observed at 20 bar, is likely due to the insufficient pressure to facilitate chain growth.   It is at 6 bar that C₂-C₅ are not observed.

We recognize the confusion in our statement. The apparent contradiction in our initial statement arises from the comparison of data collected at two different pressure values and distinct space velocities (5400 mL/h/gcat at 6 bar and 1800 mL/h/gcat at 20 bar). We recognize that the effect on selectivity is not solely dependent on pressure but is also influenced by the variation in space velocity. To address this, we have conducted additional analyses independently assessing the impact of both pressure and space velocity in the reactions, as reported above.

We have incorporated new figures (Figures 4, 5, and 6) in the revised manuscript to present these data more transparently. In these figures, we have elucidated the effects of pressure and space velocity on long-chain hydrocarbons (C₂-C₅) selectivity in the 1wt% Fe/Al₂O₃ sample at both 6 and 20 bars, specifically at a space velocity of 5400 mL/h/gcat.

We hope this clarification provides a more accurate representation of our observations, and we appreciate your diligence in reviewing our work.

 

8. Contradiction in C2-C5 Selectivity:

The catalytic composition 4% by weight Fe-Ru 3:1/Al2O3 (IL) has a high C₂-C₅ selectivity at 6 bars compared to 20 bars. Isn't this a contradiction with the pressure effect?

Thank you for highlighting the discrepancy in the C2-C5 selectivity of the 4% by weight Fe-Ru 3:1/Al2O3 (IL) catalyst at 6 and 20 bars. We appreciate your keen observation, and we would like to offer clarification. The apparent contradiction in selectivity arises from the fact, as discussed previously, that the two pressure conditions were tested at significantly different space velocities. To address this, we repeated the measurements at both 6 and 20 bars, maintaining a consistent space velocity of 5400 mL/h/gcat. Upon reevaluation, the 4% by weight Fe-Ru 3:1/Al2O3 (IL) catalyst exhibited a C2-C5 selectivity of 10.3% at 6 bars (Figure 4) and 16.5% at 20 bars (Figure 5). As anticipated, the selectivity of hydrocarbons increased with the pressure, aligning with the expected pressure effect.

 

We appreciate your constructive feedback, which has undoubtedly enhanced the quality and clarity of our manuscript.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The presented work is claimed to be a synthesis of highly selective catalysts for the direct hydrogenation of CO2 to short chain hydrocarbons (C2-C5). The results obtained do not correspond to the Title at all. Of the 10 catalysts synthesized (Table 1), only 2 showed at least some formation of C2-C5 hydrocarbons with a selectivity of less than 20%. On the remaining catalysts, the only products are CO and H2O, or CH4.  Another main idea of the work is the use of ionic liquids for the synthesis of catalysts. The authors declare that the use of ionic liquids outperforms traditional colloid-based techniques, resulting in superior selectivity for target hydrocarbons. In this case, a real comparison is carried out only for one catalyst composition Fe-Ru 3:1/Al2O3, synthesized both by the traditional colloidal method (Fe-Ru 3:1/Al2O3 (COL)) and the new one using ionic liquids (Fe-Ru 3 :1/Al2O3 (IL)). Indeed, at a pressure of 20 bar, a notable five-fold enhancement in CO2 conversion is observed with the new synthetic procedure, although C2-C5 hydrocarbons are not formed at all. If these catalysts are tested at a pressure of 6 bar, the conclusion will be the opposite. The catalyst synthesized using traditional colloidal technology is 3 times more active than the catalyst using the new technology (Figure 5). In addition, in the abstract, the authors talk about the connection between morphological changes in the catalysts and catalytic performance. At the same time, the work presents separately physicochemical and catalytic studies, the connection between which is absolutely not discussed.

Author Response

Dear Reviewer,

We appreciate your thorough review of our work, and we thank you for bringing up these important points. We recognize the concerns raised and would like to address them in detail. Please find below a point-wise response to the comments. We believe that the revised manuscript addresses the comments saEsfactorily and is now suitable for publicaEon in MDPI Catalysts.

1. Title revision:

The presented work is claimed to be a synthesis of highly selecEve catalysts for the direct hydrogenaEon of CO2 to short chain hydrocarbons (C2-C5). The results obtained do not correspond to the Title at all. Of the 10 catalysts synthesized (Table 1), only 2 showed at least some formaEon of C2-C5 hydrocarbons with a selecEvity of less than 20%. On the remaining catalysts, the only products are CO and H2O, or CH4.

We acknowledge that the obtained results did not enErely align with the iniEally claimed Etle, and we thank you for bringing this to our aSenEon. In response to your insighTul feedback, we have carefully revisited our conclusions and modified the Etle to beSer reflect the paper's focus. We aim to highlight the novel synthesis approach employed in comparison to the convenEonal colloidal method for synthesizing bimetallic catalysts.

2. Comparison of Ionic Liquid Synthesis:

Another main idea of the work is the use of ionic liquids for the synthesis of catalysts. The authors declare that the use of ionic liquids outperforms tradiEonal colloid-based techniques, resulEng in superior selecEvity for target hydrocarbons. In this case, a real comparison is carried out only for one catalyst composiEon Fe-Ru 3:1/Al2O3, synthesized both by the tradiEonal colloidal method (Fe-Ru 3:1/Al2O3 (COL)) and the new one using ionic liquids (Fe-Ru 3 :1/Al2O3 (IL)).

We appreciate your perspecEve and would like to provide addiEonal informaEon to address your concerns. In response to your comment, we have expanded our comparison beyond the Fe-Ru 3:1/Al2O3 catalyst to include a series of new catalysts. These new catalysts, prepared with Fe-Ru raEos of 1:1 and 9:1 using both tradiEonal colloidal and novel ionic liquid methods, have been thoroughly characterized and tested for catalyEc acEvity. The comprehensive details of these catalysts, including their preparaEon methods, characterizaEon results (as shown in Table 1 and Figure 1B), and acEvity tests (Figures 4, 5, 6) conducted at different pressures and space velociEes, are now presented in the revised manuscript. The extended comparison affirms our iniEal findings with the Fe-Ru 3:1/Al2O3 (IL) catalyst, demonstraEng superior acEvity in CO2 hydrogenaEon to hydrocarbons across various condiEons. We believe that this expanded analysis provides a more robust understanding of the influence of synthesis methods on catalyst performance.

3. Pressure-Dependent AcEvity:

Indeed, at a pressure of 20 bar, a notable five-fold enhancement in CO2 conversion is observed with the new syntheEc procedure, although C2-C5 hydrocarbons are not formed at all. If these catalysts are tested at a pressure of 6 bar, the conclusion will be the opposite. The catalyst synthesized using tradiEonal colloidal technology is 3 Emes more acEve than the catalyst using the new technology (Figure 5).

Upon careful consideraEon of your comment, we recognized that the comparison of the two synthesis methods was influenced by variaEons in space velocity. Specifically, the catalysts tested at 6 bar were assessed at a space velocity of 1800 GHSV, which is significantly lower than the 5400 GHSV used for the 20 bar condiEons. In response to this, we have implemented a systemaEc approach in our study. We conducted tests at both 20 bar and 6 bar, ensuring that the comparison was made at the same space velocity of 5400 GHSV. The new results are reported in figure 4 (20 bar, GHSV 5400 mL/h/gcat) and in

figure 5 (20 bar, GHSV 1800 mL/h/gcat). This systemaEc design allows for a more accurate assessment of the catalysts' acEvity under consistent condiEons, addressing the potenEal bias introduced by different space velociEes. We have also included a comparison of the two different pressures (Figure 6: 6 bar, GHSV 5400 mL/h/gcat) to provide a comprehensive understanding of the catalysts' performance across varying condiEons.

4. ConnecEon Between Morphological Changes and CatalyEc Performance:

In addiEon, in the abstract, the authors talk about the connecEon between morphological changes in the catalysts and catalyEc performance. At the same Eme, the work presents separately physicochemical and catalyEc studies, the connecEon between which is absolutely not discussed.
We appreciate your construcEve feedback regarding the connecEon between morphological changes in the catalysts and their catalyEc performance. Your observaEon has been duly considered, and we have taken steps to address this concern in the revised manuscript.

In the updated version, we have integrated a more comprehensive discussion that explicitly links the physicochemical and catalyEc studies, creaEng a more cohesive narraEve. AddiEonally, to further support and illustrate this connecEon, we have included an invesEgaEon of the X-ray-diffracEon- characterized samples in the supplemental informaEon (SI, Figure 1a and 1.b). This addiEon aims to demonstrate the presence of acEve species on the catalysts' surface and correlate these findings with the evidence derived from the catalyEc acEvity tesEng. We believe that these enhancements will provide a clearer and more integrated understanding of the relaEonship between morphological changes and catalyEc performance in our study.

Thank you for bringing this aspect to our aSenEon, and we hope these revisions meet your expectaEons. We appreciate your construcEve feedback, which will undoubtedly contribute to refining our work.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

If there is no objection from the other reviewers, I suggest that the article be published. The authors have worked through the suggested corrections appropriately.

Reviewer 2 Report

Comments and Suggestions for Authors

After corrections, the article can be published with the modifications made

Reviewer 3 Report

Comments and Suggestions for Authors

The authors took into account the reviewers' comments and made significant changes to the text of the manuscript. As presented, this manuscript may be accepted for publication in the journal.