Selective Oxidation of Furfural at Room Temperature on a TiO₂-Supported Ag Catalyst

Round 1

Reviewer 1 Report

The authors investigated the impacts of different reaction parameters on the oxidation of furfural (FF) to furoic acid (FA) using Ag/TiO2 catalyst, including temperature, nature of the atmosphere, base equivalent, and nature of inorganic bases used. Optimum conditions were identified, and durability for multiple testing were evaluated based on the FF conversion, FA yield and carbon balance before and after testing. Overall the authors did a comprehensive testing work and it is worthy to publish it on Catalyst.

A minor revision is requested for the comments below.

1.       In the Introduction section, line 40, why was Ag selected for the research? The authors mentioned the drawbacks of gold catalysts but didn’t claim the potential advantages for Ag.

2.       The cross-reference for all Figures did not work with an error message “Error! Reference source not found” in the Line 60, 65, 71, 81 and 82. The figure numbering is wrong.

Author Response

The authors investigated the impacts of different reaction parameters on the oxidation of furfural (FF) to furoic acid (FA) using Ag/TiO₂ catalyst, including temperature, nature of the atmosphere, base equivalent, and nature of inorganic bases used. Optimum conditions were identified, and durability for multiple testing were evaluated based on the FF conversion, FA yield and carbon balance before and after testing. Overall, the authors did a comprehensive testing work and it is worthy to publish it on Catalysts.

We would like to thank Referee 1 for the time spent to evaluate our manuscript and for his/her valuable remarks. We performed all required changes (highlighted in red).

A minor revision is requested for the comments below.

1. In the Introduction section, line 40, why was Ag selected for the research? The authors mentioned the drawbacks of gold catalysts but didn’t claim the potential advantages for Ag.

      The main advantages of silver is its availability, low price and the long term stability in the price during last 10 years as compared to gold. From 2017 the price for 1 kg of gold increased from 35000 USD to more than 55000 USD today, while in the same period the price of 1 kg of Ag increased from 500 to 600 USD. This is now added to the Introduction.

2. The cross-reference for all Figures did not work with an error message “Error! Reference source not found” in the Line 60, 65, 71, 81 and 82. The figure numbering is wrong.

      We would like apologize for this error. This is now corrected in the Manuscript.

Reviewer 2 Report

1. It is necessary to include «TiO2» in the keywords.

2. What was purpose of the ethanol introduction in the impregnation solution at the stage of anchoring of the AgNO3 on the TiO2?

3. line 242 (Materials and Methods): the phrase «was calculated» is repeated twice.

4. What determines the chosen silver content in the catalysts (6 wt. %)?

5. What was pretreatment of the samples before study of their texture characteristics?

6. Links in the picture are not displayed in the « Results and discussion».

7. Nowhere is it indicated in the article at what stage base was added. Was the base added in the experiments discussed in the « 2.1. Effect of the temperature» and « 2.2. Effect of the atmosphere»?

8. It the scheme reaction mechanism it is necessary to be shown more clearly the stages of the reaction, to be numbered of the steps, and to be refer to them in the text of the article.

9. It is necessary to indicate in the caption to the table 1 what base was used.

10. In the text of the article, it is worth giving a possible explanation for the decrease of the FA yield at high OH/FF ratio.

11. It is necessary to be written in more detail the explanation of the greater efficiency of the NaOH compared to NaHCO3 and Na2CO3.

 

12. What is the reason of the increase of the Ag particle size after reaction?

Author Response

We truly appreciate the work done by Referee 2. We performed required changes in the Manuscript.

1. It is necessary to include «TiO₂» in the keywords.

The Referee is right. The “TiO₂” was added to the keywords

2. What was purpose of the ethanol introduction in the impregnation solution at the stage of anchoring of the AgNO₃ on the TiO₂?

It was shown in the literature that alcoholic solution permits better dispersion of Ag on the support and permits to obtain small Ag particle size. However, we did not study this parameter. We used well know method reported earlier (DOI: 10.1021/cg901497p, 10.1186/1556-276X-7-151, 10.1021/cg901497p).

3. Line 242 (Materials and Methods): the phrase «was calculated» is repeated twice.

This is now corrected.

4. What determines the chosen silver content in the catalysts (6 wt. %)?

The silver content was chosen taking into account previous works on this catalyst in HMF oxidation (DOI: 10.1016/j.mcat.2022.112353).

5. What was pretreatment of the samples before study of their texture characteristics?

Any specific treatment was applied. Before BET the catalyst was degassed at 110°C for 2 hours. This is now mentioned in the experimental part.  

6. Links in the picture are not displayed in the « Results and discussion».

The links were removed.

7. Nowhere is it indicated in the article at what stage base was added. Was the base added in the experiments discussed in the « 2.1. Effect of the temperature» and « 2.2. Effect of the atmosphere»?

We apologize for this error. We added the new sentence to the experimental part. All experiments in the sections 2.1 and 2.2 were carried out in the presence of NaOH. This is now added to the text.

8. It the scheme reaction mechanism it is necessary to be shown more clearly the stages of the reaction, to be numbered of the steps, and to be refer to them in the text of the article.

We would like to thank Referee 2 for this remark. However, more work is needed to fully understand the mechanism of this reaction. We proposed the simplify scheme according to our observation. In the presence of a base, the attack of the hydroxide ion onto furfural occurs forming acetal. This step is followed by acetal dehydrogenation and beta hydride elimination. Ag catalyst is regenerated via oxygen. However, It should be noted here that an alternate mechanism involving furoic acid complex formation and H₂ liberation cannot be entirely ruled out. In the absence of oxygen, the catalytic cycle halts at this stage; however, when an oxygen is present, the furoate ligand detaches as product furoate salt while regenerating the Ag surface.

9. It is necessary to indicate in the caption to the table 1 what base was used.

In this study NaOH was used. This is now added to the Table caption.

10. In the text of the article, it is worth giving a possible explanation for the decrease of the FA yield at high OH/FF ratio.

We do believe that the decrease in the FA yield is due to the degradation of the products. Basic media are a source of degradation of furfural under reaction conditions. Tests in the absence of a catalyst at high pH showed furfural conversion without any formation of detectable oxidation products. In addition, a color change from yellow to brown was noticed in the reaction mixture, as well as the formation of a black precipitate when HCl was added [Eur. J. Soil Sci. 1999, 50, 687–694, Catalysts 2017, 7, 352]. This precipitate was attributed to the formation of humic acids, although the formation of humins has also been reported. Humins are insoluble in water at all pHs, and humic acids are soluble only in a basic medium. A mixture of many different acids can form during the reaction. As pointed out by some researchers, a basic medium also favors the formation of levulinic acid, formic acid, and some unknown byproducts. This behavior emphasizes how detrimental a high pH is for furfural stability in solution. According to the authors, the increase of NaOH concentration and temperature of the reaction accelerate the degradation process. In the presence of a catalyst, however, these degradation products formed in minor amounts or did not form [Eur. J. Soil Sci. 1999, 50, 687–694, Catalysts 2017, 7, 352]. The short discussion is added to the manuscript.

11. It is necessary to be written in more detail the explanation of the greater efficiency of the NaOH compared to NaHCO₃ and Na₂CO₃.

The reported results highlighted the detrimental effect of the base addition. No activity was observed under base free conditions. Subsequently, different bases were added to the system to try to promote the catalytic activity and to check if there was a change in the reaction pathway. Very good results were observed when NaOH was used as a base. Surprisingly, the use of a milder base decreased the activity of the catalyst, confirming the crucial role of NaOH. The presence of hydroxide ions plays an important role during the oxidation reaction. Davis et al. have discussed how the product distribution depends on pH [10.1039/C2GC36441G]. In principle, the buffer effect can explain the differences observed for different bases. The increase in the buffer strength represents an increase in the concentration of HO− and, consequently, can sustain the differences observed in the reaction rate. During the oxidation of aldehydes in basic aqueous solution the reversible hydration of the aldehyde to germinal diols is a main step. It is accelerated in high pH. The germinal diols will adsorb in the metal surface to form an alkoxide, that later undergoes a β-elimination to form carboxylic acid. Thus, the combination of high concentration of OH− and longer reaction times favors the oxidation to carboxylic acid. New reference was added (25).

12. What is the reason of the increase of the Ag particle size after reaction?

Very good questions. This is the phenomenon that we are actually studying in our group. It is unusual to see the increase in the particle size of metallic nanoparticles at RT. As of today, we did not find any possible explanation in the literature. 

Reviewer 3 Report

In this paper, the liquid-phase conversion of furfural to furoic acid using TiO₂ supported Ag catalyst at room temperature. The effect of temperature, atmosphere nature, and sodium-based inorganic bases are discussed and analyzed. The content of the article is detailed and sufficient. All in all, this is a valuable article. However, before the article can be published, some questions need to be answered or some errors need to be corrected by the author.

·         Why Ag/TiO2 was selected for this work instead of Ag/ZrO2 and Ag2O/Cu2O.

·         Please provide the catalytic activity of various Ag loading on TiO2 support.

·         How much Ag% loaded on TiO2 support.

·         The abbreviation of furfural and furoic acid should be unified to FF and FA, respectively.

·         References cited in the text are inappropriate.

·         All references must be formatted journal style.

·         Figure citations in the suitable text match in the "Results and discussion" section (XRD, TEM, 2.1, and 2.4, etc.)

·         Provide Figure 3b explanation and %FA yield values in section 2.1.

·         Section 2.3 and Table 2: When the basic nature of NaHCO3 to NaOH increases, FF conversion, and FA yield increase. I think pH plays a role in catalytic activity. Need to study the effect of pH in NaOH vs catalytic activity.

·         Page 4, Row 109: Figure 1 changed to Figure 4

·         Page 5, Row 132: Figure 2 changed to Figure 5

·         Page 7, Row 182: Figure 3 changed to Figure 6

·         Page 7, Row 185: Figure 4 changed to Figure 7

·         Reference is suggested to be expanded such as: liquid-phase biomass conversion advantages: New J. Chem., 2022, 46, 5907-5921.

 

·         It is noted that the manuscript needs further careful editing in aspects of grammar, spelling sentences, and typo errors to be clear enough to reader.

Author Response

In this paper, the liquid-phase conversion of furfural to furoic acid using TiO₂ supported Ag catalyst at room temperature. The effect of temperature, atmosphere nature, and sodium-based inorganic bases are discussed and analyzed. The content of the article is detailed and sufficient. All in all, this is a valuable article. However, before the article can be published, some questions need to be answered or some errors need to be corrected by the author.

We would like to thank the Referee 3 for his/her valuable remarks. We answered to all questions and performed required changes in the manuscript.

1. Why Ag/TiO₂ was selected for this work instead of Ag/ZrO₂ and Ag₂O/Cu₂O.

Ag/ZrO₂ and Ag₂O/Cu₂O catalysts were already reported in the HMF oxidation reaction as mentioned in the Introduction part (https://doi.org/10.1039/D0CY00878H, https://doi.org/10.3390/molecules13040948). We decided to focus on Ag/TiO₂ catalyst which showed good catalytic properties in phase gas ethylene glycol oxidation to glyoxal. Moreover, it was not reported before in the room temperature oxidation of furfural.

2. Please provide the catalytic activity of various Ag loading on TiO₂ support. How much Ag% loaded on TiO₂ support.

6 wt.% of Ag was used. This information was already present in the manuscript (line 58). We did not study other % Ag loadings. However, after deep literature research the 6% was the most appropriate loading for Ag catalysts. This kind of catalyst was very recently studied for HMF oxidation and photocatalytic reactions (10.1016/j.cej.2017.01.107, 10.1021/am5085447, 10.1016/j.mcat.2022.112353).

4. The abbreviation of furfural and furoic acid should be unified to FF and FA, respectively.

The referee is right. This is now corrected.

5. References cited in the text are inappropriate. All references must be formatted journal style.

All references are formatted now as required.

6. Figure citations in the suitable text match in the "Results and discussion" section (XRD, TEM, 2.1, and 2.4, etc.)

This is now corrected in the text.

7. Provide Figure 3b explanation and %FA yield values in section 2.1.

This is now corrected and the Figure 3b is discussed in the section 2.1.

8. Section 2.3 and Table 2: When the basic nature of NaHCO₃ to NaOH increases, FF conversion, and FA yield increase. I think pH plays a role in catalytic activity. Need to study the effect of pH in NaOH vs catalytic activity.

The referee is right. This is also our conclusion. And this was also studied by Davis et al. [10.1039/C2GC36441G]. The reported results highlighted the detrimental effect of the base addition. No activity was observed under base free conditions. Subsequently, different bases were added to the system to try to promote the catalytic activity and to check if there was a change in the reaction pathway. Very good results were observed when NaOH was used as a base. Surprisingly, the use of a milder base worsened the activity of the catalyst, confirming the role crucial role of NaOH. The presence of hydroxide ions plays an important role during the oxidation reaction. Davis et al. have discussed how the product distribution depends on pH [10.1039/C2GC36441G]. In principle, the buffer effect can explain the differences observed for different bases. The increase in the buffer strength represents an increase in the concentration of HO− and, consequently, can sustain the differences observed in the reaction rate. During the oxidation of aldehydes in basic aqueous solution the reversible hydration of the aldehyde to germinal diols is a main step. It is accelerated in high pH. The germinal diols will adsorb in the metal surface to form an alkoxide, that later undergoes a β-elimination to form carboxylic acid. Thus, the combination of high concentration of OH− and longer reaction times favors the oxidation to carboxylic acid. New reference was added (25).

9. Page 4, Row 109: Figure 1 changed to Figure 4

Figure number was updated.

10. Page 5, Row 132: Figure 2 changed to Figure 5

Figure number was updated.

11. Page 7, Row 182: Figure 3 changed to Figure 6

Figure number was updated.

12. Page 7, Row 185: Figure 4 changed to Figure 7

Figure number was updated.

13. Reference is suggested to be expanded such as: liquid-phase biomass conversion advantages: New J. Chem., 2022, 46, 5907-5921.

We would like to thank for this valuable remark. This reference was added to the manuscript (Reference 4).

14. It is noted that the manuscript needs further careful editing in aspects of grammar, spelling sentences, and typo errors to be clear enough to reader.

The manuscript was edited and checked.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.