Efficient Synthesis of Furfural from Corncob by a Novel Biochar-Based Heterogeneous Chemocatalyst in Choline Chloride: Maleic Acid–Water

Round 1

Reviewer 1 Report

The manuscript studies the Synthesis of Furfural (FAL) from Corncob by a biochar-Based catalyst. The work consistently shows the results of the catalytic characterization of a catalyst, SO42-/SnO2-FFS, which was used in a DESMLA–water system.

The authors also studied the effect of different DES types and the pKa value of different organic acids in DES on the FAL yield and they also analyzed the effect of different DES types and the pKa value of different organic acids in DES on the FAL 234 yield on the FAL yield.

In addition, the results of the reuse of the catalysts are shown.

The system studied in this work is novel, and the results are good.

This article can be accepted after some minor corrections.

Experimental details of biochar preparation should be added to the paper.

Please, review the legends of Figures 5 and 6.

Could the authors report the number of total acid sites obtained from the NH3-TPD curves?

The citations included in the text are underlined, please review.

Could the authors show some characterization results  of the catalyst post-reaction? Is the number of acid sites maintained after use?

Author Response

Reviewer 1#

The manuscript studies the Synthesis of Furfural (FAL) from Corncob by a biochar-Based catalyst. The work consistently shows the results of the catalytic characterization of a catalyst, SO42-/SnO2-FFS, which was used in a DESMLA–water system.

The authors also studied the effect of different DES types and the pKa value of different organic acids in DES on the FAL yield and they also analyzed the effect of different DES types and the pKa value of different organic acids in DES on the FAL yield on the FAL yield.

In addition, the results of the reuse of the catalysts are shown.

The system studied in this work is novel, and the results are good.

This article can be accepted after some minor corrections.

1. Experimental details of biochar preparation should be added to the paper.

Response: Thanks for the good suggestion.

In this study, the biobased char obtained from FFS is a crucial carrier for the prepared catalyst. Therefore, providing a detailed account of the process for producing biobased char using FFS is deemed essential. Upon a comprehensive review of the entire manuscript, a more elaborate description of the process for producing biomass-derived char using FFS has been meticulously outlined on lines 431-435 of page 14. We have revised and organized this preparation process, and the specific details are presented as follows:

FFS was thoroughly washed with clean water, followed by boiling in deionized water. Subsequently, the boiled mixture was filtered to remove gelatin, proteins, and other impurities. The obtained wet fish scale was then dried in a 60 °C oven for 16 h. The desiccated fish scale was finely ground using a milling machine to achieve particle sizes between 60 and 80 mesh. Finally, the powdered fish scale was placed in a muffle furnace and subjected to a temperature of 300 °C for 1 h, resulting in the production of biomass charcoal derived from fish scales.

2. Please, review the legends of Figures 5 and 6.

Response: Thanks for the good suggestion. The updated captions for Figures 5 and Figure 6 were given as follows:

Figure. 5. Temperature-Programmed Desorption of ammonia (NH₃-TPD) image of SO₄^2-/SnO₂-FFS.The temperature was scanned between 0-600 °C.

Figure. 6. X-ray Photoelectron Spectroscopy (XPS) image of SO₄^2-/SnO₂-FFS.The B. E. (eV) was scanned between 480-500.

 

3. Could the authors report the number of total acid sites obtained from the NH3-TPD curves?

Response: Thanks for the good suggestion.

In general, acid sites were associated with dehydration, hydrolysis and alkyl conversion, which would play a key role in catalyzing biomass to FAL. In this revised manuscript, the information about total acid sites was given as below:

"From the observation of Figure 5, it could be deduced that SO₄^2-/SnO₂-FFS possesses a predominant type of acid site (weak acid site) only at 108 °C. "

4. The citations included in the text are underlined, please review.

Response: Thanks for the good suggestion. We have thoroughly reviewed the entire manuscript, meticulously evaluating the contribution of each cited reference to the content. The specific modifications were as follows:

[9] "Yu, X., Miao, Z., Wang, H., Jia, W., Wang, Q., Sun, Q., Tang, X., Zeng, X., Yang, S., Li, Z., Wei, Z.J., Xu, F., Lin, L., 2022. Insight into the catalytic mechanism of core–shell structured Ni/Ni-N/CN catalyst towards the oxidation of furfural to furancarboxylic acid. Fuel, 317, 123579." was revised to "E. Amini, C. Valls, M.B. Roncero, 2021. Ionic liquid-assisted bioconversion of lignocellulosic biomass for the development of value-added products. Journal of Cleaner Production, 326, 129275."

[30] "Huang, C.-Y., Kuo, J.-M., Wu, S.-J., Tsai, H.-T. 2016. Isolation and characterization of fish scale collagen from tilapia (Oreochromis sp.) by a novel extrusion–hydro-extraction process. Food chemistry, 190, 997-1006." was revised to " D. Qin, S. Bi, X. You, M. Wang, X. Cong, C. Yuan, M. Yu, X. Cheng, X.-G. Chen, 2022. Development and application of fish scale wastes as versatile natural biomaterials. Chemical Engineering Journal, 428, 131102."

[43] "Li, Q., Hu, Y., Tao, Y.-Y., Zhang, P.-Q., Ma, C.-L., Zhou, Y.-J., He, Y.-C. 2021. Improving biocatalytic synthesis of furfuryl alcohol by effective conversion of D-xylose into furfural with Tin-loaded sulfonated carbon nanotube in cyclopentylmethyl ether-water media. Catalysis Letters, 151(11), 3189-3196." was revised to "Q. Jia, X. Teng, S. Yu, Z. Si, G. Li, M. Zhou, D. Cai, P. Qin, B. Chen, 2019. Production of furfural from xylose and hemicelluloses using tin-loaded sulfonated diatomite as solid acid catalyst in biphasic system. Bioresource Technology Reports, 6, 145-151."

 

5. Could the authors show some characterization results of the catalyst post-reaction? Is the number of acid sites maintained after use?

Response: Thanks for the good suggestion.

Characterizing the recovered catalyst post-reaction is indeed an insightful notion. Such an approach could potentially provide us with a deeper grasp of how the distinctive structure of FFS in our study bestows the catalyst with remarkable reusability. As per your suggestion, we endeavored to retrieve the catalyst from the reaction system and subjected it to SEM, XRD, NH₃-TPD, and other analyses. Nevertheless, our experience in catalyst recovery and separation procedures is limited, and certain instrumental resources for characterization were lacking. Anticipated completion of the corresponding characterization results is projected to take approximately one month. We sincerely apologize for the time constraints that prevented us from conducting an exhaustive characterization of the recuperated heterogeneous catalyst prior to submitting the revised manuscript. Despite the tantalizing opportunity to further validate the catalyst's specific state post-catalysis, we regret that time did not allow for its completion. As outlined in our research group's roadmap, apart from evaluating the potential reusability of the segregated catalyst and DES for recurrent catalysis, it is equally imperative to delve into the structural attributes of the reclaimed catalyst, elucidating the factors contributing to its reusability.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The article, although not overly original, seems based on sound scientific foundations, but is written in such a poor english that it is too difficult to follow, at times requiring multiple readings of the same sentence to grasp what was meant to be conveyed to the reader. Therefore, a major revision is needed before an accurate revision can be made.

As said before, the article is written in an extremely poor english, which makes reading and comprehending the meaning behind each sentence extremely difficult. A major proofreading is needed before proceeding to actually revisioning the science behind the proposed work.

Author Response

Reviewer 2#

 

Comments and Suggestions for Authors

The article, although not overly original, seems based on sound scientific foundations, but is written in such a poor english that it is too difficult to follow, at times requiring multiple readings of the same sentence to grasp what was meant to be conveyed to the reader. Therefore, a major revision is needed before an accurate revision can be made.

 

Comments on the Quality of English Language

As said before, the article is written in an extremely poor english, which makes reading and comprehending the meaning behind each sentence extremely difficult. A major proofreading is needed before proceeding to actually revisioning the science behind the proposed work.

Response: Thanks for the good suggestion. This manuscript was further edited by Native English speaker. The revised parts were marked with red color.

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

1. The function of fresh fish scale (FFS) in the catalyst design is not clearly demonstrated or discussed. Is it just a pure biobased carrier/substrate to disperse and support SnO₂, or does it provide extra catalytic sites? More control experiments are needed here. For example, can "AT-FFS" or "FFs after loading" (as shown in Figure 11) show any catalytic performance? Does SO₄^2- /SnO₂ without FFS support show similar yield under the same reaction condition?  The authors should have more explanations and discussions in the intro or conclusion part (I only found little in line 339) about the importance of FFS .  

2. It is highly suggested to shorten the introduction part (paragraph 2 and 3) for readers' convenience. The current  two paragraphs contain too much experimental details from the reported cases. The authors should try to summarize the cases in a more organized and clearer way.

3. The manuscript contains a lot of abbreviations. The authors should check the appropriate usage of these abbreviations. For example, the "ChCl" in the title is not that obvious to readers. The "FFS" has appeared several times in the text and in figure 1 before first being introduced in line 110. 

4. The Figure 1 is a summary of the main design and is very important. However, the display of Figure 1 is very insufficient. The authors should consider to have more explanations in the caption, such as the meaning of top images and bottom images. The reaction temperature, time, chloride salt, yield can be included in the figure. The authors may also consider to highlight the feature of "FFS" in the figure since it is the key improvement or modification of the design.

5. Same issues of lacking explanations in the captions of Figure 3, 4, 5. Also, the caption for Figure 5 and 6 do not match. The scale bar in Figure 2 should be displayed more clearly. 

6.  According to the XPS results, there are three Sn species and the Sn³⁺ ratio is very high (line 148). Any similar results have been reported before? What is the reason of the presence of three Sn species?

7. Figure 8b is confusing. "None" should be replaced with "0:1" or using a different way to present ratio in the figure. Also the discussion in the text (line 228) is insufficient. Does it relate to the viscosity or also the PKa as well?

8. Figure 9 is very difficult to understand. Is ZnCl2 necessary for the catalytic activity? It looks like without chloride salts (figure 9a) or no matter how much ZnCl2 added 9b), the yield is close to the maximum yield. The authors may consider how to present and discuss this part. By the way, what is the best reaction condition? The summarized best condition in the abstract and conclusion part do not match. 

The quality of English must be improved. There are a certain amount of typos and grammatical errors. To list few:

1. "which purpose in improving the yield of FAL" in line 80.

2. "when the SO4/SnO2-FFS load was rose from 0 to 3.6 wt.%, the FAL yield was raised greatly" in line 173.

3.  "yield was weakened" and " yield was declined" in line 228 and 229.

Author Response

Reviewer 3#

Comments and Suggestions for Authors

1. The function of fresh fish scale (FFS) in the catalyst design is not clearly demonstrated or discussed. Is it just a pure biobased carrier/substrate to disperse and support SnO₂, or does it provide extra catalytic sites? More control experiments are needed here. For example, can "AT-FFS" or "FFs after loading" (as shown in Figure 11) show any catalytic performance? Does SO₄^2-/SnO₂ without FFS support show similar yield under the same reaction condition? The authors should have more explanations and discussions in the intro or conclusion part (I only found little in line 339) about the importance of FFS.

Response: Thanks for the good suggestion.

 The role and significance of FFS as a crucial innovation in this study were not adequately showcased and discussed, which was an oversight. Such oversight fails to emphasize its key advantages. As a result, we have made revisions to the introduction to better elucidate the relevant role of FFS.

 "The main components of fish scales are HAP and collagen. Processed HAP possesses irregular pores. HAP, a calcium phosphate biomaterial, serves as an ideal catalyst carrier due to its ion exchange, adsorption, acid-base properties, and stability. Previous research has modified HAP via organic methods, metal ion substitution, encapsulation, and catalyst loading to create multiphase catalysts. Currently, no report exists on converting biomass or D-xylose to FAL using fish scale-based solid acid catalyst. Given the abundant, cost-effective fish scale sources, biochar preparation to adsorb active centers for solid acid catalyst creation is possible."

Designing appropriate control experiments can effectively highlight the superior catalytic performance of SO₄^2-/SnO₂-FFS. Conducting separate conversions using "AT-FFS" or "loaded FFS" as catalysts under the same conditions is a valuable and feasible suggestion. However, constrained by our limited experience, we further employed "FFS" and "SnO₂-FFS" as controls to illustrate the catalytic effects of SO₄^2-/SnO₂-FFS. The detailed information has been added to Section 2.2:

"In order to analyze the impact of the SO₄^2-/SnO₂-FFS catalyst on FAL yield, xylose aqueous solution was employed as the substrate in a DES_MLA-water system, with the addition of ZnCl₂, and the reaction was conducted at 175 °C for 15 minutes. The results were presented in Table S1 (in Support Information), revealing that SnO₂-FFS and SO₄^2-/SnO₂ could give higher FAL yields compared to the control group without catalyst. Furthermore, the FAL was achieved in the highest yield (70.5%) through the catalysis of SO₄^2-/SnO₂-FFS. This could be attributed to the synergistic effect of Lewis and Brønsted acidic sites in sulfonated SnO₂-FFS, resulting in more active acid sites [49,50]. In summary, the solid acid catalyst SO₄^2-/SnO₂-FFS employed in this study exhibited enhanced catalytic activity in biomass conversion to FAL.".

 

Table S1. The FAL production from xylose catalyzed by FFS based solid acids in DES_MLA-water (1/9, v/v) system.

Catalyst	FAL yield (%)
-	32.0
FFS	34.8
SnO2-FFS	58.6
SO42-/SnO2	60.1
SO42-/SnO2-FFS	70.5

 

 

 

 

 

 

 

 

 

 

Reaction conditions: 175 °C for 15 min in 40 mL working volume, 20 g/L Xylose, 20 g/L ZnCl₂.

 

2. It is highly suggested to shorten the introduction part (paragraph 2 and 3) for readers' convenience. The current two paragraphs contain too much experimental details from the reported cases. The authors should try to summarize the cases in a more organized and clearer way.

Response: Thanks for the good suggestion.

The introduction section is an important aspect for readers to quickly grasp the essence of the paper, and concise introductions facilitate better reader comprehension. The excessive inclusion of experimental details leads to unclear and confusing reading experience, adding unnecessary complexity for understanding this research. Therefore, it is necessary to shorten the second and third paragraphs of the introduction section. After a careful review of these paragraphs, we have summarized the past experimental cases in each of these sections in a more organized and clear manner.

The revised second paragraph of the introduction section is as follows:

"Generally, furfural (FAL) can be synthesized using both homogeneous and heterogeneous catalysts. Utilizing homogeneous catalysts such as hydrochloric acid, the highest FAL yield from xylose could reach 37.5% [14]. Combining Lewis acid (AlCl₃) and Brønsted acid (HCl) as homogeneous catalysts for the catalytic conversion of untreated cellulose resulted in a remarkable FAL yield of 75% [15]. However, the use of homogeneous catalysis might accelerate equipment corrosion and lead to difficulties in recycling, posing challenges in terms of both economic efficiency and environmental sustainability [16]. Consequently, more eco-friendly heterogeneous catalysts have been employed for FAL synthesis [17,18]. Among these, sulfonated carbon derived from biomass has garnered attention due to its low cost, favorable thermal stability, and acidity. Using biochar-based SO₄^2-/SnO₂-CS as a catalyst, corn stalk was transformed into FAL with a yield of 68% at a temperature of 170 °C for 0.5 h [19]. 62% of FAL yield from D-xylose by carbonized and sulfonated teff straw was achieved in a toluene-water system at 170 °C for 0.5 h [20]. FAL was synthesized from corncob with sulfonated tin-loaded rice husk activated carbon, and the yield of FAL was 40.9% [21]. Employing 4-BDS as a sulfonating agent, sucrose was carbonized into a solid acid catalyst, achieving a 61% FAL yield from corn stalk in 100 min at a temperature of 200 °C [22]. This highlights the significant potential of using biomass as a catalyst carrier. Therefore, utilizing sulfonated carbon derived from biomass as a catalyst or carrier is essential for achieving high-value utilization of biomass."

The modified third paragraph of the introduction section is presented below:

"Appropriate solvents also play a crucial role in enhancing FAL productivity [23]. Deep eutectic solvents (DESs) are defined as systems that combine eutectic Lewis (L) or Brönsted (B) acids with various ionic species [24]. Compared to traditional reaction systems, DESs offer several advantages including ease of preparation, non-toxicity, non-volatility, and reusability [25]. Utilizing choline chloride (ChCl)-lactic acid as a chemical catalyst and reaction medium, rice straw was transformed into FAL (12% yield) at 180 °C for 2 h [26]. Water–ChCl:oxalic acid was employed as a medium to convert corncob (CC) at 180 °C for 0.5 h, achieving a FAL yield of 46% [27]. ChCl-carboxylic acid containing H⁺ and Cl⁻ might facilitate the dehydration of D-xylose to produce FAL [28]. Therefore, to further enhance FAL yield, a synergistic catalysis approach using carboxylic acid-based DES and heterogeneous catalysts can be employed in the reaction system, providing a more environmentally-friendly and efficient means of catalyzing biomass-derived D-xylose into FAL."

 

3. The manuscript contains a lot of abbreviations. The authors should check the appropriate usage of these abbreviations. For example, the "ChCl" in the title is not that obvious to readers. The "FFS" has appeared several times in the text and in figure 1 before first being introduced in line 110.

Response: Thanks for the good suggestion.

The full names of abbreviations were clearly defined when they were firstly introduced in this manuscript, and the full names were given as below:

In the title, "ChCl" was revised to "Choline chloride".

"HA" was revised to "HAP".

"hydroxyapatite" was revised to "HAP"".

“waste fish scales" was revised to "fresh fish scales (FFS)".

"MLA" was revised to "maleic acid (MLA)".

"MA" was revised to "malic acid (MA)".

"LA" was revised to "lactic acid (LA)".

"CA" was revised to "citric acid (CA)".

"TA" was revised to "tartaric acid (TA)".

 

4. The Figure 1 is a summary of the main design and is very important. However, the display of Figure 1 is very insufficient. The authors should consider to have more explanations in the caption, such as the meaning of top images and bottom images. The reaction temperature, time, chloride salt, yield can be included in the figure. The authors may also consider to highlight the feature of "FFS" in the figure since it is the key improvement or modification of the design.

Response: Thanks for the good suggestion.

Issues like inadequate image presentation can significantly impair the reader's reading experience. Such errors should not be present in the article, and we sincerely apologize for this. Based on your valuable suggestions, we have made improvements to Figure 1, adding reaction temperature, time, chloride salt, and yield information. Additionally, explanations for the upper and lower images have been provided in the captions to clarify their meanings. Furthermore, the features of FFS have been highlighted in Figure 1.

Figure 1. The process of catalytically converting the D-xylose solution obtained from the hydrolysis of corncob at 140 °C for 40 minutes into FAL using SO₄^2-/SnO₂-FFS in a DES-water system (a); a chemical strategy for synthesis of FAL from corncob with biochar-based heterogeneous chemocatalyst (b).

 

5. Same issues of lacking explanations in the captions of Figure 3, 4, 5. Also, the caption for Figure 5 and 6 do not match. The scale bar in Figure 2 should be displayed more clearly.

Response: Thanks for the good suggestion.

Lack of explanatory captions can lead to misconceptions in readers' understanding, which is an unfortunate oversight. Following your suggestion, we have provided detailed supplementary captions for Figures 3, 4, and 5, as shown below:

Figure 3. Fourier transform infrared spectroscopy (FT-IR) images of chemocatalyst SO₄^2-/SnO₂-FFS and FFS. FT-IR was conducted by using Nicolet IS50 (Thermo Scientific, Waltham, MA, USA) in the range of 4000–500 cm⁻¹.

Figure 4. X-Ray Diffraction (XRD) images of chemocatalyst SO₄^2-/SnO₂-FFS and FFS[XRD was carried out by using a D/max-2500 (Japan) instrument (Rigaku Co., Akishima-shi, Japan). The 2θ angle was scanned between 10° and 80°.

 

Figure. 5. Temperature-Programmed Desorption of ammonia (NH₃-TPD) image of SO₄^2-/SnO₂-FFS.The temperature was scanned between 0-600 °C.

 

Misaligned captions with images can significantly affect the reader's experience and increase the difficulty of interpretation. We sincerely apologize for this. Consequently, we have rephrased the captions for Figures 5 and 6 as follows:

Figure. 5. Temperature-Programmed Desorption of ammonia (NH₃-TPD) image of SO₄^2-/SnO₂-FFS.The temperature was scanned between 0-600 °C.

 

Figure. 6. X-ray Photoelectron Spectroscopy (XPS) image of SO₄^2-/SnO₂-FFS.The B. E. (eV) was scanned between 480-500.

 

Unclear images can create confusion during readers' comprehension, leading to a lack of accurate understanding of the article's content. Ensuring clarity in the visual representation of image content is essential for enhancing reader efficiency. To address this, we have enlarged Figure 2, adjusted its clarity, and improved its display quality.

 

6. According to the XPS results, there are three Sn species and the Sn³⁺ ratio is very high (line 148). Any similar results have been reported before? What is the reason of the presence of three Sn species?

Response: Thanks for the good suggestion.

Figure. 6. X-ray Photoelectron Spectroscopy (XPS) image of SO₄^2-/SnO₂-FFS.The B. E. (eV) was scanned between 480-500.

 

Sorry for our mistake. As shown in Figure 6, no Sn³⁺ was detected. Only Sn⁴⁺, Sn²⁺, and Sn⁰ were detected. C. As displayed in XPS image (Figure. 6), SO₄^2-/SnO₂-FFS had been accompanied by Sn, and Sn had three valences (+4, +2, 0). The fraction of Sn⁴⁺3d_5/2, Sn²⁺3d_5/2, and Sn⁰3d_5/2 were 48.4, 46.1, and 5.5%, respectively.

 

7. Figure 8b is confusing. "None" should be replaced with "0:1" or using a different way to present ratio in the figure. Also the discussion in the text (line 228) is insufficient. Does it relate to the viscosity or also the PKa as well?

Response: Thanks for the good suggestion.

The unclear representation of the DES-water ratio caused confusion for readers during their reading. We have replaced the "None" in Figure 8b with "0:1".

Figure 8. Effect of different DES type and the pKa value of different organic acid in DES on the FAL yield in DES–water (20:80, v/v) system [Xylose-rich hydrolysate containing 20 g/L xylose, 3.6 wt.% SO₄^2-/SnO₂-FFS, 170 °C, 15 min, 500 rpm] (a); Effect of DES_MLA–water system (0:1–7:3, v/v) on the FAL yield [Xylose-rich hydrolysate containing 20 g/L xylose, 3.6 wt.% SO₄^2-/SnO₂-FFS, 170 °C, 15 min, 500 rpm] (b).

 

Insufficient discussion has led to a misunderstanding of the data and trends by readers. In accordance with your suggestions, we have readdressed the impact of different amounts of DES on furfural yield, with a specific focus on its relationship with the viscosity of the reaction medium. The role of pKa has been utilized to elucidate the influence of various carboxylic acid-chooline chloride DES compositions on furfural yield, as depicted in Figure 8a. The re-evaluated discussion was as follows:

"In the DES_MLA-water system, the FAL yield exhibited a significant increase with rising volumetric ratio of DES_MLA. When DES was absent from the reaction system, the FAL yield approached 60%. However, with an increase in DES_MLA volumetric ratio up to 10%, the yield of FAL exceeded 60%. The highest yield was achieved in DES_MLA-water (1:9, v/v). Furthermore, as the volumetric ratio of DES_MLA was further increased, the FAL yield displayed a noticeable decline. High content of DES_MLA (> 10%) would increase the viscosity of the reaction medium, leading to the reduction in FAL yield. Accordingly, DES_MLA-water (1:9, v/v) was regarded as the most suitable reaction system."

 

8. Figure 9 is very difficult to understand. Is ZnCl2 necessary for the catalytic activity? It looks like without chloride salts (figure 9a) or no matter how much ZnCl2 added 9b), the yield is close to the maximum yield. The authors may consider how to present and discuss this part. By the way, what is the best reaction condition? The summarized best condition in the abstract and conclusion part do not match.

Response: Thanks for the good suggestion.

 The addition of chloride salts is essential, as the appropriate amount of these salts shifts the reaction equilibrium further towards the product side, leading to a notable enhancement in FAL yield. We have made revisions to the section concerning the results, discussion, and implications of chloride salts.

Modifications have been made to the description as follows:

" Hence, different chloride salts (15 g/L) were individually supplemented to the catalytic system, which would result in the promotion of the dispersion and yield of FAL. Distinct from the control group, SnCl₄ and LiCl₂ could apparently hindered the production of FAL (Fig. 9a). Most of the chloride salts (CaCl₂, MgCl₂, NH₄Cl, AlCl₃, KCl, MnCl₂, BaCl₂, and NaCl) didn’t significantly promote the catalytic activity. However, NiCl₂ and ZnCl₂ exhibited positive effects on the enhancement of catalytic reaction. Particularly, the addition of ZnCl₂ facilitated the FAL formation and resulted in the highest FAL yield of 66.6%."

“When the content of ZnCl₂ was 20 g/L, the highest FAL yield reached 70.5% for 15 min in DES_MLA–water (170 °C). In the absence of ZnCl2, the FAL yield was only 63%.”

 

In this revised manuscript, this “2.4. Effect of chloride salts on the FAL yield” section was revised as below:

“2.4. Effect of chloride salts on the FAL yield

Chloride salts are frequently employed to stabilize the transition state and intermediate structures in catalytic processes, thereby reducing undesirable side-reactions and significantly improving the yield of FAL [55]. Hence, different chloride salts (15 g/L) were individually supplemented to the catalytic system, which would result in the promotion of the dispersion and yield of FAL. Distinct from the control group, SnCl₄ and LiCl₂ could apparently hindered the production of FAL (Fig. 9a). Most of the chloride salts (CaCl₂, MgCl₂, NH₄Cl, AlCl₃, KCl, MnCl₂, BaCl₂, and NaCl) didn’t significantly promote the catalytic activity. However, NiCl₂ and ZnCl₂ exhibited positive effects on the enhancement of catalytic reaction. Particularly, the addition of ZnCl₂ facilitated the FAL formation and resulted in the highest FAL yield of 66.6%. Moreover, different load of ZnCl₂ (0–30 g/L) were supplemented into DES_MLA–water medium (Fig. 9b), and the catalytic effect on the generation of FAL from D-xylose was tested. When the content of ZnCl₂ was 20 g/L, the highest FAL yield reached 70.5% for 15 min in DES_MLA–water (170 °C). In the absence of ZnCl2, the FAL yield was only 63%. The addition of ZnCl₂ enhanced the selectivity of FAL production from xylan-rich biomass, reducing the occurrence of side-reactions and the formation of by-products [56]. As a result, this led to a further increase in the overall FAL yield.

 

Figure 9. Effect of different chloride salts (15 g/L) on the FAL yield [Xylose-rich hydrolysate containing 20 g/L xylose, 3.6 wt.% SO₄^2-/SnO₂-FFS, DES_MLA–water system (1:9, v/v), 170 °C, 15 min, 500 rpm] (a); Effect of ZnCl₂ dosage (0-30 g/L) on the FAL yield [Xylose-rich hydrolysate containing 20 g/L xylose, 3.6 wt.% SO₄^2-/SnO₂-FFS, DES_MLA–water system (1:9, v/v), 170 °C, 15 min, 500 rpm] (b).

 

Modifications have been made to the description as follows: " The addition of ZnCl₂ enhanced the selectivity of FAL production from xylan-rich biomass, reducing the occurrence of side-reactions and the formation of by-products [54]. As a result, this led to a further increase in the overall FAL yield"

 

In “Abstract”, this best condition was provided as below:

"The corncob was initially hydrolyzed at 140 °C to obtain xylose-rich hydrolysate. Subsequently, SO₄^2-/SnO₂-FFS (3.6 wt.%) was used to catalyze the corn cob hydrolysate containing D-xylose (20.0 g/L) at a reaction temperature of 170 °C for 15 minutes. Additionally, ZnCl₂ (20.0 g/L) was added. Ultimately, furfural (106.8 mM, 70.5% yield) was produced in the deep eutectic solvent ChCl:maleic acid–water (DES_MLA–water 20:80, v/v)."

 

In “3. Conclusions”, this best condition was provided as below:

“3. Conclusions

In this work, the abundant biobased FFS was used as carrier to prepare biochar SO₄^2-/SnO₂-FFS catalyst. After the acid hydrolysis of corncob, xylose-rich corncob-hydrolysate was used as substrate to produce FAL through the catalysis with biochar SO₄^2-/SnO₂-FFS catalyst. In DES_MLA–water (DES_MLA, 10 vol%), the FAL yield was obtained at 70.5% by SO₄^2-/SnO₂-FFS (3.6 wt.%) with ZnCl₂ (20.0 g/L) for 15 min under the temperature of 170 °C. The potential catalytic mechanism for the production of FAL from corncob catalyzed by SO₄^2-/SnO₂-FFS in DES_MLA–water was proposed. An efficient method for the synthesis of biofuran from lignocellulose was successful developed by using biochar solid acid catalyst in a green reaction system”.

 

Comments on the Quality of English Language

The quality of English must be improved. There are a certain amount of typos and grammatical errors. To list few:

1. "which purpose in improving the yield of FAL" in line 80.
2. "when the SO4/SnO2-FFS load was rose from 0 to 3.6 wt.%, the FAL yield was raised greatly" in line 173.
3. "yield was weakened" and " yield was declined" in line 228 and 229.

Response: Thanks for the good suggestion.

Spelling errors and similar issues can greatly impede the reader's reading experience. Such avoidable mistakes should not be present in the article, and we sincerely apologize for this oversight. Following your valuable feedback, the listed spelling errors have been rectified.

1. "Therefore, to further enhance FAL yield, a synergistic catalysis approach using DES and heterogeneous catalysts can be employed in the reaction system, providing a more environmentally friendly and efficient means of catalyzing biomass-derived D-xylose into FAL."
2. "when the loading of SO₄^2-/SnO₂-FFS increased from 0 to 3.6 wt%, a significant enhancement in FAL yield was observed,"
3. "Furthermore, as the volumetric ratio of DES_MLA was further increased, the FAL yield displayed a noticeable decline. High content of DES_MLA (> 10%) would increase the viscosity of the reaction medium, leading to the reduction in FAL yield."

 

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

No further scientific comments.

The quality of English language can be further improved. It is highly recommended that all the authors read through the manuscript carefully. Based on the new Table S1, It seems to me that using FFS slightly improved yield. The authors should be soft or be careful about the wording when discussing the function of FFS in the catalyst design.