Review Reports - Synthesis of Sulfonated Carbon from Discarded Masks for Effective Production of 5-Hydroxymethylfurfural

Round 1

Reviewer 1 Report

The paper of Xinhua Qi et al. deals with Hydroxymethylfurfural production from fructose using a sulfonated porous carbon as a solid acid catalyst, which was prepared from discarded masks.

In the lines 44-46, authors mention about the usage of face masks which has increased exponentially. A suggestion is to include an explanation about the dimension of the number of masks disposable and scale with respect to collection and transport costs, since this material, despite being disposable, also has a cost. For example, in the reference 26, which is mentioned in this work, the authors discuss about the dimension of the number of masks disposed, which even show the importance of using this discarded material

In the line 132, authors mention the acidity amount of the functional carbon, but this method in not described in the “Material and Experimental” section. So, this method must be included.

In the line 255 in mentioned the "low-cost" of the material, but it should be suppressed, since economic analysis was not evaluated in this work.

Overally, the paper communicates well the objectives, course and conclusion of the study. So, I recommend accepting this paper for publication after a minor review.

Author Response

Reviewer 1

The paper of Xinhua Qi et al. deals with Hydroxymethylfurfural production from fructose using a sulfonated porous carbon as a solid acid catalyst, which was prepared from discarded masks.

In the lines 44-46, authors mention about the usage of face masks which has increased exponentially. A suggestion is to include an explanation about the dimension of the number of masks disposable and scale with respect to collection and transport costs, since this material, despite being disposable, also has a cost. For example, in the reference 26, which is mentioned in this work, the authors discuss about the dimension of the number of masks disposed, which even show the importance of using this discarded material

Reply: Information was given in introduction (Lines 52-53).

In the line 132, authors mention the acidity amount of the functional carbon, but this method in not described in the “Material and Experimental” section. So, this method must be included.

Reply: In this work, Boehm method was used to determine the acidity amount of the functional groups on masks-derived sulfonated carbon surface (highlighted in red, Section given in 3.2).

In the line 255 in mentioned the "low-cost" of the material, but it should be suppressed, since economic analysis was not evaluated in this work. Overall, the paper communicates well the objectives, course and conclusion of the study. So, I recommend accepting this paper for publication after a minor review.

Reply: According to your comment, introduction and conclusion were revised to illustrate the point.

Functional carbon materials as a promising low-cost heterogeneous acid catalyst….. (Introduction, line 44)

This method for the synthesis of sulfonated carbon from discarded mask provides a new idea for the environmental-friendly and value-added resource utilization for the polypropylene-based material wastes. (Conclusion)

Reviewer 2:

The work presented by Hao et al. faces two themes of great actuality, such as the synthesis of HMF, one of the most important platform chemicals, and the valorization of discarded masks, an abundant waste after the COVID-19 epidemic. However, the work presents several editing mistakes and in my opinion some deeper analysis and comments lack, thus the authors should strongly improve the manuscript according to the following points:

1) Check the English by a native speaker.

Reply: The manuscript has been carefully edited and checked for spelling, grammar and word.

2) The authors should evidence in the abstract also the importance of HMF.

Reply: According to your comment, the abstract was revised as follow:

5-hydroxymethylfurfural (HMF) as one of the top ten important platform chemicals can be used to produce 2,5-furandicarboxylic acid (FDCA), 2,5-dimethyl furan (DMF), levulinic acid and other chemicals.

3) Line 27: The authors cannot fail to mention also levulinic acid, which is, analogously to HMF, one of the most important platform-chemical. The authors can cite the following reference: D. Licursi et al., “Smart valorization of waste biomass: exhausted lemon peels, coffee silverskins and paper wastes for the production of levulinic acid”, Chem. Eng. Trans. 2018, 65, 637-642.

Reply: The suggested refs were carefully studied, and cited as ref [4]. The levulinic acid were mention in the file (line 32).

4) Lines 28-30: The authors maybe mean that the compounds derived from HMF (and not HMF) can be used as solvents, and clean fuels, but also as monomers. The authors should update the literature, reference 2, 6, and 7 are too old.

Reply: The sentence was revised as follow:

HMF is a typical acid catalytic dehydration product of hexose, which can be used as a key platform compound and compounds derived from HMF can be used as solvent and clean fuels.

The literature has been updated as:

[1] M. Sajid, Y. Bai, D. Liu, X. Zhao, Organic acid catalyzed production of platform chemical 5-hydroxymethylfurfural from fructose: Process comparison and evaluation based on kinetic modeling, Arabian Journal of Chemistry, 2020, 13: 7430-7444. https://doi.org/10.1016/j.arabjc.2020.08.019

[2] Ashok, R.P.B.; Oinas, P. et al. Techno-economic evaluation of a biorefinery to produce g-valerolactone (GVL), 2-methyltetrahydrofuran (2-MTHF) and 5-hydroxymethylfurfural (5-HMF) from spruce. Renewable energy, 2022, 190: 396-407. https://doi.org/10.1016/j.renene.2022.03.128

[3] Manishkumar S.T., Dipti W. et al. Solvent Free Upgrading of 5-Hydroxymethylfurfural (HMF) with Levulinic Acid to HMF Levulinate Using Tin Exchanged Tungstophosphoric Acid Supported on K-10 Catalyst, ACS Organic & Inorganic Au, 2022. https://doi.org/10.1021/acsorginorgau.2c00027

5) Line 34: The authors have confused homogeneous and heterogeneous catalysts. In addition, not only mineral acids (H₂SO₄ and HCl) have been adopted as homogeneous catalysts for the synthesis of HMF, but also inorganic salts, thus the authors should add some references regarding these catalysts. Among the heterogeneous catalysts, also Amberlyst-70 has been adopted by the research group of Raspolli, please add the reference: C. Antonetti et al. “Amberlyst A-70: a surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water”, Catal. Commun. 2017, 97, 146-150.

Reply: The sentence was revised as including homogenous acid catalyst (e.g. HCl, H₂SO₄, NH₄Cl) and heterogenous acid catalysts (e.g. S-doped porous carbon, Amberlyst-15, Amberlyst-70).

The suggested refs were carefully and cited as ref [15]. Other homogenous acid catalyst (NH₄Cl) were also cited as ref [11].

6) Lines 69-71: Maybe the authors should introduce how the different samples have been prepared in order to better understand the differences. Moreover, the SEM analysis of the fresh sample (the mask) should be added with the aim of evidencing the morphological differences.

Reply: Section 3.2 Catalyst Preparation was revised, to make understand easy the different between each sample.

According to your comment, the SEM image of fresh sample was added (Fig.1a). And the section 2.1 was revised as follow:

Compared to fresh mask (Fig. 1a), the ball-milling treatment of mask leaded fibrous structure slightly breaks to produce debris (Fig. 1b).

7) Line 77: "Temperature above 430 °C". Is it due to the decomposition of polypropylene?

Reply: Yes, the polymer chain of polypropylene is gradually broken about 430 °C. [1]

[1] Hu X. and Lin Z. Transforming waste polypropylene face masks into S-doped porous carbon as the cathode electrode for supercapacitors. Ionics (Kiel). 2021, 27(5): 2169-2179. DOI: 10.1007/s11581-021-03949-7.

8) The authors should add also the TGA analysis of the fresh mask, bMC, and bMC-SO₃H in order to evidence how the thermal treatment influences the thermal properties and somehow determine if there is some polypropylene not converted in the samples.

Reply: According to your comment, the TG-DTA analysis result of fresh mask, bM and mbM were given in Fig.2a. The TG-DTA analysis results of bMC and bMC-SO₃H were given in Fig.2b. The results and discussion (Section 2.1) was revised as follow:

TG-DTA results for fresh mask, ball-milling treated mask (bM) and mixed ball-milling treated mask (mbM) show that bM underwent an irreversible transformation at temperature above ~430 ºC under nitrogen conditions (Fig. 2a). The polymer chain of polypropylene is gradually broken about 430 °C. The mbM sample weight loss began below 100 ºC and the major weight loss occurred at temperature higher than ~420 ºC, suggesting that the ball-milling treated mask is thermally stable than the mixed ball-milling treated mask (mbM) (Fig. 2a). TG-DTA results for bMC(600) and bMC(600)-SO₃H show that mix ball-milling treat-ment mask carbonization at 600 ºC performs thermal stability (Fig. 2b).

9) Lines 91-93: Why disappeared?

Reply: It is possible due to the sulfonated step promote carbonization.[1]

[1] Mateo, W.; Lei, H.; Villota, E.; Qian, M.; Zhao, Y.; Huo, E.; Zhang, Q;. Lin, X.; Wang, C. One-step synthesis of biomass-based sulfonated carbon catalyst by direct carbonization-sulfonation for organosolv delignification, Bioresour Technol. 2021, 319, 124194. https://doi.org/10.1016/j.biortech.2020.124194

10) Lines 98-100: Rephrase.

Reply: The manuscript was revised as follow: The sulfonated groups in the bMC(600)-SO₃H was introduced by mix ball-milling and pyrolysis carbonization.

11) Line 107: The authors should introduce the sample bMC(400)-SO3H. Why authors have characterized this sample only through XPS technique?

Reply: The as-prepared carbon catalyst was characterized by XPS, FT-IR, XRD, SEM, BET and Raman technique. Section 2.1 was revised to understand easy.

12) The authors should perform the elemental analysis of all the reported samples in order to calculate the H/C and O/C ratios and show them on a van Kravelen diagram, thus underlining the differences between the samples.

Reply: The H/C and O/C ratios were performed by the Elemental Analyzer results. And the section 2.1 was revised as follow:

Moreover, the elemental constants were performed by Elemental Analyzer. The atomic O/C ratio of bMC(400)-SO₃H and bMC(600)-SO₃H were 0.03 and 0.19, respectively. The H/C atomic ratios of bMC(600)-SO₃H (H/C=0.04) were lowered than that of bMC(400)-SO₃H (H/C=0.17). It is suggested that the disposed masks were difficult to form carbon at 400 °C.

13) The authors should add the porosity and surface area analysis of the prepared samples because also these properties influence the catalytic performances.

Reply: The porosity and surface area analysis were added (Fig.1f). And the section 2.1 was revised as follow:

To further determine the porous structure and surface area of sulfonated mask-based functional carbon at 600 °C, nitrogen adsorption-desorption isotherm measurements were performed and the results are shown in Fig. 1f. The adsorption-desorption iso-therm can be classified to type IV isotherm with a H3 hysteresis loop (Fig. 1f). This result reveals that the bMC(600)-SO₃H shows a typical mesoporous structure. The Brunauer-Emmett-Teller (BET) surface area of bMC(600)-SO₃H is calculated to be 192.6 m²/g. The Barrett-Joyner-Halenda (BJH) pore size distributions calculated from the desorption data reveal that the average pore diameters is 9.6241 nm.

14) Line 126: In the absence of the catalyst?

Reply: Yes, it is means without solid carbon catalyst. The sentence was revised to understand easy (page 6, lines 151-153).

A low HMF yield of 6.2% with 20.9% fructose conversion was obtained at 120 °C after 1 h without added solid carbon catalyst (Entry 1, Table 1).

15) Line 128: I think that is bMC(600)

Reply: Yes, that is bMC(600). The manuscript was revised as follow: When bMC(600) was used…….

16) Line 130: the conversion and yield are reversed.

Reply: The manuscript was revised as follow:

The sulfonated carbon bMC(400)-SO3H improved the fructose conversion and HMF yield to 40.5% and 25.3%, respectively.

17) Table 1: Add the reference with Amberlytst-70 of Antonetti et al. (2017) Catal. Commun.

Reply: The suggested ref was cited in the table 1.

18) Table 1: I think that these are not the right references. Please check.

Reply: The cite reference in Table 1 was revised.

19) In order to verify if other catalyst properties, such as the surface area and porosity, influence the performances, the authors should carry out the runs with bMC(600)-SO₃H and bMC(400)-SO₃H with the same introduced acid sites and not with the same grams of catalyst.

Reply: Same acid site amount of bMC(600)-SO₃H and bMC(400)-SO₃H catalyst was results in similar catalytic activity in fructose dehydration into HMF (Entries 3 and 5, Table 1).

20) Table 1 note b: How much starting fructose has been reported in ref 2?

Reply: The ref2 reported that 44 wt% fructose and 16 wt% solid acid catalyst in water (0.5 mL) heated to 160 °C for 70 min. This information has been given in Table 1.

21) Table 1 note c: Which are the reaction conditions of ref 3?

Reply: The reaction conditions of ref3: 500 mg of fructose and 100 mg of cellulose-derived hydrothermal carbon in 10 ml dimethyl sulfoxide (DMSO) as solvent heated to 140 °C for 30 min. This information has been given in the table 1.

22) Line 154: I think that the temperature is 75 °C.

Reply: Yes, the temperature is 75 °C. The sentence was revised.

23) Line 154: Figure 4a?

Reply: Yes, the Fig. 4b was revised as Fig. 4a.

24) Lines 157-158: Please report the reaction time.

Reply: The reaction time was given and the sentence was revised as follow:

When the reaction temperature was 115 °C, the fructose conversion quickly reached 95% with the 78.4% HMF yield within 30 min.

25) Lines 162-164: The authors should report for each performed run also the yields of formic and levulinic acid and the carbon closure, in order to evaluate the humins formation.

Reply: In the case of the base catalyst of fructose at 95 °C or 115 °C, a range of 89-95% yields of liquid products (HMF, levulinic acid, formic acid) was obtained after 30 min reaction time, with an 5-11% yields of solid products (humins).

26) Figure 4: I think that Figure 4a reports the fructose conversion and Figure 4b reports the HMF yield.

Reply: Yes, the title of Figure 4 was revised.

27) Lines 172-174: This is true if fructose is only converted to HMF but it is well-known that it can be involved also in other reactions leading mainly to humins. The authors should make this assumption explicit.

Reply: Section 2.4 was revised as follow:

The following assumptions were determined: were determined:

(1) All reactions are irreversible.

(2) The main reaction is fructose to 5-HMF and ignores other possible reactions.

(3) All unidentified products are considered to be degradation products (humin).

(4) All other intermediates had negligible concentrations.

28) Figure 5: Please add the points at 90 and 120 min already reported in Figure 4.

Reply: According to your comment, the points at 90 and 120 min were given in Figure 5. Section 2.4 was also revised.

29) Line 182: Why? These are the best reaction conditions?

Reply: Yes, these are the best reaction conditions. A maximum HMF yield of 82.1% was obtained from fructose with bMC(600)-SO₃H as catalyst after 120 min at 95 °C.

30) The elemental analysis should be performed also on the recycled catalysts in order to verify the leaching phenomenon.

Reply: The XPS spectra of the recycled catalysts of bMC(600)-SO₃H were given in Fig. 7. As shown in the Fig. 7, the XPS spectra of the recycled catalysts of bMC(600)-SO3H re-vealed composition of C 1s (Fig. 7a) and S 2p (Fig. 7b) peaks. The binding energy of sulfate shifted to low energy, it is possible due to the interaction between the sulfate groups of catalyst and humins.

31) The authors should add the procedure adopted for the determination of the acidity of the catalysts in Paragraph 3.2.

Reply: The procedure adopted for the determination of the acidity of the catalysts was given in Section 3.2 (highlight in red).

32) Please report all the equations involved in the kinetic study.

Reply: The equations involved in the kinetic study was added in the manuscript (Section 3.4).

Reviewer 3

Manuscript presented by Hengyu Hao et al. shows a study about systems for the synthesis of sulfonated carbon materials to 5-hydroxymethylfurfural production. The topic is very important due to industry as well as environmental aspects.

An already well written and prepared manuscript. Easy to read and follow. Some aspects should be improved. I recommend the article to publish but first the paper should be corrected. My decision – reconsider after minor revision. Comments to be considered, in order to further improve the manuscript quality:

(1) Only major comment is that in the introduction part and in the conclusion part Authors need to elaborate in more detail the novelty of presented work and its suitability. The introduction and conclusion are too short and general. Please correct and improve sections.

Reply: According to your comment, some related refs were studied and cited. Except of introduction and conclusion, the abstract also was revised.

(2) The style of manuscript should be improving (see Catalysts template), eg, citation style in text (Abbreviated Journal Name)

Reply: The style of manuscript was revised follow the “Catalysts template” style. The citation style was revised.

(3) Superscripts and subscripts as well as commas and periods should be change and correct. Avoid extra spaces and enters. Correct in whole manuscript.

Reply: Superscripts, subscripts, commas, periods, spaces and enters was checked and revised.

(4) The English correction is necessary.

Reply: The manuscript has been carefully edited and re-checked.

Reviewer 4

The disposal of waste mask is an interesting idea under covid-19 situation however, author need to declare the significant study compare with the other works related with carbon catalyst.

Reply: The introduction was revised as follow and highlighted in red.

Because the COVID-19 epidemic has spread all over the word in the last three years, the production and usage of face masks have increased exponentially [27, 28]. Last year at least 1.07 billion masks were discarded, worth USA$ 10.76 billion [29]. The major ingredient of disposed masks is polypropylene, which is difficult to depolymer-ize and pose persistent hazard to environment [27]. Until now, most of the disposa-ble masks have not been recycled [30] and it needs more than 400 years for the poly-propylene decomposition, which results to environmental pollution [31, 32]. However, compare with the other works related with carbon catalyst from biomass, the direct synthesis of carbon materials from polypropylene based materials are still challenging and their application are rarely reported [34], because the poly-propylene-based materials are difficult to form carbon [35].

The surface functional groups especially acid site is the key factor of catalytic performance. Author should more discuss. The acid strength, acidity and acid type should be discussed.

Reply: According to your comment, the surface functional group, acidity and acid type related to the fructose conversion was discussed. For example in section 2.2, compared with many kinds of sulfonated solid catalysts in the previous reports was given.

The catalytic performance should present in the rate of production including TOF.

Reply: The catalytic performance in the rate was given in Table 1.

The bMC(600)-SO₃H sample exhibit the highest TOF value, whereas functional carbon bMC(600) and bMC(600)-SO₃H given higher TOF values than that with bMC(400)-SO₃H sample (Table 1).

Characterization of used catalyst should be added.

Reply: The XPS spectra of the recycled catalysts of bMC(600)-SO₃H were given in Fig. 7. The XPS spectra of the recycled catalysts of bMC(600)-SO3H re-vealed composition of C 1s (Fig. 7a) and S 2p (Fig. 7b) peaks. The binding energy of sulfate shifted to low energy, it is possible due to the interaction between the sulfate groups of catalyst and humins.

Author Response File: Author Response.pdf

Reviewer 2 Report

The work presented by Hao et al. faces two themes of great actuality, such as the synthesis of HMF, one of the most important platform chemicals, and the valorization of discarded masks, an abundant waste after the COVID-19 epidemic. However, the work presents several editing mistaks and in my opinion some deeper analysis and comments lack, thus the authors should strongly improve the manuscript according to the following points:

1) Check the English by a native speaker.

2) The authors should evidence in the abstract also the importance of HMF.

5) Line 34: The authors have confused homogeneous and heterogeneous catalysts. In addition, not only mineral acids (H2SO4 and HCl) have been adopted as homogeneous catalysts for the synthesis of HMF, but also inorganic salts, thus the authors should add some references regarding these catalysts. Among the heterogeneous catalysts, also Amberlyst-70 has been adopted by the research group of Raspolli, please add the reference: C. Antonetti et al. “Amberlyst A-70: a surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water”, Catal. Commun. 2017, 97, 146-150.

7) Line 77: "Temperature above 430 °C". Is it due to the decomposition of polypropylene?

8) The authors should add also the TGA analysis of the fresh mask, bMC, and bMC-SO3H in order to evidence how the thermal treatment influences the thermal properties and somehow determine if there is some polypropylene not converted in the samples.

9) Lines 91-93: Why disappeared?

10) Lines 98-100: Rephrase.

11) Line 107: The authors should introduce the sample bMC(400)-SO3H. Why authors have characterized this sample only through XPS technique?

13) The authors should add the porosity and surface area analysis of the prepared samples because also these properties influence the catalytic performances.

14) Line 126: In the absence of the catalyst?

15) Line 128: I think that is bMC(600)

16) Line 130: the conversion and yield are reversed.

17) Table 1: Add the reference with Amberlytst-70 of Antonetti et al. (2017) Catal. Commun.

18) Table 1: I think that these are not the right references. Please check.

19) In order to verify if other catalyst properties, such as the surface area and porosity, influence the performances, the authors should carry out the runs with bMC(600)-SO3H and bMC(400)-SO3H with the same introduced acid sites and not with the same grams of catalyst.

20) Table 1 note b: How much starting fructose has been reported in ref 2?

21) Table 1 note c: Which are the reaction conditions of ref 3?

22) Line 154: I think that the temperature is 75 °C.

23) Line 154: Figure 4a?

24) Lines 157-158: Please report the reaction time.

25) Lines 162-164: The authors should report for each performed run also the yields of formic and levulinic acid and the carbon closure, in order to evaluate the humins formation.

26) Figure 4: I think that Figure 4a reports the fructose conversion and Figure 4b reports the HMF yield.

28) Figure 5: Please add the points at 90 and 120 min already reported in Figure 4.

29) Line 182: Why? These are the best reaction conditions?

30) The elemental analysis should be performed also on the recycled catalysts in order to verify the leaching phenomenon.

31) The authors should add the procedure adopted for the determination of the acidity of the catalysts in Paragraph 3.2.

32) Please report all the equations involved in the kinetic study.

Author Response

Reviewer 1

The paper of Xinhua Qi et al. deals with Hydroxymethylfurfural production from fructose using a sulfonated porous carbon as a solid acid catalyst, which was prepared from discarded masks.

In the lines 44-46, authors mention about the usage of face masks which has increased exponentially. A suggestion is to include an explanation about the dimension of the number of masks disposable and scale with respect to collection and transport costs, since this material, despite being disposable, also has a cost. For example, in the reference 26, which is mentioned in this work, the authors discuss about the dimension of the number of masks disposed, which even show the importance of using this discarded material

Reply: Information was given in introduction (Lines 52-53).

In the line 132, authors mention the acidity amount of the functional carbon, but this method in not described in the “Material and Experimental” section. So, this method must be included.

Reply: In this work, Boehm method was used to determine the acidity amount of the functional groups on masks-derived sulfonated carbon surface (highlighted in red, Section given in 3.2).

In the line 255 in mentioned the "low-cost" of the material, but it should be suppressed, since economic analysis was not evaluated in this work. Overall, the paper communicates well the objectives, course and conclusion of the study. So, I recommend accepting this paper for publication after a minor review.

Reply: According to your comment, introduction and conclusion were revised to illustrate the point.

Functional carbon materials as a promising low-cost heterogeneous acid catalyst….. (Introduction, line 44)

Reviewer 2:

Check the English by a native speaker.

Reply: The manuscript has been carefully edited and checked for spelling, grammar and word.

The authors should evidence in the abstract also the importance of HMF.

Reply: According to your comment, the abstract was revised as follow:

Line 27: The authors cannot fail to mention also levulinic acid, which is, analogously to HMF, one of the most important platform-chemical. The authors can cite the following reference: D. Licursi et al., “Smart valorization of waste biomass: exhausted lemon peels, coffee silverskins and paper wastes for the production of levulinic acid”, Chem. Eng. Trans. 2018, 65, 637-642.

Reply: The suggested refs were carefully studied, and cited as ref [4]. The levulinic acid were mention in the file (line 32).

Lines 28-30: The authors maybe mean that the compounds derived from HMF (and not HMF) can be used as solvents, and clean fuels, but also as monomers. The authors should update the literature, reference 2, 6, and 7 are too old.

Reply: The sentence was revised as follow:

HMF is a typical acid catalytic dehydration product of hexose, which can be used as a key platform compound and compounds derived from HMF can be used as solvent and clean fuels.

The literature has been updated as:

Line 34: The authors have confused homogeneous and heterogeneous catalysts. In addition, not only mineral acids (H₂SO₄ and HCl) have been adopted as homogeneous catalysts for the synthesis of HMF, but also inorganic salts, thus the authors should add some references regarding these catalysts. Among the heterogeneous catalysts, also Amberlyst-70 has been adopted by the research group of Raspolli, please add the reference: C. Antonetti et al. “Amberlyst A-70: a surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water”, Catal. Commun. 2017, 97, 146-150.

Reply: The sentence was revised as including homogenous acid catalyst (e.g. HCl, H₂SO₄, NH₄Cl) and heterogenous acid catalysts (e.g. S-doped porous carbon, Amberlyst-15, Amberlyst-70).

The suggested refs were carefully and cited as ref [15]. Other homogenous acid catalyst (NH₄Cl) were also cited as ref [11].

Lines 69-71: Maybe the authors should introduce how the different samples have been prepared in order to better understand the differences. Moreover, the SEM analysis of the fresh sample (the mask) should be added with the aim of evidencing the morphological differences.

Reply: Section 3.2 Catalyst Preparation was revised, to make understand easy the different between each sample.

According to your comment, the SEM image of fresh sample was added (Fig.1a). And the section 2.1 was revised as follow:

Compared to fresh mask (Fig. 1a), the ball-milling treatment of mask leaded fibrous structure slightly breaks to produce debris (Fig. 1b).

Line 77: "Temperature above 430 °C". Is it due to the decomposition of polypropylene?

Reply: Yes, the polymer chain of polypropylene is gradually broken about 430 °C. [1]

The authors should add also the TGA analysis of the fresh mask, bMC, and bMC-SO₃H in order to evidence how the thermal treatment influences the thermal properties and somehow determine if there is some polypropylene not converted in the samples.

Lines 91-93: Why disappeared?

Reply: It is possible due to the sulfonated step promote carbonization.[1]

Lines 98-100: Rephrase.

Reply: The manuscript was revised as follow: The sulfonated groups in the bMC(600)-SO₃H was introduced by mix ball-milling and pyrolysis carbonization.

Line 107: The authors should introduce the sample bMC(400)-SO3H. Why authors have characterized this sample only through XPS technique?

Reply: The as-prepared carbon catalyst was characterized by XPS, FT-IR, XRD, SEM, BET and Raman technique. Section 2.1 was revised to understand easy.

The authors should perform the elemental analysis of all the reported samples in order to calculate the H/C and O/C ratios and show them on a van Kravelen diagram, thus underlining the differences between the samples.

Reply: The H/C and O/C ratios were performed by the Elemental Analyzer results. And the section 2.1 was revised as follow:

The authors should add the porosity and surface area analysis of the prepared samples because also these properties influence the catalytic performances.

Reply: The porosity and surface area analysis were added (Fig.1f). And the section 2.1 was revised as follow:

Line 126: In the absence of the catalyst?

Reply: Yes, it is means without solid carbon catalyst. The sentence was revised to understand easy (page 6, lines 151-153).

A low HMF yield of 6.2% with 20.9% fructose conversion was obtained at 120 °C after 1 h without added solid carbon catalyst (Entry 1, Table 1).

Line 128: I think that is bMC(600)

Reply: Yes, that is bMC(600). The manuscript was revised as follow: When bMC(600) was used…….

Line 130: the conversion and yield are reversed.

Reply: The manuscript was revised as follow:

The sulfonated carbon bMC(400)-SO3H improved the fructose conversion and HMF yield to 40.5% and 25.3%, respectively.

Table 1: Add the reference with Amberlytst-70 of Antonetti et al. (2017) Catal. Commun.

Reply: The suggested ref was cited in the table 1.

Table 1: I think that these are not the right references. Please check.

Reply: The cite reference in Table 1 was revised.

In order to verify if other catalyst properties, such as the surface area and porosity, influence the performances, the authors should carry out the runs with bMC(600)-SO₃H and bMC(400)-SO₃H with the same introduced acid sites and not with the same grams of catalyst.

Reply: Same acid site amount of bMC(600)-SO₃H and bMC(400)-SO₃H catalyst was results in similar catalytic activity in fructose dehydration into HMF (Entries 3 and 5, Table 1).

Table 1 note b: How much starting fructose has been reported in ref 2?

Reply: The ref2 reported that 44 wt% fructose and 16 wt% solid acid catalyst in water (0.5 mL) heated to 160 °C for 70 min. This information has been given in Table 1.

Table 1 note c: Which are the reaction conditions of ref 3?

Line 154: I think that the temperature is 75 °C.

Reply: Yes, the temperature is 75 °C. The sentence was revised.

Line 154: Figure 4a?

Reply: Yes, the Fig. 4b was revised as Fig. 4a.

Lines 157-158: Please report the reaction time.

Reply: The reaction time was given and the sentence was revised as follow:

When the reaction temperature was 115 °C, the fructose conversion quickly reached 95% with the 78.4% HMF yield within 30 min.

Lines 162-164: The authors should report for each performed run also the yields of formic and levulinic acid and the carbon closure, in order to evaluate the humins formation.

Figure 4: I think that Figure 4a reports the fructose conversion and Figure 4b reports the HMF yield.

Reply: Yes, the title of Figure 4 was revised.

Lines 172-174: This is true if fructose is only converted to HMF but it is well-known that it can be involved also in other reactions leading mainly to humins. The authors should make this assumption explicit.

Reply: Section 2.4 was revised as follow:

The following assumptions were determined: were determined:

(1) All reactions are irreversible.

(2) The main reaction is fructose to 5-HMF and ignores other possible reactions.

(3) All unidentified products are considered to be degradation products (humin).

(4) All other intermediates had negligible concentrations.

Figure 5: Please add the points at 90 and 120 min already reported in Figure 4.

Reply: According to your comment, the points at 90 and 120 min were given in Figure 5. Section 2.4 was also revised.

Line 182: Why? These are the best reaction conditions?

Reply: Yes, these are the best reaction conditions. A maximum HMF yield of 82.1% was obtained from fructose with bMC(600)-SO₃H as catalyst after 120 min at 95 °C.

The elemental analysis should be performed also on the recycled catalysts in order to verify the leaching phenomenon.

The authors should add the procedure adopted for the determination of the acidity of the catalysts in Paragraph 3.2.

Reply: The procedure adopted for the determination of the acidity of the catalysts was given in Section 3.2 (highlight in red).

Please report all the equations involved in the kinetic study.

Reply: The equations involved in the kinetic study was added in the manuscript (Section 3.4).

Reviewer 3

Only major comment is that in the introduction part and in the conclusion part Authors need to elaborate in more detail the novelty of presented work and its suitability. The introduction and conclusion are too short and general. Please correct and improve sections.

Reply: According to your comment, some related refs were studied and cited. Except of introduction and conclusion, the abstract also was revised.

The style of manuscript should be improving (see Catalysts template), eg, citation style in text (Abbreviated Journal Name)

Reply: The style of manuscript was revised follow the “Catalysts template” style. The citation style was revised.

Superscripts and subscripts as well as commas and periods should be change and correct. Avoid extra spaces and enters. Correct in whole manuscript.

Reply: Superscripts, subscripts, commas, periods, spaces and enters was checked and revised.

The English correction is necessary.

Reply: The manuscript has been carefully edited and re-checked.

Reviewer 4

The disposal of waste mask is an interesting idea under covid-19 situation however, author need to declare the significant study compare with the other works related with carbon catalyst.

Reply: The introduction was revised as follow and highlighted in red.

The surface functional groups especially acid site is the key factor of catalytic performance. Author should more discuss. The acid strength, acidity and acid type should be discussed.

The catalytic performance should present in the rate of production including TOF.

Reply: The catalytic performance in the rate was given in Table 1.

The bMC(600)-SO₃H sample exhibit the highest TOF value, whereas functional carbon bMC(600) and bMC(600)-SO₃H given higher TOF values than that with bMC(400)-SO₃H sample (Table 1).

Characterization of used catalyst should be added.

Author Response File: Author Response.pdf

Reviewer 3 Report

(1) Only major comment is that in the introduction part and in the conclusion part Authors need to elaborate in more detail the novelty of presented work and its suitability. The introduction and conclusion are too short and general. Please correct and improve sections.

(2) The style of manuscript should be improve (see Catalysts template), eg, citation style in text (Abbreviated Journal Name)

(3) Superscripts and subscripts as well as commas and periods should be change and correct. Avoid extra spaces and enters. Correct in whole manuscript.

(4) The English correction is necessary.

Author Response

Reviewer 1

The paper of Xinhua Qi et al. deals with Hydroxymethylfurfural production from fructose using a sulfonated porous carbon as a solid acid catalyst, which was prepared from discarded masks.

In the lines 44-46, authors mention about the usage of face masks which has increased exponentially. A suggestion is to include an explanation about the dimension of the number of masks disposable and scale with respect to collection and transport costs, since this material, despite being disposable, also has a cost. For example, in the reference 26, which is mentioned in this work, the authors discuss about the dimension of the number of masks disposed, which even show the importance of using this discarded material

Reply: Information was given in introduction (Lines 52-53).

In the line 132, authors mention the acidity amount of the functional carbon, but this method in not described in the “Material and Experimental” section. So, this method must be included.

Reply: In this work, Boehm method was used to determine the acidity amount of the functional groups on masks-derived sulfonated carbon surface (highlighted in red, Section given in 3.2).

In the line 255 in mentioned the "low-cost" of the material, but it should be suppressed, since economic analysis was not evaluated in this work. Overall, the paper communicates well the objectives, course and conclusion of the study. So, I recommend accepting this paper for publication after a minor review.

Reply: According to your comment, introduction and conclusion were revised to illustrate the point.

Functional carbon materials as a promising low-cost heterogeneous acid catalyst….. (Introduction, line 44)

Reviewer 2:

Check the English by a native speaker.

Reply: The manuscript has been carefully edited and checked for spelling, grammar and word.

The authors should evidence in the abstract also the importance of HMF.

Reply: According to your comment, the abstract was revised as follow:

Line 27: The authors cannot fail to mention also levulinic acid, which is, analogously to HMF, one of the most important platform-chemical. The authors can cite the following reference: D. Licursi et al., “Smart valorization of waste biomass: exhausted lemon peels, coffee silverskins and paper wastes for the production of levulinic acid”, Chem. Eng. Trans. 2018, 65, 637-642.

Reply: The suggested refs were carefully studied, and cited as ref [4]. The levulinic acid were mention in the file (line 32).

Lines 28-30: The authors maybe mean that the compounds derived from HMF (and not HMF) can be used as solvents, and clean fuels, but also as monomers. The authors should update the literature, reference 2, 6, and 7 are too old.

Reply: The sentence was revised as follow:

HMF is a typical acid catalytic dehydration product of hexose, which can be used as a key platform compound and compounds derived from HMF can be used as solvent and clean fuels.

The literature has been updated as:

Line 34: The authors have confused homogeneous and heterogeneous catalysts. In addition, not only mineral acids (H₂SO₄ and HCl) have been adopted as homogeneous catalysts for the synthesis of HMF, but also inorganic salts, thus the authors should add some references regarding these catalysts. Among the heterogeneous catalysts, also Amberlyst-70 has been adopted by the research group of Raspolli, please add the reference: C. Antonetti et al. “Amberlyst A-70: a surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water”, Catal. Commun. 2017, 97, 146-150.

Reply: The sentence was revised as including homogenous acid catalyst (e.g. HCl, H₂SO₄, NH₄Cl) and heterogenous acid catalysts (e.g. S-doped porous carbon, Amberlyst-15, Amberlyst-70).

The suggested refs were carefully and cited as ref [15]. Other homogenous acid catalyst (NH₄Cl) were also cited as ref [11].

Lines 69-71: Maybe the authors should introduce how the different samples have been prepared in order to better understand the differences. Moreover, the SEM analysis of the fresh sample (the mask) should be added with the aim of evidencing the morphological differences.

Reply: Section 3.2 Catalyst Preparation was revised, to make understand easy the different between each sample.

According to your comment, the SEM image of fresh sample was added (Fig.1a). And the section 2.1 was revised as follow:

Compared to fresh mask (Fig. 1a), the ball-milling treatment of mask leaded fibrous structure slightly breaks to produce debris (Fig. 1b).

Line 77: "Temperature above 430 °C". Is it due to the decomposition of polypropylene?

Reply: Yes, the polymer chain of polypropylene is gradually broken about 430 °C. [1]

The authors should add also the TGA analysis of the fresh mask, bMC, and bMC-SO₃H in order to evidence how the thermal treatment influences the thermal properties and somehow determine if there is some polypropylene not converted in the samples.

Lines 91-93: Why disappeared?

Reply: It is possible due to the sulfonated step promote carbonization.[1]

Lines 98-100: Rephrase.

Reply: The manuscript was revised as follow: The sulfonated groups in the bMC(600)-SO₃H was introduced by mix ball-milling and pyrolysis carbonization.

Line 107: The authors should introduce the sample bMC(400)-SO3H. Why authors have characterized this sample only through XPS technique?

Reply: The as-prepared carbon catalyst was characterized by XPS, FT-IR, XRD, SEM, BET and Raman technique. Section 2.1 was revised to understand easy.

The authors should perform the elemental analysis of all the reported samples in order to calculate the H/C and O/C ratios and show them on a van Kravelen diagram, thus underlining the differences between the samples.

Reply: The H/C and O/C ratios were performed by the Elemental Analyzer results. And the section 2.1 was revised as follow:

The authors should add the porosity and surface area analysis of the prepared samples because also these properties influence the catalytic performances.

Reply: The porosity and surface area analysis were added (Fig.1f). And the section 2.1 was revised as follow:

Line 126: In the absence of the catalyst?

Reply: Yes, it is means without solid carbon catalyst. The sentence was revised to understand easy (page 6, lines 151-153).

A low HMF yield of 6.2% with 20.9% fructose conversion was obtained at 120 °C after 1 h without added solid carbon catalyst (Entry 1, Table 1).

Line 128: I think that is bMC(600)

Reply: Yes, that is bMC(600). The manuscript was revised as follow: When bMC(600) was used…….

Line 130: the conversion and yield are reversed.

Reply: The manuscript was revised as follow:

The sulfonated carbon bMC(400)-SO3H improved the fructose conversion and HMF yield to 40.5% and 25.3%, respectively.

Table 1: Add the reference with Amberlytst-70 of Antonetti et al. (2017) Catal. Commun.

Reply: The suggested ref was cited in the table 1.

Table 1: I think that these are not the right references. Please check.

Reply: The cite reference in Table 1 was revised.

In order to verify if other catalyst properties, such as the surface area and porosity, influence the performances, the authors should carry out the runs with bMC(600)-SO₃H and bMC(400)-SO₃H with the same introduced acid sites and not with the same grams of catalyst.

Reply: Same acid site amount of bMC(600)-SO₃H and bMC(400)-SO₃H catalyst was results in similar catalytic activity in fructose dehydration into HMF (Entries 3 and 5, Table 1).

Table 1 note b: How much starting fructose has been reported in ref 2?

Reply: The ref2 reported that 44 wt% fructose and 16 wt% solid acid catalyst in water (0.5 mL) heated to 160 °C for 70 min. This information has been given in Table 1.

Table 1 note c: Which are the reaction conditions of ref 3?

Line 154: I think that the temperature is 75 °C.

Reply: Yes, the temperature is 75 °C. The sentence was revised.

Line 154: Figure 4a?

Reply: Yes, the Fig. 4b was revised as Fig. 4a.

Lines 157-158: Please report the reaction time.

Reply: The reaction time was given and the sentence was revised as follow:

When the reaction temperature was 115 °C, the fructose conversion quickly reached 95% with the 78.4% HMF yield within 30 min.

Lines 162-164: The authors should report for each performed run also the yields of formic and levulinic acid and the carbon closure, in order to evaluate the humins formation.

Figure 4: I think that Figure 4a reports the fructose conversion and Figure 4b reports the HMF yield.

Reply: Yes, the title of Figure 4 was revised.

Lines 172-174: This is true if fructose is only converted to HMF but it is well-known that it can be involved also in other reactions leading mainly to humins. The authors should make this assumption explicit.

Reply: Section 2.4 was revised as follow:

The following assumptions were determined: were determined:

(1) All reactions are irreversible.

(2) The main reaction is fructose to 5-HMF and ignores other possible reactions.

(3) All unidentified products are considered to be degradation products (humin).

(4) All other intermediates had negligible concentrations.

Figure 5: Please add the points at 90 and 120 min already reported in Figure 4.

Reply: According to your comment, the points at 90 and 120 min were given in Figure 5. Section 2.4 was also revised.

Line 182: Why? These are the best reaction conditions?

Reply: Yes, these are the best reaction conditions. A maximum HMF yield of 82.1% was obtained from fructose with bMC(600)-SO₃H as catalyst after 120 min at 95 °C.

The elemental analysis should be performed also on the recycled catalysts in order to verify the leaching phenomenon.

The authors should add the procedure adopted for the determination of the acidity of the catalysts in Paragraph 3.2.

Reply: The procedure adopted for the determination of the acidity of the catalysts was given in Section 3.2 (highlight in red).

Please report all the equations involved in the kinetic study.

Reply: The equations involved in the kinetic study was added in the manuscript (Section 3.4).

Reviewer 3

Only major comment is that in the introduction part and in the conclusion part Authors need to elaborate in more detail the novelty of presented work and its suitability. The introduction and conclusion are too short and general. Please correct and improve sections.

Reply: According to your comment, some related refs were studied and cited. Except of introduction and conclusion, the abstract also was revised.

The style of manuscript should be improving (see Catalysts template), eg, citation style in text (Abbreviated Journal Name)

Reply: The style of manuscript was revised follow the “Catalysts template” style. The citation style was revised.

Superscripts and subscripts as well as commas and periods should be change and correct. Avoid extra spaces and enters. Correct in whole manuscript.

Reply: Superscripts, subscripts, commas, periods, spaces and enters was checked and revised.

The English correction is necessary.

Reply: The manuscript has been carefully edited and re-checked.

Reviewer 4

The disposal of waste mask is an interesting idea under covid-19 situation however, author need to declare the significant study compare with the other works related with carbon catalyst.

Reply: The introduction was revised as follow and highlighted in red.

The surface functional groups especially acid site is the key factor of catalytic performance. Author should more discuss. The acid strength, acidity and acid type should be discussed.

The catalytic performance should present in the rate of production including TOF.

Reply: The catalytic performance in the rate was given in Table 1.

The bMC(600)-SO₃H sample exhibit the highest TOF value, whereas functional carbon bMC(600) and bMC(600)-SO₃H given higher TOF values than that with bMC(400)-SO₃H sample (Table 1).

Characterization of used catalyst should be added.

Author Response File: Author Response.pdf

Reviewer 4 Report

1. The disposal of waste mask is an interesting idea under covid-19 situation however, author need to declare the significant study compare with the other works related with carbon catalyst.

2. The surface functional groups especially acid site is the key factor of catalytic performance. Author should more discuss. The acid strength, acidity and acid type should be discussed.

3. The catalytic performance should present in the rate of production including TOF.

4. Characterization of used catalyst should be added.

Author Response

Reviewer 1

The paper of Xinhua Qi et al. deals with Hydroxymethylfurfural production from fructose using a sulfonated porous carbon as a solid acid catalyst, which was prepared from discarded masks.

In the lines 44-46, authors mention about the usage of face masks which has increased exponentially. A suggestion is to include an explanation about the dimension of the number of masks disposable and scale with respect to collection and transport costs, since this material, despite being disposable, also has a cost. For example, in the reference 26, which is mentioned in this work, the authors discuss about the dimension of the number of masks disposed, which even show the importance of using this discarded material

Reply: Information was given in introduction (Lines 52-53).

In the line 132, authors mention the acidity amount of the functional carbon, but this method in not described in the “Material and Experimental” section. So, this method must be included.

Reply: In this work, Boehm method was used to determine the acidity amount of the functional groups on masks-derived sulfonated carbon surface (highlighted in red, Section given in 3.2).

In the line 255 in mentioned the "low-cost" of the material, but it should be suppressed, since economic analysis was not evaluated in this work. Overall, the paper communicates well the objectives, course and conclusion of the study. So, I recommend accepting this paper for publication after a minor review.

Reply: According to your comment, introduction and conclusion were revised to illustrate the point.

Functional carbon materials as a promising low-cost heterogeneous acid catalyst….. (Introduction, line 44)

Reviewer 2:

Check the English by a native speaker.

Reply: The manuscript has been carefully edited and checked for spelling, grammar and word.

The authors should evidence in the abstract also the importance of HMF.

Reply: According to your comment, the abstract was revised as follow:

Line 27: The authors cannot fail to mention also levulinic acid, which is, analogously to HMF, one of the most important platform-chemical. The authors can cite the following reference: D. Licursi et al., “Smart valorization of waste biomass: exhausted lemon peels, coffee silverskins and paper wastes for the production of levulinic acid”, Chem. Eng. Trans. 2018, 65, 637-642.

Reply: The suggested refs were carefully studied, and cited as ref [4]. The levulinic acid were mention in the file (line 32).

Lines 28-30: The authors maybe mean that the compounds derived from HMF (and not HMF) can be used as solvents, and clean fuels, but also as monomers. The authors should update the literature, reference 2, 6, and 7 are too old.

Reply: The sentence was revised as follow:

HMF is a typical acid catalytic dehydration product of hexose, which can be used as a key platform compound and compounds derived from HMF can be used as solvent and clean fuels.

The literature has been updated as:

Line 34: The authors have confused homogeneous and heterogeneous catalysts. In addition, not only mineral acids (H₂SO₄ and HCl) have been adopted as homogeneous catalysts for the synthesis of HMF, but also inorganic salts, thus the authors should add some references regarding these catalysts. Among the heterogeneous catalysts, also Amberlyst-70 has been adopted by the research group of Raspolli, please add the reference: C. Antonetti et al. “Amberlyst A-70: a surprisingly active catalyst for the MW-assisted dehydration of fructose and inulin to HMF in water”, Catal. Commun. 2017, 97, 146-150.

Reply: The sentence was revised as including homogenous acid catalyst (e.g. HCl, H₂SO₄, NH₄Cl) and heterogenous acid catalysts (e.g. S-doped porous carbon, Amberlyst-15, Amberlyst-70).

The suggested refs were carefully and cited as ref [15]. Other homogenous acid catalyst (NH₄Cl) were also cited as ref [11].

Lines 69-71: Maybe the authors should introduce how the different samples have been prepared in order to better understand the differences. Moreover, the SEM analysis of the fresh sample (the mask) should be added with the aim of evidencing the morphological differences.

Reply: Section 3.2 Catalyst Preparation was revised, to make understand easy the different between each sample.

According to your comment, the SEM image of fresh sample was added (Fig.1a). And the section 2.1 was revised as follow:

Compared to fresh mask (Fig. 1a), the ball-milling treatment of mask leaded fibrous structure slightly breaks to produce debris (Fig. 1b).

Line 77: "Temperature above 430 °C". Is it due to the decomposition of polypropylene?

Reply: Yes, the polymer chain of polypropylene is gradually broken about 430 °C. [1]

The authors should add also the TGA analysis of the fresh mask, bMC, and bMC-SO₃H in order to evidence how the thermal treatment influences the thermal properties and somehow determine if there is some polypropylene not converted in the samples.

Lines 91-93: Why disappeared?

Reply: It is possible due to the sulfonated step promote carbonization.[1]

Lines 98-100: Rephrase.

Reply: The manuscript was revised as follow: The sulfonated groups in the bMC(600)-SO₃H was introduced by mix ball-milling and pyrolysis carbonization.

Line 107: The authors should introduce the sample bMC(400)-SO3H. Why authors have characterized this sample only through XPS technique?

Reply: The as-prepared carbon catalyst was characterized by XPS, FT-IR, XRD, SEM, BET and Raman technique. Section 2.1 was revised to understand easy.

The authors should perform the elemental analysis of all the reported samples in order to calculate the H/C and O/C ratios and show them on a van Kravelen diagram, thus underlining the differences between the samples.

Reply: The H/C and O/C ratios were performed by the Elemental Analyzer results. And the section 2.1 was revised as follow:

The authors should add the porosity and surface area analysis of the prepared samples because also these properties influence the catalytic performances.

Reply: The porosity and surface area analysis were added (Fig.1f). And the section 2.1 was revised as follow:

Line 126: In the absence of the catalyst?

Reply: Yes, it is means without solid carbon catalyst. The sentence was revised to understand easy (page 6, lines 151-153).

A low HMF yield of 6.2% with 20.9% fructose conversion was obtained at 120 °C after 1 h without added solid carbon catalyst (Entry 1, Table 1).

Line 128: I think that is bMC(600)

Reply: Yes, that is bMC(600). The manuscript was revised as follow: When bMC(600) was used…….

Line 130: the conversion and yield are reversed.

Reply: The manuscript was revised as follow:

The sulfonated carbon bMC(400)-SO3H improved the fructose conversion and HMF yield to 40.5% and 25.3%, respectively.

Table 1: Add the reference with Amberlytst-70 of Antonetti et al. (2017) Catal. Commun.

Reply: The suggested ref was cited in the table 1.

Table 1: I think that these are not the right references. Please check.

Reply: The cite reference in Table 1 was revised.

In order to verify if other catalyst properties, such as the surface area and porosity, influence the performances, the authors should carry out the runs with bMC(600)-SO₃H and bMC(400)-SO₃H with the same introduced acid sites and not with the same grams of catalyst.

Reply: Same acid site amount of bMC(600)-SO₃H and bMC(400)-SO₃H catalyst was results in similar catalytic activity in fructose dehydration into HMF (Entries 3 and 5, Table 1).

Table 1 note b: How much starting fructose has been reported in ref 2?

Reply: The ref2 reported that 44 wt% fructose and 16 wt% solid acid catalyst in water (0.5 mL) heated to 160 °C for 70 min. This information has been given in Table 1.

Table 1 note c: Which are the reaction conditions of ref 3?

Line 154: I think that the temperature is 75 °C.

Reply: Yes, the temperature is 75 °C. The sentence was revised.

Line 154: Figure 4a?

Reply: Yes, the Fig. 4b was revised as Fig. 4a.

Lines 157-158: Please report the reaction time.

Reply: The reaction time was given and the sentence was revised as follow:

When the reaction temperature was 115 °C, the fructose conversion quickly reached 95% with the 78.4% HMF yield within 30 min.

Lines 162-164: The authors should report for each performed run also the yields of formic and levulinic acid and the carbon closure, in order to evaluate the humins formation.

Figure 4: I think that Figure 4a reports the fructose conversion and Figure 4b reports the HMF yield.

Reply: Yes, the title of Figure 4 was revised.

Lines 172-174: This is true if fructose is only converted to HMF but it is well-known that it can be involved also in other reactions leading mainly to humins. The authors should make this assumption explicit.

Reply: Section 2.4 was revised as follow:

The following assumptions were determined: were determined:

(1) All reactions are irreversible.

(2) The main reaction is fructose to 5-HMF and ignores other possible reactions.

(3) All unidentified products are considered to be degradation products (humin).

(4) All other intermediates had negligible concentrations.

Figure 5: Please add the points at 90 and 120 min already reported in Figure 4.

Reply: According to your comment, the points at 90 and 120 min were given in Figure 5. Section 2.4 was also revised.

Line 182: Why? These are the best reaction conditions?

Reply: Yes, these are the best reaction conditions. A maximum HMF yield of 82.1% was obtained from fructose with bMC(600)-SO₃H as catalyst after 120 min at 95 °C.

The elemental analysis should be performed also on the recycled catalysts in order to verify the leaching phenomenon.

The authors should add the procedure adopted for the determination of the acidity of the catalysts in Paragraph 3.2.

Reply: The procedure adopted for the determination of the acidity of the catalysts was given in Section 3.2 (highlight in red).

Please report all the equations involved in the kinetic study.

Reply: The equations involved in the kinetic study was added in the manuscript (Section 3.4).

Reviewer 3

Only major comment is that in the introduction part and in the conclusion part Authors need to elaborate in more detail the novelty of presented work and its suitability. The introduction and conclusion are too short and general. Please correct and improve sections.

Reply: According to your comment, some related refs were studied and cited. Except of introduction and conclusion, the abstract also was revised.

The style of manuscript should be improving (see Catalysts template), eg, citation style in text (Abbreviated Journal Name)

Reply: The style of manuscript was revised follow the “Catalysts template” style. The citation style was revised.

Superscripts and subscripts as well as commas and periods should be change and correct. Avoid extra spaces and enters. Correct in whole manuscript.

Reply: Superscripts, subscripts, commas, periods, spaces and enters was checked and revised.

The English correction is necessary.

Reply: The manuscript has been carefully edited and re-checked.

Reviewer 4

The disposal of waste mask is an interesting idea under covid-19 situation however, author need to declare the significant study compare with the other works related with carbon catalyst.

Reply: The introduction was revised as follow and highlighted in red.

The surface functional groups especially acid site is the key factor of catalytic performance. Author should more discuss. The acid strength, acidity and acid type should be discussed.

The catalytic performance should present in the rate of production including TOF.

Reply: The catalytic performance in the rate was given in Table 1.

The bMC(600)-SO₃H sample exhibit the highest TOF value, whereas functional carbon bMC(600) and bMC(600)-SO₃H given higher TOF values than that with bMC(400)-SO₃H sample (Table 1).

Characterization of used catalyst should be added.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have improved the manuscript according to the referee's comments thus it is ready for publication.

Reviewer 4 Report

The revised manuscript can be accepted.