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Open AccessArticle
Peer-Review Record

Development of TiO2-Carbon Composite Acid Catalyst for Dehydration of Fructose to 5-Hydroxymethylfurfural

Catalysts 2019, 9(2), 126; https://doi.org/10.3390/catal9020126
Reviewer 1: Anonymous
Reviewer 2: Anonymous
Catalysts 2019, 9(2), 126; https://doi.org/10.3390/catal9020126
Received: 28 December 2018 / Revised: 22 January 2019 / Accepted: 29 January 2019 / Published: 31 January 2019
(This article belongs to the Special Issue Emerging Trends in TiO2 Photocatalysis and Applications)

Round 1

Reviewer 1 Report

Songo et al. have described the utilization of TiO2C as a catalyst for conversion of fructose to 5-HMF. The material is well characterized and performance is well described and comparable to similar materials. Some of the aspects that would need more explanation are given below-

- In the abstract, yield is mentioned. It is recommended that if possible clarify this in abstract itself (like in few words what is meant by yield). This is a suggestion since many readers would like to get a general idea of work from abstract.

-"were found to be promising heterogeneous catalysts OR the basis" - ON the basis?

- In the introduction the term yield is used to describe related work (by Wang & Zhao et al. etc). The authors do mention this term later, but it is recommended to explain it in the first instance it is used, since yield by itself is not a generic term and could be interpreted differently.

- "To the best of our knowledge, there have been no reports on the use of TiO2C as a catalyst for the dehydration of fructose into 5-HMF." - Why should the introduction only talk about TiO2C for fructose into 5-HMF dehydration? If there are similar class of materials with comparable or higher yield it should be mentioned, unless TiO2C has unique advantages vs. these materials. For instance, ref to https://doi.org/10.1016/j.compositesb.2018.08.044 or  RSC Adv., 2013,3, 2595-2603 as an example. It is suggested that related work is referenced. It might be relevant to mention these in table 3.

- The method describes low conversion in other solvents (other than DMSO). Is there a reason for the same and could it potentially limit it's use?


Author Response

Songo et al. have described the utilization of TiO2C as a catalyst for conversion of fructose to 5-HMF. The material is well characterized and performance is well described and comparable to similar materials. Some of the aspects that would need more explanation are given below-

Authors’ reply:  Firstly, accept our appreciation for the meticulous assessment of the manuscript. We have carefully considered the recommendations and the following changes/additions have been made.

- In the abstract, yield is mentioned. It is recommended that if possible clarify this in abstract itself (like in few words what is meant by yield). This is a suggestion since many readers would like to get a general idea of work from abstract.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

-"were found to be promising heterogeneous catalysts OR the basis" - ON the basis?

Authors’ reply:  Biomass-derived carbonaceous materials were found to be promising candidates for catalytic support application because of their low cost of production and thermal stability. Thank you for this suggestion. It has been taken into account in the revised version.

- In the introduction the term yield is used to describe related work (by Wang & Zhao et al. etc). The authors do mention this term later, but it is recommended to explain it in the first instance it is used, since yield by itself is not a generic term and could be interpreted differently.

Authors’ reply:  Thank you for this suggestion. It is actually dehydration and it has been taken into account in the revised version.

 

- "To the best of our knowledge, there have been no reports on the use of TiO2C as a catalyst for the dehydration of fructose into 5-HMF." - Why should the introduction only talk about TiO2C for fructose into 5-HMF dehydration? If there are similar class of materials with comparable or higher yield it should be mentioned, unless TiO2C has unique advantages vs. these materials. For instance, ref to https://doi.org/10.1016/j.compositesb.2018.08.044 or  RSC Adv., 2013,3, 2595-2603 as an example. It is suggested that related work is referenced. It might be relevant to mention these in table 3.

Authors’ reply:  Thank you for this suggestion. However, the work reported in reference https://doi.org/10.1016/j.compositesb.2018.08.044 focused on dehydration of fructose to 5HMF using sulfonic acid functionalized mesoporous silica, whereas the work in reference RSC Adv., 2013,3, 2595-2603) focused on the dehydration of D-xylose into furfural. Our work was based on dehydration of fructose into 5-HMF. Therefore, we did not consider them to be included in table 3.

- The method describes low conversion in other solvents (other than DMSO). Is there a reason for the same and could it potentially limit it's use?

Authors’ reply:  Thank you for this query. DMSO acted both as a solvent and a reaction medium; hence a high HMF yield was obtained when compared to other solvents

We hope you will be satisfied with our revision and that the manuscript will now be acceptable for publication in Catalysts as a regular article.


Reviewer 2 Report

Review of “Development of TiO2-Carbon Composite Acid Catalyst for Dehydration of Fructose to 5-Hydroxymethylfurfural” by Morongwa Martha Songo, Richard Moutloali , Suprakas Sinha Ray submitted to “Catalysts” journal

 

It is an interesting article; however it needs a major revision with some additional tests/experiments to clarify the presented results before it can be published. First of all, the presented FTIR and EDS results do not support that statement that the C-TiO2 materials is actually a composite and it seems to me that the materials is actually a mixture of two phases. The interpretation of the characterisation of the material need revision, as sometimes the conclusion are taken in the text which are not supported by any scientific evidence presented. The heterogeneous aspect of the catalytic  process is not clear, because the authors did not present the results of the amount of sulfonic groups on the surface of the catalysts before and after the reaction. The “hot filtration test” is not clearly described in the experimental part so it is not clear whether a new substrate was added after the removal of the catalyst or not. The yield is based on the UV measurements of HMF absorbance in the region where levulinic acid (one of the main side products of fructose dehydration, also shows absorbance. Thus, it is not clear which of the products the authors are really measuring. The blank catalytic test is missing for the best reaction conditions so it is not clear how much better the catalysts really are as compared to the activity of the solvent itself. It is known that DMSO used by the authors is active in this reaction. How is the catalytic activity and selectivity affected by the acidity of the catalysts is also not clear from the presented results, however the authors conclude that indeed the catalytic performance was affected by the catalyst acidity. Some corrections of the formatting is also needed. More detail list of corrections as follows:

 

1)      Increase in the use

2)      Repetition of increase in the first sentence

3)      Contributing to the growing number of serious health issues

4)      Using homogenous organic acids or heterogeneous solid acids

5)      What is the basis of catalytic application? Supports?

6)      Carbons are rather hydrophobic materials that is why they do not disperse in water. Only oxidised carbon materials are hydrophilic!

7)      In the mixtures of DMSO and ionic liquid

8)      Concentrated sulphuric acid or fuming surfuric acid was used for functionalization?

9)      The hot filtration test, it is not clear in the experimental section is after the filtration of the catalyst, the reaction was carried out with addition of the substrate or without ? How did the authors determine that the reaction was not catalysed by the sulphuric anions present in the solution after the catalyst was removed? Were the trials made with the substrate on its own to determine the extent of the reaction?

10)   How do the authors know that the SO3H groups are attached to the carbon not to the TiO2 in the composites?

11)   The drop of surface area with the addition of TiO2 to C can point at TiO2 blocking some micropores of the carbon.

12)   What was the type of bonding between C and TiO2? The authors say that it was a composite, however the referee thinks it could have been only a physical mixture.

13)   How did the authors measure the amount of TiO2 added to the spheres in the composites. The authors should be aware that the theoretical amount of TiO2 added does not result in the equal amount present in the final material.

14)   Did the authors try to first oxidise the carbon and later add TiO2? This way TiO2 could attached itself to the carbon surface via e.g. hydrogen bonds???

15)   It is well known that the sulfonation of carbons especially under harsh conditions results in the drop of surface area of carbons which is due to the broadening of micropores and blocking some micropores. There is no need to discuss this in such a detail in the manuscript. It is enough to add some references.

16)   Table 1, I assume that the C is after sulfonation ??? It should be explained more clearly, otherwise it can suggest that the as-made material contained sulphur.

17)   Why the composite with 5 % TiO2 has higher amount of sulphuric groups?

18)   Page 4 SO3-

19)   In Figure 1, it looks to the referee that the material is a physical mixture of carbon and TiO2. Which particles do the authors refer as “ the composite particles”??? The difference in the SEM images is that the more TiO2 was added, the less spheres of carbon are visible…The irregular particles belong clearly to TiO2… In order to support the statement “Moreover, a high amount (5 and 10%) of TiO2 nanoparticles loading promoted the formation of irregular particles in the composites”, SEM image of neat TiO2 should be added and compared with the composites.

20)   After the sulfonation, the particles become smaller, so there is an impact on the structure of the material.

21)   Are shown in Figure 2 e and f, what do the authors mean by “parts”??

22)   Was the EDS taken from various areas of the samples or only from one area of each of the samples? The authors put the composition of the materials based on the EDS spectra…was the calibration made befohand?

23)   What do the authors mean by “TiO2 coated the samples”?? Base on the EDS results it can be only confirmed that the TiO2 was present in the “ composite” samples.

24)   How did the authors confirmed that S was present in the samples in the form of SO3H? did the carry out the XPS analysis of sulphur or is it only based on the literature? If so, than it should be written that based on the literature findings (ref), sulphur is most likely present in the form of…It is impossible to confirm the type of S present based on the EDS results…

25)   The samples contained sulfonic acid or sulphur???

26)   The EDs detectors are normally not able to detect any elements with atomic number less than 5, so it is obvious that hydrogen could not be detected.

27)   Page 6, the caption should be below the figure on the same page

28)   Oxygen groups on the surface of carbon could also catalyse the reaction. Some comments should be added in the manuscript about this.

29)   Some oxygen groups should be removed during calcination. Any ideas why this did not happen?  For example carboxylic groups normally leave at the temperatures lower than 400 °C. What is the difference in the activity between the calcined C spheres and as made ones? The authors say that high temperature of calcination “destroyed the structure” of the material, but a few lines above this, the authors claim that no changes were observed in FTIR between the neat and calcined C spheres.  Please review the FTIR analysis.

30)   Please reformat the captions in the Figure 3

31)   The authors mentioned the band at 630 cm-1 which is supposed to confirm that TiO2 was chemically bonded to C, however the referee struggled to see such a band in the Figure 3 . In fact, the spectra of carbon and the composites are literately very similar…The spectra of neat carbon and carbon-TiO2 “composites” should be overlapped to clearly show the existence of the chemical bonding. Otherwise, the material should be referred to as  a mixture not a composite.

32)   Characteristic phases of the amorphous carbonaceous material

33)   Normally phases present at the quantities of less than 5% are not detectable by XRD technique

34)   The blank test (only substrate, in the absence of the catalyst)  is missing in the catalytic results

35)   Why ethanol turn out to be a better solvent than methanol?

36)   How do the authors know whether the measured UV absorbance belongs to pure HMF or to levulinic acid, which is a major product of this reaction and has a UV absorbance in the same region.

37)   The highest yield cited in the text for all of the composites is different in table 2 than in the text

38)   What is MLC in Table 3?

39)   The results obtained at 120 C are visible in the Figure but they are not mentioned in the text…It seems a bit strange that the amount of HMF does not change with the increase of the temperature…It should be discussed. What was the conversion of fructose at 100 °C and in 120°C.

40)   Blank test should have been done for the best reaction conditions and for all of the solvents not only for the reaction time study…

41)   There is lack of results for carbon material as-prepared and for TiO2 under the same reaction conditions.

42)   The authors say that the yield of HMf did not increase with the increase in catalyst dosage however the Figure 7 show that in case of the 1wt%TiO2-C the yield significantly increased…

43)   Figure 8 should be placed before the conclusions

 


Author Response

Review of “Development of TiO2-Carbon Composite Acid Catalyst for Dehydration of Fructose to 5-Hydroxymethylfurfural” by Morongwa Martha Songo, Richard Moutloali , Suprakas Sinha Ray submitted to “Catalysts” journal

 

It is an interesting article; however it needs a major revision with some additional tests/experiments to clarify the presented results before it can be published. First of all, the presented FTIR and EDS results do not support that statement that the C-TiO2 materials is actually a composite and it seems to me that the materials is actually a mixture of two phases. The interpretation of the characterisation of the material need revision, as sometimes the conclusion are taken in the text which are not supported by any scientific evidence presented. The heterogeneous aspect of the catalytic  process is not clear, because the authors did not present the results of the amount of sulfonic groups on the surface of the catalysts before and after the reaction. The “hot filtration test” is not clearly described in the experimental part so it is not clear whether a new substrate was added after the removal of the catalyst or not. The yield is based on the UV measurements of HMF absorbance in the region where levulinic acid (one of the main side products of fructose dehydration, also shows absorbance. Thus, it is not clear which of the products the authors are really measuring. The blank catalytic test is missing for the best reaction conditions so it is not clear how much better the catalysts really are as compared to the activity of the solvent itself. It is known that DMSO used by the authors is active in this reaction. How is the catalytic activity and selectivity affected by the acidity of the catalysts is also not clear from the presented results, however the authors conclude that indeed the catalytic performance was affected by the catalyst acidity. Some corrections of the formatting is also needed. More detail list of corrections as follows:

 Authors’ reply:  Firstly, accept our appreciation for the meticulous assessment of the manuscript. We have carefully considered the recommendations and the following changes/additions have been made.

1)      Increase in the use

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

2)      Repetition of increase in the first sentence

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

3)      Contributing to the growing number of serious health issues

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

4)      Using homogenous organic acids or heterogeneous solid acids

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

5)      What is the basis of catalytic application? Supports?

Authors’ reply:  Thank you for this query. Supports!

Catalyst support plays an important role for supported metal catalysts in bringing out the capability of the supported metal as a catalytically active center. Generally, materials for catalyst supports show high surface area, chemical stability as well as capability for dispersing metal particles highly over the surface.

6)      Carbons are rather hydrophobic materials that is why they do not disperse in water. Only oxidised carbon materials are hydrophilic!

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

7)      In the mixtures of DMSO and ionic liquid

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

8)      Concentrated sulphuric acid or fuming surfuric acid was used for functionalization?

Authors’ reply:  Thank you for this question. Concentrated sulfuric acid was used for functionalization under reflux conditions

9)      The hot filtration test, it is not clear in the experimental section is after the filtration of the catalyst, the reaction was carried out with addition of the substrate or without? How did the authors determine that the reaction was not catalysed by the sulphuric anions present in the solution after the catalyst was removed? Were the trials made with the substrate on its own to determine the extent of the reaction?

10)   Authors’ reply:  Hot filtration test was carried out initially with the substrate, then no substrate was added when the reaction was repeated.

11)   How do the authors know that the SO3H groups are attached to the carbon not to the TiO2 in the composites?

Authors’ reply:  Thank you! Ti in TiO2 does not have a single electron that can form a bond with the sulfur in sulfonic acid and moreover the FTIR band appeared at 1106-1168 cm-1 which was assigned to the C=S stretching confirmed that SO3H groups were attached to the carbon.

12)   The drop of surface area with the addition of TiO2 to C can point at TiO2 blocking some micropores of the carbon.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

13)   What was the type of bonding between C and TiO2? The authors say that it was a composite, however the referee thinks it could have been only a physical mixture.

Authors’ reply:  On the basis of available literature we believe this is Chemical bonding, References- dx.doi.org/10.1021/cs400080w|ACSCatal.2013, 3, 1477−1485 and TiO2-graphene composite and N.C.T. Martins et al./Applied Catalysis B: Environmental 193 (2016), 67–7.

14)   How did the authors measure the amount of TiO2 added to the spheres in the composites. The authors should be aware that the theoretical amount of TiO2 added does not result in the equal amount present in the final material.

15)  Authors’ reply:  Thank you for this query. The method which was used to add the TiO2 NPs onto the carbon spheres is highlighted in the paper. The authors are very much aware that the theoretical amount of TiO2 which was added during synthesis is not equal to the amount of TiO2 which is present in the final material.

16)   Did the authors try to first oxidise the carbon and later add TiO2? This way TiO2 could attached itself to the carbon surface via e.g. hydrogen bonds???

Authors’ reply:  Thank you for this suggestion. However, we think this is not possible.

17)   It is well known that the sulfonation of carbons especially under harsh conditions results in the drop of surface area of carbons which is due to the broadening of micropores and blocking some micropores. There is no need to discuss this in such a detail in the manuscript. It is enough to add some references.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

18)   Table 1, I assume that the C is after sulfonation??? It should be explained more clearly; otherwise it can suggest that the as-made material contained sulphur.

Thank you for this suggestion. It has been taken into account in the revised version.

19)   Why the composite with 5 % TiO2 has higher amount of sulphuric groups?

Authors’ reply:  Thank you! The sulfonic group amounts are closer to each other, (1.46, 1.55 and 1.49)

20)   Page 4 SO3-

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

21)   In Figure 1, it looks to the referee that the material is a physical mixture of carbon and TiO2. Which particles do the authors refer as “ the composite particles”??? The difference in the SEM images is that the more TiO2 was added, the less spheres of carbon are visible…The irregular particles belong clearly to TiO2… In order to support the statement “Moreover, a high amount (5 and 10%) of TiO2 nanoparticles loading promoted the formation of irregular particles in the composites”, SEM image of neat TiO2 should be added and compared with the composites.

Authors’ reply:  Thank you for the suggestion. The material is not a physical mixture of carbon and TiO2; high magnification SEM results revealed that TiO2 particles were deposited on to the surface of the carbon. These results have been taken into account in the revised version.

22)   After the sulfonation, the particles become smaller, so there is an impact on the structure of the material.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

23)   Are shown in Figure 2 e and f, what do the authors mean by “parts”??

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

24)   Was the EDS taken from various areas of the samples or only from one area of each of the samples? The authors put the composition of the materials based on the EDS spectra…was the calibration made befohand?

Authors’ reply:  The EDS was taken from various parts of the sample. The calibration was made before the analysis

25)   What do the authors mean by “TiO2 coated the samples”?? Base on the EDS results it can be only confirmed that the TiO2 was present in the “ composite” samples.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

26)   How the authors did confirmed that S was present in the samples in the form of SO3H? did the carry out the XPS analysis of sulphur or is it only based on the literature? If so, than it should be written that based on the literature findings (ref), sulphur is most likely present in the form of…It is impossible to confirm the type of S present based on the EDS results…

Authors’ reply:  Thank you for this suggestion. The confirmation of S being present in the sample in the form of SO3H was made based on the referenced literature findings.

27)   The samples contained sulfonic acid or sulphur???

Authors’ reply: Sulfonic acid.

28)   The EDs detectors are normally not able to detect any elements with atomic number less than 5, so it is obvious that hydrogen could not be detected.

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

29)   Page 6, the caption should be below the figure on the same page

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

30)   Oxygen groups on the surface of carbon could also catalyse the reaction. Some comments should be added in the manuscript about this.

31)   Authors’ reply:  Thank you! Dehydration reaction of fructose is closely associated with the acidity of the catalyst. Oxygen group on the surface of the carbon could not catalyse the reaction, the material need to be functionalized with an acid first.

32)   Some oxygen groups should be removed during calcination. Any ideas why this did not happen?  For example carboxylic groups normally leave at the temperatures lower than 400 °C. What is the difference in the activity between the calcined C spheres and as made ones? The authors say that high temperature of calcination “destroyed the structure” of the material, but a few lines above this, the authors claim that no changes were observed in FTIR between the neat and calcined C spheres.  Please review the FTIR analysis.

Authors’ reply:  Thank you for this suggestion. The FTIR results have been re-analysed, and indeed the high temperature destroyed the structure of the material. There is a difference between the FTIR spectra of the neat C spheres and the calcined C spheres.

33)   Please reformat the captions in the Figure 3

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

34)   The authors mentioned the band at 630 cm-1 which is supposed to confirm that TiO2 was chemically bonded to C, however the referee struggled to see such a band in the Figure 3 . In fact, the spectra of carbon and the composites are literately very similar…The spectra of neat carbon and carbon-TiO2 “composites” should be overlapped to clearly show the existence of the chemical bonding. Otherwise, the material should be referred to as a mixture not a composite.

 Authors’ reply:  Thank you for this suggestion.  It has been taken into account in the revised version. The FTIR results were re analysed and indeed the band at 630 cm-1 was not visible.

 

35)   Characteristic phases of the amorphous carbonaceous material

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

36)   Normally phases present at the quantities of less than 5% are not detectable by XRD technique

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

37)   The blank test (only substrate, in the absence of the catalyst) is missing in the catalytic results

Authors’ reply:  Corrections were made and the results of blank tests (non-catalytic reaction) have been written in the paper.

38)   Why ethanol turn out to be a better solvent than methanol?

Fructose has a measurable level of solubility in any given solvent, at some temperature. However, it has been reported that it is less soluble in alcohols. Ethanol could have turned out to be a better solvent than methanol due to its solubility in fructose at 100 oC. Similar results were found from the work reported in Reference ChemSusChem 2011, 4, 1745 – 1748

 

39)   How do the authors know whether the measured UV absorbance belongs to pure HMF or to levulinic acid, which is a major product of this reaction and has a UV absorbance in the same region.

 

Authors’ reply:  HMF is measured at the UV absorbance of 284 nm and levullinic acid at 254 nm. The majority of the product of this reaction was HMF because we used DMSO as a solvent. DMSO prevented the formation of by-products such as levullinic acid and humins

40)   The highest yield cited in the text for all of the composites is different in table 2 than in the text

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

 

41)   What is MLC in Table 3?

Authors’ reply:  Magnetic lignin-derived carbon (MLC). It has been taken into account in the revised version.

42)   The results obtained at 120 C are visible in the Figure but they are not mentioned in the text…It seems a bit strange that the amount of HMF does not change with the increase of the temperature…It should be discussed. What was the conversion of fructose at 100 °C and in 120°C.

Authors’ reply:  Necessary corrections have been made in revised version. The HMF dehydration at 100 to 120 oC were as follows:  82 to 84% C_S, 90 to 91% 1%TiO2C_S, 90 to 92% 5%TiO2C_S, and 94 to 95% 10%TiO2C_S, respectively

 

43)   Blank test should have been done for the best reaction conditions and for all of the solvents not only for the reaction time study…

 

Authors’ reply:  Corrections have been made and blank test results have been included in revised version

44)   There is lack of results for carbon material as-prepared and for TiO2 under the same reaction conditions.

Authors’ reply: The aim of the study was based on the use of carbon and carbon-TiO2 solid acid catalyst for catalytic dehydration of fructose to 5-HMF. The as prepared carbon results are denoted as neat carbon in the revised version. We did not test the TiO2 for catalytic application

 

45)   The authors say that the yield of HMf did not increase with the increase in catalyst dosage however the Figure 7 show that in case of the 1wt%TiO2-C the yield significantly increased…

 

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

46)   Figure 8 should be placed before the conclusions

Authors’ reply:  Thank you for this suggestion. It has been taken into account in the revised version.

We hope you will be satisfied with our revision and that the manuscript will now be acceptable for publication in Catalysts as a regular article.

 


Round 2

Reviewer 2 Report

Not all of the referees´ doubts have been addressed correctly, however significant improvements have been made in the manuscript and now it can be accepted for publication

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