Innovative Synthetic Approaches for Sulphate-Promoted Catalysts for Biomass Valorisation

Round 1

Reviewer 1 Report

 

The manuscript is devoted to synthesis of sulfated zirconia catalysts using an innovative procedure involving microwave drying and calcination. The synthesized materials were extensively characterized by several physical methods and used as catalysts for glucose hydrolysis. I believe that the manuscript could be accepted for publication in Catalysts after a major revision addressing the following comments.

 

1. My made concern is related to selection of samples used for detailed study. It is obviously not a good idea that the samples used in catalytic tests featured variations both in the synthesis method and concentration of sulfates at the same time. The concentration of sulfates on the surface of sulfated zirconia catalysts is known to have a major effect on the catalytic activity. So, I would definitely recommend varying these parameters separately, i.e. testing samples prepared by different methods with approximately similar concentrations of sulftates and prepared by the same method with different concentrations of sulfates. As a lot of experimental work has been performed, I do not insist on immediate investigation of additional samples. However, I do believe that the authors should follow my advice in their later studies if they continue studying these materials.
2. Line 94. An obvious mistyping should be corrected.
3. Although English in the manuscript is mostly acceptable, I would recommend splitting some very long sentences, e.g. in lines 97-103, 106-109. This would make the text easier to follow. There are more such excessively long sentences throughout the manuscript.
4. Figure 1 caption. What is the “final product” here? Please, add more details what further synthesis steps were made to obtain it.
5. The setup for use of graphite as a susceptor should be explained in more detail. Is it possible to use other materials for the same purpose?
6. It is not clear what final temperatures were reached in the MW-assisted calcination of samples Z1, Z3 and SZ samples. Is it possible to vary temperature in a controllable fashion by adjusting some parameters of the MW treatment?
7. What is the estimated average crystallite size of sample Z3?
8. Is it possible to prepare SZ by adding sulfuric acid to the gel prior to the MW drying and calcination?
9. Line 226. What does the phrase “platform chemicals” mean?
10. Line 260. It is better to say “spectra of activated samples”. The phrase “referred to” has a different meaning that is not appropriate here.
11. Lines 278-301. Comments regarding higher ionicity of sulfates on S1Z_5 are not convincing. In my opinion, all bands in the spectra of the samples prepared by MW calcination are very similar. I would suggest that some ionic sulfates are present on the surface of all samples, whereas some covalent are also present. At first glance, it is difficult to differentiate the samples prepared using MW calcination.
12. Figure 5a. It is not clear why the intensity of lines 1630- and 1645 cm^-1 on SZ_p increases after evacuation.
13. Line 361. It is not clear why the authors made a conclusion on the suggested order of strength of Bronsted acid sites. Their spectra seem to be very similar, except for intensity. Meanwhile, the intensity seems to be related only to the concentration of Bronsted acid sites rather than to their strength. Also, it is not clear from the text if direct comparison of the band intensity for different samples is meaningful.
14. Is it possible to determine the concentrations of various acid sites on the surface of the studies samples quantitatively using the available FTIR data?
15. Lines 399-401. The data reported in Table 2 suggest that S1Z_5 has poor selectivity rather than low catalytic activity. Conversion on this sample seems to be comparable with the other ones. 
16. Line 405. Change “a susceptor to rapidly reaches” for “a susceptor that rapidly reaches”.

 

Although English in the manuscript is mostly acceptable, I would recommend splitting some very long sentences, e.g. in lines 97-103, 106-109. This would make the text easier to follow. There are more such excessively long sentences throughout the manuscript.

Some other mistakes are also mentioned in my comments.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments

The manuscript demonstrated that an innovative and quick method for the synthesis of sulfated zirconia was proposed, wherein a sol-gel procedure followed by two steps assisted by microwaves (MW) heating was involved. To underscore the superiority of this method, a series of sulfated zirconia were also prepared by different methods. Moreover, the as-synthesized materials were characterized by various techniques. Subsequently, the as-prepared sulfated zirconia materials were developed for catalyzing dehydration of glucose into 5-HMF. Overall, the work presents convincing results and can be accepted for publication after addressing the following minor comments.

 (1) Please denote the time required to reach the specified reaction temperature (i.e., 180 °C) in the section of Materials and Methods.

(2) Although the presence and strength of L and B acid sites has been investigated by FTIR spectroscopy of adsorbed CO and 2,6-DMP probe molecules, the molar ratio of Bronsted acid to Lewis acid was not found in the manuscript. As said in the section of introduction, a suitable molar ratio of Bronsted/Lewis acid sites is important for dehydration of glucose into 5-HMF. So, it would be better if providing the molar ratio of Bronsted/Lewis acid sites.

(3) As a heterogeneous catalyst, the stability of as-synthesized sulfated zirconia material should be evaluated. Thus, reuse experiments and hot filtration experiments are suggested to be conducted.

(4) Several errors or typos are found in the manuscript.

(i) MW-assisted calcination permits to obtained tetragonal ZrO₂ (line 15).

(ii) the full name should be written when 5-HMF first appears (lines 20-21)

(iii) through dehydration in 5-HMF (line 58)

(iv) obt<<<<<<<<ained (line 94)

(v) (ii) S1Z_5 () (line 237)

No

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Interesting manuscript on biomass valorization using sulfate-promoted catalysts. The manuscript evaluates innovative synthetic approaches for sulfate-promoted catalysts and further tests them in biomass valorization. The data provided is good, however, it needs to be revised before this manuscript can become suitable for publication. My questions/suggestions are as below:

My questions/suggestions:

1.     Please correct many typos in the manuscript: e.g. line 31, ‘XX’ century; line 19, 2,6-dymethylpyridine should dimethylpyridine; line 94, obt<<<<<<<ained; SZ1 instead of S1Z; etc.

2.     Authors should improve the manuscript's clarity. Although the results provided are very good it feels too many different things were attempted rather than a systematic approach to clearly point out differences.

3.     If authors could add different reactions (main and side reactions) in the introduction, it will be helpful to better understand valorization results.

4.     Authors provide qualitative distribution of acid sites. Can authors quantify the type and distribution of acid sites based on FTIR? (weak, medium, strong, and Lewis/Bronsted).

 

5.     Can authors comment on the rapid removal of water (-OH groups) under MW irradiation that may influence Bronsted acid sites? as compared to gradual heating under a muffle furnace. 

Grammar in several places needs to be corrected.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have reasonably addressed all my comments. I beleive that the manuscript can be accepted for publication in Catalysts.