Review Reports

Round 1

Reviewer 1 Report

Dear author,

 

All comments made can be found in the annex.

 

With the best regards

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this manuscript the authors only prepared two catalysts MCM-41-SO3H-1 and MCM-41-SO3H-2. Both of them have the similar S contents, acidity properties, etc. In the reaction study only one catalyst was reported. The novelty of this manuscript is very limited. The research is not systematic. Therefore, I cannot recommend this manuscript to be processed for further review.

Also, in the Introduction part the relevant literature review and discussion was not enough, particularly about the biphasic system, the catalysts used in furfural production and the importance of mesoporous catalysts (which has been studied for many years). The sulfonic MCM-41 has been extensively studied. However, in lines 63-65 reference 13 is far from enough to represent this part of research.

The manuscript was not prepared rigorously. For examples,

(1)   Line 17 “2:1 y 2.5:1”, line 47 “process start whit”, line 59 “MCM 41”, line 74 “SO3H” (3 should be subscript), line 356 “min-1” (“-1” should be superscript), line 144 “MCM-41-SO3H 9 19”, equation (2) “(x)FUR]”.

(2)   Line 379, I cannot understand what “The conversion and selectivity at 15% of conversion” mean.

(3)   In Figure 2 it is not clear which is MCM-41-SO3H-1 or -2. Similar problems exit in other relevant figures.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this manuscript, the authors claim to have developed a method for converting hemicellulose and xylose to furfural using a heterogeneous acid catalyst MCM-41-SO₃H in a mixture of H₂O/n-BuOH solvents. It is also claimed that MCM-41-SO₃H catalyst is an inexpensive and non-toxic solid which makes the process convenient, economic, and environmental benign. In general, the heterogeneous catalyst process is considered to be preferable to processes using mineral acids because no toxic effluents are generated and the catalyst is easy to recycle.

The concept is not new, but the authors simplified the preparation of the catalyst compared to similar works by using readily available chlorosulfonic acid for sulfonation, and the catalytic process itself was carried out in a mixture of water and n-butanol, which stands out against the background of previously used mixtures and solvents such as DMSO, toluene and etc. As a result, interesting data were obtained, which, in my opinion, are worthy of publication.

However, there is a key point in the work that should be clarified first. In the course of studying the possibility of reusing the catalyst, the authors showed that in the second cycle the catalyst completely loses its selectivity for furfural, which was associated with the leaching of -SO₃H groups. Accordingly, the question arises whether the catalyst MCM-41-SO₃H is not an exotic way of delivering sulfuric acid to the reaction mixture, which then catalyzes the reaction as a typical homogeneous mineral acid? To clarify this issue, the following experiments should be carried out:

● Carry out the reaction with xylose and hemicellulose in the presence of only sulfuric acid in an amount equivalent to the number of acid sites in the catalyst. Compare the results with those for the catalyst MCM-41-SO₃H.

● Carry out a hot filtration test. Filter the catalyst approximately half way through the conversion and monitor the progress of the reaction. Continued reaction after filtration of the catalyst would indicate the homogeneous nature of the catalysis.

● Quantify the amount of sulfur in the solution after the reaction. This is desirable for both the hot filtration test filtrate and the typical reaction mixture filtrate. Estimate how many percent of the sulfonic groups are leached into the solution from the catalyst MCM-41-SO₃H during the reaction.

Experiments may reveal that the catalyst is not actually MCM-41-SO₃H, but H₂SO₄ leaching from MCM-41-SO₃H. In that case, the formulation of the main result of the work must be revised, and it will be necessary to honestly say that MCM-41-SO₃H cannot work as a heterogeneous catalyst under the studied conditions, which entails rejection of the claim that MCM-41-SO₃H is an inexpensive and non-toxic solid, which makes the process convenient, economical and environmentally friendly. Finding conditions in which the catalyst is stable can save this part of the work, for example, you can try lowering the reaction temperature. Nevertheless, a negative result is also a result and publishing data that a catalyst of this composition is unstable in such a range of conditions and the real catalyst is H₂SO₄ may be of interest to other researchers and is justified in honestly pointing out this fact.

Also, regardless of the results on the previous question, I ask for some additional improvements and clarifications to be made to the manuscript:

● First, please, identify and discuss key reaction conditions. Explain why you chose exactly those parameters that are given in the manuscript (0.6 g substrate, 0.3 g catalyst, 60 ml solvent, 10 bar N₂, 170 °C). What reaction parameters is the bottleneck in improving the overall process? Make a table comparing your catalyst to the best other catalysts in xylose and hemicellulose to furfural reactions, also include mineral acids as a baseline. Compare on key parameters and use integral characteristics of processes, for example, space-time yield.

● In connection with the previous question, please give the full reaction conditions using substrate concentration (for the total volume of the solvent mixture), catalyst amount in g and mol% (relative to acid sites) in the figure captions. Also add these in catalytic activity measurements conditions description in Materials and Methods.

● What by-products are produced during the conversion of xylose and hemicellulose to furfural? What is the main product in cases where the yield of furfural is low or near zero? Is it possible to bring the material balance of the reaction products? Please, add data to the manuscript and discuss it.

● How was the conversion of hemicellulose determined during the course of the reaction? Because it is a polymer, it cannot be easily quantified with GC-FID or GC-MS. The same question about the quantitative determination of xylose in the reaction mixture. This is a non-volatile carbohydrate that, when heated in the inlet of a gas chromatograph, decomposes instead of volatilizing. Describe the analysis method in detail.

● Please provide details of the sulfonation procedure for MCM-41. I was able to find it by reference, but I think that for the convenience of the reader it should be added, if not to the main text, then at least to SI.

● Add to the SI a photo of a typical reaction mixture before and after the reaction, as well as a photo of the catalyst itself before and after the reaction.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript has been sufficiently improved to warrant publication in Catalysts.

Author Response

Reviewer 1 is OK

Reviewer 2 Report

The authors only modified the manuscript slightly. The key problems of this manuscript stated by the reviewer previously have not been improved at all in the revised manuscript. The authors still reported two catalysts with the similar S contents, acidity properties, etc, with only one catalyst in the reaction study. The novelty of this manuscript is very limited. The research is not systematic.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Judging by the results of additional studies on the nature of catalysis on MCM-41-SO3H conducted by the authors during the revision, it cannot be said that the catalysis is heterogeneous, since most of the -SO₃H groups are leached during the reaction.

However, the authors claim that their process is more environmentally friendly and selective than the process with sulfuric acid as a catalyst. In terms of selectivity, their statement is justified, but not from the point of view of environmental friendliness. Yes, slightly less sulfuric acid is emitted per reaction cycle, but HCl gas is emitted during catalyst preparation and regeneration.

I suggest that the authors focus not on demonstrating the superiority of their process, but on identifying key challenges in catalyst development. Since when comparing the main parameters such as reaction time and furfural yield, MCM-41-SO₃H does not stand out significantly in its characteristics and also does not have high reusability.

In this work, the authors clearly showed that the main reason for the deactivation of MCM-41-SO₃H is the leaching of active acid sites. Previously, deactivation was mainly explained by the adsorption of by-products and coke deposition, which hindered access to the active sites. Undoubtedly, this also contributes to deactivation, but a significant leaching of active sites is a much more important factor.

Accordingly, this finding should be highlighted explicitly. There is no point in fighting coke deposition or scaling up the process until the active site leaching issue is resolved.

I request the following additions and changes to the manuscript before publication:

·         Add a figure showing the progress of the reaction after hot filtration, since you have already done this test.

·         Remove the statement that “the MCM-41-SO₃H catalyst is an inexpensive and non-toxic solid which makes the process convenient, economic, and environmental benign”. There is no real evidence that the process is economical and environmentally friendly with MCM-41-SO₃H catalyst in its current form.

·         Add a short discussion that the main problem with the catalyst is active site leaching and should be addressed first when developing sulfonated acid catalysts for xylose conversion. Catalyst sulfonation does not completely solve the problem in this case, because then MCM-41-SO₃H is simply an analogue of homogeneous sulfuric acid with a slightly higher selectivity.

Author Response

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Author Response File: Author Response.pdf