Combination of Autohydrolysis and Catalytic Hydrolysis of Biomass for the Production of Hemicellulose Oligosaccharides and Sugars

Round 1

Reviewer 1 Report

The authors describe in their paper the combined autohydrolysis and catalytic hydrolysis of three different kinds of biomass.

Im sum, the paper is only of modest interest as only very few new general insights are given (catalytic hydrolysis). However, it might be interesting for those working with these special kinds of biomass so that the paper might be published if the following topics are properly addressed by the authors.

1. How did the authors characterize which oligosaccharides (OS) were present in the autohydrolysis liquor? For me it is not clear in which way the different OS can be identified in the mixture by using the described analytical procedures. This needs to be clarified.
2. Table 2: Some of the values for wheat straw obviously are false.
3. Figure 4: Are the values for Miscanthus at 160 °C and 180 °C as well as corncub xylan – high MW at 140 °C and 160 °C correct? One would expect a steady increase with temperature.
4. Figure 5: corncub xylan – low MW at 180 °C gives only about 50 % monomers. Of course, degradation is plausible. Did the authors analyzed any degradation product or can they give a hint which kind of degradation products might have been produced?
5. Figure 7: Did the authors carry out a blind experiment without catalyst? As the temperature during catalytic hydrolysis is as high as during autohydrolysis, I would expect some autohydrolysis reactions parallel to catalytic hydrolysis. A blind experiment would help to assume which amount of catalytic hydrolysis contributed to the overall result.

The following sentences or words need corrections:

1. Page 6, lines 206-208: false order: wheat straw has less oligosaccharides than eucalyptus
2. Page 6, line 216: c.a.
3. Page 6, line 223: in instead of I
4. Page 6, lines 226-229: false boiling point for HMF at atmospheric pressure; adjust the meaning of the sentence accordingly
5. Page 14, line 454: Shouldn’t it read overestimated?
6. Page 14, line 455: is instead of ise

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript has some interesting features, but the experimental methods cannot be reproduced with the methods given.

Detailed comments:

1. The manuscript mentions that hemicelluloses are often neglected in process design. But industrial 2nd generation bioethanol plants do convert (also) hemicellulose into ethanol. There, enzymatic hemicellulose hydrolysis is not merely studied (line 62) but actually industrially implemented.
2. Line 84: "The hybrid combination of autohydrolysis and
   heterogeneous catalytic hydrolysis to produce sugars and furfural and HMF were investigated, without any intermediate purification between both steps". This is somewhat misleading, because it turns out that most of the water was evaporated, which would be an extremely costly intermediate treatment step at industrial scale.
3. The biomass should be specified better. Which species, which parts of residues, which particle size, which loss upon drying? Which miscanthus?
4. Line 107: Indicate how much was added. Specify for biomass if it was on wet or dry basis.
5. Indicate how long it did take to achieve the setpoint temperature. Otherwise the results cannot be compared to external results, and become meaningless.
6. The carbon analysis is interesting, but I wonder if it has been properly validated using only glucose. It is not clear how it is used for detecting HPLC effluent.
7. Eq. 3 seems to have wrong signs in the numerator. Explain what the square brackets indicate. Molar concentration?
8. Table 1 includes measurements that have not been described.
9. Table 1 should include a total % row
10. Line 223: Explain "I"
11. I wonder how sure the authors are that their XOS contains only xylose as monomer; etc. Hemicelluloses typically contains several monomers.
12. Figure 3 lacks a vertical scale
13. Table 2 comes after Table 3
14. Line 453: Not osidic but glycosidic

Author Response

Please see the attachment

Author Response File: Author Response.pdf