This review focuses on the formation, use and characterization of lignin oligomers. This topic is not very common in relation to the depolymerization of lignin into monomers, so this article is very useful and moreover well written. Except for some revisions and comments (see below), the review can be published as is.
- Figure 1 : this figure is fine except that the authors should precise a few things :
o Are the yields given in wt% on initial lignin ?
o Could the authors give more details as the presence of catalysts ?
o Also some references are not well seen as for instance in the middle left ref 40-44
The authors should also carefully check all the references given in this figure as it appears that the results given are not found in the cited references :
For instance, Ref 19 is given to report 90% of oligomers under SCF conditions at 10 bar and around 160°C ; however, those results are not reported in this article dedicated mostly to the obtention of monomers and dimers from wood. Also, ref 17 is reporting conversion of lignin and wood in oxidative conditions but is put in the inert gas part in the figure 1 !
- In the 2.1 part, the process used in traditional refinery to remove O, S or N and that was also applied to lignin deconstruction is hydrotreatment (hydrodesulfurization, hydrodeoxygenation, hydrodenitrogenation). Hydrocracking is specific for C-C cleavage.
- Line 124-125 : pyrolysis conditions are often too harsh to preserve ether bonds, the assumption that under normal pyrolysis conditions (actually what is normal pyrolysis conditions ?) lignins undergo all the routes is not correct and has to be justified.
- Put GCxGC-MS instead of GC-GC-MS
- GPC is a chromatographic technique as HPLC. It should be in the 3.1.1 part.
- Line 205-206 : higher molecular weight : than what ?
- Figure 3 : I have also many doubts about the reference numbers given in that Figure ! The authors should chech it carefully !
- Figure 4 : the same, please check the reference numbers : for instance ref 66 is OCF and not RCF !
- line 316 : the term « by different mechanism » seems inapropriate in this case
- the part 3.1.4. could be better organized as it is a mix of very different techniques : for instance it would be better to present the characterization techniques by type.

For Figure 1, I still not see the mention of units for the yields ! Otherwise that's fine now.