Lignin
is a promising feedstock for the replacement of conventional
carbon sources for the production of chemicals and fuels. In this
paper, results are reported for the depolymerization of various residual
lignins in the absence of a catalyst by utilizing ozone. Reactions
were performed in a microreactor setup ensuring high gas–liquid
mass transfer rates, a low inventory of ozone, and straightforward
scale-up possibilities. The ozonation is demonstrated using a representative
model compound (vanillin) and various lignins (pyrolytic and organosolv)
dissolved in methanol (2.5 wt %). Experiments were performed under
ambient conditions, at gas–liquid flow ratios ranging from
30 to 90 and short residence times on the order of 12–24 s.
Analyses of the products after methanol removal revealed the presence
of (di)carboxylic acids, methyl esters, and acetals. Extensive depolymerization
was achieved (i.e., up to 30% for pyrolytic lignin and 70% for organosolv
lignins). Furthermore, a two-step approach in which the ozonated lignin
is further hydrotreated (350–400 °C, 100 bar H2, 4 h, Pd/C as catalyst) showed a substantial increase in depolymerization
efficiency, yielding a 2.5-fold increased monomer yield in the product
oil compared to a hydrotreatment step only.
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