Uncatalyzed Lignin Solvolysis: Utilizing Water-Solvent Mixtures as a Reaction Medium

The success of the biorefineries lies in the efficient use of all the feedstock’s components. Combustion of lignin for energy production is an underutilization of its potential. Even though research on manufacture of value-added products from lignin is currently vivid, no real breakthroughs have been seen in this area. This underlines the need for new methods of lignin depolymerization, as well as research in the fundamental aspects of the process chemistry. This paper discusses uncatalysed solvolysis of organosolv hardwood lignin with mixtures of water and solvents – methanol, ethanol, and 2-methyltetrahydrofuran (2-MTHF). Lignin was mixed with pure water and water-solvent mixtures (1:1, volume basis), carrying out solvolysis at 270 °C (Parr 4575 batch reactor). Fractions were separated from the reaction mixture (undegrad-
ed lignin, char, depolymerized lignin, and gases), subsequently determining their yields. Molar
mass distribution of the depolymerized lignin was determined (GPC). Based on the results 2-
MTHF was selected to be looked into in more detail. Different water-solvent ratios (1:3, 1:1, 3:1)
and reaction temperatures (270, 290, 310 °C) were tested. The reaction products were analyzed
as described above. The reaction product yields showed a notable effect of the organic solvents
on char formation: while 15.8 w-% (related to the feedstock lignin) char formed in pure water,
the respective values were 2.4, 1.2, and 1.8 w-% for the 1:1 mixtures of water with methanol,
ethanol, and 2-MTHF. On one hand, the highest yield of depolymerized lignin was formed with
2-HTMF (86.8 w-%), while on the other hand the GPC measurements showed methanol to pro-
mote the most extensive depolymerization. 1:3 and 3:1 water:2-MTHF ratios produced inferior
yields of depolymerized lignin compared to the 1:1 ratio. Increasing the temperature promoted
lignin depolymerization: solvolysis at 310 °C yielded 88.6 w-% depolymerized lignin, and only
0.3 w-% char; the respective values when using water only in the same conditions were 13.1 and
12.6 w-%. Utilizing water-solvent mixtures for uncatalyzed lignin depolymerization gives prom-
ising results. Especially 2-MTHF mixed with water promotes lignin depolymerization and disso-
lution, while char formation is minimal. The reaction mechanisms and kinetics were not the
scope of this research, but in the future, these aspects will have to be looked into to enable full
utilization of the proposed protocol for lignin depolymerization.