Lignin Extraction and Condensation as a Function of Temperature, Residence Time, and Solvent System in Flow-through Reactors

Solvolytic extraction of lignin from biomass is a critical
step
in lignin-first biorefining, including the reductive catalytic fractionation
(RCF) process. Key to optimal RCF processing is the ability to rapidly
extract lignin from biomass at high delignification extents and transfer
the lignin molecules to a catalyst surface in a time frame that minimizes
lignin condensation reactions. Here, we use a flow-through reactor
to study the effects of temperature (175–250 °C), residence
time (9 to 36 min), and solvent composition (methanol and methanol–water)
on lignin extraction and condensation. We evaluated three metrics
at each condition: total delignification, delignification rate, and
extent of condensation, the latter measured by a decrease in monomer
yield for batch hydrogenolysis reactions of solvolysis liquor compared
to batch RCF reactions. We observe that delignification is predominantly
determined by temperature, while residence time dictates the lignin
condensation extent. Moreover, the extent of both extraction and condensation
increased in the methanol–water solvent system compared to
that in the methanol system. Lignin extracted in methanol is stable
up to 18-min residence times at or below 225 °C, while a majority
of the lignin extracted in methanol–water is condensed with
a 9-min residence time at 200 °C. These results can inform reactor
designs and solvent selection for lignin-first biorefining processes
that aim to physically separate the biomass and catalyst.