Counter-current chromatography for lignin monomer–monomer and monomer–oligomer separations from reductive catalytic fractionation oil

Abstract

Reductive catalytic fractionation (RCF) is a lignin-first biorefining technique that produces a polysaccharide-rich pulp and a lignin oil that is rich in aromatic monomers from aryl–ether bond cleavage and carbon–carbon linked aromatic oligomers. Separations of the lignin-derived monomers, both from one another and from the oligomers, out of these lignin oils could potentially yield high value co-products. To that end, we demonstrate that counter-current chromatography (CCC) is an effective means for simultaneous lignin monomer–monomer and monomer–oligomer separations using oils from RCF reactions with hardwood, softwood, and herbaceous feedstocks. Partition coefficient measurements of aromatic monomers from RCF of poplar, pine, and corn stover were first used to inform CCC solvent selection. We subsequently demonstrated CCC separations of those lignin oils using the HEMWat −3 solvent system and refined the measured partition coefficients using solute retention times and the cell utilized partitioning model to account for matrix effects in the following optimization experiments. Furthermore, the carbon–carbon linked oligomers in the lignin oil substrates elute together and separately from the aromatic monomers in lignin oil, resulting in an oligomer-rich product stream. Case studies of optimization of poplar RCF-derived lignin oil separations exhibited non-polar monomer yields of 95–99% with purities of 72–96%. Additionally, the same separation using a propyl-rich lignin oil produced from a H₂-free RCF process showed a nearly 46% increase in normalized productivity, exhibiting the importance of tuning feed composition to improve separation performance. Taken together, this work shows that CCC is a promising method for simultaneous lignin monomer–monomer and monomer–oligomer separations.