Converting lignocellulosic biomass to biofuels and bioproducts is significantly hindered by the innate recalcitrance of biomass to chemical and biological breakdown, and it usually requires a pretreatment stage in order to improve conversion yields. A promising novel pretreatment named Cosolvent Enhanced Lignocellulosic Fractionation (CELF) involving dilute acid treatment of biomass in a THF−water mixture was recently developed to overcome biomass recalcitrance. Detailed elucidation of physicochemical structures of the fractionated lignin that is precipitated from CELF pretreatment of hardwood poplar, also called CELF lignin, reveals transformations in its molecular weights, monolignol composition, and hydroxyl group content. Isolated CELF lignin revealed dramatic reductions in its molecular weight by up to ∼90% compared with untreated native lignin. Furthermore, CELF lignin's β-O-4 interunit linkages were extensively cleaved after CELF pretreatment as indicated by a semiquantitative HSQC NMR analysis. This is further evidenced by a 31 P NMR analysis showing a significant decrease in aliphatic OH groups due to the oxidation of lignin side chains, whereas the content of total phenolic OH groups in CELF lignin significantly increased due to cleavage of interunit linkages. In conclusion, the CELF process generated a uniquely tunable and highly pure lignin feedstock of low content aryl ether linkages, low molecular weight, and high amount of phenolic hydroxyl groups, suitable for its development into fuels, chemicals, and materials.

Lignin valorization is significantly hindered by the intrinsic heterogeneity of its complex structures and variability of biomass feedstocks. Fractionation of lignin can overcome these challenges by producing functionally distinct lignin cuts that can be further tailored to end products. Herein, lignin was extracted and depolymerized from poplar by the co-solvent enhanced lignocellulosic fractionation method with renewable THF to obtain CELF lignin. Several solvents were screened to separate soluble and insoluble fractions from the parent CELF lignin. The ethanol soluble portion was then fractionated into different molecular weight cuts via sequential precipitation of the lignin by reducing the concentration of THF. The physicochemical structures of different CELF lignin cuts were elucidated by GPC and NMR techniques. These results suggest that CELF lignin cuts with lower molecular weight contain progressively higher phenolic and carboxylic acid OH groups, which can be more suitable as green antioxidants than the parent lignin.

A robust method is needed to achieve high yield all-catalytic conversion of recalcitrant lignocellulosic biomass to transportation fuels while maximizing carbon utilization from raw substrates. To accomplish this, we developed an integrated strategy that combines homogeneous and heterogeneous reactions with a treatment-extraction step to coproduce 2-methylfuran (MF) and 2,5-dimethylfuran (DMF) directly from hardwood poplar while maintaining high catalyst activity. In the first step, poplar wood chips were treated with dilute FeCl 3 in THF−water at subpyrolytic temperature to yield 93.5% furfural (FF) from xylan and 66.0% 5-hydroxymethylfurfural (HMF) from glucan. Concurrently, a highly pure lignin powder was obtained from the liquor by precipitation upon room temperature vacuum recovery of THF from the water. Afterward, FF and HMF were extracted from water into an organic phase consisting of toluene and 1,4-dioxane treated with Ca(OH) 2. A second hydrodeoxygenation reaction using Cu−Ni/TiO 2 catalyst yielded 87.8% MF from FF and 85.6% DMF from HMF. Characterization of the lignin product showed its molecular weight to be reduced by an order of magnitude from its native state as well as complete removal of its native β-aryl ether linkages without hydrogen input or further heterogeneous catalytic processing. A 60% cumulative yield of MF, DMF, and lignin products from the available carbon (xylan+glucan+lignin) in poplar was achieved, rivaling more mature cellulosic ethanol strategies.

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