Biological lignin valorization has emerged as a major solution for sustainable and cost-effective biorefineries. However, current biorefineries yield lignin with inadequate fractionation for bioconversion, yet substantial changes of these biorefinery designs to focus on lignin could jeopardize carbohydrate efficiency and increase capital costs. We resolve the dilemma by designing ‘plug-in processes of lignin’ with the integration of leading pretreatment technologies. Substantial improvement of lignin bioconversion and synergistic enhancement of carbohydrate processing are achieved by solubilizing lignin via lowering molecular weight and increasing hydrophilic groups, addressing the dilemma of lignin- or carbohydrate-first scenarios. The plug-in processes of lignin could enable minimum polyhydroxyalkanoate selling price at as low as $6.18/kg. The results highlight the potential to achieve commercial production of polyhydroxyalkanoates as a co-product of cellulosic ethanol. Here, we show that the plug-in processes of lignin could transform biorefinery design toward sustainability by promoting carbon efficiency and optimizing the total capital cost.

The quest for converting lignin into high-value products has been continuously pursued in the past few decades. In its native form, lignin is a group of heterogeneous polymers comprised of phenylpropanoids. The major commercial lignin streams, including Kraft lignin, lignosulfonates, soda lignin and organosolv lignin, are produced from industrial processes including the paper and pulping industry and emerging lignocellulosic biorefineries. Although lignin has been viewed as a low-cost and renewable feedstock to replace petroleum-based materials, its utilization in polymeric materials has been suppressed due to the low reactivity and inherent physicochemical properties of lignin. Hence, various lignin modification strategies have been developed to overcome these problems. Herein, we review recent progress made in the utilization of functionalized lignins in commodity polymers including thermoset resins, blends/composites, grafted functionalized copolymers and carbon fiber precursors. In the synthesis of thermoset resins such as polyurethane, phenol-formaldehyde and epoxy, they are covalently incorporated into the polymer matrix, and the discussion is focused on chemical modifications improving the reactivity of technical lignins. In blends/composites, functionalization of technical lignins is based upon tuning the intermolecular forces between polymer components. In addition, grafted functional polymers have expanded the utilization of lignin-based copolymers to biomedical materials and value-added additives. Different modification approaches have also been applied to facilitate the application of lignin as carbon fiber precursors, heavy metal adsorbents and nanoparticles. These emerging fields will create new opportunities in cost-effectively integrating the lignin valorization into lignocellulosic biorefineries.

An industrial alkaline lignin preparation and poplar particles were mixed and hot pressed under different conditions. The alkaline lignin and the lignin isolated from the poplar particles were thoroughly investigated by quantitative nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and elemental analysis techniques. For the first time, it was found that the content of β-O-4′ linkages decreased accompanying with the formation of β-β′, β-5′, and β-1′ linkages at mild heat treatment temperatures (130–170 °C). However, it should be noted that most of the β-O-4′, β-β′, β-5′, and β-1′ linkages nearly disappeared at a higher temperature (180 °C). Cross-linking reactions were predominant during the hot-pressing process as the molecular weight of lignin increased at elevated temperature. Owing to the self-bonding between lignin fragments during the hot-pressing process, a green poplar wood–lignin composite was successfully prepared with poplar particles and a small amount of alkaline lignin (∼20%, w/w). Internal bond strength with 0.47 MPa was surprisingly achieved under the conditions of pressing temperature 160 °C, pressure 5 MPa. An in-depth understanding of the concerted reactions between fragmentation and cross-linking reaction in lignin during hot pressing was beneficial to a better development of self-bonding green wood–lignin composites in future.

In tensile behaviour, polymeric materials containing only methylated ball-milled lignin surpass polystyrene, while 85% w/w ligninsulphonate blends approach polyethylene.