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1. Recent developments towards performance-enhancing lignin-based polymers

   https://doi.org/10.1039/d1py00694k  

   Bass, Garrett F.; Epps, Thomas H. (July 2021, Polymer Chemistry)

   null (Ed.)

   Fossil fuels are a cheap and abundant feedstock for polymeric materials that have enabled innumerable quality-of-life improvements. Yet, their declining supply and non-renewable nature have driven the pursuit of bio-based alternatives. Lignin represents the largest natural source of aromatic carbon on the planet, and thus, lignin-derived products have emerged as critical elements in the next generation of polymers. The relative abundance, large concentration of functional handles, and thermal stability of lignin make it an attractive target for bio-based polymers. However, the valorization of lignin to high-performance and cost-competitive materials remains a challenge. In this review, developments in the translation of lignin into value-added macromolecular components are discussed. Strategies to incorporate bulk lignin in polymer blends and composites are introduced with a focus on applications. Furthermore, recent advances in the preparation of higher-value thermoplastics, thermosets, and vitrimers from deconstructed lignin products are highlighted from a synthetic perspective. Finally, key hurdles and future opportunities in lignin valorization are explored. 

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2. Valorisation of waste to yield recyclable composites of elemental sulfur and lignin

   https://doi.org/10.1039/C9TA03222C  

   Karunarathna, Menisha S.; Lauer, Moira K.; Thiounn, Timmy; Smith, Rhett C.; Tennyson, Andrew G. (January 2019, Journal of Materials Chemistry A)

   Lignin is the second-most abundant biopolymer in nature and remains a severely underutilized waste product of agriculture and paper production. Sulfur is the most underutilized byproduct of petroleum and natural gas processing industries. On their own, both sulfur and lignin exhibit very poor mechanical properties. In the current work, a strategy for preparing more durable composites of sulfur and lignin, LSx , is described. Composites LSx were prepared by reaction of allyl lignin with elemental sulfur, whereby some of the sulfur forms polysulfide crosslinks with lignin to yield a three-dimensional network. Even relatively small quantities (<5 wt%) of the polysulfide-crosslinked lignin network provides up to a 3.4-fold increase in mechanical reinforcement over sulfur alone, as measured by the storage moduli and flexural strength determined from dynamic mechanical analysis (temperature dependence and stress–strain analysis). Notably, LSx composites could be repeatedly remelted and recast after pulverization without loss of mechanical strength. These initial studies suggest potential practical applications of lignin and sulfur waste streams in the ongoing quest towards more sustainable, recyclable structural materials. 

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3. One-pot synthesis of enzyme@metal–organic material (MOM) biocomposites for enzyme biocatalysis

   https://doi.org/DOI https://doi.org/10.1039/D1GC00775K  

   Pan, Y.; Li, H.; Lenertz, M.; Han, Y.; Ugrinov, A.; Kilin, D.; Chen, B.; Yang, Z. (March 2021, Green chemistry)

   null (Ed.)

   Metal–organic frameworks/materials (MOFs/MOMs) are advanced enzyme immobilization platforms that improve biocatalysis, materials science, and protein biophysics. A unique way to immobilize enzymes is co-crystallization/co-precipitation, which removes the limitation on enzyme/substrate size. Thus far, most enzyme@MOF composites rely on the use of non-sustainable chemicals and, in certain cases, heavy metals, which not only creates concerns regarding environmental conservation but also limits their applications in nutrition and biomedicine. Here, we show that a dimeric compound derived from lignin, 5,5′-dehydrodivanillate (DDVA), co-precipitates with enzymes and low-toxicity metals, Ca2+ and Zn2+, and forms stable enzyme@Ca/Zn–MOM composites. We demonstrated this strategy on four enzymes with different isoelectric points (IEPs), molecular weights, and substrate sizes. Furthermore, we found that all enzymes displayed slightly different but reasonable catalytic efficiencies upon immobilization in the Ca–DDVA and Zn–DDVA MOMs, as well as reasonable reusability in both composites. We then probed the structural basis of such differences using a representative enzyme and found enhanced restriction of enzymes in Zn–DDVA than in Ca–DDVA, which might have caused the activity difference. To the best of our knowledge, this is the first aqueous-phase, one-pot synthesis of a lignin-derived “green” enzyme@MOF/MOM platform that can host enzymes without any limitations on enzyme IEP, molecular weight, and substrate size. The different morphologies and crystallinities of the composites formed by Ca–DDVA and Zn–DDVA MOMs broaden their applications depending on the problem of interest. Our approach of enzyme immobilization not only improves the sustainability/reusability of almost all enzymes but also reduces/eliminates the use of non-sustainable resources. This synthesis method has a negligible environmental impact while the products are non-toxic to living things and the environment. The biocompatibility also makes it possible to carry out enzyme delivery/release for nutritional or biomedical applications via our “green” biocomposites. 

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4. Facile new approach to high sulfur-content materials and preparation of sulfur–lignin copolymers

   https://doi.org/10.1039/c9ta10742h  

   Karunarathna, Menisha S.; Tennyson, Andrew G.; Smith, Rhett C. (January 2020, Journal of Materials Chemistry A)

   null (Ed.)

   This report introduces a new approach to high sulfur-content materials. This route, RASP (radical-induced aryl halide/sulfur polymerization), expands the substrate scope beyond olefins required for the traditional inverse vulcanization route to such materials. RASP allows direct reaction of two unmodified industrial waste products to give lignin–sulfur composites. 

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5. Durable composites by vulcanization of oleyl-esterified lignin

   https://doi.org/10.1039/D2RA07082K  

   Karunarathna, Menisha S.; Maladeniya, Charini P.; Lauer, Moira K.; Tennyson, Andrew G.; Smith, Rhett C. (January 2023, RSC Advances)

   Productive utilization of lignocellulosic biomass is critical to the continued advancement of human civilization. Whereas the cellulose component can be efficiently upconverted to automotive fuel-grade ethanol, the lack of upconversion methods for the lignin component constitutes one of the grand challenges facing science. Lignin is an attractive feedstock for structural applications, in which its highly-crosslinked architecture can endow composite structures with high strengths. Prior work suggests that high-strength composites can be prepared by the reaction of olefin-modified lignin with sulfur. Those studies were limited to ≤5 wt% lignin, due to phase-separation of hydrophilic lignin from hydrophobic sulfur matrices. Herein we report a protocol to increase lignin hydrophobicity and thus its incorporation into sulfur-rich materials. This improvement is affected by esterifying lignin with oleic acid prior to its reaction with sulfur. This approach allowed preparation of esterified lignin–sulfur (ELS) composites comprising up to 20 wt% lignin. Two reaction temperatures were employed such that the reaction of ELS with sulfur at 180 °C would only produce S–C bonds at olefinic sites, whereas the reaction at 230 °C would produce C–S bonds at both olefin and aryl sites. Mechanistic analyses and microstructural characterization elucidated two ELS composites having compressive strength values (>20 MPa), exceeding the values observed with ordinary Portland cements. Consequently, this new method represents a way to improve lignin utilization to produce durable composites that represent sustainable alternatives to Portland cements. 

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