New Opportunities in the Valorization of Technical Lignins

Abstract

Sugar‐based biorefineries have faced significant economic challenges. Biorefinery lignins are often classified as low‐value products (fuel or low‐cost chemical feedstock) mainly due to low lignin purities in the crude material. However, recent research has shown that biorefinery lignins have a great chance of being successfully used as high‐value products, which in turn should result in an economy renaissance of the whole biorefinery idea. This critical review summarizes recent developments from our groups, along with the state‐of‐the‐art in the valorization of technical lignins, with the focus on biorefinery lignins. A beneficial synergistic effect of lignin and cellulose mixtures used in different applications (wood adhesives, carbon fiber and nanofibers, thermoplastics) has been demonstrated. This phenomenon causes crude biorefinery lignins, which contain a significant amount of residual crystalline cellulose, to perform superior to high‐purity lignins in certain applications. Where previously specific applications required high‐purity and/or functionalized lignins with narrow molecular weight distributions, simple green processes for upgrading crude biorefinery lignin are suggested here as an alternative. These approaches can be easily combined with lignin micro‐/nanoparticles (LMNP) production. The processes should also be cost‐efficient compared to traditional lignin modifications. Biorefinery processes allow much greater flexibility in optimizing the lignin characteristics desirable for specific applications than traditional pulping processes. Such lignin engineering, at the same time, requires an efficient strategy capable of handling large datasets to find correlations between process variables, lignin structures and properties and finally their performance in different applications.

Multifunctional lignin-based nanocomposites and nanohybrids

“…During the recent years, there have been many reviews on lignin nanoparticles and their applications, 9,[20][21][22][23][24][25][26][27][28]29 chemical and microbial transformation of lignin to chemicals and polymer precursors, 30,31 and chemical functionalization and processing of lignin into various materials (not in the nanoscale). 2,[32][33][34][35][36][37][38][39][40] Though many of the prior reviews discuss formulation of lignin-based polymers and composite systems, [41][42][43][44][45][46][47][48][49] the literature is lacking a review on nanocomposites and hybrid materials based on lignin. These two classes of materials are quickly gaining momentum.…”

“…These are kraft and soda lignins (from kraft Numbers indicate the amounts of specific units per 100 Ar (i.e., mol%). 2 and soda pulping, respectively) and lignosulfonates (sulfite pulping). On the other hand, there is a big group of technical lignins from various emerging biorefining processes, such as those obtained from the different variations of acid hydrolysis (AH), steam explosion (SE), and organosolv (OS) pretreatment.…”

“…Even more importantly, this direction looks very problematic from the economic point of view due to low yields of the target products resulting in low revenue for the lignin feedstock used. 2 In contrast, the use of spherical lignin particles (LPs) in different high-value applications can overcome low lignin reactivity without complex and expensive lignin depolymerization and thus looks as a more reasonable alternative. 9 2.3.2.…”

“…Therefore, some approaches have used model films of lignin to unveil basic properties, 73 as wel as other approaches or/and a broader set of different lignin samples have been combined with more sophisticated modeling to establish these correlations. 2 2.3.3. Combination of biorefinery lignin purification, fractionation, and reactive extraction with LMNP production.…”

“…A reason for such interest relates to the impending availability of lignins from the next generation bioproduct mills or biorefi-neries, whose economic viability will hinge on finding new uses for this abundant biomacromolecule. 1,2 Lignins are in fact one of the most important streams in the forest industry. 3,4 Not surprisingly, the most recent publications related to lignin utilization include widespread opportunities going from chemical engineering, energy and fuels, green sustainable chemistry and technology, polymer science, physical chemistry, biochemistry and materials science.…”