Study toward a More Reliable Approach to Elucidate the Lignin Structure-Property-Performance Correlation.

The correlation between lignin structure, its properties, and performance is crucial for lignin engineering in high-value products. Currently, a widespread approach is to compare lignins which differ by more than one parameter (i.e., Kraft vs organosolv vs lignosulfonates) in various applications by attributing the changes in their properties/performance specifically to a certain variable (i.e., phenolic -OH groups). Herein, we suggest a novel approach to overcome this issue by changing only one variable at a time while keeping all others constant before investigating the lignin properties/performance. Indulin AT (Ind-AT), a softwood Kraft lignin, was chosen as the model substrate for this study. Selective (analytical) lignin modifications were used to mask/convert specific functionalities, such as aliphatic (AliphOH) including benzylic -OH (BenzOH) and phenolic -OH (PhOH) groups, carboxyl groups (-COOH) and carbonyl groups (CO) via methylation, acetylation, and reduction. The selectivity and completeness of the reactions were verified by comprehensive NMR analysis (31P and 2D HSQC) of the modified preparations together with state-of-the-art molar mass (MM) characterization. Methylene blue (MB) adsorption, antioxidant activity, and glass transition temperature (Tg) were used to demonstrate and compare the properties/performance of the obtained modified lignins. We found that the contribution of different functionalities in the adsorption of MB follows the trend BenzOH > -COOH > AlipOH > PhOH. Noteworthy, benzylic -OH contributes ca. 3 and 2.3 times more than phenolic and aliphatic -OH, respectively. An 11% and 17% increase of Tg was observed with respect to the unmodified Indulin by methylating benzylic -OH groups and through reduction, respectively, while full acetylation/methylation of aliphatic and phenolic -OH groups resulted in lower Tg. nRSI experiments revealed that phenolic -OH play a crucial role in increasing the antioxidant activity of lignin, while both aliphatic -OH groups and -COOHs possess a detrimental effect, most likely due to H-bonding. Overall, for the first time, we provide here a reliable approach for the engineering of lignin-based products in high value applications by disclosing the role of specific lignin functionalities.

Advanced NMR Characterization of Aquasolv Omni (AqSO) Biorefinery Lignins/Lignin-Carbohydrate Complexes.

Our recently reported AquaSolv Omni (AqSO) process shows great potential as a parameter-controlled type of biorefinery, which allows tuning of structure and properties of the products towards their optimal use in high-value applications. Herein, a comprehensive NMR (quantitative 13 C, 31 P, and 2D heteronuclear single-quantum coherence) structural characterization of AqSO lignins is reported. The effect of the process severity (P-factor) and liquid-to-solid ratio (L/S) on the structure of the extracted lignins has been investigated and discussed. Low severity (P-factor in the range 400-600) and L/S=1 led to the isolation of less degraded lignin with a higher ß-O-4 content up to 34/100 Ar. Harsher processing conditions (P-factor=1000-2500) yielded more condensed lignins with a high degree of condensation up to 66 at P-factor=2000. New types of lignin moieties, such as alkyl-aryl and alkyl-alkyl chemical bonds together with novel furan oxygenated structures have been identified and quantified for the first time. In addition, the formation of lignin carbohydrate complexes bonds has been hypothesized at low severity and L/S. Based on the obtained data we were able to formulate a possible outlook of the occurring reactions during the hydrothermal treatment. Overall, such detailed structural information bridges the gap from process engineering to sustainable product development.

High-Density Lignin-Derived Carbon Nanofiber Supercapacitors with Enhanced Volumetric Energy Density.

Supercapacitors are increasingly used in short-distance electric transportation due to their long lifetime (≈15 years) and fast charging capability (>10 A g-1 ). To improve their market penetration, while minimizing onboard weight and maximizing space-efficiency, materials costs must be reduced (<10 $ kg-1 ) and the volumetric energy-density increased (>8 Wh L-1 ). Carbon nanofibers display good gravimetric capacitance, yet their marketability is hindered by their low density (0.05-0.1 g cm-3 ). Here, the authors increase the packing density of low-cost, free-standing carbon nanofiber mats (from 0.1 to 0.6 g cm-3 ) through uniaxial compression. X-ray computed tomography reveals that densification occurs by reducing the inter-fiber pore size (from 1-5 µm to 0.2-0.5 µm), which are not involved in double-layer capacitance. The improved packing density is directly proportional to the volumetric performances of the device, which reaches a volumetric capacitance of 130 F cm-3 and energy density of 6 Wh L-1 at 0.1 A g-1 using a loading of 3 mg cm-2 . The results outperform most commercial and lab-scale porous carbons synthesized from bioresources (50-100 F cm-3 , 1-3 Wh L-1 using 10 mg cm-2 ) and contribute to the scalable design of sustainable electrodes with minimal 'dead volume' for efficient supercapacitors.

New Opportunities in the Valorization of Technical Lignins.

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.

Free-standing supercapacitors from Kraft lignin nanofibers with remarkable volumetric energy density.

We have discovered a very simple method to address the challenge associated with the low volumetric energy density of free-standing carbon nanofiber electrodes for supercapacitors by electrospinning Kraft lignin in the presence of an oxidizing salt (NaNO3) and subsequent carbonization in a reducing atmosphere. The presence of the oxidative salt decreases the diameter of the resulting carbon nanofibers doubling their packing density from 0.51 to 1.03 mg cm-2 and hence doubling the volumetric energy density. At the same time, the oxidative NaNO3 salt eletrospun and carbonized together with lignin dissolved in NaOH acts as a template to increase the microporosity, thus contributing to a good gravimetric energy density. By simply adjusting the process parameters (amount of oxidizing/reducing agent), the gravimetric and volumetric energy density of the resulting lignin free-standing carbon nanofiber electrodes can be carefully tailored to fit specific power to energy demands. The areal capacitance increased from 147 mF cm-2 in the absence of NaNO3 to 350 mF cm-2 with NaNO3 translating into a volumetric energy density increase from 949 µW h cm-3 without NaNO3 to 2245 µW h cm-3 with NaNO3. Meanwhile, the gravimetric capacitance also increased from 151 F g-1 without to 192 F g-1 with NaNO3.