From Synthesis of Functional Lignin Nanoparticles to Their Catalyst-Free Ozonolysis: Closing The Loop

Recent years have seen a growing interest in lignin nanoparticles (LNPs) based on technical lignins extracted from plant biomass. A common method to produce LNPs involves diluting a lignin solution in an aqueous organic solvent by adding water or evaporating the organic phase. Our group has contributed to overcoming obstacles of LNPs such as their instability in aqueous and organic solvents. These approaches aim to preserve the spherical shape and allow for chemical modification before or after particle preparation. Hydroxymethylation of lignin with subsequent production of colloidal particles and their hydrothermal processing gave rise to particles that remain stable at acidic media as well as alkaline pH 13 and anhydrous polar organic solvents.[1] Another means to produce LNPs with tailored functionality involves first fractionating lignin into solvent-soluble and insoluble fractions, epoxidation of one of these fractions, and their combination in original mass ratio.[2] When epoxidized high-molecular weight lignin fraction was combined with the low Mw fraction the particles could be internally cross-linked via simple heating at 100 °C in colloidal state.<br/>Moreno et al. (2023) established a method to stabilize LNPs using only natural components.[3] By co-aggregating urushi, a black oriental lacquer, with softwood kraft lignin, hybrid particles were formed. Owing to its catechol-type structure with unsaturated hydrocarbon chains linked to the aromatic ring urushi served as a renewable stabilizer through hydration barrier effects and thermally triggered internal cross-linking. The weight ratios of the components allowed adjustable stabilization levels. Hybrid particles with Urushi content &gt;25 wt% underwent interparticle cross-linking, creating multifunctional hydrophobic coatings that enhanced water resistance of wood.<br/>Covalently stabilized lignin structures present in the aforementioned LNPs also pose questions regarding their resistance to oxidative degradation that occurs in natural environment due to oxidoreductive enzymatic activities. Likewise, if these particles were found to be persistent to advanced water purification processes there would be a risk of environmental pollution in the form of nanoplastics. Ozone is a well-known decomposition agent targeting the aromatic rings present in lignin. Because of the chemical robustness of the LNPs, their degradation should be investigated in order to close the loop. For this reason, the acidic aqueous LNP dispersion (non-covalently crosslinked) was treated with ozone and their degradation was monitored by means of DLS, SEM, FTIR, UV-Vis and ³¹P-NMR within three hours and it was compared with that of the most stable LNPs, i.e., the crosslinked hydroxymethylated ones.[4] Upon ozonolysis, the phenols were linearly converted to muconic acid structures and at longer times the degradation of LNPs was accompanied with the formation of small acidic compounds as identified by LC-MS. Although the chemical composition of the materials was significantly affected even after 15 min of ozonolysis, the size and spherical morphology of the LNPs was retained for one hour followed by their aggregation or complete degradation. Interestingly, mechanically homogenized softwood kraft lignin displayed considerable slower degradation rates compared to LNPs or HLNPs under similar conditions. The results of this study demonstrate that even chemically stabilized LNPs can be degraded with industrially relevant approaches. Further work should be directed to study biodegradation of LNPs that possess a high surface area under environmentally relevant conditions.<br/><br/><b>References</b><br/>[1] Morsali et al., Biomacromolecules 2022, 23, 11, 4597–4606.<br/>[2] Ferruti et al., Green Chem. 2023, 25, 639-649.<br/>[3] Moreno et al., Urushi as a Green Component for Thermally Curable Colloidal Lignin Particles and Hydrophobic Coatings. ACS Macro Letters 12, 2023, 759-766.<br/>[4] Alexakis et al., Manuscript.