Microwave-assisted selective acetylation of Kraft lignin: Acetic acid as a sustainable reactant for lignin valorization.

Lignin acetylation, one of the most widespread chemical modifications used for improve the solubility of this biopolymer in organic solvents and increase polymer-lignin compatibility, has been performed for decades using time-consuming methodologies and acetylating agents with serious drawbacks. Moreover, traditional acetylation reactions generally conduce to non-selective acetylation of both aliphatic and phenolic groups. In this work, we demonstrated that partial and selective acetylation of kraft lignin can be carried out through a greener, simple and fast microwave-assisted process using acetic acid as solvent and acetylating agent. Structural characterization via FTIR, 1H-13C HSQC and 31P NMR demonstrated that acetylation reaction occurs selectively only in aliphatic hydroxyls, preserving the phenolic hydroxyls. Optimal reaction conditions were obtained using 1% (v/v) of H2SO4 as catalyst and only 5 min as reaction time. The acetylated Kraft lignin (AKL) obtained, have enhanced solubility in organic solvents (ethyl acetate, chloroform and dichloromethane) compared to unmodified Kraft lignin (KL) and antioxidant capacity almost 8 times higher than a commercial antioxidant BHT. These characteristics make the partially and selectively acetylated Kraft lignin a potential green antioxidant additive to be used in polymers blends.

Poly(methyl methacrylate) films reinforced with coconut shell lignin fractions to enhance their UV-blocking, antioxidant and thermo-mechanical properties.

Lignin is a high added-value product obtained from agrowastes through organosolv process to yield materials for technological applications. Here, coconut shell organosolv lignin was fractionated using green solvents (acetone and ethanol) and incorporated in poly(methyl methacrylate) (PMMA) films. The non-fractionated (WCSAL) and soluble fractions (ACT-F and EtOH-F) were completely characterized regarding their structures. The fractionation process altered lignins molecular weights, decreasing with the increased solvent polarity, although the higher polarity favored the dissolution of acylated and methoxylated fragments. PMMA films incorporated with lignin fractions were analyzed by TGA and DSC, which showed improved thermal and thermo-oxidative stabilities. DMA analyses of the films indicated that lignin soluble fractions had a plasticizer effect, while non-fractionated lignin increased PMMA films glass transition temperature (Tg). The antioxidant capacity of the films was also enhanced with the addition of lignins, in which those incorporated with soluble fractions showed the lowest IC50 values. The optical properties and photo-stability were also considerably improved, especially in the UVA and UVB regions. Therefore, solvent-fractionation represents a potential sustainable process to obtain lignins featuring different chemical structures, which can be applied effectively in the enhancement of PMMA films properties.

Organic solvent fractionation of acetosolv palm oil lignin: The role of its structure on the antioxidant activity.

Pressed palm oil mesocarp fibers (PPOMF) are by-products from oil palm industry and represents a potential source of lignocellulosic biomass. In order to add value to this agro-waste, dewaxed palm oil acetosolv lignin (DPOAL) was extracted under eco-friendly pulping method. The chemical composition and structural characteristics of DPOAL were investigated. The results showed elevated yield (48.5%) and high purity (94.3%), besides a moderate average molecular weight (1394 g mol-1) and narrow polydispersity index (1.88). Structural characterization via FT-IR, 1H13C HSQC and 31P NMR indicated that DPOAL was a typical HGS-type lignin. In addition, to increase the phenolic hydroxyl contents and improve DPOAL's antioxidant properties through a simple method, a fractionation process with methanol, ethanol and acetone was carried out, obtaining the methanol (MeOH-F), ethanol (EtOH-F) and acetone (ACT-F) soluble fractions. These were characterized by FT-IR, DSC, 1H13C HSQC and 31P NMR, which showed higher values of phenolic and aliphatic hydroxyls groups compared to DPOAL. The antioxidant activity was evaluated by the free radical scavenging activity of 2,2­diphenyl­1­picrylhydrazyl radicals (DPPH·) and compared with commercial antioxidants, such as BHT and Irganox 1010. Interestingly, lignin samples had significantly lower IC50 values compared to commercial antioxidants, what suggests a great potential as novel natural antioxidant.