Nitration of Hydrolysis Lignin in Water-Aprotic Solvent Mixtures

Abstract

Industrial lignins are formed from native lignins during chemical or biochemical processing of plant raw materials. Lignins can be modified to produce valuable products, including monomers, polymeric materials, and composites. The article presents the results of a study of hydrolysis lignin nitration under various conditions. The aim of the study was to obtain a nitrated hydrolysis lignin with a maximum yield and maximum nitrogen content. Therefore, the nitration was carried out using nitric acid in a water-aprotic solvent binary mixtures (1,4-dioxane, dimethyl sulfoxide, tetrahydrofuran, dimethylformamide, acetonitrile). Acetyl nitrate, which is a mixed anhydride of nitric and acetic acids, was also used as a nitrating agent. In this regard, the consumption of acetic anhydride in the synthesis of acetyl nitrate was used taking into account the water present in concentrated nitric acid. Acetyl nitrate was obtained by the reaction of acetic anhydride and concentrated nitric acid at room temperature for 30 min. Acetyl nitrate is a mild nitrating agent opposed to nitric acid. Nitration was carried out under reflux in a boiling water bath for 2–5 min (with nitric acid) or 1–60 min (with acetyl nitrate). Upon completion of the nitration reaction, the products were filtered, washed with distilled water and dried to constant weight without heating. When nitration was performed with nitric acid, the maximum yield of nitrated hydrolysis lignin (83–101 %) was achieved using 1,4-dioxane, acetonitrile, and tetrahydrofuran; and the maximum nitrogen content (4.3–4.5 %) was achieved using 1,4-dioxane or acetonitrile. The use of dimethyl sulfoxide and dimethylformamide leads to a decrease in the product yield to 23–35 %, to a lower nitrogen content of 1.3–3.9 % and an increased oxygen content, which indicates the occurrence of not only nitration, but also depolymerization and oxidative transformations. When nitration with acetyl nitrate, the reaction takes place for 1–3 min, herewith the product contains up to 4.7 % of nitrogen. On the IR spectra of nitrated hydrolysis lignins, new absorption bands appear at 1555 and 1710 cm–1 due to the appearance of carboxyl and nitro groups.

Low Ecological Impact Lignin-Based Flame Retardants for Polyurethane Foams

“…The nitration reaction of lignin is a straightforward chemical modification process typically conducted in non-aqueous solvents utilizing nitrating agents (see Figure 5). Well-established nitrating agents encompass nitric acid in concentrated acetic acid or sulfuric acid [37]. The resulting nitrated lignin is recognized as an amorphous powder with a color spectrum spanning from yellow to brown.…”

“…Veshnyakov et al optimized lignin nitration using nitric acid in binary mixtures of water and aprotic solvents, such as acetonitrile, dimethyl sulfoxide, 1,4-dioxane, tetrahydrofuran, and dimethylformamide. They employed a nitrating agent formed by mixing acetic acid and the anhydride of nitric acid to produce nitrolignin with a nitrogen content of 4.7% [37]. In another study, Zhang and Huang investigated the thermal stability of lignin-based PU by incorporating nitrolignin into an interconnected network, forming a star-like PU from castor oil and diisocyanate [38].…”

Lignin functionalization strategies and the potential applications of its derivatives – A Review

“…Nitration causes an extensive lignin dealkylation, and nitrogen incorporation is generally around 7 wt% (Zhang and Huang 2001;Meister 2002). Lakhmanov et al (2020) performed the nitration of lignin with nitric acid in binary water-aprotic solvent mixtures. Acetyl nitrate, a mixed anhydride of nitric and acetic acids, was also used as the nitrating agent.…”