Study of Kraft Lignin Nitration with Acetyl Nitrate in the Presence of Aprotic Solvents

A method for homogeneous nitration of kraft lignin (KL), which consists in its treatment with acetyl nitrate in 1,4-dioxane, tetrahydrofuran, or dimethyl sulfoxide, has been proposed. The effects of reaction time and consumption of acetyl nitrate on KL nitration were studied. A new absorption band with a maximum at 430 nm due to aromatic nitro groups appears in the ionization spectra upon KL nitration. The intensity of this absorption band rises with an increase in consumption of acetyl nitrate and reaction time. At the same time, the intensities of two absorption bands of phenolic hydroxyl groups in the range of 230–300 nm decrease. At the highest acetyl nitrate consumption of 71 mmol / g KL, phenolic absorption bands are practically absent in the ionization spectrum of the products, which, apparently, can be explained by KL acetylation. To analyze the ionization spectra, they were deconvolved. The spectra of reaction products can be described by 3–5 Gaussians with an error of no more than 5 %. The proposed method makes it possible to synthesize the KL nitrated with nitrogen content of up to 2.4 %. Depending on the aprotic solvent used, the kinetic curves of nitration are different. A specific feature of KL nitration in tetrahydrofuran is the presence of an induction period, the duration of which decreases with increasing temperature and which practically disappears at temperatures above 30 °C. The introduction of small additions of nitrites into the reaction mixture makes it possible to reduce the induction period. The KL nitration in dimethyl sulfoxide occurs at a low rate that also increases significantly in the presence of sodium nitrite.