Synthesis and Characterization of Carboxyethylated Lignosulfonate

Abstract

Lignosulfonate is a byproduct of the sulfite pulping process and has limited use in industry. The main objective of this study was to investigate the carboxyethylation of lignosulfonate to increase its charge density to broaden its applications. The carboxyethylation of lignosulfonate was optimized under the conditions of 30 wt % NaOH, 2.0 mol mol 2-chloropropinic acid/lignosulfonate, 90 °C, 0.5 h, and 0.03 mol 2-chloropropinic acid, which produced carboxyethylated lignosulfonate with a charge density and molecular weight of -3.51 meq g and 46 493 g mol , respectively. The mechanism of the carboxyethylation reaction using 2-chloropropinic acid by an S 1 pathway in an alkaline solution was discussed. Methylation was also used to mask the phenolic hydroxide groups of lignosulfonate to investigate if carboxyethylation occurred on aliphatic hydroxide groups of lignosulfonate. The produced carboxyethylated lignosulfonate was characterized by using FTIR spectroscopy, NMR spectroscopy, gel permeation chromatography, and elemental and functional group analyses. Basic H- H 2 D COSY NMR spectroscopy was used to record the coupled spins of the carboxyethyl group on carboxyethylated lignosulfonate. The information from 1 D H NMR and 2 D NMR COSY spectroscopy provided evidence for the existence of a 1-carboxyethyl group on the carboxyethylated lignosulfonate structure.

Grafting strategies for hydroxy groups of lignin for producing materials

“…In the meantime, sodium lactate and homopropionic acid polymer could be produced as the byproducts of carboxyethylation with 2-chloropropionic acid. Carboxyethylation of lignin would improve its hydrophilicity and generate a highly charged anionic lignin, which could be a pathway to produce pH sensitive anionic lignin 123 for use as a dispersant and flocculant in industry. This reaction reported to slightly increase the molecular weight of lignin as well.…”

“…This reaction reported to slightly increase the molecular weight of lignin as well. 123 In this reaction, lignin can be mixed with 2-chloropropionic acid, the donor of the carboxyethyl group, in a mixture of water and isopropyl alcohol in the basic environment at 60-90°C for 0.5-2 h ( Table 8). The production of sodium lactate is the undesired side reaction.…”

“…The production of sodium lactate is the undesired side reaction. 123 Due to the insolubility of the produced lignin in solvents, the reaction mixture can be mixed with ethanol for lignin isolation. 123 Alternatively, membrane dialysis and filtration can be used for isolating the product from the reaction media.…”

“…Therefore, carboxyethylation may be considered as a more influential modification pathway for lignin than carboxymethylation. 122,123 However, the main drawback of carboxyethylation is the solvent used in the reaction (e.g., isopropyl alcohol), which may not be an attractive pathway to develop green processes for lignin modification.…”

“…181 In methylation, methyl groups replace only phenolic hydroxyl groups due to their remarkably higher (about 80 times) ionization efficiency. 123,141 Table 13 shows the methylation reaction implemented on lignin. In the past, lignin was mixed with dimethyl sulfate, methyl iodide, diazomethane or tetramethylammonium hydroxide in an alkaline medium or it was dissolved in anhydrous N,N-dimethylformamide (DMF) for methylation.…”