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.

In this work, 1-carboxypropyled (1-CPRLS) and 5-carboxypentyled lignosulfonates (5-CPELS) were synthesized using 2-chlorobutanoic acid and 6-chlorohexanoic acid as carboxylate group donors via SN1 and SN2 mechanisms, respectively. 1-Carboxypropyl and 5-carboxypentyl lignosulfonates with the charge densities of −3.45 and −2.94 meq g−1 and molecular weights of 87,900 and 42,400 g·mol−1 were produced, respectively, under mild conditions. The carboxylate content and degree of substitution (DS) of the 1-CPRLS product were 2.37 mmol·g−1 and 0.70 mol·mol−1, while those of 5-CPELS products were 2.13 mmol·g−1 and 0.66 mol·mol−1, respectively. The grafting of carboxypropyl and carboxypentyl groups to lignosulfonate was confirmed by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (1H-NMR and 13C-NMR) spectroscopies. In addition, 1-CPRLS and 5-CPELS were applied as coagulants for removing ethyl violet (EV) dye from a simulated solution, and their performance was related to their charge densities and molecular weights. Furthermore, fundamental discussion is provided on the advantages of (1) producing 1-CPRLS and (2) the superior properties and performance of 1-CPRLS to carboxyethylated lignosulfonate.

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The availability and multifunctionality of lignin makes it a desirable, renewable alternative to petroleum for specialty and bulk chemicals. However, recovering lignin from black liquor is expensive, thus lowering its prospect as a raw material source. Direct conversion of unwashed lignin (UL) to chemicals eliminates unit operations and reaction steps, which makes it a more likely feedstock. We compared the selective oxidation of lignin and UL to produce carboxylic acids over vanadium pyrophosphate (VPP), HZSM-5 zeolite, and VPP/HZSM-5 catalysts at atmospheric pressure and varying temperature (from 140 to 650 °C), steam concentration (from 0 to 20%), and oxygen (from 0 to 15.5%). The main acids produced were formic acid, acetic acid, and malonic acid. VPP/HZSM-5 produced 5% more acids with UL compared to treated lignin. The catalytic reaction network comprises both anaerobic and aerobic pathways for oxidation-based valorization of free phenolic guaiacylglycerol-β-guaiacyl ether (GG) and non-phenolic veratrylglycerol β-guaiacyl ether (VG) model lignin compounds. The three main reaction types include selective catalytic oxidative depolymerization with interunit bond breakage (e.g., alkyl–O–aryl bonds), alkyl side chain modification (e.g., Cα–OH to CαO ketone), and aromatic ring oxidation and ring opening to produce benzoquinones and dicarboxylic acids.