Demethylation of Waste Alkali Lignin for Rapid and Efficient Ammonia Adsorption

Alkali lignin, a significant waste resource from the pulp and paper industry, is produced in large quantities each year, but its utilization remains limited. This study focuses on the cleavage of –OCH3 groups attached to the aromatic ring in alkali lignin to obtain demethylated alkali lignin nanospheres (DAL-NPs) enriched with aromatic hydroxyl (Ar–OH) groups. These DAL-NPs are intended for the efficient adsorption of NH3. Qualitative and quantitative characterization of alkali lignin, both pre- and post-demethylation, was performed using Fourier transform infrared spectroscopy and 1H NMR. The analysis revealed a reduction in methoxy groups and an increase in Ar–OH groups in the DAL-NPs. The NH3 adsorption experiment demonstrated that the DAL-NPs exhibited a rapid and high adsorption capacity, reaching an NH3 uptake capacity of 10.0 mmol g–1 within 0.42 min at 298 K and 1.0 bar. In addition, the DAL-NPs displayed superior selectivity based on the ideal adsorption solution theory for 10% NH3/90% N2 (3252) and 10% NH3/90% CO2 (175) at 298 K and 1.0 bar. The DAL-NPs also demonstrated reversibility and stability during 10 adsorption–desorption cycles. The DAL-NP adsorbent possesses exceptional properties, such as abundance and low cost, rapid and efficient adsorption, reversibility, and stability, making it a promising solid adsorbent for effective NH3 adsorption in the ammonia process.