(490f) Microwave Assisted Depolymerization of Lignin to Phenolics in Polar Solvents

Lignin, a natural polymer is the third major constituent of vascular plants and provides structural support. It is separated from cellulose and hemicelluloses during the Kraft process in paper industry. An estimated 50 million tonnes of lignin is produced annually worldwide which is burned to generate process heat within the pulping industry [1]. The structure and aromatic units present in lignin makes it a valuable source of wide range of products ranging from macromolecules to phenolics. The conventional methods of lignin conversion require higher temperature and pressure conditions. Non-conventional methods such as microwave energy provide alternate ways of lignin conversion which are less energy intensive. Microwaves are electromagnetic radiation in the frequency range of 300MHz to 300 GHz. The interaction of microwaves with the reaction media is based on the mechanism of dipole rotation and ionic conduction. In this study, microwave assisted conversion of lignin was carried out in solvents, with high to medium polarity such as ethylene glycol (EG), dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF). These solvents also have the ability to dissolve alkali lignin completely at 100°C. Experiments were carried out in a microwave reactor at different temperatures (100-140°C) and reaction time (0-80min). The effect of microwave treatment on the molecular weight distribution (MWD) and average molecular weights of lignin were analyzed using gel permeation chromatograph. The phenolics produced by lignin conversion were identified using MALDI-TOF mass spectrometry and high pressure liquid chromatography. In high polarity solvent such as EG, maximum reduction in weight average molecular weight (M_w) was observed at 120 ^oC, while in medium polarity solvents such as DMSO and DMF, M_w decreased with increase in temperature. The highest yield of lignin oligomers (127.74 mg g^-1_lignin) and phenolic monomers (133.7 mg g^-1_lignin) containing acetosyringone, guaiacol, syringaldehyde, anisole was obtained at 100°C in DMF and DMSO respectively. The possible reactions for the formation of phenolic monomers and oligomers occur by the cleavage of etheric C_β-O and C_α-C_β bond induced by solvent interactions. The molecular weight distribution of lignin shifted to larger molecular weights at higher temperatures and reaction time indicating the involvement of recombination reactions.

References

1. Tolbert, A.; Akinosho, H.; Khunsupat, R.; Naskar, A.K.; Ragauskas, A.J. Characterization and analysis of the molecular weight of lignin for biorefining studies. Biofuels Bioprod. Bioref. 2014, 8, 836–856.