Thermocatalytic depolymerization of kraft lignin to guaiacols using HZSM-5 in alkaline water-THF co-solvent: a realistic approach

Lignin is recognized as a renewable and rich source of aromatics, which could be an alternative to fossil fuel-based petrochemicals. However, the development of an efficient catalyst and industrially scalable process for lignin depolymerization remains a challenge. The present work is therefore dedicated towards a one-pot approach for depolymerization of commercial kraft lignin in a batch reactor with higher loading using the water-THF solvent mixture over the HZSM-5 and NaOH catalytic system. Process parameters such as time, temperature, lignin-to-solvent (w/v), water-to-THF (v/v), NaOH-to-lignin (w/w), catalyst-to-lignin (w/w), and Si/Al mole ratios of HZSM5 were chosen carefully and optimized using statistical analysis - response surface methodology (RSM). A significant yield of 61% for a particular single ring aromatic compound (i.e. guaiacols) was obtained under the optimized reaction conditions: reaction time = 2 h, temperature = 533 K, lignin-to-solvent ratio = 0.060 (1 : 16.6), water-to-THF ratio = 0.86 : 1, catalyst-to-lignin ratio = 1 : 6 (0.16), NaOH-to-lignin ratio = 0.1 and Si/Al ratio = 280. Around 77% of lignin was selectively converted to guaiacols (79%) with minimal char formation (7.7%). The Hansen solubility parameter and RSM studies indicate that the obtained results were significantly related to the solubility of lignin and guaiacol at different water-THF ratios. FT-IR and H-1-NMR studies were also performed to correlate the process outcome with the presence of functional groups in identified EtOAc soluble products. A plausible mechanism for catalytic depolymerization of lignin has been proposed considering the product distribution obtained under varying process parameters.