Microwave Assisted Degradation of Lignin to Monolignols

Abstract

Objective: To carry out lignin degradation under very mild conditions using microwave technique. Methods: Lignin salt purchased from HiMedia laboratories was dissolved in distilled water and the solution was introduced to Biotage Initiator microwave. The reaction temperature was varied between 100-150°C with average power 85-90 W. Mass spectra were recorded in ACN-H2O (3:7) solution on BrukermicroTOF QII mass spectrometer in +ve ESI mode. Acetonitrile and water were HPLC grade and were bought from Sigma Aldrich while Sodium fluoride was purchased from Spectrochem. SEM images were recorded on ZEISS EVO LS10 scanning electron microscope with prior coating of silver using Quorum Q150R ES coating machine. Results: 1 g of lignin dissolved in 10 ml distilled water was irradiated under microwaves. Aliquots of samples were withdrawn and mass spectra were recorded after every 10 min. Maximum conversion of lignin to monolignols was observed after 90 min. Effect of NaF was also observed when catalytic amount of NaF (10 mg) was used in each reaction. Surprisingly, the results obtained in the previous conditions after 90 min were achieved only after 30 min in the presence of NaF. Conclusion: Lignin was degraded to its monomeric units when its aqueous solution in presence of catalytic amount of NaF was irradiated under microwaves at 150°C for 30 min. Hence, this technique could be used to degrade lignin into various lower lignols along with monolignols. Using this technique, revolutionary change could be made in paper, pulp and leather industry. Above all, several pharmaceutical raw materials could be synthesized efficiently from important monolignols produced from lignin degradation.