Sonophotocatalytic degradation of lignin: Production of valuable chemicals and kinetic analysis

Lignin, a key by-product of paper and pulp industry, and second-generation bioethanol refinery, is a potential source of platform chemicals due to its aromatic-rich structure. This study aims to demonstrate the synergism achieved by combining photocatalysis with ultrasound to depolymerize lignin into low molecular weight compounds. Sonophotocatalysis of lignin in alkaline aqueous medium using commercial TiO2 catalyst resulted in higher rate of degradation of lignin compared to photocatalysis. The increase in ultrasound frequency from 58 kHz to 58 + 132 kHz resulted in four-fold increase in lignin degradation rate. Upto 93 % degradation of lignin was achieved within 180 min via sonophotocatalysis. The molecular weight distribution of lignin was monitored using gel permeation chromatography to determine the change in number average and weight average molecular weights with time. At 2 g L−1 of TiO2 loading, 58 + 132 kHz ultrasound frequency, and initial lignin concentration of 100 mg L−1, the rate constant of lignin depolymerization, determined using the continuous distribution kinetics model with random scission, was 2.5 × 10-6 min−1 for sonophotocatalysis, while it was 0.93 × 10-6 min−1 for photocatalysis. The products obtained from sonophotocatalysis of lignin were 3,4-dihydroxy benzaldehyde, cinnamyl alcohol, 4-methyl-3-hexanone and succinic acid. The total yield of products from sonophotocatalysis of lignin was higher compared to that from photocatalysis. Overall, sonophotocatalysis is demonstrated to be a promising technique to degrade lignin and convert it to valuable chemicals.