Hybrid microwave-ultrasound assisted catalyst-free depolymerization of Kraft lignin to bio-oil

In this study, the simultaneous effect of microwave-ultrasound (MW + US) irradiations was compared with those of microwave (MW) and ultrasound (US) applied individually for the depolymerization of Kraft lignin in a hybrid microwave-ultrasound high pressure chemical reactor. The bio-oils obtained upon depolymerization were analyzed by GC–MS, CHNS, MALDI-TOF-MS, 2D-HSQC NMR, TGA and FTIR techniques. The highest bio-oil yield of 48.2% was obtained upon depolymerization of Kraft lignin with hybrid MW + US irradiations for 1 h under autogenous pressure while only 42.1% and 25.5% bio-oil yields were obtained for MW and US irradiations, respectively. MALDI-TOF MS spectra suggested the presence of significant amount of dimers in the bio-oil obtained over hybrid MW + US process than US and MW processes. The simultaneous effect of MW and US reduced the repolymerization of reaction intermediates thus favoring relatively low residue and high bio-oil yield. 2D-HSQC NMR demonstrated considerable degradation of the representative inter-unit linkages (β-O-4 and β-5). The HHVBoie of the bio-oils obtained over US (29.28 MJ/Kg) and MW + US (29.21 MJ/Kg) irradiations were quite comparable. Although the thermal stability of the bio-oils followed the order MW > MW + US > US, the bio-oil obtained over hybrid MW + US was superior in both quality and quantity than obtained over MW and US irradiations.