Catalytic valorization of lignin to liquid fuels over solid acid catalyst assisted by microwave heating

In this study, three types of lignin feedstocks were introduced into the microwave assisted depolymerization process over solid acid catalysts. The effects of lignin structures, solvents and catalysts were investigated to improve bio-oil yields and properties. A common phenomenon was observed that the highest bio-oil yields were all obtained in methanol under optimal conditions with microwave heating. The highest bio-oil yield from alkaline lignin, dealkaline lignin and lignosulfonate was 57.4%, 82.9% and 70.9% respectively, all obtained in methanol. The promotional effect of methanol over HSZ-640 (or HSZ-660) in the depolymerization process was confirmed due to the higher polarity of methanol, and higher acidity of catalysts. The promotional effect on the elemental composition and HHV of bio-oil was observed, finding that HSZ-640 (or HSZ-660) exhibited to be effective to convert lignin to liquid fuels with higher HHV, and the HHV of obtained bio-oil increased to about 28–32 MJ/kg, confirming that hydrogenation/hydrodeoxygenation took place during the depolymerization.