Liquefaction Behavior of Lignin in Different Alcohol Solvents under the Catalysis of Heteropolyacid Salt

The liquefaction processes of lignin with and without heteropolyacid salt catalysis at 260–320 °C for 45 min were investigated; the effect of different liquefaction solvents (such as iso-propanol, ethanediol, and propylene glycol) on the chemical compositions of bio-oil was also studied. It is found that the maximum lignin conversion of 73.4% can be obtained when the catalyst content was 0.02 g in the presence of iso-propanol at 300 °C. The experimental results showed that the presence of heteropolyacid salt can improve the conversion of lignin; different chemical compositions were observed in bio-oil obtained from iso-propanol, ethanediol, and propylene glycol liquefaction because of the different hydrogen-donor capacity and reactions with intermediates. The residues were analyzed by Fourier transform infrared spectroscopy, scanning electron microscope, thermogravimetric analysis–differential thermoanalysis, and elemental analysis. The results demonstrated that the residues still maintain the microstructure of the lignin.