Microwave-Assisted Esterification of Tall Oil Fatty Acids with Methanol Using Lignin-Based Solid Catalyst

During alkaline pulping significant amounts of lignin, carbohydrates (mostly hemicelluloses), and extractives (tall oil soap and turpentine) are removed from wood feedstock. In this study, the catalytic esterification of fatty acids in tall oil with methanol to produce fatty acid methyl esters under microwave irradiation was performed at 100 °C for 10–60 min. A novel heterogeneous acid catalyst tested for this purpose was synthesized from the hardwood alkali lignin that was precipitated by acidification from the black liquor from soda-AQ pulping. The comparative reaction data were obtained by using other solid catalysts, Amberlyst 15, and p-toluenesulfonic acid. The results showed the highest esterification yields of 93, 88, and 80% with the p-toluenesulfonic acid, lignin-based, and Amberlyst 15 catalysts, respectively, at 100 °C with a reaction time of 60 min, and the corresponding yield without catalyst was 20%. It was also observed that the lignin-based catalyst could be easily recovered and reused without any notable deactivation.