Investigation on the production of formic and acetic acids from lignin by ethanol organosolv treatment at mild conditions

Cellulose and hemicellulose are usually considered the sources of formic and acetic acids that are obtained during ethanol pulping process, while our research reveals that lignin is another critical source of acids in the process. In this research, the sample lignin was purified and treated under ethanol pulping conditions and the factors that influence the yields of acids including: ethanol ratio (0-100%), residence time (30-210 min), reaction temperature (150-200 °C) and the effect of residual oxygen in the vessel, were tested separately. The yields of acids were identified using UPLC, the volatile products were characterized by GC-MS and the residual lignin was characterized by ¹³C NMR. The results indicated that the residual oxygen in the reaction vessel acted as an oxidant and the maximum yields of formic and acetic acid are 5.5% and 4.8% (g/g-lignin) from reed and aspen lignin, respectively. For understanding mechanism of the reaction, six lignin model compounds (LMCs) were treated and analyzed in the same reaction conditions; the subsequent results showed that both formic and acetic acid could be detected for all the LMCs tested. On the bases of the experimental results, the reaction pathways have been proposed and discussed.