Study On The Chemical Reactions Of Lignin In Formic Acid

Lignocellulosic biomass is the most abundant terrestrial renewable organic resource on the earth.Plant cell wall is the mass unit of lignocellulosic resources,which is chemical composed by cellulose,hemicellulose,lignin and pectin.Lignocellulose fractionation is a crucial step in biorefinery platform for the subsequent upgrading transformation.However,the traditional batch treatment employs high temperature,long reaction time,and concentrated formic acid,which leads to significant degradation of lignin and hemicellulose.Based on this fact,lignin model compound GG was used to investigate the reactions in the formic acid.Through a series of characterization of reaction products,we proposed the mechanism of GG reactions,explaining the advantages of flow-through reactor.Flow-through fractionation in a tube reactor is proposed using aqueous formic acid with concentration less than its azeotropic point,with the hope of reducing lignin and hemicellulose degradation as results of enhanced mass transfer and rapid separation of fractionation products.Comparison between batch and flow-through fractionation of wheat straw using formic acid demonstrated the advantages of flow-through treatment in the hemicellulose hydrolysis and protection of carbonhydrates.Cellulose can be proctected well in flowthrough treatment with a high retaining yield of 98.8%.Mass balance of hemicelloluse demonstrated significant loss of carbonhydrates in severity treatment conditons,which may be resulted from the degradation into furfurals,or formylation reaction.FTIR analysis suggested that water insoluble lignin(WIL)in the spent liquor had more non-conjugated carbonyl groups than milled wood lignin(MWL).The amount of non-conjugated carbonyl groups increased with intensity of treatment conditions due to formylation reaction between formic acid and hydroxyl groups in lignin side chain,which resulted in the reduction of hydroxyl groups in the side chain of lignin.Gel pemeation chromotography(GPC)analysis showed a broader molecular weight distribution and a higher polydispersity of WIL than MWL.The polydispersity of WIL showed insignificant variation among different treatment conditons.For WIL from higher treatment temperature,more fractions were observed in high molecular weight range,which suggested the enhanced delignification toward lignins with higher degree of lignification and molecular weigh.2D HSQC NMR showed a reduction of ?-O-4 of WIL compared with MWL.The amount of ?-O-4 of WIL is much higher than that of lignins from one pot treatments using ethanol or alkali,which suggested the advantage of flow-through method in preserving lignin structure.In addition,the amount and types of hydroxyl groups are similar between WIL and MWL,but the thermal stability of MWL is decreased.