Mannitol assisted oxalic acid pretreatment of poplar for the deconstruction and separation of hemicellulose

The effect of mannitol (MA) on the poplar separation using mannitol-assisted oxalic acid (M/OA) pretreatment was revealed. The effects of reaction parameters on the separation efficiency were studied. The optimized conditions were OA 7.0%, MA 7.0%, temperature 150 °C, and time 60 min. The hemicellulose separation was 79.08%. The separation of cellulose and lignin decreased from 12.43% and 21.82–8.58% and 16.11%, respectively, under hemicellulose separation similar to those of conventional OA pretreatment. Smooth fiber surfaces with less lignin deposition were observed. There were more β-O-4 and fewer β-β in lignin. Additionally, larger amounts of aliphatic hydroxyl groups and smaller amounts of phenolic hydroxyl groups were observed, which implies that MA addition prevented lignin from condensing. An effective and environmentally friendly separation of hemicellulose was achieved by M/OA pretreatment. This provides a practical approach for resolving the stubborn problems of lignin cleavage and condensation during organic acid pretreatment.