External fields enhanced glycerol pretreatment of forestry waste for producing value-added pyrolytic chemicals

Abstract

Softwood forestry waste is an abundantly available lignocellulosic feedstock which could be converted into value-added chemicals via fast pyrolysis. However, its prominent recalcitrant nature resulting from the tightly intertwining of cellulose, hemicellulose and lignin as well as the catalytic effect of alkaline and alkaline earth metals (AAEMs) in it make the valorization of target pyrolytic chemicals particularly challenging. Glycerol pretreatment is a novel process to disrupt the recalcitrant structure of lignocellulose by separating it into three fractions. However, pretreatment effect from conventional heating assisted glycerolysis is poor due to low heat transfer. Introduction of external fields such as ultrasonic and microwave radiation could effectively improve the pretreatment effect. Microwave heating enabled better pretreatment effect than conventional oil bath heating due to its rapid heat transfer and athermal effect. Turbulence effects of ultrasonic radiation enhanced uniformly mixing of feedstock with glycerol, therefore further improving the pretreatment effect. The optimal glycerol pretreatment effect was realized by ultrasonic radiation integrated with microwave heating process, with pine sawdust deconstructed into 53.09 wt.% cellulose-rich fraction, 14.64 wt.% hemicellulose-derived sugars and 12.46 wt.% organosolv lignin, simultaneously removing 91.7 wt.% AAEMs. This considerably facilitated the formation of target pyrolytic chemical (levoglucosan), whilst preventing the production of undesired low molecular weight oxygenates such as acids, ketones, aldehydes. The highest yield of levoglucosan reached up to 52.07 wt.%. Kinetic analysis revealed that, higher activation energy is required for the glycosidic cleavage-based concerted reactions of cellulose into levoglucosan than AAEM catalyzed ring-breaking into light oxygenates. This work introduces an efficient method to produce value-added chemicals from forestry waste whilst reducing its hazardous impact on environment.