Ultrasonic and microwave assisted organosolv pretreatment of pine wood for producing pyrolytic sugars and phenols

Abstract Efficient fermentable sugar release from softwood biomass is considerably hampered by the recalcitrant nature of its plant cell wall. Here, we reported an ultrasonic and microwave assisted ethylene glycol pretreatment coupled with fast pyrolysis to make total utilization of softwood (pine sawdust) for pyrolytic sugar (levoglucosan), hemicellulosic sugar and phenol production. Compared with oil bath heating, fractionation of pine sawdust into cellulose rich fraction, hemicellulosic sugars and organosolv lignin was enhanced drastically after applying ultrasonic and microwave heating. Whereas, ultrasonic radiation prior to microwave heating turned out to slightly inhibit the fractionation due to oxygenation of sulfuric acid. Mechanistic insight into levoglucosan formation pathway revealed that microwave heating promoted the cleavage of covalent bonds, aromatic ether linkages and glycosidic bonds in lignocellulose, leading to significant improvement in biomass recalcitrance alleviation as well as alkaline and alkaline earth metal removal. This considerably facilitated the heterolytic cleavage of cellulose into levoglucosan, while inhibiting the formation of microorganism inhibitors (e.g. carboxylic acids and phenols) and other light oxygenates in the pyrolysate. Levoglucosan yield exhibited a drastic increase from 3.53 (pine sawdust) to 48.13 wt.% via single microwave heating at 240 W, which was further enhanced with increasing microwave power. Hemicellulosic sugar derived from organosolvolysis and phenol from fast pyrolysis of organosolv lignin also showed an increase in yields with increasing pretreatment severity. Ethylene glycol pretreatment via single microwave heating at 480 W was a preferable process for achieving high yields of levoglucosan (55.87 wt.%), hemicellulosic sugars (19.96 wt.%) and phenols (14.04 wt.%). These findings help to provide an efficient process for maximizing the conversion of recalcitrant softwood for the production of fermentable sugars and phenols.