Maximizing production of sugar and ultrafine lignin particles from recalcitrant softwood by different acids-assisted organosolvolysis and fast pyrolysis

Abstract

Lignocellulosic biomass is an abundantly available feedstock for the sustainable production of chemicals and materials. However, total conversion of three major components within biomass (hemicellulose, cellulose and lignin) into chemicals and materials via single hydrolysis or fast pyrolysis remains a challenge because of the heterogeneous structure and recalcitrant nature of biomass, which is especially evident for softwood. In order to address these challenges, with ethylene glycol as solvent and different acids (HCl, HClO4, H2SO4, H3PO4,HCOOH, C3H6O3, or CH3COOH) as catalyst, an integrated process consisting of different acids-assisted organosolvolysis and subsequent fast pyrolysis was proposed for maximizing conversion of softwood into sugars (hemicellulosic and pyrolytic sugars) and ultrafine lignin. Mechanistic insight into acids-assisted organosolvolysis of recalcitrant pine indicated that, acid pKa and H+ concentrations were vital factors influencing the deconstruction of softwood into cellulose-rich fraction, hemicellulosic sugars and high purity ultrafine lignin. This considerably enhanced the heterolytic cleavage of cellulose into levoglucosan (LG) in subsequent fast pyrolysis. The highest LG yield of 44.45 wt% was realized by HCl due to its high pKa value and maximum H+ concentration. Total conversion efficiencies of strong acids-assisted integrated process were superior to those from medium strong and weak acids-assisted process with the highest total conversion efficiency of 61.61 wt% achieved by HClO4 and that of HCl and H2SO4 over 50.00 wt%. Moreover, prewashing feedstocks with HCl solution could effectively improve the total conversion efficiency as well as the purity and uniformity of ultrafine lignin. These findings help to provide an efficient process for maximizing the conversion efficiency of recalcitrant softwood for the production of sugars and ultrafine lignin.