Pretreatment of lignocellulosic biomass at atmospheric conditions by using different organosolv liquors: a comparison of lignins

In order to strengthen the bio-based economy, it is necessary to produce value-added products by processing all fractions of the biomass, especially lignin. Isolation of lignin at atmospheric pressure and lower temperature will make it possible to recover lignin and use it for different purposes. In the present study, lignins were recovered from wood sawdust by applying six different organosolv treatments at atmospheric pressure. The lignins (ethanol organosolv lignin (EOL), alkaline glycerol organosolv lignin (AGOL), acetic acid organosolv lignin (AAOL), formic acid/acetic acid/water organosolv lignin (FAWOL 1 (40/40/20, v/v/v), FAWOL 2 (50/30/20, v/v/v), FAWOL 3 (30/50/20, v/v/v)) were compared in terms of yield, thermal properties, weight-average molecular weight, and fast pyrolysis properties. The effect of isolation method on lignin yield and structure was remarkable. Organosolv treatments at atmospheric conditions resulted in precipitation yields between 6.6% (ethanol organosolv) and 42% (alkaline glycerol organosolv). The lignin precipitated with the highest yield (AGOL) actually had the highest ash content (3.9%) and the lignin precipitated with the lowest yield (EOL) had the lowest ash content (1.2%). According to FTIR analysis, all lignins exhibited peaks at similar wavelengths but with different intensities. The weight-average molecular weight of lignins ranged between 1373 g/mol (AGOL) and 7400 g/mol (FAWOL 1). The polydispersity index of lignins ranged between 7.3 (FAWOL 1) and 2.5 (EOL and AAOL), where the lignins isolated from formic acid/acetic acid/water liquors showed higher polydispersity index compared to other lignins. Pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) revealed that pyrolysis product distribution was strongly affected from the isolation method. The most abundant phenolic compound, which was observed in all lignins, was 2-methoxy-4-methylphenol (4-methylguaiacol), which was followed by 2-methoxy-4- (2-propenyl) phenol (eugenol) and guaiacol, respectively. It has been observed that each lignin is a good candidate for a different end-use due to their own characteristics.