Structural changes in biomass (yellow poplar and empty fruit bunch) during hydrothermal and oxalic acid pretreatments and their effects on enzymatic hydrolysis efficiency

In this study, the structural changes in hemicellulose and lignin of the yellow poplar (YP) and empty fruit bunch (EFB) during hydrothermal and oxalic acid pretreatment were investigated and their effect on enzymatic hydrolysis was evaluated. Hemicellulose and organosolv lignin were isolated from the biomass (raw material or pretreated biomass), and their structural properties were investigated using FTIR, GPC, and NMR. The weight loss of biomass was slightly higher with oxalic acid pretreatment (31.87–33.77%) than with hydrothermal pretreatment (30.13–32.46%) regardless of the biomass species. Under the same pretreatment conditions, the weight loss of YP (32.46–33.77%) was relatively higher than that of EFB (30.13–31.87%). The enzymatic hydrolysis yield of the pretreated biomass was higher for YP (88.83–93.75%) than for EFB (86.69–89.03%). According to the thermogravimetric analysis results, the temperature required to achieve the maximum rate of hemicellulose and organosolv lignin degradation was slightly higher for EFB (276.75 °C for hemicellulose and 369.29–380.24 °C for organosolv lignin) than for YP (260.99 °C for hemicellulose and 366.67–369.74 °C for organosolv lignin). Most of the hemicellulose was removed during the pretreatment process, hemicellulose could not be obtained from either of the pretreated biomass. The organosolv lignin of the EFB (G: 190.91–361.96 and S/G ratio: 1.14–1.93) had more guaiacyl units (G), thus, its S/G ratio was lower than that of YP (G: 176.35–298.28 and S/G ratio: 1.17–2.44). The organosolv lignin of the EFB (1703−1730) had relatively high molecular weight and p-hydroxyphenyl unit compared to that of YP (1753−1845) after pretreatment. The guaiacyl and p-hydroxyphenyl units in the EFB may have a negative effect on the yields following enzymatic hydrolysis. The changes in chemical composition of the biomass during pretreatment may affect the enzymatic hydrolysis yield.