Impact of Hot Compressed Water Pretreatment on the Structural Changes of Woody Biomass for Bioethanol Production

As an initial step in an alternative use of woody biomass to produce bioethanol, this work was aimed at investigating the effect of hot compressed water (HCW) pretreatment within the temperature range 100 to 200 °C in a batch-type reactor on the structural changes of Tamarix ramosissima. The untreated and pretreated solid residues were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), solid-state cross polarization/magic angle spinning (CP/MAS), C NMR spectroscopy, and thermogravimetric analysis (TGA), as well as chemical methods. The results showed that HCW pretreatment solubilized mainly hemicelluloses and resulted in enriched cellulose and lignin content in the pretreated solids. It was found that the deposition of lignin droplets on the residual surfaces was produced during pretreatment under the hot water conditions above 140 °C. In addition, the removal of hemicelluloses and lignin re-localisation as a result of condensation reactions under harsh pretreatment condition may lead to an increase in cellulose crystallinity and thermal stability of biomass solid residues, thus consequently in favor of the available accessibility for enzymes actions. Finally results showed enzyme saccharification of HCW pretreated solids at the severest conditions assayed at 200 °C resulted in an enzymatic hydrolysis yield of 88%, which was improved by 3.4-fold in comparison with the untreated material.