Influence of steaming pressure on steam explosion pretreatment of Lespedeza stalks (Lespedeza crytobotrya): Part 1. Characteristics of degraded cellulose

A two-step process based on steam explosion pretreatment followed by alkaline ethanol solution post-treatment was used to fractionate Lespedeza stalks (Lespedeza cyrtobotrya). Steam explosion pretreatment, under at 15 kg/m2 to 25 kg/m2 for 4 min, followed by post-treatment with 60% aqueous ethanol containing 1% NaOH yielded 49.6-65.5% (% dry matter) cellulose rich fractions, compared to 68.6% from non-pretreated material. It was found that the content of glucose was gradually increased from 73.7 to 86.9% as the result of elevating steaming pressure, but the solubilisation of lignin maintained the same level (about 10-11%) regardless of the severity. The average degree of polymerization increased first and then decreased, revealing that autohydrolysis reactions were dominant in different regions during the steam explosion. Scanning electron microscopy images of the cellulosic residues show that steam explosion mainly resulted in breakage of the fibres, and extraction post-treatment led to solution of lignin (and hemicelluloses) and significant defibrillation. The increase of onset degradation temperature, together with the higher pyrolysis residues suggest that the thermal stability of cellulose rich fractions was increased by steam explosion and elevated steaming pressure. All the rich-in-cellulose fractions were further characterized by FT-IR, XRD, and CP/MAS 13C NMR spectroscopy.