Physico-chemical pretreatment technologies of bioconversion efficiency of Paulownia tomentosa (Thunb.) Steud.

• Paulownia, a promising lignocellulose material was selected for bioethanol.
• Holocellulose content as high as 61% was detected from raw paulownia.
• Dilute acid treatment increased xylose yield, while alkali enhanced lignin removal.
• Total reducing sugar yield of 92.8% was achieved in ultrasonic-assisted paulownia.
• Chemicals mainly destroyed amorphous region and surface of crystalline region.

Paulownia is one of the most abundant lignocellulosic material and has great potential as a raw substrate for bioethanol production. However, the tight lignocellulose structure limits its application at present. In this study, three commercial pretreatment methods including dilute acid, alkali and ultrasonic-assisted alkali were conducted for pretreatment of paulownia under the optimal conditions, as it selectively removes the lignin and hemicelluose. Then enzymatic hydrolysis was performed to ascertain their potential for ethanol production. Component analysis suggested paulownia contained plenty of holocellulose (61.5%). In all pretreatments, the highest reducing sugar yield (42.6%) was obtained by dilute acid pretreatment, while effective delignification was achieved with ultrasonic-assisted alkali pretreatment (165.96 mg/g). Meanwhile, enzymatic hydrolysis efficiency and overall conversion efficiencies of carbohydrates (total reducing sugar yield) of the three methods were comparable. The characterization of FT-IR, XRD and SEM indicated that the three pretreatment processes damaged the amorphous region followed by crystalline region. This study provides a better understanding of the mechanism of paulownia pretreatment by acid, alkali and ultrasonic-alkali, respectively, which is essential for process optimization for bioethanol yield.