Combinatorial Study of Deep Eutectic Solvent and Ultrasonication Pretreatment in Napier Grass to Enhance Bioethanol Production.

The present study investigated the efficacy of ultrasound-assisted deep eutectic solvent (DES) pretreatment for enhancing bioethanol production from Napier grass (NG). Various pretreatment strategies, including ultrasonication (Ultra), DES, and their sequential combinations (Ultra-DES and DES-Ultra), were employed using two DES systems, including ethylene glycol: citric acid (EG: CA) and choline chloride: citric acid (ChCl: CA), as representatives of polyol-based and salt-based DESs. The effect of sonication amplitude and duration during the pretreatment step on sugar yield was assessed, revealing that optimal ultrasonic conditions (40–50% amplitude for 20 min at 20 kHz frequency) significantly improved cellulose accessibility to enzymatic hydrolysis. X-ray diffraction (XRD) analysis demonstrated an increased crystallinity index (CrI) in pretreated samples. Saccharification of pretreated biomass revealed that the combination of Ultra-DES yielded the highest glucose and xylose yields at 7.58 mg/mL and 1.76 mg/mL, respectively. These sugars were efficiently fermented into bioethanol, with the Ultra-DES (EG: CA) pretreatment achieving the maximum bioethanol yield of 0.27 g/g raw biomass, representing a 1.4-fold increase compared to the untreated sample. Mass balance analysis confirmed improved recovery and conversion of structural carbohydrates into fermentable sugars and bioethanol. This integrated approach provides the systematic evaluation of sequence-dependent pretreatment performance and offers a promising and sustainable method for valorizing Napier grass into bioethanol, contributing to the advancement of efficient biorefinery processes and renewable energy solutions.