Green and Efficient Pretreatment of Lignocellulosic Biomass for Bioethanol Production Using Deep Eutectic Solvent Synthesized from Choline Chloride, Zinc and Urea

Abstract

: Organic solvents have been the focus of numerous restrictions and legislation recently because of their detrimental impact on the environment and toxicity to human health. This research offers alternatives to the ionic liquids (ILs) as they pose danger to the environment with vehemently socioeconomic conflicts inherently in first-generation biofuels convertion. This study synthesized and evaluated two bio-based DES systems:Choline Chloride-Urea (ChCl:Urea) and Zinc Chloride-Urea (ZnCl:Urea) at a 1:2 molar ratio for the pretreatment of agricultural residues, specifically maize, rice, and corn straws. The physicochemical analysis confirmed the formation of stable, homogeneous solvent systems. The ChCl:Urea system exhibited a lower viscosity (0.94 kg/m·s) and higher conductivity (3.12 mS/cm) compared to the ZnCl:Urea system (1.15 kg/m·s and 2.61 mS/cm, respectively), suggesting superior mass transfer capabilities. Experimental results from this research work reveal that biomass pretreated with ChCl:Urea consistently achieved higher bioethanol yields across all substrates with the highest yield being observed in corn straw at 48.94%, followed by maize straw (39.24%) and rice straw (36.63%). Subsequently, the pH values of 8.95–9.73 indicate the basic nature of these and that helps facilitates the deprotonation of phenolic groups in lignin, thereby enhancing cellulose accessibility. The study also concludes that DES pretreatment, particularly using bio-based organic cations, effectively mitigates the recalcitrance of lignocellulosic matrices, providing a scalable and environmentally benign pathway for commercially viable bioethanol production.