Assessment of microwave pretreatments using acidic and alkaline DES for rice straw valorization

Developing efficient and sustainable pretreatment methods is essential to overcome lignocellulosic biomass recalcitrance and enable its full valorization. This study evaluates acidic and alkaline deep eutectic solvents (DES) for rice straw pretreatment, focusing on comparing conductive heating and microwave (MW) irradiation. Acidic DES formulated from choline chloride (ChCl) or ethylammonium chloride (EAC) with lactic acid (LA) or acetic acid (AA) were first tested under both heating methods. MW-assisted pretreatments achieved comparable yields in half the time, demonstrating greater efficiency. Based on these results, MW-assisted pretreatment was applied to compare acidic and alkaline DES, the latter prepared from ChCl or EAC with monoethanolamine (MEA) or imidazolium (IMI). Alkaline DES showed superior performance, particularly ChCl:MEA [1:6], which at 150 °C and 10% solids loading yielded 88.2% glucose, 62.4% xylose, and 79.5% total polysaccharide conversion. To enhance industrial relevance, higher solids loadings (15–20% w/w) were evaluated, achieving 75.7% glucose and 54.0% xylose at 20% w/w with ChCl:MEA [1:6]. Structural analyses by X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy confirmed reduced crystallinity and increased surface disruption relative to untreated biomass. A life cycle assessment demonstrated that MW-assisted pretreatment using ChCl:MEA [1:6] at 150 °C and 20% solids loading results in lower environmental impacts than other green solvent-based pretreatments. Overall, this work highlights the potential of alkaline DES under MW irradiation as an effective and environmentally favorable strategy for rice straw deconstruction within biorefinery systems. • DES pretreatment using MW-assisted heating outperformed conductive heating. • Alkaline DES outperformed acidic ones, reaching 88% glucose and 62% xylose yields. • Alkaline DES pretreatment at high loads (15-20% w/w) maintained high sugar yields. • LCA compared environmental impact of three pretreatments using green solvents.