The development of the global economy faces mounting challenges in sectors like energy, food and agriculture as it seeks to combat global warming and reduce fossil fuel dependence. The COVID-19 pandemic from late 2019 has exacerbated these challenges. Rising fuel costs and heightened concerns over fossil fuel environmental impacts have compelled researchers to explore renewable biofuels production from lignocellulosic biomass more urgently (Islam et al., 2020). Bio-refining of lignocellulosic biomass shows immense potential for converting plant-based wastes into industrial products and renewables (Abraham et al., 2020). Main sources are forestry and agricultural residues, dedicated energy crops, organic municipal wastes, and industrial waste streams (wood, paper, pulp) (Roy et al., 2021).
According to surveys, around 81.5 billion tons of lignocellulosic biomass is produced globally each year, but only 4.5% is effectively utilized. Of this, 3.8% comes from forests, farms and grasslands, while the remaining 0.7% are agricultural crop residues (Dahmen et al., 2019). Traditionally, lignocellulosic biomass has been used for cooking, heating, construction and paper industries. For environmental sustainability, current research aims to produce biofuels, biochemicals and other industrial products from lignocellulosic biomass, reducing dependence on fossil fuels (Biddy et al., 2016; Patel et al., 2019; Usmani et al., 2020). As the most abundant renewable feedstock, lignocellulosic biomass can potentially meet sustainable chemical and energy needs, lowering the high reliance on fossil fuels. Therefore, efficient conversion processes need to be developed to tap into this vast unused resource. With emerging bio-refining technologies, lignocellulosic biomass promises to enable a transition from fossil fuels to sustainable bio-based fuels, chemicals and materials.
Lignocellulosic biomass has a complex three-dimensional structure, with the cellulose skeleton intricately wrapped by hemicellulose and stress-resistant lignin bound by hydrogen and covalent bonds (Zhao et al., 2012). This rigid structure makes lignocellulose highly resistant to degradation, requiring targeted pretreatments to unlock biomass conversion. Currently, various pretreatment methods have been developed to overcome lignocellulose’s structural inflexibility and improve carbohydrate accessibility. These include acid (Jędrzejczyk et al., 2019), alkaline (Keshav et al., 2016), organic solvent(Capolupo and Faraco, 2016) and ammonia fiber explosion pretreatments (Baral and Shah, 2017). Physical disruption via milling or extrusion is often incorporated as the first step before chemical pretreatments to aid in biofuel conversion (Hendriks and Zeeman, 2009). Although traditional pretreatment techniques effectively decompose lignocellulosic structure, they often cause environmental pollution and carbohydrate loss (Balan, 2014). Ionic liquids have emerged as a green solvent alternative, composed of large organic cations and small anions that can selectively dissolve lignocellulose by tuning their structure. As they exist as liquids at room temperature with negligible vapor pressure and 99% recovery, ionic liquids can disrupt the intermolecular hydrogen bonds in polysaccharides and lignin (Samayam and Schall, 2010). In a typical process, biomass is pretreated with ionic liquids at 90–130°C and ambient pressure for 1–24 hours, followed by thorough washing before enzymatic hydrolysis (Brodeur et al., 2011). The anion interacts with cellulose hydrogen bonds, disrupting its crystalline structure to yield amorphous cellulose that is more readily enzymatically hydrolyzed. However, ionic liquids can cause irreversible enzyme inactivation over time, necessitating careful recycling and recovery (Turner et al., 2003).
Ionic liquids have a high capacity to dissolve and decrystallize cellulose, overcoming the recalcitrance of lignocellulosic biomass and enhancing enzymatic hydrolysis (Brandt et al., 2013). They exhibit high thermal stability unlike traditional toxic organic solvents with low cellulose solubility. Most ionic liquids also have negligible vapor pressure, making handling and recovery easier during pretreatment. Moreover, their tunable cation and anion structures allow specific designs for targeting biomass components (Olivier-Bourbigou et al., 2010). However, high costs currently limit viability. Protic ionic liquids, generated by one-step proton transfer between Brønsted acids and bases, have simpler synthesis, lower costs and faster production than aprotic types (Chen et al., 2014). This enables selective lignin extraction for value-added products (Rocha et al., 2017).
In this work, a low-cost protic ionic liquid was synthesized as a pretreatment solvent. Corn stalks (CS) were used as the raw material for pretreatment to separate lignin and obtain polysaccharide-rich pretreated CS. Optimal pretreatment conditions were determined by investigating temperature and time effects. Ethyl levulinate (EL) was then prepared by catalytically transforming the pretreated CS using a self-made ionic liquid as of [C3H6SO3Hmim]HSO4 the catalyst. Optimal reaction conditions for EL production were found by evaluating temperature, time and catalyst loading.