**1. Introduction**

As a result of being one of the largest crude palm oil producers in the world, Indonesia produces around 37 million tons of oil palm empty fruit bunch (OPEFB) yearly [1]. Indonesian oil palm plants are located and concentrated in some areas. For instance, more than 50% of the OPEFB is produced in 5 regions in Sumatra [2]. OPEFB accumulation can cause environmental problems including the proliferation of disease-causing pests and microorganisms, the requirement of extensive land for burying, and gas emissions originated from its combustion [3,4]. Therefore, handling and valorization

of OPEFB are crucial, and at a starting point, the existence of areas with higher production of palm oil can be used as an advantage for valorization by decreasing transportation costs [2].

Being a lignocellulosic material, OPEFB is mainly composed of cellulose, hemicellulose, and lignin that account to 24–65 wt%, 21–34 wt%, and 14–31 wt% of the material, respectively [5]. The holocellulosic (cellulose and hemicellulose) fragments can be enzymatically converted into simple sugars [6] for further conversion into value-added products. However, the recalcitrance of OPEFB hinders easy access to its carbohydrate polymers. In general, enzymatic digestibility of lignocellulosic materials is limited by a number of factors such as the presence of lignin, cellulose crystallinity, degree of polymerization, acetyl groups bound to hemicellulose, surface area, and biomass particle size [7,8]. Hence, pretreatment is needed to open the lignocellulosic structure of OPEFB and have access to sugar polymers for further valorization.

Pretreatment of lignocelluloses can be performed by mechanical, chemical, enzymatic, and biological processes, or by a combination of these [9]. Chemical pretreatment is a strong and effective pretreatment to improve the digestibility of lignocellulosic biomass, applying acids, bases, or other catalysts such as hydrogen peroxide and ozone [10], with dissimilar end-results. For instance, in acid pretreatment, deconstruction of the lignocellulosic structure is carried out mainly through dissolution of hemicellulose, leaving lignin in the solid fraction together with cellulose [11]. The remaining lignin will interfere with the following enzymatic hydrolysis of cellulose. Alkali pretreatment can easily break the lignin bonds and enhance the solubilization of the polymer [12]. In this method, lignin is effectively removed; however, the lignin dissolved in the liquid is difficult to recover. Ozone pretreatment is safer than alkaline and acid pretreatments and leads to efficient lignin removal. Nonetheless, lignin recovery remains a hurdle and ozone generation is very expensive [13]. Hydrogen peroxide pretreatment is efficient towards the removal of lignin and xylan [14], but it is very toxic for the environment. Therefore, none of the pretreatment methods described can efficiently recover the lignin from the lignocellulose.

Lignin has normally been considered as a barrier to properly access carbohydrate polymers. As a low-value stream in lignocellulosic biorefineries, lignin is normally used for heat and power generation through combustion [15]. However, as a result of feasibility concerns of lignocellulosic biorefineries and increasing range of high-value applications for lignin, higher attention has been devoted to the full valorization of lignocellulose-derived polymers. In fact, high purity lignin can be utilized for the production of value-added products such as resin, flavor compounds, and nanofibers with antioxidant activity [16–18]. Hence, pretreatment methods that enable both efficient lignin removal from lignocelluloses and easy recovery into a high-purity lignin fraction, can positively contribute to the feasibility of lignocellulosic biorefineries.

The use of organic solvent (organosolv) for pretreatment is a promising strategy. The organic solvent is able to extract lignin and hydrolyses the hemicellulose [19]. A high-purity cellulose with only minor degradation and a high proportion of its amorphous phase, which is susceptible to enzymatic hydrolysis, can be recovered [20]. The extracted lignin can be further precipitated by dilution with water and recovered as a solid product, while hemicellulosic sugars remain in the liquid stream [19]. The organic solvent can easily be recovered using evaporation or distillation processes.

There are several parameters affecting the success of the fractionation process, especially delignification, during organosolv pretreatment such as solvent type and concentration, catalyst type and amount, temperature, retention time, and solid to liquid ratio (S/L ratio) [21]. Ethanol is a solvent that is frequently used for organosolv pretreatment of lignocellulosic biomass due to its low price, good solubility of lignin, lower toxicity compared to other alcohol-based solvents, its miscibility with water, and ease of recovery [22]. Moreover, the production of ethanol using sugars and starch-rich substrates is an industrially mature technology well distributed throughout the world; ca. 29,100 million gallons were produced in 2019 [23]. In the organosolv pretreatment, acids and bases can be added as catalysts to increase the delignification rate, whereas comparatively lower delignification rates (≤60%) are normally obtained during non-catalyzed organosolv pretreatment of

lignocellulosic biomass [24,25]. Mineral acids have high reactivity and efficiency and sulfuric acid is the most studied catalyst for ethanol-organosolv pretreatment [26,27]. Higher S/L ratio is favorable because a smaller amount of solvent is employed and a better balance can be found among energy needed to carry out the pretreatment step and the amount of processed material. Overall, an organosolv pretreatment strategy that ensures high S/L ratio, minimum addition of acid and solvent, and optimum temperature and retention time, while attaining fractions that are rich in high-quality glucan, lignin, and hemicellulosic compounds, are of interest for the valorization of lignocellulosic materials.

The research landscape on organosolv pretreatment is characterized by a wide range of substrates and pretreatment conditions investigated, but by a lack of systematic studies to reveal unequivocally substrate-tailored pretreatment systems [28]. For instance, various organosolv strategies have been used for the pretreatment of OPEFB, as a result of extensive research on the development of efficient biorefinery systems for its valorization. These included the use of bisulfite, a mixture of acetic acid and ammonia, or ethylene glycol as solvents, where studies on ethanol organosolv pretreatment of OPEFB are scarce in literature [29–34]. Moreover, information about the influence of pretreatment parameters on lignin recovery and purity following organosolv pretreatment is still scarce in the literature, and it is common to all lignocellulosic substrates studied [28]. A recent study has shown that organosolv pretreatment conditions influence the recovery and purity of lignin from oat husks [35]. Therefore, the aim of the present study was to study the effect of acid-catalyzed ethanol organosolv pretreatment on the delignification of OPEFB. A range of parameters was studied in organosolv pretreatment of OPEFB, namely, acid type and concentration, temperature, retention time, and S/L ratio. Emphasis was given to lignin purity and lignin recovery as well as the digestibility of the glucan-rich fraction. The optimization steps carried out in this study led to high lignin purity and recovery. In addition, digestible glucan-rich fraction which has high glucan purity and recovery was obtained. The results obtained demonstrate the possibility to use very low acid concentration for deconstruction of OPEFB into high-quality fractions.
