Synthesis, pretreatment, and recovery of [TEA] [HSO4] ionic liquid for OPEFB lignocellulosic transformation

In recent years, various studies have been utilized lignocellulosic biomass from oil palm empty fruit bunches (OPEFB) to be converted into bioethanol fuels. In this study, we present the techno-economical preparation for biomass pretreatment based on triethylamine hydrogen sulfate ([TEA][HSO4]) ionic liquid. Synthesis, pretreatment, and recovery of [TEA][HSO4] ionic liquid have been carried out via the one-pot method. Based on these results, the synthesized [TEA][HSO4] has been characterized using IR spectroscopy showing the appearance chemical groups such as H, CH3, CN, and SO2. In addition, H-NMR spectroscopy was presenced the CH3CH2- structure towards low ppm. Thermal stability has also demonstrated unique physical properties of ionic liquid (IL) with a melting point of 49°C and a decomposition temperature of 274.3°C. The effectiveness to produce the chemical components shows that the useful use of [TEA][HSO4] was successfully synthesized with an optimum composition of 83% resulting in chemical levels of 45.84% (cellulose), 5.00% (hemicellulose), and 34.40% (lignin). The recovered [TEA][HSO4] with a composition of 90.90% was effective in reducing the lignin content about 80%. These results also depend heavily on the temperature and separation techniques applied during the pretreatment process.


Introduction
Nowadays, the pretreatment technology (PT) has been promoted as one of the promising methods in treating lignocellulosic biomass (Kim et al., 2016;Sindhu et al., 2016;Fithri et al., 2020). It is utilized as renewable energy sources for producing bioethanol, biopesticides, sugars, and biofuels. In addition, the PT is crucially the rst stage which plays an important role in lignocellulosic biomass transformation and to overcome the abundance of lignocellulosic waste (Naik et al., 2010;Singh et al., 2010;Arbaain et al., 2019). Based on this brief background, the PT was improved to overcome the abundance of lignocellulosic biomass waste (Natsir et al., 2018b;Arutanti et al., 2020). Unfortunately, the effectiveness of lignocellulose pretreatment has not shown optimal results because it is in uenced by differences in the raw material source (Maulidiyah et al., 2017;Natsir et al., 2018a;Maulidiyah et al., 2019). According to Kassaye et al. (Kassaye et al., 2017) reported that the chemical composition in lignocellulose biomass is certainly in uenced by differences in raw materials used, for example the lignocellulose in bamboo has a varied chemical composition with differences in sugar polymer content (cellulose and hemicellulose) in the range of 38-50% or 23-32% and lignin on 15-25%.
Especially in OPEFB waste is a potential source of cellulose which can serve as a promising raw material for the production of bioethanol (Rosli et al., 2017;Hassan et al., 2020). Approximately 70% of OPEFB is produced from palm oil production, and the remaining 25-30% produces palm oil (Fithri et al., 2020).
Based on these problems, it encourages us to develop effective, cheap, and environmentally friendly pretreatment technologies. Several pretreatment methods have been reported to be applicable in obtaining cellulose from lignocellulosic biomass, such as the use of alkaline (Nargotra et al., 2018;Song et al., 2019;Yang et al., 2019), acidic (Nargotra et al., 2018), hydrothermal (Simanungkalit et al., 2017) (Mahmood et al., 2016;Halder et al., 2019), and alkaline-ionic liquid combinations Nykaza et al., 2016;Kassaye et al., 2017). Generally, these methods have also an impact on the high use of hazardous chemical solvents during the pretreatment process which can cause new environmental problems (Fithri et al., 2020).
In order to overcome this problem, IL pretreatment is a promising method as a new technology that offers a deconstruction of lignocellulose biomass with a low melting point (Alayoubi et al., 2020;Nasirpour et al., 2020). The IL has special properties such as wider uid temperatures, high thermal stability, and negligible vapor pressure (Yuan et al., 2017). Certainly, these essential properties are needed in the lignocellulose biomass transformation. In addition, the advantages of this method are environmentally friendly, high-recovery process, better deligni cation, effectively reduce the crystallinity of cellulose and do not damage the sugar content during the fermentation process (Li et al., 2010a).
Many studies have successfully synthesized and applied ILs in lignocellulosic biomass pretreatment such as using 1-ethyl-3-methylimidazolium-diethyl phosphate (Zaitsau & Verevkin, 2020), 1-Butyl-3 methylimidazolium chloride (Xie et al., 2020), 1-Ethyl-3-methylimidazolium acetate (Liang & Liu, 2020), 1allyl-3-methylimidazolium chloride (Hmad & Gargouri, 2020;Ikeguchi et al., 2020), and 1-Butyl-3methylimidazolium bis (tri uoromethyl sulfonyl) (Busato et al., 2020). Although it also shows the higheffectiveness under the pretreatment process, the use of ILs continues to be developed and modi ed in order to reduce production costs and also to improve the stability of ILs (Zhang et al., 2017;Hallett et al., 2019 (Gschwend et al., 2018). In summary, 75.9 g (750 mmol) of triethylamine was inserted into a three-neck ask and stirred by using a magnetic stirrer under cold conditions. Under stirring, it slowly added 5M H 2 SO 4 97% and ethanol solution (1:5 w/w) for 24 hours. After that, the ethanol solution was removed using a vacuum evaporator for 2 hours and dried in a vacuum oven at 40°C  To support FTIR data, we also characterize using H-NMR spectroscopy to describe the CH 3 CH 2 -structure in [TEA] [HSO 4 ]. Based on the data ( Figure 3A), we discover two high speci cal peaks that predicted from CH 3 -at 1.106 ppm and CH 2 -N at 3.005 ppm. Based on the chemical structure proposed in Figure 1 has shown the CH 3 CH 2 -a structure formed so that the direction of the chemical shift lead to low ppm. In addition, physical properties have been identi ed based on thermal analysis using the TGA instrument is important to analyze the performance of IL which will be applied to the lignocellulose pretreatment process. In the rst step ( Figure 3B), we start the thermal analysis under ambient temperature at 37°C.  Gschwend et al. (2018) that the usage of [TEA] [HSO 4 ] is intended only to observe high cellulose content so that the optimal concentration used is 83%.
Referring to the 83% [TEA] [HSO 4 ] concentration in the pretreatment process, we also tested temperature variations for the lignocellulose pretreatment process. It has been carried out with variations of 50, 80, 100, 120, and 150°C which is intended to observe IL that can be applied in low-temperature. Figure 5 shows that good performance for producing chemical content especially in cellulose compound is shown at the temperature of 80, 100, and 120°C. This is clearly an increase in cellulose content during pretreatment using the [TEA][HSO 4 ] ionic liquid. However, at 150°C the cellulose content decreases due to damage to cellulose compounds and also the IL slowly decomposes. In addition, amorphous cellulose is easily hydrolyzed because water molecules and protons easily enter the amorphous cellulose region and damage the β-1,4-glycosidic bond (Gschwend et al., 2018). Based on this variation, it is known that the optimum cellulose from the pretreatment process using [TEA] [HSO 4 ] occurs at 120°C.

[TEA][HSO 4 ] ionic liquid recovery
The potential reuse of [TEA] [HSO 4 ] for lignocellulose repretreatment is a techno-economical way to obtain optimal pretreatment processes. At this stage, the separation of IL should be carefully because the volume obtained is small and does not apply to high-scale pretreatment. In addition, the IL recovery is needed quite time-consuming so that new innovations should be improved to streamline research time. Actually, the optimization of IL recovery is a key step in biomass pretreatment because it is cheaper and reusable (Li et al., 2010b the hemicellulose monomer hydration process so that the lignin content in the high composition will decrease (Kumar et al., 2009). In addition, there is reaction stagnation caused by the terminating of β-1,4 glycosidic bonds during the addition of 72% H 2 SO 4 . Nonetheless, the decrease of lignocellulose contents is much smaller than before the pretreatment process. Declarations