Natural Deep Eutectic Solvent-Assisted Hydrothermal Carbonization of Corn Stover for Producing Lignin-Rich Solid Fuel and Sugar-Rich Intermediates

Abstract

The sustainable conversion of agricultural waste biomass, particularly crop residues such as corn stover, into high-value products is vital for reducing their open-field burning and mitigating environmental hazards. The hydrothermal carbonization (HTC) process integrated with natural deep eutectic solvents (NADES) presents an alternative approach for valorizing biomass into lignin-rich solid fuels and fermentable sugars for bioethanol production. In this study, corn stover was subjected to HTC using deionized (DI) water, a xylose-based NADES (ChCl:Xy:W), and an oxalic acid-based NADES (ChCl:OA:W) in a 150–300 °C temperature range to optimize both solid fuel and sugar stream yields. Characterization, including fiber analysis, SEM, FTIR, EDS, and bomb calorimetry, was conducted to evaluate structural, compositional, and energetic transformations. The results explored the HTC process, restructuring the biomass, promoting extensive hemicellulose solubilization and cellulose depolymerization, as well as substantially enriching lignin and polymerized compounds with increasing temperature. In addition, the DI water at 300 °C generated a lignin-rich residue, the Xy-based NADES effectively removed ash and extractives, and the OA-based NADES produced the most carbon-dense hydrochar with the highest calorific value. Collectively, these findings demonstrate that solvent-assisted HTC may be employed as a possible strategy for the valorization of agricultural residues into high-energy solid fuels.

1. Introduction

Globally, the open-field burning of biomass, particularly crop leftovers, contributes enormous amounts of greenhouse gases (GHGs) to the atmosphere, leading to environmental degradation and ozone depletion [1,2]. Worldwide, biomass production has been estimated to be over 170 billion metric tons per annum [2,3,4]. Biomass, after pretreatment, has many possibilities for use, including as nanocomposite films [5,6,7]. Among biomass sources, corn straw (stover) offers numerous advantages, such as abundance, fast growth, a short life cycle, and low cost. Corn stover entails three types of biopolymers, including lignin (10–15%), hemicellulose (20–30%), and cellulose (40–50%) [8,9]. Increasing the value of corn stover may pave the way to alleviate the fossil fuel shortage and reduce environmental pollution.

Hydrothermal carbonization (HTC) is a technique that combines dehydration and decarboxylation processes for a corn stover sample to elevate its carbon content to achieve a higher calorific value [10]. Additionally, the HTC process encompasses converting stover into carbon-rich solids (commonly referred to as hydrochar) by employing heat and pressure in the presence of liquid water. Basically, it is wet torrefaction, involving the thermochemical conversion of wet biomass into a valuable, carbon-rich solid product using sub-critical liquid water as a reaction medium. However, to improve corn stover-based bioethanol production, pretreatment of stover is required prior to enzymatic hydrolysis to separate lignin and holocellulose (which is cellulose and hemicellulose combined) [11,12,13]. HTC is often compared to pyrolysis, but pyrolysis requires pre-drying of the biomass and requires higher temperatures [13]. Steam explosion is another possible pretreatment; however, HTC tends to be better at improving the hydrochar produced.

For thermochemical and biochemical conversion of biomass, pretreatment (PT) induces structural alterations to overcome corn stover’s recalcitrant nature [14]. The polymeric and aromatic constituents (cellulose, hemicellulose, and lignin) control the degradation ability of corn stover during the PT processes, requiring heat. As a result, a product with numerous structural changes and a carbon framework is formed, leading to a significant increase in the performance of the pretreated corn stover in bioconversion processes [15]. However, research gaps exist concerning the development of pretreatment technologies that involve minimal degradation of vital components of corn stover. Therefore, research concerning corn stover pretreatment methods specifically tailored for efficient bioconversion to facilitate the bio-refinery processes for biofuel production is required. Water HTC limits the possibilities for the obtained conversion products, and generally does not produce the same severity at a given temperature as when other components, such as acid, are available in the reaction environment [9]. Temperatures lower than 220 °C with water HTC do not significantly change the structure of corn stover, indicating that other solvents may be more effective [13]. The need for higher temperatures for water HTC to change the solid hydrochar properties suggests that further exploration of HTC using various types of solvents is needed [13]. Looking beyond corn stover, more recalcitrant biomass may require other types of solvents to keep the treatment temperature at a reasonable level [14]. This bottleneck requires further study involving less common solvents.

Other solvents, such as ionic liquids or natural deep eutectic solvents (NADES), have not been thoroughly researched in HTC pretreatment, suggesting a research gap in HTC. By virtue of high conductivity, low vapor pressure, and extensive designability, ionic liquids have been regarded as promising and potent green solvents for the PT of corn stover. However, NADES have recently been highlighted as green and efficient alternatives to ionic liquids for the conversion of biomass [16]. NADES are generally mixtures comprising two or more compounds containing hydrogen bond donors and acceptors of hydrogen bonds. This hydrogen bond interaction causes components of NADES to form a stable and uniform solvent system. NADES offer a wide range of advantages, including convenient and rapid preparation techniques, minimal or no requirements for purification, and economic feasibility [17]. Previously, a variety of NADES were synthesized using organic acids and choline chloride (ChCl) mixtures that remained effective in triggering the solubilization of lignocellulosic biomass [13]. Likewise, polyol-based NADES have proven their efficacy in improving the performance of hydrolysis by increasing enzyme activity [17]. Moreover, acid-based NADES have also been synthesized and tested, which exhibited pronounced efficiency in lignin extraction, with depolymerization reactions occurring [17,18].

The corn stover PT process involving NADES is a complex reaction system that is affected by several factors, especially NADES traits and pretreatment conditions. Biomass type and particle size of samples, types of solvents, and temperature were the most pronounced factors that affected NADES efficiency [19,20]. Hydroxyl bonds are thought to play a vital role in facilitating the removal of lignin, along with glucan recovery [21].

However, research and knowledge gaps exist concerning the efficacy of NADES for converting corn stover during the HTC process. Thus, it was hypothesized that choline chloride-based NADES hold the potential to facilitate the extraction of lignin for solid fuel production by causing glycosidic bond cleavage among the constituents of the biomass, including corn stover. The hypothesis was that NADES HTC would increase the lignin content of the hydrochar as measured by the National Laboratory of the Rockies (NREL) fiber analysis procedures. The limitations of this study, therefore, include the fact that these fiber analysis procedures do not quantify the pseudo-lignin produced (from the condensation of cellulose and other components) as separate from lignin. The primary aim of this investigation was to subject corn stover to HTC with various solvents to determine the fiber and ash content, lignin, cellulose, and hemicellulose content, along with the heat of combustion, using cutting-edge spectroscopic techniques, including bomb calorimetry and Fourier transform infrared spectroscopy (FTIR). The components chosen all naturally occur and are generally considered to be safe for addition to food products. The NADES selected both contain choline chloride (ChCl) as a hydrogen bond acceptor. ChCl is a supplement taken by people who limit foods in their diet that contain it, such as eggs. The hydrogen bond donors used in the two different NADES are xylose and oxalic acid. They are different in that xylose, commonly found in fruits, is pH neutral, and its OH groups likely interact with the OH groups in cellulose to disrupt the crystalline nature of cellulose in corn stover [16]. Oxalic acid, found in a variety of foods, has a low pH, leading to added H⁺ ion concentration when part of a NADES. Thus, the H⁺ ions would be expected to break the β-1,4 glycosidic bond between the glucose monomers that form cellulose. For these reasons, both types of NADES would be expected to disrupt cellulose, making it more soluble, and thus concentrating lignin in the solid hydrochar.

2. Materials and Methods

2.1. Materials

Milled corn stover was obtained from the Idaho National Laboratory (Idaho Falls, ID, USA). Thereafter, at the Biomass laboratory of the Chemical Engineering Department (Louisiana Tech University, Ruston, LA, USA), the milled corn stover was meshed using a Gilson orbital sieve shaker (Gilson Inc., Lewis Center, OH, USA) to obtain the fraction between mesh no. 40 and 25, giving a particle size of 0.707 mm to 0.420 mm. This particle size was chosen as it has been found to be suitable in previous studies [13]. The meshed corn stover was dried at 105 °C for 24 h prior to pretreatment. Oxalic acid dihydrate (reagent grade, ≥99%) was purchased from Flinn Scientific (Batavia, IL, USA), and choline chloride (BioReagent suitable for cell culture, ≥99%) was purchased from Sigma-Aldrich Corp. (St. Louis, MO, USA).

2.2. Methods

2.2.1. Preparation of NADES

Choline chloride (ChCl), serving as a hydrogen bond acceptor, was combined with oxalic acid (OA) or xylose (Xy), both acting as hydrogen bond donors, in a ternary mixture with water (W) at mole ratios of 2:1:2 and 5:2:14, respectively. Different amounts of water in the solvents were used to ensure the solvents had similar densities. These mixtures were placed in an orbital shaker at 80 °C for 2 h. After 1 h, the initially cloudy deep eutectic solvents became clear solutions, achieving full transparency by the end of the 2 h period. The homogeneous solutions were then sealed and stored at room temperature, where they remained clear and free of solid particles for up to several months. The physicochemical properties of the Xy-based and OA-based solvents at ambient temperature are summarized in

Table 1. Physiochemical characteristics of solvents at ambient temperature.

Solvent Name	Conductivity (mS/cm)	Viscosity (mPa·s)	Density (g/mL)	pH
ChCl:OA:W (2:1:2)	2.31	541	1.252	0.00
ChCl:Xy:W (5:2:14)	6.86	386	1.204	3.76

. The viscosity at room temperature was sufficiently low to flow during its initial entry into the reactor vessel and would decrease at the temperatures used during HTC.

2.2.2. Hydrothermal Carbonization (HTC) of Corn Stover

Hydrothermal carbonization (HTC) is a thermochemical conversion process that transforms lignocellulosic biomass, such as corn stover, into a carbon-rich solid in the presence of either plain water or a green NADES under elevated temperature. This process accelerates the natural carbonization that typically occurs over geological timescales, enabling efficient biomass transformation in a controlled environment. In this study, HTC of corn stover was performed using a 2000 mL Parr bench-top reactor (Model 4524, Moline, IL, USA), operated at four target temperatures—150 °C, 200 °C, 250 °C, and 300 °C—regulated by a Parr PID controller (Model 4848, Moline, IL, USA). For each run, 15 ± 0.02 g of oven-dried corn stover was loaded into a tempered glass reactor vessel along with 150 ± 0.08 g of solvent (biomass-to-solvent ratio 1:10). Three solvent systems were evaluated: ChCl:OA:W (2:1:2), ChCl:Xy:W (5:2:14), and deionized (DI) water. The reactor was heated to the designated temperature and held for 5 min once the set point was reached. Five minutes was chosen as a time that is sufficient for the desired reactions to occur, as shown in previous studies [13]. During heating, pressure remained low for temperatures below 100 °C but increased sharply thereafter due to water vaporization and the formation of gaseous products such as carbon dioxide and methane. Depending on the operating temperature and type of solvent used for pretreatment, reactor pressure varied from approximately 1000–1400 kPa (150–200 psi) at 150 °C to 1400–3700 kPa (205–540 psi) at 200–250 °C, reaching 4000 kPa (580 psi) at 300 °C. Initial work indicated that no reactions occurred for DI water nor Xy-based solvents at 150 °C, so these were not considered in the analysis. Representative temperature and pressure profiles for DI water pretreatment at 250 °C are shown in Figure 1.

After reaction, the vessel was cooled in a tap water bath to 30 °C. Depending on the pretreatment solvent and the temperature settings of pretreatment, the average time required to reach the desired temperature in the reactor was about 50–65 min, and the average cooling time to reach ambient temperature was about 60–70 min. Despite cooling, residual pressure remained due to non-condensable gases, stabilizing at 70–100 kPa (10–15 psi) for DI water runs and 275–340 kPa (40–50 psi) for NADES-based pretreatments. This pressure was released through the reactor’s relief valve, and mass measurements before and after HTC were used to calculate the amount of gas formed. Following pressure release, the reactor contents were filtered to separate the solid residue (hydrochar), enriched in lignin, from the sugar-laden liquid containing dissolved cellulose and hemicellulose. For OA-based NADES pretreatment, the solid residue was additionally rinsed with 500 mL of DI water to remove residual solvent. Ash and extractives were expected to remain in the liquid phase due to their solubilization during high-temperature HTC. Solid residues were filtered using 0.6 µm nylon Whatman filters, while the filtrates—rich in pentose and hexose sugars—were refrigerated for subsequent analysis. The washed hydrochar samples were dried at 105 °C for 24 h, weighed, and stored in sealed airtight bags for further characterization. Possible sources of error include the incorporation of the solvent in the pores of the hydrochar. All HTC experiments were conducted in triplicate, and results are reported in this study as mean values with standard errors.

2.2.3. Fiber Analysis

After HTC pretreatment with three different solvents across four temperature levels, the resulting solid residue, enriched primarily in lignin, was analyzed through saccharification via acid hydrolysis following the National Laboratory of the Rockies (NREL) protocol LAP/TP-510-426. In this procedure, the holocellulose fraction (cellulose + hemicellulose) in the solid residue was solubilized using 72% sulfuric acid, while the acid-insoluble components—lignin and ash/extractives—were recovered through filtration and quantified gravimetrically. The holocellulose content was then determined indirectly by subtracting the measured lignin and ash/extractive mass from the total mass of the HTC-pretreated solid residue.

Lignin in raw corn stover (CS) or HTC-pretreated CS solid residue consists of acid-insoluble residue, which includes both acid-insoluble lignin (AIL) and acid-insoluble ash (AIL), along with a small portion of acid-soluble lignin (ASL). It may also include cellulose that has reacted to become more insoluble, also known as pseudo-lignin [13]. The AIL represents the residue remaining after extensive acid hydrolysis of the raw corn stover or HTC pretreated solid residue, while ASL refers to the portion of lignin that becomes solubilized during the hydrolysis process. The acid insoluble residue (AIR) and AIL were determined gravimetrically, while ASL was quantified using a UV–Vis spectrophotometer (Shimadzu, Kyoto, Japan). In this process, approximately 0.3 ± 0.01 g of raw corn stover or HTC-pretreated solid residue (denoted as m_s) was mixed with 3 mL of 72% w/w sulfuric acid (H₂SO₄) in a test tube, placed in a water bath at 30 °C, and hydrolyzed for 2 h, with stirring every 10 min to ensure proper mixing and wetting. The mixture was then transferred to an autoclavable bottle and diluted to a 4% acid concentration by adding 84 mL of water. The sample was autoclaved for 1 h at 121 °C and allowed to cool to room temperature. The hydrolyzed solution was filtered through a pre-dried 140 mL borosilicate glass Gooch crucible with a fritted disc, using a vacuum pump. Approximately 30 mL of the filtrate was used to determine the ASL. The remaining residue, AIR, which includes both AIL and acid insoluble ash (AIA), was dried at 105 °C for 24 h to obtain a constant weight ($m_{AIL+AIA}$). The AIR in the Gooch glass crucible was combusted at 575 °C for 24 h to determine the AIA content. The percentages of AIL and AIA in the raw or HTC-pretreated corn stover solid residue were calculated using the following formulas:

$$\%AIR={\displaystyle \frac{m_{AIL+AIA}}{m_s}} \ast 100\%$$

$$\%AIL={\displaystyle \frac{m_{AIL+AIA}-m_{AIA}}{m_s}} \ast 100\%$$

$$\%AIA={\displaystyle \frac{m_{AIA}}{m_s}} \ast 100\%$$

where ${\mathit{m}}_{\mathit{s}}$ is the mass of the sample, either raw CS or HTC-pretreated CS solid residue, ${\mathit{m}}_{\mathit{A}\mathit{I}\mathit{L}+\mathit{A}\mathit{I}\mathit{A}}$ is the mass of acid-insoluble lignin (AIL) and AIA (g), ${\mathit{m}}_{\mathit{A}\mathit{I}\mathit{A}}$ is the mass of AIA (g).

UV–Vis spectroscopy was utilized to quantify acid-soluble lignin (ASL) in both raw corn stover and HTC-pretreated corn stover solid residue, as suggested by the NREL protocol. At a wavelength of 198 nm, lignin tends to demonstrate strong absorbance due to its aromatic structure, which is beyond the UV–Vis spectrometer range (200–600 nm). A 4% sulfuric acid solution was used as the reference blank. In this procedure, the corn stover samples underwent acid hydrolysis with 72% sulfuric acid, after which the solution was diluted to approximately 4% acid concentration. The lignin concentration in the resulting solution was measured using a UV–Vis spectrophotometer (Model UV-2401PC, Shimadzu Corporation, Kyoto, Japan). In this study, an absorbance band associated with acid-soluble lignin in the solid residues pretreated with DI water, Xy-based solvent, and OA-based solvent was consistently observed at 284 nm. The percentage of ASL in the sample was calculated using the formula:

$$\%ASL={\displaystyle \frac{A\times V\times D_f}{\epsilon \times m_s\times L}} \ast 100\%$$

where A is the absorbance at 284 nm, V is the volume of filtrate (87 mL), D_f is the dilution factor, ε is the extinction coefficient (30 L/g·cm for lignin), ${\mathit{m}}_{\mathit{s}}$ is the mass of the sample (g), and L is the path length of the cuvette (cm). The wavelength–absorbance profile obtained from the UV–Vis spectrometer for a representative DI-water pretreatment at 25 °C is shown in Figure 2.

The total lignin content was calculated as follows:

$$\%Lignin=\%AIR+\%ASL$$

Holocellulose (combination of cellulose and hemicellulose) analysis of raw CS or HTC-pretreated solid residue in fiber analysis aims to quantify the combined content of cellulose and hemicellulose. The percentage of holocellulose (%HC) was calculated as follows:

$$\%HC=100\%-\%AIR-\%ASL$$

2.2.4. Other Characterization Methods

Scanning electron microscopy (SEM) was used to examine the surface morphology of raw corn stover and HTC-pretreated solid residues. Samples were sputter-coated with gold to ensure conductivity and imaged using a Bruker S-4800 SEM (Billerica, MA, USA) at accelerating voltages of 3 kV with magnification of 500–10,000 times. Microstructural changes were analyzed to assess pretreatment effects. Potential sources of experimental uncertainty include the areas chosen for SEM.

Energy-dispersive X-ray spectroscopy (EDS) coupled with SEM was performed to determine the elemental composition of raw corn stover and HTC-pretreated solid residues. Samples were mounted on carbon tape and analyzed using an Oxford EDS detector (Chiyoda City, Tokyo, Japan) at 0–10 kV to quantify major elements and evaluate carbon enrichment and mineral removal.

Fourier-transform infrared spectroscopy (FTIR) was utilized to analyze the molecular bonds of the various compounds. In this study, a Nicolet 6700 FTIR (Thermo Fisher Scientific, Madison, WI, USA) was utilized to investigate the changes in the molecular structure of corn stover subjected to HTC pretreatment with three different solvents across four temperature levels. Finely ground raw corn stover, processed using a planetary ball mill, and HTC-pretreated corn stover solid residue were mixed with potassium bromide (KBr) to form pellets. These pellets were analyzed using the Nicolet 6700 FTIR, with each sample undergoing 32 scans at a resolution of 2 cm⁻¹ over the spectral range of 4000–600 cm⁻¹, measuring the vibration intensities at lower wavenumbers.

In this study, a bomb calorimeter (Model 1341EB Bomb Calorimeter, Parr Instrument Company, Moline, IL, USA) was used to evaluate the energy content of raw corn stover (CS) and HTC-pretreated CS solid residues. In this experiment, a sample of raw CS and HTC-pretreated CS solid residue, weighing between 0.4 and 0.5 g, was placed in a crucible. A copper fuse wire, 10 cm in length and weighing approximately 0.0145 g, was crimped between two electrodes so that the wire was in contact with the sample. The bomb container was then sealed and filled with oxygen at a pressure of 30 atm. The sealed container was placed inside the calorimeter vessel, which was filled with 1 L of DI water. The stirrer was continuously used to achieve a uniform and stable initial water temperature, which was recorded after 3 min of stirring. The sample was ignited using an electric spark (Model 2901EB Ignition Unit 115V, Parr Instrument Company, Moline, IL, USA), initiating the combustion reaction within the bomb. The heat generated by the combustion was transferred to the surrounding water, causing a temperature rise. This temperature increase was recorded at 30 s intervals. Initially, the temperature rose rapidly before stabilizing after approximately 10 min. The final temperature was then documented. Subsequently, the crucible and fuse wire were reweighed to determine the ash content and any remaining fuse wire. The heat of combustion (high heating value, HHV) of the sample was calculated using the following equation:

$$H_{HHV}={\displaystyle \frac{C_{p,w}\times m_w\times \left(T_f-T_i\right)-C_{p,fw}\times m_{fw}}{m_s}}$$

where $H_{HHV}$ = high heating value of the sample (kJ/kg), $C_{p,w}$ = specific heat of water (kJ/kg·K), $m_w$ = mass of water (kg), $T_f$ = final water temperature (°C), $T_i$ = initial water temperature (°C), $C_{p,fw}$ = specific heat of the fuse wire in kJ/kg, $m_{fw}$ = mass of the fuse wire (determined by the weight difference pre- and post-combustion), $m_s$ = mass of the sample (determined by the weight difference pre- and post-combustion).

3. Results and Discussion

3.1. HTC Yield

Figure 3 presents the yields of gas, primarily carbon dioxide and methane, solid residue (hydrochar), composed largely of lignin with remaining cellulose and hemicellulose fragments, and the sugar-laden liquid phase, which contained dissolved carbohydrate derivatives, generated during hydrothermal carbonization (HTC) of corn stover using DI water, Xy-based NADES, and OA-based NADES at different temperatures. Across all pretreatment systems, both temperature and solvent type exert a strong influence on product distribution, reflecting the progressive depolymerization and transformation of lignocellulosic components with increasing thermal severity.

For DI water HTC, gas production increased markedly with temperature, rising from 8.58 ± 0.58 g at 200 °C to 14.86 ± 1.51 g at 250 °C and reaching 46.64 ± 2.65 g at 300 °C. This increased gas production can be attributed to decarboxylation from the decomposition of carboxyl/carbonyl groups, which primarily produce CO₂ with some CO [22], with higher temperatures increasing this reaction. In contrast, the solid residue decreased significantly with temperature, dropping from 9.17 ± 0.45 g at 200 °C to 8.50 ± 0.34 g at 250 °C and 5.40 ± 0.24 g at 300 °C. This reduction reflected intensified devolatilization and increasing carbon loss from the solid matrix as hydrothermal severity increased. Although hydrochar remained the predominant product at 200 °C, its yield declined sharply at 300 °C due to enhanced gasification and accelerated thermal degradation of cellulose and hemicellulose, which promoted their dissolution into the aqueous phase. Correspondingly, the sugar-laden liquid fraction decreased from 147.25 ± 2.12 g at 200 °C to 141.64 ± 1.65 g at 250 °C and 112.96 ± 1.11 g at 300 °C, indicating extensive hydrolysis of structural polysaccharides and increased formation of low-molecular-weight organics under subcritical water conditions.

The Xy-based NADES system exhibited a distinctly different product distribution compared to DI water HTC. Although the initial corn stover mass was 15 g for each sample, the solid residue increased substantially to 27.90 ± 0.86 g, 32.87 ± 0.61 g, and 33.14 ± 0.44 g at 200 °C, 250 °C, and 300 °C, respectively. These values were considerably higher than those obtained with DI water at comparable temperatures, suggesting that carbon-, hydrogen-, and oxygen-containing fragments from the xylose component of the NADES may integrate into the hydrochar matrix, contributing to the elevated solid yield. Possible reactions causing this phenomenon include the condensation and polymerization of xylose, as HTC was originally performed on glucose and xylose to form hydrochars [22]. The pores produced from the removal of hemicellulose and cellulose may then entrap this hydrochar. This finding indicates a serious limitation to the usefulness of the process and the need to further characterize the hydrochar produced. Gas production also rose with temperature, increasing from 7.43 ± 0.32 g at 200 °C to 15.86 ± 0.44 g and 44.99 ± 0.87 g at 250 °C and 300 °C, respectively. Correspondingly, the sugar-laden liquid fraction decreased with increasing severity, declining from 129.67 ± 1.12 g at 200 °C to 116.27 ± 1.06 g at 250 °C and 86.87 ± 0.91 g at 300 °C. This behavior indicated that the NADES promoted selective depolymerization of polysaccharides, producing a liquid phase enriched in fermentable sugars and soluble aromatics, while simultaneously facilitating char formation and limiting excessive gasification.

For the OA-based NADES system, increasing temperature induced a pronounced shift in product distribution, characterized by decreasing solid residue and liquid yields accompanied by a substantial increase in gas formation, consistent with the strong acid-catalyzed hydrolysis promoted by oxalic acid. The solid residues yielded at 150 °C, 200 °C, and 250 °C were 9.60 ± 0.41 g, 11.13 ± 0.53 g, and 9.05 ± 0.33 g, respectively; however, at 300 °C, the solid residue increased to 16.17 ± 0.56 g, exceeding the mass of the initial biomass. This anomalous increase likely reflected secondary condensation or polymerization reactions of soluble intermediates at high temperature, as well as the retention of residual OA within the solid matrix after filtration. Organic acids have been reported to condense and polymerize [22], and this finding at the higher temperature may indicate a higher activation energy for such behavior.

These observations indicated extensive cleavage of glycosidic bonds and solubilization of biomass components under severe acidic conditions. Gas production rose steadily with temperature—from 15.26 ± 0.45 g at 150 °C to 36.25 ± 0.67 g, 43.24 ± 0.74 g, and 48.33 ± 0.81 g at 200 °C, 250 °C, and 300 °C, respectively—reflecting increased decomposition of volatile intermediates. Correspondingly, the sugar-laden liquid fraction decreased progressively with higher temperature, declining from 140.14 ± 0.46 g at 150 °C to 117.62 ± 0.45 g, 112.70 ± 0.54 g, and 100.50 ± 0.36 g at 200 °C, 250 °C, and 300 °C, respectively, further confirming the strong hydrolytic activity of the OA-based NADES system.

The recorded findings of corn stover subjected to the HTC process revealed that both solvent chemistry and reaction temperature strongly govern product distribution. DI water promoted progressive devolatilization and carbohydrate hydrolysis, shifting yields from solid toward gas and decreasing sugar-rich liquid with increasing severity during the HTC process [22,23,24,25]. The Xy-based NADES produced an unusually high solid yield due to solvent-derived carbon incorporation while simultaneously facilitating selective depolymerization of polysaccharides. In contrast, the OA-based NADES generated the strongest hydrolytic environment, resulting in extensive glycosidic bond cleavage, substantial gas formation, and reduced liquid fractions at high temperatures [26,27,28]. Overall, these results demonstrate that solvent-assisted HTC enables tunable conversion pathways, with each solvent system driving distinct mechanisms of lignocellulosic deconstruction and product formation.

3.2. pH of Sugar-Laden Liquid

Figure 4 illustrates the pH of the sugar-laden liquid obtained after pretreatment of corn stover using DI water, Xy-based solvent, and OA-based solvent across temperatures ranging from 150 to 300 °C. The results clearly indicated that both solvent chemistry and pretreatment temperature exert strong control over the acidity of the resulting liquid phase.

For the DI water system, the sugar-laden liquid remained moderately acidic and exhibited a steady decline in pH with increasing temperature. The pH decreases from 4.00 ± 0.10 at 200 °C to 3.84 ± 0.09 at 250 °C and further to 3.59 ± 0.15 at 300 °C. This trend reflects the progressive hydrothermal degradation of cellulose and hemicellulose (holocellulose) into organic acids, including acetic, formic, and levulinic acids [13]. The consistent downward shift in pH confirms increased carbohydrate hydrolysis and organic acid formation under subcritical water conditions. In the Xy-based solvent system, the pH of the liquid phase remains relatively stable, ranging narrowly between 3.50 and 3.93 across all temperatures. This minimal variation suggested that the NADES matrix provides buffering capacity—either by retaining acidic intermediates within the solid or solvent phase or by limiting their release into the aqueous fraction. Further research is needed to investigate this phenomenon and the interactions involved. Consequently, the sugar-laden liquid maintains mildly acidic yet comparatively consistent pH values over the full temperature range. In contrast, the OA-based solvent system displays an opposite temperature-dependent trend. The pH was extremely low at 150 °C (1.16 ± 0.02) due to the intrinsic acidity of oxalic acid, but increased substantially with temperature, rising to 2.82 at 200 °C, 5.22 at 250 °C, and approaching neutrality (pH ≈ 6) at 300 °C. This upward shift possibly could have resulted from thermal decomposition of biomass-derived intermediates into more basic compounds at elevated temperatures, causing the neutralization of solvent (OA-based NADES), but further investigation into this finding is needed.

It may be inferred that the pH profiles recorded during this study highlight clear solvent- and temperature-dependent differences in the chemical environment of the liquid phase. DI water produces progressively more acidic liquids with increasing temperature; the Xy-based solvent maintains a stable, mildly acidic pH; and the OA-based solvent transitions from strongly acidic to nearly neutral as temperature rises during the HTC process [29,30,31,32]. Possible reasons for this change would be the decomposition of OA into CO₂ and formic acid, which has a higher pH. These contrasting behaviors underscore the distinct hydrolytic strengths and decomposition mechanisms associated with each pretreatment system.

3.3. Fiber Analysis of HTC Pretreated Solid Residue

Figure 5 illustrates the changes in lignin, holocellulose, and ash and extractive (A&E) contents of raw corn stover and samples pretreated with DI water, Xy-based NADES, and OA-based NADES at different temperatures. The raw corn stover used in this study displayed a typical lignocellulosic composition consisting of 31.67% ± 1.68% lignin, 58.76% ± 2.23% holocellulose, and 9.57% ± 0.87% ash and extractives. It must be noted, as mentioned in Section 2.2.3, that the lignin found by fiber potentially includes other components, such as polymerized cellulose.

Under DI water pretreatment, approximately 60% of the ash and extractives dissolved into the liquid phase, reducing their content in the solid residue from ~10% to ~4% across all temperature ranges (200–300 °C). Holocellulose content decreased significantly with increasing temperature, declining from 58.76% ± 2.23% in raw biomass to 56.02% ± 1.31% at 200 °C, 43.75% ± 1.21% at 250 °C, and 2.84% ± 0.21% at 300 °C. Correspondingly, lignin content increased substantially, rising to 40.57% ± 1.21%, 51.91% ± 1.39%, and 92.23% ± 1.88% at 200, 250, and 300 °C, respectively. This enrichment reflected the preferential solubilization of hemicellulose and significant cellulose loss under severe hydrothermal conditions.

The Xy-based NADES pretreatment produced a distinct compositional shift of A&E. Nearly all A&E dissolved into the sugar-laden liquid, leaving <1% remaining in the solid residue at 200–300 °C. Holocellulose decreased progressively from 45.46% ± 1.07% at 200 °C to 32.28% ± 0.91% at 250 °C and 18.83% ± 0.51% at 300 °C, indicating substantial solubilization of hemicellulose and amorphous cellulose. Lignin content increased correspondingly to 53.74% ± 1.11%, 67.07% ± 1.27%, and 80.20% ± 1.77% at 200, 250, and 300 °C, respectively. These results confirmed the strong delignification–carbohydrate fractionation behavior of the NADES system, yielding a lignin-rich solid residue. Limitations to these findings include the lack of understanding of the absorption of the xylose-based NADES into the hydrochar.

The OA-based NADES pretreatment exhibits a different pattern. Most ash and extractives dissolved at higher temperatures, leaving approximately 7% at 150 °C, 5% at 250 °C, and 4% at 300 °C in the solid residue. Holocellulose content declines from raw levels to 51.70% ± 1.17% at 150 °C, 50.55% ± 0.45% at 200 °C, 27.47% ± 0.31% at 250 °C, and 13.10% ± 0.26% at 300 °C, indicating progressively intensified hydrolysis. Lignin content showed an initial decrease from 46.00% ± 0.45% at 150 °C to 42.32% ± 0.31% at 200 °C, but rose sharply at higher temperatures to 66.69% ± 1.12% at 250 °C and 82.95% ± 1.31% at 300 °C. These trends confirmed that oxalic acid strongly promotes hemicellulose degradation and increasingly catalyzes cellulose depolymerization with temperature, while also contributing to lignin concentration in the solid residue.

The compositional analysis shows that all three pretreatment systems substantially modified the structural composition of corn stover, with higher temperatures enhancing holocellulose and ash removal while increasing lignin concentration. DI water and Xy-based NADES effectively promoted hemicellulose solubilization, whereas OA-based NADES exhibited the strongest hydrolytic severity, resulting in pronounced cellulose depolymerization at elevated temperatures. Therefore, it may be inferred that DI water pretreatment at 300 °C provided the most efficient route for achieving near-complete holocellulose removal and producing an almost lignin-rich solid, whereas the Xy-based solvent was highly effective for eliminating A&E, which aligns with previous studies [13,33,34].

3.4. Morphology Analysis of HTC Pretreated Solid Residue

The SEM images in Figure 6 illustrate the morphological evolution of corn stover during hydrothermal pretreatment using different solvent systems and temperatures. The raw corn stover, as shown in Figure 6a, exhibits a compact and smooth structure characteristic of intact lignocellulosic biomass where cellulose, hemicellulose, and lignin remain strongly interconnected. After hydrothermal pretreatment at 200 °C, noticeable disintegration of the biomass structure occurs. The DI-treated sample, as shown in Figure 6b, demonstrated partial disruption of fiber bundles and surface roughening, indicating the beginning of hemicellulose solubilization. The xylose-based solvent pretreatment, as shown in Figure 6c, led to more pronounced particle fragmentation and the formation of irregular granules, suggesting enhanced hydrolysis and depolymerization. The oxalic acid-treated sample, as shown in Figure 6d, displayed severe structural collapse and the presence of aggregated porous fragments, demonstrating the acidic medium’s stronger catalytic role in breaking down lignocellulosic components.

When the pretreatment temperature increased to 300 °C, the morphological transformation was more prominent. DI-treated solids, as shown in Figure 6e, exhibited increased porosity, micro-fractures, and hollow structures, indicating deeper degradation of cell-wall polymers. The xylose solvent sample at 300 °C, as shown in Figure 6f, consisted largely of spherical carbonaceous particles and agglomerates, suggesting advanced carbonization and condensation reactions. Oxalic acid pretreatment at 300 °C, as shown in Figure 6g, yielded a highly porous, fragmented structure with greater surface area exposure, signifying extensive hydrolysis and structural rearrangement. Removal or recondensation of the holocellulose components is likely the reason for these changes.

From this data, it can be inferred that both temperature and solvent type significantly influenced the solid residue decomposition behavior. Elevated temperature and acidic media resulted in more intensified structural change, improved porosity, and formation of carbon-rich granules, which are beneficial for subsequent activation and adsorption applications.

3.5. Elemental Analysis of HTC Pretreated Solid Residue

The EDS spectra in Figure 7 provided insight into the elemental composition changes of corn stover before and after hydrothermal pretreatment with varying solvent types and temperatures. Raw corn stover, as shown in Figure 7a, depicted a balance of carbon (C) and oxygen (O), along with detectable amounts of inorganic elements, collectively called ash, such as potassium (K), calcium (Ca), and silicon (Si), which are naturally present in lignocellulosic biomass.

Following hydrothermal pretreatment at 200 °C, the DI water-treated sample, as shown in Figure 7b, caused a slight increase in carbon content and a reduction in inorganic ash-forming elements (K, Ca), indicating partial leaching of minerals into the aqueous medium. The xylose-based solvent treatment, as shown in Figure 7d, enhanced deoxygenation more prominently, reflected by a higher carbon-to-oxygen ratio compared to DI water. Oxalic acid pretreatment, as shown in Figure 7f, resulted in notable retention of Ca and the presence of Cl. As choline chloride is a component of both NADES, some Cl would be expected to be sorbed into pores in the solid product. Oxalic acid NADES pretreatment could have a strong interaction with structural cations, possibly forming oxalates or chloride-containing residues. Depending on the Cl-containing compounds formed, the hydrochar could be toxic or have limited applications.

At a higher temperature of 300 °C, carbon content increased substantially across all solvent systems due to intensified dehydration and decarboxylation reactions. DI pretreatment, as shown in Figure 7c, showed a pronounced carbon enrichment and reduced oxygen fraction, confirming enhanced carbonization. The xylose-treated sample, as shown in Figure 7e, exhibited similar trends along with increased chlorine incorporation, likely from solvent degradation byproducts. Oxalic acid pretreatment at 300 °C, as shown in Figure 7g, produced the highest carbon content and lowest oxygen content among all samples, highlighting its strong catalytic role in promoting carbon densification.

Therefore, based on recorded findings, it may be inferred that the hydrothermal pretreatment conditions significantly influence elemental composition. Both increasing temperature and employing acidic solvents led to higher carbon concentration and mineral redistribution, which are favorable characteristics for producing carbon-rich precursors for activated carbon applications.

3.6. FTIR of HTC Pretreated Solid Residue

Figure 8 shows the FTIR spectra of raw corn stover and solid residue after pretreating with DI water, Xy-based deep eutectic solvent (ChCl:Xy:W), and oxalic acid-based solvent (ChCl:OA:W) at temperatures from 150 to 300 °C. The major lignocellulosic absorption bands include the broad O–H stretching region at 3300–3400 cm⁻¹ (cellulose, hemicellulose, and bound moisture) and the 2920–2850 cm⁻¹ aliphatic C–H stretching of polysaccharides and lignin. The 1734 cm⁻¹ band corresponds to C=O stretching of hemicellulose acetyl/uronic ester groups and lignin–carbohydrate ester linkages, while the 1648 cm⁻¹ feature represents H–O–H bending and/or conjugated C=O/C=C structures in modified lignin. Aromatic lignin signatures appear at 1510, 1457, and 1250 cm⁻¹, and cellulose/hemicellulose contributions are observed at 1045 cm⁻¹ (C–O–C/C–O stretching) and 899 cm⁻¹ (β-1,4-glycosidic linkage of amorphous cellulose). The raw corn stover spectrum exhibits strong hemicellulose and lignin absorption bands with partially masked cellulose features, confirming an intact cellulose–hemicellulose–lignin matrix.

Figure 8a demonstrates that DI water hydrothermal pretreatment primarily targets hemicellulose. The 1734 cm⁻¹ band decreased steadily with temperature and became very weak at 250–300 °C, indicating extensive deacetylation and hemicellulose solubilization. Lignin absorption bands at 1510 and 1457 cm⁻¹ show modest reductions, reflecting limited delignification. In contrast, cellulose-related bands at 1045 and 899 cm⁻¹ became sharper with increasing temperature, consistent with cellulose enrichment following the removal of amorphous hemicellulose. It may be inferred that the narrowing of the O–H region highlighted reduced hydrogen-bonding availability as pretreatment severity increased.

Figure 8b shows that the Xy-based solvent induced the most extensive lignin removal among the three systems. The lignin-associated bands at 1510, 1457, and 1250 cm⁻¹ decreased markedly with temperature due to cleavage of aryl–ether and lignin–carbohydrate linkages. The hemicellulose carbonyl band at 1734 cm⁻¹ also declined rapidly, indicating simultaneous ester hydrolysis. This would decrease the more oxygen-rich hemicellulose fraction, leading to a higher carbon-to-oxygen ratio, as indicated by EDS data. At temperatures ≥ 250 °C, cellulose absorption bands at 1045 and 899 cm⁻¹ dominated the spectrum, reflecting substantial cellulose enrichment. Minor broadening in the 1648 cm⁻¹ region may indicate modified hydrogen bonding or formation of condensed lignin structures.

Figure 8c highlights the effects of OA pretreatment. The 1734 cm⁻¹ hemicellulose absorption band decreased sharply with temperature and nearly disappeared at 250–300 °C, confirming strong acid-catalyzed hemicellulose hydrolysis. Lignin bands diminished with increasing temperature but persisted to some extent, indicating partial delignification and lignin structural modification. Meanwhile, cellulose-related bands at 1045 and 899 cm⁻¹ intensify as cellulose becomes more exposed. The enhanced 1648 cm⁻¹ signal at high temperature may reflect increased bound water in a more porous matrix or pseudo-lignin formation.

Based on the recorded findings of the FTIR analysis, it may be inferred that pretreatment temperature strongly accelerates biomass deconstruction across all solvent systems, as previously suggested by other researchers [35,36,37,38,39,40]. DI water predominantly removes hemicellulose; the Xy-based solvent achieves the greatest lignin disruption; and oxalic acid induces the most aggressive hemicellulose hydrolysis.

3.7. Higher Heating Value of Solid Residue

Figure 9 illustrates the higher heating value (HHV) of raw corn stover, and the solid residues obtained after HTC pretreatment with DI water, Xy-based NADES, and OA-based NADES at temperatures ranging from 150 to 300 °C. Raw corn stover exhibits an HHV of 9.84 ± 0.18 MJ kg⁻¹, consistent with typical lignocellulosic biomass. Pretreatment significantly altered the HHV of the resulting solids, reflecting changes in lignin, fixed carbon, and volatile composition.

Across all pretreatment systems, the HHV of the solid residue increased with rising temperature, reflecting progressive carbon densification, dehydration, decarboxylation, and aromatic condensation reactions. In the DI water system, pretreatment at 200 °C initially lowers the HHV to 8.49 ± 0.12 MJ kg⁻¹, but a further increase in temperature elevated the HHV to 9.57 ± 0.38 MJ kg⁻¹ at 250 °C and 14.55 ± 0.31 MJ kg⁻¹ at 300 °C, representing a 1.5-fold improvement relative to raw biomass. The increased HHV at higher temperature is related to the increase in carbon-to-oxygen ratio, as found by EDS. When a fuel has a greater carbon-to-oxygen ratio, it is more highly reduced, leading to a higher fuel value. The Xy-based NADES produces consistently higher HHV values than DI water at all temperatures, increasing to 10.54 ± 0.45, 12.93 ± 0.37, and 14.38 ± 0.41 MJ kg⁻¹ at 200, 250, and 300 °C, respectively; this behavior reflects the solvent’s strong ability to remove oxygen-rich polysaccharides while enriching the solid in lignin and condensed aromatic structures. The OA-based NADES exhibits the most pronounced enhancement among all systems, with HHV increasing even at 150 °C to 10.60 ± 0.45 MJ kg⁻¹, and rising further to 12.83 ± 0.46, 13.83 ± 0.59, and 19.27 ± 0.15 MJ kg⁻¹ at 200, 250, and 300 °C, respectively. At 300 °C, OA-based pretreatment yields a hydrochar with nearly twice the HHV of raw corn stover, demonstrating the strong catalytic influence of oxalic acid in promoting dehydration, decarboxylation, and lignin-driven aromatic condensation [41,42,43,44,45], ultimately generating a highly carbon-dense and energy-rich solid. This increase can again be attributed to an increased carbon-to-oxygen ratio, as seen from EDS. A lignin-rich product may not be the only reason for an increase in HHV, as polymerized or condensed other components may have high carbon-to-oxygen ratios, enhancing the fuel value.

As per recorded findings, it became evident that both solvent type and temperature strongly influence HHV. Previously, it has been revealed that increasing temperature consistently enhanced the energy density of the solid product for all pretreatments due to progressive dehydration, aromatization, and carbon enrichment [41,42,43,44,45]. Among the solvents tested, the OA-based system yielded the highest HHV at elevated temperatures, followed by the Xy-based solvent and DI water. These differences highlight the distinct roles of solvent chemistry in promoting lignin condensation, polysaccharide degradation, and energy densification during hydrothermal pretreatment [33,41,46,47,48,49,50,51].

Further work is needed to fill the gaps in the understanding of NADES-based HTC. The filling of hydrochar pores with certain NADES must be quantified, and the mechanism explored. Methods to separate and quantify pseudo-lignin in the hydrochars when fiber analysis is performed need to be devised and tested to give a full understanding of the solid product. These and other limitations of the present study will provide ample opportunity for future work in this research area.

4. Conclusions

The qualitative trends found suggest that hydrothermal carbonization (HTC) of corn stover in the presence of DI water, Xy-based NADES, and OA-based NADES enabled solvent-dependent and temperature-driven deconstruction of lignocellulosic biomass, producing different distributions of gas, sugar-laden liquid, and hydrochar rich in lignin or condensed lignin-like (pseudo-lignin) compounds. The findings confirmed that different product distributions are found when two different NADES are used instead of DI water in HTC. DI water pretreatment primarily enhanced hemicellulose solubilization and carbohydrate hydrolysis, while Xy-based NADES facilitated selective polysaccharide depolymerization and yielded unusually high solid residues due to solvent-derived carbon incorporation. This finding requires future study and represents a severe drawback to the further implementation of this NADES. In contrast, OA-based NADES apparently generated the most severe hydrolytic environment, promoting extensive cleavage of glycosidic bonds, high gas formation, and the strongest temperature-associated compositional shifts in both solid and liquid products. Structural characterization (SEM, FTIR, EDS) confirmed extensive biomass breakdown, oxygen elimination, and formation of carbon-rich matrices with NADES-assisted pretreatment. Fiber analysis and calorimetric measurements collectively confirmed that increasing pretreatment severity enriched lignin-like (lignin combined with pseudo-lignin) content, decreased holocellulose, and significantly enhanced the energy density of hydrochar, with OA-based NADES at 300 °C producing the highest HHV—nearly double that of raw corn stover. While complete mass balances and quantitative correlations are still needed, these findings suggest NADES-assisted HTC may be an effective pathway for converting corn stover into high-calorific lignin-rich solid fuels, supporting the development of integrated biorefinery processes.