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Article

Systematic Isolation and Characterization of Regenerated Hemicellulose and Lignin from Soybean Feedstocks Using Ionic Liquids

by
Victor Essel
and
Douglas E. Raynie
*
Department of Chemistry, Biochemistry and Physics, South Dakota State University, Brookings, SD 57007, USA
*
Author to whom correspondence should be addressed.
Separations 2025, 12(2), 37; https://doi.org/10.3390/separations12020037
Submission received: 29 December 2024 / Revised: 17 January 2025 / Accepted: 23 January 2025 / Published: 4 February 2025
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Abstract

:
The use of ionic liquids in biomass pretreatment for ethanol production has seen increased attention in recent years. In this work, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), and 1-ethyl-3-methylimidazolium acetate ([Emim]Ac) were used to regenerate and recover significant amount of hemicellulose and lignin from soybean meal, flakes, and hulls. The regenerated lignin and hemicellulose were characterized using Fourier-transform infrared (FTIR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). For all three ionic liquids, the amount of regenerated hemicellulose and lignin ranged from approximately 6 to 12% and 8 to 19%, respectively. Lignin characteristic bands 1738.8, 1652.6, 1516.4, 1455.2, and 1174.9 cm−1 were identified in the FTIR spectrum. The regenerated hemicellulose showed the characteristic bands 1658.31, 1434.14, 1167.98, and 865.20 cm−1. The Py-GC/MS analysis of the regenerated lignin showed the characteristic grass lignin pyrolyzates phenol, 2-methoxyphenol, 4-methylphenol, 2-benzaldehyde, 2-methoxy-4-vinylphenol, phenol-2,6-dimethoxy, and ethylvanillin.

Graphical Abstract

1. Introduction

Biomass (lignocellulose) pretreatment can be defined as the application of an external force (chemical, physical, or biological) to disrupt or compromise the recalcitrance of hemicellulose and lignin components. This presents cellulose and hemicellulose availability for further processing into ethanol. Lignocellulosic materials are composed of polymers of cellulose, hemicellulose, and lignin held together in a complex inter-network through strong polymeric bonding. The complex internetwork structure of the polymers forms a barrier that prevents enzymatic accessibility to cellulose and hemicellulose. Lignin has been found to play a significant role in the barrier-structural formation. Despite the readily available, relatively low cost, and great abundant of lignocellulosic feedstock, the recalcitrant constraint of the structural network to enzymatic hydrolysis of the polymers in their native state is a socio-economic drawback in the biofuel industry. Scientists continue to explore economically viable approaches through various pretreatment methods to overcome the recalcitrant properties of lignocellulosic materials. Pretreatment results in differential compositional and structural changes in plant cell walls, which results in the breakdown of the internetwork barrier, the fractionation of the components, and the hydrolysis of the polysaccharides. The degree of hydrolysis and fractionation is dependent on the efficiency of the treatment. Fractionation and hydrolysis involve the breakdown of covalent bonds, hydrogen bonding, and Van der Waals forces between the cellulose, hemicellulose, and lignin polymers [1,2]. Based on the method used, pretreatment parameters and conditions can be manipulated to target different components of the biomass sample which generally contains 40–50% w/w cellulose, 20–40% w/w hemicellulose, and 10–25% w/w lignin [3,4].
Cellulose comprises linear polymers of D-glucopyranose monomer units linked through β (1→4) linkages. The average degree of cellulose polymerization in a cell wall is about 10,000–90,000 units with only about 65% being crystalline and highly oriented with no solvent accessibility [5]. The remaining 35% is partially accessible to solvent due to their orientation with lignin and hemicellulose. Hemicellulose is the second largest group of organic compounds in dry plant cell matter comprising of D-xylose, L-arabinose, 4-O-methylglucuronic acid, D-glucose, and D-galactose. Hemicellulose is highly branched, non-crystalline, and with a lower degree of polymerization. Lignin makes up the third largest component of dry plant matter. It is an amorphous, three-dimensional phenolic polymer present in plant cell walls [6]. It provides rigidity and a structural framework for plants by interconnecting with hemicellulose to form the cementing material. In the conversion of lignocellulosic materials to biofuels such as ethanol, through hydrolysis and fermentation, the presence of lignin and hemicellulose generates inhibitory products which reduce the conversion efficiency of the transformation process. The lignin degradation products cinnamaldehyde, p-hydrobenzaldehyde, and syringaldehyde units, and hemicellulose degradation products furfural and hydroxymethylfurfural [6,7,8], inhibit enzymatic activity during carbohydrate fermentation. This hemicellulose and lignin inhibitory effect creates a setback in the biofuel industry that reduces bioethanol yield and production efficiency. It is important to selectively remove lignin and/or hemicellulose from biomass prior to cellulose conversion to bioethanol to enhance enzymatic accessibility and the elimination of fermentation inhibitory products. When removed, the hemicellulose together with the lignin can be converted into various forms of consumable products.
Various pretreatment methods have been explored over the years in an effort to obtain an economically viable pretreatment technique capable of providing breakthroughs in the conversion of lignocellulosic materials into biofuel. Chemical methods such as acid, base, steam, organic solvents, and ionic liquids have been used effectively to pretreat lignocellulose materials for conversion into biofuel. These methods can be adjusted to target specific components for isolation and processing into other valuable by-products. In recent years, ionic liquids have been found to be suitable solvent replacements for the extraction of lignocellulosic material components. In addition to their solvating ability, they possess low vapor pressure and may be recycled. These solvents eliminate the issue of volatile organic compound (VOC) emissions [9]. In ionic liquids, the large cation or anion renders a high degree of asymmetry in the molecule, which tends to reduce the lattice energy of the salt resulting in a low melting point [10]. ILs present an efficient medium for the study of organic synthetic pathways, kinetics, and thermodynamics of reactions [9,11]. Structural modifications of the cation and/or simple replacement of the anion confers a corresponding physicochemical change to the properties of ionic liquids. This flexibility in ionic synthesis makes it possible to design specific ionic liquids towards a specific task [12,13]. The identified mechanism of interaction between ionic liquids and lignocellulosic materials involves the disruption and/or breakdown of the extensive intra– and extracellular hydrogen bonds of the polymeric components of the biomass by the interfering activity of the ionic salts through non-covalent bonding [14,15]. This interfering activity of the ionic liquid ions leads to the breakdown of the polymeric intra-molecular bonds between the biopolymers resulting in the breakdown of the recalcitrant network and subsequent release of the components into solution.
In this work, we report an efficient hemicellulose and lignin regeneration method using the ionic liquid (IL) pretreatment at moderate temperatures and which presents little or no co-extraction. This isolation method presents a green systematic route for selectively isolating hemicellulose and lignin from soybean meal, flakes, and hulls using [Bmim]Cl, [Amim]Cl, and [Emim]Ac. The use of these ILs, which can be recycled, presents a green chemical process which eliminates the environmental pollution factor that most hemicellulose and lignin isolation methods possess. The absence of hemicellulose and lignin from the biomass enhanced enzymatic accessibility of the cellulose during cellulosic-ethanol production, however, was not reported in this paper. Ionic liquids have been reported as green solvents in biomass conversion suitable for replacing some of the organic solvents, acids, and bases used in the extraction and conversion of lignocellulosic materials [16,17,18,19]. The ionic liquid recovery and purity of recovered solvents were not reported in this manuscript.
Isolated lignin is usually in the degraded or derivatized form. However, this degradation consideration is absent or minimal in hemicellulose isolation [17,18,19,20]. Many of these extraction processes involve the use of organic solvents, strong alkali, and/or acids which are implicated in a lot of environmental problems. Bjorkman [21,22] and Wu, and Argyropoulos [23] isolated a representative of native lignin using an acidic dioxane-water. Their work resulted in a significantly higher yield than that obtained with a corresponding milled-wood lignin preparation [23,24]. Hemicellulose is generally extracted from lignocellulosic materials with 10% NaOH [25] into an ethanol solution [26,27]. The problem with this and other hemicellulose and lignin isolation techniques which involve the use of basic, acidic and organic solvents, is the isolation of the hemicellulose-lignin complex [28], the release of these solvents into the environment, and the use of elevated temperatures resulting in high pretreatment cost [29].
The hemicellulose and lignin content of many biomass byproducts has been isolated and characterized, but none have been reported on soybean feedstocks. With the advancement of research into the possible conversion of lignin into biofuel and hemicellulose into ethanol and biodegradable polymer products, we studied the isolation of hemicellulose and lignin from soybean meal, flakes, and hulls feedstocks using ionic liquids (ILs) as part of our biofuel research projects. The most profitable uses of soybean feedstock are for human and animal feed supplements. Soybean feedstocks, especially hulls, can serve as a renewable source of significant amount of cellulose, lignin, and other carbohydrates for conversion to biofuels.

2. Materials and Methods

2.1. Materials

The ionic liquids [Bmim]Cl, [Amim]Cl, and [Emim]Ac were obtained from Sigma-Aldrich (St. Louis, MO, USA), with each having 95% purity. Concentrated hydrochloric acid (37% w/w, 12.1 N), 190 proof ethanol ((95.0%, 0.7800 g/mL), ACS reagent), and dimethyl sulfoxide (≥99.9%, ACS reagent) were obtained from Fisher Scientific (Fair Lawn, NJ, USA). Soybean meal, flakes, hulls, and soybean protein concentrate were obtained from the United Soybean Board (Solae Company, St. Louis, MO, USA). The soybean samples received from the United Soybean Board were used as received.

2.2. Dissolution of Soybean Meal, Flakes, and Hulls in ILs

For each 3 g of sample, 10 g of IL and 5 mL of dimethyl sulfoxide (DMSO) were added. These measurements were obtained through the optimization of the dissolution process. The mass of the soybean samples were varied between 1 the 5 g, 5 the 10 g for the ILs, and 3 the 6 g for the dimethyl sulfoxide. The mixture was then placed on a hot-plate at each specified temperature (50–70 °C) with continuous stirring. The dissolved hemicellulose and lignin components were regenerated using 190 proof ethanol and concentrated hydrochloric acid. Figure 1 is a schematic diagram of the regeneration processes. The addition of the DMSO reduces the viscosity of the ILs.

2.3. Hemicellulose Regeneration

To avoid contamination, we regenerated any cellulose and protein present in the IL-dissolved soybean samples prior to hemicellulose regeneration, Figure 1. After the IL dissolution of the soybean meal, flake, and hull samples, they were cooled and diluted to 100.0 mL with deionized water. The cellulose and some of the protein co-precipitated. The proteins remaining in the filtrate were regenerated using 190 proof ethanol in a filtrate to ethanol ratio of 1:1.5. The dissolved hemicellulose in the cellulose- and protein-free (CP-F) filtrate was regenerated using two parts of ethanol to one part of CP-F filtrate. In most instances, the precipitate formation was observed after the addition of the ethanol. The regenerated hemicellulose selectively precipitated upon standing and was collected as pellets by centrifugation at 1300 rpm for thirty minutes using an AccuSpinTM 3R purchased from Fisher Scientific (Pittsburgh, PA, USA). The hemicellulose pellets were washed several times with deionized water and air dried for FTIR analysis. All the filtrates from the washing process were added to the cellulose-protein- and hemicellulose-free (CPH-F) filtrate.

2.4. Lignin Regeneration

Using a drop-and-stir technique, 50% hydrochloric acid was gradually added to the CPH-F filtrate with continuous stirring. The pH of the filtrate was monitored to bring the pH to 2.0. Excessive pH, i.e., less than 2.0, can re-disperse the lignin precipitate. When lignin precipitates did not form upon the addition of the acid, forty-five minutes was allowed for complete precipitation from the solution. Lignin pellets were collected by centrifugation 1300 rpm and washed several times with deionized water.

2.5. FTIR Analysis of Regenerated Hemicellulose and Lignin

The dried, regenerated hemicellulose and lignin were analyzed using the Thermo Scientific FTIR 380 system (Pittsburgh, PA, USA). The samples were placed on a silica-crystal sample holder and analyzed in the transmittance mode with 100 scans.

2.6. Pyrolysis-Gas Chromatography/Mass Spectrometry (GC/MS) Analysis

The regenerated lignin was characterized with pyrolysis-GC/MS using a CDS Analytical 5000 series Pyroprobe (Midland, ON, Canada). The sample was pyrolyzed at a set point temperature of 600 °C at a ramp rate of 20 °C/ms with a final hold time of 15 s. The acquisition parameters for the pyrolysis analysis were oven temperature 50 °C for 5 min, ramp at 5 °C/min to 280 °C, and hold 5 min; 300 °C injection, volume = 10 µL, 24:1 split; helium carrier gas, 9.00 min solvent delay; 200 °C transfer, 200 °C EI source, scan from 14 to 300 Da; 30.0 m × 250 µm DB-5MS (Agilent Technologies, Little Falls, DE, USA) (0.25-µm film); 5% phenyl/95% dimethylpolysiloxane stationary phase (SGE Analytical Sciences, Austin, TX, USA).

3. Results and Discussion

3.1. Hemicellulose and Lignin Regeneration

Table 1 shows the percent hemicellulose and lignin recovered from the three ionic liquids. The extracted amount ranged from 6.13 to 12.37 wt% for hemicellulose and 8.46 to 18.93 wt% for lignin. We observed the appearance of the IL dissolved soybean meal and flakes to be a clear, dark yellow, viscous solution, whereas the hull had a viscous, clear, dark brown appearance. The amount of hemicellulose and lignin regenerated from the samples were the same regardless of the ILs used. The three ILs used had the same dissolution effect on the samples except the time taken for complete dissolution. From visual observation, the complete dissolution of the three samples was at approximately 12, 15, and 18 h for EmimAc, AmimCl, and BmimCl respectively. The addition of two parts of 190 proof ethanol to one part of CP-F filtrate for the regeneration of the dissolved hemicellulose was instantaneous with the formation of a hazy solution. The relatively large amounts of hemicellulose and lignin regenerated, shown in Table 1, indicate the release of cellulose during the dissolution process. This presents the IL dissolution of soybean feedstocks as an effective fractionation method for releasing cellulose, which can then be converted to ethanol.

3.2. FTIR Analysis of Regenerated Hemicellulose

FTIR spectra were generated for the hemicellulose regenerated from the IL-dissolved soybean meal, flake, and hull samples, Figure 2. These spectra were identical and completely overlaid. FTIR is a great tool for monitoring side reactions in processes [30]. Hemicellulose is primarily composed of xylose, a monomeric sugar principally composed of L-arabino-(4-O methylglucorono)-D-xylans [20,31]. The bands at 1658.31, 1434.14, 1167.98, and 865.20 cm−1 in Figure 2 are characteristic hemicellulose bands. The glucuronic acid side-chain band appeared at 1658.31 cm−1. This band is associated with carboxylate asymmetric stretching [32,33].
The prominent sharp band at 1434.14 cm−1 is a characteristic CH2 symmetric bending of a xylose ring [24]. This band, when present in a hemicellulose FTIR spectrum, can also represent either a C-O stretch and C-H or O-H bending [25]. The 1167.98 cm−1 band is a characteristic glycosidic C-O-C bridge antisymmetric stretching vibration. The H vibration associated with glucomannan absorbed at 865.20 cm−1 [34]. Another hemicellulose characteristic band at 1740 cm−1, carbonyl stretching vibrations of esters, was not presented in Figure 2. This could mean that the hemicellulose was saponified [35] or the EmimAc did not cleave the ester linkages [30], but since our method did not involve the use of base, the latter was more likely to have occurred. The characteristic sharp peak of DMSO at 1044 cm−1 derived from the S=O stretching vibration was not observed [36,37]. This indicates the absence of DMSO contamination in the isolate.

3.3. FTIR Analysis of Regenerated Lignin

Figure 3 is a representative lignin spectrum of the three spectra generated for the recovered soybean meal, flake, and hull lignin. The bands identified are in agreement with the bands identified by Samuel et al. for Alamo switchgrass [38] and Hu et al. for switchgrass [39]. Characteristic lignin bands at 1516.6 and 1652.6 cm−1 were observed. The band at 1516.6 cm−1 represents the C=C aromatic ring vibration [40,41] of the isolated lignin. This band is associated with the syringyl propane unit from which soft lignins are built [42]. The two bands at 1652.6 and 1738.8 cm−1 represent C=O stretching vibrations in carbonyl and carboxyl groups. These groups are formed through polymer degradation during the lignin or hemicellulose extraction processes [43,44]. The intensity of the band at 1738.8 cm−1 can therefore be used to predict the degree of lignin degradation during an extraction process.
From Figure 3, the sharp and more intense band 1738.8 cm−1 can be used to speculate that extensive lignin degradation took place during the IL dissolution of the soybean feedstocks. However, it cannot be emphatically stated whether the lignin degradation was due to the action of EmimAc or exposure to either the acidic or ethanol medium. The bands at 1455.2 and 1174.9 cm−1 can be attributed to phenolic ring moieties or phenolic hydroxyl group vibrations [45]. The two sharp bands at 2924.6 and 2854.1 cm−1 are characteristic bands for asymmetric CH2 stretch and symmetric CH2 stretch vibrations respectively [46,47,48]. These two bands represent the methylene stretching mode of the aromatic rings present in the isolated lignin. A characteristic glucose band was observed at 1046.4 cm−1 in Figure 3 [49,50] representing the presence of polysaccharide in the lignin isolate. This could arise from cross-contamination during the lignin isolation. This band can also represent the presence of small amounts of hemicellulose. The characteristic DMSO at 1044 cm−1 derived from the S=O stretching vibration was not observed [36,37].

3.4. Pyrolysis-GC/MS Analysis of Regenerated Lignin

The regenerated lignin was pyrolyzed and analyzed by GC/MS. Eleven characteristic monolignols (A-K) associated with softwood or grasses [51,52] were identified within the phenolic region 20.0–42.0 min (Figure 4). These fragments and their peak areas are identified in Table 2. The chromatographic peaks between 0.0 and 9.0 min usually represent small carbohydrate-derived aliphatic products [53]. Their peaks were removed from Figure 4 to enhance the resolution of the peaks of interest. Larger alcohol chains derived from syringyl moieties are retained between 42.0 and 55.0 min [51]. The characteristic lignin moieties listed in Table 2 confirmed the presence of all three precursors H, G, and S. The G moiety was in greater abundance compared to the H and S moieties. This confirms the presence of grass lignin since G-type lignin is abundantly found in grass or softwood lignin. The total peak area for all the identified G moieties (B, D, G, I, and J) was greater (9.92%) than the total peak area for the H moieties (A, C, and E) at 3.79% and the S moieties (H and K) at 1.99%. Our monolignol identification compares favorably to those identified by Ross and Mazza [54] for their grass lignin samples isolated with [Emim]Ac. The H, G, and S concentrations were the same for all three samples.
Our work showed that soybean meal, flakes, and hulls contain significant amounts of hemicellulose and lignin which can be isolated at moderate temperatures using ionic liquids. The FTIR characterization showed no derivatization of the hemicellulose and lignin during the fractionation. The chemical and physical characteristics of the hemicellulose and lignin were not compromised by the degree of depolymerization as observed by the FTIR characterization. Both polymers were isolated in the solid form with the characteristic white color for the hemicellulose and brown color for the lignin persisting. The availability of 6.13 to 12.37 wt% of hemicellulose and 8.46 to 18.93 wt% of lignin from the soybean samples after IL dissolution at moderate temperatures indicates that our isolation method was efficient and economically viable compared to conversional pretreatment methods such as acid, base, and steam explosion methods. Acid and alkaline pretreatment temperatures typically range from 140 to 250 °C for treatment time for minutes to hours. Karstens reported 10–25% xylose recovery in aqueous solutions using sulphuric acid, hydrochloric acid and nitric acid 0.05–2.5 N at 240 °C [55]. Savannah River Nuclear Solutions reported the possible recovery of 65–95% of lignin from hydrolysate. They reported about 45% of lignin extracted from switchgrass [56]. In comparison to the conventional pretreatment process, our method provided an improved efficient pretreatment method for lignin and hemicellulose recovery. We have shown that, in addition to wood and grasses, soybean meal, flakes, and hulls can serve as renewable sources of a significant amounts of hemicellulose and lignin that can be isolated and converted to biofuels.
For the first time, we report a method that systematically isolated soybean hemicellulose and lignin using ILs and the subsequent characterization using FTIR and Py-GC/MS. The identification of soybean lignin by the Py-GC/MS analysis was significant since such information will be needed to guide the further processing of the lignin into biofuel and other consumable products. The various kinds of phenolic moieties present in a lignin sample and their corresponding concentrations will inform their possible chemical reactions and characteristics. The regenerated underivatized hemicellulose can serve as a renewable resource for enzymatic conversion into ethanol. Others have reported the isolation of lignin from biomass at elevated temperatures using ILs but our work has shown that soybean lignin can be isolated using ILs at moderate temperatures. The dissolution of biomass using ILs under moderate temperature provides economic benefits compared to the use of elevated temperatures.
Table 1. Amount of regenerated hemicellulose and lignin isolated from [Emim]Ac-, [Amim]Cl-, and [Bmim]Cl-dissolved soybean meal, flakes, and hulls at 70 °C. Increased temperature decreased the time required for complete dissolution.
Table 1. Amount of regenerated hemicellulose and lignin isolated from [Emim]Ac-, [Amim]Cl-, and [Bmim]Cl-dissolved soybean meal, flakes, and hulls at 70 °C. Increased temperature decreased the time required for complete dissolution.
Ionic LiquidSoybean FeedstockRegenerated Hemicellulose (%)Regenerated Lignin (%)
EmimAcMeal6.20 ± 0.258.87 ± 0.14
Flakes6.13 ± 0.298.850 ± 0.040
Hulls12.240 ± 0.01018.03 ± 0.14
AmimClMeal6.80 ± 0.558.51 ± 0.18
Flakes6.23 ± 0.718.950 ± 0.045
Hulls12.37 ± 0.5518.41 ± 0.37
BmimClMeal6.50 ± 0.298.46 ± 0.27
Flakes6.10 ± 0.618.61 ± 0.80
Hulls11.14 ± 0.1018.93 ± 0.54
Table 2. Grass lignin phenolic fragments identified in the Py-GC/MS pyrogram for the regenerated soybean meal, flakes, and hulls lignin.
Table 2. Grass lignin phenolic fragments identified in the Py-GC/MS pyrogram for the regenerated soybean meal, flakes, and hulls lignin.
Phenolic Moieties
Identified		Identity on PyrogramArea (%) aLignin Moiety
PhenolA2.01H
2-methoxyphenolB2.63G
4-methylphenolC0.94H
2-methoxy-4-methylphenolD2.90G
3-ethylphenolE0.84H
2-methylbenzaldehydeF5.33*
2-methoxy-4-vinylphenolG3.48G
2,6-dimethoxyphenolH0.74S
VanillinI0.26G
3-tertbutyl-4-hydroxyanisoleJ0.66G
2,6-dimethoxy-4-propenylphenolK1.25S
a Peak area in the total ion chromatogram, Figure 3. * Not defined as H, G, or S.

4. Conclusions

BmimCl, AmimCl, and EmimAc successfully dissolved soybean meal, flakes, and hulls. Except for the rate of dissolution, all the three ionic liquids had the same dissolution effect on the soybean meal, flake, and hull samples. Hemicellulose characteristic IR bands at 1658.31, 1434.14, 1167.98, and 865.20 cm−1 are present in Figure 2 with no lignin contamination. The 1652.6 and 1738.8 cm−1 bands in Figure 3 showed that the lignin polymer degraded during the dissolution of the samples. The intensity of band 1738.8 cm−1 thus predicts a relatively high degree of lignin degradation. The absence of observable derivatization in the regenerated hemicellulose and lignin implied that the ionic liquids did not react with these components. ILs dissolution of soybean meal, flakes, and hulls is therefore a good biomass fractionation technique that can be used to regenerate hemicellulose and lignin with a very high degree of efficiency. The successful regeneration or isolation of hemicellulose and lignin from the soybean meal, flakes, and hulls will enhance the conversion of these feedstocks to bioethanol and other fuels. The percent recovery reported here proves that this method is comparatively highly efficient in isolating hemicellulose and lignin from lignocellulose materials and the isolates are readily available for further processing into biofuels without any further treatment or processing.

Author Contributions

Conceptualization, V.E. and D.E.R.; methodology, V.E.; formal analysis, V.E.; investigation, V.E.; resources, D.E.R.; data curation, V.E.; writing—original draft preparation, V.E.; writing—review and editing, D.E.R.; supervision, D.E.R.; project administration D.E.R.; funding acquisition, D.E.R. All authors have read and agreed to the published version of the manuscript.

Funding

Funding for this research was provided by the United Soybean Board, Project 8438.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
FTIRFourier-transform infrared spectrometry
Py-GC/MSPyrolysis-Gas Chromatography/Mass Spectrometry
[Bmim]Cl1-butyl-3-methylimidazolium chloride
[Amim]Cl1-allyl-3-methylimidazolium chloride
[Emim]Ac1-ethyl-3-methylimidazolium acetate
ILIonic liquid
DMSODimethyl sulfoxide
CP-FCellulose- and protein-free
CPH-FCellulose-, protein- and hemicellulose-free

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Separations 12 00037 g001
Figure 1. Schematic diagram of the separation procedure for the isolation of cellulose, protein, hemicellulose, and lignin from soybean products using ionic liquids and selective precipitation.
Figure 1. Schematic diagram of the separation procedure for the isolation of cellulose, protein, hemicellulose, and lignin from soybean products using ionic liquids and selective precipitation.
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Separations 12 00037 g002
Figure 2. FTIR spectrum of regenerated hemicellulose recovered from EmimAc-dissolved soybean meal, flakes and hulls. Characteristic hemicellulose bands 1658.31, 1434.14, 1167.98, and 865.20 cm−1 are present. They define the xylan, glucuronic acid, carboxylate, and methylene groups associated with hemicellulose. Spectra from all three samples’ hemicellulose regenerated from all three ionic liquids dissolution were identical and completely overlaid.
Figure 2. FTIR spectrum of regenerated hemicellulose recovered from EmimAc-dissolved soybean meal, flakes and hulls. Characteristic hemicellulose bands 1658.31, 1434.14, 1167.98, and 865.20 cm−1 are present. They define the xylan, glucuronic acid, carboxylate, and methylene groups associated with hemicellulose. Spectra from all three samples’ hemicellulose regenerated from all three ionic liquids dissolution were identical and completely overlaid.
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Separations 12 00037 g003
Figure 3. A sample spectrum of isolated lignin showing the characteristic FTIR bands at 1738.8 and 1652.6 (carbonyl and carboxyl stretching groups), 1516.4 (C=C aromatic ring vibration), and 1455.2 and 1174.9 cm−1 (phenolic hydroxyl group vibrations). The spectra generated for the soybean meal, flake, and hull lignin from all three ionic liquid dissolutions completely overlay on each other.
Figure 3. A sample spectrum of isolated lignin showing the characteristic FTIR bands at 1738.8 and 1652.6 (carbonyl and carboxyl stretching groups), 1516.4 (C=C aromatic ring vibration), and 1455.2 and 1174.9 cm−1 (phenolic hydroxyl group vibrations). The spectra generated for the soybean meal, flake, and hull lignin from all three ionic liquid dissolutions completely overlay on each other.
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Separations 12 00037 g004
Figure 4. Py-GC/MS pyrogram of regenerated lignin isolated from soybean meal, flakes and hulls. The lignin phenolic ion moieties appeared between 23.26–41.13 min. Peaks A–K are characteristic monolignols associated with softwood or grasses. Peak identifications are in Table 2.
Figure 4. Py-GC/MS pyrogram of regenerated lignin isolated from soybean meal, flakes and hulls. The lignin phenolic ion moieties appeared between 23.26–41.13 min. Peaks A–K are characteristic monolignols associated with softwood or grasses. Peak identifications are in Table 2.
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Essel, V.; Raynie, D.E. Systematic Isolation and Characterization of Regenerated Hemicellulose and Lignin from Soybean Feedstocks Using Ionic Liquids. Separations 2025, 12, 37. https://doi.org/10.3390/separations12020037

AMA Style

Essel V, Raynie DE. Systematic Isolation and Characterization of Regenerated Hemicellulose and Lignin from Soybean Feedstocks Using Ionic Liquids. Separations. 2025; 12(2):37. https://doi.org/10.3390/separations12020037

Chicago/Turabian Style

Essel, Victor, and Douglas E. Raynie. 2025. "Systematic Isolation and Characterization of Regenerated Hemicellulose and Lignin from Soybean Feedstocks Using Ionic Liquids" Separations 12, no. 2: 37. https://doi.org/10.3390/separations12020037

APA Style

Essel, V., & Raynie, D. E. (2025). Systematic Isolation and Characterization of Regenerated Hemicellulose and Lignin from Soybean Feedstocks Using Ionic Liquids. Separations, 12(2), 37. https://doi.org/10.3390/separations12020037

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