Sustainable Utilization of Phoenix Date Palm Biomass via Extraction of Lignin Using Pyridinium-Based Protic Ionic Liquid

Abstract

Lignin is a renewable biopolymer widely available in industrial crops, yet its efficient separation and valorization remain challenging due to its heterogeneous structure derived from coniferyl, sinapyl, and p-coumaryl alcohol units. Its properties vary with the biomass type and extraction method. This study investigates lignin extraction from four Saudi date palm leaf varieties (Ajwa, Safawi, Amber, and Rabia) using a newly synthesized protic ionic liquid (PIL) composed of a pyridinium cation and formate anion. Extraction was carried out at temperatures ranging from 25 to 125 °C for 1 to 6 h. The PIL achieved high lignin recovery, up to 90%, under mild conditions (75 °C, 3 h), while also enabling subsequent cellulose pulp saccharification. In the second stage, extracted lignin samples were characterized using structural, thermal, and morphological analysis techniques in a comparative approach. It was concluded that PIL-treated lignin resulted in enhanced phenolic and aromatic contents with increased thermal stability, making it a suitable biofuel. The findings highlight date palm leaves as a low-cost lignin source suitable for pharmaceutical applications and the development of bionanocomposites with enhanced thermal performance.

1. Introduction

One of the most important areas of chemical research is the exploration of various natural resource domains for cost-effective technologies in biorefineries. Date palm, or Phoenix dactylifera, is a symbol of nourishment and cultural value, particularly in the Middle East’s desert regions. According to FAOSTAT statistics, Saudi Arabia was one of the world’s top producers of dates in 2019, with an output of 1,539,756 tons of date fruits. In addition to their nutritional value, date palm trees produce many different types of agricultural byproducts. Every year, each tree produces ≈35 kg of palm residues on average. Accordingly, Saudi Arabia is estimated to produce over one million tons of waste annually from date palm biomass. The majority of date palm waste in underdeveloped countries is burned or dumped in landfills, which seriously pollutes the environment.

One key constituent of the date palm biomass is lignin. Lignin content in hardwood varies between 20% and 30%, whereas in softwood, it ranges from 26% to 32%. Lignin is complex in nature, as it possesses an irregular network of phenolic polymers. Notably, the heterogeneity is so profound that it is believed that no two lignin molecules are identical in their phenyl unit sequences. The strong bonds between lignin and polysaccharides make it challenging to isolate pure lignin without changing its original native structure. Acting as a protective barrier, lignin prevents hydrolytic enzymes from breaking down cellulose to extract sugars, posing a hindrance in biofuel production. Despite being the second-most abundant component in lignocellulosic biomass, lignin recovery has received very little attention in both research and industry, and usually, lignin is only burnt to provide energy. The hydrophobic nature of lignin further enhances this complexity, which makes lignin insoluble in many solvents as well.

Conventionally, lignin is extracted from biomass using harsh chemicals, including strong acids (acid hydrolysis), alkalis (kraft pulping), organic solvents (organosolv pulping), and enzymatic processes. , Each of these approaches has certain advantages and disadvantages, notably regarding their influence on the environment, operating conditions, and the altered structural integrity of the recovered lignin. Notably, many of these traditional methods frequently call for extreme process conditions and present environmental problems, especially regarding waste production and energy consumption. , Therefore, only 1% to 2% of lignin is utilized for valuable uses despite its value addition and appealing applications; the remainder is burned as a low-grade fuel. A few other studies have also explored green solvents for lignin extraction. , In the modern era, it is highly desirable to pursue the development of an efficient and revolutionary separation technique that is not only simple, affordable, and safer but also breaks the compact network structure of the biomass while protecting its biopolymers (cellulose, hemicellulose, and lignin) from degradation.

PILs have emerged as promising green solvents for their use in the lignin extraction process, according to recent developments and research. ,, PILs display improved selectivity in the extraction process because of their capacity for hydrogen bonding, remarkable heat stability, low vapor pressure, recyclability, and particular interactions with lignin polymers. , Achinivu et al. reported the extraction of lignin from corn stover using protic ionic liquids. A lignin extraction efficiency of 75% was achieved at 90 °C in 24 h. Brandt-Talbot et al. studied bond breakage and its mechanism when PILs interact with biomass. They reported that PILs can depolymerize lignin by cleaving β-O-4 ether, glycosidic, and ester linkages and breaking the bonds present between lignin and hemicellulose. The same research team synthesized a protic ionic liquid ([TEA]­[HSO₄]) based on triethanolamine (TEA) on a large scale. They delignified Miscanthus giganteus using [TEA]­[HSO₄]; 85% lignin removal and 100% pure hemicellulose and cellulose were achieved.

Brito et al. delignified cabbage stalks using bis­(2- hydroxyethyl) ammonium-cation-based PIL ([BHEA]­[Pr]). They reported a 90% lignin extraction efficiency from cabbage stalks. In order to investigate the toxicity of PILs, they prepared various samples of mineral acids and PILs with cabbage stalks, respectively. They reported that the proportion of toxic compounds in PIL-treated samples was far less than that in samples treated with conventional mineral acids.

Although many significant improvements in the field of lignin valorization have been brought about by employing a PIL, there is still a need to explore cheaper PILs. From our previous study, pyridinium protic PILs have been established as superior solvents for lignin dissolution and extraction. Pyridinium protic PILs have shown potential in selectively solubilizing lignin, even from complex biomass matrices. Among the several amines employed in synthesis, pyridine is a less often used reagent among PILs, while being readily accessible and reasonably priced. Our previous research established pyridinium-based PILs with alkyl side chains as highly biodegradable and efficient compared to imidazolium-based ILs.

In the present study, a new PIL containing pyridinium cation and formate anion, i.e., pyridinium formate ([Py]­[For]), is extended to study its impact on lignin valorization from Saudi date palm biomass. Phoenix dactylifera L. is a common plant in Saudi Arabia. There are many types, but we selected Phoenix–Ajwa, Phoenix–Safawi, Phoenix–Amber, and Phoenix–Rabia, which are comparatively more popular and produce more waste. The acquired PIL was utilized under mild extraction conditions (75 °C, 3 h) for delignification of Saudi date palm biomass. The results showed that pyridinium-based PIL exhibited excellent lignin extraction efficiency (%) as well as the disruption of unyielding networks of cellulose and lignin. The extracted lignin samples and cellulose-rich residues were analyzed by using Fourier Transform Infrared Spectroscopy (FTIR), Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD). To the best of our knowledge, there are currently no studies that have examined and collected parametric and analytical data on the application of pyridinium-based PIL for lignin extraction from these specific date palm (Phoenix–Ajwa, Phoenix–Safawi, Phoenix–Amber, and Phoenix–Rabia) leaf sheaths despite their potential uses. Our research aims to investigate the characteristics of the lignin obtained from different species of date palm leaf sheaths and their scope for industrial use and evaluate the effectiveness of pyridinium-based PIL in the targeted lignin extraction process to fill this knowledge gap.

2. Materials and Methods

2.1. Materials

In this study, the preliminary material consisted of 99% (w/w) formic acid, glacial acetic acid, pyridine, DMSO-d ₆, 99% (w/w) acetone, 98% (w/w) sulfuric acid, a 17.5% (w/w) solution of sodium hydroxide and sodium chlorite (chemical reagents required for the compositional analysis of the raw biomass), kraft lignin (used for comparative analysis), and other reagents. All the preliminary materials were sourced from Sigma-Aldrich and used as received. The PIL was prepared through a proton exchange and atom neutralization process described by Rashid et al.,. Karl Fischer titration standards were used for the moisture content analysis.

2.2. Methods

Phoenix-Ajwa (P-Aj), Phoenix-Safawi (P-Sa), Phoenix-Amber (P-Am), and Phoenix-Rabia (P-Ra) are four varieties of Saudi date palm that were obtained from Saudi Arabia and kept at a temperature of 25 °C. To remove grease and oil from the raw biomass, a 2 wt % detergent solution was used to wash it. To further ensure the complete removal of grease and organic contaminants, Soxhlet extraction was also performed on the washed samples. The cleaned biomass samples were ground and crushed in a power cutting mill after being dried naturally in the sun for 24 h (Pulverizette 25). The ground-up biomass samples were then sieved to produce particles with size ranges of 100–300 μm, 300–500 μm, and 500–1000 μm, respectively. For the tests, triple-distilled water was employed.

2.3. Proximate and Compositional Analysis

The following standard procedures were used to determine the composition of the Saudi date palm biomass (P-Aj), (P-Sa), (P-Am), and (P-Ra):

• (1)TAPPI T222 om-02 was employed to determine the lignin content.
• (2)The method explained by Teramoto and Lee. was used to determine holocellulose and α-cellulose.
• (3)Extractives, moisture, and ash contents were calculated using the NREL laboratory analytical procedures (LAP-010), (LAP-001), and (LAP-005), respectively.

The average of the duplicates served as the source for all the given data listed in Table

1. Composition of the Saudi Date Palm Leaf Sheath of Different Species.

% Composition
Saudi Date Palm Leaf Sheath
Components	(P-Aj)	(P-Sa)	(P–Am)	(P–Ra)
Acid insoluble lignin	20.14 ± 0.31	23.35 ± 0.34	22.25 ± 0.28	25.23 ± 0.28
Hemicellulose	26.54 ± 0.38	27.11 ± 0.40	26.47 ± 0.28	24.58 ± 0.28
α-Cellulose	40.68 ± 0.37	41.35 ± 0.34	43.15 ± 0.27	42.15 ± 0.25
Volatile Matter	4.21 ± 0.28	4.21 ± 0.32	2.62 ± 0.32	3.10 ± 0.37
Ash (dry matter)	3.83 ± 0.31	2.59 ± 0.28	2.82 ± 0.31	3.17 ± 0.34
Others	4.6	1.39	2.68	1.77

^aCalculated by difference.

2.4. PIL-Mediated Extraction of Lignin

The lignin extraction scheme from the Phoenix date palm biomass species is shown in Figure . Phoenix date palm biomass species ((P-Aj)/(P-Sa)/(P-Am)/(P-Ra)) (ground to a particle size range of 300–500 μm) were placed into a 100 mL round-bottom flask along with the necessary amount of PIL for the lignin extraction procedure. The flask was placed in an oil bath and swirled at room temperature for 2 h. After the extraction procedure was finished, lignin was regenerated by adding an acetone/water mixture in a ratio of 1:2.

1.

Lignin extraction scheme from Phoenix date palm biomass species.

Subsequently, lignin precipitated out, while the cellulose-rich residue settled down. The cellulose-rich residue was subjected to multiple washes with triple-distilled water by filtering through a ceramic funnel. The separated lignin/[Py]­[For]/water mixture underwent filtration using a vacuum filtration apparatus. Afterward, the resulting filtrate was processed through a rotary evaporator to recover it for reuse. The dried lignin and the cellulose-rich residue were heated to 60 °C until they reached a uniform weight. The lignin extraction efficiency (%) was estimated using the protocol described in our previous study. The formula below was used to determine the quantity of lignin extracted (%).

$$\mathrm{Total}\mathrm{Lignin}\mathrm{Extracted}\%=\left(\frac{\mathrm{Total}\mathrm{lignin}\mathrm{extracted}\mathrm{from}\mathrm{Saudi}\mathrm{Date}\mathrm{Palm}}{\mathrm{Total}\mathrm{lignin}\mathrm{content}\mathrm{of}\mathrm{Saudi}\mathrm{Date}\mathrm{Palm}}\right)\times 100$$ 1

2.5. Characterization of Lignin Extracted from (P-Aj)/(P-Sa)/(P-Am)/(P-Ra)

2.5.1. Fourier Transform Infrared Analysis (FTIR)

The presence of functional groups in the extracted lignin samples was confirmed by FTIR spectroscopy. The FTIR spectroscopy was carried out using a PerkinElmer spectrometer with wavenumbers in the range of 4000 to 1000 cm^–1.

2.5.2. Thermogravimetric Analysis (TGA)

The thermal behavior of extracted lignin samples (P-Aj), (P-Sa), (P–Am), and (P–Ra) was determined using a PerkinElmer Pyris V-3.81 thermal gravimetric analyzer. Samples were heated from 50 to 800 °C at 10 °C min^–1. For performing the analysis, around 4 mg of the sample was weighed into an aluminum pan under a nitrogen blanket, maintaining a nitrogen flow of 20 to 25 mL/min.

2.5.3. Differential Scanning Calorimetry (DSC)

The glass transition temperatures “Tg” of extracted lignin samples were investigated using a calibrated Waters DSC Q2000 V24.11 Build 124A instrument using indium for calibration. The samples were dried at 150 °C at a heating rate of 20 °C min^–1 before testing. Before the measurements were performed, they were cooled to a stable temperature of 20 °C. About 7 mg of the sample was placed into an aluminum crucible. All measurements were recorded at a heating rate of 10–20 °C min^–1. The values reported are based on the averages of two sets of measurements.

2.5.4. Scanning Electron Microscope (SEM)

Samples were examined for their structure using SEM (model LEO VP1430) at a voltage of 20.0 kV. To ensure the leaf sheaths were conductive and to prevent degradation and charge accumulation, the samples were gold-coated before imaging. Magnifications of 400X and 500X were used to take images of the samples.

2.5.5. X-ray Diffractometer (XRD) Analysis

The crystalline structure of pure microcrystalline cellulose (MCC) PIL-treated residue was examined using a Bruker X-ray diffraction (model Bruker D8) advance horizontal XRD equipped with a Cu anode. The samples were scanned at room temperature within the range of 10.00 ° to 80.00 ° at a 2θ scale in step mode, with an increment of 0.020° with a step time of 1 s.

2.5.6. ¹H NMR Analysis of Recycled Solvent

The ¹H NMR (Nuclear magnetic resonance) analysis of recycled solvent was performed on a Bruker AVANCE 400 NMR spectrometer at 400 MHz frequency and room temperature using DMSO-d₆ as the solvent.

3. Results and Discussion

3.1. Effect of Operating Conditions on Lignin Extraction from Saudi Date Palm Leaf

The application of [Py]­[For] for lignin extraction from the Saudi date palm leaf is investigated for the first time in this study. Therefore, initial screening experiments were performed to examine the effect of various operating conditions on lignin extraction from Saudi date palm leaves using PIL. The impact of conditions, i.e., particle size ranges 100–300 μm, 300–500 μm, and 500–1000 μm; biomass loading (5, 10, 15, 20 w/w % (i.e., a 5% (w/w) solution corresponds to 5 g of biomass dissolved in 95 g of solvent); extraction temperature (25, 50, 75, 100, 125) °C; and extraction time (1, 2, 3, 4, 5, 6) h, was investigated. The findings are discussed in the following section.

3.1.1. Effect of Biomass Loading and Particle Size

Reduced biomass loading is typically more conducive to improved pretreatment and successful delignification. , This is reasonable because PIL’s interaction with the biomass particles increases with decreasing biomass content. Figure a shows that lignin extraction significantly increased with an increase in biomass loading from 5% to 10% (w/w). Increasing solid loading leads to an increased concentration gradient, and hence more lignin diffuses from the internal cell matrix into the solvent, which favors mass transfer, and thus the extraction efficiency was enhanced. However, increasing biomass loading (i.e., >10%) results in decreased extraction efficiency; these findings align with the previously reported literature. Various types of date palm biomass samples did not show a significant impact on the extent of delignification. However, (P-Aj) showed slightly higher lignin extraction efficiency, i.e., 82 wt %, which may be due to its lower lignin content (Table ) compared to other date palm biomass samples. It is evident from Figure a that an optimum extraction efficiency (%) is achieved at 10% biomass loading. Based on the experimental findings, a 10% (w/w) biomass loading was chosen for additional experiments to prevent the use of an excessive amount of solvent.

2.

Lignin extraction from Phoenix date palm biomass species ((P-Aj)/(P-Sa)/(P-Am)/(P-Ra)) at (a) various biomass loadings and (b) a particle size range.

When biomass pretreatment is concerned, particle size selection is also crucial. Larger particle sizes are highly desirable because they eliminate the need for preprocessing, grinding, and particle selection, which reduces energy consumption and the time spent throughout the process. Various particle size ranges of 100–300 μm, 300–500 μm, and 500–1000 μm were used to study lignin extraction (Figure b). As expected, increasing the particle size from 100–300 μm to 300–500 μm decreased the lignin extraction efficiency. A possible explanation for the observed enhanced extraction at smaller particle sizes may be due to the reason that smaller particles possess higher surface area, hence shorter mass transfer distance. Similarly, larger particles require enhanced penetration of [Py]­[For] to accomplish equivalent extraction efficiency compared to smaller particles. , This may be because lignin is trapped in the complex 3D polymeric network of biomass, which leads to hindered penetration of [Py]­[For] in the case of greater particles. Moreover, it was observed that there was a minor difference between the extraction efficiency (%) at 100–300 μm and 300–500 μm; therefore, the a particle size of 300–500 μm was selected for further experiments.

3.1.2. Effect of Extraction Temperature and Time

It is well established that increasing the temperature or extending the duration of time results in an improved delignification process. , However, there exists a threshold limit beyond which the lignin extraction efficiency or pulp and glucose yield is adversely impacted. The PIL employed in our investigation also showed a similar pattern, which is consistent with the findings in the literature. Figure a shows that lignin extraction increased linearly until a threshold temperature (75 °C). However, after the threshold temperature (75 °C), it was observed that there was no significant increase in lignin extraction (%) even at a higher temperature of 100 °C. This trend decreased exponentially when a plateau in the extraction process was reached at 125 °C, as observed previously. , The maximum extraction efficiency (%) of lignin corresponds to 75 and 100 °C for 3 h. However, increasing the temperature from 75 to 100 °C resulted in only a minor rise in extraction efficiency (≈2%). According to the literature, higher extraction temperatures and extended extraction times are not favorable for lignin extraction as they may lead to the breaking of ether linkages present in lignin, which negatively affects the quality of extracted lignin and, as a result, hinders lignin valorization. In addition, for reduced energy consumption, a lower extraction temperature of 75 °C was selected for further experiments. The reason for achieving these low values of extraction temperature could be due to the high protic contents of [Py]­[For], which lead to low H-bond basicity, hence resulting in efficient lignin extraction efficiency (%). , It was also observed that (P-Aj) showed slightly higher lignin extraction efficiency (%) at all temperatures (25, 50, 75, 100, and 125 °C), i.e., 35, 77, 82, 84, and 80%, respectively. This may be due to the lesser lignin content present in (P-Aj) as compared to other date palm biomass samples.

3.

Lignin extraction from Phoenix date palm biomass species ((P-Aj)/(P-Sa)/(P-Am)/(P-Ra)) at (a) various extraction temperatures and (b) times.

Figure b shows that lignin extraction increases linearly with extraction time. However, after reaching a plateau at 3 h, there was no significant increase in lignin extraction. Figure b also depicts that increasing the time beyond 3 h further resulted in reduced lignin extraction, which is consistent with previous research. , The sustainability of the extraction process is dependent upon achieving the maximum yield in the shortest amount of time. This study revealed that the investigated PIL ([Py]­[For]) was able to extract lignin from date palm leaf sheath samples in a relatively short time.

According to Brandt et al., during prolonged incubation times, condensation reactions between lignin and solvents take place that lead to degradation of the native lignin structure. It can be concluded that [Py]­[For]-mediated lignin extraction makes the process time-effective.

3.2. Analysis of Extracted Lignin

3.2.1. FTIR Analysis

Qualitative FTIR is a typical method for determining the functional groups of lignin at specific frequencies. FTIR is often used to verify the presence of various functional groups in lignin, such as hydroxyl, carbonyl, methoxyl, and carboxyl groups. While the intensity of these bands can vary depending on the lignin’s source and the extraction method used, the FTIR spectra typically exhibit common patterns and distinct vibrational features characteristic of lignin. The presence of the lignin backbone in the fingerprint region was confirmed by comparing the extracted lignin from Saudi date palm biomass (P-Aj), (P-Sa), (P-Am), and (P-Ra) with commercially available kraft lignin (Figure ). Figure (A) displays the FTIR spectra for the extracted lignin from Saudi date palm biomass ((P-Aj), (P-Sa), (P-Am), and (P-Ra)) using [Py]­[For]. The O–H vibrations in the aromatic and aliphatic O–H groups of lignin appear at 3300–3500 cm^–1, respectively. The aromatic nature of lignin is attributed to the vibrations within its aromatic skeleton, specifically from the guaiacyl (G) and syringyl (S) components, which occur within the 1600–1300 cm^–1 frequency range. The peaks at 1600 cm^–1, 1512 cm^–1, 1450 cm^–1, and 1438 cm^–1 are attributed to the aromatic skeleton (C–C stretching). , Moreover, the bands at 1260 cm^–1 (assigned to guaiacyl ring breathing with carbonyl stretching (G) units), 1125 cm^–1 (assigned to the methoxyl group (C–O deformation)), and 1450 cm^–1 (assigned to C–H deformations of methyl and methylene groups) correspond to lignin. The fact that all distinct peaks could be seen in the spectra of the four extracted lignins indicated that the lignin’s structural integrity had not been compromised during the extraction and regeneration processes. Some substantial individual characteristics were observed because of the diversity in the morphological components of the biomass.

4.

Panel­(a) FTIR spectra for the extracted lignin from kraft lignin and Saudi date palm biomass: (a) kraft lignin, (b) (P-Aj), (c) (P-Sa), (d) (P–Am), and (e) (P–Ra) lignin from 4000 to 1000 cm^‑1. Panel (b): Detail of the fingerprint region of (a) kraft lignin, (b) (P-Aj), (c) (P-Sa), (d) (P–Am), and (e) (P–Ra) lignin from 1900 cm^–1 to 1100 cm^–1.

Figure B clearly shows that bands appear at 1717 cm^–1 and 1660 cm^–1, which are assigned to nonconjugated and conjugated carbonyl regions present in lignin. It is also observed that these bands are not present in the kraft lignin. This reveals that [Py]­[For]-treated lignin is chemically unmodified and rich in aromatics. This may lead to commercial lignin utilization from Saudi date palm biomass for the production of various chemicals that provide additional value, including lignin-derived carbon fibers, binders made of isocyanate, biological dispersing agents, and phenolic and thermosetting resins. During the delignification process using various solvents (acid, organic, ionic liquids), nonetherified phenolic OH groups are produced as the β–O–4 and α–O–4 linkages are cleaved and appear at 1371 cm^–1.

A reasonable level of comparison involving biomass from Saudi Arabian date palms (P-Aj), (P-Sa), (P-Am, and P-Ra) with commercially available kraft lignin is present, which confirms that lignin is being selectively extracted from the Saudi date palm samples using [Py]­[For]. ,

3.2.2. Thermal Analysis

The upper service limit and pyrolytic thermal stability of the material can be determined by Thermal Gravimetric Analysis (TGA). The selection of a solvent for lignin extraction is of high importance, as the thermal stability of lignin is highly dependent on the functional groups and linkages present in it. Figure a depicts the thermograms obtained for commercially available kraft lignin and Saudi date palm (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignin, respectively. In the temperature range of 30–800 °C, all samples exhibit two main thermal characteristics, i.e., degradation temperature (T_DTGmax) and onset temperature (T_onset), as summarized in Table .

5.

(a): TGA thermograms of kraft lignin, (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignin. (b): DTGA thermograms of kraft lignin, (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignin.

2. Degradation Temperature (T_DTGmax), Onset Temperature (T_onset), Glass Transition Temperature (Tg), and Residue Wt% of the Studied Lignin Samples.

Lignin Sample	Degradation Temperature (°C) TDTGmax	Onset Temperature (°C) Tonset	Glass Transition Temperature (°C) Tg	Residue wt %
Kraft Lignin	335	210	133	33.94
(P-Aj) Lignin	355	223	122	1.07
(P-Sa) Lignin	364	221	121	17.09
(P-Am) Lignin	353	235	122	8.44
(P-Ra) Lignin	352	236	124	5.39

The four extracted lignin samples ((P-Aj), (P-Sa), (P-Am), and (P-Ra)) exhibited different DTG curves, with the amount of lignin in biomass differing depending on the plant species and even between morphological parts of the same tree. Therefore, some individual differences and characteristics are noticeable. The onset temperatures (T_onset) of kraft lignin and the four extracted lignin samples ((P-Aj), (P-Sa), (P-Am), and (P-Ra)) are tabulated in Table . It is evident from Table that the T_onset of the extracted lignin samples ((P-Aj), (P-Sa), (P-Am), and (P-Ra)) is noticeably different from the T_onset of kraft lignin. The T_onset of kraft lignin was observed to be 210 °C, whereas for the extracted lignin samples ((P-Aj), (P-Sa), (P-Am), and (P-Ra)), the T_onset increased to ≥220 °C. This increase in T_onset could be due to the PIL treatment developing condensed linkages, which are extremely stable.

Observations indicated that the lignin from (P-Ra) exhibited the highest initial decomposition temperature (T_onset = 236 °C), as listed in Table . This elevated T_onset may be attributed to the presence of condensed structures within the G-type aromatic rings linked by C₅ bonds in the extracted lignin of (P-Ra), which are in greater abundance and possess considerable stability. ,

In the DTGA profile, the maximum weight loss can be divided into three main stages. The first stage, ranging from 25 to 150 °C, is indicative of the expulsion of free water and light volatiles from the lignin. The second stage, which is the initial degradation phase covering 151–565 °C, is associated with maximal weight loss due to pyrolysis mechanisms. In the third stage, secondary degradation occurs between 566 and 800 °C, during which the thermal decomposition of lignin’s aromatic rings leads to the formation of char or coke, as reported in the literature. , A minor weight loss in lignin was observed at 30 to 100 °C, attributed to the loss of light volatiles and moisture, which is not deemed a thermal event.

It was observed that the maximum weight loss in the first degradation stage was in the range of 151 to 565 °C for (P-Aj) lignin (355 °C), (P-Sa) lignin (364 °C), (P–Am) lignin (353 °C), and (P–Ra) lignin (352 °C). It can be clearly seen that the weight loss temperature of PIL-extracted lignin is higher than that of kraft lignin (335 °C). It was also observed that the residue weight (%) of the PIL-extracted lignin was lower than that of kraft lignin after the completion of the pyrolysis process at 800 °C. This shows that the PIL-extracted lignin is more thermally stable. Mohtar et al. also documented comparable results. An increase in the thermal stability of lignin may be due to enhanced branching and a higher level of condensation. The residue contents of (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignin are 1.07 wt %, 17.09 wt %, 8.44 wt %, and 5.39 wt %, respectively (Figure b). Whereas, in the case of kraft lignin, the residue contents were much higher (33.94 wt %), as shown in Table . This could be due to the presence of inorganic minerals, present as impurities, during the kraft delignification process. ,, The PIL-extracted lignin is thermally stable, as indicated by DTGA and TGA results. These results fall within the reported weight loss temperature range of PIL-extracted lignin in literature.

3.2.3. Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) is a widely used method for assessing the transition of lignin from a glass-like to an elastic, rubber-like state upon heating. “Tg” of lignin varies with its molecular weight and the amount of phenolic hydroxyl and methoxyl groups (constituents of syringyl units) it contains. “Tg” of lignin varies with its molecular weight and the amount of phenolic, hydroxyl, and methoxyl groups it contains. Figure illustrates the glass transition temperatures estimated from the DSC traces. The Tg values of four extracted lignin samples ((P-Aj), (P-Sa), (P-Am), and (P-Ra)) are tabulated in Table . It was observed that the Tg value is almost in the same range for all extracted lignin (i.e., 121 °C–124 °C).

6.

DSC thermograms of kraft lignin and (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignin.

The current findings indicate that the “Tg” values align with those in the literature, which states that the glass transition temperatures for three distinct lignins derived from biomass range between 80 and 180 °C. , Minor variations in Tg between the lignin extracted using PIL and kraft lignin may arise from delicate alterations in the hydrogen bond interactions within the molecules and the molecular weight distribution of the PIL-extracted lignin samples. , It was also observed that (P-Am) and (P-Ra) lignins exhibited small endothermic peaks in the DSC thermograms. These endothermic peaks may be attributed to the high lignin contents present in (P–Am) and (P–Ra) lignins. These lignin contents lead to slight structural differences among the samples studied, i.e., (P-Aj), (P-Sa), (P-Am), and (P-Ra) lignins. These endothermic peaks may be attributed to the structural differences and high lignin contents, due to which retention of moisture content and light volatile matters may have occurred. , These endothermic peaks are not considered phase transitions but are formed as a result of compositional differences and extraction conditions.

3.2.4. Morphological Analysis

The morphology of the untreated Saudi Date Palm leaf sheath and PIL-treated cellulose-rich residue (right) was captured using Scanning Electron Microscopy (SEM) (Figure S1). As (P-Aj) showed a maximum lignin extraction efficiency (%) for all studied conditions. Therefore, (P-Aj) was therefore used for the SEM analysis before and after [Py]­[For] treatment.

Figure S1 clearly shows a distinct morphological difference in the residue treated with PIL compared with the residue contents of (P-Aj) Saudi date palm biomass. According to Figure S1, the residue treated with [Py]­[For] exhibits a greater degree of agglomeration and morphological modifications resulting from fiber and debris dissolution during the extraction and regeneration phases, as referenced. ,, The alteration in texture, along with the formation of pores on the surface of the fibrous biomass upon [Py]­[For] treatment, confirms the dissolution process in the Saudi Date Palm leaf sheaths. These findings are consistent with previous reports on extracting polyphenolic compounds from medicinal plants under mild extraction conditions facilitated by ionic liquids. Additionally, PIL-enhanced lignin extraction and biomass saccharification are in line with earlier findings. ,

3.2.5. Effect on Crystallinity Index

The XRD patterns of PIL-treated and pure microcrystalline cellulose (MCC) are compared in Figure S2. XRD was used to compare the crystallinity index (CrI) of the [Py]­[For]-treated biomass. In this study, MCC was used only as a conceptual crystallinity reference. The X-ray diffraction pattern of MCC showed a well-defined primary peak at 2θ = 22.5° and a secondary peak at 2θ = 19°, while the [Py]­[For] treated cellulose-rich residue showed a marked decrease in the intensity of the primary and secondary peaks at the same intensities, indicating a decrease in crystallographic pattern behavior in the recovered residue. This is due to the degradation in the crystalline structure and, hence, decreased crystallinity. The XRD patterns of treated and pure cellulose (MCC) are compared in Figure S2. A crystallinity index was used to quantify the crystallinity of commercial and [Py]­[For]-treated cellulosic residue using the following equation:

$$\mathrm{Crl}\%=\frac{\mathrm{Imax\; -\; Imin}}{\mathrm{Imax}}$$ 2

where Imax is the height of the peak at 2θ = 22.5°and Imin is the height of the peak at 2θ = 19°. The estimated CrI % was found to be 91% and 62% for MCC and [Py]­[For]-treated cellulose-rich biomass residue, respectively. The results indicate that the crystallinity remarkably decreased after [Py]­[For] treatment.

3.3. Effect of Solvent Recycling on Extraction Efficiency

For biomass pretreatment to be established in an industrially viable manner, the solvent must be recyclable while maintaining its extraction efficiency. This was validated by subjecting the recovered solvent, after the first cycle of the extraction process, to a rotary vacuum evaporator for 6 h at 80 °C and 30 kPa. Approximately 98% of the mass of [Py]­[For] was recovered, indicating the potential viability and recyclability of [Py]­[For]. Three extraction cycles were performed while utilizing recovered [Py]­[For], and lignin extractions of 88%, 78%, and 73% were observed for recycling cycles 1, 2, and 3, respectively. The extraction efficiency of recycled [Py]­[For] in the first recycle was found to be quite similar to that of fresh [Py]­[For]. A slight decrease in extraction efficiency (second and third cycles) may be because the water content in the recycled solvent was not reduced to ppm levels. Decreasing the water content to a suitable parts-per-million level could improve the extraction efficiencies. Solvent purity (for the three recycled solvent samples) was analyzed by employing ¹H NMR after every cycle. The ¹H NMR spectra for pure and recycled [Py]­[For] are presented in Figure S3. Results revealed that the ¹H NMR spectra of pure [Py]­[For] and those after the first and second recyclings are comparable, indicating that there was no formation of side products during the extraction process and no modifications in the molecular structure of [Py]­[For]. However, it was observed that the signal at 8.55 reduced in the second and third recycles. This reduction of the signal after the second and third recycles may be attributed to changes in the hydrogen bonding and exchange environment of the −CHO (formate) group. A possible reason for this may be the presence of water content in the recycled solvent. Moreover, it can also be seen that the pyridinium ring signals are persistent, indicating that the fundamental structure of [Py]­[For] remains intact.

4. Conclusions

The present study presents the first phase in the research on pyridinium protic ionic liquid, having clusters around the formate anion [Py]­[For]. Various species of the Saudi Date Palm leaf sheath were studied to explore the effect of operating parameters and compare their chemical, thermal, and morphological properties. Lignin was successfully extracted using mild extraction conditions (75 °C) and a relatively short extraction time (3 h). It was observed that the initial lignin content of the biomass affected the extraction efficiency significantly. (P-Aj) exhibited an extraction efficiency of 82% at these optimum conditions using [Py]­[For]. The FTIR spectra of the extracted lignin revealed a notable difference between the compositions of kraft lignin and [Py]­[For]-extracted lignin. Compared to Kraft lignin [Py]­[For], extracted lignin exhibited improved thermal stability, which enables its use for applications that require high thermal stability, such as polymer bionanocomposites. Furthermore, the Tg of [Py]­[For]-extracted lignin was not affected by the dissolution and regeneration process. SEM analysis showed that [Py]­[For]-treated cellulose-rich residues exhibited irregular and agglomerated morphology.

Saudi Date Palm leaf sheath lignin holds potential for use in pharmaceuticals and the biorefinery industry. The current extraction method, using a pyridinium protic ionic liquid, paves the way for creating various high-value products from lignin, such as phenolic compounds, adhesives, carbon fibers, motor fuels, lignin-derived polymers, and absorbents. This method could advance the development of a lignin-based biorefinery and improve pretreatment methods for lignocellulosic biomass. Prompted by these initial findings, further investigations are underway to validate the application prospects and efficacy of the [Py]­[For] solvent in lignin extraction from actual lignocellulosic biomass.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c12443.

• The Supporting Information includes several further characterizations, such as SEM micrographs of untreated Saudi Date Palm leaf sheath and [Py]­[For]-treated cellulose-rich residue; XRD spectra of MCC and [Py]­[For]-treated cellulose-rich residue; and ¹H NMR of pure and recycled [Py]­[For] (PDF)

A.A.: Conceptualization, Methodology, Writing – Original Draft, Project Administration, Funding Acquisition. T.R.: Data Curation, Methodology, Formal Analysis, Software, Writing – Original Draft. K.M.: Validation, Investigation, Visualization. R.N.: Resources, Visualization, Project Administration. A.A.: Writing – Review & Editing, Visualization. M.A.A.: Supervision, Writing – Review & Editing. E.A.: Project Administration, Resources. S.U.I.: Supervision, Methodology, Project Administration, Writing – Review & Editing

The authors declare no competing financial interest.