
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

72078
10.1038/s41598-024-72078-8
Article
Enhanced nanocellulose extraction from date palm waste: green solvent hydrolysis with transition metal complex
Raza Mohsin
Jawaid Mohammad
Abu-Jdayil Basim babujdayil@uaeu.ac.ae

https://ror.org/01km6p862 grid.43519.3a 0000 0001 2193 6666 Chemical and Petroleum Engineering Department, College of Engineering, United Arab Emirates University, PO BOX 15551, Al Ain, United Arab Emirates
20 9 2024
20 9 2024
2024
14 2196012 3 2024
3 9 2024
© The Author(s) 2024
2024
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This study presents a novel method for nanocellulose production using [Bmim]Cl as a green solvent, with enhanced hydrolysis efficiency achieved through the addition of a transition metal complex as a catalyst. The redox capability of the transition metal complex to break the glycosidic bonds in cellulose is amplified by the addition of an oxidizing agent. This protocol represents the latest innovation in the field of nanocellulose production, resulting in improved yield and reduced particle size. Nanocellulose (NC) was extracted from date seeds using 1-butyl-3-methylimidazolium chloride [Bmim]Cl coupled with a transition metal complex comprising copper metal and pyridine as a ligand along with H2O2 as an oxidizing agent. Unlike conventional [Bmim]Cl hydrolysis, which typically yields only microcrystalline cellulose (MCC), this approach resulted in a 25% higher yield of NC than that of MCC. Dynamic light scattering analysis showed a substantial reduction in hydrodiameter from 1200 nm for MCC to 128.7 nm for NC, highlighting the remarkable efficiency of this process. Thermal analysis demonstrated the high stability of NC, which showed a Tonset of 286 °C and an activation energy (Ea) of 220.41 kJ/mol. X-ray diffraction analysis indicated that NC possessed a high degree of crystallinity (Crl= 70.28%). Furthermore, NC underwent modification with 3-aminopropyltriethoxysilane to replace free hydroxyl groups (–OH), making it redispersal and suitable for various applications. This modification was confirmed through Fourier transform infrared spectroscopy, which showed the presence of characteristic functional groups, and energy-dispersive X-ray spectroscopy, which verified the elemental composition. Zeta potential measurements revealed surface charge differences, with MCC at − 27.87 mV, NC at − 27.28 mV, and modified NC at − 44.72 mV, indicating improved colloidal stability after modification. These findings highlight the protocol's effectiveness and its potential impact on the NC production industry, offering improved yields and the production of nanosized fibers using green solvents.

Keywords

Nanocellulose
Microcrystalline cellulose
Copper metal complex
Date seeds
1-butyl-3-methylimidazolium chloride [Bmim]Cl
Subject terms

Engineering
Materials science
Nanoscience and technology
http://dx.doi.org/10.13039/501100006013 United Arab Emirates University 12R014 12R014 Raza Mohsin Abu-Jdayil Basim issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

The extraction of a natural biopolymer, nanocellulose (NC), from lignocellulosic biomass presents an opportunity to utilize low-value waste and promote the concept of circular economy1. Utilizing biomass is a cost-effective strategy that promotes permaculture by recycling organic waste into valuable products2. In recent years, research trends regarding the use of mineral acids as hydrolysis media for cellulose to produce nanocellulose have shifted3. Limitations associated with sulfuric acid (H2SO4) and hydrochloric acid (HCl) hydrolysis include equipment corrosion, lack of acid recovery, toxic-pollutant generation, costly waste disposal, excessive water consumption, and cellulose degradation, resulting in lower yields4.

Methylimidazolium-based ionic liquids (ILs), known for their lower melting point, lower volatility, and higher thermal stability compared to conventional molecular solvents, have emerged as hydrolyzing solvents for cellulose5. ILs offer the advantage of their efficient recovery after hydrolysis (e.g., 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) can be reused over 90% across four cycles without losing activity)6. Moreover, the ability of [Bmim]Cl to alter cation/anion combinations offers versatility, as exemplified by the transition from [Bmim]+ to [SBmim]+, impacting NC dimensions and morphology7. [SBmim]+ refers to the cation 1-sulfobutyl-3-methylimidazolium. It is a modified version of the [Bmim]+ cation, where the butyl group is functionalized with a sulfonate group (-SO3H). This modification can influence the properties of the ionic liquid, such as its solubility, acidity, and interaction with cellulose, potentially impacting the efficiency and outcome of cellulose hydrolysis8,9.

[Bmim]Cl, an IL comprising chloride as the anion and butyl side chains on the methylimidazolium core, shows enhanced cellulose dissolution ability10. 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) is regarded as an environmentally friendly alternative to conventional solvents due to its low volatility and potential for recyclability, which helps in reducing air pollution and waste generation. Its effectiveness in dissolving cellulose can lead to more energy-efficient processes in specific applications11. A notable advantage of [Bmim]Cl is its ability to be recovered and reused after nanocellulose (NC) production, and with appropriate waste disposal practices, this can result in minimal environmental impact compared to conventional mineral acids 12,13. Nonetheless, concerns include its toxicity to aquatic organisms, limited biodegradability, and the use of non-renewable resources in its production, highlighting the need for responsible production practices and effective waste management14,15.

Phanthong et al.7 produced microcrystalline cellulose (MCC) via [Bmim]Cl-assisted ball milling, while Jordan et al.16 produced NC using [Bmim]Cl mixed with H2SO4. Muhazeli et al.17 treated cellulose with [Bmim]Cl extracted from Mahang wood to produce MCC. Iskak et al.18 successfully produced NC via hydrolysis using [Bmim]Cl from commercial MCC. Despite its cellulose-dissolving abilities, [Bmim]Cl is constrained to producing either MCC with mechanical assistance or NC when [Bmim]Cl hydrolysis is combined with mild mineral acids. While [Bmim]Cl demonstrates significant cellulose-dissolving abilities, its capacity to produce nanocellulose is limited when used independently. The production of nanocellulose typically requires either mechanical assistance, as in the case of microcrystalline cellulose production, or the addition of mild mineral acids to enhance hydrolysis. This suggests that [Bmim]Cl alone is insufficient for efficiently generating nanocellulose without the aid of additional processes or substances.

Enhancing the hydrolysis capacity of [Bmim]Cl is vital for the efficient production of NC without mineral acid addition or mechanical co-treatment. NC offers numerous advantages over microcellulose, including superior mechanical properties, high surface area, and biodegradability, rendering it valuable for various applications19. To date, no research has investigated the improvement of the hydrolysis capacity of [Bmim]Cl as a green solvent while enhancing yield, both of which are critical factors ensuring hydrolysis media efficiency.

Transition metal complexes containing metals such as copper, iron, or chromium exhibit catalytic properties that facilitate the degradation of lignocellulosic materials20. These complexes leverage the redox capabilities of transition metals to enable electron transfer reactions, leading to oxidative reactions and the cleavage of chemical bonds in biomass21. While transition metal complexes have proven effective in fractionating lignocellulose into cellulosic and lignin streams, their catalytic role is primarily confined to this fractionation process22. This process often entails a two-step treatment involving initial delignification with sodium hydroxide (NaOH) followed by treatment with a metal complex (e.g., copper sulfate and bipyridine) in alkaline media comprising NaOH and hydrogen peroxide (H2O2)23.

Due to the presence of primary and secondary hydroxyl groups (–OH) on each β-1,4-anhydro-d-glucopyranose unit in the cellulose structure, along with strong intermolecular hydrogen bonds and high surface energy, NC fibers tend to aggregate, thereby limiting their applications24. However, H2SO4 hydrolysis addresses this problem by attaching negatively charged sulfate half-ester groups to the surface, resulting in the formation of a negative electrostatic layer that encapsulates NC and enhances its redispersing behavior25. Consequently, when employing ILs for hydrolysis, it becomes essential to modify the surface of NC to achieve optimal redispersion by replacing the free –OH groups.

Silane grafting emerges as an effective method for modifying NC surfaces. 3-aminopropyltriethoxysilane (APTES) stands out for its simple structure and cost efficiency26. The chemical grafting of APTES onto NC surfaces typically involves three steps: (i) hydrolysis of the alkoxy groups of silane in the presence of water to generate silanols; (ii) adsorption of silanol groups onto the –OH-rich surface of NC; and (iii) chemical condensation, leading to the formation of siloxane bridges (Si–O–Si) and their grafting onto NC surfaces via Si–O–C bonds27.

This study presents a new protocol for producing nanocellulose (NC) via green solvent hydrolysis, coupled with a transition metal complex. Given the redox capabilities of transition metal complexes, it was hypothesized that these complexes would enhance the disruption of glycosidic bonds in cellulose. Additionally, it was hypothesized that hydrogen peroxide (H₂O₂) would augment the redox capability of the transition metal complex. To this end, the study aimed to improve the hydrolysis efficiency of [Bmim]Cl, which had previously been limited to the extraction of microcrystalline cellulose (MCC). A transition metal complex, consisting of copper and a pyridine ligand, was used in varying weight percentages (0.5%–1.5%), in combination with H₂O₂ as an oxidizing agent.

There is no existing study in the literature where a transition metal complex has been used to improve the hydrolysis efficiency of an ionic liquid, nor has any study investigated the use of an oxidizing agent to amplify the yield of NC during hydrolysis with ionic liquids. This study is of great significance for NC production at industrial scales using ionic liquids, which is currently not available. It represents a significant advancement and the latest innovation in the field of NC production. Date seeds (DSs) were selected due to their abundance in the United Arab Emirates and the Middle East and North Africa (MENA) region. With approximately 40 million date palm trees in the UAE alone, each generating various types of waste such as date pits/stones, leaves, surface fibers, rachis, stems, and wood, utilizing these by-products becomes essential28. Effective utilization of date seed waste can significantly reduce the costs associated with waste management and foster the production of value-added products, thereby promoting the concept of a circular economy.

Production process optimization was conducted using response surface methodology with a central composite design (CCD). The study also includes analysis of variance (ANOVA) for the transition metal complex and H2O2. Finally, silane treatment was applied to NC to replace the free hydroxyl groups (–OH) to facilitate its redispersion. The results of this study are innovative and introduce a new methodology for producing NC with nanosized dimensions using green solvents with higher yield and improved redispersion behavior, rendering NC suitable for various applications.

Materials and methods

Materials

The DSs were obtained from the agricultural farm of the United Arab Emirates University. Sodium hydroxide (NaOH), sodium chlorite (NaClO2), 1-Butyl-3-methylimidazolium chloride [Bmim]Cl, copper nitrate Cu(NO3)2, 3-Aminopropyl)triethoxysilane (APTES, 99%), acetic acid glacial (≥ 99%) and pyridine were purchased from Sigma-Aldrich (Germany) and used without any further modification. Table 1 shows the chemical composition, proximate analysis, and ultimate analysis of date seeds, as reported in our previous publication29.Table 1 Chemical composition, proximate and ultimate analysis of date seeds.

Chemical composition	
 α − cellulose	27.85	
 hemicellulose	29.55	
 lignin	30.63	
Proximate analysis	
 Volatile matter (%)	78.70	
 Fixed carbon (%)	13.84	
 Ash content (%)	2.46	
 Moisture (%)	4.5	
Ultimate analysis	
 C (%)	46.95	
 H (%)	6.10	
 N (%)	0.90	
 S (%)	0.10	
 O (%)	45.95	
 HHV (MJ/kg)	19.25	

Pretreatment of DSs

The DSs were subjected to delignification with a 4 wt.% NaOH solution at 80 °C for 1 h, followed by vacuum filtration. Subsequently, the fibers underwent bleaching with a 4 wt.% NaClO2 solution at 80 °C for 1 h, followed by vacuum filtration and rinsing until a pH of 6.5–7 was achieved. The resulting delignified and bleached fibers, identified as cellulose, were dried at 105 °C for 24 h.

MCC extraction using [Bmim]Cl

[Bmim]Cl and cellulose were preheated at 100 °C for 1 h to convert [Bmim]Cl into a liquid. Subsequently, 1 g of cellulose was added to 10 ml of [Bmim]Cl, and hydrolysis was conducted at 100 °C for 3 h. The hydrolysis process was terminated by adding 500 cm3 of deionized (DI) water at 4 °C, which generated white precipitates. Following centrifugation (4000 rpm, 4 times), the microfibers were sonicated and then vacuum-filtered to eliminate nonhydrolyzed fibers. The final step involved freeze–drying at − 65 °C to obtain MCC.

Nanocrystalline cellulose extraction using [Bmim]Cl and copper metal complex

The optimized experimental conditions were determined using a CCD, where the amounts of copper catalyst (0–10 mg) and hydrogen peroxide (0–5 ml) were varied. Under the optimized hydrolysis conditions, a copper metal complex consisting of 5 mg of copper nitrate (Cu) and 5 mg of pyridine (py) as a ligand was added to a solution containing 1 g of cellulose in 10 ml of [Bmim]Cl. Additionally, 1.65 ml of 10 v/v% H2O2 was added, corresponding to 0.5 wt.% compared to the mass of cellulose. The hydrolysis process was terminated by adding 500 cm3 of deionized (DI) water at 4 °C, which generated white precipitates. Following centrifugation (4000 rpm, 4 times), the NC suspension was sonicated. The final step involved freeze–drying at − 65 °C to obtain NC. Figure 1 shows an overview of the MCC and NC extraction process.Fig. 1 Overview of the MCC and NC extraction process.

In the extraction process of NC, the use of [Bmim]Cl, as green solvent, in combination with H2O2, contributes to a greener methodology compared to conventional techniques. Ionic liquids like [Bmim]Cl are considered green due to their non-volatile nature and recyclability, making them a more environmentally friendly option for cellulose dissolution. H2O2 enhances the green credentials of this process as it acts as a green oxidant, breaking down into water and oxygen without leaving harmful residues. While pyridine is traditionally not considered green due to its toxicity, it is used in very small quantities (milligrams) to catalyse the reaction, minimizing its environmental impact. This strategic use of pyridine improves the efficiency of the process while maintaining the overall greenness of the solvent system. The addition of copper nitrate and pyridine improves the yield and particle size of NC during cellulose hydrolysis supporting the utilization of [Bmim]Cl as a green solvent.

The waste stream from delignification can be processed to isolate lignin, which can then be used for value-added applications such as in the production of bioplastics, adhesives, and as a precursor for fine chemicals30. Additionally, [Bmim]Cl, the ionic liquid used in hydrolysis, can be recovered through distillation or precipitation methods and reused to reduce costs and environmental impact. Careful disposal or recovery of metal complex residues is essential to prevent environmental contamination and ensure compliance with waste management regulations31.

The proposed mechanism indicates that the copper metal complexes, particularly in the presence of an oxidizing agent like hydrogen peroxide (H2O2), generate free radicals through redox reactions. These free radicals, such as hydroxyl radicals (•OH), have unpaired electrons, making them highly reactive. The presence of these radicals initiates radical chain reactions by abstracting hydrogen atoms from cellulose molecules, leading to the formation of new radicals on the cellulose chains. Free radicals generated from copper metal complexes together with H2O2 exhibit enhanced reactivity compared to [Bmim]Cl alone. The unpaired electron in the radical species enables them to react with cellulose molecules at specific sites, such as the glycosidic linkages, facilitating bond cleavage and fragmentation of the cellulose chains.

Following freeze–drying, MCC and NC were weighed to determine the yield using Eq. 1. The yield calculation offered insights into copper metal complex–assisted [Bmim]Cl hydrolysis efficiency.1 Yield=AmountofMCC/NCafterfreezdryinggmAmountofcelluloseusedforhydrolysisgm×100

Response surface design and optimization

The CCD implemented in the Minitab statistical software was utilized to optimize the weight percentage of the copper metal complex and H2O2. The result measured was the yield of both MCC and NC. Table 2 illustrates the CCD design, presenting the data in its original, uncoded form for enhanced comprehension. The two-factorial, two-stage CCD experimental design encompassed 13 experimental runs, featuring four factorial points, four axial points, and five mean points, to determine the pure error (experimental error).Table 2 CCD for optimizing copper metal complex–assisted [Bmim]Cl hydrolysis.

StdOrder	Cu–py (1:1) mg	H2O2 ml	Yield (%)	
1	0	0.0	72.00	
2	10	0.0	76.00	
3	0	5.0	74.55	
4	10	5.0	89.99	
5	0	2.5	75.11	
6	10	2.5	93.00	
7	5	0.0	82.10	
8	5	5.0	92.25	
9	5	2.5	91.88	
10	5	2.5	91.21	
11	5	2.5	91.97	
12	5	2.5	92.50	
13	5	2.5	92.29	

After optimizing the amount of the copper metal complex as 10 mg (Cu: py = 5 mg:5 mg) and H2O2 as 1.65 ml (10 v/v% solution of H2O2), as depicted in Fig. 2, it was observed that the yield tended to decline beyond these optimized values. Hydrolysis with [Bmim]Cl in the absence of a copper metal complex and H2O2 yielded only 72% microcellulose fibers as a substantial proportion of the particles remained unhydrolyzed and were subsequently removed by filtration. When [Bmim]Cl was combined with the copper metal complex, the yield increased to 76%. Similarly, when [Bmim]Cl was combined with H2O2, the yield reached 75%. Remarkably, the combination of copper metal complex and H2O2 together with [Bmim]Cl resulted in a significantly higher yield of 92%, leaving only a negligible amount of non-hydrolysed fibers. The combination of [Bmim]Cl with both the copper metal complex and H2O2 resulted in a significant increase in cellulose hydrolysis yield from 72 to 92%, demonstrating a clear synergistic effect. The inclusion of H2O2 improved the redox capability of the copper metal complex, enhancing its catalytic activity during the cellulose hydrolysis process. This improvement led to a more efficient breakdown of cellulose, contributing to the overall increase in yield when used in conjunction with [Bmim]Cl.Fig. 2 Optimized quantities for the copper metal complex and H2O2.

Utilizing copper metal complex–assisted [Bmim]Cl hydrolysis in conjunction with H2O2 under optimized conditions led to a substantial improvement in yield. The yield increased to 90%, marking a 25% increase compared to the uncatalyzed hydrolysis process. This enhancement underscores the efficacy of the copper metal complex together with H2O2 as a catalyst in promoting cellulose hydrolysis, resulting in a more efficient and productive reaction.

SEM and TEM analysis

Scanning electron microscopy (SEM) analysis was conducted using a JEOL, JSM-6390A instrument (Japan) to examine the morphology of DS, cellulose, MCC, and NC. Prior to the analysis, the samples were coated with gold to prevent electrostatic charging during testing. TEM analysis was performed using FEI Tecnai at 200 kV.

Particle size and zeta analyses

The average MCC and NC particle size was determined using dynamic light scattering (DLS) conducted with equipment from Otsuka Electronics (Japan). Measurements were automatically recorded at 25 °C. Prior to the analyses, MCC and NC were diluted 50 times with DI water and sonicated for 20 min.

Thermogravimetric analysis (TGA)

TGA was used to analyze the thermal decomposition characteristics of DS, cellulose, MCC, and NC using a mass range of 5–10 mg. The analysis was conducted utilizing the Q 500 series instrument from TA Instruments (USA). TGA was performed under nonisothermal heating conditions in an inert nitrogen atmosphere (60 mL/min). The heating rate was set to 20 °C/min, ranging from 25 to 700 °C for all samples.

Differential scanning calorimetry (DSC)

The thermal transition properties of cellulose, MCC, and NC were investigated using the TA instrument DSC25, USA. Samples weighing 5 and 10 mg were subjected to DSC within a temperature range of 30–300 °C under an inert nitrogen atmosphere (50 mL/min). The heating rate for the analysis was set to 10 °C/min.

X-ray diffraction (XRD)

DS, MCC, and NC were analyzed using Cu-K radiation under lamp operating conditions of 40 kV and 30 mA, with a receiving slit width of 0.15 mm. Reflection intensities were evaluated with a scanning speed of 2°/min and scanning range of 10–80°. The Segal equation was employed to determine the crystallinity index Crl for the samples32:2 Crl=I200-IamI200×100\%

where I200 denotes the diffracted intensity at the peak associated with the highest crystalline structure. Further, Iam denotes the diffraction intensity of the amorphous region.

Activation energy (Ea) analysis

The Ea analysis was conducted using the Coats–Redfern integral method with eight solid-state diffusion models. This method builds upon two previous studies28,33. Table 3 provides details regarding the diffusion models employed for the analysis.Table 3 Solid-state diffusion models34.

No	Mechanism	g(α)	Model code	
1	One-way transport (parabolic law)	α2	DM 1	
2	Two-way transport	(1 − α) − ln(1 − α) + α	DM 2	
3	Three-way transport	[[− ln(1 − α)]1/3]2	DM 3	
4	Valensi Eq. (2D diffusion)	α + (1 − α) ln(1 − α)	DM 4	
5	Ginstling–Brounshtein equation (anti-Jander equation)	(1 − 2α/3) − (1 − α)2/3	DM 5	
6	Zhuravlev equation	[(1 − α)−1/3 − 1]2	DM 6	
7	Jander Eq. (3D diffusion)	[1 − (1 − α)1/3]2	DM 7	
8	Ginstling Eq. (4D diffusion)	1 − (0.67α) − (1 − α)0.67	DM 8	

NC modification

First, 5 ml of APTES was combined with 100 ml of water to initiate the modification process. The pH of the APTES solution was adjusted to an acidic level of approximately pH = 4 by adding acetic acid. Subsequently, NC was introduced into the solution with constant stirring. The modification reaction proceeded for 120 min at 25 °C. After the reaction time was completed, the solution was centrifuged at 4000 rpm for 10 min to separate the APTES solution and obtain treated NC. Treated NC was then cured in an oven at 105 °C for 15 min. Following curing, modified NC underwent another round of centrifugation and washed with ethanol (96% purity) to eliminate any unreacted APTES. Modified NC was subsequently dried in an oven at 105 °C for 24 h27.

Fourier transform infrared (FT-IR) spectroscopy

FT-IR spectroscopy (Shimadzu, Kyoto, Japan) was used to analyze the functional groups of DS, MCC, and NC. Infrared spectra were obtained using an attenuated total-reflection FT-IR spectrometer, comprising 34 scans with a spectral resolution of 4 cm−1, spanning the range of 400–4,000 cm−1.

Energy-dispersive X-ray (EDX) analysis

SEM analysis was conducted using a JEOL, JSM-6390A instrument from Japan to analyze the elemental composition of NC and modified NC. Prior to analysis, the samples were coated with gold to prevent electrostatic charging during the test. The study was performed under a liquid nitrogen atmosphere.

Results and discussions

ANOVA

Table 4 presents the results of the ANOVA conducted to examine the impact of two factors, amount of Cu–py and H2O2, on the hydrolysis process in conjunction with the amount of [Bmim]Cl to enhance the yield of NC. The analysis indicates that the selected parameters substantially influence the final yield of both MCC and NC, as evidenced by the high F-value of the model (45.32) and the p-value of < 0.05. Moreover, the ANOVA reveals that 97% of the variability in the developed model is attributed to the linear relationship of these factors. In particular, the amount of copper and pyridine (Cu–py) demonstrates the most substantial effect, with an F-value of 64.34 (p-value < 0.05). Next, the amount of H2O2 emerges as the second most influential factor affecting the yield of NC, with an F-value of 32.89 (p-value < 0.05). The quadratic relationship (square of each factor) demonstrates profound significance on the yield of NC, indicated by an F-value of 60.16 (p-value < 0.05). Within the quadratic relationships, the amount of Cu–py exhibits a notable effect on the yield of NC, with an F-value of 54.73 (p-value < 0.05). At the same time, the amount of H2O2 shows an F-value of 21.80 (p-value < 0.05), indicating its significant influence on the variance of NC yield. Furthermore, the two-way interaction of both the factors significantly impacts the yield of NC, with an F-value of 9.06 (p-value < 0.05). Overall, the ANOVA reveals that 97% of the variability in the predicted model is explained by linear factors, 51.50% by their quadratic effects, and 3.88% by their two-way interaction. The error is only 3%. The low fitting error of 2.88% indicates that the predicted model is acceptable, fits the experimental data well, and can predict the yield of NC.Table 4 ANOVA for copper metal complex–assisted [Bmim]Cl hydrolysis.

Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-value	P-value	
Model	5	818.028	97.00%	818.028	163.606	45.32	0.000	
Linear	2	350.981	41.62%	350.981	175.490	48.61	0.000	
Cu–py	1	232.255	27.54%	232.255	232.255	64.34	0.000	
H2O2	1	118.726	14.08%	118.726	118.726	32.89	0.001	
Square	2	434.329	51.50%	434.329	217.164	60.16	0.000	
Cu–py*Cu–py	1	355.627	42.17%	197.585	197.585	54.73	0.000	
H2O2*H2O2	1	78.701	9.33%	78.701	78.701	21.80	0.002	
2-Way Interaction	1	32.718	3.88%	32.718	32.718	9.06	0.020	
Cu–py*H2O2	1	32.718	3.88%	32.718	32.718	9.06	0.020	
Error	7	25.270	3.00%	25.270	3.610			
Lack-of-Fit	3	24.301	2.88%	24.301	8.100	33.44	0.003	
Pure Error	4	0.969	0.11%	0.969	0.242			
Total	12	843.297	100.00%					

Regression model equation

Equation (3) represents the second-order polynomial model equation obtained through regression analysis using ANOVA based on the implementation of the CCD experimental design. This regression equation describes the NC yield in terms of two independent factors. The positive sign in this equation indicates that the factor positively affects the yield, while the negative sign signifies a negative effect on the yield. In the case of linear independent factors, the amount of Cu–py and H2O2 positively influence the yield, as indicated by their positive coefficients in the equation. However, in the quadratic effects, none of the quadratic independent terms positively affect the yield of NC. In addition, in two-way interactions, the interactions between the amount of Cu–py and H2O2 demonstrate positive synergistic effects on the yield of NC.2 Yield=70.52+4.056Cu-py+4.91H2O2-0.3383(Cy-py)2-0.854(H2O2)2+0.2288Cu-py×H2O2.

Table 5 summarizes the regression model developed to predict the yield of NC. In the table, S represents the standard deviation of the residuals, which are the deviations between the observed values (experimental values) and values predicted by the model. A smaller S value indicates a more accurate fit of the model to the data. R2 quantifies the proportion of variance in the dependent variable that can be predicted based on the independent variables. A higher value of R2 (coefficient of determination) indicates a better fit, and values > 95% indicate a strong correlation between predicted and experimental values. The adjusted R2 is an improved form of R2 that penalizes the inclusion of unnecessary variables that do not contribute to the model’s effectiveness. Achieving a of > 90% value in this study indicates a high potential for the accurate prediction of NC yield. Predictive R2 measures the model’s ability to predict outcomes for new, unseen data, providing important insights into its performance beyond the training dataset. An R2 value of 70.84% indicates a robust ability to predict NC yields beyond the range of experimental values.Table 5 Summary of the second-order model.

Model summary	
S (Residual standard deviation)	1.89	
R2 (Coefficient of determination):	97.0	
R2 (Adj) (Adjusted R-squared):	94.86	
R2 (pred) (Predictive R-squared):	70.84	
Model type	Quadratic	

SEM and TEM

Figure 3 presents the morphological evaluation of DS and extracted fibers at various stages of treatment. As shown in Fig. 3a, DS exhibits a coarse structure characterized by tightly encapsulated cellulose fibers embedded in lignin, a nonpolysaccharide biopolymer. This lignin is a protective barrier and makes the cellulose and hemicellulose resistant to chemical attack. Figure 3b illustrates cellulose fibers post delignification and bleaching. The fibers appear clearer and exhibit a spherical morphology. In Fig. 3c, cellulose fibers are depicted following hydrolysis with [Bmim]Cl. These fibers are reduced in size and exhibit swollen particles. Figure 3d showcases the hydrolysis of [Bmim]Cl assisted by the copper metal complex and H2O2, which substantially reduces the size of the cellulose fibers. The resultant spherical NC displays a uniform particle size distribution, indicating a notable enhancement in the hydrolysis capacity of [Bmim]Cl facilitated by the copper metal complex and H2O2. Figure 3e and f present transmission electron microscopy (TEM) images of NC, with spatial resolutions of 0.5 µm and 5 µm, respectively, confirming its nanoscale dimensions. The images reveal that the average particle size of NC ranges from approximately 50 to 80 nm.Fig. 3 SEM images of (a) DS, (b) cellulose, (c) MCC, (d) NC and (e & f) TEM images of NC.

FTIR analysis

The FTIR spectra of date seeds (DS), microcrystalline cellulose (MCC), and nanocellulose (NC) illustrate in Fig. 4 shows the transformation and purification of lignocellulosic material. In the DS spectrum, the broad O–H stretching peak at 3525 cm⁻1, along with the C-H stretching peaks at 2915 and 2849 cm⁻1, indicate the presence of hydroxyl and methylene groups, typical of cellulose, hemicellulose, and lignin35. The C=O stretching peak at 1720 cm⁻1 and the C=C aromatic peak at 1630 cm⁻1 suggest significant lignin and hemicellulose content36,37. As the material is processed into MCC, these peaks diminish, indicating reduced non-cellulosic components, while the C-O stretching peak at 1030 cm⁻1 remains prominent, reflecting retained cellulose. In NC, the spectra show a strong O–H peak, reduced C-H peaks, and an almost absent carbonyl peak, highlighting the effective removal of lignin and hemicellulose and the high purity and crystalline nature of nanocellulose. This progression demonstrates the effective purification and cellulose enrichment through each processing stage.Fig. 4 FTIR spectra of DS, MCC, and NC.

Particle size analysis

DLS is highly efficient in analyzing spherical particles, with its consistency corroborated by SEM results, reinforcing confidence in using DLS to measure the average particle size. MCC and NC exhibited average particle diameters of 1.2 µm and 128.7 nm, respectively, accompanied by a polydispersity index (PDI) of 0.429 and 0.138 respectively. The low PDI values indicate monodispersed particles with a narrow size distribution, highlighting NC’s suitability for various applications. [Bmim]Cl hydrolysis, assisted by a copper metal complex and H2O2, reduced NC particle size to approximately 100 nm. Figure 5 illustrates the particle size distribution of MCC and NC.Fig. 5 Particle size distribution of NC (LHS) and MCC (RHS).

TGA

Figure 6 illustrates the thermal decomposition curves for DS, cellulose, MCC, and NC, revealing three distinct stages: dehydration, devolatilization, and carbonization. Table 6 provides the thermal decomposition properties of the fibers at each treatment stage. As the untreated lignocellulosic material, DS exhibited an initial decomposition temperature (Tonset) of 230 °C, with a 30.78% mass loss during devolatilization. DS produced the highest biochar content (34.74%) due to their untreated nature and abundant lignin, a prominent biochar-forming component. Cellulose exhibited an enhanced Tonset at 275 °C, with a 50.78% mass loss during devolatilization, indicating cellulose accumulation. The biochar content decreased to 19.67%, which indicates effective delignification. The MCC resulting from [Bmim]Cl hydrolysis exhibited a Tonset of 270 °C, which is slightly lower than that of cellulose fibers, attributed to the cleavage of the cellulose chain, leading to shorter, more reactive fragments and alterations in the cellulose structure. Huang et al.38 similarly reported a reduction in Tonset during hydrolysis of cotton–linter-based cellulose as a precursor using [Bmim]Cl. In the devolatilization stage, the mass loss increased to 59.10%, accompanied by further cellulose accumulation, while the biochar content decreased to 8.6%. Interestingly, NC produced with copper catalyst–assisted [Bmim]Cl hydrolysis exhibited the highest Tonset at 286 °C, indicating increased thermal stability. This presence of the copper catalyst likely enables a more controlled and efficient decomposition of cellulose, resulting in a higher Tonset. The maximum mass loss in the devolatilization region (61.80%) indicates that the fibers were highly enriched with cellulose. Consequently, only 0.36% solid residue remains, indicating purity of NC. If the NC is of high purity and free from any inorganic impurities or additives, it would mean that most of the material decomposes completely at high temperatures. Specifically, the minimal residue suggests that there is an absence of lignin, as lignin typically contributes to the formation of biochar during decomposition. In the absence of lignin, which would otherwise contribute to residual mass in the post-volatilization region, the residual mass of the NC would be very low, reflecting its high purity and complete decomposition. Therefore, the low residual mass is a positive indicator of the high purity of nanocellulose achieved through [Bmim]Cl hydrolysis coupled with transition metal complex and H2O2, suggesting that the process effectively eliminates non-cellulosic components and results in a highly pure product. Figure 7 illustrates the differential thermogravimetric analysis (DTG), showing a notable trend where all fibers exhibit peak decomposition rates within a high-temperature range of 250 °C–320 °C.Fig. 6 TGA curves for DS, cellulose, MCC, and NC.

Table 6 Thermal characteristic properties of DS, B&D fibers, microfibers, and NC.

Fibers	Tonset(°C)	Tf(°C)	% Devolatilization	% Char	Tmax(°C)	
DS	230	317	30.78	34.74	253	
Cellulose	275	358	50.78	19.67	321	
MCC	270	353	59.10	8.7	303	
NC	286	357	61.80	0.36	318	

Fig. 7 DTG curves for DS, cellulose, MCC, and NC.

Eaanalysis

Table 7 represents the Ea values for MCC and NC utilizing eight solid-state diffusion models. Previous research has recognized diffusion models as effective reaction mechanisms for estimating Ea in lignocellulosic and cellulosic materials33. Ea values help understand the change in crystalline structure and resistance to chemical attacks, fire retardancy, and degradability39. The Ea analysis was performed in the main decomposition region (devolatilization region). The devolatilization region for MCC and NC were 270 °C–336 °C and 292 °C–336 °C, respectively. All diffusion models except DM2 showed a higher regression coefficient (R2>0.90) with experimental thermogravimetric data. The average estimated Ea values for MCC and NC were 139.60 and 220.41 kJ/mol, respectively. Regardless of the model used, Ea (NC) consistently surpassed Ea (MCC), indicating an increase in Ea ranging from 36 to 38%. This indicates that copper metal complex–assisted [Bmim]Cl hydrolysis together with H2O2 can yield NC with higher Ea compared to simple [Bmim]Cl hydrolysis, implying the potential use of NC in high-end applications such as composites materials with superior mechanical and fire-retardant properties.Table 7 Ea(kJ/mol) of MCC and NC extracted from DS.

Model Code	MCC	NC	
Ea(kJ/mol)	R2	Ea(kJ/mol)	R2	% Ea increase	
DM1	106.41	0.96	171.93	0.97	38.10	
DM2	53.08	0.89	86.50	0.87	38.63	
DM3	67.84	0.97	110.16	0.97	38.41	
DM4	131.77	0.98	210.34	0.99	37.35	
DM5	152.50	0.99	229.46	0.99	33.53	
DM6	288.60	0.91	451.45	0.89	36.07	
DM7	172.09	0.99	273.19	0.99	37.00	
DM8	144.54	0.99	230.29	0.99	37.23	
Avg	139.60		220.41			

It is essential to note that the Ea values for NC materials can vary substantially due to factors such as source materials and extraction methods. For instance, different lignocellulosic sources exhibit distinct Ea values, leading to variations in the properties of the extracted NC. For example, palm shell waste possesses a Ea of 150 kJ/mol40, cashew shells display a Ea of 115 kJ/mol41, while pine wood exhibits a notably higher Ea of 218 kJ/mol42. Table 8 provides a comprehensive overview of Ea values related to similar phenomena. In studies by Huang et al.43, NC extracted from corn stalks using sulfuric acid (60 wt.%) exhibited Ea values estimated values at 302 kJ/mol. Singh et al.44 extracted NC from pigeon peas using hydrochloric acid (4 N),withEa values for NC estimated at 209.20 kJ/mol. Mohsin et al.33 performed the extraction of NC from date stem using sulfuric acid (62 wt.%) and conducted Ea analysis using the Coats–Redfern method, yielding Ea values for NC at 125.6 kJ/mol. In this investigation, [Bmim]Cl yielded microfibers with a Ea value of 139.60 kJ/mol. The introduction of a copper metal complex and H2O2 alongside [Bmim]Cl in the hydrolysis process resulted in highly crystalline nanofibers characterized by a substantially high Ea of 220.41 kJ/mol.Table 8 Ea(kJ/mol) of MCC and NC compared to other cellulosic materials.

Lignocellulose	Source	Hydrolysis	Ea(kJ/mol)	Method	Ref	
MCC	Date waste (seeds)	Ionic liquid: [Bmim]Cl	139.60	Coats–Redfern	This study	
NC	Date waste (seeds)	[Bmim]Cl + metal complex + H2O2	220.41	Coats–Redfern	
NC	Date waste (stem waste)	Mineral acid: 62 wt.% H2SO4	125.60	Coats–Redfern	33	
Cellulose	Date waste (leaves waste)	Pretreatment with chlorinated solution	103.27	Coats–Redfern	32	
NC	Pigeon pea	Mineral acid: 4 N HCl	209.20	Coats–Redfern	44	
NC	Corn stalk	Mineral acid: 60 wt.% H2SO4	302	Coats–Redfern	43	

DSC

Figure 8 displays the second heating cycle of the samples using DSC, focusing on cellulose, MCC, and NC. The decision to emphasize the second cycle stems from the potential presence of residues, such as solvent residues or impurities, in the first cycle. By excluding these elements, the second cycle ensures a more stable and conditioned state for analysis, offering a clearer insight into the intrinsic thermal behavior of the samples. Notably, all four cellulose-rich fibers exhibit no thermal transition behavior, such as glass transition (Tg), cold crystallization, and melting points, within the temperature range of 25–300 °C. The absence of thermal transitions in the investigated fibers is likely due to their highly crystalline structure. The inherent molecular order and stiffness associated with crystallinity mitigate conventional thermal behaviors.Fig. 8 DSC curves for cellulose, MCC, and NC.

XRD

Figure 9 shows the X-ray diffraction (XRD) patterns of date seed (DS), microcrystalline cellulose (MCC), and nanocellulose (NC). The diffraction patterns exhibit the characteristic peaks associated with crystalline cellulose. The prominent peaks observed at 2θ values of 15°, 22°, and 34° correspond to the (110), (200), and (004) crystal planes of cellulose I, respectively45. These peaks are present in all three samples, indicating the inherent crystalline nature of the cellulose within the materials. The peak at 22° is attributed to the presence of crystalline carbon components in the samples. Additionally, the peak at 34° is associated with the hemicellulose dehydrate, which agrees with the cellulose I crystal structure, as confirmed by the JCPDS database (00 056–1718)46. Interestingly, the sample treated with the [Bmim]Cl-copper metal complex and H2O2 exhibits a sharper diffraction peak at around 22.5° compared to the only [Bmim]Cl-treated sample and the raw date seeds. The sharpness of the diffraction peaks is directly correlated with the degree of crystallinity in the fiber structure, where a sharper peak indicates a higher degree of crystallinity. The degree of crystallinity (Crl) for DS, MCC, and NC were 32.84%, 38.51%, and 69.69%, respectively. These findings are consistent with the previous work reported by Israa et al.47 which also observed similar diffraction peaks for raw date seeds and the associated nanocellulose extracted using mineral acid hydrolysis with Crl of 67–72%. The [Bmim]Cl-copper metal complex-assisted hydrolysis was more effective in removing the amorphous regions of the cellulose, resulting in the production of nanocellulose with a higher degree of crystallinity. This is because the copper metal complex can catalyze the hydrolysis process, selectively targeting and breaking down the amorphous domains while preserving the crystalline regions of the cellulose. Fig. 9 XRD spectra for DS, MCC, and NC.

EDX spectroscopy

EDX spectroscopy analysis coupled with FE-SEM was utilized for the elemental analysis of the modified NC. Figure 10 illustrates the EDX spectrum of the modified NC, revealing the presence of the elements carbon (C), oxygen (O), silicon (Si), and nitrogen (N). The peaks observed at approximately 0.277, 0.525, 0.392, and 1.739 keV correspond to C, O, N, and Si binding energies, respectively. The primary components of the NC are C and O, while the presence of Si and N can be attributed to APTES. Consequently, the EDX analysis confirms the successful attachment of APTES to NC.Fig. 10 SEM–EDX of modified NC.

Zeta potential

The zeta potential is a crucial factor in determining the dispersion stability of cellulose and nanocellulose suspensions. A high absolute zeta potential value, typically greater than ± 30 mV, signifies strong electrostatic repulsion between particles, which prevents aggregation and enhances dispersibility. For optimal dispersion stability of NC, absolute zeta potential values in the range of ± 40 to ± 60 mV are desirable, as these values provide sufficient repulsive forces to maintain a stable dispersion and minimise the risk of flocculation48. Therefore, maintaining a zeta potential within this range is key to achieving high dispersibility and stability in NC suspensions49. Table 9 shows the zeta potential values for MCC, NC, and modified NC, highlighting their dispersion stability. MCC, extracted with [Bmim]Cl, has a zeta potential of − 27.87 mV, and NC, synthesized with [Bmim]Cl, a copper metal complex, and H2O2, has a zeta potential of − 27.28 mV. Both values indicate poor dispersion stability, as a zeta potential magnitude below 30 mV suggests insufficient electrostatic repulsion to prevent particle aggregation. However, modified NC, treated with 3-aminopropyltriethoxysilane (APTES), shows a significantly higher zeta potential of − 44.72 mV, indicating much better dispersion stability due to increased electrostatic repulsion. This demonstrates that surface modification can effectively enhance the stability of nanocellulose dispersions, making them more suitable for applications requiring uniform particle distribution.Table 9 Zeta potential values of MCC, NC, and Modified NC.

Material	Zeta potential (mV)	
Microcrystalline cellulose (MCC)	 − 27.87	
Nanocellulose (NC)	 − 27.28	
Modified Nanocellulose (M.NC)	 − 44.72	

Conclusion

A transition metal complex of copper nitrate and pyridine as a ligand in combination with an oxidizing agent (H2O2), demonstrated enhanced hydrolysis efficiency of [Bmim]Cl as a green solvent for NC production. This new protocol yielded a 25% higher NC yield than using [Bmim]Cl alone while reducing particle size to approximately 100 nm in hydrodiameter. The resulting NC exhibited high thermal stability, with a Tonset of 286 °C and a Ea of 220.41 kJ/mol. Moreover, NC obtained displayed high crystallinity (Crl= 70.28%). To enhance the redispersion behavior of NC, it was modified with a silane agent to replace the free hydroxyl groups (–OH). This modification was confirmed by the absence of a peak at 3360 cm-1 in the FT-IR spectra. This study introduces a new, efficient, and environmentally friendly process for producing NC with a high yield and smaller fibers.

Legal compliance

The study complies with relevant institutional, national, and international guidelines and legislation.

Acknowledgements

All authors express their gratitude to the Chemical and Petroleum Engineering department at United Arab Emirates University, UAE, for their invaluable support.

Author contributions

Mohsin Raza: Methodology, experimental work, writing–original draft; Mohammad Jawaid: Conceptualization, Editing of draft; Basim Abu-Jdayil: conceptualization, formal analysis, methodology, funding acquisition, supervision, project administration, writing–review, and editing.

Funding

The authors acknowledge the financial support provided by research grant no. 31R272 and 12R014 from the United Arab Emirates University.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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