Statement of Novelty

Microcrystalline cellulose (MCC), a renewable biomacromolecule, is principally extracted form lignocellulosic materials (LM) using different approaches. In the present paper, MCC was successfully isolated from date palm fronds (DPF). Furthermore, several previous research works have been carried out to produce MCC without completely addressing the issue of the effect of different pretreatments on the final features of the derived MCC from LM. The current work is also focusing on the evaluation of the effect of three main delignification processes on the properties of MCC produced LM. It was shown that DPF is a prominent source of MCC and the nature of LM treatment affects sensibly the physicochemical and thermal features of the prepared MCC.

Introduction

As awareness of our impact on the planet is growing, the demand of new renewable and sustainable materials has increased swiftly in the last few decades, as revealed by the UN agenda 2030 [1]. Chemicals, biofuels, and composites derived from renewable resources, such as agricultural and wood biomass, have been considered as the most prominent alternatives for replacing the common traditional raw materials [2]. One of the most abundant and renewable biopolymer on earth, which can be obtained from agricultural waste or biomass, is cellulose with annual production of 7.5 × 1010 tons [3,4,5]. Regardless of the source, it consists if linear polymeric chain composed of 1–4 linked β-d-anhydroglucopyranose units [6, 7]. Based on the reduction of the cellulose fibers size, recent investigations have been focused on the development of micro/nano scale cellulosic biomolecules with interesting features such as low density, low environmental impact, excellent mechanical properties as well as tailorable surface characteristics [8,9,10,11]. These micro/nano materials offer significant opportunities for multiple applications in various fields such as materials science, pharmacy and biomedicine.

Microcrystalline cellulose, one of the most widely employed forms of cellulose in pharmaceutical cosmetic, food and allied industries encompassing material sciences, is a purified, white, odorless, crystalline, and partially depolymerized cellulose powder extracted from a variety of lignocellulosic biomass [3, 12, 13]. It can be extracted using different methods, comprising chemical (e.g. mineral acid hydrolysis, metal ions enhanced high temperature liquid water, ionic liquid, hydrothermal, etc.), physical (disintegration, irradiation, high-pressure homogenization), biological (e.g. enzymatic hydrolysis) and combination of two or more of these processes [3, 14]. However, acid hydrolysis method remains the most widely employed method to isolate MCC from native cellulose owing to its many advantages such as reasonable price, efficiency and short reaction duration. During the hydrolysis process, hydronium ions cause the cleavage of glycosidic bonds. The amorphous parts of cellulosic fibers are susceptible to be destroyed, whereas the crystalline regions stay intact owing to their resistance to acid penetration [15].

Various alternative sources to wood and cotton have been explored to produce MCC such as non-wood species, aquatic plants, grass and agro-wastes. The date palm tree fronds (DPF) is a an agro-waste product of date fruit during pruning, which is widely distributed in hot arid regions in an oasis agro-system of Middle East Asia, North Africa and North American desert regions [16]. The date palm (Phoenix dactylifera L.) is a monocotyledonous and dioecious species from Areaceae family [17]. It is a renewable and abundant material and its annual worldwide production is estimated to be 1.13 million of date palm fiber tons, over 1 200,000 tons of petioles, 410,000 tons of leaves and 300,000 tons of bunches [18]. Algeria is the world’s third-largest date producer, with about 1 million tons. More than 18 million date palm trees that distributed over about 100,000 farms have been identified in 2009. Approximately 210,000 tons of date palm petioles, 73,000 tons of leaves and 52,000 tons of brunches are generated in Algeria [19, 20]. The combination of these three parts constitute the DPF. Such waste is an interesting feedstock for a number of applications such as fuels, composites, activated carbons owing to its availability and its high cellulose content (30–47%) [21,22,23]. This potential agro-waste was previously utilized to isolate pure cellulose and produce some cellulosic nanomaterials [24,25,26,27,28,29,30]. To the best of our knowledge, no study on DPF microcrystalline cellulose has been found in the literature. Thus, the aim of the current investigation was to isolate and characterize MCC from DPF, which may help developing its further utilization and improve its economic feasibility.

However, the MCC characteristics, which can be tailored to specific needs, are not only depend on the raw material and the hydrolysis procedures, but also on pretreatments employed to isolate cellulose from lignocellulosic biomass. For many years, acidified sodium chlorite delignification is the common method utilized method to eliminate lignin as an initial stage [31]. Current approaches encompass the use of the total chlorine-free methods such as deep eutectic solvents, ionic liquids and hydrogen peroxide solutions [32]. However, depending on the natural lignocellulosic source, lignin, hemicellulose and cellulose are arranged in a complex hierarchical microstructure, forming recalcitrant nature against chemicals and microbial treatments. Thus, efficient separation of these components is crucial to produce value-added products using either one of the above treatments or a specific combination of them [14, 33].

The present work aimed to develop an effective approach to extract pure cellulose and MCC via acid hydrolysis from Algerian DPF. There different delignification processes (acidified NaClO2, totally chlorine free TCF and their combination) have been employed. Their effect on the physicochemical features and thermal stability of the different derived MCCs has been deeply investigated using a series of analyses, including FTIR, XRD, SEM, DSC and TGA.

Material and Methods

Materials

Date Palm frond fibers were collected from the Touggourt oasis, in Algeria. These fibers were treated with distilled water to remove dirt and other soluble substances, dried in an oven at 80 °C for 48 h, milled and sifted into particle size of 35 mesh. After that, they were preserved in a vacuum desiccator. Commercial microcrystalline cellulose (Avicel®, C-MCC) was supplied by Merck. Ethanol (96%), toluene (99.5%), hydrogen peroxide (H2O2, 30%), acetic acid glacial (CH3COOH, 96%), nitric acid (HNO3, 67%) and hydrochloric acid (HCl, 37%) were purchased from VWR Prolabo. sodium hydroxide (NaOH) pellet, sodium chlorite (NaClO2) were supplied by Sigma Aldrich. All reactants are analysis grade and are employed without prior purification.

Methods

Preparation of DPF Cellulose Samples

The preconditioned DPF fibers (20 g) are extracted with 220 mL of toluene/ethanol (2/1) mixture using a Soxhlet extractor at 120 °C for 6 h to remove organic solvent extracts [34]. Then, 15 g of the obtained fibers are treated with 200 mL of distilled water in an oil bath at 105 °C for 5 h to remove sugars, coloring matter ad starch [35]. After that, to eliminate lignin from DPF fibers, three different delignification processes have been applied using either acidified NaClO2 process, the totally chlorine free beaching or their combined approach, as schematized in Fig. 1.

Fig. 1
figure 1

Scheme of the isolation of microcrystalline cellulose from date palm fronds

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The acidified process is completed according to the method described by Ilyas et al. [36]. 10 g of the extractive-free DPF fibers are soaked in a 500 mL round bottom flask containing 325 mL of distilled water in an oil bath. 4 mL of acetic acid and 6 g of sodium chlorite are added to the mixture periodically (once every 1 h) for 7 h, and the mixture temperature was kept at 70 °C. The obtained holocellulose (holocellulose-DPF-NaClO2) is filtered, rinsed by distilled water, and dried in oven at 100 °C.

The TCF process is carried out within three stages. The first one is conducted following Kuznetsov et al. method with slight modification [37]. 10 g of the extractive-free DPF fiber is placed in a 250 mL round-bottomed flask containing 150 mL of hydrogen peroxide (6%)/acetic acid (25%) mixture at 100 °C for 5 h. The obtained residue is rinsed by distilled water. In the second stage, this residue is placed in a 250 mL round bottom containing 150 mL of nitric acid (20%) /acetic acid (25%) mixture at 120 °C for 20 min. The obtained product is filtered and washed with distilled water [38]. In the final step, the residue is treated with 200 mL of hydrogen peroxide (5%)/sodium hydroxide (4%) mixture at 50 °C for 90 min. The obtained holocellulose, named holocellulose-DPF-TCF, is filtered, washed with distilled water and dried in oven at 105 °C overnight [39].

The combined approach consists of the addition of a further treatment step to the TCF process. The dried holocellulose fibers obtained from the TCF process are mixed with 350 mL of distilled water in oil bath at 70 °C for 5 h. 6 g of sodium chlorite and 4 mL of acetic acid are added to the mixture to adjust the pH to 4.5. During the filtration, the white holocellulose, named holocellulose-DPF-combined, is washed by distilled water and dried at 105 °C overnight [40].

The different samples of holocellulose are further treated to produce celluloses according to Jiang et al. method [41]. The samples are soaked in NaOH (5%) solution with a ratio of (1/20) for 24 h under magnetic stirring at room temperature, which is then increased to 90 °C for 2 h. The produced cellulose samples are filtered, neutralized with distilled water and diluted acetic acid, and then dried at 105 °C overnight.

Preparation of Microcrystalline Cellulose

To produce date palm MCCs, the hydrolysis acid of date palm cellulose samples is performed via the identical process described in the literature with slight modification [42], using 2.5 m of hydrochloric HCl acid at 100 °C during 30 min in the ratio of 1/20 pulp over liquor. After that, large amount of distilled water was added to the mixture to stop the reaction. The resultant depolymerized cellulose was neutralized with NaOH (0.5 m) solution, rinsed with distilled water to reach a pH of 7, and dried in oven at 60 °C for 24 h. The obtained products were snowy-white in appearance. The overall steps of the extraction process of MCC and digital images of each product are displayed in Fig. 1.

Characterization

Chemical Composition

The chemical composition of date palm frond fibers was determined following the TAPPI (Technical Association of the Pulp and Paper Industry) standard methods and previous investigations [43, 44]. The ash content is measured with the standard T 211 om-07, the toluene/ethanol solvent extractives content are quantified using T 204 cm-07, the hot water extractives are evaluated with the standard T-257, the lignin content is evaluated using the standard T-222 om-06, then the α-cellulose content is measured with the standard T 203 cm-99. The holocellulose content was obtained using the method reported in the work of Wise et al. [45], whereas the hemicellulose content was calculated as the difference between holocellulose and α-cellulose contents. The measurements were carried out in triplicate.

FTIR Analysis

FTIR spectroscopic analysis was performed to investigate the changes of functional groups and chemical transformation in the structure of the different specimens. The dried samples (0.1–2 mg) were pelletized with KBr (100 mg) and were analyzed with Parkin Elmer FTIR spectrometer. The spectra were recorded in transmittance band mode, in the wave number region of 400–4000 cm−1 by averaging of 64 scans at resolution of 4 cm−1. Before each measurement, background spectra were obtained at room temperature and then subtracted automatically from the sample spectrum.

X-ray Diffraction Examination

X-ray diffraction analysis is carried out to study the structure and to collect the diffractograms of the different cellulose and microcrystalline cellulose samples. The results were collected using a PANalytical X’Pent PRO Multipurpose diffractometer with Cu Kα radiation at a generator voltage of 45 kV and current of 40 mA. An X’celerator detector was employed to record the data over a 2θ angular range of 5–50° with a step size of 0.017°/2θ and a count time of 50.1650 s at each step. The crystalline indexes of raw fibers and all cellulose and microcrystalline cellulose samples were determined from different diffractograms, according to Segal’s method [46].

$$CrI\,\left( \% \right) = \frac{{I_{{200}} - I_{{amp}} }}{{I_{{200}} }} \times 100$$
(1)

Where CrI is the crystallinity index, I200 is the maximum intensity of the 200 peak at 2θ = 22° and Iam is the intensity at 2θ = 18°.

From the different diffractograms obtained using 200 lattice plane, the size of crystallite was evaluated via Scherrer equation.

$$D_{{\left( {hkl} \right)}} = \frac{{k\lambda }}{{\beta _{{hkl}} \cos \theta }}$$
(2)

where D (hkl) is the crystallite size (nm), k is the Scherrer constant (0.9), λ is the X-ray wavelength (1.5418 cm−1), βhkl is the full width at half maximum intensity (FWHM) of the diffraction peak and θ is the half of the Bragg angle [31].

Scanning Electron Microscopy (SEM)

Surface morphologies of all cellulose and microcrystalline cellulose samples were obtained with FEI Quanta 250 scanning electron microscope tool at a 10 mm working distance and 10 kV accelerating voltage. Dry powders of different cellulosic samples are placed on double-sided conductive adhesive tape. Then, they were characterized by using the secondary electrons (SE) for morphology. The particle size of the samples was evaluated using Image J software [36].

Thermal Analysis

The thermal behavior of cellulose and microcrystalline cellulose samples was assessed using a thermogravimetric analysis (TGA) and a differential scanning calorimetry (DSC), respectively. The TGA experiments were conducted using a TGA Q500 V20.13 Build 39, whereas the DSC ones have been performed using a calibrated Perkin Elmer DSC 8000 analyzer. A Sample mass of 5 mg was used for the different analyses, which are conducted in the temperature range of 25–500 °C at a heating rate of 10 °C min−1. The different analyses were carried out under inert nitrogen gas atmosphere.

Results and Discussion

Chemical Composition

The composition of raw date palm fronds are given in Table 1. According to the obtained results, the raw fibers are composed of 31.4% cellulose followed by lignin with 25.2% and hemicellulose with 20.92%. However, the lowest percentages are referred to the organic solvent extractives, the hot water extractives and ash with 7.72%, 8.4% and 4.16%, respectively. The chemical composition of DPF exploited in the present work contains lower cellulose amount compared to DPF used in another study [22], but higher than that recently reported by Abu-Thabit (24.75%) [47]. This variation is probably due to the climate conditions and soil composition. Moreover, it can be seen that our DPF had a similar chemical composition than the date palm rachis [48], date palm leaves [49] and rice husk [50]. Furthermore, it is worth noting that date palm fibers [51], date palm rachis [52] and alfa grass fibers [42] presented a higher amount of cellulose than our DPF.

Table 1 The chemical composition of raw date palm fronds
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FTIR Analysis

FTIR spectroscopic characterizations are carried out to confirm the removal of non-cellulosic materials, in addition to the identification of the functional groups of all samples obtained during the isolation of the different microcrystalline celluloses (MCCs).

The FTIR spectra of the studied samples, i.e., raw sample (R), DPF-after Soxhlet extraction (a), DPF-after hot water extraction (b), holocellulose DPF-NaClO2 (c), holocellulose DPF-TCF (d), holocellulose DPF-combined (e), DPF-C-NaClO2 (f), DPF-C-TCF (g), DPF-C-combined (h), DPF-MCC-NaClO2 (i), DPF-MCC-TCF (j), DPF-MCC-combined (k) are exhibited in Figs. 2 and 3, respectively. The obtained results indicate that almost all samples have similar functional groups and the delignification processes employed do not have any adverse effect on the cellulose configuration. The absorbance band at 3450 cm−1 is referred to stretching vibration of –OH groups [53,54,55]. The absorbance peak at 2904 cm−1 is attributed to aliphatic saturated –CH2 stretching vibration what means that α-cellulose in the DPF is not degraded. The band centered at 2860 cm−1 is attributed to the asymmetric and symmetric stretching vibration of the CH2 group of hemicellulose in DPF. The spectra demonstrated the gradual transformation of this band, which decreased after delignification with either NaClO2 or TCF (Spectra (h) and (g), Fig. 3), and completely disappeared after the employment of the combined process. The first two pretreatments seem to be less efficient because of the complex structure of lignocellulosic fibers, whereas the combined process appeared to be more effective. In this case, the acidified sodium chlorite treatment initially allows dissolving and removing lignin without taking into account the elimination of hemicelluloses, where the phenolic rings are oxidized to provide quinonoid and muconic acid derivatives or to methyl/methylene groups in the allyric position producing carbonyl or carboxylic groups [56, 57]. However, the second part of the combined process provides higher delignification and bleaching efficiency, where the hydroperoxide anions (HOO−) are responsible for the removing of the lignin chromophore groups, selectively attacking ethylenic and carbonyl groups, whereas other radical species such as (OH−) are responsible for the solubilization of hemicelluloses to provide extremely purified cellulose [58]. The peak at 1735 cm−1, that indicate the presence of either acetyl or uronic ester group of hemicellulose or ester linkage of the carboxylic groups of the ferulic and p-coumeric acids of lignin and/or hemicellulose in the DPF, is removed after the different delignification processes. The employed treatments can cause swelling of cellulose, which in turn interrupts the intermolecular hydrogen bonds between cellulose and hemicellulose [59]. The peaks 1515 cm−1 and 1250 cm−1 are associated to C=C of aromatic vibration and C=O in lignin, respectively. The absence of these latter confirms the complete elimination of lignin after the delignification processes. The absorption at 1643 cm−1 is related to the absorbed water due to interaction in between water and cellulose. At 898 cm−1, the peak observed is corresponded to C–H rock vibration in cellulose β-glucosides. The absorption band at 1430 cm−1 is known as crystalline band that is attributed to CH2 symmetric stretching [38, 60], which increased in date palm cellulose and date palm-MCC, and indicating the elimination of the amorphous parts. According to literature and the obtained results, it can be inferred that the different cellulosic samples are successfully isolated, whatever the delignification process used [3, 42, 61]. Furthermore, the produced MCCs from date palm fronds using hydrochloric acid hydrolysis have similar chemical composition than that reported in the literature [42].

Fig. 2
figure 2

FTIR spectra of: (R) raw sample, (a) DPF-after soxhlet extraction, (b) DPF-after hot water extraction, (c) holocellulose DPF-NaClO2, (d) holocellulose DPF-TCF and (e) holocellulose DPF-combined

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Fig. 3
figure 3

FTIR spectra of: (f) DPF-C-NaClO2, (g) DPF-C-TCF, (h) DPF-C-combined, (i) DPF-MCC-NaClO2, (j) DPF-MCC-TCF and (k) DPF-MCC-combined

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XRD Analysis

Cellulose consists of both crystalline and amorphous parts, which are linked with intra and inter-molecular bonds (hydrogen bonding interactions and Vander Waals forces) [60, 62, 63]. The XRD experiments have been performed to confirm the structure of cellulose and MCC samples isolated from DPF. The obtained results are used to determine the crystallinity indexes. The X-ray diffraction patterns of (R) raw DPF, (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF and (f) DPF-MCC-combined are exhibited in Fig. 4. The diffraction patterns (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined obtained confirmed the presence of cellulose type I, with diffraction peaks at about 2θ = 14.8° (1ī0), 16.3° (110), 22.5° (200), 34.5° (004) without the presence of cellulose type II, from the fact that there is no doublet in the intensity of the main peak. These peaks become more defined upon chemical treatments as expected. Moreover, the peak presented in the diffractograms of raw date palm at 2θ = 26.3° is affected by the presence of a large amount of impurities existed in date palm biomass [64, 65]. While the absence of this peak in both cellulose and microcrystalline cellulose samples shows the total elimination of impurities with the different delignification processes used and alkali treatment of raw fibers [12, 64]. In addition, the diffraction peak at around 22.5° becomes sharper and narrower, confirming an increase of crystallinity of MCC samples compared to native celluloses. It is obvious that the three methods of delignification applied had no impact on the structure the different cellulosic samples what is also validated by the FTIR analyses.

Fig. 4
figure 4

X-ray diffraction patterns of (R) raw DPF, (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF and (f) DPF-MCC-combined

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The crystallinity indexes (CrI) of all samples are calculated and summarized in Table 2. Noticeably, the crystallinity index of the native celluloses is higher to that of the raw date palm (54.82%) because of the elimination of hemicellulose and lignin, which are amorphous in nature [66]. Lignin and hemicellulose principally influence the 2θ region of 18–25° by broadening the cellulose [200] peak and increasing the underlying background [67]. Furthermore, it is evident that the crystallinity indexes of microcrystalline cellulose samples, i.e., DPF-MCC-NaClO2 (72.73%), DPF-MCC-TCF (73.52%) DPF-MCC-combined (74.20%) are higher than those of their respective native cellulose samples, i.e., DPF-C-NaClO2 (63.46%), DPF-C-TCF (65.83%), DPF-C-combined (70.19%), what is due to further degradation of the amorphous regions caused by the acid hydrolysis [60, 61, 68, 69]. The cellulose and MCC prepared using sodium chlorite during the delignification process showed lower crystallinity indexes and crystallites sizes with respect to those obtained using the totally chlorine free process. Moreover, the combined delignification processes allowed obtaining the higher crystallinity and crystallite size values of the corresponding cellulose and MCC samples compared to the two other treatments (sodium chlorite and totally chlorine free process). During acid hydrolysis of cellulose derived from the combined process, a large amount of protons (H+) from HCl opens the fiber structure to facilitate access to cellulose microstructure, destroys the outermost amorphous cellulosic parts and then breaks down the β1,4-glycosidic bonds to reach the levelling-off degree of polymerization, leading to higher crystalline MCC. However, in the case of the two other treatments, producing cellulose containing some hemicelluloses, the acid hydrolysis will selectively cause the solubility of the amorphous hemicellulose compared to cellulose, since the former has shorter chain length (100–200) compared to cellulose (10,000–15,000), presents different ring structures and hydroxyl configuration, and contains more branched structure and more reactivity [70, 71]. Thus, the acid hydrolysis will not only be used to destroy the amorphous regions of cellulose, but also will be dedicated to remove the hemicelluloses. Consequently, lower MCC crystallinity are reached compared to that obtained with the combined process/acid hydrolysis. The increase of crystallinity value of MCC obtained by the combined pretreatment will certainly provide better physical and mechanical features for various potential applications. It is worthy to note that the obtained crystallinity values of MCC in the present work are higher than that reported by Abu-Thabit for MCC derived from acid hydrolysis from date seeds of the date palm tree, which is 70% [47], and slightly lower to that mentioned by Alotari for MCC obtained from date palm fruit bunch stalk (79.4%) [72]. Moreover, the prepared MCC samples with CrI% of falls within the crystallinity range of the commercial MCC, which is in the range of 55–80% [3].

Table 2 The crystallinity indexes, crystallite size and diameter size of the different samples
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Morphology

SEM is an important tool that has been widely used for morphological inspection. SEM images of (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF and (f) DPF-MCC-combined are presented in Fig. 5. The delignification processes applied consent eliminate the surface impurities and other non-cellulosic compounds, leading to clean smooth surface, long, individualized and uniform cellulose filaments (a, b and C samples) [73]. A similar observation was found by FTIR analysis results, confirming the dissolution of lignin and hemicellulose after delignification and bleaching treatments. The diameter of the isolated cellulose and MCC samples using different pretreatment methods was computed by ImageJ processing software (IJ 1.46) using the SEM micrographs. More than 50 fibers have been utilized to ensure reproducible data.

Fig. 5
figure 5

SEM images of all cellulose samples: a DPF-C-NaClO2, b DPF-C-TCF, c DPF-C-combined, d DPF-MCC-NaClO2, e DPF-MCC-TCF and f DPF-MCC-combined

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It is demonstrate that the diameter of native cellulose obtained using TCF treatments presents lower value compared to that derived from NaClO2 treatment (Table 2). This result can be assigned to the influence of nitric and acetic acids mixture used at high temperature, in addition to the effect of alkaline and acid hydrogen peroxide oxidation for TCF, which can cause more degradation of cellulose [39, 74,75,76]. The combination of the above two treatments generates lower diameter because of the intense elimination of hemicellulose and lignin, compared to the individual treatments, as well as the further degradation of cellulosic fibers.

With regard to the effect of acid hydrolysis, it is clear that HCl hydrolysis produces irregular rod like structure of micro-sized fibrils with slightly rough surface [3]. Such treatment generates lower average diameter for cellulose dignified by the combined process compared to the two others (Table 2), confirming that the pretreatment influences the hydrolysis process. In comparison with cellulose samples, MCC ones show modified and irregular micro sized rod shape with slightly rough surfaces, which can be due to the further degradation of the structure of cellulose fibers during acid hydrolysis. Short length particles have been obtained after hydrolysis because of the cleavage of pyranose linkages as well as the destruction of the amorphous parts of cellulose [42]. This result is in good agreement with the XRD data. It is important to note that comparable diameters to those of the produced MCCs have been reported for Alfa fibers, reeds and algae as well as commercial MCC [15, 77]. On the other hand, though the length of the different MCCs was not measured from SEM images, it is obvious that their length is much lower than their respective diameters. Some amounts of pith (a non-fibrous material) can be observed in the different produced MCCs, which may be attributed to the strong cohesion of hydrogen bonding between microfibers, which generates aggregates.

Thermal Stability

The DSC and TGA/DTG analyses were employed for obtaining the thermal characteristics as well as evaluating the thermal stability of the different cellulosic materials [3, 61, 78]. These features are crucial to assess the performance of the material and its utilization in various industries. Table 3 summarizes the thermal parameters for all samples.

Table 3 The TGA/DTG and DSC data of the different cellulose and MCC samples
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Figure 6 displays the DSC curves obtained for (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF, and (f) DPF-MCC-combined. The different samples show two endothermic phenomena. The first process is attributed to the water desorption within cellulose at the region between 6 and 140 °C. The second stage, between 250 and 380 °C, corresponds to the main decomposition due the scission of the 1,4-glycosidic bonds of cellulose biopolymer followed by the formation of levoglucosan and charring. This second endothermic peak is well resolved, which indicates the total dissolution of the non-cellulosic components after delignification, bleaching and acid hydrolysis. The obtained results revealed that the employment of a combined delignification process provides pure cellulose, which degrades at higher temperatures, with an increase of the onset and degradation temperatures compared to the use of individual treatments. This result is in good accordance with FTIR analysis, which shows the absence of the band at 2860 cm−1. Furthermore, MCC samples exhibit higher thermal decomposition temperatures with respect to those of their native celluloses what is due to the higher degree of molecular ordering of MCCs, as displayed by XRD spectra. Hence, higher crystalline structure provides higher thermal decomposition features and consequently better thermal stability. Various authors already reported such trend [15, 77, 79]. Furthermore, it is worthy to note that the use of combined treatment for delignification allows efficient acid hydrolysis and lead to better thermostable MCC, compared to the employment of NaClO2 and TCF pretreatments, respectively.

Fig. 6
figure 6

DSC curves of (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF and (f) DPF-MCC-combined

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The TGA and DTG curves of the different dried samples are exhibited in Fig. 7. It can be seen that the different samples present one thermal decomposition process at high temperature in the range of 250–380 °C, which is due to the degradation of cellulose caused by the decarboxylation, depolymerization and the decomposition of glycosidic units followed by the formation of a charred residue [3, 60]. Compared to the results obtained by DSC, similar trend is found by TGA/DTG for the different samples. From the Table 3, it can be illustrated that the microcrystalline cellulose samples presented higher thermal stability than the respective native cellulose samples. The samples delignified with either sodium chlorite (DPF-C-NaClO2 and DPF-MCC-NaClO2) or totally chlorine free (DPF-C-TCF and DPF-MCC-TCF) process exhibited lower thermal stability than MCC obtained by the combined delignification process (DPF-C-combined and DPF-MCC-combined). The lower thermal stability for the samples obtained by the first delignification processes can be attributed to the presence of hemicellulose, which can degrade at lower temperature and affects negatively the stability of cellulose. Furthermore, the effectiveness of the hydrolysis process can be inhibited by the presence of hemicellulose causing the decrease of the efficiency of the removing of the amorphous parts in cellulose, which are more susceptible to the degradation. The prepared MCC samples display comparable thermal properties of commercial MCC and MCC isolated from other parts of date palm such as date seed (353 °C) and fruit bunch stalk (364.2 °C) and fall within the reported decomposition range of cellulose (315–400 °C) [3, 47, 72]. Broadly, the obtained MCC using a combined delignification/acid hydrolysis process from date palm fronds shows important thermal stability that makes it a promising candidate to prepare high-value products.

Fig. 7
figure 7

a TGA and b DTG curves of all cellulose samples: (a) DPF-C-NaClO2, (b) DPF-C-TCF, (c) DPF-C-combined, (d) DPF-MCC-NaClO2, (e) DPF-MCC-TCF and (f) DPF-MCC-combined

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Conclusion

The date palm tree fronds are rich in cellulose and annually available agro-waste and can thus be a prominent feedstock for MCC preparation. In the present study, cellulose and MCC were successfully isolated from DPF following different delignification methods and the HCl acid hydrolysis. FTIR spectra revealed that the delignification method may affect the purity of cellulose without influencing its chemical structure. The combined process that comprises a multistep stage of acidified sodium chlorite and totally chlorine-free allowed obtained higher cellulose purity compared to the separated processes. The morphology of the different MCCs presented rod-like shape or granular of micro-fibrils with a compact and rough microstructure, similar to the commercial MCC. XRD analyses showed that the prepared MCC displayed typical cellulose I, with crystallinity index ranging from 72 to 75%. Furthermore, relatively high crystallinity, smaller dimension and high thermal stability of MCC is obtained with the combined process with respect to the two others, thus making it potential candidate for application in various fields. The present work provides prominent methodology to isolate MCC from date palm fronds, which would be an alternative way for sustainable utilization of DPF.