Easy Fabrication of Highly Thermal-Stable Cellulose Nanocrystals Using Cr(NO3)3 Catalytic Hydrolysis System: A Feasibility Study from Macro- to Nano-Dimensions

This study reported on the feasibility and practicability of Cr(NO3)3 hydrolysis to isolate cellulose nanocrystals (CNCCr(NO3)3) from native cellulosic feedstock. The physicochemical properties of CNCCr(NO3)3 were compared with nanocellulose isolated using sulfuric acid hydrolysis (CNCH2SO4). In optimum hydrolysis conditions, 80 °C, 1.5 h, 0.8 M Cr(NO3)3 metal salt and solid–liquid ratio of 1:30, the CNCCr(NO3)3 exhibited a network-like long fibrous structure with the aspect ratio of 15.7, while the CNCH2SO4 showed rice-shape structure with an aspect ratio of 3.5. Additionally, Cr(NO3)3-treated CNC rendered a higher crystallinity (86.5% ± 0.3%) with high yield (83.6% ± 0.6%) as compared to the H2SO4-treated CNC (81.4% ± 0.1% and 54.7% ± 0.3%, respectively). Furthermore, better thermal stability of CNCCr(NO3)3 (344 °C) compared to CNCH2SO4 (273 °C) rendered a high potential for nanocomposite application. This comparable effectiveness of Cr(NO3)3 metal salt provides milder hydrolysis conditions for highly selective depolymerization of cellulosic fiber into value-added cellulose nanomaterial, or useful chemicals and fuels in the future.


Introduction
For the last few decades, the energy crisis threatened the world due to the excessive utilization of the world's depleting oil reserves by the ever-increasing human population. High worldwide demand for energy, unstable and uncertain petroleum sources, and concern over global climate change has led to the resurgence in the development of alternative renewable resources that can displace fossil fuel or petroleum-based polymers. Recently, environmental awareness has become greater and the demand for sustainable plant-based raw materials for the eco-friendly economy continues to grow into the foreseeable future [1]. In response, many countries have initiated extensive research and development programs in nanocellulose production, a green, bio-based and renewable biomaterial that has the broad possibility of use in various fields of innovative material. In fact, scholar-researchers have moved towards the utilization of this fully bio-based nanomaterial as a prominent candidate to replace synthetic reinforcing fillers in biodegradable composites and polymer matrixes as well as for the production of nanotubes and thin films [2].
Cellulose is the most abundant and renewable biopolymer available on Earth, which can be obtained from a variety of sources, including woody and non-woody plants, animals and bacteria. It has been estimated that more than 10 10 -10 11 tons of cellulose are synthesized and destroyed globally per annum. Therefore, rational and sustainable utilization of these abundant lignocellulosic biomasses (H 3 PO 4 ) is recognized as the most effective to hydrolyze defective cellulose crystalline parts while leaving the highly crystalline nano-sized cellulose segments unaltered [16]. In the case of strong acid treatment, the hydronium ions (H 3 O + ) will attack and cleave β-1,4-glycosidic linkages within the single cellulose chains as well as break down the extensive network of intra-and intermolecular bonds between the cellulose chains [7,17]. Unfortunately, the yielded sulfated nanocellulose suffers from low product yield and less thermal stability compared to that of its starting material due to the introduction of active sulfate ester groups on the fiber surface that can be detrimental to the thermal stability of treated product [18]. Moreover, this process is highly corrosive, hazardous, and requires harsh conditions and the involvement of tedious neutralization treatment of concentrated acidic effluent, which further limits its industrialization. For HCl hydrolysis, the aqueous nanomaterial suspensions tend to flocculate rather than well-disperse, and only a low yield of 20% could be obtained [19]. Even if some studies reported a higher yield of 60.5% via phosphotungstic acid hydrolysis, the hydrolysis efficiency was low and the reaction process is time-consuming [20], or else mechanical activation (ultrasonication, ball milling or mechanochemical) is necessary to enhance the competence [21]. Similar to that of diluted or organic acids that have been proposed for milder hydrolysis reactions, such treatments were less effective in hydrolyzing the high recalcitrant cellulose macromolecule. For these reasons, an increasing effort has been made to use the other mineral acids for cellulose hydrolysis. Recently, the mixed acid solution (hydrochloric acid and sulfuric acid) may be a good choice for the production of CNC suspension, but this process still also took a long period.
According to some published papers, the typical hydrolysis methods, such as TEMPO-mediated oxidation and mechanical disintegration are popular in the preparation of CNF from various cellulosic feedstocks. Briefly, TEMPO process is initiated by (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, and the CNF obtained are more uniform and can be well dispersed in aqueous solution. In contrast, mechanically induced deconstructing strategies by different approaches (such as grinding, microfluidization, high-pressure homogenization, high-speed blending, refining, cryocrushing or ultrasonication) are considered as promising methods for isolating CNF. However, large-scale applications of these techniques have several restrictions. In fact, mechanical refining could efficiently separate the microfibrils, but the high crystalline ordered structure might be broken due to the lower selectivity of high shearing force, resulting in a lower crystallinity. Likewise, the TEMPO-oxidation process may turn some of the crystalline cellulose molecules into disordered structures during the oxidation and the resultant CNF product exhibited a lower CrI value, even compared to its starting material [22]. Ultrasonication is another renowned way for CNF fabrication in which 20-50 kHz of ultrasound is applied to defibrillate cellulose for cell disruption. This is a greener hydrolysis technique as no chemical is involved during the generation of nanocellulose. The high frequency is generated and converted into the mechanical energy, which can be transmitted to cellulose via a metal probe. Unfortunately, the product obtained by this technique is a mixture of nano-and microfibers since some of the fibrils tended to be peeled off from the original cellulose fibers while some still remain on the surface of fibers [23]. In addition, cellulose can be cycled through a high-pressure homogenizer to yield nanofibers. Typically, smaller and more uniform nanoparticles can be obtained with increasing number of homogenization cycles, but such high-pressure treatment is more likely to reduce and/or destroy the crystalline domains by separating the cellulose molecular mass, or instead fail to defibrillate the cellulosic pulp sufficiently [24]. It is worth mentioning that the energy demand increases significantly with the increase of homogenizing time, and this may be the main drawback of application for CNF. The main issue often experienced by a homogenizer is extensive clogging and flocculation of the nozzle as the CNF tend to aggregate, and this means that extra capital cost is required to overcome this frequent issue. Therefore, some authors proposed that an additional pre-or post-treatment is necessary in order to obtain nanofibers product, such as hydrolysis, grinding, homogenization, or other mechanical treatments.
To accomplish a technical feasibility and high selectivity controllable hydrolysis pathway, transition metal salt catalysts have been used as a potential hydrolyzing agent in the cellulose depolymerization process. Today, metal salts have been discovered to have the following notable advantages over inorganic acids or organic solvents in cellulose hydrolysis: (i) can disrupt the hydrogen bonds more efficiently and induce the degradation of cellulose; (ii) less corrosive and more environment-friendly; and (iii) better contact between molten state metal salts and solid cellulose [25]. According to literature studies, chromium-based metal salt catalysts have been found to be highly effective in direct conversion of cellulosic resources into different chemicals and liquid fuels such as glucose, hydroxymethylfurfural (HMF), xylose and levulinic acid. Binder and Raines [26] reported that CrCl 2 -catalyzed hydrolysis of cellulose could produce a high yield of HMF (ca. 54%) in the N,N-dimethyl acetamide solvent containing [EMIM]Cl and lithium chloride (LiCl) additives at 140 • C for 2 h. Su et al. [27] investigated the effect of CrCl 2 in [EMIM]Cl ionic liquid on the cellulose hydrolysis, and the results revealed that 58% of HMF was successfully achieved under the reaction conditions of 120 • C for 8 h. In recent years, a study performed by Peng et al. [28] found that CrCl 3 catalyst was exceptionally effective in the conversion of cellulose to levulinic acid with a maximum yield of 67 mol % at 200 • C for 180 min. During the catalytic hydrolysis, the multivalent metal ions act as a Lewis acid that possess a high ability for hydrolyzing the cellulose matrix by disrupting the bonding system to produce fermentable sugar molecules, and it had better efficiency compared to diluted acid alone [29]. Therefore, it is believed that the intermediate solid crystalline nanocellulose could be produced instead of water-soluble monomer molecules under controlled low severity hydrolysis settings.
In this study, we propose for the first time the use of Cr(NO 3 ) 3 hydrolysis system as an efficient and sustainable technique for producing nanocellulose. Many papers have reported the combination of metal salts and mineral salt or mechanical process for hydrolyzing the cellulosic feedstock into hydrocellulose or nanocellulose, but there is limited research on the involvement of metal ion catalyst alone to produce cellulose nanocrystals (CNC) without the addition of any mineral acid or assistance of mechanical treatment. The main objectives of this study include the following: (i) exploiting the feasibility and practicability of Cr(NO 3 ) 3 as a high potential catalyst for converting cellulose into its nanostructured morphology; (ii) optimizing the reaction conditions of Cr(NO 3 ) 3 hydrolysis system in order to produce a high performance nanocellulose; and (iii) evaluating the physicochemical changes in terms of functional groups (Fourier transform infrared; FTIR), crystalline structure (X-ray diffraction; XRD), morphological properties (Field emission scanning electron microscopy; FESEM and transmission electron microscopy; TEM) and thermal stability (Thermogravimetric analysis; TGA) during the production of nanocellulose from macro-sized native cellulose. We also attempted to produce CNC from the same starting material by concentrated H 2 SO 4 hydrolysis, in order to perform a comprehensive comparison and assessment between resulting nanocellulose (CNC Cr(NO3)3 and CNC H2SO4 ) using a similar analytic instrument systematically. We used microcrystalline cellulose (MCC) as a starting material instead of extracted cellulosic pulp from lignocellulosic biomass to better understand the role of Cr(NO 3 ) 3 hydrolysis system in the conversion of cellulose, avoiding impure bulk materials that might cause irreproducible results.

Effect of Reaction Temperature
Reaction temperature plays an important role in initiating the hydrolytic depolymerization process of cellulose and eventually enhancing the chemical degradation of cellulose macromolecule into the nano-dimensional. The crystallinity index of yielded nanocellulose affected by reaction temperature is shown in Figure 1a. The reaction temperature was set at different levels (20,40,60,80 and 100 • C) while other operating parameters were fixed, including Cr(NO 3 ) 3 concentration (0.8 M), reaction time (1.5 h) and solid-liquid ratio (1:30). The crystallinity index of nanocellulose was significantly increased (62.1% to 86.7%) with the rise of temperature from 20 to 80 • C. The increment was mainly contributed by the increased diffusion rate of H 3 O + and Cr 3+ ions in aqueous solution and penetration into loosely-packed non-crystalline regions of cellulose to cleave the glycosidic bonds. In fact, the hydrolysis reaction is generally more kinetically favorable, which means that a higher reaction temperature could significantly enhance the degree of progressive removal of defective crystalline domains in the cellulose matrix, releasing the individual crystallite segments [7]. This led to the breakage of the glycosidic linkage in cellulose long polymeric chains into a smaller dimension. However, the extremely high temperature of 100 • C resulted in the destruction of the crystalline structure of cellulose under excessive heat energy, and eventually promoted the formation of unfavorable side products, such as levulinic acid and HMF [30]. Therefore, the optimal reaction temperature of 80 • C was selected to conduct further experiments with the advantages of energy-saving, ability to produce higher crystallinity product and, most importantly, the reaction was more manageable under normal conditions of atmospheric pressure.
Surprisingly, the yield of produced CNC Cr(NO3)3 gradually decreased with increasing hydrolysis temperature, which decreased to ca. 75% when the reaction temperature rose from 20 to 100 • C. This finding suggested that less-ordered cellulosic surface chains started to disintegrate into water soluble product by the acidic solution, which in turn lowered the final yield of the nanomaterial [31]. It was observed that the yield of nanocellulose dropped sharply from 83.1% to 75.4% when the reaction was heated up from 80 to 100 • C. Therefore, the reaction parameters must be well-controlled in order to compromise between product yield and the crystallinity index of the nanocellulose particles.

Effects of Hydrolysis Time
Besides the reaction temperature, hydrolysis time also presents a significant effect on the crystallinity index of nanocellulose, which is illustrated in Figure 1b when the hydrolytic depolymerization of cellulose was conducted at 80 • C, 0.8 M Cr(NO 3 ) 3 concentration and solid-liquid ratio of 1:30. The nanocellulose yield showed a gradual decline trend while the crystallinity index of nanocellulose was increased over time. The increment reached a maximum of 86.8%, followed by a decreasing trend when the reaction was further prolonged to 2.5 h. The decrement was due to unfavorable hydrolysis process occurring in the crystalline cellulose, which led to the damage of high crystalline segments. However, insufficient reaction time (0.5 h) resulted in incomplete hydrolysis process of breaking down the strong bonding network of cellulose macromolecules, contributing to a low crystallinity index (79.6%). It is widely accepted that increasing contact time between hydrolyzing catalyst and reactant would eventually cause the swollen cellulose and even the enlargement of the intra-and inter-fiber pores of treated matrix, so that the degradation of cellulose network structure can happen effectively. This would also allow the catalyst to penetrate into the interior part of the cellulose pulp to liberate the nanoscale cellulose segments to accelerate the production of nanocellulose. For these reasons, it is very important to choose an appropriate hydrolysis time to maximize the removal of non-crystalline areas in cellulose, while preserving the crystalline parts. Therefore, hydrolysis time of 1.5 h was chosen as the optimal preparation condition and was used in the subsequent experiments, as the extension of time would cause a negative effect in enhancing the crystallinity of nanocellulose.

Effects of Metal Salt Concentration
Theoretically, higher catalyst concentration can enhance the degree of hydrolysis of cellulose. Unfortunately, the hydrolysis process is not only occurs in the cellulose defect domains but also in its crystalline structure. The defective crystalline phase in cellulose chains is more easily decomposed to its water soluble oligomers in the presence of metal ion catalyst than that of crystalline phase owing to its tightly packed structure and strong hydrogen bonding [32]. Thus, enhancing the hydrolysis degree of less-ordered cellulose phase by increasing the metal ion concentration benefits the production of high crystallinity nanocellulose. However, it is necessary to find an optimal metal ion concentration, as it is possible that some crystalline nanocellulose segments could gradually be hydrolyzed to further convert to liquid organic molecules as the secondary product when treated with an extremely high concentration of metal salt catalyst. Figure 1c reveals that when Cr(NO 3 ) 3 concentration increased from 0.2 to 1.2 M, the nanocellulose crystallinity progressively increased from 62.5% to 87.1% while maintaining reaction duration for 1.5 h, and a solid-liquid ratio of 1:30 at 80 • C. Simultaneously, the product yield decreased almost linearly with increasing concentration of metal salt. This implied that the successive degradation of defective crystalline allomorphs after the catalytic hydrolysis process induces the exposure of high crystalline segment in the nanocellulose, which leads to increasing crystallinity. However, it is worth noting that the increasing tendency of crystallinity index became slower when the metal salt concentration was higher than 0.8 M. Therefore, for better economic competence, 0.8 M metal salt catalyst was selected as the optimum concentration for hydrolysis.
In this study, Cr(NO 3 ) 3 acted as a Lewis acid to react with cellulose molecules for the production of the nanostructured product. The possible mechanism behind this phenomenon is that Cr 3+ ions can dissociate into a coordination complex structure with water molecules (H 2 O) at the initial hydrolysis stage. The H 2 O molecules tend to be polarized by this central metal ion by withdrawing its electron density, resulting in the H-atoms in hydroxyl groups (O-H) becoming more electropositive. However, the original state of these complex ions was unstable and there was a high probability of becoming deprotonated by releasing hydronium (H 3 O + ), further enhancing the acidity of the reaction mixture [33]. The detail reaction steps are as follows: Enhancing the acidity of the hydrolytic system would weaken and thus promote the breakdown of anhydro-glucopyranose units between cellulose polymeric chains, producing smaller cellulose units. Otherwise, there were insufficient hydronium ions present in the reaction system if the metal salt concentration was low. In addition, the Cr 3+ metal ions could easily interact with the oxygen atoms of C-O-C glycosidic bonds between the glucose units, leading to the formation of the oxygen-chromium complex intermediate. Due to the adsorbed metal ions, the bonding energy of oxygen atoms and carbon atoms of pyranose rings in the intermediate were reduced because of the increase of its bond length and bond angle [29]. Therefore, the activation energy of the hydrolysis process was reduced. However, as stated earlier, the operational conditions of the hydrolysis reaction must be optimized in order to prevent the carbonization process (formation of char).

Effects of Water Content
As shown in Equations (1)-(4), water molecules play a vital role in initiating the Cr 3+ -catalyzed hydrolysis process by generating H 3 O + ions. In order to accelerate the rate of the hydrolysis reaction, it was suggested that water content be increased in order to produce more hydronium ions by shifting the reaction forward to the product side (Le Chatelier's principle). Therefore, the effect of solid-liquid ratio of crystallinity index of nanocellulose was investigated by varying the ratio from 1:10 to 1:50 at a constant metal salt concentration (0.8 M), and 1.5 h reaction at 80 • C. As shown in Figure 1d, the nanocellulose yield increase when more water was added into the hydrolysis system (higher solid-liquid ratio); this might be similar to decreasing the metal salt concentration. In other words, the acidity of the reaction mixture was reduced by water molecules. However, the crystallinity of nanocellulose reached a maximum value (87.2%) at the solid-liquid ratio of 1:30 and gradually decreased when the cellulose matrix was treated with a more diluted hydrolysis system (1:40 and 1:50). This result might reflect that a small amount of H 2 O could reduce the cellulose hydrolysis efficiency due to the incompletion reaction between the cellulosic material and hydrolyzing catalyst. In addition, it was observed that some portion of the cellulosic material failed to immerse completely in the reaction solution when the solid-liquid ratio set at 1:10, which also leads to the hydrolysis process not taking place effectively. However, the catalyst concentration significantly reduced when a large amount of water was introduced to the reacting system, and this eventually led to a decrement of the crystallinity of nanocellulose as shown in Figure 1c. In fact, it is important to control the consumption of water in actual industrial production. Taking the cost and efficiency into consideration, 1:30 was chosen as a suitable loading ratio between cellulose solid material and the catalyst aqueous solution.  From the several single-factor experiments conducted, it can be summarized that the optimum operational conditions for cellulose hydrolysis were determined as 1.0 g of cellulose material and 0.8 M of Cr(NO3)3 catalyst with a solid-liquid ratio of 1:30 at the temperature of 80 °C for 1.5 h. In addition, the yield of solid product cellulose nanomaterial (CNCCr(NO3)3) was approximately 83.6% ± 0.6%. Meanwhile, the CNCH2SO4 obtained via sulfuric acid hydrolysis was determined to produce about 54.7% ± 0.3%. The yield of CNCH2SO4 was much lower than that of CNCCr(NO3)3 as an acid hydrolysis process is well-known to be a harsh treatment that results in disintegration and degradation, leading to fewer crystalline domains, causing the dramatic decrease in product yield. Both manufactured CNC products were further characterized.

Crystallinity Study-XRD Investigation
It is widely recognized that natural state cellulose consists of both less-ordered and highly-arranged crystalline regions in their molecular structure, and wide-angle X-ray scattering (WAXS) analysis was conducted to determine the crystalline index of native cellulose and optimized nanocellulose. XRD studies were performed to evaluate the crystallinity behaviors of native cellulose and yielded nanocellulose specimens. As shown in Figure 2, the XRD diffractogram profiles for all samples exhibited a similar pattern, which represented the typical semi-crystalline materials with a crystalline peak and non-crystalline broad halo. In addition, all XRD profiles showed the major peaks at around 2θ = 15.1 (1-10), 16.5 (110), 22.5 (200), and 34.6° (004), which indicated the presence of cellulose I structure [17,34]. From the X-ray curves, it was clearly observed that the XRD patterns of both nanocellulose samples and commercial MCC were similar. This From the several single-factor experiments conducted, it can be summarized that the optimum operational conditions for cellulose hydrolysis were determined as 1.0 g of cellulose material and 0.8 M of Cr(NO 3 ) 3 catalyst with a solid-liquid ratio of 1:30 at the temperature of 80 • C for 1.5 h. In addition, the yield of solid product cellulose nanomaterial (CNC Cr(NO3)3 ) was approximately 83.6% ± 0.6%. Meanwhile, the CNC H2SO4 obtained via sulfuric acid hydrolysis was determined to produce about 54.7% ± 0.3%. The yield of CNC H2SO4 was much lower than that of CNC Cr(NO3)3 as an acid hydrolysis process is well-known to be a harsh treatment that results in disintegration and degradation, leading to fewer crystalline domains, causing the dramatic decrease in product yield. Both manufactured CNC products were further characterized.

Crystallinity Study-XRD Investigation
It is widely recognized that natural state cellulose consists of both less-ordered and highly-arranged crystalline regions in their molecular structure, and wide-angle X-ray scattering (WAXS) analysis was conducted to determine the crystalline index of native cellulose and optimized nanocellulose. XRD studies were performed to evaluate the crystallinity behaviors of native cellulose and yielded nanocellulose specimens. As shown in Figure 2, the XRD diffractogram profiles for all samples exhibited a similar pattern, which represented the typical semi-crystalline materials with a crystalline peak and non-crystalline broad halo. In addition, all XRD profiles showed the major peaks at around 2θ = 15.1 (1-10), 16 cellulose I structure [17,34]. From the X-ray curves, it was clearly observed that the XRD patterns of both nanocellulose samples and commercial MCC were similar. This observation suggested that the crystalline structure of cellulose I of MCC had been well maintained after the hydrolysis process to different extents.
The crystallinity index for native cellulose, CNC H2SO4 and CNC Cr(NO3)3 were calculated based on Equation (5) and found to be 65.7%, 81.4% and 86.5%, respectively. It was noticeable that the produced CN CH2SO4 and CNC Cr(NO3)3 exhibited higher crystallinity compared with starting material. This is because of the successive removal of the less-ordered surface cellulose chains in the matrix during the catalytic hydrolysis process, leading to the exposure of solid elementary crystalline cellulose phases [35]. In this study, the untreated MCC was less crystalline than the treated ones. The higher intensity peak associated with CNC H2SO4 and CNC Cr(NO3)3 was predominantly due to the successive dissolution of the defective cellulose regions with the consequent increase in the sample crystallinity index. Although H 2 SO 4 and Cr(NO 3 ) 3 exhibited the strong catalytic effects toward the degradation of cellulose, the maximum crystallinity index was obtained with CNC Cr(NO3)3 (86.5% ± 0.3%) compared to that of CNC H2SO4 (81.4% ± 0.1%). In the process of hydrolysis, the H 3 O + ions tend to penetrate into the non-crystalline regions of the cellulose matrix, promoting the hydrolytic cleavage of glycosidic linkages and eventually releasing the individual crystallites. On the other hand, the higher crystallinity of nanocellulose product could be attained because the nanocellulose tends to be realigned into a better organized and crystalline structure, which results in the self-assembly phenomenon of nanocellulose, enabling close packing and hydrogen bond formation [36]. Besides that, there is no remarkable difference in the XRD patterns between CNC H2SO4 and CNC Cr(NO3)3 . These results confirmed that the proposed Cr(NO 3 ) 3 hydrolysis system does not have a negative effect on the crystallites structure of cellulose.
In this study, the crystallinity index of CNC H2SO4 was lower than that of CNC Cr(NO3)3 , which could be ascribed to the reasonable explanation that the strong H 2 SO 4 not only attacks the randomly-ordered structure in cellulose but also destroys the crystalline one during hydrolysis. For industrial uses, the addition of nanocellulose to a polymer matrix is one of the effective ways to reduce the oxygen transmission rate in packaging application; however, the slightly lower crystallinity of CNC H2SO4 might have a negative effect on its gas barrier property [22,37]. observation suggested that the crystalline structure of cellulose I of MCC had been well maintained after the hydrolysis process to different extents. The crystallinity index for native cellulose, CNCH2SO4 and CNCCr(NO3)3 were calculated based on Equation (5) and found to be 65.7%, 81.4% and 86.5%, respectively. It was noticeable that the produced CNCH2SO4 and CNCCr(NO3)3 exhibited higher crystallinity compared with starting material. This is because of the successive removal of the less-ordered surface cellulose chains in the matrix during the catalytic hydrolysis process, leading to the exposure of solid elementary crystalline cellulose phases [35]. In this study, the untreated MCC was less crystalline than the treated ones. The higher intensity peak associated with CNCH2SO4 and CNCCr(NO3)3 was predominantly due to the successive dissolution of the defective cellulose regions with the consequent increase in the sample crystallinity index. Although H2SO4 and Cr(NO3)3 exhibited the strong catalytic effects toward the degradation of cellulose, the maximum crystallinity index was obtained with CNCCr(NO3)3 (86.5% ± 0.3%) compared to that of CNCH2SO4 (81.4% ± 0.1%). In the process of hydrolysis, the H3O + ions tend to penetrate into the non-crystalline regions of the cellulose matrix, promoting the hydrolytic cleavage of glycosidic linkages and eventually releasing the individual crystallites. On the other hand, the higher crystallinity of nanocellulose product could be attained because the nanocellulose tends to be realigned into a better organized and crystalline structure, which results in the self-assembly phenomenon of nanocellulose, enabling close packing and hydrogen bond formation [36]. Besides that, there is no remarkable difference in the XRD patterns between CNCH2SO4 and CNCCr(NO3)3. These results confirmed that the proposed Cr(NO3)3 hydrolysis system does not have a negative effect on the crystallites structure of cellulose.
In this study, the crystallinity index of CNCH2SO4 was lower than that of CNCCr(NO3)3, which could be ascribed to the reasonable explanation that the strong H2SO4 not only attacks the randomly-ordered structure in cellulose but also destroys the crystalline one during hydrolysis. For industrial uses, the addition of nanocellulose to a polymer matrix is one of the effective ways to reduce the oxygen transmission rate in packaging application; however, the slightly lower crystallinity of CNCH2SO4 might have a negative effect on its gas barrier property [22,37].

Chemical Structures-FTIR Analysis
FTIR spectroscopy is an appropriate technique to evaluate the changes in the chemical compositions of the samples in response to native cellulose and yielded nanocellulose (CNCH2SO4 and CNCCr(NO3)3). As presented in Figure 3, the FTIR spectra of all the samples exhibited two major absorbance regions, which included the high (3500-2800 cm −1 ) and low wavenumbers (1700-500  and CNC Cr(NO3)3 ). As presented in Figure 3, the FTIR spectra of all the samples exhibited two major absorbance regions, which included the high (3500-2800 cm −1 ) and low wavenumbers (1700-500 cm −1 ) regions, consistent with previous studies [38,39]. A broad and dominant absorbance peak located in the region of 3600 to 3200 cm −1 was observed in all spectra, which corresponded to the characteristic of hydrogen bonding O-H stretching vibrations [40]. In addition, C-H stretching gave rise to the band at 2900 cm −1 [21]. A small peak was detected at 1640 cm −1 associated with -OH bending vibration of absorbed water by the fibers as the cellulose is hygroscopic in nature [41], whereas the peak at 1430 cm −1 related to the intermolecular hydrogen attraction at the C 6 group [7]. Although all the samples were subjected to a proper drying process prior to FTIR analysis, the complete elimination of moisture in the samples was very difficult due to the strong cellulose-water interaction [42]. Moreover, the stretching vibration of the C-O-C pyranose ring within the cellulose molecules was observed at a sharp peak of 1054 cm −1 [43]. Besides that, the fingerprint region of the FTIR spectra, which lies in the range within 600 to 1100 cm −1 , is a significant signal for tracking the presence of β-glycosidic linkages that contributed by the vibration of wagging, deformation and twisting modes of the anhydro-glucopyranose units [5]. Furthermore, the absence of NO 3 − peaks at 1384 cm −1 in the spectra implied the complete removal of Cr 3+ metal salt during the washing by centrifuge and dialysis process [17].

Chemical Structures-FTIR Analysis
located in the region of 3600 to 3200 cm −1 was observed in all spectra, which corresponded to the characteristic of hydrogen bonding O-H stretching vibrations [40]. In addition, C-H stretching gave rise to the band at 2900 cm −1 [21]. A small peak was detected at 1640 cm −1 associated with -OH bending vibration of absorbed water by the fibers as the cellulose is hygroscopic in nature [41], whereas the peak at 1430 cm −1 related to the intermolecular hydrogen attraction at the C6 group [7]. Although all the samples were subjected to a proper drying process prior to FTIR analysis, the complete elimination of moisture in the samples was very difficult due to the strong cellulose-water interaction [42]. Moreover, the stretching vibration of the C-O-C pyranose ring within the cellulose molecules was observed at a sharp peak of 1054 cm −1 [43]. Besides that, the fingerprint region of the FTIR spectra, which lies in the range within 600 to 1100 cm −1 , is a significant signal for tracking the presence of β-glycosidic linkages that contributed by the vibration of wagging, deformation and twisting modes of the anhydro-glucopyranose units [5]. Furthermore, the absence of NO3 -peaks at 1384 cm −1 in the spectra implied the complete removal of Cr 3+ metal salt during the washing by centrifuge and dialysis process [17]. Based on the FTIR spectra obtained from native cellulose and yielded nanocellulose, there were no significant changes in the functional groups after the cellulose hydrolyzed by H2SO4 and Cr(NO3)3 hydrolyzing catalysts, except peak intensities. Therefore, this suggested that the chemical structure of yielded nanocellulose that was not altered after the hydrolysis process, where the typical structure of the parent cellulose remained well-preserved, only different in term of morphology and crystallinity. This observation was in good agreement with other works of literature [7,17,21,44] exhibited a similar FTIR pattern compared to its corresponding cellulosic materials. It is important to note that the extracted nanocellulose exhibited absorbance signals at 1428, 1160, 1110, and 898 cm −1 , suggesting that nanocellulose was primarily in cellulose I structure [45].  Based on the FTIR spectra obtained from native cellulose and yielded nanocellulose, there were no significant changes in the functional groups after the cellulose hydrolyzed by H 2 SO 4 and Cr(NO 3 ) 3 hydrolyzing catalysts, except peak intensities. Therefore, this suggested that the chemical structure of yielded nanocellulose that was not altered after the hydrolysis process, where the typical structure of the parent cellulose remained well-preserved, only different in term of morphology and crystallinity.
This observation was in good agreement with other works of literature [7,17,21,44] exhibited a similar FTIR pattern compared to its corresponding cellulosic materials. It is important to note that the extracted nanocellulose exhibited absorbance signals at 1428, 1160, 1110, and 898 cm −1 , suggesting that nanocellulose was primarily in cellulose I structure [45]. Figure 4a illustrates that native cellulose is primarily comprised of aggregated cellulosic fibrils with an irregular shape. In nature, each cellulose fiber is made up of several to hundreds of microfibers that tend to be assembled together and lead to the formation of the compact structure of cellulose [7]. The aggregation of microfibers can contribute to the strong hydrogen bonding between the individual cellulose chains [46]. In addition, each elementary cellulose fiber appeared to be long in length with a rough surface and ow aspect ratio. The aggregation of native cellulose is due to the strong hydrogen bonding system between each individual cellulose microfibril in the macrostructure of cellulose, which is consistent with the literature [7].

Morphological Investigation-FESEM-EDX
The long cellulosic fibrils were broken down to a great extent after the hydrolysis treatment catalyzed by Cr(NO 3 ) 3 and H 2 SO 4 , which can be clearly observed in the FESEM micrographs as presented in Figure 4b,c. Smaller and more individualized fragments were successfully disintegrated and/or separated from the bundles of micro-sized cellulose fibers, which resulted in further reduction of its diameter. These findings were in excellent accordance with the reported literature [7]. The intermittent breakdown in the fibrillar structure of CNC in this study could be correlated with the successive hydrolysis treatment initiated by the Cr(NO 3 ) 3 metal salt and H 2 SO 4 in the dissolution of the less-ordered defective crystalline regions as well as the hydrolytic cleavage of β-1,4-glycosidic linkages. The highly ordered crystalline arrangements appear in nanocellulose samples due to the formation of intra-and intermolecular H-bonding between the hydroxyl groups.
In the present study, finer and shorter nanoscale fibrils were expected in the CNC H2SO4 and CNC Cr(NO3)3 ; however, this was not obvious from the FESEM micrographs, as shown in Figure 4b,c. This was accredited to the strong intermolecular hydrogen bonding within the cellulose chains. As a result, the cellulose fibrils tended to agglomerate with each other during the freeze-drying process owing to the higher number of OH groups on the cellulose fibrils. A similar morphology has also been reported for the nanocellulose derived from flax and microcrystalline cellulose [47]. In order to confirm the successive production of nanocellulose, more insight into the particle size and dimension of CNC H2SO4 and CNC Cr(NO3)3 in suspension was observed through TEM analysis.
The results of EDX analyses of the CNC H2SO4 and CNC Cr(NO3)3 are given in Table 1. The chemical analysis of both nanocellulose products revealed carbon (C) and oxygen (O) as its major elements. However, the elemental analysis by EDX identified that a small amount of sulfur (S) content was introduced on CNC specimen treated by H 2 SO 4 . The trace amounts of sulfur components probably contributed by the sulfate groups that came from H 2 SO 4 . In fact, for sulfuric acid hydrolyzed CNC, sulfur will always be present since the sulfate half esters have a role in stabilizing the CNC aqueous suspensions and are introduced during the hydrolysis step. Moreover, the presence of sulfur indicated that the chemical interaction of sulfuric acid with the surface of cellulose fiber, and, most importantly, this also further evidenced that cellulose fiber had been hydrolyzed by H 2 SO 4 during the treatment. Sulfated nanocellulose was believed to be due to the esterification process occurred during the acid hydrolysis according to the procedure as follows [7]: Most importantly, the elemental chromium was not detected in CNC Cr(NO3)3 , which indicated that Cr 3+ could be completely removed during centrifugation steps. Thus, it can be concluded that the yielded CNC Cr(NO3)3 had been washed cleanly after the hydrolysis process and metal ion was completely detached in the treated samples. This finding was in good agreement with FTIR result.  It is worth noting that the cellulose fiber is diameter greatly reduced after hydrolysis.

Structural Morphology and Particle Size Distribution-TEM Analysis
Because the manufactured nanocellulose products were matted and not individualized as seen in FESEM images, the nanoparticles size and morphology of CNCH2SO4 and CNCCr(NO3)3 were confirmed by transmission electron microscopy (TEM). This technique is considered one of the most accurate methods for direct measurement and observation at a nanoscopic scale. Under the controlled hydrolysis conditions, the non-crystalline regions of cellulose were cleaved transversely by H2SO4 and Cr(NO3)3 hydrolyzing catalysts with heat treatment. The average diameter of the nanocellulose fibers was calculated by ImageJ software for at least 200 measurements. The results showed that the macro-sized native cellulose fiber diminished to the nanometer scale after the hydrolysis reaction. However, it can be clearly observed that the morphology of the nanocellulose obtained for both hydrolysis procedures exhibited noticeably different structures: CNCH2SO4 displayed a rice-shape structure ( Figure 5a) and CNCCr(NO3)3 revealed a spider-web-network-like structure (Figure 5b), confirming a successful extraction of nanocellulose from MCC. However, part of the CNCH2SO4 and CNCCr(NO3)3 samples were observed in a somewhat agglomerated form, as bundles of particles with some dispersed individual fibrils present. In fact, it was challenging to obtain separated nanocellulose fibrils due to the presence of strong hydrogen bonding and the high surface area between these fibrils fosters agglomeration, overlapping and assembly. This phenomenon occurred mainly during the evaporation of the dispersing medium in order to dry the particles for TEM imaging [48]. Similar observation has been previously reported [49].
Besides the morphological aspects, the TEM images allowed the observation of a variation in the average particle width (d) and length (L), thus enabling determination of the aspect ratio (L/d) of  3 . It is worth noting that the cellulose fiber is diameter greatly reduced after hydrolysis.

Structural Morphology and Particle Size Distribution-TEM Analysis
Because the manufactured nanocellulose products were matted and not individualized as seen in FESEM images, the nanoparticles size and morphology of CNC H2SO4 and CNC Cr(NO3)3 were confirmed by transmission electron microscopy (TEM). This technique is considered one of the most accurate methods for direct measurement and observation at a nanoscopic scale. Under the controlled hydrolysis conditions, the non-crystalline regions of cellulose were cleaved transversely by H 2 SO 4 and Cr(NO 3 ) 3 hydrolyzing catalysts with heat treatment. The average diameter of the nanocellulose fibers was calculated by ImageJ software for at least 200 measurements. The results showed that the macro-sized native cellulose fiber diminished to the nanometer scale after the hydrolysis reaction. However, it can be clearly observed that the morphology of the nanocellulose obtained for both hydrolysis procedures exhibited noticeably different structures: CNC H2SO4 displayed a rice-shape structure ( Figure 5a) and CNC Cr(NO3)3 revealed a spider-web-network-like structure (Figure 5b), confirming a successful extraction of nanocellulose from MCC. However, part of the CNC H2SO4 and CNC Cr(NO3)3 samples were observed in a somewhat agglomerated form, as bundles of particles with some dispersed individual fibrils present. In fact, it was challenging to obtain separated nanocellulose fibrils due to the presence of strong hydrogen bonding and the high surface area between these fibrils fosters agglomeration, overlapping and assembly. This phenomenon occurred mainly during the evaporation of the dispersing medium in order to dry the particles for TEM imaging [48]. Similar observation has been previously reported [49].
Besides the morphological aspects, the TEM images allowed the observation of a variation in the average particle width (d) and length (L), thus enabling determination of the aspect ratio (L/d) of the yielded nanocellulose products. Significant differences were noticed for the average particle length. It was clearly observed that the average diameter (d) of CNC Cr(NO3)3 was comparable to that of CNC H2SO4 , the length (L) of the former product seems to be much longer than the latter product, and this gave rise to the presumably higher aspect ratio (L/d). The use of MCC as starting material hydrolyzed in the H 2 SO 4 system led to the formation of CNC H2SO4 having an average fiber width of approximately 9.9 ± 3.2 nm with an average length of 34.8 ± 14.2 nm providing an aspect ratio of 3.5. Meanwhile, the inorganic salt-treated CNC Cr(NO3)3 displayed a mean width of 29.1 ± 7.8 nm, a length of 455.7 ± 38.1 nm and an aspect ratio of 15.7. Briefly, CNC Cr(NO3)3 rendered stronger reinforcing effect (i.e., higher tensile strength and modulus) than that of CNC H2SO4 contributed by its high aspect ratio, making it become an ideal reinforcing agent for polymer matrix [50]. In addition, CNC Cr(NO3)3 with higher aspect ratio might exhibit higher optical transmittance when it is used for preparing nano-paper or reinforcement for polymer composites [37].
In this study, the aspect ratio values of CNC Cr(NO3)3 are well above 10 (~15.7), which is considered the minimum value required for a good stress transfer from the matrix to the fiber for a good reinforcement when nano-biocomposite is targeted [51]. Generally, it is known that to constitute the composites, nanocellulose structures with a higher aspect ratio have a good reinforcing capability of the final polymer products, resulting in an improvement in the thermal and mechanical properties [52]. Besides the outstanding mechanical properties, another advantage of Cr(NO 3 ) 3 -catalyzed hydrolysis over H 2 SO 4 hydrolysis is that no sulfate groups were introduced to the surface of cellulose fibrils after the hydrolysis process. the yielded nanocellulose products. Significant differences were noticed for the average particle length. It was clearly observed that the average diameter (d) of CNCCr(NO3)3 was comparable to that of CNCH2SO4, the length (L) of the former product seems to be much longer than the latter product, and this gave rise to the presumably higher aspect ratio (L/d). The use of MCC as starting material hydrolyzed in the H2SO4 system led to the formation of CNCH2SO4 having an average fiber width of approximately 9.9 ± 3.2 nm with an average length of 34.8 ± 14.2 nm providing an aspect ratio of 3.5. Meanwhile, the inorganic salt-treated CNCCr(NO3)3 displayed a mean width of 29.1 ± 7.8 nm, a length of 455.7 ± 38.1 nm and an aspect ratio of 15.7. Briefly, CNCCr(NO3)3 rendered stronger reinforcing effect (i.e., higher tensile strength and modulus) than that of CNCH2SO4 contributed by its high aspect ratio, making it become an ideal reinforcing agent for polymer matrix [50]. In addition, CNCCr(NO3)3 with higher aspect ratio might exhibit higher optical transmittance when it is used for preparing nano-paper or reinforcement for polymer composites [37]. In this study, the aspect ratio values of CNCCr(NO3)3 are well above 10 (~15.7), which is considered the minimum value required for a good stress transfer from the matrix to the fiber for a good reinforcement when nano-biocomposite is targeted [51]. Generally, it is known that to constitute the composites, nanocellulose structures with a higher aspect ratio have a good reinforcing capability of the final polymer products, resulting in an improvement in the thermal and mechanical properties [52]. Besides the outstanding mechanical properties, another advantage of Cr(NO3)3-catalyzed hydrolysis over H2SO4 hydrolysis is that no sulfate groups were introduced to the surface of cellulose fibrils after the hydrolysis process.

Thermostability Analysis
The pyrolytic study was used to investigate the thermal stability of native cellulose, CNC H2SO4 , and CNC Cr(NO3)3 . Determination of thermal properties of reinforcing materials is an important parameter in order to evaluate and identify their applicability of these cellulose nanomaterials in biocomposite applications which are always processed at high temperatures. Both TG and DTG curves were plotted and graphically presented in Figure 6 to track the thermal stability differences between native cellulose and yielded nanocellulose samples (CNC H2SO4 and CNC Cr(NO3)3 ). In fact, DTG curves provided a more precise assessment and comparison, especially in term of maximum decomposition temperature (T max ) value.
An initial weight loss was observed for all cellulosic samples in the region below 100 • C, mainly corresponding to the evaporation of moisture content or any other volatile low-molecular-weight compounds loosely bound to the surface or inside the materials [42]. The presence of absorbed water was also observed by FTIR characteristic peaks at the signal of 1640 cm −1 , which represents the bending vibration of intermolecular hydrogen bonding of water molecules. The DTG curve revealed there is a major difference in the decomposition temperature between native cellulose and CNC H2SO4 , in which the CNC H2SO4 started to undergo a degradation process much earlier (at 210 • C) than that of cellulose as well as CNC Cr(NO3)3 . This phenomenon was accredited to several possible reasons:

(i) The replacement of hydroxyl groups (O-H) by active sulfate groups (O-SO 3 H) via either
esterification or direct catalysis would initiate the dehydration reaction to occur on the sulfated nanocellulose fiber, leading to the water formation, further catalyzing the decomposition of remaining cellulose fiber into smaller particles. On the other hand, the unsulfated crystals tend to collapse at higher decomposition temperature [53]; (ii) It was believed that the concentrated H 2 SO 4 not only removes the loosely-packed defective parts but also has high potential to damage crystalline domains, making the molecules more susceptible to the degradation step in response to increased temperature [18]; (iii) The rapid reduction of molecular weight of nanocellulose during hydrolysis compared to that of its starting material contributed to its early decomposition during heating treatment [54]; (iv) The short nanocellulose chains provide a high specific surface area and result in the formation of a large number of free-end-chains in the surface that tended to decompose easily at a lower temperature. The high surface area of nanocellulose plays a significant role in diminishing their thermal stability due to the increased exposure to heat source [55].
Thermal degradation of native cellulose started at approximately 225.4 • C in an N 2 atmosphere, while the CNC Cr(NO3)3 product degradation began at ca. 238.2 • C. The increased thermal stability of CNC Cr(NO3)3 might be attributed to the removal of defective crystalline cellulose, as evidenced above by XRD results. Nevertheless, the CNC H2SO4 showed a lower thermal stability (201.2 • C) than the untreated cellulosic material. This was presumably due to the attaching of sulfate groups on fiber surface during the hydrolysis step, which clearly reduced the activation energy for the degradation of nanocellulose product, making the nanomaterial less resistant to pyrolysis. Additionally, the lower degradation temperature of CNC H2SO4 could be due to the smaller fiber dimension, resulting in more surface areas exposed to the heat source and the partial disruptions of crystal structure compared to that of cellulose precursors [22]. These findings are in good accordance with Kallel's study [56], in which the nanocellulose isolated from garlic straw by H 2 SO 4 hydrolysis started to degrade at a lower decomposition temperature (200 • C) than that of its starting material (220 • C). In summary, the introduction of the sulfated groups and massive decrement of molecular weight significantly decreased the thermal stability of CNC.
Compared to CNC H2SO4 prepared via H 2 SO 4 hydrolysis procedure (T max = 273 • C), the CNC Cr(NO3)3 produced using Cr(NO 3 ) 3 hydrolysis system rendered much higher thermal stability (T max = 344 • C) than its raw material (T max = 295 • C), which can be explained by the fact that less damage happened in crystalline regions of the cellulose matrix benefiting from the milder hydrolysis conditions. In addition, the additional thermal stability is affected by the crystalline structure of cellulose, which increased as a result of the Cr(NO 3 ) 3 hydrolysis treatments. Some studies [57] reported that the high crystallinity nanocellulose could lead to higher thermal stability, but the thermal stability of high crystallinity sulfated CNC was reduced. Similar results have been reported by Mohamed et al. [42] where the nanocellulose derived from newspaper pulp rendered a lower T max value (187.4 • C) than its starting material (346.4 • C) although it had higher crystallinity (90.15%) compared to untreated pulp (82%). In fact, the authors believed that the thermal destruction of cellulose was primarily affected by the presence of sulfate groups from H 2 SO 4 , in which the lower thermal stability was correlated to the content of sulfur impurities as well as removal of thermostable minerals during acid hydrolysis of the MCC.
As pointed above, sulfuric acid hydrolysis leads to reduced thermal stability of acid hydrolyzed CNC compared to that of neat cellulosic feedstock. However, this effect can be diminished by neutralizing the CNC H2SO4 with NaOH solution (~1% NaOH). After neutralization with NaOH, no acid sulfate groups remained, and the thermal stability of neutralized CNC Cr(NO3)3 was shifted to a higher temperature. However, the presently used method for isolating the CNC Cr(NO3)3 has some restrictions, such as prolonged production time and capital cost in order to remove the free sulfuric acid remaining on the CNC Cr(NO3)3 surface after hydrolysis process, for their use in industrial scale. Thus far, the residual sulfuric acid in the cellulosic fiber is usually eliminated by the dialysis process against water until neutral pH is achieved, which is a costly procedure and usually takes a long time (3-7 days) [58]. Some studies reported that the presence of the divalent group (SO 4 2− ) or anionic (-OSO 2 O-) of HSO 4 and sulfates on the surface of nanocellulose may significantly reduce the interaction of cellulose fibrils with other polymer and biopolymer materials [59]. Even if the chemical neutralization method is simpler with less processing steps to produce unsulfated CNC H2SO4 than time-consuming dialysis process, the large amount of waste effluents (salts) produced during the neutralization process needs to be further recycled before being discharged into the local waterway or ecological system. In fact, the sulfur content is only able to affect the thermal behavior of the nanocellulose, but maybe not influence or cause a negative effect on the crystallinity of the H 2 SO 4 hydrolyzed CNC [58]. In this study, the thermal degradation pattern of neutralized CNC H2SO4 was similar as native cellulose and CNC Cr(NO3)3 , in which the low-temperature degradation peak in the DTG curve disappeared completely after neutralization. The neutralized CNC H2SO4 started to degrade at the temperature of ca. 237 • C. As expected, the T max value of neutralized CNC H2SO4 profoundly increased to 349 • C, which can be explained by two main reasons. First, the absence of amorphous regions in the treated cellulose matrix and the highly structured crystals segments rendered the higher-temperature degradation. Second, the replacement of hydrogen ions (H + ) from acid hydrolysis by alkaline ion (Na + ) inhibited the dehydration catalyzed by H 2 SO 4 [58].
After 500 • C, the thermal deposition temperature of all the cellulosic samples leveled off, and a slow thermal degradation profile was attained. This can be accredited to the further carbonization of polysaccharide chains initiated by the cleavage of C-C and C-H bonds. According to Figure 6, it is interesting to note that the sulfated-CNC H2SO4 possesses the highest residues (black char) at 600 • C as compared to that of other cellulosic sample. This behavior was attributed to that the sulfate groups in the CNC H2SO4 tend to play the role of flame retardant by promoting the dehydration reactions, which is conducive to the production of anhydrocellulose and leads to its decomposition to carbonaceous residues, as reported by other authors as Mohamed and his group [42]. Moreover, the CNC H2SO4 product with the acid sulfate group might be decomposed by more pathways by the action of acid, and, eventually, the intermediate products obtained, which could be further decomposed into the char residue, were more complex compared to the cellulose with acid content. Therefore, it is expected that char yield of neutralized CNC H2SO4 (6.57%) is remarkably lower than sulfated CNC H2SO4 (17.4%), considering the lack of remnant surface sulfate in the CNC H2SO4 . It is quite interesting to note that the char level of neutralized CNC H2SO4 and CNC Cr(NO3)3 (6.57%-9.03%) was much lower than native cellulose (10.2%). The decreased char residue for the nanocellulose sample may be mostly related to the increased split hydrogen bonds and/or due to the absence of non-cellulosic components in the treated nanocellulose after hydrolysis treatment [60]. Furthermore, the presence of alkaline ions with highly polar field might act as the catalyst for complete degradation of glycosidic linkages via the hemolytic mechanism, which resulted in the decrement of char yield [61].
In view of the above thermogravimetric analysis, it was concluded that the produced CNC Cr(NO3)3 product from cellulosic feedstock exhibits better thermal stability than sulfated CNC H2SO4 , and it will be more suitable for the production of green bio-nanocomposites as the typical processing temperature for polymer synthesis (i.e., thermoplastics) is normally higher than 200 • C. In this sense, CNC Cr(NO3)3 with more thermal stability than that of MCC and sulfated CNC H2SO4 provide better-reinforcing capability for different applications such as cosmetics, thermal sensitive papers, and disposable products, owing to the high elastic modulus of its crystal domains [51]. Table 2 summarizes the values of T max for cellulose and its nanomaterials derived from different species and collected from the literature. degradation of glycosidic linkages via the hemolytic mechanism, which resulted in the decrement of char yield [61].
In view of the above thermogravimetric analysis, it was concluded that the produced CNCCr(NO3)3 product from cellulosic feedstock exhibits better thermal stability than sulfated CNCH2SO4, and it will be more suitable for the production of green bio-nanocomposites as the typical processing temperature for polymer synthesis (i.e., thermoplastics) is normally higher than 200 °C. In this sense, CNCCr(NO3)3 with more thermal stability than that of MCC and sulfated CNCH2SO4 provide better-reinforcing capability for different applications such as cosmetics, thermal sensitive papers, and disposable products, owing to the high elastic modulus of its crystal domains [51]. Table 2 summarizes the values of Tmax for cellulose and its nanomaterials derived from different species and collected from the literature.    2.10. Visual Appearance of Nanocellulose Figure 7 shows photographs illustrating the visual appearance of the aqueous suspension of CNC H2SO4 and CNC Cr(NO3)3 samples with a white light source. For comparison, all nanocellulose samples were dispersed in water at the same concentration. After the ultrasonication process, the electronic photos of CNC H2SO4 and CNC Cr(NO3)3 suspensions in the sample bottle are taken at 0 h, 24 h and 168 h. As can be seen in Figure 7a, two milk-like turbid suspensions were produced, suggesting that CNC H2SO4 and CNC Cr(NO3)3 both formed stable dispersions in water, with the latter being whiter than the former. After 24 h standing, the turbid phenomena continued for CNC H2SO4 colloidal suspensions, as shown in Figure 7b. As the time prolonged, however, some sedimentation from CNC Cr(NO3)3 were flocculated, whereas the good stability of CNC H2SO4 suspension could be kept at ambient temperature condition. For CNC Cr(NO3)3 , the original suspension had separated into a two-layer mixture, which consists of a clear layer and a turbid layer that can be visibly observed. This could be attributed to the fact that no surface groups have been attached to CNC Cr(NO3)3 when prepared from Cr(NO 3 ) 3 metal salt catalyst, and thus the colloidal dispersion was expected to be less stable than CNC H2SO4 hydrolyzed by sulfuric acid. The high stability of sulfated CNC H2SO4 suspension can be achieved due to the sulfate negative charges (SO 4 2− ) on the CNC H2SO4 surface being able to create a strong electrostatic repulsion between the cellulose surfaces, and these charged groups limit their ability to flocculate [24]. For this reason, CNC Cr(NO3)3 showed a higher risk for agglomerate in a water medium. Due to the unmodified surface of CNC Cr(NO3)3 with the high aspect ratio and high content of hydroxyl groups on the fiber surface, the CNC Cr(NO3)3 tends to intertwine with their neighbor fibers and these networks were caused by the physical entanglement lock between the fibers. Furthermore, the lack of imparting electrostatic stability on CNC Cr(NO3)3 resulted in poor stability, leading to agglomeration as compared to the sulfated CNC H2SO4 which remained individually dispersed. A similar observation has been reported by others [49,64]. As a result, the CNC Cr(NO3)3 suspension was unable to maintain its stability after being stored for seven days and obvious precipitation can be seen, while the CNC H2SO4 still remained a milky colloidal suspension with negligible precipitation, as displayed in Figure 7c. In summary, the excellent stability of CNC H2SO4 suspensions was beneficial to reinforce the hydrophilic polymers (such as polyvinyl alcohol). Further studies on the potential reinforcement of composites by the CNC H2SO4 and CNC Cr(NO3)3 manufactured in this study are underway.  Figure 7 shows photographs illustrating the visual appearance of the aqueous suspension of CNCH2SO4 and CNCCr(NO3)3 samples with a white light source. For comparison, all nanocellulose samples were dispersed in water at the same concentration. After the ultrasonication process, the electronic photos of CNCH2SO4 and CNCCr(NO3)3 suspensions in the sample bottle are taken at 0 h, 24 h and 168 h. As can be seen in Figure 7a, two milk-like turbid suspensions were produced, suggesting that CNCH2SO4 and CNCCr(NO3)3 both formed stable dispersions in water, with the latter being whiter than the former. After 24 h standing, the turbid phenomena continued for CNCH2SO4 colloidal suspensions, as shown in Figure 7b. As the time prolonged, however, some sedimentation from CNCCr(NO3)3 were flocculated, whereas the good stability of CNCH2SO4 suspension could be kept at ambient temperature condition. For CNCCr(NO3)3, the original suspension had separated into a two-layer mixture, which consists of a clear layer and a turbid layer that can be visibly observed. This could be attributed to the fact that no surface groups have been attached to CNCCr(NO3)3 when prepared from Cr(NO3)3 metal salt catalyst, and thus the colloidal dispersion was expected to be less stable than CNCH2SO4 hydrolyzed by sulfuric acid. The high stability of sulfated CNCH2SO4 suspension can be achieved due to the sulfate negative charges (SO4 2− ) on the CNCH2SO4 surface being able to create a strong electrostatic repulsion between the cellulose surfaces, and these charged groups limit their ability to flocculate [24]. For this reason, CNCCr(NO3)3 showed a higher risk for agglomerate in a water medium.

Visual Appearance of Nanocellulose
Due to the unmodified surface of CNCCr(NO3)3 with the high aspect ratio and high content of hydroxyl groups on the fiber surface, the CNCCr(NO3)3 tends to intertwine with their neighbor fibers and these networks were caused by the physical entanglement lock between the fibers. Furthermore, the lack of imparting electrostatic stability on CNCCr(NO3)3 resulted in poor stability, leading to agglomeration as compared to the sulfated CNCH2SO4 which remained individually dispersed. A similar observation has been reported by others [49,64]. As a result, the CNCCr(NO3)3 suspension was unable to maintain its stability after being stored for seven days and obvious precipitation can be seen, while the CNCH2SO4 still remained a milky colloidal suspension with negligible precipitation, as displayed in Figure 7c. In summary, the excellent stability of CNCH2SO4 suspensions was beneficial to reinforce the hydrophilic polymers (such as polyvinyl alcohol). Further studies on the potential reinforcement of composites by the CNCH2SO4 and CNCCr(NO3)3 manufactured in this study are underway.

Comparison between H 2 SO 4 and Cr(NO 3 ) 3 Hydrolysis System
When comparing the two nanocellulose production procedures, the important differences are noticed in terms of environmental impact, productivity, fiber quality, and economic point of view. For CNC H2SO4 production, concentrated sulfuric acid (64 wt %) is required; however, the strong acidic waste effluent is a potentially harmful hazard to the biological system or nearby environment if the acidic wastewater is being discharged without any purification treatment. Therefore, it is a necessary step to neutralize this acid waste with base chemicals (i.e., KOH, NaOH, or NaHCO 3 ) in the waste neutralization system before it can be safely released into the environment. In contrast, the Cr 3+ -containing solution produced from the Cr(NO 3 ) 3 catalytic system can be treated using several effective methods [65]. Among these techniques, chemical precipitation is worth mentioning as it is the most effective way for converting the Cr 3+ to Cr(OH) 3 via precipitation process with the presence of aqueous ammonia or sodium hydroxide. For commercial use, Cr(OH) 3 is widely used as a pigment, mordant, and catalyst for organic reactions. This means that Cr(NO 3 ) 3 catalyst can not only produce value-added nanocellulose (solid residue) from the low-cost cellulosic feedstock, the generated supernatant may also be transferred into another useful byproduct, rather than discharged into the wastewater treatment plant without any profit. Approximately 9 kg of H 2 SO 4 is required to produce 1 kg of CNC H2SO4 and the industry facing the difficulties in acid recovery economically. Therefore, the Cr(NO 3 ) 3 hydrolysis system could be sustainable and economically feasible. This will be the objective of further studies.
Additionally, the CNC Cr(NO3)3 produced via Cr(NO 3 ) 3 system rendered a better thermal stability than that of sulfated CNC H2SO4 . This could make the CNC Cr(NO3)3 a promising candidate for reinforcing agent and gas-barrier material, as well as providing convenience for polymer applications and processing performed under high temperatures, especially thermoplastics. Oppositely, the introduction of ester groups on the surface CNC H2SO4 could enhance its stability in suspension, which is beneficial for the manufacture of nanocomposites. Unfortunately, the uncontrollable degradation of cellulose into oligomers liquid fraction is a common phenomenon observed from H 2 SO 4 hydrolysis, resulting in a very low yield (~55%). In other words, extra attention is necessary during the process not only due to the highly corrosive acid treatment, most importantly to prevent the acid hydrolysis process going too far. Table 3 displays an exhaustive list of nanocellulose obtained from numerous lignocellulosic biomasses by different hydrolysis techniques, detailing the reaction conditions and presenting the physicochemical properties.

Materials
Microcrystalline cellulose (MCC) was purchased from Sigma-Aldrich (Saint Louis, MO, USA). The chemical compositions of the native cellulose determined according to the ASTM/TAPPI standard protocols are shown in Table 4. The hydrolyzing catalysts used in this study, namely sulfuric acid (H 2 SO 4 , 95%-97% purity) and chromium (III) nitrate (Cr(NO 3 ) 3 ), were bought from Merck (Kuala Lumpur, Malaysia). All chemicals were of analytical grade and used as received without further purification.

Preparation of CNC Cr(NO3)3 via Cr(NO 3 ) 3 Catalyzed Hydrolysis
In order to optimize the operating hydrolysis conditions, one-variable-at-a-time (OVAT) method was applied in this study to investigate the significance of every hydrolysis parameters in nanocellulose production. The catalytic hydrolysis reaction was initiated by reacting the native cellulosic material (1.0 g, oven dry weight) with Cr(NO 3 ) 3 solution (0.1-1.2 M) in a round-bottom flask with constant mechanical agitation using a heating mantle. The reaction was conducted at different temperatures (20-100 • C) and for different hydrolysis times (0.5-2.5 h). The solid-liquid ratio varied from 1:10 to 1:50. After the course of the reaction, the solid residue was separated from the reaction suspension via high-speed centrifugation (higher centrifuge speed can be used to improve separation) and washed thoroughly with cold deionized water until the wash water was maintained at constant pH. Afterward, the collected solid residue was subjected to lyophilization process and finally the fluffy white fiber product was obtained. Three replications were applied in each group of experiments in order to confirm the good reproducibility. The yield of CNC Cr(NO3)3 was determined by gravimetric analysis, as described elsewhere [9].

Isolation of CNC H2SO4 via H 2 SO 4 Catalyzed Hydrolysis
The procedure for isolating of CNC H2SO4 was adapted from [70] with slight modification. Briefly, 1 g of cellulose material was dispersed in 100 mL deionized water through continuous mechanical stirring before the dropwise addition of 60 mL concentrated H 2 SO 4 under an ice bath. This was to minimize a sudden temperature spike due to the exothermic reaction caused by the ion interaction between water and sulfuric acid. The suspension then heating up to 45 • C for 135 min. The reaction was terminated by diluting it with 10-fold cold water to avoid crystallization (iced water can be used to minimize dilution water usage while effectively terminating the hydrolysis reaction). The diluted suspension was centrifuged repeatedly to get the precipitate while removed the excess acid simultaneously by washing with deionized water. The supernatant was decanted off; fresh deionized water was added to the solids and mixed it well. This washing and centrifuging procedure were repeated until the supernatant was turbid, suggesting that CNC H2SO4 were being dispersed in solution. The turbid supernatant and settled solids were then mixed together and this slurry dialyzed against distilled water for 3-4 days until a constant pH was achieved, and freeze-dried to yield a white sulfate ester nanocellulose product. The dried product was stored in a vacuum for further product characterization.

Chemical Structure: Fourier-Transformed Infrared Spectral Analysis
Fourier transform-infrared spectroscopy (FTIR) of native cellulose and yielded nanocellulose were analyzed using a FTIR spectrometer (Bruker IFX 66/S, (PerkinElmer, Germany)) in order to determine the functional groups of the cellulosic product. In fact, the effects of the hydrolysis treatment on the chemical composition changes can be tracked from the FTIR spectra. The FTIR analysis was carried out in the transmittance mode with a wavelength range of 4000-400 cm −1 with a resolution of 4 cm −1 at 32 scans. The samples were ground and mixed with KBr powder with a ratio of 1:100 (w/w), then pressed into an ultra-thin pellet before the analysis.

Crystal Structure: X-ray Diffraction Studies
The impacts of the hydrolysis treatments on the crystallinity index of treated products can be investigated from the X-ray diffractograms. The crystallinities of native cellulose and isolated nanocellulose products were characterized by X-ray diffraction (XRD) study. The XRD patterns of the specimens were obtained from an X-ray diffractometer (PANalytical Empyrean) with a Cu-Kα radiation source which operated at 40 mA and 40 kV. The diffractograms were recorded from 5 • to 60 • . Crystallinity index (CrI) of samples was calculated by the peak deconvolution method as described by [71]. Generally, the crystallinity of samples was calculated by dividing the total peak area of all the crystalline peaks by the total peak of all non-crystalline plus crystalline peaks according to the Equation (5): where A c and A m are the area under the crystalline and non-crystalline regions, respectively.

Morphological Analysis: FESEM and TEM Analysis
The surface morphology analysis of cellulose and nanocellulose products was studied with a field emission scanning electron microscope (FESEM; Hitachi SU8030, (Hitachi HTA, Schaumburg, IL, USA)) at an accelerating voltage of 5 kV. The samples were mounted on double-sided adhesive carbon tapes that stick on aluminum stubs, and coated with platinum using an auto-fine coater (JFC-1600, (JEOL, Ltd., Tokyo, Japan)) to improve conductivity and avoid over-charging. The elemental analysis of each sample was determined using EDX coupled with FESEM unit. The size and dimensions of yielded nanocellulose were further investigated by a transmission electron microscope (TEM; Tecnai G2 F20 Series, (FEI company, Hillsboro, OR, USA)) performed at a 200 kV acceleration voltage. To perform the TEM analysis, a nanocellulose suspension was treated with ultrasound for 3 min to separate agglomerated fibers. A droplet of the diluted suspension was deposited on a copper grid coated with a thin carbon film, and dried in a vacuum desiccator prior to analysis to ensure the samples were completely dry. The morphology and diameter of the yielded nanocellulose fibers were determined by analyzing the micrographs with ImageJ software (National Institutes of Health, New York, NY, USA).

Thermal Property Investigation: Thermogravimetric Analysis (TGA) Study
The thermal stability of samples was determined by a thermogravimetric analyzer (Model Q500 brand TA) under a nitrogen atmosphere. The sample was put in an aluminum cup and heated from 25 to 600 • C at a constant heating rate of 10 • C·min −1 in order to prevent any thermo-oxidative degradation.

Visual Examination
The changes in dispersion of the CNC H2SO4 and CNC Cr(NO3)3 suspension after the hydrolysis process were studied through visual appearance in the aqueous medium (water). For this purpose, each nanocellulose aqueous suspension was prepared with a ratio of 2 mg/mL and the cellulose-water slurries were simultaneously treated by ultrasonication for 10 min in order to disperse CNC H2SO4 and CNC Cr(NO3)3 thoroughly. The pictures of samples were taken immediately after the ultrasonic treatment and then allowed to stand motionlessly for 1 day and 7 days at room temperature and atmospheric pressure before further photographing.

Conclusions
A comparative study on two types of nanocellulose products, namely CNC H2SO4 and CNC Cr(NO3)3 , derived from native MCC feedstock using concentrated H 2 SO 4 and Cr(NO 3 ) 3 hydrolysis system, respectively, was conducted in this work. The optimal conditions for production of CNC Cr(NO3)3 given by RSM methodology were 80 • C, 1.5 h, 0.8 M Cr(NO 3 ) 3 catalyst and solid-liquid ratio of 1:30. Under these experimental conditions, the manufactured CNC Cr(NO3)3 displayed a spider-web-like fibrous morphology with a higher crystallinity of 86.5% ± 0.3%, yield of 83.6% ± 0.6%, and T max value of 344 • C. For comparison, the CNC H2SO4 hydrolyzed by H 2 SO 4 exhibited a rice-shaped structure with the crystallinity index of 81.4% ± 0.1%, yield of 54.7% ± 0.3%, and T max value of 273 • C. Due to their crystallinity and aspect ratio differences, the isolated CNC Cr(NO3)3 with higher crystallinity, higher aspect ratio (about 15.7) and more thermal stability could potentially be used as reinforcement material for application in various fields targeting specific properties. In conclusion, the proposed Cr(NO 3 ) 3 hydrolytic system is easy to implement and permits producing more thermally-stable nanocellulose with high yield under milder hydrolysis conditions, thus eluding the customary usage of conventional concentrated H 2 SO 4 treatment for producing better quality nanocellulose.