Subcritical Water: A Method for Green Production of Cellulose

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Subcritical water: A method for green production of cellulose nanocrystals Lísias Pereira Novo, Julien Bras, Araceli Garcia, Mohamed Naceur Belgacem, and Antonio A.S. Curvelo ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 24, 2015

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ACS Sustainable Chemistry & Engineering

Subcritical water: A method for green production of cellulose nanocrystals Lísias P. Novoabc, Julien Brasab, Araceli Garcíaabd, Naceur Belgacemab, Antonio A. S. Curveloce* a

Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France

b

CNRS, LGP2, F-38000 Grenoble, France

c

Univ. de São Paulo, Departamento de Físico-química, Instituto de Química de São Carlos, Av.

Trabalhador São Carlense 400, 13560-970, São Carlos, São Paulo, Brazil d

Univ. of the Basque Country UPV/EHU, Department of Chemical and Environmental

Engineering, Plaza Europa, 1, 20018 Donostia-San Sebastián, Spain e

Laboratório Nacional de Ciência e Tecnologia do Bioetanol (CTBE), Centro de Pesquisa em

Energia e Materiais (CNPEM), Caixa Postal 6179, 13083-970 Campinas, São Paulo, Brazil *Tel.: +55 16 3373 9938; fax: +55 16 3373 9952. E-mail: [email protected] KEYWORDS. Subcritical water, cellulose nanocrystals, sulfur free, greener process, cleaner effluent process, cost management, thermal stability.

ABSTRACT

ACS Paragon Plus Environment

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In the present study, an innovative method to produce cellulose nanocrystals is proposed. The conventional production of nanocrystals uses concentrated solutions of strong acids to promote the hydrolysis of cellulose amorphous regions and hemicelluloses. However, in the conventional method, long duration washing steps and the nanocrystals low temperature resistance still limit their larger industrialization and some applications in processes or end-uses that require heat resistance, like extrusion. In this context, the use of subcritical water (120°C and 20.3 MPa for 60 minutes) allows higher diffusion, activity and ionization of water. With that, partial hydrolysis of cellulose can be attended (with 21.9 wt% NCC yield). The cellulose source, the hydrolyzed cellulose and a commercial nanocellulose were submitted to different analytical techniques to evaluate their morphology and physicochemical characteristics. The obtained cellulose nanocrystals presented high crystallinity index (79.0% by XRD), rod-like shape with similar aspect ratio as those known for classic cellulose nanocrystals but also a higher thermal stability even when compared with the original cellulosic source (onset around 300°C). The exclusive use of water as reagent is a promising process not only for its green characteristics but also for its low corrosion, low and cleaner effluent and low cost of reagents.

INTRODUCTION The use and research of nanomaterials from renewable sources has strongly increased with the growth of environmental concerns, over the past decade. In this context, cellulose shows a great potential due to its well-known structure and abundance in nature. Several types of nanocellulose can be prepared either by (i) mechanical treatment, called microfibrillated cellulose or cellulose nanofibers1 or by (ii) chemical hydrolysis, identified as cellulose nanocrystals or whiskers2. The


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field of applications of cellulose nanocrystals discovered in the 60s is very broad and strongly increased with their industrialization since 2012. Some applications are: (i) in nanocomposite materials, as a filler to change physicochemical and mechanical properties3–7; (ii) in drugdelivery systems

8,9

; (iii) in smart materials10–12; and (iv) in antimicrobial composites

formulations13. Cellulose nanocrystals are mainly obtained by hydrolyzing the less organized and more accessible fraction of cellulose. This occur because these regions (amorphous or semicrystalline) are less resistant to hydrolysis14,15. Thus, hydrolysis using strong inorganic acids, such as sulfuric acid or hydrochloric acid, is the main process for production of cellulose nanocrystals3,16,17. Such nanocrystals are usually sensitive to temperature due to the presence of acidic moieties at their surface8, which limits their use in some applications. Because of their highly concentrated media, these reactions are costly and produces high amounts of effluent that need treatment. This washing step is usually the main limitation in its industrialization18. Alternative methods to produce cellulose nanocrystals have also been studied. Different oxidative reagents have been used to hydrolyze cellulose for the production of cellulose nanocrystals: e.g., ammonium persulfate19,20, and sodium metaperiodate21. Although these methods produce cellulose nanocrystals that already have a surface modification, the reagents used are expensive, reactive, corrosive and toxic in nature. In this sense, the use of green medium to hydrolyze cellulose would improve the production of cellulose nanocrystals. Recently, ionic liquids have been successfully applied into the production of cellulose nanocrystals, because of their ability to well solvate cellulose. In fact, Lazko et al.22 used 1butyl-3-methylimidazolium chloride as solvent medium to achieve an increase in the accessibility to the hydrolytic cleavage of specific cellulose sites. Thus, decreasing the sulfuric


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acid consumption, when compared with traditional cellulose nanocrystals extraction. However, hydrolysis was still performed with aqueous sulfuric acid solution, which increases complications in recovery of the expensive solvent medium, i.e., the ionic liquid23. The use of a similar ionic liquid with the hydrogen sulfate anion allowed it to act simultaneously as solvent media and as an acidic reactant23. This concept has been recently optimized24 by promoting the initiation of the cellulose hydrolysis by the controlled addition of water. However, the use of ionic liquids is still limited because of their availability, toxicity and high recovery costs. Cellulose nanocrystals can also be obtained from natural fibers by enzymatic hydrolysis after a combination of chemical pretreatments and high mechanical shearing forces25. Different processes of enzymatic hydrolysis were described in literature to obtain globular26 and rod-like cellulose nanocrystals27,28. The viability of these reactions requires the use of both chemical and mechanical pretreatments to increase accessibility to amorphous regions and extremely long isolation times, in comparison to acid hydrolysis. In addition, the scaling up of these methods towards industrial production would imply the recovery of the enzymes, an important issue that has not yet been developed or even investigated. The ability of water to hydrolyze polysaccharides is well known, as seen in hydrothermal processes of hemicelluloses removal29. The key points for an extensive hydrolysis rate are both the presence of H3O+ species and the availability of water molecules30. Sub- and supercritical water presents lower values of Kw and, consequently, higher concentrations of ionized species31. Thus, their use is effective for the hydrolysis reactions. Several studies have used water at high temperature and pressures to hydrolyze lignocellulosics32, gasify the biomass33 and liquefy cellulose/hemicelluloses34,35. Using supercritical water and controlling temperature (up to 400°C) and pressure (up to 27 MPa),


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Cantero et al.36 achieved the complete hydrolysis of cellulose avoiding the secondary sugar reactions. Thus, incomplete hydrolysis of cellulose could be achieved by using less severe reaction conditions. However, to the best of our knowledge nobody has already tested this strategy for cellulose nanocrystals production. The present work describes a novel and greener pathway for the production of cellulose nanocrystals from cellulose powder so far reported. This innovative method, using “only” pressurized hot water (at temperature conditions between boiling and supercritical temperatures, also known as subcritical water), lead to a new, more economic and green process for cellulose nanocrystals production. The resulted nanoparticles were characterized, and their properties compared with those obtained by conventional methods. Furthermore, energy and chemical consumptions of both production processes (subcritical water and conventional hydrolysis) were theoretically evaluated considering economic and environmental concerns. The obtained results are very promising and this proof of concept might open several applications and optimizations.

EXPERIMENTAL Materials. Commercial microcrystalline cellulose Avicel® was used as raw material in all of the experiments. This cellulose powder has a nominal particle size of 20 µm and was purchased from FMC Corp. The hydrolysis reaction was performed in distilled water. Dialysis membranes with molecular weight cut-off of 6-8 kD (Spectra/Por® 1) were purchased from Spectrum Labs. A commercial cellulose nanocrystal suspension, provided by the UMaine Process Development Center (University of Maine, USA), was used as reference during the evaluation of the physicochemical properties. This suspension was 6.5 wt% of nanocrystals, extracted following


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the conventional sulfuric acid hydrolysis and measured using a moisture analyzer (MA35, Sartorius, Germany).

Subcritical hydrolysis of cellulose. The batch hydrolysis reaction was carried out in 100 mL stainless steel reactor in a SFT–250 SFE/SFR System (Supercritical Fluid Technologies, Inc.), as detailed in Supplementary Figure S1. The reaction was carried out by introducing 1 g of raw material (dry basis) and completely filling the reactor with water. The reaction conditions were: temperature of 120 °C (with heating and cooling rates of approximately 6 and 3°C.min-1 respectively); time of 60 minutes (considered only after the heating ramp, been the total reaction time 110 minutes); and pressure of 20.3 MPa. During the experiment the pressure inside the reactor was controlled by opening the restrictor valve to decrease it (due to the loss of a small volume of liquid) or by injecting water with a precision pump to increase the internal pressure of the reactor, as shown in Supplementary Figure S1. The cellulosic product was separated from the hydrolyzed sugars and the degradation products by filtration with a Pyrex® Buchner funnel with glass fritted disc with maximum pore size of 10 to 15 µm. A test to detect nanometric particles based on the laser diffraction was made on the filtrated solution to verify the absence of nanoparticles. To remove any soluble sugar and by-product, the sample was submitted to a dialysis process against distilled water for 5 days, changing the water daily. An aliquot was dried to quantify the final dry weight and the hydrolysis yield. A suspension was prepared by ultrasonic dispersion with a 250 Sonifier (Branson) for 3 min with pulses of 50 % of time and filtered with a NITEX® 3-1/1 canvas (Sefar) of 1 µm of opening to obtain the yield of cellulose nanocrystals. The product was then stored at 5 °C for further analyses and characterization.


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Size and morphological characterization. The morphological characteristics of the cellulose nanoparticles were analyzed by Dynamic Light Scattering (DLS) and Atomic Force Microscopy (AFM). Before the DLS and AFM analyses, 0.001 wt% water suspensions of cellulose nanocrystals were prepared and dispersed by ultrasound with a 250 Sonifier (Branson) for 3 min under cooling, to avoid overheating. The ultrasonic dispersion was made before each analysis to ensure minimum nanoparticles aggregation. The size of the nanoparticles was accessed using DLS in a Vasco® I particle size analyzer (Cordouan Technologies). The measures were made in triplicate using the cumulant method, with 10 acquisitions per analysis. This technic provides both the average particle size (ZD) and the polydispersity index (PDI). The DLS analysis uses the fluctuation of the intensity of the scattered light to calculate the hydrodynamic diameter and not the real dimensions of the particle. Therefore, a more precise analysis of size and shape is required. In this context, the morphology of the cellulose nanocrystals was evaluated from the images obtained by AFM. The AFM images were obtained in a Nanoscope IIIa microscope (Veeco Instruments) working in tapping air mode. One drop of about 10 µL of the previously prepared suspension was deposited on a mica substrate and oven dried at 50°C for 2h. The scanning probe used is made of silicon, with a maximum radius of curvature of 10 nm. The resonance frequencies of the cantilever used to perform the imaging were in the range of 250 to 350 kHz. The measurements of the average dimensions of the cellulose samples were made using the open-source software ImageJ. For each sample, more than 50 particles were measured to provide the average and the standard deviation.


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Furthermore, the morphology of the original cellulose source was evaluated by optical microscopy and environmental scanning electron microscopy (ESEM). The optical analysis of cellulose powder was performed in an Axio Imager M1m microscope (Zeiss) with an objective lens of 20x magnification. The ESEM analysis was performed in a Quanta200® microscope (FEI) with magnitude of 800x, high voltage of 10.0 kV and working distance of 9.4 mm.

Chemical characterization. The materials were characterized by Fourier transform infrared spectroscopy (FTIR) and solid state carbon 13 nuclear magnetic resonance (13C NMR). The infrared spectra of the samples were obtained using KBr pellets and recorded using a Spectrum One spectrometer (Perkin-Elmer). The pellets were prepared with 1–2 wt% of air dried and powdered cellulose nanocrystals in anhydrous KBr. The analysis range used was from 4000 to 600 cm-1 with a resolution of 1 cm-1 and an accumulation of 32 scans. The data were normalized considering the 1110 cm-1 peak. At least five spectra have been performed with different samples and the most representative have been considered for discussion. To evidence the absence of degradation products in the cellulose nanocrystals, 13C NMR was made. 13C NMR was performed on a Bruker AVANCE 400 spectrometer. Samples were placed in 4 mm ZrO2 rotors. All spectra were recorded using a combination of cross-polarization, high power proton decoupling and magic angle spinning (CP/MAS). 13C NMR spectra were acquired at 298 K, with a 4 mm probe operating at 100.13 MHz. The chemical shift values were measured with respect to TMS via glycine as a secondary reference with the carbonyl signal set to 176.03 ppm.


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Crystallinity measurement. The crystallinity index of the samples were calculated using both X-ray diffraction (XRD) and 13C NMR. The XRD diffractogram patterns were recorded on a PW 1720 X-ray generator (Philips) operated at 45 kV and 40 mA with Cu Kα radiation (λ = 0.154 nm) in the range 2θ = 6 – 56° with a fixed step interval. The analyzed samples were previously pulverized on an agate mortar and pestle. The crystallinity index using XRD (‫ܥ‬ூ௑ோ஽ ) was calculated according to the equation 1. The crystallinity index corresponds to the ratio between the interference of the [200] crystal planes subtracted by the amorphous contribution and the total peak height of the [200] crystal planes. The total peak height of the [200] crystal planes (‫ܫ‬ଶ଴଴ ) was observed at 2θ around 22.5° while the amorphous contribution (‫ܫ‬௔௠ ) was observed by the intensity of the baseline at 2θ around 18.0° 37,38.

‫ܥ‬ூ௑ோ஽ =

ூమబబ ିூೌ೘ ூమబబ

× 100

Eq. 1

Crystallinity index was also measured from the 13C NMR spectra (‫ܥ‬ூேெோ ). To measure ‫ܥ‬ூேெோ , acquisition time was 0.035 seconds, and sweep-width was 30 kHz. Variable amplitude cross polarization was used to minimize intensity variations of the non-protonated aromatic carbons that are sensitive to Hartmann-Hahn mismatch at higher MAS rotation rates39. MAS was performed at 6500 Hz. The number of scans was 15000 with a relaxation time of 1.0 seconds and CP time of 1.0 ms. The method to obtain ‫ܥ‬ூேெோ compare the areas for the peaks of crystalline and amorphous C4. This was done by separating the C4 region of the spectrum into crystalline (with chemical shift of 89 ppm) and amorphous peaks (with chemical shift of 84 ppm)40. The calculus was done by dividing the area of the crystalline peak (86 to 92 ppm) by the total area assigned to the C4 peak (79 to 92 ppm), according to the equation 241.


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‫ܥ‬ூேெோ =

஺೎ೝ೤ೞ ஺೎ೝ೤ೞ ା஺ೌ೘೚ೝ೛೓

× 100

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Eq. 2

Thermal stability. The thermal stability of the cellulosic materials was evaluated by thermogravimetric analysis (TGA) and differential thermogravimetry (DTG). The thermograms were obtained in a TGA-6 (Perkin-Elmer) under constant air flow of 50 mL.min-1, at a temperature ranging from 20 to 700°C and heating rate of 10°C.min-1. The cellulosic samples (between 10 and 20 mg) were in the form of powder. At least duplicates were performed.

Theoretical evaluation of nanocellulose production processes. In order to assess the energetic concerns associated to the proposed nanocrystals production pathway (Supplementary Figure S2-a), a theoretical evaluation was performed and compared with a conventional nanocrystals production process (Supplementary Figure S2-b). In both cases, 100 kg/h of raw material (cellulose) were treated. The subcritical treatment of the cellulose was performed using water with liquid to solid ratio of 10:1, at 120 ºC and 200 atm (approximately 20.3 MPa) during 1h. The resulting suspension was then cooled to 25 ºC and partially concentrated to 10% of solids before the subsequent dialysis step. For the conventional process, the cellulose was treated with 64% H2SO4 in a liquid to solid ratio of 10:1, at 45 ºC during 1h. Then, the nanocellulose suspension was filtered and washed four times with 1000 kg/h fresh water, for maximum acid removal before dialysis, totaling 4000 kg of washing water. This study was carried out using a process simulation tool, Aspen Plus® (AspenTech, Virginia). Cellulose, glucose, water and sulfuric acid were defined as process components from the Aspen data base. For the nanocellulose production step, a stoichiometric reactor was


10

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designed, where 70% of the cellulose is partially hydrolyzed to glucose and 30% recovered as nanocellulose. In both cases, the degradation of glucose is neglected, since its use is not yet discussed in this study.

RESULTS AND DISCUSSION Production of cellulose nanocrystals by subcritical water (SC-NCC). The optical and scanning electron microscopies of the cellulose powder used as raw material is shown in Supplementary Figure S3. By observing the images, it is possible to conclude that the particles do not have a preferential shape and have a broad distribution of size. From these images, the dimensions of this microcrystalline cellulose were determined, being 23.1 ± 14.2 µm in length and 13.9 ± 9.0 µm in width. The raw material treatment to obtain cellulose nanocrystals is detailed in the experimental part and presented in Supplementary Figure S1. The main innovativeness of this approach is the use of water “alone” to degrade cellulose substrates and produce nanocrystals. In this case, specific subcritical conditions (T=120°C, P=20.3 MPa) are necessary. Up to our knowledge, this is the first time reporting such a strategy. Our first results prove the concept and its potential use and extension. Indeed, the suspension obtained after the process showed both light diffraction and a very low birefringence behavior when agitated and observed through two cross polarizers right after the ultrasonic dispersion, as shown in Supplementary Figure S4. While light diffraction is a characteristic of colloidal suspensions, the formation of birefringence domains when the suspension is stirred depends on the rod-like shape of the nanoparticles. When aggregates are formed in low stability suspensions, the visualization of birefringence domains is reduced and weakened37,42.


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After this quick hydrolysis (60 minutes), a yield of 46.2 wt% is obtained, considering the total mass of solid after the process. However only about 47.5 wt% of this amount corresponds to nanocrystals, resulting in an overall yield of nanocellulose crystals of 21.9 wt%. Bondeson et al.43 have optimized the H2SO4 hydrolysis of a microcrystalline cellulose, with similar size to the one used in this study, for the production cellulose nanocrystals. They concluded that with a reaction temperature of 44°C, H2SO4 concentration of 63.5 wt% and a reaction time of 130 minutes a yield of 30 wt% of cellulose nanocrystals could be achieved. Others44 announce yield in similar range with other sources. Even if some industrial announcements (CelluForce) claimed an optimized yield of 60% using wood fibers, no detailed studies described such high yield. We can then consider that our strategy gives similar yield at laboratory scale than that observed for classic hydrolysis but without using sulfuric acid and sulfate groups at the nanocrystal surface. To prove this point, the spectra of vibrational spectroscopy of the subcritical hydrolyzed cellulose (SC-NCC) were compared to cellulose powder (raw material) and sulfuric acid hydrolyzed nanocellulose (H-NCC), as shown in Supplementary Figure S5. From the three spectra, it is possible to observe the presence of the characteristic peaks of cellulose: at around 3350 and 1640 cm-1 peaks corresponding to OH hydrogen-bonded stretching peak and absorbed water, respectively; at around 2900 and 1375 cm-1 the CH stretching and bending modes, respectively; a peak around 1110 cm-1 due to C-O-C asymmetric stretching; at around 1060 cm-1 due to C-O/C-C stretching45,46. This suggests that there was no great modification or degradation of cellulose during the hydrolysis process. To evaluate the presence of chemical degradation in the SC-NCC,

13

C NMR spectra of the

three cellulosic materials were obtained, as shown in Figure 1. The assignment of the peaks is indicated in Figure 140,41,47. Comparing the SC-NCC and the cellulose powder 13C NMR spectra


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it can be seen that both have the same peak pattern. However, when they are compared with the H-NCC 13C NMR spectrum a few different peaks can be observed, as indicated in Figure 1: (i) a peak at approximately 63 ppm; (ii) a peak of chemical shift of 76.5 ppm; (iii) and a peak of chemical shift over 107 ppm. These differences may be due to a presence of small quantities of cellulose II48 in the commercial cellulose nanocrystals.

Figure 1. 13C NMR spectra of SC-NCC, cellulose powder and H-NCC.

Morphology and crystallinity of SC-NCC. The DLS results (ZD and PDI) for nanocellulose samples are shown in table 1. The size distributions can be also seen in Supplementary Figure S6. PDI indicates the degree of size homogeneity or heterogeneity in a given sample. A zero value of PDI indicates a mono-disperse sample, while values closer to 1 indicates a wide size distribution. The low value of PDI of our SC-NCC suspension indicates a mono-dispersed system. Espino et al.49 produced cellulose nanocrystals by sulfuric acid hydrolysis from three different lignocellulosic raw materials, including microcrystalline cellulose, and obtained suspensions with very sharp size distribution (PDI values between 0.22 and 0.11). Thus, the


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higher PDI value of the H-NCC shows that our nanocrystals suspension has a very sharp distribution of sizes. The size distribution shown in Supplementary Figure S6 confirms the tendency deduced from PDI values: the produced nanocellulose have two peaks of particle sizes with little difference. On the other hand, the commercial sample has various peaks distributed on a large range of particle sizes. The much larger hydrodynamic diameter of the produced nanocellulose in comparison to the commercial nanocellulose can be a consequence of the two following characteristics: the real size of the nanocrystal is bigger; the nanocrystals tend to form aggregates easily in water suspensions. The correct assignment can be accessed by observing the AFM images. Table 1. Hydrodynamic diameter (ZD), polydispersity index (PDI), colloidal behavior, morphology and XRD results for the cellulosic materials. Cellulose powder

SC-NCC

H-NCC

ZD (nm)

---

503 ± 82

224 ± 58

PDI

---

0.13 ± 0.06

0.35 ± 0.06

Colloidal behavior

---

Low birefringence

High birefringence

Length

23.1 ± 14.2 µm

242 ± 98 nm

191 ± 58 nm

Width

13.9 ± 9.0 µm

55 ± 20 nm

37 ± 16 nm

Aspect ratio (L/W)

1.7

4.4

5.2

Agglomerates

Yes

Yes

No

Cellulose pattern

Cellulose Iβ

Cellulose Iβ

Mainly cellulose Iβ

‫ܥ‬ூ௑ோ஽

80.0

79.0

78.7

‫ܥ‬ூேெோ

55

56

66

The AFM images of both nanocelluloses are presented in Figure 2. The morphology analysis of the cellulosic materials obtained from the AFM and optical microscopy is shown in Table 1. SC-NCC exhibited a rod-like shape and similar sizes as the commercial nanocellulose. However


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few particles with higher width and length are observed, which increased their average values. The lack of sulfate groups on the cellulose surface favor the aggregation of the nanoparticles, as seen in Figure 2-a. A similar behavior occurs also when cellulose nanocrystals are produced by HCl hydrolysis, as demonstrated by Araki et al.50 who reported that the low particles stability in aqueous suspension is due to the absence of strongly charged –OSO3– groups. According to the AFM results, the much bigger hydrodynamic diameter observed on the DLS analysis of the SCNCC sample may be due to aggregation of particles. Araki et al.51 have also showed that a post treatment with H2SO4 could introduce the same groups on the surface to increase suspension stability, however enabling the control of the charge density.

Figure 2. AFM images of cellulose nanocrystals: “a” and “c” from SC-NCC; “b” and “d” from H-NCC. Figure 3 shows the X-ray diffraction patterns of SC-NCC in comparison to the original microcrystalline cellulose and H-NCC. The produced nanocellulose and the original source show the same cellulose I characteristic peaks (and correspondent reflection planes) at 2θ angles of


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around 15° (plane 1-10), 16.5°(plane 110), 20.5°(plane 102), 22.5°(plane 200) and 34.5° (plane 004)37,52,53. The diffraction pattern of H-NCC shows, other than the cellulose I diffraction peaks, the presence of two more peaks correspondent to cellulose II diffraction at 2θ angles of around 12.5° (plane 1-10) and 20° (plane 110)52–54. From these data, it is possible to obtain ‫ܥ‬ூ௑ோ஽ of each sample, as shown in Table 1. The hydrolysis process did not change significantly the original cellulose ‫ܥ‬ூ௑ோ஽ . Both cellulose powder and SC-NCC have similar crystallinity comparing to the commercial cellulose nanocrystal. Zhang et al.28 produced nanocrystals with ‫ܥ‬ூ௑ோ஽ of 78.8 % from a microcrystalline pulp of bamboo by H2SO4 hydrolysis. They28 also produced nanocrystals with ‫ܥ‬ூ௑ோ஽ of 72 % by enzymatic hydrolysis from the same microcrystalline pulp.

Figure 3. X-Ray diffraction patterns of cellulosic materials. The crystallinity index was also obtained by the 13C NMR spectra. The ‫ܥ‬ூேெோ results are shown in Table 1. Similar results of ‫ܥ‬ூேெோ can be found in the literature, usually around 60 ± 5% for both cellulose powder and nanocrystals40,41,47,55. Comparing the ‫ܥ‬ூேெோ with ‫ܥ‬ூ௑ோ஽ it can be seen that the values are very smaller, however this kind of behavior is commonly observed in literature40,41. Park et al.40,41 suggests that the much lower values for ‫ܥ‬ூேெோ , among others reasons, is due to the fact that chemical shift for carbons of the cellulose chains on the surface have changes from carbons in the crystalline region, and are counted as carbons from the amorphous regions.


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Thermal stability behavior. Considering the use of cellulose nanocrystals as reinforcement in nanocomposites, the thermal stability plays a critical role in the preparation of melt-processed composites37. Thus, the thermal degradation behavior of the cellulosic materials is shown in Figure 4, while Figure 5 shows their differential thermogravimetry curves. All materials presented an event of weight loss below 100°C due to the presence of residual humidity52,56, around 5 wt%. The thermal degradation of pure cellulose and cellulose nanocrystals without surface groups occurs in a single weight loss event with DTG usually between 300 and 350 °C56, ௖௘௟௟ indicated as ܶௗଵ in Figure 5. Differently, cellulose nanocrystals obtained from sulfuric acid ுே஼஼ ுே஼஼ and ܶௗଶ , hydrolysis generally present two weight loss events, presented in Figure 5 as ܶௗଵ

with beginning of thermal degradation in lower temperatures than that known for pure cellulose37,52.

Figure 4. TGA thermograms of the cellulosic materials.

Figure 5. Differential thermogravimetry of the cellulosic materials.


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In comparison to the original material, it is possible to observe that the main event of weight loss is observed between 300 and 400°C, having a deference of about 10 °C in the DTG peak. The thermal degradation onset of SC-NCC is 15°C higher than that of cellulose powder, as indicated by ∆t in Figure 4. Therefore, the produced cellulose nanocrystals have higher thermal stability when compared to pure commercial cellulose. Comparing the thermal behavior of H-NCC to SC-NCC, it is possible to observe a big difference in the onset thermal degradation temperature: while the acid treated nanocrystal starts to degrade around 200°C, our nanocrystals started to degrade only over 300°C. The same occurs when comparing the DTG peak of SC-NCC and the first main DTG peak of H-NCC. However, the DTG peak of SC-NCC has a value of 20 °C lower when comparing to the second thermal event of H-NCC, as shown in Figure 5. The presence of sulfate groups on the surface of cellulose nanocrystals promotes a stray decrease on thermal stability, when comparing to native cellulose3. This result is in agreement with the reported literature. Indeed, Lin and Dufresne37 demonstrated that the control of charged sulfate groups on the surface lead to an increase on the thermal stability, however, even nanocellulose particles bearing low amount of sulfate groups still have lower stability to compare with native cellulose from cotton. Zhang et al.56 verified the effect of different acid hydrolysis to produce cellulose nanocrystals, and indicated that cellulose nanocrystals from processes that did not insert charges had higher stability in comparison to nanocrystals produced via sulfuric or phosphoric acids hydrolysis, which induce the presence of anionic charges at their surface.

Theoretical evaluation of process viability. The simulation results of both processes allowed evaluating the chemicals, the water and the energy requirements in each process. Supplementary


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Figure S7 shows the Sankey diagrams for both scenarios, considering identical amounts of treated raw material, the same nanocellulose yield and reaction times. Supplementary Figure S7a illustrates the raw material and water requirements for the preparation of nanocellulose using subcritical water, whereas Supplementary Figure S7-b summarizes the inputs needed for the conventional nanocellulose sulfuric acid pathway. Regarding the specified operation conditions, in both cases 30 kg of nanocellulose would be obtained. The proposed novel process only would require water during the nanocellulose preparation, leading to 330 kg of nanocellulose suspension containing 10% of nanocrystals and 7% of dissolved sugars (glucose). For heating considerations, 424.8 MJ of energy would be theoretically required for the production of nanocrystals, and the same amount of energy should be consumed as cooling water consumption in order to temperate the resulting suspension to room temperature. Even though, the energy consumption for the classical nanocellulose production process is significantly lower than that required by the new production pathway (43.2 MJ) the conventional process requires 640 kg of acid per each 100 kg of treated cellulose. This chemical load entails a subsequent washing step in order to reduce acid content, resulting in a highly water consuming process and generating huge amounts of acid effluents that should be treated, before recycling and/or discharging. If one takes into account a full mater and energy balance of the main production stage (not the subsequent purification and conditioning steps), and considering the cost of the chemicals and associated utilities57 (Table 2), the conventional NCC production stage would produce nanocellulose with a cost of 1.54 $/kg of NCC, whereas cellulose nanowhiskers could be produced at a price of 0.02 $/kg of NCC using the proposed subcritical treatment, i.e. this new


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process could produce nanocellulose with a theoretical value 77 fold lower than that using a classical sulfuric acid hydrolysis step. Table 2. Chemical and utilities consumptions and costs57 for the new and conventional NCC production processes.

Unit cost ($/kg)

SC-NCC

H-NCC

Consumption (kg)

Cost ($)

Consumption (kg)

Cost ($)

Process water

0.0003

1000

0.3000

4360

1.3080

Sulfuric acid

0.070

-

-

640

44.80

0.00001

5059

0.0506

-

-

Cooling watera

Low pressure 0.002 193 0.3857 19.7 0.0390 steamb a cooling water temperature input and output specifications: 10 ºC and 30 ºC, respectively. b

low pressure steam of 200 kPa, 2201.56 kJ/kg heating value.

In this sense, although the proposed cellulose nanocrystals production process requires a reactor that supports high pressures and high consumption of energy, it results in a greener method in terms of water and chemicals consumption reduction, lower effluent generation and treatability of wastes.

ACKNOWLEDGMENTS The authors are thankful to the Institute of Chemistry of São Carlos from the University of São Paulo and to the Laboratory of Pulp and Paper Science and Graphic Arts that provided the structure to develop the research. The authors are also grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the scholarship, the Basque Government (Postdoctoral Development Program) and to Conselho Nacional de Pesquisa e Desenvolvimento


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(CNPq) for funding this research. LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir grant agreement n°ANR-11-LABX-0030) and of the Énergies du Futur and PolyNat Carnot Institutes (Investissements d’Avenir - grant agreements n°ANR-11-CARN-007-01 and ANR-11CARN-030-01).

SUPPORTING INFORMATION This section provides supplementary figures, available alongside the published content on the website.

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Table of Contents art

For the first time, cellulose nanocrystals were produced through subcritical water hydrolysis of cellulose powder, instead of the classical method of sulfuric acid hydrolysis.


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Subcritical Water: A Method for Green Production of Cellulose

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