Optimizing the yield and physico-chemical properties of pine cone cellulose nanocrystals by different hydrolysis time
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Abstract
Cellulose nanocrystals (CNCs) were isolated for the first time from pine cones (PC) by alkali and bleaching treatments and subsequent sulfuric acid hydrolysis (64%) at 45 °C. The influence of the hydrolytic reaction time (30, 45, and 90 min) on the yield, chemical composition and structure, and thermal stability of CNCs was evaluated. The removal of non-cellulosic constituents during the alkaline and bleaching treatment resulted in high pure cellulosic fibres. The isolation of CNCs from these cellulosic fibres at different reaction times was verified by the nano-dimensions of the individual crystals (< 3 and < 335 nm of average diameter and length, respectively). The highest yield (15%) and the optimum CNCs properties in terms of aspect ratio, thermal stability and crystallinity were obtained for an extraction time of 45 min. PC appeared to be a new promising source of cellulose fibres and CNCs with potential to be applied as reinforcement in composites and for food-packaging.
Keywords
Pine cones Cellulose nanocrystals Sulfuric hydrolysis conditions Physico-chemical properties Yield-recoveryIntroduction
During the last years, concerns about sustainable development have increased significantly leading to a major interest for the research and development of environmentally friendly materials as an alternative to petroleum-based materials. Cellulose is the most abundant biopolymer on earth and is characterized by being renewable, biodegradable and non-toxic (Haafiz et al. 2013; Peng et al. 2011). Cellulose consists of linear homopolysaccharide chains of β-d-glucopyranose units linked together by β-1,4-glycosidic bonds (Luzi et al. 2016). Intramolecular and intermolecular hydrogen bonds are established between the cellulosic chains resulting in ordered/packed crystalline structures which are intercalated with amorphous regions in the fibril structure (Jiang and Hsieh 2013; Mariano et al. 2014; Sheltami et al. 2012; Siqueira et al. 2010). These crystalline domains can be separated from each other by overcoming the extensive and strong inter-fibrillar hydrogen bonds with acid treatment (Camarero Espinosa et al. 2013; Lamaming et al. 2015; Lu and Hsieh 2012), specific enzymes (Satyamurthy et al. 2011) and/or intense mechanical forces (Chen et al. 2011; Deepa et al. 2011). Among these extractive methods, the use of sulfuric acid to provoke the cleavage of the glycosidic bonds, achieving the disintegration of the cellulose amorphous region, has been the most widely used (Lu et al. 2013; Mariano et al. 2014; Neto et al. 2013). Cellulose nanocrystals (CNCs) obtained from sulfuric acid hydrolysis have been of great scientific interest due to their high crystallinity, low density, rod-like shape, high aspect ratio (diameter/length), high specific surface area, good mechanical properties (high stiffness and elastic modulus), low coefficient of thermal expansion, stability in aggressive media, gas permeability and optical transparency (Ioelovich 2012; Jiang and Hsieh 2013; Mueller et al. 2014; Peng et al. 2011). Most of these CNCs properties (such as aspect ratio, morphology, thermal stability and degree of crystallinity) are highly dependent on the hydrolysis conditions as well as on the raw cellulosic material they are extracted from Le Normand et al. (2014). The reaction time has been identified as one of the most important parameters to consider during the CNCs extraction. Kargarzadeh et al. (2012) obtained CNCs from kenaf with different hydrolysis reaction times (20, 30, 40, 60, 90, and 120 min) and specific thermal and crystallinity properties were achieved for each time. Silvéiro et al. (2013) also studied the effect of the acid hydrolysis time (30, 60, and 90 min) on the chemical, physical and thermal properties of CNCs from corncob and the highest crystallinity and thermal stability were obtained with an hydrolytic time of 60 min. The origin of the cellulose raw materials on the particular performance of CNCs has been also evaluated through different comparative studies, where the hydrolysis conditions were fixed. CNCs were obtained from a variety of sources; however, most research efforts have been focused on partially purified versions of woods (such as microcrystalline cellulose and bleached pulp). Beck-Candanedo et al. (2005) investigated the suspension properties of CNC obtained by hydrolysis of a softwood (spruce) and a hardwood (eucalyptus) pulp. These two suspensions had similar CNCs dimensions and surface charge, implying that the basic unit of wood cellulose organization is the same for the two species. During the last decade, the use of residual lignocellulosic biomass from agriculture, food and forest to produce CNCs has become impellent due to the increasing demand of finding cheaper sources and higher extraction yields alternatives to produce nanocelluloses as reinforcing agents for composites (Maiti et al. 2013). Deepa et al. (2015) obtained CNCs from various biomass residues (sisal, kapok, banana rachis, pineapple leaf and coir) and differences in their size, crystallinity and thermal stability were detected depending on the used raw material. In previous studies, bark, pine needles, branches and woody chips were proposed as new cost-effective forest raw materials to isolate CNCs. The feasibility of obtaining CNCs with specific performances depending on the physico-chemical properties of the cellulose fibres in the forest residues was proved (Le Normand et al. 2014; Moriana et al. 2016). A comparative study of the aspect ratio, crystallinity and thermal stability of the extracted CNCs was proposed to assess their performance as reinforcing agent in composites (Moriana et al. 2016).
In this paper, we investigate for the first time the feasibility of using pine cones from Pinus Pinea (Stone pine) as a raw material to produce CNCs. Pinus Pinea (Stone pine) is one of the most important species in the Mediterranean region due mainly to the production of pine nuts, a culinary ingredient with an increasing demand (Özgüven and Vursavuş 2005). Nowadays, Spain, Portugal and Italy are the countries with the largest population of this tree species with 490,000; 175,000 and 46,000 ha respectively (Loewe et al. 2017). After collecting the pine nuts the cones are discarded, leading to a high amount of residue, which has no industrial or economic value. Currently, small amounts of pine cones are being used as home fuel, however most of this waste ends up being incinerated or thrown into the field creating a risk of fire (Almendros et al. 2015). The study and research of alternatives for the recovery of this food-waste by obtaining value-added products will contribute to move towards a circular bio-based economy and to develop a biorefinery concept with environmental, social and economic benefits (Li et al. 2016). The goals of this study are: to evaluate the feasibility of isolating CNCs from pine cones using different hydrolytic times (30, 45, and 90 min); to determine the overall recovery yield, the physico-chemical properties and thermal behavior of the produced CNCs; and, to estimate the optimal processing conditions to obtain CNCs suitable for being used as reinforcements in polymeric-based composites.
Materials and methods
Materials
Pine cones (PCs) were collected from a Pine forest (Pinus Pinea) in Alicante area (Spain). These forest residues were conditioned at 40 °C during 1 week. The raw PCs were ground in a Wiley mill (Thomas Scientific, USA) to pass through a 20 µm mesh screen. Analytical grade chemicals used for the experimental procedure were: sodium hydroxide (NaOH 99%), sulfuric acid (H2SO4 98%), sodium chlorite (NaClO2 80%), sodium acetate (C2H3NaO2 > 99%) and glacial acetic acid (CH3COOH 99.7%). All of them were purchased from Sigma-Aldrich (Sigma-Aldrich, Germany). All water used was purified by Milli-Q water (Millipore Corporate, USA). Polysaccharide standards (cellulose, starch, galactomannan, glucomannan, arabinoxylan, arabinogalactan, arabinan) for the chemical analysis were purchased from Sigma-Aldrich or Megazyme (Megazyme, Ireland).
Isolation of cellulose nanocrystals (CNCs)
The isolation of the CNCs was achieved by subjecting milled PC samples to different chemical treatments. First, an alkaline and bleaching treatment was proposed to remove the extractives, lignin and hemicelluloses and to produce the subsequent isolation of the cellulose fibres from the raw material. Second, a hydrolytic treatment with sulfuric acid was performed to remove the amorphous regions to obtain the CNCs.
Alkaline and bleaching treatment
Milled PC samples were subjected to an alkaline and bleaching treatment following the same methodology previously described by Moriana et al. (2016). Three different batches of PC (4% w/v) were treated with sodium hydroxide solution (4.5% w/v NaOH) for 2 h at 80 °C under vigorous mechanical stirring. This alkaline treatment was repeated three times and after each treatment, the resulting material was washed with water until the removal of the chemicals and subsequently dried at room temperature overnight. After alkali treatment, three different batches (4% w/v) of alkaline pine cones (APC) were bleached with a solution made up of equal parts (1:1:1) of 1.7 wt% aqueous sodium chlorite, acetate buffer (0.2 M, pH 4.8), and water. The bleaching procedure was performed at 80 °C for 4 h and repeated five times. After each bleaching treatment the bleached pine cones (BPC) were filtered and washed with Milli-Q water and dried at room temperature for 24 h.
Acid hydrolysis treatment
The acid hydrolysis treatment was performed on the BPC. BPC were dried at 40 °C for 24 h in an air-circulating oven and subsequently grinded in a Wiley Mill (Thomas Scientific, New Jersey, USA) using a 20 µm mesh screen. The milled BPC were hydrolyzed in sulfuric acid solution (65% wt) at 45 °C for 30, 45, and 90 min under mechanical stirring. At least three different batches at 4% w/v of BPC were prepared for each hydrolyzed time. The suspensions were diluted with ice cubes to stop the hydrolysis reaction. Afterwards, they were washed with Milli-Q water by successive centrifugations in an Avanti J-E centrifuge (Beckman Coulter, California, USA) at 13,000 rpm for 10 min at 4 °C until the supernatant reached a constant pH of 5. After, the suspensions were dialyzed with purified water during 1 week until the neutral pH. Finally, the suspensions were sonicated for 5 min at an amplitude of 27% using an ultrasonic homogenizer Vibra-cell Mod. VCX 750 (Sonics and Materials Inc., Connecticut, USA) in an ice bath to avoid overheating. To remove the largest particles, the suspensions were centrifuged and the sediments were discharged. The CNC suspensions were stored at 4 °C for further characterization. The obtained CNCs suspensions were labelled depending on the extraction time as follows: CNC30, CNC45 and CNC90, for the CNC suspensions extracted at 30, 45, and 90 min respectively.
Characterization of alkaline, bleached and hydrolyzed PC samples
Gravimetric analysis
The gravimetric yield of each experimental treatment (alkaline, bleaching and hydrolysis) was determined by weighing the dried samples before and after of each experimental procedure. At least three different replications of each sample and batch were considered to calculate the average and the standard deviations.
Chemical composition analysis
The chemical composition of all the studied samples (PC, APC, BPC, CNC30, CNC45 and CNC90) was evaluated following the methodology previously described by Moriana et al. (2015). The dry matter of samples was determined in a Mettler Toledo HB43 moisture analyzer (Mettler Toledo SAE, USA). The ash content was determined gravimetrically after heating the dry samples in a furnace at 525 °C for 6 h, following the TAPPI standard method T211 om-02. The determination of acid insoluble (Klason) lignin in the samples was carried out according to the TAPPI T222. The total amount of soluble extractives in the PC was determined by sequential extraction with ethanol, toluene and hot water (de Carvalho et al. 2015). The carbohydrates composition was assessed by conventional two-step Saeman hydrolysis (Saeman et al. 1954), followed by quantification of the released monosaccharides using high-pH anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). In short, 1 mg of dry sample was initially pre-hydrolyzed with 250 μL 72% H2SO4 at room temperature for 3 h, diluted until a final concentration of 1 M H2SO4, and then subjected to a second hydrolysis step at 100 °C for 3 h. The hydrolysates were centrifuged at 4 °C and subjected to analysis directly without any other purification or neutralization. The released monosaccharides were separated on a Dionex ICS3000 system (Dionex Corporation, California, USA) by 10 μL injection of the filtered hydrolysates on a Dionex Carbopac PA1 column at 30 °C at a flow rate of 1 mL min−1. Neutral sugars (glucose, mannose, xylose, arabinose, galactose, and rhamnose) and uronic acids (galacturonic and glucuronic acid) were analyzed separately using different elution profiles (Wright and Wallis 1996). The samples were analyzed in triplicate.
Scanning electron microscopy (SEM)
The morphologies of PC, APC and BPC were evaluated using a scanning electron microscopy Phenom (FEI Company, Eindhoven, the Netherlands) operated at 5 kV acceleration voltage. Prior the analysis the samples were coated with a thin layer of gold/palladium alloy in a sputter coater EMITECH model SC7620 (Quorum Technologies Ltd., East Sussex, UK).
Atomic force microscopy (AFM)
AFM imaging of CNCs with different hydrolysis times was performed in a Nanoscope IIIa Multimode scanning probe microscope (Digital Instruments Inc., New York, USA). A few drops of diluted nanocrystals suspension were deposited onto a freshly cleaved mica surface and air-dried. All images were obtained using the tapping-mode in air at room temperature with RTESP silica cantilevers (Bruker) having a tip with a radius of 8 nm and a spring constant of 20–80 N m−1 and resonance frequencies between 306 and 366 kHz. Lengths and diameters were obtained from printouts of several height mode AFM images, using the section analysis tool of the NanoScope Analysis software Version 1.40. AFM samples from each type of CNCs were prepared from the three different batches to evaluate the reproducibility of the experimental procedure. No significant differences in the CNC dimensions were detected from batch to batch. Therefore, more than a hundred CNCs of each material were randomly selected and measured to determine their average length and diameter.
Thermogravimetric analysis (TGA)
The thermal behavior of PC, APC, BPC and the different CNCs was evaluated by thermogravimetric analysis (TGA) (Le Normand et al. 2014). The samples, with an average mass of 4.5 mg, were placed in an alumina crucible (70 µL) and heated from 25 to 750 °C at a constant heating rate of 10 °C min−1 on a Mettler-Toledo TGA/DSC 1 thermobalance (Schwerzenbach, Switzerland) under nitrogen atmosphere (50 mL min−1). The onset temperature, maximum decomposition temperature and mass loss percentages, for each thermal decomposition process were obtained using the STARe Evaluation Software. At least three replicates of each sample were performed and evaluated.
X-ray diffraction spectroscopy (XRD)
Fourier transform infrared spectrometry (FTIR)
FTIR spectra of all the samples were collected at room temperature on a Spectrum 2000 FTIR spectrometer from Perkin-Elmer (Perkin-Elmer Inc., Massachusetts, USA) equipped with a golden single-reflection accessory for ATR measurements. Each spectrum was obtained as 24 individual scans at 4 cm−1 resolution in the wavenumber range comprised between 4000 and 600 cm−1. The measurements were repeated three times for each sample. The spectra were automatically baseline-corrected and smoothened using Omnic 7 Software.
Results and discussion
Visual description and gravimetric yield of the isolation procedure
Schematic representation of the pine cone CNC isolation procedure with different hydrolysis times
The effect of the specific hydrolysis times on the chemical composition, morphology, crystallinity and thermal stability of the resulting CNCs was evaluated to assess the most suitable conditions to produce CNCs to be used as reinforcing agents for polymer matrices in composites.
Chemical composition
Yield obtained after each chemical treatment and chemical composition of PC, APC, BPC and CNCs with different hydrolysis times
Treatment | PC | APC | BPC | CNC30 | CNC45 | CNC90 |
---|---|---|---|---|---|---|
Yielda | 100 | 64.9 ± 3.5 | 60.3 ± 1.5 | 32.7 ± 0.4 | 37.1 ± 0.4 | 26.8 ± 0.3 |
Arabinose | 0.23 ± 0.01 | 0.20 ± 0.00 | 0.11 ± 0.01 | < 0.1 | < 0.1 | < 0.1 |
Rhamnose | < 0.1 | < 0.1 | < 0.1 | < 0.1 | < 0.1 | < 0.1 |
Galactose | 1.31 ± 0.03 | 1.10 ± 0.00 | 0.88 ± 0.07 | < 0.1 | < 0.1 | < 0.1 |
Glucose | 45.33 ± 0.04 | 62.62 ± 0.02 | 88.99 ± 0.14 | 95.99 ± 0.07 | 95.65 ± 0.01 | 97.85 ± 0.05 |
Xylose | 0.23 ± 0.01 | 0.14 ± 0.02 | 0.11 ± 0.00 | 0.16 ± 0.03 | 0.23 ± 0.01 | < 0.1 |
Mannose | 2.91 ± 0.05 | 2.34 ± 0.03 | 2.76 ± 0.22 | 2.40 ± 0.04 | 2.50 ± 0.00 | 0.50 ± 0.02 |
Galacturonic acid | 0.39 ± 0.08 | 0.13 ± 0.04 | 0.21 ± 0.07 | < 0.1 | < 0.1 | < 0.1 |
Glucuronic acid | 0.30 ± 0.07 | 0.18 ± 0.03 | 0.30 ± 0.09 | 0.18 ± 0.02 | 0.17 ± 0.01 | < 0.1 |
Total carbohydrates | 50.71 ± 0.30 | 66.70 ± 0.13 | 93.36 ± 0.61 | 98.86 ± 0.17 | 98.66 ± 0.03 | 98.47 ± 0.10 |
Cellulose | 45.33 ± 0.04 | 62.62 ± 0.02 | 88.99 ± 0.14 | 95.99 ± 0.07 | 95.65 ± 0.01 | 97.85 ± 0.05 |
Hemicellulose/pectin | 5.38 ± 0.25 | 4.08 ± 0.12 | 4.37 ± 0.47 | 2.87 ± 0.09 | 3.01 ± 0.02 | 0.62 ± 0.02 |
Extractives | 6.51 ± 0.31 | – | – | – | – | – |
Klason lignin | 39.66 ± 0.01 | 29.76 ± 0.02 | 4.10 ± 0.06 | N/A | N/A | N/A |
Ash | 3.12 ± 0.25 | 3.54 ± 0.23 | 2.54 ± 0.32 | 1.14 ± 0.04 | 1.34 ± 0.04 | 1.53 ± 0.05 |
During the hydrolytic treatment is expected hemicelluloses and pectins hydrolyzed together with the amorphous part of the cellulose and became soluble (Moriana et al. 2015). However, it was observed that the mannose content in CNC30 and CNC45 was similar to that present in the BPC sample. This is because the mannose is more resistant to hydrolysis and dissolution during the preparation of the CNCs than other amorphous cell wall components (Hannuksela et al. 2002; Iwata et al. 1998). However, in the CNC90 the mannose content significantly decreases, thus indicating greater removal of the hemicellulose content and, maybe, the degradation of the cellulose. This fact can also be pointed out due to the small gravimetric yield (Table 1) of the CNC90 in comparison to CNC45. The CNC yields for CNC30 and CNC90 (32.7 and 26.8% respectively) are similar to those reported by Le Normand et al. (2014) for spruce bark and by Brito et al. (2012) for bamboo fibres. However, the CNC45 yields are higher and more similar to those reported by Moriana et al. (2016) for woody chips.
Morphological surface
SEM micrographs of a raw pine cone (PC), b alkaline (APC) and c bleached (BPC) samples
AFM images of CNC45 in a amplitude, and b height mode (scale bar 1.5 µm). Size distribution histograms of the CNCs at different hydrolysis time: c length and d width
Length, width and aspect ratio of CNCs with different hydrolysis times obtained by AFM analysis
Sample | Length L (nm) | Width D (nm) | Average aspect ratio (L/D) | ||||
---|---|---|---|---|---|---|---|
Max | Min | Average | Max | Min | Average | ||
CNC30 | 572.3 | 165.6 | 334.9 ± 112.3 | 5.5 | 1.6 | 3.0 ± 0.9 | 111.6 |
CNC45 | 501.2 | 223.4 | 328.9 ± 82.2 | 5.7 | 1.8 | 2.9 ± 0.7 | 113.4 |
CNC90 | 410.0 | 106.7 | 206.5 ± 64.5 | 4.4 | 1.2 | 2.4 ± 0.7 | 86.0 |
Crystallinity and crystal size
X-ray diffraction, Crystallinity Index (CrI) of PC, APC, BPC and CNCs with different hydrolysis times
During the alkaline and bleaching treatments, an increment in the CrI was observed (from 42.6 to 65%) due to the reduction and removal of amorphous non-cellulosic compounds such as lignin and hemicellulose (Chen et al. 2011; Rosli et al. 2013). During the hydrolysis treatment, an increase in the CrI with respect to the BPC due to the hydrolytic scission of the glycosidic bonds occurred. This phenomenon allowed releasing individual crystals and removing the amorphous domains (Lamaming et al. 2015; Lu et al. 2014).Comparing the CrI of the CNCs with different hydrolysis times, the same behavior as that observed for the gravimetric yield was observed; samples showed an increase in their crystallinity from 30 to 45 min and then, decreased at 90 min. Therefore, the highest crystallinity is achieved for a hydrolysis time of 45 min (88.5%) whereas the CNC90 showed the lowest CrI (80.8%). This may be due to the fact that crystalline regions can be partially destroyed with high acid hydrolysis times (Chen et al. 2009; Neto et al. 2013). The CNC crystallinity indexes obtained from PC are similar to those obtained in other agro-industrial and forest residues such as rice straw (Lu and Hsieh 2012), spruce bark (Le Normand et al. 2014) or Swedish forest residues (Scots pines and Norway spruce) (Moriana et al. 2016), being higher to those obtained in other lignocellulosic residues such as soy hulls (Neto et al. 2013) or kenaf fibres (Kargarzadeh et al. 2012).
Chemical structure
FTIR spectra of PC, APC, BPC and CNCs with different hydrolysis times
In CNCs spectra for the different hydrolysis times, it is worthy to note the appearance of a new small peak at 1205 cm−1 related to S=O vibration. This can be explained by considering the esterification reaction during the hydrolysis process (Silvério et al. 2013). This peak had a higher intensity as the hydrolysis time increased.
Thermal stability and behaviour
TGA and DTG curves of a, c PC, APC, BPC and b, d CNCs with different hydrolysis times
Figure 6b shows the thermal degradation behavior of CNCs at different acid hydrolysis times. The acid hydrolysis of the bleached fibres led to a decrease in thermal stability due to the substitution of hydroxyl groups by sulphate groups. First, evaporation of water absorbed by the CNCs occurred, being lower than in the previous steps due to the dehydration of cellulose fibres occurred during the acid hydrolysis (Roman and Winter 2004). Three overlapping processes took place at higher temperatures (> 145 °C): the first one centered at approximately 175 °C was related to the sulphate groups that catalyze cellulose dehydration; the second peak centred at 220–234 °C was associated with the breakdown of the more accessible region in the crystal interior; and the last one, with a temperature around 364 °C, was associated with the less accessible crystal interior of the CNCs (Silvério et al. 2013). The thermal degradation behavior of CNC30 was similar to CNC45, however CNC90 displayed a slightly lower thermal stability and a higher second peak due to the higher sulfate content and the degradation of the most labile region in the crystal interior. The residual mass in the CNCs was larger than in the bleached samples and could be explained by the presence of sulfuric acid that promoted the dehydration reactions and acted as a flame retardant (Kim et al. 2001; Roman and Winter 2004).
Conclusions
Cellulose nanocrystals (CNCs) were successfully isolated from pine cones (PC), an abundant and cheap biomass residue. In this work the changes in the chemical composition, morphology, crystallinity and thermal properties of the biomass during the alkaline and bleaching treatment and the influence of the acid hydrolysis time (30, 45, and 90 min) on the overall yield and on performance of the resulting CNCs have been studied. The progressive removal of lignin and hemicelluloses resulted in extraordinary high cellulosic fibre samples suitable for the subsequent CNCs isolation. The CNC45 showed similar chemical composition, aspect ratio, crystalline size and thermal behavior to those for the CNC30 however, the yield and crystallinity were higher for the CNC45. The CNC90 exhibited the lowest yield, crystal size, crystallinity index, aspect ratio and thermal stability, indicating that partial destruction of crystalline domains could occur. Therefore, these results show that PC is an effective renewable source for the production of CNCs with an optimal extraction time of around 45 min during hydrolysis at 45 °C with 65% sulfuric acid. The high aspect ratio, the high crystallinity and the good thermal stability of the obtained CNCs show great potential for their use as reinforcement in polymeric composites for different applications.
Notes
Acknowledgments
This work was supported by the Ministry of Economy and Competitiveness (MINECO) [MAT2014-59242-C2-1-R]. D. García-García wants to thanks the Spanish Ministry of Education, Culture and Sports for their financial support through an FPU Grant [FPU13/06011].
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