Abstract
Cellulosic fibers in Oil Palm Empty Fruit Bunches (OPEFB) are tightly packed with lignin, hemicelluloses, small depositions of wax, and inorganic elements. In the present work, eco-friendly reagents with low concentrations of 20% (v/v) formic acid and 10% (v/v) of 30% hydrogen peroxide were employed at 85 ºC for the extraction of cellulose from OPEFB. The yield of 64% (w/w) achieved was among the highest ever reported. Based on the XRD, the alpha-cellulose content was 93.7% with a high crystallinity of 69.9%. The average diameter was 13.5 μm with structural evidence of separated fibrils as investigated by FESEM. The TEM analysis suggested that the material was crystalline and its geometry was a monoclinic structure. The FTIR spectral peaks representing wax and hemicelluloses at 1735 cm-1 and 1375 cm-1, respectively, and lignin at 1248 cm-1 and 1037 cm-1, were not observed in the extracted OPEFB-cellulose spectra. Based on the TGA results, thermal stability at 325 ºC with a single degradation curve suggests the purity of OPEFB-cellulose.
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Eco-Friendly Extraction and Characterization of Cellulose from Oil Palm Empty Fruit Bunches
Muhammad Shahid Nazir,a Bambang Ari Wahjoedi,b Abdul Wahid Yussof,c and Mohd Azmuddin Abdullah a,*
Cellulosic fibers in Oil Palm Empty Fruit Bunches (OPEFB) are tightly packed with lignin, hemicelluloses, small depositions of wax, and inorganic elements. In the present work, eco-friendly reagents with low concentrations of 20% (v/v) formic acid and 10% (v/v) of 30% hydrogen peroxide were employed at 85 oC for the extraction of cellulose from OPEFB. The yield of 64% (w/w) achieved was among the highest ever reported. Based on the XRD, the -cellulose content was 93.7% with a high crystallinity of 69.9%. The average diameter was 13.5 μm with structural evidence of separated fibrils as investigated by FESEM. The TEM analysis suggested that the material was crystalline and its geometry was a monoclinic structure. The FTIR spectral peaks representing wax and hemicelluloses at 1735 cm-1 and 1375 cm-1, respectively, and lignin at 1248 cm-1 and 1037 cm-1, were not observed in the extracted OPEFB-cellulose spectra. Based on the TGA results, thermal stability at 325 oC with a single degradation curve suggests the purity of OPEFB-cellulose.
Keywords: Oil Palm Empty Fruit Bunch (OPEFB); Green technology; Biomaterials; Cellulose; Characterization
Contact information: a: Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750, Perak, Malaysia; b: Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750, Perak, Malaysia; c: FELCRA Nasaruddin Oil Palm Mill, Km37, Jalan Tronoh, 32600, Bota, Perak, Malaysia; *Corresponding author. Tel: +605 3687636; Fax: +605 3656176; E-mail Address: azmuddin@petronas.com.my
INTRODUCTION
In Malaysia, Oil Palm Empty Fruit Bunches (OPEFB) annually generate 19.8 million tonnes of biomass on a wet basis, or 6.93 million tonnes on a dry basis (Foo et al. 2011). This provides huge resources for the conversion into value-added products, as the cellulosic biomass is inexhaustible, renewable, biodegradable, recyclable, and a deriva- tizable biopolymer. OPEFB are lignocellulosic composites, comprised of compactly arranged cellulose, hemicelluloses, and lignin with 44.4% cellulose fraction (Sun et al. 1999), out of which as much as 60.6% can be turned into value-added biopolymer (Wanrosli et al. 2004).
Naturally, cellulose polymer is a linear chain of anhydro-glucose monomer units connected through 1-4 β-linkages. The cellulose polymer chain is stabilized at the end-terminal with non-reducing and reducing sugar units (Klemm et al. 2005). The physical and chemical properties of the cellulose chain can be attributed to reactive –OH groups that reside on positions C-2, C-3, and C-6. The chain is compactly and tightly arranged due to random twisting and the formation of interactive forces: a weaker van der waals force and stronger intermolecular and intramolecular H-bonding (Abdullah et al. 2011).
Acidified sodium chlorite (NaClO2 + H3O+) is widely used as a standard reagent for the delignification and the extraction of cellulose from wood materials (Wise et al. 1946). Chlorite (ClO2–) may produce a chlorine radical, Cl●, which reacts and fragments the lignocellulosic material into highly toxic organochlorine. The cellulose extraction from the OPEFB involving pretreatment with toluene/methanol (2:1,v/v) (Sun et al. 1999) and ethanol/benzene (1:2) (Fahma et al. 2010), with further delignification using chlorite and microcrystalline cellulose preparation (Rosnah et al. 2009), have been reported. However, the use of toluene, benzene, and chlorite are not environmentally friendly. There are now concerted efforts to “go-green” by using less solvent at low concentration, using less toxic and non-toxic feedstock selections in combination with physical treatments that are less energy intensive. The delignification of wheat straw with alkali (Sun and Tomkinson 2002), sulphuric acid hydrolysis of OPEFB (Yunus et al. 2010), the formic acid/performic acid hydrolysis of jute fiber to isolate microcrystalline cellulose (Jahan et al. 2011), and autothermal degradation of Miscanthus x giganteus to cellulose fiber using formic acid and hydrogen peroxide (Haverty et al. 2012), are among the green methods that have been explored.
Organic acids such as acetic acid and formic acid are eco-friendly reagents, less corrosive, and effective for the pretreatment of lignocellulosic biomass, and they provide a more stable medium for monosaccharide in the aqueous phase as compared to sulfuric acid (Marzialetti et al. 2011). Formic acid could be recovered easily after the complete delignification with multiple additions of water, and the process is economical and easy to control at low operating temperature and pressure (Li et al. 2012). A mixture of hexane/ethanol and water has been used to remove wax from the rice husk, with further autoclaving and ultrasound treatments for the delignification and purification of cellulose using hydrogen peroxide and acetic acid (Rosa et al. 2012). The extraction of cellulose fibers from the OPEFB by hydrogen peroxide with an ultrasound-assisted alkali extraction yields 49% of cellulose (Nazir et al. 2012).
The main objective of the present research work was to explore an eco-friendly method for cellulose extraction from the OPEFB for enhanced yield and high crystallinity. The solvents employed were formic acid and hydrogen peroxide at low concentrations. The structural characterization was investigated by Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). The chemical functional group identification was done by Fourier Transmission Infra-red spectroscopy (FTIR). The crystalline properties were studied by X-ray diffractometry (XRD), and the thermal stability of microcrystalline cellulose was investigated by Thermogravimetric analysis (TGA).
Experimental
Materials
Fresh OPEFB samples were collected from the FELCRA Nasaruddin Oil Palm Mill, Bota, Perak, Malaysia and stored at 4 oC. The chemicals, formic acid (96.0% Sigma-Aldrich), sodium hydroxide (pellet), 30% hydrogen peroxide, and ethanol (99.5% Sigma-Aldrich), of ACS grade were used as purchased. To assist in the physical treatment, an autoclave (75X, All American) was used. The commercial microcrystalline cellulose (MCC) (Cotton linter; 20 μm, Aldrich) was used as a standard for comparison.
Percentage of Moisture Content and Ash Contents
The percentage of the OPEFB moisture content was determined based on the standard method (ASTM 2003). Ten grams of the sample (Dragon 303, METTLER-TOLEDO) were placed in the crucible and left in an oven (UNB-400, MEMERT) at 100 ± 2 oC for 3 h. Samples in three replicates were then kept in a desiccator until room temperature, before the final weight was noted, and an average percentage was calculated.
For the ash content percentage (ASTM 2007), a dried sample (at 105 oC) was ground and sieved through a nylon filter (40 mesh, Kexin). Two grams were placed in a clean crucible and sent to a furnace (Carbolite RHF 1600) until a constant temperature of 600 oC was reached for 1 h. The crucible was left in a desiccator, weighed, and the ash content percentage of three replicates was calculated.
Pretreatment
OPEFB fibers were physically separated and washed several times with 1% washing detergent to remove oil, greases, and sand particles, and rinsed until the freshly used rinse water turned from dark brown to colorless. The fibers were dried at room temperature and cut into 0.2 mm by using power cutting mill (Pulverisette 25). These fibers were considered to be raw OPEFB.
For the treated OPEFB fibers, the raw fibers after washing were dried in an oven (UNB-400, Memert) at 100 ± 2 oC until they were at constant weight, and later cut into 0.2 mm size pieces.
The fibers were dewaxed in 200 mL 70% (v/v) ethanol, with a fiber to solvent ratio of 1:10 (g L-1) using soxhlet (FAVORIT) apparatus for six hours. The fibers were collected and washed with deionized water to remove alcohol traces before further drying.
Later, ten grams of dewaxed fibers were suspended in 10% of 100 mL NaOH, followed by 10% of 100 mL H2O2, covered with aluminium foil, and autoclaved (75X, All American) for one hour at 121 oC and 1.5 bar. The dark brown supernatant was separated from the fibers, followed by several rounds of washing until clear water was observed.
Extraction of OPEFB-Cellulose
The extraction method was adapted and modified from that developed for jute fibers (Jahan et al. 2011). The autoclaved fibers were soaked in a mixture of 20% formic acid and 10% hydrogen peroxide at 1:1 ratio (v/v), and placed in a water bath set at 85 oC for 2 h. Delignified fibers were then collected after filtration and washed further with 10% formic acid, followed by washing several times with distilled water. The extracted cellulose fibers were light yellow in color and re-suspended in 10% of hydrogen peroxide at pH 11 for 90 min at 60 oC. The pH was adjusted with 10% of sodium hydroxide.
The white suspension obtained was filtered, washed several times, and the insoluble fraction of cellulose was collected and the yield (w/w) calculated. The -cellulose content was determined by the TAPPI method (TAPPI 1991).
Characterization
Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron microscope (TEM)
Morphological studies of raw OPEFB and extracted cellulose were carried out with a Field Emission Scanning Electron Microscope (FESEM, Zeiss Supra 55VP). The samples were suspended in isopropyl alcohol (IPA) and sonicated for 1 h with an interval of eight minutes. One drop of the sample was placed on a carbon tap, and allowed to dry for 24 h at room temperature. The samples were then gold coated using a sputtering method to avoid charges, and the accelerated voltage was set to 2 kV. Morphological observations were made, and the mean diameter was determined by selecting ten fibers randomly from the image. An elemental analysis was performed by an energy-dispersive X-ray (EDX) coupled with FESEM. When an electron beam penetrated the sample, the X-ray produced was collected, analyzed, and detected by Silicon drift detectors (SDD), and the X-ray map was recorded.
Further structural studies were made by using a Transmission Electron Micro-scope (TEM, Libra 200 FE, ZEISS). The samples were suspended in IPA and sonicated as described above. One drop of the sample was placed on a carbon coated Cu-grid and allowed to dry at room temperature for 24 h and the observation was recorded. To investigate the crystalline structure, the Fast Fourier Transmission (FFT) image was analyzed by using ITEM (Olympus) software.
Fourier Transmission Infra-Red spectroscopy (FTIR)
The chemical investigation of functional groups in raw OPEFB and extracted cellulose were studied by using Fourier Transmission Infra-red Spectroscopy (Spectrum Perkin Elmer). The sample disc was prepared by mixing and compressing the sample and KBr at a 1:1 ratio. FTIR spectra were produced after fifty times by scanning at a 4 cm-1 resolution for transmission wavelength range of 4000 to 450 cm-1.
X-Ray Diffraction (XRD)
The phase behaviour of raw OPEFB and extracted cellulose was elucidated by using an X-ray diffractometer (D8-Advance Bruker-AXS). The analyses and instrument conditions were set at 1.540oA wavelength (CuKα radiation), with a scan speed of 2o per second and a 2θ range of 2-80o. The crystallinity index, CIr, was calculated as follows (Segal et al. 1959),
CIr = [(I002-Iam/I002)] × 100 (1)
where, I002 is the highest peak intensity of the crystalline fractions and Iam is the low intensity peak of the amorphous region.
The crystallite size was determined by using Scherrer’s equation (Patterson 1939),
D = K/Bcos (2)
where, K is the constant 0.91, θ is Bragg’s angle, and β is the intensity of the full width at half of the maximum (FWHM) corresponding to a high intensity peak of the diffraction plane 002.
Thermogravimetric Analysis (TGA)
The thermal stability of the raw OPEFB and extracted cellulose was studied by using TGA (Pyris 1, Perkin Elmer). Ten mg of sample (dried) was kept under TGA observation at a set temperature range of 30 to 700 oC in a nitrogen environment. The constant nitrogen flow was maintained at 30 cm3min-1, followed by a furnace flow rate of 150 cm3min-1, before the samples were run.
Results and Discussion
Extraction of OPEFB-cellulose
The percentage of the moisture and ash contents of the raw OPEFB fibers were 9.9% and 3.8%, respectively, and they were comparable to a previous report (Faruk et al. 2012). The ethanol extracts by the soxhlet method were 3%, containing oil, waxes, fats, and resin, and they were comparable to a previous study (Fahma et al. 2010). The α-cellulose content of the isolated OPEFB cellulose was 93.7%, which is comparable to the reported value of 97.1% (Leh et al. 2008), but higher than 62.3% (Wanrosli et al. 2011). Further delignification with performic acid effectively removed the lignin and the hemicelluloses fraction, and produced a total cellulose yield of 64% (w/w) on a dry weight basis, which was higher than the previously reported yield at 44.4% (Sun et al. 1999) and 60.6% (Wanrosli et al. 2004) using chlorite and sodium hydroxide, respectively. The moisture, ash, and ethanol extractive fraction vary with the environmental abiotic and biotic stresses, while the cellulosic content varies with the plant species, growth factors, and the choice of reagent during the extraction.
High cellulose content suggests that the method developed was efficient. A pretreatment in an autoclave produces high energy steam that dislocates fibrillar arrange-ments and assists with the penetration of sodium hydroxide and hydrogen peroxide for partial oxidation. Formic acid, with its major contributors, is very stable in an aqueous medium. A resonance system (A=B–Xˉ) may be generated in the molecule with the release of protons (H+), and stability is gained by shifting electrons from the negatively charged oxygen to a single bond and turns it into a multiple bond. On the other end, the multiple bond between the C=O shifts onto a connecting oxygen atom and the oxygen bearing the negative charge. The to and fro multiple shifting of electrons through the single bond produces equivalent structures, or major contributors, to give maximum stability. This may form performic acid by accepting an oxygen atom from hydrogen peroxide.
Stable formic acid provides the medium for the interaction and dissolution of the non-crystalline part of cellulose, while the crystalline region is stable to the acid attack, thus increasing the crystallinity. The synergistic effects remove the non-lignin parts, hemicelluloses, and lignin (by partial oxidation), converting the hemicelluloses to the corresponding salt forms, and the lignin to respective carboxylic acids. These will be removed as a water soluble fraction, leaving behind the crystalline cellulose that is insoluble and suspended in an aqueous medium. The decomposition products of reacting mixture are CO2, H2O, and O2 which may be less hazardous than the chloro-organic compounds generated with the use of chlorite, whilst the CO2 is recovered from agro-resources.
Morphological Characterization
FESEM
Figures 1 a-c show that the raw OPEFB is a lignocellulosic composite with compact fibrillar packing. The external surface suggests an irregular heavy deposition of wax, hemicelluloses, lignin, and other inorganic components (Na, Mg, K, Ca, S, Al, Si) as illustrated by EDX spectra (not shown). The extracted cellulose (Figs. 1 d-f) show an isolated fibril, having an average diameter of 13.5 μm, and it was separated from each other, suggesting the complete removal of cementing wax, hemicelluloses, and lignin. The external surface showed a mixture of a smooth surface with scars, highly likely due to the removal of inorganic particles such as silica and the metal components. This was supported by EDX spectra (data not shown), which showed only carbon and oxygen remaining.
Fig. 1. FESEM micrographs of (a-c) raw OPEFB; (d-f) extracted OPEFB-cellulose
TEM
The raw OPEFB (Figs. 2a and 2b) show an unexposed crystalline region surrounded by amorphous hemicelluloses and lignin with solid round elemental particles, most probably silica or inorganic elements. The extracted cellulose image suggests the presence of a large crystalline region and a small amount of an amorphous region (Fig. 2c). One part of the image was selected and cropped for FFT (Fast Fourier Transmission) (Fig. 2d), where the crystalline arrangement of cellulose can be observed. The diffraction pattern was recorded (Fig. 2e), and the inter-planar distances were calculated (Fig. 2f) suggesting the equivalence to a monoclinic crystal system. In a unit cell monoclinic structure, all of the lengths must be different (a≠b≠c). The image shows only β = γ (≈90o), and β ≠90o angles (Fig. 2f). The crystalline structure of the cellulose chains and the interaction between the consecutive chains are due to the H-bonding or Van der Waals forces (Zugenmaier 2007). The monoclinic structure of the extracted cellulose suggests the stability of the cellulose chain through H-bonding (Avolio et al. 2012).
Fig. 2. TEM micrograph of (a & b) raw OPEFB; (c & d) extracted cellulose; (e) the diffraction pattern; (f) the monoclinic pattern and its measurement
Chemical Characterization
FTIR
The FTIR spectra (Figs. 3a, b, and c) show the transmittance peaks at 3400 cm-1 and 2906 cm-1, which were attributable to cellulose –OH and C-H groups stretching vibrations, respectively (Khalil et al. 2001). Similarly, the peaks at 1432, 1375, and 1326 cm-1 present in the raw OPEFB are associated with the bending vibrations of -CH2, C-H, and C-O of cellulose, and they were also present in the extracted cellulose and the commercial MCC. The water absorption peak was observed at 1644 cm-1, and the peak at 895 cm-1 assigned to the β-gycosidic linkage (Le Troedec et al. 2008) was present in all of the spectra. However, the peak at 1735 cm-1 in the raw OPEFB spectra (Fig. 3a) attributable to a waxy C=O acetyl group of hemicellulose ester or carbonyl ester of the p-coumaric monomeric lignin unit, and the peak at 1248 cm-1 due to the C-O-C of aryl-alkyl ether in lignin (Alemdar and Sain 2008; Nazir et al. 2012) were not present in the extracted cellulose and the commercial MCC (Figs. 3b and 3c). The peaks at 1161, 1137, and 1105 cm-1 attributable to the deformation of the C–H rocking vibration and the C–O–C pyranose were more prominent in Figs. 3b and 3c (Kargarzadeh et al. 2012). These spectral changes are characteristics of cellulose, which further demonstrates the removal of hemicelluloses and lignin.
Fig. 3. FTIR of (a) raw OPEFB, (b) extracted OPEFB-cellulose, (c) commercial MCC
XRD
Raw OPEFB fibers are composites of amorphous (hemicellulose, lignin) and crystalline (cellulose) structures (Fig. 4a); hence the small peak intensity as the cellulose fibers may still be entrapped in amorphous hemicellulose and lignin.
Fig. 4. X-ray diffractogram of (a) raw OPEFB, (b) extracted OPEFB-cellulose, (c) commercial MCC
With the extracted cellulose (Fig. 4b), a single high intensity peak at 22.5o (002), comparable to the commercial MCC (Fig. 4c), suggests that the depolymerization of hemicellulose and delignification had been successfully achieved. The sharp high intensity peak describes the crystalline nature of the material, and the intensity value shows the amount of crystalline structure. The diffractogram of the extracted cellulose showed a high intensity crystalline peak I002 at 2θ=22.5, and an amorphous peak with an intensity Iam at 2θ=16o corresponding to the crystallographic plane (101), similar to results reported earlier (Johar et al. 2012; Sheltami et al. 2012). These characteristic peaks were attributed to cellulose based on the JCPDS data (03-0289). The crystallinity index and the crystallite size of raw fibers were calculated as 43.9% and 31.5 nm, respectively, and for the extracted cellulose, they were 70% and 9.8 nm, respectively, corresponding well with a previously reported value of 69% (Jonoobi et al. 2011).
TGA
Lignocellulosic material is a composite structure of different types of compo-nents, and it will degrade below 400 oC with wax, pectin, and hemicelluloses degraded at 180 oC, cellulose at around 300 oC, and lignin at 400 oC (Johar et al. 2012). TGA of the raw OPEFB (data no shown) showed around 10% of water losses at 110 oC, with low temperature degradation of hemicelluloses, wax, and pectin material ranging from 150 to 230 oC. After 230 oC, the sharp change was due to cellulose degradation before becoming stable at 350 oC, followed by a slow decomposition phase of lignin. In OPEFB cellulose, two phases of thermal degradation could be observed, the thermal degradation of the OPEFB-cellulose with water loss below 200 oC, and the depolymerization of the cellulose chain between 200 to 390 oC. The decomposition phases attributed to hemicelluloses (>180 oC) and lignin (>390 oC) were absent in celluloses. The sharp weight loss with linearity of the thermal curve from 325 oC and 350 oC was comparable to that of commercial MCC. Thermal weight loss curve suggests high purity of the extracted MCC, which would have otherwise produced different phases should there be any impurities. The thermal stability of the OPEFB-cellulose (325 oC) was higher than that of wheat straw (296 oC) and soy hulls (290 oC) (Alemdar and Sain 2008). The residual mass around 25% at 700 oC for the raw OPEFB may be due to the formation of ash with enriched lignin carbon compounds accumulated as char (Fahma et al. 2011). The residues of 5% char may be produced as a result of aromatization of fragmented cellulose C–C, C—O bonds at temperatures higher than 400 oC (Jahan et al. 2011).
Conclusions
- A physico-chemical method was developed for the extraction of the OPEFB-cellulose using a low concentration of formic acid (20%) and hydrogen peroxide (10%), which yielded 64% of cellulose with an α-cellulose content of 93.7% and 70% crystallinity.
- FESEM observation showed that the morphology of the extracted cellulose appeared as separate fibrils with crystallite parallel chains. The TEM diffraction pattern elucidated the unitary crystalline structure of cellulose in a monoclinic crystal pattern.
- FTIR studies demonstrated the removal of wax, lignin, and hemicelluloses functionalities. XRD had proven the crystalline nature of the cellulose extracted, and the TGA analysis indicated the purity in terms of thermal stability achieved at 325 oC, which was confirmed and validated by comparing the results with commercially available MCC.
Acknowledgements
This study was partially funded by Biomedical Technology MOR under the Bioengineering Research group provided by Universiti Teknologi PETRONAS. The scholarship support provided to Muhammad Shahid Nazir is acknowledged.
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Article Submitted: December 28, 2012; Peer review completed: March 1, 2013; Revised version received and accepted: March 6, 2013; Published: March 8, 2013.