Research Article pubs.acs.org/journal/ascecg
Morphological and Thermochemical Changes upon Autohydrolysis and Microemulsion Treatments of Coir and Empty Fruit Bunch Residual Biomass to Isolate Lignin-Rich Micro- and Nanofibrillar Cellulose Anurodh Tripathi,*,†,‡ Ana Ferrer,‡,⊥ Saad A. Khan,† and Orlando J. Rojas*,†,‡,§
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Department of Chemical & Biomolecular Engineering, NC State University, 911 Partners Way, Raleigh, North Carolina 27659, United States ‡ Department of Forest Biomaterials, NC State University, 2820 Faucette Drive, Raleigh, North Carolina 27607, United States § Department of Byproducts and Biosystems, School of Chemical Engineering, Aalto University, Vuorimiehentie 1, P.O. Box 16300, FIN-00076 Aalto, Espoo, Finland S Supporting Information *
ABSTRACT: Autohydrolysis and microemulsion treatments followed by microfluidization are employed to isolate micro- and nanofibrillar cellulose (MNFC) from coir fibers and palm tree empty fruit bunches (EFB) with residual lignin content of ∼24 and ∼31 wt %, respectively. The fibers and associated MNFC are characterized in each treatment for their chemical, structural, and thermal properties. The most significant findings include the fact that two MNFC populations are produced, with distinctive structural differences and characteristic lateral dimensions of 20−70 nm and 1−3 μm. The lignin distribution after possible recondensation occurred in the form of nanodroplets. Finally, a correlation between thermal degradation of MNFC with spatial arrangement of lignin is hypothesized and a defibrillation mechanism is proposed. The detailed structural and thermochemical analyses presented here are expected to facilitate further interest in the development of new materials from MNFC isolated from coir and EFB, two abundant bioresources that are most suitable for their valorization. KEYWORDS: Defibrillation, Microfluidization, Pretreatment, Coir fibers, Micro- and nanofibrillar cellulose, MNFC, Thermogravimetric analysis, Fiber morphology
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INTRODUCTION Lignocellulosic biomass in the form of wood, grasses, fibers, and agricultural residues are efficient components in natural biocomposites, as produced by nature after centuries of iterations and evolution. Thus, man-made materials from sustainable, biobased, biodegradable, and renewable biomass are an excellent starting point in efforts to replace fossil sources for such purposes.1−3 However, materials engineered from lignocellulosic fibers often use chemicals in their isolation from biomass that may require complex recovery processes; in addition, the extensive degree of chemical and physical deconstruction may limit a full exploitation of the properties inherent to wood fibers. There has been a large research impetus on producing fibers from natural resources using mild treatments to better preserve the original structure of the system and to reduce the environmental footprint.4,5 The natural fibers used in this study are residuals from processing the coconut husk and the empty fruit bunches of palm trees. Coir fibers from the coconut husks represent 5−6 million tons per year with roughly 80% contribution coming from Southeast Asia.6 Only 10% of this potential enters the © 2017 American Chemical Society
commercial trade and, traditionally, they are used to make mats, carpets, ropes, mattings, fishing nets, and brushes. Recently, a few studies proposed coir fiber utilization as raw material in polymer composites and efforts are underway to explore wider markets.7−10 The other nonwoody biomass studied here, empty fruit bunch (EFB) fibers, is the residual lignocellulosic material after palm oil production.4,11 Malaysia is the largest palm oil producer, contributing to 51% of the market worldwide. The global oil production reached almost 21 million tons in 2010.12 Thus, an enormous volume of residues are generated, mainly EFB fibers, which are proposed here for the production of micronano fibers by methods different than those reported so far by us4,13 and others.5,14−17 One critical issue common to coir and EFB fibers is the high lignin content, as high as 45%. Therefore, any attempt in utilization should consider preserving Received: November 23, 2016 Revised: January 6, 2017 Published: January 12, 2017 2483
DOI: 10.1021/acssuschemeng.6b02838 ACS Sustainable Chem. Eng. 2017, 5, 2483−2492
Research Article
ACS Sustainable Chemistry & Engineering
shear mixer at 22 000 rpm for 15 min. MNFC-coir and EFB fibers were obtained following microfluidization (Microfluidics M-110T). The microfibers were passed 20 times through an intensifier pump that increased the pressure (2000 bar), followed by an interaction chamber which defibrillated the microfibers by shear forces and impacts against the channel walls and colliding streams. Through this process, the microfibers were further broken up into nanosized structures forming the micronano lignocellulosic fibrils (MNFC). The unit was operating under a constant shear rate. The temperature was not controlled but fluidization was temporarily ceased when the temperature of the dispersion reached ∼90 °C to prevent pump cavitation. Processing then recommenced when the temperature was ∼45 °C. The resulting LCNF suspensions were collected and stored at 4 °C in a refrigerator. It was observed in our case that the microfluidizer got clogged after 1 pass of the coir/EFB fibers that were not subjected to microemulsion pretreatment. Fiber Composition Analysis. The chemical composition of the raw fibers was determined following TAPPI standards: T-222 for lignin, T-203 for α-cellulose, T-204 for ethanol−benzene extractives, and T-211 for ash. Holocellulose content was determined by the Wise and co-workers method.22,23 Fourier Transform Infrared Spectroscopy (FTIR). FTIR was conducted using a PerkinElmer spectrophotometer in attenuated total reflectance mode (FTIR-ATR). Samples were analyzed using the Pike Miracle accessory equipped with a germanium (GE) crystal. The spectrum was collected for 12 scans and corrected for background noise. The multipoint baseline correction was realized for each spectrum, and the curves were normalized to 20% transmittance with respect to the peak at 1027 cm−1. All samples were dried in an oven at 60 °C and kept dry in a desiccator prior to analysis. Scanning Electron Microscopy (SEM). Imaging was performed by field emission scanning electron microscopy (FESEM), FEI Verios 460L. The P-coir and P-EFB fibers were cryo-fractured under liquid N2. A sharp, clean blade was used to fracture the material and to obtain images of the radial cross sections. The remaining samples (H, M, and MNFC) were suspended in water at 0.01 wt % and freeze-dried after vitrifying in liquid N2. All the samples were mounted on an SEM stub using a double sided carbon tape. The prepared samples were coated with 5 nm layer of gold and platinum to capture secondary electrons from the surface and thus reduce charging. The imaging was carried out at the landing voltage of 2 kV and 13 pA current at a working distance of 5 mm. The Everhart−Thornley (ETD) detector was employed to detect secondary electrons from the samples. Atomic Force Microscopy (AFM). A silica wafer was carefully cut into 1 × 1 cm2 and the surface was cleaned by soaking in 0.1 N NaOH for 10 s. After rinsing in DI water, the wafer was subjected to UV light for 15 min. A 0.01 wt % of the fiber suspension was sonicated at 30% amplitude. A drop of the resulting suspension was placed on the previously cleaned silica surface, which was allowed to dry in the oven at 60 °C. The fibers dried on silica surfaces were mounted on an aluminum sample holders and examined with a Dimension 3000 scanning probe microscope from Veeco Metrology Group. Scanning was performed in the tapping mode in air using silicon cantilevers (NSCl5/AIBS) delivered by Olympus AC160TS. The drive frequency of the cantilever was about 275−325 kHz (nominal resonance of 300 kHz). The scanned areas were imaged. No image processing except flattening was made. Images were taken with a feedback loop to keep the amplitude of oscillation constant and the response of the feedback loop was measured. The response of the feedback loop was used to determine how far the scanner was moved in the Z direction in order to keep the amplitude of oscillation constant. Laser Scanning Confocal Fluorescence Microscopy (LSCM). A Zeiss LSM 710 laser scanning confocal microscope with Zen 2008 was used with the excitation laser at Ar 488 nm over an emission range of 490 to 560 nm and a 40× C-Apochromat objective (1.1 W NA) zoom to acquire multichannel fluorescence image. The fiber samples were suspended in water prior to the analysis. X-ray Diffraction (XRD) Analysis. XRD was conducted using Rigaku SmartLab X-ray diffractometer using the operating mode of Bragg−Brentano reflection geometry. A Cu Kα radiation source of
this component as the only way to make any industrial effort truly sustainable. Efforts to extract nanocelluloses from coir and EFB fibers have been reported.4,5,15−17 However, the studies so far have generally used chemical treatments that can be improved in terms of their severity and its consequences. Specifically, microand nanofibers carrying residual lignin, hemicelluloses, and extractives may present an attractive option for higher yield, low production cost, and much lower environmental impact. Moreover, the presence of lignin has been shown to enhance barrier properties and tunable surface properties of nanocellulose films and nanopapers.18,19 With the goal of retaining the native lignin in coir and EFB fibers, we report a methodology to isolate from these residual sources, ligninrich micro- and nanofibrillar cellulose (generally termed here as MNFC). Our multistage procedure involves previously reported protocols for autohydrolysis20 and microemulsion treatment,21 both of which are mild and, to a great extent, preserve the original cellulosic content. Our study includes morphological, structural, and thermal analyses which were performed after each processing step to develop a thorough understanding of defibrillation and lignin redistribution. Based on the developed understanding and wood fiber morphology, a defibrillation mechanism is proposed.
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MATERIALS AND METHODS
Materials. Coir fibers were obtained from the National Coir Research and Management Institute (NCRMI) under the Government of Kerala, India. These were retted fibers, manually separated from husk. EFB from a Malaysian oil palm mill was supplied by Straw Pulping Engineering S.L. (Zaragoza, Spain). The fiber samples were used as received. The chemicals for microemulsion pretreatment included Rlimonene sourced from Fulka, St. Louis, USA. Sodium dodecyl sulfate (SDS), n-pentanol, sodium chloride (NaCl), sodium hydroxide (NaOH), and urea were obtained from Sigma-Aldrich. DI water was used for all experimental purposes. Autohydrolysis Pretreatment. For pretreatment, the fibers (30 g of either coir or EFB fibers) were loaded together with distilled water (DI) in a 10-L vessel at a fiber-to-water mass ratio of 1:60. The fibers were digested at 180 °C for 60 min followed by mechanical disintegration in a Sprout-Bauer. The disintegrated fibers were filtered using a cheese cloth whereupon they were filtered by centrifugation at 15 000 rpm for 1 min. The pretreated samples were stored in plastic bags at 4 °C until further use. These grinded fiber samples are referred to as H-Coir or H-EFB. Microemulsion Pretreatment. A microemulsion was prepared with an anionic surfactant (sodium dodecyl sulfate, SDS) and Rlimonene, creating a thermodynamically stable, single phase surfactant−oil−water system (SOW).21 As reported before, the treatment was performed on the fiber samples to disrupt the hydrogen bonding between the cellulosic fibrils, thus facilitating the mechanical defibrillation. Briefly, a 100 mL microemulsion was prepared by adding SDS (3.0 wt %) to water (67.7 wt %) and NaCl (2.7 wt %) solution. The active agent, urea (12.5 wt %), was added followed by limonene (8.0 wt %), n-pentanol (3.1 wt %) and NaOH (3.0 wt %). The mixture was mixed under low shear (magnetic bar) to produce a clear, isotropic liquid microemulsion. The H-Coir or H-EFB fibers (3.0 wt % solid content) were soaked in this microemulsion for 12 h at room temperature and atmospheric pressure. The fibers were then filtered under vacuum and washed with DI water several times to remove the components of the microemulsion that may have remained after filtration. Thereafter, they were centrifuged at 15 000 rpm for 1 min and stored at 4 °C until further use. The fiber samples after microemulsion pretreatment are referred to as M-Coir or M-EFB. Fiber Processing and Defibrillation. The M-coir and M-EFB fibers were redispersed in water (0.15 wt % solid content) using a high 2484
DOI: 10.1021/acssuschemeng.6b02838 ACS Sustainable Chem. Eng. 2017, 5, 2483−2492
Research Article
ACS Sustainable Chemistry & Engineering wavelength 0.154 nm and operating at 40 kV and 44 mA was incident on the solid sample. The XRD patterns were recorded over the angular range 2θ ranging from 5 to 50°. The fiber samples were dried in the oven at 60 °C and manually cut into small pieces using a pair of scissors prior to the analysis. Full width at half-maximum (FWHM) was calculated by calculating peak width of I200 at half its maximum intensity. Thermogravimetric Analysis (TGA). Thermal degradation analysis was performed using a TA Instruments TGA Q500. The analysis was carried out in a N2 environment at a flow rate of 60 mL/ min with a constant heating rate of 10 °C/min over a temperature range of 40−700 °C. The fiber samples were dried in oven at 60 °C and stored in desiccator prior to the analysis. The analysis was done in triplicate for data reproducibility. The time-derivative of the weight loss curve was obtained and smoothed by adjacent averaging over 50 points. The differential curve was plotted against temperature.
content (by 3−5%) is attributed to the alkaline medium of the microemulsion system, due to presence of NaOH.21 The composition analysis after microfluidization was not attempted since it was assumed that mechanical defibrillation did not alter substantially the chemical composition of the fibers.25 With the unique combination of these mild pretreatment processes, the MNFC from coir and EFB fibers retained the original lignin, to a large degree, with final content of 31 and 24 wt %, respectively. To the best of our knowledge, this is the first study to report on the isolation of MNFC from coir and EFB fibers while retaining the native lignin. Investigating Chemical Structure. FTIR analysis was performed to investigate any changes in the chemical structure during fiber processing. The infrared spectra of cellulose, hemicellulose and lignin has been extensively studied in the literature.14,26,27 The three fiber components mainly comprise functional groups such as alcohols, ketones, ether, aromatics, saturated alkanes and esters. The FTIR spectra obtained from coir and EFB fiber samples at each processing stage is shown in Figure 1. Some of the significant peaks are highlighted in the FTIR spectra. Table S1 lists all the major peaks that are identified with their corresponding functional groups, chemical compounds and the given fiber component. Two main transmittance regions ranging from 3500 to 2900 and 1740 to 700 cm−1 are seen in all the curves. A broad and prominent peak at 3329 cm−1 and a distinct peak at 2910 cm−1 are due to O−H stretch and sp3 C−H stretch respectively, demonstrating availability of large number of acid, alcohol groups, and saturated alkanes, that are present in all three fiber components. One major difference observed during processing of coir and EFB fibers is the CO stretch at 1736 cm−1 that arises from esters present in hemicelluloses in P-coir and P-EFB. This peak is shifted to ca. 1715 cm−1 after autohydrolysis and remains following further processing. This may be due to two reasons: First, most of the hemicelluloses leach out after autohydrolysis. Second, upon hydrolysis the ester groups in the remaining hemicelluloses are most likely converted to carboxylic groups. An alkoxy C−O stretch and C−O deformation at 1027 cm−1 and an acyl C−O peak at 1240 cm−1 represents C−O−C stretching vibration arising from pyranose ring.16,28 The shoulder peaks at 1161 and 1100 cm−1 are characteristic of cellulosic components and have been reported before by Yang et al.27 These peaks are also attributed to C−O−C stretching vibrations. The C−O−C asymmetric stretching can also be attributed to ester functional group present in hemicelluloses and the absence of this peak after autohydrolysis further indicates reduction in hemicellulose content. The peaks in the range of 1470−1430 cm−1 represent methoxy groups, and the peaks between 1600 and 1460 cm−1 represent CC stretch of aromatics. Peaks in both regions are characteristic of methoxy groups and aromatic rings present in the lignin. These peaks are absent in the FTIR of pure cellulose, as shown in Figure S2. The peaks in 1400−1300 cm−1 correspond to sp3 C−H bend. Even though FTIR spectra were developed after careful drying and storage of the samples, a small peak at 1638 cm−1 was observed and attributed to the O−H bending arising from the adsorbed water on the surface. This demonstrates that water molecules are difficult to completely remove from cellulose due to strong molecular interactions. The peak at 898 cm−1 is attributed to C−O−C stretch from glycosidic linkage between glucose units. The peaks from 900 to 700 cm−1 are due to aromatic sp2 bend arising from aromatic hydrogen present in lignin.
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RESULTS AND DISCUSSION We begin by describing the procedure used for the isolation of the lignin-rich MNFC from coir and EFB fibers. During each step of processing, the fibers were characterized for chemical composition (TAPPI standard and FTIR), morphology (AFM, SEM), lignin distribution (LSCM), crystallinity (XRD), and thermal properties (TGA). Based on the results, a defibrillation model is proposed. Isolation of Lignin-Rich MNFC. The experimental procedure for the isolation of MNFC fibers (Figure S1) was designed to retain the lignin present in the coir or EFB fibers (amounting to 36−43%). The as-received or pristine coir and EFB fibers (P-coir and P-EFB) were subjected to autohydrolysis (hydrothermal treatment) followed by fiber disintegration and refining in the Sprout−Bauer producing H-coir and HEFB, respectively. A microemulsion pretreatment was performed on these fibers to delaminate and disrupt the hydrogen bonding to give M-coir and M-EFB fibers. These fibers were then microfluidized and are referred to as MNFC-coir or MNFC-EFB, respectively. The microemulsion step was necessary to prevent microfluidizer from clogging. The details of the procedure are included in the Experimental Section. Autohydrolysis is a mild fiber treatment process, in which fibers are heated in DI water at 180 °C to partially leach the soluble lignin and hemicelluloses from the fibers, while leaving behind high residual amounts of insoluble lignin and cellulose in the fibers.20,24 The chemical composition of the coir and EFB fibers after each processing stage is summarized in Table 1. Table 1. Chemical Composition of Coir and EFB Fibers (P) and after Sequential Treatment via Autohydrolysis (H) and Microemulsion Impregnation (M) sample P-coir/ P-EFB H-coir/ H-EFB M-coir/ M-EFB
extractives (wt %)
lignin (wt %)
cellulose (wt %)
hemicellulose (wt %)
0.5/0.8
43/36
30/34
23/20
