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Chapter 12
Cellulose and Nanocellulose Produced from Lignocellulosic Residues by Reactive Extrusion Flavia Debiagi, Paula C. S. Faria-Tischer, and Suzana Mali* Departament of Biochemistry and Biotechnology, State University of Londrina, Rod. Celso Garcia Cid SN, 86057-970, Londrina-PR, Brazil *E-mail:
[email protected].
The use of lignocellulosic biomass from agricultural and agroindustrial sectors has recently gained worldwide attention as a potential feedstock to obtain new bio-based products, which aligns with the concept of biorefinery, i.e., developing a sustainable economy that includes the development of new processes and technologies. Obtaining cellulose and nanocellulose from agroindustrial lignocellulosic residues has been discussed over the last few years, and reactive extrusion has emerged as an innovative and attractive technology for is purpose, combining a high-temperature, short-duration process with a high versatility and low effluents generation. Thus, this chapter discusses the use of lignocellulosic residues to obtain cellulose and nanocellulose via reactive extrusion and the advantages and challenges of using this technology that is already used for food and plastics production.
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Introduction Considering that the world population is expected to reach 9.7 billion in 2050 (1, 2), the search for new alternatives to reduce the dependence on petroleumbased products and to minimize or recycle industrial residue discharges is urgent. Thus, over the last few years, there has been increased interest in research and development on bio-based materials with the aim to valorize several residues from industrial processes to obtain new bioproducts, which aligns with the biorefinery concept. The biorefinery concept meets the vision of a sustainable economy using of bioresources to obtain bio-based products, such as biofuels, energy, and several other value-added chemical commodities (1, 3), with the goal of reducing costs, improving the competitiveness of new products, and exploiting the potential of some industrial residues (4). According to Ferreira (5), the biorefinery approach includes the development of new processes and technologies, including pretreatment of feedstock, and biorefineries can be classified based on the feedstock or technology, e.g., lignocellulosic and marine biorefineries, biochemical and thermochemical biorefineries and advanced biorefineries. A biorefinery can be classified as first or second generation with respect to the feedstock. First-generation biorefineries use edible crops for simple processes, such as biodiesel production from vegetable oils and bioethanol production from energy crops. However, these crops are available only in certain seasons, and spontaneous degradation can be an issue during long-time storage, resulting in unsustainable processes. Second-generation biorefineries use non-edible feedstocks, such as agricultural, industrial, zootechnical, fishery and forestry biowaste, as the main feedstock, which can result in sustainable production of energy and chemicals; however, the processes are usually more complex, resulting in higher operational costs (6). The agricultural sector generates approximately 140 billion tons of biomass every year worldwide, and a considerable portion is recognized as waste and does not conflict with food availability, such as leaves, roots, stalks, bark, bagasse, straw residues, seeds, wood and animal residues (2, 7, 8). Lignocellulosic residues are the most abundant source of unutilized biomass, and their use has recently gained worldwide attention (1, 3, 5). Several lignocellulosic agroindustrial residues have been studied as an attractive source for the extraction of low-cost and high-performance cellulose and nanocellulose due to their specific useful features, such as their high availability and renewable sources (9, 10). Compared to wood, which is the conventional cellulose source, lignocellulosic residues have lower lignin contents, which is an interesting characteristic and makes this residue an attractive source for the extraction of cellulose and its derivatives. To obtain cellulose and nanocellulose, a pretreatment of lignocellulosic biomass is necessary, and the effect of this pretreatment has been described as a disruption of the cell-wall matrix, including the connections among cellulose, hemicelluloses and lignin (11), which usually requires an excessive amount of reagents and generates a large amount of effluents. 228
The use of reactive extrusion to extract cellulose and nanocellulose is innovative, and there are few reports in the literature on the use of this technology for the extraction of cellulose and nanocellulose. Extrusion technology is a high-temperature, short-duration process with the advantages of a high versatility and absence of effluents (12), and it can be used to extract cellulose from lignocellulosic residues with less effluent generation than conventional methods (13). This chapter discusses the use of lignocellulosic residues to obtain cellulose and nanocellulose via reactive extrusion and the challenges and advantages of using this technology that is already largely used for food and plastics production.
Lignocellulosic Biomass from Agroindustrial Residues Agricultural-based industries produce vast amounts of lignocellulosic residues every year, and the release of these materials to the environment without proper disposal procedures can result in environmental pollution and harmful effects on human and animal health. Most of the agroindustrial residues are untreated and underutilized; therefore, in most reports, they are disposed of either by burning, dumping or unplanned landfill dispoal (14). Globally, approximately 1.5x10 (11) tons of lignocellulosic biomass are typically derived from agricultural wastes once a year (3, 8). Agroindustrial residues can be classified into two groups: agriculture residues and industrial residues. Agriculture residues can be further divided into (1) field residues, which are present in the field after the process of crop harvesting (e.g., leaves, stalks, seed pods, and stems) and (2) process residues; i.e., residues that are present after the crop is processed into alternate valuable resources, such as molasses, husks, bagasse, seeds, leaves, stem, straw, stalk, shell, pulp, stubble, peel, and roots, and used for animal feed, soil improvement, fertilizer, manufacturing, and various other processes. Industrial residues are derived from food processing industries, such as juice, chips, meat, confectionary, oil and fruit industries, and include potato peel, orange peel, cassava peel, and soybean oil cake among others (14). According to Zuin and Ramin (2), based on their available volume, low costs locally and globally, and their structures and chemical heterogeneity, all agroindustrial residues can be considered promising renewable resources to obtain new materials and energy sources; thus, research and development on new strategies to obtain high value-added chemicals from these residues can minimize the volume of non-renewable materials used today, which can greatly reduce greenhouse gas emissions and dependence on non-sustainable resources. Typically, most agroindustrial lignocellulosic residues consist mostly of three components, cellulose (30-60 %), hemicelluloses (10-40 %) and lignin (4–30 %) (3, 15–17), which are intermeshed and chemically bonded by non-covalent forces and covalent cross linkages (18). Cellulose and hemicelluloses are structural polysaccharides, and the ratio of cellulose:hemicelluloses commonly varies between 2:1 to 1:1 (16). The composition of lignocellulosic biomass varies depending on the plant species, age, and position on the stem of the plant and 229
the soil, climatic conditions and management to which the plants were subjected during their growth (19). Lignocellulosic biomass has been used as a combustion fuel source for heat and raw materials for pulps, papers, fabrics, and additives for food and plastics, and recently, it has gained attention for the production of biofuels (20). Agroindustrial lignocellulosic residues can also be used for the extraction of cellulose, which has several interesting properties, such as a high stiffness and resistance and high thermal, chemical and physical stability, and these properties allows cellulose to be applied in different processes under different operating conditions (21). In addition, there is interest in obtaining nanocellulose from cellulose. The term nanocellulose refers to cellulosic materials originating from different lignocellulosic sources that have nanometric-scale dimensions (22).
Cellulose and Nanocellulose from Agroindustrial Residues Cellulose is the major structural component of plant cell walls, and it is responsible for the mechanical strength of plant cells (17). Cellulose is a highly stable linear polymer composed of β-D-glucopyranose units linked by β-1,4 glycosidic linkages. The long cellulose chains form elemental fibrils that are linked together by hydrogen bonds and van de Waals forces (17, 23). Cellulose is a semi-crystalline polymer, and in its supramolecular organization, cellulose molecules appear as bundles of microfibrils with highly ordered regions, called the crystalline fraction. These regions alternate with less ordered regions, known as the amorphous fraction, which are more accessible to attack by reagents, enzymes or even the absorption of water (3). Cellulosic fibers have widths ranging from 5 to 20 μm and lengths in the range of 0.5 to several mm, and these fibrils are attached to each other by hemicelluloses and covered by lignin. Over the past decades, natural cellulose materials have been used as an energy source for building material, paper, textile, clothing, pharmaceutical and food industries (24). Hemicelluloses are the second most abundant polysaccharide group in plants, and depending on the tissue, hemicelluloses can be defined as cell-wall polysaccharides that are insoluble in water but can be extracted with aqueous alkali and hydrolyzed into component monosaccharides with diluted sulfuric acid (H2SO4). Hemicelluloses are grouped into four classes according to the main types of sugar residues present: xyloglucans, xylans, mannans and mixed-linkage β-glucans (16). Hemicelluloses are linked to cellulose and lignin via hydrogen and covalent bonding, respectively, and they are less crystalline than cellulose because of non-uniformity and the presence of side groups (3, 17). Lignin is generally the most complex and smallest fraction of lignocellulosic biomass (17). Lignin is composed of three major phenolic components: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, and it is synthesized by polymerization of these components. This ratio varies between different plants, wood tissues and cell-wall layers (3). Lignin is theoretically considered an adhesive, holding cellulose and hemicelluloses together (21), and it is linked to both hemicellulose and cellulose to form a physical seal that is an impenetrable 230
barrier in the plant cell wall. Lignin is present in the cellular wall to provide structural support, impermeability and resistance against microbial attack and oxidative stress (23). Because of the structural arrangement of the cellulose-hemicellulose-lignin complex and the variations among these three components depending on the plant species and other climatic and management conditions, the extraction of cellulose and conversion of lignocellulosic biomass into higher value-added products such as fine chemicals or biofuel is a challenge. Usually, these processes require multi-step processing that includes a (i) pretreatment (mechanical, chemical, biological, or combination), (ii) enzymatic hydrolysis and (iii) fermentation process (17). These pretreatments are necessary because they disrupt the cellulose-hemicellulose-lignin complex, resulting in a more accessible material for enzymatic attack or a cellulose-rich material that can be used after further purification (8). The heterogeneity of lignocellulosic biomass makes it very difficult to process, and it is very complicated to generalize pretreatment systems and the ideal equipment for them (25). According to Menon and Rao (3), most cellulose in nature is unsuitable for extraction or bioconversion unless effective and economically viable procedures (pretreatments) are developed to remove hemicelluloses and lignin. The main obstacles in the existing pretreatment processes include insufficient separation of cellulose and lignin, formation of by-products that inhibit fermentation, high use of chemicals and/or energy, and considerable waste production. It is important to highlight that wood is currently the most important industrial source of cellulose; however, non-wood plants are receiving increasing interest as a source of cellulose because they generally have a lower lignin content than wood. Thus, delignification and purification processes could be easier and less harmful (26), resulting in processes with less pollution. When lignocellulosic residues are used to obtain cellulose, the first step consists of extracting the cellulose from the lignocellulosic complex via delignification techniques and pretreatments without destroying the cellulosic fibers. In this way, it is possible to selectively separate the lignin and hemicelluloses, removing them from the fiber by chemical, thermal, physical, and biological methods or combinations thereof (19). Several techniques and pretreatments can be employed for this purpose. According to Menon and Rao (3), a mechanical process is generally required for size reduction. The most employed chemical pretreatments of lignocellulosic residues are based on the use of acid or alkaline reagents. An acid pretreatment consists of the use of concentrated or diluted acids to break the rigid structure of the lignocellulosic material and remove hemicelluloses, and the most commonly used acid is dilute sulfuric acid (H2SO4). Alkaline pretreatment generally involves the use of sodium hydroxide (NaOH) and results in a disruption of lignin from the biomass, increasing the accessibility of cellulose and hemicelluloses to enzymes. The combination of both acid and alkaline treatments in a sequence results in the removal of hemicelluloses and lignin, respectively, resulting in relatively pure cellulose. According to Ng and coworkers (27), an alkaline pretreatment results in the depolymerization of part of the lignin and hydrolysis of the hydrogen bonds 231
between the -OH groups of the hemicelluloses, and thus, it is an effective method for the removal of lignin and hemicelluloses. The pretreatment also partially removes the impurities present on the surface of the fibers, such as pectins, waxes, and oils. Peracids are another group of chemical reagents that are being studied for delignification and cellulose pulp bleaching and have shown good results in terms of resistance, indicating low levels of cellulose degradation. Peracetic acid can be prepared by the oxidation of acetic acid by hydrogen peroxide, and it is considered a highly selective delignification agent due to its capacity to oxidize structures rich in electrons, such as the aromatic rings of lignin. In the treatment of a pulp with peracetic acid, opening of the aromatic ring makes the oxidized lignin more hydrophilic, contributing to its solubilization in the bleaching liquor. In addition, the formation of acid groups also favors the solubilization of fragments of lignin, especially during the alkaline extraction stages (28). Paschoal and coworkers (29) and Nascimento and coworkers (30) reported that the use of peracetic acid can preserve the cellulosic fibers used for nanocellulose production. Lindström (31) reported that nanocellulosic materials have attracted immense interest from the research community, governmental bodies and industry for several decades due to their interesting properties. Nanocellulose is defined as cellulose material smaller than 100 nm in at least one dimension (32, 33), and bacterial cellulose (BC), cellulose nanocrystals (CNCs) and nanofibrillated cellulose (NFC) can be classified as nanocellulosic materials (34). The CNC morphology generally depends on the source of cellulose, but CNC fibers generally have diameters of 3-35 nm and lengths of 200-500 nm (26). In contrast to straight CNC, NFC consists of long, flexible and entangled cellulose nanofibers with diameters of approximately 10-100 nm and lengths on the micrometer scale, and they consist of alternating crystalline and amorphous domains (34, 35). The isolation of nanocellulose from lignocellulosic materials generally occurs in two stages; i.e., a pretreatment involving the removal of hemicelluloses and lignin, resulting in the isolation of cellulose, and a chemical or physical treatment to obtain the nanocellulose (33). The first report of the production of colloidal suspensions of cellulose crystals employing a controlled acid hydrolysis with H2SO4 was in 1949 (36). Sulfuric acid degrades the amorphous regions of cellulose and leaves the crystalline regions, which have a higher resistance to acid attack, intact. Rod-like, rigid CNCs with sulfate groups on their surface are obtained (26). CNCs have been produced by several authors employing several lignocellulosic sources, and the most reported method to obtain CNCs is still based on controlled acid hydrolysis with H2SO4. CNCs have been prepared from soybean hulls (37), wheat straw (38, 39), rice straw (40, 41), coconut fibers, branch mulberry bark (42), sweet potato residue (43), corncob (44), banana pseudosteam (45), barley straw and husk (46), sugarcane bagasse (47), and garlic straw residues (48). The typical protocol consists of several stages after the acid hydrolysis with sulfuric acid (generally employed at a concentration of 64 %), including repeated washing steps with centrifugation and dialysis against distilled water to remove 232
remaining free acid molecules. Then, a mechanical treatment such as sonication is required to disperse the CNCs in a uniform and stable suspension (33). The process is complex and requires many days of work. Also, a large amount of effluents is derived from the washing steps, and the yield can vary widely. The first report in the literature on NFC production was in 1983, and the authors reported a new form of cellulose obtained by mechanical action under heat (49). Several mechanical processes to obtain NFC have been reported in the literature, including conventional processes, such as high-pressure homogenization, grinding and microfluidization, and non-conventional processes, such as extrusion, steam explosion, ball milling, cryocrushing and high-intensity ultrasonication. These processes are efficient in disintegrating the cell wall; however, they require intensive mechanical treatments and high-energy consumption due to the cohesion of the cell wall. The use of conventional mechanical treatments without a pretreatment results in inhomogeneous materials that may contain a major fraction of poorly fibrillated fibers from a high-cost production. This can be solved by combining several sequential chemical and mechanical treatments (31, 50–52). Zimmerman and coworkers (53) applied an acid-hydrolysis step with H2SO4 before pumping the sulfite pulp through a homogenizer to obtain NFC with finer fibril structures with diameters below 50 nm and lengths in the micrometer range. Janoobi and coworkers (54) reported the production of NFC using a combination of chemical treatments (NaOH-anthraquinone followed by three-stage bleaching processes) and mechanical treatments (refining, cryocrushing and high-pressure homogenization), resulting in nanofibers 10-90 nm in diameter with lengths in the micrometer range. Qua and coworkers (55) reported the production of NFC derived from microcrystalline cellulose and flax fibers using acid hydrolysis combined with the application of ultrasound and high-pressure microfluidization. The resulting nanofibers were less than 25 nm in diameter. Paschoal and coworkers (29) obtained NFC from oat hulls via bleaching with peracetic acid, and they reported that this type of bleaching is effective in the removal of hemicelluloses and lignin but preserves cellulosic fibers. Also, this is an environmentally safe alternative for bleaching because it is a chlorine-free reagent. The bleached oat hulls were submitted to acid hydrolysis with H2SO4 followed by high-intensity ultrasonication to produce NFC with diameters ranging from 70 to 100 nm. Deepa and coworkers (56) used banana fibers to obtain NFC, and they initially treated the fibers with NaOH (2 %) under heat (110-120 °C) in an autoclave, and after neutralization, the fibers were submitted to steam explosion using a mixture of NaOH or acetic acid and sodium hypochlorite for 1 hour six times. Finally, the steam-exploded bleached fibers were treated with oxalic acid for 3 hours in an autoclave. Cherian and coworkers (57) and Abraham and coworkers (58) also reported the combination of steam explosion and chemical treatments to obtain NFC from pineapple leaf fibers (5-60 nm in diameter) and coir fibers (5-50 nm in diameter). Tuzzin and coworkers (59) produced NFC (20-70 nm in diameter) from tobacco stem waste by steam explosion followed by pulp bleaching with several chemical reagents and refining. 233
Boufi and Ckaher (52) reported the production of NFC from corn stalks by combining a pretreatment with sodium hypochlorite and acetic acid with a 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation with mechanical disintegration for 30 minutes using a high-speed blender. Valdebenito and coworkers (9) combined a chemical pretreatment based on TEMPO-mediated oxidation and a mechanical treatment carried out with a high-pressure homogenizer to obtain NFC from corn husks and oat hulls with diameters less than 20 nm and lengths on the micrometer scale. Nascimento and coworkers (30) produced NFC from rice hulls and reported that a previous treatment with NaOH was necessary to remove silica from the raw material. This was followed by a bleaching step with peracetic acid. Then, the hulls were submitted to acid hydrolysis with H2SO4 followed by high-intensity ultrasonication to produce nanofibers with diameters less than 100 nm. According to Abdul Khalil and coworkers (60), NFC can be produced on a large industrial scale, which is an advantage compared to BC or CNC, but there are still challenges to produce NFC on a commercial scale. The high need for mechanical energy is a major obstacle, but the high-energy consumption could be reduced with the selection of an appropriate chemical or enzyme pretreatment process (52).
Reactive Extrusion To Obtain Cellulose and Nanocellulose from Agroindustrial Residues Reactive extrusion is not a recent process, and it is a versatile processing technique that has advanced well beyond its original use in plastics processing. Reactive extrusion has been used for the chemical modification of existing polymers, and it consists of using an extruder (single or twin-screw) as a continuous chemical reactor (61). Reactive extrusion can provide a unique continuous reactor environment through a combination of thermo-mechanical and chemical pretreatments of lignocellulosic biomass at higher throughput and solid levels (62, 63). The extrusion process includes heating, mixing, and shearing, resulting in physical and chemical changes during the passage through the extruder (64), and when the extruder is employed as a chemical reactor, additional functions are added, such as the development and control of a chemical reaction (61). Compared to a classical batch process in solution, reactive extrusion presents several advantages: (1) the reaction is conducted with a lower quantity of solvents or in the absence of any type of solvent, resulting in a reduction in the generation of effluents; (2) an extruder can work with very viscous products, which is not the case for batch reactors; (3) the processing conditions are much wider and more flexible in an extruder; and (4) the reaction times are shorter, which results in lower production costs (61). According to Gibril and coworkers (64), extrusion can be considered an environmentally safe and scalable route for polymer modification because, generally, no hazardous effluents are generated. Extrusion is largely used in the food and plastics industry; however, considering the structural integrity and complexity of lignocellulosic materials, 234
extrusion of these materials is more complex and requires that processes be developed and standardized for each type of material. In addition, compared to food materials and plastics, lignocellulosic materials do not melt, even at high temperatures, and show poor flow properties inside the extruder barrel and die (63, 65). The screw speed and barrel temperature inside the extruder possibly result in a disruption of the lignocellulose structure, causing fibrillation and shortening of the fibers. Also, when the material passes through the extruder barrel, a high pressure develops, and when the extruded material comes out of the die, it is experiences low pressure and explodes. This causes the cellulose and hemicelluloses to be more susceptible to hydrolysis (66). The reactive extrusion process has been explored by several researchers for the pretreatment of biomass with satisfactory results. Yoo and coworkers (63) reported that the cellulose conversion from an extrusion pretreatment of soybean hulls was comparable or better than that obtained from traditional chemical pretreatments utilizing acid and alkali components. The use of reactive extrusion as a pretreatment for lignocellulosic materials was first investigated in the 1980s for crop residues, sawdust and municipal solid waste in the presence of dilute H2SO4 (66). Recently, Lee and coworkers (67) used wood biomass to obtain cellulose fibers with diameters ranging from 100 nm to 1 μm employing reactive extrusion as a pre-saccharification treatment. They employed additives with a cellulose affinity (ethylene glycol, glycerol, and dimethyl sulfoxide) to fibrillate the wood cell wall into submicron or nanoscale size components, opening up the cell-wall structure to improve enzymatic accessibility. Lamsal and coworkers (62) reported that extrusion can be used as a pretreatment prior to saccharification of lignocellulosic residues, and they worked with wheat bran and soybean hulls. These authors reported that the combination of a lower temperature and high residence time (low screw speed) or higher temperature and low residence time (high screw speed) led to higher sugar yields; however, the use of a chemical treatment (NaOH, urea, and thiourea) in combination with extrusion did not result in an improvement in the hydrolysis of the lignocellulosic substrates. They reported that washing the extruded samples was critical in maximizing the reducing sugar yields. Fan and coworkers (68) compared two methods for the pretreatment of corn cobs (reactive extrusion with dilute acid and acid hydrolysis). The authors verified that the reactive extrusion process provided a better recovery for xylose and cellulose conversion. In addition, reactive extrusion increased the efficiency of enzymatic hydrolysis. Coimbra and coworkers (69) reported that the use of an alkaline extrusion for lignocellulosic residues generates a material with a lower proportion of lignin and a larger proportion of cellulose than the untreated residues. The reactive extrusion process for the extraction of cellulose and nanocellulose is innovative, attractive and recent, and there are few reports in the literature describing the extraction of cellulose or nanocellulose from lignocellulosic materials using this process. It is believed that the screw speed and 235
temperature lead to the disruption of the structure of the lignocellulosic complex, defibrillation and greater access to cellulose (63, 70). Reactive extrusion can be effective for the deconstruction of lignocellulosic biomasses, and extrusion in the presence of an alkaline medium allows destruction of the cell-wall polymers, ensuring chemical degradation by solubilization of organic matter, especially for hemicelluloses and lignin. However, the residence time is too short to obtain direct significant saccharification (71), preserving the cellulose fibers, but the results can differ depending on the biomass source. According to Berzin and coworkers (72), lignocellulosic materials processed by reactive extrusion are subjected to a reduction in their diameter due to separation of the bundles and their length due to breakage, and these changes are related to the extrusion processing conditions, such as the screw profile, barrel temperature, screw speed, and feed rate. Hanna and coworkers (73) described in a United States Patent (US 6,228,213 B1) the production of microcrystalline cellulose by reactive extrusion employing lignocellulosic residues, and this process was based on the use of a basic solution during extrusion, followed by acid hydrolysis in the extruder. Yano and coworkers (74) in a Japanese patent described the nanofibrillation of wood biomass using water and cellulose-swelling agents. Heiskanen and coworkers (75) also reported in a United States Patent the preparation of NFC (~20 nm in diameter and lengths between 100 nm and 10 μm) from wood cellulose fibers, agricultural raw materials or waste products by reactive extrusion. The raw material has to be submitted to a pretreatment before extrusion, which can be an enzymatic pretreatment, to obtain materials with higher solid contents (up to 50 %). The obtained material can also be chemically modified. Ho and coworkers (76) reported the use of twin-screw extrusion to obtain microfibrillated cellulose from needle-leaf bleached kraft pulp. The pulp sheets were extruded 1 to 14 times at a low temperature (< 40 °C). They observed the cellulosic material after 10 - 14 passes through the extruder, and after 10 passes through the twin-screw extruder, the cellulose fiber diameters reached values of approximately 6 μm. They concluded that kraft pulp with a 28 % solid content can be disintegrated into high-quality, fibrillated cellulose fibers with 33 - 45 % solid content. Baati and coworkers (77) described the production of NFC from Eucalyptus grandis wood via several delignification steps with NaOH and sodium hypochlorite, followed by TEMPO-mediated oxidation. Then, the oxidized holocellulose pulps were processed by continuous extrusion using a twin-screw, resulting in fibrils with diameters of approximately 3 to 7 nm and lengths on the micron scale. Rol and coworkers (78) described the production of NFC at a high solid content (20 %) from eucalyptus pulp using an enzymatic pretreatment or TEMPO oxidation followed by 7 passes through a twin-screw extrusion process. They reported that the obtained NFC has properties similar to those of commercial NFC, i.e., nanometric size (20-30 nm in diameter), good mechanical properties, and transparency. Our research group has been working in recent years on the extraction of cellulose by reactive extrusion employing lignocellulosic residues. In a previous 236
work (13) it was employed reactive extrusion to obtain microcrystalline cellulose from soybean hulls. Microcrystalline cellulose was obtained after a two-step extrusion process. In the first step, the hulls were extruded in a single-screw extruder with NaOH, which was followed by an extrusion step with H2SO4, and after each extrusion step, all samples were washed with distilled water and neutralized until pH 5-6. The obtained material was composed of short and rod-shaped fibers with a cellulose content of 84 % and a crystallinity index of 70 %. Reactive extrusion proved to be an alternative and effective method for the production of microcrystalline cellulose from lignocellulosic residues and has the advantages of simplicity and less pollution than conventional methods. More recently, our research group has been working on the development of simple and less polluting processes based on reactive extrusion to produce NFC from different lignocellulosic residues using a single-screw extruder. Some previous results employing soybean hulls (data not published) were very promising, and one of these protocols was based on Merci and coworkers13 with one more extrusion step consisting of acid hydrolysis followed by an ultrasonication step. The hull samples were processed in three steps by reactive extrusion, i.e., the first step with NaOH and two sequential extrusion steps with H2SO4, and after each extrusion step, all samples were washed with distilled water and neutralized until pH 5-6. Then, they were dried in a ventilated oven at 40 °C and milled to yield particles
