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The fast and effective isolation of nanocellulose from selected cellulosic feedstocks
Accepted Manuscript Title: The fast and effective isolation of nanocellulose from selected cellulosic feedstocks ˇ Author: Matjaˇz Kunaver Alojz Anˇzlovar Ema Zagar PII: DOI: Reference:
S0144-8617(16)30444-1 http://dx.doi.org/doi:10.1016/j.carbpol.2016.04.076 CARP 11016
To appear in: Received date: Revised date: Accepted date:
16-11-2015 15-4-2016 16-4-2016
ˇ Please cite this article as: Kunaver, Matjaˇz., Anˇzlovar, Alojz., & Zagar, Ema., The fast and effective isolation of nanocellulose from selected cellulosic feedstocks.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.04.076 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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THE FAST AND EFFECTIVE ISOLATION OF NANOCELLULOSE FROM
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SELECTED CELLULOSIC FEEDSTOCKS
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Matjaž Kunaver1,*, Alojz Anžlovar1, Ema Žagar1
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1
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Hajdrihova 19, 1000 Ljubljana, Slovenia, Tel: +38614760363, Fax: +38614760300
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*
National Institute of Chemistry, Laboratory for Polymer Chemistry and Technology,
Corresponding author: Matjaž Kunaver E-mail: [email protected]
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Abstract
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A new process for the production of nanocellulose from selected cellulose-containing natural
9
materials has been developed. The liquefaction reaction, using glycols and mild acid catalysis
10
(methane sulphonic acid), was applied to four model materials, namely cotton linters, spruce
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wood, eucalyptus wood and Chinese silver grass. The process contains only four steps, the
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milling, the glycolysis reaction, centrifugation and final rinsing with an organic solvent. The
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nanocrystalline cellulose recovery was between 56% and 75%. The crystallinity index was greater
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than that of the starting materials due to the liquefaction of lignin, hemicelluloses and amorphous
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cellulose. The final product was a stable, highly concentrated nanocrystalline cellulose suspension
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in the organic medium. The liquid residue, after the liquefaction of the cotton linters contained
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significant quantities of levulinic acid. Different sugars were identified in the liquid residues that
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were derived from cellulose and hemicelluloses during the liquefaction reaction.
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1. Introduction
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Cellulose containing biomasses have become possible alternatives to fossil resources for
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chemicals manufacture and for fuel production, cellulose being the most abundant polymer in
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nature. Cellulosic resources include agricultural residues (wheat straw, corncobs, sugarcane
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bagasse, corn stover), forest residues and energy crops (hardwood and softwood sawdust, silver
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grass), municipal and industry wastes (waste paper waste cotton textiles and waste timber-based
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materials). Low cost cellulosic materials represent a feasible option for the production of
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materials that carry a higher added value.
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Most of the low-value lignocellulosic biomasses consist of cellulose, hemicelluloses and lignin
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(Fengel and Wegener 1989). The inner structure contributes to the hydrolytic stability and
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structural robustness. Cellulose, as a bulk material, acts as a framework, based on highly oriented
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cellulose fibrils. Cellulose is complex in its chemical composition and its intra- and
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intermolecular hydrogen bonding. Within the cellulose fibrils there are highly ordered regions,
1
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less well ordered regions and relatively highly disordered regions. The ordered (more crystalline)
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regions can be extracted, resulting in the creation of nanocrystalline cellulose (NCC).
34
The crystallinity index of cellulose has been used to describe its relative amount of crystalline
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material and has been determined by several methods (Terinte et al., 2011). In general, the
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crystallinity index varies from 33 to 60 (Thygesen et al. 2005). Nanocrystalline cellulose, used as
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a reinforcing material in nanocomposites, has been the subject of research worldwide (Habibi et
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al. 2010). Besides its low cost, renewability, light weight and high aspect ratio, NCC has a high
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modulus of elasticity and considerable tensile strength. These are the reasons for its incorporation
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into many promising synthetic and natural polymeric matrices (Khalil et al. 2012), for its use in
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different packaging material applications (Ridgway and Gane 2012), as well as in automobile
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transportation and drug delivery applications (Lam et al. 2012).
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The isolation of nanocellulose has been studied by many authors since the first isolation of
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cellulose nanofibrils (Turbak et al. 1983). Favier et al. (1995) have reported on the properties of
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nanocomposites containing cellulose whiskers, from which time the number of published results
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related to nanocellulose isolation, its use in different composites and in different applications has
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grown annually(Klemm et al. 2005).
48
Several processes have been developed for the NCC isolation. These include:
49
•
Mechanical pulping, mostly of wood (Chakraborty et al. 2005),
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•
Chemical pulping, where lignin is removed by a Kraft process (Jiang and Hsieh 2013),
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•
Steam explosion, resulting in the breakdown of the biomass structure, the hydrolysis of hemicelluloses and lignin and defibrillation (Abraham et al. 2011),
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•
Acid hydrolysis with sulphuric acid, hydrochloric acid and, more seldom used, phosphoric acid and hydrobromic acid (Brinchi et al. 2013),
54 55
•
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cellulose (Kos et al. 2014 and Li et al. 2011).
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The most common method of choice of processing towards NCC isolation is acid hydrolysis.
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Typically, the cellulose fibers are subjected to strong acid hydrolysis under the controlled
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conditions of temperature, acid concentration, agitation and time. Kos et al. (2014) applied
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microwaves to shorten the reaction time of the acid hydrolysis process to only ten minutes. Also,
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by varying the sulphuric acid concentration, they were able to control the NCC average
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dimensions. Experiments dealing with the isolation of NCC from different natural sources and
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details outlining experimental conditions have been published in recent years. Thus, Dai et al.
Microwave assisted hydrolysis or high intensity ultrasonication of microcrystalline
2
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(2013) isolated nanocellulose from hemp fibers using dodecyltrimethylammonium bromide and
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ultrasonic agitation. Johar et al. (2012) processed rice husks, using an alkaline pre-treatment,
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coupled with bleaching and acid hydrolysis steps. Fan and Li (2012) have studied the influence of
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the reaction temperature, the time and the sulphuric acid concentration on isolated yields of NCC
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from cotton pulp fibers. Mandal and Chakrabarty (2011) prepared NCC from waste sugarcane
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bagasse by delignification, the removal of hemicelluloses and sulphuric acid-based hydrolysis.
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Nascimento et al. (2014) used a similar process for the isolation of NCC from white coir. Also, Li
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et al., (2011) used a similar process for the isolation of NCC from bleached softwood Kraft pulp.
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The use of different acids (hydrochloric acid (Yu et al. 2013), phosphoric acid (Espinosa et al.
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2013), hydrobromic acid (Sadeghifar et.al 2011) and p-toluenesulphonic acid (Anžlovar et.al.
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2016) is quoted in the literature, but mostly aqueous sulphuric acid solutions, in concentrations
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above 60%, have been used. Although very little information concerning yields obtained is
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available, most of these authors have determined that the increase in crystallinity index of
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products have been significant, namely from 77 for cotton fibers to 91 for the isolated NCC
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(Teixeira et al. 2010); the average dimensions of the nano crystals being between 250 nm and 600
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nm in length and from 4 nm to 70 nm in diameter.
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Although attempts to convert cellulose into useful chemicals, in an aqueous medium, have been
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made over an extended period, the liquefaction of lignocellulosic biomass has been known for an
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even longer period. Liquefied lignocellulosic materials have been considered to be an important
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feedstock for the creation of polymeric blends and derivatives/composites, options having been
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described by several authors including Kunaver et al. (2010); Lin et al. (1994), and Hassan and
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Shukry (2008). The products have been used as polyols in polyurethane reactions (Chen and Lu
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2009), as adhesives (Kunaver et al. 2010a; Juhaida et al. 2010) and as a fuel for gas turbines
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(Seljak et al. 2012).
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This paper concerns the process of preparing NCC from different natural sources by a
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liquefaction reaction, using glycols as the main reactant and an acid catalyst in low concentration.
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Here, during the one step reaction, lignin, hemicelluloses and the more disordered components of
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the cellulosic fibers are liquefied, only the crystalline cellulose remaining as a solid residue.
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Compared to the well-known acid hydrolysis-based procedure, a loading of only 3% of acid was
93
used. The yields, crystallinity index and morphology of thus produced NCC were comparable to
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those NCC products that were described in recently published papers.
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2. Materials and methods
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2.1. Materials 3
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Cellulose linters and wood sawdust were provided by GGP (Gozdno Gospodarstvo Postojna,
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Slovenia). Methane sulfonic acid was supplied by Arkema, France. All of the other reagents were
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supplied by Sigma-Aldrich (>98 %, GC, Reagent Plus). The Chinese silver grass (Miscanthus
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sinensis) was obtained from the experimental plantation of Petrović U., Ljubljana, Slovenia. The
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Eucalyptus wood chips (Eucaliptus globulus) were provided by Agronelli Agroindustria, Uberaba
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Brasil. The samples were dried to 10 % (w/w) of water content in a laboratory oven and milled to
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2 mm particles using a RETSCH SM-2000 mill in conjunction with a 2 mm grinding mesh. Wood
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sawdust was dried in a laboratory oven to 10 % (w/w) in its water content and sieved through 5
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mm mesh sieve. The chemical composition of cellulose-containing materials was determined
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according to standard methods (TAPPI T222 om-11, TAPPI T211 om-12, TAPPI T204 cm-07,
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TAPPI T207 cm-08, TAPPI T203 om-09, TAPPI T223 cm-10). The composition of materials that
109
were used in these experiments is presented in Table 1.
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Table 1. The composition of the materials and their crystallinity index.
Biomass
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Hemicelluloses Cellulose (%) (%)
Lignin (%)
Extractive s (%)
Ash (%)
Crystallinity index (CrI)
Cotton linters
7.1±0.7
81.3±4.2
2.5±0.4
6.3±0.9
2.8±0.1
64±7
Spruce wood
29.6±2
42.3±3
26.9±5
0.9±0.9
0.3±0.1
47±5
Chinese silver grass
33.9±4.4
47.1±6.0 10.5±4.7
5.6±1.3
2.9±0.5
46±7
Eucalyptus wood
26.9±1.8
44.3±3.1 26.1±2.0
2.3±0.2
0.4±0.2
57±4
Data reported are on a dry matter basis.
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2.2. Biomass liquefaction and NCC isolation
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In each case, the diol or glycerol or a mixture of the two types, (300 g) and methane sulphonic
115
acid (9 g) were placed into a 1000 cm3 three-necked reactor, equipped with mechanical stirring.
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The mixture was heated to 150 oC and was stirred constantly. After the milled biomass had been
117
added to the preheated reaction mixture, the liquefaction process was carried out for 180 minutes.
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The polyhydroxy alcohols used in the liquefaction reaction were diethylene glycol, ethylene
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glycol, glycerine and mixtures of these alcohols. Typically, a ratio of 4:1 between the glycol (or
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glycerol) and the biomass was used. 3% of methane sulfonic acid was added (calculated on the
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amount of the glycol). The reaction temperature was maintained at 150 oC. These reaction 4
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conditions were chosen according to the previous experience in the liquefaction of wood and
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similar biomasses (Jasiukaitytė et al. 2012; Kunaver et al. 2012).
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The reaction mixture was cooled to room temperature, diluted 1:1 with 1,4-dioxane and
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centrifuged at 8000 rpm for 20 minutes. The sediment was dispersed in 1,4-dioxane and
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centrifuged using the same conditions. The process of re-dispersion and centrifugation was
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repeated three to five times until a clear supernatant liquid was obtained. Finally, the sediment
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was dispersed in 1,4-dioxane, with additional sonication for 10 minutes (Ultrasonic Processor
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UP400S, Hielscher). The product was a 15% to 20% (w/w) suspension of NCC in 1,4-dioxane.
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A diluted suspension of NCC was centrifuged at 8000 rpm for 30 minutes to separate the
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nanocrystals from the liquefied products in the supernatant liquid. The yield, (Y) of the NCC
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isolation was calculated according to the mass of the solid residue, obtained after rinsing and
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centrifuging the products that were obtained after the liquefaction reaction, taking into account
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the crystallinity index of the initial cellulose.
Y [%] =
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m ⋅ 100 % Sc %Cell Cr I M⋅ ⋅ . 100 100 100
Equation 1
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Here, m denotes the mass of dry product, M denotes the initial mass of the original sample, %Sc is
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the initial solids loading in the reaction mixture, %Cell is the cellulose content in the starting
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material and CrI is the crystallinity index (%) of the starting material.
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2.3.
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2.3.1. X-ray diffraction (XRD)
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The NCC samples were characterized by wide-angle X-ray diffraction using a XPert PRO MPD
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diffractometer from PANalytical with a Cu anode as the X-ray source, at wavelength Cu Kα1:
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1.5406 Å. Diffractograms were measured at 25 oC in the 2-theta range from 5o to 40o, with a step
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of 0.04o and step time of 300 s. The crystallinity index was calculated according to the Segal
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method (Segal et al. 1959) from the ratio of the height of the 002 peak (Ioo2) and the minimum
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between 002 and the 101 peaks, subtracted from the background signal that was measured
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without cellulose. It should be noted that the Segal method suffers some inaccuracy due to the
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inaccurate amorphous peak position, and, consequently the influence of the amorphous material
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present in the sample is underestimated. Four crystalline peaks are shown. However, only the
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highest (002) peak is used in calculation. None-the less, this much simplified method is suitable
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for use when comparing the relative differences between samples. In order to obtain more reliable
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results for the crystallinity index, two additional methods were applied namely, the Segal
Characterization
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calculation using “the amorphous peak method, at 21o “ (Terinte et al. 201, Park et al. 2010) and
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the amorphous subtraction method, for which alkali lignin was used as the amorphous reference
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standard (Agarwal et al. 2011). The crystallite sizes were estimated by using well known Scherrer equation (Klug and
156 157
Alexander 1974):
Dhkl =
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Kλ βhkl ⋅ cos Θ
Equation 2
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Here, Dhkl is the crystallite size, K is the Scherrer constant (being 1 for needle-like crystals),
160
(Smilgies 2009), λ is the wavelength of the radiation used (0.15406 nm) and βhkl is the width at
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half-maximum of the diffraction peak angle of the (002) crystal plane. The βhkl peak fitting was
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performed prior to calculation of crystallite size.
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2.3.2. Dynamic light scattering (DLS)
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The average hydrodynamic diameter of the species in the aqueous NCC suspensions was
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determined by dynamic light scattering at 25 oC, using a Malvern Zetasizer Nano-ZS, (Malvern
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Instruments Ltd.) The instrument was calibrated using a Thermo Scientific Nanosphere Size
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standard. For the diameter values and for the size distribution measurements, the NCC
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suspensions were diluted by 100x with deionized water and sonicated for at least 5 minutes,
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(Ultrasonic Processor UP400S, Hielscher).
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2.3.3. Scanning Transmission Electron Microscopy (STEM) and Scanning Electron
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Microscopy (SEM)
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The microtopographies of NCC samples were observed by STEM. The STEM micrographs were
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taken on a Zeiss Supra 35 VP at an acceleration voltage of 20.0 kV and a working distance of
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4.5–5.0 mm, using a STEM electron detector. Samples for the STEM measurements were
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prepared by the drop-casting of a 100x diluted NCC suspension in acetone. Prior the drop-casting
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procedure, the NCC suspensions were sonicated for 10 minutes (Ultrasonic Processor UP400S,
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Hielscher). The same suspensions were applied to a glass substrate from which the acetone
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quickly evaporated. The dried glass support with its NCC particles on the surface was then coated
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with gold and used for SEM observations.
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2.3.4. Thermogravimetric analysis (TGA)
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TGA analyses of dry samples were performed on a Mettler-Toledo TGA/DSC 1 instrument. Each
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sample (5 mg) was heated at 10 oC/min from 30 oC to 800 oC, under N2 purging (50 mL/min).
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183
2.3.5. Fourier transform infrared spectroscopy (FTIR)
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FTIR analyses of dry samples were performed on a Perkin Elmer Spectrum 1 instrument using
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KBr technique. The dried NCC powders were embedded in KBr pellets and the absorption spectra
186
were recorded in the range of 4000 cm-1 to 400 cm-1. The FTIR spectra were used for the
187
determination of the crystallinity, as described by Nelson and O’Connor (1964). Here, the ratio
188
between the 1731 cm-1 absorption band and the 2900 cm-1absorption band gives the crystallinity
189
index of the sample.
190
2.3.6. Gas chromatographic measurements
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The amount of levulinic acid in the reaction mixture was determined using an Agilent 6890N gas
192
chromatograph, coupled with the mass selective detector, Agilent 5973N. Thus, 0.1 g of the
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sample was put into a 20 mL vial. 3 mL of the BF3/methanol reagent (Merck) was added and the
194
vial sealed. The vial was heated at 80 oC for 60 minutes. 3 mL of chloroform and 5 mL of water
195
were added and the mixture well shaken. The resulting methyl ester of levulinic acid was
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extracted into the chloroform layer, which was then used for the analysis.
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The hydroxyl group-containing components of the reaction mixture were analysed as trimethyl
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silyl ether derivatives. The etherification was performed according to the method described by
199
Esposito and Swann (1969).
200
Gas chromatographic analyses were performed using a DB-35MS column of 30 m in length and
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0.25 mm in I.D. The temperature program was 4 minutes, with an initial temperature of 50 oC,
202
then heating to 250 oC at 30oC/min with holding for 10 minutes at the final temperature. 2uL of
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the sample was injected with a Split 100:1 mode. Mass spectra were collected within the 33 – 550
204
mass range. The Wiley mass spectra library (Wiley 1999) was used for the identification of the
205
methyl ester of levulinic acid and of the trimethyl silyl ether derivatives of the hydroxyl group
206
containing components.
207 208
3. RESULTS AND DISCUSSION
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3.1. Nanocellulose yields
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First, the efficiency of the biomass liquefaction process, with respect to the yields of NCC
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obtained, was studied by changing the reaction temperature, the reaction time and the reagent
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composition. The choice of the polyhydroxy alcohol, the acid catalyst loading and the glycol to
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lignocellulosic material ratio that was used was that optimized by Jasiukaitytė et al. (2012). The
214
results show that there was little deviation in the yields of the NCC recovery across the changes 7
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made to the process parameters. The temperature of the reaction was 150 oC, while the ratio of the
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glycol to cellulose-containing-material was kept constant at 1:4. The choice of polyhydroxy
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alcohol selection did not influence significantly the yields of the reactions. The acid catalyst
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loading was kept constant and was not below 3%, calculated on the glycol content. The initial
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water that was present in the wood or other biomass materials evaporated within the first few
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minutes of the reaction, since the temperature was sufficiently high. All of the samples in this
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study were obtained through the same preparation route. The % recovery of the nanocrystalline
222
cellulose, the crystallinity index of the nanocrystalline cellulose and the average crystal width are
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presented in the Table 2. The yields are moderate and differ due to different cellulose content in
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the original material. Additionally, the cell structure might also influence the efficiency of the
225
reaction. That would explain the low yield that was obtained when the Chinese silver grass was
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liquefied.
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Table 2. The % recovery of NCC, the crystallinity index, the average crystal length and crystal width.
Biomass
Average length
(%)
Crystallinity index (CrI)a
of NCC(nm)b
Average NCC crystal width (nm)c
74.5±6.0
80%
242±8
12.7±0.4
Spruce wood
61.5±3.2
63%
235±23
8.9±0.1
Chinese silver grass
55.6±4.0
62.8%
250±17
8.9±0.2
Eucalyptus wood
63.0±8.5
66%
306±13
9.0±0.1
Cotton
NCC recovery
linters
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a
230
b
231
c
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The liquefaction temperature was lower than that previously reported in the published literature
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(Kunaver et al. 2010) in order that greater yields of NCC might be obtained. The reaction yields
234
decreased on increasing the reaction temperature to greater than 150 oC.
235
3.2. DLS measurements
The crystallinity index was calculated using the Segal 21-WAX method The length of the NCC crystals was evaluated from the SEM micrographs.
The width of the NCC crystals was estimated by using the Scherrer equation.
8
236
The results of the DLS measurements can be related to the Brownian motion of the nanoparticles
237
in the medium. The value given by this technique is the radius of a sphere having the same
238
diffusion coefficient as the rod-like NCC particles. The method is suitable for the rapid evaluation
239
of the NCC average particle size. However, correlations with data obtained from the STEM
240
micrographs were needed. The average lengths are within the range observed by Fan and Li
241
(2012) and by Brinchi et al. (2013).
242
The obtained average hydrodynamic diameter of the NCC particles was similar regardless to the
243
origin of the particles. The samples were dispersed in water prior the measurements, almost
244
immediate agglomeration being observed. This process was partly avoided by the
245
ultrasonification of each sample just prior the measurement taking place. 5 % to 8 % (by
246
intensity) of the aggregates was always detected, their average size being between 3000 nm and
247
4000 nm.
248
The average particle size for different time intervals during the reactions was also measured by
249
DLS. The results are shown in Figure 1.
250 251
Figure 1. Average hydrodynamic diameter of the NCC particles size vs. the reaction time (min)
252
used: eucalyptus (a), Chinese silver grass (b), cotton linters (c) and spruce wood (d).
253
In each instance, for each polysaccharide source, the relationship is almost linear, from 292 nm
254
after 60 minutes of reaction time for wood to 125 nm at the end of the reaction. Here, only nano-
255
sized particles were taken into consideration. It is evident from these results that during the
256
liquefaction reaction, the average particle size was reduced. The average particle sizes differ at 60
257
minutes of reaction time due to the differences in the particle sizes of the starting materials, 9
258
namely that the wood particles were in a shape of needles up to 5 mm length whilst the Chinese
259
silver grass and eucalyptus particles were taken as a mixture of dust and smaller particles, up to 3
260
mm in length. Here, the lignin and hemicellulose content might have influenced the speed of the
261
size reduction during the reaction. Longer reaction times and higher temperatures could result in
262
the complete dissolution of nanocellulose particles (Jasiukaitytė et al. 2012). Large aggregates
263
and cellulose fibrils that did not dissolve in the earlier stages of the reaction were identified in the
264
spruce wood distribution patterns. As the reaction progressed, the average hydrodynamic
265
diameter distribution became narrower and more uniform. Typical DLS hydrodynamic size
266
distributions are shown in Figure 2, the small peak at 4770 nm indicating the presence of large
267
aggregates.
268
269
Figure 2. Hydrodynamic size distribution of NCC isolated from selected cellulose sources at
270
different reaction times
271
When the NCC was dispersed in water, aggregation was immediately observed. The addition of a
272
suitable surfactant, such as Tween 85 helped to keep the suspension stable during the DLS
273
measurements although it was not possible completely to avoid the presence of a small amount of
274
aggregates.
275
3.3. SEM and STEM microscopy evaluations
276
Figure 3 shows the SEM micrograph of rod-like nanocrystalline cellulose particles, as derived
277
from cotton linters, Chinese silver grass, spruce wood and eucalyptus wood. The highly diluted
278
suspension was prepared in acetone, using ultrasonification. A drop of suspension was put on the
279
glass support from which the acetone evaporated. The length of the NCC fibers was 220 nm to
280
300 nm and their diameter 14 nm to 23 nm, the average aspect ratio being 15. Although small
10
281
aggregates and nanofiber-like structures were formed during the evaporation of the acetone, the
282
individual NCC particles were able to be observed and measured.
283
Evaluations of the NCC particles’ dimensions confirmed the values that were obtained from the
284
SEM micrographs. The values are also comparable to the results emanating from the DLS study.
285
The differences between the SEM derived results and the DLS derived measurements may arise
286
from the fact that the DLS-technique measures the hydrodynamic diameter of a sphere having the
287
same diffusion coefficient as the rod like NCC in suspension, while the SEM technique gives the
288
actual particle dimensions. The morphology of NCC particles differs across the samples. The
289
NCC crystals derived from the cotton linters are more separated while the NCC crystals from
290
other three samples form a structure that is similar to those of nanofibers. The length and the
291
width of all of the NCC samples is similar. The STEM micrograph representing the cotton linters
292
NCC sample is shown in Figure 1 S1 of the Supplementary materials.
293 294
Figure 3. SEM micrographs of NCC: cotton linters (a), chinese silver grass (b), spruce wood (c)
295
and eucalyptus wood (d).
296
3.4. X-ray diffraction measurements
297
The crystallinity index and the values of the average diameter of the NCC particles are presented
298
in Table 2, the corresponding diffractograms being shown in Figure 4. The diffractograms
299
display their main peak at 22.8o, being characteristic of cellulose I. The reported crystallinity
300
index of cotton linters is 64.4%, (Morais J.P.S. et al. 2013). The crystallinity index of the NCC
301
that was isolated from cotton linters was much greater (80%). This is because all of the disordered
302
sections would have been dissolved during the reaction while the more highly ordered crystalline 11
303
regions would be more resistant to attack by the reagents. The crystallinity Miller’s indices 1Ī0,
304
110 and 200 are parallel to those of the cellulose chains and are well defined in the X-ray
305
diffraction patterns. The crystallinity index was calculated using Segal 21-WAX method.
306
Although this method gives greater values than those obtained using other methods such as the
307
peak deconvolution method or the amorphous subtraction method, it is very convenient in its use,
308
particularly when comparing the products. Similar results were obtained with Chinese silver
309
grass, spruce wood and eucalyptus, with crystallinity index values in the range from 62.8% to
310
66%, although the crystallinity reflexions other than 200 were less explicit. These results show
311
lower values for the crystallinity index, most likely because the initial nanocrystalline cellulose
312
loading was much lower in the starting material, due to the presence of a certain proportion of the
313
amorphous nanocellulose as a nanofiber and because the side products remained adsorbed on the
314
surface of the nanocrystals even after several rinsing operations had been carried out, thus
315
influencing the diffractogram in the amorphous region. This point is also indicated by slight
316
coloration of NCC suspension that was obtained from spruce wood and Chinese silver grass due
317
to the greater amount of lignin determined in the starting material (Figure 5, S1 of the
318
Supplementary materials). The data obtained from different methods for the crystallinity
319
determination are presented in the Table 1 S1of the Supplementary materials.
320 321
Figure 4. XRD diffractograms of NCC: cotton linters(a), Chinese silver grass (b), spruce wood (c)
322
and eucalyptus wood (d).
12
323
The average diameter was calculated according to the Scherrer equation. The calculated values
324
are less than those observed from the SEM micrographs. Measurements from micrographs are
325
less accurate due to the lower resolution and the fact that very few single crystallites were found.
326
The authors calculated the average diameter, (the Scherrer equation approach), after the
327
glycolysis of the products of the different cellulose sources, under the similar reaction conditions.
328
The values obtained were between 8.9 nm and 12.7 nm.
329
3.5. Thermal stability of nanocrystalline cellulose
330
All of samples gave an initial weight loss at low temperatures (below 110 oC), mainly due to
331
water loss. (Figure 2 S1 of the Supplementary materials). The thermal degradation onset
332
temperature, (To) and the maximum decomposition temperature, (Tmax) are presented in Table 3.
333
Values for raw cotton linters are included in order to show the significant difference between Tmax
334
of nanoparticles and that of the raw cotton fibers. It is clear that the nanoparticles have a greater
335
surface area compared with those of the cellulose fibers, leading to a greater surface area being
336
exposed to the heat. The Tmax value of the analysed samples is in direct correlation with the
337
measured crystallinity index, being the lowest for cotton linters (284.6 oC), with a crystallinity
338
index of 89% and the highest for spruce wood (326.1 oC), with a crystallinity index of 68%. The
339
decrease of the Tmax has been explained in some literature sources ( Li et al. 2011 ) as being due
340
to the presence of sulphate groups, which would significantly lower the degradation temperature
341
of NCC. This is because the elimination of any sulphate groups results in the occurrence of a
342
lesser activation energy. The additional elemental analyses and 1H NMR analyses proved the
343
presence of only trace quantities of sulphur and sulphonate groups in all of the NCC samples.
344
Thus, the main reason for the Tmax decrease is the larger surface area of NCC crystals. The same
345
reasoning applies to the char residues. A greater quantity of char was generated when the NCC
346
from cotton linter was analysed. The reason may be due to the greater proportion of carbon in
347
highly crystalline sample.
348
3.6. FTIR analysis of NCC dry powders
349
The relevant FTIR spectra are presented as Figure 3 S1 of the supplementary material. These
350
show clearly that neither hemicelluloses nor lignin are present in the products. The FTIR spectra
351
of all of the samples are practically identical, indicating that there are minimal differences in their
352
chemical composition. Thus, bands at 1597 cm-1 and 1506 cm-1 (aromatic C-H out-of-plane
353
vibrations of lignin) are missing. Also the band at 1735 cm-1, which is ascribed to the C=0
354
stretching of the acetyl group in hemicelluloses is missing. The absorption band at 2900 cm-1 is
355
associated with C-H stretching, signals at 1431 cm-1 and 1372 cm-1 are attributed to CH3, CH2 and
356
CH stretching and bending. The signals at 1164 cm-1, 1113 cm-1 1060 cm-1 , C-O-C stretching
357
and 898 cm-1 are attributed to glycosidic -C-O-C- deformation which is characteristic of the β13
358
glycosidic link in cellulose. The ratio between two adsorption bands, namely 2900 cm-1 and 1371
359
cm-1 was used to determine the crystallinity index determination. The results are presented in the
360
Table 1 S1 of the supplementary material. The accuracy of the method was checked by
361
calculating the crystallinity index of an amorphous lignin standard, producing a value of 21.8%,
362
which is unacceptable. The same standard sample was analyzed using XRD and was shown to be
363
100% amorphous.
364
3.7. Gas chromatographic analysis of liquid residue
365
In order to identify the major components of the liquid residues, obtained after the isolation of the
366
NCC species, the methyl esters of the acidic components and trimethyl silyl ethers of the
367
hydroxyl groups containing components were synthesised. The GC/MS chromatogram of each of
368
the methyl esters is presented in Figure 5. The main component is the methyl ester of levulinic
369
acid, identified by comparing the MS spectrum with the reference spectrum from Wiley MS
370
spectra library. When using cotton linters as the starting material, the yield of levulinic acid was
371
between 7% (w/w) and 9% (w/w).
372
373
Figure 5. The GC/MS chromatogram of the liquid residue, obtained after the isolation of NCC
374
from selected materials. The methyl ester of levulinic acid was identified at 6.95 minutes.
375
When using spruce wood or Chinese silver grass, the yield was between 3%(w/w) and 4%(w/w).
376
The lowest yield was achieved with eucalyptus wood (2.7%). The yield depends on the cellulose
377
loading of the starting material, being the greatest in the cotton linters and the least in eucalyptus
378
and spruce wood. The depolymerisation of cellulose was monitored in detail, according to the
14
379
study of Yamada and Ono (2001), who observed that the first step is the formation of diethylene
380
glycol glucoside, which further decomposes to levulinic acid. When dealing with wood or similar
381
lignocellulosic materials, some sugars derived from hemicelluloses were identified as was some
382
glucose. The major products of the depolymerisation of the hemicelluloses were xylose and
383
furfural, with mannose in smaller quantities. The GC/MS chromatogram of the trimethylsilyl
384
ethers is presented as Figure 4 S1 of the supplementary materials. Here, the trimethyl silyl ethers
385
of sugars were identified at retention times from 10.65 minutes to 11.35 minutes, by comparing
386
the MS spectra with the reference spectra given in the Wiley MS spectra library. The total amount
387
of sugars present in the residual liquid was obtained by quantitative GC/MS analysis. The results
388
are presented in Table 3. A greater value for the cotton linters sample was expected but the
389
GC/MS analysis showed that conversion into the levulinic acid predominated.
390
Table 3. The GC/MS determination of sugars present in the residual liquid and thermal
391
parameters for the NCC samples and raw cotton linter.
392
Biomass
Total sugars
To o
a
Tmax o
a
Residue
(mg/g)
( C)
( C)
(%)
Cotton linter
16.08±1.4
249.7
284.6
24.1
Spruce wood
14.59±1.2
299.3
326.1
22.3
Chinese silver grass
10.65±1.0
268.6
301.7
21.5
Eucalyptus wood
10.13±0.9
290.1
322.5
20.9
Raw cotton linter
‐
314.4
362.5
17.9
393
a
394
It is possible to isolate the levulinic acid selectively from liquid residues using alkyl phenolic
395
solvents, as indicated by Alonso et al. (2011).
396
The sugars that were identified in the liquid residue from the wood liquefaction process could be
397
used as a feedstock for several fermentation processes in ethanol production (Singh et al. 2014,
398
Koller et al. 2015). Also, the liquid residues could be used as a source of a range of valuable
399
chemicals such as gamma-valerolactone and hydroxymethyl furfural (Wettstein et al. 2012).
To and Tmax were calculated from TGA profiles.
400 15
401
4. Conclusions
402
The liquefaction with glycols, using methane sulphonic acid as a catalyst has been employed in
403
attempts at simplifying the process of NCC production. The whole process contains only four
404
steps, the milling, the glycolysis reaction, centrifugation and final rinsing with an organic solvent.
405
The yields and the crystallinity indices obtained were in the range of published methods in which
406
hydrolysis with sulphuric acid was used. During the one step glycolysis reaction, lignin,
407
hemicelluloses and the more disordered components of the cellulosic fibers were liquefied, only
408
the crystalline cellulose remaining as a solid residue. The nanocrystalline cellulose recovery was
409
between 56% and 75%. Compared to the well-known acid hydrolysis-based procedure, a loading
410
of only 3% of acid was used. Product isolation and cleaning was achieved by simple
411
centrifugation and rinsing with 1,4-dioxane. The method can be applied to different cellulose-
412
containing biomasses. The main benefit of the process arises from the ability to prepare stable
413
NCC suspensions in an organic medium at 10 times greater loadings than can be achieved in
414
aqueous suspensions. The thermal stability of all of the NCC samples is less than that of the
415
starting material due to the greater surface area of nanocrystals and is in good correlation with the
416
crystallinity index. The liquid residues contain significant quantities of levulinic acid and
417
different sugars that were derived from cellulose and hemicelluloses.
418 419
Acknowledgments
420
The authors gratefully acknowledge the Slovenian research Agency for financial support
421
(program P2-0145).
422 423
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424
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546
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547
20
547 548
List of Tables:
549
Table 1: The composition of the materials and their crystallinity index
Biomass
550
Hemicelluloses Cellulose (%) (%)
Lignin (%)
Extractives (%)
Ash (%)
Crystallinity index (CrI)
Cotton linters
7.1
81.3
2.5
6.3
2.8
64
Spruce wood
29.6
42.3
26.9
0.9
0.3
47
Chinese silver grass
33.9
47.1
10.5
5.6
2.9
53
Eucalyptus wood
26.9
44.3
26.1
2.3
0.4
57
Data reported are on a dry matter basis.
551
21
551 552
Table 2: The % recovery of NCC, the crystallinity index, the average crystal length and crystal width.
Biomass
Average length
(%)
Crystallinity index (CrI)a
of NCC(nm)b
Average NCC crystal width (nm)c
Cotton linter
74.5±6.0
80%
242±8
12.7+/‐0.4
Spruce wood
61.5±3.2
63%
235±23
8.9+/‐0.1
Chinese silver grass
55.6±4.0
62.8%
250±17
8.9+/‐0.2
Eucalyptus wood
63.0±8.5
66%
306±13
9.0+/‐0.1
553
a
554
b
555
c
556
NCC recovery
The crystallinity index was calculated using Segal 21-WAX method The length of the NCC crystals was evaluated from SEM micrographs.
The width of the NCC crystals was estimated by using the Scherrer equation.
557
22
557 558
Table 3: The GC/MS determination of sugars present in the residual liquid and thermal
559
parameters for the NCC samples and raw cotton linter
Total sugars
To
Tmax
Residue
(mg/g)
(oC)a
(oC)a
(%)
Cotton linter
16.08±1.4
249.7
284.6
24.1
Spruce wood
14.59±1.2
299.3
326.1
22.3
Chinese silver grass
10.65±1.0
268.6
301.7
21.5
Eucalyptus wood
10.13±0.9
290.1
322.5
20.9
Raw cotton linter
‐
314.4
362.5
17.9
Biomass
560
a
561
To and Tmax were calculated from TGA curves.
562 563 564 565 566
List of Tables in Supplementary materials:
567
Table 1 S1: Crystallinity index of nanocrystalline cellulose that was isolated from selected
568
cellulose-containig materials, determined by different methods:
569
23
569 570
Figure captions:
571
Figure 1. Average hydrodynamic diameter of the NCC particles size vs. the reaction time (min)
572
used: eucalyptus (a), Chinese silver grass (b), cotton linters (c) and spruce wood (d).
573
Figure 2: Hydrodynamic size distribution of NCC isolated from selected cellulose sources at
574
different reaction times
575
Figure 3: SEM micrographs of NCC: cotton linters (a), Chinese silver grass (b), spruce wood (c)
576
and eucalyptus wood (d)
577
Figure 4: XRD diffractograms of NCC: cotton linters(a), Chinese silver grass (b), spruce wood (c)
578
and eucalyptus wood (d)
579
Figure 5: The GC/MS chromatogram of the liquid residue, obtained after the isolation of NCC
580
from selected materials. The methyl ester of levulinic acid was identified at 6.95 minutes.
581
Figure captions in Supplementary materials:
582
Figure 1 S1: STEM micrograph of nanocrystalline cellulose, isolated from cotton linters
583
Figure 2 S1: Differential TGA analysis of selected cellulose-containing materials
584
Figure 3 S1: FT-IR spectra of nanocrystalline cellulose that was isolated from selected cellulose-
585
containig materials: (a): Chinese silver grass, (b): spruce wood, (c): eucalyptus, (d): cotton linter
586
Figure 4 S1: The GC/MS chromatogram of liquid residues after the isolation of NCC from spruce
587
wood. The trimethyl silyl ethers (TMS) of the sugars were identified from 9.4 minutes till 12.0
588
minutes. (The TMS ethers of glucose, xylose and galactose were observed at 9.74, 10.91 and
589
10.99 minutes, respectively).
590
Figure 5 S1: Suspension of NCC in 1,4-Dioxane: (a) spruce wood, (b) eucalyptus (c) Chinese
591
silver grass, (d) cotton linters
592 593 594
24
594
Highlights
595
•
A new process for the production of nanocrystalline cellulose was developed
596
•
The liquefaction reaction using glycols and mild acid catalysis was used
597
•
Four model cellulose containing materials were used
598
•
The yields and crystallinity index were high
599
•
Levulinic acid and different sugars were identified in the liquid residue
600
•
Nanocellulose suspension in organic medium with grossly reduced agglomeration
601
602 603 604
25
SUPPORTING INFORMATION FOR THE FAST AND EFFECTIVE ISOLATION OF NANOCELLULOSE FROM SELECTED CELLULOSIC FEEDSTOCKS Matjaž Kunaver1,*, Alojz Anžlovar1, Ema Žagar1 1
National Institute of Chemistry, Laboratory for Polymer Chemistry and Technology, Hajdrihova 19,
1000 Ljubljana, Slovenia, Tel: +38614760363, Fax: +38614760300
Figure 1 S1. STEM micrograph of nanocrystalline cellulose isolated from cotton linters.
Table 1 S1. Crystallinity index of nanocrystalline cellulose isolated from selected cellulose containig materials determined by different methods.
Amorphous subtraction
Segal 21-WAX
Segal 18-WAX
Cotton linter
53.9%
80%
89%
58.0%
Spruce wood
42.8%
63%
68%
48.0%
Chinese silver grass
43.9%
62.8%
80%
48.9%
Eucalyptus wood
45.9%
66%
79%
50.9%
Biomass
FT-IR
Figure 2 S1. Figure 2 S1 Differential TGA analysis of selected cellulose containing materials. The DTGA of the raw cotton linter was added in order to show the significant difference between Tmax of nanoparticles and raw cotton fibers.
Figure 3 S1. FT-IR spectra of nanocrystalline cellulose isolated from selected cellulose containig materials: (a) Chinese silver grass, (b) spruce wood, (c) eucalyptus, (d) cotton linters.
Figure 4 S1. The GC/MS chromatogram of liquid residues after the isolation of NCC from spruce wood. The trimethyl silyl ethers (TMS) of the sugars were identified from 9.4 minutes till 12.0 minutes. (The TMS ethers of glucose, xylose and galactose were observed at 9.74, 10.91 and 10.99 minutes, respectively).
Figure 5 S1. Suspension of NCC in 1,4-Dioxane: (a) spruce wood, (b) eucalyptus (c) Chinese silver grass, (d) cotton linters.
S0144-8617(16)30444-1 http://dx.doi.org/doi:10.1016/j.carbpol.2016.04.076 CARP 11016
To appear in: Received date: Revised date: Accepted date:
16-11-2015 15-4-2016 16-4-2016
ˇ Please cite this article as: Kunaver, Matjaˇz., Anˇzlovar, Alojz., & Zagar, Ema., The fast and effective isolation of nanocellulose from selected cellulosic feedstocks.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.04.076 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
THE FAST AND EFFECTIVE ISOLATION OF NANOCELLULOSE FROM
2
SELECTED CELLULOSIC FEEDSTOCKS
3
Matjaž Kunaver1,*, Alojz Anžlovar1, Ema Žagar1
4
1
5
Hajdrihova 19, 1000 Ljubljana, Slovenia, Tel: +38614760363, Fax: +38614760300
6
*
National Institute of Chemistry, Laboratory for Polymer Chemistry and Technology,
Corresponding author: Matjaž Kunaver E-mail: [email protected]
7
Abstract
8
A new process for the production of nanocellulose from selected cellulose-containing natural
9
materials has been developed. The liquefaction reaction, using glycols and mild acid catalysis
10
(methane sulphonic acid), was applied to four model materials, namely cotton linters, spruce
11
wood, eucalyptus wood and Chinese silver grass. The process contains only four steps, the
12
milling, the glycolysis reaction, centrifugation and final rinsing with an organic solvent. The
13
nanocrystalline cellulose recovery was between 56% and 75%. The crystallinity index was greater
14
than that of the starting materials due to the liquefaction of lignin, hemicelluloses and amorphous
15
cellulose. The final product was a stable, highly concentrated nanocrystalline cellulose suspension
16
in the organic medium. The liquid residue, after the liquefaction of the cotton linters contained
17
significant quantities of levulinic acid. Different sugars were identified in the liquid residues that
18
were derived from cellulose and hemicelluloses during the liquefaction reaction.
19
1. Introduction
20
Cellulose containing biomasses have become possible alternatives to fossil resources for
21
chemicals manufacture and for fuel production, cellulose being the most abundant polymer in
22
nature. Cellulosic resources include agricultural residues (wheat straw, corncobs, sugarcane
23
bagasse, corn stover), forest residues and energy crops (hardwood and softwood sawdust, silver
24
grass), municipal and industry wastes (waste paper waste cotton textiles and waste timber-based
25
materials). Low cost cellulosic materials represent a feasible option for the production of
26
materials that carry a higher added value.
27
Most of the low-value lignocellulosic biomasses consist of cellulose, hemicelluloses and lignin
28
(Fengel and Wegener 1989). The inner structure contributes to the hydrolytic stability and
29
structural robustness. Cellulose, as a bulk material, acts as a framework, based on highly oriented
30
cellulose fibrils. Cellulose is complex in its chemical composition and its intra- and
31
intermolecular hydrogen bonding. Within the cellulose fibrils there are highly ordered regions,
1
32
less well ordered regions and relatively highly disordered regions. The ordered (more crystalline)
33
regions can be extracted, resulting in the creation of nanocrystalline cellulose (NCC).
34
The crystallinity index of cellulose has been used to describe its relative amount of crystalline
35
material and has been determined by several methods (Terinte et al., 2011). In general, the
36
crystallinity index varies from 33 to 60 (Thygesen et al. 2005). Nanocrystalline cellulose, used as
37
a reinforcing material in nanocomposites, has been the subject of research worldwide (Habibi et
38
al. 2010). Besides its low cost, renewability, light weight and high aspect ratio, NCC has a high
39
modulus of elasticity and considerable tensile strength. These are the reasons for its incorporation
40
into many promising synthetic and natural polymeric matrices (Khalil et al. 2012), for its use in
41
different packaging material applications (Ridgway and Gane 2012), as well as in automobile
42
transportation and drug delivery applications (Lam et al. 2012).
43
The isolation of nanocellulose has been studied by many authors since the first isolation of
44
cellulose nanofibrils (Turbak et al. 1983). Favier et al. (1995) have reported on the properties of
45
nanocomposites containing cellulose whiskers, from which time the number of published results
46
related to nanocellulose isolation, its use in different composites and in different applications has
47
grown annually(Klemm et al. 2005).
48
Several processes have been developed for the NCC isolation. These include:
49
•
Mechanical pulping, mostly of wood (Chakraborty et al. 2005),
50
•
Chemical pulping, where lignin is removed by a Kraft process (Jiang and Hsieh 2013),
51
•
Steam explosion, resulting in the breakdown of the biomass structure, the hydrolysis of hemicelluloses and lignin and defibrillation (Abraham et al. 2011),
52 53
•
Acid hydrolysis with sulphuric acid, hydrochloric acid and, more seldom used, phosphoric acid and hydrobromic acid (Brinchi et al. 2013),
54 55
•
56
cellulose (Kos et al. 2014 and Li et al. 2011).
57
The most common method of choice of processing towards NCC isolation is acid hydrolysis.
58
Typically, the cellulose fibers are subjected to strong acid hydrolysis under the controlled
59
conditions of temperature, acid concentration, agitation and time. Kos et al. (2014) applied
60
microwaves to shorten the reaction time of the acid hydrolysis process to only ten minutes. Also,
61
by varying the sulphuric acid concentration, they were able to control the NCC average
62
dimensions. Experiments dealing with the isolation of NCC from different natural sources and
63
details outlining experimental conditions have been published in recent years. Thus, Dai et al.
Microwave assisted hydrolysis or high intensity ultrasonication of microcrystalline
2
64
(2013) isolated nanocellulose from hemp fibers using dodecyltrimethylammonium bromide and
65
ultrasonic agitation. Johar et al. (2012) processed rice husks, using an alkaline pre-treatment,
66
coupled with bleaching and acid hydrolysis steps. Fan and Li (2012) have studied the influence of
67
the reaction temperature, the time and the sulphuric acid concentration on isolated yields of NCC
68
from cotton pulp fibers. Mandal and Chakrabarty (2011) prepared NCC from waste sugarcane
69
bagasse by delignification, the removal of hemicelluloses and sulphuric acid-based hydrolysis.
70
Nascimento et al. (2014) used a similar process for the isolation of NCC from white coir. Also, Li
71
et al., (2011) used a similar process for the isolation of NCC from bleached softwood Kraft pulp.
72
The use of different acids (hydrochloric acid (Yu et al. 2013), phosphoric acid (Espinosa et al.
73
2013), hydrobromic acid (Sadeghifar et.al 2011) and p-toluenesulphonic acid (Anžlovar et.al.
74
2016) is quoted in the literature, but mostly aqueous sulphuric acid solutions, in concentrations
75
above 60%, have been used. Although very little information concerning yields obtained is
76
available, most of these authors have determined that the increase in crystallinity index of
77
products have been significant, namely from 77 for cotton fibers to 91 for the isolated NCC
78
(Teixeira et al. 2010); the average dimensions of the nano crystals being between 250 nm and 600
79
nm in length and from 4 nm to 70 nm in diameter.
80
Although attempts to convert cellulose into useful chemicals, in an aqueous medium, have been
81
made over an extended period, the liquefaction of lignocellulosic biomass has been known for an
82
even longer period. Liquefied lignocellulosic materials have been considered to be an important
83
feedstock for the creation of polymeric blends and derivatives/composites, options having been
84
described by several authors including Kunaver et al. (2010); Lin et al. (1994), and Hassan and
85
Shukry (2008). The products have been used as polyols in polyurethane reactions (Chen and Lu
86
2009), as adhesives (Kunaver et al. 2010a; Juhaida et al. 2010) and as a fuel for gas turbines
87
(Seljak et al. 2012).
88
This paper concerns the process of preparing NCC from different natural sources by a
89
liquefaction reaction, using glycols as the main reactant and an acid catalyst in low concentration.
90
Here, during the one step reaction, lignin, hemicelluloses and the more disordered components of
91
the cellulosic fibers are liquefied, only the crystalline cellulose remaining as a solid residue.
92
Compared to the well-known acid hydrolysis-based procedure, a loading of only 3% of acid was
93
used. The yields, crystallinity index and morphology of thus produced NCC were comparable to
94
those NCC products that were described in recently published papers.
95 96
2. Materials and methods
97
2.1. Materials 3
98
Cellulose linters and wood sawdust were provided by GGP (Gozdno Gospodarstvo Postojna,
99
Slovenia). Methane sulfonic acid was supplied by Arkema, France. All of the other reagents were
100
supplied by Sigma-Aldrich (>98 %, GC, Reagent Plus). The Chinese silver grass (Miscanthus
101
sinensis) was obtained from the experimental plantation of Petrović U., Ljubljana, Slovenia. The
102
Eucalyptus wood chips (Eucaliptus globulus) were provided by Agronelli Agroindustria, Uberaba
103
Brasil. The samples were dried to 10 % (w/w) of water content in a laboratory oven and milled to
104
2 mm particles using a RETSCH SM-2000 mill in conjunction with a 2 mm grinding mesh. Wood
105
sawdust was dried in a laboratory oven to 10 % (w/w) in its water content and sieved through 5
106
mm mesh sieve. The chemical composition of cellulose-containing materials was determined
107
according to standard methods (TAPPI T222 om-11, TAPPI T211 om-12, TAPPI T204 cm-07,
108
TAPPI T207 cm-08, TAPPI T203 om-09, TAPPI T223 cm-10). The composition of materials that
109
were used in these experiments is presented in Table 1.
110
Table 1. The composition of the materials and their crystallinity index.
Biomass
111
Hemicelluloses Cellulose (%) (%)
Lignin (%)
Extractive s (%)
Ash (%)
Crystallinity index (CrI)
Cotton linters
7.1±0.7
81.3±4.2
2.5±0.4
6.3±0.9
2.8±0.1
64±7
Spruce wood
29.6±2
42.3±3
26.9±5
0.9±0.9
0.3±0.1
47±5
Chinese silver grass
33.9±4.4
47.1±6.0 10.5±4.7
5.6±1.3
2.9±0.5
46±7
Eucalyptus wood
26.9±1.8
44.3±3.1 26.1±2.0
2.3±0.2
0.4±0.2
57±4
Data reported are on a dry matter basis.
112 113
2.2. Biomass liquefaction and NCC isolation
114
In each case, the diol or glycerol or a mixture of the two types, (300 g) and methane sulphonic
115
acid (9 g) were placed into a 1000 cm3 three-necked reactor, equipped with mechanical stirring.
116
The mixture was heated to 150 oC and was stirred constantly. After the milled biomass had been
117
added to the preheated reaction mixture, the liquefaction process was carried out for 180 minutes.
118
The polyhydroxy alcohols used in the liquefaction reaction were diethylene glycol, ethylene
119
glycol, glycerine and mixtures of these alcohols. Typically, a ratio of 4:1 between the glycol (or
120
glycerol) and the biomass was used. 3% of methane sulfonic acid was added (calculated on the
121
amount of the glycol). The reaction temperature was maintained at 150 oC. These reaction 4
122
conditions were chosen according to the previous experience in the liquefaction of wood and
123
similar biomasses (Jasiukaitytė et al. 2012; Kunaver et al. 2012).
124
The reaction mixture was cooled to room temperature, diluted 1:1 with 1,4-dioxane and
125
centrifuged at 8000 rpm for 20 minutes. The sediment was dispersed in 1,4-dioxane and
126
centrifuged using the same conditions. The process of re-dispersion and centrifugation was
127
repeated three to five times until a clear supernatant liquid was obtained. Finally, the sediment
128
was dispersed in 1,4-dioxane, with additional sonication for 10 minutes (Ultrasonic Processor
129
UP400S, Hielscher). The product was a 15% to 20% (w/w) suspension of NCC in 1,4-dioxane.
130
A diluted suspension of NCC was centrifuged at 8000 rpm for 30 minutes to separate the
131
nanocrystals from the liquefied products in the supernatant liquid. The yield, (Y) of the NCC
132
isolation was calculated according to the mass of the solid residue, obtained after rinsing and
133
centrifuging the products that were obtained after the liquefaction reaction, taking into account
134
the crystallinity index of the initial cellulose.
Y [%] =
135
m ⋅ 100 % Sc %Cell Cr I M⋅ ⋅ . 100 100 100
Equation 1
136
Here, m denotes the mass of dry product, M denotes the initial mass of the original sample, %Sc is
137
the initial solids loading in the reaction mixture, %Cell is the cellulose content in the starting
138
material and CrI is the crystallinity index (%) of the starting material.
139
2.3.
140
2.3.1. X-ray diffraction (XRD)
141
The NCC samples were characterized by wide-angle X-ray diffraction using a XPert PRO MPD
142
diffractometer from PANalytical with a Cu anode as the X-ray source, at wavelength Cu Kα1:
143
1.5406 Å. Diffractograms were measured at 25 oC in the 2-theta range from 5o to 40o, with a step
144
of 0.04o and step time of 300 s. The crystallinity index was calculated according to the Segal
145
method (Segal et al. 1959) from the ratio of the height of the 002 peak (Ioo2) and the minimum
146
between 002 and the 101 peaks, subtracted from the background signal that was measured
147
without cellulose. It should be noted that the Segal method suffers some inaccuracy due to the
148
inaccurate amorphous peak position, and, consequently the influence of the amorphous material
149
present in the sample is underestimated. Four crystalline peaks are shown. However, only the
150
highest (002) peak is used in calculation. None-the less, this much simplified method is suitable
151
for use when comparing the relative differences between samples. In order to obtain more reliable
152
results for the crystallinity index, two additional methods were applied namely, the Segal
Characterization
5
153
calculation using “the amorphous peak method, at 21o “ (Terinte et al. 201, Park et al. 2010) and
154
the amorphous subtraction method, for which alkali lignin was used as the amorphous reference
155
standard (Agarwal et al. 2011). The crystallite sizes were estimated by using well known Scherrer equation (Klug and
156 157
Alexander 1974):
Dhkl =
158
Kλ βhkl ⋅ cos Θ
Equation 2
159
Here, Dhkl is the crystallite size, K is the Scherrer constant (being 1 for needle-like crystals),
160
(Smilgies 2009), λ is the wavelength of the radiation used (0.15406 nm) and βhkl is the width at
161
half-maximum of the diffraction peak angle of the (002) crystal plane. The βhkl peak fitting was
162
performed prior to calculation of crystallite size.
163
2.3.2. Dynamic light scattering (DLS)
164
The average hydrodynamic diameter of the species in the aqueous NCC suspensions was
165
determined by dynamic light scattering at 25 oC, using a Malvern Zetasizer Nano-ZS, (Malvern
166
Instruments Ltd.) The instrument was calibrated using a Thermo Scientific Nanosphere Size
167
standard. For the diameter values and for the size distribution measurements, the NCC
168
suspensions were diluted by 100x with deionized water and sonicated for at least 5 minutes,
169
(Ultrasonic Processor UP400S, Hielscher).
170
2.3.3. Scanning Transmission Electron Microscopy (STEM) and Scanning Electron
171
Microscopy (SEM)
172
The microtopographies of NCC samples were observed by STEM. The STEM micrographs were
173
taken on a Zeiss Supra 35 VP at an acceleration voltage of 20.0 kV and a working distance of
174
4.5–5.0 mm, using a STEM electron detector. Samples for the STEM measurements were
175
prepared by the drop-casting of a 100x diluted NCC suspension in acetone. Prior the drop-casting
176
procedure, the NCC suspensions were sonicated for 10 minutes (Ultrasonic Processor UP400S,
177
Hielscher). The same suspensions were applied to a glass substrate from which the acetone
178
quickly evaporated. The dried glass support with its NCC particles on the surface was then coated
179
with gold and used for SEM observations.
180
2.3.4. Thermogravimetric analysis (TGA)
181
TGA analyses of dry samples were performed on a Mettler-Toledo TGA/DSC 1 instrument. Each
182
sample (5 mg) was heated at 10 oC/min from 30 oC to 800 oC, under N2 purging (50 mL/min).
6
183
2.3.5. Fourier transform infrared spectroscopy (FTIR)
184
FTIR analyses of dry samples were performed on a Perkin Elmer Spectrum 1 instrument using
185
KBr technique. The dried NCC powders were embedded in KBr pellets and the absorption spectra
186
were recorded in the range of 4000 cm-1 to 400 cm-1. The FTIR spectra were used for the
187
determination of the crystallinity, as described by Nelson and O’Connor (1964). Here, the ratio
188
between the 1731 cm-1 absorption band and the 2900 cm-1absorption band gives the crystallinity
189
index of the sample.
190
2.3.6. Gas chromatographic measurements
191
The amount of levulinic acid in the reaction mixture was determined using an Agilent 6890N gas
192
chromatograph, coupled with the mass selective detector, Agilent 5973N. Thus, 0.1 g of the
193
sample was put into a 20 mL vial. 3 mL of the BF3/methanol reagent (Merck) was added and the
194
vial sealed. The vial was heated at 80 oC for 60 minutes. 3 mL of chloroform and 5 mL of water
195
were added and the mixture well shaken. The resulting methyl ester of levulinic acid was
196
extracted into the chloroform layer, which was then used for the analysis.
197
The hydroxyl group-containing components of the reaction mixture were analysed as trimethyl
198
silyl ether derivatives. The etherification was performed according to the method described by
199
Esposito and Swann (1969).
200
Gas chromatographic analyses were performed using a DB-35MS column of 30 m in length and
201
0.25 mm in I.D. The temperature program was 4 minutes, with an initial temperature of 50 oC,
202
then heating to 250 oC at 30oC/min with holding for 10 minutes at the final temperature. 2uL of
203
the sample was injected with a Split 100:1 mode. Mass spectra were collected within the 33 – 550
204
mass range. The Wiley mass spectra library (Wiley 1999) was used for the identification of the
205
methyl ester of levulinic acid and of the trimethyl silyl ether derivatives of the hydroxyl group
206
containing components.
207 208
3. RESULTS AND DISCUSSION
209
3.1. Nanocellulose yields
210
First, the efficiency of the biomass liquefaction process, with respect to the yields of NCC
211
obtained, was studied by changing the reaction temperature, the reaction time and the reagent
212
composition. The choice of the polyhydroxy alcohol, the acid catalyst loading and the glycol to
213
lignocellulosic material ratio that was used was that optimized by Jasiukaitytė et al. (2012). The
214
results show that there was little deviation in the yields of the NCC recovery across the changes 7
215
made to the process parameters. The temperature of the reaction was 150 oC, while the ratio of the
216
glycol to cellulose-containing-material was kept constant at 1:4. The choice of polyhydroxy
217
alcohol selection did not influence significantly the yields of the reactions. The acid catalyst
218
loading was kept constant and was not below 3%, calculated on the glycol content. The initial
219
water that was present in the wood or other biomass materials evaporated within the first few
220
minutes of the reaction, since the temperature was sufficiently high. All of the samples in this
221
study were obtained through the same preparation route. The % recovery of the nanocrystalline
222
cellulose, the crystallinity index of the nanocrystalline cellulose and the average crystal width are
223
presented in the Table 2. The yields are moderate and differ due to different cellulose content in
224
the original material. Additionally, the cell structure might also influence the efficiency of the
225
reaction. That would explain the low yield that was obtained when the Chinese silver grass was
226
liquefied.
227 228
Table 2. The % recovery of NCC, the crystallinity index, the average crystal length and crystal width.
Biomass
Average length
(%)
Crystallinity index (CrI)a
of NCC(nm)b
Average NCC crystal width (nm)c
74.5±6.0
80%
242±8
12.7±0.4
Spruce wood
61.5±3.2
63%
235±23
8.9±0.1
Chinese silver grass
55.6±4.0
62.8%
250±17
8.9±0.2
Eucalyptus wood
63.0±8.5
66%
306±13
9.0±0.1
Cotton
NCC recovery
linters
229
a
230
b
231
c
232
The liquefaction temperature was lower than that previously reported in the published literature
233
(Kunaver et al. 2010) in order that greater yields of NCC might be obtained. The reaction yields
234
decreased on increasing the reaction temperature to greater than 150 oC.
235
3.2. DLS measurements
The crystallinity index was calculated using the Segal 21-WAX method The length of the NCC crystals was evaluated from the SEM micrographs.
The width of the NCC crystals was estimated by using the Scherrer equation.
8
236
The results of the DLS measurements can be related to the Brownian motion of the nanoparticles
237
in the medium. The value given by this technique is the radius of a sphere having the same
238
diffusion coefficient as the rod-like NCC particles. The method is suitable for the rapid evaluation
239
of the NCC average particle size. However, correlations with data obtained from the STEM
240
micrographs were needed. The average lengths are within the range observed by Fan and Li
241
(2012) and by Brinchi et al. (2013).
242
The obtained average hydrodynamic diameter of the NCC particles was similar regardless to the
243
origin of the particles. The samples were dispersed in water prior the measurements, almost
244
immediate agglomeration being observed. This process was partly avoided by the
245
ultrasonification of each sample just prior the measurement taking place. 5 % to 8 % (by
246
intensity) of the aggregates was always detected, their average size being between 3000 nm and
247
4000 nm.
248
The average particle size for different time intervals during the reactions was also measured by
249
DLS. The results are shown in Figure 1.
250 251
Figure 1. Average hydrodynamic diameter of the NCC particles size vs. the reaction time (min)
252
used: eucalyptus (a), Chinese silver grass (b), cotton linters (c) and spruce wood (d).
253
In each instance, for each polysaccharide source, the relationship is almost linear, from 292 nm
254
after 60 minutes of reaction time for wood to 125 nm at the end of the reaction. Here, only nano-
255
sized particles were taken into consideration. It is evident from these results that during the
256
liquefaction reaction, the average particle size was reduced. The average particle sizes differ at 60
257
minutes of reaction time due to the differences in the particle sizes of the starting materials, 9
258
namely that the wood particles were in a shape of needles up to 5 mm length whilst the Chinese
259
silver grass and eucalyptus particles were taken as a mixture of dust and smaller particles, up to 3
260
mm in length. Here, the lignin and hemicellulose content might have influenced the speed of the
261
size reduction during the reaction. Longer reaction times and higher temperatures could result in
262
the complete dissolution of nanocellulose particles (Jasiukaitytė et al. 2012). Large aggregates
263
and cellulose fibrils that did not dissolve in the earlier stages of the reaction were identified in the
264
spruce wood distribution patterns. As the reaction progressed, the average hydrodynamic
265
diameter distribution became narrower and more uniform. Typical DLS hydrodynamic size
266
distributions are shown in Figure 2, the small peak at 4770 nm indicating the presence of large
267
aggregates.
268
269
Figure 2. Hydrodynamic size distribution of NCC isolated from selected cellulose sources at
270
different reaction times
271
When the NCC was dispersed in water, aggregation was immediately observed. The addition of a
272
suitable surfactant, such as Tween 85 helped to keep the suspension stable during the DLS
273
measurements although it was not possible completely to avoid the presence of a small amount of
274
aggregates.
275
3.3. SEM and STEM microscopy evaluations
276
Figure 3 shows the SEM micrograph of rod-like nanocrystalline cellulose particles, as derived
277
from cotton linters, Chinese silver grass, spruce wood and eucalyptus wood. The highly diluted
278
suspension was prepared in acetone, using ultrasonification. A drop of suspension was put on the
279
glass support from which the acetone evaporated. The length of the NCC fibers was 220 nm to
280
300 nm and their diameter 14 nm to 23 nm, the average aspect ratio being 15. Although small
10
281
aggregates and nanofiber-like structures were formed during the evaporation of the acetone, the
282
individual NCC particles were able to be observed and measured.
283
Evaluations of the NCC particles’ dimensions confirmed the values that were obtained from the
284
SEM micrographs. The values are also comparable to the results emanating from the DLS study.
285
The differences between the SEM derived results and the DLS derived measurements may arise
286
from the fact that the DLS-technique measures the hydrodynamic diameter of a sphere having the
287
same diffusion coefficient as the rod like NCC in suspension, while the SEM technique gives the
288
actual particle dimensions. The morphology of NCC particles differs across the samples. The
289
NCC crystals derived from the cotton linters are more separated while the NCC crystals from
290
other three samples form a structure that is similar to those of nanofibers. The length and the
291
width of all of the NCC samples is similar. The STEM micrograph representing the cotton linters
292
NCC sample is shown in Figure 1 S1 of the Supplementary materials.
293 294
Figure 3. SEM micrographs of NCC: cotton linters (a), chinese silver grass (b), spruce wood (c)
295
and eucalyptus wood (d).
296
3.4. X-ray diffraction measurements
297
The crystallinity index and the values of the average diameter of the NCC particles are presented
298
in Table 2, the corresponding diffractograms being shown in Figure 4. The diffractograms
299
display their main peak at 22.8o, being characteristic of cellulose I. The reported crystallinity
300
index of cotton linters is 64.4%, (Morais J.P.S. et al. 2013). The crystallinity index of the NCC
301
that was isolated from cotton linters was much greater (80%). This is because all of the disordered
302
sections would have been dissolved during the reaction while the more highly ordered crystalline 11
303
regions would be more resistant to attack by the reagents. The crystallinity Miller’s indices 1Ī0,
304
110 and 200 are parallel to those of the cellulose chains and are well defined in the X-ray
305
diffraction patterns. The crystallinity index was calculated using Segal 21-WAX method.
306
Although this method gives greater values than those obtained using other methods such as the
307
peak deconvolution method or the amorphous subtraction method, it is very convenient in its use,
308
particularly when comparing the products. Similar results were obtained with Chinese silver
309
grass, spruce wood and eucalyptus, with crystallinity index values in the range from 62.8% to
310
66%, although the crystallinity reflexions other than 200 were less explicit. These results show
311
lower values for the crystallinity index, most likely because the initial nanocrystalline cellulose
312
loading was much lower in the starting material, due to the presence of a certain proportion of the
313
amorphous nanocellulose as a nanofiber and because the side products remained adsorbed on the
314
surface of the nanocrystals even after several rinsing operations had been carried out, thus
315
influencing the diffractogram in the amorphous region. This point is also indicated by slight
316
coloration of NCC suspension that was obtained from spruce wood and Chinese silver grass due
317
to the greater amount of lignin determined in the starting material (Figure 5, S1 of the
318
Supplementary materials). The data obtained from different methods for the crystallinity
319
determination are presented in the Table 1 S1of the Supplementary materials.
320 321
Figure 4. XRD diffractograms of NCC: cotton linters(a), Chinese silver grass (b), spruce wood (c)
322
and eucalyptus wood (d).
12
323
The average diameter was calculated according to the Scherrer equation. The calculated values
324
are less than those observed from the SEM micrographs. Measurements from micrographs are
325
less accurate due to the lower resolution and the fact that very few single crystallites were found.
326
The authors calculated the average diameter, (the Scherrer equation approach), after the
327
glycolysis of the products of the different cellulose sources, under the similar reaction conditions.
328
The values obtained were between 8.9 nm and 12.7 nm.
329
3.5. Thermal stability of nanocrystalline cellulose
330
All of samples gave an initial weight loss at low temperatures (below 110 oC), mainly due to
331
water loss. (Figure 2 S1 of the Supplementary materials). The thermal degradation onset
332
temperature, (To) and the maximum decomposition temperature, (Tmax) are presented in Table 3.
333
Values for raw cotton linters are included in order to show the significant difference between Tmax
334
of nanoparticles and that of the raw cotton fibers. It is clear that the nanoparticles have a greater
335
surface area compared with those of the cellulose fibers, leading to a greater surface area being
336
exposed to the heat. The Tmax value of the analysed samples is in direct correlation with the
337
measured crystallinity index, being the lowest for cotton linters (284.6 oC), with a crystallinity
338
index of 89% and the highest for spruce wood (326.1 oC), with a crystallinity index of 68%. The
339
decrease of the Tmax has been explained in some literature sources ( Li et al. 2011 ) as being due
340
to the presence of sulphate groups, which would significantly lower the degradation temperature
341
of NCC. This is because the elimination of any sulphate groups results in the occurrence of a
342
lesser activation energy. The additional elemental analyses and 1H NMR analyses proved the
343
presence of only trace quantities of sulphur and sulphonate groups in all of the NCC samples.
344
Thus, the main reason for the Tmax decrease is the larger surface area of NCC crystals. The same
345
reasoning applies to the char residues. A greater quantity of char was generated when the NCC
346
from cotton linter was analysed. The reason may be due to the greater proportion of carbon in
347
highly crystalline sample.
348
3.6. FTIR analysis of NCC dry powders
349
The relevant FTIR spectra are presented as Figure 3 S1 of the supplementary material. These
350
show clearly that neither hemicelluloses nor lignin are present in the products. The FTIR spectra
351
of all of the samples are practically identical, indicating that there are minimal differences in their
352
chemical composition. Thus, bands at 1597 cm-1 and 1506 cm-1 (aromatic C-H out-of-plane
353
vibrations of lignin) are missing. Also the band at 1735 cm-1, which is ascribed to the C=0
354
stretching of the acetyl group in hemicelluloses is missing. The absorption band at 2900 cm-1 is
355
associated with C-H stretching, signals at 1431 cm-1 and 1372 cm-1 are attributed to CH3, CH2 and
356
CH stretching and bending. The signals at 1164 cm-1, 1113 cm-1 1060 cm-1 , C-O-C stretching
357
and 898 cm-1 are attributed to glycosidic -C-O-C- deformation which is characteristic of the β13
358
glycosidic link in cellulose. The ratio between two adsorption bands, namely 2900 cm-1 and 1371
359
cm-1 was used to determine the crystallinity index determination. The results are presented in the
360
Table 1 S1 of the supplementary material. The accuracy of the method was checked by
361
calculating the crystallinity index of an amorphous lignin standard, producing a value of 21.8%,
362
which is unacceptable. The same standard sample was analyzed using XRD and was shown to be
363
100% amorphous.
364
3.7. Gas chromatographic analysis of liquid residue
365
In order to identify the major components of the liquid residues, obtained after the isolation of the
366
NCC species, the methyl esters of the acidic components and trimethyl silyl ethers of the
367
hydroxyl groups containing components were synthesised. The GC/MS chromatogram of each of
368
the methyl esters is presented in Figure 5. The main component is the methyl ester of levulinic
369
acid, identified by comparing the MS spectrum with the reference spectrum from Wiley MS
370
spectra library. When using cotton linters as the starting material, the yield of levulinic acid was
371
between 7% (w/w) and 9% (w/w).
372
373
Figure 5. The GC/MS chromatogram of the liquid residue, obtained after the isolation of NCC
374
from selected materials. The methyl ester of levulinic acid was identified at 6.95 minutes.
375
When using spruce wood or Chinese silver grass, the yield was between 3%(w/w) and 4%(w/w).
376
The lowest yield was achieved with eucalyptus wood (2.7%). The yield depends on the cellulose
377
loading of the starting material, being the greatest in the cotton linters and the least in eucalyptus
378
and spruce wood. The depolymerisation of cellulose was monitored in detail, according to the
14
379
study of Yamada and Ono (2001), who observed that the first step is the formation of diethylene
380
glycol glucoside, which further decomposes to levulinic acid. When dealing with wood or similar
381
lignocellulosic materials, some sugars derived from hemicelluloses were identified as was some
382
glucose. The major products of the depolymerisation of the hemicelluloses were xylose and
383
furfural, with mannose in smaller quantities. The GC/MS chromatogram of the trimethylsilyl
384
ethers is presented as Figure 4 S1 of the supplementary materials. Here, the trimethyl silyl ethers
385
of sugars were identified at retention times from 10.65 minutes to 11.35 minutes, by comparing
386
the MS spectra with the reference spectra given in the Wiley MS spectra library. The total amount
387
of sugars present in the residual liquid was obtained by quantitative GC/MS analysis. The results
388
are presented in Table 3. A greater value for the cotton linters sample was expected but the
389
GC/MS analysis showed that conversion into the levulinic acid predominated.
390
Table 3. The GC/MS determination of sugars present in the residual liquid and thermal
391
parameters for the NCC samples and raw cotton linter.
392
Biomass
Total sugars
To o
a
Tmax o
a
Residue
(mg/g)
( C)
( C)
(%)
Cotton linter
16.08±1.4
249.7
284.6
24.1
Spruce wood
14.59±1.2
299.3
326.1
22.3
Chinese silver grass
10.65±1.0
268.6
301.7
21.5
Eucalyptus wood
10.13±0.9
290.1
322.5
20.9
Raw cotton linter
‐
314.4
362.5
17.9
393
a
394
It is possible to isolate the levulinic acid selectively from liquid residues using alkyl phenolic
395
solvents, as indicated by Alonso et al. (2011).
396
The sugars that were identified in the liquid residue from the wood liquefaction process could be
397
used as a feedstock for several fermentation processes in ethanol production (Singh et al. 2014,
398
Koller et al. 2015). Also, the liquid residues could be used as a source of a range of valuable
399
chemicals such as gamma-valerolactone and hydroxymethyl furfural (Wettstein et al. 2012).
To and Tmax were calculated from TGA profiles.
400 15
401
4. Conclusions
402
The liquefaction with glycols, using methane sulphonic acid as a catalyst has been employed in
403
attempts at simplifying the process of NCC production. The whole process contains only four
404
steps, the milling, the glycolysis reaction, centrifugation and final rinsing with an organic solvent.
405
The yields and the crystallinity indices obtained were in the range of published methods in which
406
hydrolysis with sulphuric acid was used. During the one step glycolysis reaction, lignin,
407
hemicelluloses and the more disordered components of the cellulosic fibers were liquefied, only
408
the crystalline cellulose remaining as a solid residue. The nanocrystalline cellulose recovery was
409
between 56% and 75%. Compared to the well-known acid hydrolysis-based procedure, a loading
410
of only 3% of acid was used. Product isolation and cleaning was achieved by simple
411
centrifugation and rinsing with 1,4-dioxane. The method can be applied to different cellulose-
412
containing biomasses. The main benefit of the process arises from the ability to prepare stable
413
NCC suspensions in an organic medium at 10 times greater loadings than can be achieved in
414
aqueous suspensions. The thermal stability of all of the NCC samples is less than that of the
415
starting material due to the greater surface area of nanocrystals and is in good correlation with the
416
crystallinity index. The liquid residues contain significant quantities of levulinic acid and
417
different sugars that were derived from cellulose and hemicelluloses.
418 419
Acknowledgments
420
The authors gratefully acknowledge the Slovenian research Agency for financial support
421
(program P2-0145).
422 423
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424
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546
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547
20
547 548
List of Tables:
549
Table 1: The composition of the materials and their crystallinity index
Biomass
550
Hemicelluloses Cellulose (%) (%)
Lignin (%)
Extractives (%)
Ash (%)
Crystallinity index (CrI)
Cotton linters
7.1
81.3
2.5
6.3
2.8
64
Spruce wood
29.6
42.3
26.9
0.9
0.3
47
Chinese silver grass
33.9
47.1
10.5
5.6
2.9
53
Eucalyptus wood
26.9
44.3
26.1
2.3
0.4
57
Data reported are on a dry matter basis.
551
21
551 552
Table 2: The % recovery of NCC, the crystallinity index, the average crystal length and crystal width.
Biomass
Average length
(%)
Crystallinity index (CrI)a
of NCC(nm)b
Average NCC crystal width (nm)c
Cotton linter
74.5±6.0
80%
242±8
12.7+/‐0.4
Spruce wood
61.5±3.2
63%
235±23
8.9+/‐0.1
Chinese silver grass
55.6±4.0
62.8%
250±17
8.9+/‐0.2
Eucalyptus wood
63.0±8.5
66%
306±13
9.0+/‐0.1
553
a
554
b
555
c
556
NCC recovery
The crystallinity index was calculated using Segal 21-WAX method The length of the NCC crystals was evaluated from SEM micrographs.
The width of the NCC crystals was estimated by using the Scherrer equation.
557
22
557 558
Table 3: The GC/MS determination of sugars present in the residual liquid and thermal
559
parameters for the NCC samples and raw cotton linter
Total sugars
To
Tmax
Residue
(mg/g)
(oC)a
(oC)a
(%)
Cotton linter
16.08±1.4
249.7
284.6
24.1
Spruce wood
14.59±1.2
299.3
326.1
22.3
Chinese silver grass
10.65±1.0
268.6
301.7
21.5
Eucalyptus wood
10.13±0.9
290.1
322.5
20.9
Raw cotton linter
‐
314.4
362.5
17.9
Biomass
560
a
561
To and Tmax were calculated from TGA curves.
562 563 564 565 566
List of Tables in Supplementary materials:
567
Table 1 S1: Crystallinity index of nanocrystalline cellulose that was isolated from selected
568
cellulose-containig materials, determined by different methods:
569
23
569 570
Figure captions:
571
Figure 1. Average hydrodynamic diameter of the NCC particles size vs. the reaction time (min)
572
used: eucalyptus (a), Chinese silver grass (b), cotton linters (c) and spruce wood (d).
573
Figure 2: Hydrodynamic size distribution of NCC isolated from selected cellulose sources at
574
different reaction times
575
Figure 3: SEM micrographs of NCC: cotton linters (a), Chinese silver grass (b), spruce wood (c)
576
and eucalyptus wood (d)
577
Figure 4: XRD diffractograms of NCC: cotton linters(a), Chinese silver grass (b), spruce wood (c)
578
and eucalyptus wood (d)
579
Figure 5: The GC/MS chromatogram of the liquid residue, obtained after the isolation of NCC
580
from selected materials. The methyl ester of levulinic acid was identified at 6.95 minutes.
581
Figure captions in Supplementary materials:
582
Figure 1 S1: STEM micrograph of nanocrystalline cellulose, isolated from cotton linters
583
Figure 2 S1: Differential TGA analysis of selected cellulose-containing materials
584
Figure 3 S1: FT-IR spectra of nanocrystalline cellulose that was isolated from selected cellulose-
585
containig materials: (a): Chinese silver grass, (b): spruce wood, (c): eucalyptus, (d): cotton linter
586
Figure 4 S1: The GC/MS chromatogram of liquid residues after the isolation of NCC from spruce
587
wood. The trimethyl silyl ethers (TMS) of the sugars were identified from 9.4 minutes till 12.0
588
minutes. (The TMS ethers of glucose, xylose and galactose were observed at 9.74, 10.91 and
589
10.99 minutes, respectively).
590
Figure 5 S1: Suspension of NCC in 1,4-Dioxane: (a) spruce wood, (b) eucalyptus (c) Chinese
591
silver grass, (d) cotton linters
592 593 594
24
594
Highlights
595
•
A new process for the production of nanocrystalline cellulose was developed
596
•
The liquefaction reaction using glycols and mild acid catalysis was used
597
•
Four model cellulose containing materials were used
598
•
The yields and crystallinity index were high
599
•
Levulinic acid and different sugars were identified in the liquid residue
600
•
Nanocellulose suspension in organic medium with grossly reduced agglomeration
601
602 603 604
25
SUPPORTING INFORMATION FOR THE FAST AND EFFECTIVE ISOLATION OF NANOCELLULOSE FROM SELECTED CELLULOSIC FEEDSTOCKS Matjaž Kunaver1,*, Alojz Anžlovar1, Ema Žagar1 1
National Institute of Chemistry, Laboratory for Polymer Chemistry and Technology, Hajdrihova 19,
1000 Ljubljana, Slovenia, Tel: +38614760363, Fax: +38614760300
Figure 1 S1. STEM micrograph of nanocrystalline cellulose isolated from cotton linters.
Table 1 S1. Crystallinity index of nanocrystalline cellulose isolated from selected cellulose containig materials determined by different methods.
Amorphous subtraction
Segal 21-WAX
Segal 18-WAX
Cotton linter
53.9%
80%
89%
58.0%
Spruce wood
42.8%
63%
68%
48.0%
Chinese silver grass
43.9%
62.8%
80%
48.9%
Eucalyptus wood
45.9%
66%
79%
50.9%
Biomass
FT-IR
Figure 2 S1. Figure 2 S1 Differential TGA analysis of selected cellulose containing materials. The DTGA of the raw cotton linter was added in order to show the significant difference between Tmax of nanoparticles and raw cotton fibers.
Figure 3 S1. FT-IR spectra of nanocrystalline cellulose isolated from selected cellulose containig materials: (a) Chinese silver grass, (b) spruce wood, (c) eucalyptus, (d) cotton linters.
Figure 4 S1. The GC/MS chromatogram of liquid residues after the isolation of NCC from spruce wood. The trimethyl silyl ethers (TMS) of the sugars were identified from 9.4 minutes till 12.0 minutes. (The TMS ethers of glucose, xylose and galactose were observed at 9.74, 10.91 and 10.99 minutes, respectively).
Figure 5 S1. Suspension of NCC in 1,4-Dioxane: (a) spruce wood, (b) eucalyptus (c) Chinese silver grass, (d) cotton linters.