characterization of depolymerized residues from extremely
TRANSCRIPT
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Industrial Crops and Products 36 (2012) 560–571
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Industrial Crops and Products
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haracterization of depolymerized residues from extremely low acid hydrolysisELA) of sugarcane bagasse cellulose: Effects of degree of polymerization,rystallinity and crystallite size on thermal decomposition
eandro Vinícius Alves Gurgela, Karen Marabezia, Luiz Antonio Ramosa,ntonio Aprigio da Silva Curveloa,b,∗
Grupo de Físico-química orgânica, Departamento de Físico-química, Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São Carlense, 400, Caixa Postal80, 13560-970 São Carlos, São Paulo, BrazilLaboratório Nacional de Ciência e Tecnologia do Bioetanol (CTBE), Centro de Pesquisa em Energia e Materiais (CNPEM), Caixa Postal 6179, 13083-970 Campinas, São Paulo, Brazil
r t i c l e i n f o
rticle history:eceived 5 September 2011eceived in revised form 26 October 2011ccepted 11 November 2011vailable online 14 December 2011
a b s t r a c t
Sugarcane bagasse cellulose was subjected to the extremely low acid (ELA) hydrolysis in 0.07% H2SO4 at190, 210 and 225 ◦C for various times. The cellulose residues from this process were characterized by TGA,XRD, GPC, FTIR and SEM. A kinetic study of thermal decomposition of the residues was also carried out,using the ASTM and Kissinger methods. The thermal studies revealed that residues of cellulose hydrolyzedat 190, 210 and 225 ◦C for 80, 40 and 8 min have initial decomposition temperature and activation energy
eywords:ugarcane bagassehermal decompositionLAicrocrystalline cellulose
cid hydrolysis
for the main decomposition step similar to those of Avicel PH-101. XRD studies confirmed this findingby showing that these cellulose residues are similar to Avicel in crystallinity index and crystallite size inrelation to the 110 and 200 planes. FTIR spectra revealed no significant changes in the cellulose chemicalstructure and analysis of SEM micrographs demonstrated that the particle size of the cellulose residueshydrolyzed at 190 and 210 ◦C were similar to that of Avicel.
© 2011 Elsevier B.V. All rights reserved.
. Introduction
Batch hydrolysis of cellulose to glucose by dilute acid hydrol-sis results in yields limited to 60–65% of the potential glucoseBouchard et al., 1989). Recently, The U.S. National Renewablenergy Laboratory (NREL) developed a dilute acid hydrolysis pro-ess for lignocellulose biomass called extremely low acid (ELA)ydrolysis. The ELA conditions employ low sulphuric acid con-entration (0.07 wt.%) and high temperatures in order to improvehe yield of glucose. According to Kim et al. (2001), there are dis-inct advantages of using extremely low acid (ELA) conditions forhe hydrolysis of lignocellulosic biomass. One of them is that theorrosion characteristics of ELA conditions are very close to thosef a neutral aqueous reaction, so that standard grade stainless
teel equipment can be used instead of high nickel alloy, reducingquipment cost and maintenance. The advancement made in ELAas brought acid hydrolysis to a position where it can favorably∗ Corresponding author at: Grupo de Físico-química orgânica, Departamento deísico-química, Instituto de Química de São Carlos, Universidade de São Paulo, Av.rabalhador São Carlense, 400, Caixa Postal 780, 13560-970 São Carlos, São Paulo,razil. Tel.: +55 16 3373 9938; fax: +55 16 3373 9952.
E-mail address: [email protected] (A.A.S. Curvelo).
926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.indcrop.2011.11.009
compete with enzymatic hydrolysis in overall process economics.Besides, the process using the ELA conditions could be regardedas an environmentally friendly process, as it has minimal envi-ronmental effects. Recent findings have proven that glucose yieldsaround 62% are attainable when �-cellulose is used as feedstock ina batch reactor.
In Brazil, first generation ethanol is mainly produced by fer-mentation of the sucrose in sugarcane juice. This is pressed fromthe cane by milling, leaving large amounts of bagasse. Accordingto the last Brazilian crop survey performed by the National Com-pany of Supply (CONAB, 2010), the estimated sugarcane harvest inthe 2010/2011 season is expected to reach 651.5 million tons, 7.8%greater than in 2009/2010, which will generate about 162.9 milliontons of bagasse and 132.9 million tons of straw, given that one tonof sugarcane produces about 250 kg of bagasse and 204 kg of strawon a dry weight basis.
Sugarcane bagasse is mainly composed of cellulose, hemicel-luloses and lignin. Cellulose, the subject of this study, is a linearhomopolymer of d-glucopyranose units, which are linked by �-
(1 → 4) glycosidic bonds. It contains amorphous and crystallineregions where the microfibrils are oriented randomly or in par-allel bundles along the direction of cellulose fibrils, respectively.The amorphous region is more reactive than the crystalline, adding
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omplexity to the thermal decomposition of cellulose (Shen & Gu,009).
Numerous studies have been reported on the thermal degra-ation of cellulose, analyzed by thermogravimetry (TG) andifferential thermal analysis (DTA). Mechanisms for thermal degra-ation of cellulose have been discussed by Broido and Nelson1975), Bradbury et al. (1979), Wang et al. (2007) and Shen and Gu2009). Calahorra et al. (1989) have also studied the effect of degreef polymerization and crystallinity index on thermal decomposi-ion of cellulose. However, very little attention has been paid so faro the characteristics of the cellulose residues obtained from dilutecid hydrolysis. In this connection, Bouchard et al. (1989) were therst to investigate the thermal decomposition of cellulose residuess well as their structural changes, after dilute acid hydrolysis.
It is well known that the cellulose residues left after dilutecid hydrolysis have a lower degree of polymerization, higherrystallinity index, and large amounts of glucose decompositionroducts along their surface (Bouchard et al., 1989; Nelson &onrad, 1948; Sharples, 1957). Furthermore, these residues areesistant to further hydrolysis and 4% H2SO4 at 121 ◦C have beenuggested as a favorable set of hydrolysis conditions to produceermentable glucose from these residues (Xiang, 2002; Xiang et al.,003a,b). However, these hydrolysis conditions are unfavorableompared to the ELA conditions, in relation to both acid con-entration and hydrolysis time. Besides, according to Bouchardt al. (1989), the sugar potential of the unconverted celluloseesidues decreases with the extent of depolymerization. Consid-ring the physicochemical characteristics of cellulose residues
more economically profitable application needs to be alsonvestigated.
However, the effects of the structural changes and the pres-nce of decomposition products produced by dilute acid hydrolysisnder the ELA conditions on the thermal decomposition of cellu-
ose have not been investigated at present and, therefore, constituten impediment to suggesting a new use for these residues. Thus,he main aim of this study was to investigate the effects of degreef polymerization, crystallinity index and crystallite size on thehermal decomposition of cellulose residues obtained from ELAydrolysis conditions, by thermogravimetric analysis (TGA). A sec-nd aim of this study was to propose a commercial application forellulose residues based on their physicochemical characteristics.or these purposes, sugarcane bagasse cellulose was prepared byxtracting hemicelluloses prior to the soda-anthraquinone pulp-ng process and subjecting the cellulose to ELA hydrolysis at highemperatures for various times. Gel permeation chromatographyGPC) and X-ray diffraction (XRD) were also used to investigatehe changes in degree of polymerization, crystallinity index andrystallite size. Fourier transform infrared (FTIR) spectroscopy wassed to investigate possible structural changes in the celluloseesidues after dilute acid hydrolysis. Scanning electron microscopySEM) was employed in order to investigate the modificationf fiber structure as well as the morphology and size of thearticles.
. Experimental
.1. Chemicals
Sodium hydroxide and sulphuric acid (95–98%) were pur-hased from Qhemis (Brazil). Anthraquinone (98%) was providedy Lwarcel Cellulose (SP, Brazil). Avicel® PH-101 (microcrys-
alline cellulose, MCC) was purchased from FMC (Philadelphia,A). Lithium chloride was purchased from Synth (Brazil). N,N′-imethylacetamide (HLPC grade) was purchased from TediaBrazil).d Products 36 (2012) 560–571 561
2.2. Sugarcane bagasse cellulose preparation
Sugarcane bagasse was provided by Ipiranga alcohol and sugarmill (São Carlos, SP, Brazil). Slices of raw bagasse were mechan-ically stirred in water at 70 ◦C for 1 h to remove residual sugarsfrom the milling process. The raw bagasse was then subjected towet depithing, in order to separate the fiber and pith fractions, usinga sieve system composed by two screens, of 16 (1.19 mm) and 60(0.250 mm) mesh. Depithed bagasse was air dried to 10% moisturecontent. Hemicelluloses were extracted from the depithed bagassein a 1000 mL Parr reactor. Sugarcane bagasse (65 g), containing7.7% moisture, were loaded into the reactor with 715 mL of dis-tilled water to give a solid-to-liquid ratio of 1:12 (w:v). The reactorwas heated to 160 ◦C and maintained at this temperature for 1 h,to hydrolyze the hemicelluloses. The reactor was then depressur-ized and the content transferred to a Buchner funnel and washedwith an excess of water, to remove hydrolyzed hemicellulosesfrom the surface of the fibers. The moisture was again determinedand the prehydrolyzed depithed bagasse was subjected to soda-anthraquinone pulping. For this process, 110.4 g of the material,containing 59% moisture, were again loaded into the Parr reactor,with 9.34 g of NaOH (16 wt.% of dry bagasse as Na2O), 67.9 mg ofanthraquinone (0.15%) and 523 mL of distilled water in order togive a solid-to-liquid ratio of 1:13 (w:v). The reactor was heatedto 160 ◦C and kept at this temperature for 1 h, after which the cel-lulose pulp was transferred to a Buchner funnel and washed withan excess of water, to remove soluble lignin from the pulp untilthe washings reached pH 6. The moisture content was determinedand the cellulose pulp was chilled to −20 ◦C and stored until usedin acid hydrolysis experiments. The chemical composition of dep-ithed bagasse, prehydrolyzed depithed bagasse and cellulose pulpwas analyzed according to TAPPI standard methods for extractives(TAPPI, 1997), cellulose and hemicelluloses (TAPPI, 2000), Klasonsoluble and insoluble lignin (TAPPI, 2002), and ash (TAPPI, 1993).
2.3. Batch acid hydrolysis experiments
Samples of 0.6 g of the cellulose pulp obtained from prehy-drolyzed depithed bagasse were loaded into 316L stainless-steelreactors (25.0 mm OD × 15.7 mm ID × 83.2 mm length) with 12 mL0.07% H2SO4 (w/v), to give a solid-to-liquid ratio of 1:20 (w:v). Thereactors were equipped with self-sealed o-ring closures made ofPTFE fluoropolymer resistant to high temperatures (>250 ◦C). Sam-ples were hydrolyzed at 190, 210, and 225 ◦C for various times. Thereactors were immersed in a glycerin bath heated to the desiredreaction temperature. At the end of the experiments, the reactorswere quenched in an ice bath to stop the reaction. The reactor con-tents were separated into liquid and solid by vacuum filtration ina sintered glass funnel. The residual cellulose was rinsed in water,dried at 60 ◦C under reduced pressure (100 mbar) and stored in adesiccator prior to analysis.
2.4. Thermogravimetric analysis (TGA)
Thermogravimetric analysis was carried out in a Shimadzu TA-50WSI attached to a TGA50 module. A sample of 4 mg of celluloseresidue was placed on a platinum pan. TGA experiments were per-formed under a nitrogen atmosphere at a flow rate of 50 mL/min.Samples were heated from 25 to 800 ◦C at linear heating rates of2.5, 5, 10, and 20 ◦C/min, as stipulated in the standard test methodfor decomposition kinetics by thermogravimetry (ASTM E 1641).
2.5. X-ray diffraction (XRD)
X-ray diffraction analysis was carried out in a Carl ZeissJena URD-6 diffractometer with monochromatic Cu-K˛ radiation
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62 L.V.A. Gurgel et al. / Industrial C
� = 1.5406 A), generated at a voltage of 40 kV and a current of0 mA. The Bragg angle was varied from 3◦ to 50◦ at a scan ratef 2◦/min.
The peak profiles were resolved with Microcal OriginTM 8.5 rou-ines, Lorentz and Pearson VII functions being used for crystal peaksnd a fifth-degree polynomial function for the background profile,ollowing the method described by Kim et al. (2010). The crystal-ite size of the cellulose in the 1 1 0, 1 1 0 and 2 0 0 planes werealculated by the Scherrer equation (1), as follows:
h,k,l = K�
H cos �(1)
here K is the Scherrer constant (0.89), � (nm) is the X-ray wave-ength, H is the full-width at half-maximum (FWHM) in radians, and
is the Bragg angle. The crystallinity index (CrI) was determinedy the Segal method, from Eq. (2):
rI =(
I2 0 0 − Iam
I2 0 0
)× 100 (2)
here CrI is the crystallinity index (%), I2 0 0 the maximum intensityf the (2 0 0) reflection, at 2� of 22◦–23◦, and Iam is the intensity ofiffraction at 2� of 18◦–19◦ for cellulose I.
.6. Gel permeation chromatography (GPC)
Gel permeation chromatography analysis was carried out withShimadzu system equipped with pump (LC-10AD), system
ontroller (SCL-10A), and differential refraction index detectorRID-6A) and oven (CTO-10A). Lithium chloride (0.5 wt.% LiCl)n N,N′-dimethylacetamide (DMAc) was used as eluent (60 ◦C,.6 mL/min). The carbohydrates were separated in a series ofolumns, consisting of a pre-column of 10 �m Plgel and twoolumns of 10 �m mixed-B Plgel (7.5 mm ID × 300 mm) (Waters).he chromatogram was calibrated with Pullulan (Polymer Lab-ratories): 1,600,000, 380,000, 200,000, 107,000, 48,000, 23,700,2,200, 5,800, 738 and 342 g/mol (cellobiose, Aldrich), 180 g/mold-glucose, Aldrich), and 150 g/mol (d-xylose, Aldrich). Bothtandards and cellulose samples were previously filtered on�m–25 mm acrodisc glass fiber disks (Waters), before injectionf 100 �L into GPC column.
Cellulose residues were dissolved in LiCl/DMAc (0.5 wt.%) asollows. A sample (10 mg) was weighed and transferred to a round-ottom flask containing 0.25 g of LiCl and 5 mL of anhydrous DMAc,o give a concentration of 2 mg/mL. A reflux condenser was attachedo the reaction flask and the suspension was heated to 160 ◦C under
ild constant stirring for 90 min. After this, the heating was turnedff and the solution was stirred for a further 22.5 h, giving a totaleaction time of 24 h.
.7. Fourier transform infrared spectroscopy (FTIR)
The FTIR spectroscopy analysis was carried out on a Bomen –B spectrometer. Spectra were collected with 32 scans from 4000
o 400 cm−1 at a resolution of 2 cm−1. The sample was prepared byixing 1 mg of dried cellulose powder with 100 mg of KBr.
.8. Scanning electron microscopy (SEM)
Dry powder samples were dispersed on a graphite ribbon fixed
n an aluminum sample holder. The powder was sputter-coatedith gold in a modular high-vacuum coating system and examinedith a LEO-440 (Zeiss-Leica) scanning electron microscope, usingfilament voltage of 20 kV.d Products 36 (2012) 560–571
2.9. Theoretical approach
The rate of a heterogeneous solid-state reaction can generallybe described by Eq. (3):
d˛
dt= k(T)f (˛) (3)
where t is time, k(T) is a constant that depends on the temperature(T) and f(˛) is a function that describes how the reaction rate con-stant changes with the extent of reaction or conversion, ˛.˛ at t isdefined by Eq. (4):
˛ = w0 − wt
w0 − wash(4)
where w0 (mg) is the initial sample weight, wash (mg) is the finalsample weight, and wt is the weight at time t.
Temperature dependence of the reaction rate constant can bedescribed by the Arrhenius equation. Hence, the rate of a solid-statereaction can generally be expressed as:
d˛
dt= Ae(−Ea/RT)f (˛) (5)
where A is the pre-exponential factor, Ea is the Arrhenius activationenergy, and R is the ideal gas constant. The above rate expressioncan be transformed into a non-isothermal rate expression describ-ing the reaction rate as a function of temperature, at a constantheating rate, ˇ (Ramajo-Escalera et al., 2006):
d˛
dT= A
ˇe(−Ea/RT)f (˛) (6)
where ˇ is the heating rate. Various kinetic methods have beenderived from Eq. (6). Generally, these can divided into integralmethods, such as those of Ozawa (1965) and Flynn and Wall (1966),and differential methods such as those of Friedman (1964) andKissinger (1957). The methods of Flynn and Wall (ASTM E 1641)and Kissinger were employed to calculate kinetic parameters inthe present study.
2.9.1. Kinetic method of Flynn and Wall (ASTM E 1641)The standard method to investigate decomposition kinetics by
thermogravimetry (ASTM E 1641) is based on an integral iso-conversional method proposed by Flynn and Wall (1966). Todetermine the kinetic parameters, Arrhenius activation energy andpre-exponential factor, it is assumed that the decomposition obeysfirst-order kinetics. In this isoconversional method, the temper-ature at which a fixed conversion (˛) occurs is recorded. Then,Arrhenius activation energy can be calculated by using Eq. (7) asfollows:
Ee = −(
R
b
)�log ˇ
�(1/T)(7)
where ˇ (K/min) is the heating rate, Ee (J/mol) is the estimatedArrhenius activation energy, b (1/K) is an iteration variable withthe first iteration being fixed at 0.457/K, R is the ideal gas constant(8.314 J/K mol) and T (K) is the absolute temperature at conversion˛.
A plot of log ˇ versus 1/T for each conversion level (˛) shouldresult in a straight line. The least-squares method was used to fiteach straight line and determine the slope, �(log ˇ)/�(1/T). Theestimated activation energy, Ee, was then calculated by Eq. (7), mak-ing use of a value of 0.457/K for b in the first iteration. The value,E/RTc, was calculated from the temperature, Tc, at constant conver-
sion for the heating rate closest to the midpoint of the experimentalheating rates, i.e., 10 K/min. This initial value of E/RTc was used toobtain a new estimate of b from a table published by Doyle (1961).This procedure was repeated until the estimated activation energy
L.V.A. Gurgel et al. / Industrial Crops and Products 36 (2012) 560–571 563
Table 1Chemical composition of the materials.
Material Cellulose (%) Hemicelluloses (%) Lignin (%) Ash (%) Extractives (%)
cA
f
A
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2
ue
w
toc
aT
n
wbd
S
wtf(
3
dstpa
aHwsfrmiT
Depithed sugarcane bagasse 49.49 ± 0.31 24.34 ± 0.78Prehydrolyzed depithed bagasse 55.22 ± 0.25 17.54 ± 0.22Bagasse cellulosic pulp 88.32 ± 0.07 11.31 ± 0.09
hanged by less than 1%. The refined value, Er, is reported as therrhenius activation energy.
The pre-exponential factor was calculated by using Eq. (8) asollows:
= −(
ˇ′
Er
)R ln(1 − ˛)10a (8)
here ˇ′ is the heating rate nearest the midpoint (10 K/min), Er
J/mol) is the refined Arrhenius activation energy, and a is the valuef an approximation of the temperature integral taken from Doyle’sata for the refined value of Ee/RTc.
.9.2. Kinetic method of KissingerThe Kissinger method is a differential isoconversional method
sed to obtain kinetic parameters such as Arrhenius activationnergy, E, assuming f(˛) = (1 − ˛)n:
d˛
dT=
(A
ˇ
)e(−E/RT)(1 − ˛)n (9)
here n is the reaction order.The Kissinger method derives the activation energy from the
emperature (Tm) at which the maximum reaction rate (DTG peak)ccurs and the reaction order from the shape of the mass loss-timeurve (Jiang et al., 2010; Kissinger, 1957).
The activation energy, E, is obtained by plotting ln(ˇ/Tm2)
gainst 1/Tm. The slope, �[ln(ˇ/Tm2)]/�(1/Tm), being equal to −E/R.
he reaction order, n, is obtained from Eq. (10):
= 1.26S1/2 (10)
here S is defined by Eq. (11). The value of S, the shape index, cane calculated from the time derivative of the DTG curve (seconderivative of mass loss-time curve), as follows:
=∣∣∣∣ (d2˛/dt2)1
(d2˛/dt2)2
∣∣∣∣ (11)
here subscripts 1 and 2 refer to the values of these quantities athe inflection points, i.e., where d3˛/dt3 = 0. The pre-exponentialactor, A, can be obtained by inserting the values of n and E in Eq.9) and solving it.
. Results and discussion
The chemical composition of depithed bagasse, prehydrolyzedepithed bagasse and cellulose pulp was determined using TAPPItandard methods and is shown in Table 1. As can be seen in Table 1,he cellulose pulp used in the batch acid hydrolysis experimentsresents minimal quantity of lignin and low hemicelluloses andsh contents.
Soda-anthraquinone cellulose pulp obtained from depithed sug-rcane bagasse was hydrolyzed at 190, 210 and 225 ◦C in 0.07%2SO4 (w/v) for various times. After the reaction time, the reactoras quenched in an ice bath and the residual cellulose pulp was
eparated from the hydrolysate by filtration on a sintered glassunnel. The residual pulp was washed and dried at 60 ◦C under
educed pressure for 24 h and the weight loss after hydrolysis waseasured. The number- and weight-average degrees of polymer-zation were measured by gel permeation chromatography (GPC).he crystallinity index was calculated by the Segal method (Segal
22.69 ± 0.08 0.34 ± 0.03 1.8 ± 0.6724.08 ± 0.30 0.59 ± 0.05 –
1.45 ± 0.07 0.46 ± 0.04 –
et al., 1959) and crystallite size was calculated for the three maincrystal reflections (1 1 0, 1 1 0 and 2 0 0) by means of the Scherrerequation. The starting cellulose pulp and the residues after hydrol-ysis were subjected to dynamic thermogravimetric analysis undera nitrogen atmosphere at four different heating rates, in order todetermine the effects of hydrolysis time, temperature, degree ofpolymerization and crystallite size on thermal decomposition ofcellulose. In the following section the main results will be discussed.
3.1. Thermogravimetric analysis (TGA)
Dynamic thermogravimetric curves (TG) at four heating ratesand their first derivative curves (DTG), for the starting cellulosepulp and samples hydrolyzed at 190, 210 and 225 ◦C for 80, 40and 8 min, are shown in Fig. 1(a–d). As can be seen, these curvesare generally similar to those reported in the literature (Bouchardet al., 1989, 1990; Calahorra et al., 1989; Kim et al., 2010; Polettoet al., 2011). The initial small weight loss, which occurred at tem-peratures below 100 ◦C, represented 5% of the total weight loss onaverage and was attributed to the vaporization of bound water inthe samples. For all samples, this initial weight loss was followedby a plateau that extended to the start of the main decompositionevent causing weight loss. As can be seen in Table 2, the onset tem-perature (Tonset), which indicates thermal stability, decreased ashydrolysis time increased for cellulose residues obtained after acidcatalyzed hydrolysis at 190 ◦C in comparison with original cellulosepulp. For cellulose residues obtained after hydrolysis at 210 ◦C and225 ◦C, the Tonset initially decreased as hydrolysis time increasedin comparison with original cellulose pulp and after which it waspractically constant as hydrolysis time increased.
According to Shafizadeh (1985), the main decomposition eventconsists of the depolymerization of cellulose to various anhy-drosugar derivatives and their volatilization. The mechanism ofcellulose decomposition was recently elucidated by Shen and Gu(2009), who studied cellulose pyrolysis in a thermogravimetric ana-lyzer coupled to the FTIR spectrophotometer. According to Shenand Gu (2009), the main component detected was levoglucosan(1,6-anhydro-�-d-glucopyranose). After the main decompositionevent, a third event began, and the rate of weight loss was notconstant. Bouchard et al. (1989) attributed this event to the slowcombustion of the charred fraction, which is inhibited in a nitrogenatmosphere.
The differences in the decomposition profiles of the original cel-lulose pulp and its residues after dilute acid hydrolysis at 190, 210and 225 ◦C for 80, 40 and 8 min, respectively, indicate the differ-ences in initial decomposition temperature of these samples. Inthe following sections, the influence of degree of polymerization,crystallinity index and crystallite size will be discussed in detail.
3.2. Influence of degree of polymerization
Table 2 shows the percentage of remaining pulp after dilute acidhydrolysis in 0.07% H2SO4 (w/v) at 190, 210 and 225 ◦C, number-and weight-average degrees of polymerization (DPn and DPw), crys-
talline index (CrI), crystallite size in the 1 1 0, 1 1 0 and 2 0 0 planes,initial (Ti), maximum (Tm) and final (Tf) temperatures for the maindecomposition event, char yield and activation energy (Ea) and pre-exponential factor calculated using the ASTM E 1641 and Kissinger
564 L.V.A. Gurgel et al. / Industrial Crops and Products 36 (2012) 560–571
Fig. 1. TG and DTG curves at four heating rates for (a) starting cellulose pulp and samples of pulp hydrolyzed in 0.07% H2SO4, (b) 190 ◦C for 80 min, (c) at 210 ◦C for 40 mina
mpaf
hiaattrr
Triftar
nd (d) at 225 ◦C for 8 min.
ethods. The effect of number- and weight-average degrees ofolymerization on the thermal decomposition of cellulose can benalyzed in more detail from Fig. 2, which shows Ti, Tm and Tf as aunction of DPn and DPw.
It can be seen in Table 2 and Fig. 2, for cellulose residuesydrolyzed at 190, 210 ◦C and 225 ◦C, that Ti, which indicates the
nitial decomposition temperature, initially increased as number-nd weight-average degrees of polymerization decreased, reachingmaximum value, after which it began to decrease for all reaction
emperatures studied. Besides, the initial increase in Ti was lower ashe reaction temperature was increased from 190 to 225 ◦C. Theseesults indicate that the more severe the acid hydrolysis, the lessesistant is the cellulose residue to thermal decomposition.
From the derivative thermogravimetric curves (DTG) (Fig. 1),m values were determined as those temperatures at which theate of decomposition reaches its maximum value. As can be seenn Table 2 and Fig. 2, Tm decreased as DPn and DPw decreased
or all reaction temperatures; in other words, peak decomposi-ion temperatures were shifted to lower values with falling DPnnd DPw. However, Tf values increased as DPn and DPw decreased,eaching a maximum value, for cellulose hydrolyzed at 210 and
225 ◦C, and beyond this maximum value a small decrease wasobserved. For hydrolysis at 190 ◦C, Tf increased as DPn and DPw
decreased, reaching a maximum at the lowest values of DPn andDPw. Besides, Tf exhibited a higher rate of increase with decreas-ing DPn and DPw as the reaction temperature was increased from190 to 225 ◦C. In Table 2 it may be noted that the charred frac-tion increased as DPn and DPw decreased and also as the reactiontemperature was increased. This increase in the charred fractionbecomes more marked as the reaction temperature was increasedand consequently as DPn and DPw became more reduced. Theseresults suggest that cellulose decomposes at a lower temperatureand that char formation is favored by falling DPn and DPw.
Similar results were observed by Calahorra et al. (1989), whostudied the effect of molecular weight on thermal decompositionof cellulose, and by Bouchard et al. (1989, 1990), who studied thethermal behavior of depolymerized cellulose residues produced bydilute acid hydrolysis in 0.4% H2SO4 at 190 ◦C for various periods
of time and thermo-mechanical treatment at high temperature,using ethylene glycol as solvent. According to Bouchard, severehydrolysis produces cellulose more resistant to thermal degrada-tion. Bouchard based his observation on the results obtained from
L.V.A. Gurgel et al. / Industrial Crops and Products 36 (2012) 560–571 565
Fig. 2. Variation of the temperatures of Ti , Tm and Tf (obtained from TG curves recorded at a heating rate of 10 ◦C/min) with DPn and DPw for cellulose residues obtained fromdilute acid hydrolysis (0.07% H2SO4) at 190 ◦C (a) and (d), 210 ◦C (b) and (e) and 225 ◦C (c) and (f).
566 L.V.A. Gurgel et al. / Industrial Crops anTa
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d Products 36 (2012) 560–571
analytical techniques such as TG, DSC, FTIR and DRIFTS and con-cluded that the additional resistance to thermal decomposition wasprovided by important structural and/or chemical rearrangementsof the cellulose during acid hydrolysis.
Comparing the temperatures Ti, Tm and Tf obtained for AvicelPH-101 and bagasse cellulose samples hydrolyzed at 190, 210 and225 ◦C for 80, 40 and 8 min, respectively, it can be seen that thetemperatures Ti and Tm for these cellulose residues were close tothose of Avicel and that Tf alone deviates. To a first approximation,these results suggest that these cellulose residues exhibited similarthermal decomposition behavior to that of Avicel.
The kinetics of thermal decomposition of the cellulose pulp andresidues after acid hydrolysis was analyzed by the methods of Flynnand Wall (ASTM E 1641) and Kissinger. Table 2 shows the resultingactivation energies (Ea), pre-exponential factors (A) and reactionorders (n). Fig. 3 shows the activation energies calculated by theASTM and Kissinger methods, plotted against DPn and DPw. Accord-ing to the ASTM method, thermal decomposition obeys first-orderkinetics. For the hydrolysis temperature of 190 ◦C, the activationenergy calculated by this method initially increased as DPn andDPw decreased and achieved a maximum value, after which it fellsharply. For 210 ◦C, the activation energy initially decreased as DPn
and DPw decreased, reaching a minimum, after which it increased asDPn and DPw decreased. For 225 ◦C, the activation energy was prac-tically constant from the highest DPn (96) and DPw (1356) downto 51 and 169. At lower molecular weights, the activation energyincreased to a maximum of 211.5 kJ/mol. This value of activationenergy was the highest observed for all hydrolysis temperaturesstudied, by the ASTM method.
Employing the Kissinger method, it was possible to calculate thereaction orders for the thermal decomposition of cellulose pulp andresidues after acid hydrolysis. The reaction orders (n) showed somevariation with DPn and DPw. The lower value of n was obtained withthe starting cellulose pulp (0.97). For the cellulose residues, n var-ied from 1.03 to 1.23. These values of reaction order indicate thatboth cellulose pulp and the residues with reduced DPn and DPw
after acid hydrolysis obeyed first-order decomposition kinetics,confirming that the ASTM method can be used to calculate kineticparameters of cellulose decomposition. For the hydrolysis temper-ature of 190 ◦C, the activation energy calculated by the Kissingermethod was constant from DPn and DPw of 96 and 1356 to 64 and347, respectively, at lower values of DPn and DPw, the activationenergy increased to a maximum and after which it fell sharply. For210 ◦C, the activation energy decreased as DPn and DPw decreased,to reach a minimum value, after which it increased sharply and thenremained practically constant. For 225 ◦C, the activation energyincreased sharply as DPn and DPw decreased, achieving a maximumvalue and then decreasing rapidly.
The activation energies for thermal degradation of celluloseresidues hydrolyzed at 190, 210 and 225 ◦C for 80, 40 and 8 min,calculated by the ASTM and Kissinger methods, differed by 17.2%,8.0% and 10.0%, and 5.6%, 13.1% and 13.8%, from those obtainedfor Avicel PH-101, respectively. Considering the observed temper-atures Ti and Tm and the activation energies obtained by the ASTMand Kissinger methods, it may be suggested that cellulose residuesobtained by ELA hydrolysis at 190, 210 and 225 ◦C for 80, 40 and8 min presented similarities with Avicel. Subsequent interpreta-tion of the crystalline index and crystallite size in the next sectionwill confirm this result. One important point to emphasize is thatthe comparisons of the physicochemical properties of the celluloseresidues and Avicel have to take into consideration the differencesby which each material is obtained. Furthermore, the possibility of
using these residues in commercial applications is a matter of greatinterest in the field of biofuels and biomaterials.Comparing the values of Ea obtained by the two analytical meth-ods, it can be observed that activation energies calculated by the
L.V.A. Gurgel et al. / Industrial Crops and Products 36 (2012) 560–571 567
F ) anda
AKatteepitbdlsTCw
3
i
ig. 3. Variation of activation energy obtained by ASTM (a) and (b) and Kissinger (cnd 225 ◦C.
STM method were in general higher than those calculated byissinger method. Nonetheless, for hydrolysis temperatures of 190nd 210 ◦C, similar profiles were observed, while for 225 ◦C, con-rasting behavior was noted. The increase in activation energy ashe degree of polymerization falls was also observed by Calahorrat al. (1989). Supplementary Table 1 shows the values of activationnergy calculated by Calahorra et al. (1989) for cellulose sam-les containing various degrees of polymerization and crystallinity
ndices, employing the Freeman-Carroll and Broido methods. Whenhe results obtained in this study are compared with those obtainedy Calahorra et al. (1989), it is notable that the reaction orderecreased as the degree of polymerization decreased in the cel-
ulose samples prepared by Calahorra et al. (1989), whereas in thistudy the reaction order showed little variation, from 1.03 to 1.23.hus, the fall in the activation energies calculated by the Freeman-arroll and Broido methods, as degree of polymerization decreased,as greater than that obtained in this study.
.3. Influence of crystallinity and crystallite size
As can be seen in Table 2, Ti initially increased as the crystallinityndex and crystallite size in the 1 1 0 and 2 0 0 planes increased, for
(d) methods with DPn and DPw for cellulosic residues hydrolyzed at 190 ◦C, 210 ◦C
cellulose samples hydrolyzed at 190, 210 and 225 ◦C. These resultssuggest that an increase in the crystallinity index and crystallite sizeincreases the initial decomposition temperature of the celluloseresidues. This increase in the crystallinity index and crystallite size(in the 1 1 0 and 2 0 0 planes) may be related to the diminution inthe amorphous cellulose region that was accompanied by weightlosses of 11.6%, 23.8% and 51.9% in the first stages of the dilute acidhydrolysis at 190, 210 and 225 ◦C, respectively. Kim et al. (2010)studied various cellulose samples and noticed that an increase inthe crystallinity index and crystallite size provided higher thermalstability. Poletto et al. (2011) also studied cellulose pulp obtainedby two pulping processes and observed a displacement of the DTGpeak to higher temperatures with increasing crystallite size.
In Table 2, it is possible to observe that, for samples hydrolyzedat 190 and 225 ◦C, the activation energies calculated by both theASTM and Kissinger methods increased as crystallinity and crys-tallite size increased. For 210 ◦C, the influence of crystallinity andcrystallite size was not so clear. Kim et al. (2010) reported that
the activation energy was not affected by the crystallinity index orcrystallite size.The crystalline index and crystallite sizes (in the 1 1 0, 1 1 0and 2 0 0 planes) of cellulose residues hydrolyzed at 190, 210 and
568 L.V.A. Gurgel et al. / Industrial Crops and Products 36 (2012) 560–571
F lp hyd
2tcadst
ig. 4. SEM micrographs of (a) bagasse cellulose pulp, (b) Avicel PH-101, and (c) pu
25 ◦C for 80, 40 and 8 min were similar to Avicel PH-101. CrI forhese residues differed by 8.5%, 5.7% and 3.0% from Avicel and therystallite size differed by 2.7%, 0.9% and 5.0% for the 1 1 0 plane
nd 6.1%, 2.4% and 7.8% for the 2 0 0 plane, whereas for the unhy-rolyzed bagasse cellulose pulp, CrI differed by 33.9% and crystalliteizes by 16.7% (1 1 0) and 22.6% (2 0 0). These results, in addition tohose obtained from the degree of polymerization, thermal analysisrolyzed at 190 ◦C for 80 min, (d) at 210 ◦C for 40 min and (e) at 225 ◦C for 8 min.
and kinetic decomposition studies, also indicate that the celluloseresidues processed by ELA hydrolysis at 190, 210 and 225 ◦C havesimilar physicochemical characteristics to Avicel PH-101. There-
fore, it is suggested that these cellulose residues, representing upto 20% of the starting sugarcane bagasse feedstock (on dry basis),may be used in various commercial applications as microcrystallinecellulose (MCC). This finding represents a potential novel use of the
ops and Products 36 (2012) 560–571 569
cbcpw
3
c4ivimthetFmolb
wcSsalftFdt1fs2ih3
ce2
3
0AalbteiCiiaaOs
Fig. 5. FTIR spectra of cellulose pulp and residues hydrolyzed at (a) 190 ◦C, (b) 210 ◦Cand (c) 225 ◦C for various times.
L.V.A. Gurgel et al. / Industrial Cr
ellulose residues of the dilute acid hydrolysis process, which mayecome an important product in a biorefinery with various appli-ations in developing green materials (Das et al., 2010) and as arecursor in the production of whiskers (nanocrystalline cellulose),ith almost no weight loss (Das et al., 2009).
.4. Scanning electron microscopy (SEM)
Fig. 4 shows SEM micrographs of bagasse cellulose pulp, Avi-el PH-101 and samples hydrolyzed at 190, 210 and 225 ◦C for 80,0 and 8 min, respectively. Fig. 4a shows the bagasse pulp fibers
n their initial condition. In this figure, the fibers exhibit a greatariety of shapes and sizes, characterizing the great heterogene-ty of the bagasse pulp. Fig. 4b shows Avicel PH-101, a commercial
icrocrystalline cellulose, which is mainly composed of particleshat have a rod-shaped fibers. Fig. 4c–e shows cellulose residuesydrolyzed at 190, 210 and 225 ◦C. For 190 ◦C (Fig. 4c), the particlesxhibited smaller length/width ratio than Avicel and it is apparenthat shortening of the fibers took place, giving smaller elements.or 210 ◦C (Fig. 4d), the particles seem to be larger and more frag-ented than for 190 ◦C and Avicel, but large fragments can still be
bserved. For 225 ◦C (Fig. 4e), the particles seem to be rod-shapedike Avicel, but much larger than Avicel, and flat elements can alsoe observed.
Particle size was measured with Image-Pro PlusTM 6.0 soft-are. On average, a sample of one hundred particles was
ollected and analyzed statistically with StatisticaTM 7.0 software.upplementary Fig. 1 shows the particle length distributions of fouramples, Avicel PH-101 and the bagasse pulp samples hydrolyzedt 190, 210, 225 ◦C for 80, 40 and 8 min, respectively. The particleength distributions were best fitted by the lognormal distributionunction. As can be seen in Supplementary Fig. 1, higher hydrolysisemperatures and shorter reaction times produced longer particles.urthermore, higher temperatures and shorter reaction times pro-uced particles more heterogeneous in size, which can be seen fromhe increasing widths of the distributions. From Supplementary Fig.it can be concluded that the cellulose residue hydrolyzed at 190 ◦C
or 80 min exhibited smaller particles than Avicel and that the latterhowed particles smaller than the cellulose residues hydrolyzed at10 and 225 ◦C for 40 and 8 min, respectively. Avicel PH-101 exhib-
ted a particle length of 87.5 ± 34.2 �m, while cellulose residuesydrolyzed at 190, 210 and 225 ◦C exhibited particle lengths of8.0 ± 20.1, 213.2 ± 97.1 and 347.6 ± 114.4 �m, respectively.
The above morphology and particle size analysis leads to theonclusion that cellulose residues hydrolyzed at 190 and 210 ◦Cxhibited particle lengths similar to Avicel, while hydrolysis at25 ◦C produced much larger particles.
.5. Fourier transform infrared spectroscopy (FTIR)
The FTIR spectra of cellulose pulp and samples hydrolyzed in.07% H2SO4 (w/v) at 190 ◦C, 210 ◦C and 225 ◦C are given in Fig. 5.ccording to Bouchard et al. (1989), the appearance of a bandround 1710 cm−1 is attributed to carbonyl formation on cellu-ose. In the cellulose samples hydrolyzed at 210 and 225 ◦C, aand appeared at 1708 cm−1 and increased in intensity as reac-ion time and temperature were increased. According to Bouchardt al. (1989), as the leveling-off degree of polymerization (LODP)s reached, this band appears. All the bands related to CH and/orH2 vibrations at 1430, 1370, 1315 and 895 cm−1 decreased in
ntensity or almost disappeared as the reaction temperature wasncreased from 190 ◦C to 225 ◦C. According to Bouchard et al. (1989),
decreasing intensity or disappearance of these bands indicatesn increase in the degree of oxidation. All the bands attributed toH bending at 1455, 1336 and 1203 cm−1 are reduced in inten-
ity. The bands at 1162, 1115, 1058 and 1035 cm−1, related to
5 rops an
aC(trttea
lwotm
3l
wwmct4pHlDn7abaarsaff(D2dtotwaSlp2c
4
t(ties
70 L.V.A. Gurgel et al. / Industrial C
symmetric C–O–C bridge stretching, anhydroglucopyranose ring,–OR stretching and C–O–C pyranose ring skeletal vibrationPoletto et al., 2011), were unaltered and/or strengthened as reac-ion time was increased for all temperatures. Only for a longeaction time, such as 80 min (190 ◦C), and/or the highest tempera-ure (225 ◦C) and a reaction time of 12 min, was a fall in intensity ofhe bands at 1058 and 1035 cm−1 observed. According to Bouchardt al. (1989), these observations indicate that the pyranose ring islmost unchanged.
It can be concluded that the intensity of the carbonyl bands fol-owed the extent of cellulose depolymerization, which is correlated
ith the number- and weight-average degrees of polymerizationf cellulose residues, especially at higher temperatures. FTIR spec-ra of cellulose residues also indicated that the degree of chemical
odification of cellulose increases with the severity of hydrolysis.
.6. Comparison with bagasse microcrystalline cellulose (MCC) initerature
Tang et al. (1996) prepared MCC by treating bagasse pulpith 2.5 mol/L HCl at 105 ◦C for 15 min. This MCC from bagasseas characterized by XRD and SEM, and the DP was deter-ined by measuring the intrinsic viscosity of the cellulose in a
uproethylenediamine solution. The crystallinity index was foundo be 72%, crystallite size in the 1 0 1 and 2 0 0 lattice planes was.9 and 5.5 nm and DPv was 117. El-Sakhawy and Hassan (2007)repared MCC from bleached bagasse pulp by treating with 2 NCl and/or H2SO4 under reflux for 45 min, employing a solid-to-
iquid ratio of 1:10, and characterized it by XRD, TGA, SEM andP. Again DP was measured by intrinsic viscosity in cuprammo-ium hydroxide solution. Crystallinity was found to be 76% and5%, crystallite size in the 2 0 0 plane was 4.42 nm and DPv was 317nd 299 for HCl and H2SO4, respectively. Ti and Tm were determinedy TGA carried out at a heating rate of 10 ◦C/min under a nitrogentmosphere and were found to be 251 and 252 ◦C for HCl and 325nd 322 ◦C for H2SO4, respectively. For comparison, the celluloseesidues hydrolyzed at 190, 210 and 225 ◦C for 80, 40 and 8 minhowed crystalline indices of 76.5%, 78.5% and 80.6%, respectively,nd crystallite size in 1 0 1 and 2 0 0 planes of 6.082 and 5.070 nmor 190 ◦C, 6.331 and 5.251 nm for 210 ◦C and 4.540 and 4.990 nmor 225 ◦C, which compare well with those obtained by Tang et al.1996) and El-Sakhawy and Hassan (2007), by traditional methods.Pn and DPw were found to be 40 and 205 for 190 ◦C, 12 and 166 for10 ◦C and 14 and 137 for 225 ◦C in this study. Considering that theifference between Mv and Mw depends on sample molar mass dis-ribution and Mw > Mv, it is possible to conclude that the degreef polymerization of MCC prepared by Tang et al. (1996) is closeo that of the cellulose residues obtained in this study. Ti and Tm
ere found to be 212 and 357 for 190 ◦C, 213 and 358 for 210 ◦Cnd 206 and 348 for 225 ◦C, quite similar to those observed by El-akhawy and Hassan (2007). From the above comparisons betweeniterature data and the cellulose residues obtained in this study, it isossible to state that cellulose residues hydrolyzed at 190, 210 and25 ◦C for 80, 40 and 8 min can be considered as microcrystallineelluloses.
. Conclusion
This study showed that the initial decomposition tempera-ure (Ti) of cellulose residues after extremely low acid hydrolysisELA) increased as the degree of polymerization decreases and that
his rise in Ti diminished as hydrolysis became more severe byncreasing the reaction temperature from 190 to 225 ◦C. Activationnergies determined by the ASTM and Kissinger methods demon-trated that the severity of hydrolysis directly affects the energyd Products 36 (2012) 560–571
involved in the main decomposition step of the cellulose residues.Furthermore, XRD studies also showed that initial decompositiontemperature (Ti) increased as crystalline index and crystallite sizein the 1 1 0 and 2 0 0 lattice planes increased. The thermal, XRD andSEM studies demonstrated that the cellulose residues obtained at190, 210 and 225 ◦C for 80, 40 and 8 min, respectively, have simi-lar thermal decomposition behavior, crystallinity index, crystallitesize and particle size to Avicel PH-101 and can also be consideredmicrocrystalline celluloses.
Acknowledgements
The authors are grateful to Conselho Nacional de Pesquisae Desenvolvimento (CNPq), Fundacão de Amparo à Pesquisa doEstado de São Paulo (FAPESP) and Oxiteno for providing financialsupport for this research. The authors also thank the Instituto deQuímica de São Carlos (IQSC), USP at São Carlos (SP, Brazil).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.indcrop.2011.11.009.
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