ARTICLE pubs.acs.org/Biomac
Structural Changes in Microcrystalline Cellulose in Subcritical Water Treatment Lasse K. Tolonen,*,† Gerhard Zuckerst€atter,‡ Paavo A. Penttil€a,§ Walter Milacher,‡ Wilhelm Habicht,|| Ritva Serimaa,§ Andrea Kruse,|| and Herbert Sixta† †
Department of Forest Product Technology, School of Chemical Technology, Aalto University, Helsinki, Finland Lenzing AG, Lenzing, Austria § Department of Physics, University of Helsinki, Helsinki, Finland Institute of Catalyst Research and Technology, Karlsruhe Institute of Technology, Karlsruhe, Germany
)
‡
ABSTRACT: Subcritical water is a high potential green chemical for the hydrolysis of cellulose. In this study microcrystalline cellulose was treated in subcritical water to study structural changes of the cellulose residues. The alterations in particle size and appearance were studied by scanning electron microscopy (SEM) and those in the degree of polymerization (DP) and molar mass distributions by gel permeation chromatography (GPC). Further, changes in crystallinity and crystallite dimensions were quantified by wideangle X-ray scattering and 13C solid-state NMR. The results showed that the crystallinity remained practically unchanged throughout the treatment, whereas the size of the remaining cellulose crystallites increased. Microcrystalline cellulose underwent significant depolymerization in subcritical water. However, depolymerization leveled off at a relatively high degree of polymerization. The molar mass distributions of the residues showed a bimodal form. We infer that cellulose gets dissolved in subcritical water only after extensive depolymerization.
’ INTRODUCTION Cellulose, with the natural production of 1.5 1012 tons annually, is the most abundant natural polymer and the main component in all biomass.1 Cellulose exists as a partially crystalline homopolymer consisting of β(1f4)-linked D-glucose units. In biomass, cellulose forms a complex matrix with the other main components of biomass, hemicelluloses, and lignin. Due to its rigid structure and extensive hydrogen bond network, cellulose is insoluble in water, and only a few solvents are capable of dissolving cellulose as a polymer.1 Hydrothermal treatment in subcritical water has been proposed as a promising method to process biomass and cellulose.2 Subcritical water is pressurized, high temperature liquid water at temperatures below the critical temperature of 374 C. It is characterized by a higher ion product and thus higher Hþ and OH ions concentrations compared to ambient water, and therefore, it offers a highly interesting reaction medium for hydrolysis processes. An additional advantage is that acid neutralization is not required because the Hþ ion concentration is a function of temperature, and decreases when the temperature is lowered. Bobleter3 and Sasaki et al.4 confirmed in the 1990s that cellulose can be largely hydrolyzed in subcritical water without addition of a catalyst. In the following years, the hydrothermal treatment for cellulose was studied extensively by Saka and Ueno,5 Schacht et al.,6 and Kumar and Gupta7 to name a few. However, the conversion mechanism is not fully understood. Based on kinetic data, Sasaki et al.8 suggested that the conversion takes place by two distinct mechanisms: by surface peeling at r 2011 American Chemical Society
relatively low temperatures followed by homogeneous swelling and dissolution when exceeding a temperature of 350 C at 25 MPa. The surface peeling model has been adopted later also by Kamio et al.9 Deguchi et al.10 reported a crystalline-to-amorphous transition of crystalline cellulose at approximately 320 C. The result was based on in situ observation using an optical microscope, in which cellulose became gradually transparent and lost its birefringence. Unfortunately, the depolymerization of cellulose occurring concomitantly was not studied. It is known that cellulose is substantially depolymerized in subcritical water before dissolving as short-chain cellulose or cello-oligomers.8,11 Sasaki et al.8 reported that the degree of polymerization (DP) of the residual microcrystalline cellulose gradually decreased with an increasing degree of conversion. The depolymerization pattern was independent of the reaction temperature. Contrary to the degree of polymerization, crystallinity of cellulose remains relatively unchanged in hydrothermal treatments. Jollet et al.12 did not observe any significant change in the crystallinity by X-ray diffraction and 13C solid-state NMR. In their experiment, 35% of microcrystalline cellulose was hydrothermally solubilized at 190 C. Similarly, Sasaki et al.8 reported that the crystallinity of cellulose residue was not affected by subcritical water treatment. According to Kumar et al.,13
Received: March 14, 2011 Revised: May 20, 2011 Published: June 06, 2011 2544
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degrees of conversion, probably due to the absence of humic compounds. The second filtration was faster, taking 530 min. The moist solid cellulose residues collected from filtration were extracted with acetone in a Soxhlet extractor for 18 h to remove humins, which are typically formed under hydrothermal conditions. The extracted samples were dried at room temperature to an equilibrium moisture content of about 96% before being analyzed. The degree of conversion (X) of solid cellulose to water-soluble reaction products was analyzed gravimetrically from both unextracted and extracted cellulose residues according to eqs 2 and 3, respectively. All the weights are calculated as oven-dried cellulose. Xunextracted ¼
wðcellulose inÞ wðunextracted cellulose outÞ wðcellulose inÞ
ð2Þ
wðcelluloseinÞ wðextracted cellulose outÞ wðcellulose inÞ
ð3Þ
Xextracted ¼
Figure 1. Reactor setup.
however, crystallinity of microcrystalline cellulose increased slightly from 78 to 84% during subcritical water treatment. Parallel to the applied research on subcritical water treatment of cellulose, a more fundamental approach is desired to develop a deeper understanding of the conversion mechanism. In the present study microcrystalline cellulose was treated in subcritical water and the resulting structural changes within the cellulose residues were thoroughly investigated. Further, the parallel formation of watersoluble products was quantified as a function of different reaction conditions.
Besides the gravimetric analyses, the content of the total organic carbon (TOC) in the liquid phase was utilized to determinate the degree of conversion. TOC was analyzed by a Dimatoc 2000 analyzer. The degree of conversion based on the TOC was calculated by eq 4. The term TOCinitial was calculated based on the amount of initial cellulose concentration, and c(cellulose) and TOCliquid were analyzed from the liquid phase after the reactor.
’ EXPERIMENTAL METHODS
The viscosity average degree of polymerization was determined from the intrinsic viscosity measurement after dissolution in cupriethylenediamine according to the SCAN-CM 15:99 method. The correlation [η] = 0.42 DPv reported by Gruber and Gruber15 was used to calculate the degree of polymerization from viscosity values. The molar mass distribution was analyzed by size exclusion chromatography coupled with MALLS/RI-detection after dissolution in LiCl/ DMAc.16 The molar mass distributions curves were multiplied by the yield of residues, equal to (1 degree of conversion), as in eq 5. In this way, the molar mass fractions are compared with the fractions in the untreated sample instead of the fractions in the residues. In eq 5, M is a molar mass, w(M) is the mass fraction of polymers that have the molar mass of M or less, and X is a degree of conversion.
Two series of experiments were conducted using reaction times of 2.5 and 6 s. The reaction temperatures were ranging from 245 to 319 C. The pressure was kept constant at 25 MPa. Microcrystalline cellulose (MCC) obtained from Merck (No. 1.02330.0500) was used as received. A suspension of 2.1 wt % of MCC in water was prepared by adding MCC to 900 g deionized water having conductivity less than 1.0 μS/m. The suspension was mixed by a magnetic stirrer for at least one hour at room temperature prior to the experiments. A plug-flow reactor as shown in Figure 1 was used in the experiments. The cellulose suspension was poured into the feeding tank with a mixing system. Existing water in the system diluted the suspension to a concentration of 1.64 wt %. The suspension was pumped from the feeding tank by a high-pressure pump (Lewa EK3) through the heater unit through the vertical reactor and then to the cooling unit. The volume of the reactor part was 1.71 cm3, and the inner diameter was 1.60 mm. A back-pressure regulator (Tescom 26-1721-24A) was used to control the pressure. The reaction temperature was set by adjusting the temperature of the heating element. The reaction time treaction was calculated according to eq 1, in which Vreactor is the volume of the reactor part excluding the heater and cooler, m_ is the mass flow and Fwater is the density of water at reaction temperature and pressure. The density of water as a function of temperature and pressure was obtained from a database.14 In all calculations, cellulose suspension was assumed to behave like pure water. treaction ¼
Vreactor 3 Fwater m_
ð1Þ
The solid cellulose residue was filtered from the liquid (filter paper S&S 5892) at room temperature and atmospheric pressure using a B€uchner funnel and a vacuum pump. The filtered liquid was refiltrated using a 0.45 μm nylon filter (Whatman No. 7404-044) to recover all solid particles. Typically, the time required for the first filtration was from 30 min to two hours. The filtration was faster for the samples of low
XTOC ¼
TOCliquid cðTOCliquid Þ ¼ 72 TOCinitial cðcelluloseÞ 162
differential mass fractionðlog10 MÞ ¼
dwðMÞ ð1 XÞ d log10 M
ð4Þ
ð5Þ
The molar mass distribution curves were deconvoluted by Matlab using the curve fit toolbox. The deconvolution was done with a fit of two Gaussian distributions as illustrated in eq 6. The term a is the area of a peak, μ describes the peak position, and σ is the width of a peak as standard deviation. The Gaussian distributions are differentiated by subscripts low and high. ahigh ðlog10 M μlow Þ2 ðlog10 M μlow Þ2 alow Fitðlog10 MÞ ¼ pffiffiffiffiffiffi exp þ pffiffiffiffiffiffi exp 2 2σlow 2σ2high 2πσlow 2πσhigh
ð6Þ The w(M) value was transformed to w(DP) according to eq 7. Normalized number weighted distribution of DP was then defined according to eq 8. wðDPÞ ¼ 2545
wðMÞ 162
ð7Þ
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Table 1. Degree of Conversion vs Reaction Conditions degree of conversion (%) before extraction
after extraction
TOCa
No.
t (s)
T (C)
1.1
2.5
246
0.3
2.2
1.1
1.2
2.5
275
7.8
10.0
8.5
1.3
2.5
287
25.5
24.3
25.9
1.4
2.5
300
59.6
52.5
60.2
1.5
2.5
318
98.8
97.0
100.0
2.1
6.0
245
1.7
4.9
1.0
2.2
6.0
275
30.9
32.4
34.8
2.3 2.4
6.0 6.1
287 300
64.3 94.5
64.9 90.1
73.2 100.0
2.5
6.2
319
100.0
96.6b
100.0
gravimetric
gravimetric
Figure 2. Arrhenius plot of cellulose conversion using pseudo-firstorder reaction kinetics.
a Total organic carbon based degree of conversion. b High amount of humins.
1 dwðDPÞ differential number fractionðDPÞ ¼ DP 3 dDP 3
Z 0
¥
1 wðDPÞ dDP DP
ð8Þ A scanning electron microscope (FE-SEM DSM 982 Gemini device by Zeiss) was used for SEM-micrographs in the backscatter electron mode at 10 keV beam energy. To prevent charging of the nonconductive fibers due to beam electrons, all samples were coated by a few nanometers of Pt/Pd (Cressington Sputter Coater 208 HR). The X-ray measurements were carried out with a wide-angle X-ray scattering (WAXS) setup consisting of an X-ray generator (Siemens), an X-ray tube with a Cu anode (λ=1.541 Å) and point focus, a collimating Montel multilayer as monochromator, and a two-dimensional image plate detector (MAR345, Marresearch). The measurements were done by using perpendicular transmission geometry, having the sample powders pressed by hand in metal rings and sealed with Mylar foils. The data were corrected for absorption, measurement geometry, scattering by air and Mylar foils, and noise caused by the image reading process of the detector. The calculation of crystal sizes and crystallinities was done as described by Penttil€a et al.17 13 C CP-MAS NMR was used for the structural analysis of cellulose residue. The method is described elsewhere.1820
’ RESULTS AND DISCUSSION Conversion Kinetics. Subcritical water treatment converted microcrystalline cellulose either partially or completely to water-soluble reaction products, as listed in Table 1. As expected, the degree of conversion increased with increasing temperature and reaction time. The conversion was measurable already at 245 C, and at 300 C more than half of the cellulose was converted when a reaction time of 2.5 s was used. At 300 C the reaction time of 6.0 s resulted in a nearly complete conversion. For comparison, Sasaki et al.21 reported conversions of 43.1 and 97.7% in 2.5 and 7.3 s, respectively, at 320 C. The higher reactivity observed in this study may be partially an artifact caused by the fact that the exact temperature profiles in the heater and cooler were not known. Consequently, the time in the heater and cooler was excluded from the reported reaction times, which inevitably affects the observed reaction rate. It is also reported that the reactivity of microcrystalline
cellulose is higher in the beginning of the conversion.22 Therefore, the conversion now observed at 245 C may exaggerate the reactivity of microcrystalline cellulose compared to isothermal studies with longer reaction times. Despite the limited precision of the temperature profile, the degree of conversion was determined reliably. The gravimetrically determined and the TOC-based degree of conversion of unextracted cellulose were in good agreement with each other. The degree of conversion after the acetone extraction was somewhat higher, which may be attributed to the removal of nonsolubilized or adsorbed reaction products upon extraction. The assumption is supported by the fact that the acetone extraction improved the brightness of the cellulose residues based on visual examination. Figure 2 shows the Arrhenius plot of the present results according to the pseudo-first-order reaction kinetics. The good linear fit with the results indicates that the conversion follows the pseudo-first-order reaction kinetics provided that the assumption of an Arrhenius-type temperature dependency is accepted. The data points obtained by using different reaction times, and corresponding to different extents of conversion, follow the same plot. This suggests that the activation energy for cellulose conversion is constant in the studied temperature range. The calculated apparent activation energy is 225 kJ/mol, which is higher than previously reported 169 and 145 kJ/mol without catalyst or 100 and 144 kJ/mol using a dilute acid catalyst.6,8,23 However, due to the aforementioned limitations with the determination of the exact reaction time, and because only a limited number of the experiments were carried out in a relatively narrow temperature range, the obtained activation energy must be judged critically. The comparison with dilute acid hydrolysis at lower temperatures may give additional information about the conversion mechanisms. The dilute acid hydrolysis is commonly divided into two stages.24 In the first stage, cellulose undergoes a rapid weight loss. The high reactivity is associated with accessible amorphous regions in cellulose that are more vulnerable to chemical attacks than the crystalline regions.21 The fast initial stage is followed by a slow hydrolysis step of the remaining cellulose. The hydrolysis reaction is heterogeneous because cellulose is not soluble in dilute acid. However, several authors have shown that the weight loss in the later stage follows accurately a pseudo-first-order reaction kinetics of homogeneous reactions.2527 In this respect, the observed conversion of cellulose in subcritical water appears to be comparable to that occurring in dilute acid hydrolysis. 2546
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Figure 3. SEM micrograph of cellulose residues using magnifications of 1000 (upper) and 50000 (lower). Sample numbers and degree of conversion shown in figures. Scale bars: 20 μm (upper) and 500 nm (lower).
The reaction temperature has also an influence on the solvent properties of water.2830 It has been suggested that the shift in solvent properties affects the kinetics of cellulose conversion.8,10 However, in the present study the unchanged activation energy implies that the reaction mechanism of the hydrothermal conversion is not markedly affected. The density of water at a pressure of 25 MPa is 0.8273 kg/dm3 at 245 C, 0.7056 kg/dm3 at 319 C, and 1.008 kg/dm3 at 25 C. For 245 C, 319 and 25 C the dielectric constants of pure water are 23.7, 16.4 and 79.4, and the ion products 11.0, 11.3, and 13.9, respectively.14,29,30 Particle Size and Appearance. The acetone-extracted cellulose residues from experiments 1.11.4 and 2.12.3, as listed in Table 1, were subjected to a comprehensive analytical characterization. The size and shape of the particles were visualized by a SEM microscope using magnifications of 25050000. Selected SEM micrographs are presented in Figure 3. In low magnification images it can be seen that the average length of the untreated MCC particles was approximately 20 μm and the particle size of the biggest ones was estimated to be lower than 50 μm. The diameter of the particles decreased markedly in subcritical water treatment and after an extensive conversion the particles showed a tendency to agglomerate (sample 2.3). The drastic reduction in size of the particles suggests a disintegration of large particles rather than their peeling from the surface layer by layer. The higher magnification SEM micrographs exhibited nanosized whiskers on the surface of the untreated and treated cellulose particles. In the extensively converted residue of the sample 2.3 it appears as if the whiskers were more condensed than in the reference or in the samples 1.2 and 2.2. In the high magnification micrographs, we
Table 2. Degree of Polymerization and Polydispersity vs Degree of Conversion
a
No.
X (%)
DPva
DPwb
DPnb
polydispersityb
REF
0.0
264
203
90
2.24
2.1
1.0
214
172
76
2.25
1.1
1.1
250
176
82
2.15
1.2 1.3
8.5 25.9
195 169
141 117
68 61
2.08 1.90
2.2
34.8
167
124
65
1.92
1.4
60.2
160
104
56
1.86
2.3
73.2
157
105
62
1.68
Viscosity determination. b GPC determination.
did not find the granular surface of precipitated cellulose which has been published elsewhere.11 Depolymerization. The degree of polymerization of MCC was examined by viscosity and GPC analyses. Not surprisingly, low molecular mass crystalline MCC was depolymerized in subcritical water. As listed in Table 2, the viscosity average DP clearly decreased already at very early stages of the treatment. On the other hand, the DP remained relatively high even after extensive yield losses. The relationship between the viscosity average DP and the degree of conversion was independent of reaction time and temperature. This further supports the hypothesis that the water properties do not significantly affect the conversion mechanism in the studied temperature range but the DP in the residue is merely a function of the degree of conversion. 2547
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Figure 4. Number weighted distribution of degree of polymerization of cellulose residues. DP range of 0120 enlarged in the small diagram. Degrees of conversion in the direction of the arrow shown in figure. Thick solid line, reference sample; thin solid line, reaction time 2.5 s; and thin dashed line, reaction time 6 s.
Figure 5. Molar mass distribution of cellulose residues at different degrees of conversion. Areas under curves correspond to the yield of solid residue. Arrow indicates a possible trace of precipitated cellulose.
The weight average DP derived from the GPC analyses followed the same pattern as the viscosity average DP. Polydispersity, that is, the ratio between the weight and number average DP, gradually decreased with progressive conversion. The fast decrease in DP in the beginning of the conversion is a well-recognized phenomenon of the dilute acid hydrolysis, and it is often related to the reactive, at least partially amorphous, regions in cellulose.25,27,31 Thereafter, only less reactive crystalline cellulose regions are considered to remain. At the same time, the DP approaches the so-called leveling-off DP.25,31 In the present study, the DP remained relatively unchanged after the conversion of 25.9%, thus, demonstrating the similar leveling-off behavior than during dilute acid hydrolysis. Similarly, Yu and Wu22 reported a decreasing reactivity for microcrystalline cellulose after a conversion of 20% when treated in subcritical water at 230 C. It is plausible that 7080% of microcrystalline cellulose is less susceptible to depolymerization in subcritical water than the more reactive 2030%. While the difference in the reactivity is easy to argue, it is somewhat more challenging to elucidate how the DP remains unchanged after a far progressed conversion. Sharples26 suggested that the leveling-off DP results from the fact that noncrystalline regions are randomly distributed along the cellulose microfibrils. This leads to an exponential number weighted distribution of DP after the hydrolysis of these noncrystalline regions. Sharples
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Figure 6. Deconvolution of molar mass distribution by a fit of two Gaussian distributions for sample no. 1.2, degree of conversion 25.9%.
assumed that end-attack is the mechanism for depolymerization, that all the chain-ends are attacked at a similar rate, and that the formed monomers or short oligomers dissolve from the residue. Consequently, the molar mass distribution, and of course the average DP as well, remain largely unchanged in the cellulose residue throughout the conversion. In addition, Shaples’ model is in accordance with the widely accepted pseudo-first-order reaction kinetics. Figure 4 presents the number weighted distributions of DP from the present experiments. The nearly linear lines in the semilogarithmic diagram demonstrate that the length of the cellulose chains was nearly exponentially distributed above a DP of 100. These chains corresponded to 5369 wt % of the residues, depending on the degree of conversion. The slope of the lines in the diagram increased with progressing conversion. The exponential distribution was not valid for the short cellulose chains. For them the number weighted distribution of DP deviated markedly from the linear part of the lines showing an excess of DP 1560 chains. In Figure 4, both the reference sample and the subcritical water-treated samples show the similar linear part and the deviating part at lower DPs. Excess of short chains is also reported in the literature.32 Yu and Wu reported that chains with the DP 28 or less are soluble at 270 C.33 This offers a credible explanation why the number of chains peak around DP 25, and why the number of shorter chains is limited in the residues. We did not detect chains shorter than 10 glucose units in the residue because they are evidently completely soluble in subcritical water. Sharples’ hypothesis of the randomly distributed noncrystal regions along the cellulose microfibrils is also supported by the nearly exponential distribution of the long chains. However, the number-weighted distributions of DP shown in Figure 4 do not remain unchanged, as it would be if the depolymerization occurred exclusively at the chain ends at an identical rate.26 Figure 5 shows the molar mass distributions of cellulose residues where the area below the curve corresponds to the yield of the residue. The general shape of the molar mass distributions followed a bimodal distribution, revealing a high intensity peak at a higher DP and a low intensity peak at a lower DP, mainly formed as a shoulder. For further analysis, the curves were deconvoluted by using two Gaussian distributions, as illustrated in Figure 6. Deconvolution parameters for fitted Gaussian distributions are listed in Table 3. 2548
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Table 3. Deconvolution Parameters average μ
peak area a
standard deviation σ
goodness of fit
No.
conversion (%)
low
high
low
high
low
high
sum of squared errors
R-squared
REF
0.0
0.151
0.840
3.723
4.463
0.177
0.296
0.320
0.996
2.1
1.0
0.208
0.770
3.732
4.413
0.189
0.282
0.382
0.995
1.1
1.1
0.175
0.817
3.720
4.419
0.178
0.284
0.259
0.997
1.2
8.5
0.229
0.677
3.721
4.359
0.193
0.259
0.311
0.995
1.3
25.9
0.223
0.519
3.713
4.323
0.195
0.234
0.164
0.996
2.2
34.8
0.177
0.512
3.724
4.312
0.198
0.258
0.188
0.995
1.4
60.2
0.103
0.298
3.631
4.257
0.171
0.239
0.099
0.992
2.3
73.2
0.075
0.191
3.779
4.254
0.218
0.228
0.023
0.997
Table 4. Crystallinity, Paracrystallinity, and Sizes of Crystallites in Different Planes at Different Degrees of Conversion NMR
WAXS size of crystallites (Å)
a
No.
X (%)
crystallinity (%)
paracrystallinity (%)
crystallinity (%)
(1 1 0)
(1 1 0)
(2 0 0)
REF
0.0
55
23
52
44
50
55
2.1
1.0
53
23
55
40
55
57
1.1
1.1
53
23
55
41
55
59
1.2
8.5
52
17
57
39
61
64
1.3
25.9
51
18
57
39
63
62
2.2
34.8
53
17
57
38
67
66
1.4 2.3
60.2 73.2
52 51
15 15
55 naa
40 naa
74 naa
67 naa
Not analyzed due to too small amount of the sample.
The area of the high DP peak, ahigh, gradually decreased with increasing conversion. The area of low DP shoulder, alow, slightly increased at low degrees of conversion. Later, the shoulder gradually disappeared. The average position of the high DP peak, μhigh, was slightly shifted toward a lower DP, whereas the low DP peak retained its position. The ratio ahigh/(ahighþalow) stayed between 0.70 and 0.85 throughout the conversion. On the grounds of the good agreement between the twopeak deconvolution model and the analyzed molar mass distributions, we assume that the cellulose residues indeed consist of two populations. We can speculate that certain regions of cellulose, for example, domains located on the surface, are more accessible for depolymerization than the others, which eventually leads to a bimodal distribution. The bimodal distribution already existed in the starting material (MCC), which was probably the result of acid-catalyzed depolymerization as well. It is important to try to elucidate the origin of the low DP population. It is know that microcrystalline cellulose partly dissolves as short chain polymers in sub- or supercritical water and that the DP range of these dissolved chains is roughly comparable the low DP population we observed.11,21,34 These chains are known to adsorb relatively fast at ambient conditions.11,33 Therefore it is possible that the low DP population is formed either before the chains are dissolved, or after dissolution and back-adsorption. One way to address this question is to investigate the amount of cellulose II polymorph in the residues because it is well-known that precipitated cellulose occur as cellulose II.1,8 However, we did
not find substantial amounts of cellulose II by WAXS determination, as shown in Figure 8. The WAXS results of sample 1.4 revealed only traces of the (1 1 0) reflection of cellulose II peak can be found at 12.3 degree. It is probable that precipitation did occur to low extent when high reaction temperatures and short reaction time of 2.5 s was used because these conditions favor the formation of relatively high DP chains. Precipitated cellulose most likely caused the small peak in the MMD indicated by an arrow in Figure 5. Nonetheless, precipitation is apparently not the mechanism creating the low DP population. Thus, we hypothesize that the high DP cellulose fractions are heterogeneously attacked forming the low DP population which in turn dissolves in water under subcritical and supercritical conditions and partly precipitate upon cooling. The hypothesis will be tested in a future study by determining the molar mass distributions of the dissolved fractions. Crystallinity. Crystallinity and sizes of crystallites were investigated by 13C CP-MAS NMR and WAXS analyses. The results are tabulated in Table 4. The NMR spectra of the C4 region and WAXS scattering intensities of selected samples are presented in Figures 7 and 8, respectively. The crystallinity of the cellulose residues was almost not affected by the extent of conversion. The NMR method gave crystallinities between 51 and 55%, whereas the WAXS measurements ranged between 52 and 57%. Although the value of the crystallinity is typically method-dependent,35 the crystallinities determined by WAXS and NMR were in the same range. The paracrystallinity, which is related to the surfaces of the 2549
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Figure 7. 13C CP-MAS NMR spectra of the C4 region of selected samples. Degrees of conversion shown in figure.
Figure 8. WAXS intensities of selected samples. Degrees of conversion and peak assignments shown in figure. Arrow shows vague cellulose II reflection in sample 1.4 after conversion of 60.2%.
cellulose crystallites20 decreased with the progressing conversion. The reported NMR crystallinity includes also the paracrystalline part. The size of the crystallites in the (1 1 0) plane slightly decreased, whereas the size significantly increased in the (1 1 0) and (2 0 0) planes. The practically unchanged crystallinity is in agreement with the results reported in the literature for hot water treated celluloses.8,12,13 All the samples contained significant amounts of noncrystalline regions, also, after extensive conversion. Yu and Wu recently showed that the reactivity of microcrystalline cellulose increases in subcritical water after microcrystalline cellulose is transformed to the amorphous state by extensive ball-milling.36 This traditional concept of the higher reactivity of the amorphous regions is supported in other studies as well.27,31 However, the assumed higher reactivity of noncrystalline cellulose is in clear contradiction to experimental findings that the determined crystallinity often remains unchanged when cellulose is hydrolyzed, as pointed out by McKeown and Lyness already in 1960.37 Also, in this study, the apparent rate of conversion was equal for the crystalline and noncrystalline fractions because their ratio was not much affected by the extent of conversion. We may
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speculate that the noncrystalline regions have limited accessibility, which explains why their apparent hydrolysis rate is comparable to that of the crystalline regions. A possible arrangement in which noncrystalline regions are entrapped inside aggregated microfibrils has been presented in the literature.20 In the arrangements, the surrounding microfibrils protect the entrapped noncrystalline regions until they are dissolved. Therefore the dissolution rate of the noncrystalline regions cannot much exceed that of the crystalline regions. Consequently, also, the crystallinity remains unchanged. It is also possible that amorphous cellulose is indeed dissolved and is later reformed when hot water treated samples are dried. However, this is not plausible because it is generally thought that drying leads to the formation of new hydrogen bonds and recrystallization, which would increase the crystallinity rather than reducing it. The decreasing paracrystallinity and the increasing crystallite dimensions in the (1 1 0) and (2 0 0) planes implies that the crystallites are aggregated during the subcritical water treatment or, alternatively, small crystallites are preferably dissolved and disappear from the solid phase. The increase in the crystallite dimensions would also explain the decreasing paracrystallinity because the ratio of area-to-volume would decrease concomitantly. We also compared the ratio of high-to-low DP peaks as derived from the molar mass distributions, ahigh/(ahighþalow), to the crystallinity of the residues, but this did not reveal a clear and confident correlation.
’ CONCLUSIONS We investigated the effect of subcritical water treatment on the structure of microcrystalline cellulose at temperatures ranging from 245 to 319 C at a constant pressure of 25 MPa. We found out that the conversion kinetics apparently follow a pseudo-firstorder reaction kinetics, while the DP of the solid residue remains at a relatively high value (leveling-off DP) even after extensive conversion. This compares very well with the behavior of low temperature dilute acid hydrolysis. Further, the DP of the cellulose residue in the temperature range investigated is only dependent on the degree of conversion, which implies that the investigated reaction temperatures have only a minor effect on the depolymerization pattern. The crystallinity of microcrystalline cellulose remains practically unchanged throughout the entire treatment. The results from WAXS and solid state NMR show an increase in the size of the crystallites, which implies either an aggregation forming bigger crystals or a disappearance of small crystallites. Further examination of the results from GPC measurements reveals that the number weighted distribution of DP follows approximately an exponential distribution for the cellulose chains longer than 100 glucose units. The exponential distribution, however, is not valid for the short chains. The molar mass distributions of both the untreated microcrystalline cellulose and the cellulose residues from subcritical water treatment are characterized by a distinct bimodal shape, indicating a two-phase reaction mechanism. Based on these findings, we hypothesize a heterogeneous depolymerization of cellulose where the low molar mass fraction is gradually dissolved in subcritical water. ’ AUTHOR INFORMATION Corresponding Author
*Phone: þ358505124196. E-mail: lasse.tolonen@aalto.fi. 2550
dx.doi.org/10.1021/bm200351y |Biomacromolecules 2011, 12, 2544–2551
Biomacromolecules
’ ACKNOWLEDGMENT Forest Cluster Ltd. and the Finnish Funding Agency for Technology and Innovation TEKES as a part of the Future Biorefinery program are thanked for the funding of the work. Heikki Tulokas, Armin Lautenbach, Thomas Tietz, and Matthias Pagel are gratefully acknowledged for the excellent experimental and technical support. David Hoffman and Andrew Plowman are thanked for linguistic advice.
ARTICLE
(33) Yu, Y.; Wu, H. Energy Fuels 2010, 24, 1963–1971. (34) Ehara, K.; Saka, S. Cellulose 2002, 9, 301–311. (35) Park, S.; Baker, J. O.; Himmel, M. E.; Parilla, P. A.; Johnson, D. K. Biotechnol. Biofuels [Online] 2010, 3, Article 10 http://www.biotechnologyforbiofuels.com/content/3/1/10 (accessed May 4, 2011). (36) Yu, Y.; Wu, H. AIChE J. 2011, 57, 793–800. (37) McKeown, J. J.; Lyness, W. I. J. Polym. Sci. 1960, 47, 9–17.
’ REFERENCES (1) Klemm, D.; Heublein, B.; Fink, H.; Bohn, A. Angew. Chem., Int. Ed. 2005, 44, 3358–3393. (2) Kruse, A. Biofuels, Bioprod. Biorefin. 2008, 2, 415–437. (3) Bobleter, O. Prog. Polym. Sci. 1994, 19, 797–841. (4) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 1998, 13, 261–268. (5) Saka, S.; Ueno, T. Cellulose 1999, 6, 177–191. (6) Schacht, C.; Zetzl, C.; Brunner, G. J. Supercrit. Fluids 2008, 46, 299–321. (7) Kumar, S.; Gupta, R. B. Ind. Eng. Chem. Res. 2008, 47, 9321–9329. (8) Sasaki, M.; Adschiri, T.; Arai, K. AIChE J. 2004, 50, 192–202. (9) Kamio, E.; Sato, H.; Takahashi, S.; Noda, H.; Fukuhara, C.; Okamura, T. J. Mater. Sci. 2008, 43, 2179–2188. (10) Deguchi, S.; Tsujii, K.; Horikoshi, K. Green Chem. 2008, 10, 191–196. (11) Yu, Y.; Wu, H. Ind. Eng. Chem. Res. 2009, 48, 10682–10690. (12) Jollet, V.; Chambon, F.; Rataboul, F.; Cabiac, A.; Pinel, C.; Guillon, E.; Essayem, N. Green Chem. 2009, 11, 2052–2060. (13) Kumar, S.; Gupta, R.; Lee, Y. Y.; Gupta, R. B. Bioresour. Technol. 2010, 101, 1337–1347. (14) Standard Reference Data Program: Thermophysical Properties of Fluid Systems. http://webbook.nist.gov/chemistry/fluid/ (accessed November 29, 2010). (15) Gruber, E.; Gruber, R. Papier 1981, 35, 133–141. (16) Schelosky, N.; Roeder, T.; Baldinger, T. Papier 1999, 53, 728–738. (17) Penttil€a, P. A.; Varnai, A.; Lepp€anen, K.; Peura, M.; Kallonen, A.; J€a€askel€ainen, P.; Lucenius, J.; Ruokolainen, J.; Siika-aho, M.; Viikari, L.; Serimaa, R. Biomacromolecules 2010, 11, 1111–1117. (18) Larsson, P. T.; Wickholm, K.; Iversen, T. Carbohydr. Res. 1997, 302, 19–25. (19) Wickholm, K.; Larsson, P. T.; Iversen, T. Carbohydr. Res. 1998, 312, 123–129. (20) Zuckerst€atter, G.; Schild, G.; Wollboldt, P.; R€oder, T.; Weber, H. K.; Sixta, H. Lenzinger Ber. 2009, 87, 38–46. (21) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K. Ind. Eng. Chem. Res. 2000, 39, 2883–2890. (22) Yu, Y.; Wu, H. Ind. Eng. Chem. Res. 2010, 49, 3902–3909. (23) Mok, W. S.; Antal, M. J.; Varhegyi, G. Ind. Eng. Chem. 1992, 31, 94–100. (24) Sharples, A. I. In Degradation of Cellulose and Its Derivatives, 1st ed.; Bikales, N. M., Segal, L., Eds.; Wiley-Interscience: New York, 1971; Vol. 2, pp 9911006. (25) Millett, M. A.; Moore, W. E.; Saeman, J. F. Ind. Eng. Chem. 1954, 46, 1493–1497. (26) Sharples, A. Trans. Faraday Soc. 1957, 53, 1003–1013. (27) Nelson, M. L. J. Polym. Sci. 1960, 43, 351–371. (28) Kruse, A.; Dinjus, E. J. Supercrit. Fluids 2007, 39, 362–380. (29) Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. Data 1981, 10, 295–304. (30) Uematsu, M.; Franck, E. U. J. Phys. Chem. Ref. Data 1980, 9, 1291–1306. (31) Hakansson, H.; Ahlgren, P.; Germgard, U. Cellulose 2005, 12, 327–335. (32) Wood, B. F.; Conner, A. H.; Hill, C. G. J. Appl. Polym. Sci. 1981, 37, 1373–1394. 2551
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