Letter pubs.acs.org/macroletters
Hydrothermal Transformation of Wood Cellulose Crystals into Pseudo-Orthorhombic Structure by Cocrystallization Tomoko Kuribayashi,† Yu Ogawa,*,‡,§ Cyrille Rochas,‡,§ Yuji Matsumoto,† Laurent Heux,‡,§ and Yoshiharu Nishiyama*,‡,§ †
Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-Ku, Tokyo 113-8657, Japan CNRS, CERMAV, F-38000 Grenoble, France § Univ. Grenoble Alpes CERMAV, F-38000 Grenoble, France ‡
S Supporting Information *
ABSTRACT: The ultrastructural transformation of wood cellulose crystals by hydrothermal treatment was followed by synchrotron and standard X-ray scattering experiments. When treated at 200 °C for 2 h in the presence of an excess of water, a significant sharpening of the equatorial reflections of crystalline cellulose was observed, and the average crystallite size, estimated from the X-ray line broadening, was twice as large as that of untreated wood cellulose. During the treatment, the cellulose structure was converted from the native monoclinic form of cellulose I into a pseudo-orthorhombic system, coined as cellulose I′, a transformation occurring only with an excess of water, above 180 °C and after more than half an hour. In situ experiments indicated that the increase of crystallite size was likely due to cocrystallization of individual crystallites rather than to the crystallization of the amorphous domains of cellulose.
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We sealed samples of beech (Fagus crenata, hard wood), Japanese cedar (Cryptomeria japonica, soft wood), and bamboo (Phyllostachys heterocycla) with various water contents in glass tubes (diameter = 3 mm, wall thickness = 200 μm) for analysis with an X-ray beam of 16 keV (λ = 0.775 Å) at D2AM beamline (European Synchrotron Radiation Facility). The samples were mounted on a sample exchanger equipped with thermal control for in situ X-ray diffraction analysis. These samples were stepwise heated to 200 °C with 10 °C steps in 3 h, kept at 200 °C for up to 3 h, and then allowed to cool to room temperature in 4 h. Two-dimensional diffraction patterns were recorded with a CCD camera at each step and numerically deconvoluted into isotropic background and anisotropic components according to the method proposed by Nishiyama et al.12 Since cellulose consists of microfibrils that are roughly oriented parallel to the long axis of wood, the anisotropic component was assumed to be mainly from crystalline cellulose, while the isotropic component was attributed to scattering from water and other matrix components, as well as cellulose microfibrils that were not aligned parallel to the wood long axis. Other samples were also inserted into a thin-walled glass capillary (1 mm diameter) and annealed in a stainless-steel autoclave immersed in an oil bath at various temperatures, followed by ice quenching. These samples were then subjected
ignified plant cell walls represent the major biomass on earth and, as such, participate in the bases of our life. In many industrial applications, these cell walls experience elevated temperature in the presence of various quantities of water. Examples of these processes range from kiln drying of wood to its thermal treatment for durability increase,1,2 thermomechanical pulping,3 hydrothermal pretreatment for enzymatic saccharification,4 and torrefaction for biogas or charcoal production,5 and so on. The heating protocol and humidity control are specific to each application and, even within a given process, the hydration and temperature history can differ within the samples themselves due to limited thermal and moisture diffusion. Hydrothermal pretreatments in the context of second-generation biofuel development are achieved with excess water, while thermal treatments under relatively dry conditions confer durability to wood. The thermal treatment alters both the chemical compositions6−9 and the fine spatial organization4,10−12 of the wood cell wall. In this study, we are focusing on the latter. Under immersion conditions, a coarsening of the matrix components and an increase of lateral dimensions of cellulose crystallites are observed for different types of cellulose-based biomass by X-ray and neutron scattering experiments.4,12 While exploring wide operating conditions, we found that when never-dried beech wood (dry basis water content = 130%13) was treated at 200 °C in an autoclave, diffraction peaks became unprecedentedly sharp for wood cellulose, with significant position shifts. We, thus, decided to carefully analyze the corresponding structural evolution under different hydrothermal conditions. © XXXX American Chemical Society
Received: April 6, 2016 Accepted: May 23, 2016
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DOI: 10.1021/acsmacrolett.6b00273 ACS Macro Lett. 2016, 5, 730−734
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ACS Macro Letters to diffraction analyses using a Ni-filtered Cu Kα X-ray beam (λ = 1.5418 Å) from a Philips PW3830 X-ray generator at room temperature to investigate effects of the treatment duration and temperature. Details of diffraction peak fitting analyses and crystallite size estimations are given in the Supporting Information (Figures S1−S3). Figure 1 compares the two-dimensional wide-angle X-ray diffraction patterns, together with the equatorial and meridional profiles of wet (water content = 130%) and dry (water content = 10% conditioned at 58% relative humidity) beech wood before and after the in situ heating treatment. Before the treatment, the patterns are typical of wood cellulose samples, and the reflection positions and intensity ratios correspond to those of cellulose Iβ, with rather broad reflection widths due to the small crystallite size of wood cellulose (Figure 1a,c). On the equator two characteristic peaks of cellulose Iβ, 110/11̅0, and 200 are visible and centered at around s = 0.18 and 0.25 Å−1, respectively. After the annealing at 200 °C, the diffraction diagram from the dry sample (Figure 1d) remained almost identical to that of the untreated sample. On the other hand, the wet sample showed a spectacular modification (Figure 1b), while the broad scattering area from water (framed) became less intense. In fact, water migrated toward the tip of the capillary, which was outside the heating block of the sample exchanger. Since the macroscopic sample morphology did not change during the treatment, one can assume that the solid content inside the beam was constant throughout the experiment. The 200 reflection, corresponding to the distance between pyranosyl sheets in cellulose Iβ,14 became much sharper and shifted toward a lower diffraction angle after the annealing treatment under wet conditions. The corresponding reflection of 200 shifted to s = 0.2565 Å−1, a value higher than that of the typical one for dry annealed wood cellulose near s = 0.250 Å−1 (Figure 1d,f) and that of the wet unannealed counterpart at s = 0.2545 Å−1 (Figure 1a,e). In Figure 1b, the apparent crystallite size calculated from the peak width of 200 reflection and Scherrer equation turned to be 6.0 nm, twice as large as that of the 3.0 nm for the untreated sample (Figure 1e,f). Although the sharpening of equatorial reflections of wood cellulose upon thermal treatments had been reported on several occasions,15 such a drastic increase in crystallite size has never been observed using scattering methods to the best of our knowledge. After the hydrothermal treatment, the composite 110/11̅0 reflection became sharp and centered at s = 0.18 Å−1. This reflection can no longer be considered as arising from the merging of the two broad peaks of 110 and 110̅ (at s = 0.19 and 0.17 Å−1) of cellulose Iβ (see also Figures S1 and S3 in the Supporting Information). Thus, the γ angle at 98.0° in the untreated samples becomes closer to 90° in the hydrothermally treated samples, indicative of a transformation from the initial monoclinic system to a pseudo-orthorhombic arrangement. The transformation of cellulose crystal into a pseudoorthorhombic system has been reported as the allomorphic conversion to cellulose IVI,16 which is classically obtained via thermal treatment of cellulose IIII, another cellulose allomorph, in glycerol.17 Other examples of cellulose IVI have been reported in specific primary wall cellulose18,19 or in the nonswelling saponification of cellulose triacetate I.20 Typically, the reported diffraction position of 200 for cellulose IVI range froms 0.243 to 0.253 Å−1.18,21 These values are definitely smaller than those observed in this study for hydrothermally treated sample. The unit cell dimensions determined from the
Figure 1. Top: X-ray diffraction diagrams of wet (water content = 130%, left) and dry (water content = 10%, right) beech wood before (a, c) and after (b, d) in situ thermal treatment. The framed dashed area in diagram (a) corresponds to scattering from water. Middle: Xray diffraction profiles of the anisotropic equatorial and meridional components of beech wood before (dashed line) and after (solid line) thermal treatment with water contents of (e) 130% and (f) 10%. Scattering vector s = 1/d (Å−1). Bottom: Solid-state CP/MAS 13C NMR spectra of beech wood before (dashed line) and after (solid line) thermal treatment with water contents of (g) 130% and (h) 10%. Insets are enlargements of the C6 regions.
X-ray data in Figure 1b,e are a = 7.79 Å, b = 8.19 Å, c = 10.36 Å, and γ ≈ 90°, with a unit cell volume of 661 Å3, while the reported cell dimensions of cellulose IVI are a = 8.03 Å, b = 731
DOI: 10.1021/acsmacrolett.6b00273 ACS Macro Lett. 2016, 5, 730−734
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ACS Macro Letters 8.13 Å, c = 10.34 Å, and γ = 90°, with the cell volume of 675 Å3.16 Thus, the pseudo-orthorhombic structure obtained through the hydrothermal treatment described in the present study should be distinguished from the classical cellulose IVI allmorph and will hereafter be referred to as cellulose I′. The change in the line shape of 110/11̅0 reflection was less obvious for the dry sample (Figure 1d), and thus, the crystals retained their monoclinic structure after the treatment. A slight sharpening of the 200 reflection was observed and resulted in an increase in the crystallite size to 3.0 nm from 2.7 nm in the untreated dry sample. The γ angle decreased from 96.6° in the untreated sample to 95.1° in the treated sample. Noteworthy, the 200 reflections shifted toward a higher d-spacing in contrast to the case of the wet sample. Solid-state 13C NMR spectra of wet and dry beech wood before and after the in situ thermal treatment are shown in Figure 1g,h. While an increase in crystallinity of cellulose upon annealing was visible at the C4 (80−90 ppm) and C6 regions (60−68 ppm), as observed in previous literature,15 the spectral features of both the wet and the dry samples after the hydrothermal annealing were similar to those of native cellulose Iβ. In particular, the persistence of the crystalline C6 contribution at 66 ppm indicates that the hydroxymethyl group remained in tg conformation after the conversion to the pseudo-orthorhombic system. Note that thermally induced decomposition of matrix components, such as deacetylation (near 20 ppm), was observed in the NMR spectra. Figure 2 presents a relationship between the 200 d-spacing and crystallite size of beech wood with different water contents
cases, the d-spacings became smaller, accompanying a significant increase of lateral crystallite sizes after the treatments. On the other hand, with the initial water contents below 30%, the sample was completely dried before reaching 180 °C. In those cases, the d-spacings became larger and the increase in the crystallite size was less pronounced. The correlation between temperature and annealing time was also an important factor. When treated with an excess of water at 200 °C in autoclave, the sharpening and shift of the 200 peak was the most pronounced when treated at 2 h, and the shorter treatments resulted in intermediate peak widths and peak displacements (Figure 2). We also treated the samples for 2 h at 160 and 180 °C, and a clear transition could only be seen at 180 °C indicating, therefore, that there is a critical temperature between 180 to 200 °C where the transition to highly crystalline pseudo-orthorhombic unit cell proceeds in the time scale of about an hour. Thus, there is a clear correlation between the amount of water in the system at high temperature and the transformation into cellulose I′. At least water content between 30 and 60% during annealing is needed for the full transformation. Similar effects were observed for cedar wood and bamboo (Figures S4 and S5 in the Supporting Information). Due to the small density of cedar (generally around 0.3 g/cm3), the tube contained a large dead-volume that tends to accelerate water migration in the in situ experiment, and the clear sharpening could only be observed when the sample was filled with water, in which case the water content was approximately 200%. The sample conditioned to 100% water content was almost dry below 140 °C judging from the liquid water scattering, which resulted in an increase of d-spacing after the treatment. Bamboo, on the contrary, contained more water when conditioned in the same humidity environment, and the water remained in the sample at high temperature. A series of equatorial profiles of wet beech wood (water content = 130%) during in situ experiments is shown in Figure 3. In the heating phase, increases of the d-spacings of both reflections are seen, as expected from the thermal expansion of cellulose crystal,22 whereas they experience a d-spacings decrease during the cooling phase corresponding to cooling-
Figure 2. Relationship between d-spacings and crystallite size of the 200 reflections of beech cellulose crystal annealed with (i) various water contents, (ii) various times, and (iii) various temperatures. The dashed lines correspond to the variations during the in situ experiments.
from 3 to 130% before and after annealing with various durations (10 min to 2 h). When the sample was treated inside the autoclave immersed in an oil bath, we considered that there was no water migration and that the water content was the same as in the starting condition. For the data of the in situ experiment, we used the peak height of isotropic water scattering (s = 0.3 Å−1) normalized by X-ray transmission to estimate the water content and used the value at 180 °C as an indicator of water content during the treatment. The thermal treatment generally increased the crystallite size, but its influence on the d-spacings was strongly dependent on the water content. With high initial water content, the samples kept water contents of above 60% at high temperature. In those
Figure 3. Variation of equatorial X-ray diffraction profiles of wet beech wood (water content = 130%) taken during the program of an in situ heating, holding, and cooling. 732
DOI: 10.1021/acsmacrolett.6b00273 ACS Macro Lett. 2016, 5, 730−734
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Figure 4. Changes in peak heights, d-spacings, fwhm, integral intensities, and azimuthal width of (a) 110/110̅ and (b) 200 reflections of beech wood with water content of 130% (red) and 10% (green) as a function of time during thermal treatment.
spacing variation mainly reflects their thermal expansion that is almost reversible. The drastic structural modifications during the wet annealing experiments have never been reported so far. This is because the necessary conditions, that is, annealing time, temperature, and water content, have never been considered together. Highly crystalline cellulose such as samples such as those from tunicate mantles, or from Valonia and Glaucocystis cell walls has been subjected to similar hydrothermal conditions, leading, in particular, to intracrystalline H/D isotope exchange23 and Iα → Iβ conversion.24,25 Nevertheless, these model samples did not show the present phenomena, which appears specific to higher plant cellulose, where the lateral crystallite sizes are small. We can tentatively explain this phenomenon as cocrystallization of adjacent microfibrils, as illustrated in Figure 5. We use a simple molecular packing model for cellulose microfibril, having a squarish cross section with hydrophilic 11̅0 and 110 surfaces exposed. The pyranosyl rings in cross section are parallel to the long axis of the rectangle in the figure. A 16-chain model is used in the presentation for clarity, although the lateral dimension can be larger for wood cellulose microfibrils. In higher plant cell walls, the microfibrils are closely packed with neighboring microfibrils with statistical molecular polarity,25 so that tentatively a cocrystallization may occur between antiparallel aligned microfibrils. If the antiparallel microfibrils were interfaced with their 110 planes in contact, the 200 planes of the two microfibrils would be roughly perpendicular. (The same is true if antiparallel microfibrils were interfaced with their 110̅ planes.) This situation cannot explain the orthorhombiclike diffraction, and also, a close packing of microfibrils requires large shear deformation in the simplified model. On the other hand, a cocrystallization between 11̅0 and 110 surfaces of antiparallel microfibrils, which leads to alignment of the pyranosyl planes, requires only small deformation for a tight packing and, thus, would explain the orthorhombic structure where the 11̅0 and 110 d-spacings coincide.
induced contraction. Besides this, a drastic sharpening of 200 reflection is visible during the holding period at 200 °C and even in the cooling phase. In agreement with the data in Figures 1 and 2, when the heating/cooling cycle is complete, the d-spacing of the 200 reflection has shifted from 3.92 Å (0.2545 Å−1) to 3.89 Å (0.2565 Å−1). On the other hand, that of the 110/11̅0 reflection has shifted from 5.55 Å (0.18 Å−1) to 5.88 Å (0.17 Å−1). Figure 4 shows the evolution of peak height, d-spacing, peak width (full width at half-maximum, fwhm), integral intensity, and azimuthal width of the two major equatorial diffraction peaks of cellulose I during the thermal treatment of beech wood under wet and dry conditions. The intensities are normalized to the incident beam intensity. In the wet condition (water content = 130%), a drastic sharpening of the peaks occurs during the heating profile between 150 and 200 °C, while the integral intensities slightly decrease at the same time. Thus, it is likely that the peak-sharpening is due to cocrystallization of separate crystals rather than crystal growth, which would result in an increase in the integral intensity. At the same stage of heating, the azimuthal width of the corresponding reflections significantly decreases, indicating improved axial orientation of cellulose crystals along the longitudinal direction of the specimens. During the period where the samples were kept at 200 °C, the integral intensity increased by about 50%, while the peak width remained constant and the d-spacings became smaller. This can be interpreted as being an internal reorganization within the fusioned crystals, eliminating structural defects, or crystal growth by recruiting molecules from amorphous region. During the cooling phase, the integral intensity further increases due to the decreased thermal displacement, in a similar way as the intensity decreased during the heating phase. In the dry condition (10% water content), the peak width only slightly decreases during the annealing period and remains constant during the cooling. Thus, for these samples the d733
DOI: 10.1021/acsmacrolett.6b00273 ACS Macro Lett. 2016, 5, 730−734
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ACKNOWLEDGMENTS Y.O. thanks The Japan Society for the Promotion of Science (JSPS) for financial support. The study was funded by the French National Research Agency (ANR) as part of the project FUNLOCK ANR-13-BIME-0002. We thank Dr. Nathalie Boudet and Dr. Nils Blanc for technical assistance at the beamline. The authors appreciate the valuable help of Prof. Yukie Saito and the staffs of the University of Tokyo Chichibu Forest and the Arboricultural Research Institute for harvesting the wood samples. The authors thank H. Chanzy for his suggestions during the writing of this work.
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Figure 5. Proposed schematic drawing of cocrystallization of cellulose microfibrils in lateral view. Each small rectangle denotes the projection of a cellulose molecule perpendicular to the chain length, and the different green shades indicate different molecular polarities. The black parallelograms in (a) and rectangles in (b) represent the unit cells in projections perpendicular to the chain axis.
In summary, cellulose in woody biomass slowly underwent an irreversible phase transition into a pseudo-orthorhombic structure together with a doubling of the crystallites size in the presence of excess water at above 180 °C. The mechanism of this structural alternation of wood cellulose can tentatively be explained as cocrystallization of microfibrils packed in antiparall manner in the wood cell wall. The extreme phenomena observed in this study should give some clue to understand the variation of apparent unit cell parameters observed in native cellulose of different origins and their structural evolutions during industrial processes. An investigation of the structural changes at the mesoscale level and structural dynamics of wood under various heating conditions is currently underway and will be reported in the future.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00273. Details of peak fitting and evolution of goodness of fit (Figures S1−S3), relationship between d-spacings and crystallite sizes of cedar (Figure S4) and bamboo (Figure S5) cellulose upon hydrothermal annealing, and experimental procedure for CP/MAS 13C NMR spectroscopy (PDF).
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REFERENCES
(1) Esteves, B. M.; Pereira, H. M. BioResources 2009, 4 (1), 370−404. (2) Sandberg, D.; Haller, P.; Navi, P. Wood Mater. Sci. Eng. 2013, 8 (1), 64−88. (3) Pokhrel, D.; Viraraghavan, T. Sci. Total Environ. 2004, 333 (1−3), 37−58. (4) Langan, P.; Petridis, L.; O’Neill, H. M.; Pingali, S. V.; Foston, M.; Nishiyama, Y.; Schulz, R.; Lindner, B.; Hanson, B. L.; Harton, S.; Heller, W. T.; Urban, V.; Evans, B. R.; Gnanakaran, S.; Ragauskas, A. J.; Smith, J. C.; Davison, B. H. Green Chem. 2014, 16 (1), 63−68. (5) Rodriguez Alonso, E.; Dupont, C.; Heux, L.; Da Silva Perez, D.; Commandre, J.-M.; Gourdon, C. Energy 2016, 97, 381−390. (6) Tjeerdsma, B. F.; Militz, H. Holz als Roh - und Werkst. 2005, 63 (2), 102−111. (7) Wikberg, H.; Maunu, S. L. Carbohydr. Polym. 2004, 58 (4), 461− 466. (8) Sivonen, H.; Maunu, S. L.; Sundholm, F.; Jämsä, S.; Viitaniemi, P. Holzforschung 2002, 56 (6), 648−654. (9) Nguila Inari, G.; Petrissans, M.; Lambert, J.; Ehrhardt, J. J.; Gérardin, P. Surf. Interface Anal. 2006, 38 (10), 1336−1342. (10) Irvine, G. M. Wood Sci. Technol. 1985, 19 (2), 139−149. (11) Driemeier, C.; Mendes, F. M.; Santucci, B. S.; Pimenta, M. T. B. Cellulose 2015, 22 (4), 2183−2195. (12) Nishiyama, Y.; Langan, P.; O’Neill, H.; Pingali, S. V.; Harton, S. Cellulose 2014, 21 (2), 1015−1024. (13) Dry basis water content (Wd) is given as Wd (%) = Mw/Md × 100, where Mw and Md are weights of wet and dry samples, respectively. (14) Nishiyama, Y.; Langan, P.; Chanzy, H. J. Am. Chem. Soc. 2002, 124 (31), 9074−9082. (15) Šturcová, A.; His, I.; Apperley, D. C.; Sugiyama, J.; Jarvis, M. C. Biomacromolecules 2004, 5 (4), 1333−1339. (16) Gardiner, E. S.; Sarko, A. Can. J. Chem. 1985, 63 (9), 173−180. (17) Loeb, L.; Segal, L. J. Polym. Sci. 1954, 14 (73), 121−123. (18) Chanzy, H.; Imada, K.; Mollard, R.; Vuong, R.; Barnoud, F. Protoplasma 1979, 100 (3−4), 303−316. (19) Nishimura, H.; Okano, T.; Asano, I. Mokuzai Gakkaishi 1982, 28 (7), 484−485. (20) Roche, E. J.; O’Brien, J. P.; Allen, S. R. Polym. Commun. 1986, 27 (5), 138−140. (21) Hess, K.; Kiessig, H. Z. phys. Chem. 1941, B49 (4−5), 235−244. (22) Wada, M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40 (11), 1095−1102. (23) Nishiyama, Y.; Isogai, A.; Okano, T.; Müller, M.; Chanzy, H. Macromolecules 1999, 32 (6), 2078−2081. (24) Yamamoto, H.; Horii, F.; Odani, H. Macromolecules 1989, 21 (10), 4132−4134. (25) Sugiyama, J.; Okano, T.; Yamamoto, H.; Horii, F. Macromolecules 1990, 23 (12), 3196−3198.
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The authors declare no competing financial interest. 734
DOI: 10.1021/acsmacrolett.6b00273 ACS Macro Lett. 2016, 5, 730−734
