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Structural modifications of Tilia cordata wood during heat treatment investigated by FT-IR and 2D IR correlation spectroscopy
Journal of Molecular Structure 1033 (2013) 176–186
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Structural modifications of Tilia cordata wood during heat treatment investigated by FT-IR and 2D IR correlation spectroscopy Maria-Cristina Popescu a, Julien Froidevaux b, Parviz Navi b, Carmen-Mihaela Popescu a,⇑ a b
‘‘Petru Poni’’ Institute of Macromolecular Chemistry of Romanian Academy, 41A, Gr. Ghica Voda Alley, Ro. 700487 Iasi, Romania Bern University of Applied Sciences, Solothurnstrasse 102, 2500 Biel, Switzerland
h i g h l i g h t s " Evaluation of structural behaviour of Tilia wood heat treatment with low temperature and humidity. " Chemical modifications were investigated by FT-IR, PCA and 2D-COS spectroscopy. " HT treatment of wood revealed the formation of acetic acid, which catalyse the hydrolysis of carbohydrates. " At the beginning a higher extent of carbohydrates, then an increase of the lignin degradation was observed.
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Article history: Received 6 July 2012 Received in revised form 12 August 2012 Accepted 20 August 2012 Available online 30 August 2012 Keywords: Lime wood Hydro-thermal treatment FT-IR spectroscopy Principal component analysis 2D IR correlation spectroscopy
a b s t r a c t It is known that heat treatment of wood combined with a low percent of relative humidity causes transformations in the chemical composition of it. The modifications and/or degradation of wood components occur by hydrolysis, oxidation, and decarboxylation reactions. The aim of this study was to give better insights on wood chemical modifications during wood heat treatment under low temperature at about 140 °C and 10% percentage of relative humidity, by infrared, principal component analysis and two dimensional infrared correlation spectroscopy. For this purpose, hardwood samples of lime (Tilia cordata) were investigated and analysed. The infrared spectra of treated samples were compared with the reference ones, the most important differences being observed in the ‘‘fingerprint’’ region. Due to the complexity of this region, which have contributions from all the wood constituents the chemical changes during hydro-thermal treatment were examined in detail using principal component analysis and 2D IR correlation spectroscopy. By hydro-thermal treatment of wood results the formation of acetic acid, which catalyse the hydrolysis reactions of hemicelluloses and amorphous cellulose. The cleavage of the b-O-4 linkages and splitting of the aliphatic methoxyl chains from the aromatic lignin ring was also observed. For the first treatment interval, a higher extent of carbohydrates degradation was observed, then an increase of the extent of the lignin degradation also took place. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Wood is a complex natural material composed of cellulose, hemicelluloses and lignin, and an array of low molecular mass compounds. Cellulose, the main wood constituent, is a linear high molecular mass polymer built up of anhydro-D-gluco-pyranose units linked by b(1–4) glycosidic linkages. It is a highly polar compound due to the presence of three AOH groups for every ⇑ Corresponding author. Tel.: +40 232217454; fax: +40 232211299. E-mail address: [email protected] (C.-M. Popescu). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.08.035
structural unit. Hemicelluloses are branched polymers with molecular mass significantly lower than that of cellulose; mainly composed of glucose, mannose, galactose, xylose and arabinose. Lignin is a three dimensional, crosslinked aromatic polymer formed from phenyl propane units, with one or more methoxyl groups bonded to the aromatic ring. The different structural units are linked together mainly by b-O-4 aryl ether, or by carbon–carbon linkages [1,2]. Due to its various properties, wood has many applications from engineering to artworks. However, in addition to its biodegradability, wood is very susceptible to weathering, especially degradation
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by UV light. Furthermore, photo-oxidation occurs when oxygen reacts with wood, causing discoloration and deterioration. Thermal treatment is often used to improve the physical characteristics of wood for particular purposes, including the dimensional stability and durability of the wood if it is to be exposed to chemicals or biological agents such as fungi and bacteria, or to frequent use under natural environmental conditions [3]. Also, thermal treatment of wood was used to simulate the natural degradation of wood [4]. The treatment of wood causes transformations in its composition. Chemical modifications and degradation of wood components take place through a series of hydrolysis, oxidation, and decarboxylation reactions coupled with heat and mass transfer, its structure being reformed. The changed wood composition results in a lower hygroscopicity with a major influence on both dimensional stability and durability. Chemical reactions involved during wood heat treatment as well as the final properties of the material depend directly on the treatment temperature, relative humidity, treatment time and by the nature of the wood specie used. Different researchers reported changes in crystallinity of cellulose after heat treatment, but the studies were done under ovendried conditions. The wood properties strongly depend by the presence of moisture during the treatments, especially the crystallization of wood cellulose being affected [5]. Generally, infrared (FTIR) spectroscopy is used as a simple and useful technique to obtain rapid information about the structure of wood constituents and chemical changes taking place in wood due to different treatments. Contrary to conventional chemical analysis, this technique requires small sample sizes and short analysis time [6]. Ates et al. [7], Tuong and Li [8] and Akgul et al. [5] used IR spectroscopy to assess the effects of thermal treatment of calabrian pine, acacia hybrid wood and Scots pine and Uludag fir, respectively. This technique has also been used to analyse the chemical changes occurring during wood weathering, decay and chemical treatments [9–11] or natural ageing [12]. The principal component analysis (PCA) is a well-established technique in statistics and chemometrics, that gives a precise mathematical estimation of changes along the object and variable vectors. This method allows for visualisation of the main variability of a data set without the constraint of an initial hypothesis concerning the relationship within samples, or between samples and responses (variables) [22]. Two dimensional infrared correlation spectroscopy (2D-COS) is a powerful tool used to evaluate the differences appearing during an external perturbation. Usually, this method enhances the spectral resolution giving new information, which cannot be established through conventional infrared and its derivative spectra. The method was used by several researchers, such as: Shinzawa et al. [13] studied finely ground microcrystalline cellulose, Hinterstoisser and Salmén [14] and Hishikawa et al. [15] used this method for cellulose characterisation, Labbé et al. [16] analysed pine wood with varying amount of cellulose contents, Popescu et al. [9,10,17–19] used 2D IR to compare eucalyptus wood chips, brown stock pulp, and chlorite-bleached pulp samples, and to evaluate the degradation stages in naturally and artificially aged lime wood, while Huang et al. [20,21] identified the poplar and Eucalyptus through the Fourier transform infrared spectroscopy (FT-IR) combined with two-dimensional correlation spectroscopy (2D IR) and evaluated the differences between three hard to distinguish wood species. The aim of this study is to give better insights on lime (Tilia cordata Mill.) wood chemical modifications during heat treatment under low relative humidity percent by infrared, principal component analysis and two dimensional infrared correlation spectroscopy.
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2. Materials and methods 2.1. Materials Lime (Tilia cordata Mill.) wood samples were exposed to heating at 140 °C, and about 10% relative humidity conditions in a self-designed autoclave. The samples were removed from autoclave at different intervals of 96, 168, 288 and 504 h. All samples were compared with the reference sample. The reference sample was kept in normal environmental conditions. The main components of wood are cellulose, lignin and hemicelluloses (mainly xylan) – see Fig. 1. 2.2. Methods Powdered wood samples were sieved and the fraction with average diameter less than 200 lm was retained for analysis. FT-IR spectra were recorded on solid samples in KBr pellets by means of FT-IR DIGILAB Scimitar Series Spectrometer (USA) with a resolution of 4 cm1. The concentration of the sample was of 5 mg/500 mg KBr. Five recordings were performed for each sample after successive milling and the evaluations were made on the average spectrum obtained from these five recordings. Processing of the spectra was done by means of Grams 9.1 program (Thermo Fisher Scientific Inc.). The second derivative spectra were obtained with the Savitsky–Golay method (second-order polynomial with fifteen data points) using Grams 9.1 program (Thermo Fisher Scientific Inc.). Principal component analysis (PCA) is a multivariate statistical technique used for extraction and interpretation of the systematic variance in a data set. The underlying idea in PCA modelling is to replace a complex multi-dimensional data set (for example spectroscopic data) by a simplified version involving fewer dimensions (principal components (PCs) or factors), but still fitting the original data closely enough to be regarded as a good approximation [22]. The input data set is decomposed into two matrices of interest (scores and loadings), when data are converted into the dimensionally reduced PCA space. These new axes or principal components (PCs) correspond with the eigenvectors of the original data’s covariance matrix. The loadings matrix defines the new axes of the dimensionally reduced data set, and scores matrix describes the samples in the PC space. With PCA, the most important features of the FTIR spectra can be identified. The band shifts and non-symmetries in the bands between the samples can be quickly determined. Two-dimensional correlation spectroscopy (2D-COS) enables cross-correlation analysis of spectral series of systems that change with a modulation variable [23,24]. Two-dimensional wavenumber–wavenumber correlation analysis provides two different correlation maps. The synchronous map displays correlations between all spectral bands changing in phase in the experiment and shows whether they increase or decrease relative to each other. The asynchronous correlation map, in contrast, relates spectral bands that change at different rates and also contains information about the sequence of the occurring events. 3. Results and discussion In Fig. 2, the colour changes on treated wood samples with the time of treatment are shown. The wood colour become significantly darker with increasing treatment time, compared with reference sample. Thermal treatment, all the time, results in darkening of wood, the change in the appearance being due especially to the hemicelluloses degradation [25].
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Fig. 1. Structure of the principal wood components: cellulose (a), hemicelluloses – especially xylan (b), and lignin (c).
Fig. 2. Colour change in wood samples with increasing time of hydro-thermal treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.1. Fourier-transform infrared (FT-IR) spectroscopy FT-IR spectra of reference and hydro-thermal treated lime wood and their derivatives are shown in Figs. 3 and 4. Because of the wood spectra complexity, these were separated into two regions, namely: the hydroxyl and methyl/methylene stretching vibrations in the 3800–2750 cm1 region (Fig. 3) and the ‘‘fingerprint’’ region in 1800–800 cm1 (Fig. 4). The assignments of the bands are presented in Tables 1 and 2. In the 3800–2750 cm1 region, hydrogen bonded (OAH) stretching absorptions and CAH stretching absorptions are
observed. This region of the spectrum is useful for elucidating hydrogen-bonding patterns because each distinct hydroxyl group gives a single stretching band at a certain frequency. From Fig. 3a, a broad band with a maximum at 3407 cm1 is observed. This band shifts to lower wavenumbers with the increasing treatment time. According to literature [26,27] a mixture of intermolecular and intramolecular hydrogen bonds (inter- and intramolecular H-bonds in cellulose I and lignin, O3AH3 O5 intramolecular H-bonds, O2AH2 O6 intramolecular H-bonds, O6AH6 O30 intermolecular H-bonds in carbohydrates, aliphatic and phenolic inter- and intra-molecular hydrogen bonds in lignin)
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Fig. 3. FT-IR (a) and the second derivative (b) spectra of hydro-thermal treated lime wood samples in 3800–2750 cm1 region.
Fig. 4. FT-IR (a) and the second derivative (b) spectra of hydro-thermal treated lime wood samples in 1800–800 cm1 region.
is considered to cause the broadening of the OH band in the IR spectra. From the second derivative spectrum (Fig. 3b) it is possible to distinguish and identify different bands from this region. Thus, it can be observed a first band at 3585 cm1 assigned to intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water [10,27,28]. At 3430 cm1 it is a band
assigned to intermolecular hydrogen bonds involving the C6 position (primary hydroxyl groups) (Table 1) and results in the formation of crystalline regions in cellulose component of wood. The frequency for the O5AH5 O3 intramolecular hydrogen bonds in carbohydrates was observed at 3343 cm1[10,28] (Table 1). This last band was observed to increase in intensity with increasing treatment time. The bands at 3274 and 3220 cm1 are assigned
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Table 1 The characteristic infrared bands from second derivative spectra of the studied lime wood samples in 3800–2750 cm1 region [9,10,26–29,32,33]. Wavenumber (cm1)
Band assignment
Intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water O(2)H. . .O(6) intramolecular hydrogen bonds in cellulose O(3)H. . .O(5) intramolecular hydrogen bonds in cellulose O(6)H. . .O(3) intermolecular hydrogen bonds in cellulose Ib O(6)H. . .O(3) intermolecular hydrogen bonds in cellulose Ia Multiple formation of an intermolecular hydrogen bond between biphenol and other phenolic groups (in lignin) Asymmetric methoxyl CAH stretching Symmetric CH2 stretching
to the two crystalline allomorphs, cellulose Ib and Ia, respectively (Table 1) [10,28]. The first one, assigned to monoclinic phase Ib, is shifted to higher wavenumbers and increase in intensity with treatment time. This means an increase in the amount of monoclinic phase Ib of cellulose during the hydro-thermal treatment of lime wood, due to the proportionality of this band with the amount of Ib phase. The second band, assigned to the O6AH6 O3 intramolecular hydrogen bonds existing only in triclinic Ia cellulose, is also shifted to higher wavenumbers but no increase in intensity was observed. In this region were also observed bands at 3077 and 3017 cm1, assigned to multiple formation of an intermolecular hydrogen bond between biphenol and other phenolic groups (in lignin), and the bands at 2941, 2887 and 2855 cm1 assigned to antisymmetric and symmetric stretching vibrations of methoxyl CH, methyl, and methylene groups (Table 1) [9,29]. The first two bands decrease in intensity and are shifted to lower (2931 cm1) and higher (2904 cm1) wavenumbers, respectively. These modifications suggest the cleavage of methyl, methylene groups from the wood components, especially from hemicelluloses and then from lignin. The spectra in the ‘‘fingerprint’’ region of the untreated and hydro-thermal treated lime wood samples (Fig. 4a) are more complicated. These consist in many overlapped bands, assigned to different stretching vibration and deformation of different groups belonging to wood components. The bands can be partially separated in the second derivative spectra (Fig. 4b). Several changes can be observed in both infrared and their second derivative spectra, which can be assigned to changes in
0h
96 h
168 h
288 h
504 h
3585 3430 3343 3274 3220 3077 3017 2941 2887 2855
3585 3426 3343 3275 3222 3077 3016 2038 2889 2855
3584 3426 3343 3275 3224 3077 3016 2938 2889 2855
3578 3426 3343 3276 3226 3077 3014 2936 2896 2854
3576 3426 3343 3277 3226 3077 3014 2931 2904 2853
hemicelluloses and lignin structures. Thus, it can be seen that the band at 1739 cm1 assigned to the C@O stretching vibrations in acetyl, carbonyl and carboxyl groups (Table 2) [9,10,29] decreases with increasing treatment time, indicating the cleavage of acetyl side chains of hemicelluloses. For the wood sample treated for 96 h a small shoulder at lower wavenumber (1720 cm1) is observed, indicating the formation of new carbonyl groups. This band disappears again for the wood samples treated for longer periods. In the same time, a progressive decreases for the band at 1663 cm1 assigned to conjugated CAO in quinines coupled with C@O stretching of various groups (Table 2) was also observed. This may suggests the degradation of acetyl groups from hemicelluloses through a deacetylation reaction causing releasing of acetic acid, which further can catalyse depolymerisation of the less ordered carbohydrates and condensation and degradation reactions in lignin structure. The loss of acetyl groups is also evidenced by the decrease of the intensity of the band at 1245 cm1 assigned to CAO stretching vibration of PhAOAC coupled with aromatic ring vibration (lignin) and CAO stretching vibration in xyloglucan (Table 2) [10,29], while the increase of the intensity of the band at 1323 cm1 has been attributed to the formation of condensed structures in lignin [30]. An increase, up to 168 h of treatment, then a decreases of the band intensity at 1507 cm1, assigned to the stretching vibrations of the C@C bonds of aromatic skeletal (lignin) (Table 2) was observed. The increase of the intensity of this band suggests the degradation of amorphous carbohydrates (especially hemicelluloses), at the beginning, with the increasing of the amount of lignin in the wood samples. It can be also seen a gradual shifting of this
Table 2 The characteristic infrared bands from second derivative spectra of the studied lime wood samples 1800-800 cm1 region [9,10,26–29,32,33]. Band assignment
C@O stretching vibration of carbonyl, carboxyl and acetyl groups Conjugated CAO in quinines coupled with C@O stretching of various groups Absorbed OAH stretching C@C stretching of aromatic skeletal (lignin) C@C stretching of aromatic skeletal (lignin) CAH deformation in lignin and carbohydrates CAH deformation in lignin and carbohydrates CAH deformation in cellulose and hemicellulose CAH vibration in cellulose and ClAO vibration in syringyl derivatives – condensed structures in lignin CAO stretching in lignin CAO stretching vibration of PhAOAC coupled with aromatic ring vibration (lignin) and CAO stretching vibration in xyloglucan CAOAC stretching mode of the pyranose ring CAOAC stretching vibration in cellulose and hemicelluloses CAOAC stretching vibration in cellulose and hemicelluloses CAO stretching Glucose ring stretching vibration CAO stretching vibrations in cellulose and hemicelluloses CAO ester stretching vibrations in methoxyl and b-O-4 linkages in lignin
Wavenumber (cm1) 0h
96 h
168 h
288 h
504 h
1739 1663 1631 1594 1507 1466 1426 1375 1323 1263 1245 1226 1204 1163 1127 1112 1058 1024
1739 1663 1631 1594 1509 1466 1426 1375 1322 1263 1244 1229 1204 1163 1127 1112 1158 1025
1740 1663 1631 1594 1509 1466 1426 1375 1321 1264 1244 1229 1204 1163 1127 1113 1158 1025
1740 1662 1632 1594 1511 1466 1427 1374 1320 1266 1243 1229 1205 1163 1126 1113 1158 1030
1738 1662 1632 1594 1514 1466 1427 1374 1319 1269 1243 1229 1206 1163 1125 1113 1158 1030
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band from 1507 cm1 to 1514 cm1 with increasing the treatment time, suggesting the splitting of aliphatic side chains, the cleavage of b-O-4 linkages in lignin structure followed by the condensation reactions [31,32]. Acetic acid, from the acetyl groups of hemicelluloses, was formed and released in the reactor. This acts as a catalyst for other degradation reactions of the wood components, especially the cleavage of the b-O-4 linkages in lignin. The decrease of the intensity and shifting to higher wavenumber of the band at 1024 cm1, assigned to CAO ester stretching vibrations in methoxyl and b-O-4 linkages in lignin (Table 2) [33], also confirm the cleavage of the b-O-4 linkages and splitting of the methoxy groups from lignin structure. The intensity of the bands at 1163, and 1058 cm1, assigned to CAOAC bridges stretching vibrations, and CO stretching vibrations in cellulose and hemicelluloses, increase up to 168 h of treatment, then start to decrease. The intensity increases of the band at 1058 cm1 indicates the formation of aliphatic alcohols during treatment. The ratio of the integral area of the characteristic lignin band at 1507 cm1 against different carbohydrate characteristic bands at 1738, 1375 and 1163 cm1 is presented in Fig. 5. The band at 1507 cm1 was used for appreciation of the lignin modification content in respect with carbohydrates because belongs purely from aromatic skeletal vibration (C@C) in lignin [32]. The carbohydrates characteristic bands are assigned to C@O stretching vibrations of various groups in carbohydrates, CAH deformation in cellulose and hemicelluloses, and CAOAC stretching vibration in cellulose and hemicelluloses [9,10,29]. When analysing the integral area ratio of the characteristic lignin band to characteristic carbohydrates bands (Fig. 5), can be observed for the first 168 h of treatment an increase of the obtained values. This means the decrease of the carbohydrate bands due to degradation reactions which occur in this treatment interval. After 168 h of treatment the degradation of lignin start to be more important, so the ratio values decreases. The band at 1113 cm1, assigned to glucose ring stretching vibration (Table 2), show intensity increases up to 168 h of treatment, then starts to decrease. This effect may be due, at the beginning, to the percentage increase of crystalline cellulose due to the cleavage and dehydration of amorphous carbohydrates and/or crystallization of the paracrystalline region of cellulose. For a treatment longer than 168 h in the cellulose structure the cleavage of the glycosidic linkages take place. 3.2. Principal component analysis (PCA) The chemical changes in wood structure and its physical properties as result of the treatment are also reflected in the PC scores. Fig. 6 shows a PCA scores plot of hydro-thermal treated lime wood samples based on the time-dependent FTIR spectra. PC1 (principal component factor 1) describes 89.6% and PC2 (principal component factor 2) 8.8% of data variance. 98.4% of the existed variances in all spectra can be captured using these two dimensions instead of the initial data. The spectra of samples with 0–168 h of treatment time have a negative score on PC1 (on the left) and those for 288–504 h positive ones (the right part). PC1 is the most informative latent variable for the description of the time of hydro-thermal treatment of the wood samples. The spectral groups corresponding to 96– 168 h of treatment have negative scores on PC1 and form a tight cluster. This suggests that the fundamental structure of the wood reflected in the spectra does not show major changes by heating in this treatment interval. The spectra of the samples treated more than 288 h have PC1 score spread out from values closed to 0 (288 h) to higher values for the 504 h. This reflects progressive thermal modification of the wood components. The spectra are
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Fig. 5. Comparison of integral area ratio of lignin characteristic IR band at 1507 cm1 against different carbohydrate characteristic bands at 1738, 1375 and 1159 cm1 during hydro-thermal treatment.
Fig. 6. PC1 versus PC2 score plots of hydro-thermal treated lime wood samples, at 0 h (black), 96 h (red), 168 h (blue), 288 h (green) and 504 h (pink) of treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
separated also on PC2. The spectrum of untreated wood sample (0 h of treatment) shows negative PC2 score, while those collected at 96–288 h have positive PC2 score. Thus PC1 may be considered as the time axis, while PC2 may be considered as the axis representing the structural modification of wood components. The loadings plot (Fig. 7) shows the chemical features responsible for grouping the samples along PC1 and PC2. The loadings can be used to understand how much each wavenumber contributes to the meaningful variation in the data, and to interpret variable relationships. From this plot, it is possible to obtain informations concerning the chemical aspects involved in the hydro-thermal process. Thus, PC1 contribute to the groups separation corresponding to 0–288 h of treatment from that corresponding to 504 h, and PC2 contribute especially for the separation between the 0–96 h and the 168–288 h groups of treatment. In the 3900–2700 cm1 loading region, in PC1, the bands at 3594 and 3220 cm1 are negative and the band at 3343 cm1 is positive. It is well known, that if a variable has a positive loading, it means that all samples with positive scores have higher than average values for that variable and all samples with negative scores have lower than average values for that variable [34]. According to this rule, one can conclude that the samples with hydro-thermal
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Fig. 7. PC1 and PC2 loading plots of TH wood.
treatment for longer time period (288–504 h) have a higher amount of intramolecular hydrogen bonds in a phenolic group (in lignin) and cellulose Ia and lower O3AH3 O5 intramolecular H-bonds in cellulose, than the samples treated for a short time period (less than 288 h). Also, the negative bands located at 2937 and 2887 cm1 are assigned to CH groups present in wood components and indicate their changes during hydro-thermal treatment. PC2 present two negative bands located at 3343 and 3122 cm1 which suggest an increase of the population of O3AH3 O5 intramolecular bonds in cellulose and aromatic CH of lignin with the increase of the treatment time. Because the 3900–2700 cm1 region reflects essentially the amounts and structure of hydroxyl groups in various types of hydrogen bonding of wood components, it seems reasonable to suggest that this separation reflects the progressive loss of hydroxyl groups involved in the structure of the acetic acid released after degradation of hemicelluloses (especially xyloglucan) at short treatment time. This is followed by the loss and/or reformation of the intra- and interchain hydrogen bonds in the wood structure. The 1800–800 cm1 loadings region is the most important region which permits to evidence the influence of hydro-thermal treatment on wood structure. Additional vibrations appear in several regions of the loadings at 1204 and 1693 cm1 in both PC1 and PC2. The PC1 positive absorption band are evidenced at 1337, 1110, 1060 and 1032 cm1, and negative ones at 1648, 1595, 1507, 1468, 1421, 1385 and 1262 cm1. According to previous rule, one can conclude that the samples in the left cluster (short time hydro-thermal treatment) have lower cellulose and hemicelluloses content than the samples in the right cluster (long time thermal treatment). Based on the negative bands associated with lignin all samples with negative scores (0, 96, 168 h of treatment) have lower than average values for these variables. PC2 gives informations about the differences between the untreated and treated wood for 96, 168 and 288 h. Positive bands at 1591, 1507, 1465, 1430, 1246 cm1, assigned to different stretching vibrations in lignin structure, indicate an increase of lignin content, and the negative ones at 1693, 1623, 1204, 1157, 1106, 1058 cm1, assigned to different groups in cellulose and hemicelluloses, indicate the decrease of hemicelluloses content with increasing treatment time. These data are supported by the observations made from the infrared and second derivative spectra. 3.3. Two-dimensional correlation spectroscopy (2D-COS) The spectra of untreated and treated wood samples, especially in the ‘‘fingerprint’’ region, are very complex, being formed
through the contribution of all wood components. For evaluation of structural differences which appear after hydro-thermal treatment, the 2D IR correlation spectroscopy was applied. This method usually enhances the spectral resolution giving new informations which cannot be obtained by using conventional IR and its derivative spectra. 2D IR correlation spectra were generated from the treatment time-dependent infrared spectra of the lime wood samples. These show the synchronous and asynchronous correlation peaks among different modes of molecular vibrations. In order to pick up useful local features of the correlation profiles it was more convenient to scan only a part of the correlation maps, therefore, the contour maps in the 3850–2750, 1850–1555, and 1555–855 cm1 regions for two treatment time ranges of 0–168 h and 168–504 h were evaluated. For the first evaluated region of 3850–2750 cm1 of the 0–168 h of treatment interval (Fig. 8a), in the synchronous spectrum two important auto-peaks at 3439 and 3215 cm1 were evidenced. These form five positive cross-peaks with the bands at 2934 and 2858 cm1. The auto-peaks are assigned to O2AH2 O6 intramolecular hydrogen bonds stretching vibrations in carbohydrates and O6AH6 O30 intermolecular hydrogen bonds stretching vibrations in cellulose Ia, respectively. This indicates, for shorter treatment periods, modifications in the carbohydrate structures. In the asynchronous spectrum constructed from the treatment time-dependent IR spectral variations in the time range of 0– 168 h (Fig. 8b) five bands at 3432, 3344, 3117, 2936 and 2856 cm1 were identified. These bands are assigned to different hydroxyl and methyl, methylene stretching vibrations and form positive cross-peaks at 3432–3344 cm1, 3117–2936 cm1, and negative ones at 3344–3117 cm1, 3344–2936 cm1, 3344– 2856 cm1. Using the Noda’s fundamental rule [23,24], from the synchronous and asynchronous spectra from the first treatment time range, the following sequence of spectral changes was evidenced:
2856 > 3117 > 2936 > 3432 > 3344 cm1 This means the methylene groups are changing first, followed by intermolecular hydrogen bonds between biphenol and other phenolic groups (in lignin), CH bonds from methoxyl groups and O2AH2 O6 and O5AH5 O3 intramolecular hydrogen bonds in carbohydrates. As it was evidenced before, the bands assigned to different groups from lignin increase at the beginning (up to 168 h of treatment) due to the degradation of the carbohydrates structure. The synchronous 2D IR correlation spectrum of the second treatment time interval (of 168–504 h) (Fig. 8c), shows four autopeaks at 3585, 3222, 2929 and 2904 cm1. The shift to lower wavenumber of the auto-peak at 2929 cm1 for the treated wood for longer periods (168–504 h) and to higher wavenumbers of the auto-peak at 2904 cm1 indicate modifications in the lignin structure. This affirmation is also supported by the presence of the autopeak at 3585 cm1, assigned to intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water. In the asynchronous 2D IR correlation spectrum (Fig. 8d) six bands at 3585, 3343, 3222, 3079, 2929 and 2904 cm1 are observed. These ones form positive cross-peaks at 3585–2929 cm1, 3585–2904 cm1, 3343–3222 cm1, 3343–2929 cm1, 3343– 2904 cm1, 3222–2929 cm1, 3222–2904 cm1 and negative cross-peaks at 3585–3343 cm1, 3585–3222 cm1, 3222– 3079 cm1. For the second treatment interval (of 168–504 h), when applying the Noda’s rule [23,24], the following sequence of spectral changes was established:
3343 > 3278; 3222 > 3585 > 3079 > 2929 > 2904 cm1
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Fig. 8. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 3850–2750 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
This means, the O5AH5 O3 intramolecular hydrogen bonds in carbohydrates are changing first, then followed by O6AH6 O30 intermolecular hydrogen bonds in cellulose, intermolecular hydrogen bonds between biphenol and other phenolic groups (in lignin), CH bonds from methoxyl groups in lignin and hemicelluloses. These results indicate the order in which the hydrogen bonds are cleaved and rearrangements in the wood structure. In the second spectral region (1850–1555 cm1), in the synchronous spectrum for the 0–168 h treatment time (Fig. 9a), three auto-peaks at 1764, 1705 and 1633 cm1 are observed. These form a positive cross-peak at 1764–1705 cm1 and two negative crosspeaks at 1764–1633 cm1, 1705–1633 cm1. The bands at 1764 and 1705 cm1 vary in opposite direction with the band at 1633 cm1. The asynchronous spectrum in the same treatment interval (Fig. 9b) contain six bands at 1764, 1728, 1705, 1663, 1633, and 1594 cm1, which form positive cross-peaks at 1764– 1728 cm1, 1764–1663 cm1, 1764–1633 cm1, 1728–1663 cm1, 1728–1633 cm1, 1705–1663 cm1, 1705–1633 cm1 and negative ones at 1728–1705 cm1, and 1633–1594 cm1. After a careful interpretation of the synchronous and asynchronous spectra the following sequence of spectral changes was found:
1663; 1633 > 1764 > 1705 > 1728 > 1594 cm1
This shows the order of different bond cleavages. Thus, the absorbed water and conjugated C@O bonds in PhA(C@O)A groups in lignin is changing first, then C@O bonds in non-conjugated ketones, acetyl, carboxyl and carbonyl groups, and then C@C of aromatic skeletal in lignin. In this case the modification of lignin bands is due especially to the higher rate degradation of carbohydrates. The second treatment interval, of 168–504 h, reveal in the synchronous spectrum (Fig. 9c) only two auto-peaks, at 1739 and 1648 cm1. These bands form three positive cross-peaks at 1739– 1648 cm1, 1739–1594 cm1 and 1648–1594 cm1 and one negative cross-peak at 1739–1705 cm1. This means the bands at 1739, 1648, 1594 cm1 vary in the same direction and in opposite direction with the band at 1704 cm1, namely the first three bands decrease in intensity, while the last one increase. The asynchronous spectrum (Fig 9d) evidence a series of positive cross-peaks at 1704–1648 cm1, 1704–1594 cm1, 1648– 1594 cm1 and negative cross-peaks at 1739–1648 cm1, 1739– 1704 cm1, 1648–1625 cm1 and 1594–1575 cm1. Using the Noda’s rule [23,24], from both synchronous and asynchronous spectra in the second treatment interval, the following sequence of spectral changes was observed:
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1625; 1575 > 1648 > 1739 > 1704 > 1594 cm1 From above sequence the modification order of different bonds can be evaluated. Thus, the moment of absorbed water and C@C bonds of substituted aromatic ring coupled with conjugated CAO bonds in lignin is changing first, followed by conjugated C@O bonds in PhA(C@O)A groups in lignin, C@O bonds in carboxyl and acetyl groups, and C@C of aromatic skeletal in lignin. For longer treatment time, the structure of lignin is also changed. In the last spectral region, 1555–855 cm1, both, synchronous and asynchronous spectra are more complex. The synchronous spectrum of the shorter time treated samples (0–168 h) (Fig. 10a) evidence seven auto-peaks at 1507, 1466, 1333, 1204, 1126, 1113 and 987 cm1. All these bands form only positive cross-peaks between them, meaning all bands vary in the same direction. The asynchronous spectrum (Fig. 10b) in the same region and treatment interval form a series of positive and negative cross-peaks. After a very careful interpretation of the correlation spectra, the following spectral changes sequence was observed:
1244; 1031 > 1425; 1466 > 1374 > 1126; 1113 > 1333 > 1204 > > 1162 > 1070 > 987 > 898 > 1507 cm1
From here can be observed that the moment of CAO linkage in acetyl groups and CAO in methoxyl and b-O-4 linkages in lignin is changing first, followed by CAH bonds in carbohydrates and lignin, CAOH groups in carbohydrates, CAO and CAOAC bonds in carbohydrates, CAO and CAC bonds of >CHAOH and ACH2AOH groups in carbohydrates, CAC bonds of aliphatic rings in carbohydrates and C@C linkages in aromatic skeletal in lignin. For shorter treatment time the degradation of hemicelluloses and amorphous cellulose, followed by cleavage of the b-O-4 linkages and methoxyl aliphatic side chains from lignin was evidenced after evaluation of the correlation maps in the first 0–168 h interval. In the second treatment interval, of 168–504 h, the synchronous spectrum (Fig. 10c) contain a number of seven auto-peaks at 1512, 1466, 1375, 1265, 1126, 1080 and 987 cm1. These form, as in the previous case, only positive cross-peaks between them. The asynchronous spectrum (Fig. 10d) shows, both, positive and negative cross-peaks. After a careful interpretation of synchronous and asynchronous correlation maps, the following bands modification order was evidenced:
1204 > 1466; 1425 > 1512; 1265 > 1162 > 1226 > 1080 > 1126 > 1333 >> 1375 > 987 > 1030 cm1
Fig. 9. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 1850–1555 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
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Fig. 10. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 1555–855 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
The above sequence evidence that the moment of CAO bonds of PhAOAC group in lignin and CAO bonds in xylan is changing first, followed by CH2 groups in lignin and carbohydrates, CH and C@C bonds of substituted aromatic lignin ring, CAOAC bonds in cellulose and hemicelluloses, CAO bonds mainly from C(3)AO(3)H in cellulose I, CAOH bonds in cellulose and hemicelluloses, CAO bonds of acetate groups in hemicelluloses, CH2 and CH bonds in cellulose and hemicelluloses, and CAO and CAC bonds of >CHAOH and ACH2OH groups from cellulose. For longer treatment time the modification of lignin structure is much more pronounced starting with methoxyl aliphatic side chains splitting and formation of new bonds. In the same time, the degradation of carbohydrates continues by cleavage of hydroxyl groups and opening of the pyranosic rings and formation of new bonds between the formed radicals. 4. Conclusions The FT-IR, PCA and 2D IR correlation analysis of lime wood samples treated at 140 °C and about 10% relative humidity for a period up to 504 h indicates chemical changes induced by the hemicelluloses degradation to a greater extent and also to the other components degradation.
During heating of wood in the presence of a certain relative humidity percent the formation of acetic acid was evidenced. This further catalysis the hydrolysis reactions of hemicelluloses and to a lesser extent the amorphous cellulose. The cleavage of the b-O-4 linkages and splitting of the aliphatic methoxyl chains from the aromatic lignin ring was also observed. For the first 168 h of treatment, a higher extent of carbohydrates degradation was observed, especially, from the integral area ratio of the lignin to carbohydrates characteristic bands. After this period up to 504 h, an increase of the extent of the lignin degradation also took place. These observations are supported by the infrared and second derivative spectra and also by the principal component analysis and 2D correlation analysis. By 2D correlation spectroscopy the splitting of the methyl groups from the methoxyl side chain from lignin was observed to start from the beginning of treatment but to a lesser extent. With increasing the time of treatment the modification of lignin structure is more accentuated, the splitting of methyl groups being followed by the depolymerization and recondensation reactions. In the same time, the degradation of the carbohydrates continues by cleavage of hydroxyl groups and opening of the pyranosic rings and formation of new bonds between the formed radicals.
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Acknowledgements The authors gratefully acknowledge the financial support of the COST Action FP0904. M.C. Popescu acknowledges the financial support of CNCSIS-UEFISCSU, Project Number PN II-RU PD 460/2010. References [1] F. Bulian, J.A. Graystone, Wood Coatings: Theory and Practice, Elsevier, Oxford, 2009. [2] T.C. Chang, H.T. Chang, C.L. Wu, H.Y. Lin, S.T. Chang, Polym. Degrad. Stab. 95 (2010) 1518. [3] B.M. Esteves, H.M. Pereira, Forest Res. 4 (2009) 370. [4] K. Candelier, M. Chaouch, S. Dumarcay, A. Pétrissans, M. Pétrissans, P. Gérardin, J. Anal. Appl. Pyrol. 92 (2011) 376. [5] M. Akgul, E. Gumuskaya, S. Korkut, Wood Sci. Technol. 41 (2007) 281. [6] M.P.C. Conrad, G.D. Smith, G. Fernlund, Wood Fiber Sci. 35 (2003) 570. [7] S. Ates, M.H. Akyildiz, H. Ozdemir, BioResources 4 (2009) 1032. [8] V.M. Tuong, J. Li, BioResources 5 (2010) 1257. [9] C.-M. Popescu, M.-C. Popescu, C. Vasile, Int. J. Biol. Macromol. 48 (2011) 667. [10] C.-M. Popescu, M.-C. Popescu, C. Vasile, Microchem. J. 95 (2010) 377. [11] A.K. Moore, N.L. Owen, Appl. Spectrosc. Rev. 36 (2001) 65. [12] C.-M. Popescu, C. Vasile, M.-C. Popescu, Gh. Singurel, Cell. Chem. Technol. 40 (2006) 649. [13] H. Shinzawa, S. Morita, K. Awa, M. Okada, I. Noda, Y. Ozaki, H. Sato, Appl. Spectrosc. 63 (2009) 501. [14] B. Hinterstoisser, L. Salmén, Cellulose 6 (1999) 251. [15] Y. Hishikawa, S. Inoue, J. Magoshi, T. Kondo, Biomacromolecules 6 (2005) 2468.
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Structural modifications of Tilia cordata wood during heat treatment investigated by FT-IR and 2D IR correlation spectroscopy Maria-Cristina Popescu a, Julien Froidevaux b, Parviz Navi b, Carmen-Mihaela Popescu a,⇑ a b
‘‘Petru Poni’’ Institute of Macromolecular Chemistry of Romanian Academy, 41A, Gr. Ghica Voda Alley, Ro. 700487 Iasi, Romania Bern University of Applied Sciences, Solothurnstrasse 102, 2500 Biel, Switzerland
h i g h l i g h t s " Evaluation of structural behaviour of Tilia wood heat treatment with low temperature and humidity. " Chemical modifications were investigated by FT-IR, PCA and 2D-COS spectroscopy. " HT treatment of wood revealed the formation of acetic acid, which catalyse the hydrolysis of carbohydrates. " At the beginning a higher extent of carbohydrates, then an increase of the lignin degradation was observed.
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 12 August 2012 Accepted 20 August 2012 Available online 30 August 2012 Keywords: Lime wood Hydro-thermal treatment FT-IR spectroscopy Principal component analysis 2D IR correlation spectroscopy
a b s t r a c t It is known that heat treatment of wood combined with a low percent of relative humidity causes transformations in the chemical composition of it. The modifications and/or degradation of wood components occur by hydrolysis, oxidation, and decarboxylation reactions. The aim of this study was to give better insights on wood chemical modifications during wood heat treatment under low temperature at about 140 °C and 10% percentage of relative humidity, by infrared, principal component analysis and two dimensional infrared correlation spectroscopy. For this purpose, hardwood samples of lime (Tilia cordata) were investigated and analysed. The infrared spectra of treated samples were compared with the reference ones, the most important differences being observed in the ‘‘fingerprint’’ region. Due to the complexity of this region, which have contributions from all the wood constituents the chemical changes during hydro-thermal treatment were examined in detail using principal component analysis and 2D IR correlation spectroscopy. By hydro-thermal treatment of wood results the formation of acetic acid, which catalyse the hydrolysis reactions of hemicelluloses and amorphous cellulose. The cleavage of the b-O-4 linkages and splitting of the aliphatic methoxyl chains from the aromatic lignin ring was also observed. For the first treatment interval, a higher extent of carbohydrates degradation was observed, then an increase of the extent of the lignin degradation also took place. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Wood is a complex natural material composed of cellulose, hemicelluloses and lignin, and an array of low molecular mass compounds. Cellulose, the main wood constituent, is a linear high molecular mass polymer built up of anhydro-D-gluco-pyranose units linked by b(1–4) glycosidic linkages. It is a highly polar compound due to the presence of three AOH groups for every ⇑ Corresponding author. Tel.: +40 232217454; fax: +40 232211299. E-mail address: [email protected] (C.-M. Popescu). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.08.035
structural unit. Hemicelluloses are branched polymers with molecular mass significantly lower than that of cellulose; mainly composed of glucose, mannose, galactose, xylose and arabinose. Lignin is a three dimensional, crosslinked aromatic polymer formed from phenyl propane units, with one or more methoxyl groups bonded to the aromatic ring. The different structural units are linked together mainly by b-O-4 aryl ether, or by carbon–carbon linkages [1,2]. Due to its various properties, wood has many applications from engineering to artworks. However, in addition to its biodegradability, wood is very susceptible to weathering, especially degradation
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by UV light. Furthermore, photo-oxidation occurs when oxygen reacts with wood, causing discoloration and deterioration. Thermal treatment is often used to improve the physical characteristics of wood for particular purposes, including the dimensional stability and durability of the wood if it is to be exposed to chemicals or biological agents such as fungi and bacteria, or to frequent use under natural environmental conditions [3]. Also, thermal treatment of wood was used to simulate the natural degradation of wood [4]. The treatment of wood causes transformations in its composition. Chemical modifications and degradation of wood components take place through a series of hydrolysis, oxidation, and decarboxylation reactions coupled with heat and mass transfer, its structure being reformed. The changed wood composition results in a lower hygroscopicity with a major influence on both dimensional stability and durability. Chemical reactions involved during wood heat treatment as well as the final properties of the material depend directly on the treatment temperature, relative humidity, treatment time and by the nature of the wood specie used. Different researchers reported changes in crystallinity of cellulose after heat treatment, but the studies were done under ovendried conditions. The wood properties strongly depend by the presence of moisture during the treatments, especially the crystallization of wood cellulose being affected [5]. Generally, infrared (FTIR) spectroscopy is used as a simple and useful technique to obtain rapid information about the structure of wood constituents and chemical changes taking place in wood due to different treatments. Contrary to conventional chemical analysis, this technique requires small sample sizes and short analysis time [6]. Ates et al. [7], Tuong and Li [8] and Akgul et al. [5] used IR spectroscopy to assess the effects of thermal treatment of calabrian pine, acacia hybrid wood and Scots pine and Uludag fir, respectively. This technique has also been used to analyse the chemical changes occurring during wood weathering, decay and chemical treatments [9–11] or natural ageing [12]. The principal component analysis (PCA) is a well-established technique in statistics and chemometrics, that gives a precise mathematical estimation of changes along the object and variable vectors. This method allows for visualisation of the main variability of a data set without the constraint of an initial hypothesis concerning the relationship within samples, or between samples and responses (variables) [22]. Two dimensional infrared correlation spectroscopy (2D-COS) is a powerful tool used to evaluate the differences appearing during an external perturbation. Usually, this method enhances the spectral resolution giving new information, which cannot be established through conventional infrared and its derivative spectra. The method was used by several researchers, such as: Shinzawa et al. [13] studied finely ground microcrystalline cellulose, Hinterstoisser and Salmén [14] and Hishikawa et al. [15] used this method for cellulose characterisation, Labbé et al. [16] analysed pine wood with varying amount of cellulose contents, Popescu et al. [9,10,17–19] used 2D IR to compare eucalyptus wood chips, brown stock pulp, and chlorite-bleached pulp samples, and to evaluate the degradation stages in naturally and artificially aged lime wood, while Huang et al. [20,21] identified the poplar and Eucalyptus through the Fourier transform infrared spectroscopy (FT-IR) combined with two-dimensional correlation spectroscopy (2D IR) and evaluated the differences between three hard to distinguish wood species. The aim of this study is to give better insights on lime (Tilia cordata Mill.) wood chemical modifications during heat treatment under low relative humidity percent by infrared, principal component analysis and two dimensional infrared correlation spectroscopy.
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2. Materials and methods 2.1. Materials Lime (Tilia cordata Mill.) wood samples were exposed to heating at 140 °C, and about 10% relative humidity conditions in a self-designed autoclave. The samples were removed from autoclave at different intervals of 96, 168, 288 and 504 h. All samples were compared with the reference sample. The reference sample was kept in normal environmental conditions. The main components of wood are cellulose, lignin and hemicelluloses (mainly xylan) – see Fig. 1. 2.2. Methods Powdered wood samples were sieved and the fraction with average diameter less than 200 lm was retained for analysis. FT-IR spectra were recorded on solid samples in KBr pellets by means of FT-IR DIGILAB Scimitar Series Spectrometer (USA) with a resolution of 4 cm1. The concentration of the sample was of 5 mg/500 mg KBr. Five recordings were performed for each sample after successive milling and the evaluations were made on the average spectrum obtained from these five recordings. Processing of the spectra was done by means of Grams 9.1 program (Thermo Fisher Scientific Inc.). The second derivative spectra were obtained with the Savitsky–Golay method (second-order polynomial with fifteen data points) using Grams 9.1 program (Thermo Fisher Scientific Inc.). Principal component analysis (PCA) is a multivariate statistical technique used for extraction and interpretation of the systematic variance in a data set. The underlying idea in PCA modelling is to replace a complex multi-dimensional data set (for example spectroscopic data) by a simplified version involving fewer dimensions (principal components (PCs) or factors), but still fitting the original data closely enough to be regarded as a good approximation [22]. The input data set is decomposed into two matrices of interest (scores and loadings), when data are converted into the dimensionally reduced PCA space. These new axes or principal components (PCs) correspond with the eigenvectors of the original data’s covariance matrix. The loadings matrix defines the new axes of the dimensionally reduced data set, and scores matrix describes the samples in the PC space. With PCA, the most important features of the FTIR spectra can be identified. The band shifts and non-symmetries in the bands between the samples can be quickly determined. Two-dimensional correlation spectroscopy (2D-COS) enables cross-correlation analysis of spectral series of systems that change with a modulation variable [23,24]. Two-dimensional wavenumber–wavenumber correlation analysis provides two different correlation maps. The synchronous map displays correlations between all spectral bands changing in phase in the experiment and shows whether they increase or decrease relative to each other. The asynchronous correlation map, in contrast, relates spectral bands that change at different rates and also contains information about the sequence of the occurring events. 3. Results and discussion In Fig. 2, the colour changes on treated wood samples with the time of treatment are shown. The wood colour become significantly darker with increasing treatment time, compared with reference sample. Thermal treatment, all the time, results in darkening of wood, the change in the appearance being due especially to the hemicelluloses degradation [25].
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Fig. 1. Structure of the principal wood components: cellulose (a), hemicelluloses – especially xylan (b), and lignin (c).
Fig. 2. Colour change in wood samples with increasing time of hydro-thermal treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.1. Fourier-transform infrared (FT-IR) spectroscopy FT-IR spectra of reference and hydro-thermal treated lime wood and their derivatives are shown in Figs. 3 and 4. Because of the wood spectra complexity, these were separated into two regions, namely: the hydroxyl and methyl/methylene stretching vibrations in the 3800–2750 cm1 region (Fig. 3) and the ‘‘fingerprint’’ region in 1800–800 cm1 (Fig. 4). The assignments of the bands are presented in Tables 1 and 2. In the 3800–2750 cm1 region, hydrogen bonded (OAH) stretching absorptions and CAH stretching absorptions are
observed. This region of the spectrum is useful for elucidating hydrogen-bonding patterns because each distinct hydroxyl group gives a single stretching band at a certain frequency. From Fig. 3a, a broad band with a maximum at 3407 cm1 is observed. This band shifts to lower wavenumbers with the increasing treatment time. According to literature [26,27] a mixture of intermolecular and intramolecular hydrogen bonds (inter- and intramolecular H-bonds in cellulose I and lignin, O3AH3 O5 intramolecular H-bonds, O2AH2 O6 intramolecular H-bonds, O6AH6 O30 intermolecular H-bonds in carbohydrates, aliphatic and phenolic inter- and intra-molecular hydrogen bonds in lignin)
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Fig. 3. FT-IR (a) and the second derivative (b) spectra of hydro-thermal treated lime wood samples in 3800–2750 cm1 region.
Fig. 4. FT-IR (a) and the second derivative (b) spectra of hydro-thermal treated lime wood samples in 1800–800 cm1 region.
is considered to cause the broadening of the OH band in the IR spectra. From the second derivative spectrum (Fig. 3b) it is possible to distinguish and identify different bands from this region. Thus, it can be observed a first band at 3585 cm1 assigned to intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water [10,27,28]. At 3430 cm1 it is a band
assigned to intermolecular hydrogen bonds involving the C6 position (primary hydroxyl groups) (Table 1) and results in the formation of crystalline regions in cellulose component of wood. The frequency for the O5AH5 O3 intramolecular hydrogen bonds in carbohydrates was observed at 3343 cm1[10,28] (Table 1). This last band was observed to increase in intensity with increasing treatment time. The bands at 3274 and 3220 cm1 are assigned
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Table 1 The characteristic infrared bands from second derivative spectra of the studied lime wood samples in 3800–2750 cm1 region [9,10,26–29,32,33]. Wavenumber (cm1)
Band assignment
Intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water O(2)H. . .O(6) intramolecular hydrogen bonds in cellulose O(3)H. . .O(5) intramolecular hydrogen bonds in cellulose O(6)H. . .O(3) intermolecular hydrogen bonds in cellulose Ib O(6)H. . .O(3) intermolecular hydrogen bonds in cellulose Ia Multiple formation of an intermolecular hydrogen bond between biphenol and other phenolic groups (in lignin) Asymmetric methoxyl CAH stretching Symmetric CH2 stretching
to the two crystalline allomorphs, cellulose Ib and Ia, respectively (Table 1) [10,28]. The first one, assigned to monoclinic phase Ib, is shifted to higher wavenumbers and increase in intensity with treatment time. This means an increase in the amount of monoclinic phase Ib of cellulose during the hydro-thermal treatment of lime wood, due to the proportionality of this band with the amount of Ib phase. The second band, assigned to the O6AH6 O3 intramolecular hydrogen bonds existing only in triclinic Ia cellulose, is also shifted to higher wavenumbers but no increase in intensity was observed. In this region were also observed bands at 3077 and 3017 cm1, assigned to multiple formation of an intermolecular hydrogen bond between biphenol and other phenolic groups (in lignin), and the bands at 2941, 2887 and 2855 cm1 assigned to antisymmetric and symmetric stretching vibrations of methoxyl CH, methyl, and methylene groups (Table 1) [9,29]. The first two bands decrease in intensity and are shifted to lower (2931 cm1) and higher (2904 cm1) wavenumbers, respectively. These modifications suggest the cleavage of methyl, methylene groups from the wood components, especially from hemicelluloses and then from lignin. The spectra in the ‘‘fingerprint’’ region of the untreated and hydro-thermal treated lime wood samples (Fig. 4a) are more complicated. These consist in many overlapped bands, assigned to different stretching vibration and deformation of different groups belonging to wood components. The bands can be partially separated in the second derivative spectra (Fig. 4b). Several changes can be observed in both infrared and their second derivative spectra, which can be assigned to changes in
0h
96 h
168 h
288 h
504 h
3585 3430 3343 3274 3220 3077 3017 2941 2887 2855
3585 3426 3343 3275 3222 3077 3016 2038 2889 2855
3584 3426 3343 3275 3224 3077 3016 2938 2889 2855
3578 3426 3343 3276 3226 3077 3014 2936 2896 2854
3576 3426 3343 3277 3226 3077 3014 2931 2904 2853
hemicelluloses and lignin structures. Thus, it can be seen that the band at 1739 cm1 assigned to the C@O stretching vibrations in acetyl, carbonyl and carboxyl groups (Table 2) [9,10,29] decreases with increasing treatment time, indicating the cleavage of acetyl side chains of hemicelluloses. For the wood sample treated for 96 h a small shoulder at lower wavenumber (1720 cm1) is observed, indicating the formation of new carbonyl groups. This band disappears again for the wood samples treated for longer periods. In the same time, a progressive decreases for the band at 1663 cm1 assigned to conjugated CAO in quinines coupled with C@O stretching of various groups (Table 2) was also observed. This may suggests the degradation of acetyl groups from hemicelluloses through a deacetylation reaction causing releasing of acetic acid, which further can catalyse depolymerisation of the less ordered carbohydrates and condensation and degradation reactions in lignin structure. The loss of acetyl groups is also evidenced by the decrease of the intensity of the band at 1245 cm1 assigned to CAO stretching vibration of PhAOAC coupled with aromatic ring vibration (lignin) and CAO stretching vibration in xyloglucan (Table 2) [10,29], while the increase of the intensity of the band at 1323 cm1 has been attributed to the formation of condensed structures in lignin [30]. An increase, up to 168 h of treatment, then a decreases of the band intensity at 1507 cm1, assigned to the stretching vibrations of the C@C bonds of aromatic skeletal (lignin) (Table 2) was observed. The increase of the intensity of this band suggests the degradation of amorphous carbohydrates (especially hemicelluloses), at the beginning, with the increasing of the amount of lignin in the wood samples. It can be also seen a gradual shifting of this
Table 2 The characteristic infrared bands from second derivative spectra of the studied lime wood samples 1800-800 cm1 region [9,10,26–29,32,33]. Band assignment
C@O stretching vibration of carbonyl, carboxyl and acetyl groups Conjugated CAO in quinines coupled with C@O stretching of various groups Absorbed OAH stretching C@C stretching of aromatic skeletal (lignin) C@C stretching of aromatic skeletal (lignin) CAH deformation in lignin and carbohydrates CAH deformation in lignin and carbohydrates CAH deformation in cellulose and hemicellulose CAH vibration in cellulose and ClAO vibration in syringyl derivatives – condensed structures in lignin CAO stretching in lignin CAO stretching vibration of PhAOAC coupled with aromatic ring vibration (lignin) and CAO stretching vibration in xyloglucan CAOAC stretching mode of the pyranose ring CAOAC stretching vibration in cellulose and hemicelluloses CAOAC stretching vibration in cellulose and hemicelluloses CAO stretching Glucose ring stretching vibration CAO stretching vibrations in cellulose and hemicelluloses CAO ester stretching vibrations in methoxyl and b-O-4 linkages in lignin
Wavenumber (cm1) 0h
96 h
168 h
288 h
504 h
1739 1663 1631 1594 1507 1466 1426 1375 1323 1263 1245 1226 1204 1163 1127 1112 1058 1024
1739 1663 1631 1594 1509 1466 1426 1375 1322 1263 1244 1229 1204 1163 1127 1112 1158 1025
1740 1663 1631 1594 1509 1466 1426 1375 1321 1264 1244 1229 1204 1163 1127 1113 1158 1025
1740 1662 1632 1594 1511 1466 1427 1374 1320 1266 1243 1229 1205 1163 1126 1113 1158 1030
1738 1662 1632 1594 1514 1466 1427 1374 1319 1269 1243 1229 1206 1163 1125 1113 1158 1030
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band from 1507 cm1 to 1514 cm1 with increasing the treatment time, suggesting the splitting of aliphatic side chains, the cleavage of b-O-4 linkages in lignin structure followed by the condensation reactions [31,32]. Acetic acid, from the acetyl groups of hemicelluloses, was formed and released in the reactor. This acts as a catalyst for other degradation reactions of the wood components, especially the cleavage of the b-O-4 linkages in lignin. The decrease of the intensity and shifting to higher wavenumber of the band at 1024 cm1, assigned to CAO ester stretching vibrations in methoxyl and b-O-4 linkages in lignin (Table 2) [33], also confirm the cleavage of the b-O-4 linkages and splitting of the methoxy groups from lignin structure. The intensity of the bands at 1163, and 1058 cm1, assigned to CAOAC bridges stretching vibrations, and CO stretching vibrations in cellulose and hemicelluloses, increase up to 168 h of treatment, then start to decrease. The intensity increases of the band at 1058 cm1 indicates the formation of aliphatic alcohols during treatment. The ratio of the integral area of the characteristic lignin band at 1507 cm1 against different carbohydrate characteristic bands at 1738, 1375 and 1163 cm1 is presented in Fig. 5. The band at 1507 cm1 was used for appreciation of the lignin modification content in respect with carbohydrates because belongs purely from aromatic skeletal vibration (C@C) in lignin [32]. The carbohydrates characteristic bands are assigned to C@O stretching vibrations of various groups in carbohydrates, CAH deformation in cellulose and hemicelluloses, and CAOAC stretching vibration in cellulose and hemicelluloses [9,10,29]. When analysing the integral area ratio of the characteristic lignin band to characteristic carbohydrates bands (Fig. 5), can be observed for the first 168 h of treatment an increase of the obtained values. This means the decrease of the carbohydrate bands due to degradation reactions which occur in this treatment interval. After 168 h of treatment the degradation of lignin start to be more important, so the ratio values decreases. The band at 1113 cm1, assigned to glucose ring stretching vibration (Table 2), show intensity increases up to 168 h of treatment, then starts to decrease. This effect may be due, at the beginning, to the percentage increase of crystalline cellulose due to the cleavage and dehydration of amorphous carbohydrates and/or crystallization of the paracrystalline region of cellulose. For a treatment longer than 168 h in the cellulose structure the cleavage of the glycosidic linkages take place. 3.2. Principal component analysis (PCA) The chemical changes in wood structure and its physical properties as result of the treatment are also reflected in the PC scores. Fig. 6 shows a PCA scores plot of hydro-thermal treated lime wood samples based on the time-dependent FTIR spectra. PC1 (principal component factor 1) describes 89.6% and PC2 (principal component factor 2) 8.8% of data variance. 98.4% of the existed variances in all spectra can be captured using these two dimensions instead of the initial data. The spectra of samples with 0–168 h of treatment time have a negative score on PC1 (on the left) and those for 288–504 h positive ones (the right part). PC1 is the most informative latent variable for the description of the time of hydro-thermal treatment of the wood samples. The spectral groups corresponding to 96– 168 h of treatment have negative scores on PC1 and form a tight cluster. This suggests that the fundamental structure of the wood reflected in the spectra does not show major changes by heating in this treatment interval. The spectra of the samples treated more than 288 h have PC1 score spread out from values closed to 0 (288 h) to higher values for the 504 h. This reflects progressive thermal modification of the wood components. The spectra are
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Fig. 5. Comparison of integral area ratio of lignin characteristic IR band at 1507 cm1 against different carbohydrate characteristic bands at 1738, 1375 and 1159 cm1 during hydro-thermal treatment.
Fig. 6. PC1 versus PC2 score plots of hydro-thermal treated lime wood samples, at 0 h (black), 96 h (red), 168 h (blue), 288 h (green) and 504 h (pink) of treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
separated also on PC2. The spectrum of untreated wood sample (0 h of treatment) shows negative PC2 score, while those collected at 96–288 h have positive PC2 score. Thus PC1 may be considered as the time axis, while PC2 may be considered as the axis representing the structural modification of wood components. The loadings plot (Fig. 7) shows the chemical features responsible for grouping the samples along PC1 and PC2. The loadings can be used to understand how much each wavenumber contributes to the meaningful variation in the data, and to interpret variable relationships. From this plot, it is possible to obtain informations concerning the chemical aspects involved in the hydro-thermal process. Thus, PC1 contribute to the groups separation corresponding to 0–288 h of treatment from that corresponding to 504 h, and PC2 contribute especially for the separation between the 0–96 h and the 168–288 h groups of treatment. In the 3900–2700 cm1 loading region, in PC1, the bands at 3594 and 3220 cm1 are negative and the band at 3343 cm1 is positive. It is well known, that if a variable has a positive loading, it means that all samples with positive scores have higher than average values for that variable and all samples with negative scores have lower than average values for that variable [34]. According to this rule, one can conclude that the samples with hydro-thermal
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Fig. 7. PC1 and PC2 loading plots of TH wood.
treatment for longer time period (288–504 h) have a higher amount of intramolecular hydrogen bonds in a phenolic group (in lignin) and cellulose Ia and lower O3AH3 O5 intramolecular H-bonds in cellulose, than the samples treated for a short time period (less than 288 h). Also, the negative bands located at 2937 and 2887 cm1 are assigned to CH groups present in wood components and indicate their changes during hydro-thermal treatment. PC2 present two negative bands located at 3343 and 3122 cm1 which suggest an increase of the population of O3AH3 O5 intramolecular bonds in cellulose and aromatic CH of lignin with the increase of the treatment time. Because the 3900–2700 cm1 region reflects essentially the amounts and structure of hydroxyl groups in various types of hydrogen bonding of wood components, it seems reasonable to suggest that this separation reflects the progressive loss of hydroxyl groups involved in the structure of the acetic acid released after degradation of hemicelluloses (especially xyloglucan) at short treatment time. This is followed by the loss and/or reformation of the intra- and interchain hydrogen bonds in the wood structure. The 1800–800 cm1 loadings region is the most important region which permits to evidence the influence of hydro-thermal treatment on wood structure. Additional vibrations appear in several regions of the loadings at 1204 and 1693 cm1 in both PC1 and PC2. The PC1 positive absorption band are evidenced at 1337, 1110, 1060 and 1032 cm1, and negative ones at 1648, 1595, 1507, 1468, 1421, 1385 and 1262 cm1. According to previous rule, one can conclude that the samples in the left cluster (short time hydro-thermal treatment) have lower cellulose and hemicelluloses content than the samples in the right cluster (long time thermal treatment). Based on the negative bands associated with lignin all samples with negative scores (0, 96, 168 h of treatment) have lower than average values for these variables. PC2 gives informations about the differences between the untreated and treated wood for 96, 168 and 288 h. Positive bands at 1591, 1507, 1465, 1430, 1246 cm1, assigned to different stretching vibrations in lignin structure, indicate an increase of lignin content, and the negative ones at 1693, 1623, 1204, 1157, 1106, 1058 cm1, assigned to different groups in cellulose and hemicelluloses, indicate the decrease of hemicelluloses content with increasing treatment time. These data are supported by the observations made from the infrared and second derivative spectra. 3.3. Two-dimensional correlation spectroscopy (2D-COS) The spectra of untreated and treated wood samples, especially in the ‘‘fingerprint’’ region, are very complex, being formed
through the contribution of all wood components. For evaluation of structural differences which appear after hydro-thermal treatment, the 2D IR correlation spectroscopy was applied. This method usually enhances the spectral resolution giving new informations which cannot be obtained by using conventional IR and its derivative spectra. 2D IR correlation spectra were generated from the treatment time-dependent infrared spectra of the lime wood samples. These show the synchronous and asynchronous correlation peaks among different modes of molecular vibrations. In order to pick up useful local features of the correlation profiles it was more convenient to scan only a part of the correlation maps, therefore, the contour maps in the 3850–2750, 1850–1555, and 1555–855 cm1 regions for two treatment time ranges of 0–168 h and 168–504 h were evaluated. For the first evaluated region of 3850–2750 cm1 of the 0–168 h of treatment interval (Fig. 8a), in the synchronous spectrum two important auto-peaks at 3439 and 3215 cm1 were evidenced. These form five positive cross-peaks with the bands at 2934 and 2858 cm1. The auto-peaks are assigned to O2AH2 O6 intramolecular hydrogen bonds stretching vibrations in carbohydrates and O6AH6 O30 intermolecular hydrogen bonds stretching vibrations in cellulose Ia, respectively. This indicates, for shorter treatment periods, modifications in the carbohydrate structures. In the asynchronous spectrum constructed from the treatment time-dependent IR spectral variations in the time range of 0– 168 h (Fig. 8b) five bands at 3432, 3344, 3117, 2936 and 2856 cm1 were identified. These bands are assigned to different hydroxyl and methyl, methylene stretching vibrations and form positive cross-peaks at 3432–3344 cm1, 3117–2936 cm1, and negative ones at 3344–3117 cm1, 3344–2936 cm1, 3344– 2856 cm1. Using the Noda’s fundamental rule [23,24], from the synchronous and asynchronous spectra from the first treatment time range, the following sequence of spectral changes was evidenced:
2856 > 3117 > 2936 > 3432 > 3344 cm1 This means the methylene groups are changing first, followed by intermolecular hydrogen bonds between biphenol and other phenolic groups (in lignin), CH bonds from methoxyl groups and O2AH2 O6 and O5AH5 O3 intramolecular hydrogen bonds in carbohydrates. As it was evidenced before, the bands assigned to different groups from lignin increase at the beginning (up to 168 h of treatment) due to the degradation of the carbohydrates structure. The synchronous 2D IR correlation spectrum of the second treatment time interval (of 168–504 h) (Fig. 8c), shows four autopeaks at 3585, 3222, 2929 and 2904 cm1. The shift to lower wavenumber of the auto-peak at 2929 cm1 for the treated wood for longer periods (168–504 h) and to higher wavenumbers of the auto-peak at 2904 cm1 indicate modifications in the lignin structure. This affirmation is also supported by the presence of the autopeak at 3585 cm1, assigned to intramolecular hydrogen bond in a phenolic group (in lignin) and weakly bounded absorbed water. In the asynchronous 2D IR correlation spectrum (Fig. 8d) six bands at 3585, 3343, 3222, 3079, 2929 and 2904 cm1 are observed. These ones form positive cross-peaks at 3585–2929 cm1, 3585–2904 cm1, 3343–3222 cm1, 3343–2929 cm1, 3343– 2904 cm1, 3222–2929 cm1, 3222–2904 cm1 and negative cross-peaks at 3585–3343 cm1, 3585–3222 cm1, 3222– 3079 cm1. For the second treatment interval (of 168–504 h), when applying the Noda’s rule [23,24], the following sequence of spectral changes was established:
3343 > 3278; 3222 > 3585 > 3079 > 2929 > 2904 cm1
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Fig. 8. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 3850–2750 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
This means, the O5AH5 O3 intramolecular hydrogen bonds in carbohydrates are changing first, then followed by O6AH6 O30 intermolecular hydrogen bonds in cellulose, intermolecular hydrogen bonds between biphenol and other phenolic groups (in lignin), CH bonds from methoxyl groups in lignin and hemicelluloses. These results indicate the order in which the hydrogen bonds are cleaved and rearrangements in the wood structure. In the second spectral region (1850–1555 cm1), in the synchronous spectrum for the 0–168 h treatment time (Fig. 9a), three auto-peaks at 1764, 1705 and 1633 cm1 are observed. These form a positive cross-peak at 1764–1705 cm1 and two negative crosspeaks at 1764–1633 cm1, 1705–1633 cm1. The bands at 1764 and 1705 cm1 vary in opposite direction with the band at 1633 cm1. The asynchronous spectrum in the same treatment interval (Fig. 9b) contain six bands at 1764, 1728, 1705, 1663, 1633, and 1594 cm1, which form positive cross-peaks at 1764– 1728 cm1, 1764–1663 cm1, 1764–1633 cm1, 1728–1663 cm1, 1728–1633 cm1, 1705–1663 cm1, 1705–1633 cm1 and negative ones at 1728–1705 cm1, and 1633–1594 cm1. After a careful interpretation of the synchronous and asynchronous spectra the following sequence of spectral changes was found:
1663; 1633 > 1764 > 1705 > 1728 > 1594 cm1
This shows the order of different bond cleavages. Thus, the absorbed water and conjugated C@O bonds in PhA(C@O)A groups in lignin is changing first, then C@O bonds in non-conjugated ketones, acetyl, carboxyl and carbonyl groups, and then C@C of aromatic skeletal in lignin. In this case the modification of lignin bands is due especially to the higher rate degradation of carbohydrates. The second treatment interval, of 168–504 h, reveal in the synchronous spectrum (Fig. 9c) only two auto-peaks, at 1739 and 1648 cm1. These bands form three positive cross-peaks at 1739– 1648 cm1, 1739–1594 cm1 and 1648–1594 cm1 and one negative cross-peak at 1739–1705 cm1. This means the bands at 1739, 1648, 1594 cm1 vary in the same direction and in opposite direction with the band at 1704 cm1, namely the first three bands decrease in intensity, while the last one increase. The asynchronous spectrum (Fig 9d) evidence a series of positive cross-peaks at 1704–1648 cm1, 1704–1594 cm1, 1648– 1594 cm1 and negative cross-peaks at 1739–1648 cm1, 1739– 1704 cm1, 1648–1625 cm1 and 1594–1575 cm1. Using the Noda’s rule [23,24], from both synchronous and asynchronous spectra in the second treatment interval, the following sequence of spectral changes was observed:
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1625; 1575 > 1648 > 1739 > 1704 > 1594 cm1 From above sequence the modification order of different bonds can be evaluated. Thus, the moment of absorbed water and C@C bonds of substituted aromatic ring coupled with conjugated CAO bonds in lignin is changing first, followed by conjugated C@O bonds in PhA(C@O)A groups in lignin, C@O bonds in carboxyl and acetyl groups, and C@C of aromatic skeletal in lignin. For longer treatment time, the structure of lignin is also changed. In the last spectral region, 1555–855 cm1, both, synchronous and asynchronous spectra are more complex. The synchronous spectrum of the shorter time treated samples (0–168 h) (Fig. 10a) evidence seven auto-peaks at 1507, 1466, 1333, 1204, 1126, 1113 and 987 cm1. All these bands form only positive cross-peaks between them, meaning all bands vary in the same direction. The asynchronous spectrum (Fig. 10b) in the same region and treatment interval form a series of positive and negative cross-peaks. After a very careful interpretation of the correlation spectra, the following spectral changes sequence was observed:
1244; 1031 > 1425; 1466 > 1374 > 1126; 1113 > 1333 > 1204 > > 1162 > 1070 > 987 > 898 > 1507 cm1
From here can be observed that the moment of CAO linkage in acetyl groups and CAO in methoxyl and b-O-4 linkages in lignin is changing first, followed by CAH bonds in carbohydrates and lignin, CAOH groups in carbohydrates, CAO and CAOAC bonds in carbohydrates, CAO and CAC bonds of >CHAOH and ACH2AOH groups in carbohydrates, CAC bonds of aliphatic rings in carbohydrates and C@C linkages in aromatic skeletal in lignin. For shorter treatment time the degradation of hemicelluloses and amorphous cellulose, followed by cleavage of the b-O-4 linkages and methoxyl aliphatic side chains from lignin was evidenced after evaluation of the correlation maps in the first 0–168 h interval. In the second treatment interval, of 168–504 h, the synchronous spectrum (Fig. 10c) contain a number of seven auto-peaks at 1512, 1466, 1375, 1265, 1126, 1080 and 987 cm1. These form, as in the previous case, only positive cross-peaks between them. The asynchronous spectrum (Fig. 10d) shows, both, positive and negative cross-peaks. After a careful interpretation of synchronous and asynchronous correlation maps, the following bands modification order was evidenced:
1204 > 1466; 1425 > 1512; 1265 > 1162 > 1226 > 1080 > 1126 > 1333 >> 1375 > 987 > 1030 cm1
Fig. 9. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 1850–1555 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
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Fig. 10. Synchronous (a and c) and asynchronous (b and d) 2D correlation spectra in the 1555–855 cm1 region constructed from the treatment time-dependent IR spectra. Hydrothermal treatment time range of 0–168 h (a and b) and 168–504 h (c and d).
The above sequence evidence that the moment of CAO bonds of PhAOAC group in lignin and CAO bonds in xylan is changing first, followed by CH2 groups in lignin and carbohydrates, CH and C@C bonds of substituted aromatic lignin ring, CAOAC bonds in cellulose and hemicelluloses, CAO bonds mainly from C(3)AO(3)H in cellulose I, CAOH bonds in cellulose and hemicelluloses, CAO bonds of acetate groups in hemicelluloses, CH2 and CH bonds in cellulose and hemicelluloses, and CAO and CAC bonds of >CHAOH and ACH2OH groups from cellulose. For longer treatment time the modification of lignin structure is much more pronounced starting with methoxyl aliphatic side chains splitting and formation of new bonds. In the same time, the degradation of carbohydrates continues by cleavage of hydroxyl groups and opening of the pyranosic rings and formation of new bonds between the formed radicals. 4. Conclusions The FT-IR, PCA and 2D IR correlation analysis of lime wood samples treated at 140 °C and about 10% relative humidity for a period up to 504 h indicates chemical changes induced by the hemicelluloses degradation to a greater extent and also to the other components degradation.
During heating of wood in the presence of a certain relative humidity percent the formation of acetic acid was evidenced. This further catalysis the hydrolysis reactions of hemicelluloses and to a lesser extent the amorphous cellulose. The cleavage of the b-O-4 linkages and splitting of the aliphatic methoxyl chains from the aromatic lignin ring was also observed. For the first 168 h of treatment, a higher extent of carbohydrates degradation was observed, especially, from the integral area ratio of the lignin to carbohydrates characteristic bands. After this period up to 504 h, an increase of the extent of the lignin degradation also took place. These observations are supported by the infrared and second derivative spectra and also by the principal component analysis and 2D correlation analysis. By 2D correlation spectroscopy the splitting of the methyl groups from the methoxyl side chain from lignin was observed to start from the beginning of treatment but to a lesser extent. With increasing the time of treatment the modification of lignin structure is more accentuated, the splitting of methyl groups being followed by the depolymerization and recondensation reactions. In the same time, the degradation of the carbohydrates continues by cleavage of hydroxyl groups and opening of the pyranosic rings and formation of new bonds between the formed radicals.
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