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Thermal decomposition of hemicelluloses
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Thermal decomposition of hemicelluloses Kajsa Werner, Linda Pommer, Markus Broström ∗ Department of Applied Physics and Electronics, Umeå University, SE – 90187 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 9 May 2014 Accepted 20 August 2014 Available online xxx Keywords: Pyrolysis Thermal decomposition Hemicellulose Xylan
a b s t r a c t Decomposition modeling of biomass often uses commercially available xylan as model compound representing hemicelluloses, not taking in account the heterogeneous nature of that group of carbohydrates. In this study, the thermal decomposition behavior of seven different hemicelluloses (-glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan, and xylan) were investigated in inert atmosphere using (i) thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, (ii) differential scanning calorimetry, and (iii) pyrolysis-gas chromatography/mass spectroscopy. Results on decomposition characteristics (mass loss rate, reaction heat and evolving gas composition) were compared and summarized for the different hemicelluloses and for comparison also crystalline cellulose was included in the study. The mass loss rate characteristics differed between the polysaccharides, with cellulose and glucan-based hemicelluloses as the thermally most stable and xylan as the least stable sample. The heat flow during slow heating in nitrogen flow showed a much more exothermal decomposition of xylan compared with the other hemicelluloses. The composition of off-gases during heating showed large differences between the samples. During decomposition of xylan high levels of CO2 and lower levels of other components were formed, whereas also CO, methanol, methane, furfural, 5-hydroxymethylfurfural and anhydrosugars were formed in substantial amounts from the other polysaccharides. The formation of anhydrosugar was correlated to the monosaccharide composition of the polysaccharide chain. The results from the current study contribute to new knowledge concerning thermochemical behavior of different hemicelluloses. © 2014 Elsevier B.V. All rights reserved.
1. Introduction All terrestrial biomass (coniferous and deciduous trees as well as herbaceous plants) share the same basic cell wall architecture with a cellulose microfibril network surrounded by a matrix of other polysaccharides (hemicellulose and pectin). In secondary cell walls of woody tissues also lignin is present [1]. It is important to consider that unlike cellulose, hemicellulose is a group of structurally diverse polysaccharides with amorphous structure and lower degree of polymerization. The building blocks in hemicelluloses are pentoses (arabinose and xylose), hexoses (galactose, glucose and mannose) and hexuronic acids (glucuronic acid). The nomenclature of hemicelluloses is principally determined by the most occurring sugar unit in the backbone [1]. In conifers (softwood trees), mannans (backbone of (1 → 4)-linked -d-mannose
∗ Corresponding author. Tel.: +46 70 295 5971. E-mail addresses: [email protected] (K. Werner), [email protected] (L. Pommer), [email protected] (M. Broström).
units) and glucomannans (backbone of both -d-mannose and -d-glucose units) are the predominating hemicelluloses [1–3]. Glucomannan, together with galactoglucomannan, may occur in close association with cellulose and represents the major (10–30%) hemicellulose component in softwood, but only a minor fraction in hardwood [1]. In deciduous trees (hardwood trees) xylanbased hemicelluloses are predominating. Xylans are composed of chains of (1 → 4)-linked -d-xylose, with single 4-O-methyl-dglucuronic acid [2,3]. The hemicellulosic fraction of herbaceous plants (monocotyledons) differs significantly from that of woody biomass (dicotyledons), both in relative abundance and composition [1]. Two examples of hemicelluloses characteristic for herbaceous plants are glucuronoarabinoxylan and -glucan. Glucuronoarabinoxylan has both 4-O-methyl-d-glucuronic acid and arabinose side chain residues attached to the xylan backbone chain, whereas -glucan (also known as mixed-linked glucan) is an unbranched homopolymer of glucose having both (1 → 3) and (1 → 4) linkages, which provides an unusual non-linear structure. The primary cell wall of grasses consists of 2–12% of -glucan, whereas it cannot be found in other biomasses [1]. A hemicellulose
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present in all types of biomass is arabinogalactan. Since the distinction between pectin and hemicellulose components of the cell wall is not well-defined, arabinogalactan is sometimes classified as pectin polysaccharide because of its -(1 → 4) linkages with non-equatorial configuration of C1 and C4 [1]. Furthermore, xyloglucan is a hemicellulose common for all higher plants and the most abundant (20–25%) in primary cell walls [1,3]. It has cellulose-like backbone branched by principally xylose units and are tightly bound to the cellulosic microfibrils by hydrogen bonds [3]. The close connection between hemicelluloses and other cell wall components, mainly cellulose and lignin, makes it very hard to extract hemicelluloses without changing the chemical structure. In this study commercially available xylan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan and glucan were selected to represent hemicellulose of terrestrial biomass. Thermal degradation of biomass, and in particular decomposition of carbohydrates, is a complex process and it includes many different reactions occurring simultaneously, e.g. dehydration, depolymerization, fragmentation, rearrangement, repolymerization, condensation and carbonization [4]. The pyrolysis process results in a char residue (fixed carbon and ash) and gaseous products. The evolved gases include both condensable and non-condensable components, ranging from CO, CO2 , short chain alcohols, aldehydes and acids, to larger organic compounds such as furan-ring products and anhydrosugars. Several reaction pathways have been suggested for decomposition of cellulose [5] and xylan [6], and have been commendably summarized by Shen et al. [7,8]. Ponder and Richards [6] described an intramolecular stabilization by 1,6-glycocidic bond of anhydroglucopyranose (levoglucosan) for cellulose decomposition, and thereby explained the increased decomposition temperature interval for cellulose compared to the less stable intramolecular 1,4-glycocidic bond in anhydroxylopyranose formed during xylan decomposition. Based on these reported decomposition pathways, polysaccharides of different structure and sugar composition are expected to have different degradation pathways resulting in differences in thermal stability and product distribution. To enhance the understanding of thermochemical conversion mechanisms, decomposition modeling of different biomasses and their main components (principally cellulose, hemicellulose and lignin) is essential. Current knowledge about hemicellulose decomposition is limited and because of its commercial availability xylan has been used extensively as model substance [4,8–11]. Thermogravimetric analyses of xylan have revealed weight loss in the range 190–350 ◦ C [8,9,11,12] and DSC experiments have shown that it the decomposition is associated with exothermal reactions [9]. In contrast to the many studies on xylan decomposition, only a few have aimed at analyzing softwood hemicellulose [13,14]. The objective of this study was therefore to evaluate the thermal behavior of different commercially available hemicelluloses, potentially useful as model substances. Thermal properties were examined using thermogravimetric analysis (TGA/DTG) and differential scanning calorimetry (DSC). Gas product distribution was analyzed by using both TGA coupled to a Fourier transform infrared spectrometer (TG-FTIR) and gas chromatography/mass spectroscopy equipped with a pyrolysis unit (py-GC/MS). Inorganic compounds and contaminations have earlier been reported to cause changes in decomposition, both lowering decomposition temperature and altering product distribution by favoring formation of char, lighter organic compounds, and water [4,11,15]. Therefore, also the content and composition of inorganics in the samples were characterized. Differences in decomposition behavior with respect to mass loss rates, calorimetric information and gas compositions are discussed.
2. Materials and methods 2.1. Material characteristics In this study, commercially available hemicelluloses were studied and compared with a microcrystalline cellulose (Sigma–Aldrich). Xylan from beechwood (Sigma–Aldrich) consists of 1,4-linked -xylopyranosyl units substituted by 4-O-methyl␣-d-glucopyranosyl uronic acid units. The xylosyl-to-uronic acid ratio is 8:1 [8]. The other hemicelluloses tested were -glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan and xyloglucan (Megazyme International). All samples (Table 1) were used without further purification or processing. The low purity of arabinoxylan is the result of extra careful extraction and purification to maintain (aromatic) ferulic acid cross-linkages in native arabinoxylan. All samples were analyzed with respect to ash content and amount of fixed carbon using thermogravimetric analysis (Q5000IR, TA Instruments). Ash content was measured by heating the sample to 550 ◦ C at 50 ◦ C/min using a TGA and keeping it isothermal for 30 min while purged with air at 50 ml/min. For xylan the procedure had to be modified in order to oxidize all char, rising the isothermal temperature to 750 ◦ C. Fixed carbon was measured as the remaining weight after heating the samples at maximum heating rate of the instrument, corresponding to approximately 700 ◦ C/min, to 900 ◦ C and kept isothermal for 7 min in a 50 ml/min nitrogen purge. The remaining mass was interpreted as fixed carbon after correction for the ash content. Due to limited amounts of the samples, the main ash forming elements were identified using semi quantitative information from ESEM/EDS analysis of the remaining ashes from the TGA experiments. The instrument used was a Philips model XL30 environmental scanning electron microscope with an EDAX detector. 2.2. Thermal analysis Thermal analysis of the samples was performed by means of thermogravimetry (TGA/DTG) and differential scanning calorimetry (DSC). Detailed mass loss profiles (visualized as the negative derivate of the mass loss) were used to analyze and describe the decomposition characteristics of the samples. Small samples (0.5 mg) were used to avoid possible effects on mass transfer caused by the dense surface that was formed as the samples melted during heating. The samples, evenly spread on ceramic pans, were predried at 105 ◦ C for 15 min and were then heated at 10 ◦ C/min to 600 ◦ C in a 35 ml/min nitrogen purge. A DSC instrument (Q1000, TA Instruments) was used to refine the information on the decomposition reactions causing the weight changes in the DTG-data. For DSC measurements, samples (2 mg) were placed in open aluminum pans and were pre-dried at 105 ◦ C for 15 min, equilibrated at 10 ◦ C and then heated 10 ◦ C/min to 400 ◦ C. The instrument was purged with a 100 ml/min nitrogen flow. To also allow secondary reactions, complementary experiments were made with sample pans covered with aluminum lids that had only a small (0.5 mm) opening. Approximated reaction heats were calculated as the integral of the DSC signals over the temperature range of decomposition. This was done without taking in account the physical effects of evaporation heat and mass loss during decomposition, and therefore it serves only as a comparison between the samples, expressed as J/g of initial sample mass. 2.3. FTIR gas analysis Temperature resolved compositions of gases produced during heating of the different samples were characterized using a FTIR spectrometer (Frontier FTIR with EGA TL 8000 coupling,
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Table 1 Polysaccharides analyzed. Polysaccharide
Catalog number
Purity [%]
Monosaccharide compositiona
Xylan Arabinoxylan Arabinogalactan Galactomannan Glucomannan Xyloglucan -glucan Cellulose
X4252 P-WAXYI P-ARGAL P-GGMMV P-GLCML P-XYGLN P-BGBL 310,697
97.3 ∼80 >95 >98 >98 >97 ∼95 n.a.
Xyl:GluA = 87.5:12.5 Xyl:Ara:Glu:Man:Gal = 51:36:6.5:4.4:1.6 Gal:Ara = 85:15 Man:Gal = 62:38 Man:Glu = 60:40 Glu:Xyl:Gal:Ara = 45:36:16:3 Glu 95% Glu 100%
Not analyzed (n.a.). a Abbreviations: arabinose (Ara), galactose (Gal), glucose (Glu), glucuronic acid (GluA), mannose (Man), xylose (Xyl).
Perkin Elmer) coupled to the TGA instrument. For these experiments, larger samples (10 mg) and higher purge gas flow rates (100 ml/min) were used in order to optimize the FTIR signal. The samples, evenly spread on ceramic pans, were pre-dried at 105 ◦ C for 15 min and then heated at 10 ◦ C/min to 600 ◦ C. The FTIR, with a deuterated triglycine sulfate (DTGS) detector set to a scan rate of 0.2 cm/s and 2 cm−1 resolution resulting in totally 288 spectra recorded every 10 s during the heating ramp, was set to scan between 4000 and 450 cm−1 . The temperatures of the sampling line and the gas cell were set to 230 ◦ C and the pump of the EGA system was set to constantly extract 50 ml/min from the TGA off-gas through the FTIR gas cell. Both spectra interpretation and quantification are difficult from the complex mixture of pyrolysis gas. Interpretation can be done either by correct identification correlated to a library search [10], by in-house calibration of preferably light gases [16], or to assign different absorption to groups of species that by their composition and chemical bonds are known to absorb in specific regions. The difficulty lies in the mixture of many different, sometimes unknown, organic species evolving simultaneously and often with similar and overlapping absorption spectra. In the present study, some of the smaller compounds (H2 O, CH4 , CO2 and CO) were identified, assigned and recorded according to their well-known and characteristic absorption spectra (Table 2), whereas other absorbing species were grouped together and only assigned to corresponding bands of absorption. Absorption by H2 O can be detected in the O H stretching bands at 4000–3400 cm−1 and 1800–1300 cm−1 , here monitored by the peak at 3853 cm−1 . The variations in shape of the absorption band 3100–2600 cm−1 are due to differences in distribution between different compounds, including hydrocarbons, alcohols and aldehydes. A common interpretation is to assign the whole band to CH4 , but by carefully analyzing available reference spectra it can be seen that a more correct way to identify CH4 in that region is by its sharp peak at 3018 cm−1 since the broad absorption of CH4 overlaps with that of other hydrocarbons. In this study, CH4 was identified as the height of that distinct peak and with a two-point subtraction of the broad underlying absorption. The peaks centered around 2354 and 2320 cm−1 show the presence of CO2 in the gas, whereas absorption bands around 2190 and 2110 cm−1 represent the typical absorption of CO.
Table 2 Absorption bands and specific wave numbers used for the gas profiles. Compound
Absorption band [cm−1 ]
Identification [cm−1 ]
H2 O CH4 CO2 CO
4000–3400 and 1800–1300 peak at 3018 2391–2217 2220–2150 and 2140–2060
3853 3018a 2360 2186
a
The broad hydrocarbon absorption used as background.
2.4. Pyrolysis-GC/MS gas analysis In addition to TG-FTIR, also py-GC/MS was used to analyze evolved gases. Samples were weighed (50 g) and placed into tin cups, (PY-1-ECO Frontier lab). The samples were subjected to flash pyrolysis conditions to a temperature of 450 ◦ C for 0.2 min (AS-1020E and Py-2020iD, Frontier Lab). Compounds volatilizing during pyrolysis were analyzed using a gas chromatograph equipped with a mass spectrometer (GC/MS) (Agilent 7890A, MS 5975). The GC was equipped with an Ultra alloy-5 capillary column (5% diphenyl, 95% dimethylpolysiloxane, 30 m, I.D. 0.25 mm, film thickness 0.25 m). The initial furnace temperature was 40 ◦ C and was ramped at 10 ◦ C/min to 320 ◦ C, where it was kept isothermal for 10 min. The split ratio was 16:1 using helium as carrier gas with a column flow of 1.2 ml/min. The temperature of the transfer line to the MS was set to 320 ◦ C. Electron ionization at 70 eV and m/z range 12–400 was used for analysis. Components were tentatively identified using NIST library (NIST/EPA/NIH mass spectral library, version 2, 2009) complemented with the polysaccharide and lignin degradation libraries of Faix [17,18]. 3. Results and discussion 3.1. Material properties The samples were characterized using TGA and SEM/EDX for determination of fixed carbon, ash content and identification of the main ash forming elements (Table 3). The inorganic elements are probably residues from the extraction procedure. Sodium, potassium and chlorine are typical constituents of agents used for extracting hemicellulose from the biomass matrix. Grains of potassium chloride were seen in the ash from glucomannan during the ESEM/EDS analysis. Silicone-containing grains, probably SiO2 , were detected in the ashes from the xyloglucan, -glucan and galactomannan samples. Silicone was also seen in the ash from arabinoxylan, but in the form of fibers, probably originating from filter material used in the extraction procedure. The different amounts of fixed carbon formed from the polysaccharides (Table 3), indicated differences in char formation mechanisms. Cellulose and -glucan formed the lowest amount of fixed carbon, whereas xylan and glucomannan demonstrated the highest yields (13.1 and 8.4%, respectively). The high char formation of xylan is in agreement with previous studies, e.g. 20% char yield at 900 ◦ C [9], 22% at 540 ◦ C [8], 30% at 1000 ◦ C [12] and 25% at 900 ◦ C [11]. Minor differences in results between the studies might be due to differences in e.g. sample purity, sample preparation, heating rates, sample mass, instrument geometry and gas flows. Xylan and glucomannan were shown not only to have high char yields but also to be the samples with the highest ash content (2.0 and 2.2%, respectively). Since ash forming elements are known to favor char formation, it is likely that the observed differences in char formation between the hemicelluloses studied are at least
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Table 3 Ash content, fixed carbon and main ash forming elements for the analyzed polysaccharides. Polysaccharide
Fixed carbona [% d.s., ash free]
Ash contenta [% d.s.]
Main ash forming elementsb
Xylan Arabinoxylan Arabinogalactan Galactomannan Glucomannan Xyloglucan -glucan Cellulose
13.1 2.7 3.6 4.8 8.4 3.6 1.3 2.0
2.0 0.6 0.4 0.5 2.2 0.5 n.d. n.d.
Na, Ca Ca, Mg, P, S, Si Ca, Na Na, Si, Ca, Cl Ca, Na, K, P, Cl, Mg Na, Si, Ca, P n.a. n.a.
Not detected (n.d.), not analyzed (n.a). a Measured by TGA, % of dry substance (d.s.), and, for fixed carbon, on ash free basis. b Elements detected at >5% of the ash on molar basis measured by ESEM/EDS, in descending order.
partially caused by differences in the amount or the composition of inorganic constituents. Fixed carbon and ash contents (Table 3) had a positive correlation (R2 = 0.82). 3.2. Thermogravimetric (DTG) and calorimetric (DSC) analysis DTG profiles from the polysaccharides decomposition during heating in inert atmosphere showed differences in mass loss rate characteristics (Fig. 1). Overall, mass loss appeared in the temperature range 200–380 ◦ C. Defining the thermal stability as the temperature of the DTG peak maximum, the most stable cell wall polysaccharide was cellulose, closely followed by the glucan-based hemicelluloses -glucan and xyloglucan, and then arabinoxylan, arabinogalactan, galactomannan, glucomannan and finally xylan. The stable appearance of cellulose is explained by its partly crystalline structure. Moreover, the differences in stability among the polysaccharides can probably be explained by formation of more or less stable intermediates during decomposition, where levoglucosan (6C) is the most stable, anhydrosugar compound formed and arabinosan (5C) and xylosan (5C) the least stable ones, as described by others [6,11]. Since arabinoxylan mainly forms thermally less stable anhydrosugars (arabinosan and xylosan) it may seem peculiarly that it keeps losing mass at higher temperatures (up to 243 ◦ C). This could be due to the phenolic compounds cross-linked to arabinoxylan in the cell wall. Xylan deviated compared to the other evaluated polysaccharides by mass loss at lower temperature and in two stages, with maximum mass loss rates at 243 ◦ C and 292 ◦ C (Fig. 1). This result agrees with earlier studies performed under similar conditions [8,9]. The mass losses of the other polysaccharides were between
Fig. 1. Mass loss rate curves for the analyzed polysaccharides.
200 and 250 ◦ C with mass loss rate maxima between 309 and 326 ◦ C. For arabinoxylan the high decomposition temperature, together with the broad mass loss interval, could be explained by the presence of aromatic impurities remaining from extraction. The DSC curves (Fig. 2) show the heat flow (exothermal and endothermal behavior) of the sample during heating, both in open and closed pans. Arabinogalactan, arabinoxylan, -glucan, galactomannan, glucomannan, xyloglucan and cellulose showed overall endothermal decomposition characteristics in open pans, whereas xylan displayed a clear exothermal behavior. The endothermal behavior with open pans shows trends very similar to the devolatilization in the corresponding DTG curves (Fig. 1), and it is plausibly caused mainly by the endothermal devolatilization. The endothermal behavior of hemicelluloses, apart from xylan, has to the authors’ knowledge not previously been reported. Yang et al. 2007 [9] have published data on the exothermal behavior of xylan, which is therein assigned to the exothermal charring reactions. An alternative or complementary explanation for the exothermic behavior is the presence of glucuronic acid units in xylan. The carboxylic acids groups ( COOH) are prone to undergo decarboxylation resulting in carbon dioxide and exothermal enthalpy changes. This aspect of the uronic sugar content in
Fig. 2. Reaction heat curves for the analyzed polysaccharides in open (solid lines) and closed (dashed lines) sample pans. Heat flow in W/g of initial sample mass. Curves displayed pairwise with a −2 W/g offset.
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Fig. 3. Heat of reactions (integral of DSC peak over the temperature range of decomposition, positive values are exothermal) vs fixed carbon yield (mass remaining at 400 ◦ C corrected for the ash content) for the analyzed polysaccharides as measured by DSC in both open and closed samples pans in order to study effects of secondary reactions. The effects of adding lids are highlighted by the arrows.
biomass (applicable for hemicellulose and pectin) has previously not been emphasized in pyrolysis literature. However, exothermal behavior has been observed from fixed bed bench-scale pyrolysis of some biomasses, [19] and could be the effect of uronic sugar decomposition. Fig. 3 shows the relation between reaction heat, estimated as the integral of the DSC curves over the temperature range of the decomposition, vs. fixed carbon, measured as the residue after the experiment and compensated for the ash content. Overall, a positive correlation was observed between fixed carbon yields and heat flow. The effect of reducing rapid evaporation of volatiles by closing the pan and thereby promoting secondary char forming reactions is also shown in Fig. 3. The secondary char forming reactions are present also in an open environment, but to a much lower extent. Allowing for secondary reactions increased the char yield and exothermicity for all samples except for xylan and arabinogalactan. Xylan showed similar and high char yield and exothermicity both in open and closed environment (no arrow in Fig. 3), whereas arabinogalactan showed an increase in reaction heat that was higher in relation to the increase in char yield (almost vertical arrow in Fig. 3). A plausible reason for this could be non-equatorial configuration of the -(1 → 4) linkages, associated with the composition of pyrolysis gases, which reacted further in secondary reactions. 3.3. FTIR gas analysis The off-gases from heating the samples in the TGA were characterized by FTIR spectroscopy. For all polysaccharide samples, absorption vs. temperature profiles for H2 O, CH4 , CO2 and CO were plotted together with the corresponding DTG curves (Fig. 4a–h). Concentrations were not quantified, but since the experimental conditions were identical, differences in gas formation can be analyzed by comparing release profiles for the different samples, if also taking in account differences in mass loss rate. The relative gas compositions had similarities for all polysaccharides, except for xylan that decomposed into mainly CO2 with only small amounts of the other components (Fig. 4). The relatively high CO2 concentration from xylan decomposition is in agreement with findings by others and is therein explained by either decarboxylation of COOH groups on glucuronic acid units [4], cracking and reforming of C O and COOH groups [9] or decarboxylation of O-acetyl groups linked to xylan [20]. Moreover, the dominating
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CO2 formation provides a plausible explanation for the exothermal behavior (Fig. 2), since CO2 formation indicates that carbon has undergone full oxidation resulting in a maximized exothermal enthalpy change for the volatilized carbon containing components. At higher temperatures, during charring reactions, significant amounts of methane were detected for the two samples with the highest yields of fixed carbon (xylan and glucomannan, Fig. 4a and e). Also this observation is in agreement with the findings by others [8] and were explained by the breaking of stable methoxy group (O CH3 ) bonds leading to a dislocation to higher temperatures for the methane formation. For more detailed information on gas composition, FTIR spectra recorded at the respective time for maximum weight loss rates are presented (Fig. 5). The broad 4000–3500 cm−1 absorption band is caused by OH-groups with water as the main contributor. Alcohols absorb in the range 3200–2600 cm−1 with a maximum between 3050 and 2850 cm−1 and also have absorptions between 1450–1200 cm−1 and 1100–960 cm−1 with a distinct peak around 1060 cm−1 . In pyrolysis literature, the 3200–2600 cm−1 range is often interpreted as mainly methanol, but other hydrocarbons with OH groups, e.g. anhydrosugars, are also present in the pyrolysis gas and cannot be elucidated since their absorption spectra are very similar to methanol [21]. Also methane, one of the main low molecular weight gases formed during pyrolysis, has a broad absorption band in this region (3150–2850 cm−1 ). No methane was detected for any sample in the spectra at maximum weight loss (Fig. 5), since it reached its maxima at higher temperatures (Fig. 4). The carbonyl (C O) stretch between 1900 and 1600 cm−1 (a in Fig. 5) shows high absorption for all samples, except for xylan and cellulose. This region includes overlapping absorption bands for aldehydes, ketones, acids and esters. However, aldehydes are known to have strong absorption focused around 1746 cm−1 , which agrees well with the sharp peak maxima (mainly visible for xyloglucan and galactomannan) found in the present study. This also correlates well with the aldehyde absorption between 2840 and 2720 cm−1 seen as a bump in the spectra, especially for those two samples. Between 1400 and 1200 cm−1 is the rocking and twisting bands of OH, that contains some information on acids and aromatics. It is difficult to evaluate due to weak absorptions, low concentrations and/or the interfering broad water absorption band 2000–1300 cm−1 . The hydroxyl ( OH) region between 1300 and 1000 cm−1 (b in Fig. 5) is characteristic for absorption of carboxylic acids, esters, ethers and alcohols. Typical pyrolysis products found in this region are formic acid, acetic acid, methanol and anhydrosugars. The pyranose structures of anhydrosugars have strong absorption bands in this region due to C OH, C O C, C C and C H in their ring structures. Anhydrosugars were, together with other larger gaseous products, further evaluated in the py-GC/MS section below. 3.4. Pyrolysis-GC/MS gas analysis Analysis of condensable gases formed during heating of different hemicelluloses and cellulose (Table 4, corresponding pyrograms in supplementary material) showed the presence of aldehydes [e.g. hydroxyacetaldehyde (HAA) and 2-furaldehyde (furfural)] and ketones [e.g. propanal-2-one and 4-hydroxy-5,6-dihydro-(2H)pyran-2-one (4-HDP)] from all samples. 5-hydroxylmethylfurfural (5-HMF), (2H)-furan-3-one and 2-hydroxymethyl-5-hydroxy-2,3dihydro-(4H)-pyran-4-one (2-HHDP) were formed only from hexose-containing polysaccharides (i.e. -glucan, arabinogalactan, galactomannan, glucomannan, xyloglucan and cellulose). The formation of 5-HMF has previously been reported to be formed from hexoses but not from pentoses [22], here demonstrated to be valid also for hemicelluloses. For xylan and cellulose, low amounts of two
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Fig. 4. DTG curve and corresponding gas profiles for (a) xylan, (b) arabinoxylan, (c) arabinogalactan, (d) galactomannan, (e) glucomannan, (f) xyloglucan, (g) -glucan, and (h) cellulose.
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208 429 5 26 28 – 88 5 11 153 8 – 72 – 137 – 88 84 358 13 21 2 – – 8 26 – 12 – – 204 87 – – 189 353 7 15 2 – – – 23 – 7 – 130 207 – – 172 349 4 26 5 – – 13 31 – 21 – – – 1073 31 – 216 338 16 37 169 – 320 – 9 203 14 33 5 12 137 – 3 488 381 12 13 13 – 101 – 20 – 19 – 471 – – – 303 a
b
Pyrograms in supplementary material. Contains impurities of other monosaccharides (Table 1).
80 120 – 29 8 – – – – 7 – – – – – – – 2.35 2.62 4.60 5.08 7.55 9.33 10.00 10.70 10.91 11.23 11.95 12.21 12.80 13.74 14.37 16.10 16.20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Propanal-2-one HAA (2H)-furan-3-one Furfural 4-HDP Levoglucosenone Arabinosan 1.4:3.6-dianhydro-glucopyranose 5-HMF Xylosan 2-HHDP 2-methoxy-4-vinylphenol Levogalactosan Levomannosan Levoglucosan 1.6-anhydro--d-glucofuranose 1.6-anhydro-␣-d-galactofuranose
Xylo-glucan Gluco-mannan Galacto-mannan Beta-glucan Arabino-xylan b Arabino-galactan
Polysaccharide [abundance/10,000]
Xylan
Retention time [min]
Identified compound
7
Peak label a
Table 4 Analyzed products from decomposition of the polysaccharides.
Fig. 5. FTIR spectra recorded at the time for maximum mass loss rate for the analyzed polysaccharides. The “a” and “b” regions denote C O and OH absorption, respectively.
Cellulose
of the main components, propanal-2-one and HAA, were formed during decomposition, which is in agreement with the lower FTIR signal representing carbonyl stretch between 1900 and 1600 cm−1 (a in Fig. 5). Anhydrosugars are typical pyrolysis decomposition products formed from polysaccharides and were here formed to a high extent from all polysaccharides evaluated with the exception of xylan (Table 4). This is in agreement with the discussions in connection to Fig. 5 where the IR absorption band between 1300 and 1080 cm−1 is lower for xylan and higher for the other hemicelluloses. Accordingly, -glucan and cellulose formed high amounts of their respective anhydrosugar, which was demonstrated by an increased absorption band at 1100–1000 cm−1 (b in Fig. 5). The main anhydrosugar product from glucans, mannans and galactans were 1,6-anhydro--d-glucopyranose (levoglucosan), 1,6-anhydro--d-mannopyranose (levomannosan) and 1,6-anhydro-␣-d-galactopyranose (levogalactosan), respectively. Moreover, arabinose and xylose containing hemicelluloses formed 1,4-anhydro-arabinofuranose (arabinosan) and 1,4-anhydro-␣-dxylopyranose (xylosan), respectively. The formation of anhydrosugars from carbohydrates during pyrolysis is known to be strongly correlated to the monosaccharide composition [22] and generally pentoses form 1,4-anhydropentoses and hexoses form 1,6-anhydrohexoses [22]. This concept was here demonstrated to be valid also for cell wall hemicelluloses. Worth noting is that levoglucosenone was formed only from decomposition of cellulose. In addition to the main anhydrosugar product focused on in this study, various other anhydrosugar derivatives (different isomers) are also formed during pyrolysis of biomass polysaccharides. To be able to fully evaluate py-GC/MS data, challenges remain in improving elution behavior and product separation in order to avoid broad and overlapping peaks. Initial work utilizing different derivatization reagents and techniques have been performed by Scalarone et al. [23] and Torri et al. [24]. The only phenolic compound identified in this work was 2-methoxy-4-vinylphenol from the decomposition of arabinoxylan. This product derives from the lignin-like compound ferulic acid, which is cross-linked to arabinoxylan in the cell walls of commelinid plants (such as rice, maize, wheat, rye, barley and sorghum).
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4. Conclusions Hemicellulose is of special interest for thermal decomposition studies since it make up a substantial fraction of all biomasses and is a heterogeneous group of polysaccharides whose composition differs between biomass species. In this study, the thermal decomposition behaviors of different commercially available hemicelluloses were investigated with respect to mass loss rate, reaction heat and composition of evolved gases. The most prominent result, supported by all properties studied, were the large variation between the different hemicelluloses. For instances, it was observed that for the different polysaccharides evaluated: - The temperature for maximum mass loss rates ranged between 243 ◦ C (the first peak of xylan) and 332 ◦ C (arabinoxylan, xyloglucan and -glucan), - reaction heats ranged from being endothermal to exothermal, - yields of fixed carbon ranged from 1.3% (-glucan) to 13.1% (xylan), and - gas formation profiles (CO, CO2 , CH4 , H2 O) and decomposition products differed and were correlated to the monosaccharide composition of the polysaccharide. The results also showed that compared to other hemicelluloses xylan showed: - mass loss at lower temperature and in two steps, - an overall exothermal behavior not correlated to the increased char formation when secondary char formation reactions were promoted in closed environment. - higher char formation, and - high CO2 formation as compared to the more heterogeneous gas mixture from the other hemicelluloses. Acknowledgements We thank Bio4Energy, a strategic research environment appointed by the Swedish government, for supporting this work.
For financial support, also the Sector project within the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n◦ 282826 is acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2014.08.013. References [1] H.V. Scheller, P. Ulvskov, Annu. Rev. Plant Biol. 61 (2010) 263. [2] G.O. Aspinall, Adv. Carbohydr. Chem. 14 (1959) 429. [3] A. Ebringerová, Z. Hromádková, T. Heinze, in: Thomas Heinze (Ed.), Polysaccharides I, Structure, Characterization and Use, 186, 2005, pp. 1–67, ISBN: 978-3-540-26112-4 (Print) 978-3-540-31583-4 (Online). [4] P.R. Patwardhan, R.C. Brown, B.H. Shanks, ChemSusChem 4 (2011) 636. [5] F. Shafizadeh, Y.L. Fu, Carbohydr. Res. 29 (1973) 113. [6] G.R. Ponder, G.N. Richards, Carbohydr. Res. 218 (1991) 143. [7] D.K. Shen, S. Gu, Bioresour. Technol. 100 (2009) 6496. [8] D.K. Shen, S. Gu, A.V. Bridgwater, J. Anal. Appl. Pyrolysis 87 (2010) 199. [9] H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Fuel 86 (2007) 1781. [10] R. Bassilakis, R.M. Carangelo, M.A. Wojtowicz, Fuel 80 (2001) 1765. [11] A. Jensen, K. Dam-Johansen, M.A. Wojtowicz, M.A. Serio, Energy Fuels 12 (1998) 929. [12] K. Raveendran, A. Ganesh, K.C. Khilar, Fuel 75 (1996) 987. [13] C. Branca, C. Di Blasi, C. Mango, I. Hrablay, Ind. Eng. Chem. Res. 52 (2013) 5030. [14] A. Aho, N. Kumar, K. Eranen, B. Holmbom, M. Hupa, T. Salmi, D.Y. Murzin, Int. J. Mol. Sci. 9 (2008) 1665. [15] C. Di Blasi, A. Galgano, C. Branca, Ind. Eng. Chem. Res. 50 (2011) 3864. [16] E. Granada, P. Eguia, J.A. Vilan, J.A. Comesana, R. Comesana, Renew. Energy 41 (2012) 416. [17] O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- und Werkstoff 49 (1991) 213. [18] O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- und Werkstoff 48 (1990) 281. [19] C. Di Blasi, C. Branca, F.E. Sarnataro, A. Gallo, Energy Fuels 28 (4) (2014) 2684–2696, http://dx.doi.org/10.1021/ef500296g. [20] D.K. Shen, S. Gu, A.V. Bridgwater, Carbohydr. Polym. 82 (2010) 39. [21] Q. Liu, S.R. Wang, Y. Zheng, Z.Y. Luo, K.F. Cen, J. Anal. Appl. Pyrolysis 82 (2008) 170. [22] S. Moldoveanu, Pyrolysis of Organic Molecules with Applications to Health and Environmental Issues, Elsevier, Amsterdam, 2010, p. [23] D. Scalarone, O. Chiantore, C. Riedo, J. Anal. Appl. Pyrolysis 83 (2008) 157. [24] C. Torri, E. Soragni, S. Prati, D. Fabbri, Microchem. J. 110 (2013) 719.
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Thermal decomposition of hemicelluloses Kajsa Werner, Linda Pommer, Markus Broström ∗ Department of Applied Physics and Electronics, Umeå University, SE – 90187 Umeå, Sweden
a r t i c l e
i n f o
Article history: Received 9 May 2014 Accepted 20 August 2014 Available online xxx Keywords: Pyrolysis Thermal decomposition Hemicellulose Xylan
a b s t r a c t Decomposition modeling of biomass often uses commercially available xylan as model compound representing hemicelluloses, not taking in account the heterogeneous nature of that group of carbohydrates. In this study, the thermal decomposition behavior of seven different hemicelluloses (-glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan, and xylan) were investigated in inert atmosphere using (i) thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, (ii) differential scanning calorimetry, and (iii) pyrolysis-gas chromatography/mass spectroscopy. Results on decomposition characteristics (mass loss rate, reaction heat and evolving gas composition) were compared and summarized for the different hemicelluloses and for comparison also crystalline cellulose was included in the study. The mass loss rate characteristics differed between the polysaccharides, with cellulose and glucan-based hemicelluloses as the thermally most stable and xylan as the least stable sample. The heat flow during slow heating in nitrogen flow showed a much more exothermal decomposition of xylan compared with the other hemicelluloses. The composition of off-gases during heating showed large differences between the samples. During decomposition of xylan high levels of CO2 and lower levels of other components were formed, whereas also CO, methanol, methane, furfural, 5-hydroxymethylfurfural and anhydrosugars were formed in substantial amounts from the other polysaccharides. The formation of anhydrosugar was correlated to the monosaccharide composition of the polysaccharide chain. The results from the current study contribute to new knowledge concerning thermochemical behavior of different hemicelluloses. © 2014 Elsevier B.V. All rights reserved.
1. Introduction All terrestrial biomass (coniferous and deciduous trees as well as herbaceous plants) share the same basic cell wall architecture with a cellulose microfibril network surrounded by a matrix of other polysaccharides (hemicellulose and pectin). In secondary cell walls of woody tissues also lignin is present [1]. It is important to consider that unlike cellulose, hemicellulose is a group of structurally diverse polysaccharides with amorphous structure and lower degree of polymerization. The building blocks in hemicelluloses are pentoses (arabinose and xylose), hexoses (galactose, glucose and mannose) and hexuronic acids (glucuronic acid). The nomenclature of hemicelluloses is principally determined by the most occurring sugar unit in the backbone [1]. In conifers (softwood trees), mannans (backbone of (1 → 4)-linked -d-mannose
∗ Corresponding author. Tel.: +46 70 295 5971. E-mail addresses: [email protected] (K. Werner), [email protected] (L. Pommer), [email protected] (M. Broström).
units) and glucomannans (backbone of both -d-mannose and -d-glucose units) are the predominating hemicelluloses [1–3]. Glucomannan, together with galactoglucomannan, may occur in close association with cellulose and represents the major (10–30%) hemicellulose component in softwood, but only a minor fraction in hardwood [1]. In deciduous trees (hardwood trees) xylanbased hemicelluloses are predominating. Xylans are composed of chains of (1 → 4)-linked -d-xylose, with single 4-O-methyl-dglucuronic acid [2,3]. The hemicellulosic fraction of herbaceous plants (monocotyledons) differs significantly from that of woody biomass (dicotyledons), both in relative abundance and composition [1]. Two examples of hemicelluloses characteristic for herbaceous plants are glucuronoarabinoxylan and -glucan. Glucuronoarabinoxylan has both 4-O-methyl-d-glucuronic acid and arabinose side chain residues attached to the xylan backbone chain, whereas -glucan (also known as mixed-linked glucan) is an unbranched homopolymer of glucose having both (1 → 3) and (1 → 4) linkages, which provides an unusual non-linear structure. The primary cell wall of grasses consists of 2–12% of -glucan, whereas it cannot be found in other biomasses [1]. A hemicellulose
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present in all types of biomass is arabinogalactan. Since the distinction between pectin and hemicellulose components of the cell wall is not well-defined, arabinogalactan is sometimes classified as pectin polysaccharide because of its -(1 → 4) linkages with non-equatorial configuration of C1 and C4 [1]. Furthermore, xyloglucan is a hemicellulose common for all higher plants and the most abundant (20–25%) in primary cell walls [1,3]. It has cellulose-like backbone branched by principally xylose units and are tightly bound to the cellulosic microfibrils by hydrogen bonds [3]. The close connection between hemicelluloses and other cell wall components, mainly cellulose and lignin, makes it very hard to extract hemicelluloses without changing the chemical structure. In this study commercially available xylan, arabinogalactan, arabinoxylan, galactomannan, glucomannan, xyloglucan and glucan were selected to represent hemicellulose of terrestrial biomass. Thermal degradation of biomass, and in particular decomposition of carbohydrates, is a complex process and it includes many different reactions occurring simultaneously, e.g. dehydration, depolymerization, fragmentation, rearrangement, repolymerization, condensation and carbonization [4]. The pyrolysis process results in a char residue (fixed carbon and ash) and gaseous products. The evolved gases include both condensable and non-condensable components, ranging from CO, CO2 , short chain alcohols, aldehydes and acids, to larger organic compounds such as furan-ring products and anhydrosugars. Several reaction pathways have been suggested for decomposition of cellulose [5] and xylan [6], and have been commendably summarized by Shen et al. [7,8]. Ponder and Richards [6] described an intramolecular stabilization by 1,6-glycocidic bond of anhydroglucopyranose (levoglucosan) for cellulose decomposition, and thereby explained the increased decomposition temperature interval for cellulose compared to the less stable intramolecular 1,4-glycocidic bond in anhydroxylopyranose formed during xylan decomposition. Based on these reported decomposition pathways, polysaccharides of different structure and sugar composition are expected to have different degradation pathways resulting in differences in thermal stability and product distribution. To enhance the understanding of thermochemical conversion mechanisms, decomposition modeling of different biomasses and their main components (principally cellulose, hemicellulose and lignin) is essential. Current knowledge about hemicellulose decomposition is limited and because of its commercial availability xylan has been used extensively as model substance [4,8–11]. Thermogravimetric analyses of xylan have revealed weight loss in the range 190–350 ◦ C [8,9,11,12] and DSC experiments have shown that it the decomposition is associated with exothermal reactions [9]. In contrast to the many studies on xylan decomposition, only a few have aimed at analyzing softwood hemicellulose [13,14]. The objective of this study was therefore to evaluate the thermal behavior of different commercially available hemicelluloses, potentially useful as model substances. Thermal properties were examined using thermogravimetric analysis (TGA/DTG) and differential scanning calorimetry (DSC). Gas product distribution was analyzed by using both TGA coupled to a Fourier transform infrared spectrometer (TG-FTIR) and gas chromatography/mass spectroscopy equipped with a pyrolysis unit (py-GC/MS). Inorganic compounds and contaminations have earlier been reported to cause changes in decomposition, both lowering decomposition temperature and altering product distribution by favoring formation of char, lighter organic compounds, and water [4,11,15]. Therefore, also the content and composition of inorganics in the samples were characterized. Differences in decomposition behavior with respect to mass loss rates, calorimetric information and gas compositions are discussed.
2. Materials and methods 2.1. Material characteristics In this study, commercially available hemicelluloses were studied and compared with a microcrystalline cellulose (Sigma–Aldrich). Xylan from beechwood (Sigma–Aldrich) consists of 1,4-linked -xylopyranosyl units substituted by 4-O-methyl␣-d-glucopyranosyl uronic acid units. The xylosyl-to-uronic acid ratio is 8:1 [8]. The other hemicelluloses tested were -glucan, arabinogalactan, arabinoxylan, galactomannan, glucomannan and xyloglucan (Megazyme International). All samples (Table 1) were used without further purification or processing. The low purity of arabinoxylan is the result of extra careful extraction and purification to maintain (aromatic) ferulic acid cross-linkages in native arabinoxylan. All samples were analyzed with respect to ash content and amount of fixed carbon using thermogravimetric analysis (Q5000IR, TA Instruments). Ash content was measured by heating the sample to 550 ◦ C at 50 ◦ C/min using a TGA and keeping it isothermal for 30 min while purged with air at 50 ml/min. For xylan the procedure had to be modified in order to oxidize all char, rising the isothermal temperature to 750 ◦ C. Fixed carbon was measured as the remaining weight after heating the samples at maximum heating rate of the instrument, corresponding to approximately 700 ◦ C/min, to 900 ◦ C and kept isothermal for 7 min in a 50 ml/min nitrogen purge. The remaining mass was interpreted as fixed carbon after correction for the ash content. Due to limited amounts of the samples, the main ash forming elements were identified using semi quantitative information from ESEM/EDS analysis of the remaining ashes from the TGA experiments. The instrument used was a Philips model XL30 environmental scanning electron microscope with an EDAX detector. 2.2. Thermal analysis Thermal analysis of the samples was performed by means of thermogravimetry (TGA/DTG) and differential scanning calorimetry (DSC). Detailed mass loss profiles (visualized as the negative derivate of the mass loss) were used to analyze and describe the decomposition characteristics of the samples. Small samples (0.5 mg) were used to avoid possible effects on mass transfer caused by the dense surface that was formed as the samples melted during heating. The samples, evenly spread on ceramic pans, were predried at 105 ◦ C for 15 min and were then heated at 10 ◦ C/min to 600 ◦ C in a 35 ml/min nitrogen purge. A DSC instrument (Q1000, TA Instruments) was used to refine the information on the decomposition reactions causing the weight changes in the DTG-data. For DSC measurements, samples (2 mg) were placed in open aluminum pans and were pre-dried at 105 ◦ C for 15 min, equilibrated at 10 ◦ C and then heated 10 ◦ C/min to 400 ◦ C. The instrument was purged with a 100 ml/min nitrogen flow. To also allow secondary reactions, complementary experiments were made with sample pans covered with aluminum lids that had only a small (0.5 mm) opening. Approximated reaction heats were calculated as the integral of the DSC signals over the temperature range of decomposition. This was done without taking in account the physical effects of evaporation heat and mass loss during decomposition, and therefore it serves only as a comparison between the samples, expressed as J/g of initial sample mass. 2.3. FTIR gas analysis Temperature resolved compositions of gases produced during heating of the different samples were characterized using a FTIR spectrometer (Frontier FTIR with EGA TL 8000 coupling,
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Table 1 Polysaccharides analyzed. Polysaccharide
Catalog number
Purity [%]
Monosaccharide compositiona
Xylan Arabinoxylan Arabinogalactan Galactomannan Glucomannan Xyloglucan -glucan Cellulose
X4252 P-WAXYI P-ARGAL P-GGMMV P-GLCML P-XYGLN P-BGBL 310,697
97.3 ∼80 >95 >98 >98 >97 ∼95 n.a.
Xyl:GluA = 87.5:12.5 Xyl:Ara:Glu:Man:Gal = 51:36:6.5:4.4:1.6 Gal:Ara = 85:15 Man:Gal = 62:38 Man:Glu = 60:40 Glu:Xyl:Gal:Ara = 45:36:16:3 Glu 95% Glu 100%
Not analyzed (n.a.). a Abbreviations: arabinose (Ara), galactose (Gal), glucose (Glu), glucuronic acid (GluA), mannose (Man), xylose (Xyl).
Perkin Elmer) coupled to the TGA instrument. For these experiments, larger samples (10 mg) and higher purge gas flow rates (100 ml/min) were used in order to optimize the FTIR signal. The samples, evenly spread on ceramic pans, were pre-dried at 105 ◦ C for 15 min and then heated at 10 ◦ C/min to 600 ◦ C. The FTIR, with a deuterated triglycine sulfate (DTGS) detector set to a scan rate of 0.2 cm/s and 2 cm−1 resolution resulting in totally 288 spectra recorded every 10 s during the heating ramp, was set to scan between 4000 and 450 cm−1 . The temperatures of the sampling line and the gas cell were set to 230 ◦ C and the pump of the EGA system was set to constantly extract 50 ml/min from the TGA off-gas through the FTIR gas cell. Both spectra interpretation and quantification are difficult from the complex mixture of pyrolysis gas. Interpretation can be done either by correct identification correlated to a library search [10], by in-house calibration of preferably light gases [16], or to assign different absorption to groups of species that by their composition and chemical bonds are known to absorb in specific regions. The difficulty lies in the mixture of many different, sometimes unknown, organic species evolving simultaneously and often with similar and overlapping absorption spectra. In the present study, some of the smaller compounds (H2 O, CH4 , CO2 and CO) were identified, assigned and recorded according to their well-known and characteristic absorption spectra (Table 2), whereas other absorbing species were grouped together and only assigned to corresponding bands of absorption. Absorption by H2 O can be detected in the O H stretching bands at 4000–3400 cm−1 and 1800–1300 cm−1 , here monitored by the peak at 3853 cm−1 . The variations in shape of the absorption band 3100–2600 cm−1 are due to differences in distribution between different compounds, including hydrocarbons, alcohols and aldehydes. A common interpretation is to assign the whole band to CH4 , but by carefully analyzing available reference spectra it can be seen that a more correct way to identify CH4 in that region is by its sharp peak at 3018 cm−1 since the broad absorption of CH4 overlaps with that of other hydrocarbons. In this study, CH4 was identified as the height of that distinct peak and with a two-point subtraction of the broad underlying absorption. The peaks centered around 2354 and 2320 cm−1 show the presence of CO2 in the gas, whereas absorption bands around 2190 and 2110 cm−1 represent the typical absorption of CO.
Table 2 Absorption bands and specific wave numbers used for the gas profiles. Compound
Absorption band [cm−1 ]
Identification [cm−1 ]
H2 O CH4 CO2 CO
4000–3400 and 1800–1300 peak at 3018 2391–2217 2220–2150 and 2140–2060
3853 3018a 2360 2186
a
The broad hydrocarbon absorption used as background.
2.4. Pyrolysis-GC/MS gas analysis In addition to TG-FTIR, also py-GC/MS was used to analyze evolved gases. Samples were weighed (50 g) and placed into tin cups, (PY-1-ECO Frontier lab). The samples were subjected to flash pyrolysis conditions to a temperature of 450 ◦ C for 0.2 min (AS-1020E and Py-2020iD, Frontier Lab). Compounds volatilizing during pyrolysis were analyzed using a gas chromatograph equipped with a mass spectrometer (GC/MS) (Agilent 7890A, MS 5975). The GC was equipped with an Ultra alloy-5 capillary column (5% diphenyl, 95% dimethylpolysiloxane, 30 m, I.D. 0.25 mm, film thickness 0.25 m). The initial furnace temperature was 40 ◦ C and was ramped at 10 ◦ C/min to 320 ◦ C, where it was kept isothermal for 10 min. The split ratio was 16:1 using helium as carrier gas with a column flow of 1.2 ml/min. The temperature of the transfer line to the MS was set to 320 ◦ C. Electron ionization at 70 eV and m/z range 12–400 was used for analysis. Components were tentatively identified using NIST library (NIST/EPA/NIH mass spectral library, version 2, 2009) complemented with the polysaccharide and lignin degradation libraries of Faix [17,18]. 3. Results and discussion 3.1. Material properties The samples were characterized using TGA and SEM/EDX for determination of fixed carbon, ash content and identification of the main ash forming elements (Table 3). The inorganic elements are probably residues from the extraction procedure. Sodium, potassium and chlorine are typical constituents of agents used for extracting hemicellulose from the biomass matrix. Grains of potassium chloride were seen in the ash from glucomannan during the ESEM/EDS analysis. Silicone-containing grains, probably SiO2 , were detected in the ashes from the xyloglucan, -glucan and galactomannan samples. Silicone was also seen in the ash from arabinoxylan, but in the form of fibers, probably originating from filter material used in the extraction procedure. The different amounts of fixed carbon formed from the polysaccharides (Table 3), indicated differences in char formation mechanisms. Cellulose and -glucan formed the lowest amount of fixed carbon, whereas xylan and glucomannan demonstrated the highest yields (13.1 and 8.4%, respectively). The high char formation of xylan is in agreement with previous studies, e.g. 20% char yield at 900 ◦ C [9], 22% at 540 ◦ C [8], 30% at 1000 ◦ C [12] and 25% at 900 ◦ C [11]. Minor differences in results between the studies might be due to differences in e.g. sample purity, sample preparation, heating rates, sample mass, instrument geometry and gas flows. Xylan and glucomannan were shown not only to have high char yields but also to be the samples with the highest ash content (2.0 and 2.2%, respectively). Since ash forming elements are known to favor char formation, it is likely that the observed differences in char formation between the hemicelluloses studied are at least
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Table 3 Ash content, fixed carbon and main ash forming elements for the analyzed polysaccharides. Polysaccharide
Fixed carbona [% d.s., ash free]
Ash contenta [% d.s.]
Main ash forming elementsb
Xylan Arabinoxylan Arabinogalactan Galactomannan Glucomannan Xyloglucan -glucan Cellulose
13.1 2.7 3.6 4.8 8.4 3.6 1.3 2.0
2.0 0.6 0.4 0.5 2.2 0.5 n.d. n.d.
Na, Ca Ca, Mg, P, S, Si Ca, Na Na, Si, Ca, Cl Ca, Na, K, P, Cl, Mg Na, Si, Ca, P n.a. n.a.
Not detected (n.d.), not analyzed (n.a). a Measured by TGA, % of dry substance (d.s.), and, for fixed carbon, on ash free basis. b Elements detected at >5% of the ash on molar basis measured by ESEM/EDS, in descending order.
partially caused by differences in the amount or the composition of inorganic constituents. Fixed carbon and ash contents (Table 3) had a positive correlation (R2 = 0.82). 3.2. Thermogravimetric (DTG) and calorimetric (DSC) analysis DTG profiles from the polysaccharides decomposition during heating in inert atmosphere showed differences in mass loss rate characteristics (Fig. 1). Overall, mass loss appeared in the temperature range 200–380 ◦ C. Defining the thermal stability as the temperature of the DTG peak maximum, the most stable cell wall polysaccharide was cellulose, closely followed by the glucan-based hemicelluloses -glucan and xyloglucan, and then arabinoxylan, arabinogalactan, galactomannan, glucomannan and finally xylan. The stable appearance of cellulose is explained by its partly crystalline structure. Moreover, the differences in stability among the polysaccharides can probably be explained by formation of more or less stable intermediates during decomposition, where levoglucosan (6C) is the most stable, anhydrosugar compound formed and arabinosan (5C) and xylosan (5C) the least stable ones, as described by others [6,11]. Since arabinoxylan mainly forms thermally less stable anhydrosugars (arabinosan and xylosan) it may seem peculiarly that it keeps losing mass at higher temperatures (up to 243 ◦ C). This could be due to the phenolic compounds cross-linked to arabinoxylan in the cell wall. Xylan deviated compared to the other evaluated polysaccharides by mass loss at lower temperature and in two stages, with maximum mass loss rates at 243 ◦ C and 292 ◦ C (Fig. 1). This result agrees with earlier studies performed under similar conditions [8,9]. The mass losses of the other polysaccharides were between
Fig. 1. Mass loss rate curves for the analyzed polysaccharides.
200 and 250 ◦ C with mass loss rate maxima between 309 and 326 ◦ C. For arabinoxylan the high decomposition temperature, together with the broad mass loss interval, could be explained by the presence of aromatic impurities remaining from extraction. The DSC curves (Fig. 2) show the heat flow (exothermal and endothermal behavior) of the sample during heating, both in open and closed pans. Arabinogalactan, arabinoxylan, -glucan, galactomannan, glucomannan, xyloglucan and cellulose showed overall endothermal decomposition characteristics in open pans, whereas xylan displayed a clear exothermal behavior. The endothermal behavior with open pans shows trends very similar to the devolatilization in the corresponding DTG curves (Fig. 1), and it is plausibly caused mainly by the endothermal devolatilization. The endothermal behavior of hemicelluloses, apart from xylan, has to the authors’ knowledge not previously been reported. Yang et al. 2007 [9] have published data on the exothermal behavior of xylan, which is therein assigned to the exothermal charring reactions. An alternative or complementary explanation for the exothermic behavior is the presence of glucuronic acid units in xylan. The carboxylic acids groups ( COOH) are prone to undergo decarboxylation resulting in carbon dioxide and exothermal enthalpy changes. This aspect of the uronic sugar content in
Fig. 2. Reaction heat curves for the analyzed polysaccharides in open (solid lines) and closed (dashed lines) sample pans. Heat flow in W/g of initial sample mass. Curves displayed pairwise with a −2 W/g offset.
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Fig. 3. Heat of reactions (integral of DSC peak over the temperature range of decomposition, positive values are exothermal) vs fixed carbon yield (mass remaining at 400 ◦ C corrected for the ash content) for the analyzed polysaccharides as measured by DSC in both open and closed samples pans in order to study effects of secondary reactions. The effects of adding lids are highlighted by the arrows.
biomass (applicable for hemicellulose and pectin) has previously not been emphasized in pyrolysis literature. However, exothermal behavior has been observed from fixed bed bench-scale pyrolysis of some biomasses, [19] and could be the effect of uronic sugar decomposition. Fig. 3 shows the relation between reaction heat, estimated as the integral of the DSC curves over the temperature range of the decomposition, vs. fixed carbon, measured as the residue after the experiment and compensated for the ash content. Overall, a positive correlation was observed between fixed carbon yields and heat flow. The effect of reducing rapid evaporation of volatiles by closing the pan and thereby promoting secondary char forming reactions is also shown in Fig. 3. The secondary char forming reactions are present also in an open environment, but to a much lower extent. Allowing for secondary reactions increased the char yield and exothermicity for all samples except for xylan and arabinogalactan. Xylan showed similar and high char yield and exothermicity both in open and closed environment (no arrow in Fig. 3), whereas arabinogalactan showed an increase in reaction heat that was higher in relation to the increase in char yield (almost vertical arrow in Fig. 3). A plausible reason for this could be non-equatorial configuration of the -(1 → 4) linkages, associated with the composition of pyrolysis gases, which reacted further in secondary reactions. 3.3. FTIR gas analysis The off-gases from heating the samples in the TGA were characterized by FTIR spectroscopy. For all polysaccharide samples, absorption vs. temperature profiles for H2 O, CH4 , CO2 and CO were plotted together with the corresponding DTG curves (Fig. 4a–h). Concentrations were not quantified, but since the experimental conditions were identical, differences in gas formation can be analyzed by comparing release profiles for the different samples, if also taking in account differences in mass loss rate. The relative gas compositions had similarities for all polysaccharides, except for xylan that decomposed into mainly CO2 with only small amounts of the other components (Fig. 4). The relatively high CO2 concentration from xylan decomposition is in agreement with findings by others and is therein explained by either decarboxylation of COOH groups on glucuronic acid units [4], cracking and reforming of C O and COOH groups [9] or decarboxylation of O-acetyl groups linked to xylan [20]. Moreover, the dominating
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CO2 formation provides a plausible explanation for the exothermal behavior (Fig. 2), since CO2 formation indicates that carbon has undergone full oxidation resulting in a maximized exothermal enthalpy change for the volatilized carbon containing components. At higher temperatures, during charring reactions, significant amounts of methane were detected for the two samples with the highest yields of fixed carbon (xylan and glucomannan, Fig. 4a and e). Also this observation is in agreement with the findings by others [8] and were explained by the breaking of stable methoxy group (O CH3 ) bonds leading to a dislocation to higher temperatures for the methane formation. For more detailed information on gas composition, FTIR spectra recorded at the respective time for maximum weight loss rates are presented (Fig. 5). The broad 4000–3500 cm−1 absorption band is caused by OH-groups with water as the main contributor. Alcohols absorb in the range 3200–2600 cm−1 with a maximum between 3050 and 2850 cm−1 and also have absorptions between 1450–1200 cm−1 and 1100–960 cm−1 with a distinct peak around 1060 cm−1 . In pyrolysis literature, the 3200–2600 cm−1 range is often interpreted as mainly methanol, but other hydrocarbons with OH groups, e.g. anhydrosugars, are also present in the pyrolysis gas and cannot be elucidated since their absorption spectra are very similar to methanol [21]. Also methane, one of the main low molecular weight gases formed during pyrolysis, has a broad absorption band in this region (3150–2850 cm−1 ). No methane was detected for any sample in the spectra at maximum weight loss (Fig. 5), since it reached its maxima at higher temperatures (Fig. 4). The carbonyl (C O) stretch between 1900 and 1600 cm−1 (a in Fig. 5) shows high absorption for all samples, except for xylan and cellulose. This region includes overlapping absorption bands for aldehydes, ketones, acids and esters. However, aldehydes are known to have strong absorption focused around 1746 cm−1 , which agrees well with the sharp peak maxima (mainly visible for xyloglucan and galactomannan) found in the present study. This also correlates well with the aldehyde absorption between 2840 and 2720 cm−1 seen as a bump in the spectra, especially for those two samples. Between 1400 and 1200 cm−1 is the rocking and twisting bands of OH, that contains some information on acids and aromatics. It is difficult to evaluate due to weak absorptions, low concentrations and/or the interfering broad water absorption band 2000–1300 cm−1 . The hydroxyl ( OH) region between 1300 and 1000 cm−1 (b in Fig. 5) is characteristic for absorption of carboxylic acids, esters, ethers and alcohols. Typical pyrolysis products found in this region are formic acid, acetic acid, methanol and anhydrosugars. The pyranose structures of anhydrosugars have strong absorption bands in this region due to C OH, C O C, C C and C H in their ring structures. Anhydrosugars were, together with other larger gaseous products, further evaluated in the py-GC/MS section below. 3.4. Pyrolysis-GC/MS gas analysis Analysis of condensable gases formed during heating of different hemicelluloses and cellulose (Table 4, corresponding pyrograms in supplementary material) showed the presence of aldehydes [e.g. hydroxyacetaldehyde (HAA) and 2-furaldehyde (furfural)] and ketones [e.g. propanal-2-one and 4-hydroxy-5,6-dihydro-(2H)pyran-2-one (4-HDP)] from all samples. 5-hydroxylmethylfurfural (5-HMF), (2H)-furan-3-one and 2-hydroxymethyl-5-hydroxy-2,3dihydro-(4H)-pyran-4-one (2-HHDP) were formed only from hexose-containing polysaccharides (i.e. -glucan, arabinogalactan, galactomannan, glucomannan, xyloglucan and cellulose). The formation of 5-HMF has previously been reported to be formed from hexoses but not from pentoses [22], here demonstrated to be valid also for hemicelluloses. For xylan and cellulose, low amounts of two
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Fig. 4. DTG curve and corresponding gas profiles for (a) xylan, (b) arabinoxylan, (c) arabinogalactan, (d) galactomannan, (e) glucomannan, (f) xyloglucan, (g) -glucan, and (h) cellulose.
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208 429 5 26 28 – 88 5 11 153 8 – 72 – 137 – 88 84 358 13 21 2 – – 8 26 – 12 – – 204 87 – – 189 353 7 15 2 – – – 23 – 7 – 130 207 – – 172 349 4 26 5 – – 13 31 – 21 – – – 1073 31 – 216 338 16 37 169 – 320 – 9 203 14 33 5 12 137 – 3 488 381 12 13 13 – 101 – 20 – 19 – 471 – – – 303 a
b
Pyrograms in supplementary material. Contains impurities of other monosaccharides (Table 1).
80 120 – 29 8 – – – – 7 – – – – – – – 2.35 2.62 4.60 5.08 7.55 9.33 10.00 10.70 10.91 11.23 11.95 12.21 12.80 13.74 14.37 16.10 16.20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Propanal-2-one HAA (2H)-furan-3-one Furfural 4-HDP Levoglucosenone Arabinosan 1.4:3.6-dianhydro-glucopyranose 5-HMF Xylosan 2-HHDP 2-methoxy-4-vinylphenol Levogalactosan Levomannosan Levoglucosan 1.6-anhydro--d-glucofuranose 1.6-anhydro-␣-d-galactofuranose
Xylo-glucan Gluco-mannan Galacto-mannan Beta-glucan Arabino-xylan b Arabino-galactan
Polysaccharide [abundance/10,000]
Xylan
Retention time [min]
Identified compound
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Peak label a
Table 4 Analyzed products from decomposition of the polysaccharides.
Fig. 5. FTIR spectra recorded at the time for maximum mass loss rate for the analyzed polysaccharides. The “a” and “b” regions denote C O and OH absorption, respectively.
Cellulose
of the main components, propanal-2-one and HAA, were formed during decomposition, which is in agreement with the lower FTIR signal representing carbonyl stretch between 1900 and 1600 cm−1 (a in Fig. 5). Anhydrosugars are typical pyrolysis decomposition products formed from polysaccharides and were here formed to a high extent from all polysaccharides evaluated with the exception of xylan (Table 4). This is in agreement with the discussions in connection to Fig. 5 where the IR absorption band between 1300 and 1080 cm−1 is lower for xylan and higher for the other hemicelluloses. Accordingly, -glucan and cellulose formed high amounts of their respective anhydrosugar, which was demonstrated by an increased absorption band at 1100–1000 cm−1 (b in Fig. 5). The main anhydrosugar product from glucans, mannans and galactans were 1,6-anhydro--d-glucopyranose (levoglucosan), 1,6-anhydro--d-mannopyranose (levomannosan) and 1,6-anhydro-␣-d-galactopyranose (levogalactosan), respectively. Moreover, arabinose and xylose containing hemicelluloses formed 1,4-anhydro-arabinofuranose (arabinosan) and 1,4-anhydro-␣-dxylopyranose (xylosan), respectively. The formation of anhydrosugars from carbohydrates during pyrolysis is known to be strongly correlated to the monosaccharide composition [22] and generally pentoses form 1,4-anhydropentoses and hexoses form 1,6-anhydrohexoses [22]. This concept was here demonstrated to be valid also for cell wall hemicelluloses. Worth noting is that levoglucosenone was formed only from decomposition of cellulose. In addition to the main anhydrosugar product focused on in this study, various other anhydrosugar derivatives (different isomers) are also formed during pyrolysis of biomass polysaccharides. To be able to fully evaluate py-GC/MS data, challenges remain in improving elution behavior and product separation in order to avoid broad and overlapping peaks. Initial work utilizing different derivatization reagents and techniques have been performed by Scalarone et al. [23] and Torri et al. [24]. The only phenolic compound identified in this work was 2-methoxy-4-vinylphenol from the decomposition of arabinoxylan. This product derives from the lignin-like compound ferulic acid, which is cross-linked to arabinoxylan in the cell walls of commelinid plants (such as rice, maize, wheat, rye, barley and sorghum).
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4. Conclusions Hemicellulose is of special interest for thermal decomposition studies since it make up a substantial fraction of all biomasses and is a heterogeneous group of polysaccharides whose composition differs between biomass species. In this study, the thermal decomposition behaviors of different commercially available hemicelluloses were investigated with respect to mass loss rate, reaction heat and composition of evolved gases. The most prominent result, supported by all properties studied, were the large variation between the different hemicelluloses. For instances, it was observed that for the different polysaccharides evaluated: - The temperature for maximum mass loss rates ranged between 243 ◦ C (the first peak of xylan) and 332 ◦ C (arabinoxylan, xyloglucan and -glucan), - reaction heats ranged from being endothermal to exothermal, - yields of fixed carbon ranged from 1.3% (-glucan) to 13.1% (xylan), and - gas formation profiles (CO, CO2 , CH4 , H2 O) and decomposition products differed and were correlated to the monosaccharide composition of the polysaccharide. The results also showed that compared to other hemicelluloses xylan showed: - mass loss at lower temperature and in two steps, - an overall exothermal behavior not correlated to the increased char formation when secondary char formation reactions were promoted in closed environment. - higher char formation, and - high CO2 formation as compared to the more heterogeneous gas mixture from the other hemicelluloses. Acknowledgements We thank Bio4Energy, a strategic research environment appointed by the Swedish government, for supporting this work.
For financial support, also the Sector project within the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n◦ 282826 is acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jaap.2014.08.013. References [1] H.V. Scheller, P. Ulvskov, Annu. Rev. Plant Biol. 61 (2010) 263. [2] G.O. Aspinall, Adv. Carbohydr. Chem. 14 (1959) 429. [3] A. Ebringerová, Z. Hromádková, T. Heinze, in: Thomas Heinze (Ed.), Polysaccharides I, Structure, Characterization and Use, 186, 2005, pp. 1–67, ISBN: 978-3-540-26112-4 (Print) 978-3-540-31583-4 (Online). [4] P.R. Patwardhan, R.C. Brown, B.H. Shanks, ChemSusChem 4 (2011) 636. [5] F. Shafizadeh, Y.L. Fu, Carbohydr. Res. 29 (1973) 113. [6] G.R. Ponder, G.N. Richards, Carbohydr. Res. 218 (1991) 143. [7] D.K. Shen, S. Gu, Bioresour. Technol. 100 (2009) 6496. [8] D.K. Shen, S. Gu, A.V. Bridgwater, J. Anal. Appl. Pyrolysis 87 (2010) 199. [9] H. Yang, R. Yan, H. Chen, D.H. Lee, C. Zheng, Fuel 86 (2007) 1781. [10] R. Bassilakis, R.M. Carangelo, M.A. Wojtowicz, Fuel 80 (2001) 1765. [11] A. Jensen, K. Dam-Johansen, M.A. Wojtowicz, M.A. Serio, Energy Fuels 12 (1998) 929. [12] K. Raveendran, A. Ganesh, K.C. Khilar, Fuel 75 (1996) 987. [13] C. Branca, C. Di Blasi, C. Mango, I. Hrablay, Ind. Eng. Chem. Res. 52 (2013) 5030. [14] A. Aho, N. Kumar, K. Eranen, B. Holmbom, M. Hupa, T. Salmi, D.Y. Murzin, Int. J. Mol. Sci. 9 (2008) 1665. [15] C. Di Blasi, A. Galgano, C. Branca, Ind. Eng. Chem. Res. 50 (2011) 3864. [16] E. Granada, P. Eguia, J.A. Vilan, J.A. Comesana, R. Comesana, Renew. Energy 41 (2012) 416. [17] O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- und Werkstoff 49 (1991) 213. [18] O. Faix, I. Fortmann, J. Bremer, D. Meier, Holz als Roh- und Werkstoff 48 (1990) 281. [19] C. Di Blasi, C. Branca, F.E. Sarnataro, A. Gallo, Energy Fuels 28 (4) (2014) 2684–2696, http://dx.doi.org/10.1021/ef500296g. [20] D.K. Shen, S. Gu, A.V. Bridgwater, Carbohydr. Polym. 82 (2010) 39. [21] Q. Liu, S.R. Wang, Y. Zheng, Z.Y. Luo, K.F. Cen, J. Anal. Appl. Pyrolysis 82 (2008) 170. [22] S. Moldoveanu, Pyrolysis of Organic Molecules with Applications to Health and Environmental Issues, Elsevier, Amsterdam, 2010, p. [23] D. Scalarone, O. Chiantore, C. Riedo, J. Anal. Appl. Pyrolysis 83 (2008) 157. [24] C. Torri, E. Soragni, S. Prati, D. Fabbri, Microchem. J. 110 (2013) 719.
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