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Energy & Fuels 2009, 23, 498–501
Cross-Polarization/Magic Angle Spinning (CP/MAS) 13C Nuclear Magnetic Resonance (NMR) Analysis of Chars from Alkaline-Treated Pyrolyzed Softwood Kasi David, Yunqiao Pu, Marcus Foston, John Muzzy, and Arthur Ragauskas* School of Chemistry and Biochemistry and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed June 12, 2008. ReVised Manuscript ReceiVed October 20, 2008
Pyrolysis chemistry of alkaline-treated loblolly pine is investigated in this study. The pyrolysis experiments were accomplished under an argon atmosphere at 200-400 °C, to determine the effect of alkaline treatment on the char formation. Solid-state cross-polarization/magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy was used to characterize the chars of the treated and untreated loblolly pine. These studies showed that, in the samples treated with NaOH, there is a shift in the chemical composition of the thermally modified char from cellulosic/hemicellulosic structures to more aryl C structures. At 300 °C, carbohydrate peaks were still present in the char of untreated wood but not in the char of the alkaline-treated wood. This indicated that the addition of NaOH enhanced the thermal degradation of the sugars in the pine sawdust. Our studies also showed that there was an emergence of an aliphatic peak around 12 ppm in the treated pine, which is absent in the spectra of the untreated pine. In addition to the relative increase in aryl and aliphatic C species, there is also an increase in the signals seen for the carbonyl species centered around 210 ppm at the higher end of the temperature range in this study for the treated samples. This investigation showed that solid-state NMR provided a facile methodology to investigate the types of changes that occur to wood chars during pyrolysis.
1. Introduction The increase in global population and gross domestic product (GDP) has led to an increase in the demand for transportation fuels over the past 3 decades, and this fuel consumption is expected to increase about 60% in the next 20 years. The challenge in addressing this demand curve will need to be addressed in a multipathway approach, involving enhanced vehicle efficiencies, upgrading of heavy crude reserves, additional exploration for oil recovery, and second-generation biofuels. The conversion of biomass to biofuels can be accomplished by employing either biological conversion technologies or thermal treatments. The two main thermal biomass technologies are (i) gasification of biomass to syn-gas and Fischer-Tropsch conversion to green diesel, gasoline, dimethyl ether, and/or ethanol and (ii) fast pyrolysis of biomass to a biooil followed by upgrading.1 Each of these approaches has its own unique technical strengths and challenges that must be addressed before they become practical stand-alone operations. Fast pyrolysis has garnered much interest because it is a relatively low capital cost process that can convert a broad spectrum of bioresources to bio-oils.2 It also produces little waste because both the bio-oil produced and residual char can be used as fuel resources. In the literature, there is a wide range of conditions used for the pyrolysis of wood for bio-oil production with experimental temperatures ranging from 250 to an upper value of 1400 °C in * To whom correspondence should be addressed. Telephone: (404) 8949701. E-mail:
[email protected]. (1) Demirbas, A. Prog. Energy Combust. Sci. 2007, 33, 1–18. (2) Mohan, D.; Pittman, C. U., Jr.; Steele, P. H. Energy Fuels 2006, 20, 848–889.
some cases, although typical conditions are in the temperature range of 425-557 °C.3-6 Bio-oils are free-flowing organic liquids that have been reported to contain in excess of 300 chemical constituents, many of which contain oxygenated functionality.7,8 They are formed when the gases evolve during pyrolysis, that contain fragmented components of wood biopoloymers, are rapidly condensed. In terms of its major components, bio-oil is made up of a complex aqueous mixture of hydroxylaldehydes, hydroxyketones, carboxylic acids, phenolic compounds, lignin, and sugars.9-11 The final ratio of these compounds is dependent upon many parameters, including the water content of the chosen biomass, the pyrolysis temperature, particle size, and gas flow rates.12-14 Dependent upon the heating rate and temperature, the onset of pyrolysis occurs approximately at 190 °C and the most significant mass loss occurs between 300 and 400 °C.2,15,16 It was shown that increased temperature (3) Demirbas, A. J. Anal. Appl. Pyrolysis 2005, 73, 39–43. (4) Paris, O.; Zollfrank, C.; Zickler, G. A. Carbon 2005, 43 (1), 53–66. (5) Guerrero, M.; Ruiz, M. P.; Alzueta, M. U.; Bilbao, R.; Millera, A. J. Anal. Appl. Pyrolysis 2005, 74, 307–314. (6) Bridgewater, A. V. Chem. Eng. J. 2003, 91, 87–102. (7) Czernik, S.; Bridgewater, A. V. Energy Fuels 2004, 18, 590–598. (8) Peacocke, G. V. C.; Russel, P. A.; Jenkins, J. D.; Bridewater, A. V. Biomass Bioenergy 1994, 7, 169–178. (9) Garcia-Perez, M.; Chaala, A.; Pakdel, H.; Kretschmer, D.; Roy, C. Biomass Bioenergy 2007, 31, 222–242. (10) Ji-lu, Z. J. Anal. Appl. Pyrolysis 2007, 80, 30–35. (11) Amen-Chen, C.; Pakdel, H.; Roy, C. Biomass Bioenergy 1997, 13, 25–37. (12) Uzun, B. B.; Putun, A. E.; Putun, E. Bioresour. Technol. 2006, 97, 569–576. (13) Putun, A. E.; Apaydin, E.; Putun, E. Energy 2004, 29, 2171–2180. (14) Onay, O.; Mete Kocka, O. Fuel 2006, 85, 1921–1928. (15) Helsen, L.; van den Bulck, E.; Mullens, S.; Mullens, J. J. Anal. Appl. Pyrolysis 1999, 52, 65–86.
10.1021/ef8004527 CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
CP/MAS 13C NMR Analysis of Chars
Energy & Fuels, Vol. 23, 2009 499 Table 1. Signal Assignments for CP/MAS 13C NMR of Loblolly Pine Used in This Study28-30
Figure 1. Comparison of char yields of loblolly pine with and without NaOH treatment at various pyrolysis temperatures and heated for 30 min.
Figure 2. CP/MAS 13C NMR spectra of (a) pine wood, (b) wood char obtained by heating pine impregnated with a 4% aqueous NaOH solution for 30 min at 200 °C, and (c) wood char obtained by heating pine impregnated with a 4% aqueous NaOH solution for 30 min at 250 °C.
leads to changes in the types of reactions during the pyrolysis of wood. For example, Schroder et al. found that higher temperatures led to an increased carbon content in the char fraction, while Branca et al. found that liquid yields, consisting of tars and water, increased from ∼40 to 55% of the dry wood mass with increasing temperature.17,18 Other than changes attributable solely to the reaction temperature, it has been shown that certain additives also affect the thermal behavior of the wood. For example, certain metal salts have been shown to reduce the flammability and smoke generation of heated wood. It has also been shown that wood impregnated with metal compounds promoted the formation of char as well as water and carbon dioxide and decreased the yield of tar.19 Researchers have also discovered that the addition of NaOH has a catalytic effect on the decomposition of the wood and its constituents, where the onset of major decomposition is lowered from ∼360 to ∼270 °C.20,21 The presence of other ions, (16) Murwanashyaka, J. N.; Pakdel, H.; Roy, C. J. Anal. Appl. Pyrolysis 2001, 60, 219–231. (17) Schroder, E. J. J. Anal. Appl. Pyrolysis 2004, 71, 669–694. (18) Branca, C.; Giudicianni, P.; di Blasi, C. Ind. Eng. Chem. Res. 2003, 42 (14), 3190–3202. (19) Fu, Q.; Argyropoulos, D. S.; Tilotta, D. C.; Lucia, L. A. J. Anal. Appl. Pyrolysis 2008, 81, 60–64. (20) Wang, J.; Mingxu, Z.; Mingqiang, C.; Fanfei, M.; Suping, Z.; Zhengwei, R.; Yongjie, Y. Thermochim. Acta 2006, 444, 110–114. (21) Amen-Chen, C.; Pakdel, H.; Roy, C. Bioresour. Technol. 2001, 79, 277–299.
chemical shift (ppm)
assignment
141-152 123-140 110-123 105 102 89 84 73-76 65 62 56 21
C-OR aryl of lignin C-R aryl of lignin C-H aryl of lignin C-1 of cellulose C-1 of hemicellulose C-4 of crystalline cellulose C-4 of amorphous cellulose C-2, C-3, and C-5 of cellulose C-6 of crystalline cellulose C-6 of amorphous cellulose -OCH3 of lignin CH3CdO of hemicellulose
such as CuII and FeIII salts, has been shown to improve the production of vanillin and syringaldehyde in the degradation of lignin.22 Wang et al. suggested that, because the sodium ion is very small, it can penetrate into the biomass and break the intermolecular hydrogen bonds during swelling or heating.20 These observations suggest the possibility of lowering the temperature required for the pyrolysis of wood and related biomass with the addition of a base, such as NaOH. While there have been many studies related to the effect of metal ions on the final composition of bio-oils produced by pyrolysis, there remains a need to investigate the composition of the char remaining after pyrolyzing metal-ion-treated wood. In summary, although there have been some investigations on the thermal degradation of wood components at lower temperatures,23,24 the chemistry taking place in the char during pyrolysis has been understudied, especially with wood treated with bases, such as NaOH. To garner a greater understanding of the pyrolysis process, this study examined the residues remaining from pyrolysis of loblolly pine treated with NaOH. The wood samples were pyrolyzed in the temperature range of 200-400 °C, and the chars were characterized by solid-state cross-polarization/magic angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy.25,26 2. Experimental Section 2.1. Materials and Sample Preparation. A 15-year old Loblolly Pine tree (Pinus taeda) was acquired from a University of Georgia plot in Baldwin County, GA, and used for all studies in this report. The tree was 13-15 m high and was visually free of disease and compression wood. The wood was manually debarked, chipped, and refined with a Wiley mill through a 0.13 cm screen. The refined wood had a 10.34% moisture content, and samples were stored at slightly below 0 °C prior to use. 2.2. Thermal Treatment. A total of 25.00 mL of a 4% aqueous NaOH solution was added to 10.00 g of sawdust, and the mixture was mixed vigorously for 10.0 min at room temperature. The NaOH-impregnated wood was then dried at 75 °C to a constant weight. Pyrolysis reactions were performed in a preheated tube furnace under argon with a flow rate of 150 cc/min and at temperatures of 200, 250, 300, 350, and 400 °C for 30.0 min. Additional reactions were performed at 400 °C for 7.5 and 15.0 min. Typical sample loadings were 1.20 g of oven-dried (od) weight of wood. Pyrolysis reactions in the previously stated range were (22) Wu, G.; Heitz, M.; Esteban, C. Ind. Eng. Chem. Res. 1994, 33, 718–723. (23) Inari, G. N.; Mounguengui, S.; Dumarcay, S.; Petrissans, M.; Gerardin, P. Polym. Degrad. Stab. 2007, 92, 997–1002. (24) Wikberg, H.; Maunu, S. L. Carbohydr. Polym. 2004, 58, 461–466. (25) Hakkou, M.; Petrissans, M.; Zoulalian, A.; Gerardin, P. Polym. Degrad. Stab. 2005, 89, 1–5. (26) Sivonen, H.; Nuopponen, M.; Maunu, S. L.; Sundholm, F.; Vuorinen, T. Appl. Spectrosc. 2003, 57 (3), 266–273.
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Figure 3. CP/MAS 13C NMR spectra of wood char obtained by heating pine impregnated with a 4% aqueous NaOH solution for 30 min at (a) 300 °C, (b) 350 °C, and (c) 400 °C. Table 2. Elemental Microanalysis of Selected Chars Obtained from Untreated and NaOH-Treated Pine percentage of percentage of percentage of carbon (%) hydrogen (%) oxygen (%) temperature (°C) untreated treated untreated treated untreated treated 200 250 300 350 400
49.31 51.17 55.60 70.89 73.93
45.30 55.77 54.15 55.46 59.60
5.89 5.77 5.57 4.36 3.54
5.73 5.24 5.29 4.66 3.83
43.72 42.67 38.06 23.57 20.30
43.79 31.59 31.10 29.52 24.58
also performed on samples made by adding 25.00 mL of distilled water to 10.00 g of sawdust. After thermal treatment, the wood residues were immediately placed in a desiccator under a N2 atmosphere and allowed to cool to room temperature. Elemental analysis of the treated chars were determined by Huffman Laboratories, Inc., Golden, CO. Carbon, hydrogen, and nitrogen analyses were conducted by combustion in pure oxygen, and oxygen was determined through combustion in a carbon combustion tube. 2.3. CP/MAS 13C NMR Analysis. Solid-state CP/MAS 13C NMR was carried out using a Bruker Avance/DMX 400 NMR spectrometer operating at 100.59 MHz. The experiments were performed at ambient temperature using a Bruker 4 mm MAS probe. The samples were ground and packed into 4 mm ZrO rotors, which were spun at 5 kHz. Adamantane was used for the Hartman-Hahn matching calibration, and glycine was used as an external standard for calibrating the chemical shifts.27
3. Results and Discussion Figure 1 shows the char yields at various temperatures for the untreated and alkaline-treated loblolly pine samples. Although the chemical analysis of wood has been accomplished with a host of analytical tools, CP/MAS 13C NMR has been shown to be well-suited to characterize changes in functional groups especially in a series of closely related samples.28 The CP/MAS 13C NMR spectra of the starting Loblolly pine and thermally treated wood at 200 and 250 °C are shown in Figure 2. The starting wood shows the standard signals ascribed to the main components of wood. The CP/MAS 13C NMR signal assignments for the pine used in this study are summarized in Table 1.29-31 The shoulder appearing around 102 ppm on the C-1 signal of cellulose has been assigned to the hemiacetal carbon of hemicellulose.25,26 Upon heating to 200 °C, the hemicellulose acetyl absorption at 21 ppm and C-1 hemicellulose signal at 102 ppm are no longer present, indicating the degradation of (27) Pu, Y.; Ziemer, C.; Ragauskas, A. J. Carbohydr. Res. 2006, 341, 591–597. (28) Bardet, M.; Hediger, S.; Gerbaud, G.; Gambarelli, S.; Jacquot, J. F.; Foray, M. F.; Gadelle, A. Fuel 2007, 86, 1966–1976.
Figure 4. CP/MAS 13C NMR spectra of wood char obtained by heating pine impregnated with a 4% aqueous NaOH solution at 400 °C for (a) 7.5 min, (b) 15 min, and (c) 30 min.
this polysaccharide. A peak around 25 ppm begins to appear in this heat-treated wood, indicating the generation of aliphatic carbon components. The remaining portions of the spectra, including those peaks for crystalline cellulose and lignin, still closely resemble the spectra of the starting material. The third spectrum shows the result of heating wood at 250 °C. At this temperature, most of the signals assigned to cellulose and hemicellulose are significantly reduced in relative signal intensity. Also, the signal intensities of the aromatic region of lignin are dramatically enhanced. This indicates that at a pyrolysis temperature of ∼250 °C and under the conditions employed most of the carbohydrates are degraded, leaving lignin as the main component in the char. There is also an apparent increase in the signal in the aliphatic region at 25 ppm, which is ascribed to the formation of aliphatic structures. Figure 3 shows the spectra obtained when the alkaline-treated wood was heated at 300, 350, and 400 °C. Upon heating to 300 °C, the two resonances centered at 56 and 148 ppm ascribed to the methoxyl and oxygen-substituted aromatics become relatively smaller. These results suggest a loss of oxygen functionality from lignin structures, which is also supported by the elemental microanalysis summarized in Table 2. There is also a significant change in the maxima of the aliphatic peak centered near 25 ppm, and the chemical-shift value of the maxima of this peak shifts from 30 to 12 ppm (Figure 3). This was ascribed to increased hydrocarbon products as the pyrolysis temperature increases. In an attempt to obtain a better understanding of the processes taking place at the higher temperatures in this study, multiple treatments were performed at 400 °C using different heating times. This is illustrated in Figure 4, where the spectra display the results of pyrolysis at 400 °C for 7.5, 15.0, and 30.0 min. As the time increases, there is no significant further changes observed in the spectral data, indicating that pyrolysis at 400 °C with NaOH has yielded a relatively stable char. Figure 5 shows the spectra obtained when the pine wood was treated only with distilled water and pyrolyzed at 200, 250, 300, 350, and 400 °C. The results obtained at 200 °C were similar to what is seen occurring at 200 °C in the NaOH-treated sawdust. At (29) Sekiguchi, Y.; Frye, J. S.; Shafizadeh, F. J. Appl. Polym. Sci. 1983, 28, 3513–3525. (30) Alesiani, M.; Proietti, F.; Capuani, S.; Maraviglia, B. Appl. Magn. Reson. 2005, 28, 1–8. (31) Baldock, J. A.; Smernik, R. J. Org. Geochem. 2002, 33, 1093– 1109.
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sawdust. The spectra of the char obtained at 350 and 400 °C also resembles that of the treated sample, except there is an absence of peaks in the aliphatic and carbonyl region of the spectra. 4. Conclusions
Figure 5. CP/MAS 13C NMR spectra of wood char obtained by heating pine impregnated with distilled water for 30 min at (a) 200 °C, (b) 250 °C, (c) 300 °C, (d) 350 °C, and (e) 400 °C.
250 °C, there are more signals assigned to cellulose and hemicellulose still present in the char compared to the NaOHtreated sample at this temperature. The presence of carbohydrate peaks can also still be seen in the char at 300 °C, while they are not present in the char of the alkaline-treated wood at this same temperature. This indicates that the addition of NaOH enhanced the thermal degradation of the sugars in the pine
This study using CP/MAS 13C NMR spectroscopy has provided added insight into the transformations occurring in NaOH-treated wood during the pyrolysis process. The results show that, in this treated sample, there is a shift in the chemical composition of the thermally modified char from cellulosic/ hemicellulosic structures to more aryl C structures. This result is similar to the one seen by Sekiguchi et al. and Baldock et al. while characterizing the char produced from untreated wood.28,30 The biggest difference is the emergence of an aliphatic peak around 12 ppm, which differs from the results seen in chars formed from untreated wood. In addition to this relative increase in aryl and aliphatic C species, there is also an increase in the signals seen for the carbonyl species centered around 210 ppm at the higher end of the temperature range in this study. These results show that the addition of NaOH reduces the temperature at which decomposition occurs. The use of solid-state NMR provides a facile methodology by which the structure of wood char from pyrolysis can be readily characterized. Acknowledgment. Financial assistance from the Chevron Corp. to conduct this study is greatly appreciated and acknowledged. EF8004527
