Characterization of Pyrolytic Lignin Extracted from Bio-oil
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BIOTECHNOLOGY AND BIOENGINEERINGChinese Journal of Chemical Engineering , 18(6) 1018—1022 (2010)Characterization of Pyrolytic Lignin Extracted from Bio-oil *JIANG Xiaoxiang (姜小祥)1, Naoko Ellis 2 and ZHONG Zhaoping (仲兆平)1,**1Thermoenergy Engineering Research Institute, Southeast University, Nanjing 210096, China 2Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, CanadaAbstract Bio-oil is a new liquid fuel produced by fast pyrolysis, which is a promising technology to convert bio-mass into liquid. Pyrolytic lignin extracted from bio-oil, a fine powder, contributes to the instability of bio-oil. The paper presents the structural features of three kinds of pyrolytic lignin extracted from bio-oil with different methods (WIF, HMM, and LMM). The pyrolytic lignin samples are characterized by Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS). FTIR data indicate that the three pyrolytic lignin samples have similar functional groups, while the absorption intensity is different, and show characteristic vibra-tions of typical lignocellulosic material groups O H (3340-3380 cm −1), C H (2912-2929 cm −1) and C O (1652-1725 cm −1). Comparison in the region (3340-3380 cm −1) indicates that WIF has more O H stretch groupsthan HMM and LMM. The carbon spectra are fitted to four peaks: C1, C C or C H, BE =283.5 eV; C2, C OR or C OH, BE =284.5-285.8 eV; C3, C O or HO C OR, BE =286.10-287.10 eV; C4, O C O, BE =287.5-287.7 eV . The absence of C1, C C or C H indicates the dominant polymerization structure of aro-matic carbon in pyrolytic lignin samples. For HMM and WIF, C2a and C2b can not be separated, so there is no free hydroxyl group in the samples. The oxygen peaks are also fitted to four peaks: O1, OH, BE = 530.3 eV; O2, RC O, BE =531.45-531.72 eV; O3, O C O, BE = 532.73-533.74 eV; O4, H 2O, BE =535 eV . The absence of O1 and O4 indicates that little hydroxyl groups and adsorbed water are present in the samples.Keywords bio-oil, pyrolytic lignin, Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy1 INTRODUCTIONDevelopment of renewable energy, such as bio-mass, is an alternative way to solve the energy crisis and environmental problems [1]. It is not economical to transport low energy density biomass in remote areas a long distance for further processing. Pyrolysis is a well established thermochemical process since it can convert 60%-75% (by mass) of the original bio-mass into a crude bio-oil with an energy density around 26800 MJ·m −3, and the bio-oil can be eco-nomically transported to bio-refineries located up to 500 km from the bio-resource [2]. Furthermore, bio-oil has been regarded as a candidate to replace petroleum fuels in various applications [3].To obtain more liquid products, the process should be operated at relatively low temperature, high heating rate and short gas residence time [4], so the fast pyrolysis is not a chemical equilibrium process. The liquid product, bio-oil, is not as stable as conven-tional petroleum fuels. Czernik et al . [5] studied the aging of bio-oils. The bio-oil was exposed to elevated temperatures for different periods. The viscosity and the molecular weight of bio-oil increased with time and temperature. Diebold and Czernik [6] gave a com-prehensive overview on the stability of bio-oil, and the reactions of aldehydes to hydrates, hemiacetals, ethers, dimers were suggested to take place during the aging test of bio-oil. The bio-oil must be preheated prior to combustion in order to lower its viscosity and improve its atomization. Some problems such as plugged filters and nozzles at low temperature were observed [7]. The pyrolytic lignin contributes significantly to the insta-bility of bio-oils. It needs to identify the composition and structure of pyrolytic lignin in order to understand the aging mechanism of bio-oil during storage. To improve the stability of bio-oil, pyrolytic lignin should be removed from bio-oils. There are only a few re-ports on extracting pyrolytic lignin from bio-oils [8-10]. It may be separated by adding water to precipi-tate lignin derived hydrophobic compounds. Lignin may be a more valuable co-product if extracted effi-ciently. For example, pyrolytic lignin can be used as adhensive in wood-based panel industry.In this study, the solvent fraction method is used to extract pyrolytic lignin from bio-oil. Compared with the water precipitation method [8], the advantage of this method is that the bio-oil will not contain more water and can be used as fuel. The pyrolytic lignin is characterized and compared with the lignin extracted from bio-oil by water precipitation method. 2 EXPERIMENTAL2.1 Preparation of pyrolytic lignin2.1.1 Method IPyrolytic lignin was extracted according to the Refs. [8-10]. Five grams of aged bio-oil was added dropwise to 500 ml of ice-cooled distilled water stirred at 5000 r·min −1 with a homogenizer. After stirred for 1 h the precipitated lignin was filtered and washed. The filtered lignin was re-suspended in 500Received 2010-04-27, accepted 2010-08-05.* Supported by State Key Development Program for Basic Research of China (2007CB210208), National Science and Technol-ogy Major Project of China (2008ZX07101), China Scholarship Council (CSC), Natural Science and Engineering Research Council of Canada (NSERC), BIOCAP, and Canadian Funding for Innovations (CFI). ** To whom correspondence should be addressed. E-mail: zzhong@Chin. J. Chem. Eng., Vol. 18, No. 6, December 20101019ml of ice-cooled water and stirred for another 4 h. Fi-nally, the solution was filtered and the pyrolytic lig-nin named WIF was dried under vacuum at room temperature.2.1.2 Method II200 ml ether is slowly added into 100 ml bio-oil using a separating funnel. Shake it for several minutes, then keep it standstill for settling for about 40 min. Two layers form in the separating funnel, which are called ES (upper layer) and EIS (lower layer) after removing the ether solution by rotary evaporator. Re-move the ES (ether soluble fraction of bio-oil), add about 250 ml dichloromethane into EIS (ether insolu-ble fraction of bio-oil), shake for several minutes, and then settle for about 40 min. In the funnel, the upper layer is called DCMS and the lower layer is DCMIS after removing the dichloromethane solution by rotary evaporator. Then DCMS and DCMIS are dried using vacuum filtration and cold drying to obtain pyrolytic lignin, called low-molecular-mass pyrolytic lignin of EIS (LMM) and high-molecular-mass pyrolytic lignin of EIS (HMM), respectively.2.2 Characterization of pyrolytic ligninElemental analysis was performed using a Perkin Elmer 2400 Series II CHNS/O analyzer (CHN mode) according to the standard procedure. The higher heat-ing values (HHV) were calculated by Dulong’s for-mula [11]:()HHV[338.2C1442.8H O/8]0.001=×+×−× (1) Where HHV is in MJ·(kg dry basis)−1, and C, H, O as percent on a dry basis. Fourier transformed infrared spectroscopy (FTIR) was performed on a Perkin Elmer Spectrum One FTIR spectrophotometer. 2 mg of pyrolytic lignin was ground with 150 mg of KBr and the mixture was pressed into a pellet. All spectra were normalized to the height of the aromatic skeletal vibrations peak at 1432 cm−1. Each sample had 32 spectra between 4000 and 400 cm−1.XPS measurements were performed with a ES-CALAB MK II-MICROLAB 500 spectrometer using a non-monochromatized Mg Kα radiation. Survey scans were taken with a 1.0 eV step and 80 eV ana-lyzer pass energy while the high resolution regional spectra were recorded with a 0.1 eV step and 20 eV pass energy. The base pressure was typically <10−9 Pa. The pressure was less than 2×10−8 Pa and normally remained much lower. The X-ray power was 200 W. Internal calibration was referenced to the C compo-nent of the C1s spectra at the binding energy (BE) of 284.8 eV. After subtraction of a nonlinear background, the spectra were analysed by using the peak fitting software.3 RESULTS AND DISCUSSIONProperties of crude bio-oil are shown in Table 1. Density is an important parameter on storage, trans-portation and utility of bio-oil. The density of bio-oil is 1219 kg·m−3 at 25 °C and increases to 1223 kg·m−3at 20 °C, larger than those of water and some heavy fuels (usually 940 kg·m−3). It may affect the atomiza-tion burning of bio-oil. Kinematic viscosity is an in-dicator of the fluidity and stickiness of oils. The ki-nematic viscosity of the bio-oil in this study is 92.5 mm2·s−1 at room temperature, which is much larger than that of traditional fuels (3-4 mm2·s−1). Bio-oils usually have large water content, which may cause the stratification of bio-oil and accelerate its deterioration. In this study, the water mass content is 22.49% and the absolute error of the measurement is 0.51%. The acid number of the bio-oil at room temperature is 49.71 KOH mg·g−1. The high acidity may be resulted from the acidic materials in the bio-oil.Elemental analysis of pyrolytic lignin is given in Table 2. The data for selected milled wood lignin is also presented. Pyrolytic lignin contains more carbon and hydrogen than wood, while wood has more oxy-gen. Among the three lignin samples extracted from bio-oils, LMM contains more carbon and hydrogen but has the least oxygen. LMM has the largest high heating value, while HMM has the least.Table 2 Elemental analysis and high heat value (HHV) ofpyrolytic lignin samplesElemental analysis/% (by mass) SamplesHHV/MJ·kg−1 C H O N WIF 26.27164.936.5328.330.21LMM 27.35366.586.6726.550.20HMM 23.21159.816.2733.620.30 MWL (spruce) 59.78 5.79 34.09Table 1 Physicochemical properties of fresh bio-oilProperties Analysismethod Values density DMA 35N EX Petro digital density meter 1223 kg·m−3 (at 20 °C)1219 kg·m−3 (at 25 °C)kinematic viscosity HAAKE rotary viscometer 92.5 mm2·s−1 (at 25 °C) water content K-F Titration Metrohm (22.49±0.51)% (by mass)acid number K-F Titration Metrohm (49.71±0.31)KOHmg·g−1Chin. J. Chem. Eng., Vol. 18, No. 6, December 20101020 The pyrolytic lignin samples were measured as self-supported pellets by FTIR spectroscopy. FTIR spectra for the three pyrolytic lignin samples are pre-sented in Fig. 1. They have similar functional groups, while their absorption intensities are different. Peaks of interest are listed in Table 3 (assignments based on data from [12, 13]). The FTIR spectra show character-istic vibrations of typical lignocellulosic material groupsO H (3340-3380 cm −1), C H (2912-2929 cm −1) and C O (1652-1725 cm −1). We can also find peaks re-lated to C C bonds of aromatic skeleton (1422-1515 cm −1) and some signals associated with syringyl (S) and guayacyl (G) groups in the lignin samples (1120- 1155 cm −1). The bands observed at 500-1030 cm −1 are attributed to hemicelluloses and silicates.Figure 1 FTIR spectra of three pyrolytic lignin samplesComparison of the region (3340-3380 cm −1) in-dicates that WIF has more O H stretch groups than HMM and LMM. This may be due to the initial β-O -4 ether bond homolysis and the phenol formation from the phenoxy radical. Because of the homolysis of ini-tial β-O -4, there are many unconjugated.and conju-gated C O at 1652-1725 cm −1. The intensity of sub-stitution reaction of aromatic ring causes differenttransmittance in 1266-1270 cm −1.C detail spectra give the information on the chemical nature of the pyrolytic lignin samples. These spectra are fitted to four peaks: C1, C C or C H, BE =283.5 eV; C2, C OR or C OH, BE =284.5- 285.8 eV; C3, CO or HO C OR, BE =286.10- 287.10 eV; C4, O C O, BE =287.5-287.7 eV . C1 peak is for C atom with bonds to C or H, and the other three peaks are for C atoms with one, two and three peaks to O atoms.Figure 2 shows the deconvoluted high resolution carbon spectra. The deconvolution uses Gaussian peak shapes and integrated background subtraction [14]. For the fitting, the peak positions are fixed according to tabulated chemical shifts [15] and guidelines estab-lished [16]. What is of interest here is the aliphatic carbon region, designated as C1 centered at 283.5 eV . C1 is not found in the spectra, which may be due to the dominant polymerization structure of aromatic carbon of pyrolytic lignin.Table 4 shows the binding energy of each kind of C and relative peak area (%). The peak of C2b is not found in HMM and WIF, which may be due to the combination of the C atoms in the side chain of pyro-lytic lignin with O atoms, so that the two peaks (C2a and C2b) can not be separated. It also indicates that almost no free hydroxyl group is present in the sample.Additional information on the chemical nature of the three pyrolytic lignin samples can be obtained from the O detail spectra (Fig. 3). These peaks are alsofitted to four peaks: O1, OH, BE =530.3 eV; O2, RC O, BE =531.45-531.72 eV; O3, O C O, BE = 532.73-533.74 eV; O4, H 2O, BE =535 eV . Table 5 shows the binding energy of each kind of O and rela-tive peak area (%). Only O2 and O3 are found. The absence of O1 indicates that little hydroxyl groups are in the pyrolytic lignin. The absence of O4 peak shows that little adsorbed water is in the samples.Table 3 FTIR peaks of interest of three pyrolytic lignin samplesWave numbers of samples/cm −1Wave numbers/cm −1Band origin LMM HMM WIF 3340-3380 O H stretch 33343335 3335 2912-2929 C H stretch 2935 2934 2936 1700-1725 CO stretch1710170917101550-1630 ring stretches 1601 1602 16031490-1515 ring stretches 1514 1514 1515 1446-1473 CH 3, CH 2 deformations1463146314631398-1443 ring stretch 1431 1432 1432 1280-1300CC bridge bond stretch1297 1297 1297 1266-1270 C O +guaiacyl ring 1275127412751221-1234 CC plus CO 1235 1234 12341140 aromatic C H deformation in guaiacyl ring 1154 1154 1154 1125-1128 aromatic C H deformation in syringyl ring112411241124Chin. J. Chem. Eng., Vol. 18, No. 6, December 20101021(a) WIF(b) HMM(c) LMMFigure 2 C 1s XPS spectra of three pyrolytic lignin samples4 CONCLUSIONSThree ptrolytic lignin samples ware prepared with two methods. FTIR data indicate that the sampleshave similar functional groups, while the absorption intensity is different. The characteristic vibrations of typical lignocellulosic material groups are for O H (3340-3380 cm−1), C H (2912-2929 cm −1) and CO(a) HMM(b) LMM(c) WIFFigure 3 O 1s XPS spectra of three pyrolytic lignin samplesTable 4 Binding energy and ratio of C 1s area (A ) of three pyrolytic lignin samplesC2C1 (C H/CC)C2a(C OR) C2b(C OH) C3 (CO/HO COR) C4 (O C O) SamplesBE/eVABE/eVABE/eV A BE/eV ABE/eV A WIF — — 284.982 76.47 — — 286.871 23.53— — LMM — — 284.685 50.36 285.583 28.54 286.746 21.10——HMM——284.828 61.21——286.207 22.90 287.503 15.89Chin. J. Chem. Eng., Vol. 18, No. 6, December 20101022 (1652-1725 cm −1). WIF has more O H stretch groups than HMM and LMM. The carbon spectra are fitted to four peaks. The absence of C1, C C or C H indi-cates the dominant polymerization structure of aro-matic carbon in pyrolytic lignin samples. In HMM and WIF, C2a and C2b are unable to be separated, so there is no free hydroxyl group in the samples. The oxygen peaks are also fitted to four peaks: O1, OH, BE =530.3 eV; O2, RC O, BE =531.45-531.72 eV; O3, O C O, BE = 532.73-533.74 eV; O4, H 2O, BE =535 eV . The absence of O1 and O4 indicates that there are little hydroxyl groups and adsorbed water in the samples. These results are promising for better application of bio-oil and pyrolytic lignin. REFERENCES1 Bridgwater, A.V ., Peacocke, G.V .C., “Fast pyrolysis processes forbiomass”, Renew . Sustain . 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Technol ., 12, 735-743 (1978).Table 5 Binding energy and ratio of O 1s area (A ) ofthree pyrolytic lignin samplesO2 (RC O) O3 (O C O)Samples BE/eVA BE/eVAWIF——533.026 100LMM 531.700 33.22 533.473 66.78HMM 531.520 28.59533.245 71.41。
