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DETERMINATION OF LIGNIN CONTENT IN PRESSURISED HOT WATER EXTRACTS USING DIFFERENT LIGNIN DETERMINATION METHODS Conference Paper · August 2012 CITATIONS
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3 authors: Risto I. Korpinen
Andrey V. Pranovich
Natural Resources Institute Finland (Luke)
Åbo Akademi University
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DETERMINATION OF LIGNIN CONTENT IN PRESSURISED HOT WATER EXTRACTS USING DIFFERENT LIGNIN DETERMINATION METHODS Risto Korpinen*1, Andrey Pranovich1,2, Stefan Willför1 1 Åbo Akademi University, Process Chemistry Centre c/o Laboratory of Wood and Paper Chemistry, Porthansgatan 3–5, FI-20500 TURKU/ÅBO, FINLAND 2 Finnish Forest Research Institute P.O. Box 18, FI-01301, VANTAA, FINLAND *E-mail:
[email protected] ABSTRACT Different conventional lignin determination methods have been applied to hemicellulose-rich extracts from spruce and birch obtained by pressurised hot-water extraction and subsequent membrane ultrafiltration. Mass balances of the overall processes were calculated in respect to dry solids content and lignin content. The different lignin determination methods were evaluated according to the lignin mass balances obtained by the applied methods. The amount of lignin in the raw extracts and filtrates varied significantly depending on the used method. Nevertheless, the lignin mass balance calculations yielded similar results in general. I. INTRODUCTION Pressurised hot-water extraction is an environmentally sound process to extract hemicelluloses from different wood species. The purity of these hemicellulose-rich fractions is an important aspect for further usage and the major impurity is usually lignin, or fragments thereof. Lignin can be reactive when these hemicellulose fractions are processed into new products. Also, lignin gives undesired colour and lignin-derived products even smell. There are several studies where the lignin content of the hemicellulose-rich extracts has been determined using various methods. However, comparative studies are very few and none have been done earlier on hot-water extracts. Additionally, the lignin mass balances obtained by different lignin determination methods have not been studied appropriately. II. EXPERIMENTAL Industrial Norway spruce and Scandinavian birch sawdusts were extracted using a flow-through pressurised hot-water extraction (PHWE) process (Leppänen et al 2011). Thereafter the obtained hemicellulose-rich raw extract fractions, called feed fractions, were ultrafiltered (UF) using a regenerated cellulose membrane (10 kDa cut-off). The mass balances of the UF processes can be seen in Table 1. The UF process resulted in two fractions, i.e. a concentrate and a permeate. The concentrate is a fraction containing high-molar-mass hemicelluloses. Also, the dry solids content in the concentrate is significantly higher than in the feed. The permeate is a fraction containing low-molar-mass hemicelluloses with low dry solids content. Table 1. Mass balance on the ultrafiltration for Norway spruce and Scandinavian birch extracts. Spruce feed Spruce concentrate Spruce permeate Difference Birch feed Birch concentrate Birch permeate Difference
Amount extract, kg 1570 30 1540
Dry solids content, % 1.02 19.16 0.69
1570 30 1540
1.57 25.00 1.05
Amount dry solids, kg 16.01 5.75 10.63 -0.36 24.65 7.50 16.17 0.98
The obtained fractions were freeze-dried and analysed for lignin content. The following lignin determination methods were applied: determination of chlorine consumption or chlorine number (ISO 3260), acetyl bromide method (Yokoyama et al. 2002) and modifications of Klason lignin
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(KCL N:o 115b:82, TAPPI T 222, TAPPI UM 250, Sluiter et al. 2011, Goldschmid 1971). In the Goldschmid method, TAPPI T 222 was used for determining the acid-insoluble lignin content. Lignin content from chlorine number was obtained by multiplying the chlorine number by 0.9 (Kyrklund and Strandell 1969). In the acetyl bromide method (AcBr), spruce and birch milled wood lignin (MWL) was used for UV-spectrophotometer calibration. The Klason lignin methods give gravimetric (acid-insoluble) lignin content and acid-soluble lignin content determined by UVspectrophotometry. The different wavelength and extinction coefficients can be seen in Table 2.
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Table 2. Wavelengths and extinction coefficients used to determine acid-soluble lignin by Klason method Method KCL
Wavelength, nm 203
Absorbance 0.2–0.7
TAPPI LAP
205 240
0.2–0.7 0.7–1.0
Goldschmid*
215 and 280
Extinction coefficient, L/gcm 128, softwood 110, hardwood 110 12, softwood 25, hardwood
*) lignin concentration, g/L = (4.53 * A215 nm – A280 nm) / 300. KCL: KCL N:o 115b:82, TAPPI: TAPPI UM 250, LAP: Sluiter et al. 2011, Goldschmid: Goldschmid 1971. Furfural and hydroxymethylfurfural (HMF) was determined for the Klason lignin hydrolysates using reversed-phase HPLC with a diode array detector. Prior analysis the pH value of the hydrolysates was adjusted to approximately 3 by adding 2 M NaOH solution and filtrated. Authentic furfural and HMF compounds were used for HPLC calibration. III. RESULTS AND DISCUSSION The acid-insoluble lignin content in the spruce feed fraction determined by different methods was comparatively the same. Moreover, these methods gave also even values for the spruce concentrate and permeate fractions, as seen in Figures 1–2. The same phenomenon was also observed for the studied birch fractions. However, different modifications of Klason lignin method resulted in more substantial differences in the acid-soluble lignin content. KCL, TAPPI and Goldschmid methods gave similar results, but the LAP method gave significantly higher acid-soluble lignin content, for both spruce and birch.
Figure 1. Lignin content in dry solids of spruce extract and filtrates (left) and birch extract and filtrates (right). As seen in Table 3, substantial amounts of furfural and HMF were found in the hydrolysates when the LAP method was applied. Furfural and HMF have the absorbance maxima at 277 nm and 285 nm, respectively (Chi et al. 2009). Although their absorbance maxima differ from 240 nm, which is used in the LAP method, these low-molar-mass compounds have high extinction coefficients and still absorb UV-light at this wavelength as well. Consequently, the significantly high furfural and HMF content will increase the UV-absorbance and contribute to the acid-soluble lignin content.
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Table 3. Furfural and HMF content (mg/l) in Klason lignin filtrates. KCL Spruce feed Spruce concentrate Spruce permeate Birch feed Birch concentrate Birch permeate
Furfural 2.0 0.5 2.8 8.1 6.2 6.6
HMF 1.1 1.6 1.2 0.2 0.1 0.2
TAPPI Furfural HMF 5.5 2.1 1.1 1.9 5.2 1.6 17.1 0.3 11.6 0.2 17.9 0.3
LAP Furfural 42.5 14.8 65.3 178.0 130.1 175.6
HMF 17.7 25.8 15.7 2.3 1.9 2.2
The lignin content in the spruce raw hot-water extract and filtrates obtained by chlorine number and AcBr methods were higher than those obtained by Klason lignin methods, except the lignin content obtained by the LAP method. The same phenomenon was also observed for the birch feed fraction. In birch concentrate and permeate fractions, chlorine number and AcBr methods resulted in lignin contents higher than obtained by the LAP method. In case of AcBr method, the explanation of high lignin value can be related to assumption that the isolated MWL used for calibration is generally believed to be similar to the true native lignin, i.e. it is less oxidised and degraded compared to lignin release from wood at high temperature (Li et al. 2000). Thus, the structure and UV-spectra properties of model MWL and lignins in extracts can also be different. Also, xylan degradation products formed during the analysis will contribute to higher lignin content than expected (Hatfield et al. 1999). During the chlorine number method, chlorine gas is liberated and it reacts with lignin. The higher lignin content obtained by the chlorine number method may be explained by the fact that the chlorine gas also can react with carbohydrates and extractives (Fukayama 1986; Gullichsen 1999) and therefore result in higher lignin content. The amount of lignin in different fractions was calculated using the data in Tables 1–2. Although the different lignin determination methods resulted in different amounts of lignin in the feed, concentrate and permeate fractions, the obtained lignin mass balances yielded comparatively same results, as seen in Tables 4–5. However, the lignin mass balance for birch extracts determined by AcBr method deviated substantially from the rest. Table 4. The amount of total lignin in spruce hot-water extract and filtrates obtained by different lignin determination methods. Cl number AcBr KCL Tappi LAP Goldschmid
Feed, kg 3.15 3.41 2.36 2.58 4.20 2.57
Concentrate, kg 0.82 1.03 0.74 0.77 1.13 0.77
Permeate, kg 2.17 2.32 1.74 1.54 3.31 1.51
Difference, kg* 0.16 0.05 -0.12 0.27 -0.24 0.28
Difference, %** 94.83 98.40 105.04 89.51 105.64 88.94
*) Difference = Feed - (Concentrate + Permeate) **) Difference = (Concentrate + Permeate) / Feed * 100% Table 5. The amount of total lignin in birch hot-water extracts and filtrates obtained by different lignin determination methods. Cl number AcBr KCL Tappi LAP Goldschmid
Feed, kg 6.06 6.72 5.03 5.15 7.18 5.21
Concentrate, kg 2.23 2.93 2.04 2.13 2.57 2.16
Permeate, kg 3.25 5.30 2.90 2.84 4.15 2.93
Difference, kg* 0.59 -1.50 0.09 0.17 0.46 0.12
Difference, %** 90.34 122.36 98.26 96.63 93.63 97.73
*) Difference = Feed - (Concentrate + Permeate) **) Difference = (Concentrate + Permeate) / Feed * 100% Based on the calculated total lignin mass balances, the methods have no significant differences, except the lignin mass balance in birch hot-water extract and filtrates since the values cannot be determined correctly by the AcBr method. The gross chemical composition of the studied materials should be included when the methods are further evaluated. IV. CONCLUSIONS The different lignin determination methods resulted in different amounts of lignin in the studied Norway spruce and Scandinavian birch extracts and filtrates obtained by pressurised hot water extraction followed by ultrafiltration. However, the lignin mass balances in respect to the total dry
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solids content and the total lignin content of the studied fractions showed only minor differences, only the total lignin content in birch extract and corresponding filtrates could not be determined precisely. Due to the fact that Klason lignin methods are sample demanding and time consuming we suggest that, by using more rapid lignin determination methods, satisfactory information is gained for determining the lignin mass balances of the extracts obtained by pressurised hot-water extraction and ultrafiltration.
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V. ACKNOWLEDGEMENT This research was part of the activities at the Åbo Akademi Process Chemistry Centre. This study was financed by Forestcluster Ltd. and the Finnish Funding Agency for Technology and Innovation (Tekes) within the framework of the Future Biorefinery (FuBio) Joint Research 2 programme. Finnish Forest Research Institute is acknowledged for performing the pressurised hot water extractions and Lappeenranta University of Technology, Laboratory of Membrane Technology and Technical Polymer Chemistry for performing the membrane filtrations. A gratitude is also expressed to Ms. Aura Silvaani is for assisting with the lignin determinations. VI. REFERENCES Chi, C., Zhang, Z., Chang, H. and Jameel, H. Determination of Furfural and Hydroxymethylfurfural Formed From Biomass Under Acidic Conditions, J. Wood Chem. Technol. 2009, 29(4), 265–276. Fukayama, M.Y., Tan, H., Wheeler, W.B. and Wei, C.I. Reactions of Aqueous Chlorine and Chlorine Dioxide with Model Food Compounds, Environ. Health Perspect. 1986, 69(Nov), 267– 274. Goldschmid, O. Ultraviolet spectra in “Lignins - Occurrence, formation, structure and reactions”, edited by Sarkanen, K.V. and Ludwig, C.H. Wiley-Interscience, New York, pp. 241–266 (1971). Gullichsen, J. Fiber line operations in “Papermaking Science and Technology, Book 6A, Chemical Pulping”, edited by Gullichsen, J. and Fogelholm, C.-J. Fapet Oy, Helsinki, pp. A17–A243 (1999). Hatfield, R.D., Grabber, J., Ralph, J. and Brei, K. Using the Acetyl Bromide Assay To Determine Lignin Concentrations in Herbaceous Plants:ௗ Some Cautionary Notes, J. Agric. Food Chem. 1999, 47(2), 628–632. Kyrklund, B. and Strandell,G. Applicability of the chlorine number for evaluation of the lignin content in pulp, Pap. Puu 1969, 51(4a), 299–305. Leppänen, K., Spetz, P., Pranovich, A., Hartonen, K., Kitunen, V. and Ilvesniemi, H. Pressurized hot water extraction of Norway spruce hemicelluloses using a flow-through system, Wood Sci. Technol. 2011, 45(2), 223–236. Li, S., Lundquist, K. and Westermark U. Cleavage of arylglycerol ȕ-aryl ethers under neutral and acid conditions, Nord. Pulp Pap. Res. J. 2000, 15(4), 292–299. Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D. and Crocker, D. Determination of Structural Carbohydrates and Lignin in Biomass - Laboratory Analytical Procedure (LAP), Technical Report NREL/TP-510-42618, U.S. Department of Energy (2011). Yokoyama, T., Kadla, J.F. and Chang, H.-M. Microanalytical method for the characterization of fiber components and morphology of woody plants, J. Agric. Food. Chem. 2002, 50(5), 1040– 1044.
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