Hemicellulose Extraction of Mixed Southern Hardwood with Water at

Aug 16, 2008 - Hemicellulose Extraction of Mixed Southern Hardwood with Water at 150 °C: Effect of Time. M. Sefik Tunc* and Adriaan R. P. van Heining...
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Ind. Eng. Chem. Res. 2008, 47, 7031–7037

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GENERAL RESEARCH Hemicellulose Extraction of Mixed Southern Hardwood with Water at 150 °C: Effect of Time M. Sefik Tunc* and Adriaan R. P. van Heiningen Department of Chemical and Biological Engineering, UniVersity of Maine, Orono, Maine 04469

Hemicelluloses derived from biomass are presently underutilized. In order to develop more profitable biorefinery processes, the mechanism responsible for hemicellulose removal by pretreatments has to be further explored. The hydrothermal dissolution profile of the wood components cellulose, hemicelluloses, and lignin of a hardwood mixture during autohydrolysis in a modified Dionex ASE-100 is described. Well-closed material balances were obtained for lignin-free yield, xylan, and glucomannan when comparing the solid and liquid phases. Xylo-oligomers were the predominant component in the extract. Xylan initially dissolved as oligosaccharides and then slowly depolymerized into monomeric xylose. The residual xylan in wood was only slightly deacetylated. A smaller amount of glucomannan was removed as oligosaccharides. Arabinose and galactose were completely removed from wood as monomers at the end of the extraction process. Initially all acetyl groups were removed while still bound to oligosaccharides. Then, acetic acid was released by deacetylation of the dissolved oligosaccharides. Introduction Plant biomass is the only sustainable source for liquid fuels and renewable chemicals.1 It has been estimated that the cost of lignocellulosic biomass in the U.S. varies from $12 to $24 per barrel of oil equivalent,2 which is significantly below the cost of crude oil of over $100/bbl reached in early 2008. However, we do not as yet possess the technology to economically convert plant biomass to liquid fuels and bulk chemicals in a greenfield biorefinery. Woody biomass, the feedstock for the forest products industry, holds great potential for production of biofuels and renewable chemicals in a socalled integrated forest products biorefinery (IFBR).3 Challenges exist with the collection and processing of cellulosic biomass to a greenfield biorefinery. However, many of these challenges have been solved by the forest products industry, given that this industry already collects and processes woody biomass at centralized facilities. Other advantages of the IFBR are the availability of wastewater treatment and boiler systems and the presence of environmental permits and experienced labor. Furthermore, since a “state of the art” Kraft pulp mill will have an excess energy production of about 30%,4 energy integration with energy consuming biofuels production will lead to important synergies. Conversely, the added revenue will increase the profitability of forest products companies that are faced with increased wood cost and decreasing forest products prices. Hemicelluloses are the second major sugar component in lignocellulosic materials. However, their complex character and linkage to lignin presents a major hurdle. Presently, the hemicelluloses are underutilized in the pulp and paper industry because they are mostly degraded during Kraft pulping, the major chemical pulping process. Since the value obtained from hemicelluloses in the Kraft process is mostly limited to recovery of the low heating value by combustion, a more economical use would be to extract them by suitable pretreatment methods, followed by conversion to higher value-added products. * To whom correspondence should be addressed. Tel.: 1(207)5812210. Fax: 1(207) 581-2323. E-mail: [email protected].

Interest in the isolation of hemicelluloses from biomass has greatly increased in recent years.5–10 Hemicelluloses are linked to cellulose and lignin via hydrogen and covalent bonds respectively.10 Highly branched hemicelluloses are easily watersoluble. However, more uniform hemicelluloses (with a low degree of side-chain substitution) are tightly bound to cellulose and thus are less water-soluble.12 Removal of hemicelluloses in a pure form from wood involves hydrolysis of ester and ether bonds which link the hemicelluloses to lignin.13 There are several pretreatment technologies suggested for removal of hemicelluloses from biomass such as steam explosion, organic solvents, alkali, dilute acid, enzyme treatment, and water extraction (autohydrolysis). Autohydrolysis is of interest because water is the only reagent making it an environmentally friendly and inexpensive process compared to dilute mineral acid prehydrolysis.14 During the autohydrolysis process at elevated temperatures, acetic acid, which is released from the acetylated polysaccharides in the wood, lowers the pH of the extract to a range of 3-4.15 Depending on the intensity of acid hydrolysis, sugar dehydration reactions also take place16 leading to hydroxyfurfural (5-(hydroxymethyl)-2-furaldehyde or HMF) and furfural from hexose and pentose mono sugars, respectively. Low molar-mass cellulose, lignin, and uronic acids are also extracted from hardwoods by autohydrolysis.17–19 Garrote et al.20 proposed a kinetic model which takes into account the removal of high and low molecular weight oligomers from pentoses (xylan and arabinan) in Barley husks. In order to correlate experimental data, several kinetic models have been proposed for xylan removal.20–23 Kinetic models which are based on the existence of two xylans reacting at different reaction rates are widely employed.23–28 It was reported that about 60-70% of xylan is hydrolyzed according to first order fast reaction kinetics, while the remaining xylan is removed by a slower first order reaction.29,30 The existence of two different reaction rates (fast and slow) for xylan removal is explained by accessibility rather than polydispersity of the removed xylan.27 Thus, the slow reaction rate may be due to a

10.1021/ie8007105 CCC: $40.75  2008 American Chemical Society Published on Web 08/16/2008

7032 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 Table 1. Chemical Composition of Original Extractives-Free Southern Hardwood Mixture (percent on Wood) chemical component arabinose galactose glucose xylose mannose a

amount (% by wt)

chemical component

amount (% by wt)

0.52 ( 0.01 1.00 ( 0.01 43.66 ( 0.61 15.48 ( 0.03 2.18 ( 0.05

Klason lignin acid sol. lignin ash AcGa UAGb

25.78 ( 0.20 3.20 ( 0.05 0.34 ( 0.07 2.76 ( 0.08 4.83 ( 0.10

Acetyl groups. b Uronic acid groups.

portion of lignocellulosic xylan which is embedded within lignin or is linked to lignin via lignin carbohydrate bonds.31 It was reported that more than one type of these bonds exist31,32 and that a fraction of these bonds are lignin-xylan ester linkages via the uronic acid side group33,34 Our objective of the forest biorefinery concept is to utilize each of the three main components of wood in the most profitable manner. Xylose-based oligosaccharides can be isolated from lignocellulosic materials, and they have potential use in pharmaceutical, food, agriculture, and papermaking industries.35–42 However the cellulose fraction must be retained in the form of mostly undegraded fibers and at a high yield since the fibers represent the highest product value.3 Therefore, a relatively low temperature of 150 °C was selected to study the kinetics of xylan removal. Also, at higher prehydrolysis temperatures resinous deposits of lignin are formed43 and these deposits prevent commercial implementation of the water prehydrolysis technology. In order to investigate the possibility to obtain xylan in high yield from hardwood, a series of hydrothermal pretreatments of a mixture of southern hardwoods was carried out at 150 °C from 15 to 500 min in an accelerated solid extraction system, namely in a modified Dionex ASE-100 extractor. This type of extractor is commercially available and well suited to rapid laboratory extraction and pretreatment experiments under elevated temperatures and pressures. Usually, neutral solvent extractions are performed with ASE. However in the present work, water is used in order to examine whether the ASE is suited for rapid prescreening experiments of lignocellulosic autohydrolysis before larger scale tests. Cellulose, hemicelluloses, lignin, acetyl groups, and uronic acid groups remaining in the wood were determined from analysis of the solids, while the content of oligosaccharides and monosaccharides, lignin, acetic acid, furfural, and HMF were determined in the extract. Materials and Methods Materials. A mixture of southern hardwood chips (SHM) consisting of sweet and black gum (35%), oak (35%), maple (15%), poplar and sycamore (12%), and southern magnolia (3%) were used. The SHM were air-dried (unsuitable chips were removed), ground in a Wiley mill, and the fraction passing 2 mm holes was stored in double plastic bags for later experimental use. The ground wood particles were extracted with dichloromethane to eliminate the interference of wood extractives on the analysis of carbohydrate and lignin in the extracted wood. Only extractives-free wood particles were used for the extraction experiments. The chemical composition of extractives-free SHM is summarized in Table 1. The amounts of cellulose and hemicelluloses of extractivesfree SHM were 40.94 ( 0.38 and 29.72 ( 0.38%, respectively. The extractives content of the mixed southern hardwood was determined by both the standard Soxhlet method and ASE-100 using CH2Cl2 as solvent. The results from the two techniques

were comparable (2.00 and 2.25%, respectively). However, the ASE-100 method was considerably more rapid (15 min) compared to that of the Soxhlet method (6 h). Experimental Procedures. The experimental analysis scheme for the hemicellulose extraction of the SHM is summarized in Figure 1. The solid phase was analyzed for cellulose, hemicellulose, lignin, uronic acid groups, acetyl groups, and ash content, while the liquid phase was analyzed for cellulose, hemicellulose, lignin, furfural, and acetic acid concentration. Details about the analysis and calculation procedure are given in the following sections. Analysis of Wood Particles. The moisture content of the milled wood particles was determined by drying a representative sample at 100 ( 5 °C in a convection oven overnight. The ash content was determined according to TAPPI standard method T211 om-85. The acid insoluble lignin content, Klason lignin, was determined according to a method by Effland,44 while the acid soluble lignin content is determined by Tappi method 250. The uronic anhydride content was determined using the chromophoric group analysis method developed by Scott.45 The mono sugar content was determined by high performance anion exchange chromatography with pulse amperometric detection (HPAEC-PAD) of the hydrolysate which was produced by a two-step hydrolysis with 72 and 4% sulfuric acid.46 Acetic acid in the hydrolysate was determined by HPLC using a refractive index detector and BIO-RAD Aminex HPX-87H column. The mobile phase used was 5 mM H2SO4 with a flow rate of 0.6 mL/min and the oven temperature was 60 °C. Autohydrolysis of Wood. Hemicelluloses were extracted with water (autohydrolysis) using different extraction times at 150 °C in a modified Dionex ASE-100 extractor. The time at temperature is corrected for heatup time in the Dionex program of the modified ASE-100. The liquor to wood ratio (L/W) was approximately 3.7:1. The amount of water required to displace the extract in the cell after autohydrolysis in the ASE-100 was found to be 1.5 cell volumes. The use of the ASE-100 equipment simplifies the extraction process due to the elevated pressures used. Penetration of the voids inside and outside wood particles by the extraction liquor is completed in a short time at the high pressure (∼15 MPa) used. The Dionex ASE-100 was modified to stabilize the pressure at the set point by connecting a pressurized expansion tank during extraction. This modification allows the ASE-100 to operate at constant volume during extraction regardless of thermal expansion or gases generated during operation. The modified ASE-100 is schematically represented in Figure 2. A heat exchanger after the extraction cell was also added to condense any volatile products such as methanol, which may form during the extraction. Analysis of Solid and Liquid Phases after Autohydrolysis. The solid residue after washing was air-dried and subjected to the same analysis as applied to the original wood. The extraction yield was calculated from the difference in oven dry wood weight before and after autohydrolysis. The pH of the displaced liquid phase after autohydrolysis was recorded. The extraction liquid was centrifuged, the supernatant recovered, and the dried solid residue weighed. The supernatant, called hemicellulose extract, was analyzed for monosugar content by direct injection in the HPAEC-PAD and after one hour hydrolysis with 4% H2SO4 at 121 °C in an autoclave. The yield of oligomers was calculated from the increase in monosugar anhydride content due to acid hydrolysis (see Figure 1). The solid content of the hemicellulose extract was determined, and the extraction yield based on original wood was calculated.

Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 7033

Figure 1. Experimental design for pre-extraction of hemicellulose.

Figure 2. Modified Dionex ASE-100.

Furfural and acetic acid in the hemicellulose extract were determined using the HPLC system. The lignin content of the hemicellulose extract was determined from the UV-Vis absorbance at 280 nm using an extinction coefficient of 20.3 (L g-1 cm-1) for hardwoods.47 Pectins were not determined. However, this results in a relatively small mass balance error since the content of pectins in hardwoods is less than 1%.16 The amounts of xylan and glucomannan in the extracted wood were calculated using the calculation procedure described in

the Appendix. The same formulas were also used for the original wood and the solids in the extracts. Results and Discussion Analysis of the Extracted Solids. The amount of cellulose and hemicellulose remaining in wood during the autohydrolysis of SHM at 150 °C is shown in Figure 3. It is apparent from Figure 3 that only a very small amount of cellulose was removed

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Figure 3. Cellulose and hemicellulose retentions of southern hardwood mixture with water versus time. Table 2. Molar Ratios of Acetyl and Uronic Acid Groups to 10 mol of Xylose in Wood per 10 xylose (remaining in wood) time (min)

acetyl groups

uronic acid groups

wood 15 30 60 100 200 300 500

5.1 4.7 4.7 4.7 4.5 4.5 4.0 4.2

2.2 2.1 2.1 1.8 1.7 1.5 1.5 1.0

during extraction compared to hemicellulose. Similar results were reported by others.12,19 This can be explained by the fact that the glycosidic linkages in xylan are degraded 1500 times faster than that of cellulose.48 The initial fast and later slow dissolution rate of the hemicelluloses may be explained based on the molecular weight and chemical structure of hemicelluloses as well as by the association of hemicelluloses with lignin. Low-molar-mass hemicelluloses dissolve easier than high-molarmass hemicelluloses,49,50 and highly branched hemicelluloses are more soluble than the less branched hemicelluloses that remain tightly linked to celluloses.13 Therefore, the fast initial dissolution rate of hemicelluloses was probably due to dissolution of highly branched low molecular weight hemicelluloses, while the later slower dissolution rate of hemicelluloses was caused by dissolution of less branched higher molecular weight hemicelluloses in wood. Covalent bonds between lignin and hemicellulose13 may also explain the relatively slow dissolution rate during the final extraction phase. The molar ratios of acetyl groups and uronic acid groups to 10 mol of xylose remaining in wood are shown in Table 2. The acetyl to xylose ratio decreases only slightly during autohydrolysis. This indicates that xylan in wood remains acetylated. The ratio of uronic acid groups to 10 xylose units in wood decreases by more than a factor 2 after 500 min autohydrolysis. This is surprising because it is known that the glycosidic bond between methylglucoronic acid and xylan is more stable than the glycosidic bond between the xylose monomers in the xylan polymer.51 Hence, it may be that the uronic acid groups were still attached to extracted oligomeric xylosaccharides, while a more linear xylan was retained in wood. Composition of Hemicellulose Extract. The content of monomeric and oligomeric anhydromono sugars in the hemicellulose extract based on the original wood weight is summarized in Table 3. The sugar loss from the solid phase is also

summarized in Table 4. The amount of each sugar in Table 4 was expected to be similar to sum of the corresponding monomeric and oligomeric sugars in Table 3. It can be seen in Figure 4 that this was true except for glucose. A comparison of Tables 3 and 4 also shows that the amount of arabinose in the liquid phase is much higher than that removed from the solid phase. Similarly, the arabinose content of the original SHM of 0.5% (see Table 1) was much smaller than the maximum amount of arabinose in the extract of 1.2%. This could be interpreted that the amount of arabinan measured in the liquid phase was too high. The likely explanation is that arabinan degraded during acid hydrolysis of the solids during the monosugar analysis,14 while the monomeric arabinose in the hemicellulose extract which was directly measured without acid hydrolysis do not. Thus, the arabinose content measured in the extracted solids and original SHM is underestimated since a portion of arabinose is degraded to furfural during the acid hydrolysis analysis procedure. This also explains the negative values of oligomeric arabinose in Table 3 which are calculated after secondary hydrolysis of the extract. Comparison of the data in Tables 3 and 4 also shows that all arabinose (1.2%) and galactose (1.0%) in SHM are in monomeric form in the extract after 500 min. It is clear from Table 3 that xylooligosaccharides are the most abundant component in the extract. Finally, the results in Table 3 show that the majority of dissolved glucose is in polymeric form and a minor amount is in monomeric glucose. The polymeric glucose most likely represented low-molar-mass cellulose. It is clear from Table 3 that furfural formation starts at around 100 min and then increases with time. Since the amount of extracted oligomeric xylose was much higher than that of oligomeric mannose (see Table 3), it can be assumed that all acetyl groups were associated with xylooligosaccharides. On the basis of this assumption, the molar ratio of acetyl groups bound to the dissolved xylooligosaccharide may be calculated, and the results are summarized in Table 5. It is apparent from the table that initially the extracted oligosaccharides were not deacetylated since the molar ratio of acetyl groups to 10 moles of xylose in the original wood was 5.1. However, significant deacetylation took place later in the autohydrolysis process. The final pH of the hemicellulose extract at the end of autohydrolysis is plotted against extraction time in Figure 5. It is clear that the acidity of extract increases with time. Comparison of Solid and Extract Analysis. The total extraction yield and extracted lignin and lignin-free yields determined from analysis of both the extracted wood and the extract are shown in Table 6. It can be seen that the total extraction yield determined from the solid phase was higher than that from the liquid phase. However, the lignin-free yields determined from both phases were approximately the same. This suggests that the difference in total yields obtained from the solid and liquid phases was due to the lignin analysis. The amount of lignin removed determined from the wood analysis was higher than that found in the liquid phase. The explanation may be cleavage of lignin-carbohydrate (LC) bonds13 in the extract leading to precipitation of lignin43 which was discarded after centrifugation of the extract (see Figure 1). This also explains why the total extraction yields determined from the liquid phase were lower than that determined from the solid phase (see Table 6). The data in Table 6 also shows that the lignin extraction was significant, with the highest lignin removal of approximately 5% (which correspond to 15% of the original lignin) after around 300 min extraction.

Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 7035 Table 3. Oligomeric/Monomeric Sugars and Furfural of Hemicellulose Extract from Liquid Phase monomeric anhydro-monosugars (g/100 g od wood) time (min)

arabinose 0.069 ( 0.004 0.140 ( 0.024 0.399 ( 0.011 0.690 ( 0.020 0.991 ( 0.020 1.085 ( 0.019 1.222 ( 0.022

15 30 60 100 200 300 500

g/100 g od wood

galactose

glucose

xylose

mannose

furfural

0.029 ( 0.003 0.044 ( 0.008 0.123 ( 0.004 0.234 ( 0.010 0.463 ( 0.033 0.643 ( 0.011 0.919 ( 0.019

0.021 ( 0.002 0.179 ( 0.007 0.074 ( 0.004 0.225 ( 0.008 0.272 ( 0.012 0.325 ( 0.001 0.415 ( 0.006

0.013 ( 0.002 0.020 ( 0.003 0.071 ( 0.001 0.196 ( 0.010 0.868 ( 0.015 1.656 ( 0.020 3.645 ( 0.033

0.007 ( 0.002 0.008 ( 0.001 0.016 ( 0.001 0.025 ( 0.002 0.049 ( 0.001 0.086 ( 0.001 0.185 ( 0.004

0.000 0.000 0.000 0.015 ( 0.001 0.025 ( 0.002 0.042 ( 0.009 0.071 ( 0.014

oligomeric anhydro-monosugars (g/100 g od wood) 15 30 60 100 200 300 500

0.000 0.000 -0.152 -0.361 -0.555 -0.617 -0.795

0.000 0.049 ( 0.005 0.056 ( 0.000 0.088 ( 0.004 0.158 ( 0.011 0.127 ( 0.007 0.000

0.396 ( 0.009 0.400 ( 0.036 0.742 ( 0.020 0.958 ( 0.037 1.186 ( 0.088 1.287 ( 0.073 1.237 ( 0.064

0.277 ( 0.018 0.337 ( 0.058 1.040 ( 0.053 2.092 ( 0.098 5.652 ( 0.265 7.227 ( 0.253 7.729 ( 0.185

0.068 ( 0.005 0.087 ( 0.008 0.135 ( 0.003 0.243 ( 0.009 0.458 ( 0.025 0.575 ( 0.025 0.630 ( 0.039

Table 4. Composition of Hemicellulose Extract from Solid anhydro-monosugars (g/100 g o.d wood) time (min)

arabinose

galactose

glucose

xylose

mannose

15 30 60 100 200 300 500

0.102 ( 0.003 0.136 ( 0.005 0.300 ( 0.019 0.389 ( 0.042 0.465 ( 0.067 0.480 ( 0.115 0.488 ( 0.151

0.157 ( 0.005 0.178 ( 0.008 0.296 ( 0.004 0.423 ( 0.013 0.662 ( 0.032 0.795 ( 0.028 0.902 ( 0.076

0.000 0.000 0.218 ( 0.002 0.862 ( 0.004 0.987 ( 0.003 1.063 ( 0.000 1.267 ( 0.002

0.255 ( 0.002 0.465 ( 0.004 1.030 ( 0.016 2.190 ( 0.020 6.127 ( 0.023 8.618 ( 0.042 10.988 ( 0.036

0.000 0.000 0.108 ( 0.003 0.177 ( 0.003 0.372 ( 0.024 0.604 ( 0.010 0.706 ( 0.049

The total lignin-free, xylan, glucomannan, and cellulose extraction yields determined from analysis of both the solid and liquid phases are shown in Figure 6. It can be seen that the lignin-free, xylan, glucomannan, and cellulose yields calculated from the solid and liquid phases generally agree. This confirms the accuracy of the analysis and calculation procedures. It should be noted that the cellulose dissolution estimated from the liquid phase was slightly higher than that from the solid phase. A possible explanation may be that the ratio of mannose to glucose used in eq 3 (see the Appendix) to calculate the amount of

polymeric cellulose in the liquid was different from that of the solid phase. In this study the same ratio (b ) 1.6 in eq 3) was assumed for the extract and solid phases. The total lignin-free yield increased sharply with increasing time up to 300 min, and then the increase slowed down. The same behavior can be seen

Figure 5. Dissolution of major hemicelluloses of southern hardwood mixture with water versus pH. Table 6. Total Extraction, Lignin, and Lignin-Free Yields from Solid and Liquid Phases solid phase, g/100 g od wood

liquid phase, g/100 g od wood

time (min)

total ext yield

lignin removal

ligninfree yield

total ext yield

lignin removal

ligninfree yield

15 30 60 100 200 300 500

3.3 4.2 6.3 9.0 16.8 22.0 26.4

2.1 2.3 2.7 2.3 3.6 4.7 5.1

1.3 1.9 3.6 6.7 13.2 17.3 21.2

3.0 3.6 5.6 8.0 15.5 20.0 23.3

1.0 1.1 1.4 1.7 2.5 3.0 3.6

2.0 2.5 4.2 6.4 13.0 16.9 19.7

Figure 4. Monosugar extraction yields of southern hardwood mixture determined from liquid and solid phases. Table 5. Molar Ratio of Acetyl Groups to 10 mol of Xylose (As Xylooligosaccharides) in Hemicellulose Extract time (min)

acetyl groups/10 xylose

60 100 200 300 500

6 3 3 3 3

7036 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008

b ) 1.6 for hardwood53 Hemicellulose content of extracted solid 132 162 ( 150 ) + (Gal + Glu + Man)( 180 ) + Ac + 190 132 UA[( + 0.6( - C (4) 176 ) 176 )]

H(%) ) (Ara + Xyl)

Nomenclature

Figure 6. Total lignin-free, xylan, and glucomannan extraction yields versus time.

for xylan dissolution. However, the amount of dissolved glucomannan reached a plateau at approximately 200 min. It is also clear from Figure 6 that the cellulose dissolution reached approximately 1% after about 100 min, and no more cellulose was extracted beyond 100 min. Conclusions This study shows that xylan dissolves as oligosaccharides during autohydrolysis of hardwood at 150 °C, and then it depolymerizes slowly into monomeric xylose at longer extraction times. Xylo-oligosaccharides are the most abundant component in the reaction medium. No significant amount of furfural was generated under the present extraction conditions. The percentage of uronic acid groups in xylan remaining in wood, decreases significantly with increasing extraction time. Arabinose and galactose are completely removed from wood as monomers at the end of the autohydrolysis process. During the first 30 min, all acetyl groups were removed while still bound to oligosaccharides. Then acetic acid was released by deacetylation of the dissolved oligosaccharides. The results show that the modified ASE is a useful instrument to study autohydrolysis of lignocellulosic materials and that xylan may be efficiently extracted from hardwood (SHM) without significant removal of cellulose. Acknowledgment This research has been made possible by financial support of the US Department of Energy and International Paper Corporation under contract FC36-04GO14306 and the National Science Foundation under Grant No. 0554545. Appendix: Calculation Procedure Xylan content of extracted solid 132 132 ( 150 ) + 0.6UA( 176 )

Xn(%) ) Xyl

(1)

For an exaplanation of the value of 0.6, see ref 52. Glucomannan content of extracted solid 162 162 ( 180 )[1 + b1 ] + Gal( 180 )

Gm(%) ) Man

(2)

Cellulose content of extracted solid 162 162 ) - Man ( 180 b ( 180 )

C(%) ) Glu

(3)

Ac ) g acetyl groups per 100 g od wood Ara ) g arabinose per 100 g od wood b ) empirical constant (see eqs 2 and 3) C ) cellulose content of extracted solid (%) Gal ) g galactose per 100 g od wood Gm ) glucomannan content of extracted solid (%) Glu ) g glucose per 100 g o.d wood H ) hemicellulose content of extracted solid (%) Man ) g mannose per 100 g od wood UA ) g uronic anhydride per 100 g od. wood Xn ) xylan content of extracted wood (%) Xyl ) g xylose per 100 g od wood

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ReceiVed for reView November 13, 2007 Accepted June 13, 2008 IE8007105