ARTICLE pubs.acs.org/IECR
Fractionating Pentosans and Hexosans in Hybrid Poplar Chunhui Zhang†,‡ and Troy Runge*,† † ‡
Department of Biological System Engineering, University of Wisconsin;Madison, Madison, Wisconsin 53706, United States State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, China ABSTRACT: In this paper, a study on the fractionation of pentosans and hexosans from poplar chips was performed using a circulation reactor. The study’s methodology created kinetic models of both pentosan and hexosan hydrolysis/degradation to predict the fractionation yield of both substances. The conditions were varied to include a temperature range of 140170 °C, a sulfuric acid concentration range of 0.10.9 wt %, and a constant liquor-to-wood ratio of 6:1. The yields of both substances were favored at high acid concentration and temperature, with pentosan being considerably more reactive. Under optimal conditions, 91% of pentosan (as monomers, oligmers, and degradation products) could be recovered, with more than 93% of hexosan retained in the solids. This study demonstrates that pentosans and hexosans may be fractionated from this biomass using acid hydrolysis, which will enable further processing of relatively homogeneous saccharide streams to create fuels or chemicals.
1. INTRODUCTION A dramatic increase in crude oil prices in the past decade has prompted renewed interest for renewable transportation fuels. Apart from economics, it is desired to curtail traditional petroleumbased fossil fuel usage for ecological reasons, including habitat destruction from fuel extraction and global warming caused by the release of carbon dioxide and other greenhouse gases. While corn ethanol was initially viewed as an ideal renewable fuel alternative, concerns around energy efficiency, land use, and food prices have increased research on biofuels from lignocellulosic material. Lignocellulosic biomass offers the potential to provide sustainable sugar streams from a variety of materials, including agricultural and forest residuals and high-yielding bioenergy crops such as switchgrass, Miscanthus, and hybrid poplar.1 Production of biobased fuels and material can be accomplished through biological, chemical, and thermochemical pathways.2 Biological routes utilize enzymes from microorganisms to convert sugars through fermentation to ethanol and, less commonly, to propanol and butanol. These agents carry out the fermentation of saccharides from sugar, starch, or lignocelluloses biomass. If lignocellulosic biomass is utilized, the process must start with the hydrolysis of polysaccharide components to fermentable monomeric, reducing sugars.3 A hydrolysis step is required and may consist of multiple steps to hydrolyze the biomass to break the recalcitrant material’s polysaccharides to its constituent sugars, which is usually catalyzed by acids or enzymes.4 Although pentose and hexose can both be fermented, most fermenting organisms are adapted for six-carbon hexoses.5 Furthermore, furfural and acetic acid formed as hemicellulose degradation products can inhibit fermentation organisms, suggesting the need to fractionate the material prior to this step.6 Chemical production of biobased fuels can also benefit from fractionating saccharides into hexoses and pentoses. Separate liquid fuel processes have been reported, with hexoses going through a levulinic acid pathway7 and pentoses going through a furfural pathway.8,9 Catalytic processing on a heterogeneous feedstock consisting both pentoses and hexoses can be performed, but it introduces an additional complexity for catalyst r 2011 American Chemical Society
fouling, product separation, and intermediate reactions, thus lowering yields. Fractionation of lignocelluloses has traditionally been used to separate the polysaccharides from lignin with a pulping process.10 In this process the biomass was treated to strong caustic or acidic conditions with additional nucleophiles to degrade and solubilize the lignin, allowing it to be removed in a washing step. A fraction of the saccharides was typically degraded, with the resulting product typically being predominantly cellulose in nature.11,12 Research into pretreating biomass prior to biofuel production has investigated enzymatic hydrolysis, acid and alkaline hydrolysis, autohydrolyis, solvent extraction, and combinations of these methods. Dilute acid hydrolysis appears to be economically efficient at hydrolyzing the hemicellulose fraction in a soluble stream, while minimizing the amount of cellulose that is solublized.1315 The dilute acid hydrolysis pretreatment is typically done to make both the pentose and hexose sugars more accessible for saccharification and fermentation, not fractionation, as the hydrolyzed streams are not separated from the biomass. This research will take an alternative track to optimize an acid hydrolysis step to enable the biomass to be separated into two separate streams, for discrete processing in separate processes. Choosing a mild acid hydrolysis as the fractionation step, optimal extraction conditions must be determined. One approach to determining these conditions is to create kinetic models of both pentosan and hexosan hydrolysis and maximize the ratio. The model must consider two reactions including (1) the depolymerization of both polysaccharides to smaller extractable oligosaccharides or monosaccharides and (2) the formation of subsequent monosaccharide degradation products such as furfural, 5-hydroxymethylfurfural, and humins.1618 Many models13,15,17,1923 have been developed based on the hydrolysis kinetics of hemicelluloses from biomass, but a suitable model has not been found Received: June 2, 2011 Accepted: November 17, 2011 Revised: November 9, 2011 Published: November 17, 2011 133
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Figure 2. Schematic diagram of reactor.
hydroxide (VWR). Deionized (DI) water was used in all the experiments. 2.2. Raw Material Preparation and Characterization. The wood logs were manually debarked, chipped, and screened, airdried, and cold stored at 4 °C in plastic bags until they were used. A 500.0 g sample of air-dried chips was milled (pass 40 mesh) using a Wiley mill for characterization. Benzeneethanol (2:1, v/v) extraction was utilized to obtain extractive free wood, which was then milled to a fine meal using a 40 mesh screen size. The ash content was determined gravimetrically following TAPPI standard T211. Klason lignin was determined according to a modified TAPPI T222 standard by subjecting the meal to a 72 wt % H2SO4 solution for 2 h and further autoclaving it at 121 °C for 1 h, in a 3.0 wt % H2SO4 solution. The hydrolysate from this procedure was retained for acid-soluble lignin measurements. Acidsoluble lignin was determined by absorbance at 205 nm.26 Sample preparation for carbohydrate analysis is based on the method described in TAPPI standard T-249. These samples were neutralized with 0.6 mol/L NaOH, diluted with deionized water, and filtered through 0.22 μm nylon filters. The samples were analyzed using a Dionex HPLC system (ICS-3000) equipped with an integrated amperometric detector and Carbopac PA20 guard and analytical columns at 20 °C.25,27 Eluent was provided at a rate of 0.6 mL/min, according to the following gradient: 020 min, 100% water; 20.130 min, 30% water and 70% 0.1 mol/L NaOH; 30.135 min, 100% water, with 0.5 mol/L NaOH used as a postcolumn eluent. Simple organic acids and furans generated, such as acetic acid, formic acid, furfural, levulinic acid, and 5-hydroxylmethylfurural (HMF), were diluted with deionized water, filtrated through 0.22 μm nylon filters, and analyzed using the Dionex ICS-3000 equipped with a Supelcogel C-610H column at a temperature of 50 °C, the UV detector at 210 nm, and with 0.1% phosphoric acid used as an eluent at a constant rate of 0.6 mL/min. 2.3. Mild Acid Extraction and Characterization. Mild acid extraction was performed using two interconnected 10 L stainless steel reactors with heated circulation, shown schematically in Figure 2. A liquor to dry wood ratio of 6 (L/W 6) was used in all experiments. A 1000 g amount of wood chips (oven dry) was placed into the chip tank, while the acid solution was in the liquor tank, heated to set points, and pumped into the chip tank. Samples were taken at different time intervals from the sampling port during extraction for HPLC analysis. After the extraction, the chip tank was cooled and depressurized, and extracted chips were collected. The extraction conditions were a temperature range of 140170 °C, a sulfuric acid range of 0.10.9 solution wt %, and a liquor-to-wood ratio of 6:1.
Figure 1. Simplified fractionation scheme of hybrid poplar chips into valued chemicals and liquid/solid fuel.
to determine optimal conditions to fractionate pentoses and hexoses. It was hypothesized that with the use of pentosan and hexosan hydrolysis kinetic models the conditions to effectively fractionate these materials would be found. The methodology creates kinetic models for both pentosan and hexosan hydrolysis and then uses a mathematical optimization routine to create the highest theoretical ratio of pentosan/hexosan hydrolyzed. Utilizing these conditions, biomass can be fractionated through a hydrolysis reaction followed by a filtering step, leaving the hexosan in a solid state for further processing, such as another acid hydrolysis with harsher conditions to hydrolyze and extract the hexoses from the acid-insoluble lignin. The simplified scheme for converting carbohydrates of hybrid poplar chips into liquid/solid fuel and valued chemicals is illustrated in Figure 1. In this paper, only the mild acid extraction to fractionate hexose and pentoses will be discussed.
2. MATERIALS AND METHODS 2.1. Materials. The biomass chosen for this study was a hybrid poplar, Populus, which is a wood genus that has a high pentosan content, primarily xylans.24 Additionally, poplar is a promising energy crop that offers high annual yields, low inputs, and high density storage. The specific hybrid used for these experiments was NM-6 hybrid poplar, Populus maximowiczii nigra, which was harvested from northern Wisconsin at 10 years of age and seasoned for 3 months. Five monosaccharides, all of which were purchased from Acros Organics, were used as standards for carbohydrate analysis: glucose, arabinose, galactose, xylose, and mannose. Other trace sugars that may be present were not analyzed. The subsequent degradation of monosaccharides was assessed by quantifying the four primary degradation products of 2-furaldehyde (furfural, F), 5-hydroxymethyl-2-furaldehyde (HMF), formic acid (FA), and levulinic acid (LA), with standards purchased from Acros Organics. Acetic acid from EMD was used to estimate the deacetylation of hemicelluloses. Various solutions were prepared using sulfuric acid (Fisher Scientific), phosphoric acid (Fisher Scientific), and 50 wt % sodium 134
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deionized water, and filtered through 0.22 μm nylon filters and analyzed using Dionex ICS-3000 for monosaccharides and their degradation products following the procedure and conditions mentioned above. In order to determine the content of oligmers that were solubilized in the liquid samples, the supernatant was further autoclaved with 3 wt % H2SO4 solution at 121 °C for 60 min and then prepared and analyzed following the same procedure and conditions as those of the unautoclaved.
The liquid samples taken at different time intervals from the sampling port during the acid hydrolysis were centrifuged at 4000 rpm for 5 min to separate suspended materials. The supernatant was neutralized with 0.6 mol/L NaOH, diluted with Table 1. Characterization of Raw Material pentosicsa arabinose
15.4 ( 0.78b 0.3 ( 0.02
xylose
13.3 ( 0.67
furfural
1.8 ( 0.09
hexosicsa glucose
40.7 ( 2.04
galactose
0.5 ( 0.03 3 ( 0.15
Klason lignin
0.7 ( 0.04 28 ( 1.40 22.2 ( 1.11
acid-soluble lignin
3.6 ( 0.18
ash
2.2 ( 0.11
total yield of measured components
3.1. Characterization of Raw Material. Samples from the hybrid poplar chips were analyzed for saccharides, lignin, and ash to characterize the material. Additionally, since the acidolysis used in the sugar analysis creates organic acid and furans, the major hydrolysis/degradation products of formic acid (FA), acetic acid (AA), levulinic acid (LA), 5-hydroxymethylfurfural (HMF), and furfural (F) were also characterized to provide a fuller perspective on the biomass composition. The results shown in Table 1 are as expected for a poplar species. The principal hemicellulose species present in this hybrid poplar is xylan, constituting about 15.0% of the dry weight. 3.2. Effects of Temperature and Acid Concentration on Hemicellulose Hydrolysis. A total of eight batch experiments were performed in a broad range of extraction conditions (T = 140170 °C, CH2SO4 = 0.10.9%, duration 0120 min). The pentosan and hexosan released into the liquor are in different forms, including saccharide monomers and oligomer, dehydration, and degradation products. To provide an indication of the composition of these varied products, the compositions of liquor samples taken at various times under typical extraction conditions (150 °C, 0.5 wt % H2SO4, L/W 6) are plotted in Figure 3. The results indicate the majority of species present are pentosan monomers and oligomers and acetic acid from deacetylation of arabinoxylan hemicelluloses. It is worthwhile to take notice of the low concentration of dehydration products of furfural and hydroxymethylfurfural. To create a kinetics model to optimize fractionation of pentose and hexose materials, a simplifying method was utilized to separate the various hydrolysis degradation products according to whether they derived from pentosan or hexosan material. In this scheme, arabinose, xylose, and furfural were calculated as “pentosics”, while galactose, glucose, mannose, LA, and HMF were calculated as “hexosics”. In order to determine the concentrations of oligomers that were present in the liquor sample,
44.9 ( 2.26
mannose hydroxymethylfurfural lignin and ash
3. RESULTS AND DISCUSSION
88.3 ( 4.44
a
Pentosics and hexosics include both the saccharides and their degradation products formed during acid hydrolysis from the test procedure. b Value represents 95% confidence interval.
Figure 3. Mild acid hydrolysis liquor composition.
Figure 4. Effects of temperature and acid concentration on pentosan hydrolysis. 135
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Figure 5. Effects of temperature and acid concentration on hexosan hydrolysis.
The concentration of pentosics (CP) as a function of time may be represented by the differential equations (1) and (2), to account for both the formation and loss to insoluble degradation products such as humins. The concentrations of hexosics (CH) and acetic acid (CAA) as a function of time are represented by differential equations (3)(6). Due to the low concentrations of hexosics in this system, no degradation products were considered. Figure 6. Hydrolysis and degradation of pentosan and hexosan.
samples were autoclaved with 3 wt % sulfuric acid at 121 °C for 60 min to further hydrolyze the oligomers. Degradation products from this autoclave step such as furfural, HMF, and LA, were calculated as nonspecific pentose or hexose sugars. Data shown in Figures 4 and 5 depict the results from these calculations as the total concentrations of pentosics and hexosics in the liquor. The data indicate, as expected, that higher temperature and acid concentration will promote the hydrolysis and release of both pentosan and hexosan moieties. 3.3. Kinetics Models Development. As shown in Table 1, the main component of the pentosic fraction of hybrid poplar is the biopolymer xylan, a polysaccharide made from xylose units that can be hydrolyzed to its monomer by mineral acids or xylanase. Compared to crystalline cellulosic components of biomass, xylan is much easier to hydrolyze because of its relatively low degree of polymerization and heterogeneous structure. Several kinetic models have been developed on the dilute acid hydrolysis of biomass.19,23,28 However, the models primarily focus on the xylan portion of hemicellulose, and are not coupled with the cellulose hydrolysis and degradation products. In order to maximize the yield of pentosics and minimize the hydrolysis/degradation of hexosan, a simple model was developed to predict the extraction yields of both pentosics and hexosics. The extraction conditions were a temperature range of 140170 °C, a sulfuric acid range of 0.10.9 wt %, and a liquor-to-wood ratio of 6:1. Assuming that the hydrolysis reactions of pentosan and hexosan are independent, the release of pentosics and hexosics can be modeled by the two separate first-order reactions shown in Figure 6. This model assumes the hydrolysis reactions of pentosan and hexosan are independent, and side reaction products, formed during the release of acetic acid and hexosics from fibers into the liquor during extraction, are negligible.
dCpr ¼ KP, F Cpr dt
ð1Þ
dCP ¼ KP, F Cpr KP, L CP dt
ð2Þ
dChr ¼ Khr CH dt
ð3Þ
dCaar ¼ Kaar CAA dt
ð4Þ
dCH ¼ K H CH dt
ð5Þ
dCAA ¼ KAA CAA dt
ð6Þ
where Cpr is the pentosan content (g/L) retained in the biomass and Chr and Caar are the hexosan and acetyl contents (g/L) retained in the biomass. The reaction rate constant Ki is represented by modified Arrhenius equations, including the effects of temperature (T) and acid concentration (CA), as represented by eq 7. Ei mi Ki ¼ ki ½CA exp ð7Þ RT where ki is the frequency factor, mi is the reaction order in sulfuric acid, R is the ideal gas constant, and Ei is the activation energy. A total of eight batch experiments gave 80 sets of experimental data, where each set consists of the concentrations of pentose (arabinose, xylose), hexose (galactose, glucose, mannose), HMF, furfural, and levulinic acid at a certain reaction time. The kinetic parameters were estimated using a maximum likelihood approach, which is based on minimization of errors between the 136
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Table 2. Kinetic Parameters of the Saccharide Hydrolysis and Degradation R2 temp (°C)
CA (wt %)
KP,F 100 (min1)
KP,L 1000 (min1)
KP 100 (min1)
KH 1000 (min1)
KAA 100 (min1)
KP
KH
KAA
140
0.7
1.39
1.34
1.26
0.59
1.30
0.98
0.97
0.99
140
0.9
1.60
1.81
1.42
0.67
1.57
0.97
0.97
0.98
150
0.5
1.87
1.72
1.70
0.74
1.38
0.95
0.95
0.95
150
0.7
2.33
2.19
2.11
0.78
2.18
0.94
0.96
0.93
160
0.3
1.96
1.29
1.83
0.74
1.40
0.97
0.96
0.91
160
0.5
3.41
2.26
3.18
1.00
2.22
0.98
0.94
0.90
170
0.1
1.11
0.15
1.09
0.54
0.42
0.98
0.96
0.94
170
0.3
2.59
2.39
2.35
0.94
1.28
0.97
0.91
0.94
experimental data and the kinetic model. Minimization of objective function is initiated by providing initial guesses for each kinetic parameter. The best estimates were obtained using the MATLAB toolbox fminsearch, which is based on the NelderMead optimization method. The results are given in Table 2 with the kinetic rates in eqs 812. 77:731 KP, F ¼ ð1:350 108 Þ½CA 0:948 exp RT R 2 ¼ 0:88
ð8Þ
1:426
KP, L ¼ ð2:091 10 Þ½CA 8
86:973 exp RT
R 2 ¼ 0:87 KP ¼ KP, F KP, L
ð9Þ ð10Þ
47:170 KH ¼ ð6:630 102 Þ½CA 0:524 exp RT R 2 ¼ 0:95
Figure 7. KP/KH as a function of T and CA.
can allow optimal conditions to be determined to fractionate materials into primarily pentose and hexose streams through hydrolysis. Inspection of Figure 7 indicates KP/KH is enhanced by increasing the temperature and acid concentration. Based on the models above, optimization of dilute acid extraction was done using the fmincon function in the MATLAB optimization toolbox. The fmincon function finds a minimum of a constrained nonlinear multivariable function, and by default is based on the SQP (sequential quadratic programming) algorithm. Optimized values were determined to be 170 °C, 43 min, and 1.0 wt % H2SO4. Under these conditions, the yield of pentosics was 20.7 g/L, while that of hexosics was 4.9 g/L. That meant that, under those conditions, 91.0% of pentosan was released into the liquor during extraction, while about 93.0% of hexosan was retained in the chips. The weight-based yields of pentosan and hexosan were calculated by dividing the concentration of all pentosics and hexosics in the extraction liquor, which include monomers, oligomers, and degradation products, by the initial concentrations of pentoses (22.67 g/L) and hexoses (44.2 g/L) from the biomass charged to the reactor. The relationship between KAA/KP,F and CA and temperature is shown in Figure 8, which indicates that CA has larger effect on KAA/KP,F than temperature. KAA/KP,F is less than 1.0, which means the deacetylation rate is lower than the formation rate of all pentosics. Additionally, as the ratio is close to 1.0 the rates are
ð11Þ
61:690 KAA ¼ ð1:284 106 Þ½CA 1:190 exp RT R 2 ¼ 0:86
ð12Þ
The overall kinetic rates indicate that acetic acid extraction occurs the fastest, but at a similar rate to the pentose extraction. The calculated rates also clearly indicate that pentoses are extracted significantly faster than hexoses, as expected. Generally, the extraction rate of the pentoses is an order of magnitude greater than the degradation of pentoses under the conditions studied. Interestingly, the activation energy EH is lower than EP,F, which suggests that pentosan is chemically more resistant to the hydrolysis and extraction than hexosan. It is the pentosan’s high frequency factor that drives the faster rate. This observation is consistent with the model that pentoses in the branched hemicellulose portion of the biomass are more accessible to reactions than the crystalline cellulose where the majority of hexoses exist. The relationship between KP/KH as a function of acid concentration and temperature is shown in Figure 7. The relative rate of pentosan to hexosan hydrolysis/degradation is of interest, as it 137
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Figure 8. KAA/KP,F as a function of T and CA.
relatively close, which suggests that separation of acetic acid from pentose would be difficult through an acidic hydrolysis scheme, utilizing the conditions studied.
4. CONCLUSIONS To effectively process biomass into fuels and materials, fractionating pentose and hexose into separate streams is desired. To assess whether acid hydrolysis could be utilized for this fractionation, an experimental and modeling study on the fractionation of pentoses and hexoses in hybrid poplar chips was performed. The experiments used a two-vessel reactor with heated circulation with low sulfuric acid concentrations between 0.1 and 0.9 wt % and temperatures between 140 and 170 °C. Kinetic models, using modified Arrhenius equations that include the effects of temperature (T) and acid concentration (CA), were developed for the deacetylation, pentosan hydrolysis, pentose degradation, and hexose hydrolysis. A maximum likelihood approach has been applied to estimate the kinetic parameters which resulted in a good fit between experimental data and modeling results. The pentose-to-hexose hydrolysis rate was favored at high acid concentration and temperature, with the most efficient pentosics fractionation being achieved at conditions of 170 °C, 43 min, and 1.0 wt % H2SO4. Under these optimal conditions, 91 wt % of pentosan could be extracted from the wood chips, with more than 93 wt % of hexosan was retained in the solids for further processing into biofuels or biomaterials. These results indicate that it is possible to utilize acid hydrolysis under optimized conditions to effectively fractionate biomass saccharides.
ARTICLE
’ LIST OF VARIABLES F = furfural HMF = 5-hydroxymethyl-2-furaldehyde FA = formic acid LA = levulinic acid CP = concentration of pentosics CH = concentrations of hexosics CAA = concentrations of acetic acid Cppr = pentosan content (g/L) retained in the biomass Chr = hexosan content (g/L) retained in the biomass Caar = acetyl content (g/L) retained in the biomass Ki = reaction rate constant KP,F = reaction rate constant of pentosics formation from pentosans KP,L = reaction rate constant of pentosics degradation KP = net reaction rate constant of pentosics formation KAA = reaction rate constant of deacetylation or acetic acid formation KH = reaction rate constant of hexosics formation T = temperature CA = acid concentration k = frequency factor m = reaction order in sulfuric acid R = ideal gas constant E = activation energy ’ REFERENCES (1) Perlack, R. D.; et al. U.S. Department of Energy; U.S. Department of Agriculture; Oak Ridge National Laboratory. Biomass as feedstock for a bioenergy and bioproducts industry the technical feasibility of a billion-ton annual supply. http://www.ornl.gov/∼webworks/cppr/ y2001/rpt/123021.pdf. (2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. ChemInform 2006, 37 (52), DOI: 10.1002/chin.200652240. (3) Kumar, P.; Barrett, D. M.; Delwiche, M. J.; Stroeve, P. Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production. Ind. Eng. Chem. Res. 2009, 48 (8), 3713–3729. (4) Alvira, P.; Tomas-Pejo, E.; Ballesteros, M.; Negro, M. J. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: A review. Bioresour. Technol. 2010, 101 (13), 4851–4861. (5) Chandel, A. K.; Chandrasekhar, G.; Radhika, K.; Ravinder, R.; Ravindra, P. Bioconversion of pentose sugars into ethanol: A review and future directions. Biotechnol. Mol. Biol. Rev. 2011, 6 (1), 008–020. (6) Lynd, L. R.; Wyman, C. E.; Gerngross, T. U. Biocommodity Engineering. Biotechnol. Prog. 1999, 15 (5), 777–793. (7) Bond, J. Q.; Alonso, D. M.; Wang, D.; West, R. M.; Dumesic, J. A. Integrated catalytic conversion of valerolactone to liquid alkenes for transportation fuels. Science 2010, 327 (5969), 1110. (8) Weingarten, R.; Cho, J.; Conner, J. W. C.; Huber, G. W. Kinetics of furfural production by dehydration of xylose in a biphasic reactor with microwave heating. Green Chem. 2010, 12 (8), 1423–1429. (9) Zeitsch, K., The Chemistry and Technology of Furfural and Its Many By-Products; Elsevier: New York, 2000; Vol. 13. (10) Ragauskas, A. J.; Nagy, M.; Kim, D. H.; Eckert, C. A.; Hallett, J. P.; Liotta, C. L. From wood to fuels: integrating biofuels and pulp production. Ind. Biotechnol. 2006, 2 (1), 55–65. (11) Sj€ostr€om, E. Wood Chemistry: Fundamentals and Applications, 1st ed.; Academic Press: New York, 1981. (12) Al-Dajani, W. W.; Tschirner, U. W. Pre-extraction of hemicelluloses and subsequent kraft pulping. Part I: alkaline extraction. Tappi J. 2008, 7 (6), 3–8. (13) Shen, J.; Wyman, C. E. A novel mechanism and kinetic model to explain enhanced xylose yields from dilute sulfuric acid compared to
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors are grateful to the Wisconsin Bioenergy Initiative at the University of Wisconsin for financial support of this work. Additionally, the authors want to thank Dr. JunYong Zhu of the USDA Forest Products Lab for his assistance with this research. 138
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