Dilute Acid Hydrolysis of Loblolly Pine: A ... - ACS Publications

Aug 26, 2008 - A comprehensive study of the acid hydrolysis of the softwood species, Loblolly pine (Pinus taeda), using different hydrolysis condition...
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Ind. Eng. Chem. Res. 2008, 47, 7131–7140

7131

APPLIED CHEMISTRY Dilute Acid Hydrolysis of Loblolly Pine: A Comprehensive Approach Teresita Marzialetti, Mariefel B. Valenzuela Olarte, Carsten Sievers, Travis J. C. Hoskins, Pradeep K. Agrawal,* and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe NW, Atlanta, Georgia 30332-0100

A comprehensive study of the acid hydrolysis of the softwood species, Loblolly pine (Pinus taeda), using different hydrolysis conditions is presented. The effect of the type of acid, pH, reaction temperature, and reaction time on hydrolysis products such as monosaccharides (mannose, glucose, galactose, xylose, and arabinose) and the subsequent degradation products, 5-hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (furfural) is reported using a batch reactor. Trifluoroacetic acid (TFA) is found to yield the highest amount of overall soluble monosaccharides (∼70% yield from the hemicelluloses fraction) at 150 °C at pH 1.65. The mineral acids (HCl, H2SO4, HNO3, and H3PO4) gave a slightly lower yield of monosaccharides from hydrolyzed hemicellulose (∼60%). At 200 °C, cellulose is hydrolyzed by the mineral acids as evidenced by higher levels of solid dissolution and higher soluble hexose (relative to pentose) yields. Larger amounts of degradation products are also noted at higher temperatures. Furthermore, an increased amount of HMF and furfural is noted in the liquid product as compared to lower temperatures. TFA was found to be the most “gentle” acid, leading to limited monosaccharide degradation among the acids used. The presence of soluble oligosaccharides in solution after hydrolysis was confirmed by applying a secondary acid hydrolysis to the solid-free liquid hydrolysate. Good closure of mass balances was possible using total organic carbon (TOC) analysis. 1. Introduction Rapidly growing worldwide energy demand has triggered a renewed interest in producing fuels from biomass to add to worldwide energy supplies. Governments around the world are seeking the use of indigenous biomass, a readily available and renewable resource, not only for energy supply security but also as a means to support local economies. The added advantage that biomass is a CO2-neutral feedstock in the midst of mounting concerns about society’s carbon footprint further enhances the attractiveness of its conversion.1 Technologies that are under investigation for biomass utilization include thermal,2-6 thermochemical,7-9 chemical,9-12 and biological/enzymatic conversion methods.13 Among these, production and utilization of bioethanol is dominant, especially in the United States, where it accounts for about 99% of the current biofuel consumption.13-15 In 2000, the Biomass Technical Advisory Committee in the United States established a vision with regards to bioenergy and biobased products.14 Included in the vision is the substitution of 20% of petroleum transportation fuels with fuels from biomass.14 This puts increased pressure on ethanol production and consequently on corn, a staple food from which ethanol is currently mainly produced. As a means to alleviate this concern, production of ethanol and other biobased fuel from nonfood sources such as lignocellulosic biomass is being pushed forward. The various challenges toward utilization of lignocellulosics for liquid biofuels and ways to address them were the focus of a recently concluded workshop.16 Lignocellulosic biomass is a complex feedstock. It consists of three major groups of polymers: (a) cellulose, (b) hemicellulose, and (c) lignin. Cellulose and hemicellulose comprise the carbohydrate fraction of biomass while lignin consists of * To whom correspondence should be addressed. E-mail: cjones@ chbe.gatech.edu; [email protected].

poly(aromatic) moieties from phenylpropanoid building blocks.17 Minor constituents include organic extractives (mainly terpenes) and inorganic compounds (ash).17 Current biofuel technologies focus on the utilization of the carbohydrate fraction, such as in ethanol production.14 In the production of ethanol from lignocellulosic biomass, the most common method of pretreating biomass is through chemical hydrolysis.13,18-20 However, several other techniques are being developed to either extract or expose the carbohydrate fraction of lignocellulosics. These include steam explosion, ammonia percolation, hot water pretreatment, and steam pretreatment.13,18-22 Chemical hydrolysis uses agents such as acid or base to effect the solubilization of the saccharides.19,23,24 Of the types of chemical hydrolysis mentioned, acid-catalyzed reactions are predominant. The first detailed study of the kinetics of dilute acid hydrolysis of wood at elevated temperatures has been attributed to Saeman in 1945.25 However, acid and alkali hydrolysis of wood, as well as attempts to quantify the hydrolyzates, have been done as early as the 1920s.26-28 Today, commercial processes exist for the dilute acid hydrolysis of lignocellulosic biomass.29 Two classes of reactions are of importance during acid hydrolysis (as shown in Scheme 1): (1) the depolymerization of both hemicellulose and cellulose to their oligosaccharide or monosaccharide components and (2) the formation of subsequent monosaccharide degradation products that can cause inhibition in the fermentation stage.17,25,30 Acid hydrolysis of several biomass sources has been studied. These include agricultural residues31-36 (such as corn stover and switchgrass) and wood,32-34,37-39 both hardwoods and softwoods. However, a comprehensive study of Loblolly pine acid hydrolysis using different acids and under systematically varied conditions has not been reported. Loblolly pine is an important and prevalent species of softwood in the United States, especially in the southern region. It is naturally found in 14

10.1021/ie800455f CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

7132 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Scheme 1. Carbohydrate Reactions in the Presence of Acid

Table 1. Carbohydrate Analysis of Loblolly Pine Using HPAEC-PAD Technique with a CarboPac PA10 Columna monosaccharide glucose arabinose galactose xylose mannose total mmono-cellb mmono-hemic mfurfurald mHMFe

mass, mg

mtheoretical ) m/mdry-wood × 100, %

4539 (391 [hem.] + 4148 [cel.]) 140 306 796 1174 6955 4148 2807 502 3532

41.3 1.3 2.8 7.3 10.7 63.4 37.8 25.6 5.4 38.4

a Maximum values from the complete acid hydrolysis of 9202 ( 10 mg of dry wood. b Calculated using eq 2. c Calculated using eq 3. d Calculated from C5 sugars following Scheme 3. e Calculated from C6 sugars following Scheme 2.

states, from southern New Jersey to central Florida to eastern Texas.40 According to statistics from the USDA, timber volume from this species and shortleaf pine accounted for about 70% of the entire southern U.S. timber supply and more than half of the production in the eastern United States in 2005.41 In this work, dilute aqueous acid hydrolysis of Loblolly pine was performed using different acids, reaction temperatures, acid concentrations, and reaction times. The goal of the study was to generate a body of information regarding Loblolly pine hydrolysis such that the type and proportions of products evolved can be determined on the basis of hydrolysis conditions. Optimization of conditions for a specific target slate was not performed; rather, a comprehensive overview of the biomass dissolution as a function of a prescribed set of reaction conditions is presented. Products in the liquid phase, such as monosaccharides, HMF, and furfural were quantified. Analysis of the total organic carbon (TOC) of both solid and liquid products helped in achieving good material balances of the process. The results shed light on possible approaches to the solublization of woody biomass for subsequent production of alcohols from fermentation or hydrogen from reforming processes.7-9 2. Experimental Details 2.1. Materials. Five monosaccharides were used as standards for carbohydrate analysis: glucose, arabinose, galactose, xylose, and mannose, all of which were purchased from Aldrich. The

subsequent degradation of aqueous-phase dissolved monosaccharides was assessed by identifying and quantifying two degradation products: 2-furaldehyde (Furfural) and 5-hydroxymethyl-2-furaldehyde (HMF), with both compounds again purchased from Aldrich. Various acid-hydrolysis solutions were prepared using sulfuric acid (H2SO4) (Fisher Scientific), trifluoroacetic acid (TFA) (EMD), nitric acid (HNO3) (SigmaAldrich), phosphoric acid (H3PO4) (Acros Organic), and hydrochloric acid (HCl) (BDM). The water used in all experiments was deionized (DI) but remained slightly acidic, with a pH of 5.45. Loblolly pine logs were obtained from Oglethorpe, Georgia. 2.2. Raw Material Preparation and Characterization of Raw Material, Liquid and Solid Samples. The raw material used in this study was Loblolly pine. Initially, the logs were manually debarked and then milled using a Wiley mill. Subsequently, the milled material was screened and sieved. The -35 + 60 mesh fraction of sawdust was stored in zipped bags in a freezer until they were used in experiments. Note that the raw material was not treated before reaction, that is, it was not extractive-free. The characterization of raw material, Loblolly pine sawdust, was performed following various TAPPI methods for the determination of extractives and ash content, acid soluble and insoluble lignin, and carbohydrate analysis, as noted below. For the moisture content of the raw material, two samples were weighed onto separate pretared Petri dishes and were heated overnight in an oven set at 105 °C. Afterward, they were equilibrated in a desiccator for an hour before weighing. The ash content was determined by heating a sample at 525 °C for 2 h in accordance with TAPPI T211 om-93. The extractives were obtained following the procedure described in TAPPI T 264 cm-97, using dichloromethane for 24 h in a Soxhlet apparatus. Acid-soluble and acid-insoluble lignin were obtained following the method outlined in TAPPI T222 om-98. In this method, the raw material sawdust was hydrolyzed using a 72% H2SO4 solution. The insoluble residue was filtered and weighed determining the Klason lignin by mass difference. The acidsoluble fraction of lignin was then determined by a spectrophotometric method based on absorption of ultraviolet radiation. The monosaccharides dissolved in the aqueous phase such as glucose, arabinose, galactose, xylose, and mannose were measured using the procedure described in TAPPI T 249. Accordingly, after the acid hydrolysis of the raw material described above, the liquid product obtained after filtration was

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7133 Scheme 2. Acid-Catalyzed Dehydration of Glucose (Hexoses) to Levulinic Acid and Formic Acid with HMF as Detectable Intermediate

analyzed for monosaccharide content using high-performance anion-exchange chromatography with a pulsed amperometric detector (HPAEC-PAD, Dionex) and a CarboPac PA10 column. The carbohydrate analyses include an internal standard sugar, fucose, which cannot be found in woody biomass and thus serves as a good calibration agent. For the monosaccharide degradation product characterization, a Shimadzu HPLC model (LC Avp-10) was employed with 0.005 N H2SO4 as the eluent. Additionally, all liquid and solid samples were analyzed for total organic carbon (TOC) content in an effort to close the carbon mass balance and account for products other than soluble monosaccharides, HMF, and furfural. Thus, the TOC analysis of liquid samples, including the wash, was carried out using Shimadzu TOC-V analyzer with the combustion catalytic oxidation/NDIR method. TOC of solid residues was analyzed using a CM150 (TC/TIC/TOC) analyzer (UIC, Inc.). Dilute acid solutions were prepared adding the corresponding acid dropwise to DI water until the desired pH was reached. The pH was measured using a Fisher Scientific pH/mV meter (AP62). Similar to the characterization of raw material, the solid residue after each reaction was analyzed by proximate analysis using the TAPPI methods mentioned above for extractives and ash content, acid soluble and insoluble lignin determination, and carbohydrate analysis. 2.3. Aqueous Acid Hydrolysis Protocol and Reaction Conditions. All reactions were performed in a 300 mL Parr reactor equipped with a Teflon liner. The corresponding diluteacid solution or (deionized water) and the raw material were weighed in the liner of the reactor. A liquid to dry wood ratio of 9 (L/W ) 9) was used in all tests. A typical mass of Loblolly pine sawdust used was 10 g, of which ∼8% was moisture. Then, the liner with the wood and the liquid were placed into the reactor, and the reactor was closed and heated to the desired reaction temperature. After the reaction, the reactor was cooled down and slowly depressurized. The liner contents were carefully emptied into a vacuum filtration apparatus. Two aliquots of the liquid sample were collected from the filtrate for HPLC and HPAEC analyses. The solid residue was washed with 900 mL of deionized water, and two aliquots of the wash liquid were collected for HPLC and HPAEC analyses. All liquid samples were sealed in a vial and stored closed in a freezer, at -5 °C, to limit further degradation. The solid residue was carefully collected in a clean, preweighed Petri dish and put in an oven at 105 °C overnight. All dried solid residue samples were then weighed at room temperature with a precision balance, and they were stored in a refrigerator until further characterization. Various reaction variables were evaluated during this study such as reaction temperature, pH, reaction time, and acid type. The reaction temperatures used were 120, 150, and 200 °C. The pH25°C of the dilute acid solutions was kept at 0.95 (chosen because it represents ∼5% H2SO4), 1.65 (chosen because it represents ∼1% H2SO4), or 2.23. The standard reaction time (hold time at reaction temperature after heating ramp) was 60 min, but reaction times of 0, 15, 45, and 120 min were also investigated for comparison. The heating and cooling times of the reactor were not considered as part of the reaction time.

Scheme 3. Acid-Catalyzed Dehydration of Xylose (Pentoses) to Furfural

Five acids including H2SO4, HNO3, TFA, H3PO4, and HCl were tested, as well as DI water without added acid. In this work, the expression “acid hydrolysis” was used when dilute-acid solutions of H2SO4, HNO3, TFA, H3PO4, or HCl were used in the hydrolysis of the raw material, and “autohydrolysis” was used to describe when only water was utilized. 2.4. Methods of Calculation and Characterization of Reaction Products. The dried raw material and dried solid residue were weighed to determine the total mass dissolved (Yd, %) defined by Yd )

mwood,dry - mresidue × 100 mwood,dry

(1)

The soluble monosaccharides from hydrolyzed cellulose (mmono-cell, mg) and from hydrolyzed hemicellulose (mmono-hemi, mg) may be estimated as follows:42 1 mmono-cell ) glucose - × mannose 3

(2)

4 mmono-hemi ) × mannose + arabinose + galactose + xylose 3 (3) where glucose, mannose, arabinose, galactose, and xylose are the masses of the monosaccharides dissolved in the aqueous phase after hydrolysis of Loblolly pine sawdust. Therefore, the yield of monosaccharides derived from either hydrolyzed cellulose (Ymono-cell, %) or hydrolyzed hemicellulose (Ymono-hemi, %) is defined by eq 4 and eq 5: Ymono-cell ) Ymono-hemi )

mmono-cell mmono-cell-max

× 100

mmono-hemi mmono-hemi-max

× 100

(4) (5)

where mmono-cell (or mmono-hemi) represents the mass of soluble monosaccharide measured in the liquid phase from each tested reaction condition, calculated using eq 2 (or eq 3). The value mmono-cell-max (or mmono-hemi-max) is the maximum value achieved assuming complete hydrolysis of cellulose (or hemicellulose) (see Table 1). The maximum yields of furfural and HMF were calculated according to the formation mechanisms proposed in the literature43,44 where one mole of either pentose and hexose is dehydrated, losing three water molecules to become furfural and 5-hydroxymethylfurfural (HMF), respectively. The acidhydrolyzed dehydration of hexoses and pentoses are represented by Schemes 2 and 3, respectively.

7134 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Chart 1. Representation of Galactoglucomannan

Table 2. Characterization of Raw Material, Loblolly Pine Sawdust, Using Proximate Analysis raw material

m/mdry-wood × 100, %

monosaccharides extractives and ash acid insoluble lignin (Klason) acid soluble lignin

63.4 2.3 31.6 0.3

3. Results and Discussion 3.1. Characterization of Raw Material. To the best of our knowledge, previous work on acid hydrolysis of woody biomass has not explicitly reported the yields of monosaccharides produced from hemicellulose and from cellulose, that is, Ymono-hemi and Ymono-cell. This is because of the difficulty in determining the amount of the glucose derived from cellulose vs hemicellulose, as this monomer is found in both saccharide polymers. We employ here a simple relation to quantify the extent of hydrolysis of hemicellulose and cellulose. Using an empirical relation described below, we estimate the fraction of glucose derived from each polysaccharide.17 The principal hemicellulose species present in softwoods (e.g., Loblolly pine) is galactoglucomannan, represented in Chart 1, constituting about 20% of the dry weight. The galactoglucomannan consists of a linear β-1,4-linked D-glucopyranose and D-mannopyranose backbone with R-1,6-linked D-galactopyranose residues as single side chain substituents. The ratio of galactose to glucose to mannose can range from about 0.1:1:4 (low galactose content) to 1:1:3 (galactose-rich fraction) depending on the total galactose content.42 Consequently, knowing the ratio of at least two of the main components of galactoglucomannan, it is possible to estimate the fraction of soluble glucose coming from cellulose, as well as the overall soluble monosaccharide yield from hemicellulose. The carbohydrate analysis of the complete hydrolysis of our Loblolly pine sawdust, summarized in Table 1, shows the galactose to mannose ratio to be ∼0.8:3. Thus, on the basis of the assumption of the pine sample being galactose-rich, we estimate a hemicellulose content of 25.6% of the total raw material. The quantification of the total amount of monosaccharides was part of the overall characterization of our raw material. Thus, a complete characterization of raw material using the TAPPI methods described above is given in Table 2. 3.2. Effect of Acid Type. Five acids were chosen to catalyze the hydrolysis of Loblolly pine sawdust. H2SO4 has been commonly used for acid hydrolysis of biomass because it is a low cost, strong acid (pKa1 ) -3.0, pKa2 ) 1.99). Three other mineral acids were tested including HCl, H3PO4, and HNO3. HCl was chosen because it is another low cost, strong acid (first ionization; HCl pKa ) -8). However, HCl is very corrosive, requiring special alloys for processing vessels. H3PO4 is a weaker acid than H2SO4 (pKa1 ) 2.12, pKa2 ) 7.21, pKa3 ) 12.32); it is triprotic acid and is less corrosive than HCl. H3PO4 (∼0.8%, e185 °C) has been used in tandem with Ru/C by Robinson et al.45 for production of a C5/C6 polyol mixture from both model compounds and raw biomass. HNO3 is a strong acid (pKa ) -1.3) that has been commonly used for the pretreatment

Figure 1. Total biomass dissolved vs the acid used at pH25 °C ) 1.65 at 150 and 200 °C for 60 min. Also shown is autohydrolysis (water at pH25 °C ) 5.45) of Loblolly pine sawdust. Table 3. Ymono-cell, Ymono-hemi, and Yfurf+HMF during Acid Hydrolysis of Loblolly Pine Sawdust at 150 °C Using Various Dilute Acid Solutions (pH ) 1.65) and Water (pH ) 5.45) hydrolysis agent

Ymono-cell (%)

Ymono-hemi (%)

YHMF-fur (%)

water TFA H3PO4 HNO3 H2SO4 HCl

0.1 4.3 1.9 3.2 2.1 2.1

9.7 70.0 59.2 56.8 59.6 63.4

0.4 2.5 3.0 4.0 2.9 2.6

of biomass as well.46,47 This acid should be used with caution, considering its potential to decompose to gaseous nitrogen oxides under harsh conditions. For this reason, this acid was not used at 200 °C. Previously, trifluoroacetic acid (TFA) was proposed as a better acid for the hydrolysis of polysaccharides compared to H2SO4,48 as it causes lower degradation of the monosaccharides produced during the hydrolysis under certain reaction conditions. It is an organic acid (pKa ) 0.5) that can be completely removed by evaporation, in principle, unlike most mineral acids. It has been reported that the hydrolysis of cellulose, pulp, or wood requires concentrated TFA in homogeneous solution.49 Hydrolysis of Loblolly pine sawdust using DI water at pH ) 5.45 was tested for comparison with the dilute acid solutions. It is established that the hydrolysis in the presence of steam is an autocatalytic process, where the acetic acid liberated by deacetylation of hemicellulose catalyzes subsequent hydrolysis.50 The Loblolly pine sawdust was hydrolyzed at two temperatures, 150 and 200 °C, for 60 min using the five dilute acids described above (pH25 °C ) 1.65). The only exception was that HNO3 was not used at 200 °C to avoid significant acid decomposition. The autohydrolysis gave the lowest yield of dissolved mass at both temperatures (Figure 1). The autohydrolysis of both the cellulose and hemicellulose fractions at low temperature (Table 3) produced oligosaccharides that were soluble in the aqueous phase, but these were not considered in the monosaccharide quantification because they are difficult to reliably quantify. However, the presence of oligosaccharides was confirmed through the application of a second acidic hydrolysis to the aqueous solution collected from the first hydrolysis. The secondary hydrolysis was performed following the NREL procedure51 with 4% H2SO4 for 60 min at 121 °C. Analysis of the soluble products from the secondary hydrolysis showed a marked increase in the soluble monosaccharide yield for the primary autohydrolysis at 150 °C (vide infra). It should be noted that this secondary treatment also dehydrates some monosaccharides to HMF, furfural, and other products, and therefore,

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7135 Table 4. Ymono-cell, Ymono-hemi, and Yfurf+HMF during Acid Hydrolysis of Loblolly Pine Sawdust at 200°C Using Various Dilute Acid Solutions (pH ) 1.65) and Water (pH ) 5.45) hydrolysis agent

Ymono-cell (%)

Ymono-hemi (%)

Yfurf+HMF (%)

water TFA H3PO4 H2SO4 HCl

2.3 3.0 17.7 14.3 17.7

2.6 1.2 2.2 1.2 1.5

14.7 23.1 12.0 7.2 not determined

the exact amount of sugar monomers present as oligosaccharides could not be quantified. At 200 °C, the low proton concentration (pH ) 5.45) in the autohydrolysis was enough to completely dissolve the hemicelluloses, along with a very small amount of cellulose, perhaps the amorphous cellulose. In the acid hydrolysis at 150 °C, the total mass dissolved did not show any significant differences among the acids, being in a narrow range of 30-33% (Figure 1). However, at 200 °C the total mass dissolved almost doubled to 52-53% using strong acids such as H2SO4 and HCl. Moreover, an increasing trend of the total mass dissolved with decreasing pKa of the mineral acid was observed at the higher temperature. Note that the organic acid, TFA, was a little less productive than the mineral acids even though its pKa is smaller than that of H3PO4. These differences may be, at least in part, explained by the fact that increasing temperatures favors the association of aqueous Brønsted acid over their dissociation.52,53 Data published in a review by Chen et al. suggest that H2SO4 and HCl are still mostly in their dissociated form at 200 °C, whereas the proton concentration from other acids is reduced significantly.52 Moreover, Zhao et al.54 reported that at elevated temperatures esterification of TFA monomers on the external surface of crystalline cellulose and agglomeration of cellulose macrofibrils can inhibit the diffusion rate of TFA into the cellulose and decrease the ability of TFA to swell the sample. Therefore, the small increase in the total mass dissolved using TFA at high temperature here might be a consequence of these factors hindering the hydrolysis of crystalline cellulose. As an alternative, the volatility of TFA might also play a role, partitioning more of the acid into the vapor phase. However, no direct evidence for either of these effects was obtained. It should be noted that the basic cations in the ash portion of lignocellulosic biomass can partially neutralize the added acid in all cases.21,34,36 In this report, the pH of the dilute acids was measured at room temperature before the addition of biomass. The Ymono-cell, Ymono-hemi, and furfural+HMF yield (Yfurf+HMF) are presented in Tables 3 and 4 with respect to acid type at pH ) 1.65 at both 150 and 200 °C, respectively. Furfural and HMF are key degradation products of monosaccharides under acidic, dehydrating conditions.55,56 In fact, industrial routes for furfural production can use conventional mineral acids, such H2SO4, as catalyst.57 According to the literature, the reaction mechanism for the dehydration of xylose to furfural involves irreversible formation of enediol intermediates.43,58,59 On the other hand, the acid-catalyzed dehydration of hexoses such as glucose produces HMF (Scheme 2), which is further converted to levulinic acid and formic acid.44 At 150 °C (Table 3), the Ymono-hemi increased slightly with the pKa (∼57% to 63%) when mineral acids were used. However, the highest yield was obtained using TFA (Ymono-hemi ∼70%). TFA appeared to be the most selective catalyst tested for hydrolyzing hemicellulose and preventing further degradation of dissolved monosaccharides. On the other hand, very low Ymono-cell (less than 5%) was achieved in all reactions. It is wellknown that the amorphous structure of hemicellulose favors its hydrolysis, compared with cellulose.

Table 5. Degradation of Hexoses and Pentoses into HMF and Furfural, Respectively, as a Function of Dilute Acid (pH ) 1.65) and DI Water (pH ) 5.45) for the Hydrolysis of Loblolly Pine Sawdust at 150°C for 60 min hydrolysis agent

Yhexoses (%)

Ypentoses (%)

Yfurfural (%)

YHMF (%)

water TFA H3PO4 HNO3 H2SO4 HCl

1.3 22.9 19.2 15.7 19.6 20.3

21.3 70.3 61.4 50.8 61.5 67.6

0.0 11.8 0.1 0.2 11.9 0.1

0.1 1.1 1.6 1.9 1.6 1.3

Table 6. Degradation of Hexoses and Pentoses into HMF and Furfural, Respectively, as a Function of Dilute Acid (pH ) 1.65) and DI Water (pH ) 5.45) in the Hydrolysis of Loblolly Pine Sawdust at 200 °C for 60 min hydrolysis agent

Yhexoses (%)

Ypentoses (%)

Yfurfural (%)

YHMF (%)

water TFA H3PO4 H2SO4

2.5 2.6 12.6 10.3

1.4 0.5 1.5 0.1

0.4 57.1 0.4 33.8

11.7 18.2 7.4 3.5

At 200 °C, Ymono-hemi dropped dramatically, reflecting the high degree of degradation of the soluble monosaccharides derived from hemicellulose (Table 4). In parallel, Ymono-cell increased to ∼20% when using mineral acids at higher temperature. The dramatic decrease in Ymono-hemi was expected considering that the monosaccharide decomposition should increase with reaction temperature.46 The increase in Ymono-cell might be explained by the effect of both the better accessibility of the acids to the β(1f4)-glycosidic bonds in cellulose at high temperatures and a lower decomposition rate of the glucose as compared with xylose, galactose, arabinose, and mannose.46 Table 4 also shows that TFA produced the largest amount of combined furan-containing degradation products at 200 °C, followed by water, H3PO4, and H2SO4. This is another manifestation of the unique reactivity of TFA, which has a pKa intermediate between phosphoric and nitric acid but shows reactivity trends throughout this work that do not correlate simply with acidity. For a better understanding of the degradation pathway of monosaccharides, the hexose degradation (glucose, mannose, and galactose) to HMF, and pentose degradation (xylose and arabinose) to furfural as a function of hydrolysis temperature for all dilute acids and DI water are shown in Table 5. As described previously, the maximum yield of HMF was calculated on the basis of the complete degradation of hexoses in the raw material; likewise, the maximum furfural yield was obtained assuming complete degradation of pentoses in the raw material. Surprisingly, a significant decrease in soluble pentoses was observed using dilute HNO3 as compared with other mineral acids. Considering that all reactions with dilute mineral acid gave similar yields of undissolved solids, this suggests a faster degradation of pentoses in HNO3 under these conditions. In fact, considerable enhancement in the furfural yield was also observed at the same time. The pentose yields were very similar for H3PO4, HCl, and H2SO4. A similar trend was observed for furfural yields. The previous suggestion that dilute TFA preserves soluble monosaccharides from further degradation can be supported by the higher pentose and hexose yields with this acid. The autohydrolysis at these reaction conditions did not significantly degrade the soluble sugars from the partial dissolution of hemicellulose. Table 6 shows the hexose, pentose, furfural, and HMF yields for reactions conducted at 200 °C. It is evident that a dramatic reduction of YMono-Hemi was observed. This correlates with the

7136 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008

Figure 2. Klason lignin content determined from proximate analysis (% based on dry raw material) vs acid used for the hydrolysis of Loblolly pine sawdust at 150 and 200 °C for 60 min.

Figure 3. Yd, Ymono-hemi, Ymono-cell, and Yfurf+HMF as a function of pH, for the acid hydrolysis of Loblolly pine sawdust using dilute TFA at 150 °C for 60 min.

high concentration of furfural and the low yields of pentoses. Among the different acids, dilute TFA gave higher furfural and HMF yields. TFA appears less active in all acid-catalyzed reactions including the hydrolysis of cellulose as well as the dehydration of both HMF and furfural. This hypothesis is reinforced by the low total mass dissolved observed using dilute TFA at 200 °C compared to other acids (see Figure 1). The lower HMF and furfural yields exhibited by the mineral acids (Table 6) may be attributed to more extensive degradation of the furan compounds to smaller organic molecules. This increase in both furfural and HMF yields with increasing temperature observed in our study agrees with previous kinetic studies that showed that the decomposition rates of both furfural and HMF increase with the reaction temperature and the solution acidity.46,60 Baugh et al.46 reported that monosaccharide decomposition rates decrease in order: xylose, mannose, galactose, and glucose, with xylose the least stable carbohydrate under the studied reaction conditions (pH between 2 and 4 and temperature between 170 and 210 °C). Given this, one might expect the furfural yield to be larger than the HMF yield because of the faster decomposition of the pentoses as compared with the decomposition of the hexoses. However, the reverse is seen in Table 6. We attribute these observations to either (i) faster degradation of furfural to smaller degradation products in the acid catalyzed series or (ii) production of pentoses from hemicellulose occurring earlier than the majority of the hexoses from cellulose during the course of the reaction. In this latter case, hexoses are formed later and are thus transformed into HMF later, with the 60 min reaction capturing this result. The above results describe the characterization of soluble products and the total amount of biomass dissolution. However, an analysis of the solid, undissolved biomass is also necessary to gain a complete understanding of the overall process. The solid residues were analyzed using the proximate analysis method, and the Klason lignin fractions of the solid residue are shown in Figure 2. At 150 °C, the Klason lignin mass in the residual solid is similar to the amount of Klason lignin in the raw material. However, at 200 °C the apparent amount of Klason lignin in the residual solid determined from proximate analysis was higher than in the raw material. A similar effect has been observed by Li et al., who suggested that during harsh treatments such as steam explosion, the lignin fraction can depolymerize and nearly simultaneously repolymerize under acidic conditions.61 We suggest that the lignin can combine with other reactive species such as sugar degradation products to yield an increase in the apparent lignin fraction under our hightemperature (200 °C) acid catalysis conditions. A complete analysis of the solid residue from the acid hydrolysis of Loblolly

pine using solid-state NMR is discussed in a forthcoming publication.62 3.3. Effect of pH in Acid Hydrolysis of Loblolly Pine Sawdust. The effect of proton concentration on the acid hydrolysis of Loblolly pine sawdust was studied using dilute TFA at three pH values. TFA was chosen for further testing because of its selectivity in the acid hydrolysis of hemicellulose and its ability to preserve the monosaccharides produced during the hydrolysis at 150 °C. It is well-known that the hydrolysis of woody biomass is accelerated with increasing acid concentration of the solution.63 However, high acid concentrations can also degrade dissolved monosaccharides into secondary products such as furfural and HMF or even further to tertiary degradation products (Schemes 2 and 3). In this study, the reaction conditions included treatment at 150 °C for 60 min at TFA solution pHs of 0.95, 1.65, or 2.23. As expected, the total mass dissolved increased from 26% to 38% with increasing proton concentration, as shown in Figure 3. The highest Ymono-hemi was reached at a pH of 1.65. In the case of Ymono-hemi ∼70% at pH < 1.65, it is assumed that the remaining 30% corresponds to both dissolved oligosaccharides, which were not quantified, and degradation products from monosaccharide decomposition. The formation of furfural and HMF increases with the proton concentration, showing that under these conditions more of these products were produced from monosaccharides than were consumed by the series reactions leading to levulinic acid and other products. It should be noted that the cellulose hydrolysis increased with the proton concentration, indicating a higher solubility at more acidic conditions, as expected. Figure 4 shows the proximate analysis results for the solid residue obtained from TFA hydrolysis at three different pHs for 60 min at 150 °C. As expected, the largest component of the solid residue is lignin. The Klason lignin in the residue remained nearly constant at the three different pH values and was consistent with the lignin fraction in the raw material. 3.4. Effect of Hold Time at Reaction Temperature. Another variable considered in this study was the hold time at the reaction temperature. All batch experiments were carried out with a heating rate that varied slightly with the final desired reaction temperature. However, the final hold time at the highest reaction temperature was controlled for 15, 45, 60, and 120 min. Time zero (t ) 0) was considered to be when the reaction mixture reached the desired final reaction temperature, and the heating and cooling times of the reactor were not considered as part of the hold time. For reference, a typical heating time from 25 to 150 °C was ∼30 min, while the cooling time from reaction temperature to ambient was ∼60 min. On the basis of

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7137

Figure 4. Total mass dissolved, Klason lignin content in the solid residue after hydrolysis, and ash + extractives + soluble lignin percentages relative to the dry raw material, as a function of solution pH for the acid hydrolysis of Loblolly pine sawdust using dilute TFA at 150 °C for 60 min.

Figure 5. Yd, Ymono-hemi, Ymono-cell, and Yfurf+HMF as a function of reaction time for the acid hydrolysis of Loblolly pine sawdust using dilute H2SO4 (pH ) 1.65) at 150 °C.

the biomass dissolution yield achieved at time zero, it is suggested that hydrolysis has already started during heating. The effect of reaction time was studied using dilute H2SO4 at pH ) 1.65 and 150 °C, covering a wide range of reaction times. Figure 5 shows that the total mass dissolved increased from 27 to 33% with increasing reaction time, and it reached a plateau after ∼45 min. On the other hand, Ymono-hemi had a maximum value (59%) at 45-60 min, with the onset of dominating degradation of soluble monosaccharides at reaction times over 100 min. Wyman et al. 35 also observed a maximum yield for xylose and glucose as a function of pretreatment time in the dilute acid pretreatment of corn stover. On the other hand, Ymono-cell, as well as the Yfurf+HMF slightly increased with the reaction time. Overall, the reaction time does not seem to substantially enhance the hydrolysis efficiency of raw material, its main effect being the degradation of soluble monosaccharides. The hydrolysis of holocellulose clearly started during the heating time, as shown in Figure 5 by the data at time zero. Here, the total mass dissolved (∼27%) is even larger than the total mass dissolved during the autohydrolysis at 150 °C for 60 min (∼21%), meaning the acid hydrolysis began at low temperature. 3.5. Effect of Hydrolysis Temperature. The effect of reaction temperature on the acid hydrolysis or Loblolly pine sawdust was studied using dilute TFA at pH ) 1.65 for 60 min. As shown in Figure 6, the temperatures tested included 120, 150, and 200 °C. TFA was chosen for this study because of its ability to preserve the monosaccharides produced during the hydrolysis,

Figure 6. Yd, Ymono-cell, Ymono-hemi, and Yfurf+HMF as a function of reaction temperature for the acid hydrolysis of Loblolly pine sawdust using dilute TFA at pH ) 1.65 for 60 min.

Figure 7. Yield of soluble hexoses and pentoses and degradation into HMF and furfural, respectively, as a function of hydrolysis temperature in dilute TFA (pH ) 1.65) for 60 min.

as noted above. The total mass dissolved increased with increasing temperature, and reached a plateau of 38%. It is noteworthy that the maxima for both Ymono-cell and Ymono-hemi occurred at 150 °C (Figure 6). This is suggested to be due to the high degree of monosaccharide degradation at 200 °C and low poly(saccharide) hydrolysis rates at 120 °C. The Ymono-hemi doubled when the temperature was increased from 120 to 150 °C but dropped to ∼1% at 200 °C. The significant loss of Ymono-hemi at the higher temperature suggests the almost complete degradation of soluble monosaccharides under these conditions. In parallel, the Yfurf+HMF dramatically increased with the treatment temperature. Figure 7 shows a similar trend in the degradation of hexoses and pentoses to HMF and furfural, respectively. The values for Ymono-cell and for Ymono-hemi were calculated accounting for only soluble monosaccharides in the liquid samples, that is, no oligo(saccharides) were considered, as noted above. Therefore, a value of 70% for Ymono-hemi means that 30% of the hemicellulose has been transformed into soluble oligo(saccharides), degradation products, or left as unreacted solid poly(saccharides). More complete analysis to close the entire carbon balance is presented in the sections below. 3.6. Test for the Presence of Soluble Oligosaccharides. As mentioned earlier, in addition to soluble monosaccahrides and degradation products, we expected that soluble oligosaccharides might be present in some samples. To verify the presence of oligosaccharides in our liquid samples after the primary hydrolysis, the solid residue was separated, and a secondary hydrolysis of the liquid products was conducted. The secondary

7138 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 7. Ymonosaccharides of Sample 1, Sample 2, and Sample 3 from Primary and Secondary Hydrolysesa Ymonosaccharides (%) sample sample 1 sample 2 sample 3

primary hydrolysis

secondary hydrolysis

1.7 6.9 16.5

7.9 20.0 16.7

a (Sample 1) Primary hydrolysis of sample 1: DI water (pH ) 5.45), 150°C, 60 min hold time. (Sample 2) Primary hydrolysis of sample 2: dilute TFA (pH ) 2.23), 150°C, 60 min hold time. (Sample 3) Primary hydrolysis of sample 3: dilute H2SO4 (pH ) 1.65), 150°C, 60 min hold time.

hydrolysis procedure followed the NREL protocols for “Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples”.51 An amount of 72% H2SO4 was added to an aliquot of liquid samples to bring the total acid concentration to 4%. Next, the 4% acid solutions were placed in an autoclave at 121 °C for 1 h. Three samples of aqueous phase products from a primary hydrolysis, excluding the wash, were chosen for description here, and the results are given in Table 7. In sample 1, primary hydrolysis was performed in DI water (pH ) 5.45) at 150 °C for 60 min; in sample 2, primary hydrolysis was carried out in dilute TFA (pH ) 2.23) at 150 °C for 60 min; for sample 3, primary hydrolysis was performed in H2SO4 (pH ) 1.65) at 150 °C for 60 min. Note in Table 7 that the amount of soluble monosaccharides increased for both sample 1 and sample 2 after a secondary hydrolysis. This is consistent with the presence of oligosaccharides in both primary hydrolysis solutions. If the amount of soluble monosaccharides increases after a secondary hydrolysis in the absence of the solid residue, one may attribute it to the hydrolysis of soluble oligosaccharides, as no other source of monomeric sugars is present in the sample. However, in sample 3 the overall amount of soluble monosaccharides in solution was similar after a secondary hydrolysis. In this case, we cannot confirm or exclude the presence of soluble oligosaccharides. We must take into account the competing series reactions that can produce monosaccharides from oligomers and consume monosaccharides in producing HMF, furfural, and other degradation products. In general, we detected oligosaccharides only in samples produced under mild conditions. 3.7. Mass Balance on Total Organic Carbon (TOC). The presence of oligosaccharides as well as unquantified degradation products (other than furfural and HMF, such as levulinic acid, formic acid, acetic acid, etc.), makes carbon balance closure

difficult. Therefore, total organic carbon analysis was carried out to seek mass balance closure. The liquid and solid residues tested for TOC are described in Table 8. Total mass balance, soluble sugars, HMF+furfural as well as the unidentified liquid phase organic products are calculated based on the maximum total organic carbon of the dry raw material. It is noteworthy that the mass balance based on TOC closed in all cases studied (∼100%) within the expected bounds of accumulated analytical errors. The fact that most samples yielded mass balances of >100% suggests the impact of some minor systematic error that leads to overestimation of one or more fractions of the carbon material. A fraction of liquid phase products were not identified by liquid chromatography techniques. This liquid fraction was larger in the reactions using strong hydrolysis conditions such as application of H2SO4 and H3PO4 at 200 °C, as well as dilute TFA at pH ) 0.95, suggesting that these compounds are degradation products derived from soluble monosaccharides as well as from HMF and furfural. Thus, under mild conditions, this undetected fraction was associated with soluble oligosaccharides and limited amounts of degradation products, whereas under harsh conditions this fraction was composed mostly of soluble sugar and HMF/furfural degradation products. Monosaccharides formed here by the acid- or autohydrolysis of Loblolly pine can be used in the production of biofuels and high-value chemicals. Glucose is the main lignocellulosicbiomass product used in the production of bioethanol. However all hexoses and pentoses produced by acidic hydrolysis of holocellulose may be used, in principle, in bioethanol production. Furthermore, furan derivatives such as furfural and HMF, also produced during the acidic hydrolysis of our raw material, may be used for the production of fine chemicals, resins, and plastics, or as a feedstock for fuel additives. Finally, it should be noted that a mixture of soluble oxygenated organic compounds from lignocellulose hydrolysis may be a suitable raw material for hydrogen generation via aqueous phase reforming.7-9 However, in this work, no attempt was made to optimize the conditions for a particular product (monosaccharide, furan, etc.). 4. Conclusions A comprehensive study of the acid hydrolysis of Loblolly pine sawdust, with systematic manipulation of a wide range of variables, was completed. The total mass dissolved, the yield of soluble monosaccharides estimated to be derived from hemicellulose hydrolysis and cellulose hydrolysis, yield of HMF and furfural,

Table 8. Mass Balance Closure via Total Organic Carbon Analysis of Acid Hydrolysis and Autohydrolysis Samples from Different Reaction Conditions hydrolysis conditions

percentages of organic carbon on raw material

entry

hydrolysis

temp (°C)

pH

time (min)

total mass balance of TOC

soluble sugars in liquid phase

HMF+furfural in liquid phase

unidentified liquid phase products

solid residue

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

none none TFA TFA TFA TFA TFA H2SO4 H2SO4 H2SO4 H2SO4 H2SO4 H3PO4 H3PO4 HCl

150 200 120 150 150 200 150 150 150 150 150 200 150 200 150

5.45 5.45 1.65 1.65 1.65 1.65 0.95 1.65 1.65 1.65 1.65 1.65 1.65 1.65 1.65

60 60 60 60 120 60 60 0 45 60 120 60 60 60 60

104 106 109 100 109 104 112 99 103 106 109 99 105 105 109

2 2 8 15 13 1 18 11 13 13 14 7 14 6 15

0 8 1 1 3 13 2 0 1 1 2 5 2 7 1

15 14 16 9 17 13 19 6 15 15 16 23 12 22 16

86 82 85 75 75 77 73 82 74 77 77 63 77 71 77

Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7139

and the overall amount of unidentified organic carbon were quantitatively determined. The yield of monosaccharides compared with their degradation products as well as the amount of acid added to the system are important factors in comparing different pretreatments, with consideration given to the downstream processes expected for biofuel production. Consequently, the quantitative assessment of Ymono-cell, Ymono-hemi, and Yfurf+HMF were crucial to this study. Under the reaction conditions tested, the highest total mass dissolved was achieved using dilute H2SO4 and HCl (both at pH ) 1.65) at 200 °C for 60 min. Here, we suggest that all the solid hemicellulose as well as nearly all the cellulose was dissolved. However, high hydrolysis temperatures resulted in a significant amount of the soluble monosaccharides being converted into both primary (e.g., HMF and furfural) and secondary degradation products. On the other hand, at mild acid hydrolysis conditions (pH ) 1.65, 150 °C, and 60 min of reaction time), all acid solutions were nearly similar in their ability to dissolve the pine, with the exception of the autohydrolysis where only small amounts of soluble compounds were obtained. Even though softwoods like pine are harder to hydrolyze than hardwoods, at these mild acid hydrolysis conditions (pH ) 1.65, 150 °C and 60 min), only hemicellulose and a small amount of cellulose were dissolved. A secondary hydrolysis of the liquid phase products from primary hydrolyses carried out under mild reaction conditions confirmed the presence of soluble oligosaccharides. Quantifying the amount of oligosaccharides was not possible because of the simultaneous conversion to monosaccharides as well as the subsequent degradation reactions. Additionally, mass balances on organic carbon in all reactions tested were closed. Proximate analysis of solid residue after hydrolysis allowed showed that the amounts of Klason lignin, ash, soluble lignin, and extractives were consistent with the content in the raw material. However, some changes in the apparent Klason lignin content under harsh hydrolysis conditions were observed, with an increase in the apparent Klason lignin content. Overall, this study presents a comprehensive view of the reactivity of Loblolly pine using a variety of acidic treatment conditions identifying a range of important products. Acknowledgment This work was supported by Chevron through the Georgia Tech Strategic Energy Institute. We thank Prof. Art Ragauskas for acquiring the loblolly pine feedstock and Kathleen Poll for the proximate analysis of the loblolly pine feedstock. Literature Cited (1) Lange, J. Lignocellulose conversion: an intorduction to chemistry, process and economics. Biofuels, Bioprod. Bioref. 2007, 1, 39–48. (2) Demirbas, A. Hydrogen production via pyrolytic degradation of agricultural residues. Energy Sources 2005, 27, 769–775. (3) Chum, H. L.; Overend, R. P. Biomass and renewable fuels. Fuel Process. Technol. 2001, 71, 187–195. (4) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass gasification in near- and supercritical water: Status and prospects. Biomass Bioenergy 2005, 29, 269– 292. (5) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of wood/biomass for bio-oil: A critical review. Energy Fuels 2006, 20, 848–889. (6) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. Hydrothermal gasification of biomass and organic wastes. J. Supercrit. Fluids 2000, 17, 145–153.

(7) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002, 418, 964–967. (8) Valenzuela, M. B.; Jones, C. W.; Agrawal, P. K. Batch aqueousphase reforming of woody biomass. Energy Fuels 2006, 20, 1744–1752. (9) Huber, G. W.; Dumesic, J. A. An overview of aqueous-phase catalytic processes for production of hydrogen and alkanes in a biorefinery. Catal. Today 2006, 111, 119–132. (10) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-phase catalytic processing of biomass-derived oxygenated hydrocarbons to fuels and chemicals. Angew Chem., Int. Ed. 2007, 46, 7164–7183. (11) Corma, A.; Huber, G. W.; Sauvanaud, L.; O’Connor, P. Processing biomass-derived oxygenates in the oil refinery: Catalytic cracking (FCC) reaction pathways and role of catalyst. J. Catal. 2007, 247, 307–327. (12) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem. ReV. 2006, 106, 4044–4098. (13) Hahn-Hagerdal, B.; Galbe, M.; Gorwa-Grauslund, M. F.; Liden, G.; Zacchi, G. Bio-ethanol - the fuel of tomorrow from the residues of today. Trends Biotechnol. 2006, 24, 549–556. (14) Perlack, R. D., Wright, L. L., Turhollow, A. F., Graham, R. L., Stokes, B. J. and Erbach, D. C. Biomass as Feedstock for Biofuels and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply; US Department of Energy and US Department of Agriculture: Washington, DC, 2005. (15) Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones, A. D.; O’Hare, M.; Kammen, D. M. Ethanol can contribute to energy and environmental goals. Science 2006, 311, 506–508. (16) Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next Generation Hydrocarbon Biorefineries; National Science Foundation: Washington, DC, 2008. (17) Sjo¨stro¨m, E., Wood Chemistry, 2nd ed.; Academic Press: New York, 1999. (18) Wyman, C. E. Biomass ethanol: Technical progress, opportunities, and commercial challenges. Annu. ReV. Energy EnViron. 1999, 24, 189– 226. (19) Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y. Y. Coordinated development of leading biomass pretreatment technologies. Bioresour. Technol. 2005, 96, 1959–1966. (20) Sun, Y.; Cheng, J. Y. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour. Technol. 2002, 83, 1–11. (21) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673– 686. (22) Excoffier, G.; Toussaint, B.; Vignon, M. R. Saccharification of Steam-Exploded Poplar Wood. Biotechnol. Bioeng. 1991, 38, 1308–1317. (23) Bouchard, J.; Nguyen, T. S.; Chornet, E.; Overend, R. P. Analytical Methodology for Biomass Pretreatment 0.2. Characterization of the Filtrates and Cumulative Product Distribution as a Function of Treatment Severity. Bioresour. Technol. 1991, 36, 121–131. (24) Garrote, G.; Dominguez, H.; Parajo, J. C. Hydrothermal processing of lignocellulosic materials. Holz Als Roh-Und Werkstoff 1999, 57, 191– 202. (25) Saeman, J. F. Kinetics of Wood Saccharification - Hydrolysis of Cellulose and Decomposition of Sugars in Dilute Acid at High Temperature. Ind. Eng. Chem. 1945, 37, 43–52. (26) Sherrard, E. C.; Gauger, W. H. Effect of salts upon the acid hydrolysis of wood. Ind. Eng. Chem. 1923, 15, 63–64. (27) Sherrard, E. C.; Blanco, G. W. Some of the products obtained in the hydrolysis of white spruce wood with dilute sulfuric acid under steam pressure. Ind. Eng. Chem. 1923, 15, 611–616. (28) Hagglund, E.; Klingstedt, F. W.; Rosenqvist, T.; Urban, H. The contents of carbohydrate in simply hydrolisable hemicellulosis of the spruce. Hoppe-Seylers Zeitschrift Phys. Chem. 1928, 177, 248–263. (29) Dilute Acid Hydrolysis. U.S. Department of Energy - Energy Efficiency and Renewable Energy. http://www1.eere.energy.gov/biomass/ printable_versions/dilute_acid.html (accessed 06/27/08). (30) Larsson, S.; Palmqvist, E.; Hahn-Hagerdal, B.; Tengborg, C.; Stenberg, K.; Zacchi, G.; Nilvebrant, N. O. The generation of fermentation inhibitors during dilute acid hydrolysis of softwood. Enzyme Microb. Technol. 1999, 24, 151–159. (31) Chen, Y.; Sharma-Shivappa, R. R.; Keshwani, D.; Chen, C. Potential of agricultural residues and hay for bioethanol production. Appl. Biochem. Biotechnol. 2007, 142, 276–290. (32) Brink, D. DeVelopment of Alternate Pretreatment and Biomass Fractionation Processes; National Renewable Energy Laboratory: Golden, CO, 1998.

7140 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 (33) Torget, R.; Walter, P.; Himmel, M.; Grohmann, K. Dilute-Acid Pretreatment of Corn Residues and Short-Rotation Woody Crops. Appl. Biochem. Biotechnol. 1991, 75–86. (34) Esteghlalian, A.; Hashimoto, A. G.; Fenske, J. J.; Penner, M. H. Modeling and optimization of the dilute-sulfuric-acid pretreatment of corn stover, poplar and switchgrass. Bioresour. Technol. 1997, 59, 129–136. (35) Lloyd, T. A.; Wyman, C. E. Combined sugar yields for dilute sulfuric acid pretreatment of corn stover followed by enzymatic hydrolysis of the remaining solids. Bioresour. Technol. 2005, 96, 1967–1977. (36) Lloyd, T. A.; Wyman, C. E. Predicted effects of mineral neutralization and bisulfate formation on hydrogen ion concentration for dilute sulfuric acid pretreatment. Appl. Biochem. Biotechnol. 2004, 113, 1013–1022. (37) Taherzadeh, M. J.; Eklund, R.; Gustafsson, L.; Niklasson, C.; Liden, G. Characterization and fermentation of dilute-acid hydrolyzates from wood. Ind. Eng. Chem. Res. 1997, 36, 4659–4665. (38) Grethlein, H. E.; Converse, A. O. Common Aspects of Acid Prehydrolysis and Steam Explosion for Pretreating Wood. Bioresour. Technol. 1991, 36, 77–82. (39) Nguyen, Q. A.; Tucker, M. P.; Keller, F. A.; Eddy, F. P. Twostage dilute-acid pretreatment of softwoods. Appl. Biochem. Biotechnol. 2000, 84, 561–576. (40) Baker, J. ; Langdon, O. Pinus taeda L. http://www.na.fs.fed.us/ pubs/silvics_manual/Volume_1/pinus/taeda.htm (accessed Feb. 2008). (41) Agricultural Conservation and Forestry Statistics. http://www.nass.usda.gov/Publications/Ag_Statistics/2005/05_ch12.PDF (accessed Feb. 2008). (42) Roberts, J. C., Paper Chemistry, 2nd ed.; Blackie Academic: London, 1996. (43) Antal, M. J.; Leesomboon, T.; Mok, W. S.; Richards, G. N. Kineticstudies of the reactions of ketones and aldoses in water at high-temperature. 3. Mecanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 1991, 217, 71–85. (44) Girisuta, B.; Janssen, L.; Heeres, H. J. Kinetic study on the acidcatalyzed hydrolysis of cellulose to levulinic acid. Ind. Eng. Chem. Res. 2007, 46, 1696–1708. (45) Robinson, J. M.; Burgess, C. E.; Bently, M. A.; Brasher, C. D.; Horne, B. O.; Lillard, D. M.; Macias, J. M.; Mandal, H. D.; Mills, S. C.; O’Hara, K. D.; Pon, J. T.; Raigoza, A. F.; Sanchez, E. H.; Villarreal, J. S. Biomass Bioenergy 2004, 26, 473. (46) Baugh, K. D.; McCarty, P. L. Thermochemical pretreatment of lignocellulose to enhance methane fermentation. 1. Monosaccharide and furfurals hydrothermal decomposition and product formation rates. Biotechnol. Bioeng. 1988, 31, 50–61. (47) Baugh, K. D.; Levy, J. A.; McCarty, P. L. Thermochemical pretreatment of lignocellulose to enhance methane fermentation. 2. Evaluation and application of pretreatment model. Biotechnol. Bioeng. 1988, 31, 62–70. (48) Albersheim, P. D. J. N.; English, P. D.; Karr, A. Carbohydr. Res. 1967, 5, 340. (49) Paice, M. G. L. J.; Desrochers, M. Pretreatment of lignocellulosic materials for efficient bioethanol production. TAPPI J. 1982, 65.

(50) Jakobsons, J. B. H.; Erins, P.; Sundquist, J. Characterization of alkali-soluble fraction fo steam exploded birch wood. Holzforschung 1995, 49 (1), 51–59. (51) Sluiter, A.; Haimes, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D. Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples; Laboratory Analytical Procedure; National Renewable Energy Laboratory: Golden, CO, 2005. (52) Chen, X. M.; Izatt, R. M.; Oscarson, J. L. Thermodynamic Data For Ligand Interaction With Protons And Metal-Ions In Aqueous-Solutions At High-Temperatures. Chem. ReV. 1994, 94, 467–517. (53) Frank, H. S.; Evans, M. W. Free Volume and Entropy in Condensed Systems 3. Entropy in Binary Liquid Mixtures - Partial Molar Entropy in Dilute Solutions - Structure and Thermodynamics in Aqueous Electrolytes. J. Chem. Phys. 1945, 13, 507–532. (54) Zhao, H. B.; Holladay, J. E.; Kwak, J. H.; Zhang, Z. C. Inverse temperature-dependent pathway of cellulose decrystallization in trifluoroacetic acid. J. Phys. Chem. B 2007, 111, 5295–5300. (55) Newth, F. H. The formation of furan compounds from hexoses. AdV. Carbohydr. Chem. 1951, 6, 83–106. (56) Haworth, W. N.; Jones, W. G. M. The conversion of sucrose into furan compounds. Part I. 5-hydroxymethylfurfuraldehyde and some derivatives. J. Chem. Soc. 1944, 667–670. (57) Kamm, B.; Gruber, P. R.; Kamm, M. Biorefineries-Industrial Processes and Products, Status Quo and Future Directions, 1st ed.; WileyVCH: Weinheim, Germany, 2006. (58) Zeitsch, K. J. the Chemistry and Technology of Furfural and Its Many by-Products, 1st ed.; Elsevier Science: Ko¨ln, Germany, 2000. (59) Antal, M. J.; Leesomboon, T.; Mok, W. S.; Richards, G. N. Kineticstudies of the reactions of ketoses and aldoses in water at high-temperature. 3. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 1991, 217, 71–85. (60) Asghari, F. S.; Yoshida, H. Kinetics of the decomposition of fructose catalyzed by hydrochloric acid in subcritical water: Formation of 5-hydroxymethylfurfural, levulinic, and formic acids. Ind. Eng. Chem. Res. 2007, 46, 7703–7710. (61) Li, J. B.; Henriksson, G.; Gellerstedt, G. Lignin depolymerization/ repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour. Technol. 2007, 98, 3061–3068. (62) Sievers, C., Hoskins, T., Marzialetti, T., Agrawal, P. K., Jones, C. W., Quantitative characterization of residues from aqueous phase acid hydrolysis by solid state NMR spectroscopy. Submitted for publication. (63) Galbe, M.; Zacchi, G., Pretreatment of lignocellulosic materials for efficient bioethanol production. In Biofuels; Springer-Verlag: Berlin, 2007; Vol. 108, pp 41-65.

ReceiVed for reView March 20, 2008 ReVised manuscript receiVed July 9, 2008 Accepted July 10, 2008 IE800455F