Reaction Kinetic Model of Dilute Acid-Catalyzed Hemicellulose

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Reaction Kinetic Model of Dilute-Acid Catalyzed Hemicellulose Hydrolysis of Corn Stover under High-Solid Condition Suan Shi, Wenjian Guan, Li Kang, and Yoon Y. Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01768 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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Industrial & Engineering Chemistry Research

Reaction Kinetic Model of Dilute-Acid Catalyzed Hemicellulose Hydrolysis of Corn Stover under High-Solid Condition Suan Shi1*, Wenjian Guan2, Li Kang3, Y. Y. Lee2, Hawaii Natural Energy Institute, University of Hawaii at Manoa, HI 96822 2 Department of Chemical Engineering, Auburn University, AL 36849 3 Henkel Corporation, Rocky Hill, CT, USA 06067 1

*Corresponding author, 1680 East-West Road, University of Hawaii at Manoa, Honolulu, HI 96822 (Ph): 808-956-4207, E-mail: [email protected] Abstract High solid conditions are desirable in pretreatment of lignocellulosic biomass. An advanced dilute-acid pretreatment reactor has been developed at National Renewable Energy Laboratory (NREL). It is a continuous auger-driven reactor that can be operated with high-solid charge at high temperature and with short residence time resulting high productivity and high sugar concentration. We investigated the kinetics of the reactions associated with dilute-acid pretreatment of corn stover, covering the reaction conditions of the NREL reactor operation: 155-185 oC, 1-2 wt% sulfuric acid concentration, and 1:2 solid to liquid ratio. The experimental data were fitted to a first-order biphasic model which assumes that xylan is comprised of two different fragments: fast and slow reacting fractions. Due to the high solid loading condition, significant amount of xylose oligomers was observed during the pretreatment. The oligomers were included as an intermediate entity in the kinetic model. The effect of acid concentration was incorporated into the pre-exponential factor of Arrhenius equation. The kinetic model with bestfit kinetic parameters has shown good agreement with experimental data. The kinetic parameter values of the proposed model were noticeably different from those previously reported. The activation energies of xylan hydrolysis are lower and the acid exponents are higher than the average of literature values. The proposed model can serve as a useful tool for design and operation of pretreatment system pertaining to corn stover.

Key words: hemicellulose, pretreatment, dilute-acid hydrolysis, high solid loading, kinetic model, Corn stover.

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Introduction Corn stover is a renewable and abundant biomass resource. The hydrolysis of corn stover to produce sugars and conversion to fuels and chemicals has been studied extensively

1-4

.

Hemicellulose accounts for about 20% of corn stover. Its utilization is an important segment in overall bioconversion process. Pretreatment is a necessary step in bioconversion process. The overall purpose of pretreatment is to break down the shield formed by lignin and hemicellulose, disrupt the crystalline structure, and reduce the degree of polymerization of cellulose 5, 6. Diluteacid pretreatment is one of the most advanced and widely accepted pretreatment technologies 7. It can effectively recover most of the hemicellulose sugars, and at the same time increase the yield of cellulose in subsequent enzymatic hydrolysis. The dilute-acid pretreatment accompanies hydrolysis of the hemicellulose fraction in the biomass often termed as prehydrolysis. The prehydrolysis has advanced to the point where the hemicellulose sugars are obtained with yields above 80% and with sugar concentrations high enough to be directly applied to the bioconversion process 7, 8. The mechanism of lignocellulosic biomass hydrolysis in acid environment is yet to be fully elucidated. Due to its heterogeneous characteristics with multi-stage rate processes involved, such as diffusion of catalyst and transportation of products, simplified kinetic models have been postulated. Kinetic modeling based on pseudo-homogeneous reactions has been commonly applied and found to be useful in predicting experimental outcome with reasonable accuracy 9. The simplest kinetic model of acid-catalyzed hemicellulose hydrolysis is adapted from that of Saeman that was originally developed for hydrolysis of cellulose

10

. It is a two-step

homogeneous pseudo-first-order reaction: xylan is first hydrolyzed to xylose, and the xylose is then degraded to furfural and other decomposition products.


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An improved model was later proposed by Kobayashi and Sakai

(1) 11

. It was developed

from observation of biphasic pattern during hemicellulose hydrolysis. There are two different fractions of hemicellulose that hydrolyze in a parallel mode at different rates: a portion that hydrolyzes rapidly, the remainder hydrolyzes slowly. Since then, most of the kinetic studies have incorporated this modification 12-17.

⎼

" !⎼

(2)

Another modification of the simple two-step model (Eq. 1) has been applied that a reaction intermediate (xylose oligomer) is included between xylan and xylose

18-21

.

The

intermediate product, oligomer, is defined as the water soluble polymer with DP between 1 and 10 22. This model is applicable for reactions carried out under relatively mild conditions or high solid loading conditions where buildup of oligomers becomes significant 18, 23, 24.

%

#$ A more comprehensive model was suggested by Mehlberg and Tsao

25

(3)

in the following

pattern:

⎼

&

"

!⎼ →

%

#$ (4)

In all cases, the reactions are assumed to be pseudo-homogeneous following a first-order dependence on reactant concentrations. All the rate constants have Arrhenius-type temperature 3 ACS Paragon Plus Environment


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dependence with inclusion of acid concentration term as a part of the pre-exponential factor as shown in Eq. 5 13, 17, 24: ) = )+ ∗ -. ∗ /0/23 where

k0: pre-exponential factor (min-1);

E: activation energy (kJ mol-1);

n:

A: acid concentration (%);

acid exponent;

R: 8.314 × 10-3 (kJ mol-1 K-1);

(5)

T: temperature (K)

Previous studies have led to development of various kinetic models with associated kinetic parameters. These models and parameters only apply to specific substrates and normally cover a narrow range of reaction conditions. To our knowledge, there is only one published paper that deals with hydrolysis of hemicellulose in corn stover applying the comprehensive model that incorporates biphasic hydrolysis as well as formation of xylo-oligomers

26

. Even in

that work, the solid loading was far below that of this investigation and it lacks the in-depth discussion on the kinetic parameters. Shen and Wyman studied a novel mechanism and kinetic model to explain enhanced xylose yields from dilute sulfuric acid compared to hydrothermal pretreatment of corn stover

27

. They applied a biphasic model including oligomers but with

reversible reactions. Also, the definition of fast and slow xylan in their model is different from the most of models that have been proposed. In our preliminary test, none of the models and parameters from previous studies has shown satisfactory agreement with the experimental data obtained from hydrolysis of corn stover hemicellulose under high-solid conditions. A high-solid loading is preferred from an economic standpoint because it reduces the reactor size, lowers the process energy, and water usage. Saeman reported that the solid to liquid (S/L) ratio over the range of 5 to 20 had a relatively low but discernible effect on the rate of cellulose hydrolysis 10. Solid loading effect was reported to be an indicator of deviation from first-order kinetics 4 ACS Paragon Plus Environment

28

.

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Jensen et al., suggested that the effect of solid loading could potentially be incorporated into the Arrhenius expression for the kinetic constants 24. Proto-type continuous auger-driven pretreatment reactors that can be operated with highsolid charge and short residence time have been developed and tested in various research laboratories and manufacturing facilities including NREL. In the actual operation of such reactor, it has been observed that the experimental data deviated considerably from those of batch reactor. The deviation may stem from the non-ideal behavior of the reactor, perhaps due to axial backmixing of the moving-bed reactor, and/or the high-solid/low-water experimental conditions. From our separate study on the non-ideal behavior of the reactor employing the residence time distribution data, we found that the NREL reactor behaves closely to a PFR, which led us to believe that the deviation is primarily due to the high-solid condition in the reactor. With this understanding this study was undertaken to investigate the kinetics of the reactions occurring in dilute-acid pretreatment of corn stover, covering the reaction conditions applicable to screwdriven continuous reactor including the NREL continuous pretreatment reactor paying special attention to the high-solid condition. Materials and Methods Feedstock Kramer Corn Stover (Pioneer variety 33A14) sample was provided by NREL. It was then milled at NREL through a Mitts & Merrill rotary knife mill to pass a 2 mm screen. Upon receipt of the KCS sample, it was stored in sealed bag at room temperature. Before subjecting to the hydrolysis experiments, KCS was oven-dried at 40oC to moisture content below 10% and further ground and screened, particles with size between 20-60 mesh were collected. The chemical composition of KCS was analyzed according to NREL’s standard procedure 5 ACS Paragon Plus Environment

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. The KCS was


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found to contain 37.3% glucan, 20.6% xylan, 3.6% arabinan, 0.6% galactan, 16.03% insoluble lignin, 1.01% acid soluble lignin and 6.3% ash. Neutralization Capacity of Feedstock Since the mineral content of biomass can neutralize some of the acid during dilute acid pretreatment

14, 30

. The concentration of acid should be corrected according to feedstock’s

neutralization capacity (NC). The NC of KCS was measured by titration with 1N [H+] sulfuric acid solution (ACS Reagent grade, purchased from VWR). Five grams of KCS (dry weight) was put into 100 ml DI water, the control being 100 ml blank DI water. Sulfuric acid solution was added to the each beaker while the pH change was monitored. From the pH difference taken after reaching steady level, the [H+] value was calculated to determine the NC. Dilute Acid Hydrolysis Batch experiments were performed in duplicates to obtain the time progression of all of the components involved in the reaction sequence. The acid hydrolysis reactions were carried out in small-scale tubular reactors (provided by NREL) with 4 cm3 total working volume (ID=0.75 cm and length=2.5 cm). The tubular reactors are made out of Hastelloy alloy to prevent corrosion and especially formation of metallic ions by sulfuric acid. Some of the metallic ions, especially chromium ion which is one of the major components in SS-316, are known to be a strong catalyst for sugar decomposition. The temperature inside the reactor was traced by a thermocouple thermometer with digital recording. Accurate kinetic experiment requires a short heat-up time. The reactants must be quickly brought up to desired temperature and remain at isothermal condition. To initiate the reaction, a set of reactors were placed into an oil bath (Haake FS2 model) in which the temperature was pre-adjusted at a level 50 °C higher than the 6 ACS Paragon Plus Environment

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desired reaction temperature. The ramp-up time was less than 1 minute. When the center section of the reactor reached the desired reaction temperature, it was transferred into another oil bath pre-set at the desired reaction temperature. The reaction temperatures were set at 155, 170 and 185 oC. The S/L ratio of 1:2 (equivalent to 33% solid consistency) was applied uniformly. Three levels of acid loading (after NC correction) were used: 1.0%, 1.5% and 2.0% (made from 98% sulfuric acid solution, ACS Reagent grade, purchased from Sigma Aldrich). Totally nine sets of experiments were performed. The experimental design was set up within the practical reaction conditions. For example, the acid level below 1% at 155 oC was not covered because of low yields; whereas the acid level above 2% at 185 oC was not tested because of too high xylose degradation. To ensure complete wetting and diffusion of acid through the biomass, the air-dried and screened KCS was presoaked with acid solution for 30 minutes before being placed into the reactor. Each reactor contained 3.0 grams of wet feedstock. Reactors were withdrawn from the oil bath with preset interval and immediately quenched in an ice-water bath to stop the reaction. Because of the high S/L ratio, it was difficult to squeeze out the liquid from the pretreated KCS. Thus, each collected samples was diluted 10times by DI water. The diluted slurries were filtrated to separate the solid and liquid. The liquid portion was collected for the subsequent analysis of monomer and oligomer sugar components. The oligomer value was determined following the NREL standard protocol of TP 510-42623 by taking the difference of xylose monomer values in the liquid before and after secondary hydrolysis done by autoclaving at 4 % sulfuric acid and 121 oC for 1 hour (Eq. 6) widely used method in measuring xylose monomer and oligomer value = Xylose after autoclaving – Xylose before autoclaving

(6)


21, 32-34

31

. This is a

. Xylose-oligomer


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The solid portion was then washed and air-dried for the subsequent composition analysis. These data were used to determine the associated kinetic parameters of the proposed model. HPLC Analysis Solid and liquid samples were analyzed by two separate HPLCs equipped respectively with Bio-Rad’s Aminex HPX-87P and Aminex HPX-87H columns and refractive index detectors. The sugars were determined following the NREL standard protocol of TP 510-42623 using Aminex HPX-87P column, and the degradation products were analyzed following the NREL TP 510-42618 using Aminex HPX-87H column. Kinetic Model Development In consideration of the high-solid conditions of this work, the biphasic model with inclusion of oligomer was applied:

⎼

"

!⎼ →

#$ 9

(7)

For this kinetic model, the variation of each individual component are theoretically determined by the following set of differential equations: ⁄ = −) ∗

(8)

" ⁄ = −)" ∗ "

(9)

#⁄ = ) ∗ + )" ∗ " − ) ∗ #

(10)

⁄ = ) ∗ # − ) ∗

(11)

⁄ = ) ∗ 8 ACS Paragon Plus Environment

(12)

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With initial conditions at = 0, = ∗ + , " = " ∗ + , # = 0, = 0, = 0. Where Xf: fast-xylan

Xs: slow-xylan

O: xylose oligomer

X: xylose monomer

F: furfural

Ff: fraction of fast-xylan

Fs: fraction of slow-xylan

X0: initial xylan

Solving these linear differential equations with their respective initial conditions, the solutions for each component as function of reaction time are obtained as follows:

# () = () =

FC ∗ GH ∗ C

I / C

FC ∗ GH ∗ C ∗ I

( I / C )( L / C )

− M

() = ∗ + ∗ / C ∗D

(13)

" () = " ∗ + ∗ / E ∗D

(14)

J / C ∗D − / I ∗D K +

FE ∗ GH ∗ E

I / E

( / E ∗D − / I ∗D )

F ∗ G ∗ ∗

(15)

J / C ∗D − / L ∗D K + ( E/ H)( E / I ) J / C ∗D − / L ∗D K

I

L / I

I

NO

FC ∗ GH ∗ C

I / C

+

FE ∗ GH ∗ E

I / E

E

L

E

P ( / I ∗D − / L ∗D )

(16)

The total amount of xylan remaining in the solids residue can be expressed as: Q = + " = ∗ + ∗ / C ∗D + (1 − ) ∗ + ∗ / E ∗D Where XR: total xylose equivalents remaining in the solids residue.


(17)


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The experimental data were fitted to the proposed kinetic model using Nonlinear Regression Program in Mathematica. The experimental data of remaining xylan in solids were first fitted to Eq. (17) to determine Ff, kf and ks simultaneously. The average of the nine Ff values thus obtained was used for subsequent regression. The rate constants were calculated by nonlinear regression procedure given in Mathematica. After acquiring the rate constants at each set of reaction conditions (temperature and acid concentration), the kinetic parameters within Arrhenius equation (k0, n and E) were determined by multiple linear regression using Eq. 18. ()) = ()+ ) + ∗ (-) – S/QT

(18)

Results and Discussion Neutralization Capacity Lignocellulosic biomass has a significant amount of mineral content. It is well known that these minerals can neutralize some of the acid during dilute acid pretreatment, reducing the overall acidity 14, 30. The ash in biomass contains cations such as potassium, calcium, magnesium, sodium, manganese, and ammonium; possible anions are sulfates, phosphates, chloride and nitrate 35-37. When combined with biomass, mineral acids such as H2SO4 are neutralized through an ion-exchange reaction between inorganic cations and hydronium ions35. The neutralizing capacity of biomass has a minor effect at low solid loading and/or high acid loading conditions. However, with high solid loading, it becomes a factor that can significantly reduce the effective acid catalyst level. For sulfuric acid, an equilibrium shift to formation of bisulfate during neutralization can further reduce hydrogen ion concentrations and compound the effect of neutralization

38

. Because the equilibrium shift has a more pronounced effect at lower acid

concentrations, additional acid is needed to compensate. Coupled with the effect of temperature 10 ACS Paragon Plus Environment

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on acid dissociation, these effects increase acid requirements to achieve a particular reaction rate unless minerals are removed prior to hydrolysis. Neutralization of sulfuric acid by buffering action by minerals in biomass reduces the hydrogen ion activity and must be taken into consideration in kinetic models involving acid-catalyzed reactions such as dilute-acid hemicellulose hydrolysis. By the procedure of the experimental methods section, the NC of KCS was determined to be 0.0137, meaning that each gram of KCS neutralizes 0.0137 grams of H2SO4. This value was subtracted from the total acid input to calculate the effective H2SO4 in the dilute-acid pretreatment experiments. Model Fitness Significant amount of xylose oligomers was seen to accumulate in all of the pretreatment runs involving high solid loading. The maximum oligomer concentration was in the range of 41 53% of initial xylan. Presence of high oligomer content justifies inclusion of it as reaction intermediate in the reaction pathway of hemicellulose hydrolysis. It is also observed that xylose oligomer concentration increased at the early stage of the reaction and declined after reaching a maximum, a typical pattern of an intermediate product in a sequential reaction. The only exception was a run made under low temperature and low acid. These data collectively indicate that the proposed kinetic model of Eq. 7 is appropriate. Figure 1 shows the representative reaction pattern of acid hydrolysis of xylan in corn stover. It is a semi-log plot of remaining xylan in hemicellulose vs. reaction time, the straight line in the plot indicating the first-order reaction. The sharp breakage in the plots confirms that the xylan in KCS can indeed be taken as a biphasic substrate. This finding is in agreement with 11 ACS Paragon Plus Environment


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conventional kinetic model as described previously. One exception was the straight line at lowest temperature and lowest acid concentration (155 oC & 1% H2SO4). Under this condition, it is believed that 120 minutes of reaction time was not sufficient to remove the entire fast-xylan fraction. The percentage of the fast-xylan (Ff) in KCS estimated from the plots varied between 0.59 and 0.85 depending on the pretreatment conditions. There was no apparent relationship between the Ff and pretreatment condition. Therefore, the average value of 0.70 was chosen and applied throughout modeling study. This value is close to the value of 0.65 reported for corn stover by Esteghlalian et al

16

. We also note that the clear-cut two-part distinction of fast/slow

xylan is not a true representation of intrinsic property of xylan, but only a conceptual parameter applicable for kinetic model. The percentage data of fast-xylan in various biomass species from literature are shown in Table 1. The biphasic behavior of hemicellulose can be explained in many ways. First, a portion of xylan is located in the cell wall and is easily accessible to the reagent, while the remaining xylan is located at a greater depth and is firmly retained within cellulose chains. Second, the slow reaction is due to a part of xylan that is embedded within or attached to the lignin by lignin-carbohydrate bonds

39

.

Third, the difference in the hydrolysis rate is

attributed to the variation in the polymeric structure of xylan as the acetyle and uronic acid ratios to xylose change 40. Each set of experimental data were fitted to Eq. (13) to (17) to statistically estimate ki’s for each run. The rate constants thus determined for each of the conditions are shown in Table 2. The statistical output for each of the rate constant shows that all of the p-Values are far below 0.01 and the standard errors are very small (data not shown), which proves our regression results are valid. The data indicate that the xylan hydrolysis rates are much higher than xylose degradation rates, which implies that high yield of xylose and its oligomers are expected within 12 ACS Paragon Plus Environment

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the pretreatment conditions explored. Figure 2 to 4 show the comparison between the experimental data and model prediction. These plots are representatives ones out of nine sets of experimental runs made in this study. The model predictions have shown good agreement with the experimental data confirming the validity of the model. From the experimentally determined rate constants the Arrhenius parameters (preexponential factor, activation energy and the acid exponent) were determined by the procedure described in the Methods section. The statistically determined activation energy and acid exponent for each individual reaction are listed in Table 3. The R2-value for each of the parameter is high (0.96 - 0.99), indicating a fine fit to the experimental data. Samples of the Arrhenious plots using these parameters are shown in Figures 5 (activation energy plots) and Figure 6 (acid exponent plots). It is important to note that the lines and the points in these plots are independently determined; the line not being the best-fit of the three kinetic constant. The activation energies and the acid exponents (the slope of the line in the plot) are determined using all nine points, not the three points for a given acid concentration or temperature. Such model values are again seen to be in reasonable agreement with the experimental ki’s. Table 4 summarizes the comparison of kinetic parameters experimentally determined in this study with the values reported in the literature. A direct comparison of the kinetic parameters from this study with the ones from literature is difficult because of the differences in kinetic models, substrates materials, and experimental conditions. The hydrolysis of slow xylan fraction requires more energy since it has higher activation energy value than the fast hydrolysis step. This is in line with the findings of previous research along these lines

13, 15, 16

. Discrepancy was

found in both activation energy and the acid exponent between this study and those of previous work reported for corn stover; activation energies of kf, ks and k2determined in this study are low 13 ACS Paragon Plus Environment


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compare to literature values and the acid exponent are higher

13, 16, 26

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. We believe that the

discrepancy is primarily due to the difference in solid loading, which is 3-5 times higher in this study than previous investigation. We also note that formation of xylose oligomers was not taken into consideration in the reaction pathway in most of the previous studies. The model difference also brings about significant differences in the kinetic parameter values. Also to be noted in this result is that all of the hydrolysis reactions are more sensitive to temperature and acid concentration than the decomposition reaction as indicated by the magnitude of activation energies and acid exponents. Thus, the higher the temperature and acid concentration, the higher yield of xylose is expected. Schell et al., was the only one reported the parameters of hemicellulose hydrolysis in corn stover using the comprehensive model which includes both the biphasic pattern and oligomers as intermediate 26. But the acid exponent was not applied in their parameter analysis. Conclusion The reaction kinetics of acid-catalyzed hydrolysis of xylan in corn stover was investigated under high-solid condition. The kinetic pattern under high-solids condition is different from that of low-solid condition in that the significant amount of oligomers accumulates, thus included as a reaction intermediate. The kinetic parameter values of the proposed model were noticeably different from those previously reported partly due to the model difference. The activation energies of xylan hydrolysis are lower and the acid exponents are higher than the average of literature values. Yet both of these parameters for hydrolysis reactions are higher than those of decomposition reaction, indicating that the hydrolysis reactions are more sensitive to temperature and acid concentration than the decomposition reaction. Thus, the higher the temperature and acid concentration, the higher yield of xylose is expected. The validity of the 14 ACS Paragon Plus Environment

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proposed kinetic model was verified by the experimental data. The proposed model with provision of fitted parameters can therefore serve as a tool to predict the reaction progress during dilute-acid pretreatment of corn stover under high-solid condition.



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Acknowledgements The authors gratefully acknowledge the financial support provided for this research by the National Renewable Energy Laboratory (Subcontract LCO-9-99343-01) and by Alabama Center for Paper and Bioresource Engineering.


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References 1. Kadam, K. L.; McMillan, J. D., Availability of corn stover as a sustainable feedstock for bioethanol production. Bioresource Technology 2003, 88, (1), 17-25. 2. Lau, M. W.; Dale, B. E., Cellulosic ethanol production from AFEX-treated corn stover using Saccharomyces cerevisiae 424A (LNH-ST). Proceedings of the National Academy of Sciences 2009, 106, (5), 1368-1373. 3. Öhgren, K.; Rudolf, A.; Galbe, M.; Zacchi, G., Fuel ethanol production from steam-pretreated corn stover using SSF at higher dry matter content. Biomass and Bioenergy 2006, 30, (10), 863-869. 4. Kazi, F. K.; Fortman, J. A.; Anex, R. P.; Hsu, D. D.; Aden, A.; Dutta, A.; Kothandaraman, G., Techno-economic comparison of process technologies for biochemical ethanol production from corn stover. Fuel 2010, 89, S20-S28. 5. Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y.; Holtzapple, M.; Ladisch, M., Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresource technology 2005, 96, (6), 673-686. 6. Zhang, Y.-H. P.; Ding, S.-Y.; Mielenz, J. R.; Cui, J.-B.; Elander, R. T.; Laser, M.; Himmel, M. E.; McMillan, J. R.; Lynd, L. R., Fractionating recalcitrant lignocellulose at modest reaction conditions. Biotechnology and Bioengineering 2007, 97, (2), 214-223. 7. Torget, R. W.; Kim, J. S.; Lee, Y., Fundamental aspects of dilute acid hydrolysis/fractionation kinetics of hardwood carbohydrates. 1. Cellulose hydrolysis. Industrial & engineering chemistry research 2000, 39, (8), 2817-2825. 8. Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y., Coordinated development of leading biomass pretreatment technologies. Bioresource technology 2005, 96, (18), 1959-1966. 9. Garrote, G.; Domı́nguez, H.; Parajo, J. C., Kinetic modelling of corncob autohydrolysis. Process Biochemistry 2001, 36, (6), 571-578. 10. Saeman, J. F., Kinetics of wood saccharification-hydrolysis of cellulose and decomposition of sugars in dilute acid at high temperature. Industrial & Engineering Chemistry 1945, 37, (1), 43-52. 11. Kobayashi, T.; Sakai, Y., Hydrolysis rate of pentosan of hardwood in dilute sulfuric acid. Journal of the Agricultural Chemical Society of Japan 1956, 20, (1), 1-7. 12. Conner, A. H., Kinetic modeling of hardwood prehydrolysis. Part 1. Xylan removal by water prehydrolysis. Wood Fiber;(United States) 1984, 16, (2). 13. Maloney, M. T.; Chapman, T. W.; Baker, A. J., Dilute acid hydrolysis of paper birch: Kinetics studies of xylan and acetyl‐group hydrolysis. Biotechnology and Bioengineering 1985, 27, (3), 355-361. 14. Grohmann, K.; Torget, R.; Himmel, M. In Optimization of dilute acid pretreatment of biomass, Biotechnology and bioengineering symposium, 1986; Wiley: 1986; pp 59-80. 15. Kim, S.; Lee, Y., Kinetics in acid-catalyzed hydrolysis of hardwood hemicellulose. 1986. 1986 16. 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. Bioresource Technology 1997, 59, (2-3), 129-136. 17. Nabarlatz, D.; Farriol, X.; Montane, D., Kinetic modeling of the autohydrolysis of lignocellulosic biomass for the production of hemicellulose-derived oligosaccharides. Industrial & engineering chemistry research 2004, 43, (15), 4124-4131. 18. Abatzoglov, N.; Bouchard, J.; Chornet, E.; Overend, R., Dilute acid depolymerization of cellulose in aqueous phase: experimental evidence of the significant presence of soluble oligomeric intermediates. The Canadian Journal of Chemical Engineering 1986, 64, (5), 781-786.



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19. Abatzoglou, N.; Chornet, E.; Belkacemi, K.; Overend, R. P., Phenomenological kinetics of complex systems: the development of a generalized severity parameter and its application to lignocellulosics fractionation. Chemical Engineering Science 1992, 47, (5), 1109-1122. 20. Liu, C.; Wyman, C. E., The effect of flow rate of very dilute sulfuric acid on xylan, lignin, and total mass removal from corn stover. Industrial & engineering chemistry research 2004, 43, (11), 2781-2788. 21. Morinelly, J. E.; Jensen, J. R.; Browne, M.; Co, T. B.; Shonnard, D. R., Kinetic characterization of xylose monomer and oligomer concentrations during dilute acid pretreatment of lignocellulosic biomass from forests and switchgrass. Industrial & engineering chemistry research 2009, 48, (22), 9877-9884. 22. Lee, Y.; McCaskey, T., Hemicellulose hydrolysis and fermentation of resulting pentoses to ethanol. Tappi;(United States) 1983, 66, (5). 23. Conner, A. H.; Lorenz, L. F., Kinetic modeling of hardwood prehydrolysis. Part III. Water and dilute acetic acid prehydrolysis of southern red oak. Wood Fiber Sci 1986, 18, (2), 248-263. 24. Jensen, J.; Morinelly, J.; Aglan, A.; Mix, A.; Shonnard, D. R., Kinetic characterization of biomass dilute sulfuric acid hydrolysis: Mixtures of hardwoods, softwood, and switchgrass. AIChE journal 2008, 54, (6), 1637-1645. 25. Mehlberg, R.; Tsao, G. In Low liquid hemicellulose hydrolysis of hydrochloric acid, 178th ACS National Meeting, Washington, DC, 1979; 1979. 26. Schell, D. J.; Farmer, J.; Newman, M.; McMILLAN, J. D., Dilute-sulfuric acid pretreatment of corn stover in pilot-scale reactor. In Biotechnology for Fuels and Chemicals, Springer: 2003; pp 6985. 27. Shen, J.; Wyman, C. E., A novel mechanism and kinetic model to explain enhanced xylose yields from dilute sulfuric acid compared to hydrothermal pretreatment of corn stover. Bioresource Technology 2011, 102, (19), 9111-9120. 28. Jacobsen, S. E.; Wyman, C. E., Xylose monomer and oligomer yields for uncatalyzed hydrolysis of sugarcane bagasse hemicellulose at varying solids concentration. Industrial & engineering chemistry research 2002, 41, (6), 1454-1461. 29. Crocker, D. Determination of structural carbohydrates and lignin in biomass; NREL/TP-51042618 NREL Laboratory Analytical Procedure. National Renewable Energy Laboratory, Golden. http://www. nrel. gov/biomass/pdfs/42618. pdf: 2008. 30. Malester, I. A.; Green, M.; Shelef, G., Kinetics of dilute acid hydrolysis of cellulose originating from municipal solid wastes. Industrial & engineering chemistry research 1992, 31, (8), 1998-2003. 31. Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D., Determination of sugars, byproducts, and degradation products in liquid fraction process samples. National Renewable Energy Laboratory 2008. 2008 32. Bunnell, K.; Lau, C.-S.; Lay Jr, J. O.; Gidden, J.; Carrier, D. J., Production and fractionation of xylose oligomers from switchgrass hemicelluloses using centrifugal partition chromatography. Journal of Liquid Chromatography & Related Technologies 2015, 38, (7), 801-809. 33. Yat, S. C.; Berger, A.; Shonnard, D. R., Kinetic characterization for dilute sulfuric acid hydrolysis of timber varieties and switchgrass. Bioresource technology 2008, 99, (9), 3855-3863. 34. Jensen, J. R.; Morinelly, J. E.; Gossen, K. R.; Brodeur-Campbell, M. J.; Shonnard, D. R., Effects of dilute acid pretreatment conditions on enzymatic hydrolysis monomer and oligomer sugar yields for aspen, balsam, and switchgrass. Bioresource technology 2010, 2010, 101, (7), 2317-2325. 35. Canettieri, E. V.; Rocha, G. J.; Carvalho, J. A.; Silva, J. B., Evaluation of the kinetics of xylose formation from dilute sulfuric acid hydrolysis of forest residues of Eucalyptus grandis. Industrial & Engineering Chemistry Research 2007, 46, (7), 1938-1944. 36. Springer, E. L.; Harris, J. F., Procedures for determining the neutralizing capacity of wood during hydrolysis with mineral acid solutions. Industrial & engineering chemistry product research and development 1985, 24, (3), 485-489. 18 ACS Paragon Plus Environment

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37. Chen, S.-F.; Mowery, R. A.; Scarlata, C. J.; Chambliss, C. K., Compositional analysis of watersoluble materials in corn stover. Journal of agricultural and food chemistry 2007, 55, (15), 59125918. 38. Lloyd, T. A.; Wyman, C. E. In Predicted effects of mineral neutralization and bisulfate formation on hydrogen ion concentration for dilute sulfuric acid pretreatment, Proceedings of the Twenty-Fifth Symposium on Biotechnology for Fuels and Chemicals Held May 4–7, 2003, in Breckenridge, CO, 2004; Springer: 2004; pp 1013-1022. 39. Conner, A. H., Kinetic modeling of hardwood prehydrolysis. Part I. Xylan removal by water prehydrolysis. Wood and fiber science 2007, 16, (2), 268-277. 40. Nikitin, N. I. e., The chemistry of cellulose and wood. 1966. 1966 41. Simmonds, F.; Kingsbury, R.; Martin, J., Purified hardwood pulps for chemical conversion. II. Sweetgum prehydrolysis-sulfate pulps. Tappi 1955, 38, (3), 178-186. 42. Springer, E. L.; Harris, J.; Neill, W., Rate studies of the hydrotropic delignification of aspenwood. Tappi 1963, 46, (9), 551. 43. Springer, E.; Zoch, L., Hydrolysis of xylan in different species of hardwoods. Tappi 1968, 51, (5), 214.



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Figure Captions Figure 1 Hydrolysis of xylan in KCS during dilute acid pretreatments (S/L = 1:2) .......................................... 25 Figure 2 Experimental and fitted data profiles of KCS hydrolysis, 155 oC and 1% H2SO4 ............................ 26 Figure 3 Experimental and fitted data profiles of KCS hydrolysis, 170 oC and 1.5% H2SO4 ......................... 27 Figure 4 Experimental and fitted data profiles of KCS hydrolysis, 185 oC and 2% H2SO4 ............................ 28 Figure 5 Activation energy plots for rate constant ki ................................................................................................... 30 Figure 6 Acid exponent plots for rate constant ki .......................................................................................................... 32


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Tables Table 1 Fast-xylan fraction in various biomass species References

Feedstock

Kobayashi & Sakai 11

Buna

Simmonds et al. 41 Springer et al. 42

Springer & Zoch 43 Conner 12

Maloney et al. 13

Grohmann et al. 14 Kim & Lee 15

Esteghlalian et al. 16 Schell et al. 26 This study

Sweetgum Aspen

Aspen, Birch, Elm, Maple Southern red oak Paper birch Red maple Quaking aspen American elm Paper birch Aspen Wheat straw Southern red oak Corn stover Switchgrass Popler Corn stover Corn stover


Fast-xylan fraction (%) 70.0 70.0 60.0 60.0 73.9 71.6 80.3 76.0 84.3 68.4 76.0 67.0 69.7 64.4 76.8 83.8 72.0 70.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

Table 2 Fitted kinetic constants during the hydrolysis at various pretreatment condition T (oC) 155 170 185 155 170 185 155 170 185

H2SO4 (%) 1 1 1 1.5 1.5 1.5 2 2 2

kf

ks

0.0277 0.1331 0.2185 0.0803 0.3163 0.5811 0.2850 0.7140 1.6778

0.0070 0.0080 0.0300 0.0038 0.0241 0.0372 0.0106 0.0559 0.1265


k1

0.0094 0.0390 0.0707 0.0233 0.0650 0.0968 0.0707 0.1187 0.3673

k2

0.0124 0.0518 0.1100 0.0134 0.0306 0.0443 0.0173 0.0273 0.0687

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Table 3 Fitted Arrhenius parameters for each rate constant ki

kf ks k1 k2

k0 (min-1)

n

E (kJ mol-1)

R2

3.28E+11

2.15

105.63±2.19

0.99

2.07E+09

2.02

92.09±1.66

0.96

1.96E+11 1.47E+08

1.68

112.49±2.87

0.88

82.82±1.92


0.98 0.97


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

Table 4 Comparison of parameters results of this study with the literature values Reference Kim & Lee

Feedstock 15

Maloney et al. 13 Esteghlalian et al. 16

k0 (min-1)

Parameter n

E (kJ mol-1)

kf ks k2

1.04 x 1014 6.00 x 1012 1.77 x 1011

1.54 1.19 1.07

120.1 118.0 112.5

kf ks

2.27 x 1016 1.16 x 1019

-

126.7 156.6

kf ks k2

1.9 x 1021 4.2 x 1023 3.8 x 1010

0.4 2.0 1.45

169.0 210.7 99.5

kf ks k2

3.3 x 1021 3.3 x 1022 8.5 x 1010

0.4 1.5 0.55

176.7 192.0 102.0

kf ks k2

6.7 x 1016 6.9 x 1019 3.7 x 1010

1.5 1.6 0.5

129.8 167.6 98.4

kf ks k1 k2

2.61 x 1025 1.00 x 1015 1.03 x 1028 1.00 x 1015

-

240.32 250.79 245.35 139.84

kf ks k1 k2

3.28 x 1011 1.96 x 1011 2.07 x 109 1.47 x 108

2.15 1.68 2.02 0.88

105.63 112.49 92.09 82.82

Southern red oak

Paper birch Switchgrass

Poplar

Corn stover

Schell et al. 26

This study

Corn stover

Corn stover


Page 25 of 33

Figures

0.0 155 C, 1% acid 185 C, 1% acid 170 C, 1.5% acid 185 C, 2% acid

-0.5

LN (Remaining Xylan, w/w)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.0

-1.5

-2.0

-2.5

-3.0 0

20

40

60

80

100

120

time (min) Figure 1 Hydrolysis of xylan in KCS during dilute acid pretreatments (S/L = 1:2)



100

% Initial Xylan (Xylose Equivalent)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

Remaining Xylan Xylose Xylose Oligomer

80

60

40

20

0 0

20

40

60

80

100

120

140

time (min) Figure 2 Experimental and fitted data profiles of KCS hydrolysis, 155 oC and 1% H2SO4


Page 27 of 33

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



80

60

40

20

0 0

10

20

30

40

50

60

70

time (min) Figure 3 Experimental and fitted data profiles of KCS hydrolysis, 170 oC and 1.5% H2SO4



100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33


80

60

40

20

0 0

5

10

15

20

25

time (min) Figure 4 Experimental and fitted data profiles of KCS hydrolysis, 185 oC and 2% H2SO4


Page 29 of 33

(a) 1

2 % acid 1.5 % acid 1 % acid

0

LN kf

-1

-2

-3

2.20

2.24

2.28

2.32

1/T x 1000

(b) -2

2 % acid 1.5 % acid 1 % acid

-3

LN ks

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

-5

-6 2.20

2.24

2.28

1/T x 1000


2.32


(c) -1

2 % acid 1.5 % acid 1 % acid -2

LN k1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-3

-4

2.20

2.24

2.28

2.32

1/T x 1000

Figure 5 Activation energy plots for rate constant ki (a) activation energy plots for kf (b) activation energy plots for ks (c) activation energy plots for k1


Page 30 of 33

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(a) 1

185 C 170 C 155 C

0

LN kf

-1

-2

-3

0.0

0.2

0.4

0.6

0.8

0.6

0.8

LN (acid concentration)

(b) -1

185 C 170 C 155 C

-2

-3

LN ks

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

-5

-6

-7 0.0

0.2

0.4




(c) 0

185 C 170 C 155 C

-1

-2

LN k1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

-3

-4

-5

-6 0.0

0.2

0.4

0.6


Figure 6 Acid exponent plots for rate constant ki (a) acid exponent plots for kf (b) acid exponent plots for ks (c) acid exponent plots for k1


0.8

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For Table of Contents Only


Reaction Kinetic Model of Dilute Acid-Catalyzed Hemicellulose

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