Detailed Mechanism of Xylose Decomposition in Near-Critical and

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Detailed mechanism of xylose decomposition in near critical and supercritical water Nattacha Paksung, Ren Nagano, and Yukihiko Matsumura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00918 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 15, 2016

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Detailed mechanism of xylose decomposition in near critical and supercritical water Nattacha Paksung1, Ren Nagano1, Yukihiko Matsumura2* 1

Department of Mechanical Sciences and Engineering, Hiroshima University

2

Division of Energy and Environmental Engineering, Hiroshima University

*

To whom correspondence should be addressed. 1-4-1 Kagamiyama, Higashi-Hiroshima 7398527 Japan, Fax: +81-82-422-7193. E-mail: [email protected].

Abstract: The aim of this study was to determine the complete reaction network of xylose decomposition in subcritical and supercritical water, including small-molecule intermediates like organic acids, which are thought to be the final intermediates in the formation of gaseous products. Solutions of xylose in water were heated under sub- and supercritical conditions in the temperature range 350–450 °C in a continuous reactor at a controlled pressure of 25 MPa. The intermediates found in the liquid phase were xylulose, furfural, retro-aldol products (glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde), and organic acids (acetic acid and formic acid). The reaction types involved were classified according to Arrhenius behavior: the ionic reaction (not showing Arrhenius behavior in the supercritical region) and the free-radical reaction (showing Arrhenius behavior in the supercritical region). Formic acid was the final intermediate before gasification, while acetic acid and formaldehyde were not gasified in the temperature range of this study. Keywords: xylose, supercritical water gasification (SCWG), biomass, reaction network, organic acid

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Nomenclatures kij

rate constant of reaction ij [s−1]

nC (X )

amount of carbon in product X [mol]

nC 0

initial amount of carbon in xylose feedstock [mol]

t

residence time [s]

YC (X )

product yield of compound X [-]

Subscript ag

gasification of acetic acid

dgl

isomerization of dihydroxyacetone to glycolaldehyde

dt

decomposition of dihydroxyacetone

fag

gasification of formic acid

fog

gasification of formaldehyde

ft

decomposition of furfural to TOC

gct

decomposition of glycolaldehyde to TOC

gld

isomerization of glycolaldehyde to dihydroxyacetone

glgc

retro-aldol condensation of glyceraldehyde to glycolaldehyde and formaldehyde

glt

decomposition of glyceraldehyde to TOC

ta

decomposition of TOC to acetic acid

tfa

decomposition of TOC to formic acid

tg

gasification of TOC

xf

dehydration of xylose to furfural


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xgl

retro-aldol condensation of xylose to glyceraldehyde and glycolaldehyde

xt

decomposition of xylose to TOC

xxy

isomerization of xylose to xylulose

xyf

dehydration of xylulose to furfural

xygc

retro-aldol condensation of xylulose to glycolaldehyde and dihydroxyacetone

xyt

decomposition of xylulose to TOC

xyx

isomerization of xylulose to xylose

1. Introduction Supercritical water gasification (SCWG) is a promising technology for converting wet biomass into fuel gas because water is both a reactant and the reaction medium of the process. Thus, no drying treatment before use of the wet biomass in this technology is necessary, which results in savings on operating costs. Supercritical water is hot compressed water, whose temperature and pressure are above the critical values of 374 °C and 22.1 MPa, respectively (1). Motivated by the achievements of Michael J. Antal

(2)- (6)

, who initiated research in the field of

supercritical water utilization for biomass gasification, many researchers (including our research group), have been interested in exploring different approaches to the SCWG of biomass . Many actual biomasses have been studied, along with the effects of various factors, such as type of feedstock, temperature, pressure, catalyst, and feedstock concentration on the characteristics of the gasification products. Plant biomass, also known as lignocellulosic biomass, has always been a source for hydrogen-rich gas production due to its less complex constituents (mainly organic elements consisted of C, H, and O). Biomass materials, such as wood sawdust,


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rice straw, rice shell, wheat stalk, peanut shell, corn stalk, corn cob and sorghum stalk were parametrically investigated in the work of Guo’s group

(7)- (10)

. The by-products or waste from

industry, such as glycerol (11), black liquor (12)-(13), stockbreeding waste (14) and sewage sludge (15) can also be converted to energy. Another feedstock is microalgae, which is also rapidly gaining the attention of researchers

(16)

. A detailed investigation of the reactions in algae was done by

Guan et al (17)- (18). Yanik et al. have studied the effect of the nature of the biomass, as well as the effect of catalysts, on real biomass conversion (19)-(20). They used various kinds of feedstock with different components and compositions. They found that even slight differences in the compounds in the feedstock could affect the product distribution. Using model compounds to elucidate the reaction schemes can help us understand the more complex reaction schemes in real biomass. For lignocellulosic biomass, glucose was employed as a model compound of cellulose. Its decomposition behavior had been investigated by many research groups and the reactions that take place have been elucidated (21)-(24). However, not every compound enhances gasification. Char and tarry material formation is undesirable because these products can block the reactor. 5-Hydroxymethyl furfural (5-HMF), which is an intermediate in glucose decomposition, is believed to induce char and tar formation

(25)-(27)

. Our

group investigated the char formation mechanism, both under subcritical and supercritical conditions, and found that the char formation occurred in the subcritical range, and its rate of formation increases with temperature and residence time

(28)-(31)

. Although, phenolic and

aromatic compounds in lignin potentially form char, competitive gas formation can occur

(32)

.

Another type of biomass, for example protein, was also studied using amino acids as model compounds (33).


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Some of Antal’s work reported on xylose decomposition at a lower temperature of 250 °C and suggested some major products in the liquid effluent. The two major acids formed under these conditions were formic acid and lactic acid

(34)

. His other work was on the

investigation of the reaction pathways of lactic acid in supercritical water: decarbonylation, decarboxylation, and dehydration (35). In our previous study, we employed a model compound in the kinetic study of lignocellulosic biomass in supercritical water. Xylose was used as a model for hemicellulose, and its intermediates were investigated (36). The reactions occurring during the SCWG process with xylose are mainly retro-aldol condensation reactions and, to a less extent, the dehydration to furfural, which is in good agreement with research done by Aida et al. (37) It is to be noted that Goodwin and Rorrer predicted the production of liquid intermediates and gas using a xylose model for the decomposition kinetics

(38)

. However, the other liquid products,

specified as total organic carbon (TOC) were still found in significant amounts. Those products could possibly be organic acids, presumably the decomposition products of xylose and other intermediates. Thus, overall reaction network for xylose decomposition in supercritical water is close at hand, and will complete the work Antal started. Therefore, the purpose of this study was to define the complete reaction network of xylose decomposition in subcritical and supercritical water as an extension of Antal’s work. In addition, a mechanism for each reaction was investigated based on the types of reaction; freeradical reaction (showing Arrhenius behavior in the supercritical region) and ionic reactions (not showing Arrhenius behavior in the supercritical region). 2. Experiment A continuous reactor made of SS316 stainless steel was employed in this work. The reactor scheme and operating procedure has been described previously


(25)

. The experimental

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conditions are shown in Table 1. In brief, a 7.5 wt % of xylose (obtained from Nacalai Tesque, purity >98 %) solution was prepared and fed into the reactor to be mixed with preheated water at a ratio of 1:4. All experimental runs were conducted under the pressure of 25 MPa, controlled by a back-pressure regulator. The temperature was varied between 350 and 450 °C, which covered both subcritical and supercritical regions. The residence time was controlled by the flow rate. Water at room temperature was fed to the exit of the reactor to quench the products and immediately terminate the reaction. Solid products were separated by inline filters, and then the liquid and gas species were collected at the final sampling ports. The gaseous product was quantitatively analyzed by gas chromatography (GC, Shimadzu GC-14B); CO2 and CO were detected by thermal conductivity detector (TCD) using He as the carrier gas; CH4, C2H4, and C2H6 were detected by GC equipped with flame ionization detector (FID) using He as the carrier gas; H2 was detected by TCD using N2 as the carrier gas. A total organic carbon (TOC) analyzer (Shimadzu TOC-V CHP) was used to quantify the amounts of carbon compounds in the feedstock, liquid product and dissolved gaseous products to confirm a carbon balance in each experiment. High-performance liquid chromatography (HPLC) was used to identify compounds in the liquid effluent using different methods depending on the chemical and employing an appropriate standard. An SCR102H column (Shimadzu), with 0.005 M HClO4 aqueous solution as the mobile phase (column temperature = 40 °C, flow rate = 0.7 ml/min, RID detector), was used for analyzing xylose, xylulose, glyceraldehyde, glycolaldehyde, formaldehyde, and acetic acid; an RSpak DE-413L column (Shodex) with 0.01 M H3PO4 as the mobile phase (column temperature = 50 °C, flow rate = 0.7 ml/min, RID detector), for dihydroxyacetone, and formic acid; an RSpak DE-413L column (Shodex), with aqueous 0.005 M


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HClO4 solution and acetonitrile in a 1:1 volume ratio (column temperature = 40 °C, flow rate = 0.5 ml/min, SPD detector), for furfural. 3. Results and discussion 3.1.

Product yield The product yield of compound X was evaluated based on the carbon amount using the

equation

YC ( X )[−] =

nC ( X ) nC 0

(1)

Where nC (X ) and nC 0 denote the amount of carbon in product X and that in the xylose feedstock, respectively.

Table 2 shows the carbon yields obtained for gaseous and liquid products; the solid phase was ignored in this study because it was rarely observed. The yield of gas products was comparatively low due to the very short residence time observed in this study. The gas yield increased slightly when the experiments were conducted at a temperature higher that the critical value of water (374 °C). 3.2. Intermediate compounds The intermediate compounds in the liquid phase are the key to understanding the mechanisms of the reactions of biomass in supercritical water. In our study, xylulose was found as an isomerized product of xylose. These reactions are similar to those of glucose and fructose, which were suggested in a previous study when glucose was decomposed in supercritical water (28)

. This similarity could be the result of the homologous molecular structures of xylose and

glucose, except that xylose has one less CH(OH). Therefore, the decomposition mechanism of xylose is probably similar to that of glucose. A reaction converting xylose to furfural is


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considered to be analogous to the dehydration of glucose to 5-HMF. In addition, retro-aldol condensation reactions play important roles under supercritical conditions

(36)-(38)

. Additionally,

small molecules such as organic acids are thought to be the final intermediates before gas formation. Contrary to the reports of Antal (34)-(35), lactic acid and its derivatives, acrylic acid and propionic acid, were not found in our study probably due to the higher temperature employed in this study, but formic acid and acetic acid could be detected. In summary, the intermediates found in the liquid phase were xylulose, furfural, retro-aldol products (glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde), and organic acids (acetic acid and formic acid). The other products were specified as TOCs, which represented unknown products in the liquid effluent. 3.3. Kinetic model Based on the intermediate compounds mentioned above, the detailed reaction kinetic model for the decomposition of xylose in supercritical water proposed in this study is presented in Fig. 1. All the reactions were assumed to be the first order. Hence, the rate equations can be written as follows:

dYc (xylose) = k xyx Yc (xylulose) − (k xxy + k xf + k xgl + k xt )Yc (xylose) dt dYc (xylulose) = k xxy Yc (xylose) − (k xyx + k xyf + k xygc + k xyt )Yc (xylulose) dt dYc (furfural) = k xf Yc (xylose) + k xyf Yc (xylulose)- k ft Yc (furfural) dt dYc (glyceraldehyde) 3 = k xgl Yc (xylose) + k dgl Yc (dihydroxyacetone) dt 5 - (k glgc + k glt )Yc (glyceraldehyde)

(1) (2) (3) (4)

dYc (glycolaldehyde) 2 2 2 = k xgl Yc (xylose) + k xygc Yc (xylulose) + k glgc Yc (glyceraldehyde) (5) dt 5 5 3 - k gct Yc (glycolaldehyde)


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dYc (formaldehyde) 1 = k glgcYc (glyceraldehyde) − k fog Yc (formaldehyde) dt 3 dYc (dihydroxyacetone) 3 = k xygc Yc (xylulose) + k gld Yc (glyceraldehyde) dt 5 - (k dgl + k dt )Yc (dihydroxyacetone) dYc (formic) = k tfa Yc (TOC) - k fag Yc (formic) dt dYc (acetic) = k ta Yc (TOC) − k ag Yc (acetic) dt dYc (TOC) = k xt Yc (xylose) + k xyt Yc (xylulose) + k ft Yc (furfural) dt

+ k glt Yc (glyceraldehyde) + k gct Yc (glycolaldehyde)

(6) (7)

(8) (9)

(10)

+ k dt Yc (dihydroxyacetone) - (k tg + k fa + k ta )Yc (TOC) dYc (gas) = k fog Yc (formaldeh yde) + k fag Yc (formic) + k ag Yc (acetic) + k tg Yc (TOC) dt

(11)

Where Yc, t, and k denote the carbon yield of each compound, reaction time, and reaction rate constant, respectively. The kinetic reaction rates were determined by the least square of error (LSE) method that gave the best fitting of the empirical data and the calculated values from Eqs. 1 to 11. The calculated kinetic rate constants of all reactions are summarized in Table 3. The best fittings are shown in Fig. 2 and Fig. 3. Fig. 2 shows xylose decomposition was comparatively slower at a temperature of 350 °C than at temperatures above the critical temperature of water (400 °C and 450 °C). The same trend was found with a xylulose yield. Furfural, a dehydration product of xylose and its isomer, xylulose, was produced in lower yield when the temperature was increased. This indicates that the dehydration reaction is favored at subcritical temperatures, where the ionic product is high. Hence, it could be concluded that the dehydration reaction is an ionic reaction. As for the yields of the products from retro-aldol condensation reactions – glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde – the highest yield was found with glycolaldehyde. This is because glycolaldehyde is a derivative of the retro-aldol condensation


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reaction of xylose, which also produces glyceraldehyde. The glyceraldehyde is further decomposed via the same mechanism to glycolaldehyde and formaldehyde. In addition, xylulose can also react through the same mechanism to form glycolaldehyde as well. Fig. 3 shows the yield of TOC, gas, acetic acid, and formic acid. The highest yield of formic acid was found at a temperature of 350 °C; the yield of formic acid decreased dramatically at higher temperatures until the product yield was zero. On the other hand, the TOC yield dominates at supercritical temperatures, which indicates that some other unknown reactions were preferred under these conditions. At subcritical temperatures, the gasification of formic acid was apparently dominant. The gasification rate of formic acid is supposed to increase with temperature because formic acid is gasified by a free-radical mechanism, promoted by high temperature and low density in supercritical water

(39)

. However, the yield of formic acid was so low at supercritical

temperatures that accurate calculation of the gasification rate of formic acid was not possible. Further investigation of formic acid gasification alone is essential to explain its behavior. The gasification rates of formaldehyde and acetic acid were found to be zero at all the temperatures used in this study. It can be concluded that these compounds are stable species and are not further converted into gaseous products. 3.4. Arrhenius behavior of the reactions Previously, Promdej and Matsumura observed that the properties of water, namely the ion products, can be used to classify reaction types into ionic reactions and free-radical reactions (29)(30)

. The former depends on the ionic products under those conditions; it is disfavored when ion

products are lower, under supercritical conditions. The latter reaction type does not depend on this property. The suppression of ionic reactions makes free-radical reactions dominate under supercritical conditions. Fig. 4 shows the Arrhenius plots of each reaction in the proposed


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reaction network in the Fig. 1. Note that zero values of kinetic reaction constants were excluded from the plot because the logarithm value of zero is not defined. The ionic reactions show nonlinear plots for the following reactions: the isomerization between xylose and xylulose (kxxy, kxyx), the dehydration of xylose and xylulose to furfural (kxf, kxyf), and the isomerization between glyceraldehyde and dihydroxyacetone (kgld, kdgl). The decomposition reactions related to unknown intermediate compounds (kxt, kxyt, kglt, ktfa, kta) were classified as non-Arrhenius behavior too but their reaction mechanisms are unclear. In contrast, the free-radical reactions gave linear plots for the retro-aldol condensation reactions of: xylose to glyceraldehyde and glycolaldehyde (kxgl), xylulose to glycolaldehyde and dihydroxyacetone (kxygc), glyceraldehyde to glycolaldehyde and formaldehyde (kglgc), and decomposition of glycolaldehyde (kgct). The gasification of formic acid (kfag) cannot be predicted in this study because the product yield of formic acid was too low in the supercritical region for accuracy. The activation energies and pre-exponential factors of these reactions are shown in

Table 4. The other reaction constants were found to be zero for at all temperatures (kfog, kdt, kag), and it can be concluded that dihydroxyacetone, formaldehyde and acetic acid are stable and do not decompose further into gaseous products. A plausible explanation for the stagnation of acetic acid is that the thermal stability of acetic acid is high compared with other low-molecular weight carboxylic acids (40). The overall xylose decomposition is interesting in terms of comparison with previous studies. It was found to demonstrate Arrhenius behavior as shown in Fig. 5. The result from this study seems to be in a good agreement with the others (37), (38), (41).


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The main gasification source was formic acid when the temperature was below the critical temperature. The gasification rate constant of TOCs was only slightly higher at supercritical temperatures than at a subcritical temperature, suggesting that some unspecified compounds were formed instead of formic acid under supercritical conditions that does not easily form gas.

4. Conclusion Xylose was used as a model compound for hemicellulose and its decomposition in suband supercritical water using continuous reactor was elucidated. The detailed network of reactions in the decomposition of xylose in sub- and supercritical water was proposed to be mainly isomerization, dehydration, retro-aldol condensation reactions and gasification. The kinetic parameters of each reaction were calculated based on the assumption of first order kinetics for all reactions. The effect of temperature was used to classify the reactions into two types; the ionic reaction (not showing Arrhenius behavior in the supercritical region) and the free-radical reaction (showing Arrhenius behavior in the supercritical region). In the final reactions, formic acid and TOC were gasified, while other small molecules such as acetic acid and formaldehyde were not further gasified in this range of temperatures.

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(34) Antal, M. J.; Leesomboon, T.; Mok, W. S.; Richards, G. N. Carbohydr. Res. 1991, 217, 71–85. (35) Mok, W. S. L.; Antal, M. J.; Jones, M. J. Org. Chem. 1989, 54 (19), 4596–4602. (36) Paksung, N.; Matsumura, Y. Ind. Eng. Chem. Res. 2015, 54 (31), 7604–7613. (37) Aida, T. M.; Shiraishi, N.; Kubo, M.; Watanabe, M.; Smith, R. L. J. Supercrit. Fluids

2010, 55 (1), 208–216. (38) Goodwin, A. K.; Rorrer, G. L. Chem. Eng. J. 2010, 163 (1-2), 10–21. (39) Chakinala, A. G.; Kumar, S.; Kruse, A.; Kersten, S. R. A.; van Swaaij, W. P. M.; Brilman, D. W. F. J. Supercrit. Fluids 2013, 74, 8–21. (40) Quitain, A. T.; Faisal, M.; Kang, K.; Daimon, H.; Fujie, K. J. Hazard. Mater. 2002, 93 (2), 209–220. (41) Jing, Q.; Lü, X. Chin. J. Chem. Eng. 2007, 15 (5), 666–669.


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Page 16 of 33

Table Captions Table 1. Experimental conditions Table 2. Product yields for the xylose experiments at the pressure of 25 MPa with varying temperature and residence time

Table 3. Calculated kinetic rate constant of each reaction in the network Table 4. Activated energy and pre-exponential factor of Arrhenius reactions Figure Captions Figure 1. Detailed reaction network of xylose decomposition in sub- and supercritical water Figure 2. Product yield of xylose, xylulose, furfural, and retro-aldol products (glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde) at temperature of (a) 350 oC, (b) 400 oC, (c) 450 oC and pressure of 25 MPa as a function of residence time

Figure 3. Product yield of organic acids (acetic acid and formic acid), TOC and gas at temperature of (a) 350 oC, (b) 400 oC, (c) 450 oC and pressure of 25 MPa as a function of residence time

Figure 4. Arrhenius plot of (a) kxxy (b) kxf (c) kxgl (d) kxt (e) kxyx (f) kxyf (g) kxygc (h) kxyt (i) kft (j) kglgc (k) kgld (l) kglt (m) kgct (n) kdgl (o) ktfa (p) kta (q) ktg (r) kx

Figure 5. Arrhenius plot of total xylose decomposition comparing with previous study (37), (38), (41)


Page 17 of 33

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

Energy & Fuels

Tables and Figures Table 1. Experimental conditions Feedstock

d-xylose

Temperature

350 - 450 C

Pressure

25 MPa

Concentration of feedstock

7.5 wt%

Feedstock : water ratio by volume

1:4

Residence time

0.5-5 s

o


Energy & Fuels

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 18 of 33

Table 2. Product yields for the xylose experiments at the pressure of 25 MPa with varying temperature and residence time Temperature

Residence

(°C)

time (s)

Liquid

Gas

Solid

balance (-)

350

0.5

1.08

0.01

-

1.09

1

0.94

0.01

-

0.95

3

0.96

0.07

-

1.03

5

0.94

0.07

-

1.01

0.5

0.97

0.04

-

1.01

1

0.94

0.08

-

1.02

3

0.96

0.09

-

1.05

5

0.90

0.06

-

0.96

1

0.90

0.06

-

0.96

2

0.94

0.06

-

1.00

3

0.87

0.11

-

0.98

4

0.86

0.14

-

1.00

5

0.75

0.10

-

0.85

400

450

Product yield (-)

Carbon


Page 19 of 33

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

Energy & Fuels

Table 3. Calculated kinetic rate constant of each reaction in the network k [s-1] Kinetic parameter

Reaction 350 °C

400 °C

450 °C

kxxy

Isomerization

1.09E+00

2.77E-01

1.55E-01

kxf

Dehydration

3.86E-01

1.87E-01

1.73E-01

kxgl

Retro-aldol condensation

1.85E+00

3.93E+00

2.29E+01

kxt

Decomposition

0.00E+00

3.56E+00

1.91E+01

kxyx

Isomerization

1.88E-01

0.00E+00

0.00E+00

kxyf

Dehydration

0.00E+00

1.88E-01

5.27E-02

kxygc


0.00E+00

0.00E+00

2.77E-01

kxyt

Decomposition

0.00E+00

0.00E+00

2.44E-04

kft

Decomposition

4.95E-05

0.00E+00

0.00E+00

kglgc


1.08E+00

1.88E+01

1.15E+02

kgld

Isomerization

4.31E-01

2.55E+00

0.00E+00

kglt

Decomposition

2.48E+00

0.00E+00

0.00E+00

kgct

Decomposition

1.52E-01

2.54E-01

3.23E-01

kfog

Gasification

0.00E+00

0.00E+00

0.00E+00

kdgl

Isomerization

1.28E-01

3.91E+00

2.59E-01

kdt

Decomposition

3.68E-05

3.16E+00

0.00E+00

kfag

Gasification

5.08E-02

N/A

N/A

ktfa

Decomposition

3.70E+02

6.37E-03

7.09E-03

kta

Decomposition

7.97E+01

3.28E-02

3.01E-02

kag

Gasification

0.00E+00

0.00E+00

0.00E+00

ktg

Gasification

2.00E-02

5.04E-02

6.49E-02

kx

Total xylose decomposition

3.33E+00

7.95E+00

4.24E+01


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Page 20 of 33

Table 4. Activated energy and pre-exponential factor of Arrhenius reactions Activated Pre-exponential Reaction

energy

95% reliability

95% reliability factor (s-1)

(kJ.mol-1) xgl

9.32E+01

8.73E+01±3.59E+01

1.01E+08

1.15E+09±1.20E+09

glgc

1.76E+02

1.70E+02±4.89E+02

6.39E+14

1.91E+17±1.25E+17

gct

2.85E+01

3.11E+01±2.69E+01

3.89E+01

7.05E+03±4.96E+03

tg

4.46E+01

4.04E+01±1.74E+01

1.21E+02

4.29E+04±3.30E+04

x

9.44E+01

2.36E+08


Page 21 of 33

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

Energy & Fuels

Figure 1. Detailed reaction network of xylose decomposition in sub- and supercritical water


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

yield [-]

Energy & Fuels

Page 22 of 33

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

xylose xylose cal xylulose xylulose cal furfural furfural cal glyceraldehyde glyceraldehyde cal glycolaldehyde glycolaldehyde cal dihydroxyacetone dihydroxyacetone cal

0

1

2 3 4 residence time [s]

5

(a) 350 oC


formaldehyde formaldehyde cal

Page 23 of 33

1

yield [-]

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

Energy & Fuels

xylose

0.9

xylose cal

0.8

xylulose

0.7

xylulose cal furfural

0.6

furfural cal

0.5

glyceraldehyde

0.4

glyceraldehyde cal

0.3

glycolaldehyde

0.2

glycolaldehyde cal dihydroxyacetone

0.1

dihydroxyacetone cal

0 0

1


5

(b) 400 oC



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

yield [-]

Energy & Fuels

Page 24 of 33

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

xylose xylose cal xylulose xylulose cal furfural furfural cal glyceraldehyde glyceraldehyde cal glycolaldehyde glycolaldehyde cal dihydroxyacetone dihydroxyacetone cal

0

1


5


(c) 450 oC

Figure 2. Product yield of xylose, xylulose, furfural, and retro-aldol products (glyceraldehyde, glycolaldehyde, dihydroxyacetone, and formaldehyde) at temperature of (a) 350 oC, (b) 400 oC, (c) 450 oC and pressure of 25 MPa as a function of residence time


Page 25 of 33

1 0.9 0.8

yield[-]

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

Energy & Fuels

0.7

acetic acid

0.6

acetic cal

0.5

formic acid formic cal

0.4

TOC

0.3

TOC cal

0.2

gas

0.1

gas cal

0 0

1

2 3 residence time [s]

4

(a) 350 oC


5

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

yield[-]

Energy & Fuels

Page 26 of 33

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

acetic acid acetic cal formic acid formic cal TOC TOC cal gas gas cal

0

1


4

(b) 400 oC


5

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

Energy & Fuels

yield[-]

Page 27 of 33

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

acetic acid acetic cal formic a formic cal TOC TOC cal gas gas cal

0

1


4

5

(c) 450 oC

Figure 3. Product yield of organic acids (acetic acid and formic acid), TOC and gas at temperature of (a) 350 oC, (b) 400 oC, (c) 450 oC and pressure of 25 MPa as a function of residence time


Energy & Fuels

kxf

0.5

0

0

-0.5

-0.5

130 130

140

150

160

170

-1

ln k [s-1]

ln k [s-1]

kxxy

140

150

160

170

-1

-1.5

-1.5 -2

-2

1/T [10-5 K-1]

3.5 3 2.5 2 1.5 1 0.5 0

(b)

kxgl

kxt

y = -0.1122x + 18.427 R² = 0.9287

130

1/T [10-5 K-1]

(a)

ln k [s-1]

ln k [s-1]

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

140 150 160 -5 -1 1/T [10 K ]

170

3.5 3 2.5 2 1.5 1 0.5 0 135

(c)

140 145 -5 1/T [10 K-1]

(d)


150

Page 29 of 33

kxyx

200

1/T [10-5 K-1]

140

145

150

1/T [10-5 K-1]

(e)

(f)

kxygc

kxyt

100

200 ln k [s-1]

0 -0.2 0 -0.4 -0.6 -0.8 -1 -1.2 -1.4

0 -0.5 135 -1 -1.5 -2 -2.5 -3 -3.5

ln k [s-1]

100

ln k [s-1]

0 -0.2 0 -0.4 -0.6 -0.8 -1 -1.2 -1.4 -1.6 -1.8

kxyf

ln k [s-1]

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

Energy & Fuels

1/T [10-5 K-1]

0 -1 0 -2 -3 -4 -5 -6 -7 -8 -9

(g)

100

1/T [10-5 K-1]

(h)


200

Energy & Fuels

kft

kglgc

0

6 100

200

-4 -6 -8

y = -0.2113x + 34.091 R² = 0.9927

5 ln k [s-1]

ln k [s-1]

-2 0

-10

4 3 2 1

-12

0

1/T [10-5 K-1]

130

140 150 160 1/T [10-5 K-1]

(i)

kglt 1

1

0.8 ln k [s-1]

1.5

0.5 0

-0.5 -1

145

150

170

(j)

kgld

ln k [s-1]

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 30 of 33

155

160

1/T [10-5 K-1]

165

0.6 0.4 0.2 0 0

(k)

100 1/T [10-5 K-1]

(l)


200

Page 31 of 33

kgct

kdgl

130

140

150

160

170

ln k [s-1]

-0.5 -1

y = -0.0344x + 3.6603 R² = 0.9732

-1.5

2 0 -2 145 -4 -6 -8 -10 -12

1/T [10-5 K-1]

150

140

150

(n)

kdt

ktfa

155

160

165

1/T [10-5 K-1]

8 6 4 2 0 -2 130 -4 -6

160

170

1/T [10-5 K-1]

(m)

ln k [s-1]

-2

2 1.5 1 0.5 0 -0.5 130 -1 -1.5 -2 -2.5

ln k [s-1]

0

ln k [s-1]

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

Energy & Fuels

140

(o)

150

1/T [10-5 K-1]

(p)


160

170

Energy & Fuels

kta 5 4 3 2 1 0 -1 130 -2 -3 -4

140

ktg

150

160

170

0 -0.5 130 -1 -1.5 -2 -2.5 -3 -3.5 -4 -4.5

140

ln k [s-1]

ln k [s-1]

1/T [10-5 K-1]

150

160

170

y = -0.0537x + 4.7929 R² = 0.9258

1/T [10-5 K-1]

(q)

(r)

kx

ln k [s-1]

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

4 3.5 3 2.5 2 1.5 1 0.5 0

y = -0.1136x + 19.278 R² = 0.9511

130

140 150 160 -5 -1 1/T [10 K ]

170

(s)

Figure 4. Arrhenius plot of (a) kxxy (b) kxf (c) kxgl (d) kxt (e) kxyx (f) kxyf (g) kxygc (h) kxyt (i) kft (j) kglgc (k) kgld (l) kglt (m) kgct (n) kdgl (o) kdt (p) ktfa (q) kta (r) ktg (s) kx


Page 33 of 33

15 10 5 ln(k )[s-1]

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

Energy & Fuels

this study Aida(2010)

0 120

170

220

270

-5

Goodwin(2010) Jing(2007)

-10 -15

1/T [10-5 K-1]

Figure 5. Arrhenius plot of total xylose decomposition comparing with previous study (37), (38), (41)


Detailed Mechanism of Xylose Decomposition in Near-Critical and

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