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Aug 20, 2012 - Division of Energy and Environmental Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima,. 739-8527 Japan...
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Reaction Kinetics of the Lignin Conversion in Supercritical Water Tau Len-Kelly Yong† and Yukihiko Matsumura*,

‡

†

Department of Mechanical Systems Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527 Japan ‡ Division of Energy and Environmental Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527 Japan ABSTRACT: The effect of temperature (390−450 °C) and residence time (0.5−10 s) at a pressure of 25 MPa was investigated for lignin conversion in supercritical water (SCW) using a continuous flow apparatus designed to rapidly heat the system to the desired reaction temperature. Conversion of lignin in SCW occurs rapidly, and complete depolymerization can be achieved within a 5 s residence time. A high degree of depolymerization is achieved from rapid heating to supercritical temperatures. In addition, supercritical conditions result in a high yield of solid that does not significantly change with an increase in temperature or residence time. To test the suggested hypothesis that the formation of low molecular weight fragments and cross-linking of these fragments forms higher molecular weight fragments, the yield of char, gaseous products, phenolic compounds (phenol, guaiacol, catechol, o-cresol, m-cresol, and catechol) and aromatic hydrocarbons (benzene, toluene, and naphthalene) were determined. The formation of phenolic compounds at short residence time indicates that ether bonds in lignin are easily degraded under supercritical conditions. A reaction network model was proposed, and the subsequent kinetic parameters for the conversion pathways were determined by assuming a first-order reaction. It is observed that the rate constant of overall lignin conversion obeys Arrhenius behavior. The individual rate constants of each reaction in the network are evaluated to determine conformity to Arrhenius behavior.

1. INTRODUCTION Lignins, nature’s second most abundant raw material and most abundant aromatic (phenolic) polymer, can be defined as amorphous three-dimensional polymeric substances consisting of phenylpropane. Their precursors are three aromatic alcohols (monolignols): p-coumaryl, coniferyl, and sinapyl alcohols.1,2 The compositions of lignin polymers are unique to each species of plant but can generally be classified into three types. For example softwood (coniferous trees) lignins are formed from coniferyl alcohol. On the other hand, hardwood (nonmonocotyledon angiosperm trees) lignins have both coniferyl and sinapyl alcohol, and grass lignins contain all three of the above-mentioned monolignols.3 The various extraction techniques that are used to separate and purify lignin samples from biomass often result in degradation and modification of the macromolecule, ultimately providing a different overall structure to the final product.4 As explained by Chakar and Ragauskas,5 during lignin separation by hot akaline (sulfate) treatment, the hydroxide and hydrosulfide anions react with the lignin to cause the polymer to decompose into smaller water/alkali soluble fragments. However, despite the difference and complexity of the structures, the dominant linkage in softwood lignin arises from phenylpropane β-aryl ether (β-O-4) linkers, which approximately account for 45−50% of the overall linkages that connect the phenylpropane units in softwood lignin.5 Lignin conversion into more meaningful products for energy generation and chemicals application is a challenge. One preferred method for this is gasification and liquefaction of the lignin in subcritical water and supercritical water (SCW). Water in the sub- and supercritical state exhibits a wide range of chemical and physical properties, varying from gas-like to © 2012 American Chemical Society

liquid-like behavior. This ability of water to obtain different properties by manipulations of its temperature and density has led to increased attention on its use as a medium for reaction chemistry. However, several researchers6−9 have discussed complications with its application, most notably the importance of operating variables such as residence time, water density, solid loading, temperature, heating rate, and pressure. Information on the reaction pathways of lignin conversion in SCW is limited, while what is known is complicated by the differences observed for each lignin due to their diverse structures obtained from the various methods of isolation from biomass. It is essential to obtain a better understanding of the reaction pathways and subsequent kinetic parameters to elucidate the formation and degradation of intermediate compounds during such reactions. Among the earliest studies carried out in this field, Bobleter and Concin10 proposed a simplified mechanism consisting of a two-phase reaction for the hydrothermal degradation of poplar lignin. Therein, a very fast reaction phase initially occurs where the lignin is degraded into soluble fragments followed by a slower reaction phase where the soluble fragments react with one another through repolymerization. In addition, Zhang et al.11 discovered that lignin degradation occurs rapidly (within 0.4 min) near critical conditions. The rapid depolymerization of lignin and the simultaneously higher char yield obtained from the reaction is an interesting feature that requires further study, while the Received: Revised: Accepted: Published: 11975

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ensure the desire immediate reaction temperature was met by directly measuring the temperature at both mixing points. The reactor was immersed in a molten salt bath at the desired reaction temperature and a continuous air bubble provided to maintain a uniform temperature. To ensure steady-state conditions, the feedstock mixture was fed continuously for at least 30 min before sampling was conducted. After removal from the reactor, the reaction products were immediately cooled by the direct addition of cold water and delivered into a cooling jacket. The pressure was controlled using a backpressure regulator. Large solid particles were trapped inside the inline filters, and the liquid effluent (with dispersed char particles) was collected at the liquid sampling point. The liquid sample was collected from the liquid sampling port via a threeway valve with further analysis by TOC and HPLCs carried out to identify its constituents. The gaseous sample was subsequently collected from the gas-sampling port (79.55 mL volume; 1.65% error) using the following procedure. Initially, the gas sampling port is put under vacuum before the three-way valve is then opened to allow the liquid and gaseous products to flow into the sampling port. A conical flask is used to collect the liquid product, and the gas product is collected in a gas sample vial and subsequently used for analysis. The change in pressure inside the sampling port before and after collection is noted before the valve is turned back to divert the direction of the flow. The volume of the liquid product stored in the flask is then determined. Subsequently, the gas generation rate can be calculated by applying the ideal gas law

studies mentioned did not identify or quantitatively determine the key intermediate compounds in the liquid effluent. Several studies have been conducted using lignin model compounds in supercritical conditions, e.g., guaiacol and catechol, from which, their reaction kinetics were determined.12,13 However, due to the diversity in the reactions, there has thus far been no attempt made to elucidate a detailed mechanism. Providing a detailed mechanism and its kinetics could potentially be beneficial in providing an understanding on behavior of lignin under supercritical conditions. This study therefore attempts to elucidate lignin conversion behavior in SCW and determine the kinetics of the reactions involved.

2. EXPERIMENTAL PROCEDURES 2.1. Reactors and Materials. Lignin behavior in SCW is investigated by elucidating the kinetic parameters under supercritical conditions (390−450 °C) with very short residence times (0.5−10 s). The short residence time utilized in this study allows for the determination of the initial reactions when the lignin is immersed under supercritical conditions and for the investigation into how quickly it is degraded into lower molecular weight products. From this, we can formulate a kinetic model capable of describing the initial behavior of lignin conversion under supercritical conditions. The continuous flow system utilized in this study is capable of rapidly heating the lignin to the desired supercritical conditions through rapid mixing with preheated water. This system avoids the complication of a slow heating rate, thus not allowing intermediate reactions to occur at subcritical conditions before the actual supercritical conditions are achieved. The schematic of the experimental apparatus shown in Figure 1 has been previously reported elsewhere.14 The reactor

n=

(Pf − Pi)Vg RTt

(1)

where n = gas generation rate [mol/s], Pi = initial pressure [Pa], Pf = final pressure [Pa], Vg = gas volume inside the gas sampling port (=V0 − Vl), V0 = volume of the gas sampling port (79.55 cm3), Vl = liquid product volume in the gas-sampling port [m3], R = ideal gas constant (8.315 J /(K. mol)), T = temperature during sampling [K], and t = sampling time duration [s]. The lignin used in this experimental work was alkali lignin (low sulfonate content), obtained in powder form from SigmaAldrich Japan Co. (catalog number 471003). The lignin is isolated from a commercial mill predominantly using raw material from Norway Spruces. Due to its extraction using sulfite process, the lignin is completely soluble in water at room temperature. The average molecular weight (Mw) of the lignin was 10 kg/mol, and deionized water (374 °C), in this study, a higher degree of depolymerization is achievable at even shorter residence times. According to Li et al.,18 the rapid depolymerization at supercritical conditions is mainly due to the cleavage of ether bonds from the abundant β-aryl ether (β-O-4) linkage in the softwood lignin. Faravelli et al.4 discovered that the weakest bond inside the lignin structure favor radical formation and initiate the radical reaction. The low disassociation enthalpies of the ether bond in the β-O-4 linkage initiate reaction that forms a phenoxy radical and a secondary alkyl aromatic radical. Furthermore, Roberts et al.19 and Chen et al.20 suggested that thermal cleavage of lignin ether bonds in the early stages of pyrolysis follows the behavior of a free radical reaction. However, it has also been suggested by Wahyudiono et al.13 and Man et al.21 that depolymerization of lignins occurs through different mechanisms of hydrolysis and dealkylation to produce low-molecular weight fragments (formaldehyde, etc.) and phenolic compounds with reactive functional groups. However, this is unlikely under supercritical conditions. As the reaction temperatures utilized in this study are under supercritical conditions, it may be reasonable to assume that the low dielectric constant leads to free radical reactions. 11981

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Figure 6. Products yields from the experiment (symbol) and first-order model (line) at temperature of 390 (×), 420 (▲), and 450 °C (■).

minimal at 390 °C but subsequently increased with an increase in temperature. It has been deduced by Pandey and Kim23 that CH4 is produced from cracking of the methoxyl group. As indicated, the methoxyl group is the weakest of the substituent bonds in guaiacyl compounds, and its cleavage is enhanced at higher temperatures. Other gases such as C2H4 and C2H6 were only detected in trace amounts, while no CO was detected for any of the experiments carried out herein. The zero yield of CO and high yields of CO2 and H2 even at 0.5 s is an indication that the water-gas shift reaction (i.e., CO + H2O → H2 + CO2) may have gone to completion under the supercritical conditions. However, based on study by Resende et al.7,24 the water-gas shift reaction is not a dominant reaction during the early minutes of lignin gasification. They concluded that SCW gasification of lignin appears to take place in 2 stages. During the first stage, gases are formed directly from solid and liquid species. They are formed directly from the conventional gasification through pyrolysis of the unhydrolyzed lignin. Nunn et al.25 also concluded that carbon gaseous compound is

evolved from the primary conversion of lignin during pyrolysis. During pyrolysis, the conversion of the substituted groups and aliphatic structures in lignin leads to CO2 release from the carboxyl group and H2 from the aliphatic and methoxyl group.26 Furthermore, the gas yields obtained at 0.5 s residence time are 0.05 (390 °C), 0.08 (420 °C), and 0.09 (450 °C), respectively. This observation further indicates that the evolution of carbon compounds from lignin to form gas is a rapid and direct reaction instead of the initial assumption that it is formed through liquid intermediates. However, with longer residence time (2−10 s), decomposition and steam reforming of the liquid intermediates will contribute significantly to the gas formation apart from the initial yield produced directly from lignin.25 It is noted that at 420 °C as shown in Figure 7(b), the yield of H2 gas is similar to that at 390 and 450 °C; however, its concentration is low compared to the higher yields of carbon gases such as CO2, CH4, and C2H6. 11982

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Figure 7. Gas product compositions from lignin conversion at temperatures of 390−450 °C and residence times of 0.5−10 s.

the reaction probably consisted of phenolic and alcoholic groups in its structure, similarly to the lignin feedstock. This is consistent with findings by Pinkowska et al.27 that suggested the solid residue produced from SCW decomposition of alkali lignin is mainly phenolic char. To examine the suggested hypothesis that there is formation of low-molecular weight fragments and that cross-linking of these fragments forms higher-molecular weight fragments, it is important to determine the yield of phenolic compounds (phenol, guaiacol, catechol, o-cresol, m-cresol, and catechol) and aromatic hydrocarbons (benzene, toluene, and naphthalene). At high temperatures, numerous C−C bonds in a lignin are likely cleaved to produce single-ring compounds. From Figure 6, it is observed that the main phenolic compound from lignin decomposition is guaiacol, which is followed by minor compositions of other phenolic compounds such as phenol, catechol, o-cresol, and m-cresol. The formation of phenolic compounds at short residence times indicates that ether bonds in the lignin are easily degraded under supercritical conditions. In all cases, the yield of all phenolic compounds decreases with an increase in residence time. This can be attributed to the repolymerization that occurs through cross-linking of the phenolic compounds to form higher molecular weight compounds. The determination of guaiacol yield is vital because Hosoya et al.26 discovered that during pyrolysis, a substantial amount of char is only formed from compounds containing a guaiacyl unit, particularly guaiacol. This was further confirmed by Wahyudiono et al.13 who discovered that the formation of substances with high molecular weights reformed to char is important for guaiacol conversion to reach equilibrium. The high yield for the guaiacol formation (Figure 6a), even at the shortest residence time, is expected because guaiacol is the main structure within the softwood lignin. However, subsequent conversion of guaiacol into other compounds for all temperatures indicates

It is observed in Figure 5(d) that high char yield is obtained when reactions are conducted under supercritical conditions. However, the yield does not change significantly with the increase in temperature and residence time. The formation of char occurs due to cross-linking reactions between the reactive degradation fragments and residual lignin, resulting in the production of higher molecular-weight fragments.21 The constant char yield from short to long residence times suggests that this cross-linking occurs instantaneously. The increase in temperature in the range of 390−450 °C does not affect char formation. This fact may imply that, in supercritical conditions, the amount of reactive degradation fragments produced is determined by the type of feedstock and is irrespective of the temperature. In addition, we obtained the results of elemental analysis of the lignin feedstock and char obtained from the experiment as shown in Table 2. Roberts et al.19 reported that the carbon Table 2. Elemental Analysis of Lignin Feedstock and Char Obtained from Experiment elemental analysis [kg/kg-dry] types of compounds

C

H

N

S

O + ash (by balance)

lignin feedstock char

0.474 0.433

0.047 0.045

0.001 0.001

0.004 0.004

0.474 0.517

content from CHN analysis of lignin before and after hydrothermal treatment shows slight changes (5.2 wt % higher carbon content), indicating that lignin undergoes not only a structural but also a minor chemical modification during depolymerization. This was also observed in this study. The CHNS/O analysis of the lignin feedstock and char yielded 47.4 wt % and 43.3 wt % carbon, respectively. The small changes in carbon and hydrogen content indicate the char formed from 11983

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Figure 8. The proposed lignin conversion pathways in SCW (all reactions are assumed to be in first order.).

that it also has a fast decomposition rate. This is consistent with findings of Wahyudiono et al.13 who found that guaiacol had a fast decomposition rate, and almost complete conversion can be achieved under supercritical conditions. The formation of catechol (Figure 6b), phenol (Figure 6c), and o-cresol (Figure 6d) most likely results from guaiacol and compounds containing a guaiacyl unit, based on their relatively slower rate of formation at short residence times compared to guaiacol. Catechol, phenol, and o-cresol are single ring phenolics that are comparatively more stable than guaiacol types of phenolic rings. Therefore, they are selectively preferred under supercritical conditions. The guaiacol ring structure has an OH group in addition to a methoxyl (O−CH3) group. It has been established that the methoxyl group contains the weakest bond in the guaiacol unit and is therefore susceptible to being cleaved. In SCW, the methoxyl group undergoes hydrolysis to produce catechol. Phenol is likely produced from catechol through cleavage of the bond between the OH groups and the benzene ring. Formation of o-cresol likely results from the radical reactions deduced by Wahyudiono et al.13 and Hosoya et al.26 Under supercritical conditions, guaiacol radicals are easily formed after undergoing scission at the methoxyl group. These guaiacol radicals will induce a rearrangement reaction to produce o-cresol. The formation of m-cresol (Figure 6e) arises from the alkyl rearrangement of alkylphenol. Meta- and para type compounds are typically derived from ortho type compounds at high temperature, in this case from o-cresol.12 Aromatic hydrocarbons (naphthalene, benzene, and toluene) are precursors to the formation of aromatic char. As explained by Fang et al.9 aromatic chars likely result from lignin that has heterogeneously degraded further to form highly condensed char with cross-linked aromatics. These are different from phenolic char. Phenolic chars are mainly from phenolic compounds that are produced from the initial lignin degradation and which are repolymerized with aldehydes to form heavier cross-linked structures. Therefore, through the identification of these aromatic hydrocarbons and their subsequent change to aromatic char, we can distinctly identify the behavior under supercritical conditions.

As observed from Figure 6(f), there is an increased formation of aromatic hydrocarbons with an increase in residence time. However, the yield is reduced with an increase in temperature. This can be used to deduce that the formation of aromatic chars is not enhanced by supercritical conditions. Therefore, the high yield of char in this study is most likely phenolic char, as discussed previously, in which we found that formation of other phenolic compounds is enhanced under supercritical conditions. To further validate this deduction, we conducted a preliminary experiment under subcritical conditions of 300 and 350 °C, with similar residence times. It is interesting to note that the results obtained for these subcritical conditions show a higher yield of aromatic hydrocarbons and a significantly reduced yield of char. This indicates that supercritical conditions suppress the formation of aromatic hydrocarbons, which in turn suppresses the formation of aromatic char, and that lignin chars are likely to primarily be phenolic chars. As discussed previously, results from elemental analysis of the char showed that its carbon and hydrogen content only changes slightly. This is important as it indicates that the char maintains its phenolic and alcoholic group in its structure, similarly to lignin feedstock, consistent with findings by Pinkowska et al.27 that suggested the solid residue produced from SCW decomposition of alkali lignin is mainly phenolic char. 3.2. Modeling of Lignin Conversion. Analysis of the kinetics was carried out for the conversion of lignin from the reaction pathways shown in Figure 8. The rate equation for each reaction in the pathways was assumed to be first order with the lignin conversion fitted to the resultant differential equations d[lignin] = −(klt + klgu + klga + klch)[lignin] dt

(3)

d[guaiacol] = klgu[lignin] − (kguoc + kgut + kguc + kgup) dt [guaiacol] 11984

(4)

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d[catechol] = kguc[guaiacol] − (kct + kcp + kcoc)[catechol] dt

Table 3. Kinetic Parameters for Lignin Conversion in Supercritical Conditions Obtained from the First-Order Model

(5)

temperature, (°C)

d[phenol] = kcp[catechol] + kgup[guaiacol] dt

−1

− (kpt + kpch)[phenol]

(6)

d[m‐cresol] = kocmc[o‐cresol] − kmct[m‐cresol] dt

(7)

d[o‐cresol] = kguoc[guaiacol] dt = kcoc[catechol] − (koct + kocmc)[o‐cresol] (8)

d[aromatic] = ktac[TOC] − kacch[aromatic] dt

(9)

d[TOC] = klt[lignin] + kgut[guaiacol] + kct[catechol] dt + kpt[phenol] + koct[o‐cresol] + kmct[m‐cresol] − (ktga + ktac + ktch)[TOC] (10)

d[gas] = ktga[TOC] + klga[lignin] dt

kinetic parameters, k (s )

390

420

450

lt lgu lga guc gup gut guoc coc cp ct pt pch ocmc mct oct tch tga tac acch lch

1.0141 0.2790 0.2539 0.3218 0.7838 0.0000 1.2152 0.0000 0.0000 0.0000 0.0000 2.4438 0.6268 0.5167 0.0000 0.0418 0.0000 0.1781 0.0172 2.8755

1.0255 0.4160 0.5296 0.3579 0.5251 0.0000 1.1208 0.1281 0.0000 0.1382 0.0000 2.4499 0.6964 0.6588 0.1401 0.0647 0.0029 0.1431 0.0550 3.8074

1.1943 0.4269 0.6477 0.6659 0.1587 0.0000 0.3922 0.3889 0.0000 0.2271 0.0000 2.5844 1.0399 0.5936 0.0000 0.0000 0.0000 0.0924 0.0599 5.1457

(11)

d[char] = kpch[phenol] + kacch[aromatic] + ktch[TOC] dt + klch[lignin]

(12)

where [lignin] = lignin concentration [mol-C/dm3], [guaiacol] = guaiacol concentration [mol-C/dm3], [catechol] = catechol concentration [mol-C/dm3], [phenol] = phenol concentration [mol-C/dm3], [m-cresol] = m-cresol concentration [mol-C/ dm3], [o-cresol] = o-cresol concentration [mol-C/dm3], [aromatic] = aromatic hydrocarbons (benzene, naphthalene, and toluene) concentration [mol-C/dm3], [TOC] = lumped carbon concentration of the other liquid products [mol-C/ dm3], [gas] = carbon in gas phase [mol-C/dm3], [char] = carbon in char [mol-C/dm3], and t = residence times [s]. The rate constants were then calculated using the nonlinear regression with least-squares of error (i.e., the difference between the experimental and calculated values) as a criterion to fit the model with the experimental data. A trial and error procedure was used using the solver add in with Microsoft’s spreadsheet, Microsoft Excel. Figures 5 and 6 illustrate the comparison between the calculated (solid line) and experimental product yields. As Figures 5 and 6 show and as suggested by the high r2 (coefficient of determination) values obtained, the model is able to reproduce trends for most of the product yields in terms of variation in residence time and temperature. Table 3 shows the kinetic parameters identified for the reaction pathways at all reaction temperatures (390− 450 °C). Figure 9 shows the Arrhenius plot of the reaction rate constant of lignin conversion (kl) and its comparison with the work conducted by Zhang et al.11 They determined the reaction kinetics for the hydrothermal treatment of Kraft pine lignin, a type of softwood, at temperatures of 300 and 374 °C, which are similar conditions to those utilized in this study. As

Figure 9. Arrhenius plot of the rate constants for lignin conversion in SCW (kl) in comparison with work by Zhang et al.11

observed, the reaction rate constants for both studies display similar trends. As illustrated by Figure 9, the lignin conversion behavior in SCW can be reasonably described by an Arrhenius equation. These parameters are consistent with those of the previous studies shown in the same figure. The overall conversion rate of lignin, kl, is directly proportional to the increase in reaction temperature ascribed by the equation. From the Arrhenius plot in Figure 9, we obtained a pre-exponential factor of 2.24 × 103 s −1 and activation energy of 34.34 kJ/mol for the overall reaction. The activation energy of lignin degradation in suband supercritical conditions can vary based on differences in the lignin itself in regards to its structural type (softwood, hardwood, or grass lignin) and also from the employed method of isolation from biomass. However, Zhang et al.11 determined the activation energy of softwood lignin degradation to be 37 kJ/mol for the temperature range of 300−374 °C. Therefore, the estimated activation energy (34.34 kJ/mol) obtained in this study is reasonable. 11985

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Figure 10. Arrhenius plot of the reaction rate constants identified based on the first order proposed lignin conversion pathways.

The increasing rate of lignin conversion to gas, char, and TOC with the increase in temperature is as discussed previously. It is interesting to note the high reaction rate of direct char formation from the lignin, which supports the suggestion that, at short residence times and higher temperatures, lignin depolymerization and repolymerization occur rapidly under supercritical conditions and at higher rates than the cross-linking of the simple phenolic compounds. In addition, the gas formed is mainly from the lignin, which suggests that it is produced during the early period of lignin depolymerization under supercritical conditions. The gasification of lignin likely results from cleavage of lignin bonds particularly in the aromatic rings and methoxyl group of the guaiacyl compounds. As mentioned previously, the increased rate of guaiacol formation from the lignin with an increase in temperature is expected because guaiacol is the main structure in the softwood lignin. Overall, the study of lignin conversion under supercritical conditions at very short residence times offers interesting insight into its behavior and holds the key to predicting its overall pathways. It is observed that even at short residence times, the lignin are converted into various organic liquid and

Figure 10 shows the Arrhenius plot for each reaction rate constant in the reaction network. The rate of reactions of klt (lignin → TOC), klga (lignin → gas), klgu (lignin → guaiacol), kguc (guaiacol → catechol), kcoc (catechol → o-cresol), kct (catechol → TOC), kocmc (o-cresol → m-cresol), kacch (aromatic hydrocarbons → char), and klch (lignin → char) increases with an increase in reaction temperature under supercritical conditions. However, the rates of the reactions corresponding to kgup (guaiacol → phenol), kguoc (guaiacol → o-cresol), and ktac (TOC → aromatic hydrocarbons) do not conform to Arrhenius behavior since it is observed that the rate decreases when the temperature is increased. As discussed previously by Promdej and Matsumura,27 both ionic and radical reactions can take place under hydrothermal conditions. These reactions can be differentiated through examining the conformity to Arrhenius behavior. Radical reactions will not be affected by a change in dielectric constant or ion product, hence Arrhenius behavior is expected. On the other hand, if the reaction is ionic in nature, a change in the dielectric constant and the ion product will significantly affect the stability of the ions in the reaction, and, as a result, deviations from Arrhenius behavior can be expected. 11986

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gaseous compounds, in addition to its repolymerization to form solid. Furthermore, we also discovered that there was rapid conversion of these intermediate phenolic compounds during the reaction. The importance of temperature on the lignin conversion is another integral part of the study. It is interesting to note that although the overall lignin conversion follows Arrhenius behavior, as shown Figure 10, some individual pathways may be diverted. In this case, the rate of reaction for kgup (guaiacol → phenol), kguoc (guaiacol → o-cresol), and ktac (TOC → aromatic hydrocarbons) did not increase with temperature. The decreasing rate of the formation of aromatic hydrocarbons with temperature, as explained previously, indicates that formation of aromatic char is suppressed at supercritical conditions while phenolic compounds yield are enhanced. Therefore the high yield of char observed is likely to be from phenolic compounds forming phenolic chars.

4. CONCLUSIONS The conversion of lignin was studied in SCW at 390−450 °C and 25 MPa at very short residence times (0.5−10 s). Under these conditions, we conclude that lignin is rapidly converted in SCW. Supercritical conditions resulted in a high yield of solid and the formation of char occurred due to cross-linking reactions between the reactive degradation fragments and residual lignin to produce higher molecular- weight fragments. The constant formation of char from short to long residence times suggests that this cross-linking occurs instantaneously. The formation of phenolic compounds at short residence times indicates that ether bonds in the lignin are easily degraded under supercritical conditions. In addition, the formation of gas mainly arises from the lignin, suggestive that it is produced during the early period of lignin depolymerization under supercritical conditions. The reaction rate of lignin conversion under supercritical conditions was also determined, and its behavior can be reasonably explained by a first-order, serial kinetic model. The conversion rate increases with temperature, as prescribed by the Arrhenius equation, and complete conversion of lignin could be obtained even at short residence times. Furthermore, most of the individual pathways conform to Arrhenius characteristics.

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Corresponding Author

*Phone: +81 (0) 82-424-7561. Fax: + 81 (0) 82-422-7193. Email: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie300921d | Ind. Eng. Chem. Res. 2012, 51, 11975−11988

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(27) Promdej, C.; Matsumura, Y. Temperature effect on hydrothermal decomposition of glucose in sub- and supercritical water. Ind. Eng. Chem. Res. 2011, 50, 8492−8497.

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dx.doi.org/10.1021/ie300921d | Ind. Eng. Chem. Res. 2012, 51, 11975−11988