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Interaction among glucose, xylose, and guaiacol in supercritical water Nattacha Paksung, and Yukihiko Matsumura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02813 • Publication Date (Web): 24 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017
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Energy & Fuels
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Interaction among glucose, xylose, and guaiacol in
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supercritical water
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Nattacha Paksung, Yukihiko Matsumura*
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Department of Mechanical Sciences and Engineering, Hiroshima University
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*
To whom correspondence should be addressed. Fax: +81-82-422-7193. E-mail:
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[email protected].
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Abstract: A mixture of three model compounds of lignocellulosic biomass, namely glucose,
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xylose, and guaiacol, was treated in supercritical water to investigate the interactions taking
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place between the model compounds.
All experiments were carried out at 450 °C and
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25 MPa, with varying residence times of 5–40 s. The inclusion of guaiacol resulted in high
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yields of both 5-hydroxymethylfurfural and furfural. In addition, reaction rate constants were
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determined for the reaction network, and a comparison with literature values indicated that
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guaiacol addition suppressed radical reactions, thus increasing the yields of products derived
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from ionic reactions.
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Keywords: Lignocellulosic biomass; glucose; xylose; guaiacol; supercritical water
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gasification; interaction
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1. Introduction
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Biomass is one of the most abundant raw materials on Earth, and is considered to be a
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sustainable energy resource due to its carbon neutral characteristics. Lignocellulosic biomass
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is of particular interest as it is non-edible, and as such, does not compete with the food
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supply.1 Other than energetic production, biomass has a potential as a platform for chemical
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production such as furfurals and phenolic compounds, which are important starting material
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for many industries. However, when biomass contains a high moisture content, costly drying
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pretreatment processes is required, thus rendering its use economically infeasible. In this
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context, the supercritical water gasification of biomass is an effective process, as water is
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used both as the reaction medium and as the reactant, and so it is not necessary to dry the
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biomass prior to use. Supercritical water is present as a fluid phase when the temperature and
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pressure of water are above their critical points of 374 °C and 22.1 MPa, respectively. In
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supercritical water gasification, biomass can be homogeneously dissolved and efficiently
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decomposed due to the high reactivity of the supercritical water.2
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To elucidate the reaction mechanism taking place during this process, the use of
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model compounds is particularly effective. Glucose, for example, is often used to represent
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biomass3-9 because it is a monomer of cellulose. Indeed, the gasification of glucose is
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favored in supercritical water, because gasification proceeds mainly via radical reactions,
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which dominate in supercritical water.8,
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supercritical water produces 5-hydroxymethylfurfural (5-HMF), which subsequently
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polymerizes to form char11-13. Similarly, xylose is used to represent hemicellulose, with the
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major product of its decomposition in subcritical water being furfural. In contrast, the retro-
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aldol condensation of ᴅ-xylose is dominant in near critical and supercritical water.14-17
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Furthermore, in the work of Kanetake et al., guaiacol was used as a model compound for
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lignin.18
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As such, the gasification of glucose in
Under the hydrothermal conditions examined, the main products of guaiacol
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gasification were catechol, phenol, and o-cresol. Following the kinetic analysis of guaiacol
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conversion in sub- and supercritical water by Yong and Matsumura,19 it was reported that
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char formation from guaiacol was enhanced under supercritical conditions.
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To date, the majority of studies have focused on the behavior of single components in
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supercritical water. However, this is not sufficient to reach an understanding of how a
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biomass sample containing multiple components produces gas or undesirable char products.
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In this context, Yanik et al. examined the effect of differences in biomass composition,20 and
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found that even for feedstocks sharing similar components, small differences in the
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component mixture could result in vastly different results.
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understand the interactions taking place between the various components present in biomass.
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As such, Yoshida and Matsumura investigated the interactions between cellulose, xylan, and
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lignin,21 and they showed that the addition of lignin resulted in low gas yields due to
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interactions between the three components.
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Magdenoglu et al., who investigated the hydrothermal gasification of a mixture of cellulose
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and lignin alkali.22 They found that lignin suppressed the formation of both hydrogen and
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methane. Furthermore, Weiss-Hortlata et al. studied the interactions between glucose and
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phenol, and found that the hydrogen yield and the total volume of gas produced from the
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conversion of glucose was reduced. Moreover, in the presence of phenol, the total organic
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carbon (TOC) removal and residual phenol components of the liquid phase were higher.23
It is therefore important to
A similar finding was also reported by
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However, to date, no studies have focused on the interactions between biomass
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components in terms of the reaction kinetics. We therefore aimed to kinetically investigate
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the interactions between the various compounds present in lignocellulosic biomass, focusing
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on cellulose, hemicellulose, and lignin through the use of model compounds.
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2. Experimental
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A tubular reactor similar to that reported in our previous study was also employed
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herein.19 This reactor was composed of SS316 steel with inner and outer diameters of 1 and
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1.59 mm, respectively. Feedstock solutions of glucose, xylose, and guaiacol were prepared
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and mixed with the preheated water at the entrance of the reactor (1:4 mass ratio of
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feedstock:water) to avoid feedstock decomposition prior to reaching the reactor. As such, the
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feedstock solution was diluted by 5 times inside the reactor, and was heated rapidly to the
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target temperature. The residence time was varied between 5 and 40 s by adjusting both the
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length of the reactor and the flow rate of the preheated water/model compound solution.
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After the desired residence time, the effluent from the reactor was cooled rapidly by the
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addition of an equal volume of deionized water, after which it was further cooled using a
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double tube heat-exchanger containing running water.
The pressure of the system was
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maintained at 25 MPa using a back-pressure regulator.
For char recovery, the reaction
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products were passed through a 7 µm solid filter. In addition, the liquid effluent and gaseous
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products were collected at the liquid and gas sampling ports under ambient conditions. To
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ensure the steady state, sampling was made after the time longer than that calculated by
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dividing volume of all the tubes in the system by the flow rate of each part. Note that the
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flow rate varied by the density of solution at different temperature. Normally, the waiting
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time from the introduction of feedstock solution until sample collection was 30−60 min.
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After the experimental run, the reactor was thoroughly washed with large amount of water
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and mixture of acetone and methanol, but tarry material left in the reactor was negligible.
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The various experimental conditions and feedstock concentrations employed are summarized
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in Table 1.
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The gaseous products were analyzed by gas chromatography (GC) using a gas
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chromatograph equipped with a thermal conductivity detector and a flame ionization detector.
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Helium was employed as the carrier gas in all cases except during the analysis of the
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produced hydrogen, where N2 was instead employed. A total organic carbon (TOC) analyzer
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was used to quantify the carbon compounds present in both the feedstock and the product
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effluent in the liquid (non-purgeable organic carbon: NPOC) and dissolved gaseous products
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(inorganic carbon:
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chromatography (HPLC) was employed to identify the compounds present in the liquid
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effluent. Acids, aldehydes, and ketones were quantitatively measured using either an SCR-
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102H column (Shimadzu) with a 0.005M aqueous HClO4 solution as the mobile phase, or an
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RSpak DE-413L (Shodex) column with 0.01M H3PO4 as the mobile phase. In addition, the
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phenolic compounds were measured using the RSpak DE-413L column with a 1:1 (vol.)
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mixture of acetonitrile and 0.005M aqueous HClO4 as the mobile phase. The solid particles
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trapped in the filter were considered as char. The carbon content in the char is assumed to be
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similar to that in 5-HMF 24. The product yield YC(X) of carbon compound X was evaluated
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based on the carbon content as outlined in Equation (1):
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YC ( X)[−] =
IC) to confirm the carbon balance.
nC (X ) nC 0
High performance liquid
(1)
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where nC(X) and nC0 denote the amount of carbon in product X and in the feedstock,
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respectively.
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3. Results and discussion
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3.1.
Product distribution
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The distribution of the gaseous, liquid, and solid products of the supercritical water
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gasification reactions of the lignocellulosic biomass model compounds is shown in Figure 1,
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where it is apparent that the highest yields were obtained for the liquid products. Although
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char formation from sugar (i.e., glucose and xylose) is generally suppressed under
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supercritical conditions,8,
16, 17
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conditions.19
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amounts, even when guaiacol was present in the feedstock mixture. This indicates that
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glucose and xylose may influence char formation during the decomposition of guaiacol. We
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also observed that the gasification normally promoted under supercritical conditions was also
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suppressed.
char formation from guaiacol is enhanced under these
However, in our experiments, char formation was only observed in trace
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The compositions of the gaseous products obtained from the various experiments are
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shown in Figure 2. In general, the main components were H2 and CO2, although CO was
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also observed after a residence time of 20 s. Upon increasing the residence time to 40 s, the
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CO content decreased, indicating that the water-gas shift reaction took place at this point to
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produce H2 and CO2. In addition, minimal quantities of hydrocarbon gases containing more
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than one carbon atom per molecule were observed. Although the formation of CH4 was
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likely due to methanation, i.e., the secondary reaction between H2 and either CO or CO2, CH4
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was only detected in the gaseous product produced from the guaiacol-containing feedstock.
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As such, an alternative route to CH4 formation may involve the cracking of the aliphatic C–O
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bond of the methoxy group in guaiacol.19
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As in previous studies,8, 16, 17 the decomposition of glucose and xylose in supercritical
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water was rapid, and neither glucose nor xylose were observed in the liquid product. Isomers
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of these sugar compounds (i.e., fructose and xylulose) were also not observed. The obtained
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yields of observed major constituents of the liquid product (i.e., 5-HMF, furfural, catechol,
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and phenol) are shown in Figure 3. The solid lines indicate the expected product yield in the
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absence of interactions determined by substituting values reported in the previous studies8, 16,
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17
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the calculation because of consideration in this study that the dehydration of glucose to form
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furfural also produces single-carbon-atom TOC. The dashed lines show the calculated yields
. Note that kinetic parameters reported in Promdej and Matsumura8 were modified prior to
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obtained using the rate constants determined from the experimental yields (see Section 3.2).
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In addition, dehydration to produce 5-HMF and furfural was previously reported to be an
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ionic reaction,8 and so we would expect the formation of these products to be suppressed
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under the supercritical conditions employed herein. However, unexpectedly high yields of 5-
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HMF and furfural were obtained, with the furfural yield reaching 0.07–0.17. Furthermore,
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the major components of the liquid product were phenolic compounds, with catechol and
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phenol being obtained in addition to 5-HMF and furfural. These phenolic products are
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derived from the decomposition of guaiacol. Previously, Yong and Matsumura19 reported
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that the hydrolysis and pyrolysis reactions responsible for catechol formation were largely
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influenced by the properties of the water employed in the reaction. For example, in the
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supercritical region, where the dielectric constant drops significantly, the ionic hydrolysis
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reaction is suppressed, and as such, the formation of catechol via guaiacol pyrolysis (i.e., a
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radical reaction) is favored under the temperatures employed herein. However, phenols can
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also form through the radical decomposition of guaiacol or via the scission of a catechol O–H
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bond, which is also a radical reaction. This is in a good agreement with our higher product
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yield of phenol compared to catechol. Furthermore, existence of phenol as an intermediate
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compound could imp;y the existence of radical scavenging effect23 that demoted radical
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reactions and consequently increased the product yield from ionic reactions. Nonetheless,
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more clarification of the radical scavenging role of phenolic compounds is still lacking and
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needs further investigation.
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3.2.
Kinetic study of the interaction of model compounds
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According to the previously proposed reaction pathways of the three model
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compounds, namely glucose,8 xylose,17 and guaiacol,19 gasification occurs for the three
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model compounds, while char formation occurs only for glucose and guaiacol.
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reactions were incorporated into a single network as outlined in Figure 4. As shown, furfural
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was formed from the dehydration of both a six-carbon sugar and a five-carbon sugar. As
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such, the gas yield obtained herein reflected the sum of all gasified products from the three
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model compounds. In addition, the char yield reflected the sum of all solid products from
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glucose and guaiacol, while the furfural yield was derived from both glucose and xylose.
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Furthermore, TOCs, which are unknown intermediates in the liquid effluent, were defined as
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TOC1, TOC2, and TOC3 according to their source model compound, i.e., glucose, xylose,
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and guaiacol, respectively.
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assumption can be expressed as follows:
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dYc (glucose) dt
The kinetic rate equations using the first order reaction
= − k gf + k gfu + k gt1 + k g5 Yc (glucos e)
(
(2)
)
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dYc (fructose) = k gf Yc (glucose) − k f5 + k ffu + k ft1 Yc (fructose) dt
(3)
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dYc (5 − HMF) = k g5 Yc (glucose) + k f5 Yc (fructose) − k 5t1 + k 5c Yc (5 − HMF) dt
(4)
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dYc (furfural) 5 5 = k gfu Y c (glucose) + k ffu Yc (fructose) dt 6 6 + k xf Yc (xylose) + k xyf Yc (xylulose) − k fut1 + k fuc + k ft2 Yc Yc (furfural)
(5)
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dYc (TOC1) 1 1 = k gt1 + k gfu Yc (glucose) + k ft1 + k ffu Yc (fructose) + k 5t1Yc (5 − HMF) + k fut1Yc (furfural) dt 6 6 − k t1c + k t1g Yc (TOC1)
(
)
(
)
(6)
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dYc (char) = k fuc Yc (furfural) + k 5c Yc (5 − HMF) + k t1c Yc (TOC1) dt + k pch Yc (phenol) + k bch Yc (benzene) + k t3ch Yc (TOC3) + k guch Yc (guaiacol)
(7)
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dYc (gas) = k t1g Yc (TOC1) + k fog Yc (formaldehyde) + k t2g Yc (TOC2) dt + k t3ga Yc (TOC3) + k guga Yc (guaiacol)
(8)
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dYc (xylose) = k xyx Yc (xylulose)− (k xxy + k xf + k xgl + k xt2 )Yc (xylose) dt
(9)
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dYc (xylulose) = k xxy Yc (xylose)− (k xyx + k xyf + k xygc + k xyt2 )Yc (xylulose) dt
(10)
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dYc (furfural) = k xf Yc (xylose)+ k xyf Yc (xylulose)- k ft2 Yc (furfural) dt
(11)
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dYc (glyceraldehyde) 3 = k xgl Yc (xylose)+ k dgl Yc (dihydroxyacetone) dt 5 - (k glgc + k glt2 )Yc (glyceraldehyde)
(12)
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dYc (glycolaldehyde) 2 2 2 = k xgl Yc (xylose) + k xygc Yc (xylulose)+ k glgc Yc (glyceraldehyde) dt 5 5 3 - k gct2 Yc (glycolaldehyde)
(13)
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dYc (formaldehyde) 1 = k glgc Yc (glyceraldehyde) − k fog Yc (formaldehyde) dt 3
(14)
dYc (dihydroxyacetone) 3 = k xygc Yc (xylulose) + k gld Yc (glyceraldehyde) dt 5 - (k dgl + k dt2 )Yc (dihydroxyacetone)
(15)
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189
190
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dYc (TOC2) = k xt2 Yc (xylose) + k xyt2Yc (xylulose) + k ft2 Yc (furfural) dt + k glt2Yc (glyceraldehyde) + k gct2Yc (glycolaldehyde)
(16)
+ k dt2 Yc (dihydroxyacetone) - k t2g Yc (TOC2)
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dYc (guaiacol) = k guoc + k gut3 + k guc + k gup + k guch + k guga Yc (guaiacol) dt
(17)
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dYc (catechol) = k guc Yc (guaiacol) - k ct + k cp + k coc Yc (catechol) dt
(18)
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dYc (phenol) = k cp Yc (catechol) + k gup Yc (guaiacol) + k pt3 Yc (TOC3) − k pch Yc (phenol) dt
(19)
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dYc (m - cresol) = k ocmc Yc (o - cresol) + k t3mc Yc (TOC3) dt
(20)
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dYc (o - cresol) = k guoc Yc (guaiacol) + k coc Yc (catechol) − k oct3 + k ocmc Yc (o - cresol) dt
(21)
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dYc (benzene) = k t3b Yc (TOC3) + k gub Yc (guaiacol) − k bch Yc (benzene) dt
(22)
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dYc (TOC3) = k gut3 Yc (guaiacol) + k ct3 Yc (catechol) + k oct3 Yc (o − cresol) dt − k t3ga + k t3b + k t3ch + k t3p + k t3mc Yc (TOC3)
(23)
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where YC, t, and k denote the carbon yield of each compound, the reaction time, and the
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reaction rate constant, respectively. The least square error (LSE) estimation was employed to
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determine the values that gave the best fit with the experimental data. Figure 5 reveals a
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parity plot comparing the experimental values with predicted values of the product yield of
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every compounds at all experimental conditions.
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Table 2 shows calculated kinetic rate constants of each reaction in the glucose, xylose,
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and guaiacol reaction networks defined previously. When no interaction takes place between
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the various model compounds, the kinetic rate constants should remain constant.
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previously calculated values for each reaction as described in the literature19 are therefore
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shown alongside our reported values to determine whether any interactions took place.
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Under the experimental conditions employed herein, glucose and fructose were not observed
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in the final products, and the isomerization rate from glucose to fructose could not be
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determined theoretically. As both glucose and fructose produce the same products (i.e., 5-
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HMF, furfural, and TOC1), the kinetic parameters employed for the isomerization of glucose
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to fructose (gf), the dehydration of fructose to 5-HMF (f5) and furfural (ffu), and the
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decomposition of fructose (ft1) were set to zero. The numbers in bold type are the reaction
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rate constants that were reduced by
