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Effect of acetic acid addition on decomposition of xylose in supercritical water Tanawan Chalermsaktrakul, and Yukihiko Matsumura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02720 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018
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Energy & Fuels
Effect of acetic acid addition on decomposition of xylose in supercritical water Tanawan Chalermsaktrakul, Yukihiko Matsumura* Department of Mechanical Science and Engineering, Hiroshima University *
To whom correspondence should be addressed. 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 Japan,
Fax: +81-82-422-7193. E-mail:
[email protected].
Abstract: The supercritical water gasification of xylose, a model substrate for hemicellulose, was carried out at 400 and 450 °C and at a constant pressure of 25 MPa in the presence of acetic acid using a continuous flow reactor. More specifically, we aimed to compare the reaction rate constants of xylose decomposition both in the presence and absence of acetic acid. Upon the application of a residence time of 0.5–5 s, a xylose concentration of 1.5 wt%, and an acetic acid concentration of 1.5 wt%, we successfully elucidated the effect of acetic acid on each reaction within the reaction network for the first time. In the presence of acetic acid, the retro-aldol reactions and carbon gasification production (i.e., the radical reactions) were suppressed, while the acetic acid-catalyzed dehydration of xylulose to furfural (i.e., an ionic reaction) was enhanced by two orders of magnitude. As such, reaction control through the addition of chemical species to either stabilize ions or react with radicals appears possible.
Keywords: xylose, supercritical water gasification (SCWG), biomass, acetic acid, catalyst
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1. Introduction
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Supercritical water gasification (SCWG) is a promising technology for gasifying biomass
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containing a high moisture content 1-3, and is typically carried out at temperatures and pressures above the
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critical points of 374.2 °C and 22.1 MPa, respectively.
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supercritical water, water can act as both a reactant and as a reaction medium, thereby rendering this
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method suitable for wet biomass, as the costly drying step can be omitted. In addition, the cellulose,
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hemicellulose, and lignin components of biomass are homogeneously dissolved in supercritical water 4-6,
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thereby allowing gasification to take place rapidly without any issues relating to mass transfer between
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phases.
As the gasification process takes place in
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As an improved understanding of the decomposition mechanisms of the various biomass
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components can lead to improvements in the SCWG process, our focus has primarily been on the use of
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model compounds to elucidate the key reaction mechanisms taking place. In this context, glucose7-9,
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guaiacol10, and xylose11 have been employed as model substrates for cellulose, lignin, and hemicellulose,
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respectively. Indeed, we previously succeeded in proposing a potential reaction network, in addition to
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determining the reaction rate of each reaction in the network. Upon investigation into the effect of
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temperature on the various reactions, we found that two types of reaction existed, namely one that
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followed Arrhenius law, and one that did not12. Considering that the dielectric constant (i.e., a reflection
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of ion stability) of water varies with temperature under a constant pressure, the former was assumed to be
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a radical reaction, while the latter was assumed to be an ionic reaction. This assumption accounts for the
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fact that rapid heating during the SCWG results in a high gasification efficiency, while slow heating
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results in a low gasification efficiency and increased char production13. This can be explained that rapid
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heating inhibits the ionic reactions that produce tarry materials, and slow heating promotes such ionic
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reactions, thereby reducing the gasification efficiency.
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We therefore expected that this assumption could explain other interesting characteristics of the
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SCWG process.
As previously reported, under hydrothermal conditions, formic acid catalyzes the
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decomposition of xylose, the formation of furfural, and the decomposition of furfural14 between 160 and
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200 °C. In addition, Chen et al. reported that the transformation of xylose into furfural was catalyzed by ACS Paragon Plus Environment
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acetic acid under hydrothermal conditions at 170–210 °C15. It was therefore proposed that the addition of
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an organic acid could both enhance and retard specific reactions within the network, thereby causing the
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observed effects. A similar effect should therefore be expected in the case of supercritical water, although
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interestingly, acetic acid is a by-product of the SCWG of xylose. However, to date, this approach to
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explain the SCWG characteristics based on the effect of additives on the various reactions within the
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network has not yet been examined. Thus, the purpose of this study is to investigate the effect of acetic
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acid addition on the SCWG of xylose with a specific focus on the effect of this organic acid on each
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reaction within the overall network.
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2. Experimental ᴅ-(+)-xylose (>98%) and acetic acid (>99.9%) were purchased from Nacalai Tesque, Inc., and Sigma Aldrich Co., respectively.
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All experiments were carried out using a tubular flow reactor, as shown in Figure 1. The reactor
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was composed of SS316 steel, and had inner and outer diameters of 1 and 1.59 mm, respectively. Three
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high pressure pumps (Nihon Seimitsu Kagaku Co., Ltd, Japan) were employed, and the reactor length and
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flow rate were varied as required to adjust the residence time. The reactor and a section of the pre-heater
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were soaked in the molten salt, water was fed into the pre-heater, and the pre-heated water was fed and
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mixed with the feedstock containing xylose and aqueous acetic acid at room temperature in a 4:1 mass
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ratio just before entering the reactor. Following rapid heating of the feedstock to the reaction temperature
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by this mixing, the reaction took place in the reactor, and then the product was mixed with an equal
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quantity of cold water upon passing through the reactor to quench the reaction. Product samples were
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taken after 1 h to ensure steady state conditions. Filters with a replaceable sintered element of 7 µm
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nominal pore sizes were placed to protect the back pressure regulator
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The reaction was conducted at either 400 or 450 °C, and at a pressure of 25 MPa. Prior to mixing
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with pre-heated water, the initial acetic acid concentration was changed between 0 and 12.5 wt%. In
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addition, the residence time was varied between 0.5 and 5 s.
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Initially, the effect of acetic acid
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concentration was investigated at a constant residence time of 3 s. Subsequently, the effect of residence
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time was studied with an initial acetic acid concentration of 7.5 wt%.
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The liquid effluents were analyzed by total organic carbon (TOC) analysis and high-performance
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liquid chromatography (HPLC). More specifically, the quantities of carbon in the liquid products (non-
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purgeable organic carbon) and in the dissolved gas products (inorganic carbon) of the feedstock and the
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liquid products were quantified using the TOC analyzer (Shimadzu TOC-V CHP) to confirm the carbon
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balance. The xylose, xylulose, glyceraldehyde, glycolaldehyde, formaldehyde, and acetic acid contents of
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the liquid effluents were analyzed by HPLC using an SCR102H column (Shimadzu), with an aqueous
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0.005 M HClO4 solution as the mobile phase (column temperature = 40 °C, flow rate = 0.7 mL/min,
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equipped with a refractive index detector (RID)). In addition, an RSpak DE-413L column (Shodex) with
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an aqueous 0.01 M H3PO4 solution as the mobile phase (column temperature = 40 °C, flow rate =
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0.7 mL/min, equipped with an RID) was used to determine the dihydroxyacetone and formic acid
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contents. Furthermore, an RSpak DE-413L column (Shodex) with a mixture of aqueous 0.005 M HClO4
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and acetonitrile (1:1, v/v) (column temperature = 40 °C, flow rate = 0.5 mL/min, equipped with an SPD
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detector) was used for the analysis of furfural.
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Gas chromatography (GC, Shimadzu GC-14B) was employed to analyze the gaseous products.
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More specifically, CO2 and CO were detected by GC-TCD (GC equipped with a thermal conductivity
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detector) using He as the carrier gas, while CH4, C2H4, and C2H6 were detected by GC-FID (GC fitted
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with a flame ionization detector), again using He as the carrier gas. In addition, H2 was detected by GC-
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TCD using N2 as the carrier gas.
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The product yield of compound X, YC (X) , was calculated based on the number of carbon atoms using the following equation: YC ( X ) =
nC ( X ) nC 0
(1)
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where nC (X) and nC0 denote the molar amount of carbon atoms in product X and the initial molar
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amount of carbon atoms in the xylose feedstock, respectively. The solid phase was rarely observed
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throughout these experiments, and so it was disregarded in this study. In addition, the carbon balance was ACS Paragon Plus Environment
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always >80%. Following a preliminary experiment, we found that no acetic acid decomposition took
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place at 400 °C and 25 MPa with a 3 s residence time and at acetic acid concentrations of 0.5–2 wt%.
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This observation agrees with the zero decomposition of acetic acid in the absence of an MnO2 catalyst but
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in the presence of oxygen, as observed by Yu and Savage16, in addition to the acetic acid stability in
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supercritical water as reported by Watanabe et al.17 Indeed, acetic acid decomposition was previously
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observed only at 600 °C, but this was promoted by catalysis from the metal reactor wall18, and so the
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conversion was expected to be
