Article pubs.acs.org/IECR
Kinetics of Glycerol Dehydration with WO3/TiO2 in Supercritical Water Makoto Akizuki* and Yoshito Oshima Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan ABSTRACT: Dehydration of glycerol on TiO2 and WO3/TiO2 in supercritical water at 400 °C and 33 MPa was examined using a fixed bed flow reactor. The structure of TiO2 was changed from anatase type to rutile type in supercritical water, and addition of WO3 to TiO2 suppressed the change of structure. Main products were acrolein and acetaldehyde. In the reaction on TiO2, a large amount of propionic acid was also produced. The reaction route was proposed on the basis of the products distribution from glycerol and intermediate products. From kinetic analysis, the reaction route could explain the experimental data well. Because of high water density, degradation of acrolein to acetaldehyde is favorable in supercritical water. Reaction rate for dehydration of glycerol to acrolein considerably increased with an increase in WO3 content of the catalyst, and this dependence is attributed to higher surface area and stronger acidity of WO3/TiO2 catalysts.
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INTRODUCTION Glycerol is known as a byproduct of the biodiesel synthesis process. As the producing amount of biodiesel increases, conversions of glycerol to other useful chemicals are attracting much attention. In particular, dehydration of glycerol to acrolein is industrially important because acrolein is known as a raw material for the synthesis of valuable chemicals such as acrylic acid and methionine. Many researchers investigated this reaction.1−7 Most of researches were performed in gas phase using solid acid catalysts. Researches were also performed in suband supercritical water because reaction conditions can be controlled more specifically in sub- and supercritical water and crude glycerol from biodiesel synthesis process has high content of water. Previously, glycerol reactions in sub- and supercritical water without catalysts and with homogeneous catalysts had been investigated. It is reported that acrolein was produced using H2SO42,8 and ZnSO4,3 and lactic acid was produced using NaOH.9 Sub- and supercritical water is also considered to be suitable for heterogeneous reactions because its high diffusion rate can reduce the effect of mass transfer. In addition, heterogeneous catalysts can avoid the corrosion problem of the reactor, which severely occurs when using homogeneous acid/ base catalysts. Therefore, many researchers have examined performances of solid acid/base catalysts for conversions of biomass derived materials10,11 and organic syntheses12−15 in suband supercritical water. Yet, until now, detailed examination of glycerol reactions using solid acid in supercritical water has not been reported. In this study, we investigated solid acid catalyzed dehydration of glycerol in supercritical water. We used TiO2 and WO3/TiO2 as solid acid catalysts. TiO2 is a well-known catalyst for acidcatalyzed reactions in supercritical water.13,16 WO3/TiO2 is known as a strong solid acid for reactions in the gas phase.17 In subcritical water, it is reported that WO3 deposited on ZrO2 showed good activity to hydration of cyclohexene.14 Recently, it is reported that WO3/TiO2 is effective for dehydration of glycerol to acrolein in the gas phase,7 but the effect of WO3 on © 2012 American Chemical Society
respective reactions has not been reported. The purpose of this study is to investigate solid acid catalyzed glycerol reactions in supercritical water and discuss effects of WO3 to catalytic properties and reactions.
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EXPERIMENTAL METHODS Reagents. Glycerol (Kanto Chemical Co., Inc.), acrolein (Tokyo Chemical Industries Co., Ltd.), and hydroxyacetone (Wako Pure Chemical Industries, Ltd.) were used as purchased. Distilled water was prepared by distillation equipment (RFD240HA; Advantec Toyo Kaisha, Ltd.). Aqueous organic solutions and distilled water were degassed by N2 gas bubbling prior to use. Catalysts Preparation. For the TiO2 catalyst, anatase type TiO2 powder (Wako Pure Chemical Industries, Ltd.) was used. This anatase type TiO2 contained ca. 10 wt.% rutile type TiO2. It was calcined at 500 °C for 6 h. WO3/TiO2 was prepared by the impregnation method as follows. TiO2 powder was added to an aqueous solution of (NH4)10W12O41·5H2O (Wako Pure Chemical Industries, Ltd.) and stirred for 4 h. After evaporating the water, the catalyst was dried in a vacuum oven and then calcined at 600 °C for 6 h. According to the WO3 content, we abbreviate the WO3/TiO2 catalysts as xWTi (x is the WO3 content [wt %]) in this paper. All catalysts were shaped in granular forms of 0.3−0.5 mm or 0.5−0.7 mm in diameter. Prior to use, catalysts were conditioned in supercritical water at 450 °C and 25 MPa to prevent transformation of anatase type to rutile type during the reaction, which is described in a later section. Procedure. Experiments were conducted using an isothermal, isobaric tubular plug flow reactor (Figure 1) at 400 °C and 33 MPa. Glycerol solution and distilled water were Received: Revised: Accepted: Published: 12253
July 9, 2012 September 3, 2012 September 5, 2012 September 5, 2012 dx.doi.org/10.1021/ie301823f | Ind. Eng. Chem. Res. 2012, 51, 12253−12257
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hand, the change of structure from anatase type to rutile type was suppressed with an increase in WO3 content, and almost no transformation of anatase to rutile took place with the addition of more than 5 wt % WO3 to the catalyst. It is reported that W reduced microstrains in the structure of TiO2 and inhibited transformation from anatase type to rutile type.19 Peaks of WO3 disappeared until the WO3 content reached 17 wt %, which indicates that some of WO3 and TiO2 made solid solutions. Table 1 shows the surface area of each catalyst and results of the acidity measurement of TiO2 and 5WTi. Surface areas of Table 1. BET Surface Area and Acidity of Catalysts
Figure 1. Schematic diagram of experimental equipment.
Acidity [mmol/g]
pumped separately using HPLC pumps (PU-2080 and PU-2085; JASCO Corp.), and water was preheated so that the temperature at the reactor entrance was 400 °C. These two streams were mixed and fed into a fixed bed reactor (SUS316 tubing of 2.17 mm or 3.87 mm i.d. packed with catalysts) set in a GC oven. Initial concentration of glycerol at the reactor entrance was 0.05 mol/dm3. The stream emitted from the reactor was cooled immediately in a water-cooled heat exchanger and depressurized by a back pressure regulator (SCF-Bpg; JASCO Corp.). The effluents were collected in sampling vials and kept in a refrigerator until analysis. Analysis. Organics in the samples were analyzed by HPLC equipped with RI detector (RID-10A; Shimadzu Corp.) and a UV detector (SPD-10A; Shimadzu Corp.). Two columns (ULTRON PS-80H; Shinwa Chemical Industries, Ltd. and SCR-102H; Shimadzu Corp.) were used in series for separation. GC-MS (GC-MS-QP2010; Shimadzu Corp.) with a capillary column (InertCap Pure-Wax; GL Sciences, Inc.) was also used for analyzing the reaction products. Catalysts were characterized by XRD (SmartLab; Rigaku Corp.), surface area analyzer (Jemini 2360; Shimadzu Corp.), and acidity measurement (Benesi method18 using methyl red, dicinnamalacetone, and anthraquinone as Hammet indicators.).
#
catalyst name
WO3 content [wt.%]
Surface area [m2/g]
−8.2 < H0 ≤ −3.0
−3.0 < H0 ≤ 4.8
1 2 3 4 5
TiO2 1WTi 5WTi 10WTi 17WTi
− 1 5 10 17
12.1 15.2 39.3 44.4 41.7
0.00 − 0.11 − −
0.09 − 0.04 − −
WO3/TiO2 catalysts were higher than that of the TiO2 catalyst due to a higher surface area of anatase type TiO2. Concerning the acidity of the catalyst, the TiO2 catalyst only had weaker acidic sites (−3.0 < H0 ≤ 4.8). On the other hand, the number of stronger acidic sites (−8.2 < H0 ≤ −3.0) increased and weaker acidic sites decreased with the addition of WO3. Reaction Rate and Products. Figure 3 shows glycerol conversion on each catalyst at 400 °C and 33 MPa. W/F, defined
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RESULTS Characterization of Catalysts. Figure 2 shows XRD patterns of each catalyst after conditioning in supercritical water. The sructure of the TiO2 catalyst was largely changed from anatase type to rutile type in supercritical water. On the other
Figure 3. Glycerol conversion at 400 °C and 33 MPa. (●) TiO2, (▲) 1WTi, (□) 5WTi, (○) 10WTi, (⧫) 17WTi, and (- - -) fitting results.
as the amount of catalyst divided by volumetric flow rate, was used as an indicator of reaction time. All catalysts promoted glycerol conversion in supercritical water. Reaction rate increased with an increase in WO3 content. In addition, reaction rates on 10WTi and 17WTi were almost same. Regardless of catalyst species, acrolein and acetaldehyde were mainly produced. A considerable amount of propionic acid was also produced in the reaction on the TiO2 catalyst, but the production was suppressed with the addition of WO3 to the catalyst. Hydroxyacetone and lactic acid was produced in relatively high selectivity in an initial stage of the reaction, but they decreased as the reaction proceeded. In addition to these products, allyl alcohol, propionaldehyde, acetone, acetic acid, and acrylic acid were also produced as byproducts. Figure 4 shows the effect of WO3 on the products yield of acrolein, acetaldehyde, propionic acid, hydroxyacetone, and lactic acid. As the WO3 content of the catalyst increased, acrolein selectivity was increased and acetaldehyde selectivity was
Figure 2. XRD patterns of catalysts. (○) TiO2 of anatase type, (⧫) TiO2 of rutile type, and (▲) WO3. 12254
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Figure 4. Product yields of glycerol reactions at 400 °C and 33 MPa. (a) TiO2, (b) 1WTi, (c) 5WTi, (d) 10WTi, and (e) 17WTi. Experimental data: (●) acrolein, (▲) acetaldehyde, (□) hydroxyacetone, (○) lactic acid, and (⧫) propionic acid. Fitting results: (―) acrolein, (− −) acetaldehyde, (•••) hydroxyacetone, and (−•−•) lactic acid.
Tsukuda et al. reported the reaction route of glycerol dehydration in gas phase catalyzed by solid acids.4 They speculated that glycerol is dehydrated into 3-hydroxypropionaldehyde (3-HPA) or hydroxyacetone, and 3-HPA is readily dehydrated into acrolein or decomposed into acetaldehyde and formaldehyde through the retro-aldol reaction. Because acrolein and acetaldehyde were the main products in supercritical water and hydroxyacetone selectivity in an initial stage of the reaction was relatively high, these reactions (R1, R2, R3, R4) are taken into consideration in the proposed reaction route. In addition, our experimental data indicated that degradation of acrolein occurred. We conducted reactions of acrolein on TiO2 at 400 °C and 33 MPa to investigate the behavior of acrolein, and the products were only acetaldehyde and formaldehyde. Hall et al. reported that acrolein was hydrated into 3-HPA in aqueous phase using H2SO4 catalyst.20 Therefore, we consider that acrolein degraded into acetaldehyde through 3-HPA as an intermediate (R−2, R3). In addition to these three products, lactic acid was produced in an initial stage of the reaction. One possible production route for lactic acid is direct production from glycerol (R5) catalyzed by basic sites of metal oxide catalysts. Kishida et al. reported that glycerol was converted into lactic acid through pyruvaldehyde as an intermediate in subcritical water using NaOH catalyst.9 Additionally, we conducted reactions of hydroxyacetone on TiO2 at 400 °C and 33 MPa, and a large portion of the product in
decreased. Similar to the reaction rates, products selectivity on 10WTi and 17WTi were almost the same. Figure 5 shows the relationships between glycerol conversion and carbon balance in liquid phase in the reaction on each
Figure 5. Carbon balance at 400 °C, 33 MPa. (●) TiO2, (▲) 1WTi, (□) 5WTi, (○) 10WTi, and (⧫) 17WTi.
catalyst. It indicates that almost all products were recovered in liquid phase. Loss of carbon balance in the high conversion region was probably due to formation of formaldehyde, which was not analyzed quantitatively due to a separation problem.
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DISCUSSION Reaction Route. A plausible reaction route is shown in Figure 6 on the basis of the results of the previous section. 12255
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Figure 6. Main reaction route in supercritical water.
Table 2. Pseudo-First-Order Kinetic Rate Constants of Each Reaction kinetic rate constants [m3/kg-cat s] catalyst
k1
k2
k−2
k3
k4
k5
k6
k7
TiO2 1WTi 5WTi 10WTi 17WTi
6.5 × 10−5 1.9 × 10−4 8.5 × 10−4 1.2 × 10−3 1.3 × 10−3
7.8 × 10−2 5.0 × 10−1 9.3 × 10−1 4.4 × 10−1 4.3 × 10−1
3.0 × 10−4 1.7 × 10−3 3.6 × 10−3 1.7 × 10−3 1.6 × 10−3
6.8 × 10−2 1.5 × 10−1 1.0 × 10−1 6.9 × 10−2 6.9 × 10−2
2.1 × 10−5 3.1 × 10−5 9.7 × 10−5 1.2 × 10−4 9.8 × 10−5
6.7 × 10−6 6.2 × 10−6 2.6 × 10−5 3.6 × 10−5 1.9 × 10−5
1.7 × 10−3 2.6 × 10−3 1.8 × 10−3 1.4 × 10−3 9.7 × 10−4
2.5 × 10−5 2.2 × 10−5 2.6 × 10−5 2.8 × 10−5 2.2 × 10−5
(Yacrolein, Yacetaldehyde, Yhydroxyacetone, Ylactic acid) can be described as a function of W/F and ki. Kinetic rate constants of each reaction (ki) were evaluated by fitting equations of Xglycerol, Yacrolein, Yacetaldehyde, Yhydroxyacetone, and Ylactic acid to the experimental data. Because a restriction for fitting is not enough, equilibrium constants between R2 and R−2 (K2) was used for evaluating the relationships between k2 and k−2. The value of K2 was estimated using THERGAS,22 which is a computer program for estimating thermodynamic properties. Fitting results are shown in Figures 3 and 4. Experimental data are explained well by proposed reaction route. Kinetic rate constants derived from fitting are shown in Table 2. As the WO3 content of a catalyst increased, the kinetic rate constant of glycerol dehydration to 3-HPA (k1) largely increased. In addition, the dependence of the kinetic rate constant of 3-HPA degradation to acetaldehyde (k3) on the WO3 content of the catalyst was smaller than that of 3-HPA dehydration to acrolein (k2). As a result, a higher acrolein yield was obtained in the reaction on the WO3/TiO2 catalyst than on the TiO2 catalyst. One reason for the dependence of k1 on the WO3 content of catalyst is that WO3/TiO2 catalysts have larger surface areas than TiO2 catalysts (Table 1). However, the rate constant per surface area (k1 devided by surface area) also increased as the WO3 content of the catalyst increased, which suggests that the change in surface area is not the only reason for the dependence of k1 on the WO3 content. The other reason may relate to the acidity strength of WO3/TiO2. Compared to the acidity of the TiO2 catalyst, total acidity of 5WTi was 1.7 times higher, and 5WTi had large amounts of stronger acidic sites (−8.2 < H0 ≤ −3.0) (Table 1). Because k1 per surface area of the 5WTi catalyzed reaction was 4.0 times higher than that of the TiO2 catalyzed reaction, dehydration of glycerol to 3-HPA is probably more promoted on stronger acidic sites (−8.2 < H0 ≤ −3.0) than
an initial stage of the reaction was lactic acid. Therefore, we consider that the production route of hydroxyacetone from lactic acid also existed (R6). Propionic acid, which was produced in the reaction on the TiO2 catalyst, is considered to be produced from lactic acid (R7). Mok et al. reported formation of propionic acid from lactic acid in supercritical water.21 A major difference between reactions in supercritical water and reactions in gas phase is that degradation of acrolein to acetaldehyde (R−2, R3) proceeds favorably in supercritical water due to high water concentration. In fact, acetaldehyde yields of this study were higher than that previously reported in gas phase reactions using TiO2 and WO3/TiO2 as catalysts.7 To obtain high acrolein yield in supercritical water, suppressing hydration of acrolein to 3-hydroxypropionaldehyde (3HPA) and/or retro-aldol reaction of 3-HPA to acetaldehyde are very important. Kinetic Analysis and Effect of WO3 on Reactions. In order to discuss characteristics of glycerol reactions in supercritical water on TiO2 and WO3/TiO2, kinetic analysis was conducted on the basis of the proposed reaction route (Figure 6). R1, R4, and R5 are considered to be first-order reactions to glycerol because glycerol conversions were independent of initial glycerol concentration. Other reactions are also assumed as firstorder reactions to the reactant. Although water concentration may affect some reactions such as R−2, these reactions are assumed as pseudo-first-order reactions to the reactant because water concentration is considerably high. The reaction rate of Ri (ri) can be described as follows ri = k iCreactant(i = 1 ∼ 7, −2)
(1)
where ki is the pseudo-first-order kinetic rate constant, and Creactant is the concentration of each reactant. Using these equations, glycerol conversion (Xglycerol) and product yields 12256
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weaker acidic sites (−3.0 < H0 ≤ 4.8) in supercritical water. It is reported that solid acid catalysts that possess acidic sites of −8.2 < H0 ≤ −3.0 showed high acrolein selectivity in gas-phase dehydration of glycerol at 315 °C,23 and our results in supercritical water are consistent with the previous report. The small dependence of k3 on the WO3 content of the catalyst suggests that the retro-aldol reaction of 3-HPA to acetaldehyde per surface area was suppressed on WO3/TiO2 catalysts. Because retro-aldol reaction is a base-catalyzed reaction, suppression of R3 may be attributed to lower basicity per surface area of WO3/ TiO2 catalysts. Ramis et al. reported that addition of WO3 to TiO2 reduces basicity of the catalyst due to the interaction of acidic WO3 with the basic sites of TiO2,24 which is consistent with our findings.
(3) Ott, L.; Bicker, M.; Vogel, H. Catalytic dehydration of glycerol in sub- and supercritical water: A new chemical process for acrolein production. Green Chem. 2006, 8, 214. (4) Tsukuda, E.; Sato, S.; Takahashi, R.; Sodesawa, T. Production of acrolein from glycerol over silica-supported heteropoly acids. Catal. Commun. 2007, 8, 1349. (5) Suprun, W.; Lutecki, M.; Haber, T.; Papp, H. Acidic catalysts for the dehydration of glycerol: Activity and deactivation. J. Mol. Catal. A: Chem 2009, 309, 71. (6) Corma, A.; Huber, G.; Sauvanaud, L.; Oconnor, P. Biomass to chemicals: Catalytic conversion of glycerol/water mixtures into acrolein, reaction network. J. Catal. 2008, 257, 163. (7) Ulgen, A.; Hoelderich, W. F. Conversion of glycerol to acrolein in the presence of WO3/TiO2 catalysts. Appl. Catal., A 2011, 400, 34. (8) Ramayya, S.; Brittain, A.; DeAlmeida, C.; Mok, W.; Antal, M. J., Jr Acid-Catalysed dehydration of alcohols in supercritical water. Fuel 1987, 66, 1364. (9) Kishida, H.; Jin, F.; Zhou, Z.; Moriya, T.; Enomoto, H. Conversion of glycerin into lactic acid by alkaline hydrothermal reaction. Chem. Lett. 2005, 34, 1560. (10) Watanabe, M.; Aizawa, Y.; Iida, T.; Nishimura, R.; Inomata, H. Catalytic glucose and fructose conversions with TiO2 and ZrO2 in water at 473 K: Relationship between reactivity and acid−base property determined by TPD measurement. Appl. Catal., A 2005, 295, 150. (11) Chareonlimkun, A.; Champreda, V.; Shotipruk, A.; Laosiripojana, N. Catalytic conversion of sugarcane bagasse, rice husk and corncob in the presence of TiO2, ZrO2 and mixed-oxide TiO2-ZrO2 under hot compressed water (HCW) condition. Bioresour. Technol. 2010, 101, 4179. (12) Sridhar, T.; Mahajani, S. M.; Shanna, M. M. Direct Hydration of propylene in liquid phase and under supercritical conditions in the presence of solid acid catalysts. Chem. Eng. Sci. 2002, 57, 4877. (13) Tomita, K.; Oshima, Y. Enhancement of the catalytic activity by an ion product of sub- and supercritical water in the catalytic hydration of propylene with metal oxide. Ind. Eng. Chem. Res. 2004, 43, 2345. (14) Yuan, P.-Q.; Liu, Y.; Bai, F.; Xu, L.; Cheng, Z.-M.; Yuan, W.-K. Hydration of cyclohexene in sub-critical water over WOx−ZrO2 catalysts. Catal. Commun. 2011, 12, 753. (15) Akizuki, M.; Tomita, K.; Oshima, Y. Kinetics of solid acid catalyzed 1-octene reactions with TiO2 in sub- and supercritical water. J. Supercrit. Fluids 2011, 56, 14. (16) Watanabe, M.; Iida, T.; Aizawa, Y.; Ura, H.; Inomata, H.; Arai, K. Conversions of some small organic compounds with metal oxides in supercritical water at 673 K. Green Chem. 2003, 5, 539. (17) Yamaguchi, T.; Tanaka, Y.; Tanabe, K. Isomerization and disproportionation of olefins over tungsten oxides supported on various oxides. J. Catal. 1980, 65, 442. (18) Benesi, H. A. Acidity of catalyst surfaces. II. Amine titration using hammett indicators. J. Phys. Chem. 1957, 61, 970. (19) Depero, L. E. Influence of vanadium and tungsten substitution on the stability of anatase. J. Solid State Chem. 1993, 104, 470. (20) Hall, R. H.; Stern, E. S. Acid-catalysed hydration of acraldehyde. kinetics of the reaction and isolation of β-hydroxypropaldehyde. J. Chem. Soc. 1950, 490. (21) Mok, W. S. L.; Antal, M. J.; Jones, M. Formation of acrylic-acid from lactic-acid in supercritical water. J. Org. Chem. 1989, 54, 4596. (22) Muller, C.; Michel, V.; Scacchi, G.; Come, G. M. THERGAS − A computer program for the evaluation of thermochemical data of molecules and free radicals in the gas phase. J. Chim. Phys. Phys. Chim. Biol 1995, 92, 1154. (23) Tao, L.-Z.; Chai, S.-H.; Zuo, Y.; Zheng, W.-T.; Liang, Y.; Xu, B.-Q. Sustainable production of acrolein: Acidic binary metal oxide catalysts for gas-phase dehydration of glycerol. Catal. Today 2010, 158, 310. (24) Ramis, G.; Busca, G.; Cristiani, C.; Lietti, L.; Forzatti, P.; Bregani, F. Characterization of tungsta-titania catalysts. Langmuir 1992, 8, 1744.
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CONCLUSION Solid acid catalyzed reactions of glycerol in supercritical water at 400 °C and 33 MPa with TiO2 and WO3/TiO2 catalysts were investigated using a fixed bed flow reactor. In supercritical water, the structure of the TiO2 catalyst was changed from anatase type to rutile type. With addition of WO3 to the catalyst, the change in structure from anatase type to rutile type was suppressed. As a result, the WO3/TiO2 catalyst showed a higher surface area than that of the TiO2 catalyst due to the high surface area of the anatase phase. Similar to solid acid catalyzed reactions of glycerol in gas phase, dehydration of glycerol (R1, R4) proceeded as main reactions in supercritical water. In addition, production of lactic acid (R5, R6) also occurred in our experimental condition. Because of high water density, degradation of acrolein to acetaldehyde (R−2, R3) is favorable in supercritical water, and suppressing these reactions is important for high acrolein yield. The reaction rate for dehydration of glycerol to 3hydroxypropionaldehyde (R1) considerably increased with the addition of WO3 to the catalysts. Stronger acidic sites (−8.2 < H0 ≤ −3.0) are considered to promote the reaction strongly. Degradation of 3-hydroxypropionaldehyde to acetaldehyde (R3) was not promoted with the addition of WO3 to the catalysts, and this may be attributed to lower basicity per surface areas of the WO3/TiO2 catalysts than to those of the TiO2 catalyst.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel./Fax: +81 4 7136 4694. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS XRD analysis was performed in the Institute for Solid State Physics, The University of Tokyo, and BET surface area was measured in the Energy Technology Research Institute, the National Institute of Advanced Industrial Science and Technology, all of which are very much appreciated.
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REFERENCES
(1) Ederer, H. J.; Buhler, W.; Dinjus, E.; Kruse, A.; Mas, C. Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water. J. Supercrit. Fluids 2002, 22, 37. (2) Watanabe, M.; Iida, T.; Aizawa, Y.; Aida, T. M.; Inomata, H. Acrolein synthesis from glycerol in hot-compressed water. Bioresour. Technol. 2007, 98, 1285. 12257
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