Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes

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JIEC 3463 1–8 Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes

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Inho Kima,b , Adid Adep Dwiatmokoa,b,c , Jae-Wook Choia , Dong Jin Suha,b,d , Jungho Jaea,b , Jeong-Myeong Haa,b,d,* , Jae-Kon Kime

3 4 5 Q1 6 7 8 9

a

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea Research Center for Chemistry, Indonesian Institute of Sciences (LIPI), Kawasan Puspiptek, Serpong, Tangerang, Indonesia d Green School (Graduate School of Energy and Environment), Korea University, Seoul 02841, Republic of Korea e Research Institute of Petroleum Technology, Korea Petroleum Quality & Distribution Authority, Chungcheongbuk-do 28115, Republic of Korea b c

A R T I C L E I N F O

Article history: Received 28 March 2017 Received in revised form 5 June 2017 Accepted 5 June 2017 Available online xxx Keywords: Hydrodeoxygenation Bio-oil Upgrading Catalyst Coke

10

A B S T R A C T

Using a continuous flow reactor, bio-oil prepared by the pyrolysis of sawdust was upgraded to produce petroleum-like deoxygenated hydrocarbon fuels. Dispersed tiny bio-char particles in the raw bio-oil were removed by extraction using diethyl ether to suppress solid-particle-nucleated coking. Noble metal catalysts, instead of molybdenum-based catalysts, were selected to avoid the continuous addition of sulfur compounds. Among the catalysts, tungstate-zirconia-supported Ru catalysts (Ru/WZr) exhibited high hydrodeoxygenation activity and less formation of cokes, which characteristics are important in the development of feasible upgrading processes. The investigation of catalysts used in this study demonstrates that the larger quantity of Brønsted acid sites compared to Lewis acid sites suppresses the formation of cokes. © 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Introduction

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Lignocellulose-derived pyrolysis oil or bio-oil, because it can be obtained from abundant cheap resources including wood, grass, and other inedible biomass, is a potential feedstock to produce transportation fuels and valuable organic chemicals [1]. In spite of its potential value, crude pyrolysis oil requires further upgrading if it is to be used as a transportation fuel or a feedstock of an oil refinery because of its high acidity, high viscosity, and low heating value [2–4]. These challenging issues of bio-oil can be attributed to the high oxygen content; upgrading processes necessary to perform deoxygenation, including ketonization [5], decarboxylation [6], and hydrodeoxygenation (HDO) [7–12], have been investigated. Although upgrading processes to convert pyrolysis bio-oil to deoxygenated hydrocarbons have been attempted [13– 16], feasible upgrading processes are still under development and continuous operation of those processes is difficult because of the formation of cokes and tars, which cause plugging in the reaction

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

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* Corresponding author at: Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea. Fax: +82 2 958 5209. E-mail address: [email protected] (J.-M. Ha).

systems (Fig. 1) [17,18]. The significant formation of cokes on the catalyst is a major issue in the upgrading of bio-oil. Cobalt- and nickel-molybdenum sulfide catalysts for the HDO of bio-oil, although these materials have been used as industrial hydroprocessing catalysts for decades, are deactivated by the formation of coke [19–21]. Sulfur leached from the sulfide catalysts during the reaction may also contaminate the products [22]. Bifunctional catalysts composed of metals and solid acids have been used for the upgrading of bio-oil [3,20,23,24], and possible deactivation by the formation of cokes has also been studied [8]. Multi-step upgrading [25,26] and esterification [27,28] were attempted to stabilize the bio-oil reactant and to reduce the formation of cokes. In addition to the catalysts studied for the upgrading of bio-oil, a tungsten-based catalyst is another potentially efficient catalyst for the upgrading. Tungsten-based catalysts have been investigated for hydrocracking, dehydrogenation, isomerization, reforming, alcohol dehydration, and olefin oligomerization [29]. In addition, WOx-added catalysts including WOx-Al2O3 [30] and WOx-TiO2 [31] have been used for the catalytic upgrading of model compounds for bio-oil [30–32], which process emphasizes the importance of tungstate monolayers formed on the support, improving the catalytic activity and surface acidity. Because of Brønsted acid sites created by tungsten oxide species, the WOx-ZrO2 catalyst promotes

http://dx.doi.org/10.1016/j.jiec.2017.06.013 1226-086X/© 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Please cite this article in press as: I. Kim, et al., Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.06.013

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Fig. 1. (a) Sticky solid residue at the pipelines and (b) rigid coke formed at the catalyst bed surface. 50

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dehydration step in the upgrading reaction pathway [32]. These tungsten-based catalysts, however, have not yet been used for the upgrading of actual bio-oil. As for the noble metals, Ru is selected as a tungstate-zirconia supported catalyst; Ru catalysts have mainly been studied in long-term operation of bio-oil upgrading, especially for hydrotreating, which is known as a stabilization step at mild temperature (125–300  C) [4]. The objectives of this work are (i) to screen the several catalysts exhibiting catalytic activity for the upgrading of bio-oil; (ii) to select catalysts suitable for continuous operation of the upgrading process; (iii) to understand the relationship between the formation of cokes and acid sites on the catalysts; and (iv) to understand the effects of tungsten on the upgrading activity.

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Experimental section

64

Materials

65

Commercially available catalysts including 10 wt% Pd/C, 5 wt% Pd/C, 5 wt% Pt/C, 5 wt% Ru/C, and 5 wt% Ru/Al2O3 were purchased from Aldrich (Milwaukee, Wisconsin, USA). Palladium(II) acetylacetonate (Pd(C5H7O2)2, 99 wt%), ruthenium chloride hydrate (RuCl3
xH2O, 99.99 wt%), nickel(II) nitrate hexahydrate (Ni (NO3)2
6H2O, 99.999 wt%), activated carbon (Darco, 60–100 mesh), silica-alumina (grade 135), zirconium(IV) hydroxide (Zr(OH)4, 97 wt%), and ammonium metatungstate hydrate ((NH4)6H2W12O40
xH2O, 99.99 wt%) were also purchased from Aldrich (Milwaukee, Wisconsin, USA). Aluminum oxide (g-phase) was purchased from Alfa Aesar (Ward Hill, Massachusetts, USA). Tungstate zirconia was purchased from Wako (Osaka, Japan). Diethyl ether (99 wt%) was purchased from Junsei (Tokyo, Japan).

51 52 53 54 55 56 57 58 59 60 61

66 67 68 69 70 71 72 73 74 75 76 77

Pretreatment of bio-oil

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Crude bio-oil prepared from pine sawdust was supplied by Daekyung Esco Co., Ltd (Incheon, Korea). The biochar particles dispersed in the crude bio-oil, which were not completely collected at the cyclone of fluidized bed pyrolysis system, were removed by the extraction of liquid fraction of bio-oil using diethyl ether (Table 1). The crude bio-oil containing small biochar particles and diethyl ether (1:2 v/v) were mixed and agitated at room temperature. The ether-soluble fraction (upper layer) was collected and diethyl ether solvent was removed using a rotary evaporator. The prepared ether-extracted bio-oil was stored in a refrigerator prior to use.

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Preparation of tungstate-zirconia supports

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The tungstate-zirconia supports were prepared by hydrothermal impregnation of zirconium hydroxide with aqueous solutions of ammonium metatungstate hydrate to obtain tungsten loadings of 10.9 and 14.6 wt%. The prepared slurry was stirred for 3 h at room temperature and heated in an autoclave at 180  C for 12 h. The slurry was then evaporated at 70  C, and subsequently calcined in air at 800  C for 2 h. The obtained powder was described as WZr (x), where x is the W loading (wt%). Commercial tungstate-zirconia from Wako is denoted as WZr(com) [33].

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Preparation of catalysts

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Tungstate-zirconia-supported Pd (Pd/WZr(10.9)), silica-alumina supported Pd (Pd/SiAl), carbon-supported Ni (Ni/C), aluminasupported Ru (Ru/Al2O3), silica-alumina-supported Ru (Ru/SiAl), zirconia-supported Ru (Ru/ZrO2), and tungstate-zirconia-supported Ru (Ru/WZr(14.6), Ru/WZr(10.9), Ru/WZr(com)) were prepared by an incipient wet impregnation method using aqueous solutions of ruthenium chloride hydrate, palladium chloride, or nickel nitrate hexahydrate for the corresponding metals (Table S1). The support was added to the metal precursor solutions, and the mixture was stirred for 12 h under ambient conditions, followed by drying at 70  C in vacuum. Solid residue was further dried at 105  C for 12 h and then reduced for 2 h at 350  C for Pd and Ru and at 550  C for Ni using H2/Ar (5% v/v). Reduced catalysts were passivated using O2/N2 (0.5% v/v) at room temperature for 30 min prior to storage.

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Upgrading of bio-oils to hydrocarbon fuels

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The catalytic testing system is depicted in Fig. S1. A fixed bed reactor (36 mL of net inner volume with 1.8 cm of internal diameter and 14 cm in length), made from 316 stainless steel, was used in this study. A double metal filter (0.5 mm pore size) was placed at the bottom of reactor to support the catalyst. Hydrogen flow was

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Table 1 Compositions of the bio-oils. Feed Entry Entry Entry Entry Entry Entry Entry Entry a

1 1 2 2 3 3 4 4

after extraction with ether after extraction with ether after extraction with ether after extraction with ether

80 81 82 83 84 85 86 87 88 89

92 93 94 95 96 97 98 99

102 103 104 105 106 107 108 109 110 111 112 113 114 115

118 119 120 121

Q4 Q5 b

C (wt%) H (wt%) N (wt%)

S (wt%)

O (wt%)

O/C (atom/atom) H/C (atom/atom) Water content (wt%) HHV (MJ/kg)

46.4 56.1 47.7 57.2 44.4 56.2 48.3 57.7

<0.3(0)c <0.3(0) <0.3(0) <0.3(0) <0.3(0) <0.3(0) <0.3(0) <0.3(0)

43.2 35.4 41.7 33.4 45.2 34.7 39.2 29.6

0.70 0.47 0.66 0.44 0.76 0.46 0.61 0.39

7.3 6.9 7.1 7.7 7.6 7.6 7.2 8.2

1.7 1.0 1.8 1.5 0.24 (<0.3)b 0.7 1.0 0.7

1.87 1.47 1.79 1.61 2.04 1.61 1.80 1.71

20.0 6.2 19.5 6.4 19.5 6.4 20.0 8.5

18.3 22.5 18.8 24.4 17.7 23.6 19.7 25.9

Measured without removal of moisture. b Measured by the oxygen detector. c Less than the detection limit.

Please cite this article in press as: I. Kim, et al., Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.06.013

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controlled using a gas mass flow controller, while the pressure inside the reactor was maintained using a back-pressure regulator (BPR). Cooling water circulated in the condenser was maintained at 2  C to condense the vapor to a liquid. Dry gas meters (0.5 L/rev) were connected directly to the outlet of the gas line to measure the gas flow rate. The temperature of the reactor was monitored at three or more points using a thermocouple around the catalyst bed. Using this reaction system, the hydrodeoxygenation of bio-oil was performed at 300–350  C and 100 bar. A certain volume of catalyst was loaded into the catalyst bed. Before the bio-oil was fed into the reactor, the catalyst was reduced using H2 flow at 350  C for 2 h. The liquid product was weighed. The solid residue (catalyst and coke) in the catalytic bed was collected and weighed. Collected solid residue was washed with 20 mL of acetone and centrifuged three times using 10 mL of acetone to remove soluble organic compounds that remained on the spent catalyst. After the centrifugation, the solid residue was dried at 60  C for 24 h. The amount of coke formed during the reaction was determined by subtracting the weight of fresh catalyst from the weight of solid residue after the reaction.

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Characterization of reactants and products

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Reactants and products were identified using a gas chromatograph–mass spectrometer combination (GC/MS, Agilent 7890A with 5975C inert MS XLD) with an HP-5MS capillary column (60 m  0.25 mm  250 mm). Elemental analysis of reactants and products was performed at the KIST advanced analysis center (Seoul, Korea) using a Flash 2000series CHNS Organic Elemental Analyzer (Thermo Scientific), while oxygen analysis was performed using a Fisons Instruments EA 1108 (Thermo Scientific). The water content in the bio-oils was measured using a Karl Fischer Moisture Titrator MKV-710 with the addition of HYDRANALComposite-5 and a solvent mixture of methanol (75 vol%) and chloroform (25 vol%). The quantity of coke formed during the

123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140

144 145 146 147 148 149 150 151 152 153 154

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reaction was measured using a TGA SDT Q600 instrument (TA Instruments) under air flow. Heating values of the reactant and product were calculated using the DuLong equation [34]: HHV (higher heating value, MJ/kg) = 33.742  [C] + 143.905  ([H]  [O]/ 8) + 9.396  [S], where [C], [H], [O], and [S] are the mass fractions as measured through elemental analysis. Clarus 600 GC-FID equipment installed with MXT-1HY SimDist column was used to measure the weight distributions, along with the boiling points. Obtained chromatograms were analyzed by ASTM D2887 method. The total acid number (TAN) of the product oil analysis was measured using KOH.

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Characterization of catalysts

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Powder X-ray diffraction (powder XRD) results of the catalysts were obtained using a Rigaku X-ray diffractometer with a CuKa1 (l = 0.15406 nm) radiation source operated at 40 kV and 30 mA N2 adsorption-desorption isotherms, pore volume, and the Brunauer–Emmett–Teller (BET) surface area were measured using a Micromeritics ASAP 2020. NH3-temperature-programmed desorption (NH3-TPD) analyses and CO-chemisorption were performed using a BELCAT-B catalyst analyzer (BEL Japan, Inc.) equipped with a thermal conductivity detector (TCD) and mass spectrometer (MS). For the NH3-TPD, the catalyst powder (50– 100 mg) was pretreated with He flow (50 mL/min) at 400  C (500  C for alumina-supported catalysts) for 80 min and subsequently at 100  C for 10 min. The catalyst powder was treated with NH3/He (7.5% v/v) at 100  C for 30 min and flushed with He flow at 100  C for 10 min. The desorption of NH3 was observed in the He flow (30 mL/min) using TCD and MS at 100–900  C with a heating rate of 10  C/min. The FT-IR spectra of the catalysts which absorbed pyridine were obtained using an FT-IR 4100 type spectrometer (JASCO International Co., Ltd) equipped with an MCT-detector with a resolution of 4 cm 1. Catalysts were grinded and self-pelletized using an Autocrush-IR (PIK Technologies). In

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Table 2 Hydrodeoxygenation results of extracted bio-oils on commercial and synthesis catalysts. Run (catalyst)

LHSV (h 1)

TOS (h)

Yield of liquid product (g/g of feed oil)

Yield of solid residue (g/g of feed oil)

Increasing gas flow (L/g of Temperature of catalyst bed feed) ( C)

Run 1 (5 wt% Pd/C)

0.22

0.30 0.31 0.20 0.35 0.17 0.31 0.22 (aqueous phase) 0.28 0.43 0.24 0.30 0.35

0.16

0.27a

4.70 4.48 4.39 4.32 n. a. n. a. 5.52 4.88 4.95 3.37 4.23 2.99

0.03

2.13 3.12

Run 2 (5 wt% Pd/C) Run 3 (10 wt% Pd/C) Run 4 (5 wt% Pt/C)

0.17 0.27 0.22

Run 5 (5 wt% Ru/C) Run 6 (20 wt% Ni/C)

0.24 0.22

Run 7 (5 wt% Pd/SiAl)

0.37

Run 8 (5 wt% Pd/WZr (10.9))

0.90

1.0–4.5 4.5–8.5 1.0–5.5 1.2–5.0 1.0–4.4 4.4–7.0 1.6–4.8 1.0–4.5 4.5–7.0 1.0–3.0 3.0–6.0 1.0–4.0

Run. 9 (3 wt% Ru/WZr (com))

0.24

4.0–6.0 1.0–3.0

0.41 0.38

3.0–6.2 6.6–9.6 9.6– 12.4 12.4– 19.6 20.6– 27.6 27.6– 31.4

0.34 n. a. 0.38

5.02 n. a. 5.99

0.47

7.35

0.38

6.75

0.32

4.17

a b

a

0.47 0.23 0.16

<0.03b 0.18 0.05

345–350 290–300 345–350 340–355 345–350 350–350 350–350 295–305

350–355

The liquid reactants and products on the catalysts could not be fully washed. Coke was not formed by observation.

Please cite this article in press as: I. Kim, et al., Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.06.013

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an IR cell (Harrick Scientific), the pellets were evacuated at 250  C for 3 h to remove physically adsorbed moisture. Pyridine vapor was adsorbed onto the catalyst surface for 20 min, and the IR cell was thermally evacuated at 150  C for 1 h. All spectra were recorded at 50  C under pure nitrogen condition. Raman spectra analysis was performed using a T64000 Triple Raman Spectrometer (HORIBA Scientific) at the National Center for Inter-university Research Facilities (NCIRF) at Seoul National University (Seoul, Korea).

197

Results and discussion

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Removal of tiny biochar particles dispersed in the bio-oil

188 189 190 191 192 193 194 195

199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219

Prior to the upgrading of the bio-oil, the tiny biochar particles dispersed in the crude bio-oil were removed by extraction using diethyl ether, as described in the Experimental Section (Fig. S2) [35]. The biochar particles nucleated to large cokes causing plugging in the continuous flow reaction systems [25], which accumulated at valves and pipelines of the reaction system and suppressed the continuous flow of oil reactants and products (Fig. 1). Heavy molecules including tar may also be removed because of their low solubility in ether and water, which improved the feeding of bio-oil to the reactor. The ether-extraction also removed the water-soluble molecules, including aldehydes and ketones, which are to be dehydroxygenated. Filtration was attempted at the beginning of this study, but the crude bio-oil did not flow through the filter paper because of its high viscosity. The yield of ether-extracted bio oils was 75% after three consecutive extraction steps, and the distribution constant was 0.14 (kg/L in diethyl ether)/(kg/L in water); the HHVs slightly increased from 17.7 to 19.7 to 22.4 to 25.9 MJ/kg because of the simultaneous removal of non-inflammable water (Table 1). All catalytic upgrading processes discussed in this study used these ether-extracted bio-oil as reactants.

Upgrading of bio-oils

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The upgrading of bio-oils was performed using several supported noble metal catalysts. Although molybdenum-based catalysts exhibit high catalytic activity, suppressing the saturation of phenyl rings, we attempted to avoid the use of molybdenumbased catalysts because they require additional sulfidation to activate the catalysts and because their alumina supports can be degraded by acidic bio-oil reactants [36]. Catalysis was performed at 350  C and 100 bar based on the literature [37]; the variation of the reaction conditions was not shown in this study because the objective of this study is the stable operation of the continuous flow reaction to suppress the formation of cokes. The formation of undesirable coke and tar, whose quantities were measured as solid residue, was sufficiently significant to deactivate the catalysts, although tiny biochar particles were removed from the reactant feedstock by ether extraction before the bio-oil was upgraded (Tables 3 and 4). In spite of coking, the catalysts used in this study, including Pd/C, Pt/C, Ru/C, Ni/C, Pd/SiAl, Pd/WZr(10.9), and Pd/WZr (com), exhibited good hydrodeoxygenation activity, producing biphasic mixtures composed of an oil phase upper layer and an aqueous phase lower layer. The gas products were not analyzed because they contained overwhelmingly large amounts of excess hydrogen gas with tiny amounts of methane, ethane, propane, carbon monoxide, and carbon dioxide [25]. The GC/MS measurement of an aqueous phase lower layer found a broad peak of water and negligible peaks of other compounds. The elemental analysis indicated that the aqueous phase contained carbon atoms at a level smaller than 1 wt%. The Karl–Fischer titration indicated that the moisture content was almost 100 wt%. Based on these observations, the aqueous phase lower layer must be water containing tiny amounts of organic compounds; this layer will not be discussed in this study. The elemental analysis of the oil phase upper layer indicated O/C = 0.01–0.19 (atom/atom), which is significantly lower than O/C = 0.39–0.47 (atom/atom) of bio-oils (Tables 2 and 3). The

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Table 3 Composition of hydrodeoxygenated bio-oils.a Run (catalyst)

TOS (h)

C (wt %)

H (wt %)

N (wt%) S (wt%)

Run 1 (5 wt% Pd/C)

1.0–4.5

82.4

11.7

1.3

4.5–8.5 1.0–5.5 1.2–5.0 1.0–4.4 4.4–7.0 1.6–4.8 1.0–4.5 4.5–7.0 3.0–6.0 1.0- 4.0

81.4 11.8 1.4 68.2 11.9 1.3 82.9 14.4 1.0 78.4 13.1 0.9 78.7 13.0 1.1 Aqueous phase only 85.9 13.7 <0.3 (0) 85.5 13.7 1.2 84.9 13.1 1.2 80.6 13.9 1.1

4.0–6.0 1.0–3.0

78.0 85.4

12.8 13.6

3.0–6.2 6.6–9.6 9.6–12.4 12.4– 19.6 20.6– 27.6 27.6– 31.4

86.3 83.8 82.8 82.6

Run 2(5 wt% Pd/C) Run 3 (10 wt% Pd/C) Run 4 (5 wt% Pt/C) Run 5 (5 wt% Ru/C) Run 6 (20 wt% Ni/C)

Run 7 (5 wt% Pd/SiAl) Run 8 (5 wt% Pd/WZr (10.9)) Run. 9 (3 wt% Ru/WZr(com))

a b c

O (wt %)b

O/C (atom/ atom)

H/C (atom/ atom)

Moisture in an oil phase (wt HHV (MJ/kg) %)

4.6

0.04

1.70

0.5

43.8

3.6 17.5 1.4 6.3 7.8

0.03 0.19 0.01 0.06 0.07

1.74 2.09 2.09 2.00 1.98

0.5 15.3 <10 ppm (0)b 1.0 1.4

43.8 37.0 48.5 44.2 43.9

<0.3 (0)

1.6

0.01

1.91

n. a.

48.4

<0.3 (0) <0.3 (0) <0.3 (0)

1.0 1.1 2.2

0.01 0.01 0.02

1.92 1.85 2.07

0.8 0.4 0.4

48.3 47.3 46.8

1.3 1.0

<0.3 (0) <0.3 (0)

3.6 1.2

0.04 0.01

1.97 1.92

0.2 0.4

44.1 48.2

14.3 13.8 13.8 13.6

1.2 1.3 1.0 1.0

<0.3 <0.3 <0.3 <0.3

(0) (0) (0) (0)

0.7 0.6 0.4 0.8

0.01 0.01 0.01 0.01

1.98 1.98 1.99 1.98

0.1 0.2 <10 ppm (0)b 0.1

49.5 48.1 47.7 47.4

85.7

13.9

0.9

<0.3 (0)

0.6

0.01

1.95

<10 ppm (0)b

48.8

84.2

13.6

1.5

<0.3 (0)

0.7

0.01

1.94

2.8

47.8

<0.3 (0)c <0.3 <0.3 <0.3 <0.3 <0.3

(0) (0) (0) (0) (0)

Measured after the removal of aqueous phase but without removal of moisture in the oil phase. Measured by the oxygen detector. Less than the detection limit.

Please cite this article in press as: I. Kim, et al., Upgrading of sawdust pyrolysis oil to hydrocarbon fuels using tungstate-zirconia-supported Ru catalysts with less formation of cokes, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.06.013

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Table 4 Hydrodeoxygenation results of extracted bio-oils with higher LHSVs. Run (catalyst)

LHSV (h 1)

TOS (h)

Yield of liquid product (g/g of feed oil)

Yield of liquid Yield of solid Increasing gas residue residue flow (g/g of total feed oil) (g/g of total feed oil) (L/g of feed)

Temperature of catalyst zone ( C)

Run. 10 (3 wt% Ru/Al2O3)

2.27

1.0–4.0

0.37

0.15

0.07

2.16

350–360

Run. 11 (3 wt% Ru/SiO2Al2O3)

2.33

4.0–7.8 1.0–3.8

0.41 0.29

0.16

0.09

2.08 1.91

350–360

Run. 12 (3 wt% Ru/ZrO2)

2.32

3.8–5.7 1.0–5.0

n. a. 0.30

0.10

0.06

n. a. 2.25

350–360

Run. 13 (3 wt% Ru/WZr (10.9))

2.10

5.0–6.3 1.5–3.7

0.45 0.39

0.12

0.04

n. a. n. a.

350–355

0.49 0.50

2.02

3.7–10.9 10.9– 13.1 1.0–4.5

4.5–10.2 10.2– 14.2

0.51 0.50

Run. 14 (3 wt% Ru/WZr (14.6))

254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293

0.38

2.79 3.15 0.07

HHVs of the upgraded oils were in a range from 37.0 to 49.5 MJ/kg, which values are significantly larger than those of bio-oil reactants. We note that the temperatures of the catalyst bed were depicted as a range, not as single values, because the measured temperature changed through the thick catalyst bed. We also note that, for the reliable analysis, the liquid products were collected during certain ranges of TOS. The LHSVs slightly changed between runs because of minute operation control of the lab-scale processes. Among the metal catalysts in this study, Pd, Pt, Ni, and Ru produced biphasic mixtures of aqueous and oil layers, while only the aqueous phase product was observed for Ru/C. For the biphasic mixture products, light brown and light yellow liquids were observed for the non-aqueous and aqueous layers, respectively. Note that the decreasing quantity of oxygen atoms (35.4 wt% to 3.6–4.6 wt%) removed using Pd/C at 350  C was larger than that (35.4 wt% to 17.5 wt%) at 300  C (runs 1 and 2 in Tables 2 and 3), indicating that improved hydrodeoxygenation can be achieved at the higher reaction temperature. Although the O/C ratios of the liquid products decreased to 0.01–0.07 (atom/atom) for several of the good catalysts (Tables 2 and 3), hard lumps of carbon powder and sticky tarlike materials formed on the catalyst bed surface during the upgrading reaction. The significant pressure drop caused by the plugging at the catalyst bed was observed during the operation, which could be attributed to the formation of these cokes or tar materials. Based on the results described in Tables 3 and 4, the yields of liquid product and solid residue were plotted and Ru/WZr (com) was found to exhibit the largest yield of liquid product and the smallest yield of solid residue (Fig. 2). Using Ru/WZr(com), the reaction was performed for up to 31 h and the O/C ratio decreased to 0.01 (mol/mol). The pressure drop was 15 bar at TOS 31.4 h, at which point the reaction was forced to stop. The upgraded oils prepared using 3 wt% Ru/WZr(com) (run 9 in Tables 2 and 3) became completely transparent and were identified using a GC/MS (Fig. S2 and Table S2). While the bio-oil contained phenolic compounds, including benzenediols, eugenol, and substituted phenols, and saturated cyclic hydrocarbons, including cyclopentenones, prior to the upgrading reaction, the upgraded oil contained deoxygenates, including substituted cyclohexanes and

0.05

2.45

350–355

2.20 2.34

Fig. 2. Yields of liquid products and solid residues depending on catalysts.

other compounds. The simulated distillation of hydrodeoxygenated bio-oil indicated that the product oil consists of gasoline and diesel fractions (Fig. S3). Total acid numbers (TANs) of liquid product at TOS 6.6–27.6 h for run 9 were 0.36–0.49 mg KOH/g. We also note that the term “hydrodeoxygenation” has been used to describe the removal of oxygen atoms by the reaction with hydrogen molecules or hydrogen atoms in many research works [7–11,13–15]: this process may, however, be depicted as “hydrocracking” because of the removal of C-O bonds during the reaction [36,37].

294

Catalytic activity and coke deposition with higher LHSV

304

The catalytic activities of Ru/Al2O3, Ru/SiO2-Al2O3, Ru/ZrO2, and Ru/WZr were further investigated using the higher LHSV, which was selected to clarify the differences in catalyst activity. The upgrading results for the bio-oil at higher LHSVs (2.01–2.33 h 1) are presented in Tables 4 and 5. The combined yield of products,

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Table 5 Composition of upgraded bio-oils with higher LHSVs.a Run (catalyst)

TOS (h)

C (wt %)

H (wt %)

N (wt %)

S (wt%)

Run. 10 (3 wt% Ru/Al2O3)

1.0–4.0

80.5

10.2

1.3

4.0–7.8 3.8–5.7

76.0 70.2

9.5 8.8

1.0–5.0

72.3

5.0–6.3 1.5–3.7

Run. 11 (3 wt% Ru/SiO2Al2O3) Run. 12 (3 wt% Ru/ZrO2) Run. 13 (3 wt% Ru/WZr (10.9))

Run. 14 (3 wt% Ru/WZr (14.6))

a b c

310

O (wt %)b

O/C ratio (atom/ atom)

H/C ratio (atom/ atom)

Moisture in oil phase (wt HHV (MJ/kg) %)

0.08

1.51

2.7

40.3

1.0 0.7

<0.3 8.4 (0)c <0.3 (0) 14.5 <0.3 (0) 18.9

0.14 0.20

1.50 1.50

2.6 15.0

36.7 32.9

8.8

1.0

<0.3 (0) 18.0

0.19

1.46

n. a.

33.8

73.9 85.2

9.2 12.6

1.2 1.5

<0.3 (0) 15.9 <0.3 (0) 1.5

0.16 0.01

1.50 1.77

5.0 0.4

35.4 46.8

3.7–10.9 10.9– 13.1 1.0–4.5

85.7 83.3

11.5 10.2

1.1 0.9

<0.3 (0) 3.1 <0.3 (0) 6.1

0.03 0.06

1.61 1.47

0.4 1.1

44.9 41.7

86.7

12.0

0.5

<0.3 (0) 2.1

0.02

1.66

0.9

46.1

4.5–10.2 10.2– 14.2

83.8 79.3

10.4 9.5

1.0 <0.3 (0)

<0.3 (0) 6.5 <0.3 (0) 10.6

0.06 0.10

1.49 1.43

<10 ppm (0) 0.3

b

42.1 38.5

Measured after the removal of aqueous phase but without removal of moisture in the oil phase. Measured by the oxygen detector. Less than the detection limit.

probably because of the sintering of their Ru particles. Ru particles of Ru/ZrO2 and Ru/WZr(10.9) did not significantly grow, exhibiting almost the same Ru dispersions when the spent catalysts were calcined. The measured BET surface areas and the total pore volumes of the spent catalysts also decreased; they were restored when the catalysts were calcined in air, confirming the coking on the catalyst surface. XRD results were analyzed to observe the stability of the catalyst structures during bio-oil upgrading (Figs. S4 and S5). While the alumina-supported catalysts exhibited the formation of aluminum hydroxide oxide (AlO(OH)) during the reaction, indicating the instability of the alumina support, the zirconiasupported catalysts exhibited identical results for both fresh and spent catalysts, indicating the stability of the zirconia-supported catalysts.

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324

residue, and solids was approximately 60%, in which the gas products and the loss during the process made up 40%. For Ru/ZrO2, the catalyst bed exhibited stable pressure up to TOS 6.0 h; 10 bar of pressure drop suddenly occurred at TOS 6.3 h. For Ru/SiO2-Al2O3, the operation was stopped at TOS 5.7 h because complex multiple layers formed in the upgraded oil, indicating poor upgrading activity. The O/C ratios were 0.08–0.20 (atom/atom) for Ru/Al2O3, Ru/SiO2-Al2O3, and Ru/ZrO2, while Ru/WZr(10.9) exhibited a ratio of O/C = 0.01–0.06 (atom/atom). The yield of solid residue was also lower for Ru/WZr(10.9) than it was for the other catalysts, indicating less plugging. Based on these results, Ru/WZr(10.9) was found to exhibit the highest upgrading activity and to produce the smallest quantities of cokes and tars. Ru/WZr(10.9) also exhibited better activity and less production of coke compared to Ru/WZr(14.6).

325

Deactivation of upgrading process

Coking-resistance of tungstate-zirconia supported catalysts

362

326

The quantities of cokes formed on the catalyst bed were measured using thermogravimetry measurements (Fig. 3), which poison the active sites on the catalysts and plug the catalyst bed. Spent Ru/Al2O3 and Ru/SiO2-Al2O3, whose fresh catalysts deoxygenated the oil less than did the tungstate-zirconia-supported catalysts, exhibited the formation of a large amount of coke, which was burnt at 300–400  C in air flow (Fig. 3(i, j)). Ru/WZr(10.9) produced less coke compared to Ru/WZr(14.6), indicating that Ru/ WZr(10.9) may suppress the formation of cokes. Ru/ZrO2 also produced a smaller amount of cokes (Table 4), but it exhibited poor deoxygenation activity. In addition to the thermogravimetry measurements, the deactivation of the catalysts was studied by observing the active sites of the Ru particles (CO-chemisorption) and the pore structures of the supports (N2-physisorption) (Table 6). All spent catalysts used in this study exhibited a significant decrease of Ru dispersion ([CO]/[Ru]) compared to the fresh catalysts. The measured [CO]/[Ru] ratio was restored when the spent catalysts were calcined at 350  C, indicating that the cokes poisoned the Ru surface. The Ru dispersions of calcined spent Ru/Al2O3, Ru/SiO2Al2O3, and Ru/WZr(14.6) were, however, not fully restored,

A greater number of acid sites increases the catalytic HDO or upgrading activity [38], although these sites may also improve the formation of cokes [39]. This study, however, indicated that the upgrading performance of bio-oil did not depend on the total quantity of acid sites (Table 7 and Figs. S6 and S7). The low temperature acid sites (LTA) of the catalysts, as measured by NH3 desorbed at 100–450  C, did not exhibit any correlation with the catalytic upgrading activity or with the quantity of cokes formed. High temperature acid sites (HTA), which were measured using NH3 desorbed at 450–850  C, were observed only for alumina- and silica-alumina-supported catalysts, which formed cokes during the upgrading. These results suggest that high temperature acid sites should be associated with the formation of cokes. As the strongly adsorbed bio-oil components, according to the literature [39], can more easily form carbocations, which can further be converted to cokes, silica-alumina-supported Ru with a greater quantity of acid sites was found to exhibit a greater formation of cokes. In contrast, neither Ru/Al2O3 nor Ru/WZr exhibited a significantly different quantity of acid sites, but Ru/Al2O3 produced a larger quantity of cokes compared to Ru/WZr(10.9) and Ru/WZr (14.6). The formation of cokes exhibited the strong

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327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346

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Table 7 Quantities of acid sites measured by NH3-TPD and pyridine-FT-IR. Catalyst

SiO2-Al2O3 Al2O3 ZrO2 WZr(10.9) WZr(14.6) a

Fig. 3. Thermogravimetry results of (a) fresh 3 wt% Ru/ZrO2, (b) fresh 3 wt% Ru/WZr (10.9), (c) fresh 3 wt% Ru/WZr(14.6), (d) spent 3 wt% Ru/ZrO2, (e) spent 3 wt% Ru/ WZr(10.9), (f) spent 3 wt% Ru/WZr(14.6), (g) fresh 3 wt% Ru/Al2O3, (h) fresh 3 wt% Ru/SiO2-Al2O3, (i) spent 3 wt% Ru/Al2O3, and (j) spent 3 wt% Ru/SiO2-Al2O3. 384 385 386

correlationswith quantities of Lewis and Brönsted acid sites (Table 7 and Fig. S8). Greater quantities of Lewis acid sites (LAS, observed as an FT-IR peak at 1450 cm 1), which are known to be

The amount of acidic sites (mmol/g-catalyst) NH3-TPD

Pyridine-FT-IR (Evacuated at 150  C)

Low-temperature acid sites (Desorbed at 100–450  C)

BAS

LAS

BAS/LAS

0.59 0.21 0.07 0.18 0.16

0.10 <0.01 n. a.a 0.07 0.06

0.04 0.01 n. a.a 0.01 0.01

2.5 <1.0 n. a.a 7.0 6.0

Negligible integrated peak area.

reactant acceptors on the catalyst surface [40], compared to Brönsted acid sites (BAS, observed as an FT-IR peak at 1545 cm 1), were observed on the silica-alumina and alumina supports; the molar ratio of BAS/LAS was smaller than 2.5 (Table 7 and Fig. S8) [41–43]. Tungstate-zirconia-supported Ru catalysts exhibited more Brønsted acid sites, with a molar ratio of BAS/LAS larger than 6.0 (Table 7). These observations indicate the relatively deficient number of BAS compared to LAS initiates the formation of more cokes. The higher tungstate loadings (14.6 wt%) on zirconia led to a lower number of acid sites compared to that of the 10.9 wt% tungstate loaded support (Table 7) because of the formation of tungsten oxide particles (WOx) [44–46]. The increase of tungstate loading above the optimum value decreases the acidity of the catalysts via the formation of WOx particles beyond the polytungstate monolayer coverage [33,47]. In our study, the formation of WOx or WO3 particles for WZr(14.6), and of saturated polytungstate species on zirconia, was verified using XRD (Figs. S4 and S5) and Raman spectroscopy results (Fig. S9). ZrO2 without a tungsten additive mostly exhibited diffraction patterns characteristic of monoclinic phases, which phases have been known to be thermodynamically stable [47], while the synthesis of tungstate-zirconia influenced by supported tungsten species favored tetragonal zirconia crystals (Figs. S4, S5, and S9) [48,49]. The saturated monolayer tungsten species did not present any W-O bands for WO3 [50] or terminal W = O bands [51] in the Raman spectra (Fig. S9), which indicates that, because of the Zr-O-W bond network, WOx interacting with zirconia was not able to change the molecular level motion. On the other hand, the tungstate-zirconia with WO3 crystals on its surface had the same Raman peaks as those of pure tungsten-oxide.

Table 6 CO-chemisorption and N2-physisorption results of catalysts. Catalyst

[CO]/[Ru] (mol/mol)

BET surface area(m2/g)

Total pore volume(cm3/g)

3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt% 3 wt%

0.141 0 0.096a 0.064 0.010 0.033a 0.076 0 0.075a 0.114 0.024 0.113a 0.097 0.018 0.048a

59 27 54b 457 116 210b 18 12 15b 62 37 57b 61 30 50b

0.26 0.14 0.24b 0.62 0.22 0.54b 0.07 0.04 0.06b 0.20 0.10 0.16b 0.17 0.07 0.16b

Ru/Al2O3 (fresh) Ru/Al2O3 (spent) Ru/Al2O3 (spent, after calcination) Ru/SiO2-Al2O3 (fresh) Ru/SiO2-Al2O3 (spent) Ru/SiO2-Al2O3(spent, after calcination) Ru/ZrO2 (fresh) Ru/ZrO2 (spent) Ru/ZrO2 (spent, after calcination) Ru/WZr(10.9) (fresh) Ru/WZr(10.9) (spent) Ru/WZr(10.9) (spent, after calcination) Ru/WZr(14.6) (fresh) Ru/WZr(14.6) (spent) Ru/WZr(14.6) (spent, after calcination)

Calcined at 350  C for 15 min in O2/He (5% v/v). Calcined at 350  C for 1 h in air. The calcination temperature of 350  C was selected in order to avoid the sintering at the high temperature although the coke can be fully removed at the temperature above 400  C. a

b

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Conclusion

434

To produce petroleum-like deoxygenated hydrocarbon fuels with less formation of cokes, the catalytic upgrading of ether extracted bio-oil was attempted using a continuous flow fixed-bed reactor. To remove solid particles and to reduce the water content, sawdust pyrolysis bio-oil was extracted with diethyl ether. The tungstate-zirconia-supported catalyst was selected as the most efficient catalyst in this study. The upgrading process using tungstate-zirconia supported Ru catalysts was successfully operated without significant formation of cokes for a long time-onstream. Catalytic HDO or upgrading activity may not require strong acid sites or a large quantity of acid sites; rather, middle or weak acid sites may suffice for the deoxygenation of bio-oil with less formation of coke. Larger quantity of Lewis acid sites compared to that of Brönsted acid sites appears to cause significant formation of cokes, and tungstate-zirconia-supported catalysts with larger molar ratio of BAS/LAS were found to be appropriate for a continuous upgrading operation.

435

Acknowledgments

436 Q3

441

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of Republic of Korea (No. 20143010091790 and No. 20163010092210). The authors thank Daekyung ESCO Co., Ltd. (Incheon, Korea) for the supply of the sawdust pyrolysis oil.

442

Appendix A. Supplementary data

443

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jiec.2017.06.013.

419 420 421 422 423 424 425 426 427 428 429 430 431 432 433

437 438 439 440

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446

447 448

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