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Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts
Renewable Energy xxx (2018) 1e8
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Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts Hoda Shafaghat a, Ji Man Kim b, In-Gu Lee c, Jungho Jae d, e, Sang-Chul Jung f, Young-Kwon Park a, * a
School of Environmental Engineering, University of Seoul, Seoul 02504, South Korea Department of Chemistry, Sungkyunkwan University, Suwon 16410, South Korea Biomass and Wastes to Energy Laboratory, Korea Institute of Energy Research, Daejeon 34129, South Korea d Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, South Korea e Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, South Korea f Department of Environmental Engineering, Sunchon National University, Suncheon 57922, South Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2018 Received in revised form 17 June 2018 Accepted 24 June 2018 Available online xxx
Pyrolysis oil (bio-oil) consists of high water content and vast variety of oxygenates (acids, alcohols, aldehydes, esters, ketones, sugars and phenols), causing some undesirable properties that prevent the direct use of bio-oil as a transportation fuel. Bio-oil upgrading to decrease its oxygen content provides a sustainable fuel that can be considered a valuable substitution for depleting fossil fuels. Catalytic hydrodeoxygenation (HDO) is an efficient method for bio-oil upgrading. This paper presents the HDO of crude bio-oil in supercritical fluid (ethanol, methanol, and 2-propanol) using a batch high pressure reactor. Supercritical fluids have unique physicochemical properties of liquid-like density and gas-like high diffusivity and low viscosity. The upgrading efficiency was evaluated by measuring the elemental composition (CHNSeO), water content, carbon residue, and high heating value (HHV) of the bio-oil upgraded over Ni/HBeta catalyst. Compared to ethanol and 2-propanol, supercritical methanol resulted in a higher decrease in the oxygen content of bio-oil. The activity of Ni/HBeta was examined by varying the Ni loading (5e20 wt%), initial hydrogen pressure (10e30 bar), and reaction time (2e6 h). Meanwhile, effects of support materials (HZSM-5, HBeta, HY, Al-SBA-15, and silylated HBeta) on the performance of nickel catalyst in bio-oil upgrading were investigated using supercritical methanol. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Supercritical methanol Bio-oil hydrodeoxygenation Carbon residue Deoxygenation degree High heating value (HHV)
1. Introduction Bio-oil derived from the pyrolysis of renewable energy source, such as lignocellulosic biomass, is a carbon-neutral fuel which has the potential to substitute for fossil fuels [1e3]. However, the high oxygen content of bio-oil, causing its serious negative properties of high viscosity, low solubility in other hydrocarbons, low volatility, corrosiveness, and low calorific value, limits its direct use as a transportation fuel [1,4,5]. Another problem related to the use of bio-oil in the fuel market is that bio-oil is composed of highly reactive compounds such as acids, alcohols, ketones, aldehydes, and phenols, which have very low stability during the storage process, and even storing bio-oil for a few weeks leads to a
* Corresponding author. E-mail address: [email protected] (Y.-K. Park).
remarkable change in the bio-oil properties [1,6,7]. Therefore, biooil needs to be deoxygenated to improve its quality and make it suitable for the fuel industry. In recent years, various upgrading processes, such as catalytic cracking, hydrodeoxygenation (HDO), esterification, and steam reforming have been used to decrease the oxygen content of bio-oil [8e11]. Among them, catalytic hydrodeoxygenation (HDO) is an efficient upgrading method that uses high pressure of hydrogen to convert the unstable oxygenated compounds of bio-oil to stable deoxygenated or partially deoxygenated components [9,12e15]. Finding a suitable catalyst with a high capability of activating both bio-oil oxygenates and hydrogen gas is one of the main research subjects in the HDO upgrading of bio-oil. In addition, a range of process parameters, such as temperature, reaction time, co-feeding a solvent with bio-oil as either reactant or reaction medium, agitation rate of the reaction mixture, and hydrogen pressure can influence the performance of a catalyst in bio-oil HDO. For instance, the role of solvent on the HDO
https://doi.org/10.1016/j.renene.2018.06.096 0960-1481/© 2018 Elsevier Ltd. All rights reserved.
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performance cannot be ignored because of the direct impact of solvent on catalysts, substrates, intermediates, and products. These direct interactions can positively or negatively affect the reaction rate and product yield/selectivity [16]. Various properties of the solvent, such as polarity and acidity/basicity, can significantly alter the reaction efficiency. Behtash et al. [17,18] reported that the polarity of solvents can alter the HDO performance of Pd(111); compared to the nonpolar solvent of octane, the polar solvent of water enhanced the activity of Pd(111) for the HDO of propionic acid. In addition, it was reported by Munshi et al. [19] that the acidity of alcohol solvents increased the hydrogenation rate of carbon dioxide using ruthenium trimethylphosphine complexes. In recent years, supercritical solvents have attracted a great attention for the HDO of bio-oil [13,20e25]. The use of supercritical solvents in the catalytic HDO of bio-oil can potentially remove the limitation of the hydrogen solubility by preparing a single phase of reactants and hydrogen [26]. The supercritical fluid provides a reaction system with unique liquid-like (high density and dissolving power) and gas-like (low viscosity and high diffusivity) properties [23,24,27]. Xu et al. [21] indicated that over Ru/C catalyst at 280 C, the HDO upgrading of bio-oil using supercritical 1-butanol resulted in greater improvement in the bio-oil properties (viscosity, 5.7 cSt; deoxygenation degree, 68%) compared to the HDO upgrading of bio-oil in subcritical 1-butanol (viscosity, 6.6 cSt; deoxygenation degree, 53%). In addition, Peng et al. [28] demonstrated that supercritical ethanol (260 C) was more effective than subcritical ethanol (238 C) for bio-oil HDO over HZSM-5. The main aim of this study was to reduce the oxygen content of a pyrolysis bio-oil through a HDO upgrading process using supercritical fluid as the reaction medium. Initially, the influence of the concentration of supercritical solvent on the deoxygenation degree of bio-oil was examined. The capability of supercritical methanol (MeOH), ethanol (EtOH), and 2-propanol (2-PrOH) for the HDO of bio-oil was tested over Ni (10 wt%)/HBeta using the optimized biooil to solvent ratio. Because supercritical methanol resulted in the higher deoxygenation of bio-oil in the reaction conditions of this study, further examination of the catalytic activity was carried out in supercritical methanol. Subsequently, the effect of the nickel loading on the HDO efficiency of the Ni/HBeta catalyst was investigated. After finding the suitable concentration of nickel impregnated on HBeta for the HDO upgrading of bio-oil, the effects of the reaction time and hydrogen pressure on the catalytic efficiency of Ni/HBeta were studied. In addition to HBeta, HZSM-5, HY, silylated HBeta (Si-HBeta), and Al-SBA-15 were used as a Ni carrier to study the role of the support type on the catalytic activity of nickel catalyst for bio-oil HDO.
2. Materials and methods 2.1. Materials Pyrolysis oil (bio-oil) provided by Korea Institute of Energy Research (KIER) was used as a feedstock. Methanol (MeOH, SIGMAALDRICH, 99.9%), ethanol (EtOH, SAMCHUN, 99.9%), and 2propanol (2-PrOH, SAMCHUN, 99.5%) were used as solvent. Purified hydrogen gas (99.99%) was used as the hydrogen source of the HDO reaction. Nickel(II) nitrate hexahydrate (trace metal basis, 99.999%) was purchased from SIGMA-ALDRICH and used as nickel precursor. Microporous Beta (SiO2/Al2O3: 38), ZSM-5 (SiO2/Al2O3: 30), and HY (SiO2/Al2O3: 30) zeolites were provided by Zeolyst International. All zeolites were calcined at 550 C for 9 h and then used as the catalyst support.
2.2. Catalyst preparation For silane functionalization of the HBeta surface, 10 g of calcined Beta zeolite (HBeta) was dispersed in 100 ml hexane (SIGMAALDRICH, laboratory reagent, 95%). Then, 2 ml tetraethylorthosilicate (TEOS, SAMCHUN, 98%) was added to the 100 ml solution of hexane and HBeta. The provided suspension was stirred (250 rpm) at room temperature for 24 h. Hexane was separated from the sample by evaporation. The silylated HBeta (Si-HBeta) was then collected after drying at 110 C for 3 h and calcining at 550 C for 5 h. The mesoporous Al-SBA-15 support was synthesized based on the procedure reported by Jeon et al. [29]. The Ni/HZSM-5, Ni/ HBeta, Ni/HY, Ni/Si-HBeta and Ni/Al-SBA-15 were prepared by wetness impregnation method using an aqueous solution of nickel(II) nitrate hexahydrate. Before impregnation, all the support materials were calcined at 550 C for 9 h. Different amounts of Ni (5e20 wt%) were loaded on HBeta and the catalysts were called xNi/HBeta which x shows the nickel loading; for example, 5Ni/ HBeta means 5 wt% Ni loaded on HBeta. 2.3. Catalyst characterization BET surface area and pore volume of the catalysts were determined using nitrogen isothermal adsorption-desorption method. Crystalline structure of the catalysts was analyzed by X-ray diffraction (XRD). XRD patterns of the catalysts were recorded at 2q range of 5 e80 with the scan rate of 0.05 /s. Density and distribution of acid sites of the catalysts were analyzed by temperatureprogrammed desorption of ammonia (NH3-TPD). Meanwhile, the reducibility of catalysts was investigated by hydrogen temperatureprogrammed reduction (H2-TPR) using the same device as NH3TPD. 2.4. Catalytic activity measurement A 100 ml autoclave reactor equipped with a magnetic drive stirrer and water cooling solenoid was used for the all HDO experiments. A 40 g sample of a mixture of bio-oil and solvent (with bio-oil to solvent ratios of 1:1 and 1:2 w/w) was loaded into the reactor. Thereafter, catalyst (1 g) was added to the reactor under the inert atmosphere of nitrogen. Before each experiment, the catalyst was reduced in a mixed flow of 10 mol% H2/90 mol% N2 (100 ml/ min) at 320 C for 3 h and subsequently passivated in a mixed flow of 1 mol% O2/99 mol% He (100 ml/min) at room temperature for 1 h; passivation was carried out to create a protective oxide layer on catalyst surface in order to prevent the subsurface oxidation during catalyst loading. After loading the reactor with bio-oil, solvent, and catalyst, it was sealed, and the air inside the reactor was replaced with hydrogen gas by pressurizing/depressurizing the reactor for four times. Afterward, the reactor was pressurized with hydrogen gas (10e30 bar) and heating was started. The supercritical HDO of bio-oil was carried out at 265 C and a stirring rate of 400 rpm for 2e6 h (with an operating pressure of 88e116 bar). After the reaction, the reactor was cooled rapidly to room temperature and the solid catalyst was separated from the liquid product by settlement. The yields of liquid (bio-oil and solvent) and solid (char and coke) products were determined by weighing the separated liquid and solid phases. The gas yield was calculated by the difference. The coke content of the spent catalyst was determined by thermogravimetric analysis (TGA) in flowing air (20 ml/min). The spent catalyst was heated from room temperature to 750 C (10 C/min) and held at that temperature for 30 min. The weight loss at the temperature range of 300e750 C was considered as catalyst coke. The solvent fraction of the refined bio-oil was separated by vacuum distillation at 60 C. The water content and elemental composition
Please cite this article in press as: H. Shafaghat, et al., Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts, Renewable Energy (2018), https://doi.org/10.1016/j.renene.2018.06.096
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(CHNS) of the solvent-free bio-oil were analyzed using Karl Fischer Titration (870 KF Titrino plus, Metrohm) and Flash EA 2000 series (Thermo Fisher), respectively. The qualitative distribution of product compositions of the upgraded bio-oil was determined by GC-MS (7890A Agilent Technologies). The carbonization potential (carbon residue) of the bio-oil was measured using TGA under a nitrogen flow (20 ml/min), in which the bio-oil sample was heated until 900 C (10 C/min) and held at that temperature for 1 h. The weight percentage remaining is considered as the fraction of nondistillable and heavy residues (carbon residue) of bio-oil. 3. Results and discussion 3.1. Catalyst properties Table 1 lists the textural properties of HBeta, Si-HBeta, 10Ni/ HBeta, 10Ni/Si-HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 catalysts determined by nitrogen adsorption-desorption isotherms. Silane functionalization of HBeta caused decreases in the BET surface area and pore volume of this zeolite. Both the micro- and mesopore volumes of HBeta were also reduced by silylation treatment, but the reduction of mesopore spaces was greater than that of the micropore spaces, indicating the occupation of mostly mesoporous spaces of HBeta by the silane agent. The incorporation of nickel oxide on HBeta and Si-HBeta led to further decreases in the BET surface area and pore volume. The BET surface area and pore volume of the supported nickel catalysts decreased in the order of 10Ni/HY > 10Ni/HBeta >10Ni/Si-HBeta > 10Ni/Al-SBA15 > 10Ni/HZSM-5, and 10Ni/Al-SBA-15 > 10Ni/HY > 10Ni/HBeta >10Ni/Si-HBeta > 10Ni/HZSM-5, respectively. The high mesoporous volume of the Al-SBA-15-supported Ni catalyst indicates the mesoporous nature of this support. In contrast, the zeolite-supported nickel catalysts have mainly a microporous structure. Fig. 1a shows XRD patterns of the HBeta, Si-HBeta, 10Ni/HBeta, 10Ni/Si-HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 catalysts. The similar XRD profiles of HBeta and Si-HBeta indicates that the structure of HBeta did not change after silane functionalization of its surface. The crystalline structure of HBeta and Si-HBeta remained unchanged after the impregnation of 10 wt% Ni over these zeolites. Reflections appeared at 2Theta values of 37.19 , 43.28 , and 63.12 in the XRD results of Ni-impregnated catalysts belong to the nickel oxide (NiO) phase [5]. As shown in Fig. 1a, there were no peaks for Al-SBA-15 highlighting the amorphous structure of this support, while the well-resolved XRD patterns of the zeolites display their highly crystalline nature [30]. Although the silane functionalization of the HBeta surface did not alter its crystalline structure, a significant decrease in the acidity of this zeolite was observed after the silylation treatment (Fig. 1b). Considering that the area of the ammonia desorption peak is proportional to the density of acid sites, it can be inferred that the density of acid sites of Ni/Al-SBA-15 is significantly lower than that of zeolite-supported nickel catalysts. Meanwhile, 10Ni/HBeta, 10Ni/HZSM-5, and 10Ni/ HY catalysts had similar density of acid sites, which was 1.3 times
Fig. 1. XRD patterns (a), NH3-TPD curves (b) and H2-TPR profiles (c) of the catalysts.
Table 1 Textural properties of the catalysts. Catalyst
SBET (m2/g)
Smicro (m2/g)
Vmicro (m3/g)
Vmeso (m3/g)
Vtotal (m3/g)
HBeta Si-HBeta 10Ni/HBeta 10Ni/Si-HBeta 10Ni/HZSM-5 10Ni/HY 10Ni/Al-SBA-15
636.50 539.57 514.01 460.55 316.84 728.46 432.18
511.09 448.18 400.53 394.52 255.71 568.13 61.48
0.218 0.193 0.172 0.172 0.113 0.248 0.021
0.103 0.080 0.100 0.079 0.111 0.214 0.590
0.321 0.273 0.272 0.251 0.224 0.462 0.611
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higher than the concentration of acid sites of 10Ni/Si-HBeta. Fig. 1c shows the reducibility of supported nickel catalysts determined by H2-TPR. The peaks appeared in the TPR profiles are related to the reduction of NiO to metallic Ni. The reduction peaks of 10Ni/HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 appeared at the temperature range of 315e330 C. Silylation of HBeta shifted the reduction peak of 10Ni/Si-HBeta to a higher temperature compared to 10Ni/HBeta.
3.2. Effect of solvent on the HDO activity of 10Ni/HBeta Bio-oil was upgraded over 10Ni/HBeta using supercritical methanol as a solvent at 265 C for 2 h with bio-oil to methanol ratios of 1:1 and 1:2. The properties of crude bio-oil and upgraded bio-oil (elemental composition, high heating value (HHV), water content and carbon residue) are listed in Table 2. A comparison of elemental composition of crude bio-oil (33.43 wt% C, 8.15 wt% H and 58 wt% O) and upgraded bio-oil (bio-oil to methanol ratio of 1:1) (55.63 wt% C, 6.69 wt% H and 37.60 wt% O) indicates an increase in carbon content (66.41%) and a decrease in oxygen content (35.17%). The HHV of bio-oil (12.61 MJ/kg), which is very low compared to the petroleum oils (41e43 MJ/kg [31,32]), was also improved to 21.69 MJ/kg through a 35.17% decrease in its oxygen content during the supercritical HDO process. A remarkable decrease in the water content of bio-oil was also observed after the HDO treatment in supercritical methanol. The high water content in bio-oil imparts a polar nature to this fuel, causing the immiscibility of bio-oil in the fossil fuels. Carbon residue, which shows the heavy and non-volatile fraction of bio-oil, was also decreased via supercritical HDO upgrading. The carbon residue has the potential to form carbonaceous deposits in a combustion chamber, thereby causing the damage to engine. Decreasing the bio-oil/methanol ratio in the feedstock to 1:2 improved the deoxygenation degree (42.71%) and heating value (24.17 MJ/kg) of bio-oil. Since the methanol reacts with the bio-oil components during the HDO treatment, increasing its content can enhance the contact of bio-oil with methanol, leading to a higher deoxygenation efficiency. Measurements of the coke content deposited on the spent catalyst showed that increasing the solvent concentration decreased the coke yield by 28.07%. A remarkable decrease in the yield of solid products (char and coke) was achieved by the increase in solvent amount, shifting the reaction to the formation of more liquid and gas components (Fig. 2). In addition to methanol, ethanol and 2-propanol were also used as supercritical solvents in bio-oil HDO. When methanol or ethanol (primary alcohols) was used as a solvent, the refined bio-oil showed a remarkable increase in deoxygenation degree and heating value
Fig. 2. Effects of bio-oil to solvent ratio and solvent type on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical conditions using 10Ni/HBeta.
(Table 2). On the other hand, the HDO ability of 2-propanol (secondary alcohol) was not significant under the experimental conditions of this work. As presented in Fig. 2, using the supercritical 2propanol led to a large amount of carbon loss due to the significant formation of solid and gas products. Polarity and acidity of the solvents, which are reduced in the order methanol > ethanol > 2propanol, can be considered as the main properties affecting the bio-oil HDO efficiency. Supercritical conditions increase the polarity and acidity of alcohols. The high polarity of solvents can improve the solubility and decomposition of bio-oil components, increasing their accessibility to the catalytic active sites and thereby enhancing the reaction rate. As shown in Table 2, water content of the upgraded bio-oil was lower when methanol was used in the HDO than when ethanol or 2-propanol was used. Owing to the high water content in the bio-oil upgraded using supercritical ethanol and 2-propanol, the carbon residue of these refined bio-oils was lower than that of the bio-oil upgraded using the supercritical methanol. In addition, the amount of coke deposited on the catalyst during the HDO process was also dependent on the solvent type; the lower coke deposition on the catalyst in supercritical methanol and ethanol compared to supercritical 2-propanol might attribute to the good solubility of coke precursors in the former two alcohols, causing the extraction of these precursors from the catalyst pores and preventing coke formation.
Table 2 Properties of the bio-oil upgraded over 10Ni/HBeta catalyst using supercritical methanol, ethanol and 2-propanol. Reaction conditions: feedstock, 40 g; bio-oil/solvent ratios, 1:1 and 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 2 h and rpm, 400. Product properties
Bio-oil
Bio-oil/MeOHa
Bio-oil/MeOHb
Bio-oil/EtOHb
Bio-oil/2-PrOHb
C (wt%) H (wt%) N (wt%) O (wt%)c Deoxygenation (%) HHV (MJ/kg)d Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 36.15 17.08 e
55.63 6.69 0.08 37.60 35.17 21.69 14.07 13.15 23.44
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
57.10 8.09 0.07 34.74 40.10 24.72 28.59 4.90 17.21
46.65 7.47 0.40 45.48 21.58 18.35 20.37 7.20 34.75
a b c d
Bio-oil to solvent ratio of 1:1. Bio-oil to solvent ratio of 1:2. Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
Please cite this article in press as: H. Shafaghat, et al., Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts, Renewable Energy (2018), https://doi.org/10.1016/j.renene.2018.06.096
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3.3. Effect of the nickel loading on the HDO activity of Ni/HBeta in supercritical methanol Table 3 lists the effect of nickel loading (5, 10, 15, and 20 wt%) on the catalytic performance of Ni/HBeta in bio-oil HDO using supercritical methanol. The highest catalytic activity of Ni/HBeta (deoxygenation degree of 42.71% and HHV of 24.17 MJ/kg) was achieved with a 10 wt% Ni loading. The Ni/HBeta, which exhibited the lowest deoxygenation degree (35.89%) and HHV (22.04 MJ/kg) of the biooil, contained 5 wt% nickel, providing a smaller number of active sites compared to 10 wt% Ni. The high loadings of Ni (15 and 20 wt %) also led to a decrease in catalyst activity probably due to the reduction of nickel surface area caused by nickel particles agglomeration. Furthermore, the enhanced agglomeration of nickel species on catalyst can increase coke formation and accelerate catalyst deactivation; the coke yields in the HDO of bio-oil over 15Ni/HBeta and 20Ni/HBeta were similar and higher than the coke yields on 5Ni/HBeta and 10Ni/HBeta. The minimum coke amount of 16.86 wt% was deposited on 10Ni/HBeta as a result of the higher activity of this catalyst for converting the HDO intermediates into light chemicals rather than polymerizing them into heavy compounds. Besides, the minimum water content of 10.69 wt% was achieved after the HDO upgrading of bio-oil using the Ni/HBeta with a 10 wt% Ni loading. As shown clearly in Fig. 3, changing the nickel content on HBeta resulted in no considerable change in the yields of liquid, solid and gas products. Over the 10Ni/HBeta catalyst, both the gas and solid yields were the lowest, whereas the liquid yield was the highest.
Fig. 3. Effect of Ni loading on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol.
3.4. Effects of the reaction time and hydrogen pressure on the HDO activity of 10Ni/HBeta in supercritical methanol The reaction time and hydrogen pressure, as the two major factors that influence the HDO efficiency of 10Ni/HBeta in supercritical methanol, were increased from 2 to 6 h and 10e30 bar, respectively. The data presented in Fig. 4 show that prolonging the reaction time resulted in a decrease in the liquid product and an increase in the solid and gas products. Since the catalyst coke was not considerably changed by increasing the reaction time from 2 to 4 h (Table 4), it could be inferred that a larger amount of char was produced at a longer reaction time of 4 h. On the other hand, further increase in the reaction time to 6 h resulted in an increase in both char and catalyst coke contents. In contrast, rising the hydrogen pressure from 10 to 30 bar did not considerably change the liquid yield, whereas a decrease in solid formation and an increase in gas formation were observed (Fig. 4). According to the coke data, which show an increase with increasing hydrogen pressure from 20 to 30 bar, reduction of solid formation is favorably due to the suppression of char formation at higher hydrogen pressures. As shown
Fig. 4. Effects of reaction time and hydrogen pressure on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol using 10Ni/HBeta.
in Table 4, the quality of the treated bio-oil was enhanced further by increasing the reaction time; for instance, when the reaction time was increased from 2 to 4 h, the oxygen content was decreased from 33.23 to 31.96 wt%, the heating value was increased from 24.17 to 25.96 MJ/kg, the water content was decreased from 10.69 to 5.38 wt% and the carbon residue was increased from 14.78 to 15.32 wt%. Indeed, the high deoxygenation degree of bio-oil
Table 3 Effect of Ni loading on the HDO efficiency of HBeta supported nickel catalyst in bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 2 h and rpm, 400. Product properties
Bio-oil
5Ni/HBeta
10Ni/HBeta
15Ni/HBeta
20Ni/HBeta
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
55.94 6.81 0.07 37.18 35.89 22.04 13.43 13.87 19.13
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
57.99 6.66 0.08 35.35 39.05 22.85 10.72 15.60 21.33
56.16 6.93 0.08 36.83 36.50 22.35 13.49 14.09 21.12
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
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Table 4 The effects of reaction time and hydrogen pressure on the HDO efficiency of 10Ni/HBeta catalyst in bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; catalyst, 1 g; temperature, 265 C and rpm, 400. Product properties
Bio-oil
t: 2 h PH2: 10 bar
t: 4 h PH2: 10 bar
t: 6 h PH2: 10 bar
t: 4 h PH2: 20 bar
t: 4 h PH2: 30 bar
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
60.05 7.91 0.08 31.96 44.90 25.96 5.38 15.32 17.17
59.09 7.91 0.15 32.85 43.37 25.47 7.31 10.79 25.41
60.73 7.70 0.10 31.11 46.37 26.04 5.27 13.65 22.55
59.16 6.81 0.07 33.96 41.45 23.71 10.22 12.91 22.71
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
(44.90%) achieved after 4 h reaction was accompanied by a concomitant decrease in liquid yield and an increase in solid yield. A further increase in reaction time to 6 h declined the HDO performance of the 10Ni/HBeta catalyst. Catalyst deactivation due to the increased amount of coke formation (up to 1.5 times) by prolonging the reaction time from 4 to 6 h could be a cause of the decrease in HDO performance of 10Ni/HBeta. Increasing the hydrogen pressure from 10 to 20 bar enhanced the HDO efficiency of the process by reaching to the bio-oil deoxygenation degree and HHV value of 46.37% and 26.04 MJ/kg, respectively. On the other hand, a further increase in the hydrogen pressure from 20 to 30 bar resulted in a decrease in deoxygenation degree, HHV, and carbon residue and an increase in water content. At supercritical state, a small change in the pressure can significantly alter the thermochemical properties of supercritical solvent and reactants. For example, the density and viscosity of methanol go up when the operating pressure is raised at a constant temperature. 3.5. Effect of the catalyst support on the supercritical HDO of bio-oil over Ni-based catalysts Fig. 5 depicts the yields of liquid, solid, and gas produced from the supercritical HDO of bio-oil at 265 C using the catalyst of Ni (10 wt%) impregnated on different supports. The highest liquid, solid, and gas yields were obtained with Ni supported on HY, AlSBA-15, and HZSM-5, respectively. Compared to the catalysts of Ni supported on microporous zeolites of HZSM-5, HBeta, Si-HBeta,
Fig. 5. Yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol using Ni supported on HZSM-5, HY, HBeta, Si-HBeta and Al-SBA-15.
and HY, which yielded 14.5e15.9 wt% char/coke, the catalyst of Ni supported on mesoporous Al-SBA-15 produced a considerable amount of char/coke (23.48 wt%) during the HDO of bio-oil in supercritical methanol. As shown in Fig. 5, although the yield of products is strongly dependent on the support type, the use of all support materials except for HZSM-5 led to the formation of almost similar liquid yields (in a narrow range of 67.37e75.42 wt%). The remarkable gas yield of 30.22 wt% obtained over HZSM-5supported nickel catalyst indicates the high potential of the HZSM-5 support for the gasification reaction. As shown clearly in Table 5, 10Ni/HBeta resulted in a higher HDO efficiency compared to 10Ni/HZSM-5 and 10Ni/HY, even though the acidity of these catalysts was similar. The different performance of the HZSM-5, HY and HBeta-supported Ni catalysts might be caused by the difference in pore size of the zeolite supports. The catalytic performance of 10Ni/HZSM-5 was lower than that of 10Ni/HBeta due probably to the smaller pore size of ZSM-5 (0.51 0.55/0.53 0.56 nm) compared to Beta (0.66 0.67/0.56 0.56 nm) [33], which might decrease the diffusion rate of reactants/intermediates inside the catalyst channels. Meanwhile, 10Ni/HY with a larger pore size of the catalyst support (0.74 0.74 nm) [33] compared to that of 10Ni/ HBeta, resulted in lower HDO efficiency. The large pore size of HY could cause a high polymerization degree of the bio-oil intermediates and coke production. Coke precursors cover the active sites of catalyst and reduce their accessibility for the reactants. While the highest deoxygenation degree belongs to 10Ni/HBeta, the lowest deoxygenation activity belongs to 10Ni/Al-SBA-15. The weak performance of Al-SBA-15 in bio-oil HDO could be due to its lower acidity compared to the zeolite supports (refer to the TPD results, Fig. 1b), even though the mesostructure of Al-SBA-15 makes the catalytic sites easily accessible for reactants [34]. Meanwhile, although functionalizing the surface of HBeta with silane can give a hydrophobic character to it and decline the destructive effect of hot water on this support [35], silane modification of HBeta made the activity of this support for bio-oil HDO in supercritical methanol decrease by reducing the number of its surface acid sites (refer to the TPD results, Fig. 1b). A comparison of the moisture content of the bio-oil upgraded over 10Ni/HBeta and that treated over 10Ni/ HZSM-5, 10Ni/HY, 10Ni/Si-HBeta and/or 10Ni/Al-SBA-15 showed larger decrease in the water content was during the HDO over 10Ni/ HBeta. As listed in Table 5, the lowest carbon residue of the refined bio-oil was obtained using the HZSM-5-supported nickel catalyst. As shown in this table, the extent of catalyst coke depends considerably on the support material. The coke contents of 10Ni/ HZSM-5 (13.85 wt%), 10Ni/HBeta (17.17 wt%), and 10Ni/Si-HBeta (17.26 wt%) are much lower than that of 10Ni/Al-SBA-15 (33.82 wt %) and 10Ni/HY (36.75 wt%). The lowest coke formation over the HZSM-5-supported Ni catalyst might be related to HZSM-5 having the smallest pore size compared to the other supports.
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Table 5 Effect of support type on the HDO efficiency of nickel catalyst for bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 4 h and rpm, 400. Product properties
Bio-oil
10Ni/Al-SBA-15
10Ni/HZSM-5
10Ni/HY
10Ni/Si-HBeta
10Ni/HBeta
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
54.45 8.31 0.23 37.01 36.18 23.73 12.50 12.14 33.82
56.41 8.06 0.15 35.38 39.00 24.33 9.25 10.36 13.85
55.73 8.37 0.14 35.76 38.34 24.48 10.73 11.68 36.75
58.09 8.05 0.12 33.74 41.83 25.18 8.97 13.33 17.26
60.05 7.91 0.08 31.96 44.90 25.96 5.38 15.32 17.17
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
Distribution of the organic compounds of crude bio-oil and upgraded bio-oil determined by GC-MS analysis is illustrated in Fig. 6. The bio-oil used in this study was composed mainly of acids (18.6%; acetic acid and formic acid), methoxy-phenols (16.9%; guaiacol and alkylated guaiacols), and sugars (22%; levoglucosan and D-allose). The result of bio-oil HDO experiment with supercritical methanol showed the complete disappearance of sugars after a 4 h reaction over 10Ni/HBeta. Alkyl-phenols (2%; methyland dimethyl-phenol) and esters (42.6%; propanoic acid 2methoxy-methyl ester, butanedioic acid dimethyl ester and etc.) were produced during the HDO process. The appearance of alkylphenols in the refined bio-oil indicates the activity of HBetasupported Ni catalyst for hydrocracking of lignin-derived components. Alkyl-phenols are value-added platform compounds that are used for the manufacture of pharmaceuticals, detergents, stabilizers, wood-adhesives, resins, polymers, etc. [36]. The conversion of acid compounds of bio-oil into their analogous esters through esterification with methanol was the dominant reaction occurred in the supercritical HDO of bio-oil. Li et al. [37] reported that supercritical methanol favorably enhances the esterification of the high boiling carboxylic acids of bio-oil. Compared to acids, esters are more preferred in the fuel composition and have a less corrosive effect on the engine surface [11]. The catalyst acidity plays a key role in the esterification reaction. The data showed that the lower acidity of 10Ni/Al-SBA-15 compared to 10Ni/HBeta resulted in a lower selectivity to esters in the refined bio-oil (data is not shown).
The strong acid sites of 10Ni/HBeta provide protons as Brønsted acid, accelerating the esterification reaction. 4. Conclusions In this study, the HDO upgrading of crude bio-oil was carried out using a HBeta-supported nickel catalyst (10 wt%) in supercritical methanol, ethanol, and 2-propanol (265 C/2 h). Supercritical methanol and 2-propanol resulted in the highest (42.71%) and lowest (21.58%) degree of bio-oil deoxygenation, respectively. The heating value of the crude bio-oil was improved from 12.61 MJ/kg to 24.17 MJ/kg through HDO upgrading over 10Ni/HBeta using supercritical methanol as a solvent. A study of the catalytic activity of Ni/HBeta with different Ni loadings (5e20 wt%) in bio-oil HDO in supercritical methanol showed that the highest HDO efficiency of bio-oil was achieved over 10 wt% Ni/HBeta. These data showed that, in addition to the solvent role, methanol is involved in the esterification reaction with the acids in the bio-oil. Although the sugars and acids were the most abundant compounds in crude bio-oil, esters formed the main fraction of the bio-oil upgraded over 10Ni/HBeta. The degree of deoxygenation and HHV of bio-oil were enhanced by prolonging the reaction time from 2 to 4 h and increasing the initial hydrogen pressure from 10 to 20 bar; the maximum deoxygenation degree (46.37%) and HHV (26.04 MJ/kg) of the bio-oil were obtained at a hydrogen pressure of 20 bar and a reaction time of 4 h. A comparison of the activity of Ni impregnated
Fig. 6. Distribution of organic compounds in the crude bio-oil (a) and the one upgraded over 10Ni/HBeta using supercritical methanol (b). Reaction conditions: feedstock, 40 g; biooil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; 10Ni/HBeta, 1 g; temperature, 265 C; reaction time, 4 h and rpm, 400.
Please cite this article in press as: H. Shafaghat, et al., Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts, Renewable Energy (2018), https://doi.org/10.1016/j.renene.2018.06.096
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on the different supports (mesoporous Al-SBA-15 and microporous zeolites of HZSM-5, HBeta and HY) for the supercritical HDO of biooil indicated that the support acidity plays a decisive role in the HDO efficiency. Al-SBA-15, with lower acidity than zeolites, exhibited a lower degree of deoxygenation of bio-oil. Meanwhile, HBeta proved to be a more effective support for nickel sites than HZSM-5 and HY in bio-oil HDO upgrading. On the other hand, by functionalizing the HBeta surface with silane, the activity of this support for bio-oil HDO was decreased due to the decrease in the number of its surface acid sites. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03029131). Also, this work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CAP-16-05-KIMM). References [1] H. Lee, Y.M. Kim, I.G. Lee, J.K. Jeon, S.C. Jung, J.D. Chung, W.G. Choi, Y.K. Park, Recent advances in the catalytic hydrodeoxygenation of bio-oil, Kor. J. Chem. Eng. 33 (2016) 3299e3315. [2] H. Kim, H. Shafaghat, J. Kim, B.S. Kang, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Stabilization of bio-oil over a low cost dolomite catalyst, Kor. J. Chem. Eng. 35 (2018) 922e925. [3] Y. Lee, H. Shafaghat, J.K. Kim, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Upgrading of pyrolysis bio-oil using WO3/ZrO2 and Amberlyst catalysts: evaluation of acid number and viscosity, Kor. J. Chem. Eng. 34 (8) (2017) 2180e2187. [4] K.A. Rogers, Y. Zheng, Selective deoxygenation of biomass-derived bio-oils within hydrogen-modest environments: a review and new insights, ChemSusChem 9 (2016) 1750e1772. [5] H. Shafaghat, P.S. Rezaei, W.M.A.W. Daud, Catalytic hydrodeoxygenation of simulated phenolic bio-oil to cycloalkanes and aromatic hydrocarbons over bifunctional metal/acid catalysts of Ni/HBeta, Fe/HBeta and NiFe/HBeta, J. Ind. Eng. Chem. 35 (2016) 268e276. [6] S. Oh, H.S. Choi, U.J. Kim, I.G. Choi, J.W. Choi, Storage performance of bio-oil after hydrodeoxygenative upgrading with noble metal catalysts, Fuel 182 (2016) 154e160. [7] N.S. Tessarolo, R.V.S. Silva, G. Vanini, A. Casilli, V.L. Ximenes, F.L. Mendes, ~o, E.V.R. Castro, C.R. Kaiser, D.A. Azevedo, Characterization A.R. Pinho, W. Roma of thermal and catalytic pyrolysis bio-oils by high-resolution techniques:1H NMR, GC GC-TOFMS and FT-ICR MS, J. Anal. Appl. Pyrolysis 117 (2016) 257e267. [8] S. Wang, Q. Cai, X. Wang, L. Zhang, Y. Wang, Z. Luo, Biogasoline production from the co-cracking of the distilled fraction of bio-oil and ethanol, Energy Fuels 28 (2014) 115e122. [9] M. Zhou, Y. Wang, Y. Wang, G. Xiao, Catalyticconversionofguaiacoltoalcoholsforbio-oilupgrading, J. Energy Chem. 24 (2015) 425e431. [10] X. Jiang, N. Ellis, Upgrading bio-oil through emulsification with biodiesel: thermal stability, Energy Fuels 24 (2010) 2699e2706. [11] Y. Liu, Z. Li, J.J. Leahy, W. Kwapinski, Catalytically upgrading bio-oil via esterification, Energy Fuels 29 (2015) 3691e3698. [12] P.M. Mortensen, D. Gardini, C.D. Damsgaard, J.D. Grunwaldt, P.A. Jensen, J.B. Wagner, A.D. Jensen, Deactivation of Ni-MoS2 by bio-oil impurities during hydrodeoxygenation of phenol and octanol, Appl. Catal., A 523 (2016) 159e170. [13] T. Yang, Y. Jie, B. Li, X. Kai, Z. Yan, R. Li, Catalytic hydrodeoxygenation of crude bio-oil over an unsupported bimetallic dispersed catalyst in supercritical ethanol, Fuel Process. Technol. 148 (2016) 19e27. [14] H. Shafaghat, P.S. Rezaei, W.M.A.W. Daud, Using decalin and tetralin as
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Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts Hoda Shafaghat a, Ji Man Kim b, In-Gu Lee c, Jungho Jae d, e, Sang-Chul Jung f, Young-Kwon Park a, * a
School of Environmental Engineering, University of Seoul, Seoul 02504, South Korea Department of Chemistry, Sungkyunkwan University, Suwon 16410, South Korea Biomass and Wastes to Energy Laboratory, Korea Institute of Energy Research, Daejeon 34129, South Korea d Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, South Korea e Division of Energy & Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, South Korea f Department of Environmental Engineering, Sunchon National University, Suncheon 57922, South Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2018 Received in revised form 17 June 2018 Accepted 24 June 2018 Available online xxx
Pyrolysis oil (bio-oil) consists of high water content and vast variety of oxygenates (acids, alcohols, aldehydes, esters, ketones, sugars and phenols), causing some undesirable properties that prevent the direct use of bio-oil as a transportation fuel. Bio-oil upgrading to decrease its oxygen content provides a sustainable fuel that can be considered a valuable substitution for depleting fossil fuels. Catalytic hydrodeoxygenation (HDO) is an efficient method for bio-oil upgrading. This paper presents the HDO of crude bio-oil in supercritical fluid (ethanol, methanol, and 2-propanol) using a batch high pressure reactor. Supercritical fluids have unique physicochemical properties of liquid-like density and gas-like high diffusivity and low viscosity. The upgrading efficiency was evaluated by measuring the elemental composition (CHNSeO), water content, carbon residue, and high heating value (HHV) of the bio-oil upgraded over Ni/HBeta catalyst. Compared to ethanol and 2-propanol, supercritical methanol resulted in a higher decrease in the oxygen content of bio-oil. The activity of Ni/HBeta was examined by varying the Ni loading (5e20 wt%), initial hydrogen pressure (10e30 bar), and reaction time (2e6 h). Meanwhile, effects of support materials (HZSM-5, HBeta, HY, Al-SBA-15, and silylated HBeta) on the performance of nickel catalyst in bio-oil upgrading were investigated using supercritical methanol. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Supercritical methanol Bio-oil hydrodeoxygenation Carbon residue Deoxygenation degree High heating value (HHV)
1. Introduction Bio-oil derived from the pyrolysis of renewable energy source, such as lignocellulosic biomass, is a carbon-neutral fuel which has the potential to substitute for fossil fuels [1e3]. However, the high oxygen content of bio-oil, causing its serious negative properties of high viscosity, low solubility in other hydrocarbons, low volatility, corrosiveness, and low calorific value, limits its direct use as a transportation fuel [1,4,5]. Another problem related to the use of bio-oil in the fuel market is that bio-oil is composed of highly reactive compounds such as acids, alcohols, ketones, aldehydes, and phenols, which have very low stability during the storage process, and even storing bio-oil for a few weeks leads to a
* Corresponding author. E-mail address: [email protected] (Y.-K. Park).
remarkable change in the bio-oil properties [1,6,7]. Therefore, biooil needs to be deoxygenated to improve its quality and make it suitable for the fuel industry. In recent years, various upgrading processes, such as catalytic cracking, hydrodeoxygenation (HDO), esterification, and steam reforming have been used to decrease the oxygen content of bio-oil [8e11]. Among them, catalytic hydrodeoxygenation (HDO) is an efficient upgrading method that uses high pressure of hydrogen to convert the unstable oxygenated compounds of bio-oil to stable deoxygenated or partially deoxygenated components [9,12e15]. Finding a suitable catalyst with a high capability of activating both bio-oil oxygenates and hydrogen gas is one of the main research subjects in the HDO upgrading of bio-oil. In addition, a range of process parameters, such as temperature, reaction time, co-feeding a solvent with bio-oil as either reactant or reaction medium, agitation rate of the reaction mixture, and hydrogen pressure can influence the performance of a catalyst in bio-oil HDO. For instance, the role of solvent on the HDO
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performance cannot be ignored because of the direct impact of solvent on catalysts, substrates, intermediates, and products. These direct interactions can positively or negatively affect the reaction rate and product yield/selectivity [16]. Various properties of the solvent, such as polarity and acidity/basicity, can significantly alter the reaction efficiency. Behtash et al. [17,18] reported that the polarity of solvents can alter the HDO performance of Pd(111); compared to the nonpolar solvent of octane, the polar solvent of water enhanced the activity of Pd(111) for the HDO of propionic acid. In addition, it was reported by Munshi et al. [19] that the acidity of alcohol solvents increased the hydrogenation rate of carbon dioxide using ruthenium trimethylphosphine complexes. In recent years, supercritical solvents have attracted a great attention for the HDO of bio-oil [13,20e25]. The use of supercritical solvents in the catalytic HDO of bio-oil can potentially remove the limitation of the hydrogen solubility by preparing a single phase of reactants and hydrogen [26]. The supercritical fluid provides a reaction system with unique liquid-like (high density and dissolving power) and gas-like (low viscosity and high diffusivity) properties [23,24,27]. Xu et al. [21] indicated that over Ru/C catalyst at 280 C, the HDO upgrading of bio-oil using supercritical 1-butanol resulted in greater improvement in the bio-oil properties (viscosity, 5.7 cSt; deoxygenation degree, 68%) compared to the HDO upgrading of bio-oil in subcritical 1-butanol (viscosity, 6.6 cSt; deoxygenation degree, 53%). In addition, Peng et al. [28] demonstrated that supercritical ethanol (260 C) was more effective than subcritical ethanol (238 C) for bio-oil HDO over HZSM-5. The main aim of this study was to reduce the oxygen content of a pyrolysis bio-oil through a HDO upgrading process using supercritical fluid as the reaction medium. Initially, the influence of the concentration of supercritical solvent on the deoxygenation degree of bio-oil was examined. The capability of supercritical methanol (MeOH), ethanol (EtOH), and 2-propanol (2-PrOH) for the HDO of bio-oil was tested over Ni (10 wt%)/HBeta using the optimized biooil to solvent ratio. Because supercritical methanol resulted in the higher deoxygenation of bio-oil in the reaction conditions of this study, further examination of the catalytic activity was carried out in supercritical methanol. Subsequently, the effect of the nickel loading on the HDO efficiency of the Ni/HBeta catalyst was investigated. After finding the suitable concentration of nickel impregnated on HBeta for the HDO upgrading of bio-oil, the effects of the reaction time and hydrogen pressure on the catalytic efficiency of Ni/HBeta were studied. In addition to HBeta, HZSM-5, HY, silylated HBeta (Si-HBeta), and Al-SBA-15 were used as a Ni carrier to study the role of the support type on the catalytic activity of nickel catalyst for bio-oil HDO.
2. Materials and methods 2.1. Materials Pyrolysis oil (bio-oil) provided by Korea Institute of Energy Research (KIER) was used as a feedstock. Methanol (MeOH, SIGMAALDRICH, 99.9%), ethanol (EtOH, SAMCHUN, 99.9%), and 2propanol (2-PrOH, SAMCHUN, 99.5%) were used as solvent. Purified hydrogen gas (99.99%) was used as the hydrogen source of the HDO reaction. Nickel(II) nitrate hexahydrate (trace metal basis, 99.999%) was purchased from SIGMA-ALDRICH and used as nickel precursor. Microporous Beta (SiO2/Al2O3: 38), ZSM-5 (SiO2/Al2O3: 30), and HY (SiO2/Al2O3: 30) zeolites were provided by Zeolyst International. All zeolites were calcined at 550 C for 9 h and then used as the catalyst support.
2.2. Catalyst preparation For silane functionalization of the HBeta surface, 10 g of calcined Beta zeolite (HBeta) was dispersed in 100 ml hexane (SIGMAALDRICH, laboratory reagent, 95%). Then, 2 ml tetraethylorthosilicate (TEOS, SAMCHUN, 98%) was added to the 100 ml solution of hexane and HBeta. The provided suspension was stirred (250 rpm) at room temperature for 24 h. Hexane was separated from the sample by evaporation. The silylated HBeta (Si-HBeta) was then collected after drying at 110 C for 3 h and calcining at 550 C for 5 h. The mesoporous Al-SBA-15 support was synthesized based on the procedure reported by Jeon et al. [29]. The Ni/HZSM-5, Ni/ HBeta, Ni/HY, Ni/Si-HBeta and Ni/Al-SBA-15 were prepared by wetness impregnation method using an aqueous solution of nickel(II) nitrate hexahydrate. Before impregnation, all the support materials were calcined at 550 C for 9 h. Different amounts of Ni (5e20 wt%) were loaded on HBeta and the catalysts were called xNi/HBeta which x shows the nickel loading; for example, 5Ni/ HBeta means 5 wt% Ni loaded on HBeta. 2.3. Catalyst characterization BET surface area and pore volume of the catalysts were determined using nitrogen isothermal adsorption-desorption method. Crystalline structure of the catalysts was analyzed by X-ray diffraction (XRD). XRD patterns of the catalysts were recorded at 2q range of 5 e80 with the scan rate of 0.05 /s. Density and distribution of acid sites of the catalysts were analyzed by temperatureprogrammed desorption of ammonia (NH3-TPD). Meanwhile, the reducibility of catalysts was investigated by hydrogen temperatureprogrammed reduction (H2-TPR) using the same device as NH3TPD. 2.4. Catalytic activity measurement A 100 ml autoclave reactor equipped with a magnetic drive stirrer and water cooling solenoid was used for the all HDO experiments. A 40 g sample of a mixture of bio-oil and solvent (with bio-oil to solvent ratios of 1:1 and 1:2 w/w) was loaded into the reactor. Thereafter, catalyst (1 g) was added to the reactor under the inert atmosphere of nitrogen. Before each experiment, the catalyst was reduced in a mixed flow of 10 mol% H2/90 mol% N2 (100 ml/ min) at 320 C for 3 h and subsequently passivated in a mixed flow of 1 mol% O2/99 mol% He (100 ml/min) at room temperature for 1 h; passivation was carried out to create a protective oxide layer on catalyst surface in order to prevent the subsurface oxidation during catalyst loading. After loading the reactor with bio-oil, solvent, and catalyst, it was sealed, and the air inside the reactor was replaced with hydrogen gas by pressurizing/depressurizing the reactor for four times. Afterward, the reactor was pressurized with hydrogen gas (10e30 bar) and heating was started. The supercritical HDO of bio-oil was carried out at 265 C and a stirring rate of 400 rpm for 2e6 h (with an operating pressure of 88e116 bar). After the reaction, the reactor was cooled rapidly to room temperature and the solid catalyst was separated from the liquid product by settlement. The yields of liquid (bio-oil and solvent) and solid (char and coke) products were determined by weighing the separated liquid and solid phases. The gas yield was calculated by the difference. The coke content of the spent catalyst was determined by thermogravimetric analysis (TGA) in flowing air (20 ml/min). The spent catalyst was heated from room temperature to 750 C (10 C/min) and held at that temperature for 30 min. The weight loss at the temperature range of 300e750 C was considered as catalyst coke. The solvent fraction of the refined bio-oil was separated by vacuum distillation at 60 C. The water content and elemental composition
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(CHNS) of the solvent-free bio-oil were analyzed using Karl Fischer Titration (870 KF Titrino plus, Metrohm) and Flash EA 2000 series (Thermo Fisher), respectively. The qualitative distribution of product compositions of the upgraded bio-oil was determined by GC-MS (7890A Agilent Technologies). The carbonization potential (carbon residue) of the bio-oil was measured using TGA under a nitrogen flow (20 ml/min), in which the bio-oil sample was heated until 900 C (10 C/min) and held at that temperature for 1 h. The weight percentage remaining is considered as the fraction of nondistillable and heavy residues (carbon residue) of bio-oil. 3. Results and discussion 3.1. Catalyst properties Table 1 lists the textural properties of HBeta, Si-HBeta, 10Ni/ HBeta, 10Ni/Si-HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 catalysts determined by nitrogen adsorption-desorption isotherms. Silane functionalization of HBeta caused decreases in the BET surface area and pore volume of this zeolite. Both the micro- and mesopore volumes of HBeta were also reduced by silylation treatment, but the reduction of mesopore spaces was greater than that of the micropore spaces, indicating the occupation of mostly mesoporous spaces of HBeta by the silane agent. The incorporation of nickel oxide on HBeta and Si-HBeta led to further decreases in the BET surface area and pore volume. The BET surface area and pore volume of the supported nickel catalysts decreased in the order of 10Ni/HY > 10Ni/HBeta >10Ni/Si-HBeta > 10Ni/Al-SBA15 > 10Ni/HZSM-5, and 10Ni/Al-SBA-15 > 10Ni/HY > 10Ni/HBeta >10Ni/Si-HBeta > 10Ni/HZSM-5, respectively. The high mesoporous volume of the Al-SBA-15-supported Ni catalyst indicates the mesoporous nature of this support. In contrast, the zeolite-supported nickel catalysts have mainly a microporous structure. Fig. 1a shows XRD patterns of the HBeta, Si-HBeta, 10Ni/HBeta, 10Ni/Si-HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 catalysts. The similar XRD profiles of HBeta and Si-HBeta indicates that the structure of HBeta did not change after silane functionalization of its surface. The crystalline structure of HBeta and Si-HBeta remained unchanged after the impregnation of 10 wt% Ni over these zeolites. Reflections appeared at 2Theta values of 37.19 , 43.28 , and 63.12 in the XRD results of Ni-impregnated catalysts belong to the nickel oxide (NiO) phase [5]. As shown in Fig. 1a, there were no peaks for Al-SBA-15 highlighting the amorphous structure of this support, while the well-resolved XRD patterns of the zeolites display their highly crystalline nature [30]. Although the silane functionalization of the HBeta surface did not alter its crystalline structure, a significant decrease in the acidity of this zeolite was observed after the silylation treatment (Fig. 1b). Considering that the area of the ammonia desorption peak is proportional to the density of acid sites, it can be inferred that the density of acid sites of Ni/Al-SBA-15 is significantly lower than that of zeolite-supported nickel catalysts. Meanwhile, 10Ni/HBeta, 10Ni/HZSM-5, and 10Ni/ HY catalysts had similar density of acid sites, which was 1.3 times
Fig. 1. XRD patterns (a), NH3-TPD curves (b) and H2-TPR profiles (c) of the catalysts.
Table 1 Textural properties of the catalysts. Catalyst
SBET (m2/g)
Smicro (m2/g)
Vmicro (m3/g)
Vmeso (m3/g)
Vtotal (m3/g)
HBeta Si-HBeta 10Ni/HBeta 10Ni/Si-HBeta 10Ni/HZSM-5 10Ni/HY 10Ni/Al-SBA-15
636.50 539.57 514.01 460.55 316.84 728.46 432.18
511.09 448.18 400.53 394.52 255.71 568.13 61.48
0.218 0.193 0.172 0.172 0.113 0.248 0.021
0.103 0.080 0.100 0.079 0.111 0.214 0.590
0.321 0.273 0.272 0.251 0.224 0.462 0.611
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higher than the concentration of acid sites of 10Ni/Si-HBeta. Fig. 1c shows the reducibility of supported nickel catalysts determined by H2-TPR. The peaks appeared in the TPR profiles are related to the reduction of NiO to metallic Ni. The reduction peaks of 10Ni/HBeta, 10Ni/HZSM-5, 10Ni/HY, and 10Ni/Al-SBA-15 appeared at the temperature range of 315e330 C. Silylation of HBeta shifted the reduction peak of 10Ni/Si-HBeta to a higher temperature compared to 10Ni/HBeta.
3.2. Effect of solvent on the HDO activity of 10Ni/HBeta Bio-oil was upgraded over 10Ni/HBeta using supercritical methanol as a solvent at 265 C for 2 h with bio-oil to methanol ratios of 1:1 and 1:2. The properties of crude bio-oil and upgraded bio-oil (elemental composition, high heating value (HHV), water content and carbon residue) are listed in Table 2. A comparison of elemental composition of crude bio-oil (33.43 wt% C, 8.15 wt% H and 58 wt% O) and upgraded bio-oil (bio-oil to methanol ratio of 1:1) (55.63 wt% C, 6.69 wt% H and 37.60 wt% O) indicates an increase in carbon content (66.41%) and a decrease in oxygen content (35.17%). The HHV of bio-oil (12.61 MJ/kg), which is very low compared to the petroleum oils (41e43 MJ/kg [31,32]), was also improved to 21.69 MJ/kg through a 35.17% decrease in its oxygen content during the supercritical HDO process. A remarkable decrease in the water content of bio-oil was also observed after the HDO treatment in supercritical methanol. The high water content in bio-oil imparts a polar nature to this fuel, causing the immiscibility of bio-oil in the fossil fuels. Carbon residue, which shows the heavy and non-volatile fraction of bio-oil, was also decreased via supercritical HDO upgrading. The carbon residue has the potential to form carbonaceous deposits in a combustion chamber, thereby causing the damage to engine. Decreasing the bio-oil/methanol ratio in the feedstock to 1:2 improved the deoxygenation degree (42.71%) and heating value (24.17 MJ/kg) of bio-oil. Since the methanol reacts with the bio-oil components during the HDO treatment, increasing its content can enhance the contact of bio-oil with methanol, leading to a higher deoxygenation efficiency. Measurements of the coke content deposited on the spent catalyst showed that increasing the solvent concentration decreased the coke yield by 28.07%. A remarkable decrease in the yield of solid products (char and coke) was achieved by the increase in solvent amount, shifting the reaction to the formation of more liquid and gas components (Fig. 2). In addition to methanol, ethanol and 2-propanol were also used as supercritical solvents in bio-oil HDO. When methanol or ethanol (primary alcohols) was used as a solvent, the refined bio-oil showed a remarkable increase in deoxygenation degree and heating value
Fig. 2. Effects of bio-oil to solvent ratio and solvent type on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical conditions using 10Ni/HBeta.
(Table 2). On the other hand, the HDO ability of 2-propanol (secondary alcohol) was not significant under the experimental conditions of this work. As presented in Fig. 2, using the supercritical 2propanol led to a large amount of carbon loss due to the significant formation of solid and gas products. Polarity and acidity of the solvents, which are reduced in the order methanol > ethanol > 2propanol, can be considered as the main properties affecting the bio-oil HDO efficiency. Supercritical conditions increase the polarity and acidity of alcohols. The high polarity of solvents can improve the solubility and decomposition of bio-oil components, increasing their accessibility to the catalytic active sites and thereby enhancing the reaction rate. As shown in Table 2, water content of the upgraded bio-oil was lower when methanol was used in the HDO than when ethanol or 2-propanol was used. Owing to the high water content in the bio-oil upgraded using supercritical ethanol and 2-propanol, the carbon residue of these refined bio-oils was lower than that of the bio-oil upgraded using the supercritical methanol. In addition, the amount of coke deposited on the catalyst during the HDO process was also dependent on the solvent type; the lower coke deposition on the catalyst in supercritical methanol and ethanol compared to supercritical 2-propanol might attribute to the good solubility of coke precursors in the former two alcohols, causing the extraction of these precursors from the catalyst pores and preventing coke formation.
Table 2 Properties of the bio-oil upgraded over 10Ni/HBeta catalyst using supercritical methanol, ethanol and 2-propanol. Reaction conditions: feedstock, 40 g; bio-oil/solvent ratios, 1:1 and 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 2 h and rpm, 400. Product properties
Bio-oil
Bio-oil/MeOHa
Bio-oil/MeOHb
Bio-oil/EtOHb
Bio-oil/2-PrOHb
C (wt%) H (wt%) N (wt%) O (wt%)c Deoxygenation (%) HHV (MJ/kg)d Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 36.15 17.08 e
55.63 6.69 0.08 37.60 35.17 21.69 14.07 13.15 23.44
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
57.10 8.09 0.07 34.74 40.10 24.72 28.59 4.90 17.21
46.65 7.47 0.40 45.48 21.58 18.35 20.37 7.20 34.75
a b c d
Bio-oil to solvent ratio of 1:1. Bio-oil to solvent ratio of 1:2. Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
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3.3. Effect of the nickel loading on the HDO activity of Ni/HBeta in supercritical methanol Table 3 lists the effect of nickel loading (5, 10, 15, and 20 wt%) on the catalytic performance of Ni/HBeta in bio-oil HDO using supercritical methanol. The highest catalytic activity of Ni/HBeta (deoxygenation degree of 42.71% and HHV of 24.17 MJ/kg) was achieved with a 10 wt% Ni loading. The Ni/HBeta, which exhibited the lowest deoxygenation degree (35.89%) and HHV (22.04 MJ/kg) of the biooil, contained 5 wt% nickel, providing a smaller number of active sites compared to 10 wt% Ni. The high loadings of Ni (15 and 20 wt %) also led to a decrease in catalyst activity probably due to the reduction of nickel surface area caused by nickel particles agglomeration. Furthermore, the enhanced agglomeration of nickel species on catalyst can increase coke formation and accelerate catalyst deactivation; the coke yields in the HDO of bio-oil over 15Ni/HBeta and 20Ni/HBeta were similar and higher than the coke yields on 5Ni/HBeta and 10Ni/HBeta. The minimum coke amount of 16.86 wt% was deposited on 10Ni/HBeta as a result of the higher activity of this catalyst for converting the HDO intermediates into light chemicals rather than polymerizing them into heavy compounds. Besides, the minimum water content of 10.69 wt% was achieved after the HDO upgrading of bio-oil using the Ni/HBeta with a 10 wt% Ni loading. As shown clearly in Fig. 3, changing the nickel content on HBeta resulted in no considerable change in the yields of liquid, solid and gas products. Over the 10Ni/HBeta catalyst, both the gas and solid yields were the lowest, whereas the liquid yield was the highest.
Fig. 3. Effect of Ni loading on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol.
3.4. Effects of the reaction time and hydrogen pressure on the HDO activity of 10Ni/HBeta in supercritical methanol The reaction time and hydrogen pressure, as the two major factors that influence the HDO efficiency of 10Ni/HBeta in supercritical methanol, were increased from 2 to 6 h and 10e30 bar, respectively. The data presented in Fig. 4 show that prolonging the reaction time resulted in a decrease in the liquid product and an increase in the solid and gas products. Since the catalyst coke was not considerably changed by increasing the reaction time from 2 to 4 h (Table 4), it could be inferred that a larger amount of char was produced at a longer reaction time of 4 h. On the other hand, further increase in the reaction time to 6 h resulted in an increase in both char and catalyst coke contents. In contrast, rising the hydrogen pressure from 10 to 30 bar did not considerably change the liquid yield, whereas a decrease in solid formation and an increase in gas formation were observed (Fig. 4). According to the coke data, which show an increase with increasing hydrogen pressure from 20 to 30 bar, reduction of solid formation is favorably due to the suppression of char formation at higher hydrogen pressures. As shown
Fig. 4. Effects of reaction time and hydrogen pressure on the yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol using 10Ni/HBeta.
in Table 4, the quality of the treated bio-oil was enhanced further by increasing the reaction time; for instance, when the reaction time was increased from 2 to 4 h, the oxygen content was decreased from 33.23 to 31.96 wt%, the heating value was increased from 24.17 to 25.96 MJ/kg, the water content was decreased from 10.69 to 5.38 wt% and the carbon residue was increased from 14.78 to 15.32 wt%. Indeed, the high deoxygenation degree of bio-oil
Table 3 Effect of Ni loading on the HDO efficiency of HBeta supported nickel catalyst in bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 2 h and rpm, 400. Product properties
Bio-oil
5Ni/HBeta
10Ni/HBeta
15Ni/HBeta
20Ni/HBeta
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
55.94 6.81 0.07 37.18 35.89 22.04 13.43 13.87 19.13
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
57.99 6.66 0.08 35.35 39.05 22.85 10.72 15.60 21.33
56.16 6.93 0.08 36.83 36.50 22.35 13.49 14.09 21.12
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
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Table 4 The effects of reaction time and hydrogen pressure on the HDO efficiency of 10Ni/HBeta catalyst in bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; catalyst, 1 g; temperature, 265 C and rpm, 400. Product properties
Bio-oil
t: 2 h PH2: 10 bar
t: 4 h PH2: 10 bar
t: 6 h PH2: 10 bar
t: 4 h PH2: 20 bar
t: 4 h PH2: 30 bar
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
59.86 6.87 0.04 33.23 42.71 24.17 10.69 14.78 16.86
60.05 7.91 0.08 31.96 44.90 25.96 5.38 15.32 17.17
59.09 7.91 0.15 32.85 43.37 25.47 7.31 10.79 25.41
60.73 7.70 0.10 31.11 46.37 26.04 5.27 13.65 22.55
59.16 6.81 0.07 33.96 41.45 23.71 10.22 12.91 22.71
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
(44.90%) achieved after 4 h reaction was accompanied by a concomitant decrease in liquid yield and an increase in solid yield. A further increase in reaction time to 6 h declined the HDO performance of the 10Ni/HBeta catalyst. Catalyst deactivation due to the increased amount of coke formation (up to 1.5 times) by prolonging the reaction time from 4 to 6 h could be a cause of the decrease in HDO performance of 10Ni/HBeta. Increasing the hydrogen pressure from 10 to 20 bar enhanced the HDO efficiency of the process by reaching to the bio-oil deoxygenation degree and HHV value of 46.37% and 26.04 MJ/kg, respectively. On the other hand, a further increase in the hydrogen pressure from 20 to 30 bar resulted in a decrease in deoxygenation degree, HHV, and carbon residue and an increase in water content. At supercritical state, a small change in the pressure can significantly alter the thermochemical properties of supercritical solvent and reactants. For example, the density and viscosity of methanol go up when the operating pressure is raised at a constant temperature. 3.5. Effect of the catalyst support on the supercritical HDO of bio-oil over Ni-based catalysts Fig. 5 depicts the yields of liquid, solid, and gas produced from the supercritical HDO of bio-oil at 265 C using the catalyst of Ni (10 wt%) impregnated on different supports. The highest liquid, solid, and gas yields were obtained with Ni supported on HY, AlSBA-15, and HZSM-5, respectively. Compared to the catalysts of Ni supported on microporous zeolites of HZSM-5, HBeta, Si-HBeta,
Fig. 5. Yield of liquid, solid (char and coke) and gas products obtained from HDO of bio-oil in supercritical methanol using Ni supported on HZSM-5, HY, HBeta, Si-HBeta and Al-SBA-15.
and HY, which yielded 14.5e15.9 wt% char/coke, the catalyst of Ni supported on mesoporous Al-SBA-15 produced a considerable amount of char/coke (23.48 wt%) during the HDO of bio-oil in supercritical methanol. As shown in Fig. 5, although the yield of products is strongly dependent on the support type, the use of all support materials except for HZSM-5 led to the formation of almost similar liquid yields (in a narrow range of 67.37e75.42 wt%). The remarkable gas yield of 30.22 wt% obtained over HZSM-5supported nickel catalyst indicates the high potential of the HZSM-5 support for the gasification reaction. As shown clearly in Table 5, 10Ni/HBeta resulted in a higher HDO efficiency compared to 10Ni/HZSM-5 and 10Ni/HY, even though the acidity of these catalysts was similar. The different performance of the HZSM-5, HY and HBeta-supported Ni catalysts might be caused by the difference in pore size of the zeolite supports. The catalytic performance of 10Ni/HZSM-5 was lower than that of 10Ni/HBeta due probably to the smaller pore size of ZSM-5 (0.51 0.55/0.53 0.56 nm) compared to Beta (0.66 0.67/0.56 0.56 nm) [33], which might decrease the diffusion rate of reactants/intermediates inside the catalyst channels. Meanwhile, 10Ni/HY with a larger pore size of the catalyst support (0.74 0.74 nm) [33] compared to that of 10Ni/ HBeta, resulted in lower HDO efficiency. The large pore size of HY could cause a high polymerization degree of the bio-oil intermediates and coke production. Coke precursors cover the active sites of catalyst and reduce their accessibility for the reactants. While the highest deoxygenation degree belongs to 10Ni/HBeta, the lowest deoxygenation activity belongs to 10Ni/Al-SBA-15. The weak performance of Al-SBA-15 in bio-oil HDO could be due to its lower acidity compared to the zeolite supports (refer to the TPD results, Fig. 1b), even though the mesostructure of Al-SBA-15 makes the catalytic sites easily accessible for reactants [34]. Meanwhile, although functionalizing the surface of HBeta with silane can give a hydrophobic character to it and decline the destructive effect of hot water on this support [35], silane modification of HBeta made the activity of this support for bio-oil HDO in supercritical methanol decrease by reducing the number of its surface acid sites (refer to the TPD results, Fig. 1b). A comparison of the moisture content of the bio-oil upgraded over 10Ni/HBeta and that treated over 10Ni/ HZSM-5, 10Ni/HY, 10Ni/Si-HBeta and/or 10Ni/Al-SBA-15 showed larger decrease in the water content was during the HDO over 10Ni/ HBeta. As listed in Table 5, the lowest carbon residue of the refined bio-oil was obtained using the HZSM-5-supported nickel catalyst. As shown in this table, the extent of catalyst coke depends considerably on the support material. The coke contents of 10Ni/ HZSM-5 (13.85 wt%), 10Ni/HBeta (17.17 wt%), and 10Ni/Si-HBeta (17.26 wt%) are much lower than that of 10Ni/Al-SBA-15 (33.82 wt %) and 10Ni/HY (36.75 wt%). The lowest coke formation over the HZSM-5-supported Ni catalyst might be related to HZSM-5 having the smallest pore size compared to the other supports.
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Table 5 Effect of support type on the HDO efficiency of nickel catalyst for bio-oil upgrading. Reaction conditions: feedstock, 40 g; bio-oil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; catalyst, 1 g; temperature, 265 C; reaction time, 4 h and rpm, 400. Product properties
Bio-oil
10Ni/Al-SBA-15
10Ni/HZSM-5
10Ni/HY
10Ni/Si-HBeta
10Ni/HBeta
C (wt%) H (wt%) N (wt%) O (wt%)a Deoxygenation (%) HHV (MJ/kg)b Water (wt%) Carbon residue (wt%) Catalyst coke (wt%)
33.43 8.15 0.42 58.00 e 12.61 37.85 17.08 e
54.45 8.31 0.23 37.01 36.18 23.73 12.50 12.14 33.82
56.41 8.06 0.15 35.38 39.00 24.33 9.25 10.36 13.85
55.73 8.37 0.14 35.76 38.34 24.48 10.73 11.68 36.75
58.09 8.05 0.12 33.74 41.83 25.18 8.97 13.33 17.26
60.05 7.91 0.08 31.96 44.90 25.96 5.38 15.32 17.17
a b
Calculated by difference. Calculated from Dulong equation: HHV ¼ 0.3383C þ 1.442 (HeO/8); C, H and O obtained from elemental analysis of the bio-oil.
Distribution of the organic compounds of crude bio-oil and upgraded bio-oil determined by GC-MS analysis is illustrated in Fig. 6. The bio-oil used in this study was composed mainly of acids (18.6%; acetic acid and formic acid), methoxy-phenols (16.9%; guaiacol and alkylated guaiacols), and sugars (22%; levoglucosan and D-allose). The result of bio-oil HDO experiment with supercritical methanol showed the complete disappearance of sugars after a 4 h reaction over 10Ni/HBeta. Alkyl-phenols (2%; methyland dimethyl-phenol) and esters (42.6%; propanoic acid 2methoxy-methyl ester, butanedioic acid dimethyl ester and etc.) were produced during the HDO process. The appearance of alkylphenols in the refined bio-oil indicates the activity of HBetasupported Ni catalyst for hydrocracking of lignin-derived components. Alkyl-phenols are value-added platform compounds that are used for the manufacture of pharmaceuticals, detergents, stabilizers, wood-adhesives, resins, polymers, etc. [36]. The conversion of acid compounds of bio-oil into their analogous esters through esterification with methanol was the dominant reaction occurred in the supercritical HDO of bio-oil. Li et al. [37] reported that supercritical methanol favorably enhances the esterification of the high boiling carboxylic acids of bio-oil. Compared to acids, esters are more preferred in the fuel composition and have a less corrosive effect on the engine surface [11]. The catalyst acidity plays a key role in the esterification reaction. The data showed that the lower acidity of 10Ni/Al-SBA-15 compared to 10Ni/HBeta resulted in a lower selectivity to esters in the refined bio-oil (data is not shown).
The strong acid sites of 10Ni/HBeta provide protons as Brønsted acid, accelerating the esterification reaction. 4. Conclusions In this study, the HDO upgrading of crude bio-oil was carried out using a HBeta-supported nickel catalyst (10 wt%) in supercritical methanol, ethanol, and 2-propanol (265 C/2 h). Supercritical methanol and 2-propanol resulted in the highest (42.71%) and lowest (21.58%) degree of bio-oil deoxygenation, respectively. The heating value of the crude bio-oil was improved from 12.61 MJ/kg to 24.17 MJ/kg through HDO upgrading over 10Ni/HBeta using supercritical methanol as a solvent. A study of the catalytic activity of Ni/HBeta with different Ni loadings (5e20 wt%) in bio-oil HDO in supercritical methanol showed that the highest HDO efficiency of bio-oil was achieved over 10 wt% Ni/HBeta. These data showed that, in addition to the solvent role, methanol is involved in the esterification reaction with the acids in the bio-oil. Although the sugars and acids were the most abundant compounds in crude bio-oil, esters formed the main fraction of the bio-oil upgraded over 10Ni/HBeta. The degree of deoxygenation and HHV of bio-oil were enhanced by prolonging the reaction time from 2 to 4 h and increasing the initial hydrogen pressure from 10 to 20 bar; the maximum deoxygenation degree (46.37%) and HHV (26.04 MJ/kg) of the bio-oil were obtained at a hydrogen pressure of 20 bar and a reaction time of 4 h. A comparison of the activity of Ni impregnated
Fig. 6. Distribution of organic compounds in the crude bio-oil (a) and the one upgraded over 10Ni/HBeta using supercritical methanol (b). Reaction conditions: feedstock, 40 g; biooil/MeOH ratio, 1:2; hydrogen pressure, 10 bar; 10Ni/HBeta, 1 g; temperature, 265 C; reaction time, 4 h and rpm, 400.
Please cite this article in press as: H. Shafaghat, et al., Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts, Renewable Energy (2018), https://doi.org/10.1016/j.renene.2018.06.096
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on the different supports (mesoporous Al-SBA-15 and microporous zeolites of HZSM-5, HBeta and HY) for the supercritical HDO of biooil indicated that the support acidity plays a decisive role in the HDO efficiency. Al-SBA-15, with lower acidity than zeolites, exhibited a lower degree of deoxygenation of bio-oil. Meanwhile, HBeta proved to be a more effective support for nickel sites than HZSM-5 and HY in bio-oil HDO upgrading. On the other hand, by functionalizing the HBeta surface with silane, the activity of this support for bio-oil HDO was decreased due to the decrease in the number of its surface acid sites. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03029131). Also, this work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CAP-16-05-KIMM). References [1] H. Lee, Y.M. Kim, I.G. Lee, J.K. Jeon, S.C. Jung, J.D. Chung, W.G. Choi, Y.K. Park, Recent advances in the catalytic hydrodeoxygenation of bio-oil, Kor. J. Chem. Eng. 33 (2016) 3299e3315. [2] H. Kim, H. Shafaghat, J. Kim, B.S. Kang, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Stabilization of bio-oil over a low cost dolomite catalyst, Kor. J. Chem. Eng. 35 (2018) 922e925. [3] Y. Lee, H. Shafaghat, J.K. Kim, J.K. Jeon, S.C. Jung, I.G. Lee, Y.K. Park, Upgrading of pyrolysis bio-oil using WO3/ZrO2 and Amberlyst catalysts: evaluation of acid number and viscosity, Kor. J. Chem. Eng. 34 (8) (2017) 2180e2187. [4] K.A. Rogers, Y. Zheng, Selective deoxygenation of biomass-derived bio-oils within hydrogen-modest environments: a review and new insights, ChemSusChem 9 (2016) 1750e1772. [5] H. Shafaghat, P.S. Rezaei, W.M.A.W. Daud, Catalytic hydrodeoxygenation of simulated phenolic bio-oil to cycloalkanes and aromatic hydrocarbons over bifunctional metal/acid catalysts of Ni/HBeta, Fe/HBeta and NiFe/HBeta, J. Ind. Eng. Chem. 35 (2016) 268e276. [6] S. Oh, H.S. Choi, U.J. Kim, I.G. Choi, J.W. Choi, Storage performance of bio-oil after hydrodeoxygenative upgrading with noble metal catalysts, Fuel 182 (2016) 154e160. [7] N.S. Tessarolo, R.V.S. Silva, G. Vanini, A. Casilli, V.L. Ximenes, F.L. Mendes, ~o, E.V.R. Castro, C.R. Kaiser, D.A. Azevedo, Characterization A.R. Pinho, W. Roma of thermal and catalytic pyrolysis bio-oils by high-resolution techniques:1H NMR, GC GC-TOFMS and FT-ICR MS, J. Anal. Appl. Pyrolysis 117 (2016) 257e267. [8] S. Wang, Q. Cai, X. Wang, L. Zhang, Y. Wang, Z. Luo, Biogasoline production from the co-cracking of the distilled fraction of bio-oil and ethanol, Energy Fuels 28 (2014) 115e122. [9] M. Zhou, Y. Wang, Y. Wang, G. Xiao, Catalyticconversionofguaiacoltoalcoholsforbio-oilupgrading, J. Energy Chem. 24 (2015) 425e431. [10] X. Jiang, N. Ellis, Upgrading bio-oil through emulsification with biodiesel: thermal stability, Energy Fuels 24 (2010) 2699e2706. [11] Y. Liu, Z. Li, J.J. Leahy, W. Kwapinski, Catalytically upgrading bio-oil via esterification, Energy Fuels 29 (2015) 3691e3698. [12] P.M. Mortensen, D. Gardini, C.D. Damsgaard, J.D. Grunwaldt, P.A. Jensen, J.B. Wagner, A.D. Jensen, Deactivation of Ni-MoS2 by bio-oil impurities during hydrodeoxygenation of phenol and octanol, Appl. Catal., A 523 (2016) 159e170. [13] T. Yang, Y. Jie, B. Li, X. Kai, Z. Yan, R. Li, Catalytic hydrodeoxygenation of crude bio-oil over an unsupported bimetallic dispersed catalyst in supercritical ethanol, Fuel Process. Technol. 148 (2016) 19e27. [14] H. Shafaghat, P.S. Rezaei, W.M.A.W. Daud, Using decalin and tetralin as
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Please cite this article in press as: H. Shafaghat, et al., Catalytic hydrodeoxygenation of crude bio-oil in supercritical methanol using supported nickel catalysts, Renewable Energy (2018), https://doi.org/10.1016/j.renene.2018.06.096