Hydrotreatment of Vegetable Oils to Produce Bio-Hydrogenated

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Hydrotreatment of Vegetable Oils to Produce Bio-Hydrogenated Diesel and Liquefied Petroleum Gas Fuel over Catalysts Containing Sulfided Ni
Mo and Solid Acids Yanyong Liu,*,† Rogelio Sotelo-Boyas,‡ Kazuhisa Murata,† Tomoaki Minowa,† and Kinya Sakanishi† †

Biomass Technology Research Center, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Center 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Instituto Politecnico Nacional, Mexico ESIQIE, Mexico D.F. 07738, Mexico ABSTRACT: Biohydrogenated diesel (BHD) and liquefied petroleum gas (LPG) fuel were produced by the hydrotreatment of vegetable oils over Ni
Mo-based catalysts in a high-pressure fixed-bed flow reaction system at 350 °C under 4 MPa of hydrogen. Because triglycerides and free fatty acids underwent the hydrogenation and deoxidization at the same time during the reaction, various vegetable oils (jatropha oil, palm oil, and canola oil) were converted to mixed paraffins by the one-step hydrotreatment process although they contained quite different amounts of free fatty acids. Ni-Mo/SiO2 formed n-C18H38, n-C17H36, n-C16H34, and n-C15H32 as predominant products in the hydrotreatment of jatropha oil. These long normal hydrocarbons had high melting points and thus gave the liquid hydrocarbon product over Ni-Mo/SiO2 a high pour point of 20 °C. Either Ni-Mo/H-Y or Ni-Mo/H-ZSM-5 was not suitable for producing BHD from jatropha oil because a large amount of gasoline-ranged hydrocarbons was formed on the strong acid sites of zeolites. When SiO2-Al2O3 was used as a support for the Ni-Mo catalyst, the pour point of the liquid hydrocarbon product decreased to
10 °C by converting some C15
C18 n-paraffins to iso-paraffins and light paraffins on SiO2-Al2O3. Because SiO2-Al2O3 had a proper solid acidic strength, both the chemical composition and the pour point of liquid hydrocarbon product over Ni-Mo/SiO2-Al2O3 were similar to those of a normal diesel bought from a petrol station. Meanwhile, the glycerin groups in the vegetable oils were converted to propane over Ni-Mo/SiO2-Al2O3 by the hydrogenation and deoxidization. Therefore, the liquid hydrocarbon product can be directly used as a BHD fuel for the current diesel engines, and the gas hydrocarbon product can be used as a liquefied petroleum gas (LPG) fuel in the hydrotreatment of vegetable oils over Ni-Mo/SiO2-Al2O3.

1. INTRODUCTION The development of alternative fuels from renewable resources to substitute fossil fuels has received considerable attention because of the global climate change and the depletion of fossil fuel resources.1 Biodiesel is an important alternative fuel made from renewable resources such as vegetable oils.2 Although biodiesel is a renewable fuel with environmental benefits, it is necessary to find cheap raw materials without competition with arable land and food in the development of biodiesel.3 Jatropha (Jatropha Curcas L.) has gained a worldwide attraction from the views of the environment and energy owing to some strong points. First, jatropha is easy to establish, grows relatively quickly, and can grow almost in any wastelands, even on stony, gravelly, sandy, and saline soils.4 Second, the seed harvest of jatropha is much higher than those of canola, sunflower, and soybean in one hectare.5 Third, using Jatropha oil for producing fuels does not compete with food uses because of the toxic substance in Jatropha seeds.6 Without competition with food uses gives Jatropha oil a low and stable price. In a word, Jatropha is an ideal candidate either for producing biofuel or for covering wastelands and desert. There are three main methods for using vegetable oils as a fuel in diesel engines. Straight vegetable oil (SVO) is the simplest method because it uses vegetable oils in diesel engines just after filtration.7 However, SVO just can be used in some special diesel engines only and it is not suitable for the majority of current r 2011 American Chemical Society

diesel engines due to high viscosity and low fluidity. Fatty acid methyl ester (FAME) is the first generation of biodiesel and it is produced by the transesterification of vegetable oil with methanol.2,8 However, FAME has some shortages as a fuel in the current diesel engines because both CdC bonds and CdO bonds have been remained in the molecules of FAME.2 The antioxidation ability of FAME is low due to unsaturated CdC double bonds, and the flash point of FAME is high because FAME is less flammable than paraffins in normal diesel. Biohydrogenated diesel (BHD), which is known as the next generation of biodiesel, is produced from the hydrotreatment of vegetable oil, instead of the transesterification of vegetable oil.9,10 A paraffin mixture is produced from vegetable oil after all unsaturated CdC double bonds have been hydrogenated and all oxygen atoms have been eliminated during the hydrotreatment process. Hence, the chemical compositions are mixed paraffins for both the BHD from vegetable oils and the normal diesel from crude oil.9 In recent years, the hydrotreatment of vegetable oils to produce hydrocarbons has been studied worldwide and an extensive research has been performed to search suitable reactors and catalysts.9
20 When vegetable oils are treated at high temperatures and high pressures without a catalyst, vegetable oils can be Received: June 17, 2011 Revised: August 24, 2011 Published: August 24, 2011 4675

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Energy & Fuels converted to the mixtures of paraffins, cycloparaffins, and aromatic hydrocarbons but a relatively large amount of fatty acids remains in the products.11 Solid acids (such as H-ZSM-5, SO4/ZrO2, and so on) can convert vegetable oils to the mixtures of gasoline, kerosene, light gas oil, gas oil, and long residue in the hydrocracking of vegetable oils.12,13 Industrial fluid catalytic cracking (FCC) catalysts can convert vegetable oils to gasoline distillated hydrocarbons in the FCC unit under the FCC conditions.14 As for the BHD production, two types of catalysts have been reported as effective catalysts in converting vegetable oils to diesel distillated hydrocarbons: noble metal catalysts (such as supported Pd, Pt, and so on)15
18 and sulfided bimetal catalysts (such as Ni-Mo, Co-Mo, Ni-W, and so on).18
20 The yield of C15
C18 n-paraffins usually has been emphasized in literature for the hydrotreatment of vegetable oils to BHD, and it seems that the amount of C15
C18 n-paraffins determines the quality of BHD fuel.15
19 Actually, an overwhelming amount of C15
C18 n-paraffins must give BHD a low fluidity and hinder the use of BHD in the current diesel engines. A post-treatment of BHD for fitting for the current diesel engines will increase the cost of BHD production. We think that the character of normal diesel from crude oil should be a goal for the BHD fuel because the current diesel engines have been designed for using the normal diesel as a fuel.20 We proposed the concept that BHD fuel should has a chemical composition and physical properties similar to those of the normal diesel for the first time in the previous work.20 However, it needs further research to investigate how to achieve the concept satisfactorily. Many aspects for the reaction and catalysts should be clarified, such as the influences of various acidic supports, the influences of various vegetable oils, the influences of various reaction conditions, and so on. We had tried to combine Pt with various solid acids to achieve the catalyst’s multiple functions (hydrogenation-dehydrogenation, hydroisomerization, and hydrocracking) in the hydrotreatment of long-chain hydrocarbons.21,22 This paper focuses on the combination of sulfided Ni-Mo and various solid acids to achieve hydrogenation, deoxygenation, hydroisomerization, and hydrocracking for the hydrotreatment of vegetable oils. The influences of solid acidity, oil composition, and reaction conditions have been thoroughly investigated, and the chemistry in the hydrotreatment of vegetable oil has also been discussed in this work.

2. EXPERIMENTAL SECTION 2.1. Materials. Jatropha oil was purchased from Nippon Biodiesel Fuel Company, which imported the refined jatropha oil (gum eliminated) from Indonesia. Canola oil and palm oil were supplied by Lion Oleo Chemical Company, Japan. The standard reagents of fatty acids and triglycerides were purchased from Aldrich Chemical Co. and Tokyo Kasei Chemical Company. Gas cylinders were purchased from Sumitomo Seika Chemical Company, and the purity was higher than 99.995% for each gas. Chemical reagents for synthesizing catalysts were purchased from Wako Pure Chemical Industries Company, and the purity was higher than 99% for each reagent. Distilled water was used throughout the synthesis process. 2.2. Catalyst Preparation. SiO2-Al2O3 (SiO2/Al2O3 = 8.0) was purchased from Fuji Silysia Chemical Company. SiO2 (JRC-SIO-5; BET surface area, 192 m2 g
1) and γ-Al2O3 (JRC-ALO-4; BET surface area, 167 m2 g
1) were distributed by the Catalysis Society of Japan as reference catalysts. H-Y

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Figure 1. Flowchart of the high-pressured flowed fixed-bed reaction system for experimental investigation: (1) H2 cylinder, (2) pressure regulator, (3) mass flow controller, (4) tank of vegetable oil, (5) highpressured microfeeder, (6) stainless steel tubular reactor, (7) catalyst layer, (8) furnace, (9) cold trap, (10) tank of ice water, (11) outlet of liquid product, (12) back-pressure regulator, and (13) online GCs.

zeolite (SiO2/Al2O3 = 5.5) was purchased from Wako Pure Chemical Industries Company. H-ZSM-5 zeolite (SiO2/Al2O3 = 23.2) was supplied by Tosoh Chemical Company. Ni-Mo-based catalysts were synthesized using a wet coimpregnation method similar to the method reported in the literature.23 The support (SiO2, γ-Al2O3, SiO2-Al2O3, H-Y, or H-ZSM-5) was added to a mixed aqueous solution of (NH4)6Mo7O24 3 4H2O and Ni(NO3)2 3 6H2O to form a slurry. The slurry was stirred at 60 °C until formation of a solid sample by evaporating water. The solid sample was then dried at 110 °C for 24 h and finally calcined in air at 500 °C for 4 h. The loading of MoO3 was 15 wt % and the loading of NiO was 3 wt % in each catalyst. Prior to the reaction, the catalysts were sulfided and reduced in a mixed gas containing 10% H2S and 90% H2 (flow rate, 60 mL min
1) at 400 °C for 10 h. 2.3. Characterization. Acid values of vegetable oils and liquid products were determined by an acid
base titration technique using a KOH aqueous solution (ASTM D 664). Iodine values of the oils were measured by a titration technique using ICl and Na2S2O3 solutions (ASTM D 1959). The density of the oils was determined at 20 °C using a density/specific gravity meter (Kyoto Electronics DA-130N). The viscosity of the oils was determined at 40 °C using a vibro viscometer (A&D Co. Lim. Japan, SV-10). The chemical composition of vegetable oil was analyzed using an Agilent 6890 N gas chromatograph-flami ionization detector (GC-FID) and an Omnistar Q-mass. A HP-624 capillary column was used to separate the free fatty acids and an UA-TRG capillary column was used to separate triglycerides, diglycerides, and monoglycerides in the vegetable oils. Temperature-programmed desorption of ammonia (NH3-TPD) was carried out using a BELCAT-B automatic system equipped with a thermal conductivity detector (TCD) and an Omnistar Q-mass. A part of 0.05 g of the solid sample was pretreated at 400 °C for 1 h in a He flow with a flow rate of 50 mL min
1. After the temperature was decreased to 100 °C, ammonia was adsorbed onto the sample’s surface, followed by evacuation for 1 h at 100 °C to eliminate the weakly adsorbed ammonia. Then, NH3-TPD was recorded from 100 to 600 °C with a rate of 8 °C min
1. 2.4. Experimental Setup and Procedure. Figure 1 shows the flowchart of the high-pressured flowed fixed-bed reaction system used in this study. 4676

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Table 1. Compositions and Properties of Various Vegetable Oils composition (wt %)

Table 2. Composition of Free Fatty Acid in Jatropha Oil from Indonesiaa structureb

wt %

palmitic

C16:0

15.9

palmitoleic stearic

C16:1 C18:0

0.9 6.9

C18H34O2

oleic

C18:1

41.1

C18H32O2

linoleic

C18:2

34.7

112

C18H30O2

linolenic

C18:3

0.3

0.92

0.91

C20H38O2

eicosenoic

C20:1

0.2

48.7

56.1

jatropha oil

palm oil

canola oil

formula

triglyceride

80.4

91.5

96.6

C16H32O2

diglyceride monoglyceride

2.1 2.5

1.6 0.8

1.3 1.1

C16H30O2 C18H36O2

free fatty acid

14.9

6.1

1.0

acid value (mg of KOH/g of oil)

27.2

11.5

1.8

iodine value (g of I2/100 g of oil)

105

58

density at 25 °C (g/mL)

0.92

viscosity at 30 ° (mPa/s)

49.4

A stainless steel tubular reactor (i.d., 1 cm; length, 50 cm) was used for loading catalyst and a furnace was used for heating the tubular reactor. The vegetable oil was pushed into the reactor in a constant rate by a JP-H type high-pressured microfeeder (Furue Science Company). In the meanwhile, a mixed gas containing 90% H2 and 10% Ar (using as an internal standard) was introduced into the reactor from a high-pressure H2 cylinder and the flow rate was controlled by a mass flow controller. The pressure in the reaction system was controlled by a back-pressure regulator. A cold trap (soaked in a tank of ice water) was set between the reactor exit and the back-pressure regulator to collect liquid products. The standard reaction conditions were shown as follows: catalyst amount, 1 g; reaction temperature, 350 °C; H2 pressure, 4 MPa; liquid hourly space velocity (LHSV), 7.6 h
1; ratio of H2 to oil in feed, 800 mL/mL, in which the H2 volume was described in the conditions of standard temperature and pressure (STP). 2.5. Product Analysis. The gas products were continuously analyzed by two online GCs during the reaction. Inorganic gases (H2, Ar, CO, and CO2) were analyzed using a Shimadzu 8A GC-TCD equipped with MS-5A and Porapak-Q columns. Gas hydrocarbons (C1
C4) were analyzed using an Agilent 6890 N GC-FID equipped with a RT-QPLOT capillary column. The factors of various gases were obtained using a standard mixed gas (with known concentration for each component) from a cylinder. The liquid products were taken out from the cold trap after the reaction. After removal of the water layer in the bottom using a separation funnel, a certain amount of 1-methylnaphthalene was added to the organic phase as an internal standard. Then, the organic phase was analyzed by an Agilent 6890 N GC-FID equipped with three capillary columns. A UA-DX30 capillary column was used to analyze C5+ hydrocarbons, a HP-624 capillary column was used to analyze fatty acids, and a UA-TRG capillary column was used to analyze triglycerides, diglycerides, and monoglycerides. The yields of C1
C4 hydrocarbons, CO, and CO2 in the gas products were calculated using an Ar internal standard. The yields of organic compounds in the liquid products were calculated using 1-methylnaphthalene internal standard. The yield of water was calculated from the weights of formed water and introduced vegetable oil. The carbon mass balance had an error H-Y > SiO2-Al2O3 > γ-Al2O3 > SiO2 (no acidity). This order coincided with the results in the literature.28
30 The SiO2-Al2O3 samples with various SiO2/ Al2O3 ratios usually exhibit various acidic properties.28 A SiO2Al2O3 sample with a low SiO2/Al2O3 ratio shows a relatively strong solid acidity because it contains a large amount of Al2O3. The SiO2-Al2O3 sample used in this study had a relatively high SiO2/Al2O3 ratio (SiO2/Al2O3 = 8.0) among various SiO2Al2O3 samples and thus it is a weak solid acid. The H-ZSM-5 sample used in this study had a high SiO2/Al2O3 ratio (SiO2/ Al2O3 = 23.2), but it showed the strongest acidity among various catalysts (Figure 3). The microporous structure of H-ZSM-5 determines the position of Al sites and gives H-ZSM-5 a strong solid acidity. Hence, the solid acidity is mainly determined by the structure of catalysts, and the Al amount (SiO2/Al2O3 ratio) influences the acidity of the catalysts which had the same structure. The isomerization of n-paraffin proceeds via a carbenium ion intermediate which formed on the acid sites.31
33 When solid acid (such as H-ZSM-5, H-Y, and SiO2-Al2O3, and so on) alone is used for the isomerization of n-paraffin, the initial normal carbenium ion is formed from n-paraffin through hydride elimination or proton addition followed by hydrogen elimination. The formed normal carbenium ion is then isomerized to a branched carbenium ion, and the branched carbenium ion is finally converted to an iso-paraffin on the acid sites. By addition of metal (such as Pt, Ni-Mo, and so on) in the solid acid, n-olefin is rapidly formed by the dehydrogenation of n-paraffin on the metal sites. The initial normal carbenium ion is formed by the addition of proton to the n-olefin. Then, the formed normal carbenium ion is isomerized to a branched carbenium ion on the acid site, followed by the elimination of proton to form an iso-olefin. Finally, the iso-olefin is hydrogenated to form an iso-paraffin on the metal sites. In a word, metal achieves a function of hydrogenation
dehydrogenation and solid acid achieves a function of isomerization. The isomerization of n-paraffin is greatly enhanced by combination of metal with solid acid because the carbenium ion is much easier to be formed from olefin (by

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proton addition) in comparison with from paraffin (by hydride elimination). Because a large amount of carbenium ion can be formed on the strong acid sites, the acidic strength of solid acid determines the activity of catalytic isomerization when the same metal is used. As shown in Table 4, the iso/n ratio of the liquid hydrocarbon products over various catalysts was in the order of Ni-Mo/H-ZSM-5 (1.21) > Ni-Mo/H-Y (0.87) > Ni-Mo/SiO2Al2O3 (0.26) > Ni-Mo/γ-Al2O3 (0.08) > Ni-Mo/SiO2 (0.03). This order is the same as the order of the acidic strength of solid acids in various catalysts. The cracking of paraffin is a competitive reaction with respect to the isomerization because of the common carbenium ion intermediate. The cracking of a carbenium ion forms a light carbenium ion and an olefin by β-scission. All n-paraffins in the catalytic system (including reactants and products) undergo the cracking in parallel with the isomerization. The acidic strength of solid acid is crucial for the product distribution in the cracking of paraffin. Ni-Mo/SiO2 hardly formed light paraffins (eC14) because SiO2 did not have acid sites on the surface. The acidity of γ-Al2O3 was too weak, and thus the amount of light paraffins was not enough in the product over Ni-Mo/γ-Al2O3. Either Ni-Mo/ H-Y or Ni-Mo/H-ZSM-5 produced a large amount of gasolineranged hydrocarbons because H-Y and H-ZSM-5 contained strong acid sites on the surfaces. The liquid hydrocarbon product over Ni-Mo/SiO2-Al2O3 contained a proper amount of light paraffins, indicating that SiO2-Al2O3 had a proper acidic strength for the cracking. On the other hand, the relative reactivity in the hydrocracking of paraffin greatly increases with increasing the carbon number in the carbon chain.34,35 For example, the speed of n-C17H36 hydrocracking is 2.4 times faster than that of n-C16H34 hydrocracking and is 4.0 times faster than that of n-C15H32 hydrocracking over the catalysts containing metal and solid acid.34 As shown in Figure 2, the amount of C15
C18 n-paraffins was in the order n-C15H32 < n-C16H34 < n-C17H36 < n-C18H38 over Ni-Mo/SiO2 but was in the order n-C15H32 > n-C16H34 > n-C17H36 > n-C18H38 over Ni-Mo/SiO2-Al2O3. The high cracking reactivity caused a remarkable decrease of large paraffins (such as n-C18H38) in the product over the catalysts containing solid acids (such as Ni-Mo/SiO2-Al2O3). The disproportionation of paraffin also occurs in the reaction systems containing solid acids. The disproportionation is an alkylation-cracking reaction via a part of “bimolecular mechanism”. That is, a large carbenium ion is formed by the alkylation of a carbenium ion with an olefin molecule, followed by the cracking of the large carbenium ion on acid sites to achieve the disproportionation. Hence, the disproportionation can form a paraffin product which is larger than the paraffin reactant. As shown in Figure 2, the liquid hydrocarbon product over Ni-Mo/SiO2Al2O3 contained some C19+ paraffins. These C19+ paraffins have been formed from the disproportionation of C15
C18 n-paraffins in the catalytic system. Because the formed C19+ large paraffins rapidly cracked relative to light paraffins in the catalytic system during the reaction, only a small amount of C19+ paraffins remained in the final products. 3.3. Chemistry of Hydrotreatment Process. Figure 4 shows the photographs of the products collected in the cold trap for the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. The photographs were taken at room temperature (25 °C). jatropha oil was a liquid at room temperature because its main components were unsaturated triglycerides and unsaturated free fatty acids (Tables 1
3). After the reaction was carried out at 150 °C, a solid wax was formed and no liquid phase could 4680

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Figure 4. Photographs of the products collected in the cold trap in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. (A) Before reaction; (B) after reaction at 150 °C; (C) after reaction at 250 °C; and (D) after reaction at 350 °C. Reaction conditions: H2 pressure = 4 MPa, H2/oil in feed = 800 mL/mL; LHSV = 7.6 h
1.

be found in the product at room temperature. This indicates that the hydrogenation of CdC double bonds in unsaturated triglycerides and free fatty acids was the first step for the hydrotreatment of jatropha oil. The mixture of the formed saturated triglycerides and free fatty acids became a solid wax at room temperature. After the reaction was carried out at 250°C, the product was separated into two layers at room temperature. By GC-MS analysis, the upper liquid phase was hydrocarbons and the bottom solid phase was saturated triglycerides and fatty acids. Therefore, the deoxygenation of saturated triglycerides and fatty acids occurred on the Ni-Mo active phases as the second step for the hydrotreatment of jatropha oil. After the reaction was carried out at 350 °C, the product became two liquid layers and no solid phase could be found in the product at room temperature. The upper layer (about 90%) was hydrocarbons, and the bottom layer (about 10%) was water. Free fatty acids occupied 14.9 wt % in jatropha oil (Table 1). After the first hydrogenation step in the hydrotreatment process, all C16-acids were converted to palmitic acid (C15H31COOH) and all C18-acids were converted to stearic acid (C17H35COOH) due to extinction of unsaturated CdC double bonds. By calculation with the data in Table 2, stearic acid occupied 83.0 wt % and palmitic acid occupied 16.8 wt % in total fatty acids after the first hydrogenation step. Then, the formed saturated fatty acids underwent the second deoxygenation step to form n-paraffins through three paralleled paths: reduction, decarbonylation, and decarboxylation.19 C17 H35 COOH þ 3H2 ¼ C18 H38 þ 2H2 O ðreductionÞ

ð1Þ

C17 H35 COOH þ H2 ¼ C17 H36 þ CO þ H2 O ðdecarbonylationÞ

ð2Þ

C17 H35 COOH ¼ C17 H36 þ CO2 ðdecarboxylationÞ

ð3Þ

As shown in Table 2, all free fatty acids in jatropha oil had the carbon chains with even carbon numbers (16, 18, or 20). For a

fatty acid with an even carbon number, the reduction produces a paraffin that has an even carbon number plus water; the decarbonylation produces a paraffin that has an odd carbon number plus water and CO; and the decarboxylation produces a paraffin that has an odd carbon number plus CO2. Both the decarbonylation and the decarboxylation occurred during the reaction because both CO and CO2 were detected in the gas product over each Ni-Mo catalyst. Hence, in the second deoxygenation step, n-C16H34 and n-C18H38 were formed by the reduction of palmitic acid and stearic acid, and n-C15H32 and n-C17H36 were formed by the decarbonylation and the decarboxylation of palmitic acid and stearic acid on Ni-Mo sites. Therefore, C15
C18 n-paraffins became the predominant products in the liquid hydrocarbon product over Ni-Mo/SiO2 and they gave a high pour point of the liquid hydrocarbon product (Table 4). In order to decrease the pour point of the liquid product, we designed the third step for the isomerization/cracking of C15H32
C18H38 n-paraffins on solid acid catalysts. SiO2-Al2O3 had been proved as the most suitable solid acid for supporting the Ni-Mo catalyst in the hydrotreatment of jatropha oil because of proper acidic strength. Triglycerides occupied 80.4 wt % in jatropha oil (Table 1). For the hydrotreatment of triglycerides, the unsaturated triglycerides at first saturated on their CdC bonds in the carbon chains by the hydrogenation on the Ni-Mo sites. Then, the formed saturated triglycerides decomposed by the scission of the CdO bonds, leading to the formation of diglycerides, monoglycerides, and carboxylic acids (fatty acids) in that order. Then, the formed carboxylic acids underwent the deoxygenation on the Ni-Mo sites to form C15H32
C18H38 n-paraffins. Finally, the C15H32
C18H38 n-paraffins underwent the isomerization/cracking on the solid acid sites to form iso-paraffins and light paraffins. As discussed above (Tables 2 and 3), the amount of C18-esters (68.0 wt %) was larger than that of C16-esters (32.0 wt %) and the amount of free C18-acids (83.0 wt %) was larger than that of free C16-acids (16.8 wt %) in jatropha oil. Hence, the amounts of n-C17H36 and n-C18H38 (formed from C18-esters and C18-acids) 4681

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were larger than the amounts of n-C15H32 and n-C16H34 (formed from C16-esters and C16-acids) in the liquid hydrocarbon product over Ni-Mo/SiO2 (Figure 2). With adjustment of the composition and properties using solid acid SiO2-Al2O3, the composition and properties of the liquid hydrocarbon product over Ni-Mo/SiO2-Al2O3 became similar to those of normal diesel from a petrol station (Table 4). Figure 5 shows the GC-FID chart (RT-QPLOT capillary column) of gas hydrocarbons formed from the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Triglycerides produce C15
C18 n-paraffins and propane at the same time on the Ni-Mo sites after all CdO bonds in triglycerides have been cut down during the hydrotreatment process. As shown in Figure 5, propane is the predominant product (>95 wt %) in the gas hydrocarbon hydrocarbon over Ni-Mo/SiO2-Al2O3. During the reaction, the formed propane and butane can be easily collected by a liquefaction method under several atmospheres of pressure at room temperature, while the other gases (H2, CO, CO2, CH4, and C2H6) are flowed out from the catalytic system. The collected propane and butane can be used as an LPG fuel.

Figure 5. GC-FID chart (RT-QPLOT capillary column) of the gas hydrocarbons formed from the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3 (reaction conditions: same as those in Table 4).

Figure 6. Chemistry of the hydrotreatment of Jatropha oils over Ni-Mo/SiO2-Al2O3.

Figure 6 sums the chemistry of the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Ni-Mo sites achieved the hydrogenation and deoxidization of jatropha oil to form C15
C18 n-paraffins and propane. Solid acid sites on SiO2-Al2O3 achieved the isomerization/cracking of C15
C18 n-paraffins to form branched paraffins and light paraffins. Hence, Ni-Mo/SiO2Al2O3 is a trifunctional catalyst with abilities of hydrogenation, deoxygenation, and isomerization/cracking in the hydrotreatment of jatropha oil. Because Ni-Mo has a strong ability in the deoxygenation step and SiO2-Al2O3 has a proper acidic strength in the isomerization/cracking step for the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3, the liquid hydrocarbon product (Figure 2) can be directly used as a green BHD fuel in current diesel engines and the gas hydrocarbon product (Figure 5) can be used as a high-purity LPG fuel. Table 5 shows the product yield and BHD property for the hydrotreatment of various vegetable oils over Ni-Mo/SiO2Al2O3. The conversion was 100% and did not decrease even after reaction for 10 h because triglycerides could not be detected in the products for the hydrotreatment of each vegetable oil over Ni-Mo/SiO2-Al2O3. The BHD yield was in a narrow range of 81.4
83.5 wt %, and the LPG yield ranged in a narrow range of 4.9
5.7 wt % for the hydrotreatment of various vegetable oils. The LPG yield from the hydrotreatment of jatropha oil was relatively low (4.9 wt %) because jatropha oil contained a large amount of free fatty acids. The free fatty acids did not produce propane in the hydrotreatment process. Because palm oil contained a relatively large amount of C16-acids and C16-esters, the BHD product from palm oil had a relatively low pour point (
15 °C) in comparison with those from jatropha oil and canola oil. The BHD fuels produced from various vegetable oils had similar properties (including iso/n ratio, pour point, density, and viscosity) to each other. Because the CdC double bonds were saturated in the first hydrogenation step and both free fatty acids and triglycerides were deoxygenated at the same time in the second deoxygenation step, the vegetable oils which contained various amounts of free fatty acids and CdC double bonds could be converted to BHD and LPG over Ni-Mo/SiO2-Al2O3 in the one-step hydrotreatment process. 3.4. Influences of Reaction Conditions. Figure 7 shows the effect of reaction temperature in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. The yield of LPG increased with increasing reaction temperature from 275 to 325 °C and was almost kept at a constant value when the reaction temperature was above 325 °C. Hence, a reaction temperature of 325 °C was necessary for the deoxygenation of triglycerides in jatropha oil. On the other hand, the yield of BHD greatly increased with increasing reaction temperature from 275 to 325 °C but slightly decreased when the reaction temperature was higher than 325 °C. An increase of gas hydrocarbon products (especially CH4) caused a slight decrease

Table 5. Product Yield and BHD Property for the Hydrotreatment of Various Vegetable Oils over NiMo/SiO2-Al2O3a yield (wt %)

BHD property

oil

BHDb

LPGc

fuel gasd

COxe

H2O

iso/n

pour point (°C)

density at 25 °C (g/mL)

viscosity at 30 °C (mPa/s)

jatropha

83.5

4.9

0.2

2.4

8.9

0.26


10

0.78

4.13

canola

81.4

5.7

0.3

2.1

9.5

0.25


10

0.79

4.22

palm

82.1

5.4

0.3

2.3

9.3

0.27


15

0.78

3.82

Reaction temperature, 350 °C; H2 pressure, 4 MPa; H2/oil ratio in feed, 800 mL/mL; LHSV, 7.6 h
1. b Liquid hydrocarbon products. c C3H8 + C4H10. d CH4 + C2H6. e CO + CO2. a

4682

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Figure 7. Effect of reaction temperature in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. H2 pressure, 4 MPa; H2/oil ratio in feed, 800 mL/mL; LHSV, 7.6 h
1. (b) Yield of BHD (liquid hydrocarbons); (9) yield of LPG (C3 + C4); (() acid value of BHD.

Figure 8. Effect of H2 pressure in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Reaction temperature, 350 °C; H2/oil ratio in feed, 800 mL/mL; LHSV, 7.6 h
1. (b) Yield of BHD (liquid hydrocarbons); (9) yield of LPG (C3 + C4); (() acid value of BHD.

of BHD yield at high reaction temperatures (>325 °C). Moreover, the composition and the pour point of BHD changed at above 325 °C because the amount of C5
C10 hydrocarbons increased with increasing reaction temperature. Hence, the reaction temperature can also be used for adjusting the composition and property of the BHD product. On the other hand, the acid value in BHD rapidly decreased with increasing reaction temperature and could not be detected when the reaction temperature was higher than 350 °C. On the whole, the most suitable reaction temperature was 350 °C in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Figure 8 shows the effect of H2 pressure in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. The yield of LPG increased with increasing H2 pressure from 0.5 to 3 MPa and was almost kept at a constant value when the H2 pressure was above 3 MPa. Hence, a H2 pressure of 3 MPa was the minimum value for the deoxygenation of triglycerides in jatropha oil. On the other hand, the yield of BHD greatly increased with increasing H2 pressure from 0.5 to 3 MPa and then just slowly increased with increasing the H2 pressure above

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Figure 9. Effect of H2/oil in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Reaction temperature, 350 °C; H2 pressure, 4 MPa, LHSV, 7.6 h
1. (b) Yield of BHD (liquid hydrocarbons); (9) yield of LPG (C3 + C4); (() acid value of BHD.

3 MPa. As shown in eq 1
3, the deoxygenation of fatty acid contained three paralleled reactions: reduction, decarbonylation, and decarboxylation. The reduction is favorable under high H2 pressures because it consumes more H2 molecules than the decarbonylation and decarboxylation. Meanwhile, the reduction does not produce COx, while either the decarbonylation or the decarboxylation produces COx and loses one C in the carbon chain. The COx yield decreased with increasing H2 pressure from 3 to 8 MPa, which gave a slightly increase of BHD yield. As for the acid value of BHD product, it rapidly decreased with increasing H2 pressure and could not be detected when H2 pressure was higher than 4 MPa. Therefore, a H2 pressure of 4 MPa was necessary for the hydrotreatment of jatropha oil over Ni-Mo/ SiO2-Al2O3. Figure 9 shows the effect of H2/oil in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. The H2/oil was a ratio of H2 feed rate to liquid (jatropha oil) feed rate during the reaction. The LPG yield increased with increasing H2/oil from 400 to 700 and almost stayed constant when H2/oil was larger than 700. On the other hand, the BHD yield greatly increased with increasing H2/oil from 400 to 700 (due to improvement of the deoxygenation) and slowly increased with increasing H2/oil at above 700 (due to improvement of the reduction). The acid value of BHD product rapidly decreased with increasing H2/oil and could not be detected when H2/oil was larger than 800. Therefore, a H2/oil of 800 was necessary for the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. The H2 consumption for converting 1 mL of jatropha oil to saturated hydrocarbons could be estimated using the data in Tables 1
3. From Table 2, the average molecular formula of the free fatty acids in jatropha oil was calculated as C17.668H33.086O2, and this molecule contained two oxygen atoms and 1.125 CdC double bonds. From Table 3, the average molecular formula of the triglycerides in jatropha oil was calculated as C55.08H102.16O6, and this molecule contained six oxygen atoms and 2.0 CdC double bonds. As for diglyceride and monoglyceride, we assumed that one diglyceride molecule contained one saturated C16-ester and one unsaturated C18-ester with a CdC double bond and assumed that one monoglyceride molecule contained one unsaturated C18-ester with a CdC double bond. This assumption was relatively rational and should not bring a large error in calculation because the amounts of diglyceride and monoglyceride were very low in 4683

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Figure 10. Effect of contact time in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. Reaction temperature, 350 °C; H2 pressure, 4 MPa; H2/oil ratio in feed, 800 mL/mL. (b) Yield of BHD (liquid hydrocarbons); (9) yield of LPG (C3 + C4); (() acid value of BHD.

jatropha oil (Table 1). According to this assumption, the molecular formula of the diglyceride in jatropha oil was C37H70O5, and this molecule contained five oxygen atoms and 1.0 CdC double bonds. The molecular formula of the monoglyceride in jatropha oil was C21H40O4, and this molecule contained four oxygen atoms and 1.0 CdC double bonds. With the use of all the data above, the average molecular formula of jatropha oil used in this study was calculated as C48.219H89.494O5.327, and 1 mol of jatropha oil contained 5.327 mol of oxygen atoms and 1.821 mol of CdC double bonds. Because one CdC double bond consumed one H2 molecule to achieve hydrogenation and one oxygen atom consumed one H2 molecule to achieve deoxygenation, 1 mol of jatropha oil consumed 7.148 mol of H2 to form saturated hydrocarbons in the hydrotreatment process. The density of jatropha oil was 0.92 g/mL (Table 1), and the average molecular weight of jatropha oil was calculated as 753.4. Finally, we obtained the conclusion that 1 mL of Jatropha oil consumed about 196 mL of H2 (at standard temperature and pressure (STP) conditions) to form saturated hydrocarbons through hydrogenation and the deoxygenation. As shown in Figure 9, in order to ensure the quality of BHD from the hydrotreatment of jatropha oil, an H2/oil ratio of 800 mL/mL (about 4 times as much as the ratio of consumed H2 to oil) was needed in the high-pressure fixed-bed flow reaction system. Figure 10 shows the effect of contact time in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. A large LHSV value means feeding jatropha oil with a fast rate through an invariable amount of catalyst during the reaction. The increase of LHSV improves the productivity of BHD within a certain period of time by shortening the contact time of jatropha oil with the catalyst. As shown in Figure 10, both the LPG yield and the BHD yield are almost kept constant when LHSV ranged from 1.9 to 9.5 h
1 but slightly decreased with increasing LHSV at above 9.5 h
1. On the other hand, the acid value of the BHD product could be detected at a LHSV of 9.5 h
1 and greatly increased at LHSV values above 9.5 h
1. Therefore, a LHSV of 7.6 h
1 was the maximum acceptable value for the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3. For a further improvement of BHD productivity without increasing the acid value, a large reactor filled with a large amount of catalyst is recommended (instead of unreasonable increase of LHSV) in the hydrotreatment of jatropha oil.

Figure 11. Time course in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3 at a large LHSV of 30.4 h
1. Reaction temperature, 350 °C; H2 pressure, 4 MPa; H2/oil ratio in feed, 800 mL/mL. (A) Fresh catalyst; (B) used catalyst after reducing by H2 at 400 °C for 2 h; (C) used catalyst after treating by 10% H2S (in 90% H2) at 400 °C for 2 h.

Figure 11 shows the time course in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3 at a large LHSV of 30.4 h
1. We used a low LHSV of 7.6 h
1 as a standard reaction condition in this study because we want to obtain a high-quality BHD which does not contain fatty acids and triglycerides. Here, we carried out the reaction at a large LHSV of 30.4 h
1 for a long reaction time (72 h) to investigate the deactivation and regeneration of the Ni-Mo/SiO2-Al2O3 catalyst. As shown in Figure 11A, the fresh catalyst showed an initial BHD yield of 81.1% and the BHD yield slowly decreased to 75.2% after 72 h on stream. The used catalyst was reduced by H2 at 400 °C for 2 h and then was used again for the reaction at 350 °C (Figure 11B). As shown in Figure 11B, the initial catalytic activity did not recover (compared to Figure 11A) and the initial BHD yield was about 75.0%. This indicates that the oxidation of Ni-Mo (by the formed steam) was not the reason for the catalyst deactivation of Ni-Mo/SiO2-Al2O3. Figure 11C shows the time course over the used catalyst after treating by 10% H2S (in 90% H2) at 400 °C for 2 h. The catalytic activity recovered (compared to Figure 11A) because the initial BHD yield was about 81.0%. Hence, a loss of sulfur from sulfided Ni-Mo during the reaction caused the catalyst deactivation, and a sulfided treatment could regenerate the deactivated catalyst in the hydrotreatment of jatropha oil over Ni-Mo/SiO2-Al2O3.

4. CONCLUSIONS Vegetable oils (jatropha oil, palm oil, and canola oil) were convert to green BHD and LPG fuel by a one-step hydrotreatment process over the catalysts containing Ni-Mo and solid acids. SiO2-Al2O3 was a suitable support for Ni-Mo to produce BHD from vegetable oils. Ni-Mo/SiO2-Al2O3 was a trifunctional 4684

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Energy & Fuels catalyst with abilities of hydrogenation, deoxygenation, and isomerization/cracking. After all CdC bonds were hydrogenated in the first step and both free fatty acids and triglycerides were deoxygenated in the second step, vegetable oils were converted to C15
C18 n-paraffins and propane. SiO2-Al2O3 had a proper acidic strength for the isomerization/cracking of C15
C18 n-paraffins in the third step. Vegetable oils can be convert to BHD and LPG over Ni-Mo/SiO2-Al2O3 no matter how many CdC unsaturated bonds and how many free fatty acids they contained. The liquid hydrocarbon product can be directly used as a BHD fuel in current diesel engines, and the gas hydrocarbon product can be used as a LPG fuel. The suitable reaction conditions for the hydrotreatment of jatropha oil were listed as follows: a reaction temperature of 350 °C, a H2 pressure of 4 MPa, a H2/oil ratio of 800, and a LHSV of 7.6 h
1.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: (81) 29-861-4826. Fax: (81) 29-861-4776. E-mail: [email protected].

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