Role of Support in Hydrotreatment of Jatropha Oil over Sulfided NiMo

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Effect of Support on Performance of NiMo Catalysts in Hydrotreating of Jatropha Oil Shaofeng Gong, Akira Shinozaki, and Eika W. Qian Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 11 Oct 2012 Downloaded from http://pubs.acs.org on October 12, 2012

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Industrial & Engineering Chemistry Research

Role of Support in Hydrotreating of Jatropha Oil over Sulfided NiMo Catalysts Shaofeng Gong, Akira Shinozaki, Eika W. Qian* The Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, Nakacho 2-24-16, Koganei, Tokyo 184-8588, Japan E-mail: [email protected]

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ABSTRACT

Hydrotreatment of jatropha oil over a series of sulfided NiMo/SiO2-Al2O3 and NiMo/ZSM-5-Al2O3 catalysts using a fixed-bed reactor was carried out. Effect of support on various reactions occurring in hydrotreatment was investigated. For NiMo/SiO2-Al2O3 series catalysts, the rates of decarboxylation and/or

decarbonylation

increased

with

increasing

Si/(Si+Al)

ratio,

while

the

ratio

of

hydrodeoxygenation decreased with increasing Si/(Si+Al) ratio. For NiMo/ZSM-5-Al2O3 series catalysts, with increasing addition amount of ZSM-5, the rate of decarboxylation and/or decarbonylation versus hydrodeoxygenation did not change. These results could be attributed to that the total acidic sites of catalyst could have a positive effect on the decarboxylation and/or decarbonylation pathways. NiMo/SiO2-Al2O3 catalysts showed much higher isomerization/cracking ratio than NiMo/ZSM-5-Al2O3 catalysts. It was suggested that isomerization reaction was favorable for weak and middle acidic site but cracking reaction was favorable for strong acidic site.


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1. Introduction Diminishing fossil resources and growing concerns about world environment necessitate the production of green fuels to substitute conventional fossil-derived ones from renewable resources. 1 Recently, hydrotreatment of vegetable oils to produce green paraffinic fuels has attracted great attention and been considered to be one of the best routes for producing biofuels.2,3 Many studies have been conducted on the hydrotreating of vegetable oils and some model compounds, such as palm oil, 4 rapeseed oil, 5,6,7,8 sunflower oil, 9,10 jatropha oil, 11,12,13 fatty acid 14,15 and ester. 16,17 In these studies, traditional hydrodesulfurization catalysts such as sulfided NiMo/Al2O3, CoMo/Al2O3 and NiW/Al2O3 catalysts were mostly used, which showed high hydrodeoxygenation activity and complete conversion of vegetable oil into hydrocarbons can be achieved. It was well known that there are three deoxygenation pathways in hydrotreating of triglycerides9,13, which were the main constituent of all vegetable oils. The first was hydrodeoxygenation, removing the oxygen by forming H2O only and keeping the carbon atom number of the corresponding fatty acids. The second was decarbonylation, removing the oxygen by producing both CO and H2O and leading to one carbon atom loss of corresponding fatty acid. The third was decarboxylation, removing the oxygen by formation of CO2 and leading to one carbon atom loss of fatty acid. Senol et al 17 proposed a reaction mechanism for hydrotreating of methyl heptanoate in which heptanal was considered to be an intermediate leading selectivity to three deoxygenation pathways over sulfided NiMo and CoMo/Al2O3 catalysts. But they did not find any direct evidence and the deoxygenation mechanism remained to be investigated. Kubička et al7 investigated the effect of active metals on deoxygenation pathway in hydrotreating of rapeseed oil over sulfided Ni/Al2O3, Mo/Al2O3 and NiMo/Al2O3 catalysts and found that the deoxygenation pathway over Ni/Al2O3 catalyst was only compiled through decarboxylation and/or decarbonylation but the oxygen removal pathway over Mo/Al2O3 catalyst was executed primarily by hydrodeoxygenation. They also found that deoxygenation over NiMo/Al2O3 proceeded via both decarboxylation/decarbonylation and hydrodeoxygenation, and the ratio of Ni/Mo did not affect the deoxygenation pathway obviously. Hydrotreating of jatropha oil over reduced PtPd/Al2O3 and sulfided ACS Paragon Plus Environment

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NiMoP/Al2O3 catalysts was reported in our previous paper.13 It was found that the oxygen removal of jatropha oil over PtPd/Al2O3 catalyst proceeded primarily through decarboxylation and/or decarbonylation, but over NiMoP/Al2O3 catalysts, the oxygen removal proceeded primarily through hydrodeoxygenation. Although it was known that deoxygenation reactions were sensitive to the support of catalysts18, the support effect in hydrotreating of vegetable oils is ambitious. Because all the corresponding fatty acids in vegetable oil were straight chain and their carbon numbers center at 16 and 18,19 the deoxygenated products of vegetable oil mainly consisted of n-alkanes from C15 to C18. These normal alkanes had a high cetane number (above 98) but their cold flow properties such as cloud point were so unfavorable that their application as diesel fuels was strictly limited. To resolve problems about the cold flow properties, isomerization of n-paraffin to their branched isomers, which has been widely applied in petroleum industries such as dewaxing of lubricating oil, was regarded as an extremely feasible method.20,21 Hydrotreating of vegetable oil over Pt/zeolites8 and PtRe/H-ZSM-522 catalysts in a batch reaction was studied. Results showed that high metal loading amount was needed to achieve complete deoxygenation, and that the ratio of cracking/isomerization was very high on these catalysts. At the same time, it was found that HZSM-5 catalysts showed higher ratio of iso-paraffin/n-paraffin and yield of hydrocarbon products than other zeolites such as HY, Beta, and MOR. Using NiW/SiO2-Al2O3 catalysts to hydrotreat waste soya-oil23 and using NiMo/SiO2-Al2O3 catalysts to hydrotreat vegetable oils24 were also studied. Part of n-alkanes could be converted into iso-alkanes over these catalysts but the selectivity of iso-alkanes was not enough to resolve the problems of poor cold flow properties, and that the effects of the ratio of SiO2/Al2O3 and role of acidic sites in isomerization and cracking were not concerned. Although lots of studies about hydrotreatment of vegetable oils and model compounds were reported and interesting results were observed, the effect of support on deoxygenation pathway is yet ambitious. Moreover, there are no papers to study the effect of support for isomerization and cracking reactions in hydrotreating vegetable oils. Thus, in the present study, a series sulfided NiMo/SiO2-Al2O3 and NiMo/ZSM-5-Al2O3 catalysts were prepared and hydrotreatment of jatropha oil over these catalysts ACS Paragon Plus Environment

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under suitable reaction conditions was conducted. Then the effect of support on deoxygenation, isomerization and cracking reactions was further investigated.

2. Experimental Section 2.1. Feedstock and Catalysts The feedstock used for catalyst activity evaluation was a kind of crude jatropha oil supplied by CREATA-IPB and produced in Bogor, Indonesia. The γ-Al2O3, two kinds of SiO2-Al2O3 (SA-1 and SA-2), and SiO2 were commercial supports supplied by Nippon Ketjen. The three other supports, which were denoted by Z-5, Z-10 and Z-30 (5, 10 and 30 indicate the addition weight percent of ZSM-5 in the supports), were prepared according to the following procedure: Firstly, ZSM-5 (Supplied by Catalysis Society of Japan, No. JRC-Z5-70H, SiO2/Al2O3=70, specific surface area: 322 m2/g), γ-Al2O3 and binder (boehmite, 20 wt %) were milled into powder and mixed. The mixture and a small amount of deionized water were put into a Teflon beaker and followed by intensively stirring to make a mud-bike solid. And then the mud-like solid was pressed and shaped into small cylinders with an experimental scale model. At last, the small cylinders were dried at room temperature for 12 h and calcined in a Muffle furnace at 550 °C for 3h. All the supports were crushed and sieved to a 20-80 mesh particle size fraction before metal loading. All catalysts were prepared following conventional successive impregnation method procedure as reported.25 The respective support was loaded with Mo and Ni with aqueous solutions of (NH4)6Mo7O24 and Ni(NO3)2 to achieve a loading amount of 20 wt % MoO3 and 3.5 wt % NiO, in which Mo was introduced first. After impregnation, the samples were dried at 105°C for 2 h and then calcined at 450°C for 10 h under air flow.

2.2 Catalyst Characterization Elemental analysis was conducted using an X-ray fluorescence instrument (EDX-800, Shimadzu). The samples were pressed into disks before analysis. Specific surface areas and pore sizes were ACS Paragon Plus Environment

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determined from the nitrogen adsorption and desorption isotherms and recorded at –196 °C using a Belsorp-mini II device (Bel Japan). Before the measurements, the samples were degassed under vacuum at 400 °C for 1 h. The specific surface areas and pore volumes were calculated using the BET (Brunauer–Emmett–Teller) method. Then, the average pore sizes were calculated from desorption isotherm branches using the BJH (Barrett–Joyner–Halenda) method. The NH3-TPD (ammonia temperature programmed desorption) experiments were carried out using a Chemisorption Physisorption Analyzer (ChemBET PULSAR TPR/TPD, Quantachrome) according to the following procedure. First, about 200 mg of sample was put into the sample cell and pretreated at 500 °C under helium flow for 3 h at 15mL/min. And after cooling to 100 °C, ammonia adsorption was carried out for 40 min under ammonia flow at 15mL/min. Then, physically adsorbed ammonia was removed with blowing helium at 100 °C for 2 h. At last, the NH3-TPD of the samples was carried out by increasing the cell temperature linearly from 100 °C to 650 °C with a heating rate of 10 °C min-1 and a helium flow rate of 15 mL/min.

2.3. Activity Test of Catalysts Details of the apparatus were also described in an earlier report. 26 In short, the reactions were conducted with a fixed-bed flow microreactor (ø 8 mm i.d.). About 3 mL of catalyst was loaded and sandwiched by quartz sands in the reactor. The feedstock was injected by a high-pressure pump into the hydrogen stream. The reaction was conducted at 330–370 °C, 3 MPa of total pressure, LSHV (Liquid hourly space velocity) =2 h–1, H2/feed ratio of 600 (v/v). Before reaction, all the catalysts were sulfided in situ at 400 °C and atmospheric pressure by a mixture of 5 vol % H2S and 95vol % H2 at a rate of 50 mL/min for 3 h. The catalytic activity was measured after stabilization of the catalyst (all the time on stream was about 10 h). In all experiments, no significant deactivation of the various catalysts was observed during 10 h reaction. Reaction products were separated into gas and liquid first; then, the liquid products were separated into water and oil phases. Two gas chromatographs (GC-17A and GC-14B; Shimadzu), equipped with an FID (flame ionization detector) and a commercially available column (DB-1, 0.25 mm × 60 m), were used to ACS Paragon Plus Environment

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analyze the hydrocarbons in the gaseous products and liquid products, and a gas chromatograph mass spectrometer (GCMS-QP5050A; Shimadzu) equipped with the same capillary column (DB-1, 0.25 mm × 60 m) was used to identify the components of the reaction hydrocarbon products. Another gas chromatograph with a TCD (thermal conductivity detector) and a commercial column (Unibeads C, 3 mm × 3 m) was used to determine other components, such as CO and CO2, in gaseous products. Additionally, a CP-TAP CB for triglycerides column (0.25 mm × 25 m) was used to analyze the heavy products such as monoglycerides, diglycerides, and triglycerides.

3. Results and Discussion 3.1. Feedstock Analysis The composition of the feedstock to be hydrotreated has a direct effect on the consumption of hydrogen and the distribution of the hydrocarbon products. Some physical and chemical properties of the crude jatropha oil were determined and the results are presented in Table 1. Density and Dynamic viscosity were determined using a Hydrometer (JIS-II) and a Dynamic Viscometer (SV-10A, Vibro). The acid value was determined by titration using a standard solution of potassium hydroxide in ethanol according to the ASTM-D974. Fatty acid compositions of the feedstock were determined using gas chromatography after transesterification and esterification with methanol. The fatty acid composition of jatropha oil and their contents are also presented in Table 1. From the analysis results, the jatropha oil had a high dynamic viscosity of 25.32 mPa·s at 25 °C which generated by the triglycerides and a high acid value of 7.86 mg KOH/g which caused by the free fatty acid. Fatty acids mainly found in jatropha oil were palmitoleic, linoleic, oleic, palmitic and stearic acid, each of which had a straight chain structure. The fatty acids with a carbon number of 16 and 18 had carbon molar percentages of 14.55 % and 85.45 %, respectively. The saturated and unsaturated fatty acid contents were respectively 19.47 % and 80.53 %. -----Table 1----


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3.2. Characterization of Catalysts All calcined catalysts were prepared according to the same procedure and try to achieve a same loading amount. Although different supports were employed, final metal oxide loadings determined by XRF in all catalysts (as presented in Table 2) were approximately the same each other. For all catalysts, cobalt oxide loading varied within range of 3.2–3.5 wt %, whereas loadings of molybdenum oxide ranged from 16.9 to 19.6 wt %. The SiO2 contents of the NiMo/Al2O3, NiMo/SA-1, NiMo/SA-2, NiMo/SiO2, NiMo/Z-5, NiMo/Z-10 and NiMo/Z-30 were 0.2 wt %, 4.2 wt %, 9.1 wt %, 79.6 wt %, 3.4 wt %, 6.8 wt % and 20.3 wt %, respectively. The textural properties derived from adsorption-desorption isotherms of nitrogen on the calcined catalysts are also shown in Table 2. NiMo/Al2O3, NiMo/SA-1, NiMo/SA-2 and NiMo/SiO2 catalysts had a similar specific surface areas (220±5 m2/g). But due to the addition of binder, NiMo/Z-5, NiMo/Z-10 and NiMo/Z-30 catalysts had a lower specific surface area (184–187 m2/g) than the catalysts mentioned above. It was also found that the pore volume and average pore size of the catalysts increased with the addition of SiO2 but decreased with the addition of ZSM-5. -----Table 2--------Figure 1--------Table 3---The surface acidity properties of all oxided NiMo catalysts were investigated by means of NH3TPD. The NH3-TPD profiles of all catalysts are illustrated in Figure 1. As shown in Figure 1, all catalysts showed different distribution of acidic sites. NiMo/Al2O3 and NiMo/SiO2 catalysts showed a broad peak, while the other catalysts showed two partially overlapping peaks. To compare the acidity distribution among the catalysts, the NH3-TPD profiles were separated into two or three peaks by curves fitting with Gaussian functions. These peaks were divided into three categories: 216–231 °C, 278– 336 °C, and 433–448 °C, which represented weak, medium and strong acidity, respectively. According to the published study27,28, three kinds of adsorbed ammonia species could be assigned to NH3 weakly adsorbed on the Lewis acid site, NH4+ on the Brønsted acid site, and strong adsorbed on and/or ACS Paragon Plus Environment

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interacted with the dislodged Al, respectively. The amounts of desorbed ammonia for each type of acidic site were calculated and listed in Table 3. All catalysts showed different quantity for each type acidic site. The quantity of weak and medium acidic sites decreased in the same order: NiMo/SA-2 > NiMo/SA-1 > NiMo/SiO2 > NiMo/Al2O3 > NiMo/Z-5 > NiMo/Z-10 > NiMo/Z-30. However the quantity of strong acidic sites decreased in the order: NiMo/Z-30 > NiMo/Z-10 > NiMo/Z-5 >> other catalysts. Namely, the addition of ZSM-5 brought an increase of strong acidic sites; however, the addition of SiO2 brought an increase of weak and medium acidic sites.

3.3. Hydrotreating of Jatropha Oil over NiMo/SiO2-Al2O3 Catalysts

In the present study, the hydrocarbon products under all hydrotreatment conditions were determined by two GCs and divided into four ranges: gaseous hydrocarbon products C1–4, cracked liquid hydrocarbon products C5–14, deoxygenated hydrocarbon products C15–18 and heavy hydrocarbon products C18+. The hydrocarbon products represented a simple mixture containing mainly n-paraffins (mainly n-C15–18). Beside from n-paraffins, the liquid hydrocarbon product also contained small amounts of iso-paraffins and trace amounts of cycloalkanes and aromatics. Some oxygenates, e.g., fatty acid, fatty alcohol, monoglycerides, which were considered to be the intermediates in HDO of jatropha oil, were found in some cases. However, the contents of these oxygenates were very low and they were donated into C18+. A CHNS/O elemental analyzer was also used to determine the elemental composition of liquid hydrocarbon products. It was found that the oxygen content was lower than 0.05% for all samples. Because the deoxygenation was higher than 99.5% in all cases, the deoxygenation would not be described in the following discussion. We also detected some special products including CO, CO2 and water. All yields were defined on the basis of molar number of carbon. Hydrogenation of jatropha oil over several catalysts with different Si/(Si+Al) ratio (NiMo/Al2O3, NiMo/SA-1, NiMo/SA-2, NiMo/SiO2) were conducted under conditions of T = 350 °C, LHSV = 2h–1, P = 3.0 MPa, H2/oil = 600 mL/mL. Figure 2 shows the yields of different fractions of hydrocarbon ACS Paragon Plus Environment

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products. The yield of C1–4 products ranged from 6.1 % to 6.4 %. Most of this fraction was propane (about 5.7 %) which was mainly generated from the hydrogenation of glyceryl group in jatropha oil13. The yield of C5–14 products, which was produced by the cracking reactions, ranged from 0.9 % to 3.0 %. The C15–18 fraction was the major product with carbon yield from 82.4 % to 88.2 %. This fraction was generated from the deoxygenation of free fatty acid group in jatropha oil. The yield of C18+ products varied from 2.4 % to 5.0 %. Some identified results (as determined by GC-MS) showed that these fractions mainly contained heavy alkanes (C19–24) and trace amount of oxygenates (such as fatty acid, fatty alcohol, monoglycerides). Triglycerides and diglycerides were not determined in this fraction. The total hydrocarbon yield of C1–18 products over four catalysts were summarized, and it was high up to 93±2 %. Because the carbon mole percents in CO, CO2 and C18+ are still not included, we can safely conclude that almost all triglycerides in the jatropha oil were converted into hydrocarbons. It was found that the Si/(Si+Al) ratio of NiMo/SiO2-Al2O3 catalysts only had a slight impact on the yields of different fraction. The yield of C1–4, C5–14 and C18+ products increased as the Si/(Si+Al) ratio increased from 0.002 to 0.11, and then slightly decreased as the Si/(Si+Al) ratio increased further to 0.99. However, the yield of C15–18 products showed an opposite tendency as the Si/(Si+Al) ratio increased. Comparing with the NH3-TPD results (Table 3), the decrease of C15–18 products and increase of C1–4 and C5–14 products could be attributed to the increase in acidity of catalyst, which resulted in increasing the cracking activity of catalyst. ----- Figure 2---To get a better understanding of reaction mechanism before the subsequent discussion, it is necessary to describe the pathway of triglyceride deoxygenation. According to the previous works9,13,16,17 , it is considered that three deoxygenation pathways exist in triglyceride hydrotreatment, as shown in equations. (1–3). One mole of Cn (where n denotes the carbon number) fatty acid group in the triglyceride generates one mole Cn–1 hydrocarbon by decarboxylation (Eqs. (1)) or decarbonylation (Eqs. (2)), but generates one mole Cn hydrocarbon by hydrodeoxygenation (Eqs. (3)). The oxygen is removed by generating one mole of CO2, one mole of CO and one mole of H2O, and two mole of H2O ACS Paragon Plus Environment

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in pathway of decarboxylation, decarbonylation, and hydrodeoxygenation, respectively. It should be noted that the H2 consumption increases as decarboxylation (one mole of H2 required) < decarbonylation (two mole of H2 required) < hydrodeoxygenation (four mole of H2 required). From an economic perspective, decarboxylation is considered to be the best reaction pathway R–CH2–COOA + H2 → AH + R–CH3 + CO2

(1)

R–CH2–COOA + 2H2 → AH + R–CH3 + CO + H2O

(2)

R–CH2–COOA + 4H2 → AH + R–CH2–CH3 + 2H2O

(3)

)

(A:

Additionally to the deoxygenation reactions, some other reactions such as water gas shift (Eqs. (4)) and methanation (Eqs. (5) and (6)) may occur among CO, CO2, H2O derived from the deoxygenation reactions and H2. CO2 + H2 ↔ CO + H2O

(4)

CO2 + 4H2 ↔ CH4 + 2H2O

(5)

CO + 3H2 ↔ CH4 + H2O

(6)

Figure 3a shows the yields of total alkanes of C15, C16, C17 and C18. Alkanes C15, C16, C17 and C18 were the most abundant products. This was because jatropha oil comprised of fatty acid group with C18 and C16 mainly, which formed C17 and C15 alkanes by decarbonylation and/or decarboxylation pathways, and formed C18 and C16 alkanes by hydrodeoxygenation pathway (as described above). With increasing the Si/(Si+Al) ratio, the yields of C15 and C17 alkanes increased, but the yields of C16 and C18 decreased. It meant that the deoxygenation pathway was affected by the support. As described above, because CO, CO2 and H2O were the products formed in three deoxygenation pathways, it was interesting to study the deoxygenation pathway by studying the formation rates of CO, CO2 and H2O. It was found that the formation rate of methane was much lower than those of carbon ACS Paragon Plus Environment

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oxides and water (see Figure 3b, Figure 5b and Figure 7). In addition, part of methane was also formed via cracking reaction. This indicated that the methanation reactions (Eqs. (5) and (6)) were much slower than deoxygenation reactions and could be ignored compared with deoxygenation reactions (Eqs. (1), (2) and (3)). To further investigate the deoxygenation pathway, the methanation reactions were ignored in the following discussion. Because both equation (2) and (4) generate one mole H2O at the same time generate one mole CO while equation (3) generates two mole H2O but does not generate CO, (H2O–CO) could be regarded as the generated H2O of equation (3). And furthermore, (H2O–CO)/2 could be regarded as the reaction rate of hydrodeoxygenation (Eqs. (3)). However, due to the presence of water gas shift reaction, the formation rates of CO2 and CO could not be regarded as the rates of decarboxylation and decarbonylation, respectively. Therefore, the formation rates of CO2 and CO were added together to used to study the total rate of decarboxylation and/or decarbonylation. The formation rates of total carbon oxides and (H2O–CO)/2 were calculated and are showed in Figure 3b. With increasing Si/(Si+Al) ratio, the formation rates of carbon oxides increased but formation rate of (H2O– CO)/2 decreased. These results indicated that the rates of decarbonylation and hydrodeoxygenation increased with increasing Si/(Si+Al) ratio, while the ratio of hydrodeoxygenation decreased with increasing Si/(Si+Al) ratio. ----- Figure 3---The normal alkanes C15–18 produced from deoxygenation of jatropha oil can be ideal additives for diesel fuel due to their high cetane numbers. However, large amounts of normal alkanes in diesel fuel may also cause bad cold flow properties, and partial isomerization of the normal alkanes is desired. The yields of isomerized products and cracked products are shown in Figure 3c. All alkanes except normal alkanes in C15–18 and all alkanes from C1 to C14 saving propane were separately regarded as the isomerized products and cracked products. From Figure 3c, the yield of cracked products was at a low level (below 3.84 %) over all NiMo/SiO2-Al2O3 catalysts. The yield of isomerized products increased up to 22.91 % as the Si/(Si+Al) ratio increasing from 0.002 catalyst to 0.11 catalyst, and then decreases as the Si/(Si+Al) ratio increasing to 0.99. ACS Paragon Plus Environment

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In this case, the yields of isomerized products were too low to improve the cold flow properties (the cloud point of liquid hydrocarbon products over NiMo/SA-2 catalyst was 9°C). Thus, further work should focus on how to develop more efficient catalysts to obtain higher branched isomers during the hydrotreating of vegetable oil.

3.4. Hydrotreating of Jatropha Oil over NiMo/ZSM-5-Al2O3 Catalysts To investigate the effect of another solid acid support––ZSM-5 for hydrotreating of vegetable oil, hydrotreatment of jatropha oil over various ZSM-5 addition NiMo/ZSM-5-Al2O3 catalysts (NiMo/Z-5, NiMo/Z-10, NiMo/Z-30) was carried out under conditions of T = 350 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600 mL/mL. Figure 4 shows the yields of different fractions. The yields of C1–4 and C5–14 fractions increased with increasing addition of ZSM-5, while the yields of C15–18 and C18+ fractions decreased with increasing addition of ZSM-5. The yields of C1–4, C5–14 and C15–18 fractions were respectively 16.62 %, 41.24 %, and 40.34 % over NiMo/Z-30 catalyst. It was concluded that the triglycerides in jatropha oil were almost converted into alkanes over all NiMo/ZSM-5-Al2O3 catalysts by looking at the low yield of C18+ and high total hydrocarbon yield of C1-18 products (96±2 %). ----- Figure 4-------- Figure 5---Figure 5a shows the products yields of C15, C16, C17, and C18 alkanes. With increasing addition amount of ZSM-5, the yields of C15, C16, C17 and C18 alkanes decreased significantly. This was because of ZSM-5 with high cracking activity converted the deoxygenation products C15–C18 alkanes into light hydrocarbons C1–C14 alkanes as shown in Figure 4. It was interesting that the ratio of C15+17/C16+18 did almost not change with changing the addition of ZSM-5, which meant that the decarbonylation plus decarboxylation versus hydrodeoxygenation did not change with addition of ZSM-5 (if the difference of cracking rate between C15, C16, C17, and C18 are ignored). From Figure 5b, it was observed that the formation rate of methane was much lower than those of carbon oxides and water even NiMo/ZSM-5Al2O3 catalysts had high cracking active and could generate a lot of methane. This result indicated that ACS Paragon Plus Environment

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the methanation reaction was still much slower than deoxygenation reactions on NiMo/ZSM-5-Al2O3 catalysts. The formation rates of carbon oxides and (H2O–CO)/2 did almost not change with increasing addition amount of ZSM-5, which indicated that the rate of decarboxylation and/or decarbonylation versus hydrodeoxygenation did not change with increasing the addition of ZSM-5. This result was consistent with the similar ratio of C15+17/C16+18. Figure 5c shows the yields of isomerized products and cracked products. The yields of isomerized products remained at low level (below 4.50 %) over all NiMo/ZSM-5-Al2O3 catalysts. But the yields of cracked products increased sharply as the addition of ZSM-5 increasing. We attempted to reduce the cracking and increase isomerization by decreasing reaction temperature over NiMo/Z-30 catalyst. At 330 °C, the yield of cracked products was still high. At 310 °C, a solid white wax product was formed, which was a mixture of saturated triglycerides, free fatty acid and other oxygenates (as determined by GC-MS). It meant that cracking could not be simply reduced by lowering the reaction temperature over NiMo/Z-30 catalyst because lower reaction temperature also caused the loss of deoxygenation activity. Both hydrocracking and hydroisomerization reactions were widely recognized to process essentially through carbenium ion mechanism, in which acid properties of a support played a key role.20,29 The mechanism of isomerization and cracking of a linear paraffin started with a first step of formation of a carbenium ion. The carbenium ion was likely firstly isomerized by skeletal rearrangement on an acidic site. And then the branched carbenium ion could continue to be cracked into a smaller alkylcarbenium ion and alkane by β-scission on the acidic site or be desorbed to generate an isomer product. Cracking was also considered to be a consecutive reaction which was favored for multibranched alkanes. It was believed that the isomerization/cracking ratio depended on facts such as strength of the acid sites, type of active metal, porous structure of support, reaction conditions, etc., in which the density and strength of acidic sites were the most important, and an adequate balance between these parameters was critical for catalyst activity.20 In the present study, it could be found that the yields of isomerized products increased with middle acidic site and the yields of cracked products increased with strong acidic site (As shown in Table 3, Figure 3 and Figure 5). And NiMo/SiO2-Al2O3 catalysts ACS Paragon Plus Environment

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which had more medium acidic sites but no strong acidic sites showed much higher isomerization/cracking ratio than NiMo/ZSM-5-Al2O3 catalysts which had more strong acidic sites but few medium acidic sites. Consequently, it could be concluded that isomerization reaction was favorable for middle acidic site but cracking reaction was favorable for strong acidic site for hydrotreating of vegetable oil under our experiment conditions.

3.5. Effects of Support on Catalytic Performance Catalytic actives of NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10 catalysts were investigated under the condition of T = 330–370 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600 mL/mL. Figure 6 portrays the yields of isomerized products and cracked products over NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10. The high reaction temperature favored isomerization and cracking reactions, resulting in an increase in yields of both isomerized products and cracked products over three catalysts. This was because high temperature could activate the acid function of these three catalysts to achieve a higher isomerization and cracking activity.20 However, these catalysts showed different isomerized and cracked products yields. NiMo/SA-2 catalyst showed much higher isomerized products yield than NiMo/Z-10 catalyst and NiMo/Al2O3 catalyst. While NiMo/Z-10 catalyst showed the highest cracked products yield between three catalysts. This result could also be contributed to the different acidity of these three catalysts. NiMo/Z-10 catalyst had most strong acidic sites causing large cracking reactions, whereas NiMo/SA-2 only had most medium and weak acidic sites leading to high yields of isomerized products. Another probable reason for low isomerization active of NiMo/Al2O3-ZSM-5 catalysts was the diffusion-limit. It was reported that the critical diameter of the triglyceride molecule was around 2 nm,30,31 whereas the ZSM-5 only had medium pores of about 0.56 nm × 0.53 nm,32 which was much smaller than triglyceride molecule. Therefore the molecules of triglycerides obviously could not diffuse through the ordered micropores of the ZSM-5 and that efficient contact between triglycerides molecule and active site was mainly possible only on the external surface of ZSM-5. The triglycerides molecule must be converted to hydrocarbons by deoxygenation firstly and then react with the active site on the ACS Paragon Plus Environment

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inert surface of ZSM-5. But NiMo/SiO2-Al2O3 catalysts had average pore diameter of 11.9–15.2 nm, which was much bigger than triglyceride molecules. This meant that the diffusion of triglyceride molecules on the NiMo/SiO2-Al2O3 catalysts was much easier than NiMo/Al2O3-ZSM-5 catalysts. ----- Figure 6---To investigate the effect of reaction temperature and support kinds on the deoxygenation pathway, Figure 7a and b show the formation rates of Carbon oxides and (H2O–CO)/2 over NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10 catalysts at various temperatures. On the one hand, higher temperatures resulted in higher formation rates of carbon oxides but lower formation rate of (H2O–CO)/2. According to the mention above, it was suggested that a high temperature was favorable for decarboxylation and/or decarbonylation pathway, which could be attribute to that high temperature had a positive effect on cleavage of C–C bond13. On the other hand, a difference of deoxygenation pathway on three catalysts was observed. It was found that NiMo/SA-2 showed higher rates of decarboxylation and/or decarbonylation but lower rate of hydrodeoxygenation than NiMo/Al2O3 and NiMo/Z-10, which was consistent with change in quantity of total acidic sites: NiMo/SA-2 had much more acidic sites than NiMo/Al2O3 and NiMo/Z-10. Thus the total acidic sites of catalyst could have a positive effect on the decarboxylation and/or decarbonylation pathway. ----- Figure 7----

4. Conclusions A series of sulfided NiMo/SiO2-Al2O3 and NiMo/ZSM-5-Al2O3 catalysts were prepared and used in hydrotreatment of jatropha oil in a fixed-bed flow reactor. A remarkable support effect on the deoxygenation pathways was observed. For NiMo/SiO2-Al2O3 catalysts, the rates of decarboxylation and/or

decarbonylation

increased

with

increasing

Si/(Si+Al)

ratio,

while

the

ratio

of

hydrodeoxygenation decreased with increasing Si/(Si+Al) ratio. For NiMo/ZSM-5-Al2O3 catalysts, the rate of decarboxylation and/or decarbonylation versus hydrodeoxygenation did not change with increasing the addition of ZSM-5. Through comparing the rates of three deoxygenation pathway on ACS Paragon Plus Environment

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NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10 catalysts, it was suggested that the total acidic sites of catalyst could have a positive effect on the decarboxylation and/or decarbonylation pathways. It was found that NiMo/SiO2-Al2O3 catalysts had the highest yields of isomerized products while NiMo/ZSM5-Al2O3 catalysts had the highest yields of cracked products. These results were attributed to that isomerization reaction was favorable for weak and middle acidic site but cracking reaction was favorable for strong acidic site.


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Table 1. Properties and Fatty Acid Composition of Jatropha Oil Properties

Jatropha oil

density (g/cm3) dynamic viscosity (mPa·s/25 °C) acid value (mg KOH/g) fatty acid composition, C mass% palmitoleic acid (C16:1) palmitic acid (C16:0) linoleic acid (C18:2) oleic acid (C18:1) stearic acid (C18:0)

0.91 25.32 7.86 0.93 13.62 36.21 43.39 5.85

Table 2. Chemical Compositions and Textural Properties of Several NiMo Catalysts

catalyst

chemical composition (wt %)

textural parameter

SiO2

NiO

MoO3

SBET a(m2/g)

Vtotal b(cm3/g)

davec(nm)

NiMo/Al2O3

0.2

3.2

19.3

219

0.615

11.2

NiMo/Z-5 NiMo/Z-10 NiMo/Z-30 NiMo/SA-1 NiMo/SA-2 NiMo/SiO2

3.4 6.8 20.3 4.2 9.1 79.6

3.4 3.3 3.4 3.5 3.4 3.2

19.6 18.8 17.5 18.1 17.6 16.9

187 184 187 219 215 225

0.503 0.463 0.377 0.651 0.687 0.852

10.8 10.0 8.1 11.9 12.8 15.2

Table 3. Acidity Properties of Several NiMo Catalysts catalyst NiMo/Al2O3 NiMo/SA-1 NiMo/SA-2 NiMo/SiO2 NiMo/Z-5 NiMo/Z-10 NiMo/Z-30

weak acidity T(°C) (μmol/g) 231 18.0 221 29.5 223 33.9 216 23.5 217 13.3 221 12.7 221 12.0

medium acidity T/(°C) (μmol/g) 314 38.2 330 67.0 336 77.0 278 43.9 291 37.3 288 37.0 291 36.8

strong acidity T(°C) (μmol/g)

433 441 448

10.5 16.1 24.8

total acidity (μmol/g) 56.2 96.5 110.9 67.4 61.1 65.8 73.6


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NiMo/ZSM-30

NiMo/ZSM-10

Signal (a.u.)

NiMo/ZSM-5

NiMo/SiO2

NiMo/SA-2

NiMo/SA-1

NiMo/Al2O3

100

200

300

400

500

600

Temperature (oC)

Figure 1. NH3-TPD profiles of Several NiMo catalysts.

10

100 C5-14

C15-18

C18+

8

80

6

60

4

40

2

20

0

Yield of C 15-18 (%)

C1-4

Yields of C 1-4, C5-14 and C18+ (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0.002

0.05

0.11

0.99

Si/(Si+Al)

Figure 2. Hydrocarbon product yields in hydrotreating of jatropha oil over several NiMo/SiO2-Al2O3 catalysts (T = 350 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600). ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 12

60

Yields of C 15 and C16 (%)

C16

C17

C18

10

50

8

40

6

30

4

20

2

10

(b)

0.002

16

Formation Rate (mmol/h)

Yield of C 17 and C18 (%)

C15

(a)

Yields of Isomerized & Cracked Products (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.05 0.11 CH4 CO+CO Ratio of Si/Al 2

0.99 (H2O-CO)/2

12

8

4

0 30

(c) Isomerized Products

Cracked Products

25 20 15 10 5 0 0.002

0.05

0.11

0.99

Si/(Si+Al)

Figure 3. Yields of C15-C18 alkanes (a), formation rate of deoxygenation products (b) and yields of isomerized and cracked products (c) in hydrotreating of jatropha oil on several NiMo/SiO2-Al2O3 catalysts (T = 350 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600).


20

50

100 C1-4

Yields of C 1-4, C5-14 and C18+ (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C5-14

C15-18

C18+

40

80

30

60

20

40

10

20

0

Yield of C 15-18 (%)

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0 0

10 20 Addtion Amount of ZSM-5 (%)

30

Figure 4. Hydrocarbon products yields in hydrotreating of jatropha oil over several NiMo/ ZSM-5Al2O3 catalysts (T = 350 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600 mL/mL).


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Industrial & Engineering Chemistry Research 12

60

Yields of C 15 and C16 (%)

C16

C17

C18

10

50

8

40

6

30

4

20

2

10 0.002

15

(b) Formation Rate (mmol/h)

Yield of C 17 and C18 (%)

C15

(a)

CH4

0.05 0.11 Ratio of Si/Al CO+CO2

0.99 (H2O-CO)/2

12

9

6

3

0

Yields of Isomerized and Cracked Products (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 50 (c)

10 Isomerized Products

20

30

Cracked Products

40

30

20

10

0 0

10

20

30

Addition Amount of ZSM-5 (%)

Figure 5. Yields of C15–C18 alkanes (a), formation rate of deoxygenation products (b), and yields of isomerized and cracked products (c) in hydrotreating of jatropha oil over several NiMo/ZSM-5-Al2O3 catalysts (T = 350 °C, LHSV = 2h–1, P = 3 MPa, H2/oil = 600).


22

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Yields of Isomerized Products (%)

30

(a)

Al2O3

SA-2

Z-10

(b)

Al2O3

SA-2

Z-10

25 20 15 10 5 0 30

Yields of Cracked Products (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 20 15 10 5 0 330

340

350 360 o Temperature ( C)

370

Figure 6. Effect of reaction temperature on yields of isomerized products (a) and cracked products (b) over NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10 catalysts (LHSV = 2h–1, P = 3 MPa, H2/oil = 600).


23

Formation Rate of (H2O-CO)/2 (mmol/h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Formation Rate of Carbo oxides (mmol/h)


Page 24 of 26

14

(a)

Al2O3

SA-2

Z-10

(b)

Al2O3

SA-2

Z-10

12

10

8

6

4 15

12

9

6

3

0 330

340

350 360 o Temperature ( C)

370

Figure 7. Effect of reaction temperature on formation rates of Carbon oxides (a), and (H2O–CO)/2 (b) over NiMo/Al2O3, NiMo/SA-2 and NiMo/Z-10 catalysts (LHSV = 2h–1, P = 3 MPa, H2/oil = 600).


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Role of Support in Hydrotreatment of Jatropha Oil over Sulfided NiMo

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