Role of Solvent in Catalytic Conversion of Oleic ... - ACS Publications

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Role of Solvent in Catalytic Conversion of Oleic Acid to Aviation Biofuels Qiurong Tian,†,‡ Zihao Zhang,†,‡ Feng Zhou,§ Kequan Chen,∥ Jie Fu,*,‡ Xiuyang Lu,‡ and Pingkai Ouyang‡,∥ ‡

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Fushun, Liaoning 113001, People’s Republic of China ∥ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical, Nanjing Tech University, Nanjing, Jiangsu 211816, People’s Republic of China S Supporting Information *

ABSTRACT: The role of solvents in the conversion of oleic acid over Pt/C was studied. Three solvent systems (solvent-free, water, and dodecane systems) were employed for the conversion of oleic acid over Pt/C at 350 °C. Decarboxylation, hydrogen transfer, and aromatization were observed in these three reaction systems. In comparison to the non-solvent reaction system, much slower decarboxylation and aromatization rates and fewer heptadecane and aromatic products were observed in the hydrothermal and dodecane reaction systems. The decarboxylation and aromatization rates and yields of heptadecane and aromatics decreased with increased dodecane loading in the dodecane reaction system, and the decarboxylation and aromatization rates and yields of heptadecane and aromatics significantly decreased with the increase of water in the hydrothermal reaction system. The effects of solvent loading, catalyst loading, and reaction time on the reactions (decarboxylation, hydrogen transfer, and aromatization) were investigated. The reaction behaviors of 1-heptadecene with different solvents were studied, and N2 adsorption−desorption and thermogravimetric analysis of fresh and spent Pt/C in the three reaction systems were also performed. The results indicate that the competition of dodecane for the Pt/C active sites is mainly responsible for the slow decarboxylation and aromatization rates. In addition to the similar influencing factor to that in the dodecane system, H+ released from water and hydrogen bonding, which inhibited the ionization of carboxyl groups, was the key influencing factor for the slower decarboxylation and aromatization rates obtained under hydrothermal conditions. alkanes and aromatics, respectively.7−13 Lercher and coworkers14 have reported a route to convert crude microalgae oil into diesel-range alkanes over heterogeneous catalysts. The selectivity for paraffins and aromatics in the conversion of triglycerides has also been comprehensively studied by changing the distribution of Lewis and Brønsted sites of hierarchical catalysts.15 However, most studies focus only on the production of long-chain paraffins from fatty acids.16−18 For unsaturated fatty acids, the conversions of unsaturated fatty acid in the hydrothermal, organic solvent, and non-solvent systems have been reported in the previous studies. For the non-solvent system, in 2008, Murzin and co-workers19 first reported the successful deoxygenation of unsaturated fatty acids over Pt/C. In 2012, Na et al.20 found that oleic acid can be converted to heptadecane and heptadecene with a total yield of 72% over 5 wt % Pt/C in the solvent-free system, and aromatics were also detected. In 2015, Carron et al.21 have reported that the selectivity for heptadecane from oleic acid reached 66% under solvent-free conditions using a Pt-SAPO-34 catalyst. In 2016, Fu et al.22 systematically studied the reaction pathway and mechanism for the conversion of oleic acid to heptadecane

1. INTRODUCTION Renewable and green liquid fuel sources are being aggressively explored, owing to depleting petroleum reserves, increased fuel demand, and environmental concerns, and one of the vital fuel sources is biofuels.1 Bioethanol and biodiesel are currently the most prominent biofuels in commercial production and use, and biodiesel can be further deoxygenated (upgraded) to be an environmentally friendly alternative liquid fuel for aviation fuel.2−4 Jet fuel, with real prices almost tripling from approximately $1.30/gallon in 2000 to approximately $3.00/ gallon in 2012, has accounted for approximately 10% of the U.S. petroleum refinery production over the past 2 decades.5 In addition to the challenge of the increasing and volatile price of jet fuel, the aviation industry faces environmental concerns associated with aviation fuel, including its impact on air quality and greenhouse gas emissions.5 Therefore, the development of alternative liquid fuels in aviation is becoming increasingly important and useful.6 Aviation fuel consists of a complex mixture of C8−C17 paraffins, aromatics, and naphthenes; as a result, many researchers are interested in the synthesis of biomass-derived long-chain alkanes and aromatics, which can be further refined into jet fuel. Fatty acids and their derivatives, which can be extracted from animal fat and vegetable oil in large quantities, have been regarded as candidates for producing long-chain © 2017 American Chemical Society

Received: February 27, 2017 Revised: May 22, 2017 Published: May 22, 2017 6163

DOI: 10.1021/acs.energyfuels.7b00586 Energy Fuels 2017, 31, 6163−6172

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Figure 1. Typical GC/FID chromatogram of the oleic acid in situ hydrogen transfer, decarboxylation/decarbonylation, and aromatization products under the conditions of non-solvent. TCI. Acetone (analytic reagent grade) was obtained from Sinopharm Chemical Reagent Co., Ltd. Heptadecane and dodecane (>98% purity) were obtained from Aladdin Industrial Corporation. Commercial 5% Pt/C and stearic acid (>98.5% purity) were obtained from Sigma-Aldrich, St. Louis, MO, U.S.A. All of the above chemicals and the catalyst were used as received. 2.2. Experimental Procedure. The oleic acid conversion was carried out in a microbatch reactor (1.67 cm3 volume) assembled from one 3/8 in. tube and two 3/8 in. caps, and the reactors were purchased from Swagelok, Solon, OH, U.S.A. In a typical experiment, 75 mg of reactant was added to the reactor, followed by 15 mg of Pt/C and 0.75 mL of solvent. After being loaded, each reactor was sealed by attaching and then tightening the reactor cap. Then, the reactor was placed in a fluidized sand bath (TECHNE SBL-2D) that was already at the desired reaction temperature. As soon as the reaction time reached the desired value, the reactor was submerged in ambient temperature water to quench the reaction. The sample in the cooled reactor was rinsed with acetone and transferred to a centrifuge tube; the reactor was rinsed until the total volume reached 15 mL; and after being filtered, the sample was injected into a gas chromatography (GC) vial for analysis. 2.3. Characterization. N2 adsorption−desorption was performed at 77 K in a static volumetric apparatus (Micromeritics 3Flex). Samples were degassed at 300 °C for 10 h before N2 adsorption. The specific surface area was determined by the Brunauer−Emmett−Teller (BET) equation, and the pore size and volume were calculated according to the Barrett−Joyner−Halenda (BJH) method. All calculations were achieved using the adsorption isotherms. The thermogravimetric characteristics were measured using thermogravimetric analysis (TGA, TA-Q500). The fresh and spent catalyst were heated to 600 °C at 10 °C min−1 with flowing air at 50 cm3 min−1. To detect the depletion of Pt, the liquid sample after reaction was rinsed with acetone until the total volume reached 15 mL, and then 1 mL of sample was transferred to a volumetric flask and diluted with water until the total volume was 100 mL. The metal in the solution was determined by inductively coupled plasma mass spectrometry (ICP−MS), X Series II (Thermo Fisher Scientific). 2.4. Analysis Method. The samples were analyzed by a gas chromatograph (GC, Agilent 7890B) equipped with a 30 m × 0.25 mm × 0.25 μm HP-5MS capillary column, a mass spectrometer (MS, Agilent 5977A MSD), a flame ionization detector (FID), and a thermal conductivity detector (TCD). A 1 μL sample was injected into the GC at a split ratio of 10:1, where the temperature of the injector was 280 °C and the carrier gas (nitrogen) flow rate was 11.383 mL min−1. The remaining sample was split into three equal portions, which flowed to the MS, FID, and TCD. The temperature of these three detectors was 280 °C. The FID temperature was 280 °C, with a H2 flow of 30 mL min−1, an air flow of 400 mL min−1, and a makeup N2 flow of 25 mL min−1. The MS had a solvent delay of 2.75 min and scanned masses

and aromatics. For the system of dodecane, catalytic deoxygenation of oleic acid in dodecane was performed by Lamb and co-workers;23 the results showed that only 12% heptadecane was obtained without added H2 over Pt/C from the conversion of oleic acid, but no data about the aromatics were reported. Murzin et al.24 have also studied the conversion of oleic acid in dodecane over 1 wt % Pd/C under the atmosphere of 1% hydrogen in argon. The products were stearic acid and little aromatics. Fu and co-workers25 suggested that it was not clear whether hydrocarbon solvents (e.g., dodecane) play a key role in activity and selectivity for fatty acid decarboxylation. Thereafter, in 2011, Fu et al. proposed a new approach to convert saturated and unsaturated fatty acids to hydrocarbons in near or supercritical water, and a less than 20% selectivity for heptadecane from unsaturated fatty acids was obtained over Pt/C after 2.5 h without added H2. In 2015, Yeh et al.26 discovered that Pt3Sn/C showed higher selectivity for the decarboxylation of oleic acid, with the selectivity to heptadecane of 60%. The decarboxylation of fatty acids in organic solvent and water and without solvent has been previously reported, and numerous systematic studies27−30 on the influence of the reaction time, temperature, pressure, atmosphere, catalyst, and support on the decarboxylation of fatty acids have been performed over many years. However, to the best of our knowledge, the different reaction behaviors in dodecane, hydrothermal, and non-solvent systems on the conversion of oleic acid at the same reaction conditions (reactors, catalysts, reaction atmosphere, etc.) were never compared before. In addition, the influential factor of different solvents on the conversion of oleic acid is unclear and not systematically studied. The aim of this work is to demonstrate the role of solvents (water, dodecane, and solvent-free systems) on the conversion of oleic acid using Pt/C. The effects of solvent loading, catalyst loading, and reaction time along with the reaction of 1heptadecene with different solvents were investigated. Additionally, the catalytic activity and stability were also examined to determine the relationship between the solvent and catalyst. We observed different phenomena when using different solvents and, for the first time, deduced the role of solvent in the conversion of oleic acid.

2. EXPERIMENTAL SECTION 2.1. Materials. Oleic acid (>99.0% purity), undecylbenzene (>98% purity), and 1-heptadecene (>99.5% purity) were purchased from 6164


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Energy & Fuels from 50.00 to 300 amu. The oven temperature program consisted of a 4 min soak at 50 °C, followed by a 20 °C min−1 ramp to 250 °C, and was finally held for 2 min. The reaction products were identified by fragmentation patterns from the MS detector and calibration with known standards. Quantitative analysis of each compound was completed using calibration curves obtained from FID signals. The reactant molar conversion was calculated as the moles of oleic acid consumed divided by the initial moles of oleic acid loaded into the reactor. Yields were calculated as the moles of product recovered divided by the theoretical moles of product, while oleic acid is consumed completely. Uncertainties reported herein are standard deviations, which were determined by replicating the experiments. Each data point represents the mean result from three independent experiments.

3. RESULTS AND DISCUSSION 3.1. Reaction Behavior of Oleic Acid with Different Solvents. The reaction behavior in non-solvent, hydrothermal,

Figure 2. Yields of heptadecane, aromatics, and stearic acid from oleic acid in the non-solvent,22 dodecane and water systems over Pt/C. Reaction conditions: T = 350 °C, t = 80 min, oleic acid loading = 75 mg, Pt/C loading = 15 mg, and solvent loading = 0.75 mL. Figure 3. Effect of the solvent (a, dodecane; b, water) loading on the conversion of oleic acid. Reaction conditions: T = 350 °C, t = 80 min, oleic acid loading = 75 mg, Pt/C loading = 15 mg, and solvent loading = 0−1 mL.

and dodecane reaction systems was studied. An oleic acid loading of 75 mg, Pt/C loading of 15 mg, reaction temperature of 350 °C, and reaction time of 80 min were used in the experiments in section 3.1. Table S1 of the Supporting Information showed the performance of 14 different kinds of catalysts on the decarboxylation and aromatization of oleic acid. Pt/C showed the best catalytic performance on the decarboxylation and aromatization of oleic acid, and then Pt/ C was chosen as the model catalyst. Figure 1 shows a typical GC/FID chromatogram of the in situ hydrogen transfer, decarboxylation/decarbonylation, and aromatization products from oleic acid under the condition of non-solvent. The major products were heptadecane and aromatics, and the aromatics were mainly identified as 1-methydecylbenzene, 1,1-dimethylnonylbenzene, undecylbenzene, and 2-undercylphenol. In these systems, similar kinds of aromatics were detected, but the yields of aromatics were significantly different. Figure 2 shows the conversion of oleic acid in non-solvent,22 hydrothermal, and dodecane solvent systems over Pt/C. Figure S1 of the Supporting Information shows the conversion of oleic acid in methanol, formic acid, and cyclohexane solvent systems over Pt/C. For the solvent-free reaction, a 71% yield of heptadecane and 19% yield of aromatics were obtained.22 When 0.75 mL of dodecane was added as the solvent, a 41% yield of heptadecane, 4% yield of aromatics, and 31% yield of stearic acid were obtained. When 0.75 mL of water was added as the solvent, only a yield of 12% heptadecane, 3% aromatics, and 68% stearic acid was obtained. The results indicate that decarboxylation,

hydrogen transfer, and aromatization occurred in these three reaction systems. However, the yields of heptadecane and aromatics both decreased remarkably with the addition of dodecane and water. The addition of solvent did not accelerate the decarboxylation and aromatization rates but restricted the reactions of decarboxylation and aromatization. To determine the reason for the restrained conversion of oleic acid by the solvent, a series of experiments were carried out and described in the following sections. 3.2. Effect of the Solvent Loading. To determine the influence of the solvent, the conversion of oleic acid was performed with different solvent loadings. The same oleic acid loading (75 mg), Pt/C loading (15 mg), reaction temperature (350 °C), and reaction time (80 min) as before were used in the experiments in section 3.2. The solvent loadings ranged from 0 to 1 mL. Figure 3 shows the effect of the solvent loading on the conversion of oleic acid. Figure 3a shows that the effect of the dodecane loading on the conversion of oleic acid was not significant. As the loading of dodecane increased from 0 to 1 mL, the yield of hydrogenated product (stearic acid) increased from 0 to 83%, the yield of heptadecane decreased from 71 to 13.3%, and the yield of aromatics decreased from 19 to 2%. It indicates that the in situ hydrogen transfer was not influenced by the increase of dodecane but decarboxylation and 6165


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Figure 4. Effect of the catalyst loading on the conversion of oleic acid with different solvents (a, non-solvent;22 b, dodecane; and c, water). Reaction conditions: T = 350 °C, t = 80 min, oleic acid loading = 75 mg, solvent loading = 0.75 mL, and Pt/C loading = 5−20 mg.

Figure 5. Effect of the reaction time on the conversion of oleic acid with different solvents (a, non-solvent;22 b, dodecane; and c, water). Reaction conditions: T = 350 °C, oleic acid loading = 75 mg, Pt/C loading = 15 mg, solvent loading = 0.75 mL, and t = 1−6 h.

aromatization rates were restrained. The influential way of dodecane might compete with oleic acid for the active sites of the catalyst. The more dodecane added, the less active sites remained for oleic acid. Therefore, the decarboxylation and aromatization rates were slower; the yields of aromatics and heptadecane were also lower; and the reaction was detained at the hydrogenated product (stearic acid). Figure 3b shows that the effect of the water loading on the conversion of oleic acid was significant. As the loading of water increased from 0 to 0.25

mL, the yield of hydrogenated product (stearic acid) increased from 0 to 79%, the yield of heptadecane decreased from 71 to 18%, and the yield of aromatics decreased from 18 to 2%. In contrast, the yields of heptadecane and aromatics were kept stable when the loading of water increased from 0.25 to 1 mL. It indicates that decarboxylation and aromatization rates were also restrained under the hydrothermal conditions, similar to the reaction behavior in dodecane. Furthermore, the inhibition 6166


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Figure 6. Reaction results of (a) oleic acid and (b) 1-heptadecene in the non-solvent,22 dodecane and water systems. Reaction conditions: T = 350 °C, t = 80 min, solvent loading = 0.75 mL, reactant loading = 75 mg, and Pt/C loading = 15 mg.

large amount of water-released H+ and hydrogen bonding existing between oleic acid and water restricted the ionization of carboxyl groups, which may be another influencing factor.

effect of water was much more significant than that of dodecane. The remarkable influence of water on the reaction of oleic acid can be attributed to the following possible reasons. One is the competition for the active sites of the catalyst, and another is that the existence of H+ restricted the ionization of carboxyl groups, which may be the key impacting factor. Furthermore, Shurvell et al.31 reported that hydrogen bonding exists between acetic acid and water. Therefore, hydrogen bonding may also exist between oleic acid and water, and this hydrogen bonding may inhibit the ionization of carboxyl groups, further slowing the decarboxylation and aromatization rates in the hydrothermal system. To further prove that H+ released from water and hydrogen bonding existing between oleic acid and water is the influential factor, the effect of the solvent on the reaction of 1-heptadecene without a carboxyl group was investigated in section 3.5. 3.3. Effect of the Catalyst Loading. To determine the influencing factors and causes, the conversion of oleic acid was conducted with different catalyst loadings. The same oleic acid loading (75 mg), solvent loading (0.75 mL), reaction temperature (350 °C), and reaction time (80 min) as before were used in the experiments of section 3.3. The Pt/C loading ranged from 5 to 20 mg. Figure 4 shows the effect of the catalyst loading on the conversion of oleic acid with different solvents (a, non-solvent;22 b, dodecane; and c,water). Under the conditions of non-solvent, the loading of the catalyst did not have significant influence on the yields of heptadecane and aromatics, which has been reported in our previous research22 (Figure 4a). When dodecane was used as the solvent, the decarboxylation and aromatization rates and yields of heptadecane and aromatics obviously increased with increased Pt/C loading, especially the yield of heptadecane, increasing from 10.1 to 58.1% (Figure 4b). It proves that the supposition that dodecane competed for active sites is reasonable. With the increase of catalyst loading, the competition from dodecane for the active sites of the catalyst decreased. When water was used as the solvent, the decarboxylation and aromatization rates and yields of aromatics and heptadecane mildly increased with the increase of Pt/C loading (the yield of heptadecane increased from 7.4 to 20.0%; Figure 4c). Considering the different trends in the hydrothermal and dodecane reaction systems, there was likely another factor in the hydrothermal reaction system that inhibited the decarboxylation and aromatization reactions. The

RCOOH ↔ RCOO− + H+ H 2O ↔ OH− + H+

(small amount)

(large amount)

In conclusion, the phenomenon that the decarboxylation and aromatization rates and yields of aromatics and heptadecane increased with an increase in Pt/C loading when solvents were added may be proof that competition for the active sites of the catalyst is an influencing factor. The restriction of carboxyl group ionization by H+ and hydrogen bonding from water needs further research. 3.4. Effect of the Reaction Time. The conversion of oleic acid with respect to the reaction time was examined. The same oleic acid loading (75 mg), Pt/C loading (15 mg), solvent loading (0.75 mL), and reaction temperature (350 °C) as before were used in the experiments in section 3.4. The reaction time ranged from 1 to 6 h. Figure 5 shows the effect of the reaction time on the conversion of oleic acid with different solvents (a, non-solvent;22 b, dodecane; and c, water). Figure 5a22 shows that oleic acid was quickly and completely converted within 15 min. On the contrary, the in situ hydrogen transfer product of oleic acid (stearic acid) increased to 40% within 15 min and then decreased continuously with a prolonged reaction time. Finally, under the condition of nonsolvent, a heptadecane yield of 71% and aromatic yield of 19% were achieved within 80 min.22 Figure 5b shows that, in the dodecane reaction system, stearic acid was transformed to heptadecane over time. When the reaction time increased to 3 h, stearic acid (intermediate) transformed completely and the yield of heptadecane increased to the maximum value of 84%, while the yield of aromatics remained stable at 4%. Figure 5c shows that, in the hydrothermal reaction system, oleic acid was hydrogenated to stearic acid very quickly. When the reaction time increased to 4 h, stearic acid transformed completely and the yields of heptadecane and aromatics increased to 59 and 9%, respectively, and then the reaction remained stable. In the dodecane and hydrothermal reaction systems, the oleic acid and intermediate products could be completely consumed as the reaction time elapsed. Although the yield of aromatics was only 6% after 6 h in the dodecane reaction system, the yield of 6167


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Table 2. Physical Properties of Pt/C in the Reactions with Fresh Catalyst and Catalysts Recycled from (RF) Nonsolvent, Dodecane, and Water Reactions catalyst

SBET (m2/g)

Vtotal (cm3/g)a

pore size (nm)b

fresh RF non-solvent RF dodecane RF water

1321.67 475.27 492.85 546.03

1.19 0.58 0.65 0.61

4.8 5.5 5.5 5.7

a

Single-point adsorption total pore volume of pores less than 193.4646 nm in diameter at P/Po = 0.99. bBJH adsorption average pore diameter (4V/A).

oleic acid as the reactant over Pt-based catalysts. It might also be a reasonable explanation for why the carbon balance is low in the hydrothermal reaction system. In the dodecane reaction system,23 hydrogen transfer from dodecane can ensure the saturation of unsaturated fatty acid. The reactant is not used as a source of hydrogen; therefore, the carbon balance in the dodecane reaction system is relatively higher than that in the hydrothermal reaction system. Unsaturated fatty acid is easier to be hydrogenated in both hydrothermal and dodecane reaction systems; as a result, the aromatization reaction is suppressed in comparison to the non-solvent reaction system. Therefore, as the reaction time elapsed, fatty acids could still decarboxylate slowly in hydrothermal and dodecane reaction systems, while the restriction for aromatization could not be eliminated by prolonging the reaction time. 3.5. Effect of the Solvent on the Reaction of 1Heptadecene. To prove that the restricted ionization of the carboxyl groups from H+ of water and hydrogen bonding was an influencing factor on the reaction, experiments using 1heptadecene as the reactant were carried out. The same 1heptadecene loading (75 mg), Pt/C loading (15 mg), solvent loading (0.75 mL), reaction temperature (350 °C), and reaction time (80 min) as before were used in the experiments of section 3.5. Figure 6 shows the solvent effect on the conversion of oleic acid and 1-heptadecene. Using oleic acid as a reactant, the yields of heptadecane and aromatics were 71 and 19%, respectively, in the non-solvent system,22 41 and 4%, respectively, in the dodecane system, and 12 and 3%, respectively, in the water system (Figure 6a). It indicates that dodecane and water inhibited the decarboxylation and aromatization of oleic acid compared to the data under nonsolvent conditions. However, the influence of water was much more obvious than that of dodecane, suggesting that water possessed more influential factors than dodecane. Using 1heptadecene as a reactant, the yields of heptadecane and aromatics were 65 and 13%, respectively, in the non-solvent system,22 38 and 3%, respectively, in the dodecane system, and 37 and 4%, respectively, in the water system (Figure 6b). Namely, dodecane and water had the same influence on the conversion of 1-heptadecene compared to the non-solvent reaction system, indicating that the influential way of water was the same as dodecane (competing with the catalyst) in the reaction system of 1-heptadecene. The difference between oleic acid and 1-heptadecene is the carboxyl group, whose ionization can be restricted by H+ released from water and can also be used to form hydrogen bonding. The different experimental results in the reaction system of oleic acid and 1-heptadecene indicate that, in addition to the similar influencing factor to that in the dodecane system, H+ released from water and hydrogen bonding in the hydrothermal reaction system, inhibiting the

Figure 7. Reaction results of oleic acid over recycled Pt/C with different solvents (a, non-solvent; b, dodecane; and c, water). Reaction conditions: T = 350 °C, t = 80 min, solvent loading = 0.75 mL, oleic acid loading = 75 mg, and Pt/C loading = 15 mg.

Table 1. Loss of Pt in the Reactions with the Conditions of Non-solvent, Dodecane, and Water solvent system

Pt depletion (%)a

non-solvent dodecane water

0.00025 0.00055 0.00030

a

The mass ratio of Pt lost in the liquid product and Pt added to the reactor.

heptadecane could rise to 83% and the mole balance of the liquid sample was still as high as 89%. In the hydrothermal reaction system, the yields of heptadecane and aromatics were 61 and 7% after 6 h and the mole balance was about 75%, which might be caused by more coupling and polymerization reactions occurring in water. Savage and co-workers26 have reported that the hydrogen source added to the unsaturated fatty acids during the hydrothermal treatment is from hydrothermal gasification of both H2O and reactant with 6168


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Figure 8. (a) N2 adsorption−desorption isotherms and (b) BJH pore diameter distributions of Pt/C in the reactions with fresh catalyst and catalyst recycled from (RF) non-solvent, dodecane, and water reactions.

significantly affect the efficiency of decarboxylation, consistent with the results in panels b and c of Figure 4. As the Pt/C loading increased from 5 to 10 mg, the yield of heptadecane increased remarkably from 10 to 35% in the dodecane system, but it only rose from 7 to 11% in the hydrothermal system. As a result, the decarboxylation rates in the water did not change remarkably and the yield of heptadecane varied from 12.5 to 10.0% when Pt/C was used for the third time. (b) The deactivation of catalyst in the hydrothermal system was not as serious as that in the other two systems. Characterizations (ICP−MS, BET, and TGA) results were used for explaining a relatively slighter deactivation of catalyst in the hydrothermal system. To prove the Pt leaching, the liquid sample after reaction was rinsed and diluted and the metal in the solution was determined by ICP−MS, X Series II (Thermo Fisher Scientific). The loss of Pt during the reactions under the conditions of non-solvent, dodecane, and water was shown in Table 1. The Pt depletions in these solvent systems were all less than 0.0006%, suggesting that the deactivation of Pt/C was not related to the depletion of Pt. Table 2 shows the physical properties of Pt/C in reactions with fresh catalyst and catalysts recycled from (RF) nonsolvent, dodecane, and water reactions. In comparison to fresh Pt/C, Pt/C recycled from the non-solvent, dodecane, and water reactions exhibited much smaller surface areas and volumes and larger pore sizes. It is noticeable that Pt/C recycled from the hydrothermal system had a slightly larger surface area compared to that recycled from the dodecane or non-solvent

ionization of carboxyl groups, was the key influential factor for the slower decarboxylation and aromatization rates under hydrothermal conditions. 3.6. Reusability of the Catalyst. The reusability of the catalyst was examined in the different solvent systems. The same oleic acid loading (75 mg), Pt/C loading (15 mg), solvent loading (0.75 mL), reaction temperature (350 °C), and reaction time (80 min) as before were used in the experiments in section 3.6. Figure 7 shows the reaction results of oleic acid over spent Pt/C with different solvents (a, non-solvent; b, dodecane; and c, water). In the non-solvent system, the yields of heptadecane decreased from 69 to 18% when Pt/C was used for the third time, while the intermediates 8-heptadecene and stearic acid increased from 2 to 12% and from 0 to 43%, respectively. In the dodecane system, the yields of heptadecane decreased from 41 to 15% when Pt/C was used for the third time, while the intermediates 8-heptadecene and stearic acid increased from 1 to 8% and from 30 to 50%, respectively. These results indicate that the activity of the catalyst for the decarboxylation reaction decreased after use, which might be caused by the depletion of Pt from the catalyst, carbon deposition, or adsorption of products on the catalyst. While in the hydrothermal system, the yields of stearic acid (59.7− 68.1%) and heptadecane (12.5−10.0%) were relatively stable when Pt/C was used for the third time. The possible reason was suspected as follows: (a) In the hydrothermal system, the decarboxylation rate was slow, owing to H+ released from water and hydrogen bonding between water and oleic acid. The changes in the active site of the catalyst after use did not 6169


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Figure 10. TGA result of (a) fresh Pt/C and (b) fresh Pt/C wetted by the products (heptadecane and undecylbenzene), followed by rinsing 3 times (all of the processing steps were the same as the sample treatment).

The variation of the physical properties of Pt/C might be caused by the coking or adsorption of products. Figure 9 shows the TGA results of Pt/C recycled from the non-solvent,22 dodecane, and hydrothermal systems. The first peak at 165 °C represents the heat released from the combustion of coking or desorption of products, and the second peak at approximately 360 °C might represent the heat released from the combustion of the carbon support of Pt/C. To figure out whether the first peak at 165 °C stands for the combustion of coking or the desorption of products, TGA of (a) fresh Pt/C and (b) fresh Pt/C wetted by products was conducted, as shown in Figure 10. The sample of fresh Pt/C wetted by products was obtained as follows: fresh Pt/C was wetted by the solution of products (heptadecane and undecylbenzene), followed by rinsing 3 times (all of the operation processes were the same as the sample treatment) with acetone. Figure 10a (fresh Pt/C) shows that the peak at approximately 360 °C represented the heat released from the combustion of the carbon support of Pt/C. Figure 10b shows that there is no peak from 150 to 200 °C, meaning that the recycle method can wash all of the products off. Therefore, the first peak at 165 °C in Figure 9 should be attributed to the combustion of coking rather than the desorption of products. Figure 9 shows that the coking in the non-solvent, dodecane,

Figure 9. TGA results of Pt/C in the reaction using catalyst recycled from (a) non-solvent,22 (b) dodecane, and (c) water reactions.

systems. A relatively larger surface area may be another reason why the decarboxylation and aromatization rates decreased slighter after the third use when the solvent was water. The N2 adsorption−desorption isotherms and pore diameters of fresh catalyst and catalyst recycled from (RF) non-solvent, dodecane, and water reactions are shown in panels a and b of Figure 8, respectively. Adsorption−desorption isotherms show similar sharp hysteresis loops for all examined catalysts, and their pore diameter distributions were the same. 6170


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and hydrothermal systems was 12, 23, and 14%, respectively. The amount of coking in the dodecane system was the highest, because the coking might be achieved from dodecane. The amount of coking in the non-solvent system was similar to that in the hydrothermal system, because the amount of organics was the same in these two systems and water had no significant influence for the formation of coking. The relatively less coking was also a reason why the decarboxylation rate decreased slightly in the hydrothermal system when Pt/C was used for the third time compared to that in the dodecane and non-solvent systems. Overall, Pt leaching was not detected in these three solvent systems. However, coking and the decrease of surface area were observed in all solvent systems, especially in the dodecane system. Therefore, decarboxylation rates in the non-solvent and dodecane systems decreased when Pt/C was used for a third time. In the hydrothermal system, a relatively slighter deactivation of catalyst and low decarboxylation rates resulting from the addition of water might be the reasons why decarboxylation results did not vary remarkably when Pt/C was used for a third time.

†

Qiurong Tian and Zihao Zhang contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21436007 and 21676243) and the Zhejiang Provincial Natural Science Foundation of China (LR17B060002 and LZ14B060002), with funding from the Boeing Company.

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4. CONCLUSION Three solvent systems (solvent-free, water, and dodecane) were employed for the conversion of oleic acid over Pt/C at 350 °C to study the influence of the solvent. Decarboxylation, hydrogen transfer, and aromatization were observed in these three reaction systems. In comparison to the non-solvent reaction system, much slower decarboxylation and aromatization rates and fewer aromatic products were observed in the hydrothermal and dodecane reaction systems. The decarboxylation and aromatization rates and yield of aromatics decreased with increasing dodecane loading in the dodecane reaction system, and the decarboxylation and aromatization rates and yield of aromatics significantly decreased in the hydrothermal reaction system. The competition of dodecane for the active sites of Pt/C is largely responsible for the slow decarboxylation and aromatization rates. In addition to a similar influencing factor to that in the dodecane system, in the hydrothermal reaction system, H+ released from water along with hydrogen bonding inhibited the ionization of carboxyl groups and also played a large role in contributing to the slower decarboxylation and aromatization rates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00586. Performance of 14 different kinds of catalysts on the decarboxylation and aromatization of oleic acid (Table S1) and yields of heptadecane, aromatics, and stearic acid from oleic acid with methanol, formic acid, and cyclohexane over Pt/C (Figure S1) (PDF)

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REFERENCES

(1) Verma, D.; Kumar, R.; Rana, B. S.; Sinha, A. K. Aviation fuel production from lipids by a single-step route using hierarchical mesoporous zeolites. Energy Environ. Sci. 2011, 4 (5), 1667−1671. (2) Smith, B.; Greenwell, H. C.; Whiting, A. Catalytic upgrading of tri-glycerides and fatty acids to transport biofuels. Energy Environ. Sci. 2009, 2 (3), 262−271. (3) Gupta, K. K.; Rehman, A.; Sarviya, R. M. Bio-fuels for the gas turbine: A review. Renewable Sustainable Energy Rev. 2010, 14 (9), 2946−2955. (4) Biller, P.; Riley, R.; Ross, A. B. Catalytic hydrothermal processing of microalgae: decomposition and upgrading of lipids. Bioresour. Technol. 2011, 102 (7), 4841−8. (5) Davidson, C.; Newes, E.; Schwab, A.; Vimmerstedt, L. An Overview of Aviation Fuel Markets for Biofuels Stakeholders; National Renewable Energy Laboratory (NREL): Golden, CO, 2014; Technical Report NREL/TP-6A20-60254. (6) Sacia, E. R.; Balakrishnan, M.; Deaner, M. H.; Goulas, K. A.; Toste, F. D.; Bell, A. T. Highly Selective Condensation of BiomassDerived Methyl Ketones as a Source of Aviation Fuel. ChemSusChem 2015, 8 (10), 1726−1736. (7) Šimácě k, P.; Kubička, D.; Šebor, G.; Pospíšil, M. Hydroprocessed rapeseed oil as a source of hydrocarbon-based biodiesel. Fuel 2009, 88 (3), 456−460. (8) Meng, X.; Yang, J.; Xu, X.; Zhang, L.; Nie, Q.; Xian, M. Biodiesel production from oleaginous microorganisms. Renewable Energy 2009, 34 (1), 1−5. (9) Fu, J.; Shi, F.; Thompson, L. T.; Lu, X.; Savage, P. E. Activated Carbons for Hydrothermal Decarboxylation of Fatty Acids. ACS Catal. 2011, 1 (3), 227−231. (10) Fu, J.; Lu, X.; Savage, P. E. Hydrothermal decarboxylation and hydrogenation of fatty acids over Pt/C. ChemSusChem 2011, 4 (4), 481−6. (11) Simakova, I.; Simakova, O.; Mäki-Arvela, P.; Murzin, D. Y. Decarboxylation of fatty acids over Pd supported on mesoporous carbon. Catal. Today 2010, 150 (1), 28−31. (12) Ping, E. W.; Pierson, J.; Wallace, R.; Miller, J. T.; Fuller, T. F.; Jones, C. W. On the nature of the deactivation of supported palladium nanoparticle catalysts in the decarboxylation of fatty acids. Appl. Catal., A 2011, 396 (1), 85−90. (13) Mäki-Arvela, P.; Kubickova, I.; Snåre, M.; Eränen, K.; Murzin, D. Y. Catalytic deoxygenation of fatty acids and their derivatives. Energy Fuels 2007, 21 (1), 30−41. (14) Peng, B.; Yuan, X.; Zhao, C.; Lercher, J. A. Stabilizing catalytic pathways via redundancy: selective reduction of microalgae oil to alkanes. J. Am. Chem. Soc. 2012, 134 (22), 9400−5. (15) Chen, H.; Wang, Q.; Zhang, X.; Wang, L. Quantitative conversion of triglycerides to hydrocarbons over hierarchical ZSM-5 catalyst. Appl. Catal., B 2015, 166−167, 327−334. (16) Wu, J.; Shi, J.; Fu, J.; Leidl, J. A.; Hou, Z.; Lu, X. Catalytic Decarboxylation of Fatty Acids to Aviation Fuels over Nickel Supported on Activated Carbon. Sci. Rep. 2016, 6, 27820.

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Jie Fu: 0000-0002-3652-7715 6171


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Energy & Fuels (17) Yang, L.; Tate, K. L.; Jasinski, J. B.; Carreon, M. A. Decarboxylation of Oleic Acid to Heptadecane over Pt Supported on Zeolite 5A Beads. ACS Catal. 2015, 5 (11), 6497−6502. (18) Al Alwan, B.; Salley, S. O.; Ng, K. Y. S. Biofuels production from hydrothermal decarboxylation of oleic acid and soybean oil over Nibased transition metal carbides supported on Al-SBA-15. Appl. Catal., A 2015, 498, 32−40. (19) Snåre, M.; Kubičková, I.; Mäki-Arvela, P.; Chichova, D.; Eränen, K.; Murzin, D. Y. Catalytic deoxygenation of unsaturated renewable feedstocks for production of diesel fuel hydrocarbons. Fuel 2008, 87 (6), 933−945. (20) Na, J.-G.; Yi, B. E.; Han, J. K.; Oh, Y.-K.; Park, J.-H.; Jung, T. S.; Han, S. S.; Yoon, H. C.; Kim, J.-N.; Lee, H.; Ko, C. H. Deoxygenation of microalgal oil into hydrocarbon with precious metal catalysts: Optimization of reaction conditions and supports. Energy 2012, 47 (1), 25−30. (21) Ahmadi, M.; Nambo, A.; Jasinski, J. B.; Ratnasamy, P.; Carreon, M. A. Decarboxylation of oleic acid over Pt catalysts supported on small-pore zeolites and hydrotalcite. Catal. Sci. Technol. 2015, 5 (1), 380−388. (22) Tian, Q.; Qiao, K.; Zhou, F.; Chen, K.; Wang, T.; Fu, J.; Lu, X.; Ouyang, P. Direct Production of Aviation Fuel Range Hydrocarbons and Aromatics from Oleic Acid without an Added Hydrogen Donor. Energy Fuels 2016, 30 (9), 7291−7297. (23) Immer, J. G.; Kelly, M. J.; Lamb, H. H. Catalytic reaction pathways in liquid-phase deoxygenation of C18 free fatty acids. Appl. Catal., A 2010, 375 (1), 134−139. (24) Simakova, I.; Rozmysłowicz, B.; Simakova, O.; Mäki-Arvela, P.; Simakov, A.; Murzin, D. Y. Catalytic Deoxygenation of C18 Fatty Acids Over Mesoporous Pd/C Catalyst for Synthesis of Biofuels. Top. Catal. 2011, 54 (8−9), 460−466. (25) Fu, J.; Lu, X.; Savage, P. E. Catalytic hydrothermal deoxygenation of palmitic acid. Energy Environ. Sci. 2010, 3 (3), 311−317. (26) Yeh, T. M.; Hockstad, R. L.; Linic, S.; Savage, P. E. Hydrothermal decarboxylation of unsaturated fatty acids over PtSnx/ C catalysts. Fuel 2015, 156, 219−224. (27) Mäki-Arvela, P.; Snåre, M.; Eränen, K.; Myllyoja, J.; Murzin, D. Y. Continuous decarboxylation of lauric acid over Pd/C catalyst. Fuel 2008, 87 (17−18), 3543−3549. (28) Vardon, D. R.; Sharma, B. K.; Jaramillo, H.; Kim, D.; Choe, J. K.; Ciesielski, P. N.; Strathmann, T. J. Hydrothermal catalytic processing of saturated and unsaturated fatty acids to hydrocarbons with glycerol for in situ hydrogen production. Green Chem. 2014, 16 (3), 1507. (29) Shim, J. O.; Jeong, D. W.; Jang, W. J.; Jeon, K. W.; Kim, S. H.; Jeon, B. H.; Roh, H. S.; Na, J. G.; Oh, Y. K.; Han, S. S.; Ko, C. H. Optimization of unsupported CoMo catalysts for decarboxylation of oleic acid. Catal. Commun. 2015, 67, 16−20. (30) Kandel, K.; Anderegg, J. W.; Nelson, N. C.; Chaudhary, U.; Slowing, I. I. Supported iron nanoparticles for the hydrodeoxygenation of microalgal oil to green diesel. J. Catal. 2014, 314, 142−148. (31) Ng, J. B.; Shurvell, H. Application of factor analysis and band contour resolution techniques to the Raman spectra of acetic acid in aqueous solution. J. Phys. Chem. 1987, 91 (2), 496−500.

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