Hydrocracking of Algae Oil into Aviation Fuel-Range Hydrocarbons

Oct 30, 2014 - (1-3) Most algae produce and accumulate oils as triglycerides,(4, 5) which must be transformed into hydrogenated biodiesel (HBD) for ...
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Hydrocracking of Algae Oil into Aviation Fuel-Range Hydrocarbons Using a Pt−Re Catalyst Kazuhisa Murata,*,† Yanyong Liu,† Makoto M. Watanabe,‡ Megumu Inaba,† and Isao Takahara† †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan S Supporting Information *

ABSTRACT: Renewable green aviation and diesel-type alkanes can be produced by hydrocracking of Botryococcus braunii oil (Bot-oil) and squalane oils with Pt−Re/SiO2−Al2O3 (SA) catalysts, which are active at 310−340 °C and the weight ratio of the oil/catalyst of 10. For purified Bot-oil on Pt−3 wt % Re/SA at 330 °C, the highest aviation fuel-range hydrocarbon (C10−C15) yield of 50.2% was achieved, with diesel-range hydrocarbons (C16−C20) of 16.7% yield. The re-modified Pt/SA catalysts are also active for hydrocracking of squalane as the model compound of algae oil, and the C10−C20 hydrocarbon yield of 68.8% was obtained. The Pt−Re catalyst was found to be stable for the second regeneration cycle, even with the use of crude Bot-oil. Investigation of catalyst natures indicates that metallic Pt and Re are independently present on the surface, but synergism of these two metals could play an important role in the hydrocracking reaction. The reaction pathway involves the C−C bond scission reaction via a carbenium ion formed from one of the double-bond carbons over the SA catalyst, followed by hydrogenation of the CC and C−C bonds by the Pt−Re catalyst and partial aromatization over the SA catalyst.

1. INTRODUCTION Algae oils represent a promising alternative feedstock for biofuels.1−3 Most algae produce and accumulate oils as triglycerides,4,5 which must be transformed into hydrogenated biodiesel (HBD) for producing hydrocarbon fuels, which are good candidates for automobile and aviation fuels. However, these hydrogenation processes would require a lot of hydrogen because of oxygen removal as well as hydrogenation of olefin structures. Thus, there is still a requirement to further improve the HBD production process to reduce costs, for example, by lowering hydrogen consumption. In contrast to other algae, Botryococcus braunii oils (abbreviated as Bot-oil) are composed of non-oxygenated triterpenic hydrocarbons (mainly C34H58),6−8 and therefore, this species has attracted significant interest as a potential candidate for the manufacture of automobile fuels. However, the physical properties of Bot-oil make it unsuitable for use as the fuel in its natural state, and therefore, it will be necessary to develop cracking technologies for this material to generate a fuel with the required properties. To date, several studies concerning Bot-oil cracking have been reported. Hellene al.9 carried out hydrocracking of Bot-oil to produce transport fuels. In this study, hydrocracking was performed using conditions of 400 °C and 20 MPa, producing oil with a 67% gasoline fraction in a yield of 80%. Milne et al.10 investigated the catalytic cracking of algae and vegetable oils to obtain gasoline. Kitazato et al.11 investigated the catalytic cracking of Bot-oil to obtain gasoline. The resulting oils had a 62% gasoline fraction within an octane number of 95 at 500 °C. Tracy et al.12and Garciano et al.13 have reported a production of gasoline-range fuel from squalane. Yamamoto et al. investigated a production of diesel fuel from squalene using USY zeolite under mild conditions.14 Both of these cracking reactions were performed for using © XXXX American Chemical Society

zeolites without any hydrogen for manufacture of gasoline (C5−C9) and diesel fuels (C16−C20) but not for aviation fuel (C10−C15). These processes contain catalytic cracking using zeolite NiMo or CoMo catalysts. In our previous study, we found that the Pt/H-ZSM-5 catalyst was effective for propane formation from hydrocracking of glycerol.15 Also, a re-modified Pt/zeolite catalyst was found to be effective for the deoxyhydrogenation of Jatropha oil into diesel-range hydrocarbons.16 Thus, the aim of the present study is to examine the effect of a re-modified Pt catalyst on the activity of hydrocracking of Bot-oil as well as squalane and, additionally, to shed light on the effect of impurities in Bot-oil on the catalyst recycle with the physical properties of liquid products. Now, we report on the catalyst nature of the remodified Pt/SiO2−Al2O3 (SA) system for the hydrocracking reaction.

2. MATERIALS AND METHODS 2.1. Materials. 2.2.1. Catalyst. SA was purchased from JGC C&C Co., Ltd., Japan. Pt(NH3)4Cl2·H2O and NH4ReO4 were purchased from Soekawa Chemicals, Japan. The Pt/SA catalyst used in this study was prepared by immersion impregnation of the SA support with Pt(NH3)4Cl2·H2O, followed by drying at 100 °C and calcination for 5 h at 500 °C. Pt−Re/SA was prepared by immersion impregnation of SA with Pt(NH3)4Cl2·H2O and NH4ReO4, followed by drying at 100 °C and calcination for 5 h at 500 °C. The concentration of platinum metal was 1 wt %. 2.2.2. Algal Oil. Bot-oil was used as an algal oil. The oil was isolated by Makoto M. Watanabe. Briefly, the oil fraction was extracted from air-dried B. braunii cells by rinsing several times with n-hexane, and Received: August 25, 2014 Revised: October 13, 2014

A

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g of Bot-oil or squalane was introduced without any solvent and the autoclave was pressurized with 5.5 MPa of a H2/N2 gas mixture (H2/ N2 = 91:9, vol %). The reaction was carried out at the prescribed temperature for 12 h. After reaction, the gaseous products were collected in a plastic bag and then the autoclave was rinsed with CH2Cl2 solvent containing the internal standard, dioxane. The used catalyst was removed using a disposable filter, and the obtained product solution and gaseous products were subjected to off-line flame ionization detector (FID) and TCD gas chromatography (GC) analyses, which were equipped with a CP-Al2O3/KCl plot (50 m, 0.25 mm inner diameter, and 4 μm film thickness) for C1−C4 hydrocarbons and UA-DX30 columns (30 m, 0.25 mm inner diameter, and 0.15 μm film thickness) for C5+ hydrocarbons for the FID and Porapak Q and MS 5A columns for the TCD. TCD analysis was performed using N2 as the internal standard. The Shimadzu GC-8A was used as the GC− TCD, while the Agilent 7890A was used as the GC−FID. The column temperature was kept at 45 °C for 5 min, raised to 400 °C with 20 °C/ min, and kept for 10 min at 400 °C. The carrier gas is He, with the inlet temperature of 320 °C, split ratio of 50, and injection volume of 1 μL. Here, from off-line GC analysis, the division and integration of the chromatographic data of the carbon fractions were successfully performed. The division parameters were slope sensitivity = 1, peak width = 0.04, area reject = 10, and height reject = 1. The selectivity was classified, for convenience, as seven fractions, such as gas [C1−C4], [C5−C9], [C10−C15], [C16−C20], [C21+], [squalane or Bot-oil], and [bottom]. The squalane or Bot-oil conversion = (1 − residual [squalane or Bot-oil](mol)/initial [squalane or Bot-oil](mol)) × 100. Each carbon selectivity is defined as the moles of carbon in each product divided by the total carbon in the oil initially introduced. The amount of carbon deposit over the catalyst surface was estimated by TGA under air conditions. [Bottom] = 100 − ([C1−C4] + [C5−C9] + [C10−C15] + [C16−C20] + [C21+] + [squalane or Bot-oil]). Therefore, [bottom] includes carbon deposit and the carbonaceous compounds not detected by GC analysis. Catalyst regeneration was carried out at calcination at 500 °C for 5 h, followed by reduction similar to the fresh catalyst. For flow reaction, the catalyst (1 mL) was introduced into a stainless-steel-type fixed-bed reactor custom designed with an inner diameter of 10 mm and a length of 400 mm, equipped with a highpressure trap kept at 50 °C to collect heavy hydrocarbons formed during hydrocracking reaction before analyzing the effluent gas. The simplified flow scheme is shown in Figure S1 of the Supporting Information. Prior to the reaction, fresh catalyst was pretreated by reduction with 2 MPa of H2 at 250 °C for 5 h. After reduction, H2 gas and squalane were introduced into the catalyst bed. Squalane was introduced with a Shimadzu LC10-ADvp pump, where the H2 (mL h−1)/squalane (mL h−1) ratio was 500 and liquid hourly space velocity (LHSV) was 2.45 h−1. The typical reaction conditions were T = 310 °C and P = 1.9 MPa. The online gas chromatograph was equipped with Porapak Q and MS 5A columns for the TCD for inorganic gases (H2, CO, CH4, and N2) and the same plot capillary column for the FID for C1−C4 hydrocarbons and CBP-1 capillary column (50 m, 0.22 mm inner diameter, and 0.25 μm film thickness) for another FID for C5−C16 hydrocarbons. From online TCD and FID analyses and offline analysis of liquid product trapped inside a high-pressure trap, Botoil conversion and carbon-based selectivity were estimated. Here, from GC analysis, the selectivity was classified, for convenience, as seven fractions, such as [C1−C4], [C5−C9], [C10−C15], [C16−C20], [C21+], [Bot-oil], and [bottom]. The used catalysts were obtained after 12 h of reaction. To obtain liquid products for gas chromatography/mass spectrometry (GC/MS) analysis, a 5-fold-scale reaction was carried out at 330 °C using Bot-C and Bot-P (5 g) and the catalyst (0.5 g) and autoclavetype reactor, under the same hydrogen pressure and reaction time as above. The solid catalyst was filtered out, and liquid product was obtained. GC/MS analysis of the liquid product was performed on an Agilent model 7890A gas chromatograph interfaced with an Agilent model 5975C mass spectrometer. The product was separated on an Ultra Alloy DX-30, 30 m capillary column, 0.25 inner diameter, and

after hexane was evaporated, crude oil (Bot-C) was obtained. The hydrocarbon fraction was purified using a silica gel column. The eluate resulting from the column purification process was Bot-P, and these two oils (Bot-C and Bot-P) were used as the oils for subsequent catalytic conversion. The main component of Bot-oil was C34H58. For some work in this study, squalane (C30H62) was used as a Bot-oil substitute, because of a limited supply of Bot-oil. Preliminary studies of the catalytic conversion were performed using squalane and both batch- and flow-type reactors, while the conversion of Bot-oil was carried out using only a batch-type reactor. Proximate and ultimate analyses of Bot-oil are given in Table 1, where no nitrogen (N) was present for both Bot-C and Bot-P. The experimental methods are described in the Analysis section.

Table 1. Proximate and Ultimate Analyses of Algae Oil Bot-C moisture volatile matter ash content fixed carbon C H N O

Proximate Analysis (%) 0 89.1 0 5.08 Ultimate Analysis (%) 84.34 12.31 0 3.35

Bot-P 0 98.7 0 1.30 87.53 12.47 0 0

2.2. Analysis. X-ray diffraction (XRD) patterns of the fresh catalysts were recorded on a Mac Science M18XHF22-SRA diffractometer fitted with a monochromator using Cu Kα radiation (1.5418 Å). The reducibility of the prepared catalysts was studied by H2 temperature-programmed reduction (TPR) using the BEL-CAT apparatus. Before the analysis, 50 mg of sample in a quartz tube was purged with He at 200 °C for 1 h to remove the impurities on the surface of the catalyst. After the sample was cooled to room temperature in flowing He, TPR was carried out using a 5% H2/Ar (v/v) mixture with a flow rate of 30 mL/min at a heating rate of 5 °C/ min from room temperature to 750 °C and the H2 consumption was recorded with a thermal conductivity detector (TCD). Brunauer−Emmett−Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) pore size distributions of all of the catalysts were determined by N2 physisorption using a BELSORP MAX apparatus (Nippon Bell Co., Ltd.) at a liquid-N2 temperature of −196 °C. The dispersion and particle size of the reduced ruthenium particles were measured by CO chemisorption methods, which were carried out using again the BEL-CAT apparatus. Before chemisorption of CO, the catalysts (50 mg) were pretreated in He for 35 min and then reduced for 30 min in a 5% H2/Ar gas flow of 50 mL/min and in He for 15 min at 400 °C in a reaction chamber. After this pretreatment, the samples were cooled at 50 °C under He gas flow and CO pulse measurements were carried out using a 5% CO/He gas flow of 50 mL/min. Finally, the particle size of Pt was determined from CO pulse data. For comparison, the particle size of Re is estimated on the basis of the assumption that Pt and Re are independently present on the SA surface. Thermogravimetric analysis (TGA) of Bot-oil17 and used catalysts was carried out using a TG 2000, Bruker AXS. Approximately 15 mg of sample was heated from 40 to 800 °C at a typical ramp rate of 10 °C/ min in a N2 flow rate of 50 mL/min. The moisture content was approximately zero for the weight loss when the sample was heated to 180 °C. Two-step devolatilizations began once the temperature was at 180 °C and was completed at 630 °C. An ultimate analysis was performed using a CE Instruments EA1110 analyzer. 2.3. Methods. For the batch reaction, the catalyst (0.1 g) was introduced into a 100 cm3 autoclave-type reactor and first pretreated by reduction with 2 MPa of H2 at 473 K for 5 h. After the reduction, 1 B

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Figure 1. GC chromatograms of Bot-oil. 0.15 mm film thickness. The GC oven was programmed with the following temperature regime: hold at 40 °C for 3 min, ramp to 200 °C at 5 °C min−1, ramp to 400 °C at 10 °C min−1, and hold at 400 °C for 5 min. The inlet temperature was 320 °C, with a split ratio of 50, and the injection volume was 2 μL. Mass chromatograms were taken at m/z 57 (alkanes), m/z 55 (alkenes), and m/z 91 (alkylbenzenes).18

catalytic conversion were performed using squalane, while conclusive confirmation trials were performed using Bot-oil. 3.2. Catalyst Properties. Table 3 shows properties of 1 wt % Pt-based catalysts. The BET surface area of re-modified Pt/ SA catalyst is higher than that of Pt/SA, except for Pt−5 wt % Re/SA. CO adsorption measurements of the catalysts make it possible to estimate Pt dispersions and particle sizes. As shown in Table 3, the values of Pt particle sizes for re-modified Pt/SA are 0.24−0.29 nm, lower than that of Pt/SA (1.68 nm). The particle size of Re is estimated as 0.37−1.51 nm, and the size increases with the increase in the Re content. Figure 2 shows the XRD patterns of the Pt−Re/SA catalysts, where the data are taken after hydrogenation. The reduced catalysts show diffraction peaks corresponding to only metallic platinum (2θ = 39.7° and 46.3°) with no Re peak (2θ = 43.0°). As shown in Table 3, lattice constants (Lc) of Pt were 0.68− 2.32 nm, being consistent with values from CO adsorption. Figure 3 shows TPR profiles of 1 wt % Pt−xRe/SA. The reduction profile of Pt−Re/SA shows an uniform reduction peak at ca. 234.8−259.5 °C. The peak intensity of the peak increased with the increase in the Re content, indicating reduction from Re7+ to Re0,20 while the Pt reduction peak (from Pt4+ to Pt0) would not be clear, probably because of a small content of Pt of 1 wt %. 3.3. Hydrocracking of Squalane. The hydrocracking of squalane was carried out using the 1 wt % Pt−5 wt % Re/SA catalysts under the conditions at 290−330 °C for 12 h under 5.5 MPa of H2/N2 (90:10, vol %). As shown in Figure 4, the squalane conversion reached 100% at 310 °C. The yield of C10−C15 hydrocarbons was 44.4% at 310 °C but decreased to 38.6% at 330 °C, while the yield of C5−C9 and C1−C4 hydrocarbons increased. The C16−C20 yield was 21.7% at 310

3. RESULTS AND DISCUSSION 3.1. Properties of Original Bot-oil. Analysis of Bot-oil by GC demonstrated botryococcene (C34H58) as a major component. GC chromatograms are shown in Figure 1. In Bot-C, small amounts of free fatty acids (FFA), such as nhexadecanoic acid and cis-13-octadecenoic acid, were detected with Bot-oil impurities, such as carotenoid (C40H56).19 In BotP, FFA and Bot-oil impurities decreased. Table 2 compares the Table 2. Physical Properties of Bot-oil, Squalane, and Liquid Products physical properties (25 °C) raw materialsa

density (g cm−3)

kinematic viscosity (cSt)

Bot-C Bot-P squalane squalane (Pt2Re/SA, F350 °C) squalane (Pt2Re/SA, F310 °C) squalane (Pt2Re/SA, F290 °C) Bot-C (Pt3Re/SA, B330 °C) Bot-P (Pt3Re/SA, B330 °C)

0.6825 0.7265 0.642 0.7116 0.7165 0.6950 0.7360 0.7895

23.6 (22.3 °C) 77.4 (21.8 °C) 61.9 (22.5 °C) 1.82 (22.5 °C) 2.50 (23.4 °C) 30.0 (22.4 °C) 1.01 (22.5 °C) 2.03(24.5 °C)

a

F, fixed-bed reactor; B, batch reactor.

properties of Bot-oil and squalane. Kinematic viscosities are in the order: Bot-P > squalane > Bot-C. Preliminary studies of the Table 3. Catalyst Properties

XRDa

physical properties catalyst Pt/SA Pt−1 wt Pt−2 wt Pt−3 wt Pt−5 wt

% % % %

b

Re/SA Re/SA Re/SA Re/SA

SBET (m2 g−1)

pore volume (cm3 g−1)

pore diameter (nm)

284 340 428 313 277

0.223 0.464 0.156 0.432 0.105

2.50 5.46 1.46 5.52 1.52

Pt dispersion (%) [nm] 67.3 389 392 396 484

[1.68] [0.29] [0.29] [0.29] [0.24]

c

Re dispersion (%) [nm] 372 187 126 92

[0.37] [0.74] [1.10] [1.51]

c

d (nm)

Lc

0.23 0.23 0.23 0.23 0.23

0.68 1.09 1.62 2.32 2.22

a Obtained from XRD analyses of Pt metal. The d and Lc denote the lattice constant and crystallite size. d = λ/2 sin θ, where λ and θ denote the Xray wavelength and diffraction angle (deg). Lc = Kλ/(β cos θ), where K and β denote the constant (=0.9) and half-value width (radian) of the diffraction peak. bPt = 1 wt %. cThe Pt and Re dispersions were estimated by CO adsorption. The numbers in brackets denote Pt particle sizes.

C

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Figure 4. Effect of the reaction temperature on the hydrocracking of squalane. Conditions: squalane, 1 g; 1 wt % Pt−5 wt % Re/SA catalyst, 0.1 g; P, 5.5 MPa (H2/N2 = 10:1); and time, 12 h. Catalyst pretreatment: reduction at 250 °C under 2 MPa of hydrogen for 4 h.

Figure 2. XRD patterns of pre-reduced Pt−Re/SA catalysts (Pt = 1 wt %).

shown in Figure 6, the squalane conversion increased with the increase in the temperature from 290 to 350 °C and the conversion reached 100% at 350 °C. The yields of C10−C15 and C5−C9 hydrocarbons increased with the temperature increase and were 48.7 and 29.7% at 350 °C. The C16−C20 yield was 20.5% at 310 °C but dramatically decreased to 1.76% at 350 °C. The C1−C4 yield gently increased with the increased temperature. Thus, for fixed-bed reaction, the yield of C10− C20 hydrocarbons was 59.5% at 310 °C, which was consistent with the results of batch reactions. As shown in Table 2, the kinematic viscosity of product liquid after reaction at 310 °C was 2.50 cSt, close to that of diesel fuel commercially available. 3.4. Hydrocracking of Bot-oil. The hydrocracking of Botoil was carried out using the 1 wt % Pt−3 wt % Re/SA catalyst at 320−340 °C analogous to the conditions of the hydrocracking of squalane. The GC chromatogram of the liquid product is shown in Figure 7, where the peak of the chromatogram is poorly resolved, but each peak was approximately divided by a suitable chromatographic parameter setup (see the Methods section). In this case, FFA were not detected after reaction. As shown in Table 4, the Bot-C conversion reached almost 100% at 320−340 °C. The yield of C10−C15 hydrocarbons was 40.3% at 330 °C but decreased to 34.2% at 340 °C, and the C16−C20 yield was 21.6% at 320 °C. The bottom yield was between 9.1 and 20.2%. As a result, the C10−C20 yield at 320 °C was 60.1%, a little lower than the value from squalane (68.8%). On the contrary, in purified Bot-P, the C10−C15 yield at 330 °C was found to be 50.2%, while the bottom yield was only 0.38%. The C10−C20 yield is 66.9%, which is comparable to the value from squalane. Five chromatogram data in Table 4 are shown in Figures S2.1 and S2.2 of the Supporting Information, where retention times of C5−C33 are additionally given. GC/MS analysis of the volatiles recovered from hydrocracking of Bot-C and Bot-P revealed the presence of C8−C30 alkanes (m/z 57) and alkenes (m/z 55) and C 8 −C 18 alkylbenzenes (m/z 91) (panels a and b of Figure 8). For liquid products from Bot-C, the alkane/alkene ratio was 1.54, close to that of Bot-P, as shown in Table 5. However, the ratio of alkylbenzene/(alkane + alkene) of Bot-C was 1.60, much

Figure 3. TPR profiles of Pt−Re/SA. Pt, 1 wt %; Re, x wt %.

°C but dramatically decreased to 2.41% at 330 °C. Thus, the yield of C10−C20 hydrocarbons was 66.1% at 310 °C. Figure 5 shows the effect of the Re content on the converstion into hydrocarbons and selectivities at a squalane/ catalyst weight ratio of 10. The squalane conversion increased from 59.9 to 100% with the increase in the Re content from 0 to 5 wt %. The yield of C10−C15 hydrocarbons increased from 21.2 to 47.0% with the increase in the Re content from 0 to 3 wt % and decreased with a further increase up to 5 wt % Re. Similar trends are true for the C5−C9 yield, and the highest yield at Re of 3 wt % was 17.5%. The C15−C20 yield increased from 17.8 to 25.2% with the increase in Re from 0 to 3 wt % but thereafter decreased to 21.1%. The gaseous hydrocarbon yield (C1−C4) increased with the increased Re content. Thus, the yield of C10−C20 hydrocarbons was 68.8% at 3 wt % Re at a squalane/catalyst weight ratio of 10. The hydrocracking of squalane was performed using a fixedbed reactor, where H2/squalane, 800 (mL/mL); LHSV, 1.6 h−1; 1 wt % Pt−2 wt % Re/SA, 1 mL; and H2, 1.9 MPa. As D

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Figure 5. Effect of the Re content on the hydrocracking of squalane. Catalyst = Pt−Re/SA (Re = 0, 1, 2, 3, and 5 wt %). For other conditions, see Figure 4.

Figure 7. GC chromatogram of the liquid product from Bot-C oil. Figure 6. Effect of the reaction temperature on the hydrocracking of squalane using a fixed-bed reactor. Conditions: H2/squalane, 800 (mL/mL); LHSV, 1.6 h−1; 1 wt % Pt−2 wt % Re/SA catalyst, 1 mL; and H2, 1.9 MPa.

up to 5 wt % Re. The C15−C20 yield increased from 21.5 to 24.2% with the increase in Re from 0 to 1 wt % but thereafter gradually decreased to 20.0%. The C21+ yield decreased from 20.4 to 11.2% with the increase in Re from 0 to 3 wt % but thereafter increased to 19.4% at Re = 5 wt %. The bottom yield was between 6.8 and 9.1% at the Re content from 0 to 3 wt %, but the yield increased to 16.8% at Re = 5 wt %. These trends below Re = 3 wt % are similar to those of squalane (Figure 5), but at Re = 5 wt %, the trends of the C1−C4, C21+, and bottom yields, in particular, are not the same. Thus, the C10−C20 hydrocarbons can be formed with an acceptable yield at 2−3 wt % Re content, even from Bot-C without any purification. 3.5. Effect of the Catalyst Regeneration. The activity of the regeneration catalyst was examined using the 1 wt % Pt−3 wt % Re/SA catalyst and Bot-oil or squalane. As shown in Table 6, the squalane conversion at 310 °C decreased from 97.4 to 87.9% with the increase in the regeneration cycle but the yield of C10−C20 hydrocarbons slightly decreased from 64.4 to 62.3%. In the hydrocracking of Bot-C at 330 °C using the same catalyst, the conversion was almost 100%, even after the second

smaller than 6.60 for Bot-P, indicating that, under the same conditions, aromatization of alkane and alkene would occur more easily for Bot-P than for Bot-C. As already mentioned at Figure 1, Bot-C contains FFA with high boiling organic compounds, such as carotenoid. It seems likely that successive aromatization could be depressed by these organic impurities, but hydrocracking of these impurities would occur to form alkanes and alkenes, as seen by more clear C16/C18 alkane/ alkene formation for Bot-C than for Bot-P (panels a and b of Figure 8). Figure 9 shows the effect of the Re content on the converstion into hydrocarbons and selectivities at the Bot-C/ catalyst weight ratio of 10. The Bot-C conversion was 100%, except Re = 0 wt %. The yield of C10−C15 hydrocarbons increased from 23.7 to 38.5% with the increase in the Re content from 0 to 3 wt % and decreased with a further increase E

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Table 4. Catalytic Hydrocracking of Bot-oil into Hydrocarbonsa yield (%)b raw material

c

Bot-C Bot-C Bot-C Bot-P Bot-P

temperature (°C)

Bot-oil conversion (%)

320 330 340 320 330

100 100 98.1 88.2 100

d

C1−C4

C5−C9

C10−C15

C16−C20

C21+

bottome

4.34 4.15 4.14 3.01 4.63

15.2 17.1 16.2 9.74 17.2

38.5 40.3 34.2 37.6 50.2

21.6 16.8 17.7 22.7 16.7

11.3 4.57 5.62 14.6 10.9

9.12 17.1 20.2 0.57 0.38

a Catalyst = 1 wt % Pt−3 wt % Re/SA (0.10 g). For other preteatment and reaction conditions, see the Methods section of the text. bYield = [(conversion) × (carbon selectivity)]/100. cBot-C, botryococcene crude; Bot-P, botryococcene purified. dFor Bot-oil conversion, see the Methods section. eCarbon selectivity = (moles of hydrocarbons × number of carbon atoms in the hydrocarbons)/∑(moles of carbon atoms in hydrocarbon products) × 100. Bottom = 100 − (∑each hydrocarbon yield + residual [Bot]).

Figure 9. Effect of the Re content on the hydrocracking of Bot-C oil. Conditions: Bot-C, 1 g; 1 wt % Pt−x wt % Re/SA catalyst, 0.1 g; temperature, 330 °C; P, 5.5 MPa (H2/N2 = 50:5); and time, 12 h.

17.6%. In the hydrocracking of Bot-C at 330 °C using 1 wt % Pt−2 wt % Re/SA catalyst, the conversion was approximately 100% but the yield of C10−C20 hydrocarbons moderately decreased from 55.9 to 50.4%. In the hydrocracking of Bot-P at 330 °C using 1 wt % Pt−2 wt % Re/SA catalyst, the conversion was constant at 95.0−96.9% and the yield of C 10−C20 hydrocarbons moderately decreased from 55.3 to 50.2%. Thus, the catalyst activity could be slightly affected by the regeneration cycle, and a further study is required to check catalyst stability under fixed-bed conditions. 3.6. TGAs of the Upgraded Oil from Squalane. Volatility is one of the important indexes for evaluating the quality of a fuel as well as a comparison to GC fraction data shown in Tables 4 and 6. Figure 10a and Table 7 show the thermogravimetric curves of original squalane and the upgraded oil. For original squalane, it can be seen that the first and second weight losses accounted for above 97% of the squalane

Figure 8. Mass chromatograms of aliphatic and aromatic hydrocarbons from hydrocracking of (a) Bot-C and (b) Bot-P, with m/z 57 (alkanes), m/z 55 (alkenes), and m/z 91 (alkylbenzenes).

regeneration, but the yield of C10−C20 hydrocarbons decreased from 57.1 to 46.6%, while the C21+ yield increased from 4.57 to

Table 5. Product Ratios of Alkanes, Alkenes, and Alkylbenzenes Estimated by GC/MS Dataa

a

raw material

alkane/alkene

alkylbenzene/ (alkane + alkene)

polyaromatic/monoaromatic

Bot-C Bot-P

1.54 1.41

1.60 6.69

0 0.028

These ratios were estimated by area % of each peak. F

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Table 6. Catalytic Hydrocracking of Bot-oil into Hydrocarbons Using Fresh and Regenerated Catalystsa yield (%)b raw material

c

d

catalyst /temperature (°C)

Bot-oil conversion (%)

fresh/310 re-1/310 re-2/310 fresh/330 re-1/330 re-2/330 freshe/330 re-1e/330 re-2e/330 freshf/330 re-1f/330 re-2f/330

97.4 93.6 87.9 100 98.1 100 98.1 100 100 95.0 96.9 96.8

squalane squalane squalane Bot-C Bot-C Bot-C Bot-C Bot-C Bot-C Bot-P Bot-P Bot-P

b

C1−C4

C5−C9

C10−C15

C16−C20

C21+

bottom

2.28 1.73 1.84 4.15 2.58 2.76 2.29 3.50 2.72 2.25 2.46 2.64

9.01 7.75 8.38 17.1 11.5 11.8 12.3 13.4 11.4 11.8 11.8 10.9

38.9 39.5 38.5 40.3 29.0 28.6 31.5 31.9 28.3 36.5 35.5 32.0

25.5 27.9 23.8 16.8 22.5 18.0 24.4 22.4 22.1 18.8 21.0 18.2

14.1 16.2 14.0 4.57 16.5 17.6 19.0 15.1 20.9 13.6 15.4 14.8

7.58 0.58 1.40 17.1 16.0 21.3 8.56 13.7 14.6 12.0 10.8 18.1

a

Catalyst = 1 wt % Pt−3 wt % Re/SA (0.10 g). For other preteatment and reaction conditions, see the Methods section of the text. bFor Bot-oil conversion and yield, see Table 4. cFor Bot-C and Bot-P, see footnote c of Table 4. dre-1 and re-2 denote the reactions after the first and second regenerations. eCatalyst = 1 wt % Pt−2 wt % Re/SA (0.10 g). fCatalyst = 1 wt % Pt−2 wt % Re/SA (0.025 g).

Figure 10. Thermogravimetric curves of original squalane and upgraded oils at different reaction temperatures: (a) thermogravimetry (TG) data and (b) GC fraction data. For conditions, see Figure 6.

findings could be roughly consistent with GC fraction data (Figure 10b). 3.7. Posutulated Reaction Pathways. A reaction pathway for conversion of Bot-oil into hydrocarbons is shown in Figure 11. In the first step of this reaction pathway, the CC double bonds in Bot-oil are protonated on SA to form carbenium intermediates21 and then broken down into lower hydrocarbons. These hydrocarbons undergo hydrocracking on Pt−Re sites and partial aromatization on SA to produce aviation fuels (C10−C15) and diesel fuels (C16−C20) with light hydrocarbons (C1−C9). Part of organic impurities, such as FFA and carotenoid, could also undergo hydrocracking to give hydrocarbons. Thus, a re-modified Pt/SA catalyst might be effective for hydrocracking of organic impurities as well as Bot-oil. Further work is now underway to elucidate the influence of organic impurities on Bot-oil hydrocracking.

Table 7. Thermogravimetric Curves of the Original Squalane and Upgraded Oilsa TG (%)b

squalane (original)

290 °Cc

310 °Cc

350 °Cc

first second third residue

6.61 90.55 2.81 0.03

36.36 48.94 13.77 0.93

53.70 42.10 0.74 3.46

78.60 0.84 0 20.56

a

Catalyst = 1.0 wt % Pt−2.0 wt % Re/SA. bTG (%) is estimated from a normalized TG weight loss (Figure 9). “First” denotes the first step weight loss. cReaction temperature.

below 400 °C. After hydrocracking at 290 °C using a fixed-bed reactor, the first weight loss above 30% shows a formation of the upgraded oil (C5−C21+), whereas the second and third weight losses above 60% show remaining squalane. After reaction at 310 °C, the first weight loss above 50% shows a formation of the upgraded oil (C5−C20), while the second and residue weight losses above 40% would exhibit C21+ and highmolecular-weight compounds (bottom) as well as remaining squalane. On the contrary, after reaction at 350 °C, the first weight loss of ca. 80% displayed a predominant formation of C5−C20 hydrocarbons, with 20% of the bottom. All of these

4. CONCLUSION Re-modified Pt/SA catalysts, which was active for hydrocracking of Jatropha oil into diesel-range hydrocarbons, were found to be active for hydrocracking of Bot-oil to form C10−C20 hydrocarbons directly. For purified Bot-oil on Pt−3 wt % Re/ G

dx.doi.org/10.1021/ef5018994 | Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

Article

(4) Chisti, Y. Biodiesel from microalgae. Biotechnol. Adv. 2007, 25, 294−306. (5) Schenk, P. M.; Thomas-Hall, S. R.; Stephens, E.; Marx, U. C.; Mussgnug, J. H.; Posten, C.; Kruse, O.; Hankamer, O. Second generation biofuels: High efficiency microalgae for biodiesel production. Bioenergy Res. 2008, 1, 20−43. (6) Banerjee, A.; Sharma, R.; Chisti, Y.; Banerjee, U. C. Botryococcus braunii: A renewable source of hydrocarbons and other chemicals. Crit. Rev. Biotechnol. 2002, 22, 245−279. (7) Metzger, P.; Largeau, C. Botryococcus braunii: A rich source for hydrocarbons and related ether lipids. Appl. Microbiol. Biotechnol. 2005, 66, 486−496. (8) Nagano, S.; Yamamoto, S.; Nagakubo, M.; Atsumi, K.; Watanabe, M. M. Physical properties of hydrocarbon oils produced by Botryococcus braunii: Density, kinematic viscosity, surface tension, and distillation properties. Procedia Environ. Sci. 2012, 15, 73−79. (9) Hillen, L. W.; Pollard, G.; Wake, L. V.; White, N. Hydrocracking of the oils of Botryococcus braunii to transport fuels. Biotechnol. Bioeng. 1982, 24, 193−205. (10) Milne, T. A.; Evance, R. J.; Nagle, N. Catalytic conversion of microalgae and vegetable oils to premium gasoline, with shapeselective zeolites. Biomass 1990, 21 (3), 219−232. (11) Kitazato, H.; Asaoka, S.; Iwamoto, H. Catalytic cracking of hydrocarbons of microalgae. Sekiyu Gakkaishi 1989, 32, 28−34. (12) Tracy, N. I.; Crunkleton, D. W.; Price, G. L. Catalytic cracking of squalene to gasoline-range molecules. Biomass Bioenergy 2011, 35, 1060−1065. (13) Garciano, L. O., II; Tran, N. H.; Kamali Kannangara, G. S.; Milev, A. S.; Wilson, M. A.; Volk, H. Developing saponite supported cobalt−molybdenum catalysts for upgrading squalene, a hydrocarbon from the microalgae Botryococcus braunii. Chem. Eng. Sci. 2014, 107, 302−310. (14) Yamamoto, S.; Mandokoro, Y.; Nagano, N.; Nagakubo, M.; Atsumi, K.; Watanabe, M. M. Catalytic conversion of Botryococcus braunii oil to diesel fuel under mild reaction conditions. J. Appl. Phycol. 2014, 26, 55−64. (15) Murata, K.; Takahara, I.; Inaba, M. Propane formation by aqueous-phase reforming of glycerol over Pt/H-ZSM5 catalysts. React. Kinet., Mech. Catal. 2008, 93 (1), 59−66. (16) Murata, K.; Liu, Y.; Inaba, M.; Takahara, I. Production of synthetic diesel by hydrotreatment of Jatropha oils using Pt−Re/HZSM-5 catalyst. Energy Fuels 2010, 24 (4), 2404−2409. (17) Ross, A. B.; Jones, J. M.; Kubacki, M. L.; Bridgeman, T. Classification of macroalgae as fuel and its thermochemical behaviour. Bioresour. Technol. 2008, 99, 6494−6504. (18) Garciano, L. O., II; Tran, N. H.; Kannangara, G. S. K.; Milev, A. S.; Wilson, M. A.; McKirdy, D. M.; Hall, P. A. Pyrolysis of a naturally dried Botryococcus braunii residue. Energy Fuels 2012, 26, 3874−3881. (19) Eroglu, E.; Okada, S.; Melis, A. Hydrocarbon productivities in different Botryococcus strains: Comparative methods in product quantification. J. Appl. Phycol. 2011, 23, 763−775. (20) Simonetti, D. A.; Kunkes, E. L.; Dumesic, J. A. Gas-phase conversion of glycerol to synthesis gas over carbon-supported platinum and platinum−rhenium catalysts. J. Catal. 2007, 247 (2), 298−306. (21) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Process; McGraw-Hill: New York, 1979; Chapter 1.

Figure 11. Postulated reaction pathway.

SA at 330 °C, the highest jet fuel-range hydrocarbon (C10−C15) yield of 50.2% was achieved, with diesel-range hydrocarbons (C16−C20) of 16.7% yield. For crude Bot-oil at 330 °C, the C10−C15 yield of 40.3% was achieved with C16−C20 of 16.8% yield. The re-modified Pt/SA catalysts are also active for hydrocracking of squalane as a model compound of algae oil, and the C10−C20 hydrocarbon yield of 68.8% was obtained. The Pt−Re catalyst was found to be stable for the second regeneration cycle, even with the use of crude Bot-oil. The kinetatic viscosity of the liquid product was approximately 1− 2.5 cSt, being comparable to commercial-grade diesel fuel. The reaction pathway involves the C−C bond scission reaction via a carbenium ion formed from one of the double-bond carbons over the SA catalyst, followed by hydrogenation of the CC and C−C bonds by the Pt−Re catalyst and partial aromatization over the SA catalyst.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Simplified flow scheme (Figure S1) and GC chromatograms of products from Bot-oil with each peak division (Figures S2-1 and S2-2). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Telephone: +81-29-861-9395. Fax: +81-29-861-4776. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Agriculture, Forestry and Fisheries Research Council (AFFRC) grant of the Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF).

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REFERENCES

(1) Watanabe, M. M. Handbook of AlgaeTheir Diversity and Utilization; NTS: Tokyo, Japan, 2012; p 490. (2) Tran, N. H.; Bartlett, J. R.; Kannangara, G. S. K.; Milev, A. S.; Volk, H.; Hilson, M. A. Catalytic upgrading of biorefinery oil from micro-algae. Fuel 2010, 89, 265−274. (3) Zhang, Y. In Hydrothermal Liquefaction To Convert Biomass Intocrude Oil; Boaschek, H. P., Ezeji, T. C., Scheffran, J, Eds.; Blackwell: Oxford, U.K., 2010; pp 201−232. H

dx.doi.org/10.1021/ef5018994 | Energy Fuels XXXX, XXX, XXX−XXX