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Journal of Biomedicine and Biotechnology
Volume 2012 (2012), Article ID 637125, 9 pages
Conversion of Isoprenoid Oil by Catalytic Cracking and Hydrocracking over Nanoporous Hybrid Catalysts
1Graduate School of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0135, Japan
2Environmental Geochemistry, China University of Geosciences, 29 Xueyuan Lu, Beijing 100083, China
3Research Institute of Tsukuba Bio-Tech Corporation, 2-10-1 Matsushiro, Ibaraki Tsukuba 305-0035, Japan
Received 20 February 2012; Revised 15 April 2012; Accepted 29 April 2012
Academic Editor: Claudio M. Soares
Copyright © 2012 Toshiyuki Kimura et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In order to produce petroleum alternatives from biomass, a significant amount of research has been focused on oils from microalgae due to their origin, which would not affect food availability. Nanoporous hybrid catalysts composed of ns Al2O3 and zeolites have been proven to be very useful compared to traditional catalysts in hydrotreating (HT), hydrocracking (HC), and catalytic cracking (CC) of large molecules. To evaluate the reaction scheme and products from model isoprenoid compounds of microalgae oil, nanoporous hybrid catalyst technologies (CC: ns Al2O3/H-USY and ns Al2O3/H-GaAlMFI; HC: [Ni-Mo/γ-Al2O3]/ns Al2O3/H-beta) were studied. The major product from CC on ns Al2O3/H-USY was highly aromatic gasoline, while the product from HC was half-isoparaffinic/olefinic kerosene. Although more than 50 wt% of the products from HT/CC on the USY catalyst was liquefied petroleum gas due to overcracking, the product from HT/CC on the MFI catalyst was high-octane-number gasoline. Delightfully, the product from HT/HC was kerosene and its average number was 11, with more than 80 wt% being isoparaffinic. As a result, it was demonstrated that hydrotreating may convert isoprenoid oil from microalgae over nanoporous hybrid catalysts into a variety of products.
For more than three centuries, fossil fuels have been used in various aspects of our lives; however, these fuels are not renewable and will eventually be depleted. In fact, over 80% of the energy we use comes from petroleum, coal, and natural gas. About 98% of carbon emissions are due to fossil fuel combustion, resulting in large amounts of carbon dioxide being discharged into the atmosphere . It is possible to reduce carbon dioxide emissions by reducing the amount of fossil fuels used. Biomass is one of the most potentially energetic organic resources to replace fossil fuels, because it is renewable and neutral with regard to carbon dioxide emissions. Recently, many researchers have studied synthetic hydrocarbon fuels from various biomass sources, for example, wheat, soy beans, sugar cane, corn, rapeseed and palm oils [2–9], or woody cellulose [10–12]. However, biomass has become a problem since it may affect food availability. Therefore, attention has turned to microalgae oil as a new renewable energy source. Production of biofuels from microalgae consists of culturing microalgae and producing hydrocarbon fuels from the resulting oil. This paper will focus on the latter stage of fuel production from microalgae. Recently, microalgae oil has been used to produce hydrocarbon fuels and gas or other related products [13–19].
Microalgae oil can be divided into two types: a triglyceride series and an isoprenoid series. The triglyceride series is produced by Euglena and others, and the isoprenoid series is produced by Botryococcus braunii and others. Conversion of triglyceride fatty oil to fuels has generally been performed by transesterification using an alkali catalyst to produce biodiesel fuels (BDF) [20–22]. The main component in BDF is fatty acid methyl ester (FAME). FAME can be used directly in diesel engines, but it cannot replace gasoline and jet fuels. In practice, BDF is used by mixing about 3% BDF with gasoline and jet fuels. Because BDF tends to contain some impurities, for example, oxygen and nitrogen, it causes damage to engines. Additionally, FAME is costly because it requires a large amount of methanol, and this must be separated from the final product. Thus, some problems still remain in using BDF as a renewable new energy source to replace gasoline and jet fuels.
In contrast, the microalgae oil produced by Botryococcus braunii is botryococcene, an isoprenoid hydrocarbon of 27–34 carbon atoms with a relatively high melting point, which is easy to handle and does not corrode equipment. The coauthor previously studied the catalytic cracking of isoprenoid oil extracted from Botryococcus braunii using the traditional catalytic cracking catalyst . The main products were gasoline, light cycle oil, and heavy cycle oil. It was confirmed that the reactivity and products correlated with the catalytic cracking of squalene. Recently, studies on the fuel synthesis process and catalyst from botryococcene and/or squalene have been reported [24–27].
Our current research has been intentionally focused on composites containing unimodal nanoporous nanosized (defined as 5–50 nm, and hereafter referred to as ns) oxides with pore sizes ranging from about 5 nm to several tens of nanometers . This research yielded some advances in catalysis, including hydrodesulfurization [29, 30], hydrocracking [31–35], reforming [36–38], and catalytic cracking [39, 40]. Various ns alumina matrices with different properties were studied for catalytic cracking catalysts. The activity was found to be dependent on the pore size of the matrix because precracking proceeds independently from the zeolite component. An inverse relationship was also demonstrated between the activity and the rate of degradation by coking, both of which were analyzed as a function of the pore diameter of the matrix. For hydrocracking and isomerization catalysts, a tricomponent catalyst consisting of beta zeolite, ns oxide, and metal/γ-Al2O3 was studied. This catalyst has relatively high selectivity for hydroreforming large n-paraffins to lighter isoparaffins. The enhanced catalytic performance is due to the combined effects of zeolite, ns oxide, and metal/γ-Al2O3. Catalysts containing dealuminated beta zeolite with ns Al2O3 have high conversion and isoselectivity; their catalytic performances are suitable for producing isoparaffins for gasoline. Therefore, the conversions of squalene and squalane, selected as models of microalgae oil and its hydrotreated version, respectively, were studied by catalytic cracking and hydrocracking on our own catalysts.
2.1. Catalyst Preparation
Three types of catalysts for catalytic cracking were prepared using the following reagents [37–40]. Two kinds of zeolites, the commercial USY zeolite (SiO2/Al2O3 molar ratio = 12) (JGC Catalysts and Chemicals, LTD.) and lab-made H-GaAlMFI zeolite (SiO2/Al2O3 molar ratio = 30) were chosen as the zeolite catalyst component. The H-GaAlMFI zeolite was synthesized using sodium silicate, aluminum sulfate 14–18 hydrate (Al2(SO4)3·14–18H2O), gallium nitrate n hydrate (Ga(NO3)2·nH2O), tetrapropylammonium bromide (TPABr), sodium chloride, sulfuric acid, and deionized water. These reagents were mixed sequentially. The mixture was aged overnight at room temperature, then the zeolite was hydrothermally synthesized at 150°C for 24 h. The synthesized zeolite was washed with H2O until the pH reached 7, then dried overnight at room temperature. Further drying was conducted at 120°C for 3 h, followed by calcination in air at 550°C for 3 h. A moderate developed kind of dry unimodal boehmite sol powder was prepared by the hydrothermal aging method. The powder can be converted to a type of ns Al2O3 matrix material with a unimodal pore diameter of 14 nm by calcination, as reported previously [39, 40]. Two of these solid powders were mixed with water to form a thick paste and this paste was mechanically kneaded. After kneading, the catalysts were extruded into pellets, then dried overnight at room temperature and heated at 120°C for 3 h. Finally, they were calcinated at 550°C for 3 h. The ns Al2O3/GaAlMFI catalysts pellets were converted into H-type zeolites by a conventional ion exchange method with an aqueous solution of 2.2 mol/L NH4NO3. The catalysts were made from ns Al2O3 and zeolite (dry weight ratio: 70 : 30).
[NiMo/γ-Al2O3]/ns Al2O3/H-beta zeolite catalyst was prepared for hydrocracking with isomerization [31–35]. The NiMo/γ-Al2O3 catalyst was prepared by coimpregnation using the incipient wetness method. Two catalysts were loaded with 0.39 g/g-(γ-Al2O3) and 0.36 g/g-(γ-Al2O3) of Ni(NO3)2·6H2O and (NH4)6Mo7O24·4H2O in the following manner. A laboratory-produced Al2O3 extrudate with 11 nm unimodal pores and a high-surface area of 225 m2/g was used as the metal carrier. 100 g of the alumina extrudate was impregnated with the Ni-Mo metal solution. A solution of 39 g of nickel nitrate hexahydrate [Ni(NO3)2·6H2O] was prepared in 40 mL of water. Similarly, 36 g of hexaammonium heptamolybdate tetrahydrate [(NH4)6Mo7O24·4H2O] was dissolved in 40 mL of water and heated at 40~50°C to dissolve the solid. The two solutions were mixed and quickly added to the extrudates. They were then mixed well to ensure that the alumina was homogeneously impregnated by the solution. The mixing and impregnation were performed quickly to avoid complex formation between the two solutions. The impregnated extrudate was aged overnight in a sealed container at room temperature. It was then dried at 120°C for 5 h and calcined at 550°C for 2 h. A metal-impregnated alumina (NiMo/γ-Al2O3) powder (particle size: 15 μm) was obtained by grinding the catalyst. Cataloid AP-1 (JGC Catalyst and Chemical, LTD.) was used as the ns Al2O3 precursor for the hydrocracking catalyst. The precursor consists of 71.0 wt% alumina, 11.0 wt% acetic acid, and 18.0 wt% water and has an average particle size of 5.4 nm. H-beta zeolite (SiO2/Al2O3 molar ratio = 25) is a commercial zeolite (Sud-Chemie Catalysts Japan, Inc.). [NiMo/γ-Al2O3]/ns Al2O3/H-beta zeolite catalyst was synthesized by wet mixing followed by calcination. Three of these solid powders were mixed with water to form a thick paste and this paste was mechanically kneaded. After kneading, the catalysts were extruded into pellets. They were then dried overnight at room temperature and heated at 120°C for 3 h. Finally, they were calcined in air at 550°C for 3 h. The tri-component catalysts were made from NiMo/γ-Al2O3, ns Al2O3, and H-beta zeolite (dry weight ratio of 2 : 1 : 2).
2.2. Reaction Test and Catalyst Evaluation
The performances of the catalysts were evaluated using a feedstock of squalene (iso-C30H50) and its hydrotreated form, squalane (iso-C30H62), which have normal boiling points of 285°C and 350°C, respectively. These were selected as model compounds of microalgae oil.
A continuous-flow reactor with a down flow-type fixed bed catalyst (volume: 0.5 mL) was used for the reaction tests. Reaction conditions were as follows: temperature, 250~550°C; liquid hourly space velocity (LHSV), 7.2–40 h−1 (contact time (1/LHSV), 0.025~0.50 h); pressure, 0.12 MPa under nitrogen or hydrogen atmosphere; molar ratio of iso-C30H50 or iso-C30H62 : N2 or H2, 1 : 15.
The acid amount was measured by the NH3-TPD method . Mesoporosity was measured using nitrogen adsorption-desorption isotherms . Elemental composition of the catalyst surface layer was determined by X-ray photoelectron spectroscopy (XPS) using a KRATOS equipped with a mono Al source operating at 450 W. The spectra were acquired at room temperature with narrow scans using rather high 40 eV pass energy for the samples. The spectrometer energy scale was calibrated with Ag 3d5/2. The binding energies and atomic concentrations of the catalysts were calculated from the XPS results using the total integrated peak areas of the Al 2p, Si 2p, Ga 2p, O 1s, and C 1s regions.
3. Results and Discussion
3.1. Catalytic Cracking of Nonhydrotreated Oil
The ns Al2O3/USY zeolite catalyst was developed as a fluid catalytic cracking catalyst in our previous research [39, 40]. It was confirmed that a catalyst with a middle pore size of around 14 nm made from two different pore sizes of ns Al2O3 of 7 nm and 35 nm exhibited higher cracking activity and a lower rate of degradation by coking. The properties of the catalyst are shown in Table 1. Catalytic cracking of nonhydrotreated oil, squalene, was investigated over the ns Al2O3/USY zeolite catalyst at temperatures in the range 400 to 500°C. The results are shown in Figures 1 and 2.
As shown in Figure 1, conversions were almost 100% and olefins were produced at about 50% selectivity at all temperatures. Aromatics were produced at about 40% selectivity at 500°C. As shown in Figure 2, the product distribution of carbon number peaked at nine with the main products being aromatics. It is considered that C8 and C9 olefins caused cyclization and aromatization reactions, resulting in hydrogenation of the highly unsaturated olefins by hydrogen transfer. The equilibrium between the hydrogenation and dehydrogenation reactions was balanced such that hydrogen gas was not used substantially in the reaction system.
3.2. Hydrocracking of Nonhydrotreated Oil
In our previous research, it was confirmed that the tri-component catalyst consisting of NiMo/γ-Al2O3, ns Al2O3, and H-beta zeolite had relatively high selectivity for hydroreforming large n-paraffins to lighter isoparaffins . As shown in Figure 3, H-beta zeolite bound with ns Al2O3 had more acid content than only H-beta zeolite, and the increased acids consist of mild-moderate acid and mild-strong acid. On the other hand, H-beta zeolite bound with ns SiO2 had almost the same acid content of only H-beta zeolite. It is considered that the generated acid sites at the boundary between ns Al2O3 and nanoporous zeolite were derived from pseudoboehmite and H-beta zeolite. Hydrocracking of nonhydrotreated squalene was investigated over [NiMo/γ-Al2O3]/ns Al2O3/H-beta zeolite catalyst at temperatures in the range 250 to 350°C. The results are shown in Figures 4 and 5.
As shown in Figure 4, the selectively for isoparaffins was about 35%, while aromatics selectivity was about 40% at 350°C. As shown in Figure 5, the product distribution of carbon number in hydrocracking also had a peak at nine, similar to catalytic cracking. The main aromatics were xylenes and trimethylbenzenes. Thus, large amounts of aromatics were produced despite the hydrocracking reaction. It was considered that the cracking reaction was suppressed because the reaction temperature of hydrocracking is lower than that of catalytic cracking. Therefore, aromatization progressed much further, resulting in highly unsaturated products of 8~9 carbon atoms. The reaction scheme of catalytic cracking and hydrocracking of nonhydrotreated oil based on Figures 2 and 5 is summarized in Figure 6.
As shown in Figure 6, in the case of catalytic cracking of nonhydrotreated oil, aromatics are produced by aromatization and transhydrogenation of cracked olefins with hydrogen transfer as a result of the high temperature and lack of hydrogen in the reaction system. In the case of hydrocracking, aromatics are similarly produced by aromatization of cracked olefins. In this reaction system, olefins were converted to paraffins, and hydrogen transfer proceeded more often than in catalytic cracking. It was revealed that a large amount of olefins and aromatics was inevitably produced from nonhydrotreated oil of highly unsaturated olefins.
3.3. Catalytic Cracking of Hydrotreated Oil
3.3.1. ns Al2O3/H-USY Zeolite Catalyst
As shown in Figure 7, this catalyst showed very high activity (more than 90% conversion at 350°C) and aromatics were not produced at any of the reaction temperatures. It was revealed that isoparaffins used as feedstock were easier to decompose at low temperatures. In addition, olefins were produced by the cracking reaction without aromatization because there were enough paraffins in the reaction system to keep the equilibrium far from highly unsaturated olefins.
As shown in Figure 8, the product distribution of carbon number in catalytic cracking of hydrotreated oil at 350°C still had a peak at nine. On the other hand, overcracking occurred at the higher temperature. The product distribution of carbon number at 550°C had a peak at three (data not shown). It was demonstrated that the optimum reaction temperature to obtain useful products in catalytic cracking of hydrotreated oil on ns Al2O3/USY catalyst was 350°C.
3.3.2. ns Al2O3/H-GaAlMFI Zeolite Catalyst
ns Al2O3/H-GaAlMFI zeolite catalyst was developed for cracking reforming of heavier n-paraffins . The properties of the catalyst are shown in Table 2. The catalyst generated new acid sites at the boundary between ns Al2O3 and GaAlMFI zeolite. In addition, as shown in Figure 9, the Ga species can be extracted to the surface layer by calcination, protonation, and composition. The extracted Ga species has a relatively high activity for aromatization.
Catalytic cracking of hydrotreated oil was investigated over ns Al2O3/H-GaAlMFI zeolite catalyst with contact times in the range 0.025 to 0.1 h and at temperatures in the range 400 to 500°C. The results are shown in Figures 10, 11, and 12.
As shown in Figure 10, at the longer contact time, the aromatics yield was increased and many polycyclic aromatics were produced in 0.1 h. n-paraffins also increased due to hydrogen transfer reactions. Therefore, 0.05 h was selected as the optimum contact time.
As shown in Figure 11, comparing the conversions among reaction temperatures, the maximum value was about 80% at 500°C. Typically, conversion increases with increasing reaction temperature. However, the conversion decreased at 550°C. A large amount of polycyclic aromatics were observed in the product oil at high temperatures, especially at 550°C. Therefore, it was considered that a large amount of coke was produced, which caused rapid degradation of the catalyst. It was demonstrated that the optimum reaction temperature for the catalyst was 500°C.
As shown in Figure 12, the main aromatic products were toluene, xylenes, and benzene. The reaction scheme of catalytic cracking of hydrotreated oil based on Figures 8 and 12 is summarized in Figure 13. As shown in Figure 13, aromatics were produced by oligomerization and aromatization of cracked olefins. ns Al2O3/USY zeolite catalyst does not cause aromatization, whereas MFI-type zeolite does and Ga promotes dehydrogenation. Therefore, ns Al2O3/H-GaAlMFI could have produced most of the aromatics in this study.
3.4. Hydrocracking of Hydrotreated Oil
As shown in Figure 14, the catalytic performance was very high. The conversion was over 90% and isoparaffins selectivity was about 90% at 300°C. As shown in Figure 15, the cracking reaction was suppressed and the product distribution of carbon number at 275°C had a peak at 10. At higher temperatures, the cracking reaction progressed and the product distribution of carbon number at 350°C had a peak at 7 (data not shown). Thus, the product distribution could be controlled by changing the reaction temperature on [NiMo/γ-Al2O3]/ns Al2O3/H-beta zeolite catalyst. It was demonstrated that the optimum reaction temperature was 275~300°C.
3.5. Evaluation of Processes
These four processes (CC: catalytic cracking; HC: hydrocracking; HT-CC: hydrotreating-catalytic cracking; HT-HC: hydrotreating hydrocracking) were compared in terms of conversion and product selectivities under optimum reaction conditions. The results are shown in Table 3. Comparison of the obtained fuel fraction and hydrogen consumption is shown in Table 4 and the evaluation summary of these processes is shown in Figure 16.
As shown in Table 3, the products of CC were olefinic because the material contains only a small amount of hydrogen. It was considered that hydrocracking used hydrogen in the carrier gas, but selectivity for olefins was still less than in CC. The products from HT-CC on ns Al2O3/USY zeolite catalyst were isoparaffins and olefins. Aromatics were not produced due to low temperatures and the mild cracking ability of this catalyst. The products from HT-CC on ns Al2O3/H-GaAlMFI zeolite catalyst were mostly aromatics because this catalyst has high aromatization and dehydrogenation abilities. On the other hand, the products of HT-HC were isoparaffins due to the relatively mild cracking ability of the catalyst.
As shown in Table 4 and Figure 16, the major fuel fractions from CC on ns Al2O3/H-USY were gasoline and kerosene, while those from HC were half-isoparaffinic/olefinic kerosene and gasoline. Although more than 50 wt% of the products from HT-CC on the USY catalyst were isoparaffinic/olefinic gasoline, the products from HT-CC on the MFI catalyst at ca. 500°C were high-octane-number gasoline including aromatics. Furthermore, an external hydrogen supply was not required in HT-CC on the MFI catalyst. Attractively, the carbon number distribution of the products from HT/HC at 275~300°C populated the kerosene region and its average carbon number was between 10 and 11, while more than 80 wt% was isoparaffinic. It was demonstrated that the fuel produced can be used as kerosene.
Various hybrid zeolite catalysts with ns Al2O3 had high performance for HT-CC and HT-HC from isoprenoid oil derived from microalgae to afford gasoline, aromatics, and kerosene, while required hydrogen consumption increased in the order CC<HT-CC(MFI)<HC<HT-CC(USY)<HT-HC. Isoparaffinic/olefinic gasoline could be produced by hydrotreating-catalytic cracking on ns Al2O3/H-USY catalyst; high-octane-number aromatics-rich gasoline could be produced without an external hydrogen supply by hydrotreating-catalytic cracking on ns Al2O3/H-GaAlMFI catalyst. While hydrotreating hydrocracking on [NiMo/γ-Al2O3]/ns Al2O3/H-beta catalysts required a good supply of hydrogen, isoparaffins-rich kerosene could be produced at low temperatures.
The authors gratefully acknowledge the financial support from CREST-JST (Japan Science and Technology Agency) and the assistance of their coworkers, H. Sata and S. Sudo.
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