Article pubs.acs.org/EF
Catalytic Cracking Reaction of Heavy Oil in the Presence of Cerium Oxide Nanoparticles in Supercritical Water Mehdi Dejhosseini,†,‡ Tsutomu Aida,∥ Masaru Watanabe,§ Seiichi Takami,‡ Daisuke Hojo,# Nobuaki Aoki,# Toshihiko Arita,‡ Atsushi Kishita,⊥ and Tadafumi Adschiri*,‡,∥,# †
Graduate School of Engineering, Tohoku University, 6-6 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Research Center of Supercritical Fluid Technology, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ∥ New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8579, Japan ⊥ Department of Environmental Science and Technology, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8579, Japan # World Premier International Research Center-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡
ABSTRACT: Catalytic cracking of Canadian oil sand bitumen in supercritical water was performed to clarify the effect of CeO2 nanoparticles. The cracking was performed at 723 K to promote a redox reaction between the water, bitumen, and catalyst for the production of hydrogen and oxygen. As the catalyst, CeO2 with two different morphologies was employed because the redox reaction of CeO2 with water and organics is expected and its activity can be controlled by its structure. In this study, two roles of water were considered as well. Water is attractive as a high potential medium with low dielectric constant and density at near the critical point (374 °C, 22.1 MPa) that allows formation of highly crystalline smaller metal oxides particles. However, the chemical effects of water are investigated with heavy oil catalytic cracking. Transmission electron microscopy images indicated that CeO2 nanoparticles with cubic and octahedral shape were synthesized using a plug-flow reactor under hydrothermal conditions. The particles sizes were 8 and 50 nm for cubic and octahedral CeO2, respectively. At 773 K, it was found that the oxygen storage capacity (OSC) of the cerium oxide nanoparticles with cubic {100} facets was nearly 3.4 times higher than that of the cerium oxide nanoparticles with octahedral {111} facets. Heavy oil fractions of bitumen were cracked in a batch-type reactor at 723 K in order to produce as much light oil as possible, and the effect of the catalyst loading and reaction conditions on the conversion rate and coke formation were investigated. As a result, it was demonstrated that it is possible to obtain a high conversion rate by increasing the exposed surface area and reducing the particle size of the catalyst. The highest conversion was obtained in the presence of 20 mg loading of cubic CeO2 nanoparticles (8 nm) with reaction time of 1 h.
1. INTRODUCTION
Accordingly, the use of water for the production of lighter fuels from heavy oils could be a promising method if water could act as a hydrogen sources. Supercritical water (SCW), which is defined as the thermodynamic state above its critical temperature and critical pressure (647 K and 22.1 MPa, respectively), is expected to provide a new reaction medium for heavy oil upgrading because of the solubility of heavy hydrocarbons in SCW and the reactivity of SCW as a radical source. SCW forms a single phase with hydrocarbons that have a relatively high molecular weight.2 It is also known that SCW works as a radical source in oxidation reaction,3 and thus, SCW is expected to be a hydrogen sources in the thermal decomposition of hydrocarbons, which is also a radical reaction. There have been, in fact, several reports on bitumen upgrading in SCW. Kishita et al. revealed that, even in SCW, bitumen forms two or more phases depending on the temperature and pressure.4 Watanabe et al. reported that the rate of coke formation was enhanced in high density water (200 kg/m3) at 723 K. In this report, however, the probability of
Heavy oils such as Canadian oil sand bitumen are alternative petroleum resources. However, they have high density, low hydrogen/carbon ratio, high carbon residue, large amounts of asphaltenes, large amount of heteroatoms (sulfur and nitrogen), and significant heavy metals contents (V, Ni, Cr, and so on).1 Conventionally, primary upgrading of such heavy oils (such as bitumen) and petroleum residues to distillate products are achieved by either coking or catalytic hydroconversion. In both types of processes, formation of coke limits the possible conversion to distillable liquid products. An improvement in the operability and efficiency of these processes requires control or management of the coke formation, which can only be achieved through a better understanding of the fundamental mechanisms involved. Increasing the production of hydrogen gas particularly in hydrocracking can reduce coke formation; however, much hydrogen is derived from steam cracking of methane, and hydrogen production is a much greater expense. Hence, a new process for the formation of hydrogen gas is necessary. Thereby, in situ hydrogen and oxygen species generated from water molecule were concerned for cracking the heavy oil fractions of the residual oil. © 2013 American Chemical Society
Received: May 8, 2013 Revised: July 6, 2013 Published: July 19, 2013 4624
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water as a hydrogen source was assumed to be negligible.5 Morimoto et al. showed that the upgrading of bitumen in the presence of SCW proceeded with a higher olefin yield than reaction in the absence of SCW. In this case, heavy fractions were considered to be highly dispersed in the SCW, and the dispersed heavy fractions were converted into olefins via intramolecular dehydrogenation. Once again, hydrogen provision from water appears to have been neglected.6 On the other hand, catalytic cracking has been noticed in heavy oil reactions in the presence of water. Hydrogen and oxygen species can promote heavy fraction cracking,7 and thus, the source of these species should be considered as well. Fumoto et al. applied iron oxide catalysts for oxidative cracking of heavy oils in steam at atmospheric pressure and reported that large amounts of active oxygen species were generated from the steam that suppressed coke formation.7 Oxidation of the hydrocarbons by active oxygen species from the iron oxide must have progressed, and the water must have acted as the oxygen supplier in order for continuous oxidation to occur. However, in the report, there was little evidence of hydrogenation because of the very low hydrogen concentration at the lower water concentration used in the study. To improve catalytic hydrogenation using the hydrogen generated from water molecules, the concentration of water must be increased, and thus, SCW is expected to be an effective reactant for catalytic cracking because of its high concentration. In addition, a catalyst with a highly exposed area and greater activity for oxidative reactions is necessary. Nanoscale materials have attracted extensive academic and industrial interest. Nanoscale cerium(IV) dioxide (CeO2), commonly known as ceria, is an important material for catalytic applications because of its enhanced capability for absorbing and releasing oxygen via the Ce4+/Ce3+ redox cycle.8−10 For example, the oxygen storage capacity and release properties of nanostructure cerium oxide are often applied to three-way catalysts in automobile exhaust systems, where the oxygen concentration needs to be maintained in order to maximize the catalytic performance of the noble metal catalysts.11,12 Supercritical hydrothermal techniques, in which supercritical water is the reaction medium, have been studied as a promising green method to synthesize metal oxide nanoparticles with narrow size distributions.13,21 In the present study, we synthesized cerium oxide nanoparticles with two morphologies under supercritical hydrothermal conditions. The crystallization of nanoparticles involves the precipitation of a solid phase from solution, which consists of a nucleation step followed by crystal growth. The properties of supercritical water, such as density and dielectric constant, can be widely changed by pressure and temperature, resulted in accelerated nucleation rate, crystals growth, and smaller size with narrow distribution. On the other hand, the solubility of the metal oxides drastically decreases, which leads to the crystallization of metal oxide nanoparticles.14 One key requirement during catalytic optimization of these nanoscale cerium-oxide-based catalysts is the control of the size and morphology of the cerium oxide. For example, recently, the oxygen storage capacity (OSC) of cerium oxide has been enhanced by modifying its morphology.15,17 In particular, the {100} facet of cerium oxide is the best catalytic candidate for highly reactive surfaces.17,18 The six {100} planes have the highest surface energy among the low-index crystal planes. This high surface energy originates from the instability of the surface oxygen atoms, which are located at bridging positions between two cerium ions.16 Interestingly, Zhang et al. demonstrated that
cerium oxide nanoparticles with six {100} facets show enhanced OSC performance at lower temperature compared to those with irregular morphologies.19 Catalysts based on cerium oxide are also promising for low temperature CO oxidation, soot oxidation, and water−gas shift reactions.20 In that study, nanoscale catalysts with different morphologies exhibited different performance and activities corresponding to the exposed surface area of the CeO2 structure planes. Specifically, two types of CeO2 nanoparticles with octahedral {111} facets and cubic {100} facets were employed. These two types of CeO2 were denoted as octahedral and cubic CeO2, respectively. In this study, octahedral and cubic CeO2 nanoparticles were synthesized in a plug flow reactor, and their performance in the catalytic cracking of bitumen with SCW was investigated with the goal of obtaining a larger light oil yield and conversion of the heavier fractions.
2. EXPERIMENTAL METHODS 2.1. Synthesis and Properties of Octahedral and Cubic CeO2. Octahedral cerium oxide nanoparticles were synthesized in the reported manner.21 Cubic cerium oxide nanoparticles were synthesized using the following procedure, as shown in Figure 1. The procedure
Figure 1. Schematic of the experimental apparatus used to synthesize cubic CeO2. can be briefly described in three steps: (1) preparation of precursor solutions in toluene, (2) synthesis of cubic CeO2 nanoparticles under supercritical water conditions using a modifier, and (3) removal of the modifier without changing the morphology of the cubic CeO2. Preparation of the cubic cerium oxide nanoparticles was achieved as follows. A precursor solution was prepared by dissolving Ce(OH)4 (Aldrich Chemicals, 0.050 mol/L) and hexanoic acid (99%, Wako Chemicals, 0.30 mol/L) as an organic modifier in toluene (99.5%, Wako Chemicals). The precursor was then mixed under continuous stirring for 40 min to obtain clear solution. The precursor solution was fed using a high-pressure pump (Nihon Seimitsu Kagaku, NP- KX540) with a flow rate of 7.0 mL min−1. Simultaneously, deionized water was fed using another pump at a rate of 3.0 mL min−1. The precursor solution was mixed with the deionized water at a junction and rapidly heated to 653 K by using a furnace. The residence time in the heated zone was ca. 95 s, which was estimated from the following parameters: reactor volume, total flow rate, density of the water and toluene mixture at the mixing point, reaction temperature, and pressure. The mixture was then cooled using a water jacket. A back pressure regulator (TESCOM, 26-1700 series) controlled the pressure of the system, which was maintained at 30 MPa. The cubic 4625
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The conversion of asphaltene was defined as the yield of asphaltene converted to maltene and gas after reaction of the bitumen.
cerium oxide nanoparticles were obtained as a dispersion in a mixture of water, toluene, and unreacted raw materials. The sample was left to stand overnight in order to allow separation of the water and toluene phases. The nanoparticles in the toluene phase were then purified by adding ethanol and subjecting the mixture to three cycles of centrifugation and decantation in order to remove any unreacted organic molecules. Finally, the particles were dispersed in cyclohexane and freeze-dried under vacuum for 8 h. The morphology and size of the nanoparticles were observed via transmission electron microscopy (TEM, Hitachi H7650) at an acceleration voltage of 100 kV. Fourier transform infrared (FTIR) spectra were acquired using a JASCO FT/ IR-680 spectrometer to investigate the chemical bonding and functional groups on the surface of nanoparticle. The transmission IR spectra were collected from 400 to 4000 cm−1. The crystallinity and purity of the particles were identified using X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Kα radiation in a 2θ−θ setup. The 2θ angle was scanned between 20° and 70°. In order to remove any organic ligands from surface of the particles, the collected nanoparticles were calcined at 300 °C for 2 h in air in a temperature-programmed muffle furnace at a ramp rate of 2 °C min−1. The calcined nanoparticles were purified in ethanol several times, and any unreacted molecules were removed by centrifugation and decantation. Finally, the particles were vacuum-dried for 6 h, after which, OSC measurements were performed on the calcined of the nanoparticles. All of the samples were alternately exposed to O2 and He every 20 min for determination of the oxygen storage (oxygen association with CeO2) and O2− release (dissociation from CeO2) capability. 2.2. Bitumen Pyrolysis in a Batch-Type Reactor. Canadian oil sand bitumen obtained from the Athabasca area was used in this study. The bitumen, which consists of maltene and asphaltene, is a sticky liquid at room temperature and is completely soluble in toluene. In this study, maltene is defined as the component soluble in n-pentane, while asphaltene is insoluble in this solvent. The maltene and asphaltene were found to be 82 and 18 wt % of the bitumen, respectively. The bitumen was diluted with 1-methylnaphthalene to reduce its viscosity, and a 10 wt % solution was used as the feedstock. Experiments were conducted in a pressure-resistant tube reactor (SUS 316) with an inner volume of 6.3 mL. The reactor was loaded with bitumen solution (1.00 g), water (1.00 mL), and synthesized CeO2 nanoparticles (10−20 mg) as the catalyst, and then capped tightly and placed in an electric furnace adjusted at 723 K. This temperature was selected in order to encourage hydrogen and oxygen generation via the redox reaction of water and the catalyst.7 (After 60 min), the reactor was removed from the furnace and rapidly cooled in a cold water bath to terminate the reaction. Liquid and solid products were recovered by rinsing the reactor with toluene. The toluene-insoluble fraction (coke) was separated by filtration using a membrane filter (pore size: 25 nm), and the toluene-soluble fraction was recovered using a rotary evaporator. The toluene-soluble components were then separated into n-pentane soluble and insoluble components (maltene and asphaltene, respectively). The asphaltene and coke yields were evaluated on the basis of the weight of the loaded sample and the products, as follows: asphaltene (or coke) yield (wt%) ⎧ weight of asphaltene (or coke) ⎫ ⎬ × 100 =⎨ ⎩ weight of loaded bitumen (g) ⎭
3. RESULTS AND DISCUSSION It has been already reported that carboxylic acids have a high tendency to hybridize with nanoparticles via the carboxyl group and for controlling size and morphology of nanoparticles.22−24 There are several effective parameters for demonstration of the final morphology of nanoparticles during two vital steps: nucleation and growth, for instance, the crystalline phase of seeds at the nucleating stage, the intrinsic surface energy of different crystallographic surfaces, and the role of capping agent in surface-selective.25 The addition of organic surface modifiers in the reaction medium can inhibit crystal growth. For example, modifiers with an amine group usually act as shape-determining agents and can control the nanocrystal growth to certify the formation of anisotropic nanostructures.26,27 Zhang et al. reported that organic ligand molecules have a pronounced effect on the morphology of the nanocrystals formed in the SCW process, because of the lower dielectric constant of water under supercritical conditions provided a suitable environment for the interaction of the organic ligand molecules with the surface of CeO2 nanocrystals.28 Sahraneshin et al. reported that trapezohedron-shaped HfO2 nanoparticles were synthesized under subcritical conditions; however, the particle shapes changed to oval-like under supercritical conditions. In addition, in the presence of decanoic acid particle size drastically reduced.29 Shape-dependent OSC behavior strongly addressed that high OSC of CeO2 materials can be designed by tuning their shapes with specific surface crystallographic facets, that is, how to enhance the fraction of more reactive {100} and {110} planes, together with decreasing the fraction of less-reactive {111} planes in the nanocrystalline catalysts.16 3.1. Crystalline Structure and Morphology. Figure 2 shows XRD patterns of the produced CeO2 nanoparticles. Comparing the XRD patterns of the nanoparticles with the Joint Committee on Powder Diffraction Standards (JCPDS) card from the International Center for Diffraction Data (00034-0394), the obtained nanoparticles were observed to have a
(1)
The amount of coke includes the summation of toluene insoluble product and remaining coke on the catalyst surface. The coke on the catalyst surface was calculated by weight loss with calcination of recovered catalyst at 650 °C. Because the maltene is extracted with n-pentane and it is difficult to separate all of the maltene species from n-pentane to evaluating the weight of the maltene, the maltene yield was then determined on the basis of the calculated asphaltene and coke yields as follows:
Figure 2. XRD patterns of the synthesized CeO2 nanoparticles: (a) synthesized CeO2 in the presence of hexanoic acid (C5H11COOH), (b) cubic CeO2 nanoparticles after calcination at 300 °C, (c) cubic CeO2 nanoparticles after reaction at 450 °C, (d) synthesized octahedral CeO2 nanoparticles, (e) octahedral CeO2 nanoparticles after reaction at 450 °C.
maltene yields (wt%) = 100 − asphaltene yield − coke yield (2) 4626
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Figure 3. TEM images of the synthesized CeO2 nanoparticles (a, b) without hexanoic acid, (c) with hexanoic acid, (d) after calcination at 300 °C, and (e) after calcination at 650 °C. Scale bars: 50 nm.
CeO2 crystalline structure. The XRD peaks in Figure 2a−c are broader than the peaks in Figure 2d−e, indicating that the size of the nanoparticles represented in Figure 2a−c is smaller than that of the nanoparticles represented in 2d−e. The crystalline size evaluated by Scherrer’s equation was ca. 8 and 50 nm for the nanoparticles represented by Figure 2 (a−c and d−e, respectively). The size and morphology of the particles were analyzed using TEM. Figure 3 depicts the morphology of the synthesized cerium oxide nanoparticles. Figure 3a−b shows the TEM images of the cerium oxide nanoparticles synthesized without hexanoic acid at 613 K; Figure 3c shows an image of the cerium oxide nanoparticles synthesized with hexanoic acid at 653 K;
Figure 3d indicates the shape of the cerium oxide after calcination at 573 K; and Figure 3e depicts used cubic cerium oxide nanoparticles after calcination at 923 K. Two types of particles are shown in the images: octahedral particles enclosed by eight {111} planes and cubic particles enclosed by six {100} planes. The evolution in particle shape from octahedral to cubic is because of the preferential interaction of the hexanoic acid ligand molecules with the truncated octahedral {001} planes, which greatly reduced the growth rate of the crystals in the {001} direction; crystal growth in the {111} direction became predominant, eventually leading to the formation of nanocubes.28 Notably, the size obtained on the basis of the XRD 4627
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nanoparticles (100 μg−O g−1) at 723 K. Zhang et al. reported that the OSC of cerium oxide with a cubic morphology was 2.6 times higher than irregular-shaped CeO2 at 400 °C and also implied that cubic cerium oxide has an OSC even at 150 °C.19 In addition, cubic cerium oxide nanoparticles with a smaller size and active {100} facets have a larger OSC. This result is attributed to the larger exposed surface area in the smaller nanoparticles and indicates that the oxygen molecules involved in the oxygen storage/release process are located mainly on the surface of the CeO2.31,32 3.3. Catalytic Cracking of Bitumen over CeO2. Next, the cerium oxide nanoparticles were used to promote the oxidative cracking of heavy oil. The two types of cerium oxide (octahedral and cubic) were used for bitumen cracking in the presence of water. Figure 5 shows the product distributions when the bitumen was cracked at 723 K using the CeO2 catalysts. The bitumen used in this study contained 18%
measurements matched that determined from the TEM analysis. To verify the existence of the organic molecules chemically bonded to the surface of the cubic nanoparticles, FTIR spectra were obtained. As shown in Figure 4, CH2 stretching peaks
Figure 4. FTIR spectra of cubic cerium oxide: (a) modified particles and (b) particles calcined at 300 °C.
appeared in the region of 2900−2970 cm−1 in the surfacemodified nanoparticles. These peaks were assigned to the C−H stretching mode of the methyl and methylene groups in the hexanoic acid and are present in the FTIR spectra of the neat modifiers, indicating the existence of organic molecules on the surface of the nanoparticles. In the spectra of the hexanoic-acidmodified nanoparticles, the two major peaks at 1531 and 1444 cm−1 were assigned to the asymmetric and symmetric modes of the carboxylate group, respectively. This indicates that hexanoic acid is chemically bonded to the cerium oxide nanoparticles surface through its carboxylate group.35−37 Prior to using the cerium oxide nanoparticles as catalyst in bitumen catalytic cracking, it was necessary to remove the organic ligands attached to the catalyst surface without changing the morphology of the particles, because the organic ligands that are bound to the catalyst surface can serve as reactants in the reaction. In addition, they can inhibit interaction of the catalyst surface with the heavy oil fractions. Accordingly, the organic molecules should be removed from the catalyst surface. Thermal treatment was selected as a common method for the removal of the organic ligands from the surface of particles. The organic molecules easily decompose to CO2 and H2O during combustion in an air flow. The FTIR spectrum in Figure 4b shows that the presence of the organic molecules decreased after the nanoparticles were calcined. The morphology was then investigated using TEM analysis, and it was confirmed that no change occurred during calcination (Figure 3d). However, the used CeO2 nanoparticles were also calcined at 923 K in order to remove formed coke on the catalyst, and it showed a little change occurred with calcination (Figure 3e). Consequently, it means the CeO2 nanoparticles shape can be fixed under reaction temperature (723 K), because it is lower than calcination temperature. 3.2. Oxygen Storage Capacity of the CeO2 Nanoparticles. OSC is defined as the amount of oxygen stored in and released from catalysts. The OSC of the cubic and octahedral CeO2 nanoparticles was measured at 723 K to determine the total available OSC and assess their potential activity for catalytic reactions. The results indicate that the OSC of the cubic cerium oxide nanoparticles was 340 μg−O g−1, nearly 3.4 times higher than that of the octahedral cerium oxide
Figure 5. Effect of catalyst loading on the asphaltene and coke yields in the catalytic cracking of bitumen at 450 °C: (a) asphaltene yield in the presence of 20 mg catalyst (■ without catalyst; △ with octahedral CeO2; ○ with cubic CeO2), (b) asphaltene yield in the presence of 20 mg of used catalyst, (c) maltene yield in the presence of 20 mg of catalyst. 4628
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asphaltene and 82% maltene and almost no toluene-insoluble fraction (coke). As can be seen in Figure 6, after cracking in SCW, the asphaltene yield decreased and coke was formed when no
The catalyst effectiveness was also observed through the change in color mixture of the recovered oil and 1-methyl naphthalene after the reaction. Figure 7 shows unreacted
Figure 7. Bitumen and recovered oil mixed with 1-methyl naphthalene: (a) unreacted bitumen, (b) recovered oil without using catalyst at 450 °C, (c) recoverd oil using octahedral CeO2 at 450 °C, and (d) recoverd oil using cubic CeO2 at 450 °C.
Figure 6. Product yield after the reaction of bitumen with SCW over octahedral and cubic CeO2 nanoparticles at 450 °C.
catalyst was added. When a catalyst was loaded into the reactor, the asphaltene consumption gradually increased and the coke formation was reduced. Accordingly, the catalyst attenuated coke formation, while the asphaltene was mostly converted to light oils. The asphaltene yield with both the octahedral and cubic catalysts after 1 h of reaction was reduced to 5.1 wt % and 2.8 wt %, respectively, compared to 9.2 wt % without the catalyst. In addition, comparison of the catalytic cracking results clearly demonstrated that the catalyst with cubic morphology exhibited higher activity, probably because of its higher oxygen storage capacity and small particle size. In the case of the cubic nanoparticles, the exposed surface area is higher than that of the octahedral nanoparticles and thus, more oxygen can be exposed, therefore reacting with the heavy fractions of the bitumen. Consequently, the asphaltene was more rapidly consumed in the presence of the cubic CeO2, and the asphaltene conversion reached its highest value in the presence of the cubic CeO2. Furthermore, the rate of C−C bond cleavage was enhanced in the catalytic cracking, a greater amount of hydrogen and oxygen species were generated from the splitting of the water, and these species reacted with the heavy fractions at high temperature.33 Next, we tried to estimate the catalyst activity in the second sequence of reaction. Therefore, in some experiments, the used catalyst from the first experiment was collected by filtration, dried in a vacuum at 60 °C for 6 h, and then reloaded into the reactor to estimate its activity. As can be observed in Figure 5b, after the second sequence of catalytic cracking, the asphaltene yield in the presence of both types of reused CeO2 catalysts was lower than that for the reaction without catalyst. This result is likely because the asphaltene and maltene underwent further upgrading in the presence of the catalyst, leading to a reduction in the conversion of the asphaltene to coke. Notably, the largest maltene yield was achieved by loading 20 mg of cubic CeO2. In this reaction, the heavy oil of the bitumen decomposed to generate maltene. Subsequently, some portions of the maltene were decomposed and converted to light fractions and gas. Hence, the yield of gas products was enhanced as the reaction time increased.34
bitumen and recovered oils that were prepared in this study. All feedstocks were mixed with 1-methyl naphthalene to reduce viscosity. The ratio of 1-methyl naphthalene to bitumen was 9.0 in all samples. As can be observed in the Figure 7, in the absence of catalyst, the color of the recovered oil did not change considerably, and thus, the conversion of heavy fractions was not significant. However, on using the CeO2 catalysts, the color changed toward that of light fractions, particularly after reaction at 723 K using the cubic catalyst. 3.4. Effect of Catalyst on the Gas Composition. To examine the activity of CeO2 nanocatalyst in cracking of bitumen, we measured the gas composition in products. Figure 8 shows the gas composition of bitumen cracking without using catalyst and in presence of cubic CeO2 nanocatalyst for 1 h. The gas products collected in a sampling gas bag were quantitatively analyzed by gas chromatographs with thermal conductivity and flame ionization detectors. The main gas products were methane and alkene in reaction of unused catalyst, although carbon dioxide (CO2) and alkane were the
Figure 8. Gas composition after the reaction of bitumen in presence and absence of cubic CeO2 nanoparticles at 450 °C in SCW. Reaction conditions: catalyst loading = 20 mg; reaction time = 1 h. 4629
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Figure 9. Schematic illustration of hydrogen transfer in SCW reaction medium.
4. CONCLUSIONS In this study, cubic and octahedral CeO2 nanoparticles were synthesized under hydrothermal conditions using plug-flow reactors and short reaction times. By using hexanoic acid as a modifier, cubic particles with a size of 8 nm were synthesized, while in the absence of the modifier, octahedral particles with a size of 50 nm were obtained. Performance of the nanocatalysts was investigated for heavy oil cracking under supercritical conditions. In the presence of both types of CeO2, the conversion drastically increased, whereas the coke yield decreased. The asphaltene conversion reached its highest value in the presence of the cubic CeO2 nanoparticles, which exhibited higher activity, a larger oxygen storage capacity, and a greater exposed surface area. Our findings demonstrate that the use of different morphologies of CeO2 with varying oxygen storage capacities and exposed surface areas can lead to major differences in the catalytic properties of CeO2 for oxidative catalytic cracking of heavy oil in SCW.
main products with cubic CeO2 nanocatalyst. Hydrogen gas was also detected, although not as much as other gases. This meant the hydrogen generated from water and water−gas shift reaction was consumed by free alkyl chain molecules to form light molecules. The results of the reaction with CeO 2 nanocatalysts represent that CeO2 with active crystal plane and regular morphology promoted the cracking of bitumen to light oils. Thus, CeO2 exhibits activity to produce active oxygen and hydrogen species from water. The existing CO2 gas indicates that oxygen species react with heavy oil fractions to yield light oils and gas. On the other hand, heavy oil reacted with oxygen species generated from water in the CeO2 nanoparticles, and the active hydrogen species generated from water transferred to the lighter oils. Hence, the maltene yield was enhanced using CeO2 nanocatalyst at 450 °C. 3.5. Role of Water in the Catalytic Cracking of Bitumen in the Presence of CeO2. From the above results, it can be concluded that coke formation was suppressed with the addition of both types of catalysts. According to the literature,7 cracking of heavy oil is promoted by redox reactions between the oil, water, and a zirconia-alumina-iron oxide catalyst. By analogy, therefore, it is assumed that active oxygen species generated from the water led to the improved cerium oxide catalytic activity in this study. The presence of the additional oxygen species enabled the enhanced absorption and release of oxygen via the Ce4+/Ce3+ redox cycle. In addition, the oxygen on the surface of the cerium oxide catalyst became unstable, and therefore, cracking of the heavy oil took place via oxidation. At the same time, the increased presence of hydrogen in cracked radicals suggests that cracking of the −C−C− bonds between the aromatic groups in both maltene and asphaltene was promoted in the presence of the cerium oxide catalysts, also probably because of the presence of oxygen produced from the CeO2 or the water via the redox reaction. The above-mentioned redox reaction among the cerium oxide, water, and heavy oil would also generate hydrogen as a byproduct that could participate in stabilization of the cracked radicals and formation of molecules. Indeed, the oxidation of hydrocarbons results in the formation of CO2 as one main gas. Detecting CO2 demonstrates that oxygen species were transferred to decompose heavy components into lighter fractions and gases. Alternatively, hydrogen could be abstracted from the alkyl chain group via β-scission, forming an internal olefin.30 During SCW catalytic cracking, produced olefins could rapidly react with hydrogen generated from the water and thus would not be detected in the cracked products. The overall result would be a reduction in the coke yield and an increase in the yield of light oils. Figure 9 shows a schematic of hydrogen transferring in presence of CeO2 nanoparticles in SCW reaction medium.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +81 22 217 6321. Fax: +81 22 217 6321. E-mail: ajiri@ tagen.tohoku.ac.jp. Notes
The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ef400855k | Energy Fuels 2013, 27, 4624−4631
