Characterization of an Iron-Oxide-Based Catalyst Used for Catalytic

Jan 23, 2018 - Characterization of a complex metal oxide catalyst (Fe, Zr, and Al) used for catalytic cracking of petroleum residual oil with steam wa...
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Article Cite This: Energy Fuels 2018, 32, 2834−2839

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Characterization of an Iron-Oxide-Based Catalyst Used for Catalytic Cracking of Heavy Oil with Steam Eri Fumoto,* Shinya Sato, and Toshimasa Takanohashi Research Institute of Energy Frontier, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ABSTRACT: Characterization of a complex metal oxide catalyst (Fe, Zr, and Al) used for catalytic cracking of petroleum residual oil with steam was examined. Heavy oil fractions were oxidatively cracked on the catalyst, producing light oil, gas, and carbonaceous residue. Vacuum residue (VR) conversion was maintained for 2−6 h using adequate contents of Fe, Zr, and Al in the catalyst. Although consumption of lattice oxygen within the catalyst caused partial reduction of iron oxide in the initial stage of the reaction, oxygen species were effectively incorporated into the iron oxide lattice from steam, which reacted with heavy oil, suppressing further reduction of iron oxide. The contents of Fe, Zr, and Al in the catalyst were not homogeneous. Some coke was deposited in the region where the Fe content was low, resulting in pore plugging. This pore plugging slightly decreased light oil yields after 6 h, but VR conversion was maintained. Almost no coke was deposited in the region where iron oxide was the main component, and Zr and Al contents were low in the complex metal oxide catalyst. The heavy oil fractions were oxidatively cracked effectively in this region.

1. INTRODUCTION The petroleum industry requires upgrading of heavy oil, such as petroleum residual oil. However, there are some difficulties associated with the upgrading of heavy oil as a result of its high specific gravity, high viscosity, low H/C ratio, high carbon residue, and high concentrations of sulfur and metals. Coke generation causes serious problems, such as catalyst deactivation and plugging of process equipment. Conventional upgrading processes are coking, visbreaking, hydrocracking, and residue fluidized catalytic cracking (RFCC).1 Hydrocracking is useful to convert heavy oil to light fractions that have a high H/C ratio with a low coke yield; however, hydrogen is expensive. Water has good potential as an alternative hydrogen source, and several studies have reported upgrading with supercritical water (SCW)2−12 and steam cracking.13−24 Heavy oil was cracked with and without catalyst in SCW. Reported studies of oxidative cracking in SCW aided by catalysts used cerium oxide4 and iron oxide catalysts.7,8,11 Steam cracking uses non-noble transition metals13−15 and metal oxide catalysts.16−24 Fumoto et al., Funai et al., and Kondoh et al. developed ZrO2-supporting iron oxide catalysts for the upgrading of petroleum residual oil and oil sand bitumen in a steam atmosphere.18,20−23 Lattice oxygen of iron oxide reacts with heavy oil, producing light fractions. Oxygen species are generated from steam and incorporated into the iron oxide lattice. ZrO2 promotes the generation of oxygen species from steam. Hence, heavy oil fractions are oxidatively cracked using ZrO2-supporting iron oxide catalysts in a steam atmosphere. The catalyst is useful for the catalytic cracking of oil sand bitumen in SCW.11 Hosseinpour et al. reported catalytic cracking of petroleum residual oil in SCW using SiO2supported α-Fe2O3.7,8 They also suggested oxidative cracking of hydrocarbons. Lee et al. and Nguyen-Huy et al. studied steam catalytic cracking of petroleum residual oil with ZrO2impregnated macro−mesoporous red mud.16,17 The catalysts © 2018 American Chemical Society

showed high oxidative cracking activity as a result of the large surface area of the macro−mesoporous structure. Iron-oxide-based catalysts could be useful for heavy oil upgrading with steam as a result of their usefulness and low cost. Coke formation is a key factor for catalyst durability in the upgrading process. Nguyen-Huy and Shin reported that the activity of ZrO2-impregnated macro−mesoporous red mud decreased after 3 h as a result of an increase in coke.17 The catalytic activity was regenerated by air calcination. Fumoto et al. modified the catalyst durability by the addition of Al to iron oxide.20 When lattice oxygen of iron oxide was consumed during the reaction, the catalyst was calcined and used for heavy oil cracking repeatedly. The ZrO2-supporting iron oxide catalyst without Al was deactivated as a result of a phase change of iron oxide between α-Fe2O3 and Fe3O4 and subsequent peeling of ZrO2.25 On the other hand, the complex metal oxide catalysts of Fe, Zr, and Al, prepared by co-precipitation, showed durable activity for cracking petroleum residual oil, and the α-Fe2O3 structure of the catalyst was maintained after 4 h without regeneration. Funai et al. reported that the domain size of αFe2O3 showed negligible change after 6 h with an adequate composition of Fe, Zr, and Al. Furthermore, Kondoh et al. reported that a complex metal oxide catalyst of Fe, Ce, Zr, and Al was durable after 6 h in SCW.11 The reactions of the consumption of lattice oxygen and the incorporation of oxygen species to iron oxide lattice from steam cycled effectively because coke formation was suppressed under high-pressure conditions. Consequently, to maintain catalytic activity, there is a need to inhibit coke formation and the phase change from αFe2O3 to Fe3O4. Special Issue: 18th International Conference on Petroleum Phase Behavior and Fouling Received: January 5, 2018 Published: January 23, 2018 2834

DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839

Article

Energy & Fuels The previous work revealed that the complex metal oxide of Fe, Zr, and Al inhibited the phase change of iron oxide and less coke formation did not deactivate the catalyst. However, factors affecting coke formation have not been elucidated. In this study, we examined steam catalytic cracking of heavy oil under atmospheric pressure for 2−6 h and characterized the fresh and used catalysts to investigate factors affecting coke formation. The catalysts were complex metal oxides of Fe, Zr, and Al because the addition of Al inhibited the phase change of iron oxide.

Table 1. Product Yields, Conversion, and H/C Ratios of the Residue from Catalytic Cracking of AR after 2−6 h and AR Composition before reaction (AR composition) after 2 h after 4 h after 6 h yield (mol % C) CO2 CH4 C2−C4 alkane C2−C4 alkene light oil VGO VR residue VR conversion (mol % C) H/C ratio of residue (mol/mol)

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. Complex metal oxide catalysts containing Fe, Zr, and Al were prepared by a coprecipitation method using a water solution of iron chloride, aluminum chloride, and zirconium oxychloride. The catalysts were treated at 873 K for 1 h in a steam atmosphere and sieved to obtain particles of 300− 850 μm. The extensive preparation procedure has been described previously.22 The primary catalyst molar composition was Zr/Fe = 0.063 and Al/Fe = 0.13. Various other compositions were Al/Fe = 0− 0.26 at Zr/Fe = 0.063, Zr/Fe = 0−0.13 at Al/Fe = 0.13, and Zr/Fe = 0 at Al/Fe = 0. The structures of the catalysts were analyzed by X-ray diffraction (XRD, D2 PHASER, Bruker AXS) and transmission electron microscopy (TEM, EM-002B, TOPCON). Focused elemental analysis of the region of the catalysts was performed using an energy-dispersive spectroscopy (EDS, Thermo Fisher Scientific) system incorporated in TEM. The specific surface area and pore volume distributions of the catalysts were evaluated on the basis of nitrogen adsorption isotherms measured at 77.4 K (BELSORP-mini II, BEL Japan) using Brunauer− Emmett−Teller (BET) and Dollimore−Heal (DH) methods, respectively. 2.2. Feedstock. We used an atmospheric residue (AR) of Middle East crude oil, with the following composition: 4.2% light oil (boiling point of 773 K). AR was diluted with toluene to reduce viscosity. A preliminary experiment confirmed toluene to be an inert solvent for this catalytic reaction. A 50% solution of AR with toluene was used as the feedstock. 2.3. Catalytic Cracking. Catalytic cracking of AR with steam was performed using a down-flow-type fixed-bed reactor loaded with 1.5 g of catalysts at 748 K, under atmospheric pressure. The AR solution and steam were fed into the reactor using syringe pumps, and nitrogen flow was controlled to 5 cm3 standard temperature and pressure [(STP)]/min. The time factor of the ratio of catalyst weight/AR solution flow rate (W/F ) was 1.3 h, and the ratio of steam/AR solution (FS/F) was 3.0 (g/g). Liquid and gas products were separated through an ice trap. The operation time was 2−6 h. Gas products collected in a sampling bag were quantitatively analyzed by gas chromatographs (GC-12A and GC-14A, Shimadzu Corp.) with thermal conductivity and flame ionization detectors equipped with Porapak-Q and Unibeads 3S columns, respectively. Liquid products were analyzed by gas chromatographic distillation (7890B GC, Agilent Technologies) using a wide-bore capillary column according to ASTM D2887. The carbonaceous residue on the catalysts was subjected to elemental analysis (EA1110, Finnigan Mat). The detailed experimental procedure was described previously.22

4.2 42.2 53.6

0.57 0.71 0.94 1.1 25.1 42.6 14.8 14.1 45.9

0.41 0.62 0.85 1.0 25.5 46.1 15.7 9.7 52.5

0.34 0.54 0.72 0.83 22.4 48.0 19.8 7.3 49.4

0.52

0.43

0.43

⎛ VR yield + residue yield ⎞ VR conversion = ⎜1 − ⎟ × 100 composition of VR in AR ⎠ ⎝ (1)

The heavy oil fractions were decomposed to produce light oil, gas, and residue. The amounts of VR decreased after the reaction, and VR conversion showed negligible change after 2, 4, and 6 h. A previous study found that catalytic activity was durable after 4 h when a feedstock of 10% AR solution was used for catalytic cracking of AR under atmospheric pressure,20 and the activity was durable for 6 h in heavy oil upgrading in SCW.11 In the present study, our results indicate that catalytic activity to decompose VR was durable for 6 h in the reaction using a 50% AR solution under atmospheric pressure. This reaction produced CO2 for each reaction time, indicating that heavy oil fractions were oxidatively decomposed. The highest CO2 yield was observed after 2 h. Relatively large amounts of lattice oxygen of iron oxide were consumed in the initial stage of the reaction, producing a large amount of CO2. After the consumption of lattice oxygen, oxygen species derived from steam were incorporated into the iron oxide lattice to react with heavy oil fractions.22 The residue yield decreased with an increasing reaction time. The H/C atomic ratio of the residue was higher after 2 h than after 4 and 6 h (Table 1). The residue, a carbonaceous deposit on the catalyst after the reaction, included both coke and heavy hydrocarbons. The residue obtained after 2 h included relatively large amounts of heavy hydrocarbons. In the initial reaction stages, the heavy hydrocarbon deposits on the catalyst were decomposed slowly to light fractions. Hence, the residue yield and H/C ratio of the residue decreased after 4 h. These results indicate that the coke yield was less than 10%. The lower coke yield resulted in durable activity for 6 h. Kondoh et al. described a similar trend, in which heavy hydrocarbons were deposited on the catalyst in the initial reaction stage and the complex metal oxide catalyst showed excellent durability with less than 10% coke formation.11 There was no change in the light oil yield after reacting for 2 and 4 h and a slight decrease after 6 h, although VR conversion was maintained for 6 h. Consumption of lattice oxygen of the iron oxide catalysts and coke deposition on the catalysts might result in catalyst deactivation. When lattice oxygen is wholly consumed and α-Fe2O3 is reduced to Fe3O4 during the reaction, the activity of oxidative cracking decreases.20 Large

3. RESULTS AND DISCUSSION 3.1. Product Distribution of AR Cracking with Steam. Catalytic cracking of AR with the complex metal oxide catalyst (Zr/Fe = 0.063, and Al/Fe = 0.13) was conducted for 2, 4, and 6 h in a steam atmosphere. The product yields and VR conversion are shown in Table 1. The VR conversion was calculated by eq 1. 2835

DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839

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To examine coke deposition on the catalyst, TEM observations were conducted. Figure 2 shows TEM images of fresh and used catalysts for AR cracking with steam. Figure 2a shows that approximately 10 nm of particles was observed over the surface of the fresh catalyst. The morphology of the fresh catalyst showed a negligible change after reaction for 2−6 h. Some coke deposits were observed around the particles, whereas no coke was deposited on the agglomerate after the reaction. The region of the catalyst with no coke deposits was maintained after 6 h. The catalytic activity may have been maintained as a result of the regions with no coke deposits. To examine the effect of the partial coke deposition on catalytic activity, the surface area and pore volume distribution were analyzed, as shown in Table 3 and Figure 3. There were minor changes in the surface area of fresh and used catalysts after 2−6 h. Figure 3 shows the changes in the pore volume distribution. Pores greater than 30 nm decreased in volume after the reaction compared to those of the fresh catalyst as a result of the partial phase change of iron oxide. Although the pores of 10−30 nm were maintained after reaction for 2 h, the pores decreased in volume after 4 h. The pore structure was maintained from 4 to 6 h. These results suggest that coke formation led to plugging of 10−30 nm pores, causing a slight decrease in the light oil yield after 6 h, although VR conversion was maintained. No pore structure change from 4 to 6 h may expect durable activity for heavy oil cracking above 6 h. Air calcination is useful to remove coke and oxidation of Fe3O4.11,17 Therefore, the catalyst could be regenerated by air calcination when the coke deposits deactivate the catalysts. 3.3. Effect of Catalyst Components on Coke Formation. To examine the difference between regions with and without coke deposits, TEM/EDS analyses of the catalysts used for 6 h were conducted. Figure 4 shows the TEM images and EDS spectra at regions around the agglomerate (I), particles on the agglomerate (II), and particles (III). Region I had almost no coke deposited, while some coke was deposited around regions II and III. A larger amount of coke was observed around region III than around region II. The distribution of Fe, Zr, and Al in the catalyst was not homogeneous. The peaks in Zr and Al were small at region I and clearly increased at region III. Table 4 shows the elemental ratios of Zr/Fe, Al/Fe, and C/ (Zr + Al + Fe) calculated from the EDS spectra at each region. The ratios of Zr/Fe, Al/Fe, and C/(Zr + Al + Fe) increased in the following order: region I < region II < region III. The average composition of the catalyst calculated from the amount of chemicals used in the preparation procedure was Zr/Fe = 0.063 and Al/Fe = 0.13. The main component was iron oxide, and the Zr and Al contents were lower than the average composition at region I. The ratios of Zr/Fe and Al/Fe almost corresponded to the average composition at region II. The contents of Zr and Al were higher than the average composition at region III. The carbon content increased with increasing Zr and Al and decreasing Fe. The results suggest that heavy oil fractions were oxidatively cracked effectively using lattice oxygen of iron oxide to prevent coke deposition around region I. The TEM/EDS analyses showed that coke deposition was prevented around the region where the main component was iron oxide. However, when an iron oxide catalyst without Zr and Al was used, the activity to decompose heavy oil fractions was low and iron oxide was reduced as a result of consumption of lattice oxygen of iron oxide.23 Furthermore, Funai et al. reported that, for effective decomposition of heavy oil fractions

amounts of coke deposited on the active sites of catalysts generally cause catalyst deactivation. Hence, characterization of the used catalysts was conducted to investigate the consumption of lattice oxygen and factors affecting coke formation. 3.2. Structure of the Used Catalyst. We previously reported that the α-Fe2O3 structure of the complex metal oxide was maintained, although some α-Fe2O3 was reduced to Fe3O4 as a result of partial consumption of lattice oxygen during the oxidative cracking of heavy oil fractions.22 Figure 1 shows the

Figure 1. XRD patterns of fresh and used catalysts for catalytic cracking of AR after 2−6 h.

XRD patterns of fresh and used catalysts. The patterns of the fresh catalysts correspond to that of α-Fe2O3, and the patterns of α-Fe2O3 and Fe3O4 were observed after the reaction. There are no peaks corresponding to ZrO2, indicating high dispersion of ZrO2 in the α-Fe2O3 matrix.16,26 The structure of α-Fe2O3 and Fe3O4 did not change from 2 to 6 h. The crystallite size of α-Fe2O3 and Fe3O4 was calculated from the XRD data (2θ = 33.2° and 35.5°, respectively) using the Scherrer formula, shown in Table 2. The crystallite size of α-Fe2O3 and Fe3O4 did Table 2. Crystallite Size of Fresh and Used Catalysts for Catalytic Cracking of AR after 2−6 h crystallite size (nm)

fresh

used for 2 h

used for 6 h

α-Fe2O3 Fe3O4

39.5

25.9 30.9

25.9 30.9

not change between 2 and 6 h, although the crystallite size of αFe2O3 was lower in used catalysts than in fresh catalysts as a result of the partial phase change of α-Fe2O3 to Fe3O4 in the initial stage of the reaction. These results suggest that the reaction of lattice oxygen and the heavy oil fractions of AR caused partial reduction of α-Fe2O3 at the surface of the catalysts in the initial stage of the reaction because the incorporation of oxygen species from steam to the iron oxide lattice is slower than the consumption of lattice oxygen. Hence, the CO2 yield was highest after 2 h (Table 1). After the initial stage, oxygen species are effectively incorporated into the iron oxide lattice from steam and react with heavy oil fractions, suppressing further reduction of iron oxide. 2836

DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839

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Table 3. Specific Surface Areas of Fresh and Used Catalysts for Catalytic Cracking of AR after 2−6 h BET surface area (m2/g)

fresh

used for 2 h

used for 4 h

used for 6 h

17.5

18.6

17.0

16.2

Figure 3. Pore volume distributions of fresh and used catalysts for catalytic cracking of AR after 2−6 h.

0.064) and 7.0 wt % Al2O3 (Al/Fe = 0.13).21 To examine the effect of the catalyst composition on the catalytic activity and coke formation, catalysts with various compositions of Fe, Zr, and Al were prepared. Figure 5 shows the carbonaceous residue yields and VR conversion for catalytic cracking of AR for 2 h using catalysts with various compositions. Although the lowest residue yield was achieved using an iron oxide catalyst without Zr and Al, the conversion was low. Conversion and residue yields increased with increased Zr/Fe at Al/Fe = 0.13 (Figure 5a). Coke formation generally increases with increased conversion. The active site for oxidative cracking on iron oxide slightly decreases with increased Zr, although increased Zr promotes generation of oxygen species from steam and conversion increases.23 The high conversion and the decrease in active sites may have increased the carbonaceous residue. An excess Zr content (Zr/Fe = 0.13) causes a decrease in conversion because the active site on iron oxide is covered by ZrO2.23 The catalyst comprised of Zr/Fe = 0.063 and Al/Fe = 0 showed the highest conversion and low residue yields (Figure 5b). However, iron oxide was reduced, and catalyst durability was low.20,21 When Al/Fe increased at Zr/Fe = 0.063, the conversion was almost constant and residue yields increased. A slight decrease in available active sites for oxidative cracking on iron oxide with a high Al content may have increased residue yields. Some coke may have been generated on ZrO2 and Al2O3 as a result of their acidic and basic properties. Consequently, residue yields increased with increases in Zr and Al. However, the durability of the catalyst with low Zr and Al contents was low. The Zr/Fe = 0.063 and Al/Fe = 0.13 catalysts showed the highest activity and durability. This result corresponds to the results reported by Funai et al.21 The heterogeneous distribution of Fe, Zr, and Al in the catalyst with adequate composition provided durable activity for 6 h of AR cracking because no coke deposited at the region where the main component was iron oxide.

Figure 2. TEM images of (a) fresh catalyst, (b) used catalyst after 2 h, and (c) used catalyst after 6 h for AR cracking with steam.

without deactivation of the catalysts, the appropriate composition of the catalyst was 8.3 wt % ZrO2 (Zr/Fe = 2837

DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839

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Figure 4. (a) TEM images and (b) EDS spectra of used catalysts for catalytic cracking of AR after 6 h.

from steam and reacted with heavy oil fractions, suppressing further reduction of iron oxide. The used catalysts had regions with and without coke deposits. Although light oil yields decreased slightly after 6 h as a result of coke formation, leading to plugging of 10−30 nm pores, VR conversion was maintained after 6 h. The distribution of Fe, Zr, and Al in the catalyst was not homogeneous. Iron oxide was the main component, and Zr and Al contents were low in the region where no coke was deposited. The regions without coke deposits were maintained after 6 h, and heavy oil fractions continued to be effectively oxidatively cracked. When various contents of Fe, Zr, and Al catalysts were used, the Zr/Fe = 0.063 and Al/Fe = 0.13 catalysts showed the highest activity and durability. The iron oxide catalyst without Zr and Al showed low activity, and the durability of the complex metal oxide catalysts with low Zr and Al was low, although coke formation was low. Accordingly, catalyst durability for AR cracking is maintained using a

Table 4. Ratios of Zr/Fe, Al/Fe, and C/(Zr + Al + Fe) at Regions I, II, and III of Catalysts Used for Catalytic Cracking of AR after 6 h Zr/Fe (mol/mol) Al/Fe (mol/mol) C/(Zr + Al + Fe) (mol/mol)

I

II

III

0.0060 0.091 0.011

0.041 0.15 0.78

0.11 0.29 1.6

4. CONCLUSION Catalytic cracking of AR with complex metal oxide catalysts comprised of Fe, Zr, and Al in a steam atmosphere was conducted to examine factors affecting coke formation. VR conversion was maintained for 2−6 h. Consumption of lattice oxygen caused a partial reduction of α-Fe2O3 to Fe3O4 in the initial stage of the reaction. After the initial stage, oxygen species were effectively incorporated into the iron oxide lattice 2838

DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839

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(10) Sato, T.; Sumita, T.; Itoh, N. J. Supercrit. Fluids 2015, 104, 171− 176. (11) Kondoh, H.; Nakasaka, Y.; Kitaguchi, T.; Yoshikawa, T.; Tago, T.; Masuda, T. Fuel Process. Technol. 2016, 145, 96−101. (12) Gai, X. K.; Arano, H.; Lu, P.; Mao, J. W.; Yoneyama, Y.; Lu, C. X.; Yang, R. Q.; Tsubaki, N. Fuel Process. Technol. 2016, 142, 315−318. (13) Fathi, M. M.; Pereira-Almao, P. Energy Fuels 2011, 25, 4867− 4877. (14) Nguyen-Huy, C.; Shin, E. W. Fuel 2016, 169, 1−6. (15) Pinilla, J. L.; Arcelus-Arrillaga, P.; Puron, H.; Millan, M. Fuel 2013, 109, 303−308. (16) Lee, H. S.; Nguyen-Huy, C.; Pham, T.-T.; Shin, E. W. Fuel 2016, 165, 462−467. (17) Nguyen-Huy, C.; Shin, E. W. Fuel 2016, 179, 17−24. (18) Kondoh, H.; Hasegawa, N.; Yoshikawa, T.; Nakasaka, Y.; Tago, T.; Masuda, T. Energy Fuels 2016, 30, 10358−10364. (19) Kondoh, H.; Tanaka, K.; Nakasaka, Y.; Tago, T.; Masuda, T. Fuel 2016, 167, 288−294. (20) Fumoto, E.; Tago, T.; Masuda, T. Chem. Lett. 2006, 35, 998− 999. (21) Funai, S.; Fumoto, E.; Tago, T.; Masuda, T. Chem. Eng. Sci. 2010, 65, 60−65. (22) Fumoto, E.; Sugimoto, Y.; Sato, S.; Takanohashi, T. J. Jpn. Pet. Inst. 2015, 58, 329−335. (23) Fumoto, E.; Tago, T.; Tsuji, T.; Masuda, T. Energy Fuels 2004, 18, 1770−1774. (24) Nguyen-Huy, C.; Shin, E. W. Fuel 2017, 192, 149−157. (25) Fumoto, E.; Tago, T.; Masuda, T. Energy Fuels 2006, 20, 1−6. (26) Fumoto, E.; Mizutani, Y.; Tago, T.; Masuda, T. Appl. Catal., B 2006, 68, 154−159.

Figure 5. Carbonaceous residue yields and conversion amounts for catalytic cracking of AR after 2 h with various (a) Zr/Fe and (b) Al/Fe catalysts.

heterogeneous distribution of Fe, Zr, and Al in the catalyst with adequate composition as a result of the region of no coke deposit where the main component was iron oxide.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (C) (25420822) from the Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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DOI: 10.1021/acs.energyfuels.8b00054 Energy Fuels 2018, 32, 2834−2839