Production of Lighter Fuels by Cracking Petroleum Residual Oils with

All of the catalysts were confirmed to be inactive to benzene in advance.7,9 Catalytic ... We have previously reported that ZrO2-supporting iron oxide...
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VOLUME 20, NUMBER 1

JANUARY/FEBRUARY 2006

© Copyright 2006 American Chemical Society

Articles Production of Lighter Fuels by Cracking Petroleum Residual Oils with Steam over Zirconia-Supporting Iron Oxide Catalysts Eri Fumoto,* Teruoki Tago, and Takao Masuda DiVision of Chemical Process Engineering, Graduate School of Engineering, Hokkaido UniVersity, N 13 W 8, Kita-ku, Sapporo, 060-8628, Japan ReceiVed April 15, 2005. ReVised Manuscript ReceiVed October 27, 2005

Zirconia-supporting iron oxide catalysts were developed for recovery of lighter fuels in a steam atmosphere from residual oils in petroleum refinery processes. In these processes, steam is first decomposed on zirconia, yielding active hydrogen and oxygen species. These oxygen species spill over to the surface of iron oxide and react with heavy oil molecules, producing lighter molecules and carbon dioxide. The remaining active hydrogen species are then added to the lighter molecules. This reaction therefore proceeds by oxidative degradation. It was found in the present study that the catalysts exhibited catalytic activity to decompose the residual oil without any carbonaceous residue. The catalysts were, however, deactivated when the sequence of reaction and regeneration was repeated, which is attributed to a change in the iron oxide, namely, between hematite and magnetite, and subsequent peeling of zirconia from the catalyst. To avoid this phase change, Al2O3 was added to the iron oxide lattice. The second-order reaction rate constant of this catalyst was almost the same value 0.16 as 0.18 of the catalyst without Al2O3 and increased to 0.22 after the third sequence of the reaction and regeneration, where the rate constant of the catalyst without Al2O3 decreased to 0.11.

1. Introduction In the petroleum industry, the demand for light fuel products such as gasoline, kerosene, and gas-oil has grown, even though half of the primitive petroleum deposits have already been consumed, according to the BP Statistical Review of World Energy. To meet this demand, refineries convert their residual oils into lighter hydrocarbons. Although there are large quantities of heavy oils such as atmospheric- and vacuum-distilled residual oils generated as byproducts in the refinery process, it is not easy to convert these residual oils into useful hydrocarbons. Conventional methods for recovering light hydrocarbons from heavy oil are thermal cracking,1 hydrocracking, and catalytic cracking.2-4 In all of these methods, however, a carbonaceous * To whom correspondence should be addressed. E-mail: fumoto@ eng.hokudai.ac.jp. (1) Zaykin, Y. A.; Zaykina, R. F. Radiat. Phys. Chem. 2004, 71, 469472.

residue is formed both in the reactor and on the catalysts. It has also been reported that heavy metals such as vanadium and nickel contained in the heavy oil can deactivate catalysts.5 This formation of carbonaceous residues and the catalytic deactivation cause serious problems. There have been several reported attempts to compensate for these problems. Catalysts resistant to the deposition of heavy metals and regeneration of the deactivated catalyst have been developed,5 and the formation of carbonaceous residues has been suppressed at high hydrogen pressures.4 The resource of this hydrogen, however, is produced from the petroleum deposits remaining as half of their primitive (2) Ali, M. A.; Tatsumi, T.; Masuda, T. Appl. Catal., A 2002, 233, 7790. (3) Dehkissia, S.; Larachi, F.; Chornet, E. Fuel 2004, 83, 1323-1331. (4) Matsumura, A.; Kondo, T.; Sato, S.; Saito, I.; Souza, W. F. Fuel 2005, 84, 411-416. (5) Cho, S. I.; Jung, K. S.; Woo, S. I. Appl. Catal., B 2001, 33, 249261.

10.1021/ef050105t CCC: $33.50 © 2006 American Chemical Society Published on Web 12/02/2005

2 Energy & Fuels, Vol. 20, No. 1, 2006

amount. We have therefore attempted to utilize the catalytic cracking of heavy oil with steam to provide an alternative hydrogen source. In our previous papers, we have shown that poly(ethylene terephthalate) can be decomposed over iron oxide catalysts in a steam atmosphere6,7 and that ZrO2-supporting iron oxide catalysts are more effective for catalytic cracking of oil palm waste8 and atmospheric-distilled residual oil9 in a steam atmosphere. ZrO2 exhibits activity to produce active oxygen and hydrogen species from H2O.9 Therefore, in the catalytic cracking of the residual oil with steam over ZrO2-supporting iron oxide catalysts, hydrogenation and oxidative decomposition of the residual oil occur. As such, the yield of useful light hydrocarbons is increased without any carbonaceous residue. The stability of the catalyst for this process has not, however, been examined, and the possibility of applying this catalyst to a heavier residual oil, namely, vacuum-residual oil, has not yet been investigated. The main objective of the present study was to improve the durability of the ZrO2-supporting iron oxide catalyst and to apply this catalyst to the catalytic cracking of vacuum-residual oils with steam.

Fumoto et al.

Figure 1. (a) Liquid product yield and (b) gas compositions after the sequence of the reaction of AR with steam over Zr/FeOx catalyst and regeneration. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

2. Experimental Section 2.1. Catalyst Preparation and Characterization. Iron oxide catalysts (abbreviated to FeOx) were obtained by aging iron(III) hydroxide (first grade, Wako Chemicals, Ltd.) at 773-873 K for 1 h in a steam atmosphere. To allow the catalysts to decompose steam to active hydrogen and oxygen species,8 zirconia was supported on this FeOx catalyst by a conventional impregnation technique using ZrOCl2‚8H2O solution (concentration ) 3.2 wt % solution), followed by steam treatment, to yield zirconia-supporting iron oxide catalysts (denoted as Zr/FeOx). The amount of the loaded ZrO2 was 7.7 wt %. As Al2O3 is a highly steam-tolerant material, FeOx containing Al2O3 was prepared by a coprecipitation method using aqueous iron(III) chloride and aluminum sulfate (concentration of FeCl3‚6H2O ) 4.8 wt %; Al2(SO4)3‚14∼18H2O ) 4.8 wt %). The supports were treated with steam at 773-873 K for 1 h. The atomic fraction of Al in the support (Al/(Fe + Al)) was 0.079. Zirconia was loaded on the supports by a conventional impregnation technique, yielding zirconia-supporting complex metal oxide of Fe and Al catalyst (denoted to Zr/Al-FeOx catalyst). All of the catalysts were pelletized without any binders and crushed and sieved to yield catalyst particles 300-850 µm in diameter. These catalysts were again treated in steam at 773-873 K for 1 h and were used in further experiments. The structures of the catalysts were analyzed using a transmission electron microscope (JEM 1010; JEOL) and an X-ray diffractometer (JDX-8020; JEOL). The activity of the catalysts to generate active hydrogen and oxygen species was analyzed using temperature thermal desorption spectroscopy techniques (EMD-WA 1000S; ESCO), denoted as TDS. The surface area and pore volume distributions of the catalysts were evaluated from nitrogen adsorption isotherms measured at 77.4 K (BELSORP-mini; BEL Japan). The amounts of impurities such as V and Ni were measured by inductively coupled plasma analysis (ICP 1000IV; Shimadzu Co. Ltd.). 2.2. Catalytic Cracking. Residual oils of atmospheric and vacuum distillations from a petroleum process (denoted as AR and VR, respectively) were diluted with benzene in order to reduce the (6) Masuda, T.; Niwa, Y.; Tamagawa, A.; Mukai, S. R.; Hashimoto, K.; Ikeda, Y. Polym. Degrad. Stab. 1997, 58, 315-320. (7) Masuda, T.; Niwa, Y.; Hashimoto, K.; Ikeda, Y. Polym. Degrad. Stab. 1998, 61, 217-224. (8) Masuda, T.; Kondo, Y.; Niwa, M.; Shimotori, T.; Mukai, S. R.; Hashimoto, K.; Takano, M.; Kawasaki, S.; Yoshida, S. Chem. Eng. Sci. 2001, 56, 897-904. (9) Fumoto, E.; Tago, T.; Tsuji, T.; Masuda, T. Energy Fuels 2004, 18, 1770-1774.

viscosity of the residual oils. Two kinds of benzene solutions containing AR or VR at 10 wt % thus obtained were used as feedstock. All of the catalysts were confirmed to be inactive to benzene in advance.7,9 Catalytic cracking with steam was carried out in a fixed-bed reactor loaded with approximately 1 g of the catalyst at a reaction temperature of 773 K and 1 atmospheric pressure. Time factor W/F ranged from 1.2 to 1.5 h, where F is the flow rate of the feedstock and W is the amount of the catalysts. The range of the W/F corresponds to that from 12 to 15 of another time factor W/FR, where FR is the flow rate of AR or VR without benzene. A mixture of steam and nitrogen was introduced into the reactor as a carrier gas at flow rates of 72 cm3/min steam and of 5 cm3/min nitrogen. The liquid products were collected by use of an ice trap and were analyzed by a liquid chromatograph (CTO-10A; Shimadzu Co.Ltd.). The analysis of gaseous products was quantitatively performed using gas chromatographs with thermal conductivity and flame ionization detectors (GS20B; Shimadzu Co. Ltd.) with activated carbon and Porapak Q columns, respectively. We have previously provided a detailed description of the experimental procedure, apparatus, and product analysis.9 The oxidative degradation of heavy oil proceeds by the reaction of heavy oil with active oxygen species supplied from the lattice of FeOx and the decomposition of water over ZrO2 on FeOx. When the activity of ZrO2 is insufficient, the phase transfer of iron oxide from hematite to magnetite easily proceeds, because of consumption of lattice oxygens of FeOx in hematite, as reported in our previous paper.9 Therefore, to examine the activity of ZrO2, the sequence of reaction and regeneration was repeated using the same sample. The reaction with the residual oil was allowed to proceed for 2 h, and the used catalyst was then regenerated at 773-873 K in an air stream for 1 h followed by treatment in steam for 1 h at the same temperature as that for air treatment. The regenerated catalyst was used again for the catalytic cracking with steam. This sequence of reaction and regeneration was repeated two or three times.

3. Results and Discussion 3.1. Catalytic Cracking of AR over Zr/FeOx Catalysts. We have previously reported that ZrO2-supporting iron oxide (Zr/FeOx) catalysts show a high activity for cracking of AR with steam.9 In this study, the durability of the catalysts during a repeat of the sequence of reaction and regeneration was examined. Figure 1 shows the typical product yields and gas compositions for the reaction of AR over Zr/FeOx catalyst. The liquid

Production of Lighter Fuels

Energy & Fuels, Vol. 20, No. 1, 2006 3

Figure 2. Change in the reaction rate constant over FeOx and Zr/ FeOx catalyst with the reaction and regeneration sequence number. The feedstock of the reaction is AR. Tpre is the pretreated temperature in the steam atmosphere. Reaction conditions: T ) 773 K, W/F ) 1.21.5 h. Regeneration conditions: T ) 773-873 K, calcination in air followed by steam treatment.

products are classified into four lumps: gas, gasoline + kerosene (carbon number of 7-18), gas-oil (19-29) (denoted as C19C29), and heavy oil (above 30) (denoted as C30+). The Zr/FeOx catalyst was pretreated at 873 K. The experimental results without catalysts and the composition of AR are shown in the figure for comparison. The main products of the reaction over the Zr/ FeOx catalyst were gasoline and kerosene without any carbonaceous residue. Because ZrO2 exhibits activity to produce active oxygen and hydrogen species from H2O, active oxygen species oxidized the carbonaceous residue as well as inducing the oxidation cracking of heavy oil. These results indicate that the oxidative catalytic cracking proceeds over the catalyst, as expected. The yield of the heavy fraction is, however, increased after the second or third sequence of reaction and regeneration over the Zr/FeOx catalyst. The yield of CO2 is also decreased with the number of the sequence. These results suggest that the catalytic activity to generate active oxygen species decreases. The rate equation of the reaction of residual oil is of second order in terms of the weight fraction of heavy oil, fC30+, and is given by,9,10

dfC30+

) -kC30+ fC30+2

Figure 3. TEM photographs of Zr/FeOx catalysts. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

Figure 4. TEM photographs of Zr/Al-FeOx catalysts. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.21.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

The reaction rate constant kC30+ was calculated from the weight fraction of heavy oil and is shown in Figure 2. The temperature values (Tpre) in the figure represent the pretreated temperature of the catalysts in a steam atmosphere. The activity of the FeOx catalyst does not change with the number of reaction and regeneration sequence. The Zr/FeOx catalysts show higher activity than that of the FeOx catalyst. The activities of the Zr/ FeOx catalysts were, however, reduced with the number of reaction and regeneration sequence. After the second or third sequence of the Zr/FeOx catalyst pretreated at 873K, the activity of the catalyst was deactivated to that of the FeOx catalyst. 3.2. Improvement of the Zr/FeOx Catalyst. To examine the mechanism of catalyst deactivation, transmission electron microscopy (TEM) observation of the catalysts was carried out. Figure 3 provides TEM photographs showing the change in the morphology of the Zr/FeOx catalysts with the number of the sequence of reaction and regeneration. The catalysts were pretreated at 873 K, and the feedstock of the reaction was AR. Prior to the reaction, many pores can be observed on the outer surface and within the FeOx phase of the catalyst.8,9 Most pores,

however, disappear and the supporting ZrO2 peels off from the FeOx with increases in the sequence number. In our previous study, a partial phase change from hematite to magnetite in the FeOx phase resulted from the reaction of AR with the lattice oxygen in hematite crystals but not with active oxygen species produced from H2O over ZrO2.9 Therefore, the sequence of reaction and regeneration causes subsequent phase transfer among the hematite and magnetite, leading to subsequent shrinkage and expansion in the FeOx phase. These phenomena induce a disappearance of pores and a peeling of ZrO2 particles from FeOx. As a consequence, the Zr/FeOx catalysts are deactivated. The durability of the Zr/FeOx catalyst must be enhanced by preventing phase transfer from hematite to magnetite. Fe2O3 is alloyed with Al2O3 homogeneously, according to the phase diagram of ferrite.11 It is known that Al2O3 is inexpensive and stable under the steam condition. Therefore, Al2O3 is one of the candidate materials for addition in the FeOx phase in order to prevent the phase transfer. Figure 4 shows the TEM photographs of the Zr/Al-FeOx catalysts prior to and after the sequence of the reaction of AR and the regeneration. The catalysts were pretreated at 873 K. Unlike in the case of the Zr/FeOx catalysts, the change in the morphology was negligibly

(10) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Energy Fuels 1994, 8, 131-135.

(11) Kubaschewski, O.; Schmid-Fetzer, R. In Ternary Alloys; Petzow, G., Effenberg, G., Eds.; VCH: Weinheim, Germany, 1992; Vol. 5, p 334.

d(W/FR)

(1)

4 Energy & Fuels, Vol. 20, No. 1, 2006

Figure 5. (a) Liquid product yield and (b) gas compositions after the sequence of the reaction of AR with steam over Zr/Al-FeOx catalyst and regeneration. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

Fumoto et al.

Figure 7. XRD patterns of the catalysts prior to and after the reaction. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

Figure 8. Mechanism of the deactivation and durability of the catalysts.

Figure 6. Change in the reaction rate constant over Zr/Al-FeOx catalyst with the reaction and regeneration sequence number. The feedstock of the reaction is AR. Tpre is the pretreated temperature in the steam atmosphere. Reaction conditions: T ) 773 K, W/F ) 1.21.5 h. Regeneration conditions: T ) 773-873 K, calcination in air followed by steam treatment.

small prior to and after the sequence of reaction and regeneration. These results suggest that the Zr/Al-FeOx catalyst is not deactivated. 3.3. Catalytic Cracking of AR with Zr/Al-FeOx Catalyst. The activity of the Zr/Al-FeOx catalysts to decompose AR with steam was investigated. Figure 5 shows the product yields and gas composition of the reaction of AR with steam with the increase in the number of changes in the sequence of the reaction and regeneration. The Zr/Al-FeOx catalyst was pretreated at 873 K. The fraction of the unreacted heavy component decreased, and the gaseous products, which include CO2 and H2 mainly, are increased with the number of the sequence. These results indicate that the activity for producing active oxygen and hydrogen species from H2O is stable. To reveal the stability of the Zr/Al-FeOx catalysts, the reaction rate constant appearing in eq 1 was calculated, as shown in Figure 6. The values of temperature in the figure (Tpre) represent the pretreated temperature of the catalysts in the steam atmosphere. The activity of the Zr/Al-FeOx catalyst was enhanced as the number of the sequence of reaction and regeneration increased. However, the activity of the Zr/AlFeOx catalyst decreased with the sequence of the reaction without regeneration.

The pretreated temperature of the catalysts in a steam atmosphere influences the catalytic activity. It was found that the activity of the Zr/FeOx catalyst pretreated at 773 K is higher than that pretreated at 873 K (Figure 2). On the other hand, the Zr/Al-FeOx catalyst pretreated at 873 K exhibits the highest activity (Figure 6). These results indicate that the addition of Al2O3 into FeOx improves the durability of the catalyst. 3.4. Characterization of Zr/FeOx and Zr/Al-FeOx Catalysts. The reaction of AR proceeds by the following sequence: the spilling over of active oxygen and hydrogen species generated from H2O on ZrO2, oxidative cracking of AR with active oxygen species on FeOx to yield light hydrocarbons, and the addition of active hydrogen species to the light hydrocarbons. Therefore, Zr/FeOx and Zr/Al-FeOx have two kind of active sites, ZrO2 and FeOx. To clarify the mechanism of catalyst deactivation and durability, a detailed characterization of the catalysts was conducted. The X-ray diffraction (XRD) patterns of the Zr/FeOx and Zr/Al-FeOx catalysts prior to and after the reaction of AR are shown in Figure 7. There are not any peaks corresponding to Al2O3 and ZrO2, including high dispersion of Al2O3 in the FeOx matrix and fine particles of ZrO2.9 In our previous study, the phase transfer from hematite to magnetite of the Zr/FeOx catalyst resulted from the reaction of AR with the lattice oxygens in hematite crystals.9 The patterns of the Zr/Al-FeOx catalyst prior to and after the reaction show the same transfer from hematite to magnetite as that of the Zr/FeOx catalyst. The peaks of the Zr/Al-FeOx catalyst are broader than those of the Zr/ FeOx catalyst peaks. The broad peaks of the Zr/Al-FeOx catalyst indicate that the domain size of the FeOx lattice is small. Figure 8 shows the mechanism of the deactivation and durability of the catalyst. The domain of the FeOx lattice of Zr/FeOx

Production of Lighter Fuels

Energy & Fuels, Vol. 20, No. 1, 2006 5 Table 1. Loading of V and Ni on Catalystsa Zr/FeOx

V [wt%] Ni [wt%]

Zr/Al-FeOx

prior to reaction

after 3rd sequence

prior to reaction

after 4th sequence

0.76 0

0.95 0

0.49 0

0.79 0

a The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 773 K, calcination in air followed by steam treatment.

Figure 9. Surface area of the Zr/FeOx and Zr/Al-FeOx catalysts prior to and after the reaction with and without regeneration. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.21.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

Figure 11. TDS spectra of (a) Zr/FeOx and (b) Zr/Al-FeOx catalyst prior to and after the reaction. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 773 K, calcination in air followed by steam treatment.

Figure 10. Pore volume distributions of (a) Zr/FeOx and (b) Zr/AlFeOx catalysts prior to and after the reaction and regeneration sequence. The feedstock of the reaction is AR. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

shrinks and expands during the sequence of reaction and regeneration due to the removal and addition of oxygen in the FeOx lattice. This phenomenon causes a peeling off of the supported ZrO2, leading to the deactivation. In the case of Zr/ Al-FeOx, the insertion of Al2O3 makes the domain of the FeOx lattice small, as shown in Figure 7. These small domains were found to lead to a reduction in the degree of expansion and shrinkage during the phase transfer of FeOx, thus preventing deactivation of the catalyst when the sequence of reaction and regeneration is repeated. XRD patterns show the deactivation mechanism attributed to the peeling of ZrO2 particles. At the remaining FeOx active sites, oxidative cracking of AR with active oxygen species occurs. This activity could be proportional to the surface area of FeOx. Hence, the Brunauer-Emmett-Teller (BET) surface areas and the pore volume distributions of the Zr/FeOx and Zr/ Al-FeOx catalysts prior to and after the reaction of AR were measured, as shown in Figures 9 and 10, respectively. The surface area of the Zr/FeOx catalyst decreases after the third

sequence of reaction and regeneration. This decrease exhibits a similar trend to that of the catalytic activity (Figure 2). The pore volume of the Zr/FeOx catalyst after the third sequence is decreased, followed by a decrease in the surface area. This result coincides with the pore disappearance of the Zr/FeOx catalyst, shown in Figure 3. In contrast, the pore volume distribution of the Zr/Al-FeOx catalyst is shifted toward a smaller pore size range after repeats of the reaction and regeneration sequence (Figure 10b). The surface area of the Zr/Al-FeOx catalyst is then increased with the sequence number (Figure 9). Accordingly, our results indicate that the pore openings and active sites grow and that the activity of the Zr/Al-FeOx catalyst increases gradually with the sequence number. The increase in the CO2 yield with the sequence number provides evidence that the active oxygen species are generated from H2O on the active sites. The lack of correlation between the activity of the Zr/FeOx and Zr/ Al-FeOx catalysts and the V and Ni content of the AR was confirmed by ICP analysis, as shown in Table 1. To verify the effects of the supported ZrO2, thermal desorption spectroscopy (TDS) analysis was carried out. In this method, steam was preadsorbed on the catalyst at room temperature and the catalyst was evacuated, followed by heating at 30 K min-1. The desorption rate of H2 from the catalyst was measured by a quadrupole mass spectrometer. Figure 11 shows the desorption rate of H2 from the Zr/FeOx and Zr/Al-FeOx catalysts. Prior to the reaction, the Zr/FeOx catalyst had a remarkable ability to generate H2 from the adsorbed H2O. After that reaction of

6 Energy & Fuels, Vol. 20, No. 1, 2006

Figure 12. (a) Liquid product yield and (b) gas compositions after the sequence of the reaction of VR with steam over Zr/Al-FeOx catalyst and regeneration. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

AR, desorbed H2 from the Zr/FeOx catalyst, however, decreased. Because of the peeling of supporting ZrO2 from FeOx, the activity for producing active oxygen and hydrogen species from H2O was decreased. Hence, the activity of the Zr/FeOx catalyst for oxidative cracking of AR decreased, resulting in an increase in the heavy fraction and a decrease in the CO2 yield (Figure 1). In contrast, the Zr/Al-FeOx catalyst after the reaction of AR has almost the same ability to generate H2 from H2O as that prior to the reaction. As such, the Zr/Al-FeOx catalyst shows high activity and durability. 3.5. Catalytic Cracking of VR. Catalytic cracking of VR over the Zr/Al-FeOx catalyst with steam was carried out at 773 K. The catalyst was pretreated with steam at 873 K. Figure 12 shows the product yields and gas composition, respectively, after the reaction and regeneration sequence. The experimental results without catalyst and the composition of VR are shown in the figure for comparison. Approximately 90 wt % of VR is the heavy fraction. The heavy fraction in VR is larger than that in AR (Figure 5). The primary gaseous product of the catalytic cracking of VR with steam is CO2. In the catalytic cracking of VR, the H2 yield is less than that in the catalytic cracking of AR. These results indicate that active oxygen and hydrogen species are generated from H2O in the catalytic cracking of AR and VR. These active species are consumed in the cracking of AR and VR, and excess species are produced as CO2 and H2. In the catalytic cracking of VR, most active hydrogen species are consumed in the cracking of VR because there is a larger heavy fraction; as such, the H2 yield is less than that of AR (Figure 5).

Fumoto et al.

Figure 13. Change in the reaction rate constant over Zr/Al-FeOx catalyst with the reaction and regeneration sequence number. The feedstock of the reaction is AR and VR. Tpre is the pretreated temperature in the steam atmosphere. Reaction conditions: T ) 773 K, W/F ) 1.2-1.5 h. Regeneration conditions: T ) 873 K, calcination in air followed by steam treatment.

After the first reaction and regeneration sequence, the yield of the heavy fraction decreases to approximately 45 wt %. Furthermore, the yield of the heavy fraction is reduced to approximately 40 wt % after the second sequence. Figure 13 shows the reaction rate constant calculated using eq 1 when the sequence of the reaction of VR with the Zr/Al-FeOx catalyst and the regeneration was repeated. The activity of the Zr/Al-FeOx catalysts for the cracking of VR was found to increase with the sequence number, as expected from the results obtained in the case of AR. It is therefore concluded that the Zr/Al-FeOx catalyst shows high durability and is able to crack a heavier residual oil, VR. 4. Conclusion Oxidative cracking of residual oils such as AR and VR with steam was studied in an attempt to convert the oil into useful lighter hydrocarbons. Zr/FeOx and Zr/Al-FeOx catalysts were found to be active in decomposing the residual oil without any carbonaceous residue. The Zr/FeOx catalyst was, however, deactivated during repeated sequence of reaction and regeneration because of the phase transfer of FeOx and the subsequent peeling of supported ZrO2 particles. On the other hand, the activity of the Zr/Al-FeOx catalyst increased as the sequence was repeated. It was considered that the small domain size of FeOx in the Zr/Al-FeOx catalyst reduced the influence of the phase transfer, thus preventing the supported ZrO2 particles from peeling off the Al-FeOx catalyst. Acknowledgment. The support of the Japan Petroleum Energy Center is gratefully acknowledged. EF050105T