SO2 Removal and Regeneration of MgO-Based Sorbents Promoted

Jan 21, 2009 - Company 453-2 Seongnae-dong, Gangdong-gu, Seoul 134-848, South Korea, and Korea Electric Power. Research Institute, Daejon 305-380, ...
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Ind. Eng. Chem. Res. 2009, 48, 2691–2696

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SO2 Removal and Regeneration of MgO-Based Sorbents Promoted with Titanium Oxide Soo Jae Lee,† Suk Yong Jung,† Soo Chool Lee,† Hee Kwon Jun,†,‡ Chong Kul Ryu,†,§ and Jae Chang Kim*,† Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 702-701, Korea, GS Fuel Cell Company 453-2 Seongnae-dong, Gangdong-gu, Seoul 134-848, South Korea, and Korea Electric Power Research Institute, Daejon 305-380, Korea

To improve the deactivation of the sorbent for the removal of SO2 in residue fluid catalytic cracking plants, a Ce-Fe-MgO sorbent with excellent sulfur removal capacity and regeneration ability was modified by adding new promoters such as Ti, Al, and Zr. The sorbents were prepared by a coprecipitation method, and their sulfur removal capacities and regeneration abilities were measured in a fixed-bed reactor during multiple cycles. In particular, the sulfur removal capacity of the CeFeMgTi sorbent showed a high absorption value (41.2 g of sulfur/g of absorbent × 100) and excellent regeneration ability during multiple cycles. These results can be explained by the difference in their crystal structures and the physical and textural properties. 1. Introduction Sulfur dioxide (SO2) is a pollutant gas produced by various human activities such as the operation of industrial boilers and the burning of oil and coal at power plants. The discharge of waste gas streams with high levels of toxic compounds into the atmosphere is environmentally undesirable; this condition is frequently encountered in conventional operations. In residue fluid catalytic cracking (RFCC) and fluid catalytic cracking (FCC) units, about 45-55% of the sulfur in the hydrocarbon feedstock is converted to hydrogen sulfide in the reactor units and about 35-45% remains as a liquid product. The rest of the sulfur (5-10%) is deposited on the FCC catalyst.1-3 It has been proven that sulfur can promote the deactivation of cracking catalysts, while the catalytic cracking of hydrocarbons takes place in the reaction zone. Up to now, the deactivated catalyst must be regenerated in the presence of oxygen in the regeneration zone. The SOx emissions containing 90% SO2 and 10% SO3 are usually produced from the regeneration units, and they should be removed before their emission into atmosphere in order to prevent environmental contamination. In order to reduce the emissions, about 5-10 wt % of one or more metal oxide sorbents is added to the cracking catalyst. In the RFCC unit, the functions of the sorbents are the absorption of SOx in the catalyst regeneration zone and transformation of SOx back to H2S in the cracking reaction zone, which can then be treated directly in a Claus plant. The mechanisms generally involve the following reactions: SOx (SO2 and SO3) is generated from the coke burning in the catalyst regenerator: S (sulfur compounds in coke) + O2 f SO2

(1)

1 SO2 + O2 f SO3 2

(2)

SOx is removed by the metal oxide sorbent: * To whom correspondence should be addressed. Tel.: +82-53-9505622. Fax: +82-53-950-6615. E-mail: [email protected]. † Kyungpook National University. ‡ GS Fuel Cell Co. § Korea Electric Power Research Institute.

1 MeO + SO2 + O2 f MeSO4 2

(3)

MeO + SO3 f MeSO4

(4)

Regeneration takes place in the cracking catalyst reactor: MeSO4 + 4H2 f MeO + H2S + 3H2O

(5)

MeSO4 + 4H2 f MeS + 4H2O

(6)

MeS is hydrolyzed to form H2S and MeO in the stripper: MeS + H2O f MeO + H2S

(7)

This technique is known as an inexpensive process compared with the stack-gas scrubbing or feed hydrodesulfurization techniques, and it is, from economic and technical viewpoints, a very practical and attractive technique.2,3 It is also known that basic oxides like magnesium oxide in the presence of an oxidation promoter like CeO2 have very high potential for SO2 removal following reactions 1 and 2. Different material promoters such as V, Ce, Co, and Pt have been claimed as SO2 oxidation promoters,4-7 but CeO2 is the most commonly used today.8-12 In addition, numerous materials have been proposed for removing SOx. Al2O3-based sorbents promoted with Ce showed low SOx removal capacity because the Al2(SO4)3 formed is very unstable at the regenerator temperature. It releases the sulfate species as produced in the SOx absorption condition.6 CaO-based sorbents have been the leading candidate materials for several decades. However, they are not suitable in the RFCC and FCC units because the material requires a very high temperature in the range of 565-790 °C for the removal of SO2 and their reuse is nearly impossible due to the very stable CaSO4 formed during SO2 absorption. MgO-based sorbents have been developed for the absorption of SO2 even at a lower temperature similar to the FCC conditions. Many promoters were added to the MgO in order to promote the SO2 transformation to SO3, which would be easily absorbed into the MgO. Even in these cases, however, the MgSO4 formed during SO2 absorption is very stable and it has not been completely regenerated under the cracking conditions.1,2 Recently, hydrotalcite-type or magnesium-rich hydrodtalcite material promoted

10.1021/ie801081u CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

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with various metal oxides such as Ce, Fe, Cu, Mn, Li, Pt, and Zn have been used.4,8-11,13-18 In our previous research, the Ce-Fe-MgO sorbent showed an excellent sulfur removal capacity and regeneration ability.17 However, the total sulfur removal capacity of this sorbent was deactivated during multiple cycles of SO2 absorption and regeneration. This sorbent had some problems regarding the regeneration after SO2 absorption during multiple cycles. The objective of this work was to improve the deactivation of the sorbent Ce-Fe-MgO, which was modified by adding new promoters such as Zr, Al, and Ti, and to improve regeneration abilities and sulfur removal efficiency. 2. Experimental Section 2.1. Preparation of Sorbent by Coprecipitaion Method. 2.1.1. Preparation of Ce-Fe-MgO Sorbent. Two metal nitrate solutions such as Ce(NO3)3 · 6H2O and Fe(NO3)3 · 9H2O (Ce, 15 wt %, and Fe, 5 wt %) were added to a 1.5 M concentration magnesium nitrate solution. After mixing for 10 min, NaOH solution (1.5 M) was added to the Mg solution at a flow rate of 2 mL/min, until the pH value reached 10.5. The resultant gel was aged for 18 h at 80 °C. Then the product was filtered and washed; the sodium ion in the washed water was not detected by AAS (atomic absorption spectrophotometry). Then it was dried and calcined in air at 750 °C for 4 h. 2.1.2. Preparation of CeFeMgX Sorbent (X ) Ti, Al, Zr). A CeFeMgTi sorbent was prepared by the coprecipitation method. Ce(NO3)3 · 6H2O (Ce, 15 wt %), Fe(NO3)3 · 9H2O (Fe, 5 wt %), and TiCl4 (Ti, 20 wt %) solutions were added to a 1.5 M concentration of Mg(NO3)3 · 6H2O solution. After mixing for 10 min, the NaOH solution with a concentration of 1.5 M was added at a flow rate of 2 mL/min to the Mg solution until the pH value reached 10.5. The resultant gel was aged for 18 h at 80 °C. Then the product was filtered and washed. The resultant samples were then dried and calcined in air at 750 °C for 4 h. The CeFeMgAl and CeFeMgZr sorbents were prepared by the same coprecipitaion method using Al(NO3) 3 · 9H2O (Al, 20 wt %) and ZrCl4 (Zr, 20 wt %) solutions, respectively, instead of TiCl4 solutions. 2.2. Apparatus and Procedure. Multiple cycles of sulfation at 700 °C and regeneration at 530 °C were performed in a fixedbed quartz reactor with a diameter of 1 cm in an electric furnace. A 0.25 g sample of sorbent was packed into the reactor and the space velocity (SV) was maintained at 5000 h-1 to minimize severe pressure drops and the channeling phenomena. All of the volumetric flows of gases were calculated at the standard temperature and pressure (STP) conditions. The temperature of the inlet and outlet lines of the reactor was maintained above 80 °C to prevent condensation of water vapor in the sulfation processes. The outlet SO2 and H2S gases from the reactor were automatically analyzed every 8 min by a gas chromatograph (thermal conductivity detector) equipped with an autosampler (Valco). Its detection limitation of SO2 and H2S is about 200 ppm. The column used in the analysis was a 1/8-in. Teflon tube packed with Chromosil 310. The sulfation and regeneration conditions and the compositions of mixed gases are shown in Table 1. When the outlet SO 2 gas concentration level reached 5000 ppm, the concentration of SO2 in the inlet stream of mixed gases, inert nitrogen gas without SO2, was introduced to purge the system, until it reached the regeneration temperature. Sulfurized sorbents were regenerated by H2 gas until H2S was not detected. 2.3. Characterization of Sorbent. The nitrogen adsorption analysis using an Autosorb I (Quantachrome) was used to

Figure 1. Sulfur removal capacities of various MgO-based sorbents promoted with Zr, Al, and Ti during multiple sulfation/regeneration cycles. Table 1. Experimental Conditions for MgO-Based Sorbents SO2 absorption temperature (°C) pressure (atm) flow rate (mL/min) gas composition

SO2 O2 N2

700 1 50 5000 ppm 5.2 vol % balance

regeneration

H2 N2

530 1 50 50 vol % balance

determine the BET surface area and pore volume of the sorbent. The regeneration ability of the spent sorbent was determined by the temperature programmed reduction (TPR) method using a quartz reactor and a thermal conductivity detector. A 0.2 g sample of the sorbent was heated at a rate of 1 °C/min (400-700 °C) and 50 mL/min of 50 vol % H2 in an N2 balance was fed into the reactor. In addition, in order to identify the crystalline phases in the mixed oxides, an X-ray diffraction (XRD) study was performed with a Philips XPERT instrument using a Cu KR radiation source at the Korea Basic Science Institute. The particle size analysis of the sorbent was performed using transmission electron microscopy (TEM; Model h-7100, HITACHI Corp.) at the Korea Basic Science Institue at a magnification of 100 000. 3. Results and Discussion 3.1. SO2 Absorption and Regeneration Performance of the MgO-Based Sorbents. Figure 1 shows the sulfur removal capacities of the various Ce-Fe-MgO-based sorbents during multicycles. When both sulfation and regeneration are considered as a one-cycle process, the horizontal axis indicates the number of cycles repeated. The vertical axis indicates the sulfur absorbed per gram of sorbent until the SO2 concentration in the outlet gas of the reactor reaches 300 ppmv. The sulfur removal capacity of the Ce-Fe-MgO sorbent was 44 g of sulfur/g of absorbent × 100 in the first cycle, and it was 31.2 g of sulfur/g of absorbent × 100 in the eighth cycle. The regeneration ability of the MgO-based sorbents was improved by adding the Fe promoter.4,10,17,18 However, the sulfur removal capacity of the Ce-Fe-MgO sorbent was gradually decreased during multiple cycles. The stability during multiple cycle tests, as well as the sulfur removal capacity, is a very important factor in the RFCC process. To improve this problem, new materials such as Zr, Al, and Ti were added into the Ce-Fe-MgO sorbent by the coprecipitation method. These sorbents were denoted CeFeMgZr, CeFeMgAl, and CeFeMgTi, respectively. As shown in Figure 1, the sulfur removal capacities of CeFeMgTi, CeFeMgAl, and the CeFeMgZr were maintained during mul-

Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2693 Table 2. SO2 Removal Capacity and Efficiency for Various CeFeMg-Based Sorbent Promoted with Zr, Al, and Ti sorbent name (wt %) Ce(15)Fe(5)MgO(80) Ce(15)Fe(5)MgO(60) Zr(20) Ce(15)Fe(5)MgO(60) Al(20) Ce(15)Fe(5)MgO(60) Ti(20) a

theoretical sulfur sulfur removal sulfur removal removal capacitya capacityb efficiency (%) A

B

B/A × 100

63.6 47.7

44.1 18.1

69.4 38.1

47.7

25.7

58.5

47.7

41.2

86.4

Based on active site MgO. b Based on sulfur atom.

ticycles unlike Ce-Fe-MgO. However, the sulfur removal capacities of CeFeMgAl and CeFeMgZr sorbents were lower than that of the CeFeMgTi sorbent. The Al-containing MgObased sorbents with a lower total sulfur absorption capacity compared with that of the MgO-based sorbents were reported previously.9,10,18 Table 2 shows the initial sulfur removal capacities and efficiencies of the Ce-Fe-MgO, CeFeMgZr, CeFeMgAl, and CeFeMgTi sorbents. The sulfur removal capacities of these sorbents were 44.1, 18.1, 25.7, and 41.2 g of sulfur/g of sorbent × 100, respectively. It was found that the sulfur removal capacity of the CeFeMgTi sorbent was higher than those of CeFeMgZr and CeFeMgAl, and was similar to that of Ce-Fe-MgO. Table 2 also shows the initial sulfur removal efficiencies of the MgO-based sorbents. The sulfur removal efficiency (%) was defined by sulfur removal capacity/ theoretical sulfur removal capacity (B/A × 100), unlike the sulfur removal capacity, which was the sulfur absorbed per gram of sorbent. The theoretical sulfur removal capacities were calculated through assuming that only MgO participated in SO2 absorption and that MgO of 1 mol reacted with SO2 of 1 mol. The sulfur removal efficiencies of the Ce-Fe-MgO, CeFeMgZr, CeFeMgAl, and CeFeMgTi sorbents were 69.4%, 38.1%,

58.5%, and 85.18%, respectively. The sulfur removal efficiency of CeFeMgTi was higher than those of the other MgO-based sorbents. Figure 2 shows the XRD patterns of the various MgObased sorbents before and after SO2 absorption. The XRD patterns of the Ce-Fe-MgO and CeFeMgAl sorbents before SO2 absorption show the MgO (JCPDS No.43-1022) and CeO2 (JCPDS No.81-0792) phases. However, unlike Ce-Fe-MgO and CeFeMgAl, CeFeMgTi before SO2 absorption sorbent shows MgTiO3 (JCPDS No. 1-079-0831) alloy structure, in addition to the CeO2 and MgO phases. In the case of CeFeMgZr sorbent, MgO, CeO2, and Mg2Zr5O12 (JCPDS No. 80-0967) phases were observed before SO2 absorption. Also, in a separate experiment, it was identified that the uncalcined samples showed mainly the Mg(OH)2 phase. After SO2 absorption at 700 °C, the separate MgO of the Ce-Fe-MgO and CeFeMgAl sorbents was transformed to the MgSO4 (JCPDS No.74-1364) phase. In the case of CeFeMgTi, MgO and MgTiO3 were transformed to the MgSO4 and TiO2 (JCPDS No. 1-076-0319) phases. It was thought that the additional MgO phase separated from MgTiO3 participated in SO2 absorption. In the case of the CeFeMgZr sorbent, the main crystal structure of the initial Mg2Zr5O12 phase remained after SO2 absorption and did not participate in SO2 absorption during the sulfation process. The decrease in sulfur removal capacity of the CeFeMgZr sorbent was thought to be due to the formation of the inactive phase of Mg2Zr5O12. In addition to the sulfur removal capacity, regeneration characteristics are among the most important factors to be considered. To investigate regeneration abilities of the Mg-based sorbents, the H2S emission concentration was measured during the regeneration process. Nitrogen gas mixed with 50 vol % hydrogen was introduced to regenerate the spent sorbents until the H2S concentration reached a value of less than 200 ppm. Figure 3 shows the H2S breakthrough curves of various MgObased sorbents during the regeneration process at 530 °C. The

Figure 2. XRD patterns of various Ce-Fe-MgO sorbents promoted with Zr, Al, and Ti before and after SO2 absorption: (a) Ce-Fe-MgO; (b) CeFeMgZr; (c) CeFeMgAl; (d) CeFeMgTi.

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Figure 3. Regeneration performance of Ce-Fe-Mg-based sorbent promoted with physical promoters such as Al, Zr, and Ti with 50 vol % H2 at 530 °C.

Figure 5. Desorption amount of sulfur and sulfur removal capacity of Ce-Fe-MgO sorbent during the regeneration process.

Figure 4. TPD results of Ce-Fe-MgO and CeFeMgTi sorbents.

H2S concentration of Ce-Fe-MgO sorbent was increased until 84 min and then gradually decreased, while that of the CeFeMgTi sorbent was rapidly increased until 42 min, and then dramatically decreased. It was found that the regeneration of the CeFeMgTi sorbent was much faster than that of Ce-Fe-MgO at our regeneration conditions. To identify the effect of the Ti promoter on the regeneration ability of sorbent, temperature programmed desorption (TPD) tests were performed after SO2 absorption. The tests were carried out by measuring the concentration of H2S desorbed by the introduction of nitrogen gas containing 50 vol % hydrogen when the ramping rate of the temperature was 1 °C/min. The TPD experimental results are shown in Figure 4. In the case of the Ce-Fe-MgO sorbent, H2S was desorbed in the narrow temperature range of 525-640 °C. In the case of the CeFeMgTi sorbent, two peaks were obserbed. The first peak was desorbed in the temperature range between about 490 and 600 °C, and the second peak was desorbed in the temperature range between about 525 and 656 °C. The initial peak around 540 °C was due to that of MgSO4, which was formed by SO2 absorption of MgTiO3. The second peak around 620 °C was consistent with that of MgSO4 resulting from separated MgO in the Ce-Fe-MgO sorbent. Considering the different crystal structures verified by XRD in Figure 2, the shift to the low temperature and the excellent regeneration ability of the sorbents promoted with Ti additives could be explained by the structural effect of MgTiO3 phase formed in the CeFeMgTi sorbent. Even though the details of the structural effect of the regeneration process have not been verified yet, it is clear that the MgTiO3 alloy structure formed during preparation showed better regeneration properties than Ce-Fe-MgO with a separate MgO phase. Figure 5 shows the desorption

Figure 6. XRD patterns of Ce-Fe-MgO and CeFeMgTi sorbents before SO2 absorption and those after eight cycle regeneration: (a) fresh state of Ce-Fe-MgO; (b) after eight cycle regeneration of Ce-Fe-MgO; (c) fresh state of CeFeMgTi; (d) after eight cycle regeneration of CeFeMgTi.

amount of sulfur during the regeneration process. In the case of the Ce-Fe-MgO sorbent, the desorption amount of sulfur was reduced with the cyclic numbers. About 4-6% of sulfur absorbed was not desorbed even after 1000 min of hydrogen reduction. Figure 6 shows the XRD patterns of the Ce-Fe-MgO and CeFeMgTi sorbents before SO2 absorption and those after eight cycles of regeneration. In the case of the Ce-Fe-MgO sorbent before SO2 absorption, the main structures were MgO and CeO2, whereas those of the CeFeMgTi sorbent were MgO, MgTiO3, and CeO2. After eight cycles of the regeneration process, most of the MgSO4 phase of the CeFeMgTi sorbent was converted into the initial MgO phase. In the case of the Ce-Fe-MgO sorbent, MgSO4 was not completely transformed to the MgO phase even after a long time hydrogen treatment

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Figure 7. H2S breakthough curves of Ce-Fe-MgO and CeFeMgTi sorbent during regeneration after the first and eighth sulfation processes: (a) Ce-Fe-MgO; (b) CeFeMgTi. Table 3. Physical Properties of Various Ce-Fe-MgO and CeFeMgTi Sorbents Fresh and Regenerated after Eight Cycles sorbents

cycle no.

surface area (m2/g)

total pore vol (mL/g)

Ce-Fe-MgO

fresh 8 cycle fresh 8 cycle

32.15 13.02 42.07 28.51

0.152 0.038 0.249 0.177

CeFeMgTi

for 1000 min. It was thought that the remaining MgSO4 resulting from the separate MgO phase gradually decreased the sulfur removal capacity during the multiple cycles. Figure 7 shows the H2S breakthrough curves for the Ce-Fe-MgO and CeFeMgTi sorbents after the first and eighth sulfations. Even though the regeneration rates of both were reduced by increase of the cyclic numbers, the regeneration rate of CeFeMgTi after eight cycles remained a constant value as shown in Figure 1 and faster than those of the initial or eighth cycle of Ce-Fe-MgO. It was concluded that the CeFeMgTi sorbent was an excellent material with a large SO2 removal capacity and an ability for multiple cycle operations. To investigate these differences between the Ce-Fe-MgO and CeFeMgTi sorbents, their textural properties such as surface area and pore volume were compared. Table 3 shows the surface area and pore volume for fresh and regenerated sorbents after eight cycles. After eight cycles and deactivation of Ce-Fe-MgO (Figure 5), as shown in Table 3, the total pore volume of the Ce-Fe-MgO sorbent dramatically decreased by about 75% (from 0.152 to 0.038 mL/g) compared to that of the fresh state. The total pore volume of the CeFeMgTi sorbent only decreased by about 29% (from 0.249 to 0.177 mL/g). Considering the constant absorption capacities of CeFeMgTi (Figure 1) with cycles, it could be concluded that the textural properties of the CeFeMgTi sorbent such as surface area and total pore volume

Figure 8. TEM images for fresh, after SO2 absorption, and regeneration of Ce-Fe-MgO sorbent: (a) fresh; (b) after SO2 absorption; (c) after regeneration.

were more stable than those of the Ce-Fe-MgO sorbent and that the steady sulfur removal capacity of CeFeMgTi sorbents was explained by the stable textural properties maintained during the multiple cycles. Figure 8 shows TEM images of the Ce-Fe-MgO sorbent for the fresh state and after sulfation and regeneration. The particle sizes of the fresh state and after regeneration were about 30-48 nm while that after sulfation was 40-60 nm, respectively. It was observed that the particle sizes of sorbents after SO2 absorption were larger than those of fresh state and regeneration sorbents. The change of textual property by expansion/contraction of sorbent during multiple cycles is considered to be a reason for the cracking of sorbent. The variation in particle size of sorbent was related to expansion/ contraction of the sorbent during sulfation/regeneration. Moreover, volume expansion generally occurred during the sulfation process as the metal oxide was converted into metal sulfate, while contraction occurred during the regeneration process as the sorbent returned to the original oxide phase. In the case of MgO-based sorbent, only MgO works as an active site during the sulfation process. Thus, when MgO (molar volume 11.26

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mL/mol) is converted into MgSO4 (molar volume 41.01 mL/ mol), the ratio of its molar volume expansion is about 4 times. The molar volume could be obtained from the density calculated from X-ray measurements by the NBS EXAIDS83 program. The same degree of volume contraction occurs during the regeneration process. The results indicated that the volume expansion/contraction of MgO-based sorbent is sufficient for the sorbent to be broken during multiple cycles.19-22 These results considered that the expansion/contraction of the sorbent during sulfation/regeneration of the desulfurization sorbent would be thought to be a possible reason for the significant changes to the textural properties and deactivation of the MgObased sorbent. The steady sulfur removal capacity of the CeFeMgTi sorbent could be explained by the new structure of MgTiO3, which has a better regeneration property than that of the separate MgO phase, and the stable textural property explained by adding the Ti additives unlike the Ce-Fe-MgO sorbent. 4. Conclusions New Ce-Fe-Mg-based sorbents modified by adding new physical promoters such as Ti, Al, Ca, and Zr were prepared by the coprecipitation method, and the effects of the promoters on the deactivation were tested during multiple cycles. In particular, the CeFeMgTi sorbent showed excellent sulfur removal capacity and regeneration properties, which were maintained without deactivation even after eight cycles of sulfation and regeneration, compared with the Ce-Fe-MgO sorbent. These results could be explained by regeneration ability, which originated from the new structure of MgTiO3 formed and the textural properties such as large surface area and total pore volume. The MgTiO3 alloy structure showed better regeneration ability than that of the separated MgO phase. The sorbent was an excellent material for SO2 removal in the RFCC condition, and it satisfied the requirement of multiple cycles of sulfation and regeneration performance. Acknowledgment We gratefully acknowledge financial support from the Carbon Dioxide Reduction & Sequestration Research Canter (DA2-202), one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of Korean government. Literature Cited (1) McCauley, J. R. Catalytic cracking with reduced emission of sulfur oxide. U.S. Patent 6,129,833, 2000. (2) Wang, J. A.; Chen, L. F.; Li, C. L. Roles of cerium oxide and the reducibility and recoverability of the surface oxygen species in the CeO2/ MgAl2O4 catalysts. J. Mol. Catal. A 1999, 139, 315–323. (3) Wang, J. A.; Li, C. L. SO2 adsorption and thermal stability reducibility of sulfates formed on the magnesium-aluminate spinel sulfurtransfer catalyst. Appl. Surf. Sci. 2000, 161, 406–414.

(4) Wang, J. A.; Zhu, A. L.; Li, C. L. Pathway of the cycle between the oxidative adsorption of SO2 and the reductive decomposition of sulfate on the MgAl2-xFexO4. J. Mol. Catal. A 1999, 139, 31–41. (5) Johannessen, T.; Koutsopoulos, S. One-Step Flame Synthesis of an Active Pt/TiO2 Catalyst for SO2 OxidationsA Possible Alternative to Traditional Methods for Parallel Screening. J. Catal. 2001, 205, 404–408. (6) Dawody, J.; Skoglundh, M.; Fridell, E. The effect of metal oxide additives (WO3, MoO3, V2O5, Ga2O3) on the oxidation of NO and SO2 over Pt/Al2O3 and Pt/BaO/Al2O3 catalysts. J. Mol. Catal. A 2004, 209, 215– 225. (7) Wang, Y.; Liu, Z.; Zhan, L.; Huang, Z.; Liu, Q.; Ma, J. Performance of an activated carbon honeycomb supported V2O5 catalyst in simultaneous SO2 and NO removal. Chem. Eng. Sci. 2004, 59, 5283–5290. (8) Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A. Advanced DeSOx catalyst: Mixed solid silution spinels with cerium oxide. Appl. Catal., B 1992, 1, 169–189. (9) Corma, A.; Palomares, A. E.; Rey, F. Optimization of SOx additives of FCC catalysts based on MgO-Al2O3 mixed oxides produced form hydrotalcites. Appl. Catal., B 1994, 4, 29–43. (10) Yoo, J. S.; Bhattacharyya, A. A.; Radlowski, C. A. De-SOx catalyst: The role of iron in iron mixed solid solution spinels, MgO · MgAl2-xFexO4. Ind. Eng. Chem. Res. 1992, 31, 1252–1258. (11) Carla, M. S. P.; Cristiane, A. H.; Arnaldo, A. N.; Jose, L. F. M. Syntheis, characterication and evaluation of CeO2/Mg, Al-mixed oxides as catalysts for SOx removal. J. Mol. Catal. A 2005, 184–193. (12) Wang, J. A.; Chen, L. F.; Limas-Ballesteros, R. Evaluation of crystalline structure and SO2 storage capacity of a series of compositionsensitive De-SO2 catalysts. J. Mol. Catal. A 2003, 194, 181–193. (13) Kim, G. SOx control compositions. U.S. Patent 5,627,123, 1997. (14) Gabriele, C.; Siglinda, P. Behaviour of SOx-trap derived from ternary Cu/Mg/Al hydrotalcite materials. Catal. Today 2007, 127, 219– 229. (15) Strehlau. W. Storage material for sulfur oxides. U.S. Patent 6,338,945, 2002. (16) Chen, S. L. F. Layered SOx tolerant NOx trap catalysts and methods of making and using the same. U.S. Patent 6,923,945, 2005. (17) Lee, S. J.; Jun, H. K.; Jung, S. Y.; Lee, T. J.; Ryu, C. K.; Kim, J. C. Regenerable MgO-based SOx removal sorbents promoted with Cerium and Iron oxide in RFCC. Ind. Eng. Chem. Res. 2005, 44, 9973–9978. (18) Cantu, M.; Salinas, E. L.; Valente, J. S. SOx removal by calcined MgAlFe hydrotalcite-like materials: Effect of the chemical composition and the cerium incorporation method. EnViron. Sci. Technol. 2005, 39, 9715– 9720. (19) Jun, H. K.; Jung, S. Y.; Lee, T. J.; Ryu, C. K.; Kim, J. C. Decomposition of NH3 over Zn-Ti-based desulfurization sorbent promoted with cobalt and nickel. Catal. Today 2003, 87, 3–10. (20) Jun, H. K.; Lee, T. J.; Ryu, S. O.; Yi, C. K.; Ryu, C. K.; Kim, J. C. A study of Zn-Ti-based H2S removal sorbents promoted with cobalt and nickel oxides. Energy Fuels 2004, 18 (1), 41–48. (21) Jung, S. Y.; Lee, S. J.; Lee, T. J.; Ryu, C. K.; Kim, J. C. H2S removal and regeneration properties of Zn-Al-based sorbents promoted with various promoters. Catal. Today 2006, 111 (3), 217–222. (22) Jung, S. Y.; Jun, H. K.; Lee, S. J.; Lee, T. J.; Ryu, C. K.; Kim, J. C. The improvement of the desulfurization and regeneration properties through the control of pore structures of the Zn-Ti-based H2S removal sorbents. EnViron. Sci. Technol. 2005, 39 (23), 9324–9330.

ReceiVed for reView July 15, 2008 ReVised manuscript receiVed November 24, 2008 Accepted December 9, 2008 IE801081U