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Desulfurization of Odorant-Containing Gases by Sorption on Cu/ZnO/Al2O3: Effects of Sulfur Compounds Hyung-Tae Kim,† Ki-Won Jun,*,† Hari Shankar Potdar,† Young-Seek Yoon,‡ and Myung-Jun Kim‡ Micro-Chemical Technology Research Team, Korea Research Institute of Chemical Technology, Yu-Seong, Daejeon 305-600, Korea, and Corporate R&D Center, SK Corporation, 140-1 Wonchon-dong Yuseong-gu, Daejeon 305-712, Korea ReceiVed July 26, 2006. ReVised Manuscript ReceiVed October 16, 2006

The removal of tetrahydrothiophene (THT), dimethylsulfide (DMS), and the ordorant mixture of t-butylmercaptan and tetrahydrothiophene (TBM plus THT) from gas fuel has been investigated by using a coprecipitated Cu/ZnO/Al2O3 sorbent. It was found that the sorption capacities of THT, DMS, and TBM plus THT on Cu/ZnO/Al2O3 increased with the sorption temperature, and the maximum sorption capacities of 0.89 and 0.78 mmol/g at 350 °C and 0.50 mmol/g at 250 °C were obtained, respectively, and then decreased with a further increase of the temperature. The Cu/ZnO/Al2O3 adsorbent had a high selective sorption capacity for TBM in the mixture of (TBM and THT). The Brunauer-Emmett-Teller surface areas were decreased after saturation with sulfur compounds. The morphology of the sorbent was changed to rod-shaped grains at 300 °C because of the formation of sulfides after THT sorption. The desorption peak (m/e ) 32) for DMS on the sorbent during TPD was observed in the temperature range of 350-460 °C. The TPD peak (m/e ) 45) for THT was detected in the temperature range of 470-540 °C. This broad TPD peaks may be attributed to the decomposition of copper sulfide clusters formed on Cu/ZnO/Al2O3 after sorption of sulfur compounds.

Introduction Fuel cells are expected to be one of the most effective alternatives to supply power for both mobile vehicles and stationary power stations.1,2 Gaseous hydrocarbon fuels are considered to be the promising fuels to generate a clean hydrogen fuel for fuel cells that are able to convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants.3-5 Organic sulfur compounds in the parts per million (ppm) level as odorants are added intentionally into gaseous hydrocarbon fuels such as city gas and liquefied petroleum gas (LPG) to give people a warning of a leak. However, sulfur compounds in the hydrocarbon fuels have a detrimental effect on the performance of catalysts used in reforming and water gas shift reactors as well as electrode catalysts used in fuel-cell stacks.6,7 Therefore, the desulfurization of hydrocarbon fuels is absolutely essential before the introduction of these gases to the reforming process for fuel-cell applications.6-8 * To whom correspondence should be addressed. Telephone: +82-42860-7671. Fax: +82-42-860-7388. E-mail: [email protected]. † Korea Research Institute of Chemical Technology. ‡ SK Corporation. (1) Satokawa, S.; Kobayashi, Y.; Fujiki, H. Appl. Catal., B 2005, 56, 51-56. (2) Jayne, D.; Zhang, Y.; Haji, S.; Erkey, C. Int. J. Hydrogen Energy 2005, 30, 1287-1293. (3) Wakita, H.; Tachibana, Y.; Hosaka, M. Microporous Mesoporous Mater. 2001, 46, 237-247. (4) Ma, X.; Sun, X.; Song, C. Catal. Today 2002, 77, 107-116. (5) Kim, J. H.; Ma, X.; Zhou, A.; Song, C. Catal. Today 2006, 111, 74-83. (6) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207-238. (7) Novochinskii, I.; Song, C.; Ma, X.; Liu, X.; Shore, L.; Lambert, J.; Farrauto, R. J. Energy Fuels 2004, 18, 576-583. (8) Song, C. Catal. Today 2003, 86, 211-263.

A catalytic hydrodesulfurization (HDS) process combined with the adsorption of H2S on zinc oxide has been attempted at elevated temperatures and pressures by using CoMo or NiMo catalysts for the removal of sulfur compounds in hydrocarbon fuels.3 In this process, it is necessary to keep both the catalyst and the adsorbent at high temperatures and to add hydrogen for HDS. In addition, zinc oxide periodically has to be replaced. To overcome this particular problem, one of the promising new approaches, i.e., selective adsorption, which can be performed at ambient temperature and pressure, has been proposed.1-3,9 Activated carbon and manganese oxide have been used as adsorbents for the desulfurization of hydrocarbon fuel at ambient temperature.10,11 However, these materials do not have high enough sulfur capacities as well as thermal stability.10 Also, zeolites with high thermal stability have been tried as adsorbents for the removal of sulfur compounds at ambient temperature and pressure.3,11,12 However, it is difficult to apply zeolites under water-containing conditions because they have a strong affinity for water. Modified zeolite-based adsorbents using transition metals are also tried to improve sorption capacity and adsorptivity for sulfur compounds. It has been reported that copper and silver metals on zeolites acted as active materials to remove sulfur compounds under ambient conditions.1,4,9,13 Considering (9) Herna´ndez-Maldonado, A. J.; Yang, F. H.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 56, 111-126. (10) Futami, H.; Hashizume, Y. Proc. 1989 Int. Gas Res. Conf. 1990, 1592. (11) Roh, H. R.; Jun, K.-W.; Kim, J. Y.; Kim, J. W.; Park, D. R.; Kim, J. D.; Yang, S. S. J. Ind. Eng. Chem. 2004, 10, 511-515. (12) Bu¨low, M.; Micke, A. Fundamentals of Adsorption; Kluwer Academic Publishers: Boston, MA, 1990; p 1592. (13) Yang, R. T.; Herna´ndez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79-81.

10.1021/ef060342b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006

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the above-mentioned facts, it is necessary to find new adsorbents having a high selective adsorption capacity for different odorants. Therefore, the main objective of the present study is to see the utility of coprecipitated Cu/ZnO/Al2O3 material as a sorbent and measure the desulfurization capacity for a model gas containing different odorants, namely, dimethylsulfide (DMS), tetrahydrothiophene (THT), and the ordorant mixture of tbutylmercaptan and tetrahydrothiophene (TBM plus THT) at various temperatures. Another aspect is to perform the characterization of sorbents after desulfurization of different odorants. All of these results related to the optimization of sorption capacities and characterization are presented in this paper. Experimental Section The sorbents for desulfurization of odorant-containing gases were prepared by a conventional coprecipitation method.14 For the preparation of Cu/ZnO/Al2O3, a mixed aqueous solution of copper, zinc, and aluminum nitrates (Cu/Zn/Al molar ratio ) 1:1:0.3) and an aqueous solution (2.45 M) of sodium carbonate were dropped simultaneously with constant stirring into 500 mL of deionized water.15 The precipitation was carried out at room temperature, and the pH was kept at ∼7.0. The precipitate thus obtained was aged for 2 h, followed by filtration and washing with deionized water. The precipitate was washed several times with hot water to avoid the contamination of “Na” ions. The sample was extruded for the sorption experiment, dried at 120 °C overnight, and finally calcined in air at 300 °C for 12 h. The calcined sample showed a surface area ∼ 104.26 m2/g with a total pore volume of 0.43 cm3/g and an average pore diameter of 16.54 nm. The effect of temperature on sorption of THT, DMS, and TBM plus THT was examined over the wide range of temperatures (50400 °C). The sorption runs were performed in a fixed bed tubular reactor containing 1 mL of sorbent at ambient pressure. The sorbent was activated by 5% H2 diluted with nitrogen at 200 °C for 3 h. The model gases of THT (100 ppm), DMS (100 ppm), and TBM plus THT (24 and 56 ppm) diluted with methane were used to determine the sorption capacity of sulfur on the sorbent. The inlet concentrations were maintained the same during the experiments. The gas flow rate was 100 mL/min, and the GHSV was 6000 h-1. The concentrations of THT, DMS, and TBM plus THT in the inlet and outlet were analyzed by an online gas chromatograph equipped with a pulsed flame photometric detector (PFPD) using a SPB-1 fused silica capillary column (Supel Co.). The sorption capacity of sulfur on the sorbent was determined by the time of breakthrough when the concentration of sulfur at the moment of the first detection of sulfur was below about 10 ppb in the outlet gas. The sorbed samples of sulfur compounds were characterized by various physicochemical methods. The Brunauer-Emmett-Teller (BET) surface area and pore-size distribution of the samples were determined using a Micrometrics instruments (ASAP 2400), with nitrogen adsorption at -196 °C. To observe the changes in the morphology for sulfur-containing samples, scanning electron microscopic (SEM) studies were performed on a JEOL scanning electron microscope (JSM 6700F JEOL Japan, Inc.). Temperatureprogrammed desorption (TPD) of the sulfur-containing compounds was carried out by an automatic TPD apparatus equipped with a quadrupole mass detector (GENESYS 422). Sorption runs were performed at room temperature for 3 days using vaporized DMS, TBM, and THT gases. Then, the samples (0.1 g) were put in a quartz tube, and the temperature was elevated from 30 to 700 °C (14) Masuda, M.; Okada, O.; Tabata, T.; Hirai, Y.; Fujita, H. U.S. Patent 6,042,798, 2000. (15) Jun, K.-W.; Shen, W. J.; Rama Rao, K. S.; Lee, K. W. Appl. Catal., A 1998, 174, 231-238. (16) Rodriguez, J. A.; Kuhn, M.; Hrbek, J. Surf. Sci. 1997, 380, 397407. (17) Rodriguez, J. A.; Jirsak, T.; Chaturvedi, S.; Hrbek, J. Surf. Sci. 1998, 407, 171-188.

Figure 1. Effect of the temperature in THT, DMS, and TBM plus THT sorption on Cu/ZnO/Al2O3. Sulfur-containing gases: 100 ppm of THT, 100 ppm of DMS, and 24 ppm of TBM plus 56 ppm of THT in CH4, respectively.

at a rate of 10 °C/min in He flow. The surface of the sulfurcontaining sample was analyzed using ESCA (ESCA-LAB MK II) equipped with a hemispherical electron energy analyzer with multichannel detection, a Mg KR X-ray source, and optics for lowenergy electron diffraction (LEED).

Results and Discussion The sorption runs of THT, DMS, and TBM plus THT diluted with methane on Cu/ZnO/Al2O3 were carried out to estimate the sorption capacity of sulfur. Figure 1 shows the effect of temperature for the removal of THT, DMS, and TBM plus THT at ambient pressure. The sulfur capacities on Cu/ZnO/Al2O3 were determined by the time of breakthrough for THT, DMS, and TBM plus THT. As shown in Figure 1, the sorption capacities of THT and DMS were found to be increased with an increase of the temperature. The maximum amounts of sulfur compounds in THT and DMS were sorbed at 350 °C with capacities of 0.89 and 0.78 mmol/g, respectively. Also, the sulfur sorption capacity for TBM plus THT was determined by the time of THT breakthrough. The maximum amount of TBM plus THT was sorbed at 250 °C with a maximum capacity of 0.50 mmol/g. It is found that the sorption capacity for TBM plus THT on Cu/ZnO/Al2O3 was decreased up to 82.0% when compared with a previous study on the removal of TBM.18 It was found out during our earlier study18 that the sorption capacity of TBM increased with temperature and the maximum amount of sulfur compound was absorbed at 250 °C with a capacity of 8.84 mmol/g. This observation indicates that each odorant is sorbed competitively on Cu/ZnO/Al2O3 in coexistence of sulfur compounds and also shows that Cu/ZnO/Al2O3 has a selective sorption capacity for TBM. The typical breakthrough curves of THT and DMS for sorption on Cu/ZnO/Al2O3 at 250 °C are shown in Figure 2. The BET data of Cu/ZnO/Al2O3 after sorption of THT, DMS, and TBM plus THT are tabulated in Tables 1-3, respectively. In comparison with a reduced sample, BET surface areas of the samples sorbed at 350 °C were 81.6 m2/g (THT sorption) and 67.9 m2/g (DMS sorption). The surface area values were found to be decreased by about 2.7 and 18.9% after sorption of THT and DMS, respectively. After TBM plus THT sorption, the BET surface area of the sample sorbed at 250 °C was 64.2 m2/g and decreased by about 23.5%. As the sulfur sorption capacities for THT, DMS, and TBM plus THT were increased, (18) Kim, H.-T.; Kim, S. M.; Jun, K.-W.; Yoon, Y.-S.; Kim, J. H. Int. J. Hydrogen Energy 2006, manuscript submitted.

Desulfurization of Odorant-Containing Gases

Energy & Fuels, Vol. 21, No. 1, 2007 329 Table 4. Comparison of Adsorbed Amounts of Sulfur Compounds on Various Sorbents

Figure 2. Breakthrough curves of THT and DMS on Cu/ZnO/Al2O3 at 250 °C. Sulfur-containing gases: 100 ppm of THT and 100 ppm of DMS in CH4, respectively.

adsorbent

DMS (mmol/g)

TBM (mmol/g)

Na-Y (ref 3) H-β (ref 3) Ag(18)Na-Y (ref 1) activated carbon (ref 1) present paper

1.10 0.89 1.90 0.20 0.78

0.95 0.94 0.60 0.60 8.80

compounds. Table 4 provides the comparison of data of the amount of sulfur compounds adsorbed on various sorbents. Although the experimental conditions during measurements were not exactly similar, the data shown in Table 4 definitely give some idea in evaluating the capability of material in adsorbing sulfur compounds. The X-ray diffraction analysis (XRD) patterns of calcined and reduced samples are shown in Figure 3. The calcined sample showed reflections corresponding to CuO

Table 1. Characteristics of Cu/ZnO/Al2O3 Samples after THT Saturation

sample reduced saturated with THT at 150°C saturated with THT at 250°C saturated with THT at 300°C saturated with THT at 350°C saturated with THT at 400°C

BET surface area (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

83.8 85.1

0.45 0.44

21.4 22.9

87.4

0.43

19.6

85.2

0.43

20.2

81. 6

0.42

20.5

83.9

0.47

22.2

Table 2. Characteristics of Cu/ZnO/Al2O3 Samples after DMS Saturation

sample reduced saturated with DMS at 150°C saturated with DMS at 250°C saturated with DMS at 300°C saturated with DMS at 350°C saturated with DMS at 400°C

BET surface area (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

83.8 86.1

0.45 0.46

21.4 21.2

68.4

0.40

23.1

66.4

0.38

22.9

67.9

0.37

21.8

73.2

0.38

20.5

Table 3. Characteristics of Cu/ZnO/Al2O3 Samples after TBM Plus THT Saturation

sample reduced saturated with TBM plus THT at 150°C saturated with TBM plus THT at 200°C saturated with TBM plus THT at 250°C saturated with TBM plus THT at 300°C

BET surface area (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

83.8 74.4

0.45 0.40

21.4 21.6

65.8

0.39

23.7

64.2

0.32

19.9

73.9

0.38

20.5

total pore volumes were also found to be decreased. All of these observations indicate that the decrease of both the surface area as well as the total pore volume may probably be attributed to the formation of metal sulfides from the sorption of sulfur

Figure 3. XRD pattern of a calcined adsorbent (a) and a reduced adsorbent (b).

and ZnO phases. No reflections corresponding to any of the reported Al2O3 were observed in Figure 3, which confirms the noncrystalline nature of Al2O3. The XRD peak at 39.14° assigned to CuO because of a {200} reflection is broader than that of a peak at 31.77° assigned to ZnO because of a {100} reflection, which confirms the nanocrystalline nature of CuO particles. The reduced sample, however, showed a very broad reflection at 43.3° corresponding to a {111} plane, thereby suggesting the nanosize nature as well as better dispersion of Cu metal particles in the ZnO matrix. Thus, careful analyses of XRD of reduced samples indicate that the higher sorption capacities of Cu/ZnO/Al2O3 may result because of the nanosize nature and better dispersion of copper metal crystallites in the ZnO matrix. Thus, XRD results support observations of the results mentioned in Table 3. SEM photomicrographs of Cu/ZnO/Al2O3 material after sulfur sorption are depicted in parts A-C of Figure 4, respectively. The morphology of Cu/ZnO/Al2O3 sorbed by THT at different temperatures is found to be changed slowly from spherical shape grains to one containing rod-shaped grains as is evident from Figure 4A. However, in parts B and C of Figure 4, the morphology of the samples after sorption of DMS and TBM plus THT indicated a slight increase in the spherical grain size and extent of agglomeration with an increase of the temperature. Figure 5 shows the TPD profiles of DMS on Cu/ZnO/Al2O3 after sorption in DMS. The TPD profile of m/e ) 44 (CH2S) showed three peaks at 95, 147, and 250 °C, respectively. The peak of m/e ) 28 was observed at 150 °C, and its fragment was mainly attributed to C2H4. As shown in Figure 4, the TPD peak of m/e ) 32 (S) was observed in the temperature range of

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Energy & Fuels, Vol. 21, No. 1, 2007 331

Figure 4. (A) SEM micrographs of Cu/ZnO/Al2O3: (a) saturated with THT at 50 °C, (b) saturated with THT at 150 °C, (c) saturated with THT at 300 °C, and (d) saturated with THT at 350 °C. (B) SEM micrographs of Cu/ZnO/Al2O3: (a) saturated with DMS at 50 °C, (b) saturated with DMS at 150 °C, (c) saturated with DMS at 250 °C, and (d) saturated with DMS at 350 °C. (C) SEM micrographs of Cu/ZnO/Al2O3: (a) saturated with TBM plus THT at 50 °C, (b) saturated with TBM plus THT at 150 °C, (c) saturated with TBM plus THT at 200 °C, and (d) saturated with TBM plus THT at 250 °C.

Figure 5. TPD behavior of Cu/ZnO/Al2O3 after sorption of DMS.

350-460 °C. This observation indicates that the DMS is sorbed strongly on Cu/ZnO/Al2O3. The TPD profiles of TBM on the sorbent after sorption in TBM are shown in Figure 6. From the TPD of TBM, the peaks of m/e ) 15, 27, 29, 39, 41, 55, and 57 were observed at 110 °C and these fragments were assigned to CH3, (CH3)C, (CH3)CH2, C3H3, C3H5, (CH3)2CHC, and (CH3)3C, respectively. As shown in Figure 5, m/e ) 41 (C3H5, the main fragment of TBM) showed two peaks at 110 and 125

Figure 6. TPD behavior of Cu/ZnO/Al2O3 after sorption of TBM.

°C, respectively. The TPD curve of m/e ) 39 was also similar to that of m/e ) 41. In addition, the TPD peaks of m/e ) 39 and 41 were continuously detected in the temperature range (∼415 °C) after the second peaks were detected at 125 °C.

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Figure 7. TPD behavior of Cu/ZnO/Al2O3 after sorption of THT.

Kim et al.

sample sorbed of sulfur, S 2p peak remained at 171.8 eV. All of these results confirm the formation of sulfides because of the interaction of sulfur during the sorption process. The probable mechanism involves a chemical reaction between the sulfur compound and Cu/ZnO/Al2O3.16,17 This result includes the breaking of the C-S bond and the formation of a new bond, S-Cu (or Zn) bond, at high temperatures. The sulfur compounds are decomposed at higher temperatures, liberating S, and then, S binds with Cu and Zn in Cu/ZnO/Al2O3, leading to the formation of sulfides.1,17 The higher sorption capacity for THT, DMS, and TBM plus THT on Cu/ZnO/Al2O3 at high temperatures is attributed to an easier decomposition of the S-H and C-S bonds. In our earlier study,19 it was found out that the activation with hydrogen is an essential step to obtain higher sorption capacities in the case of coprecipitated reduced Cu/ ZnO/Al2O3 sorbents for the removal of TBM plus THT from the natural gas. The sorption capacities of 0.26 and 0.5 mmol/g were obtained for calcined and reduced samples at 200 °C in the case of TBM plus THT. The sorption capacity is found to be lower in calcined material as compared to the 200 °C reduced sample. These observations indicate that the copper metal surface provides better sorption sites compared to the copper oxide surface. Although coprecipitated reduced Cu/ZnO/Al2O3 sorbents showed a higher sorption capacity for THT, DMS, and TBM plus THT, however, the regeneration of these sorbents was found to be really difficult. The efforts are underway to find suitable sorbents that will not only give a high sorption capacity for sulfur compounds but can also be regenerated, which is an essential requirement for the sorbent to be useful for industrial applications. 4. Conclusion

Figure 8. Al 2p, Cu 2p, Zn 2p, and S 2p XPS spectra of Cu/ZnO/ Al2O3 acquired before and after sorption of DMS.

Figure 7 shows TPD profiles on Cu/ZnO/Al2O3 after sorption in THT. Upon the TPD of THT, the peaks of m/e ) 27, 39, 41, 45, 60, and 88 were observed at a low-temperature range (the peak temperature of 35 °C) compared with the desorption temperature of DMS and TBM. These fragments were assigned to (CH3)C, C3H3, C3H5, SCH, C2H4S, and C3H8S, respectively. Especially, the peak of m/e ) 45 was observed at a hightemperature range (470-540 °C). This broad TPD peak may be attributed to the decomposition of copper sulfide clusters formed on Cu/ZnO/Al2O3 after sorption in THT.1 Figure 8 displays Al 2p, Cu 2p, Zn 2p, and S 2p spectra taken before and after sorption of the sulfur compound (DMS) on Cu/ZnO/Al2O3. Upon the sorption of sulfur, there was a negative binding energy shift of ∼1.5 eV in the Cu 2p. This result implies that copper present on the surface of the sorbent is strongly combined with sulfur.16 A similar binding energy shift was also observed in the Al 2p spectrum acquired after sulfur sorption. In the case of the Zn 2p spectrum, the sorption of sulfur brought an increase of ∼0.6 eV in the Zn 2p peak position. From the

The sorption temperature plays an important role in the removal of sulfur compounds using Cu/ZnO/Al2O3. THT, DMS, and TBM plus THT are removed efficiently from methane by sorption on Cu/ZnO/Al2O3 at high temperatures. The removal of THT using Cu/ZnO/Al2O3 is more effective than that of DMS. Especially, the Cu/ZnO/Al2O3 shows the selective sorption capacity for TBM in coexistence with TBM and THT gases. The morphology change was observed on Cu/ZnO/Al2O3 after THT sorption over 300 °C. The morphology change of Cu/ZnO/ Al2O3 is related to the formation of metal sulfides because of the interaction of sulfur during the sorption process. The chemical reaction between the sulfur compound and Cu/ ZnO/Al2O3 includes the breaking of C-S and the formation of the bond S-Cu (or Zn) at high temperatures. The sulfur compounds are decomposed, and then, S binds with Cu and Zn in Cu/ZnO/Al2O3. The higher sorption capacity for THT, DMS, and TBM plus THT on Cu/ZnO/Al2O3 at high temperatures is attributed to the easier decomposition of the S-H and C-S bonds. Acknowledgment. The authors acknowledge the financial support of the “National RD&D Organization for Hydrogen and Fuel Cell” under “New and Renewable Energy R&D Programs” of the Ministry of Commerce Industry and Energy, Korea. EF060342B (19) Kim, H.-T.; Jun, K.-W.; Kim, S. M.; Potdar, H. S.; Yoon, Y.-S. Energy Fuel 2006, 20, 2170-2173.