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Energy & Fuels 2006, 20, 2170-2173
Co-Precipitated Cu/ZnO/Al2O3 Sorbent for Removal of Odorants t-Butylmercaptan (TBM) and Tetrahydrothiophene (THT) from Natural Gas Hyung-Tae Kim,† Ki-Won Jun,*,† Seung-Moon Kim,† Hari Shankar Potdar,† and Young-Seek Yoon‡ Micro-Chemical Technology Research Team, Korea Research Institute of Chemical Technology, Post Office Box 107, Yu-Seong, Daejeon 305-600, Korea, and Corporate R&D Center, SK Corporation, 140-1 Wonchon-dong Yuseong-gu, Daejeon 305-712, Korea ReceiVed May 24, 2006. ReVised Manuscript ReceiVed July 27, 2006
The removal of sulfur-containing odorants from natural gas has been investigated by using sorption. The CuO/ZnO/Al2O3 sorbents were prepared by a coprecipitation/aging method. A reduced sorbent, namely, Cu/ ZnO/Al2O3, has a sorption capacity of 0.5 mmol/g at 250 °C when the sodium content is kept below 0.027 wt %. The high sorption capacity of the sorbent is due to the better dispersion of nanocrystalline copper metal particles in a zinc oxide matrix as confirmed by X-ray diffraction analysis. The Brunauer-Emmett-Teller surface area of calcined material (Cu/ZnO/Al2O3) increased systematically from 18.38 to 104.26 m2/g with the lowering of the residual “Na” concentration. The temperature-programmed reduction study revealed that high incorporation of “Na” made the reduction of the CuO phase difficult in unwashed sorbent. However, as the “Na” content is lowered below 0.027 wt %, the reduction of CuO particles occurred at much lower temperatures because of the strong interaction of nanosized CuO with ZnO particles.
Introduction Natural gas containing methane as a main constituent is generally used to produce a clean hydrogen fuel for fuel cells that are expected to become one of the most effective energysaving power-generating systems.1,2 However, supported Ruor Ni-based catalysts applied for the steam-reforming process to produce hydrogen are poisoned by sulfur compounds, e.g., t-butylmercaptan (TBM) and tetrahydrothiophene (THT), that are added intentionally in parts per million level as odorants to give people a warning of a leak in case they have to be removed.3 Therefore, the desulfurization of hydrocarbon fuel is absolutely essential before the introduction of the same to the reforming process in the fuel cell system.4-6 For this purpose, the catalytic hydrodesulfurization to H2S on zinc oxide has been attempted.3 However, in this process, it is necessary to keep both the catalyst and the absorbent at ∼300 °C and an addition of hydrogen for hydrodesulfurization is needed. In addition, zinc oxide periodically has to be replaced. To solve this problem, a process comprising the adsorption of sulfur compounds at room temperature and the regeneration of the adsorbent has been proposed and activated carbon and manganese oxide have been tried.7,8 However, these materials do not * 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) Lampert, J. J. Power Sources 2004, 131, 27-34. (3) Wakita, H.; Tachibana, Y.; Hosaka, M. Microporous Mesoporous Mater. 2001, 46, 237-247. (4) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207-238. (5) Ma, X.; Sun, X.; Song, C. Catal. Today 2002, 77, 107-116. (6) Song, C. Catal. Today 2003, 86, 211-263. (7) Futami, H.; Hashizume, Y. Proc. Int. Gas Res. Conf. 1990, 1592.
have a high stability and enough sulfur adsorption capacities.7 Zeolite-based materials are also tried as adsorbents at ambient temperature and pressure.3,8 However, it is difficult to apply these materials under water-containing conditions because they have a strong affinity for water. It is reported that the removal of sulfur-containing compounds is also achieved by catalytic processes operated at elevated temperatures > 300 °C and H2 pressures of 20-100 atm using Co-Mo/Al2O3 or Ni-Mo/Al2O3 catalysts.9 Considering the above-mentioned facts, there is a need to find better selective adsorbents having a high adsorption capacity for odorants, namely, TBM and THT. The goal of the present investigation is to find the suitability of the coprecipitated reduced Cu/ZnO/Al2O3 sorbent in desulfurization of TBM and THT in the fuel. The other aspect of the present investigation is to optimize desulfurization activity by controlling sodium content, surface area, pore volume, pore-size distribution and interaction of CuO with ZnO particles in the catalyst, and operating temperature. For this purpose, various characterization techniques are employed to characterize calcined as well as reduced sorbents. All of these results pertaining to synthesis, characterization of sorbents, and their performance in desulfurization of TBM and THT are presented in this paper. Experimental Section The sorbents for desulfurization of TBM and THT are synthesized by a conventional coprecipitation/aging process.10 For the preparation of the Cu/ZnO/Al2O3 sorbent, a mixed aqueous solution (8) 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. (9) Hernandez-Maldonado, A. J.; Yang, F. H.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 56, 111-126. (10) Masuda, M.; Okada, O.; Tabata, T.; Hirai, Y.; Fujita, H. U.S. Patent 6,042,798, 2000.
10.1021/ef060236+ CCC: $33.50 © 2006 American Chemical Society Published on Web 08/23/2006
Cu/ZnO/Al2O3 Sorbent for RemoVal of Odorants of copper, zinc, and aluminum nitrates in the molar ratio of 1:1:0.3 and an aqueous solution of sodium carbonate are dropped simultaneously into a flask containing 500 mL of deionized water while stirring to form a precipitate at room temperature.11 The precipitate thus obtained is aged for 2 h at room temperature. The pH ∼7.0 is maintained during the experiment. The precipitates are either used without washing with water, i.e., unwashed sample, or washed several times with deionzed water by varying the ratio of deionized water (DIW) to the slurry solution to control the Na+ content in the precursors, i.e., washed sample. The precipitate precursors are dried at 120 °C overnight and finally calcined at 300 °C/12 h in air to obtain the CuO/ZnO/Al2O3 sorbent. The experiments are performed to study the effect of the sodium content in the sorbents and the temperature on sorption of TBM and THT. For this purpose, sorption runs were taken in a fixed bed tubular reactor containing 1 mL of sorbent at normal pressure. In a typical experiment, the sorbents (CuO/ZnO/Al2O3) are activated by the reduction process at 200 °C for 3 h in a flow of 5% H2 diluted with nitrogen. The model gas of 80 ppm diluted with methane is used to determine the sulfur sorption capacities on the Cu/ ZnO/Al2O3 sorbent. The gas flow rate of 100 mL/min with GHSV of 6000 h-1 is maintained during the experiment. The concentration of (TBM plus THT) in the inlet and outlet is 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 sulfur sorption capacity on Cu/ZnO/Al2O3 is determined by the time of breakthrough when the concentration of sulfur at the moment of first detection of (TBM plus THT) is below 10 ppb in the outlet gas. Powder X-ray diffraction patterns were recorded on a Rigaku D/MAX 2000V X-ray diffraction meter with a copper target operating at 40 kV and 40 mA. The spectra were recorded with a step size of 0.04 and step time of 15 s. The X-ray diffraction (XRD) study of as-dried, calcined, and reduced samples was undertaken to know the phase composition and dispersion in these materials. The Brunauer-Emmett-Teller (BET)-specific surface area, pore size, and its distribution in these sorbents are measured by nitrogen adsorption at -196 °C using a Micrometrics instrument (ASAP 2400 USA, Inc.). The atomic absorption spectroscopy (AAS) analysis technique is employed to determine the sodium content in the as-dried precipitates by using a Perkin-Elmer 2380 atomic absorption spectrometer. For temperature-programmed reduction (TPR) experiments, the sorbent samples were first activated under He flow at 400 °C for 1 h, followed by cooling to 50 °C. A gas mixture of 5% H2/argon is passed over the samples at a flow rate of 30 mL/min. The temperature of the samples was linearly increased at a rate of 10 °C/min. Any water that is formed during the reduction was trapped by a molecular Cu. The hydrogen consumption was continuously monitored by a thermal conductivity detector. The TPR study was performed to determine the interaction of Cu metal particles with ZnO and to understand the extent in reduction temperature because of the interaction of these particles.
Results and Discussion A bluish colored precursor of mixed hydroxides containing Cu, Zn, and Al cations is precipitated at pH ∼7.0 when sodium carbonate and nitrate salts solution are dropped in deionized water while stirring at room temperature by following the chemical reaction:
Cu(NO3)2 + Zn(NO3)2 + Al(NO3)3 + 3.5Na2CO3 + 7H2O f {Cu(OH)2 + Zn(OH)2 + Al(OH)3} + 7NaNO3 + 3.5CO2 + 3.5H2O (1) The precipitate thus obtained by the chemical reaction (eq 1) is aged at room temperature, which helped to homogenize it (11) Padmaja, P.; Pillai, P. K.; Warrier, K. G. K. J. Porous Mater. 2004, 11, 147-155.
Energy & Fuels, Vol. 20, No. 5, 2006 2171
Figure 1. Effect of sodium content in Cu/ZnO/Al2O3 for desulfurization of sulfur-containing gas. Feed: 24 ppm of TBM plus 56 ppm of THT in CH4.
Figure 2. Effect of temperature on (TBM plus THT) sorption on Cu/ ZnO/Al2O3. Feed: 24 ppm of TBM plus 56 ppm of THT in CH4.
because of the slow-ripening process. The precipitate is further processed without washing and with washing with deionized water to reduce the contamination of sodium ions. The AAS analysis technique is employed to determine the sodium content in as-dried precipitates. The effect of the sodium content in the Cu/ZnO/Al2O3 sorbent for desulfurization of sulfur compounds is depicted in Figure 1. It is seen from Figure 1 that, as the sodium content is lowered, the sulfur sorption capacity is increased and reached a maximum value of 0.5 mmol/g when the sodium content associated with it is kept below 0.027 wt % in the adsorbent. Thus, it is clear from Figure 1 that the coprecipitated sorbent is effective in removing sulfur from TBM and THT when the sodium content is kept below 0.027 wt % and a further decrease in the sodium content does not help in improving the sorption capacity. This is an important observation that sodium plays a crucial role in controlling the sorption capacity of sulfur compounds in Cu/ZnO/Al2O3-based sorbents. It is inferred from these observations that the Cu/ZnO/Al2O3 sorbents are effective in lowering sulfur levels from natural gas, which are harmful in poisoning the reforming catalyst. The effect of temperature on sorption of (TBM plus THT) in Cu/ZnO/ Al2O3 sorbents is shown in Figure 2. It is seen from Figure 2 that the sorption capacity is increased with temperature, reached a maximum value of 0.5 mmol/g at 250 °C, and then decreased with a further increase in the temperature. All of the above experimental results indicate that the sorption capacity of 0.5 mmol/g is maintained when the sodium level is kept below 0.027 wt % and the operating temperature is kept around 250 °C. To
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Figure 3. XRD patterns of Cu/ZnO/Al2O3. Fresh samples: (a) DIW/ solution ) 0, (b) DIW/solution ) 2, (c) DIW/solution ) 4, (d) DIW/ solution ) 8, and (e) DIW/solution ) 10. Reduced samples: (f) DIW/ solution ) 0 (reduced at 330 °C) and (g) DIW/solution ) 10 (reduced at 200 °C).
correlate the high sorption capacity of the Cu/ZnO/Al2O3 sorbents with physical and chemical properties, the systematic characterization of these materials was carried out by using various techniques, and the results are analyzed carefully to correlate them with the observed property of these materials, e.g., surface area, pore size and its distribution, the nature of phases present and their interaction in lowering the reduction temperature, etc. The XRD patterns of calcined and reduced samples without and with washing with deionized water are shown in parts a-g of Figure 3, respectively. The unwashed calcined material (Figure 3a) showed the reflections corresponding to CuO, ZnO, and NaNO3 phases. However, the reflection assigned to the NaNO3 phase is found to be absent in washed samples (parts b-e of Figure 3). This observation indicates that the concentration of NaNO3 could be below the detection limit of XRD. To quantify the contamination level of the sodium content, it is estimated by AAS analysis of unwashed/washed samples, and its effect on desulfurization activity is discussed earlier. The XRD peak assigned to the CuO phase because of a {200} reflection appearing at 39.14° is found to be broader as compared to the ZnO peak corresponding to a {100} reflection appearing at 31.77° in calcined material, which indicates the nanocrystalline nature of CuO particles. The reduced sample showed a very broad reflection of Cu metal particles appearing at 43.3° because the {111} plane is very broad, thereby confirming the presence of nano-sized Cu metal crystallites and its better dispersion in the ZnO matrix. No lines corresponding to any of the reported Al2O3 phases are seen in XRD patterns in parts a-g of Figure 3, thereby indicating its noncrystalline nature. The pretreatment run was performed at 330 °C for an unwashed sample to confirm the reduction of CuO to Cu metal particles (Figure 3f). From the comparison of the XRD pattern with those of other samples, it is apparent that all CuO is converted into Cu metal particles completely during the reduction process at 330 °C because reflections corresponding to the Cu metal are seen in Figure 3f. The variation in surface area, total pore volume, and average pore diameter in both calcined and reduced samples is presented in parts a and b of Figure 4. The data are compiled in Table 1. In the case of washed samples, a definite trend is observed from the Table 1. The BET surface areas are increased from 18.38 to 104 m2/g. The pore vol-
Figure 4. (a) N2 adsorption/desorption isotherms of Cu/ZnO/Al2O3 with washing conditions. (b) Bopp-Jancso-Heinzinger (BJH) poresize distribution of Cu/ZnO/Al2O3 with washing conditions. Table 1. Characteristics of Cu/ZnO/Al2O3 Samples
samples )0 DIW/solution ) 2 DIW/solution ) 4 DIW/solution ) 8 DIW/solution ) 10 reduced sample (DIW/solution ) 10) DIWa/solution
a
BET surface area (m2/g)
total pore volume (cm3/g)
average pore diameter (nm)
18.38 60.32 99.67 100.20 104.26 83.81
0.088 0.332 0.428 0.398 0.431 0.449
19.20 22.05 17.20 15.91 16.54 21.41
DIW: deionized water.
ume is found to be increased from 0.088 to 0.431 cm3/g with the washing treatment. The pore size in the range of 19.2016.54 nm is obtained. However, the maximum surface area of ∼104.26 m2/g is obtained after the fourth washing when the sodium content is reduced to 0.006 wt % as measured by AAS analysis. The nature of the N2 adsorption and desorption isotherm remains type IV as seen from Figure 4a, with changes in hysteresis thereby indicating changes in the pore structure in these materials.11 The reduced sample showed the similar trend in the adsorption isotherm with surface area, ∼83.81 m2/g; pore diameter, ∼21.41 nm; and pore volume, ∼0.449 cm3/g. Parts a-e of Figure 5 show the TPR profiles of the sorbents without and with washing treatment. The reduction peak appearing at 450 °C assigned to CuO is lowered to 250 °C after washing with water. From the TPR results, it is readily seen that the samples are reduced at much lower temperatures because of the removal of Na cations associated with the adsorbents. This may be related to the synergy effect between CuO and ZnO as
Cu/ZnO/Al2O3 Sorbent for RemoVal of Odorants
Energy & Fuels, Vol. 20, No. 5, 2006 2173 Table 2. Sulfur Sorption Capacities of Cu/ZnO/Al2O3 with Reduction Conditions samples
reduction temperature (°C)
sulfur capacity (mmol/g)
Cu/ZnO/Al2O3-w-0La Cu/ZnO/Al2O3-w-0L Cu/ZnO/Al2O3-w-5Lb Cu/ZnO/Al2O3-w-5L Cu/ZnO/Al2O3-w-5L
200 330 none 200 330
0.0075 0.0054 0.2630 0.5000 0.3352
a w-0L: identical sorbent with DIW/solution ) 0, unwashed sample. w-5L: identical sorbent with DIW/solution ) 10, washed sample with DIW 5L.
b
Figure 5. TPR profiles of the sorbents with varying sodium contents. (a) DIW/solution ) 0, (b) DIW/solution ) 2, (c) DIW/solution ) 4, (d) DIW/solution ) 8, and (e) DIW/solution ) 10.
well as the support effect of Al2O3.12 The ease of reduction of CuO may be related to the interaction of nano-sized particles of CuO with ZnO and Al2O3. The reduction peak shifts to a lower temperature after washing the samples with water, suggesting that the presence of sodium nitrate causes some difficulty in the reduction of CuO to Cu0. Furthermore, the TPR of the unwashed sample showed two overlapped reduction peaks, thereby indicating the presence of two kinds of CuO-related species that coexist. The low-temperature peak may be assigned to CuO species that are interacting with ZnO and Al2O3, and the high-temperature peak may be related to uninteracted CuOrelated species.12 Thus, the careful analysis of these results confirm that the sodium nitrate inhibits the interaction of CuO with ZnO and Al2O3 particles, creating some difficulty in the reduction of the CuO phase that decreases the sorption capacity of reduced Cu/ZnO/Al2O3 sorbents. To reduce CuO to Cu metal particles completely, the activation runs of both unwashed and washed samples were carried out at higher temperature, i.e., 330 °C for 3 h. The measured sorption capacities are tabulated in Table 2. It is clear from the Table 2 that the sorption capacity in the water-washed sample is found to be 0.3352 mmol/g, which is lower than that observed for the sample reduced at 200 °C, i.e., 0.5000 mmol/g. However, the unwashed sample gave a capacity on the order of 0.0054 mmol/g after reduction. Furthermore, the XRD of these samples (parts f and g of Fig(12) Jun, K. W.; Shen, W. J.; Rama Rao, K. S.; Lee, K. W. Appl. Catal., A 1998, 174, 231-238.
ure 3) indicated the presence of copper-metal particles with a better dispersion in washed samples because peaks corresponding to them are found to be broad in water-washed samples. The washed sample without any reduction treatment gave a sorption capacity of 0.2630 mmol/g, which is much lower than that observed in the reduced washed samples at two different temperatures as seen from Table 2. All of these above results confirm that the residual sodium plays an important role in controlling the dispersion of copper metal particles, its interaction with ZnO and Al2O3, and its reducibility. The observation that the water-washed reduced samples showed a higher sorption capacity on the order of 0.5000 mmol/g than in the washed sample without any reduction treatment, i.e., 0.2630 mmol/g, indicates that the copper metal surface provides better sorption sites compared to the copper oxide surface. Conclusion TBM and THT from natural gas can be removed using sorbent Cu/ZnO/Al2O3 at low as well as high temperature. The sorbent Cu/ZnO/Al2O3 prepared by a coprecipitation/aging method has a maximum sulfur sorption capacity of 0.5 mmol/g at 250 °C. The enhanced sorption capacity of TBM and THT in these materials is due to the controlled pore size and its distribution, high surface area and better dispersion of fine CuO particles, and their strong interaction with ZnO and Al2O3 matrixes when Na content is maintained lower than 0.027 wt % in the sorbent. 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. EF060236+
