Adsorptive Desulfurization by Copper Species within Confined Space

Sep 30, 2010 - highly dispersed on SBA-15 by use of SPA strategy. Such CuO forms a smooth layer coated on the internal walls of SBA-15, which contribu...
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Adsorptive Desulfurization by Copper Species within Confined Space Wen-Hang Tian, Lin-Bing Sun, Xue-Lin Song, Xiao-Qin Liu,* Yu Yin, and Gu-Se He State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China Received May 10, 2010. Revised Manuscript Received September 13, 2010 Copper species were incorporated into SBA-15 by solid-state grinding precursor with as-prepared mesoporous silica (SPA). The obtained materials (CuAS) were well-characterized by XRD, TEM, N2 adsorption, H2-TPR, IR, and TG and compared with the material derived from calcined SBA-15 (CuCS). Surprisingly, CuO up to 6.7 mmol 3 g-1 can be highly dispersed on SBA-15 by use of SPA strategy. Such CuO forms a smooth layer coated on the internal walls of SBA-15, which contributes to the spatial order and results in less-blocked mesopores. However, the aggregation of CuO takes place in CuCS material containing 6.7 mmol 3 g-1 copper, which generates large CuO particles of 21.4 nm outside the mesopores. We reveal that the high dispersion extent of CuO is ascribed to the abundant silanols, as well as the confined space between template and silica walls provided by as-prepared SBA-15. The SPA strategy allows template removal and precursor conversion in one step, avoids the repeated calcination in conventional modification process, and saves time and energy. We also demonstrate that the CuAS material after autoreduction exhibits much better adsorptive desulfurization capacity than CuCS. Moreover, the adsorption capacity of regenerated adsorbent can be recovered completely.

Introduction Deep desulfurization of transportation fuels has become very urgent for the petroleum refining industry due to the increasing stringent environmental regulations and future applications in fuel cells.1-5 Hydrodesulfurization is a conventional method for sulfur elimination and is highly efficient in removing thiols and sulfides, but is less effective for thiophenic compounds. Fortunately, deep desulfurization via π-complexation adsorption can overcome the shortcomings of hydrodesulfurization, since it can capture thiophenic compounds selectively at ambient conditions. Cuprous species dispersed on various supports including alumina,6,7 zeolite,8-12 and activated carbon13 have been used as π-complexation adsorbents and are active in adsorptive desulfurization. The formation of π-complexes between adsorbate molecules and cuprous ions has been proven to be an essential step for the removal of thiophenic compounds. Therefore, two factors are believed to correlate well with the adsorption capacity of an adsorbent. The first factor is the amount of cuprous species, since more cuprous species mean the possible formation of more π-complexes. The second factor is the dispersion extent of cuprous species, taking into account that a thiophene molecule *Corresponding author. Telephone: þ86-25-83587178; fax: þ86-2583587191; e-mail: [email protected].

(1) Song, C. Catal. Today 2003, 86, 211. (2) Sun, Y.; Prins, R. Angew. Chem., Int. Ed. 2008, 47, 8478. (3) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207. (4) Yang, Y.; Lu, H.; Ying, P.; Jiang, Z.; Li, C. Carbon 2007, 45, 3042. (5) Kim, J. H.; Ma, X.; Zhou, A.; Song, C. Catal. Today 2006, 111, 74. (6) Hernandez-Maldonado, A. J.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 61, 212. (7) Yang, X.; Erickson, L. E.; Hohn, K. L.; Jeevanandam, P.; Klabunde, K. K. Ind. Eng. Chem. Res. 2006, 45, 6169. (8) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79. (9) Hernandez-Maldonado, A. J.; Yang, R. T. J. Am. Chem. Soc. 2004, 126, 992. (10) Zhang, Z. Y.; Shi, T. B.; Jia, C. Z.; Ji, W. J.; Chen, Y.; He, M. Y. Appl. Catal., B 2008, 82, 1. (11) King, D. L.; Li, L. Catal. Today 2006, 116, 526. (12) Shan, J. H.; Liu, X. Q.; Sun, L. B.; Cui, R. Energy Fuels 2008, 22, 3955. (13) Ania, C. O.; Bandosz, T. J. Carbon 2006, 44, 2404.

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can form a π-complex only with the “touchable” cuprous ion located on the surface. The host with a high surface area and large pore volume is thus desirable from the point of view of accommodation and subsequent dispersion of cuprous species. Since the discovery of mesoporous silicas M41S, a series of mesoporous materials with various pore symmetries (for example, hexagonal, cubic, lamellar, and wormhole) have been synthesized by the surfactant templating method.14-18 These mesoporous silicas have high surface areas and large pore volumes and are of great interest for adsorption, sensing, and catalysis.19-25 As a result, mesoporous silicas are promising hosts for the preparation of π-complexation adsorbents.26-28 Many attempts have been made to disperse guest species on mesoporous silicas up to now. Chen et al.29 investigated the dispersion behavior of CuO on mesoporous silica systematically. They found that about 10 wt % of CuO can be well-dispersed on SBA-15 by either grafting or deposition precipitation, while impregnation led to the generation of CuO particles. Wang et al.30 employed a solvent-free method and realized the dispersion of 10 wt % of Cr2O3 or 15 wt % of CuO on SBA-15. These studies provide interesting approaches to (14) Asefa, T.; MacLachan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (15) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Science 1998, 282, 1302. (16) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500. (17) Liu, X. Y.; Tian, B. Z.; Yu, C. Z.; Gao, F.; Xie, S. H.; Tu, B.; Che, R. C.; Peng, L. M.; Zhao, D. Y. Angew. Chem., Int. Ed. 2002, 41, 3876. (18) Wan, Y.; Zhao, D. Chem. Rev. 2007, 107, 2821. (19) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615. (20) Davis, M. E. Nature 2002, 417, 813. (21) Stein, A. Adv. Mater. 2003, 15, 763. (22) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (23) Ma, X.; Wang, X.; Song, C. J. Am. Chem. Soc. 2009, 131, 5777. (24) Sen, T.; Sebastianelli, A.; Bruce, I. J. J. Am. Chem. Soc. 2006, 128, 7130. (25) Wang, Y. M.; Wu, Z. Y.; Shi, L. Y.; Zhu, J. H. Adv. Mater. 2005, 17, 323. (26) Dai, W.; Zhou, Y.; Li, S.; Li, W.; Su, W.; Sun, Y.; Zhou, L. Ind. Eng. Chem. Res. 2006, 45, 7892. (27) Wang, Y. H.; Yang, R. T.; Heinzel, J. M. Chem. Eng. Res. 2008, 63, 356. (28) Wang, Y. H.; Yang, R. T.; Heinzel, J. M. Ind. Eng. Chem. Res. 2009, 48, 142. (29) Chen, L. F.; Guo, P. J.; Zhu, L. J.; Qiao, M. H.; Shen, W.; Xu, H. L.; Fan, K. N. Appl. Catal., A 2009, 356, 129. (30) Wang, Y. M.; Wu, Z. Y.; Zhu, J. H. J. Solid State Chem. 2004, 177, 3815.

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disperse guest species on mesoporous silicas, but the dispersion amount is relatively low. It should be stated that mesoporous silicas after the removal of template are usually used as hosts, and less attention has been paid to the as-prepared materials containing template. By incorporating amine into as-prepared MCM-41 occluded with template, Yue et al.31 produced a new kind of CO2 capturer, which is highly efficient in the adsorption of CO2. Apparently, there exists an extraordinary microenvironment between template and silica walls in as-prepared mesoporous materials. The design and fabrication of π-complexation adsorbents by use of such a confined space is highly expected. The cuprous species used for π-complexation adsorption can be obtained either by the direct introduction of cuprous compounds or by the reduction of supported cupric compounds. According to the recent report of Yang’s group,27 the adsorbents prepared from cupric compounds reduction have much better regeneration ability as compared with those derived from cuprous compounds. Therefore, we first introduce cupric species to SBA15 by solid-state grinding precursor with as-prepared mesoporous silica (SPA) in the present study. The obtained materials (denoted by CuAS) are well-characterized by various approaches and compared with those prepared from SBA-15 after template removal (denoted by CuCS). Surprisingly, 6.7 mmol of CuO is highly dispersed per gram of SBA-15 (namely, 35 wt %) by use of the confined space between silica walls and template. To the best of our knowledge, it is the first report of such a high dispersion amount of CuO. This SPA strategy allows the template removal and precursor conversion in one step, avoids the repeated calcination in conventional modification process, and saves time and energy. We also demonstrate that the CuAS material after autoreduction exhibits much better adsorptive desulfurization capacity than CuCS, and that the adsorption capacity of regenerated adsorbent can be recovered completely.

Experimental Section Materials Synthesis. Mesoporous silica SBA-15 was synthe-

sized according to the procedure reported by Zhao et al.32 In a typical synthesis, 2 g of triblock copolymer P123 (EO20PO70EO20) was dissolved in 75 g of 1.6 M HCl aqueous solution with stirring at 40 °C. Then, 4.25 g of tetraethylorthosilicate (TEOS) was added to the homogeneous solution and stirred at this temperature for 24 h. Finally, the temperature was heated to 100 °C and held at this temperature for 24 h under static condition. The as-prepared sample was recovered by filtration, washed with water, and airdried at room temperature. Thermogravimetry (TG) analysis shows a weight loss of 50% in the range 150-550 °C due to the decomposition of template. This weight loss is consistent with the reported value (52%),33 indicating that the template is preserved in the pores of SBA-15. The removal of template was carried out in an air flow at 500 °C for 5 h with a heating rate of 2 °C 3 min-1. The precursor Cu(NO3)2 3 3H2O was incorporated into asprepared SBA-15 by solid-state grinding at ambient conditions for 30 min according to the reported method.25,34 The homogeneous powder was calcined in an air flow at 500 °C for 5 h with a heating rate of 2 °C 3 min-1. The obtained material was denoted as CuAS(n), where n represents the molar amount of copper per gram of SBA-15. For comparison, Cu(NO3)2 3 3H2O was also introduced to templatefree SBA-15 followed by calcination as described above. The (31) Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Y.; Wang, Z., J.; Zhu, J. H. Chem.; Eur. J. 2008, 14, 3442. (32) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (33) Kleitz, F.; Schmidt, W.; Schuth, F. Microporous Mesoporous Mater. 2003, 65, 1. (34) Jiang, Q.; Wu, Z. Y.; Wang, Y. M.; Cao, Y.; Zhou, C. F.; Zhu, J. H. J. Mater. Chem. 2006, 16, 1536.

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Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of SBA-15, CuAS, and CuCS samples. resulting material was denoted by CuCS(n), where n represents the molar amount of copper per gram of SBA-15. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded using a Bruker D8 Advance diffractometer with Cu KR radiation in the 2θ ranges from 0.5° to 10° and 10° to 80° at 40 kV and 40 mA. Transmission electron microscopy (TEM) was performed on a JEM-2010 UHR electron microscope operated at 200 kV. The N2 adsorption-desorption isotherms were measured using a Belsorp II system at -196 °C. The samples were degassed at 300 °C for 3 h prior to analysis. The BrunauerEmmett-Teller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The pore diameter was calculated from the adsorption branch by using the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared (IR) measurements were performed on a Nicolet Nexus 470 spectrometer by means of the KBr pellet technique. The spectra were collected with a 2 cm-1 resolution. TG analysis was conducted on a thermobalance (STA-499C, NETZSCH). About 10 mg of sample was heated from 35 to 900 °C in an air flow (25 mL 3 min-1). H2-temperature programmed reduction (TPR) experiments were conducted on a BELSORP BEL-CAT-A apparatus. About 50 mg of sample was pretreated at 200 °C under He for 2 h. After cooling to room temperature in a He atmosphere, the gas was switched to 10% H2/He mixed gas (30 mL 3 min-1). The sample was heated to 600 °C at a rate of 10 °C 3 min-1, and the amount of H2 consumed was detected by a thermal conductivity detector (TCD). Adsorptive Test. Thiophene was used as the representative of sulfur contaminants. The model fuel containing 550 ppmw (parts per million by weight) sulfur was prepared by mixing thiophene with isooctane. The desulfurization capacity of materials was evaluated on the basis of breakthrough curves. Experiments were performed in a vertical quartz column with a supporting quartz grid. All adsorptive tests were conducted at room temperature. The testing fuel was pumped up with a mini creep pump. The materials CuAS and CuCS were autoreduced in situ at 700 °C for 12 h in an Ar flow prior to adsorption. After cooling to room temperature, the feed was switched to the model fuel and the feed rate was kept at 10 mL 3 h-1. Effluent solutions were collected at regular intervals until saturation was reached. The sulfur content in effluent solutions was determined with a Varian 3800 gas chromatograph (GC) equipped with a pulsed-flame photometric detector (PFPD). A calibration curve was prepared to correct the GC results. Breakthrough curves were generated by plotting the normalized sulfur concentration versus the cumulative fuel volume. The normalized concentration (c/c0) was obtained from the detected content (c) divided by the initial content (c0), and the cumulative fuel volume was normalized by the adsorbent weight. The adsorption capacity was calculated by integral calculus DOI: 10.1021/la101856d

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sample

2

-1

SBET (m 3 g )

Vp (cm3 3 g-1)

Dp (nm)

a0a (nm)

dCuOb (nm)

H2 consumedc mmol H2 3 (g SBA-15)-1

SBA-15 781 1.118 8.0 11.3 1.5 CuAS(1.7) 582 0.955 8.0 11.9 n.d.d CuAS(3.3) 509 0.799 8.0 11.9 n.d. 3.2 CuAS(6.7) 389 0.707 8.0 11.9 n.d. 6.4 CuAS(10.0) 322 0.570 7.1 11.9 3.5 10.6 CuCS(6.7) 419 0.631 8.0 11.3 21.4 6.7 a Unit cell constant calculated according to a0 = 2  3-1/2  d100. b CuO crystallite size calculated by the Scherrer formula. c Calculated from TPR results. d Below the detection limit of XRD.

Figure 2. TEM images of (A) CuAS(6.7) and (B) CuCS(6.7) samples. according to the reported method.6 Regeneration of the spent adsorbent was carried out in situ. The saturated adsorbent by model fuel was heating in flowing air at 450 °C for 5 h, followed by treating in flowing Ar at 700 °C for 12 h. The regenerated adsorbent was then cooled to room temperature for the desulfurization of model fuel again.

Results Figure 1A shows the low-angle XRD patterns of SBA-15 and copper-containing samples. All samples have an intense diffraction line accompanied with two weak lines indexed as (100), (110), and (200) reflections, respectively, corresponding to a two-dimensional hexagonal pore regularity of a p6mm space group. As compared with parent SBA-15, the samples CuAS(10.0) and CuCS(6.7) possess obviously weaker intensity of the (100) reflection. Surprisingly, the (100) reflection intensity of CuAS samples containing 1.76.7 mmol 3 g-1 copper is stronger than that of SBA-15. It can also be seen from Figure 1A that the d-spacings of CuAS samples shift toward higher values, while that of CuCS is identical with SBA-15. Further calculation shows that the unit cell constant (a0) is 11.3 nm for SBA-15 and CuCS and 11.9 nm for CuAS as listed in Table 1. The wide-angle XRD pattern of parent SBA-15 presents a single broad diffraction line centered at 23°, which can be assigned to amorphous silica (Figure 1B). No new diffraction lines appear on CuAS(1.7), CuAS(3.3), and CuAS(6.7). However, some weak peaks originated from CuO (JCPDS 80-1916) emerge on CuAS(10.0). These results mean that CuO up to 6.7 mmol 3 g-1 can be well-dispersed on SBA-15 by the SPA strategy. The sample CuCS(6.7) derived from calcined SBA-15 exhibits an intense crystalline phase of CuO, indicating that large amounts of CuO are impossible to disperse by the conventional method. Table 1 displays the crystallite size calculated by the Scherrer formula. The crystallite size of CuO in CuAS(10.0) is 3.5 nm, while CuO in CuCS(6.7) exhibits a much larger crystalline size of 21.4 nm. 17400 DOI: 10.1021/la101856d

TEM provides another important technique to characterize the long-range channel ordering. For comparison, the TEM image of calcined SBA-15 was first recorded and shown in Figure S1 in the Supporting Information. Such an image indicates the highly ordered mesostructure of calcined SBA-15. Also, the ordered hexagonal arrays of mesopores with uniform pore size and wall thickness can be judged from the white-dark contrast. As illustrated in Figure 2A, the periodic mesostructure is wellpreserved in the CuAS sample, being consistent with the result of XRD. No particles can be observed, which means that CuO is highly dispersed on the host SBA-15. On the contrary, the TEM image of CuCS in Figure 2B shows some particles, which implies the aggregation of CuO. These aggregated particles may give rise to XRD diffraction lines of CuO in the wide-angle range. Also, the sample CuAS(6.7) exhibits a better contrast between pore walls and pore space than CuCS(6.7). Figure 3A depicts the N2 adsorption-desorption isotherms of SBA-15 before and after copper modification. The isotherms of CuAS(1.7), CuAS(3.3), and CuAS(6.7) are of type IV with an H1 hysteresis loop, which is characteristic of materials with cylindrical mesopores. These three samples have a steep increase in adsorption at the relative pressure (p/p0) of 0.6-0.8, indicative of the similar large mesopores and narrow pore size distributions as that of parent SBA-15. Interestingly, no shift of the inflections toward lower p/p0 values occurs, corresponding to an identical pore diameter (8.0 nm) of SBA-15 and CuAS samples containing 1.7-6.7 mmol 3 g-1 copper as shown in Figure 3B and Table 1. For the sample CuAS(10.0), however, the desorption branch of the isotherm is apparently delayed because of the aggregation of CuO. It is worth noting that the sample CuCS(6.7) exhibits the same isotherms of type IV as SBA-15 despite the poor dispersion of guest CuO. Although a constant pore diameter is observed for SBA-15, CuAS(1.7), CuAS(3.3), and CuAS(6.7), both surface areas and pore volumes decrease gradually with the increase of copper content (Table 1). Langmuir 2010, 26(22), 17398–17404

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Figure 3. (A) N2 adsorption-desorption isotherms and (B) pore size distributions of SBA-15, CuAS, and CuCS samples. Curves are plotted offset for clarity.

Figure 5. IR spectra of SBA-15, CuAS(6.7), and CuCS(6.7) samples (A) before and (B) after calcination.

Figure 4. H2-TPR profiles of CuAS and CuCS samples.

Figure 6. (A) TG and (B) DTG curves of SBA-15, CuAS(6.7), and CuCS(6.7) before calcination.

Figure 4 presents the H2-TPR profiles of CuAS and CuCS samples. All supported CuO can be reduced to metallic copper before 420 °C, and the amount of H2 consumed is in good agreement with the copper content of samples (Table 1). Nevertheless, the reduction temperature varies with the state of CuO. The samples CuAS(1.7), CuAS(3.3), and CuAS(6.7) give a single reduction peak at about 230 °C, indicating the identical CuO state in these samples. In addition to the reduction peak at 230 °C, another peak at 260 °C emerges on CuAS(10.0). The sample CuCS(6.7) exhibits quite different H2-TPR profiles from CuAS. An intense reduction peak appears at a high temperature of 360 °C along with a quite weak one at 230 °C, which means that CuO in CuCS(6.7) is much difficult to be reduced. The IR spectrum of as-prepared SBA-15 shows several infrared bands at around 2850-3000 and 1350-1500 cm-1, which is caused by C-H stretching and bending vibrations of the template P123, respectively (Figure 5A).35 The band at 960 cm-1 ascribed to the bending vibration of Si-OH is also observed.35,36 After calcination, the bands of P123 disappear and the relative intensity of the Si-OH band decreases due to the decomposition of the template and the condensation of silanol groups (Figure 5B). Besides the bands of P123, the vibration bands at 1760, 1380, and 825 cm-1 assigned to nitrate appear on CuAS(6.7) after the (35) Tian, B.; Liu, X.; Yu, C.; Gao, F.; Luo, Q.; Xie, S.; Tu, B.; Zhao, D. Chem. Commun. 2002, 1186. (36) Sun, L. B.; Gu, F. N.; Chun, Y.; Yang, J.; Wang, Y.; Zhu, J. H. J. Phys. Chem. C 2008, 112, 4978. (37) Sun, L. B.; Yang, J.; Kou, J. H.; Gu, F. N.; Chun, Y.; Wang, Y.; Zhu, J. H.; Zou, Z. G. Angew. Chem., Int. Ed. 2008, 47, 3418. (38) Sun, L. B.; Gong, L.; Liu, X. Q.; Gu, F. N.; Chun, Y.; Zhu, J. H. Catal. Lett. 2009, 132, 218.

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incorporation of Cu(NO3)2 into as-prepared SBA-15.37,38 These bands become invisible on the calcined sample, suggesting that both template decomposition and precursor conversion can be realized in a single calcination step. Furthermore, the band at 960 cm-1 degrades as a negligible shoulder of the band at 1090 cm-1, and is much weaker than that on calcined SBA-15. This indicates the extensive consumption of silanol groups during the generation of CuAS sample. The conversion of Cu(NO3)2 supported on calcined SBA-15 is also observable by comparing the CuCS sample before and after calcination. The detailed decomposition process of various samples is studied by the TG technique. As shown in Figure 6A, the weight loss below 163 °C is only 2% for as-prepared SBA-15, indicative of a small amount of physisorbed water. The decomposition of P123 takes place between 163 and 280 °C with a large weight loss of 45%, corresponding to a sharp DTG peak at 168 °C (Figure 6B). The temperature of this decomposition process is lower than that for pure P123 (about 210 °C), which demonstrates the catalysis of silica frameworks on the decomposition of block copolymer.39 The subsequent broad step up to around 800 °C with a weight loss of 5% can be attributed to the removal of residual carbonaceous species and the dehydroxylation of silanols.33 The conversion of Cu(NO3)2 3 3H2O supported on calcined SBA-15 initiates the removal of water followed by the decomposition of Cu(NO3)2,40 which corresponds to two predominant DTG peaks as shown in Figure 6B. An interesting (39) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (40) Yu, Q.; Ma, X.; Lan, Z.; Wang, M.; Yu, C. J. Phys. Chem. C 2009, 113, 6969.

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Figure 8. Functionalization of mesoporous silica SBA-15 with copper species by (A) conventional method and (B) SPA strategy.

Figure 7. Breakthrough curves of thiophene in a fixed-bed adsorber with SBA-15, CuAS, and CuCS after autoreduction.

decomposition behavior of Cu(NO3)2 3 3H2O supported on asprepared SBA-15 is observed. The weight loss below 195 °C is calculated to be 14%, which is derived from the liberation of water and the partial decomposition of Cu(NO3)2. The great weight loss between 195 and 220 °C is 42%, corresponding to a remarkable DTG peak at 208 °C, which is due to the simultaneous conversion of P123 and residual Cu(NO3)2. The total weight loss of 56% is measured for uncalcined CuAS(6.7) and is in good agreement with the theoretical value (57%). The weight remains constant above 220 °C, which significantly differs from that observed on SBA-15. This indicates the extensive consumption of silanols during the conversion of Cu(NO3)2, thus confirming the results of IR. It is noticeable that the decomposition temperature of P123 in uncalcined CuAS(6.7) (208 °C) is higher than that of P123 in SBA-15 (168 °C), but is consistent with that of pure P123 (210 °C). This gives evidence of the incorporation of copper species into as-prepared SBA-15, which separate the template from silica walls. Hence, the silica walls can no longer catalyze the decomposition of P123. Figure 7 presents the breakthrough curves of thiophene with SBA-15 and copper-containing samples after autoreduction. The parent SBA-15 exhibits the worst performance and is capable of removing 0.14 mmol 3 g-1 of thiophene at saturation. The introduction of copper species to SBA-15 improves the desulfurization capacity evidently. The amount of thiophene captured can reach 0.25 mmol 3 g-1 over CuAS(6.7), which is much higher than that over CuCS(6.7) (0.17 mmol 3 g-1). We also compared the adsorption performance of CuAS and CuCS samples with a lower copper content. As shown in Figure S2 in the Supporting Information, the adsorption capacity of CuAS(3.3) is obviously higher than that of CuCS(3.3). It is believed that the active species for thiophene adsorption are cuprous species, and the adsorption proceeds through the formation of π-complexes between adsorbate and adsorbent molecules.6,8,9 The content of cuprous species on various adsorbents is thus measured. The results show that CuAS(6.7) after autoreduction possesses an apparently larger amount of cuprous species (2.6 mmol 3 g-1) than CuCS(6.7) (0.7 mmol 3 g-1), despite the fact that the copper content is identical in the two materials. On the basis of the results described above, it is clear that the well-dispersed CuO is readily able to generate cuprous species during autoreduction, which function as active species in desulfurization and can capture thiophene by π-complexation adsorption. The autoreduction of CuO supported on SiO2 to Cu2O has been reported by various groups. Anpo et al.41 (41) Anpo, M.; Nomura, T.; Kitao, T.; Giamello, E.; Che, M.; Fox, M. A. Chem. Lett. 1991, 5, 889.

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showed that Cu2þ/SiO2 could be converted to Cuþ/SiO2 by evacuation at 500 °C. DeJong et al.42 found that CuO loaded on a SiO2 support could be autoreduced to Cu2O in Ar. Similarly, CuO supported on SBA-15 can be reduced to Cu2O at a high temperature in Ar, as shown in Figure S3 in the Supporting Information. In the present study, the adsorptive desulfurization is obtained by π-complexation between Cu(I) and thiophene.9 As presented in Figure S2 in the Supporting Information, through the π-complexation mechanism the Cu(I) can form the usual σ bonds with their empty s-orbitals and, in addition, their d-orbitals can back-donate electron density to the antibonding π-orbitals (π*) of the sulfur rings. A π-complex is thus produced between adsorbate and adsorbent, and desulfurization by adsorption is realized. After saturation by thiophene, regeneration of the spent CuAS(6.7) is conducted. It is worth noting that the regenerated adsorbent can still remove 0.25 mmol 3 g-1 thiophene at saturation. This means that 100% adsorptive desulfurization capacity of the used adsorbent can be recovered.

Discussion Dispersion Behavior and Characteristics of CuO in Different Samples. It is interesting to note that different samples exhibit quite different dispersion behaviors of the guest CuO on the same host SBA-15. In CuAS samples with the CuO content ranging from 1.7 to 6.7 mmol 3 g-1, the guest can be well-dispersed without any XRD-detectable crystalline phase. In the case of CuAS(10.0), the aggregation of CuO takes place with a crystallite size of 3.5 nm, while large CuO particles of 21.4 nm appear on the CuCS(6.7) sample. Because the pore diameter of SBA-15 is 8.0 nm, the CuO particles in CuAS(10.0) may locate in the pores of SBA-15, whereas most of the CuO particles in CuCS(6.7) should situate outside the pores. The N2 adsorption-desorption isotherms give additional evidence of the position of CuO particles. The desorption branch of CuAS(10.0) is obviously delayed as compared with parent SBA-15, while the adsorption branch is slightly affected. This adsorption-desorption behavior can be attributed to the structure comprising open and closed cylindrical mesopores.43 It is known that SBA-15 possesses a pore system with completely open mesopores. After introducing CuO with a copper content of 10.0 mmol 3 g-1, CuO begins to aggregate and subsequently block part of the mesopores. As a result, some closed cylindrical mesopores were produced. In the above pore structure, capillary condensation in both open and closed sections occurs at the same relative pressure, when the metastable adsorption film on the cylindrical surface loses its stability.44 However, the open and closed pores behave differently in the process of desorption. In the open sections, the desorption occurs at (42) DeJong, K. P.; Geus, J. W.; Joziass, J. J. Catal. 1980, 65, 437. (43) Khodakov, A. V.; Zholobenko, V. L.; Bechara, R.; Durand, D. Microporous Mesoporous Mater. 2005, 79, 29. (44) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105, 6817.

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equilibrium conditions, whereas in the closed sections, the desorption is delayed until vapor pressure is reduced below the limit of stability of condensed nitrogen.43,45 This interpretation is fully supported by nonlocal density functional theory and molecular simulations of adsorption and hysteresis in cylindrical pores.44,46 Although the aggregation of CuO is observed, the sample CuCS(6.7) possesses similar isotherms of type IV as SBA-15. This finding confirms the XRD results and indicates that most of the copper species are loaded outside the SBA-15 channels.47 Therefore, CuO in the present samples exists in three states, namely, CuO highly dispersed in pores (for CuAS(1.7), CuAS(3.3), and CuAS(6.7) samples), CuO aggregated in pores (for CuAS(10.0) sample), and CuO aggregated outside pores (for CuCS(6.7) sample). The H2-TPR results show that these three states of CuO can be reduced to metallic copper under different conditions. CuO welldispersed in the pores of SBA-15 can be reduced at a low temperature of 230 °C. A higher temperature of 260 °C is demanded for the reduction of CuO aggregated in pores. CuO aggregated outside the pores is the most difficult to reduce, and the reduction temperature reaches as high as 360 °C. These results confirm three existence states of CuO in different samples and point out that CuO with a higher dispersion is more easily reduced. It is noticeable that the (100) reflection intensity in CuAS(1.7), CuAS(3.3), and CuAS(6.7) samples is obviously stronger than that in SBA-15 (Figure 1A). In general, the intensity of diffraction lines should decline after the introduction of metal oxides, because of the decreasing scatter contrast between pore walls and pore space. However, the formation of a smooth layer on the internal walls of SBA-15 can enhance the reflection intensity.48,49 Such a smooth layer contributes to the spatial order giving rise to the diffraction lines itself. Also, it was reported that the formation of a smooth layer could result in less-blocked mesopores. The CuAS samples containing 1.7-6.7 mmol 3 g-1 copper possess the pore diameter identical to SBA-15 (Figure 3B and Table 1), which confirms the XRD results, pointing out the formation of a smooth CuO layer. Similar results were also found on yttria or calcium oxide modified SBA-15.48,50 Therefore, the conclusion can be tentatively drawn that CuO in CuAS samples forms a smooth layer coated on the internal walls of SBA-15. The thickness of the CuO layer can be estimated by the difference between the wall thicknesses of CuAS and SBA-15. The wall thickness of CuAS(6.7) is 3.9 nm (wall thickness = a0 - pore diameter), while that of SBA-15 is 3.3 nm. As a result, the thickness of CuO layer should be 0.6 nm. Besides the formation of a smooth layer that leads to lessblocked mesopores, there is another cause for the identical pore diameters of SBA-15 and CuAS samples containing 1.76.7 mmol 3 g-1 copper. That is, the incorporation of copper species avoids the shrinkage of mesoporous frameworks during calcination. The unit cell constant of calcined SBA-15 is 11.3 nm and is smaller than that of as-prepared SBA-15 (11.9 nm), indicating the shrinkage of frameworks during calcination. However, the calcined samples CuAS have a unit cell constant of 11.9 nm, which is (45) Van Der Voort, P.; Ravikovitch, P. I.; De Jong, K. P.; Benjelloun, M.; Van Bavel, E.; Janssen, A. H.; Neimark, A. V.; Weekhuysen, B. M.; Vansant, E. F. J. Phys. Chem. B 2002, 106, 5873. (46) Neimark, A. V.; Ravikovitch, P. I.; Vishnyakov, A. Phys. Rev. E 2000, 62, R1493. (47) Guo, X.; Yin, A.; Dai, W.; Fan, K. Catal. Lett. 2009, 132, 22. (48) Sauer, J.; Marlow, F.; Schuth, F. Phys. Chem. Chem. Phys. 2001, 3, 5579. (49) Wang, Y. M.; Wu, Z. Y.; Wei, Y. L.; Zhu, J. H. Microporous Mesoporous Mater. 2005, 84, 127. (50) Sun, L. B.; Kou, J. H.; Chun, Y.; Yang, J.; Gu, F. N.; Wang, Y.; Zhu, J. H.; Zou, Z. G. Inorg. Chem. 2008, 47, 4199.

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Article

identical with as-prepared SBA-15. This means that no framework shrinkage occurs even if the removal of the template is performed at high temperature. The existence of copper species between template and silica walls is considered to hinder the shrinkage of frameworks, and subsequently avoid the reduction of pore diameter. Hence, the constant pore diameter can be ascribed to the comprehensive effect of unshrinking mesoporous frameworks and formation of the smooth layer. Dispersion Mechanism for CuO within Confined Space. To examine the factors affecting the dispersion of CuO, the abundant silanols in as-prepared SBA-15 is first taken into consideration, because the interaction of guest species with silanols is beneficial to the dispersion of the guest.29 An evident bending vibration band of silanols (960 cm-1, Figure 5) is observed in as-prepared SBA-15 with or without copper species incorporation. After calcination, the band of silanols becomes weak, which indicates that the calcined SBA-15 possesses fewer silanols as compared with the as-prepared one, and that calcination is unable to consume the silanols completely in the absence of guest species. In contrast to SBA-15, almost all of the silanols in the CuAS sample are consumed during calcination. This consumption of silanols gives evidence for the interaction between guest and host, which is favorable to the dispersion of CuO. TG results also provide evidence for the interaction of guest with host. After decomposition of the template centered at 168 °C, a gradual weight loss persists up to 800 °C in as-prepared SBA-15. The weight loss at high temperature is related to dehydroxylation of silanols. However, the TG curve of the CuAS sample gives a plateau above 220 °C, suggesting that silanols are consumed in the process of Cu(NO3)2 decomposition. By combining the results of IR and TG, it is clear that the abundant silanols in as-prepared sample enhance the interaction between copper species and silica walls. These silanols can be consumed during the decomposition of copper precursor and are helpful for the dispersion of the resulting oxide. The confined space between template and silica walls is considered to be another factor influencing the dispersion of CuO. The results of TG show that the decomposition temperature of P123 in CuAS(6.7) is 208 °C, which is obviously higher than that of P123 in SBA-15 (168 °C) but in good agreement with that of pure P123 (210 °C). This demonstrates that copper species are successfully introduced to the confined space and form an interlayer between template and silica walls. As a result, the silica walls can no longer catalyze the decomposition of P123. It should be stated that the decomposition sequence of copper precursor and template also plays an important role in the dispersion of CuO. If the decomposition of template is finished before the decomposition of copper precursor, the aggregation of CuO is able to take place despite the fact that the copper precursor locates within the confined space. In the case of the CuAS sample, the majority of Cu(NO3)2 is decomposed between 195 and 220 °C along with the decomposition of the minor below 195 °C. Interestingly, the decomposition of P123 is also completed between 195 and 220 °C. This means that most of the copper precursor is decomposed simultaneously with the template. Therefore, the confined space between template and silica walls can work for the dispersion of the resulting guest oxide. On the basis of the description above, it can be concluded that the high dispersion of CuO can be ascribed to the abundant silanols and the confined space between template and silica walls existed in asprepared samples. (51) Fujdala, K. L.; Drake, I. J.; Bell, A. T.; Tilley, T. D. J. Am. Chem. Soc. 2004, 126, 10864.

DOI: 10.1021/la101856d

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The high dispersion of guest oxide is extremely desirable for the fabrication of efficient adsorbents and catalysts with various applications.47,51-53 As for crystalline transition metal compounds, normal loading on the order of 1 wt % is sufficient to give rise to sharp XRD peaks.54 Although great efforts, such as deposition precipitation, grafting, and molecularly designed dispersion, have been dedicated,28-30,55 so far the amount of dispersed CuO has never exceeded 25 wt %. In the present study, we realize the high dispersion of CuO up to 6.7 mmol 3 g-1 (namely, 35 wt %) on SBA-15 by use of the SPA strategy. To the best of our knowledge, this is the first report of such a high dispersion amount of CuO. This SPA strategy permits the template removal and precursor decomposition in one step, and the repeated calcination in the conventional modification process is thus avoided (Figure 8). Moreover, the CuAS material after autoreduction exhibits much better adsorptive desulfurization capacity than CuCS, and the adsorption capacity of regenerated adsorbent can be recovered completely. In comparison with the aforementioned methods, our strategy provides a convenient, energy-saving, and efficient approach for the dispersion of guest oxides. This strategy may open up a route for the design and fabrication of new functional materials.

Conclusions By adjusting the copper content in as-prepared or template-free SBA-15, three states of CuO can be obtained. When the copper (52) Krishnan, C. K.; Hayashi, T.; Ogura, M. Adv. Mater. 2008, 20, 2131. (53) Liu, J. H.; Chi, Y. S.; Lin, H. P.; Mou, C. Y.; Wan, B. Z. Catal. Today 2004, 93-95, 141. (54) Xie, Y. C.; Tang, Y. Q. Adv. Catal. 1990, 37, 1. (55) Chmielarz, L.; Kustrowski, P.; Dziembaj, R.; Cool, P.; Vansant, E. F. Appl. Catal., B 2006, 62, 369.

17404 DOI: 10.1021/la101856d

species that incorporated into as-prepared SBA-15 are less than 6.7 mmol 3 g-1, the resulting CuO can be well-dispersed in the pores of SBA-15. The increase of copper content to 10.0 mmol 3 g-1 leads to the aggregation of CuO in pores. However, the aggregation of CuO outside pores occurs for the sample derived from template-free SBA-15. The high extent of dispersion of CuO can be attributed to the extraordinary microenvironment existing in as-prepared SBA-15, which is absent for calcined SBA-15. Such a microenvironment includes the abundant silanols as well as the confined space between template and silica walls. As compared with the aggregated CuO, the well-dispersed CuO can more easily generate cuprous species during autoreduction, which is active in thiophene capture through π-complexation adsorption. Moreover, 100% of adsorptive desulfurization capacity of the spent adsorbent can be recovered. Acknowledgment. The National Science Foundation of China (Nos. 20976082 and 21006048), the Major Basic Research Project of Natural Science Foundation of Jiangsu Province Colleges (No. 08KJA530001), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20093221120001), the Natural Science Foundation of Jiangsu Province Colleges (No. 09KJB530004), and the Academic Foundation for Young Teachers in Nanjing University of Technology are acknowledged for their financial support of this research. Supporting Information Available: TEM image of calcined SBA-15, adsorptive performance of CuCS(3.3) sample, and schematic diagram for the generation of Cu2O and thiophene capture by π-complexation adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(22), 17398–17404