Adjusting Host Properties to Promote Cuprous Chloride Dispersion

Jul 6, 2011 - By use of such a strategy, properties of the host SBA-15 were successfully adjusted. The enhancement of host–guest interaction and the...
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Adjusting Host Properties to Promote Cuprous Chloride Dispersion and Adsorptive Desulfurization Sites Formation on SBA-15 Gu-Se He, Lin-Bing Sun, Xue-Lin Song, Xiao-Qin Liu,* Yu Yin, and Yu-Chao Wang State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

bS Supporting Information ABSTRACT: Deep desulfurization via π-complexation adsorption is a promising method for the purification of transportation fuels. The desulfurization performance of an adsorbent has been proven to strongly depend on the dispersion extent of adsorption active species. In this paper, we report a strategy to promote the dispersion of active species CuCl on mesoporous silica SBA-15 by incorporating alumina. By use of such a strategy, properties of the host SBA-15 were successfully adjusted. The enhancement of host guest interaction and the improvement of surface hydrophilicity were realized simultaneously. Furthermore, the solid-state ion exchange between CuCl and formed Br€onsted acid sites (H+) was observed, which leads to the generation of isolated cuprous species. As a result, the dispersion of guest CuCl on the host was efficiently promoted. We also demonstrated that the obtained material, CuCl supported on SBA-15 incorporated with 10 wt % of alumina, can capture 0.240 mmol 3 g 1 thiophene, which is obviously higher than that over CuCl/SBA-15 (0.167 mmol 3 g 1). Our materials may provide a potential candidate for application in adsorptive desulfurization.

’ INTRODUCTION Deep desulfurization of transportation fuels has become very urgent for the petroleum refining industry because of increasingly rigorous environmental regulations. As a well-known process, hydrodesulfurization is highly efficient in removing some kinds of sulfides, but not aromatic sulfur compounds such as thiophene and its derivatives.1 In addition, hydrogen technology requires high temperature (300 350 °C) and hydrogen pressure (2 10 MPa).2 Recently, deep desulfurization via π-complexation adsorption has attracted much attention, because it can capture thiophenic compounds selectively and be operated at ambient conditions in the absence of hydrogen.3 Cuprous species dispersed on various supports including silica,4 alumina,5 and zeolite6 have been employed as π-complexation adsorbents. The formation of π-complexes between adsorbate molecules and cuprous ions is demonstrated to be a crucial step for the removal of thiophenic compounds.7 Taking into consideration that a thiophene molecule can form a π-complex only with the outlayer cuprous ion located on the surface of adsorbent,8 the dispersion extent of cuprous species correlates well with the adsorptive desulfurization performance. A host with a high surface area and a large pore volume is thus expected from the point of view of dispersing cuprous species. Since the discovery of mesoporous silica M41S, a variety of mesoporous materials has been synthesized through the surfactant templating method.9 11 Such mesoporous silicas possess high surface areas and large pore volumes and are of great interest for application in adsorption. Dai et al. has reported the dispersion of cuprous chloride (CuCl) on mesoporous silica SBA-15.12 The synthesized material was active in the selective removal of thiophene from simulated oils. By use of copper nitrate as the precursor, Wang et al. prepared cuprous oxide-modified SBA-15 and MCM-41 and applied them to the adsorptive desulfurization r 2011 American Chemical Society

of a fuel sample of JP-5 light fraction.4 These studies indicate that mesoporous silicas are promising hosts for the fabrication of π-complexation adsorbents. Despite the efforts made to develop functional materials derived from mesoporous silicas, the efficient dispersion of guest species is hindered by two inherent factors. The first factor is the weak host guest interaction for silica-supported materials, as demonstrated previously on silica modified by potassium nitrate.13 Such a weak interaction is unfavorable to the dispersion and, subsequently, decomposition of guest potassium nitrate on silica, which differs from potassium nitrate supported on metal oxides alumina and zirconia. The second factor is the poorer hydrophilicity of silica as compared with other oxides such as alumina,14 which leads to the difficulty in the dispersion of hydrophilic guest (e.g., CuCl).15 Aiming at promoting the dispersion of cuprous species on mesoporous silicas, it is extremely desirable to enhance the host guest interaction as well as improve the hydrophilicity of silica surface. In the present study, we chose mesoporous silica SBA-15 as a starting material due to its high surface, and developed a strategy to adjust the properties of SBA-15 by incorporating alumina before CuCl modification. By use of such a strategy, the enhancement of host guest interaction and the improvement of host hydrophilicity were realized simultaneously. Moreover, the solid-state ion exchange between CuCl and formed Br€onsted acid sites took place. The dispersion of CuCl on SBA-15 was thus successfully promoted after incorporating alumina. The obtained materials were well characterized by various methods. On the basis of the experimental results, the possible dispersion mechanism was proposed. We also demonstrated that CuCl supported Received: April 1, 2011 Revised: July 5, 2011 Published: July 06, 2011 3506

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Energy & Fuels on alumina-containing SBA-15 exhibited much better adsorptive desulfurization performance than that on pure SBA-15, and that the adsorption capacity of regenerated adsorbent can be well recovered.

’ EXPERIMENTAL SECTION Materials Synthesis. Mesoporous silica SBA-15 was synthesized according to the reported method using triblock copolymer EO20PO70EO20 (Pluronic P123) as the organic template.16 In a typical procedure, 3.0 g of P123 was dissolved in 22.5 g of H2O and 90.0 g of aqueous HCl solution (2 M) at 313 K, followed by the addition of 6.38 g of tetraethylorthosilicate (TEOS) as the silica source with stirring. The mixture was continuously stirred at 313 K for 24 h, then transferred to a Teflon-lined autoclave and subjected to a hydrothermal process at 373 K for another 24 h. After cooling to room temperature, the resultant solid product was filtered, washed with deionized water, and dried under ambient conditions. The removal of template was carried out in an air flow at 823 K for 5 h. Alumina-incorporated SBA-15 (AS) were prepared by wet impregnation. In a typical synthesis, a required amount of Al(NO3)3 3 9H2O (0.38 1.84 g) was dissolved in 10 cm3 of deionized water, followed by the addition of SBA-15 (1.0 g). After stirring at room temperature for 24 h, the mixture was evaporated at 353 K, dried at 373 K for 4 h, and calcined at 823 K for 5 h in air. The resulting sample, alumina-incorporated SBA-15, is denoted as AS-x, where x is the mass percentage of alumina. Adsorption active species CuCl was introduced to AS hosts by the spontaneous monolayer dispersion approach.17 An identical amount of CuCl (namely 4.0 mmol 3 g 1) was used for all samples. Typically, 4.0 mmol CuCl and 1.0 g of host were ground for 0.5 h. The mixture was then heated at 653 K (the temperature between the Tammann temperature and the melting point of CuCl) for 2 h in a dry Ar atmosphere. After being cooled to room temperature in Ar, the adsorbent was obtained and denoted as CuCl/AS-x, where x is the mass percentage of alumina in the host AS. As a comparison, CuCl-modified pure SBA-15 (CuCl/SBA-15) with the same CuCl content was also prepared via a similar process. To detect a possible product HCl during sample preparation, the outlet gas was absorbed by water. The adsorbed HCl was detected by pH test paper and confirmed by titration of silver nitrate solution. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded using a Bruker D8 Advance diffractometer with Cu KR radiation in the 2θ range from 0.7 to 8° and 5 to 80° at 40 kV and 40 mA. The N2 adsorption desorption isotherms were measured using a Belsorp II system at 77 K. The samples were degassed at 423 K for 4 h prior to analysis. The Brunauer Emmett Teller (BET) surface area was calculated using adsorption data in a relative pressure range 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 water-vapor adsorption desorption isotherms were determined using the intelligent gravimetric analyzer (IGA-100, Hiden) at 298 K. Transmission electron microscopy (TEM) measurements were taken on a JEM-2010 electron microscope with an acceleration voltage of 200 kV. Solid state 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded at 9.37 T on a Bruker Avance 400D spectrometer at a frequency of 104.17 MHz using a 4-mm zirconia rotor. The spin rate of the sample was 9.0 kHz, and a 4.5-μs pulse width was used with a pulse delay of 1.5 s. AlCl3 3 6H2O was used as the reference for chemical shifts. Temperatureprogrammed desorption (TPD) of NH3 experiments were conducted on a BELSORP BEL-CAT-A apparatus. The sample was pretreated at 473 K for 2 h prior to the adsorption of NH3 at 323 K. After the physically adsorbed NH3 was purged by a He flow at 323 K, the sample

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was heated to 873 K at the rate of 10 K 3 min 1, and the NH3 liberated was detected by an online OmniStar mass spectrometer. Quantitative analysis of Cu(I) content in the samples was performed by using a wet chemistry method. Cuprous chloride in the sample was dissolved in 10 mL of a ferric chloride reagent and 50 mL of H2O. The ferric chloride reagent was prepared by dissolving 75 g of ferric chloride in 150 mL of HCl (37%), 400 mL of H2O, and 5 mL of H2O2 (30%), and the reagent was heated until boiling to remove excess H2O2. The reaction of Cu+ with Fe3+ leads to the formation of stable Cu2+ and Fe2+. The amount of Fe2+ was then determined by titration with ceric sulfate solution (0.0889 mol 3 L 1), where phenanthroline was employed as an indicator. Adsorptive Test. Thiophene was used as the representative of sulfur contaminants. The model fuel containing 564 ppmw (parts per million by weight) sulfur was prepared by mixing thiophene with isooctane. For competitive adsorption experiments, an extra 8 wt % of toluene was added to the model fuel above. 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. The testing fuel was pumped up with a mini creep pump. The adsorbents after thermal dispersion in situ at 653 K in an Ar flow were cooled to room temperature. The feed was then switched to the model fuel and the feed rate was kept at 3 cm3 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 according to the reported method.18 Regeneration of the spent adsorbent was carried out in situ. The saturated adsorbent by model fuel was swept with Ar at room temperature for 12 h, followed by treating in flowing Ar at 653 K for 2 h. The regenerated adsorbent was then cooled to room temperature for the desulfurization of model fuel again.

’ RESULTS Adjusting Host Properties by Alumina Incorporation. Figure 1A displays the low-angle XRD patterns of SBA-15 before and after alumina incorporation. All samples exhibit one strong diffraction peak at 2θ of 0.9° accompanied with two weak ones at 1.4° and 1.6°, which can be indexed as (100), (110), and (200) reflections. These reflections correspond to a two-dimensional hexagonal pore regularity of a p6mm space group. This suggests that the ordered mesostructure of SBA-15 is well preserved after alumina incorporation. It is interesting to note that the intensity of (100) reflection becomes stronger after introducing alumina, which can be ascribed to the formation of a smooth alumina layer on the internal walls of SBA-15. Such a smooth layer contributes the spatial order giving rise to the diffraction pattern itself and results in less-blocked mesopores. It is also observable that the d-spacings shift toward lower values after alumina incorporation, corresponding to the decline of unit cell constant (a0) from 12.4 (for SBA-15) to 11.6 (for AS-20) as shown in Table 1. This indicates some contraction of siliceous frameworks caused by the partial substitution of aluminum. It should be stated that the effect of frameworks contraction on the structural ordering is less than that of the formation of a smooth layer. As a result, the intensity of XRD peaks increases after introducing aluminum. This phenomenon has also been reported by other groups.19 22 Also, the introduction of other metal oxides such as calcium oxide 3507

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Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of (a) SBA15, (b) AS-5, (c) AS-10, and (d) AS-20 samples.

Figure 2. (A) Nitrogen adsorption desorption isotherms and (B) BJH pore size distributions of (a) SBA-15, (b) AS-5, (c) AS-10, and (d) AS20 samples. Curves are plotted offset for clarify.

Table 1. Physicochemical Properties of Different Samples SBET sample

2

a0a

Vp 1

3

1

(m 3 g ) (cm 3 g ) (nm)

Cu(I) amount 1

(mmol 3 g )

(nm)

SBA-15

816

1.16

12.4

AS-5

650

0.92

12.2

AS-10 AS-20

603 442

0.83 0.53

12.1 11.6

CuCl/SBA-15

547

0.75

12.4

3.8

29.5 ( 0.2

CuCl/AS-5

440

0.67

12.2

3.9

29.1 ( 0.1

CuCl/AS-10

366

0.59

12.1

3.7

27.5 ( 0.3

CuCl/AS-20

251

0.37

11.6

3.8

Unit cell constant calculated according to a0 = 2  3 crystallite size calculated by the Scherrer formula. a

dCuClb

29.0 ( 0.2 1/2

 d100. b CuCl

and yttria into SBA-15 can also result in the formation of a smooth layer, which subsequently enhances the intensity of XRD diffraction peaks.23,24 As presented in Figure 1B, only a broad peak with 2θ at 22° assigned to amorphous silica is detected on SBA-15. After the incorporation of alumina, no new diffraction peak emerges, implying that alumina is well dispersed on SBA-15. The intensity of diffraction peaks is dependent on the type of metal oxides. Alumina is a kind of oxide that can be easily dispersed. The dispersion of 20% of alumina giving no evidence in XRD patterns has already been reported in the literature.25,26 Actually, Cheralathan et al. found that no XRD diffraction peaks can be observed until the amount of alumina reached 63%.27 As shown in Figure 2A, the isotherms of AS-5 and AS-10 samples are of type IV with two sharp inflections and an H1 hysteresis loop, quite similar to that of SBA-15. This confirms that the ordered mesostructure is well preserved. Further increasing the alumina content to 20 wt %, a small tail appears in the hysteresis of isotherm, corresponding to a small pore size distribution at 4.8 nm (Figure 2B). Table 1 lists the textual parameters calculated from N2 adsorption isotherms. It can be seen that with the increase of alumina content, both surface areas and pore volumes decline gradually. Figure 3 presents the water-vapor adsorption results of different samples. As compared with N2 adsorption, the hysteresis loops of water-vapor adsorption extend to higher relative pressure due to the difference in adsorbate properties. Nonetheless, the shape of isotherms for different samples is quite similar, mirroring the maintenance of mesostructure after alumina incorporation. As

Figure 3. Water-vapor adsorption desorption isotherms of (a) SBA15, (b) AS-5, (c) AS-10, and (d) AS-20 samples. Curves are plotted offset for clarify.

shown in Table S1, the total pore volume of SBA-15 determined by water-vapor adsorption (VH2O) is apparently lower than that by N2 adsorption (VN2), corresponding to a VH2O/VN2 value of 0.76. This reveals the poor surface hydrophilicity of silica. Interestingly, the incorporation of 5 wt % of alumina leads to an obvious increase of the VH2O/VN2 value to 0.90. Further increasing the amount of alumina, such a VH2O/VN2 value keeps increasing. For the sample AS-20, the value of VH2O is quite close to that of VN2 and the ratio of VH2O/VN2 reaches as high as 0.98. These results evidently demonstrate that the surface of SBA-15 becomes more hydrophilic after incorporating alumina. Figure 4 depicts the solid-state 27Al NMR spectra of AS samples. Two distinct signals assigned to tetrahedral and octahedral aluminum atoms are detected in all samples. Tetrahedral aluminum indicates framework aluminum that is covalently bound to four silicon atoms via oxygen bridges, whereas octahedral coordination corresponds to extraframework aluminum. The introduction of aluminum in the siliceous frameworks changes the composition of the inorganic walls and results in the formation of Br€onsted acidity which is ion-exchangeable.28 With the increase of aluminum content, the resonance ascribed to pentahedral aluminum becomes obvious. Aluminum located at the interface between tetrahedral aluminosilicate frameworks and octahedral alumina phases is responsible for pentahedral coordination. The existence of pentahedral aluminum implies that the local 3508

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Figure 4. Solid-state 27Al NMR spectra of (a) SBA-15, (b) AS-5, (c) AS-10, and (d) AS-20 samples.

Figure 5. NH3-TPD profiles of (a) SBA-15, (b) AS-5, (c) AS-10, and (d) AS-20 samples.

arrangement of aluminum atoms is different from that in bulk γ-alumina, which excludes the formation of a large number of the separate alumina phase.29 As displayed in Figure 5, the NH3-TPD profile of pure silica SBA-15 shows no evident peak, indicative of its negligible acidity. The incorporation of alumina leads to the generation of two desorption peaks at about 430 and 570 K, corresponding to weak and strong acid sites, respectively. The peak at 430 K is related to NH3 desorbed on the surface Al OH groups,30 whereas the peak at 570 K corresponds to NH3 desorbed on Br€onsted acid sites created by the framework aluminum species.31 With the increase of alumina content, the amount of desorbed NH3 increases, indicating the augment of acid sites. Apparently, acid sites with different strength are derived from different environment of aluminum, and the NH3-TPD profiles are thus well supported by the results of solid-state 27Al NMR. Alumina-Promoted Dispersion of CuCl on SBA-15. Taking into consideration that Cu(I) species are possible to be oxidized during the sample preparation, the Cu(I) amount was first examined. As displayed in Table 1, the Cu(I) amount in different samples ranges from 3.7 to 3.9 mmol 3 g 1, which is in good agreement with the theoretical value (4.0 mmol 3 g 1). These results indicate that CuCl is successfully introduced to the samples and, more important, the cuprous state is well maintained. Figure 6A shows low-angle XRD patterns of the samples

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Figure 6. (A) Low-angle and (B) wide-angle XRD patterns of (a) CuCl/SBA-15, (b) CuCl/AS-5, (c) CuCl/AS-10, (d) CuCl/AS-20, and (e) CuCl samples.

Figure 7. (A) Nitrogen adsorption desorption isotherms and (B) BJH pore size distributions of (a) CuCl/SBA-15, (b) CuCl/AS-5, (c) CuCl/ AS-10, and (d) CuCl/AS-20 samples. Curves are plotted offset for clarify.

after CuCl modification. All samples possess three well-defined diffraction peaks, suggesting that the ordered mesostructure is well preserved after introducing adsorption active species. In contrast with the hosts, the unit cells of CuCl/SBA-15 and CuCl/ AS samples keep constant. This means that CuCl does not lead to the structure shrinkage, which differs from what occurs in the process of alumina incorporation. The three diffraction peaks, namely (111), (220), and (311), can be observed in wide-angle XRD of the samples containing CuCl (JCPDS 06-0344).32 It is worth noting that CuCl dispersed on different hosts displays quite different full width at half-maximum (fwhm) of the diffraction peaks in wide-angle XRD patterns (Figure 6B). Corresponding CuCl crystallite sizes estimated by using the Scherrer equation are listed in Table 1. The crystallite size of CuCl is 29.4 nm in the sample CuCl/SBA-15, which is obviously smaller than that of unsupported CuCl (40.8 nm). The incorporation of alumina results in the decline of crystallite size, and the crystallite size of the sample CuCl/AS-10 is as low as 27.6 nm. These results reflect that the incorporation of alumina promotes the dispersion of active species CuCl on the host. Figure 7A depicts the N2 adsorption desorption isotherms of CuCl-modified samples. These samples display a typical IV isotherm with two sharp inflections and an H1-type hysteresis loop. The capillary condensation between the two inflections indicates the preservation of ordered mesoporous channels, which 3509

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Figure 8. TEM images of the sample (A) CuCl/SBA-15 and (B) CuCl/ AS-10. Figure 10. Effect of toluene on the adsorption capacity of different samples. The model fuel was prepared by mixing thiophene (564 ppmw S) with isooctane. For competitive adsorption, 8 wt % of toluene was added. LHSV = 2.3 h 1.

Figure 9. Breakthrough curves of thiophene in a fixed-bed adsorber with (a) SBA-15, (b) CuCl/SBA-15, (c) CuCl/AS-5, (d) CuCl/AS-10, and (e) CuCl/AS-20 samples. The model fuel was prepared by mixing thiophene (564 ppmw S) with isooctane; LHSV = 2.3 h 1.

can be confirmed by the pore size distributions (Figure 7B). As presented in Table 1, both surface areas and pore volumes diminish after CuCl modification, implying the successful introduction of adsorption active species to the hosts. TEM provides another important technique to characterize the periodic ordering of the mesostructure. As displayed in Figure 8, an ordered mesoporous structure can be observed for the samples CuCl/SBA-15 and CuCl/AS-10, consistent with the results of low-angle XRD and N2 adsorption. This means that the introduction of alumina and CuCl does not affect the structural integrity of SBA-15. Only sporadic dark spots ascribed to CuCl are visible in the sample CuCl/SBA-15. However, more CuCl particles with a smaller size can be identified in the sample CuCl/ AS-10. These results reveal that the guest CuCl tends to aggregate in pure SBA-15, while the incorporation of alumina promotes the dispersion of CuCl, which is in good agreement with the wide-angle XRD results. Adsorptive Desulfurization Performance of CuCl/AS. Figure 9 gives the breakthrough curves of thiophene with different adsorbents. The pure silica SBA-15 exhibits a poor performance and can only capture 0.091 mmol 3 g 1 thiophene at saturation (Figure 10). The introduction of Cu(I) to SBA-15 improves the desulfurization capacity obviously, and thiophene captured by the adsorbent CuCl/SBA-15 reaches 0.167 mmol 3 g 1. It is worth noting that the incorporation of alumina leads to a further increase of adsorptive desulfurization performance. The sample CuCl/AS-10 is capable of removing 0.240 mmol 3 g 1 of thiophene, which is evidently higher than that over the sample CuCl/ SBA-15 despite the identical Cu(I) content of two samples. As a

comparison, we also ran the adsorptive desulfurization experiments of Al2O3 and CuCl/Al2O3 samples. The content of CuCl in CuCl/Al2O3 sample is identical to that in CuCl/AS-10 (4.0 mmol 3 g 1). As shown in Figure S1 in Supporting Information, the samples Al2O3 and CuCl/Al2O3 can capture 0.066 and 0.125 mmol 3 g 1 thiophene at saturation, respectively, which is obviously lower than CuCl/AS-10 (0.240 mmol 3 g 1). The excellent adsorption performance of CuCl/AS-10 can be ascribed to the high surface area and pore volume as well as ion-exchangeable acid sites of modified support, which are absent in pure Al2O3. It is known that Cu(I) species play an important role in the π-complexation adsorption of thiophene.33 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 backdonate 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.7 Because a thiophene molecule can form a π-complex only with the out-layer cuprous ion located on the surface of adsorbent,8 the dispersion extent of Cu(I) species is essential to the adsorptive desulfurization performance. As described above, the incorporation of alumina promotes the dispersion of CuCl, and the adsorptive desulfurization performance of CuCl/AS samples is thus enhanced as compared with CuCl/SBA-15. Moreover, the best dispersion extent of CuCl is obtained on the host AS-10, which is in good agreement with the largest adsorption capacity of CuCl/AS-10. As for the rate of desulfurization, the feed rate of fuel and the liquid hourly space velocity (LHSV) were considered. In the present study, the feed rate is 3 cm3 3 h 1, corresponding to an LHSV of 2.3 h 1. Similar rates were also reported by other groups. The application of CuCl/SBA-15 and PdCl2/SBA-15 as adsorbents for the desulfurization was studied.34 The feed rate was controlled at 3 cm 3 h 1 with an LHSV of 1.2 h 1. For the adsorbent Cu2O/MCM-41, a flow rate of 6 cm3 3 h 1 and an LHSV of 1.1 h 1 have been used.4 By employing activated carbon (AC) and PdCl2/AC for desulfurization, Wang et al. reported that the feed rate was 3 cm3 3 h 1, which corresponds to an LHSV of 2.3 h 1.35 Actually, the mesoporous adsorbents employed in the present study have large pore diameters around 8 nm, which may allow faster diffusion as compared with some 3510

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Energy & Fuels microporous materials such as zeolite. Of course, much work will be conducted aiming at the practical applications of this kind of adsorbent in the future. In addition to organic sulfur compounds, there also exists a certain amount of aromatics in commercial gasoline. Also, because of the structural similarity of organic sulfur compounds to aromatics,36 it is necessary to investigate the influence of aromatics on sulfur removal. As shown in Figure 10, the adsorption ability of all samples decreased in the presence of toluene. Further calculation indicates that the decreased magnitude ranged from 47% to 68%. Similar results were also reported by other groups, and the adsorption capacity can be reduced by 50 75% due to the introduction of aromatics. Yang et al.37 studied the desulfurization of transportation fuels by using the zeolite CuY. The saturation adsorption capacity was reduced by 65% after introducing toluene. Bu et al.38 reported the adsorption of 4,6-dimethyldibenzothiophene (4,6-DMDBT) with activated carbons. They found that the presence of aromatic hydrocarbon can decrease the adsorption amount to a half. The results of Dai et al. suggested that the introduction of benzene can lead to a 75% decrease of adsorption amount.12 This indicates that aromatics can reduce sulfur compounds adsorption and with the similar adsorption mode of aromatics and sulfur compounds, sulfur adsorption via π electronic interaction can be seriously replaced by aromatics adsorption. It should be noted that our adsorbents can also capture a considerable amount of thiophene in the presence of toluene, and the adsorption capacity of adsorbents containing alumina is higher than that of the samples without alumina. After saturation, regeneration of the used sample CuCl/AS-10 was carried out. It is noticeable that the regenerated adsorbent can still remove 0.217 mmol 3 g 1 thiophene at saturation. That means,90% of adsorption capacity of the spent adsorbent can be recovered. The regeneration experiments were run for three times, and the adsorption capacity is listed in Table S2 of the Supporting Information. As shown in Table S2, the adsorption capacity decreases with the increase of regeneration times. However, about 80% of adsorption capacity can also be recovered after three times regeneration. It can be seen from Table S2 that the Cu(I) content declines gradually with the increase of regeneration times. Furthermore, some amount of metal copper can be identified from XRD patterns in regenerated samples (Figure S2 in Supporting Information). This means that the partial deactivation of adsorbents is due to the formation of Cu(0) from Cu(I) reduction. The possible explanation is that some organics remained in adsorbents after adsorption. In the process of regeneration, these organics or their derivatives may act as reducing agents, leading to the conversion of Cu(I) to Cu(0). The regeneration of metal chlorides is generally difficult as indicated by Hernandez-Maldonado et al.5 They found that the activity of CuCl/γ-Al2O3 was hard to be recovered even with the aid of ultrasound. Through investigating the regeneration of PdCl2/SBA-15, Wang et al. reported that only 48% of adsorption capacity can be recovered.34 The comparison of these results implies the superior reusability of our materials, which may provide a potential candidate for use as adsorbent in deep desulfurization.

’ DISCUSSION On the basis of above-mentioned results, it is clear that the incorporation of alumina into mesoporous silica SBA-15 promotes

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the dispersion of guest CuCl. Moreover, CuCl with a better dispersion extent is beneficial to the removal of aromatic sulfur thiophene via π-complexation adsorption. To examine the factors promoting the dispersion of CuCl, the enhancement of host guest interaction by alumina incorporation is first taken into consideration, because a strong interaction is favorable to the dispersion of guest on the surface of host.13 It is known that zeolites and aluminosilicates are good supports for catalysts and ion exchangers. They have a hydrophilic affinity toward polar molecules as a result of existence of aluminum atoms in their structure. Alumina-containing mesoporous silica has been reported to serve as supports to immobilize manganese complex.28 The interaction between manganese complex and supports was enhanced by incorporation of aluminum into frameworks. Wojcieszak et al.39 prepared nickel-based catalysts derived from mesoporous aluminosilicate. They found that the introduction of alumina to mesoporous silica led to a strong metal support interaction, which favors the loading of nickel. Therefore, the incorporation of alumina generated a strong affinity toward cuprous ions and enhances the interaction of guest with host, which subsequently promotes the dispersion of CuCl. The surface hydrophilicity of host is considered to be another factor influencing the dispersion of guest CuCl. From the point of view of dispersing a hydrophilic guest, the host with hydrophilic surface is desirable. As described above, pure silica SBA-15 shows a poor hydrophilicity, which hinders the efficient dispersion of CuCl. The incorporation of alumina is demonstrated to improve the H+-donor behavior of SBA-15, and it is also reported that alumina enhances the density of surface polar centers and creates Si(OH)Al hydroxyl structures.14 As a result, the surface of alumina-containing samples becomes more hydrophilic in contrast with pure silica SBA-15, which can be quantitatively characterized by comparing the adsorption amount of watervapor with N2. The improvement of host surface hydrophilicity favors the dispersion of hydrophilic guest, and a better dispersion extent of CuCl is therefore obtained on alumina-incorporated samples. As demonstrated by the results of 27Al NMR and NH3-TPD, some aluminum incorporated into the siliceous frameworks, which results in the generation of Br€onsted acidity, that is Si(OH)Al similar to zeolites.40 It is reported that thermal treatment of CuCl supported on zeolite will lead to the solid-state ion exchange between CuCl and Br€onsted acid sites (H+).41 Kuroda et al.42 has proved the exchange of CuCl with H+ in the zeolite ZSM-5 after heating at 573 K. In the present study, thermal dispersion of CuCl on the hosts AS was conducted at the temperature of 653 K. Under the conditions of thermal dispersion of CuCl, therefore, the solid-state ion exchange between CuCl and Br€onsted acid sites in the hosts AS may take place. That means some Si(OH)Al structures are converted to Si(OCu)Al ones, which leads to the formation of isolated Cu(I) species. Such isolated Cu(I) species can be regarded as adsorption active sites with an extreme dispersion extent, and are highly active in the capture of thiophene. Taking into account that HCl would be produced in the process of solid-state ion exchange from CuCl and H+, the gases liberated during thermal dispersion were analyzed. As expected, HCl was detected in the liberated gases, which gives evidence of the occurrence of solid-state ion exchange. Apart from the aforementioned factors, the impact of surface areas of AS hosts is also evaluated. According to the discussion above, it is known that an AS host with a large alumina content is 3511

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Energy & Fuels favorable to the enhancement of host guest interaction, the improvement of surface hydrophilicity, and the formation of ionexchangeable sites. Consequently, the dispersion extent of CuCl supported on AS-20, the host with the largest alumina content, should be the highest among the alumina-incorporated samples investigated. Nevertheless, the crystalline size of CuCl on the host AS-20 is bigger than that on AS-10 (Table 1). As can be seen from Table 1, the surface area of the sample AS-20 is 442 m 3 g 1 and is obviously lower that of than AS-10 (603 m2 3 g 1). This should be responsible for the worse dispersion extent of CuCl on AS-20, since a high surface area has an important effect on the accommodation and subsequently dispersion of guest. The high dispersion of guest compound is extremely desirable for the fabrication of efficient adsorbents for fuel desulfurization as well as other applications.43 46 Although there are a large number of literature reports on the development of novel π-complexation adsorbents,3 5,7,18 research regarding the adjustment of host properties to promote the dispersion of adsorption active species is very scarce, if any. Mesoporous silica SBA-15 possesses a large surface area, which is beneficial to the accommodation of guest. Unfortunately, the weak host guest interaction and the poor hydrophilicity obstruct the efficient dispersion of guest CuCl on SBA-15. In the present study, we designed a strategy to adjust the properties of pure silica SBA-15 by incorporating alumina. By use of such a strategy, the enhancement of host guest interaction and the improvement of host hydrophilicity were realized simultaneously. Furthermore, the formation of Br€onsted acid sites leads to the exchange of CuCl with H+ and subsequently the generation of isolated Cu(I) species. Hence, the dispersion of guest CuCl on the host was successfully promoted after incorporating alumina. The obtained alumina-containing adsorbents exhibited much better adsorptive desulfurization performance as compared with CuCl supported on pure silica SBA-15. Our strategy offers a convenient and effective approach to promote the dispersion of guest compounds and enhance the adsorption performance of target materials. This strategy may open up an avenue for the design and fabrication of new functional materials for applications in adsorption as well as in catalysis and sensing.

’ CONCLUSIONS A strategy was utilized to modify mesoporous silica SBA-15 by incorporating alumina before the introduction of adsorption active species CuCl. Such a strategy can promote the dispersion of guest CuCl efficiently. The incorporation of alumina enhances the host guest interaction, improves the surface hydrophilicity, and creates ion-exchangeable Br€onsted acid sites, which are responsible for the dispersion of guest CuCl. The increase of alumina content generally favors the dispersion of CuCl, while the decrease of surface area resulted from too much alumina is harmful to the dispersion. The optimal content of alumina in SBA-15 is 10 wt % from the point of view of dispersing CuCl. The adsorptive desulfurization performance of an adsorbent is well related to the dispersion extent of active species CuCl. The material CuCl/AS-10 exhibited the best adsorptive performance, and is obviously superior to the material CuCl/SBA-15 without the incorporation of alumina.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Comparison of total pore volumes detected by adsorption of N2 and water-vapor, as well as adsorption capacity, Cu(I) content, and XRD patterns of regenerated adsorbents. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-25-83587178; fax: +86-25-83587191; e-mail: liuxq@ njut.edu.cn.

’ ACKNOWLEDGMENT The National Science Foundation of China (20976082 and 21006048), the Major Basic Research Project of Natural Science Foundation of Jiangsu Province Colleges (08KJA530001), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20093221120001), the Natural Science Foundation of Jiangsu Province Colleges (09KJB530004), and China Postdoctoral Science Foundation (20110491406) are acknowledged for their financial support of this research. ’ REFERENCES (1) Kim, H.; Lee, J. J.; Moon, S. H. J. Phys. Chem. B 2003, 44, 287. (2) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207. (3) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79. (4) Wang, Y.; Yang, R. T.; Heinzel, J. M. Ind. Eng. Chem. Res. 2009, 48, 142. (5) Hernandez-Maldonado, A. J.; Qi, G. S.; Yang, R. T. Appl. Catal., B 2005, 61, 212. (6) Shan, J. H.; Liu, X. Q.; Sun, L. B.; Cui, R. Energy Fuels 2008, 22, 3955. (7) Hernandez-Maldonado, A. J.; Yang, R. T. J. Am. Chem. Soc. 2004, 126, 992. (8) Tian, W. H.; Sun, L. B.; Song, X. L.; Liu, X. Q.; Yin, Y.; He, G. S. Langmuir 2010, 26, 17398. (9) Asefa, T.; MacLachan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (10) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Science 1998, 282, 1302. (11) 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. (12) Dai, W.; Zhou, Y. P.; Li, S. N.; Li, W.; Su, W.; Sun, Y.; Zhou, L. Ind. Eng. Chem. Res. 2006, 45, 7892. (13) Sun, L. B.; Gu, F. N.; Chun, Y.; Yang, J.; Wang, Y.; Zhu, J. H. J. Phys. Chem. C 2008, 112, 4978. (14) Szczodrowski, K.; Prelot, B.; Lantenois, S.; Douillard, J.; Zajac, J. Microporous Mesoporous Mater. 2009, 124, 84. (15) Lee, B.; Kim, Y.; Lee, H.; Yi, J. Microporous Mesoporous Mater. 2001, 50, 77. (16) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (17) Xie, Y.; Tang, Y. Adv. Catal. 1990, 37, 1. (18) Hernandez-Maldonado, A. J.; Yang, F. H.; Qi, G.; Yang, R. T. Appl. Catal., B 2005, 56, 111. (19) Gao, L.; Fu, F. N.; Zhou, Y.; Yang, J.; Wang, Y.; Zhu, J. H. J. Hazard. Mater. 2009, 171, 378. (20) Vinu, A.; Murugesan, V.; B€ ohlmann, W.; Hartmann, M. J. Phys. Chem. B 2004, 108, 11496. (21) Baca, M.; Rochefoucauld, E.; Ambroise, E.; Krafft, J.; Hajjar, R.; Man, P.; Carrier, X.; Blanchard, J. Microporous Mesoporous Mater. 2008, 110, 232. 3512

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