Enhanced adsorption selectivity of dibenzothiophene on ordered mesoporous carbon-silica nanocomposites via copper modification

Microporous and Mesoporous Materials 212 (2015) 137e145

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Enhanced adsorption selectivity of dibenzothiophene on ordered mesoporous carbon-silica nanocomposites via copper modification Jieling Cheng, Shuangling Jin*, Rui Zhang, Xia Shao, Minglin Jin* School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 November 2014 Received in revised form 18 February 2015 Accepted 17 March 2015 Available online 27 March 2015

In this study, copper-modified ordered mesoporous carbon-silica nanocomposites (MCSs) were synthesized by incipient wetness impregnation of copper nitrate. The desulfurization performance of the asprepared adsorbents was evaluated by the selective adsorption of dibenzothiophene (DBT) as model sulfur compound from model fuels at ambient conditions. For comparison, the adsorptive desulfurization capacity for copper species supported on pure silica SBA-15 and pure carbonaceous CMK-3 was also investigated, respectively. The results indicate that MCS with 10wt.% of copper performed highest selectivity for DBT in competition with benzene. The significantly enhanced desulfurization performance of MCS could be attributed to its hybrid mesoporous carbon-silica nature which is favorable for high dispersion extent of copper and synergistic effect between the carbon substrate and the supported copper species. © 2015 Elsevier Inc. All rights reserved.

Keywords: Carbon-silica nanocomposite Copper modification Dibenzothiophene Selective adsorption Desulfurization

1. Introduction Fossil fuels as the largest source of energy are refined into various petroleum products such as gasoline, diesel and jet fuel widely applied in many areas. The sulfur compounds in the fuel are converted to SOx during combustion, which not only results in acid rain, but also poisons catalysts in catalytic converters for reducing CO and NOx [1e3]. Many countries have implemented more stringent regulations for refineries to produce diesel or gasoline with low sulfur content. On the other hand, liquid hydrocarbon fuels are candidate fuels for producing H2 for use in automotive and portable fuel cells due to their higher energy density and safety for transportation and storage. However, the sulfur compounds in the fuel and H2S produced from them within hydrocarbon reforming process are poisons to reforming and shift catalysts and the electrode catalysts. Thus, sulfur concentration in the fuel needs to be reduced to less than 1 ppmw for proton exchange membrane fuel cell (PEMFC) and less than 10 ppmw for solid oxide fuel cell (SOFC) [4,5]. Current approaches for desulfurization mainly include hydrodesulfurization (HDS), oxydesulfurization (ODS), biodesulfurization (BDS), and adsorptive desulfurization (ADS) [3e6]. At present, HDS

* Corresponding authors. Tel.: þ86 021 60873070; fax: þ86 021 60873407. E-mail addresses: [email protected] (S. Jin), [email protected] (M. Jin). http://dx.doi.org/10.1016/j.micromeso.2015.03.016 1387-1811/© 2015 Elsevier Inc. All rights reserved.

is a conventional technology to produce low-sulfur fuels under high temperature and high pressure using hydrogen gas. Besides the severe conditions of operation, HDS is not effective for removing heterocyclic sulfur compounds such as DBT and its derivatives due to steric hindrance, which makes HDS more complicated and difficult, not suitable for deep desulfurization. In comparison with the HDS process, the selective adsorption for removing sulfur (SARS) considered to be a more promising technology owing to such advantages as they react at atmospheric pressure, ambient temperature and without hydrogen consumption. Various types of adsorbents such as zeolites [7,8], activated carbons [9e11], metal oxides [12e17], metal-organic frameworks (MOFs) [18e20] have been investigated during the past decade. For efficient removal, not only adequate porosity/pore size but also specific adsorption sites are required. In particular, adsorption of thiophenic sulfur compounds using the adsorbent modified with transition metal like Cu, Ag, Pd and Pt which employ the function of p-complexation has attracted more attention [8e11]. Because only those active species dispersed on the outermost layer are accessible to interact with adsorbate molecules, the adsorption efficiency is strongly dependent on the dispersion degree of active species. Because of the high surface areas and large pore volumes, mesoporous silicas such as SBA-15 and MCM-48 used as adsorbent supports have received much attention. To disperse guest species on those mesoporous silicas, several interesting approaches have been developed, which include adjusting the properties of SBA-15

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or MCM-48 by incorporating alumina [12], and using assynthesized mesoporous silica SBA-15 as support that own confined space between template and silica walls [13e17]. However, most of commercial fuels consist of 70e80 wt.% aliphatics components and 20e30 wt.% aromatics. The aromatics can strongly compete with thiophenic sulfur compounds, due to their similar adsorption mode as sulfur compounds toward active sites via p electronic interaction. Therefore, the enhancement of the selectivity of mesoporous silica for sulfur compounds is still limited. Compared with other adsorbents, activated carbon is proposed to be better support for p-complexation adsorbents to remove sulfurcontaining compounds, due to the geometric effect between the carbon substrate and the supported p-complexation species [7]. Nevertheless, the extremely inert and hydrophobic carbon surface is unfavorable for introducing the guest species by impregnation method and subsequent heattreatment can lead to aggregation and pore blockage. Hence, the sulfur-adsorption capacity and selectivity of adsorbents need to be further improved by modifying the properties of the hosts. In this study, copper species supported on MCS adsorbents were prepared by incipient wetness impregnation of copper nitrate. The desulfurization performance of the as-prepared adsorbents was evaluated by the selective adsorption of dibenzothiophene (DBT) as model sulfur compound from model fuels at ambient conditions. To understand the effect of pore wall compositions on the copper distribution and hence the desulfurization performance, the introduction of copper species into the pure silica SBA-15 and pure carbonaceous CMK-3 and their desulfurization performance are also investigated. Among the tested materials, the carbon-silica nanocomposites with 10wt.% copper performed highest selectivity ability for DBT in competition with benzene. The significantly enhanced sulfur selectivity could be attributed to its highly ordered hybrid mesoporous carbon-silica nature which is favorable for high dispersion extent of copper and synergistic effect between the carbon substrate and the supported copper species.

8.5 g of TEOS was added and the resulting mixture was stirred at  38 C for 20 h. The milky mixture was transferred into an auto clave and aged at 100 C for 48 h. The solid was harvested by filtration, washed with deionized water, dried at room temperature, and calcined in air at 550  C for 6 h to obtain the SBA-15 sample. The synthesis of the host structure CMK-3 was carried out using SBA-15 silica as the hard template and sucrose as the carbon source following the synthesis procedure described by Jun et al. [23]. Briefly, 1 g of SBA-15 was impregnated with an aqueous solution obtained by dissolving 1.25 g of sucrose and 0.14 g of H2SO4 in 5.0 g of deionized water. The mixture was then dried at 100  C for 6 h, and subsequently at 160  C for 6 h. The silica sample, containing partially polymerized and carbonized sucrose, was treated again at 100  C and 160  C after the addition of 0.8 g of sucrose, 90 mg of H2SO4 and 5.0 g of deionized water. The sucrose/silica composite  was then heated at 900 C for 6 h under nitrogen to complete the carbonization. The silica template was dissolved with 1 M NaOH  (50% vol ethanol and 50% vol H2O) at 80 C. The template-free carbon product thus obtained was filtered, washed with deionized water and ethanol, and dried. A definite quantity of Cu(NO3)2 was incorporated into asprepared MCS powder by incipient wetness impregnation method for 24 h. Then the mixture was placed inside a quartz reactor and heated to 400  C at a rate of 1  C/min and maintained at this temperature for 2 h in pure Ar atmosphere. Then the reactor was cooled in the pure Ar atmosphere to room temperature, and the complex adsorbent nCu/MCS was obtained, where the n represents the mass percentage of copper in the samples. In contrast to the previous series, pure mesoporous carbons CMK-3 and pure mesoporous silicate SBA-15 were used as supports. 10Cu/CMK-3, 10Cu/SBA-15 samples were prepared by the same procedures as descripted above.

2. Experimental section

Thermogravimetric (TG) measurement was carried out on a SDT Q600 thermal analyzer in the temperature range from 25 to 800  C at a heating rate of 10  C/min in air. X-ray diffraction (XRD) patterns were recorded on a Rigaku-DMAX2200PC diffractometer using Cu Ka (l ¼ 0.1506 Å) radiation over the range 0.5 to 5 (low angle) and 10 to 80 (wide angle). Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F field emission source transmission electron microscope operated at 200 kV. Energy dispersive X-ray spectroscopy (EDX) was performed on a Philips EDAX instrument. TEM samples were ultrasonically treated in a solution of ethanol and then deposited on molybdenum grids coated with carbon. N2 adsorptionedesorption isotherms were measured at 77 K with an ASAP 2020 instrument. The samples were separately degassed at 200  C in a vacuum environment for a period of at least 4 h prior to measurements. Experimental adsorption data in the relative pressure (P/P0) range of 0.05e0.2 was used to calculate surface area values using the Brunauer-Emmett-Teller (BET) equation. The pore size distribution was determined by applying density functional theory (DFT) model, assuming slit-pore geometry for the pores less than 2 nm and cylindrical pore geometry for the pores in the range of 2e30 nm.

2.1. Material synthesis The MCS is prepared by a direct tri-constituent co-assembly method with soluble phenolic resins as carbon precursor, triblockcopolymer F127 as the soft template and tetraethoxysilane (TEOS) as silica precursor according to the procedure reported by the Zhao group [21]. Resols preparation: 5 g of phenol was melted at 42  C, and then 1.06 g of 20 wt.% NaOH aqueous solution was added. After stirring for 10 min, 8.85 g of 37 wt.% formaldehyde aqueous solution was added, and the mixture was stirred at 70  C for 1 h. The mixture was further neutralized by 2 M HCl solution after naturally cooling down to room temperature. Water was removed under  vacuum at 45 C. Then the mixture was made up of 20 wt.% resols ethanolic solution. MCS was prepared as following. 1.6 g of F127  was dissolved in 7 g of ethanol at 40 C, followed by the addition of 1 g of 0.2 M HCl solution. Next, 2.08 g of TEOS and 5 g of 20 wt.% resols ethanolic solution were added in sequence and stirred for 2 h. Then the mixture was transferred to several Petri dishes and evaporated at room temperature for 8 h, followed by thermal  polymerization at 100 C for 24 h. The as-made product was calcined at 900  C for 4 h under N2 flow to obtain sample MCS, with the heating rate of 1  C/min below 600  C and 5  C/min above 600  C. The preparation of the SBA-15 silica is based on the synthesis procedure first reported by Zhao et al. [22]. The typical synthetic process is as follows. First, 4 g of P123 was dissolved in 30 g of  deionized water and 120 g of 2 M HCl with stirring at 38 C. Then,

2.2. Characterization

2.3. Adsorptive test DBT was used as the representative of sulfur contaminants. The model fuel with different sulfur concentrations from 32 to 960 ppmw was prepared by mixing DBT and n-octane. For competitive adsorption experiments, an extra 20 wt.% of benzene

J. Cheng et al. / Microporous and Mesoporous Materials 212 (2015) 137e145

was added to the model fuel above. The desulfurization performance of adsorbents was evaluated by static saturation tests. About 0.1 g of tested adsorbent and 10 g of model were added into a glass tube. The tube was capped and placed in a shaking  bath at 30 C for 48 h. Then the mixture was filtered, and the treated model fuel samples were analyzed to estimate the sulfur adsorption capacity of the adsorbents. The amount of sulfur adsorbed on per gram of adsorbent, qe (mg/g), was calculated with the following equation:

qe ¼

ðC0  Ce Þm m

(1)

where C0 and Ce are the initial and equilibrium sulfur concentrations in the model fuel (mg/g), respectively, m* is the weight of model fuel (g) and m is the mass of the adsorbent (g). In order to facilitate the quantitative discussion of the adsorptive selectivity, a selectivity factor K was used in the present study, which is defined as:

K¼

q1e  q2e q1e

(2)

where q1e is the sulfur adsorptive capacity for the model fuel without benzene (mg/g) and q2e is the sulfur adsorptive capacity for the model fuel with benzene (mg/g). K represents the percentage of reduced adsorption capacity in the presence of benzene. Note that the larger K value, the lower selectivity. The adsorption equilibrium data were fitted using the Langmuir equation:

Ce 1 Ce ¼ þ qe Kl qm qm

(3)

where qm is the maximum adsorption capacity (mg/g), Kl is the adsorption equilibrium constant (g/mg), characteristic of the affinity between the adsorbent and adsorbate. Kl and qm can be obtained by linear regression of (Ce/qe) vs. Ce data. The adsorption data were also fitted using the Freundlich isotherms. The Freundlich isotherm is an empirical model that can be applied for non-ideal adsorption on heterogeneous surfaces as well as for multilayer adsorption. The Freundlich isotherm is expressed by the following empirical equation:

1=n

qe ¼ Kf Ce

3. Results and discussion 3.1. Mesostructure and chemical compositions of copper-modified hosts TG measurement was used to reveal the precise chemical composition of the as-obtained MCS. The TG curve (Fig. 1) shows a significant weight loss of 40 wt.% in the temperature range from 400 to 600  C in air, and the weight residue is 60 wt.%, implying that MCS is composed of silica and carbon with the weight ratio of about 1.5:1. Fig. 2a displays the low-angle XRD patterns of MCS, SBA-15, CMK-3 samples before and after loading copper species. The MCS sample exhibits an intense diffraction peak at 2q of 0.95 indexed as (100) reflection, and a weak peak at 2q of 1.85 indexed as (110), corresponding to a two-dimensional hexagonal pore regularity of a p6mm space group. The SBA-15 sample also shows three wellresolved peaks at 2q of 0.95 , 1.6 and 1.8 that can be indexed as (100), (110), and (200) reflections associated with hexagonal symmetry. Compared with SBA-15, the diffraction peaks of the carbonization material CMK-3 in the low-angle range shift somewhat to higher angles with three peaks at 2q of 1.0 , 1.8 and 2.0 assigned to (100), (110), and (200), and the width of peak (100) increased slightly, which should be due to structural defects and shrinkage happened in the carbonization and the silica removal steps. In contrast with the XRD patterns of pristine samples, after copper incorporation, the characteristic peaks of the samples shift to higher angles and the intensity decreased, indicating copper modification results in the contraction of frameworks and the decrease of the mesostructural ordering. The shrinkage of mesoporous frameworks is caused by the calcination during the incorporation of copper species, and the declined intensity of diffraction lines is attributed to the pore-filling effects that can reduce the scattering contrast between pore walls and space. Similar results have been reported previously [13,24e26]. These changes in the diffraction patterns suggest that the copper species was successfully introduced into the mesochannels of the hosts. And the intensity of peak (100) of sample 15Cu/MCS and 10Cu/CMK-3 decreases sharply compared with their parent hosts, implying the introduced copper species is blocking the pore or the ordered mesostructure is somewhat destroyed. Wide-angle XRD patterns of different samples were also obtained, as shown in Fig. 2b. All MCS samples possess a broad diffraction peak centered at 22.5 , which

(4)

where Kf is the Freundlich adsorption constant ((g/mg)1/n), which is an indicator of the adsorption capacity, while n refers to the adsorption tendency. In all cases, a double-log plot of qe vs. Ce data resulted in a good linear relationship, allowing for the estimation of the model parameters Kf and n by linear regression. The sulfur content of the treated model fuels was determined using a gas chromatograph (Shimadzu 2014C) equipped with a flame ionization detector (FID), a capillary column (MXT-5, Restek, 60 m length, 0.25 mm inner diameter, 0.25 mm film thickness) and a split mode injector (ration: 100:1). Ultrahigh purity helium was used as a carrier gas. The column temperature was set at 140  C for   5 min, 10 C/min from 140 to 220 C, hold for 7 min. The temper ature of both injector and detector was 280 C. The injection volume of sample was 1 mL. In this analysis, phenyl sulfide was used as an internal standard.

139

Fig. 1. TG analysis curves of sample MCS in air.

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Fig. 2. Low-angle (a) and wide-angle (b) XRD patterns of MCS, SBA-15, CMK-3 samples before and after loading copper species.

can be assigned to amorphous carbon and silica walls [14,24]. The diffraction peaks of MCS samples with different copper content locate at 2q of 43.2 , 50.4 and 74.0 match well with those of the metallic Cu (JCPDS No. 04-0836), and the peaks become stronger when copper amount is enhanced. For the sample 15Cu/MCS, besides prominent peaks for elemental Cu, a weak peak at 2q of 36.5 originated from Cu2O (JCPDS No. 65-3288) is also detected, implying the copper species begin to aggregate and is difficult to be reduced with the increasing of loading amount. During heat treatment in inert atmosphere, copper nitrate decomposed to CuO and then was reduced to Cu by the carbon wall. And if the generated CuO particles aggregated severely, they are difficult to be reduced. For the sample 10Cu/CMK-3, the presence of the apparent diffraction peak of Cu2O also suggests that some large copper-derived agglomerates on the exterior of CMK-3 cannot be fully reduced due to their poor contact with carbon framework. The sample 10Cu/ SBA-15 shows intense diffraction peaks originated from CuO (JCPDS No. 44-0706). It has been reported that the CuO supported on SiO2 can be partially autoreduced to Cu2O under inert condition [12,14,16], which function as active species in desulfurization and can capture organo-sulfur compounds by p-complexation adsorption. These results suggest that the support and copper loading degree have a significant effect on the existence state of copper species. Corresponding crystallite sizes of copper species estimated by using the Scherrer formula are listed in Table 1. It should be noted that the calculated crystallite size is the average value, and incipient wetness impregnation method usually results in the some extra species which may exist in the spaces of the bulk particles of the supports [27]. TEM was used to directly observe the periodic ordering of the mesostructure and the existing state of the copper species. The images of different samples are shown in Fig. 3. For all of the samples, the ordered hexagonal arrays of mesopores can be judged from the white dark contrast, as shown in Fig. 3a, f and h, which is consistent with the low-angle XRD patterns. The difference of copper content leads to the different dispersion state of copper

particles inside the pore channels of MCS adsorbents. The dispersed small copper particles inside mesochannels of 5Cu/MCS sample can be seen in Fig. 3b. In the low-magnification TEM image of 10Cu/MCS (Fig. 3c), no bulk particles are observable, while the EDAX probe reveals the presence of copper (Fig. 3e). From the enlarged image (Inset of Fig. 3c), we can see that the copper nanoparticles are dispersed in the mesochannels of host MCS. Further increasing the copper loading level to 15wt.%, the copper particles begin to aggregate in the pore channels, as shown in Fig. 3d. On the contrary, bulk aggregated particles can be easily found on the external surface of CMK-3 (Fig. 3g). In addition, the TEM image of 10Cu/SBA-15 in Fig. 3i shows that the aggregation of CuO nanoparticles occurs in some regions of the mesochannels. These results reveal that the guest tends to aggregate in pure SBA-15, and the hydrophobic carbon surface of CMK-3 restricts the impregnation of copper nitrate aqueous solution, while the nanocomposite MCS favors the introduction of guest solution and promotes the dispersions of copper nanoparticles. N2 adsorptionedesorption isotherms and the corresponding pore size distributions obtained using a DFT model of different samples are shown in Fig. 4. The pore parameters are listed in Table 1. All samples display a typical IV isotherm with two sharp inflections and an H1-type hysteresis loop [28]. The capillary condensation between the two inflections indicates the preservation of ordered mesoporous channels after introduction of copper, which can be confirmed by the pore size distributions. As presented in Table 1, the surface area is found to decrease gradually from 333 m2/g for MCS to 327 m2/g for 5Cu/MCS, 291 m2/g for 10Cu/MCS, and 276 m2/g for 15Cu/MCS. Similarly, the pore volume of pores in the range of 2e30 nm (V2e30nm) decreases also in this order, and the pore size distribution curve of 10Cu/MCS in the range of 6e8 nm shift toward the smaller pores. All these results suggest that the successful introduction of copper species to the mesochannels of the MCS hosts. It is worth noting that the sample 15Cu/ MCS exhibits similar V2e30nm as 10Cu/MCS, indicating incorporation of copper by high concentration of copper nitrate solution via

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141

Table 1 Microstructure parameters of MCS, SBA-15 and CMK-3 samples before and after loading copper species. Samples

SBET (m2/g)

V<2nm (cm3/g)

V2e30nm (cm3/g)

dCu(nm)

dCu2O (nm)

dCuO (nm)

MCS 5Cu/MCS 10Cu/MCS 15Cu/MCS SBA-15 10Cu/SBA-15 CMK-3 10Cu/CMK-3

333 327 291 276 595 345 1000 928

0.060 0.051 0.055 0.050 0.053 0.030 0.171 0.171

0.329 0.307 0.291 0.292 1.341 0.818 1.011 0.832

e 26.0 35.2 23.0 e e e 24.7

e e e 8.0 e e e 24.5

e e e e e 29.8 e e

SBET: BET surface area. V<2nm: pore volume of pores less than 2 nm. V2e30nm: pore volume of the pores in the range of 2e30 nm. dCu, dCu2O and dCuO: Cu, Cu2O and CuO crystallite sizes calculated by the Scherrer formula.

incipient wetness impregnation make copper difficult to enter into the mesopores. For CMK-3, the surface area decreases from 1000 m2/g to 928 m2/g, and V2e30nm decreases from 1.011 cm3/g to 0.832 cm3/g after incorporation of copper, however, the micopore volume is unchanged, implying some copper species is introduced into the mesochannel not in the micopore. And the incorporation of copper also leads to the diminution of surface areas and pore volumes of SBA-15. All these results are in good agreement with above XRD and TEM results. For clarity, a comparison mechanism for the incorporation of copper species on different supports is given in Fig. 5. The silanol groups on the surface of SBA-15 can interact with the copper solution, which favors the mass transfer of copper solution inside mesochannels. However, when the nucleation of copper oxides occurs at some domains in the confined mesochannels, the precursors in other areas transfer and nucleate together. In contrast, the inert and hydrophobic carbon surface of CMK-3 makes it difficult to introduce the aqueous solution of copper nitrate into the mesopores. And the copper ions/nanoparticles in the mesopores easily aggregate due to their weak interaction with carbon surface. The MCS clearly demonstrates great advantages over its singlecomponent counterparts when acting as the support. The introduction of silica in the framework favors the introduction of the aqueous solution of copper nitrate. Moreover, copper ions may have strong affinity with the hydrophilic surface of silica, while the inert, hydrophobic component carbon possibly plays a role in separating them. The synergistic effect of hybrid silica and the carbon nature of the hosts lead to the formation of relatively dispersed copper nanoparticles after annealing. 3.2. Adsorptive desulfurization performance The MCS samples with different copper loading levels were screened for their adsorptive desulfurization ability for DBT from the model oil with the sulfur concentrations of 320 ppmw. The results of adsorptive desulfurization experiments are displayed in Fig. 6. Cu/MCS adsorbents adsorb more DBT than pristine MCS, but the uptake amounts are not proportional to the loading amount of copper. This nonlinear response could be a consequence of variations in the structure and composition of domains on the Cu/MCS surface or inaccessibility to the active copper sites for some domains. It is found that after loading 5wt.% copper, the sulfur uptake capacity of 5Cu/MCS is increased compared with that of MCS, which is due to p-complexation between DBT and copper. Further increasing the copper loading level increases the amount of the active copper sites, which results in the better desulfurization capacity of 10Cu/MCS than that of 5Cu/MCS in the presence of benzene, indicating there are more active copper sites in 10Cu/MCS than 5Cu/MCS. However, with more copper species introduced into

the support, the copper particles begin to aggregate and subsequently block the mesopores, leading to the decreased amount of accessible active copper sites to form the p-complexation with DBT. Thus, the adsorbent with higher copper loading (15Cu/MCS) shows a smaller sulfur uptake capacity than that with lower copper loading (10Cu/MCS). Therefore, among all the samples, 10Cu/MCS shows the highest desulfurization capacity. In addition, it has been proved that the particle size and dispersion state of metallic copper have important effect on the utilization of p-complexation [29], and the aggregated copper particles are not effective for the adsorption of thiophenic sulfur compounds. To compare the desulfur function of copper species on supports with different properties, desulfurization by 10Cu/CMK-3 and 10Cu/SBA-15 were also tested under the same desulfurization conditions. DBT adsorption capacities of different substrates before and after loading of copper are shown in Fig. 7. It is shown that the adsorption capacities of the substrates decrease in the following sequence, CMK-3>MCS > SBA-15. Moreover, the selectivity factor K of the samples, which represents the decreased magnitude of adsorption capacity in the presence of benzene is listed in Table 2. For the MCS samples, K is decreased with the increase of copper content until 10 wt.%, indicating the introduction of copper enhanced the selectivity of MCS with its p-complexation effect. However, for the CMK-3, K has no change after incorporation of copper. Previous results reported by other groups show that the K value is in the range of 40e75% by introducing of aromatics. He et al. [14] reported the adsorption of thiophene with the CuClmodified SBA-15 and Alumina-incorporated SBA-15, and the K value ranged from 47% to 68% after introducing 8 wt.% of toluene. The results of Dai et al. [30] suggested the introduction of 20 wt.% benzene can lead to a 75% decrease of capacity for thiophene by using Cu(I)/SBA-15. Yang et al. [8] studied the desulfurization performance of zeolite CuY by adsorption of thiophene, and the saturation adsorption capacity was reduced by 65% after introducing 20 wt.% benzene. Khan et al. [31] found that the presence of 25 wt.% toluene can decrease the adsorption amount by 70% for the adsorption of benzothiophene by using Cuþ/activated carbon. Dai et al. [18] also reported the sulfur adsorption capacity of DBT of Cu(I)/MOF was reduced by 38% due to the introduction of 20 wt.% benzene. Those results indicate that aromatics can reduce sulfur compounds adsorption seriously. It is worth noting that the Cu/ MCS adsorbents are superior to those of copper species supported on SBA-15, zeolites, activated carbons and MOFs reported previously. The sample 10Cu/MCS exhibits the highest adsorption capacity per unit of surface area among all samples, as shown in Table 2. For the pure carbon substrate CMK-3, the excellent sulfur capacity is due to its larger BET surface area and higher micropore volume ratio. The adsorption driving force of DBT on carbon is mainly due

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to the pep dispersion interactions between the aromatic ring of DBT and the aromatic structure of the graphene layers [32], and the DBT adsorption capacity governed by micropore volume of carbon has been observed in previous study [33e36]. After copper species are loaded onto CMK-3, the agglomeration of copper-derived particles may block pores, and they are not effective for the adsorption of DBT due to their low activity, resulting in the decline of adsorption capacity. And it is found that although SBA-15 has a relatively high surface area of 595 m2/g, it could not fix DBT due to the weak interaction (probably by van der Waals forces) between DBT and silica surface [30,37]. Compared with SBA-15, the adsorption capacity of the 10Cu/SBA-15 increase to 0.6 mg/g, but it can only capture 0.2 mg/g in the presence of benzene. This may be because the CuO can be partially autoreduced to Cu2O, and the Cu2O can form p-complexation with DBT, but benzene with the similar adsorption mode can reduce the DBT adsorption capacity. Moreover, the aggregated CuO in the pores is more difficult to be reduced to cuprous species which function as active species and thus the enhancement of the adsorption capacity is limited. 3.3. Adsorption isotherms of DBT Adsorption isotherm models have been widely adopted to assess the relationship between adsorbate and adsorbent at equilibrium. The fitting parameters obtained by different models can help to explain the adsorption mechanism, heterogeneity of the adsorbent surface and affinities of the adsorbents which are important to optimize the design of an adsorption system to remove the adsorbate [38,39]. In the present investigation, the experimental data of adsorbing DBT were fit by the Langmuir and Freundlich models to study the adsorption characteristics and mechanism. Fig. 8 shows the experimental data points along with Langmuir and Freundlich fitting cures of sample MCS and 10Cu/MCS. The higher adsorption capacities of DBT on the 10Cu/MCS than on their original counterparts MCS with and without benzene are obviously seen. Since the surface area and pore volume all decrease after loading of copper, the increased adsorption on the 10Cu/MCS adsorbent suggests favorable p-complexation between copper and DBT. The fitting parameters of the experimental adsorption equilibrium data are given in Table 3. The data calculated from the Langmuir equation reveal an increase of qm and Kl upon the sample after incorporation of copper, which is an indication of the enhanced affinity between DBT and adsorbent surface. Moreover, Kl and qm are all decreased in the presence of benzene, maybe owing to the benzene with the similar adsorption mode toward the adsorbent as DBT occupy the active site. The same tendency of copper modification facilitating the adsorption process is also observed from the fitting of Freundlich models. It shows that after loading of 10wt.% copper, Kf and n increased, indicating that the adsorption potential between adsorbent and DBT enhanced with the incorporation of copper. In any cases, when considering the value of correlation coefficient square (R2), it seems that the experimental data of 10Cu/MCS is better fitted to Freundlich model than Langmuir model, possibly implying that the introduction of copper had resulted in a more heterogeneous surface of the adsorbents. 3.4. Mechanism of selective adsorption for copper-modified MCS

Fig. 3. TEM images of MCS (a), 5Cu/MCS (b), 10Cu/MCS (c), 15Cu/MCS (d), CMK-3 (f), 10Cu/CMK-3(g), SBA-15(h) and 10Cu/SBA-15 (i) and the EDAX pattern of 10Cu/MCS (e). Insets in (a) and (c) are the enlarged images.

The significantly enhanced sulfur selectivity of MCS compared with that of SBA-15 after copper incorporation is attributed to its synergistic effect between the carbon substrate and the supported copper species. There are a large number of peripheral sites on the edges of the supported copper which are dispersed on the MCS substrates [10], and the edge sites provide an ideal combination of sites for the DBT molecule, as illustrated in Fig. 9. The thiophene

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143

Fig. 4. N2 adsorptionedesorption isotherms (a,c,e) and the corresponding DFT pore size distributions (b, d, f) of MCS, SBA-15 and CMK-3 samples before and after loading copper species.

Fig. 5. Schematic illustration of the preparation strategy.

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J. Cheng et al. / Microporous and Mesoporous Materials 212 (2015) 137e145 Table 3 Isotherm model parameters for DBT on MCS and 10Cu/MCS under different adsorption conditions. Samples

With benzene

MCS(B) 10Cu/MCS(B) Without benzene MCS 10Cu/MCS

Langmuir

Freundlich

qm

Kl

R2

n

Kf

R2

10.739 11.143 13.190 13.954

0.861 1.147 1.276 1.915

0.986 0.980 0.992 0.970

1.465 1.592 1.636 1.929

4.923 5.975 7.446 9.259

0.999 0.990 0.982 0.988

Fig. 6. Adsorption capacities of MCS samples with different copper loading levels.

Fig. 9. Depiction of synergistic effect in adsorption of DBT on copper-modified MCS samples.

Fig. 7. Adsorption capacities of SBA-15 and CMK-3 samples before and after loading copper species. Table 2 The selectivity factor K and sulfur capacities based on per unit of surface area of samples. Samples

MCS 5Cu/MCS 10Cu/MCS 15Cu/MCS SBA-15 10Cu/SBA-15 CMK-3 10Cu/CMK-3

K(%)

38.8 36.8 27.6 28.0 e 69.0 32.7 32.6

2

ring of DBT molecule adsorbs strongly at well-dispersed copper active sites on the substrate, while benzene rings of DBT molecule are bonded to the surface of carbon by the pep dispersive interaction between the aromatic rings and the graphene layers of carbon. By binding cooperatively in this manner, stronger adsorption and hence higher adsorbed amounts were achieved. Based on above edge sites theory, after incorporation of copper, CMK-3 should have superior adsorption performance, but the copper aggregation and pore blockage restrict the mentioned synergistic effect. The hybrid carbon-silica nature of the MCS support may play a role in generating relatively dispersed copper nanoparticles. Thus, it is shown in this work that MCS is a relatively effective support for p-complexation adsorbent. 4. Conclusion

3

Sulfur capacity (mg/m  10 ) Without benzene

With benzene

14.7 17.4 19.9 18.1 0.0 1.7 15.3 14.5

9.0 11.0 14.4 13.0 0.0 0.5 10.3 9.8

Copper species supported on MCS adsorbents were prepared by an incipient wetness impregnation method and the desulfurization properties for model fuels were evaluated under ambient conditions. It is shown that the pore wall nature has great influence on the dispersion of copper species and the interaction between adsorbate and substrate, in turn, the desulfurization performance. The selectivity and sulfur capacity based on per unit of surface area for MCS with 10wt. % copper are higher than that of SBA-15 and CMK-3 with same copper loading amount. The significantly

Fig. 8. Langmuir (a) and Freundlich (b) isotherms for DBT on MCS and 10Cu/MCS under different adsorption conditions. (symbols: experimental data points, solid lines: fitting curves).

J. Cheng et al. / Microporous and Mesoporous Materials 212 (2015) 137e145

enhanced desulfurization performance of MCS could be ascribed to its highly ordered hybrid mesoporous carbon-silica nature which is favorable for high dispersion extent of copper and synergistic effect between the carbon substrate and the supported copper species. The application of hybrid silica-carbon as metal carriers would open up an avenue for the design and fabrication of efficient adsorbents for fuel desulfurization as well as other applications. Acknowledgments This work was supported by the Capacity Building Program of Shanghai Local Universities (No. 12160503600), Youth Innovation Fund from Shanghai Institute of Technology (No. YJ2013-35), the First-class Discipline Construction Fund of Shanghai Municipal Education Commission (No. J201212), Nature Science Foundation of China (No. U1332107) and Key Discipline Construction Fund of Composite Materials of Shanghai Institute of Technology (No. 10210Q140001). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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