Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene

FUPROC-03558; No of Pages 9 Fuel Processing Technology xxx (2012) xxx–xxx

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Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene N. Farzin Nejad a, E. Shams a, M.K. Amini a,⁎, J.C. Bennett b a b

Department of Chemistry, University of Isfahan, Isfahan 81746‐73441, Iran Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada B4P2R6

a r t i c l e

i n f o

Article history: Received 24 May 2012 Received in revised form 27 June 2012 Accepted 2 September 2012 Available online xxxx Keywords: Adsorbent Desulfurization Dibenzothiophene Fuel Magnetic mesoporous carbon Mesoporous silica

a b s t r a c t We synthesized magnetic mesoporous carbon (Ni-CMK-3) as an adsorbent for removal of sulfur from model oil (dibenzothiophene, DBT, in n-hexane). X-ray diffraction and transmission electron microscopy analyses revealed the presence of face-centered cubic Ni nanoparticles with an average size of 17± 3 nm, and indicated that the carbon support retained its mesoporosity and morphology after immobilization of the metal nanoparticles. Nitrogen adsorption measurements indicated that the resultant Ni-CMK-3 possesses high surface area (705 m2 g−1), large pore-volume (0.87 cm3 g−1) and average pore-size of 4.5 nm. The resulting magnetic mesoporous carbon afforded a maximum adsorption capacity of 62.0 mg DBT g−1 of Ni-CMK-3 at the optimized conditions (Ni loading, 20%; adsorbent dose, 5 g L−1; contact time, 1 h; temperature, 40 °C). Magnetic measurement revealed the ferromagnetic property of Ni-CMK-3 at room temperature with saturation magnetization, remanent magnetization and coercive force of 13.8 emu g−1, 38.0 Oe and 2.2 emu g−1, respectively, which made it desirable for separation under an external magnetic field. Following adsorption of DBT, Ni-CMK-3 could be separated by a magnet and regenerated by extraction with toluene. The regenerated adsorbent afforded 97%, 94% and 80% of the initial adsorption capacity after the first three regeneration cycles, respectively. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Sulfur oxides (SOx) resulting from combustion of sulfur compounds in fuels have become one of the increasingly serious environmental problems in the world as they are a major cause of acid rains, global warming effect, and atmospheric pollution. Thus, in recent years considerable attention has been paid to the deep desulfurization of gasoline and diesel fuels due to increasingly stringent environmental regulations being imposed to reduce the sulfur content to very low levels. The removal of sulfur compounds from liquid fuels is carried out industrially by catalytic hydrodesulfurization (HDS). While conventional HDS has been highly effective for the reduction of sulfur levels, aromatic sulfur compounds such as thiophene, benzothiophene, DBT and their derivatives as the major objectionable sulfur components present in the petroleum fractions, are less reactive to this process [1]. Further improvement of the HDS process for deep desulfurization is limited to increasingly severe operating conditions at high cost. Moreover, the deep HDS processes require considerably increased energy and hydrogen consumptions, and substantially improved reactivity and selectivity of the necessary catalysts. Most of the present HDS catalysts cause undesirable side reactions, which can result in decrease of the octane number of gasoline [2]. Therefore, from both environmental and economic considerations, various alternative deep desulfurization ⁎ Corresponding author. Tel.: +98 311 7932708; fax: +98 311 6689732. E-mail address: [email protected] (M.K. Amini).

processes have been recently investigated, including: oxidative desulfurization (ODS) using different catalysts [3–7], biodesulfurization [8]; extractive desulfurization with different types of ionic liquids [9–11] and desulfurization based on electrochemical methods [12]. A relatively new approach for removing sulfur compounds from fuels is adsorptive desulfurization, which seems very promising with regard to energy consumption, because the adsorption process can be performed at ambient temperature and pressure. Moreover, some adsorbents can efficiently remove refractory aromatic sulfur compounds [13]. Therefore, a great deal of research has been devoted to develop new adsorbents with improved adsorption capacity, high selectivity and regenerability, as well as elucidation of adsorption mechanism [14]. A wide variety of novel adsorbent materials have been reported, including various types of zeolites and metal loaded zeolites such as MCM-22 zeolite [15], cation exchanged Y-zeolites [16], heteroatom zeolites [17], nanocrystalline NaY zeolite synthesized using carbon nanotube templated growth [18], lanthanum loaded mesoporous MCM-41 sorbents for diesel fuel [19], Ce(IV)Y zeolites [20], ion-exchange zeolites of Cu(I)- and Ag(I)-beta [21], Ni/ZSM-5 and Ni/Al2O3 extrudates [22]; activated carbons and their modified forms including modified activated carbons and carbon cloths [13,23,24]; adsorbents based on mesoporous silica, alumina and zirconia, and their metal-loaded derivatives including xerogel-derived zinc-based nanocrystalline aluminum oxide [14], activated alumina [25], magnetic mesoporous silica composite [26], metal halides supported on MCM-41 and SBA-15 mesoporous materials [27], copper-supported zirconia [28], CuO on mesoporous SBA-15 [29],

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Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

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Ni-supported mesoporous AlMCM-41 [30], mesoporous CeMCM-41 [31], and different types of zirconia-based adsorbents [32]. Among different adsorbents, mesoporous carbons such as CMK-1, CMK-3 and CMK-5, because of their large specific surface area, highly ordered mesoporous structure and high thermal stability, have attracted much attention as adsorbents for different compounds [33–35]. Both non-functionalized and functionalized ordered mesoporous carbons have been used for desulfurization applications [36]. For example, the mesoporous carbon, CMK-3, prepared using hexagonal AlSBA-15 mesoporous silica template, was used for adsorption of DBT from petroleum fuels [37]. Also, mesoporous carbons, prepared by using mesoporous silica HMS as the hard template and phenolic resin as the carbon source, were used to remove DBT and 4,6dimethyldibenzothiophene from diesel [38]. The mesoporous carbons are also effective supports for transition metal catalysts in HDS processes [39,40]. An important step after adsorption of the desired compounds from the liquid sample, is separation of the spent adsorbent from the medium, which is conventionally performed by filtration or centrifugation. As an alternative, a convenient method is to prepare magnetic adsorbents and attract them to an external magnetic field. Several studies have shown that incorporation of magnetic nanoparticles into different types of carbons make this feasible [41–44]. Typical examples on the preparation and use of magnetic mesoporous carbons include: mesoporous magnetic iron oxide-carbon encapsulates for removal of arsenic [45], ordered mesoporous carbon/nanoparticle nickel and iron composites for removal of bulky dye molecules [44,46], and multifunctional mesoporous carbon Fe-CMK-3 for removal of toxic organic compounds from waste-water [47]. In the present work we prepared magnetic mesoporous carbon (Ni-CMK-3) and studied its performance for the first time for adsorption of DBT from liquid samples. The adsorbent was prepared by synthesis of SBA-15 template, followed by pore-filling with the carbon precursor, carbonization, removing the silica template and deposition of Ni nanoparticles by impregnation. The structural order, textural properties and magnetic properties of the adsorbent have been characterized by powder X-ray diffraction (XRD), N2 adsorption–desorption isotherms (Brunauer–Emmett–Teller, BET technique), magnetometry and high resolution transmission electron microscopy (HRTEM). Magnetization of the mesoporous carbon particles enables convenient isolation of the adsorbent from the liquid phase by an external magnet, which provides a simple alternative to filtration or centrifugation of the spent adsorbent. The effect of such factors as the adsorbent dose, initial DBT concentration and contact time on the adsorption efficiency of DBT by Ni-CMK-3 have been studied. The kinetics of adsorption has been studied by testing the experimental data with pseudo-first-order and pseudo-second-order models. Adsorption equilibrium study has been performed by fitting the experimental data to various isotherm equations, and thermodynamic parameters were evaluated. 2. Experimental 2.1. Materials All chemicals including HCl, H2SO4, NaOH, tetraethyl orthosilicate TEOS, sucrose, Ni(NO3)2·6H2O and DBT were purchased from Merck. The non-ionic triblock copolymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) surfactant (pluronic P123, MW= 5800), as a structure-directing agent or soft template, was supplied by Aldrich. Double distilled deionized water was used throughout. 2.2. Instruments The XRD analysis of the samples was performed with a Bruker D8-Advance powder diffractometer using Ni filtered Cu–Kα radiation (λ =1.54056 Å) to investigate the structure of mesoporous adsorbents

and nickel nanoparticles. The diffraction patterns were recorded over a 2θ range of 0.5–6° with a 0.04° step size and 2 s step time for smallangle and 2θ range of 1–90° with a 0.08° step size and 0.2 s step time for wide-angle measurements. N2 adsorption–desorption isotherms were obtained at −196 °C using a Belsorp instrument. Before adsorption measurement, the sample was degassed for 3 h at 300 °C under vacuum. The specific surface areas were estimated by the BET method. The pore-size distributions were derived from the adsorption branches of isotherms by using the Barett–Joyner–Halenda (BJH) model. The total pore volumes (Vt) were determined from the amount of N2 uptake at P/P0 of 0.99. The morphologies of the adsorbents were characterized by a HRTEM instrument (Philips CM30) operating at 300 kV. Sample preparation for HRTEM examination involved the ultrasonic dispersion of the sample in ethanol and placing a drop of the suspension on a copper grid covered with perforated carbon film. The magnetization of the magnetic adsorbents was measured on an alternating gradient force magnetometer (AGFM, Kavir Kashan Co., Iran). The amount of Ni was determined by using an atomic absorption spectrophotometer (AAS) model Analyst 200 (Perkin Elmer). A UV–vis spectrophotometer (JASCO model V-670) was used for determining the amount of analyte (DBT) retained by the adsorbent and recording the spectra as required. A water bath shaker (Nuve model ST402) was used for mixing the solutions. An Astra ultrasonic bath (Tecno-Gaz) was used for synthesis of magnetic mesoporous carbon. 2.3. Synthesis of magnetic mesoporous carbon, Nix-CMK-3 The mesoporous carbon CMK-3 was prepared using the SBA-15 template, which in turn was synthesized according to the method proposed by Zhao et al. [48]. Typically, 4 g of the surfactant (P123) was dissolved in 150 mL of 2 M HCl solution at 40 °C with stirring. To the solution was added 9.0 g TEOS dropwise and the mixture was kept in a water bath at 40 °C under stirring for 20 h. The resulting mixture was hydrothermally treated by transferring into a sealed container and aging at 100 °C for 48 h. The solid product was filtered, washed with water, dried at 60 °C and finally calcined at 550 °C in air for 6 h to remove the template P123. The synthesis of Nix-CMK-3 was performed by two different methods: one-step route (co-impregnation) and two-step route (consecutiveimpregnation). In the first method, SBA-15 was impregnated with the carbon source (sucrose) and the metal source (Ni(II) nitrate) simultaneously. For this purpose, a solution containing 1.25 g sucrose, 0.14 g sulfuric acid and 5 g water was prepared, and then a known volume of 0.5 M solution of Ni(II) nitrate to afford Nix-CMK-3 with a predetermined concentration of Ni (x= 10, 20, 30, 40, 50 or 70 wt.%) was added. Then, 1 g SBA-15 as the hard template was added gradually to the stirring solution. After complete mixing, the mixture was placed in the oven at 100 °C for 6 h and then at 160 °C for another 6 h, which afforded a dark brown sample due to partial polymerization and carbonization of sucrose. In order to better fill the pores of mesoporous silica with carbon source and produce a full hexagonal structure with the same p6mm space groups as that of SBA-15, the sample was treated again with a solution containing 0.8 g sucrose, 0.09 g H2SO4, 5 g water and the desired amount of Ni(II) solution as described above. Then the mixture was subjected to the same heating steps. Complete carbonization of the sample was performed under pure nitrogen gas by ramping from room temperature to 800 °C at a heating rate of 5 °C min−1 and kept at this temperature for 6 h. Finally, the silica template was removed by treating the resulting composite with aqueous 6 M NaOH solution at 70 °C. The silica-free carbon product thus obtained was thoroughly washed with water and dried in oven for 6 h at 60 °C. In the consecutive-impregnation method, the carbon and metal sources were impregnated separately. The first step was impregnation of SBA-15 with carbon source (sucrose) to produce mesoporous carbon

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

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CMK-3 according to Jun et al. [49]. In the second step, the Ni-CMK-3 with different Ni contents were prepared by incipient wetness impregnation method, using calculated quantities of aqueous solution of Ni(II) to afford 10, 20, 30, 40, 50 and 70% Ni on CMK-3. The samples were sonicated for 1 h and then oven dried at 50 °C. To reduce Ni(II), the resulting solids were heat treated under Ar/H2 (20% H2) by ramping from room temperature to 800 °C at a heating rate of 5 °C min−1 and kept at this temperature for 1 h. 2.4. DBT adsorption studies using magnetic mesoporous carbon A DBT solution (1000 mg kg−1) in n-hexane was used for adsorption studies. The optimum procedure for adsorption of DBT on Ni-CMK-3 included adding 50 mg of the magnetic mesoporous adsorbent into 10 mL of 1000 mg kg−1 DBT in n-hexane and shaking at 40 °C for 1 h. Then, the adsorbent was separated from the solution by a magnet (1.4 T) and the solution was subjected to UV–vis measurement. The measurement was performed using the UV–vis spectrophotometer in the range of 190–500 nm. The equilibrium concentration of DBT at different stages was measured from the calibration curve obtained at λmax of 326 nm (R2 = 0.999), and the amount of DBT adsorbed (qe) by the adsorbent was calculated using the equation qe = (C0 − Ce)w / m, where C0 and Ce are the initial and equilibrium concentrations (mg kg −1) of DBT in the bulk phase, respectively, w is the amount of the liquid phase (kg) and m is the amount of adsorbent (g). The amount of DBT adsorbed onto the magnetic mesoporous carbon at any time (qt) was obtained from qt = (C0 − Ct)w / m, where Ct is the bulk concentration (mg kg−1) of DBT at any time t. 3. Results and discussion 3.1. Choice of the synthesis method

3

some pore blocking, besides a proportional reduction in concentration of the active adsorbing ingredient (CMK-3) which arises from incorporation of Ni nanoparticles. These effects significantly reduce the specific surface area of the adsorbent as shown in Fig. 1 (see also the discussion on N2 adsorption–desorption isotherms). The adsorbent prepared by the consecutive-impregnation method also shows a considerable decrease in adsorption capacity at high Ni loading (Fig. 1), which mainly results from a proportional decrease of the active adsorbing ingredient (mesoporous carbon). However, it can be clearly seen that the performance of the adsorbents prepared by this method is superior to those prepared by the co-impregnation method at all Ni loadings, hence, the adsorbent prepared by the consecutive-impregnation method was used for further studies. It can be seen from Fig. 1 that at 10–20% metal loading the adsorption capacity of this adsorbent is still high and close to that of CMK-3. Our experimental results showed that Ni-CMK-3 with 20% metal loading affords complete separability of the spent adsorbent from the liquid sample, and moreover, its carbon content is high enough to enable good separation of DBT. Metal loadings of b20% made separation of the spent adsorbent difficult and higher concentrations resulted in a considerable reduction in adsorption capacity of the adsorbent. In fact, a 20% metal loading was chosen as a compromise between the adsorption capacity and separability of the spent adsorbent under the external magnetic field. For simplicity, the adsorbent with 20% Ni loading is hereafter denoted as Ni-CMK-3. 3.2. Physical characterization and structural studies Small-angle XRD patterns of the synthesized SBA-15, CMK-3 and Ni-CMK-3 were recorded in the 2θ range of 0.5–6° and the results are shown in Fig. 2a. The CMK-3 displays three peaks, which are indexed to the (100), (110) and (200) diffraction planes characteristic of well ordered mesoporous structure, suggesting that 2D hexagonal symmetry (p6mm) of the SBA-15 template has been retained in

The adsorbents (Nix-CMK-3) were prepared by two different methods: one-step route (co-impregnation) and two-step route (consecutive-impregnation), using several Ni loadings (10, 20, 30, 40, 50 and 70%) in each method, as described in Section 2.3. The performance of the resulting composites for adsorption of DBT was studied according to the procedure described in Section 2.4. Fig. 1 shows the plots of qe versus Ni loading for the two series of adsorbents. The results clearly show a significant decrease in adsorption capacity of both adsorbents, mainly because the concentration of the active adsorbing ingredient (CMK-3) is reduced proportionally to the metal loading. It can be seen that the consecutive-impregnation is superior to the co-impregnation method, as it shows much less decrease in adsorption amount (qe) compared to its non-metallated precursor (CMK-3). It seems that simultaneous impregnation of the mesoporous silica (SBA-15) with the carbon source and nickel salt, significantly reduces the adsorption capacity of the resulting mesoporous carbon due to

Intensity (a.u.)

a

Ni-CMK-3

CMK-3 110 200 100 0

SBA-15

1

2

3

4

5

6

2θ (degree) (111)

50 Co-impregnation Consecutive-impregnation

b

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30

600

(200) 400

C (200) C (002)

Intensity (counts)

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200

20

(220)

0 10

0 0

10

20

30

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50

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70

Ni content (%) Fig. 1. Effect Ni loading of Nix-CMK-3 on the amount of DBT adsorbed for the adsorbents synthesized by co-impregnation and consecutive-impregnation methods.

10

20

30

40

50

60

70

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90

2θ (degree) Fig. 2. (a) Small-angle XRD patterns of SBA-15, CMK-3 and Ni-CMK-3, and (b) wide-angle XRD pattern of Ni-CMK-3. The crystalline structure of Ni was examined according to JCPDS card no. 040850 for pure Ni.

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

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the carbon replica and that the support used for the fabrication of the nanoparticles is highly ordered [50]. The d100 spacings for SBA-15 and CMK-3 samples were ca. 9.3 and 8.8 nm, respectively, corresponding to the respective unit cell parameters (a0 = 2d100 / (3)1/2) of about 10.7 and 10.2 nm. The lower cell parameter of the carbon replica compared to the parent SBA-15 is due to some structural shrinkage of the carbon/silica composite material during pyrolysis at high-temperature (800 °C) [51]. Weaker (100) peak of CMK-3 is also the result of structural shrinkage, which causes some decrease in the structural ordering of the carbon replica. An analogous XRD pattern was observed for Ni-CMK-3, which is also assigned to the P6mm symmetry, indicating that the support retained its mesoporosity and morphology even after immobilization of metal nanoparticles. However, the diffraction peak intensities of Ni-CMK-3 are weaker than that of the support, which is probably due to some pore-filling and pore-blockage of the host CMK-3 which can reduce the scattering contrast between the pores and the walls of the mesoporous material [52]. These results correlate well with the following N2 adsorption measurements. The wide-angle XRD pattern of Ni-CMK-3 (20% Ni, Fig. 2b) shows three prominent peaks at 44.2°, 51.4° and 76.0° for Ni, indicating the characteristics of face-centered cubic (fcc) corresponding to (111), (200) and (220) planes of Ni crystals (JCPDS card no. 040850) [53]. The broad peak around 2θ = 22° corresponds to the reflection of amorphous carbon framework of Ni-CMK-3 composite. In addition, the peak around 2θ = 26° can be assigned to the (002) diffraction of graphite-like carbon from some catalytic graphitization of carbon by Ni nanoparticles. No characteristic peaks of nickel oxide were detected. The formation of Ni was realized through reduction reaction of Ni(II) nitrate during the heat treatment at high temperature under Ar/H2. The resulting fcc-Ni introduces magnetic property into the Ni-CMK-3 composite as will be discussed shortly. The final concentration of Ni in Ni-CMK-3, as measured by AAS after dissolving the sample in a mixture of HCl and HNO3 (3:1) at 50–60 °C was found to be 20.4%, which is very close to the nominal value of 20%. The N2 adsorption–desorption isotherms for the SBA-15, CMK-3 and Ni-CMK-3 are shown in Fig. 3a and the corresponding pore structure parameters, including specific surface areas (SBET), total pore volumes (Vt) and average pore-sizes (DBJH) are compiled in Table 1. All the samples exhibit type IV isotherms with H1 hysteresis loop (according to the IUPAC classification) which are typical for mesoporous materials with 2-D hexagonal ordered structure [49]. The sharp inflection of the SBA-15 isotherm in P/P0 ranging from 0.5 to 0.7 is characteristic of capillary condensation within uniform pores. The capillary condensation is very steep, resulting in a narrow pore-size distribution in the range of 5.5–8 nm centered at 7.0 nm (Fig. 3b). The average pore-size is 6.4 nm. The nitrogen uptake step for the carbon replica (CMK-3) is broader, and as a consequence, compared to SBA-15 shows a relatively broader pore-size distribution centered at 3.7 nm with an average pore-size of 4.5 nm. The mesoporous carbon sample shows considerably higher surface area and pore volume than the silica template (Table 1) which may be due to contribution from the microporosity within the carbon walls and the extra porosity due to the incomplete replication process. Since the lattice parameter (a0) of SBA-15 was determined to be 10.7 nm, its wall thickness is calculated to be approximately 4.3 nm, which is close to the pore diameter of CMK-3 (4.5 nm), although due to some shrinkage that inevitably occurs during the carbonization process and subsequent dissolution of the silica framework, a smaller pore diameter is expected for the carbon replica. As can be seen in Table 1, after Ni incorporation into CMK-3 (Ni-CMK-3), SBET and Vt decrease from 947 to 705 m 2 g −1 and from 1.24 to 0.87 cm3 g−1, respectively, but there is no significant change in DBJH (4.5 nm). A close examination of the specific surface areas (SBET) reveals a decrease of 25% for Ni-CMK-3 (705 m 2 g−1) with respect to CMK-3 (947 m2 g−1). This is close to the amount of Ni incorporated into the mesoporous carbon (20.4%, as determined by AAS). The difference of about 5% can be related to

800

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CMK-3 Ni-CMK-3 SBA-15

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Ni-CMK-3 CMK-3 SBA-15

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Pore size (nm) Fig. 3. (a) Nitrogen sorption isotherms and (b) BJH pore-size distribution of SBA-15, CMK-3 and Ni-CMK-3.

pore-blocking by the metal particles. The above results indicate that most of the nanometer-scale void spaces of the host mesoporous carbon are open in Ni-CMK-3 with 20% metal loading. The morphologies of the as-prepared samples were characterized by TEM. Fig. 4 shows the TEM images of SBA-15, CMK-3 and Ni-CMK-3. It can be seen that SBA-15 has a hexagonal array of uniform channels of about 7 nm in diameter (Fig. 4A) and CMK-3 almost completely retains the inverse replica of SBA-15 (Fig. 4B), exhibiting an ordered array of mesoporous tubes (white lines) separated by carbon rods (black lines). The carbon nanorods have a diameter of about 6.7 nm, which is in reasonable agreement with the average pore-size of the SBA-15 template obtained from N2 adsorption results. The TEM micrograph of the Ni-CMK-3 (Fig. 4C) exhibits the presence of well dispersed Ni nanoparticles on the surface of the carbon adsorbent. The micrograph reveals almost intactness of the host structure after performing the impregnation and reduction steps for deposition of the metal nanoparticles. The corresponding particle size histogram indicates a mean Ni particle size of 17 ± 3 nm (Fig. 4E). Some big agglomerates of the Ni particles are also observed. The corresponding selected-area electron diffraction pattern (Fig. 4D) is consistent with metallic Ni (fcc, a = 0.352 nm). Magnetization of Ni-CMK-3 was measured in a magnetic field of ±10 kOe at room temperature using AGFM. The magnetization

Table 1 Porosity characteristics of the mesoporous silica, mesoporous carbon and magnetic mesoporous carbon. Sample

SBET (m2 g−1)

Vt (cm3 g−1)

DBJH (nm)

SBA-15 CMK-3 Ni-CMK-3

664 947 705

0.76 1.24 0.87

6.4 4.5 4.5

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

N. Farzin Nejad et al. / Fuel Processing Technology xxx (2012) xxx–xxx

E

25

Frequency (%)

5

20 15 10 5 0 10

15

20

25

Particle size (nm) Fig. 4. TEM images of (A) SBA-15, (B) CMK-3 viewed along and perpendicular to the direction of the hexagonal pore arrangements, and (C) Ni-CMK-3 (20% Ni) at two magnifications. The selected-area electron diffraction pattern (D) and particle size histogram (E) of Ni nanoparticles.

curve (Fig. 5) exhibits a hysteresis loop, which means a weak ferromagnetic property. The saturation magnetization strength (MS) for the adsorbent with 20% Ni content is 13.8 emu g −1, which is lower than that of the bulk Ni (51.3 emu g−1) [54] mainly due to the nanosize effect and the presence of non-magnetic carbon as the main constituent of the composite. The coercivity (HC) and remanent magnetization (Mr) are 38.0 Oe and 2.2 emu g −1, respectively. Relatively low values of 16 12 8

coercive force and remanence magnetization indicated that the sample has soft ferromagnetic characteristics desirable for the application in adsorption and separation under an external magnetic field. The magnetic separability of the composites was tested by placing a magnet near the glass bottle containing the reagents as will be discussed in Section 3.4. The powder samples are easily attracted by the magnet and separated from the liquid medium, suggesting an easy separation process. The magnetic strength of the composite increases with increasing Ni content, but as the adsorption capacity of the adsorbent decreases proportional to the Ni loading, the composite with 20% metal content which afforded complete separation with the external magnet was used in this work.

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3.3. Adsorption of DBT onto Ni-CMK-3 and effect of different parameters

0 -4 -8 -12 -16 -10

-8

-6

-4

-2

0

2

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Magnetic field (kOe) Fig. 5. Magnetization curve of the Ni-CMK-3 measured at room temperature.

The effect of temperature on the adsorption of DBT on Ni-CMK-3 was investigated by adding 25 mg of the magnetic adsorbent into 10 mL of 1000 mg kg −1 DBT and shaking the mixture for 90 min at different temperatures of 10, 20, 30 and 40 °C in a water bath. The results indicated that the equilibrium adsorption amount increases with increasing temperature (Fig. 6), indicating that the adsorption process is endothermic. The increase in the amount of DBT adsorbed on the mesoporous carbon with temperature can be attributed to an

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

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45

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40 40

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30 32

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Time (min) Fig. 6. Influence of temperature in the range of 10–40 °C on the adsorption of DBT by Ni-CMK-3.

7

b

6

increased mobility of the adsorbate molecules in solution and within the sorbent porous structure overcoming the activation energy barrier. This results in the enhancement of the adsorption capacity of Ni-CMK-3 for the DBT molecules [25]. The effect of adsorbent dosage (m) on the amount of DBT adsorbed at equilibrium (qe, mg g−1), was investigated at 40 °C and DBT concentration (C0) of 1000 mg kg−1 and the results are shown in Fig. 7. As can be seen, the removal of DBT from 10 mL of solution (w=6.6×10−3 kg) increases by increasing m from 0.01 to 0.05 g, then remains almost unchanged from 0.05 to 0.08 g, and finally decreases at m>0.08 g. The increase in the amount of DBT adsorption with Ni-CMK-3 dosage can be attributed to the availability of greater surface area and more adsorption sites. At mb 0.05 g, the adsorbent surface becomes saturated with DBT and the residual DBT concentration in the solution is significant. Based on the results of this study 50 mg was chosen as the optimum amount of Ni-CMK-3 adsorbent for 10 mL solution of 1000 mg kg−1 DBT. The effect of shaking time on the amount of DBT adsorbed on Ni-CMK-3 was investigated in the range of 5–240 min at 40 °C using the optimum amount of adsorbent (50 mg) for 10 mL of 1000 mg kg −1 DBT and the results are shown in Fig. 8a. It seems that the adsorption process has a two-stage kinetic behavior; rapid initial adsorption followed by a second stage with a much lower adsorption rate. The results indicate that most of the DBT is removed within 30 min at which the adsorption amount (qt) is ~ 40 mg DBT/g of Ni-CMK-3, and almost levels off after this period, so that the DBT uptake remains almost unchanged with increasing the contact time, indicating that all adsorption sites have been saturated. This can be related to the large number of vacant macropore and mesopore sites that are available for adsorption of DBT during the initial stage of adsorption. During the later period of adsorption, the DBT molecules have to traverse farther and deeper into the micro-pores encountering much larger resistance, which results in slowing down the adsorption process [25]. We used 1 h of continuous shaking, at which a steady 40

30

20

10

0

0

20

40

60

80

100

120

140

Adsorbent (mg) Fig. 7. Effect of the amount of adsorbent on DBT adsorption capacity of Ni-CMK-3 at 40 °C.

5 4 3 2 1 0 0

40

80

120

160

200

240

Time (min) Fig. 8. (a) Effect of the shaking time on DBT adsorption capacity onto Ni-CMK-3 at 40 °C. The symbol and the line represent the experimental data points and the predicted pseudo-second-order model, respectively. The inset shows Weber–Morris intra-particle diffusion plot of qt versus t1/2. (b) Pseudo-second-order linear plot for the removal of DBT.

state approximation can be assumed and a quasi-equilibrium situation can be accepted. Accordingly, all the following experiments were conducted with a contact time of 1 h under vigorous shaking. The kinetic data of DBT adsorption have been tested by the pseudofirst-order and pseudo-second-order models using the following equations, respectively: lnðqe −qt Þ ¼ ln qe −k1 t

ð1Þ

t 1 1 ¼ − t qt k2 ðqe Þ2 qe

ð2Þ

where k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first- and pseudo-second-order rate constants. Other parameters were defined previously. The calculated parameters obtained by plotting ln(qe–qt) versus t for the pseudo-first-order model and t/qt versus t for the pseudo-second-order model, together with the corresponding correlation coefficients (R2) values are presented in Table 2. The results indicate that there is no linear correlation between ln(qe–qt) and t (R2 = 0.160), so that the kinetic of DBT adsorption onto Ni-CMK-3 cannot be described by the pseudo-first-order model. However, the data are very well fitted to the pseudo-second-order model (Fig. 8b) and the correlation coefficient is close to unity (R2 = 0.9988), indicating that the adsorption process can be described by this kinetic model. The adsorption kinetic data were further processed to explore the possibility of intra-particle diffusion using the Weber–Morris equation [55]: qt =kidt1/2 +c, where, kid is the intra-particle diffusion rate constant and c is the film thickness. A linear plot of qt versus t1/2 indicates that the adsorption process is only controlled only by the intra-particle diffusion. The deviation of the plot from linearity signifies that two or more steps influence the overall adsorption process. As can be seen from the inset of Fig. 8a, the plot of qt versus t1/2 is not linear over the whole time range, indicating that the adsorption is at least a two step process with

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

N. Farzin Nejad et al. / Fuel Processing Technology xxx (2012) xxx–xxx

7

Table 2 Pseudo-first-order and Pseudo-second-order kinetic model for adsorption of DBT on magnetic mesoporous carbon. Initial DBT concentration (mg kg−1)

Pseudo-first-order model

Pseudo-second-order model

k1 (min−1)

R2

Exp. qe (mg g−1)

Cal. qe (mg g−1)

k2 (g mg−1 min−1)

R2

1000

0.0069

0.160

40.0

41.0

0.0120

0.9988

the first step being the film diffusion. The second step can be attributed to pore diffusion. The overall kinetic survey suggests that the pseudosecond-order adsorption mechanism is predominant and that the rate of DBT adsorption process is controlled by more than one-step.

3.4. Adsorption isotherms and effect of initial DBT concentration The effect of initial DBT concentration in the range of 100 to 1500 mg kg −1 on the extent of its adsorption onto 5 g L −1 Ni-CMK-3 at 40 °C using contact time of 60 min is shown in Fig. 9a. As can be seen from this figure, the equilibrium adsorption amount is dependent 60

a

50

qe ¼ q max

40

KLCe 1 þ K L Ce

ð3Þ

where KL (kg mg −1) represents Langmuir constant that relates to the affinity of binding sites which describes the intensity of the adsorption process, and qmax is maximum adsorption capacity. The characteristic constants of the Langmuir model can be determined from the linearized form of Eq. (3):

30 20 10 0 0

250

500

750

1000

1250

1500

b 20 15 10 5

0

Ce 1 C ¼ þ e : qe qmax K L q max

200

400

600

800

1000

ln qe ¼ ln K F þ 4.0

1 ln C e n

ð5Þ

where KF and n are Freundlich constants indicative of adsorption capacity and adsorption intensity, respectively. Fig. 9c shows the Freundlich plot (lnqe verses lnCe) for the adsorption data, and the fitted results. As shown in Table 3, the linear regression analysis by the Freundlich isotherm shows a correlation coefficient (R2) of 0.978, indicating that the adsorption isotherm fitted slightly better with the Langmuir model (R2 = 0.989). The Temkin isotherm was also applied to the adsorption of DBT on Ni-CMK-3. This model is given by the following linearized form:

c

3.5

3.0

2.5

qe ¼ BT ln K T þ BT ln C e 3.0

ð4Þ

Fig. 9b shows the Langmuir plot (Ce/qe verses Ce) for the adsorption data of DBT on Ni-CMK-3 and the fitted results (solid line) by linear regression analysis. The Langmuir isotherm shows a good fit to the adsorption data with correlation coefficient (R2) of 0.989, supporting that the adsorption of DBT on Ni-CMK-3 follows this model. The values of qmax and KL were obtained from the slope and intercept of the linear plot, respectively, are presented in Table 3. The maximum adsorption capacity (qmax) was found to be 62.0 mg g−1 at 40 °C. The Freundlich isotherm is represented by an empirical model that describes heterogeneous adsorption and assumes that the adsorption energy decreases exponentially with surface coverage. This isotherm is given by the following linearized form:

25

0

on the initial DBT concentration and the equilibrium uptake has been increased with increase in the adsorbate concentration. The equilibrium adsorption of DBT increased from 10.69 to 55.27 mg g −1 adsorbent when the initial concentration of DBT increased from 100 to 1500 mg kg−1. The increase of DBT adsorption with respect to its initial concentration is a result of the increase in mass transfer driving force due to concentration gradient developed between the bulk solution and surface of the adsorbent [37]. To better elucidate the mechanism of DBT adsorption onto Ni-CMK-3, the equilibrium experimental adsorption data were fitted by three adsorption isotherms including Langmuir, Freundlich and Temkin models and the summary of parameters calculated from the fitting results are listed in Table 3. The equilibrium expression of the Langmuir model is:

3.5

4.0

4.5

5.0

5.5

6.0

6.5

ð6Þ

7.0

Fig. 9. Effect of initial concentration of DBT on adsorption (a). Experimental adsorption data fitted to Langmuir (b) and Freundlich (c) models.

where BT and KT are the Temkin constants. This model afforded somewhat lower correlation coefficient (R2 = 0.967) compared to the other isotherms. The results indicate that the Langmuir isotherm model provides the most satisfactory fit to the experimental data.

Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002

8

N. Farzin Nejad et al. / Fuel Processing Technology xxx (2012) xxx–xxx

Table 3 Summary of parameters calculated from fitting the results of adsorption isotherm of DBT on Ni-CMK-3 to different models. Langmuir

Freundlich

Temkin

qmax (mg g−1)

KL (kg mg−1)

R2

KF (mg g−1)(kg mg−1)1/n

n

R2

KT (kg mg−1)

BT (J mol−1)

R2

62.0

5.38 × 10−3

0.989

3.59

2.50

0.978

9.11 × 10−2

11.13

0.967

By using the Langmuir constant (KL), the Gibbs free energy change (ΔG 0) can be calculated according to the following equation [35]: 0

ΔG ¼ −RT ln K L :

ð7Þ

The Gibbs free energy is also related to the heat of adsorption (ΔH 0) and entropy change (ΔS 0) at a constant temperature (T) by the following equation: 0

0

0

ΔG ¼ ΔH −TΔS :

ð8Þ

The value of ΔG 0 was calculated to be − 18.0 kJ mol −1, indicating that the DBT adsorption by Ni-CMK-3 adsorbent was a spontaneous and favorable process. The adsorption of DBT onto Ni-CMK-3 is endothermic in nature, giving a positive value of ΔH 0. Therefore, ΔS 0 is positive, confirming a high preference of DBT molecules for the adsorbent surface. 3.5. Regeneration of the adsorbent For the industrial applications, the regeneration and subsequent recycling of the adsorbent are of vital importance. Removal of the sulfur compounds from the adsorbent was performed by two methods; heating the adsorbent and solvent extraction. The regeneration experiments were performed after saturating the adsorbent with DBT at the optimized conditions and separating the adsorbent with the magnet. In the heating method, regeneration of the spent adsorbent was performed at 350 °C for 2 h under Ar atmosphere. After the regeneration was completed, the adsorbent was mixed with a fresh DBT solution and the second adsorption cycle was performed. The results showed that 62% of the desulfurization capacity was recovered after the first regeneration cycle, which was decreased to 47% in the second cycle. In the solvent extraction method, regeneration was performed by adding the spent adsorbent to 10 mL of each extracting solvent (toluene, acetonitrile, methanol or ethanol) and the mixture was shaken at room temperature for 1 h. The results of adsorption study indicated that the desulfurization capacity of the regenerated adsorbent after the first regeneration varies in the following order: toluene (97%) ≫ acetonitrile ≅ methanol (36%) > ethanol (23%). Consequently, toluene was chosen as a suitable extractant for regeneration of the used adsorbent. The regeneration afforded 97, 94 and 80% of the initial adsorption capacity after the first three regeneration cycles, respectively. 4. Conclusions In this research, Ni-supported mesoporous carbon (Ni-CMK-3, 20% Ni) was synthesized by preparation of the SBA-15 template, followed by pore-filling, carbonization of the carbon precursor, removing of the silica template and deposition of Ni nanoparticles. The magnetic mesoporous carbon was used for adsorption of the aromatic sulfur compound (DBT). Adsorption of DBT onto Ni-CMK-3 was found to be suitable at 20% Ni loading, at which the adsorbent retained its mesoporosity and morphology. The adsorbent is easily attracted by a magnet and can be separated from the liquid medium which enables an easy separation process. The overall kinetic survey suggests that the pseudo-second-order adsorption mechanism is predominant and that the rate of DBT adsorption process is controlled by at least

two steps; the first step being the film diffusion and the second step can be attributed to pore diffusion. Langmuir isotherm best represented the equilibrium adsorption data. The value of ΔG0 was calculated to be −18.0 kJ mol−1, indicating that the DBT adsorption by Ni-CMK-3 is a spontaneous and favorable process. This, together with the endothermic nature of the process, gives a positive ΔS 0 confirming a high preference of DBT molecules for the adsorbent surface. The results show that Ni-CMK-3 is a highly efficient and promising adsorbent for separation of DBT from liquid samples. This adsorbent provides an easy separation process because it can be attracted by a magnet and separated from the medium. The spent adsorbent can be reused after regeneration by solvent extraction with toluene.

Acknowledgments This work was financially supported by the Research Council and Center of Excellence for Catalyst and Fuel Cell of the University of Isfahan, and Iranian Nanotechnology Initiative Council.

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Please cite this article as: N. Farzin Nejad, et al., Synthesis of magnetic mesoporous carbon and its application for adsorption of dibenzothiophene, Fuel Processing Technology (2012), http://dx.doi.org/10.1016/j.fuproc.2012.09.002