Preparation and characterization of octyl-modified ordered mesoporous carbon CMK-3 for phenol adsorption

Preparation and characterization of octyl-modified ordered mesoporous carbon CMK-3 for phenol adsorption

Microporous and Mesoporous Materials 121 (2009) 173–177 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homep...

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Microporous and Mesoporous Materials 121 (2009) 173–177

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Preparation and characterization of octyl-modified ordered mesoporous carbon CMK-3 for phenol adsorption Jianguo He, Kun Ma, Jun Jin, Zhengping Dong, Juanjuan Wang, Rong Li * College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China

a r t i c l e

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Article history: Received 19 November 2008 Received in revised form 20 January 2009 Accepted 24 January 2009 Available online 6 February 2009 Keywords: SBA-15 CMK-3 Phenol Adsorption Octyl

a b s t r a c t Mesoporous carbon CMK-3 was prepared by using SBA-15 silica mesoporous molecular sieves as hard template. Then, octyl modified mesoporous carbon molecular sieves C8-CMK-3 was prepared through chemical oxidation of CMK-3 by ammonium persulfate and H2SO4 and esterification reaction with 1-octanol in m-xylol. Fourier transform infrared spectra (FT-IR) showed the presence of carboxyl and octyl group on C8-CMK-3. Powder XRD and TEM images indicated that the pore structure of CMK-3 was almost not destroyed after modification. Nitrogen adsorption–desorption measurements showed that surface areas, pore size, and pore volume had evident change after CMK-3 was modified by octyl group. The phenol adsorption test showed that the adsorption performance of CMK-3 was significantly improved after it was modified with alkyl chain. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction In recent years, ordered mesoporous molecular sieves such as SBA-15 [1], MCM-48 [2], CMK-1 [3], CMK-3 [4] and CMK-5 [5] have attracted more and more attention owing to their particular properties, such as high surface areas, regular frameworks and narrow pore size distributions, etc. These materials have potential application in many fields, such as catalysis, adsorption, and biomedical engineering, etc. [6–10]. Mesoporous carbon molecular sieves can be prepared by using silica mesoporous molecular sieves as hard template. In 1999, Ryoo and co-workers synthesized mesoporous carbon CMK-1 using MCM-48 as hard template [3]. From then on, the door was opened for the synthesis of various ordered porous carbon molecular sieves. SBA-15 mesoporous molecular sieves possess tunable pore diameter, thicker walls and high hydrothermal stability. Mesoporous carbon CMK-3 has been synthesized using SBA-15 as hard template [4]. It was reported that CMK-3 can be used as adsorbents for L-histidine, Alkylphenol ethoxylate surfactants, Cytochrome C, Vitamin E, Biomaterials, and Fullerenes, etc. [11–16]. Later, researchers began to study how to modify the ordered mesoporous carbon surface for more application. For example, Ajayan Vinu functionalized CMK-3 with carboxyl groups through oxidation reaction using a solution of ammonium persulfate in H2SO4 for adsorbing proteins, and the modified order mesoporous carbon CMK-3 showed excellent adsorption capability [17]. * Corresponding author. Tel.: +86 931 891 2311; fax: +86 931 891 2582. E-mail address: [email protected] (R. Li). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.01.028

As it is well known, phenol is one of the harmful compounds for public health and environmental quality. Various physical and chemical processes have been adopted to remove phenolic compounds from wastewater, such as oxidation, photocatalytic, solvent extraction and adsorption [18–21]. Adsorption is considered as one of the most effective ways [22]. In this work, mesoporous carbon CMK-3 was prepared by using SBA-15 as hard template. Then, octyl modified mesoporous carbon molecular sieves C8-CMK-3 were prepared through chemical oxidation of CMK-3 by ammonium persulfate and H2SO4 and esterification reaction with 1-octanol in m-xylol. The resulting mesoporous carbon C8-CMK-3 showed excellent adsorption capability for phenol. 2. Experimental 2.1. Chemicals Tri-block copolymer P123 (EO20PO70EO20, EO = ethylene oxide, PO = propylene oxide, 5800) was obtained from Aldrich. TEOS (Si(OCH2CH4)4) was purchased from Sinopharm chemical Reagent Co. Ltd. Ammonium persulfate ((NH4)2S2O8), 1-octanol (C8H17OH), m-xylol (C6H4(CH3)2), and p-toluenesulfonic acid (CH3C6H4SO3H  H2O) were purchased from Tianjin Chemical Reagent Co. Ltd. All the chemicals were used as-received. 2.2. Preparation of silica template SBA-15 and CMK-3 SBA-15 was synthesized as reported by Zhao et al. [1]. In a typical synthesis, 2 g P123 was dissolved in 75 ml 2 M HCl solution

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In each adsorption experiment, 4 mg adsorbent was added in 8 ml phenol solution. The resulting mixture was continuously shaken in a shaking bath with a speed of 160 shakes/min at 293 K for 24 h until equilibrium was reached. The phenol concentration in the supernatant was analyzed with a UV spectrophotometer (UV1700, Shimadzu) with the wavelength at 270 nm. Prior to the analysis, centrifugation was used to avoid potential interference from suspended scattering particles in the UV analysis. The amount of phenol adsorbed qe (lmol/g) was calculated as

with stirring, followed by addition of 4 ml TEOS to the homogeneous solution(staring mole ratio: TEOS/P123/HCl/H2O = 1/0.019/ 8.4/233). This gel was stirred at 313 K for 24 h, and then crystallized at 373 K for 24 h under static condition. The resulting solid was filtered, washed, dried overnight at 373 K and calcined at 823 K in air for 6 h. Thus, SBA-15 was obtained. CMK-3 carbon was prepared according to the process described in the literature [4]. 1 g SBA-15 was added to 5 ml aqueous solution containing 1.25 g sucrose and 0.14 g H2SO4. The resulting mixture was heated in an oven at 373 K for 6 h and then 433 K for another 6 h. In order to obtain fully polymerized sucrose inside the pores of the SBA-15 template, 5 ml aqueous solution containing 0.8 g sucrose and 0.09 g H2SO4 were added again and the mixture was subjected to the thermal treatment described above one more time. Then, it was carbonized in an argon flow at 1173 K for 6 h with a heating rate of 5 K min1. Finally, the resulting solid was washed with 1 M NaOH solution (50 vol% ethanol-50 vol% H2O) twice to remove the silica template, filtered, washed with ethanol until pH = 7, and dried at 373 K for 4 h. Thus, mesoporous carbon CMK-3 was obtained.

where C0 (lmol/l) is the initial concentration of phenol solution, Ce (lmol/l) is the equilibrium concentration of phenol solution, V (l) is the volume of the solution and W (g) is the weight of the adsorbent. To determine Ce, the working curve of the UV adsorbency of the standard phenol solution with different known concentrations was first measured. Then, the adsorbency of the residual phenol solution was measured and Ce was calculated based on the working curve.

2.3. Chemical oxidation of CMK-3 and surface grafting

3. Results and discussion

CMK-3 carbon was carboxylated using a simple procedure 300 mg CMK-3 was immersed into 18 ml solution of 1.75 M ammonium persulfate ((NH4)2S2O8) and 2 M H2SO4 and then the mixture was stirred at 313 K for 24 h [17]. Carboxylated CMK-3 carbon is denoted as carboxyl-CMK-3. The CMK-3 with by carboxyl groups makes it possible to further expand its surface properties by grafting several other functional groups [23]. An esterification reaction with 1-octanol was chosen for this purpose. In a typical esterification process, 0.3 g carboxyl-CMK-3 was mixed with 3 ml 1-octanol, 0.124 g p-toluenesulfonic acid and 210 ml m-xylol in a flask. The reaction was carried out under stirring at 353 K for 4 h. In order to completely remove the physically adsorbed organic species, the obtained mesoporous carbon (denoted as C8-CMK-3) were washed with ethanol several times, filtered and dried under vacuum.

3.1. FT-IR

2.4. Characterizations Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet NEXUS 670 FT-IR spectrometer (Nicolet, USA) by the standard KBr disk method. Low angle X-ray diffraction (XRD) analyses were performed on a Rigaku D/Max-2400 diffractometer (Rigaku, Japan), using CuKa radiation over the range of 0.5–10°. Transmission electron microscopy (TEM) measurements were taken on a Hitachi-600 electron microscope, with an accelerating voltage of 100 kV. The surface morphology of the samples was observed by JSM6701F scanning electron microscope (Jeol, Japan). The nitrogen adsorption/desorption experiments were performed at 77 K in a Micromeritics ASAP 2010 (USA). The samples were degassed at 373 K overnight before the measurement.

qe ¼

ðC 0  C e Þv w

As shown in Fig. 1, compared with the FT-IR spectrum of CMK-3 (Fig. 1a), there were two peaks at 1580 and 1735 cm1 assigned to carbonyl ([email protected]) stretching vibration for carboxyl-CMK-3 (Fig. 1b). In Fig. 1c, the peaks appearing around 2920, 2853 and 1461 cm1 can be ascribed to the methylene (–CH2–) asymmetric and symmetric stretching, and methylene dC–H, respectively. The results are closely in agreement with published data [24]. The results indicated that octyl group was connected to the surface of CMK-3 successfully through a chemical bond. 3.2. XRD Fig. 2 shows the powder X-ray diffraction patterns (XRD) of SBA-15, CMK-3 and C8-CMK-3. It can be seen that all the samples show three diffraction peaks in the 2h range of 0.5–2°, which can be assigned to (1 0 0), (1 1 0) and (2 0 0) reflections. The result indicates that these samples have an ordered two dimensional (2D) hexagonal structure. Moreover, the XRD patterns of CMK-3 and C8-CMK-3 are similar to that of SBA-15, indicating that CMK-3 is

2.5. Adsorption performance of CMK-3 for phenol in aqueous solution The influence of pH and initial concentration of phenol solution on adsorption performance of CMK-3 for phenol was investigated. A series of phenol solutions with the same initial concentration and their pH ranging from 3 to 11 were prepared by adding different amount of HCl or NaOH. Then, a series of phenol solutions with the same pH and their concentration ranging from 100 to 1200 lmol/l were prepared by dissolving different amount of phenol in the deionized water.

Fig. 1. FTIR spectra of CMK-3 (a), carboxylated CMK-3 (b) and C8-CMK-3 (c).

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length and 300–500 nm in diameter (Fig. 4a). The CMK-3 and C8CMK-3 almost maintain a rodlike shape with only a few deformations, and their diameter and length are almost the same as that of SBA-15 (Fig. 4b and c). Compared with CMK-3, the size of C8-CMK3 is not decreased in both microscopic and macroscopic scale. This suggests the shape of particles is not changed under mechanical stirring even for a long time. 3.5. Nitrogen adsorption–desorption measurements

Fig. 2. Powder XRD of SBA-15 (a), CMK-3 (b) and C8-CMK-3 (c).

a true replica of the mesoporous silica SBA-15 and the oxidation process almost does not damage the structure of CMK-3. 3.3. TEM Fig. 3 shows the TEM images of SBA-15, CMK-3 and C8-CMK-3. It can be seen that SBA-15 has a hexagonal array of uniform channels of about 6 nm in diameter (Fig. 3a) and CMK-3 is exactly an inverse replica of SBA-15 (Fig. 3b and c). More information can be obtained from Fig. 3b that carbon nanorod of CMK-3 is about 6 nm in diameter and the center of adjacent rods is about 10 nm. The carbon nanorods are interconnected by spacers, which are constituted by the carbon that filled the channel-interconnecting micropores within the SBA-15 wall. As shown in Fig. 3c, C8-CMK-3 retains the uniformity of the mesopores from the original inorganic wall structure of the parent CMK-3 carbon. The result indicates that the modification happens in the pore of CMK-3 and the structure of CMK-3 is not destroyed. 3.4. SEM The surface morphology of the obtained SBA-15, CMK-3 and C8CMK-3 was observed by SEM, as shown in Fig. 4. The image reveals that the calcined SBA-15 is rodlike particle of 700–800 nm in

Fig. 5 shows the nitrogen adsorption isotherms of SBA-15, CMK3 and C8-CMK-3. Pore structure parameters of materials are shown in Table 1. All isotherms are of type IV, according to the IUPAC classification, and exhibit a H1 hysteresis loop. For SBA-15, capillary condensation occurs at around P/P0 = 0.6–0.8, whereas condensation occurs at about P/P0 = 0.35–0.6 in both case of the CMK-3 and C8-CMK-3. The CMK-3 and C8-CMK-3 obtained from SBA-15 possess pores with diameter of about 3.8 and 3.4 nm respectively, which are smaller than that of SBA-15 (5.5 nm). However, their BJH surface areas, 1346 and 1043 m2/g respectively, were significantly higher than that of SBA-15 (727 m2/g). The results indicate that the CMK-3 and C8-CMK-3 were more suitable for adsorbent compared with the SBA-15. From Table 1, it can be seen that the pore volume decreases with alky chain group loading, indicating that octyl is bonded inside the mesoporous channels of CMK-3 successfully [25]. 3.6. Adsorption isotherms The effect of pH on phenol adsorption by CMK-3 and C8-CMK-3 was examined at an initial phenol concentration of 600 lmol/l. In the pH range of 3.0–11.0, as shown in Fig. 6, the amount of phenol adsorbed passed through a maximum and the optimum pH for phenol adsorption was 4.0. Fig. 7 shows the adsorption isotherms of phenol on C8-CMK-3 in comparison with CMK-3. 4 mg adsorbent and 8 ml phenol solution of different initial concentration (pH 4) are used to determine the maximum amount of the adsorbed phenol. The maximum adsorption amount of phenol over the C8-CMK-3 was approximately 720 lmol g1, which was markedly higher than that of CMK-3 (620 lmol g1). The result indicates that the adsorptive amount almost decreased along with increasing pH. Why did it happen? The explanation is as follows: It is well known that the solubility of phenol in water increases with increasing pH. There is phenol in water

Fig. 3. TEM images of SBA-15 (a), CMK-3 (b) and C8-CMK-3 (c).

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Fig. 5. Nitrogen adsorption–desorption isotherms of SBA-15, CMK-3 and C8-CMK-3.

Table 1 Pore structure parameters of SBA-15, CMK-3 and C8-CMK-3. Sample

SBET (m2 g1)

SBJH (m2 g1)

VBJH (cm3 g1)

DBJH (nm)

SBA-15 CMK-3 C8-CMK-3

679.0 1147.7 746.3

727.4 1346.2 1043.6

1.0 1.3 0.9

5.5 3.8 3.4

Fig. 6. The amounts of adsorbed phenol as a function of pH in the 6  104 M solutions.

Fig. 4. SEM images of SBA-15 (a), CMK-3 (b) and C8-CMK-3 (c).

in the form of phenolate at high pH values. The solubility of phenolate in water is higher than of phenol, therefore it is not easily adsorptive by absorbent. Since the solubility of phenol is very small and easily be adsorption at low pH values. The result also indicates that the CMK-3 modified by alkyl chain increases its hydrophobic

ability. So, the hydrophobic ability of C8-CMK-3 was higher than CMK-3. Based on rule that like is like, phenol is hydrophobic, it should be easier to adsorb on C8-CMK-3. Because of hydrogen bonding between hydroxyl groups of phenols and CMK-3 surface, it is possible to make phenol into the pore. Compared with CMK-3, the hydrophobic interaction between phenol molecule and the surface of C8-CMK-3 is higher and favors the adsorption of phenol molecules on the mesopores of C8-CMK-3. The experimental results indicate that increasing hydrophobic force of CMK-3 surface is very helpful to achieve excellent phenol adsorption performance.

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Cx-CMK-3 can be used to remove harmful organic compounds in aqueous solution. References

Fig. 7. Adsorption isotherms of phenol over the CMK-3 and C8-CMK-3 at 25 °C.

4. Conclusion This study described the preparation of CMK-3 using hexagonal mesoporous silica SBA-15 as template and the modification of CMK-3 by octyl through esterification in m-xylol. The structural order and textural properties of all the materials are characterized by XRD, TEM, SEM and nitrogen adsorption. The characteristic results indicated that the modification was successful and the pore structure of CMK-3 was almost not changed. The phenol absorption experiment showed that the adsorption performance of CMK-3 was significantly improved after it was modified with alkyl chain.

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